synthesis & characterization of the cubic form of tantalum nitride
TRANSCRIPT
1
SYNTHESIS AND
CHARACTERIZATION
OF THE CUBIC FORM OF
TANTALUM NITRIDE
Student: Angel Alfonso Lopez
Project Supervisor: Mats Johnsson
Department of Materials and Environmental chemistry
(Stockholm University)
2
INDEX 1 – Abstract ............................................................. Page 4
2 – Nitrides
2.1 - Tantalum Nitride (TaN) ............................ Page 5
2.2 - Titanium nitride (TiN) .............................. Page 9
2.3 - Silicon nitride (Si3N4) .............................. Page 12
3 – Carbides
3.1 - Tantalum Carbide (TaC) .......................... Page 17
3.2 Titanium Carbide (TiC) ............................ Page 19
3.3 - Tungsten Carbide (WC) ........................... Page 22
4 – Solid Solutions ................................................ Page 26
5 – Grinding Mill .................................................... Page 28
6 – Spark Plasma Sintering (SPS) ....................... Page 31 7 – Archimedes Principle …………………………. Page 34 8 – X-Ray Diffraction ………………………………. Page 36 9 – Scanning Electron Microscopy (SEM) …….. Page 43 10 – Experimental procedure
10.1 – Mixture preparation ………………… Page 48
10.2 - Powder mixing ……………………….. Page 50
3
10.3 - Spark Plasma Sintering (SPS) ……… Page 51
10.4 - Powder X-Ray analysis ……………… Page 55
10.5 – Theoretical Density calculation …….. Page 55
10.6 – Archimedes method ………………… Page 57
11 – Results and discussion 11.1 SPS sintering results ………………….. Page 59
11.2 SPS sintering discussion ……………… Page 62
11.3 - Powder X-Ray diffraction results …. Page 63
11.4 - X-Ray results discussion ……………. Page 81
11.5 - Density measurements results ……… Page 82
11.6 - Density measurements discussion ….. Page 85
11.7 - SEM images …………………………. Page 86
11.8 - SEM discussion …………………….. Page 103
12 – Conclusions …………………………………. Page 105
13 – Bibliography …………………………………. Page 107
4
1- Abstract
The aim of this study is to optimize a method to turn the typical lattice of TaN at
normal conditions (hexagonal) into its cubic counterpart using Spark Plasma Sintering (SPS).
The effect of some parameters such as synthesis temperature and pressure on the cubic
conversion is studied, as well as the role that plays the addition of some dopants (TaC, TiN) in
low quantities. On the other hand, some physical parameters such as density and compactness
were assessed for each of the samples, in an attempt to relate them to the presence of cubic
and hexagonal lattice. To carry out the study, different characterization techniques such as X-
Ray diffraction were used to determine the hexagonal and cubic present lattices, as well as
density measurements. Some SEM images were also obtained providing information about
the formation of the lattices, their physical properties and the grain sizes of the different
samples.
2 - Nitrides
Nitrides are compounds consisting of Nitrogen and another element which is less
electronegative than Nitrogen, in which N has a -3 valence in the bonding. Since they cover a
large variety of compounds, their properties, applications and synthesis methods are different
for each compound. Apart from the ordinary nitrides, there are other compounds formed with
nitrogen ions such as pernitrides (N2-2
) and azides (N3-)
Uses of Nitrides
Nowadays, nitrides are most widely used for the following applications:
1) As abrasives ( Cubic Boron Nitride)
2) As lubricants ( Hexagonal Boron Nitride)
3) As refractory materials ( like Silicon Nitride )
4) For the fabrication of cutting tools ( Silicon Nitride)
5) For metallic coatings ( Titanium Nitride )
6) For hydrogen storage, because of their high reactivity with H2 (Lithium Nitride),
creating the species LiNH2 when the hydrogen is stored
5
2.1 - Tantalum Nitride (TaN)
Properties
Tantalum Nitride (TaN) is an inorganic compound consisting of crystals whose tonality
is within a range from brown to black and has a Co-Sn structure type. In fact, its colour is
dependent on the lattice arrangement, which can be either hexagonal or cubic when the moles
ratio is 1:1.
Although naturally TaN presents itself in an hexagonal lattice, the cubic phase can also
be formed when applied high pressures and temperatures to the synthesis process. TaN is also
insoluble in water and possesses a high melting point (around 3360 ºC) as well as the fact that
it is a refractory material capable of retaining its strength at high temperatures.
Some of its main physical properties are presented in the following table
Molecular Weight (g/mol) 195
Density for the Hexagonal form (g/cm3) 13.7
Color Brown to Black
Melting Point 3360ºC
Appearance Crystalline Solid
Synthesis
Various methods can be used to synthesize solid TaN
1) Chemical Vapor Deposition (CVD)
CVD is a set of chemical reactions that turn gaseous molecules (called
precursors) into a solid material, creating either a powder mixture or a thin film on the surface
of the substrate. When choosing the precursor different considerations must be taken into
account, such as its bonds strength (which will affect the operating temperature and the purity
of the product), its thermal stability and reactivity. For example, tantalum hydrides (TaH,
TaH2) and tantalum halides (TaCl4, TaCl5) could be used as a precursor.
6
Energy can be applied as thermal energy (from furnaces and lamps) or as photovoltaic energy
(from UV and laser) to the reactor. Apart from the solid, also gaseous products are formed,
which means they must be treated carefully if hazardous. In the process, the precursor is
densified by a thermically enhanced nitrating gas made up of oxygen, nitrogen and ammonia.
A metal nitride layer is subsequently formed and densified thanks to the nitrating gas action,
even though sometimes a metal layer must be deposited over the substrate prior to the gas
exposure. To create semiconductor devices such as electrodes, by depositing dielectric layers
between the tantalum nitride layers hence creating a stack.
Solidification process in CVD
2) Atomic layer deposition (ALD)
ALD is a technique to create thin films out of gas reactants by a chemical process.
This process allows the creation of metal nitrides layers with ease. Mostly 2 different
precursors are used. The reaction takes place between the precursors and the substrate surface
(one precursor at a time) in a sequential way. It differs from CVD in the fact that ALD breaks
the CVD into two half reactions, meaning that each precursor is used in a different half
reaction. Since the precursor amount in each reaction cycle is set, the reaction is self-limiting
which makes it easy to control and so is the film thickness, which can be as low as 0.1Å per
monolayer. The amount of precursor is controlled by means of a purged gas (nitrogen or
argon) which removes the remaining gas from the former reaction stage.
7
The process consists of four major steps which are repeated subsequently
1) Application of the 1st precursor
2) Purge of the precursors and the gaseous products of the 1st stage
3) Application of the 2nd
precursor
4) Purge of the precursors and the gaseous products of the 2nd
stage
The main advantage of ALD over CVD is its ease to control film thickness only by setting the
number of cycles, and also that the mixture needn’t to be homogeneous which simplifies the
process because it allows larger contact area and higher reproducibility. The only drawback to
this technique is its slowliness since only a layer is deposited at a time.
3) Magnetron Sputtering deposition technique.
This is a type of physical vapor deposition (PVD). In this technique material from a
source (which is called target) is resputtered or reejected into the substrate where the thin
films are created by means of a sputtering gas which provides the atoms of the target with
the necessary energy. The sputtering gas has a wide energy distribution which goes up to
10eV. Some of the sputtering gas atoms can cause the atoms from the target to be ionized
and head for the substrate and impact energetically to create the thin film. This effect is
called ―resputtering‖ and is the main effect which produces the film formation.
The sputtering gas must be inert (Argon for example) so as not to react with the
substrate. Its energy can be controlled by setting the pressure conditions of the sputtering
gas. The closer the atomic weights of the sputtering gas and that of the substrate are, the
better the momentum is transferred to the atoms on the substrate. That’s why neon is more
likely to be used when spluttering light elements and krypton is usually used for heavier
elements. There are a lot of parameters to take into account in this process which make it
complex. However, its complexity also allows a high control over features such as film
growth and microstructure if the parameters are well controlled. All its features make it
suitable for fields such as electronics, (especially when depositing thin films of various
materials to create integrated circuits) or coating.
8
The main advantage over other techniques is that it can even sputter high
melting point substrates if the sputtering gas beam is energetic enough, the fact that the
film composition is very close to the former substrate, and better adhesion of the films. Its
main disadvantage is that the sputtered atoms can’t be directed exactly to the substrate,
which may lead to chamber contamination.
Magnetron Sputtering deposition technique.
Applications
Tantalum Nitride (TaN) has its main applications in the electronics field. Some of its most
usual applications are listed below:
1) Barrier Layers
TaN is used in the electronics industry to create thin barriers which are placed between
copper and silicon-based components such as interlevel dielectrics (ILDs) made of silicon
dioxide, in integrated circuits. TaN based layers are effective when it comes to avoiding
copper or silicon diffusion between the layers. This non-diffusive properties are kept even at
high electric fields and voltages when TaN is used. This barrier also presents a good adhesion
between the metallic copper layer and the silicon-based ILDs.
9
2) Resistors
A resistor is a device that produces a voltage between two electronic components. This
voltage is directly proportional to electric current according to Ohm’s law.
Together with nickel-chrome, tantalum nitride is used to manufacture resistors
for precission applications due to the reduction of thermal and electronic noise that this
material provides in the shape of thin films, being the most widely used material for that
purpose. Unlike its nickel-chrome counterparts, tantalum nitride films are resistant to
humidity even at high application potentials. This occurs thanks to a tantalum oxide layer
formed espontaneously on the surface which prevents it from corroding, process known as
passivation.
2.2 - Titanium nitride (TiN)
Properties
It’s a ceramic and hard material stable at room temperature (oxidizes at 800ºC) and reactive to
strong acids. It also has good IR properties in the range of gold (Au) which give it a golden
colour. Most commonly it presents the Na-Cl type crystal structure when the stechiometry is
1:1 that is a cubic lattice. Some other stechiometrys also present stable structures for titanium
nitride.Its good properties make it suitable for applications such as coating or superinsulators
among others.
Some other of its properties are
Vickers hardness = 18-21 GPa
Thermal conductivity = 19.1W/(m* ºC)Modulus of elasticity 251 GPa
Modulus of elasticity = 251GPa
Thermal expansion coefficient = 9.35*10-6
K-1
Superconducting transition temperature = 5.6 K
10
Synthesis
TiN films are usually synthesized by physical vapor deposition (PVD)
techniques such as sputtering, electron beam heating or cathodic deposition as well as by
chemical vapor deposition (CVD). Titanium reacts with highly energetic nytrogen in vacuum
conditions to create titanium nitride. As explained, PVD is mostly used when TiN is
combined with higher melting point materials like stainless steel or to create titanium alloys.
TiN can also be obtained by packing powdered titanium by compression in a nitrogen
atmosphere (nitrogen or ammonia) at high temperature (1200ºC) . The heat is provided by the
exothermal reaction between titanium and nitrogen, having as a product a compact and hard
material. TiN coatings can also be obtained by thermal spraying.
Applications
Electronics
Thin films of TiN are used for semiconductors. They’re used in coper based
chips as a conductive barrier placed between the silicon parts and the metallic ones. Its role is
to block copper diffusion and allow electrons to flow through at the same time, acting as a
metal diffusion blocker. It’s also used to improve transistor performances. In fact, when
combined with gate dielectrics such as HfSiO posseses better performance than pure SiO2
providing them with a better threshold voltage and a lower leakage
Coatings
TiN is used to avoid corrosion, specially on tools such as drills milling cutters
and even improving their lifetime at least trice. It’s also used to coat another metals such as
chromium or nickel on plumbing and hardware applications. Since it’s not toxic, can also be
applied to medical tools such as scalpels or orthopedic saw blades which provide the
necessary cutting sharpness for medical operations. In fact, TiN can also be used to make up
prothesis.It is also used for jewellery and decorative motifs because of its golden colour.
11
As a constituent in steel making
Titanium is added intentionally to some steel alloys to form TiN within the steel
alloy. It’s produced at high temperatures in the form of micrometric cubic particles and has a
low solubility in steel. The titanium helps to stabilise the steel by hindering possible
corrosion, since titanium has a higher affinity for carbon, oxygen and nitrogen rather than
chromium, thus producing stable carbides, oxides and nitrides in the steel formation process.
All these species could have formed detrimental compounds if they had not reacted with
titanium, which could lead to reduced mechanical properties. However, this kind of steels are
prone to cracks formation after welding.
12
2.3 - Silicon nitride (Si3N4)
Properties
Silicon Nitride is a hard ceramic material which is resistant over a broad range of
temperatures. It also has a low thermal expansion coefficient, moderate thermal conductivity,
a high elastic modullus and quite a high fracture toughness (unlike most ceramics). All these
properties provide Si3N4 with the capabilities of supporting high loads succesfully and with
excellent thermal shock resistance, which make it suitable for applications such as
manufacturing devices which operate at high temperatures (turbines, car engines ..) as well as
cutting tools and metal working. Silicon nitride films are also used in the electronics industry.
