Nonequilibrium melting and crystallization of a model Lennard-Jones system

Nonequilibrium melting and crystallization of a model Lennard-Jones system were investigated with molecular dynamics simulations to quantify the maximum superheating/supercooling at fixed pressure, and over-pressurization/over-depressurization at fixed temperature. The temperature and pressure hystereses were found to be equivalent with regard to the Gibbs free energy barrier for nucleation of liquid or solid. These results place upper bounds on hysteretic effects of solidification and melting in high heating- and strain-rate experiments such as shock wave loading and release. The authors also demonstrate that the equilibrium melting temperature at a given pressure can be obtained directly from temperatures at the maximum superheating and supercooling on the temperature hysteresis; this approach, called the hysteresis method, is a conceptually simple and computationally inexpensive alternative to solid-liquid coexistence simulation and thermodynamic integration methods, and should be regarded as a general method. We also found that the extent of maximum superheating/supercooling is weakly pressure dependent, and the solid-liquid interfacial energy increases with pressure. The Lindemann fractional root-mean-squared displacement of solid and liquid at equilibrium and extreme metastable states is quantified, and is predicted to remain constant (0.14) at high pressures for solid at the equilibrium melting temperature.     

On asymmetry between superheating and supercooling in solid-liquid transitions: Landau models

We investigated the asymmetry between superheating and supercooling in solid-liquid transitions using two Landau-type models for the first-order phase transitions, the Landau-Devonshire and Landau-de Gennes models. The Landau models reproduced the asymmetric behavior described by the classical nucleation theory.     

Supercooled states in the benzene-naphthalene system

Precrystallization supercooled states in the benzene-naphthalene system were studied by thermal analysis methods. A sharp change in the character of the crystallization of benzene and naphthalene, from quasi-equilibrium without supercooling to nonequilibrium-explosive with supercooling depending on liquid phase superheating with respect to the temperature of fusion was observed. Such a transition occurred monotonically for alloys. The phase diagram of the benzene-naphthalene system including metastable regions with supercooling under normal crystallization conditions was constructed. The rules governing Gibbs energy changes and mean supercoolings in crystallization depending on the concentrations of the components were determined. The supercooling values were also used to calculate such nucleation parameters as the size of critical nuclei, work of their formation, and number of unit cells and molecules in them. The molecular structure of benzene and naphthalene crystals and the structural changes of these substances during fusion that influenced the character of their subsequent crystallization were described.     

Comparison between superheating of crystals in melting and supercooling of melts in crystallization

The influence of surface tension forces and the dislocation structure of crystals on the formation of new phase nuclei in melting and crystallization is considered. It is shown that the main cause of the dramatic difference between superheating of the solid phase in melting and supercooling of the liquid phase in crystallization is surface defects.     

Solid-liquid transitions of sodium chloride at high pressures

We investigate solid-liquid transitions in NaCl at high pressures using molecular dynamics simulations, including the melting curve and superheating/supercooling as well as solid-solid and liquid-liquid transitions. The first-order B1-B2 (NaCl-CsCl type) transition in solid is observed at high pressures besides continuous liquid structure transitions, which are largely analogous to the B1-B2 transition in solid. The equilibrium melting temperatures (T(m)) up to megabar pressure are obtained from the solid-liquid coexistence technique and the superheating-supercooling hysteresis method. Lindemann's vibrational and Born's mechanical instabilities are found upon melting. The Lindemann frequency is calculated from the vibrational density of states. The Lindemann parameter (fractional root-mean-squared displacement) increases with pressure and approaches a constant asymptotically, similar to the Lennard-Jones system. However, the Lindemann melting relation holds for both B1 and B2 phases to high accuracy as for the Lennard-Jonesium. The B1 and B2 NaCl solids can be superheated by 0.18T(m) and 0.24T(m), and the NaCl liquid, supercooled by 0.22T(m) and 0.32T(m), respectively, at heating or cooling rates of 1 K/s and 1 K/ps. The amount of maximum superheating or supercooling and its weak pressure dependence observed for NaCl are in accord with experiments on alkali halides and with simulations on the Lennard-Jones system and Al.     

Epitaxial clusters in single crystal hosts

It is a general phenomenon for single crystal hosts that insoluble materials precipitate in the form of epitaxially aligned clusters. Such precipitates (or inclusions) are therefore coherently aligned and offer a unique model system for X-ray diffraction studies. In this paper, the general phenomenon of epitaxial clusters is discussed, and specific X-ray experiments on krypton as well as lead clusters in aluminum are presented. A hypothesis for the origin of epitaxial alignment is described and interfacial roughening is discussed. Also discussed is superheating and supercooling at the melting transition.     

The Influence of the Thermal Behaviour of AgGaS2 on the Crystal Growth Process

Differential thermal analysis has been carried out on AgGaS₂ samples in order to investigate the relationship between the superheating of the melt and the supercooling behaviour of the material leading to an improvement of crystal growth conditions. The knowledge gained will be correlated to the crystal growth experiments which had been carried out by using the gradient freezing method.This revised version was published online in November 2005 with corrections to the Cover Date.     

