Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates,depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH₄ mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH₄ mixture at high pressures, phase transition sII-sI can occur.     

Raman Spectroscopy of Nitrogen Clathrate Hydrates

在253 K和16 MPa的压力下,于实验室内合成了氮气水合物,用显微共焦拉曼光谱对其N-N和O-H键伸缩振动的光谱特征进行了研究．结果表明,氮气水合物中的N-N和O-H键的拉曼峰分别为2322.4和3092.1 cm^-1,与天然的空气水合物中的数据十分接近．另外,还测定了液氮和溶解于水中的氮分子中N-N键的拉曼峰值,分别为2326.6和2325.0 cm^-1．氮气笼型水合物分解的拉曼谱图表明,氮分子同时进入水合物的大笼和小笼中,但由于氮分子在大、小笼中的环境氛围十分接近,其拉曼位移相差不大,故拉曼谱图只能显示N-N键伸缩振动一个峰．     

Influence of gravitational potential on the thermodynamic stability of pure and mixed clathrate hydrates

Clathrate hydrates are an ice-like material consisting of gas molecules confined within cavities in a crystalline water lattice. Phase equilibria of clathrate hydrates systems was described using the statistical mechanical theory of van der Waals and Platteeuw. This theory makes use of the fractional occupancy of cavities within the clathrate hydrate lattice in the determination of chemical equilibria. Classical density functional theory with intermolecular interactions restricted to the first hydration shell was employed to determine the fractional occupancy. In addition to the external field describing the gas-water interactions, the effect of a gravitational field was introduced. The results of the calculations show that although the gravitational potential term may be orders of magnitude smaller than the thermal kinetic energy of the gas species or the hydrogen-bond energy holding the clathrate lattice together, it can nevertheless influence the phase equilibrium of the clathrate hydrate system to some degree. The effect of the magnitudes of both the gravitational potential and the local gravitational field are considered too.     

Characterization of Salting-Out Processes during CO2-Clathrate Formation Using Raman Spectroscopy: Planetological Application

Clathrate hydrates are particular solids that planetologists study in detail because those solids may be present in several bodies of the solar system, such as Mars, comets, and the icy satellites. The solids are formed by solid H₂O, like common water ice, but adopt open structures with cavities containing gas molecules. Clathrate hydrates are usually stable at relatively low temperature and high pressure, which are the typical conditions present inside these planetary objects. Their interest for astrobiology is that they represent potential sources of liquid water and gases when they decompose. The present work is focused on the crystallization of clathrates in Europa's (icy satellite of Jupiter) interior conditions. We postulate that clathrate hydrates may play an important role in its crust mineralogy and that it can explain some features of the satellite's surface due to their formation/destabilization. An in situ kinetic study by Raman Spectroscopy of the clathrate formation from salty solutions was performed in our laboratory. The chemical composition that we used mimics those obtained from Europa's surface during the Galileo mission. An effect of the salting-out process in the solution was monitored through the clathrate formational path. Our results demonstrate that this process may have geological consequences on Europa and confirm the suitability of Raman spectroscopy for planetary detection of clathrate hydrates and other ices.     

甲烷水合物的固体核磁共振碳谱与激光拉曼光谱研究

甲烷水合物（CH₄·nH₂O）是主要由甲烷和水分子构成的冰状笼型化合物,在自然界储量巨大．固体核磁共振（NMR）波谱和激光拉曼光谱是在分子水平分析甲烷水合物的重要手段．该文利用低温固体核磁共振碳谱（¹³C NMR）对合成的甲烷水合物结构进行了研究,分别使用¹³C交叉极化（¹³C CP）和高功率质子去偶（¹H HPDEC）2种脉冲程序采集甲烷水合物的¹³C NMR谱图,结合实验结果分析及理论推导可知,使用¹H HPDEC方法得到的¹³C NMR谱图信号更强,更利于定量分析;甲烷气体与冰粉合成的甲烷水合物为I型,其大笼和小笼占有率分别为0.988和0.824,水合数为6.07;甲烷气体与SH2站位沉积物和冰粉合成的甲烷水合物也为I型,其大笼和小笼占有率分别为0.987和0.887,水合数为5.98;SH2站位沉积物使合成的甲烷水合物的小笼占有率提高、水合数降低、水合物饱和度提高．激光拉曼光谱结果证实了上述结果的准确性．该文为甲烷水合物测试提供了重要的方法参考．     

