Abstract: In this paper, studies are carried out on W-Re mixed matrix cathodes developed in house, will be discussed. It has been observed that a proper choice of rhenium particle size is essential for obtaining good emission density. The cathodes have been prepared by impregnating W-Re (50% – 50% by weight) mixed metal matrices with 5:3:2 barium calcium aluminates. A current density of 20 A/cm2 has been achieved at 1360K. Life testing of these cathodes has been carried out in closely spaced diode configuration. Variation of work function and emission current density after 35000 Hrs of life will be presented Keywords: Thermionic Emission, dispenser cathodes, mixed metal matrix

Introduction Mixed metal matrix cathodes can offer longer life and higher resistance to ion bombardment when compared to M-type dispenser cathodes used in microwave tubes. Iridium, Osmium, and Rhenium are the materials which are used in mixed metal matrix type cathodes. Osmium is a toxic materials need very special care for handling. W-Ir dispenser cathode current density is only slightly better than B- type cathodes [1] On the other hand Rhenium is capable of enhancing the emission as much as osmium and also it is nontoxic. Taran et al developed Re-W “sponge” matrix (75%-Re/25%-W mass ratio) and 3BaO:0.5CaO:Al2O3 as the emissive active substance [2] Additional advantage of rhenium, though not with the same effectiveness of tungsten, is that its reaction with barium calcium aluminates which can release free barium[3]. Price advantage was the factor tilting the scale towards rhenium.

Experimental W-Re mixed metal matrix cathodes of

composition (50% W and 50% Re by weight) were pressed and sintered at 1830OC.During sintering, alloying of the mixture takes place along with the densification resulting in two- phase structure. W and Re are taken in equal proportions to form the mixed metal matrix pellets. As a result of this, it can be seen from the optical micrograph, formation of alloy phase is uniform on the surface. This uniformity will reflect in the emission uniformity of W –Re cathodes. For testing of MM type cathode in closely spaced diode configuration, glass bulbs were mounted in a UHV

station and evacuated to < 5x10-8 Torr. The Glass bulbs were baked at 250OC and after completing baking, the cathode has been activated. High voltage pulse will be applied between the anode and the cathode. The temperature of the cathode is measured using a disappearing filament pyrometer. Typical Results of a W-Re MM type cathode is shown below in figure 1. The zero field emission density (Jo) or Average work function (�) of cathode at a given temperature is an accepted parameter for characterizing dispenser cathodes. The extrapolation from the Schottky region to V = 0 gives the zero- field current density J0. Work function values were calculated from the Richardson-Dushman equation.

Results The typical zero field current density achieved at 1360 K is 20 A/cm2 and the Average work function achieved at this temperature is 1.90 eV. Our results were compared with reported results in Ref [2]. From the figure 2, It is clear that W-Re(50%-50%) with 5BaO:3CaO:2Al2O3 impregnant mix gives lower work function and higher emission density than the (75%-Re/25%-W mass ratio) and 3BaO·0.5CaO·Al2O3. Also the obtained work function values are better than the values reported by Raju et al [4].

Further, to study the life of the cathode, it was kept on continuously using uninterrupted power supply. The cathode was mounted in the closely spaced diode configuration in the life test vehicle. A continuous current at 2A/cm2 loading is being drawn while cathode is on life test. To evaluate cathode, V- I characteristics measured after 35000 Hrs of life are compared with V-I characteristics measured during the early life of the cathode.

Conclusions

W-Re(50:50) mixed metal matrix cathodes have been successfully developed in the laboratory. Emission current density of 20 Acm2 was achieved at 1360 K. As shown in Fig.3, there is no measurable emission degradation of emission in the temperature limited region, even after kept on life test for more than 35000 Hours. With good emission current density and long life capability, these cathodes can be used in microwave tube applications.

3.3: Tungsten-Rhenium Mixed Metal Matrix CathodesM. Ravi*, K. Santhosh Kumar and K.S. Bhat

Microwave Tube Research & Development Centre, Bangalore, Karnataka-560013, IndiaE-mail: *mravi@mtrdc.drdo.in

978-1-4244-8660-1/11/$26.00 ©2011 IEEE 39

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Fig 1. V-I Characteristics of W-Re Cathodes

Fig 2. Comparison of Work function & Current Density data with Ref [2]

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Fig 3. Comparison of V- I Characteristics after 35000 Hours of life

Acknowledgements

Authors are thankful to DRDO, Ministry of Defence for funding this work. Guidance and inspiration given by Dr. Lalit Kumar, Director,MTRDC are thankfully acknowledged.

References

1. K.Santosh Kumar, P.Durga Devi, M.Ravi and K.S.Bhat,

Applied Surface Science, 252(2006) pp 5632-5635 2. Taran A., Plankovskyy S., Voronovich D. and Abashin

S., IEEE International Vacuum Electronics Conference, 2009. IVEC '09. pp 407 - 408

3. W.L. Ohlinger and C.V. French “A Study of Rhenium as an alternate substrate material for impregnated dispenser cathodes” Air Force Wright Aeronautical Laboratories, US Report No. AFWAL-TR-84-4141

4. R.S.Raju, IEEE International Vacuum Electronics Conference, 2008. IVEC 2008, pp 40 - 41

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