Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide （GAP） from zinc-blende （ZB） to a rocksalt （RS） structure is investigated by the plane-wave pseudopotential density functional theory （DFT）. Lattice constant a0, elastic constants cij, bulk modulus B0 and the pressure derivative of bulk modulus B0 are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/Vo, the Debye temperature 8, the heat capacity Cv and the thermal expansion coefficient a are also discussed in a pressure range from 0 CPa to 40 GPa and a temperature range from 0 K to 1500 K.

Phase transition and thermodynamic properties of SrS via first-principles calculations

The phase transition of SrS from NaCl structure （B1） to CsCl structure （B2） is investigated by means of ab initio plane-wave pseudopotential density functional theory, and the thermodynamic properties of the B1 and the B2 structures are obtained through the quasi-harmonic Debye model. It is found that the transition phase from the B1 to the B2 structures occurs at 17.9 GPa, which is in good agreement with experimental data and other calculated results. Moreover, the thermodynamic properties （including specific heat capacity, the Debye temperature, thermal expansion and Griineisen parameter） have also been obtained successfully.

Phase transition and thermodynamic properties of BiFeO_3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.

Phase transition and thermodynamic properties of ThO2： Quasi-harmonic approximation calculations and anharmonic effects

Formation control and obstacle avoidance for multi-agent systems have attracted more and more attention. In this paper, the problems of formation control and obstacle avoidance are investigated by means of a consensus algorithm. A novel distributed control model is proposed for the multi-agent system to form the anticipated formation as well as achieve obstacle avoidance. Based on the consensus algorithm, a distributed control function consisting of three terms(formation control term, velocity matching term, and obstacle avoidance term) is presented. By establishing a novel formation control matrix, a formation control term is constructed such that the agents can converge to consensus and reach the anticipated formation. A new obstacle avoidance function is developed by using the modified potential field approach to make sure that obstacle avoidance can be achieved whether the obstacle is in a dynamic state or a stationary state. A velocity matching term is also put forward to guarantee that the velocities of all agents converge to the same value. Furthermore, stability of the control model is proven. Simulation results are provided to demonstrate the effectiveness of the proposed control.

Phase transition,structural and thermodynamic properties of Mg_2 Si polymorphs

The plane-wave pseudo-potential method within the framework of first principles is used to investigate the structural and elastic properties of Mg2Si in its intermediate pressure （Pnma） and high pressure phases （P63/mrnc）. The lattice constants, the band structures. The bulk moduli of the Mg2Si polymorphs are presented and discussed. The phase transition from anti-cotunnite to Ni2In-type Mg2Si is successfully reproduced using a vibrational Debye-like model. The phase boundary can be described as P = 24.02994 ＋ 3.93 × 10^-3T -- 4.66816 × 10^-5T2 -- 2.2501 × 10^-9T3＋ 2.33786 × 10^-11T4. To complete the fundamental characteristics of these polymorphs we have analysed thermodynamic properties, such as thermal expansion and heat capacity, in a pressure range of 1-40 GPa and a temperature range of 0-1300 K. The obtained results tend to support the available experimental data and other theoretical results. Therefore, the present results indicate that the combination of first principles and a vibrational Debye-like model is an efficient scheme to simulate the high temperature behaviours of Mg2Si.

Phase transition and thermodynamic properties of TiO_2 from first-principles calculations

The pressure induced phase transitions of TiO2 from anatase to columbite structure and from rutile to columbite structure and the temperature induced phase transition from anatase to rutile structure and from columbite to rutile structure are investigated by ab initio plane-wave pseudopotential density functional theory method （DFT）, together with quasi-harmonic Debye model. It is found that the zero-temperature transition pressures from anatase to columbite and from rutile to columbite are 4.55 GPa and 19.92 GPa, respectively. The zero-pressure transition temperatures from anatase to rutile and from columbite to rutile are 950 K and 1500 K, respectively. Our results are consistent with the available experimental data and other theoretical results. Moreover, the dependence of the normalized primitive cell volume V/Vo on pressure and the dependences of thermal expansion coefficient α on temperature and pressure are also obtained successfully.

Phase transition, thermodynamic and elastic properties of ZrC

First principles calculation and quasi-harmonic Debye model were used to obtain more physical properties of zirconium carbide under high temperature and high pressure.The results show that the B1structure of ZrC is energetically more favorable with lower heat of formation than the B2structure,and that mechanical instability and positive heat of formation induce the inexistence of the B2structure at normal pressure.It is also found that the B1structure would transform to the B2structure under high pressure below the critical point of V/V0=0.570.In addition,various thermodynamic and elastic properties of ZrC are obtained within the temperature range of0-3000K and the pressure range of0-100GPa.The calculated results not only are discussed and understood in terms of electronic structures,but also agree well with corresponding experimental data in the literature.

