Study Objectives 1. To know about the processes of atomic diffusion 2. To know about the densification mechanisms 3. To know about the driving forces for the densification 4. To review some fundamentals about ceramics (vapor pressure, curvature, ionic crystal, covalent crystal points defects and their characterization)
Grain growth during the firing step Review Primary Recrystallization Nucleated grains and normal growth Secondary Recrystallization Excessive grain growth Consuming the uniform-grain-size-matrix harmful to mechanical properties Concerns on Changes in grain size Normally, fusion occurs during firing As for solid state sintering, what is the most concern ????
Solid-vapor Interface Originally Porous Compact 25 - 60% porosity Expected Dense Compact sintering Progressive changes in grain shape, pore shape, and pore size Elimination of porosity by materials transfer
Driving Force for Densification Decrease in surface area and lowering of the surface free energy Solid-Solid Interface Fine Particle size Small Radius of curvature High Pressure High Reactive Activity The sintering rate is roughly proportional to the inverse of the particle size Helpful for the expending reading ? Specific surface area
Mechanisms of Mass Transfer Evaporation and Condensation Viscous Flow Surface diffusion Grain-boundary or lattice diffusion Plastic deformation Surface atoms evaporation and sink to the neck due to the pressure difference Liquid diffusion Capillarity forces surface atoms from one part to another Hot pressing
Alternate Paths for Matter Transport 1. Surface diffusion 2. Lattice diffusion from surface 3. Vapor transport 4. Boundary diffusion 5. Lattice diffusion from G.B. 6. Lattice diffusion from dislocations
Review Please indicate the types of points defects in an ionic crystal How to characterize the points defects? High Resolution Transmission Electronic Microscope To show the atomic arrangement Frenkel disorder Schottky disorder Problems in Class
1. Grain contact 2. Neck formation 3. Neck growth moving of the interface 4. Resultant grown grains Real Grain Growth