Determination of barium and calcium evaporation rates from impregnated tungsten dispenser cathodes

Applications of Surface Science 16 (1983) 25-39 North-Holland Publishing Company

DETERMINATION OF BARIUM AND CALCIUM EVAPORATION RATES FROM IMPREGNATED TUNGSTEN DISPENSER CATHODES G.L. JONES

25

*

a n d J.T. G R A N T

Research Institute, University of Dayton, Dayton, Ohio 45469, USA Received 28 June 1982; accepted for publication 11 October 1982

The evaporation rates of barium and calcium from impregnated tungsten dispenser cathodes have been determined by both a vapor-collect method and line-of-sight mass spectrometry. Cathodes having molar ratios of BaO, CaO, and AI203 of 4:1 : 1, 5:3:2, and 1 : 1 : 1 have been studied. All measurements were conducted in ultrahigh vacuum. For the vapor-collect method, a W(110) collector was found to be suitable for monolayer growth. The procedure involved plotting the ratio of the adsorbate Auger peak-to-peak height to that from the collector as a function of collection time. Break-points in these plots characterized the collection of one monolayer of adsorbate. A geometrical correction then allowed the evaporation rates to be determined. Evaporation rates were determined at cathode temperatures of 1050, 1100, and 1150°C. For the line-of-sight mass spectrometry an Extranuclear quadrupole was used. The quadrupole was capable of measuring species up to 300 amu. Measurements with the quadrupole were made on a 5 : 3 : 2 and a 4 : 1 : 1 cathode. Results obtained using these two methods are compared.

1. Introduction C a t h o d e s a r e t h e sole a c t i v e e l e m e n t s i n t r a v e l i n g w a v e t u b e s ( T W T ) a n d are, t h e r e f o r e , t h e life l i m i t i n g c o m p o n e n t s i n t h e s e t u b e s . L i t t l e is k n o w n about the basic materials and processes that affect cathode performance and l i f e t i m e . M o s t c a t h o d e s u s e b a r i u m o x i d e as t h e a c t i v e i n g r e d i e n t a n d t h i s paper describes studies that have been made on a number of commercial cathodes to determine the evaporation rate of barium at different operating temperatures. The evaporation rate of calcium from the impregnant has also b e e n m e a s u r e d . E v a p o r a t i o n r a t e is o n e p a r a m e t e r t h a t will a f f e c t l i f e t i m e . Measurements were made on impregnated tungsten dispenser cathodes which a r e o f t h e t y p e p l a n n e d t o b e u s e d i n s o m e s a t e l l i t e s i n t h e l a t e 1980's. T w o methods were used to determine evaporation rates, namely, a vapor-collect method and line-of-sight mass spectrometry. * This work was sponsored by the Air Force Wright Aeronautical Laboratories, Materials Laboratory, Air Force Systems Command, United States Air Force, Wright-Patterson Air Force Base, Ohio 45433, USA. 0378-5963/83/0000-0000/$03.00

© 1983 N o r t h - H o l l a n d

26

G.L Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

2. Experimental Impregnated tungsten dispenser cathodes having B a O : C a O : A 1 2 0 3 molar concentrations o f 4 : 1 : 1, 5 : 3 : 2 , and 1 : 1:1 were purchased from Spectra-Mat, Inc. a n d / o r Semicon Associates, Inc. The diameters of the emitting surfaces were 0.134 + 0.001 inch for the Spectra-Mat cathodes and 0.126 + 0.001 inch for the Semicon cathodes. Measurements were performed in the ultra high vacuum chamber of a Physical Electronics Model 545 Scanning Auger Microprobe. The base pressure was about 3 x 10 -8 Pa. Cathodes were activated (with no current draw) by raising the temperature slowly to 1190°C, holding it at this temperature for 5 min, and then holding it at an equilibrating temperature of 1150°C for 4 h. This procedure cleans the surface of any excess impregnant. Cathode temperature was measured using a P t / 6 % R h versus P t / 3 0 % R h thermocouple which was spot welded onto the cathode body about 2 m m back from the emitting surface.

