Liquid hydrogen

laboratory in 1955 in Brook Park, Ohio, in 1967 tank for liquid hydrogen at the Museum Autovision in Altlußheim, Germany, in 2008 placards indicating the presence of hazardous materials, which are used with liquid hydrogen In 1885, Zygmunt Florenty Wróblewski published hydrogen's critical temperature as ; critical pressure, ; and boiling point, . Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929. ==Spin isomers of hydrogen==

The two nuclei in a dihydrogen molecule can have two different spin states. Parahydrogen, in which the two nuclear spins are antiparallel, is more stable than orthohydrogen, in which the two are parallel. At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is only metastable when liquefied at low temperature. It slowly undergoes an exothermic reaction to become the para isomer, with enough energy released as heat to cause some of the liquid to boil. ==Uses==

Liquid hydrogen is a common liquid rocket fuel for rocketry application and is used by NASA and the U.S. Air Force, which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons). In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer, usually liquid oxygen, and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2–O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can increase specific impulse, despite the incomplete combustion. Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. The MF Hydra ferry is powered by liquid hydrogen. Various submarines, including the Type 212 submarine, Type 214 submarine, and others, and concept hydrogen vehicles have been built using this form of hydrogen, such as the DeepC, BMW H2R, and others. Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). Liquid hydrogen is being investigated as a zero carbon fuel for aircraft. Because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses. Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in many bubble chamber experiments. The first thermonuclear bomb, Ivy Mike, used liquid deuterium, also known as hydrogen-2, for nuclear fusion. ==Properties==

The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done. Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes to global warming (to a lesser extent than CO2). Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel. The density of liquid hydrogen is only 70.85 kg/m3 (at 20 K), a relative density of just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetric energy density, many fold lower. Liquid hydrogen requires cryogenic storage technology such as special thermally insulated containers and requires special handling common to all cryogenic fuels. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day ==Safety==

Due to its cold temperatures, liquid hydrogen is a hazard for cold burns. Hydrogen itself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures. An indirect safety risk exists due to the cryogenic temperature being lower than the boiling point of oxygen. Exposure of insufficiently thermally insulated liquid hydrogen containers can result in air condensing on the outside of the container, leading to oxygen enrichment that can spontaneously ignite flammable materials. ==See also==