Poisoning of LaB6 cathodes

Poisoning of LaB6 cathodes received in tinal form 21 February 1977

A A Avdienko and M D Malev, Institute of Nuclear Physics, Novosibirsk 90, USSR

This paper describes an investigation of the thermionic emission decay due to chemical interaction between lanthanum hexaboride and the residual gases in a vacuum system. The results of the thermochemical analysis are in good agreement with the experimental data on poisoning. It is shown that oxygen poisoning is possible only at a low cathode temperature ( ~ 1300°C) and relatively high gas pressure, but the LaB6 cathode is much more sensitive to carbon poisoning. The carbon poisoning takes place at cathode temperatures up to 1500°C and at low gas pressures ( 10 -7 torr) if the residual atmosphere contains heavy hydrocarbons or vapours of the organic solvents. It is found that carbon poisoning may be offset by increase of the partial pressure of carbon dioxide.

! . Introduction

Lanthanum hexaboride combines a high chemical stability and a low rate of evaporation with a relatively small work function, and LaB6 cathodes are used in demountable vacuum systems at pressures up to 10 -5 tort. The LaB6 cathodes are atmospherically stable and they are used in the electron guns of high voltage accelerator tubes, electron-beam furnaces, etc. There are two points of view about the nature of LaB6 emission. Many investigators, following Lafferty I, believe-that lanthanum hexaboride is a thin-film emitter with a monoatomic electropositive layer of free La on the cathode surface. In such a model, the work function depends upon the stoichiometric excess of lanthanum, i.e. upon the equilibrium between the diffusion of La atoms, evaporation of these atoms and their chemical bonding with the active gases. Then rise of the partial pressure of gases containing oxygen leads at a certain cathode temperature to decrease of the free La concentration and the emission current falls 2. Sometimes the film cathode poisoning is explained also by the adsorption of electronegative gases (O.,,CO2, etc.) on the cathode surface. The adsorption mechanism of the LaB6 poisoning is analysed in detail by Gallacher 3. Unfortunately, it is difficult to estimate quantitatively the poisoning action of various gases since the values of the dipole moments and other adsorption parameters are unknown3. The alternative hypothesis considers the LaB6 cathode as a semiconductor with acceptor levels '*'5. The work function of the extrinsic semiconductor is determined by the impurity concentration and by the level structure. In the well-activated cathode the work function is the least and any disturbance of the cristalline lattice leads to a decrease of the emission. To all appearance, this phenomenon takes place during the first stages of the cathode-residual gases interaction, when the solid reaction products are impurities in the near-surface region of the cathode. Eventually, the reaction products accumulate on the surface and the emission current falls to zero. Thus the poisoning mechanism is a result of the chemical interaction between LaB6 and the residual gases in a vacuum Vacuum/volume 27/numbers 10/11.

system. The poisoning action of various gases in such a model has to depend not upon their adsorption characteristics, but on the thermodynamical equilibrium of the corresponding chemical reactions. LaB6 poisoning by residual gases was investigated experimentally by Buckingham 2 and Gallacher a. Both authors showed that nitrogen- and oxygen-containing gases reduce the emission current. The poisoning depth is a function of both pressure and temperature, so that at a certain cathode temperature there is a critical pressure below which no poisoning occurs. Experimental poisoning characteristics are usually given by the current ratio I/Io =f(P,T)where I is the current emitted when the cathode is poisoned and Io is the unpoisoned emission (see, for example, Figure 1). For a particular temperature and for Pc < P < Po, this dependence may be described by the empirical formula: In P/Po

I/I0

=

In

PJ-~-~o'

