多元醇二元体系固-固相变机理的研究

多元醇在一定温度下发生固-固相变,大量文献都报道了它们的固-固相变焓、相变温度以及相图.本文以红外光谱测试结果以及量热实验结果为基础,探讨了新戊二醇/季戊四醇二元体系的固-固相变焓与氢键的关系:二元体系中部分分子间氢键变弱,变弱的氢键更易于断裂;温度上升到一定值,新戊二醇在季戊四醇中的相向塑性晶体相相变时保留下来的氢键断裂,形成二元体系的量热曲线的第二个吸收峰;高新戊二醇浓度的二元体系,可能保留下来的氢键数量少,第二个吸收峰对应的焓值小,但保留的氢键受新戊二醇影响更大,第二个吸收峰对应的温度更低.     

三羟甲基甲胺、季戊四醇及其二元体系固-固相变的变温红外光谱研究

利用傅里叶变温红外光谱仪分别测定了三羟甲基甲胺（TAM）、季戊四醇（PE）及其二元体系变温红外谱。实验表明，多元醇分子中羟基吸收峰随温度升高耐发生位移向高波数移动，此结果既能反映多元醇及其二体系固－固相变的温度区间，又与转变热相对应，从而揭示了多元醇及其二元体系固－固相变贮热的机理。     

基于焓法的固-液相变格子Boltzmann模型

潜热储能系统中的相变材料固液相变问题是一个重要的多相流动问题.本文研究了在具有不同类型源项下的焓法格子Boltzmann相变模型在还原温度场分布上的特性.为了消除外力项对温度场的额外影响,本文对源项进行了修正,并通过求解单区域相变问题和自然对流相变问题,验证了由Chapman-Enskog展开得到的焓守恒方程中源项的影响.结果 表明,在单区域相变问题中,改进模型的温度分布更加准确.另外,在有相变的自然对流问题中,没有潜热附加项的改进模型的熔化过程比其他模型略慢,同一时刻的最大液相率与最小液相率之差小于0.01.     

考虑非傅立叶效应的固液相变分子动力学模拟

采用耦合双温度模型的分子动力学方法对飞秒激光照射金箔的固液相变过程进行了模拟研究,利用序参数法对固液原子进行判定从而确定了金箔发生相变时的固液界面位置和温度,对基于傅立叶定律的抛物线模型和考虑非傅立叶效应的双曲线模型模拟得到的结果进行对比分析,在此基础上采用耦合双曲线模型的分子动力学方法研究了激光能量密度和脉冲宽度对金箔相变过程的影响.结果表明,当激光作用于金箔时,金箔上表面首先熔化,固液界面随时间不断向金箔底部移动,并且在相同条件下,双曲线模型下的金箔熔化深度和固液界面温度均大于抛物线模型的结果.当考虑非傅里叶效应时,激光能量密度越大,固液界面温度越高,金箔熔化时间越短;激光脉冲宽度越小,固液界面温度越大,金箔熔化速度越快.     

高分子凝胶体积相变中的氢键作用

我们把关联模型(association models)推广应用到高分子凝胶体系,研究高分子与溶剂分子间的氢键和溶剂分子与溶剂分子间的氢键在高分子凝胶体积相变中的作用.首先通过分析凝胶体积分数与温度的关系发现,由于两种氢键作用,随着温度变化高分子凝胶出现连续、不连续体积相变,结果表明在体积相变过程中两种氢键都起着重要作用.其次,对不同氢键分数条件下的旋节线的研究发现,对于高分子凝胶体积相变中出现的UCST和LCST(上临界共溶温度和下临界共溶温度)现象也是由于高分子与溶剂分子间氢键和溶剂分子与溶剂分子间氢键共同作用的结果.     

