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Surface studies on the low work function surface complex of barium on an osmium-ruthenium substrate
127
Applied Surface Science 29 (1987) 127-142 North-Holland, Amsterdam
SURFACE COMPLEX Ralph
STUDIES ON THE LOW WORK FUNCTION OF BARIUM ON AN OSMIUM-RUTHENIUM
SURFACE SUBSTRATE
FORMAN
ANALEX
Corporation,
21000 Brookpark
Received
12 November
1986; accepted
Road. Mail-Stop for publication
55-I. Clewlund,
21 February
OH 44135, USA
1987
Barium and oxygen on an osmium-ruthenium (Os/Ru) surface is the surface complex responsible for the excellent electron emission properties of commercial M-type thermionic cathodes. A computerized Auger technique was used to study the surface properties of this surface complex and compare it with results obtained earlier for BaaO-W surfaces, which characterize the properties of commercial barium impregnated tungsten thermionic cathodes. Barium desorption, electron emission and barium, barium oxide evaporation were measured at elevated temperatures for this experimental surface study. Desorption measurements, at 1100 o C, on the barium on osmium-ruthenium substrate show that the energy for desorption was higher than that for the lowest work function Ba-O-W surface (5.4 to 4.8 eV). Oxygen was always present on the sputtered Os/Ru substrate and contributed to the strong bonding of barium to the Os/Ru surface. The barium and barium oxide evaporative products from such a surface at 1100 o C were found to be mainly barium with the Ba/BaO ratio equal to approximately 100.
1. Introduction Ever since their invention early in the 20th Century [l], low temperature electron emitting cathodes with barium as a major constituent have been viewed as a form of “black magic” by their users. Their employment as oxide cathodes in the 1920’s was a major impetus to the early success of the electronic industry. The need for higher current devices less susceptible to poisoning in the 1950-1960 era led to the development of the tungsten-based barium dispenser cathodes [2-51. Although their development has been a boon, to the maturation of the electron tube industry, the persistence of sporadic cathode problems, unexplained failures and lack of a firm scientific understanding of cathode breakdown is still a major burden to the industry. Extensive literature on these problems has been published. Only in recent years, with the development of sophisticated surface analysis equipment, has it been possible to formulate plausible scientific models which can be experimentally verified and understood. The investigation reported in this paper is one of a series [6-111 whose objective is understanding the mechanism of operation of barium impregnated thermionic cathodes. 0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
128
R. Formm
/ Low work function surface complex of Ba on OS - Ru
In earlier research [ll] on the role of oxygen in the Ba-O-W surface complex, which is characteristic of the impregnated tungsten cathode, we found that high temperature studies of thermal desorption of barium and electron emission suggested a physical-chemical model consistent with the experimental data. The same technique was employed in this investigation of the properties of a barium on oxygen on an Os/Ru surface. Electron emission and high temperature barium desorption were measured for a barium monolayer on a sputter-deposited Os/Ru surface, similar to that used on the M-type cathode.
2. Experimental
details
The experimental apparatus used in this study has been described earlier [9-111. It employs a computerized scanning Auger microprobe spectrometer for Auger and desorption studies and a VG Spectralab SX200 mass analyzer for barium and barium oxide evaporation measurements. The two samples employed in these experiments were both tungsten foils, 0.0025 cm thick and approximately 0.13 cm wide, bent into a U-shape with the center portion of the U coated with a 0.5 pm thick sputter deposited Os/Ru layer. The legs of the tungsten U were - 0.8 cm and the center portion was 0.7 cm. The samples were mounted on a carousel which could be rotated, to alternatively face a barium getter source, the quadrapole of a mass spectrometer and/or the Auger spectrometer, as described in ref. [ll]. Optical pyrometry through a vacuum window was used to calibrate the sample temperature as a function of heater current through the tungsten foil. The Os/Ru substrate was identical to that used in conventional M-cathodes, a sputter deposited layer that was nominally 80% OS and 20% Ru. Auger measurements on the virgin “base” sputtered Os/Ru surface showed oxygen was always present on the surface and therefore all these experiments describe our measurements of barium on an oxygenated osmium-ruthenium surface. However, since this sputtered coating is identical to that used in M-cathode fabrication, its usage conforms to the objective of this investigation; namely to employ surface study techniques to better understand the mechanism of operation of an M-type thermionic cathode. The technique employed to generate a barium monolayer on the Os/Ru surface was similar to that employed in ref. [ll]. The experimental sequence was: (a) The Os/Ru surface was cleaned by heating to 1150°C for 3-5 min. (b) Multilayered barium was then deposited on the Os/Ru surface by rotating the sample to expose it to the heated barium source. (c) A barium monolayer was obtained by heating the sample to - 500 o C for 20-30 min.
R. Forman / Low work Junction surface complex of Ba on OS - Ru
129
The limit of 1150°C in process (a) was to minimize any alloying action between osmium and tungsten that could invalidate the study. Van Stratum and Kuin [12] have shown that alloying at 1050”~ brightness for 5000 h eliminated the advantages of the osmium-coated tungsten surface. It was feared that continued operation above 1150 o C could produce a similar effect. However, this procedure had its limitations. In the course of these experiments we found that the desorption energy for barium on the Os/Ru surface was so from the surface high that we could not remove more than - 0.5 monolayers at 1400 K in any reasonable time. This meant that once we put a monolayer of barium on the samples we were never able to completely desorb the barium by this high temperature treatment. Repeated monolayer desorption experiments then consisted of putting a monolayer of barium on a surface that started with - 0.5 monolayers before the new deposition. Another variable beyond our control was the initial surface oxygen concentration which was always present. Whether its appearance on the surface is due to its existence in the tungsten substrate or from the sputtered Os/Ru coating could not be determined. Its presence, however, is probably of similar origin to its occurrence on an M-type cathode surface and therefore does not conflict with our primary objective, namely to obtain insight into the surface chemistry and physics of M-type cathodes. Monolayer desorption at a given temperature was measured in a manner identical to that described in ref. [ll] by means of the computerized scanning Auger microprobe spectrometer in the EN(E) mode. Barium and barium oxide evaporation data were obtained by rotating the sample, having a monolayer of barium on its surface, in front of the mass analyser (distance separation l-2 mm) and heating the sample to a given temperature in a manner similar to that described in ref. [ll].
