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.     

金属铜升温熔化过程的分子动力学模拟

采用分子动力学方法模拟了金属铜的升温熔化过程.原子间作用势采用FS (Finnis-Sinclair)势，结构分析采用双体分布函数(PCF)、均方位移(MSD)等方法.计算结果表明,在连续升温过程中，金属铜在1444 K熔化，在该熔化点的扩散系数为4.31×10－9 m2•s－1.上述结论与实验值相当接近，并且比之采用EAM镶嵌原子势所作模拟得到的结果更佳，说明FS势可以用来处理象液铜这样较复杂的无序体系.本文指出了升温速率在金属熔化过程中所起的作用.     

PMMI/PVDF共混体系相分离的时温依赖性

用小角激光光散射(SALLS)研究了聚(N-甲基甲基丙烯酰亚胺)/聚偏氟乙烯(PMMI/PVDF)共混体系的相分离行为.通过升温法获得了PMMI/PVDF体系的浊点温度,发现体系的浊点温度强烈依赖于升温速率,呈明显的非线性关系,即升温速率大于1 K.min-1时,升温速率是影响I(t)的唯一因素;升温速率较小时,高分子链松弛速率的差别得以体现,则该情况下升温速率与松弛速率共同影响浊点温度.该体系具有典型的最低临界共溶温度(LCST)特征.恒温法相分离结果表明,在实验温度范围内该体系相分离行为对温度的依赖性遵循时温叠加(time-temperature superposition,TTS)原理.     

金属Cu低指数表面熔化行为的分子动力学模

采用Mishin镶嵌原子势, 通过分子动力学方法模拟了金属Cu的低指数表面在不同温度的表面熔化行为, 分析了熔化过程中系统结构组态的变化以及固-液界面迁移情况. 金属Cu的(100)和(110)表面在低于熔点发生预熔化, 而(111)表面存在明显的过热现象. 准液体层的厚度随温度升高而增加, 热稳定性与表面的密排顺序一致, 按(111)、(100)、(110)顺序增大. 当温度高于热力学熔点时, 固液界面的移动速度与温度成正比, 外推得到热力学熔点约为1360～1380 K, 与实验结果1358 K吻合良好. 动力学系数定义为界面移动速度与过热程度的比值, 表现为明显的各向异性: k100=39 cm•s−1•K−1, k110=29 cm•s−1•K−1, k111=20 cm•s−1•K−1. k100与k110之间的比例符合collision-limited理论, (111)密排面有与其它低指数表面不同的熔化方式.     

松木屑与褐煤共气化的TG-FTIR实验研究

利用TG-FTIR对松木屑、褐煤及其混合物的共气化过程及气化产物进行了分析，研究了掺混比例、升温速率以及反应气氛对共气化过程的影响。结果表明，松木屑加入后提高了试样的反应活性，随松木屑比例增加，气化失重速率逐渐降低，CO的开始析出温度及析出峰面积A_CO呈降低趋势；较低的升温速率有利于CO和CH₄的析出，随着升温速率的增加，DTG曲线向高温侧移动，最大失重速率显著增加，褐煤热解对应的峰逐渐消失；CO₂气氛对挥发分析出阶段的失重行为影响不明显；空气气氛时挥发分析出阶段的两个失重峰分别对应挥发分的燃烧和固定碳燃烧，该气氛下没有明显的焦炭气化阶段。     

煤-焦炉气共热解热重研究Ⅱ.热解半焦燃烧反应性

采用高压热天平在终态温度为８００℃，压力为０．１～５ＭＰａ，升温速率为５～２５℃／ｍｉｎ的实验条件下，对先锋、黄县褐煤和兖州烟煤在真实焦炉气气氛中共热解特性进行了系统的ＴＧ／ＤＴＧ研究，主要考察了反应压力、升温速率对热解特性的影响，并与可比条件下加氢热解作比较。结果表明，压力和升温速率对热解有较大影响，压力升高热解总失重量增加，升温速率减慢有利于提高热解总转化率；压力与升温速率互相促进又互相制约共同影响热解特性。在给定较高压力下，减慢升温速率能明显提高热解总转化率；在慢速升温条件下，提高压力也能明显提高热解总转化率，通过降低升温速率可以实现在较低压力下达到甚至超过高压下的热解效果。与可比条件下的加氢热解相比，煤焦炉气共热解具有相近的热解特性，但总转化率略有降低且失重峰温向高温区移动。     

