Kinetics of crystal growth of strontium tungstate from solutions in sodium tungstate melts by continuous cooling

The crystallisation kinetics of strontium tungstate from unstirred saturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallisation temperatures T₀ = 1000° to 800°C to room temperature at cooling rates R_T = 0.67° to 3.3°C min⁻¹. The main crystal growth was diffusion rate-controlled; the final crystal growth was rate-controlled by the development rate of excess solute concentration. The estimated diffusion rate constant (k_D) values increased with cooling rates and initial crystallisation temperatures. They are higher than the rate constants for diffusion-controlled growth of calcium tungstate from sodium tungstate melts, but very much smaller than those for strontium tungstate from lithium chloride melts.     

The crystallisation of chromite-magnesiochromite spinels from a calcium magnesium aluminosilicate glass: Nucleation,crystal growth and final crystal sizes

The crystallisation of chromite-magnesiochromite spinels was studied from a calcium magnesium aluminosilicate glass (a simulated slag) containing 3 to 12 percent total iron oxides and 0.3 to 1.5 percent chromium(III) oxide, at temperatures from 1400° to 700 °C. – Spinel crystallisation occurred in glasses with 3–7 percent FeO and 0.7–1.1 percent Cr₂O₃. At temperatures 1100 °C and above, the nucleation was rapid and crystal numbers very high, at FeO contents above 3 percent and Cr₂O₃ contents above 0.7 percent; at 1056° and 1000 °C however, the crystal numbers reached some optimum values but then decreased as clinopyroxene crystals grew onto and enveloped the spinel microcrystals. In these glasses, the crystal lengths varied with growth time according to the relation, l_t = 2 k_g t^α = R_g1 t^α, where α = 0.7–1.0: this time dependence was a compromise between a relation for dendritic growth and one for facetted growth. The growth rates generally increased about five to seven times for 160 °C temperature rise: the energy of activation for the spinel crystal growth was then estimated as 180 ± 60 kJ mole⁻¹. – No spinel crystals were observed in glasses with more than 7 percent FeO content, only clinopyroxene crystals. Probably, these latter had nucleated rapidly and grown onto spinel microcrystals, while the spinel microcrystals were still of < 0.1 μm size.     

The Crystallisation of Calcium Molybdate and Tungstate from Lithium Chloride Melts by Continuous Cooling. The Kinetics of Crystal Growth in Alumina Crucibles

The kinetics of crystallisation of calcium molybdate and tungstate from unstirred supersaturated solutions in lithium chloride melts — in alumina crucibles — was studied by continuous cooling from initial temperature T₀ = 800°C down to room temperature at cooling rates R_T = 20° to 200°C hr⁻¹. The solutions were analysed chemically and the crystals were examined by optical microscopy. Crystal growth started practically immediately after the onset of cooling: at first, the amount of material deposited onto crystals was far less than the amount of excess solute developed within the supersaturated solutions but crystallisation rates then increased as the crystal sizes increased. Then, after some time t* (at about seventy percent crystallisation), all excess solute was deposited onto growing crystals.     

Differential thermal analysis studies on the crystallization of strontium tungstate melts. Variation of rate constants with crystallization temperatures and cooling rates

Kinetics of strontium tungstate crystallization from sodium tungstate melts were studied in platinum crucibles (by DTA) by continuous cooling from initial crystallization temperatures T₀ = 800° to 1000° to below the eutectic temperature at cooling rates R_T = 0.67° to 3.3° min⁻¹. Heterogeneous nuclei first formed slowly onto metal platinate particles within the solution during induction periods (t ); the main crystal growth then started after the development of some exces solute concentration (ΔC ) at the induction temperature (T ). The actual growth after t was diffusion rate-controlled. The diffusion rate-constants (k_Dt) for growth after the induction periods along the major axis were estimated; the increased with T₀ and R_T. These values were higher than those for diffusion-controlled crystal growth of strontium tungstate from sodium tungstate melts in alumina crucibles but much smaller than the real diffusion rate-constants (k_Dl)_real.     

The crystallisation of alkaline-earth metal metasilicates from metal chloride melts: Preliminary experimental studies

The crystallisation of alcaline-earth metal metalsilicates was studied by slow cooling of saturated solutions in the metal chloride melts at T₀ = 830° to 1300°C, down to ambient temperature; cooling rates were varied from 20° to 200°C hr⁻¹. Calcium metasilicate crystallised as β-CaSiO₃ prisms, strontium metasilicate as α-SrSiO₃ platelets and barium metasilicate as α-BaSiO₃ platelets. Final crystal lengths increased with reduction in cooling rate and with increase in initial crystallisation temperature.     

