关键词： Creep   Brittle fracture   Plastic flow collapse   Material nonlinearity   Fractals   Dimensional transition  

摘要： The transition from extremely brittle to very ductile behaviours of creeping materials is discussed, where analogies with power-law hardening materials are pointed out. Considering Norton's Law as a viscous constitutive law, it is possible to define a generalized stress-intensity factor K-c -characterizing the intermediate asymptotic behaviour under steady-state creep conditions- with physical dimensions depending upon the Norton stress exponent n . In the two limit cases of creep resistant materials (n similar or equal to 1) and creep sensitive materials (n >> 1), K-c assumes respectively the dimensions of an elastic stress-intensity factor ( F L-3/ 2) and of a stress (FL-2). Such a dimensional transition, with consequent stress-singularity attenuation, is completely analogous to that occurring through the introduction of a fractal stress-intensity factor (K-c) *, when the influence of microstructural disorder is considered. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Shape memory polymer   Thermoplastic polyurethane   Montmorillonite layered silicate   Halloysite nanotube   Hybrid filler  

摘要： Herein, the effect of simultaneous incorporation of two silicate-based nanofillers, including montmorillonite (Mt) and halloysite nanotube (Hal), on mechanical and shape memory performance of thermoplastic polyurethane (TPU) nanocomposites was studied. Different amounts (1, 3, and 5 wt%) of Hal, Mt. and hybrid Hal:Mt. were embedded into TPU matrix, utilizing a melt mixing method. Morphological studies on the hybrid nano composites elucidated that appropriate dispersion and distribution were achieved. Higher transition temperatures, as well as lower degrees of crystallinity, were observed for TPU/Hal:Mt. nanocomposites, compared to the other counterparts. Dynamic-mechanical-analysis (DMA) results illustrated superior storage modulus in the hybrid nanocomposites in both glassy and rubbery regions owing to superlative polymer-filler interactions. According to the tensile measurements, the hybrid nanocomposites demonstrated remarkable improvements in Young's modulus as well as ultimate strength and elongation at break than those of solo-nanocomposites due to better stress transfer from the hybrid fillers to the TPU. Superior ameliorations of storage modulus, both in glassy and rubbery regions, consequently effectuated the shape memory performance of the hybrid nanocomposites. For instance, 5 wt% incorporation of the hybrid filler within the matrix enhanced shape fixity ratio, shape recovery ratio, and recovery speed by 6.1%, 2.9% and 2.3 s, respectively, compared to pristine TPU.

关键词： 水泥水化   HYMOSTRUC3D模型   矿物掺合料   微观结构   CH含量   水化放热  

摘要： 现代水泥基材料大量使用矿物掺合料,使得关于水泥石微观结构演变特征的研究变得更加复杂。本实验充分考虑材料氧化物组成和含量,计算出硅酸盐水泥、粉煤灰和矿渣的水化速率参数,并代入HYMOSTRUC3D模型中建模,同时考虑水灰比和矿物掺合料的影响,建立了水泥、C-S-H、CH、水化热、孔隙率、孔径分布等随水化龄期的变化关系,获取了水泥水化过程中微观结构的演变信息。基于在HYMOSTRUC3D中所建模型获取了CH和C-S-H的含量,并与XRD测试结果和CEMHYD3D模拟结果进行了对比;考虑了CH在水泥水化过程中的产生以及与FA或BFS混配形成的水泥基材料中的消耗;并模拟研究了不同掺量的活性矿物掺合料对水泥水化放热的影响;同时,预测了水灰比对孔隙率和孔径分布的影响以及孔隙率和孔径分布随时间的变化规律。研究结果表明:HYMOSTRUC3D模拟结果与XRD实验测试结果、CEMHYD3D模拟结果整体吻合较好,验证了所计算水化速率参数和所建立模型的准确性。同时还发现,FA或BFS的掺入可消耗水泥水化产生的CH,且可有效控制水泥基材料的水化放热,掺量越大,消耗的CH越多,控热效果越明显。此外,水灰比越大的水泥浆孔隙率越大,大孔越多,孔径分布越宽。

关键词： 激光器   光纤光栅   氢气传感器   Pt-WO3   纳米棒   飞秒激光  

摘要： 制备了一种纳米棒Pt-WO3螺旋微结构光纤布拉格光栅(FBG)氢传感器。采用飞秒激光对光纤布拉格光栅包层进行了螺旋微结构的制备,通过水热法合成了纳米棒WO3,然后分解Pt(acca)2前驱体,合成Pt-WO3纳米棒颗粒,将两种金属的原子比控制在Pt∶W=1∶5,将纳米棒Pt-WO3镀在FBG微结构包层上。相比镀Pt-WO3膜的标准FBG,Pt-WO3纳米棒微结构FBG氢传感器灵敏度提高了1.5倍,传感器在1%氢气体积分数下的响应时间为15~30 s,该传感器在氢气传感领域具有良好的应用前景。

