Thermodynamic Properties of the Quasibinary System Bi2Se3 - Bi2Te3 over a Broad Temperature Range

We have studied the heat capacity and enthalpies of solid solutions in the quasibinary system Bi₂Se₃ - Bi₂Te₃ at low and high temperatures. Based on low-temperature measurements of the heat capacity, we have calculated the values of the enthalpy, entropy, and reduced Gibbs energy of the compounds under standard conditions. We have determined the temperature dependences of the thermodynamic functions for the solid solutions Bi₂Se₃ - Bi₂Te₃ (50, 70, and 80 mole% Bi₂Te₃) in the temperature range 298.15 K - T_mp. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 80–85, July–August, 2005.     

Heat Capacity and Enthalpy of Bi2Si3 and Bi2Te3 in the Temperature Range 58-1012 K

We have studied the heat capacity and enthalpy of Bi₂Si₃ and Bi₂Te₃ at low and high temperatures. Based on low-temperature measurements of the heat capacity, we have calculated the enthalpy, entropy, and reduced Gibbs energy of the compounds under standard conditions. We have determined the temperature dependences of the thermodynamic functions of Bi₂Si₃ and Bi₂Te₃ in the range 298.15-968 K and 298.15-874 K, respectively. We have determined the melting points and the enthalpies and entropies of melting for the compounds.     

Effect of Annealing Temperature on the Thermoelectric Properties of the Bi0.5Sb1.5Te3 Thin Films Prepared by Radio-Frequency Sputtering

The effect of annealing temperature on the crystallinity, thermoelectric properties, and surface morphology of the Bi_0.5Sb_1.5Te₃ thin films prepared on SiO₂/Si substrate by radio-frequency (RF) magnetron sputtering was investigated using X-ray diffraction (XRD), the four-point probe method, and scanning electron microscopy (SEM). XRD results show that the crystallite structure of the Bi _x Sb_2–x Te₃ thin films belong to Bi_0.5Sb_1.5Te₃. When the Bi _x Sb_2–x Te₃ thin films were annealed between 423 K and 523 K (150 °C and 250 °C) for 10  minutes, the crystallinity of the thin films continuously increases with the temperature increase. In addition, the (015) reflection plane as the preferred orientation and the oxidation compound of Bi_3.73Sb_1.5O₃ first appeared when the Bi_0.5Sb_1.5Te₃ thin films were annealed at 523 K (250 °C) for 10 minutes. An activation energy of 51.66 kJ/mol for crystallite growth of Bi_0.5Sb_1.5Te₃ thin films annealed between 423 K and 523 K (150 °C and 250 °C) for 10 minutes was obtained. The resistivity was 2.69 × 10² and 5.93 × 10  μΩ·m, respectively, for the as-deposited Bi_0.5Sb_1.5Te₃ thin films and annealed at 523 K (250 °C) for 10 minutes. The maximum values of the Seebeck coefficient and power factor were 256.5 μV/K and 1.12 × 10³ μW/m·K², respectively, for the Bi_0.5Sb_1.5Te₃ thin films annealing treatment at 523 K (250 °C) for 10 minutes.     

Thermodynamic properties of bismuth sesquiselenide and sesquitelluride and their solid solutions

The Gibbs free energies, enthalpies and entropies of formation of bismuth selenide Bi₂Se₃ between 308 and 408 K, bismuth telluride Bi₂Te₃ between 308 and 413 K, and their solid solutions between 670 and 840 K are measured using the e.m.f. method. Positive deviations of the activities of components in the solid solutions from the ideal behavior are found. The possibility of ordering effects in the solid solutions at lower temperatures is assumed.     

Phase diagram of Er-Sn-Te system for diluted magnetic semiconductor developments

Phase diagrams of the RE (rare earth)-IV-VI systems are very important for the design of rare earth doped diluted magnetic semiconductors (DMSs), but related information is very limited. In this work, ...     

