# ch19 - Thermal Properties of Materials Chapter 19 What you...

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1 Thermal Properties of Materials Chapter 19 What you should understand by the end of this chapter: ± The main parameters that characterize the thermal behaviour of materials. ² Heat capacity and specific heat. ² Thermal expansion coefficient. ² Thermal conductivity. ± How these concepts are applied to thermal protection systems (e.g. space shuttle tiles).

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2 I. Heat Capacity ¾ Energy content in a material depends on. . . ± … temperature. ± … the material. ¾ Heat capacity: ± C represents energy needed to raise one mole of a solid by one degree (J/mol-K). ± It can be measured either at constant volume C v , or constant pressure C p . ² C p is somewhat larger than C v . ¾ We also define the specific heat ( c ) as energy per unit mass per degree (J/kg-K). ¾ The heat capacity measures the energy content of a material. dT dQ C =
3 Physical Basis ¾ Atoms vibrate within the solid. ± They are constrained by surrounding atoms because of bonding. ± Energy propagates like an elastic wave. ² This is the basis for sound transmission. Simplest model: 3-D solid Î 3 vibrational modes Heat content 3 kT per atom or 3 RT per mole

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4 Heat Capacity – Temperature Dependence ¾ Heat capacity is sensitive to temperature. ¾ At low T ( < θ D ) Î C v = A T 3 ¾ Above θ D Î C v = 3R --- 25 J/mol-K Debye temperature (usually less than T room ) T (K) Heat capacity, C v 3R θ D C v = constant gas constant = 8.31 J/mol-K
5 Specific Heat – Dependence on Material 128 Tungsten 138 Gold 486 Steel 900 Aluminum

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ch19 - Thermal Properties of Materials Chapter 19 What you...

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