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{{Short description|Type of nuclear battery which generates electric current}}
{{about|devices that generate electricity directly from beta particles|devices that use photocells|optoelectric nuclear battery}}
A '''betavoltaic device''' ('''betavoltaic cell''' or '''betavoltaic battery''') is a type of [[nuclear battery]] which generates [[electric current]] from [[beta particle]]s ([[electron]]s) emitted from a [[radioactive]] source, using [[semiconductor junction]]s. A common source used is the [[hydrogen]] [[isotope]] [[tritium]].  Unlike [[radioisotope thermoelectric generator|most nuclear power sources]] which use nuclear radiation to generate heat which then is used to generate electricity, betavoltaic devices use a non-thermal conversion process, converting the [[Carrier generation and recombination|electron-hole pairs]] produced by the ionization trail of beta particles traversing a semiconductor.<ref name=tr20091109>{{cite web|url=http://www.technologyreview.com/energy/23959/|title=A 25-Year Battery: Long-lived nuclear batteries powered by hydrogen isotopes are in testing for military applications|date=2009-11-17|author=Katherine Bourzac|archive-url=https://web.archive.org/web/20120119073609/http://www.technologyreview.com/energy/23959/|archive-date=2012-01-19|website=[[Technology Review]]|publisher=MIT}}</ref>

Betavoltaic power sources (and the related technology of [[Atomic battery#Alphavoltaic conversion|alphavoltaic]] power sources<ref>NASA Glenn Research Center, [http://rt.grc.nasa.gov/power-in-space-propulsion/photovoltaics-power-technologies/technology-thrusts/alpha-and-beta-voltaics/ Alpha- and Beta-voltaics] {{Webarchive|url=https://web.archive.org/web/20111018100731/http://rt.grc.nasa.gov/power-in-space-propulsion/photovoltaics-power-technologies/technology-thrusts/alpha-and-beta-voltaics/ |date=2011-10-18 }} (accessed October 4, 2011)</ref>) are particularly well-suited to [[Low-power electronics|low-power electrical applications]] where [[Service life|long life]] of the energy source is needed, such as [[Implant (medicine)|implantable medical devices]] or [[military]] and [[Outer space|space]] applications.<ref name="tr20091109" />

== History ==
Betavoltaics were invented in the 1970s.<ref>{{Cite web|url=http://large.stanford.edu/courses/2017/ph241/park-j1/|title=Review and Preview of Nuclear Battery Technology|website=large.stanford.edu|access-date=2018-09-30}}</ref> Some [[pacemaker]]s in the 1970s used betavoltaics based on [[promethium]],<ref name=olsen1973>{{cite journal | last = Olsen | first = L.C. | title = Betavoltaic energy conversion | journal = Energy Conversion | volume = 13 | issue = 4 | pages = 117–124, IN1, 125–127 | publisher = Elsevier Ltd. | date = December 1973 | doi = 10.1016/0013-7480(73)90010-7 }}</ref> but were phased out as cheaper lithium batteries were developed.<ref name="tr20091109"/>

Early [[semiconductor|semiconducting]] materials weren't efficient at converting [[electron]]s from [[beta decay]] into usable current, so higher energy, more expensive—and potentially hazardous—[[isotope]]s were used. The more efficient semiconducting materials used {{as of|2019|lc=y}}<ref>{{Cite journal|last1=Maximenko|first1=Sergey I.|last2=Moore|first2=Jim E.|last3=Affouda|first3=Chaffra A.|last4=Jenkins|first4=Phillip P.|date=December 2019|title=Optimal Semiconductors for 3H and 63Ni Betavoltaics|journal=Scientific Reports|volume=9|issue=1|page=10892|doi=10.1038/s41598-019-47371-6|issn=2045-2322|pmc=6659775|pmid=31350532|bibcode=2019NatSR...910892M}}</ref> can be paired with relatively benign isotopes such as tritium, which produce less radiation.<ref name="tr20091109"/>

The [[Betacel]] was considered the first successfully commercialized betavoltaic battery.

