过渡金属Mo高压熔化曲线的分子动力学模拟

基于经典的分子动力学模拟,运用嵌入原子势模型,研究了Mo的熔化曲线。通过分子动力学模拟得到的热状态方程与早期的实验及第一性原理计算结果相一致,验证了我们采用的EAM势的可靠性。单相模拟的结果表明钼在冲击熔化之前并没有固固相变的发生。通过固液共存的两相模拟方法获得了Mo的熔化温度数据,利用Simon函数拟合得到Mo的熔化曲线。两相模拟有效的考虑了过热效应,模拟结果证实了动高压实验的结论。     

过渡金属Mo高压熔化曲线的分子动力学模拟

基于经典的分子动力学模拟,运用嵌入原子势模型,研究了 Mo 的熔化曲线。通过分子动力学模拟得到的热状态方程与早期的实验及第一性原理计算结果相一致,验证了我们采用的 EAM 势的可靠性。单相模拟的结果表明钼在冲击熔化之前并没有固固相变的发生。通过固液共存的两相模拟方法获得了 Mo 的熔化温度数据,利用 Simon 函数拟合得到Mo 的熔化曲线。两相模拟有效的考虑了过热效应,模拟结果证实了动高压实验的结论。
     

FeO熔化曲线的分子动力学模拟

利用壳层模型分子动力学方法,研究了高温高压条件下FeO的熔化温度,同时还计算了温度在300K及压强上升到140 GPa时FeO的状态方程.作者在研究中,考虑了分子动力学模拟熔化存在的过热现象,通过晶体的现代熔化理论,对FeO的分子动力学模拟熔化温度进行了修正,获得了高温高压下FeO正确的熔化温度.因此,为常压下利用壳层模型分子动力学研究物质熔化提供了一种较好的方法,该方法亦可进一步推广应用到其它物质的高压熔化研究工作.     

高压下ε-Fe的磁性，热力学性质及熔化曲线

在密度泛函（DFT）理论的框架内, 我们利用平面波赝势及准谐德拜模型研究了六角结构铁的磁性,热力学性质和融化曲线. 磁性对六角结构铁的影响较大,影响的压力范围从0 GPa到72.9 GPa. 利用准谐德拜模型, 很好的得到了热胀系数, 体模量随压力和温度的关系. 计算得到的晶格热容, 熵与德拜温度的数据与实验值符合较好。利用Lindemann熔化定律,得到了六角铁在地核压力范围内的熔化曲线. 在地核压力（约330GPa）下,铁的熔化温度为6160 K.     

变壁温水平圆管内接触熔化新析

对变壁温水平圆管内接触熔化过程进行了研究.通过求解液膜边界层的控制方程,得到熔化速度的准确解析表达式,纠正了文献[10]存在的错误.熔化的理论分析结果同样适用于定壁温水平圆管内接触熔化问题.分析了管壁温度分布的影响规律,研究发现:管壁温度分布多项式的一次项对熔化影响显著;Ste数较小时(Ste<0.2),增加圆管内壁温度是缩短完全熔化时间的有效方法;Ste数较大时,改变圆管内壁温度分布对加强熔化,缩短完全熔化时间更为有效.结果对热储能系统的设计和研究具有一定参考价值.     

原位合成Mo5SiB2的热力学分析

计算了Mo-Si-B三元系中各化合物不同温度下标准生成自由焓和合成Mo5SiB2(T2相)反应在不同开始温度下的绝热温度及反应产物的熔化比.结果表明:用Mo、Si和B三种元素粉末混合物来原位合成Mo5SiB2在热力学上是完全可行的;合成Mo5SiB2不宜用燃烧合成的自蔓延模式,宜采用燃烧合成的热爆模式(原位反应热压工艺);反应的绝热温度及反应产物的熔化比与开始温度有关.     

