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The tungsten powder study of the dispenser cathode
Applied Surface Science 252 (2006) 5873–5876 www.elsevier.com/locate/apsusc
The tungsten powder study of the dispenser cathode Bao Ji-xiu*, Wan Bao-fei The Institute of Electronic Beam and Vacuum Technology, Shanghai Jiao Tong University, Huashan Rode 1954, Shanghai 200030, China Received 13 June 2005; received in revised form 3 August 2005; accepted 8 August 2005 Available online 4 October 2005
Abstract The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 8C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment. # 2005 Elsevier B.V. All rights reserved. Keywords: Dispenser cathodes; Tungsten powder; Porosity; Granules; Size of porous; Cathode life
1. Introduction In the vacuum electronic apparatus the cathode is the important components, which provides the apparatus the requirable electronic beams. The characteristics and the lifetime of the apparatus are influenced by the cathode performance. Basic requirements for modern tungsten cathodes are the long lifetime and large current density. The possibility of realization of these requirements is limited by * Corresponding author. Tel.: +86 21 62932848; fax: +86 21 62933571. E-mail address: [email protected] (J. Bao).
factors such as the high operating temperature and considerable evaporation rate of the Ba emission activator [1,2]. Operating temperature decreases if Os, Ir or Ru coating of cathode surface or a Scandate cathode is used. In both cases the operating temperature decrease is ensured by introduction of some Ba compound containing impregnant in the porous base material (tungsten). The Ba is necessary to the cathode activation. The cathode has the excellent performance until the enough Ba is produced and dispersed to the cathode surface. In the whole cathode activation there are complicated physics–chemistry reaction. It is dominating that some Ba compound containing impregnant in the porous base material (tungsten) is
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.012
5874
J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
decompounded to BaO and BaO is deoxidized by the tungsten to the emission Ba. It is simply as follows:
3. Results and discussion
BaXO ! BaO þ BaY
(1)
6BaO þ W ! Ba3 WO4 þ 3Ba
(2)
Although in the normal temperature the tungsten powder does not react on air and oxygen, for a long time in the humidity air the tungsten powder will be oxidized to WO3 [9]. The tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 8C, which is the gas–solid multiphase reaction, as follows:
It is seen that the tungsten is very important. The porosity and morphology is the major factor determining the tungsten matrix properties. The tungsten matrix is made of the tungsten powder .The grain size, distribution and morphology of which is the major factor determining the cathode performance [1–3]. In our work, the possibility of cathode characteristic improvement by means of achievement of optimal size of open pore channel, and hence, optimal emission activator consumption in a geometrically stable framework without cathode composition changing, is investigated. Among problems during longlifetime dispenser cathode development one can note insufficient information, concerning selection of the particles size of the tungsten powder for emitter matrix manufacturing. The influence of the homogeneity degree of particle size distribution and their morphology on the emission longevity of cathodes is also unknown. There is no consensus of opinion concerning influence of the cathode porosity on dispenser cathode longevity. Frameworks, used by various firms, differ by porosity, for example, 23–27% and 18% in [3–8].
WO3 þ 3H2 ! W þ 3H2 O Cathode frameworks were manufactured of industrial powders, some characteristics of which are shown in the Fig. 1. It is shown that the porosity of the tungsten matrix distributes more even and the closed pore fewer the average granule size of the tungsten powder distribute more convergent. In the dispenser cathodes a porous tungsten pellet is impregnated with the activator. The tungsten matrix such as grain size, distribution and morphology directly affect the cathode parameters like the lifetime, the activator evaporation, the emission current density and distribution, etc. In work, the super-pure tungsten powder that the shape of particle is semi-spherical and the particle size
2. Experimental The tungsten powder is cleaned in the high temperature hydrogen furnace before it is pressed into the flat tungsten matrix. The tungsten matrix sintered in the high temperature hydrogen furnace is made into the cathode, which is installed to a diode to do the comparative test of the cathode performance. In the high quality microscope the morphology of the tungsten matrix can be observed carefully. The emission performance of the cathode is evaluated in the diode by setting a thermo-couple to measure the cathode temperature on the nearby emission location of the cathode. The anode is used by Mo and the cathode–anode distance is from 1.1 to 1.3 mm.
Fig. 1. The particle size distribution of the tungsten powder.
J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
5875
Fig. 2. Correlation of porosity and the tungsten powder.
