GB2106707A - Electrodes for thermionic valves - Google Patents

Abstract

In a large high power valve the grid may be given a black, microscopically- rough surface of pyrolytic graphite to enhance the dissipation of heat therefrom. The present invention seeks to reduce, if not eliminate, the likelihood of the graphite layer separating from the grid material by forming on the grid an intermediate layer (between grid and graphite) which is itself tightly held by the grid surface, and which has an outer surface that is microscopically rough and forms a good bond to the pyrolytic graphite deposited thereon. In particular, it provides a pyrolytic-graphite- coated electrode for use in a thermionic valve, which electrode has, between the electrode metal (21) and the pyrolytic graphite coating (23), a separately- applied intermediate layer (22) of the carbide of one or more Group IVb, Vb or VIb metal, e.g. Ta, Mo, W but especially zirconium, which layer is conveniently from 0.01 to 0.05 mm thick. The carburised region 24 of the electrode metal 21, e.g. Ta, Mo or W, is shallower than it would be if there was no intermediate coating 22. <IMAGE>

Description

SPECIFICATION Electrodes for thermionic valves This invention relates to electrodes for thermionic valves, and concerns in particular the construction of grid electrodes for use in valves of the type employed in industrial heating equipment and in the output stages of high power ratio transmitting equipment.

Though semiconductor amplifying devices are now widely used for low power electronic equipment, the handling of large amounts of power (of the order of Kilowatts) in, say, industrial heaters and television transmitters necessitates the utilisation of thermionic valves (vacuum tubes). For the purposes of this Specification, such a valve may be defined as an evacuated sealed envelope within which there is a cathode, an anode, and one or more control grids (so called because they are usually grid-like networks of wires). A standard structural form of grid is that known as a squirrel cage, which may be regarded as an orthogonal grid of wires curved to form a tube and given a circular plate cap at one end.

In operation, the cathode is heated till it emits electrons, and these are then drawn to the anode via the control grid(s). Applied to the grid, in the form of a potential difference between it and the cathode, is the signal the valve is to amplify; the effect of this grid potential is to modify the cathode-anode current in correspondence with the signal changes - and thus the relatively small valve input signal causes the desired relatively large valve output current.

As can be appreciated, the control signal applied to the grid will on occasion be such that electrons are drawn to the grid itself; these electrons constitute what is known as the grid current. In a large high power valve this grid current may be sufficient to result in the grid absorbing a considerable proportion of the power being handled by the valve, possibly as high as 2%. For example, in a 20 Kilowatt valve the grid may have to deal with a grid current amounting to a power absorption of 400 watts overall, or the same as half a conventional 1 bar electric fire.It will be clear that this will cause the grid to become very hot - and that, if it is not to become too hot, this heat energy must be removed (a grid that runs too hot will not only emit electrons itself, so nullifying the effect it is intended to have, but it will also soften and sag, upsetting the carefully-planned spacing of the various components, and leading almost inevitably to sudden and rather catastrophic failure). One conventional method of keeping the grid cool is to allow it to radiate the heat away, and in order to enhance its radiation at low temperatures - 400 watts to be radiated away from a wire grid of active region surface area (that part adjacent the cathode) of the order of 20 to 30 cms2 is about 15 watts/cm2 - the grid is usually given a black, microscopically-rough surface (such surfaces emit heat radiation much better than, say, smooth, shiny surfaces).The enhancement of the heat emissivity of the surface can be achieved in a number of ways, but generally it is preferred to coat the grid with a cohesive thin layer of a suitable substance in very finely-divided form. One such substance is pyrolytic graphite. The present invention is concerned with grids provided with pyroiytic graphite coatings to enhance their heat energy emissivity.

Pyrolytic graphite is a form of molecularly-ordered carbon which is produced by vapour deposition of carbon particles resulting from the decomposition of a hot carbonaceous gas. Although the material is referred to as pyrolytic graphite it is not a true graphite in the crystallographic sense (its properties are described in the article "Pyrolytic Graphite" by W.H. Smith and D.H. Leeds published in Modern Materials, Volume 7 at page 139 et seq., Academic Press Inc. New York and London 1970).

Pyrolytic graphite is particularly suitable for use as a grid electrode coating in high powerthermionic valves, and has now been employed as such for some years. However, while valves utilising pyrolytic-graphite-coated grids have performed very satisfactorily, it is felt that there is still some room for improvement, especially in the field of the adhesion of the graphite coating to the grid material itself. The problem seems to be two-fold; firstly there is the physical nature - specifically, the roughness - of the material surface, and secondly there is the considerable disparity in thermal expansion coefficients between the material and the pyrolytic graphite layer.