Crystallizaton Characteristics of the Alternating Tetrafluoroethylene-Ethylene Copolymer

The alternating tetrafluoroethylene-ethylene (TFE-E) copolymer has been studied with respect to its crystallization by using differential scanning calorimetry and optical microscopy. The value of the specific surface energy σ at the melt/crystal interface is calculated to be about 2 · 10⁻³ J/m². The nucleating activity Φ of TiO₂ particles introduced into TFE-E copolymer is estimated to be approximately 0.9. The temperature dependence of the nucleation rate and of the linear growth velocity are constructed in the whole region from the melting temperature T_m to the vitrification temperature T_g. The minimum cooling rate for the formation of a crystal-free TFE-E copolymer glass is calculated and its value amounts to 10⁴ K · min⁻¹.     

Hyperfine structure of EPR spectra of Cr5+ ions in irradiated sodium dichromate crystals

EPR spectra are studied of X-irradiated sodium dichromate crystals grown from an aqueous solution by evaporation at 31°C. Doublet lines of EPR-absorption are attributed to the Cr⁵⁺ ions in the CrO₄³⁻ and CrO₃⁻ radicals resulting from radiation decomposition of Cr₂O₇²⁻ and being in the lattice in two unequivalent positions. Hyperfine structure caused by interaction of an unpaired electron with Cr⁵³ nucleus were observed both at liquid nitrogen and room temperatures. For the line, caused by CrO₄³⁻ radical, g_y and A_y directions coincide and angles both between A_x, g_x and A_z, g_z make up ∼ 25°. The spectrum is described by usual spin Hamiltonian for S = 1/2 with following parameters (T = 77 K): for CrO: g_z = 1.984, g_y = 1.970, g_x = 1.961, |A_z| = 8.2 · 10⁻⁴ cm⁻¹, |A_y| = 13.7 × 10⁻⁴ cm⁻¹, |A_x| = 36.1 · 10⁻⁴ cm⁻¹; for CrO₃⁻: g_z = 1.915, g_y = 1.975, g_x = 1.985, |A_z| = 32.2 · 10⁻⁴ cm⁻¹.     

Structures of vapour-deposited amorphous films of arsenic chalcogenides

The structures of vapour-deposited amorphous films of composition As_xSe_1?x′, with x between 0 and 1, have been investigated by X-ray diffraction. In all cases the structure of the freshly deposited film differs very considerably from that of the corresponding bulk glass. For the elemental amorphous films the structure is highly disordered and contains voids which for Se cannot be annealed out below the crystallization temperature of 70°C or, for As, below the temperature ($\text{?130°}\text{C}$) where the As film re-evaporated. Annealing of the arsenic selenide films at temperatures below T_g causes the structures to relax towards those of the bulk, with a distribution of activation energies around 25 kJ mol^?1. The composition of the vapour has been examined by mass spectroscopy and it is concluded that even if some molecular identity is retained on condensation there must be considerable cross-linking to give the observed structural behaviour.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

Optical absorption of Fe3+ Ion in LiF crystals

The absorption spectrum of LiF—Fe³⁺ single crystals was investigated at liquid nitrogen temperature. Six new absorption bands, unobserved earlier in other works, were found, which were located at 11 200, 16 000, 22 700, 27300, 30 500, and 41 900 cm⁻¹. The spectrum was interpreted in the cubic cristalline field approximation with D_q = 1397 cm⁻¹, B = 657 cm⁻¹, C = 3226 cm⁻¹. Above mentioned bands were assigned to the transitions from a ground state ⁶A_1g to the levels ⁴T_1g(G), ⁴T_2g(G), ⁴A_1g(G) (⁴E_g), ⁴E_g(D), ²T_1g(F), and ⁴T_1g(F), respectively.     

Thermal dehydration of the double salts K2Be(XO4)2·2H2O (X = S,Se)