Fig. 4. Correlation of the evaporation of the cathode and the tungsten powder.
is uniform is made into the tungsten matrix, which is turned into the cathode. The porosity of the tungsten matrix made of the tungsten powder that the particle is distributed as Fig. 1(a) and the particle size is different each other in the same condition is different, some characteristics of which are shown in Fig. 2. It is shown that the porosity of the tungsten matrix is 22% when the average granule size of the tungsten powder is 3.5 mm, but the porosity of the tungsten matrix is 32% when the average granule size of the tungsten powder is 5.6 mm. Namely, the porosity of the tungsten matrix is bigger the average granule size of the tungsten
powder is bigger [10]. In the high quality microscope the morphology of the tungsten matrix is shown in Fig. 3, from which the morphology of the tungsten matrix is more rough and the pore and the aperture of the tungsten matrix is bigger with the big size of particle. In the dispenser cathodes made of a porous tungsten pellet impregnated with Ba compound, the evaporation of Ba is the major factor determining the cathode lifetime. The decrease of the evaporation can improve the reliability and prolong longevity of the electronic apparatus. The tungsten powder of different average granule size is made
Fig. 3. Morphology of the tungsten matrix and the size of the tungsten powder are (a) 2.2 mm, (b) 3.5 mm, (c) 4.8 mm.
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J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
emission current density of the cathode made of the tungsten matrix of small size is bigger because the surface area of the tungsten matrix of small size is bigger than that of the big size; (ii) the tungsten matrix aperture made of big size of the tungsten powder is bigger than that of the small size of the tungsten powder is bigger. Because of the cathode made of the big pore matrix the activator not only is easily vaporized but also poisoned by bad gas such as O2 and CO2 and CO and so on and the emission current density is small. Fig. 5. Correlation of the emission current density and the tungsten powder.
4. Conclusion into the same porous tungsten matrix where the activator evaporation rate is also different. In experiment, the evaporator thickness of the anode wall is measured in the dissection diode, as shown Fig. 4. It is seen that the evaporator thickness on the anode wall is 0.25 mm when the size of the tungsten powder is 2.2 mm, but the evaporator thickness on the anode wall is 0.7 mm when the particle size of the tungsten powder is 4.8 mm. That is to say, the evaporation rate in the tungsten matrix made of the big tungsten powder is ever bigger than that of the small tungsten powder. It is explained that the aperture of the tungsten matrix made of the big size tungsten powder is bigger than that of the small size tungsten powder. The evaporation rate is bigger the aperture is bigger. In conclusion in the limited scope the pore and aperture of the tungsten matrix are diminished in order to decrease the evaporation rate of the cathode [11]. In the same porosity 28% the emission current density of the cathode made of the tungsten powder of the different size is not the same (Fig. 5). It is shown that the emission current density of the cathode is 16 A/cm2 when the particle size of the tungsten powder is 3.5 mm, but the emission current density of the cathode is 11.2 A/cm2 when the size is 5.6 mm. So the emission current density of the cathode of the big size tungsten powder is bigger than that of the small size tungsten powder. And the emission current density of the cathode is smaller the porosity is bigger. The explanation: (i) the activator and the
A fully cleaning step of the tungsten powder is necessary and the size of the tungsten powder is to sure to obtain the cathode of the even emission surface and the big emission current density and longevity. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger.
References [1] Hong-wei Zhang, Zhen-gang Bai, Shi-ji Yu, Vac. Electron. 3 (2002) 67. [2] V.A. Smirnov, IEEE International Vacuum Electron Sources Conference 7, Florida, 2002, p. 93. [3] D.H. Tomich, J.A. Mescher, J.T. Grant, Appl. Surf. Sci. 28 (1) (1987) 34–52. [4] A. Palluel, A.M. Shroff, J. Appl. Phys. 51 (1980) 2894. [5] T. Aida, H. Tahuma, S. Sasaki, T. Yaguchi, S. Taguchi, N. Koganezawa, Y. Nonaka, Appl. Phys. 4 (11) (1993) 6482– 6487. [6] N.B. Zhukova, E.V. Tolstik, V.I. Kozlov, Electronics technique S6, Materials 9 (182) (1983) 1–23. [7] A.M. Shroff, P. Palluel, Revue Technique Thomson CSF 14 (1982) 3. [8] N.N. Chubun, L.N. Sudakova, Appl. Surf. Sci. 111 (1997) 81– 83. [9] Peng Shaofang, The Tungsten Metallurgy, The Metallurgy Industry Publishing Company, 1981, pp. 25–28. [10] Irina P. Melnikova, Victor G. Vorozheikin, Dmitry A. Usanov, Appl. Surf. Sci. 215 (2003) 59–64. [11] J.M. Roquais, F. Poret, R. le Doze, Appl. Surf. Sci. 215 (2003) 5–17.
