MPa Nitrogen Gas Research Articles

Test of a 400-kV Compact Bushing

A two-stage compact bushing has been developed by CEA-Cadarache and installed on its 1-MV testbed. This bushing acts as high-voltage feedthrough between a transmission line under pressurized nitrogen or sulfur hexafluoride (SF6) gas and a vacuum tank with a volume of 30 m3. The bushing has been successfully tested up to 420 kV. Electron field emission causes a dark current to flow between the electrodes in the vacuum, both inside or outside the bushing in the vacuum tank. High-voltage conditioning leads to a significant reduction in this dark current. The dark current can be further reduced or even suppressed by adding low-pressure (~1 mPa) nitrogen or SF6 gas to the vacuum.

Simulation of the effect of internal pressure on the integrity of hydrogen pre-charged BCC and FCC steels in SSRT test conditions

Three effects of hydrogen have been reported on an austenitic stainless steel (Type-316L), a Cr-Mo steel (JIS-SCM435) and a carbon steel (JIS-SGP) during slow strain rate tensile (SSRT) tests performed in air on H-charged, smooth round-bar specimens. Two H-charging conditions were considered: exposure to 100-MPa hydrogen gas (270 °C, 200 h for Type-316L), and immersion in a NH4SCN solution (40 °C, 48 h for other steels). Modifications of the micro-void coalescence (MVC) mechanism were observed for each steel: decrease of dimple size (Type-316L), increase of dimple size (JIS-SGP), and formation of quasi-cleavage (QC) surfaces (JIS-SCM435). To clarify the contribution of the hydrogen-induced cracking (HIC) mechanism to these failures, the pressure build-up in preexisting cavities and its impact on the material strength was simulated by finite difference method (FDM) and finite element method (FEM). The failure criterion was defined based upon the elastoplastic fracture mechanics parameter: J-integral. For Type-316L, no effect of internal pressure on the fracture was expected. For JIS-SCM435 and JIS-SGP, although an effect of internal pressure exists, its relatively low value cannot lead to failure. SSRT tests were performed on Type-316L under the following conditions: (I) non-charged in vacuum; (II) H-charged in vacuum; (III) H-charged in 115 MPa nitrogen gas; (IV) non-charged in 115 MPa nitrogen gas. The experimental results successfully supported the simulation-based conclusions.

Synthesis of Silicon Nitride Ceramic Fibers and the Effect of Nitrogen Atmosphere on their Morphology

The effect of nitrogen gas flow rate on the morphology of silicon nitride fibers obtained via carbothermal nitridation heat treatment method was investigated. A precursor containing silicon, oxygen and carbon was obtained by a sol-gel method from a mixture of tetraethyl orthosilicate, polyvinyl alcohol, H2O and ethanol. A white wool-like product was obtained by heat treating the precursor placed in an alumina crucible under a 0.5 MPa nitrogen gas pressure at 1500oC with different nitrogen gas flow rates. The mass-based production rates of the samples obtained from the precursor powder were 20-30% for the different nitrogen gas flow rates. X-ray diffraction analysis revealed that the samples contained α-Si3N4 as the major phase along with β-Si3N4, Si2N2O and a small amount of amorphous product as minor phases. Unique twisted fibers with diameters of several hundreds of nanometers were found among the straight fibers by SEM observation. Elemental analysis using energy dispersive X-ray spectroscopy indicated that silicon and nitrogen were contained in the twisted fibers along with approximately 68 at.% of oxygen and several at.% of aluminum, which might have come from the crucible material. The SiAlON-like structures might have been formed by the partial dissolution of Al and O in the Si3N4 fibers. It was considered that the twisted morphology of some fibers might be formed by co-existing of β-SiAlON and/or amorphous phase regions in the Si3N4 fiber and resultant distortion of the fibers.

