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DISPENSER CATHODES: THE CURRENT STATE OF THE TECHNOLOGY

L.R. Falce

Ceradyne Electron Sources 3169-A Red Hill Avenue Costa Mesa, California 92626 ABSTRACT This paper describes the present technology of dispenser cathodes with emphasis on emission characteristics and life capabilities of the various tvpes. The differences between the "B", the "S", the "L", the "M", the "MK", the "CD", the "MMM" (Mixed Metal Matrix), the scandate and "CPD" Cathodes are described. Work is going on in several locations in the LLS., the LLK., Germany, Holland and France on dispenser cathodes. The primary purpose of this paper Is to distinguish between the characteristics of the dispenser cathodes that are presently or soon to be available to potential users such as tube designers and the update the status of the technology. INTRODUCTION Driven by the demands of microwave tubes for higher power and/or higher frequencies and/or longer life and/or combinations of these, pressure has been placed on dispenser cathodes to supply higher current densities for longer life times This pressure has resulted in the expenditure of considerable effort in several locations in the U.S. and other countries to develop improved performance dispenser cathodes. Dispenser cathodes types can be classified into two groups. The first are cavity reservoir dispenser cathodes and the second are impregnated dispenser cathodes. Both basic classification have had extensive development efforts in the past few years which have resulted in improved performance. Cavity Reservoir Dispenser Cathode The first stage of the development of dispenser cathodes began at the Philips Research Laboratories in Holland around 1950 (1) (2). The result was the 'IL" cathode which consisted of a thin porous tungsten plug over a cup or reservoir of barium carbonate. (Figure 1) The object was to dispense barium to the emitting surface through the pores of this tungsten plug. In order to generate free barium it was required to decompose or convert the carbonate during tube processing similar to oxide cathode conversion which was the common cathode prior to this time. This resulted in exceedingly long tube processing since the by-product carbon dioxide (CO2) had to come out through the tungsten pores. The cathode work function was a relatively low 2.0 e.v. Philips eventually evolved to the impregnated cathode to be described later.

Figure 1 L type dispenser cathode. The next cavity reservoir dispenser cathode of significances is the Metall-Kapillar-Kathoden or MK Cathode developed at Siemens in Munich Germany by Helmet Katz~") This approach also uses a porous tungsten disc and reservoir but with two improvements. The reservoir material is barium oxide that was preconverted prior to insertion and a tungsten wool placed over the BaO to take up the reaction products in the release of free barium which keeps the porous tungsten dispenser disc clean. This cathode comes in two versions, osmium coated and uncoated. The uncoated cathode work function is approximately that of an 'L" but the coated is approximately 1.8. The most recent variation of the cavity reservoir cathode is the CPD Cathode (4) (5) (6) conceived at the U.~ Naval Research Laboratory and developed at Varian Associates and Hughes Aircraft Company. (Figure 2) Instead of a randomly porous tungsten disc a thin tungsten foil 25 to 50 micorns thick with a uniform array of laser drilled holes is used as the dispenser/emitting surface. The reservoir structure is integrally grown by chemical vapor deposition (CVD). The reservoir contains a barium calcium aluminate compound mixed with 5 micron diameter tungsten powder. The approach provides a constant path length for barium migration and no reaction products that could clog the pores. The reservoir, as in the Land MK, can be as large as required for long life and constant cathode current with life is expect~ The CPD cathode can have either an all tungsten emitter or alloy surface which can be co-deposited by CVD or by overcoating the CVD tungsten foiL The tungsten surface CPD has a work function comparable to the original 'L" of approximately 2.0 e.v. The alloy surfaces have between 1.8 and 1.9 e.v. work functions.

