Unusual Thermal Behaviour of Mercurous Chloride

A precise determination of the lattice parameters and the coefficients of thermal expansion of mercurous chloride has been made at different temperatures ranging from 30 °C to 260 °C with a Unicam 19 cm high-temperature powder camera and FeKα radiation. The ‘a’ parameter increases non-linearly, whereas the ‘c’ parameter decreases linearly with temperature. Both the c/a value and the unit-cell volume are found to decrease with increasing temperature. The coefficient of thermal expansion along the ‘c’ axis, α∥, is found to have a constant negative value throughout the range of temperature studied. The positive value of α⊥ increases while the negative value of volume coefficient (β) decreases linearly with increasing temperature indicating an unusual and interesting thermal behaviour.     

近零膨胀Zr0.5Hf0.5V1.4P0.6O7材料的制备及其性能研究

采用传统的固相法合成了近零膨胀氧化物功能陶瓷材料Zr_0.5Hf_0.5V_1.4P_0.6O₇,用X射线衍射(XRD)、Raman光谱和热膨胀法对Zr_0.5Hf_0.5V_1.4P_0.6O₇的热膨胀系数、各向同性和相变进行了测试,通过Hf⁴⁺/P⁵⁺共掺杂使得材料具有较低的热膨胀系数,研究发现合成的Zr_0.5Hf_0.5V_1.4P_0.6O₇具有Pa3立方相结构,从334 K附近到673 K较宽的温度范围内的线性热膨胀系数为-1.56×10^-6 K^-1,表现出稳定的近零热膨胀特性。由于固溶体内部微结构的影响造成膨胀仪实验结果与变温X射线衍射结果存在一定的差距。Zr_0.5Hf_0.5V_1.4P_0.6O₇具有的近零膨胀特性为通过负热膨胀材料合成膨胀系数可控的材料提供了基础。     

Thermal properties of sodium silicate glasses at low temperatures

The heat capacity and the thermal expansion coefficient have been measured at low temperatures for vitreous silica containing 0, 10, 20, 30 and 40% soda. The heat capacity per gram is not much affected at 4 K, but is reduced by up to 30% at 10 K by addition of soda. The effect on the expansion coefficient is much greater; the negative values shown by pure silica below 150 K largely disappear with the addition of {10% soda. Below 10 K the coefficient remains negative until 25% soda is added. The Grüneisen function γ(T) is calculated for the various compositions.     

Thermal Expansion Coefficient of Binary Semiconductors

The linear thermal expansion coefficient of tetrahedrally coordinated A^IIB^VIand A^IIIB^Vsemiconductors has been calculated using plasmon energy data. A simple relation between the bond length and plasm on energy has been derived. The calculated values of thermal expansion coefficient and bond length have been compared with the experimental values and the values reported by differentworkers. An excellent experiment has been obtained between them.     

Empirical methods for estimating the linear coefficient of expansion of oxide glasses from their composition

Empirical methods for estimating the linear coefficient of expansion (α) of oxide glasses have been derived. New parameters (β_i) are define for the glass modifiers. The β_i are calculated from the ionic radius R, charge Z, and the electronegativity EN of the modifiers. β_i parameters and their normalized values (γ_i) with respect to a standard modifier, classify the modifiers according to their contribution to the thermal expansion. Two semi-emperical relations were derived for the calculation of α. Calculated expansion coefficient for over 500 glass compositions are within ± 15% of the measured values. The application of the methods to ternary, quaternary and multicomponent industrial glasses is treated.     

X-Ray Studies of Some Europium Compounds at Elevated Temperatures

The temperature variation of the lattice constants of europium iron garnet (Eu₃Fe₅O₁₂), europium sulphide (EuS) and europium fluoride (EuF₂) has been studied using an X-ray powder diffractometer. The lattice constant of Eu₃Fe₅O₁₂ increases linearly upto 800 °C with an expansion coefficient of 10.4 × 10⁻⁶ °C⁻¹. In the case of EuS, the lattice constant increases non-linearly with temperature. At room temperature the expansion coefficient has a value of 14.3 × 10⁻⁶ °C⁻¹. In EuF₂ the lattice constant increases non-linearly upto 140 °C. At higher temperatures, the lattice constant decreases with increasing temperature with a negative expansion coefficient of −29 × 10⁻⁶ °C⁻¹ over the range 170–235 °C. The cause of the anomalous behaviour observed in EuF₂ is yet to be understood.     

