C/C多孔体的高温热处理对C/SiC复合材料结构及力学性能的影响

以三维针刺碳毡作为预制体,先采用树脂单向加压浸渍-热解工艺制备出C/C多孔体,再通过反应熔体浸渗法获得C/SiC复合材料。重点研究了C/C多孔体的高温热处理对C/SiC复合材料结构和力学性能的影响。结果表明,C/C多孔体的高温热处理不会改变C/SiC复合材料的相组成,但可使复合材料中的SiC含量提高,C含量降低;高温热处理有利于熔融Si浸渗,使复合材料致密度增大,孔隙率降低,从而使其弯曲断裂强度提高约28%;高温热处理还可使纤维-基体界面结合强度降低,改善复合材料的断裂韧性。     

炭纤维热处理对C/C复合材料力学性能的影响

采用化学气相沉积工艺对未处理和2 500℃热处理的炭纤维预制体进行致密化,对致密化后的C/C复合材料进行弯曲力学性能测试,借助偏光显微镜和扫描电子显微镜观察热解炭的组织、纤维的表面和弯曲试样断口的形貌.结果显示:高温热处理后,纤维表面变的更加光滑,表面出现很多沿纤维轴向的沟槽;致密化后的两种C/C复合材料的基体消光角约为21°,均为高织构热解炭;与未处理纤维增强C/C复合材料相比,经高温热处理后纤维增强的C/C复合材料的弯曲强度和模量均大幅下降,断裂特征由脆性转变为典型的假塑性,断口处有大量纤维拔出,纤维表面未粘附热解炭,表明对纤维进行高温热处理显著降低了纤维和热解炭基体的界面结合强度,导致材料强度降低,断裂呈假塑性.     

C/C多孔体的高温热处理对C/C-SiC复合材料摩擦磨损行为的影响

低密度C/C多孔体的结构与性能调控是制备具有优异摩擦磨损性能的C/C-SiC复合材料的关键。本研究采用化学气相渗积法制备了C/C多孔体,并对其进行2100℃高温热处理,再通过反应熔渗法制备了C/C-SiC复合材料,研究了C/C多孔体高温热处理对C/C-SiC复合材料微观结构、导热性能和摩擦磨损性能的影响。结果表明,经2100℃热处理的C/C多孔体孔隙率和石墨化程度增加,用其制备的C/C-SiC复合材料比C/C多孔体未经热处理的密度更大(2.22 g/cm3),孔隙率由5.1%降低至3.4%, SiC陶瓷相含量比热处理前提高11.9%。石墨化程度越高,声子的平均自由程越大,因此其室温的导热率提升到3.1倍, 1200℃导热率提升到1.2倍。经过热处理的热解炭更软,摩擦面易形成连续且稳定的摩擦膜,因此摩擦系数更稳定,并且在测试载荷为3、6和9 N下磨损率均显著降低,下降幅度达到47.8%、41.9%和11.7%,平均摩擦系数分别为0.47、0.38和0.39。综上所述,对C/C多孔体进行高温热处理可使C/C-SiC复合材料的导热性能提升,更耐磨并且表现出更稳定的摩擦系数。     

C/C多孔体对C/C-SiC复合材料制备及性能的影响

以准三维针刺碳纤维预制体,经化学气相渗透(CVI)法制备了4种密度的C/C多孔体,利用先驱体浸渍裂解法(PIP)制备了C/C-SiC复合材料,研究了C/C多孔体对C/C-SiC复合材料制备和最终性能的影响。结果表明:C/C多孔体密度越低,最终得到的C/C-SiC复合材料开孔隙率及SiC含量较高。SiC的存在使C/C-SiC材料具有较高的弯曲强度,纤维和基体界面也是影响弯曲强度的关键因素,其中密度为1.35g/cm3的C/C多孔体所制备的C/C-SiC复合材料纤维和基体之间形成较好的结合界面,其弯曲强度最大。同时,SiC含量增加可显著提高C/C-SiC复合材料的抗烧蚀性能。     

中间热处理对薄壁C/CSiC构件制备及力学性能的影响

采用“化学气相渗透法+先驱体浸渍裂解法”（CVI+PIP）混合工艺制备了薄壁C/C-SiC复合材料构件,研究了C/C多孔体的热处理对C/C-SiC构件密度、变形量及力学性能的影响。研究结果表明：中间热处理可提高C/C的开孔率,有利于SiC的渗入,制备出密度较高的C/C-SiC复合材料构件;中间热处理对构件的层间剪切性能影响不大,但影响构件面内拉伸强度和整体承压性能;中间热处理会导致薄壁C/C-SiC构件在内外径和高度方向发生变形;合适的热处理温度（1600~1800℃）使C/C-SiC构件界面结合强度适中,面内拉伸强度及整体承压性能有了极大的提高;而较高的热处理温度（2100~2300℃）使碳纤维强度下降,使构件拉伸强度及整体承压性能大幅下降。     

乙醇热解制备C／C复合材料探索研究

以可再生的资源无水乙醇为前驱体,在负压条件下,沉积温度为900℃～1200℃,采用压力梯度CVI工艺制备C/C复合材料.考察了沉积时间与密度的变化规律,采用偏光显微镜和扫描电镜观察了材料的组织结构和断口形貌,利用三点弯曲测定了材料的弯曲强度.结果表明:采用乙醇为前驱体,可大幅度提高致密化效率,96h制备出密度为1.47g/cm3的C/C复合材料;易于获得高织构的组织,制备试样的热解炭组织以粗糙层为主,断裂方式为假塑性断裂.乙醇是一种很有应用前景的制备C/C复合材料的前驱体.     