Synthesis
There are three different routes to obtain Silicon Nitride (Si3N4)
1) By reaction between silicon and nitrogen at high temperatures (1300 —1400ºC)
3 Si(s) + 2 N2(g) → Si3N4(s)
2) By diimide synthesis:
SiCl4 + 6 NH3 (g) → Si (NH)2(s) + 4 NH4Cl(s) at 0 °C
3 Si(NH)2(s) → Si3N4(s) + N2(g) + 3 H2(g) at 1000 °C
3) By carbothermal reduction in nitrogen atmosphere at 1400–1450 °C:
3 SiO2(s) + 6 C(s) + 2 N2(g) → Si3N4(s) + 6 CO(g)
13
The simple nitridation of silicon was formerly the main large scale production method.
However, silicates are also formed and contaminate the process.
The diimide synthesis produces amorphous silicon nitride which needs further annealing with
nitrogen at high temperatures to make it crystalline. The carbothermal reduction method and
presents high cost and S3N4 purity performances
Si3N4 films can be formed by chemical vapor deposition CVD,being one of its variants,
Plasma enhanced chemical vapor deposition (PECVD) the most effective method.The process
complies with the following reactions
3 SiH4(g) + 4 NH3(g) → Si3N4(s) + 12 H2(g)
3 SiCl4(g) + 4 NH3(g) → Si3N4(s) + 12 HCl(g)
3 SiCl2H2(g) + 4 NH3(g) → Si3N4(s) + 6 HCl(g) + 6 H2(g)
If what is needed is to place films between semiconductor substrates (made of silicon), also
Low pressure chemical vapour deposition (LPCVD) can be used. In this process, deposition
parameters are important since if not controlled correctly, it could lead to stress appearance,
due to the fact that Si2N4 and pure silicon don’t have the same lattice parameters.
Applications
The main drawback when using Si3N4 is not its technical performance but rather
its price. Since it has become cheaper, new horizons have opened to this promising material,
being the most important
1) Car industry
One of the major applications of sintered silicon nitride is in automobile
industry as a material for engine parts. Those include, in Diesel engines, glowplugs for faster
start-up; precombustion chambers (swirl chambers) for lower emissions, faster start-up and
lower noise; turbocharger for reduced engine lag and emissions.
14
In spark-ignition engines, silicon nitride is used for rocker arm pads for lower
wear, turbocharger for lower inertia and less engine lag, and in exhaust gas control valves for
increased acceleration. As examples of production levels, there is an estimated more than
300,000 sintered silicon nitride turbochargers made annually.
2) Bearings
Different types of bearings
Si3N4 bearings can be either pure or ceramic hybrids altogether with steel. The
silicon nitride’s good shock resistance makes it perfect for such an application. Since Silicon
Nitride is harder than metal, when such a material is used for bearings the contact with the
bearing track is radically reduced. This leads to lower friction (80% lower than when its metal
counterpart is used), longer lifetime, higher operation temperature and to a reduction in
corrosion.
3) High-temperature material
Due to its excellent behaviour at high temperatures, shown by its resistance to high
thermal shocks and thermal gradients, this material can play an important role when used in
devices which operate at high temperatures such as rocket engines.
15
4) Metal working
Silicon Nitride is perfect to play roles such as milling, grinding and piercing of metals.
In fact, is used for cutting and abrasive tools because of its excellent thermal and mechanical
performance. S3N4 can cut through cast iron and high steel at a speed 25 times higher than
when conventional materials are used. In fact, Silicon Nitride has had a great importance to
assemble well-finished metal products.
5) Electronics
S3N4 is used in electronics as both an insulator and a chemical barrier for
integrated circuits and to electrically isolate circuit parts. As a chemical barrier is better than
SiO when blocking species such as water and sodium ions which can cause corrosion and
instability. It can also be used as dielectric between polysilicon layers in capacitors.
3 - Carbides
A carbide is a binary compound made of carbon and a more electropositive
element. Carbides can be classified into different groups, and their properties, synthesis and
applications are dependent on the group to which they belong to. The simplest classification
is:
1) Salt-Like carbides: They are compounds formed by carbon and the compounds from
groups I,II A and III B (except Actinium).depending on the lattice arrangement, these
carbides can be represented by the anions C4-
, C34-
,C22-
2) Covalent carbides: These are silicon (SiC) and boron carbides (B4C). Both materials
are hard and refractory, which makes them important industrially speaking.
16
3) Insterstitial carbides: These group includes the transition metals from the groups IV,
V and VI B (except chromium). They all are refractory and possess metallic properties.
Some of them (Titanium carbide, tantalum carbide and tungsten carbide) are used to coat
cutting tools.
4) Molecular carbides: These are metal complexes containing carbon. There are
molecules with carbon fragments or most commonly clusters in a metal-centered body.
One example of a molecular carbide is the compound Fe5C(CO)15
Fe5C(CO)15 molecular carbide
.
Uses of Carbides
1) Abrasives and cutting tools ( Silicon carbide)
2) Steelmaking (Calcium Carbide)
3) Coatings on Iron and Steels ( Titanium Carbide)
4) High temperature applications (Tantalum Carbide)
5) Machine tools and military purposes (Tungsten Carbide)
17
3.1 - Tantalum Carbide (TaC)
Properties
TaC is a hard and refractoryceramic material (with a hardness around 9 – 10 mohs).
It’s one of the hardest materials, and only diamond has a higher hardness.This makes its use
suitable for cutting tools and for the manufacturing of ceramic—metallic materials among
others. It appears to be a brownish powder and its crystal arrangement is cubic and equal to
that of NaCl. In fact, is the stechiometrically binary compound (1:1) with the highest melting
point, only excedeed by its counterpart TaC0.89, with a melting point of 4000ºC. It’s also a
flammable compound when it’s presented in the form of a dust, because it can form
flammable gases.
Some other of its physical properties are exposed in the following table:
Molecular weight 192.96 g/mol
Density 13.9 g/cm3
Melting Point 3880ºC
Boiling Point 5500ºC
Appearance Solid brownish powder
Solubility Not soluble in water
Crystal Structure Cubic
Risks None
Synthesis
There are various routes to synthesize TaC:
1) By mixing tantalum pentoxide (Ta2O5) with tantalite [(Fe, Mn) (Ta,Nb)2O6] in the p
resencepresence of carbon ( C ) and sodium carbonate (Na2CO3) at 1500ºC, leaving 6% of
djdjdjdunreacted carbon.
18
2) By means of a simple reaction among metallic magnesium (Mg), sodium carbonate
(Na2CO3) and tantalum pentachloride (TaCl5) at 600ºC, thus obtaining good tantalum
carbide with good thermal stability and oxidation resistance when used below 450ºC
in an air environment.
3) By the reaction of metallic Tantalum powder (Ta) with carbon (C) at 2000ºC in a
graphite furnace. This method leaves around 6% of unreacted carbon.
4) Carbidization of tantalum powder and tantalum oxides in the presence of carbon and a
hydrogen stream at 1600ºC
Applications
1) Oxidation and thermal protection coating:
TaC layers can be deposited on materials such as carbon, refractory metals and alloys to
protect them from thermal impacts and oxidation with a very good performance. The
advantages of this carbide for this use are the possibility to coat the material at a low
temperature (10% of the melting point of the carbide), the lack of porosity in the coating and
its adaptation to difficult shapes and surfaces. TaC offers great protection in very corrosive
and high temperature environments.
2) Cutting tools
When TaC is used to manufacture cutting tools it provides very good mechanical
properties such as high hardness, low tool deformation even at high temperatures, good wear
resistance and low brittleness. All of these properties are paramount when cutting at high
speeds. In fact, tantalum carbide coated tools cut three times faster than high-speed steels.
Tantalum carbide is often used with a mixture of other carbides to obtain the necessary
mechanical properties. It’s specially used to manufacture high-speed machine tool bits,
mining equipment, cutting tools and teeth for construction.
19
3) Cermet material (metallic-ceramic) composites
This composite can be created with the ceramic material Tantalum Carbide. The main
reason is to combine its properties with those of metals, such as malleability. For this
application the most interesting properties provided by TaN are its resistance to high
temperatures and its good wear resistance.
4) Tantalum carbide/graphite composite material
This hybride is one of the hardest materials ever manufactured by man, and has a
meltng point of 3738ºC. This material combines the best properties of each material, being the
best properties of one of the material the worst of the other, thus neutralising them. In other
words, carbon has very good ablation and thermal shock resistance, but erodes easily.
Tantalum carbide resists erosion perfectly, but its ablation and thermal shock resistance is
poor. In the output composite materials, all of these properties have been improved when
compared to the former materials. This improvement opens a new horizon of applications
where excellent thermal and mechanical properties are required, such as bearings, turbines or
rocket nozzles.
3.2 Titanium Carbide (TiC)
Properties
Titanium carbide is a refractory ceramic material, with a hardness around 9.5 in the
Mohs scale. It’s a crystalline black powder with a cubic centered structure like NaCl. This
material is resistant to wear and corrosion, and preserves its hardness at high temperatures.
It’s not flammable in the solid state, although grinded dust can be flammable and also cause
skin and eyes irritation. It’s also a chemically stable compound which only dissolves in acids
or alkalis. All this properties make it useful for applications such as cermets assembly, cutting
tools and alloying.
20
Some of its properties are listed below:
Molecular weight 59.9 g/mol
Density 4.93 g/cm3
Melting Point 3160 ºC
Boiling Point 4820 ºC
Appearance Black powder
Solubility Not soluble in water
Crystal Structure Cubic
Risks Can cause irritation
Synthesis
Different methods can be used to synthesise TiC, among which the following were remarked:
1) Self-propagating High-temperature Synthesis (SHS)
This synthesis takes place inside of a reactor and can be easily reproduced industrially. In this
process, titanium powder (Ti), carbon black (C), and an inert diluent such as titanium carbide
(TiC) form the starting mixture. The TiC diluent plays an important role: it reduces the
combustion rate, decreases the necessary synthesis temperature around 200K and enables the
obtention of TiC in a powder form. Different gases are formed during the synthesis due to the
impurities present in the starting powders, such as TiO2 layers that may have been formed,
which need to be purged to avoid product contamination.
2) Mechanochemical process
By means of mechanical grinding, the reaction between black carbon and titanium in powder
form can take place at ambient temperature. This process is relatively cheap and fast, and
provides ultrafine powder that is perfect to manufacture cutting tools due to its improved
fracture toughness.
21
3) Chemical Vapor Deposition (CVD)
In this synthesis, the used precursors are TiCl4 gaseous molecules. These solidify to form
solid TiC thin films over a quartz substrate after the reaction with toluene (the carbon source)
takes place. The depositing temperature must be set over 900ºC and nickel must be present to
play an important role as a catalyst. Otherwise, only amorphous carbon would be deposited.
The reaction to obtain TiC is the following:
2
º900.),(
3564 10877 ClHClTiCCHHCTiClCTcatNi
Applications
1) Tool bits and cutting tools
Titanium carbide is widely used for cutting tools and tool bits because of its good
mechanical properties. When it is added to hard metals it provides excellent wear resistance
and hardness (since it is one of the hardest carbides) and oxidation stability, being usually
combined with a mixture of other carbides to improve the mechanical properties. Titanium
carbide is used for cutting tools such as grooves, cutting teeth and milling slots. This material
is also used for surgery tools as coating.
2) Titanium carbide – nickel-cobalt Cermets (ceramic-metallic composite)
These cermets can be prepared with a wide range of metals (nickel, cobalt, chromium,
molybdenum, stainless steel …). Their main characteristic is their high toughness and
hardness although they’re dependent on the material base. And so then, nickel based cermets
with titanium carbide are thermally and abrasive wear resistant, making them suitable for
engineering applications such as turbine blades and electrical brushes. Chromium-Titanium
carbide cermets possess high temperature erosion resistance, which makes them suitable for
metal coating.
22
3) Coatings
By coating different metals such as steel and iron with Titanium Carbide their
wear and corrosion resistances are improved, as well as their hardness. It also acts as a heat
barrier which allows titanium carbide to be used for applications in which the thermal shock is
important such as aerospacial components, ball bearings, compressor blades and metal cutting
tools coatings. Its high hardness also makes it suitable for scratch-proof coating applications
such as clinical dentistry and watches.
