SiC纤维直流电阻加热CVD工艺研究

对SiC纤维的直流电阻加热CVD工艺进行了研究,实验采用将两种硅烷的比例混合液体通过液体流量计计量供液并即时完全汽化后与氢气混合输入到反应管并在水冷水银封入气口通入顶吹氢的供气方案,从而简化了工艺并解决了反应气体冷凝问题.在CVD工艺研究中发现影响纤维沉积质量的因素主要有沉积温度、反应气体组分及流量、走丝速度等工艺参数,此外还发现直流电阻加热CVD工艺中,反应管前后存在约200℃的温差,采用双沉积室工艺可减缓温差.在一定的沉积参数下,可沉积出直径60～100μm、抗拉强度3100～4080MPa的SiC纤维.     

化学气相沉积法制备钨芯SiC纤维的装置和工艺

分析比较了两种不同类型化学气相沉积(CVD)法制备钨芯SiC纤维的反应器,提出了改进方法,设计了一种立式冷壁反应器,并对用CVD法制备钨芯SiC纤维的工艺进行了研究.运用扫描电镜观察了制备纤维形貌,并用Weibull统计方法分析了纤维强度与其形貌之间的关系.结果表明,在不同的氢气流量和氮气流量比下,沉积速率均随沉积温度的升高而增大.总反应在较低温度区受表面反应控制,在较高温度区受质量传输控制.纤维的断裂强度大致可分为高、中、低3个区,CVD-SiC纤维的强度与其微观形貌有密切关系,缺陷的多少决定纤维强度的高低.     

CVD SiC涂层SiC纤维增强SiC复合材料的研究

本文采用CVD技术对KD-1 SiC纤维作涂层处理,再通过聚碳硅烷浸渍裂解法制备单向SiCf/SiC复合材料.研究了不同沉积时间的CVDSiC涂层对SiCf/SiC复合材料性能的影响,同时运用SEM研究了SiC纤维表面SiC涂层的形貌.结果表明:经过5小时CVDSiC涂层SiCf/SiC复合材料具有良好的力学性能和抗氧化性能.     

化学气相沉积法连续SiC纤维的研究现状和发展趋势

综述了采用化学气相沉积法制备连续SiC纤维的研究进展,介绍了国内外化学气相沉积法制备的大直径、致密和均匀的SiC纤维的制备装置、制备工艺、性能、微观组织以及表面处理等热点研究方向,讨论了SiC纤维的制备工艺、性能和微观组织之间的关系以及利用表面处理如何弥补SiC纤维的缺陷,指出了今后采用化学气相沉积法制备连续SiC纤维的研究重点和发展趋势.     

涂碳前后SiC纤维的强度测试和评价

测定了室温下涂碳前后化学气相沉积法(CVD)制备的国产SiC纤维的抗拉强度,发现威布尔分布可以较好地描述SiC纤维的抗拉强度的统计分布.分析得出以下结论,涂碳后与涂碳前的纤维抗拉强度的Weibull模数,及其平均抗拉强度相比,前者明显高于后者.涂碳后SiC纤维的表面缺陷大大减小.随着标距和应变速率的增加,纤维的平均强度逐渐下降,而Weibull模数基本不变.并对断口进行分析,结果表明SiC纤维呈明显的脆性断裂.     

SiC纤维表面C涂层的制备及介电性能研究

采用化学气相沉积(CVD)法,在SiC纤维表面沉积了100nm厚的C涂层,研究了制备温度对C涂层微观结构、单丝纤维体电导率及纤维编制体介电性能的影响.采用SEM和RAM显微技术(Raman microscopy)对C涂层的表面形貌和微观结构进行分析.结果表明:保持C涂层厚度一致,当沉积温度由800℃升到900℃后,C涂层的石墨化程度提高,晶粒变大,SiC纤维单丝体电导率由0.745Ω~(-1)·cm~(-1)升到6.289Ω~(-1)·cm~(-1);SiC纤维编制体的复介电常数实部由90升到132,介电损耗由0.95升到1.14,其中虚部由87升到150.实部增大与载流子浓度增大有关,虚部增大与材料漏导电有关.认为这是SiC纤维表面沉积的C层使纤维电导率增大所致.直流电导损耗足其主要损耗机制.     

