Monday, 31 December 2012

Talk - A Diamond is Forever

The East Midlands Materials Society recently hosted a fascinating talk by Paul Butler-Smith entitled “A Diamond is Forever”

Paul, who has spent a long career in the diamond industry, began by pointing out the phenomenal success of the diamond marketing machine, as illustrated by the fact that the percentage of brides who received a diamond engagement ring had increased from 20% in the 1940’s to 80% in the 1990s

Diamonds can be natural (either mined directly from diamond containing rocks, or from their eroded alluvial deposits) or synthetic.

One of the reference documents used in the talk was a marketing analysis by Bain and Company - it can be found here and is a fascinating document that concisely presents a lot of information about the formation, processing and sales of natural and synthetic diamonds.

Some 95% of the value of natural diamond is used in jewellery, while industrial applications are served overwhelmingly by synthetic diamonds.

Whilst most people are aware of the harness of diamonds, the fact that they have a thermal conductivity five times greater than that of Copper is less well known. Indeed, the combination of Diamonds thermal and chemical stability, low thermal expansion and high optical transparency makes them ideal for a number of applications in laser optics.

Natural Diamonds are formed in geological structures known as “Kimberlitic pipes” in which magma from unusually deep in the earth’s mantle is forced up to the earths surface.

Schematic of a Kimberlitic Pipe

Synthetic Diamonds are produced by two main techniques, the High Pressure, High Temperature Process (HPHT) - in which a small seed diamond is surrounded by graphite and catalysts and then subjected to temperatures of up to 2500C and pressures up to 60,000 atmospheres.

HPHT diamonds (note large size!)

and the Chemical Vapour Depositions Process (CVD) - in which diamonds can be grown by depositing a chemical vapour onto a prepared surface. This technique can produce diamonds that are single or multicrystalline and also discs and domes as well as particulate diamonds.

Cut CVD diamond - comparable in quality to natural gemstones

In both techniques, additives can be incorporated to change the colour and conductivity of the resulting diamonds. Using the CVD process in particular, it is possible to produce diamonds that are almost indistinguishable from natural stones.

Whilst the majority of synthetic diamonds are used for abrasive or tooling applications, there are a number of other niche areas where high quality synthetic diamonds can be used. You can see some of these at the website of the synthetic diamond manufacturer Element 6

One slightly strange development in the synthetic diamond market is the emergence of companies who will take the hair or ashes of a deceased relative (or pet) and then convert them to a diamond - and example of this being the company DNA2Diamonds.

Synthetic diamonds are becoming very price competitive with other abrasive materials - with the result that volumes have dramatically increased over recent years and applications have move into the mainstream. For example, a diamond coated grinding disc may be a routine purchase for a DIY’er today - but would have been a specialist item for industry only 20 years ago.

Paul presented data that showed how the ratio of natural to synthetic diamond production had changed over time:

1950 : Natural 20million carats, Synthetic 0 carats
1970 : Natural 30million carats, Synthetic 200million carats
1990 : Natural 40million carats, Synthetic 800million carats
2010 : Natural 50million carats, Synthetic 3000million carats

Continuing with a focus on synthetic diamonds, Paul described the work he was currently involved with in. This is looking at the laser ablation of diamond coated cutting micro tools to produce micro tools that can cut surfaces 5 times smoother than conventional diamond coated tools.

By a happy co-incidence, the Royal Institution Christmas Lectures mentioned carbon and diamond a number of times - in particular, the very high thermal conductivity of diamond was explained by the analogy of a bunch of people staning in a line with their arms on the shoulders of the person in front.

If the arms were loose and relexed then pushing the person at the back of the queue did not affect the person at the front because the energy was absorbed the the loose arms along the line.

In contrast, if the people held their arms out in front rigidly and held on tightly to the person in front then a push to the back of the queue was transmitted all the way through to the front by the rigid arms (analagous to the strong carbon-carbon bonds in diamond).

Image sources
Kimerlitic Pipe, HPHT diamonds, CVD diamond

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