CVI Method Research Articles

Yttria-stabilized zirconia thin films deposited on NiO–(Sm 2O 3) 0.1(CeO 2) 0.8 substrates by chemical vapor infiltration

Fabrication of YSZ films deposited on NiO–samaria-doped ceria (SDC) substrate was studied by the chemical vapor infiltration method (CVI). A NiO–SDC substrate was used as oxygen source. The main mechanism of YSZ growth was electrochemical vapor deposition (EVD), while the contribution of oxygen in the carrier gas increased with increasing NiO content of the substrate above 60.6 mol%. The YSZ film on SDC used as the anode proved effective in obtaining high cell performance. In particular, a YSZ film thickness of 1 μm yielded the highest cell performance in the temperature range from 973 to 1073 K. The CVI method was useful for preparing a dense and strong YSZ film on the complex-shaped NiO–SDC substrate.

Fabrication of YSZ Thin Film on (Sm2O3)0.1(CeO2)0.9 Substrate by CVI Method and its Electrical Properties

not Available.

Evaluation of Fiber/Matrix Interfacial Strength of Neutron Irradiated SiC/SiC Composites Using Hysteresis Loop Analysis of Tensile Test

Advanced SiC/SiC composites composed of highly-crystalline and near-stoichiometric SiC fibers, SiC matrix by CVI method, and PyC interphase were irradiated up to 1:0 � 10 25 n/m 2 (E > 0:1 MeV, � 1 dpa) at 1273 K. Unload/reload cyclic tensile test and hysteresis loop analysis were carried out in order to identify neutron irradiation effects on interfacial properties of advanced SiC/SiC. The composites exhibited good irradiation resistance in ultimate tensile strength (� 10% increase), but there were slight reduction of elastic modulus and proportional limit stress (PLS) (� 10%). Wider hysteresis loops and lower gradient of the curves beyond PLS from the strain–stress curves, and longer pullouts of fiber from the SEM observation were observed, which imply weaker F/M interactions after neutron irradiation. From the hysteresis loop analysis, degradation of interfacial sliding stress and negative increment of misfit stress after neutron irradiation were obtained.

Effect of a SiC whisker formation on the densification of Tyranno SA/SiC composites fabricated by the CVI process

SiC whiskers were introduced into the SiC fiber preform by the CVI method with and without an Fe catalyst. The whisker growing process resulted in a composite with a higher density than the conventional-CVI composite when assuming similar infiltration times. Multiple cycles of the whisker growing and the matrix infiltration steps were required to effectively reduce the size of the interbundle voids, due to the limited length of the whiskers grown without the catalyst. Although the adoption of the metallic catalyst increased the length and the homogeneity of the whiskers, the early clogging of the composite surface restricted the effective infiltration of the center region of the composite. A gradient of the whisker concentration in the thickness direction was suggested for a uniform densification.

Development of Evaluation and Application Techniques of SiC/SiC Composites for Fusion Reactors

Evaluation techniques and mechanical properties of silicon carbide composites (SiC⁄SiC composites) reinforced with highly crystalline fibers are reviewed for fusion applications. The SiC⁄SiC composites used were fabricated by means of the CVI method. The evaluation includes in-plane tensile strength by in-plane tensile test, transthickness tensile strength by transthickness tensile test and diametral compression test and shear strength by compression test using double-notched specimen. All tests were successfully conducted using small specimens for neutron irradiation experiment. As application technique, the novel tungsten(W) coating technique on SiC is reviewed. The W powder melted by high power lamp in a few seconds and formed coating on SiC. No thick reaction layers of WC and W5Si3, which are formed by the other coating methods, were formed by this method.

Temperature dependence of internal friction and elastic modulus of SiC/SiC composites

Although SiC/SiC composites are expected to be used as advanced composite materials due to their superiorities in strength-to-weight and stiffness-to-weight ratios, there have been few reports on the internal friction of SiC-based composite materials. In this study, PIP, HP and CVI methods were employed as fabrication processes of SiC-based composites. In the case of the composite produced by the CVI process, by controlling the conditions of manufacturing and thermal annealing, a stable peak of internal friction due to the grain boundary relaxation was obtained at about 1200 K. It was found that this peak was affected not by the property of the fiber but the microstructure of the matrix and the temperature of the peak moved to the lower side after heat treatment at 1773 K. Also, it is expected that SiC/SiC composites may have a greater high-damping property by crystallizing the matrix.

