Shulou(Shulou.com)11/24 Report--

As the saying goes,

People go up, water flows down.

since ancient times,

People have an endless yearning for high places.

For physicists,

In addition to being able to climb geographical peaks

We should also continue to climb the heights of science.

This means that more extreme experimental environments need to be created constantly.

Part I is also afraid of Qionglou Yuyu. It is well known that people living in plains or low altitude areas are prone to altitude sickness due to lower air pressure, lower oxygen content, ultraviolet rays, low temperature and other factors after entering the plateau.

But you know what? In addition to people, the daily necessities we usually use will also be highly negative.

Items such as toothpaste and facial cleanser will gush out by themselves, and bags of puffed foods such as potato chips will swell and burst.

Photo source: Weibo, these phenomena are all caused by the pressure difference between the gas inside the object and the outside world caused by the decrease of external pressure.

So, the first question is, of course, why the higher the altitude, the lower the air pressure?

Let's first consider a small layer of thin atmosphere at a certain height, which is dz in thickness. High school physics tells us what to do with an object when we see it.

Of course, the force analysis is carried out first!

This atmosphere receives its own gravity, the pressure of the gas on the upper and lower surface, and we can assume that the pressure on the lower surface is p, and since the atmospheric pressure certainly varies with height, we can set the pressure on the upper surface to p+dp.

According to the force balance, there can be a formula (where An is the bottom area of the gas column and ρ (z) is the atmospheric density) [1]:

Simplify it to:

Here we can temporarily assume that the humidity of the atmosphere does not change with the temperature. Although this assumption is rough, it can be considered approximately valid at the altitude of 11-12km [2].

Bring into the ideal gas equation of state: pV=nRT, that is, pM= ρ (z) RT, where M is the molar mass of atmospheric molecules.

The variation of atmospheric pressure p with altitude z can be obtained by integral.

This model is also known as the isothermal atmosphere model.

Of course, our daily feeling is that the temperature tends to decrease with the increase of altitude.

Because the relationship between air pressure and temperature is very complex, we can use multi-state processes to describe:

Where γ is the specific heat ratio of the atmosphere, and p0 and T0 are the atmospheric pressure and temperature on the surface.

By integrating the above formula, we can get a more accurate variation of atmospheric pressure p with altitude z:

Through the calculation, we can know that when γ is 1.235, the change of atmospheric pressure with altitude is more accurate.

The variation curve of atmospheric pressure with altitude (Picture Source: Atmospheric pressure-Wikipedia) Part II if you want to see the scenery of thousands of miles, please climb another tall building. From the above discussion, we can know that in the plateau, the "high reaction phenomenon" of many objects comes from low pressure.

In other words, the expansion of potato chips and toothpaste comes from a relatively "high" pressure on the inside compared to the outside world. In addition, we all know that in plateau areas, the boiling point of water is low due to low air pressure, which leads to undercooking, and pressure cookers can solve this problem.

In addition to the pressure cooker used in our daily life, in fact, in the laboratory, we can also create some high-pressure environment by some means, so that all kinds of materials are under high pressure, thus we can find a lot of interesting phenomena.

The easiest way to achieve high pressure is to compress a gas or liquid. For example, for a gas, according to the previously mentioned equation of state of the gas: pV=nRT.

From this, it can be known that the pressure can be increased by compressing the volume of the gas in the isothermal compression process.

Therefore, the earliest high pressure scientific device was the piston-cylinder device designed and manufactured by the British physicist Parsons at the end of the 19th century.

Schematic diagram of piston-cylinder device and sealing method [3]

Due to the large volume of the pressurized cavity and the limitation of the material, this device can not apply very high pressure. Now it is often used to generate a pressure environment below 5GPa (about 50,000 atmospheric pressure).

Then in the mid-20th century, the American physicist Bridgman introduced a "anvil" (pronounced duck gizzard) device to achieve higher pressure, which is called the Bridgman press.

