doped titania project zinc focus - george j. ferko v

*

Undergraduate Student – Department of Materials Science and Engineering – Lehigh University

Tel.: 610-597-8007

E-mail:[email protected]

Sintering Behavior of 1% and 10% concentrations of

Zinc, Copper, and Boron Doped Titania

Under Conventional Sintering

George J. Ferko V*

Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA, 18015, United States

April 24, 2009

Abstract

The role of dopants in the sintering behavior of ceramics has been a major theme in the studying of ceramics for the last 50 years. The

industrial popularity of titania has prompted this study of its sintering behavior under 1% and 10% concentrations of zinc (Zn), copper

(Cu), and boron (B) dopant, conventionally sintered at temperatures of 1000oC and 1200

oC for sintering times of 0, 2, and 6 hours.

The study has provided insight into the formation of second phase, the growth of abnormal grains, and the effect of the level of

doping, sintering temperature, and sintering time on the densification and grain size in the final samples. The Cu doped samples were

found to have second phase present in all samples. The Zn doped samples had second phase in all samples sintered at 1200oC. The B

doped samples were all found to have abnormal elongated grains. Longer sintering time always resulted in larger grains. Higher

doping levels always lead to larger grains except in the Zn doped samples where the formation of second phase inhibited grain growth.

Higher sintering temperature always lead to larger grains in the samples tested. The doped samples typically exhibited a higher

densification than the undoped samples. Densification was inhibited in the B doped samples due to the elongated shape of the grains.

Densification increased with an increase in sintering time in the Zn doped samples that were sintered at 1000oC.

Introduction (background and justification)

(1)

Titania has many applications through a broad range of

industries. Titania has the greatest use in products which

involve white pigments. The high refractive indices and high

reflectance of anatase and rutile make them perfect for making

pigments white and bright. White pigment applications for

TiO2 exist in the coatings, paper, ink, paints, plastics, rubbers,

ceramics, fibers, ultraviolet light protection, food, and

cosmetics industries. TiO2 is considered to be thermally

stable, nontoxic, noncorrosive, and noncombustible making it

irreplaceable in the many sectors of industry to which it is

applied. Titania offers a high amount of opacity meaning the

color of other aspects of a product cannot be seen through it.

TiO2 is also used as a photocatalyst because of the ability of

its band gaps to absorb ultraviolet radiation through excitation

of valance electrons. Photocatalytic applications include

cleaning of contaminated water and self cleaning windows and

tiles [1][2]. TiO2 is of particular importance for the modeling

of ceramics because of the broad range of industry to which it

is applied. The study of the effects of different dopants on the

sintering behavior of TiO2 will allow for better production of

the products made with TiO2. The modeling of the sintering

behavior of TiO2 anatase will also provide insight into the

sintering behavior of other ceramics with the tetragonal crystal

structure [3].

(2)

It is important to control the microstructure (grain size

and shape) and the densification (percent porosity) in sintering

because the final microstructure and porosity will have strong

effects on the properties of the ceramic. The mechanical,

thermal, electrical, magnetic, and optical properties are all

strongly affected by final densification and microstructure.

Being able to control the grain growth and the densification

allows for the creation of advanced ceramics with properties

better than those of conventional metals and ceramics [4][5].

(3)

Mehdi Mazaheri, et al, have done significant research into

the various sintering methods of commercially pure titania and

their effect on grain size and densification in 2008 [4]. Shen

Dillion, et al, have done research and created theories about

the role of grain boundary complexions into grain boundary

mobility and the formation of abnormal grains in 2007 and

2008 [6][7]. Chak Chan, et al, have done research on the

effects of different calcinations temperature on the

microstructure of sintered ceramics in 1999 [7]. Yu-hong

2 George J. Ferko V

Zhang, et al, have done research on the effects of various

metal dopants and the ability to inhibit grain growth in titania

in 2002 [9][10].

(4)

The goal of this experiment is to study the effects of

doping elements (Zn, Cu, and B), doping level, sintering

temperature, and holding time on the densification and grain

growth during the sintering of TiO2 ceramics [11].

