材料物理教材ch17 thermal propertiess.pptVIP

* * * * * * * * * * * * * * * * * * 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 do materials respond to the application of heat? ? How do we define and measure... -- heat capacity? -- thermal expansion? -- thermal conductivity? -- thermal shock resistance? ? How do the thermal properties of ceramics, metals,  and polymers differ? Chapter 17:Thermal Properties * ? Quantitatively: The energy required to produce a unit rise in  temperature for one mole of a material. heat capacity (J/mol-K) energy input (J/mol) temperature change (K) 17.2 Heat Capacity ? Two ways to measure heat capacity: Cp : Heat capacity at constant pressure. Cv : Heat capacity at constant volume. Cp usually Cv ? Heat capacity has units of The ability of a material to absorb heat * Atomic Vibrations Atomic vibrations are in the form of lattice waves or phonons The vibrational thermal energy for a material consists of a series of elastic waves, which have a range of distributions and frequencies. Only certain energy values are allowed (the energy is said to be quantized), and a single quantum of vibrational energy is called a phonon. (A phonon is analogous to the quantum of electromagnetic radiation, the photon.) * ? Heat capacity.