This is the roundest object in the world, worth tens of millions, and its existence can change the world

Which object on Earth is closest to being a true "circle"?

The answer is not a gift from nature, but the pinnacle of human technology - a sphere made of ultra-pure silicon.

Image source: nist

How round is it? If the silicon sphere were the size of the Earth, the distance between the deepest ravine and the highest mountain on the sphere would be only 5 meters... almost perfect.

This sphere was no random experimental product, but a scientific miracle whose name was written into the legend of physics: the Avogadro Project.

Avogadro Project Source: sci-galary

Currently, there are 7 silicon spheres of the Avogadro Project in the world. These spheres are extremely expensive to manufacture, and each is worth about 3 million US dollars. These 7 silicon spheres are used to accurately measure the Avogadro constant to redefine the mass standard of the kilogram.

Their extremely high precision is essential to ensuring the consistency and stability of the International System of Units, and these spheres are also among the most sophisticated scientific instruments in the world, representing the highest level of human achievement in scientific measurement.
The story begins in the 1990s. The definition of the International System of Units was facing an unprecedented crisis: the original kilogram standard, a cylinder made of platinum-iridium alloy stored in Paris, had changed its mass slightly over time. In short, 1 kilogram was no longer 1 kilogram. It was no longer accurate.

The original kilogram standard (Big K) Image source: courtesy of BIPM

This drift is unpredictable and uncontrollable, which is unacceptable for scientific fields that rely on precise measurements. Especially in fields such as pharmaceuticals, material sciences, and precision manufacturing, small quality changes can lead to inconsistent experimental results, which in turn affects product quality and safety. These fields require absolutely stable quality standards to ensure consistent measurement and production worldwide.

So, a group of scientists from around the world decided to find a new quality standard that would not drift over time.

Dr. Andreas, a German physicist, is one of the core figures of this project. He worked as a senior researcher at the German National Metrology Institute (PTB), focusing on precision measurement and metrology research. In the Avogadro Project, Dr. Andreas led the manufacturing and measurement of silicon spheres to ensure unprecedented accuracy. He once mentioned in an interview, "Our goal is not just to make an object, but to make it part of the natural constant." Thus, a bold idea was born: to redefine the quality standard by making a perfect silicon sphere, to be precise, a silicon-28 sphere.

Image source: MDPI

So why choose the silicon-28 isotope?

Because silicon has extremely high thermal stability and mechanical strength, it is not easily affected by environmental changes (such as temperature, humidity or pressure). In addition, silicon itself has good chemical inertness and will hardly react with oxygen or other chemicals in the air after purification, ensuring that its surface and volume remain stable during long-term storage.

Silicon exists in nature in three main isotopes: silicon-28 (92.23%), silicon-29 (4.67%), and silicon-30 (3.10%). Silicon-28 is the absolute majority, so purifying silicon-28 is more economical and technically feasible than other isotopes.

Image source: Wikipedia

The purified silicon-28 is composed entirely of the same type of atoms, eliminating errors caused by differences in mass and atomic radius between different isotopes. This uniformity is crucial for calculating the number of atoms and lattice parameters.

Image source: springer

This silicon ball is made of 99.9999% ultra-pure silicon, but why not 100%?

This is because in reality, true 100% purity is almost impossible to achieve. Even the most advanced purification technology is difficult to completely eliminate trace impurities, because at the atomic level, any material will be affected by other atoms in the external environment. These tiny impurities may enter the material during extraction, processing, and even storage. Reaching 99.9999% purity is already a limit. Further purification is not only extremely costly, but also faces technical challenges such as atomic interactions and the physical limits of equipment.

In contrast, although platinum-iridium alloys have high density and corrosion resistance, their surfaces absorb pollutants in the air, such as moisture, carbon dioxide, and organic compounds, due to long-term exposure to the environment. These pollutants can cause small mass losses. This change in mass is unpredictable, and after many years, even small changes can lead to inconsistencies in the definition of mass.

Scientific research has shown that the platinum-iridium alloy kilogram prototype exhibits non-negligible mass drift in different environments, and these drifts are believed to be caused by the chemical instability of the material and long-term surface oxidation.

Why was a sphere chosen as the shape of the silicon benchmark object in the Avogadro Project?

The perfect symmetry of a sphere allows its surface area and volume to be accurately calculated using mathematical models, which is crucial in measurement. Compared to other shapes, such as cubes or irregular objects, the symmetry of a sphere minimizes errors, ensuring highly consistent and repeatable measurements.

If other shapes are chosen, the complexity of measuring surface area and volume will increase dramatically, resulting in more accumulated errors. In addition, the spherical shape also minimizes the impact of surface inhomogeneities, which is crucial for calculating the exact number of atoms.

More importantly, the shape of a sphere is uniform when subjected to force, which means that it is not easy to deform during transportation and storage, ensuring that its physical properties remain stable. This stability is exactly the foundation needed when redefining the International System of Units.

The production process of silicon balls is extremely complicated.

First, the silicon-28 isotope is separated from natural silicon. This step requires the use of high-precision equipment such as gas centrifuges to increase the purity of silicon to 99.9999% to ensure the quality and stability of the final product.

After that, high-purity silicon-28 gas is deposited on the seed crystal by chemical vapor deposition (CVD) to grow a large-sized single-crystal silicon ingot. This process needs to be carried out in an ultra-clean environment to prevent the introduction of impurities.

Image source: Internet

Next, the single crystal silicon ingot is cut into rough billets that are close to spherical. High-precision diamond tools are used during the cutting process to ensure the accuracy of the size. The rough billets are precisely ground and polished multiple times to gradually approach a perfect spherical shape. During this process, nano-level abrasives and polishing fluids are used to ensure that the surface smoothness and roundness reach the limit.

In order to ensure that every part of the ball is nearly perfect, scientists also used advanced technologies such as laser interferometry and X-ray crystal analysis for measurement. Based on the measurement results, the sphere was fine-tuned until its deviation was controlled at the nanometer level. These technologies not only helped them ensure the perfect shape of the ball, but also paved the way for defining Avogadro's constant by calculating the number of atoms.

Based on this silicon ball, scientists were able to accurately calculate a nearly perfect Avogadro constant, thus providing a stable basis for the new definition of the International System of Units. In this way, the definition of the kilogram bid farewell to the drift error of the platinum-iridium alloy and achieved a truly global unified standard.

On November 16, 2018, a historic scientific conference was underway at the Palace of Versailles in France. On this day, scientists from all over the world bid farewell to a "abdicated": the International Prototype Kilogram - the platinum-iridium alloy cylinder that has defined "1 kilogram" for the past 139 years.

Instead, the world's roundest ball, which is introduced today, became the new definition of the kilogram based on the constant of nature.

From the corners of the earth to the atomic world, humans have never stopped pursuing the ultimate. This silicon ball not only fulfills the dream of physics, but also allows us to see the infinite possibilities of science itself.

This is not only a story about units of mass, but also an exploration of the human mind - an unremitting pursuit of ultimate beauty, a great journey from micro to macro.

References

[1https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project

[2https://www.scientificamerican.com/article/sphere-made-to-redefine-kilogram-has-purest-silicon-ever-created/

Planning and production

Source: Bring Science Home (ID: steamforkids)

Author: Su Chengyu

Editor: Yang Yaping

Proofread by Xu Lai and Lin Lin