弯曲纤维对SiO2f/SiO2复合材料拉伸强度的影响

以SiO_(2f)织物作为增强相,采用循环浸渍固化工艺,制备了SiO_(2f)/SiO_2复合材料。在制备过程中,通过对SiO_(2f)织物进行模压处理,使SiO_(2f)呈现出不同程度的弯曲,测定了纤维弯曲后复合材料的拉伸强度,研究了纤维弯曲时复合材料的断裂过程。结果表明:弯曲纤维将导致复合材料的拉伸强度下降,最低拉伸强度仅为5.5 MPa,纤维弯曲时复合材料的断裂过程为逐层断裂,断裂应变增加,最大断裂应变达到1.19%。     

Microstructure and high-temperature oxidation behavior of plasma-sprayed Si/Yb2SiO5 environmental barrier coatings

An environmental barrier coating (EBC) consisting of a silicon bond coat and an Yb₂SiO₅ top-coat was sprayed on a carbon fibers reinforced SiC ceramic matrix composite (CMC) by atmospheric plasma spray (APS). The microstructure of the coating annealed at 1300 °C and its high-temperature oxidation behavior at 1350 °C were investigated. The significant mass loss of silica during the plasma spray process led to the formation of Yb₂SiO₅ and Yb₂O₃ binary phases in the top-coat. Eutectics of Yb₂SiO₅ and Yb₂O₃ were precipitated in the top-coat, and channel cracks were formed in the top-coat after 20 h annealing because of the mismatch between the coefficients of thermal expansion (CTEs) of Yb₂SiO₅ and the SiC substrate. The EBC effectively improved the oxidation resistance of the CMC substrate. The channel cracks in the Yb₂SiO₅ top-coat provided inward diffusion channels for oxygen and led to the formation of oxidation delamination cracks in the bond coat, finally resulting in spallation failure of the coating after 80 h oxidation.     

Oriented molybdenum disulfide-silica/hydrogenated nitrile butadiene rubber composites: Effects of nanosheets on mechanical and dielectric properties

A series of non-covalently functionalized molybdenum disulfide-silica (f-MoS₂-SiO₂) nanocomposites was prepared by an in-situ assembled method and used to fabricate the oriented molybdenum disulfide-SiO₂/Hydrogenated Nitrile Butadiene Rubber (f-MoS₂-SiO₂/HNBR) composites. The characterization results show the synergistic dispersion between the functionalized molybdenum disulfide (f-MoS₂) nanosheets and SiO₂ nanoparticles. The addition of f-MoS₂ nanosheets can improve the dispersion of fillers in the rubber matrix and weaken the filler network. The non-covalently functionalization improves the interface interaction between f-MoS₂ nanosheets and the rubber matrix. Furthermore, the tensile strength of f-MoS₂-SiO₂/HNBR is 65.9% higher than that of SiO₂/HNBR by adding 1.0wt% of f-MoS₂. At the same time, the dielectric constant of f-MoS₂-SiO₂/HNBR is increased by 23.7% compared to SiO₂/HNBR due to the micro-capacitor structure of parallel f-MoS₂ nanosheets in the rubber matrix. Our work provides new ideas for the development of high-performance elastomer materials.     

不同介孔结构的SiO2 对PMMA 性能的影响

分别采用二维六方孔结构SiO2(SBA-15) 和蠕虫状孔结构SiO2(MSU-J) 与MMA 原位聚合制备

介孔SiO2 / PMMA 杂化材料。采用XRD、N2 吸附-脱附、SEM、DSC 和TGA 等方法研究了材料的微观结构、力学

性能、热性能和介电性能。结果表明:介孔SiO2 对PMMA 有增强增韧作用,同时也有利于杂化材料热性能和介

电性能的提高。杂化材料的热稳定性和耐热性均高于PMMA,含4wt% 的SBA-15/ PMMA 和7wt% 的MSU-J/

PMMA 杂化材料的介电常数由2. 91 分别降至最低2. 73 和2. 64。

     

溶胶共凝法制备准连续密度梯度SiO2气凝胶

针对气凝胶在航空航天领域的应用需求,为了进一步拓展单一密度气凝胶的应用范围,对密度梯度SiO2气胶的制备进行了研究.采用自建密度梯度制备成型装置,以正硅酸乙酯为有机硅源,经溶胶共凝工艺,结合CO2超临干燥技术,获得密度范围在60~160 mg/cm3的准连续密度梯度SiO2气凝胶.采用光学显微镜、扫描电子显微镜、X线相...     

