Investigation of the dielectric properties and thermal expansion of the ferroelectric ceramic PKR-1 in the phase-transition region

The permittivity and thermal expansion of polarized and unpolarized samples of the ferroelectric ceramic PKR-1 have been investigated in the temperature range 300–700 K. Anomalous behavior of the thermal expansion coefficient is found in the range of ferroelectric phase transitions. The results obtained are analyzed taking into account the specific features of the structure of polar ceramics.     

Effect of maceoscopic structure on the thermal expansion of synthetic graphite

The predominant-orientation parameters and grain defectiveness (microscopic cracks and pores) have been evaluated for extruded coke-pitch samples subjected to various types of heat treatment (220–2770 °K). The calculations are based on a previously reported theoretical model and experimental thermal-expansion coefficients for temperatures in the range 300–1070 °K. The results are used in a discussion of the effect of macroscopic structure on thermal expansion.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 52–56, August, 1969.     

Thermal expansion of nanostructured PbS films and anharmonicity of atomic vibrations

The dependences of the coherent scattering region size and thermal expansion coefficient α of a PbS nanofilm on the annealing temperature in the range of 293–473 K and on the duration of annealing at a constant temperature of 423 K have been measured. It has been found that the thermal expansion coefficient α of the PbS nanofilm is almost twice as much as the coefficient α of coarse-grained lead sulfide. It has been shown that the large difference in the coefficients α is associated with the small size of particles in the film, which leads to an increase in the anharmonicity of atomic vibrations. The contribution from the small size of particles to the thermal expansion coefficient of the PbS nanofilm has been evaluated theoretically.     

Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics

The thermal expansion of the CaCu₃Ti₄O₁₂ ceramics has been measured over a wide temperature range 120?C1200 K. The high quality of the samples under study has been confirmed by good agreement of the results of measurements of the heat capacity in the range 2?C300 K and in the vicinity of the phase transition of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion that can be associated with the phase transition at 726?C732 K assumed by other investigators have been found. The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere and in air has been investigated.     

Magnetic and electrical properties of bismuth cobaltite Bi24(CoBi)O40 with charge ordering

The compound Bi₂₄(CoBi)O₄₀ has been synthesized using the solid-phase reaction method. The temperature and field dependences of the magnetic moment in the temperature range 4 K < T < 300 K and the temperature dependences of the EPR line width and g-factor at temperatures 80 K < T < 300 K have been investigated. The electrical resistivity and thermoelectric power have been measured in the temperature range 100 K < T < 1000 K. The activation energy has been determined and the crossover of the thermoelectric power from the phonon mechanism to the electron mechanism with variations in the temperature has been observed. The thermal expansion coefficient of the samples has been measured in the temperature range 300 K < T < 1000 K and the qualitative agreement with the temperature behavior of the electrical resistivity has been achieved. The electrical and structural properties of the compound have been explained in the framework of the model of the electronic-structure transition with inclusion of the exchange and Coulomb interactions between electrons and the electron-phonon interaction.     

Elastic properties of nanocrystalline forsterite under high pressure and high temperature

A simple theoretical model is developed to study the pressure–volume–temperature relationship and applied for nanocrystalline forsterite in the temperature range 300–1573 K and pressure range 0–9.6 GPa. The results obtained with the present model are in quite close agreement to the experimental values. The model is therefore extended to study the variation of bulk modulus and the coefficient of volume thermal expansion under high pressure and high temperature. The present study also reveals that the quasi-harmonic approximation, i.e., the product of bulk modulus and the coefficient of volume thermal expansion as constant, is valid at least up to the temperature 1573 K and pressure 9.6 GPa in case of nanocrystalline forsterite.     

Heat capacity anomaly in UO2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO₂ were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO₂ and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.     

U_(1-x)Pu_xO_2热膨胀性质分子动力学模拟研究

基于部分离子势函数的分子动力学方法,研究了U_(1-x)Pu_xO_2(x=0.07,0.15,0.25和0.5)在300—3000 K温度范围和0—1.5 GPa下的热力学性质,研究发现,在等压和不同温度下,随着钚(Pu)比率的增加,U_(1-x)Pu_xO_2的晶格常数线性减小,线性膨胀系数有所增大,等温压缩系数减小,通过拟合得到了零压下U_(1-x)Pu_xO_2晶格常数和线膨胀系数随Pu比率变化的经验表达式。     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

Thermal properties of PCR-1 and PCR-37 piezoceramics

The temperature dependences of the thermal conductivity (300–800 K), specific heat, and thermal expansion coefficient of PCR-1 and PCR-37 piezoceramics have been experimentally determined. Anomalies are found near the Curie temperature, which are caused by changes in the phase composition variations and structure rearrangement.     

Thermal physical properties of ferroelectric ceramics PKR-7M near the diffuse phase transition

The thermal physical properties (heat capacity, thermal expansion coefficient, and deformation) of a relaxor piezoelectric ceramics based on the lead zirconate titanate PKR-7M have been studied in the temperature range 200–800 K. Diffuse anomalies have been revealed in the temperature dependences of the heat capacity and thermal expansion over wide temperature ranges 270–650 and 450–600 K, respectively. It has been shown that the anomalous behavior of the heat capacity is due to the manifestation of two-level states (Schottky anomalies). The results of the study have been discussed together with the data of structural studies.     

