关键词： latent heat   Field's metal   Wood's metal   thermal cycling   thermal stability   passive cooling  

摘要： Metallic phase change materials (mPCMs) have been demonstrated as potential passive cooling solution for pulse power applications. The possibility of integrating a metallic PCM directly on top of a heat source, reducing the thermal resistance between the device and the cooling system, could result in a significant improvement in thermal management for transient applications. However, many thermo-physical properties of these alloys are still unknown;furthermore, their microstructural stability with repetitive melting/solidification cycles is not warrant. In this work, we provide a series of potential mPCMs for thermal management of electronics operating on a wide range of application temperatures, followed by an experimental investigation of microstructural and thermo-physical stability of these materials under repetitive melting solidification cycles. The results of the effect of cyclic thermal loading of theses alloys on the melting behavior and latent heat of fusion are discussed. Thermal stability of 51.0wt % In-32.5wt %Bi-16.5wt %Sn and 50wt %Bi-26.7wt %Pb-13.3wt %Sn-10wt %Cd alloys, as potential midtemperature mPCM, has been evaluated. The results suggest that these mPCMs maintain their thermo-physical stability over large periods of thermal cycles.

关键词： Thin films   Plasma-enhanced chemical vapor deposition   Optical properties   Microstructural properties   Substrate material  

摘要： Plasma-enhanced chemical vapor deposition (PECVD) technique is well suited for fabricating optical filters with continuously variable refractive index profiles;however, it is not clear how the optical and microstructural properties of thin films differ when deposited on different substrates. Herein, silicon oxynitride films were deposited on silicon, fused silica, and glass substrates with two stoichiometric ratios by PECVD, using silane, ammonia and nitrous oxide gasses, to investigate the effects of the substrate used on the optical properties and microstructures of the films. All the deposited films were amorphous. Further, the types and amounts of Si-centered tetrahedral O-x-Si-N(y )bonds formed were based upon the substrates and stoichiometric ratios used;O-x-Si-N-y bonds with higher elemental nitrogen content were formed on Si substrates, which lead to obtaining higher refractive indices. The relationship between the surface roughness of the film and type of substrate was discussed. The extinction coefficients of all the films were very small, and the films exhibited low absorption. The refractive indices of the films formed on the different substrates had a maximum difference of 0.03. The deposition rates of the films increased in the order of fused silica, glass, and silicon.

关键词： SiC颗粒   复合材料   超声处理   微结构   性能  

摘要： 利用超声波预处理颗粒与合金熔体的复合工艺制备SiC颗粒增强铝基复合材料,研究七种不同的超声预处理工艺对SiC颗粒表面清洁度与润湿性的影响规律,研究搅拌过程中不同超声功率、施振时间对增强体颗粒分布、界面形态与材料原生性能的影响规律及作用机制。结果表明:高温焙烧工艺可以显著改善颗粒的清洁度与润湿性;增大超声功率能够增强对SiC颗粒团聚的抑制作用,提高其在基体组织分布的均匀性,在功率一定的条件下,施振时间不宜过长,否则超声的驻波效应将形成颗粒的二次偏聚。

关键词： Fatigue   Welding   Microstructure   WAAM   Mechanical properties  

摘要： Many engineered components on subjecting to critical environments require properties that slightly differs with location in the fabricated part. The inadequate understanding of wire arc additive manufacturing (WAAM) process-microstructural features correlation brings interests on the structural reliability of functionally graded materials (FGM), which limits the extensive use of this cutting edge technology. Gas metal arc welding (GMAW) based WAAM process was adopted to fabricate the FGM of super austenitic stainless steel SS904L and Hastelloy C-276 with excellent bonding. The micrographs revealed that the WAAM processed FGM was mainly composed of columnar and equiaxed dendrites and had noticeable inhomogeneous features along the built direction. Tensile test results of sample along the built direction revealed a yield strength of 311.08 MPa and tensile strength of 680.73 MPa. Cyclic loading results depicted that the WAAM processed FGM has a fatigue resistance of 156 Mpa after sustaining 2 x 10(6) cycles. As a result of the varying microstructural characteristics along the built direction, the failure was initiated and fractured at the SS904L weaker region.

关键词： 微结构设计   网状碳纳米管蒸发膜   界面蒸发   光热转换   太阳能蒸发器   重盐水  

摘要： 目前,太阳能海水淡化领域通过光子管理、纳米尺度热调控、开发新型光热转换材料、设计高效光吸收太阳能蒸馏器等方法实现了界面太阳能驱动蒸汽生成,这种绿色、可持续的脱盐技术已成为近年来的研究热点。碳基材料如碳纳米管、石墨烯、炭黑、石墨等都有涵盖整个太阳光光谱的光吸收能力,是一类新型的光热转换材料。本文通过对材料进行微结构设计,使用化学气相沉积(CVD)技术,在不锈钢网状骨架上生长碳纳米管形成光热转换活性区,以实现高效光吸收、光热转换,并进一步设计了房屋型太阳能蒸发器,其中盐水表面被微米网状-碳纳米管蒸发膜覆盖,利用光热转换过程产生的热量驱动重盐水中的水蒸发产生水蒸气,最后对水蒸气进行冷凝回收实现脱盐。实验结果表明,当光照强度为1个太阳光(1 kW·m-2)时,膜表面温度迅速升高并稳定于84.37°C,对于重盐水(100 g·L-1 NaCl)的脱盐率达到99.92%,可实现稳定持续的重盐水脱盐。这种方法可用于构建多孔界面光热转换脱盐系统,对设计界面光蒸汽转化膜材料及器件,实现规模化海水淡化具有重要的意义。