乙烯基单体光聚合动力学与光固化流变行为

概述了光引发体系、光照强度、单体平均官能度、液晶含量、反应温度、助剂等对乙烯基单体/液晶混合物光聚合动力学及光固化流变行为的影响.在染料和胺组成的二元Ⅱ型(夺氢型)光引发体系中,引入适当的第三组分能显著提高反应速率和转化率;增加光照强度和升高温度也能显著提高反应速率和转化率.随着单体平均官能度的增加,体系的反应速率和转化率呈现先增加后降低的趋势.对于I型(裂解型)光引发剂,液晶含量增加,单体反应速率降低,但转化率基本不变;但对于Ⅱ型光引发剂,液晶含量增加能大幅提高单体反应速率和转化率.加入使体系黏度降低的助剂也能提高体系的反应速率和转化率.在体系相同、温度固定的条件下,反应速率越高,凝胶化时间越短;相同体系,温度升高,凝胶化时间提前;相同温度下,单体官能度愈高,体系凝胶时间亦愈短.     

Mg(OH)2热分解反应的非等温动力学研究

用非等温动力学方法对氢氧化镁的热分解动力学进行了研究. 分解反应机理符合晶核形成及生长机理A，且随着升温速率的升高，机理由A2转变为A1.5. 根据Kissinger非机理方程计算和数值回归方法验证所得的分解反应活化能结果相互印证，约为148 kJ•mol^-1. 进一步研究发现，水蒸气的存在对氢氧化镁热分解反应具有非常明显的影响，可能是其动力学机理随升温速率升高而改变的主要影响因素.     

孤岛减压渣油窄馏分的热反应动力学：——反应级数的计算

用超临界流体萃取精密分馏技术将孤岛减压渣油分成１５个窄馏分，选取其中８个在热重装置上进行了反应动力学研究。所得数据用（经本文作者改进）Ｚｓａｋｏ法进行动力学处理，计算了各窄馏分的热反应级数。结果发现，升温速率的改变对各窄馏分在同一转化率范围的反应级数影响不大，而在每一升温速率下，反应级数随反应温度和转化深度不同而改变。同时，不同升温速率下，随着转化深度的增加，各窄馏分的表观活化能升高。     

双酚A聚砜对PBT的结晶与多重熔化行为的影响

本文研究了PBT结晶的熔化行为及非晶性组分PSF的加入对PBT结晶度及熔化行为的影响。结果表明:PBT结晶熔化时的DSC双峰图为多重熔化行为而非多晶型的结果;PSF的加入对PBT结晶度影响不大,但对多重熔化行为有明显影响。此乃PSF无定形组分加入后,使PBT结晶速率减慢而形成更完善的PBT结晶之缘故。     

Melting kinetics of it-polypropylene crystals over wide heating rates

Melting kinetics of it-polypropylene crystals has been examined over wide heating rates of 0.6 K min^?1?10⁴ K s^?1 using a standard DSC and a fast-scan DSC. With fast-scan DSC, we have an access to the melting of crystals obtained at low temperatures, which are susceptible to re-organization at the heating rates applicable with standard DSC. It is clearly discernible that the appearance and disappearance of multiple melting peaks are strongly influenced by the applied heating rates and dependent on the crystallization temperatures. By examining the heating rate dependence of superheating of melting, we have determined the melting points of as-grown crystals formed under wide crystallization temperatures.     

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.     

Effect of x-rays on extended chain crystals of polyoxymethylene

Extended chain crystals of polyoxymethylene were irradiated with x-rays of about 10⁵ r./min. An immediate decrease in superheating on melting was noted. After 90 min. of irradiation the melting point level, refractive indices, and density also decreased. After 480 min. the DTA melting peak at a heating rate of 20°C./min. had decreased 25°C., the melting point decreased about 18°C., and the density calculated from refractive indices decreased 0.031 g./cm.^?3. These effects are interpreted as indication of chain scission and formation of amorphous defects.     