冲击加载过程中苯的液—固相转变

利用一级轻气炮加载技术和最近发展的一种透光性测量技术,在线观测到液态苯在多次冲击压缩过程中透光性随时间变化特征.分析表明,导致透光性下降的原因是局部发生液—固相变而引起的光散射效应,且散射特征反映了相变过程的时间弛豫和空间积累特性.结果澄清了苯在冲击条件下液—固相变是否发生的争论,对冲击相变动力学研究提供了重要实验依据. 关键词： 苯 多次冲击压缩 液—固相变 透光性     

固相硝基甲烷相变的第一性原理计算

研究极端条件下固相分子晶体含能材料的相变机理,对于人们认识固相含能材料的爆轰反应有着重要的意义.采用基于校正密度泛函理论的第一性原理方法研究固相硝基甲烷在静水压下的行为.分析晶格参数a,b和c轴随压强的变化,发现在1 GPa到12 GPa时晶格参数出现不连续的变化,表明体系发生相变.在相变时最大的二面角从155.3?增加到177.5?,二面角的增加限制CH3官能团自由旋转,使得C-N和C-H键的键长发生变化.在相变之前,体系主要存在由C-H···O组成的分子间的氢键,而在相变之后存在分子内的H···O和分子间C-H···O组成的氢键.此外通过对硝基甲烷体系的电子结构进行计算,发现相变会影响带隙随压强的变化,而且还会影响费米能级附近的态密度结构.     

静电场中水分子的扩散与液固相变

扩散与相变是本科阶段基础物理教学的重要内容.本文以体相水分子系统为例,基于分子动力学模拟,研究静电场环境下水分子的扩散和相变.在较小电场强度情况下,水分子表现出显著的大角度跳跃转动,且转动频次随电场呈先增加后减小的非单调行为.这种大角度跳跃转动的反常电场依赖性与水分子扩散系数随电场强度变化的非单调性存在较强相关性.在电场强度较大时,水分子系统发生液固相变,即电致结冰,并观察到了两种冰相沿电场方向的层状混合堆叠结构.该分子动力学模拟有助于学生从微观层次理解分子扩散与相变等物理现象的分子机制,可作为教学实例培养本科生的科研实践能力.     

方腔内相变材料固液相变传热研究

基于相变材料(PCM,phase change material)的相变储能设备具有储能密度高的特点。本文建立了基于相变储能元件伪焓模型的固液相变格子Boltzmann模型,研究了内部管道位置、方腔倾斜角度对PCM融化过程的影响规律。结果表明,在内管道靠近方腔上部时,由于上部界面(固液相变界面或上壁面)对自然对流阻碍作用,使PCM的融化速率减慢。但是,在此时使方腔发生倾斜,会改变管道热流体到上部界面的距离,强化PCM的热质传递过程,使融化加快。     

基于伪焓法的多孔介质固液相变格子Boltzmann方法

格子Boltzmann方法是求解相变材料(PCM,phase change material)固液相变界面上复杂质量、能量和动量传递的有效方法。本文将伪焓LB模型推广至多孔介质固液相变中,构建相应的LB模型,并通过焓和温度的关系,获得下一时间步的焓、温度和对应液相率。为验证多孔介质固液相变LB模型的正确性,本文分别对一维Stefan问题和二维方腔多孔介质内固液相变问题进行求解。结果表明,在无自然对流作用下(一维Stefan问题),本文所构建模型能精确还原温度分布,与解析解相对误差控制在2%以内。除此之外,本文模型能反映自然对流对固液相变过程的影响,与已有工作差别较小,能准确追踪固液相变界面位置。     

Study on Phase‐Change Characteristics of PET‐PEG Copolymers

Polyethylene glycol (PEG) was selected as a phase‐change material (PCM) and the phase‐change fibers of its copolymers with polyethylene terephalate (PET), PET‐PEG, were successfully prepared by melt spinning. The PET‐PEG copolymers have solid‐solid phase change characteristics at 10–60°C without obvious liquid substance appearing, while PET/PEG blends will lose their phase‐change characteristics since the PEG of the blends may melt and leak under high temperature. By controlling the molecular weight and relavent proportion of PEG added, the phase‐change temperature range and the enthalpy can be adjusted.     

