synonymLiquid helium(Liquid helium) generally refers to liquid helium
Liquid helium is a liquefied form of helium.Colorless and transparent, odorless and tasteless.It can obtain mKUltralow temperatureIs the main source of low temperature.Nature: it will never solidify under normal pressureThree phase pointThe solid phase only occurs when the pressure exceeds 2.5 MPa;Exists inphase transitionPhenomenon, at melting point (2.172K)specific heat, density, etc;existenceSuperfluidityCreeping film phenomenon and superconductivity,Coefficient of viscosityNear zero.[1]
Helium changes from gaseous helium to liquid helium at very low temperature.becauseHelium atomInteraction between（Van der Waals）AndAtomic massThey are very small, difficult to liquefy and solidify.Isotope richfourGas liquid phase transition curve of Hecritical temperatureandCritical pressure5.20K and 2.26 respectivelypressure, oneStandard atmospheric pressureThe temperature under normal pressure drops from the critical temperature toAbsolute zeroHelium is always kept as a liquid and will not solidify. Only when the pressure is greater than 25 atmospheres can it become solid.
At 2.18K, there will be obvious property changes, such asSuperfluidity, called He II, to distinguish it from ordinary liquid helium (He I).
Helium is usually a colorless and odorless gas;Melting point - 272.2 ℃ (25 atmospheres), boiling point - 268.785 ℃;Density 0.1785g/L, critical temperature - 267.8 ℃,critical pressure2.26 atmospheric pressure;in the watersolubility8.61cmthree/Kg water.Helium is the only material that cannot be solidified under standard atmospheric pressure.When the temperature of liquid helium drops to 2.18K（HeII), property mutationSuperfluidIt can flow upward along the vessel wall, and its thermal conductivity is 800 times that of copper;hisSpecific heat capacity、surface tension、CompressibilityThey are all abnormal.
The density of liquid helium at one atmospheric pressure is 0.125g/mL.Helium has two kinds of naturalisotope：helium-3 、Helium-4Helium in nature is basically helium-4.Ordinary liquid helium is easy to flowcolorless liquid , its surface tension is very small,Refractive indexIt is similar to gas, so it is not easy to see it.liquid statefourHe includes two phases with different properties, called He Ⅰ and He Ⅱ respectivelytransition temperature, density of liquid heliumPermittivityAnd the specific heat capacity increased abnormally.The transition temperature between the two liquid phases He Ⅰ and He Ⅱ is calledLambda point，Saturated vapor pressureThe λ point under is 2.172K. When the pressure increases, the λ point moves to a lower temperature. The phase transition curve of the two liquid phases is a straight line, called the λ line.
Superfluid
Liquid helium has a series of remarkable characteristics, mainly in the following aspects.
SuperfluidityThe viscosity of ordinary liquid increases with the decrease of temperature. In contrast, when the temperature drops to about 2.6K, the viscosity of He Ⅰ is almost independent of temperature, and its value is about 3 × 10-6Pascal second, which is much less viscous than ordinary liquids.Below 2.6K, the viscosity of He Ⅰ decreases rapidly with the decrease of temperature.The viscosity of He Ⅱ immediately drops to a very small value (<10-12Pascal seconds), this kind ofViscosityThe characteristic of is called superfluidity.Use different thicknesscapillaryWhen doing experiments, I found thatFlow tubeThe finer, the more obvious the superfluidity is. When the diameter is less than 10-5In a flow tube with a diameter of cm, the velocity is almost independent of the pressure difference and the length of the flow tube, but only depends on the temperature, and does not lose kinetic energy when flowing.
The helium film is covered with a layer on the wall of any device in contact with He Ⅱliquid filmThe liquid film only contains the non viscous superfluid component, which is called helium film.The existence of helium film enables liquid helium to move as low as possible along the vessel wall.EmptyBeakerWhen partially immersed in He Ⅱ, the liquid helium outside the beaker will climb up the beaker mouth along the outer wall of the beaker and enter the beaker until the liquid level inside the beaker is equal to that outside the beaker.On the contrary, when the beaker containing liquid helium is lifted out of the liquid helium surface, the liquid helium in the beaker will continuously transfer to the outside of the beaker along the vessel wall and drip down.This transfer rate of liquid helium is related to the liquid levelHeight differenceThe distance is independent of the barrier height.
