in excess of 55 h of continuous operation. It was found that the life of such cathodes depends mainly on the adhesion of the LaBfi coating to the core wire instead ...
Processing and characterization of LaB6coated hairpin cathodes Rajendra S. Khairnar, P. W. Mahajan, Dilip S. Joag, A. S. Nigavekar, and P. L. Kanitkar Citation: Journal of Vacuum Science & Technology A 3, 398 (1985); doi: 10.1116/1.573228 View online: http://dx.doi.org/10.1116/1.573228 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Simple, high current LaB6 cathode Rev. Sci. Instrum. 60, 964 (1989); 10.1063/1.1140305 Evaluation of single crystal LaB6 cathodes J. Vac. Sci. Technol. B 1, 100 (1983); 10.1116/1.582529 Brightness of LaB6 cathodes J. Appl. Phys. 51, 5566 (1980); 10.1063/1.327284 A Method of Preparation of LaB6 Cathodes Rev. Sci. Instrum. 42, 1765 (1971); 10.1063/1.1685003 Poisoning of LaB6 Cathodes J. Appl. Phys. 40, 44 (1969); 10.1063/1.1657092
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Processing and characterization of
laB6~coated
hairpin cathodes
Rajendra S. Khairnar, P. W. Mahajan,a) DHio S. Joag, A. S. Nigavekar. and P. L. Kanitkar Department of Physics, University of Poona. PUNE-41 iOO7, India
(Received 27 August 1984; accepted 30 November 1984) Lanthanum hexaboride (LaB6J was cataphoretically deposited on a filamentary hairpin support of tantalum. Such filaments were further processed to form cathodes. Various procedures involved during the processing of cathodes, such as carburization of the core tantalum wire in hairpin shape, deposition of LaB6 onto it, and sintering of such cathodes are described. These cathodes were characterized for thermionic work function measurements, life tests, and thermal shocks in a diode configuration. The ultrahigh-vacuum (URV) sintering gives better emission stability, compact coating, low porosity, etc., as compared to high-vacuum sintering. The work function values were found to be in the range of 2.6-2.8 eV for various cathodes whereas, the preexponential factor seemed to vary in the range of 30-200 A cm - 2K -- 2. These cathodes showed life in excess of 55 h of continuous operation. It was found that the life of such cathodes depends mainly on the adhesion of the LaBfi coating to the core wire instead of the evaporation rate ofLaB 6 at operating temperatures. The cathodes sustained a larger number of thermal shocks i.n DHV than in a high-vacuum environment.
I,
INTRODUCTION
Lanthanum hexaboride (LaB6) is known as an excellent thermionic emitter 1,2 which finds extensive use in a large number of devices like high-density plasma guns,-] mass spectrometers,4,5 etc. LaB6 cathodes have become very popular on account of their unique quality of high brightness,6 due to which they have found extensive applications in electron probe-forming instruments like electron microscopes. The LaB6 cathodes used in microscopes at present are either in the form of pressed, sintered rods 7-11 or single crystals. 12-15 Considerable development has already been made in these forms to the extent that they are now being commercially produced. Indirectly heated LaBo pressed, sintered rods consume a considerable amount of power and make the gun design complicated. Also, the single-crystal pointed cathodes are very fragile. On the other hand, the coated cathodes have an advantage of ease offabrication and low power consumption, as compared to the rod cathodes. There have been quite a few attempts at making LaB 6 coated refractory metal cathodes for a few applications like ion gauges4 and ion lasers,16 but no attempts seem to have been reported to make coated cathodes for electron microscopes. We, in this paper, present various aspects offabrication of LaB 6 -coated tantalum hairpin cathodes and their characterization in diode configuration, which indicate the feasibility of such cathodes for use in electron microscopes.
