processing of ceramics. powdercompact or “green” ceramic forming sintering or densification or...
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Processing of ceramics
Processing of ceramics
powder compact or“green”
ceramic
Forming
Sintering ordensification or
firing
T 2Tm/3
ConventionalHot-pressing
Hot isostatic pressingSpark plasma sintering
Starting powders
Milling
Mix with binder and other additives
Dry/Granulate
Powder compaction/shaping:Dry pressing
Injection moulding
Binder burnout
Sintering
Machining and finishing
Slurry/Slip/Paste
Shape forming:Slip casting
Tape castingExtrusion
Dry
Binder burnout
Sintering
Finishing
Powder compact:green
Dense polycrystalline body: ceramic
Processing of ceramics
Solvents:WaterOrganic solvents (Tb= 100-140°C)EtOH-MEKToluene-EtOH
Particle packing and granulation
“perfect” powder Maximum packing
In practice powders are composed of particles with a size distribution and often the particle shape is not spherical.Random packing of spheres with a log-normal distribution 67%.
Schematic diagram of a spray-drier
Particle packing and granulation
Angular shape of milled alumina
Sketch of an agglomerate with uncontrolled shape
Particles with sharp edges or formed by irregular aggregates do not flow and do not pack efficiently (r.d. <50%). Granulation is essential.
Granules can be obtained by forcing a rigid ceramic paste through the mesh of a sieve or, better, using a spray-drier
Powder consolidation and shaping
Uniaxial pressing Isostatic pressing
Slip castingTape casting
Dry powder, very simple shapes.Die-wall friction introduces density gradients which lead to differential densification and distortions during sintering
Uniform pressure gives uniform green density and limits lamination. Used for mass production of spark plugs and high-voltage insulators
Viscous slip (50% solid)
Plaster of Paris mould
Slip surplus
Water
Extrusion
A ceramic paste containing binders and lubricants is forced through the orifices of a die. Components with uniform section and high length/diameter ratio, such as rods and tubes. Used also for thick dielectric substrates.
Traditional pottery industry and technical ceramics (zirconia, Si3N4, SiC)
Injection moulding
Ceramic powder + 40% thermoplastic, need careful burnout. Complex shapes, high shrinkage (15-20%).
Moving band
Up to 30% of organic additives (deflocculant, binder and plasticizer). Water or organic solvents. Used for electronic substrates, multilayer ceramic capacitors and actuators.
Powder consolidation and shaping
Powder consolidation and shaping
•Binder: gives the dry shape (green) sufficient strength for handling before sintering (starch, cellulose ethers, polyvinyl alcohol, polymethacrylates, polyvinylbutyral).
•Deflocculant/dispersant: gives the suspension a high stability (electrostatic and electrosteric stabilization) against sedimentation/flocculation required for casting (ammonium polyacrylate, citric acid).
•Plasticizer: gives flexibility to tapes and deformability to granules by lowering the Tg (glass transition temperature) of binders (glicerine, butyl benzyl phthalate, poly(ethylen) glycol)
•Lubricant: decreases die-powder and granule-granule friction (salts of stearic acid)
Powder consolidation and shaping
The stages of dry pressingDry-bag isostatic pressing
Extrusion Double-gated injection moulding device
Powder consolidation and shaping
Compact tape casting unit
Drying chamber
Casting head
Tape
Non-continuously working laboratory casting unit
Casting head
IR lamp
Continuously-working (20 cm/min) industrial casting units
Schematic of a doctor blade casting unit
Sintering: removal of pores between particles accompanied by shrinkage (densification) and grain growth.
Driving force for sintering: reduction of surface area and lowering of surface energy. High energy solid-gas surfaces are replaced by low energy solid-solid interfaces (grain boundaries).At microscopic level, the driving force is related to the difference in surface curvature and consequently of partial pressure and chemical potential between different parts of the system.
Sintering and grain growth
Types of sintering
Solid-state sintering (SSS)only in high-purity compounds
Liquid phase sintering (LPS) <20% liquid; impurities or specific additives
Viscous glass sintering or viscous flow (VGS)Densification of glass powders
Viscous composite sintering or vitrification (VCS)>20% liquid: whitewares, porcelains
Neck formation Pore removal and shrinkage
Effect of particle size: the smaller the particles, the higher the radius of curvature and the chemical potential higher sintering rate.