(I)

where Po is the pressure at which a current falls to zero and Pc is the critical pressure at which poisoning commences, i.e. emission current starts to fall. The equation (l) gives the possibility of obtaining the value of the poisoning pressure Po from experimental data by Gallacher 3 (see Table 1). The ratio Po/Pc is nearly 50-100. Poisoning data of various authors are in good agreement except in the case of hydrogen. Gallacher 3 established that at a temperature of 1200-1400°C the emission characteristics are not changed up to hydrogen pressures of about 10 -2 torr. However, Buckingham 2 showed that the emission current decreased by a factor of 2-3, when hydrogen at a pressure of 10 -6 torr was introduced into the uhv-system (To ----"1235°C). A t the same time, the components with molecular masses M = 2 8 - 7 0 appeared in the residual atmosphere. The author explains this fact by the formation o f gaseous boranes (mainly B2H6): the losses of boron atoms produces the trapping sites for lanthanum near the cathode surface, and excess of free lanthanum diminishes 2. It is seen from Table I and Figure 1 that in the temperature

Pergamon Press/Printed in Great Britain

583

A A Avdienko and M D Malev: P o i s o n i n g o f LaB s c a t h o d e s

_

°~°ko

° °°%,

ok

o

d c

O

O

O

1400% 0

I IO-"

I

I0-6

I0"~

I0 -a

IO"3

lO 2

3

Pco2

Figure 1. Carbon dioxide poisoning of a LaB6 cathode. O--Experimental data3; straight lines--calculation from the equation (I),

Table 1. Poisoning by oxygen.containing gases and nitrogen ~ (/'0, torr) Tc (°C)

1100

1200

1300

1400

1500

02

--

--

CO2 Air Nz

--l x l 0 -4

5 x 1 0 -5 5 x 1 0 -s 4 x 1 0 -3

6×10 -5 l x l 0 -3 1 x l 0 -3 --

4 x 1 0 -4 1x10 -2 l x l 0 -2 --

3 x l 0 -3 ----

H2

--

> 1:0 - 2

__

__

__

A.r

--

>10 -2

--

--

--

range 1400-1500°C, if the residual atmosphere consists of nitrogen-, hydrogen- and oxygen-containing gases at pressure below 10-* tort, the poisoning effect is neglected. T h e case is quite different when the residual atmosphere contains the heavy hydrocarbons (molecular mass M > 40). W e observed unusual behaviour of LaB6 cathode under testing the high-voltage accelerator tube ELV-11°. The electron gun was switched on at a pressure < ( 1 - 2 ) x 10 -6 torr only; nevertheless, considerable poisoning occurred at a cathode temperature of 1450°C. The emission current was not restored even after over-heating up to 1600°C. The poisoning effect became w e a k after baking of the ceramic and metal components of the accelerator tube, and it was practically eliminated after replacement of the rubber seals by Viton ones. The main effect of baking consisted in moving away the traces of the organic solvents (ethyl alcohol, acetone, trichlorethylene) which were used for the cleaning. The present w o r k was carried out to study systematically this p h e n o m e n o n which we called the ' c a r b o n ' poisoning of the LaB~ cathode.

2.

Experimental

procedure

and

results

The experiments were undertaken with the electron gun f r o m the accelerator tube ELV-1. T h e electron gun was m o u n t e d in an all-metal vacuum system with a 250 1 s - ~ getter-ion p u m p (Figure 2). The l a n t h a n u m hexaboride fiat tablet (10-ram diameter) was fixed tightly into the tantalum cup that was heated by a tungsten filament. The electron b e a m was formed by quasi-Pierce optics. The emission current was equal to 0.5-1 A at a working t e m p e r a t u r e nearly 1400-1450°C and the filament power was