Magnetocaloric and magneto-transport properties of Gd10Co20Si70 alloy

We report the results of magnetic, thermodynamic, transport and magnetocaloric effect (MCE) studies of newly synthesized Gd_(10)Co_(20)Si_(70) alloy. These measurements confirm an antiferromagnetic transition at T_N=9 K. Both MCE and magnetoresistance (MR) show quadratic dependence on the applied magnetic field, indicating the presence of spin fluctuations in the alloy. The maximum values of the magnetic entropy change determined from the isothermal magnetization data for magnetic field change of 7 and9 T are found to be 10.5 and 15.6 J/kg·K, respectively. As a consequence of the spin fluctuations effect, the MCE peaks are pulled towards high temperature side as asymmetrically broadened peak. The MR attains a large positive value of 73%at 2 K in 8 T. The large MR and reversible MCE make this alloy an attractive multifunctional magnetic material.     

Phase composition and thermodynamic properties of alloys of the Te—PbTe—Sb2Te3 system

The phase composition and thermodynamic properties of Te−PbTe−Sb ₂Te₃ alloys are determined by means of X-ray phase analysis and measurements of electromotive forces in the temperature range 540–660 K. The formation of the Pb₂Sb₆Te₁₁ ternary compound in this temperature range has not been confirmed. The results of the electrochemical measurements confirm the mutual solubility of the components of the quasibinary PbTe−Sb₂Te₃ system. The activities of PbTe are determined in the (PbTe+Sb₂Te₃) phase field. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 66–69, May–June, 1999.     

Liquidus Projection of the Ternary Bi-Sb-Te Thermoelectric Material System

The ternary Bi-Sb-Te system is important for thermoelectric applications. Its liquidus projection is determined, except for the Bi-rich corner with various unconfirmed binary Bi-Te compounds. Fifty Bi-Sb-Te ternary alloys are prepared. Primary solidification phases are identified using metallographic observations and X-ray diffraction techniques. Phase transformation temperatures are determined using differential thermal analysis. No ternary compound is found. (Bi,Sb)₂Te₃ and (Bi,Sb) are continuous solid solutions. The Bi₂Te₃ and Sb₂Te₃ phases form a pseudo-binary section. Alloys of the primary solidification phases of Te, (Bi,Sb)₂Te₃, γ, δ, and (Bi,Sb) are determined and the boundaries of various primary solidification phases are delineated. The Bi-Sb-Te liquidus projection is also calculated with the CALPHAD method using preliminary thermodynamic models, and the calculated results are qualitatively in agreement with the experimental determinations.     

Magnetic and specific heat studies of the frustrated Er2Mn2O7 compound

A new Er2Mn2O7 compound was synthesized by the ceramic method and its crystal structure was characterized using powder X-ray diffraction (XRD) and observed by scanning electron microscopy (SEM). The magnetic properties were investigated using a BS2 magnetometer and the heat capacity was studied using a quantum design (PPMS). The structural study revealed that this compound was monophasic and crystallized in the monoclinic system with the P2/M space group. Magnetization measurements were carried out in the temperature range of 1.8-200 K under an applied magnetic field of 0.05 T. A crossover from a room temperature paramagnetic phase to an antiferromagnetic one at low temperature was detected from the magnetic study. The magnetic susceptibility, in the paramagnetic region above 40 K, was found to present a simple Curie-Weiss type behavior. From the specific heat (CP) measurements in magnetic fields up to 5 T, we noted the presence of a wide peak characteristic of a second order mag-neto-structural transition.     

Sol-gel preparation of La-doped bismuth ferrite thin film and its low-temperature ferromagnetic and ferroelectric properties

Bi0.85La0.15FeO3 thin film was prepared on ATO glass substrates by sol-gel technique. The effect of La doping on phase structure, film surface quality, ion valence, and ferroelectric/magnetic properties of Bi0.85La0.15FeO3 film were investigated. La dop-ing suppressed the formation of impurity phases and the transition of Fe3+ to Fe2+ ions at room temperature. Compared with the un-doped BiFeO3, La-doping also increased the average grain size and the film density, which resulted in the decrease of film leakage current density. The remanent polarization and saturation magnetization were enhanced significantly by La doping. The remanent polarization of Bi0.85La0.15FeO3 films gradually decreased while saturation magnetization increased with the decrease of measuring temperature within a range from 50 to 300 K.     

