关键词： Particle aggregates   Multi-scale modelling   FEM-DEM coupled simulations   Micro-structural features  

摘要： A multi-scale model for the analysis of granular systems is proposed, which combines the principles of a coupled FEM-DEM approach with a novel servo-control methodology for the implementation of appropriate micro-scale boundary conditions. A mesh convergence study is performed, whereby the results of a quasi-static biaxial compression test are compared with those obtained by direct numerical simulations. The comparison demonstrates the capability of the multi-scale method to realistically capture the macro-scale response, even for macroscopic domains characterized by a relatively coarse mesh;this makes it possible to accurately analyse large-scale granular systems in a computationally efficient manner. The multi-scale framework is applied to study in a systematic manner the role of individual micro-structural characteristics on the effective macro-scale response. The effect of particle contact friction, particle rotation, and initial fabric anisotropy on the overall response is considered, as measured in terms of the evolution of the effective stress, the volumetric deformation, the average coordination number and the induced anisotropy. The trends observed are in accordance with notions from physics, and observations from experiments and other DEM simulations presented in the literature. Hence, it is concluded that the present framework provides an adequate tool for exploring the effect of micro-structural characteristics on the macroscopic response of large-scale granular structures.

关键词： Ceramics   dental crown   mechanical properties   polymers  

摘要： Purpose To determine and compare the microstructure, flexural strength, flexural modulus, fracture strength, and microhardness of four types of computer-aided design/computer-aided manufacturing (CAD/CAM) materials for monolithic dental restorations. Materials and Methods A lithium disilicate (LD;IPS *** CAD), a zirconia-reinforced lithium silicate (ZLS;VITA Suprinity), a hybrid high-performance polymer (HPP) composite resin (GC Cerasmart), and a hybrid polymer-infiltrated ceramic network (PICN) material (VITA Enamic) were used to manufacture monolithic ceramic posterior crowns (n = 10) that were adhesively cemented on resin-based composite dies and loaded until fracture. In addition, 40 rectangular bars (n = 10) were milled and polished for three-point flexural strength testing. Microhardness (Vickers indentation), as well as quantitative (energy dispersive spectroscopy) and qualitative (scanning electron microscopy) structural analysis were conducted on fracture surfaces. Data were analyzed by one-way ANOVA and Tukey HSD post-hoc test (p = 0.05). Results Mechanical testing results showed that the material type has a significant effect on the fracture strength (p < 0.0001) of the monolithic crowns with ZLS and LD presenting significantly higher fracture strength than the PICN and HPP hybrid materials. LD showed the highest flexural strength (p < 0.0001) followed by ZLS, HPP, and PICN, respectively. The lowest flexural modulus and hardness were presented by HPP whereas ZLS had the highest flexural modulus and hardness. The LD presented the highest modulus of resilience and the PICN the lowest. Conclusions All CAD/CAM crown materials exhibited high values of fracture and flexural resistance, making them suitable materials for posterior full-crown restorations. Glass-ceramics suffered more from catastrophic and nonreparable fracture patterns, whereas minimal chipping and type II fracture patterns were more common in hybrid materials. The combination of more flexibil

关键词： Hydrogel   Microfabrication   Microstructuring   Microfluidics   Aqueous two-phase systems  

摘要： Growing importance of hydrogels in various areas of human life has led to increasing need in controlling their properties, which is generally achieved by adjusting hydrogels shape and microstructure. Even though standard microfabrication and microstructuring techniques can be currently applied in hydrogel research, the variety of properties of hydrogel materials makes it difficult to employ any of these techniques universally. Furthermore, to produce hydrogel structures complex enough to mimic biological tissues, several structuring and microfabrication approaches on various length scales would need to be combined. The complexity and diversity of problems associated with such processes raises a whole set of multidisciplinary challenges. This doctoral dissertation explores novel approaches to structuring and fabrication of polymeric and supramolecular hydrogels by combining modern microfabrication techniques with molecular self-assembly and/or exploiting mutual incompatibility of certain hydrophilic polymers. Chapter 1 introduces the main types and properties of hydrogels followed by a brief review of the literature on their microfabrication and structuring. Additionally, chapter 1 summarizes the challenges of existing techniques and outlines the goals of this doctoral project. Chapter 2 describes a method for continuous microfluidic production of hydrogel capsules with a hydrogel shell and a liquid core from an all-aqueous double emulsion. While most previous work on water-in-water emulsions focused on active droplet formation, here double emulsion droplets were spontaneously generated at a three-dimensional flow-focusing junction through the break-up of a double jet formed by immiscible aqueous solutions of polyethylene glycol and cross-linkable dextrans. The capsules obtained with this method displayed excellent stability in a variety of harsh conditions, as well as good permeability to polar solutes. Chapter 3 presents a general method for compartmentalizing supr

