Performance of CVR Coatings for PBR Fuels

1993 ◽  
Vol 327 ◽  
Author(s):  
Jay W. Adams ◽  
R. E. Barletta ◽  
J. Svandrlik ◽  
P. E. Vanier
Keyword(s):  
Pyrolytic Graphite ◽  
Chemical Vapor ◽  
Screening Tests ◽  
Coated Materials ◽  
Before And After ◽  
Component Development ◽  
Flowing Hydrogen ◽  
Fuel Particles ◽  
Particle Bed ◽  
Surrogate Fuel

AbstractAs a part of the component development process for the particle bed reactor (PBR), it is necessary to develop coatings for fuel particles which will be time and temperature stable. These coatings must not only protect the particle from attack by the hydrogen coolant, but must also help to maintain the bed in a coolable geometry and mitigate against fission product release. In order to develop these advanced coatings, a process to produce chemical vapor reaction (CVR) coatings on fuel for PBRs has been developed.The initial screening tests for these coatings consisted of testing in flowing hot hydrogen at one atmosphere. Surrogate fuel particles consisting of pyrolytic graphite coated graphite particles have been heated in flowing hydrogen at constant temperature. The carbon loss from these particles was measured as a function of time. Exposure temperatures ranging from 2500 to 3000 K were used and samples were exposed for up to 14 minutes in a cyclical fashion, cooling to room temperature between exposures. The rate of weight loss measured as a function of time is compared to that from other tests of coated materials under similar conditions. Microscopic examination of the coatings before and after exposure was also conducted and these results are presented.

scholarly journals Performance of CVD and CVR Coated Carbon-Carbon in High Temperature Hydrogen

1993 ◽  
Vol 327 ◽  
Author(s):  
J. W. Adams ◽  
R. E. Barlettia ◽  
J. Svandrlik ◽  
P. E. Vanier
Keyword(s):  
Development Process ◽  
Chemical Vapor ◽  
Screening Tests ◽  
Carbon Structure ◽  
Carbon Substrates ◽  
Component Development ◽  
Flowing Hydrogen ◽  
Deposition Processes ◽  
Particle Bed ◽  
Coated Carbon

AbstractAs a part of the component development process for the particle bed reactor (PBR), it is necessary to develop coatings which will be time and temperature stable at extremely high temperatures in flowing hydrogen. These coatings must protect the underlying carbon structure from attack by the hydrogen coolant. Degradation which causes small changes in the reactor component, e.g. hole diameter in the hot frit, can have a profound effect on operation. The ability of a component to withstand repeated temperature cycles is also a coating development issue. Coatings which crack or spall under these conditions would be unacceptable. While refractory carbides appear to be the coating material of choice for carbon substrates being used in PBR components, the method of applying these coatings can have a large effect on their performance. Two deposition processes for these refractory carbides, chemical vapor deposition (CVD) and chemical vapor reaction (CVR) have been evaluated.Screening tests for these coatings consisted of testing of coated 2-D and 3-D weave carbon-carbon in flowing hot hydrogen at one atmosphere. Carbon loss from these samples was measured as a function of time. Exposure temperatures up to 3000 K were used and samples were exposed in a cyclical fashion, cooling to room temperature between exposures. The results of these measurements are presented along with an evaluation of the relative merits of CVR and CVD coatings for this application.

scholarly journals The Development of CVR Coatings for PBR Fuels

1993 ◽  
Vol 327 ◽  
Author(s):  
Robert. E. Barletta ◽  
P. E. Vanier ◽  
M. B. Dowell ◽  
J. A. Lennartz
Keyword(s):  
Pyrolytic Graphite ◽  
Chemical Vapor ◽  
Graphite Substrate ◽  
Refractory Carbide ◽  
Carbide Coatings ◽  
Large Numbers ◽  
Fuel Design ◽  
Fuel Particles ◽  
Particle Bed ◽  
Surrogate Fuel

