New ²⁸Si Single Crystal for the Avogadro Project of the PTB

The PTB wants to realize the mass unit kilogram using silicon spheres after the redefinition that is awaited for 2018. The first single-crystal of isotopic enriched silicon for the PTB was delivered in March 2015 [1].  On 01 June 2016 the second single-crystal arrived at the PTB, s.figure 1.

Figure 1: The second 6 kg ²⁸Si single-crystal for the Avogadro-Projekt of PTB

The isotope enrichment was performed again at the Electrochemical Plant in Zelenogorsk (Russia). The Institute of Chemistry of High-Purity Substances (IChHPS) of the Russian Academy of Sciences in Nishniy Novgorod converted the enriched silicon tetrafluoride (²⁸SiF₄) into silane (²⁸SiH₄), purified this and then deposited the ²⁸Si as polycrystalline silicon onto a thin, electrically heated silicon rod. The polycrystalline rod was delivered by the export trading company ISOTOPE (Moscow, Russia) to Germany in August 2015. Finally, the Leibniz Institute for Crystal Growth (IKZ) in Berlin grew the single-crystal. The new crystal with a mass of about 5.6 kg has an isotope enrichment of 99.9995%, significantly higher than the former single-crystals.

Literature:

[1] ptb news 2015 Issue 1, PTB Annual Report 2015

Contact person:

Horst Bettin, FB 1.8, AG 1.83, E-Mail: horst.bettin(at)ptb.de