A METHOD OF PRODUCING DEVICES WHICH GENERATE ELECTRICAL ENERGY

Abstract: A method of constructing a photovoltaic diode capable of generating electrical energy from exposure to radiation, the method comprising the steps of: (a) preparing dry metal powders of non symmetrical shaped particles, having sizes in the range of 1 to 145 microns; (b) preparing the surface of a supporting substrate such that the surface is substantially chemically clean and electrically conductive; (c) cooling the surface of the supporting substrate to a temperature within the range -200 to -20 DEG C; and (d) heating the dry metal powders to a temperature at which the particles thereof become partially molten, thus at least partially oxidising the metal, depositing the molten oxides onto said surface of the supporting substrate to form a laminate of oxide matrices having alternate layers of n type and p type characteristics and depositing an electrically conducting layer onto the oxide laminate.

Description: A METHOD OF PRODUCING DEVICES WHICH GENERATE ELECTRICAL ENERGY The present invention relates to a method of constructing photo voltaic diodes, particularly but not exclusively to radiation hard solid state diodes constructed from laminations of n and p type metals or metallic oxides such that electron hole pairs are formed in the depletion layer under the influence of incident radiation. Devices for converting radiation directly into electrical energy can be classified roughly into three types:- (a) Equipment having a metal collector set in a vacuum tube which collect the electrons given off by a radiating source and which generate a potential between the source and the collector. Such devices are well known and over a suitable period of time will generate high potential voltages between the emitting source and the collector. (b) Gas ionisation electric cells, whereby radiation causes ionisation in a gaseous medium, generating ion electron pairs which are subsequently directed to electrodes by an electric field generated by the device itself, thus producing an electric current which can be harnessed for use. (c) Solid state ionisation devices, along the lines of photo voltaic cells, where the nuclear radiation replaces light photons as the generating source. The present invention is concerned with this latter type of energy producing device. Current solid state devices based on silicon are not radiation hard and have a very limited life in environments where intense levels of radiation energy is present. It is an object of the present invention to provide an alternative means of forming solid state photo voltaic diodes which are radiation hard and will generate electrical energy from exposure to nuclear radiation over prolonged periods of several years. According to the present invention there is provided a method of constructing a photo voltaic diode capable of generating electrical energy from exposure to radiation the method comprising the steps of: (a) preparing dry metal powders of non symmetrical shaped particles having sizes in the range of 1 to 145 microns; (b) preparing the surface of a supporting substrate such that the surface is substantially chemically clean and electrically conductive; (c) cooling the surface of the supporting substrate to a temperature within the range -200 to -20° C ; and (d) heating the dry metal powders to a temperature at which the particles thereof become partially molten, thus at least partially oxidising the metal, depositing the molten oxide onto said surface of the supporting substrate to form at least one layer of an oxide matrix having n or p type characteristics and depositing an electrically conducting layer onto the oxide laminate. The basis of the new process is to utilise a hot spraying technique to deposit a series of layers of different metals or metallic oxides so as to produce n and p type junctions. The hot spraying technique consists of passing a stream of very finely powdered metal particles through a heat source, during which process the particles become molten and are chemically changed to the relevant oxide and which when projected onto an appropriate substrate form a homogeneous oxide matrix layer. The heating sources may take the form of a plasma device, or an arc struck between two wires, such that the wires melt and the molten droplets are projected as a stream onto a surface, or a flame source formed from the reaction of oxygen with hydrocarbon gases or liquids. The most cost effective devices are of the flame spray type utilising combustion gases, such as hydrogen, propane or acetylene. The degree to which the sprayed metal oxide matrices demonstrate their n or p type properties depends upon the degree to which the metal particles are chemically converted to an oxide whilst passing through the heat source. This reaction is time, temperature dependent and also requires the presence of free oxygen in the heat source. For the flame spray process it has been determined that the ratio of oxygen to the combustion gases, in volumetric terms, must be twice that required for stoichiometric combustion conditions. In addition, for the reaction to proceed satisfactorily during the short time it takes the metal particles to pass through the heating source, the particles must reach a temperature at which they become fully molten and remain so to the point where they impact onto the substrate surface. Conventional metal flame spraying equipment is not fully suitable in meeting the required conditions and will only do so when modified. In conventional equipment the combustion flame is unrestrained and loses a great deal of its heat to the surrounding