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

Original title: "Cold hands and scalded mouth, holding a thermos cup I fantasize..."

Ding ~ Your winter will arrive soon. Please go to the door to collect it.

Winter is coming and the temperature is getting colder

Xiao Bian's hands are frozen to the point of losing consciousness, and it's even difficult to hit the keyboard.

At this critical moment, if you can hold a cup of hot water to warm your hands and heart

Thinking of this, Xiao Bian immediately picked up a thermos cup and poured a cup of hot water into his palm. As a result, he only felt the coolness of the metal.

Why can't I feel warm when I'm holding hot water?

Temperature So what is temperature? At this time, someone wanted to answer first. Hot was high temperature, cold was low temperature.

There is nothing wrong with this statement. Temperature is a physical quantity that indicates how cold or hot an object is. Microscopically, what is temperature?

Temperature is the violent degree of molecular motion of an object, and it is a sign of the average kinetic energy of molecules of an object.

For example, this is a high temperature ~:

Source: bilibili This is low temperature ~:

Source: bilibili heat transfer heat transfer is achieved by heat conduction, heat convection and heat radiation.

Heat transfer process in which particles move relative to each other in a convective fluid.

Thermal radiation All objects whose temperature is above absolute zero produce thermal radiation, which radiates heat away in the form of radiation.

Heat conduction When two objects with different temperatures contact each other or there is a temperature difference inside the same object, energy transfer occurs through microscopic vibrations, displacements and collisions of molecules, atoms and electrons inside the object, making the two parts tend to be the same.

The hot water in the cup mainly transfers heat through heat conduction with the air and the wall of the cup. The temperature of the hot water drops and the temperature of the cup rises, making the cup of water in the hand feel hot.

The vacuum cup has a double-layer structure, and a vacuum layer is added between the shell and the inner container. Due to the low gas density in the vacuum layer, the collision between gas molecules and the inner container is lacking, thereby reducing heat conduction and heat convection; at the same time, the inner container of the vacuum cup is plated with copper or silver, which can reflect thermal radiation and effectively reduce the heat lost through radiation.

There's a hundred million points between me and this cup of hot water in my hand ~

The word vacuum is not unfamiliar, and the state of gas below one atmosphere in a given space is called vacuum.

There are different degrees of vacuum (vacuum degree):

Low vacuum range: 105~103 Pa

Medium vacuum field: 103~10-1 Pa

High vacuum field: 10-1~10-5 Pa

Ultra-high vacuum field: 10-5~10-8 Pa

Ultra-high vacuum field: < 10-8 Pa

When you think of vacuum, what comes to mind first?

Xiao Bian guessed that everyone must think of space and astronauts. The gas in space is thin and cannot breathe, so astronauts need to wear space suits and maintain breathing through oxygen cylinders.

Could it be that the thermos cup was made by astronauts in space and then transported back to Earth? Is that rounded up and small series also a person who has been to space?

Hey, if that's the case, Xiaobian should not be able to afford a thermos cup!

So how is the vacuum used in everyday life achieved? Driven by curiosity, Xiao Bian discovered this artifact_vacuum pump!

Vacuum pump refers to the use of mechanical, physical, chemical or physicochemical methods to exhaust gas from a vacuum chamber at high speed to achieve the purpose of generating, improving and maintaining vacuum. According to the different working principles, it can be divided into two types: gas transportation mode and adsorption mode.

oil-sealed rotary vane pump

Principle of oil-sealed rotary vane pump;1-eccentric rotor;2-rotary wing;3-pump body; Source: Reference 1 oil-sealed rotary vane pump belongs to low vacuum pump, working range is 10-1~10-5 Pa. It is mainly composed of eccentric rotor, rotating wing and pump body. An eccentric rotor is installed in the cavity of the rotary vane pump, the rotary wing is connected with the rotor through a spring, and the top end of the rotary vane is kept in contact with the inner wall of the pump cavity by virtue of centrifugal force and tension of the spring.

Oil seal rotary vane pump schematic motor drive eccentric rotor rotation, pump body and rotary wing surrounded by two parts of the volume of space changes constantly. When the volume of the space communicated with the suction port is increased, the air pressure is reduced, and gas is sucked from the suction port; when the volume of the space communicated with the exhaust port is reduced, the gas is exhausted from the exhaust port, and the vacuum environment is obtained by continuously extracting the gas in the cavity.