关键词： ZnO   水热法   微结构   光学性能  

摘要： ZnO是一种典型的稀磁半导体材料,具有独特的光学和压电等优良特性,可广泛应用在新型的电子器件中。采用水热法生长出了本征ZnO纳米材料,用X射线衍射分析和扫描电子显微镜对其微结构进行了表征,采用光致发光谱分析其光学性能并从能级缺陷图解释了发光机理,结果发现ZnO纳米材料在紫外区的发光峰是因为高能级的电子跃迁到低能级与空穴进行复合后释放能量所引起的;而可见光区域的发光峰则是因为晶体中的缺陷产生,氧反应引起的。

关键词： 声子晶体   拓扑优化   带隙性质   遗传算法   多相微结构  

摘要： 声子晶体的带隙特性可以通过调整其周期性的微结构来控制,具有很强的可设计性,因此在材料的隔振、隔声和滤波等领域具有潜在的应用价值。合理的控制材料的布局和比例,有望得到具有特殊带隙性质的复合材料。通过遗传算法和拓扑优化技术研究了二维三相声子晶体的优化设计,并给出了三个算例研究。结果表明:三相材料能够得到更好的隔振效果;对比两相材料,三相声子晶体能够进一步降低禁带频率,并得到更大的相对带宽。可见,利用进化算法进行多相声子晶体的拓扑优化设计是可行的。

关键词： Grain-boundary diffusion   Nonisothermal conditions   Numerical solution   Concentration distribution   Triple junction  

摘要： The paper presents a two-dimensional model of grain-boundary diffusion with consideration for triple junctions. A diffusant is assumed to come from an amorphous film containing an active component. In heating, the diffusant is transferred from the film into the crystalline material, which is composed of the two phases: grain (volume) phase and grain-boundary phase. The material structure is specified explicitly, which allows an investigation of the influence of the phase size and triple junctions on diffusion dynamics. Boundary conditions suggest a perfect contact between the phases. Consideration is given to a temperature dependence of the diffusion coefficient. A variation in temperature with time is specified explicitly. The problem is solved numerically. We plot concentration-time dependences at points of the computational domain as well as the diffusant concentration distribution over the phases to illustrate the effect of grain size on diffusion. The diffusant is found to behave nonmonotonously, accumulating and consuming, at the triple junction during diffusion. The higher the diffusion coefficient ratio between the phases and the smaller the grain, the faster the concentration variation at the triple junction.

关键词： cement-based materials   correlation   desert sand   fly ash   mechanical property   PVA fiber  

摘要： In order to study the engineering application of desert sand and PVA fiber, a single factor test scheme was adopted to explore the influence of fiber length, fiber content, fly ash content and sand type on the mechanical properties of cement-based materials reinforced by fiber. The correlation between compressive strength, splitting tensile strength and bending strength was analyzed. The internal structure of the material was investigated by scanning electron microscopy and mercury injection. The results show that the compressive strength, splitting tensile strength and flexural strength of the material increase with the decrease of fiber length and the increase of fiber content, and there is a linear correlation among them. With the increase of fly ash content, the compressive strength of the material continuously decreases, splitting tensile strength and flexural strength first increase and then decrease, and the addition of fly ash improves the fiber-cement matrix interface. Compared with quartz sand, the splitting tensile strength and flexural strength of the material prepared from desert sand are improved. Due to the small particle size of desert sand, the proportion of the cumulative volume of medium and large pores in the materials increases. © 2020 Bulletin of the Chinese Ceramic Society Press. All Rights Reserved.

关键词： 聚合物   涂层   表面微结构   分子扩散   内应力  

摘要： 聚合物材料表面微结构对其功能化的实现具有至关重要的作用。在过去几十年的时间里,通过电纺、光刻、等离子处理等经典方法制备了各种各样的结构功能表面,实现在光、电、生物、化学等领域的广泛应用。然而为满足技术发展需求,实现表面微结构调控新策略的开发势不可挡。本文主要从分子扩散、材料内应力、外力的施加/释放,以及多种机制协同作用四个角度出发对聚合物表面结构调控的新策略进行介绍,并对今后聚合物表面结构调控的发展方向进行简要论述。

关键词： 微纳米测量   纳米测量机   三维微接触测头   高深宽比   侧壁倾角  

摘要： 针对当前微纳米测量中存在的微结构跨尺度、高精度测量及高深宽比结构多参数表征问题,基于纳米测量机和微接触测头构建了纳米坐标测量系统。通过对测头与定位平台机械、电气及软件接口的设计,实现测头与平台的集成,并利用标准球对测量系统进行校准。为保证测量结果的可溯源性,对定位平台三轴激光干涉仪的激光器进行了拍频。最后,利用搭建的测量系统对高度10μm,2 mm的超高台阶及硅臂卡爪的侧壁倾角进行了测量,表明系统具备大尺寸结构的高精度测量和复杂MEMS器件特征尺寸的精确表征能力。