Experimental Methods

(1)

3 grams of TiO2 anatase powder from Alfa Aesar with a

starting particle size of about 150 nm were measured on a

Mettler P1200 scale in a Nalgene low density polyethylene

(LDPE) tray and poured into a Nalgene LDPE bottle. 1 mol/L

of Zn(NO3)2 in ethanol is measured in a graduated cylinder

and poured into the LDPE bottle. The amount of dopant

needed is determined by equation 1 seen on page 3. An extra

10 mL of ethanol is poured in the bottle and the solution is

mixed and un-conglomerated ultrasonically in the VWR

B3500A-DTH for 10 minutes. The bottle was labeled and

placed under a Labconco fume hood overnight to evaporate

the ethanol. Some samples that remained wet had their drying

accelerated by putting a light over them. The sample was

poured into a double bag and excess was scrapped out of the

bottle with a clean plastic knife. The agglomerates were

broken by rolling a cylindrical metal weight over the bags.

The sample was put into an alumina crucible and placed in the

Lindberg/Blue M box furnace model # BF51866A-1 where it

was calcined at 600oC for 2 hours with a heating and cooling

rate of 5oC/min. The total time of the calcinations comes out

to six hours; however, the samples were left in the furnace

overnight and removed the next day after which they were

poured back into their plastic bag. 0.8 grams of powder was

poured from the 10%Zn-TiO2 onto the Mettler P1200 scale in

a Nalgene HDPE tray. Agglomerates were broken with a

plastic fork which had been kept in a bag designated for the

10% Zn doped powders. The powder was poured into a tool

steel die and placed into the Carver Laboratory Press model:M

and pressed at very low pressure. The pellet was released and

die was held carefully while the cylindrical ring used to push

out the pellet was put on and the die and ring were put back in

the press. Pressure was reapplied slowly until the pellet

emerged and was placed into a latex glove. The air was

sucked out of the glove using a Welch – Chem Star – Model:

1400N – vacume pump so that isostatic pressure could be

applied. During pressing the glove was tied shut and the

pellets were isostatically pressed at 45000 psi for 3 minutes in

the Fluitron cold isostatic press. The pressure is built up and

released slowly. After pressing the pellet it will be 40-60% of

the theoretic density. Excess oil was washed off the gloves

with hot water and pellets were cut out of each finger in the

glove. The pellet was placed in a Pall sample holding plastic

tray. A layer of TiO2 anatase powder was poured into an

alumina crucible as a base layer under the Zn-TiO2 pellets and

an additional layer of TiO2 anatase powder was poured on top.

The pellets were placed into the Thermo Scientific –

Thermolyne – TSM furnace where they will be held at

1000oC for 2 hours with a heating and cooling rate of 5

oC/min.

The following day the samples were removed from the furnace

and returned to their Pall sample holding plastic trays. An

epoxy mounting container was cleaned with ethanol and

release agent was swabbed onto the mounting surfaces of both

parts of the two part mounting container. Hardener was mixed

with the epoxy resin for 2 minutes until it was clear and then

poured into the mounting mold on top of the sample. The

sample was placed into a vacuum and vacuum was created and

released twice (for 2 cycles). The sample was placed under a

fume hood to allow it to cure for 24 hours. A Buehler Ltd.

Metallurgical Apparatus grinding wheel was used on low

speed for the grinding and polishing of the sample. The

sample was hand grinded and polished using 320, 400, 600,

8µm, and then 3µm SiC paper followed by 1µm diamond

paste. The samples were cleaned with ethanol and then

placed, while in ethanol, into an F350 ultrasonic for 5 minutes

to be cleaned. Vibratory polishing was done with the Buehler

Vibromet I polisher using a 0.15 μm SiO2 colloidal suspension

for 24 hours. Sample was removed and sprayed with ethanol.

The epoxy sample was then heated in the L and L box furnace,

so that fumes would be carried away by the hood. Pellet was

removed from the epoxy while it was still brittle. The pellet

was returned to an alumina crucible for thermal etching. The

Thermo Scientific – Thermolyne – TSM furnace was heated

to 900oC where temperature was held for 30 min with a

heating and cooling rate of 5oC/min. After thermal etching the

sample was removed and placed on SEM mounting stubs

using carbon tape. The samples were coated with Iridium in

the Electron Microscopy Sciences – Model: EMS5575X –

peltier cooled Iridium coating machine for 8 sec. Using the

JEOL 840 SEM with an accelerating voltage of 15kV and a

working distance of 15mm micrographs of the sample were

taken at the top edge, middle, and bottom edge. Magnification

was determined subjectively by considering how well the

images would provide accurate data. Print outs of these

micrographs were used to graphically analyze the sample for

grain size, using equation 5, and percent porosity. The

abnormal or normal grain growth was observed subjectively.

