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Desorption of a thorium monolayer
Desorption of a Thorium Monolayer W. E. DANFORTH Bartol Research Foundation of the Franklin Institute, Swarthmore Pennsylvania Introduction
permits thorium vapor from this source to condense upon the filament and also upon the helices. As is well known, deposition of thoruim on the tungsten filament causes its emission to rise by many orders of magnitude to a maximum value. If the filament temperature is not too high, and if the vacuum is good, this maximum emission will, of course, continue indefinitely. If, however, the filament is raised for a period of time to some higher temperature, and then returned again to the original low temperature, it may be found that the high temperature has caused the evaporation of some of the thorium, and the electron emission will then be lower than it was before the high temperature treatment. Fig. 2 shows data of this kind taken with the tube of Fig. 1.
The material to be presented in this paper pertains to that class of devices in which cathode emitters of relatively high temperature are used. This class includes microwave magnetrons and also more recently, those devices known as thermionic converters or plasma thermocouples which are designed for the conversion of heat into electricity. A type of cathode which seems well adapted for these high temperature applications is that in which the emitting area is a refractory metal, usually tungsten, on whose surface some kind of monolayer is maintained for the purpose of reducing the work function. Naturally the tube engineer is immediately interested in whatever circumstances may exist which might cause the removal or desorption of this monolayer film from the cathode surface. The two major causes of such desorption are of course, (1) bombardment of the film by one kind of particle or another, and (2) ordinary thermal evaporation of the film. Recent studies at Bartol have been concerned with various aspects of the physics of the desorption of thorium atoms from a tungsten surface and some of these studies are here reported. The major portion of this paper is concerned with thermal evaporation of a thorium layer. Brief reference will be made at the end to some previously unreported results concerning desorption by electron bombardment. Evaporation
measurements
In Fig. 1 is shown a tube which has yielded some results in both of these respects. It was designed primarily for studies of desorption by electron bombardment. In the course of those studies the evaporation data were taken which are here discussed. In this tube a 10 mil tungsten filament about 3 in. long is mounted coaxially with three tungsten helices. The center one of these three serves as anode for measurement of thermionic emission from the filament, in which measurement the two adjoining helices serve as guard electrodes. In the measurements having to do with desorption by electron bombardment which will be mentioned below, the helices are heated by passage of current and serve as sources of bombarding electrons. The filament and the helices are surrounded by a tantalum shield can which was intended to prevent bombardment of the glass by stray electrons with possible poisoning effects. As shown in the diagram, a thorium-impregnated tungsten matrix, which is heated by passage of current, serves as a source of thorium vapor. An opening in the shield can
LFilamen+ ii
Thorium-
FIG. 1. Tube for studies concerning
desorption
of a thorium
monolayer.
The temperature at which the electron emission was measured was 1045’Ca. At this temperature the maximum emission from the filament (whose area under the anode was Q cmz) was 2 x 10-4 A or 1.6 mA/cm2. The horizontal line corresponds to that value of emission. Having thus activated the filament by laying down an optimum thorium layer the experiment consisted in raising 80
81
Desorption of a Thorium Monolayer
I
DEACTIVATION
OF
A
THORIUM
MONOLAYER
BY
BARTOL
EVAPORATION
DATA
BRATTAIN
AND
BECKER
LANGMUlR CALCVLATEO
FOR
!!,kT‘6--
dt
0
20
40
60
60
100 TIME
FIG. 2. Deactivation
by evaporation
remarks
Time will not permit discussion of evaporation theory in this paper. It has been found, however, that within the existing uncertainties, the data conform to an expression obtained by integrating the equation shown in Fig. 2, where f is the relative coverage, E the binding energy to the surface, Y the repulsion energy between thorium atoms adsorbed on nearest neighbor sites, and Bm is the fraction of total number of sites occupied when the thermionic emission is a maximum. This expression was derived from material in the book : “Theory of Rate Processes “, by Glasstone, Laidler, and Eyring.
.-4v&f kT
h
140
v’ .OS6
160
160
200
220
MINUTES
of a thorium
the filament to some higher temperatures for a given time, and then returning it to 1045 ‘X&tand observing the emission. The uppermost curve in this figure shows the rate of deactivation which is caused by holding the filament at 1600°Cnz periodically returning it to 1045°C for measurements of emission. Below this is a group of curves which show observed deactivation phenomena due to evaporation at 1750°C~. The solid curves were all taken with the tube just described and one sees that the reproducibility is not all one might desire. The spread of data could not be ascribed to temperature uncertainties. We are inclined to attribute it to very minute traces of impurities still remaining in the tungsten after many days of electron bombardment ; impurities whose presence on the surface in varying degrees may be the cause of the differing values of evaporation rate. Below the 175O”CB data is a group of curves showing deactivation at 1850°C~. Here too considerable lack of reproducibility is evident. One sees that our data, given by solid curves, show an approximate sort of agreement with the observations made by Brattain and Becker1 and by Langmuir some thirty years ago. Theoretical
120 IN
C * 7.50,
monolayer
at different
temperatures.
