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Advances in Mo–RE2O3 thermionic cathode materials
Applied Surface Science 146 Ž1999. 117–119
Advances in Mo–RE 2 O 3 thermionic cathode materials Jiuxing Zhang ) , Zuoren Nie, Meiling Zhou, Jinshu Wang, Danmin Liu, Chaogen Yao, Tieyong Zuo Department of Materials Science and Engineering, Beijing Polytechnic UniÕersity, 100 Pingleyuang, Chaoyuan District, Beijing 100022, China
Abstract This paper presents recent advances in Mo–RE 2 O 3ŽLa 2 O 3 , Y2 O 3 , Sc 2 O 3 . heated cathode materials. It is shown that Mo–La 2 O 3 –X ŽX s Y2 O 3 , Sc 2 O 3 , Y2 O 3 , qSc 2 O 3 . have better elongation than Mo–La 2 O 3. At operating temperatures of 1623–1673 K, the average saturation current of the Mo–La 2 O 3 cathode is 118 mA, the maximum current reaching 174 mA; and the corresponding average current density is 367 mArcm2 , and the average emission efficiency is 11.8 mArw. The lifetime is more than 2000 h when the stable emission current is 75–80 mA. These results show that the Mo–La 2 O 3 cathode is closer to practical application. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum; Rare earth oxide; Cathode; Thermionic emission
1. Introduction Mo wires doped with rare earth oxides such as La 2 O 3 , Y2 O 3 , and Sc 2 O 3 are promising for use as heated cathodes to replace the radioactive Th–W cathode. In previous research, it was shown that the Mo–La 2 O 3 cathode not only has high-temperature strength and good ambient ductility, but also possesses very good electron-emission capacity, high emissive efficiency, and a pulse ratio superior to that of the Th–W cathode w1x. However, it was found that the emission current of the Mo–La 2 O 3 electron tube attenuated with time rapidly. In order to obtain stable emission of Mo–La 2 O 3 tubes, much research work on the material itself, the tube design, and its suitable
)
Corresponding author. Tel.: q86-10-67392169, 67392591; Fax: q86-10-67395281; E-mail: [email protected]
technical parameters and operating mechanisms has been done in recent years w2–4x. Great progress has been made in the emission stability of Mo–La 2 O 3 tubes. In this paper, we report the main results obtained recently.
2. Experimental procedure A total of 2–4 wt.% RE 2 O 3 ŽLa 2 O 3 , Y2 O 3 , Sc 2 O 3 . was added to Mo oxide powders as aqueous solutions of LaŽNO 3 . 3 , YŽNO 3 . 3 and ScŽNO 3 . 3 . The doped Mo oxide powder was reduced to RE 2 O 3doped metal molybdenum in dry hydrogen. The powder was then pressed in steel dies and sintered by electric resistance heating in hydrogen. The sintered bars were swaged and drawn into wires 0.26– 1.04 mm in diameter with the reduction of 99.9% in the temperature range from 1573 K to 773 K.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 2 3 - 9
J. Zhang et al.r Applied Surface Science 146 (1999) 117–119
118
The experimental diodes were specially designed with straight wire or screw cathodes of Mo–La 2 O 3 wires of 0.26 mm in diameter and a cylinder Mo anode. The basic emitting properties and lifetime have been systematically examined, and the phases and microstructure were observed or analyzed by means of XRD, SEM, and TEM.
3. Results and discussion 3.1. Mechanical properties of Mo–La2 O3 –X (X s Y2 O3 , Sc2 O3 , Y2 O3 , qSc2 O3 ) materials XRD shows that rare earth dopants exist in the form of single rare earth oxide and compounds in Mo powders and sintered ingot. The changes of phases in powders and ingots is shown in Table 1. The strength of Mo–La 2 O 3 –X wires of 1.04 mm in diameter is more than 600 MPa when annealed in a temperature range of 1473–1973 K. It is noted that the strength of Mo–La 2 O 3 –X is a little lower than that of Mo–La 2 O 3 , but much better elongation is obtained in Mo–La 2 O 3 –X wires, as shown in Fig. 1. A combination of higher strength and better ductility can be obtained after annealing at temperatures above 1773 K. 3.2. Design of standard diode and emission properties of Mo–La2 O3 cathode A ‘standard’ diode with a straight wire cathode 0.26 mm in diameter has been designed and tested. The anode is of inner diameter 75 mm and 10 mm
Table 1 XRD results of Mo–La 2 O 3 –X powders and ingots Dopant
Powder
Sintered ingot
La 2 O 3
La 2 O 3 , La 2 MoO6 , La 2 Mo 3 O12 La 2 O 3 , La 2 Mo 3 O12 , La 2 Mo 4 O15 , Y2 O 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
La 2 O 3
La 2 O 3 , Y2 O 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
La 2 O 3 , LaYO 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
Fig. 1. Elongation of Mo–Re 2 O 3 –X wires of 1.04 mm in diameter annealed at different temperatures in hydrogen for 30 min.
