UV/ozone cleaning of platinum/iridium kilogram mass prototypes

Abstract

Platinum–iridium (Pt/Ir) kilogram mass prototypes are known to gain contamination from the environment in which they are stored. The current method of cleaning these mass prototypes is called nettoyage–lavage and involves the physical rubbing of a kilogram with a chamois leather cloth soaked in a solvent followed by removal of any solvent residue using a jet of steam water. The manual nature of the technique means the effectiveness of the cleaning process is reliant on the human operative. An alternative cleaning method involving exposure to ultraviolet light and ozone (UV/O3) has been tested on Pt/Ir foils and kilogram mass prototypes. The changes to the surface of the Pt/Ir foils as a result of this process have been quantified using x-ray photoelectron spectroscopy and have shown a clear reduction in the quantity of carbonaceous contamination. Variation of the UV intensity, ozone concentration and exposure duration enabled the optimum cleaning conditions to be established. The UV/O3 cleaning method was then used to clean two Pt/Ir kilogram mass prototypes and gravimetric weighing of the kilograms before and after cleaning gave the amount of contamination removed. These gravimetric weighing results demonstrated that UV/O3 cleaning was as effective as the nettoyage–lavage process.

Cleaning of mass standards: II. A comparison of new techniques applied to actual and potential new materials for mass standards

“…A direct comparison of hydrogen plasma and UV/ozone revealed that hydrogen also reduces species of oxygen while UV/ozone results in an oxidation [11]. The latter result is in agreement with earlier findings [5] while Davidson et al found no evidence for an oxidation [8,9]. However, the various results clearly demonstrated that both modern processes lead to different but fully reproducible initial surface chemical states, and thus are most preferable for a cleaning process and for the periodic re-initialization of primary mass standards.…”

“…The same variable findings about oxidation upon UV/ozone treatment were reported for PtIr. Earlier measurements [6] found that the surface was oxidized while newer experiments [9] give no indication of an oxidation.…”

“…First attempts to overcome this problem and to find a reproducible method have been made one century after the establishment of the nettoyage-lavage. UV/ozone treatment, a standard method in semiconductor technology, was first applied to PtIr by Cumpson and Seah [6] and continued by Davidson [7][8][9]. Another technique using low-pressure plasma with hydrogen and oxygen was implemented and first tested for PtIr, Au and AuPt [10][11][12].…”

Practical requirements for the successful implementation and subsequent dissemination of the redefined kilogram

“…Both techniques have the advantage that they are noncontact so the effectiveness and repeatability of the cleaning achieved should be governed solely by the parameters set (UV intensity and ozone concentration or plasma intensity and pressure). Both METAS (Switzerland) [16,17] and NPL (UK) [18,19] have evaluated UV/ozone and H-plasma non-contact cleaning methods in parallel with the traditional nettoyage-lavage method. The results from the two laboratories are shown in Figs.…”

Removal of mercury contamination on primary mass standards by hydrogen plasma and thermal desorption

“…Among the potential materials it is one of the chemically most resistant and forms no oxides under normal conditions. Physisorbed contaminations can largely be removed by various cleaning methods such as nettoyage-lavage [12], UV/ozone [13][14][15] or hydrogen plasma [16,17]. These characteristics make PtIr quite favourable as a material for mass standards of highest stability.…”