The Prototype Kilogram is losing weight, prompting efforts to redefine the base unit for measuring mass

The International Prototype Kilogram (IPK) is a physical block weighing as close to 1kg as is physically possible to engineer. It is used as the primary standard for virtually all units of mass and is the mass upon which the weight of a kilogram is based and all traceability mass measurements are measured. However, a strange phenomenon has been observed – the IPK is getting lighter. The reason, however, is unknown.

Now researchers are seeking alternatives to the platinum-iridium alloy artifact that is stored in a safe in Paris. The basic plan is to redefine the kilogram and in future, a physical constant will replace the material kilogram.

Here, the Avogadro experiment is to be used to determine the number of atoms in an almost perfect silicon sphere. Researchers at the Fraunhofer Institute have recently succeeded in homogeneously coating a spherical surface to make the measurements, with a level of certainty to within 10µg.

Thus, a team of scientists from the Physikalisch-Technische Bundesanstalt (PTB) (the national metrology institute of Germany) is conducting experiments with spheres of isotope-enriched silicon, which could be used as a new calibration standard. The experts, therefore, must determine the Avogadro constant, which indicates the number of atoms in one mole.

“We calculate the number of atoms in a sphere and use mathematical methods to obtain the number of atoms per mole,” says Dr. Ingo Busch, physicist at the PTB in Braunschweig. “In simple terms, we find out how much a silicon atom weighs and through inverse conclusion can thus determine how many silicon atoms are needed for a kilogram. The mole is the mediator between the atomic mass scale and the kilogram.”

During the production of these spheres at the PTB, a natural oxide layer of silicon dioxide (SiO2) is formed. This also has an influence on the mass and volume of the silicon spheres. The problem is that the native layer grows slowly and, in part, unevenly. This makes it very difficult to measure the actual weight of both the oxide layer and the sphere. Therefore, an alternative, homogeneous coating is required to reduce measurement inaccuracies and precisely determine the volume and mass of the sphere.

Alternative SiO2 layer

Researchers of the neighbouring Fraunhofer Institute for Surface Engineering and Thin Films IST have recently succeeded in bringing forward a solution on coating a silicon sphere with such an SiO2 surface that it meets highest standards.

“With our method, we can apply a layer of SiO2 with a precisely defined roughness and an adjustable layer thickness to the sphere,” says Tobias Graumann, a scientist at IST. “In addition, the layer is also stoichiometric, which means that the ratio of the individual atoms remains constant among each other or the ratio between silicon and oxygen.”

For the coating, the researchers selected Atomic Layer Deposition (ALD). The advantage of this method is that is offers a reproducible, extremely thin oxide layer where a homogeneous thickness can be applied to the sphere.

Potential impurities, such as carbon or nitrogen are below the limit of detection. The roughness of the layers also remains below a nanometer.

“The sphere’s roughness is not significantly increased by the coating,” says Graumann. “This is a factor which keeps the measurement inaccuracy below 10µg. Even a fingerprint weighs more.”

And the time factor also plays an important role. By applying SiO2, the manufacturing process can be accelerated. In contrast, the growth of the native oxide layer can take several months.

Coating in clean room

The ALD coating plant installed at the Institute was specially adapted and prepared for the project so that all work to do with the coating could take place in a clean room atmosphere. The main focus over the years of research was on how the silicon sphere was mounted in the reactor. Since the sphere has to be coated over the full area, the researchers decided to use a three-point mounting system, meaning the sphere being measured makes contact at three points.

“Here, we take advantage of the ALD’s mechanism,” says Grauman. “The gaseous chemicals typically diffuse between the sphere and the three contact surfaces of the mount, which are also coated in the process.”

The coatings of the silicon sphere have been concluded and the measurements are being performed at the PTB.

The researchers at the Fraunhofer IST and their colleagues at the PTB hope that the silicon spheres will become the new calibration standard with metrology institutes and calibration laboratories being given the opportunity to acquire copies of the spheres.

The results will be presented at the Conference on Weights and Measures in autumn 2018 when the new sphere will replace the original kilogram as standard.

The International Prototype Kilogram

The International Prototype Kilogram is made of a platinum alloy known as Pt‑10Ir, which is 90% platinum and 10% iridium (by mass). It is machined into a right-circular cylinder where the height is equal to the diameter of about 39mm to minimise its surface area.

The addition of 10% iridium improved upon the all-platinum Kilogram of the Archives by greatly increasing hardness while still retaining platinum’s many virtues: extreme resistance to oxidation, extremely high density (almost twice as dense as lead and more than 21 times as dense as water), satisfactory electrical and thermal conductivities, and low magnetic susceptibility.

The IPK and its six sister copies are stored at the International Bureau of Weights and Measures in Paris in an environmentally monitored safe in the lower vault. Three independently controlled keys are required to open the vault. Official copies of the IPK were made available to other nations to serve as their national standards. These are compared to the IPK roughly every 40 years, thereby providing traceability of local measurements back to the IPK.

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