high. The relationship between the standard diode current and voltage is given as: Ia s 35.89 = 10y3 Ua3r2 Ž mA . . Ž 1. Theoretical calculation and experimental results are compared in Table 2, showing good agreement. At the operating temperature of 1483 K, the average saturation current of Mo–La 2 O 3 standard diode is 30 mA, the corresponding current density reaching 367 mArcm2 , and the emission efficiency is 13.6 mArw. Up to now, the lifetime of these diodes is near 500 h, as shown in Fig. 2, indicating that thermionic emission is stable. 3.3. Emission properties of practical diode of Mo– La2 O3 cathode On the basis of a ‘standard’ diode, the practical diode was made with a screw cathode and tested. At operating temperature of 1623–1673 K, the average saturation current of 10 tubes is 118 mA, the maximum current reaching 174 mA, and the corresponding average current density is 367 mArcm2 , and the average emission efficiency is 11.8 mArw. In particular, the lifetime tested in the standard procedure is more than 2000 h when the stable emission current Table 2 Relationship of current vs. voltage in Mo–La 2 O 3 standard diode Ia ŽmA.
Ua ŽV.
Tubes
20
25
30
40
50
60
80
100
Theoretical 1a 2a 3a 4a
3.2 r r r r
4.5 4.5 4.5 4.5 4.5
5.9 5.8 5.7 5.8 6.0
9.1 8.8 8.5 9.0 9.2
12.7 12.2 12.0 12.5 12.8
16.7 15.5 15.2 15.8 16.2
25.7 23.5 22.9 24.0 24.8
35.9 31.5 30.7 32.5 32.5
J. Zhang et al.r Applied Surface Science 146 (1999) 117–119
119
Thus, a higher, stable emission current is obtained in Mo–La 2 O 3 diodes. The more detailed mechanism is under investigation.
4. Summary
Fig. 2. Relationship of lifetime with emission current in Mo– La 2 O 3 tubes.
is 75–80 mA. These results show that the Mo–La 2 O 3 cathode is closer to practical application.
The Mo–La 2 O 3 –X materials show much better combined mechanical properties than Mo–La 2 O 3 material. The basic emission properties of the Mo– La 2 O 3 cathode are essential to design practical electron tubes. The breakthrough in emission stability indicates that Mo–La 2 O 3 is nearest to practical industry application.
3.4. Emission mechanisms of Mo–La2 O3 cathode
Acknowledgements
The existing form and content of La on the surface of Mo–La 2 O 3 thermionic cathode wire at high temperature have been examined by XPS. The results indicate that elemental La cannot be produced by reduction and thermal decomposition at high temperature. However, the concentration of La 2 O 3 on the surface obviously increased with temperature. With semiquantitative calculations at 1623–1723 K, it is about 4 times as much as RT w5x. Therefore, the emission mechanism of the Mo–La 2 O 3 cathode is much different from the monatomic layer mechanism of the Th–W cathode because no elemental La is found. La 2 O 3 particles distributed dispersedly on the surface of the cathode may be arranged in some preferred orientation in the function of the electrical field and form a large numbers of emission center.
This research was supported by China National High-Tech Program 863 and the Beijing Science and Technology Star Program.
References w1x M.L. Zhou, Z.C. Cheng, J.X. Zhang, J. Li, T.Y. Zuo, High Temperature–High Pressure 26 Ž1994. 145. w2x M.L. Zhou, J. Li, T.Y. Zuo, Transactions of Nfsoc 4 Ž1994. 57. w3x M.L. Zhou, J.S. Wang, J.X. Zhang, Z.R. Nie, T.Y. Zuo, Trans. Nonferrous Met. Soc. China 6 Ž1996. 57. w4x Z.R. Nie, J.X. Zhang, M.L. Zhou, J.S. Wang, T.Y. Zuo, in: G. Kneringer, P. Hodlhammer, P. Wilhartitz ŽEds.., Proc. of 14th Int. Plansee Seminar, Vol. 1, Plansee, 1997, p. 76. w5x Z.R. Nie, H.Z. Huan, J.X. Zhang, M.L. Zhou, J.S. Wang, T.Y. Zuo, Met. Func. Mater. 5 Ž1998. 32, in Chinese.
