多孔氮化硅陶瓷透波材料研究进展

多孔氮化硅陶瓷透波材料具有优异的机械性能、耐热性能及介电性能,成为透波材料科学研究领域中的热点之一。本文介绍了多孔氮化硅陶瓷的主要制备技术,并对国内外多孔氮化硅陶瓷透波材料的应用研究进展进行了综述。     

多孔氮化硅陶瓷的制备及应用现状

综述了多孔氮化硅陶瓷目前的研究进展,介绍了添加造孔剂法,成型和烧结工艺中造孔法,干燥及其它工艺造孔法等方法制备多孔氮化硅陶瓷,最后展望了多孔氮化硅陶瓷在催化剂载体,气体过滤器以及航空透波材料等领域的应用。     

固相含量对多孔Si3N4显微结构及性能的影响

以氮化硅为原料,以叔丁醇为溶剂,采用凝胶注模成型工艺,在无压烧结温度1780℃、保温时间1.5h、流动氮气气氛下,成功制备出具有高强度和高气孔率的多孔氮化硅陶瓷透波材料.通过对材料的密度、孔隙率、压缩强度和介电性能测试及显微组织观察,系统分析了固相含量对透波多孔陶瓷性能的影响.研究结果表明:随着固相含量的提高,孔隙率与...     

高比强多孔Si_3N_4陶瓷透波材料的研究

通过采用Y_2O_3,Al_2O_3作为烧结助剂,气氛压力烧结工艺,成功地制备出了高强度多孔Si_3N_4陶瓷透波材料。通过SEM对材料微观结构进行了研究。实验结果表明:在适当的工艺下可以制得弯曲强度大于160 MPa,气孔率50%的多孔氮化硅陶瓷。并讨论了多孔氮化硅形成的机制和高强度的主要原因。     

透波材料在微波冶金领域的应用进展

综述了透波材料的研究现状,探讨了目前在不同的微波冶金温度区间所需的微波透波材料。聚四氟乙烯等可作为低温微波透波材料,xMeO.yAl2O3.zSiO2陶瓷体系已经作为高温微波冶金透波材料,继续研究适合微波加热不同温段的透波材料体系是目前微波冶金发展的迫切需要。     

氮化硅陶瓷在四大领域的研究及应用进展

氮化硅陶瓷不仅具有较高的力学性能还具有良好的透波性能、导热性能以及生物相容性能,是公认的综合性能最优的陶瓷材料。作为轴承球的致密氮化硅陶瓷广泛应用在机械领域;作为透波材料的多孔氮化硅陶瓷广泛应用在航空航天领域;随着对氮化硅陶瓷材料的深入研究,其在导热性和生物相容性方面的优异特性逐渐被科研工作者认识并得到开发和应用。本文详细阐述了氮化硅粉体的制备方法,并综述了氮化硅陶瓷作为结构陶瓷在机械领域和航空航天领域的研究进展,此外还介绍了其作为功能陶瓷在半导体领域、生物制药领域的研究和应用现状,最后对其未来发展进行了展望。     

影响多孔氮化硅结构陶瓷材料性能的因素

多孔氮化硅陶瓷是一种新型功能陶瓷,它密度低,气孔率高,相对介电常数较小,抗腐蚀耐热性能良好,使用寿命长,且相对介电常数可以根据气孔率的多少进行调节,可在较大温度范围内正常使用,优异的性能使其在航天透波天线罩材料方面有很大的应用空间,是一种理想的新型高性能天线罩候选材料,已被广泛应用于航空航天,民用设备等领域。本文主要分析了多孔氮化硅结构陶瓷材料性能的影响因素。     

多孔氮化硅陶瓷制备及其性能研究进展

多孔氮化硅(Si_3N_4)陶瓷由于具有优异的力学性能、良好的抗热震和低介电常数等特点,在极端力/热环境下具有很大的应用潜力。研究表明:不同的制备工艺对多孔氮化硅陶瓷晶粒尺寸、微结构有很大影响,从而影响材料的力学性能;介电性能受气孔率、相组成影响;渗透率受气孔率、气孔尺寸、弯曲度的影响。综述了多孔Si_3N_4陶瓷的烧结工艺、成型工艺及其相关性能研究,并结合目前的研究热点,指出了未来多孔氮化硅陶瓷研究的发展方向。     

烧结助剂含量对多孔Si3N4/BN复合陶瓷力学性能和介电性能的影响

以Si3N4和BN为原料,叔丁醇为溶剂,SiO2、Y2O3和Al2O3为烧结助剂,采用凝胶注模成型工艺制备具有高强度、低介电常数多孔Si3N4/BN复合陶瓷。研究了Y2O3和Al2O3含量对多孔陶瓷气孔率、孔径分布、物相组成、显微结构、抗弯强度和介电常数的影响。结果表明:通过调节Y2O3和Al2O3含量,多孔Si3N4/BN复合陶瓷的气孔率由55%增加到68%,气孔尺寸呈单峰分布,平均孔径为0.89～1.02μm;抗弯强度和相对介电常数随Y2O3和Al2O3含量的增加而单调增大,抗弯强度和相对介电常数的变化范围分别为29.9～60.9 MPa和2.30～2.85;通过调节Y2O3和Al2O3含量调控气孔率,能够获得介电性能和力学性能可调的高性能透波材料。     