CVI-iC/TaC改性C/C复合材料的力学性能及其断裂行为

以针刺碳纤维整体毡为预制体,采用化学气相渗透工艺对预制体纤维进行PyC/SiC/TaC的多层复合模式的涂层改性, 然后采用化学气相渗透和热固性树脂浸渍-化进行增密,制备出新型C/C复合材料。对复合材料的微观结构和力学性能进行了研究。结果表明:包覆在碳纤维表面的PyC/SiC/TaC多层结构均匀致密、无裂纹,在C/C复合材料中形成空间管状网络结构;改性后C/C复合材料的抗弯强度和韧性均大大提高, 平均抗弯强度达到522 MPa,断裂位移达到1.19mm;复合材料弯曲断裂形式表现为脆性断裂,经过2000℃高温热处理以后,复合材料的抗弯强度下降,但最大断裂位移增大,弯曲断裂形式由脆性断裂转变为良好的假塑性断裂。      

热处理温度对双连续β-氮化硅增强铝基复合材料性能的影响

通过常压烧结法制备了气孔率为53.43%的多孔氮化硅预制体,并利用挤压铸造法制备双连续β-氮化硅增强铝基复合材料,并研究了热处理温度对复合材料显微组织和力学性能的影响。利用位错增殖强化理论和界面反应理论分析复合材料的显微组织和力学性能,结果表明,随着热处理温度的升高,铝合金基体发生位错增殖,β-氮化硅增强体和铝合金基体的界面反应程度增强,复合材料的显微硬度增高,断裂韧性降低,弯曲强度先增加后降低,复合材料断裂模式从沿晶韧性断裂转变为沿晶脆性断裂。当热处理温度为850℃,复合材料的界面层厚度约为20~50 nm,其综合力学性能达到最佳。     

CVI-SiC/TaC改性C/C复合材料的力学性能及其断裂行为

以针刺碳纤维整体毡为预制体,采用化学气相渗透工艺对预制体纤维进行PyC/SiC/TaC的多层复合模式的涂层改性,然后采用化学气相渗透和热固性树脂浸渍-炭化进行增密,制备出新型C/C复合材料.对复合材料的微观结构和力学性能进行了研究.结果表明:包覆在碳纤维表面的PyC/SiC/TaC多层结构均匀致密、无裂纹,在C/C复合材料中形成空间管状网络结构;改性后C/C复合材料的抗弯强度和韧性均大大提高,平均抗弯强度达到522MPa,断裂位移达到1.19mm;复合材料弯曲断裂形式表现为脆性断裂,经过2000℃高温热处理以后,复合材料的抗弯强度下降,但最大断裂位移增大,弯曲断裂形式由脆性断裂转变为良好的假塑性断裂.     

热处理对含CSiCTaCC界面C/C复合材料力学性能的影响

以准三维针刺炭纤维毡为预制体, 采用化学气相渗透工艺在预制体中炭纤维/基体炭之间制备C-SiC-TaC-C复合界面, 利用树脂浸渍-炭化工艺对材料进一步增密, 获得含C-SiC-TaC-C界面的C/C复合材料。研究了1400～2500℃不同温度热处理前后复合材料的微观结构和力学性能。结果表明: 热处理前, SiC-TaC界面为管状结构, 复合材料的抗弯强度为241.6 MPa, 以脆性断裂为主; 经1400～1800℃热处理后, TaC界面破坏呈颗粒状, 复合材料的平均抗弯强度下降到238.9～226.1 MPa, 其断裂方式不变, 但断裂位移由0.7 mm增至1.0 mm; 经2000～2500℃热处理后, SiC、 TaC界面均受到破坏, 复合材料平均抗弯强度急剧下降至158.7～131.8 MPa, 断裂方式由脆性断裂转变为假塑性断裂。      

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Measurement of the thermal expansion of metal and FRPs

Thermal expansion is an important parameter for characterization of different binding forces, lattice dynamics, band and crystal structure of any solids. Many investigators have focused their attention to study this property theoretically and experimentally at different temperatures. It is one of the important properties of metals and its alloys, which helps to calculate the thermal stress. This parameter is also used to determine the compatibility of an insulator as load bearing materials. Different experimental setups have been developed to study thermal expansions of the materials using different techniques namely capacitance method, interferometric principle, LASER, optical, quartz tube etc. This paper reviews most of the experimental setups available to measure thermal expansion of metals, alloys, polymers and fibre-reinforced plastics at temperature ranging from 1 to 1100 K.     