== Proposals ==
The primary use for betavoltaics is for remote and long-term use, such as [[spacecraft]] requiring electrical power for a decade or two. Recent progress has prompted some to suggest using betavoltaics to [[trickle charging|trickle-charge]] conventional batteries in consumer devices, such as [[cell phones]] and [[laptop computers]].<ref>{{cite web|title=betavoltaic.co.uk|url=http://www.betavoltaic.co.uk/trickle_charge.html|access-date=21 February 2016|archive-url=https://web.archive.org/web/20191231194527/http://www.betavoltaic.co.uk/trickle_charge.html|archive-date=31 December 2019}}</ref>{{unreliable source?|date=August 2023}} As early as 1973, betavoltaics were suggested for use in long-term medical devices such as [[artificial pacemaker|pacemakers]].<ref name=olsen1973/>

In 2018 a Russian design based on 2-micron thick [[Isotopes of nickel|nickel-63]] slabs sandwiched between 10 micron diamond layers was introduced. It produced a power output of about 1 μW at a [[power density]] of 10 [[Microwatt|μW]]/cm³. Its energy density was 3.3 kWh/kg. The half-life of nickel-63 is 100 years.<ref>{{cite journal |last1=Bormashov |first1=V.S. |last2=Troschiev |first2=S.Yu. |last3=Tarelkin |first3=S.A. |last4=Volkov |first4=A.P. |last5=Teteruk |first5=D.V. |last6=Golovanov |first6=A.V. |last7=Kuznetsov |first7=M.S. |last8=Kornilov |first8=N.V. |last9=Terentiev |first9=S.A. |last10=Blank |first10=V.D. |title=High power density nuclear battery prototype based on diamond Schottky diodes |journal=Diamond and Related Materials |date=April 2018 |volume=84 |pages=41–47 |doi=10.1016/j.diamond.2018.03.006 |bibcode=2018DRM....84...41B |doi-access=free }}</ref><ref>{{Cite web|title=Prototype nuclear battery packs 10 times more power|url=https://mipt.ru/english/news/prototype_nuclear_battery_packs_10_times_more_power|access-date=2020-09-01|website=Moscow Institute of Physics and Technology|language=en|archive-date=2020-10-27|archive-url=https://web.archive.org/web/20201027160223/https://mipt.ru/english/news/prototype_nuclear_battery_packs_10_times_more_power}}</ref><ref>{{Cite web|last=Irving|first=Michael|date=June 3, 2018|title=Russian scientists pack more power into nuclear battery prototype|url=https://newatlas.com/nickel-nuclear-battery-design/54884|access-date=2018-06-14|website=newatlas.com}}</ref>

In 2019 a paper indicated the viability of betavoltaic devices in high-temperature environments in excess of {{convert|733|K|C F|abbr=on}} like the surface of [[Venus]].<ref name="oconnor2019">{{cite journal |last1=O'Connor |first1=Andrew |last2=Manuel |first2=Michele V. |last3=Shaw |first3=Harry |title=An extended-temperature, volumetric source model for betavoltaic power generation |journal=Transactions of the American Nuclear Society |date=November 2019 |volume=121 |pages=542–545 |doi=10.13182/T30591 |pmid=34248155 |pmc=8269951 }}</ref>

A prototype betavoltaic battery announced in early 2024 by the ''Betavolt'' company of China contains a thin wafer providing a source of [[beta particle]] electrons (either [[Carbon-14]] or [[nickel-63]]) sandwiched between two thin [[Crystallographic defects in diamond#Interaction between intrinsic and extrinsic defects|crystallographic]] diamond semiconductor layers.<ref name=NA>[https://newatlas.com/energy/betavolt-diamond-nuclear-battery/ "Betavolt says its diamond nuclear battery can power devices for 50 years"]. David Szondy for New Atlas, 16 January 2024. Accessed 17 January 2024.</ref><ref>"[https://www.betavolt.tech/359485-359485_645066.html  贝塔伏特公司成功研制民用原子能电池]" ('Betavolt successfully develops atomic energy battery for civilian use'), on Betavolt website (in Chinese). Accessed 17 January 2024.</ref> The Chinese startup claims to have the miniature device in the pilot testing stage.<ref name=AC>{{Cite news |author= Anthony Cuthbertson |title= Nuclear battery produces power for 50 years without needing to charge |newspaper=[[The Independent]] |url= https://www.independent.co.uk/tech/nuclear-battery-betavolt-atomic-china-b2476979.html |date= 12 January 2024 |access-date= 14 January 2024}}</ref> Unveiled in January 2024, it is allegedly generating 100 microwatts of power and a voltage of 3V and has a lifetime of 50 years without any need for charging or maintenance.<ref name=AC/> Betavolt claims it to be the first such miniaturised device ever developed.<ref name=AC/> It gains its energy from a sheet of nickel-63 located in a module the size of a very small coin.<ref name=NA/><ref name=AC/> Once the decay period has passed, the isotopes turn into [[Stable nuclide|stable, non-radioactive]] [[isotopes of copper]], which pose no environmental threat.<ref name=AC/>

