Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Processing of ZrO2 scaffolds coated by glass–ceramic derived from 45S5 bioglass

Three dimensional, highly porous, ZrO₂ scaffolds coated by glass–ceramic derived from 45S5 bioglass were fabricated. The surface reactivity of 45S5 in aqueous solution was investigated as a function of the immersion time. The influence of the solid loading on the rheological behavior of 45S5 aqueous slips with ammonium polyacrylate (NH₄PA) was studied; besides the effect of poly(vinyl)alcohol (PVA) on the relative viscosity was determined. The structure and microstructure of uncoated and coated ZrO₂ scaffolds were characterized. The high ionic exchange capability of 45S5 was demonstrated by the pH rise, the significant weight loss and the amorphous calcium phosphate nucleation, upon its immersion in aqueous solution. The addition of PVA did not affect the dispersion properties of the 45S5 powder, which were basically controlled by its negative surface charge. 30 wt% 45S5 slips with 4 wt% PVA exhibited a yield stress and an adequate viscosity in the low shear rate range, to produce a bioglass coating into the ZrO₂ scaffold. The glass-ceramic coating was distributed along the strut surfaces, forming a thin film without altering the porosity and the strut thickness of the original ZrO₂ scaffold structure.     

Synthesis and Evaluation of Bis-quaternary-Based Surfactants as Additives for Water-Based Mud

Two series of cationic gemini surfactants, alkanediyl-α,ω-bis[N,N-dimethyl alkyl (octyl or dodecyl)ammonium] dibromide (R-s-R; s = 6, 10, 12 and R = 8 and 12) were prepared and evaluated as additives for water-based mud. The chemical structures of the prepared surfactants were confirmed using FTIR and mass spectroscopy. Surface activity of these compounds has been studied and their surface properties including surface tension, emulsification power, critical micelle concentration, effectiveness, maximum surface excess and minimum surface area were determined. The results showed that the prepared compounds have significant surface activity, especially those of longer hydrophobic chain length. The prepared cationic gemini surfactants were evaluated as viscosifiers and filter loss additives for water-based mud formulated from local Na-montmorillonite clay. XRD analysis was carried out to the Na-montmorillonite clay to determine the interaction of the surfactants with inter layers of the clay structure. Rheological properties, gel strength, thixotropy, filtration properties and the effect of temperature on rheological properties of the water-based mud were studied. The results indicated that the gemini surfactants have a positive effect on the rheological and filtration properties of the Na-montmorillonite clay according to American Petroleum Institute specifications.     

基于行为自适应的台钻主轴轴径视觉测量策略

鉴于视觉测量过程行为的多样性与影响关系的复杂性,提出基于行为自适应的台钻主轴视觉测量新策略。采用周向分工位采集图像与轴向分段提取轮廓结合的组合采样法获取被测信息,并建立了主轴轴径视觉测量系统;设计曝光自适应图像采集、图像批处理、误差反馈闭环测量等功能集成的行为自适应视觉测量流程并开发相应软件。以某型号台钻主轴为对象,开展曝光区间自适应优选实验,多工位、多轴、多采样方案测量实验误差分析与稳定性评定。结果表明：所提出的行为自适应测量策略通过获取测量行为变化对测量误差的影响,形成闭环测量,可有效提高测量的稳定性,不仅满足台钻主轴高精度轴段测量需求,还有利于推动测量智能化发展。     

石膏矿山顶板破坏机理探讨

近几年来，国内的石膏矿山中，大规模采空区顶板冒落事故频繁发生。目前，对于石膏矿山大规模顶板冒落发生机理研究的还很少。根据近几年对石膏矿山的现场调查，结合岩体力学的相关理论和数值模拟技术，得出石膏矿采场顶板冒落及大规模采空区顶板冒落主要是由于石膏矿柱流变特性使其强度变低及随着采区的扩大，石膏矿柱应力增大等而引起的。     

Rheological behavior of the epoxy/thermoplastic blends during the reaction induced phase separation

The correlation between rheological behavior and time evolution of the phase separation patterns was investigated in the epoxy/thermoplastic blends. Before and during the induction period of phase separation, the storage and loss modulus initially increased with epoxy curing reaction and the concentration fluctuation. At the late stage of phase separation, the modulus values also increased and showed a sharp enhancement around the epoxy gel point. However, the time evolution during the reaction induced phase separation process differed a lot at various thermoplastic (TP) concentrations. At the low TP concentrations, the rheological parameters decreased with the coarsening of sea-island structure. At the high TP concentrations, the TP-rich continuous structures initially formed and maintained until the end, resulting in a continuous increase for the rheological characters. At middle TP concentrations, formation and evolution of the three-layered structure displayed a complicated rheological behavior. It was found that the storage modulus quickly increased, reached a vertex, then rapidly decreased, reached a minimum, and increased again afterwards. Although the rheological behaviors were almost phenomenologically similar as that in the normal dynamically symmetric system, driving force for the variation was fundamentally different. Especially for the case of middle TP concentrations, the behavior of the holistic volume shrinking of the slow dynamic TP-rich network and the flowing out of the fast dynamic epoxy-rich phase from the network during this period, as radically transformed the nature of the matrix from an elastic network to a macro-phase separated layer structure and caused the dramatic change of the rheological behaviors.     

空间目标几何与行为一体化建模方法

空间目标的三维建模包括几何建模和行为建模技术,是构建虚拟空间环境的基础。物体的几何建模方法目前比较成熟,但行为建模技术相对滞后。为实现虚拟空间环境中航天器的在轨运行仿真,该文提出了基于对象层次结构的实体三维几何与行为一体化建模语言（GBML）。该语言在定义基本图元的基础上,使用树状组件结构将空间目标的几何与行为参数进行表达、管理和绘制,以实现空间目标的几何与行为一体化仿真。最后基于OpenGL基本函数库,实现了可重用的支持GBML语言的模型处理组件ModelReaderCom。通过试验结果表明,该方法是有效的、可行的,可以广泛应用于各种虚拟环境中。     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

信息共享的记忆被囊群算法

针对基本被囊群算法求解精度较低、收敛速度较慢、容易陷入局部最优的缺陷,提出信息共享的记忆被囊群算法.首先,将整个种群分为执行信息共享搜索和喷气推进搜索两个子群,引入动态自适应调整策略,用于平衡算法的全局开拓能力和局部开发能力.然后,在执行信息共享搜索模式时,部分个体向同伴所在领域相互获取信息,实现种群个体之间信息的充分交流与共享.另一部分个体引入历史最优位置,用于引导学习,增强算法搜索的有效性.在20个基准测试函数上的实验表明,文中算法的收敛速度、求解精度、鲁棒性等都较优.