Experimental research on mass determination of non-metal weights

With the revision of the definition of the kilogram, more and more weights made of non-metal material, such as silicon spheres and PM 2.5 film, need to be determined with high accuracy. As conventional mass is commonly used in mass metrology, this paper discusses the mass-measuring process for silicon and PM 2.5 film. Electrostatic eliminators are used to eliminate the electrostatic effects of non-metal material to improve the stability of mass measurement. Due to the big difference of weights’ densities, the air buoyancy correction and its uncertainty evaluation are also conducted.

Investigation of the repeatability of a cleaning method for silicon spheres by gravimetric measurements

After a short description of the metrological relevance of silicon spheres and the applied cleaning method, mass measurements of silicon spheres in vacuum and the evaluation of the results are outlined. The results show that the mass of silicon spheres in vacuum stabilises in the microgram range within two to three days after cleaning and that the repeatability of the applied cleaning method can be characterised by a standard deviation in the order of two micrograms.

A Computational Comparative Study of the Lithium Diffusion in Amorphous Silicon Spheres, Rods, and Circular Disks

We study through extensive finite element analysis the lithium diffusion in small elements of Si anodes under the forms of spheres, rods, and circular disks for Li-ion batteries. Elastoplastic properties of the amorphous silicon are assumed to be lithium concentration-dependent. Effects of the normalized flux of Li-ions on the lithium concentrations, stresses, and total equivalent plastic strains are considered. Effects of the disk's thickness are also included. At a given normalized flux, the heterogeneity of the lithiation, stresses, and plastic deformation increases in the order: disk, sphere, and rod. The thinner disk the better performance is. Below a critical value of the normalized flux of Li-ions, silicon spheres and disks exhibit linear elasticity and homogeneous distribution of Li-ions, whereas silicon rods undergo always plastic deformation after lithiation. When the radii of these three structures are smaller than several micrometers and the normalized flux is taken as 95% of their critical value, the charge time falls in the range from minutes to several hours. Our findings will help to optimize the charge and geometrical parameters for silicon anodes.

Investigation of a cleaning procedure for silicon spheres used in the realization and dissemination of the redefined kilogram via combined spectroscopic and gravimetric measurements

The new definition of the SI kilogram requires new methods of realizing this unit. The X-ray crystal density method is a primary realization method and uses silicon spheres. The spheres get cleaned before each measurement, in order to remove surface contaminations and thus reduce their uncertainty contribution to the realization. Therefore, cleaning is an inherent part of the realization and dissemination of the kilogram. A cleaning method for silicon spheres is investigated, concerning its suitability as a part of the realization of the redefined kilogram. Six silicon spheres were used to determine the repeatability of the established cleaning method. Measurements of the spheres' mass and the quantification of their surface layer mass after cleaning were carried out in several cycles resulting in 29 mass and surface measurements. The repeatability of the cleaning method applied shows a standard deviation in the order of two micrograms for both the mass and the surface layer. The cleaning method therefore sufficiently fulfils these requirements.

The molar mass of a new enriched silicon crystal: maintaining the realization and dissemination of the kilogram and mole in the new SI

The local distribution of the isotopic composition and molar mass M of a new silicon crystal (Si28-24Pr11) highly enriched in the 28Si isotope is reported, with focus on the experimental methods as well as on the associated uncertainties. The crystal was used in 2018 for the production of two additional silicon spheres for the realization and verification of the Avogadro constant NA using the “X-ray-crystal-density (XRCD) method” which is a primary method for the dissemination of the revised SI units mole and kilogram. 17 subsamples have been investigated (from five different axial and in several radial positions) by isotope ratio mass spectrometry using a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS). The average molar mass of the crystal is M = 27.976 933 787(77) g/mol with a relative combined uncertainty uc,rel(M) = 2.7 × 10−9. The mean amount-of-substance fraction of 28Si is x(28Si) = 0.999 993 104 (66) mol/mol indicating that this crystal has the highest enrichment in this isotope which has ever been used for the determination of NA. No local variations in M and x(iSi) (i = 28, 29, and 30) could be identified due to material properties. The results are compared with those from two previous enriched crystals.

Electrochemical Sensor of Double-Thiol Linked PProDOT@Si Composite for Simultaneous Detection of Cd(II), Pb(II), and Hg(II)

Heavy metal ions in water, cosmetics, and arable land have become a world-wide issue as they cause a variety of diseases and even death to humans and animals when a certain level is exceeded. Therefore, it is necessary to development a new kind of sensor material for the determination of heavy metal ions. In this paper, we present an electrochemical sensor based on composite material (thiol(–SH) grafted poly(3,4-proplenedioxythiophene) (PProDOT(MeSH)2)/ porous silicon spheres (Si) composite, denoted as PProDOT(MeSH)2@Si) from the incorporation of thiol(–SH) grafted poly(3,4-proplenedioxythiophene) (PProDOT(MeSH)2) with porous silicon spheres (Si) for the electrochemical detection of heavy metal ions (Cd(II), Pb(II), and Hg(II)). The PProDOT(MeSH)2@Si composite was synthesized via a chemical oxidative polymerization method. The structure and morphology of PProDOT(MeSH)2@Si composite were characterized by Fourier transform infrared (FT-IR), Ultraviolet–visible spectroscopy (UV–Vis), X-ray diffraction (XRD), scanning electron microscope (SEM), Transmission electron microscope (TEM), and Brunauer−Emmett−Teller (BET). Furthermore, the electrochemical performance of PProDOT(MeSH)2@Si was evaluated by detecting of Cd(II), Pb(II), and Hg(II) ions using the differential pulse voltammetry (DPV) method. The relationship between structural properties and the electrochemical performance was systematically studied. The results showed that the entry of two thiol-based chains to the monomer unit resulted in an increase in electrochemical sensitivity in PProDOT(MeSH)2, which was related to the interaction between thiol group(-SH) and heavy metal ions. And, the combination of PProDOT(MeSH)2 with Si could improve the electrocatalytic efficiency of the electrode material. The PProDOT(MeSH)2@Si/GCE exhibited high selectivity and sensitivity in the rage of 0.04 to 2.8, 0.024 to 2.8, and 0.16 to 3.2 μM with the detection limit of 0.00575, 0.0027, and 0.0017 µM toward Cd(II), Pb(II), and Hg(II), respectively. The interference studies demonstrated that the PProDOT(MeSH)2@Si/GCE possessed a low mutual interference and high selectivity for simultaneous detection of Cd(II), Pb(II), and Hg(II) ions.