CH5716Processing of Materials

Prof. J.T.S. Irvine

Dr M. Cassidy

Lecture JI1 – Solid State Reaction& Chemie Douce

Course Aims1. Understand the mechanism and kinetic limitations of conventional solid state reactions.

Understand the chemistry of sol gel processing and to provide examples of the use of

alkoxide and carboxylate precursors to prepare ultrafine oxide powders. To understand the

chemistry of intercalation/deintercalation processes, and to provide examples of their

importance.

2. Understand sintering mechanisms and how raw materials and green body processing

influence this. Discuss engineering of microstructures to achieve desired functionality.

3. Understand driving forces for sintering, mass transport mechanisms, and atomic mobility.

Solid-state sintering: mass transport mechanisms. Discuss and understand stages of

sintering and relation with microstructure development.

4. Understand liquid-phase sintering: thermodynamic and phase equilibria, sintering models,

and transient-liquid phase sintering.

5. Reactive sintering processes: reactive sintering, reaction bonding, and reactive hot

compaction.

6. Pressure-assisted sintering: effect of pressure in sintering, deformation mechanisms,

densification maps, and pressure-assisted sintering processes. Secondary phenomena:

phase transformations, Review the effects of constraint on sintering and final

microstructure. Application of sintering aids both solid state and liquid phase

Understand the mechanism and kinetic limitations of conventional

solid state reactions. Understand the chemistry of sol gel

processing and to provide examples of the use of alkoxide and

carboxylate precursors to prepare ultrafine oxide powders. To

understand the chemistry of intercalation/deintercalation processes,

and to provide examples of their importance.

Lecture 1, 2

Standard solid state synthesis

• Direct reaction of a mixture of solid startingmaterials:- most widely used method for thesynthesis of polycrystalline solids

• Need high temperature, typically > 800 C

• Thermodynamics and kinetics both important

• Why are solid state reactions difficult ?

• MgO + Al2O3 MgAl2O4 1500C >48 hours

ABC ABA ABC

1. Formation of MgAl2O4 nuclei:- difficult

(a) usually considerable differences betweenreactants and products

(b) large amount of structural reorganisationinvolved in forming the product

2. Growth of product layer:- may be moredifficult

-need counterdiffusion of Mg2+ and Al3+ ionsthrough the existing MgAl2O4 product layer tothe new reaction interfaces

MgO Al2O3MgAl2O4

MgO /MgAl2O4

2Al3+ -3 Mg2+ + 4MgO -> MgAl2O4

Al2O3 /MgAl2O4

-2Al3+ +3 Mg2+ + 4Al2O3 -> 3MgAl2O4

x 3x

Wagner reaction mechanism

Three important factors influence rate ofreaction between solids

1. Area of contact between the reacting solidsand hence their surface areas

2. Rate of nucleation of the product phase

3. Rate of diffusion of ions through the variousphases and especially through the productphase

Surface Area of solids

-area of contact between grains of reacting solidsdepends approximately on the total surface area

-surface area greatly increases with decreasingparticle size

Nucleation and diffusion

Nucleation aided by structural similarity betweenproduct and one or more of the reactants -reduces the amount of structural reorganisationnecessary

Diffusion of ions greatly enhanced by the presenceof crystal defects, e.g. vacancies

Kinetics

Rate determining step?

a) Transport of matter to the reaction interface

b) Reaction at the interface

c) Transfer of matter away from the reactioninterface

Slow kinetics can lead to metastable products

Calcination and reaction

Carbonates, nitrates, sulphates decompose onheating to yield oxide precursors.

Reaction profiles

E

t/T

Oxides

Carbonates

Ball Milling

Milling ProceduresSpeed of Rotation – Theaction of the grinding inside amill is determined be the speedat which the mill cylinder isrotating.Cascading occurs when the millspeed is such that the mediacharge breaks away from themill wall at an angle of 45 to 60degrees above the horizontal.The impact fractures the grains orthe charge.

High Density Alumina Spheres and Cylinders –Specific

gravities from 3.6 to 3.8 and an alumina content between 85

and 99 percent.