The theoretical research on the properties of He Ⅱ was first made by F. London.fourHe atom has integral spinBosonLondon regards He Ⅱ as composed of bosonsBose gas, observe BoseStatistical lawBose statistics allow different particles to be in the samequantum stateMedium.London proved that there is a critical temperatureTcWhen the temperature is lower than Tc, some particles will be in the zero vibration energy state at the same time (i.eground state）, called condensation, the lower the temperatureNumber of particlesThe more, inAbsolute zeroAll particles condense to the state of zero vibrational energy, which is called Bose Einstein condensation.50. Tisa believes that the superfluidity of He Ⅱ is caused by Bose Einstein condensation.Since the He Ⅱ atom condensed to the ground state has the lowest zero point vibrational energyMean free pathIt can pass through extremely thin capillary tubes almost without obstruction.Tisa first proposed the second-class body, and later LD. Landau modified and supplemented this model.Two fluid modelIt is considered that He Ⅱ is composed of two independent and mutually permeable fluids. One is the condensed part in the ground state, with entropy equal to zero and no viscosity, and it is a superfluid;The other is inexcited stateIncoherentNormal fluidEntropy is not equal to zero, it has viscosity.The sum of the densities of the two fluids is equal to the total density of He Ⅱ. When the temperature drops to λ, the normal fluid begins to partially transform into superfluid. The lower the temperature, the greater the density of superfluid, while the density of normal fluid is smaller. At absolute zero, all atoms are in a condensed state, and all fluids are superfluid.This two fluid model can be used to explain many mechanical and thermal properties of liquid helium.
Thermal conductivity
He Ⅰ with ordinary fluidThermal conductivityTherefore, when the pressure is reduced, the liquid helium will undergo intense boiling.The thermal conductivity of He Ⅱ is 10% higher than that of He ⅠsixTimes, 10 times higher than copperfourTimes.When the temperature exceeds λ point and He Ⅰ changes to He Ⅱ, the liquid helium changes from very badThermal conductorIt suddenly became the best thermal conductor so far.Since the thermal conductivity of He Ⅱ is unusually high, there is no temperature difference in its interior, so it is impossible to vaporize inside, that is, to boil.When air extraction method is used to reduceVapor pressureAt the beginning, intense boiling occurs. When the temperature drops below λ point, He Ⅰ changes to He Ⅱ, and the boiling stops suddenly. The liquid surface is as calm as a mirror, and vaporization only occurs on the liquid surface.Thermal conductivity of normal fluid vstemperature gradientIt has nothing to do with. It is purely a quantity reflecting the nature of the material, but the thermal conductivity of He Ⅱ is related to the temperature gradient and even thegeometryof
heat effect
Thermal effects include mechanical thermal and thermal mechanical effects.The two containers containing liquid helium are connected with extremely thin capillary C, and liquid helium is injected. The temperature is lower than λ point, and the liquid level on the right side is higher than that on the left side, forming a pressure difference Δ p. In liquid heliumLow entropySuperfluidComponents can pass through from the right sideCapillary transferTo the left, the normal component with high entropy cannot pass through the capillary.This causes the entropy of liquid helium on the right side to increase and the entropy on the left side to decrease, which means that the temperature on the right side increases and the temperature on the left side decreases.This heat transfer caused by mechanical force is called mechano thermal effect.The inverse process of mechanical thermal effect is called thermal mechanical effect.After the right liquid helium is heated (endothermic Q), the low entropy superfluid component decreases. The superfluid component in the left liquid helium flows to the right through the capillary tube, while the normal component cannot, which causes the right liquid level to rise and form a pressure difference.The "fountain" device of thermo mechanical effect.With capillaryinjectorBottomlessglass tubeFill forEmeryPowder P, plug the bottom with cotton C, and immerse it in liquid helium.Illuminate the glass tube with light so thatLiquid helium temperatureWhen it rises, the superfluid component is excited into the normal component.The superfluid component outside the tube is transferred to the tube through the cotton plug, forming the pressure difference between inside and outside, and liquid helium is ejected from the nozzle.
