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Properties and manufacture of top-layer scandate cathodes
173
Applied Surface Science 26 (1986) 173-195 North-Holland, Amsterdam
PROPERTIES AND MANUFACTURE SCANDATE CATHODES
OF TOP-LAYER
J. HASKER, J. VAN ESDONK and J.E. CROMBEEN Philips Research Laboratories, Eindhoven, The Netherlandr
Received 1 October 1985; accepted for publication 9 April 1986
The life and/or the staying power against ion bombardment of scandate cathodes can be improved by using a top layer of W + Sc,Os or W + W/ScH, on a tungsten matrix. The latter is impregnated with the usual 4-l-l impregnant. Even at high voltage pulses the current densities are so high that the deviation from space charge limitation is small. The manufacture is discussed and the cathode life at the operating temperature of 1220 K is shown to be very long. Moreover, the relationship between processing parameters and emission recovery after ion bombardment is examined with the aid of combined sputter- and scanning-Auger measurements. It is shown that thin-layer coverage of tungsten by scandia is important to the high emission. This coverage is related to the impregnation process. After extended sputtering it cannot be completely recovered by reactivation. Consequently, the top layer cathodes cannot withstand sufficiently the usual processing and operation of television display tubes. On the other hand, they can improve the life and performance of electron devices with good vacuum and/or relatively low accelerating potentials. Moreover, activated top-layer scandate cathodes are relatively insensitive to exposure to (moist) air.
1. Introduction
Until about 5 years ago there were two types of scandate cathodes: the pressed scandate cathode, which is made by pressing and sintering a mixture of W and Ba,Sc,O,, and the impregnated scandate cathode, which is a usual impregnated cathode with scandium compounds in the impregnant. Some properties of these cathodes and our considerations for making a new type of scandate cathode - the top-layer scandate cathode - have already been discussed before [l]. A schematic representation is shown in fig. 1. This cathode should have a longer life than the pressed scandate cathode and a better recovery after ion bombardment than the impregnated scandate cathode. It is noted that the latter shows no recovery after bombardment [2]. Fig. 2 indicates the justification for our interest in scandate cathodes. It shows the emission capability of a top layer cathode in comparison with that of an OS-Ru coated impregnated cathode, the so-called M cathode. The latter can be considered as the best and most stable impregnated cathode till now. 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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J. Hasker et al. / Properties and manufacture of scandate cathodes
impregnant Fig. 1. Schematic representation of a top-layer scandate cathode. The plug consists of about 0.4 mm porous W with a porous top layer of about 0.1 mm. The plug is impregnated with the usual 4-l-l impregnant without additives.
Papers by Yamamoto and coworkers [3,4] show that the interest in scandate cathodes is not confined to our laboratory. They describe an impregnated cathode with a mixed matrix of W and Sc,O, and mention an emission anomaly at low applied voltages (in a diode) as a disadvantage which is ascribed to nonuniformly distributed low work function patches [4,5]. In the course of our investigations it became clear that a mixture of relatively big grains of W and Sc,,O, may not be the most favorable geometry. From the viewpoint of homogeneity of emission and recovery after ion
jl A/cm2)
1
V”2, V In volts Fig. 2. Current density versus square root of anode voltage for both a top-layer scandate cathode and an OS-Ru coated cathode in diode configuration with a distance of about 0.3 mm between cathode and anode.
J. Hasker et al. / Properties and manufacture of scandate cathodes
175
bombardment a top layer consisting of W grains which are partially covered with an SqO, layer of about 0.5 pm will do better. Other options are of course layers deposited by sputtering, evaporation or chemical vapor deposition. In the present paper we shall confine ourselves to the geometry of fig. 1 and its closest relative, i.e. a top layer consisting of tungsten which is partially covered with S%O,. In section 2.1 the diode characteristics, as already shown in fig. 2, are considered in more detail, together with the characterization of emission capability and a life test result. Our top layer cathodes do not show an emission anomaly at low applied voltages. In section 2.2 some remarks are made with respect to surface composition and emission mechanism. In section 3 the manufacture of both types of top layer cathodes and some inherent problems are considered in connection with emission experiments. It is shown that top layer cathodes may be expected to live about twice as long as the mixed matrix cathodes. Section 4 deals with the recovery of emission after ion bombardment in relation to processing parameters as, for instance, the temperature during sintering. Also considered is the influence of exposure of an activated cathode to air. In section 5 some results of investigations with the aid of scanning Auger microscopy (SAM) are presented and discussed. Both the influence of ion sputtering and the relationship between surface composition and processing (section 3) are considered. Finally, some conclusions are summarized in section 6. If, in combination with the usual processing, top layer cathodes are to be used in television display tubes some further improvement will be necessary. However, in devices with good vacuum they can substantially improve the performance.
2. Emission and surface properties 2.1. Diode characteristics
and emission characterization
All experiments were performed on indirectly heated planar cathodes of 1.8 mm diameter. All cathode temperatures in the following are MO brightness temperatures (after correction for absorption through the viewing window) measured on the sleeve just below the plug. At relatively low emitted current density the heating power is about 2 W. It will be clear that the characteristics in fig. 2 were measured in single-pulse operation (5 ps pulse length): at a continuous load of 100 A/cm2 the emission cooling would be of the same order of magnitude as the heating power. Let us now consider these characteristics in some more detail.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
“space charge”
I “saturation”
(a)
lb) Fig. 3. (a) Schematic voltage). (b) Schematic
representation representation
of the diode characteristic of the potential distribution
(current density versus within the diode.
diode
The transition between “space charge” and “saturation” region of the characteristic (see fig. 3) is given by zero Poisson field strength at the cathode. j, has The corresponding current density is j,. A method for determining been described before [6]. Moreover, it has been shown that for current densities greater than j, space charge remains important up to the greatest values of V used in practice. This is one of the reasons why - in particular for scandate cathodes - the usual Schottky extrapolation as applied in ref. [3] delivers erroneous values for j, [6]. For both OS-Ru and scandate cathodes the saturation current density distribution over the surface is inhomogeneous. Nevertheless, for j rj, these inhomogeneities are smoothed by the space charge in front of the cathode and j can be calculated accurately with the aid of Langmuir’s theory for the ideal space-charge limited diode. Hence, for j V, (V, being the anode
J. Hasker et al. / Properties and manufacture of scandate cathodes
171
anode/ potential
500/v/
Fig. 4. Cathode with a top layer of W + 5%Sc,03, scanned along a diameter by means of an anode with a 20 pm hole. The distance between cathode and anode is about 0.5 mm. The current through the hole versus position shows irregularities at V = 500 V. These decrease with decreasing V and have been smoothed by space charge at V = 200 V.
potential at which/j = jO) the deviation from the space-charge limitation is small when compared with the situation in the case of an M cathode. This can be clearly seen in fig. 9 of ref. [6]. Considering the behavior described above it will be clear that there is no question of anomalous emission at low applied voltages for our top-layer scandate cathodes. When dealing with relatively large numbers of diodes the emission characterization by means of j,,, determined as referred to above, is rather laborious. Therefore, in that case the current density at V= 1000 V is used. Its value will be denoted by j(lOO0). Obviously, for a fair mutual comparison, the distance d between cathode and anode must always be the same in our diodes. However, with respect to the face value of about 0.25 mm, deviations occur. To correct for them, the anode voltage at which i = 35 mA is measured. This value of the anode voltage will be denoted by V(35). At V= V(35) the diodes operate in the space charge region, i.e. at j
(1)
Fig. 5 shows j,(lOOO) for top-layer scandate cathodes as a function of age for
178
J. Hasker et al. / Properties and manufacture of scandate cathodes 120 100 G
~~1000~ (Alc,,,2)
t
'
2*
L
6
-
'
,OOr---------
s-u
8060LO20I 10
I
20
I
40 60 100
I
1
I
103 -time
I
I
10L
i
(h)
Fig. 5. Life test results of W+SWSc,O, top-layer cathodes. Cathode temperature 1220 K, continuous load 1.4 A/cm’. The current density has been measured from time to time with a single 1000 V pulse. The plotted values have been corrected for differences in cathode to anode distance. Number of diodes 10. The bar indicates the standard deviation of the mean (u,,, = e/L/;;, e being the series standard deviation and n the number of diodes).
