Nitrogen trifluoride

Description

Nitrogen trifluoride is a colorless gas with little odor. Nitrogen trifluoride is an oxidizer that is thennodynamically stable except at elevated temperatures. At temperatures up to about 482°F (250°C), its reactivity is comparable to oxygen. At higher temperatures, its reactivity is similar to fluorine owing to appreciable dissociation into NF2 and F-. The thennal dissociation of nitrogen trifluoride has been studied by a number of investigators and has been found to peak in the temperature range of 1100K to 1500K. In handling nitrogen trifluoride, conditions should be avoided that can result in high temperatures such as adiabatic compression from the rapid pressurization of a system.

Nitrogen trifluoride acts primarily upon the elements as a fluorinating agent, but not a very active one at lower temperatures. At elevated temperatures, nitrogen trifluoride pyrolyzes with many of the elements to produce nitrogen tetrafluoride and the corresponding fluoride. The pyrolysis of nitrogen trifluoride over copper turnings produces nitrogen tetrafluoride in a 62 percent to 71 percent yield at 707°F (375°C). Pyrolysis over carbon is more complete.

Chemical Properties

Nitrogen trifluoride is a colorless gas. Moldy odor. Shipped as a nonliquefied compressed gas.

Physical properties

Colorless gas; moldy odor; liquefies at -128.75°C; density of liquid 3.116 g/mL; vapor pressure at -158°C 96 torr; solidifies at -206.8°C; critical temperature -39.15°C; critical pressure 44.02 atm; critical volume 126 cm³/mol; very slightly soluble in water.

Uses

Nitrogen trifluoride is an etchant and chamber cleaning agent.
Oxidizer for high-energy fuels, chemical synthesis.

Preparation

Nitrogen trifluoride is prepared by electrolysis of either molten ammonium fluoride, NH₄F, or melted ammonium acid fluoride, NH₄HF₂ (or ammonium fluoride in anhydrous HF). While the NH₄F method is preferred because it forms nitrogen trifluoride as the only product, electrolysis of ammonium acid fluoride yields a small amount of dinitrogen difluoride, N₂F₂,and NF₃.
Also, nitrogen trifluoride can be prepared by reaction of ammonia with fluorine diluted with nitrogen in a reactor packed with copper. Other nitrogen fluorides, such as N₂F₂, N₂F₄, and NHF₂ also are produced. The yield of major product depends on fluorine/ammonia ratio and other conditions.

Production Methods

Nitrogen trifluoride can be formed from a wide variety of chemical reactions. The commercial process for production involves direct fluorination of ammonia with fluorine gas in the presence ofammonium fluoride.

Reactions

Hydrogen reacts with nitrogen trifluoride with the rapid liberation of large amounts of heat and is the basis for the use of nitrogen trifluoride in high-energy chemical lasers. The flammability range for nitrogen trifluoride-hydrogen mixtures is 9.4 mole percent to 95 mole percent nitrogen trifluoride. Nitrogen trifluoride reacts with organic compounds, but generally an elevated temperature is required to initiate the reaction. Under these conditions, the reaction will often proceed explosively, and great care must be exercised when exposing nitrogen trifluoride to organic compounds. Therefore, nitrogen trifluoride has found little use as a fluorinating agent for organic compounds.

General Description

A colorless gas with a moldy odor. Very toxic by inhalation. Slightly soluble in water. Corrosive to tissue. Under prolonged exposure to fire or heat the containers may rupture violently and rocket. Used to make other chemicals and as a component of rocket fuels.

Air & Water Reactions

Slightly soluble in water.

Reactivity Profile

Nitrogen trifluoride is a very powerful oxidizing agent. Presents dangerous fire hazard in the presence of reducing agents. Etches glass in the presence of moisture. Emits toxic and corrosive fumes of fluoride when heated to decomposition [Lewis, 3rd ed., 1993, p. 937]. Can react violently with hydrogen, ammonia, carbon monoxide, diborane, hydrogen sulfide, methane, tetrafluorohydrazine, charcoal. Explosive reaction with chlorine dioxide. A severe explosion may occur when exposed to reducing agents under pressure [Bretherick, 5th ed., 1995, p. 1427].