AbstractParticle bed reactors (PBRs) are being developed for both space power and propulsion applications. These reactors operate with exhaust gas temperatures in the range of 2500 to 3000 K and fuel temperatures which may be hundreds of degrees higher. One fuel design for these reactors consists of uranium carbide encapsulated in either carbon or graphite. This fuel kernel must be protected from the coolant gas, usually H2, both to prevent attack of the kernel and to limit fission product release. Refractory carbide coatings have been proposed for this purpose. The typical coating process used for this is a chemical vapor deposition. Testing of other components have indicated the superiority of refractory carbide coatings applied using a chemical vapor reaction (CVR) process, however technology to apply these coatings to large numbers of fuel particles with diameters on the order of 500 gim were not readily available.A process to deposit these CVR coatings on surrogate fuel consisting of graphite particles is described. Several types of coatings have been applied to the graphite substrate. These include NbC in various thicknesses and a bilayer coating consisting of NbC and TaC with a intermediate layer of pyrolytic graphite. These coated particles have been characterized prior to test and the results of this characterization will be presented.

Early Growth in the Chemical Vapor Deposition of Tin and TiC and Monitoring by Laser Scattering

1989 ◽  
Vol 168 ◽  
Author(s):  
Max Klein ◽  
Bernard Gallois
Keyword(s):  
Pyrolytic Graphite ◽  
Chemical Vapor ◽  
Early Growth ◽  
Columnar Growth ◽  
Laser Scattering ◽  
Fine Grained ◽  
Steady Rate ◽  
Columnar Grains ◽  
State Growth

AbstractThe early growth of chemically vapor deposited TiN and TiC coatings on pyrolytic graphite was studied in the kinetic- and mass transport-controlled regimes. While steady-state growth of these coatings results in columnar grains, such morphologies do not originate at the substrate/coating interface. Rather, TiC deposition begins on the substrate as fine grains less than 100 nm in diameter. Early TiN growth occurs in layers of 50 nm grains. In both cases, early fine-grained growth occurs at a lower rate than the linear, steady rate observed for columnar growth. A laser scattering technique has been developed as a tool for characterizing early growth through surface roughness. This noncontact method can be used as an in-situ diagnostic to detect changes in the surface of the growing deposit.

scholarly journals Field Emission from Carbon Films Deposited by Controlled-Low-Energy Beams and CVD Sources

1999 ◽  
Vol 585 ◽  
Author(s):  
Douglas H. Lowndes ◽  
Vladimir I. Merkulov ◽  
L. R. Baylor ◽  
G. E. Jellison ◽  
D. B. Poker ◽  
...  
Keyword(s):  
Carbon Nanotubes ◽  
Surface Morphology ◽  
Field Emission ◽  
Metal Ion ◽  
Ion Beam ◽  
Pyrolytic Graphite ◽  
Carbon Film ◽  
Chemical Vapor ◽  
Carbon Films ◽  
Damage Site

AbstractThe principal interests in this work are energetic-beam control of carbon-film properties and the roles of doping and surface morphology in field emission. Carbon films with variable sp3-bonding fraction were deposited on n-type Si substrates by ArF (193 nm) pulsed-laser ablation (PLA) of a pyrolytic graphite target, and by direct metal ion beam deposition (DMIBD) using a primary Cs+ beam to generate the secondary C- deposition beam. The PLA films are undoped while the DMIBD films are doped with Cs. The kinetic energy (KE) of the incident C atoms/ions was controlled and varied over the range from ∼25 eV to ∼175 eV. Earlier studies have shown that C films' sp3-bonding fraction and diamond-like properties can be maximized by using KE values near 90 eV. The films' surface morphology, sp3–bonding fraction, and Cs-content were determined as a function of KE using atomic force microscopy, TEM/EELS, Rutherford backscattering and nuclear reaction measurements, respectively. Field emission (FE) from these very smooth undoped and Cs-containing films is compared with the FE from two types of deliberately nanostructured carbon films, namely hot-filament chemical vapor deposition (HF-CVD) carbon and carbon nanotubes grown by plasma-enhanced CVD. Electron field emission (FE) characteristics were measured using ∼25-μm, ∼5-μm and ∼1-μm diameter probes that were scanned with ∼75 nm resolution in the x-, y-, and z-directions in a vacuum chamber (∼5 × 10-7 torr base pressure) equipped with a video camera for viewing. The hydrogen-free and very smooth a-D or a-C films (with high or low sp3 content, and with or without ∼1% Cs doping) produced by PLD and DMIBD are not good field emitters. Conditioning accompanied by arcing was required to obtain emission, so that their subsequent FE is characteristic of the arc-produced damage site. However, deliberate surface texturing can eliminate the need for conditioning, apparently by geometrical enhancement of the local electric field. But the most promising approach for producing macroscopically flat FE cathodes is to use materials that are highly nanostructured, either by the deposition process (e.g. HF-CVD carbon) or intrinsically (e.g. carbon nanotubes). HF-CVD films were found to combine a number of desirable properties for FE displays and vacuum microelectronics, including the absence of conditioning, low turn-on fields, high emission site density, and apparent stability and durability during limited long-term testing. Preliminary FE measurements revealed that vertically aligned carbon nanotubes are equally promising.