The electrodes - and particularly the grids - of thermionic valves are conventionally constructed of Group Vb and Vlb metals such as tantalum, molybdenum and tungsten and their alloys (these are all strong, high melting point metals with low vapour pressures at the intended electrode operating temperature). Their coefficients of thermal expansion are higher than that of any pyrolytic graphite deposited thereon, and this difference means that there is a tendency for the graphite layer to separate from the electrode material, becoming - where the material is used in wire form - a tube surrounding each wire but spaced from it around a large fraction of its circumference. This is not necessarily detrimental per se, but where for some reason there is excessive strain in the graphite coating due either to internai or external forces it may crack and flake off.

This is obviously undesirable, and necessitates careful inspection during manufacture to ensure that the likehood of flaking is minimised during the operating life of the electrode.

The present invention seeks to reduce, if not eliminate, the likelihood of this graphite layer separation by forming on the electrode an intermediate layer (between electrode and graphite) which is itself tightly held by the electrode surface, and which has an outer surface that is microscopically rough and forms a good bond to the graphite deposited thereon.

In one aspect, therefore, this invention provides a pyrolytic-graphite-coated electrode for use in a thermionic valve, which electrode has, between the electrode metal and the pyrolytic graphite coating, a separately-applied intermediate layer, having a microscopically rough outer surface, which is stable under the electrode's intended operating conditions and exhibits good adhesion to the electrode metal.

The electrode of the invention may have had its pyrolytic graphite coating applied thereto in any of the ways used, or suggested, in the art. Generally, the coating is deposited by a method in which the electrode is placed in an evacuated oven, the whole is heated to the pyrolysis temperature of a chosen carbonaceous "gas" (for example, acetylene), and the gas is then fed into the oven, where it is pyrolised, the formed colloidal carbon particles setting out onto the electrode. Various improving modifications to this general method are available.

One such is described and claimed in the Complete Specification of English Electric Valve Company British Letters Patent No. 1,444,419; here, the electrode is mounted between massive thermallyconductive bodies, and then ohmically heated (by passing an electric current through it), so that it gets hot only where it is not near, or contacted by, the massive bodies. In this method the carbonaceous gas is only pyrolised - and carbon is only deposited exactly where it is needed, and not (wastefully) all over the interior of the oven chamber.

Atypical carbonaceous gas useable to form the pyrolytic graphite coating is, as stated above, acetylene (another suitable gas is methane), and while naturally the required thickness of the deposited coating depends on many factors - the use type of the electrode (e.g., a grid) the physical and dimensional structure of the electrode (e.g., a contra helically-wound wire cage 10 cm tall by 5 cm diameter, with a diamond-shaped "mesh" of size 0.5 cm) and the dimensions of the electrode core material (e.g., wire of 0.5 mm diameter) - neverthe less it is reasonable to say that in general it will be from 0.01 to 0.05 mm thick.

The electrode may, of course, be any type of electrode on which a pyrolytic graphite coating is useful (say, to increase its heat radiation emissivity), but it is primarily envisaged that the electrode will be a control and/or screen grid - specifically, the G1 or G2 grid - of the valve.

The electrode metal may, of course, be any of the metals used, or suggested for use, in the art. Typical such metals - as mentioned above, are the Group Vb and Vlb metals tantalum, molybdenum and tungsten (and alloys of these).

The intermediate layer used in the inventive electrode between the electrode metal and its coat ing of pyrolytic graphite has a microscopically rough outer surface, and is made of a material that is stable under the electrode's intended operating temperature. The preferred material is a layer of the carbide of one or more Group IVb, Vb or Vlb metal. In particular, this carbide is zirconium (especially), tantalum, molybdenum or tungsten carbide (or a combination of two or more of them). Each of these carbides can be formed upon the electrode metal as an adherent stable layer having a microscopically rough surface to which the subsequent pyrolytic graphite exhibits excellent adhesion.

Though materials other than the Group IVb, Vb and Vlb carbides can also result in layers having the desired surface character, these carbides are of particular value because they are especially compati ble in other relevant ways with the valve and the components. Thus, these carbides are particularly physically and chemically stable under the likely use conditions (they are non-volatile, they do not break down chemically - and certainly not into atomic or molecular fragments themselves detrimental to valve operation - and they appear not to be dissolved to any significant extent into the electrode metal to form "alloy" compositions with inferior or otherwise undesirable properties).Moreover, the carbides appear to act as a barrier layer to the pyrolyticallyformed carbon, preventing or reducing its dissolution in the electrode metal ("carburisation"), which dissolution could also result in the metal's properties changing for the worse. This carburisation reduction might allow there to be used as the electrode material cheaper metals - such as molybdenum hitherto considered less suitable because of the effect carburisation has had on them.