70 MPa水素ステーション実証試験で使用されたプレクーラー冷却パイプの事例解析

The case study on cooling pipe of pre-cooler, which had been used on a verification test of 70 MPa hydrogen station, was carried out. Cooling pipe consisted of main pipe, mechanical joint pipe and mechanical joint. Main pipe and mechanical joint pipe were joined by TIG welding. Chemical composition analysis, microstructure observation and Vickers hardness measurement showed that the main pipe and the mechanical joint pipe were manufactured from SUS316L, and that the filler metal of TIG welding was 316L. Round specimens were machined out from main pipe to investigate tensile properties of the base metal. On the other hand, round specimens and square specimens having reinforcement were machined out form the weld joint. Using three types of specimens, slow strain rate tests were conducted in 0.1 MPa nitrogen gas and 115 MPa hydrogen gas at −40 °C. Reductions of area for the base metal round specimen, the weld joint round specimen and the weld joint square specimen were 83.5, 71.3 and 81.4 % in nitrogen gas, whereas the values were 60.1, 61.3 and 40.1 % in hydrogen gas. That is, reductions of area for three types of specimens were smaller in hydrogen gas than in nitrogen gas. Dimples were formed on fracture surfaces of three types of specimens in nitrogen gas, whereas dimples and quasi-cleavages were formed in hydrogen gas. These results suggest that cooling pipe of pre-cooler can be “embrittled” in high-pressure hydrogen gas at low temperature.

Al-Mg Deoxidation of High Nitrogen Steels in VIF

Abstract: The present experiment was conducted to a steel, Fe-20%Cr-10%Mn-6%Ni, in a 25kg intermediate frequency vacuum induction furnace at 1873K under protection of 0.08 MPa nitrogen gas. The inclusions in the steel were studied both before and after Al, Mg was added. The results revealed that the inclusions were removed more efficiently after treated by Al, Mg and the cleanness of metal was promoted to a higher level. The Al treated inclusions had more Mn, Cr and Fe at the core and a significant amount of Al around periphery according to line scanning analysis. After Mg was introduced, it concentrated at the surface of inclusions. The remaining Al content was as low as 0.01% which was lower than expected. The reaction between the Al and MgO crucible should be responsible for it and trace amount of Mg was found in the Al treated inclusions.

Fabrication of Bionic Superhydrophobic Manganese Oxide/Polystyrene Nanocomposite Coating

A superhydrophobic manganese oxide/polystyrene (MnO 2/PS) nanocomposite coating was fabricated by a facile spraying process. The mixture solution of MnO 2/PS was poured into a spray gun, and then sprayed onto the copper substrate using 0.2 MPa nitrogen gas to construct superhydrophobic coating. The wettability of the composite coating was measured by sessile drop method. When the weight ratio of MnO 2 to PS is 0.5:1, the maximum of contact angle (CA) (140°) is obtained at drying temperature of 180 °C. As the content of MnO 2 increases, the maximum of CA (155°) is achieved at 100 °C. Surface morphologies and chemical composition were analyzed to understand the effect of the content of MnO 2 nanorods and the drying temperature on CA. The results show that the wettability of the coating can be controlled by the content of MnO 2 nanorods and the drying temperature. Using the proposed method, the thickness of the coating can be controlled by the spraying times. If damaged, the coating can be repaired just by spraying the mixture solution again.

Fatigue Crack Growth of Aluminum Alloy A6061-T6 in High Pressure Hydrogen Gas and Failure Analysis on 35 MPa Compressed Hydrogen Tanks VH3 for Fuel Cell Vehicles

In order to ensure the safety of carbon fiber reinforced tanks for 35 and 70 MPa fuel cell vehicles, it is necessary to clarify the fatigue crack growth behavior of A6061-T6 aluminum alloy for the tank liner. In this study, the fatigue crack growth tests were carried out under the conditions of test frequencies f = 0.001-10 Hz in 90 MPa (≒70 MPa×125 %) hydrogen gas, 90 MPa nitrogen gas, air and deionized water at room temperature. The fatigue crack growth rates in hydrogen and nitrogen gas at f = 1-10 Hz were lower than those in air and deionized water. However, the fatigue crack growth rates in hydrogen and nitrogen gas at f = 0.001-0.1 Hz were nearly the same as those in air and deionized water. It was found from those results that the fatigue crack growth rate was not accelerated in 90 MPa hydrogen gas. The results of fatigue crack growth tests in this study were applied to the failure analysis of 35 MPa hydrogen tanks. In JARI (Japan Automotive Research Institute), using 35 MPa hydrogen tanks, the pressure cycling tests were conducted at -40, 25 and 85°C by water or PEPF (Perfluoroalkylpolyether) under 125 % filling pressure. The tanks fractured by LBB (Leak Before Break). Increasing in the test temperature increased in the number of cycles to LBB failure. Such a behavior was predicted by the fatigue crack growth properties of A6061-T6 obtained in this study.

