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At least 397 records · Page 22

Micromachined peristaltic pump

A micromachined pump including a channel formed in a semiconductor substrate by conventional processes such as chemical etching. A number of insulating barriers are established in the substrate parallel to one another and transverse to the channel. The barriers separate a series of electrically conductive strips. An overlying flexible conductive membrane is applied over the channel and conductive strips with an insulating layer separating the conductive strips from the conductive membrane. Application of a sequential voltage to the series of strips pulls the membrane into the channel portion of each successive strip to achieve a pumping action. A particularly desirable arrangement employs a micromachined push-pull dual channel cavity employing two substrates with a single membrane sandwiched between them.

Hartley, Frank T.↗

Foreign Object Damage in a Yb2Si2O7 Environmental Barrier Coating

Foreign object damage (FOD) is one of the key damage/failure modes in environmental barrier coatings (EBCs) developed for gas turbine engines. A limiting factor of EBC development is the growth of a SiO2 thermally grown oxide (TGO) layer which exacerbates spallation. While there has been extensive research on FOD in metals, ceramics, and ceramic matrix composites (CMCs), there is a limited understanding of the effects of FOD in EBC systems. Previous work studied the effects of TGO growth on FOD in NASA’s Generation II Ytterbium Disilicate EBC. Recently, it has been shown that the addition of oxide modifiers to the Ytterbium Disilicate EBC resulted in significant reduction in TGO growth. The present work will compare FOD in a modified Ytterbium Disilicate EBC to that of the baseline Ytterbium Disilicate EBC. FOD testing was conducted on both as-processed and steam cycled samples at room temperature using a 1.59mm steel ball projectile with particle velocities ranging from 50–300 m/s at a normal incidence angle. The impact damage was characterized by optical profilometry and scanning electron microscopy (SEM) of the cross-sections.

Leland Hoffman↗

Feasibility Assessment of Thermal Barrier Seals for Extreme Transient Temperatures

The assembly joints of modem solid rocket motor cases are generally sealed using conventional O-ring type seals. The 5500+ F combustion gases produced by rocket motors are kept a safe distance away from the seals by thick layers of phenolic insulation. Special compounds are used to fill insulation gaps leading up to the seals to prevent a direct flowpath to them. Design criteria require that the seals should not experience torching or charring during operation, or their sealing ability would be compromised. On limited occasions, NASA has observed charring of the primary O-rings of the Space Shuttle solid rocket nozzle assembly joints due to parasitic leakage paths opening up in the gap-fill compounds during rocket operation. NASA is investigating different approaches for preventing torching or charring of the primary O-rings. One approach is to implement a braided rope seal upstream of the primary O-ring to serve as a thermal barrier that prevents the hot gases from impinging on the O-ring seals. This paper presents flow, resiliency, and thermal resistance for several types of NASA rope seals braided out of carbon fibers. Burn tests were performed to determine the time to burn through each of the seals when exposed to the flame of an oxyacetylene torch (5500 F), representative of the 5500 F solid rocket motor combustion temperatures. Rope seals braided out of carbon fibers endured the flame for over six minutes, three times longer than solid rocket motor burn time. Room and high temperature flow tests are presented for the carbon seals for different amounts of linear compression. Room temperature compression tests were performed to assess seal resiliency and unit preloads as a function of compression. The thermal barrier seal was tested in a subscale "char" motor test in which the seal sealed an intentional defect in the gap insulation. Temperature measurements indicated that the seal blocked 2500 F combustion gases on the upstream side with very little temperature rise on the downstream side.

Steinetz, Bruce M.↗

High Temperature Materials for Chemical Propulsion Applications

Radiation or passively cooled thrust chambers are used for a variety of chemical propulsion functions including apogee insertion, reaction control for launch vehicles, and primary propulsion for planetary spacecraft. The performance of these thrust chambers is limited by the operating temperature of available materials. Improved oxidation resistance and increased operating temperatures can be achieved with the use of thermal barrier coatings such as zirconium oxide (ZrO2) and hafnium oxide (HfO2). However, previous attempts to include these materials showed cracking and spalling of the oxide layer due to poor bonding. Current research at NASA's Marshall Space Flight Center (MSFC) has generated unique, high temperature material options for in-space thruster designs that are capable of up to 2500 C operating temperatures. The research is focused on fabrication technologies to form low cost Iridium,qF_.henium (Ir/Re) components with a ceramic hot wall created as an integral, functionally graded material (FGM). The goal of this effort is to further de?celop proven technologies for embedding a protective ceramic coating within the Ir/Re liner to form a robust functional gradient material. Current work includes the fabrication and testing of subscale samples to evaluate tensile, creep, thermal cyclic/oxidation, and thermophysical material properties. Larger test articles have also being fabricated and hot-fire tested to demonstrate the materials in prototype thrusters at 1O0 lbf thrust levels.

