Callister - An introduction - 8th edition

WHY STUDY the Thermal Properties of Materials?
Of the three primary material types, ceramics are the
most susceptible to thermal shock—brittle fracture
resulting from internal stresses that are established
within a ceramic piece as a result of rapid changes in
temperature (normally upon cooling). Thermal shock is
normally an undesirable event, and the susceptibility of
a ceramic material to this phenomenon is a function of
its thermal and mechanical properties (coefficient of
thermal expansion, thermal conductivity, modulus of
elasticity, and fracture strength). From a knowledge of
the relationships between thermal shock parameters
and these properties, it is possible (1) in some cases, to
make appropriate alterations of the thermal and/or
mechanical characteristics in order to render a ceramic
more thermally shock resistant; and (2) for a specific
ceramic material, to estimate the maximum allowable
temperature change without causing fracture.
Learning Objectives
After studying this chapter you should be able to do the following:
1. Define heat capacity and specific heat.
2. Note the primary mechanism by which thermal
energy is assimilated in solid materials.
3. Determine the linear coefficient of thermal
expansion given the length alteration that
accompanies a specified temperature change.
4. Briefly explain the phenomenon of thermal
expansion from an atomic perspective
using a potential-energy-versus-interatomicseparation
plot.
5. Define thermal conductivity.
6. Note the two principal mechanisms of heat
conduction in solids, and compare the relative
magnitudes of these contributions for each of
metals, ceramics, and polymeric materials.
19.1 INTRODUCTION
Thermal property refers to the response of a material to the application of heat. As
a solid absorbs energy in the form of heat, its temperature rises and its dimensions
increase. The energy may be transported to cooler regions of the specimen if temperature
gradients exist, and ultimately, the specimen may melt. Heat capacity, thermal
expansion, and thermal conductivity are properties that are often critical in the
practical utilization of solids.
19.2 HEAT CAPACITY
heat capacity
Definition of heat
capacity—ratio of
energy change
(energy gained or
lost) and the
resulting
temperature change
specific heat
A solid material, when heated, experiences an increase in temperature signifying
that some energy has been absorbed. Heat capacity is a property that is indicative
of a material’s ability to absorb heat from the external surroundings; it represents
the amount of energy required to produce a unit temperature rise. In mathematical
terms, the heat capacity C is expressed as follows:
C dQ
dT
(19.1)
where dQ is the energy required to produce a dT temperature change. Ordinarily, heat
capacity is specified per mole of material (e.g., J/mol#
K, or cal/mol#
K). Specific heat
(often denoted by a lowercase c) is sometimes used; this represents the heat capacity
per unit mass and has various units (J/kgK, cal/g#
K, Btu/lb m
# F).
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