The tungsten powder study of the dispenser cathode Bao Ji-xiu*, Wan Bao-fei The Institute of Electronic Beam and Vacuum Technology, Shanghai Jiao Tong University, Huashan Rode 1954, Shanghai 200030, China Received 13 June 2005; received in revised form 3 August 2005; accepted 8 August 2005 Available online 4 October 2005
Abstract The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 8C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment. # 2005 Elsevier B.V. All rights reserved. Keywords: Dispenser cathodes; Tungsten powder; Porosity; Granules; Size of porous; Cathode life
1. Introduction In the vacuum electronic apparatus the cathode is the important components, which provides the apparatus the requirable electronic beams. The characteristics and the lifetime of the apparatus are influenced by the cathode performance. Basic requirements for modern tungsten cathodes are the long lifetime and large current density. The possibility of realization of these requirements is limited by * Corresponding author. Tel.: +86 21 62932848; fax: +86 21 62933571. E-mail address: [email protected] (J. Bao).
factors such as the high operating temperature and considerable evaporation rate of the Ba emission activator [1,2]. Operating temperature decreases if Os, Ir or Ru coating of cathode surface or a Scandate cathode is used. In both cases the operating temperature decrease is ensured by introduction of some Ba compound containing impregnant in the porous base material (tungsten). The Ba is necessary to the cathode activation. The cathode has the excellent performance until the enough Ba is produced and dispersed to the cathode surface. In the whole cathode activation there are complicated physics–chemistry reaction. It is dominating that some Ba compound containing impregnant in the porous base material (tungsten) is
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.012
5874
J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
decompounded to BaO and BaO is deoxidized by the tungsten to the emission Ba. It is simply as follows:
3. Results and discussion
BaXO ! BaO þ BaY
(1)
6BaO þ W ! Ba3 WO4 þ 3Ba
(2)
Although in the normal temperature the tungsten powder does not react on air and oxygen, for a long time in the humidity air the tungsten powder will be oxidized to WO3 [9]. The tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 8C, which is the gas–solid multiphase reaction, as follows:
It is seen that the tungsten is very important. The porosity and morphology is the major factor determining the tungsten matrix properties. The tungsten matrix is made of the tungsten powder .The grain size, distribution and morphology of which is the major factor determining the cathode performance [1–3]. In our work, the possibility of cathode characteristic improvement by means of achievement of optimal size of open pore channel, and hence, optimal emission activator consumption in a geometrically stable framework without cathode composition changing, is investigated. Among problems during longlifetime dispenser cathode development one can note insufficient information, concerning selection of the particles size of the tungsten powder for emitter matrix manufacturing. The influence of the homogeneity degree of particle size distribution and their morphology on the emission longevity of cathodes is also unknown. There is no consensus of opinion concerning influence of the cathode porosity on dispenser cathode longevity. Frameworks, used by various firms, differ by porosity, for example, 23–27% and 18% in [3–8].
WO3 þ 3H2 ! W þ 3H2 O Cathode frameworks were manufactured of industrial powders, some characteristics of which are shown in the Fig. 1. It is shown that the porosity of the tungsten matrix distributes more even and the closed pore fewer the average granule size of the tungsten powder distribute more convergent. In the dispenser cathodes a porous tungsten pellet is impregnated with the activator. The tungsten matrix such as grain size, distribution and morphology directly affect the cathode parameters like the lifetime, the activator evaporation, the emission current density and distribution, etc. In work, the super-pure tungsten powder that the shape of particle is semi-spherical and the particle size
2. Experimental The tungsten powder is cleaned in the high temperature hydrogen furnace before it is pressed into the flat tungsten matrix. The tungsten matrix sintered in the high temperature hydrogen furnace is made into the cathode, which is installed to a diode to do the comparative test of the cathode performance. In the high quality microscope the morphology of the tungsten matrix can be observed carefully. The emission performance of the cathode is evaluated in the diode by setting a thermo-couple to measure the cathode temperature on the nearby emission location of the cathode. The anode is used by Mo and the cathode–anode distance is from 1.1 to 1.3 mm.
Fig. 1. The particle size distribution of the tungsten powder.
J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
5875
Fig. 2. Correlation of porosity and the tungsten powder.
