Papers by Keyword: CVI

Abstract: SiC_f/SiC composites are one of the candidates for high temperature structural applications because of their high strength and corrosion resistance under severe conditions and stability under neutron irradiation [1~3]. A silicon carbide fuel cladding for the light water cooled reactors (LWRs) may allow a number of advances, including: the increased safety margins under transients and accident scenarios, such as loss of coolant accident; the improved resource utilization via a higher burn-up beyond the present limit of 62 GWd/MTU; and improved waste management [3~5]. Some components of SiC_f/SiC composite will be applied as tubular geometry for the high-temperature core parts. The proposed design of an advanced LWR fuel cladding, referred to as Triplex, consists of three layers: an inner SiC monolith, a central SiC_f/SiC composite, and an outer dense SiC evrionmental barrier coating. The inner SiC layer provides the strength and hermeticity to contain fission products. The SiC_f/SiC composite layer fabricated by the CVI process provides a pseudo-ductile failure mode. The outer SiC thin coating layer protects against corrosion [5]. The chemical vapor deposition (CVD) technique is an effective approach for the fabrication of SiC_f/SiC composite and coated SiC monolith [6]. To increase the homogeneity of the microstructure and the deposition rate of a SiC tube, the process parameters should be optimized and modified.

Abstract: C/C composites are being used as high-temperature structural material due to the superior characteristics of low density, high specific strength, high thermal conductivity, low thermal expansion coefficient and excellent anti-ablation. The state of research into the manufacturing processes of Carbon-Carbon composites is critically reviewed with emphasis in this paper. The processes include several kinds of methods, for example, CVD, CVI, Liquid Impregnation and etc. And the usages of some corresponding methods in manufacturing Carbon-Carbon materials as ablation resistant materials are shown in this paper too. It will provide the references for the further development of weaponry materials in the coming years.

Abstract: A reduced model was used to simulate the CVI process from methane. The scale of the preform was 120mm in diameter and 90mm in height. A random pore model was used to describe the evolution of pores in preform. Parameters such as temperatures and pressures were studied to research the deposition process. Effects of these two parameters are similar but have different impact mechanisms according to the analysis of results. Temperature impacts the reaction rate to improve the efficiency of deposition, while pressure impacts the concentrations of all hydrocarbon and then improves the total amount of carbon which also results in an increase of density.

Abstract: Carbon fiber reinforced silicon carbide (C/SiC) composites are considered as one of the most potential thermal structure materials. However, the non-machinability of the three dimension woven fabric restrict the wide application of the c/sic composites. In this paper, we discuss the effect of machinability on the properties of 3D-c/sic composites, such as the modulus, mechanical properties, and so on. The results show that c/sic composites exhibit excellent mechanical properties after machinability, an extensive microstructure study is also carried out to understand the properties of the composites.

Abstract: To obtain high performance ceramic matrix composites (CMCs), fiber coatings are often fabricated as the interphase between fiber and matrix. The SiC coating was synthesized at low temperature and reduced pressure in the present experiment. SiC was derived from a gaseous methyltrichlorosilane (MTS)/H2 precursor by chemical vapor infiltration (CVI). The thickness of the coating was inspected by SEM. The correlation between the coating thickness and the depositing conditions, i.e. the deposition temperature, the pressure, the deposition time per pulse and the pulse number were investigated. Based on these work, the C/SiC double-layer coating was fabricated.

Abstract: Titanium diboride (TiB2) appears to be an attractive candidate for a high-temperature fiber reinforced composites. Chemical Vapor Infiltration (CVI) has been carried out for the preparation of TiB2/C composites. Titanium diboride has been deposited from the gas mixtures of TiCl4, BCl3, and H2 in the furnace at the reaction temperatures between 850 to 950 °C and about 20 torr. Effects of infiltration parameters such as temperature, reaction time, and concentrations of BCl3 and H2 have been studied. Analyses with SEM, TGA and XPS were carried out. The amount of deposition in the preform increased with the increases of the reaction time, temperature and the flow rate of BCl3. The activation energy of the whole deposition reaction could be estimated from the Arrhenius plot.

