Research | IKZ | 01-01-2020

Original kilogram replaced – New International System of Units entered into force

Along with the ampere, kelvin, and mole, the kilogram is now defined by a natural constant. The single crystals grown from the highly enriched isotopic silicon-28 at the Leibniz-Institut für Kristallzüchtung (IKZ) have made it possible.

The new International System of Units (SI) was adopted at the 26th General Conference on Weights and Measures in Paris on November 16, 2018, and officially came into force on May 20, 2019, World Metrology Day. From then on, seven natural constants form the foundation of all measures. Hereafter, a new definition for the kilogram is valid using the Planck constant and thus this unit is no longer determined through the mass of the “original kilogram.” The scientific and high-technology communities mostly benefit from this. The IKZ played a decisive role in replacing the almost 130-year-old artificial object of the original kilogram, because the structurally perfect crystals of isotopically-pure silicon-28 (28Si, enrichment up to 99.9995 %) grown at the IKZ were of decisive importance for this project.

In these crystals, almost all the atoms have the same mass and are arranged in a regular three-dimensional lattice, which makes a very exact assignment possible between the mass of the crystal and the number of its atoms. From this relation, the value of the Avogadro constant could be derived with unprecedented precision and thus used as a fundamental natural constant for the definition of the kilogram, since the Plank constant could be determined more precisely with the help of the Avogadro constant. In the new SI system, the value of the Avogadro constant is determined and one mole therefore contains exactly 6.02214076×1023 individual particles.

Within the framework of the “KILOGRAM” projects led by the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, several very precise spheres with shape deviations of less than 20 nanometers at a diameter of about 94 millimeters and with a defect-free polished surface were prepared from the 28Si crystals grown at IKZ using the float-zone method (FZ). Under these preconditions, PTB succeeded in determining the number of 28Si atoms in a crystal sphere of one-kilogram total mass, with the required uncertainty of less than 2 x 10-8.

It amounts to: 2.152538397 x 1025 atoms of silicon-28

To guarantee the necessary purity of the crystals grown from this material, various material-intensive molten-zone cleaning steps are necessary. The special challenges were therefore the approximately 1,000 times higher material price compared to conventional silicon as well as the limited material availability. Silicon is regarded as a very comprehensively investigated semiconductor material that dominates microelectronics and thus communication technologies worldwide. The IKZ will continue to work on the extreme requirements for the further improvement of material properties in order to enable future applications such as artificial intelligence and quantum technologies. “IKZ´s expertise on isotope pure Si crystals, developed during this metrology project, will allow us to play in the next round a key role as materials science institute for the development of innovative quantum technologies,” states Professor Thomas Schröder, the scientific director at IKZ.

Leibniz-Institut für Kristallzüchtung (IKZ)
Dr. Nikolay Abrosimov
Phone +49 30 6392-3010

Stefanie Grüber
Presse and Public Relations
Phone +49 30 6392-3263