Forced Chemical Vapor Infiltration of Tubular Geometries: Modeling, Design, and Scale-Up

In advanced indirectly fired coal combustion systems and externally fired combined cycle concepts, ceramic heat exchangers are required to transfer heat from the hot combustion gases to the clean air that drives the gas turbines. For high efficiencies, the temperature of the turbine inlet needs to exceed 1,100 C and preferably be about 1,260 C. The heat exchangers will operate under pressure and experience thermal and mechanical stresses during heating and cooling, and some transients will be severe under upset conditions. Silicon carbide-matrix composites appear promising for such applications because of their high strength at elevated temperature, light weight, thermal and mechanical shock resistance, damage tolerance, and oxidation and corrosion resistance. The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter

Thermochemical modeling of nuclear waste glass

The development of assessed and consistent phase equilibria and thermodynamic data for major glass constituents used to incorporate high-level nuclear waste is discussed in this paper. The initial research has included the binary Na{sub 2}O-SiO{sub 2}, Na{sub 2}O-Al{sub 2}O{sub 3}, and SiO{sub 2}-Al{sub 2}O{sub 3} systems. The nuclear waste glass is assumed to be a supercooled liquid containing the constituents in the glass at temperatures of interest for nuclear waste storage. Thermodynamic data for the liquid solutions were derived from mathematical comparisons of phase diagram information and the thermodynamic data available for crystalline solid phases. An associate model is used to describe the liquid solution phases. Utilizing phase diagram information provides very stringent limits on the relative thermodynamic stabilities of all phases which exist in a given system

An approach to thermochemical modeling of nuclear waste glass

This initial work is aimed at developing a basic understanding of the phase equilibria and solid solution behavior of the constituents of waste glass. Current, experimentally determined values are less than desirable since they depend on measurement of the leach rate under non-realistic conditions designed to accelerate processes that occur on a geologic time scale. The often-used assumption that the activity of a species is either unity or equal to the overall concentration of the metal can also yield misleading results. The associate species model, a recent development in thermochemical modeling, will be applied to these systems to more accurately predict chemical activities in such complex systems as waste glasses

Fiber-matrix interfaces in ceramic composites

The mechanical properties of ceramic matrix composites (CMCs) are governed by the relationships between the matrix, the interface material, and the fibers. In non-oxide matrix systems compliant pyrolytic carbon and BN have been demonstrated to be effective interface materials, allowing for absorption of mismatch stresses between fiber and matrix and offering a poorly bonded interface for crack deflection. The resulting materials have demonstrated remarkable strain/damage tolerance together with high strength. Carbon or BN, however, suffer from oxidative loss in many service environments, and thus there is a major search for oxidation resistant alternatives. This paper reviews the issues related to developing a stable and effective interface material for non-oxide matrix CMCs

Uranium nitride-silicide advanced nuclear fuel: Higher efficiency and greater safety

The development of new nuclear fuel compositions is being driven by an interest in improving efficiency/lowering cost and increasing safety margins. Nuclear fuel efficiency is in large measure a function of the atomic density of the uranium, that is, the more fissionable uranium available per unit volume the less fuel volume that is required. Proliferation concerns limit the concentration of fissile 235U, and thus attention is directed to higher overall uranium content fuel. Among the options are the high temperature phases U3Si2 and composite UN- U3Si2 where the design would have the more water-stable U3Si2 surround the more soluble, but higher uranium density UN grains. (Uranium metal of course has the highest atomic density, however its low melting point, high degree of swelling under irradiation, and chemical reactivity eliminate it from consideration.) Another advantage of the nitride and silicide phases are their high thermal conductivity, greatly exceeding the current standard UO2 fuel, with the high conductivity potentially allowing the fuel to operate at a higher power density. Please click Additional Files below to see the full abstract

Oxidation-resistant interfacial coatings for continuous fiber ceramic composites

Developing an oxidation-resistant interfacial coating for continuous fiber ceramic composites (CFCCs) continues to be a major challenge. CFCCs` mechanical behavior are influenced by the interfacial bonding characteristics between the fiber and the matrix. Finite element modeling studies suggest that a low-modulus interfacial coating material will be effective in reducing the residual thermal stresses that are generated upon cooling from processing temperatures. Nicalon/SiC composites with carbon, alumina and mullite interfacial coatings were fabricated with the SiC matrix deposited using a forced-flow chemical vapor infiltration process. Composites with mullite interfacial coatings exhibited considerable fiber pull-out even after oxidation and have potential as a composite system

Assessing corrosion in oil refining and petrochemical processing

This paper summarizes the development of an information system used to manage corrosion of metals and alloys by high temperature gases found in many different oil refining, petrochemical, power generation, and chemical processes. The database currently represents about 7.9 million h of exposure time for about 5,500 tests with 89 commercial alloys for a temperature range of 200 – 1,200 °C. The system manages corrosion data from well-defined exposures and determines corrosion product stabilities. New models used in the analysis of thermochemical data for the Fe-Ni-CrCo-C-O-S-N-H system are being compiled. All known phases based upon combinations of the elements have been analyzed to allow complete assessments of corrosion product stabilities. Use of these data allows prediction of stable corrosion products and hence identification of the possible dominant corrosion mechanisms. The system has the potential to be used in corrosion research, alloy development, failure analysis, lifetime prediction, and process operations evaluations. The corrosion mechanisms emphasized are oxidation, sulfidation, sulfidation/oxidation, and carburization

Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb\u3csub\u3e4\u3c/sub\u3eGe\u3csub\u3e5\u3c/sub\u3eO\u3csub\u3e9\u3c/sub\u3eF\u3csub\u3e6\u3c/sub\u3e:Nb

A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthe-sized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystal-lizes in the orthorhombic space groupPbcnwith lattice parametersa= 6.98430(10)Å,b= 11.7265(2) Å,andc= 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9units connectedby GeO3F3octahedra. In its pure form, Rb4Ge5O9F6shows neither luminescence nor scintillation butwhen doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. Theisostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space groupPbcnwith lattice parametersa= 6.9960(3) Å,b= 11.7464(6) Å, andc= 19.3341(9) Å. X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggestthat the niobium is located in an octahedral coordination environment. Optical measurements informus that the niobium dopant acts as the activator. The synthesis, structure, and optical properties arereported, including radioluminescence (RL) measurements under X-ray irradiation

Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb\u3csub\u3e4\u3c/sub\u3eGe\u3csub\u3e5\u3c/sub\u3eO\u3csub\u3e9\u3c/sub\u3eF\u3csub\u3e6\u3c/sub\u3e:Nb

A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation

Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb\u3csub\u3e4\u3c/sub\u3eGe\u3csub\u3e5\u3c/sub\u3eO\u3csub\u3e9\u3c/sub\u3eF\u3csub\u3e6\u3c/sub\u3e:Nb

A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation