The invention relates to nuclear batteries and in particular nuclear batteries using the interaction of radioactive particles with an electroluminescent phosphor to produce photons which are detected by photovoltaic cells to generate electricity.

Nuclear batteries have been known since the early 20 ^th century and have been used for a wide variety of applications including high power applications. Due to the health, environmental and security concerns regarding the use of nuclear materials, much of the utility of these devices has been in extra-terrestrial applications. One area in which nuclear material has been readily accepted into the terrestrial environment is that of smoke detectors where the power released from the radioactive decay of the americium source in a domestic smoke detector is approximately 0.5μ\¥. Traditionally these low power levels would have been dismissed as negligible when considering the power supply requirements of conventional electronic systems. However, these levels are significant when considering nuclear batteries for powering micro-systems.

The operation of a nuclear battery is conceptually very simple. When a radioactive nuclide decays, energy is released by the ejection of a charged particle, usually an a or β particle. The kinetic energy of the particle may then be extracted to undertake useful work. In a simple situation the emitted particles can be used to stimulate either directly or indirectly a photovoltaic cell. Direct stimulation is possible but there is the possibility of radiation damage to the PN junction of the photovoltaic cell, which could lead to a limitation on the useful life of the battery.

Indirect stimulation involves the interaction of the radioactive material with an electroluminescent phosphor. Here the emitted particles stimulate the phosphor and the emitted photons so emitted stimulate the PN junction in the photovoltaic cell. This has the advantage of separating the PN junction from possible radiation damage by the nuclear source. Such nuclear batteries are being considered for powering condition monitoring sensors that are embedded in systems and are not accessible for changing batteries.

Gas-based systems using tritium have been previously produced. However, a problem of such sources is having to constrain the gas. Moreover, the half-life of tritium is relatively short (approximately 12.3 years) reducing the use of that radionucleotide as a long term power source. Promethium-147 has also been used as a power source and was used in "Betacel" batteries used in heart pacemakers in the 1970's. A half-life of only 2.6 years, the production of gamma radiation by the material and its toxicity limited its use. Technetium-99 has also been used in nuclear batteries, but again its usefulness has been limited by its half- life of 2.1 years.

Nickel-63 is a radioactive isotope that emits β particles that is suitable for interaction with a phosphor material to create a light source. Ni-63 has a large energy density, which is released slowly, and it has a half-life of almost 100 years so it is particularly suitable for use in inaccessible sensors. However, if the Ni-63 layer is too thick it absorbs the β particles before they are emitted, heating the Ni layer. This self-ab sorption severely limits the flux available at the external surface of a Ni-63 isotope layer and the efficiency and energy density of a nuclear battery.

The problem of self-absorption also occurs with other isotopes. There is therefore a need for a reliable and efficient way of harnessing the energy released by radioactive particles to power maintenance-free sensors for prolonged service life.

A first aspect of the invention provides a nuclear battery comprising nanoparticles of a radioactive isotope mixed with an electroluminescent phosphor layer; and a photovoltaic cell arranged to receive photons from the electroluminescent phosphor layer.

Typically, the radioactive isotopes are solid at operating temperature and typically emit alpha or beta particles. Typically, they have a half-life of at least 10, 50, 90 or at least 100 years to reduce the need for maintenance and extend the working life of the battery. Radioactive isotopes having reduced half-lives may also be used. Typically, Ni-63 or Am-241 is used. However, other radioactive isotopes having extended half-lives include Si-32, Sr-90, Sn- 121m, Sm-151, Eu-152, Po-209, Ac-227, Th-229, Pu-238 and Cf-249.

Typically, substantially all (e.g. more than 95%)of the nanoparticles have a diameter of less than ΙΟμιη diameter. More typically, diameters of less than 1 μπι, such as lOOnm may be used. The inventors have found that peak efficiency of transfer of energy from the radioisotope occurs as the radius tends to zero. For nickel spherical sources with a radius of approximately 0.5 pm almost 80% of the available energy in the β particles is able to escape the isotope. Using the isotope in the form of nanoparticles therefore ensures high efficiency and dispersity of the nanoparticles within the phosphor material to maximise the conversion of β particle energy to photon output. The use of such nanoparticles not only improves the efficiency and effective energy density of the battery, but also increases the particle flux entering the phosphor and so increases the effective power density of the battery.

By coupling the phosphor material with a photovoltaic cell, the photons emitted by the phosphor are converted to electrical energy to provide a source of power. The electrical power so-produced can be directly used to power devices or stored for use when required. Such a nuclear battery may, for example, have a volume of 10 nano litre, more typically 1 nano litre in size and may be capable of providing levels of 1.0 μ\ν per cell and depending on the radioisotope used. They may have an in-use lifetime in excess of 100 years. Nuclear batteries of this type can then be used to power embedded sensor systems that are particularly suited for use in powering embedded sensor system that are inaccessible, for example, within buried water pipes.