3.3 - Tungsten Carbide (WC)
Properties
Tungsten carbide is an inorganic compound made up of Tungsten (W) and
Carbon (C) in a molar ratio 1:1. It has got the appearance of a grey powder which is usually
pressurized into different shapes for industrial purposes. WC has very high strength as shown
by its high stiffness (three times stiffer than steel) and hardness. Only few compounds among
(SiC or diamond among others) can polish it. Its impact resistance performance is good even
at low temperatures and also possesses high thermal and electrical conductivities. Its hardness
is maintained up to 760ºC and has got a low coefficient of friction. This material is also more
corrosion resistant than noble metals and more wear resistant than steel as well as resistant to
heat and oxidation. At high temperatures it decomposes to tungsten (W) and carbon (C) and is
oxidized over 600ºC. The most normal crystal structure is the hexagonal (α-WC) although at
high temperatures a cubic structure can be formed (β-WC)
Some of the WC properties are the following:
Molecular weight 195.9 g/mol
Density 15.8 g/cm3
Melting Point 2870ºC
Boiling Point 6000ºC
Appearance Grey Solid
Solubility Insoluble in water
Crystal Structure Hexagonal
Risks Can cause irritation, inhalation may cause
lung damage (fibrosis)
23
Synthesis
Tungsten carbide can be prepared in many different ways.
1) Reaction of tungsten metal and carbon at high temperatures:
)()()( º20001400 sWCsCsW C
2) A fluid bed process which entails the reaction of tungsten metal or blue tungsten oxide with
carbon oxides and hydrogen at high temperatures in a patented fluid bed
OHsWCgHgCOCOsW C
2
º1200900
22 )()()(/)(
3) By Chemical vapour deposition (CVD) WC can be synthesized in two different reactions:
3.1) Tungsten hexachloride reacts with methane in the presence of a reducing agent (H2)
HClsWCgHCHsWCl C6)()()(
º670
246
3.2) Tungsten hexafluoride reacts with methanol in the presence of H2
OHHFsWCgHOHCHsWF C
2
º350
236 6)()()(
24
Applications
Tungsten Carbide is used in a wide range of applications. Those most important
are described below
1) Machine tools
Tungsten carbide is used in machinery parts, cutting tools, drills and abrasives.
When manufacturing these tools, the used tungsten carbide has been bound specially with
cobalt, which is known as cemented carbide and belongs to the group of cermet materials
(ceramic – metal matrices). For example, WC is used to manufacture both huge and tiny drills
with a wide range of applications. Tungsten Carbide is the hardest material besides diamond
and the second most efficient when it comes to cutting. In fact, a tungsten-copper-steel alloy
is used to bind millimetric diamond pieces that would not be of further industrial use as a
grinding tool, thus increasing the diamond lifetime twice. When applied to rotatory cutting
tools, it cuts five times faster and also has a much longer lifetime than a steel-made one.
Tungsten Carbide tools can also be used at higher temperatures and produce better finish to
products than their steel counterparts. WC drills hold a great significance when exploiting
earth resources such as gas and petroleum because of its great hardness, which allows piercing
deep layers of soil made up of different materials.
Generally speaking, tungsten Carbide cutting tools are important when operating
on hard materials such as steel or large material quantities, where most of the tools would
wear out. This wearing out resistance is what makes it suitable for applications such as
bearings and machinery parts.
2) Cemented Carbide synthesis
Cemented carbide is a cermet material (a composite made of a ceramic and a
metal) obtained by binding the ceramic material (WC) with a metallic compound, mostly with
cobalt (Co) and sometimes with nickel (Ni).
25
This material combines the hardness and temperature resistance of the tungsten carbide with
the malleability of the metal which allows stress application without breaking. In other words,
the metal addition reduces brittleness. This composite can be easily synthesized and also
provides magnificent properties. During the sintering, tungsten carbide is soluble in cobalt at
high temperatures, and the used cobalt-based liquid precursor also spreads out properly
throughout tungsten carbide. This leads to a dense and pore free product cermet which
possesses high hardness, toughness and strength at the same time.
3) Military
Both its high density and hardness make it a perfect material (especially when
uranium is not available) for military machines parts, piercing ammunition and projectiles due
to its high penetration power. In fact, tungsten carbide is especially used in the cores of
ammunition made up of two different parts in which the two of them come off on impact. WC
is also widely used in the nuclear weapons industry due to its capability to reflect neutrons,
which allows nuclear chain reactions.
4) Sports
The tungsten carbide hardness allows its use in poles which are meant to hit
hard surfaces and poles used in trekking and skiing simulation in hard surfaces. This carbide
allows the traction and duration that such poles need. This material is even used in tires studs
to provide a better adherence on ice, being its performance better than that of steel studs.
5) Jewellery
Tungsten carbide’s remarkable properties make it perfect for jewellery. This
material provides jewellery with contrast and bright mirror-like finishes with an hematite-like
colour, apart from improving the jewels´ resistance to marks and scratches. However, due to
its high hardness they can barely be removed. In fact, only specialised tools can cut through it
without jeopardizing the hand’s wellbeing.
26
4 - Solid Solutions
Introduction
A solid solution is a mixture of two solid compounds that form a single
crystalline solid. There are various methods to produce solid solutions. Therefore, they can be
formed by melting two different solids, mixing them and cooling down the mixture or by
dissolving both solutes in a common solvent and letting it dry up. Vapor deposition over a
substrate also leads to solid solutions in the shape of a thin film.
Solid solutions are most likely to be produced when the solutes are close to each
other in the periodic table (especially for metals). In fact, solids and liquids behave alike in
solubility. That is, depending on their chemical properties and crystalline structure, they have
different degrees of mutual solubility which determine the positions of the mixed atoms in the
final crystal.
Types of solutions
These type of solutions can be formed either by a substitutional mechanism (in
which the atoms of one species replace some from the other) or interstitial (in which the
atoms occupy free gaps in the solvent lattice). The solubility of the species is dependent on
their concentrations in the mixture and in the solids nature. Different properties of both
compounds regarding chemical composition and crystal lattice must be taken into account to
determine whether a solid solution will be formed, some of which are listed below.
1) Both must have similar radii with less than 15% difference.
2) Their valency must be alike
3) The electronegativities of the compounds must be close
4) Their lattice arrangement must be similar.
27
When the solid solution conditions are not met, the solid solution is reversed and two separate
phases make their appearance. This process is called exsolution.
Examples
Some examples of such solutions are minerals created naturally at high
temperatures and pressures like the solid solution composed of two olivines, fayalite
(Fe2SiO4) and forsterite (Mg2SiO4). Both species form mutual solid solutions with a wide
range of different iron and magnesium concentration which provides the mixture with slightly
different physical properties than that of the starting materials.
Therefore, solid solutions can provide if used with the proper concentrations
better properties than that of the former materials which leads to potential commercial and
industrial applications such as metal alloying. Most of the alloys such as brass (Co-Zn) are
considered solid solutions formed by a substitutional mechanism, whereas in steel carbon is
interstitially forming a solid solution in an iron matrix.
Solid solutions are also a great asset in the semiconductors industry because
they allow the selection of intermediate electrical properties between those of the former
materials which make them suitable for optical and electronical applications. An example is
the band gap of the solid solution formed by InAs and GaAs which can be set somewhere
between the values for InAs (0.36eV) and GaAs (1.4eV).
28
5 - Grinding Mill
A grinding mill is a mechanical device that breaks the fed solid into smaller particles, thus
reducing the particle size of the solid which was introduced. In the mill, the machine’s
mechanical forces try to surpass those of the internal bonds of the matter itself, in order to
eliminate such bonds. After this, the grain size and their shapes are totally modified, as well as
their arrangement.
The grinding purpose may be to extract valuable constituents in different ore materials or to
prepare the raw materials for an industrial process, such as grinding coal for furnaces or clay
for pottery. In engineering, it helps to increase the surface area of a solid and also to create
products with the desired grain size.Various kinds of grinding mills can be found. In the very
beginning they were operated by manual rotation (like mortars), animal force, and the action
of wind and water. Later on electrical ones were introduced.
The main types of grinding mills are listed below according to their operation principle:
Ball mill
It consists of a tilt or horizontal cylinder which rotates around its horizontal axis, filled up
with balls, either metallic or made of stone. By impact, friction and abrasion these balls grind
the fed solid material to a certain degree of grain size. These ball mills are used in industry
(for example to manufacture cement) in a large scale. There can be also found little versions
used in research laboratories which are used to grind sample material in order to ensure the
quality of the sample.
Laboratory ball mill
29
Rod mill
They’re also usually referred to as ―slitting mills‖. In this type of mills a rotating vessel helps
to create the necessary mechanical friction between steel rods and the solid particles. Its rods
are usually made of iron or some other metal.
Laboratory Rod Mill
SAG mill
SAG or Semi-Autogenous Grinding are comprised by mills that use for grinding a mixture of
both steel balls and big rocks. Its ball charge ranges from 6 to 15%. In this mill, both the balls
and the rocks are thrown down at the solid and this action causes the break down into smaleer
pieces of the solid samples by impact and compression while the drum is rotating. These
drums are usually large in diameter and short in length. There are lifts installed on the inside
to lift the solid particles up and around the mill, after which it falls off the plates boosting
particle movement throughout the mill. Some of its industrial applications are the grinding of
precious metals such as gold, platinum and silver among others.
30
Principle of SAG Mill operation SAG Mill
Autogenous mill
In this system large rocks are thrown down thus breaking down the solid
particles by impact and compression. In fact, it is very similar to the SAG system, apart from
the fact that it does not use metal balls. It can be used when contamination by metals is to be
avoided.
High pressure grinding rolls mill
The solid meant to be grinded is put between two rollers located very close to
each other. These rolls rotate in opposite directions thus crushing the solid material.In fact,
while it is spinning, the solid fits into a tiny gap in between, and it reaches pressures so high
that are capable of breaking it down into finer particles, even fracturing the material at the
grain size level.
Operation diagram of a high pressure grinding rolls mill
31
6 - Spark Plasma Sintering (SPS)
Definition
SPS (Spark Plasma Sintering) is a sintering process that can assemble compact
samples from ceramics and metal powders at a different range of temperatures and pressures.
The technique is especially interesting due to its capability of doing so at low temperatures. It
is yet to be discovered which are the mechanisms taking place in the densification and grain
growth processes as no direct observations of a plasma have been made. In this process
uniaxial pressure is applied while heating. In order to heat, a pulsed direct current goes
through the electrically conducting plates and sometimes also through the sample, unlike
other heating systems. This means that the sample is heated from both the inside and the
outside.
SPS Sample Holder
Mechanism
When operating, the SPS applies pulsed direct current. When the voltage is
applied to the powdered material the energy is transferred and dispersed homogenously all
over the powder, thus presenting highly energy efficient sintering conditions. This method
focuses the energy on the intergranular bondings which leads to synthesis improvements
when compared to other methods such as hot-press sintering. As explained, because of the
spark discharges between the particles of the material, localized and high temperatures are
achieved. This leads to both vaporization and melting of the powder which causes plastic
transformations of the material.
32
In the end, a dense tablet of over 99% compactness is obtained. As a matter of
fact, since only the surface of the particles is heated a better control over the grain growth is
held, and a rapid and precise sintering is possible.
SPS instrumentation
Features
What makes this process special is the fact that fast heating rates in short periods of
time can be used to produce quite dense samples. There are three main factors which
contribute to this purpose:
1) The mechanical pressure: It helps the powder to diffuse therefore avoiding porosity
2) The heat transfer: The equipment presents incredible heat transfer efficiency from
the plates to the sample. In fact, the plates act as a heat source themselves.
3) The use of direct current: The presence of an electrical field generates spark
discharges and plasma appearance within the powder. This helps to dispose of
adsorbed substances such as CO2 and H2O in the first place. Afterwards, both the lack
of contaminants and the spark discharges boost material diffusion all over the sample
increasing its density.
33
However, since this is a rapid densification process the grain growth is difficult to
control, and that is the reason why the conditions must be set appropriately. In fact, conditions
like holding time, applied pressure, heating rate and initial and final temperature influence the
grain growth and the mechanical properties of the sample such as brittleness and fracture
toughness.
Laboratory SPS sintering equipment
Main Advantages
The advantages offered by SPS over other sintering methods are:
• Tight control over temperature, pressure and cooling down.
• Fast and uniform synthesis
• Obtention of highly dense tablets with low porosity
• Efficient method, which leads to low costs.
• Easy to operate.
• Elimination of present contamination
• Low grain growth
• Low effect of the sintering on the material microstructure.
Besides, it also allows the attachment of some accessories such as automation systems and
robotic interfaces.
34
7 - Archimedes Principle
This principle allows the volume determination of an object when it is immersed
in a liquid media. Therefore it can be used to determine densities.
Such as shown by the density formula, the density is the relation between a mass
and the volume it occupies ( = m/V). The mass can be easily weighed by a set of scales,
whereas the volume determination cannot be performed accurately most of the times,
especially when the objects to be measured present irregular shapes. It is in such cases when
the Archimedes principle becomes useful.
The volume of an object or a liquid can be easily calculated applying the
principle of buoyancy force, which simply states that an object in a liquid is subjected to
forces from all directions, caused by hydrostatic pressure. However, the value of the vertical
forces caused by the liquid, which point towards both directions (up and down) do not usually
cancel each other out. The reason is that the gravitational force of the water must also be
taken into account in the vertical axis, being directly proportional to the depth at which the
object is located in water.
