US3458749A - Dispenser cathode made of tungsten powder having a grain size of less than three microns - Google Patents

Description

y 1969 A. J. A. VAN STRATUM ET AL 3,458,749 DISPENSER CATHODE MADE OF TUNGSTEN POWDER HAVING A GRAIN SIZE OF LESS THAN THREE MICRONS Filed June 6, 1967 AGE United States Patent US. Cl. 313-346 5 Claims ABSTRACT OF THE DISCLOSURE A pressure dispenser cathode of tungsten and barium aluminate using tungsten powder with a grain size less than 3 microns to reduce the barium evaporation rate.

The invention relates to a dispenser cathode comprising a pressed mixture of tungsten power and barium aluminate-containing powder.

From publications of Hughes and Coppola, see US. Patent 2,929,133, and in particular part II of the article in Philips Technical Review, vol. 19, No. 6, pp. 177190 (1957-8), whose contents are hereby incorporated and made a part hereof, it is known that pressed dispenser cathodes may be manufactured from a mixture of powdery tungsten and powdery tribariumaluminate, which preferably contains, in addition, calcium oxide, pressed by a high pressure (11,000 kg./cm. into a cathode body, and heated in vacuum or in a hydrogen atmosphere at 1800 to 1900 C. at which the aluminate melts and the gases are expelled from the cathode. Owing to the use of aluminates the cathode can be preserved in air only to a restricted extent.

These cathodes exhibited a high rate of barium evaporation and had a short lifetime. The active metal in this cathode was tungsten. An alloy of tungsten and molybdenum was used afterwards for reducing the evaporation of barium from the cathode. It appeared to be necessary to use an alloy of about 75% of Mo and 25% of W, but this reduced the emission as compared with the use of pure tungsten powder. The metal powders employed were mixtures of grains having a size lying between about 4 and 1212.

The disadvantage of excess evaporation of barium involved in the use of pure tungsten powder is found to be almost completely avoidable in accordance with the present invention by using metal powders, the particles of which have maximum granular size of less than 3 preferably less than 0.5a, and using a powdery mixture obtained from coprecipitated barium carbonate, calcuim carbonate and aluminum oxide. The tungsten and carbonate and oxide powders are thoroughly mixed and compressed at a pressure of at least 25,000 kg./cm. It was a surprise to find that the evaporation of barium from such a cathode is even considerably lower than in the known molybdenum-containing pressed cathodes in spite of the use of pure tungsten without the molybdenum diluent. The electron emission obtained is equivalent to that of the so-called L-cathode, in which the active barium oxide is accommodated in a cavity beneath a porous tungsten body. An advantage of the inventive cathode is that degassing and activation thereof takes considerably less time than in the case of an L-cathode. The pressure of 25,000 l g./cm. required for compressing a cathode according to the invention is more than twice ice that required for the known pressed cathodes using tungsten powders of a grain size lying between 4 and 12;/., but technically this involves no particular difliculties. The higher the pressure, the less the evaporation. Pressures below 20,000 kg./cm. are unsuitable because the Ba evaporation becomes too high.

It is found that in spite of the very fine pores produced in the complete cathode structure, degassing and activation do not take more time than the known pressed cathodes. The cathode according to the invention has the same properties as the L-cathode, owing to the absence of molybdenum, but it can be manufactured at much lower cost.

The invention will now be described more fully with reference to the accompanying drawing, the sole figure of which is a cross-sectional view of one form of a cathode according to the invention.

The cathode shown comprises a pressed emissive body 1, accommodated in a cylindrical molybdenum foil 2. The cylinder 2 is provided with strips 3 for securing same in the electrode system of the usual electron t-ube, e.g., a cathode-ray tube. The cylinder 2 accommodates a heating element 4 and a partition 5 of molybdenum in order to avoid emission from the body 1 to the heater 4.

The body 1 consists of a powdery coprecipitated mixture of barium carbonate, calcium carbonate, and aluminum oxide in a molecular ratio of 5:312 (SBaCO 3CaCO 2Al O nH O), and tungsten powder having a grain size of less than 0.5 ,u. Such fine tungsten powder cannot be obtained by grinding, but is obtainable for example by the reduction of gaseous tungsten chloride (WCI with hydrogen, which is known as such. The mixture contains furthermore a small supply of carbon, preferably in the form of a soluble carbon compound such as sugar (C H O since this ensures a homogeneous distribution of the carbon in the mixture. The ratio in weight of the powder mixture to be compressed is: 93% of tungsten, 6.8% of coprecipitated barium-calcium-aluminate compound, and 0.2% of sugar. The mixture is introduced into a mold comprising the molybdenum cylinder 2 and the partition 5 and it is compressed with a pressure of 25,000 kg./cm. while the upper rim of the cylinder 2 is slightly pressed inwardly, so that the cathode body 1 is firmly held in place. Then the supporting strips 3 are welded to the cylinder 2 and the heating body 4 is slipped into it.

