Superheating and induced melting at semiconductor interfaces

We present ab initio density-functional simulations of the state of several semiconductor surfaces at temperatures near the bulk melting temperatures. We find that the solid-liquid phase-transition temperature at the surface can be altered via a microscopic (single-monolayer) coating with a different lattice-matched semiconducting material. Our results show that a single-monolayer GaAs coating on a Ge(110) surface above the Ge melting temperature can dramatically reduce the diffusion coefficient of the germanium atoms, going so far as to prevent melting of the bulk layers on the 10 ps time scale. In contrast, a single-monolayer coating of Ge on a GaAs(110) surface introduces defects into the bulk and induces melting of the top layer of GaAs atoms 300 K below the GaAs melting point. To our knowledge, these calculations represent the first ab initio investigation of the superheating and induced melting phenomena.     

Peculiarities of gallium crystallization in confined geometry

The freezing and melting phase transitions for gallium embedded into a porous glass with a pore size of about 8 nm were studied using acoustic, NMR, and x-ray techniques. It was shown that the broadened solidification and melting transitions upon deep cooling up to complete freezing at 165 K were due to the formation of β-Ga within pores. The offset of confined β-Ga melting was lowered by about 21 K compared to the bulk β-Ga melting point. Both melting and freezing in pores were irreversible. The fulfillment of some special thermal conditions led to gallium crystallization into other modifications. The role of heterogeneous crystallization in freezing of confined gallium is discussed.     

Surface theory of melting

We demonstrate that melting is a surface initiated process. The surface becomes unstable before the bulk and the process of melting consists in the unstable surface that proceeds into the otherwise stable bulk.     

低孔隙度疏松锡的高压声速与相变

为研究微孔洞对锡的高压相变的影响, 对含亚微米孔洞的疏松锡(疏松度m=1.01)进行了冲击加载-卸载实验. 利用DPS(Doppler pins system)测得了31.8-66.1 GPa冲击压力下疏松锡/LiF界面粒子的速度剖面, 获得了各压力下的纵波声速与体波声速, 给出了该疏松锡的冲击熔化起始压力约为49.1 GPa, 获得了各压力下的剪切模量与泊松比. 结合密实锡与疏松锡的高压纵波声速、体波声速与剪切模量, 界定密实锡的冲击熔化压力在53.5-62.3 GPa之间, 高于疏松锡的值, 表明微孔洞明显降低了冲击熔化压力. 对密实锡准确的冲击熔化压力值还需要进一步的实验数据. 测试的固态压力范围内的声速数据没有明显奇异点, 表明疏松锡没有类似密实锡的固态bcc 相变发生.     

Phase diagrams of multilayer adsorbed methane

We propose multilayer phase diagrams based on recent observations of methane adsorbed on graphite and gold (111). Methane wets graphite at all temperatures observed, but it fully wets gold only above the bulk triple point. Possible reasons for this difference in behavior are discussed. The methane-graphite phase diagram also includes layer by layer condensation with critical points leading to a predicted bulk roughening temperature, and an extension of the bulk melting curve into the multilayer film region where it closes at a triple point with a compressed incommensurate first layer solid phase. Experimental evidence for these features is presented and discussed.     

Hot and solid gallium clusters: too small to melt

A novel multicollision induced dissociation scheme is employed to determine the energy content for mass-selected gallium cluster ions as a function of their temperature. Measurements were performed for Ga(+)(n) (n=17 39, and 40) over a 90-720 K temperature range. For Ga+39 and Ga+40 a broad maximum in the heat capacity-a signature of a melting transition for a small cluster-occurs at around 550 K. Thus small gallium clusters melt at substantially above the 302.9 K melting point of bulk gallium, in conflict with expectations that they will remain liquid to below 150 K. No melting transition is observed for Ga+17.     

Reversible self-propelled droplet movement: a new driving mechanism

Reversible self-propelled movements of droplets of long chain alkanes are presented and analyzed. Slightly above the bulk melting temperature, solid alkane multilayers melt into droplets which move in a self-avoiding, random path. While moving, the droplets consume the solid alkane and leave behind a widening groove. At temperatures slightly below bulk melting this process can be reversed. Now the droplets move backwards leaving behind a narrowing solid trail which is nourished by the shrinking droplet. The speeds of the droplets are analyzed quantitatively and the melting enthalpy is identified as energetic source for their movement.     

