Applied Surface Science 40 (1989) 97-101

North-Holland 97

EMISSION POISONING STUDIES ON IMPREGNATED TUNGSTEN DISPENSER CATHODE UNDER CO, AND 0, ENVIRONMENT *

A.K. SHARMA, A.K. CHOPRA and Roy MATHEW

Microwave Tubes Area, Central Electronics Engineering Research Institute, Pilani, Rajasthan, India

Received 11 January 1989; accepted for publication 7 June 1989

An impregnated cathode of 18% porosity has been fabricated using barium-calcium-aluminates of molar ratio 5 : 3 : 2. Effects of

carbon dioxide and oxygen on emission of this cathode have been investigated experimentally. After poisoning, recovery of emission

and activation time have also been studied. The results are discussed and compared with the work of other investigators.

1. Introduction

Modem high emission density dispenser cath- odes consist of porous tungsten plugs impregnated with a mixture of barium oxide, calcium oxide and aluminium oxide. Experimental studies [1,4], e.g. Auger electron spectroscopy (AES), X-ray photo- electron spectroscopy (XPS), surface extended X- ray absorption fine structure (EXAFS), etc. have determined that at the operating temperature of these cathodes, i.e. 950-1100 o C,, approximately a stoichiometric monolayer of BaO covers the tungsten surface and lowers the work function of tungsten - 2 eV, causing high electron emission densities. Several studies [4-71 have revealed that the work function of this active layer has been modified by adsorption of certain gases, particu- larly if the gas is electronegative. In this case the work function will increase and results in a de- crease of electron emission usually termed as emission poisoning. Oxidizing gases such as CO,, 0, and water vapour have been identified which poison the cathode emission when present beyond a certain level and also cause reactivation delays

PA. Studies of the influence of ambient gases on

cathode emission have then become quite signifi-

* Paper presented in a National Seminar on “Electromagnet-

its and their Applications” at Banaras Hindu University,

Varanasi, India in December 1988.

0169-4332/89/$03.50 0 Elsevier Science Publishers B.V.

(North-Holland)

cant. Therefore we report here the behaviour of CEERI developed type-B dispenser cathode emis- sion under the influence of CO, and 0, gases at the operating temperature, i.e. 1050 o C n.

2. Experimental procedures

Emission studies of the cathode have been car- ried out in a closed space test diode. Our cathode is an impregnated tungsten pellet of 18% porosity and of 2.8 mm diameter. The impregnation of barium oxide, calcium oxide and aluminum oxide in 5 : 3 : 2 ratios respectively was carried out in a hydrogen atmosphere at 17OO’C. The cathode temperature was measured during emission studies by optical pyrometer through a fine hole in the anode block. The brightness temperature ( o C,) has been mentioned in the text. At higher anode voltages anode dissipation became excessive, caus- ing evolution of gases and thermal radiation back to the cathode. However, for anode degassing an insulated heater was also coiled around it.

The test diode was mounted on a sputter ion pump station having a pump speed of 30 e/s. The schematic experimental set-up for emission poi- soning studies is shown in fig. 1. The vacuum system can be baked upto 450°C. Residual gases have been analysed through a Supavac Residual Gas Analyser of VG. Research purity gases from Matheson have been admitted through a calibrated

A.K. Sharma et al. / Emission porsomng studies on impregnated W dispenser cathodes

Fort?

RGA

Puma

rll m clJHV Leak Valve.

UI \ ’ u

ES3 @I

1 1

Fig. 1. Schematic experimental set-up.

fine leak valve which can control a leak of 1 X

10p’o Torr L/s. Gas lines were flushed and evacuated through a moisture and oil trapped rotary pump. Before emission poisoning studies, the system was fully baked and degassed. It at- tained an ultimate vacuum on the order of lo-’ Torr. The cathode was then activated fully. A representative RGA spectrum of the ambient gases

just before the poisoning run, is shown in fig. 2. Partial pressures of CO, and 0, were on the order of lo-lo mbar before emission poisoning studies of the cathode. These studies were carried out when the cathode emission was in partially space-

1%

Atomic Mass llnits --a

Fig. 2. RGA spectrum of ambient gases before emission poi-

soning run.

charge limited condition. Emission measurements were then recorded by an X-Y recorder.

3. Results and discussion

Figs. 3 and 4 depict the behaviour of cathode emission with the increase of partial pressures of carbon dioxide and oxygen respectively. Rapid decrease of cathode emission with the increase of partial pressures of CO, and 0, has been found from 4.5 X 10P6 and 5.0 x 10e5 Torr respectively. Such partial pressures have been termed as critical partial pressures. These results have been com- pared in table 1 with the works of other investiga-

06

05

OLI 16’ 166 105

IvL(rlldl I~rt.,“xl., T”,. +

Fig. 3. Emission versus partial pressure of carbon dioxide at

1050°c,.

