Z. Silvestri

Volume magnetic susceptibility of gold–platinum alloys: possible materials to make mass standards for the watt balance experiment

In 2000, the Bureau National de Metrologie (BNM, France) decided to develop a new watt balance experiment. Among numerous design studies, the choice of the transfer mass is particularly important. Because of the proximity to a source of high magnetic intensity, this mass must have a magnetic susceptibility as weak as possible. Gold–platinum alloy seems to meet this requirement, as well as additional requirements for density and hardness values, making it a possible candidate for mass standard realization. Five different gold–platinum alloys were studied, their volume magnetic susceptibility ranging from −2.8×10−5 to −2.1×10−5 for two of them and from +1.1×10−5 to +8.8×10−5 for the other three.

Study and comparison of two polishing methods for platinum?iridium surfaces, by means of three characterization techniques

Machining the surface of mass standards is still of great importance. This paper details a comparative study of the roughness of two plane surfaces of a platinum?iridium alloy (90% of platinum and 10% of iridium). Using the BIPM processes, the surfaces were initially machined on a lathe using diamond tools; in addition, one of these was manually polished with a diamond paste.Three techniques for surface characterization are used: shear-force microscopy (ShFM), optical roughness-meter and x-ray reflectometer. The first technique uses the shear-force interaction between the probe and the sample. The ShFM is part of a scanning near-field optical microscope that has the advantage of providing two images simultaneously, a topographical one and a near-field optical one. Only the topographic images will be presented in this paper; the results obtained in optical near-field will be the subject of another publication. To avoid any confusion, we will speak here of ShFM. The topographic images and their associated statistical and physical parameters, such as power spectral densities (PSDs), root-mean-square height, etc, are discussed in this paper. The PSDs of the surface are also determined experimentally from x-ray and visible light scattering measurements using angle-resolved scattering theory. This theory, which is well adapted for visible radiation, is extended to x-rays. The measurements made with the three instruments demonstrate that the two surfaces present a roughness of the same order of magnitude. However, the defects that contribute in a preponderant manner to their roughness spectrum do not belong to the same spatial bandwidth but depend on the polishing process.

Present and future mass standards for the LNE watt balance and the future dissemination of the mass unit in France

The value of the Planck constant h was determined in 2014 by means of the LNE watt balance experiment. The relative standard uncertainty was 31 parts in 108. This first determination was performed in air with a 500 g mass standard made from XSH Alacrite. The main uncertainty components in air associated with the mass involve the calibration, the mass stability, the buoyancy correction and the magnetic interaction correction. The combined relative uncertainty due to the mass is 7.2 parts in 108. The use in 2016 of a mass standard made from platinum iridium alloy significantly reduces the component of uncertainty arising from the mass standard for a Planck constant measurement either in air or under vacuum. The relative uncertainty due to this contribution is estimated to be about 3 parts in 108 in air and one part in 108 under vacuum. The future system for the dissemination of the mass unit using the LNE watt balance will be based on a primary realization with three 500 g mass standards made from platinum–iridium alloy, pure iridium and Udimet 720 respectively, coupled with a pool of kilograms made from different materials. Pure iridium and Udimet 720 are new materials to make reference mass standards proposed by CNAM and LNE respectively and have never been used by any NMI for manufacturing mass standards until now. Some new results concerning their surface behavior are given.