·
Linear Formula NH2NH2 ·
xH2O
·
Molecular Weight 32.05 (anhydrous
basis)
Hydrazine is an inorganic compound with the chemical
formula N2H4 (also written H2NNH2).
A simple pnictogen hydride, it is a colorless flammable liquid
with an ammonia-like odor.
Hydrazine is highly toxic and dangerously unstable unless handled in solution.
Detailed description
As of
2000, approximately 120,000 tons of hydrazine hydrate (corresponding to a 64%
solution of hydrazine in water by weight) were manufactured worldwide per year.[9] Hydrazine
is mainly used as a foaming agent in
preparing polymer foams, but significant applications also include its
uses as a precursor to polymerization catalysts
and pharmaceuticals. Additionally,
hydrazine is used in various rocket fuels and to
prepare the gas precursors used in air bags. Hydrazine is
used within both nuclear and conventional electrical power plant steam
cycles as an oxygen scavenger to control concentrations of dissolved oxygen in
an effort to reduce corrosion. Anhydrous hydrazine is corrosive towards glass,
in a manner similar to hydrofluoric
acid.
Applications
Main uses
The majority use of hydrazine is as a precursor
to blowing agents. Specific compounds include azodicarbonamide and azobisisobutyronitrile,
which yield 100-200 mL of gas per gram of precursor. In a related
application, sodium azide, the gas-forming agent in air bags, is produced from
hydrazine by reaction with sodium nitrite.[9]
Hydrazine is also used as a propellant on board
space vehicles, and to both reduce the concentration of dissolved oxygen in and
control pH of water used in large industrial boilers. The F-16 fighter jet and U-2 Spy Plane use hydrazine
to fuel their emergency power units.[15]
Precursor to pesticides and pharmaceuticals
Hydrazine is a precursor to several pharmaceuticals
and pesticides. Often these applications involve conversion of hydrazine
to heterocyclic rings such as pyrazoles and pyridazines. Examples of commercialized bioactive
hydrazine derivatives include cefazolin, rizatriptan, anastrozole, fluconazole, metazachlor, metamitron, metribuzin, paclobutrazol, diclobutrazole, propiconazole, and triadimefon.[9]
Small-scale, niche, and historic uses
Rocket fuel
Hydrazine was first used during World War II as a component
in rocket fuel mixtures. A 30% mix by weight with 57% methanol (M-Stoff in German) and
13% water was called C-Stoff by the Germans.[16]The mixture was used
to power the Messerschmitt Me 163B rocket-powered fighter plane. Hydrazine was also used as a
propellant with the German high test peroxide T-Stoff oxidizer. Unmixed hydrazine was referred to as B-Stoff by the
Germans, a designation also used later for the methanol/water fuel for
the V-2 missile.
Hydrazine is used as a low-power monopropellant for the
maneuvering thrusters of spacecraft, and was used to power the Space Shuttle's auxiliary power
units (APUs). In addition, monopropellant hydrazine-fueled rocket engines are
often used in terminal descent of spacecraft. Such engines were used on
the Viking program landers in the 1970s as well as the Phoenix lander and Curiosity roverwhich landed on Mars
in May 2008 and August 2012, respectively.
In all hydrazine monopropellant engines, the
hydrazine is passed by a catalyst such as iridium metal
supported by high-surface-area alumina (aluminium
oxide) or carbon nanofibers,[17] or more
recently molybdenum nitride on alumina,[18] which causes
it to decompose into ammonia, nitrogen gas, and hydrogen gas according to the following
reactions:[19]
1.
3 N2H4 →
4 NH3 + N2
2.
N2H4 →
N2 + 2 H2
3.
4 NH3 +
N2H4 → 3 N2 + 8 H2
Reactions 1 and 2 are extremely exothermic (the catalyst
chamber can reach 800 °C in a matter of milliseconds,[17]) and they produce
large volumes of hot gas from a small volume of liquid,[18] making
hydrazine a fairly efficient thruster propellant with a vacuum specific impulse of about 220
seconds.[20] Reaction 3
is endothermic and so reduces the temperature of the products, but also
produces a greater number of molecules. The catalyst structure affects the
proportion of the NH3 that is dissociated in Reaction 3; a
higher temperature is desirable for rocket thrusters, while more molecules are
desirable when the reactions are intended to produce greater quantities of gas[citation
needed].
Other variants of hydrazine that are used as rocket
fuel are monomethylhydrazine,
(CH3)NH(NH2) (also known as MMH), and unsymmetrical
dimethylhydrazine, (CH3)2N(NH2)
(also known as UDMH). These derivatives are used in two-component rocket fuels,
often together with dinitrogen tetroxide,
N2O4. These reactions are extremely exothermic, and the
burning is also hypergolic (it starts burning without any external ignition).
There are ongoing efforts in the aerospace industry
to replace hydrazine and other highly toxic substances. Promising alternatives
include hydroxylammonium nitrate, 2-Dimethylaminoethylazide (DMAZ) and energetic ionic liquids.
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