OA18273A - Electrical generator system. - Google Patents

Abstract

A power battery using the energy from a radioactive material. The arrangement uses ZnO as a semiconductor, with energy generated a metal-semiconductor junction. The ZnO is arranged in thin layers. This allows for good durability and relatively high power production.

Description

ELECTRICAL GENERATOR SYSTEM

Technical Field [0001] The présent Invention relates to the field of electrical génération, and in particular, to electrical energy converted from the energy from radioactive émissions.

Background of the Invention [0002] Power cells provide a self-contained source of electrical energy for driving an external load. A common example of an electrical power cell is an electrochemical battery. While electrochemical batteries are effective at providing power needs for a period of time at a relatively low cost, the lîmitîng factor Is the available energy defined by the material type 10 and weight. Due to the limited energy storage and energy density of electrochemical batteries with regard to their mass, there hâve been various attempts at producing alternative power cells, such as batteries powered by radioactive isotopes due to the higher theoretical limits of energy density.

[0003] There are several different types of radîoîsotope-powered batteries. Once such 15 type is a radio thermal generator (RTG) which uses the heat produced during decay of radioactive material to produce electrical energy. These devices hâve low conversion efficiency of the heat energy to electrical energy. Accordingly, RTGs are generally used with very high energy radioisotopes to produce a source of electrical power and usually require substantial shielding. In addition, the electrical power output is low.

[0004] Another type of radioisotope-powered battery Is an indirect conversion device which uses a radioisotope, luminescent material and a photovoltalc cell. The decay particles emitted by the radioisotope excite the luminescent material. The light emitted by the luminescent material Is absorbed by the photovoltalc cells to generate electricity. This type of battery generally has low efficiency because of the two step conversion and a relatively short 25 llfespan because the luminescent material Is damaged by the émissions.

[0005] Another example of a radioisotope powered battery is a direct conversion device which uses a radioisotope and semiconducting material. Conventional semiconductors are of only limited use in this application, as they suffer collateral radiation damage from the radioisotope decay products. In particular, incident high-energy beta particles create defects

In the semiconductor that scatter and trap the generated charge carriers. The damage accumulâtes and thereby over time reduces the performance of the battery.

[0006] US 5,260,621 discloses a solid state nuclear battery comprising a relatively high energy radiation source, with concomitant heat génération, and a bulk crystalline semiconductor such as AIGaAs, which is characterised by defect génération In response to the radioisotope. The material is chosen so that radiation damage is repalred by annealing at the elevated operating température of the battery. This device suffers from low efficlency, which nécessitâtes the use of a high energy radiation source and also requires elevated operating températures to function.

[0007] US 5859484 teaches a solid state radioisotope-powered semiconductor battery comprising a substrate of crystalline semiconductor material such as GalnAsP. This battery preferably uses a radioisotope that emlts only low energy particles to minimise dégradation of the semiconductor material In order to maximise lifetlme. The effect of using a lower energy radiation source is a lower maximum power output [0008] A further such device ls disclosed in US 6479919, which describes a beta cell Incorporatlng Icosahedral boride compounds, for example Bi₂P₂ or B_t2As₂, a beta radiation source and a means for transmitting electrical energy to an outside load. Manufacturing boron arsenide and boron phosphlde is expensive, which Increases the cost of producing these types of devices. Further, the production of such devices has increased health, safety and environmental risks associated with handling the arsenide and phosphlde materials.

[0009] In summary, problems with currently available radioisotope powered cells include Inefficiency of conversion of the emttted energy to electrical energy, radiation damage affecting the device materials, shielding requirements for high energy nuclear sources and semiconductor material that is subject to dégradation.

[0010] It is an object of the présent invention to provide a radioisotope power cell which exhibits an improved balance between durability and power output.

Summary of the Invention [0011] According to the présent invention there ls provided an electrical generator system Including: a radionuclide material; a thin layer of zinc oxide; métal électrodes contacting the zinc oxide and forming a metal-semiconductor junction therebetween, wherein radioactive émissions received from the radionuclide material are converted Into electrical energy at the metal-semiconductor junction; and electrical contacts connected to the électrodes which facilitate the flow of the electrical energy when connected to a load.

