The kilogram

This is in contrast to the watt balance projects which are favored by some countries. Hereby, the Avogadro constant is determined by measuring the crystal properties (molar mass, crystalline lattice parameter, and impurity concentration) as well as the sphere properties (mass, volume, and surface layers) via which the kilogram can be traced. Early in 2015, the international Avogadro Project (BIPM, INRIM, NMIJ, NIST, PTB) published a new value of the Avogadro constant: N_A = 6.022 140 76(12) ∙ 10²³ mol^–1. This value is consistent with the results of the watt balance projects and provides a relative uncertainty of 2.0 ∙ 10^–8 for the realization of the new kilogram.

PTB has meanwhile started an Avogadro project of its own which is aimed at a further validation of the measurement uncertainty and for which high-purity silicon-28 single-crystals with an isotopic purity of 99.998 % were required. For this purpose, PTB had, in 2012, concluded a contract for the delivery of two 6 kg ²⁸Si polycrystals with the export trading company ISOTOP in Moscow. First of all, natural solid silicon was converted into gaseous SiF₄ at the “Electrochemical Plant” in Zelenogorsk. The highly complicated isotope enrichment to ²⁸SiF₄ (which took several months) was performed in the gas centrifuges at the “Electrochemical Plant” in Zelenogorsk. In sophisticated processes, the “Institute of Chemistry of High-Purity Substances” of the Russian Academy of Sciences in Nishniy Novgorod finally converted the isotopically enriched ²⁸SiF₄ into silane, purified it chemically over several months, and fabricated two ²⁸Si polycrystals (weight: 6 kg each) in specially developed deposition plants. In August 2014 and in August 2015, 6 kg each of polycrystalline material were delivered. From the first polycrystal, the “Leibniz Institute for Crystal Growth” in Berlin grew a single-crystal which was handed over to PTB on 25 March 2015. The second single-crystal is expected early in 2016. From these single-crystals, a total of four spheres with a mass of approx. 1 kg each are to be manufactured at PTB with the highest precision. Meanwhile, all the necessary measuring techniques have also been developed at PTB with the greatest time and effort.

Early in 2015, PTB concluded a new contract – this time directly with the “Electrochemical Plant” in Zelenogorsk – for the delivery of three further isotopically enriched ²⁸Si polycrystals of 6 kg. The plan is that PTB will have a total of ten (10) 1 kg ²⁸Si singlecrystal spheres at its disposal by 2018 at the latest. An even higher accuracy of the Avogadro constant is thus expected. This is important, as the accuracy of the unit of mass is of the utmost significance to industry.