4

~

[

1

>< I q Figure 2. Schematic diagram of electron gun and vacuum system. I. Electron gun: (a) LaBo tablet, (b) tantalum cup, (c) tungsten filament, (d) heat screen, (e) pierce electrode, (f) electron collector; 2,4. ion gauges; 3. mass-spectrometer; 5. 30 ls -~ slit; 6. vacuum valve; 7. 250 I s - l getter-ion pump; 8. leak valve. about 60 W. The perveance of the electron gun is equal to 2.4 × 10 . 4 m A V -a/2 and the anode current was not greater than 40-50 m A during all measurements ( U , = 1-3 kV). Such working conditions permitted the anode current to be measured continuously without any effect on the residual atmosphere by gases evolving from the collector under electron bombardment. The vacuum system was made from stainless steel with copper seals. The electron gun was separated from the pumping system by a slit of conductance 30 1 s-1 (for nitrogen). The total pressure was measured by two ionization gauges, one on each side of the slit. The pressures as measured by the two gauges differed by a factor of 1-1.5 depending on the gaseous flow. The pressure near the electron gun was used in all our experimental data. Because of lack of accurate calibration data for heavy hydrocarbons, all pressure data are cited as nitrogen equivalents, although the real pressures for heavy gases may be less by a factor of 5 to 6 ~. An omegatron was m o u n t e d near the cathode; its resolution was not less than 20--40 in the mass-range from 2 to 180, The poisoning gas or vapour was admitted to the test cathode through a metal leak valve. A constant leak rate was balanced against the pumping to maintain a constant pressure near the electron gun. All c o m p o n e n t s of the vacuum system (and of the electron gun) were cleaned in the supersonic baths with trichlorethylene, water a n d ethyl alcohol and then they were dried by baking at 300°C. The vacuum system was not baked out in situ. Before any gas o r v a p o u r was introduced into the v a c u u m system, the pressure near t h e cathode was less than l0 - 6 torr. The main components of the residual atmosphere were hydrogen, carbon monoxide, nitrogen and water; a fraction o f the heavy gases with m a s s numbers M exceeding 30 was less

A A Avdienko and M D Malev: P o i s o n i n g o f L a B s c a t h o d e s

than 5-10%~ (see curvc I in Figure 3). W h e n air, H_,,N,,H_,O a n d the v a p o u r of the organic solvents (ethyl alcohol, acetone, toluene, triclflorethylene) were introduced, the mass spectrum was cbanged but it coincided with s t a n d a r d one T. A piece of v a c u u m r u b b e r (with a surface area of a b o u t 8 cm-') was used as an origin of heavy h y d r o c a r b o n s . As it is seen from curve 3 on Figure 3, in this case the c o m p o s i t i o n of the light gases was the same as in the pure v a c u u m system, but the total pressure was due a l m o s t entirely to heavy h y d r o c a r b o n s of mass n u m b e r s M 6-160. Since the resolutior of an o m e g a t r o n is insufficient to

4a) or after decreasing the pressure to I • 10 -5 t o r t (Figure 4b). The fall of emission is much stronger in the a t m o s p h e r e of heavy h y d r o c a r b o n s or the v a p o u r of organic solvents. It is seen from Figure 5 that, at the working temperature, the LaB6 c a t h o d e is poisoned for a pressure of tricblorethylene v a p o u r of a b o u t 10- 5 torr. The c u r r e n t - v o l t a g e d i a g r a m s of the electron gun are s h o w n in Figure 6. Curves I in the all d i a g r a m s of

(o)

k/-

T

I~

J2Z

I

-?

IO

5

15

(b)

05

L I\

3

20

40

60

80

~00

i20

140

160

I0

i80

15

M

F i g u r e 3. M a s s - s p e c t r u m o f the residual gases in the v a c u u m w i t h LaB~, c a t h o d e (T~ = 1 4 0 0 ' C ) . 1. P u r e system P = 2 t o r r ; 2. V i t o n P 8 - 10 - 6 t o r r , q - 1.5 - 10 - 5 t o r r I c m 3. v a c u u m r u b b e r P 5 . 10 - s t o r r , q = 1.5 • 10 -'~ t o r r

system • 10 -~' -z s-t; I cm -z

S-I "

