High Temperature Powder Camera Research Articles

Thermal Expansion Discontinuities of Mullite

Thermal expansion data of mullite were collected between 25° and 800°C with high accuracy by means of an X‐ray high‐temperature Guinier powder camera, and high‐resolution single‐crystal Bond techniques. 3/2‐type mullites, synthesized from chemically coprecipitated precursors and 2/1‐type fused‐mullite single crystals, were used for the measurements. Mullite displays low and nonlinear thermal expansions along the crystallographic a, b, and c axes below about 200°C, and linear expansions between 200° and 400°C. Between about 400° and 500°C, expansion curves are discontinuous. Above 500°C, a, b, and c expansions are linear again. The temperature‐induced expansion discontinuities are relatively strong parallel to a, but are much weaker parallel to b and c. The slopes of the a and volume expansion curves are higher above the expansion discontinuity than below, while there is no significant tendency for those of b and c. Thermal expansion coefficients and discontinuity effects are reversible without any hysteresis. The increase of the a and volume expansion curves above the expansion discontinuities may be due to possible deformations, rotations, and tiltings of the [AlO6] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [Al⋆O4] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400° and 500°C.

An X-ray determination of the thermal expansion of α-phase Cu–Al alloys at high temperature

The lattice expansion of four Cu–Al alloys in the solid-solution range containing 3.7, 6.7, 12.3 and 18.4 at.% Al have been determined by X-ray diffraction in the temperature range 303–923 K using a Unicam 19 cm high-temperature powder camera. In all cases the measured lattice. parameters have been found to increase non-linearly with increasing temperature, from which the linear thermal expansion coefficients (α) have been determined. The α values have been found to decrease with increasing temperature; but the average α values increase slowly with the increase of Al concentration.

High temperature lattice parameters and thermal expansion of Y 1− xCe xAl 2 ( x = 0, 0.15, 1) intermetallics

Precise determination of the lattice parameters of the quasi-binary compounds Y 1− x Ce x Al 2 was made from room temperature to about 800 K using a Unicam (diameter, 19 cm) high temperature powder camera and Cu Kα radiation. The coefficients of thermal expansion at different temperatures were evaluated by an analytical method. It was observed that the lattice parameter increased non-linearly with increasing temperature, whereas the coefficient of thermal expansion decreased with increasing temperature in these compounds. This result is similar to that observed in many of the isotypic RAl 2 intermetallics.

X-ray studies of the thermal expansion and Debye temperatures of Pd-Ag-Au ternary alloys

Using a Unicam high temperature powder camera 19 cm in diameter, the precision lattice parameters and the coefficients of thermal expansion of Pd-Ag-Au ternary alloys of the type Pd x -Ag (1−x) 2 -Au (1−x) 2 have been determined at different temperatures ranging from room temperature to 900 ° by the X-ray method. From the X-ray diffractograms, obtained on a Philips Diffractometer at room temperature, the Debye-Waller factors, Debye temperatures and the root-mean-square amplitudes of thermal vibration of these alloys have been evaluated. The composition dependence of the lattice parameter is examined. The lattice parameter increases with decreasing palladium concentration, but shows a negative deviation from Vegard's law as in the case of the binary systems Pd-Ag and Pd-Au. Both the thermal parameters, the thermal expansion coefficient and the r.m.s. amplitude of thermal vibration show a systematic variation with composition and no anomalous character is found as in the case of the hardness data for these alloys. A gradation is observed between the lattice parameter and the mean expansion coefficient on the one hand and between the lattice parameter and the Debye temperature on the other hand.

Thermal expansion and Debye temperatures of KCl-KBr mixed crystals by an x-ray method

The precision lattice parameters and the coefficients of thermal expansion of KCl, KBr, and their solid solutions have been determined as a function of temperature by an X-ray method using a high-temperature powder camera. From the X-ray diffractograms obtained at room temperature the Debye-Waller factors, Debye temperatures and the root mean square amplitudes of vibration of these mixed crystals have been evaluated. A linear relationship is obtained between the lattice parameter and the composition of the solid solution in accordance with Vegard's law. Both of the thermal parameters, the thermal expansion coefficient and the root mean square amplitude of thermal vibration, vary non-linearly with increasing mole percentage of KBr, the deviation from linearity being maximum around equimolar concentration. The composition dependence of these parameters is discussed in relation to a number of physical properties of these mixed crystals available in the literature.

Anomalous thermal expansion of osmium dioxide

AbstractThe precision lattice parameters of osmium dioxide have been determined at different temperatures, in the temperature range 30–444 °C, using a Unicam high temperature powder camera 19 cm in diameter and CuKα radiation. The data have been used to evaluate the coefficients of thermal expansion at various temperatures by a graphical method. The ‘a’ parameter increases non‐linearly with increasing temperature while the ‘c’ parameter remains constant throughout the range of temperature studied indicating a zero coefficient of expansion along the c‐direction. The abnormal thermal behaviour of this compound is explained in terms of the electronic configuration of the d‐shell of the cation.

