Thermal shielding of multilayer walls with phase change materials under different transient boundary conditions

A numerical model is presented to determine the thermal shielding performance of an exterior wall (e.g., building envelope) containing layers of PCMs. The model is exploited to perform a parametric study to assess the influence of the position and melting temperature of one PCM layer. Results showed that benefits are to be expected when the interior and exterior temperatures are close. Then, the wall composition has been optimized with a genetic algorithm based on a yearly analysis with the possibility of including several PCM layers. Idealized weather conditions and measured weather conditions (including solar radiation) have been considered. Results showed that for Québec City, optimal south-facing wall includes one PCM layer when real weather data are considered. Its effect is to shield the heat transfer in the summer. This paper provides a fundamental understanding of multilayer walls with PCMs.     

Investigation of methane steam reforming in planar porous support of solid oxide fuel cell

Adopting the porous support in integrated-planar solid oxide fuel cell (IP-SOFC) can reduce the operating temperature by reducing thickness of electrolyte layer, and also, provide internal reforming environment for hydrogen-rich fuel gas. The distributions of reactant and product components, and temperature of methane steam reforming for IP-SOFC were investigated by the developed physical and mathematical model with thermodynamic analysis, in which eleven possible reaction mechanisms were considered by the source terms and Arrhenius relationship. Numerical simulation of the model revealed that the progress of reforming reaction and the distribution of the product, H₂, were influenced by the operating conditions, included that of temperature, ratio of H₂O and CH₄, as well as by the porosity of the supporting material. The simulating results indicate that the methane conversion rate can reach its maximum value under the operating temperature of 800 °C and porosity of ε = 0.4, which rather approximate to the practical operating conditions of IP-SOFC. In addition, characteristics of carbon deposition on surface of catalyst were discussed under various operating conditions and configuration parameters of the porous support. The present works provided some theoretical explanations to the numerous experimental observations and engineered practices.     

Size dependent strengthening in particle reinforced aluminium

The tensile behaviour of composites produced by infiltrating ceramic particle beds with high purity (99.99%) Al is studied as a function of reinforcement size and chemistry (Al₂O₃ and B₄C). The yield stress is higher in composites containing B₄C particles, increasing with decreasing interparticle distance in both composite systems. The flow stress of the composites, when corrected for damage, displays the same dependence on interparticle distance as the yield stress. The overall strain hardening exponent, however, is independent of the microstructural scale. These observations are rationalized based on the theory of geometrically necessary dislocations.     

Multifunctional architectured materials for electromagnetic absorption

A sandwich structure involving a honeycomb core filled with a carbon nanotube-reinforced polymer foam and glass fiber-reinforced composite face sheets has been developed in order to combine high electromagnetic absorption and high mechanical performance. The large electromagnetic absorption is attained by simultaneously minimizing the reflection and transmission, which, in terms of effective material properties, requires a low dielectric constant and a conductivity around 1 S m^?1. The sandwich offers also high stiffness versus density performance.     

Microstructure Evolving Behaviors of Undercooled Ultrafine Austenite Grains During Deformation

 Samples with ultrafine grained austenite were prepared by repetitive rapid heating and quenching for three times and were used to investigate the dynamic microstructural evolving behaviors at different temperatures. A simultaneous development of dynamic strain induced transformation (DSIT) and austenite grain growth was detected at the deformation temperatures above Ar3, while only DSIT happened as the deformation proceeded at lower temperatures close to and below Ar3. In addition, a reverse ferrite to austenite transformation was also observed. Most of the strain induced ferrite nucleated on the boundaries of ultrafine prior austenite grains, especially at the corners and no evidence about intragranular nucleus was obviously obtained.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

Mechanical and thermal expansion properties of β-eucryptite prepared by sol–gel methods and hot pressing

The microstructures, mechanical properties and thermal expansion behavior of monolithic lithium aluminosilicate glass–ceramics, prepared by sol–gel method and hot pressing, were investigated by using X-ray diffraction, scanning and transmission electron microscopies, three-point bend tests and dilatometry. β-eucryptite appeared as main phase in the monolithic lithium aluminosilicate glass–ceramics. The glass ceramics exhibited high relative densities and the average flexural strength and fracture toughness values were 154 MPa and 2.46 MPa m^1/2, respectively. The lithium aluminosilicate glass–ceramics hot pressed 1300 and 1350 °C demonstrated negative coefficient of thermal expansion, which was affected by amount and type of crystalline phases.     

Near surface martensitic transformation and recrystallization in a Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating by plasma spraying

Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrate have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the influence of the high temperature process on substrates can not to be ignored, especially for metastable β type titanium alloy substrates. In this paper, martensitic transformation and recrystallization occurred in near surface of the low-modulus Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating. Various analyses revealed that the martensitic transformation and recrystallization were mainly related to the high temperature and cooling process. The different microstructures can be attributed to inhomogeneous temperature and cooling rate distributions in the substrate. Corresponding to the microstructure, the Young's modulus and micro-hardness of the near surface of the alloy showed a similar graded distribution along the direction perpendicular to the coating/substrate interface. In addition, the martensitic transformation and recrystallization caused remarkable changes of the entire alloy.     

Generation and characterization of T40/A5754 interfaces with lasers

Laser-induced reactive wetting and brazing of T40 titanium with A5754 aluminum alloy with 1.5 mm thickness was carried out in lap-joint configuration, with or without the use of Al5Si filler wire. A 2.4 mm diameter laser spot was positioned on the aluminum side to provoke spreading and wetting of the lower titanium sheet, with relatively low scanning speeds (0.1–0.6 m/min). Process conditions did not play a very significant role on mechanical strengths, which were shown to reach 250–300 N/mm on a large range of laser power and scanning speeds. In all cases considered, the fracture during tensile testing occurred next to the TiAl₃ interface, but in the aluminum fusion zone. The interfacial resistance was then evaluated with the LASAT bond strength tester, based upon the generation and propagation of laser-induced shock waves. A 0.68 GPa uniaxial bond strength was estimated for the T40/A5754 interface under dynamic loading conditions.     

焊接热输入对800MPa超级钢焊接接头组织性能的影响

对800 MPa超级钢不同焊接热输入作用下HAZ组织和性能进行了研究.结果表明,800 MPa超级钢在焊接热循环作用下HAZ晶粒明显长大,随着焊接热输入的增大,晶粒长大的趋势越显著;HAZ包含有超低碳贝氏体和细小铁素体等组织,过热区组织在焊接热循环的作用下变化明显,是断裂裂纹的发源地;不同焊接热输入下,整个HAZ的硬度...