Diamond wheel grinding characteristics of 3D orthogonal quartz fiber reinforced silica ceramic matrix composite

Fiber-reinforced silica ceramic matrix composites (SiO_2f/SiO₂) have gained extensive attention in recent years for its applications in aeronautics field such as radar radome and window. However, the machining properties and mechanism of the material remain unclear. The features and mechanical properties of the material itself have a significant influence on both its machining characteristics and surface integrity. Thus, a full-factor grinding experiment is conducted using a 3D orthogonal SiO_2f/SiO₂ aiming to obtain its machining characteristics. The effects of grinding parameters and tools on the grinding force, surface roughness, and material damage type are investigated using a dynamometer, Scanning Electron Microscope (SEM), and Acoustic Emission (AE) analysis. The AE frequency band is analyzed, and a semi-analytical force model is established to study the difference between a single grain and wheel grinding. It was found that the changes in surface roughness correlate with the changes in grinding force, with fiber fracture being the main reason behind the increase in grinding force. Finally, the material removal mechanism was studied based on the AE analysis. It was found that the removal mechanism is fiber fracture dominated with matrix crack and debonding, and the primary sources of energy consumption are fiber fracture and friction.     

Al2 O3 掺杂对SiO2 纳米透波/ 隔热材料性能的影响

在溶胶-凝胶过程中加入纳米Al2O3 颗粒制备Al2O3 掺杂的SiO2 纳米透波/ 隔热材料,研究了

Al2O3 掺杂对材料高温热稳定性、介电性能及隔热性能的影响。结果表明,Al2O3 掺杂阻碍了SiO2 颗粒之间的

烧结,提高了材料的使用温度;材料介电常数符合Lichtenecker 对数法则;室温热导率略有提高,通过组合结构

改善了材料的隔热性能。

     

Interconnected Ni（OH）2 and polyether amine on carbon fiber surface for simultaneously improving interfacial adhesion of epoxy composites and electrochemical properties

Interconnected Ni（OH）₂ nanoflakes and polyether amine（PEA） were deposited on carbon fiber tows via a facial and effective process of chemical bath deposition and dip coating. Based on this, a win–win benefit of simultaneously improvements in interfacial shear strength（IFSS） of carbon fiber/epoxy composites and the electrochemical activity has been achieved. Compared with CF and CF-Ni（OH）₂ composites, the IFSS of CF-Ni（OH）₂-PEA/epoxy composite respectively increas...     

Joining of SiO2 ceramic and TC4 alloy by nanoparticles modified brazing filler metal

Nano-Al₂O₃ particles modified AgCuNi filler was adopted to braze the SiO₂ ceramic and TC4. The effects of filler size as well as the brazing temperature on the interfacial microstructure and mechanical property of the joints were investigated. Nanoscale filler reduced the phases dimension and promoted the homogeneous distribution of microstructure, obtaining a higher joint strength when compared to microscale filler. The increase of brazing temperature made the accelerating dissolution and diffusion of Ti, which promoted the increase of thickness of Ti₄O₇ + TiSi₂ layer adjacent to SiO₂ ceramic and diffusion layer zone nearby TC4 alloy. The hypoeutectic structure was produced in the brazing seam due to the high Ti content. The maximum shear strength of ∼40 MPa was obtained at 950 °C for 10 min.     

石墨烯/二氧化硅三维杂化材料的制备及其在改性酚醛泡沫中的应用

为了得到结构和力学性能良好的酚醛泡沫,利用原位生成的方法制备了石墨烯/二氧化硅(GNPs/SiO_2)杂化材料,并将其用于酚醛泡沫的制备当中。对杂化材料进行了红外光谱分析,透射电镜以及X射线衍射分析,验证了石墨烯表面SiO_2球体(粒径在150~180 nm)的存在。对泡沫的结构和压缩性能进行对比分析,发现三维杂化材料相比二维石墨烯能够更好的改善酚醛泡沫的泡孔结构,杂化材料为0.5wt%时,GNPs/SiO_2改性酚醛泡沫的泡孔尺寸更小,结构更均匀;同时杂化材料改性酚醛泡沫表现出更优异的力学性能,其压缩强度和弹性模量分别达到0.22和4.1 MPa,比纯酚醛泡沫分别提高了91%和86%。     

SCC evaluation of a 2297 Al-Li alloy rolled plate using the slow-strain rate technique