II, EXPERIMENTAL
Ao Cathode processing In the present work LaB6-coated cathodes were shaped in hairpin filamentary form with the intention of testing them in electron microscopes. The various stages involved in processing such coated cathodes were: shaping of a length of core wire, carburization, cataphoretic deposition of LaB 6 , and sintering of the coated filaments. 398
J. Vac. Sci. Techno!. A 3 (2), Mar/Apr 1985
A tantalum (Ta) wire of U5-,um diam of desired length was shaped in the form of a hairpin using a shaping jig. It was then spot-welded to the electrodes of a filament base. This filament, along with the base, was then held in a flashing jig, and was momentarily flashed in high vacuum for a few times to white-hot temperature by resistive heating. This caused initial degassing and relaxation of stresses in the core wire, so that its shape did not change by heating during later stages. The filament was then carburized so as to arrest boron diffusion from LaB 6 into the Ta core wire. 1 The carburization was carried out in butane gas at 1 atm by resistive heating at temperatures above 1173 K for a certain period. At these temperatures, butane cracks in the vicinity of the filament surface, and carbon thus formed diffuses into the core wire. The filament temperature was monitored using an optical pyrometer within the accuracy of ± 10 K. The carburized filament was momentarily flashed at elevated temperatures in high vacuum two or three times to cause the degassing of impurities appearing on the surface during carburization. The filament surface was then examined under optical and scanning electron microscopes. A large number of trials with the temperature in the range of 1173-1373 K, and the heating periods in the range of 30-180 s were taken to select these parameters for carburization of the filaments. Cataphoretic depositing of LaB6 powder, having a grain size of 3 /-ill, was done on the carburized Ta filaments using a technique very similar to that described by Nasini 16 and Favreau.17 In particular, a voltage difference of 15 volts de across the electrodes in the cataphoretic cell with 1 rnA current resulted in the LaB6 coating with the overall filament thickness of 200 pm. The quality of the coating was observed under the optical microscope and the scanning electron microscope (SEM). It is known that the quality of the coating depends on the electrolyte used in the cataphoretic process. 18 • 19 Our results on cataphoretic deposition using various electrolytes viz. He!, NH4 N0 3 , La(N0 3 h, and RNO} have shown that RCl gives better adhesion and a smoother and more uniform LaB6 coating as compared with the rest.
0734-2101185/020398-05$01.00
@ 1985 American Vacuum Society
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Khairnar et alo : Processing and characterization
LaB(i grains, when deposited cataphoretically, adhere rather loosely to each other and to the core wire. It is desirable to make their packing dense with low porosity, a smooth surface, and a strong adherence to each other and to the core wire. This should result in stable electron emission, long life, and should sustain a large number of thermal shocks. This was achieved by sintering the cathodes in a vacuum of 1 X 10-- 8 Torr for 1 hat 1923 K. A large number of cathodes were processed to optimize the set of parameters involved in the above processing. Cathodes having coating thickness in the range 150-250 jtm, processed under identical conditions, were chosen for characterization.
399
a
B. Work function measurements
Thermionic work function of the sintered LaB6 coated cathodes was measured in a diode geometry. J- V characteristics were taken at various temperatures ranging from 1173-1573 K. The temperature of the filament was measured with an optical pyrometer. All temperatures were corrected for the spectral emissivity of LaB6 at a wavelength of 0.65 jtm. 2 The values of the work function were determined from the Richardson plots.
C. Cathode life The sintered coated cathodes were tested for their active life in a diode geometry in the vacuum of 1 X to-II Torr. Each cathode was kept at 1923 K and the anode potential was adjusted (200-250 dc) to extract a constant emission current. Continuous emission of 75 rnA was drawn and the current was recorded during the life test using a chart recorder. The cathode life was defined to be over when the emission current changed by ± 5% ofits original value. The life tests were also carried out in high vacuum of the order of 5 X 10- 5 Torr for the sake of comparison.
