相分离液体在玻璃转变过程中的动力学性质(英文)

近年来,相分离动力学一直受到大家的关注,人们从技术和理论角度进行了研究.一个二元系统从混溶温度开始进行快速冷却,这个过程驱使两个相的形成和生长.这种自发生长过程遵从幂规律R(t)∝t~n,R(t)是在t时刻的相分离域尺寸大小,增长指数n是一个重要的量,有效的反映了相分离域的增长机制.但是对于相分离液体的玻璃转变过程的研究相对较少.本文采用分子动力学(MD)模拟方法,提出了一个新的模型——相分离液体来研究玻璃转变过程.相分离在玻璃转变过程中,过冷液体分离为两个相.我们主要研究了体系的动力学不均匀性.发现两个相的动力学性质是不均匀的,两个相的玻璃转变温度是不一样的.分析得出,相分离液体在玻璃转变过程中存在着一个动力学奇异点.说明相分离可能是引起玻璃转变系统动力学奇异性的一个原因.     

小尺寸铜团簇冷却与并合过程中结构变化的原子尺度研究

研究Cu_N（N=57,58,59）熔融铜团簇在冷却过程以及300 K时两个具有二十面体结构Cu₅₅团簇在并合过程中的结构变化.对这些小尺寸团簇的结构变化采用基于嵌入原子方法的正则系综分子动力学进行计算机模拟.通过对模拟结果的分析表明,小团簇的冷却和并合过程存在阶段变化的特点.降温过程中Cu_N（N=57,58,59）团簇的原子运动及其微观结构变化表现出较大差异,由此导致这三类团簇内原子排布的不同,其中Cu₅₉团簇结构的有序程度最低.在两个Cu₅₅团簇并合早期阶段,这两个团簇相接触后发生变形导致原子位置出现较大改变,在随后的并合过程中,原子扩散引起原子局部位置调整导致所并合体系的结构发生变化.远离两个团簇接触区的原子仍保持其并合前的结构. 关键词： 团簇 分子动力学 计算机模拟 表面     

超薄膜生长的Monte Carlo模拟研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

钙原子4p_(1/2,3/2)ng(J=3)自电离Rydberg光谱的研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

MgO（001）表面上沉积MgO薄膜过程的分子动力学模拟

采用分子动力学方法模拟了MgO分子连续沉积于MgO（001）表面上的薄膜生长过程,分析了衬底温度和分子入射能对MgO分子在衬底表面上的扩散能力以及对衬底表面覆盖率的影响.模拟结果表明,随着衬底温度的升高,在衬底表面上沉积的MgO分子扩散能力增强,MgO薄膜层中空位缺陷变少.低温下,分子入射能的增大有助于提高衬底表面覆盖率;高温下,表面覆盖率随入射能增大到3.0 eV时达到最大值,入射能继续增大,表面覆盖率减小. 关键词： MgO薄膜生长 分子动力学 计算机模拟 表面扩散     

横向外延过生长磷化铟材料的生长速率模型

分别考虑气相扩散和掩膜表面扩散过程，建立了金属有机化学气相沉积条件下横向外延过生长的速率模型. 在砷化镓衬底上外延磷化铟条件下，模拟得到了生长速率随掩膜/窗口宽度(m/w)变化的关系. 通过讨论掩膜/窗口宽度的影响，说明了掩膜宽度、窗口宽度以及有效掩膜宽度是决定生长速率的关键因素. 以上结论与实验结果一致. 关键词： 横向外延 生长模型 扩散 生长速率     

[111]晶向银纳米杆结构稳定性的分子动力学研究

采用三维分子动力学模拟方法和Finnis-Sinclair型多体势,以[111]晶向银纳米杆为研究对象,模拟研究不同尺寸纳米杆在不同温度弛豫过程中的动态平衡变化过程,分析研究弛豫后银纳米杆的稳态结构变化、平均势能的变化及其在不同时刻结构的演变过程.结果表明,温度对银纳米杆结构稳定性将产生重大影响,银纳米杆存在一临界失稳温度,当温度小于临界失稳温度时,体系保持完好线状晶态,当温度大于临界失稳温度小于熔点时,体系坍塌熔化后发生重结晶,体系形成由(111)和(100)面围成的多面体;随银纳米杆截面尺寸增大,其临界失稳温度、熔点均增大,当截面尺寸大于2 nm时,临界失稳温度趋近于熔点,失稳现象只在一很窄温度区域内存在.     

