Experimental study and thermodynamic calculation of Au–Bi–Sb system phase equilibria

Phase equilibria in the Au–Bi–Sb ternary system have been studied experimentally and calculated by the CALPHAD method. Three calculated isopleths with molar ratios Au:Bi=1, Bi:Sb=1 and Au:Sb=1 were compared with the DTA results from this work. The liquidus projection has been calculated. Two ternary invariant reactions were noted. Calculated phase diagram of isothermal section at 573 K was compared with the results of SEM/EDX analysis.     

Experimental investigation and thermodynamic calculation of the Bi-Ga-Sn phase equilibria

Binary thermodynamic data, successfully used for phase diagram calculations of binary systems Bi-Ga, Bi-Sn, and Ga-Sn, were used for prediction of phase equilibria in ternary Bi-Ga-Sn system. The thermodynamic functions, such as enthalpy of formation and activity, were calculated using the Redlich-Kister-Muggianu model and compared with experimental data reported in the literature. The liquidus surface, invariant equilibria and three vertical sections with molar ratio Ga:Sn=1, Bi:Sn=1 and Bi:Ga=1 of the Bi-Ga-Sn ternary system were calculated by the CALPHAD method. Alloys, situated along three calculated vertical sections, were investigated by Differential Scanning Calorimetry (DSC). The experimentally determined phase transition temperatures were compared with calculation results and good mutual agreement was noticed.     

Thermodynamic modeling of Fe–Ni pentlandite

Gibbs energy modeling of iron–nickel pentlandite has been performed using experimental data of ternary phase equilibria. A three-sublattice approach in the framework of the Compound Energy Formalism is developed to refine a two-sublattice model of pentlandite recently applied within a complete assessment of the Fe–Ni–S system. Experimental data about the iron site fraction on the octahedral sublattice at 523.15 K for the composition Fe₅Ni₄S₈ as well as the enthalpy of formation at 298.15 K for the composition Fe_4.5Ni_4.5S₈ are predicted satisfactorily by the novel model. New possibilities to interpret experimental phase equilibrium data on complex phase relations with pentlandite are discussed together on the basis of the recent extension of a second high-temperature heazlewoodite phase to a ternary solution phase.     

Pyroelectric properties of tricyclohexylmethanol (TCHM) single crystal

The pyroelectric effect is measured in tricycloheylmethanolmethanol (TCHM) crystal around the second-order phase transition at 104 K. The presented results confirm that the spontaneous polarization exists in the low temperature phase of TCHM and is reversible in external electric field. It is suggested that TCHM is an improper ferroelectric below 104 K.     

Thermodynamic and kinetic properties of Ho1−xTixCo2-hydrogen system

The hydrogen absorption behavior of Laves phase Ho_1−xTi_xCo₂ (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho_1−xTi_xCo₂-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed.     

Molecular alloys as phase change materials (MAPCM) for energy storage and thermal protection at temperatures from 70 to 85 °C

Molecular alloys, that combine a relatively high heat of melting with a suitable melting temperature adapted to the application temperature, are excellent materials for thermal protection and for thermal energy storage. Of special interest is the fact that, by making alloys of molecular materials; the range of melting can be adjusted over a range of temperatures. The present paper reports on the design of MAPCMs to be used for energy storage and thermal protection at temperatures from 70 to 85 °C. The aim is to use these materials for thermal protection in the catering sector in order to avoid proliferation of micro organisms; the minimal temperature required is higher than 65 °C. The work illustrates how some fundamental studies are helpful in choosing the right composition that is able to work at the temperature required for an application. Several molecular alloys using the n-alkanes are elaborated and characterized. The preparation of mixed crystals, their crystallographic and thermodynamic properties and stability, phase change behaviour, and their use in practical applications are reported.     

