Effect of Ca additions on microstructure and microhardness of an as-cast Mg-5.0 wt.% Al alloy

The effects of Ca additions (0.5-2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg-5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg₁₇Al₁₂ phase was completely replaced by a (Al, Mg)₂Ca phase. The Vickers microhardness of the as-cast Mg-Al-Ca alloys increased with increasing Ca content. Tests on the Mg-5.0Al-2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law.     

Comments on the paper “Modification of composite hardness models to incorporate indentation size effects in thin films”, D. Beegan, S. Chowdhury and M.T. Laugier, Thin Solid Films 516 (2008), 3813-3817

Since the original work of Bückle concerning the substrate influence on the hardness measurement of thin film, more than 20 models were proposed to separate the contribution of the substrate. Subsequently to the development of these numerous models, a question arises: Which is the most relevant models among them? Indeed, the authors usually consider that their proposed model leads to the best prediction of the film hardness which is probably correct for a given experimental condition applied to a particular material. In addition, the authors also assume that the other models are not so relevant. But to have a sound discussion about the existing models, it is necessary to correctly apply them according to the author statement. In this paper, we better specified the application of the Jönsson and Hogmark model and that of Chicot and Lesage applied to the results obtained on copper films by Beegan et al. Contrarily to these authors, we show that the above-mentioned models lead to a good representation of the experimental data and a good predicted value of the film hardness.     

X-ray absorption spectroscopy of PbMoO<Subscript>4</Subscript> single crystals

X-ray absorption spectra of PbMoO₄ (LMO) crystals have been investigated for the first time in literature. The measurements have been carried out at Mo absorption edge at the dispersive EXAFS beamline (BL-8) of INDUS-2 Synchrotron facility at Indore, India. The optics of the beamline was set to obtain a band of 2000 eV at 20,000 eV and the channels of the CCD detector were calibrated by recording the absorption edges of standard Mo and Nb foils in the same setting. The absorption spectra have been measured for three LMO samples prepared under different conditions viz. (i) grown in air from stoichiometric starting charge, (ii) grown in argon from stoichiometric starting charge and (iii) grown in air from PbO-rich starting charge. The results have been explained on the basis of the defect structure analysed in LMO crystals prepared under different conditions. The Mo absorption edge is significantly influenced by the deviations in crystal stoichiometry.     

Extrapure WO<Subscript>3</Subscript> for the preparation of CdWO<Subscript>4</Subscript> single crystals

A process has been developed for the preparation of extrapure WO suitable for crystal growth. The method involves thermal decomposition of ammonium paratungstate (APT) powder at 230–250° C, resulting in the formation of soluble ammonium metatungstate (AMT). The AMT solution is then purified and hydrolyzed. The resultant purified APT is converted to WO₃. The reaction yield is at a level of 95%. The contents of metallic impurities, especially those of polyvalent impurities (Fe, Mn, Cu, Pb, V, Cr, and others), capable of influencing the optical properties of crystals, was comparable to their detection limit: 10^?6 to 10^?5 wt %. The purification process was tested using a pilot-scale system. The purity was confirmed by the synthesis of high-quality CdWO₄ crystals.     

X-ray dosimetric properties of vapor-grown CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

CdGa₂S₄ single crystals have been grown from a presynthesized source material by closed-tube iodine vapor transport, and their X-ray dosimetric properties have been studied. Their X-ray sensitivity coefficient K ranges from K = 1.26 × 10⁻¹¹ to 1.39 × 10⁻¹⁰ A min/(V R) at effective X-ray hardnesses V _a = 25–50 keV and dose rates E = 0.75–78.05 R/min, and increases with X-ray dose. The K(V _a) curve has a negative slope, in contrast to the K(E) curve. The photocurrent-dose curves of the CdGa₂S₄ single crystals demonstrate that the steady-state X-ray photocurrent is a power-law function of X-ray dose rate: ΔI _E,0 ∼ E ^α . With increasing V _a, the slope of the curves sharply decreases and α approaches unity.     

The effect of illumination on the current-voltage characteristics and conductivity of MnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

The effect of illumination on the current-voltage (J-U) characteristics and the temperature dependence of conductivity σ(T) of MnIn₂S₄ single crystals has been studied. Under illumination, the J-U curves can be divided into three parts corresponding to the linear (J ∼ U), “3/2” (J ∼ U ^1.5) and “5/2” (J ∼ U ^2.5) laws. This behavior is explained by an increase in the density of free charge carriers under the joint action of electric field and illumination. The current J ₁ in the illuminated sample is 10⁴–10⁵ times the dark J ₂ value. The positions of energy levels in the semiconductor crystal under study are determined.     