Vertical forces caused by hydrostatic pressure in an immersed object
35
In fact, the resulting force (Buoyancy force) in the vertical axis (F2 – F1) is
proportional to the gravitational force applied by the mass of liquid displaced to the object as
follows:
gVFgVF loB
or
llB
Where: Vl = Volume of liquid displaced by the object.
Vo = Volume of the immersed object.
Eventually, since the volume of the object is the same as that of displaced water
(Vl ) the value of the object’s volume can be easily worked out from the former formula. Once
both values (weight and volume) are known the density value can be easily assessed.
36
8 - X-Ray Diffraction
Introduction
In this technique, X rays interact with matter thus preceding interferences (which can
either be positive or negative). The interaction of different wavelength rays are plotted and
later used for analysis. The software measures all the atom positions in the crystal. Once the
cell parameters are known it’s possible to determine the matter structure and the
corresponding angles.
X-ray diffractometry are physical techniques used for the identification of substances,
and for other types of analysis, principally for crystalline materials in the solid state. In these
techniques, a monochromatic beam of X-rays is directed onto a polycrystalline (powder)
specimen, producing a diffraction pattern that is recorded on film or with a diffractometer.
This X-ray pattern is a fundamental and unique property resulting from the atomic
arrangement of the diffracting substance. Different substances have different atomic
arrangements or crystal structures, and hence no two chemically distinct substances give
identical diffraction patterns. Identification may be made by comparing the pattern of the
unknown substance with patterns of known substances.
.
In modern age, X-ray diffractography is undoubtedly one of the largely used
techniques because the experimental measurement is relatively short in time and allows
finding out the compounds and phases present in an unknown sample and also all sort of data
regarding its structure and even the processes the phases have gone through in the past. It
must be taken into account that even though in powder diffraction the material is not forming
a simple crystal in its own right, the analysis is still possible since this powder is still capable
of diffraction thanks to the degree of crystallinity it possesses.
37
Lattice geometry
The atoms in crystalline substances are arranged in a symmetrical three-dimensional
pattern: some atomic arrangement is repeated by the symmetry of the crystal along straight
lines throughout the crystal. The smallest group of atoms which has the symmetry of the
entire pattern is called the unit cell. The traces of the various lattice planes (normal to the
drawing) are indicated by heavy lines. To identify the planes, one must count the number of
planes crossed from one lattice point to the next along a, then repeat the procedure along b
and c are known as the Miller indices of that set of planes, and assignment of indices to each
line is called indexing. The spacings d between the planes are related to the Miller indices and
the unit cell dimensions.
In crystals of the cubic system (such as cubic TaN), the crystallographic axes are
normal to each other and have the same length, a = b = c; and spacing d is given by Bragg’s
law:
38
Bragg’s law:
Sindn 2
Where λ is the wavelength of the incident X-Rays.
The angle between the two spacing is θ.
2θ is the angle between impact and detection line.
Thus the conditions for x-ray reflection are very restrictive because these is only one
angle θ at which the X-rays of a given wavelength are reflected by a particular set of atomic
planes of spacing d.
Characteristics of powder patterns
Many materials are not available in the form of large single crystals, and moreover it
is impractical to obtain all the X-ray reflections from single crystals for identification
purposes.
If the sample does not already exist in polycrystalline form, it may be pulverized.
When a fine grained powder consisting of thousands of small, randomly oriented crystallites
is exposed to the X-ray beam, all the possible reflections from the various sets of atomic
planes can occur simultaneously.
Pieces of information can be derived from the experimental pattern:
(1) The 2θ value from which the d-spacing can be calculated;
(2) The absolute intensity, from which relative intensities can be calculated;
(3) The peak width;
(4) The form of the background.
The complexity of the pattern is determined primarily by the symmetry of the
substance rather than by its chemical composition.
39
If the crystallites do not have a completely random orientation, the line shapes and
relative intensities will change accordingly. Comparison of the random and oriented X-ray
patterns shows the degree of orientation in the sample. Since the two types of patterns are so
different, the method is ideally suited to distinguish between crystalline and amorphous
substances and to determine the degree of crystallinity of substances between the two
extremes.
There are also many smaller changes in the X-ray pattern which may reveal important
information. In substitutional solid solutions, for example, atoms of different elements may
substitute for one another and occupy the same relative positions as in the pure metals. The
substitutions of solute atoms occur on the same lattice sites occupied by the solvent atoms, but
are randomly distributed. If the atoms are of different size, the average unit cell size will
change accordingly. In simple cases, it is possible to determine the chemical composition of
intermediate members by measuring the unit cell dimensions because there is often a nearly
linear relationship between the two. In interstitial solid solutions, atoms are added to the
empty spaces in the structure and there are little, if any, changes in the dimensions.
Diffraction databases
There are a number of databases available for X-ray diffraction work. The majority of these
databases are designed and maintained for the single-crystal community rather than for the
powder community. However, some Powder Diffraction File (PDF) are calculated from single
crystal data of the type contained in the other databases.
The Powder Diffraction Files (PDF) are collections of single-phase x-ray powder diffraction
patterns in the form of tables of the interplanar spacings (d) and relative intensities
characteristic of the compound. These databases have proven their usefulness in a wide range
of applications because every crystalline material gives, at least in principle, a unique x-ray
diffraction pattern, study of diffraction patterns from unknown phases offers a powerful
means of qualitative identification by comparing an X-ray pattern from the material to be
analyzed with a file of single phase reference patterns.
40
The ability to recognize a reference pattern in an unknown material strongly depends
on the quality of the d’s and I’s in both the reference material and the unknown sample. One
of the major problems in the identification of materials by comparison of an experimental
pattern with reference patterns is the variability in the quality of the data. As the quality of
both reference and experimental patterns improves, the problem of pattern recognition
becomes easier.
Instrumentation
There are many types of powder diffractometer available ranging from simple
laboratory instruments to versatile and complex instruments using a synchrotron source.
Completely automated equipment for X-ray analysis is available. Most laboratory instruments
consist of a high-voltage generator which provides stabilized voltage for the X-ray tube, so
that the X-ray source intensity varies by less than 1%. X-rays are produced using a
molybdenum source.
Electronic circuits use an X-ray detector to convert the diffracted X-ray photons to
measurable voltage pulses, and to record the diffraction data. The number of elements useful
for X-ray tube targets is limited to a few, of which tungsten is the most commonly used.
During the tests, either the X-ray tube or the detector move, depending on the settings, thus
covering all the 2θ range
X-ray diffractometer
41
Optical Instrumentation in a X-Ray diffractometer
Signal Instrumentation in a X-Ray diffractometer
42
The Rietveld Refinement
This analysis technique is used to analyze the presence of different phases in
powder X-Ray diffractograms. In fact, powder diffraction is used for the analysis of the great
majority of materials since they cannot be synthesized in the form of big crystals. However,
one of its main drawbacks is the overlapping of data, which makes difficult a proper
determination of the material’s structure.
These difficulties can be overcome by using the Rietveld refinement, which
indeed separates the overlapping data, therefore making it possible to determine the structure
accurately. Its accuracy relies on the fact that it analyses the overall diffractogram as a whole,
and takes into account the height, widths and positions of the reflections by means of a least
squares approach. This approach tries to reproduce the data profile mathematically and as
approximately as possible.
Ever since the application of this technique to analyze powder diffraction huge
improvements like never before have been made in the reliability of the so commonly
overlapped data. This method is so trustworthy that nowadays structures can be determined
almost as well by powder diffraction as they are in its single crystal counterpart. Some other
of its applications are the determination of different components in mixturesl, also known as
phase analysis and therefore find some industrial uses in fields such as the oil or cement
industry.
Figure showing a Rietveld calculated profile (in blue), its divergence with the diffractogram
data (black line) and the determined structure (red lattice)
43
9 - Scanning Electron Microscopy (SEM)
Definition
The Scanning Electron Microscope technique is an instrument that allows the study of
organic and inorganic materials, providing morphological information. It’s used to observe
surfaces and analyze their relief, texture, grain sizes, grain shapes and chemical composition
(EDS) from biological and mineral simples. That is why it is a technique with a wide range of
applications in different fields concerning science and technology.
It allows the carrying out of qualitative and quantitative analysis by using the Energy
Dispersive X-ray Spectroscopy (EDS) technique. Results regarding the atomic mass
percentage and percentages from the different elements present in the sample are obtained.
Theoretical basis
In SEM, the image appears when the electrons beam goes down through the
column until the sample. A scanning generator is responsible for the beam movement. When
the electrons come into contact with the sample, some signals are emitted, which are then
picked up by their corresponding detectors (each signal has its own detectors). The detector
turns the received signal into an electronic one. Afterwards the latter is sent out to a Cathode
Ray Tube (CRT), which allows either the formation of the image or the chemical analysis.
Interaction Incident Beam – SEM sample
When the beam comes into contact with the sample, interactions between the
electrons and the sample atoms take place. From this interaction signals such as secondary
electrons, retrodispersed electrons, Auger electrons and X-rays are obtained, each of which is
captured by a different detector. For example, the secondary electrons detector detects signals
at low voltages (50eV) because those signals are emitted due to ionizations caused by
inelastic interactions.
44
Scanning Electron Microscope Signals
Main characteristics
The SEM’s main characteristics are its high resolution (~100 Å), its great field’s
depth which provides 3D images and the sample’s preparation simplicity. This equipment also
allows images obtention in a wide range of operation pressures (from vacuum to high pressure
conditions). Good images can also be obtained at low potentials, which sometimes can spare
sample pretreatments.
SEM’s Instrumentation
.
A Scanning Electron Microscope consists of different parts:
An optical-electronical unit, which generates the electrons beam.
A sample holder, with different movement degrees.
A signal detection unit with an attached amplification system.
An images display system
A cathode Ray Tube (CRT)
A vacuum system, a refrigeration system and an electrical power system, similar to
those of MET
A photographic, magnetic and video recording system
An image processing system (optional)
Different types of detectors depending on what to detect (secondary electrons, back-
scattered electrons, X-Rays, Auger electrons, transmitted Electrodes and light
cathodoluminescence)
45
Applications
The Scanning Electron Microscope allows the main following applications:
1. Observation at a high magnification rate
The image resolution is much more higher than that of its optical counterpart, since electrons (
with lower wavelength) rather than light are used.
2. Fractographic studies:
Due to its great field depth, fracture surfaces can be observed at high magnifications. .
3. Chemical analyses of small areas, such as intermetallic phases, precipitates, pollutant
particles …
4. Morphological and analytical surface characterization of a wide range of materials
5. Diffusion, segregation, quality analysis and irregularity studies processes.
Energy Dispersive X-ray Spectroscopy (EDS).
In EDS the microanalysis is carried out by measuring the energy, intensities and
signals distribution obtained from X-ray signals generated by the electrons beam. This allows
to find out the material’s chemical composition from areas down to 1mm2. Nevertheless, to
carry out the analysis, the signals intensities must be compared with that of a pattern, which
requires the sample’s surface to be flat and well polished.
In the EDS analyzer different analyses are carried out, which are later on plotted
in histograms and graphs that show the chemical elements distribution in the sample. The
EDS spectrum is obtained by using a software which collects photons per minute emitted by
the sample and classifies them according to energy. In the spectrum Energy in KeV (X axis)
versus intensity (Y axis) is plotted. Then the software automatically identifies and analyzes
the elements both quantitatively and qualitatively from the peaks of the histogram.
46
Energy Dispersive X-Ray spectrogram
Comparison between SEM and Optical microscope
The optical and the electronic microscope are almost identical. Both allow to
magnify details invisible to the naked eye. The main difference is the illumination source.
Therefore, the Optical Microscope uses a light beam whose wavelength is within the visible
range, whereas the SEM emits an electrons beam of very short wavelength which allows a
higher resolution to be obtained.
The main advantages that the electronic microscope has over its optical counterpart are:
• Higher field depth: The field depth is the capability of focusing on two different points at a
different height. This allows the analysis of fracture surfaces at high magnification, which
can’t be done using optical microscopy.
• Higher image resolution: The resolution boundary in an optical microscope is located
around a wavelength of 2000 Å because of the visible light nature.
47
In the electronic microscope the electrons possess a wavelength lower than 0.5 Å, which
makes it possible to reach magnifications up to 800.000x. However, due to instrumental
parameters, it only reaches up to 75.000x.
• Energy Dispersive X-ray Spectroscopy (EDS): Unlike the optical microscope, the
electronical microscope has also an EDS detector which allows to carry out chemical analyses
of the sample both qualitatively and quantitatively.
Scanning Electron Microscope Optical Microscope
48
10 – Experimental procedure
10.1 – Mixture preparation
In order to transform the hexagonal form of TaN and also to stabilize the cubic
form, solid mixtures with little amounts of similar compounds that already show a cubic form
lattice (known as solid solutions) must be carried out.
