Water-driven structure transformation in nanoparticles at room temperature

The thermodynamic behaviour of small particles differs from that of the bulk material by the free energy term gammaA--the product of the surface (or interfacial) free energy and the surface (or interfacial) area. When the surfaces of polymorphs of the same material possess different interfacial free energies, a change in phase stability can occur with decreasing particle size. Here we describe a nanoparticle system that undergoes structural changes in response to changes in the surface environment rather than particle size. ZnS nanoparticles (average diameter 3 nm) were synthesized in methanol and found to exhibit a reversible structural transformation accompanying methanol desorption, indicating that the particles readily adopt minimum energy structural configurations. The binding of water to the as-formed particles at room temperature leads to a dramatic structural modification, significantly reducing distortions of the surface and interior to generate a structure close to that of sphalerite (tetrahedrally coordinated cubic ZnS). These findings suggest a route for post-synthesis control of nanoparticle structure and the potential use of the nanoparticle structural state as an environmental sensor. Furthermore, the results imply that the structure and reactivity of nanoparticles at planetary surfaces, in interplanetary dust and in the biosphere, will depend on both particle size and the nature of the surrounding molecules.     

Formation of ordered ice nanotubes inside carbon nanotubes

Following their discovery, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary, mould or template in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions. Particularly intriguing is the conjecture that matter within the narrow confines of a carbon nanotube might exhibit a solid-liquid critical point beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid-liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.     

Vacancies in solids and the stability of surface morphology

Determining how thermal vacancies are created and destroyed in solids is crucial for understanding many of their physical properties, such as solid-state diffusion. Surfaces are known to be good sources and sinks for bulk vacancies, but directly determining where the exchange between the surface and the bulk occurs is difficult. Here we show that vacancy generation (and annihilation) on the (110) surface of an ordered nickel-aluminium intermetallic alloy does not occur over the entire surface, but only near atomic step edges. This has been determined by oscillating the sample's temperature and observing in real time the response of the surface structure as a function of frequency (a version of Angstr?m's method of measuring thermal conductivity) using low-energy electron microscopy. Although the surface-exchange process is slow compared with bulk diffusion, the vacancy-generation rate nevertheless controls the dynamics of the alloy surface morphology. These observations, demonstrating that surface smoothing can occur through bulk vacancy transport rather than surface diffusion, should have important implications for the stability of fabricated nanoscale structures.     

Electrochemically induced annealing of stainless-steel surfaces

Modification of the surface properties of metals without affecting their bulk properties is of technological interest in demanding applications where surface stability and hardness are important. When austenitic stainless steel is heavily plastically deformed by grinding or rolling, a martensitic phase transformation occurs that causes significant changes in the bulk and surface mechanical properties of the alloy. This martensitic phase can also be generated in stainless-steel surfaces by cathodic charging, as a consequence of lattice strain generated by absorbed hydrogen. Heat treatment of the steel to temperatures of several hundred degrees can result in loss of the martensitic structure, but this alters the bulk properties of the alloy. Here we show that martensitic structures in stainless steel can be removed by appropriate electrochemical treatment in aqueous solutions at much lower temperature than conventional annealing treatments. This electrochemically induced annealing process allows the hardness of cold-worked stainless steels to be maintained, while eliminating the brittle martensitic phase from the surface. Using this approach, we are able to anneal the surface and near-surface regions of specimens that contain rolling-induced martensite throughout their bulk, as well as those containing surface martensite induced by grinding. Although the origin of the electrochemical annealing process still needs further clarification, we expect that this treatment will lead to further development in enhancing the surface properties of metals.     

Metastable garnet in oceanic crust at the top of the lower mantle

As oceanic tectonic plates descend into the Earth's lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite-the 'post-garnet' and 'post-spinel' transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at approximately 800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.     

