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An analysis of the surface of the Ni–W layer of a tungsten film coating cathode
Applied Surface Science 146 Ž1999. 47–50
An analysis of the surface of the Ni–W layer of a tungsten film coating cathode Takuya Ohira a
a,)
, Hiroyuki Teramoto a , Masato Saito a , Takashi Shinjo
b
AdÕanced Technology R & D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Display DeÕices Business DiÕision, Mitsubishi Electric Corporation, 1 Babazusho, Nagaokakyo, Kyoto 617-8550, Japan
Abstract Superior characteristics of a cathode operated with higher current density are required for achieving higher brightness and higher resolution of CRTs. An improved Sc 2 O 3-dispersed-oxide cathode Žconventional cathode. with tungsten film coating Žnew cathode. has been developed. The capability of the new cathode for high-current density operation is 1.8 times that of the conventional Sc 2 O 3-dispersed-oxide cathode w1x. Investigation of W behavior as well as typical characteristics of the new cathode in CRTs have been reported. This new type of oxide-coated cathode will contribute to higher brightness and higher resolution of CRTs. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cathodes; CRT; Tungsten; Life
1. Introduction We developed a Sc 2 O 3-dispersed-oxide cathode in 1986 w2–4x, employing a new technique of dispersing several weight percent of Sc 2 O 3 in the emission material layer of ŽBa,Sr,Ca.O. The current density of this cathode was improved to 2.0 Arcm2 , 4 times higher than that of the conventional oxidecoated cathode. Our new approach has been based on the analysis of our conventional Sc 2 O 3-dispersedoxide cathode. The emission mechanism model of the new cathode is shown in a separate report w1x. The details, however, had not been clarified. In this paper, we will report on the surface and the crosssection of the base metal in the new cathode, which
was examined in detail using scanning electron microscopy ŽSEM., electron probe microanalysis ŽEPMA. and Auger electron spectroscopy ŽAES..
2. New cathode structure Fig. 1 shows the structure of the new cathode. The only difference from the conventional cathode is a W film coated layer between the Ni base metal and
)
Corresponding author. Tel.: q81-6-497-7516; Fax: q81-6497-7549; E-mail: [email protected] 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 6 4 - 1
Fig. 1. Cross-section of new cathode.
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T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
Fig. 2. Decay of new and conventional cathode emission current.
the emission material layer. This new cathode preparation is basically the same as that of the conventional cathode, except for the addition of a W film coating process. The new cathode does not need any additional processes in CRT manufacturing. The new cathode features are low-cost compared to impregnated cathodes.
Fig. 4. X-ray diffraction pattern of Ža. Ni base surface, Žb. Ni base surface with W film coating, and Žc. Ni base surface with W film coating after heating.
The W film thickness of this new cathode is about 1 mm. It clearly shows that the new cathode results in little deterioration of emission current compared to that of the conventional cathode. W behavior in the new cathodes has been studied through the basic experiments and surface analysis.
3. Decay of emission current 4. The Ni base surface with W film coating We studied numerous variables of the W film coating, such as film coating method, film thickness, rate of film forming, and thermal treatment. Film thickness is one of the important factors in determining cathode properties. Fig. 2 shows the decay of emission current of the new cathode and the conventional cathode under 2.5 Arcm2 current density in CRTs. This current density is under DC mean loading. The heater voltage throughout lifetime is 6.3 V.
Fig. 3. SEM image of Ni base surface with W film coating area and noncoated area.
We prepared a sample cathode, which had a W-film-coated area and non-W-film-coated area on its Ni base. Fig. 3 shows a SEM image of the Ni base surface after 4000 hours. A Ni–W alloy is
Fig. 5. X-ray images ŽBa,Si. of Ni base cross-section.