separate the individual peaks it is possible only to give i n f o r m a tion a b o u t the m a i n c o n t e n t s of the heavy gases, not a b o u t their c o m p o s i t i o n (see Figure 3). F o r the same reason one c a n n o t see any difference between the mass spectra of the various kinds of r u b b e r including silicone rubber. But a piece o f V i t o n with the same surface gives a gaseous flow less by a factor of 10, a n d the ratio of the heavy to light masses is appreciably less than for the v a c u u m r u b b e r (see curve 2 in Figure 3). The experimental results for air, nitrogen a n d h y d r o g e n are close to G a l l a c h e r ' s d a t a : these gases exert no appreciable influence o n the emission c u r r e n t if the pressure is less t h a n l 0 - 4 torr a n d the c a t h o d e t e m p e r a t u r e exceeds 1400°C. As is seen from T a b l e 2 a n d Figure 4, the c a t h o d e sensitivity to water p o i s o n i n g is roughly the same as for o t h e r oxygenc o n t a i n i n g gases. The emission current falls at 1400°C at a w a t e r - v a p o u r pressure of 1 x l0 -'~ t o r r a n d it recovers immediately after increasing the t e m p e r a t u r e to 1500°C (Figure

Table 2. Poisoning by water and carbon compounds (Po tow)

Water Acetone Ethyl alcohol Rubber Viton

M (g mole- ') 1300

Tc (°C) 1400

1450

1600

18 58

--

5 × 10 -4

--

--

--

--

1 × 10 - s

2 × 10- 5

46 60-160 80-120

5 × 10-5 ---

__ 2 × 10 -~ --

__ -> 10- s

__ -_

Figure

4. LaB~,

poisoning

by

20 I, rnin

25

30

35

water.

(a)

I

II

Ill

IV

P,,zO(torr): Tc (~C) : (b) PH.,O (torr) : Tc CC) :

I . 10 -5 1400 I 5 - 1 0 -6 1420

5 . 10 -5 1400 11 2 x 1 0 -'~ 1420

l x l 0 -'~ 1400 III I x l 0 -5 1420

1;<10 -'t 1500

u

HI

I

~

~r

I

I0 l,

chin

Figure 5. LaB6 poisoning by trichlorethylene. I II Ill IV P(torr) I x 10 -6 4 x 10 -5 3 x 10 -5 2 x 10 -5 Tc(°C) 1450 1450 1450 1450

V 2 x 10 -5 1550

Figure 6 are the characteristics o f the u n p o i s o n e d cathode. I n the t e m p e r a t u r e range 1300-1600°C, the influence o f a c e t o n e a n d ethyl alcohol v a p o u r o n a LaB6 c a t h o d e is imperceptible at pressures as low as 1 × 10 - 6 torr (Figures 6a a n d b). But heavy h y d r o c a r b o n s evolving from r u b b e r p o i s o n a c a t h o d e at pressures of 5 x 10 -7 t o r r a n d Tc = 1450°C (Figure 6c). Thus, c a r b o n p o i s o n i n g occurs at a c a t h o d e t e m p e r a t u r e o f 1400-1500°C a n d at a gas pressure o f 1 0 - 7 - 1 0 - 6 torr. O u r experimental values for Po are cited in T a b l e 2. T h e c a t h o d e p o i s o n i n g by heavy h y d r o c a r b o n s differs f r o m p o i s o n i n g by oxygen not only by a high sensitivity, b u t also by some o t h e r peculiarities. T h e critical pressure Pc rises w i t h t e m p e r a t u r e very slowly; after r e m o v i n g the source o f poisoning, the emission c u r r e n t is n o t restored (or it is restored only after 585

A A Avdienko and M D Malev: Poisoning of LaB6 cathodes [a)

(c)

4o I

,< E 3c 2

20

3

~

2

Va,

z

3

Vo,

kV

kV

3

i

2

3

v~, kv

Figure 6. The current-voltage diagrams of a LaB~, electron gun in a hydrocarbon atmosphere. (a) Acetone vapour. I. 1600 C, 1.5 - 10 -6 torr; 2. 1450°C, 1.5× 10 -~' torr; 3. 1600~C, 3,'.~ 10 -5 torr; 1300C, 7>: l0 -v torr. (b)Ethyl alcohol vapour. I. 1450 C, 4 . 10 -s torr; 2. 1300 C, 4 x 10 -5 torr; 3. 1300°C, 2 > 10 -6 torr. (c) Hydrocarbons evolving from rubber. P 5 :,: 10 -'~ tort; q - 5 • 10 -~' torr " cm--' s- '. I. 1500 C; 2. 1450°C; 3. 1400°C.

