Papers by Keyword: High Pressure

Abstract: We report a phenomenological model based calculation of pressure-induced structural phase transition and elastic properties of ZrN compound. Gibb’s free energy is obtained as a function of pressure by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. From the present study, we predict a structural phase transition from NaCl structure (B1) to the CsCl structure (B2). The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

Abstract: The high-pressure technique is useful to understand physical properties because the technique can directly control bond length and phase transition. As a general trend, the pressure-induced phase transition causes an increase of coordination number with a drastic change of their physical properties. Here, we attempt to explore the pressure-induced phase transitions from the sixfold-coordinated NaCl structure (B1) to the eightfold-coordinated CsCl structure (B2) in Mg_xCd_1−xO by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. Assuming that both the ions are polarizable, the Slater-Kirkwood variational method is employed to estimate the vdW coefficients for parent compounds. The estimated values of the phase transition pressure (P_t) increase with Mg concentration. The vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from B1 to B2 structure. The results obtain from the present calculations requires the complete understanding of many physical interactions that are essential to ternary oxides, containing elements with size and chemical mismatch, will lead to a consistent explanation of the documented structural properties.

Abstract: In the present paper we have calculated the phase transition and volume collapse of Samarium Bismuthide under pressure using a three body interaction potential model which includes long range columbic interaction, three body interactions and short range overlap repulsive interaction operative up to second nearest neighbour. This compound undergoes transition from NaCl structure to body-centred tetragonal (BCT) structure (distorted CsCl-type P4/mmm). Our calculated results of phase transitions and volume collapses of SmBi are found to be close to the experimental results.

Abstract: The structural properties of rare earth compound in NaCl-structure are studied in the present investigation. To study these properties of erbium telluride (ErTe), the Realistic Interaction Potential Approach (RIPA) model has been used. Present compound shows NaCl-type to CsCl-type structural phase transformation. The structural properties, including phase transition pressure and volume collapse are obtained and compared with available literature. The calculated equilibrium structural parameters are in good agreement with the available experimental and theoretical results. Further more to study the electronic properties, band structure and density of state are reported.

Abstract: A theoretical study of the elastic behavior in IrN compound using effective interionic interaction potential is carried out. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to CsCl structure (B2). C₁₁, C₁₂ and C₄₄ increase nearly linearly with pressure. At phase transition pressure IrN has shown a discontinuity in second order elastic constants, which is in accordance with the first-order character of the phase transition.

Abstract: High-pressure studies on thermoelectric materials allow the study of the relationship between structural, elastic, and electronic properties. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, CrSi₂ is an indirect band gap semiconductor that has potential applications in solar cells.

Abstract: Amorphous solids prepared from their melt state exhibit glass transition phenomena upon heating. Derivatives of volume like viscosity, specific heat and thermal expansion coefficient show rapid changes at the glass transition temperature (T_g). In general, application f high pressure increases the T_g (a positive dT_g/dP). This positive dTg/dP has been well understood with the Free Volume and Entropy models. However, there are few exceptions where a negative dT_g/dP has been observed. It has been proposed that the glasses which undergo negative thermal expansion can exhibit a negative dT_g/dP. In this study, electrical resistivity of semiconducting Ge₂₀Te₈₀ glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at T_g. The pressure dependence of T_g (dT_g/dP) shows a decreasing trend (-dT_g/dP). Chalcogenide glasses like Se, As₂Se₃ and As₃₀Se₃₀Te₄₀ show a positive dT_g/dP in contradiction to the present observation of negative dT_g/dP. A model proposed by deNeufville and Rockstad finds a linear relationship between T_g and the optical band gap (E_g) when they are grouped according to their connectivity (Z_av).Application of high pressure decreases the interatomic distance which in turn decreases the separation between the valence and conduction bands (optical band gap). This reduction in optical band gap shifts the glass transition to lower values. It is also suggested that the sign of the pressure derivative of T_g can be negative (-dT_g/dP) if the thermal expansion coefficient is negative. Inelastic neutron diffraction studies show a negative thermal expansion coefficient for most of the Te based chalcogenide glasses. Hence, Ge₂₀Te₈₀ glass is uniquethat its pressure dependence of T_gobeys both thermodynamic and the T_g-E_g-Z_av models.

Abstract: To resolve the materials corrosion problems during exploiting high H₂S/CO₂ oil and gas field, the corrosion behavior of 718 nickel base alloy for 720 hours was studied, the conditions of the experiment is about temperature=177°C, P(H₂S)=3.0MPa, P(CO₂)=13.5MPa, solution mixed NaCl 20%wt.. The Electronic Parts Manufacturers Association (EPMA) and electrochemical methods was applied to characterize the microstructure, chemical composition and impedance characteristics of corrosion film. The results show that the surface of corrosion film is mainly composition by sulphide of Ni, Cr and Fe, the inside of corrosion film is mainly composition by oxide of Ni, Cr and Fe, electrochemical analysis found the alloy could resist corrosion in this environment.

Abstract: Phase stability and elastic properties of seven one dimensional long period structures (1D-LPSs) of Al₃Ti under high pressure have been systematically investigated by first-principles calculations. The enthalpy differences indicate that Al₃Ti will undergo a phase transition from 1D-LPSs to L1₂ structure at high pressure. With increase of antiphase boundary period parameter M’, the enthalpy initially decreases and then increases, and the enthalpy for D0₂₃ is the smallest. Oppositely, the phase transition pressure firstly increases and then decreases, and the maximum is for D0₂₃. The elastic constants and elastic moduli B, G and E increase monotonically with increase of pressure, and the corresponding second-order polynomial fits are also obtained. Interestingly, the pressure dependence of Poisson’s ratio show similar tendency with that of B/G ratio. Both the B/G ratios and the Cauchy pressures reveal that these 1D-LPSs exhibit brittleness at high pressure.

Abstract: In this paper, the corrosion tests of glass fiber reinforced polymer (GFRP) in high temperature and high pressure acid environment were carried out. The surface morphology and glass transition temperature were observed by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and the mechanical property of GFRP was tested. The results indicated that after being exposed to acid corrosion environment, the structure and organization of GFRP changed, and a variety of defects produced on the surface and interior of GFRP, bending strength and tensile strength of GFRP decreased. The surface analysis also proved that there were some etch pits occurred on the GFRP pipes. Furthermore, their barcol hardness became poor.