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

2. Materials change size when heating. CTE: coefficient of thermal expansion (units: 1/K) THERMAL EXPANSION

3. Bond length, r Bond energy, E o Flashback: PROPERTIES FROM BONDING: Energy versus bond length

4. Melting Temperature, T m T m is larger if E o is larger. PROPERTIES FROM BONDING: T M

5. Elastic modulus, E E ~ curvature at r o E is larger if curvature is larger. PROPERTIES FROM BONDING: Elastic Properties E similar to spring constant

6. Coefficient of thermal expansion,   ~ symmetry at r o  is larger if E o is smaller and very asymmetric. PROPERTIES FROM BONDING: CTE or 

7. T0T0 T2T2 T3T3 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. Why does  generally decrease with increasing bond energy? Selected values from Table 19.1, Callister 6e. THERMAL EXPANSION: COMPARISON Thermal expansion mismatch is a major problem for design of everything from semiconductors to bridges. Particularly an issue in applications where temperature changes greatly (esp. engines).

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? -9.2 mm

11. 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.

12. General: The ability of a material to transfer heat. Quantitative: temperature gradient k= thermal conductivity (J/m-K-s): Defines material’s ability to transfer heat. heat flux (J/m 2 -s) Atomic view: Electronic and/or Atomic vibrations in hotter region carry energy (vibrations) to cooler regions. In a metal, electrons are free and thus dominate thermal conductivity. In a ceramic, phonons are more important. THERMAL CONDUCTIVITY Fick’s First Law

13. THERMAL CONDUCTIVITY Fick’s Second Law Non-Steady State: dT/dt is not constant.

14. Selected values from Table 19.1, Callister 6e. K=k l +k e : Again think about band gaps: metals have lots of free electrons (k e is large), while ceramics have few (only k l is active). THERMAL CONDUCTIVITY

15. 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

16. 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

17. Occurs due to: --uneven heating/cooling --mismatch in thermal expansion. Example Problem -- A brass rod is stress-free at room temperature (20C). --It is heated up, but prevented from lengthening. --At what T does the stress reach -172MPa? Answer: 106C THERMAL STRESSES -172MPa 100GPa20 x 10 -6 /C 20C Strain (ε) due to ∆T causes a stress (σ) that depends on the modulus of elasticity (E):

18. THERMOELECTRIC COOLING & HEATING Two different materials are connected at the their ends and form a loop. One junction is heated up. There exists a potential difference that is proportional to the temperature difference between the ends.

19. THERMOELECTRIC COOLING & HEATING Reverse of the Seebeck effect is the Peltier Effect. A direct current flowing through heterojunctions causes one junction to be cooled and one junction to be heated up. Lead telluride and or bismuth telluride are typical materials in thermoelectric devices that are used for heating and refrigeration.

20. Why does this happen? When two different electrical conductors are brought together, e- are transferred from the material with higher E F to the one with the lower E F until E F (material 1)= E F (material 2). Material with smaller E F will be (-) charged. This results in a contact potential which depends on T. e- at higher E F are caused by the current to transfer their energy to the material with lower E F, which in turn heats up. Material with higher E F loses energy and cools down.

21. Peltier–Seebeck effect, or the thermoelectric effect, is the direct conversion of thermal differentials to electric voltage and vice versa. The effect for metals and alloys is small, microvolts/K. For Bi 2 Te 3 or PbTe (semiconductors), it can reach up to millivolts/K. Applications: Temperature measurement via thermocouples (copper/constantan, Cu-45%Ni, chromel, 90%Ni-10%Cr,…); thermoelectric power generators (used in Siberia and Alaska); thermoelectric refrigerators; thermal diode in microprocessors to monitor T in the microprocessors die or in other thermal sensor or actuators.

22. THERMOELECTRIC COOLING & HEATING http://www.sii.co.jp/info/eg/thermic_main.html