Thermal conductivity and heat capacity of Si3N4/BN fiber monoliths

This paper reports on measurements within the 5-300-K temperature interval of the thermal conductivity of Si3N4 and BN polycrystalline ceramic samples and Si3N4/BN fiber monoliths (FM) with different fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5-300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si3N4 and BN samples and in Si3N4/BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions-with minor assumptions, it can be maintained that, in the case of the Si3N4/BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si3N4 and BN components with due account of their contributions to formation of the Si3N4/BN FM. It has been established that in the low-temperature domain (5-25 K), phonons in Si3N4/BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislocations. This effect is not observed in ceramic Si3N4 and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si3N4 and BN samples and the effective mean free path length in the Si3N4/BN [0] FM.