Atoms for the kilogram

According to the Comité International des Poids et Mesures (CIPM), the International System of Units (SI) is to be defined on the basis of fundamental constants. Especially the base units the kilogram, the ampere, the kelvin and the mole are in the focus of the redefinition. Hereby, clearly determined numerical values are attributed to the fundamental constants concerned. Prior to this, however, these values have to be measured as accurately as possible. For the redefinition of the kilogram, PTB is pursuing a very promising approach by “counting” atoms in a silicon single-crystal using the so-called ”X-ray Crystal Density (XRCD) method“. Hereby, the Avogadro constant is determined by measuring the crystal properties (the molar mass and the crystalline lattice parameter, among other things) as well as the sphere properties (the mass and the volume, among other things).

However, to be able to measure the molar mass with sufficient accuracy, it is necessary to use a crystal made of practically monoisotopic silicon. For this purpose, PTB concluded a contract concerning the delivery of 12 kg of ²⁸Si with the export trading company ISOTOPE in Moscow. The materials specifications stipulated a high degree of enrichment of the isotope ²⁸Si of more than 99.99 % as well as an exceptionally high chemical purity. The silicon isotope is enriched by means of centrifuges of the Electrochemical Plant in Zelenogorsk – the largest supplier of any type of isotopes worldwide. The gaseous silicon tetrafluoride (²⁸SiF₄) is then transformed into silane (²⁸SiH₄) at the Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences in Nishniy Novgorod; it is then painstakingly purified and eventually deposited as polycrystalline silicon onto a thin, electrically heated silicon rod, a socalled “slim rod”.

The first polycrystal with a mass of 6 kg was delivered from Russia in August 2014. From this polycrystal, the Leibniz Institute for Crystal Growth (IKZ) in Berlin grew a single-crystal which was handed over to PTB in March 2015. From this single-crystal, PTB will manufacture – by means of a complex production line which was especially developed for this task – a total of two spheres having a mass of approx. 1 kg each for the determination of the Avogadro constant and of Planck's constant. The production process has meanwhile been improved to such an extent that clean oxide layers and a roundness clearly better than 50 nm can be obtained. The delivery of a second polycrystal is planned for mid-2015. Then, four silicon spheres in total will be available to determine the Avogadro constant and Planck's constant with the greatest accuracy. In addition, when the system of units has been redefined, the "silicon kilogram" will allow the unit of mass to be realized and disseminated in a feasible way.