石墨烯纳米片/AZ91镁基复合材料的显微组织与力学性能

采用铸造工艺制备了石墨烯纳米片(GNPs)增强的AZ91镁基复合材料,测试了复合材料的力学性能,并利用光学显微镜、X射线衍射仪、透射电子显微镜、扫描电子显微镜和能谱仪对复合材料的微观组织、界面结合和断口形貌进行了表征和分析,讨论了复合材料的强化机理。结果表明:石墨烯纳米片可有效细化镁基体的晶粒组织,在添加少量石墨烯纳米片时(0.1%),复合材料的屈服强度、延伸率和显微硬度分别为(164±5)MPa、(7.7±0.1)%和(74.2±2)HV,比基体分别提高了37.8%、13.2%和24.7%。GNPs与镁基体形成了强界面结合,这更有利于发挥应力转移强化、细晶强化等作用,提高镁合金强度、塑性等力学性能。     

石墨烯微片对尼龙6的改性研究

采用共混法制备尼龙6/石墨烯微片(GNPs)复合材料,研究了其导电性能、摩擦磨损性能及力学性能,并利用扫描电镜观察分析了材料磨损表面形貌,同时将其结果与炭黑(CB)体系进行了比较。结果表明,PA6/GPNs的渗滤阀值为15%(质量分数,下同),远低于PA6/CB的30%;GNPs的加入降低了材料的摩擦系数和磨损率,并在其含量为10%时达到最佳,分别降低30%和50%;提高了材料的拉伸强度、断裂伸长率、硬度,但冲击强度下降。CB的加入提高了材料的耐磨性、硬度,但摩擦性能、拉伸强度、断裂伸长率和冲击强度均下降。     

多道次热挤压制备Al2O3/AZ31复合材料的微观组织与力学性能

采用多道次热挤压制备Al_2O_3颗粒增强AZ31镁基复合材料,利用OM,SEM,TEM对Al_2O_3/AZ31复合材料进行组织观察,利用维氏硬度仪、电子万能拉伸试验机对Al_2O_3/AZ31复合材料进行力学性能测试。结果表明:经过多道次热挤压后,Al_2O_3颗粒均匀地分散在AZ31镁基体中,Al_2O_3颗粒对基体组织的晶粒细化作用得到增强,复合材料的晶粒尺寸随着道次的增加而显著减小。经过4道次热挤压后,Al_2O_3/AZ31复合材料的力学性能显著提高,其硬度,抗拉强度和屈服强度分别达到89HV,305MPa和198MPa,相比于第1道次热挤压后,其硬度,抗拉强度和屈服强度分别提高了19.2%,14.8%和14.1%。     

粉体石墨烯对铝基复合材料微观结构和性能的影响（英文）

采用机械合金化与电场压力激活辅助烧结工艺相结合的方式,分别制备纯Al和GNPs/Al复合材料,探究粉体石墨烯对铝基复合材料微观结构和性能的影响。结果表明:通过优化烧结工艺有效地抑制化合物Al_4C_3在GNPs/Al复合材料中的形成,提高石墨烯与Al基体的界面结合强度。石墨烯添加量为0.5wt.%时,在Al基体晶界处能够均匀的分散,由于石墨烯与Al基体有良好的界面润湿性,促进声子在基体材料中的移动,降低材料的界面热阻,在GNPs/Al复合材料表面形成导电网络,提高电子的迁移率和平均自由程,使GNPs/Al复合材料的热导率和电导率分别提升7.1%和4%;添加石墨烯能改变Al基体材料的晶体结构,在石墨烯周围形成晶格畸变的应力场,该应力场与位错应力场产生交互作用,使位错运动受阻,GNPs/Al复合材料的强度和硬度分别提升30.6%和44%;石墨烯能降低基体材料界面电容的介电损耗,在Al基体材料表面形成致密平整的膜层,提高GNPs/Al复合材料的电荷传递电阻,降低材料表面在电化学腐蚀过程中的弥散效应,使GNPs/Al复合材料耐腐蚀性能提高31%。石墨烯含量超过0.5 wt.%时,团聚在基体晶界的石墨烯,降低复合材料的界面结合强度,使GNPs/Al复合材料导带中的能带宽度变窄,电子的局域性增强,导致GNPs/Al复合材料的性能下降。综上所述,粉体石墨烯的最佳添加量为0.5wt.%。     