The intermediate layer of the invention may be applied in any convenient manner, such as dipping or spraying followed by a high temperature sintering step to fix it into place. The preferred carbide layers are in fact employed on other types of grid electrode to reduce electron emissivity and increase heat emissivity, in place of or together with platinum black (for many applications pyrolytic-graphitecoated grid electrodes have replaced these, being cheaper and more effective), so the techniques for applying them are generally well understood.Preferably, however, an inventive carbide layer is initially formed, as a separate step in the construction of the electrode, by conventional cataphoretic deposition techniques (in which the electrode metal is made the cathode of a pair of electrodes in an "electroplating" arrangement in which the "electrolyte" is a suspension of the chosen carbide the particles of which tend to acquite a net positive charge and are therefore attracted to, and deposited out on, the "cathodic" electrode metal). As an example, the cataphoretic deposition might involve a 20-50 gpl suspension of zirconium carbide (particle size about 1-5 micron) and aluminium nitrate in methanol at a cross cell potential of 20-50 volts and a current of 1-5 amps. This is then followed by air-drying, and vacuum ovening at about 2000"C to sinter the deposited layer and fit it firmly in position.

The inventive intermediate layer may be of any appropriate thickness - to a large extent this is, as will be well understood, dependent upon the type and dimensions (and so on) of the electrode itself, in the same manner as the thickness of the subsequent pyrolytic graphite coating is so dependent. Generally, however, the layer will be from 0.01 to 0.05 mm thick, and for an average control grid or screen electrode of the sort specified hereinbefore an intermediate layer thickness of 0.025 mm is satisfactory. However, if the electrode material itself is liable to carburisation, resulting in significant degradation of its desired properties, then the intermediate layer may be applied somewhat thicker than normal in orderto mitigate this possibility.

The invention extends, of course, to a thermionic valve whenever having an electrode - specifically, a control grid - which is one of those inventive intermediate layer pyrolytic-graphite-coated elec trodes as described and claimed herein.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic part cut-away perspective view of the relevant parts of a ther mionic valve having a control grid according to the invention; and Figures 2A and 2B show diagrammatic representations of cross-sections through respectively a wire of the grid used in the valve of Figure 1 and a wire of a grid used in a comparable Prior Art grid.

The valve shown in Figure 1 is a water-cooled high power triode suitable for use as the output stage of an industrial heater. It is generally cylindrical in shape, and comprises, mounted upon a support structure (10; not shown in detail) a water jacket with outer and inner walls (11,12) surrounding and coaxial with an anode (13) itself surrounding and coaxial with a grid (14) which in turn coaxially surrounds a cathode filament (15).

The cathode filament 15 of this particular valve is made from 0.4 diameter thoriated tungsten wire in the form of a cylindrical mesh tube with a nominal outside diameter of 75 mm. During valve operation the filament is at a temperature of approximately 1 800 C.

The grid electrode (14) is an electrode according to the invention. It is spaced from the filament 15 by a nominal 2 mm gap, and in this particular instance is made up of about 90 vertical tantalum wires (as 16) equally spaced and welded at one end to a molybdenum cap (17) and atthe other end to a molybdenum tube (not shown) within the support structure 10. A continuous tantalum wire helix (18) is welded onto the vertical wires 16 to increase the rigidity of the grid 14 and to provide it with the desired electrical characteristics.

The grid 14 is surrounded by a copper anode 13 spaced about 9 mm therefrom, and the anode 13 is surrounded by a water jacket with inner and outer walls 12,11.

The grid wires 16, 18 are, in accordance with the invention, coated first with zirconium carbide and then with pyrolytic graphite. Figure 2a shows a typical section - about 200x actual size - through one of these wires. As can be seen, there is a metal core (21) overlaid by a thin layer of zirconium carbide (22) itself coated with pyrolytic graphite (23), and the metal core 21 has a shallow carburised region (24).

By contrast, Figure 2b shows a comparable Prior Art wire. It has a metal core 21 and graphite coating 23, but it has no zirconium carbide layer. Moreover, it has a much deeper carburised region 24.

The following Example and Test Result Data are now given, though again only by way of illustration, to show certain aspects of the invention.

Example Preparation of an inventive pyrolytic-graphitecoated grid electrode for a 100 KW industrial heating triode.