Elam, Sandra↗

Aircraft System Noise Assessment of the NASA D8 Subsonic Transport Concept

Avehicle-level noise assessment has been performed for theNASAD8 concept aircraft (ND8) in the NASA Advanced Air Transport Technology Project portfolio. The NASA research-level Aircraft NOise Prediction Program (ANOPP-Research) was used to predict the noise from each source component on the ND8 to build up a noise estimate for the full aircraft. The propulsion airframe aeroacoustic (PAA) effects of the ND8, namely boundary layer ingestion (BLI) with its influence on fan noise, and the noise shielding, reflection, and diffraction mechanisms of the unconventional airframe, were empirically modeled using experimental data. Noise reduction technologies appropriate to the 2025-2035 time frame were included in this study. Including all technologies and PAA effects, the ND8 is predicted to have a cumulative margin to the Stage 4 certification metric of only 7.4 EPNdB. Boundary layer ingestion is predicted to have a detrimental impact on cumulative noise levels on the order of 15 EPNdB. Fan noise is seen to be the primary noise source at all three certification points, even if the BLI noise impact could be entirely suppressed. The impact of engine noise shielding by the airframe is limited by a lack of aft shielding and the presence of horizontal tail reflections in the aft direction. The physical constraint on engine size by the pi-tail is seen as a potential barrier to engine noise reduction through the corresponding limitation on fan bypass ratio. Mildly reduced climb performance (compared to similar reference aircraft) does not provide any benefit through increased noise propagation distance. If the boundary layer ingestion noise penalty could be suppressed such that BLI would have no effect on noise, the cumulative margin to Stage 4 would increase to 22.4 EPNdB, still below the NASA Mid Term goal of 32-42 EPNdB.

Clark, Ian A.↗

Further industrial tests of ceramic thermal barrier coatings

The NASA Lewis Research Center made technical assistance arrangements (contracts) with several commercial organizations under which Lewis designed plasma-sprayed thermal-barrier coatings (TBC) for their products. Lewis was then furnished with the test conditions and evaluations of coating usefulness. The coating systems were developed and sprayed at Lewis. All of the systems incorporated a two-layer, ceramic-bond coating concept. Coating thickness and chemical composition were varied to fit three applications: the leading edges of first-stage turbine vanes for an advanced gas turbine engine; the flame impingement surfaces of a combustor transition section; and diesel engine valves and head surfaces. The TBC incorporated yytria-stabilized zirconia, which lowered metal temperatures, protected metal parts, and increased metal part life. In some cases metal burning, melting, and warping were eliminated. Additional benefits were realized from these endeavors: hands-on experience with thermal-barrier coatings was provided to industry; the success of these endeavors encourages these and other organizations to accelerate the implementation of TBC technology.

Liebert, C. H.↗

Thermal barrier coating system having improved adhesion

The adherence between a ceramic thermal barrier coating and a metal bond coating is improved by ion sputtering a ceramic film on the bond cost. A ceramic thermal barrier coating is then plasma-sprayed onto this primer film. This improves the integrity and strength of the interface between the plasma-sprayed ceramic layer and metallic bond coat which insures stronger adherence between the metal and the ceramic.

Bill, R. C.↗

New heterojunction LWIR detector options

Researchers investigate the heterojunction internal photoemission (HIP) approach that potentially offers long wavelength infrared (LWIR) photovoltaic detector performance (single pixel) that is competitive with the best of other approaches being considered. Most significantly, this approach offers a relatively simple device technology that promises producible and uniform FPA's. Researchers emphasize an exciting process based on intervalence band absorption. They investigate both III-V and Si-based heterojunctions grown by molecular beam epitaxy (MBE) in which the barrier can be tailored to the desired cutoff wavelength. In addition, MBE allows one to optimize the device structure with precise control of doping profiles and layer thicknesses, and perform band structure engineering by control of composition and heterojunction strain. Researchers also consider free carrier absorption in heterojunctions. Acceptable absorption coefficients can be achieved in very heavily n(exp +) doped semiconductor layers (approx. equals 10(exp 20)cm(exp -3). However, in this case the appreciable filling of conduction band states leads to a Schottky-like photoresponse with a gradual (quadratic) turn-on above threshold. A more satisfactory approach would be to use p(exp +) doping so that with the higher density of states in the heavy hole valence band there would be a narrow band of occupied states. This gives the desirable effect of a more rapid (linear) turn-on above threshold. Unfortunately, the higher hole effective mass also reduces (inversely) the free carrier absorption. For this and other reasons, the intervalence band absorption process looks much more promising.

