Aggregated diamond nanotubes (ADNR) are the hardest and least compressible material known, with a mass modulus of 491 GPa. They were first synthesized in 2005 and consist of highly compressed and interconnected carbon nanotubes. ADNR can scratch both diamond and ultrahard fullerite, and are denser than diamond. They have potential to replace diamonds as an industrial abrasive and tooltip material.
The answer to this question depends on how you define “strong,” but the hardest, or least compressible, material known as of this writing (December 2007) is aggregated diamond nanotubes (ADNR), an allotrope (variety ) carbon consisting of highly compressed and interconnected nanotubes. Aggregate diamond nanotubes have a mass modulus, or measure of hardness, of 491 gigapascals (GPa), whereas conventional diamond only has a modulus of 442 GPa. Aggregated diamond nanotubes can scratch both diamond and ultrahard fullerite, another carbon allotrope that was the previous hardness record holder.
Aggregated diamond nanotubes were first synthesized by physicists in 2005 at the University of Bayreuth in Germany. The team, led by Natalia Dubrovinskaia, used a custom-designed 5,000-ton (5 million kilograms) anvil press on a sample consisting of conventional fullerenes (also known as buckyballs, element C60). By compressing these buckyballs and heating them to 2500 degrees Kelvin, this new carbon allotrope could be created. The material consists of carbon nanotubes with diameters between 5 and 20 nanometers and lengths of about one micrometer each.
The physical appearance of the aggregated diamond nanotubes is similar to that of a metal which scatters different colors of light, giving it a slightly rainbow-like surface. That it appears as a metal is unusual because the other allotropes of carbon (soot, graphite, diamond, etc.) rarely do, except perhaps graphite.
Aggregate diamond nanotubes are also denser than diamond by a factor of 0.2-0.4%, making them the densest form of carbon known. A contributing factor to the material’s hardness is thought to be the random orientation of the nanotubes that make it up. Because the physical structure of nanotubes is a fine mesh, like Kevlar®, the material is also unbreakable, unlike diamond.
Tests have shown that using diamond nanotube tipped aggregate tools to machine steel results in a tool piece that wears slower than diamond and allows for higher accuracy. When it becomes economically feasible to mass-produce aggregated diamond nanotubes, they can effectively replace diamonds as an industrial abrasive and tooltip material.
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