3. Experimental results and discussion An illustration of the type of Auger signals obtained in these studies is shown in fig. 1. These are Auger scans from 130 to 630 eV in the EN(E) mode (fig. la) and the d(EN)/dE mode (fig. lb). In fig. la peaks a, b, c and d are respectively osmium and tungsten [13], ruthenium, oxygen and barium. Curve 1 refers to a multilayered barium adsorbate on the Os/Ru substrate. Curve 2 is that for a monolayer of barium on the Os/Ru surface and curve 3 is the result of heating the surface described by curve 2 fo 1 h at 1100 o C. The data of fig. la in the EN(E) mode are the most interesting. The multilayered barium curve 1 shows a very prominent barium peak and some oxygen. The osmium-ruthenium substrate peaks are concealed by the multilayer barium. The ordinate scale of curve 1 was reduced by one third, as compared to curves 2 and 3, in order to obtain the complete Auger spectrum
130
b
d
v
I 130
230
330
430
530
, 630
CNA. v
Fig. 1. Auger output multilayered barium the surface described counts proportional d( EN)/d E. E
(b) signals as a function of electron energy over the range of 130-630 eV for: (1) on the Os/Ru substrate; (2) monolayer barium on the Os/Ru substrate: (3) by curve 2 after it was heated to 1400 K for 1 h. The ordinate for (a) is III to EN(E) and the ordinate for (b) is in counts/V in the differentiated mode is the Auger energy and N is the electron density at a given Auger energy.
on the printout when the abscissa scale varied from 130 to 630 eV. A quantitative comparison between the curves would require the barium peak of curve 1 to be increased by a factor of three. The differences between curves 2 and 3 are a major subject of the report. One interesting feature is that the loss of barium from the monolayer surface, from desorption at a high temperature. increases the oxygen and osmium peaks but does not seem to affect the ruthenium peaks. In the differentiated mode, fig. 1 b, the osmium and ruthenium peaks between curves 2 and 3 are indistinguishable but loss of barium by desorption and increases in oxygen are observed. Quantitative Auger data, at a given time and position on the surface, were obtained by measuring the peak minus background counts [ll] from data similar to that of fig. la (e.g., 166. 268, 508 and 583 eV for the peaks and 185.
R. Forman / Low’ work
function
f!ONOLAYER FORMATlOW
surface complex
of Ba on OS-
Ru
131
DESORPTIOM
t 1000
.
* r
0
33
66
99 132 TIME PERIODS
165
198
Fig. 2. The change in surface concentration of barium (0) and oxygen (A) as a function of time when the surface of the Os/Ru substrate, initially covered with a barium multilayer, is heated to 700 K and then 1375 K. Desorption of barium from a monolayer (100-200 time period) was done at 1375 K. One time period corresponds to 30 s.
283, 522 and 612 eV for the backgrounds of OS, Ru, 0 and Ba, respectively). Typical desorption results obtained in the experiments are shown in fig. 2. This figure shows the results for surface barium (circles) and oxygen (triangles) when a multilayer of barium is heated to first form a monolayer (O-99 time periods) and then desorbed at 1375 (100-200 time periods). The 1375 K temperature was obtained by applying a 75 K correction to a brightness temperature reading of 1300 K on the Os/Ru surface. Details of the experimental technique employed to obtain “correct” data for a given element as a
132
R. Forman / Low work functionsurfaceromplerof Ba on OS - Ru
function of time are given in ref. [ll]. In fig. 2, at time zero, the multilayered barium is heated to - 700 K for - 1 h to obtain a stable monolayer of barium on the Os/Ru substrate. The barium count starts at about 2600 (off-scale) and drops to - 750 (a monolayer) in - 25 min. The oxygen count starts at - 200 (small oxygen peak in (d) of fig. la) and increases to - 500 as the monolayer is formed. At time period 100 the temperature of the substrate is increased to 1375 K and rapid barium desorption occurs in a 1 h period. The most interesting phenomenon in fig. 2 is the abrupt increase in oxygen accompanying the rapid desorption.This data is very similar to that observed for barium desorption from a highly oxygenated tungsten surface as reported in ref. [ll] (figs. 4 and 5). The obvious explanation for this effect is that the underlying oxygen is exposed as the barium desorbs. However, the abruptness of the oxygen transition, before the barium is completely desorbed, indicates other phenomena are also present, e.g., high temperature oxygen diffusion from the bulk of the substrate [9,11]. One major problem in the interpretation of the data is the reliability and reproducibility of the results. The primary variable in the experiment is the beam current to the sample. The data in fig. 2 were obtained over a 2 h period. Over that time period any variations in beam current affect the background as well as the peak signals. Under such circumstances the difference count variation which is the ordinate of fig. 2, could be interpreted as a chemical change. To minimize this effect, the electron gun was turned on at least 2 h before any computerized tests were run and the maximum beam current variation, observed before and after the experiment, was in the range of 4%‘. Fig. 3 illustrates the peak and background data obtained for the barium desorption results of fig. 2. The curve A points are peak values (582 eV) and the curve B lower points are background data (613 eV). Initially the multilayer count difference is 2600. As the sample is heated to 700 K (O@lOO time periods) the background signal changes rapidly, by - 2600 counts, until it reaches a maximum. This change in background signal, at an electron energy of 613 eV, is associated with the change in secondary yield on transformation from a barium multilayer to a barium monolayer on the Os/Ru substate. The scatter in the data points are noise in the measurements. The difference values at a monolayer remain quite constant until desorption is started and the drop in this difference count is readily observed after time period 100. The stability of the background data from an abscissa value of 22 to 200 time periods shows the constancy of the beam current. The total variation in counts over this period is 400, which is only a 3% variance. Another important parameter needed to interpret these experiments is the variation of oxygen with barium desorption as the temperature is increased. In fig. 2, the barium count for a monolayer is obtained rapidly and then remains constant until desorption starts; whereas the oxygen count is changing throughout this period and only reaches some constant values ( - 500 counts)
R. Forman / Low work functionsurfacecomplex
of Ba on OS-Ru
133
16 200
. 13 600
13 000
-
I
I 33
66
99
132
165
198
TIME PERIODS
Fig. 3. Barium peak and background count data as a function of time for the barium desorption results of fig. 2. Curve A demonstrates the peak and curve B the background measurements.