Maserjian, Joseph↗

Novel Si(1-x)Ge(x)/Si heterojunction internal photoemission long wavelength infrared detectors

There is a major need for long-wavelength-infrared (LWIR) detector arrays in the range of 8 to 16 microns which operate with close-cycle cryocoolers above 65 K. In addition, it would be very attractive to have Si-based infrared (IR) detectors that can be easily integrated with Si readout circuitry and have good pixel-to-pixel uniformity, which is critical for focal plane array (FPA) applications. Here, researchers report a novel Si(1-x)Ge(x)/Si heterojunction internal photoemission (HIP) detector approach with a tailorable long wavelength infrared cutoff wavelength, based on internal photoemission over the Si(1-x)Ge(x)/Si heterojunction. The HIP detectors were grown by molecular beam epitaxy (MBE), which allows one to optimize the device structure with precise control of doping profiles, layer thickness and composition. The feasibility of a novel Si(1-x)Ge(x)/Si HIP detector has been demonstrated with tailorable cutoff wavelength in the LWIR region. Photoresponse at wavelengths 2 to 10 microns are obtained with quantum efficiency (QE) above approx. 1 percent in these non-optimized device structures. It should be possible to significantly improve the QE of the HIP detectors by optimizing the thickness, composition, and doping concentration of the Si(1-x)Ge(x) layers and by configuring the detector for maximum absorption such as the use of a cavity structure. With optimization of the QE and by matching the barrier energy to the desired wavelength cutoff to minimize the thermionic current, researchers predict near background limited performance in the LWIR region with operating temperatures above 65K. Finally, with mature Si processing, the relatively simple device structure offers potential for low-cost producible arrays with excellent uniformity.

Lin, T. L.↗

Near-equilibrium growth of thick, high quality beta-SiC by sublimation

A close spaced near-equilibrium growth technique was used to produce thick, high quality epitaxial layers of beta-silicon carbide. The process utilized a sublimation method to grow morphologically smooth layers. The beta silicon carbide growth layers varied from about 200 to 750 microns in thickness. Chemical vapor deposition grown, 2-10 microns, beta silicon carbide films were used as seeds at 1860 and 1910 C growth temperatures. The respective average growth rates were 20 and 30 microns per hour. The layers are p-type with a 3.1 x 10 exp 17/cu cm carrier concentration. Electrical measurements indicate considerable improvement in the breakdown voltage of Schottky barriers on growth samples. Breakdown values ranged from 25 to 60 V. These measurements represent the highest values reported for 3C-SiC.

Shields, Virgil B.↗

Electrodeposited CuInSe2 Thin Film Junctions

We have investigated thin films and junctions based on copper indium diselenide (CIS) which have been grown by electrochemical deposition. CIS is a leading candidate for use in polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for producing thin-film CIS. We have produced both p and n type CIS thin films from the same aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited using a step-function potential. Stoichiometry of the single layer films was determined by energy dispersive spectroscopy. Carrier densities of these films increased with deviation from stoichiometry, as determined by the capacitance versus voltage dependence of Schottky contacts. Optical bandgaps for the single layer films as determined by transmission spectroscopy were also found to increase with deviation from stoichiometry. Rectifying current versus voltage characteristics were demonstrated for the Schottky barriers and for the pn junction.

Raffaelle, R. P.↗

Electrodeposited CulnSe2 Thin Film Junctions

We have investigated thin films and junctions based on copper indium diselenide (CIS) which have been grown by electrochemical deposition. CIS is a leading candidate for use in polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for producing thin-film CIS. We have produced both p and n type CIS thin films from the same aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited using a step-function potential. Stoichiometry of the single layer films was determined by energy dispersive spectroscopy. Carrier densities of these films increased with deviation from stoichiometry, as determined by the capacitance versus voltage dependence of Schottky contacts. Optical bandgaps for the single layer films as determined by transmission spectroscopy were also found to increase with deviation from stoichiometry. Rectifying current versus voltage characteristics were demonstrated for the Schottky barriers and for the pn junction.