air and in addition the powder stream passing through the combustion flame forms a divergent cone, whereby the outer streams of metallic particles reach a lower temperature than those in the centre. The optimum conditions for the production of oxidised particles from a metal powder is to utilise a narrow combustion flame, so constrained as to minimise the heat loss to the surrounding air, and to propagate as narrow a stream of particles through it, as possible. These conditions are achieved by using a heat resistant tube, having an internal diameter of twice the flame diameter and a length of 50% to 70% of the flame length, positioned coaxially around the flame such that the stream of molten particles do not impinge onto the tube inner surface. The location of the tube around the combustion flame and particle stream is critical and can be adjusted by using a suitable mechanical device fixed to the combustion nozzle. The effect of this tube on the combustion system is two fold, firstly it promotes laminar gas flow conditions within the flame and molten particle stream and secondly it greatly reduces heat losses to the surrounding environment promoting a much more even temperature within the particle stream. The promotion of laminar flow and constriction of the dispersion of the molten particle stream is also enhanced by the provision of a ring of small inclined nozzles, situated above the top edge of the heat resistant tube and impinging onto the flame and particle stream. These nozzles may be used to deliver a separate gas than that used in the combustion process, either inert or reactive as required. The temperature at which metals become molten varies with the individual metal and therefore there must be a balance between the rate at which each type of metal powder is processed and the total heat from the combustion source. For the flame spraying process this is achieved by varying the volumetric flow of the combustion gases, whilst maintaining the correct volumetric ratios. The first step in the process is to chemically prepare the substrate such that it presents a clean surface to the molten particles. The substrate may be either a metal, ceramic; glass, tiles, bricks, etc, or a composite, glass fibre laminates, mica, plastic, or concrete, as the flexibility of the process is such that the oxide matrices may be made to adhere satisfactorily to a wide range of materials, either of a flat or shaped profile. The second step in the precess is to accurately align the substrate to the heating source at such a distance that the oxide particles are still molten at the point of impact with it. This may be done in either vertical or horizontal planes. This spray distance has been found to be dependent upon several factors which include, the type of substrate to be coated, combustion flame and molten particle temperatures, required deposit thickness and rate of particle deposition. Empirical trials have shown that this varies from 5 mm to 40 mm from the extreme edge of the heat resistant tube support. The third step in the process is to pass the stream of molten metal oxide particles rapidly over the substrate in a plurality of overlapping passes to form a homogeneous matrix layer of interlocking oxide particles of either an n type or p type oxide. The thickness of the oxide matrix may be predetermined by varying the process parameters, such as, rate of feed of the metal powder into the combustion flame and the speed and plurality of the passes of the molten oxide particle stream over the substrate. One factor which has a great influence on both the rate of oxidation of the particles and the homogeneity of the deposited matrix is the form of the original metal powder particles and their size range. For a rapid rate of oxidation it is necessary that the ratio of particle surface area to volume is as high as possible and that the total surface area per unit volume of the powder is similarly high. Spherically shaped metal powder particles, as are produced by gas atomisation, have been found to be unsuitable. The success of the process depends to a large extent on the original metal particles having a non symmetrical shape, with re¬ entrant profiles, such as are produced by the water atomisation process and being widely dispersed in size over the range of 45 Microns to +1 Micron. The large particle surface area enhances oxide formation and the non symmetrical shape promotes interlocking with both the substrate and other particles, ensuring good interparticulate adhesion of the oxidised surfaces. As previously mentioned, the degree to which the sprayed oxide matrices demonstrate their n or p type properties is dependent upon the degree of oxidation of the molten metal particles. The more oxidised the matrix, the greater is the degree of n or p type property. This property may be varied as required by adjusting the various process parameters, i.e. temperature, total flameheat, particle through put and size range, ratio of oxygen to combustion gases, and spray distance. The deposited oxide matrices are polycrystalline in nature and are capable of analysis, by conventional methods, including 'X' ray diffraction. Where the matrices are less than fully oxidised, examination by optical techniques shows them to contain a random dispersion of metal particles distributed within the oxide. The capability of the process to produce a cohesive, homogeneous matrix with a proportion of randomly dispersed metallic particles is of great importance, as is the capability to vary the proposition of metallic particles to matrix oxide and will be explained later. The metals and their oxides used in this