Серы гексафторид

Описание

Sulfur hexafluoride is a colorless, odorless, nontoxic, nonflammable gas that has a high dielectric strength and serves widely as an insulating gas in electrical equipment. At atmospheric pressures it sublimes directly from the solid to the gas phase and does not have a stable liquid phase unless under a pressure of more than 32 psia (221 kPa, abs). It is shipped as a liquefied compressed gas at its vapor pressure of 298 psig at 70°F (2050 kPa at 21.1°C).
One of the most chemically inert gases known, it is completely stable in the presence of most materials to temperatures of about 400°F (204°C) and has shown no breakdown or reaction in quartz at 900°F (482°C). Sulfur hexafluoride is slightly soluble in water and oil. No change in pH occurs when distilled water is saturated with sulfur hexafluoride.

Химические свойства

Chemical properties of sulfur hexafluoride are very stable. And compared to selenium hexafluoride, the hydrolysis rate of sulfur hexafluoride is extremely low, this is due to the small radius sulfur atom, which resulting in six fluorine atoms form a larger steric hindrance around. However, the fluorine atom radius is not big, so the repulsive force between the six fluorine atoms is not too large, S-F bond is not easy to dissociate. Enthalpy of formation of sulfur hexafluoride is-1220 kJ/mol, but enthalpy of formation of sulfur hexafluoride is-74 kJ/mol. Thus, the radius of fluorine atom and sulfur atom radius cause the very stable of sulfur hexafluoride molecule together---the molecules themselves are difficult to disconnect bond and break down and the attack group is difficult to approach to the central atom, in the thermodynamics and kinetics, they are both stable. Studies have said sulfur hexafluoride can be stably present in the atmosphere for thousands of years.

Физические свойства

Colorless, odorless gas; density 6.41 g/L; about five times heavier than air; liquefies at -50.7°C (triple point); density of liquid 1.88 g/mL at -50.7°C; sublimes at -63.8°C; critical temperature 45.54°C; critical pressure 37.13 atm; critical volume 199 cm3/mol; slightly soluble in water; soluble in ethanol.

Использование

Sulfur hexafluoride is used extensively as a gaseous dielectric in various kinds of electrical power equipment, such as switchgear, transformers, condensers, and circuit breakers. It has also been used as a dielectric at microwave frequencies and as an insulating medium for the power supplies of high-voltage machines.
Sulfur hexafluoride is also gaining use in nonelectrical applications, including blanketing of molten magnesium, leak detection, and plasma etching in the semiconductor industry. Sulfur hexafluoride also has some limited medical applications.

Подготовка

Sulfur hexachloride may be prepared by reacting fluorine with sulfur or sulfur dioxide.

Профиль реактивности

This substance undergoes chemical reactions only under relatively severe circumstances. They are resistant to ignition, although they may become flammable at very high temperatures. They may be resistant to oxidation reduction, except in the most severe conditions. These materials may be nontoxic. They can asphyxiate. Contact of very cold liquefied gas with water may result in vigorous or violent boiling of the product and extremely rapid vaporization due to the large temperature differences involved. If the water is hot, there is the possibility that a liquid "superheat" explosion may occur. Pressures may build to dangerous levels if liquid gas contacts water in a closed container [Handling Chemicals Safely 1980].

Опасность

Asphyxiant.

Угроза здоровью

Vapors may cause dizziness or asphyxiation without warning. Vapors from liquefied gas are initially heavier than air and spread along ground. Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite. Fire may produce irritating, corrosive and/or toxic gases.

Пожароопасность

Some may burn but none ignite readily. Containers may explode when heated. Ruptured cylinders may rocket.

Использование в материалах

Sulfur hexafluoride is noncorrosive to all metals. It may be partially decomposed if subjected to an electrical discharge. Some of the breakdown products are corrosive; this corrosion is enhanced by the presence of moisture or at high temperature. Sulfur hexafluoride decomposes very slightly in the presence of certain metals at temperatures in excess of 400°F (204°C); this effect is most pronounced with silicon and carbon steels. Such breakdown, presumably catalyzed by the metals, is only several tenths of 1 percent over 1 year. Decomposition at elevated temperatures does not occur with aluminum, copper, brass, and silver.
Most common gasket materials, including Teflon, neoprene, and natural rubber are suitable for sulfur hexafluoride service.

Профиль безопасности

This material is chemically inert in the pure state and is considered to be physiologcally inert as well. However, as it is ordinarily obtainable, it can contain variable quantities of the lowsulfur fluorides. Some of these are toxic, very reactive chemically, and corrosive in nature. These materials can hydrolyze on contact with water to yield hydrogen fluoride, which is highly toxic and very corrosive. In high concentrations and when pure it may act as a simple asphyxiant. Incompatible with disilane. Vigorous reaction with disilane. May explode. When heated to decomposition emits highly toxic fumes of Fand SOx.

Возможный контакт

May contain highly toxic sulfur pentafluoride as an impurity. SF6 is used in various electric power applications as a gaseous dielectric or insulator. The most extensive use is in high-voltage transformers. SF6 is also used in waveguides, linear particle accelerators; Van de Graaff generators; chemically pumped continuous-wave lasers; transmission lines; and power distribution substations. Nonelectrical applications include use as a protective atmosphere for casting of magnesium alloys and use as a leak detector or in tracing moving air masses. Several sources note that vitreous substitution of SF6 in owl monkeys results in a greater ocular vascular permeability than that caused by saline. This implies that SF6 could have an important use in retinal surgery.

хранилище

All ofthe precautions necessary for the handling of any nonflammable gas must be taken.

Перевозки

UN1080 Sulfur hexafluoride, Hazard Class: 2.2; Labels: 2.2-Nonflammable compressed gas. Cylinders must be transported in a secure upright position, in a wellventilated truck. Protect cylinder and labels from physical damage. The owner of the compressed gas cylinder is the only entity allowed by federal law (49CFR) to transport and refill them. It is a violation of transportation regulations to refill compressed gas cylinders without the express written permission of the owner.

Несовместимости

May contain impurities that cause it to hydrolyze on contact with water, forming corrosive and toxic hydrogen fluoride. Vigorous reaction with disilane.

Утилизация отходов

Return refillable compressed gas cylinders to supplier. Seal unused cylinders and return to suppliers.