Novel results on structural investigations of natural minerals of clathrate hydrates

High-resolution powder X-ray diffraction measurements were carried out on gas hydrates in the angle-dispersive mode using a synchrotron radiation source. In contrast to the structural studies of laboratory-grown gas hydrates, this study has been performed on naturally grown clathrate hydrates obtained from the sea floor at different geographic locations. While the hydrate samples of the Cascadia Margin exhibit a preponderance of structure I, those from the Gulf of Mexico consist of mixed structures, namely structure II, structure I and structure H. Ice in structure Ih is inherently present in all the clathrate hydrate samples. PACS 61.10.Nz; 61.50.Ah; 61.66.Fn; 91.50.-r     

笼形水合物的科学与技术研究以及在能源和环境领域中的应用

能源与环境是人们越来越关注的问题,而笼形水合物同时在这两方面展示出了巨大的应用前景:(1) 在广阔的海底大陆架中蕴藏着大量的以甲烷为主体的笼型水合物——天然气可燃冰;(2) 将大气中日益增多的二氧化碳装入笼型水分子中并沉于海底以降减温室效应;(3) 氢气可燃冰作为一种清洁的能源载体具有很高的能量密度、可重复再生、可快速充氢;(4) 利用笼形水合物的形成过程,可以有效地分离气体、降低能耗、减轻污染.虽然笼形水合物在工程应用以及科学研究中有着重要的价值, 但其晶体结构, 成键机制, 温压相图, 热化学与力学稳定性, 合成与分解的反应动力学, 声学弹性, 与海底沉积的反应, 以及扩散和输运性质等都有待深入的研究.目前,科学家已经研发或者正在探讨将高压和低温装置与中子衍射技术以及激光光谱、热学测量、超声技术等有机的结合在一起, 从而能够进行一系列的实验研究来解决诸多的基本科学问题. 高压及低温环境下的中子衍射装置在氢气、甲烷以及二氧化碳气体水合物体系研究中显示了巨大的优势, 并且在确定水合物晶体结构, 气体分子在水合物笼格中的占据情况, 以及气体分子在笼格中的分布状态等方面取得了巨大的进展.     

水合物研制、结构与性能及其在能源环境中的应用

天然气水合物是与能源和环境相关的物质,可以进行甲烷等能源气体的存储和提取,也可以用于对二氧化碳等废气的封存.天然气水合物主要分为三种结构:sI, sII和sH,在本文中对其稳定性、水笼类型和大小以及可俘获气体进行了论述.中子衍射技术是研究水合物的重要手段之一,有着独特的优势.如中子的穿透性可以研究在高压状态下压力腔体内的大块样品;中子对于轻元素的敏感性可以很好地确定水合物当中的碳、氢、氧元素.通过中子衍射和非弹散射可以得到水合物中H/D原子的位置、各向异性振动因子、不同温度压力下的客体分子的水笼占据率、客体分子在水笼中的无序分布、原子核密度分布(通过最大熵方法);通过时间分辨中子,可以检测水合物形成及分解过程的热力学和动力学过程.而利用非弹中子可以得到气体分子平移和旋转振动模式以及分子的量子态转变.通过二氧化碳气体注入对天然气水合物的开采可以实现能源气体甲烷的开采和废气二氧化碳的水合物封存,在减小地质灾害和开采成本上有着独特的优势.     

Energy landscape of clathrate hydrates

Clathrate hydrates are nanoporous crystalline materials made of a network of hydrogen-bonded water molecules (forming host cages) that is stabilized by the presence of foreign (generally hydrophobic) guest molecules. The natural existence of large quantities of hydrocarbon hydrates in deep oceans and permafrost is certainly at the origin of numerous applications in the broad areas of energy and environmental sciences and technologies (e.g. gas storage). At a fundamental level, their nanostructuration confers on these materials specific properties (e.g. their ??glass-like?? thermal conductivity) for which the host-guest interactions play a key role. These interactions occur on broad timescale and thus require the use of multi-technique approach in which neutron scattering brings unvaluable information. This work reviews the dynamical properties of clathrate hydrates, ranging from intramolecular vibrations to Brownian relaxations; it illustrates the contribution of neutron scattering in the understanding of the underlying factors governing chemical-physics properties specific to these nanoporous systems.     

