US3307063A - Grid electrode made of pyrolytic graphite - Google Patents

Description

Feb. 28, 1967 v J. M. SARROIS 3,307,063

GRID ELECTRODE MADE OF PYROLYTIC GRAPHITE Filed April 2, 1965 zlimfm Attorneys United States Patent ()fi1ce 3,3d7,053 Patented Feb. 2 8, 1967 3,307,063 GRID ELECTRGDE MADE OF PYRGLYTIC GRAPEHTE Jean Marcel Sarrois, Saint-Cloud, France, assignor to Compagnie Francaise Thomson Houston-Hotchkiss Brandt, a French body corporate Filed Apr. 2, 1963, Ser. No. 269,919 Claims priority, appiication France, Mar. 2, 1962,

889,759, Patent 1,344,220 I 7 Claims. (Cl. 313-348) This invention relates to grid electrodes for electron discharge devices, and has among its objects the provision of an improved grid electrode possessing very low values of primary and secondary emission, high thermal conductivity and high mechanical strength and dimensional stability.

In the operation of electron discharge tubes, serious limitations are imposed on the power rating of the tube, and the practical operating characteristics of the tube are frequently impaired due to primary (thermal) emission and secondary emission effects occurring in the grid eleca trodes. Primary emission usually occurs as a result of the fact that cathodic emissive substances are deposited over the surfaces of the grid during manufacture and/ or operation, and produce electron emission when the grid is subsequently heated due to its bombardment by electrons from the cathode of the tube, and to a lesser extent the heat radiated from the cathode. Whereas the primary emission rate of the grid is an increasing function of grid temperature, secondary emission is practically independent of temperature and depends only on the nature of the materials at the surface of the grid and the velocity of the primary electrons impinging thereon from the cathode.

Heretofore, grid electrodes designed for operation under high power and heat loading conditions were almost exclusively made from metals such as molybdenum, tantalum and tungsten, having high mechanical strength at elevated temperatures. These metallic materials however have certain definite drawbacks for such use. They are poor conductors of heat, so that the heat received by the grid cannot be dissipated as fast as it is supplied through electron bombardment and a considerable amount of forced cooling becomes necessary. Moreover, the aforesaid metals when coated with a thin film of cathodic deposits as mentioned above greatly increase the primary emission rate of the grid. This emission rate is increased by the temperature elevation which results from the poor heat conductivity just mentioned. Secondary emission effects are simultaneously increased, the secondary emis sion factor assuming a value greater than unity as the electron accelerating voltage exceeds a certain threshold value within the range of from 150 to 250 volts.

It is well-known to reduce the undesirable electronemissive properties of grid electrodes made from the above mentioned metals by coating the metallic surface with substances possessing low secondary emission factors and simultaneously able to diminish the emission characteristic of the cathodic deposits built up thereon. Such improving coatings have included Zirconium, tantalum carbide and carbon, and are applied in the form of a suspended powder and then bonded to the underlying surface by sintering. This process increases the fragility of the resulting grid structure, and the fragility further rises considerably in service. Moreover, such coatings do not improve the heat conductivity of the grid which remains low with the serious disadvantages this entails as mentioned earlier.

Objects of this invention are to eliminate these drawbacks and the consequent limitations on the power rating and other operating characteristics of the electron discharge devices in which grid electrodes are incorporated.

These objects are attained by the provision of a grid electrode characterized in that it comprises, at least at the surface thereof, oriented graphite so applied that its maximal thermal conductivity is directed substantially parallel to the surface of the electrode. In a preferred embodiment of the invention, the grid electrode is composed throughout its full depth of oriented graphite applied as just specified.