The electroluminescent phosphor layer comprises a phosphor that interacts with a decay product from the radioactive isotope, such as an alpha particle or beta particle. This interaction results in the release of a photon that is then detected by the photovoltaic cell. Electroluminescent phosphors are generally known in the art. They include, for example, europium-doped Y2O3 phosphor. This produces an emission line at around 611 nm.

Other phosphors include zinc sulphides doped with dopants such as copper, silver or manganese. Others examples include semiconductors such as indium phosphide, gallium arsenide and gallium nitride. It may also be possible to use organic semiconductors such as MEH poly(phenylene vinylene).

Such photons are detected by the photovoltaic cell and are converted into electricity. Potentially any photovoltaic cell may be used. Typically, the photovoltaic cell comprises a polycrystalline semi-conducting material, such as silicon. Photons from the phosphor are absorbed by the semi conducting material. Electrons are released from their atoms causing an electric potential difference. Due to the arrangement of the semiconducting layer within the photovoltaic cell, electrons are directed to move in a single direction producing a direct current of electricity. Such photovoltaic cells are generally known in the art.

The mixture of radioisotope and electroluminescent phosphor may be deposited directly upon the surface of the photovoltaic cell. However, alternatively, the mixture of radioisotope and electroluminescent phosphor may be arranged as a layer of material on an optically transparent material. That is, a material which allows photons from the phosphor to pass through to the photovoltaic cell. Suitable optically transparent materials include, for example, polyimides. Such materials are advantageously flexible, allowing the material to be rolled or otherwise shaped. Use of such optically transparent material allows, for example, a layer of phosphor and radioisotope mixture to be sandwiched between two layers of optically transparent material. Photovoltaic cells may be placed on each side of the sandwich to detect the maximum number of photons produced from the mixture. The optically transparent material may also reduce damage to photovoltaic cells from the alpha or beta particles.

Typically, the thickness of the phosphor layer is 10 micrometers to 100 micrometers.

In order to improve the resilience of the mixture of the radioactive isotope and the electroluminescent phosphor, the components may be mixed with a binder. Suitable binders include electro-neutral polymers that are also optically transparent and chemically inert. These include Topas (TP), polyvinylidene fluoride (PVDF) and polystyrene (PS) for example

Typically, the phosphor layer comprises powdered particles of phosphor mixed with the nanoparticles of the radioactive isotope.

The production of the nanoparticles is typically related to standard methods for producing nickel nanoparticles whereby the nickel is dissolved in sulphuric acid to form nickel sulphate and then reduced with sodium borohydride to form the nickel. This is commonly done in the presence of a surfactant such as water/SDBS (sodium dodecylbenzene sulfonate)/n- pentanol/n-heptane to facilitate the formation of nanoparticles. The photovoltaic cells may be connected by a suitable electrical contact to one or more electrical devices, such as a sensor. Accordingly, the invention provides an electrical device comprising a nuclear battery according to the invention.

The battery and/or electrical device may comprise a container. The container surrounding the battery and/or the electrical device. The container may comprise shielding, such as aluminium, to reduce the amount of radiation escaping from the nuclear battery. Alpha and beta particles can be shielded by relatively thin thicknesses of metal such as aluminium. Other metals or plastics materials may also be used.

The invention also provides a mixture comprising radioactive isotope nanoparticles and electroluminescent phosphor. The nanoparticles and electroluminescent phosphor may be as defined above.

A still further aspect of the invention provides: a method of combining an electroluminescent phosphor material with a radioactive isotope comprising providing a substrate; covering at least a surface of the substrate with a liquid comprising a suspension of radioactive isotope nanoparticles, optionally in combination with a binder; dispersing a powder of electroluminescent phosphor on the surface of the liquid; and evaporating the liquid to deposit the radioactive isotope nanoparticles and electroluminescent phosphor on the surface of the substrate.

Typically, the substrate is an optically transparent material and/or a surface of a photovoltaic cell. The electroluminescent phosphor, radioactive isotope and binder are typically as defined above. Any suitable liquid may be used. Typically, the liquid is an aqueous liquid such as water or for example ethanol.

Description of the invention

The invention will now be described by way of example with reference to the accompanying figures in which: Figure 1 is a diagrammatical representation of the configuration of a low volume mixer for production of nanoparticle precursor.

Figure 2 is a diagrammatical representation of the configuration of a low volume mixer with a quenching port.

Figure 3 is a diagrammatical representation of the low volume mixer assembly.

Figure 4 is a schematic diagram of a cross section through a portion of a nuclear battery.

Production of nanoparticles

The Ni-63 nanoparticles are produced through a low volume, continuous flow micro-mixer to provide a flexible, environmentally safe production process. The low cost of the equipment makes it suited to 'scale-out' by adding more units if there is a need to increase the volume of nanoparticles manufactured. The production of the nanoparticles is related to standard methods generally known in the art for producing nickel nanoparticles whereby the nickel is dissolved in sulphuric acid to form nickel sulphate and then reduced with sodium borohydride to form the nickel. This is commonly done in the presence of a surfactant such as water/SDBS (sodium dodecylbenzene sulfonate)/n-pentanol/n-heptane to facilitate the formation of nanoparticles. This has been adapted by the applicant to allow the production of radioactive nano particles.