Theory of the magnetic susceptibility of the nitric oxide clathrate

The magnetic properties of the nitric oxide clathrate are studied with the hindered rotation model such as was successfully employed in the much simpler problem of the O₂ clathrate. Meyer's experimental measurements in the preceding paper show that the susceptibility at low temperatures is about 40 per cent higher than for the free molecule and is practically independent of temperature below about 20°K. This behavior is hard to explain since NO is an odd molecule, and hence in most crystalline potentials its susceptibility has a term in 1/T arising from the Kramers doublet. The best results are achieved by using a crystalline potential of trigonal symmetry, in accord with the crystallographic symmetry of the clathrates. By proper, though arbitrary, combination of matrix elements of the crystalline potential of the form Δm = ± 3 and Δm = 0, we obtain the requisite 40 per cent increase in the susceptibility, but we have not been entirely successful in suppressing the 1/T term, which we estimate to be about 6 per cent of the main term at 1°K. Conceivably the spin-lattice relaxation time is so long that this term would not be detected in Meyer's experiments made at 175 c/s. Further experimentation is highly desirable to guide and clarify the theory.     

Realization of 3D random Voronoi mosaic cell in hydrate phases of clathrate compositions

It is shown that the cages in water frameworks of clathrate structures of crystalline gas hydrates are characterized by averaged values of the numbers N _V (polyhedron vertices), N _E (polyhedron edges), N _F (polyhedron faces) and N _VF (vertices per face), close to the values calculated for the unit cell of a 3D random Voronoi mosaic.     

递推与迭代在量子力学近似计算中的应用

综述了非简并微扰论和简并微扰论的递推形式，一维阶梯位势透射系数的递推计算及变化法的迭代形式，在给定的精度下，利用上述方法可以得到与精确解完全一致的效果。     

Tensile properties of structural I clathrate hydrates: Role of guest–host hydrogen bonding ability

Clathrate hydrates (CHs) are one of the most promising molecular structures in applications of gas capture and storage, and gas separations. Fundamental knowledge of mechanical characteristics of CHs is of crucial importance for assessing gas storage and separations at cold conditions, as well as understanding their stability and formation mechanisms. Here, the tensile mechanical properties of structural I CHs encapsulating a variety of guest species (CH₄, NH₃, H₂S, CH₂O, CH₃OH, and CH₃SH) that have different abilities to form hydrogen (H-) bonds with water molecule are explored by classical molecular dynamics (MD) simulations. All investigated CHs are structurally stable clathrate structures. Basic mechanical properties of CHs including tensile limit and Young’s modulus are dominated by the H-bonding ability of host–guest molecules and the guest molecular polarity. CHs containing small CH₄, CH₂O and H₂S guest molecules that possess weak H-bonding ability are mechanically robust clathrate structures and mechanically destabilized via brittle failure on the (1 0 1) plane. However, those entrapping CH₃SH, CH₃OH, and NH₃ that have strong H-bonding ability are mechanically weak molecular structures and mechanically destabilized through ductile failure as a result of gradual global dissociation of clathrate cages.     

Efficient computation of the sound fields above a layered porous ground

An efficient computation of sound fields due to a monopole source placed above a porous layer is presented. This paper examines an improved scheme whereby the steepest descent path is selected for the numerical evaluation of the Sommerfeld integral. Along the steepest descent path, a standard Gaussian-Hermite quadrature can be used to calculate the sound fields effectively. The suggested numerical scheme is accurate at all frequencies except in the very near field. The proposed method is more numerically efficient than other computational schemes, especially at long ranges and high source frequencies.     

