Scientists do stupid looking things sometimes though not too unsafe if they made the material carefully enough

Scientists do stupid looking things sometimes though not too unsafe if they made the material carefully enough PowerPoint PPT Presentation


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Scientists do stupid looking things sometimes though not too unsafe if they made the material carefully enough

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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

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