# Thermal Conductivity & Coefficient of Expansion

Thermal conductivity is the propensity for any material to transfer heat from one point to another. Of course for heat to "flow," it is necessary for a difference of temperature to exist within a continuous section of the material. Thermal conductivity is analogous to electrical conductivity. Similarly, thermal resistance is the inverse of thermal conductivity as electrical resistance is the inverse of electrical conductivity.

Coefficient of expansion is the rate at which a material will grow in length with an increase in temperature. Most material grow in a fairly linear fashion, particularly within a defined range of temperatures. A positive coefficient of expansion indicates that the material gets longer as its temperature increases. Most metals are like that. Ice is a well-known example of a negative coefficient of expansion, since it contracts in length with increasing temperature (in other words, ice expands as it gets colder).

 Substance Thermal Conductivity(W/cm·°C) Coefficient ofThermal Expansion(ppm/°C) Density(g/cm3) Specific Thermal Conductivityb(W/cm·°C) Air (still) 0.0003 Alumina 0.276 Alumina (85%) 0.118 Aluminum 2.165 0.23 2.7 0.81 Beryllia (99.5%) 1.969 Beryllia (97%) 1.575 Beryllia (95%) 1.161 Beryllium 1.772 Beryllium-Copper 1.063 Boron Nitride 0.394 Brass (70/30) 1.220 Copper 3.937 0.17 8.9 0.45 Copper/Invc/Copper 1.64 0.084 8.4 .020 Copper/Mod/Copper 1.82 0.060 9.9 0.18 Copper/Mod-Cu/Copper 2.45-2.80 0.60-0.10 9.4 0.26-0.30 Diamond (room temp) 6.299 Epoxy 0.002 Epoxy (thermally conductive) 0.008 FR-4  (G-10) 0.003 GaAs 0.591 Glass 0.008 Gold 2.913 Heatsink Compound 0.004 Helium (liquid) 0.000307 Invar 0.11 0.013 8.1 0.014 Iron 0.669 Kovar 0.17 0.59 8.3 0.020 Lead 0.343 Magnesium 1.575 Mica 0.007 Molybdenum 1.299 Monel 0.197 Mylar 0.002 Nickel 0.906 Nitrogen (liquid) 0.001411 Phenolic 0.002 Platinum 0.734 Sapphire (a-axis) 0.32 Sapphire (c-axis) 0.35 Silicon (pure) 1.457 Silicon (0.0025 Ω-cm) 1.457 Silicon Carbide 0.90 Silicon Dioxide (amorphous) 0.014 Silicon Dioxide (quartz, a-axis) 0.059 Silicon Dioxide (quartz, c-axis) 0.11 Silicone Grease 0.002 Silicone Rubber 0.002 Silicon Nitride 0.16 - 0.33 Silver 4.173 Stainless Steel (321) 0.146 Stainless Steel (410) 0.240 Steel (low carbon) 0.669 Teflon 0.002 Tin 0.630 Titanium 0.219 0.086 4.4 0.016 Tungsten 1.969 Water 0.0055 Zinc 1.024 a: Approximate values from 0 °C to 100 °Cb: Thermal conductivity divided by specific gravity    (introduced by Dr. Carl Zweben & K.A. Schmidt)c: Invard: Molybdenum