CVD SiC纤维断口及表面微观形貌的研究

一、前言 近年来,多种陶瓷纤维相继被作为金属基复合材料的增强剂来研究,为了提高复合材料性能,解决陶瓷纤维和金属基体相容性问题,许多学者从不同角度致力于陶瓷纤维表面涂层工程的设计。作为铝(钛)基体增强剂CVD SiC纤维,由于它具有高强度、高模量和耐热抗氧化等优异性能而受到人们的瞩目,但由于CVD SiC纤维是由β-柱状晶粒(直径D=40nm—100nm)所组成,裸露在纤维表面的晶界最易与基体铝反应,致使纤维受到较严重损伤,降低复合材料性能。美国Textron公司对CVD SiC纤维表面的涂层沉积工艺进行精心设计,推出以SCS为系列的多品种碳化硅纤维,不同涂层适合于不同类型的金属基体的需要。本文借助扫描电镜高放大倍率对几种碳化硅(CVD)纤维的断口和纤维表面进行观察,清楚地显示出SCS—6纤维在沉积过程中不同层次结构的微观形貌,从而解释SCS—6纤维综合性能优异的机理;清楚地显示出裸露在无涂层碳化硅纤维(SiC(∑))表面亚晶界的微观形貌;本文观察的结果对纤维涂层工艺     

低温化学气相沉积SiC涂层沉积机理及微观结构

由MTS-H2体系在1000～1300℃沉积了SiC涂层,研究了SiC涂层沉积速率和温度之间的关系,MTS-H2体系沉积反应的平均活化能为114kJ/mol,用理论模型证明了低温化学气相沉积SiC为动力学控制过程.SiC涂层表面的显微结构随沉积温度变化而呈现规律的变化:沉积温度T<1150℃时,CVD SiC涂层表面致密、光滑;T≥1150℃时,CVD SiC涂层表面变得疏松、粗糙.随着沉积温度的升高,CVD SiC涂层的结晶由不完整趋向于完整;当沉积温度T≥1150℃,CVD SiC涂层的XRD谱图中除了β-SiC占主体外还出现了少量α-SiC.     

CVD工艺中气体流型的研究

化学气相沉积(CVD)法可制备高纯度、近似元件形状、大面积的块体材料,与制备薄膜的CVD过程相比,该过程存在长时间沉积稳定性、厚度均匀性及光学质量均匀性等问题.本文采用简化装置对CVD工艺过程进行物理模拟,探讨不同工艺参数下沉积室内部气体流型的变化,分析流型对沉积过程的影响.     

碳化硅纳米纤维/炭纤维共增强毡体的制备

以电镀Ni颗粒为催化剂,采用化学气相沉积(CVD)法,在炭纤维表面原位生长SiC纳米纤维(SiC-NF),制备出SiC纳米纤维/炭纤维共增强毡体.XRD和SEM分析表明生成的SiC纳米纤维物相为β-SiC,平均长度可达几十微米,直径在几十到几百个纳米之间.通过改变电镀镍的时间,研究了催化剂Ni颗粒的大小、形态及分布对SiC-NF生长情况的影响,研究结果表明,催化剂Ni颗粒分布越细小、均匀,催化活性越大,所生长的纳米SiC纤维也越细长,分布越均匀.     