is started. Is at a time period of - 90, just before the desorption experiment this plateau in the oxygen data a true phenomenon or would the oxygen count of the sample were maincontinue rising to - 750 even if the temperature tained at 700 K? To answer this question experimental data was obtained from a multilayered barium on Os/Ru sample when it was heated over a 2 h period at 700 K. Fig. 4 illustrates the result and compares it with the data of fig. 2. The barium (circles) and oxygen (triangles) data of the figure 2 desorption experiment are presented in fig. 4 with the additional oxygen results (squares) obtained when the sample was maintained at 700 K for the 2 h period. The barium counts at 700 K for 2 h is not included in fig. 4, although as expected, after an initial change in the O-30 time period, it remained constant (700-750 counts) for the duration of the test indicating monolayer coverage. The results of fig. 4 show that the oxygen plateau, at - 500 counts in fig. 2, is a real one. The changes in surface concentration of the substrate elements osmium and ruthenium, during the 1375 K barium desorption, are illustrated in fig. 5. The barium results (circles), identical to that of fig. 2, are shown in this figure in order to compare it with the data for osmium (squares) and ruthenium (triangles) in the same experiment. Initially, with a multilayer of barium, the osmium and ruthenium counts are negative and outside the ordinate range of the figure. This arises because our technique involves using a peak measure-
0
40
80
120
160
200
240
TIME PERIODS
Fig. 4. Barium
(0)
and oxygen data obtained
1375 K desorption data of fig. 2 compared when the sample is heated to 700 K for 2 h.
(A)
with oxygen
(Cl)
ment at a lower energy than the background measurement and the EN(E) curve of fig. la for multilayered barium has a positive slope between 130 and 300 eV. As the barium desorbs the osmium and ruthenium peaks appear and rapidly increase to a plateau value established at barium monolayer coverage. Starting with the time period 100, when the sample temperature was raised to 1375 K, the barium desorbed rapidly, the osmium count increased and after an initial drop the ruthenium count also increased with time. The initial apparent small drop in ruthenium was not a reproducible result. In this case it may have been due to either a secondary yield slope change in the 250-300 eV range, caused by the barium desorption, or to a displacement of the beam to an adjacent area when the sample expanded upon heating. An interesting feature of figs. 2 and 5 is the fact that even after heating the sample at 1375 K for 1 h, the barium count is reduced by only approximately
R.Forman
/ LAW work
fun&m surfacecomplex
-*
1000
of
Ba on
OS -
Ru
135
. 1 l
900
-
.
800
700
600 2 5 3
500
400
-
300I -
zocI-
lotI -
L 0
33
66
99
132
165
198
TIME PERIODS
Fig. 5. Barium (0), osmium (Cl) and ruthenium (A) data obtained from the desorption of barium from an Os/Ru surface at 1375 K. The barium data is identical to that shown in fig. 2.
one half. This implies that a half monolayer of barium still exists on the surface, which leads to the conclusion that the surface bond is very strong between the barium adsorbate and the underlying oxygen on the Os/Ru surface. This circumstance would correspond to a very high activation energy for desorption whose value an be obtained by using the theoretical relation: E, (nW/kT)
- E, (nV/kT)
= (&T/h)
exp( - E,/kT)
(1)
and a computerized technique employing a least squares fit [9]. E, is the exponential integral, 19is the fractional coverage as a function of time t, k and h are the Boltzmann and Planck constants respectively and T is the surface temperature in K. E, is the activation energy for surface desorption, V is the repulsive energy per pair of adatoms and n the number of nearest neighbor
0
8
16
25
33
41
50
TIME, HIN
Fig. -6. Barium
desorption substrate
at 1375 K from a monolayer of harlum on an osmium-ruthenium (0) and an oxygenated (2 L) tungsten substrate (A).
adatoms. The upper curve (squares) of fig. 6 is a theoretical curve of the barium desorption data at 1375 K from fig. 2. The calculated activation energy for desorption (E,) is equal to 5.4 eV. For comparison, we have included in fig. 6 results for barium desorption from a Ba-O-W surface at 1375 K. These data (triangles), taken with a 2 L (Langmuir units) oxygen exposure of the tungsten surface before barium adsorption, are similar to those described in fig. 8 of ref. [ll] and give an activation energy for desorption of 4.8 eV. Earlier studies [ll], whose motivation was to understand the basic properties of impregnated tungsten cathodes, have shown that the 2 L oxygen exposure, described by the data of fig. 6, provides the most stable barium adsorbate bonding for a Ba-O-W surface complex. The even higher stability of the Ba-0-Os/Ru surface bond, as shown in fig. 6, is obviously one of the major reasons for the better performance of the M-type cathode as compared to the normal impregnated tungsten cathode. Another interesting feature of the data of figs. 2 and 5 is the information one an infer about the structure of the barium-oxygen-substrate bond. The desorption curves of fig. 2 can be interpreted as showing that the barium
R. Forman
/ Low work function
surface complex
of Ba on OS ~ Ru
137
r 1.5
.5
0
30
60 TIME,
Fig. 1. Evaporation osmium-ruthenium scale are arbitrary
90 SEC
120
150
of barium and barium oxide from a monolayer of barium deposited on an surface, heated to 1375 K, as a function of time. The units for the ordinate and are based on the original calibration of the instrument using molecular nitrogen (28 AMU) for a calibration standard.
overlies the oxygen; the loss of barium is accompanied by a sharp increase in the surface oxygen concentration. This type of Ba-O-substrate surface bond is also characteristic of the Ba-O-W surface bond inherent in the tungsten impregnated cathode [ll]. The major difference between the two is the stronger Ba-0-Os/Ru surface bond with a resultant stronger dipole moment and lower work function. Another aspect of the stability of the Ba-0-Os/Ru surface bond can be studied by analysing the barium evaporation product from a desorption experiment, of the type illustrated in fig. 2. By means of mass spectrometry and the use of a technique described earlier [ll], we were able to determine the ratio of barium to barium oxide desorbed at 1375 K. The data, shown in fig. 7, were obtained by monitoring the atomic mass units (AMU) 138 (Ba) and 154 (BaO) as a function of desorption time. It should be noted that the ordinate scale for the barium pressure is 100 X that for barium oxide and the ratio of the Ba/BaO peak is 120.
I 0
30
60
90
120
150
180
210
240
I
270
TIME, FUN
Fig. 8. Comparison of the electron emission and desorption at 1375 K from both a monolayer of barium on a Os/Ru substrate and a monolayer of barium on an oxygenated tungsten (2 L) surface. The desorption data for the Os/Ru substrate (0) and the oxygenated tungsten substrate (A) are similar to the data in fig. 6 and the electron emission curves are normalized to show the decrease in emission as a function of time, at 1375 K. from the initial electron current values.