纳米铜团簇扩散性质研究

本文通过分子动力学模拟研究了纳米铜团簇的自扩散性质,结果表明Nc8949铜团簇自扩散系数随温度的升高而增大,在温度为1000 K时纳米铜团簇的扩散系数随团簇半径的倒数基本呈线性增加.同时指出在常温下团簇几乎无扩散行为,而某些文献中关于常温下晶粒扩散分子动力学模拟结果是模拟体系宏观转动造成的虚假现象.?     

聚乙烯/银纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚乙烯/银纳米颗粒复合物的结构、极化率和红外光谱、热力学性质、力学特性进行计算, 分析其随模拟温度和银颗粒尺寸的变化规律. 模拟结果表明: 聚乙烯/银纳米颗粒复合物为各向同性的无定形结构, 温度升高可提高银纳米颗粒的分散均匀性; 银纳米颗粒表面多个原子层呈现无定形状态, 并在银颗粒和聚乙烯基体的界面形成电极化层, 界面区域随颗粒尺寸和温度的增加分别减小和增加; 与聚乙烯体系相比, 聚乙烯/银纳米颗粒复合物的极化率高很多, 且随温度的升高和银颗粒尺寸的减小而增大; 银颗粒尺寸直接影响界面电偶极矩的强度和振动频率, 红外光谱峰强度和峰位随颗粒尺寸发生变化; 聚乙烯/银纳米颗粒复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数, 热容随颗粒尺寸的变化较小, 但随温度的升高而明显减小, 具有显著的温度效应; 热压力系数随温度的变化较小, 但随颗粒尺寸的增加而减小, 具有明显的尺度效应, 温度稳定性更好; 聚乙烯/银纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚乙烯体系更高的杨氏模量和泊松比, 并且都随温度的升高和银颗粒尺寸的增大而减小, 加入银纳米颗粒可有效改善聚乙烯的力学性质. 关键词： 分子动力学模拟 聚合物纳米复合物 纳米颗粒     

聚酰亚胺/铜纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚酰亚胺/铜纳米颗粒复合物的形态结构、 热力学性质、力学特性进行计算, 分析其随模拟温度和纳米颗粒尺寸的变化规律. 模拟结果表明, 聚酰亚胺/铜纳米颗粒复合物为各向同性的无定形态结构, 铜纳米颗粒与聚酰亚胺基体之间通过较强的范德华作用结合在一起使结构更加稳定, 铜纳米颗粒表面多个原子层呈现无定形状态, 在铜颗粒和聚酰亚胺基体之间形成界面层, 界面区域随颗粒尺寸和温度的增加分别减小和增加. 聚酰亚胺/铜纳米颗粒复合物的等容热容随着颗粒尺寸增大而明显增高, 随温度变化比聚酰亚胺体系更为缓慢, 在较低温度下较小颗粒尺寸复合物的热容比聚酰亚胺体系更低. 聚酰亚胺/铜纳米颗粒复合物的热压力系数随颗粒尺寸增加而显著增大, 比聚酰亚胺体系的热压力系数更小, 且随温度升高而减小的程度要小得多. 聚酰亚胺/铜纳米颗粒复合物的热力学性质表现出明显的尺度效应, 温度稳定性明显高于聚酰亚胺体系. 聚酰亚胺/铜纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚酰亚胺体系更低的杨氏模量和泊松比, 随温度升高分别减小和增大, 与聚酰亚胺体系随温度的变化趋势相反, 且杨氏模量的温度稳定性显著提高, 同时泊松比随纳米颗粒尺寸增大而减小, 具有明显的尺度效应. 加入铜纳米颗粒形成复合物可获得与聚酰亚胺体系显著不同的力学新特性. 关键词： 分子动力学模拟 聚合物纳米复合物 聚酰亚胺 纳米颗粒     

On the local structure of the phase separation line in the two-dimensional Ising system