碳纳米管和石墨烯纳米片复合增强AZ91镁基复合材料组织与力学性能

目的 解决纳米碳材料在镁基体中分散难的瓶颈问题,制备出力学性能优异的镁合金复合材料。方法 采用超声工艺将质量分数为3.0%的碳纳米管插入到质量分数为0.5%的石墨烯纳米片的片层之间,添加到AZ91镁合金基体中,借助粉末冶金技术+热挤压工艺制备了0.5%GNS+3.0%CNTs复合增强的镁基复合材料。采用光学显微镜和透射电子显微镜观察和分析了复合材料的显微组织和界面结合。测试了复合材料的力学性能,并利用扫描电子显微镜观察了复合材料的拉伸断口形貌。结果 复合材料的屈服强度、伸长率和显微硬度分别为（274±5.0）MPa,（8.4±0.2）%,HV（90.5±1.8）,与基体合金相比,分别提高了63.1%,20.0%,20.1%。结论 GNS+CNTs的加入有效细化了基体合金的晶粒组织,且与镁基体形成了较好的界面结合,促使细晶强化、应力转移强化等各种强化机制的共同作用,使复合材料力学性能显著提高。     

镀铜石墨烯增强钛基复合材料的组织及性能研究

本实验通过超声搅拌加球磨的方式制备了镀铜石墨烯(GNPs)增强Ti6Al4V(TC4)钛基混合粉体,将粉体压制后采用微波烧结制备GNPs-Cu/Ti6Al4V复合材料.通过X射线衍射、扫描电子显微镜、能谱分析、显微硬度、室温压缩和摩擦磨损等测试手段,研究了石墨烯含量对钛基复合材料微观组织及力学性能的影响.研究结果表明:各石墨烯含量的钛基复合材料均出现Ti2 Cu、TiC相,当石墨烯含量为0.5％时出现GNPs相,且含量越高GNPs相的峰越高.随着石墨烯含量增加,钛基复合材料的相对密度、显微硬度、室温压缩强度和耐磨性先增加后降低,其中石墨烯含量为0.8％时复合材料的性能最好.与未加入石墨烯的Ti6Al4V基体相比,石墨烯含量为0.8％的GNPs-Cu/Ti6Al4V复合材料的显微硬度和压缩强度分别提高80.9％、69.9％.GNPs/Ti6Al4V和GNPs-Cu/Ti6Al4V复合材料的压缩强度分别比Ti6Al4V基体高33.2％和69.9％.微波烧结制备GNPs-Cu/Ti6Al4V复合材料的压缩强度分别比真空烧结和热压烧结高41.6％、22.9％.GNPs-Cu/Ti6Al4V复合材料的磨损机制为磨粒磨损与粘着磨损共存.     

石墨烯添加量对铜基复合材料性能的影响

采用一步化学还原法结合放电等离子烧结工艺制备石墨烯增强铜基复合材料,利用XRD、SEM、拉曼光谱、拉伸试验机、纳米压痕仪、涡流电导率仪等研究石墨烯含量对复合材料微观组织、力学性能和导电性能的影响。结果表明:石墨烯在复合材料基体中均匀分布,石墨烯的添加能显著增强铜基体的力学性能。与纯铜相比,添加0.025%(质量分数)的氧化石墨烯,可使其屈服强度提高219.8%,抗拉强度提高35.9%,弹性模量提高6.9%,此外,其导电率仍有93.1%IACS。随着石墨烯含量的增加,复合材料的屈服强度、抗拉强度及弹性模量均有所下降,这是因为高石墨烯含量复合粉体中部分石墨烯纳米片未能被铜颗粒包覆,其与铜基体界面结合强度低,石墨烯的剪切应力转移强化效果降低。     