operation at 1220 K with a continuous load of 1.4 A/cm2. It can be seen that under these conditions - reasonably good vacuum and no ion energies above about 50 eV - there is only a modest deterioration during 2.5 years. It should be noted that such a stability is not a matter of course. Obviously, because of their pronounced saturation, the above method used to correct j for variations in d may not be applied to M cathodes when j >j,,. For that reason the characteristic of the M cathode in fig. 2 is as has been measured while that of the scandate cathode has been corrected towards the d value of the M cathode. 2.2. Surface composition and emission mechanism Bakhtiyarov and Shishkin performed electron-microscope studies on pressed scandate cathodes [7,8]. From the combination with local emission measurements and X-ray micro-analysis they conclude that the major part of the emission originates from BaO globules on the W grains. This means that the emission is not due to monolayer coverage of Ba or Ba-0 on bulk Sc,O, - as concluded by Van Oostrom and Augustus [9] - but originates from the tungsten regions. The latter is also in agreement with the emission experiments
J. Hasker et al. / Properties and manufacture of scandate cathodes
179
described by Hasker and Stoffelen [l]. Moreover, Yamamoto et al. [3] ascribe the emission of mixed matrix scandate cathodes to a Ba-SC-0 complex on tungsten. It is noted, however, that the affinity of oxygen to scandium is greater than that to barium: for the reaction 3BaO+2Sc%S$OJ+3Ba,
(2) AG” = - 26 kcal/(mol Ba) at 1200 K [lo]. This supports the model of ref. [I] for an impregnated top-layer scandate cathode, which consists of a substrate of W and Sc,O, and Ba-0 + excess 0 as the adsorbate. The Sc,O, is thought to regulate the oxygen concentration on the surface. This view is supported by the fact that, at cathode temperature, in a mixture of W, WO, and SqO, the scandia can reduce WO, with the formation of Sc6W0,, [ll]. In the following some experiments with the latter SC compound will be discussed. Further, it should be noted that the model in ref. [l] does not exclude thin-layer coverage of W by Sc,O,. In fact, sputter experiments - to be described in section 5 will show that such a coverage is important to the emission. 3. Manufacture and emission measurements A disadvantage of mixed matrix cathodes is that - when compared with a tungsten matrix - the reaction of W and impregnant, i.e. the Ba and BaO production during cathode operation, may be affected. Generally, the barium production can be separated sufficiently from the surface composition of the substrate by applying a top layer on the usual tungsten matrix. Top layers of W and Sc,O, and top-layers containing tungsten grains with a partial coverage of Sc,,O, will be considered in sections 3.1 and 3.2, respectively. To control the manufacture and to study the influence of various processing parameters, measurements on diodes of I735) and j(lOOO), defined in section 2.1, are helpful. A diode is obtained by mounting a cathode on a support, with a revolving anode, into a glass tube. The tube diameter is 35 mm while its height is about 10 cm. The diode tube is baked out at 400°C during pumping for - 2 h. After cooling down, the anode - turned away from the cathode - is degas& by RF heating. Next, filament and cathode are degassed by filament heating - keeping the pressure below about lo-’ Torr - up to 1435 K cathode temperature. The cathode remains at this temperature for only 1 min and is then set to 1400 K. Now the anode is brought into the planar-diode position and current is drawn with V= 100 V until the emission has become stable. Generally, the latter takes a few minutes. Then the tube is sealed off and a barium getter is evaporated. Subsequently, between t = 0 h and t = 2 h the cathodes are burnt at 1400 K with a continuous load of 1.4 A/cm2. The measurements of 1/(35) and j(lOO0) during these first 2 h are carried out at 1220 K, this being the temperature at which the cathodes are set afterwards while maintaining the continuous load.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
For surface analysis unactivated cathodes are mounted into the Auger apparatus (section 5). The activation is carried out by applying a temperature treatment as described above. 3.1. Top layer consisting of a mixture of W and Sc,O, To allow homogeneous mixing of the tungsten and scandia powders, the mean diameters of both materials are taken approximately equal (- 6 pm). Because the emission does not originate from monolayer coverage of bulk Sc,O, (section 2.2), the mixture must be expected to have an optimum content of Sc,O,. To start with, mixtures with 5 and 10 wt% of Sc,O, will be considered. It is noted that with 5 wt% of scandia its volume occupation in the mixture is about 20%. After deposition and slight pre-pressing of the tungsten in the pressing mould, the mixture is deposited on top of it. Then the pressing is carried out. Various pressures will be considered. After removal from the pressing mould, the plugs are sintered in dry hydrogen (dew point < - 60 o C) for - 1 h at a temperature which is set in the range between about 1500 and 2000°C. For fair mutual comparison the combination of pressure and sinter temperature is such that the resulting porosity is constant. This is controlled by measuring both the gas leakage through the plug and its increase in weight by impregnation. Just as for our B- and M-type cathodes, the impregnation is carried out in hydrogen atmosphere via the bottom of the plug. The temperature at impregnation depends on the ratio BaO : CaO : Al,O,. In our first top-layer scandate cathodes the molar ratio 5-3-2 was used. Later on we changed to 4-l-l for reasons to be discussed below. Before welding the impregnated plug to the MO sleeve, impregnant remnants are removed from the surface by means of ultrasonic cleaning. It was a question whether other treatments, in particular polishing, might be useful. Therefore we started by making four cathodes with 5% and four cathodes with 10% Sc,O, in the top layer. The pressure and sinter temperature were 2 atm and 19OO”C, respectively. Of each four cathodes two were ultrasonically cleaned and two were polished with the aid of an alumina disc. Table 1 shows the values of j,(lOOO) at various ages, obtained from measurements on diode tubes with a continuous load of 1.4 A/cm2. Let us first consider the situation after 24 h of operation. Obviously, polishing can both decrease and increase the emission. This is not very surprising because impregnation causes thin-layer coverage by Sc,O, of the tungsten between the Sc,O, grains (section 5). Considering emission, there is an optimum “thickness” of this coverage. It is not to be expected that this optimum can be obtained with good reproducibility by means of polishing. Another argument against polishing is the emission
J. Hasker et al. / Properties and manufacture of scanabe
181
cathodes
Table 1 Values of j,(lOOO) at various ages Operation time (h)
j,(loOC9 (A/cm*) lO%Sc,O,
%sC,O,
24 2ooo 20,ooo 4t-w@
Ultrason. cleaned
Polished
Ultrason. cleaned
Polished
65 115 120 115
25 35 70 75
25 35 120 100
40 45 65 65
behavior on aging as presented in the table: for the polished cathodes the emission remains substantially lower than for the ultrasonically cleaned samples. It will also be clear from the data in table 1 that we decided already at an early stage to proceed with the 5% mixture. Life test results averaged over 10 diodes with cathodes having top layers containing 5% Scz03, pressed at 2 atm, sintered at 1900°C and impregnated with the 5-3-2 composition are shown in fig. 6. This figure shows the disadvantage already suggested by the data in table 1: it takes nearly 2000 h before maximum emission is obtained. On the other hand, it can be seen that a high emission level has been maintained over a period of about five years. It is noted that the jump between 120 and 122 h is due to the fact that, after 120 h of operation, another 2 h operation at 1400 K was inserted.
1
/
60-
,
/
/
20 1
I
10
I
I
20
LO 60 100 -time
L
ihl
I
lo3
0 0 \
I
0
I
loL
I
5.10*
Fig. 6. Life test results of W + S%Sc,O~ top-layer cathodes impregnated with the 5-3-2 composition. Cathode temperature 1220 K, continuous load 1.4 A/cm2, number of diodes 10. The bar indicates the standard deviation of the mean.
182
J. Hasker et al. / Properties and manufacture of scandate cathodes
1.0 -
relative emission I 0.8-
0.40.2-
10
I 20
I I 40 60
103
100
-time
IOk :
(h)
Fig. 7. Relative emission at 1000 V pulse versus age for top layer cathodes with 4-l-l and 5-3-2 impregnant. Both curves were obtained from averaging over 10 diodes. Cathode temperature 1220 K, continuous load 1.4 A/cm*.
The gradual emission increase during the first 2000 h is not observed when 4-l-l impregnant is used. Immediately after the first 2 h at 1400 K the end level has been reached. Its height is practically the same as in the case of the 5-3-2 impregnant. A life test result has already been shown in fig. 5. The emission behavior for plugs of the same batch with 4-l-l and 5-3-2 impregnant is shown in fig. 7. In this figure j, (1000) has been normalized at its value after 20,000 h because the latter was not exactly the same for both impregnant compositions. These results may justify that from now on only cathodes with 4-l-l impregnant will be considered. The porosity of the top-layer plugs is made such that the weight increase due to impregnation is about 4%. In the case of a mixed matrix of W and 5% Sc,O, with the same gas leakage as the top-layer plug, this increase is only
0
2 L -weight
6
8 10 % Scz03
Fig. 8. Emission of cathodes with a top layer of W + Sc,O, for various scandia contents, at both 24 h and 17,500 h. The points were obtained from averaging over 5 diodes. Cathode temperature 1220 K, continuous load 1.4 A/cm’.
183
J. Hasker et al. / Properties and manufacture of scandate cathodes
0
2
4
6 -weight
Fig. 9. Emission of cathodes 140 h of operation. Cathode
8
10
12
11
16
% Scg WO,z
with a top layer of W + Sc,WO,, temperature 1220 K, continuous
for various Sc,WO,, load 1.4 A/cm’.
contents,
after
about 2%. This is attributed to reaction of the impregnant with Sc,O, and may be related to the fact that the molar volume of Ba,Sc,O, is much greater than that of Sc,O, so that pores may be cut off. Obviously, the life of a mixed matrix scandate cathode [3,4,12] will be about two times shorter than that of a top-layer scandate cathode. Because - as has been shown in figs. 5 and 7 - the emission of top layer cathodes with 4-l-l impregnant is very stable, it makes sense to reconsider the influence of the composition of the top layer. The result is shown in fig. 8. Since the standard deviation of the mean values presented is only about 4%, it may be concluded that there is a shallow maximum near to the 5% composition. It can be seen in fig. 9 that fairly high emissions can also be obtained with a mixed scandia oxide. Though not shown, similar possibilities have also been mentioned in ref. [12]. However, considering the remark with respect to the role of scandia in section 2.2 of the present paper, it may be expected that the emission recovery after ion bombardment will be worse for a cathode with a top layer of W + Sc+WO,, than for a cathode with a top layer of W + Sc,O,. This will be shown in section 4 (see the last row of table 2 where R is a measure for the recovery). 3.2. Top layer containing partially covered tungsten Considering at an early stage of our investigations the results summarized in section 2.2 and the low mobility of Sc,O, over the surface, it was expected that - from the viewpoints of homogeneity of emission and recovery after ion bombardment - a surface layer of partially covered tungsten grains might behave better than the mixture of tungsten and scandia grains examined above. Anyhow, it was decided that examination of the properties of this geometry would provide useful additional information.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
(Al
ao60-
10
20
LO 60 100 -time
(h)
Fig. 10. Life test results of cathodes with a top layer containing partially covered tungsten. Cathode temperature 1220 K, continuous load 1.4 A/cm’. Curve (1): sinter temperature 1800°C. Curve (2): sinter temperature again 18OO“C but 50% of W added to the top layer. For a sinter temperature of 1500°C with this mixture in the top layer the results are similar to curve (1). The curves were obtained from averaging over 5 diodes.
Consultation with P. Hokkeling of the Materials Section of our laboratory gave us the assurance that tungsten grains which are partially covered with ScH, can be readily made. Moreover, at room temperature the latter compound is sufficiently resistant to air. It is noted that decomposition occurs at about 800°C [13]. Hence, at least partial oxidation may occur during sintering and impregnation. The partially covered tungsten grains are made as follows. From tungsten, similar to the powder used for the W + Sc,O, top layers, a plug is pressed with a density of about 60% of the density of solid tungsten. Metallic scandium is deposited on top of the plug and melted in vacuum. Because the surface energy of SC is much lower than that of W [14], the tungsten is wetted all over its surface within the pores. Upon cooling down in hydrogen SC is converted to ScH, which has a substantially greater volume than scandium so that the system becomes brittle enough to be pulverized. After baking the resultant powder for about 15 min in hydrogen at 800°C it can be ground to the wanted size. The weight increase of the tungsten caused by the coverage is about 3%. Pressing, sintering and impregnation are carried out in the way described in section 3.1. Pressure and sinter temperature are again such that the weight increase due to impregnation with the 4-l-l composition is about 4%. Curve (1) in fig. 10 shows the emission during the first 20,000 h of life for cathodes sintered at 18OOOC.It can be seen from comparison with fig. 5 that
J. Hasker et al. / Properties and manufacture of scandate cathodes
185
this emission behavior is certainly satisfactory. It should be mentioned however that, due to the sintering and cooling down in hydrogen , the difference in volume of SC and ScH, gives rise to unflatness of the (emitting) surface. At the center of the cathode the distance to the anode in a planar diode configuration is about 25 pm smaller than at the edge. This problem can be avoided by using a mixture of, e.g., 50% W and W/ScH, for the top layer. It can be seen from curve (2) in fig. 10 that, again for a sinter temperature of 1800 “C, the emission is about 10% below curve (1). This may be due to SC evaporation during sintering. Obviously, this loss can be reduced by using a higher pressure and a lower sinter temperature during the manufacture of the plug, in particular when this temperature is kept below the melting point of SC (1541°C). The emission result for a sinter temperature of 1500°C is, within narrow limits, similar to curve (1) in fig. 10. It will be shown in the next section that similarity does not apply to the recovery of emission after ion bombardment.
4. Ion bombardment and exposure to air The recovery of the emission of a cathode after ion bombardment is of major importance for its application in electron tubes. Obviously, the bombardment depends on the tube processing applied. For instance, after the usual processing of television display tubes serious bombardment ‘with highenergy ions still occurs. It is the purpose of the present section to look at the effect of ion bombardment without making expensive tubes. This facilitates both the cathode choice for tubes and the selection of experiments to be carried out in our scanning Auger apparatus for better understanding of the mechanisms. The influence of the sinter temperature on the recovery will be considered for sintering in hydrogen of both types of top layer cathodes. For this purpose, first the experimental procedure will be described. The sputter procedure has been chosen such that it causes a noticeable emission decrease. On the other hand, to allow the observation of differences between various cathodes, the conditions are such that the cathodes are not completely destroyed by the ion bombardment. Argon is used as the sputter gas to avoid chemical interactions. The cathodes are mounted into an ultra-high vacuum vessel containing a movable anode on an oven. After pumping (pressure about lo-” Torr) the anode - moved away from the cathode - is degassed at about 700°C for 15 min. The cathodes are activated in the way described in section 3. After activation the current density is measured with a 1500 V pulse over a diode distance of 300 pm. Its value is denoted by &(lSOO). With a pulse frequency of 10 Hz and a pulse length such that the anode dissipation is 5 W at j(l500) =ji(lSOO), the cathode is operated in lo-’ Torr Ar for 40 min. Fig. 11 shows the gradual decrease of j(1500)/~‘~(1500) for both
186
J. Hasker et al. / Properties and manufacture of scandate cathodes
T = 1220K
scandate
1
10
0
-time
20 (minutes)
30
cathode
40
Fig. 11. Decrease of j(1500)/ji(1500) during sputtering. The cathodes were operated with a 1500 V pulse over 300 pm in 10e5 Torr argon. The pulse frequency was 10 Hz and the pulse length was chosen such that the anode dissipation is 5 W at j(l500) = ji(1500). The circles show the decrease of j(1500)/ji(1500) during a second 40 min sputtering, carried out after the recovery presented by the drawn curve in fig. 12.
an M cathode and a cathode with a top-layer of W + 5% Scz03, at a cathode temperature of 1220 K. The recovery in vacuum at this temperature and a continuous load of 1 A/cm2 during 2 h is shown in fig. 12. It can be seen that total recovery occurs for the M cathode. As confirmed by Auger experiments, this is due to recovery of the Ba and 0 occupation of the surface. Obviously, the latter is not sufficient for the scandate cathode because it hardly shows emission recovery at 1220 K. This will be considered in more detail in the next section. Fig. 12 also shows the subsequent recovery of the scandate cathode during 1 h at 1320 K, again with a continuous load of 1 A/cm’. During this hour the measurements of j(l500) were performed at 1220 K. The circles in figs. 11 and 12 show the decrease of j(lSOO)/j~(l500) during a second 40 min sputtering in the way described above and the subsequent recovery in vacuum. It can be seen that this second bombardment has increased the damage. The current density at 1500 V after the second bombardment and recovery is denoted by j,(lSOO). The ratio R =j~(l500)/~~(1500)
(3)
will be used as a measure of the recovery of emission. Table 2 shows the values of R for various top layer cathodes made with different pressures and similar temperatures which have been taken such that
187
J. Hasker et al. / Properties and manufacture of scandate cathodes
I
T=1220K;
T=1320K
60-
I 0
I
I
I
60
120
180
-time
(minutes)
Fig. 12. Recovery in vacuum after the sputtering shown in fig. 11. During recovery the cathodes were continuously loaded with about 1 A/cm’. All measurements of j(lSO0) were performed at 1220 K. The circles show the recovery after a second 40 min sputtering.
in all cases the weight increase due to impregnation is 4.2%. The influence of these processing parameters can be clearly seen. The difference for both top layers with ScH, must be due to excessive SC evaporation during sintering at 1800°C, while the difference for both W + Sc,O, layers may be caused by a difference in tungsten oxidation. According to table 2 the best results are obtained with W + W/ScH, top-layers pressed at 4 atm and sintered at 15OOOC. However, after another 70 h of sputtering in the way as described above a recovery up to 58% at 1320 K was the best that could be obtained. The remaining damage in the described experiments indicates that in particular the recovery of the W + Sc,O, cathodes is not good enough for
Table 2 Values of R for various Top layer
top layer cathodes Sinter temperature
Pressure
(“C)
(atm)
;“w )
w+sssc,o, W + SOXW/ScHr W + 50%W/ScH2
1900 1500 1800 1500
2 3.5 2.5 4
65 75 55 80
w + 9%sc,WO,,
1900
2
52
w+s%sc,o,
J. Hasker et al. / Properties and manufacture of scandate cathodes
188
j
___________________
0
5 -time
lA/cm*
10
15
20
(minutes)
Fig. 13. Emission rise in vacuum after switching on the heater power for cathodes exposed to (moist) air after activation. Before the exposure the anode voltages were set at “space-charge” limited emission with a cathode load of 1 A/cm’, at cathode temperatures of 1320 and 1220 K for the M and the scandate cathode, respectively.
application in television display tubes in the case of the usual processing. This has been confirmed by experiments in tubes carried out by our colleagues at the Philips Elcoma Tube Development Department. On the other hand, the use of top-layer scandate cathodes can improve the performance of, e.g., camera tubes, oscilloscope tubes and klystrons. For a number of applications fast emission recovery after exposure of the (cold) activated cathode to moist air is important. To provide some information in this respect an M cathode and a top-layer W + S%Sc,O, cathode were mounted into the vacuum system and activated. The anode voltages were set at space-charge limited emission with a cathode load of 1 A/cm2. Next, the heater supplies were switched off and - after cooling down - air (humidity - 50%) was let into the system. After pumping the heater supplies were switched on again and the emissions versus time measured. The result is shown in fig. 13. The scandate cathode shows immediate recovery, whereas the M cathode has been poisoned. It is noted that the recovery of the scandate cathode is not confined to the “space-charge” region (at v = 900 V - see fig. 2 - it occurs as well) and that several repetitions do not affect this result. The difference with the M cathode must be due to a different bonding of barium on the surface. According to Auger measurements [l], even repetition of the activation of the M cathode does not completely restore the surface composition. It was found that after exposure to air and reactivation both the Ba-0 and the excess 0 concentration are about 30% lower than after activation of a fresh cathode.
J. Hasker et al. / Properties and manufacture of scandate cathodes
189
5. Surface analysis
The surface composition of the top layer cathodes was examined with a PHI model 590 SAM. The elemental mappings in the following were measured with a primary beam energy of 5 keV and a spot width which, depending on the magnification, ranges between 0.2 and 1 pm. It should be noted that the contrast and intensity setting is not the same in all pictures. The raster consists of 100 x 100 points. The total measuring time per point is 100 ms. Only the tungsten and scandium mappings will be shown because the oxygen pictures and to a lesser extent the Ba pictures too - are congruent with the scandium pictures. More quantitative information can be obtained from a display of the various pph’s (peak-to-peak heights in the dN/d E versus E spectrum) during selected line scans. When considering and comparing pph ratios in the following, it may be useful to realize that - as can easily be calculated - e.g. for tungsten with one monolayer of Sc,O, the pph ratio Sc(333 eV)/W(179 eV) = 2.1, while with two monolayers its value is 7.2 and for a homogeneous mixture of equal volumes of SQO; and tungsten the ratio is 2.3. Hence pph ratios provide useful but not complete information. In fact, similar sets of pph ratios can give rise to very different emission behavior. The effect of argon-ion sputtering will be examined with the aid of a differentially pumped ion gun, an angle of incidence of the ion beam on the sample of about 0.2 rad and 1.5 keV ion energy. Fig. 14 shows the W and SC mappings for a W + SSSc,O, top-layer “cathode” which has not been impregnated. The expected grainy structure can be clearly seen. To avoid disturbances by charging of the biggest SQO, grains, the measurement was carried out at 1220 K cathode temperature. On a very small grain surrounded by tungsten an Auger measurement on Sc,O, could be carried out without disturbances at room temperature. The pph ratio O/SC
Fig. 14. SAM mappingsof Sc (left) and W (right) for a non-impregnated “cathode”, measured at 1220 K. Scanned area 50 pmx40 pm.