several dozens o f h o u r s at 1600°C). After d e m o u n t i n g the v a c u u m system, one can see o n the surface o f the poisoned c a t h o d e a distinctive green-grey film. W i t h a c a r b o n dioxide filling the b e h a v i o u r of tile c a r b o n poisoned c a t h o d e was quite unexpectedly on the surface. The emission recovered very quickly on increasing the CO2 partial pressure (Figure 7). T h e value of the c o m p e n s a t i n g pressure d e p e n d s slightly u p o n the a m o u n t a n d c o m p o s i t i o n of the c a r b o n - c o n t a i n i n g gases.

(o)

I 5 t,

I,

~ I0 rain

I 15

mJn

Figure 7. Emission recovery of carbon poisoned LaB6 cathod by COz. (a) Acetone poisoning. I II Ill IV V Tc (°C) 1600 1350 1350 1300 1300 P (torr) 5 x 1 0 -~ 5 × 1 0 -~ 5 × 1 0 -~ 2 × 1 0 -6 2 × 1 0 -6 Pco2 (torr) 1 x l 0 -8 2x10 -s 2 × 1 0 -5 1>-10 -8 2 × 1 0 -s (b) Rubber poisoning, Tc = 1500cC. I lI Ill P (torr) 5x10 -s 5 × 1 0 -5 3 x 1 0 -5 Pcoa (tort) l x l O - s 2 x l O -¢ 6 x l O -5

IV 1 × 1 0 -5 3>:10 -5

V l x l 0 -5 1 × 1 0 -8

The film emitter m o d e l c a n n o t explain all these facts: similarity o f the p o i s o n i n g characteristics u n d e r the influence o f quite diverse h y d r o c a r b o n s , the a p p e a r a n c e o f the c a r b o n p o i s o n e d c a t h o d e , the emission recovery in t o u c h with electronegative CO2. N e i t h e r a d s o r p t i o n n o r a free l a n t h a n u m b i n d i n g t h e o r y c a n explain why h y d r o c a r b o n s p o i s o n LaBs c a t h o d e m o r e strongly t h a n oxygen. 586

It is possible to suppose that LaB6 p o i s o n i n g results from the chemical reactions between l a n t h a n u m h e x a b o r i d e a n d tile residual gases. As a result of such reactions, the solid c o m p o u n d s ( l a n t h a n u m a n d b o r o n carbides, hydrides, oxides, nitrides, etc.) may be formed. If these c o m p o u n d s are stable, tile work function rises a n d the c a t h o d e is being poisoned.

3. Thermochemical analysis

T o check the chemical hypothesis, it is necessary to c o m p u t e the t h e r m o d y n a m i c potentials a n d the equilibrium c o n s t a n t s of the chemical reactions between LaB6 a n d various c o m p o n e n t s o f the residual a t m o s p h e r e , a n d to c o m p a r e the calculated values of the equilibrium pressures of the particular gases in such reactions with the experimental Po d a t a in Tables l a n d 2. A t h e r m o d y n a m i c a l estimation enables us to say if a chemical interaction is possible at the particular pressure a n d temperature, but it says n o t h i n g a b o u t the rate o f the reactions. F o r t u n a t e l y , for the p o i s o n i n g analysis there is no need to k n o w the reaction kinetics. If any interaction leads to the work function increasing a n d it is possible t h e r m o d y n a m i c a l l y u n d e r the same conditions, p o i s o n i n g c o m m e n c e s m u c h earlier t h a n when total equilibrium has set in. In fact, a rise o f the work function by only 1 0 - 2 0 % causes a fall of the emission current by a factor o f 10-100. So if the pressure of a n y gas is m o r e (or less) t h a n the equilibrium pressure in a chemical reaction at the specific c a t h o d e temperature, p o i s o n i n g by this gas is possible (or impossible). But the p o i s o n i n g m e c h a n i s m m a y be identified by certain chemical reactions only if the Po value coincides with the equilibrium pressure over the whole t e m p e r a t u r e range. Such a c o n d i t i o n is necessary because the errors of the equilibrium pressure calculation reach a factor o f 10-100 because of inaccuracy o f the t a b u l a t e d t h e r m o d y n a m i c a l data. T o calculate the LaB6 p o i s o n i n g reactions it is necessary to have first o f all the t h e r m o d y n a m i c a l p a r a m e t e r s of l a n t h a n u m hexaboride. T h e heat of f o r m a t i o n a n d the t h e r m a l capacity o f LaB6 are k n o w n s'8, but e n t r o p y d a t a are absent. One can estimate e n t r o p y by integration of the t e m p e r a t u r e d e p e n d e n c e o f the t h e r m a l capacity, but the D e b y e t e m p e r a t u r e of LaB6 is u n k n o w n also. W e assume that b o r o n a n d LaB6 crystals have the same D e b y e t e m p e r a t u r e , a n d so the s t a n d a r d e n t r o p y value of 10.2 cal m o l e - ~ °C-~ was o b t a i n e d .