[0012] The use of zinc oxide was found by the inventors to hâve surprising results. While zinc oxide Is an Intrinsic n-type semiconductor, It has limited or no commercial applications as a semi-conductor material due to the lack of stable doped p-type ZnO materials. Consequently, it is considered a poor choice of semiconductor material for forming p-n junctions, which has been the primary direction for structuring radioisotope powered cells.

[0013] Traditionally accepted choices of semiconductor materials, such as GaAs, GalnAs; or Si, Si-C; or CdTe; etc, hâve been found to structurally dégradé when exposed to high levels of radiation.

[0014] The Inventor has discovered that zinc oxide, when employed at an appropriate thickness, could withstand high radiation levels and could, when employed as part of a metal-semiconductor junction (as opposed to a p-n junction), give favourable electrical génération output.

Brief Description of the Drawings [0015] Embodiments of the présent Invention will now be described with reference to the accompanying drawings, in which:

[0016] Fig. 1 is a graph showing the variation In generated current with the variation in zinc oxide thickness in tests with an applied voltage of 3V;

[0017] Fig. 2 Is a graph showing the variation in generated current with the variation In zinc oxide thickness with different electrode materials and configurations In tests with an applied voltage of 3V;

[0018] Fig. 3 is a graph showing variation of generated current against applied voltage with varying distance of radionuclide from the zinc oxide layer;

[0019] Fig. 4 is schematic view of a first embodiment of a power supply device;

[0020] Fig. 5 is a schematic of an alternative embodiment of a power supply device;

[0021] Fig. 6 is a schematic of a further alternative embodiment of a power supply device.

Detalled Description of the invention [0022] The présent Invention will be principally described with reference to particular illustrative examples. It will be understood that the principles of the présent Invention may be Implemented using variations of features on the particular implémentations Illustrated and described. The examples should be considered as illustrative and not limitative of the broad Inventive concepts disclosed herein.

[0023] One Implémentation of the présent Invention Is an electrical génération system employing an n-type semlconductor material having métal électrodes In contact with the semiconductor material, and exposing the arrangement to radiation from a radionuclide material. The radioactive émissions are converted Into electrical energy at the metalsemiconductor junction formed between the électrodes and the semiconductor material. For flow of generated electrical energy, it is important that there is a potential différence between the électrodes. Hence, there needs to be a significant différence In métal to semiconductor contact area between the électrodes In order that greater charge génération Is created at one electrode compared with the other. The electrode having greater charge accumulation effectively becomes the négative terminal and the other electrode becomes the positive terminal.

[0024] To maximise electrical génération in a radioisotope power cell, it is désirable to use a relatively high energy level radiation source and a high activity density. However, most semiconductor materials cannot withstand such high energy levels and structurally dégradé with exposure.

[0025] Zinc oxide is an n-type semiconductor, but is dismissed In the fieîd as being a very poor semiconductor material. However, the présent inventor has discovered that zinc oxide does demonstrate a capacity to withstand relatively high energy levels of radiation and high activity density.

[0026] Initial tests employing zinc oxide in the proposed electrical génération system unfortunately gave the disappointing results predicted by the accepted opinion In the field, which was that ZnO is a poor semiconductor material. Despite the capacity to withstand high levels of radiation, the generated electrical output was negligible.

[0027] However, when tests were conducted on varying the thickness of zinc oxide employed in the proposed electrical génération system, surprisingly favourable results were found when the zinc oxide was provided In the form of a sufficiently thin layer or film. For the purposes of the présent description and claims, 'thin* means less than about 15pm, and preferably less thanlOpm.

[0028] Figure 1 Is a graph showing the variation In generated current with the variation In zinc oxide thickness In tests with an applied voltage of 3V. In this test, the optimal current was at 1000 nm.

[0029] In practical experiments, a thin film of zinc oxide was formed on a substrate, by rf magnetron sputter or electrochemical vapour déposition, having a 5cm x 5cm surface. The substrate consisted of a first layer of glass. In this regard, sapphire and quartz are also considered suitable for this first layer. The substrate further consisted of a layer of a doped métal oxide material, which formed the surface upon which the zinc oxide was deposited.

