Eutectic solidification in the NaF-MgF2 system

The compound NaMgF₃ occurs in the system NaF-MgF₂ and forms a eutectic with either component. The eutectic between NaF and NaMgF₃ has been unidirectionally solidified and the phase morphology and crystallography examined by optical and electron microscopy coupled with electron diffraction. Solidification with the complex-regular morphology from faceted cellular projections on the solid/liquid interface occurs for compositions on both sides of the eutectic point. Extensive regions of rod-like morphology occur in specimens where one of the macrofacet planes, constituting the faceted projections, is perpendicular to the direction of heat extraction.An analysis of twinning observed in the NaMgF₃ phase shows that it occurs in accommodation of lattice strain associated with a distortion of the unit cell axes on cooling from the growth temperature. These small distortions involve a progressive reduction in crystal symmetry from a cubic high temperature form to orthorhombic at ambient temperatures. The twins are not associated with the eutectic growth process in providing re-entrant edges to facilitate coupled growth of the two phases.     

Eutectic solidification in the system Al2O3/Y3Al5O12

The eutectic solidification in the system Al₂O₃/Y₂Al₅O₁₂ has been investigated. A Bridgman-type crystal-growing furnace was used in this investigation. A temperature gradient of 190° C/cm and growth rates which were varied between 2 and 12 cm/h were employed in the directional solidification studies. Three types of microstructure were observed depending upon the composition and the growth rate. At a growth rate of 4 cm/h and at compositions removed from the eutectic composition, a mixture of primary phase and fine eutectic dispersion was found. At growth rates between 2 and 12 cm/h at the eutectic composition, a colony type microstructure was most commonly observed. At growth rates above 4 cm/h at the eutectic composition, regions in the solidified ingot were found to have a highly oriented eutectic microstructure consisting of both rods and platelets. These eutectic microstructures indicate that coupled growth can occur in this system. The method of Sunquist and Mondolfo [15] was used to determine whether Y₃Al₅O₁₂ was the first phase to nucleate at the eutectic.     

Non-ohmic behaviour of the binary ZnO-Nb2O5 system

Studies have been made of the non-ohmic behaviour of the system ZnO-Nb₂O₅, as a function of composition, in the range 0.1 to 0.5 wt % of Nb₂O₅ and sintering temperature varying from 900 to 1300° C. It is found that the non-linearity coefficient varies with composition and sintering temperature. The maximum value of ( 8) is achieved for the samples containing 0.2 wt% Nb₂O₅, sintered at 1100°C. These results are interpreted in terms of the variation of barrier height with composition.     

The solidification behaviour of melts in the system Zn2SiO4-Mg2SiO4

The melting and solidification behaviour of various compositions across the system Zn₂SiO₄- Mg₂SiO₄ (willemite-forsterite) was studied by means of photoemission electron microscopy, X-ray diffraction and DTA. The results revealed the peritectic type of the phase system instead of the earlier assumed eutectic one. During the formation of willemite solid solutions by rapid solidification of zinc-silicate-rich melts, grain-boundary enrichment of zinc oxide besides magnesium silicate takes place. From melts richer in magnesium silicate, forsterite solid solution forms primarily. The sluggish process of its peritectic decomposition during cooling and the preferred subsolidus formation of zinc silicate give rise to the formation of a ternary non-equilibrium phase assemblage even at slow cooling.     

Structure and microwave dielectric properties of 5BaO–2V2O5 binary ceramic system

5BaO–2V₂O₅ ceramic has been prepared by conventional solid state reaction method. The X-ray diffraction and Laser Raman studies have been carried out to ascertain the phase purity and crystal structure of the prepared composition. The microwave dielectric properties of the well sintered ceramic compacts were measured by Hakki & Colemann post resonator and cavity perturbation techniques using a vector network analyzer. The 5BaO–2V₂O₅ ceramic exhibits a low dielectric constant of 12.1, relatively good quality factor of 26,790 GHz, and a small temperature variation of resonant frequency of 7 ppm/°C in the microwave frequency region.     

Thermoelectric properties of Bi2Te3-PbTe pseudo binary near the eutectic composition

(Bi2Te3)(1-x)(PbTe)(x) binary systems near eutectic composition were prepared by melting of elemental metals and a sequential water quenching process and their microstructures and thermoelectric properties were investigated. Multiple phases such as Bi2Te3, BiPbTe and PbTe were observed due to phase separation when the composition x was higher than the eutectic point. Also the electrical conduction type of the alloys converted from p-type to n-type in the phase separated alloys. The lattice thermal conductivities in the phase-separated alloys are lower than those in alloys without phase separation, attributable to increased boundary scattering.     

An analysis of the electrical conductivity of the Ag2SO4-K2SO4 binary system

Electrical conductivity of the Ag₂SO₄-K₂SO₄ binary system shows three maxima at 20, 70 and 90 mole% of K₂SO₄ added to Ag₂SO₄. The first and the third maxima have been explained in the light of intragrain percolation due to lattice distortion, whereas, the second maximum by the surface percolation. The limit of solid solubility has been set at 20 mole% on the basis of evidences obtained from XRD, DTA and SEM techniques.     