The thermal dehydration of the title compounds was studied by TG, DTA and DSC methods and the enthalpies of dehydration were calculated (87.6 kJ mol^–1 and 167.5 kJ mol^–1 for the sulfate and selenate compound, respectively). The larger value of ΔH_deh of K₂Be(SeO₄)₂·2H₂O is due to the stronger hydrogen bonds formed in the selenate as compared to those formed in the respective sulfate owing to the stronger proton acceptor capabilities of the SeO₄^2– ions. The enthalpies of formation (ΔH_f⁰) of the dihydrates are also calculated from the DSC measurements (– 4467.4 kJ mol^–1 and – 3447.1 kJ mol^–1 for the sulfate and selenate compound, respectively). The anhydrous double salt, K₂Be(SO₄)₂, forms tetragonal crystals with lattice parameters: a = 7.232(2) Å; c = 14.168(2) Å; V = 741.0 ų, while the anhydrous salt, K₂Be(SeO₄)₂, forms monoclinic crystals with lattice parameters: a = 9.217(3) Å; b = 10.645(3) Å; c = 8.989(2) Å; β = 108.52(4)°; V = 836.2 ų. Vibrational spectra (infrared and Raman) of both the dihydrates and the anhydrous compounds are also presented and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absorption and electrical conductivity of CaMoO4 crystals

The electrical conductivity σ of CaMoO₄ crystals was investigated between room temperature and 850°C. The mobility v_v and the diffusion coefficient D_v of the O⁻ ion vacancies have been derived from σ: v_vT = 3300 exp (−1.52 eV/kT) cm² K/Vs, D_v = (0.1…1) × exp (−1.52 eV/kT) cm²/s. An absorption occuring in crystals which are reduced or X-irradiated at low temperatures is dichroitic and caused by Mo⁵⁺ ions. For measurement in c direction the oscillator strength of the 680 nm absorption band is found to be about 0.1.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Plastic Deformation of Cesium Iodide Single Crystals at High Temperatures

The plastic deformation of CsI crystals of three crystallographic orientations 〈100〉. 〈110〉. and 〈111〉 at temperatures from 423 to 773 K (0.5 to 0.86T_m) and strain rates from 2 × 10⁻³ to 10⁻⁵ s⁻¹ is studied. Four-stage stres-strain curves were found, three stages being more and more distinct with an increase in temperature up to 623 K above which stage III prevails. For all the temperatures, strain rates and oreintations studied the superplasticity features (jerky flow, deformation localization, active dynamical recovery etc.) were observed. The thermal activatoin analysis has shown that the rate of high temperature deformation of CsI is controlled by dislocation climb limited in its turn by mobility of cation vacancies (quasiviscous creep).     

The precipitation of alkaline-earth metal polymetaphosphate hydrate powders from aqueous solution. Nucleation and crystal growth processes,final precipitate compositions and crystal sizes

The precipitation of barium, strontium, calcium and magnesium polymetaphosphate hydrates was studied from aqueous solutions of initial metal salt concentrations from 0.001 to 3 M at 20 °C; equivalent sodium polymetaphosphate solutions were added to the alkaline-earth metal chloride solutions. Precipitate compositions were determined by chemical analysis, paper chromatography, potentiometric analysis, thermogravimetric and differential thermal analysis and infra-red spectrophotometry; final crystallite morphologies and sizes were studied by scanning electron microscopy and X-ray powder diffraction. Nucleation rates and nucleus numbers (at the end of the induction periods) were very high; crystal numbers varied from 10¹⁴ to 10¹⁵ at the critical concentrations to above 10¹⁷ per 1. solution. Crystal growth rates were also very high and varied as the fourth power of the initial metal salt concentration. High molecular-weight metal polymetaphosphate hydrates were precipitated from the more dilute solutions (0.001 to 0.025 M) while increasing amounts of the more soluble intermediate and low molecular-weight products were precipitated from the more concentrated solutions. Washing with cold water removed the tri- and tetralinear and cyclic phosphate products. The magnesium salts were not precipitated even from 3 M aqueous solutions. The precipitates from aqueous (NaPO₃(I))_n (n = 12) solutions had the compositions (BaP₂O₆ · 2.5 H₂O)₆, (SrP₂O₆ · 3 H₂O)_n and (CaP₂O₆ · 4 H₂O)_n while the magnesium salt precipitate from 20 percent aqueous acetone solution had the composition (MgP₂O₆ · 4 H₂O)_n, the precipitate n values varied from 19 to 13. The precipitates from aqueous (NaPO₃(II))_n (n = 20) solutions contained 0.5n to n additional adsorbed water molecules; these precipitate n values varied in turn from 40 to 26. The final precipitate powders consisted of ‘spherules’ of highly microcrystalline or amorphous polymer glass; the spherule diameters were about 0.2 μm at the critical concentrations and decreased to below 0.05 μm with increasing solution concentrations.     