基于 NiTi 温敏形状记忆纳米结构调控的润湿性研究

NiTi 合金是常见的形状记忆合金,有着良好的形状记忆效应、超弹性、耐腐蚀性和生物相容性,在生物医疗、 航空航天和微机电等领域有着广泛的应用。 目的 研究 NiTi 合金表面不同纳米级结构的润湿性能,改善 NiTi 合金 在工作环境下的磨损情况,提高 NiTi 合金的使用寿命。 方法 使用分子动力学方法模拟周期性边界条件下 NiTi 合 金的形状记忆效应,研究在不同温度下 NiTi 合金原子的微观结构演化;同时基于 NiTi 合金温度响应下不同的微观 结构,建立液滴静态接触角仿真,研究 NiTi 合金表面在不同微观结构下的润湿性能。 结果 发现 NiTi 合金在不同温 度响应下发生相变,从而使 NiTi 合金的表面原子排列发生改变,展示了温度诱发 NiTi 合金的相变和逆相变行为以 及在原子尺度下的微观结构演化,再以不同温度响应下发生相变和逆相变的 NiTi 合金表面作为基底,发现其表面 的润湿性也发生了改变和恢复。 结论 NiTi 合金随温度响应发生相变,微观结构发生改变,其表面的润湿性能也发 生改变;而且 NiTi 合金随温度逆相变,微观结构恢复到初始状态,其表面润湿性也能随之恢复到初始状态;NiTi 合 金在相变过程中的奥氏体和马氏体含量会影响 NiTi 合金表面润湿性能,因此能够通过温度进行 NiTi 合金表面在 微观结构下润湿性的自适应调控。     

Relationship between structural order and the anomalies of liquid water

In contrast to crystalline solids--for which a precise framework exists for describing structure--quantifying structural order in liquids and glasses has proved more difficult because even though such systems possess short-range order, they lack long-range crystalline order. Some progress has been made using model systems of hard spheres, but it remains difficult to describe accurately liquids such as water, where directional attractions (hydrogen bonds) combine with short-range repulsions to determine the relative orientation of neighbouring molecules as well as their instantaneous separation. This difficulty is particularly relevant when discussing the anomalous kinetic and thermodynamic properties of water, which have long been interpreted qualitatively in terms of underlying structural causes. Here we attempt to gain a quantitative understanding of these structure-property relationships through the study of translational and orientational order in a models of water. Using molecular dynamics simulations, we identify a structurally anomalous region--bounded by loci of maximum orientational order (at low densities) and minimum translational order (at high densities)--in which order decreases on compression, and where orientational and translational order are strongly coupled. This region encloses the entire range of temperatures and densities for which the anomalous diffusivity and thermal expansion coefficient of water are observed, and enables us to quantify the degree of structural order needed for these anomalies to occur. We also find that these structural, kinetic and thermodynamic anomalies constitute a cascade: they occur consecutively as the degree of order is increased.     

包覆状态下的大块Pb晶体的动力学过热

研究了非自由表面大块晶体Pb的过垫热，主要介绍通过有效的方法制得试样，然后，用DSC-7准确测量大块晶体Pb在不同加热速度条件下的过热度，从而得出大块晶体Pb能够产生动力学过热。     

Dislocation-driven surface dynamics on solids

Dislocations are line defects that bound plastically deformed regions in crystalline solids. Dislocations terminating on the surface of materials can strongly influence nanostructural and interfacial stability, mechanical properties, chemical reactions, transport phenomena, and other surface processes. While most theoretical and experimental studies have focused on dislocation motion in bulk solids under applied stress and step formation due to dislocations at surfaces during crystal growth, very little is known about the effects of dislocations on surface dynamics and morphological evolution. Here we investigate the near-equilibrium dynamics of surface-terminated dislocations using low-energy electron microscopy. We observe, in real time, the thermally driven nucleation and shape-preserving growth of spiral steps rotating at constant temperature-dependent angular velocities around cores of dislocations terminating on the (111) surface of TiN in the absence of applied external stress or net mass change. We attribute this phenomenon to point-defect migration from the bulk to the surface along dislocation lines. Our results demonstrate that dislocation-mediated surface roughening can occur even in the absence of deposition or evaporation, and provide fundamental insights into mechanisms controlling nanostructural stability.     

Water conduction through the hydrophobic channel of a carbon nanotube.

