Nano Gear groups of molecules

Expending Reading Due by Week 11 1. The unique properties of Nano materials 2. Problems related to nano composites preparation 3. Illustrate several kinds nano materials and their applications Due by Week 11

Solid State Sintering

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)

Just try

Review Grain growth during the firing step Nucleated grains and normal growth Primary Recrystallization Secondary Recrystallization Excessive grain growth Consuming the uniform-grain-size-matrix harmful to mechanical properties Normally, fusion occurs during firing Concerns on Changes in grain size As for solid state sintering, what is the most concern ????

Solid-vapor Interface Originally Porous Compact 25 - 60% porosity sintering Expected Dense Compact Elimination of porosity by materials transfer Progressive changes in grain shape, pore shape, and pore size

Driving Force for Densification Decrease in surface area and lowering of the surface free energy Solid-Solid Interface Fine Particle size The sintering rate is roughly proportional to the inverse of the particle size Small Radius of curvature Specific surface area High Pressure Helpful for the expending reading ? High Reactive Activity

Mechanisms of Mass Transfer Surface atoms evaporation and sink to the neck due to the pressure difference Evaporation and Condensation Viscous Flow Liquid diffusion Capillarity forces Surface diffusion surface atoms from one part to another Grain-boundary or lattice diffusion Plastic deformation 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 Problems in Class Please indicate the types of points defects in an ionic crystal Frenkel disorder Schottky disorder Problems in Class How to characterize the points defects? High Resolution Transmission Electronic Microscope To show the atomic arrangement

Real Grain Growth 1. Grain contact 2. Neck formation 3. Neck growth moving of the interface 4. Resultant grown grains