(2)

3 George J. Ferko V

Volume of the doping solution is a function of c, x, m,

and M, where c is the molar concentration of the doping

solution, x is the doping level, m is the mass of the TiO2

powder, and M is the TiO2 molar mass. The equation used to

calculate V, the volume of the doping solution is shown below

in equation 1.

(1)

(3)

The doped TiO2 powders must be calcined in order to

remove any volatile elements that may remain from the doping

process in week one. In week one the doping agents are added

to the TiO2 anatase powder in solution to produce a good

distribution of dopant throughout the anatase powder. The

ethanol in the solution is evaporated from the solution and

then calcination takes place according the reactions below,

shown in equations 2, 3, and 4.

(2)

(3)

(4)

As can be seen, the doping elements are not the only thing

initially added to the TiO2 anatase powder, but additional

atoms remain for each dopant atom. The calcination process

removes the additional atoms by raising the temperature of the

sample so that those additional atoms become volatile and

leave the sample. The calcination temperature is chosen by

finding a temperature that is higher than the decomposition

temperature so that the free energy of the system after the

reaction is lower than the free energy before. The

decomposition temperature for the various dopant precursors

is determined by thermal gravitational analysis (TGA). In this

test the sample mixed with dopant is heated while on a scale

until the weight of the sample decreases. The temperature that

is chosen must be as low as possible to prevent the formation

of agglomerates. The temperature must be below the

temperature at which anatase goes through a phase change into

rutile to prevent rutile phase from forming. The temperature

must also be below the temperature at which the crucible

begins to significantly diffuse into the TiO2 [8][12]. The same

temperature must be used for all the samples so that the initial

conditions for sintering are the same for all dopant-TiO2

systems.

(4)

Samples are embedded in TiO2 anatase powder because of

the high energy state that the samples will be in when heated

to their respective temperatures. If the samples were not

embedded in powder particles from the sample would be

excited and released into the air around the sample in the

furnace. The samples are embedded to prevent any diffusion

of the alumina crucible into the titania sample. Also the cost

of maintaining an inert atmosphere in a furnace for sintering is

very high. If the doped TiO2 samples are embedded in titania

powder than a non-inert atmosphere will not have an

opportunity to react with the actual sample. The embedding of

the samples allows them to be sintered without the conditions

inside the furnace, other than temperature, effecting the final

microstructure and densification.

(5)

Samples must be etched after polishing in order to reveal

their grain boundaries so that their microstructure may be

studied. The thermodynamic principle working here is that

when the surface of the sample is excited using high

temperature the highest energy material will diffuse to a lower

energy state outside of the sample. The highest energy

material happens to be on the samples grain boundaries‟, so

heating the sample proves to be a good way to reveal the

microstructure.

(6)

The sample pellet does not need to be embedded in titania

powder during thermal etching. This is so because the

sintering of the sample was performed at a higher temperature

than the thermal etching. This means that the bulk of the

material in the sample energetically prefers to stay in the

sample at temperatures even higher than the thermal etching

temperature. The only material that will leave the sample

during thermal etching is the material on the grain boundaries.

The temperature of thermal etching is not high enough for

diffusion from the crucible into the sample to occur at

significant levels. If the sample were embedded in powder

than some of the powder would remain on the surface of the

sample making valuable imaging in the SEM impossible.

(7)

The equation used to calculate grain size from the SEM

images is shown below in equation 5. The method used is

referred to as the linear interception method. The value

obtained from this two dimensional analysis is slightly skewed

from the actual three dimensional value. The skew of the data

is not taken into consideration because the two dimensional

analysis is just as accurate at comparing relative grain size.

180mm is the length of the measuring line, n is the number of

times the measuring line hit a grain boundary, µm is the length

of the micron bar as it is read from the image, and mm is the

length of the micron bar on the print. The grain size from this

equation will be in units of µm.

(5)

4 George J. Ferko V

Results and Discussion

(1)

a. b.

c. d.

