Study of materials for the realisation of mass standards
Project Description
The goal of this project is to select a material having the best criteria to realize mass standard used as transfer standard in the watt balance experiments.
Final Report 2008-02-28
The project has been completed and the report can be downloaded here>>
Progress Report 2005-12-31
1) Study of platinum-10% iridium alloy provided by the Johnson Matthey company and used as reference material
a) The study of platinum-10% iridium alloy in terms of surface finishing techniques, surface cleaning method effects and surface contaminants has been completed. The results have been reported in a thesis written by Zaccari Silvestri (CNAM-october 2005). In addition, some results carried out at LPUB concerning topographical and optical surface study have also been reported in a thesis written by Youssef Haidar (University of Burgundy – June 2005).
Some articles have been published in 2005 concerning these surface studies.
b) At present, a gravimetric study combined with a chemical surface analysis on this alloy is being carried out by METAS and a gravimetric study concerning the mass stability of a Pt-Ir-1-kg artefact when transferring between air and vacuum is planned at LNE in 2006.
2) Materials for the realisation of transfer mass for the watt balance experiments
a) In 2005, we have come into contact with PXGroup company which proposes several interesting ternary and quaternary gold alloys. Their metallographical study allowed one to select a quaternary gold alloy. Some results are presented in a thesis written by Paul-André Meury (ENSMP – December 2005). The corrosion and oxidation behavior of silver and copper will be studied by LNE in 2006.
b) A study is also in progress at LNE-INM and METAS concerning pure iridium. The main manufacturers propose this noble metal in the laminated form only. So its grain structure presents some heterogeneities due to the laminating processing. It seems very difficult to find manufacturers or laboratories having the capability to forge iridium. Nevertheless we have decided first to undertake a gravimetric study on a set of laminated iridium discs in order to estimate the short term mass stability in air and when transferring between air and vacuum.
c) Another interesting possibility could be to use amorphous metal alloys which have been discovered about ten years ago. Amorphous metal alloys are twice as strong as steel, have greater wear and corrosion resistance, are tougher than ceramics and yet have greater elasticity. Usually they are very difficult to machine, but they can be ground or polished. So, an amorphous platinum alloy has been elaborated by PXGroup. This alloy is composed of platinum, copper, nickel and phosphorus. The magnitude order of its magnetic susceptibility has been evaluated by LNE.
In addition, Richard Davis (BIPM) has recently read an article about gold metallic glass. It is based on gold, copper and silicon. We plan to pursue our initial contact with Liquidmetal Technologies, a company that specializes in amorphous metals.
Progress Report 2005-01-31
1) Materials for the realisation of transfer mass for the watt balance experiments
a) CECM in collaboration with ENSMP, CNAM-INM and LNE and has elaborated a specific Au-12%Pt alloy to be used for watt balance experiments. This new alloy presents a good homogeneous microstructure without porosity and free of any precipitation but its maximum hardness is too low (about 120 HV) for the WB application. So the work concerning gold-platinum alloys is now stopped.
b) The possibility to elaborate a gold iridium alloy is being examined and a study on pure iridium has just started. The latter material seems to be a good candidate for WB experiments but its metallurgy and its machining are not easy. The best commercial purity proposed by Johnson Matthey company is only 99.9% with about 10 ppm of iron.
2) Study of platinum-10%iridium alloy provided by the Johnson Matthey company and used as reference material
Metallographic structure and surface quality :
a) ENSMP has carried out metallographic study using electron microscopy techniques and microprobe analysis device. This study shows that platinum iridium alloy presents a good homogeneity of structure. It is a single phase alloy with fine grain size between 2 and 3 µm and grain boundaries free of any precipitation.
b) A surface quality study has been made by means of optical roughnessmeter (CNAM-INM), shear force microscope (LPUB) and X-ray reflectometer (CNAM-INM and LPCNAM). The best surface quality is obtained by using diamond paste polishing with a grain size down to 1 mm. The surface quality after this kind of polishing can be destroyed by using a smaller diamond grain size. It seems that there is a hole forming phenomenon probably due to grain incorporation. The diamond tool machining surface quality shows typical defaults because of the diamond tool chattering mainly.