The schematic diagram of the pressure anvil shows that the internal pressure does not change after the edges and corners of the elastic material are cut off. The basic structure of modern high pressure technology is based on Bridgman press. The principle is to use anvil to support a large mass material, and the stress is transferred to the surface of uniform anvil to achieve uniform pressure distribution. The anvil material is generally cemented carbide.

Bridgman press [3] due to the large cavity of the piston-cylinder device, the limit pressure is low. The sample in the anvil device is thin, so it is easy to deform seriously.

In order to improve their shortcomings, Drickamar and Balchan combine the anvil and cylinder to design a Drickamar device. Because the side of the anvil is also supported, the ultimate pressure can be greatly increased.

Drickamar device [3] in order to apply a uniform pressure in the cavity, people have developed a polyhedral anvil to design the high-pressure cavity into a regular polyhedron shape, so as to achieve a more uniform static pressure environment in the cavity.

For example, with the anvil configuration in a regular hexahedral cavity device [3] it is well known that diamond is the hardest material known, so using diamond as anvil material is bound to achieve extremely high ultimate pressure. This is the diamond anvil.

The principle of high pressure experiment with diamond anvil is very simple: the sample is placed in the middle of two diamond faces, and then the sealing material in the middle is extruded to produce high pressure. The small hole of the sealing material is filled with liquid or solid pressure transfer medium, so that the sample in the sealing material is subjected to hydrostatic pressure or quasi-hydrostatic pressure.

Diamond chamber schematic [3] in more detail, a diamond chamber usually consists of a cavity body (including base, piston, cap with screws to increase pressure and control separation), anvil, diamond anvil and gasket, of which the diamond structure and gasket are the most important parts.

It is worth noting that because the texture of the diamond is very brittle, a huge force (about 10,000 N) needs to be applied to the diamond under pressure, so it requires extremely precise mechanical control to avoid direct contact between the two diamonds. the control accuracy must reach the micron level [4].

Anvil bases are often made of durable materials (such as tungsten carbide) to withstand the enormous forces that need to be applied. The gasket is a metal sheet with a hole, which is used to carry the sample. Its function is to better control the sample in a small area to ensure that the pressure gradient applied on it is uniform, while protecting the diamond from direct contact leading to fracture.

So far, the diamond anvil has reached the high pressure of 550GPa (5.5 million times atmospheric pressure).

Because diamond is a transparent material, it can pass through electromagnetic radiation such as visible light, near infrared light, x-ray and so on, so it can be widely used in high pressure science.

The physical figure of the diamond anvil [6] after the realization of the high pressure, the next step is how to calibrate the high pressure, that is to say, what is the method to know the value of the pressure applied?

The most reliable way to measure ultra-high pressure is to use the equation of state of known materials.

The equations of state of some compounds with simple structures under high pressure have been obtained by shock wave experiments. However, the biggest disadvantage of this method is that it needs to be measured by X-ray, which is not easy to achieve [4].

Schematic diagram of plane shock wave produced by light bubbles [3] in diamond anvil, the most commonly used method is that Forman et al first proposed a spectral method for pressure calibration [5]. The most commonly used pressure calibration material is ruby.

The main component of ruby is alumina doped with Cr3+ ions. There are a series of energy levels in Cr3+ ions. After absorbing a certain energy of light, electrons will first jump to a higher energy level, and then spontaneously transition to another lower energy level, thus emitting fluorescence.

The electronic level structure of ruby [3] the fluorescence wavelength of radiation varies with the change of pressure. Therefore, the current pressure state of ruby can be determined by measuring the wavelength of radiated fluorescence of ruby.

Laser ruby pressure measurement system [3] at the same time, rubies can be processed into micron size and can produce good signals, so the realization of this technology has greatly promoted the development of high-voltage experimental technology.

Part III one day I will climb to your top and have a clear view of the small mountains around me. When matter is in a high-pressure environment, they will also show some different properties from those under normal pressure. We can call them high-pressure reactions (or high reactions for short).

Our most common function of high pressure should be a pressure cooker, which can make food more easily cooked, such as making meat stew softer and rotten. This is because the boiling point of water rises under high pressure. The temperature of boiling water is higher, so cooking food at a higher temperature will certainly get twice the result with half the effort.

