随机双指数记忆耗散系统的非马尔可夫扩散

针对具有双指数耗散记忆核函数的两自由度耦合系统, 本文利用Laplace变换导出了热宽带噪声激励下该系统响应二阶矩的解析表达式. 并观察到位移二阶矩不同于单自由度情形下的反常扩散:<x²(t)> ∝ t^α (0<α<2, α≠1), 而是随时间及噪声等参数变化呈现普遍的振荡扩散现象.分析可得, 阻尼耦合因子B使粒子远离简谐势场的束缚, <x²(t)>随B的增大扩散加剧而摩擦系数增大却使其趋于平稳状态.进一步, 若两热噪声互关联时, 较小的互关联时间对二阶矩的影响较大, 反之作用较小. 伴随互关联强度递增, 位移二阶矩的扩散加剧, 位移间的相关性加强, 与物理直观相符. 关键词： 热噪声 非马尔可夫扩散 广义朗之万方程 关联性     

热对流条件下颗粒沉降的格子Boltzmann方法模拟

采用格子Boltzmann方法模拟了在热对流条件下的颗粒沉降问题, 在研究单颗粒在等温流体、热流体和冷流体中运动的基础上, 进一步模拟了两个不同温度的颗粒在流体中的沉降.结果表明:两等温颗粒的沉降方式与雷诺数Re以及格拉晓夫数Gr密切相关, 而两不同温度的颗粒与两等温颗粒的沉降规律有显著不同.无论初始位置如何, 冷颗粒最终总位于热颗粒下方运动, Re较大时, 发生连续的拖曳、接触现象, 而Re较小时, 冷颗粒会以较大的沉降速度远离热颗粒. 关键词： 格子Boltzmann方法 颗粒沉降 热对流     

Al掺杂四针状ZnO纳米结构的制备及其光致发光和场发射特性

采用热蒸发法成功制备了Al掺杂四针状ZnO纳米结构(T-AZO),利用扫描电子显微镜、X射线衍射仪、荧光光谱仪和场发射测试系统分别研究了不同Al摩尔分数对T-AZO纳米结构表面形貌、微结构、光致发光谱和场发射特性的影响。实验结果表明:T-AZO纳米结构呈现六角纤锌矿结构,Al掺杂对四针状ZnO纳米结构的形貌产生明显影响并且使紫外发射峰产生蓝移。实验中,当Al掺杂摩尔分数为3%时,场发射性能最好,其开启场强为1.33 V/μm,场增强因子为8 420。     

Dynamic Rebound Behavior of Silica/Natural Rubber Composites: Fly Ash Particles and Precipitated Silica

This article investigated the elastic response of natural rubber (NR) compounds filled with silica from fly ash particles (FASi) and commercial precipitated silica (PSi), through a dynamic rebound test. The effects of silica content and initial drop‐height on the height and number of rebounds, dynamic stiffness, and the energy loss were of interest. The results suggested that the unfilled NR vulcanizates exhibited a greater elastic response than the FASi and PSi‐filled vulcanized composites. For given silica contents, the NR compounds with FASi had better elastic response than those with PSi, where the elastic response decreased with an increase in silica content. The greater the silica contents, the higher the dynamic stiffness of the composites. The initial drop‐height had no effects on the elastic response change for the unfilled NR compound, but resulted in an increase in the energy loss for the silica‐filled NR composites. The differences in the elastic responses for the NR compounds filled with silica from FASi and PSi were associated with the differences in crosslink density and the filler–filler interaction influenced by content of bis(3‐triethoxysilylpropyl) tetrasulfane (designated as Si69) used.     

Influence of Thermal Treatment on the Thermal Behavior of Poly‐L‐lactide

The influences of thermal treatment on cold crystallization and the thermal behavior of poly‐L‐lactide (PLLA) were investigated by DSC and polarizing microscopy. Both the cooling and heating rates had effects on cold crystallization. Double peaks were observed for the samples on subsequently heating at 10°C min^?1 after cooling between 5 and 20°C min^?1. The degrees of crystallinity dramatically increased with decreasing cooling rate, and the size of PLLA spherulites increased with a decrease in the cooling rate. Double cold crystallization peaks were also observed during heating traces at higher rates for this material after fast cooling (20°C min^?1) from the melt. The competition between the crystallization from the nuclei formed during cooling, and that from spontaneous nucleation might be responsible for the appearance of double peaks.     