Fig. 4. Correlation of the evaporation of the cathode and the tungsten powder.
is uniform is made into the tungsten matrix, which is turned into the cathode. The porosity of the tungsten matrix made of the tungsten powder that the particle is distributed as Fig. 1(a) and the particle size is different each other in the same condition is different, some characteristics of which are shown in Fig. 2. It is shown that the porosity of the tungsten matrix is 22% when the average granule size of the tungsten powder is 3.5 mm, but the porosity of the tungsten matrix is 32% when the average granule size of the tungsten powder is 5.6 mm. Namely, the porosity of the tungsten matrix is bigger the average granule size of the tungsten
powder is bigger [10]. In the high quality microscope the morphology of the tungsten matrix is shown in Fig. 3, from which the morphology of the tungsten matrix is more rough and the pore and the aperture of the tungsten matrix is bigger with the big size of particle. In the dispenser cathodes made of a porous tungsten pellet impregnated with Ba compound, the evaporation of Ba is the major factor determining the cathode lifetime. The decrease of the evaporation can improve the reliability and prolong longevity of the electronic apparatus. The tungsten powder of different average granule size is made
Fig. 3. Morphology of the tungsten matrix and the size of the tungsten powder are (a) 2.2 mm, (b) 3.5 mm, (c) 4.8 mm.
5876
J. Bao, B. Wan / Applied Surface Science 252 (2006) 5873–5876
emission current density of the cathode made of the tungsten matrix of small size is bigger because the surface area of the tungsten matrix of small size is bigger than that of the big size; (ii) the tungsten matrix aperture made of big size of the tungsten powder is bigger than that of the small size of the tungsten powder is bigger. Because of the cathode made of the big pore matrix the activator not only is easily vaporized but also poisoned by bad gas such as O2 and CO2 and CO and so on and the emission current density is small. Fig. 5. Correlation of the emission current density and the tungsten powder.
4. Conclusion into the same porous tungsten matrix where the activator evaporation rate is also different. In experiment, the evaporator thickness of the anode wall is measured in the dissection diode, as shown Fig. 4. It is seen that the evaporator thickness on the anode wall is 0.25 mm when the size of the tungsten powder is 2.2 mm, but the evaporator thickness on the anode wall is 0.7 mm when the particle size of the tungsten powder is 4.8 mm. That is to say, the evaporation rate in the tungsten matrix made of the big tungsten powder is ever bigger than that of the small tungsten powder. It is explained that the aperture of the tungsten matrix made of the big size tungsten powder is bigger than that of the small size tungsten powder. The evaporation rate is bigger the aperture is bigger. In conclusion in the limited scope the pore and aperture of the tungsten matrix are diminished in order to decrease the evaporation rate of the cathode [11]. In the same porosity 28% the emission current density of the cathode made of the tungsten powder of the different size is not the same (Fig. 5). It is shown that the emission current density of the cathode is 16 A/cm2 when the particle size of the tungsten powder is 3.5 mm, but the emission current density of the cathode is 11.2 A/cm2 when the size is 5.6 mm. So the emission current density of the cathode of the big size tungsten powder is bigger than that of the small size tungsten powder. And the emission current density of the cathode is smaller the porosity is bigger. The explanation: (i) the activator and the
A fully cleaning step of the tungsten powder is necessary and the size of the tungsten powder is to sure to obtain the cathode of the even emission surface and the big emission current density and longevity. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger.
References [1] Hong-wei Zhang, Zhen-gang Bai, Shi-ji Yu, Vac. Electron. 3 (2002) 67. [2] V.A. Smirnov, IEEE International Vacuum Electron Sources Conference 7, Florida, 2002, p. 93. [3] D.H. Tomich, J.A. Mescher, J.T. Grant, Appl. Surf. Sci. 28 (1) (1987) 34–52. [4] A. Palluel, A.M. Shroff, J. Appl. Phys. 51 (1980) 2894. [5] T. Aida, H. Tahuma, S. Sasaki, T. Yaguchi, S. Taguchi, N. Koganezawa, Y. Nonaka, Appl. Phys. 4 (11) (1993) 6482– 6487. [6] N.B. Zhukova, E.V. Tolstik, V.I. Kozlov, Electronics technique S6, Materials 9 (182) (1983) 1–23. [7] A.M. Shroff, P. Palluel, Revue Technique Thomson CSF 14 (1982) 3. [8] N.N. Chubun, L.N. Sudakova, Appl. Surf. Sci. 111 (1997) 81– 83. [9] Peng Shaofang, The Tungsten Metallurgy, The Metallurgy Industry Publishing Company, 1981, pp. 25–28. [10] Irina P. Melnikova, Victor G. Vorozheikin, Dmitry A. Usanov, Appl. Surf. Sci. 215 (2003) 59–64. [11] J.M. Roquais, F. Poret, R. le Doze, Appl. Surf. Sci. 215 (2003) 5–17.