The oil-sealed rotary vane pump is simple to operate and can be used at atmospheric pressure, but the oil used in the rotary vane pump will volatilize and enter the vacuum chamber. Xiao Bian recalled that when doing the experiment, the samples were contaminated by engine oil!

turbo dry pump

Turbo-type dry pump is composed of two vortex rotor and vortex stator with conjugate vortex lines and phase aberration of 180°. A series of crescent compression chambers are formed between the two vortices. An exhaust port is arranged at the center of the vortex body, and an air suction port is arranged at the periphery of the vortex body. With the movement of the scroll rotor, the volume of the compression chamber gradually decreases and moves toward the central exhaust port, thereby realizing the process of sucking gas, compressing and discharging gas. During the rotor movement, the compression cavity periodically changes to achieve continuous pumping.

Turbo-type dry pump schematic molecular pump

Molecular pump principle; Source: Reference 1 The core components of molecular pump are rotating blades and fixed blades. The rotating speed of rotating blades is as high as 1800~90000 rpm, and the speed of outer ends of blades can reach hundreds of m / s, which is close to the average speed of molecules. There are multiple groups of rotating blades and fixed blades in the pump, which are placed at a certain angle. The larger the blade angle, the greater the exhaust speed.

Molecular pump schematic diagram; source: bilibili When gas molecules collide with high-speed rotating blades, they gain momentum perpendicular to the direction of the blade surface, and collide with fixed blades to fly to the next rotating blade again, and are finally ejected from the exhaust port, so that the probability of gas molecules moving to the exhaust port is greater than the probability of moving to the suction port, thus achieving the purpose of pumping.

Molecular pump is a high-speed rotation of the mechanical pump, can achieve extremely high vacuum. However, the molecular pump cannot work at atmospheric pressure and is very fragile to solid foreign matter, so a protective net is installed at the suction port to filter impurities.

Sublimation pump Sublimation pump pumping mechanism is chemical adsorption. The main component is a filament made of active metal titanium (Ti). The filament is heated by current, so that the metal titanium sublimates and adsorbs and deposits on the cold pump wall surface to form a titanium film. Due to the high chemical activity of titanium, it has a strong adsorption effect on active gases such as nitrogen, oxygen and carbon monoxide, and generates stable compounds such as titanium nitride, titanium carbide and titanium oxide, thus achieving the purpose of exhaust.

Sublimation pump principle: 1-vacuum chamber;2-sublimation pump small chamber;3-sublimation pump; source: reference 1 titanium film has adsorbed gas molecules on the position can not be adsorbed gas. Therefore, the filament in the sublimation pump must be continuously sublimated to deposit a new titanium film on the pump wall surface in order to achieve the purpose of continuous pumping.

ion pump

Ion pump is composed of strong magnet, honeycomb anode and titanium (Ti) cathode. The basic principle of ion pump is to apply a large enough voltage between the two electrodes to ionize the gas molecules to produce ions, which hit the cathode and are captured by the cathode, and also produce sputtering effect. The cathode material is made of metal Ti, and after the metal Ti is sputtered, an active film is continuously formed on the inner wall of the chamber, and the reaction with gas molecules is continued, so that exhaust is continuously performed. Due to the presence of a magnetic field, the cathode discharge can be maintained at extremely low pressures.

Cryopump Cryopump working principle is the use of gas in the ultra-low temperature surface physical adsorption to achieve the effect of exhaust, is an effective and fast way to obtain clean vacuum.

Cryopump principle;1-first stage cold stage;2-second stage cold stage;3-cryogenic cover;4-shielding cover;5-baffle plate; Source: Reference 1 The first stage cold stage of cryopump is equipped with shielding cover and baffle plate, and the temperature is maintained at 80 K. The second stage cold stage is maintained at 15 K, and a low temperature cover and activated carbon adsorbent are arranged on it.

Water vapor solidifies on the 80 K shield and 80 K baffle; nitrogen, oxygen and argon solidify on the 15 K cryoshield; hydrogen, helium and neon are adsorbed on the adsorbent inside the 15 K cryoshield, and extremely high vacuum (10^(-11 Pa)) can be achieved.

Unexpectedly, there are so many ways to create vacuum, Xiaobian is really eye-opening, but different vacuum pump use conditions are not the same, need to choose according to the actual situation, otherwise it is difficult to achieve the vacuum effect, there may be damage to the pump body.

seems

Wants to warm hands and hearts

I can't use a thermos.

You can't have it both ways.

so

References:

[1]Guo Fangzhun. Practical vacuum technology: vacuum technology [M]. Dalian University of Technology Press, 2012.

[2]Sun Songgang. Design Theory and Structure Improvement of Multi-cavity Vortex Vacuum Dry Pump [D]. Northeastern University.

[3]HiPace Turbomolecular Pump-Zhihu (zhihu.com)

[4]Titanium sublimation pump_Baidu.com

[5]Cryopump_Baidu.com

[6]Heat conduction and convection_Bilibili_bilibili

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), Author: Litchi Jelly

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