According to this, two different types of mixtures were prepared by mixing TaN
(hexagonal) with little amounts of TaC (cubic) or TiN (cubic).
1) TaN + TaC TaCxN1-x
2) TaN + TiN Ta1-x TixN
These mixtures were prepared by using different molar rates of the compounds (x = 0.05, x =
0.03). The necessary amounts that needed to be weighted are the following:
A) Mixture of TaCxN1-x with molar ratio x = 0.05:
If 30g of TaN are needed in the mixture, so as to have enough powder for preparing
different samples:
The moles in 30g TaN: TaNofmolmolg
gn 154.0
/95.194
30
If this amount entails 95% in moles of the sample (1-x), the remaining 5% is due to the
presence of TaC. This quantity is:
TaCofmolmol
n 310*1.895.0
05,0*154.0
49
The weight in 8.1 *10-3
mol TaC:
mol
gTaCPm 95.192)( TaCofg
mol
gmolm 564.195.192*10*1.8 3
B) Mixture of Ta1-x TixN with molar ratio x = 0.05
If 30g of TaN are needed in the mixture, so as to have enough powder for preparing
different samples:
The moles in 30g TaN: TaNofmolmolg
gn 154.0
/95.194
30
If this amount entails 95% of all moles in the sample (1-x), the remaining 5% is due to the
presence of TiN. This quantity is:
TiNofmolmol
n 310*1.895.0
05.0*154.0
The weight in 0.154 mol TiN:
mol
gTaCPm 867.61)( TiNofg
mol
gmolm 501.0867.61*10*1.8 3
50
10.2- Powder mixing
Once known which amount to take from each of the compounds, the mixtures
can be prepared. One example of prepared mixtures is that corresponding to Ta1-xTixN with x
= 0.05, prepared by weighing the following amounts:
m (TaN) = 30,0074 g
m (TiN) = 0,5018 g
Afterwards, the mixture was dispersed in 2-Propanol (addition of 75g) and
stirred up in a grinding mill during 30min. at a speed of 300 rpm. To help the mixing of the
compounds, balls made up of ZrO2 were added during the grinding process.
Once this is over, the suspension was dried in an oven at a temperature of around 80°C
overnight. The ZrO2 balls must also be cleaned up in the mill in water and dried before used
again.
Lab weighing scales Lab oven used to dry the solid solution
51
10.3 - Spark Plasma Sintering (SPS)
This process was run to turn the powder into a compact solid state.
1) Sample preparation
The sample has to be in a little container that is able to resist high pressures and
temperatures and does not allow the sample to leak. The following method fulfils these
objectives:
- A 39 mm carbon layer was assembled in the shape of a hollow cylinder and
introduced within the die, a graphite-based cylindrical container which had a
12mm hole in the shape of a cylinder. Afterwards, a tiny graphite cap was
put on the bottom of the cylinder. Moreover, 4g of the mixture were poured
within the die and another tiny graphite cap put at the top of the powder.
- After that, both parts of the hole are covered with graphite punches with
pressurize to make the powder more compact. Finally, an insulating surface
is attached to the outer part of the die with some threads. An image showing
the final assembly is displayed:
SPS Sample Holder
52
2) SPS sintering
The sample was introduced within the experiment chamber and fixed from the
top to the bottom with the help of some carbon plates. The experiment was run in vacuum
conditions. Different samples were run for each mixture, with different concentrations,
pressure and temperature conditions for each one. The starting temperature is set at 600ºC
because at lower temperatures it would be easier to control some of the SPS parameters. This
is because at lower temperatures the difference of the obtained signal’s wavelength from the
sample (due to the radiation emission at different temperatures) is lower than at higher
temperatures and thus more difficult to control by the SPS’s detector.
The following parameters were set on the SPS:
- Starting temperature: 600°C
- Heating rate: 100°C/ min.
- Final temperatures: 1550ºC, or 1600°C, or 1650ºC, or 1700°C, or 1750 °C or 1800°C
- The final temperatures were kept for 3 min, after which the samples were cooled down until
1500 °C and later on the equipment was turned off.
- Operation pressure: Either 50 or 75 MPa
When the final temperatures are reached, the sintering process is kept at these
temperatures for 3min., after which the sintering is complete and the temperature is slowly
decreased until 1500°C in order to reduce the building up of thermal tensions that leads to
brittleness in the sample. Afterwards the equipment can be safely disconnected and let cool
down. A graph plotting the change in the operating temperature for some final temperatures is
displayed:
53
SPS sintering conditions
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time (min.)
Tem
pera
ture
(°C
)
Final T = 1600°C
Final T = 1700°C
Final T = 1800°C
Heating ratio = 100°C/min
Graph plotting the temperature sintering conditions
Once the experiment has been run, the different parameters have been plotted in
the computer software. The broad peaks that are drawn by the displacement parameter
correspond to the sample sintering. The sample must be held into the chamber until it cools
down to approximately 500°C.
3) Sample conditioning for Powder X-Ray Measurements
For X-ray characterization, only the material corresponding to the sample can be
present, thus being necessary to remove the graphite layer from it. Therefore the samples were
polished and sawed so as to use only a part of it and leave the rest intact for further tests and
measurements. Eventually, the fragment of the solid sample which was to be used for the
upcoming X-ray test was turned into powder for that purpose by smashing it with a
sledgehammer and stored safely afterwards. Once the powder is obtained, it must be grinded
down to the micrometric scale using a mortar and spread out over a thin silicon plate which
does not reflect the X-ray beams. Now the X-ray test can be carried out.
54
Laboratory polishing equipment
Laboratory Saw blade
Laboratory Sledgehammer and mortar
55
10.4 - Powder X-Ray analysis
Once the 2θ range of measurement was set from 20 to 120 and the test run for
30min. approximately. When the test is over, an X-ray diffractogram is displayed. After
selecting the most important peaks, these must be compared with the database to carry out a
so called ―phase analysis‖. This process will provide the phases present in the material and
further characterization.
All the samples were analyzed via X-Ray diffraction to study mainly the
presence of both cubic and hexagonal lattice in the sample. To do so the different peaks from
the diffractogram were classified as belonging to either the cubic or the hexagonal form by
comparing them when TaN cubic and hexagonal databases. Once the peaks were identified,
the percentages could be found accurately from the distribution of the peaks by carrying out
Rietveld Analysis, which takes into account the whole diffractogram. The results from the
Rietveld Analysis are presented in the tables of the next chapter.
10.5 – Theoretical density calculation
The theoretical density values have been looked up in different articles. Since different
density values were found for the same compound, it was decided to calculate an average
value of them all for the different compounds.
Hexagonal Tantalum Nitride (TaN)
Title 1 of its Authors Density (g/cm3) Average (g/cm
3)
Ta N, eine neue
Hochdruckform von
Tantalnitrid
Brauer, G.P 14.31
14.31 Die Nitride des
Tantals Zapp, K.H. 14.29
An X-ray study of
the tantalum-nitrogen
system
Schoenberg, N. 14.34
56
Cubic Tantalum Nitride (TaN)
Title 1 of its
Authors
Density
(g/cm3)
Average
(g/cm3)
Das kubische Tantalmononitrid(B1-
Typ) und seine Mischbarkeit mit den
isotypen Uebergangsmetallnitriden
und -carbiden
Gatterer, J. 15.93
15.79
The crystal structures of new
superconducting materials obtained
by high pressure treatment
Popova, S.V. 15.65
Cubic Tantalum Carbide (TaC)
Title 1 of its Authors Density (g/cm3) Average (g/cm
3)
Formation of cubic
solid solutions in the
Mo-Nb-C and
Mo-Ta-C systems
by the carbonization of
oxides in the plasma
arc
Matsumoto, O. 14.5
14.49
Hochschmelzende
Systeme mit
Hafniumkarbid
und -nitrid
Nowotny, H. 14.5
Thermal expansions of
some carbides and
tessellated stresses in
steel
Stuart, H. 14.5
The pseudo-binary
systems of uranium
carbide with zirconium
carbide, tantalum
carbide, and niobium
carbide
Brownlee, L.D. 14.5
57
Cubic Titanium Nitride (TiN)
Title 1 of its Authors Density (g/cm3) Average (g/cm
3)
The preparation of Na
Cl-type Til-x Alx N
solid solution
Inamura, S. 5.32
5.38
Non-stoichiometry of
titaniumnitride plates
prepared by chemical
vapour deposition
Takashi Goto 5,39
X ray diffraction study
of dynamic
characteristics of
crystal lattices of some
interstitial phases.
Samsonov, G.V. 5.39
Coefficients of thermal
expansion of titanium
carbonitrides
Alyamovskii, S.I 5.4
10.6 – Archimedes method
This method consists of weighing with a set of scales the weight of the sample
when it is both dry and wet. To determine the density, a relation between the weight of the dry
sample and that of the sample when it is immersed in water is used:
weightwetweightdry
weightdrycmgdcalc )/( 3
.
Once the experimental density is known, it can be compared with the theoretical value to
obtain a degree of compactness of the sample.
DensitylTheoretica
densityCalculatedsCompactnes
58
In order to measure the compactness, the proportions of each compound used to prepare the
mixtures must be taken into account when it comes to calculate the theoretical density of each
tablet.
Set of scales used for the Archimedes Method
To measure the theoretical densities, the density values from cubic and
hexagonal TaN must be used as well as the values of the dopant which is supposed to solve
within the TaN cubic lattice. Therefore, the hexagonal phase is supposed to be formed of pure
TaN whereas the cubic presents a 5% amount of the cubic dopant (either TaC or TiN). The
percentages corresponding to the hexagonal and the cubic lattices were obtained from X-Ray
diffraction by using Rietveld Refinement and exposed in the X-Ray diffraction tables.
Therefore, a formula to calculate the theoretical density for each of the samples is displayed
below:
..... %***)1(%* CubdopantTaNCubHexTaNHexTheor dxdxdd
59
11 - Results and discussion
In this chapter all the obtained results are shown as well as their conclusion
which could be drawn from them. These results are presented in the form of graphs, tables
and images for the different techniques applied to the samples (SPS sintering, X-Ray
diffraction and Scanning Electron microscopy).
11.1 SPS sintering results
In the following graphs both the sample displacement (shrinkage or expansion) and the
sintering temperature are plotted against time for different processes which go up to different
final temperatures as shown in the following plots taken as examples of such a process:
SPS sintering ABCR TaN at 75 MPa and 1800 °C
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 5 10 15 20
Time (min.)
Tem
pera
ture
(°C
)
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Temperature
Displacement
Sample synthesized from the starting powder ABCR TaN at 75MPa and a final temperature of
1800 ° C
60
SPS sintering ALFA TaC0.05N0.95 at 75MPa and 1800°C
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 5 10 15 20
Time (min.)
Tem
pera
ture
(°C
)
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Temperature
Displacement
Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar
ratio, at 75MPa and a final temperature of 1800 ° C
SPS sintering ALFA TaC0.05N0.95 at 75 Mpa and 1500°C
0
200
400
600
800
1000
1200
1400
1600
0 2 4 6 8 10 12 14
Time (min.)
Tem
pera
ture
(°C
)
-0.2
0
0.2
0.4
0.6
0.8
1
Temperature
Displacement
Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar
ratio, at 75MPa and a final temperature of 1500 ° C
61
SPS sintering ALFA TaN at 75 Mpa and 1500°C
0
200
400
600
800
1000
1200
1400
1600
0 2 4 6 8 10 12 14
Time (min.)
Tem
pera
ture
(°C
)
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Temperature
Displacement
Sample synthesized from the starting powder ALFA TaN mixed at 75MPa and a final
temperature of 1500 ° C
SPS sintering ALFA TaC0.05N0.95 at 75 Mpa and 1550°C
0
200
400
600
800
1000
1200
1400
1600
1800
0 2 4 6 8 10 12 14 16
Time (min.)
Tem
pera
ture
(°C
)
-0.5
0
0.5
1
1.5
2
2.5
Temperature
Displacement
Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar
ratio, at 75MPa and a final temperature of 1550 ° C
62
SPS sintering ABCR Ta0.95Ti0.05N at 75MPa and 1550°C
0
200
400
600
800
1000
1200
1400
1600
1800
0 2 4 6 8 10 12 14 16
Time (min.)
Tem
pera
ture
(°C
)
-0.5
0
0.5
1
1.5
2
Temperature
Displacement
Sample synthesized from the starting powder ABCR TaN mixed with TiN with a 5% molar
ratio, at 75MPa and a final temperature of 1550 ° C
11.2 SPS sintering discussion
The displacement plotted in the different graphs can give an overview of the
processes taking place to the sintering powder at each of the stages and therefore this
parameter will be taken into account in this discussion. In the very beginning of the sintering,
it can be seen that in some graphs there is a negative slope which is thought to be related to
the graphite expansion of the dyes used in the SPS equipment. This tendency is seen in all
graphs except for number 5.