Nanophases in mechanochemically synthesized AgI-CuI system: structure, phase stability and phase transitions

Nanoscale crystallites of Ag-rich (Ag_1−xCu_xI, x=0.05, 0.10, 0.15 and 0.25), Cu-rich (Cu_1-yAg_yI, y=0.05, 0.10, 0.15 and 0.25) and intermediate Ag_1-xCu_xI (x=0.50) solid solutions and end members AgI, CuI with sizes in the range of 46-13 nm were synthesized by attrition at ambient temperature in a soft mechanochemical reaction (MCR) of Ag, Cu and I. Monophasic γ-AgI (zincblende, ) with disordered Ag⁺ sublattice and the crystallite size of about ∼31 nm was realized in the case of Ag_0.75Cu_0.25I (x=0.25) composition. Lattice parameter decreases linearly from 649 to 604 pm with increasing Cu concentration in the AgI-CuI system validating Vegard's law. Smallest size (∼13 nm) agglomerated nanocrystals were realized in the Cu-rich composition Cu_0.75Ag_0.25I (), while unagglomerated uniform-sized (∼17 nm) and spherical shape nanocrystallites of Ag_0.50Cu_0.50I () with maximum strain were synthesized for sensor applications using MCR. Differential scanning calorimetry study shows the systematic changes in the phase transition temperature with Cu substitution. Ag-rich composition posses less enthalpy (ΔH (x or Cu=0.05, 0.10, 0.15, 0.25)=6.0, 6.11, 6.6, 6.3 in kJ/mol) and entropy (ΔS (y or Ag=0.05, 0.10, 0.15, 0.25)=14.15, 14.1, 15.03, 13.6 in J/mol K) when compared to undoped AgI () implying greater thermal stability of γ-phase due to Cu-strengthened Ag-I bond. Enhanced entropy () in Cu_0.75Ag_0.25I (Cu-rich) solid solutions relative to CuI () indicates Ag-induced cation disorder. Fifteen percent Ag-doped CuI (Cu_0.85Ag_0.15I) nanocrystals apparently behave like microscopic p-n junctions with currents in the range of 10⁻⁶-10⁻⁸ A characterized by a non-linear I-V curve.     

Theoretical study of metal-insulator transition in rhombohedral vanadium sesquioxide

The electronic structure and the metal-insulator transition (MIT) of V₂O₃ are investigated in the framework of density functional theory and GGA+U. It is found that, both the insulating and metallic phases can be realized in rhombohedral structure by varying the on-site Coulomb interaction, and the MIT in V₂O₃ can take place without any structure phase transition. Our calculated energy gap (0.63 eV) agrees with experimental result very well. The metallic phase exhibits high spin (S=1) character, but it becomes S=1/2 in insulating phase. According to our analysis, the Mott-Hubbard and the charge-transfer induce the MIT together, and it supports the mechanism postulated by Tanaka (2002) [11].     

Brillouin scattering study of phase transitions in LiK0.80(NH4)0.20SO4 mixed crystals

Brillouin spectroscopy was used to study the phase transitions of LiK_0.80(NH₄)_0.20SO₄ mixed crystals in the temperature range 10-300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ₁₆ and the quasi-transverse γ₁₇ mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO₄ and LiK_0.96(NH₄)_0.04SO₄ crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.     

Berezinsky-Kosterlitz-Thouless transition in phase fluctuating superconductors with inhomogeneous Gaussian distributed couplings

Within the framework of phase fluctuation picture for the pseudogap state of cuprate superconductors, we study the effects of both spatial inhomogeneity of coupling strength and thermal phase fluctuations on the superconducting transition temperature. Such a Berezinsky-Kosterlitz-Thouless (BKT) transition is characterized by a two-dimensional (2D) classical XY model, in which the bond coupling is assumed to be roughly proportional to the superconducting bond order parameter. In recent STM experiments with lattice-tracking spectroscopy technique, a Gaussian-like spatially distributed pairing strength is observed. Our Monte Carlo simulations using Wolff cluster update on such 2D classical XY model, in which the bond coupling obeys a similar spatial Gaussian distribution, indicate that the enhancement of the variance of Gaussian distribution may suppress the BKT transition temperature. In addition, we calculate the related physical quantities, including the spin stiffness, free energy, specific heat, magnetization and magnetic susceptibility, by changing the inhomogeneity variance.     

