High‐temperature oxidation of monolithic boron nitride (BN) is examined at 900–1200°C. Hot‐pressed BN and both low‐ and high‐density chemically vapor‐deposited BN are studied. The oxidation product is B2O3(l) and the oxidation kinetics are sensitive to crystallographic orientation, porosity, and impurity levels. The B2O3 product also reacts readily with ambient water vapor in the test furnace (ppm levels) to form the volatile species HBO2(g), leading to overall paralinear kinetics. The linear rate constant extracted from these experiments agreed with that predicted from diffusion of HBO2(g) across a static boundary layer.

The eutectic architecture of a continuous reinforcing phase within a higher volume fraction phase or https://ntrs.nasa.gov/search

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The NASA Glenn Research Center is developing both a novel cell design (BSC) and a novel ceramic fabrication technique to produce fuel cells predicted to exceed a specific power density of 1.0 kW/kg. The NASA Glenn cell design has taken a completely different approach among planar designs by removing the metal interconnect and returning to the use of a thin, doped LaCrO 3 interconnect. The cell is structurally symmetrical. Both electrodes support the thin electrolyte and contain micro-channels for gas flow--a geometry referred to as a bi-electrode supported cell or BSC. The cell characteristics have been demonstrated under both SOFC and SOE conditions. Electrolysis tests verify that this cell design operates at very high electrochemical voltage efficiencies (EVE) and high H 2 O conversion percentages, even at the low flow rates predicted for closed loop systems encountered in unmanned aerial vehicle (UAV) applications. For UAVs the volume, weight and the efficiency are critical as they determine the size of the water tank, the solar panel size, and other system requirements. For UAVs, regenerative solid oxide fuel cell stacks (RSOFC) use solar panels during daylight to generate power for electrolysis and then operate in fuel cell mode during the night to power the UAV and electronics. Recent studies, performed by NASA for a more electric commercial aircraft, evaluated SOFCs for auxiliary power units (APUs). System studies were also conducted for regenerative RSOFC systems. One common requirement for aerospace SOFCs and RSOFCs, determined independently in each application study, was the need for high specific power density and volume density, on the order of 1.0 kW/kg and greater than 1.0 kW/L. Until recently the best reported performance for SOFCs was 0.2 kW/kg or less for stacks. NASA Glenn is working to prototype the light weight, low volume BSC design for such high specific power aerospace applications.

Thin film Ho-YAG and EroyAG emitters with a platinum substrate exhibit high spectral emitlance in the emission band ( _.'_0.75,4115/2 "4113/2, for Er -YAG and E_. _ 0.65, 517 -518 for Ho-YAG) at 1500K. In addition, low out-of-band spectral emittance, ¢_. < 0.2, suggest these materials would be excellent candidates for high efficiency selective emitters in thermophotovoltaic (TPV) systems operating at moderate temperatures (1200-1500K).Spectral emittance measurements of the thin films were made (1.2<_.<3.0p.m) and compared to the theoretical emittances calculated using measured values of the spectral extinction coefficient.In this paper we present the results for a new class of rare earth ion selective emitters. These emitters are thin sections ( < I mm) of yttrium aluminum garnet (YAG) single crystal with a rare earth substitutional impurity. Selective emitters in the near IR are of special interest for thermophotovoltaic (TPV) energy conversion 1,2. The most promising solid selective emitters for use in a TPV system are rare earth oxides. Early spectral emittance work 3 on rare earth oxides showed strong emission bands in the infrared (.9 -3 microns). However, the emittance outside the emission band was also significant and the efficiency of these emitters was low. Recent improvements in efficiency 4,5 have been made with emitters fabricated from fine (5-10_m) rare earth oxide fibers similar to the Welsbach mantle used in gas lanterns. However, the rare earth garnet emitters are more rugged than the mantle type emitters.A thin film selective emitter 6 on a low emissivity substrate such as gold, platinum etc., is rugged and easily adapted to a wide variety of thermal sources. The garnet structure and its many subgroups have been successfully used as hosts for rare earth ions, introduced as substitutional impurities, in the development of solid state laser crystals 7. Doping, dependent on the particular ion and crystal structure, may be as high as 100 at. % (complete substitution of yttrium ion with the rare earth ion). These materials have high melting points, 1940 C for YAG (Yttrium Aluminum Gamet), and low emissivity in the near infrared making them excellent candidates for a thin film selective emitter.As previously stated, the spectral emittance of a rare earth emitter is characterized by one or more well defined emission bands. Outside the emission band the emittance(absorptance) is much lower. Therefore, it is expected that emission outside the band for a thin film selective emitter will be dominated by the emitter substrate. For an efficient emitter (power in the emission band/total emitted power) the substrate must have low emittance, _s" Within the emission band the spectral emittance is governed by the index of refraction, nf, and the spectral extinction coefficient, _. (sum of the absorption coefficient, aX, and scattering coefficient, c_X),the emitter temperature, T E, and thickness, d. In reference 6 the emitter emittance and efficiency are derived as a function of the optical depth, K_,= _d...

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In-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al2O3; 39.2 m/o ZrO2 (9.5 m/o Y2O3)) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al2O3.

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