Forced-flow Chemical Vapor Infiltration Research Articles

Modeling Forced Flow Chemical Vapor Infiltration Fabrication of SiC-SiC Composites for Advanced Nuclear Reactors

Silicon carbide fiber/silicon carbide matrix (SiC-SiC) composites exhibit remarkable material properties, including high temperature strength and stability under irradiation. These qualities have made SiC-SiC composites extremely desirable for use in advanced nuclear reactor concepts, where higher operating temperatures and longer lives require performance improvements over conventional metal alloys. However, fabrication efficiency advances need to be achieved. SiC composites are typically produced using chemical vapor infiltration (CVI), where gas phase precursors flow into the fiber preform and react to form a solid SiC matrix. Forced flow CVI utilizes a pressure gradient to more effectively transport reactants into the composite, reducing fabrication time. The fabrication parameters must be well understood to ensure that the resulting composite has a high density and good performance. To help optimize this process, a computer model was developed. This model simulates the transport of the SiC precursors, the deposition of SiC matrix on the fiber surfaces, and the effects of byproducts on the process. Critical process parameters, such as the temperature and reactant concentration, were simulated to identify infiltration conditions which maximize composite density while minimizing the fabrication time.

Advanced Reactor Concepts and Fuel Cycle Technologies

Nuclear power is an abundant source with zero CO 2 output, and expanded fission power is an attractive option especially in growing countries. Various reactor concepts are being studied to comply with requirements of future nuclear fuel cycle such as safety, sustainability, cost-effectiveness, and proliferation risk reduction. These generation IV reactor concepts include both thermal and fast reactors and are being developed in conjunction with advanced reprocessing technology. In addition, beyond-generation IV type reactors are being developed, which mitigate the concern on the nuclear waste issue by adopting very high-burnup fuel and long-life core. Consideringmovement towards future nuclear cycle, we have collected papers which cover the topical areas of new activities in advanced reactors and fuel cycle technologies and published these in this special issue on advanced reactor concepts and fuel cycle technologies. This special issue contains 10 research articles, including seven papers on advanced reactor concepts. The advanced reactor concept covers sodium-cooled fast reactor (SFR), gascooled fast reactor (GFR), supercritical water reactor (SCWR), and pressurized water reactor (PWR).The research topics include an advanced Korean SFR system design and analysis, improvement of Japanese SFR core performance by moderators in the blanket region, use of moderating materials to compensate the drawback ofminor actinide containing transmutation fuel in European fast reactors, a multiphysics modeling technique for in situ breeding and burning reactors, an ultralong fuel cycle compact GFR system design and analysis, a simulation strategy for the evaluation of neutronic properties of a Canadian SCWR fuel channel, and a thorium-plutonium fuel cycle for operating cycle extension of a PWR. The special issue also includes research results on the modeling forced-flow chemical vapor infiltration for fabrication of SiC-SiC composite materials, development of inactive demonstration facility for a proliferation-resistant pyroprocessing technology, and the economic analysis of a closed fuel cycle.

Modeling studies on the effects of the process parameters in forced-flow chemical vapor infiltration reactor for the preparation of C/C composites

The mathematical modeling for the preparation of C/C composites from propane by F-CVI (Forced-flow chemical vapor infiltration) was studied. Experimental data were fitted with the modeling calculations and an adjusted reaction rate constant was obtained. Effects of many operation parameters such as temperature, the inlet concentration of propane, flow rate, and the initial porosity were observed. The decrease of the deposition rate due to the decrease of porosity and the depletion of propane in the middle of the preform was compensated with the increase of the lateral surface area of fibers. It was confirmed that a slow reaction rate with a low temperature and a low concentration is necessary for a uniform infiltration. As the gas flow rate and the initial porosity increase, the amount of deposition increases.

Mathematical Modeling of the Actual Infiltration Process for the Preparation of C/C Composites

The mathematical modeling for the preparation of C/C composites from propane by F-CVI (Forced-flow Chemical Vapor Infiltration) was studied. The modeling for the actual processes including overturning the preform in the middle of the deposition process was carried out. Effects of the interval and the number of overturning processes on the time changes of porosity distribution were observed. The actual deposition process could be continued longer by overturning the preform. Furthermore, the total amount of deposition increased twice when several times of overturning were applied. It was confirmed that a low concentration and a slow reaction rate are necessary for a uniform infiltration even when the preform is overturned in the middle of the process.

Mechanical properties of advanced SiC/SiC composites after neutron irradiation

The effect of neutron irradiation on tensile properties in advanced 2D-SiC/SiC composites was evaluated. The composites used were composed of a SiC matrix obtained by the forced-flow chemical vapor infiltration (FCVI) process and either Tyranno™-SA Grade-3 or Hi-Nicalon™ Type-S fibers with single-layered PyC coating as the interphase. Neutron irradiation fluence and temperature were 3.1 × 10 25 n/m 2 ( E > 0.1 MeV) and 1.2 × 10 26 n/m 2 at 740–750 °C. Tensile properties were evaluated by cyclic tensile test, and hysteresis loop analysis was applied in order to evaluate interfacial properties. Both composites exhibited excellent irradiation resistance in ultimate and proportional limit tensile stresses. From the hysteresis loop analysis, the level of interfacial sliding stress decreased significantly after irradiation to 1.5 × 10 26 n/m 2 at 750 °C.