Second sound wave
The acoustic wave in ordinary fluid is formed by the alternate change of density, which is called density wave.Landau developed the quantum liquidfluid dynamicsIt is predicted that in addition to the ordinary density wave (called the first sound wave), there is another sound wave in He Ⅱ, which is composed of the superfluid component (low entropy, low temperature) in liquid helium and the normal fluid component (high entropy, high temperature)relative motionFormed, calledTemperature waveOr entropy wave (second sound wave).The existence of temperature wave is confirmed by experiments.
isotope
3He is an isotope of 4He, accounting for less than 10% of natural helium-7, by manualnuclear reactionA sufficient number ofthreeHe。threeCritical temperature andCritical pressure3.34K and 1.17 atmosphere respectively.AndfourHe is the same, liquid under normal pressurethreeHe will not be cured, and 34 atmospheres must be added near absolute zero to cure.In 1972, D.D. Osherov et al. found twothreeThe new liquid phase of He is calledthreeHe-A andthreeHe-B，They are superfluid.Complete when liquid 3He and 4He are above 0.87Kmutually dissolvableBelow this temperature, they are separated into two phases, which are respectively calledDense phase(includingthreeHe is more) andDilute phase(includingthreeHe is less), and the dense phase floats above the dilute phase (due tothreeHe ratiofourHe light).threeHe atom passes through the dense phaseInterfaceHeat absorption is required when entering the dilute phase, which isDilution refrigerator Operating principle of (seeUltra low temperature technology）。threeThe total electron spin of He atom is zero,Nuclear spinIt is 1/2, so it belongs to the same category as electronFermion, comply withFermi Dirac statistics, liquidthreeHe is called Fermi liquid, a normal liquidthreeThe properties of He can be used as Landau's Fermi liquidTheoretical description。
chemical property
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Heliumchemical propertyIt is stable and hardly combines with any other elements.
Theoretically, some helium compounds can exist at extremely low temperature and high pressure.
HeH can be observed in the spectrum+(the strongest acid known), and the excited state of HeH can be used asexcimer Exists.
Helium was used as the driving force for hot air balloons and airships
Helium is the most inactive element and extremely difficult to liquefy.Helium is mainly used asShielding gasAir coolednuclear reactorOfWorking fluidAnd ultra-low temperature refrigerant, etc.Helium is used in satellite launch, missile weapon industryCryogenic superconductivityResearch and semiconductor production have important applications.
Balloon and airship
Helium was used asHot Air BalloonAnd airshipdriveThe density of helium is much smaller than that of air, so if helium is filled into balloons and airships, balloons and airships will rise gradually, allowing us to realize our dream of flying into the air without taking a plane.Because hydrogen will explode when mixed with airHydrogen balloonAnd hydrogen airships are not safe.The hydrogen airship was once used as a large manned aircraft, but in 1937, Germany“Hindenburg”After the airship accidentally caught fire and exploded when landing in the United States, it completely withdrew from the stage of history.However, hot air balloons andHot airshipIt is relatively safe, and the cost of a flight is relatively cheap.
Artificial air
Divers often use artificial air made of helium and oxygen.This is because under the high-pressure underwater environment, nitrogen will dissolve in the blood. When the diver floats up, the pressure will decrease, and the nitrogen in the blood will escape one after another, forming bubbles to block the blood vessels, making the diver very uncomfortable“Decompression syndrome”。Helium is also difficult to dissolve in water under high pressure, so it can be used to replace nitrogen to solve this problem.But if we don't have helium, we can also use neon, which is difficult to dissolve in water under high pressure.