scholarly journals Characteristics of Graphene Oxide Films Reduced by Using an Atmospheric Plasma System

Nanomaterials ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 802 ◽  
Author(s):  
Chii-Rong Yang ◽  
Shih-Feng Tseng ◽  
Yu-Ting Chen
Keyword(s):  
Graphene Oxide ◽  
Sheet Resistance ◽  
Functional Groups ◽  
Pyrolytic Graphite ◽  
Chemical Oxidation ◽  
Atmospheric Plasma ◽  
Graphene Oxides ◽  
Plasma Irradiation ◽  
Before And After ◽  
Bond Peak

The chemical oxidation method can be used to mass-produce graphene oxides (GOs) from highly oriented pyrolytic graphite. However, numerous oxygen-containing functional groups (hydroxyl, epoxy, carbonyl, etc.) exist in typical GO surfaces, resulting in serious electrical losses. Hence, GO must be processed into reduced graphene oxide (rGO) by the removal of most of the oxygen-containing functional groups. This research concentrates on the reduction efficiency of GO films that are manufactured using atmospheric-pressure and continuous plasma irradiation. Before and after sessions of plasma irradiation with various irradiation times, shelters, and working distances, the surface, physical, and electrical characteristics of homemade GO and rGO films are measured and analyzed. Experimental results showed that the sheet resistance values of rGO films with silicon or quartz shelters were markedly lower than those of GO films because the rGO films were mostly deprived of oxygen-containing functional groups. The lowest sheet resistance value and the largest carbon-to-oxygen ratio of typical rGO films were approximately 90 Ω/sq and 1.522, respectively. The intensity of the C–O bond peak in typical rGO films was significantly lower than that in GO films. Moreover, the intensity of the C–C bond peak in typical rGO films was considerably higher than that in GO films.

Electrical Properties of Thin Film and Bulk Diamond Treated in Hydrogen Plasma

1989 ◽  
Vol 162 ◽  
Author(s):  
Sacharia Albin ◽  
Linwood Watkins
Keyword(s):  
Natural Diamond ◽  
Hydrogen Plasma ◽  
Diamond Films ◽  
Chemical Vapor ◽  
Deposition Process ◽  
Silicon Substrates ◽  
Free Standing ◽  
Before And After ◽  
Type Ia ◽  
Bulk Diamond

ABSTRACTCurrent-voltage characteristics of type Ia synthetic diamond, type IIb natural diamond and free-standing diamond films were measured before and after hydrogenation. The diamond films were polycrystalline, deposited on sacrificial silicon substrates using a microwave chemical vapor deposition process. On hydrogenation, all the samples showed several orders of magnitude increase in conductivity. Hydrogenation was carried out under controlled conditions to study the changes in the I-V characteristics of the samples. The concentration of electrically active hydrogen was determined from the I-V data. Hydrogen passivation of deep traps in diamond is clearly demonstrated.

