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Caractérisation physico-chimique de surface des étalons de masse

Abstract : Today the kilogram is the only remaining base units of the International System of Units (SI) still defined by a material artefact. Known as the international prototype of the kilogram, it is a cylinder made from the alloy 90% platinum, 10% iridium and its mass is defined to be exactly 1 kg. This prototype (as well as all other artefact mass standards) is, despite precautions, exposed to many contamination sources which lead to changes in its mass. The present definition is thus problematic for both the long-term stability and the universality of the unit of mass. For this reason, new lines of research are attempting to link the unit of mass to a fundamental constant of physics. These experiments could lead to a new definition. The French watt-balance project was launched in 2000 with the goal of linking the unit of mass to the Planck constant. A number of technical problems must be addressed. In particular, there will be severe constraints on the artefact mass standard used for the transfer. This artefact will be in the fringing field of a magnetic circuit delivering an induction of 1 T. Consequently, it must be made of a material not only having the smallest possible volumetric magnetic susceptibility (of the order of 10-5) but must also possess the other material properties that are indispensable to the best artefact mass standards (good hardness, high density, homogeneity,...). Certain alloys such as gold-platinum or certain other materials such as pure iridium might satisfy these criteria. In order to evaluate the performance characteristics of new materials, platinum-iridium was chosen as the reference alloy. To understand why the mass of an artefact may change with time, taking into account its surface characteristics as well as the conditions of its cleaning and storage, a number of experimental techniques must be used. These include: methods to determine surface roughness, spectroscopic methods to characterize surface contaminants and gravimetric tests of mass stability. Within the framework of these research tasks, we use techniques already available at the Institut National de Métrologie such as: a mirage-effect apparatus for studying adsorption of cleaning solvents, an optical roughness meter to study polishing techniques and a 100 g mass comparator for determining the mass stability after cleaning. In addition to these, a new device was developed to apply the technique of Thermo Desorption Mass Spectrometry to the study of physisorbed contaminants.
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Contributor : Zaccaria Silvestri <>
Submitted on : Monday, December 19, 2005 - 10:30:09 AM
Last modification on : Thursday, September 19, 2019 - 3:40:03 PM
Long-term archiving on: : Saturday, April 3, 2010 - 7:35:53 PM

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Zaccaria Silvestri. Caractérisation physico-chimique de surface des étalons de masse. Analyse de données, Statistiques et Probabilités [physics.data-an]. Conservatoire national des arts et metiers - CNAM, 2005. Français. ⟨tel-00011235⟩

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