Room-temperature positive magnetoresistance in (Fe0.64Cu0.06Co0.3)-graphite granular nanocomposites

(Fe_0.64Cu_0.06Co_0.3)_x-C_100″x (x = 10, 20, 30, and 50%) nanocomposites were prepared from ball milled Fe-Cu-Co alloy nanoparticles and micron sized graphite powders. The Structural characterization has been carried out using X-ray diffraction and FESEM measurements, while the electrical transport measurements were carried out by conventional four probe technique. Magnetoresistance (MR) measurements were performed at room temperature up to 45 kOe magnetic field. Magnetic measurements on the composites show all the composites to be ferromagnetic. Although magnetic properties show significant changes in magnetization values, comparable variations could not be detected in MR measurements, suggesting the origin of large positive magnetoresistance is non-magnetic. The existing models for the electron transport behaviour fail to explain the H ^3/2 dependence of the MR. The possible mechanism for the origin of positive MR in these composites has been discussed.     

Theoretical investigation of the precipitation of δ' in Al-Li

This paper contains the results of a theoretical investigation of the equilibrium between a disordered fee solution and an Ll₂ phase in a model binary alloy and the transformation paths that may be followed when the disordered phase is quenched into the two-phase field. The results are specifically applied to binary Al-Li alloys, in which case the ordered phase is the metastable Al₃Li(δ') phase that precipitates from the disordered solid solution (α). The thermodynamic model assumes that the atoms interact in pairs with an interaction potential that is independent of the temperature and composition, and uses the “mean field approximation” for the entropy of mixing. The assumptions confine its applicability to temperatures well below the ordering temperature of the Ll₂ phase. The model is used to compute the two-phase field that separates the disordered solution and the Ll₂ phase. For the specific case of Al-Li, it provides results that fit the available experimental data and offer a simple explanation for the observed deviation from stoichiometry of the δ' phase. The model predicts that the disordered solution orders congruently on quenching, but is then unstable with respect to decom-position by a spinodal mechanism that leads ultimately to a state of ordered Ll₂ precipitates in a disordered matrix. The results provide plausible interpretations for the transformations observed in quenched Al-Li alloys.     

Nucleation in undercooled Bi-Te alloys

Thermal analysis of bulk samples of Bi-Te alloys as a function of compositional change from hypo-eutectic to hyper-eutectic composition has proved that the intermediate phase Bi₇Te₃ with a rhombohedral crystal structure is a very efficient nucleant for bismuth. This technique is a modification of an earlier one applied by the authors as a critical experiment to test Jackson’s objection to Sundquist and Mondolfo’s droplet technique for determination of nucleation catalysis. The modified method described in this paper is unique and eliminates another possible source of error in the determination of nucleation capacity in eutectic systems.     

Multiferroic properties of Bi5.75R0.25Fe1.4Ni0.6Ti3O18 (R = Eu,Sm, Nd,Bi and La) ceramics

An improved technique was used to prepare the ceramic samples of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈(BRFNT,R=Eu,Sm,Nd,Bi,and La).The five-layer Aurivillius phase of the samples was confirmed by X-ray diffraction(XRD)without detectable impurities.BRFNT samples exhibit the pseudo-tetragonal except the orthorhombic BSmFNT samples.The characteristic plate-like morphology was revealed by field emission transmission electron microscopy(FETEM)images.At ambient tempe rature,all the samples present both ferroelectric and magnetic properties and BEuFNT shows the best ferroelectric behavior with its remanent polarization as high as 14.9μC/cm²under 155 kV/cm,which is three times higher than that of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈.Moreover,the remanent magnetization of BEuFNT is increased up to 1.20 emu/g compared to that of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈which is only 0.53 emu/g.With increasing radius of the introduced A-site ions,the ferroelectric phase transition temperature(T_CE)is decreased while the magnetic phase transition temperature(T_CM)fluctuates.The decrease in both T_CEand T_CM corresponds to the upward shift of the related Raman modes and vice versa,indicating that the T_CEand the T_CM depend not only on the t factor,but also on the strength of the covalent bonds.     

