Materials: A Very Short Introduction (Very Short Introductions) by Christopher Hall

Author:Christopher Hall [Hall, Christopher]
Language: eng
Format: epub, azw3, mobi
ISBN: 9780199672677
Publisher: Oxford University Press
Published: 2014-09-19T00:00:00+00:00

Burn and bang

There are materials whose job is to provide us with energy. That we can routinely raise a passenger aircraft (say 200 tons) 10,000 metres into the air and slide it across the Atlantic is testament to the energy density of jet fuel. The fuel, which is a refined hydrocarbon liquid, burns in the turbofan engine to produce a hot gas of CO2 and water. This chemical reaction releases about 43 megajoules (MJ) of heat energy for each kilogram (kg) of fuel. Since the hydrocarbon molecules are the same in composition as the string molecules of polyethylene (PE), the specific energy of polyethylene plastic and aviation fuel are much the same. That heat is released if PE is caught up in a building fire. The specific energy of cellulose (and hence of wood) is only 17 MJ/kg, because the cellulose polymers already have a lot of oxygen in them, so are partially pre-oxidized. The specific energy of pure hydrogen is much higher, 140 MJ/kg. When it burns in air it produces only water, but of course we have to make the hydrogen in the first place, and storing a gas is difficult. The specific energy of other hydrocarbons is much the same. Natural gas is slightly lower, 37–40 MJ/kg, because it contains a few per cent of nitrogen and CO2 which contribute nothing to the combustion heat. Acetylene is a little higher, because the carbon‒carbon ‘triple bond’ packs some additional energy. Mixed with pure oxygen, it produces an intensely hot flame for welding and cutting steel.

In explosives, the aim is to have one big bang, and to achieve the greatest possible energy-release rate. Nearly all high explosives, civil and military, are organic substances with C, H, O, and N atoms, usually in ring structures. So the fuel and the oxidant are combined in the same molecule. These materials are chosen to be as stable as possible, except when deliberately detonated. Then, the explosive decomposes in milliseconds to generate CO2, water, and nitrogen gas. The specific energy of explosives such as TNT is only one-tenth that of jet fuel, but the rate of energy release is much higher.

At the other extreme are batteries, from which we want low power for a long time, combined with the ability to recharge. Lithium-ion batteries do both these things. The internal lithium chemistry has a high energy density, not much different from jet fuel, but the specific energy of the battery calculated from the weight of the packaged battery, rather than the active ingredients inside it, seems rather low, say 1 MJ/kg.

A nuclear fission reaction releases hugely more energy than any chemical process. A uranium dioxide fuel pellet has a specific energy about 70,000 times greater than jet fuel, but it is much more difficult to liberate it.

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