Investigation and application of impregnated scandate cathodes

Ji Lia,*, Suqiu Yana, Wensheng Shaoa, Qilue Chena, Min Zhub

aCathode Electronics Laboratory, Beijing Vacuum Electronics Research Institute, P.O. Box 749, Beijing 100016, ChinabNational High Power Key Laboratory, Beijing Vacuum Electronics Research Institute, P.O. Box 749, Beijing 100016, China

Received 23 September 2002; received in revised form 10 December 2002

Abstract

This paper describes the investigation and application of scandate cathodes in Beijing Vacuum Electronics Research Institute

(BVERI). The data of emission performance and lifetime were acquired both in closed-spaced diodes and in microwave tubes.

Evaporation rates and gas poisoning properties have been determined. From the results of a series of diode experiments and the

feedback information of practical usage in tubes, there are three key factors that in our opinion limit the practical application of

scandate cathodes in general. These are no obvious saturation onset of I/U characteristics, weak ion bombardment resistivity and

non-uniform emission. The authors conclude that to a certain extent, impregnated scandate cathodes (I-Sc) can be used as

alternative electron sources instead of M-type cathode in microwave tubes.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Scandate cathodes; Impregnated cathodes; Emission characteristics; Lifetime

1. Introduction

Since scandate cathodes were originally described

in 1967 [1], there have been a large number of

publications related to emission mechanism and cath-

ode capability. Generally, scandate cathodes fall into

one of three categories, mixed pressed, impregnated

and ‘‘top-layer’’. It has been reported that maximum

emission current density of up to 460 A/cm2 (at

1030 8CW–B) are achievable in a UHV set-up with

‘‘top-layer’’ scandate cathodes prepared by laser

ablation deposition [2].

The investigation of scandate cathodes in Beijing

Vacuum Electronics Research Institute (BVERI) began

in 1981 and has included studies of all three classes of

cathode described above. Considering technology, cost

and performance, impregnated scandate cathodes (I-Sc)

were chosen for use in linear beam devices.

Our investigation of impregnated scandate cathodes

encompassed the following topics: tungsten substrate,

impregnation materials, emission properties, lifetime,

barium evaporation rate, gas poisoning, bulk and

surface analysis and fault models. The I-Sc cathode

has, thus far, been widely used in electron beam

devices, such as grid-controlled TWT (Travelling

Wave Tube), EMC TWT, mm-wave TWTand klystron

in BVERI.

2. Cathode technology

2.1. Preparation of tungsten matrix

Tungsten powder (with a grain size of around 5 mm)

was isostatically pressed and then sintered under

Applied Surface Science 215 (2003) 49–53

* Corresponding author. Tel.: þ86-10-64361731x2440;

fax: þ86-10-64362878.

0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00273-3

hydrogen to yield rough bars. After filling with copper

(or methyl methacrylate) the bars were machined to

give pellets of the desired dimensions. The filler was

then removed by chemical erosion, deoxidized in

hydrogen and evaporated under vacuum. After sinter-

ing under hydrogen at 2000 8C, the porosity of the

tungsten body is in the range of 24–28%.

2.2. Active impregnate materials

The active impregnate comprises the following com-

ponents, BaO, CaO, Al2O3 and Sc2O3 (2–6 wt.%).

Whilst a total of 22 variants were initially tested, this

was eventually narrowed down to three with the follow-

ing molar ratio compositions, 3:0.5:1; 4:0.5:1;

4:1:0:5 þ 3wt.% Sc2O3 that were used in different

kinds of devices.

The active materials were prepared by co-precipita-

tion. The advantage is that scandium can be dispersed

more homogeneously by co-precipitation than by

mechanical mix. The co-precipitation reaction process

[3] refers to the following:

BaðNO3Þ2 þ CaðNO3Þ2 þ AlðNO3Þ3 þ ScðNO3Þ3

þ ðNH4Þ2CO3 ! BaCO3 þ CaCO3 þ AlðOHÞ3

þ Sc2ðCO3Þ þ BaCaSc2ðCO3Þ5 þ BaSc2ðCO3Þ4

þ CaSc2ðCO3Þ4

The final precipitation product is very fine white

powder after baking.