== Drawbacks ==
As radioactive material emits radiation, it slowly decreases in activity (refer to [[half-life]]). Thus, over time a betavoltaic device will provide less power. For practical devices, this decrease occurs over a period of many years. For [[tritium]] devices, the half-life is 12.32 years. In device design, one must account for what battery characteristics are required at end-of-life, and ensure that the beginning-of-life properties take into account the desired usable lifetime.

Liability connected with environmental laws and human exposure to tritium and its [[beta decay]] must also be taken into consideration in [[risk assessment]] and [[product development]]. Naturally, this increases both [[Time to market|time-to-market]] and the already high cost associated with tritium. A 2007 report by the UK government's [[Health Protection Agency]] Advisory Group on Ionizing Radiation declared the health risks of tritium exposure to be double those previously set by the [[International Commission on Radiological Protection]] located in Sweden.<ref>{{Cite news |url=https://www.newscientist.com/article/dn12984-tritium-hazard-rating-should-be-doubled.html |work=NewScientist |title=Tritium hazard rating 'should be doubled' |date=29 November 2007 |first=Rob |last=Edwards}}</ref>

As radioactive decay cannot be stopped, sped up or slowed down easily, there is no way to "switch off" the battery or regulate its power output. For some applications this is irrelevant, but others will need a backup chemical battery to store energy when it isn't needed for when it is. This reduces the advantage of high power density.

== Availability ==
Betavoltaic nuclear batteries can be purchased commercially. Devices available as per 2012 included a 100 μW tritium-powered device weighing 20 grams<ref>{{Cite web|date=2012-08-16|title=Commercially-available NanoTritium battery can power microelectronics for 20+ years|url=https://newatlas.com/city-labs-nanotritium-betavoltaic-battery/23720/|access-date=2020-09-01|website=New Atlas|language=en-US}}</ref>

== Safety ==
Although betavoltaics use a radioactive material as a power source, the beta particles are low energy and easily stopped by a few millimetres of [[Radiation protection|shielding]]. With proper device construction (that is, proper shielding and containment), a betavoltaic device would not emit dangerous radiation. Leakage of the enclosed material would engender health risks, just as leakage of the materials in other types of batteries (such as [[lithium]], [[cadmium]] and [[lead]]) leads to significant health and environmental concerns.<ref>{{cite web|last=Maher|first=George|date=October 1991|title=Battery Basics|url=http://www.ag.ndsu.edu/pubs/ageng/safety/ae1022w.htm|access-date=August 29, 2011|work=County Commissions, North Dakota State University and U.S. Department of Agriculture|publisher=North Dakota State University}}</ref> Safety can be further increased by transforming the radioisotope used into a chemically inert and mechanically stable form, which reduces the risk of dispersal or [[bioaccumulation]] in case of leakage.

