Technical Glossary Coefficient of thermal conductivity

[Coefficient of thermal conductivity]

If a temperature difference is present in a substance, then the movement of heat will occur from portions with higher temperatures to those with lower temperatures. Coefficient of thermal conductivity is a coefficient that indicates how easily this movement of heat occurs, so when a temperature difference of 1℃ per unit length (thickness) exists, this is the amount of heat that moves a unit area in a unit time. In other words, when the difference in temperatures for surface A (TA) and surface B (TB) in a 1m³ cube with a volume as indicated in Figure A is 1℃ (TA ＞ TB) , the amount of heat to move 1 m (Aright arrowB) in 1 second is the coefficient of thermal conductivity. Units are indicated in J · m/s · m2 · ℃, W/m · K, or W/m · ℃. The larger the value for coefficient of thermal conductivity, the greater the amount of heat that moves, which means that heat will be easily transferred. Thus, a substance with a large thermal conductivity is preferred as a heat conductor. However, thermal conductivity is given in values for unit length, so even if a substance has a large thermal conductivity, heat-conducting action will decrease if the length (thickness) actually used is greater.
For example, heat-conducting action will be the same for a heat conductor where the thickness is 10 mm with a coefficient of thermal conductivity of 10 W/m · K and a heat conductor where the thickness is 1 mm with a coefficient of thermal conductivity of 1 W/m · K. In addition, coefficient of thermal conductivity is a specific value (physical property) for a substance, and it generally varies according to the temperature. For example, the coefficient of thermal conductivity of air increases with temperature, while the coefficient of thermal conductivity of a metal tends to decrease with a rise in temperature.

(単位: W・m-1・K-1)
ダイヤモンド（C）	1000 - 2000
銀（Ag）	420
銅（Cu）	398
鉄（Fe）	84
水晶（SiO2）	8
ガラス	1
ポリエチレン	0.41
シリコーン（Qゴム）	0.16
木材	0.15 - 0.25
空気	0.0241