Another important feature of this study is correlating the surface desorption phenomena with electron emission from the surface. This is illustrated in fig. 8 which compares the desorption data of fig. 6 with the electron emission obtained from the respective Ba-0-Os/Ru and Ba-0-W (2 L oxygen) surfaces at 1375 K with time. The electron current data of fig. 8 was obtained in the following manner for both surfaces. The multilayered barium samples were heated to - 700 K for 30 min to obtain a monolayer and then cooled down before they were placed in front of an anode structure. At time zero the temperature of the samples was raised to 1375 K and the current to the anode monitored with time. The electron emission curves are normalized by taking the ratio of the current observed at any given time to the initial current measured when the sample reached 1375 K. The electron emission data correlates with the barium coverage data: the barium coverage for the Os/Ru surface stabilizes at - 0.5 and so does its emission current with a similar correspondence for the Ba-O-W surface. The fact that the electron emission
R. Forman
/ Low work function
surface complex
of Ba on OS - Ru
139
curve reaches a stable value after a much longer interval than the barium coverage curve can be attributed to features peculiar to each individual experiment. The desorption measurements are made at a point in the center of the sample which reaches the maximum temperature at the earliest time and then remains relatively stable. The electron emission, however, comes from the whole surface of the sample exposed to the anode and this surface temperature is not uniform. At any point, the temperature is strongly dependent on its proximity to the relatively heavy current leads used to support the U-shaped sample. From the electron emission data, it appears that a stable temperature fact distribution is only reached after - 3-4 h. Again the most significant obtained from the data of fig. 8 is confirmation of the higher stability of the
600
l
t
d
I 0
33
I
I 66
99
132
165
198
TIME PERIODS Fig. 9. Barium desorption at 1375 K from a sulphur-contaminated region of the Os/Ru substrate (0) and from an adjacent sulfur-free area of the same Os/Ru substrate sample (A). The surface concentration of oxygen (0) is shown as a function of time as the barium (A) desorbs from the sulphur-free region.
140
R. Forman / Low work functm
surface complex of Bo on OS - Ru
M-type cathode surface (Os/Ru) as compared to the impregnated tungsten cathode surface. As discussed earlier two Os/Ru substrate samples were tested in this study. Inadvertently the second provided us with the opportunity to study cathode poisoning. There were sulphur-contaminated as well as sulphur-free areas on its surface. Wherever surface sulphur was present, oxygen was absent and vice-versa. This enabled us to study cathode poisoning from sulphur contamination and compare the results with similar experiments performed on adjacent sulphur-free areas. The major parameter examined was barium desorption which is illustrated in the data of fig. 9. The (0) results are desorption data at 1375 K from the sulphur-contaminated region. For this run the transition from barium multilayered to monolayer on the sulphur-contaminated surface is not shown. Initially 20 data points of monolayer coverage are shown before the temperature of the sample is raised from 700 to 1375 K and rapid barium desorption occurs. Barium desorption from a sulphur-free region on the same sample are shown by the (A) points in fig. 9. This experiment was initiated at the multilayer stage for 5 time periods. then its
0
L
I 8
16
25
33
111
50
TIME. PUN
Fig. 10. Barium desorption at 1375 K from a monolayer of barium on a sulfur-contaminated region (0) of the Os/Ru substrate and on a sulphur-free (A) area of the Os/Ru substrate.
R. Formnn / Low work function surface complex of Ba on OS- Ru
141
temperature was raised to 700 K for 50 min before it was raised to 137.5 K at 100 time periods. Also included in fig. 9 are the data for surface oxygen (squares) on the sulphur-free region during the desorption run. Similar to the earlier results (fig. 2) the oxygen concentration abruptly increased as the barium initially desorbed at 1375 K. The major difference between the sulphur-free data of fig. 9 and that of fig. 2 is that the former results indicate a lower surface oxygen concentration than the latter. In the sulphur-contaminated area no oxygen was observed and when barium (circles) desorbed the surface sulphur concentration rose in a manner similar to what one would expect from a barium on sulphur on Os/Ru surface complex; the barium desorption uncovered the underlying sulphur. It is obvious from the data of fig. 9 that the barium-sulphur-Os/Ru surface bond is much weaker than the barium-oxygen-Os/Ru surface bond. At 1375 K practically all the barium desorbs from the sulphur-contaminated surface during the 50 min desorption experiment. To determine the quantitative difference between the two surface bonds, we plotted the data of fig. 9 in a manner similar to fig. 6, using the same computerized techniques. The results are illustrated in fig. 10. The energies of desorption for the two cases are obtained in a similar manner to that for fig. 6 by using eq. (1) and the computerized technique described in ref. [9]. The value of E, for the sulphurfree region is 5.4 eV and is 4.8 eV for the sulphur-contaminated area.
4. Conclusions Experiments for studying the chemical and physical surface properties of barium as an adsorbate on an osmium-ruthenium substrate have been carried out. It is part of a continuing investigation aimed at understanding the mechanism of operation of barium impregnated thermionic cathodes. The samples studied were simulated M-type cathodes and this program’s objective was to study their chemical and physical surface properties and compare the results with those obtained earlier for barium on oxygen on tungsten surfaces, which simulate the behavior of the impregnated tungsten thermionic cathode. The desorption energy of barium monolayers for an osmium-ruthenium surface was measured at 1375 K as 5.4 eV. Similar measurements on the most stable barium on oxygen on tungsten surface showed that its desorption energy was - 4.7 eV. The higher desorption energy for the barium on osmium-ruthenium substrate as compared to the barium on tungsten substrate is consistent with the fact that the M-cathode has a lower work function and is a better electron emitter than the conventional tungsten impregnated cathode. The stronger surface bond of barium to the Os/Ru substrate leads to a higher dipole moment and a lower work function than the barium to tungsten substrate. Measurements of barium and barium oxide evaporation
from the barium on Os/Ru substrate at elevated that the evaporation products are predominantly to barium oxide evaporant is - 100.
temperatures (1375 K) show barium. The ratio of barium
Acknowledgements
The program was sponsored and supported by the Space Communications Division of NASA, Lewis Research Center. The author would also like to acknowledge and thank G. Lesny for his excellent technical support on this program.
References [l] 12) (31 [4] [5] [6] [7] [X] [9] [lo] [ll] [12] [13]
A. Wehnelt, Ann. Phys. 19 (1906) 138. R.C. Hughes, P.P. Coppola and ES. Rittner, US Patent 270011X. November R. Levi and R.C. Hughes, US Patent 2700000. February 1952. R. Levi, J. Appl. Phys. 26 (1955) 639. P. Zalm and A.J.A. van Stratum. Phillps Tech. Rev. 27 (1966) 69. R. Forman. J. Appl. Phys. 47 (1976) 5272. R. Forman. Appl. Surface Sci. 2 (1979) 258. R. Forman and G.G. Lesny, Appl. Surface Sci. 14 (1982-1983) 157. R. Forman, Appl. Surface Sci. 17 (1984) 429. R. Forman, Appl. Surface Sci. 24 (1985) 587. R. Forman, Appl. Surface Sci. 25 (1986) 13. A.J.A. van Stratum and P.N. Kuin, J. Appl. Phy,. 42 (1971) 4436. L.R. Fake. IEDM Tech. Digest (1983) 448.
