ENERGY CONVERSION DEVICES

The present invention relates to energy conversion devices, and, in particular, to energy conversion devices that make use of thermionic emitter devices which convert heat energy to electrical energy.

Background of the invention

Thermionic devices which convert heat energy into useful electrical energy are well known and described. For example, the articles "Measured Thermal Efficiencies of a Diode Configuration of a Thermo Electron Engine of a Thermo Electron Engine", Hatsopoulus and Kaye, MIT, Journal of Applied Physics, 1958, pages 1124 to 1125, and "Theoretical Efficiency of the Thermionic Energy Converter", Houston, GE Research Laboratory, NY, Journal of Applied Physics, vol. 30, No. 4, April 1959, both describe thermionic emission devices having a cathode emitter spaced apart from an anode collector. The cathode emitter emits electrons when heated, and these electrons are collected by the collector, thereby giving an electrical current. However, such devices have been found to be inefficient in their energy conversion. In particular, such devices only operate at high temperatures, typically 1000°C to 1500°C.

Accordingly, attempts have been made to improve the efficiency, mainly by changing the structure of the cathode emitter. Such an approach is described in US Patent 5,981 ,071 , and in its divided US Patent 6,214,651. These patents disclose the use of nitrogen-doped carbonaceous material for the emitter electrode. Since the devices described are primarily intended for cooling semiconductor devices, the efficiency of energy conversion is low. Indeed, the use of nitrogen as a dopant leads to a low electrical conductivity, and a high workfunction for the material. A high workfunction means that electron emission is restricted and undesirably low.

Accordingly, it is desirable to provide a energy conversion device based on thermionic electron emission techniques, but which has higher, and, hence, more useful, conversion efficiency. It is also desirable to provide a device which is able to operate successfully at lower temperatures than previous devices, for example, at 500 ⁰ C or lower.

All thermionic devices are based upon the Richardson equation for the current per unit area emitted by a metal surface with a workfunction φ _e at a temperature T _e ,

J _R {φ _e ,T _e ) = ATϊ e ^eφ / ^'k ° ^τ ' (1)

and

Most thermionic generators operate at T = 1500K with cesium metal surfaces whose workfunction is about φ _e = 2eV . Using these values gives J _R = 52 A/cm ² .

The efficiency of a thermionic diode converter is given by the relation:

( ₂ )

where φ _e andφ _c are respectively the workfunctions of the emitter and the collector,

T _e and T _c are respectively the temperatures of the emitter and collector, σ is the Stefan- Boltzmann constant, Q _e is the thermal energy loss per second from the emitter via the electrical conductors, and J ₅ is the Richardson saturation current of the emitter. The efficiency according to equation (2) is reduced due to radiation loss and thermal conductivity of the conductors.

Equation (1 ) indicates that the work function and temperature of the emitter are the key parameters determining the operational performance of a thermionic emitter, while equation (2) indicates that a collector with a lower workfunction than the emitter can significantly boost efficiency.

Summary of the present invention

According to one aspect of the present invention, there is provided an energy conversion device comprising: a thermionic emitter for emitting electrons therefrom when exposed to a radiation source, the emitter comprising a substrate, a metallic layer, carried by the substrate, and a nanoparticle layer containing lithium-processed diamond nanoparticles, the nanoparticle layer being in electrical contact with the metallic layer; a collector, spaced apart from the thermionic emitter, and arranged for collecting electrons emitted by the thermionic emitter; and first and second electrical connections, arranged to be in electrical contact with the metallic layer of the emitter and the collector respectively.

According to another aspect of the present invention, there is provided a method of manufacturing an energy conversion device, the method comprising: processing diamond nanoparticles with lithium to produce lithium-processed diamond nanoparticles; depositing a metallic layer onto a substrate; depositing the lithium processed diamond nanoparticles onto the metallic layer; locating the coated substrate and a collector in a sealable enclosure, the collector being spaced apart from the coated substrate; and providing electrical connections connected with the metallic layer and collector respectively, which connections extend outside of the enclosure.