We investigate the structure of the phase separation line between the pure phases in the two-dimensional Ising model, the liquid and vapor phase in lattice gas language, at low temperatures. The fluctuations in the location of this line are known to diverge in the thermodynamic limit, something which is also believed to happen to the continuum liquid-vapor interface in three dimensions (in the absence of the gravitational field). We show that despite this global divergence it is possible to define precisely the local structure of the phase separation line. This has a finite, exponentially small, width at low temperatures which is related by a central limit theorem⁽¹⁾ to the width of the global fluctuations on the appropriate (divergent) length scale. The latter has been computed explicitly⁽²⁾ for all temperatures below the critical temperatureT _c, where it diverges as (T _c –T)^–1/2. We also prove a Gibbs formula for the surface tension at low temperature, which relates it to the local structure of the phase separation line.Supported in part by NSF grant No. M^rPHY 78-15920 and MCS78-01885.On leave from: Departement de Physique Théorique, Université de Louvain, Belgium.     

Large‐Scale Fluctuation Behavior Prior to the Crystallization of Poly(ethylene terephthalate) Glass: A Small‐Angle Light Scattering Study

The crystallization processes of amorphous, glassy‐state poly(ethylene terephthalate) (PET) at two temperatures, a low temperature near T _g where PET has a slow crystallization speed and a middle temperature (about 55°C above T _g ) where PET crystallization is rapid, were monitored in situ by a time‐resolved small‐angle light scattering (SALS) device. It was found that large‐scale fluctuations happened prior to the crystallization at both temperatures, but the kind of fluctuation had a temperature dependence: at the middle temperature, pure density fluctuation took place during the induction period, whereas at low temperature, both density fluctuation and orientation fluctuation occurred, but the latter was the dominant factor. Analyses of the kinetics of these two kinds of fluctuation processes demonstrated that the spinodal decomposition (SD) type of phase‐separation character was undistinguishable in the SALS scale, while the nucleation‐growth (NG) type of phase behavior could describe the scattering results as well.     

The Mott transition in cold semiconductors

We discuss the Mott transition when a semiconductor is externally excited at low temperatures. We demonstrate that hysteresis is inevitable unless a phase separation occurs at higher temperatures or lower average excitation densities than necessary for the predicted ionization catastrophe. The critical temperature and pair density for hysteresis in Ge is of the order 6K and 3 · 10¹⁶ cm^-3, respectively, for a model involving dynamical screening.     

Phase separation in the strongly correlated Falicov-Kimball model in infinite dimensions

Phase separation in the strongly correlated Falicov-Kimball model in infinite dimensions is examined. We show that the phase separation can occur for any values of the interaction constant J^* when the site energy of the localized electrons is equal to zero. Electron-poor regions always have homogeneous state and electron-rich regions have chessboard state for , chessboard state or homogeneous state in dependence upon temperature for 0<J ^* <0.03 and homogeneous state for J ^* =0. For J ^* =0 and T=0, phase separation (segregation) occurs at .The obtained results are exact for the Bethe lattice with infinite number of the nearest neighbours. Received 1 December 1998 and Received in final form 12 April 1999     

Dynamical mean-field theory of a simplified double-exchange model

Simplified double-exchange model including transfer of the itinerant electrons with spin parallel to the localized spin in the same site and the indirect interaction J of kinetic type between localized spins is comprihensively investigated. The model is exactly solved in infinite dimensions. The exact equations describing the main ordered phases (ferromagnetic and antiferromagnetic) are obtained for the Bethe lattice with (z is the coordination number) in analytical form. The exact expression for the generalized paramagnetic susceptibility of the localized-spin subsystem is also obtained in analytical form. It is shown that temperature dependence of the uniform and the staggered susceptibilities has deviation from Curie-Weiss law. Dependence of Curie and Néel temperatures on itinerant-electron concentration is discussed to study instability conditions of the paramagnetic phase. Anomalous temperature behaviour of the chemical potential, the thermopower and the specific heat is investigated near the Curie point. It is found for J=0 that the system is unstable towards temperature phase separation between ferromagnetic and paramagnetic states. A phase separation connected with antiferromagnetic and the paramagnetic phases can occur only at . Zero-temperature phase diagram including the phase separation between ferromagnetic and antiferromagnetic states is given. Received 28 May 1999 and Received in final form 14 July 1999     

The Shear‐Dependent Phase Separation Behavior of the Ternary Blend Polystyrene/Polyvinyl Methyl Ether/Styrene–Acrylonitrile Copolymer