On the other hand, the steep slope detected within the time gap from 6 minutes
to 10 minutes in most graphs is due to the shrinkage of the powder when it is sintered as a
compact solid tablet. There are also sudden ups and downs which actually make the slope
change wit no apparent reason. These are present all over the plotting and appear only
because the pressure was adjusted manually at some stages (both increased and decreased) so
as not to put a lot of strain in the equipment and carry out the sintering properly.
63
To complete the assessment, it can also be seen that the sintering is over at
temperatures close to 1550ºC, as can be seen from the graphs which reach up to this final
temperature. At this stage there is no change in the displacement and therefore a horizontal
tendency is observed in the displacement when the temperature is stabilized at 1550ºC. There
can also be seen some steep decreases at the final stage in the displacement due to the fact that
the equipment was turned off at this very moment making the displacement value drop
radically.
11.3 - Powder X-Ray diffraction results
1) Starting powders
The different starting TaN powders from both companies (ABCR and ALFA)
and the used dopants (TaC and TiN) were analysed with X-Ray diffraction and from the
corresponding diffractograms the cubic lattice percentage of the TaN powders could be found
out by Rietveld Refinement, since it is logical to know which is the cubic lattice presence in
the raw material when the main objective is to assess its synthesis. The diffractograms and
corresponding results are the following.
Sample Cubic
TaN (%)
Hexagonal
TaN (%) Ta2N
Starting TaN powder (ABCR) 27.3 72.7 0
Starting TaN powder (ALFA_1) 100 0 0
Starting TaN powder (ALFA_2) 10.4 69.8 19.8
Percentages of the different phases present in the TaN starting powders from ABCR and
ALFA companies
64
X-Ray diffractograms carried out to the powders of the pure Tantalum Nitrides and dopants
used to create the samples
Two different powder bottles of 50g each were bought from the ALFA
company. It can be seen from the percentage results that the first powder bought from that
company (labelled as ALFA 1) showed better properties since it presented no cubic phase
whatsoever and therefore the cubic phase could be sintered completely from the hexagonal
TaN phase. However, the second bottle (called ALFA 2) presented some cubic phase and
what is more it presented a vast amount of a different phase (Ta2N) whose behaviour in the
sintering was totally unknown. Therefore it was decided that using this powder to synthesize
samples should be avoided, and the samples were synthesized using both ABCR and ALFA
powder (the first bottle). However, not as many samples as wanted could be synthesized with
this ALFA powder because there were only 50 grams and as seen with the second bottle it
was difficult to find another powder with the same phases from the same company ( ALFA 1
and ALFA 2 presented different phases even though they were acquired from the same
manufacturer).
65
2) SPS sintered samples
The following are some tables with the cubic and hexagonal percentages of the TaN phases
detected by X-Ray diffraction and assessed using the Rietveld method. From these tables,
different graphs were also plotted to compare different variables that influence the cubic
transformation of the hexagonal TaN which allow some conclusions to be drawn. Eventually,
it was also considered relevant to plot diffractograms for each of the sintered series at
different temperatures and for the starting materials as follows.
ABCR POWDER (TixTa1-xN)
Sample Cubic % Hexagonal % Ti
TixTa1-xN
(x =0.05, 1600°C, 50MPa)
(ABCR TaN)
37.3 59.5 3.2
TixTa1-xN
(x =0.05, 1700°C, 50MPa)
(ABCR TaN)
93.9 6.1 0
TixTa1-xN
(x =0.05, 1800°C, 50MPa)
(ABCR TaN)
97.1 2.9 0
TixTa1-xN
(x =0.05, 1550°C, 75MPa)
(ABCR TaN)
26.6 73.4 0
TixTa1-xN
(x =0.05, 1600°C, 75MPa)
(ABCR TaN)
59.5 37.3 3.2
TixTa1-xN
(x =0.05, 1650°C, 75MPa)
(ABCR TaN)
86.6 12.3 1.1
66
TixTa1-xN
(x =0.05, 1700°C, 75MPa)
(ABCR TaN)
97.5 2.5 0
TixTa1-xN
(x =0.05, 1800°C, 75MPa)
(ABCR TaN)
98.2 1.8 0
TixTa1-xN
(x =0.03, 1600°C, 75MPa)
(ABCR TaN)
29.4 70.6 0
TixTa1-xN
(x =0.03, 1700°C, 75MPa)
(ABCR TaN)
96.2 3.8 0
TixTa1-xN
(x =0.03, 1800°C, 75MPa)
(ABCR TaN)
100 0 0
ABCR POWDER (TaCxN1-x)
Sample Cubic % Hexagonal % TaC %
TaCxN1-x
(x =0.05, 1600°C, 50MPa)
(ABCR TaN)
57.2 40.8 2
TaCxN1-x
(x =0.05, 1700°C, 50MPa)
(ABCR TaN)
95.9 2.2 1.9
TaCxN1-x
(x =0.05, 1800°C, 50MPa)
(ABCR TaN)
97.1 0 2.8
TaCxN1-x
(x =0.05, 1550°C, 75MPa)
(ABCR TaN)
57.6 42.4 0
67
TaCxN1-x
(x =0.05, 1600°C, 75MPa)
(ABCR TaN)
44.2 55.2 0.6
TaCxN1-x
(x =0.05, 1650°C, 75MPa)
(ABCR TaN)
51.9 48.1 0
TaCxN1-x
(x =0.05, 1700°C, 75MPa)
(ABCR TaN)
98 2 0
TaCxN1-x
(x =0.05, 1800°C, 75MPa)
(ABCR TaN)
96.3 0.1 3.6
PURE ABCR POWDER
Sample Cubic % Hexagonal %
TaN (ABCR) starting powder 27.3 72.7
Pure TaN (ABCR)
(1600°C, 75Mpa) 32.2 67.8
Pure TaN (ABCR)
(1700°C, 75Mpa) 97.1 2.9
Pure TaN (ABCR)
(1800°C, 75Mpa) 100 0
68
ALFA 1 POWDER (TaCxN1-x)
Sample Cubic % Hexagonal % TaC % Ta
TaCxN1-x
(x =0.05, 1500°C, 75MPa)
(ALFA TaN)
23.2 76.8 0 0
TaCxN1-x
(x =0.05, 1550°C, 75MPa)
(ALFA TaN)
30 69.1 0 0.9
TaCxN1-x
(x =0.05, 1600°C, 75MPa)
(ALFA TaN)
60.6 38.5 0 0.9
TaCxN1-x
(x =0.05, 1700°C, 75MPa)
(ALFA TaN)
96.6 3.4 0 0
TaCxN1-x
(x =0.05, 1800°C, 75MPa)
(ALFA TaN)
100 0 0 0
PURE ALFA 1 POWDER
Sample Cubic % Hexagonal %
Pure TaN (Alfa) 0 100
TaN Alfa
(1500°C, 75Mpa) 8.1 91.9
TaN Alfa
(1600°C, 75Mpa) 12.1 87.9
TaN Alfa
(1700°C, 75Mpa) 93.4 6.6
TaN Alfa
(1750°C, 75Mpa) 98 2
69
3) Plotted graphics with observations
GRAPH 1
Cubic percentages of Ta0.95Ti0.05N at different
pressures (with ABCR TaN)
0
10
20
30
40
50
60
70
80
90
100
1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
TaTiN at 75MPa
TaTiN at 50 MPa
Observations 1:
There is a trend which shows that the higher the temperature the higher the
conversion into the cubic phase. Moreover, this effect seems to be more significant at lower
temperatures due to the fact that at higher temperatures both the sample temperature and
exposition time have been higher. That is why from 1700 ºC forward there seem to be no
significant effects on the cubic percentage.
There also seems that a higher pressure helps considerably to boost the cubic conversion,
especially at lower temperatures for the reasons mentioned before.
70
GRAPH 2
Cubic percentages of TaC0.05N0.95 at different
pressures (with ABCR TaN)
0
10
20
30
40
50
60
70
80
90
100
1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
TaCN at 75MPa
TaCN at 50 MPa
TaCN(1550ºC,75MPa)
Observations 2:
A higher synthesis temperature always entails a higher cubic percentage in the
sample. It must be pointed out that the samples taken at 75MPa at 1600ºC and 1650ºC
respectively belonged to a different mixture than the remaining three from the same series at
1550ºC, 1700ºC and 1800ºC.
On the one hand maybe the value of 1550 ºC at 75 MPa should be discarded
because it seems to be wrong. On the other hand, it seems like the input pressure is not an
important asset over 1700 ºC and comparison at 1600 ºC of both pressures just does not add
up since it should be the other way round (75MPa higher than 50MPa). However, the trend
was perfect with the TiTaN series (graphic 1), and not much more information can be
extracted from this graph.
71
GRAPH 3
Cubic percentages of TaxTi1-xN at 75MPa with
different dopant concentrations x
(with ABCR TaN)
0
10
20
30
40
50
60
70
80
90
100
1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
as
e (
%)
x = 0.05
x = 0.03
Observations 3)
A higher synthesis temperature always entails a higher cubic percentage in the
sample. Nevertheless, no considerable changes occur at temperatures over 1700 ºC.
The dopant (TiN with a cubic arrangement) seems to help the TaN adopt a
cubic arrangement, as can be seen in the graph, especially at the low temperature of 1600C,
where the fact that both the temperature and the reaction time have been considerably low
allow the dopant concentration to show its effect on the cubic transformation. Therefore, there
seems to be a trend which exposes that the higher the dopant concentration the higher the
cubic percentage in the sample.
72
GRAPH 4
Comparison of the Cubic percentages of
TaC0.05N0.95 and Ta0.95Ti0.05N at 50MPa
(with ABCR TaN)
0
10
20
30
40
50
60
70
80
90
100
1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
TaCN at 50MPa
TaTiN at 50 MPa
Observations 4:
A higher synthesis temperature always entails a higher cubic percentage in the
sample. Nevertheless, no considerable changes occur at temperatures over 1700 ºC.
It can be assessed from the graph that the type of dopant (TaC or TiN) is not
significant when assessing the cubic percentage. The reason for that seems to be both present
a cubic arrangement, and, in the same concentrations on the sample have the same effect on
the final cubic percentage.
73
GRAPH 5
Comparison of the Cubic percentages of TaC0.05N0.95
,Ta0.95Ti0.05N and pure TaN powder at 75MPa (with
ABCR TaN)
0
10
20
30
40
50
60
70
80
90
100
1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
TaCN at 75 MPa
TaTiN at 75 MPa
Pure TaN at 75MPa
TaCN(1550ºC,75MPa)
Observations 5:
A higher synthesis temperature always entails a higher cubic percentage in the
sample, apart from the value at 1500ºC which could be wrong. Nevertheless, no considerable
changes occur at temperatures over 1700 ºC.
It can be seen that the values of the cubic percentages for TaN to which dopant
has been added are higher than those of the pure TaN, which shows the dopant has actually an
effect on the conversion especially at low temperatures. Moreover, for all samples but that at
1550ºC, whose value maybe is wrong, it seems that the cubic percentage is higher for those
samples doped with TiN instead of TaC, but no conclusions could be drawn up from this fact
since as mentioned before two different mixtures were used at different times to prepare the
TaCN series and maybe those could just be experimental errors.
74
GRAPH 6
Comparison of the Cubic percentages of the two
TaN powders (ABCR and ALFA) at 75MPa and their
pure forms with no SPS sintering.
0
10
20
30
40
50
60
70
80
90
100
1450 1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
ABCR TaN powder
ALFA TaN powder
ALFA TaN with no
SPS sintering
ABCR TaN with no
SPS sintering
Observations 6:
On the one hand, it was determined by X-Ray diffraction that the starting TaN
powder from the ABCR company had considerable cubic content (around 27%) whereas the
TaN from the ALFA company did not present any cubic phase whatsoever. Therefore, it is
only logical that at low temperatures (where the effect of temperature and reaction time is not
that significant) the cubic percentage is higher for the ABCR powder, as can be seen for both
powders at 1600C. At higher temperatures the values of the percentages become stable at high
values and this former presence of a cubic phase in the ABCR powder does not seem so
significant. It must also be remarked that without the help of any dopant both powders
reached very high values in cubic percentage, even the ALFA powder with no cubic phase in
the very beginning. Therefore it seems logical to say that temperature and overall reaction
time seem to be the most significant assets for the synthesis of the cubic form of TaN.
75
GRAPH 7
Comparison of the Cubic percentages of TaC0.05N0.95 at
75MPa with ABCR and ALFA TaN
0
10
20
30
40
50
60
70
80
90
100
1450 1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase
(%
)
ABCR TaCN
ALFA TaCN
TaCN (1550ºC ,75MPa)
Observations 7:
Both series show again very similar values at temperatures over 1700C showing
temperature is a considerable asset. Unexpectedly, it seems that the cubic percentage seems to
be higher for the ALFA series than for its ABCR counterpart ( see samples at 1600C), which
just does not add up with the premonitions since ABCR had a higher cubic content in the
beginning and that tendency should be charted throughout the synthesis, especially at low
temperatures. However, it is unclear and filling up with more samples would be necessary to
make such a statement.
