GB2268325A - Thermionic cathode structure. - Google Patents

Abstract

A thermionic cathode comprises a body (3) of refractory electrically insulating material, such as boron nitride, with an activator-containing region or layer (2, 5) provided by deposition onto or implantation into said body, and with an electrical heating element on a further surface of the body, and an electrically conductive polycrystalline coating (1) through which the activator, eg. one or more of Ir, Ru; Rh, Re, W, Mo, Os, eg. barium, may diffuse overlies the region or layer. The electrical heating element (10) is affixed to the refractory material by means of a flame-sprayed or plasma-sprayed ceramic (11). Electrical contact to the structure is provided by the crimping of a heat choke (4). The insulating body 3 may alternatively be formed of aluminium nitride, silicon nitride or silicon carbide. The activator-containing region 25 may comprise co-deposited Ba and W or Ba is ion implanted into the boron nitride block 3 from a barium iodide source. The Ba may be replaced or augmented by Ca, Sr, Na, K, La or the like. <IMAGE>

Description

ThERMIONIC CATHODE STRUCTZTRE This invention relates to thermionic cathode structures, and particularly but not exclusively to cathode structures for use with high current densities in DC or pulsed operation.

High current cathodes generally require to be machined into complex shapes and operate at temperatures consistent with the use of a separate heater.

In the past, the so-called "dispenser" cathode has been widely used for such applications. One such structure disclosed in US patent number 3 201 633, and known as an "impregnated" cathode uses a ceramic typically containing mixed oxides impregnated into a porous tungsten plug. These impregnated cathodes sometimes have metallic coatings (see EP-A-0019992). Such cathodes have an emitting surface in dynamic equilibrium with the vacuum in the device and an activator dispensed by diffusion from some reservoir. They produce useful current densities at 900-1200"C and have a long life. They have the disadvantage, however, of being expensive to manufacture.

Such cathodes are generally made from electrically conductive material, and fabrication is complicated by the requirement to provideselectrical isolation between neighbouring portions of an adjacent heating element.

An object of the present invention is to enable the above disadvantages to be mitigated.

According to the invention there is provided a thermionic cathode structure comprising a body of electrically insulating material provided with an activator-containing region or layer within or on a first portion of its surface, and with an electrical heating element on a second portion of its surface, and an electrically conductive polycrystalline material through which the activator may diffuse covering the activator-containing region or layer.

Such a cathode structure can be easier to manufacture and lower in cost than conventional dispenser cathodes.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: Figure 1 shows a schematic cross-section through a portion of a first cathode structure close to its emitting surface; Figure 2 shows a schematic cross-section through a portion of a second cathode structure close to its emitting surface; and, Figure 3 shows a cross-section through a complete cathode structure.

The same reference numerals have been used for corresponding items;in the various figures.

In a first embodiment (see Figure 1) a body 3.of'electrically insulating material, in the present example a block of boron nitride, is used as the body of the cathode. (Use of a material which does not contain oxygen is desirable so that reactions with the activator may be avoided to a large extent). On a first portion 12 of the surface of this block an activator-containing layer 2 has been provided by deposition, for example by evaporation of barium from a Knudsen cell and simultaneous sputtering of tungsten to form a mixed layer containing elemental barium as one component. The layer 2 has a thickness between a fraction of a micrometer and several tens of micrometers.This activatorcontaining layer is covered by a layer 1 of electrically conductive polycrystalline material through which the barium may diffuse, in the present embodiment a sputtered polycrystalline osmium and tungsten alloy layer approximately one micrometer thick.

In a second embodiment (see Figure 2) a block 3 of refractory electrically insulating material, in the present example boron nitride, is used as the body of the cathode. The block 3 has an activator-containing region formed adjacent a first portion 12 of its surface by ion implantation from a barium iodide or other barium source. The region 5 has a maximum barium concentration approximately one half a micrometer below the surface 12. A layer 1 of electrically conductive polycrystalline material through which the barium may diffuse, in the present embodiment a sputtered osmium layer, is provided on the portion 12 so as to cover the region 5.

The diameter of the cathodes described above is typically 1 to 30 mm, and they work in the temperature range 900-1100 C. In operation the bodies 3 thereof have to be heated. In accordance with the invention such heating is achieved by providing an electrical heating element on a second portion of the surface of the body 3. One such heating element is shown schematically in figure 3. In figure 3 the heating element takes the form of a double start spiral 10 affixed to.a second portion 6 of the surface of the electrically insulating material 3 by flame or plasma spraying of a suitable ceramic or electrical insulator 11 having a thermal expansion coefficient matching that of the electrically insulating material 3. It will be noted that surface portions 6 and 12 face away from each other.

