lecture A
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Transcript of lecture A
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?