New Simple Method Used to Synthesize Amorphous Diamond for First Time

Earlier this year, a technique involving moderately high temperatures, high pressures, and a small amount of glassy carbon as starting material was used to synthesize amorphous diamond for the first time. A father-son team from Clemson University has now successfully calculated many basic physical properties for this new substance, including elastic constants and other related quantities. The study findings are reported this week in AIP Publishing’s Applied Physics Letters.

Diamond is a type of pure carbon where the atoms are positioned in a crystal lattice, with each carbon atom enclosed by four other carbons at the corners of a tetrahedron. The carbon-carbon bonds in diamond are sp3 bonds. The arrangement of tetrahedral structures is repeated over long distances in a diamond crystal, generates a solid material with high temperature stability. Therefore, diamond is a valuable gemstone as well as a material with many technological uses.

On the other hand, amorphous carbon has varying fractions of sp3-bonded carbon in an amorphous or disordered matrix. The amorphous structure generates extremely desirable mechanical properties. The level of bonding in amorphous carbon is not very as in pure diamond. The portion of the carbon-carbon bonds are of sp2-type, found in other carbon types such as graphite.

Sp3-bonded germanium and amorphous silicon have been known for many years and are commonly employed in transistors, thin film sensors and photovoltaics, and other high-tech applications. It is great interest, then, to identify ways to generate amorphous diamond that maintains a high fraction of sp3 bonds. Although the work that was reported earlier this year did just that, samples are not yet commonly available for testing. Initial test did reveal that these amorphous diamonds are rather dense, optically transparent and strong.

The father-son team, Arthur and John Ballato, have stepped into this knowledge gap in order to calculate some physical properties that are not yet measured for this new type of diamond. "We employed a modeling approach by which one can use the properties of crystalline diamond to deduce the properties of the glassy diamond analog," stated Ballato. "In this work, we inferred the elastic properties of this new phase of diamond from measured properties of crystalline diamond."

The process they used involves a computer model of a crystal that is computationally homogenized to make an amorphous version of the substance. The crystal model uses simple, classical physics and explains the carbon-carbon bonds as springs. The homogenization process used is called the Voigt-Reuss-Hill (VRH) technique.

The Ballatos computed many important bulk properties, including Poisson's ratio, Young's modulus, and other elastic constants for the substance by using this technique.  They used the VRH homogenization technique in earlier works to study glassy sapphire and other materials of interest for use in high power lasers. The VRH approach is more straightforward and simpler than sophisticated quantum mechanical techniques that are currently available, but the properties determined in this work can serve as a baseline, for more sophisticated, but costly modeling, and for future experimental measurements.

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