A.K. Sharma et al. / Emission poisoning studies on impregnated W dispenser cathodes 99

Table 1

Comparison of critical partial pressures of carbon dioxide and oxygen between present work and of other investigators

Gases Critical partial pressure (Torr)

Present work; Ref. [5];

18% porosity 20% porosity

Ref. [5];

45% porosity

Ref. [9];

commercial cathode

(332 4.5 x 10-6 - 10-6 - 10-7 - 10-7

02 5.0 x 10-s - 10-7 - 10-s - 10-7

1.0 ’

-7

Fig. 4. Emission versus partial pressure of oxygen at 1050 o C a.

tors [5,9]. Figs. 5 and 6 show the behaviour of cathode emission with time at a particular partial pressure where emission starts degrading. De-

crease in cathode emission under the influence of CO, and 0, with time has been termed as emis- sion poisoning and the emission recovery to the improvement of emission after closing the supply of CO, and 0,. In both the cases more than 90% of emission has been recovered within 5 min.

It has been known for some time [l,lO,ll] that the work function of tungsten, coated with an approximate monolayer of an alkali metal, causes a decrease in the work function of tungsten by dipole effect. The work function of the surface is further modified due to adsorption of gases onto it, particularly if the gases are electronegative. In this case the work function will increase and result in a decrease in emission. Shih and Haas [6] have attributed emission poisoning of BaO layers by CO, and 0, to first filling the thermally generated oxygen vacancy donor centres thus changing the Fermi level and when it is completed then by

I POISONING -

\ I

- RECOVERY

s- V I

0 2 Ii 6 6 10

Fig. 5. Emission poisoning and recovery versus time in presence of carbon dioxide at constant partial pressure of 5 X 10e6 Torr at 1050°ca.

100 A.K. Sharma et al. / Emission poisoning studies on impregnated W dispenser cathodes

Fig. 6. Emission poisoning and recovery versus time in presence of oxygen at constant partial pressure of 5.6 x 10eh Torr at 1050°c,.

increasing the electron affinity due to dipole changes brought about by accumulation of these gases on the surface. Thermal reactivation is possi- ble with a return to the original electron affinity.

The cathode in the present study has been exposed with 1440 and 1880 langmuir (L) of CO, and 0,. It has been found [12] that 20 L of CO, and 0.7 L of 0, exposure is sufficient to react a monolayer of barium. Thus in our case sufficient quantity of gas is available to react with an active BaO/Ba layer and to result in emission poisoning mainly by forming a dipole layer on the surface of the cathode.

In the present studies critical partial pressures (figs. 3 and 4) of poisoning are higher than in other investigations (table 1). Experimental pieces of evidence [5,11] have suggested that the emission of an impregnated cathode is contributed by the electron emissions from the open pores and the

activated tungsten layer. If the surface porosity is less than the emission will be dominated by the activated tungsten layer. Hence, emission poison- ing characteristics of our cathode of lower poros- ity (18%) could be attributed to the poisoning characteristics of an activated tungsten layer.

Comparison of figs. 5 and 6 reveals that at constant partial pressure and temperature, poison- ing of emission is faster with time under the influence of CO, atmosphere than 0,. This may

be understood if we consider the gettering proper- ties of Ba [13] and the chemical kinetics of Ba with CO, and 0, [14]. A metallic film of Ba has been observed on the walls of the test diode owing to its evaporation/sublimation during the cathode acti- vation, the anode degassing and at the cathode operating conditions. It is well known that at a particular temperature, e.g. at 20’ C, Ba absorbs a higher quantity of 0, (50 &/mg) than CO, (0.60 &/mg), thus at the cathode partial pressures of CO, and 0, at the RGA. Also, reaction constants for Ba and CO, are higher than Ba and 0,. Hence emission poisoning has been observed faster with time under the influence of CO, environment.

4. Conclusions

(1) The influence of carbon dioxide and oxygen on cathode emission has been studied. Higher critical partial pressures, e.g. 4.5 X lop6 Torr (CO,) and 5.0 X 10K5 Torr (0,) have been ob- served and attributed owing to the low cathode porosity.

(2) Rapid emission poisoning has been found with time under the influence of CO, compared to 0, at constant partial pressure and temperature. This has been accounted for owing to the gettering

A.K. Sharma et al. / Emission poisoning studies on impregnated W dispenser cathodes 101

effect in the test diode and higher reaction con- stant of Ba and CO, than Ba and 0,.

Acknowledgements

The authors acknowledge the financial support for the above work given by the National Radar Council, Department of Electronics, Government of India. We also express our gratitude to Dr. S.S.S. Agarwala and Dr. G.N. Acharya for their encouragement.

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