Papers by Keyword: SF₆

Abstract: Plasma chemical vaporization machining (PCVM) is a high-rate etching method that uses atmospheric-pressure plasma. Its application to the plasma dicing of SiC wafers is anticipated. However, since the reaction is mainly driven by neutral radicals, it is difficult to maintain anisotropy, and issues such as side etching are of concern. In this study, PCVM processing was performed using SF₆ gas with a Ni mask to investigate vertical and lateral etching behaviors. We achieved vertical etching of 100 µm within approximately 35 minutes, and lateral side etching of about 50 µm. The lateral etch rate remained nearly constant, whereas the vertical etch rate was initially high but decreased as the etching progressed, approaching the lateral rate. Finite element-based electrostatic field analysis revealed that, as the etching depth increased, electric field shielding by the mask weakened the field at the bottom of the trench, leading to a transition toward neutral radical-dominated reactions.

Abstract: Trench structure etching is one of the most important processes for the fabrication of 4H-SiC Trench MOSFETs. This paper introduced Al₂O₃ as an etching mask for the fabrication of trench structures. The effect of dry etching parameters to the shape of trench structures were studied systematically. Micro trenches were successfully eliminated from trench structure etching process and preliminary trench MOSFET test structures were fabricated and characterized.

Abstract: The formation of the carbon microcoils could be achieved under the low temperature (550°C). Ni powders were used as the catalyst on the alumina substrate. C₂H₂ was used as a source gas and a few amount of SF₆ gas were used as an additive gas under the thermal chemical vapor deposition system. The surface morphologies of as-grown carbon materials at the low temperature (550°C) were also investigated according to the total pressure and the injection time of SF₆ flow. At 80Torr and 10min injection time of SF₆ flow, the carbon microcoils were highly developed on the entire surface of the sample. The lower or higher total pressure and the longer injection time of SF₆ flow deteriorated the formation of CMCs. The head area of the carbon microcoils was closely investigated and the growth mode for the initiation of the carbon microcoils at the low temperature was suggested.

Abstract: Mg especially in the molten state is well known for its high affinity to O₂. When O₂ content of the atmosphere is larger than 4%, molten Mg will burn! To avoid this, melt protection is necessary. At present mostly SF₆ is used during primary production and processing of Mg and its alloys. Unfortunately SF₆ is a very potent greenhouse gas that is > 23,000 times more effective than CO₂. This also affects life cycle considerations e.g. for the use of Mg alloys in transportation. However, other protective gases like SO₂ or fluorinated hydrocarbons like HFC134a, Novec 612, or AMCover (=HFC134a) have been suggested to replace SF₆. Additionally fluxes mixed from different salts may be used again as well to protect molten Mg. But fluxes and feasible replacements of SF₆ have also disadvantages. Moreover SF₆ and other fluorinated hydrocarbons are under discussion especially in Europe. There is an existing EU legislation that will ban SF₆ from 2018 and there are similar discussions regarding all other fluorinated hydrocarbons. Due to this, new innovative ways have to be found or old methods have to be renewed to allow Mg industries further safe processing of molten magnesium. This contribution will report the state of the art in protecting molten Mg and alternatives to the use of SF₆.

Abstract: On-line monitoring device for SF₆ humidity based on ZigBee was composed of processors, signal conditioning units (SCU), data-collection devices, ZigBee wireless network communication technology, control equipment of ventilation and alarm and powering circuits. Data information of humidity, temperature and pressure of SF₆ gas can be monitored by the device. The application of information blending technique decreased the error of measuring, and achieved auto-calibration of measurement systems, compensation of measured data and signal output control, and improved precision of the reliability of measurement, and extended the service life.