Thermal hydraulic test with 6 MPa nitrogen gas loop for developing the Korean He cooled test blanket

In considering the requirements for a DEMO-relevant blanket concept, Korea (KO) has proposed a Helium Cooled Molten Lithium (HCML) Test Blanket Module (TBM) for testing in the International Thermonuclear Experimental Reactor (ITER). The performance analysis for thermal-hydraulics and safety analysis for an accident caused by loss of a coolant for the KO TBM has been carried out with a commercial CFD code, ANSYS-CFX V11 and system code, GAMMA (GAs Multicomponent Mixture Analysis). In order to verify the codes, a basic thermal-hydraulic test with a high pressure nitrogen gas loop up to 6 MPa pressure and 950 °C temperature was performed. In the experiment, single TBM First Wall (FW) mock-up made from the same material as the KO TBM, Ferritic Martensitic Steel, was used and the test was performed under the conditions of pressures of 20 and 36 bar and flow rates of 0.75 and 0.92 kg/min. As one-side of the mock-up was heated to 230 °C, the wall temperatures were measured by installed thermocouples. The measured temperatures show a strong parity with codes’ results simulated with the same test conditions. An additional test with higher pressure and temperature has been prepared for the future.

Influence of Testing Frequency on Fatigue Crack Growth of 6061-T6 Aluminum Alloy in Hydrogen Gas Environment

In order to investigate the effects of testing frequency on the fatigue crack growth rate of 6061-T6 aluminum alloy in hydrogen gas environment, fatigue crack growth tests were carried out on specimens with small artificial holes in 0.18 MPa hydrogen gas or in 0.18 MPa nitrogen gas. It takes long time to test at low testing frequency, so in this study an accelerated test method was proposed and fatigue tests were carried out using this method. The fatigue crack growth rate in hydrogen gas environment accelerates compared with in nitrogen gas environment. The crack growth rate at lower testing frequency tends to higher.

Preparation and Characterization of Rod-Like β-Si<sub>3</sub>N<sub>4</sub> Seeds

The purpose of this paper is to demonstrate a method of preparation of rod-like β-Si3N4 seeds for self-reinforced silicon nitride ceramics. β-Si3N4 seeds with different size and aspect ratios have been well developed by means of heating the mixture of α-Si3N4 containing various contents of β-Si3N4 powder with certain additives at 1800°С for holding different time under 0.5MPa nitrogen gas. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were performed to investigate the morphology, phase transformation and secondary phase. The influence of the proportion of β-Si3N4 in the initial α-Si3N4 powder on seeds size and morphology was investigated. The uniform β-Si3N4 seeds with controllable size and aspect ratios obtained through the β-Si3N4 addition can be explained by an increased heterogeneous nucleation, and MgO was more effective for enhancing the grain growth by the decreased viscosity.

Effects of Hydrogen Gas Environment on Non-Propagation Phenomena of a Type 304 Austenitic Stainless Steel

In order to investigate the hydrogen gas effect on non-propagation phenomena of a type 304 austenitic stainless steel, fatigue tests with in-situ observation using a Scanning Laser Microscope were performed in air, in 0.18MPa hydrogen gas and in 0.18MPa nitrogen gas. A nonpropagating crack was observed during the fatigue test in air. At almost the same stress level of non-propagating in air, non-propagating cracks were also observed in fatigue tests in hydrogen and in nitrogen. Stress level of the non-propagation is not sufficiently different in the three environments. However, the process up to non-propagation differs from each other, for example, the crack path and debris.

Microstructural characterization of gas-pressure-sintered α-silicon nitride containing β-phase seeds

Abstract The starting α-Si 3 N 4 powders with or without the addition of β-phase seeds using Y 2 O 3 and Al 2 O 3 as sintering aids were gas-pressure-sintered with the conditions, 0.9 MPa nitrogen gas at 1850 °C for 1 h. The microscopic evidence indicates that β-Si 3 N 4 seeds incorporated in the starting α-Si 3 N 4 powders play an important role in microstructural development and mechanical properties for silicon nitride. The growth of β-Si 3 N 4 grains is initiated from the seeds, resulting in a core/shell microstructure was observed in the abnormal grain growth. As a result, the fracture toughness of silicon nitride was improved from 5.5 to 7.6 MPa m 1/2 by adding β-phase seeds.

