5 Matching Results

Search Results

Advanced search parameters have been applied.

Task 6.7.3 - Interfacial Mass Transport Effects in Composite Materials

Description: A series of TiNX and Ti(N,O)X films formed from a TiC, N2, H2, and (N2 + 02 ) gas mixture was deposited on aluminum ceramics at 1073 K for 5 hr at atmospheric pressure, according to the procedure described in Dekker et al. These conditions were selected because they offer a reasonable deposition rate and low chlorine content in films. The TiNX films, with two different nonstoichiometries controlled by the ratio of to H2 and two Ti(N,O)X films with different nitrogen and oxygen content were prepared. The N2 and H2 gases were first purified in alkaline pyrogallol and dried over silica gel and further over a copper and palladium catalyst. TiCld concentration in flowing N2 and H2 (70/30) corresponded to its vapor pressure over liquid at 300 K. Film thickness was determined using a multiple interferometer. All of the films obtained were thicker than 1000 nm. Phase analysis and interatomic distances were determined by x-ray diffraction analysis (XRD) at room temperature.
Date: August 1, 1997
Creator: Nowok, Jan W.
Partner: UNT Libraries Government Documents Department

Task 6.7.3 - Interfacial Mass Transport Effects in Composite Materials

Description: Advanced metal-matrix composites (MMCS) consisting of titanium-based alloys possess some unique mechanical, physical, and chemical characteristics that make them highly desirable for aircraft and gas turbine engines. Tailoring MMC properties is essential for advanced product design in materials processing. The main factors that affect materials processing and, further, the nature of a metal-ceramic interface, its structure, and morphological stability is liquid surface mass transport related to adhesional wetting (physical effect) and reactive wetting (chemical effect).' Surfaces and interfaces dominate many of the technologically important processes in composite materials such as liquid-solid sintering and joining. The objective of this work is threefold: 1) to get insight into the role of the nonstoichiometry of chemical composition in ceramic materials used as reinforcement components in MMC processing, 2) to extend previous energetic analysis of mass transport phenomena to wetting behavior between liquid metal and the quasi-solidlike skin resulting from the presolidification of liquid on nonstoichiometric solids on a scale of interatomic distance, and 3) to provide experimental verification of our concept.
Date: February 1, 1998
Creator: Nowok, Jan W.
Partner: UNT Libraries Government Documents Department

Task 6.7.3 - Interfacial Mass Transport Effects in Composite Materials

Description: Advanced metal-matrix composites (MMCS) consisting of titanium-based alloys possess some unique mechanical, physical, and chemical characteristics that make them highly desirable for aircraft and gas turbine engines. Tailoring MMC properties is essential for advanced product design in materials processing. The main factors that affect materials processing and, further, the nature of a metal-ceramic interface, its structure, and morphological stability is liquid surface mass transport related to adhesional wetting physical effect) and reactive wetting (chemical effect). Surfaces and interfaces dominate many of the technologically important processes in composite materials such as liquid-solid sintering and joining. The objective of this work is threefold: 1) to get insight into the role of the nonstoichiometry of chemical composition in ceramic materials used as reinforcement components in MMC processing, 2) to extend previous energetic analysis of mass transport phenomena to wetting behavior between liquid metal and the quasi-solid like skin resulting from the presolidification of liquid on nonstoichiometric solids on a scale of interatomic distance, and 3) to provide experimental verification of our concept.
Date: February 1, 1998
Creator: Nowok, Jan W.
Partner: UNT Libraries Government Documents Department

DEVELOPMENT OF AN ADHESIVE CANDLE FILTER SAFEGUARD DEVICE

Description: In order to reach the highest possible efficiencies in a coal-fired turbine-based power system, the turbine should be directly fired with the products of coal conversion. Two main types of systems employ these turbines: those based on pressurized fluidized-bed combustors and those based on integrated gasification combined cycles. In both systems, suspended particulates must be cleaned from the gas stream before it enters the turbine so as to prevent fouling and erosion of the turbine blades. To produce the cleanest gas, barrier filters are being developed and are in use in several facilities. Barrier filters are composed of porous, high-temperature materials that allow the hot gas to pass but collect the particulates on the surface. The three main configurations of the barrier filters are candle, cross-flow, and tube filters. Both candle and tube filters have been tested extensively. They are composed of coarsely porous ceramic that serves as a structural support, overlain with a thin, microporous ceramic layer on the dirty gas side that serves as the primary filter surface. They are highly efficient at removing particulate matter from the gas stream and, because of their ceramic construction, are resistant to gas and ash corrosion. However, ceramics are brittle and individual elements can fail, allowing particulates to pass through the hole left by the filter element and erode the turbine. Preventing all failure of individual ceramic filter elements is not possible at the present state of development of the technology. Therefore, safeguard devices (SGDs) must be employed to prevent the particulates streaming through occasional broken filters from reaching the turbine. However, the SGD must allow for the free passage of gas when it is not activated. Upon breaking of a filter, the SGD must either mechanically close or quickly plug with filter dust to prevent additional dust from reaching the ...
Date: January 1, 2002
Creator: Hurley, John P.; Henderson, Ann K.; Nowok, Jan W. & Swanson, Michael L.
Partner: UNT Libraries Government Documents Department

SiAlON COATINGS OF SILICON NITRIDE AND SILICON CARBIDE

Description: The need for new engineering materials in aerospace applications and in stationary power turbine blades for high-efficiency energy-generating equipment has led to a rapid development of ceramic coatings. They can be tailored to have superior physical (high specific strength and stiffness, enhanced high-temperature performance) and chemical (high-temperature corrosion resistance in more aggressive fuel environments) properties than those of monolithic ceramic materials. Among the major chemical properties of SiAlON-Y ceramics are their good corrosion resistance against aggressive media combined with good thermal shock behavior. The good corrosion resistance results from the yttria-alumina-garnet (YAG), Al{sub 5}Y{sub 3}O{sub 12}, formed during the corrosion process of SiAlON-Y ceramics in combustion gases at 1300 C. The interfacial chemical precipitation of the YAG phase is beneficial. This phase may crystallize in cubic and/or tetragonal modifications and if formed in SiAlON-Y ceramic may simultaneously generate residual stress. Also, this phase can contain a large number of point defects, which is a consequence of the large unit cell and complexity of the YAG structure because it has no close-packed oxygen planes. Therefore, the need exists to elucidate the corrosion mechanism of a multilayered barrier with respect to using SiAlON-YAG as a corrosion-protective coating. Stress corrosion cracking in the grain boundary of a silicon nitride (Si{sub 3}N{sub 4}) ceramic enriched in a glassy phase such as SiAlON can significantly affect its mechanical properties. It has been suggested that the increased resistance of the oxynitride glass to stress corrosion is related to the increased surface potential of the fracture surface created in the more durable and highly cross-linked oxynitride glass network structure. We expect that either increased or decreased surface potential of the intergranular glassy phase is brought about by changes in the residual stress of the SiAlON-Y ceramic and/or creation of a space-charge region at the SiAlON-YAG interface. Both ...
Date: June 1, 2000
Creator: Nowok, Jan W.; Hurley, John P. & Kay, John P.
Partner: UNT Libraries Government Documents Department