Confinement of matter on the nanometre scale can induce phase transitions not seen in bulk systems. In the case of water, so-called drying transitions occur on this scale as a result of strong hydrogen-bonding between water molecules, which can cause the liquid to recede from nonpolar surfaces to form a vapour layer separating the bulk phase from the surface. Here we report molecular dynamics simulations showing spontaneous and continuous filling of a nonpolar carbon nanotube with a one-dimensionally ordered chain of water molecules. Although the molecules forming the chain are in chemical and thermal equilibrium with the surrounding bath, we observe pulse-like transmission of water through the nanotube. These transmission bursts result from the tight hydrogen-bonding network inside the tube, which ensures that density fluctuations in the surrounding bath lead to concerted and rapid motion along the tube axis. We also find that a minute reduction in the attraction between the tube wall and water dramatically affects pore hydration, leading to sharp, two-state transitions between empty and filled states on a nanosecond timescale. These observations suggest that carbon nanotubes, with their rigid nonpolar structures, might be exploited as unique molecular channels for water and protons, with the channel occupancy and conductivity tunable by changes in the local channel polarity and solvent conditions.     

Si对高耐磨模具钢9Cr6W3Mo2V2过冷奥氏体转变动力学的影响

测定了9Cr6W3Mo2V2钢的TTT和CCT曲线,研究了Si对该钢过冷奥氏体转变动力学的影响.试验表明,Si有促进珠光体转变的趋势,而对贝氏体转变,则在等温和连续冷却条件下,其影响不同.从动力学和热力学两个方面对这些现象进行了解释.     

Developing rigid constraint for the estimation of pose and structure from a single image

Pose and structure estimation from a single image is a fundamental problem in machine vision and multiple sensor fusion and integration. In this paper we propose using rigid constraints described in different coordinate frames to iteratively estimate structural and camera pose parameters. Using geometric properties of reflected correspondences we put forward a new concept, the reflected pole of a rigid transformation. The reflected pole represents a general analysis of transformations that can be applied to both 2D and 3D transformations. We demonstrate how the concept is applied to calibration by proposing an iterative method to estimate the structural parameters of objects. The method is based on a coarse-to-fine strategy in which initial estimation is obtained through a classical linear algorithm which is then refined by iteration. For a comparative study of performance, we also implemented an extended motion estimation algorithm (from 2D-2D to 3D-2D case) based on epipolar geometry.     

Unusual phase transitions in ferroelectric nanodisks and nanorods

Bulk ferroelectrics undergo structural phase transformations at low temperatures, giving multi-stable (that is, multiple-minimum) degenerate states with spontaneous polarization. Accessing these states by applying, and varying the direction of, an external electric field is a key principle for the operation of devices such as non-volatile ferroelectric random access memories (NFERAMs). Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAMs 10,000-fold. But this anticipated benefit hinges on whether phase transitions and multi-stable states still exist in low-dimensional structures. Previous studies have suggested that phase transitions are impossible in one-dimensional systems, and become increasingly less likely as dimensionality further decreases. Here we perform ab initio studies of ferroelectric nanoscale disks and rods of technologically important Pb(Zr,Ti)O3 solid solutions, and demonstrate the existence of previously unknown phase transitions in zero-dimensional ferroelectric nanoparticles. The minimum diameter of the disks that display low-temperature structural bistability is determined to be 3.2 nm, enabling an ultimate NFERAM density of 60 x 10(12) bits per square inch-that is, five orders of magnitude larger than those currently available. Our results suggest an innovative use of ferroelectric nanostructures for data storage, and are of fundamental value for the theory of phase transition in systems of low dimensionality.     

Transparent nematic phase in a liquid-crystal-based microemulsion

Complex fluids are usually produced by mixing together several distinct components, the interactions between which can give rise to unusual optical and rheological properties of the system as a whole. For example, the properties of microemulsions (composed of water, oil and surfactants) are determined by the microscopic structural organization of the fluid that occurs owing to phase separation of the component elements. Here we investigate the effect of introducing an additional organizing factor into such a fluid system, by replacing the oil component of a conventional water-in-oil microemulsion with an intrinsically anisotropic fluid--a nematic liquid crystal. As with the conventional case, the fluid phase-separates into an emulsion of water microdroplets (stabilized by the surfactant as inverse micelles) dispersed in the 'oil' phase. But the properties are further influenced by a significant directional coupling between the liquid-crystal molecules and the surfactant tails that emerge (essentially radially) from the micelles. The result is a modified bulk-liquid crystal that is an ordered nematic at the mesoscopic level, but which does not exhibit the strong light scattering generally associated with bulk nematic order: the bulk material here is essentially isotropic and thus transparent.     

