三维石墨烯-碳纳米管/水泥净浆的压敏性能

为了探索三维石墨烯-碳纳米管(G-CNTs)/水泥净浆的压敏性能,采用四电极法研究了荷载作用下GCNTs/水泥净浆的电阻率变化,并分析不同G-CNTs掺量、加载幅度、加载速度以及恒定荷载对电阻率变化的影响。研究表明:随着G-CNTs掺量的增加,电阻率呈先减小后稳定的变化趋势,在G-CNTs掺量由0.2wt%增加至1.6wt%时,电阻率下降51.8%;电阻率与温度呈负相关;G-CNTs掺量高于0.8wt%时可以显著提高水泥净浆的压敏性能,且电阻率变化率与应力应变有明显的对应关系,1.2wt%G-CNTs掺量下试件的应力灵敏系数和应变灵敏系数分别为2.3%/MPa和291;G-CNTs/水泥净浆电阻率变化率幅值随着加载幅度增大而相应增加,其电阻率变化率曲线在不同加载速度以及恒定荷载作用下均与应力-应变曲线一一对应,具有良好的压敏特性。     

Transport properties in spinel ferrite/graphene oxide nanocomposites for electromagnetic shielding

Herein, $\left({\text{Co}}_{0.5}{\text{Ni}}_{0.5}\right){\text{Cr}}_{0.3}{\text{Fe}}_{1.7}{\text{O}}_4$/graphene oxide nanocomposites were fabricated by ultrasonication technique, using pure spinel ferrite and graphene oxide synthesized by sol-gel method and modified Hummers' method, respectively. The effect of graphene incorporation with ferrite nanoparticles was studied by X-ray diffraction (XRD), electrical and dielectric measurements. XRD analysis revealed the spinel phase for the ferrite sample and confirmed the formation of graphene oxide. The crystallite size was found in the range of $37-43\text{ nm}$ and the porosity increased with the increase in the concentration of graphene oxide in the composites. The DC electrical resistivity of spinel ferrite was found equal to $3.83\times {10}^9\text{ Ω}.\text{cm}$ and it substantially decreased with the increase in the percentage of graphene oxide at room temperature. The real and imaginary part of relative permittivity followed the Maxwell-Wagner type of interfacial polarization. AC conductivity confirmed the conduction by hopping mechanism and increased on increasing the GO content. The coupling of magnetic ferrite with graphene oxide tunes the magneto-electrical properties for potential applications at high frequencies.     

In‐plane shear damage behaviours of 2D needled C/SiC composites

Understanding the in‐plane shear behaviour of composites is essential to establish the design basis for practical applications. This study aims to investigate the shear damage behaviours of 2D needled C/SiC composites by various characterization techniques. The effect of layer arrangement on shear modulus and strength was discussed via shear stress‐strain responses. The shear strain field evolution and uniformity variation were studied by digital image correlation. It shows that the uniformity of shear strain field changes with the shear load, and the shear strain field evolution consist of 5 stages. The electrical resistivity measurement results indicate that structural deformation and damage evolution caused the electrical resistivity change. Furthermore, the damage evolution has a double effect on the electrical resistivity variation. The acoustic emission monitoring shows that the shear damage evolution is a 3‐stage nonlinear process before failure. The shear damages were categorized via acoustic characteristics. Besides, the postfailure behaviours were also discussed in this study.     

Study of the influence of notch radii and temperature on the probability of failure: A methodology to perform a combined assessment

The fracture assessment of notched components based on cracked components approaches leads to over‐conservative failure predictions. In the research literature, several approaches are proposed to overcome this problem using an apparent fracture toughness, . Nevertheless, most of these approaches are based on deterministic assumptions despite the large and variable scatter exhibited by for different notch radii (ρ) or temperatures (T). This paper proposes a methodology for deriving a probabilistic field including the effect of temperature on the failure of notched components. First, the theory of critical distances is applied to transform each apparent fracture toughness into the equivalent fracture toughness for ρ = 0. Then, the temperature is supposed to act as a scale effect in the Weibull cumulative distribution function of the equivalent fracture toughness, and the corresponding scale effect function is derived. Finally, the applicability of the proposed methodology is illustrated by an example using two ferritic‐pearlitic steels: S275JR and S355J2.     