W + %%Sc,O, top-layer
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J. Hasker et al. / Properties and manujacture of scandate cathodes
Fig. 15. SAM mappings of SC (left) and W (right) for a W + 5%Sc,O, top-layer cathode, measured at room temperature, after activation (1) and subsequently 30 min of sputtering (2), another 30 min of sputtering (3) and reactivation (4). Scanned area 50 /.Im X 40 pm.
191
J. Hasker et al. / Properties and manufacture of scandate cathodes
was found to be equal to 1.14. This result has already been used in ref. [l]. The measured ratio does not change in the case of simultaneous argon-ion sputtering or when the temperature is increased up to 1220 K. For the sputter-cleaned sample the value of the SC/W pph ratio found from scanning over an area of 400 pm x 400 pm is 0.7. Then, taking selected Sc,O, and W regions as an internal standard, the composition of the top layer can be, calculated from the spectrum. The result agrees fairly well with the W + 5%Sc,O, mixture used. The SAM mappings of SC and W for a cathode with a top layer made of W + 5!%Scsc,0,,sintered at 19OO’C and impregnated with the 4-l-l composition, are shown in fig. 15. Comparison of the pictures (1) with fig. 14 shows the influence of impregnation and activation. The SC distribution has become rather homogeneous, i.e. at the surface the grainy structure has gone. Pictures (2), (3) and (4) in fig. 15 show qualitatively the influence of sputtering and subsequent reactivation. More quantitative information is shown in table 3. The data in the first three rows of the table are mean values of three cathodes and have been averaged over the scanned area. The fourth row was obtained from our measurement shown in fig. 4 of ref. [l]. Though the latter was carried out with another apparatus on another W + S%Sc,O, top-layer cathode, it can be seen that the result agrees fairly well with the first row. The results presented so far strongly indicate that reaction during impregnation gives rise to a thin-layer coverage of the surface by scandia: most of it can be removed by sputtering. Reactivation gives only partial recovery, it cannot reproduce the initial state. Hence - when compared with the bulk composition of the top layer - there is scandium depletion near to the surface which is caused by impregnation. It is noted that after reactivation the value of j(lOOO)/‘i(lOOO) was 0.6. This compares quite well with the results presented in table 2. Comparison of the scandium mappings in fig. 15 with those of ref. [3] shows that in our case the scandium distribution after activation is much more homogeneous. As can be seen in the last row of table 3, this results in an about 5 times higher SC/W pph ratio.
Table 3 pph ratios for top layers of W + 5%Sc,O,: W(179 eV), Sc(333 eV), O(510 ev), Ba(584 eV) O/Ba
Sc/Ba
W/Ba
k/w
O/SC
After activation After sputtering After reactivation
4.5 8.3 3.2
2.3 1.7 1.4
0.5 9.4 1.6
4.8 0.2 0.9
1.9 4.8 2.2
Ref. [l] after activation
4.7
2.6
0.6
4.4
1.8
2.5 ‘)
1.5 a)
1.8 ‘)
0.9 a)
1.7 a)
‘) Mixed matrix cathode of ref. [3], after activation.
J. Hasker et al. / Properties and manufacture of scandate cathodes
192
Line scans of W, SC, Ba and 0 for one of the cathodes of the first rows of table 3 are shown in fig. 16. They show that there are tungsten regions with a thin-layer scandia coverage and tungsten regions without scandia. The barium signal from the scandia regions is greater than that from the tungsten regions. Apart from differences in barium concentration, this may be due to matrix effects. Whether the scandia spots or the adjacent W spots - where, in addition to Ba, 0 is also present - are the sources of high emission is not yet clear. The SAM mappings of Sc and W for a cathode with a top layer made of W + SO%W/ScH, and sintered at 1500°C are shown in fig. 17. Comparison of both tungsten pictures shows the scandium coverage of the tungsten grains. This is again a thin layer, as can be seen from both scandium pictures (after sputtering SC is left only in the pores) and, more quantitatively, from the SC and Ba pph decreases during sputtering. The latter are shown in fig. 18. This
wh t
0
Lo
80
120
160
200
;- microns Fig. 16. pph’s of W, SC, Ba and 0 during a line scan. The measurement applies to one of the three cathodes of the first row in table 3. The units are different and arbitrary.
J. Hasker et al. / Properties and manufacture of scandate cathodes
193
Fig. 17. SAM mappings of Sc (left) and W (right) for a W + 501!ScH2 top-layer cathode, measured at room temperature, after activation (1) and subsequently 1 h sputtering and reactivation (2). Scanned area 20 pm x 16 pm.
-sputter
time lmin)
Fig. 18. pph’s of SC, Ba and 0 during sputtering for the cathode of fig. 17.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
means that the original partial coverage of the tungsten grams, with a thickness of about 0.5 pm, is no longer present at the surface. Nevertheless, the scandium distribution left after the various processing steps is more favorable than in the case of the W + 5%&O, top layer. This is reflected in the value of j(lOOO)/ji(lOOO) after sputtering and reactivation which is 0.76 for the cathode of fig. 17 instead of 0.6 for the cathode of fig. 15.
6. Conclusions and discussion For both types of top-layer scandate cathodes considered in the present paper the emissions are at least as high as for the known pressed and impregnated scandate cathodes. Moreover, the life and/or the staying power against ion bombardment have been improved. The top layer cathodes do not show an emission anomaly at low applied voltages. At current densities smaller than j,, the current-voltage characteristic is similar to that of, e.g., an M cathode. However, at current densities higher than j,, the deviation from the space-charge limitation is much smaller than for the M cathode. When compared with a mixed matrix, the top layer facilitates impregnation while the reaction of the impregnant with tungsten is not affected. For both the emission after a few hours of life and the emission after 40,000 h of life, polishing of the cathode surface after impregnation cannot be recommended. On the other hand, ultrasonic cleaning after impregnation gives rise to good reproducibility. With 5-3-2 impregnant it takes - 2000 h before the emission is maximum. Therefore - though a high emission level can be maintained for nearly 5 years _ the use of 4-l-l impregnant is preferred. The observed difference in emission behavior for both compositions will be a subject of further investigation. Auger experiments on impregnated tungsten plugs, carried out by H.J.H. Stoffelen of our laboratory, already indicated that - after the usual activation procedure - the excess oxygen concentration [l] on the surface is greater in the case of the 5-3-2 impregnant than in the case of the 4-l-l impregnant. The combination of sputter- and scanning-Auger experiments shows that in agreement with our emission experiments [l] - thin-layer coverage of tungsten by scandia is important to the high emission. Moreover, when compared with the bulk composition of the top layer, there is scandium depletion near to the surface. Both thin-layer coverage and depletion are related to the impregnation process. They imply remaining damage after extended ion sputtering: reactivation does not result in complete recovery of the surface composition. Though recovery occurs after moderate sputtering, this takes too much time at the operating temperature of 1220 K. These properties make that - in case of the usual processing - the cathodes are not
J. Hasker et al. / Properties and manufacture of scandate cathodes
195
yet suited for application in television display tubes. Therefore, further improvement is under investigation. On the other hand, given a sufficiently good vacuum and/or relatively low accelerating potentials, the emission capability and the very long life of the new top-layer scandate cathodes can improve the performance and the life of electron devices, such as camera tubes, oscilloscope tubes and klystrons. In addition, for some applications it may be advantageous that the cathodes in activated state are insensitive to exposure to (moist) air. The processing, and in particular the temperature during sintering, is important to the emission recovery after ion bombardment. This is very pronounced for the top layers of W + W/ScH,: R = 80% for sintering at 1500”C, whereas R is only 55% for sintering at 1800°C. From the considerations and results of sections 2.2, 4 and 5, it will be clear that more details with respect to the emission mechanism are required. This is another subject for further investigation.
Acknowledgement
The authors are indebted to J.J. van Lith of our Cathode Technological Department for making cathodes and performing life test measurements.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
J. Hasker and H.J.H. Stoffelen, Appl. Surface Sci. 24 (1985) 330. C.R.K. Marrian, A. Shih and G.A. Haas, Appl. Surface Sci. 16 (1983) 1. S. Yamamoto, S. Taguchi, T. Aida and S. Kawase, Appl. Surface Sci. 17 (1984) 504. S. Taguchi, T. Aida and S. Yamamoto, IEEE Trans. Electron Devices ED-31 (1984) 900. S. Yamamoto, S. Taguchi, T. Aida and S. Kawase, Appl. Surface Sci. 17 (1984) 517. J. Hasker, Appl. Surface Sci. 16 (1983) 220. R.S. Bakhtiyarov and B.B. Shishkin, Radio Eng. Electron. Phys. 16 (1971) 384. R.S. Bakhtiyarov and B.B. Shishkin, Soviet Phys.-Tech. Phys. 17 (1973) 1752. A. van Gostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. Calculated from the data in: I. Bar-in, 0. Knacke and 0. Kubaschewski, Thermochemical Properties of Inorganic Substances+ Supplement (Springer, Berlin, 1973 and 1977). [ll] V.A. Levitskii, Inorg. Mater. 16 (1981) 1489. [12] S. Taguchi, T. Aida, S. Yamamoto and Y. Honda, UK Patent Application GB 2 116 356 A. [13] E. Fromm and E. Gebhardt, Gase und Kohlenstoff in Metallen (Springer, Berlin, 1976). [14] AR. Miedema, Inst. Phys. Conf. Ser. 55 (1981) ch. 7.
