Helium is one of the elements that most people see (well, not see since it is a colorless gas) in everyday life. We are familiar with it because it used to fill toy balloons so that they rise in air. We shall get around to the calculation about that later.
Helium also is used for large balloons, blimps, and dirigibles for the same reason: it is lighter than air and so provides lift (not aerodynamic lift, which is provided by air passing over a wing surface) and so causes objects to rise, but these large craft have a payload where toy balloons do not, at least in most cases.
We also know it as the gas that causes voices to take on a bizarre, high pitch. We shall also discuss the reasons for that. But for me at least, helium is of extreme interest because of its quantum mechanical properties.
Helium is the second and final element of the first row of the periodic table, one of the most succinct and brilliant pieces of scientific work ever elucidated. The first row, because of quantum mechanical reasons, can accommodate only two elements. Hydrogen has but one proton in the nucleus (in technical language, we say that for H, Z = 1), and helium has two. The number of protons dictates the number of electrons in a neutral atom.
This number of two electrons is crucial to understanding the properties of helium. Electrons are arranged in shells around a nucleus, and the K shell is so small and close to the nucleus that two and only two electrons can reside there because of electrostatic repulsion. Another result of quantum mechanics is that having two electrons in the K shell allows a very stable electronic state to be had, and that is why helium is essentially completely nonreactive chemically. We shall revisit this overgeneralization later. Helium is the first of the so called Noble Gases, named that decades ago because they were once thought not to form any chemical compounds.
There are two stable isotopes of helium, the rare 3He and the ordinary 4He. Heavier ones have been created, but their half lives are so short that they are of little value except for pure research. 3He occurs at about 14 parts per million (ppm) from terrestrial sources of helium, the rest being 4He.
Helium has the distinction of being first discovered in the sun by spectroscopic methods in 1868. It was 14 years later that it was detected terrestrially, also by spectroscopic methods. It was not for another 13 years that it was actually isolated by the great Scot pioneer in gas physical chemistry, Sir William Ramsey.
After hydrogen, helium is the most abundant element in the cosmos. This is not surprising since stars fuse hydrogen into helium, but there is more to it than that. 4He has an incredibly stable nuclear configuration, and much of the helium present was formed just after the Big Bang, right after hydrogen.
On earth, there is not that much helium. The atmosphere contains around five parts per million (ppm). This sounds like a lot, especially when compared to, say, xenon, which is present at only around 87 parts per billion (ppb). However, xenon is used at only around 6000 standard cubic meters annually, and helium is used at around 170 million standard cubic meters annually! The atmosphere is not a good source for helium.
So, where do we get it? Ultimately, from radioactive decay of heavy elements like uranium, thorium, radium, and the like. Many of these elements decay by the alpha process, and an alpha particle is nothing but a bare helium nucleus, stripped of its electrons. The alpha soon picks up the two missing electrons and becomes elemental helium. It is estimated that 16.8 million standard cubic meters of helium are produced annually that way, or very near 10% of annual consumption. Thus, we are depleting helium ten times faster than it can regenerate.
But it is actually worse than that. Much of the newly created helium is trapped in solid rock and only slowly diffuses out, so only a very small amount of new helium is available. Our immediate source of helium is certain natural gas wells (most in the US), where the helium concentration can be as high as 7% (most sources are not nearly so rich). The gas is liquefied by refrigeration and most of the components allowed to boil off, essentially a distillation. Helium, having the lowest boiling point, is enriched in the boiled off portion, and is caught and the process repeated until only helium and a little neon (another low boiler) remain. The energy requirements to cool the gases are huge.
For most purposes, this helium is pure enough to use. So just what are the uses of helium? There are LOTS, and lifting is one of the original ones. Interestingly, lifting is becoming less and less important due to technological advances in unmanned aerial vehicles and satellite technology. Blimps and large balloons were once very important as silent spy platforms, but are quickly falling into disuse. However, since lifting was the original main use for helium, let us do some maths.
The density of air at standard conditions is 1.2 kg/m3, and that for helium is only about 0.179. Thus, helium is light enough that it will lift objects if it can be contained and the payload attached since it is only 15% as dense as air. That works very well with toy balloons because the latex or mylar balloon is very light. It gets dicier when larger structures are involved, because the mass of the supporting elements for the helium container and the payload are high. The next time that you see the Goodyear blimp on TeeVee notice how small the gondola underneath it is compared with the helium containment compartment! In other words, it takes LOTS of helium to lift anything other than a toy balloon.