1951
Applied Surface Science 29 (1987) 127-142 North-Holland, Amsterdam
SURFACE COMPLEX Ralph
STUDIES ON THE LOW WORK FUNCTION OF BARIUM ON AN OSMIUM-RUTHENIUM
SURFACE SUBSTRATE
FORMAN
ANALEX
Corporation,
21000 Brookpark
Received
12 November
1986; accepted
Road. Mail-Stop for publication
55-I. Clewlund,
21 February
OH 44135, USA
1987
Barium and oxygen on an osmium-ruthenium (Os/Ru) surface is the surface complex responsible for the excellent electron emission properties of commercial M-type thermionic cathodes. A computerized Auger technique was used to study the surface properties of this surface complex and compare it with results obtained earlier for BaaO-W surfaces, which characterize the properties of commercial barium impregnated tungsten thermionic cathodes. Barium desorption, electron emission and barium, barium oxide evaporation were measured at elevated temperatures for this experimental surface study. Desorption measurements, at 1100 o C, on the barium on osmium-ruthenium substrate show that the energy for desorption was higher than that for the lowest work function Ba-O-W surface (5.4 to 4.8 eV). Oxygen was always present on the sputtered Os/Ru substrate and contributed to the strong bonding of barium to the Os/Ru surface. The barium and barium oxide evaporative products from such a surface at 1100 o C were found to be mainly barium with the Ba/BaO ratio equal to approximately 100.
1. Introduction Ever since their invention early in the 20th Century [l], low temperature electron emitting cathodes with barium as a major constituent have been viewed as a form of “black magic” by their users. Their employment as oxide cathodes in the 1920’s was a major impetus to the early success of the electronic industry. The need for higher current devices less susceptible to poisoning in the 1950-1960 era led to the development of the tungsten-based barium dispenser cathodes [2-51. Although their development has been a boon, to the maturation of the electron tube industry, the persistence of sporadic cathode problems, unexplained failures and lack of a firm scientific understanding of cathode breakdown is still a major burden to the industry. Extensive literature on these problems has been published. Only in recent years, with the development of sophisticated surface analysis equipment, has it been possible to formulate plausible scientific models which can be experimentally verified and understood. The investigation reported in this paper is one of a series [6-111 whose objective is understanding the mechanism of operation of barium impregnated thermionic cathodes. 0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
128
R. Formm
/ Low work function surface complex of Ba on OS - Ru
In earlier research [ll] on the role of oxygen in the Ba-O-W surface complex, which is characteristic of the impregnated tungsten cathode, we found that high temperature studies of thermal desorption of barium and electron emission suggested a physical-chemical model consistent with the experimental data. The same technique was employed in this investigation of the properties of a barium on oxygen on an Os/Ru surface. Electron emission and high temperature barium desorption were measured for a barium monolayer on a sputter-deposited Os/Ru surface, similar to that used on the M-type cathode.
2. Experimental
details
The experimental apparatus used in this study has been described earlier [9-111. It employs a computerized scanning Auger microprobe spectrometer for Auger and desorption studies and a VG Spectralab SX200 mass analyzer for barium and barium oxide evaporation measurements. The two samples employed in these experiments were both tungsten foils, 0.0025 cm thick and approximately 0.13 cm wide, bent into a U-shape with the center portion of the U coated with a 0.5 pm thick sputter deposited Os/Ru layer. The legs of the tungsten U were - 0.8 cm and the center portion was 0.7 cm. The samples were mounted on a carousel which could be rotated, to alternatively face a barium getter source, the quadrapole of a mass spectrometer and/or the Auger spectrometer, as described in ref. [ll]. Optical pyrometry through a vacuum window was used to calibrate the sample temperature as a function of heater current through the tungsten foil. The Os/Ru substrate was identical to that used in conventional M-cathodes, a sputter deposited layer that was nominally 80% OS and 20% Ru. Auger measurements on the virgin “base” sputtered Os/Ru surface showed oxygen was always present on the surface and therefore all these experiments describe our measurements of barium on an oxygenated osmium-ruthenium surface. However, since this sputtered coating is identical to that used in M-cathode fabrication, its usage conforms to the objective of this investigation; namely to employ surface study techniques to better understand the mechanism of operation of an M-type thermionic cathode. The technique employed to generate a barium monolayer on the Os/Ru surface was similar to that employed in ref. [ll]. The experimental sequence was: (a) The Os/Ru surface was cleaned by heating to 1150°C for 3-5 min. (b) Multilayered barium was then deposited on the Os/Ru surface by rotating the sample to expose it to the heated barium source. (c) A barium monolayer was obtained by heating the sample to - 500 o C for 20-30 min.
R. Forman / Low work Junction surface complex of Ba on OS - Ru
129
The limit of 1150°C in process (a) was to minimize any alloying action between osmium and tungsten that could invalidate the study. Van Stratum and Kuin [12] have shown that alloying at 1050”~ brightness for 5000 h eliminated the advantages of the osmium-coated tungsten surface. It was feared that continued operation above 1150 o C could produce a similar effect. However, this procedure had its limitations. In the course of these experiments we found that the desorption energy for barium on the Os/Ru surface was so from the surface high that we could not remove more than - 0.5 monolayers at 1400 K in any reasonable time. This meant that once we put a monolayer of barium on the samples we were never able to completely desorb the barium by this high temperature treatment. Repeated monolayer desorption experiments then consisted of putting a monolayer of barium on a surface that started with - 0.5 monolayers before the new deposition. Another variable beyond our control was the initial surface oxygen concentration which was always present. Whether its appearance on the surface is due to its existence in the tungsten substrate or from the sputtered Os/Ru coating could not be determined. Its presence, however, is probably of similar origin to its occurrence on an M-type cathode surface and therefore does not conflict with our primary objective, namely to obtain insight into the surface chemistry and physics of M-type cathodes. Monolayer desorption at a given temperature was measured in a manner identical to that described in ref. [ll] by means of the computerized scanning Auger microprobe spectrometer in the EN(E) mode. Barium and barium oxide evaporation data were obtained by rotating the sample, having a monolayer of barium on its surface, in front of the mass analyser (distance separation l-2 mm) and heating the sample to a given temperature in a manner similar to that described in ref. [ll].