Brief description of the drawings

Figure 1 is a pictorial side cross sectional view of a device embodying one aspect of the present invention;

Figure 2 is a pictorial side cross sectional view of a first field emission device suitable for use in the device of figure 1 ;

Figure 3 is a pictorial side cross sectional view of a second field emission device suitable for use in the device of figure 1 ;

Figure 4 is a pictorial side cross sectional view of a third field emission device suitable for use in the device of figure 1 ; and Figure 5 is a pictorial side cross sectional view of a fourth field emission device suitable for use in the device of figure 1.

Detailed Description of the Preferred Embodiments

Figure 1 illustrates an energy converter device 1 which embodies one aspect of the present invention.

The converter device 1 comprises a thermionic emitter 10, and a collector 20. The collector 20 is separated from the thermionic emitter 10 by a gas or vacuum cavity 30. In one possible embodiment of the present invention, an optional radiation concentrator 40 is provided on which the converter module is mounted. A pair of electrical connections 50 is provided; one of the connections is connected to the thermionic emitter 10 and the other to the collector 20.

In embodiments of the present invention, the thermionic emitter 10 comprises a field emitter device having a substrate 11, a metallic layer 12, and a nanoparticle layer 13 of lithium-processed diamond nanoparticles. The substrate may be formed from an electrically insulating material having a high thermal conductivity, for example a CVD diamond wafer, a glass or ceramic plate. One possible emitter device structure is illustrated in International Patent Application No. WO2005/022579. In this prior art document, the emitter structure was described for use as a field emitter. However, it has now been recognized that the same emitter can be used as a thermionic emitter, with surprisingly good results.

As described in WO2005/022579, a cold cathode emitter can be fabricated using a substrate to provide a base upon which emission areas can be fabricated and this substrate is a relatively flat area composed of glass or quartz. A continuous cathode metal layer is deposited upon the substrate. This relatively thin film comprises a metal alloy of approximately 80-120nm depth (and that is matched to glass). The cathode metal layer can be one of a group of conductive metal oxides such as indium tin oxide (ITO), zinc oxide (ZnO), aluminium-doped zinc oxide (ZnOiAI), indium-doped zinc oxide (ZnO: In), gallium- and aluminium- codoped zinc oxide (ZnO:Ga,AI) or one of a group of metal alloys such as aluminium-doped lithium (UrAI), silver-doped lithium (Li:Ag), nichrome (Ni-Cr) or one of a group of metals such as silver (Ag), gold (Au), platinum (Pt) and nickel (Ni). A mono-layer of the lithium-doped nanodiamond particles is disposed on the cathode metal contact. The device is then thermally treated in air, inert gases or a vacuum to allow the nanoparticles to become mechanically and electrically connected with the cathode metal contact. This contact is consolidated by subsequent vacuum processing to package the cold cathode emitter into a device. Due to the manner in which the lithium is accommodated on the surfaces and within the bulk of the nanodiamond particles, the conductivity is markedly improved.

Therefore, the electrical interface formed between a cathode metal contact and each

nanodiamond particle will be enhanced compared with undoped nanodiamond materials.

The lithium-doped nanodiamond particles may be formed from nanodiamond particles according to the process described below. The term nanodiamond particles refers to diamond particles in which the domain size is in the range from 2nm-50nm.

The nanodiamond material used as the starting material for the process can be readily obtained from commercial sources, and may be composed of single crystal or polycrystalline particles. This starting material is first graded prior to use to obtain a powder with an average particle size of 25 microns or less. The material is next thermally cycled to stabilise it at a temperature in the range of 950-115O ⁰ C in an ambient of hydrogen/deuterium, helium or inert gas or in an ultra-high vacuum. In a subsequent process cycle, lithium is introduced as a vapour and made to react with the nanodiamond particles at temperature. Alternatively, a lithium compound such as lithium fluoride or lithium carbonate, or preferably lithium hydride is either applied as a conformal coating to each particle beforehand, or introduced into the reaction vessel containing the nanodiamond. Specifically, lithium hydride placed in a crucible with the nanodiamond particles in an atmosphere of argon is heated to around 68O ⁰ C. The chamber is then evacuated and the mixture heats up to around 850-900 ⁰ C, as there is no longer any convection of heat away from the mixture. The mixture is then pulse heated to a temperature of 950°C-115O ⁰ C, for example around 1100 ⁰ C, in order to control the process and protect the diamond structure. This particular technique increases the amount of lithium decorating and diffused into each nanoparticle. After the lithium or lithium compound has been allowed to decorate and also diffuse into the diamond nanoparticles, the vessel is purged with either helium, neon or argon gases and subjected to a further anneal at temperature. Afterwards the material is thermally quenched in an inert gas of ambient argon.