纳米石墨烯片-正十八烷复合相变材料制备及热物性研究

本文分别制备了纳米石墨烯片质量分数为0%, 0.5%, 1%, 1.5%, 2%的纳米石墨烯片-正十八烷复合相变材料,并通过扫描电镜测试、红外光谱分析、差示扫描量热实验及导热分析等实验对其形貌结构及热物性进行表征和研究.实验表明本文制备的纳米石墨烯-正十八烷复合相变材料具有很好的相变稳定性;当纳米石墨烯片的质量分数达到2%时,复合相变材料的导热系数相对于纯十八烷高出了89.4%.     

黏弹性流体充模过程中凝固现象的数值模拟

建立了黏弹性流体在充模过程中带有相变的气一液两相模型,该模型分别由气、液两相的质量守恒方程、动量守恒方程、能量守恒方程描述,并通过引入Heaviside函数将气一液两相的方程组统一为一个方程组;建立了一个对型腔内熔体和气体都适用的修正的焓方法来描述充模过程中的相变,采用基于同位网格的有限体积方法对模型进行求解,水平集方法捕捉充模过程中的界面演化,模拟出了黏弹性流体在充模过程中的凝固现象,得出了充模过程中型腔内的温度、压力、第一法向应力差等随时间的变化;并讨论了型腔壁面温度、熔体温度、注射速度对充模过程中凝固现象的影响,研究结果表明:型腔壁面温度越高,凝固层越薄;熔体温度越高,凝固层越薄;注射速度越高,凝固层越薄,故提高型腔壁面温度、熔体温度、注射速度可以减少或消除型腔壁面附近的凝固层。     

计算饱和醇异构体标准生成焓的新方法

基于极性叠加原理,在成功设计烷烃异构体和多氯代烷烃生成焓计算新方法的基础上,设计了一种计算多元醇异构体生成焓的新方法,并合理地假定任一异构体的原子化焓等于三种键（C-C、C-H和C-O-H键）的键能、极性叠加能项以及氢键能项的加和。用这一模型拟合24种原子化粹数据,得到了标准生成焓的估算公式。为了检验预测的精确性,又设计了一种预测方法,使用在排除液预测的化合物条件下回归得到的参数,预测该化合物的生成焓。按这种方法,预测了24种异构体的生成焓。通过该5参数预测的相对于实验值的各种误差（平均绝对误差、均方根误差和最大绝对误差）不仅比7参数的基团法预测的对应误差小得多,而且比相应实验数据的误差还要小。与键加和法比较,该方法的模型包含了极性叠加能和氢键能量,该两项代表了主要的非键相互作用能,表征了不同异构体的结构差异,并大大减少了参数。     

Crystal structure and thermodynamic properties of phase change material bis(1-dodecylammonium) tetrachlorochromate (C12H25NH3)2CdCl4(s)

A novel complex bis(1-dodecylammonium) tetrachlorochromate (C₁₂H₂₅NH₃)₂CdCl₄(s) (abbreviated as C₁₂Cd(s)) was synthesized by liquid phase reaction. Crystal structure and composition of the complex were determined by X-ray crystallography, chemical analysis, and elemental analysis. It is triclinic, the space group is P?1 and Z = 2. Lattice potential energy (LPE) of the complex was calculated to be kJ·mol^?1 from crystallographic data. Low-temperature heat capacities were measured by a precise automatic adiabatic calorimeter over the temperature range from 78 to 370 K. The temperature, molar enthalpy, and entropy of the phase transition of the complex were determined to be 331.88 ± 0.02 K, 55.79 ± 0.46 kJ·mol^?1, and 168.10 ± 1.38 J·K^?1·mol^?1, respectively. Two polynomial equations of the heat capacities as a function of temperature were fitted by least-square method. Smoothed heat capacities and thermodynamic functions of the complex were calculated.     