The stress corrosion cracking (SCC) susceptibility of 2297 Al-Li alloy in 1 M NaCl + 0.01 M H₂O₂ solution (CP solution) and 1 M NaCl + 0.01 M H₂O₂ + 0.6 M Na₂SO₄ solution (CPS solution) was investigated by slow-strain rate tests at various strain rates ranging from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹. The roles of H₂O₂ and SO₄²⁻ in the corrosion process were estimated by potentiodynamic polarization and electrochemical impedance spectroscopy. 2297 Al-Li alloy does not fracture ascribed to SCC in CP solution, while it undergoes SCC in CPS solution. In CPS solution, with a decreasing strain rate from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹, the SCC susceptibility firstly rises and then declines exhibiting a peak value at a strain rate of 10⁻⁶ s⁻¹. H₂O₂ promotes the active dissolution while SO₄²⁻ lowers the corrosion rate. The SCC fracture is associated with a decline in the dissolution rate of the crack tip by SO₄²⁻, which leads to stress concentration. In CPS solution, a reduction in the local dissolution rate of the crack tip leads to stress concentration, resulting in SCC fracture. As the preferred initiation site for a crack, pits also show a noteworthy effect on SCC of 2297 Al-Li alloy.     

柔性SiO2 气凝胶的制备

以甲基三乙氧基硅烷为前驱体,通过溶胶-凝胶法和超临界干燥制备块状柔性SiO2气凝胶.研究了前驱体浓度对材料化学组成、微观结构及柔韧性、热稳定性的影响.结果表明,甲基的存在降低了分子网络的交联密度,赋予了材料柔韧性.随着前驱体浓度降低,微观结构中颗粒堆积紧密程度下降,材料的柔韧性增强;同时分子中硅羟基增多,热稳定性提高.     

石英纤维增强聚酰亚胺复合材料超低温铣削实验

石英纤维增强聚酰亚胺复合材料是一种非均匀的各向异性材料,采用传统铣削方法对其进行加工时存在刀具磨损严重、切削力较大、加工效率低等问题。为此本文采用超低温冷却铣削方法对石英纤维增强聚酰亚胺复合材料进行铣削实验,并与传统干铣削方式进行了对比,分析了包括加工表面形貌、粗糙度、切削力和刀具磨损等切削性能。结果表明:两种工况下,表面粗糙度随主轴转速的提高而降低,随切深的增加呈先降低后增大趋势;相对于干铣削,不同切削速度下超低温冷却铣削有效抑制了低速干铣削纤维起毛、高速干铣削黏结剂烧蚀缺陷,表面质量都得到改善,刀具耐用度得到提高。超低温冷却引起的复合材料切削力增大,纤维断屑方式的改变以及切削热的有效降低是提高加工质量的主要原因。     

SCC investigation of low alloy ultra-high strength steel 30CrMnSiNi2A in 3.5wt%NaCl solution by slow strain rate technique

To evaluate stress corrosion cracking(SCC) mechanism of low alloy ultra-high strength steel 30CrMnSiNi2 A in environment containing NaCl, SCC behavior of the steel in 3.5wt% NaCl solution is investigated by slow strain rate technique(SSRT) with various strain rates and applied potentials, surface analysis technique, and electrochemical measurements. SCC susceptibility of the steel increases rapidly with strain rate decreasing from 1 · 10 5s 1to 5 · 10 7s 1, and becomes stable when strain rate is lower than 5 · 10 7s 1. SCC propagation of the steel in the solution at open circuit potential(OCP) needs sufficient hydrogen which is supplied at a certain strain rate.Fracture surface at OCP has similar characteristics with that at cathodic polarization 1000 mVSCE, which presents characteristic fractography of hydrogen induced cracking(HIC).All of these indicate that SCC behavior of the steel in the solution at OCP is mainly controlled by HIC rather than anodic dissolution(AD).     

纤维方向角对超低温冷却石英纤维复合材料铣削性能的影响

石英增强聚酰亚胺树脂基复合材料是一种非均匀的各向异性材料,其加工性能高度依赖于纤维铺层方向与加工进给方向所成角度,即纤维方向角。本文通过一系列不同纤维方向角的干切削和超低温冷却铣削实验,研究了纤维方向角对表面形貌、表面粗糙度、铣削力及刀具磨损的影响。结果表明:不同纤维方向角,剪应力形式不同,切削断屑形式也不同。纤维方向角为锐角时铣削表面质量均良好,但当纤维方向角增大到90°时,切削表面质量下降,切削力变化幅度增大。相同铣削时间内,在干切削工况下,刀具磨损严重,涂层脱落面积约为测量面积的70%;而在低温切削工况下,涂层未遭到严重破坏,刀具仍处于稳定磨损阶段,刀具耐用度优于干切削工况。     