A general process for in situ formation of functional surface layers on ceramics

Ceramics are often prepared with surface layers of different composition from the bulk, in order to impart a specific functionality to the surface or to act as a protective layer for the bulk material. Here we describe a general process by which functional surface layers with a nanometre-scale compositional gradient can be readily formed during the production of bulk ceramic components. The basis of our approach is to incorporate selected low-molecular-mass additives into either the precursor polymer from which the ceramic forms, or the binder polymer used to prepare bulk components from ceramic powders. Thermal treatment of the resulting bodies leads to controlled phase separation ('bleed out') of the additives, analogous to the normally undesirable outward loss of low-molecular-mass components from some plastics; subsequent calcination stabilizes the compositionally changed surface region, generating a functional surface layer. This approach is applicable to a wide range of materials and morphologies, and should find use in catalysts, composites and environmental barrier coatings.     

Electronic reconstruction at an interface between a Mott insulator and a band insulator

Surface science is an important and well-established branch of materials science involving the study of changes in material properties near a surface or interface. A fundamental issue has been atomic reconstruction: how the surface lattice symmetry differs from the bulk. 'Correlated-electron compounds' are materials in which strong electron-electron and electron-lattice interactions produce new electronic phases, including interaction-induced (Mott) insulators, many forms of spin, charge and orbital ordering, and (presumably) high-transition-temperature superconductivity. Here we propose that the fundamental issue for the new field of correlated-electron surface/interface science is 'electronic reconstruction': how does the surface/interface electronic phase differ from that in the bulk? As a step towards a general understanding of such phenomena, we present a theoretical study of an interface between a strongly correlated Mott insulator and a band insulator. We find dramatic interface-induced electronic reconstructions: in wide parameter ranges, the near-interface region is metallic and ferromagnetic, whereas the bulk phase on either side is insulating and antiferromagnetic. Extending the analysis to a wider range of interfaces and surfaces is a fundamental scientific challenge and may lead to new applications for correlated electron materials.     

铜固液两相共存系统的分子动力学模拟

基于Mishin势函数,用分子动力学方法模拟了金属铜的固液两相共存系统.在熔化过程中固液界面逐层推进,由collision-limited理论确定到零压下（001）表面熔点为1392.9±9.2 K,动力学系数为82.3 cm·s-1·K-1.     

具有次近邻相互作用的面心立方格子的Ising(自旋S=1)反铁磁薄膜在外场下的基态自旋结构

将文献[5]的方法推广到反铁磁薄膜,变S=1的Ising体系成一个粒子数不守恒的费米体系,严格得到fcc格子的反铁磁薄膜在外场下的基态,并给出了其交换积分在一定范围内的零温相图。从中可见其自旋状态与大块铁磁体的差别,表面自旋排列存在与内部不同的方式。这是由于系统不再具有垂直于表面方向的平移对称性而产生的非均匀效应。     

狭义相对论与运动速度上限

论证了对于具有静止参考系特性的粒子, 以绝对时空观为基础的伽利略变换是唯一允许粒子运动速度为任意大的线性时空变换, 因而对于任何非伽利略型的线性时空变换, 必然要求粒子运动速度存在上限。通过引进运动速度上限vm, 针对静质量不为零的粒子, 给出一种无需利用光速不变假设和具体的时空变换关系, 而完全在动力学范围内得到相对论质速关系和质能关系的新推导, 并进一步确定了相应线性时空变换的广义洛伦兹变换公式。质速关系和质能关系以及广义洛伦兹变换的新形式已不再直接与光速相关, 而是由更一般的普适速度上限vm替代了光速c。 对于经典伽利略变换则可作为广义洛伦兹变换在vm→∞时的一种极限特例, 而速度上限vm的具体取值可由实验测量来确定。