Frequency-dependent dielectric properties and electrical conductivity of CdIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

The real (ɛ) and imaginary (ɛ″) parts of complex dielectric permittivity and ac conductivity (σ_ac) of CdIn₂S₄ single crystals (cubic structure) have been measured in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the crystals has a relaxation nature. In the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz, the ac conductivity of single-crystal CdIn₂S₄ follows the relation σ_ac ∼ f ^0.8, characteristic of hopping conduction through localized states near the Fermi level.     

Photoconductivity and luminescence anisotropy in ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> and ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript>:Cu<Superscript>2+</Superscript> single crystals

ZnIn₂S₄ and ZnIn₂S₄:Cu²⁺ single crystals are shown to exhibit considerable photoconductivity and luminescence anisotropy. The mechanism of carrier recombination in the crystals is polarization-dependent. Their spectra can be understood in terms of optical transitions involving donor and acceptor levels. The degree and spectral distribution of the anisotropy can be tuned via doping and alternating growth of the three- and one-layer polytypes.     

The effect of illumination on the current-voltage characteristics and conductivity of MnGaInS<Subscript>4</Subscript> single crystals

We have studied the effect of illumination on the current-voltage characteristics and the temperature dependence of conductivity of MnGaInS₄ single crystals. Under illumination, the current in a sample was 10⁴–10⁵ times the dark value. It is established that the light-induced current buildup is due to the carrier release from the traps filled under illumination. The energy positions of levels are determined.     

Growth,structure, and thermal expansion anisotropy of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

Single crystals of the ternary compound FeIn₂Se₄ have been grown by directional solidification in a vertical Bridgman geometry, and their composition, structure, and melting point have been determined. The relative length change of FeIn₂Se₄ has been measured on a dilatometer using oriented single-crystal samples, and its thermal expansion coefficients have been determined. The thermal expansion of FeIn₂Se₄ is shown to be markedly anisotropic.     

On contamination of Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystals with <Superscript>207</Superscript>Bi isotope

Two versions of generation of ²⁰⁷Bi radionuclide are known from the literature: natural phenomena (cosmic rays) and technogenic factors (high-power nuclear weapons tests). The amounts of the generated radionuclide are so large that it can be detected even in Bi₄Ge₃O₁₂ (BGO) scintillator. Previously we demonstrated that natural phenomena cannot generate these amounts of ²⁰⁷Bi. The identification of ²⁰⁷Bi of cosmic origin in BGO is doubtful. BGO crystals are inevitably contaminated with isotopes from natural radioactive series, which are significant components of the background radiation of the Earth. Contamination with ²²⁷Ас due to several factors is possible. The decay of ^211+211mPo to the energy levels of ^207mPb can be misinterpreted as the ²⁰⁷Bi decay.     

Acousto-optic properties of LiBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystals grown at low temperature gradients

This paper reports the starting charge composition and process parameters for low thermal gradient (<1°C/cm) Czochralski growth of uniform bulk LiBi(MoO₄)₂ crystals. The acousto-optic figure of merit (M ₂) of a LiBi(MoO₄)₂ crystal has been measured in different directions.     

High pressure effect on MoS<Subscript>2</Subscript> and MoSe<Subscript>2</Subscript> single crystals grown by CVT method

Single crystals of MoS₂ and MoSe₂ were grown by chemical vapour transport method using iodine as a transporting agent and characterized by optical microscopy, energy dispersive analysis (EDAX), X-ray powder diffraction (XRD) and Hall mobility at room temperature. The variation of electrical resistance under pressure was monitored in a Bridgman anvil set-up up to 6.5 GPa to identify occurrence of any structural transition. MoS₂ and MoSe₂ do not undergo any structural transitions under pressure.     

Growth and thermal expansion of In<Subscript>2</Subscript>S<Subscript>3</Subscript> single crystals

In₂S₃ single crystals have been grown by the Bridgman-Stockbarger method. Their composition has been determined by electron probe x-ray microanalysis, and their phase-transition temperatures have been evaluated by differential thermal analysis. The thermal expansion coefficient of In₂S₃ has been determined in the range 80–1000 K by dilatometry.     

