Thermal Conductivity of Boron and Some Boron Compounds

Abstract

Thermal-conductivity measurements have been made on crystals of $\beta$-boron, ${\mathrm{B}}_{12}$ ${\mathrm{P}}_2$, ${\mathrm{B}}_{12}$ ${\mathrm{As}}_2$, and Y${\mathrm{B}}_{66}$ from 3 to 300 K. Phonons are the dominant carriers of heat, and their mean free paths range from ${10}^{-5\mathrm{}}$ cm for temperatures near the Debye temperature to ${10}^{-1\mathrm{}}$ cm near 4 K. A model in which only the phonons in the acoustic branches carry the heat is used. This model explains the large decreases observed in thermal conductivity as the crystal structure becomes more and more complex in the series ${\mathrm{B}}_{12}$ ${\mathrm{As}}_2$, $\beta$-boron, Y${\mathrm{B}}_{66}$. At the lowest temperatures the thermal conductivity of single crystals of Y${\mathrm{B}}_{66}$ behaves somewhat like that of a glass, presumably because of a substantial amount of frozen-in disorder in the melt-grown crystals.