Hazard

Severe explosion hazard. Corrosive to tissue. Methemoglobinemia, liver and kidney damage.

Health Hazard

Inhaling nitrogen trifluoride can reduce the capacity of red blood cells to carry oxygen. This causes cyanosis, or a bluish discoloration of the skin. Breathing nitrogen trifluoride can also lead to headache, dizziness, weakness and confusion. After prolonged exposure to high concentrations, breakdown of red blood cells and changes in the liver, kidneys, spleen and heart muscle may occur as secondary effects. In fresh air, the initial red blood cell changes will clear over several hours, but the person should still be monitored for secondary effects.

Fire Hazard

Substance does not burn but will support combustion. Some may react explosively with fuels. May ignite combustibles (wood, paper, oil, clothing, etc.). Vapors from liquefied gas are initially heavier than air and spread along ground. Runoff may create fire or explosion hazard. Containers may explode when heated. Ruptured cylinders may rocket.

Industrial uses

Nitrogen trifluoride has been used successfully in large quantities as a fluorine source for high-energy chemical lasers. It is preferred over fluorine because of its comparative ease of handling at ambient conditions.
Recently, an increasing amount of nitrogen trifluoride is being used in the semiconductor industry as a dry etchant, showing significantly higher etch rates and selectivities when compared to carbon tetrafluoride and mixtures of carbon tetrafluoride and oxygen.
Nitrogen trifluoride was also used as an oxidizer in rocketry in the early 1960s, but this application was not commercialized.

Materials Uses

At temperatures less than 482°F (250°C), nitrogen trifluoride has a reactivity similar to that of oxygen and is relatively inert to most materials of construction. At ambient temperatures, brass, aluminum, copper, steel, and stainless steels can be used because corrosion rates of less than 0.1 mil/yr. at 160°F (71.1°C) have been determined for these materials. Nitrogen trifluoride is also compatible with fluorinated materials such as Teflon at ambient temperatures. At increased temperatures and pressures, nitrogen trifluoride's reactivity increases becoming more like that of fluorine, with nickel and Monel being the preferred materials of construction.

Safety Profile

A poison. Mildly toxic by inhalation. Prolonged absorption may cause mottling of teeth, skeletal changes. Severe explosion hazard by chemical reaction with reducing agents, particularly when under pressure. A very dangerous fire hazard; a very powerful oxidner; otherwise inert at normal temperatures and pressures.

Potential Exposure

This material has been used in chemical synthesis and as an oxidizer for high-energy fuels (as an oxidizer in rocket propellant combinations).

storage

Nitrogen trifluoride cylinders must be securely supported while in use to prevent movement and straining of connections. Full cylinders must be stored in a well-ventilated area, protected from excessive heat (125°F or 51.7°C), located away from organic or flammable materials, and secured. Valve protection caps and valve outlet caps must be securely in place at all times when the cylinder is not in use.

Shipping

UN2451 Nitrogen trifluoride, Hazard Class: 2.2; Labels: 2.2-Nonflammable compressed gas; 5.1-Oxidizer. Cylinders must be transported in a secure upright position, in a well-ventilated 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.

Incompatibilities

The gas is a powerful oxidizer. Presents dangerous fire hazard in the presence of reducing agents. Etches glass in the presence of moisture. Reacts with oil, grease, reducing agents and other oxidizable materials; combustibles, organics, ammonia, carbon monoxide; methane, hydrogen, hydrogen sulfide; activated charcoal; diborane, water. Can react violently with hydrogen, ammonia, carbon monoxide, diborane, hydrogen sulfide, methane, tetrafluorohydrazine, charcoal. Nitrogen trifluoride will increase intensity of an existing fire.

Waste Disposal

Return refillable compressed gas cylinders to supplier. Vent into large volume of concentrated reducing agent (bisulfites, ferrous salts or hypo) solution, then neutralize and flush to sewer with large volumes of water.

Raw materials

Preparation Products