材料物理性能-热学.ppt

1、Physical Properties of Materials,Prof. Xiaoqin Yan 2012.08,Contents,Optical Properties Electrical Properties Thermal Properties Magnetic Properties,Other Properties of Materials (For Self-Study) Electrical Properties： Thermoelectric, Pyroelectric and Magnetoelectric Property (热电、热释电、磁电性能) Magnetic P

2、roperties： Magnetostriction and Magnetoresistance (磁致伸缩、磁电阻) Optical Properties: Electrooptic, Photoelectric and Magnetooptic Property (电光、光电、磁光性能) AcousticProperties: Propagation, Absorption and Electroacoustic Property (声音的传播、吸收、电声性能) Elasticity: Anelasticity and Internal Friction (滞弹性与内耗),3. Ther

3、mal Properties,3.1 HEAT CAPACITY Heat capacity (热容) C: ratio of energy change (energy gained or lost) and the resulting temperature change. C = dQ/dT (3.1) Specific heat (比热) c: the heat capacity per unit mass. Cv: maintaining the specimen volume constant. Cp: constant external pressure. Vibrational

4、 Heat Capacity,The vibrations may be thought of as elastic waves or sound waves, having short wavelengths and very high frequencies, which propagate through the crystal at the velocity of sound. Only certain energy values are allowed (to be quantized), and a single quantum of vibrational energy is c

5、alled a phonon (声子).,Lattice waves in a crystal by means of atomic vibrations,Temperature Dependence of the Heat Capacity Dependence of heat capacity (at constant volume) on temperature, at low temperatures (near 0 K): Cv = AT3 (3.2) Above the Debye temperature (德拜温度, D): Cv 3R (R is the gas constan

6、t).,The temperature dependence of the heat capacity at constant volume,Other Heat Capacity Contributions Electrons absorb energy by increasing their kinetic (for example, free electrons excited from filled states to empty states above the Fermi energy). Energy-absorptive processes occur at specific

7、temperatures (for example, the randomization of electron spins in a ferromagnetic material as it is heated through its Curie temperature). In most instances, these are minor relative to the vibrational contribution.,3.2 THERMAL EXPANSION Most solid materials expand upon heating and contract when coo

8、led. (lf - l0) / l0 = l (Tf - T0) or l / l0 = l T (3.3) l is the linear coefficient of thermal expansion. V / V0 = vT (3.4) vsymbolizes the volume coefficient of thermal expansion. From an atomic perspective, thermal expansion is reflected by an increase in the average distance between the atoms. Th

9、ermal expansion is really due to the asymmetric curvature of this potential energy trough, rather than the increased atomic vibrational amplitudes with rising temperature.,(a) Potential energy versus interatomic distance, demonstrating the increase in interatomic separation with rising temperature.

10、(b) For a symmetric potential - energy versus - interatomic-distance curve, there is no increase in interatomic separation with rising temperature,For each class of materials (metals, ceramics, and polymers), the greater the atomic bonding energy, the deeper and more narrow this potential energy tro

11、ugh. The increase in interatomic separation with a given rise in temperature will be lower, yielding a smaller value of l. Metals: linear coefficients of thermal expansion for the common metals range between about 510-6 and 2510-6; these values are intermediate between those for ceramic and polymeri

12、c materials. Ceramics: comparatively low coefficients of thermal expansion; values typically range between about 0.510-6 and 1510-6 . Polymeric materials: very large thermal expansions upon heating, as indicated by coefficients that range from approximately 5010-6 to 40010-6. l is isotropic or aniso

13、tropic.,3.3 THERMAL CONDUCTIVITY Thermal conduction is the phenomenon by which heat is transported from high- to low-temperature regions of a substance. q = - k dT/dx （3.5） q denotes the heat flux, or heat flow, per unit time per unit area, k is the thermal conductivity (热导率), and dT/dx is the tempe

14、rature gradient. Mechanisms of Heat Conduction: Heat is transported in solid materials by both lattice vibration waves (phonons) and free electrons, usually one or the other predominates. k = kl + ke (3.6) Metals: In high-purity metals, the electron mechanism is much more efficient because electrons

15、 are not as easily scattered as phonons and have higher velocities. Metals are extremely good conductors of heat because relatively large numbers of free electrons participating in thermal conduction. The thermal conductivities generally range between about 20 and 400 W/mK.,Free electrons are respon

16、sible for both electrical and thermal conduction in pure metals, so WiedemannFranz law: L = k / (T) (3.7) is the electrical conductivity, T is the absolute temperature, and L is a constant. The theoretical value of L, 2.4410-8 W/(K)2. Alloying metals with impurities results in a reduction in the the

17、rmal conductivity, for the same reason that the electrical conductivity is diminished.,Thermal conductivity versus composition for copperzinc alloys,Ceramics: Nonmetallic materials are thermal insulators inasmuch as they lack large numbers of free electrons. Thus the phonons are primarily responsibl

18、e for thermal conduction. Room-temperature thermal conductivities range between approximately 2 and 50 W/mK.,The phonons are not as effective as free electrons in the transport of heat energy as a result of the very efficient phonon scattering by lattice imperfections. Glass and other amorphous cera

19、mics have lower conductivities than crystalline ceramics, because the phonon scattering is much more effective when the atomic structure is highly disordered and irregular.,Thermal conductivity on temperature for several ceramic materials,The scattering of lattice vibrations becomes more pronounced

20、with rising temperature; hence, the thermal conductivity of most ceramic materials normally diminishes with increasing temperature. The conductivity begins to increase at higher temperatures, which is due to radiant heat transfer. Porosity in ceramic materials may have a dramatic influence on therma

21、l conductivity; increasing the pore volume will, under most circumstances, result in a reduction of the thermal conductivity. Polymers: Thermal conductivities for most polymers are on the order of 0.3 W/mK. Energy transfer is accomplished by the vibration and rotation of the chain molecules. The the

22、rmal conductivity depends on the degree of crystallinity; a polymer with a highly crystalline and ordered structure will have a greater conductivity than the equivalent amorphous material. Polymers are often used as thermal insulators, and their insulative properties may be further enhanced by the i

23、ntroduction of small pores.,3.4 THERMAL STRESSES Thermal stresses are stresses induced in a body as a result of changes in temperature. Stresses Resulting from Restrained Thermal Expansion and Contraction Dependence of thermal stress on elastic modulus (E), linear coefficient of thermal expansion (l

24、 ), and temperature change: = El (T0 - Tf) = El T (3.8) Stresses Resulting from Temperature Gradients Temperature gradients are caused by rapid heating or cooling, in that the outside changes temperature more rapidly than the interior; differential dimensional changes restrain the free expansion or contraction of adjacent volume elements within the