Real gas choked flow conditions at low reduced-temperatures

Real gas choked mass flux is calculated for a frictionless stream expanding isentropically until it reaches the speed of sound and without phase changes. The other parameters associated with the choked state are the pressure, density, temperature ratios, and the speed of sound. Departure of the choked mass flux from the ideal gas model is discussed first in absolute terms and then in relative terms, using the Principle of Corresponding States, for gases with boiling points in the low temperature range. Reduced-stagnation pressures are examined up to values of 30 for hydrogen, neon, nitrogen, argon, methane, krypton, xenon, and R-14 and up to 100 for ⁴He. The corresponding reduced-stagnation temperatures go down to 1.4 and in some cases down to 1.2 for nitrogen and argon. Also discussed are the limiting values of stagnation parameters for which no phase change occurs in the choked state. Compared to the ideal gas, the mass flux may almost double and the critical pressure ratio may decrease by an order of magnitude. The relevance of results is discussed qualitatively and quantitatively for Joule-Thomson cryocooling.     

Analysis of optimum configuration of two-stage thermoelectric modules

In this paper, the theoretical analysis and stimulating calculation were conducted for a basic two-stage thermoelectric module, which contains one thermocouple in the second stage and several thermocouples in the first stage. The study focused on the configuration of the two-stage thermoelectric module, especially investigating the influences of some parameters, such as the allocation of the junction temperature difference in the module, the length of thermocouples and the number of thermocouples, on the cooling performance of the module. The obtained analysis results indicate that changing the junction temperature difference in the second stage, the length of thermocouples and the number of thermocouples in the first stage can improve the cooling performance of the module. These results can be used to optimize the configuration of the two-stage thermoelectric module, and provide guides for the design and application of thermoelectric cooler.     

Development of No-Vent™ liquid hydrogen storage system for space applications

A number of technological advances required to store and maintain normal-boiling-point and densified cryogenic liquids, including liquid hydrogen, under zero boil-off conditions in-space, for long periods of time, have been developed. These technologies include (1) thermally optimized compact cryogen storage systems that reduce environmental heat leak to the lowest-temperature cryogen, which minimizes cryocooler size and input power, and (2) actively-cooled shields that surround the storage systems and intercept heat leak. The processes and tools used to develop these technologies are discussed. A zero boil-off liquid hydrogen storage system technology demonstrator for validating the actively-cooled shield technology is presented.     

Development of a miniature cryogenic fluid circuit and a cryogenic micropump

This article presents the development of a miniaturized cryogenic fluid circuit for distributed cooling of low-temperature tracking detectors in high-energy physics (HEP). The heart of the circuit is a prototype cryogenic micropump. This volumetric pump is compatible with cooling powers of about 10-100 W, and capable of producing pressure heads of up to around 0.3 MPa. Besides detector and electronics cooling in HEP, potential applications are found in the field of superconductor technology.     

Effect of different treatments of copper surface on its total hemispherical absorptivity bellow 77 K

Total hemispherical absorptivity of copper surfaces treated with standard industrial methods was measured in dependence on the temperature of thermal radiation, varying from 25 K to 300 K. The sample temperature was typically from 5 K to 40 K and did not exceed 70 K. Usability of chemical and mechanical Cu surface finishing as well as Cu plating with Ni and Au for cryogenic design is discussed. As an example of practical application of our results, the cryogenic design of a LN₂ trap is presented.     

Percolation and the resistive transition of the critical temperature Tc of Nb3Sn

The relationship between the distribution of the critical temperature, the percolation function, and the resistive transition of the critical temperature is explored for polycrystalline Nb₃Sn. In the neighborhood of the critical temperature, Nb₃Sn is assumed to be a random mixture of superconducting and normal grains. Percolation concepts are applied to a study of the resistivity. A general analysis is made showing that the onset and shape of the resistive transition for composite conductors are determined by the percolation function and the distribution of the critical temperature. An approximate form of the percolation function is determined based on a linear FEM analysis. Example resistive transitions are calculated for an assumed normal distribution of the critical temperature. An argument is presented that relates grain orientation and strain dependence in Nb₃Sn. It is noted that a dependence of the distribution of T_c with strain, in addition to the usual shift in T_c with strain, would be the result of a strain dependence that is a function of grain orientation. The analysis shows the extent to which the slope of the resistive transition is a measure of the distribution of the critical temperature, and therefore a measure of the grain orientation strain sensitivity. Finally, a method is described to determine the percolation function experimentally.     

Effect of cryogenic treatment on microstructure, mechanical and wear behaviors of AISI H13 hot work tool steel

This paper focuses on the effects of low temperature (subzero) treatments on microstructure and mechanical properties of H13 hot work tool steel. Cryogenic treatment at −72 °C and deep cryogenic treatment at −196 °C were applied and it was found that by applying the subzero treatments, the retained austenite was transformed to martensite. As the temperature was decreased more retained austenite was transformed to martensite and it also led to smaller and more uniform martensite laths distributed in the microstructure. The deep cryogenic treatment also resulted in precipitation of more uniform and very fine carbide particles. The microstructural modification resulted in a significant improvement on the mechanical properties of the H13 tool steel.