As an alternative, the body 1 may be compressed to form a pellet which is subsequently fastened in a holder.

It appears that in spite of the very fine pores of the body 1, the degassing and the formation of the aluminates, from the carbonate and oxide mixture, which aluminates are partly converted into barium oxide which reacts with the tungsten to form free barium, are performed quite rapidly. The evaporation of barium in operation appears to be even smaller than in the known pressed cathodes using a tungsten-molybdenum alloy.

A representative example of one manufacturing method is as follows:

The coprecipitated mixture is formed as follows: Dissolve 52.3 g. Ba(NO in 500 ml. H 0; 60 g.

Al(NO -3.9H O

in 500 ml. H 0 and 28.3 g. Ca(NO 3:4H O in 500 ml. H O. Mix these solutions thoroughly and add it under vigorous stirring to a solution of 65 g. (NH CO in 500 ml. H O. Thereby a thorough mixture of BaOOg, CaCO, and Al O -nH O in the above mentioned ratio is precipitated. The precipitate is filtered and washed with H 0 and ethanol, dried at 120 C. during 1 hour. g. of this mixture powder is mixed with a solution of 3 g.- of sugar (C H O) in 50 ml. distilled H O to form a slurry, which is dried at 120 C. during 1 hour and ground. This powder mixture is then mixed with fine tungsten powder and the resultant powder mixture compressed as described. The cathode-bodies can now be stored in air for a long time. Afterwards before building the cathode body into a vacuum-tube, the mixture is heated during an hour at about 850 C. to remove the CO from the carbonates and then gradually heated to 1650 C. to complete the aluminate-conversion, reduces any tungsten-oxide and sinter the mixture. This heating must be done in a hydrogen-atmosphere. Thereafter the bodies can be mounted into a discharge tube where activation takes place in ordinary way. This activation is carried out at about 1200 C. and the operating temperature is between 1000 C. and 1-150 C. The technique just described is not critical. Other methods for coprecipitating the carbonate and oxide constituents, which as such are well known, can be substituted for the method described. The proportions of the barium carbonate, calcium carbonate and aluminum oxide can be varied over the wide range described in U.S. Patent 3,201,639. That is to say, the mole ratio of BaO:Al O should exceed 1:1, with the CaO addition present in the ratio exceeding 0.1 mole. The preferred ratios is 5:223 or 3:1:1 of BaO'A1 O -CaO. The carbon compound can be varied over a fairly wide range. The amount does not appear too critical. Amounts in excess of 6% should preferably not be used to avoid carburization of the tungsten, which reduces emission. Generally, at least about 0.1% is needed to increase the emission. While sugar is preferred, as convenient and inexpensive, other carbon containing compounds soluble in suitable solvents and not completely evaporating during heating, are usable, such as solutions in benzol of stearic acid, glycerol in H 0 poly-alcoholic solvents, and even carbon-suspensions such as aquadag. For completeness sake, it is noted that the n in the formula Al O -nH O refers to the fact that the aluminum oxide contains water of crystallization whose content is uncertain. Any water of course is evolved during the subsequent firing.

While we have described our invention in connection with specific embodiments and applications, other modi fications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A thermionic dispenser cathode comprising a pressed and sintered mixture of barium-calcium aluminate powder and tungsten powder, said tungsten powder having a maximum grain size of less than 3 microns, said aluminate being formed in the pressed cathode-body by conversion of coprecipitated barium carbonate, calcium carbonate and aluminum oxide in a hydrogen atmosphere.

2. A cathode as set forth in claim 1 wherein the tungsten powder maximum grain size is less than 0.5 micron.

3. A cathode as set forth in claim 2 wherein the coprecipitated mixture comprises barium oxide, calcium oxide and aluminum oxide in a mole ratio of about 523:2.

4. A method of manufacturing a pressed tungsten dispenser cathode, comprising the steps of coprecipitating barium carbonate, calcium carbonate, and aluminum oxide to form a homogeneous mixture thereof, drying the mixture, adding a solution of a carbon containing compound thereto to form a slurry, drying the carboncontaining slurry and grinding the resulting carbon-containing mixture, mixing said mixture with fine tungsten powder having a maximum grain size below 3 microns, subjecting the mixture to a pressing operation at a pressure of at least 25,000 kg./cm. and then heating the mixture to remove gases, to convert the coprecipitated mixture into aluminuates and to sinter the mixture.

5. A method as set forth in claim 4 wherein the tungsten powder grain size is less than 0.5 micron, the coprecipitated constituents are present in a mole ratio of about 5 :3 :2 of barium carbonate to calcium carbonate to aluminum oxide, and the proportions by weight of the powder mix is about 93% tungsten, 6.8% coprecipitated mixture and 0.2% of sugar as the carbon compound.

References Cited UNITED STATES PATENTS 2,912,611 11/1959 Beck et al 3l3-346 2,975,320 4/1961 Knauer 313346 X 3,076,916 2/ 1961 Koppius 3 l 3-346 JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner U.S. Cl. X.R. 313-345

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