Ge掺杂n型Sn基Ⅷ型单晶笼合物的制备及热电传输特性

采用Sn自熔剂法制备了具有n型传导的Ⅷ型Ba₈Ga_16-xGe_xSn₃₀ （0 ≤ x ≤ 1.0）单晶笼合物,并对其结构和热电特性进行研究. 研究结果表明：Ge在单晶中的实际含量较少,随着掺杂量的增加样品的晶格常数略有减小,Ge掺杂后样品的载流子浓度较掺杂前低,迁移率增加;所有样品的Seebeck系数均为负值,且绝对值较未掺杂样品低,但Ge掺杂后样品的电导率提高了62%;x=0.5的样品在500 K附近取得最大ZT值1.25. 关键词： Ⅷ型笼合物 n型传导 热电性能     

Equalization of Erbium Gain Spectrum Using the Multiport Lattice Filter

We propose an accurate modeling technique of concatenated long-period fiber gratings. The proposed technique is then applied to the synthesis of long-period fiber gratings for the erbium gain equalization using the simulated annealing and the steepest descent minimization technique. We have obtained design parameters of the piecewise-uniform long-period fiber gratings that fit the inverted erbium gain spectrum of a commercially available erbium-doped fiber amplifier (EDFA) over the entire 1525 nm to 1570 nm range.     

高压下氢基高温超导体研究的新进展

富氢材料被认为是室温超导体的最佳候选体系,是物理学、材料科学等多学科的热点研究领域之一。理论和实验研究发现的新型共价氢化物H3S和笼状氢化物LaH10的超导转变温度(T_c)均超过200 K,进一步推动了对富氢化合物超导电性的探索。最近,通过高压实验合成的碳质硫氢化物在288 K的室温下实现了零电阻,让人们看到了室温超导的曙光。本文结合课题组在此领域的主要成果,介绍了3类典型富氢化合物的结构及超导特性,包括近期首次在层状氢化物中发现的具有类五角石墨烯结构的富氢超导体HfH₁₀,其超导转变温度高达213~234 K。     

显微激光拉曼光谱测定甲烷水合物的水合指数

甲烷水合物是由甲烷气体分子与水分子在低温高压下形成的一种笼型结构化合物,广泛存在于海底陆架区和陆地冻土区,被认为是一种潜在的能源资源。在水合物的晶格中,水分子在氢键的作用下形成大小不同的笼子,甲烷分子可分别进入大笼(51262)和小笼(512)中。在自行研制的实验装置上,分别合成了一系列不同体系下的甲烷水合物,包括十二烷基硫酸钠(SDS)水溶液-甲烷体系、冰粉-甲烷体系以及冰粉-不同粒度砂-甲烷体系。对这些甲烷水合物样品进行了激光拉曼光谱分析,测定了其水合指数,笼占有率等结构参数。结果表明,这些甲烷水合物都为Ⅰ型结构,其水合指数和笼占有率基本不受沉积物粒径大小的影响。在3种体系中生成的水合物,大笼中甲烷分子基本占满,占有率大于97%;小笼中甲烷分子占有率为80%～86%,测得的水合指数为6.05～6.15。     

Level set method for the inverse elliptic problem in nonlinear electromagnetism

An inverse problem of inhomogeneity identification inside a nonlinear magnetic material from the local measurements of the magnetic induction is investigated. The representation of the shape of the inhomogeneity and its evolution during an iterative reconstruction process is achieved by the level set method. The reconstruction is realized by the minimization of a cost function using the steepest descent method. The gradient directions are evaluated using the sensitivity equation and the adjoint variable method. Simulations has been performed showing the robustness of the algorithm and its ability to reconstruct single inhomogeneities, convex and non-convex, as well as multiple inhomogeneities.     

Clathrate hydrates: recent advances on CH4 and CO2 hydrates,and possible new frontiers

Gas hydrates continue to attract enormous attention throughout the energy industry, as both a hindrance in conventional production and a substantial unconventional resource. Scientists continue to be fascinated by the hydrates’ ability of sequestering large amounts of hydrophobic gases, unusual thermal transport properties and unique molecular structures. Technologically, clathrate hydrates promise advantages in applications as diverse as carbon sequestration and water desalination. The communities interested in hydrates span traditional academic disciplines, including earth science, physical chemistry and petroleum engineering. The studies on this field are equally diverse, including field expeditions to attempt the production of natural gas from hydrate deposits accumulated naturally on the seafloor, to lab-scale studies to exchange CO₂ for CH₄ in hydrates; from theoretical studies to understand the stability of hydrates depending on the guest molecules, to molecular simulations probing nucleation mechanisms. This review highlights a few fundamental questions, with focus on knowledge gaps representing some of the barriers that must be addressed to enable growth in the practical applications of hydrate technology, including natural gas storage, water desalination, CO₂ – CH₄ exchange in hydrate deposits and prevention of hydrate plugs in conventional energy transportation.