Influence of lanthanum on enhancement of mechanical and electrical properties of Cu-Al_2O_3 composites

Two kinds of Cu-Al_2O_3 composites(with and without La) were prepared via mechanical alloying-spark plasma sintering(MA-SPS) method. Microstructure, mechanical properties and electrical resistivity were investigated systematically using metallography, scanning electron microscopy, transmission electron microscopy, mechanical and electrical properties testing. The results indicate that an appropriate amount of La can homogenize the distribution of Al_2O_3. As such, yield strength, ultimate tensile strength and elongation of Cu-Al_2O_3-La are greatly increased. Some semi-coherent interface between Cu and Al_2O_3 is found, which means a low interface energy. The grain shape of Cu changes to irregular band with the addition of La. This change results in a density decrease of grain boundary and reduces electrical resistance. Lanthanum may exist in the form of La_2O_3.     

氮气对复采工作面遗煤复燃的防治效能

为探究不同体积分数N₂对复采工作面遗煤复燃过程的防治效果，采用自主研发的程序升温试验装置，对煤样分别通入体积分数为10%、20%、30%和40%的N₂，以氧化反应过程中耗氧速率V_O2、一氧化碳产生率V_CO和表观活化能E的变化情况来反映煤自燃的发展程度。试验表明:不同体积分数的N₂对煤氧化升温过程均具有抑制作用，相比于煤在纯空气条件下的氧化自燃，通入的N₂体积分数越大，煤初次自燃和二次氧化升温过程的V_O2和V_CO更低、E更高，说明通入的氮气体积分数越大，对煤氧化升温过程的抑制效率越好。其中，在煤初次自燃阶段，当N2体积分数C_N2≥20%时抑制效果更佳；在煤二次氧化复燃阶段，当N₂体积分数C_N2≥40%时抑制效果更佳。     

柔性电极材料对织物电阻测试的影响

为寻找适合测试织物电阻的电极,选用铝箔、铜箔、导电膏和导电胶作为柔性电极材料,结合ZC36型高阻计研究这4种柔性电极材料对织物电阻测试的影响。试验结果表明:轻压下,4种电极材料测试体积比电阻率差异较大,导电膏和导电胶测试差异较小,适合轻压下织物体积比电阻率的测试;4种电极测试的织物表面比电阻率也存在较大差异,在保证良好接触的前提下(100~200 Pa),铜箔和铝箔适合织物表面比电阻率测试。织物表面平整硬挺,在压力较大时,4种导电材料均可作为测试用电极材料。     

具有高耐蚀性和导电性的掺Cr类金刚石非晶碳的化学式解析

Cr掺杂的类金刚石非晶碳具有良好的导电性和耐腐蚀性,这对于燃料电池金属双极板涂层改性特别重要。使用团簇加连接原子模型对其非晶结构进行了详细解析,在该模型中,良好的玻璃形成材料由覆盖特征性最近邻团簇加上几个下一壳层原子的结构单元来表述。根据文献,在Cr掺杂的类金刚石非晶碳中占优势的团簇是Cr中心和C壳层的[Cr-C4]四面体团簇,然后将该团簇与适当的连接原子匹配,以满足电子轨道饱和原理。由此推导出了两个最优组成式,即[Cr-C4]CrC3(22.2%Cr)和[Cr-C4]Cr3C2(40%Cr),它们显示出良好的非晶态结构稳定性。实验结果显示,所提出的这两组化学组成的涂层材料兼具低电阻率(低至10-4Ω·cm)和优异的耐腐蚀性(腐蚀电流密度~10^-2μA/cm^2)。在sp2键含量和渗流理论的框架内讨论了导电和耐蚀的协同行为。这项工作验证了团簇加连接原子模型在具有高耐腐蚀性和高导电性的涂层材料成分设计中的可行性。     

Effect of phase maldistribution on performance of two-phase catalytic monolith reactor and its comparison with trickle bed reactor

Monolith reactors are widely considered as an alternative to the conventional trickle bed reactor. For the commercial deployment of monolith reactors, comparative performance studies are required. Reliable comparative and performance studies require a detailed understanding of the effect of phase distribution/maldistribution on the performance studies. In this work, performance and comparative studies were carried out in a relatively large column that was 4.8 cm in diameter. Experiments were performed in the same conditions that were used in studies for which phase distribution data were available. Since the properties of the catalyst used were different in both the reactors, the apparent kinetics were studied to facilitate the comparison. The hydrogenation of alpha-methyl styrene (AMS) was used as a test reaction. From the performance studies, it was found that the effect of maldistribution on the performance was stronger than the catalyst availability. From the comparative studies, it was found that the monolith reactor with maldistributed flow conditions provides higher productivity than the trickle bed reactor.