Molecularly oriented graphite, also sometimes known as pyrolytic graphite, is known in the art. It is a form of crystalline carbon obtainable through thermal decomposition of a gaseous carbon compound such as a gaseous hydrocarbon, on the surface of a supporting base carried to a suitable high temperaure, e.g. of the order of 2000" C. It deposits on the surface of the base in the form of successive layers or strata which can be built up to considerable depths. Each layer is of extremely high mechanical strength and due to the oriented crystalline buildup of the layers the resulting sheet of graphite exhibits a very high degree of anisotropy in its physical characteristics. This is especially true in regard to its heat con ductivity. Thus, the heat conductivity of such a foliated layer of oriented graphite when measured at a temperature below about 1000 C. in a direction parallel to the general surface of the layer, is very high, being about the same as that of copper sheet, about three times higher than that of ordinary commercial graphite as obtained e.g. by recrystallization from a conglomerate of microcrystalline carbon. In a direction normal to the general surface of the layer, the heat conductivity of oriented graphite is considerably less than that of ordinary graphite. In regard to electron emission properties, the secondary emission factor of oriented graphite is substantially equal to that of ordinary graphite, and is therefore less than unity regardless of the velocity of the impinging elec trons. Its primary emission properties are also satisfactory in that it acts greatly to reduce the primary emission of cathodic substances deposited thereover just as would a sub layer of ordinary graphite or carbon, as earlier mentioned.

Thus in a grid structure according to the invention, the oriented graphite constituting at least a surface layer of the electrode, is just as favourable in eliminating unwanted secondary electron emission as the best coatings known for a similar purpose in prior-art electrodes. But in addition, due to the greatly heightened heat conduction of the oriented graphite material in directions along the surface thereof, the inherent rate of heat dissipation is much increased so that, under comparable conditions of external forced cooling (if any), the operating temperature of the grid is lowered, thereby providing an additional factor for reducing the rate of primary emission from the grid surface, besides the normal reduction in primary emission due to the properties of carbon in general as earlier mentioned.

In the improved grid structure, the individual grid elements, such as strips or bars, exposed to heating from electron bombardment, are rapidly cooled by dissipation of their heat content in directions parallel to the layers or folia of oriented graphite towards the supporting elements of the electrode. Due to the extraordinarily high value of heat conductivity of oriented graphite in the aforementioned directions, this heat dissipating effect is highly effective even in cases where only a relatively thin surface layer of the grid structure is formed from the oriented graphite material; it is substantially more effective still where the grid electrode structure is composed of oriented graphite throughout its depth in accordance with the aforementioned preferred embodiment of the invention.

The mechanical strength characteristics of the electrode of the invention are also remarkably good, owing to the high mechanical strength of the elementary layers or folia of the oriented graphite especially in regard to tensile stresses in directions parallel to the general surface of the folia. In the improved electrode wherein the. oriented graphite is applied as a coating overlying electrode elements made of some other material, such as molybdenum or tantalum, there is provided in effect a tough, compact sheath around the grid elements, which greatly increases their resistance to impacts and vibrations. Where the grid is entirely formed of oriented graphite as is preferred, the over-all mechanical strength is even better, and is generally considerably higher than that of comparable electrodes made of conventional metallic materials such as molybdenum and tantalum. This is due in part to the fact that the quotient of tensile strength to specific gravity is substantially higher in the case of oriented graphite than in that of most metals. Moreover, the mechanical rigidity and stability of the electrode structure comprising oriented graphite remains practically unimpaired throughout long periods of service at high temperatures, in contrast with what is observed with electrodes made from materials of the prior art.