T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
49
5. Distribution of cathode reduction materials after operation Fig. 5 shows X-ray images ŽBa,Si. of the Ni base cross-section in the new cathode and the conventional cathode. The operating time of the cathodes is several thousand hours. In the new cathode, the concentrations of Si and Ba are dispersed throughout the Ni–W layer. Ba and Si exist more uniformly concentrated, close to the surface of the Ni base in the conventional cathode. It is thought that the observed Ba and Si signals in Fig. 6 correspond to Ba 2 SiO4 formed by the following reaction Ž1.: 2BaO q 1r2Sis 1r2Ba 2 SiO4 q Ba
Fig. 6. X-ray images ŽBa,Si. of Ni base surface of new cathode.
formed in the W coated area. The Ni–W layer forms a fine-grained structure. This structure occurs in the cross-sectional direction. It is thought that the contact area of the emission material layer and the Ni base metal increase by this structure. Fig. 4 shows the X-ray diffraction pattern of Ža. Ni base surface, Žb. Ni base surface with W film coating and Žc. Ni base surface with W film coating after heating. A different peak of Ni 4W is seen in profile Žc. compared to that of profile Žb.. This different peak is thus formed after heating.
Ž 1.
Fig. 6 shows X-ray images ŽSi,Mg. of the Ni base surface of the new cathode. The operating time of the cathodes is several thousand hours. As the film becomes thicker, it is clear that the amount of Si and Mg decreases on the surface because they are dispersed in the Ni–W layer. This shows that the Ni–W layer disperses Si and Mg. Fig. 7 shows a SEM image and an Auger image ŽMg. of the Ni base surface of the sample cathode. For the purpose of investigation, we prepared a sample cathode which had a W-film-coated area and non-W-film-coated area on its Ni base. Mg usually diffuses along the Ni grain boundary. This behavior is shown at right in Fig. 7. However, it is seen that Mg has not only diffused along the grain boundary in the W coated area Žleft side of image.. It is
Fig. 7. SEM image and Auger image ŽMg. of Ni base surface of sample cathode.
50
T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
thought that this phenomenon is also a result of the effect of Mg on dispersion in the Ni–W layer.
6. Discussion Based on the results for the Ni base surface and the cross-section, the following mechanism was indicated. The Ni–W layer forms a fine-grained structure ŽFig. 3. that increases the opportunity for reaction Ž1. because of the increased contact area with the emission material layer. It is therefore thought that much free Ba are forms in the Ni–W layer. The uniform concentration of Ba 2 SiO4 considerably restricts the emission current. In the new cathode, Si is dispersed in the Ni–W layer, as is Ba 2 SiO4 Žan insulator. ŽFigs. 5 and 6.. The diffusion of the reduction material is therefore not obstructed by a Ba 2 SiO4 layer during long operation. It is thought that both Si and Mg of the proper amount and Mg play a role as a reduction material with the growth of the Ni–W layer, and this is added to the reduction effect of W in the Ni–W layer by reaction Ž1. and reactions Ž2. and Ž3. w1x: BaO q Mg s MgO q Ba 4r3BaOq 1r3W s 1r3BaWO4 q Ba
Ž 2. Ž 3.
This is why, in Fig. 2, the new cathode is excellent in comparison to the conventional cathode.
7. Conclusion Improvement of the Sc 2 O 3-dispersed-oxide cathode in higher current operation has been attained by adopting a W film coating on a Ni base. In this new cathode, we can conclude the following. Ž1. The Ni–W layer forms a fine-grained structure. As a result, sufficient free-Ba forms within this structure. Ž2. Si and Mg are dispersed in the Ni–W layer, as is Ba 2 SiO4 . As a result, Si, Mg and W of the proper amount in the Ni–W layer apparently contribute to free-Ba production during long operation. The new cathode therefore makes high current density operation possible throughout its life. Acknowledgements We thank Dr. S. Yamamoto and Dr. K. Watanabe for their invaluable advice. References w1x M. Saito, T. Ohira, H. Teramoto, K. Watanabe, T. Shinjo, H. Yamaguchi, SID Int. Symp. Dig. Tech. Pap. 28 Ž1997. 351. w2x M. Saito, M. Ishida, K. Fukuyama, K. Watanabe, T. Kamata, K. Sano, H. Nakanishi, NTG Fachberichte 95 Ž1986. 165. w3x M. Saito, R. Suzuki, K. Fukuyama, K. Watanabe, K. Sano, H. Nakanishi, IEEE Trans. Electron. Dev. 37 Ž12. Ž1990. 2609. w4x H. Nakanishi, K. Sano, R. Suzuki, M. Saito, Proc. SID 31 Ž31. Ž1990. 165.