(A) Preliminary coating of the grid with zirconium carbide A 0.24 mm tantalum squirrel cage grid of the type shown in Figure 1 was first degreased, ultransonical ly cleaned, and vacuum furnaced at 1000"C, and then placed as the cathode in an electrophoresis cell in which the anode was nickel, and the electrolyte was a suspension of 40 gpl zirconium carbide (max.

particle size 5 micron) and 0.04 gpl aluminium nitrate in methyl alcohol. The electrophoresis was effected with a voltage of 40 volts and a current of 1 amps for a period of three minutes. At the end of this time the coated grid was removed, and the zirconium carbide brushed off the molybdenum top cap and skirt at the top and bottom of the grid. After drying thegrid in a hot cupboard (35"C) for a period of 1 hour the grid was examined and found to have a uniform, microscopically rough, black coating of zirconium carbide approximately 0.02 mm thick. The grid was then vacuum furnaced at 2000"C to sinter the coating.

(B) Formation of a pyrolytic graphite coating The grid was then mounted in a suitable electrode arrangement (see, for example, the aforementioned British Patent Specification No. 1,444,419), and surrounded by a water-cooled envelope evacuated to a pressure of around 10-3 mm Hg. Acetylene gas was introduced into the system at a pressure of 2.5-3.0 mm of Hg, and the grid was heated directly by passing a current of 500 Amps (A.C.) through it for 4 minutes. The current was then increased to 530 Amps. As the carbon deposited the current rose; when it had reached 560 amps the power was cut off, and the acetylene was stopped.

After cooling, the grid was removed and found to have a pyrolytic-graphite-coated surface with an average finished diameter of about 0.3 mm.

Test Results The grid according to the preceding Example was sectioned and examined under the scanning electron microscope. It was found to have a coating of pyrolytic graphite which was well adhered to the zirconous carbide coating which in turn was well adhered to the tantalum wire.

Having established that the adhesion was good on the first sample, 4 further samples were made. They were tested in the following manner.

Firstly, the surfaces were examined visually for signs of the coating having poor adhesion; none were found. Secondly, using a scalpel attempts were made to remove the coating; this ciould only be done with relative difficulty when compared with pyrolytic-graphite-coated grids made in accordance with the present day preferred Prior Art method (very much the same as in the Example, but without the preliminary coating with zirconium carbide).

Further, in no case was the coating seen to come away in large flakes - which was observed when the same test was done on the Prior Art pyrolyticgraphite-coated grids.

It was clear that at least prior to use in a valve the grids of the invention (with the intermediate zirconium carbide layer) were superior to those of the Prior Art in that they had significantly less surface defects and that the coating itself was more rugged.

A further three inventive pyrolytic-graphite-coated grids were made (again as in the Example), and each of these grids was used in the construction of a 100KW industrial heating triode. Another similar series of conventional Prior Art pyrolytic-graphitecoated grids was also made, and each of these grids was used in the construction of a 100KW industrial heating triode. The two sets of valves were then run, and their operating characteristics studied.

Electrical characteristics (such as R and Gm). were similar in both cases. The major differences that were noted were on high voltage performance, where the conventional Prior Art pyrolytic-graphitecoated gridded tubes were markedly inferior to the invention grids. The frequency of flash arcing was used as a major indication of valve quality. When the Prior Art conventional valves were operated with an anode voltage in excess of 12KV their performance deteriorated rapidly, and the frequency of flashovers was quite unacceptable (no better than every four minutes). The inventive valves, on the other hand, operated satisfactorily at voltages as high as 13KV without any significant degradation of performance occurring over the entire period of the test (several hours).

Claims (9)

1. A pyrolytic-graphite-coated electrode for use in a thermionic valve, which electrode has, between the electrode metal and the pyrolytic graphite coating, a separately-applied intermediate layer, having a microscopically rough outer surface, which is stable under the electrode's intended operating conditions and exhibits good adhesion to the electrode metal surface.

2. An electrode as claimed in claim 1,wherein the intermediate layer is the carbide of one or more Group IVb, Vb or Vlb metal.

3. An electrode as claimed in claim 2, wherein the intermediate carbon layer is zirconium carbide.

4. An electrode as claimed in any of the preceding claims, wherein the intermediate layer is from 0.01 to 0.05 mm thick.

5. An electrode as claimed in any of the preceding claims, wherein the intermediate layer is initially formed, as a separate step in the construction of the electrode, by a cataphoretic deposition technique followed by drying and sintering.

6. An electrode as claimed in any of the preceding claims which is a wire mesh control and/or screen grid.

7. An electrode as claimed in any of the preceding claims, wherein the thickness of the deposited pyrolytic graphite coating is from 0.01 to 0.05 mm thick.

8. A pyrolytic-graphite-coating electrode as claimed in any of the preceding claims and substan tiaily as described hereinbefore.

9. A thermionic valve whenever having an electrode which is one of those intermediate layer pyrolytic-graphite-coated electrodes as claimed in any of the preceding claims.