76
GRAPH 8
Comparison of the Cubic percentages of TaC0.05N0.95
and pure TaN powder at 75MPa (with ALFA TaN)
0
10
20
30
40
50
60
70
80
90
100
1450 1500 1550 1600 1650 1700 1750 1800
Temperature (°C)
Cu
bic
ph
ase (
%)
ALFA TaCN
ALFA TaN
Observations 8:
In this graph the effect of the dopant in the sintering can be seen, being more
significant at low temperatures. In fact, at the lowest temperature in the graph (1500C) the
doped TaN has already transformed more of its hexagonal lattice into cubic than the pure
TaN. Moreover, it can be seen that when temperature is increased (up to 1600C) the
increasement of the cubic percentage is radical for the doped TaN whereas its pure form
remains still stable. The data plotting seems relevant and therefore it must be emphasized that
the dopant is a considerable asset for the transformation of the hexagonal lattice into cubic
within a considerably low range of temperatures. This effect is especially considerable taking
into account that the starting TaN powder (Alfa) had no present cubic lattice whatsoever.
77
X-Ray diffractograms from the series TaC0,05N0,95 synthesized at 50MPa with ABCR TaN
X-Ray diffractograms from the series TaC0,05N0,95 synthesized at 75MPa with ABCR TaN
78
X-Ray diffractograms from the series Ta0,95Ti0,05N synthesized at 50MPa with ABCR TaN
X-Ray diffractograms from the series Ta0.95Ti0.05N synthesized at 75 MPa with ABCR TaN
79
X-Ray diffractograms from the series Ta0.97Ti0.03N synthesized at 75 MPa with ABCR TaN
X-Ray diffractograms from the series of pure ABCR TaN synthesized at 75MPa
80
X-Ray diffractograms from the series TaC0.05N0.95 synthesized at 75 MPa with ALFA TaN
X-Ray diffractograms from the series of pure ALFA TaN synthesized at 75 MPa
81
11.4 - X-Ray results discussion
1) The reaction temperature is the main asset, since all the cubic percentages go up
radically at temperures over 1700°C and higher, whereas there is usually a
considerable mixture of both phases (cubic and hexagonal) when the reaction takes
place at temperatures close to 1600°C. Another reason is that at higher temperatures
the overall exposure time of the sample to high pressures and temperatures is
increased, thus boosting the transformation.
2) It was also noticed there was a correlation between the pressure and the cubic
percentage (the higher the pressure, the higher the percentage). However this
parameter only had an important effect on the transformation at low temperatures
around 1600 °C. and therefore should be considered less important than parameters
such as temperature and exposure time.
3) The added dopants (TaC, TiN) were supposed to boost the synthesis into the cubic
form of TaN in all the range of temperatures. However, like pressure, they had a low
effect on the cubic percentages at temperatures over 1700°C and higher, and only were
significant at lower temperatures around 1600°C. There were questions as to whether
the reason for this was that the starting powder presented a significant cubic
percentage before the synthesis which would allow the cubic phase to grow without
the need of a dopant with a cubic lattice arrangement such as TaC or TiN. However,
this was clearly not the case since those samples synthesized with the starting ALFA
powder (which presented no cubic phase at all) behaved likewise.
4) This SPS sintering method seems reliable since all the samples had a considerably
high cubic percentage as shown in the X-Ray diffractrograms after the synthesis. It
should also be remarked that special consideration must be taken at the final
temperature reached in the sintering because it seems to be the main asset in the
transformation along with exposure time.
82
11.5 - Density measurements results
These measurements were carried out using the Archimedes method. The dry and wet weight
were obtained by weighing the sample in an air environment and drawn in water respectively..
The different types of densities along with the compactness were obtained by application of
the formulas exposed in the experimental chapter (Archimedes method).
ABCR POWDER (TixTa1-xN)
Sample name
Dry
weight
( g )
Wet
weight
( g )
Calculated
Density
( g/cm3 )
Cubic
TaN
( % )
Hexagonal
TaN
( % )
Theoretical
Density
( g/cm3 )
Compactness
( % )
Ta0.95Ti0.05N
(50MPa,1600°C)
(ABCR TaN)
1.3827 1.2731 12.61 39.7 60.3 14.90 84.6
Ta0.95Ti0.05N
(50MPa,1700°C)
(ABCR TaN)
2.7072 2.4977 12.92 93.9 6.1 15.21 84.9
Ta0.95Ti0.05N
(50MPa,1800°C)
(ABCR TaN)
0.9060 0.8368 13.09 97.1 2.9 15.24 85.9
Ta0.95Ti0.05N
(75MPa,1550°C)
(ABCR TaN)
2.2220 2.0493 12.87 26.6 73.4 14.70 87.5
Ta0.95Ti0.05N
(75MPa,1600°C)
(ABCR TaN)
1.5311 1.4127 12.93 61.4 38.6 14.90 86.8
Ta0.95Ti0.05N
(75MPa,1650°C)
(ABCR TaN)
1.8847 1.7393 12.96 87.6 12.4 15.61 83.1
Ta0.95Ti0.05N
(75MPa,1700°C)
(ABCR TaN)
1.1185 1.0325 13.01 97.5 2.5 15.25 85.3
Ta0.95Ti0.05N
(75MPa,1800°C
(ABCR TaN))
1.4425 1.3304 12.87 98.2 1.8 15.25 84.4
Ta0.97Ti0.03N
(75MPa,1600°C)
(ABCR TaN)
1.3356 1.2339 13.13 29.4 70.6 14.65 89.6
Ta0.97Ti0.03N
(75MPa,1700°C)
(ABCR TaN)
1.3632 1.2595 13.15 96.2 3.8 15.43 85.2
Ta0.97Ti0.03N
(75MPa,1800°C)
(ABCR TaN)
2.1380 1.9742 13.05 100 0 15.48 84.3
83
ABCR POWDER (TaCxN1-x)
Sample name
Dry
weight
( g )
Wet
weight
( g )
Calculated
Density
( g/cm3 )
Cubic
TaN
( % )
Hexagonal
TaN
( % )
Theoretical
Density
( g/cm3 )
Compactness
( % )
TaC0.05 N0.95
(50MPa,1600°C)
(ABCR TaN)
2.5442 2.3524 13.26 57.2 40.8 14.83 89.4
TaC0.05 N0.95
(50MPa,1700°C)
(ABCR TaN)
2.5955 2.4001 13.28 95.9 2.2 15.40 86.3
TaC0.05 N0.95
(50MPa,1800°C)
(ABCR TaN)
2.3946 2.2150 13.33 97.1 0 15.27 87.3
TaC0.05 N0.95
(75MPa,1550°C)
(ABCR TaN)
2.0093 1.8588 13.35 57.6 42.4 14.86 89.8
TaC0.05 N0.95
(75MPa,1600°C)
(ABCR TaN)
1.9590 1.812 13.33 44.2 55.2 14.85 89.8
TaC0.05 N0.95
(75MPa,1650°C)
(ABCR TaN)
1.6966 1.5698 13.38 51.9 48.1 15.08 88.7
TaC0.05 N0.95
(75MPa,1700°C)
(ABCR TaN)
1.624 1.5024 13.36 98 2 15.70 85.1
TaC0.05 N0.95
(75MPa,1800°C)
(ABCR TaN)
1.5781 1.4598 13.33 96.3 0.1 15.16 87.3
PURE ABCR POWDER
Sample name
Dry
weight
( g )
Wet
weight
( g )
Calculated
Density
( g/cm3 )
Cubic
TaN
( % )
Hexagonal
TaN
( % )
Theoretical
Density
( g/cm3 )
Compactness
( % )
Ta N
(75MPa,1600°C)
(ABCR TaN)
1.7658 1.6392 13.95 34.5 59.4 13.95 100.0
TaN
(75MPa,1700°C)
(ABCR TaN)
1.7870 1.6528 13.32 90.4 2.9 14.69 90.7
TaN
(75MPa,1800°C)
(ABCR TaN)
0.9889 0.9143 13.26 100 0 15.79 84.0
84
ALFA 1 POWDER (TaCxN1-x)
PURE ALFA 1 POWDER
Sample name
Dry
weight
( g )
Wet
weight
( g )
Calculated
Density
( g/cm3 )
Cubic
TaN
( % )
Hexagonal
TaN
( % )
Theoretical
Density
( g/cm3 )
Compactness
( % )
TaN
(75MPa,1500°C)
(ALFA 1 TaN)
1.6853 1.5553 12.96 8.1 91.9 14.39 90.1
TaN
(75MPa,1600°C)
(ALFA 1 TaN)
2.3062 2.1410 13.96 12.1 87.9 14.43 96.8
TaN
(75MPa,1700°C)
(ALFA 1 TaN)
1.5992 1.4862 14.15 93.4 6.6 15.21 93.1
TaN
(75MPa,1750°C)
(ALFA 1 TaN)
1.0132 0.9413 14.09 98 2 15.25 92.4
Sample name
Dry
weight
( g )
Wet
weight
( g )
Calculated
Density
( g/cm3 )
Cubic
TaN
( % )
Hexagonal
TaN
( % )
Theoretical
Density
( g/cm3 )
Compactness
( % )
TaC0.05 N0.95
(75MPa,1500°C)
(ALFA 1 TaN)
1.4533 1.3489 13.92 23.2 76.8 14.53 95.8
TaC0.05 N0.95
(75MPa,1550°C)
(ALFA 1 TaN)
1.8047 1.6752 13.93 30.2 69.8 14.60 95.4
TaC0.05 N0.95
(75MPa,1600°C)
(ALFA 1 TaN)
1.8450 1.7147 14.15 61 38.7 14.85 95.3
TaC0.05 N0.95
(75MPa,1700°C)
(ALFA 1 TaN)
1.3512 1.2552 14.08 96.6 3.4 15.24 92.4
TaC0.05 N0.95
(75MPa,1800°C)
(ALFA 1 TaN)
2.0982 1.9495 14.11 100 0 15.27 92.4
85
11.6 - Density measurements discussion
These density measurements were mainly done to assess whether the
compactness of the samples was high enough to allow the carrying out of some mechanical
tests to the samples such as hardness measurements. In fact, this test needs a sample
compactness of 95% or higher, otherwise the cracks will stop at the pores thus providing
wrong values for fracture toughness.
On the other hand, unexpectedly the compactness is significantly low for most of the samples.
The SPS sintering is theoretically supposed to provide with samples of at least 95%
compactness but as assessed most of the samples could barely reach high enough
compactness. This fact has no relevance on neither the phase analysis of the samples by X-
Ray diffraction nor the visualization of the grain sizes from the images obtained by SEM.
However, special considerations must be taken into account when selecting the samples if
hardness measurements are to be carried out, since most of the samples are not eligible
because they do not meet the necessary conditions in terms of compactness.
86
11.7 - Scanning Electron Microscopy (SEM) images
Different samples were visualized by SEM. The TaN powders from the
chemical companies ABCR and ALFA were observed along with different samples
created from these powders trying to cover the maximum possible range of different cubic
percentages. The sintered samples were previously fractured, being this fracture surface
the one visualized with the microscope.