关键词： Powder metallurgy   Titanium   Chromium oxide   Solid solution   Grain refinement   Zener pinning   Solute drag  

摘要： The pre-mixed pure Ti and Cr2O3 powder was consolidated by spark plasma sintering (SPS) and hot extrusion to fabricate alpha-titanium (Ti) materials with oxygen (O) and chromium (Cr) elements by powder metallurgy (PM) process. The Cr2O3 particles were completely decomposed during SPS, and then O and Cr atoms were dissolved in alpha-Ti matrix. Oxygen atoms were remarkably improved the mechanical strength of PM Ti-O-Cr alloys by their solid solution hardening effect. The Cr solution and Ti4Cr precipitates had important roles to obstruct the grains coarsening behavior by the solute drag and Zener pinning effects, respectively. The solid solution strengthening effect by Cr atoms, however, was very limited due to a small Cr solubility of about 0.35 at% in alpha-Ti phase of Ti-O-Cr alloys. Since Ti4Cr precipitates with 10-20 mu m diameters were not so fine, they hardly contributed to the precipitation hardening of Ti-O-Cr alloys.

关键词： Additive manufacturing   Metallic meta-biomaterials   Heat treatment   Surface modification   Fatigue behaviour  

摘要： Recently, lattice titanium manufactured by additive manufacturing (AM) techniques has been utilized in various applications, including biomedical. The effects of topological design and processing parameters on the fatigue behaviour of such meta-biomaterials have been studied before. Most studies show that the fatigue life of additively manufactured lattice structures is limited. Post-processing techniques could play a major role in improving the fatigue of these promising biomaterials. This study aims to provide an in-depth investigation into the effects of heat treatments, hot isostatic pressing (HIP), sand blasting, and chemical etching on the microstructure, surface morphology, strength and fatigue resistance of selective laser melted titanium meta-biomaterials. It was found that the combination of microstructural design and surface engineering, induced by HIP and sand blasting respectively, allows to increase the endurance limit of these lattice meta-biomaterials by a factor of two. HIP treatment substantially decreased the internal porosity and transformed the microstructure to a more ductile mixture of alpha + beta phases. Sand blasting allowed to eliminate surface imperfections and induced favourable compressive stress in the surface layer of the struts. Statement of Significance Additively manufactured metallic meta-biomaterials are progressively being used as bone replacement orthopedic implants. While there is a great amount of research related to topological designs and their effect on mechanical (e.g. stiffness), physical (e.g. mass transport), and biological (e.g. osseointegration) properties, fatigue lifetime of such structures remains limited. This study provides fundamental investigation into the combined effect of microstructural design and surface engineering of titanium metabiomaterial, enabled through various post treatment methods ranging from heat treatments to physical and chemical surface modifications. The findings show that fatigue life is

