Monthly Archives: December 2014

Diamond Types and Colours

Diamonds can be classified as one of four types (Type Ia, Type Ib, Type IIa and Type IIb) according to the impurities that they contain. These impurities lend colour to the diamond, (though this colour can be artificially enhanced, for example by treating with ionising radiation).

diamond-colours

Type Ia diamonds are most common, making up about 98% of all natural diamonds. They contain small concentrations (0.3%) of nitrogen impurities. Type IaA diamonds contain pairs of nitrogen atoms, and are therefore colourless, and Type IaB diamonds contain large “clumps” of nitrogen atoms and absorb blue light, giving them a pale yellow/brown colour depending on the concentration of nitrogen.

Type Ib diamonds make up about 0.1% of natural diamonds, and contain a much lower concentration (0.05%) of nitrogen impurities, with the impurities clustered rather than widespread as in Type Ia diamonds. They absorb both blue and some green light, giving them a stronger yellow or brown colour. Synthetic diamonds are generally of Type Ib.

Type IIa diamonds (2% of natural diamonds) contain almost no impurities at all, and are therefore usually colourless.

Type IIb diamonds (0.1% of natural diamonds) contain almost no nitrogen impurities, but do contain significant quantities of boron which yields a light blue or grey colour. They are among the most expensive gem diamonds due to their rarity.

During the formation of any type of diamond a deformation of the lattice of carbon atoms that makes up the diamond can occur, which can lead to a wide range of colourings: red, pink, orange, yellow, brown and purple. This colouring can be removed by treating the diamond with high temperatures and pressures to remove the deformation, but this is often not done to retain the “fancy” colouring, which can demand the highest prices.

The Vacuum Airship

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Archimedes’ Principle states that the (upthrust) force on an object that is displacing a fluid is equal to the weight of the fluid displaced. For example, a cube with sides of one metre, fully submerged in water, will experience an upward force of 9810 newtons, as this is the weight of one cubic metre of water. The material that the object is made of has no effect on this force, so if the object weighs more than 9810 newtons it will sink, and if it weighs less than 9810 newtons it will float.

The upthrust force on an object therefore depends only on the relative densities of the object and the fluid it is displacing. A bigger difference means a bigger force.

Hydrogen is the least dense gas, at 0.0898 kilograms per metre cubed, but hydrogen is rarely used in airships as it is highly flammable and therefore dangerous. Hydrogen was used in the Hindenburg because  helium was difficult to produce and the United States, the only country with significant reserves, had banned its export. Helium has a density of 0.179 kg/m3 and therefore produces only 93% of the lift of hydrogen, but it is far, far safer.

vaccum-airship

Ideally, to create the maximum upthrust force, we would want our balloon or airship’s envelope to be filled with something with the lowest possible density. The lowest possible density would be a vacuum, the total absence of anything, which would create a lifting force of 12.7 newtons per cubic metre (as opposed to 11.0 N/m3 for helium).

The problem with using a vacuum to lift an airship, is how to contain the vacuum. If a difference in pressure exists between two regions, then a difference in force exists between those two regions. In the case of a vacuum airship being used on Earth’s surface, that force would be 101325 newtons per square metre, the equivalent of more than ten tonnes pushing down on every square metre. No material on Earth is strong enough to withstand this force without being so heavy as to negate the point of the vacuum lifting effect in the first place. In order to still have lifting capability and withstand the stresses involved, we can calculate the minimum required ratio of Young’s modulus to density, and this yields a figure of around 450000 Pa/(kg/m3)2. Unfortunately, even the strongest materials, like diamond, have ratios that are only one-fifth of this, so it looks like we won’t be creating vacuum airships any time soon.