Hermetic Sealing of Stainless Steel Packages by Seam Seal Welding

2010 ◽  
Vol 2010 (1) ◽  
pp. 000720-000730
Author(s):  
Thomas Marinis ◽  
Berj Nercessian
Keyword(s):  
Stainless Steel ◽  
Laser Welding ◽  
Process Development ◽  
Screening Tests ◽  
Seal Integrity ◽  
Flange Thickness ◽  
Before And After ◽  
Designed Experiment ◽  
Weld Current ◽  
Seam Sealing

Welded stainless steel packages offer a number of advantages relative to those fabricated from kovar or aluminum metals and braze sealed. They are highly resistant to corrosion, especially in aqueous or elevated temperature environments, are mechanically stronger, dramatically so at temperatures above 100°C, exhibit lower outgassing rates, and have better vibration characteristics. Since welded stainless steel packages do not require over plating to facilitate braze sealing, combined material and fabrication costs are lower than kovar or aluminum packages. Also, unlike kovar, stainless steel is nonmagnetic, which is advantageous in many electronic applications. For sealing stainless steel packages, we elected to use seam sealing rather than laser welding for two reasons. Seam sealing subjects the package contents to lower thermal excursions than laser welding because less material is melted to achieve a weld in seam sealing. Second, seam sealing confines the molten material between the cover and package, where as laser welding produces a surface filet. Consequently, there is more opportunity for molten metal to splatter or react with the atmosphere in laser welding than in seam sealing. We successfully developed a process to weld a 2 millimeter thick cover onto a box with dimensions of 75 mm long by 50 mm wide by 15 mm deep using a seam sealer. The box walls were 1 mm thick and were penetrated by a dozen glass insulated feedthrus on one side. Both cover and box were fabricated from 316L stainless steel. A combination of analytical and finite element modeling were used in conjunction with a designed experiment to optimize the process variables of roller angle, speed and pressure, weld current, pulse shape, duration and spacing, number of weld passes, and sealing atmosphere. Weld quality and seal integrity were evaluated by leak testing before and after environmental stressing, mechanical testing and metallographic cross sectioning. The effects of component dimensions and tolerances on seal integrity were also investigated. Particular attention was paid to cover flatness, flange thickness, and tightness of fit between the cover and box. The process development was concluded by conducting a qualification experiment that used the optimized process parameters with controlled variation about their nominal values. A 100% yield of sealed boxes was obtained. These test articles were then subjected to various environmental screening tests, which were all passed with no failures.

scholarly journals Performance of Flexible Chemoresistive Gas Sensors after Having Undergone Automated Bending Tests

Sensors ◽  
2019 ◽  
Vol 19 (23) ◽  
pp. 5190 ◽  
Author(s):  
Miriam Alvarado ◽  
Silvia De La Flor ◽  
Eduard Llobet ◽  
Alfonso Romero ◽  
José Luis Ramírez
Keyword(s):  
Metal Oxide ◽  
Gas Sensing ◽  
Flexible Substrate ◽  
Test Procedure ◽  
Chemical Vapor ◽  
Bending Test ◽  
Bending Tests ◽  
Ag Electrodes ◽  
Different Types ◽  
Before And After

Many sensors are developed over flexible substrates to be used as wearables, which does not guarantee that they will actually withstand being bent. This work evaluates the gas sensing performance of metal oxide devices of three different types, before and after having undergone automated, repetitive bending tests. These tests were aimed at demonstrating that the fabricated sensors were actually flexible, which cannot be taken for granted beforehand. The active layer in these sensors consisted of WO3 nanowires (NWs) grown directly over a Kapton foil by means of the aerosol-assisted chemical vapor deposition. Their response to different H2 concentrations was measured at first. Then, they were cyclically bent, and finally, their response to H2 was measured again. Sensors based on pristine WO3-NWs over Ag electrodes and on Pd-decorated NWs over Au electrodes maintained their performance after having been bent. Ag electrodes covered with Pd-decorated NWs became fragile and lost their usefulness. To summarize, two different types of truly flexible metal oxide gas sensor were fabricated, whereas a third one was not flexible, despite being grown over a flexible substrate following the same method. Finally, we recommend that one standard bending test procedure should be established to clearly determine the flexibility of a sensor considering its intended application.

scholarly journals Nanocrystalline Ga–Zn Oxynitride Materials: Minimized Defect Density for Improved Photocatalytic Activity?