微量Al 掺杂对2D C/ SiC 性能的影响

以二维编织碳纤维碳布为预制体,采用聚铝碳硅烷(PACS)为聚合物前驱体,应用化学气相渗透(CVI)结合聚合物浸渗-裂解(PIP)工艺制备微量Al掺杂2D C/SiC复合材料。研究微量Al掺杂对C/SiC微观结构、力学、热膨胀和氧-乙炔焰烧蚀性能的影响。结果表明:掺杂微量Al未改变C/SiC的微观结构和热膨胀性能,也未降低其韧性和强度;但微量Al掺杂提高了C/SiC的抗烧蚀性能,含微量Al的SiC氧化形成微量Al熔于SiO2的固熔体,微量Al提高了SiO2的黏度和致密度,减小SiO2挥发,较未掺杂Al的C/SiC相比,线烧蚀率降低了26%。     

飞机刹车盘用炭/炭复合材料新型防氧化复合涂层

 研究了一种工艺简单、成本低廉的飞机刹车盘用炭/炭复合材料防氧化复合涂层。该复合涂层由两层组成，其中内层由硼酸制备而成，外层由硼粉、炭化硼、氧化硅玻璃粉、有机硅树脂以及铝粉和铁粉制备成。恒温氧化和热震实验的结果表明：在700 ℃下空气中恒温氧化50 h后，带该涂层试样的氧化失重率只有0.08％，同时在700 ℃下空气中热循环50次后，试样的氧化失重率也只有0.12％。另外，用扫描电镜(SEM)、X射线能量分散谱(EDX)、Raman光谱和差热分析(DTA)等方法分析了涂层的防氧化机理。     

不同刀具车削加工高硅氧玻璃纤维/酚醛树脂切削力研究

为探索高硅氧玻璃纤维/酚醛树脂复合材料的切削加工性能,对该类材料进行大直径薄壁回转类零件的车削加工。采用四种不同刀具进行实验研究,获得了不同切削参数及不同刀具材料对切削力的影响规律。试验结果表明:切削用量三要素中,切削深度对切削力的影响最大,其次是进给量,而切削速度的影响很小。当切削速度为119.32 mm/min、进给量为0.1 mm/r、背吃刀量为0.5 mm时,为最优切削参数。Ti-Al-Si-N纳米涂层硬质合金和超硬材料F2HX无涂层硬质合金刀具适合于低速加工,而PCD刀具则适合于高速加工。     

Experimental Investigation into Characteristics of Plasma Aerodynamic Actuation Generated by Dielectric Barrier Discharge

This article carries out synthetic measurements and analysis of the characteristics of the asymmetric surface dielectric barrier discharge plasma aerodynamic actuation.The rotational and vibrational temperatures of an N2 (C3Пu) molecule are measured in terms of the optical emission spectra from the N2 second positive system.A simplified collision-radiation model for N2(C) and N2+(B) is established on the basis of the ratio of emission intensity at 391.4 nm to that at 380.5 nm and the ratio of emission intensity at 371.1 nm to that at 380.5 nm for calculating temporal and spatial averaged electron temperatures and densities.Under one atmosphere pressure,the electron temperature and density are on the order of 1.6 eV and 1011cm-3 respectively.The body force induced by the plasma aerodynamic actuation is on the order of tens of mN while the induced flow velocity is around 1.3m/s.Starting vortex is firstly induced by the actuation;then it develops into a near-wall jet,about 70 mm downstream of the actuator.Unsteady plasma aerodynamic actuation might stimulate more vortexes in the flow field.The induced flow direction by nanosecond discharge plasma aerodynamic actuation is not parallel,but vertical to the dielectric layer surface.     

Small scale structure of magnetospheric electron density through on-line tracking of plasma resonances

The plasma resonance phenomena observed at f _pe, nf _ce, and f _qn by the GEOS-1 S-301 relaxation sounder are identified through a pattern recognition software process implemented in a mini-computer which receives on-line the compressed data. First, this processing system distributes in real time f _pe and f _ce measurements to the ground media. Second, it drives and controls automatically the S-301 on-board experiment by sending appropriate telecommands: the tracking of resonances is performed by shortening the frequency sweeps to a narrow range centered on the resonance location. Examples of such tracking sequences are presented, exhibiting sampling rates of the electron density measurements from once every 22 s (slowest rate) to once every 86 ms (highest rate available). The results give evidence of the existence of very small scale structures in the magnetospheric density, having characteristic sizes of the order of a few 10² m or/and a few 10^-1 s. The relative amplitude of these density fluctuations is typically 1%. Because of satellite spinning, fixed frequency sounding sequences allow to measure in a few seconds the directivity features of the plasma resonance signals. Examples of directional patterns in the plane perpendicular to the geomagnetic field are presented: the electrostatic nature of the waves received at f _pe, nf _ce, and f _qe being consistent with these patterns, the corresponding k vector orientations become available. The Bernstein modes properties are used to interpret the cf _ce and f _qe results.