Therefore the following samples ordered by cubic content percentage were tested:
Sample Cubic
%
Starting powder TaN (ABCR) 27,3
Starting powder TaN (ALFA) 100
TaN
(x =0.05, 1500°C, 75MPa)
(ALFA TaN)
8.1
TixTa1-xN
(x =0.03, 1600°C, 75MPa)
(ABCR TaN)
29.4
TaCxN1-x
(x =0.05, 1600°C, 75MPa)
(ABCR TaN)
44.2
TaCxN1-x
(x =0.05, 1600°C, 75MPa)
(ALFA TaN)
60.6
TixTa1-xN
(x =0.05, 1800°C, 75MPa)
(ABCR TaN)
61.4
TaCxN1-x
(x =0.05, 1800°C, 75MPa)
(ALFA TaN)
100
87
The following images were obtained from the SEM:
ABCR TaN POWDER
SEM Figure 1. ABCR TaN Starting powder 500X Backscattered electrons Image
SEM Figure 2. ABCR TaN 3000X (1) Secondary Electrons Image
88
SEM Figure 3. ABCR TaN 3000X BackScattered Electrons Image
SEM Figure 4. ABCR TaN 30000X Backscattered Electrons Image
89
ALFA TaN Powder
SEM Figure 5. Alfa TaN 500X Backscattered Electrons Image
SEM Figure 6. Alfa TaN 3000X Backscattered Electrons Image
90
SEM Figure 7. Alfa TaN 30000X Backscattered Electrons Image
ABCR TaC0.05 N0.95 , 1600C,75MPa
SEM Figure 8. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 500X, BackScattered
Electrons Image
91
SEM Figure 9. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 3000X, Secondary Electrons
Image
SEM Figure 10. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 3000X, BackScattered
Electrons Image
92
SEM Figure 11. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 30000X, Secondary
Electrons Image
SEM Figure 12. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 30000X, BackScattered
Electrons Image
93
SEM Figure 13. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 500X, Backscattered
Electrons Image
SEM Figure 14. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 3000X, Secondary
Electrons Image
94
SEM Figure 15. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 3000X, Backscattered
Electrons Image
SEM Figure 16. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 30000X, Secondary
Electrons Image
95
SEM Figure 17. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 30000X, Backscattered
Electrons Image
ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa)
SEM Figure 18. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 500X, Secondary Electrons
Image
96
SEM Figure 19. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 500X, Backscattered
Electrons Image
SEM Figure 20. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 3000X, Backscattered
Electrons Image
97
SEM Figure 21. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 3000X, Backscattered
Electrons Image
SEM Figure 22. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 10000X, Secondary Electrons
Image
98
SEM Figure 23. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 10000X, Backscattered
Electrons Image
ALFA TaN (1500°C, 75Mpa)
SEM Figure 24. ALFA TaN (1500°C, 75Mpa), 150X, Backscattered Electrons Image
99
SEM Figure 25. ALFA TaN (1500°C, 75Mpa), 500X, Backscattered Electrons Image
SEM Figure 26. ALFA TaN (1500°C, 75Mpa), 3000X, Secondary Electrons Image
100
SEM Figure 27. ALFA TaN (1500°C, 75Mpa), 3000X, Secondary Electrons Image
ALFA TaC0.05 N0.95 (1600°C, 75Mpa)
SEM Figure 28. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 150X, Backscattered Electrons
Image
101
SEM Figure 29. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 500X, Backscattered Electrons
Image
SEM Figure 30. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 3000X, Secondary Electrons
Image
102
ALFA TaC0.05 N0.95 (1800°C, 75Mpa)
SEM Figure 31. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 500X, Secondary Electrons
Image
SEM Figure 32. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 1000X, Secondary Electrons
Image
103
SEM Figure 33. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 3000X, Secondary
Electrons Image
11.8 - SEM Discussion
Images for both the starting powders and different sintered samples have been
taken at different magnifications in order to assess features such as the grain size, the fracture
surface and their composition.
It must be remarked that the grain size of both starting powders (ALFA and
ABCR) seems to be around 1 micrometer, although it can easily gather up to create bigger
conglomerates as can be seen from the pictures. Taking a closer look at the images of the
synthesized samples, it can be seen that their grain size has grown when compared to the
starting powders. Therefore, it seems that both factors pressure and temperature enhance the
growth of the grains. However, when comparing the different samples it seems these
parameters are not directly proportional to the grain growth. In other words, it seems that the
lattice content (whether hexagonal or cubic) is the major asset to determine the grain size.
104
As it can be seen from the images, it is in those samples that present mostly one
phase (either cubic or hexagonal) that the grain size is bigger, being at the same time bigger
when the phase is mostly hexagonal rather than cubic. In fact, the grain size seems to be
bigger mainly for the samples created with ALFA TaN at 1500 °C and 1800 °C (with 8.1 %
and 100% cubic content respectively). When comparing them to one another, the grain size of
the former one seems to be around 8 micrometers whereas it is around 5 micrometers for the
latter. The main reason for such a change seems to be that the hexagonal lattice occupies a
higher volume than the cubic. On the other hand, the reason why those samples with almost
only one phase possess bigger grain size seems to be that when there is a phase mixture one
phase hinders the other from growing. In our case, what happens is that as the reaction goes
by, the hexagonal lattice turns into cubic and when both phases are in considerable
proportions the hexagonal phase hinders somehow the cubic from growing. As time goes by
and the cubic lattice develops, the hexagonal one becomes less significant and no other phase
hinders the cubic from growing.
When assessing the fracture surface, it must be said that both types of fractures
took place in the samples: fracture through the grains and fractures through the boundaries.
An example in which both mechanisms can be seen is the SEM Figure 27. Another interesting
asset that can be seen from the fracture surface is the presence of porosity in the sample. The
porosity distribution observed in the images seems to fit the results obtained by the density
measurements. A good example is the sample synthesized at 1500°C with ALFA TaN powder
whose compactness was assessed to be close to 100%. In fact, there seems to be very low
porosity for this sample (see SEM Figure 24). However, there does not seem to be a relation
between the sintering conditions and the porosity of the samples.
On the other hand, those images obtained by backscattered electrons give an
overview of the sample’s composition with tonalities of grey and black colours. As expected,
those images present mainly bright grains due to the fact that Ta is a heavy element and is
displayed quite brightly. The EDS analysis confirmed these grains consist of TaN. It can also
be expected there are traces of unreacted dopant in the samples (TaC or TiN). This seems to
be the case at least for the samples with TiN as a dopant, since such samples present a great
deal of dark-coloured spots, which could easily be unreacted TiN (see SEM Figures 21 and
23). Therefore, it seems the samples consist of TaN with some unreacted dopant impurities.
105
12 - Conclusions
Different mixtures to which different amounts of dopants (TaC and TiN) were
added have been used to synthesize samples at different pressures and temperatures to
cover a broad range of sintering conditions, thus being able to predict the effect of the
variables on the cubic transformation.
After sample conditioning for powder X-Ray diffraction it seems that
temperature and reaction time were the main assets which boosted the cubic
transformation. In fact, the sample’s cubic lattice content was 90% or higher for all
samples from 1700°C forward, whereas the pressure and the amount of added cubic
dopant only had an effect at lower temperatures, being quite significant at temperatures
close to 1600°C and lower. Moreover, both dopants seemed to play a very similar role.
The SEM visualization also confirmed the grain growth boosted by the SPS
sintering which turned a nanometric starting powder into a tablet of bigger grain size, thus
creating a compact. It was also confirmed by EDS that most of the tablet content was
TaCN or TiTaN as expected, and some of them also presented traces of unreacted dopant,
which could also be noticed by SEM in the case of TiN.
However, one of the most considerable encountered difficulties was faced when
trying to control the porosity of the sintered samples. Actually, there seemed to be no
connection between the applied sintering conditions and the compact samples, and what’s
more, in some cases the compactness turned out to be lower than expected theoretically by
SPS sintering (samples should possess a compactness of 95% or higher).
Although this asset entails no problems for the assessment of the cubic
transformation and the phase analysis, it could be problematic when carrying out
foreseeable mechanical tests to the samples. For instance, if fracture toughness tests were
to be realized to samples with porosity, the cracks would stop at the pores and wrong
values for hardness would be provided.
106
Therefore hardness tests could only be carried out to few of these samples, those
which possess very low porosity as assessed by density measurements. Actually, such
tests are scheduled to be carried out in the foreseeable future.
On the other hand, while cutting the samples to tear them asunder (one part was
saved for SEM imaging and further hardness measurements whereas the other was
pulverized to carry out X-Ray diffraction) it could be noticed that those samples
containing mainly one type of phase (either cubic or hexagonal) seemed much easier to
cut than those in which there was a mixture of both phases. Therefore, it seems that the
presence of two phases truly reinforces the material’s fracture toughness although no
experimental values to back up such a statement were recorded.
Summing up, the SPS sintering technique turns out to be a reliable method since
the synthesis parameters can be easily controlled and the sample’s results were quite
promising in final cubic content. In fact, the hexagonal lattice was easily turned into cubic
with quite ease of operation, making the application of such a process quite feasible to
carry out such a transformation.
107
13 - Bibliography
Articles:
1) M. Tokita, ―Trends in Advanced SPS Spark Plasma Sintering System
Technology‖. Powder Technology, Volume 30, Issue 11, Pages 790-804
2) Toshihiro Isobe, ―Spark Plasma Sintering technique for reaction sintering of
Al2O3/Ni nanocomposite and its mechanical properties‖. Ceramics International,
Volume 34, Issue 1, January 2008, Pages 213-217
1) Tokita Masao, ―Introduction of an emerging compaction technology – spark
Plasma Sintering (SPS) method and systems‖. Materials science and technology-
Association for iron and steel technology-, Year 2007, Volume 5, pages 3065-3076
2) K. S. Kutateladze, ―Nitride refractories obtained by the method of Alumino-
Nitro-Silicothermy‖. Scientific-Research Institute of Building Materials, Year
1965, pages 8-13
3) H.B.Nie, ―Structural and electrical properties of tantalum nitride thin films
fabricated by using reactive radio frequency magnetron sputtering‖. Center for
Superconducting and Magnetic Materials, Institute of Engineering Science and
Department of Physics, National University of Singapore.
4) H.B.Nie, ―Metalorganic chemical vapour deposition of tantalum nitride by
tertbutylimidotris(diethylamido)tantalum for advanced metallization. American
Institute of Physics, Year 1995,
5) S.K.Kim, ―Deposition of tantalum nitride thin films by D.C. magnetron
sputtering‖. School of Materials Science and Engineering, University of Ulsa,
South Korea, Year 2004
6) Zhao Kewen, ―Titanium Nitride Inclusions in Ti-Stabilized Stainless Steel‖,
Published in ACTA METALL SIN (CHINESE EDN), Volume 23, Issue 3, pages
B145—B150, Year 1988
108
7) Jianhua Ma, ―One simple synthesis route to nanocrystalline tantalum carbide via
the reaction of tanthalum pentachloride and sodium carbonate with metallic
magnesium‖, Materials Letters, Volume 61, Issue 17, pages 3658—3661, Year
2007
8) Byung—Ryang, Kim ―Mechanical properties and rapid consolidation of
binderless nanostructured tantalum carbide‖, Ceramics International, Volume
35, Issue 8, pages 3395-3400, Year 2009
9) T.Shikama, ―Properties of titanium carbide and vanadium carbide mixtures as
first wall coatings‖, Journal of Nuclear Materials, Volumes 133-134, pages 765-
768, Year 1985
10) Yong Choi, ―Stress corrosion behavior of nickel base titanium carbide cermets
prepared by direct consolidation during combustion reaction in 50% aqueous
sodium hydroxide solution‖, Journal of Materials Science, Volume 39, Issue 3,
Pages 1041 – 1045, Year 2004
11) V.F.Loskutov, ‖Application of Titanium Carbide coatings on iron and steels‖,
Metal Science and Heat Treatment, Volume 21, Issue 10, pages 779-782, Year
1979
12) Essaki, Kenji ―Synthesis of nanoparticulate tungsten carbide under
microwave irradiation‖ , Journal of the American Ceramic Society, Volume 93,
Issue 3, pages 692-695, Year 2010
13) A.G.Merzhanov, ―Titanium carbide produced by self-propagating high-
temperature synthesis – Valuable abrasive material‖, Powder Metallurgy and
Metal Ceramics, Volume 20, Issue 10, pages 709-713, Year 1981
14) V.M.Maslov, ―Carbon interaction with oxygen during titanium carbide
synthesis‖, Combustion,Explosion, and Shockwaves, Volume 19, Issue 5, pages
634-637, Year 2004
15) Lirong Tong, ―Synthesis of titanium carbide nano-powders by thermal plasma‖,
Scripta Materialia, Volume 52, Issue 12, pages 1253 – 1258, Year 2005.
16) S. López-Romero, ―Synthesis of TiC thin fils by CVD from toluene and titanium
tetrachloride with nickel as catalyst‖, Matéria (Rio Janeiro), Volume 12, Issue 3,
Year 2007
109
Webpages:
www.substech.com (Boron Nitride)
www.azom.com (Silicon Nitride), (Tungsten Carbide)
www.accuratus.com ( Silicon Nitride), (Boron Nitride)
www.brycoat.com (Titanium Nitride)
www.reade.com (Tantalum Nitride)
www.americanelements.com (Carbides)
www.britannica.com (Carbides)
www.transtutors.com (Carbides)
www.ultramet.com (Carbides)
www.streetdirectory.com (Carbides)
www.tungstenchina.com ( Tungsten Carbide)
www.hydrocarbide.com (Tungsten Carbide)
www.panalytical.com (X-Ray diffraction)
http://www.mrl.ucsb.edu/ (X-Ray diffraction)
http://www.icdd.com/ (X-Ray Diffraction)
http://imr.chem.binghamton.edu/ (X-Ray diffraction)
http://serc.carleton.edu/ (X-Ray diffraction)
http://home.planet.nl/~rietv025/ (Rietveld Method)
http://epswww.unm.edu/xrd/xrdclass/09-Quant-intro.pdf (Rietveld Method)
http://www.mos.org (Scanning Electron Microscopy)
http://www.unl.edu (Scanning Electron Microscopy)
http://www.purdue.edu (Scanning Electron Microscopy)
http://mee-inc.com (Scanning Electron Microscopy)
http://www.sdm.buffalo.edu (Scanning Electron Microscopy)
www.askmehelpdesk.com (Scanning Electron Microscopy)
www.answers.com (Grinding Mill)
www.britannica.com (Archimedes Principle)
www.physicsprinciples.tripod.com (Archimedes Principle)