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Physics student makes sparkling discovery with diamond substitute

It was a long-held belief that cotunnite, the highest-known pressure phase of the mineral zirconia, was mechanically hard enough to be used for industrial abrasives, and therefore could possibly replace expensive diamond. It turns out, that is not the case.

The Diamond-Anvil Cell, or DAC, is a device that allows very small materials, such as zirconia, to undergo extreme pressure between two diamonds. (Photo courtesy Yahya Al-Khatatbeh and Stefanie Japel)

New Mexico State University Physics Ph.D. student Yahya Al-Khatatbeh found the material has a low mechanical hardness and does not qualify as superhard?a property needed when being used for industrial purposes such as drill bits, saws or other cutting tools. His discovery was published in Physical Review B, an American Physical Society journal, earlier this month.

Al-Khatatbeh explains in a synopsis of his research that it has long been speculated that materials synthesized at high pressures provide a promising route to develop materials that can replace diamond as an industrial abrasive. Zirconia (ZrO2) is one of these materials. It possesses a very dense high-pressure phase called cotunnite.

Through experiments and theoretical computations, Al-Khatatbeh found that, against common belief, hardness does not necessarily increase in high-pressure phases. The relationship between hardness and incompressibility is more complicated than previously thought.

"From a practical point of view, this suggests that scientists and engineers must look for other materials, most likely non-oxides, to replace diamond as an abrasive," said Boris Kiefer, physics professor and Al-Khatatbeh's co-advisor. "One of the major goals of material sciences is to design materials with desired properties. An example is the search for design criteria on how to make superhard materials."

Al-Khatatbeh said zirconia's initial structure, its phase at normal conditions, is actually better for mechanical applications because of its hardness. Future testing will include a material called hafnia, which should have properties close to that of zirconia.

Testing the zirconia, compressing it to its cotunnite phase, takes an instrument called a Diamond-Anvil Cell, or DAC. It is a hand-held compression device that allows sub-millimeter materials to undergo extreme pressure between two diamonds. In his experiments, Al-Khatatbeh used diamonds 0.2 to 0.3 millimeters in surface diameter.

"Yahya's work highlights often what can happen to our intuition when we try to predict behavior of materials at extreme conditions. It can fail," said Kanani Lee, Al-Khatatbeh's primary advisor who now works in the Department of Geology and Geophysics at Yale University. She has been advising his experimental work remotely for the past two years. Al-Khatatbeh received financial support from research grants administered through Yale as well as NMSU.

He presented his findings at the annual American Geophysical Union meeting in San Francisco last December as well as at the APS meeting in March.

For more information on Al-Khatatbeh's discovery, his research paper can be viewed at http://prb.aps.org/abstract/PRB/v81/i21/e214102