While impregnating scandate materials into the

porous tungsten body under hydrogen, the reaction

equation [3] is

BaCO3 þ CaCO3 þ AlðOHÞ3 þ Sc2ðCO3Þþ BaCaSc2ðCO3Þ5 þ BaSc2ðCO3Þ4

þ CaSc2ðCO3Þ4 ! Ba2CaAl2O6 þ Ba2CaSc4O9

þ Ba3Al2O6 þ Ba3Sc4O9 þ CO2 "

3. Emission characteristics and lifetime

3.1. Closed-spaced diode test

The saturated emission current was defined as the

point at which a log(I)–log(V) curve begins to deviate

from linearity (the so-called ‘knee curve’). In order to

compare results, all cathodes were fabricated from the

same size pellets (3.0 mm diameter and 0.8 mm thick-

ness) impregnated with the compositions outlined

above and then surface cleaned and pre-treated on

exhaust. The anode–cathode distance was fixed at

about 1.0 mm. Given that the anode was water-cooled,

this meant that both CW and pulsed measurements

were possible. Nonetheless, the anode effect is still

present when very high currents as being delivered in

the CW mode test Fig. 1.

The diode emission current densities extracted from

I-Sc cathodes were above 13 A/cm2 at 1000 8CB

(practical temperature at 900 8CB because of electron

cooling effect) in the CW mode and above 55 A/cm2 at

900 8CB in pulsed mode (t ¼ 20 ms, f ¼ 1000 Hz),

respectively. The life tests were stopped after 20,000

and 8000 h while delivering 5 and 10 A/cm2 current

density in CW mode at practical temperatures of 950

Fig. 1. Water-cooled diode for cathode life test.

Fig. 2. Electron gun for cathode life test.

50 J. Li et al. / Applied Surface Science 215 (2003) 49–53

and 910 8CB, respectively. The degradations of current

were less than 3% [4].

3.2. Practical electron gun test

A cathode control electron gun (see Fig. 2) was used

to examine cathode emission capability and lifetime in

real tube operating environment. The cathode with

5.6 mm in diameter and 8.0 mm in spherical radius

was mounted into the electron gun.

The lifetimes were above 1000 and 2000 h at

current densities of 4.2 A/cm2 at 980 8CB [5] in CW

mode and 11 A/cm2 at 1000 8CB in pulsed mode

(t ¼ 10 ms, f ¼ 2000 Hz), respectively. The life tests

were suspended in 1996.

4. Evaporation rate

The barium evaporation rate was measured by the

Becker method. Fig. 3 shows the configuration of the

standard Becker diode in BVERI.

As shown in Fig. 4, for engineering applications the

evaporation rate [6] of I-Sc cathode is at the same level

as M-type cathode.

5. Gas poisoning studies

All I-Sc cathodes used for poisoning experi-

ments have the ability of delivering more than

700 mA (�10 A/cm2) current (knee point of log(I)

versus log(U) plot) at 1000 8CB (no compensation)

under CW mode. Before admission of the gas, the

current was set at 350 mA (5 A/cm2) at a tempera-

ture of 950 8CB. The emission degradation and

Fig. 3. Configuration of Becker diode.

Fig. 4. Evaporation rate of practical cathodes.

J. Li et al. / Applied Surface Science 215 (2003) 49–53 51

recovery was then determined for the various sam-

ples.

Fig. 5 shows emission poisoning thresholds (90% of

initial emission) for several gases [7].

6. Application in practical tubes

Since 1986, I-Sc cathodes have been used in TWT

applications in BVERI. Following improvements in

technology and engineering considerations, I-Sc cath-

odes have also found application in linear beam

devices including grid controlled TWT, broad-band

TWT, mm-wave TWT and klystron.

Generally, the lifetime is in the range of 1000–

10,000 h. It is necessary to point out that lifetime largely

depends on the operating condition and environment.

The I-Sc cathode was also successfully used in a

multi-beam klystron (MBK), which requires a current

density above 28 A/cm2 and lifetime not less than 500 h

under a duty cycle of 33.3%. An example is shown in

Fig. 6. Compared with M-type cathode, the operating

temperature of I-Sc cathode decreased from 1150 to

1050 8CB. It can be expected that longer life will be

achieved by I-Sc using cathode instead of M-cathode.