A A Avdienko and M D Malev: Poisoning of LaB s cathodes

The nature of the reaction products depends upon the relative amotml of the reagents. Also, lanthanum hexaboride reacts not only with the main poisoning factor, but also with the other components of the residual atmosphere. In such a complex system, a number of the rcagents can change within broad limits; the possible set of reaction products are not simple either. In this situation, we selected the most probable thermodynamical version, i.e. reactions lhat have the minimum equilibrium pressures of the poisoning factor though otherwise under the same conditions. Nevertheless, in some cases a process is not realized because of the influence of parallel reactions. For example, the hexaboride-oxygen interaction begins at an 02 partial pressure below 10-~3 torr (7",. = 1400), but the poisoning oxygen pressure rises to 10-s-10 -'~ torr through the oxides reduction by hydrogen and carbon oxide [the reactions (4a) and (4b), Table 3]. The equilibrium pressures of the poisoning reactions are cited in Table 3. Thermodynamic potentials were computed without correction to the thermal capacity, since this correction is much less than the errors due to the scattering of the tabulated values of the heats of formation and entropies of lantlmnum and boron compounds 8'°. I f several gases take part in the reaction the partial pressures were assumed to be equal to their typical values in a vacuum system (Figure 3):

IO-e

3

•

10 s l0 .z i0 -5

O5

tO "

O6 \

07 IXl0

3

i0~ 10-.~ i0-~, ~0-6

05

06

07 lXlO ~

Figure 8. Equilibrium pressures of LaBc, reactions (straight lines) and experimental Po values (dots). 1. N_,; 2. CO-,; 3. H20; 4. O-,; 5. acetone; 6. ethyl alcohol.

Pn, = Pco = PH,O = 10-6 t o r r ; Pco, = 10-8 tort. T h e d a t a o f T a b l e 3 p o i n t to lack o f the h y d r o g e n - h e x a b o r i d e i n t e r a c t i o n . Because l a n t h a n u m a n d b o r o n h y d r i d e s a r e u n s t a b l e h y d r o g e n p o i s o n i n g is possible o n l y at very high h y d r o g e n pressures (106-108 torr). In fact, PH2 m 10 - 6 t o r r a n d the e q u i l i b r i u m o f the r e a c t i o n (5) (Table 3) has b e e n m o v e d

It is seen from Figure 8 that the equilibrium pressures are in good agreement with the experimental data of Po. So the chemical mechanism of the LaB6 poisoning is corroborated experimentally. At the same time, this is important evidence of the solid state nature of the lanthanum hexaboride emission.

Table 3. Gas interaction with LaBo Equilibrium pressure In P (torr)

Poisoner

Reaction

Equilibrium constant

1. Nitrogen

LaB6 + 7/2N-, = LaN + 6 BN

In PN2

dioxide2" Carbon

_9 LaB6 ~ 21/2 COz = La203 ~ 6B,O3_ -{- -~1/9C-

In Pco,.