Figure 1: a. TiO2 doped with 10% Zn and sintered for 2 hours at 1000oC, b. TiO2 undoped and sintered for 2 hours at 1000

oC, c. TiO2

doped with 1% Zn and sintered for 2 hours at 1000oC, d. TiO2 doped with 10% Zn and sintered for 2 hours at 1200

oC.

(2)

The 10% Zn – TiO2 sample that was sintered for 2 hours

at 1000oC can be seen in figure 1a. Grains in the sample have

grown to a size of 0.88 µm with the peak in grain growth rate

occurring somewhere near 2 hours, as can be seen in figure 2

below on page 5. The sample has the 10th

smallest grain size

of all of the 42 samples tested and the 4th

smallest grain size

out of all of the 12 Zn doped samples. The 10% Zn – TiO2

sample that was sintered for 2 hours at 1000oC is the 17

th most

dense sample over all of the 42 samples tested and the 5th

most

dense over all of the 12 Zn doped samples with a percent

porosity of 5%. Although the porosity data may be inaccurate

do to the small number of measurements taken, it can be

assumed that this sample is somewhere in the middle range as

5 George J. Ferko V

far as relative densification goes. For a description of the

effects of temperature, sintering time, and doping levels on

densification refer to the results and discussion section,

subsection (5) on page 6. Original particle size as well as the

presence of agglomerates plays a large role in the densification

process. The compaction of the green pellet also plays a large

role in the final amount of densification. These variables have

not been taken into consideration in this experiment. The

grain growth of the 10% Zn – TiO2 sample that was sintered

for 2 hours at 1000oC was much less than those performed at

higher temperatures and for longer times, figures 4 and 5.

This does fit the grain coarsening theory which states that

higher temperature and longer time result in larger grains. All

of the undoped samples for the sample temperature have a

smaller final grain size indicating that the Zn doping facilitates

grain growth in some way by changing the energy associated

with the grain boundary interface and the diffusivity across the

grain boundaries [4].

(3)

Throughout the lab there was ample opportunity for the

introduction of impurities into the sample. Impurities consist

of oil from students‟ hands, oil from the isostatic press, other

ceramic materials in the lab rooms that were worked in, and

atmospheric gases from the furnaces that did not have inert

gas atmospheres. The rooms that the sample was handled in

were not clean rooms and the hood it was left under to

evaporate ethanol out of the original doping solution was not a

clean hood. The containers that the sample was stored in may

not have been cleaned as well as is possible. Any time the

sample was subject to heating it was not in an inert

atmosphere thus there may have been some reaction with the

atmospheres in the furnaces used for calcination, removal of

the epoxy sample mount, and thermal etching. Accidental

dropping the sample or contamination of equipment used

throughout the lab, such as plastic knives, plastic forks, plastic

bags, crucibles, and sample trays could have led to

contamination of the sample. Accidentally touching the

sample while trying to mount it on the SEM stub or while

moving it between containers may also have caused the

sample to be contaminated. Contamination after sintering is

not as important as contamination before sintering.

Contamination before sintering may have affected the

diffusion properties of the sample. It is likely that some

contaminates would have sat on the grain boundaries causing a

change in grain growth and densification. The contaminates

would act as impurity defects and change the local driving

forces around them. The effect of the contamination has been

ignored for the most part because of high concentrations of

dopant being used in the doping process. Because such a large

concentration of dopant is used little to no effect from

contaminates should be noticeable in the samples. These

effects that are present may have been marginally avoided by

looking for consistency in microstructure across the samples

during the taking of micrographs in the SEM. By avoiding

analysis of abnormal sections of the pellet and taking

micrographs of the microstructures that appeared to be most

common in the sample any abnormal grains caused by

contamination should not have been included in the porosity

and grain size data.

(4)

Figure 2: Plot of the grain size versus the sintering time for TiO2 samples that were undoped, doped with 1% Zn, and doped with 10%

Zn and sintered at 1000oC, as well as samples that were undoped, doped with 1% Zn, and doped with 10% Zn and sintered at 1200

oC.

