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

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

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

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

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

14. HEAT CAPACITY Capacity at constant volume = C V Capacity at constant pressure = C P C P is typically > C V , but the difference is small for solids. When heated, materials experience an increase in T. This means that heat is absorbed. Heat capacity represents the amount of energy required to produce a unit temperature rise. H 2 O has a higher heat capacity

15. HEAT CAPACITY 3 N 0 k b  D =Temperature at which  D (Cu)  D (Al)  D (Pb)

16. THERMAL CONDUCTIVITY • General: The ability of a material to transfer heat. • Quantitative: 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. Fick’s First Law temperature gradient k=thermal conductivity (J/m-K-s): Defines material’s ability to transfer heat. heat flux (J/m 2 -s)

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

18. 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

22. THERMAL STRESSES • Occurs due to: --uneven heating/cooling --mismatch in thermal expansion. • Example Problem 19.1, p. 666, Callister 6e . --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 -172MPa 100GPa 20 x 10 -6 /C 20C Strain ( ε ) due to ∆ T causes a stress ( σ ) that depends on the modulus of elasticity (E):

23. THERMAL SHOCK RESISTANCE • Thermal shock is fracture of brittle ceramics due to asymmetric thermal expansion. • Occurs due to: uneven heating/cooling. • The change in T with position leads to built in strain and thus stress. • Ex: Assume top thin layer is rapidly cooled from T 1 to T 2 : Tension develops at surface Critical temperature difference for fracture (set  =  f ) Temperature difference that can be produced by cooling: • Result: • Large thermal shock resistance when is large. set equal

24. THERMAL PROTECTION SYSTEM • Application: Space Shuttle Orbiter Fig. 23.0, Callister 5e . (Fig. 23.0 courtesy the National Aeronautics and Space Administration. Fig. 19.2W, Callister 6e . (Fig. 19.2W adapted from L.J. Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher, "The Shuttle Orbiter Thermal Protection System", Ceramic Bulletin , No. 11, Nov. 1981, p. 1189.) • Silica tiles (400-1260C) : --large scale application Fig. 19.3W, Callister 5e . (Fig. 19.3W courtesy the National Aeronautics and Space Administration. --microstructure: ~90% porosity! Si fibers bonded to one another during heat treatment. Fig. 19.4W, Callister 5e . (Fig. 219.4W courtesy Lockheed Aerospace Ceramics Systems, Sunnyvale, CA.)

25. 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.

26. THERMOELECTRIC COOLING & HEATING Reversion 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 .

27. THERMOELECTRIC COOLING & HEATING 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 .

28. THERMOELECTRIC COOLING & HEATING 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.

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