Tuning hydrogen storage properties and reactivity: Investigation of the LiBH4-NaAlH4 system

Tuning the hydrogen storage properties of complex metal hydrides is of vast interest. Here, we investigate the hydrogen release and uptake pathways for a reactive hydride composite, LiBH₄−NaAlH₄ utilizing in situ synchrotron radiation powder X-ray diffraction experiments. Sodium alanate transforms to sodium borohydride via a metathesis reaction during ball milling or by heating at T∼95 °C. NaBH₄ decomposes at ∼340 °C in dynamic vacuum, apparently directly to solid amorphous boron and hydrogen and sodium gas and the latter two elements are lost from the sample. Under other conditions, T=400 °C and p(H₂)=∼1 bar, NaBH₄ only partly decomposes to B and NaH. On the other hand, formation of LiAl is facilitated by dynamic vacuum conditions, which gives access to the full hydrogen contents in the LiBH₄−NaAlH₄ system. Formation of AlB₂ is observed (T∼450 °C) and other phases, possibly AlB_x or Al_1−xLi_xB₂, were observed for the more Li-rich samples. This may open new routes to the stabilization of boron in the solid state in the dehydrogenated state, which is a challenging and important issue for hydrogen storage systems based on borohydrides.     

Thermochemical studies on RE2O2CO3 (RE=Gd, Nd) decomposition

Thermochemistry in the decomposition of gadolinium di-oxycarbonate, Gd₂O₂CO₃(s) and neodymium di-oxycarbonate, Nd₂O₂CO₃(s) was studied over the temperature region of 774-952 K and 775-1105 K, respectively. The equilibrium properties of the decomposition reactions were obtained by tensimetric measurement of the CO₂(g) pressure over the biphasic mixture of RE₂O₂CO₃(s) and RE₂O₃(s) at different temperatures (RE=Gd, Nd) and also by thermogravimetric analysis of the decomposition temperature at different CO₂ pressures. The temperature dependence of the equilibrium pressure of CO₂ thus measured could be given by

ln p_CO2/Pa (±0.13)=−22599.1/T+35.21 (774≤T (K)≤952) for Gd₂O₂CO₃ decomposition and

ln p_CO2/Pa (±0.19)=−23824.7/T+33.14 (775≤T (K)≤1105) for Nd₂O₂CO₃ decomposition.

From the above vapor pressure expressions, the median enthalpy and entropy of the decomposition of the oxycarbonates were calculated by the second law analysis and their thermodynamic stabilities were derived. The results are discussed in the light of available thermochemical data of the compounds.     

Effect of lateral fluorination in antiferroelectric and ferroelectric mesophases: synchrotron X-ray diffraction,dielectric spectroscopy and electro-optic study

Effect of lateral fluorination in the rigid core on several macroscopic and microscopic properties of a terphenyl based mesogenic chiral ester has been studied by synchrotron X-ray, dielectric and electro-optic techniques. Correlation lengths across the smectic planes, in para-, ferro- and antiferroelectric phases, are found to be significantly less in the fluorinated compound. Para to ferroelectric transition is found to be tricritical in nature in both the compounds. Fluorination resulted in slower response under a square pulse. Collective mode relaxation behaviour, with and without bias field, in all the phases are also found to be different in the fluorinated compound.     

Dielectric, thermal and optical study of an unusually shaped liquid crystal

Dielectric measurements have been carried out for the determination of real and imaginary parts of the permittivity of a newly synthesized, unusually shaped liquid crystal. The sample has been investigated in the frequency range from 100 Hz to 10 MHz within a temperature range 80-130 °C. The dielectric measurements in the smectic A phase indicate a Cole-Cole type of dispersion, and the activation energy was found to be 5.5 meV by using the Arrhenius plot of relaxation time. In addition to this, thermal and optical transmittance studies have also been conducted in the above mentioned temperature range, and the temperature dependence of these parameters has been discussed in detail. The phase transition temperature obtained from a differential scanning calorimetry (DSC) study matches within 2 °C that was obtained from an optical transmittance study. The dielectric and optical behavior of the unusually shaped liquid crystal has been explained on the basis of a proposed theoretical model in which a sample possesses two different conformers having induced polarizations in opposite directions.     