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20

Sin

terin

g T

ime

(hrs

)

Grain Size (µm)

Grain Size vs. Sintering Time

undoped 1000

undoped 1200

1%Zn 1000

1%Zn 1200

10%Zn 1000

10%Zn 1200

6 George J. Ferko V

(5)

The temperature at which sintering takes place seems to

play a large role on densification. All of the Zn doped

samples that have a lower percent porosity then the 10% Zn –

TiO2 sample that was sintered for 2 hours at 1000oC have been

sintered at 1200oC, figures 4 and 5. This indicates that a

greater amount of densification can be seen with temperatures

above 1000oC. None of the undoped samples achieved a

lower percent porosity than the 10% Zn – TiO2 sample that

was sintered for 2 hours at 1000oC. This indicates that the

dopant had some effect as to increase the densification;

however, because of the inaccuracy of the data this effect

cannot be pin-pointed. Among the Zn doped samples those

with a longer sintering time most often have a higher

densification than those with a shorter sintering time. Overall,

the Zn and Cu doped samples have greater densification than

the undoped and B doped samples, the samples with longer

sintering times have greater densification than those with short

sintering times, and the samples with higher sintering

temperatures have greater densification than those with lower

sintering temperatures. This is interesting data because it is

expected that grain and pore coarsening will occur at higher

temperatures and longer sintering times. In the samples tested

this does not occur.

(6)

The doping level will change the position on the phase

diagram that is dealt with during the sintering process. This

change in position does not affect the phase changes that the

Zn-TiO2 goes through. The micrographs of the 10%Zn-TiO2

taken of the samples that were sintered at 1200oC indicate that

a second phase forms in the grain boundaries of the sample. It

is unclear what the composition of the second phase is because

no analysis was done on it, but it is clear that the second phase

forms due to the level of doping and the sintering temperature.

The doping level will affect the concentration of defects that

take occur and thus affect the diffusivity in the sample. A

higher concentration of dopant makes extrinsic defects,

equation 6, become more likely relative to intrinsic defects,

equation 7, shown below. We know that for all practical

ceramics the defect reaction in equation 6 is the more likely to

occur.

(6)

(7)

Increase in the number of defects is known to lead to an

increase in the diffusivity of a ceramic and thus will change

the sintering behavior. From the micrographs of the samples

we can see that increasing the amount of dopant seems to

Figure 3: Phase diagram for the ZnO-TiO2 system showing all

phase transformations above the transformation from anatase

to rutile crystal structure [13].

increase the amount of abnormal grains, figures 4 and 5. The

amount of Zn dopant does have some effect on densification.

The low temperature Zn doped samples decrease in porosity

over time compared to the undoped low temperature samples

which do not change a large amount over time, figure 4. The

high temperature Zn doped samples have been highly

densified previous to the times at which they are observed and

the undoped samples have densified much less, figure 5. The

data here indicates that doping facilitates the densification of

TiO2 in some way. It is likely that this occurs because the

additional defects that occur due to the presence of dopant

effect the diffusion through the lattice and the interfacial

energy. The effect of diffusion through the lattice and

interfacial energy on densification rate can be deduced from

the Herring scaling law seen below in equation 8.

(8)

Grain size increases with the addition of Zn dopant. The grain

growth kinetics that apply to grain size are very similar to

7 George J. Ferko V

those that apply to densification. The addition of dopant

causes an increased number of defects and thus changes the

interfacial energy at the grain boundary as well as the

diffusivity across the grain boundary. This concept is shown

in equation 9.

(9)

The concentration of dopant will effect two terms in the

numerator of the equation so we can expect the effect of

concentration of dopant on grain growth and final grain size to

be exponential. In the high temperature 10% Zn doped sample

the grain growth is restricted by the presence of a second

phase. For the high temperature the sample with the lower

concentration of Zn has a larger average grain size than the

sample with a high concentration of Zn. The second phase

lowers the diffusivity that occurs in the perpendicular

direction across the grain boundary and changes the interfacial

energy between the grains causing the change in grain growth.

A significant amount of impingement may have also occurred

to inhibit grain growth. The two temperatures the samples are

sintered at are both in the Zn2TiO4+Rutile phase on the phase

diagram. This should show that the temperature has no effect

on the phase that is present during sintering. This is not the

case. The high temperature 10% Zn doped sample has a

second phase present in the microstructure where lower

temperature sample does not. The increase in temperature will

increase the defect concentration causing similar effects to

grain growth and densification as the increase in dopant

concentration. The equation that shows the effect of

temperature on the extrinsic defect concentration is shown

below in equation 10.