Impregnated Dispenser Cathodes The second classification of dispenser cathode is the impregnated cathode which bad its beginnings at North American Philips Company in New York (7) with the 'IA" cathode developed by Levi and Hughes when they were trying to overcome the difficulties encountered in the processing and activation of the "L'I cathode. After first attempting to press tungsten, BaCO3 and A1203 together and sintering, they found that first making a porous tungsten matrix and then impregnating this with the barium aluminate compound was more satisfactory. The A cathode however, even though easier to process, could not provide the same emission as the L with the work function as high as 2.2 to 2.3 e.v. Levi found that the addition of calcuim to the impregnate mixture reduced the work function to a more reasonable 2.1 e.v. The actual role that the calcium plays in the system is still not well understood to this day. This version with calcium became known as the B cathode and has been the workhorse dispenser cathode for almost 30 years. The molar ratio of 5 BaO, 3 CaO and 2 A1203 (5:3:2) makes up the impregnate composition. Koppius at Semicon, Inc. later in the 1950's modified the molar ratio to 4 BaO, 1 CaO and 1 A1203 (4:1:1) this is known as the S cathode. Two commercial suppliers Spectra-Mat and Varian Semicon offer a third version of 3:1:1 as well as B and ~ The tungsten impregnated dispenser cathode in its various forms however, has been found to be life limited (9) (10) (11). Because of the high temperatures required to support high current densities the surface coverage of barium decreases with time since the supply of activating material recedes back into the pores which can also become clogged with reaction by-products cutting off the required supply of barium. Figure 3 is a photo micrograph of a typical impregnated cathode cross-section. An improvement of major significance was made to the impregnated cathode in the late 1960s at the Philips Laboratories in Holland by ZaIm and Van Stratum. By overcoating the tungsten surface with either osmium, iridium, ruthenium or rhenium a dramatic reduction in work function to about 1.8 e.v. was achieved. This meant that a cathode could deliver the same emission density as the B cathode at 1000C lower temperature. This cathode is the M cathode. In its present commercial state it is a B cathode overcoated with approximate 5000 angstroms of an 80 - 20 osmium-ruthenium alloy. It is replacing the B type cathode in many tube applications particularly where longer life is a requirement. Even though the same chemistrv is involved in the generation of the activating material, barium, it is taking place an order of magnitude slower because of the reduced temperature. There has not been complete agreement among scientists as to the mechanism of enhancement. Originally it was thought that the enhancement was due to an increased dipole due to the higher work function of osmium. It is also suggested that increased dwell time of barium due to a higher sticking coefficient has significantly increased the surface coverage. But whatever the reasons, life tests have substantiated that the M-cathode has improved life performance below 4 amps/cm2 emission density. l2J

There had been fears from the beginning that the M cathode would revert to a B in time due to the dilution of the osmium-ruthenium film by diffusion of tungsten. Very recent work at Hughes has shown that this truly happens but not any where near as catastrophically as originally thought. Twelve M-cathode samples were analyzed by Auger Electron Spectroscopy for surface composition as well as composition below the surface. A standard sample consisted of a tungsten sheet coated with a known thickness of osmium-ruthenium. Eight samples were from close spaced diode life test vehicles aged at IOOO0CB for 7,000 hours and the third was aged at ii lO0CB For 7,000 hours, and the third sample was activated only and not aged at aIl. This study has shown that tungsten diffuses to the surface of M-cathodes from virtually the beginning of life. Either sintering of the coating during cathode fabrication or initial activation is sufficient to bring between 25 and 35% tungsten to the surface. It has also shown that an alloy forms within the region of the original coating thickness that is quite stable. The composition of this alloy changes very slowly at 1000 CB. In fact, in over 50,000 hours, the alloy is still only 60% tungsten. Alloying is much more rapid at temperatures like 1110 º CB, where 60% tungsten is at The surface in only 7,000 hours. At 10400CB

after 7,000 hours, there is 40% tungsten at the surface. It was concluded that a M-cathode operating temperatures (IOOO0CB and below) capable of supplying current densities of up to 4 amps/cm2, the effect of changing surface composition should not influence cathode performance for well beyond 50,000 hour~ Figure 4 shows the results of the study showing the relationship of temperature, time and coating thickness~ Other accelerated tests at Hughes indicate that a reasonable lifetime 10,000 hours or more at higher current densities is also feasible with M cathodes.