3. Experimental results and discussion An illustration of the type of Auger signals obtained in these studies is shown in fig. 1. These are Auger scans from 130 to 630 eV in the EN(E) mode (fig. la) and the d(EN)/dE mode (fig. lb). In fig. la peaks a, b, c and d are respectively osmium and tungsten [13], ruthenium, oxygen and barium. Curve 1 refers to a multilayered barium adsorbate on the Os/Ru substrate. Curve 2 is that for a monolayer of barium on the Os/Ru surface and curve 3 is the result of heating the surface described by curve 2 fo 1 h at 1100 o C. The data of fig. la in the EN(E) mode are the most interesting. The multilayered barium curve 1 shows a very prominent barium peak and some oxygen. The osmium-ruthenium substrate peaks are concealed by the multilayer barium. The ordinate scale of curve 1 was reduced by one third, as compared to curves 2 and 3, in order to obtain the complete Auger spectrum
130
b
d
v
I 130
230
330
430
530
, 630
CNA. v
Fig. 1. Auger output multilayered barium the surface described counts proportional d( EN)/d E. E
(b) signals as a function of electron energy over the range of 130-630 eV for: (1) on the Os/Ru substrate; (2) monolayer barium on the Os/Ru substrate: (3) by curve 2 after it was heated to 1400 K for 1 h. The ordinate for (a) is III to EN(E) and the ordinate for (b) is in counts/V in the differentiated mode is the Auger energy and N is the electron density at a given Auger energy.
on the printout when the abscissa scale varied from 130 to 630 eV. A quantitative comparison between the curves would require the barium peak of curve 1 to be increased by a factor of three. The differences between curves 2 and 3 are a major subject of the report. One interesting feature is that the loss of barium from the monolayer surface, from desorption at a high temperature. increases the oxygen and osmium peaks but does not seem to affect the ruthenium peaks. In the differentiated mode, fig. 1 b, the osmium and ruthenium peaks between curves 2 and 3 are indistinguishable but loss of barium by desorption and increases in oxygen are observed. Quantitative Auger data, at a given time and position on the surface, were obtained by measuring the peak minus background counts [ll] from data similar to that of fig. la (e.g., 166. 268, 508 and 583 eV for the peaks and 185.
R. Forman / Low’ work
function
f!ONOLAYER FORMATlOW
surface complex
of Ba on OS-
Ru
131
DESORPTIOM
t 1000
.
* r
0
33
66
99 132 TIME PERIODS
165
198
Fig. 2. The change in surface concentration of barium (0) and oxygen (A) as a function of time when the surface of the Os/Ru substrate, initially covered with a barium multilayer, is heated to 700 K and then 1375 K. Desorption of barium from a monolayer (100-200 time period) was done at 1375 K. One time period corresponds to 30 s.
283, 522 and 612 eV for the backgrounds of OS, Ru, 0 and Ba, respectively). Typical desorption results obtained in the experiments are shown in fig. 2. This figure shows the results for surface barium (circles) and oxygen (triangles) when a multilayer of barium is heated to first form a monolayer (O-99 time periods) and then desorbed at 1375 (100-200 time periods). The 1375 K temperature was obtained by applying a 75 K correction to a brightness temperature reading of 1300 K on the Os/Ru surface. Details of the experimental technique employed to obtain “correct” data for a given element as a
132
R. Forman / Low work functionsurfaceromplerof Ba on OS - Ru
function of time are given in ref. [ll]. In fig. 2, at time zero, the multilayered barium is heated to - 700 K for - 1 h to obtain a stable monolayer of barium on the Os/Ru substrate. The barium count starts at about 2600 (off-scale) and drops to - 750 (a monolayer) in - 25 min. The oxygen count starts at - 200 (small oxygen peak in (d) of fig. la) and increases to - 500 as the monolayer is formed. At time period 100 the temperature of the substrate is increased to 1375 K and rapid barium desorption occurs in a 1 h period. The most interesting phenomenon in fig. 2 is the abrupt increase in oxygen accompanying the rapid desorption.This data is very similar to that observed for barium desorption from a highly oxygenated tungsten surface as reported in ref. [ll] (figs. 4 and 5). The obvious explanation for this effect is that the underlying oxygen is exposed as the barium desorbs. However, the abruptness of the oxygen transition, before the barium is completely desorbed, indicates other phenomena are also present, e.g., high temperature oxygen diffusion from the bulk of the substrate [9,11]. One major problem in the interpretation of the data is the reliability and reproducibility of the results. The primary variable in the experiment is the beam current to the sample. The data in fig. 2 were obtained over a 2 h period. Over that time period any variations in beam current affect the background as well as the peak signals. Under such circumstances the difference count variation which is the ordinate of fig. 2, could be interpreted as a chemical change. To minimize this effect, the electron gun was turned on at least 2 h before any computerized tests were run and the maximum beam current variation, observed before and after the experiment, was in the range of 4%‘. Fig. 3 illustrates the peak and background data obtained for the barium desorption results of fig. 2. The curve A points are peak values (582 eV) and the curve B lower points are background data (613 eV). Initially the multilayer count difference is 2600. As the sample is heated to 700 K (O@lOO time periods) the background signal changes rapidly, by - 2600 counts, until it reaches a maximum. This change in background signal, at an electron energy of 613 eV, is associated with the change in secondary yield on transformation from a barium multilayer to a barium monolayer on the Os/Ru substate. The scatter in the data points are noise in the measurements. The difference values at a monolayer remain quite constant until desorption is started and the drop in this difference count is readily observed after time period 100. The stability of the background data from an abscissa value of 22 to 200 time periods shows the constancy of the beam current. The total variation in counts over this period is 400, which is only a 3% variance. Another important parameter needed to interpret these experiments is the variation of oxygen with barium desorption as the temperature is increased. In fig. 2, the barium count for a monolayer is obtained rapidly and then remains constant until desorption starts; whereas the oxygen count is changing throughout this period and only reaches some constant values ( - 500 counts)
R. Forman / Low work functionsurfacecomplex
of Ba on OS-Ru
133
16 200
. 13 600
13 000
-
I
I 33
66
99
132
165
198
TIME PERIODS
Fig. 3. Barium peak and background count data as a function of time for the barium desorption results of fig. 2. Curve A demonstrates the peak and curve B the background measurements.