2.1. Vapor-collect method The vapor-collect method used in this work is based on that developed by Verhoeven and Van Doveren which uses Auger electron spectroscopy to determine the number and identity of particles condensed onto a suitable collector in front of the evaporating surface [1]. Whereas Verhoeven and Van Doveren used the attenuation of Auger electrons from the collector to determine the amount of material collected, attempts were made in this work to determine this amount by detecting the point of monolayer coverage on the collector by the method of break-points. This method is valid for deposits which grow in a layer fashion and break-points can usually be seen in plots of the Auger signal from the condensate or the collector against time [2]. Three common growth mechanisms are illustrated in fig. 1 - the break-point method to determine first monolayer coverage can be used if either Stranski-Krastanov or F r a n k - V a n der Merwe growth occurs. For the vapor-collect method the collector was mounted on a manipulator and could be rotated in front of the cathode for collection and then in front of the Auger spectrometer for analysis. Three collectors were tried, namely, tungsten and tantalum foils and a W(110) crystal. The foils were cleaned by electron bombardment heating and the crystal by resistive heating. Barium was readily desorbed following each run by heating. A stainless steel heat shield with an aperture of 3.2 m m diameter was mounted 11 m m in front of the cathode. At a distance of 3 m m significant heating of the collector occurred; and this was found to affect the results. The collector was positioned 1 m m from the heat shield. The deposited region was readily identified in the specimen current imaging mode of the scanning Auger microprobe. Auger analysis was performed at the center of the deposit using a 0.1-0.2/~A, 5 keV

G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

27

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electron beam that was rastered over an area of 0.3 mm 2. Rastering was required to eliminate electron beam effects [3,4]. Data were recorded in the first derivative mode using 10 kHz sinusoidal modulation on the analyzer with peak-to-peak values of 3 or 6 eV.

2.2. Line-of-sight mass spectrometry With this method, the species leaving a cathode were identified from their mass-to-charge ratios, and their intensities were measured as a function of cathode temperature. For this work, a cathode was mounted on the manipulator and could be rotated in front of the Auger spectrometer for surface analysis or in front of an Extranuclear Model 162-8 quadrupole for mass analysis. The quadrupole was tuned for an m / e range of 3-200, and was operated in the analog mode using an Extranuclear Model 031-1 preamplifier-electrometer.

28

G.L. Jones, J. 7". Grant / Determination of Ba and Ca evaporation rates

The experimental conditions for Auger analysis were the same as used in the vapor-collect studies. An attempt to independently calibrate the Ba evaporation rates using the quadrupole was made by measuring the signal from a known pressure of xenon gas, which has approximately the same atomic mass as barium. Matching the m / e ratios is necessary as the transmission of the quadrupole varies with m / e . N o measurements were made on Ca evaporation rates with the quadrupole due to the overlap of the main Ca + peak with Ar + from the background gas in the vacuum chamber.

3. Results and discussion

3.1. Vapor-collect method N o break-points were observed in the barium Auger data when either the tungsten or tantalum foil was used as the collector, the growth corresponding to the Volmer-Weber type (see fig. 1). No tungsten products from the cathode were observed when the tantalum foil was used as the collector. Break-points were observed for monolayer coverage when the tungsten crystal was used for the collector. For convenience, plots of the ratios of the barium M N N to tungsten N N N Auger peak-to-peak heights were made as a function of deposition time, rather than barium only, to eliminate any variations in the electron beam current between runs. The tungsten Auger peaks may also include some N N O and N N V transitions [5]. For the 1 : 1 : 1 cathode the sum of the barium M N N and calcium L M M Auger signals was used. Typical Auger spectra of the collector following depositions at 1150°C for 20 min from 4 : 1 : 1, 5 : 3 : 2, and 1 : 1 : 1 cathodes are shown in fig. 2. Note that the barium to calcium ratio on the collector decreases with decreasing barium to calcium molar ratio of the impregnant.

3.1.1. The 4 : 1 : 1 a n d 5 : 3 : 2 cathodes Under the experimental conditions used here the Auger sensitivity factor of calcium is about three times that of barium [6], so for the 4 : 1 : 1 and 5 : 3 : 2 cathodes the amount of calcium collected is considerably less than the amount of barium. No significant changes in the relative barium to calcium concentrations on the collector occurred for different collection times. These relative concentrations are listed in table 1 and were calculated using the elemental relative sensitivity factors from ref. [6]. These concentrations are quite reasonable, the results from the 4 : 1 : 1 cathodes comparing quite favorably with spectroscopic analysis of the evaporated products from a 6 : 1 : 2 cathode by Brodie and Jenkins who found about 10% calcium [7]. The standard deviation in the data was 4 at%, indicating that there were no significant changes in the