These days, the most common use for helium is in cryogenics. It turns out that liquid helium has the lowest boiling point of any known substance, 4.22 kelvins (K), a very chilly negative 452 degrees F! Hydrogen, even lighter than helium, boils at 20.3 K, 16 K higher (and also 16 degrees C) than helium. This makes helium extremely important in magnetic resonance imagery, because most superconducting materials are not superconductive at liquid nitrogen temperatures (77 K), and liquid hydrogen has an extreme explosive danger. Besides, the two most common superconductive alloys are not superconductive at liquid hydrogen temperatures. If you have ever had an MRI, you were very near the coldest liquid known to exist!
After that, it is most used now for pressure vessel work. Pressure vessels are a very diverse group of things, but their intent is always the same: keep things in for a particular purpose, and then vent or open to allow the reaction products to escape, either to the atmosphere or to be kept for other treatment. Anyone who drives a gasoline or Diesel vehicle has direct experience with a pressure vessel, because the cylinders become pressure vessels during the compression and power strokes.
Some pressure vessels have to be sealed essentially completely, and helium is the perfect material for that. Since in its gaseous state it is just about the most penetrating material known, even microscopic breaches in piping and valves can be detected. I just does not “want” to be contained at all, and will get out of the most tiny hole. That is why toy balloons get flat fast, because both latex and Mylar have flaws, and the knot is quite permeable.
Helium has another purpose in pressure vessels, and that is to purge out other, more reactive gases before charging the vessel with the reactants. Helium has the advantage of being almost completely inert, easily diffused, and so just gets into every nook and cranny. Most of the helium is recovered from these often very large scale uses, but for small vessels it is cheaper just to let it escape into the atmosphere and be lost forever. That brings up another point that we shall discuss later.
Another large use is maintaining controlled atmospheres, and that includes welding applications. Since helium is essentially inert, it does not react with anything with which it is in contact. That makes it ideal for preventing unwanted reactions in many applications. In welding it prevents oxidation and is used in many situations where the cheaper argon or even cheaper nitrogen are not suitable.
There are a few more technical points about helium before I get into the political aspects of it. The first is a very personal relationship that I have had with that gas for many decades. My relationship is that helium is just about the most perfect gas to use as an eluant for gas/liquid chromatography, also called gas chromatography. I spent many years using this marvelous technique.
Helium has many advantages for this technique. First, it is very mobile and so carries lot of components quickly. Second, it is pretty much inert and does not react with the often fragile organic molecules that are bathed in it. Third, it has a very large thermal conductivity, making it perfect for a carrier when a thermal conductivity detector is used. It is also very good when a mass spectrometer is used as the detector as well. I have posted about those before, so no link. You have to do the heavy lifting to root out my post from several years ago!
Like all gases, helium can be liquified, and it is some cold stuff! Some of the strangest manifestations of quantum mechanical effects are shown in liquid helium. It takes two forms, one pretty much one as a classical liquid, and one very much different than being classical. This gets a little Geeky, but that is why we are here! In addition, the boiling points of 3He and 4He are quite different. For the lighter isotope, the boiling point at atmospheric pressure is 3.2 kelvins and for the heavier 4.2 K. This is a huge difference. Neither kind of helium can be solidified at atmospheric pressure.
When liquid 4He is cooled to 2.17 K, it undergoes a very interesting transition from a normal liquid to a superfluid. Superfluids are unlike any other known substance (except for Bose-Einstein condensates). 3He will also form a superfluid, but only around 0.1 K. There is a reason for this, and we shall discuss it later.
The superfluid form of 4He is called helium-II, and has a large amount of Bose-Einstein condensate character. Unlike other liquids, it has no viscosity, or “thickness”. For example, water has less viscosity than honey, but water still can be held by rather coarse screens. Helium-II can pass through the tiniest pore, and it is very difficult to design proper containers for it. It also has the ability to “creep” up straight sides of containers. If you have a little container of helium-II with a hole in the top of it floating in a larger, sealed container of helium-II, the superfluid will creep either into the little one (if the level of helium-II in the smaller one is lower than in the large one), or creep out of it into the larger one (if the level is higher than the larger one) until both containers have the same level, or until all of the helium-II is out of the little one. It is just like syphoning water, except that no hose is required!
Another interesting property of helium-II is that it can not boil! This is because it has the highest thermal conductivity of any known substance. It conducts heat so fast that the interior of the volume of helium-II never gets hot enough to boil, but rather the heat gets transferred to the surface where rapid evaporation occurs. The “normal” form of liquid helium behaves as a conventional liquid and does boil.