Analysis of a Lennard-Jones fcc structure melting to the corresponding frozen liquid: Differences between the bulk and the surface

We computed a Lennard-Jones frozen liquid with a free surface using classical molecular dynamics. The structure factor curves on the free surface of this sample were calculated for different depths knowing that we have periodic boundary conditions on the other parts of the sample. The resulting structure factor curves show an horizontal shift of their first peak depending on how deep in the sample the curves are computed. We analyze our resulting curves in the light of spatial correlation functions during melting. The conclusion is that the differences between bulk and surface are quite small during melting and that at the end of melting, only the very surface happens to be less dense than the bulk. This result is intrinsic to the shape of the Lennard-Jones potential and does not depend on any other parameter.     

Solid clusters above the bulk melting point

The fact that the melting points of nanoparticles are always lower than those of the corresponding bulk material is a paradigm supported by extensive experimental data for a large number of systems and by numerous calculations. Here we demonstrate that tin cluster ions with 10-30 atoms remain solid at approximately 50 K above the melting point of bulk tin. This behavior is possibly related to the fact that the structure of the clusters is completely different from that of the bulk element.     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.     

Defects induced melting in alkali halides

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature.     

Effect of voids and pressure on melting of nano-particulate and bulk aluminum

Molecular dynamics simulations are performed using isobaric–isoenthalpic (NPH) ensembles to study the effect of internal defects in the form of voids on the melting of bulk and nano-particulate aluminum in the size range of 2–9 nm. The main objectives are to determine the critical interfacial area required to overcome the free energy barrier for the thermodynamic phase transition, and to explore the underlying mechanisms for defect-nucleated melting. The inter-atomic interactions are captured using the Glue potential, which has been validated against the melting temperature and elastic constants for bulk aluminum. A combination of structural and thermodynamic parameters, such as the potential energy, Lindemann index, translational-order parameter, and radial-distribution functions, are employed to characterize the melting process. The study considers a variety of void shapes and sizes, and results are compared with perfect crystals. For nano aluminum particles smaller than 9 nm, the melting temperature is size dependent. The presence of voids does not impact the melting properties due to the dominancy of nucleation at the surface, unless the void size exceeds a critical value beyond which lattice collapse occurs. The critical void size depends on the particle dimension. The effect of pressure on the particulate melting is found to be insignificant in the range of 1–300 atm. The melting behavior of bulk aluminum is also examined as a benchmark. The critical interfacial area required for the solid–liquid phase transition is obtained as a function of the number of atoms considered in the simulation. Imperfections such as voids reduce the melting point. The ratio between the structural and thermodynamic melting points is 1.32. This value is comparable to the ratio of 1.23 for metals like copper.     

Manifestation of the heterogeneous mechanism upon melting of low-dimensional systems

A melting process that is always heterogeneous in semi-infinite systems having a surface has been analyzed. It has been shown in terms of the classical thermodynamics that, in real one-component systems, a liquid layer on the solid-phase surface is formed at temperatures lower than the reference melting temperature of the bulk material at which the system is completely melted. Depending on the temperature, a liquid layer of particular thickness on the surface is in equilibrium with the other crystalline phase. The heterogeneous melting is shown to influence a number of processes and mechanisms, such as the dispersion of a thin film into droplets, the mechanism of vapor-liquid-solid epitaxy, the mechanism of layer-by-layer crystal growth, and the mechanism of growth of carbon nanotubes.     

MAS NMR studies of nanoporous matrices filled with sodium nitrite

The sodium nitrite NaNO2 incorporated into MCM-41 molecular sieves with pore sizes of 20, 26, and 37 Å has been investigated by 23Na magic-angle spinning NMR spectroscopy. It has been demonstrated that the structure of the crystalline phase of the nitrite in a confined geometry is similar to the structure of bulk nitrite NaNO2. The direct proof of the diffuse melting of sodium nitrite in the pores has been obtained. The NMR signal of the molten sodium nitrite phase has been observed at temperatures close to the completion of the melting.     

