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Hydrogen-induced degradation in NiCuZn ferrite-based multilayer chip inductors
Materials Letters 59 (2005) 1636 – 1639 www.elsevier.com/locate/matlet
Hydrogen-induced degradation in NiCuZn ferrite-based multilayer chip inductors W.P. Chena,b,T, J.Q. Qib, Y. Wangb, X.H. Wangc, Z.W. Mac, L.T. Lic, H.L.W. Chanb a Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, PR China c Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
b
Received 26 August 2004; received in revised form 17 January 2005; accepted 19 January 2005 Available online 2 March 2005
Abstract Hydrogen-induced degradation in Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors was studied through an electrochemical hydrogen charging method, in which the silver electrodes of the inductors were made a cathode in 0.01 M NaOH solution to evolve hydrogen by the electrolysis of water. After the treatment, the inductance and the quality factor of the inductors at high frequencies were dramatically decreased. The degradation showed a little spontaneous recovery at room temperature and could be mostly recovered through a heat-treatment of 4 h at 250 8C in N2. It is proposed that hydrogen generated by the electrolysis of water is incorporated into the ferrite lattice and exists as an interstitial proton after reducing Fe3+ to Fe2+. The stability of hydrogen in ferrites decreased with increasing temperature and its outdiffusion resulted in the recovery. Hydrogen-induced degradation is important to ferrite-based chip inductors in fabrication and in operation. D 2005 Elsevier B.V. All rights reserved. PACS: 72.10.d; 75.50.Gg; 78.30.Hv Keywords: Ferrite; Hydrogen; Inductor; Degradation
1. Introduction Ferrites are one of the most important materials for multilayer ceramic chip inductors due to their high permeability in the RF frequency region, high electrical resistivity and high stability [1]. Ferrite chip inductors are applied in large amounts in various electronic circuits and they have greatly benefited the miniaturization of many latest electronic products, including mobile phones, notebook computers, and video cameras. Now extensive studies are being conducted to further improve their properties and to lower their sintering temperatures [2,3]. With the wide application of ferrite chip inductors, the influence of hydrogen on ferrites has also attracted more and more attention. Cao et al. recently reported that in electroplating hydrogen leads to a serious decrease in T Corresponding author. Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China. Tel.: +86 27 6131 2396; fax: +86 27 6875 2569. E-mail address: [email protected] (W.P. Chen). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.01.031
the inductance of NiCuZn ferrite-based multilayer chip inductors, and sometimes also causes a short-circuit failure as electroplating occurs on the ferrite substrate surface beyond the silver termination electrodes [4]. Electroplating is widely adopted to form surface-mountable three-layer termination electrodes (Ag/Ni/Sn–Pb) for various chip components [5]. Some other researchers have shown that water vapor can have a strong impact on the properties of electronic devices through the electrolysis of water [6], in which the reaction of hydrogen should be a key factor. This is especially important for ferrites chip inductors because they have no polymer coating to prevent the condensation of water on them. Obviously, the influence of hydrogen is important to ferrite-based chip inductors and should be studied in depth.
2. Experimental procedure A group of Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors were used in the present inves-
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
3. Results and discussion Fig. 1 shows a representative SEM micrograph taken on a fractured surface of a chip inductor. It is clear that the inductor has a very dense microstructure. For comparison, some inductors had been immersed in the NaOH solution at 20 8C for 100 h with no hydrogen charging. No discernible changes occurred in the properties of the samples after the immersion, indicating that the dilute solution did not influence the samples through permeation, which was in agreement with the microstructural observation. In contrast, even a very short period of electrochemical hydrogen charging resulted in great decreases in inductance and in quality factor of the inductors at high frequencies. Some representative results obtained for a chip inductor are shown in Fig. 2. A DC voltage of 3 V had been applied between its silver
Fig. 1. SEM micrograph taken for a fractured surface of a multilayer chip inductor.