X-ray studies of the thermal expansion of CeAl 2 and HoAl 2

A precise determination of the lattice parameters of the rare earth Laves phase compounds CeAl 2 and HoAl 2 was made in the temperature range 30–670°C using a Unicam 19 cm high temperature powder camera and Cu Kα radiation. The data were used to evaluate the coefficient of thermal expansion at various temperatures by an analytical method. It was found that the lattice parameter increases while the coefficient of expansion decreases with increasing temperature in both compounds.

Temperature dependence of the lattice parameter and the coefficient of thermal expansion of CsCdF 3

A 19 cm Unicam high temperature powder camera was used with Cu Kα radiation to collect high temperature lattice parameter data in the temperature range 28–450°C. The coefficient of thermal expansion at various temperatures was evaluated from these data using Cohen's least-squares method. Both the lattice parameter and the coefficient of thermal expansion increased with increasing temperature. The mean value of the coefficient of expansion of CsCdF 3 over the temperature range investigated was 15.29 × 10 −6° C −1.

Temperature dependence of the lattice parameters and the thermal expansion of CdCr 2X 4 (X ≡ S, Se)

A precise X-ray determination of the lattice parameters of the ferro-magnetic chromium chalcogenide spinel compounds CdCr 2S 4 and CdCr 2Se 4 was made in the temperature range 28–685 °C using a Unicam 19 cm high temperature powder camera. These data were used to evaluate the coefficients of thermal expansion at various temperatures by a graphical method. The coefficient of thermal expansion increases non-linearly with increasing temperature in both the compounds. The thermal expansion behaviour of these compounds is discussed in relation to their structure and other physical properties.

Lattice thermal expansion of rubidium calcium fluoride

Fluorides of the type AMF3 (where A is K, Rb, T1 or Cs and M is a 3d transition element) crystallize in the perovskite structure. Rubidium calcium fluoride (RbCaFa) has a cubic perovskite type structure (see Fig. 1) at room temperature with the space group Pm3m (0~) [1]. This compound has been of great interest because of its large dielectric constant which gives rise to a host of properties having commercial applications [2]. During the last few years, extensive studies have been made on the experimental and theoretical aspects of structural phase transitions occurring in the compounds of perovskite structure [3, 4]. However, very little information is available about the high temperature lattice thermal behaviour of these compounds. Since these studies are of great importance in the study of a large number of problems involving lattice vibrations and anharmonic effects in solids, it is thought worthwhile to undertake a general programme of studying the lattice thermal behaviour of perovskites at high temperatures. The present note, which is a part of this programme, gives an account of the precise determination of the lattice parameters and the coefficients of thermal expansion of RbCaF3 at different temperatures by an X-ray method. The powder sample of RbCaFa, used in the present investigation, was kindly supplied by Dr M. Miillner of the Institut fiir Kernphysik der J. W. Goethe Universit~it, FRG. The compound was grown by the melt method. The details of the growth and purity of this compound have already been described by Maetz et at [5]. For X-ray powder diffraction studies the powder was passed through a 325 mesh sieve. The specimen was prepared by putting this powder in a 0.05 cm diameter thin-walled quartz capillary. It was found necessary to anneal the powder at about 400 ° C to obtain well resolved sharp lines in the high angle region. Using a Unicam 19 cm high temperature powder camera and CuKa radiation from a PW 1730 Philips X-ray Generator, powder

Anomalous thermal expansion of silver gallium telluride

A precise X-ray determination of the lattice parameters of silver gallium telluride, AgGaTe2, has been made in the temperature range 301 to 667 K with a Unicam 190 mm high-temperature powder camera. The data have been used to evaluate the coefficients of thermal expansion, α⊥ and α\parallel, at various temperatures. It has been observed that the `c' parameter decreases while the `a' parameter increases with increasing temperature as in the case of AgGaSe2 and AgGaS2, which also have a chalcopyrite-type structure. The magnitudes of both α⊥ and α\parallel increase non-linearly with increasing temperature.

Thermal expansion anomaly in silver indium selenide at elevated temperatures

A precise determination of the lattice parameters of silver indium selenide was made in the temperature range 28–478 °C using a 19 cm Unicam high temperature powder camera and Cu Kα radiation. Cohen's least-squares method was adopted for determining the lattice parameters at each temperature. The data were used to evaluate the coefficients of thermal expansion, perpendicular ( α ⊥ and parallel ( α ∥) to the principal axis, at various temperatures. The parameter a decreases non-linearly with increasing temperature whilst the parameter c increases from room temperature to about 360 °C and then decreases non-linearly. The unit cell volume decreases non-linearly with temperature throughout the temperature range studied. The negative value of α ⊥ increases linearly with increasing temperature; the positive value of α ∥ decreases non-linearly, becomes zero at about 360 °C and above 360 °C it becomes negative and increases numerically with increasing temperature.

