Supercooling release of micro-size water droplets on microporous surfaces with cooling

The gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells plays a key role in controlling moisture in these cells. When the GDL is exposed to a cold environment, the water droplets or water nets in the GDL freeze. This work observed the supercooling and freezing behaviors of water droplets under low temperature. A GDL made of carbon fiber was coated with a waterproof material with 0%, 40%, and 60% PTFE (polytetrafluoroethylene) contents. The cooling process was investigated according to temperature, and the water droplets on the GDL were supercooled and frozen. Delay in the supercooling release was correlated with the size of water droplets on the GDL and the coating rate of the layer. Moreover, the supercooling degree of the droplets decreased as the number of freeze-thaw cycles in the GDL increased.

     

Oxidative stabilization of C/C tow pregs studied by light microscopy

Tow pregs obtained from polyacrylonitrile-based carbon fibres and a commercial binder coal-tar pitch were stabilized by oxidation at 548, 588 and 623 K for periods ranging between 1 and 15 h. The microstructural changes involved in the stabilization of tow pregs on the carbonized laminates were studied by light microscopy. Under the conditions studied, the oxidative stabilization of tow pregs produced a significant increase in carbon yield. At the same time, the development of porosity on carbonization was avoided and the size of the optical texture was reduced. However, the extreme oxidation of pitch components at the edges of the carbonized laminate adversely affected pitch binding capability and the sinterizability of the laminates.     

A parametric study on the deformation of gas diffusion layer in PEM fuel cell

A well-designed proton exchange membrane (PEM) fuel cell requires a robust design of cell components. Therefore, the understanding of the deformation mechanism of each component is essential for a successful design. This study deals with the induced deformation of gas diffusion layer (GDL) due to the deformation of the membrane during fuel cell operation. A parametric study with six cases is constructed and the corresponding responses of a stress variation, distribution of contact pressure and a channel intrusion are selected to assess the effect of design parameters on the deformation of the GDL. The variation in the thickness of the GDL is considered as a parameter of the study in addition to the material model of the membrane. The variation in stress levels in the GDL induced due to the cyclically deformed membrane is calculated with different material models of the membrane. Through-plane stiffness of the GDL represented by the thickness of the GDL affects the three responses of the model considered. However, the material model of the membrane influences the planned responses selectively. An observation of contact loss at the interface is a strong indication of the importance of through-plane stiffness of the GDL on the deformation between the GDL and the membrane. In order to come up with a way that improves the cell’s performance, we numerically show that the manipulation of through-plane stiffness of the GDL is needed by employing an appropriate thickness of the GDL. The results from this study assist in understanding the interrelation between the GDL and the membrane for the deformation of the GDL.

     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

[工程应用]纤维增强树脂基复合材料增材制造技术与装备研究

在综述纤维增强树脂基复合材料增材制造技术的国内外研究现状基础上,分析了短纤维、长纤维、连续纤维增强树脂基复合材料的成形方法、工艺及性能。针对高性能的连续纤维增强树脂基复合材料的增材制造成形,研究了连续纤维增材制造成形机理及工艺,揭示了其成形性能的影响规律。指出了纤维增强树脂基复合材料增材制造技术与装备的未来发展趋势：亟需开展纤维增强复合材料的增材制造成形机理、成形工艺及装备研究,更好地推进纤维增强树脂基复合材料的广泛应用。     

树脂基碳纤维复合材料栅格天线反射面制造技术

介绍了树脂基碳纤维复合材料(CFRP)栅格天线反射面的制造技术.纤维采用日本东丽公司的T700,基体为3234树脂体系,采用球墨铸铁制造模具,真空袋热压罐成型工艺.通过认真的工艺研究,突破了模具设计与制造、铺层设计、成型工艺等关键技术,制造的栅格天线反射面栅格条宽度和厚度均匀,外表光洁,型面精度高,满足了设计要求.     