An analysis of the surface of the Ni–W layer of a tungsten film coating cathode Takuya Ohira a
a,)
, Hiroyuki Teramoto a , Masato Saito a , Takashi Shinjo
b
AdÕanced Technology R & D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Display DeÕices Business DiÕision, Mitsubishi Electric Corporation, 1 Babazusho, Nagaokakyo, Kyoto 617-8550, Japan
Abstract Superior characteristics of a cathode operated with higher current density are required for achieving higher brightness and higher resolution of CRTs. An improved Sc 2 O 3-dispersed-oxide cathode Žconventional cathode. with tungsten film coating Žnew cathode. has been developed. The capability of the new cathode for high-current density operation is 1.8 times that of the conventional Sc 2 O 3-dispersed-oxide cathode w1x. Investigation of W behavior as well as typical characteristics of the new cathode in CRTs have been reported. This new type of oxide-coated cathode will contribute to higher brightness and higher resolution of CRTs. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cathodes; CRT; Tungsten; Life
1. Introduction We developed a Sc 2 O 3-dispersed-oxide cathode in 1986 w2–4x, employing a new technique of dispersing several weight percent of Sc 2 O 3 in the emission material layer of ŽBa,Sr,Ca.O. The current density of this cathode was improved to 2.0 Arcm2 , 4 times higher than that of the conventional oxidecoated cathode. Our new approach has been based on the analysis of our conventional Sc 2 O 3-dispersedoxide cathode. The emission mechanism model of the new cathode is shown in a separate report w1x. The details, however, had not been clarified. In this paper, we will report on the surface and the crosssection of the base metal in the new cathode, which
was examined in detail using scanning electron microscopy ŽSEM., electron probe microanalysis ŽEPMA. and Auger electron spectroscopy ŽAES..
2. New cathode structure Fig. 1 shows the structure of the new cathode. The only difference from the conventional cathode is a W film coated layer between the Ni base metal and
)
Corresponding author. Tel.: q81-6-497-7516; Fax: q81-6497-7549; E-mail: [email protected] 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 6 4 - 1
Fig. 1. Cross-section of new cathode.
48
T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
Fig. 2. Decay of new and conventional cathode emission current.
the emission material layer. This new cathode preparation is basically the same as that of the conventional cathode, except for the addition of a W film coating process. The new cathode does not need any additional processes in CRT manufacturing. The new cathode features are low-cost compared to impregnated cathodes.
Fig. 4. X-ray diffraction pattern of Ža. Ni base surface, Žb. Ni base surface with W film coating, and Žc. Ni base surface with W film coating after heating.
The W film thickness of this new cathode is about 1 mm. It clearly shows that the new cathode results in little deterioration of emission current compared to that of the conventional cathode. W behavior in the new cathodes has been studied through the basic experiments and surface analysis.
3. Decay of emission current 4. The Ni base surface with W film coating We studied numerous variables of the W film coating, such as film coating method, film thickness, rate of film forming, and thermal treatment. Film thickness is one of the important factors in determining cathode properties. Fig. 2 shows the decay of emission current of the new cathode and the conventional cathode under 2.5 Arcm2 current density in CRTs. This current density is under DC mean loading. The heater voltage throughout lifetime is 6.3 V.
Fig. 3. SEM image of Ni base surface with W film coating area and noncoated area.
We prepared a sample cathode, which had a W-film-coated area and non-W-film-coated area on its Ni base. Fig. 3 shows a SEM image of the Ni base surface after 4000 hours. A Ni–W alloy is
Fig. 5. X-ray images ŽBa,Si. of Ni base cross-section.