SiC含量对SiCw/Cu复合材料组织与力学性能的影响

为了研究SiCw/Cu复合材料的制备工艺、形成机理,进一步研究SiC含量对材料的组织结构、力学性能的影响.采用热压法和热等静压法制备了不同SiCw含量的SiCw/Cu复合材料,并对复合材料的致密度、显微组织和物相组成、维氏硬度、拉伸和压缩性能进行了研究,对拉伸断口进行分析.结果表明:SiCw有效阻碍Cu基体晶粒的长大,随着SiCw含量的增加,热压制备的SiCw/Cu复合材料的致密度、断后伸长率、拉伸屈服强度下降,而气孔率、维氏硬度与压缩屈服强度显著增加,抗拉强度先增加后降低.热压制备得到的1wt%SiCw/Cu复合材料,具有相对最优的综合力学性能:抗拉强度为156.9 MPa,拉伸屈服强度为112.5 MPa.采用热等静压法制备的3wt%SiCw/Cu复合材料,各方面性能都要优于相同组分的热压材料,抗拉强度达到175.6 Mpa,拉伸屈服强度达到123.2 MPa,维氏硬度达到101.8 HV.复合材料的强度是SiCw的增强作用与孔隙的弱化作用共同作用的结果,SiCw/Cu复合材料的断裂行为既表现出一定的韧性特征,又表现出一定的脆性特征.     

CNTs/SiC_p/AZ91D镁基复合材料的力学性能研究

采用搅拌铸造法制备了碳纳米管(CNTs)与碳化硅颗粒(SiC_p)增强AZ91D镁基复合材料,对复合材料的力学性能进行了测试,对其显微组织进行观察和分析。结果表明:增强相使复合材料的晶粒细化,与基体合金相比,当CNTs、SiC_p含量分别是1.1 vol%、3 vol%时,复合材料的弹性模量为70.8 GPa,比基体提高了57.3%。当SiC_p含量一定时,复合材料的延伸率随CNTs含量的增加先增加后减小;对于只含镀镍CNTs的复合材料而言,当CNTs的体积分数为1.1%时,复合材料的延伸率最大,最高值达到了13.81,比基体材料提高了近60.6%,而1.1 vol%CNTs/2 vol%SiC_p/Mg的延展率比基体略有降低。当CNT体积分数一定,复合材料的延伸率均随SiC_p含量的增加而减小。     

纳米TiC颗粒弥散增强超细晶钨基复合材料的组织结构与力学性能

采用高能球磨和热压烧结的方法成功制备了纳米TiC颗粒弥散增强超细晶W基复合材料,并对其组织结构、室温力学性能进行了研究.研究结果表明,当纳米TiC颗粒含量较小时,高能球磨可以使TiC颗粒均匀分散到W基体中,烧结后,TiC颗粒尺寸约100nm,当纳米TiC颗粒含量较高时,局部出现团聚现象;纳米TiC的加入强烈的阻碍了W晶粒的长大并使复合材料的断裂模式由沿晶断裂为主向穿晶断裂为主转变,提高了材料的力学性能;在TiC含量为1%(质量分数,下同)时,材料的致密度、维氏显微硬度、弹性模量、抗弯强度分别达到98.4%、4.33、396GPa、1065MPa.纳米TiC颗粒对复合材料的强化机制主要是细晶强化和晶界强化.     

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.     

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.     

Chitosan-collagen/organomontmorillonite scaffold for bone tissue engineering

A novel composite scaffold based on chitosan-collagen/organomontmo-rillonite (CS-COL/OMMT) was prepared to improve swelling ratio, biodegradation ratio, biomineralization and mechanical properties for use in tissue engineering applications. In order to expend the basal spacing, montmorillonite (MMT) was modified with sodium dodecyl sulfate (SDS) and was characterized by XRD, TGA and FTIR. The results indicated that the anionic surfactants entered into interlayer of MMT and the basal spacing of MMT was expanded to 3.85 nm. The prepared composite scaffolds were characterized by FTIR, XRD and SEM. The swelling ratio, biodegradation ratio and mechanical properties of composite scaffolds were also studied. The results demonstrated that the scaffold decreased swelling ratio, degradation ratio and improved mechanical and biomineralization properties because of OMMT.     

Capacitance based mass metering for cryogenic fluids

This paper describes a method for measuring the mass of cryogenic fluids in on-board rocket propellant tanks or ground storage tanks. Linear approximations to the Clausius-Mossotti relationship serve as the foundation for a capacitance based mass sensor, regardless of fluid density stratification or tank shape. Sensor design considerations are presented along with the experimental results for a capacitance based mass gage tested in liquid nitrogen. This test data is shown to be consistent with theory resulting in a demonstrated mass measurement accuracy of ±0.75% and a deviation from linearity of less than ±0.30% of full scale mass. Theoretical accuracies are also shown to be ±0.73% for hydrogen and ±1.39% for oxygen for a select range of pressures and temperatures.     