(10)

Equations 8 and 9 indicate that the increase in temperature

should lower densification and grain growth due to the

temperature term in the numerator. The effect that

temperature has on the defect concentration, diffusivity, and

interfacial energy is great and so the grain growth and

densification actually increase with temperature. This can be

seen in the micrographs taken of the samples, figures 4 and 5.

The effect of sintering time on grain size can be found by

taking the integral of equation 9. This new equation can be

seen below in equation 11.

(11)

This equation shows that the grain size relative to the grain

size at a sintering time of zero increases linearly with

increasing sintering time. When the grains are inhibited from

growing due to a second phase or impingement the grain

growth is slowed. Sintering time increases the densification

and can be seen in the micrographs of the low temperature Zn

doped samples. The densification of the Zn doped samples at

high temperature does not seem to vary with sintering time;

however, the densification in the doped samples is much

greater than in the undoped samples. This occurs in the high

temperature samples because the samples have already

densified to a large extent and if there is very little porosity

remaining than the sample cannot continue to densify at a high

rate.

(7)

The undoped samples increase in grain size with

increasing sintering time and increase in densification with

increasing temperature, figures 4 and 5. The increase in

sintering time does not appear to have an effect on

densification. Grain size increases with increasing

temperature. The Cu doped samples, figures 6 and 7, increase

in grain size with increasing sintering time. The effect of

sintering time on densification is unclear because inaccuracy

of the data. The higher sintering temperature increases the

grain size in the Cu doped samples a large amount, 10 to 20

micrometers in most cases. Higher sintering temperature does

not appear to have a significant effect on densification for the

Cu doped samples. The concentration of dopant in the Cu

doped samples has a large effect on the sintering behavior.

Second phase in the high temperature sample appears to be

slowing down the rate of grain growth. This can be deduced

by noting that the difference in grain size between the 10% Cu

doped sample and the 1% Cu doped sample is much less than

the difference in grain size between the 1% Cu doped sample

and the undoped sample. According to equation 11 this is an

indication that the grain boundary mobility is decreasing due

to formation of second phase. Concentration of dopant

doesn‟t have a clear relationship to densification; however, the

Cu doped samples are clearly less porous than the undoped

samples. The B doped samples, figures 8 and 9, increase grain

size with increasing sintering time. The effect of sintering

time on densification in the B doped samples is unclear from

the data. The grain size of the B doped samples increases with

increasing sintering temperature and the effect of sintering

temperature on densification is unclear from the data. The

grain size increases with concentration of dopant and the

effect on densification due to concentration of dopant is

unclear from the data. It should be noted that the abnormal

needle shaped grains that develop for the B doped samples

likely keep them from densifying with a trend connected to

higher temperature, sintering time, or concentration of dopant.

The abnormally shaped grains in the B doped sample occur

8 George J. Ferko V

Sintering time: 0hours 2 hours 6 hours

a.

Porosity: 20.80%, Grain size: 0.17µm Porosity: 20.30%, Grain size: 0.32µm Porosity: 20.80%, Grain size: 0.47µm

b.


c.

Porosity: 24.89%, Grain size: 0.54µm Porosity: 5.00%, Grain size:0.88µm Porosity: 5.89%, Grain size: 1.21µm

Figure 4: a. undoped samples sintered at 1000oC, b. 1%Zn doped samples sintered at 1000

oC, c. 10% Zn doped samples sintered at

1000oC.

9 George J. Ferko V


a.


b.


Figure 5: a. 1% Zn doped samples sintered at 1200oC, b. 10% Zn doped samples sintered at 1200

oC.


a.


b.


Figure 6: a. 1% Cu doped samples sintered at 1000oC, b. 10% Cu doped samples sintered at 1000

oC.

10 George J. Ferko V


a.


b.


Figure 7: a. 1% Cu doped samples sintered at 1200oC, b. 10% Cu doped samples sintered at 1200

oC.


a.


b.


Figure 8: a. 1% B doped samples sintered at 1000oC, b. 10% B doped samples sintered at 1000

oC.



a.


b.


Figure 9: a. 1% B doped samples sintered at 1200oC, b. 10% B doped samples sintered at 1200

oC

due to a grain boundary complexion transition that must occur

at some point in the sintering process. For a given dopant

there is a certain concentration of that dopant necessary to

cause a complexion transition. The likelihood of excess

dopant and transition increases with increasing grain growth.