Advances in Mo–RE 2 O 3 thermionic cathode materials Jiuxing Zhang ) , Zuoren Nie, Meiling Zhou, Jinshu Wang, Danmin Liu, Chaogen Yao, Tieyong Zuo Department of Materials Science and Engineering, Beijing Polytechnic UniÕersity, 100 Pingleyuang, Chaoyuan District, Beijing 100022, China
Abstract This paper presents recent advances in Mo–RE 2 O 3ŽLa 2 O 3 , Y2 O 3 , Sc 2 O 3 . heated cathode materials. It is shown that Mo–La 2 O 3 –X ŽX s Y2 O 3 , Sc 2 O 3 , Y2 O 3 , qSc 2 O 3 . have better elongation than Mo–La 2 O 3. At operating temperatures of 1623–1673 K, the average saturation current of the Mo–La 2 O 3 cathode is 118 mA, the maximum current reaching 174 mA; and the corresponding average current density is 367 mArcm2 , and the average emission efficiency is 11.8 mArw. The lifetime is more than 2000 h when the stable emission current is 75–80 mA. These results show that the Mo–La 2 O 3 cathode is closer to practical application. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum; Rare earth oxide; Cathode; Thermionic emission
1. Introduction Mo wires doped with rare earth oxides such as La 2 O 3 , Y2 O 3 , and Sc 2 O 3 are promising for use as heated cathodes to replace the radioactive Th–W cathode. In previous research, it was shown that the Mo–La 2 O 3 cathode not only has high-temperature strength and good ambient ductility, but also possesses very good electron-emission capacity, high emissive efficiency, and a pulse ratio superior to that of the Th–W cathode w1x. However, it was found that the emission current of the Mo–La 2 O 3 electron tube attenuated with time rapidly. In order to obtain stable emission of Mo–La 2 O 3 tubes, much research work on the material itself, the tube design, and its suitable
)
Corresponding author. Tel.: q86-10-67392169, 67392591; Fax: q86-10-67395281; E-mail: [email protected]
technical parameters and operating mechanisms has been done in recent years w2–4x. Great progress has been made in the emission stability of Mo–La 2 O 3 tubes. In this paper, we report the main results obtained recently.
2. Experimental procedure A total of 2–4 wt.% RE 2 O 3 ŽLa 2 O 3 , Y2 O 3 , Sc 2 O 3 . was added to Mo oxide powders as aqueous solutions of LaŽNO 3 . 3 , YŽNO 3 . 3 and ScŽNO 3 . 3 . The doped Mo oxide powder was reduced to RE 2 O 3doped metal molybdenum in dry hydrogen. The powder was then pressed in steel dies and sintered by electric resistance heating in hydrogen. The sintered bars were swaged and drawn into wires 0.26– 1.04 mm in diameter with the reduction of 99.9% in the temperature range from 1573 K to 773 K.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 2 3 - 9
J. Zhang et al.r Applied Surface Science 146 (1999) 117–119
118
The experimental diodes were specially designed with straight wire or screw cathodes of Mo–La 2 O 3 wires of 0.26 mm in diameter and a cylinder Mo anode. The basic emitting properties and lifetime have been systematically examined, and the phases and microstructure were observed or analyzed by means of XRD, SEM, and TEM.
3. Results and discussion 3.1. Mechanical properties of Mo–La2 O3 –X (X s Y2 O3 , Sc2 O3 , Y2 O3 , qSc2 O3 ) materials XRD shows that rare earth dopants exist in the form of single rare earth oxide and compounds in Mo powders and sintered ingot. The changes of phases in powders and ingots is shown in Table 1. The strength of Mo–La 2 O 3 –X wires of 1.04 mm in diameter is more than 600 MPa when annealed in a temperature range of 1473–1973 K. It is noted that the strength of Mo–La 2 O 3 –X is a little lower than that of Mo–La 2 O 3 , but much better elongation is obtained in Mo–La 2 O 3 –X wires, as shown in Fig. 1. A combination of higher strength and better ductility can be obtained after annealing at temperatures above 1773 K. 3.2. Design of standard diode and emission properties of Mo–La2 O3 cathode A ‘standard’ diode with a straight wire cathode 0.26 mm in diameter has been designed and tested. The anode is of inner diameter 75 mm and 10 mm
Table 1 XRD results of Mo–La 2 O 3 –X powders and ingots Dopant
Powder
Sintered ingot
La 2 O 3
La 2 O 3 , La 2 MoO6 , La 2 Mo 3 O12 La 2 O 3 , La 2 Mo 3 O12 , La 2 Mo 4 O15 , Y2 O 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
La 2 O 3
La 2 O 3 , Y2 O 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
La 2 O 3 , LaYO 3 La 2 O 3 , Sc 2 O 3 La 2 O 3 , Y2 O 3 , Sc 2 O 3
Fig. 1. Elongation of Mo–Re 2 O 3 –X wires of 1.04 mm in diameter annealed at different temperatures in hydrogen for 30 min.