Finite element simulation of the effects of process parameters on deposition uniformity of chemical-vapor-deposited silicon carbide

A two-dimensional model was developed to simulate chemical vapor deposition process for preparing SiC coating by MTS + H₂ system in a vertical hot-wall reactor. The effects of process parameters, including deposition temperature, the flux of mixed gases, the ratio of H₂ and Ar, and the volume ratio of MTS and mixed gases, on deposition uniformity of SiC coating were calculated by finite-element method. The CVD process was optimized by an orthogonal L₉(3)⁴ test to deposit uniform SiC coating. The results show that the deposition uniformity of SiC is influenced greatly by the deposition temperature and the ratio of H₂ and Ar, and little by the flux of mixed gases and the volume ratio of MTS and mixed gases. The optimal deposition uniformity of SiC can be obtained under the operating condition as follows: deposition temperature 900 °C, the flux of mixed gases 0.6 l/min, H₂: Ar = 1:0, and the volume ratio of MTS and mixed gases 1:10. Part of calculated results is validated by corresponding experimental data, which implies that this model is valid and reasonable to characterize CVD process of SiC coating.     

EVALUATION OF SIC MATRIX COMPOSITES FOR HIGH TEMPERATURE APPLICATIONS

SiC matrix composite components have been fabricated by infiltrating and overcoating fiber preforms of graphite, alumina, and SiC via chemical vapor deposition (CVD). The degree of CVD densification could be controlled to yield vacuum tight components as well as porous open mesh structures where only the fiber was coated. These components have been fabricated in a wide variety of shapes including baskets, tubes and corrugated panels. The CVD process has been successfully scaled up to produce panels 80 cm by 80 cm. The morphology, chemistry, and geometry of the fiber was found to have a significant effect on the deposition process. Thus, process conditions had to be modified based on the fiber being infiltrated. These components were subjected to 1400 C temperature treatments including operation in gas-fired furnaces. After 2000 hours of testing in a gas-fired furnace at 1300 C, the SiC composite tubes have remained intact and un cracked. This suggests that the monolithic SiC coating is the controlling material and not the fibers. Hence, in light-load applications these structures still have useful lifetimes.     

SiC 纤维的强度测试和评价

测定了 SiC(W 芯)纤维的抗张强度,实验表明:对用 CVD 法制备的 SiC(W 芯)纤维的抗张强度测试值随试样标距的增大而降低,其室温抗张强度呈正态分布状态。当纤维拉伸试样标距为50mm 和25mm 时,SiC(W 芯)纤维室温抗张强度分别为3584.2±403.7MPa 和3669.9±348.1MPa,其 Weibull 模数分别为9.8和11.9。     

Homoepitaxial 6H-SiC thin films by vapor-liquid-solid mechanism

Silicon carbide (SiC) is a IV-IV compound semiconductor with a wide energy band gap. Because of its outstanding properties, SiC can be used in high-power, high-temperature devices with high radiation resistance. In this study, a two-step vapor-liquid-solid (VLS) method was proposed for homoepitaxial growth of high quality 6H-SiC thin films, combining VLS growth and conventional chemical vapor deposition (CVD) processes. VLS growth was used to eliminate the micro-pipes (MPs) in the first step, and the subsequent step based on the CVD process was employed to improve the surface roughness. The morphology and structure of the as-grown thin films were investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, atomic force microscopy and high-resolution X-ray diffraction, showing that thin films grown by two-step method have good crystalline quality and small surface roughness.     

In situ growth carbon nanotube reinforced SiCf/SiC composite

Carbon nanotube (CNT) reinforced SiC_f/SiC composite was prepared by in situ chemical vapor deposition (CVD) growth of CNTs on SiC fibers then following polymer impregnation pyrolysis (PIP) process. The nature of CNTs and the microstructure of the as prepared CNT-SiC_f/SiC composite were investigated. The mechanical properties of the as prepared CNT-SiC_f/SiC composite were measured. The results reveal that the in situ CVD growth of CNTs on SiC fibers remarkably promotes the mechanical properties of SiC_f/SiC composite. The secondly pull-out of CNTs from matrix during the pull-out of the SiC fibers from matrix consumes the deformation energies, resulting in promotion of the mechanical properties for composite.     

