纳米隔热材料的孔隙结构特征与气体热传输特性

为研究纳米隔热材料孔隙结构内部的气体热传输特性, 采用溶胶-凝胶工艺结合超临界干燥技术, 制备了一系列具有不同孔隙结构特征的样品, 通过热导率、氮气吸-脱附和真密度测试, 全面、准确获取了其孔隙结构信息, 并专门、系统研究了孔隙结构特征与气体热传输特性之间的关系.研究结果表明: 与气相贡献热导率相对应, 材料具有双尺度孔隙结构特征, 并且当大孔隙尺度不及小孔隙的10倍时, 可进一步等效为单尺度孔隙.考虑气固耦合传热的本征气相贡献热导率随孔隙尺度的增大而升高, 与气相热导率变化类似且成一定的比例关系, 孔隙尺度小于200 nm和大于500 nm时的比例系数分别为2.0和1.5, 200~500 nm时则为2.0~1.5.当大、小孔隙尺度的比值不超过10时, 或者这一比值为100~1000且大孔隙含量低于10%时, 气相贡献热导率随环境气压的降低依次呈现快速下降、缓慢下降和无变化三个阶段; 当这一比值超过3000时, 即使大孔隙含量很低(不超过10%), 气相贡献热导率也会依次呈现快速下降、缓慢下降、快速下降和无变化四个阶段.      

Improved radiation damage tolerance of titanium nitride ceramics by introduction of vacancy defects

TiN and TiN_0.7 were irradiated using a 100 keV Ar-ion beam at 600 °C to target doses of 3 × 10¹⁷ ions cm⁻². SRIM estimation, GIXRD and fluorescence analysis have been performed to evaluate the effect of pre-existing vacancy defect on the radiation tolerance. The lattice parameter of TiN increased after irradiation due to interstitial atoms and vacancies in as-irradiated TiN. In contrary, the lattice parameter decreased for as-irradiated TiN_0.7, which indicates that the nitrogen atom vacancies in TiN_0.7 acted as sinks for displacement atoms generated by irradiation to limit interstitial atoms existing. The intensity of peaks in fluorescence spectrum of as-irradiated TiN was higher than that of as-irradiated TiN_0.7. That attributed to the presence of color centers formed by Frenkel defects in as-irradiated TiN. All of the results indicate that introducing vacancy defect in materials would offer capability to realize self-heal of irradiation damage.     

Cf/SiC复合材料低温铣削工艺优化研究

对超硬金刚石刀具低温铣削Cf/SiC复合材料进行工艺优化研究,主要研究聚晶金刚石刀具低温铣削Cf/SiC复合材料过程中,低温冷却介质(干冰、液氮)和冷却工艺参数(阀体出口压力和喷嘴喷射角度)对切削力、加工表面完整性和切屑的影响规律。研究表明:在干冰、液氮低温介质的冷却作用下,工件的加工表面质量得到了较大提高,主切削力显著降低。     

液态SiBCN前驱体转化陶瓷的抗氧化性能及机制

前驱体转化法制备的SiBCN陶瓷具有优异的耐高温抗氧化性能,有望作为高温热结构材料应用于航空航天领域。本文主要对前驱体转化法制备的SiBCN陶瓷在1 200、1 400℃下的抗氧化性能进行研究。采用XPS对陶瓷氧化前后化学键结合方式进行表征,分析了氧化前后化学结构的变化;采用XRD和SEM对陶瓷氧化前后表面相组成、微观形貌和截面氧化层进行分析,并通过氧化层厚度随时间变化对高温抗氧化动力学进行了研究。结果表明:SiBCN陶瓷经高温氧化后在表面形成了致密的氧化膜,氧化层主要以无定形态存在,且与基体结合紧密;陶瓷的高温氧化速率受氧扩散控制,其在1 200、1 400℃下的氧化动力学常数分别为0.0224 μm2/h和0.1045 μm2/h,小于SiC陶瓷的0.0449 μm2/h和0.1288 μm2/h。由于SiBCN陶瓷形成的BN （C）结构以及高温氧化后形成的SiO_xN_y致密氧化膜降低了氧气在氧化层中的扩散速率以及反应活性,使得SiBCN陶瓷具有比SiC陶瓷更加优异的高温抗氧化性能。      