Shielding gas
Helium in electric weldingSilicon waferIt can also be used in productionShielding gasIt can isolate oxygen and avoid welding workpiece, simple silicon and oxygenchemical reaction。according toU.S. governmentAccording to the statistics of relevant departments, 18% of all helium consumed in the United States in 2000 was used for welding, and 16% was used as shielding gas for other industries.However, if there is no helium, argon can also provide excellent service, and it is much cheaper.
Low temperature superconductivity technology
The most serious consequence of the lack of helium is nothing but a serious hindranceCryogenic technologyAmong which low temperature is the most affectedsuperconductivityTechnical.All knownSuperconductive materialThe superconducting properties can only be displayed at low temperatures below - 130 ℃, among which the most widely used ones (such as NbthreeSn)liquid hydrogenAt this time, only liquid helium can easily realize such extremely low temperature.Although we can use other methods to achieve the same low temperature, it is not as affordable as liquid helium.Obviously, if we do not have helium, the popularization of low-temperature superconductivity will be seriously hindered;If the cryogenic superconducting technology cannot be popularized, hospitals will not be able to afford to use nuclear magnetic resonance imaging devices (it needs superconducting materials to create strong magnetic fields).
resource distribution
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Source distribution
The main source of helium is not air, but natural gas.It turns out that the helium content in dry air is extremely small, averaging only 5 parts per million. The highest helium content in natural gas is 7.5%, 15000 times that of air.However, there are not many natural gas deposits with high helium, because the helium in natural gas is uranium, etcradioactive elementThe product of decay.Only near the natural gas mineUranium oreOnly then can helium be collected in natural gas.
Even natural gas with very low helium content is tens of thousands of times higher than that in air, so it is still the main source of helium in the world.Among them, the helium resources of the United States account for more than 50%, and China only accounts for 0.2%.
Helium in natural gas is the product of the decay of radioactive elements such as uranium.Only when there is a uranium mine near the natural gas mine can helium collect in the natural gas.Helium produced in the United States accounts for the worldtotal outputMore than 80%.
Although China also has some natural gas resources, so far, only SichuanNeijiangWeiyuanOfgas fieldOnce obtainedHelium extractionThe helium content is only 0.2%, and it has been exhausted.
In recent years, China'srequirementIt's getting bigger and bigger.Restricted by the lack of helium resources and the high cost of extracting helium, China has always relied on imports for demand.
In 2007, the United States approved helium asStrategic materialsThe limitation of crude helium production has led to the global price of liquid helium rising from 60~80 yuan/liter to more than 200 yuan/liter at present.
The high price of liquid helium makes it difficult to carry out research work widely.Experts predict that helium import will be more restricted in the future, and many existing scientific research projects involving helium and liquid helium in China may not be implemented due to lack of liquid helium supply.
Three Ways to Relieve Helium Crisis
The most direct way is to reduce expenditure.Now in the hospitalNuclear magnetic resonance instrumentMany liquid helium devices with good airtightness and evaporation prevention have greatly reduced the demand for liquid helium. Some previous instruments that consume large amounts of liquid helium have been gradually eliminated.
More scientists try to use otherCooling modeInstead of liquid helium refrigeration.For example, using liquid helium freerefrigeratorTo reachsuperconducting magnetOfworking temperature 。Compared with liquid helium refrigeration, the helium demand of the refrigerator is very low (used as the refrigeration gas of the refrigerator). The refrigerator mainly usesCold bridgeAndmagnetConnected byheat conductionThe liquid helium mainly immerses the magnet in it, which plays a great role in convection refrigeration.However, this method has not yet been really used in medical MRI.Some experts said that the advantages of liquid helium refrigeration are now obvious: the refrigeration effect is stable, which is very important for medical equipment with strict imaging requirements.The stability of the refrigerator is not as good as that of liquid helium, and it is vulnerable to disturbance, which is unfavorable for accurate imaging.However, he also said that with the further development and maturity of technology, it is not impossible for refrigerators to replace liquid helium refrigeration.