The result is the formation of lithium-doped nanodiamond particles, that is, nanodiamond particles, in which lithium has diffused into at least a part of at least a surface layer of the nanodiamond particles, or in which lithium is present on at least a part of a surface of the nanodiamond particles.

The lithium content in the lithium-doped nanodiamond particles may be between 5x10 ¹⁹ and 8x10 ²⁰ atoms per cubic centimetre.

In the embodiment illustrated in Figure 1 , the metallic layer 12 may be formed by any electrically conductive metal such as copper, silver, platinum, gold, titanium, nickel, cobalt, etc, or a metal alloy. However, other materials may be used, as described above.

One aspect of the invention is that, while the lithium-doped nanodiamond particles have a low workfunction, the workfunction of the emitter as a whole is reduced still further, as a result of the interaction between the lithium-doped nanodiamond particles and the conductive material. This effect may be due at least in part to the short distance between the emission surface and the junction between the nanodiamond particles and the conductive material, and the diffusion of lithium into the depletion region of this junction.

Figure 2 illustrates a first thermionic emitter device suitable for use in the converter device of Figure 1. The thermionic emitter device comprises a substrate 11 which carries a metallic layer 12. The metallic layer 12 carries a nanoparticle layer 13 of diamond nanoparticles which are in electrical connection with the metallic layer 12. In embodiments of the present invention, the diamond nanoparticles are lithium- processed diamond nanoparticles. The lithium-processed diamond nanoparticles are conductive, and are electrically connected with the metallic layer 12.

Figure 3 illustrates a second structure suitable for use in the device of Figure 1. The substrate 11 and metallic layer 12 are provided, as before, and the nanoparticle layer 13 comprises a plurality of lithium-processed diamond nanoparticles 131. In the Figure 3 example, boron-processed nanoparticles 132 are distributed across the metallic layer 12, and are in electrical communication with that metallic layer 12. The lithium- processed diamond nanoparticles are distributed across the metallic layer and across the boron-processed nanoparticles 132, and are in electrical connection with both the boron-processed nanoparticles and the metallic layer, where they are in contact with one another.

Figure 4 illustrates a third structure for use in the present invention, in which the nanoparticle layer 13 comprises lithium-processed diamond nanoparticles 133 distributed across the metallic layer 12, and in electrical connection with the metallic layer 12, together with boron-processed diamond nanoparticles 134 which are

distributed across the lithium-processed diamond nanoparticles 133, and which are in electrical connection with those lithium-processed nanoparticles 133.

Figure 5 illustrates a fourth structure suitable for use in the device embodiment of the present invention. In the Figure 5 structure, a layer of boron-processed diamond nanoparticles 136 is provided on the metallic layer 12, the boron-processed diamond nanoparticles 136 being in electrical contact with the metallic layer 12. In addition, a layer of lithium-processed diamond nanoparticles 135 is provided on the boron- processed diamond nanoparticles 136 such that an electrical connection is established between those two layers.

The boron-processed diamond nanoparticles can be obtained by a thermal diffusion technique similar to that used to obtain the lithium-processed diamond nanoparticles, employing boron nanoparticles that are mixed by ball milling to make a blended powder. Other materials, such as hydrogen, or lithium as a substitutional dopant, can be introduced to form the p-type diamond nanoparticles.

Referring back to Figure 1 , when the energy converter device 1 is exposed to radiation such as heat or light, the radiation concentrator 40 serves to concentrate the incoming radiation onto the substrate 11 of the thermionic emitter device. It is to be realized that the radiation concentrator 40 is optional, and that a device not having the concentrator 40 still embodies the present invention. The temperature of the thermionic emitter device rises, which, in turn, stimulates thermionic emission of electrons from the metallic layer and from the lithium-processed diamond nanoparticle layer. The collector device is spaced apart from the emitter device at a distance intended to maximise electron transfer between the emitter and collector devices. As the electrons move from the emitter device to the collector device, an electrical current can be provided through the electrical connections 50 of the converter device.