99000 --T i 11.5

3. Water

2 L a B ~ , + 2 1 H 2 0 = La203 + 6 B 2 O a + 21 H2

lnP'2° P,2

9000_~ 8.4

4. Oxygen

-

~

20000

42000 ~ + 11.7

(b) 9 LaBf, + ~21 + 2n 02 a_ It H , + it CO ' -

In

= La-,Oa + 6 BzOa + n H , O + n CO_,

= 123 + 13.8 n

(a) LaB0 -F 17/2 H2 = LaH_, + 3/2 B.~H.o

1020 InP.= = T +8

(b) LaB6 + 10 H2 = LaH2 4- 3 B2Ht,

PB2,, In pitlo --

6. Carbon monoxide

LaB~

Pco= In Pc2o

7. Acetone

LaB6 + I/2(CHa)2CO + 9/2 CO - 3/2 B,,C + LaC2 + 5/2 CO., + 3/2 H ,

7 C O = 3/2B.~C + LaC2 + 7/2 CO_,

Pco2 P,2o 66

n

n

21/2

+"

1850

44000 + 26500n T

57

T

56000 T

P ~ P~o2 In.PcoP(cH~)aCO 9

0.2

22000 ----T + 11.5

0.02 1×10 -3

T

InPo2

P'J2Pc°P°2

20000 -- ~ + 11.2

9000 . 2.4

T

(a) 2LaB6 ~- 21/20,~ = La-,O3 + 6 B 2 0 3

5. Hydrogen

-F 11.2

P at T, = 1400°C

11.2 16250 T

7.7

42000 - - ~ + 11.7

4×10-t*

at n ---+ co 11.8 - - -26500 T

1 × 10-'*

1020 8 + T

3 x 10s

IPI%H6 = 10-6

PH= = 10-6

~PBalI6

PH2 =

10-128

10-6

-- -28000 T + 1.6

5 × 10 - 4

-- -16250 T + 3.7

1 × 10 -6

587

A A Avdienko and M D Malev: Poisoning of LaB s cathodes

wholly to the left (P,~.,. -< 10-~2a torr). That is why Buckingham 2 could not see the boranes in the mass-spectrum. However, the molecular weight of B2H6 is equal to 27.69 and it is impossible to distinguish peaks with at 26 and 28 by an omegatron. To all appearances, the poisoning compounds were formed in the reactions of hydrogen with free carbon on the cathode surface :

I 0 '~ i 0 -~

ru ;

I0"

2

I0 '

I0 "

I0 • I 0 '~

IO"

I 0 q~ _

I 0 :~'

2C + H 2 = C 2 H 2 - acetylene M = 26 Pc~.~ = 10-7 torr Tk = 1235~C

I0:" I 0 e~ i0-2~

2C + 2H z = C2H 4 - ethylene M = 28 P c , , , = 10-a torr.

I 0 ~"

Let us consider some peculiarities of the LaB6 reactions with carbon-containing gases. Boron and lanthanum carbides are stable at the cathode working temperature, but they can decompose in the carbon dioxide atmosphere: 3/2 B4C + LaC2 + 7/2 CO2 = LaB6 + 7CO.

(2)

A direct proof of the carbide composing and decomposing is the visual picture of the LaB6 cathode after carbon poisoning. The green-grey spot on the acetone poisoned cathode surface disappeared and the emission recovered completely after 2-3 h at a carbon dioxide pressure of 10-5 torr (I~ = 140ffC). Under acetone and alcohol poisoning, one must take into consideration the opposite reaction of carbide decomposing (see Table 3 and Figure 8). Difficulties arise in calculating the equilibrium pressure for hydrocarbons evolving from rubber, because the composition of these gases is unknown. In this case, it is possible only to estimate the influence of the molecular weight upon the poisoning sensitivity for any of the simplest hydrocarbons series. Figure 9 demonstrates the results of the equilibrium pressure calculations for the saturated hydrocarbons of the aliphatic series C. H2n + 2 : LaB 6 + 2/n C, H 2 , + 2 + 3CO = 3/2 B4C + LaC2 + 3/2 C O 2 + 2

n+l

H 2.