Raffaelle, R. P.↗

Radiation Insulation

The Apollo and subsequent spacecraft have had highly effective radiation barriers; made of aluminized polymer film, they bar or let in heat to maintain consistent temperatures inside. Tech 2000, formerly Quantum International Corporation used the NASA technology in its insulating materials, Super "Q" Radiant Barrier, for home, industry and mobile applications. The insulation combines industrial aluminum foil overlaid around a core of another material, usually propylene or mylar. The outer layer reflects up to 97 percent of heat; the central layer creates a thermal break in the structure and thus allows low radiant energy emission. The Quantum Cool Wall, used in cars and trucks, takes up little space while providing superior insulation, thus reducing spoilage and costs. The panels can also dampen sound and engine, exhaust and solar heat.

Source record↗

NASA Lewis Thermal Barrier Feasibility Investigated for Use in Space Shuttle Solid-Rocket Motor Nozzle-to-Case Joints

Assembly joints of modern solid-rocket motor cases are usually sealed with conventional O-ring seals. The 5500 F combustion gases produced by rocket motors are kept a safe distance away from the seals by thick layers of insulation and by special compounds that fill assembly split-lines in the insulation. On limited occasions, NASA has observed charring of the primary O-rings of the space shuttle solid-rocket nozzle-assembly joints due to parasitic leakage paths opening up in the gap-fill compounds during rocket operation. Thus, solid-rocket motor manufacturer Thiokol approached the NASA Lewis Research Center about the possibility of applying Lewis braided-fiber preform seal as a thermal barrier to protect the O-ring seals. This thermal barrier would be placed upstream of the primary O-rings in the nozzle-to-case joints to prevent hot gases from impinging on the O-ring seals (see the following illustration). The illustration also shows joints 1 through 5, which are potential sites where the thermal barrier could be used.

Steinetz, Bruce M.↗

Feasibility Assessment of Thermal Barriers for RSRM Nozzle Joint Locations

Solid rockets, including the Space Shuttle solid rocket motor, are generally manufactured in large segments which are then shipped to their final destination where they are assembled. These large segments are sealed with a system of primary and secondary 0-rings to contain combustion gases inside the rocket which are at pressures of up to 900 psi and temperatures of up to 5500 F. The seals are protected from hot combustion gases by thick layers of phenolic insulation and by joint-filling compounds between these layers. Recently, though, routine inspections of nozzle-to-case joints in the Shuttle solid rocket motors during disassembly revealed erosion of the primary O-rings. Jets of hot gas leaked through gaps in the joint-filling compound between the layers of insulation and impinged on the O-rings. This is not supposed to take place, so NASA and Thiokol, the manufacturer of the rockets, initiated an investigation and found that design improvements could be made in this joint. One such improvement would involve using NASA Lewis braided thermal barriers as another level of protection for the O-ring seals against the hot combustion gases.

Steinetz, Bruce M.↗

TBCs for better engine efficiency

State-of-the-art thermal barrier coatings (TBCs) developed for aircraft engines can achieve both hot-section component operating temperature reductions and superior oxidation resistance. Such TBCs typically consist of two layers: a metallic, often NiCrAlY 'bond' inner layer in contact with the superalloy structural component, and an outer, insulating ceramic layer. A ceramic frequently used in this role due to its high durability is plasma-sprayed ZrO2, partially stabilized with 6-8 wt pct Y2O3. TBCs can also be useful in nonaircraft gas turbines, which frequently use highly contaminated fuels.

Brindley, William J.↗

Molecular-Beam Epitaxy Of IrSi3

Molecular-beam epitaxy grows layers of iridium silicide (IrSi3) on silicon at temperatures of 630 to 800 degrees C. Particularly useful as photodetector material because it forms Schottky diodes having potential barriers of only 0.12 to 0.15 eV - lowest of any metal on silicon. Photodiodes sensitive to infrared radiation at wavelengths as large as 8 to 10 micrometers. New, lower formation temperature expected to enable growth of arrays of IrSi3/Si infrared detectors on Si wafers without thermally damaging image-processing circuitry integrated on wafers.

Lin, True-Lon↗

Wolf-Rayet stars of type WN/WC and mixing processes during core helium burning of massive stars

Consequences of the recent finding that most WN/WC spectra probably originate from individual Wolf-Rayet stars for the internal structure of massive stars are discussed. Numerical models including the effect of slow-down or prevention of convective mixing due to molecular weight gradients are presented, in which a transition layer with a composition mixture of H- and He-burning ashes is formed above the convective He-burning core. These models are able to qualitatively account for the observed WN/WC frequency and agree quantitatively with the only WN/WC-composition determination so far. It is argued that the same transition layer may be responsible for the final blue loop which the SN 1987 A progenitor performed some 10,000 yr before explosion. These results indicate that composition barriers may be efficient in restricting convection during central helium burning, in contrast to computations relying on the Schwarzschild criterion for convection, with or without overshooting.

Langer, N.↗