Types of Materials• Metals

• High density

• Medium to high melting point

• Medium to high elastic modulus

• Reactive

• Ductile

• Ceramics

• Low density

• High melting point

• Very high elastic modulus

• Unreactive• Brittle

• Polymers• Very low

density• Low melting

point• Low elastic

modulus• Very reactive• Ductile and

brittle types

Ceramics• Covalent or ionic

interatomic bonding• Often compounds;

usually oxides• New “engineering

ceramics” can also be carbides, nitrides and borides

Materials Data• Hard brittle solids - no

unique failure strength because it depends on crack size

• Data can vary markedly from manufacturer to manufacturer

• Strength may depend on history after manufacture (surface damage)

• Some data are invariant - structure insensitive e.g. Melting point, Density, Elastic Modulus

• Others are highly structure sensitive e.g. Tensile Strength, Fracture Toughness, Thermal conductivity, Thermal Expansion Coefficient

Processing of Advanced Materials –ceramic particle processing

• Ceramic powder synthesis

• Ceramic particle interaction in liquid suspension

• Ceramic powder packing

• Ceramic forming and sintering

Reading List

• Y. M. Chang, Physical Ceramics, John Willey 1997

• D. Segal, Chemical Synthesis of Advanced Ceramic Materials (Cambridge University Press 1991).

• J. S. Reed, Principles of Ceramic Processing (Wiley Interscience 1995)

• T. A. Ring, Fundamentals of Ceramic Powder Processing and Synthesis, Academic Press (1996)

Conventional Routes to Ceramic Powder

• Precipitation from solution

• Powder mixing

• Fusion

Example 1: alumina production via The Bayer Process:

• Alumina occurs as the mineral bauxite and is refined in the Bayer process whereby ore is initially dissolved under pressure in sodium hydroxide so that solid impurities ( SiO2, TiO2, Fe2O3) separate from sodium alumina solution

• Al2O3+2OH-+3H2O=2[Al(OH)4]-

OHNaAlONaOHOHAl2

223

This solution is either seeded with gibbsite (β-Al2O3.3H2O) after its neutralisation with CO2 gas.

NaAlO2 + 2H2O =Al(OH)3 + NaOH2Al(OH)3= 2Al2O3 + 3H2O

Temperature, alumina supersaturation and amount of seed affect particle size during crystallisation but, as for other precipitation reactions, the production is agglomerated.

Production of yttria stablised zirconia from the minerals baddeleyite (ZrO2) and zircon sand (ZrO2.SiO2)

Zirconia exists in three crystallographic forms while monoclinic phase exists at room temperature for pure zirconia phase. The monoclinic zirconia is a brittle material, and toughening of zirconia can be achieved by incorporation of oxide such as MgO, Y2O3 and CaO into zirconia to achieve cubic phase and tetragonal phase.

Precipitation from solution

Powder mixing

1) Blending different oxide powders

2) The mixture is grinded or milled

3) Calcination

4) Firing with intermediate grinding

Example: YBa2Cu3O7-δ

From mixing Y2O3, BaCO3 and CuO, grinding and heating at 1223K in air. Powder was then pressed into pellets, sintered in flowing O2, cooled to 473K in P2 and removed from the furnace.

Fusion Route to Ceramics

Reactants are mixed as powders and melted, after which nucleating agents were added. The temperature is then raised until crystallisation occurs and the microstructure is developed.

These methods are limited by the melting point of reactants while high temperature can lead to loss of volatile oxides.

Need for improved synthetic routes for advanced ceramics

1. Produce fine powder with low degree of agglomeration which commonly occurs in traditional methods.

2. Coatings and fibre productions which can not achieved using conventional methods.

3. Remove inhomogeneity and voids which occurs in ceramics produced with conventional methods

4. Other starting materials for manufacture of monolithic ceramics

Questions:1. Compare different conventional ceramic processing

techniques and describe their limitations.

2. What materials can be produced using conventional ceramic processing methods?

Liquid Phase synthesis by precipitation

Advantage: making pure products

Disadvantages:

1. Drying and calcination

2. Agglomeration and aggregation

Synthesis by precipitation:

Precipitation and Solubility

1. Precipitation due to evaporation

2. Precipitation due to high pressure

3. Precipitation due to reaction-induced super-saturation.

When a substance is transformed from one phase to another, the change in the molar Gibbs free energy at constant pressure and temperature is given by  where is the chemical potential of phase 1 (solute) and phase 2 (solid).

12 G

The molar Gibbs energy change can also be expressed as

 

SRTaaRTLnG ln

0

There are three main categories of nucleation: 1.      Primary homogeneous2.      Primary heterogeneous3.      Secondary

Homogeneous nucleation occurs in the absence of a solid interface; heterogeneous nucleation occurs in the presence of a solid interface of a foreign seed; and secondary nucleation occurs in the presence of a solute particle interface

 

Homogeneous nucleation

aSRTVvG ln

23

ln rSRTVrrG a

v

*stdr When S=e=2.718

Standard Critical size

*stdr

Angstrom Surface energy (J/m2)

802 1.00

401 0.50

40 0.10

20 0.05

2

maxlog

SAJ

Jlong

Nucleation rate ~ super-saturation ratio as 1/S=0

Heterogeneous nucleation

heterooT JJJ hom

Crystal Shape

i

ihhh

2

2

1

1

The shape of a crystal can be controlled by either thermodynamics or kinectics. Only for crystals grown under very, very low supersaturation ratio is a crystal habit established by thermodynamics. For most other crystal growth conditions, the kinetics of slowest growing crystal faces give rise to a crystal shape.

Kinetic shape

Diffusion shape

Aggregate shape Particles can aggregate by either Brownian or shear induced aggregation.

Need for improved synthetic routes for advanced ceramics

• Produce fine powder

• Coatings and fibre productions

• Remove inhomogeneity and voids

• Other starting materials for manufacture of monolithic ceramics

Questions:

• Define super-saturation S and explain why S >>1 for nucleation in most of cases.

• For producing crystal with equilibrium shape, what level of S should be and explain why?