G. G. Gadzhiev

The Thermal and Elastic Properties of Zinc Oxide-Based Ceramics at High Temperatures

The thermal conductivity, thermal expansion coefficient (TEC), and the propagation velocity of longitudinal and transverse ultrasonic waves in ZnO-based ceramics are investigated in the temperature range from 300 to 1200 K with a porosity from 1.5 to 21%. The Young, shear, and bulk moduli and the Poisson ratio are calculated from the data on the propagation velocities of ultrasonic waves. Formulas are suggested to calculate the investigated parameters as a function of temperature and porosity.

Thermophysical Properties of Sulfides of Lanthanum, Praseodymium, Gadolinium, and Dysprosium

The temperature dependence of the thermal conductivity, electrical conductivity, thermoelectromotive force, and thermal expansion coefficient for sulfides of lanthanum, gadolinium, praseodymium and dysprosium of the composition Ln3 – xVxS4 is investigated in the temperature range from 300 to 1200 K. It is shown that the transfer phenomena and thermoelectrical properties of the investigated compositions depend on the concentration of current carriers, cation vacancies, and mobility. Gadolinium sulfide is found to have the highest thermoelectrical efficiency. The scattering from ions of rare-earth elements has a noticeable effect on the magnitude and temperature dependence of the lattice thermal conductivity and electrical resistance.

Thermal and Electrical Properties of Gadolinium Sulfides at High Temperatures

A study is made into the temperature dependence of the thermal conductivity, electrical conductivity, thermoelectromotive force, thermal expansion coefficient, and heat capacity in the temperature range from 300 to 1200 K for polycrystalline gadolinium sulfides GdSy(y= 1.495–1.487) produced both by recrystallization pressing and by crystallization from a melt. The role of the mechanisms of heat and charge transfer is estimated depending on the composition. The reasons for changes in their electrical and thermal properties are analyzed. The thermoelectric efficiency is calculated. It is demonstrated that Z≥ 0.6 × 10–3K–1at T≥ 1000 K.

Heat capacity and dielectric properties of multiferroics Bi1 − xGdxFeO3 (x = 0–0.20)

The heat capacity and the permittivity of multiferroics Bi1 − xGdxFeO3 (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.

Properties of Na0.875Li0.125NbO3 ceramics

The structural, dielectric, and thermal properties of the Na0.875Li0.125NbO3 solid solution doped with strontium and other elements have been studied in wide temperature and frequency ranges. The material has been shown to undergo a sequence of phase transitions accompanied by anomalies in its structural, dielectric, and thermal properties. The observed low-frequency dispersion of its dielectric permittivity is attributed to the effect of electrical conductivity.

Heat capacity of BiFeO3-based multiferroics

The heat capacity of Bi1 − xRexFeO3 (Re = La, Eu, Ho; x = 0, x = 0.05) multiferroics has been studied in the temperature range of 120–800 K. The substitution of a small amount of rare-earth elements for bismuth leads to a significant increase in the heat capacity in the broad temperature range studied. It is established that the temperature dependence of the excess heat capacity is related to the Schottky effect for three-level states certain that appear as a result of structure distortions in the rare-earth-doped compositions.

Phase transformations and properties of Ag1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics

We have studied the phase transformations, microstructure, and dielectric, piezoelectric, and thermophysical properties of Ag1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics. Within its homogeneity range (y ≤ 0.10), silver niobate undergoes a complex sequence of phase transformations, accompanied by anomalies in its physical properties. The observed dispersion effects are interpreted in terms of electrical conductivity above the Curie temperature and in terms of the motion of domain walls and interfaces below the Curie temperature.

Thermal conductivity of silicon carbide ceramics doped with beryllium oxide

The thermal conductivity of silicon carbide ceramics containing up to 3 wt % BeO was measured in the temperature range 300–1300 K. The effective thermal conductivity was found to rise notably with increasing BeO content in the range 1.3–1.5 wt % BeO and to decrease exponentially with increasing porosity.

Phase composition, microstructure, and properties of Na1 − yNbO3 − y/2 ceramics

We have studied the phase composition, microstructure, and dielectric and thermophysical properties of Na1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics and identified a complicated sequence of phase transformations within the homogeneity range of sodium niobate (y ≤ 0.10), accompanied by anomalous variations in its physical properties. At low y values, discontinuous secondary recrystallization occurs. We conclude that dielectric effects above the Curie temperature are related to the electrical conductivity of the material and that those at low temperatures are governed by the motion of domain walls and interfaces. The temperature-dependent structural, dielectric, and thermophysical properties of the materials studied are shown to correlate.

Second-order phase transitions under pressure in polycrystalline compounds and rocks

The dependence of thermal conductivity in mono-and polycrystalline tellurium and rocks at hydrostatic pressures of up to 400 MPa in the temperature range of 273–523 K is studied. The confining pressure in polycrystalline materials affects their volume and elastic properties, leading to second-order phase transitions.