Another version of the M-cathode, the CD cathode, was developed in EngIand at Thorn-EMI-Varian by Green, Skinner and Tuck.(1 6) This cathode again uses a conventional B cathode onto which a film of tungsten and osmium is co-deposited, The object being to obtain an optimum alloy from the very beginning of life. Varian has continued work in this area and has developed alloy coated cathodes capable of very high current densities such as 16 amp/cm2. In 1974, the author while at Varian seeking an improved version of the M cathode demonstrated that very high current densities 10 amps/cm2 and above was achieveable when an impregnated cathode had one of the enhancing metals of the M-cathnde i.e., osmium, iridium, ruthenium or rhenium dispersed throughout the matrix with tungsten.(13) A systematic study revealed that there was an optimum range of composition for the matrix as shown in Figure 5 for the iridium-tungsten system. The optimum composition range is 20 to 40% of the enhancing metal with tungsten. Telefunken in Germany has reported success with a tungsten-osmium mixed metal cathode.(14) Schroff at Thomson-CSF in France has reported on his work with a number of mixed metal system. The last of the impregnated dispenser cathodes to be described is the Scandate cathode developed at Philips in Holland.(17) This is an all tungsten matrix cathode that has a barium calcuim aluminate impregnant similar to the B cathode with an addition of from 2% - 7% by weight of scandium oxide (SC203). The developers and users have claimed emission characteristics comparable to M-cathodes. This cathode is sold commercially by Spectra-Mat under license by Philips. Summary Table I lists all the cathodes discussed above showing their class, type, description and work function.

Performance Comparison

Figure 6 compares performance in maximum space charge limited current density (1~OC above the transition between temperature limited and space charge limited emission) and brightness temperature. Band I includes the M, the coated MK, the MMM, the coated CPD and the scandate. Band II includes the L, the uncoated MK and CPD. Band III includes the B and S cathodes. Life Expectancy Figure 7 shows how the impregnated cathodes perform with current density vs life. Much of this data is based on life tests cited at a number of facilities. Other data is supplied by the deveiopers. It is assumed that the cavity reservoir types can have life corresponding to the size of their reservoirs. Conclusion The situation today is considerably brighter than it was five years ago. There are a number of improved dispenser cathodes available or soon to be available to meet both the high current density and extended life requirements of today's microwave tubes.

References 1.. Lemmens, H. J, U.S. Patent 2,543,728, 1948. 2. Lemmens, H. J., Jansen, M. J and Loosjes, ~, "A New Thermionic Cathodes for Heavv Uoads," Philips Technical Review 1950, II, pp 341-350. 3. Hubner, E, '"Kathoden Hoher Stromdichte" Vortrage Der NTG/IEE MAI 1980. 4. Falce, L R, and Thomas, R.E, "The Control Porosity Dispenser Cathodes: Iridium-Barium Oxide," IEDM Tech Digest 1981, pp 156-159. 5. Thomas, R.E., Tri Services Cathode work shop 1980.

6. Falce, L. R., Tri Services Cathode work shop 1980. 7. Levi, R- and Hughes, R. C., U.S. Patent 2,700,000. 8. Levi, R.J. AppL Phys. 26,639 1955. 9. Forman, R.,J App. Phys. 47, p 5272, 1976. 10. Longo, R. T., IEDM p 152, 1978. 11. Schroff, A. M., and Paluel, P.J., J Appl. Phys. 22, p 2894, 1979. 12. Forman, R. IEEE Trans. Elect. Dev. 26, p 1567, 1979. 13. Falce, L.R., U.S. Patent 4,165,473, Aug 1979. 14. Cornfield, G., Tri Service Cathode workshop, 1982. 15. Schraff, A. M., and Paluel, P., Extrait de Ia Revue Technique -Thomson-CSF, Vol 14, No.3, SepL 1982 16. Green, M. C, Skinner, H. B., and Tuck, R. A.,"Osmium Tungsten alloys and their Relevence to Improved M-tvpe Cathodes" Tri Services Cathode workshop, 1980. 17. Van Strotum, A., Van Os, J.G., Blatter, J. R., and ZaIm, P., U.S. Patent 4,007,393, Feb. 1977

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