Applied Surface Science 26 (1986) 173-195 North-Holland, Amsterdam
PROPERTIES AND MANUFACTURE SCANDATE CATHODES
OF TOP-LAYER
J. HASKER, J. VAN ESDONK and J.E. CROMBEEN Philips Research Laboratories, Eindhoven, The Netherlandr
Received 1 October 1985; accepted for publication 9 April 1986
The life and/or the staying power against ion bombardment of scandate cathodes can be improved by using a top layer of W + Sc,Os or W + W/ScH, on a tungsten matrix. The latter is impregnated with the usual 4-l-l impregnant. Even at high voltage pulses the current densities are so high that the deviation from space charge limitation is small. The manufacture is discussed and the cathode life at the operating temperature of 1220 K is shown to be very long. Moreover, the relationship between processing parameters and emission recovery after ion bombardment is examined with the aid of combined sputter- and scanning-Auger measurements. It is shown that thin-layer coverage of tungsten by scandia is important to the high emission. This coverage is related to the impregnation process. After extended sputtering it cannot be completely recovered by reactivation. Consequently, the top layer cathodes cannot withstand sufficiently the usual processing and operation of television display tubes. On the other hand, they can improve the life and performance of electron devices with good vacuum and/or relatively low accelerating potentials. Moreover, activated top-layer scandate cathodes are relatively insensitive to exposure to (moist) air.
1. Introduction
Until about 5 years ago there were two types of scandate cathodes: the pressed scandate cathode, which is made by pressing and sintering a mixture of W and Ba,Sc,O,, and the impregnated scandate cathode, which is a usual impregnated cathode with scandium compounds in the impregnant. Some properties of these cathodes and our considerations for making a new type of scandate cathode - the top-layer scandate cathode - have already been discussed before [l]. A schematic representation is shown in fig. 1. This cathode should have a longer life than the pressed scandate cathode and a better recovery after ion bombardment than the impregnated scandate cathode. It is noted that the latter shows no recovery after bombardment [2]. Fig. 2 indicates the justification for our interest in scandate cathodes. It shows the emission capability of a top layer cathode in comparison with that of an OS-Ru coated impregnated cathode, the so-called M cathode. The latter can be considered as the best and most stable impregnated cathode till now. 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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J. Hasker et al. / Properties and manufacture of scandate cathodes
impregnant Fig. 1. Schematic representation of a top-layer scandate cathode. The plug consists of about 0.4 mm porous W with a porous top layer of about 0.1 mm. The plug is impregnated with the usual 4-l-l impregnant without additives.
Papers by Yamamoto and coworkers [3,4] show that the interest in scandate cathodes is not confined to our laboratory. They describe an impregnated cathode with a mixed matrix of W and Sc,O, and mention an emission anomaly at low applied voltages (in a diode) as a disadvantage which is ascribed to nonuniformly distributed low work function patches [4,5]. In the course of our investigations it became clear that a mixture of relatively big grains of W and Sc,,O, may not be the most favorable geometry. From the viewpoint of homogeneity of emission and recovery after ion
jl A/cm2)
1
V”2, V In volts Fig. 2. Current density versus square root of anode voltage for both a top-layer scandate cathode and an OS-Ru coated cathode in diode configuration with a distance of about 0.3 mm between cathode and anode.
J. Hasker et al. / Properties and manufacture of scandate cathodes
175
bombardment a top layer consisting of W grains which are partially covered with an SqO, layer of about 0.5 pm will do better. Other options are of course layers deposited by sputtering, evaporation or chemical vapor deposition. In the present paper we shall confine ourselves to the geometry of fig. 1 and its closest relative, i.e. a top layer consisting of tungsten which is partially covered with S%O,. In section 2.1 the diode characteristics, as already shown in fig. 2, are considered in more detail, together with the characterization of emission capability and a life test result. Our top layer cathodes do not show an emission anomaly at low applied voltages. In section 2.2 some remarks are made with respect to surface composition and emission mechanism. In section 3 the manufacture of both types of top layer cathodes and some inherent problems are considered in connection with emission experiments. It is shown that top layer cathodes may be expected to live about twice as long as the mixed matrix cathodes. Section 4 deals with the recovery of emission after ion bombardment in relation to processing parameters as, for instance, the temperature during sintering. Also considered is the influence of exposure of an activated cathode to air. In section 5 some results of investigations with the aid of scanning Auger microscopy (SAM) are presented and discussed. Both the influence of ion sputtering and the relationship between surface composition and processing (section 3) are considered. Finally, some conclusions are summarized in section 6. If, in combination with the usual processing, top layer cathodes are to be used in television display tubes some further improvement will be necessary. However, in devices with good vacuum they can substantially improve the performance.
2. Emission and surface properties 2.1. Diode characteristics
and emission characterization
All experiments were performed on indirectly heated planar cathodes of 1.8 mm diameter. All cathode temperatures in the following are MO brightness temperatures (after correction for absorption through the viewing window) measured on the sleeve just below the plug. At relatively low emitted current density the heating power is about 2 W. It will be clear that the characteristics in fig. 2 were measured in single-pulse operation (5 ps pulse length): at a continuous load of 100 A/cm2 the emission cooling would be of the same order of magnitude as the heating power. Let us now consider these characteristics in some more detail.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
“space charge”
I “saturation”
(a)
lb) Fig. 3. (a) Schematic voltage). (b) Schematic
representation representation
of the diode characteristic of the potential distribution
(current density versus within the diode.
diode
The transition between “space charge” and “saturation” region of the characteristic (see fig. 3) is given by zero Poisson field strength at the cathode. j, has The corresponding current density is j,. A method for determining been described before [6]. Moreover, it has been shown that for current densities greater than j, space charge remains important up to the greatest values of V used in practice. This is one of the reasons why - in particular for scandate cathodes - the usual Schottky extrapolation as applied in ref. [3] delivers erroneous values for j, [6]. For both OS-Ru and scandate cathodes the saturation current density distribution over the surface is inhomogeneous. Nevertheless, for j rj, these inhomogeneities are smoothed by the space charge in front of the cathode and j can be calculated accurately with the aid of Langmuir’s theory for the ideal space-charge limited diode. Hence, for j
J. Hasker et al. / Properties and manufacture of scandate cathodes
171
anode/ potential
500/v/
Fig. 4. Cathode with a top layer of W + 5%Sc,03, scanned along a diameter by means of an anode with a 20 pm hole. The distance between cathode and anode is about 0.5 mm. The current through the hole versus position shows irregularities at V = 500 V. These decrease with decreasing V and have been smoothed by space charge at V = 200 V.
potential at which/j = jO) the deviation from the space-charge limitation is small when compared with the situation in the case of an M cathode. This can be clearly seen in fig. 9 of ref. [6]. Considering the behavior described above it will be clear that there is no question of anomalous emission at low applied voltages for our top-layer scandate cathodes. When dealing with relatively large numbers of diodes the emission characterization by means of j,,, determined as referred to above, is rather laborious. Therefore, in that case the current density at V= 1000 V is used. Its value will be denoted by j(lOO0). Obviously, for a fair mutual comparison, the distance d between cathode and anode must always be the same in our diodes. However, with respect to the face value of about 0.25 mm, deviations occur. To correct for them, the anode voltage at which i = 35 mA is measured. This value of the anode voltage will be denoted by V(35). At V= V(35) the diodes operate in the space charge region, i.e. at j
(1)
Fig. 5 shows j,(lOOO) for top-layer scandate cathodes as a function of age for
178
J. Hasker et al. / Properties and manufacture of scandate cathodes 120 100 G
~~1000~ (Alc,,,2)
t
'
2*
L
6
-
'
,OOr---------
s-u
8060LO20I 10
I
20
I
40 60 100
I
1
I
103 -time
I
I
10L
i
(h)
Fig. 5. Life test results of W+SWSc,O, top-layer cathodes. Cathode temperature 1220 K, continuous load 1.4 A/cm’. The current density has been measured from time to time with a single 1000 V pulse. The plotted values have been corrected for differences in cathode to anode distance. Number of diodes 10. The bar indicates the standard deviation of the mean (u,,, = e/L/;;, e being the series standard deviation and n the number of diodes).