It is possible that some abnormal grains may form due to

extrinsically caused inhomogeneity, but in the case of the B

doped TiO2 the abnormal grains occur very frequently

throughout the sample. This confirms that the abnormal grain

growth is due to at least two grain boundary complexions

coexisting during grain growth in the sample [6][7].

(8)

The function of dopants in sintering and the mechanics by

which they affect the microstructure of ceramics has been a

long debated issue. The concept of complexion has begun to

solve many discrepancies in the models that predict the

behavior of ceramics with the addition of different

concentrations of dopant. The concentration of dopant has

been found to cause a certain complexion on the grain

boundary which increases grain boundary mobility. The

change in grain boundary mobility due to different doping

concentrations as well as the effect of changing sintering

temperature and sintering time on the grains is the basis for

this lab. The complexion of the grain boundaries not only

explains the sintering behavior and the change in sintering

behavior with sintering time and temperature, but it also

explains the formation of abnormally shaped grains and the

interaction of grains with a second phase. The concept of

complexions can explain why the Zn doped TiO2 grains have

not become abnormal at the contact points with the second

phase. The same explanation can be used for the grains that

are in contact with second phase in the Cu doped sample.

Complexions explain that it is not the saturation of dopant

along the grain boundary that causes abnormality in the grain.

It is the complexion of the grain boundary that will affect its

growth. The grain boundary mobility of all of the samples is

directly related to the growth rate of the grains. This shows

that studying the complexions of Cu, Zn, and B doped TiO2

would help to explain the mechanisms by which their grains

grow [6][7].

Conclusions

This study has provided insight into the formation of second

phase, the growth of abnormal grains, and the effect of the

level of doping, sintering temperature, and sintering time on

the densification and grain size in the final samples. The

following conclusions were made from observing the

micrographs of the doped TiO2 as well as porosity and grain

size data:

1. The Cu doping of titania samples caused second

phase to be present in all samples.


2. The Zn doping of the samples caused second phase in

all samples sintered at 1200oC.

3. The B doing of samples will cause formation of

abnormal grains.

4. Longer sintering time always leads to larger grains.

5. Higher doping level always leads to larger grains

except in the Zn doped samples where the formation

of second phase inhibits grain growth.

6. Higher sintering temperature always leads to larger

grains in the samples tested.

7. The doped samples will typically exhibit a higher

densification than the undoped samples.

8. Densification will be inhibited in the B doped

samples due to the elongated shape of the grains.

9. Densification will increase with an increase in

sintering time in the Zn doped samples when sintered

at 1000oC.

10. Elongated grains in the B doped samples will be

caused by the coexistence of two or more grain

boundary complexions making one side of the grain

have greater grain boundary mobility than the other.

This occurs due to a transition in grain boundary

complexion during the sintering process.

Future Work

(1)

It is suggested that before future analysis of the Cu, Zn,

and B dopants in TiO2 is done that more data be collected.

Data should be collected for more concentrations of the

dopants by diffusing the dopants through the samples to

develop a concentration gradient of the dopant. The samples

should also be sintered for shorter intervals of time as well as

longer times. The sintering temperatures for the samples

should vary more. Specifically the samples could be sintered

at a higher temperature if alumina crucibles were not used and

zirconia crucibles were used instead. After data is collected

for all of these variables analysis of complexions could be

done in the TEM by milling out sub-100 nm films using the

FIB and taking images of the grain boundaries. By this

method the different types of grain boundary complexion and

the conditions for transitions in grain boundary complexion

could be found for the TiO2 systems. Discovery of the

different types of complexion in the TiO2 systems would

explain the actual mechanics behind the sintering of these

samples. Hot pressing may also be used as the sintering

process to study other effects of the dopants.

(2)

On the atomic level complexion is caused by the different

atomic radii and valance charges between the dopant and the

material to be doped. Using this scenario it can be assumed

that other dopants that will be good for inducing grain growth

will have similar valance charges and atomic radii to those

used in this lab. This is assumed because the dopants used in

this lab caused increased grain growth suggesting that the

complexions created increase grain boundary mobility. To

form similar complexion to that created by Zn, Cu, and B

dopants such as Silver (Ag), Cadmium (Cd), Indium (In),

Gallium (Ga), Aluminum (Al), Silicon (Si), and Carbon (C).