Cooperative dynamics and self-diffusion in superheated crystals

Molecular dynamics simulations have been used to study the atomistic scale dynamics of superheated crystals under different temperature and pressure conditions. The limit of superheating was determined by monitoring a suitable order parameter. The occurrence of homogeneous melting was related to the generation of structural defects characterized by the presence of pairs of particles having defective coordination. At temperatures close to the homogeneous melting point such particles formed extended stringlike clusters. Particles involved in clusters change continuously as a result of local structural rearrangements. These can result in the displacement of particles from one lattice site to another, thus providing a mechanism for self-diffusion.     

Mechanistic aspects of homogeneous and heterogeneous melting processes

Molecular dynamics simulations have been employed to explore the response of crystalline Ar systems with and without a free surface to a gradual temperature rise. The surface-free crystalline bulk undergoes a homogeneous melting process at the limit of superheating, whereas the semicrystal terminating with a free plane surface melts with a heterogeneous mechanism at a temperature corresponding to the equilibrium melting point. Numerical findings suggest that the gradual disordering of the crystalline lattice as well as the homogeneous and heterogeneous melting processes are mediated by atoms with defective coordination. Their concentration in the regions close to the semicrystal surface at the equilibrium melting point is found to be approximately the same as in the surface-free bulk at the limit of superheating.     

Differences between solid superheating and liquid supercooling

The thermodynamic and kinetic behaviors for solid superheating and liquid supercooling were critically examined and compared via molecular-dynamics simulations. It is shown that the large elastic energy associated with internal melting and solid-liquid interface disorder play important roles in superheating. The growth rate is anisotropic for supercooling, but isotropic for superheating. Supercooling can be well described by the classical nucleation theory, whereas superheating shows many exceptions. The underlying mechanisms for these differences are discussed.     

Melting behavior of drawn films from polyethylene single-crystal mats

Drawing of mats of linear polyethylene single crystals prepared from dilute solution is possible at temperatures above about 90°C. The structure and properties of the drawn specimens are much different from those ordinary drawn bulk polymer. Drawn mats have been investigated by differential scanning calorimetry. The characteristic experimental results are: (a) a broad melting curve, (b) considerable superheating depending on the rate of heating, (c) constancy of the melting point and the heat of fusion with annealing, (d) deviation from the relation between the heat of fusion and the density obtained for the drawn bulk specimens, (e) appearance of two melting peaks in samples annealed at temperatures above about 130°C. These results imply that the structure of the drawn mat is characterized by a larger number of the tie chains connecting the neighboring crystals (the structure postulated in earlier papers) than is the case in ordinary drawn bulk polymer. It can be concluded that the transformation of a fringed micellar type of structure to the folded lamellar structure may be difficult during annealing unless crystals melt and then recrystallize during cooling.     

Temperature Modulated DSC of Irreversible Melting of Nylon 6 Crystals

A new method is presented to analyze the irreversible melting kinetics of polymer crystals with a temperature modulated differential scanning calorimetry (TMDSC). The method is based on an expression of the apparent heat capacity, , with the true heat capacity, mc_p, and the response of the kinetics, . The present paper experimentally examines the irreversible melting of nylon 6 crystals on heating. The real and imaginary parts of the apparent heat capacity showed a strong dependence on frequency and heating rate during the melting process. The dependence and the Cole-Cole plot could be fitted by the frequency response function of Debye's type with a characteristic time depending on heating rate. The characteristic time represents the time required for the melting of small crystallites which form the aggregates of polymer crystals. The heating rate dependence of the characteristic time differentiates the superheating dependence of the melting rate. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is argued that the ‘reversing’ heat flow extrapolated to ω → 0 is related to the endothermic heat flow of melting and the corresponding ‘non-reversing’ heat flow represents the exothermic heat flow of re-crystallization and re-organization. The extrapolated ‘reversing’ and ‘non-reversing’ heat flow indicates the melting and re-crystallization and/or re-organization of nylon 6 crystals at much lower temperature than the melting peak seen in the total heat flow. This revised version was published online in July 2006 with corrections to the Cover Date.