First-principles calculations for transition phase and thermodynamic properties of GaAs

The transition phase of GaAs from the zincblende （ZB） structure to the rocksalt （RS） structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model. It is found that the transition from the ZB structure to the RS structure occurs at the pressure of about 16.3 GPa, this fact is well consistent with the experimental data and other theoretical results. The dependences of the relative volume V/V0 on the pressure P, the Debye temperature Θ and specific heat Cv on the pressure P, as well as the specific heat CV on the temperature T are also obtained successfully.

Phase Transition and Thermodynamic Properties of BaS:An Ab initio Study

The hydrostatic-pressure-induced transition phase of BaS from the NaCl-type structure （B1） to the CsCl-type structure （B2） is investigated by ab initio plane-wave pseudopotential den-sity functional theory method. It is found that the transition pres-sure from B1 to B2 phases is 8.2 GPa according to the usual con-dition of equal enthalpy. Through the quasi-harmonic Debye model,the dependences of the relative volume V/V0 on the pres-sure P,the thermal expansion parameter ratio on pressure P,and the Debye temperature Θ and heat capacity CV on pressure P and temperature T are estimated.

Low Temperature Heat Capacities and Thermodynamic Properties of Zinc L-Threonate Zn(C_4H_7O_5)_2(s) by Adiabatic Calorimetry

Low-temperature heat capacities of the solid compound Zn（C4H7O5）2（s） were measured in a temperature range from 78 to 374 K, with an automated adiabatic calorimeter. A solid-to-solid phase transition occurred in the temperature range of 295？322 K. The peak temperature, the enthalpy, and entropy of the phase transition were determined to be （316.269±1.039） K, （11.194±0.335） kJ？mol-1, and （35.391±0.654） J？K-1？mol-1, respectively. The experimental values of the molar heat capacities in the temperature regions of 78？295 K and 322？374 K were fitted to two polynomial equations of heat capacities（Cp,m） with reduced temperatures（X） and [X = f（T）], with the help of the least squares method, respectively. The smoothed molar heat capacities and thermodynamic functions of the compound, relative to that of the standard reference temperature 293.15 K, were calculated on the basis of the fitted polynomials and tabulated with an interval of 5 K. In addition, the possible mechanism of thermal decomposition of the compound was inferred by the result of TG-DTG analysis.

Structural and thermodynamic properties of OsN_2 from first-principles calculations

We investigate the structural and thermodynamic properties of OsN2 by a plane-wave pseudopotential density functional theory method. The obtained lattice constant, bulk modulus and cell volume per unit formula are consistent with the available theoretical data. Moreover, the pressure-induced phase transition of OsN2 from pyrite structure to fluorite structure has been obtained. It is found that the transition pressure of OsN2 at zero temperature is 67.2 GPa. The bulk modulus B as well as other thermodynamic quantities of fluorite OsN2 （including the Griineisen constant γ and thermal expansion α） on temperatures and pressures have also been obtained.

High-pressure dynamic,thermodynamic properties,and hardness of CdP_2

Using a pseudopotential plane-waves method,we calculate the phonon dispersion curves,thermodynamic properties,and hardness values of α-CdP;and β-CdP;under high pressure.From the studies of the phonon property and enthalpy difference curves,we discuss a phase transform from β-CdP;to a-CdP;in a pressure range between 20 GPa and 25 GPa.Then,the thermodynamic properties,Debye temperatures,and heat capacities are investigated at high pressures.What is more,we employ a semiempirical method to evaluate the pressure effects on the hardness for these two crystals.The results show that the hardness values of both α-CdP;and β-CdP;increase as pressure is increased.The influence mechanism of the pressure effect on the hardness of CdP;is also briefly discussed.

Thermodynamic properties of 3C-SiC

In the present paper, we report on the results of various thermodynamic properties of 3C-SiC at high pressure and temperature using first principles calculations. We use the plane-wave pseudopotential density functional theory as im- plemented in Quantum ESPRESSO code for calculating various cohesive properties in ambient condition. Further, ionic motion at a finite temperature is taken into account using the quasiharmonic Debye model. The calculated thermody- namic properties, phonon dispersion curves, and phonon densities of states at different temperatures and structural phase transitions at high pressures are found to be in good agreement with experimental and other theoretical results.

Crystallization kinetic characteristics of oxygen-induced I-phase and Zr_2Cu phase in Zr_(65)Cu_(27.5)Al_(7.5) glassy alloy

Based on the construction of TTT diagrams by isothermal DSC measurements, the thermal stabilities of Zr65Cu27.5Al7.5 glassy alloys containing 0.68% and 0.06% [O] (molar fraction) were compared. The changing tendencies of the thermal stabilities reflected in the TTT-diagrams were validated by XRD analyses and TEM observations. The crystallization kinetic characteristics of oxygen-induced I-phase and Zr2Cu phase in Zr65Cu27.5Al7.5 glassy alloy were discussed. It is found that oxygen promotes the precipitation of I-phase and retards the formation of Zr2Cu phase. If the pre-crystallization event for oxygen-induced I-phase is permitted and the main-crystallization event for Zr2Cu phase is taken as the thermal stability criterion, the alloy with higher oxygen content has longer onset time for crystallization within a rather large supercooling temperature range. The possibility for the preparation of Zr65Cu27.5Al7.5 glassy alloy-based composite containing oxygen-induced I-phase was also forecasted.