Zirconia Cylinders and balls

specific gravity of 5.5 equivalently sized zirconia grinding

media generally will mill twice as fast as high alumina. Zirconia

media is 1.6 times denser than high alumina, has a hard nonporous

surface that is chip resistant and easily cleaned. It is unaffected by

most chemicals, is nonconductive and nonmagnetic, and has

outstanding resistance to mechanical and thermal shock.

Grinding MediaWet or dry grinding is usually accomplished by the use of high-density

alumina spheres or cylinders and zirconia cylinders.

The choice between spheres and cylinders depends upon the application and millingneeds such as particle uniformity and hardness of the material being ground.

Specialty grinding media is also available in alumina and zirconia for ultra finegrinding and dispersing. Sizes starting at .2mm..

MILLING RULES OF THUMB:Media – Media size is a key factor in mill performance. The mediashould be 4 to 10 times the size of the largest agglomerate to havesufficient flatness for the hammer-like effect required.The most effective grinding will be accomplished by the smallest mediathat will do the job. Small media offer more contact per mill revolution.Larger media will have greater impact energy and may generateexcessive heat in the mill if this energy is not efficiently consumed in thegrinding action.Good practice calls for mills to be filled from 45 to 55% of their totalvolume.

TRANSITION FROM LAB TO PRODUCTION:The transition from grinding inlaboratory mill jars to grinding in production sized mills is a fairly straightforward one.

For the growth of ZnO nanorods mechanochemical reactions were carried outin a planetary ball-milling apparatus (Retsch, Germany). Zinc acetate [Zn(CH 3COO) 2 ], N-cetyl, N, N, N-trimethyl ammonium bromide (CTAB), a cationicsurfactant and sodium hydroxide pellets were used as starting materials, whichwere procured from Sigma-Aldrich, Germany. Millings were performed at 300rpm for the time durations 30 min, 2 h and 5 h. The ball to starting materialmass ratio was kept at a fixed ratio of 10:1. After the mechanochemicalreaction, the product was washed severaltimes by DI water (Millipore)

Mechanosynthesis

CHIMIE DOUCE

Gentle (relatively) chemical route to solid stateproduct

My definition: (much) lower reactiontemperature than conventional solid statesynthesis

Rouxel and Livage mid 70's

Variety of Chemical Reactions

-Cation exchange, dehydration,dehydroxylation, hydrolysis, redox,intercalation, deintercalation +...

CHIMIE DOUCE

Features

• Lower temperature routes, even roomtemperature possible

• Metastable products possible/likely

• Small particle size 10-500 nm

• Can be high cost, small scale

TiO2 (B) synthesised by hydrolysis of K2Ti4O9

(less dense than other forms of TiO2)

K Ti O H Ti O .H O2 4 9 2 4 9 2HNO3

H Ti O .H O TiO2 4 9 2500

2

o C Vacuum B, ( )

Filter

(Hydrolysis productH3OTi4O8(OH) - Ti4O9 sheet retained)

TiO2 B

anatase rutile

CHIMIE DOUCE

Gentle (relatively) chemical route to solid stateproduct

My definition: (much) lower reactiontemperature than conventional solid statesynthesis

Rouxel and Livage mid 70's

Variety of Chemical Reactions

-Cation exchange, dehydration,dehydroxylation, hydrolysis, redox,intercalation, deintercalation +...

CHIMIE DOUCE

Features

PRECURSOR METHODS

Mixing on atomic scale

Ferrites MFe2O4 (spinels)

FeC2O4.2H2O Co-precipitated from boilingsolutions with the corresponding oxalates ofMn, Co, Ni, Zn (II)

Finely crystallised, free flowing powderscontaining metals mixed on atomic scale

MFe2(C2O4)3.6H2O+2O2 -> MFe2O4 +6H2O+6CO2

Ratio of Fe:M +/- 1% at best

Better stoichiometry with

Ni3Fe6(AcO)17O3OH.12C5H5N

Hexagonal WO3

Na2WO3.2H2O + HCl -> tungstic acid gel precursor

hydrothermal treatment

dehydration and heating to 400oC

WO3 (hexagonal)

120oC