Two chunks of the purest crystal of silicon ever grown are sitting in a lab in Australia. In the next few months, if all goes to plan, these chunks will become the roundest spheres in the world.
The man charged with grinding and polishing the two A$1 million (US$875,000) pieces of crystal is Achim Leistner, who came to Australia from Germany as a refugee more than 40 years ago. Today, Leistner is based at the Australian Centre for Precision Optics in Sydney, and he鈥檚 renowned as the world鈥檚 top creator of 鈥渟ilicon balls鈥�.
Leistner has made about 40 of them so far. Most have been created for weights and measures institutes around the world, where they are used as density and volume standards. The aim of this latest project is to create a based on the mass of the silicon atom, by estimating the number of atoms in the silicon sphere, then weighing it against the standard kilo. The purer the crystal 鈥� and the rounder the sphere 鈥� the more accurate that estimate will be.
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To create a pure crystal, a team in Russia converted natural silicon into a gas and then spun it in a centrifuge to isolate a single isotope, silicon-28. The silicon-28 was then sent to Berlin in Germany, where it was grown into a near-perfect crystal. Now two sections have been delivered to Leistner.
To turn them into 93-millimetre-diameter balls, he will grind the spheres with micrometre-sized particles of an abrasive agent, such as aluminium oxide. He鈥檒l coat his polishing tools with the same pitch that Isaac Newton used to make his first mirror 鈥� and which Leistner reckons has never been bettered. Then he will polish the balls with nano-sized particles of titanium dioxide, a process he invented. The polishing involves chemical as well as physical modification of the surface of the silicon. It is based on complex science, but there鈥檚 also an art to it, he says.
Other teams have created silicon balls, but the roundness of their spheres has been out of true by about 600 nanometres. Leistner can get that error down to about 35 nanometres. If his spheres were the size of Earth, the difference between the highest and lowest point would be only about 4 metres. He can make the balls so smooth that the bumps on the surface are only the size of a single atom.
Of course, Leistner doesn鈥檛 achieve this alone. Colleagues at CSIRO, Australia鈥檚 national research organisation, have developed new techniques for imaging and measuring the balls, to show him exactly which regions need more attention.
Might it be possible to beat a roundness error of 35 nanometres? For this new project, Leistner will try. In six months, he hopes the world鈥檚 roundest objects will be in his lab.