G.L. Jones, J.T. Grant

29

/ Determination of Ba and Ca evaporation rates (a)

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G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

30

Table 1 R e l a t i v e c o n c e n t r a t i o n s of calcium and b a r i u m present on W(110) collector for several activated c a t h o d e s o p e r a t i n g at 1050, 1100, and 1150°C; c o n c e n t r a t i o n s are in at% Cathode

1050°C

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l IO0°C

1150°C

Calcium

Barium

Calcium

Barium

Calcium

Barium

9 16 20 24 26 96

91 84 80 76 74 4

10 12 23 26 23 87

90 88 77 74 77 13

11 19 28 27 79

89 81 72 73 21

may be somewhat larger than the values listed in table 1 as the calcium and barium are oxidized to various extents. This is because the relative sensitivity factor for calcium in calcium oxide is smaller than for its elemental form, but the relative sensitivity factor for barium is essentially the same in both forms

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G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

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Fig. 4. Ratio of barium MNN to tungsten NNN Auger peak-to-peak heights as a function of collection time, for several cathodes operating at 1100°C. For the Spectra-Mat 5 : 3 : 2 cathodes the line shown is the best fit for the erect triangles. Plots of the ratios of barium to tungsten Auger peak-to-peak heights I B a / I w with collection time are shown in figs. 3 through 5 for cathode temperatures of 1050, 1100, and 1150°C, respectively. The results from four cathodes are shown, namely, a Spectra-Mat 4 : 1 : 1, a Semicon 5 : 3 : 2, and two Spectra-Mat 5 : 3 : 2 cathodes. Note the difference between the two makes of the 5 : 3 : 2 cathodes. The lines drawn in the figures were obtained using a linear regression fit to the data. Break-points could be identified in all cases at an I a a / I w ratio of about 1. It can be seen that the evaporation rate of barium is low and that the measurements are very time consuming, with most collection times being more than 1 h. For cathode temperatures of 1050°C, collection times of more than one day were sometimes required. Due to the rather long collection times required, the barium and calcium will be oxidized to various degrees and it will be assumed that at the break-points a monolayer composed of barium and calcium exists, and that the oxygen is on top of this monolayer. This assumption would tend to give upper limits for the evaporation rates as some oxygen will be incorporated in the monolayer both from adsorption from the background gases in the vacuum chamber and due to some barium (and possibly calcium) oxide evaporation from the cathodes (see section 3.2).

G.L Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

32

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The barium and calcium evaporation rates have been calculated from the break-points in figs. 3 through 5, using the compositions in table l, and assuming that a monolayer is c o m p o s e d of 5.3 × 1014 a t o m s / c m 2. The geometrical factor [ 1] to allow for the c a t h o d e - c o l l e c t o r g e o m e t r y used in this work was 0.01. It is also assumed that the barium and calcium sticking coefficients on tungsten at r o o m temperature are unity, which is a reasonable assumption [1]. The results are listed in table 2. The barium and calcium evaporation rates for the two Spectra-Mat 5 : 3 : 2 cathodes are within a factor of about 2, whereas the rates from the two Spectra-Mat 4 : 1 : 1 cathodes differed by a factor of about 10 at 1050°C but only a factor of about 2 at l l 0 0 ° C . The Spectra-Mat cathodes were from the same lot, but the one that gave unusually high evaporation rates at 1050°C also showed a large variation in evaporation rates during the equilibrating period of the activation procedure. The evaporation rates obtained from the 5 : 3 : 2 cathodes compare favorably with those obtained by Brodie and Jenkins who used a thermionic emission m e t h o d to measure the evaporation rate [7]. At 1100°C their evaporation rate (barium plus calcium) for a 5 : 3 : 2 cathode was 1.1 x 1012 atoms cm -2 s - i . The inclusion of calcium oxide in the impregnant is k n o w n to reduce the evaporation rate of barium [7], and it can be seen from table 2 that the

G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

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G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

Table 3 Heats of evaporation of calcium and barium from several activated cathodes; units are k c a l / m o l e Cathode

Calcium

Barium

Spectra-Mat 4:1 : 1 Spectra-Mat 4:1 : 1 Semicon 5 : 3 : 2 Spectra-Mat 5 "3 : 2 Semicon 1 : 1 : 1