Thermal expansion of cadmium fluoride

The thermal expansion of CdF₂ single crystal has been measured over the temperature range 80–300°K. From the latter data, the thermal Grüneisen constant as a function of temperature has been evaluated, and compared with the elastic Grüneisen constant. The correlation between the two sets of data, and with possible lattice interactions is discussed.     

Estimation of Thermal Expansion Coefficients of Rare-Earth Garnets in a Wide Temperature Range

Simple expressions were obtained for the thermal expansion coefficients of the rare-earth garnets, which allow to estimate their values in a wide temperature range in cases, when either the lattice unit cell parameters or the density of the material are known at 300 K. The expression giving the relationship between the thermal expansion and thermal conductivity coefficients for the rare-earth garnets has also been obtained. It is shown that the calculated values obtained with the use of these expressions are in a good agreement with the available experimental data for the garnets of different composition.     

Heat capacity, root-mean-square displacements of atoms and thermal expansion coefficient of europium hexaboride

The temperature dependences of the specific heat capacity c _p(T), thermal expansion coefficient α(T) of europium hexaboride, and root-mean-square displacements of Eu and B atoms are determined in the temperature range from helium to room temperature (5–300 K).     

Temperature dependence of oxidation rate in clean Ge(111)

The temperature dependence of the sticking coefficient of oxygen on a clean Ge(111) surface has been investigated over a wide temperature range from 300 to 1100 °K using three methods. In the interval 300–600 °K a flash technique was used, the desorbed germanium oxide being detected by the time of flight mass-spectrometer. In the range from 500 to 1000 °K the sticking coefficient was measured from the pumping speed of oxygen by the sample surface, and in the range from 800 to 1100 °K the temperature dependence of the etching speed by oxygen was determined.The measured temperature dependence of the sticking coefficient is complex. It increases between 300 and 400 °K, remaining virtually constant from 400 to 500 °K with a new increase in the range from 500 to 1000 °K. A rapid fall in the sticking coefficient was observed at temperatures above 1000 °K.The dependence of the adsorption coverage on exposure has also been obtained for sample temperatures of 300, 350, 400 and 500 and 600 °K. The form of the adsorption curves differs considerably from a theoretical one based on a decrease in the sticking coefficient with coverage given by s = s₀(1 ? θ)². At 600 °K the sticking coefficient decreases more slowly than predicted by this equation. On the contrary, at 300 °K it begins to decrease rapidly at low coverages less than 0.1 of a monolayer.To explain the results it is assumed that oxygen molecules adsorb on the surface structural defects. At 300 °K such defects may be in the form of steps or other morphological disturbances on the surface, and above 500 °K they are probably equilibrium thermal defects, for example, surface vacancies.     

An apparatus for the measurement of thermal expansion of solids at low temperatures

A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y₁Ba₂Cu₃O_6.9 compound in the vicinity of superconducting transition temperature,T _c are also described. The system can detect relative changes in length Δl/l ₀ of about 10⁻⁸. Attempts are being made to improve the sensitivity.     

Accurate measurement of thermal expansion of solids between 77 K and 350 K by 3-terminal capacitance method

The construction of a three-terminal capacitance cell and a bath type cryostat to measure thermal expansion of solids in the temperature range 77 K to 350 K is described. Calculation of the thermal expansion coefficients by using spline approximation is discussed along with the various errors involved in the measurement. The capacitance cell is calibrated by using aluminium and germanium as standard reference materials. The cell has an accuracy of 4% in the measurement of thermal expansion coefficient and a resolution of 1 Å change in length of sample of length 1 cm.     

The effect of hydrogen on the electrical properties of n-type Pb0.9Cd0.1Te thin films

Measurements of the Hall coefficient and the d.c. conductivity were made on flash evaporated n-type Pb_0.9Cd_0.1Te epitaxial films exposed to molecular hydrogen gas at high pressures (upto 500 psi) in the temperature range 77–300 K. It has been found that the effect of hydrogen on the films is to reduce the concentration of the extrinsic carriers and to increase their mobility. It is believed that hydrogen gas removes cadmium ions and neutral cadmium atoms from these films.     

强磁场下Cernox^TM温度传感器的磁致电阻效应研究

以Cernox-1070温度传感器为研究对象,研究了其在磁场强度0-16 T和4.2-300K温区下的磁致电阻效应.实验结果表明：CX-1070温度传感器受磁场的影响较小,在16T、4.2-300K的磁效应值为-0.19-2.38%,产生的最大测量误差为0.11K;15-50K温区为磁效应最低的温度区间,在50-300...     

Low thermal expansion behavior and transport properties of Ni and Ge co-doped manganese nitride materials at cryogenic temperatures

A series of Ni and Ge co-doped manganese nitride materials were fabricated by mechanical ball milling followed by solid-state sintering. Their thermal expansion properties and electrical and thermal conductivities were investigated in the temperature range of 77–300 K. The results show that Ni and Ge co-doped manganese nitride materials have negative thermal expansion (NTE), and the operation-temperature window of NTE shifts toward the lower temperature region and the variation of linear thermal expansion (ΔL/L _(300K)) in the operation-temperature window of NTE decreases with increasing Ni content. The combination of these two factors results in a low coefficient of thermal expansion (CTE) at cryogenic temperatures. The average CTE of Mn₃(Cu_0.2Ni_0.4Ge_0.4)N drops to ‘zero’ in the temperature range of 190–77 K. The values of electrical and thermal conductivities of the Ni and Ge co-doped manganese nitride materials are in the ranges of 2–3×10³ (ohm cm)⁻¹ and 1.6–3.4 W (m K)⁻¹, respectively.