Effect of prestress level on the strength of CFRP composite laminate

The effect of fiber prestressing has been investigated on the tensile strength, modulus strength, flexural properties and residual stresses of Carbon fiber reinforced polymer (CFRP) composites. Unidirectional carbon fiber in an epoxy resin has been examined under different prestressing levels (0, 5, 10, 15, 20, 30, 45, 60, 80 MPa) at ambient temperature and 50 % fiber volume fraction. A new method was implemented to produce the prestressed laminates for several standard tests. The results showed that prestressing on 3-ply unidirectional carbon fibers improved the fiber tensile strength to 99 % and the tensile modulus to 31 %. Similarly on 8-ply unidirectional carbon fibers has improved the fiber flexural strength to 63 % and flexural modulus to 81 %. A new technique was used to measure the residual stresses and tensile modulus of the composite laminate by recording the final extension and the remaining load directly after the curing process and releasing the applied load.

     

基于超声复解析信号的碳纤维层压复合材料检测方法研究

采用超声波检测碳纤维层压复合材料时,换能器将接收到层间产生的反射信号。为了有效利用层间反射信号表征碳纤维层压复合材料内部树脂及纤维铺层的几何状态,开展基于超声复解析信号的检测方法研究。介绍超声复解析信号的基本理论,阐述其相对原始A型信号的优势;定义用于成像的复解析信号表征参量,设计基于表征参量的检测成像算法,采用简化声学模型分析碳纤维层压复合材料微观结构超声响应对表征参量的影响;针对包含冲击损伤的碳纤维增强复合材料层压板开展阵列超声检测试验,试验结果表明基于超声复解析信号的检测成像方法可清晰观测出碳纤维层压复合材料微观结构内部的纤维铺层几何形态,基于原始A型信号的检测成像方法无法有效观测,超声复解析信号具有用于表征评价复合材料脱层、褶皱等缺陷的巨大潜力。     

碳纤维喇叭天线制造工艺研究

喇叭天线是雷达系统中重要的功能件,碳纤维材料由于具有金属材料不具备的某些优异性能,在雷达系统中的应用越来越广泛。文中介绍了一种碳纤维喇叭天线的制造工艺过程,对喇叭天线原型结构的工艺改进设计满足碳纤维材料热压罐成型工艺和成型模具设计要求;成型模具结构设计满足零件尺寸精度、脱模以及零件批量生产要求;碳纤维喇叭天线表面金属化达到了金属层致密、均匀、光顺以及导电率指标。通过以上工艺过程研究,制造出满足指标要求的碳纤维喇叭天线,可为此类碳纤维喇叭天线制造提供工艺借鉴。     

等离子表面渗铬渗碳耐磨合金层

利用铬是碳化物形成元素且与α-Fe无限互溶的条件,在廉价低碳钢或中碳钢表面进行等离子渗铬,形成表面高铬合金层,当表面含铬量达到40%以上并在一定范围内渗层成分呈平缓的梯度分布时,再进行等离子渗碳,形成高铬高碳合金层。合金层中的含铬碳化物在固体状态下形成,属于二次碳化物。由于渗碳温度低,合金层的含铬量高,所形成的铬碳化物弥散、细小、均匀,当表面含碳量达2.8%以上时没有共晶莱氏体碳化物组织,表面高铬高碳合金层经淬火及回火处理后表面硬度达到HV1800。磨损试验表明,与淬火GCr15材料相比其耐磨性能提高8倍以上。     

碳纤维制备过程中纤维微观组织的变化

利用透射电镜、元素分析仪、X射线衍射仪等分析方法,研究了碳纤维制备过程中纤维微观组织的变化。结果表明：纤维皮层组织致密,芯部疏松;原丝制备时形成的皮芯结构在预氧化、炭化过程中明显改善,但是不能消除;在碳纤维制备过程中,纤维中始终存在沿轴向的层状结构。预氧化和高温炭化阶段是成分变化较大的阶段,也是化学反应最剧烈的阶段。     

Microstructural,Mechanical, and Tribological Properties of Rice Husk–Based Carbon: Effect of Carbonizing Temperature