Periodically Modulated Driving Force Applied with TMDSC to the Crystallization and Melting Kinetics of Ice Crystals Confined in a Porous Silica Gel

The application of a periodically modulated driving force has been examined in the melting and crystallization kinetics of ice crystals confined in a porous media. The kinetic response of transformation gives the real and imaginary parts of the ‘apparent’ heat capacity obtained with a temperature modulated differential scanning calorimetry (TMDSC). Based on a modelling of the kinetics, the detailed examination of the frequency dispersion and its dependence on underlying heating/cooling rate enables us to evaluate the transformation rate and the dependence of the rate coefficient on the driving force, i.e. the degree of supercooling or superheating. The experimental results indicate that the transformation processes are limited by heat diffusion from the growth interface of each crystallite to surroundings. This revised version was published online in July 2006 with corrections to the Cover Date.     

镧化合物改性超细碳酸钙对玻纤增强PET结晶性能和力学性能的影响

采用双螺杆挤出制备了玻璃纤维增强PET(FRPET)/稀土镧化合物改性超细碳酸钙(UCaCO3-La3 )复合材料,将UCaCO3-La3 对FRPET性能的影响与有机钠盐类成核剂NA做了对比.DSC分析结果表明,各成核剂均使FRPET的结晶温度和结晶度提高,结晶峰半峰宽变窄,熔融峰半峰宽变宽,且镧化合物对FRPET的双熔融峰有引发作用.根据过冷温度和过热温度,成核剂的结晶成核能力大小顺序为UCaCO3-5La3 >UCaCO3-10La3 >NA>UCaCO3-1La3 .UCaCO3-La3 的加入明显提高了FRPET的力学性能,并对FRPET综合性能的改善具有显著效果.     

金属银在高升温速率下的熔化和过热行为

采用分子动力学方法和QSC(quantum Sutton-Chen)力场研究了升温速率对金属银的熔化和过热行为的影响.模拟中考虑了缺陷和表面对熔化和过热行为的影响.研究结果表明,升温速率对金属银的熔化和过热行为有很大影响,随着升温速率的升高,金属银的熔点有所升高.高的升温速率会导致金属银体系内部无序化程度增加,降低了熔化相变的能垒.升温速率导致的银完美晶体的过热极限大约为1450 K.     

升温速率对金属铅的熔化和过热行为的影响

采用分子动力学方法和QSC(quantum Sutton-Chen)力场研究了升温速率对金属铅的熔化和过热行为的影响.模拟中考虑了缺陷和表面对熔化和过热行为的作用.结果表明，升温速率对金属铅的熔化和过热行为影响很大，随着升温速率的升高，金属铅的熔点有所升高.快的升温速率会导致金属铅体系内部无序化程度增加，进而使体系能量增加，降低了熔化相变的能垒.升温速率导致的金属铅的过热极限大约为780 K.     

Quasi-isochoric superheating of nanoparticles embedded in rigid matrixes

A thermodynamic model for pressure-induced quasi-isochoric superheating of nanoparticles embedded in rigid matrixes was established quantitatively by introducing the size dependence of melting enthalpy. The accuracy of the developed model was verified with the reported experimental data of Sn and Pb nanoparticles encapsulated in fullerene-like graphitic shells (FGS) as well as Ge nanoparticles embedded in SiO2. The mechanism behind the smaller superheating for Al nanoparticles embedded in Al2O3 was also studied. It was found that the extent of the superheating is determined by the pressure, which is in turn related to the confinement effect and to the size of the nanoparticles. Through the knowledge obtained in this study, it can be concluded that the extreme superheating of nanoparticles can be achieved on the proviso that they are encased in a sufficiently rigid matrix, while the size of nanoparticles is small enough.     

Superheating Limit of Boiling n-Hexane Spinodal Decomposition Versus Nucleation Process

Two predicative theories for superheating limits of a boiling liquid are considered in this work. In the nucleation picture, classical homogeneous nucleation theory is used to calculate superheating temperature at various pressures. In spinodal decomposition picture, stability limits are taken as the superheating temperature. A perturbed-hard chain equation of state was developed and used for the purpose of calculating mechanical stability limit. Calculations are done for the case of normal hexane at different pressures and compared with experimental results. Classical nucleation theory gives good prediction at negative and smaller pressures. While near critical pressure, spinodal picture seems to be more accurate.     