D. Thermal shocks Thermal shock tests were carried out for sintered coated cathodes in a diode geometry in the vacuum of 1 X 10- 8 Torr by instantaneously raising the filament temperature to 1923 K, operating the cathode for a certain time, and then suddenly bringing the filament down to room temperature. During this process, the emission current was monitored at a constant level of75 mA The thermal shock tests were also carried out with LaB6-deposited tungsten and tantalum core wires (both uncarburized). The cathodes were also tested for thermal shocks in high vacuum environment. III. RESUI..TS AND DiSCUSSION
A. Carburization
The filaments carburized at 1373 K for various heating periods resulted in a high degree of embrittlement. The surface appeared to be nonuniform and lumpy as observed under SEM. On the other hand, the filaments carburized at 1173 K showed hardly any observable change in their surface texture. The filaments carburized at 1273 K showed gradual changes with time of carburization within the range
b FIG. l. (a) Ta filament carhurizcd for 60 sat 1273 K. (h) Ta filament carburized for 180 s at 1273 K showing swelling.
of 30-180 s. Figures 1(a) and 1(b) show SEM micrographs of filaments carburized at 1273 K for 60 sand 180 s, respectively. Figure l(b) shows the swelli.ng of tantalum to be about 8%. This swelling can be attributed to the precipitation of carbon in tantalum, mainly at the grain boundaries,2o.2! although interstitial precipitation of carbon 22 in a single crystal cannot be completely ignored. After a large number of trials, the carburization period of about 90 s at 1273 K was found to be suitable for further work. B. Cataphoretic deposition
Figures 2(a) and 2(b) show the SEM micrographs of the filaments deposited using RCI and RN0 3 as electrolytes, respectively. The filaments deposited using HCI resulted in a uniform and compact coating whereas those coated using HN03 were nonuniform and had a lumpy appearance to the surface. HCI is used as a cleaning agent in the purification process of LaB 6 and to wash away excess boron present in LaB6 powder. This type of cleaning process could have made the LaB6 surface free from various impurities and resulted in a uniform and compact coating, as seen in Fig. 2(a). On the other hand the nonuniform and rough surface of the filament [Fig. 2(b)] seems to be due to the chemical reaction of LaB6 with HN0 3 . It is, in fact, known to reace 3 with LaB 6 , forming a compound with boron releasing free lanthanum. More details on this issue have been published elsewhere. 19
J< Vac. Sci. Techno!. A, Vol. 3, No.2, Mar/Apr 1985
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Khairnar sf 8/. : Processing and characterization
a
b
a
b
FIG. 2. LaB6 ; coated filaments using (a) Hel. and (b) RNO, as electrolytes.
FIG. 3. SEM micrograph of high-vacuum-sintered LaBo; coated Ta (a) showing dendritic type of growth; (b) nonuniform grain size.
c. Sintering
D. Work function
The cathodes sintered in high vacuum resulted in the formation ofa dendritic type of surface which could be attributed to the poisoning of LaB6 due to residual hydrocarbons backdiffused from the pumping system, and to the oxidation of the surface La atoms. 24 The growth of such newly formed compounds on the surface and within the pores through the coating, may form dendritic structures, as seen in Fig. 3(a). The grains of these cathodes were found to be highly nonuniform in size. A number of voids and pits were also present in the surface structure [Fig. 3(b)]. This type of sintering also showed instability in thermionic emission. The LaB 6 coating was found to flake off the core very easily during the operation. The cathodes sintered in a vacuum of 1 X 10- 8 Torr resulted in the formation of a more compact surface with a rather uniform grain size and low porosity, as seen in Fig. 4. This type of sintering gave uniform emission during operation. The gases evolved during sintering in the vacuum of 1 X 10- 8 Torr were analyzed by the quadrupole mass spectrometer (Balzer's QMG 112). The gases that evolved at various temperatures up to 1923 K during sintering were mainly H 2 , CH4 • H 20, 02' CO 2 , CO, N z, and some hydrocarbons. Figure 5 depicts the mass spectra taken at the beginning and the completion of sintering at 1923 K.
The work function of the cathodes tested was found to be in the range of2.6-2.8 eV. It was found that the preexponential facter A varied largely in the range of 30-200 A cm- 2 K - 2. The value of A is known to vary with the porosity and density of packing of coated cathodes. 25 •26 Figure 6 depicts the Richardson plots for the LaB 6 -coated and a conventional
FIG. 4. SEM micrograph of ultrahigh-vacuum-sintered LaBo-coated Ta showing uniform grain size.