反应器长径比对化学气相沉积SiC沉积动力学的影响

以三氯甲基硅烷(MTS)和H₂为前驱体,在沉积温度900~1 050℃,H₂和MTS摩尔比为4~20和滞留时间0.4~1 s下,采用化学气相沉积(CVD)工艺研究沉积反应器长径比分别为7∶6和7∶2时的碳化硅(SiC)沉积动力学。结果发现,不同尺寸反应器中SiC沉积速率随工艺参数变化的规律性差异明显。长径比7∶6的反应器中SiC平均沉积速率随着温度的增加而增加,而长径比7∶2的反应器中SiC平均沉积速率随着温度先增加后降低,且长径比7∶6的沉积反应器中沿程SiC沉积存在多重稳态的特征。不同H₂/MTS摩尔比下SiC沉积速率变化规律在两种反应器中基本一致,尽管长径比7∶6的反应器中出现了SiC沿程的多重择优沉积位置,但整体来说H₂对SiC沉积的抑制作用远大于反应器尺寸效应所带来的影响。长径比7∶6反应器中SiC平均沉积速率随滞留时间的增加而降低,但沿程沉积速率受反应器尺寸效应并没有出现单调降低的规律;长径比7∶2反应器中SiC平均沉积速率和沿程沉积速率均随滞留时间增加而降低后趋于稳定。利用COMSOL软件对两种长径比反应器的流场和温度场进行了数值模拟分析发现,长径比7∶6的反应器产生明显的径向流速差,而且轴向和径向流速差和温差较大,而长径比7∶2的沉积反应器流场和温度场较为均匀,这种反应器尺寸效应引起的实际工艺参数和理论工艺参数之间的偏差,正是实验中不同长径比反应器中SiC沉积动力学规律差异的原因。      

Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases

In order to tailor the fiber–matrix interface of continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites for improved fracture toughness, alternating pyrolytic carbon/silicon carbide (PyC/SiC) multilayer coatings were applied to the KD-I SiC fibers using chemical vapor deposition (CVD) method. Three dimensional (3D) KD-I SiC_f/SiC composites reinforced by these coated fibers were fabricated using a precursor infiltration and pyrolysis (PIP) process. The interfacial characteristics were determined by the fiber push-out test and microstructural examination using scanning electron microscopy (SEM). The effect of interface coatings on composite mechanical properties was evaluated by single-edge notched beam (SENB) test and three-point bending test. The results indicate that the PyC/SiC multilayer coatings led to an optimum interfacial bonding between fibers and matrix and greatly improved the fracture toughness of the composites.     

Surface nano-structured silicon carbide thin film produced using hot filament decomposition of ethylene at low temperature on silicon wafer

The structure and spectroscopic properties of nano-structured silicon carbide (SiC) thin films were studied for films obtained through deposition of decomposed ethylene (C₂H₄) on silicon wafers via hot filament chemical vapor deposition method at low temperature followed by annealing at various temperatures in the range 300-700 °C. The prepared films were analyzed with focus on the early deposition stage and the initial growth layers. The analysis of the film's physics and structural characteristics was performed with Fourier transform infrared spectroscopy and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. The conditions for forming thin layer of cubic SiC phase (3C-SiC) are found. X-ray diffraction and Raman spectroscopy confirmed the presence of 3C-SiC phase in the sample. The formation conditions and structure of intermediate SiC layer, which reduces the crystal lattice mismatch between Si and diamond, are essential for the alignment of diamond growth. This finding provides an easy way of forming SiC intermediate layer using the Si from the substrate.     

Friction and erosive wear of controlled nucleation thermochemically deposited W-C alloys and SiC and chemically vapor-deposited Si3N4

The controlled nucleation thermochemical deposition (CNTD) process differs from the conventional chemical vapor deposition (CVD) process in that CNTD results in a fine-grained non-columnar deposit with superior mechanical properties. Materials made by this technique include CM 500 (a WC alloy) and CM 4000 (CNTD SiC). These two materials, together with CVD Si₃N₄, were evaluated for their erosion and sliding wear characteristics and the results were compared with those obtained for conventional refractory and ceramic materials. It is shown that the application of a dense CVD or CNTD coating significantly improves the erosion resistance of substrate materials.