液体介质处理对纳米隔热材料细观结构的影响

为研究液体介质处理对纳米隔热材料细观结构的影响,并掌握材料结构对毛细管力的承受性,将纳米隔热材料分别在无水乙醇和去离子水中进行了浸泡和常温常压干燥处理。采用热导率测试结合SEM、N₂吸附-脱附、光学显微镜等对材料处理前后的细观结构进行了表征,并以Kaganer双孔模型对气相热导率随环境气压的变化进行了描述。研究结果表明,材料固体骨架颗粒尺度和孔隙结构均不受无水乙醇处理的影响;去离子水处理后的材料固体骨架颗粒尺度虽未改变,但颗粒之间的接触面积有所增大,并且孔隙结构发生了两极分化,一部分变大,另一部分变小;处理前以及无水乙醇处理后,材料的孔隙结构均可以等效为70 nm和300 nm两种尺度的典型孔隙,占比分别约为81%和19%,而去离子水处理后的材料孔隙结构需要等效为30 nm和60 μm两种尺度的典型孔隙,占比分别约为58%和42%。      

Deposition and characterization of hex-BN coating on carbon fibres using tris(dimethylamino)borane precursor

Boron nitride thin films were deposited on carbon fibres by chemical vapor deposition from the single source precursor tris(dimethylamino)borane (TDMAB). Hydrogen was used as carrier gas and additional nitrogen was supplied in the form of ammonia. The heating of the substrate was performed by Joule effect. Detailed TEM structural and chemical analyses reveal a meso-graphitic structure with a stoichiometric B/N ratio as confirmed by Auger Electron Spectroscopy (AES). A penetration of carbon element from the fibre was observed in the BN layer close to the interface.     

Fatigue behavior of Zr–Ti–Ni–Cu–Be bulk-metallic glasses

The high-cycle fatigue (HCF) behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (in at.%) bulk-metallic glass (BMG) was studied. Two batches of samples that are from different lots (Batches 59 and 94) are employed in present experiments. The HCF experiments were conducted, using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 in air at room temperature and under tension-tension loading, where R=σ_min./σ_max.. (σ_min. and σ_max. are the applied minimum and maximum stresses, respectively). A high-speed and high-sensitivity thermographic-infrared (IR) imaging system was employed for the nondestructive evaluation of temperature evolutions during fatigue testing. No distinct sparking phenomenon was observed at the final fracture moment for this alloy. The fatigue lifetime of Batch 59 is longer than that of Batch 94 at high stress levels (maximum stresses >864 MPa). Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Batch 94 (615 MPa). The vein pattern and liquid droplets were observed in the apparent-melting region along the edge of the fractured surfaces. The fracture morphology suggests that fatigue cracks initiated from casting defects, such as porosities and inclusions, which have an important effect on the fatigue behavior of BMGs.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrC composites

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C.     

Low cost preparation of high thermal conductivity carbon blocks with ultra-high anisotropy from a commercial graphite paper

A carbon block with ultra-high anisotropy was produced from a commercial graphite paper as the thermal reinforcement and a thermosetting phenolic resin as the binder. Hot-pressing at a maximum temperature of 200 °C was used to densify and integrate the graphite paper stacks. It has been found that the graphite paper blocks have high thermal conductivities in the paper direction and low ones perpendicular. An anisotropy of 98.8% and a thermal conductivity of 197.8 W m^?1 K^?1 in the paper direction were achieved when the density was 1.1 g cm^?3. The thermal conductivity increased to 284.8 W m^?1 K^?1 with a decrease of anisotropy to 98.3% with a density of 1.56 g cm^?3.     

Novel one-step formed composite reinforcement of "Spider web like" SiCnw networks and "Z-pins like" SiC rods for ablation resistance improvement of C/C-ZrC-SiC

A novel composite reinforcement with horizontal multilayer "Spider web like" SiC nanowire networks and vertical interconnected "Z-pins like" SiC rods was designed and prepared by facile one-step figuration. The linear ablation rate of "Spider web like" SiC nanowire networks and "Z-pins like" SiC rods collectively reinforced C/C-ZrC-SiC composites at 2610 ± 20 ℃ was 0.4 ± 0.03 μm/s with a 74.19 % reduction. The improved ablation resistance was attributed to a denser gradient oxide layer composed of central ZrO₂ layer, transitional ZrO₂-SiO₂ layer and marginal SiO₂ layer generated under the initial sticky net effect from SiCnw networks and subsequent oxide compensation from "Z-pins like" SiC rods.