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Chapter 19: Thermal Properties. 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.
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Chapter 19:Thermal Properties 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?
Scientists do stupid looking things sometimes (though not too unsafe if they made the material carefully enough)
Heat Capacity • Two ways to measure heat capacity:Cp : Heat capacity at constant pressure. Cv : Heat capacity at constant volume. Cp> Cv • Specific heat has typical units of • General: The ability of a material to absorb heat. • Quantitative: The energy required to increase the temperature of the material. energy input (J/mol) heat capacity (J/mol-K) temperature change (K)
Heat Capacity vs T Cv= constant 3R gas constant = 8.31 J/mol-K Cv Adapted from Fig. 19.2, Callister 7e. 0 T (K) 0 q D Debye temperature ) (usually less than T room • Heat capacity... -- increases with temperature -- reaches a limiting value of 3R • Atomic view: -- Energy is stored as atomic vibrations. -- As T goes up, so does the avg. energy of atomic vibr.
Energy Storage How is the energy stored? Phonons – thermal waves - vibrational modes Adapted from Fig. 19.1, Callister 7e.
Energy Storage • Other small contributions to energy storage • Electron energy levels • Dominate for ceramics & plastics • Energy storage in vibrational modes Adapted from Fig. 19.3, Callister 7e.
Heat Capacity: Comparison c (J/kg-K) p material at room T • Polymers 1925 Polypropylene c : (J/kg-K) p 1850 Polyethylene C : (J/mol-K) p 1170 Polystyrene 1050 Teflon • Why is cp significantly larger for polymers? • Ceramics M agnesia (MgO) 940 Alumina (Al O ) 775 2 3 p Glass 840 increasing c • Metals Aluminum 900 Steel 486 Tungsten 138 Selected values from Table 19.1, Callister 7e. Gold 128
Thermal Expansion T init L init T final L final • Atomic view: Mean bond length increases with T. Bond energy Adapted from Fig. 19.3(a), Callister 7e. (Fig. 19.3(a) adapted from R.M. Rose, L.A. Shepard, and J. Wulff, The Structure and Properties of Materials, Vol. 4, Electronic Properties, John Wiley and Sons, Inc., 1966.) r(T1) r(T5) Bond length (r) T 5 bond energy vs bond length increasing T curve is “asymmetric” T 1 • Materials change size when heating. coefficient of thermal expansion (1/K or 1/°C)
Thermal Expansion: Comparison a(10-6/K) Material at room T • Polymers Polypropylene 145-180 Polyethylene 106-198 Polystyrene 90-150 Teflon 126-216 • Metals Aluminum 23.6 Steel 12 Tungsten 4.5 Gold 14.2 • Ceramics Magnesia (MgO) 13.5 Alumina (Al2O3) 7.6 Soda-lime glass 9 Silica (cryst. SiO2) 0.4 Selected values from Table 19.1, Callister 7e. Polymers have smaller because of weak secondary bonds • Q: Why does a generally decrease with increasing bond energy?
Thermal Expansion: Example • Answer: For Cu rearranging Eqn 19.3b Ex: A copper wire 15 m long is cooled from 40 to -9°C. How much change in length will it experience?
Thermal Conductivity • General: The ability of a material to transfer heat. • Quantitative: temperature gradient Fourier’s Law heat flux (J/m2-s) thermal conductivity (J/m-K-s) T > T T 2 1 1 x x 1 2 heat flux • Atomic view: Atomic vibrations in hotter region carry energy (vibrations) to cooler regions.
Thermal Conductivity: Comparison Material k (W/m-K) Energy Transfer • Metals Aluminum 247 By vibration of Steel 52 atoms and Tungsten 178 motion of Gold 315 electrons • Ceramics Magnesia (MgO) 38 By vibration of Alumina (Al2O3) 39 increasing k atoms Soda-lime glass 1.7 Silica (cryst. SiO2) 1.4 • Polymers Polypropylene 0.12 By vibration/ Polyethylene 0.46-0.50 rotation of chain Polystyrene 0.13 molecules Teflon 0.25 Selected values from Table 19.1, Callister 7e.
THERMAL CONDUCTIVITY Good heat conductors are usually good electrical conductors. (Wiedemann & Franz, 1853) Metals & Alloys: free e- pick up energy due to thermal vibrations of atoms as T increases and lose it when it decreases. Insulators (Dielectrics): no free e-. Phonons (lattice vibration quanta) are created as T increases, eliminated as it decreases.
Thermal conductivity optimization To maximize thermal conductivity, there are several options: • Provide as many free electrons (in the conduction band) as possible • free electrons conduct heat more efficiently than phonons. • Make crystalline instead of amorphous • irregular atomic positions in amorphous materials scatter phonons and diminish thermal conductivity • Remove grain boundaries • gb’s scatter electrons and phonons that carry heat • Remove pores (air is a terrible conductor of heat)
Thermal Stress T room DL L room T 100GPa 20 x 10-6 /°C compressive keeps L = 0 20°C -172 MPa Answer: 106°C • Occurs due to: -- uneven heating/cooling -- mismatch in thermal expansion. • Example Problem 19.1, Callister 7e. -- A brass rod is stress-free at room temperature (20°C). -- It is heated up, but prevented from lengthening. -- At what T does the stress reach -172 MPa?
Thermal Shock Resistance (TSR) rapid quench s T2 tries to contract during cooling T1 resists contraction Critical temperature difference for fracture (set s = sf) Temperature difference that can be produced by cooling: set equal • Result: • Large thermal shock resistance when is large. • Occurs due to: uneven heating/cooling. • Ex: Assume top thin layer is rapidly cooled from T1 to T2: Tension develops at surface
Thermal Protection System Re-entry T Distribution reinf C-C silica tiles nylon felt, silicon rubber (1650°C) (400-1260°C) coating (400°C) ~90% porosity! Si fibers bonded to one another during heat treatment. 100mm • Application: Space Shuttle Orbiter Chapter-opening photograph, Chapter 23, Callister 5e (courtesy of the National Aeronautics and Space Administration.) Fig. 19.2W, Callister 6e. (Fig. 19.2W adapted from L.J. Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher, "The Shuttle Orbiter Thermal Protection System", Ceramic Bulletin, No. 11, Nov. 1981, p. 1189.) • Silica tiles (400-1260C): --large scale application --microstructure: Fig. 19.3W, Callister 5e. (Fig. 19.3W courtesy the National Aeronautics and Space Administration.) Fig. 19.4W, Callister 5e. (Fig. 219.4W courtesy Lockheed Aerospace Ceramics Systems, Sunnyvale, CA.)
Summary • A material responds to heat by: -- increased vibrational energy -- redistribution of this energy to achieve thermal equil. • Heat capacity: -- energy required to increase a unit mass by a unit T. -- polymers have the largest values. • Coefficient of thermal expansion: -- the stress-free strain induced by heating by a unit T. -- polymers have the largest values. • Thermal conductivity: -- the ability of a material to transfer heat. -- metals have the largest values. • Thermal shock resistance: -- the ability of a material to be rapidly cooled and not crack. Maximize sf k/Ea.