Grid electrodes according to the invention can be provided in a great variety of over-all geometrical shape, including fiat, cylindrical, spherical, etc. The individual grid elements, such as wires or strips, may also be made to a wide diversity of forms and dimensions. However, in the case of a grid electrode made entirely from oriented graphite, it is preferred according to the invention to provide said elements by forming uniformly spaced perforations, e.g. slots, in the sheet of oriented graphite. Thus, in the French Patent 1,323,676, corresponding to copending patent application No. 261,461, assigned to the assignee of the present application, there is described an electrode in the form of a skew-slotted tubular cylindrical element wherein the slots are conveniently provided as elliptical curves defined by the intersection of the cylindrical surface with angularly spaced planes inclined at a common angle to the generatrices of the cylinder. Such a form of electrode is especially suitable when made entirely from oriented graphite in accordance with a preferred aspect of the present invention. Another suitable shape is that of a cup-shaped element having a flat end or bottom wall in which slots in the form of equispaced circular arcs are cut, as disclosed in said application 261,461. To produce such electrodes in accordance with this invention, a tubular or flat blank made from oriented graphite may first be prepared by known techniques as by depositing the oriented graphite over a core, which may conveniently comprise ordinary commercial graphite, through heating the core in a suitable carbon containing gaseous atmosphere as earlier indicated, and then separating the deposited coating of oriented graphite from the core, an operation easily performed where the core comprises ordinary graphite.

An exemplary embodiment of the invention will now be described with reference to the drawing, which illustrates in axial cross-section a grid electrode made entirely out of oriented graphite in accordance with a preferred form of this invention.

By way of example, the electrode shown is generally similar in form with that illustrated in copending application No. 261,461 mentioned above, and comprises a part in the form of a surface of revolution including a frustoconical base section 1, a cylindrical main body section 2 and a flat top 3. The cylindrical body section 2 is formed with a series of slots 4 angled with respect to the generatrices of the cylindrical surface. Specifically the slots 4 and hence also the mid-lines of the inter-slot strips 6 forming the elements of the grid electrode, are elliptical arcs defined by the intersection with the cylinder 2 of a family of angularly-spaced planes inclined at a common angle to the axis of the cylinder. For clarity, both the angle of inclination of the slots, indicated at 7, and the pitch spacing between them, are shown larger than would actually be used in most practical applications. The average inclination of the slots with respect to the generattrices of the cylinder may advantageously be selected within the range of from 10 to 30.

The electrode shown is entirely constituted of a sheet of oriented graphite and is prepared by depositing carbon from a gaseous compound upon a suitably shaped core, made e.g. of ordinary commercial graphite, at a suitable temperature of the order of 2000 C. After the deposited oriented graphite has built up to the desired depth, the assembly is allowed to cool and the deposited layer is separated from the underlying graphite core. The resulting oriented graphite element is then machined to provide the inclined slots 4 in its cylindrical body part 2, e.g. by the procedure described in the aforementioned application No. 261,461, and which in brief consists of indexing the blank to successive angular positions about its axis, and cutting into the body 2 by means of a thin rotary milling cutter or grinding wheel whose axis of rotation is held at a constant angle to the axis of the blank, at each indexed position. The resulting slotted electrode can then be mounted on a support 8, e.g. of ordinary graphite, as by brazing the periphery of base section 1 to the support 8 by means of a suitable brazing medium such as titanium carbide, zirconium carbide, or pure titanium, as shown at 9.

In one practical embodiment of the illustrated electrode, which has operated satisfactorily, the following dimensional characteristics were used:

Wall thickness, oriented graphite, in

cylindrical body section 2 0.20 to 0.25 mm. Diameter of cylindrical body section 2 40 mm. Axial height of body section 2 30 mm.

The slots 4 extended over about 20 mm.

of said axial height. Slope angle of slots 4 to the generatrices 20. Slot width as determined by grinder thickness 0.6 mm. Inter-slot spacing 0.25 mm.

In most cases such an electrode can be machined without having to introduce any support or core within the blank, and this makes it possible to machine two similar grid electrodes in a single operation. The resulting electrodes will have their slots and inter-slot strips aligned with a high degree of accuracy, especially where the angle of the slots is comparatively low. In this way assemblies including e.g. a grid electrode and a screen electrode for incorporation into a common electron discharge tube can be simultaneously prepared.

As earlier indicated, an electrode according to an alternative embodiment of the invention can be produced by providing a coating of oriented graphite over the surfaces of a base grid structure of any suitable material and shape. An exemplary procedure relating to this aspect of the invention will now be described.