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Khairnar tit Ill: Processing and characterization
401
401
FIG. 5. Mass analysis ofthermally desorbed species from the coated cathodes at 1923 K (-) at the beginning, and (---) after sintering for 1 h at 1923 K.
Ar'
_/,~ 48
40
O·
32 ____ Mass ( emu) - - -
tungsten hairpin cathodes. It shows that for a given constant emission density of, say, IS A em - 2, the LaB/) cathode can be operated at 1873 K, whereas the tungsten cathode must be operated at a much higher temperature of 3020 K. Since the temperature is the factor deciding the life, the thermal noise, the emission stability, and the emission energy spread, low -4--
11
LaB6
"-
~
\
ij
~
.l J 3
\
E. Life tests and thermal shocks
The life of the coated cathodes was terminated mainly due to flaking of the LaB 6 coating off the core wire in the region of highest temperature (i.e., at the filament apex). Thus the life was decided by adherence of the coating to the filament rather than excessive evaporation of La and B from the hottest point. 7 Twenty test cathodes showed the average life to be in excess of 55 h of continuous operation. The mean life of a sintered-rod cathode in Broers' type LaB/) gun 27 has been found to be 100 h, and the cathode expires due to excessive evaporation of LaB6 at the hottest point, which is not the electron emitting area of the tip. The quoted values of life of such press~sintered LaB 6 -rod cathodes varies over a large range of 80-150 n, mainly because of the uncertainty in thickness of the rods and the operating temperature. 3 Quoted values of life for a single crystal cathode in high resolution electron microscopes (HREM) with accelerating potentials of a few hundreds of kV are reported to be in excess of 200 h. Although the life reported for the present cathodes is less, these may be more desirable from the point of view of simpli-
CO
w
i:
temperature operation of the cathode is always desirable. In this context, LaB 6 -coated cathodes appear to be far superior to those of conventional tungsten, and are quite comparable to the LaB(, sintered rods. 2
., TABLE L Thermal shocks for carburized and uncarburi:red core wires.
,
, •
\
7
11/ Tlx 10 4 K-' - - -
FIG. 6. Comparison of Richardson plots of LaB6 -coated Ta and conventional hairpin W cathodes.
Core material LaB. coated W LaB. coated carburized W LaB6 coated Ta LaB. coated carburized Ta
High vacuum 4 X 10- 6 Torr
Ultrahigh vacuum 1 X 10- 8 Torr
15 28 21 30
22 50 48
125
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Khalrnar et al. : Processing and characterization
TABLE II. Comparison of performance of LaB6 -coatcd cathodes with conventional hairpin tungsten at constant emission density. Specifications Operating temperature Saturated current density Work function Life Thermal shocks Pressure in test diode
W
3023 K 8Acm -2 4--4.3 eV 30 min 5X 10- 8 Torr
LaB 6 -c()ated Ta 1873 K 8Acm - ? 2.6-2.8 eV 52 h 125 ± 5 5xlO-"Torr
city in replacement, preparation, and perhaps lower cost. The life of cathodes was also tested in a vacuum of the order of 1 X 10- 5 Torr. This observation showed the life to be 40% less, as compared to that under the vacuum of 1 X 10- 8 Torr. The adhesion between the LaB 6 coating and the core metal could be tested by applying a thermal shock to the coated filaments. Because of the thermal differential coefficient of expansion, it was expected that the coating could be loosened and then would get flaked off after a certain number of thermal shocks. Thus the smaller the difference in the thermal coefficient of expansion between the core wire and the coating, the stronger adhesion could increase the ability of the cathode to sustain a large number of thermal shocks. [The thermal coefficients of expansion for Ta and LaB 6 are nearly the same (z4X 10- 6 deg- 1).] Table I shows the comparison for the thermal shocks sustained by carburized and uncarburized LaB/) coated filaments in the vacuum of 1 X 10- 5 and 1 X 10- 8 Torr. The thermal shock tests taken at high vacuum were seen to sustain fewer shocks, as compared to that in the vacuum of 1 X 10- 8 Torr, mainly because of poisoning of the cathodes in high vacuum. IV. CONCLUSIONS
(1) Vacuum sintering of the cathodes provides a smooth and compact surface, and causes better adhesion. Also, most of the gaseous impurities are evolved. (2) The work function and the preexponential term A in the Richardson equation are comparable to those reported earlier in the literature, related to the press-sintered LaB6 cathodes. (3) The filaments exhibited life in excess of 55 h and sustained about 125 thermal shocks. (4) The performance of cathodes in the vacuum of 1 X 10.- 8 Torr from the point of view of current stability, life, and thermal shocks was better than in high vacuum.
