05.12.2019
Due to the redefinition of the kilogram and due to the new possibilities this offers to disseminate the unit kilogram for a practice-oriented and stable traceability chain, the demand of national metrological institutes, calibration laboratories and balance manufacturers for silicon spheres is very high. In this context, the "Si-kg" transfer project, together with the participating companies, has developed a turnkey infrastructure to establish a complex production chain, to handle high-purity silicon spheres and to store and transport them. For the companies involved, this transfer project is thus the first step into an attractive emerging market which they would not have been able to open up without the transfer of the respective know-how.
The Physikalisch-Technische Bundesanstalt (PTB) was significantly involved in the redefinition of the International System of Units. In the field of mass, the 130-year-old "international prototype of the kilogram" has been replaced, among other things, by several unique spheres made of monocrystalline silicon. A major challenge of this new approach was the question of how to produce and supply spheres with unprecedented accuracy.
Within the scope of the three-year Si-kg transfer project, the know-how required to manufacture spheres and that necessary to handle them have each been transferred to a medium-sized company and have now been placed on the market with great success. For this purpose, first the manufacturing equipment and the manufacturing procedure were first transferred to J. Hauser GmbH & Co KG [Metrologia, 2019]. The polishing procedure, which has been patented, makes it possible to manufacture the spheres completely round, with just very slight form errors. This means that solid angles and edges have to be removed without damaging the soft surfaces. In addition, the procedure allows atoms to be removed from the surface in a purely physical manner, so that neither scratches occur nor is the monocrystalline silicon structure destroyed. The monocrystalline and scratch-free surface prevents the penetration and outgassing of impurity atoms. As a result, mass standards made of silicon achieve unprecedented long-term stability. Industry thus has a manufacturing process at its disposal that enables it to provide the national and international markets with spheres with a form error that is significantly smaller (RONt ~ 10 nm) and very low roughness (Ra < 0.5 nm) [CPEM, 2018]. The unique feature is that these specifications are met over the entire surface of the sphere.
The manufacturing procedure for spheres is a complex, multi-stage process chain [Euspen, 2015] which consists of the individual steps of cutting, turning, lapping and polishing (see 
https://www.ptb.de/cms/fileadmin/internet/video_podcast/allg_video/avogadro/Trailer_Avogadro.mp4).
The know-how that is necessary to handle silicon spheres has been transferred to Häfner GmbH. In trials and practical tests conducted over several years, PTB has identified materials with which silicon spheres can be safely handled, stored and transported. In order to ensure extremely high mass stability, the silicon spheres are mounted on special rings that neither scratch the sphere nor leave any deposits on its surface. This is particularly important when transporting the spheres, since during transport, the mass standards are subjected to constant shocks and micro-movements. In addition, based on PTB's preliminary developments, a joint transport container and transport case as well as a comprehensive calibration concept have been developed [IMEKO, 2018]. The spheres can be cleaned easily, efficiently and cost-effectively with special cloths and a recommended universal cleaner. Here, too, impurities must be removed without damaging the surface of the sphere. Meanwhile, the Häfner company offers, besides the spheres themselves and the cleaning and handling utensils, also training courses for the professional handling of silicon spheres. By transferring the sphere production to Hauser and the handling to Häfner, a unique, worldwide turnkey infrastructure has been created that allows highly stable and exact mass standards to be provided. The Si-kg transfer project was awarded the 2019 Technology Transfer Prize of IHK Braunschweig [IHK, 2019].
Bild 1: Technologieempfangende Unternehmen J. Hauser GmbH & Co. KG und Häfner Gewichte GmbH sowie die Preisträger der PTB. Von links nach rechts: Wolfgang Hauser, Stephan Hauser, Katharina Lehrmann, Thomas Wiedenhöfer, Martin Häfner, Katrin Häfner, Dr. Frank Härtig, Dr. Rudolf Meeß.
Literature:
[Metrologia, 2019] Knopf, Dorothea; Wiedenhöfer, Thomas; Lehrmann, Katharina; Härtig, Frank; A quantum of action on a scale? Dissemination of the quantum based kilogram; Metrologia, 2019. DOI: 10.1088/1681-7575/ab0851
[CPEM, 2018] Lehrmann, Katharina; Knopf, Dorothea; Härtig, Frank; Status of the realization and dissemination of the unit kilogram via silicon spheres; Conference on Precision Electromagnetic Measurements (CPEM); Paris, 08-13, July, 2018. DOI: 10.1109/CPEM.2018.8501162
[IMEKO, 2018] Härtig, Frank; Hackel, Siegfried; Hutzschenreuter, Daniel; Lehrmann, Katharina; Meeß, Rudolf; Scheibner, Alexander; Wiedenhöfer, Thomas; Mass standards made of silicon and their machine-readable Digital Calibration Certificate; XXII World Congress of the International Measurement Confederation (IMEKO 2018); Belfast, 03-06, September, 2018. DOI: 10.1088/1742-6596/1065/8/082005
[Euspen, 2015] Meeß, Rudolf, Hinzmann, Günter; Lück, Alexander; Improved manufacturing process chain for silicon spheres; Euspen's 15th International Conference & Exhibition of the European Society for Precision Engineering & Nanotechnology, Leuven, Belgium, 01-05, June, 2015
[IHK, 2019] 35. Technologietransferpreis der IHK, IHK Wirtschaft, Ausgabe Dezember 2019
[Patent] Meeß, Rudolf; Hinzmann, Günther; Lück, Alexander; Müller, Michael; Maschine zum Herstellen von Kugeln, Kugel und Verfahren zum Herstellen einer Kugel; DE 10 2015 110 712 A1; Priorität 02.07.2015
Contact:
Katharina Lehrmann, FB 1.1, AG 1.15, E-Mail: 
katharina.lehrmann(at)ptb.de