烧结矿粘结相的熔化特性

以高碱度烧结矿中的铁酸盐粘结相为研究对象,考察了粘结相的熔化特性随粘结相组成的变化规律.通过试验探讨了不同nCaO∶nFe2O3以及不同的w(Mg),w(MgO2),w(SiO2)和w(Al2O3)铁酸盐粘结相的熔化特性.结果表明,nCaO∶nFe2O3=1∶1时,粘结相的熔化温度最低,熔化时间最短;添加MgO粘结相的熔化温度升高,熔化时间变长;当w(SiO2)或w(Al2O3)为3%时,粘结相的熔化温度最低,熔化最快.     

ZnO岩盐结构熔化特性的分子动力学模拟

利用分子动力学方法和经验势模型对岩盐结构ZnO高压下的熔化特性进行了研究．对ZnO闪锌矿结构常压下的熔化进行模拟，发现存在过热熔化现象，通过与实验比较得到其过热48％的结论，然后利用该结论修正得到了ZnO岩盐结构0-50GPa的高压熔化相图．其中岩盐结构ZnO高压熔化曲线在压力低于7GPa时和由Lindemann熔化方程得到的结果符合很好．     

球形金属纳米颗粒熔化温度、熔化熵及熔化焓的尺寸效应

根据结合能的键能模型,研究了球形金属纳米颗粒熔化相关热力学量的尺寸效应.在该模型中引入3个因子:晶体致密度k因子(晶胞中原子总体积与晶胞体积的比值);β因子(晶体中表面原子与内部原子结合能的比值)和qs因子(晶面被表面原子占据的比值).在考虑上述因子后,通过模型预测具有自由表面球形金属纳米颗粒的熔化温度、熔解熵、熔解焓等热力学量的尺寸依赖性,并将所得结果与相关实验数据、分子动力学模拟结果、液滴模型以及前人提出的模型进行了比较.结果显示,对于Au和Al纳米粒子熔化温度的尺寸效应,该模型相比之前的模型更接近实验结果,与Ag和Cu纳米粒子熔化熵与熔化焓的分子动力学结果吻合较好.     

青海胶凝原油熔化特性研究

利用流变测量技术首次对青海胶凝原油的熔化特性进行了详细的研究.结果表明:胶凝原油的熔化温度要远高于其胶凝温度,同时原油熔化过程中结构参数的变化规律要远复杂于其胶凝过程.对经历一定熔化历史的胶凝含蜡原油而言,随着终冷温度的升高,其临界线性剪切应变减小,临界线性剪切应力增加,且二者与终冷温度在半对数坐标系上均满足线性关系;同时胶凝原油的屈服值与屈服应变也随终冷温度的升高而逐渐增大,但增加趋势逐渐变缓.最后根据蜡晶形态随温度的变化规律,提出了相应的胶凝原油熔化机理.     

难熔金属钽、钼、钨的高温高压状态方程及声速的第一性原理研究

难熔金属的高压熔化曲线在动-静高压实验之间存在巨大争议,而在发生冲击熔化之前是否存在固-固相变是目前的研究热点问题.本文以3种典型难熔金属钽、钼、钨为例,通过第一性原理晶格动力学方法,计算了钽、钼、钨的声子色散曲线.采用准谐近似的方法,获得了Hugoniot状态方程以及Hugoniot声速.对于钽和钨的声速计算表明,其基态体心立方结构在高压下一直保持其稳定性;而钼的晶格动力学计算表明其基态结构的稳定性在高压下消失,而钼的Hugoniot声速在175–275GPa区域内发生了拐折,这一结果证实了冲击波实验中对于钼的声速测量的结果:在210GPa压力附近声速发生间断.     

Stability of the body-centred-cubic phase of iron in the Earth's inner core

Iron is thought to be the main constituent of the Earth's core, and considerable efforts have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.     

Electronic and structural transitions in dense liquid sodium

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure. It was predicted from theory--and later confirmed by experiment--that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states--an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.     

