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1. Scientists do stupid looking things sometimes (though not too unsafe if they made the material carefully enough)
2. THERMAL EXPANSION
3. Flashback: PROPERTIES FROM BONDING: Energy versus bond length
4. PROPERTIES FROM BONDING: TM
5. PROPERTIES FROM BONDING: Elastic Properties
6. PROPERTIES FROM BONDING: CTE or a
7. Atomic positions and vibrations The minimum in an atomic energy vs. interatomic distance curve yields the near neighbor distance (bond length).
The width of the curve is proportional to the amplitude of thermal vibrations for an atom.
If the curve is symmetric, there is no shift in the average position of the atom (the center of the thermal vibrations at any given T).
The coefficient of thermal expansion is negligible for symmetric energy wells.
8. Thermal Expansion If the curve is not symmetric, the average position in which the atom sits shifts with temperature.
Bond lengths therefore change (usually get bigger for increased T).
Thermal expansion coefficient is nonzero.
9. THERMAL EXPANSION: COMPARISON
10. Thermal expansion example Example
An Al wire is 10 m long and is cooled from 38 to -1 degree Celsius. How much change in length will it experience?
11. Small/Negative thermal expansion Invar (Ni-Fe alloy) is the most common low thermal exp material: a = 1.6*10-6 / degree
Some materials have a<0 in one dimension and >0 in others.
It is possible, though not intuitive, for materials to have a negative thermal expansion in all dimensions.
An increase in temperature causes the crystal to shrink.
ZrW2O8: contracts continuously and linearly from 2 to 1050K
Composites could allow zero thermal expansion components (superb for optics, engine parts, etc).
12. Heat and Atoms Heat causes atoms to vibrate.
Vibrating in synch is often a low energy configuration (preferred).
Generates waves of atomic motion.
Often called phonons, similar to photons but atomic motion instead of optical quanta.
13. HEAT CAPACITY
14. HEAT CAPACITY – The Dulong-Petit Law
15. THERMAL CONDUCTIVITY
16. THERMAL CONDUCTIVITY
18. Good heat conductors are usually good electrical conductors.
(Wiedemann & Franz, 1853)
Thermal conductivity changes by 4 orders of magnitude (~25 for electrical conductivity).
Metals & Alloys: free e- pick up energy due to thermal vibrations of atoms as T increases and lose it when it decreases.
Insulators (Dielectrics): no free e-. Phonons (lattice vibration quanta) are created as T increases, eliminated as it decreases. THERMAL CONDUCTIVITY
19. Thermal conductivity is temperature dependent.
Analagous to electron scattering.
Usually first decreases with increasing temperature
Higher Temp=more scattering of electrons AND phonons, thus less transfer of heat.
Then increases at still higher temperatures due to other processes we haven‘t considered in this class (radiative heat transfer—eg. IR lamps). THERMAL CONDUCTIVITY
20. Thermal conductivity optimization To maximize thermal conductivity, there are several options:
Provide as many free electrons (in the conduction band) as possible
free electrons conduct heat more efficiently than phonons.
Make crystalline instead of amorphous
irregular atomic positions in amorphous materials scatter phonons and diminish thermal conductivity
Remove grain boundaries
gb’s scatter electrons and phonons that carry heat
Remove pores (air is a terrible conductor of heat)
21. THERMAL STRESSES
22. THERMAL SHOCK RESISTANCE
23. THERMAL PROTECTION SYSTEM
24. THERMOELECTRIC COOLING & HEATING
25. THERMOELECTRIC COOLING & HEATING
26. THERMOELECTRIC COOLING & HEATING
27. THERMOELECTRIC COOLING & HEATING
28. THERMOELECTRIC COOLING & HEATING
29. THERMAL IMAGING