关键词： 全疏膜   水处理   碳捕获   氧化锌奈米球   油水分离  

摘要： 全球暖化和水资源短缺现象日趋严重，水资源因气候暖化和工业废油水排放而不断受到威胁，我们需要发展突破性的水处理与碳捕捉技术来解决问题。为了解决水资源短缺等问题，相关薄膜海水淡化和碳捕捉技术已经发展多年，例如:薄膜接触器、油水分离…等。传统的多孔性薄膜应用在薄膜蒸馏程序只能处理相对干净的水质；但是对于含有微量界面活性剂的废水则完全无法应用薄膜蒸馏进行处理，因此其应用受到极大的限制。有监于此，一种可应用在薄膜蒸馏程序中处理含有界面活性剂的污水的薄膜是一亟需开发的材料技术，以应用在解决纺织、石化、科技厂等会产生低表面张力的含盐废水的相关产业领域。另外，能应用于油水分离的薄膜逐渐成为处理这些废油水的首选方法。因此，本研究致力于制备全疏膜与油水分离膜，探讨改质参数对薄膜结构之影响，证实全疏膜于薄膜接触器程序与超亲水膜于油水分离程序之适用性，提升全疏膜与超亲水膜本身应用价值。在本研究中的第一部分是全疏膜的制备、监定与薄膜蒸馏程序之应用，利用薄膜表面改质程序来制备不同润湿性的薄膜，并探讨凹角结构、氟化与表面氟素高分子涂布对薄膜效能之影响，借此在薄膜的孔洞创造气液界面以利于质量传送。在薄膜蒸馏程序中，是以直接接触式薄膜蒸馏（direct contact membrane distillation, DCMD）进行海水淡化之应用并配制低表面张力的海水为进料，期望借由全疏膜之特性，提升水通量稳定性及盐阻挡率。第二部分是探讨全疏膜于二氧化碳吸收程序之适用性，并进行长时间连续操作，并比较其与疏水膜的效能特性，最后一部分是制备超亲水膜于油水分离程序之研究，探讨奈米结构对油水分离效能之影响。在本研究中的第一部分是以化学槽沉积法制备全疏膜，并应用于薄膜蒸馏程序之研究，将氧化锌奈米粒子沉积在多孔性的玻璃纤维，借此形成层状凹角结构，后进行该结构的表面氟化程序，最后再使用高分子降低薄膜的表面能制备全疏膜。研究方法乃利用孔径分析仪（CFP）、扫描电子显微镜（SEM）、X射线光电子能谱（XPS）、原子力显微镜（AFM）与接触角测角仪等来分析全疏膜的性质，并比较其与疏水膜的特性。实验结果发现全疏膜表面具有特别的凹角结构型态，且在其表面上含有高浓度的氟(F)，上述之全疏膜的疏水与疏油程度是借由接触角的测量数值进行评估，其水与乙醇接触角分别高达152.8 ± 1.1°和110.3 ± 1.9°。全疏膜应用于薄膜蒸馏程序时，对于低表面张力的进料液体具有良好的抗湿润效果，使其水通量提升并维持稳定，盐阻挡率高达99.99％。在本研究中的第二部分是以全疏膜进行二氧化碳吸收程序之研究。结果指示，因为全疏膜对醇胺类吸收剂有更良好的抗润湿能力，在二氧化碳吸收程序进行长期的连续操作，全疏膜的二氧化碳吸收通量皆高于疏水膜。本研究的最后一部分是以化学槽沉积法制备超亲水膜，并应用于油/水分离程序之研究。使用化学槽沉积法制备氧化锌奈米粒子于玻璃纤维表面，借此形成多层次结构之超亲水膜。以扫描式电子显微镜、X射线光电子能谱仪与能量色散谱(EDS)监定薄膜表面形貌与组成，使用原子力显微镜证实氧化锌奈米粒子可增加薄膜表面奈米级之粗糙度，从接触角测量仪证实薄膜的疏油性，其水下油接触角大于150°。使用重力驱动的过滤装置来进行油水分离程序，并探讨多层次结构对超亲水膜的分离效果之影响。测试结果显示，薄膜的水通量可以大于250 L/m2-h，分离效率高于98 ％。另外，在长效性与再生性的油水分离程序中，实验结果发现所制备的薄膜具有良好的油水分离效率与重复使用性。最后总结前面章节之研究结果，针对全疏膜于薄膜蒸馏和二氧化碳吸收程序的未来发展以及可能性给予建议。由结果指出，本论文成功制备具有较高抗润湿能力之全疏膜，提供全疏膜之制备参数对薄膜结构及其于薄膜接触器应用端之影响作为参考依据，其具有处理低表面张力的含盐废水的功效，克服了习知技术无法解决的问题瓶颈。首次将全疏膜延伸应用至二氧化碳吸收程序中，进一步发掘全疏膜之发展潜力，未来能利用全疏膜于火力发电厂进行大规模的二氧化碳吸收程序。另外，本论文提出以化学槽沉积法制备奈米级结构于薄膜表面，成功制备超亲水膜并应用于油水分离程序，由结果指出，超亲水薄膜具有特殊的水下超疏油特性。因此，薄膜具有优异的油水分离效率且具有处理废油水的广泛应用性。