The most direct effect of Pixabay high pressure on matter is to compress the volume of the object, that is, to reduce the distance between atoms in the matter. As a result, the gas becomes liquid, the liquid becomes solid, and the density of solid increases.

For example, under higher pressure, water will not only turn into ice, but as the pressure increases, people find that the properties of ice are also changing. Under 0.2GPa, water will form an unstable ice structure under atmospheric pressure, which is actually a new ice structure. Under higher pressure, water also has more than a dozen different structures [3].

Phase diagram of water [3] in addition, changes in atomic spacing may cause changes in the arrangement of atoms.

For example, an atom can interact with an atom that is far away before, thus increasing the coordination number. Or the interaction between atoms changes, thus affecting the strength of the chemical bond. In other words, the pressure causes the structural phase transition of the crystal. For example, graphite can be turned into diamond under high temperature and high pressure.

Furthermore, due to the reduction of the atomic spacing, the energy levels between different atoms overlap more easily, which is shown in the crystal that the pressure leads to the change of the crystal energy band.

For semiconductors or insulators, high voltage can induce the overlap of conduction bands and valence bands, thus turning semiconductors or insulators into conductors.

For example, I2 changes from insulated state to metal state under 16GPa pressure [6]. NaYbSe2 changes from insulator to metal under 50GPa pressure. How to achieve high temperature superconductivity is regarded as the pearl in the crown of condensed matter physics, and high pressure science provides a means to pick this pearl.

In the BCS theory of conventional superconductors, the transition temperature of superconductors is inversely proportional to the atomic mass of the superconductors.

This means that the crystal composed of elements with smaller atomic mass is more likely to have a higher superconducting transition temperature.

So the hydrogen atom, as the lightest atom, may have a very high superconducting transition temperature if the hydrogen in the solid state can be prepared.

Unfortunately, even under the current extremely high pressure of hundreds of GPa (millions of times atmospheric pressure), the possibility of solidifying hydrogen has not been found. But in the second place, scientists have prepared hydrogen-rich compounds: SH3 (203K) and LaH10 (250K), which have high superconducting transition temperatures [4].

In addition to conventional superconductors, the superconducting transition temperature of high temperature copper oxide superconductors can also be increased under high pressure. The superconducting transition temperature of La-Ba-Cu-O system increases from 35K to 52K under 1.4GPa. This inspired people to choose yttrium with smaller atomic radius to replace lanthanum, thus raising the superconducting transition temperature to 93K. The mercury-barium-copper-oxygen system can even reach the transition temperature of 164K under high pressure.

The development of high-pressure high-temperature superconductors [4] in short, ultra-high pressure technology, as an extreme experimental environment, is the embodiment of physicists constantly climbing the scientific peak.

The realization of ultra-high pressure technology has also opened up a wide range of fields for the study of new materials and the discovery of new physics.

References:

[1] Qin Yunhao, Thermal Science. Higher Education Press, third Edition, 2011

LV Jun, Wang Xia, use the improved genetic algorithm to calculate the atmospheric pressure formula. College Physics, Volume 23, No. 3, end of 2004

[3] Liu Zhiguo, Qian Zheng male, high voltage technology. Harbin Institute of Technology Press, 2012

[4] J. A. Flores-Livas, M. Eremets etc. A perspective on conventional high-temperature superconductors at high pressure: Methods and materials, Phys. Rep. 856, 1 (2020)

[5] Forman RA, Piermarini GJ, Dean Barnett J, Block S. Pressure measurement made by the utilization of ruby sharp-line luminescence. Science. 1972 Tan 176 (4032): 284-5.

Guo Siyang, study on phase transition and elastic properties of carbon dioxide and silicon dioxide under high pressure. Doctoral thesis of Jilin University, 2021

[7] Ya-Ting Jia etc. Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe2. Chin.Phys.Lett, 37, 097404 (2020)

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). Author: Xiao Ming, who doesn't like physics.

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