Study of contamination by cleaning solvents or water vapour :
a) Cnam-INM uses photothermal and thermodesorption devices. The mass coverage of molecules adsorbed from cleaning solutions onto platinum iridium alloy surfaces were found to be :
of the order of 0.5 µg/cm2 (corresponding to about 10 ng/cm2 for a monolayer) for ether and ethanol after cleaning with this solvents. Their adsorbability seems to depend on their hydrophilic affinity;
of 2 µg/cm2 at least with a fast adsorption kinetic (adsorbed in 40 min in air at 20°C and about 0.50 relative humidity) for water after thermodesorption at about 400°C and 10 µPa during 8 hours;
and of about 0.5 µg/cm2 for molecules chemisorbed or physisorbed with higher activation energy after a first thermodesorption.
The correlation between adsorbed mass on the surface and the expanded area due to the surface roughness is being examined by CNAM-INM, LPUB and LPCNAM.
b) SP in collaboration with CNAM-INM has analysed Pt-Ir samples by TOF-SIMS. The studies related to stability in time after cleaning according to the BIPM protocol or by UV/ozone cleaning as compared to non treated samples. The measurements showed organic surface contamination due to polymers present in plastics and the chamois leather used in the BIPM cleaning and confirmed the effectiveness of UV/ozone cleaning for removal of the organic contamination.
c) BIPM is developing Pt-Ir sorption artefacts for air-vacuum reference. In addition a study of water vapour adsorption on mass standards measured by ellipsometry has been carried out on several materials. The measurements showed that the water vapour sorption effects are smaller for gold than for Pt-Ir samples.
3) New device to study surface behaviour of transfer mass for WB experiments
METAS has designed a high-resolution vacuum mass comparator in order to study surface effects of mass standards. The Instrument will enable characterization of the change in mass when a mass standard is brought from normal ambient conditions into vacuum. In vacuum, this gravimetric method will be combined with a quantitative chemical surface analysis (ARXPS and RGA). In this way the change in mass will be related with the change in chemical composition and the thickness of the adsorption layers. This device will be operating in the course of 2005.
Progress Report 2004-04-30
The start point of this EUROMET project in 2003 was the study of dental gold-platinum alloys. The BNM began last year several collaborations with French research laboratories such as CECM, ENSMP, LEMHE, LPCNAM and LPUB. Measurements made by BIPM, BNM-LNE and METAS allowed to select two dental alloys having a weak magnetic susceptibility (see Metrologia 40 (2003) 172-176). From metallographical study made by ENSMP, these alloys were proved unsatisfactory due to their bad homogeneity. A new gold platinum alloy is being elaborated by CECM.
For this study, platinum iridium alloy, well-known by most of the national mass laboratories, will be used as reference material. Several platinum iridium samples were realised for the different methods and techniques of study and analysis. For instance, fifteen square-shaped samples are now at SP in Sweden to be analysed by means of XPS, AES and TOF-SIMS techniques. A roughness study has been carried out by BNM-INM, LPCNAM and LPUB on a platinum iridium disc provided by BIPM with one side machined by diamond tool and the other one polished by diamond paste. In addition, BIPM proposes to fabricate platinum iridium artefacts to be used for the gravimetric study when transferring these artefacts between air and vacuum.
Of course, identical samples made from the chosen material will be realised in order to apply the same methods and techniques and to compare the results obtained on the two materials. Gold platinum alloy is not the only possible choice of materials. For instance, some colleagues suggest to use silicon which presents a very low magnetic susceptibility and of which the behaviour of surface studied by several laboratories seems satisfaying for the watt balance experiment conditions. In addition, this should lead to an interesting relation between the experiments of the Avogadro constant and the watt balance, giving an experimental determination of the molar Planck constant.
A file transfer site reserved for this project is available in order to facilitate the information exchanges between the participating laboratories and to establish common understanding and interpretation of the results obtained by each laboratory. A first meeting concerning this project was held in Vienna.