Theory
Betavoltaics is the science of directly converting beta radiation into useable electrical current. Beta radiation is the product of radioactive beta decay — high-speed, high-energy electrons which are produced by the decay of a neutron in an atomic nucleus into a proton, an electron, and an electron anti-neutrino. As with other forms of nuclear power, beta decay provides an enormous amount of energy from a very small package. If these electrons could somehow be captured for use in an electric circuit, they could be used to do work.
Experiments began in the 1950s to capture the power of beta particles ( β- ), but it wasn't until the development of the semiconductor diode that it really started to become practical. Recent new developments in semiconductors have renewed interest in the field, and have started to make real progress in developing a useful nuclear battery.
Development
One of the early practical models involved the use of tritium (a radioactive isotope of hydrogen with two neutrons) fuel and a silicon semiconductor diode. When the tritium undergoes beta decay, it scatters high-energy electrons in all directions. The electrons which impact the semiconductor P-N junction create a forward bias on the diode; that is, it adds electrons to the N side of the semiconductor, which induces a current from the P side to the N side (electrical current being defined as the opposite direction of the actual flow of the charge carrying electrons, which exit the P side). This current can then be fed into an electrical circuit to do work.
The tritium power source has a half-life of 12.3 years and can produce a power output of 24 watts per kilogram. Since it decays into a harmless, non-radioactive substance, there are no inherent complications with disposal. For comparison, a kilogram of gasoline burned at a constant rate of 24 watts would be gone in about 20 days.
There were four main problems with this model:
- Beta radiation is energetic enough to wear down silicon after constant, long-term exposure.
- Only a small amount of the beta particles were aimed at the planar silicon diode, so the efficiency was low.
- The power output is low.
- Beta radiation is somewhat dangerous and must be shielded from the user.
- This 30-fold increase in energy density comes at 10 million times the price.
The first problem can be solved with the use of stronger, more durable semiconductor materials, and research continues into developing these.
The second problem was solved by etching microscopic pits into a p-type semiconductor. These pits, only 1 micron wide and 40 microns deep, were then coated with an n-type semiconductor 0.1 microns thick and filled with tritium gas, so that it is surrounded on all sides but the top. This way, only the beta particles that are emitted straight up are not captured. This increased the efficiency tenfold. However, since a beta decaying atom is positively charged by the addition of a proton, efficiency is still limited by the fuel's tendency to reabsorb the emitted electron to restore charge neutrality. The best efficiency claimed so far is 25%.
The third problem is the most difficult to overcome, since the emission of beta particles from a radioactive source is a constant value, directly dependent on the half-life of the source material. Research into artificially stimulating and enhancing the decay rates, however, could be one solution. Another is the development of a hybrid battery, which uses a standard Lithium Ion battery constantly trickle charged by the betavoltaic cell.
The fourth problem can be easily overcome with a thin shielding (just a few millimeters) of aluminum, lead, or other material which blocks beta particles. Beta radiation is minimally harmful, being a weaker ionizing radiation than alpha particles, but are able to penetrate deeper. It is most harmful if ingested or inhaled.
It is an important point that tritium decays by beta decay only, and emits no radioactive particles or rays except for beta particles. It does not emit alpha particles, neutrons, or gamma radiation, all of which are more dangerous than beta particles. It is not capable of entering into a chain reaction state, since the beta particles it emits do not create additional reactions in the tritium. It will not meltdown, explode, go supercritical, or otherwise do any of the scary things uranium or plutonium do. Tritium is so safe that it is used in novelty glowing keychains, watch faces, and exit signs due to its faint luminosity (things for which the more dangerous radium is no longer used).
However this does nothing to alleviate the fifth problem: cost. Tritium is relatively rare, and is only manufactured in nuclear reactors. Its high cost is not likely to come down any time soon and in fact would likely only skyrocket should tritium become a widely-used fuel. Tritium also has applications in nuclear weaponry, and any significant quantities of it would be highly regulated.
Applications
Betavoltaics were first looked into as a power source for the space program, where long duration, reliable power sources are necessary for things like deep-space probes beyond the orbit of Mars, where solar panels are relatively ineffective. Today, the market is looking more closely at betavoltaics for portable electronic devices. Their long life, safe operation, reliability, and compact size are ideally suited for cellular phones, laptop computers, personal digital assistants, and other portable devices. Unfortunately their low power output limits their usefulness.
However, betavoltaics can be used in conjunction with standard battery technology to create a hybrid power source with the best features of both. The betavoltaic power cell, although not powerful enough to directly power the device, would provide a constant trickle charge to the battery, recharging it between uses. The battery would then provide the high-wattage output to power the device. Unlike today's rechargeable batteries, this hybrid cell would recharge itself over time without needing to be plugged into a wall outlet, drawing power from the enormous, but slowly released, reserves of the nuclear battery. With a half-life of 12.3 years, a tritium battery could serve a portable electronic device for its entire useful life without ever needing to be replaced.
Sources:
http://www.betavoltaic.com
http://www.automorrow.com/articles/betavoltaic.html
http://www.physorg.com/news4081.html