A patent on a material harder than diamond has been awarded to Marvin Cohen and Eugene Haller, physicists in LBL's Materials Sciences Division (MSD), and William Hansen of the Engineering Division.
The new material, a crystalline compound of carbon and nitrogen, confirms the predictions of a theoretical model developed in the 1980s by Cohen.
Hard materials are much valued in industry for use in cutting tools and abrasives, and as wear-resistant protective coatings. Diamond is the standard by which all other materials are judged and has long been considered the ultimate measure of hardness. The new superhard carbon- nitrogen compound could serve as an inexpensive substitute for diamond, or it could be used to carve diamonds into intricate shapes for use as semiconductors in electronic devices. Diamonds can be operated at higher temperatures than can silicon and are more resistant to radiation damage.
The new material was created in the laboratory by Haller and Hansen, based on Cohen's calculations. It was made in a manner similar to the way in which industrial diamonds are synthesized. A plasma containing carbon and nitrogen ions was created and then deposited as a thin film over the surface of a silicon wafer. This produced a lot of what Haller chacterizes as "polymeric goop," but embedded within the goop were tiny (about half a micron) crystals made up of three carbon atoms and four atoms of nitrogen (C3N4).
Says Haller, "The trick is to get the carbon and nitrogen ions to recombine so that their bond lengths are quite short and the stiffness of the bonds (the energy it takes to break them) is as high as that of diamond. To do this, we created a condition under which it was favorable for the carbon and nitrogen atoms to sit in the proper position."
Haller credits the theoretical investigations of Cohen and his then graduate student, Amy Liu, with pointing the way for the efforts of his experimental team.
"The work of Cohen and Liu showed that if you put carbon and nitrogen atoms into a silicon nitride configuration, you get a stable structure. Nobody would have expected this to be the case," Haller says.
It was to learn whether materials harder than diamond were even possible that Cohen and Liu began their investigations. To find a good candidate, they first screened a number of hypothetical crystals using a relatively simple empirical model devised by Cohen. The model predicts the "bulk modulus" of a solid from the lengths of its chemical bonds. Bulk modulus is the inverse of compressibility.
Says Cohen, "The hardness of a material depends on many factors, including sample defects. However, for an ideal crystal, hardness is generally proportional to bulk modulus, which, in turn, is dependent upon the properties of the chemical bonds between atoms in the material. Low compressibilities are achieved with short bonds and low ionicity."
Cohen and Amy Liu determined that a carbon-nitrogen compound, despite being partially ionic, would have a bond length short enough to give the material a higher bulk modulus than diamond. Next, using another model that Cohen developed, they performed a quantum-mechanical "first- principles" computer calculation (a calculation based only on known values for constituent atoms, such as atomic number or atomic weight) to determine what structure this superhard hypothetical compound would require.
As a prototype, Cohen chose silicon-nitride, a solid material that features a hexagonally arranged "beta phase" crystal, and substituted carbon for silicon in the crystal structure. Calculations showed that the carbon- nitrogen compound would also assume a beta-phase crystal structure and would have a bulk modulus of 4.3 megabars, compared to the 4.4 megabar bulk modulus of diamond. A megabar is one million times atmospheric pressure at sea level.
In testing this theoretical prediction, Haller and Hansen deposited thin films of carbon and nitrogen on germanium as well as silicon wafers. Carbon-nitride crystals were produced on both, but only the crystals on the silicon were in the predicted beta-phase. Crystals on the germanium appeared in the alpha-phase, a structure similar to the beta phase but with much longer chemical bonds.
Crystal structures were determined by MSD staff scientist Kin Man Yu and In Chin Wu, a graduate student of Haller's, through analyses of both x-ray and electron diffraction patterns, and through transmission electron micrographs. The recorded patterns matched the predicted patterns of the computer model.