is started. Is at a time period of - 90, just before the desorption experiment this plateau in the oxygen data a true phenomenon or would the oxygen count of the sample were maincontinue rising to - 750 even if the temperature tained at 700 K? To answer this question experimental data was obtained from a multilayered barium on Os/Ru sample when it was heated over a 2 h period at 700 K. Fig. 4 illustrates the result and compares it with the data of fig. 2. The barium (circles) and oxygen (triangles) data of the figure 2 desorption experiment are presented in fig. 4 with the additional oxygen results (squares) obtained when the sample was maintained at 700 K for the 2 h period. The barium counts at 700 K for 2 h is not included in fig. 4, although as expected, after an initial change in the O-30 time period, it remained constant (700-750 counts) for the duration of the test indicating monolayer coverage. The results of fig. 4 show that the oxygen plateau, at - 500 counts in fig. 2, is a real one. The changes in surface concentration of the substrate elements osmium and ruthenium, during the 1375 K barium desorption, are illustrated in fig. 5. The barium results (circles), identical to that of fig. 2, are shown in this figure in order to compare it with the data for osmium (squares) and ruthenium (triangles) in the same experiment. Initially, with a multilayer of barium, the osmium and ruthenium counts are negative and outside the ordinate range of the figure. This arises because our technique involves using a peak measure-
0
40
80
120
160
200
240
TIME PERIODS
Fig. 4. Barium
(0)
and oxygen data obtained
1375 K desorption data of fig. 2 compared when the sample is heated to 700 K for 2 h.
(A)
with oxygen
(Cl)
ment at a lower energy than the background measurement and the EN(E) curve of fig. la for multilayered barium has a positive slope between 130 and 300 eV. As the barium desorbs the osmium and ruthenium peaks appear and rapidly increase to a plateau value established at barium monolayer coverage. Starting with the time period 100, when the sample temperature was raised to 1375 K, the barium desorbed rapidly, the osmium count increased and after an initial drop the ruthenium count also increased with time. The initial apparent small drop in ruthenium was not a reproducible result. In this case it may have been due to either a secondary yield slope change in the 250-300 eV range, caused by the barium desorption, or to a displacement of the beam to an adjacent area when the sample expanded upon heating. An interesting feature of figs. 2 and 5 is the fact that even after heating the sample at 1375 K for 1 h, the barium count is reduced by only approximately
R.Forman
/ LAW work
fun&m surfacecomplex
-*
1000
of
Ba on
OS -
Ru
135
. 1 l
900
-
.
800
700
600 2 5 3
500
400
-
300I -
zocI-
lotI -
L 0
33
66
99
132
165
198
TIME PERIODS
Fig. 5. Barium (0), osmium (Cl) and ruthenium (A) data obtained from the desorption of barium from an Os/Ru surface at 1375 K. The barium data is identical to that shown in fig. 2.
one half. This implies that a half monolayer of barium still exists on the surface, which leads to the conclusion that the surface bond is very strong between the barium adsorbate and the underlying oxygen on the Os/Ru surface. This circumstance would correspond to a very high activation energy for desorption whose value an be obtained by using the theoretical relation: E, (nW/kT)
- E, (nV/kT)
= (&T/h)
exp( - E,/kT)
(1)
and a computerized technique employing a least squares fit [9]. E, is the exponential integral, 19is the fractional coverage as a function of time t, k and h are the Boltzmann and Planck constants respectively and T is the surface temperature in K. E, is the activation energy for surface desorption, V is the repulsive energy per pair of adatoms and n the number of nearest neighbor
0
8
16
25
33
41
50
TIME, HIN
Fig. -6. Barium
desorption substrate
at 1375 K from a monolayer of harlum on an osmium-ruthenium (0) and an oxygenated (2 L) tungsten substrate (A).
adatoms. The upper curve (squares) of fig. 6 is a theoretical curve of the barium desorption data at 1375 K from fig. 2. The calculated activation energy for desorption (E,) is equal to 5.4 eV. For comparison, we have included in fig. 6 results for barium desorption from a Ba-O-W surface at 1375 K. These data (triangles), taken with a 2 L (Langmuir units) oxygen exposure of the tungsten surface before barium adsorption, are similar to those described in fig. 8 of ref. [ll] and give an activation energy for desorption of 4.8 eV. Earlier studies [ll], whose motivation was to understand the basic properties of impregnated tungsten cathodes, have shown that the 2 L oxygen exposure, described by the data of fig. 6, provides the most stable barium adsorbate bonding for a Ba-O-W surface complex. The even higher stability of the Ba-0-Os/Ru surface bond, as shown in fig. 6, is obviously one of the major reasons for the better performance of the M-type cathode as compared to the normal impregnated tungsten cathode. Another interesting feature of the data of figs. 2 and 5 is the information one an infer about the structure of the barium-oxygen-substrate bond. The desorption curves of fig. 2 can be interpreted as showing that the barium
R. Forman
/ Low work function
surface complex
of Ba on OS ~ Ru
137
r 1.5
.5
0
30
60 TIME,
Fig. 1. Evaporation osmium-ruthenium scale are arbitrary
90 SEC
120
150
of barium and barium oxide from a monolayer of barium deposited on an surface, heated to 1375 K, as a function of time. The units for the ordinate and are based on the original calibration of the instrument using molecular nitrogen (28 AMU) for a calibration standard.
overlies the oxygen; the loss of barium is accompanied by a sharp increase in the surface oxygen concentration. This type of Ba-O-substrate surface bond is also characteristic of the Ba-O-W surface bond inherent in the tungsten impregnated cathode [ll]. The major difference between the two is the stronger Ba-0-Os/Ru surface bond with a resultant stronger dipole moment and lower work function. Another aspect of the stability of the Ba-0-Os/Ru surface bond can be studied by analysing the barium evaporation product from a desorption experiment, of the type illustrated in fig. 2. By means of mass spectrometry and the use of a technique described earlier [ll], we were able to determine the ratio of barium to barium oxide desorbed at 1375 K. The data, shown in fig. 7, were obtained by monitoring the atomic mass units (AMU) 138 (Ba) and 154 (BaO) as a function of desorption time. It should be noted that the ordinate scale for the barium pressure is 100 X that for barium oxide and the ratio of the Ba/BaO peak is 120.
I 0
30
60
90
120
150
180
210
240
I
270
TIME, FUN
Fig. 8. Comparison of the electron emission and desorption at 1375 K from both a monolayer of barium on a Os/Ru substrate and a monolayer of barium on an oxygenated tungsten (2 L) surface. The desorption data for the Os/Ru substrate (0) and the oxygenated tungsten substrate (A) are similar to the data in fig. 6 and the electron emission curves are normalized to show the decrease in emission as a function of time, at 1375 K. from the initial electron current values.