Non-isothermal kinetic studies for binary TeO2-P2O5 glass system

A study of TeO₂–P₂O₅ glass system has been carried out by Differential Thermal Analysis (DTA) to elucidate the kinetics of crystallization for these glassy samples. The results of DTA performed at different heating rates are discussed. The values of the glass transition temperature, T _g , as well as the glass crystallization temperature, T _c , are found to be dependent upon the heating rate. From this dependence, the values of activation energy for both the glass transition and crystallization are evaluated and discussed     

Percolation Superconductivity in the GaSb–V2Ga5 Eutectic

The resistivity of the GaSb–V₂Ga₅ eutectic is measured in a wide temperature range. It is found that, if the electric current is parallel to the whisker growth direction, the samples show metallic conductivity and undergo a superconducting transition at 4.1 K, independent of their geometry. If the electric current is perpendicular to the growth direction, the resistivity depends on the sample geometry and shows semiconducting behavior in the range 4.1–300 K, with a sharp drop or, in some of the samples, a superconducting transition at 4.1 K. The results are interpreted in terms of separate regions formed by superconductor–semiconductor–superconductor Josephson contacts, V₂Ga₅ superconducting whiskers, and infinite clusters of their combinations. It is suggested that, in a sequence of Josephson contacts, resonance tunneling of Cooper pairs along resonance percolation paths occurs.     

Recovery of 1 2 5Sb from Neutron-Irradiated 1 2 4Sn

Recovery of ^{1 2 5}Sb from metallic tin (96% enrichment with ^{1 2 4}Sn) irradiated in a nuclear reactor (specific activity 5 mCi g^{- 1}) was studied. Irradiated tin was dissolved in concentrated HCl with addition of H₂O₂ or Br₂; dissolution was monitored with ^{1 1 9 m} Sn tracer. The separation of ^{1 2 5}Sb and tin was studied using strongly basic (Dowex-50, AV-17), moderately basic (AN-2FN), and weakly basic (AN-31) anion exchangers with HCl, HCl + HBr, and HNO₃ eluents. In the case of strongly basic anion exchangers, the separation coefficients of ^{1 2 5}Sb from tin reached 10⁴-10⁵ in a single cycle. From for AN-31 weakly basic anion exchanger, tin was almost completely removed with 0.8 M HCl; the yields of tin and ^{1 2 5}Sb were 99 and 98%, and the content of ^{1 2 5}Sb in tin was no more than 0.1% of the initial activity.     

Mutual phase stabilization of aluminium phosphate and titania in AlPO4-TiO2 binary system

Crystallization behaviour of amorphous aluminium phosphate (AlPO₄) and titania (TiO₂) in a mixed system of the two (5:1) has been reviewed in the light of our recent results. The polymorphous aluminium phosphate in such a binary system grows exclusively in a single phase over a temperature range 500–1150°C. The phase is reported to have a tridymite-like structure belonging to orthorhombic system with cell parametersa=9·638±0·0019,b=8·664±0·0017 andc=18·280±0036Å. Titania in the system preferentially retains its anatase phase morphology up to a temperature (950°C) well beyond its normal anatase → rutile transformation temperature showing a phenomenon of stabilization of this phase in such mixture. An interfacial reaction mechanism that can explain the observed phenomenon of mutual phase stabilization has been discussed and implications of this result towards the use of such technique for stabilization of various polymorphous compounds in a single phase has been pointed out.     

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

Interfacial energies of solid CuAl2 in the CuAl2-Ag2Al pseudo binary alloy

Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy are important parameters for people doing comparisons between experimentally observed solidification morphology and predictions from theoretical models. Thus the Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy of solid CuAl₂ in the CuAl₂-Ag₂Al pseudo binary system have been determined from observed grain boundary groove shapes. The variation of thermal conductivity of solid CuAl₂ with temperature has also been measured by using a radial heat flow apparatus.     

Li2SO4: LiNbO3 binary system for solid state battery applications

Different compositions in the Li₂SO₄-LiNbO₃ system have been prepared by adopting the twin roller quenching technique. The electrical conductivity has been measured and the sample with 20 mole% LiNbO₃ exhibited maximum conductivity at 573 K. The results have been explained on the basis of XRD, SEM data and vacancy concentration. A number of solid state electrochemical cells were fabricated by varying the electrolyte composition, using Li-metal and MnO₂ as anode and cathode respectively. The 80 Li₂SO₄:20 LiNbO₃ composition gave the best cell performance.     

Revisiting dynamics and models of microsegregation during polycrystalline solidification of binary alloy

Microsegregation formed during solidification is of great importance to material properties.The conventional Lever rule and Scheil equation are widely used to predict solute segregation.However,these models always fail to predict the exact solute concentration at a high solid fraction because of theoretical assumptions.Here,the dynamics of microsegregation during polycrystalline solidification of refined Al-Cu alloy is studied via two-and three-dimensional quantitative phase-field simulations.Simulations with different grain refinement level,cooling rate,and solid diffusion coefficient demonstrate that solute segregation at the end of solidification (i.e.when the solid fraction is close to unit) is not strongly correlated to the grain morphology and back diffusion.These independences are in accordance with the Scheil equation which only relates to the solid fraction,but the model predicts a much higher liquid concentration than simulations.Accordingly,based on the quantitative phase-field simulations,a new analytical microsegregation model is derived.Unlike the Scheil equation or the Lever rule that respectively overestimates or underestimates the liquid concentration,the present model predicts the liquid concentration in a pretty good agreement with phase-field simulations,particularly at the late solidification stage.