== Efficiency==
Due to the high [[power density]] of radioisotopes and the need for reliability above all else in many applications of betavoltaics, comparatively low efficiencies are acceptable. Current technology allows for single digit percentages of [[energy conversion efficiency]] from beta particle input to electricity output, but research into higher efficiency is ongoing.<ref>{{Cite web|url=http://large.stanford.edu/courses/2013/ph241/harrison2/|title = Betavoltaic Devices}}</ref><ref>{{cite journal |last1=Sachenko |first1=A. V. |last2=Shkrebtii |first2=A. I. |last3=Korkishko |first3=R. M. |last4=Kostylyov |first4=V. P. |last5=Kulish |first5=M. R. |last6=Sokolovskyi |first6=I. O. |title=Efficiency analysis of betavoltaic elements |journal=Solid-State Electronics |date=1 September 2015 |volume=111 |pages=147–152 |doi=10.1016/j.sse.2015.05.042 |arxiv=1412.7826 |bibcode=2015SSEle.111..147S |s2cid=94359293 }}</ref> By comparison [[thermal efficiency]] in the range of 30% is considered relatively low for new large scale thermal power plants and advanced [[combined cycle]] power plants achieve 60% and more efficiency if measured by electricity output per heat input.<ref>{{Cite web|url=https://www.williamson.edu/2018/05/the-most-efficient-thermal-power-generation-plants-in-america/|title = The Most Efficient Thermal Power Generation Plants in America}}</ref> If the betavoltaic device doubles as a [[radioisotope heater unit]] it is in effect a [[cogeneration]] plant and achieves much higher total efficiencies as much of the [[waste heat]] is useful. Similar to [[photovoltaics]], the [[Shockley–Queisser limit]] also imposes an absolute limit for a single [[Band gap|bandgap]] betavoltaic device.<ref>{{cite journal |last1=Maximenko |first1=Sergey I. |last2=Moore |first2=Jim E. |last3=Affouda |first3=Chaffra A. |last4=Jenkins |first4=Phillip P. |title=Optimal Semiconductors for 3H and 63Ni Betavoltaics |journal=Scientific Reports |date=26 July 2019 |volume=9 |issue=1 |page=10892 |doi=10.1038/s41598-019-47371-6 |pmid=31350532 |pmc=6659775 |bibcode=2019NatSR...910892M }}</ref>

=== Ultimate efficiency ===
Since the highest energy that can possibly be extracted from a single EHP is the bandgap energy, the ultimate efficiency of a beta-battery can be estimated as:

<math>\eta_{max}={E_g \over E_{EHP}}</math>

where <math display="inline">E_g</math> and  <math display="inline">E_{EHP}</math> are semiconductor band gap and electron-hole pair creation eneregy respectively. The energy to generate a single EHP by a beta-particle is known to scale linearly with the bandgap as <math display="inline">E_{EHP}=AE_g+B</math>  with A and B depending on the semiconductor characteristics.<ref>{{Cite journal |last1=Evstigneev |first1=Mykhaylo |last2=Afkani |first2=Mohammad |last3=Sokolovskyi |first3=Igor |date=November 2023 |title=Limit Efficiency of a Silicon Betavoltaic Battery with Tritium Source |journal=Micromachines |language=en |volume=14 |issue=11 |page=2015 |doi=10.3390/mi14112015 |doi-access=free |pmid=38004872 |issn=2072-666X |pmc=10673167 }}</ref>

== See also ==
* [[Atomic battery]]
* [[Diamond battery]]
* [[Optoelectric nuclear battery]]
* [[Polyvinylidene fluoride]] (PVDF) thin film technology
* [[Radioisotope thermoelectric generator]]
* [[Radioisotope piezoelectric generator]]
* [[List of battery types]]

== References ==
{{Reflist}}

== External links ==
{{Library resources box}}

* [http://www.rochester.edu/news/show.php?id=2154 University of Rochester news release]
* [http://www.citylabs.net/ City Labs]
* [http://www.widetronix.com/ Widetronix]
*{{cite web |author=Armando Antoniazzi |url=http://blog.kinectrics.com/nuclear-energy/the-dawn-of-nuclear-batteries/ |title=The Dawn of Nuclear Batteries |publisher=Blog.kinectrics.com |access-date=2012-08-22 |archive-url=https://web.archive.org/web/20120321184926/http://blog.kinectrics.com/nuclear-energy/the-dawn-of-nuclear-batteries/ |archive-date=2012-03-21 }}
*{{cite web|url=http://www.gizmag.com/city-labs-nanotritium-betavoltaic-battery/23720/ |title=Commercially-available NanoTritium battery can power microelectronics for 20+ years |publisher=Gizmag.com |date=2012-08-16 |access-date=2012-08-22}}

[[Category:Nuclear technology]]
[[Category:Battery (electricity)]]