Infrared transmitting chalcogenide glass ceramics

Infrared glass ceramics transmitting light from 2 to 15 μm have been produced from a Ga-Ge-Sb-Se glass. The glass composition is selected in the middle of the glass forming region and the annealing temperature must be close to T_g to avoid excessive crystal growth. A heating time of the order of two weeks is necessary to see evolution of the thermal expansion coefficient of the composite while keeping the optical transmission properties of the glass.     

Coefficient of volume expansion and thermoacoustic parameters in N-(P-hexyloxybenzylidene)-P-toluidine

The Sharma constant S_o, the isochoric temperature coefficient of volume expansivity, the isochoric temperature coefficient of internal pressure, the reduced compressibility, the reduced volume and a few other acoustic parameters are evaluated for N-(P-Hexyloxybenzylidene)-P-Toluidine (HBT) using only the coefficient of volume expansion. The Sharma constant which was shown by Sharma to be a constant in a number of non-mesomorphic systems is bound to be a constant in the liquid crystal HBT also, investigated by the authors with the same character constant value both in the isotropic and nematic phases with anomalous behaviour and pretransitional effects at and in the immediate vicinity of the isotropic-nematic phase transition temperature. The various other acoustic parameters estimated for liquid crystal HBT investigated in the present studies are compared and discussed with the values reported by Sharma, and by Sharma and Reddy for certain other non-mesomorphic systems.     

Temperature variation of the structural and thermal characteristics of YBa2Cu3O7−x in the range from 90 to 400 K

The X-ray studies of the tetragonal YBa₂Cu₃O_7−x compound are performed at different temperatures from 400 to 90 K and the temperature variation of the thermal expansion coefficients and the Debye characteristic temperature is determined. The anisotropy of thermal expansion is investigated. It is found that the two dynamical characteristics monotonously decrease with temperature lowering whereas for the superconducting orthorhombic modification their temperature dependences are anomalous. The mean thermal expansion coefficient as well as the overall Debye temperature for the tetragonal phase are smaller than those for the orthorhombic one.     

Optimization of the rapid supercritical extraction process for aerogels

A partial differential equation is derived that describes the pressure developed in the pores of a gel during the rapid supercritical extraction process. A comparative analysis of the strains caused by syneresis and expansion of the fluid, respectively, suggests that the latter is the dominant effect for this process. Experimental results indicate that the rate of leakage from the mold is equal to the rate of volumetric expansion of the fluid, so this was used as the boundary condition for the calculation. An analytical solution is obtained for the strain produced in a purely elastic gel. The strain is found to develop most rapidly at high temperatures, where the thermal expansion of the fluid increases sharply. The model predicts a temperature dependent heating rate that can be used to avoid irreversible strains by compensating for the increase in thermal expansion coefficient.     

Temperature variation of lattice parameter and thermal expansion of TmAl2

The lattice parameter of TmAl₂ has been measured from room temperature to 637 K using CuKα-radiation. The data have been used to evaluate the coefficient of thermal expansion at various temperatures. It was found that the lattice parameter increases while the coefficient of expansion remains constant throughout the range of temperature studied.     

The nature of the glass transition in a columnar hexagonal ordered phase

The glass transition taking place in a columnar discotic phase is connected with the freezing-in of the correlated rotation of the disc-like molecules in the columns about the columnar axis. X-ray investigations on the temperature dependence of the linear expansion coefficients along the columnar axis and perpendicular to this axis reveal that the glass transition leads to a stepwise change of the expansion coefficient along the columnar axis and to a continuous variation for the corresponding coefficient perpendicular to this direction. The glass transition in such ordered systems is highly anisotropic both with respect to dynamic and to thermodynamic aspects.     

Contraction of amorphous CoP alloys during structural relaxation

The structural relaxation of amorphous alloys is accompanied by an increase of their mechanical density. This is shown by measurements of the thermal expansion of electrodeposited CoP alloys between room temperature and crystallization. Above 80°C, the temperature of preparation, the linear thermal expansion coefficient is apparently reduced as a consequence of a superimposed shrinkage, whereas the true thermal expansion coefficient is not affected by any preceding thermal treatment. The velocity of contraction increases with temperature when the material is continuously heated. But the value of the contraction per degree increase in temperature is constant above a certain temperature T₁. The higher the heating rate, the higher T₁. The mathematical treatment of these experimental results is difficult because the relaxation property, the contraction, depends not only on temperature but also on time and on the preceding thermal treatment.     