In this study, we investigated the microstructural, mechanical, and tribological properties of rice husk (RH)-based carbon carbonized at various carbonizing temperatures under dry conditions. All samples exhibited amorphous carbon structures and the X-ray diffraction spectra of the samples carbonized at 1300 and 1400?°C indicated the presence of a polymorphic crystals of silica. The hardness increased with temperature due to the densification of the structure and the presence of the hard crystalline silica. At low normal loads, the mean friction coefficient of the material decreased as the carbonizing temperature was increased from 600 to 800?°C and slightly decreased as the carbonizing temperature was further increased from 800 to 1400?°C. At the highest load, all samples, except for that carbonized at 600?°C, exhibited extremely low friction coefficients (around 0.05). The wear rates of the all samples were smaller than 10^?5 mm³/N·m, indicating that RH carbon exhibits sufficient wear resistance. A Raman spectroscopic analysis of the worn surface of a steel ball revealed that the transfer layer at 600?°C had a less graphitic structure compared to the other carbonizing temperature. Based on these findings, we recommend an optimal carbonizing temperature for applications of sliding materials exposed to dry sliding contact.     

Refractive index and absorption coefficient as measures of structural organization in carbonized bitumen

Microscope photometry has been used to determine reflectances, measured in air and in stable immersion oil, of the carbonized products of the asphaltite bitumen, gilsonite. Comparison of values for parameters (n' and k'), analogous to refractive index and absorption coefficient which are derived from the reflectances, with several X-ray parameters of the carbonized samples suggests that microscope photometry offers a sensitive alternative method for following changes that take place in the molecular-structural organization of organic materials during coalification and carbonization processes. Although direct measurement of lattice dimensions is not possible, the optical method is more rapid than X-ray procedures, is less expensive in terms of capital equipment, while the reflectance measurements can be made on extremely small, discrete and categorically identified field areas, not on the finely ground bulk samples required for X-ray procedures. Wider application of the microscope photometric method to other carbonized materials and the possibility of absolute correlations between optical and X-ray data must await more detailed study. The preliminary data are sufficiently encouraging to warrant further extended investigations.     

Effect of carbon nanotube addition on the tribological behavior of carbon/carbon composites

Carbon nanotube composite coatings were applied onto carbon/carbon composites to improve wear properties. Carbon nanotubes have been prepared by catalytic pyrolysis of hydrocarbons. The nanotube slurry was prepared by addition of phenolic resin and solvent to infiltrate into C/C composites. The nanotube added composites were then carbonized in a nitrogen atmosphere. Ball-on-disc type wear tests were performed to evaluate the tribological properties of the carbon nanotube added carbon composites. The result showed that addition of nanotube has the potential to increase the wear resistance of carbon composites. Changes in Raman spectra, morphology and surface damage were studied to explain observed wear behavior.     

Thermal influence on surface layer of carbon fiber reinforced plastic (CFRP) in grinding

In this study, we investigated thermal influence on surface layer of CFRP in grinding with heat conduction analysis using grinding temperature at wheel contact area on dry and wet condition. Moreover, the thermal affected layer was analyzed through an experiment to examine the temperature of glass transition and thermal decomposition of the matrix resin that composes the CFRP used in this study. The influence of thermal effect on grinding of CFRP was verified based on observation of ground surface finish after grinding using SEM and the measurement of surface roughness. From the measurement result of DSC (Differential Scanning Calorimetry),TG-DTA (Thermogravimetry-Differential Thermal Analysis), It was found that the thermal affected layer of CFRP includes a layer in which the matrix resin is changed in quality by exceeding the glass transition temperature and a layer in which the matrix resin is thermally decomposed by exceeding the thermal decomposition temperature. In addition, it was found that the surface roughness was significantly reduced if the thermal affected layer with thermal decomposition was generated. In each grinding atmosphere, it tended to increase of grinding temperature at wheel contact area with increasing in the setting depth of cut. In the case of dry grinding, grinding temperature at wheel contact area increased up to t thermal decomposition temperature of the matrix resin. However, in the case of the wet grinding, grinding temperature at wheel contact area did not increase until thermally decomposition temperature. From the result of simulation about thermal affected layer, influence of grinding heat increased with increasing in the setting depth of cut. Ultimately, the thermal affected layer with thermal decomposition was generated in dry grinding. Moreover, from the results of SEM observation, it was confirmed that the surface finish properties deteriorated significantly due to thermal decomposition of the matrix resin in the case of Δ = 400 μm in the setting depth of cut at fiber angle θ = 0°. On the other hand, it was confirmed that the micro damage of carbon fiber was occurred in wet grinding at each setting depth of cut.     