high. The relationship between the standard diode current and voltage is given as: Ia s 35.89 = 10y3 Ua3r2 Ž mA . . Ž 1. Theoretical calculation and experimental results are compared in Table 2, showing good agreement. At the operating temperature of 1483 K, the average saturation current of Mo–La 2 O 3 standard diode is 30 mA, the corresponding current density reaching 367 mArcm2 , and the emission efficiency is 13.6 mArw. Up to now, the lifetime of these diodes is near 500 h, as shown in Fig. 2, indicating that thermionic emission is stable. 3.3. Emission properties of practical diode of Mo– La2 O3 cathode On the basis of a ‘standard’ diode, the practical diode was made with a screw cathode and tested. At operating temperature of 1623–1673 K, the average saturation current of 10 tubes is 118 mA, the maximum current reaching 174 mA, and the corresponding average current density is 367 mArcm2 , and the average emission efficiency is 11.8 mArw. In particular, the lifetime tested in the standard procedure is more than 2000 h when the stable emission current Table 2 Relationship of current vs. voltage in Mo–La 2 O 3 standard diode Ia ŽmA.
Ua ŽV.
Tubes
20
25
30
40
50
60
80
100
Theoretical 1a 2a 3a 4a
3.2 r r r r
4.5 4.5 4.5 4.5 4.5
5.9 5.8 5.7 5.8 6.0
9.1 8.8 8.5 9.0 9.2
12.7 12.2 12.0 12.5 12.8
16.7 15.5 15.2 15.8 16.2
25.7 23.5 22.9 24.0 24.8
35.9 31.5 30.7 32.5 32.5
J. Zhang et al.r Applied Surface Science 146 (1999) 117–119
119
Thus, a higher, stable emission current is obtained in Mo–La 2 O 3 diodes. The more detailed mechanism is under investigation.
4. Summary
Fig. 2. Relationship of lifetime with emission current in Mo– La 2 O 3 tubes.
is 75–80 mA. These results show that the Mo–La 2 O 3 cathode is closer to practical application.
The Mo–La 2 O 3 –X materials show much better combined mechanical properties than Mo–La 2 O 3 material. The basic emission properties of the Mo– La 2 O 3 cathode are essential to design practical electron tubes. The breakthrough in emission stability indicates that Mo–La 2 O 3 is nearest to practical industry application.
3.4. Emission mechanisms of Mo–La2 O3 cathode
Acknowledgements
The existing form and content of La on the surface of Mo–La 2 O 3 thermionic cathode wire at high temperature have been examined by XPS. The results indicate that elemental La cannot be produced by reduction and thermal decomposition at high temperature. However, the concentration of La 2 O 3 on the surface obviously increased with temperature. With semiquantitative calculations at 1623–1723 K, it is about 4 times as much as RT w5x. Therefore, the emission mechanism of the Mo–La 2 O 3 cathode is much different from the monatomic layer mechanism of the Th–W cathode because no elemental La is found. La 2 O 3 particles distributed dispersedly on the surface of the cathode may be arranged in some preferred orientation in the function of the electrical field and form a large numbers of emission center.
This research was supported by China National High-Tech Program 863 and the Beijing Science and Technology Star Program.
References w1x M.L. Zhou, Z.C. Cheng, J.X. Zhang, J. Li, T.Y. Zuo, High Temperature–High Pressure 26 Ž1994. 145. w2x M.L. Zhou, J. Li, T.Y. Zuo, Transactions of Nfsoc 4 Ž1994. 57. w3x M.L. Zhou, J.S. Wang, J.X. Zhang, Z.R. Nie, T.Y. Zuo, Trans. Nonferrous Met. Soc. China 6 Ž1996. 57. w4x Z.R. Nie, J.X. Zhang, M.L. Zhou, J.S. Wang, T.Y. Zuo, in: G. Kneringer, P. Hodlhammer, P. Wilhartitz ŽEds.., Proc. of 14th Int. Plansee Seminar, Vol. 1, Plansee, 1997, p. 76. w5x Z.R. Nie, H.Z. Huan, J.X. Zhang, M.L. Zhou, J.S. Wang, T.Y. Zuo, Met. Func. Mater. 5 Ž1998. 32, in Chinese.