process to produce photo voltaic diodes are those situated in the transition region of the periodic table of elements, and are commonly known as the transition metals. The oxide matrices of such transition metals in the as sprayed polycrystalline state have either a p or n type characteristic and therefore a lamination of the appropriate oxides; one p type, the second n type, of any two metals within the transition metal series may be used to produce a photo voltaic diode, by this process. For example a polycrystalline as sprayed oxide matrix of nickel demonstrates the n type property, whereas the polycrystalline oxide matrices of iron, cobalt, or copper, demonstrate a p type property. Thus a combination of the oxides of nickel and iron, nickel and cobalt, or nickel and copper can be used. Furthermore, it is known that the susceptibility of these oxides to generate charge carriers is enhanced by the addition of small amounts of the alkali metals and alkaline earth metals [Group IA and IIA] and other elements from the lanthanide and actinide periods of the periodic table. For example, nickel oxide mixed with a small proportion of lithium oxide will have a more stable behaviour. Thus the process may be used not only to produce oxide matrices from the transition metals, but also oxide matrices, made of the transition metals and additions of the alkali metals and alkaline earth metals and other elements from the lanthanide and actinide periods of the periodic table, to manufacture photo voltaic diodes. The basic format of a photo voltaic diode produced by the above process consists of a metal substrate onto which is deposited a thin oxide matrix of one metal to give a p or n type layer. Onto this is deposited a second oxide matrix to give the alternate type layer, which is in turn covered by a metallic layer. A depletion zone is formed at the junction of the n and p type matrices with consequent development of a potential across it. The metal substrate has two functions in that it provides the necessary strength to support the two laminated oxides, but also acts as a charge collector to which a contact may easily be made. The outer metallic layer also acts as a charge collector. A typical diode will consist of a plate of mild steel onto which is first deposited an iron oxide matrix, p type, over which is laid a nickel oxide matrix, n type, and which is in turn coated with an electro-deposited layer of pure nickel. The points of contact of the iron and nickel oxides forms the depletion layer, the mild steel substrate and electro-deposited nickel form the collectors and contacts. Exposure of the diode to radiation, principally of the Beta and Gamma forms results in the generation of electron hole pairs in the depletion layer which migrate to the appropriate collectors to generate an external current. The thickness of the depletion layer has a considerable effect on the efficiency and operation of the diode, in that the thinner it is, the greater is the voltage field across it. However, the collection of incoming radiation and generation of electron hole pairs is improved by increasing the volume/thickness of the depletion layer. The presence of increasing amounts of metallic inclusions will assist the passage of the charge carriers to the collectors. the metallic inclusions may also assist in producing an electron cascade effect. The diodes capable of being manufactured by this process are intended to generate electron-hole pairs not only from direct penetration by radiation particles, but also from the cascade effect; whereby the primary radiation is made to penetrate an initial substrate, such as the metal charge collectors, or even the dispersed metal particles within the oxide matrices and generate a cascade of secondary electrons, which in turn enter the depletion zone creating further electron-hole pairs and thereby increasing the efficiency of the diode. Examples of typical configurations of the diodes and emitting sources can be as follows; (a) As planar laminates, whereby successive central substrates coated on both sides with an oxide matrix laminate of n and p type materials, covered in turn by an outer metal collector layer, are interspersed by thin plates of radioactive material. The electrical connections being taken successively from the central substrates and outer metal collector layers. In this configuration each diode is irradiated from both sides and such an arrangement would be used in the fields of measurement, instrumentation and control, or as reserve power supplies in remote or hostile environments such as nuclear plants or in space. (b) As a tubular arrangement whereby a metal tube is coated internally and externally with firstly a laminate of n and p type oxide matrices and then a metal collector layer and arranged coaxially around a rod of radioactive material. With such a configuration the electrical connections would be made to the tube and the metal collector layers. (c) In a spherical configuration a metal sphere, coated internally and externally with a laminate of n and p type oxide matrices and then a metal collector layer, with the radioactive source placed at the centre, irradiating uniformly onto the inside surface. The electrical connections being made to the metal sphere and metal collector layers, with suitable insulation where they passed through the shell and oxide layers. The potential uses for such photo voltaic diode devices are extremely wide and range from power sources for electronic devices, to detectors of various forms and intensities of nuclear and other types of radiation in the fields of measurement instrumentation and control.