Thermal and Thermo-Oxidative Degradation of Biodegradable Poly(Ester Urethane) Containing Poly(L-Lactic Acid) and Poly(Butylene Succinate) Blocks

A novel biodegradable poly(ester urethane; PEU) was synthesized by chain extension reaction of dihydroxylated poly(L-lactic acid; PLLA) and poly(butylene succinate; PBS) using diisocyanate as a chain extender. The kinetics of thermal and thermo-oxidative degradation of PEU containing PLLA and PBS blocks were studied by thermogravimetric analysis (TGA). TGA results indicated that PEU was more stable in air than in nitrogen and went through a two-stage degradation process irrespective of the experimental atmosphere. Activation energy of each stage was calculated by means of Kissinger, Kim-Park, Friedman, Flynn-Wall-Ozawa, and Kissinger-Akahira-Sunose methods. For the first stage, the activation energy value obtained in air was slightly higher than the corresponding value obtained in nitrogen; and for the second stage, the activation energy showed a much higher value in air than in nitrogen. The Coats-Redfern method was employed to study the degradation mechanism of each stage. The results indicated that the degradation of the first stage follows the P3/4 mechanism irrespective of the experimental atmosphere; the degradation of the second stage of PEU obeys the P1 mechanism in nitrogen while P3/2 in air.     

A Comparative Study on the Reinforcing Effect of Aramide and PET Short Fibers in a Natural Rubber-Based Composite

Four types of chopped fibers have been studied as reinforcement additives in a standard natural rubber based, carbon black filled formulation. The fibers studied were aramide (2 types) and polyester (2 types). The chopped fibers were added on top of the carbon black filled rubber compound at 2, 4, and 8 phr levels. The extra reinforcing effect in the modulus, especially at low elongation, the increase in hardness, the anisotropic properties, and the stiffening effects have been studied together with the evaluation of the mechanical hysteresis in strain and in compression. The permanent set of the resulting rubber compounds have been evaluated as well. The best compromise in performances and price was found for a certain type of polyester fiber.     

Mechanical,Thermal, and Surface Properties of Ultrahigh Molecular Weight Polyethylene/Polypropylene Blends

Various compositions of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends were prepared in decalin, with the rheological, mechanical, thermal, and surface properties of the blends being determined using the solution cast film. Viscosity and mechanical properties of the blends decreased below the additivity value with increasing PP content implying that PP molecules disturb the entanglement of UHMWPE. Contact angle of the blend films with a water drop increased with increasing content of PP. The atomic force microscope (AFM) images showed that the surface of cast UHMWPE was very smooth whereas that of cast PP was very uneven. For blends, the surface became rough and uneven with increasing content of PP. The melting temperature of PP (T _mP) decreased in the blends with increasing UHMWPE content while that of UHMWPE (T _mU) remained almost constant in blends.     

The Mechanical Properties,Compatibility, and Thermal Stabilities of POE-Graft-Methyl Methacrylate and Acrylonitrile(POE-g-MAN)/ Styrene-Acrylonitrile Copolymer (SAN Resin) Blends

POE-graft-methyl methacrylate and acrylonitrile (POE-g-MAN) was prepared by graft copolymerization of methyl methacrylate (MMA) and acrylonitrile (AN) onto polyethylene-octene copolymers (POE) with suspension polymerization. POE-g-MAN/SAN resin blends (AOMS) were prepared by blending POE-g-MAN with styrene-acrylonitrile copolymer (SAN resin). The mechanical properties, compatibility, and thermal stabilities of AOMS were studied. The notched impact strength of the blends reached 54.0 kJ/m² when the AN/(MMA + AN) ratio (f_AN) of POE-g-MAN, benzoyl peroxide dosage, and POE content in AOMS were 15 wt%, 1.0 wt%, and 25 wt%, respectively. Transmission electron microscopy analysis showed that the highest toughness occurred when the size of POE-g-MAN particles and the surface-to-surface inter-particle distance were proper. Scanning electron microscopy analysis indicated that the AOMS fracture surface had plastic flow visible, which looked like a fibril morphology when the AN/(MMA + AN) ratio (f_AN) of POE-g-MAN was 15 wt%. The toughening mechanism of AOMS was shear yielding of the matrix, which endowed AOMS with remarkable toughness. Dynamic mechanical thermal analysis showed that the compatibility of the POE phase and SAN phase improved after graft copolymerization of MMA and AN onto POE. When the grafting chain polarity was appropriate, the miscibility between POE-g-MAN and SAN resin was the best. Thermogravimetry analysis showed that thermal stability of AOMS increased with increasing AN units in POE-g-MAN.