[0030] This layer of a doped métal oxide material allowed the smaller positive electrode to be formed thereupon, thereby separating the positive electrode from the zinc oxide but providing a current path due to the semlconductive properties of the doped métal oxide. Suitable doped métal oxide materials Include, but are not limited to, fluorine doped tin oxide and tin-doped indium oxide.

[0031] A number of métal materials were tested for suitability as électrodes, namely gold, copper, aluminium and silver. In addition, different electrode configurations were examined, a first whereby the electrode covered an entire surface of the zinc oxide layer and a second whereby a comb-like or finger-like grid formation was used on the zinc oxide surface. The general thickness of the métal electrode material was In the range of 1001000nm, and preferably 150 nm.

[0032] Gold and copper were deposited by using sputtering techniques, while aluminium and silver were deposited using thermal évaporation techniques.

[0033] The different samples were exposed to Sr-90. Results found that gold, aluminium and silver produced linear and symmetric current-voltage curves at the métal18273 semiconductor junction suggesting a désirable degree of ohmic contact between these metals and the zinc oxide.

[0034] Copper produced non-linear and asymmetric results, Indicative of a Schottky barrier, which suggests that It is unsultable for the présent purposes.

[0035] In respect of the different configurations, a negligible différence In results was noted. This suggests that the comb-like grid configuration, which uses less métal, Is a viable option. It will be appreciated that other geometries and configurations are contemplated within the scope of the présent Invention.

[0036] Similarly, It will be understood that the présent Invention could be implemented with different metals, Including alloys, In the metal-semiconductor junction.

[0037] Tests were conducted with different thicknesses of the zinc oxide layer between 150nm and 1500nm.

[0038] The surprising results found that as thickness Increased from 150nm the generated electrical output also increased until an optimum thickness, after which, increasing the thickness caused a réduction In generated electrical output. Beyond approximately 1500nm, the output became too low for practical purposes. Consequently, the tests suggested an idéal thickness range for the zinc oxide to be between 150nm and 1500nm. The optimum thickness did vary depending upon sélection of materials.

[0039] The optimum thickness did vary depending upon sélection of materials. Figure 2 illustrâtes the variation in current with thickness at a constant voltage and radiation source, but with different materials and thicknesses of material. The material included silver in a finger electrode configuration; silver In full electrode; aluminium In a finger electrode configuration; aluminium in full coverage; and gold In full coverage.

[0040] in certain tests the optimum thickness was 1000nm while in other tests the optimum thickness was 1250nm, see Figs 1 and 2. Nevertheless, the overall useful range of thicknesses stayed reasonably constant. It Is expected that the optimum thickness could also vary, within the range, depending upon the choice of radionuclide material.

[0041] Alternative beta emitting materials which could be used In implémentations of the présent invention include Pm-147, Ni-63 and Tritium, or any other suitable beta emitting material. The présent Invention is In principle able to use other kinds of radioactive material, for example x-ray sources, gamma sources, or any other suitable material. The radionuclides may be In any suitable chemical form, and the material could In principle be a mixture of different radionuclide or with other materials.

[0042] Tests were also conducted on varying the distance and angle of incidence of the Sr-90 material to the zinc oxide layer, varying between 2mm and 350mm, shown In figure 3. Figure 3 Is a graph showing variation of generated current against applied voltage, with varying distances of the radionuclide from the zinc oxide layer.

[0043] As expected, the best output occurred at the smallest distance with output decreasing as distance was Increased. Nevertheless there was still appréciable output throughout the tested range, particularly up to approximately 300mm and an angle of <45°. Given the thickness dimensions of the generator, this is a large space and suggested that a number of generator arrangements could be arranged in a layered structure with the same radionuclide material, thereby Increasing the electrical output capacity from a single radionuclide source.

[0044] Examples of power supply devices employing the electrical generator system will now be described.

