Callister - An introduction - 8th edition

Summary • 795
• Coefficient-of-thermal-expansion values for polymers are typically greater than
for metals, which in turn are greater than for ceramic materials.
Thermal Conductivity
• The transport of thermal energy from high- to low-temperature regions of a material
is termed thermal conduction.
• For steady-state heat transport, flux may be determined using Equation 19.5.
• For solid materials, heat is transported by free electrons and by vibrational lattice
waves, or phonons.
• The high thermal conductivities for relatively pure metals are due to the large numbers
of free electrons and the efficiency with which these electrons transport thermal
energy. By way of contrast, ceramics and polymers are poor thermal conductors
because free-electron concentrations are low and phonon conduction predominates.
Thermal Stresses
• Thermal stresses, which are introduced in a body as a consequence of temperature
changes, may lead to fracture or undesirable plastic deformation.
• One source of thermal stresses is the restrained thermal expansion (or contraction)
of a body. Stress magnitude may be computed using Equation 19.8.
• The generation of thermal stresses resulting from the rapid heating or cooling of
a body of material result from temperature gradients between the outside and
interior portions and accompanying differential dimensional changes.
• Thermal shock is the fracture of a body resulting from thermal stresses induced
by rapid temperature changes. Because ceramic materials are brittle, they are especially
susceptible to this type of failure.
Equation Summary
Equation
Page
Number Equation Solving for Number
19.1 C dQ
dT
Definition of heat capacity 782
19.3a
19.3b
l f l 0
l 0
a l 1T f T 0 2
¢l
l 0
a l ¢T
Definition of linear coefficient of thermal
expansion
785
19.4
¢V
V 0
a y ¢T
Definition of volume coefficient of thermal
expansion
786
19.5 q k dT
dx
Definition of thermal conductivity 789
19.8
s Ea l 1T 0 T f 2
Ea l ¢T
Thermal stress 792
19.9
s f k
TSR
Ea l
Thermal shock resistance parameter 794