So why does 4He form a superfluid at 2.17 K and 3He have to be cooled to 0.001 K? This is a direct consequence of the concept of quantum mechanical spin. To oversimplify grossly, atomic nuclei can act as if they are spinning spheres. 4He acts like it has zero spin, where 3He has a spin of 1/2. This means that 4He is not affected by a magnetic field, but 3He has to align either parallel or antiparallel to a magnetic field. This seems like a small thing, but the consequences are huge. Nuclei with integral spin are called bosons (after Satyendra Nath Bose who described their statistics), whilst those with half-integer spin are called fermions (after Enrico Fermi, who described their statistics).
You can not make a Bose-Einstein condensate from fermions, so the lighter isotope has to use a different mechanism to become a superfluid. The forces involved in the superfluidicity of 3He are much weaker than for the heavier isotope, so the temperature has to be much colder, in this case around around 2000 times colder.
Before we leave, it is appropriate to discuss our promiscuous use of helium. During the Cold War the US built the helium version of the Strategic Petroleum Reserve. We had collected a billion standard cubic meters of helium stored there when Congress got downright silly. In 1996, when the 104th Congress was in session, Republicans controlled both the House and the Senate. They passed The Helium Privatization Act of 1996 and that law told Interior to begin selling off the contents of the reserve by 2005. Now we are using around 84 million standard cubic meters per year in the US, and the reserve is being depleted. I could not find hard figures about how much helium is left there (this information might be classified), but at 84 million standard cubic meters per year, even if it were full it would last only around 12 years. My understanding is that the plan is to tap out the reserve sometime in 2015. Now, there are other sources, but since helium is so important it makes sense to conserve it. Remember, unlike the other noble gases, once helium enters the atmosphere it is gone.
Prices for helium are already rising since the cheap, subsidized helium is disappearing. Prices should have been much higher in the past to discourage trivial uses and to encourage alternative, renewable substitutes. My personal feeling is that a tenfold increase in price will be seen over the next couple of years, and that will cut down on wasting it on toy balloons. I cringe whenever I see one of those outdoor balloon mass releases. Hydrogen would be perfect for that use, because all of those balloons either rupture or deflate before coming back to earth.
Even for “important” uses, helium is often used simply because it is cheap and convenient. Other shielding gases can be used for many welding applications, for pressurizing and purging as well as other controlled atmospheres other gases can be used either completely replacing helium or replacing a significant amount of it, and the list goes on.
Cryogenics, particularly those involved with superconductivity, is a bit more difficult. So far, most superconductors commercially used still require liquid helium temperatures, but high temperature superconductors, which I define as having a temperature at of above (the temperature of liquid nitrogen) are being developed and I believe someday soon will replace the older superconductors.
I promised to tell you why helium makes the pitch of the voice rise. There are two reasons. First, the speed of sound is much faster in helium than in air. Thus, everything being equal, the same sound source will sound higher pitched in helium. Second, it has to do with the density of the gas. However, that is not the entire reason or maybe not even the more important reason. It turns out that our vocal cords vibrate at a frequency dependent on our unique anatomy AND the density of the gas causing them to vibrate. Since helium is much less dense than air, our vocal cords vibrate faster in it than in air, raising the pitch. Back in the day when kids used to huff Freon, their voices were very much lower than in air. The speed of sound argument does not explain why, when you are meters away from someone who is speaking after breathing sounds higher in pitch, since air the the medium betwixt you. The vocal cord frequency does explain that.
This concludes our little discussion of helium. It is indispensable for some applications, convenient for many others, and is being used very wastefully currently. Remember, it is one of our LEAST renewable resources, because almost every atom that is used is lost to space. We should treat it like the old chlorofluorocarbon refrigerants, and recovered and reused whenever possible. It is truly a shame to squander such a valuable resource.
Well, you have done it again! You have wasted many more einsteins of perfectly good photons reading this airy piece. And even though the 21 Century insurance folks realize that their adverts are not funny when they read me say it, I always learn much more writing this series than I could possibly hope to teach, so keep those comments, questions, corrections, and other feedback coming, please. Tips and recs are also always welcome.
I would like to take the opportunity to wish all of the fathers reading tonight a very HAPPY FATHERS’ DAY!
Two health related notes: the former Mrs. Translator had a total knee replacement procedure done Thursday and is recovering beyond expectations and is home now. Please keep her in your thoughts! Secondly, my wrist is almost 100% now and I have not worn the splint is days. I still have a little residual muscle weakness from disuse, but that gets better every day. I have complete range of motion and normal sensation everywhere now. I am very happy! I was worried for a while that it would never come back. No more laying my head whislt blogging late for me!
Doc, aka Dr. David W. Smith