[0045] In Fig. 4 there Is shown a baslc 'single layer¹ device 10. As shown, the device 10 Includes a housing 12, within which at Its centre 1s a layer of a sealed radionuclide 14, for example, Sr-90, Pm-147, Ni-63 or H-3. The housing 12 can be formed of various suitable materials, such as aluminium, steel, etc., and encloses an atmosphère of air 28. The seal 16 can be aluminium, plastic, Mylar, other suitable métal alloy or similar low Z-material (Z being atomic weight). On each side of the radionuclide 14 are substrates 18 (for example, glass substrates) having a layer of tin-doped indium oxide 20 and a thin layer of zinc oxide 22 formed thereupon. An alternative to tin-doped Indium oxide can be indium tin fluoride. The main négative electrode 24 is formed on the other surface of the zinc oxide 22 and the smaller positive electrode 26 is formed on a surface of the tin-doped indium oxide 20. Conductive leads 30 are connected to both électrodes 24, 26 and lead to exterior of the housing 12 for connection to a load.

[0046] In Fig 5 there Is a shown a 'double layer¹ device 110. Each side of the central radionuclide 114 has an arrangement of two zinc oxide layers 122, each with corresponding électrodes 124,126, doped métal oxide layers 120 and separated byan insulating substrate 132.

[0047] In Fig. 6 there is shown a 'triple layer* device 210, In which layers of substrate and ZnO are arranged In a sandwich arrangement. Similarly to the other examples, a 5 central sealed radionuclide 214 has an arrangement of 3 layers of substrate 232 either side, with ZnO layers 222, doped métal oxide layers 220 and électrodes 224, 226.

[0048] As will be appreciated, It is possible to keep Increasing the number of layers and, as a conséquence, Increase generated electrical output. The limit to how many layers can be employed is dictated by how far away from the radionuclide material the furthest 10 layer Is.

[0049] It will be appreciated that structures with more than one layer of radionuclide may be used, with multiple sandwich structures added to provide a desired power level. It will also be understood that although the structure described Is generally square In shape, the structure could be of any desired shape, and could be curved in a suitable 15 Implémentation, assumlng appropriate spacings can be maintained.

Claims (15)

1. An electrical generator system including:

a radionuclide material;

a thin layer of zinc oxide;

métal électrodes, at least one of which being In direct contact with said zinc oxide and forming a metal-semiconductor junction therebetween; wherein radioactive émissions received from said radionuclide material are converted into electrical energy at said metal-semiconductor junction; and electrical contacts connected to saîd électrodes which facilitate the flow of said electrical energy when connected to a load.

2. The electrical generator system of claim 1, wherein said zinc oxide layer is formed on a substrate material.

3. The electrical generator system of claim 2, wherein said substrate material Is selected from glass, sapphire or quartz.

4. The electrical generator system according to claim 2 or claim 3, wherein a layer of a doped métal oxide material is disposed between the zinc oxide layer and the substrate.

5. The electrical generator system according to claim 6, wherein one of said métal électrodes Is disposed in direct contact with said doped métal oxide material.

6. The electrical generator system according to any one of the preceding claims, wherein said thin layer of zinc oxide Is formed by an RF magnetron sputter process.

7. The electrical generator system according to any one of the preceding claims, wherein said métal électrodes are formed of gold, silver or aluminium.

8. The electrical generator system according to any one of the preceding claims, wherein said métal électrodes are deposited on said zinc oxide by a sputtering process or electrochemical vapour déposition.

9. The electrical generator system according to any one of the preceding claims, wherein said radionuclide material is encased in a seal material.

10. The electrical generator system according to claim 9, wherein said seal material Is selected from aluminium, a métal alloy, plastic or Mylar.

11. The electrical generator system according to any one of the preceding claims, wherein said radionuclide material is selected from Sr-90, Pm-147, Ni-63 or H-3.

12. The electrical generator system according to any one of the preceding claims, wherein the thin layer of zinc oxide has a thickness between 150-1500nm.

13. The electrical generator system according to claim 12, wherein the thickness of the zinc oxide layer is equal to or less than 1250nm.

14. An electrical power supply device including a housing enclosing an electrical generator system according to any one of the preceding claims.

15. The device of claim 14, wherein there are multiple layers of zinc oxide, each layer having corresponding meta! électrodes and electrical contacts, wherein adjacent layers are separated by an insulating substrate material.