90 89 63 57 81

81 47 59 67 153

evaporation rates of barium are lower from the 5 : 3 : 2 than from the 4 : 1 : 1 cathodes. These types of cathodes normally operate in the range 1025-1125°C. Based on the amount of impregnant used in the Semicon 5 : 3 : 2 cathode (6.2 mg), it would last for a period of seven years at a barium evaporation rate of 1 × 1012 atoms c m - 2 s-1, assuming the evaporation rate does not decrease with time and that all the Ba is available for evaporation. For satellite applications, traveling wave tubes should have a mean time between failure of 7 to l0 years, but all the barium will not be available for evaporation as such tubes are no longer useful when the electron emission drops by 5-10%. The equilibrium vapor pressures of barium and calcium were calculated from their evaporation rates and these pressures were then plotted against reciprocal temperatures to give the heats of evaporation [1]. The results are listed in table 3. For the 4 : 1 : 1 and 5 : 3 : 2 cathodes the heats are higher than for the pure metals and are similar to the activation heat of 78 kcal/mole determining the lifetime of a 5 : 3 : 2 cathode [8], but somewhat smaller than other work on the heat of evaporation for 5 : 3 : 2 cathodes, namely 104 kcal/mole [7]. The value of 78 kcal/mole found by Rittner for barium was found to be consistent with the following reaction for activation [8]: 5BaO- 3CaO. 2A120 3 + ~ W = 3 Ca2BaWO6 + ¼ Ca2BaA1206 + -54BaA1204 + 9 Ba. 3.1.2. The 1 : 1 : 1 cathode Only one 1 : 1 : 1 cathode was examined in this work and, contrary to the behavior of the 4 : 1 : 1 and 5 : 3 : 2 cathodes, more calcium than barium evaporated over the temperature range studied, namely 1050-1150°C. The fraction of barium collected increased with increasing temperature from 4 at% at 1050°C to 21 at% at 1150°C as shown in table 1. Plots of the ratios of the sum of the barium and calcium, to tungsten, Auger peak-to-peak heights (IBa + l c ~ ) / I w with collection time for cathode temperatures of 1050, 1100, and 1150°C are plotted in fig. 6. The calcium and barium evaporation rates are

G.L. Jones, J.T. Grant

/ Determination of Ba and Ca evaporation rates

35

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given in table 2, and the heats of evaporation in table 3. The barium heat of evaporation is substantially higher than for the other cathodes and could indicate that a different mechanism is involved in barium formation. Unfortunately, only one 1 : 1 : 1 cathode was studied, and as there is some variation between the calculated heats for the same types of cathodes, no definite conclusion about the mechanism can be drawn. 3.2. Line-of-sight mass spectrometry 3.2.1. The 5 : 3 : 2 cathode Auger measurements were made on a Spectra-Mat 5 : 3 : 2 cathode during activation. Before heating, barium, calcium, oxygen, and tungsten were all observed, together with carbon and nitrogen (fig. 7a). Some aluminum and silicon may also have been present. As the cathode was heated, the carbon decreased and some sulfur appeared, the spectrum obtained after heating to 800°C being shown in fig. 7b. On further heating, the carbon, nitrogen, and sulfur decreased and were essentially gone at 1000°C, fig. 7c. At 1100°C, the calcium had decreased significantly and at 1150°C was not readily detected,

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G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

37

Ba

z_

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MASS/CHARGE RATIO 50 Fig. 8. Mass spectrum of species emitted by a Spectra-Mat 5 : 3 : 2 cathode over an m / e range of 130-160. The cathode had been activated and was operating at ll50°C. Some peaks due to residual Xe + in the vacuum system are also present.

fig. 7d. N o Ba ÷ or BaO ÷ ions were detected using line-of-sight mass spectrometry with the ionizer off, in agreement with other work on a similar cathode [9], although some N a + ions were observed. After activation, mass spectra of species emitted by the cathode were recorded, a typical spectrum in the B a + - B a O + range being shown in fig. 8. Some small peaks due to Xe + at 131, 132, 134, and 136 also appear in this spectrum due to residual Xe gas in the vacuum chamber from earlier sputtering work. For the Spectra-Mat 5 : 3 : 2 cathode, calibration with xenon was not made but the relative Ba + evaporation rates at 1050, l l00, and l lS0°C were measured using the quadrupole and were in the ratio 0.22:0.54: 1.00, which compares favorably with the ratio of barium obtained from a different 5 : 3 : 2 cathode using the vapor-collect method, namely 0.22 : 0.43 : 1.00. 3.2.2. The 4 : 1 : 1 cathode Calibration studies were attempted using a Spectra-Mat 4 : 1 : 1 cathode. Xenon gas was chosen as a calibration source as it has isotopes near barium, is readily available in high purity form and because the ionization gauge factor for it is known. The evaporation rate of barium (atomic weight M in g) from a cathode at temperature T (K) is related to the barium equilibrium vapor pressure p* (Pa) by h = 2.64