2019 ◽  
Vol 0 (0) ◽  
Author(s):  
Sasa Lukic ◽  
Gerdina Wilhelmina Busser ◽  
Siyuan Zhang ◽  
Jasper Menze ◽  
Martin Muhler ◽  
...  
Keyword(s):  
Photocatalytic Activity ◽  
Defect Density ◽  
Chemical Vapor ◽  
Band Gap Energy ◽  
Lower Probability ◽  
Nitridation Time ◽  
Carrier Recombination ◽  
Synthesis Strategy ◽  
Before And After ◽  
Time Band

Abstract We present an alternative synthesis strategy for developing nanocrystalline (Ga1−xZnx)(N1−xOx) semiconductors known to be very efficient photoabsorbers. In a first step we produce mixtures of highly crystalline β-Ga2O3 and wurtzite-type ZnO nanoparticles by chemical vapor synthesis. (Ga1−xZnx)(N1−xOx) nanoparticles of wurtzite structure are then formed by reaction of these precursor materials with ammonia. Microstructure as well as composition (zinc loss) changes with nitridation time: band gap energy, crystallite size and crystallinity increase, while defect density decreases with increasing nitridation time. Crystallite growth results in a corresponding decrease in specific surface area. In the UV regime photocatalytic activity for overall water splitting can be monitored for samples both before and after nitridation. We find a significantly lower photocatalytic activity in the nitrided samples, even though the crystallinity is significantly higher and the defect density is significantly lower after nitridation. Both properties should have led to a lower probability for charge carrier recombination, and, consequently, to a higher photocatalytic activity.

scholarly journals Troglitazone Induces CYP3A4 Activity Leading to Falsely Abnormal Dexamethasone Suppression Test

2003 ◽  
Vol 88 (7) ◽  
pp. 3113-3116 ◽  
Author(s):  
Eleni V. Dimaraki ◽  
Craig A. Jaffe
Keyword(s):  
Breath Test ◽  
Plasma Corticosterone ◽  
Dexamethasone Suppression Test ◽  
Screening Tests ◽  
Cyp3a4 Activity ◽  
Abnormal Response ◽  
Before And After ◽  
Erythromycin Breath Test ◽  
Suppression Test ◽  
Dexamethasone Suppression

After evaluating a patient who appeared to have a falsely abnormal response to the dexamethasone suppression test while taking troglitazone, we examined the effects of troglitazone on the activity of hepatic CYP3A4 and the screening tests for Cushing’s syndrome. We studied five healthy women and three healthy men, aged 25 ± 2 yr, before and after treatment with troglitazone (600 mg daily) for 28 d. Baseline 0800 h cortisol and corticosterone were similar before and after troglitazone treatment. Before troglitazone treatment, all subjects suppressed 0800 h cortisol below 1.8 μg/dl (mean, 0.66 ± 0.08 μg/dl) during the 1-mg overnight dexamethasone suppression test (DST), whereas during troglitazone treatment none of the subjects suppressed 0800 h cortisol below 1.8 μg/dl (mean, 9.0 ± 1.8 μg/dl). Serum dexamethasone levels decreased by 66 ± 4%, and the erythromycin breath test measurements increased by 27 ± 8%, indicating increased CYP3A4 activity during troglitazone treatment. The hydrocortisone suppression test (HST) was performed by administering 50 mg hydrocortisone at 2300 h. Using the criterion of suppression of 0800 h plasma corticosterone by more than 50%, the specificity of the HST was 100% both before and after troglitazone treatment. In conclusion, troglitazone induced the activity of CYP3A4 leading to falsely abnormal DST. HST is a useful alternative to the DST in patients taking medications that increase the activity of CYP3A4.

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