Mechanical Properties and Oxidation Resistance of Silicon Nitride Produced from Low Purity β-Powder

Low-cost and low-purity β-Si3N4 powder was sintered at 1900°C for 8h in 1 MPa nitrogen gas with 2mol% Y2O3-2mol%Al2O3 additives. Fully densified bodies were obtained by the gas-pressured-sintering. This material had a strength of 689 MPa and a fracture toughness of 6.4 MPa·m1/2. Furthermore, the high temperature strength, oxidation resistance, fatigue parameter were measured. The degradation of strength at elevated temperature was less than 5% up to 900°C. No fatal defect was created in 100h-oxidation in 900°C-air. The fatigue parameter was on a level with general material produced from α-rich powder. These properties are adequate for structural material application.

Sintering Phenomena of Si<sub>3</sub>N<sub>4</sub> Bonded Ceramics Containing Al<sub>2</sub>O<sub>3</sub> Particles

To investigate the sintering behavior of reaction bonded ceramics containing good self-sintering ceramic powders as starting materials, Al2O3 and Si powder mixtures were heated to 1350°C in a 0.88MPa nitrogen gas atmosphere. Shrinkage of abouf 0.3% was observed for the samples containing 20 to 70vol%Al2O3. This value was a few times larger than that of Si3N4 bonded ceramics containing non-self-sintering powders such as SiC, TiN and ZrN. It was proved that the grain growth of Al2O3 powders by self-sintering increased the shrinkage. TEM observation of the sintered body showed that micro-cracks were present at the grain boundaries of Al2O3 and Si3N4 grains. These cracks seemed to form due to stress of the Al2O3 sintering on the Si3N4 skeleton formed from the Si powders. This skeleton might depress the free sintering shrinkage of Al2O3 powders. The TEM observation also showed that Si3N4 grains grew epitaxially on the Al2O3 surface, which meant Si3N4 could be bonded to inorganic compounds with ionic bond character.

Development and Characterization of Electro-Conductive Si<sub>3</sub>N<sub>4</sub> Bonded TiN Ceramics

Electro-conductive Si3N4 bonded TiN ceramics were successfully produced by sintering green compacts composed of Si and TiN powders in nitrogen gas atmosphere. These ceramics showed little dimensional change in the sintering stage, which was very favorable to get near-net-shape ceramic components in as-sintered state. Si and TiN powders were mixed in a ball mill and molded using thermoplastic resin binder. The green body was dewaxed and then sintered in 0.88MPa nitrogen gas atmosphere. The dimensional change in the sintering stage was under 0.3%, and electrical resistivity of 26μΩm was obtained when TiN content was 70vol%. The bending strength of 460MPa was obtained for the ceramics containing 20vol% of TiN. Other properties of these ceramics, such as hardness, thermal expansion coefficient and oxidation resistance, were measured and discussed.

常圧焼結およびＨＩＰ処理した窒化ケイ素の諸特性におよぼす焼結助剤の影響

Si3N4 powders mixed with 6wt%Y2O3-2-6wt%Al2O3 and 6wt%Y2O3-1.5wt%MgO as sintering aids, were pressed and normally sintered at 1600°C. The sintered compacts were HIP'ed without capsule using in 100 MPa nitrogen gas at 1700°C. Density, weight change, a to β phase transformation and flexural strength were studied, and the results obtained were summarized as follows:(1) The amount of weight loss during sintering was relatively small in case of Y2O3-Al2O3 series, so that the sinterability was better than in Y2O3-MgO series.(2) In case of both the series, the amount of a to β transformation in sintered compacts was decreased compared with the compacts which contained 6 wt%Y2O3 only, and the tendency in Y2O3-Al2O3 series was more remarkable than in Y2O3-M90 series.(3) The mean strength of Y2O3-Al2O3 series was improved from 340 to 700 MPa by HIP treatment. It was found that the improvement of strength depended on not only density, but also a toβ transformation and fibrous structure formation in Si3N4 during HIP treatment.

The crystal growth of δ-VN by floating zone and zone annealing techniques

Single crystals of δ-VN have been prepared using a floating zone technique. Crystals grown in a nitrogen ambient gas pressure of up to 2 MPa had a composition in the range VN 0.74 to VN 0.86. The nitrogen content of the crystals was increased by zone annealing at 2000°C in 2 MPa nitrogen gas to the composition range VN 0.84 to VN 1.0.