Gel–liquid crystal phase transition in dry and hydrated SOPC phospholipid studied by differential scanning calorimetry

We study the thermodynamic properties of pure and hydrated samples of SOPC (stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) membrane via Differential Scanning Calorimetry. We estimate the fundamental thermodynamic quantities, such as enthalpy and entropy, of the bilayer phosphatidylcholine. It is found that the gel-liquid crystal phase transition is driven by the van’t Hoff enthalpy, revealing the occurrence of an intermediate phase transition. We discuss the influence of the heating rate on the enthalpy and on the gel ? liquid crystal phase transition temperature by introducing the adequate thermodynamic Gibbs potential. The effect of hydrogen bonding of the water molecules with the polar head and polar-apolar interface on the energetics of the bilayer membrane matrix is analysed. The obtained transition temperature was found to vary between 3 and 4°C depending on the hydration level. A result corroborated by the behaviour of the heat capacity of SOPC computed via Molecular dynamics simulation.     

Induced Smectic X Phase Through Intermolecular Hydrogen-Bonded Liquid Crystals Formed Between Citric Acid and <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis>-(Octyloxy)Benzoic Acid

Hydrogen-bonded liquid crystal (HBLC) is synthesized from citric acid (CA) and 4-(octyloxy)benzoic acid (8OBA) with different mole ratios. Fourier transform infrared spectroscopy (FT-IR) confirms the presence of hydrogen bond between CA and 8OBA. Nuclear magnetic resonance (NMR) spectroscopic studies validate the intermolecular complementary, cyclic type of hydrogen bond, and molecular environment in the designed HBLC complex. Powder X-ray diffraction analysis reveals the monoclinic nature of liquid crystal complex in solid phase. Liquid crystal parameters such as phase transition temperature and enthalpy values for the corresponding mesogenic phases are investigated using a polarizing optical microscope (POM) and differential scanning calorimetry (DSC). It is observed that the change in chain length and steric hindrance while increasing the mole ratio in HBLC complex induces a new smectic X (Sm X) along with higher-order smectic G (Sm G) phases by quenching of smectic C (Sm C). From the experimental observations, induced Sm X phase has been identified as a finger print texture. Also, Sm G is a multi-colored mosaic texture in 1:1, 1:2, and 1:3 mol ratios. The optical tilt angle, thermal stability factor, and enhanced thermal span width of CA + 8OBA complex are discussed.     

Thermal and dielectric studies of self-assembly systems formed by hydroquinone and alkyloxy benzoic acids

Self-assembly systems formed by hydroquinone and alkyloxy benzoic acids are isolated and characterized. Hydroquinone formed double hydrogen bonds with p-n-alkyloxy benzoic acids. Various hydrogen bonded complexes have been synthesized with hydroquinone and pentyloxy to dodecyloxy benzoic acid, respectively. FTIR studies confirm the hydrogen bond formation in the complex. Polarizing Optical Microscopic (POM) studies revealed the textural information while the transition and enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. Phase diagram has been constructed from the POM and DSC data. The members of the present homologous complexes are characterized by POM, DSC, optical tilt angle and dielectric studies. Odd-even effect has been evinced in enthalpy values and transition temperatures corresponding to the isotropic to nematic phase transition. Optical tilt angle in Smectic C phase data has been fitted to a power law and it has been observed that the temperature variation of the tilt angle follows Mean Field theory prediction. Results of Fourier Transform Infrared Spectra (FTIR), POM, DSC, tilt angle and dielectric studies are discussed.     

Effects of Fe－Fe bond length change in NaZn13-type intermetallic compounds on magnetic properties and magnetic entropy change

In this paper the effects of Fe－Fe bond length change on magnetic properties and magnetic entropy change have been investigated on LaFe_{12.4-x}Si_xCo_{0.6} and LaFe_{12.3-x}Al_xCo_{0.7} intermetallic compounds. According to the analyses of Fe－Fe bond length change, the variation of Curie temperature and the unusual magnetic phase transition which results in the large magnetic entropy change were explained. The effects of the substitution of Co and Si for Fe on magnetic entropy change and field-induced itinerant-electron metamagnetic transition in LaFe_{12.4-x}Si_xCo_{0.6} compounds were also studied and the considerable magnetic entropy change has been achieved.