Figure 1 shows the mixer device 10 comprising a flat plate 1 1 of suitable material that has been micro machined using micro fabrication techniques. Flat plate 11 can be made of any material with a structure that is capable of being micro-machined to take very fine features needed for microfluid devices and is resistant to the precursor materials. Examples of suitable materials include ceramics, glass, silicon and polymers. The flat plate 11 is machined to provide precursor inlet ports 12 and 13 for inputting the precursor materials appropriate to produce the required isotope nanoparticles.

The inlet ports transfer the precursor materials through micro channels 14 and 15 to the extended mixing chamber 16 in which the precursor materials are combined through complex flow patterns controlled by flow channel geometry features such as small perturbations or surface roughening to create vortex shedding before the fluid containing isotope nanoparticles exits at the output 17. Flexibility in the design of flow channel geometry provides great control over the kinetics of the nanoparticle formation.

A top cover (not shown) is placed onto the top surface 18 of the flat plate 11 to seal the channels. Inlet and outlet ports are provided in the top plate at the appropriate positions to match with the features in plate 1 1.

An alternative mixing device 20 is presented in Figure 2. The mixer device 20 comprises a flat plate of ceramic material 21 a first mixing channel 22 in which the precursor materials are mixed. A quenching input port 23 feed quenching material into the micro-channel 24 which feed the mixed materials from the first mixing channel 22 into a further mixing and quenching channel 25. The materials flow through mixing channel 25 and exit through output port 26.

A top cover (not shown) is placed onto the top surface 28 of the flat plate 21 to seal the channels. Inlet and outlet ports are provided in the top plate at the appropriate positions to match with the features in plate 21.

The small path lengths that can be achieved with micro fluidic devices provide the opportunity to both change and quench the chemical reactions used to form the nano particle using the device of Figure 2. This provides the facility to both start and stop chemical reactions and so produce inhomogeneous particles. The small path lengths are achievable through the use of micro machining and so lead to the ability to modulate the chemical reaction at small time scales. For example with flow rates of lcm/s and distance between inlet ports of ΙΟμιη a temporal resolution of 10ms is possible.

The top plates are clamped to the mixing devices 10 and 20 of Figure 1 and 2 respectively by a clamp assembly 30 of Figure 3. Steel plates 31 comprising apertures to provide access to inlet and outlet ports are fixedly joined together on the top and bottom sides of the mixer device by bolt means 32 to clamp together plates 31 , thereby applying pressure the top cover plate 33 to close the mixer device flow channels. Once the Ni-63 nanoparticles have been produced they are held in suspension in a suitable suspension medium such as deionised water or ethanol until needed for embedding in the phosphor material.

Embedding nanoparticles in electro-luminescent phosphor

A layer of phosphor material is made by floating phosphor material powder onto the surface of water, preferably deionised water, in a suitable container. The water is then drained away and/or evaporated to deposit the phosphor material on a flat plate at the bottom of the container.

The isotope nanoparticles are combined with the phosphor material by adding the nanoparticles to the phosphor powder layer of by suspending the nanoparticles in the water. As the water evaporates the phosphor material with nanoparticles are deposited onto a flat plate at the bottom of the container. Optionally, a binder material can also be suspended or dissolved in the water to improve the bonding of the phosphor and isotope particles to the plate. Alternative fluids, for example ethanol, can be used to suspend the nanoparticles and float the phosphor particles.

The plate with the phosphor layer containing isotope nanoparticles is then positioned in close proximity to a photovoltaic cell to create the nuclear battery which can be connected to a suitable electronic circuit or power storage means. The light emitted by the phosphor material as a result of the interaction of phosphor particles with the radiation emitted by the isotope is converted to electricity by the photovoltaic cell.

Such a nuclear battery with a volume of 1.0 nano litre in size can provide power levels of 1.0μ\ν per cell with an in-use lifetime in excess of 100 years. This level of power is suitable for micro-sensors and is particularly suited for application in micro-sensors in embedded systems that are inaccessible for maintenance to provide maintenance free power supplies throughout the sensor life.

Figure 4 shows a schematic diagram of a cross section through a portion of a nuclear battery according to the invention. Central layer (42) is a mixture of an electroluminescent phosphor with nanoparticles of the radioactive isotope. This is optionally sandwiched between two layers of an optically transparent material (44). Either side of this material is provided photovoltaic cells (46). Alpha or beta particles from the radioisotope strike particles of the phosphor releasing photons which pass through the transparent material (44) and are converted into electricity by interacting with semi conducting material within the photovoltaic cell. This produces electricity which is then used to provide electrical power to, for example, a sensor attached to the nuclear battery.