As described above, embodiments of the present invention make use of conductive, lithium-processed diamond emitters that are found to exhibit low electron affinity surfaces and, typically, a threshold of less than 1V/micron for field induced cold cathode operation. Low or negative electron affinity helps to mitigate space charge which can be beneficial to the operation of a thermionic vacuum diode device. It also makes a thermionic diode more efficient and allows higher output power than a conventional device.

Embodiments of the present invention may incorporate a gas/vacuum spacing of less than 50 microns in order to mitigate space charge under high vacuum operatioη. This small separation also allows emission from the doped diamond nanoparticles ^" at high gas pressures. In other embodiments, there may be a gas/vacuum spacing of less than 5 millimetres for ease of manufacture. In order to mitigate space charge under high vacuum operation in this case, a sub-threshold electric field may be applied across the vacuum space, to accelerate electrons towards the collector. Alternatively, a metal vapour of cesium, potassium, lithium or another easily ionisable element may be introduced to generate a low working pressure of, say, 0.5 torr, and creating a source of ions to neutralise the space charge build-up in front of the emitter.

The cold filed emission from lithium-processed diamond nanoparticles is controlled by electric field enhancement at the metal-diamond-vacuum junction rather than geometrical field enhancement at the tips of these nanoparticles. Unlike, other n-type diamond materials, lithium-processed diamond has been found to be highly conductive at room temperature. When a junction is made between the metal layer 12 and lithium- processed diamond nanoparticles, a number of effects contribute to the observed emission property. Within lithium-processed diamond nanoparticles, lithium is incorporated at tetrahedral interstitial lattice sites as an activated donor, and when the layer is reverse biased, the ionised donors near the junction of the metallic layer 12 and the nanoparticle layer 13 will cause an electric field enhancement that results in a narrower barrier for electrons tunnelling into the conduction band of the diamond. Lithium is also incorporated at substitutional sites and vacancies in the lattice that can result in the formation of lithium atom clusters that are not electrically active. Where these clusters are located near the junction with the cathode, a further local electric field enhancement will also occur. Once in the conduction band of diamond the electrons travel ballistically to reach the junction with the vacuum where they can be emitted very easily due to the low electron affinity of the diamond nanoparticle surface.

A further feature of embodiments of the present invention relates to altered emission performance at room temperature following heat treatment of the lithium-processed diamond nanoparticle emitter. It is observed that when lithium-processed diamond nanoparticles are disposed on a cathode containing copper, silver, gold or other noble metal or another metal such as nickel, cobalt, or titanium, and subsequently heat treated, the effective workfunction of the emitter is significantly reduced to a stable

value that is typically much lower than 2 eV. It is believed that the lithium atom clusters near the metal-vacuum-diamond junction diffuse to the surface upon high temperature heating under vacuum conditions, and once there are subject to an alloying reaction with mobile cathode metal atoms, resulting in a lowering of the workfunction of the diamond nanoparticle surfaces. This lowering of the workfunction facilitates thermionic emission operation from a lithium-processed diamond nanoparticle emitter at temperatures much lower than previously recorded.

The use of p-type diamond nanoparticle particles (that is, the boron-processed diamond nanoparticles mentioned above) to form junctions with the lithium-processed diamond nanoparticle particles results in structures that absorb higher energy photons and generating electron-hole pairs. Due to the wide band gap of diamond this process remains efficient at elevated temperatures. As a consequence, the number of electrons present in the conduction band of the diamond that are emitted thermionically at the emitter surface will be supplemented by those formed in the diamond nanoparticle junctions. Such a structure is advantageous for increasing the efficiency of a diamond thermionic converter and offers a means to achieve low temperature operation without the need for an externally applied field to "gate" the emitter into operation.

The collector can be of the same structure as the emitter. That is, the collector can also have a layer of lithium-processed diamond nanoparticles carried by a metallic layer on a substrate.

In contrast to previously considered multi oxide coated emitters, lithium-processed diamond nanoparticles exhibit low workfunction, and are stable under poor vacuum conditions. The lithium-processed diamond nanoparticle emitter is more uniform structurally and chemically. It is inert during processing which can be an advantage during fabrication processes. The diamond emitter has nanoscale structure and it is easier to control surface layer thickness and roughness. This is an important aspect, since it allows large area, uniform coatings needed for the emitter and collector electrodes to be manufactured more simply and cheaply.