is the thermal diffusivity, ∇ 2 {\displaystyle \nabla ^{2}}

The phosphoate glass has a low thermal diffusivity which is in the range of 0.27 to 0.39 (mm2/sec).

A phosphate glass having a thermal diffusivity of not more than 0.31 (mm2/sec) is particularly suitable for use in the substrate in the thermal head.

The lampshade has simple structure, small volume, long lifetime and good thermal diffusivity, and overcomes the disadvantages of conventional lampshades in above-mentioned aspects.

A graphite film that excels in thermal diffusivity and has a high density, and that at its surface is free from flaw, dents and creases and excels in planarity.