Influence of dopants and preparation method on the thermoluminescence properties of corundum crystals

Thermoluminescence (TL) of several types of corundum crystals is described. Undoped crystals showed a weak TL at 15 and 110°C with emission at 330 and 495 nm. Good TL response at 275°C is shown by Cr doped (0.03 wt%) crystals grown by Verneuil method in the oxygen cone of the flame annealed in hydrogen at 1500–1800°C. The most intensive TL response is shown by crystals doped with Ti and Mo (∼ 10⁻³wt.%) grown by Czochralski method in reducing conditions. The former may be used to the detection of γ-rays dose ≥ 10⁻³ − 10⁻² Gy, the latter ≥ 10⁻⁷ − 10⁻⁶ Gy.     

An EPR Study of Scapolite

In single crystals of scapolite from two different localities, three paramagnetic centres are detected by electron paramagnetic resonance (EPR): 1. One isotropic singlet with g_iso = 2.005; 2. One triclinic singlet with g‖ = 2.005 ± 0.001 and g⟂= 2.009; 3. One triclinic sextet with g‖= 2.005 ± 0.001, g⟂ = 2.011; A‖ = 85.4 × 10⁻⁴ cm⁻⁴, A⟂ = 85.3 × 10⁻⁴ cm⁻¹. Centres 1 and 2 can be attributed to colour centres as they are bleached after annealing. Centre 3 can be due to Mn²⁺ (only the central M_s = ± 1/2 transition is observable) most likely substituting for Ca²⁺ The site symmetry must be triclinic but due to Al, Si disorder and mixed Na, Ca composition the different components from magnetically non-equivalent sites are averaged out for many orientations.     

Scandium thin films-structure and electrical resistance (I). A study on films obtained in a vacuum of 10−5 up to 10−6 Torr

A structure as well as a specific electrical resistance of scandium films having a various thickness (300 up to 2800 Å) obtained at a condensation rate: w⋍ 1 up to 155 Å/s have been studied in the present work. Depending on condensation conditions in films a hydride ScH_x, h.c.p. — Sc, or ScH_x + Sc alternatively, is formed, a phase containing a distorted hexagonal lattice evolving into the ScH_x over a period of a few hours. The ScH_x and h.c.p-Sc change to Sc₂O₃ when annealed in a vacuum of 5 · 10⁻⁵ Torr at a T ≈ 800°C, while in a vacuum of 3 · 10⁻⁸ Torr a change to h.c.p.-Sc with negligible inclusions of oxidic Sc₂O₃ particles takes place. Formation of oxide is observed also when heating the films with an electron beam directly in a microscope under vacuum of ∼ 8 · 10⁻⁵ Torr. The specific electrical resistance ϱ of the film just after a condensation has taken place is approximatly by an order higher as against the ScH_x- and h.c.p.-Sc solid specimens. An irreversible gain in said resistance is observed while holding at a room temperature or annealing in a vacuum of 5 · 10⁻⁵ Torr, this being associated with absorption of H₂, O₂ and, possibly, of water steam, and then with formation of Sc₂O₃. Annealing in a vacuum of 5 · 10⁻⁸ up to 10⁻⁷ Torr involves a temperature relationship for ϱ of an intricated nature, this being accounted for by a structure transition as follows: A diode-effect diminishing with a temperature rise is observed in annealed films.     

TEM Investigation of Stacking Faults in the Superconducting Compound YNi2B2C

To evaluate the influence of composition and high-temperature heat treatment on the occurrence of stacking faults and intergrowths in the layered superconducting compound YNi₂B₂C a series of polycrystalline samples and a single crystal were investigated by TEM. Stacking faults were found in the most inhomogeneous sample only, which was annealed at 1100°C. Homogenisation at temperatures of 1450°C and higher results in a recovery of stacking faults. The observed faults should develop during crystallisation by formation of extra planes of YC or Ni₂B, resulting in a local deviation from the stoichiometric composition. In highly faulted areas the distance of stacking faults was about 50nm. These faults have apparently no influence on the superconduction transition temperature T_c.     

On the decomposition behaviour of Al-Zn(4.5 at.%)-Mg(2;3 at.%) alloys during continuous cooling from the temperature range of homogeneity

The decomposition processes taking place in the Al-Zn (4.5)-Mg(2;3) alloys were studied by means of XSAS and resistivity measurements and by TEM investigations after continuous slow cooling to certain temperatures T_i and during an isothermal ageing at T_i following the continuous cooling procedure. During the cooling between 380 °C and 320 °C mostly particles of the η-phase are growing. The formation of particles homogeneously distributed in the matrix starts at 165 °C in the alloy with 2 at.% Mg and at 175 °C in the one with 3 at.% Mg. Below these temperatures new subcritical GP zones are formed and particles already present in the matrix are growing. The dominating process depends on T_i.