多孔氮化硅结构陶瓷材料性能影响因素分析

多孔氮化硅陶瓷是一种新型功能陶瓷,它密度低,气孔率高,相对介电常数较小,抗腐蚀耐热性能良好,使用寿命长,且相对介电常数可以根据气孔率的多少进行调节,可在较大温度范围内正常使用,优异的性能使其在航天透波天线罩材料方面有很大的应用空间,是一种理想的新型高性能天线罩候选材料,已被广泛应用于航空航天,民用设备等领域。文中分析了多孔氮化硅结构陶瓷材料性能的影响因素。     

结构炭/炭复合材料力学性能及微观结构研究

采用四向编织、快速化学气相渗透致密化新工艺制备了炭／炭复合材料,其弯曲强度达３２０ＭＰａ。分析研究了这种材料的力学性能特征。利用ＳＥＭ和高分辨ＴＥＭ分析了基体炭、炭纤维／基体灰界面的精细结构,发现炭纤维呈单根被基体炭包围,基体现灰呈层片状,为二维有序的乱层石墨结构;在炭纤维与基体炭之间存在着过渡相,这一过渡相厚度的约几十纳米,随着与炭纤维之间距离的增大,它们之间形成的夹角由小变大,这一过渡相即为炭     

Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.     

A reduced reaction model for carbon CVD/CVI processes

Carbon-carbon composites are produced by chemical vapor deposition/chemical vapor infiltration (CVD/CVI) processes. Models of carbon-carbon composite production processes will help reduce production costs. Reliable process models must, however, include details of the gas phase kinetics in order to identify optimal conditions. We have combined detailed gas phase kinetics, surface kinetics, and a pore closure model to predict pore geometry changes with respect to time. To determine the dominant gas phase kinetics, we reduced a large set of reactions to a minimal set using a sensitivity, rate, and dimensional analysis approach. These robust and relatively fast techniques can be used under a variety of conditions, including those within the pores of the composite. The process model shows that the deposition profile depends on the kinetic model chosen. Using the more realistic reaction model, conditions for uniform, or inside-out, densification can be suggested.     

Chemical Vapor Infiltration Rates of ZrO2 into MoSi2 Porous Bodies and Comparison with Model Calculation

Zirconia (ZrO₂) was chemical vapor infiltrated (CVI) into a partially sintered MoSi₂ body (preform) by using zirconium n -propoxide (Zr(OC₃H₇)₄) as a gas precursor. Infiltration distances at different conditions were compared with the calculated results. Chemical vapor deposition (CVD) film growth rates of ZrO₂ were measured, and the data were incorporated into the model calculations. Two models were used to analyze the observed infiltration distances. Initially a conventional model assuming a pore with constant radius (SP model) was used. With this model, it was possible to predict the approximate infiltration distance. However, the model cannot predict pore closure and the infiltration distances for a variety of CVI conditions. Secondly, a newly proposed model (PC model) from a previous paper was applied to calculate the infiltration distance. Using this model, it was possible to predict the occurrence of pore closure or the formation of the deposition layer on the preform surface.     

Experimental studies and computer simulation of the preparation of nanoporous silicon-carbide membranes by chemical-vapor infiltration/chemical-vapor deposition techniques

Preparation of asymmetric nanoporous silicon-carbide membranes using chemical-vapor infiltration/chemical-vapor deposition (CVI/CVD) is described. We use macroporous SiC disks and tubes as supports, and tri-isopropylsilane as the precursor. Experimental data for the permeation and selectivity of the membranes are presented. We also develop two dynamic models to describe the preparation of the membranes. The models are shown to provide accurate predictions for the experimental data for the permeation characteristics of the membranes, as a function of the preparation conditions.     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.     

热梯度CVI制备炭/炭复合材料及其研究进展

按预制件内部的温度分布不同，可以认为，均热法及热梯度法是化学气相沉积制备炭／炭复合材料的两种基本工艺。对于圆筒或圆盘形工件，热梯度CVI具有增密快，炭的有效利用率高，可实现工业规模化生产的优点，是一种很有前景的CVI工艺。本文介绍了热梯度CVI制备炭／炭复合材料的工艺原理、工艺特点及其最新研究进展。     

炭／炭复合材料CVI致密化技术的研究与发展

详细介绍了CVI理论的建立和发展,并在此基础上,对化学气相渗透中的气相组成与显微结构变化的关系,即气相组成变化对热解炭显微结构的影响进行了全面的分析。在介绍化学气相渗透技术发展的同时对化学气相渗透技术的最新进展进行了综述,特别是详细介绍了强制流动CVI、限域变温强制流动CVI、等离子体增强等温(或热梯度)低压CVI、HCVI技术、液相气化CVI等CVI新技术近几年来的最新发展,并在文后对未来化学气相渗透技术的发展进行了展望。     

碳/碳复合材料等温化学气相渗透工艺模糊系统建模

等温化学气相渗透(chemical vapor infiltration，CVI)是制备陶瓷基和碳基复合材料重要的传统工艺，该工艺主要的不足之处是周期极长，因此，优化工艺参数、提高沉积效率是目前等温CVI工艺研究的重点。在实验样本的基础上，利用遗传算法来自动获取和优化模糊规则，从而建立了碳／碳复合材料等温CVI工艺模糊系统。通过系统对训练样本和测试样本的输出，可以看出：系统具有较高的精度和泛化能力。利用该系统，得到了沉积温度、纤维体积分数和沉积室压强等参数对等温CVI工艺的影响规律，对实际生产中CVI工艺的制定有指导意义。     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection