Shulou(Shulou.com)12/24 Report--

When it comes to vacuum, what comes to mind first?

Do you need vacuum packaging to send goose legs to friends?

No, no, change your mind. What else can you think of?

Is it the "sound cannot be transmitted in a vacuum" learned in the second grade of primary school, or the "Torricelli experiment" learned in the second grade of junior high school, or the legendary "Madeburg hemispheric experiment"?

Whatever you think of, a vacuum is no stranger to us.

If you understand the definition of vacuum, you will be surprised to find that vacuum is not mysterious at all, and you often deal with it.

The definition of vacuum comes from the Greek word "vacuum", which means "empty", but vacuum is not a state of emptiness, and there are a large number of matter molecules even at the edge of the Milky way. The general definition of vacuum in the scientific community is: vacuum refers to "the state of gas whose pressure is lower than an atmospheric pressure".

So think about it, when we drink milk, drink oral liquid, the low pressure in the bottle can be called a "vacuum"?

To put it more simply, plug the part of the needle with your hand and pull the piston. There is a vacuum inside the needle.

it is beyond logic and above reason?

It doesn't matter. Let's take a look at the history of vacuum discovery.

As early as 1643, Italian physicist Torricelli did a famous atmospheric pressure experiment-the Torricelli experiment.

He filled a closed glass tube with mercury (Hg) and then buckled the glass tube upside down in a small trough containing mercury. The mercury in the glass tube dropped under the force of gravity. He found that when the height of the mercury column dropped to 760mm, it no longer fell. Torricelli believed that the gap at the top of the glass tube was a "vacuum". At the same time, this experiment also concluded that a standard atmospheric pressure is about 760mm mercury.

Torricelli experiment because this experiment revealed the existence of a "vacuum", in memory of Torricelli, "Torr" became a unit to measure vacuum.

Time flies. In 1654, Otto von Gehrik, a German physicist and mayor of Madeburg, did the famous Madeburg hemispheric experiment to further verify the existence of atmospheric pressure.

In the Madeburg hemispheric experiment, he put two metal hemispheres about 50 centimeters in diameter together, emptied them into a vacuum, and then pulled them in the opposite direction with eight tall horses, but could not pull them apart. This experiment vividly and contagiously proves the existence of atmospheric pressure and makes us realize the charm of vacuum. Because the experiment was carried out in the city of Madeburg in Germany, it is called the Madeburg hemispheric experiment.

The unit and classification of vacuum, then according to Torricelli's experiment, we can get the following relationship.

1 standard atmospheric pressure (atm) = 760mmHg=760Torr

According to the theory of the motion of gas molecules, molecules are constantly moving irregularly. In motion, molecules will constantly collide with each other, or collide with the wall of containers from time to time. These collisions produce macroscopic phenomena such as temperature and pressure in a statistical sense. The number of molecules in the container can be measured by pressure, and the number of molecules reflects the vacuum. So the vacuum can be measured by pressure.

The relationship between vacuum and pressure is that the higher the vacuum is, the smaller the pressure is; the lower the vacuum is, the greater the pressure is. Because of this, the unit of vacuum is the unit of pressure.

A standard atmospheric pressure is expressed in Pa as

1atm=1.013 × 105Pa

In this way, we get the conversion relationship between different vacuum units.

With the unit, we can divide the vacuum degree. There is no unified standard of division in the world, and the commonly used division is like this.

Rough vacuum: 760Torr~1Torr

Low vacuum: 1Torr~10-3Torr

General vacuum: 10-3Torr~10-6Torr

High vacuum: 10-6Torr~10-9Torr

Ultra high vacuum: 10-9Torr~10-12Tor

So with this standard, the vacuum in the syringe we mentioned above and the vacuum in Torricelli's experiment belong to the category of rough vacuum.

So how is the ultra-high vacuum commonly used in the laboratory realized?

This is related to our core content-vacuum pump.

Vacuum pumping equipment-the vacuum pumps commonly used in vacuum pump laboratories include mechanical pumps, turbine molecular pumps, ion pumps, titanium sublimation pumps, cryogenic adsorption pumps and hydrogen suction pumps. It is these different kinds of pumps that can be pumped step by step in order to achieve ultra-high vacuum.

Molecular pump We know that air is made up of a large number of gas molecules, the highest content is nitrogen, oxygen, as well as a small amount of carbon dioxide, hydrogen, water vapor, rare gases and so on.

The fewer gas molecules in an airtight container, the higher the vacuum, so the purpose of vacuum is to reduce the number of gas molecules in the cavity as much as possible, which is what these vacuum pumps do.

Hydrogen suction pumps these vacuum pumps can be roughly divided into two categories according to their working principle. There are pumps that continuously inhale and discharge gases out of the body, such as mechanical pumps and turbine molecular pumps, and pumps that adsorb gas molecules on the inner wall, such as titanium sublimation pumps, cryogenic adsorption pumps and hydrogen suction pumps. Regardless of the working principle, the ultimate goal of these pumps is to reduce the number of gas molecules in the cavity.

Due to the different structure of different pumps, each pump has its own working range, which can work normally only within a certain range of pressure, otherwise it will be damaged. The working pressure range of different pumps is shown in the following figure.

From the picture above, we can see that it is impossible to achieve ultra-high vacuum with only one kind of pump, which requires different kinds of pump relay, which is very much like our scientific research process, isn't it? Teamwork.

In the laboratory, we first use the mechanical pump to pump the coarse vacuum, then turn on the molecular pump to pump the high vacuum, and finally use the ion pump or titanium sublimation pump to pump the vacuum needed for the experiment.

Why do laboratories need ultra-high vacuum conditions in small classes?

Imagine you become the same size as a molecule, like in Ant Man, and you will see the number of molecules of the order of 1023 in the air, which fall on the surface of the sample and contaminate the sample and cannot detect the properties of the surface of the sample.

So how many molecules are there in an ultra-high vacuum cavity? Let's do a simple calculation.

In the course of statistical mechanics, we have learned that by using the laws of classical mechanics and regarding gas molecules as rigid balls, the formula of gas pressure can be obtained by statistical method.

Where n is the numerical density of gas molecules and m is the molecular mass. According to the international system of units, the unit of pressure is Pa.

According to the law of equipartition of energy, each gas molecule is regarded as a particle (that is, simplified to a point with mass). There are three vibrational degrees of freedom, and the average energy of each degree of freedom is

Among them, kink is 1.38 × 10-23J / K, which is Boltzmann constant. Then

This gives the relationship between pressure P and molecular density n as well as temperature T.

If we vacuum up to 5 × 10-8Pa at 500 ℃, that is, Tunable 773K, then

It can be seen that although we have pumped an ultra-high vacuum, there are still a lot of molecules in the cavity, which is why we have to experiment in an ultra-high vacuum.

Having said so much, I wonder if you have ever thought, how do we know how high the vacuum in the cavity is?

Instruments for measuring vacuum-vacuum gauges Yes, just as we have thermometers and hygrometers, we also have instruments for measuring vacuum-vacuum gauges.

We already know that the vacuum is measured by the pressure, so the vacuum gauge reflects the vacuum by measuring the pressure.

According to its working principle, vacuum gauges can be roughly divided into two categories: one is to measure the pressure directly, which is called absolute vacuum gauge, such as U-shaped vacuum gauge and compression vacuum gauge; the other is to indirectly calibrate the value of pressure by measuring physical quantities related to pressure, which is called relative vacuum gauge, such as heat conduction vacuum gauge and ionization vacuum gauge. The counterpart vacuum gauge is commonly used in the laboratory.

Heat conduction vacuum gauge is a kind of vacuum gauge for measuring low vacuum. As its name implies, it is an instrument that uses heat change to measure vacuum. Its principle is to heat a wire electrically, because the movement of gas molecules takes away part of the heat, so the higher the temperature when reaching the heat balance, the less heat the gas conducts and the smaller the density of the gas molecules, that is, the lower the pressure is, the higher the vacuum.

Ionization vacuum gauge is the most widely used vacuum gauge at present. it is a kind of vacuum gauge for measuring high vacuum, which can be opened only when the vacuum is pumped to a certain order of magnitude. Its working principle is that electrons with sufficient energy collide with gas molecules to ionize gas molecules and produce positive ions and electrons, and the number of collisions between electrons and molecules is proportional to the molecular number density, that is, pressure P. therefore, the number of positive ions produced is proportional to the pressure P, and the vacuum can be calibrated by measuring the number of positive ions.

Summary if you can see here, then congratulations, you already know quite a lot about vacuum.

In a word, the vacuum needs to be pumped at one stage, and the vacuum pumping instrument is the vacuum pump. The vacuum degree is actually measured by the pressure, and the equipment used is the vacuum gauge.

So, after a big meal in a vacuum ocean, don't forget the goose leg in your hand, your stomach is still waiting for a big meal.

References:

Zhao Baosheng, vacuum Technology, Science Press

Guo Fangzhun, practical vacuum Technology, Dalian University of Technology Press

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), by A Bai.

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