The quantity f, we should perhaps explain, is proportional to the actual, absolute coverage, but up to this point in this paper, the constant of proportionality remains undetermined. By definition, f is equal to unity for that value of coverage, whatever it may be, for which the thermionic emission is at its maximum. The dashed curves in Fig. 2 were obtained by integrating the equation shown, and then transforming the resulting coverage-vs.-time relation into an emission-vs.-time relation, using for this purpose the experimentally determined relationship between emission and the relative coverage f. A reasonable fit with existing data was obtained using a binding energy of 7.5 V and a value of the product v em of about 0.1 v. Evaporation data presented Thus far, nothing has been said regarding the absolute significance of these deactivation curves in terms of the actual amount of thorium on the surface, or the actual evaporation rates in terms of grams leaving a square centimeter per unit of time. During the past year we have carried through a direct experimental determination of that surface density of thorium which corresponds to maximum thermionic emission. Using this information and using the mathematical expression iust referred to, we are now in a position to make some plots of actual evaporation rates in terms of temperature and absolute coverage and to relate evaporation data to electron emission. In Fig. 3 are placed, side by side, charts of emission vs. coverage, for several values of temperature, and evaporation vs. coverage and temperature. These evaporation data are plotted from the mathematical expression which yielded the dashed curves of Fig. 2, and cover the range between 0.1 mg/cmz per century at 1250” up to 0.1 mg/cm2 per hour at 2000°Cn.
82
W. E. DANFORTH
FIG. 3. Electron emission and thorium evaporation rate from a thorium monolayer on tungsten, as a function of absolute coverage for several values of temperature.
FIG. 4. Electron emission and thorium evaporation from a thorium monolayer, plotted for convenient comparison, as a function of temperature.
Desorption of a Thorium Monolayer A more pictorial comparison of evaporation and emission is given in Fig. 4 and in these diagrams is also presented our estimate of the uncertainty in the evaporation data which exists at the present time. The upper boundary of the shaded area is the data of Ahearn and Becker3 of 1938 which data seem to be the highest in the literature. The lower boundary is the data given by our own mathematical expression. Figures 4a and 4b both show emission and evaporation from a thorium monolayer. Figure 4a shows these properties when the thorium coverage is that which corresponds to maximum thermionic emission ; in Fig. 4b the coverage is one-half of that amount. Emission and evaporation from tungsten are included on the same diagrams. Examination of this figure shows that, for a thorium monolayer with no other means of retarding evaporation, the evaporation becomes excessive for most applications when the lOOA/cm2 emission level is approached. At 10 A/cmz, evaporation will be somewhere between 0.6 and 6.0 x lo-10 g/cm&c or (2-20) x 10-4 mg/cm2 hr. At 10 A/cm2 the evaporation from 1 cm2 would be in the neighborhood of 1 mg in a period of time somewhere between 500 and 5000 hr depending upon what surface conditions are present in the case being considered. Desorption
by electron bombardment
We turn now to the subject of desorption by electron bombardment. This matter was previously reported upon in March of 1959 at the MIT. Conference on Physical Electrons at which time we had concluded that bombardment
83
of the monolayer by electrons of 350 V energy or less did not remove adsorbed thorium atoms at a rate which we could detect. We calculated at that time that if any desorption of this kind existed it was too small to be of any importance in the matter of thorium dispenser cathodes. Since that report we have carried the experiment further, using the tube of Fig. 1, and I am now able to speak even more assuringly in this matter of electron desorption of the thorium layer. Using the tube shown, we were able to continue the bombardment for a total of 80 A hr/cmz. And it is concluded that such desorption as might exist must have been less than 4 x lo-11 electrons removed per bombarding electron. This total bombardment of 80 A hr/cmz is of the order of magnitude of the total bombardment which might be experienced in reasonable life of a CW magnetron. And during this period the number of atoms/cm2 removed, if any, was not more than 1013 or a very small fraction of one complete monolayer. It may therefore be concluded that removal of electrons by bombardment is of no practical importance even though the cathode concerned be a single and non-replenished layer of thorium atoms. In practical cases the loss of thorium by electron bombardment is probably considerably less than that lost either by thermal evaporation or by the residual ion bombardment from the imperfect vacuum. It is evident that desorption of thorium by electron bombardment occurs at such a small rate that it need not be considered by the cathode engineer. References
1 W. H. Brattain and J. A. Becker ;Phys.Rev., 43, 428 (1933). * I. Langmuir ; J. Franklin Inst., 217, 543 (1934). 3 A. J. Ahearn and J. A. Becker ;Pfiys. Rev., 54, 448 (1938).
permits thorium vapor from this source to condense upon the filament and also upon the helices. As is well known, deposition of thoruim on the tungsten filament causes its emission to rise by many orders of magnitude to a maximum value. If the filament temperature is not too high, and if the vacuum is good, this maximum emission will, of course, continue indefinitely. If, however, the filament is raised for a period of time to some higher temperature, and then returned again to the original low temperature, it may be found that the high temperature has caused the evaporation of some of the thorium, and the electron emission will then be lower than it was before the high temperature treatment. Fig. 2 shows data of this kind taken with the tube of Fig. 1.
