Artificial diamonds for industrialists

Simple Science

ROGER HANSON
Last updated 08:34 11/12/2012

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OPINION: On November 14 the Archduke Joseph diamond, a 76.02 carat (15 gram), colourless gem, was sold for $26 million by the auction house Christies in New York, a world record price per carat for a colourless diamond.

The combination of beauty and rarity makes diamonds desirable and expensive, but it isn't beauty alone that makes diamonds special. Diamond has the highest heat conductivity, the greatest clarity and is the hardest material known. These properties mean that diamond has some very useful industrial applications. For decades researchers have been trying to mass-produce so-called industrial diamonds, so that this unique material can be made more widely available.

Part of what follows summarises a PBS documentary, Super Carbon, reporting on progress towards making industrial diamonds.

The first step in making diamonds is to examine how nature does it. Diamonds are made of carbon and their toughness stems from the way the carbon atoms are bonded in a single strong network, a crystal, in which each carbon atom is bonded to four others. Natural diamonds are forged 200 kilometres below the surface of Earth in the mantle where the temperature is 1500 degrees Celsius and the pressure is thousands of times that on the surface. The diamonds are transported by molten volcanic rock, which snakes its way to the surface where it erupts, and when it cools, leaves a rock mound containing diamonds.

The first approach to making industrial diamonds therefore was based on simulating these conditions. Constructing a device that will generate pressures of many thousands of times that at the Earth's surface is extremely difficult. It was achieved by the GE corporation in the United States who went on to make the first industrial diamonds in 1955. The quantities were tiny. Even so, there was an immediate application in providing diamond tips to drill bits used in the mining industry. The unrivalled heat conduction properties of diamond mean that the heat generated by drilling is quickly conducted away. This, along with their hardness, substantially prolongs the drill bit life.

The process of computing generates a lot of heat, but printed circuit boards used in computers are made of silicon which is a poor heat conductor. The heat has to be removed using bulky copper strips or the circuits cook. If circuit boards could be made from a thin layer of diamonds, the superb heat conduction properties of diamond would revolutionise computer design; but, industrial diamonds are very expensive.

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In South America and Central Africa, deposits of black diamonds called carbonada have been found. Unlike clear diamond, carbonada is made up of millions of separate diamond crystals tightly bonded together. This crystalline matrix makes carbonada even tougher than clear diamonds. Carbonada was formed in the vacuum of space and arrived on Earth as meteorites. The black is due to specs of mineral dust in space becoming entrained during the formation process.

Carbonada shows that diamonds can be synthesised in a vacuum as well as under huge pressures. This led researchers to develop a process called Chemical Vapour Decomposition (CVD) in which a seed diamond is placed in a chamber just below atmospheric pressure, the temperature is raised to 1500 degrees, and methane - a source of carbon atoms - is pumped into the vacuum chamber, followed by hydrogen gas. These gases are then energised by microwaves that cause the hydrogen and methane to react and release a cloud of carbon atoms which slowly deposit on the seed crystal, causing it to grow. By seeding a wafer, a thin layer of diamonds can be built up. Diamond wafers can be used in applications such as semi-conductors used in electronic equipment (TVs and mobile phones). The properties of diamond mean that the electronics in this equipment can be made extremely thin and is super tough. But, the cost is prohibitive.

Diamond has the greatest clarity of any bulk material. Diamonds sparkle because the light is reflected internally by the crystal but the properties of the light, uniquely in diamond, remain almost completely unaltered. This means that diamond windows would be super clear.

The goal of researchers is to find a way of cheaply mass-producing diamonds so that these applications can be realised.

An advantage of industrial diamonds is that they are flawless, whereas natural diamonds have areas of weakness called cleavage planes. When the flawless industrial diamonds can be made cheaply one wonders if that will affect the market for beautiful natural diamonds like the Archbishop Joseph.

- © Fairfax NZ News

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