Lecture 17 - ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 – Advanced Materials Engineering Lecture 17 Thermal Properties Ch 19; Callister 8/e Dr. A. Keshavarz - ENGR 4250U
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ISSUES TO ADDRESS. .. How does a material respond to heat ? 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 19: Thermal Properties Dr. A. Keshavarz - ENGR 4250U
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General: The ability of a material to absorb heat. Quantitative: The energy required to increase the temperature of the material. heat capacity (J/mol-K) energy input (J/mol) temperature change (K) Heat Capacity Two ways to measure heat capacity: C p : Heat capacity at constant pressure. C v : Heat capacity at constant volume. C p > C v • Specific heat has typical units of K kg J dT dQ C = Dr. A. Keshavarz - ENGR 4250U
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Energy Storage How is the energy stored? Phonons – thermal waves - vibrational modes Adapted from Fig. 19.1, Callister 7e . Dr. A. Keshavarz - ENGR 4250U
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Phonon • Vibrational thermal energy: quantum nature • Only certain energy values are allowed • Single quantum is Phonon • Analogous to quantum of em radiation: photon • Sometimes waves are called phonons • Cause thermal scattering of free electrons during conduction Dr. A. Keshavarz - ENGR 4250U
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Heat capacity. .. -- increases with temperature -- reaches a limiting value of 3 R Atomic view: -- Energy is stored as atomic vibrations. -- As T goes up, so does the avg. energy of atomic vibr. Heat Capacity vs T Adapted from Fig. 19.2, Callister 7e . gas constant 3 R = 8.31 J/mol-K C v = constant Debye temperature (usually less than T room ) T (K) θ D 0 0 C v = AT 3 Dr. A. Keshavarz - ENGR 4250U
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Other Heat Capacity Contributions Electronic Contribution – The electrons (those that contribute to current) absorb thermal energy by increase in K.E. – Only free electron above 0K (above Fermi energy) – Less in semiconductors, ceramics, and polymers • Insignificant at higher T Randomization of Electron orbits – Ferromagnetic materials – Above Curie point Dr. A. Keshavarz - ENGR 4250U
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increasing c p Selected values from Table 19.1, Callister 7e . 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 900 486 138 128 c p : (J/kg-K) C p : (J/mol-K) material 940 775 840 Heat Capacity: Comparison Dr. A. Keshavarz - ENGR 4250U
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Dr. A. Keshavarz - ENGR 4250U
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Thermal Expansion • In many materials is anisotropic • Depends upon crystallographic directions • For isotropic materials is approximately = • Thermal expansion increase in average distance between atoms v α v l 3 Dr. A. Keshavarz - ENGR 4250U
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Bond length , r Bond energy , E o Melting Temperature , T m T m is larger if E o is larger. Properties From Bonding:
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Lecture 17 - ENGR 4250 Advanced Materials Engineering...

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