In any of the cathodes previously described, electrical contact may be made round the perimeter of the electrically conductive material layer, for example by crimping into a thin walled cylinder of metal (typically Mo:Re or Mo) which constitutes a so-called "heat choke", as indicated by dashed lines 4 in figure 3. In this case it is advantageous for the electrically conductive material layer 1 to extend beyond the barium-containing region or layer 5 or 2. The layer 1 may extend onto adjacent sides of the cathode structure to facilitate electrical contact. Such an arrangement is shown schematically in figure 3.

Although in the examples previously described manufacture has been considered to proceed from the cathode body 3 to layer 1 (which forms the emitting surface) by means of deposition or implantation followed by deposition of the layer 1, as an alternative, manufacture may proceed in reverse. In such a case a former may be provided onto which an electrically conductive polycrystalline material layer (c.f. layer 1) is deposited, followed by deposition of an activator-containing layer as described in the first example above, or a thin layer of refractory material (for example by sputtering or CVD) into which barium is then ion implanted as described in the second example above. The body of the cathode is then formed by subsequent deposition of a thicker refractory electrically insulating material layer.After this the former may be removed either mechanically, by etching, by evaporation upon heating at reduced pressure, by burning off in a suitable gas ambient or by a combination of these methods. The heating element 10 is then affixed to the surface portion 6.

Although boron nitride has been quoted-as the material of the block 3, there are many other materials which may be used with equal facility. Thus the body of the cathode may be made from aluminium nitride or silicon nitride or silicon carbide or any other. suitable electrically insulating material or mixtures thereof. Single crystal, polycrystalline or amorphous forms of such materials may be used to form the body of the cathode. Preferably the material used should have a good thermal conductivity. Boron nitride slabs grown by chemical vapour deposition may have specific orientations which have high thermal conductivity in which case the cathode structure may be machined in such a way as to maximize its thermal conductivity between the first (12) and second (6) portions of the surface.

Although in the embodiments described above the electrically conductive layer 1 is deposited by sputtering of tungsten and/or other metals, other methods of physical vapour deposition such as e-beam or thermal evaporation may be used. Alternatively, the layer 1 could be deposited by electroplating or by chemical vapour deposition from suitable precursors (such as tungsten hexafluoride with or without the presence of a reducing agent such as hydrogen).

In a like manner the activatorcontaining layer 2 may be deposited in many different ways. For example it may be caused to take the form of sequential single metal layers, or of a mixed metal layer, by simultaneous or sequential evaporation of barium from a Knudsen cell and sputtering or evaporation of other metals such as for example molybdenum or tungsten, or indeed by evaporation from a Knudsen cell in combination with sputtering of a refractory non-metallic material such as that which makes up the body of refractory material from which the cathode is made. As another alternative the activator-containing layer may be formed by chemical vapour deposition to give mixed activator and ceramic or tungsten layers.As yet another alternative the layer 2 may be provided by screen printing an ink containing a reducing agent and an activator-containing compound such as barium oxide or barium carbonate, and heating or firing this to produce an activator-containing layer, in the present example an elemental barium-containing layer.

Although the activator described in the embodiments above has been barium, for certain applications (such as operation at higher or lower temperatures) the barium may be replaced by or augmented with other materials such as Ca, Sr, Na, K, La or other alkali metals or alkaline earth metals or lanthanoids or actinoids or mixtures thereof.

Although as described the layer 1 takes the form of sputtered polycrystalline tungsten and osmium alloy or sputtered polycrystalline osmium alone, other materials may be used alternatively or in addition. Thus, for example, layer 1 may take the form of polycrystalline layers of one or more of Ir, Ru, Rh, Re, W, Mo, Os.

Claims (11)

1. A thermionic cathode structure comprising a body of electrically insulating material provided with an activator-containing region or layer within or on a first portion of its surface, and with an electrical heating element on a second portion of its surface, and an electrically conductive polycrystalline material through which the activator may diffuse covering the activatorcontaining region or layer.

2. A structure as claimed in claim 1 in which the insulating material is a ceramic which does not contain oxygen.

3. A structure as claimed in claim 1 in which the insulating material is boron nitride.

4. A structure as claimed in any one of claims 1, 2 or 3 in which the electrical heating element is affixed by covering the element and second portion of the surface with a deposited electrical insulator.

5. A structure as claimed in any one of claims 1, 2 or 3 in which the electrical heating element is affixed by plasma or flame spraying an electrical insulator.

6. A structure as claimed in any one of claims 1 - 5 in which the activatorcontaining region or layer is constituted by a layer deposited on said first portion of the surface.

7. A structure as described in any one of claims 1 - 5 in which the activator-containing region or layer is constituted by a region containing ionimplanted activator formed below said first portion of the surface.

8. A structure as claimed in any one of claims 1 - 7 in which the electrically conductive polycrystalline material is a metal.

9. A structure as claimed in claim 8 in which the metal comprises one or more of Mo, W, Os, Ir, Ru, Rh, Re.

10. A structure as claimed in any preceding claim, wherein the activator comprises elemental barium.

11. A thermionic cathode structure substantially as described herein with reference to figure 1 or figure 2 or figure 3 of the drawings.