ch17_thermal

ch17_thermal - Chapter 17: Thermal Properties A white-hot...

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Unformatted text preview: Chapter 17: Thermal Properties A white-hot cube of a silica fiber insulation material, which, only seconds after having been removed from a hot furnace, can be held by its edges with the bare hands. Initially, the heat transfer from the surface is relatively rapid; however, the thermal conductivity of this material is so small that heat conduction from the interior [maximum temperature approximately 1250C (2300F)] is extremely slow. This material was developed especially for the tiles that cover the Space Shuttle Orbiters and protect and insulate them during their fiery reentry into the atmosphere. Other attractive features of this high-temperature reusable surface insulation (HRSI) include low density and a low coefficient of thermal expansion. ISSUES TO ADDRESS... How does a material respond to heat ? 1 How do we define and measure...--heat capacity--coefficient of thermal expansion--thermal conductivity--thermal shock resistance How do ceramics, metals, and polymers rank? CHAPTER 17: THERMAL PROPERTIES 17.1 Introduction Thermal property: Response of materials to the application of heat 2 General: The ability of a material to absorb heat. Quantitative: The energy required to increase the temperature of the material. C = dQ dT heat capacity (J/mol-K) energy input (J/mol) temperature change (K) Two ways to measure heat capacity:-- C p : Heat capacity at constant pressure.-- C v : Heat capacity at constant volume. 17.2 HEAT CAPACITY c17f01 c17f01 Vibrational Heat Capacity Generation of lattice waves in a crystal by atomic vibrations. The phonon versus photon c17f02 The temperature dependence of the heat capacity at constant volume. D = Debye temperature D = max /k D < T room C v = constant = ~3R Heat Capacity vs T--increases with temperature--reaches a limiting value of 3R Atomic view:--Energy is stored as atomic vibrations.--As T goes up, energy of atomic vibration goes up too gas constant = 8.31 J/mol-K 4 Why is c p significantly larger for polymers? Selected values from Table 19.1, Callister 6e . HEAT CAPACITY: COMPARISON Polymers Polypropylene Polyethylene Polystyrene Teflon c p (J/kg-K) at room T Ceramics Magnesia (MgO) Alumina (Al 2 O 3 ) Glass Metals Aluminum Steel Tungsten Gold 1925 1850 1170 1050 900 486 128 138 increasing c p c p : (J/kg-K) C p : (J/mol-K) material 940 775 840 5 Materials change size when heating. Atomic view: Mean bond length increases with T. L final- L initial L initial = (T final- T initial ) coefficient of thermal expansion (1/K) T init T final L final L init Bond energy Bond length (r) increasing T T 1 r(T 5 ) r(T 1 ) T 5 bond energy vs bond length curve is asymmetric Adapted from Fig. 19.3(a), Callister 6e ....
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ch17_thermal - Chapter 17: Thermal Properties A white-hot...

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