The cost of photovoltaics continues to decline and a new generation of cells strains at the limits for efficiency - it therefore seems likely that energy storage is the great technological challenge that lies ahead of us before we can generate and use power in a sustainable way. Now, new research led by scientists at the US Army Research Laboratory may pave the way to a new form of isotopic battery.
For the Army specifically, the energy density of batteries is a huge concern; higher energy density can extend the running time of vital equipment in the field, where recharging is not always easy. Soldiers seem ever-more likely to use a plethora of high-tech equipment. However, chemical batteries are a relatively mature technology and are unlikely to see substantial improvements in power density as more and more materials are explored.
Thus, there is clearly a strong motivation to expand the Army's access to energy and new power sources. For example, this drives research into improved chemical batteries with Army-specific capabilities to lighten the burden for Soldiers; however, research is also underway to determine the feasibility of accessing energy stored by isotopes at 100,000 times the density that chemicals can provide. One can say we are trying to push beyond a 'chemical limit' for entirely new types of batteries.
Dr. James Carroll, Team leader - U.S. Army Research Laboratory's Power Components Branch
The isotopic effect that the researchers are looking at has been hypothesized for a while, but the paper – published as a letter to the February 8th edition of Nature - is the first demonstration of the energy storage-and-release mechanism. The method uses non-fissionable radioisotopes; in the study, radioactive isotopes of molybdenum were used. The nuclei of these isotopes can become excited into a ‘metastable’ state that lasts for approximately seven hours; when this happens, the nuclei store energy.
The nuclei can be made to de-excite in a particular way. First, the atom is ionized, leaving a hole in one of its electronic orbitals. If that gap is filled with an electron with precisely the right amount of energy, some of that energy is transferred to the nucleus, which causes the metastable state to de-excite, releasing its energy. One could perhaps envision many primed molybdenum nuclei which could be de-excited simultaneously by passing a small current of electrons through the material, releasing the stored-up power. The main advantage to this is that the energy storage – which is, after all, fundamentally a nuclear process – has an energy density that is orders of magnitude higher.
The physical effect of electron capture causing a stimulated de-excitation of the nucleus had been proposed for more than 40 years, but the Army’s desire to find novel fundamental ways of storing and releasing energy motivated the study. Nuclear scientists from Russia’s Joint Institute for Nuclear Research, as well as Universities from Europe and Australia, collaborated on the study, while the actual demonstration experiment was performed in December 2015 at the ATLAS facility at Argonne National Laboratory.
The concept of nuclear batteries has been around since 1913 when Henry Mosely demonstrated the first betavoltaic device – a battery that merely uses the natural decay of radioactive substances which emit electrons to drive a current. The costly nature of nuclear batteries, when compared to their chemical cousins, has limited their use, but the fact that they can provide power that’s long-lasting and at a very high energy density has seen them being used in specific medical applications, as well as in the military and space. This is reminiscent of how solar panels were used initially – limited at first to particular applications before the price fell sufficiently to allow widespread use.
Now that the first experimental demonstrations of this effect have taken place, they can be used to test and perhaps validate the theoretical models that already exist – which will, in turn, allow for better prototypes to be constructed. The US Army Research Laboratory is concentrating on producing technologies that might one day be useful in the field – but even if this particular type of nuclear battery proves impractical, the fundamental physics research is still valuable to the nuclear physics community. More than a century after the first rumblings about radiation occurred, we’re still unlocking the secrets of the atom – and trying to determine if we can use them to our advantage.
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