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Research paper : A new density standard replaced from water (K. Fujii)−190 Synthesiology - English edition Vol.1 No.3 (2009) provider certified according to ISO/IEC17025 standard used in the Japan Calibration Service System (JCSS).(5) To ensure that the highest order standard of registered calibration service is sufficiently stable, and does not require frequent calibration by the primary standard owned by the AIST.When working on (1) of the above policies, we considered designating water as the standard of density by determining the purification method, purity analysis, and isotropic composition measurement of water. However, despite maximum technological effort, it is still extremely difficult to have a relative uncertainty of better then 1×10-6. On the other hand, densimetric technology for silicon single-crystals, which was developed at the AIST for determination of the Avogadro constant, had already reached the level of 1×10-7. Therefore, we selected the density standard system with silicon single-crystals as the top of traceability. Item (5) was an important factor when considering the necessary work at the registered calibration service providers. We discussed this point with candidates of service providers, and reached conclusion that even if the initial investment for calibration facilities was somewhat high, in middle to long-term, it would be easier for the service providers to have their own stable density standard for maintaining reliable calibration, as they would be freed from frequent calibrations of their own density standard. Based on these considerations, AIST started working on the new density standard system.3 Development of new density standard systemSince density of silicon single-crystals is extremely stable, the National Institute of Standards and Technology (NIST) of the USA considered using it as a solid density standard for first time in the 1970s[24]. When the CSIRO developed the technology of polishing a 1 kg sphere from silicon single-crystals in 1987[25], it became possible to directly determine the density from mass and dimensional measurements, and thus it opend a new way for substantially reducing the uncertainty of density measurement. Conventionally, the density of silicon single-crystals had been calculated from buoyancy force measurement in a liquid[24], using the volume standard realized by steel spheres whose volumes had been determined from dimensional measurements. By polishing silicon single-crystals into spherical form, the absolute value of the density could be obtained directly without buoyancy force measurement. In order to use the silicon sphere for determination of the Avogadro constant and not just for a density standard, CSIRO developed a new manufacturing process using mechano-chemical polishing as well as mechanical polishing at the final phase, to prevent crystal damage near subsurface[26]. In cross-sectional observation near the sphere surface by transmission electron microscope, it was verified that crystal structure was maintained until it transformed to surface oxide layers. Using this polishing technology, it is now possible to obtain a sphere with a diameter about 94 mm, mass about 1 kg, sphericity (maximum deviation from mean diameter) 50 nm, and surface roughness 0.1 nm. AIST noticed the excellent characteristic of the silicon solid density standard when the polishing technology was first developed at the CSIRO, and started working on a new density standard system that will replace water[27]-[30].3.1 Characteristic of silicon solid density standardSilicon is a fundamental material in the semiconductor industry, and highly pure, dislocation-free, large size single-crystals are obtained readily. Since there are three isotopes 28Si, 29Si and 30Si, the density of individual silicon crystal may relatively vary about 1×10-5 due to variations in natural isotropic compositions and mass fractionation effect during the crystal growth, but their mean density value is about 2329 kg/m3 at 20.000 °C and 101.325 kPa. Followings are outstanding characteristics when silicon single-crystals are used as a density standard.(1) Since silicon has near-perfect crystalline structure, its density is extremely stable once it is measured.(2) While water and mercury are liquid, and silicon single-crystals are solid, there is much less effect for silicon single-crystals due to degradation of chemical purity and changes in isotropic composition when they are used for calibrating densities of other materials.(3) Although the surface is covered with oxide layers, the density of the oxide layers is close to that of substrate silicon single-crystals, and thus the density change due to progressive oxidation is extremely small.Particularly, (2) is major motivation in promoting the development of a new density standard system replaced from water. Silicon solid density standard is not only highly precise, but it also has excellent usability unseen in the liquid density reference materials in terms of maintenance and management as well as actual operation conducted by the calibration service provider.3.2 Development of absolute measurement technology for densityFigure 1 shows the laser interferometer developed for absolute measurement of the volume of silicon single-crystals[31]. Since the size is selected so that the mass of the silicon sphere is to be about 1 kg to realize mass measurement traceable to the Prototype Kilogram of Japan, the diameter is about 94 mm. The volume of the sphere with low sphericity can be calculated accurately from the mean diameter by measuring its diameters from many orientations. Therefore, silicon spheres with a sphericity better than 100 nm are used for the solid density standard.The SI base unit for length, meter (m), is defined by the speed of light and the wavelength of a light beam determined by (36)−

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