The material to be presented in this paper pertains to that class of devices in which cathode emitters of relatively high temperature are used. This class includes microwave magnetrons and also more recently, those devices known as thermionic converters or plasma thermocouples which are designed for the conversion of heat into electricity. A type of cathode which seems well adapted for these high temperature applications is that in which the emitting area is a refractory metal, usually tungsten, on whose surface some kind of monolayer is maintained for the purpose of reducing the work function. Naturally the tube engineer is immediately interested in whatever circumstances may exist which might cause the removal or desorption of this monolayer film from the cathode surface. The two major causes of such desorption are of course, (1) bombardment of the film by one kind of particle or another, and (2) ordinary thermal evaporation of the film. Recent studies at Bartol have been concerned with various aspects of the physics of the desorption of thorium atoms from a tungsten surface and some of these studies are here reported. The major portion of this paper is concerned with thermal evaporation of a thorium layer. Brief reference will be made at the end to some previously unreported results concerning desorption by electron bombardment. Evaporation
measurements
In Fig. 1 is shown a tube which has yielded some results in both of these respects. It was designed primarily for studies of desorption by electron bombardment. In the course of those studies the evaporation data were taken which are here discussed. In this tube a 10 mil tungsten filament about 3 in. long is mounted coaxially with three tungsten helices. The center one of these three serves as anode for measurement of thermionic emission from the filament, in which measurement the two adjoining helices serve as guard electrodes. In the measurements having to do with desorption by electron bombardment which will be mentioned below, the helices are heated by passage of current and serve as sources of bombarding electrons. The filament and the helices are surrounded by a tantalum shield can which was intended to prevent bombardment of the glass by stray electrons with possible poisoning effects. As shown in the diagram, a thorium-impregnated tungsten matrix, which is heated by passage of current, serves as a source of thorium vapor. An opening in the shield can
LFilamen+ ii
Thorium-
FIG. 1. Tube for studies concerning
desorption
of a thorium
monolayer.
The temperature at which the electron emission was measured was 1045’Ca. At this temperature the maximum emission from the filament (whose area under the anode was Q cmz) was 2 x 10-4 A or 1.6 mA/cm2. The horizontal line corresponds to that value of emission. Having thus activated the filament by laying down an optimum thorium layer the experiment consisted in raising 80
81
Desorption of a Thorium Monolayer
I
DEACTIVATION
OF
A
THORIUM
MONOLAYER
BY
BARTOL
EVAPORATION
DATA
BRATTAIN
AND
BECKER
LANGMUlR CALCVLATEO
FOR
!!,kT‘6--
dt
0
20
40
60
60
100 TIME
FIG. 2. Deactivation
by evaporation
remarks
Time will not permit discussion of evaporation theory in this paper. It has been found, however, that within the existing uncertainties, the data conform to an expression obtained by integrating the equation shown in Fig. 2, where f is the relative coverage, E the binding energy to the surface, Y the repulsion energy between thorium atoms adsorbed on nearest neighbor sites, and Bm is the fraction of total number of sites occupied when the thermionic emission is a maximum. This expression was derived from material in the book : “Theory of Rate Processes “, by Glasstone, Laidler, and Eyring.
.-4v&f kT
h
140
v’ .OS6
160
160
200
220
MINUTES
of a thorium
the filament to some higher temperatures for a given time, and then returning it to 1045 ‘X&tand observing the emission. The uppermost curve in this figure shows the rate of deactivation which is caused by holding the filament at 1600°Cnz periodically returning it to 1045°C for measurements of emission. Below this is a group of curves which show observed deactivation phenomena due to evaporation at 1750°C~. The solid curves were all taken with the tube just described and one sees that the reproducibility is not all one might desire. The spread of data could not be ascribed to temperature uncertainties. We are inclined to attribute it to very minute traces of impurities still remaining in the tungsten after many days of electron bombardment ; impurities whose presence on the surface in varying degrees may be the cause of the differing values of evaporation rate. Below the 175O”CB data is a group of curves showing deactivation at 1850°C~. Here too considerable lack of reproducibility is evident. One sees that our data, given by solid curves, show an approximate sort of agreement with the observations made by Brattain and Becker1 and by Langmuir some thirty years ago. Theoretical
120 IN
C * 7.50,
monolayer
at different
temperatures.