(3)

II

As is seen from Figure 6, the equilibrium pressure decreases continuously (and the cathode sensitivity to poisoning increases) with the rise of the molecular weight at constant CO2 pressure. Apparently such dependence has a quite common character since it is observed for other groups of carboncontaining gases too. For example, at 1400°C, the LaB6 poisoning by CO ( M = 28) begins if the pressure is more than 10 -3 torr, but for acetone vapour (M = 58) the poisoning pressure is less than 10 -6 torr [Table 3, reactions (6) and (7)]. The equilibrium pressure for the reactions (3) depends weakly upon temperature [see Table 3, reaction (9)]. A similar picture is observed experimentally: a temperature rise by 50-100°C restores completely the emission current after oxygen poisoning while after carbon poisoning the cathode does not recover even after the temperature rise by 300--400°C. The compensating CO2 pressure rises with the molecular weight more slowly than the cathode sensitivity to poisoning (Figure 9, curve 2). This fact is also corroborated experimentally. As is seen from Figure 7, the poisoning pressures of the acetone vapour and the gases evolving from the rubber differ by factors of 10-100, but in both cases the compensating C02 pressure is equal to about 10 -5 torr. Of course, comparing the experimental results and the data of Figure 9 gives only roughly qualitative information because 588

°

I0

I0

'

i0"~ IO

rO ' 20

40

60

80

I00

120

140

M

Figure 9. Equilibrium pressure of reactions (3) between LAB,, and aliphatic hydrocarbons C,.~,+ ~. Mass mtmber M ~ 14n -b 2: n I 10; P"2 Pep - 10-~' torr. I. Pco~ -- 10-8 tort; 2. Pc, u2,+2 - 10-s lorr. of the difference in the hydrocarbon composition. But apparently such an estimation is sufficient to explain the occurrence of the carbon poisoning. 4. Conclusions I. The LaB6 cathode becomes poisoned as a result of chemical interaction between lanthanum hexaboride and the residual gases in a vacuum system. These reactions lead to an increase of the work function because of the generation of lanthanum and boron oxides, nitrides and carbides on the cathode surface. 2. The cathode sensitivity to poisoning depends strongly on the residual atmosphere composition. The poisoning pressure at a cathode temperature of 1400°C is equal to about 10 --~10- 1 torr for nitrogen, 10-4-10 -3 torr for the oxygen-containing gases and 10- 7-10- 5 torr for hydrocarbons. 3. The carbon poisoning takes place if the vacuum system comprises rubber seals, vapour from organic solvents a trace of oil, etc. The cathode sensitivity to hydrocarbon poisoning rises with the molecular weight and it diminishes with increase of the CO2 partial pressure. For heavy hydrocarbons (M ~ 60) at 1400°C and Pco~ = l0 -8 torr, the poisoning pressure is equal to (I-2) × l0 -7 torr. The emission current recovers completely in such conditions if the CO2 pressure is increased to 10-5 torr. References J Lafferty, Phys Rev 82, 1951, 573. 2 j D Buckingham, BrJ ApplPhys 16, 1965, 1821. 3 H E Gallacher, J Appl Phys 40, 1969, 44. 4 M I Elinson and G F Vasiljev, Radiotekh Elektron 2, 1957, 348; 3, 1958, 945. 5 G V Samsonov and Y B Paderno, Borides of the Rare Earth Metals. Akad. Nauk Ukr. SSR, Kiev (1961). 6 S Dushman, Scientific Foundations of Vacuum Technique. Wiley, New York (1962). 7 Ch Biguenet, Vide No. 159-160, 1972, 161. 8 A P Zefirov (editor), Thermodynamical Properties of hwrganic Compotolds. Atomizdat, Moscow (1965). 9 M X Karapetjanz and M L Karapetjanz, General Thermodynamical Constants of hlorganic and Oryanic Compollnds. Chemistry, Moscow (1968). ~o A A Avdienko, A N Krjutschkov, N K Kuksonov, M D Malev. R A Salimov and A N Sharapa, 7th hit Symp Discharges and Electrical Insulation in Vacuum, Novosibirsk, August 1976, pp. 399-402.