The phase behavior and phase separation dynamics of a PS/PVME/SAN ternary blend using light scattering under a shear rate range of 0.1～40 s^?1 were investigated. The cloud point temperature first increases and then decreases with the increase of shear rates. At higher shear rates, the cloud point temperature again increases. The phase separation behavior in the early and later stages under shear field can be explained by the Cahn–Hilliard theory and the exponential growth law, respectively. The delay time τ _d ?, the apparent diffusion coefficient D _app, the growth rate R(q), and the exponent term show strong dependence on the difference between the experimental temperature and the cloud point temperature (ΔT), and on the shear rates. Compared with PS/PVME binary blends at lower shear rates, τ _d for a PS/PVME/SAN ternary blend is smaller, while at higher shear rates τ _d is larger. At higher shear rates, the introduction of the third component SAN to a PS/PVME binary blends slows the phase separation process.     

Far infrared and phase transitions in ferroelectric materials

Abstract

A number of far infrared absorption bands are found in molecular and hydrogen-bonded crystals when there are several formula units in the primitive cell. They are very sensitive to the crystal structure and constitute a convenient probe for looking at small changes in the structure, especially at low temperatures where there is a lack of X-Ray data. As well known examples, the case of LTT (Lithium Thallium Tartrate) where the absorption band observed at 21 cm^?1 at room temperature softens down to 8 cm^?1 at the Curie temperature (T_c = 12 K), and LiNH₄SO₄ which has a phase transition at 26 K, will be considered.

It is less known that some ferroelectric crystals can be studied as very thick single crystal plates (thickness t up to 10 mm) at 4 K, and they show in some cases a far IR transmission that is much higher than expected from the study of thin plates (i.e. t = 20 μm). In fact the transmission does not decrease very much when thickness is higher than some specific value t ₀/2. The crystal is not homogeneous at 4 K. A model with a far IR absorbing surface layer (thickness t ₀/2) and a transparent bulk is a good first approximation.

At some temperature T ₀ located between liquid helium and liquid nitrogen temperatures, the center of the bulk undergoes a phase transition from the absorbing phase into the transparent one. When temperature is still lowered, the transparent phase is extended towards the surface with an incommensurate phase between the surface and the bulk.     

Structure and phase transformation of alkali silicate melts analysed by Raman spectroscopy

Raman spectra of binary alkali silicates were measured at various temperatures from 1300°C to room temperature to investigate the relation between structural change and phase transformation phenomena. Distribution of structural units of Q ⁿ was estimated at each temperature by the deconvolution of spectra based on the equilibrium 2Q³ ? Q² + Q⁴. The Q ⁿ distributions of sodium and potassium silicate systems strongly depend on temperature and the equilibrium shifts to the left-hand side with decreasing temperature, but those of lithium silicate system were less sensitive to the temperature variation. In alkali disilicates (33?mol%?R₂O–67?mol%?SiO₂, where R=Li, Na or K), the Q ⁿ distributions near the melting point were independent of alkali ion species, and they held the relation [Q²]?=?[Q⁴]?≈?[Q³]/4. This means that two of 6Q³ units (six-memberd ring) in crystals are transformed into a pair of Q² and Q⁴ in the melting process. Below the melting point, the Q ⁿ distribution in lithium disilicate melt remained while, in sodium and potassium disilicate melts, [Q³] increased with decreasing temperature. Crystallization of the alkali disilicate melts is discussed considering the configuration entropy of Q ⁿ units. In 25?mol%?Li₂O–75?mol%?SiO₂, which is in the range of the immiscibility dome, the Q ⁿ distribution was maintained even when phase separation occurs in the cooling process.     

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr³⁺ ion as a replacement of Sm³⁺ in Sm_0.5Sr_0.5MnO₃ is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm_0.5−xPr_x)Sr_0.5MnO₃ show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.     

The crystal fields inα-alums—A high pressure magnetic resonance study

The origin of the crystal field and its variation with temperature in severalα-alums have been studied bynmr of²⁷Al andepr of Cr³⁺ at high hydrostatic pressures and low temperatures. The results lead to an explanation of the anomalous temperature variation of the axial crystal field at the trivalent ion site. The mechanism of the phase transition in ammonium alum is also explained. A correlation between the axial crystal field as determined bynmr (e ² qQ/h) and that determined byepr (D) has been obtained.