M.J. Kenny

Ion-implanted graphitic carbons

Abstract Ion implantation of glassy carbon and other graphitic forms of carbon leads to significantly increased resistance to wear. The effect is observed for a variety of ion species. Nitrogen ion doses as low as 5 × 10 15 ions/cm 2 at 50 keV are effective and the enhancement is related to the damage produced by the incident ions. New and previous data show the importance of ion energy and dose in determining the modified (damaged) zone. The general applicability of implantation as a modification tool for graphite-based carbon is supported by work with highly ordered pyrolytic graphite (HOPG), electrode carbon and carbon fibre. Aspects of possible mechanisms responsible for the enhanced wear resistance are discussed.

Precision determination of the density of a single crystal silicon sphere and evaluation of the Avogadro constant

Density measurements with relative uncertainty of 1/spl times/10/sup -7/ have been made on a highly polished 1-kg single crystal silicon sphere with out-of-roundness <40 nm. Roundness was profiled using a 2-D Talyrond machine and 3-D profiles were produced. The diameter was obtained using optical interferometry for a series of breadth measurements at carefully selected points and by combining them with roundness data. The mass was obtained relative to a 1-kg stainless steel reference with appropriate corrections for air buoyancy and for convection currents due to small temperature differentials. Surface oxide thickness was measured using optical ellipsometry and the data were corrected for this oxide thickness. The molar mass, crystal quality and lattice parameter have been measured elsewhere, enabling a determination of the Avogadro constant to be made. The purpose is to obtain a definition of the kilogram in terms of a specific number of /sup 12/C atoms.

The influence of Young's modulus on roundness in silicon sphere fabrication [Avogadro constant]

Silicon single-crystal spheres for use in accurate determination of the Avogadro constant are fabricated by optical grinding and polishing. Surface profiling of the deviation from sphericity of the spheres shows a strong cubic symmetry on the scale of (20 to 40) mm, which is well correlated with the orientation of the silicon crystal axes and sets a limit on the ultimate shape that can be obtained. This deviation from sphericity can be explained in terms of the variation of Youngs modulus with crystal orientation.

The influence of Young's modulus on roundness in silicon sphere fabrication

Silicon single crystal spheres for use in accurate determination of the Avogadro constant are fabricated by optical grinding and polishing. Surface profiling of the deviation from sphericity of the spheres shows a strong cubic symmetry on the scale of 20-40 nm, which is well correlated with the orientation of the silicon crystal axes. This deviation from sphericity can be explained in terms of the variation of Youngs modulus with crystal orientation.

Germanium implanted with high dose oxygen and its optical properties

Abstract Single crystal n-type Ge samples are implanted with 1 × 10 17 to 1.5 × 10 18 cm −2 oxygen ions at 45 keV. Infrared and Rutherford backscattering measurements indicate that germanium dioxide is formed. The atomic ratio of oxygen to germanium is near the GeO 2 stoichiometric value of 2.0 from the surface down to a depth of 550 A for germanium samples implanted to 1.5 × 10 18 cm −2 . The excess oxygen is redistributed during the implantation. The results of optical reflectivity measurements indicate that the reflectivity of germanium in the 0.2–1.4 μm wavelength region is greatly reduced after high dose oxygen ion implantation. The reflectivity value at about 0.7 μm is near zero for germanium implanted to a dose of 1.5 × 10 18 cm −2 .

Density of a single crystal natural silicon sphere

The density of a single-crystal natural silicon sphere identified as ldquoAvo#3rdquo has been redetermined at the National Measurement Institute, Australia (NMIA), as part of an international collaboration to determine the value of the Avogadro constant. A systematic review of the measurement system found that a significant bias was introduced in the measurement of the volume of the sphere, which is calculated from the interferometric measurements of the mean diameter. It was found that the dynamic response of the laser that is used to generate the frequency steps for the phase-shifting interferometry introduced a bias to the returned phase value. We determined the magnitude of the bias and then applied corrections to obtain the final measured value. The density reported here is based on some new and some recalculated measurements taken between April 2000 and May 2006 during the ongoing development of the measurement system.

Surface layer impurities on silicon spheres used in determination of the Avogadro constant

The Avogadro constant is required to be determined with an uncertainty of less than 1/spl times/10/sup -8/ in order to allow an atomic definition of the kilogram. A single-crystal silicon sphere 93.6 mm diameter is used for this determination. A thin surface layer (typically 2 nm to 5 nm thick on flats and 10 nm or more on spheres) of contaminants such as oxide, water and hydrocarbons on the sphere can significantly affect the measurements due to corrections for density changes and to phase change on reflection in the diameter measurement by optical interferometry. The stability of this surface layer as a function of time is also of importance because of ongoing measurements. The nature of this contamination has been investigated using optical ellipsometry and ion beam analysis. It is concluded that the composition and structure of the surface layer are affected by a number of parameters and that the most appropriate method of achieving the desired accuracy is to remove the surface layer by etching and to form a hard stable coating of controlled thickness and composition. This coating may be either silicon dioxide or silicon nitride.

A Web-based database for the international programme to improve the Avogadro constant along the silicon route

A number of measurement institutes are working together toward a determination of the Avogadro constant with a relative combined standard uncertainty of 10/sup -8/. This will enable a definition of the kilogram based on the mass of a specific number of /sup 12/C atoms rather than an artefact. In order to bring together all the data obtained in the various laboratories and maintain accountability and traceability across the collaborating institutes, a Web-based database has been set up. This database forms part of the BIPM website, although access is currently limited to project participants. Any data used in publications on the Si-route, together with supporting evidence, can be found in this database.

A web-based database of information relating to the evaluation of the Avogadro constant using the XRCD method

A number of laboratories are working towards an evaluation of the Avogadro constant with combined relative uncertainty of 1 in 10/sup 8/. This will enable a definition of the kilogram based on a specific number of /sup 12/C atoms rather than an artefact based definition. In order to coordinate the data obtained by the participating laboratories, a web-based database has been set up. This database forms part of the BIPM website, although access is limited to participants.

Determination of the Avogadro constant from precision density measurements on a silicon sphere

Density measurements with relative uncertainty of 1/spl times/10/sup -7/ have been made on a highly polished 1 kg single crystal silicon sphere. The molar mass, crystal quality and lattice parameter have been measured elsewhere, enabling a determination of the Avogadro constant to be made. The purpose is to obtain a definition of the kilogram in terms of a specific number of /sup 12/C atoms.