Structural traits of destruction of tungsten base cathode materials

Abstract

STRUCTURAL TRAITS OF DESTRUCTION OF TUNGSTEN BASE CATHODE MATERIALS P. Ya. Radchenko, V. V. Panichkina, A. V. Krainikov, and A. V. Perepelkin UDC 621.762.5001 Beams of heavy ions are widely used in basic physical research as well as in the solu- tion of a number of technical and applied problems [i]. This became possible thanks to ac- celeration techniques and the use of sources of multiply charged ions (MCI). One of the most efficient sources is the arc source with heated cathode [2]. The rate of destruction of the cathode on the side of the discharge in ion bombardment determines the length of continuous operation of such a source. A change of the geometry of the discharge and also atoms of the deteriorating cathode affect the intensity of the flux of MCI. The operating conditions of the source of MCI (voltage 500-1000 V, current 5-20 A) dic- tate the basic demands on the material of the cathode: high melting point, low coefficient of sputtering, good thermal conductivity, and large work function [2]. These requirements are most fully met by tungsten, and cathodes are therefore usually made by machining forged tungsten rods. The complex shape of the cathode and the brittleness of tungsten at medium and low temperatures make such machining difficult. Sintering would greatly simplify the production technology of tungsten cathodes [3]. Unless sintering materials are subjected to forming, they always contain residual pores. We therefore studied the effect of residual porosity on the serviceability of sintered cath- odes in comparison with forged ones. The cathodes were made from tungsten powder with particle size 0.5-i Dm. To enhance the density of the products and to reduce the sintering temperature, an activator (nickel) was added to the powder. After pressing in molds and one sintering of the compacts in hy- drogen at different temperatures the cathodes had density of 16.51 to 18.68 g/cm 3, i.e., they contained between 2.7 and 14.1% pores (Fig. i). They were tested on a stand of ion sources and in the cyclotron U-300 of the Laboratory of Nuclear Reactions of the Joint In- stitute of Nuclear Research, and also on the stand and in the cyclotron U-240 of the Insti- tute of Nuclear Research of the Academy of Sciences of the UkrSSR [3]. The parameters of discharge of the ion source were determined from the maximal detachment of MCI of argon (At +6, Ar+7). The cathode sputtering was estimated by the loss of weight of the cathodes during the testing time. The sources operated with 100 Hz frequency and arc current pulse width of approximately 1.7"10 -s sec. The current of sextuply and heptuply charged argon ions depends on the density of the sintered cathodes: it is the higher, the denser the material is (Table i). However, when porosity is lower than 5%, the characteristics of sintered cathodes become comparable with the characteristics of forged cathodes because the tungsten rods also have some porosity. Analogously the weight loss of the cathode also depends on the porosity. Sintered tungsten with porosity of less than 5% is therefore suitable as cathode material for sources of MCI. And since the technology of powder metallurgy provides parts with the required configura- tion with dimensional fluctuations of 0.1 mm, sintered cathodes do not require additional machining. During the tests it was found that the durability of cathodes also depends on their microstructure, and not only on their density. In some cases cathodes with equal density had different durability. Cathodes with coarse-grained structure had the smallest weight loss. The photomicrographs of the working surfaces of cathodes show distinctly (Fig. 2) that destruction occurs along the weakest link of the structure, viz., the grain boundary (GB). In coarse-grained material, where the grain size is comparable with or larger than institute of Materials Science, Academy of Sciences of the Ukraine. Translated from Poroshkovaya Metallurgiya, No. ii (347), pp. 59-65, November, 1991. Original article sub- mitted November i0, 1989. 952 0038-5735/91/3011-0952512.50 9 1992 Plenum Publishing Corporation