What is Helium? HELIUM, deriving from the Greek word ‘Helios’, Ήλιος’, meaning ‘Sun’, and with the elementary symbol ‘He’ (in the elements periodic table) is a colourless, odourless gaseous element. Helium is considered one of the noble gases; how could it not be? It is the element of the Sun itself!
Helium has a peculiar and very interesting fact attached to it: it is the only element discovered out of Earth before it was discovered on our planet. It was eventually discovered and isolated around 1895 by Sir William Ramsay, a British chemist, who recovered it from a uranium-bearing mineral. It was concluded that it can usually be found in places where deposits of natural gas exist. The real difficulty lies with the fact that separation and extraction of helium from other gasses is a bit complicated. The element has some unique qualities. Being non-combustible, helium is preferred to hydrogen as the lifting gas in lighter-than-air itself. Helium possesses 92% of the lifting power of hydrogen, although it bears double the weight of hydrogen. Liquid helium boils at approximately -268.93 Centigrade (4.2 Kelvin) and will not freeze at atmospheric pressure conditions. Solid helium will form when pressures above 20 times atmospheric are provided. Liquid helium, due to its extremely low boiling point, can be used in cryogenic systems when temperatures below the boiling point of nitrogen are required. The way to cool down objects is to submerge them in liquid helium or liquid nitrogen. Liquid helium and nitrogen are usually stored in vacuum insulated flasks (we commonly know these containers as ‘thermos’), officially called ‘Dewars’, named after their inventor, Sir James Dewar. Liquid helium is probably the coldest fluid that exists in nature. That being said, the unwanted substances that lie within liquid helium, or ‘impurities’, will be in a frozen, solid condition which makes it relatively simple to eliminate and get ‘optically clean’ liquid. Within the cryogenics scientific community, they distinguish the different kinds of helium. The main distinction is between the two naturally occurring isotopes, Helium 3 and Helium 4. Helium 4 makes up over 99% of the natural Helium, therefore this is the Helium we refer to when we do not specify the isotope. Helium 3 (He3) is in fact the rarer isotope, and boils at 3.2 Kelvin, one degree less than Helium 4. Both isotopes can be cooled to below their boiling temperatures by regulating pressure, reaching points below atmospheric pressure. Liquid helium, behaves like water boiling at lower temperatures if pressure is lower.
But there is also the curious case of Helium 2, an extremely unstable – and still very interesting – isotope. In order to explain its strange behaviour, scientists established the two-fluid model. Helium 2 is depicted as a combination of two fluids: normal helium and superfluid helium. At temperatures just below the lambda point, the temperature at which normal fluid helium makes the transition to superfluid helium, the mixture is almost entirely normal. While temperatures are dropping, more and more of the substance becomes superfluid.
Here are some of the very interesting properties of the superfluid helium:
It carries no thermal energy, which means no entropy;
It has no viscosity, no thickness, and therefore is can be deformed instantly (flow through gaps, for example);
It naturally ‘seeks’ heat, which converts the superfluid to normal. The flow of superfluid into any heated area will cool that area and restore the uniform mixture of normal and superfluid.