Laplace equation for a spherical droplet
rP
2r Pressure difference across a curved
interface. For a planar surface, ΔP = 0
Sintering and grain growth
Effect of curvature on vapour pressure (Thomson’s equation)
rRT
V
rP
P 2ln V: molar volume
: surface tension
r (micron) P/P(r=)
1 1.002
0.1 1.02
0.01 1.21
0.001 7.03
Effect of curvature on chemical potential
r
Vr iii
2
2121
112
rrVrr iii r1
r2
Particles with different curvature have different vapour pressure and chemical potential. Therefore they are not in equilibrium and the larger one will grow at the expense of the smaller one.
Ostwald ripening
For a cavity (r < 0), P < P(r=)
r
If r > 0, P > P(r=)
Effect of curvature on thermodynamic properties
Negative curvature
Positive curvature
Nul curvature
Pore
Grain
Grain
Sintering and grain growth
Stages of sintering(a, b) Initial stage sintering. Formation of strong bonds and necks between particles at the contact points. Moderate decrease of porosity (initial 40-50%) from particle rearrangement. (c) Intermediate stage sintering. The size of the necks increases and the amount of porosity decreases. The sample shrinks (the centers of the grains move towards each other. The grains transforms from spheres to truncated octahedra (tetrakaidecahedra). This stage continues until pores are closed (r.d. 90%).(d) Final stage sintering. Pores are slowly eliminated and major grain growth can occur.
In hot-pressing and hot isostatic pressing an additional driving force is provided by the external stress/pressure.
Initial stage Intermediate stage Final stage
tetrakaidecahedron6s+8h faces
Sintering and grain growth
Mechanism Source Sink Densification
1 Surface diff. Surface Neck No
2 Evaporation-condensation
Surface Neck No
4 Volume diff. Grain boundary
Neck Yes
6 Grain boundary diffusion
Grain boundary
Neck Yes
In ionic materials, the mobility of the slowest moving species dominates the diffusion process and sintering rates. This explain the strong dependence of sintering kinetics on nature and amount of uncontrolled impurities, dopants and sintering aids. Grain boundary diffusion is the most important densification mechanisms in many oxides.
Sintering mechanisms
Surface diffusion &evaporation-condensation
Volume and grain boundary diffusion
Negative curvature
Positive curvature
Nul curvature
Grain boundary
Sintering and grain growth
Driving force for grain growth: difference of chemical potential (Gibbs’ free energy) across a curved interface
rVG gbi
The grain boundaries with mobility Mgb migrate towards their centre of curvature at a velocity
Grains with 6 sides: no grain boundary migrationGrains with <6 sides: the grains grow smallerGrains with >6 sides: the grains grow larger
Grain boundary
Atoms
r
Grain growth
gb
gbgb
120°
Convex boundaries
Concave boundaries
rMv gbgb
1
Sintering and grain growth
Grain growth
General relationship: tkdd mm 0 m = 2-3; m = 2 if
Grain growth in undoped and Mg-doped alumina
Grain growth is inhibited by pores, second phase inclusions and solid solution impurities. Pores and solid inclusions act as pinning centres for weakly curved grains. The critical grain size at which grain growth stops is given by (Zener):
i
if V
dD
Df: limiting grain sizedi: diameter of inclusionsVi: volume fraction of inclusions
Dopants in solid solution affect depress grain growth because they segregate at grain boundaries reducing: - the interfacial energy - the grain boundary mobility
Grain boundary pinned by a pore
Dragging and agglomeration of pores determined by grain boundary migration
d
k
dt
dd
)(
Liquid phase sintering
• Enough liquid phase must be present (1-5 vol.%).• The liquid must wet the solid (contact angle θ<<90°).• The solid must be partially soluble in the liquid.•Driving forces are higher for small particles (stronger capillary forces) with high surface energy and high solubility
> 90°: nonwetting < 90°: wetting
= 0°: spreading
Particle rearrangement: the liquid spread on the particles which rotate and slip. Significant densification occurs, up to 70%, without modification of particle and pore morphology.Solution-precipitation:(1) Ostwald ripening. Small particles dissolve in the liquid and the material precipitates on bigger particles because solubility depends on the radius of curvature. (2) Dissolution occurs in the neck region because of Laplacian compressive force and material redeposit away of the neck region.(3) Sharp corners dissolve and material precipitates on regions of lower curvature.Coalescence. When enough grain growth has occurred, a solid skeleton is formed and the pores becomes closed (at 90% re. dens.). Pore elimination can proceed by solid-state diffusion.