14.3.Thermal Conduction 279 portional to the temperature gradient dT/dx.The proportionality constant is called the thermal conductivity K(or A).We thus write: J0=-K dx (14.11) The negative sign indicates that the heat flows from the hot to the cold end (Fourier Law,1822).Possible units for the heat con- ductivity are J/(m.s-K)or W/(m.K).The heat flux Jo is measured in J/(m2.s).Table 14.3 gives some characteristic values for K. The thermal conductivity decreases slightly with increasing tem- perature.For example,K for copper or Al2O3 decreases by about 20%within a temperature span of 1000C.In the same temper- ature region,K for iron decreases by 10%. For the interpretation of thermal conduction we postulate that the heat transfer in solids may be provided by free electrons as TABLE 14.3.Thermal conductivities at room temperature W Substance ·K ms-K Diamond Type IIa 2.3×103 SiC 4.9×102 Silver 4.29×102 Copper 4.01×102 Aluminum 2.37×102 Silicon 1.48×102 Brass (leaded) 1.2×102 Iron 8.02×101 GaAs 5×10 Ni-Silver 2.3×101 Al2O;(sintered) 3.5×101 SiO2 (fused silica) 1.4 Concrete 9.3×10-1 Soda-lime glass 9.5×10-1 Water 6.3×10-1 Polyethylene 3.8×10-1 Teflon 2.25×10-1 Snow(0°C) 1.6 ×10-1 Wood (oak) 1.6×10-1 Sulfur 2.0×10-2 Cork ×10-2 Glass wool 5 ×10-3 Air 2.3×10-4 a See also Figure 14.1.Source:Handbook of Chemistry and Physics,CRC Press,Boca Raton,FL (1994). b62%Cu,15%Ni,22%Zn.portional to the temperature gradient dT/dx. The proportionality constant is called the thermal conductivity K (or ). We thus write: JQ
K d d T x . (14.11) The negative sign indicates that the heat flows from the hot to the cold end (Fourier Law, 1822). Possible units for the heat con￾ductivity are J/(msK) or W/(m  K). The heat flux JQ is measured in J/(m2  s). Table 14.3 gives some characteristic values for K. The thermal conductivity decreases slightly with increasing tem￾perature. For example, K for copper or Al2O3 decreases by about 20% within a temperature span of 1000°C. In the same temper￾ature region, K for iron decreases by 10%. For the interpretation of thermal conduction we postulate that the heat transfer in solids may be provided by free electrons as 14.3 • Thermal Conduction 279 TABLE 14.3. Thermal conductivities at room temperaturea Substance K m W  K   m J sK  Diamond Type IIa 2.3 103 SiC 4.9 102 Silver 4.29 102 Copper 4.01 102 Aluminum 2.37 102 Silicon 1.48 102 Brass (leaded) 1.2 102 Iron 8.02 101 GaAs 5 101 Ni-Silverb 2.3 101 Al2O3 (sintered) 3.5 101 SiO2 (fused silica) 1.4 Concrete 9.3 101 Soda-lime glass 9.5 101 Water 6.3 101 Polyethylene 3.8 101 Teflon 2.25 101 Snow (0°C) 1.6 101 Wood (oak) 1.6 101 Sulfur 2.0 102 Cork 3 102 Glass wool 5 103 Air 2.3 104 a See also Figure 14.1. Source: Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL (1994). b 62% Cu, 15% Ni, 22% Zn.