Molecular dynamics simulations of melting and the glass transition of nitromethane

Molecular dynamics simulations have been used to investigate the thermodynamic melting point of the crystalline nitromethane, the melting mechanism of superheated crystalline nitromethane, and the physical properties of crystalline and glassy nitromethane. The maximum superheating and glass transition temperatures of nitromethane are calculated to be 316 and 160 K, respectively, for heating and cooling rates of 8.9 x 10(9) Ks. Using the hysteresis method [Luo et al., J. Chem. Phys. 120, 11640 (2004)] and by taking the glass transition temperature as the supercooling temperature, we calculate a value of 251.1 K for the thermodynamic melting point, which is in excellent agreement with the two-phase result [Agrawal et al., J. Chem. Phys. 119, 9617 (2003)] of 255.5 K and measured value of 244.73 K. In the melting process, the nitromethane molecules begin to rotate about their lattice positions in the crystal, followed by translational freedom of the molecules. A nucleation mechanism for the melting is illustrated by the distribution of the local translational order parameter. The critical values of the Lindemann index for the C and N atoms immediately prior to melting (the Lindemann criterion) are found to be around 0.155 at 1 atm. The intramolecular motions and molecular structure of nitromethane undergo no abrupt changes upon melting, indicating that the intramolecular degrees of freedom have little effect on the melting. The thermal expansion coefficient and bulk modulus are predicted to be about two or three times larger in crystalline nitromethane than in glassy nitromethane. The vibrational density of states is almost identical in both phases.     

Supercooling of aqueous NaCl and KCl solutions under acoustic levitation

The supercooling capability of aqueous NaCl and KCl solutions is investigated at containerless state by using acoustic levitation method. The supercooling of water is obviously enhanced by the alkali metal ions and increases linearly with the augmentation of concentrations. Furthermore, the supercooling depends on the nature of ions and is 2-3 K larger for NaCl solution than that for KCl solution in the present concentration range: Molecular dynamics simulations are performed to reveal the intrinsic correlation between supercoolability and microstructure. The translational and orientational order parameters are applied to quantitatively demonstrate the effect of ionic concentration on the hydrogen-bond network and ice melting point. The disrupted hydrogen-bond structure determines essentially the concentration dependence of supercooling. On the other hand, the introduced acoustic pressure suppresses the increase of supercooling by promoting the growth and coalescence of microbubbles, the effective nucleation catalysts, in water. However, the dissolved ions can weaken this effect, and moreover the degree varies with the ion type. This results in the different supercoolability for NaCl and KCl solutions under the acoustic levitation conditions.     

Preparation and study of separated single crystals of extended-chain polyethylene

Etching of extended-chain crystals of polyethylene is shown to permit separation of the polycrystalline aggregate into single crystals. The single crystals are analyzed by density, molecular weight (lamellar crystal thickness), melting temperature, birefringence, superheating, and linear melting rate determination. The linear melting rate depends linearly on superheating and can be described by simple rate theory.     

Communication: High speed optical investigations of a character of boiling-up onset

In this communication, we discuss the phenomenon of attainable superheat of liquid and the peculiarities of its release by spontaneous boiling-up. We have combined the apparatus for superheating, namely, bubble chamber, with a high speed micro-optical method for detailed monitoring of the initial stage of boiling-up. In experiments on the isothermal pressure drop, it was found that the boiling-up onset of n-hexane is accompanied by characteristic step signal. The signal has proved to be typical of the heterogeneous character of boiling-up onset in a whole range of superheating degrees. The performance of the method for investigation of the refractive index and density for superheated liquids as functions of temperature and pressure has been revealed. The experimental error is estimated to be 0.1%.     

Calculation of cryoscopic data

The accuracies of determinations of purity and freezing point based upon cryometric freezes suffer from the scatter of data, from the failure of systems ever to recover from the effects of supercooling, and from complex phenomena that elevate temperature during the later parts of runs. Methods are here proposed for decreasing the errors caused by scatter of data and failure to recover from supercooling. These methods utilize the optical projection of calculated curves upon the actual data and involve a new interpretation of the nature of recovery from supercooling.     

Mass spectrometric study of superheated vapors of lanthanide tris(hexafluoroacetylacetonates)

Superheated vapors of La(hfa)₃, Nd(hfa)₃, Sm(hfa)₃, Gd(hfa)₃, Dy(hfa)₃, Ho(hfa)₃, Yb(hfa)₃, and Lu(hfa)₃ have been studied by mass spectrometry using a double two-temperature effusion cell in order to identify the molecular species present in the vapor at different degrees of superheating and the temperature of complete decomposition of the complexes. For equal temperatures of the upper and low compartments of the cell and for moderate superheating, the vapors of the listed complexes are highly oligomerized (mono-, di-, and trimers were detected). Vapor superheating shows low thermal stability of oligomers, whose stability decreases along the series La-Lu. The metal nature is shown to have an effect on the volatility and the fragmentation pattern of the chelates. The conditions of existence of monomeric complexes are identified.     

醇类低温保护剂水溶液过冷和水合性质研究

A differential scanning calorimeter was used to study the thermal behaviors of polyalcohols aqueous solutions, such as supercooling degree of heterogeneous nucleating temperature, hydration properties. The experimental results show that the variation of supercooling and hydration behavior does not have obvious rules at the low concentrations. However, the supercooling degree and the content of unfrozen water increased with the solution concentration at the high concentrations. The difference of hydration properties shows the important effects of function groups (methyl groups and hydroxyl groups).