Equipment using a “static-analytic” method for solubility measurements in potentially hazardous binary mixtures under cryogenic temperatures

A new apparatus designed to study, at cryogenic temperatures, thermodynamic equilibria of potentially explosive binary systems such as hydrocarbon-oxygen mixtures is described herein. This equipment has an equilibrium cell which was especially designed to minimize hazards while allowing accurate phase equilibrium measurements. Reliability of results, obtained with this equipment has been verified by working on the nitrogen-propane system, for which data are already available in literature, over a large range of compositions and at various temperatures. Four isothermal curves describing liquid phase compositions at 109.98, 113.77, 119.75 and 125.63 K have been determined. Our experimental data are represented within 2% in compositions and in pressures through the Peng-Robinson equation of state implying Mathias-Copeman alpha function and Huron-Vidal mixing rule. Comparisons to literature allow pointing out: good agreement is observed with Kremer and Knapp data (1983) while the three sets of Poon and Lu data (1974) presenting systematic positive deviation are consequently judged as suspicious.     

Gas gap thermal switches using neon or hydrogen and sorption pump

The very low pressure obtained thanks to adsorption phenomenon at low temperature can be used to build cryogenic heat switches, which offer the possibility to make or break thermal contact between two parts of a cryogenic system. The ON (conducting) and OFF (insulating) states of the switch are obtained by varying the gas pressure between two copper blocks separated by a gap of 100 μm. This pressure is controlled by acting upon the temperature of a small sorption pump (activated charcoal) connected to the gap space. For a “high” sorption pump temperature, the gas previously adsorbed in the sorption pump is released to the gap between the two blocks, allowing a good thermal conduction through the gas (ON state). On the opposite, cooling the sorption pump allows a very good vacuum between the copper blocks, which efficiently break the thermal contact (OFF state). Experimental thermal characteristics (Conductance in the ON and OFF state, ON-OFF switching temperature) of such a “Gas Gap Heat Switch” are described using hydrogen or neon as exchange gas and are compared with theoretical calculations.     

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.     

Integrated Resonant Self-Pumped Loop and pulse tube cryocooler for cryogenic cooling distribution

Cooling distribution is a vital technology concerning cryogenic thermal management systems for many future space applications, such as in-space, zero boil-off, long-term propellant storage, cooling infrared sensors at multiple locations or at a distance from the cryocooler, and focal-plane arrays in telescopes. These applications require a cooling distribution technology that is able to efficiently and reliably deliver cooling power (generated by a cryocooler) to remote locations and uniformly distribute it over a large-surface area. On-going efforts by others under this technology development area have not shown any promising results.This paper introduces the concept of using a Resonant Self-Pumped Loop (RSPL) integrated with the proven, highly efficient pulse tube cryocooler. The RSPL and pulse tube cryocooler combination generates cooling power and provides a distributive cooling loop that can be extended long distances, has no moving parts, and is driven by a single linear compressor. The RSPL is fully coupled with the oscillating flow of the pulse tube working fluid and utilizes gas diodes to convert the oscillating flow to one-directional (DC) steady flow that circulates through the cooling loop. The proposed RSPL is extremely simple, lightweight, reliable, and flexible for packaging. There are several requirements for the RSPL to operate efficiently. These requirements will be presented in this paper. Compared to other distributive cooling technologies currently under development, the RSPL technology is unique.     

Helium liquefaction with a commercial 4 K Gifford-McMahon cryocooler

This paper describes helium liquefaction using a commercial cryocooler with 1.5 W cooling power at 4.2 K (Sumitomo model RDK415D with compressor CSW-71D, consuming 6.5 kW electrical power), equipped with heat exchangers for precooling the incoming gas. No additional cooling power of cryoliquids or additional Joule-Thomson stages were utilized. Measurements of the pressure dependence of the liquefaction rate were performed. A maximum value of 83.9 g/h was obtained for 2.25 bar stabilized input pressure. Including the time needed to cool the liquefied helium to 4.2 K at 1 bar after filling the bottle connected to the cold head, and correcting for heat screen influences, this results in a net liquefaction rate of 67.7 g/h. Maintaining a pressure close to 1 bar above the bath during liquefaction, a rate of 55.7 g/h was obtained. The simple design enables many applications of the apparatus.