Fabrication of carbon/carbon composites by an electrified preform heating CVI method

Carbon/carbon composites are manufactured using the electrified preform producing directly heat CVI process. The preforms are prepared by laminating the carbon fiber felts with crossply reinforcement, and infiltrated with carbon using natural gas or propylene as a reactant, with nitrogen as diluent at atmospheric pressure. The relations between the resistivity of samples and infiltration time are determined under the operating conditions. The results indicate that the preforms have gained a high infiltration rate by this technology, and the samples have higher densities using natural gas rather than propylene. Their highest average bulk densities are up to 1.71 g/cm 3 after the preforms of 1100×500×35 mm size have been densified for 80 h using natural gas. The carbon fibres in the preforms have not been damaged by this technology as yet, and the composites prepared have sufficiently high flexural properties. As the brake angular velocity is increased with the constant brake moment inertia and specific pressure, the average coefficient of friction for the composites prepared using natural gas is linearly and greatly decreased, but the variations of the brake moment inertia have a slight influence on the average coefficient of the friction when the brake angular velocity and specific pressure are kept constant. Their average thickness wear is 13×10 −4 mm/surface per stop.

Recent progress of SiC-fibers and SiC/SiC-composites for fusion applications

Recent progress in R&D of SiC fibers and reinforced SiC matrix (SiC/SiC) composites in Japan, especially focusing on the activities of CREST-ACE program, is presented. Firstly, the present status of high performance SiC fiber development, such as Hi-Nicalon Type-S and Tyrano-SA, is provided. The high performance SiC matrix production by reaction sintering (RS) method improved in both strength and thermal conductivity are accomplished. The efforts to make appropriate fiber-matrix interfacial microstructure by CVI and PIP methods have been successful, resulting in the production of high strength and high fracture toughness SiC/SiC composites. Several joining processes using PIP, RS and mechanical fastener for composites are introduced. Dimensional stability under radiation damage has been studied by neutron and charged particle irradiation. The SiC/SiC composites prepared with Type-S SiC fiber with a stoichiometric composition did not exhibit mechanical property degradation. Based on the development of SiC composites, test module concepts to verify the advanced fluid systems including SiC/SiC/Be/He coolant blanket are presented.

Room- and Elevated-Temperature Mechanical Properties of SiC Fiber-Reinforced SiC Composite Fabricated by CVI and PIP Methods.

Room- and Elevated-Temperature Mechanical Properties of SiC Fiber-Reinforced SiC Composite Fabricated by CVI and PIP Methods.

Enhanced Thermal Conductivity of CVD SiC via Beryllium and Boron Dopings

ABSTRACTSiC-SiC composites exhibit excellent mechanical and corrosion properties, which are desired for a variety of advanced applications including fusion reactors, heat exchangers and advanced turbine engines. However, SiC-SiC composites fabricated by the CVI methods have unacceptably low through-the-thickness thermal conductivity, which limits their use in these applications. CVD SiC co-deposition with either beryllium or boron compounds significantly enhanced the thermal conductivity of CVD SiC at temperatures up to 1000°C. Results pertinent to the doping processes and the measured thermal conductivity of doped CVD SiC are reported. Mechanisms for the enhanced thermal conductivity of the CVD SiC are discussed based on the microstructural characteristics of the doped specimens.

On the development of low-activation materials for fusion reactors

The induced radioactivity of first wall materials was evaluated by the IRAC code. Co, Ni, Cu, Nb, and Mo are strictly limited in concentration. The long-lived nuclide, 26A1, from Al can not be ignored for a long operation of ≧ 10 MW y m −2. The low activation materials are selected by the evaluation and materials properties of candidate alloys designed from the viewpoint of low activity were studied. The phase stability and its effect on toughness of Fe-Cr-W steels were investigated. δ-ferrite was shown to be eliminated for improving toughness. The concentration of W should be below 3% in the 9Cr-0.1C and 1.4% in the 12Cr-0.1C steels. The lowest activity material, C/SiC composite, was produced by the CVI method. CVI was shown as one of the methods to improve the toughness of ceramics composites.

Low activation first wall materials for fusion reactors

The induced activity of first wall materials was calculated and evaluated from the viewpoints of surface dose rate and waste disposal. Several properties of selected candidate materials, such as FeCrWV alloys, W single crystals, and C/SiC components, were examined. The optimum composition of ferritic steel is 9Cr-1W with respect to phase stability and toughness. Tungsten single crystals prepared with a secondary recrystallization showed a sufficient bend ductility at room temperature. It was also confirmed that a CVI method could be used to produce C/SiC composites with an improved ductility.