The Influence of Temperature on the Wear of Carbon Fiber Reinforced Resins

Wear of carbon fiber composites increases with increasing temperature. At temperatures below about 100 C, wear is influenced by the type of carbon fiber and the presence of transfer films on the counterface. Beyond 100 C, transfer films no longer exist and wear becomes independent of the type of fiber. Increasing rates of wear at elevated temperatures are attributed to progressive desorption of water from the carbon fibers.

Reduced wear at elevated temperatures may be achieved by incorporation of CdI₂ in the composite. However, volatility of this material restricts the temperature capability of composites to approximately 350 C.     

Effect of the addition of carbonaceous fibers on the tribological behavior of a phenolic resin sliding against cast iron

The tribological behavior of novolac phenolic resin matrix composites reinforced with three kinds of carbonaceous fibers was studied in sliding contact against cast iron. Slow pyrolysis was used to obtain carbonaceous fibers from Colombian plantain fiber bundles (crops residues from Urabá region). After the carbonization process the samples were heated up to either 1200 or 1400 °C ensuring that many morphological aspects of the natural fibers were retained. Then, novolac phenolic resin with HMTA as curing agent and the carbonaceous fibers were used to obtain a composite material by compression molding process. Samples with different type and volume fraction of carbonaceous fibers were prepared and tested in sliding contact against cast iron in a pin-on-disc wear testing machine. At the end of the tests, the worn surfaces and the debris were analyzed by SEM.A decrease in both friction coefficient and wear of composites was observed with the increase in fiber volume fraction, which was associated to a beneficial effect of the detachment of carbonaceous material from the worn surface. Under the tested conditions, this material remains at the interface between the composite and the cast iron and helps reduce the shear resistance of the interface. On the other hand, surface fatigue and adhesion wear was identified as the dominant wear mechanism of the phenolic resin matrix.     

碳纤维复合材料补强输电铁塔腐蚀受损地脚螺栓的试验研究

为实现输电铁塔腐蚀受损地脚螺栓的补强,采用碳纤维管及碳纤维树脂对地脚螺栓模拟试样进行修复,并对试样进行拉伸试验,获得应力-应变曲线和断裂特征,从而确定了所需碳纤维管尺寸及需灌注的碳纤维树脂厚度.试验结果表明:试样均断裂在母材部分,说明碳纤维复合材料具有良好的补强效果;碳纤维树脂厚度在3 mm以上就能取得良好效果.根据试...     

碳纤维机载电磁屏蔽机箱制造工艺研究

某机载电磁屏蔽机箱对重量的控制要求严苛,因此采用碳纤维复合材料制造该机箱。文中论述了该碳纤维机载电磁屏蔽机箱的制造工艺过程,即对该整体成型机箱的制造工艺流程进行了介绍,并对模具设计、铺层设计、金属预埋件精确定位、表面金属化等关键技术进行了详细讨论。与相同结构的铝制机箱相比,该碳纤维机箱的质量减轻了约30%,并通过了电磁屏蔽性能测试以及振动、盐雾、湿热、温度冲击等环境适应性试验验证,完全满足设计和使用要求。     

盐浴等温法测定渗碳层深度

盐浴等温法是将渗碳件重新奥氏体化后，分别在不同的Ms点温度处盐浴等温，通过测量马氏体与非马氏体转变层深度来测定相应含碳量处的渗层深度。