Free-space microwave characteristics of polypyrrole coated glass fibre

A series of polypyrrole coated glass fibre fabrics having different sheet resistivities has been chemically prepared from aqueous solution. The free-space microwave reflective properties of these materials has been measured, and their electrical characteristics described by a parallel RC network. The effects of rotation and fabrication into an epoxy resin matrix on the microwave reflective properties of polypyrrole coated glass fibre fabric have been measured. Polypyrrole coated glass fibre fabrics have also been used to construct a Salisbury screen absorber, and showed promising characteristics for their possible use as radar absorbing materials.     

Creep of SiC/Ti-1100 composite in a vacuum environment

This study examines the longitudinal creep behavior of a unidirectionally reinforced SiC fiber/Ti-1100 composite at 540°C in a vacuum. Primary creep behavior is predicted by the McLean creep model for elastic fibers in a creeping matrix provided the initial damage in the composite is taken into account and no damage accumulates during creep. At higher stresses, where the composite accumulates damage and eventually fails, both the strain and time to rupture can be predicted using the Bundle model, a modification of the McLean/Curtin creep model. In predicting rupture, the strength distribution of the weakest fibers in the composite is the controlling factor.     

The effects of the variations of carbon nanotubes on the micro-tribological behavior of carbon nanotubes/bismaleimide nanocomposite

Two kinds of original multiwalled carbon nanotubes (MWCNTs) with different diameters, and one carboxyliated MWCNTs were used to prepare three kinds of MWCNTs/bismaleimide (BMI) nanocomposites. The effects of the diameter, concentration and functional group of MWCNTs used in the composites on the micro-tribological behavior of the MWCNTs/BMI nanocomposites were investigated in this paper. The microhardness, the morphology of the worn surface, the glass transition temperature and dynamic mechanical properties of the MWCNTs/BMI nanocomposites were also measured to figure out the possible main wear mechanism of the composites. Results show that the addition of MWCNTs in BMI resin decreases the friction coefficient of the resin no matter what kind of MWCNTs is added. Moreover, the wear loss rate of all nanocomposites considerably decreases with the increasing of nanotube content until the content reaches 2.5 wt%. Functionalization of MWCNTs changes the main wear mechanism of the MWCNTs/BMI composite from adhesive wear (for pure BMI resin) to abrasive attrition by changing the self-lubricating property of the wore surface, the dispersion of MWCNTs in the BMI matrix, the interfacial strength between MWCNTs and the matrix as well as the internal strength of the materials.     

Enhanced wave-absorbing performances of silicone rubber composites by incorporating C-SnO2-MWCNT absorbent with ternary heterostructure

SnO₂ and amorphous carbon were simultaneously introduced onto the surface of multi-walled carbon nanotube (MWCNT) by a simple and green hydrothermal process followed by heat treatment, finally to obtain the C-SnO₂-MWCNT absorbent with ternary heterostructure. Subsequently, the C-SnO₂-MWCNT/silicone rubber wave-absorbing composites were prepared. XRD, Raman, XPS, SEM, TEM and TGA indicated the C-SnO₂-MWCNT absorbent with ternary heterostructure was fabricated successfully. When the mass fraction of C-SnO₂-MWCNT was 30 wt% and the thickness was 2.65 mm, the minimum reflection loss (RL_min) and effective absorption bandwidth (EAB) of the C-SnO₂-MWCNT/silicone rubber wave-absorbing composites could reach −53.5 dB and 3.16 GHz, respectively. Excellent wave-absorbing performance was due to the synergistic effect of multiple interface & dipole polarization and conduction loss. Furthermore, the corresponding heat resistance index (T_HRI) of the C-SnO₂-MWCNT/silicone rubber wave-absorbing composites with 30 wt% C-SnO₂-MWCNT reached 209.9 °C, higher than that of neat silicone rubber (187.4 °C).     