Some optical characteristics of powdered tellurites of rare-earth elements

Conclusions As a result of the crystallization of La tellurite specimens accompanying the raising of heat treatment temperature, the refractive index of the compound increases, isotropic particles with cubic structure symmetry become replaced by grains of long prismatic shape, birefringence appears, and the refractive index becomes dependent on orientation. Judged by the character of their reaction with light, compounds of the Ln₂Te₃O₉ type may be classed as semiconductors with forbidden zone of widths h ₀3.8 eV. It is shown, taking La₂Te₃O₉ as an example, that the heat treatment of air-dry specimens at temperatures above 400°C reduces the width of the forbidden zones of the compounds investigated and increases their light absorption coefficients at photon energies h 4 eV.Translated from Poroshkovaya Metallurgiya, No. 9(237), pp. 53–57, September, 1982.     

Structural and magnetic properties of Bi0.7Gd0.3FeO3 ceramics

The effect of Gadolinium (Gd) substitution on the structural and magnetic properties of BiFeO₃ is studied. Both BiFeO₃ and (Bi_0.7Gd_0.3)FeO₃ were synthesized in two steps: by high-energy ball milling of oxides for a total of 5 hours at 400 RPM and then sintering as-milled powders of BiFeO₃ at 700°C/24 hours and (Bi_0.7Gd_0.3)FeO₃ at 800°C /12 hours, to get the required phase. A structural transition is observed from rhombohedral (R3c) to orthorhombic (Pnma) as evident from the X-Ray Diffraction (XRD) study. The orthorhombic structure of Gd-substituted sample is iso-structural with GdFeO₃. (Bi_0.7Gd_0.3)FeO₃ shows a soft ferromagnetic behaviour when compared to antiferromagnetic BiFeO₃ at room temperature.     

The yield stress and saturization magnetization of Ir3Cr

Yield stress measurements were performed on polycrystalline Ir₃Cr at temperatures between 673 and 1673 K. It was found that the yield stress measured at elevated temperature was essentially temperature independent while the yield stress measured at 773 K on samples quenched from temperatures up to 1973 K showed a yield stress peak at a quench temperature of 1633±10 K. Magnetic measurements combined with Debye-Scherrer X-ray measurements on quenched samples showed that Ir₃Cr disorders at elevated temperature with all evidence for order disappearing at 1633±10 K. The saturization magnetization of fully ordered Ir₃Cr is 5.8 emu/g and that of disordered material is approximately zero. Based on the shape of the magnetizationvs applied magnetic field behavior of partially ordered samples, Ir₃Cr appears to order homogeneously for small departures from equilibrium and heterogeneously for large departures. DANIEL R. DiMICCO, formerly Graduate Student, Department of Materials Science and Engineering, University of Pennsylvania     

Electrospinning synthesis and luminescent properties of Lu2O3:Eu3+ nanofibers

One-dimensional Lu₂O₃:Eu³⁺ nanofibers were prepared by electrospinning followed by high-temperature calcinations. Thermogravimetric and differential thermal analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, photoluminescent spectra and decay curves were used to characterize the samples. Results showed that samples began to crystallize at ～500 °C and crystallized completely around 1000 °C. The average diameter of nanofibers (1000 °C annealed) was about 55 nm and the particle size of Lu₂O₃:Eu³⁺ increased with increasing annealing temperature. Under ultraviolet excitation, nanofibers exhibited typical red emission of Eu³⁺ in Lu₂O₃. The effect of heat-treatment temperature on luminescent properties of nanofibers was also discussed.     

Effect of high magnetic fields on the martensite transformation

The effect of high magnetic fields up to 132 kOe on the martensite transformation has been investigated in two alloy steels, 52100 bearing steel and a type 410 stainless steel. In both cases the martensite start temperature is raised by the application of a magnetic field, and the increase inM _s is linear with field. The rate of formation of martensite is not affected by the field. Numerical values for the entropy of the austenite-martensite reaction can be obtained from the experimental results, and are in reasonable agreement with previous results and with theoretical calculations.