高压下BCC金属钨和钼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第一性原理方法研究了体心立方金属钨和钼的体积、弹性常数、弹性模量、声子色散曲线以及广义层错能在0~100GPa压强下随压强的变化关系,并讨论了高压下两种材料的力学结构稳定性以及高压对材料韧脆性和剪切形变难易程度的影响.首先,通过0~100GPa压强下的弹性常数发现,两种材料在不同压强下的弹性常数皆满足材料力学稳定性的判定条件,而且两种材料在100GPa下的声子色散曲线中并没有出现虚频,因此两种材料的结构在0~100GPa压强下都是力学稳定的.此外,通过研究不同压强下体模量与剪切模量的比值B/G发现,两种材料的韧性随压强的增加而增强,并且Mo的韧性强于W.最后,通过研究两种材料的广义层错能、沿111密排方向的剪切模量G111以及材料的各向异性比A发现,随着压强增加,广义层错能和G111逐渐增大,A整体趋于1,这说明高压会使得111密排方向的剪切形变变得困难,而且同时也削弱了材料的各向异性.     

多相Mo-Si-B合金材料的性质

多相Mo-Si-B合金是非常具有吸引力的新型高温结构材料. 它熔点高、抗氧化性、抗蠕变性和室温断裂韧性好.阐述了Mo-Si-B微观结构对多相Mo-Si-B合金性能的影响,举例说明了多相Mo-Si-B合金的抗氧化性及断裂韧性比钼或钼的硅化物有明显提高.指出了存在的问题和今后努力方向.     

自蔓延高温合成法制备 Mo-Al 系金属间化合物

以单质Ｍｏ粉和Ａｌ粉通过自蔓延高温合成方法（ＳＨＳ）制取了金属间化合物Ｍｏ３Ａｌ８．采用Ｘ射线衍射、差热分析和扫描电镜等分析手段对其自蔓延高温合成的过程及产物的特征进行了研究．结果表明,Ｍｏ３Ａｌ８的合成反应为扩散控制的固－液反应,Ａｌ粉的熔化促使合成反应迅速发生;Ｍｏ－Ａｌ间自蔓延过程发生之前有一个孕育期．     

高压条件下几种简单金属的溶化性质

熔化熵和熔化体积是物质的重要热学参量,研究物质在高压条件下的熔化性质有助于推动高压物理、材料科学和凝聚态物理等学科的发展.应用分子动力学方法计算铝、铜、镍、钽、硅5种单质分别在2、4、6和8GPa压强下与熔化有关的参量.计算结果表明:铝、铜、镍、钽4种单质的熔化温度随压强增大而增大,熔化体积增率和熔化熵随压强增大而减小;硅单质的熔化温度和熔化熵随压强增大而减小,熔化体积增率随压强增大而增大.并对硅单质在高压条件下所表现出的特殊熔化性质进行理论解释.     

Analysis of melting driven by friction and temperature difference under sliding plate

The contact melting processes of phase change material (PCM) under a sliding plate, which is driven by the friction and temperature difference AT, are studied. By using film theory, the fundamental equations for the melting process are derived. The thickness of boundary layer, the pressure distribution inside boundary layer and the mass melting rate of PCM are also obtained with the numerical method. It is found that (1) the larger the temperature difference is, the larger the thickness of boundary layer and the mass melting rate are, and the more asymmetric the pressure distribution is; (2) the quicker the sliding velocity is, the larger the mass melting rate is, the flatter the thickness of boundary layer is, and the more symmetric the pressure distribution is; (3) the results of the contact melting driven together by the friction and temperature difference cannot be obtained by the addition of those driven respectively by the friction and temperature difference.     

Analysis of melting driven by friction and temperature difference under sliding plate

The contact melting processes of phase change material (PCM) under a sliding plate, which is driven by the friction and temperature difference ΔT, are studied. By using film theory, the fundamental equations for the melting process are derived. The thickness of boundary layer, the pressure distribution inside boundary layer and the mass melting rate of PCM are also obtained with the numerical method. It is found that (1) the larger the temperature difference is, the larger the thickness of boundary layer and the mass melting rate are, and the more asymmetric the pressure distribution is; (2) the quicker the sliding velocity is, the larger the mass melting rate is, the flatter the thickness of boundary layer is, and the more symmetric the pressure distribution is; (3) the results of the contact melting driven together by the friction and temperature difference cannot be obtained by the addition of those driven respectively by the friction and temperature difference.