Inductance ( µH )
10
Quality Factor
tigation. The inductors were in the size of 2.11.30.8 mm3 with two silver termination electrodes. Their fabrication and sintering had been reported in detail in another paper [4]. Some of the chip inductors were placed in a 0.01 M NaOH solution and DC voltages were applied between the silver electrodes of the chip inductors and a counter Pt electrode in the solution. The silver electrodes of the samples acted as cathode and the Pt electrode acted as anode. The applied DC voltages led to the electrolysis of water with hydrogen deposited on the silver electrodes of the samples and this treatment is referred to hereafter as electrochemical hydrogen charging [7,8]. The DC voltages were removed after some designated periods of time and the samples were taken out, cleaned with de-ionized water and dried. An Agilent 4294 A impedance analyzer was used to measure the frequency spectra of inductance and quality factor of the chip inductors. Microstructural analyses were conducted on a scanning electron microscope STEROSCAN 440.
1637
5
as -sintered Hydrogen -charged 10 min 20 min 30 min
0
10
1
0.1 10
3
10
4
10
5
10
6
10
7
Frequency ( Hz ) Fig. 2. Frequency spectra of inductance and quality factor of a chip inductor before and after electrochemical hydrogen charging.
termination electrodes and the anode in the solution. One can see that after 30 min electrochemical hydrogen charging, the inductance and the quality factor were decreased at 10 MHz by more than 20 and 100 times, respectively, while they almost remained unchanged at low frequencies. It is well known that chip inductors usually work at high frequencies so the hydrogen charging had greatly degraded the properties of the chip inductor. Obviously, the changes in the hydrogen-charged chip inductors should have resulted from the reaction of hydrogen. Through X-ray photoelectron spectroscopy (XPS) analysis, Cao et al. showed that Fe3+ in Ba3Co2Fe24O41-based ferrites was partially reduced to Fe2+ after electroplating. They proposed the reduction was due to atomic hydrogen generated in electroplating [9]. It is reasonable to assume that hydrogen had induced a similar reduction to the samples in this study, which can be expressed as: H2 O þ e YOH þ Hads ;
ð1Þ
Hads þ Hads YH2 ;
ð2Þ
Hads þ FeFe YH!i þ FeV Fe;
ð3Þ
where Hads represents an adsorbed hydrogen atom, H!i represents an ionized hydrogen in an interstitial site and FeFeV represents a divalent iron in the lattice site of a trivalent iron. The formation of interstitial hydrogen in oxides has recently been proven through FTIR analysis on rutile single crystals [8]. It is well known that the appearance of Fe2+ leads to electrical conduction through hopping of electrons between Fe2+ and Fe3+ in the octahedral sites of spinel lattice [10]. So the resistance of hydrogen-charged inductors must have been greatly decreased and the quality factor was thus greatly decreased. A systematic investigation on the properties of hydrogen-charged ferrites still needs to be conducted.
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
Quality Factor
Inductance ( µH )
10
as-sintered
10
Quality Factor
It has been demonstrated that the hydrogen-induced degradation of chip inductors in electroplating can be eliminated through a heat-treatment of 2 h at 500 8C in air [4,9]. In practice, however, such a treatment cannot be adopted as the layers of Ni and Sn(Pb) will be oxidized and the solderability will be seriously affected. For hydrogen-degraded TiO2 capacitors, we have observed a spontaneous recovery through aging at room temperature [8]. It is necessary to have an in-depth investigation on the stability of hydrogen in ferrites. For the hydrogen-charged chip inductor of Fig. 2, its frequency spectra of inductance and quality factor did show some spontaneous recovery at room temperature. The inductance and quality factor at high frequencies increased with increasing aging time, as shown in Fig. 3. It can be seen that after an aging of 283 h at 20 8C in air, the inductance at high frequencies has recovered quite well, while the quality factor at high frequencies was still much lower than its as-sintered value. Fig. 4 shows the relationship between the quality factor at 10 MHz and the aging time. At the early stage of aging, the quality factor increased very quickly with time. When the aging time was over 10 h, however, the recovery speed was gradually slowed down. It would take a very long period of time to further increase the quality factor through aging at 20 8C. To speed up the recovery process, we had subsequently placed the chip inductor in a chamber that was kept at 130 8C in air and the results are shown in Figs. 3 and 4, respectively. The quality factor was indeed further increased quickly. The aging at 130 8C was quite similar to that at 20 8C, namely, the recovery was fast at the early stage of aging, but became slower as aging time increased. We had finally heated the chip inductor in a tube furnace with flowing N2 at 250 8C for 4 h. N2 was helpful to avoid the oxidation of Ni and Sn(Pb) layers on the termination electrodes. Both the inductance and the quality factor were further increased, though aging had
O
250 C in N 2
Quality Factor
1638
O
1.5h 20 C in air O 7.5h 20 C in air O 283h 20 C in air O 44h 130 C in air O 4h 250 C in N 2
0
10
1
0.1 10
3
10
4
10
5
10
6
10
7
Frequency ( Hz ) Fig. 3. Frequency spectra of inductance and quality factor of a chip inductor with aging at different temperatures after 30 min of electrochemical hydrogen charging.