operation at 1220 K with a continuous load of 1.4 A/cm2. It can be seen that under these conditions - reasonably good vacuum and no ion energies above about 50 eV - there is only a modest deterioration during 2.5 years. It should be noted that such a stability is not a matter of course. Obviously, because of their pronounced saturation, the above method used to correct j for variations in d may not be applied to M cathodes when j >j,,. For that reason the characteristic of the M cathode in fig. 2 is as has been measured while that of the scandate cathode has been corrected towards the d value of the M cathode. 2.2. Surface composition and emission mechanism Bakhtiyarov and Shishkin performed electron-microscope studies on pressed scandate cathodes [7,8]. From the combination with local emission measurements and X-ray micro-analysis they conclude that the major part of the emission originates from BaO globules on the W grains. This means that the emission is not due to monolayer coverage of Ba or Ba-0 on bulk Sc,O, - as concluded by Van Oostrom and Augustus [9] - but originates from the tungsten regions. The latter is also in agreement with the emission experiments
J. Hasker et al. / Properties and manufacture of scandate cathodes
179
described by Hasker and Stoffelen [l]. Moreover, Yamamoto et al. [3] ascribe the emission of mixed matrix scandate cathodes to a Ba-SC-0 complex on tungsten. It is noted, however, that the affinity of oxygen to scandium is greater than that to barium: for the reaction 3BaO+2Sc%S$OJ+3Ba,
(2) AG” = - 26 kcal/(mol Ba) at 1200 K [lo]. This supports the model of ref. [I] for an impregnated top-layer scandate cathode, which consists of a substrate of W and Sc,O, and Ba-0 + excess 0 as the adsorbate. The Sc,O, is thought to regulate the oxygen concentration on the surface. This view is supported by the fact that, at cathode temperature, in a mixture of W, WO, and SqO, the scandia can reduce WO, with the formation of Sc6W0,, [ll]. In the following some experiments with the latter SC compound will be discussed. Further, it should be noted that the model in ref. [l] does not exclude thin-layer coverage of W by Sc,O,. In fact, sputter experiments - to be described in section 5 will show that such a coverage is important to the emission. 3. Manufacture and emission measurements A disadvantage of mixed matrix cathodes is that - when compared with a tungsten matrix - the reaction of W and impregnant, i.e. the Ba and BaO production during cathode operation, may be affected. Generally, the barium production can be separated sufficiently from the surface composition of the substrate by applying a top layer on the usual tungsten matrix. Top layers of W and Sc,O, and top-layers containing tungsten grains with a partial coverage of Sc,,O, will be considered in sections 3.1 and 3.2, respectively. To control the manufacture and to study the influence of various processing parameters, measurements on diodes of I735) and j(lOOO), defined in section 2.1, are helpful. A diode is obtained by mounting a cathode on a support, with a revolving anode, into a glass tube. The tube diameter is 35 mm while its height is about 10 cm. The diode tube is baked out at 400°C during pumping for - 2 h. After cooling down, the anode - turned away from the cathode - is degas& by RF heating. Next, filament and cathode are degassed by filament heating - keeping the pressure below about lo-’ Torr - up to 1435 K cathode temperature. The cathode remains at this temperature for only 1 min and is then set to 1400 K. Now the anode is brought into the planar-diode position and current is drawn with V= 100 V until the emission has become stable. Generally, the latter takes a few minutes. Then the tube is sealed off and a barium getter is evaporated. Subsequently, between t = 0 h and t = 2 h the cathodes are burnt at 1400 K with a continuous load of 1.4 A/cm2. The measurements of 1/(35) and j(lOO0) during these first 2 h are carried out at 1220 K, this being the temperature at which the cathodes are set afterwards while maintaining the continuous load.
180
J. Hasker et al. / Properties and manufacture of scandate cathodes
For surface analysis unactivated cathodes are mounted into the Auger apparatus (section 5). The activation is carried out by applying a temperature treatment as described above. 3.1. Top layer consisting of a mixture of W and Sc,O, To allow homogeneous mixing of the tungsten and scandia powders, the mean diameters of both materials are taken approximately equal (- 6 pm). Because the emission does not originate from monolayer coverage of bulk Sc,O, (section 2.2), the mixture must be expected to have an optimum content of Sc,O,. To start with, mixtures with 5 and 10 wt% of Sc,O, will be considered. It is noted that with 5 wt% of scandia its volume occupation in the mixture is about 20%. After deposition and slight pre-pressing of the tungsten in the pressing mould, the mixture is deposited on top of it. Then the pressing is carried out. Various pressures will be considered. After removal from the pressing mould, the plugs are sintered in dry hydrogen (dew point < - 60 o C) for - 1 h at a temperature which is set in the range between about 1500 and 2000°C. For fair mutual comparison the combination of pressure and sinter temperature is such that the resulting porosity is constant. This is controlled by measuring both the gas leakage through the plug and its increase in weight by impregnation. Just as for our B- and M-type cathodes, the impregnation is carried out in hydrogen atmosphere via the bottom of the plug. The temperature at impregnation depends on the ratio BaO : CaO : Al,O,. In our first top-layer scandate cathodes the molar ratio 5-3-2 was used. Later on we changed to 4-l-l for reasons to be discussed below. Before welding the impregnated plug to the MO sleeve, impregnant remnants are removed from the surface by means of ultrasonic cleaning. It was a question whether other treatments, in particular polishing, might be useful. Therefore we started by making four cathodes with 5% and four cathodes with 10% Sc,O, in the top layer. The pressure and sinter temperature were 2 atm and 19OO”C, respectively. Of each four cathodes two were ultrasonically cleaned and two were polished with the aid of an alumina disc. Table 1 shows the values of j,(lOOO) at various ages, obtained from measurements on diode tubes with a continuous load of 1.4 A/cm2. Let us first consider the situation after 24 h of operation. Obviously, polishing can both decrease and increase the emission. This is not very surprising because impregnation causes thin-layer coverage by Sc,O, of the tungsten between the Sc,O, grains (section 5). Considering emission, there is an optimum “thickness” of this coverage. It is not to be expected that this optimum can be obtained with good reproducibility by means of polishing. Another argument against polishing is the emission
J. Hasker et al. / Properties and manufacture of scanabe
181
cathodes
Table 1 Values of j,(lOOO) at various ages Operation time (h)
j,(loOC9 (A/cm*) lO%Sc,O,
%sC,O,
24 2ooo 20,ooo 4t-w@
Ultrason. cleaned
Polished
Ultrason. cleaned
Polished
65 115 120 115
25 35 70 75
25 35 120 100
40 45 65 65
behavior on aging as presented in the table: for the polished cathodes the emission remains substantially lower than for the ultrasonically cleaned samples. It will also be clear from the data in table 1 that we decided already at an early stage to proceed with the 5% mixture. Life test results averaged over 10 diodes with cathodes having top layers containing 5% Scz03, pressed at 2 atm, sintered at 1900°C and impregnated with the 5-3-2 composition are shown in fig. 6. This figure shows the disadvantage already suggested by the data in table 1: it takes nearly 2000 h before maximum emission is obtained. On the other hand, it can be seen that a high emission level has been maintained over a period of about five years. It is noted that the jump between 120 and 122 h is due to the fact that, after 120 h of operation, another 2 h operation at 1400 K was inserted.
1
/
60-
,
/
/
20 1
I
10
I
I
20
LO 60 100 -time
L
ihl
I
lo3
0 0 \
I
0
I
loL
I
5.10*
Fig. 6. Life test results of W + S%Sc,O~ top-layer cathodes impregnated with the 5-3-2 composition. Cathode temperature 1220 K, continuous load 1.4 A/cm2, number of diodes 10. The bar indicates the standard deviation of the mean.
182
J. Hasker et al. / Properties and manufacture of scandate cathodes
1.0 -
relative emission I 0.8-
0.40.2-
10
I 20
I I 40 60
103
100
-time
IOk :
(h)
Fig. 7. Relative emission at 1000 V pulse versus age for top layer cathodes with 4-l-l and 5-3-2 impregnant. Both curves were obtained from averaging over 10 diodes. Cathode temperature 1220 K, continuous load 1.4 A/cm*.
The gradual emission increase during the first 2000 h is not observed when 4-l-l impregnant is used. Immediately after the first 2 h at 1400 K the end level has been reached. Its height is practically the same as in the case of the 5-3-2 impregnant. A life test result has already been shown in fig. 5. The emission behavior for plugs of the same batch with 4-l-l and 5-3-2 impregnant is shown in fig. 7. In this figure j, (1000) has been normalized at its value after 20,000 h because the latter was not exactly the same for both impregnant compositions. These results may justify that from now on only cathodes with 4-l-l impregnant will be considered. The porosity of the top-layer plugs is made such that the weight increase due to impregnation is about 4%. In the case of a mixed matrix of W and 5% Sc,O, with the same gas leakage as the top-layer plug, this increase is only
0
2 L -weight
6
8 10 % Scz03
Fig. 8. Emission of cathodes with a top layer of W + Sc,O, for various scandia contents, at both 24 h and 17,500 h. The points were obtained from averaging over 5 diodes. Cathode temperature 1220 K, continuous load 1.4 A/cm’.
183
J. Hasker et al. / Properties and manufacture of scandate cathodes
0
2
4
6 -weight
Fig. 9. Emission of cathodes 140 h of operation. Cathode
8
10
12
11
16
% Scg WO,z
with a top layer of W + Sc,WO,, temperature 1220 K, continuous
for various Sc,WO,, load 1.4 A/cm’.
contents,
after
about 2%. This is attributed to reaction of the impregnant with Sc,O, and may be related to the fact that the molar volume of Ba,Sc,O, is much greater than that of Sc,O, so that pores may be cut off. Obviously, the life of a mixed matrix scandate cathode [3,4,12] will be about two times shorter than that of a top-layer scandate cathode. Because - as has been shown in figs. 5 and 7 - the emission of top layer cathodes with 4-l-l impregnant is very stable, it makes sense to reconsider the influence of the composition of the top layer. The result is shown in fig. 8. Since the standard deviation of the mean values presented is only about 4%, it may be concluded that there is a shallow maximum near to the 5% composition. It can be seen in fig. 9 that fairly high emissions can also be obtained with a mixed scandia oxide. Though not shown, similar possibilities have also been mentioned in ref. [12]. However, considering the remark with respect to the role of scandia in section 2.2 of the present paper, it may be expected that the emission recovery after ion bombardment will be worse for a cathode with a top layer of W + Sc+WO,, than for a cathode with a top layer of W + Sc,O,. This will be shown in section 4 (see the last row of table 2 where R is a measure for the recovery). 3.2. Top layer containing partially covered tungsten Considering at an early stage of our investigations the results summarized in section 2.2 and the low mobility of Sc,O, over the surface, it was expected that - from the viewpoints of homogeneity of emission and recovery after ion bombardment - a surface layer of partially covered tungsten grains might behave better than the mixture of tungsten and scandia grains examined above. Anyhow, it was decided that examination of the properties of this geometry would provide useful additional information.
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(Al
ao60-
10
20
LO 60 100 -time
(h)
Fig. 10. Life test results of cathodes with a top layer containing partially covered tungsten. Cathode temperature 1220 K, continuous load 1.4 A/cm’. Curve (1): sinter temperature 1800°C. Curve (2): sinter temperature again 18OO“C but 50% of W added to the top layer. For a sinter temperature of 1500°C with this mixture in the top layer the results are similar to curve (1). The curves were obtained from averaging over 5 diodes.