Relation of the Hydrostatic Electrostrictive Coefficient of Cubic Dielectrics with other Material Constants

A simple relation is derived that links the hydrostatic coefficient of quadratic electrostriction with the volume compressibility and an expression involving the dielectric constant. The first part of the derivation is mainly macroscopic and applicable to dielectrics with cubic symmetry. Making use of the Born-Mayer potential, an expression of the derivative of the ionic polarizability with respect to volume is given in the second part. A negative sign of the hydrostatic coefficient is predicted for typical alkali halides. This result is in agreement with recent experimental findings reported in the literature, if the observed values are corrected for the influence of electrostatic stresses in an alternative way.     

Thermal Expansion of SrF2 at Elevated Temperatures

The X-ray method was used to determine the linear thermal expansion coefficient of SrF₂ in the temperature range from 301 to 1173 K. A comparison of the thermal expansion coefficients of CaF₂, SrF₂ and BaF₂ at T > 300 K shows that the degree of lattice an–harmonicity in the alkaline earth fluorides decreases with increasing mass of the alkaline earth atom. The results for the thermal expansion of SrF₂ are used to discuss the temperature dependence of the lattice mismatch in the SrF₂/InP film–substrate system.     

Experimental study of stress relaxation during annealing

In order to calculate the residual stresses in a flat sheet of glass during annealing on the basis of its thermal history, the viscosity and coefficient of thermal expansion must be determined throughout the transformation range. The structural modifications of the material are taken into account by using fictive temperature concept. Three constitutive assumptions for the rate of the fictive temperature are discussed. A very good agreement is obtained between measured and calculated residual stresses.     

Low expansion in [(Fe0.9Co0.1)0.72B0.24Nb0.04]95.5Y4.5 bulk metallic glass

The thermal expansion behavior of an iron-based bulk metallic glass (BMG) [(Fe_0.9Co_0.1)_0.72B_0.24Nb_0.04]_95.5Y_4.5 is studied. The expansion coefficient in the wide temperature range of 100–300 K keeps in a constant, which is close to that of the Kovar alloy. Furthermore, the expansion coefficient of the BMG can be tuned by annealing below glass transition temperature. The results have implication for understanding the origin of low expansion, and the BMG may have a potential for commercial low expansion material.     

Some characteristics of glass in the Al2O3---B2O3---SiO2 system from the gel

The colorless and transparent glasses in the Al₂O₃---B₂O₃---SiO₃ system with high B₂O₃ and SiO₂ content were prepared from gels at low temperature. Their IR spectra not only revealed the evolution of the gel to glass conversion, but also showed that the formation of mixed bonds in the glasses obtained did not show any effect due to the B₂O₃ content. The accuracy of the glass composition is dependent upon the SiO₂/B₂O₃ molar ratio. The higher the ratio, the less the deviation of the analyzed compositions of the resulting glasses from their original calculated values. It is obvious that the higher the ratio, the lower the thermal expansion coefficient and the higher the transformation temperature of the glass, and the temperature at which the thermal contraction reaches an equilibrium is higher.     

Thermo-optical characterization of tellurite glasses by thermal lens,thermal relaxation calorimetry and interferometric methods

In this work the thermal lens, thermal relaxation calorimetry and interferometric methods are applied to investigate the thermo-optical properties of tellurite glasses (in mol%: 80TeO₂–20 Li₂O(TeLi), 80TeO₂–15Li₂O–5TiO₂ (TeLiTi-5) and 80TeO₂–10Li₂O–10TiO₂ (TeLiTi-10)). Thermal diffusivity, thermal conductivity, specific heat and the temperature coefficients of refractive index, optical path length, thermal expansion and electronic polarizability were determined. The use of three independent methods was useful for a complete characterization of the studied tellurite glasses. In addition, our results showed that the thermal expansion coefficient and the temperature coefficient of the optical path length (dS/dT) were significantly modified with the introduction of titanium, which may be relevant for the application of these glasses in the photonic area.     

Phase Transition Studies in N(p-n-Heptyloxy Benzylidene)p-Toluidine, 70.1, Across I-N,N-SA and SA-SB Phases

The results of density and ultrasonic velocity variation with temperature in N(p-n-heptyloxy benzylidene)p-toluidine, HYBT, 70.1 exhibiting nematic, smectic-A and crystalline smectic-B phases, are presented. The isotropic-nematic, nematic-smectic-A phase transitions are found to be first order while the smectic-A crystal smectic-B transition is weak first order. The thermal expansion coefficient α computed from the molar volume data is presented. The molar sound velocity and molar compressibility of HYBT computed from the density and ultrasonic velocity data in isotropic phase are compared with the values of the other compounds in a homologous series. They are found to be in good agreement with theory.