关键词： Binary alloys  

摘要： Aluminothermic reactions between Al microparticles and preoxidized Ni particles were carried out to fabricate mixed compounds of NixAly containing in situ generated Al2O3. The mixed composites were obtained by applying a thermal treatment to either a mixture of NiO and Al powders or to a stacking of powders. The effects of the particle size, thickness of oxide, heating ramp and temperature on the microstructure of the NiAl-Al2O3 coating are discussed. The results indicate that Ni3Al forms first during the synthesis, followed by NiAl and Al3Ni2 while Al3Ni forms on cooling. Moreover, the alumina was observed to form a continuous network surrounding NixAly intermetallic compounds. It derives that these composites have a potential to form self-healing coatings.

关键词： Hempcrete   Phase Change Materials   Micro-encapsulation   Thermal properties   Energy consumption   CFD  

摘要： Biocomposite materials offer the potential to reduce the embodied energy in the construction of buildings significantly. This study develops a new low-carbon heat storage material composed of hempcrete and microencapsulated phase change material (MPCM) that is capable of reducing energy consumption and improving indoor thermal comfort in buildings. Eight hempcrete-MPCM (HPCM) composites were created using: (a) two MPCM types, (b) four MPCM melting temperatures, and (c) two MPCM concentrations. A set of experiments enabled thermophysical and microstructural characterization of MPCM, hempcrete, and HPCM samples. Furthermore, numerical simulations allowed the extension of the experimental results by comparing the performance of timber-frame walls made of hempcrete and HPCM infills with different MPCM concentrations. The findings show that, on the one hand, the developed HPCM formulas have superior heat storage potential compared to the hempcrete owing to their 35% to 85% higher maximum specific heat capacity than hempcrete. On the other hand, HPCM formulas, on average, have lower thermal conductivity than hempcrete due to the low thermal conductivity of MPCMs. As a result, HPCM wall types achieve approximately 5% to 16% and 1% to 36% higher heating and cooling energy savings, respectively, compared to the hempcrete wall. However, the results also indicate that an increase in the percentage share of MPCMs from 9% to 18% in the hempcrete mixture reduces total energy savings and, in particular cooling savings. Therefore, there is a need for a thorough consideration of the operating temperature and percentage share of MPCMs within the hempcrete concerning the specific application and performance objectives. The optimal integration of HPCMs into building envelope might also require changes in the operation of heating and cooling systems.

关键词： Shape memory polymers   Polymer nanocomposite   Thermoplastic polyurethane (TPU)   Hybrid filler   Carbon nanotube (CNT)   Montmorillonite (MMT)  

摘要： The recent rapid development of technology has demanded smart materials with tailoring a bridge between macro properties and sophisticated micro and nano characteristic. Principally, shape memory polymers (SMPs) will come to play as an indispensable part of numerous aspects of human activity. Nevertheless, the low mechanical strength and thermal conductivity of SMPs have primarily restricted their applications. To impart shape memory behaviour and mechanical properties, we fabricated a series of composites by a feasible and commercial melt-mixing method. Thus, a series of fast heat-actuated shape memory polymer composite with greatly enhanced stretch-ability, mechanical stiffness, dynamic-modulus, rheological qualities, recovery and fixity ratio was prepared by incorporating multi-walled carbon nanotubes (CNT), montmorillonite (MMT) and CNT:MMT hybrid into thermoplastic polyurethane (TPU). Noteworthy, CNT-based specimens exhibited superior mechanical properties than those of MMT-based samples, and interestingly, the hybrid composites featured a synergistic effect due to the sacrificial role of MMT nanoplatelets for adjusting the dispersion of CNT nanotubes. Microstructural observations indicated that the crystallization percentages of the composites were generally higher than that of pristine TPU;therefore, the shape-memory performance of the specimens improved notably in the case of the hybrid composites owing to creating more interfacial zone with CNT:MMT nanoparticles as compared to other simple composites. This study proved that the simultaneous incorporation of CNT and MMT nanoparticles not only granted outstanding mechanical properties, but also improved the overall shape memory behaviour of the composites by systematical localization of the nanoparticles without any functionalization or modification.

关键词： Additive manufacturing   Selective laser melting   316L stainless steel   High-throughput experiments   Processing maps  

摘要： In the present study, a combination of high-throughput (HT) and low-throughput (LT) techniques was used to rapidly determine the processing window and generate processing maps for Selective Laser Melting (SLM) of 316L stainless steel. The HT method includes the fabrication of hundreds of hex nut-shaped specimens, each processed with a unique combination of laser power, scanning speed, and hatch spacing. An easily removable scaffolding permitted rapid sample extraction from the base plate, thus saving machining cost and time. Hardness and immersion density measurements were used for HT characterization to identify a processing window for maximum strength and density. Within the defined processing window, a low-throughput (LT) microstructural interrogation of specimens were performed. The microstructural analysis included quantification at various length scales (i.e., grains size and morphology, texture, primary dendrite arm spacing, and melt pool geometry analysis). Microstructure-based processing maps as a function of volumetric energy density were generated. The combination of HT and LT methods produced a predictive relationship between hardness and primary dendrite arm spacing using a Hall-Petch relationship. A model is proposed to explain the dependence of microstructure on the melt pool geometry. The HT method can be applied for the microstructural design of SLM-fabricated components in other alloys.