Computer Simulation of the Preparation of C/SiC Composites in the F-CVI Reactor

Computer simulation for the preparation of C/SiC composites from methyl-trichlorosilane and hydrogen by F-CVI (Forced-flow Chemical Vapor Infiltration) reactor was studied. The mathematical modeling for the actual processes was carried out. With one overturning, the process time could be continued for 50 min longer than that without overturning. Additionally, the total amount of deposition increased 1.4 times with several times of overturning. Additionally, effects of many operation parameters were observed for the process with four overturnings. Effects of the flow rate, the initial porosity, and the inlet concentration were observed.

High-temperature tensile strength of near-stoichiometric SiC/SiC composites

In an attempt to characterize mechanical properties of near-stoichiometric SiC/SiC composites, tensile tests were conducted at room temperature in air and at elevated temperature under mild oxidizing gases atmosphere. SiC/SiC composites were fabricated by forced-flow chemical vapor infiltration method using two-dimensional fabrics of carbon coated near-stoichiometric Tyranno™SA fibers. Tensile tests were conducted on composites with two types of lay-up schemes using edge-loading small specimens. The effect of lay-up orientation on the mechanical properties and fracture behavior of composites were also examined. Tensile strength of composite was slightly decreased at 1573 K, while it retained approximately 80% of the strength at room temperature. Porosity dependence on elastic modulus was clearly exhibited.

Texturing of polycrystalline SiC films on graphitic carbon in laminated composites grown by forced-flow chemical vapor infiltration

The growth and orientation of SiC on pyrocarbon using forced-flow thermal gradient chemical vapor infiltration to deposit alternate layers of these materials has been examined by transmission electron microscopy. The SiC layers were determined to be predominantly of the cubic (β) polytype, and the close-packed {111} SiC planes were found to be mostly oriented parallel and next to the highly dense {0001} C basal planes of the pyrocarbon layers at the interface. A model involving close-packed plane sequencing is proposed for the C/SiC interface.

The Stability of BN Interfacial Coatings in CFCC Systems During Oxidation and Exposure to Moisture

Abstract The mechanical reliability of ceramic matrix composites (CMCs) at elevated temperatures in oxidative environments is primarily dependent upon the chemical and structural stability of the fiber/matrix interface. Graphitic carbon coatings have traditionally been used to control the interfacial properties in CMCs, however, their use is limited in high temperature oxidative environments due to the loss of carbon and subsequent oxidation of the fiber and matrix. Thus, BN is being investigated as an alternative interfacial coating since it has comparable room temperature properties to carbon with improved oxidation resistance. The stability of BN interfaces in SiC/SiC composites is being investigated at elevated temperatures in either flowing oxygen or environments containing water vapor. The effect of several factors on BN stability, including crystallographic structure, extent of BN crystallization, and impurity content, are being evaluated. Nicalon™ fiber preforms were coated with ≈ 0.4 μm of BN by CVD using BCl3, NH3, and H2 at 1373 K. The coated preforms were densified using a forced-flow chemical vapor infiltration (FCVI) technique developed at ORNL.

Multidimensional Transport Effects on Forced-Flow Chemical Vapor Infiltration

A general multidimensional transport and reaction model that was formulated for the chemical vapor infiltration process in a previous study is used to carry out a comprehensive study of multidimensional transport effects in forced-flow chemical vapor infiltration. Results are obtained on the effects of the anisotropy and geometry of the preform, thermal gradients, and the distribution of flow at the face of the preform. For a simple, slab-shaped preform geometry, all these parameters are found to influence strongly the distance from the entrance of the preform up to which significant multidimensional transport effects are encountered in the partial pressure and density profiles. The key factor controlling the appearance and extent of multidimensional transport effects is the distribution of flow over the entrance face of the preform.

The Role of Gas‐Phase Reactions in Modeling of the Forced‐Flow Chemical Vapor Infiltration Process

An analytical model is presented, which includes the effects of both gas‐phase and surface reactions, and the pressure changes due to the chemical reactions in the forced‐flow chemical vapor infiltration (FCVI) process. For the FCVI process controlled by the gas‐phase reactions, improvements of the process by using the forced‐flow are limited. However, for the FCVI process controlled by the surface reactions, the process can be improved with the use of the forced‐flow technique. For the surface‐reaction‐controlled FCVI process, the degree of improvement increases with increasing permeability of the composite preform. In other words, advantages due to the forced‐flow are higher at the initial stage of the process when the porosity of the composite preform is high.

Contribution of Gas-Phase Reactions to the Deposition of SiC by A Forced-Flow Chemical Vapor Infiltration Process

AbstractA model, incorporating both gas-phase and surface reactions, for simulating thickness profile of SiC, deposited from trichloromethylsilane (TMS), along the longitudinal direction of a single pore is presented in this paper. The transport mechanisms considered include both forced-flow and diffusion. With the nonlinear nature of this model, a finite element model was developed to solve the problem numerically. Simulation results were in good agreement with the reported experimental data by Fedou et al. (1990). Effects of critical parameters, such as deposition temperature, ratio of sticking coefficients of TMS and intermediate species, and forced-flow, on the deposition thickness profile were investigated. Forced-flow effect was found to be small for the chemical vapor infiltration (CVI) processes at high deposition temperatures.