高压下Fe_(2)S的相变及热力学性质的第一性原理研究

本文采用基于密度泛函理论的赝势平面波方法研究了高压下Fe_(2)S的结构相变、弹性性质和热力学性质.计算表明,当压强增加到124±10 GPa时,Fe_(2)S(Co_(2)P型)的晶格常数发生了突变,轴向比c/a从1.20跃迁到1.35,向C37结构(Co_(2)Si型)转变.弹性模量的计算结果也证实了Fe_(2)S的这一结构相变.根据弹性模量,获得了高压下Fe_(2)S的声速,相对于纯Fe,S元素提升了纵波声速.通过准谐德拜模型预测了200 GPa,2000 K范围内Fe_(2)S的热力学性质.

Phase structure and hydrogen storage properties of LaMg_(3.93)Ni_(0.21) alloy

The phase structure and hydrogen storage property of LaMg3.93Ni0.21 alloy were studied. XRD and SEM results exhibited that LaMg3.93Ni0.21 alloy consisted mainly of LaMg3, La2Mg17 and LaMg2Ni phases; after hydriding/dehydriding process, all the three phases transformed, La3H7 phase existed and the actual hydrogen absorption phases were Mg and Mg2Ni phases. Pressure-composition-temperature （P-C-T） measurement showed that the reversible hydrogen storage capacity of LaMg3.93Ni0.21 alloy was 2.63 wt.%, and the absorption time for reaching 90% of the storage capacity was 124 s at 523 K, and it was 1850 s for deabsorbing 90% of the maximum dehydrogen capacity. The hydriding process of LaMg3.93Ni0.21 alloy followed the nucleation and growth mechanisms. The enthalpy and entropy for hydriding and dehydriding reactions of the Mg phase in LaMg3.93Ni0.21 alloy were calculated to be 456.38±1.10 kJ/mol H2, -100.96±1.96 J/（K·mol） H2 and 68.50-x3.87 kJ/mol H2, 98.28 ±5.48 J/（K-mol） H2, respectively. A comparison of these data with those of MgHz （-74.50 kJ/mol H2, -132.30 J/K.mol H2） suggested that the hydride of LaMg3.93Nio.21 alloy was less stable than MgH2. The existence of La hydride and synergetic effect of multiphase led to higher reversible hydrogen storage capacity and better kinetic property at lower temperature for LaMg3mNi0.21 alloy.

脉冲电流对低碳微合金钢力学性能的影响

研究了脉冲电流对一种低碳微合金钢的组织和力学性能的影响.结果表明,脉冲电流处理可以使该钢的晶粒显著细化,从而使其力学性能提高.细化后的组织热稳定性好,在 400 ℃长时间保温不会出现晶粒长大.脉冲电流加热速度快,作用时间短和降低相变势垒的作用是晶粒细化的原因.

GaAs相变和热力学性质的密度泛函理论研究(英文)

利用平面波赝势密度泛函方法 ,对GaAs从闪锌矿结构到CsCl结构的相变进行了理论研究。通过Birch Murnaghan状态方程拟合闪锌矿结构GaAs的能量和体积 ,得到了GaAs的热力学性质。我们发现对GaAs来说 ,闪锌矿结构通常比CsCl结构稳定 ,由闪锌矿结构到CsCl结构的相变压力在 37.0 1 9GPa左右。计算所得到的晶格常数。

淡水鱼低温相变区热特性参数预测模型的建立

为了给淡水鱼冷冻冷藏加工、品质控制及设备开发提供基础数据,应用差示扫描量热仪(DSC)测定鲢鱼、鳙鱼、鲤鱼以及草鱼背部和腹部肌肉在低温相变区(-40～10℃)的冰点、表观比热和热焓,建立淡水鱼低温相变区热特性参数预测模型。结果表明,建立的基于淡水鱼肌肉水溶性成分含量的线性组合冰点预测模型、低温相变区表观比热的两段式预测模型和推导的淡水鱼肌肉热焓预测模型都有很高的拟合精度。鱼肌肉中含水率、可溶性固形物含量、蛋白质含量和脂肪含量等对冰点和冰点以上的表观比热有不同程度影响,其中可溶性固形物和含水率分别对冰点和常温表观比热影响最大,而热焓和冰点以下的表观比热仅是温度的函数。

变温下NiTi合金相变热力学特征

将试验测定的 Ni Ti合金热容值用于计算不同电流值加热 (不同的加热速度 )的 Ni Ti合金丝温度的变化历程 .由于相变潜热的存在 ,当加热电流适当大时 ,会使升温曲线产生拐点而形成台阶 .实验发现加热速度的快慢对合金相变所需要的时间影响很大 .使用调制式差式扫描量热器研究了以不同速度加热或冷却合金时合金各相相变温度的变化情况 .随着加热速度的增大 ,相变温度偏离平衡温度的趋势增大 .经过数值处理之后 。