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Thermal Properties (Heat Conductivity (Graphite (Good conductivity btwn C…
Thermal Properties
Heat Conductivity
Thermal Diffusivity
How quick a material can transmit heat
Transmitted through phonons & radiation
Equation (Fig 10)
Electron Conductivity + Phonon Conductivity + Radiation
Graphite
Good conductivity btwn C network
Poor conductivity btwn layers
Light Elements
BeO, SiC, ALN
Good Thermal Conductivity
Small Scattering of phonons
Heavy Large Elements
TiO2, Al2O3, MgO
Poor Thermal Conductivity
Large Scattering of phonons
Solid Solutions
Decrease Thermal Conductivity
Thermal Expansion
Ionic Compounds
Weak ionic bonds => Large Thermal Expansion
Covalent Compounds
Strong covalent bonds => Small Thermal Expansion
Types
Isotropic
Expansion same in all directions
Cubic Structure
Polycrystalline
Anisotropic
Expansion different in different directions
Non-Cubic Structure
Single Crystal
Heat Capacity
Ability to absorb heat from external surroundings
Vibrational HC
Atoms vibrating at high frequencies & low amplitudes + Atomic Bonding
Debye Temp
HC independent of T
1/5-1/2 MP
Creep
Fracture
Temp
Low
Brittle Fracture
High
Creep Rupture
Vacancy Diffusion
Reduction in free energy of atom: Compression => Tension
High Temp
Coble Creep
GB Diffusion
Porosity
Decrease thermal conductivity
Thermal Shock