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

COIVF-9 %Ob 6Y-- N EAR-N ET-SHAPE FABRICATION BY FORCED-FLOW, THERMAL-G RAD1 ENT CVI*

Forced-flow, thermal gradient chemical vapor infiltration (FCVI) has been developed for the rapid densification of ceramic matrix composites. For preforms of >3 mm thickness FCVl can produce a near-net-shape part in less than one day as opposed to isothermal, isobaric CVI which requires several weeks to densify such a component. Efforts at ORNL and elsewhere have resulted in capability to produce prototypical thick-walled heat exchanger tubes and turbine disk blanks. This paper will review recent modeling and experimental efforts related to the FCVl of cylindrical forms

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

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 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

A data‐driven approach for predicting nepheline crystallization in high‐level waste glasses

High-level waste (HLW) glasses with high alumina content are prone to nepheline crystallization during the slow canister cooling that is experienced during large-scale production. Because of its detrimental effects on glass durability, nepheline (NaAlSiO4) precipitation must be avoided; however, developing robust, predictive models for nepheline crystallization behavior in compositionally complex HLW glasses is difficult. Using overly conservative constraints to predict nepheline formation can limit the waste loading to lower than the achievable capacity. In this study, a robust data-driven model using five compositional features has been developed to predict nepheline formation. A new descriptor is introduced called the difference based on correlation , which has higher accuracy compared to previous descriptors and also has more balanced false positive and false negative rates. The analysis of the model and the data show an overlap, instead of a distinct compositional boundary, between glasses that form and do not form nepheline. As a result, the model\u27s predictive accuracy is not the same throughout the feature space and instead is dependent on the location of the glass composition in the dimensionally reduced feature space