The apparatus used may comprise a casing made of ordinary graphite, formed with an inlet and an outlet for a suitable carbon containing gas, such for example as methane, acetylene, benzene or other hydrocarbon. The casing is positioned within an electric heating device such as a cage of closely-spaced heating resistors. Within the casing and adjacent to the gas inlet is a gas diffuser member. The electrode structure to be coated and having any suitable shape as earlier indicated, is mounted centrally within the casing. When the gas is circulated through the casing and the heater is operated to bring the internal temperature to a value of about 1500 C. or more, fol iated layers of oriented graphite deposit over the surfaces of the electrode elements and build up to a depth depending on the time the process is continued. The oriented graphite layer is found to settle with great uniformity, even if the underlying support includes irre gular areas such as spots of solder or the like, so that there are practically no reasonable restrictions on the shape and structure of the electrodes achievable by this method.

It will be apparent that various modifications of the invention may be conceived Within the scope of the claims.

For general reference regarding the properties of pyrolytic graphite, the reader is referred to an article by Pappis and Blum in the Iounal of the American Ceramic Society, vol. 44, No. 12 (December 1961) p. 592-597.

I claim:

1. A grid electrode having low secondary emission, said grid electrode having a configuration to define a surface and comprising at least in a surface area thereof, a layer of pyrolytic graphite having a direction of maximum thermal and electrical conductivity thereof directed substantially parallel to the general surface of said area.

2. A grid electrode having low secondary emission which comprises a sheet element defining a surface of pyrolytic graphite having its maximum thermal and electrical conductivity directed substantially parallel to the surface of the sheet, and spaced perforations formed in said sheet element.

3. A grid electrode having low secondary emission which comprises a tubular cylindrical body portion defining a cylindrical surface and slots formed through the wall of said body and inclined with respect to the 'generatrices of the cylindrical surface thereof, said body comprising at least in the surface area thereof a layer of pyrolytic graphite having its maximum thermal and electrical conductivity directed parallel to the general surface of said area.

4. A grid electrode having low secondary emission which comprises a cup-shaped element with a flat end Wall defining an end surface, and regularly spaced slots formed in said end wall, said end Wall comprising at least in the surface area thereof a layer of pyrolytic graphite having its maximum thermal and electrical conductivity directed parallel to the general plane of said end wall.

5. A grid electrode as claimed in claim 2, wherein said sheet element comprises pyrolytic graphite throughout its thickness.

6. A grid electrode as defined in claim 3 wherein said body comprises pyrolytic graphite throughout its thickness.

7. A grid electrode as claimed in claim 4 wherein said end wall comprises pyrolytic graphite throughout its thickness.

References Cited by the Examiner UNITED STATES PATENTS 1,612,835 1/1927 Schottky 313-348 1,985,087 12/1934 Glaser et a1. 313-407 X 2,054,234 9/1936 Allen 313-'107 X 3,138,434 6/1964 Diefendorf. 3,172,774 3/ 1965 Diefendorf 313-357 3,187,502 6/1965 Stover 3 13357 FOREIGN PATENTS 1,323,676 3/1963 France.

830,532 3/ 1960 Great Britain.

918,880 2/ 1963 Great Britain.

JOHN W. HUCKERT, Primary Examiner. A. J. JAMES, Assistant Examiner.

Claims (1)

1. A GRID ELECTRODE HAVING LOW SECONDARY EMISSION, SAID GRID ELECRODE HAVING A CONFIGURATION TO DEFINE, A SURFACE AND COMPRISING AT LEAST IN A SURFACE AREA THEREOF, A LAYER OF PYROLYTIC GRAPHITE HAVING A DIRECTION OF MAXIMUM THERMAL AND ELECTRICAL CONDUCTIVITY THEREOF DIRECTED SUBSTANTIALLY PARALLEL TO THE GENERAL SURFACE OF SAID AREA.

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