NotesThis sphere was made for the Avogadro project, an international effort to redefine the kilogram in terms of the Avogadro constant (a fundamental physical constant). Perfect spheres of single-crystal silicon (such as this) are produced and the exact volume and number of atoms in the sphere measured, allowing the value of the Avogadro constant to be very precisely measured. The sphere was chosen as the ideal shape because it has no corners or edges (to minimise chipping and wear), and because – provided a sufficiently perfect sphere can be manufactured – the volume can be calculated from a single measured parameter (the diameter). Silicon was chosen over other elements (such as carbon) as the material for the sphere, as there already exist mature processes for the production and manipulation of ultra-high purity silicon from the electronics industry.
The fabrication of the spheres is a multi-step process. To begin with an ingot or boule of single-crystal silicon is produced. This is 'grown' through a procedure known as the Float zone (FZ) process. From there, the production of the sphere is carried out in three main steps: (1) rough shaping using diamond tools and course loose abrasives, (2) fine grinding using loose abrasives down to a precision of 1 micrometre, and (3) polishing and finishing. The boules are manufactured in Germany and Japan, while the spheres are produced (through the grinding and polishing procedure described above) at CSIRO in Australia.
There are several parameters and factors which need to be carefully measured or controlled during the production. This includes: roundness characterisation, surface impurities, diameter measurement, mass measurement, molar mass, lattice parameter, and crystal structure. Achieving sufficient roundness, and characterising any imperfections are critical if the volume is to be derived directly from the diameter. The diameter of the sphere is measured using a technique called optical interferometry and the mass was measured against a 1 kg stainless steel working standard, using a high precision balance. The molar mass of the silicon sphere is determined using a technique called mass spectrometry, and the lattice parameters were measured using X-ray crystal diffraction.
There are 7 main laboratories around the world involved in the Avogadro project and in the measurement of the various parameters listed above. This includes IRMM (Belgium), PTB (Germany), IMGC (Italy), NRLM (Japan) NIST (USA), NPL (UK), and CSIRO/NML (Australia). Some parameters are measured and verified by all 7 laboratories, while others (e.g. the molar mass and lattice parameters) are measured by just one or two (depending on their capabilities) and shared with the other organisations in the project. Australia and the CSIRO, however, have played a critical role, as CSIRO is currently the only organisation which manufactures the spheres used in the project.
This sphere was made by Achim Leistner at CSIRO. Leistner has made his career in crafting precision optics, and was the Master Optician for the Avogadro project. Leistner's fabrication method involves precision handcrafting, to ensure minimisation of any irregularities. In addition to scientific instruments he uses his hands to feel for any imperfections in the surface of the spheres, and his fabrication method has been found to be superior to any purely machine-based method.
This sphere is one of the prototypes produced in the development and testing of the methodology. It was considered the very first meaningful sphere produced for the project, in terms of its precision in roundness and surface quality. While the final spheres are made of a single isotope of silicon, this one is made of naturally-occurring silicon, which comprises a mixture of isotopes. However, as far as the roundness is concerned, the quality of the sphere is as good as any of the final spheres made for the Avogadro Project.