It is very possible that some of these suggested dopants will

not facilitate grain growth at all, however, because the actual

complexions have not been evaluated in this lab no conclusion

can be made about exactly what dopants will enhance grain

growth.

(3)

In three separate sintering papers on Si-doped TiO2 the Si-

base precursor was tetraethyl orthosilicate, Si(OC2H5)4

[9][10][14]. This precursor is mixed with the TiO2 powder in

butanediol, C4H10O2, and calcined. The proper method for

selecting a dopant is to check if the doping will result in the

release of toxins. If that does not occur than the solubility of

the dopant in the solution and material to be doped should be

checked. If only dopants that release toxins are soluble with

the material to be doped than they must be used. Once these

major factors help to narrow down dopants other factors such

as availability and cost may be factored into a decision about

choosing a dopant. The dopant picked above was not arrived

at by this process. This dopant was found in literature on

doping TiO2. The precursor chosen for doping with alumina is

aluminum nitrate, Al(NO3)3. This precursor produces no

toxins and is soluble in water. The precursor can be mixed

with titania in water and dried to produce an even distribution

of dopant. During calcinations no toxins will be released. The

precursor is also readily available from Alfa Aesar.

(4)

This lab was very interesting. The amount of outside

research required and the length of the lab really helped to

reinforce many of the concepts learned about in the class. My

only wish is that the lab was coming to an end at a slightly

more convenient time.

References

[1] Richard Walton, "Titanium oxides", in

AccessScience@McGraw-Hill,

http://www.accessscience.com, DOI 10.1036/1097-

8542.801320.

[2] “Chemistry Sectors: Colorants & Fillers – Titanium

Dioxide Manufacturers Association (TDMA) – TiO2

– Uses and Properties”, cefic, 2009.


http://www.cefic.be/Templates/shwAssocDetails.asp?

NID=473&HID=25&ID=173.

[3] “Titanium Dioxide”, Wikipedia, 2009, <

http://en.wikipedia.org/wiki/Titanium_dioxide>.

[4] Mehdi Mazaheri, et al, “Sintering of Titania Nanoceramic:

Densification and Grain Growth”, Ceramics

International 35, 2009, pp 685-691.

[5] J. Rubio, et al, “Characterization and Sintering Behavior

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Titanium Tetrabutoxide” Journal of Materials

Science 32, 1997, pp 643-652.

[6] Shen J. Dillon, et al, “Complexion: A New Concept for

Kinetic Engineering in Materials Science”, Acta

Materialia 55, 2007, pp. 6208-6218.

[7] Shen J. Dillon, et al, “Demystifying the Role of Sintering

Additives with „Complexion‟”, Journal of the

European Ceramic Society 28, 2008, pp. 1485-1493.

[8] Chak K. Chan, et al, “Effects of Calcination on the

Microstructures and Photocatalytic Properties of

Nanosized Titanium Dioxide Powders Prepared by

Vapor Hydrolysis”, J. Am. Ceram. Soc. 82 (3), 1999,

pp 566-572.

[9] Yuhong Zhang, et al, “Nanotubes in Si-Doped Titanium

Dioxide”, Chem. Commun. 2002, pp 606-607.

[10] Yu-Hong Zhang, et al, “Phase Transformation and Grain

Growth of Doped Nanosized Titania”, Materials

Science and Engineering C 19, 2002, pp 323–326.

[11] ShuaiLei Ma, “Sintering of TiO2 – Spring 2009 -

Objective”, pp. 1.

[12] “Calcination”, Wikipedia, 2009,

<http://en.wikipedia.org/wiki/Calcination>.

[13] J. Yang, et al, “The Phase Stability of Zn2Ti3O8”,

Materials Characterization 37, 1996, pp 153-159.

[14] Jeerapong Watthanaarun, et al, “Titanium (IV) Oxide

Nanofibers by Combined Sol–gel and

Electrospinning Techniques: Preliminary Report on

Effects of Preparation Conditions and Secondary

Metal Dopant”, Science and Technology of

Advanced Materials 6, 2005, pp 240–245.

doped titania project zinc focus - george j. ferko v

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