The thermal conductivity of highly oriented pyrolytic boron nitride

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Abstract
The thermal diffusivity of BN has been measured from 100 K-800 K using a 'flash' method; published specific heat data has been used to evaluate the thermal conductivity. Over a limited temperature range the results have been confirmed by steady state measurements. The thermal conductivity parallel to the deposition plane has been found to have a maximum value of about 2.5 W cm-1 K-1 at 235 K, while perpendicular to it the thermal conductivity shows no maximum but decreases from about 2.5*10-2 W cm-1 K-1 at 100 K to 1.7*10-2 W cm-1 K-1 at 800 K. The conductivity parallel to the deposition plane has been analysed using a two dimensional mode in which the vibrations in and out of the plane are independent and the latter depend only upon the resistance of the layer to bending. Characteristic temperatures for the two modes of 2290 K and 1200 K respectively have been estimated. Above 200 K the conductivity can be explained in term of Umklapp processes and boundary scattering from crystallites estimated to have average diameters in the deposition plane of 580 AA and 290 AA for the two samples investigated.