X-ray studies on the precision lattice parameters and the coefficients of thermal expansion of PdAgAu equiatomic per cent alloy

Using a Unicam high temperature powder camera 19 cm in diameter and Fe K α radiation, powder photographs of PdAgAu equiatomic per cent alloy were taken at various temperatures ranging from room temperature to 710 °C. The precision lattice parameters were determined from these photographs by Cohen's analytical method. The data were used to evaluate the coefficients of thermal expansion at various temperatures by a graphical method. The lattice parameter increases linearly with increasing temperature. The coefficient of thermal expansion is found to be constant in the temperature range studied within the limits of experimental error.

A high-temperature focusing X-ray powder camera with new features

A film-type X-ray high-temperature (up to about 2250 K) powder camera using the Guinier focusing principle in transmission is described. Use is made of a germanium monochromator. The powder specimen is spread without adhesive on a horizontal supporting film of refractory material, and rotates about a vertical axis. The support is surrounded by a cylindrical furnace of thin tantalum sheet enclosed in a chamber which can be evacuated or filled with an inert gas. The X-radiation enters and leaves the chamber through beryllium windows. The film holder is situated outside the chamber. The sample temperature is measured by thermocouples and optical pyrometry. Problems due to the coating by evaporated material of the silica glass window used to observe the specimen are obviated by the use of a transparent shield which can be rotated to give fresh surfaces when needed.

Temperature dependence of the lattice parameter and the thermal expansion of AgAu (50 AT.%) alloy by an X-ray method

Using a Unicam high temperature powder camera 19 cm in diameter and Cu Kα radiation, powder photographs of Ag-Au (50 at.%) alloy were taken at various temperatures ranging from room temperature to 900 °C. The precision lattice parameters have been determined from these photographs by Cohen's analytical method. The data have been used to evaluate the coefficients of thermal expansion at various temperatures by a graphical method. The expansion coefficient increases linearly with increasing temperature.

X-ray determination of the effect of temperature on the lattice parameters and the coefficients of thermal expansion of palladium-silver-gold alloys

With a 19 cm Unicam high temperature powder camera, the precision lattice parameters and the coefficients of thermal expansion of three different compositions of Pd-Ag-Au ternary alloys have been studied at various temperatures ranging from room temperature to 899 °C by the X-ray method. It is found that, for all three alloys, the lattice parameter increases with increasing temperature. However, for all three alloys the variation of the coefficient of thermal expansion with temperature is found to be constant within the limits of experimental error. The composition dependence of the lattice parameter was also examined. The lattice parameter increases with decreasing palladium concentration, but shows a negative deviation from Vegard's law.

An X-ray determination of the thermal expansion of α-phase Ag–Ga alloys at high temperatures

The lattice parameters of four Ag-Ga alloys in the solid-solution range have been determined by X-ray methods in the temperature interval of 26-522°C using a Unicam 19 cm high-temperature powder camera. In all the cases the lattice parameters have been found to increase in a non-linear manner in the measured range of temperature and the relationship has been expressed in analytical form from the least-squares fitting of data. The linear thermal expansion coefficients have, however, been found to decrease with increasing temperature.

Precision lattice parameters and the coefficients of thermal expansion of hexagonal germanium dioxide

With a Unicam 19 cm high-temperature powder camera, precision lattice parameters of hexagonal germanium dioxide have been determined at various temperatures in the range 36 to 516 °C by the X-ray method. From these data the coefficients of thermal expansion have been evaluated by a graphical method. The temperature dependence of the coefficients of thermal expansion, || and , parallel and perpendicular to the principal axis respectively, are represented by the following equations: where T is the numerical value of the temperature expressed in °C. The thermal expansion of hexagonal GeO2 has been compared with that of the isomorphous -quartz and the polymorphous tetragonal rutile-type GeO2.

X-ray studies on the thermal expansion of tellurium dioxide

Using a Unicam 19 cm high temperature powder camera, the precision lattice parameters of tellurium dioxide have been determined at different temperatures in the range 30 to 461 °C. Using this data, the coefficients of thermal expansion, α∥ and α⊥ parallel and perpendicular to the principal axis respectively, have been evaluated. The temperature dependence of the coefficients of thermal expansion is represented by the following equations: α∥=29.673×10−6+1.552sx10−8T+1.069sx10−10T2 α⊥=9.875sx10−6−5.440sx10−9T+4.572sx 10−12 T2 HereT is the temperature in °C. The thermal expansion of tellurium dioxide has been explained in terms of the interionic distances.

Lattice parameters and thermal expansion of zinc telluride and mercury selenide

The lattice parameters of zinc telluride and mercury selenide were measured with a Unicam 19 cm high-temperature powder camera; the following equations represent the results: ZnTe, at = 6.1016 + 54.63 × 10−6t + 6.82 × 10−9t2 + 5.28 × 10−12t3; HgSe, at = 6.0854 + 28.61 × 10−6t + 4.93 × 10−9t2 + 3.74 × 10−12t3. The expressions for thermal expansion coefficients are also given.