关键词： 固体氧化物燃料电池(SOFC)   合金连接体   涂层   粉末还原法   纳米微结构  

摘要： 降低固体氧化物燃料电池(SOFC)工作温度,可使用高电导、高热导、高强度的合金作为连接体。其中含Cr的铁素体不锈钢合金,具有与SOFC其他部件材料匹配的热膨胀系数,此外还有易于加工及成本低廉等优点,成为中低温板式SOFC连接体材料应用与研究的重点。但这类合金表面高温氧化所带来的界面电阻变化,及Cr挥发进而在阴极沉积所带来的诸多问题,成为影响板式SOFC长期稳定的关键因素,因此必须进行有效的表面处理。从SOFC合金连接体引起的电堆性能衰减机理出发,阐明了降低或防止阴极Cr中毒的几类方法,论述了合金连接体涂层的必要性。结合笔者开发设计的尖晶石粉末还原法在合金连接体表面制备纳米微结构Mn_(0.9)Y_(0.1)Co_2O_4(MYC)防护涂层方面的工作,综述了国内外SOFC合金连接体涂层材料及涂层制备方法的研究进展。对各类涂层材料及涂层制备方法优缺点进行比较的同时,重点介绍了电导率高、实用性较强的钙钛矿结构及尖晶石结构涂层材料。最后展望了合金连接体涂层的发展前景。

关键词： 仿生微结构   皮革   转移细菌   抑菌  

摘要： 目的研究具有仿生微结构皮革新材料对细菌的转移抑制效果。方法采用激光共聚焦显微镜观察法和细菌定量检测方法,对皮革新材料仿生微结构和对细菌转移抑制作用进行研究。结果激光共聚焦显微镜观察显示,皮革表面微结构符合产品设计要求。经含有菌液的滤纸染菌后,金黄色葡萄球菌、大肠杆菌、铜绿假单胞菌和白色念珠菌从含有微结构的皮革表面转移至接触碟的菌量与对照组相比,分别减少85. 1%、78. 3%、81. 6%和77. 2%。结论含有仿生微结构的皮革新材料显著抑制不同细菌在已污染表面的转移和传播。

关键词： Photovoltaic   Phase change material   Cooling   Nanoparticles  

摘要： This work focused on experimental investigation of using a novel phase change material (PCM) in a PV module to enhance its cooling performance. Phase change material by absorbing a lot of heat from the surface of the PV module and control the heat capacitance of the system causes to raising its overall efficiency. In order to postpone the melting of the PCM material, copper microchannel tubes in which cold water is flowing is used and located in a chamber at the backside of the PV module. At first step of experiments, sheep fat has been used as a novel PCM and secondly;in order to increase the cooling efficiency of the sheep fat, CuO nanoparticles (0.004 (w/v)) have added to it. The results of the pure sheep fat and sheep fat + CuO nanoparticels have been compared with the layout of using paraffin wax as a conventional PCM. The obtained results of surface temperature, maximum power increase and electrical efficiency of the PV module for using each PCM materials have been compared. Results show that using both PCMs (sheep fat and paraffin wax) can enhance the cooling performance of the studied PV module;however, sheep fat is more efficient rather than the paraffin wax. In addition, results depicted that adding CuO nanoparticels in the sheep fat causes to significantly decrease in the average temperature of PV module surface. The research experiments of using sheep fat + CuO nanoparticels confirm that the maximum generated power is increased about 24.6% to 26.2% compared with the layout of no cooling system and 5.3% to 12% compared to paraffin wax.

关键词： Stable crack growth   Homogenized multigrid XFEM   Updated Lagrangian approach   von-Mises yield criterion   Multiple microstructural defects  

摘要： In this work, stable crack growth behavior of SA508 carbon steel has been evaluated through experimental and numerical investigations. The effect of various types of flaws has been observed on the load carrying capacity of the structures. A homogenized multigrid XFEM approach has been proposed to simulate the stable crack growth in ductile material using finite strain plasticity. The geometric nonlinearity has been modeled by updated Lagrangian approach whereas the material nonlinearity has been modeled by von-Mises yield criterion using isotropic strain hardening. The accuracy and effectiveness of the proposed approach has been verified by solving several stable crack growth problems. (C) 2016 Elsevier Ltd. All rights reserved.