Thermal energy storage performance of hierarchical porous kaolinite geopolymer based shape-stabilized composite phase change materials

Phase change materials (PCMs) are prospective energy materials that are widely applied in building energy conservation, waste heat recovery, infrared stealth technology and solar dynamic power system. The enhancement of heat transfer and leak-proof performance are critical to PCMs. Although geopolymers have been applied in thermal energy storage, meanwhile, hierarchically porous geopolymers have already shown superb performance in various functional applications, to the authors’ knowledge, no report concerning the application of hierarchical porous ones have been issued. This paper concerns the preparation of a shape-stabilized composite PCMs, consisting of hierarchically porous kaolinite-based geopolymer (PKG) embedding polyethylene glycol 4000 (PEG4000), which shows promising prospects in thermal energy storage. Optimized porous geopolymer matrices feature high porosity (>83%), combined with high specific surface area (4.7 m²/g) and thermal conductivity (TC, 1.324 W·m⁻¹·K⁻¹). Furthermore, the shape-stabilized composite PCMs show excellent thermal energy storage properties: loading rate of 80.93 wt%, latent heat of 168.80 J g⁻¹ and TC of ∼0.36 W·m⁻¹·K⁻¹ at 20–30 °C, which is 1.64 times of the TC of pure PEG4000. Finally, the photothermal conversion performances of the shape-stabilized composite PCMs were also simulated.     

Prototyping Na0.5Bi0.5TiO3-based multilayer ceramic capacitors for high-temperature and power electronics

Currently, research on capacitor materials for high-temperature and power electronics focuses on achieving new record-breaking limits for dielectric properties or energy storage densities, with little regard for the stability of key parameters during operation or component reliability. While environmental conditions usually do not exceed 300 °C, the voltage ratings of capacitors are still unclear. Within this work, multilayer ceramic capacitors based on lead-free sodium bismuth titanate with AgPd inner electrodes have exhibited exceptional stability of properties and capacitance at high temperatures and voltages during operation. Despite the lack of precise requirements and limits specified by manufacturers and industry, these prototype MLCCs can help to open up the field of high-temperature and power electronics. Their extensive stability of dielectric properties allows for a rather universal application.     

风机用集成式机液伺服液压缸性能分析

对风机风量调整集成式机液伺服液压缸的动态性能进行了研究。分别从改变机液伺服阀芯的阀口面积梯度以及系统泄漏量等方面对集成式机液伺服液压缸的结构参数进行优化分析。仿真分析结果表明:将机液伺服阀芯面积梯度维持在0.5 m或将系统泄漏系数增加到3×10-9,能够提高系统的响应速度且系统有较好的稳定性,系统获得良好的动态特性。     

Effects of NH4+ doping on the hydrogen storage properties of metal hydrides

Doping can modify the properties of metal hydrogen storage materials significantly. Currently, the metal doping is a frequent strategy, while the non-metal cation doping has not been examined extensively so far. In this study, the effects of NH₄⁺ doping on the hydrogen storage properties of different metal hydrides, including TiH₂, Ti_0·25V_0·25Nb_0·25Zr_0·25H₂, Ti_0·5V_0·5H₂ and VH₂, are investigated by first-principles calculations. It is found that the NH₄⁺ presents a good affinity for metal hydrides and the NH₄⁺ incorporation leads to charge redistribution and formation of dihydrogen bond. Furthermore, the NH₄⁺ doping in metal hydrides is favorable for enhancing the hydrogen storage capacity and decreasing the thermal stability simultaneously. The possible reason for the NH₄⁺ doping induced destabilization in metal hydrides is the relatively weak interaction between NH₄⁺ and hydrogen atoms.     

Study on mass transfer enhancement of locally improved structures and the application in serpentine and parallel flow fields of PEM fuel cells

Oxygen transport and water management within proton exchange membrane (PEM) fuel cells can be enhanced by locally improved structures in the flow field. Structures such as the new variable diameter structure and the block structure can significantly improve the PEM performance. In this paper, a three-dimensional numerical model is established to compare different types of locally improved structures. The structures, which employed the same compression volume, consist of a block structure, long block structure, and the new variable diameter structure. The mass transfer characteristics and the optimal layout of the locally improved structures are studied. Then, the new variable diameter structure and the block structure are incorporated into the serpentine flow field (SFF) and the parallel flow field (PFF) to form new flow fields, and their effects on the SFF and PFF are analyzed. The new variable diameter structure improves the uniformity of oxygen, and the block structure enhances the local mass transfer. The presence of the locally improved structure in the middle of the FF gives the best performance. In addition, the improved SFF has very limited performance and poor water management, but the oxygen distribution in the variable diameter PFF (PFF@VD) was significantly improved, and the peak power was increased by 26%. The energy consumption caused by pressure drop (ECP) is adopted to evaluate the performance of the flow field with locally improved structures; the PFF@VD has a higher proportion of net output power compared to the unmodified PFF.