Claims: CLAIMS 1. A method of constructing a photo voltaic diode capable of generating electrical energy from exposure to radiation the method comprising the steps of: (a) preparing dry metal powders of non symmetrical shaped particles, having sizes in the range of 1 to 145 microns; (b) preparing the surface of a supporting substrate such that the surface is substantially chemically clean and electrically conductive; (c) cooling the surface of the supporting substrate to a temperature within the range -200 to -20° C; and (d) heating the dry metal powders to a temperature at which the particles thereof become partially molten, thus at least partially oxidising the metal, depositing the molten oxide onto said surface of the supporting substrate to form at least one layer of an oxide matrix having n or p type characteristics and depositing an electrically conducting layer onto the oxide laminate. 2. A method as claimed in claim 1, wherein the metal powders have re-entrant angles. 3. A method as claimed in claim 1 or 2, wherein the metallic powders have sizes in the range of 1 to 45 microns. 4. A method as claimed in any preceding claim, wherein the surface of the supporting substrate is cooled to a temperature within the range -200 to -100°C. 5. A method as claimed in any preceding claim, wherein the metal powders are partially melted by a plasma, an arc between two metal wires, a wire and an electrode, or flame spraying equipment. 6. A method as claimed in any preceding claim, wherein at least one n type and one p type oxide matrix is deposited onto the surface of the supporting substrate. 7. A method as claimed in claim 6, wherein the molten oxides are deposited onto the substrate to form a laminate of oxide matrices having alternate layers of n type and p type characteristics. 8. A method as claimed in any preceding claim, wherein the molten oxides are deposited onto the surface of the supporting substrate in a plurality of passes over the substrate. 9. A method as claimed in any preceding claim, wherein the electrically conducting layer is deposited onto the oxide laminate by electro deposition, in fluid or vacuum, or by mechanical means. 10. A method as claimed in any preceding claim, wherein the dry metal powders are formed by blasting a molten mixture of the metal into water and subsequently crushing and drying the resulting water atomised powders. 11. A method as claimed in any preceding claim, wherein the metal powders may be any of those elements classified as transition metals in the periodic table of the elements, having atomic numbers ranging from 21 to 30, 39 to 48, 57 to 80 and 89 to 107 inclusively, either in their pure state or alloyed in varying proportions with one or more of the elements in other groups of the periodic table of the elements, having the atomic number 3, 4, 11, 12, 13, 14, 15, 16, 19, 20, 31 to 34 inclusive, 37, 38, 49 to 52 inclusively, 55, 56, 81 to 88 inclusively, and 58 to 71 (lanthanides) and 90 to 103 (actinides) inclusively. 12. A method as claimed in any preceding claim, wherein the degree to which the oxide matrices demonstrate their n and p type properties is governed by adjusting the amount of oxidation on the surfaces of the metallic particles of the powders, either by adjusting the process parameters, or varying the location of the heat resistant tube with respect to the heat source and the molten particle stream, or by selecting the size range of the particles used, within the range of 1 to 45 microns. 13. A method as claimed in any preceding claim, wherein the ability of the oxides of the metals in the periodic table of elements having the atomic numbers ranging from 21 to 30 inclusively, 39 to 48 inclusively, 57 to 80 inclusively and 89 to 107 inclusively, to generate charge carriers from the incidence of radiation is enhanced by the addition to them in varying proportions of one or more of the elements in the periodic table of elements having the atomic numbers 3, 4, 11, 12, 13, 14, 15, 16, 19, 20, 31 to 34 inclusively, 37, 38, 49 to 52 inclusively, 55, 56, 81 to 88 inclusively, 58 to 71 inclusively and 90 to 103 inclusively. 14. A method as claimed in any preceding claim, wherein the thickness of the sprayed oxide matrices, whether of n or p type, and their conductivities may be adjusted by varying the degree of oxidation on the surface of the molten metal particles and the plurality and speed of passes over the supporting substrate. 15. A method as claimed in any preceding claim, wherein when the oxide matrices are produced by flame spraying, the volumetric ratio of oxygen to the associated fuel in the combustion flame, is twice that required for stoichiometric combustion. 16. A method as claimed in any preceding claim, wherein the deposition of a laminate of two oxide matrices, one being n type and the other being p type, onto a suitably prepared supporting substrate and subsequent deposition onto the oxide laminate of a suitable collecting surface will produce a photo voltaic diode capable of generating electrical energy directly from the incidence of nuclear radiation. 17. A method as claimed in any preceding claim, wherein the laminate of two oxide matrices, n and p type, and suitable collecting surface may be applied to both surfaces of a suitably prepared supporting substrate, either in flat, tubular or spherical form, or any combinations of these forms, to produce photo voltaic diode combinations which will directly generate electrical energy from nuclear radiation incident onto the relevant surface from a variety of radioactive sources. 18. A method as claimed in any preceding claim, wherein the photo voltaic diode will generate electrical energy not only from the formation of eletron-hole pairs by the direct incidence of nuclear radiation, but also from the cascade effect, where the primary radiation is made to penetrate an initial substrate and produce a cascade of secondary electrons, which in turn enter the depletion zone to create further electron-hole pairs, thereby increasing the efficiency of the device.

Inventors: BOARDMAN JEFFERY, BOARDMAN, JEFFERY
Applicants: BOARDMAN JEFFERY, BOARDMAN, JEFFERY
Application year: 1993
Main classification: H01L31
IPC codes: G21H1, H01L31