1. Thermal Properties of Materials
Chapter 22: Thermal Properties of Materials
Chapter 22:
Thermal Properties
of Materials
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2. © 2011 Cengage Learning Engineering. All Rights Reserved.
Chapter 22: Thermal Properties of Materials
Learning Objectives
Heat capacity and specific heat
Thermal expansion
Thermal conductivity
Thermal shock
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3. Heat Capacity and Specific Heat
Chapter 22: Thermal Properties of Materials
Heat Capacity and Specific Heat
Phonons
Packets of elastic waves. They are characterized by their energy, wavelength, or frequency, which transfers energy through a material.
The energy of the phonon can be expressed in terms of the wavelength or frequency.
Specific heat
The energy required to raise the temperature of one gram of a material by one degree.
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Chapter 22: Thermal Properties of Materials
Figure 22.1
Heat capacity as a function of temperature for metals and ceramics.
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Chapter 22: Thermal Properties of Materials
Figure 22.2
The effect of temperature on the specific heat of iron. Both the change in crystal structure and the change from ferromagnetic to paramagnetic behavior are indicated.
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Chapter 22: Thermal Properties of Materials
Thermal Expansion
Linear co-efficient of thermal expansion
Describes the amount by which each unit length of a material changes when the temperature of the material changes by one degree.
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Chapter 22: Thermal Properties of Materials
Figure 22.4
The linear coefficient of thermal expansion of iron changes abruptly at temperatures where an allotropic transformation occurs.
(b) The expansion of Invar is very low due to the magnetic properties of the material at low temperatures.
Thermal stresses are related to the coefficient of thermal expansion, the modulus of elasticity E of the material, and the temperature change T:
thermal = ET
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Chapter 22: Thermal Properties of Materials
Figure 22.5
When one end of a bar is heated, a heat flux Q/A flows toward the cold end at a rate determined by the temperature gradient produced in the bar.
The thermal conductivity k is a measure of the rate at which heat is transferred through a material.
Q/A = k T/x
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Chapter 22: Thermal Properties of Materials
Thermal Conductivity
Metals
where L is the Lorentz constant - The constant that relates electrical and thermal conductivity.
Ceramics
The transfer of heat in ceramics occurs primarily by lattice vibrations (or phonons).
Semiconductors
Heat is conducted in semiconductors by both phonons and electrons.
Polymers
The thermal conductivity of polymers is very low—even in comparison with silicate glasses.
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Chapter 22: Thermal Properties of Materials
Figure 22.6
The effect of temperature on the thermal conductivity of selected materials. Note the log scale on the vertical axis.
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Chapter 22: Thermal Properties of Materials
Thermal Shock
Failure of a material caused by stresses introduced by sudden changes in temperature is known as thermal shock.
Thermal shock behavior is affected by several factors:
Coefficient of thermal expansion: A low coefficient minimizes dimensional changes and reduces the ability to withstand thermal shock.
Thermal conductivity: The magnitude of the temperature gradient is determined partly by the thermal conductivity of the material.
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Chapter 22: Thermal Properties of Materials
Thermal Shock
Modulus of elasticity: A low modulus of elasticity permits large amounts of strain before the stress reaches the critical level required to cause fracture.
Fracture stress: A high stress required for fracture permits larger strains.
Phase transformations: Additional dimensional changes can be caused by phase transformations.
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Chapter 22: Thermal Properties of Materials
Figure 22-7
The effect of quenching temperature difference on the modulus of rupture of sialon. The thermal shock resistance of the ceramic is good up to about 950°C.
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Chapter 22: Thermal Properties of Materials
Thermal Shock
Thermal shock parameter or
where
f fracture stress of the material
v Poisson’s ratio
k thermal conductivity
E modulus of elasticity
 linear coefficient of thermal expansion
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Chapter 22: Thermal Properties of Materials
Key Terms
Phonons
Heat capacity or specific heat
Linear coefficient of thermal expansion
Thermal stresses
Thermal conductivity
Lorentz constant
Thermal shock
© 2011 Cengage Learning Engineering. All Rights Reserved.