Abstract: Magnesium alloys have recently attracted increased attention as structural materials owing to their low specific gravity. However, they must typically be casted, formed, melted, and even heated under a protective shielding gas to prevent them from oxidizing and igniting. So-called “ECO-Mg” alloys have recently been developed by adding CaO to conventional Mg alloys. ECO-Mg alloys exhibit higher oxidation resistance during melting and superior castability, even when beryllium is not added or toxic protective gasses such as SF₆ are not used. We laser-welded AZ31 Mg alloys with various CaO contents and examined the mechanical properties of the welds. Increasing the CaO content in the AZ31 increased the burning resistance of the ECO-Mg alloys, thereby suppressing plume generation during laser welding; thus the ECO-Mg alloy welds could be fully penetrated faster than conventional Mg alloy welds. The weld strength also increased with increasing CaO content in the AZ31 because the CaO refined the grains.

Abstract: Analysis the 500kV H-GIS equipment failure, determine for the circuit breaker chamber internal discharge fault according to the protection action and the use of the analysis of SF₆ gas composition, a detailed analysis of the possible causes of failures, and according to the disintegration test results, to confirm the final reason for internal foreign bodies caused by discharge. Finally, put forward some prevention measures and suggestions for the accident.

Abstract: This paper describes application of carbon nanotube (CNT) gas sensor to chemical detection of sulfur hexafluoride (SF6) decomposition products generated by AC or DC corona discharge, aiming to develop a new diagnosis method of gas-insulated switchgear (GIS) filled with high pressure SF6 gas. Currently, most of GIS are designed and operated for conventional high voltage AC (HVAC) power transmission lines. Moreover, in recent years, the electrical power industry has shown a trend shifting from HVAC to high voltage DC power transmission, which has many advantages such as high power capacity and low power loss.This technological trend motivated us to explore and expand the application of CNT gas sensor to detection of SF6 decomposition products generated by AC and DC corona discharges. It was found that the CNT gas sensor exhibited significant response to AC and DC corona discharges and its dependency on the DC voltage polarity. In order to elucidate the mechanism of the polarity effect, SF6 decomposition products were analyzed by Fourier transform infrared spectroscopy as well as using a gas detection tube. Based on comparison between the polarity effects on the CNT sensor response and the decomposition products, a possible contribution of hydrogen fluoride to the CNT gas sensor response was suggested.

Abstract: Effect of plasma treatments (power, pressure and treatment time) on hydrophobicity (contact angle and water absorption time) and sorption isotherm of methylcellulose (MC) film using sulphur hexafluoride (SF6) were investigated. MC film specimens were precondition in desiccators containing silica gel at 25 ±2°C for 48 h. MC film specimens were plasma treated by an inductivity coupled plasma discharged (ICP) at 13.56 MHz with operating powers (25-75 W) and gas pressures (20-200 mTorr). The treatment times (10-300 second). Hydrophobicity of treated methylcellulose film outstandingly increased. The best plasma treatment condition was power 50 W and 100 mTorr. The treated MC film had higher contact angle and water absorption time than untreated sample at all operated treatment times; contact angle was increased up to 56-80°. These results were corresponding to sorption isotherm. The treated film was absorbed moisture higher than untreated film.

Abstract: Sulfur hexafluoride (SF6) is widely used by the magnesium industry as a component of cover gas mixtures, which reacts with the melt and forms a protective film on the melt surface. In this study the dew points of the air that acts as carrier gas for SF6 was varied from 203K to 273K, and the effect of the SF6 cover gas humidity and concentration on the efficient protection of molten AZ91D magnesium alloy was investigated in temperatures ranging from 903K to 973K. Gas chromatography (GC) combined with ion chromatography (IC) were used to identify the chemical constituents of the by-products present in the protective atmosphere, while. X-ray photoelectron spectroscopy (XPS) and Auger electron Spectroscopy (AES) were used in the characterization of the protective films formed on the melt. The results indicate that the quantity of SF6 to protect molten Mg from burning decreased with decrease in the humidity of the cover gas XPS showed that the films formed in all atmospheres with different dew points were composed of MgO, MgF2 and MgSO4. However, in cover gases with higher dew point, the amount of MgO of the film tended to increase, due to the dissociation of MgSO4 of the film into H2 and SO4-2 in the presence of moisture, which lead to the destruction of the protective film and oxidization of the melt.