Presently, we are preparing to equip another MBK

with an I-Sc cathode, which can satisfy the require-

ments of 40 A/cm2 current density and not less than

1000 h lifetime.

Although I-Sc cathode has been used in many

kinds of tubes, there are still some issues to be solved.

The main problem encountered in practice is the

emission degradation after breakdown occurring

inside tubes.

In order to make I-Sc cathodes operate normally,

some measures have to be taken, such as electrode

polishing, carefully degassing parts inside tube, use of

a gas-selective getter, etc. The goal is to lower the

probability of breakdown happening inside tubes.

7. Discussion

Whether I-Sc cathode can sustain ion bombardment

depends on bombardment energy, ion beam current

and Sc replenishment ability from tungsten body. It is

related to the I-Sc cathode operating mechanism.

For M-type cathode, the coating of Os/or Ir/or

Os/Ru (3000–6000 A) has two main functions:

(i) To lower the work function of emission surface

and make it acquire more uniform barium and

oxygen distribution.

(ii) To withstand the ion bombardment unless the

coating is destroyed by ion beam of very high

energy.

In the case of I-Sc cathodes, scandium migration

to the cathode surface (assuming the Ba/Sc/O/W

Fig. 5. Poisoning thresholds of several gases.

52 J. Li et al. / Applied Surface Science 215 (2003) 49–53

monolayer mechanism) serves a similar function to

the sputtered coating in M-type cathodes. It may be

visualized as if to the coating of M-type cathode a very

‘‘thin’’ monolayer level of scandium is added. This

should prove beneficial provided:

(i) The scandium can be uniformly dispersed

throughout the tungsten body and/or the cathode

surface.

(ii) The tungsten body and/or surface scandium layer

(or tungsten/scandium alloy) has sufficient re-

sistance to ion bombardment.

Since it is difficult for scandium to migrate from

body matrix to the emission surface, the next work will

be mainly concerned with improvement of the physi-

cal performance of base matrix containing uniformly

dispersed scandium.

Recently, a series of experiments have been done to

check emission recovery of I-Sc cathode after ion

bombardment. The content of scandium in impregnant

is varying from 0.5 to 10 wt.%. All tested cathodes

were ion sputtered and 8–12 mm were removed on the

surface. Pulsed emission measurements were made

before and after ion sputtering. For cathodes with low

levels of scandium, the emission was similar to that

obtained for common tungsten cathodes, even after

reactivation and aging. For cathodes with higher levels

of scandium (above 8%), although the emission did

not fully recover to its original pre-ion sputtering

level, the emission was, nevertheless better than that

obtained with Os-coated cathodes run under the same

conditions and at the same temperature.

8. Summary

After more than 20 years studying and using I-Sc

cathode, we can draw the following conclusions:

(1) To a certain extent, I-Sc cathode can be used as

alternative to the M-type cathode.

(2) For tubes where high current densities are needed

but lifetime is not a critical factor, I-Sc cathodes

can be the optimum choice.

(3) Long life in tubes can be expected provided some

measures are taken such as electrode polishing,

fully degassing of parts inside tube, use of gas-

selective getter, etc. The goal is to lower the

probability of a breakdown.

(4) Emission uniformity and degradation are still

issues, so future work will pay more attention to

tungsten matrix containing scandium, in which

scandium is dispersed uniformly and hence this

kind of matrix (or alloy containing scandium)

will be more resistive to ion bombardment.

References

[1] A.I. Figner, et al., US Patent 3,358,178 (December 1967).

[2] G. Gaertner, P. Geittner, H. Lydtin, A. Ritz, Appl. Surf. Sci.

111 (1997) 11.

[3] Technical Report AG269/11-16-11023-3e, 1986.

[4] Technical Report AG269/11-16-11023-3a, 1986.

[5] Technical Report AG15.482/11–22(11037)-3b, 1996.

[6] Technical Report AG15.482/11–22(11037)-3a, 1996.

[7] Technical Report AG269/11-16-11023-3c, 1986.

Fig. 6. (a) 19-beam cathode. (b) 18-beam cathode.

J. Li et al. / Applied Surface Science 215 (2003) 49–53 53