GTBL042-17 GTBL042-Callister-v2 September 14, 2007 9:36
Revised Pages17.4 Thermal Conductivity • 715Temperature (°C)Thermal conductivity (W/m-K)(Btu/ft-h-°F)Thermal conductivity (cal/cm-s-K)0 400 800 1200Dense stabilized ZrO 2Pure dense Al 2 O 3Pure dense MgOPure dense BeOGraphite1600 20001.0101001.00.10.010.0011.010100400 1200 2000Temperature (°F)
2800 3600Figure 17.5
Dependence of
thermal conductivity
on temperature for
several ceramic
materials. (Adapted
from W. D. Kingery,
H. K. Bowen, and
D. R. Uhlmann,
Introduction to
Ceramics, 2nd
edition. Copyright
©c1976 by John
Wiley & Sons, New
York. Reprinted by
permission of John
Wiley & Sons, Inc.)Porosity in ceramic materials may have a dramatic influence on thermal conduc-
tivity: increasing the pore volume will, under most circumstances, result in a reduction
of the thermal conductivity. In fact, many ceramics that are used for thermal insula-
tion are porous. Heat transfer across pores is ordinarily slow and inefficient. Internal
pores normally contain still air, which has an extremely low thermal conductivity—
approximately 0.02 W/m-K. Furthermore, gaseous convection within the pores is also
comparatively ineffective.Concept Check 17.3The thermal conductivity of a single-crystal ceramic specimen is slightly greater than
a polycrystalline one of the same material. Why is this so?[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]Polymers
As noted in Table 17.1, thermal conductivities for most polymers are on the order
of 0.3 W/m-K. For these materials, energy transfer is accomplished by the vibration
and rotation of the chain molecules. The magnitude of the thermal conductivity
depends on the degree of crystallinity: a polymer with a highly crystalline and ordered
structure will have a greater conductivity than the equivalent amorphous material.
This is due to the more effective coordinated vibration of the molecular chains for
the crystalline state.