A metrological challenge is currently underway to replace the present definition of the kilogram. One prerequisite for this is that the Avogadro constant, N A , which defines the number of atoms in a mole, needs to be determined with a relative uncertainty of better than 2 × 10 −8 . The method applied in this case is based on the x-ray crystal density experiment using silicon crystals. The first attempt, in which silicon of natural isotopic composition was used, failed. The solution chosen subsequently was the usage of silicon highly enriched in 28 Si from Russia. First, this paper reviews previous efforts from the very first beginnings to an international collaboration with the goal of producing a 28 Si single crystal with a mass of 5 kg, an enrichment greater than 0.9999 and of sufficient chemical purity. Then the paper describes the activities of a follow-up project, conducted by PTB, to produce a new generation of highly enriched silicon in order to demonstrate the quasi-industrial and reliable production of more than 12 kg of the 28 Si material with enrichments of five nines. The intention of this project is also to show the availability of 28 Si single crystals as a guarantee for the future realisation of the redefined kilogram.

Absfracf-Stress-annealed nanocrystalline FeCuNbSiB ribbons show correlation between induced magnetic anisotropy and magnetoimpedance. Two types of crystallization process were used in order to induce a transverse magnetic anisotropy: the first one was performed submitting the original amorphous samples to an applied tensile stress of a=150 MPa. In the second one, samples are nanocrystallised in a first stage and submitted to stress annealing at ~2 9 0 M P a afterwards. The maximum of the magnetoimpedance can be obtained for de fields larger than the anisotropy field of the sample or close to the irreversibility field. This behavior can be explained based in the simultaneous switching of two different magnetization processes taking place in the samples with high transverse magnetic anisotropy.Index Terms-Magnetization processes, magnetoimpedance, nanocrystalline ribbons, induced magnetic anisotropy. LINTRODUCTIONAs recently shown, soft magnetic materials exhibit the socalled giant magnetoimpedance (MI) effect [I-21, which consists in field-induced variation of its impedance through changes in the classical skin depth 6 and magnetic circular permeability p+ when a high frequency current flows through the magnetic conductor. In this framework, the frequency dependence of the MI value can be very informative due to the relation G=(p/7~p$)'~, where p is electrical resistivity and f is the frequency of the alternating, ac current. MI effect is sensitive to the anisotropy of the material [3-4]. On the other hand it is well known that suitable annealing of amorphous FeCuNbSiB ribhons under mechanical stress leads to nanoscale grain structure formation and induces a magnetic anisotropy whose easy magnetization axis is transverse to the direction of the tensile stress [5]. Some results on the MI effect in stress-annealed FeCuNbSiB ribbons were presented recently [4]. The ultrafine grain structure and the induced Mnnuscript received February 25,1999, revisedMay 10, 1999 M. Vizquez, (+34) 91-630 1724, fax (+34) 91.630 1625, mvazque@enfe.cs; G.V. Kurlyandskaya, (+34) 94.601 25 54, fax (+34) 94-485 8139, gnlina@we.lc.ehu.es, pmmnent address: ln&tute of Metal Physics UD RAS, Kovnlevskaya s t r 18, 620219 Ekntorinburg, Kussia, present address IWwrsidnd del Pais Vasoa UPV-EHU, Depto. Elwtricidnd y Eleotmnica, Aptdo. 644, 48080 Bilbao, Spain, J.P. Shocker, present nddresses: Instihlto de Fisica Uniwrsidnde Federal do Rio de Jnnoim C.P. 68528: 21945470 -Rio de,Janoiro -RI, Brasil. This work wns supported by The Basque Government, The Brazilian Agency FAPERJ, and The Sponish CICYT.magnetic anisotropy depend critically on the annealing conditions and the MI effect can be used as a probe to obtain information on the differentmicrostructural and anisotropy states In the present work the frequency dependence of the MI is studied for the nanocrystalline Fe73 ,Cu,Nb3SiI3 ,B9 ribbons with high value of stressinduced magnetic anisotropy in correlation with the magnetization process 11 EXPERlMEWCAL TECHNIQUES Amorphous Fe73 sCuINb& sB9 ribbo...

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An international comparison study on the accurate determination of the molar mass M(Si) of silicon artificially enriched in 28Si (x(28Si) > 0.9999 mol mol−1) has been completed. The measurements were part of the high level CCQM-P160 pilot study assessing the ability of National Metrology Institutes (NMIs) and Designated Institutes (DIs) to make such measurements at the lowest possible levels of measurement uncertainty and to identify possible difficulties when measuring this kind of sample. This study supports the molar mass measurements critical to disseminating the silicon route to realizing the new definitions for the kilogram and the mole. Measurements were also made by one external research institute and an external company. The different institutes were free to choose their experimental (mass spectrometric) set-ups and equipment, thereby enabling also the comparison of different techniques. The investigated material was a chemically pure, polycrystalline silicon material. The subsequent modified single crystalline secondary product of this material was intended for the production of silicon which was used for two additional spheres in the context of the redetermination of the Avogadro constant NA, required for the revision of the International System of Units (SI) via fundamental constants which came into force from May 2019. The CCQM pilot study was organized by Physikalisch-Technische Bundesanstalt (PTB). Aqueous silicon solutions were shipped to all participating institutions. The data analysis as well as the uncertainty modelling and calculation of the results was predefined. The participants were provided with an uncertainty budget as a GUM Workbench® file as well as a free software license for the duration of the comparison. The agreement of the values of the molar mass (M(Si) = 27.976 942 577 g mol−1) was excellent with ten out of 11 results reported within the range of relative uncertainty of 1 × 10−8 required for the revision of the SI.

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