Interaction of phonons with spherical nanoparticles embedded densely in simple crystalline matrix. Case of nitrogen-palladium nanocomposite

Abstract

The results of experimental investigations of thermal conductivity of nanocomposites built of palladium spherical nanoparticles embedded in the structure of crystalline nitrogen are presented in this paper. The investigations were carried out on the samples containing the Pd nanospheres of diameters of 6, 8, 10, 12, 18, and 24 nm at the palladium fraction amounting to 15% of the nanocomposite volume. The measurements were performed with a steady-state heat flow method in the temperature interval of 2–35 K. For the analysis of the experimental results, the relaxation time approximation in the frame of the thermal conductivity Debye model was used. The analysis shows that the nitrogen matrix phonons can effectively interact with spherical nanoparticles in, at least, four different mechanisms. Two of those mechanisms, a diffuse scattering by the boundary of two media (which are the matrix and the material of the nanoparticle) and an oscillating resultant of interaction of nanoparticle phonons with the matrix phonons, result in a decrease of the nanocomposite thermal conductivity. The remaining two mechanisms would be considered nonthermal resistive scattering processes: The first of them is the specular scattering of the matrix phonons by the matrix and the nanoparticle material interface. The second one is a forward scattering, in which the matrix phonon penetrates the nanoparticle and afterward penetrates another one without any resistive interaction with phonons of the crystalline matrix. The analysis shows that the nonresistive processes are significantly more frequent in the investigated nanocomposites than the resistive ones. Such a conclusion is in perfect agreement with the results of the analysis of the phonon mean free path in the nitrogen crystal-palladium nanospheres composite.