First-principles study of small aluminum clusters: Oxygen adsorptions, oxidation and phase stability

It is important to understand the properties of individual nanometals before we can exploit full potential of their applications, for example, as energetic materials, enhancing additives, or catalysts. Here, we present a density functional theory study of the structure and properties of clean Al₁₃ clusters, oxygen adsorptions on the cluster surface, and the completely oxidized clusters. The relative stability of various phases at various oxygen pressures and temperatures is investigated based on the so-called “atomistic thermodynamics”, which was previously employed for studying metals. The effect of temperature and oxygen pressure on the phase stability is taken into account via the oxygen chemical potential and reflected in the (P, T) phase diagram. Our results show that only intact and completely oxidized clusters are thermodynamically stable, and that the O adsorption phases are never thermodynamically stable. Also, our results show that the Al₁₃ clusters are extremely easy to get oxidized. The present study provides valuable insight into the basic behaviors of small Al clusters in the presence of oxygen and a theoretical basis for exploring practical applications of these clusters.     

Effect of Dichroic dye on dielectric properties of ferroelectric liquid crystals: A comparative study

In order to study the effect of mixing dye molecules in ferroelectric liquid crystals, we have investigated two ferroelectric liquid crystal samples CS1016 and Felix 17/000 along with their mixture with Anthraquinone dye. The measurements have been made in the frequency range 100 Hz-10 MHz, with the variation of temperature from 30 to 90 °C. The dielectric behaviour of dye mixed CS1016 is quite different from that of Felix 17/000. This different behaviour has been explained by determining other parameters like distribution parameter, dielectric strength and relaxation frequency, etc. The different nature shown by two different samples has also been explained by electro-optical measurements.     

Empirical correlation between melting temperature and cohesive energy of binary Laves phases

A list of 143 binary Laves phases with their melting temperature and melting type is collected, and used to study a correlation between melting temperature and cohesive energy. It is found that the melting temperature of Laves phases is roughly proportional to its cohesive energy calculated by Miedema's empirical model from their intrinsic atomic properties. The average predicted error of melting temperature of compounds is as low as 8.0%. This empirical rule is consistent with the result of the universal binding energy theory of solids.     

Phase stability in the LnCa2Mn2O7 (Ln=Pr, Nd, Sm and Gd) Ruddlesden-Popper series

The synthesis of the Ruddlesden-Popper series, LnCa₂Mn₂O₇, (Ln=Pr, Nd, Sm and Gd) is described and their structure and electronic properties investigated. The reduction in size of the A-site cation causes an increase in the distortion of their orthorhombic structures (space group Cmcm). All of these compounds form with a perovskite impurity, the amount of which increases on reduction of the cation size. The synthesis temperature also alters the amount of perovskite impurity in the phase, but only to a lower limit, implying the perovskite phase is intrinsic to the material and that a phase equilibrium exists between the layered Ruddlesden-Popper and perovskite phases, which is controlled by the cation size. The magnetic susceptibility show transitions characteristic of the perovskite phase, therefore little direct information can be obtained about the Ruddlesden-Popper phases, except that ferromagnetism is not observed in any of these materials.     

Experimental constraints on the phase diagram of titanium metal

The phase transformations of titanium metal have been studied at temperatures and pressures up to 973 K and 8.7 GPa using synchrotron X-ray diffraction. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 345 K/GPa, and the transition pressure at room temperature is located at 5.7 GPa. The volume change across the α-ω transition is ΔV=0.197 cm³/mol, and the associated entropy change is ΔS=0.57 J/mol K. Except for ΔV, our results differ substantially from those of previous studies based on an equilibrium transition pressure of 2.0 GPa at room temperature. The α-ω-β triple point is estimated to be at 7.5 GPa and 913 K, which is comparable with previous results obtained from differential thermal analysis and resistometric measurements. An update, more accurate phase diagram is established for Ti metal based on the present observations and previous constraints on the α-β and ω-β phase boundaries.     

Temperature dependence of thermal property for lead nitrate crystal

Thermal property was measured in a lead nitrate crystal, Pb(NO₃)₂, at temperatures from 90 to 340 K by use of ac calorimetry technique. The heat capacity derived from the measurements showed temperature dependence with thermal hysteresis, in the temperature region from 240 to 300 K. The anomaly of the heat capacity was found in the vicinity of 275.22 K. The broad temperature variation in the heat capacity was observed in the region from 235 to 260 K.