NMR of mercury in porous carbon and silica gel

The temperature dependences of the integrated intensity and of the Knight shift of ¹⁹⁹Hg NMR signals are measured for liquid and solid mercury introduced into porous carbon and silica gel. A decrease in the temperature of completion of crystallization and a small temperature hysteresis (from 4 to 9 K) between melting and crystallization are observed. The melting temperature of mercury in pores coincides with that in the bulk. The ¹⁹⁹Hg NMR signal from crystalline mercury under the condition of restricted geometry is observed for the first time. It is established that the Knight shift for liquid and crystalline mercury in pores is smaller than in the bulk.     

Extreme superheating and supercooling of encapsulated metals in fullerenelike shells

Nanometer-sized tin and lead crystals exhibit drastically altered melting and solidification behavior when encapsulated in fullerenelike graphitic shells. The melting transitions of encapsulated Sn and Pb nanocrystals are shown in an in situ electron microscopy study to occur at unexpectedly high temperatures, significantly higher than the melting point of the corresponding bulk materials. Atomistic simulations are used to show that the driving force for superheating is a pressure buildup of up to 3 GPa, that prevails inside graphitic shells under electron irradiation.     

冲击压缩下单晶LiF高压声速及卸载路径研究

 采用二级轻气炮加载手段，在5~79.1 GPa冲击压力范围内，利用加窗VISAR技术对氟化锂（LiF）单晶样品进行了研究，得到了不同冲击压力下的纵波声速、体积声速以及卸载路径。实验结果显示：单晶LiF沿冲击绝热线的Lagrange纵波声速随粒子速度呈线性变化；在5~79.1 GPa冲击压力下，单晶LiF并未发生熔化及其它相变；卸载路径及速度剖面弹塑性明显。     

Studies of pulsed laser melting and rapid solidification using amorphous silicon

Pulsed laser melting of ion implantation-amorphized silicon layers, and the subsequent solidification of undercooled liquid silicon, have been studied experimentally and theoretically. Measurements of the time of the onset of melting of amorphous silicon layers, during an incident laser pulse, have been combined with measurements of the duration of melting, and with modified melting model calculations to demonstrate that the thermal conductivity, K_a, of amorphous silicon is very low (K_a0.02 W/cm K). K_a is also found to be the dominant parameter determining the dynamical response of amorphous silicon to pulsed laser radiation; the latent heat of fusion and melting temperature of amorphous silicon are relatively unimportant. Transmission electron microscopy indicates that bulk (volume) nucleation occurs directly from the highly undercooled liquid silicon that can be prepared by pulsed laser melting of amorphous silicon layers at low laser energy densities. A modified thermal melting model has been constructed to simulate this effect and is presented. Nucleation of crystalline silicon apparently occurs at a nucleation temperature, T_n, that is higher than the temperature, T_a, of the liquid-to-amorphous phase transition. The model calculations demonstrate that the release of latent heat by bulk nucleation occurring during the melt-in process is essential to obtaining agreement with experimentally observed depths of melting. These calculations also show that this release of latent heat accompanying bulk nucleation can result in the existence of buried molten layers of silicon in the interior of the sample after the surface has solidified. It is pointed out that the occurrence of bulk nucleation implies that the liquid-to-amorphous phase transition (produced using picosecond or ultraviolet nanosecond laser pulses) cannot be explained by purely thermodynamic considerations.     

Surface transport in premelted films with application to grain-boundary grooving

We present a new model of surface transport in premelted films that is applicable to a wide range of materials close to their melting points. We illustrate its use by applying it to the evolution of a grain-boundary groove in a high vapor pressure material and show that Mullins's classical equation describing transport driven by gradients in surface curvature is reproduced asymptotically. The microscopic contact angle at the groove root is found to be modified over a thin boundary layer, and the apparent contact angle is determined. An explicit transport coefficient is derived that governs the evolution rate of systems controlled by surface transport through premelted films. The transport coefficient is found to depend on temperature and diverges as the bulk melting temperature is approached.     

The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations

The melting mechanism for Pd_0.25Ni_0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd–Ni NP including core-related solid-like regions melts at once.