Phase equilibria in the Tl<Subscript>2</Subscript>Se-PbSe system and growth and properties of Tl<Subscript>4</Subscript>PbSe<Subscript>3</Subscript> single crystals

The T-x phase diagram of the Tl₂Se-PbSe system has been mapped out. The system contains a congruently melting compound (803 K) of composition Tl₄PbSe₃, which forms a continuous series of solid solutions with Tl₂Se. Tl₄PbSe₃ single crystals have been grown by directional solidification, and their physicochemical and thermoelectric properties have been studied. Tl₄PbSe₃ crystals have high thermopower (α _T ) and thermoelectric figure of merit (Z _T ).     

Optical and X-ray photoelectron spectroscopy of PbGeO<Subscript>3</Subscript> and Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> single crystals

Pb₅Ge₃O₁₁ crystals are found to exhibit pale yellow colouration while PbGeO₃ are colourless. X-ray photoelectron spectroscopy (XPS) measurements show lead deficiency in both the crystals. The results also reveal a stronger ionic character for PbGeO₃ as compared to Pb₅Ge₃O₁₁ crystal. The binding energy of Ge3d core level in the case of Pb₅Ge₃O₁₁ crystal is found to be smaller than the binding energy of germanium oxide, thereby indicating the incomplete oxidation of Ge ions in the crystal lattice. On gamma ray irradiation, the transmission of both the crystals is observed to deteriorate uniformly over the entire wavelength range, which has been attributed to the oxidation of some of the lattice Pb ions. On gamma irradiation the changes observed in O1s core level energies for both the crystals are seen to be consistent with the changes noted in the Pb4f _7/2 and Ge3d spectra. Interestingly, the results reveal oxidation of surface Ge atoms with atmospheric oxygen under gamma irradiation.     

Photoelectric and X-ray dosimetric properties of TlGa<Subscript>0.97</Subscript>Mn<Subscript>0.03</Subscript>S<Subscript>2</Subscript> single crystals

It is established that the partial substitution of gallium for manganese (3 mol %) in TlGaS₂ single crystals leads to a decrease in width of the band gap (from 2.62 to 2.5 eV), broadening of the peak of intrinsic photocurrent, and appearance of a broad band of extrinsic photocurrent over the photon energy range hν = 1.7–2.4 eV. Upon the partial substitution Ga → Mn in TlGaS₂, the X-ray sensitivity coefficient increases significantly (by a factor of 24–57) within an irradiation dose of 0.75–78 R/min, and the current-dose characteristics of TlGa_0.97Mn_0.03S₂ have good reproducibility.     

Effect of cadmium,silver, and tellurium doping on the properties of single crystals of the layered compounds PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript> and PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

We have studied the effects of doping and excess Te on the thermoelectric properties of single crystals of the layered ternary compounds PbBi₄Te₇ and PbSb₂Te₄. X-ray diffraction characterization has shown that the crystals have highly disordered structures. The nature of the possible point defects in the compounds has been analyzed. The silver atoms in Ag-doped PbBi₄Te₇ seem to reside in the van der Waals gaps between the slabs. The introduction of excess Te is shown to markedly reduce the lattice thermal conductivity of both compounds.     

Absorption and luminescence spectra of CeF<Subscript>3</Subscript>-doped BaF<Subscript>2</Subscript> single crystals and nanoceramics

We have studied the radiation resistance of BaF₂:Ce³⁺ scintillator single crystals and ceramics at gamma doses from 10⁶ to 10⁸ rad. It has been shown that the ceramics produced using the self-fluorinating precursor BaF₂ ? HF is more radiation-resistant.     

Growth and properties of Zr-doped KTiOPO<Subscript>4</Subscript> crystals

Single crystals of KTi_{1 – x}Zr_xOPO₄ (0.001 < x < 0.044) solid solutions were grown, and their properties were studied. The results indicate that Zr doping has little effect on the ferroelectric transition temperature of the material but markedly reduces its low-temperature conductivity. Moreover, the nonlinear optical susceptibility of the solid solutions exceeds that of KTiOPO₄ by more than a factor of 2.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1505–1507.Original Russian Text Copyright © 2004 by Voronkova, Yanovskii, Leonteva, Agapova, Kharitonova, Stefanovich, Zverkov.