Consultation with P. Hokkeling of the Materials Section of our laboratory gave us the assurance that tungsten grains which are partially covered with ScH, can be readily made. Moreover, at room temperature the latter compound is sufficiently resistant to air. It is noted that decomposition occurs at about 800°C [13]. Hence, at least partial oxidation may occur during sintering and impregnation. The partially covered tungsten grains are made as follows. From tungsten, similar to the powder used for the W + Sc,O, top layers, a plug is pressed with a density of about 60% of the density of solid tungsten. Metallic scandium is deposited on top of the plug and melted in vacuum. Because the surface energy of SC is much lower than that of W [14], the tungsten is wetted all over its surface within the pores. Upon cooling down in hydrogen SC is converted to ScH, which has a substantially greater volume than scandium so that the system becomes brittle enough to be pulverized. After baking the resultant powder for about 15 min in hydrogen at 800°C it can be ground to the wanted size. The weight increase of the tungsten caused by the coverage is about 3%. Pressing, sintering and impregnation are carried out in the way described in section 3.1. Pressure and sinter temperature are again such that the weight increase due to impregnation with the 4-l-l composition is about 4%. Curve (1) in fig. 10 shows the emission during the first 20,000 h of life for cathodes sintered at 18OOOC.It can be seen from comparison with fig. 5 that
J. Hasker et al. / Properties and manufacture of scandate cathodes
185
this emission behavior is certainly satisfactory. It should be mentioned however that, due to the sintering and cooling down in hydrogen , the difference in volume of SC and ScH, gives rise to unflatness of the (emitting) surface. At the center of the cathode the distance to the anode in a planar diode configuration is about 25 pm smaller than at the edge. This problem can be avoided by using a mixture of, e.g., 50% W and W/ScH, for the top layer. It can be seen from curve (2) in fig. 10 that, again for a sinter temperature of 1800 “C, the emission is about 10% below curve (1). This may be due to SC evaporation during sintering. Obviously, this loss can be reduced by using a higher pressure and a lower sinter temperature during the manufacture of the plug, in particular when this temperature is kept below the melting point of SC (1541°C). The emission result for a sinter temperature of 1500°C is, within narrow limits, similar to curve (1) in fig. 10. It will be shown in the next section that similarity does not apply to the recovery of emission after ion bombardment.
4. Ion bombardment and exposure to air The recovery of the emission of a cathode after ion bombardment is of major importance for its application in electron tubes. Obviously, the bombardment depends on the tube processing applied. For instance, after the usual processing of television display tubes serious bombardment ‘with highenergy ions still occurs. It is the purpose of the present section to look at the effect of ion bombardment without making expensive tubes. This facilitates both the cathode choice for tubes and the selection of experiments to be carried out in our scanning Auger apparatus for better understanding of the mechanisms. The influence of the sinter temperature on the recovery will be considered for sintering in hydrogen of both types of top layer cathodes. For this purpose, first the experimental procedure will be described. The sputter procedure has been chosen such that it causes a noticeable emission decrease. On the other hand, to allow the observation of differences between various cathodes, the conditions are such that the cathodes are not completely destroyed by the ion bombardment. Argon is used as the sputter gas to avoid chemical interactions. The cathodes are mounted into an ultra-high vacuum vessel containing a movable anode on an oven. After pumping (pressure about lo-” Torr) the anode - moved away from the cathode - is degassed at about 700°C for 15 min. The cathodes are activated in the way described in section 3. After activation the current density is measured with a 1500 V pulse over a diode distance of 300 pm. Its value is denoted by &(lSOO). With a pulse frequency of 10 Hz and a pulse length such that the anode dissipation is 5 W at j(l500) =ji(lSOO), the cathode is operated in lo-’ Torr Ar for 40 min. Fig. 11 shows the gradual decrease of j(1500)/~‘~(1500) for both
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J. Hasker et al. / Properties and manufacture of scandate cathodes
T = 1220K
scandate
1
10
0
-time
20 (minutes)
30
cathode
40
Fig. 11. Decrease of j(1500)/ji(1500) during sputtering. The cathodes were operated with a 1500 V pulse over 300 pm in 10e5 Torr argon. The pulse frequency was 10 Hz and the pulse length was chosen such that the anode dissipation is 5 W at j(l500) = ji(1500). The circles show the decrease of j(1500)/ji(1500) during a second 40 min sputtering, carried out after the recovery presented by the drawn curve in fig. 12.
an M cathode and a cathode with a top-layer of W + 5% Scz03, at a cathode temperature of 1220 K. The recovery in vacuum at this temperature and a continuous load of 1 A/cm2 during 2 h is shown in fig. 12. It can be seen that total recovery occurs for the M cathode. As confirmed by Auger experiments, this is due to recovery of the Ba and 0 occupation of the surface. Obviously, the latter is not sufficient for the scandate cathode because it hardly shows emission recovery at 1220 K. This will be considered in more detail in the next section. Fig. 12 also shows the subsequent recovery of the scandate cathode during 1 h at 1320 K, again with a continuous load of 1 A/cm’. During this hour the measurements of j(l500) were performed at 1220 K. The circles in figs. 11 and 12 show the decrease of j(lSOO)/j~(l500) during a second 40 min sputtering in the way described above and the subsequent recovery in vacuum. It can be seen that this second bombardment has increased the damage. The current density at 1500 V after the second bombardment and recovery is denoted by j,(lSOO). The ratio R =j~(l500)/~~(1500)
(3)
will be used as a measure of the recovery of emission. Table 2 shows the values of R for various top layer cathodes made with different pressures and similar temperatures which have been taken such that
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J. Hasker et al. / Properties and manufacture of scandate cathodes
I
T=1220K;
T=1320K
60-
I 0
I
I
I
60
120
180
-time
(minutes)
Fig. 12. Recovery in vacuum after the sputtering shown in fig. 11. During recovery the cathodes were continuously loaded with about 1 A/cm’. All measurements of j(lSO0) were performed at 1220 K. The circles show the recovery after a second 40 min sputtering.
in all cases the weight increase due to impregnation is 4.2%. The influence of these processing parameters can be clearly seen. The difference for both top layers with ScH, must be due to excessive SC evaporation during sintering at 1800°C, while the difference for both W + Sc,O, layers may be caused by a difference in tungsten oxidation. According to table 2 the best results are obtained with W + W/ScH, top-layers pressed at 4 atm and sintered at 15OOOC. However, after another 70 h of sputtering in the way as described above a recovery up to 58% at 1320 K was the best that could be obtained. The remaining damage in the described experiments indicates that in particular the recovery of the W + Sc,O, cathodes is not good enough for
Table 2 Values of R for various Top layer
top layer cathodes Sinter temperature
Pressure
(“C)
(atm)
;“w )
w+sssc,o, W + SOXW/ScHr W + 50%W/ScH2
1900 1500 1800 1500
2 3.5 2.5 4
65 75 55 80
w + 9%sc,WO,,
1900
2
52
w+s%sc,o,
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188
j
___________________
0
5 -time
lA/cm*
10
15
20
(minutes)
Fig. 13. Emission rise in vacuum after switching on the heater power for cathodes exposed to (moist) air after activation. Before the exposure the anode voltages were set at “space-charge” limited emission with a cathode load of 1 A/cm’, at cathode temperatures of 1320 and 1220 K for the M and the scandate cathode, respectively.
application in television display tubes in the case of the usual processing. This has been confirmed by experiments in tubes carried out by our colleagues at the Philips Elcoma Tube Development Department. On the other hand, the use of top-layer scandate cathodes can improve the performance of, e.g., camera tubes, oscilloscope tubes and klystrons. For a number of applications fast emission recovery after exposure of the (cold) activated cathode to moist air is important. To provide some information in this respect an M cathode and a top-layer W + S%Sc,O, cathode were mounted into the vacuum system and activated. The anode voltages were set at space-charge limited emission with a cathode load of 1 A/cm2. Next, the heater supplies were switched off and - after cooling down - air (humidity - 50%) was let into the system. After pumping the heater supplies were switched on again and the emissions versus time measured. The result is shown in fig. 13. The scandate cathode shows immediate recovery, whereas the M cathode has been poisoned. It is noted that the recovery of the scandate cathode is not confined to the “space-charge” region (at v = 900 V - see fig. 2 - it occurs as well) and that several repetitions do not affect this result. The difference with the M cathode must be due to a different bonding of barium on the surface. According to Auger measurements [l], even repetition of the activation of the M cathode does not completely restore the surface composition. It was found that after exposure to air and reactivation both the Ba-0 and the excess 0 concentration are about 30% lower than after activation of a fresh cathode.
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5. Surface analysis
The surface composition of the top layer cathodes was examined with a PHI model 590 SAM. The elemental mappings in the following were measured with a primary beam energy of 5 keV and a spot width which, depending on the magnification, ranges between 0.2 and 1 pm. It should be noted that the contrast and intensity setting is not the same in all pictures. The raster consists of 100 x 100 points. The total measuring time per point is 100 ms. Only the tungsten and scandium mappings will be shown because the oxygen pictures and to a lesser extent the Ba pictures too - are congruent with the scandium pictures. More quantitative information can be obtained from a display of the various pph’s (peak-to-peak heights in the dN/d E versus E spectrum) during selected line scans. When considering and comparing pph ratios in the following, it may be useful to realize that - as can easily be calculated - e.g. for tungsten with one monolayer of Sc,O, the pph ratio Sc(333 eV)/W(179 eV) = 2.1, while with two monolayers its value is 7.2 and for a homogeneous mixture of equal volumes of SQO; and tungsten the ratio is 2.3. Hence pph ratios provide useful but not complete information. In fact, similar sets of pph ratios can give rise to very different emission behavior. The effect of argon-ion sputtering will be examined with the aid of a differentially pumped ion gun, an angle of incidence of the ion beam on the sample of about 0.2 rad and 1.5 keV ion energy. Fig. 14 shows the W and SC mappings for a W + SSSc,O, top-layer “cathode” which has not been impregnated. The expected grainy structure can be clearly seen. To avoid disturbances by charging of the biggest SQO, grains, the measurement was carried out at 1220 K cathode temperature. On a very small grain surrounded by tungsten an Auger measurement on Sc,O, could be carried out without disturbances at room temperature. The pph ratio O/SC
Fig. 14. SAM mappingsof Sc (left) and W (right) for a non-impregnated “cathode”, measured at 1220 K. Scanned area 50 pmx40 pm.