Abstract: Various technology programmes in Europe are concerned with preparing for future propulsion technologies to reduce the costs and increase the life time of components for liquid rocket engine components. One of the key roles to fulfil the future requirements and for realizing reusable and robust engine components is the use of modern and innovative materials. One of the key technologies which concern various engine manufacturers worldwide is the development of fibrereinforced ceramics – CMC's (Ceramic Matrix Composites). The advantages for the developers are obvious – the low specific weight, the high specific strength over a large temperature range, and their good damage tolerance compared to monolithic ceramics make this material class extremely interesting as a construction material. Different kind of composite materials are available and produced by EADS ST, the standard material SICARBON® (C/SiC made by Liquid Polymer Infiltration) and the new developed and qualified composite materials SICTEX® (C/SiC made by Liquid Silicon Infiltration) and CARBOTEX® (C/C made by Rapid Chemical Vapour Infiltration). The composites are based on textile techniques like weaving, braiding, stiching and sewing to produce multiaxial preforms, the SICTEX® material is densificated by the cost effective Liquid Silicon Infiltration (LSI). Over the past years, EADS Space Transportation (formerly DASA) has, together with various partners, worked intensively on developing components for airbreathing and liquid rocket engines. Since this, various prototype developments and hot firing-tests with nozzle extensions for upper and core stage engines and combustion chambers of satellite engines were conducted. MBDA France and EADS-ST have been working on the development of fuel-cooled composite structures like combustion chambers and nozzle extensions for future propulsion applications.

Abstract: Isothermal, isobaric chemical vapor infiltration of carbon fiber felts with fiber volume fractions of 7.1% and 14.2% were investigated at infiltration times from 20 to 120 hours, using a constant temperature of 1095 oC and a methane pressure of 22.5 kPa. Bulk densities and the density profiles as well as porosity at various densification stages were determined. Inside–outside densification was obtained in the most infiltrations, the gradients of densification along the infiltration depth decrease with increasing of residence time and infiltration times. Outside–inside densification occurs only in the felt with the lower fiber volume fraction at final infiltration stage and at longer residence times. Microstructure of the obtained matrix carbon was analyzed with a polarized light microscopy. Abruptly change from low/medium textured carbon to medium/high textured carbon are observed in both of the carbon fiber felts, whereas the thickness of the first lower textured layer is about 14 micros in the felt with a fiber volume fraction of 7.1%, whereas it is only 2 micros in the felt with a fiber volume fraction of 14.2%, which is caused by an increasing of initial surface area / volume ratio, [A/V], from 33 to 71 mm-1. Results are completely in agreement with the previous simulations studies on the influence of [A/V] ratios.

Abstract: Ceramic matrix composites (CMCs) comprise a fiber reinforcement embedded in a ceramic matrix, the two main constituents being bonded through an interphase, which is a thin layer of a compliant material with a low shear stress, arresting and deflecting the matrix microcracks formed under load. Non-oxide CMCs, such as C/C ; C/SiC or SiC/SiC, are fabricated from a suitable precursor of the matrix, following a gaseous (CVI-process), a liquid (PIP and RMI processes) or a slurry (SI-HPS) routes. Each of these routes is briefly depicted focusing on fundamental aspects and its advantages and drawbacks discussed. Possible extensions of the processes to new composites are suggested. Finally, a comparison of these techniques, in terms of processability and composites properties is presented.

Abstract: The densification of carbon fiber substrates by chemical vapor deposition (CVD) is a process quite used since the fifties, for manufacturing carbon reinforced with carbon fiber composites (CRCF). The process is based in thermal decomposition of a gas which contains carbon in its molecules and the resulting pyrolytic carbon is continually deposited onto de carbon fiber substrate. For this experiment the substrate material was made of carbon fiber felt. The deposition was performed by isothermal process at atmospheric pressure and at temperature of 1050 oC. Methane (CH4) gas was used as carbon bearer and nitrogen (N2) as the carrier gas. Different volumetric ratio of N2 and CH4 (N2:CH4) were used in order to get an optimum densification of the substrate. Optical and electron scanning microscopy and density measurements were used to characterize the impregnated material. An increase in ratio of N2 and CH4 increase the overall deposition for the same time interval.