T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
49
5. Distribution of cathode reduction materials after operation Fig. 5 shows X-ray images ŽBa,Si. of the Ni base cross-section in the new cathode and the conventional cathode. The operating time of the cathodes is several thousand hours. In the new cathode, the concentrations of Si and Ba are dispersed throughout the Ni–W layer. Ba and Si exist more uniformly concentrated, close to the surface of the Ni base in the conventional cathode. It is thought that the observed Ba and Si signals in Fig. 6 correspond to Ba 2 SiO4 formed by the following reaction Ž1.: 2BaO q 1r2Sis 1r2Ba 2 SiO4 q Ba
Fig. 6. X-ray images ŽBa,Si. of Ni base surface of new cathode.
formed in the W coated area. The Ni–W layer forms a fine-grained structure. This structure occurs in the cross-sectional direction. It is thought that the contact area of the emission material layer and the Ni base metal increase by this structure. Fig. 4 shows the X-ray diffraction pattern of Ža. Ni base surface, Žb. Ni base surface with W film coating and Žc. Ni base surface with W film coating after heating. A different peak of Ni 4W is seen in profile Žc. compared to that of profile Žb.. This different peak is thus formed after heating.
Ž 1.
Fig. 6 shows X-ray images ŽSi,Mg. of the Ni base surface of the new cathode. The operating time of the cathodes is several thousand hours. As the film becomes thicker, it is clear that the amount of Si and Mg decreases on the surface because they are dispersed in the Ni–W layer. This shows that the Ni–W layer disperses Si and Mg. Fig. 7 shows a SEM image and an Auger image ŽMg. of the Ni base surface of the sample cathode. For the purpose of investigation, we prepared a sample cathode which had a W-film-coated area and non-W-film-coated area on its Ni base. Mg usually diffuses along the Ni grain boundary. This behavior is shown at right in Fig. 7. However, it is seen that Mg has not only diffused along the grain boundary in the W coated area Žleft side of image.. It is
Fig. 7. SEM image and Auger image ŽMg. of Ni base surface of sample cathode.
50
T. Ohira et al.r Applied Surface Science 146 (1999) 47–50
thought that this phenomenon is also a result of the effect of Mg on dispersion in the Ni–W layer.
6. Discussion Based on the results for the Ni base surface and the cross-section, the following mechanism was indicated. The Ni–W layer forms a fine-grained structure ŽFig. 3. that increases the opportunity for reaction Ž1. because of the increased contact area with the emission material layer. It is therefore thought that much free Ba are forms in the Ni–W layer. The uniform concentration of Ba 2 SiO4 considerably restricts the emission current. In the new cathode, Si is dispersed in the Ni–W layer, as is Ba 2 SiO4 Žan insulator. ŽFigs. 5 and 6.. The diffusion of the reduction material is therefore not obstructed by a Ba 2 SiO4 layer during long operation. It is thought that both Si and Mg of the proper amount and Mg play a role as a reduction material with the growth of the Ni–W layer, and this is added to the reduction effect of W in the Ni–W layer by reaction Ž1. and reactions Ž2. and Ž3. w1x: BaO q Mg s MgO q Ba 4r3BaOq 1r3W s 1r3BaWO4 q Ba
Ž 2. Ž 3.
This is why, in Fig. 2, the new cathode is excellent in comparison to the conventional cathode.
7. Conclusion Improvement of the Sc 2 O 3-dispersed-oxide cathode in higher current operation has been attained by adopting a W film coating on a Ni base. In this new cathode, we can conclude the following. Ž1. The Ni–W layer forms a fine-grained structure. As a result, sufficient free-Ba forms within this structure. Ž2. Si and Mg are dispersed in the Ni–W layer, as is Ba 2 SiO4 . As a result, Si, Mg and W of the proper amount in the Ni–W layer apparently contribute to free-Ba production during long operation. The new cathode therefore makes high current density operation possible throughout its life. Acknowledgements We thank Dr. S. Yamamoto and Dr. K. Watanabe for their invaluable advice. References w1x M. Saito, T. Ohira, H. Teramoto, K. Watanabe, T. Shinjo, H. Yamaguchi, SID Int. Symp. Dig. Tech. Pap. 28 Ž1997. 351. w2x M. Saito, M. Ishida, K. Fukuyama, K. Watanabe, T. Kamata, K. Sano, H. Nakanishi, NTG Fachberichte 95 Ž1986. 165. w3x M. Saito, R. Suzuki, K. Fukuyama, K. Watanabe, K. Sano, H. Nakanishi, IEEE Trans. Electron. Dev. 37 Ž12. Ž1990. 2609. w4x H. Nakanishi, K. Sano, R. Suzuki, M. Saito, Proc. SID 31 Ž31. Ž1990. 165.