Yang Yan   Li Jian   Wang Man   Wang Yi   Yang Qing

  1 Historical Evolution and Physicochemical Properties In 1900, Moissan and Lebeau first obtained sulfur hexafluoride by burning sulfur in fluorine gas. In 1930, Schumb and Gamble also proposed a method for manufacturing sulfur hexafluoride. The UK proposed the use of sulfur hexafluoride in transformers in 1941, and the Soviet Union used it in cables and capacitors in 1942. The United States began industrial production of sulfur hexafluoride in 1948. Currently, countries such as the United States, the United Kingdom, France, Germany, Italy, Russia, and Japan can produce sulfur hexafluoride. Notably, Japan's Kanto Denka Kogyo Co., Ltd. and Asahi Glass Co., Ltd. had a production capacity of 1000 t/a by the 1970s. Sulfur hexafluoride is a colorless, odorless, non-toxic, non-flammable, and non-corrosive gas under normal temperature and pressure, with a gas density of 6.139 g/L. It has strong chemical stability, does not decompose at 500-600°C, and does not react with acids, alkalis, salts, ammonia, or water. Under the action of an electric arc (several thousand degrees), it decomposes into atomic sulfur and fluorine, but once the arc is removed, it recombines into SF6 within 10^-5 to 10^-6 seconds. Sulfur hexafluoride has excellent electrical insulation properties and superior arc-extinguishing performance. Its dielectric strength is 2.15 times that of nitrogen at the same pressure, its breakdown voltage is 2.15 times that of air, and its arc-extinguishing capability is 100 times that of air. It is a new generation of ultra-high voltage insulating medium material superior to air and oil.

  2 Industrial Preparation Methods and Comparisons Industrially, sulfur hexafluoride is typically prepared by reacting fluorine, produced by electrolysis, with sulfur at medium to high temperatures. In addition to sulfur hexafluoride, small amounts of by-products such as sulfur tetrafluoride (SF4) are also generated. The direct synthesis of sulfur hexafluoride from fluorine and sulfur can be divided into three methods: fluorine gas reacting with solid sulfur, molten sulfur, and sulfur vapor. The reaction of fluorine gas with solid sulfur is highly exothermic, making temperature control difficult, leading to increased by-products and complicating post-processing. Additionally, continuously adding solid sulfur to a sealed reactor is challenging, making this method unsuitable for industrial production. The reaction of fluorine gas with sulfur vapor involves heating sulfur above 445°C (its boiling point) to turn it into vapor, which is then fed into the reactor to react with fluorine. This method also suffers from difficulty in controlling the reaction temperature, which can exceed 1600°C, requiring special reactor materials, making it unsuitable for industrial production. The reaction of fluorine gas with molten sulfur involves maintaining the sulfur in the reactor at 85-105°C in a molten state and reacting it with fluorine gas. This method allows for easier temperature control and higher reaction yields by ensuring that the fluorine gas is introduced parallel to the surface of the molten sulfur. This method is widely adopted by sulfur hexafluoride manufacturers both domestically and internationally.

  3 Comparison of Domestic and International Standards The development of sulfur hexafluoride in China began in 1960. After more than 30 years of development, there are currently seven sulfur hexafluoride production plants in China, with an annual output of several hundred tons. The product quality has surpassed the standards of the International Electrotechnical Commission (IEC). Among them, CNNC Honghua Special Gas Co., Ltd. leads the domestic industry in production and sales scale. The company primarily produces sulfur hexafluoride, carbon tetrafluoride, nitrogen trifluoride, and other fluorine series products, with hydrogen and nitrogen as supplementary products. Its main product, sulfur hexafluoride, has an annual production capacity of 1400 tons, with a purity of 99.99%. It is the preferred product for export equipment among manufacturers and consistently ranks high in domestic market share. The product is exported to more than a dozen countries and regions, including Southeast Asia, Europe, America, Australia, Hong Kong, and Taiwan. The company has established good cooperative relationships with internationally renowned companies such as Union Carbide Corporation (USA), Asahi Glass Co., Ltd. (Japan), Messer Group (Germany), and DuPont (USA). A comparison of China's sulfur hexafluoride standard (GB12022-89) with certain international standards is shown in Table 1.

  4 Applications and Prospects of Sulfur Hexafluoride Sulfur hexafluoride (SF6) is a gaseous material widely used in electrical appliances, aerospace, electronics, meteorology, metallurgy, machinery manufacturing, refrigeration, lasers, medicine, and other fields. In the military sector, it is used as an etchant and cleaning agent in torpedo or submarine propellants. Particularly, due to its excellent insulation, arc-extinguishing, and chemical stability, sulfur hexafluoride is extensively used in high-voltage accelerators and capacitors. It not only ensures the miniaturization and lightweight of devices, saving materials and reducing costs, but also ensures the safety of device operation, reduces noise, and improves the working environment. Sulfur hexafluoride is the third-generation insulating medium, following the first-generation air and second-generation oil. The development of sulfur hexafluoride electrical appliances in China started relatively late. In 1970, Shanghai Switch Factory successfully developed China's first 110 kV sulfur hexafluoride gas-insulated switchgear, which was put into use at Danjiangkou Power Station in Hubei in 1973 and has been operating normally for over 20 years. In 1984, Pingdingshan High Voltage Switch Factory in Henan successfully developed 550 kV sulfur hexafluoride gas-insulated switchgear, which quickly entered the international market. Currently, major switch factories in China are introducing or forming joint ventures with Japanese, European, and North American companies to produce sulfur hexafluoride electrical appliances, not only promoting the rapid development of China's power industry but also bringing opportunities for the rapid development of China's sulfur hexafluoride industry. Therefore, sulfur hexafluoride special gas has a very promising future!

  Author's Biography: Yang Yan, Information Director of CNNC Honghua Special Gas Co., Ltd., MBA graduate, Associate Senior Researcher, Council Member of the China Nuclear Information Society, Member of the Book and Information Professional Title Evaluation Committee of the China National Nuclear Corporation, and Member of the Information Professional Committee of the Sichuan Computer Society.

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