Kinetics of thermal phonons and the structure of nanodispersed iron-containing corundum-based cermets at liquid-helium t
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PROPERTIES OF SOLIDS
Kinetics of Thermal Phonons and the Structure of Nanodispersed Iron-Containing Corundum-Based Cermets at Liquid-Helium Temperatures O. V. Karban’a,b, E. I. Salamatova, A. V. Taranovb, E. N. Khazanovb, and O. L. Khasanovc a
Physicotechnical Institute, Ural Division, Russian Academy of Sciences, Izhevsk, 426000 Russia
b Institute of Radio-Engineering and Electronics, Russian Academy of Sciences, ul. Mokhovaya 11-7, Moscow, 125009 Russia c
e-mail: [email protected] Spektr Research Center of Promising Technologies, Tomsk Polytechnical University, Tomsk, 620219 Russia Received November 7, 2008
Abstract—The kinetics of weakly nonequilibrium subterahertz thermal phonons is studied in nanodispersed iron-containing corundum-based cermets at liquid-helium temperatures. For the chosen method of fabricating cermets (which restricts grain growth), iron inclusions are shown to be described as point trapping centers of phonons. The transport of nonequilibrium phonons is analyzed in a ceramic matrix containing metallic trapping centers. PACS numbers: 61.46.-w DOI: 10.1134/S1063776109040141
INTRODUCTION Interest in the materials synthesized from nanopowders is caused by a wide spectrum of their physical and structural properties. A high melting temperature, a high hardness, high wear and chemical resistance, and a low density represent the properties of ceramics that excel those of metals. However, the absence of plasticity (brittleness) and a low fracture toughness restrict the use of ceramics as a structural material. The introduction of a metallic fraction into a dielectric ceramic matrix made it possible to create composite materials (cermets) that combine the advantages of ceramics and metals. As plastic-metal inclusions, researchers usually apply Ag, Al, Ni, Fe [1–4]. Important requirements imposed on cermets consist in the facts that metallic inclusions should closely bind a ceramic matrix and that their characteristic sizes should not exceed a critical size at which thermal stresses can induce cracks around metallic grains. The smaller the metal grain size in a dielectric ceramic matrix, the larger the difference in the thermal expansion coefficients of the metal and matrix material, since 3 thermal stresses are proportional to R Fe [5], where R is the average size of the ceramic structural fragment (iron grain in this case). When a material transforms into an ultradispersed state, its physical and structural properties can change substantially. In [6], we showed that the thermophysical properties of (ZrO2:Y2O3) + 14% Al2O3 nanocomposites at liquid-helium temperatures change sharply as a result of the formation of a “gap” in their phonon spec-
trum. This effect is related to the fact that the sizes of some structural fragments are comparable with the thermal-phonon wavelengths. The presence of a metallic fraction in nanocomposites can substantially change the transport properties of subterahertz phonons due to both resonance scattering by structural fragments and an effective electron–phonon intera
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