×

102°p*M-I/ET -1/2 atoms cm -2 S - 1 ,

(1)

assuming that the evaporation coefficient is unity [1]. The pressure p* can be related to the pressure of barium PBa (Pa) in the ionizing chamber of the quadrupole at a distance r from the cathode by P* = ~rr2A -lpBa,

(2)

38

G.L. Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

where A is the area of the cathode surface [10]. Combining eqs. (1) and (2) and substituting for the known values gives h = 5.05 x 1021T - l/2pB a. The pressure of barium PBa was determined by comparing the Ba ÷ quadrupole signal with that of Xe ÷ at a known pressure of xenon and correcting for their different first ionization cross sections. The first ionization cross sections of barium and xenon were calculated using a semiclassical method [11] and are 8.1 x 10-16 and 5.1 x 10-16 c m 2, respectively for an ionizing electron beam of energy 100 eV. The Ba ÷ and Xe + quadrupole signals were taken as the peak heights of the 138 and 136 isotopes, respectively, and corrected for the relative abundances of these isotopes [12]. The Xe ÷ spectrum was recorded at a partial pressure of 3.1 x 10 -7 Pa using an ionization gauge sensitivity factor of 2.72 relative to nitrogen. N o Ba + peaks were present when the Xe ÷ reference spectrum was recorded. The electron multiplier detection efficiency varies as the inverse square root of the ion mass [12], so no correction for this effect was needed. Using the above procedure, the Ba + evaporation rates from the 4 : 1 : 1 cathode were found to be 8.5 X 1011, 3.3 X 1012, and 8.4 x 1012 atoms c m - 2 s - 1 at 1050, 1100, and 1150°C, respectively. These values are less than the total barium evaporation rates found for the 4 : 1 : 1 cathodes by the vapor-collect method, but the ratios of the evaporation rates are similar. The heat of evaporation for Ba was calculated to be 87 k c a l / m o l e and compares favorably with one of the values obtained for barium by the vapor-collect method, namely 81 kcal/mole. The other 4 : 1 : 1 cathode studied by the vapor-collect method had unusually high barium evaporation rates, and a relatively low heat of activation (47 kcal/mole). It can be seen from fig. 8 that a significant amount of BaO also evaporates from the surface of a 5 : 3 : 2 cathode at 1150°C. A similar spectrum was obtained from the 4 : 1 : 1 cathode at 1150°C. This means that the evaporation rates for barium (Ba + BaO) using the vapor-collect method would be expected to be higher, as observed, than the evaporation rate for Ba found using the quadrupole; unfortunately, the BaO evaporation rate could not be calibrated. The discrepancy in the rates of barium evaporation obtained using the quadrupole may also be due to the smaller efficiency for ionization in the case of a beam compared with that for a static gas.

4. Conclusions The barium evaporation rate has been determined for a number of impregnated tungsten dispenser cathodes at 1050, 1100, and 1150°C using two methods, namely, a vapor-collect method and line-of-sight mass spectrometry.

G.L Jones, J.T. Grant / Determination of Ba and Ca evaporation rates

39

The geometry of the vapor-collect method used in this work was very time consuming, taking about one month to complete the measurements on each cathode, whereas the mass spectrometry method was relatively rapid with the measurements taking a few days. However, the vapor-collect method was found to be quantitative as break-points representing monolayer coverage on the collector could be readily determined. The time to acquire data using the vapor-collect method could be reduced significantly by cooling the collector and moving it closer to the cathode. The diameter of the aperture could also be reduced to minimize heating of the collector. An independent method to calibrate the evaporation rate using a mass spectrometer was moderately successful considering the assumptions involved, and allowing for the fact that BaO evaporation rates were not included. Variations in evaporation rates between cathodes having the same molar concentrations of impregnant were noted. The two techniques used are also somewhat complementary in that the vapor-collect method includes all barium species in the measurements whereas the mass spectrometer allows the individual species to be resolved.

Acknowledgments We would like to thank M.F. Koenig for valuable discussions and L. Grazulis and M. Laufersweiler for their excellent technical support.

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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