A new kilogram standard made of silicon?
In the pursuit of creating standards that really are constant by definition, the General Conference on Weights and Measures has agreed to define four basic physical quantities by creating a reference to natural constants. These four constants are Ampere, Mol, Kelvin and the kilogram. The reason is that the current definition might be subject to variability over time – after all physical and chemical processes modify things over time, and the International Prototype Kilogram (IPK) – that iridium-platinum cylinder stored in Paris – is no exception.
The new definition for the kilogram will be based on the Avogadro constant and the Planck constant. In order to determine these two constants in a complex manufacturing and analysis chain, the scientists have to count the number of atoms in a silicon sphere. This shape of this sphere has to be extremely exact – the deviation from the "ideal" geometric sphere needs to be less than 100 nanometres (nm). The PTB scientists can measure the diameter of such a sphere at an accuracy of three atom diameters; an UHV reflectrometre at the Braunschweig, Germany based institute can determine the thickness of oxide layers on the surface with an exactness of 1 nm.
Also very high requirements apply as to the purity of the material, provided by the Electrochemical Plant in Zelenogorsk (Russia). This silicon will exhibit a purity of better than 99.998 percent with respect to its isotope substance; in use is polycrystalline silicon-28. Out of a large silicon-28 crystal, the PTB will produce two spheres. During the course of the next couple of months, the Leibniz Institute in Berlin will receive another six-kilogram crystal of the same material. Out of this crystal, the Berlin scientists will create a monocrystal with extremely "clean" internal structure and deliver it to the PTB. At the end, the PTB will have material for four spheres at its disposal. These four crystal balls with their extremely spherical shape will enable the scientists to deduct the combination of the mass of the spheres and the mass of an atom: They will measure mass and volume of their Si sphere as well as the order of the atoms in the crystal and the incidence of the three existing silicon isotopes – which will enable them to deduct the Avogadro constant and the Planck constant. They can count the number of atoms at high exactness: The deviation is 2 atoms on 100 million atoms counted.
For the definition of the kilogram, it theoretically would suffice to determine the Avogadro constant. However, the experts for mass definition also want to serve the needs of their colleagues working in the area of electric measurements. These experts are currently busy to redefine the Ampere through the value of the electric charge of the electron. Since Avogadro and Planck constant are linked, the international scientist community has agreed to define the Ampere as well as the kilogram through these constants.
The goal of these efforts is to have a reproducible standard that does not vary over time. The "new" kilogram however will only be defined if at least two scientific approaches and the experiments of three researchers independently of each other will achieve the same result of defining the Planck constant with sufficient accuracy. The approach of the PTB to determine this constant through a silicon-28 sphere is only one possible way to achieve this goal.
Scientists in Switzerland, the U.S., Canada, and France, prefer a different approach – they utilise the Watt balance that enables them to determine the weight of a test object very precisely by measuring an electric current and a voltage. Both approaches currently exhibit progress; the science community therefore is confident to have the new definition of the kilogram ready by 2018.
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