W + %%Sc,O, top-layer
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J. Hasker et al. / Properties and manujacture of scandate cathodes
Fig. 15. SAM mappings of SC (left) and W (right) for a W + 5%Sc,O, top-layer cathode, measured at room temperature, after activation (1) and subsequently 30 min of sputtering (2), another 30 min of sputtering (3) and reactivation (4). Scanned area 50 /.Im X 40 pm.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
was found to be equal to 1.14. This result has already been used in ref. [l]. The measured ratio does not change in the case of simultaneous argon-ion sputtering or when the temperature is increased up to 1220 K. For the sputter-cleaned sample the value of the SC/W pph ratio found from scanning over an area of 400 pm x 400 pm is 0.7. Then, taking selected Sc,O, and W regions as an internal standard, the composition of the top layer can be, calculated from the spectrum. The result agrees fairly well with the W + 5%Sc,O, mixture used. The SAM mappings of SC and W for a cathode with a top layer made of W + 5!%Scsc,0,,sintered at 19OO’C and impregnated with the 4-l-l composition, are shown in fig. 15. Comparison of the pictures (1) with fig. 14 shows the influence of impregnation and activation. The SC distribution has become rather homogeneous, i.e. at the surface the grainy structure has gone. Pictures (2), (3) and (4) in fig. 15 show qualitatively the influence of sputtering and subsequent reactivation. More quantitative information is shown in table 3. The data in the first three rows of the table are mean values of three cathodes and have been averaged over the scanned area. The fourth row was obtained from our measurement shown in fig. 4 of ref. [l]. Though the latter was carried out with another apparatus on another W + S%Sc,O, top-layer cathode, it can be seen that the result agrees fairly well with the first row. The results presented so far strongly indicate that reaction during impregnation gives rise to a thin-layer coverage of the surface by scandia: most of it can be removed by sputtering. Reactivation gives only partial recovery, it cannot reproduce the initial state. Hence - when compared with the bulk composition of the top layer - there is scandium depletion near to the surface which is caused by impregnation. It is noted that after reactivation the value of j(lOOO)/‘i(lOOO) was 0.6. This compares quite well with the results presented in table 2. Comparison of the scandium mappings in fig. 15 with those of ref. [3] shows that in our case the scandium distribution after activation is much more homogeneous. As can be seen in the last row of table 3, this results in an about 5 times higher SC/W pph ratio.
Table 3 pph ratios for top layers of W + 5%Sc,O,: W(179 eV), Sc(333 eV), O(510 ev), Ba(584 eV) O/Ba
Sc/Ba
W/Ba
k/w
O/SC
After activation After sputtering After reactivation
4.5 8.3 3.2
2.3 1.7 1.4
0.5 9.4 1.6
4.8 0.2 0.9
1.9 4.8 2.2
Ref. [l] after activation
4.7
2.6
0.6
4.4
1.8
2.5 ‘)
1.5 a)
1.8 ‘)
0.9 a)
1.7 a)
‘) Mixed matrix cathode of ref. [3], after activation.
J. Hasker et al. / Properties and manufacture of scandate cathodes
192
Line scans of W, SC, Ba and 0 for one of the cathodes of the first rows of table 3 are shown in fig. 16. They show that there are tungsten regions with a thin-layer scandia coverage and tungsten regions without scandia. The barium signal from the scandia regions is greater than that from the tungsten regions. Apart from differences in barium concentration, this may be due to matrix effects. Whether the scandia spots or the adjacent W spots - where, in addition to Ba, 0 is also present - are the sources of high emission is not yet clear. The SAM mappings of Sc and W for a cathode with a top layer made of W + SO%W/ScH, and sintered at 1500°C are shown in fig. 17. Comparison of both tungsten pictures shows the scandium coverage of the tungsten grains. This is again a thin layer, as can be seen from both scandium pictures (after sputtering SC is left only in the pores) and, more quantitatively, from the SC and Ba pph decreases during sputtering. The latter are shown in fig. 18. This
wh t
0
Lo
80
120
160
200
;- microns Fig. 16. pph’s of W, SC, Ba and 0 during a line scan. The measurement applies to one of the three cathodes of the first row in table 3. The units are different and arbitrary.
J. Hasker et al. / Properties and manufacture of scandate cathodes
193
Fig. 17. SAM mappings of Sc (left) and W (right) for a W + 501!ScH2 top-layer cathode, measured at room temperature, after activation (1) and subsequently 1 h sputtering and reactivation (2). Scanned area 20 pm x 16 pm.
-sputter
time lmin)
Fig. 18. pph’s of SC, Ba and 0 during sputtering for the cathode of fig. 17.
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J. Hasker et al. / Properties and manufacture of scandate cathodes
means that the original partial coverage of the tungsten grams, with a thickness of about 0.5 pm, is no longer present at the surface. Nevertheless, the scandium distribution left after the various processing steps is more favorable than in the case of the W + 5%&O, top layer. This is reflected in the value of j(lOOO)/ji(lOOO) after sputtering and reactivation which is 0.76 for the cathode of fig. 17 instead of 0.6 for the cathode of fig. 15.
6. Conclusions and discussion For both types of top-layer scandate cathodes considered in the present paper the emissions are at least as high as for the known pressed and impregnated scandate cathodes. Moreover, the life and/or the staying power against ion bombardment have been improved. The top layer cathodes do not show an emission anomaly at low applied voltages. At current densities smaller than j,, the current-voltage characteristic is similar to that of, e.g., an M cathode. However, at current densities higher than j,, the deviation from the space-charge limitation is much smaller than for the M cathode. When compared with a mixed matrix, the top layer facilitates impregnation while the reaction of the impregnant with tungsten is not affected. For both the emission after a few hours of life and the emission after 40,000 h of life, polishing of the cathode surface after impregnation cannot be recommended. On the other hand, ultrasonic cleaning after impregnation gives rise to good reproducibility. With 5-3-2 impregnant it takes - 2000 h before the emission is maximum. Therefore - though a high emission level can be maintained for nearly 5 years _ the use of 4-l-l impregnant is preferred. The observed difference in emission behavior for both compositions will be a subject of further investigation. Auger experiments on impregnated tungsten plugs, carried out by H.J.H. Stoffelen of our laboratory, already indicated that - after the usual activation procedure - the excess oxygen concentration [l] on the surface is greater in the case of the 5-3-2 impregnant than in the case of the 4-l-l impregnant. The combination of sputter- and scanning-Auger experiments shows that in agreement with our emission experiments [l] - thin-layer coverage of tungsten by scandia is important to the high emission. Moreover, when compared with the bulk composition of the top layer, there is scandium depletion near to the surface. Both thin-layer coverage and depletion are related to the impregnation process. They imply remaining damage after extended ion sputtering: reactivation does not result in complete recovery of the surface composition. Though recovery occurs after moderate sputtering, this takes too much time at the operating temperature of 1220 K. These properties make that - in case of the usual processing - the cathodes are not
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195
yet suited for application in television display tubes. Therefore, further improvement is under investigation. On the other hand, given a sufficiently good vacuum and/or relatively low accelerating potentials, the emission capability and the very long life of the new top-layer scandate cathodes can improve the performance and the life of electron devices, such as camera tubes, oscilloscope tubes and klystrons. In addition, for some applications it may be advantageous that the cathodes in activated state are insensitive to exposure to (moist) air. The processing, and in particular the temperature during sintering, is important to the emission recovery after ion bombardment. This is very pronounced for the top layers of W + W/ScH,: R = 80% for sintering at 1500”C, whereas R is only 55% for sintering at 1800°C. From the considerations and results of sections 2.2, 4 and 5, it will be clear that more details with respect to the emission mechanism are required. This is another subject for further investigation.
Acknowledgement
The authors are indebted to J.J. van Lith of our Cathode Technological Department for making cathodes and performing life test measurements.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
J. Hasker and H.J.H. Stoffelen, Appl. Surface Sci. 24 (1985) 330. C.R.K. Marrian, A. Shih and G.A. Haas, Appl. Surface Sci. 16 (1983) 1. S. Yamamoto, S. Taguchi, T. Aida and S. Kawase, Appl. Surface Sci. 17 (1984) 504. S. Taguchi, T. Aida and S. Yamamoto, IEEE Trans. Electron Devices ED-31 (1984) 900. S. Yamamoto, S. Taguchi, T. Aida and S. Kawase, Appl. Surface Sci. 17 (1984) 517. J. Hasker, Appl. Surface Sci. 16 (1983) 220. R.S. Bakhtiyarov and B.B. Shishkin, Radio Eng. Electron. Phys. 16 (1971) 384. R.S. Bakhtiyarov and B.B. Shishkin, Soviet Phys.-Tech. Phys. 17 (1973) 1752. A. van Gostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. Calculated from the data in: I. Bar-in, 0. Knacke and 0. Kubaschewski, Thermochemical Properties of Inorganic Substances+ Supplement (Springer, Berlin, 1973 and 1977). [ll] V.A. Levitskii, Inorg. Mater. 16 (1981) 1489. [12] S. Taguchi, T. Aida, S. Yamamoto and Y. Honda, UK Patent Application GB 2 116 356 A. [13] E. Fromm and E. Gebhardt, Gase und Kohlenstoff in Metallen (Springer, Berlin, 1976). [14] AR. Miedema, Inst. Phys. Conf. Ser. 55 (1981) ch. 7.