Another important feature of this study is correlating the surface desorption phenomena with electron emission from the surface. This is illustrated in fig. 8 which compares the desorption data of fig. 6 with the electron emission obtained from the respective Ba-0-Os/Ru and Ba-0-W (2 L oxygen) surfaces at 1375 K with time. The electron current data of fig. 8 was obtained in the following manner for both surfaces. The multilayered barium samples were heated to - 700 K for 30 min to obtain a monolayer and then cooled down before they were placed in front of an anode structure. At time zero the temperature of the samples was raised to 1375 K and the current to the anode monitored with time. The electron emission curves are normalized by taking the ratio of the current observed at any given time to the initial current measured when the sample reached 1375 K. The electron emission data correlates with the barium coverage data: the barium coverage for the Os/Ru surface stabilizes at - 0.5 and so does its emission current with a similar correspondence for the Ba-O-W surface. The fact that the electron emission
R. Forman
/ Low work function
surface complex
of Ba on OS - Ru
139
curve reaches a stable value after a much longer interval than the barium coverage curve can be attributed to features peculiar to each individual experiment. The desorption measurements are made at a point in the center of the sample which reaches the maximum temperature at the earliest time and then remains relatively stable. The electron emission, however, comes from the whole surface of the sample exposed to the anode and this surface temperature is not uniform. At any point, the temperature is strongly dependent on its proximity to the relatively heavy current leads used to support the U-shaped sample. From the electron emission data, it appears that a stable temperature fact distribution is only reached after - 3-4 h. Again the most significant obtained from the data of fig. 8 is confirmation of the higher stability of the
600
l
t
d
I 0
33
I
I 66
99
132
165
198
TIME PERIODS Fig. 9. Barium desorption at 1375 K from a sulphur-contaminated region of the Os/Ru substrate (0) and from an adjacent sulfur-free area of the same Os/Ru substrate sample (A). The surface concentration of oxygen (0) is shown as a function of time as the barium (A) desorbs from the sulphur-free region.
140
R. Forman / Low work functm
surface complex of Bo on OS - Ru
M-type cathode surface (Os/Ru) as compared to the impregnated tungsten cathode surface. As discussed earlier two Os/Ru substrate samples were tested in this study. Inadvertently the second provided us with the opportunity to study cathode poisoning. There were sulphur-contaminated as well as sulphur-free areas on its surface. Wherever surface sulphur was present, oxygen was absent and vice-versa. This enabled us to study cathode poisoning from sulphur contamination and compare the results with similar experiments performed on adjacent sulphur-free areas. The major parameter examined was barium desorption which is illustrated in the data of fig. 9. The (0) results are desorption data at 1375 K from the sulphur-contaminated region. For this run the transition from barium multilayered to monolayer on the sulphur-contaminated surface is not shown. Initially 20 data points of monolayer coverage are shown before the temperature of the sample is raised from 700 to 1375 K and rapid barium desorption occurs. Barium desorption from a sulphur-free region on the same sample are shown by the (A) points in fig. 9. This experiment was initiated at the multilayer stage for 5 time periods. then its
0
L
I 8
16
25
33
111
50
TIME. PUN
Fig. 10. Barium desorption at 1375 K from a monolayer of barium on a sulfur-contaminated region (0) of the Os/Ru substrate and on a sulphur-free (A) area of the Os/Ru substrate.
R. Formnn / Low work function surface complex of Ba on OS- Ru
141
temperature was raised to 700 K for 50 min before it was raised to 137.5 K at 100 time periods. Also included in fig. 9 are the data for surface oxygen (squares) on the sulphur-free region during the desorption run. Similar to the earlier results (fig. 2) the oxygen concentration abruptly increased as the barium initially desorbed at 1375 K. The major difference between the sulphur-free data of fig. 9 and that of fig. 2 is that the former results indicate a lower surface oxygen concentration than the latter. In the sulphur-contaminated area no oxygen was observed and when barium (circles) desorbed the surface sulphur concentration rose in a manner similar to what one would expect from a barium on sulphur on Os/Ru surface complex; the barium desorption uncovered the underlying sulphur. It is obvious from the data of fig. 9 that the barium-sulphur-Os/Ru surface bond is much weaker than the barium-oxygen-Os/Ru surface bond. At 1375 K practically all the barium desorbs from the sulphur-contaminated surface during the 50 min desorption experiment. To determine the quantitative difference between the two surface bonds, we plotted the data of fig. 9 in a manner similar to fig. 6, using the same computerized techniques. The results are illustrated in fig. 10. The energies of desorption for the two cases are obtained in a similar manner to that for fig. 6 by using eq. (1) and the computerized technique described in ref. [9]. The value of E, for the sulphurfree region is 5.4 eV and is 4.8 eV for the sulphur-contaminated area.
4. Conclusions Experiments for studying the chemical and physical surface properties of barium as an adsorbate on an osmium-ruthenium substrate have been carried out. It is part of a continuing investigation aimed at understanding the mechanism of operation of barium impregnated thermionic cathodes. The samples studied were simulated M-type cathodes and this program’s objective was to study their chemical and physical surface properties and compare the results with those obtained earlier for barium on oxygen on tungsten surfaces, which simulate the behavior of the impregnated tungsten thermionic cathode. The desorption energy of barium monolayers for an osmium-ruthenium surface was measured at 1375 K as 5.4 eV. Similar measurements on the most stable barium on oxygen on tungsten surface showed that its desorption energy was - 4.7 eV. The higher desorption energy for the barium on osmium-ruthenium substrate as compared to the barium on tungsten substrate is consistent with the fact that the M-cathode has a lower work function and is a better electron emitter than the conventional tungsten impregnated cathode. The stronger surface bond of barium to the Os/Ru substrate leads to a higher dipole moment and a lower work function than the barium to tungsten substrate. Measurements of barium and barium oxide evaporation
from the barium on Os/Ru substrate at elevated that the evaporation products are predominantly to barium oxide evaporant is - 100.
temperatures (1375 K) show barium. The ratio of barium
Acknowledgements
The program was sponsored and supported by the Space Communications Division of NASA, Lewis Research Center. The author would also like to acknowledge and thank G. Lesny for his excellent technical support on this program.
References [l] 12) (31 [4] [5] [6] [7] [X] [9] [lo] [ll] [12] [13]
A. Wehnelt, Ann. Phys. 19 (1906) 138. R.C. Hughes, P.P. Coppola and ES. Rittner, US Patent 270011X. November R. Levi and R.C. Hughes, US Patent 2700000. February 1952. R. Levi, J. Appl. Phys. 26 (1955) 639. P. Zalm and A.J.A. van Stratum. Phillps Tech. Rev. 27 (1966) 69. R. Forman. J. Appl. Phys. 47 (1976) 5272. R. Forman. Appl. Surface Sci. 2 (1979) 258. R. Forman and G.G. Lesny, Appl. Surface Sci. 14 (1982-1983) 157. R. Forman, Appl. Surface Sci. 17 (1984) 429. R. Forman, Appl. Surface Sci. 24 (1985) 587. R. Forman, Appl. Surface Sci. 25 (1986) 13. A.J.A. van Stratum and P.N. Kuin, J. Appl. Phy,. 42 (1971) 4436. L.R. Fake. IEDM Tech. Digest (1983) 448.
1951