In the early decades of the last century, all countries in the world were looking for helium resources, mainly for fillingAirship。But today, helium is not only used in flight,Cutting-edge scienceResearch, modernizationIndustrial technologyIt is inseparable from helium, and liquid helium is often used instead of gaseous helium.Liquid helium leads people to a new field - the cryogenic world.
stayLiquid airHe and neon are still gases at a temperature of;stayLiquid hydrogenNe becomes a solid, but helium is still a gas.
To what extent will helium become liquid?
British physicist John Dewar first obtained liquid hydrogen in 1898.In the same year,NetherlandsKamelin Ones, a physicist at the University of California, also got liquid hydrogen.The boiling point of liquid hydrogen is - 253 ℃. At such a low temperature, other gases not only become liquids, but also become solids.Only helium is the last gas that refuses to become a liquid.Kamelin Ones was determined to turn helium into liquid.
In July 1908, Kamelin Ones succeeded in turning helium into liquid.He got 320 for the first timeCubic centimeterLiquid helium.
To get liquid hydrogen, you must firstHydrogen compressionAnd cool it to the temperature of liquid air, and then let it expand to further reduce the temperature, so that hydrogen becomes liquid.
Liquid helium is transparent and easy to flow, as if openedCorkLike soda water, small bubbles are splashed continuously.
Liquid helium is an unusual liquid. It boils at minus 269 ℃.At such a low temperature, hydrogen also becomes a solid. Do not let liquid helium contact air, because air will immediately freeze on the surface of liquid helium into a hard cover.
For many years, only the laboratory of Kamelin Ones in the Netherlands could produce liquid helium in the world.Until 1934, before studying in Rutherford, EnglandSoviet UnionThe scientist Kabiza invented a new type of liquid helium machine, which can produce 4 liters of liquid helium per hour.Later, liquid helium was widely studied and applied in laboratories in various countries.
Today, liquid helium has been widely used in modern technology.For example, to receivespacecraftFax photos sent or satellite broadcast receivedTV signalYou have to use liquid helium.receiving antennaTerminalParametric amplifierKeep it at the low temperature of liquid helium, otherwise the image will not be received.
Physicists have not only obtained liquid helium, but also solid helium. They are going toAbsolute zeroMarch (Physics calls - 273.15 ℃ absolute zero. This temperature scale is calledAbsolute temperature scale, denoted by K.0K is - 273.15 ℃, while 273.15K is 0 ℃).Theoretically, absolute zero can not be reached, but it can be approached constantly.The boiling point of liquid hydrogen is 20.2K on the absolute temperature scale, and that of liquid helium is 4.2K on the absolute temperature scale.At the absolute temperature scale of 2.18K, helium I becomesHelium II。In 1935, using“Adiabatic demagnetization”Method, cooling liquid helium to 0.0034K absolute temperature scale;In 1957, it reached the absolute temperature scale of 0.00002K;Up to now, it has reached 2.4 × 10-11K.
Astronomers continue to studySolar element。The hydrogen on the sun "burns" into helium. What's the future fate?They found that there are some stars in the universe that are hotter than the sun,Center temperatureReach several hundred million degrees.At the core of these stars, hydrogenNucleusHas becomeHelium nuclei Helium nuclei collide with each other, forming carbon nuclei and oxygen nuclei, and releasing a lot of energy at the same time.Like the heart of this kind of star, it expands and contracts regularly.Why is that?This is also because helium is at work.
Astronomers also studiedGalaxyThe ratio of hydrogen content to helium content.According to this ratio, someone estimated that the age of the Milky Way is 120 billion years.
The history of helium is not over, and the history of human understanding of helium is not over, but our booklet about helium has to end.
Who has made the greatest contribution to the discovery and research of helium?Are they the astronomers Jason and Rocky?Is a chemistRamsayAnd the physicist Krux?It's an inventionSpectroscopeAre our students and Kirchhoff?Of course, the credit of Hampson, Kamelin Ones and others who liquefied air, hydrogen and helium should also be considered.
It's hard to say.In the history of human understanding helium, they all have their own contributions.Helium is only one element, but its discovery and understanding are the common victories of many sciences - physics, astronomy, chemistry, geology, etc., which can never be accomplished by one person's strength.[2]