The quantity f, we should perhaps explain, is proportional to the actual, absolute coverage, but up to this point in this paper, the constant of proportionality remains undetermined. By definition, f is equal to unity for that value of coverage, whatever it may be, for which the thermionic emission is at its maximum. The dashed curves in Fig. 2 were obtained by integrating the equation shown, and then transforming the resulting coverage-vs.-time relation into an emission-vs.-time relation, using for this purpose the experimentally determined relationship between emission and the relative coverage f. A reasonable fit with existing data was obtained using a binding energy of 7.5 V and a value of the product v em of about 0.1 v. Evaporation data presented Thus far, nothing has been said regarding the absolute significance of these deactivation curves in terms of the actual amount of thorium on the surface, or the actual evaporation rates in terms of grams leaving a square centimeter per unit of time. During the past year we have carried through a direct experimental determination of that surface density of thorium which corresponds to maximum thermionic emission. Using this information and using the mathematical expression iust referred to, we are now in a position to make some plots of actual evaporation rates in terms of temperature and absolute coverage and to relate evaporation data to electron emission. In Fig. 3 are placed, side by side, charts of emission vs. coverage, for several values of temperature, and evaporation vs. coverage and temperature. These evaporation data are plotted from the mathematical expression which yielded the dashed curves of Fig. 2, and cover the range between 0.1 mg/cmz per century at 1250” up to 0.1 mg/cm2 per hour at 2000°Cn.
82
W. E. DANFORTH
FIG. 3. Electron emission and thorium evaporation rate from a thorium monolayer on tungsten, as a function of absolute coverage for several values of temperature.
FIG. 4. Electron emission and thorium evaporation from a thorium monolayer, plotted for convenient comparison, as a function of temperature.
Desorption of a Thorium Monolayer A more pictorial comparison of evaporation and emission is given in Fig. 4 and in these diagrams is also presented our estimate of the uncertainty in the evaporation data which exists at the present time. The upper boundary of the shaded area is the data of Ahearn and Becker3 of 1938 which data seem to be the highest in the literature. The lower boundary is the data given by our own mathematical expression. Figures 4a and 4b both show emission and evaporation from a thorium monolayer. Figure 4a shows these properties when the thorium coverage is that which corresponds to maximum thermionic emission ; in Fig. 4b the coverage is one-half of that amount. Emission and evaporation from tungsten are included on the same diagrams. Examination of this figure shows that, for a thorium monolayer with no other means of retarding evaporation, the evaporation becomes excessive for most applications when the lOOA/cm2 emission level is approached. At 10 A/cmz, evaporation will be somewhere between 0.6 and 6.0 x lo-10 g/cm&c or (2-20) x 10-4 mg/cm2 hr. At 10 A/cm2 the evaporation from 1 cm2 would be in the neighborhood of 1 mg in a period of time somewhere between 500 and 5000 hr depending upon what surface conditions are present in the case being considered. Desorption
by electron bombardment
We turn now to the subject of desorption by electron bombardment. This matter was previously reported upon in March of 1959 at the MIT. Conference on Physical Electrons at which time we had concluded that bombardment
83
of the monolayer by electrons of 350 V energy or less did not remove adsorbed thorium atoms at a rate which we could detect. We calculated at that time that if any desorption of this kind existed it was too small to be of any importance in the matter of thorium dispenser cathodes. Since that report we have carried the experiment further, using the tube of Fig. 1, and I am now able to speak even more assuringly in this matter of electron desorption of the thorium layer. Using the tube shown, we were able to continue the bombardment for a total of 80 A hr/cmz. And it is concluded that such desorption as might exist must have been less than 4 x lo-11 electrons removed per bombarding electron. This total bombardment of 80 A hr/cmz is of the order of magnitude of the total bombardment which might be experienced in reasonable life of a CW magnetron. And during this period the number of atoms/cm2 removed, if any, was not more than 1013 or a very small fraction of one complete monolayer. It may therefore be concluded that removal of electrons by bombardment is of no practical importance even though the cathode concerned be a single and non-replenished layer of thorium atoms. In practical cases the loss of thorium by electron bombardment is probably considerably less than that lost either by thermal evaporation or by the residual ion bombardment from the imperfect vacuum. It is evident that desorption of thorium by electron bombardment occurs at such a small rate that it need not be considered by the cathode engineer. References
1 W. H. Brattain and J. A. Becker ;Phys.Rev., 43, 428 (1933). * I. Langmuir ; J. Franklin Inst., 217, 543 (1934). 3 A. J. Ahearn and J. A. Becker ;Pfiys. Rev., 54, 448 (1938).