A plasma jet reactor is used in place of the conventional microwave reactor to speed up artificial diamond synthesis by 10-100 times. Diamond layers are used for wear-resistant coatings, infrared cameras, drill bits and laser diodes.
For the past 20 years scientists have successfully been making thin films of ‘artificial’ diamond in the laboratory by a process called chemical vapour deposition (CVD). Applications for such material include tough coatings for drill bits, windows for infrared cameras such as those used to detect survivors buried in rubble after earthquakes and heat dissipators for high power electronic components like laser diodes.
Now, Professor Mike Ashfold and his team in the School of Chemistry at the University of Bristol and De Beers Industrial Diamonds, are unravelling some of the complex chemistry involved in the rapid growth of diamond films.
Microwave Production of Synthetic Diamonds
Diamond is a form of carbon and in nature it is usually found as single crystals. However, CVD diamond is ‘polycrystalline’ – it consists of many very small crystals fused together. One way of making polycrystalline diamond in the laboratory is to place a mixture of methane (which contains the carbon) and hydrogen in a microwave reactor. The microwave energy splits the hydrogen into highly reactive atoms which, in turn, react with the methane, generating reactive carbon-containing molecular fragments called radicals. This gas mixture, which also includes many electrically charged or ionised species is called a plasma. The carbon-containing radicals alight on the material that is to be coated and given the right conditions, the carbon atoms form the appropriate chemical bonds to create crystalline diamond.
Using A Plasma Jet Reactor to Increase Deposition Rates
“In this way it takes about an hour to make a layer of diamond a micrometre thick,” says Professor Mike Ashfold. “For making wear-resistant coatings, one might typically want a layer tens of micrometres thick; for a window on an infrared camera millimetre-thicknesses would be preferable.” One way to accelerate the process is to use a device called a plasma jet reactor. A jet of argon plasma is passed at high velocity through an aperture of about 2 millimetres in diameter. Hydrogen and methane are added. The system produces a very high velocity jet of plasma at very high temperature. The plasma jet offers a brute force approach: the gas pressures and flow rates are higher, and the power input is larger, resulting in diamond film growth at rates 10-100 times greater than with the microwave reactor. However, the crystalline quality is generally less good.
Improving The Process
Now, using sophisticated spectroscopic techniques the team has been able to map the concentrations of various of the key carbon-containing species within the plasma jet in order to gain a better understanding of the process, and thereby optimise the experimental conditions needed to produce higher quality diamond layers at many times the rate of conventional systems.