E. N. Khazanov

Scattering of acoustic phonons by rare earth substitutional atoms in yttrium aluminum garnets

The diffusive maxima of phonon signals, and in particular their arrival timestm are examined for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The phonon pulses are generated by metallic films of the characteristic lengthlh heated by current pulses to the temperatureTh slightly higher than the ambient temperatureT. The injected phonons travel in wafers of the thicknessLz. They are scattered by substitutional atoms of rare earth occupying the yttrium dodecahedral sites, rare earth and yttrium atoms occupying the aluminum octahedral sites and by another lattice imperfections generated in the process of sample growing. The qualitative analysis based on our exact formula for the diffusion coefficientD allows us to extract the contribution of rare earth atoms substituting the Y atoms toD. Considering the dependence oftm/Lz2 on the temperature and the ratiolh/Lz we conclude thatD∼Th−4 and that the energy of phonons forming the diffusive maxima ranges from 3.2kBTH to 4.2kBTH, which is in reasonable agreement with the existing estimates.

Propagation of phonons in nanocrystalline ZrO2: Y2O3 ceramics

Phonon transfer in yttrium-oxide (Y2O3) stabilized ZrO2 ceramics is studied experimentally in the range of helium temperatures (1.7–3.8 K). A model of the structure of the intergranular layer in the ceramic is considered, which explains the temperature dependence of the phonon diffusion coefficient, makes it possible to determine the intergranular layer parameters (density, velocity of sound, and thickness), and gives an idea about its structure. Scattering of injected phonons from resonance vibrations of nanoceramic grains is discussed.

The scattering of non-equilibrium phonons in Al2O3 nanoceramics

Abstract The processes of phonon transport in Al 2 O 3 ceramics are studied in the region of helium temperatures. For the ceramic grain size R within two orders of magnitude and more (up to nanosizes), the phonon diffusion coefficient D eff ∝ R and exhibits a decreasing temperature dependence. In nanoceramics with R D eff decreases abruptly by two orders of magnitude, and its temperature dependence changes sign. A model of temperature dependence of the diffusion coefficient is suggested.

27Al nuclear magnetic resonance studies of the Y3-xLuxAl5O12 mixed garnets

The character of the substitutional order, the chemical shift, and the nuclear quadrupole coupling in the mixed yttrium-lutetium aluminium garnets Y3-xLuxAl5O12 (0≤x≤3) were studied for the first time using 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS nuclear magnetic resonance (NMR). The relatively narrow distribution of the isotropic NMR line shifts, revealed by MQ MAS NMR, in crystals with compositions x = 0.75 and x = 1 compared to mixed garnets of other compositions showed that these crystals are partially ordered solid solutions. The aluminium quadrupole coupling constants and isotropic chemical shifts were evaluated for all garnets under study.

Resonant scattering of nonequilibrium phonons (λph = 10–50 nm) in nanostructured ceramics based on YSZ + Al2O3 composites

Specific features of the transport of weakly nonequilibrium thermal phonons (λph = 10–50 nm) in nanoscale ceramics at a transition from micro-to nanosizes have been investigated. On the basis of the model of spherical shells randomly distributed in space and modeling grain boundaries, whose elastic properties differ from the elastic properties of grains, features of the phonon spectrum in the wavelength range λph ∼ Rg have been studied. The conditions leading to the occurrence of a gap in the phonon spectrum of nanoscale materials are analyzed. It is shown that the position of the top gap edge in the phonon spectrum is determined to a large extent by the structure of phase boundaries, while the presence of inclusions (pores, other phases) with characteristic sizes smaller than that of grains of the main ceramic material shifts the gap to high frequencies in the phonon spectrum. Temperature dependences of the diffusion coefficient of nonequilibrium phonons near the top gap edge in the phonon spectrum have been measured for multiphase ceramics based on YSZ + 14.3% Al2O3 composites.

Investigation of Nanoceramics Based on Aluminum and Zirconium Oxides Using the Heat Pulse Method

The processes of phonon transport in ceramics and composites based on aluminum and zirconium oxides are studied experimentally in the region of helium temperatures. It is demonstrated that, for the ceramic grain size R within two orders of magnitude and more (up to nanosizes), the phonon diffusion coefficient Deff ∝ R and exhibits a decreasing temperature dependence. In aluminum-based nanoceramics with R < 200 nm, the phonon diffusion coefficient decreases abruptly by two orders of magnitude, and its temperature dependence becomes increasing. A model of temperature dependence of the diffusion coefficient is suggested.

27Al and 45Sc NMR studies of the Y3ScxAl5-xO12 mixed garnets

The substitution of aluminum with scandium in the mixed yttrium aluminum scandium garnets, Y 3 Sc x Al 5-x O 12 (x = 0, 0.05, 0.5, 1.5), was studied for the first time using 27 Al magic angle spinning (MAS), multiple quantum (MQ) MAS NMR, and 45 Sc MAS NMR. The aluminum and scandium quadrupole coupling constants and isotropic chemical shifts were evaluated for all garnets under study. Al 2D spectra evidenced variation of the isotropic shifts over the aluminum sites in the mixed garnets.

Phonon spectroscopy of submicron ceramic materials based on Ce1–xGdxO2–y solid solutions

The specific features of the kinetic characteristics of subterahertz phonons in ceramic samples based on Ce1–x GdxO2–y electrolyte solid solutions have been investigated in the range of liquid-helium temperatures. It has been demonstrated that the observed anomalies of the transport characteristics of thermal phonons in this system are caused by the formation of structural defects associated with the position of vacancies in the anion sublattice with respect to impurity cations. The activation energy of the two-level system Δ = 8.53 K has been determined from an analysis of the experimental results.

Kinetics of thermal phonons and the structure of nanodispersed iron-containing corundum-based cermets at liquid-helium temperatures

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.

Kinetic characteristics of subterahertz phonons in optically transparent Y3Al5O12 ceramics with twinning elements in their structure

The relationship between the kinetic characteristics of subterahertz thermal phonons and the structural features of grains and grain boundaries in optically transparent yttrium aluminum garnet (YAG) ceramics is investigated. The effect of plastic deformation on the formation of the structure of cubic oxide ceramics is analyzed. It is shown that the main mechanism of plastic deformation responsible for the formation of natural crystallographic boundaries in samples is the twinning effect.