(5) Comparison of conventional tungsten and LaB 6 -coated filament was made at constant emission density of 8 A em - 2; the coated filament operated at 1873 K versus 3023 K for tungsten. ACKNOWLEDGMENTS
Financial support by the Department of Science and Technology, New Delhi, is gratefully acknowledged. Thanks are due to the Head of the Department of Physics, University of Poona, for providing facilities.
Present address: Bharat Electronics Ltd., Pashan Road, Pashan, PUNE411008, India. 1J. M. Lafferty, J. App!. Phys. 22, 299 (1951). 2H. Ahmed and A. N. Broers, J. App!. Phys. 43, 2185 (1972). "D_ M. Goebel, J. T. Crow, and A. T. Forrester, Rev. Sci. lnstrum. 49, 469 (1978). 4J. D. Buckingham, Bf. J. Appl. Phys. 16, 1821 (1965). 'L. Kelner, H. M. Fales, and S. P. Markey, Inst. J. Mass Spectrum and 1011 Phys. 43, 249 (1982). 6H. Ahmed, in Proc. 5th European Congress Elect. Microscopy, 10 (1972). 7A. N. Broers, J. Phys. E 2,273 (1969). 8H. Ahmed and W. C. Nixon, in Proc. 6th SEM Symp., IITRI Chicago, 217 (1973). 9A. N. Broers, Rev. Sci. Instrum. 40,1040 (1969). wAkira Yonezawa, Seiichi Nakagawa, and Mikio Suzuki, J. Electron Microse. 28, 43 !1979). "Alec. N. Broers, J. Vac. Sci. Techno!. 16 (6),1692 (1979). I2T. Gibson and J. Verhoven, J. Phys. (E) 8, 1003 (1975). LlJ. Verhoven, E. Gibson, and M. Noack, J. App!. Phys. 47, 5105 (1976). l4F. J. Hohn, T. H. P. Chang, A. N. Broers, G. S. Frankell, E. T. Peters, and D. W. Lee, 1. App!. Phys. 53,1283 (1982). "T. Takigawa, I. Sasaki, T. Megufo, and K. Motoyarna, J. App!. Phys. 53, 5891 (1982). 16M. Nasini and G. Redaelli, Rev. Sci. lnstrum. 42, 1765 (1971). 17L. J. Favreau, Rev. Sci. lnstrum. 36, 856 (1965). 1HJ. E. Dalrnore, Rev. Sci. lnstrum. 54,158 (1983). 19Rajendra S. Khairnar, D. S. Joag, S. K. Kulkarni, A. S. Nigavekar, and P. L. Kanitkar, Rev. Sci. lnstrum. 55, 1505 (1984). 2°R. Suzuki, S. Kimura, T. Hase, and Y. Tsuchie, Bull. Tokyo. Inst. Techno!. 90,105 (1969). 2lp. Son, S. Ihara, M. Miyake, and T. Sano, 1. Jpn. Inst. Metals 30, 1137 (1966). 22T. S. Ke, Phys. Rev. 74, 9 (1948). 23T. Aida, M. Futamoto, and U. Kawabe, Jpn. J. App\. Phys. 18, 1393 -I
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A. N. Broers, J. Appl. Phys. 38, 1991 (1967).
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