O
250 C in N2 1 O
20 C in air
O
130 C in air
0 10 2 0 30 4 0 50
Aging Time ( h ) 1 O
20 C in air
0
10
20
30
40
50
60
70
80
90
100
Aging Time ( h ) Fig. 4. Quality factor (10 MHz) vs. aging time for a hydrogen-charged chip inductor with three stages of aging: 283 h at 20 8C in air, 44 h at 130 8C in air and 4 h at 250 8C in N2. Inset: Quality factor (10 MHz) vs. aging time for another hydrogen-charged chip inductor with two stages of aging: 20 h at 20 8C in air and 4 h at 250 8C in N2.
been conducted at 20 8C and 130 8C for long periods of time. To verify whether the recovery achieved through aging at low temperatures makes any contribution to the final recovery, we had studied the recovery behavior of another chip inductor, which had been hydrogen charged under the same condition, or with the application of a 3 V DC voltage for 30 min. It was stored at 20 8C in air for 20 h and then heat-treated at 250 8C for 4 h in N2. As shown in the inset of Fig. 3, its final quality factor was almost the same as that of the previous sample, though its recovery obtained at lower temperatures was much smaller. It indicates that the aging at the highest temperature, including the duration time and the temperature, decides the degree of recovery. For some hydrogen-degraded oxides, it has been found that the degradation is recovered when hydrogen is driven out or diffused out from the oxides [8,11]. We believe that the recovery observed in this study is by the same mechanism, which can be expressed as: 2H!i þ 2FeV Fe Y2FeFe þ H2 :
5
10
ð4Þ
The outdiffusion of hydrogen results in the conversion of Fe2+ to Fe3+ and the various properties can thus be recovered. In proton-exchanged waveguides of LiNbO3 and LiTaO3, two states of hydrogen have been identified that correspond to a sharp absorption peak at 3510 cm1 and a broad absorption band centered at 3280 cm1 in the infrared absorption spectra [12], respectively. The peak of 3510 cm1 is due to the stretch mode of OUH bond, indicating that its hydrogen forms a bond with oxygen in the lattices and it is indeed very stable in the lattices. On the other hand, hydrogen corresponding to the broad band of 3280 cm1 is found to be only metastable in the lattices. Recently, metastable hydrogen corresponding to an absorption peak at 3280 cm1 was also observed in
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
electrochemically hydrogen-charged rutile single crystals and it was proposed that this kind of hydrogen is in an interstitial site and it is only metastable in the lattices as it does not form a bond with oxygen in the lattices [8]. Electrochemically charged hydrogen must be in the same metastable state in the inductors as it is in rutile, so the degraded inductors started to recover at room temperature. The stability of hydrogen in ferrites seems to decrease greatly with increasing temperature, and hydrogen-induced degradation can be more quickly recovered through a mild heat-treatment. In practice, a mild heat-treatment is always adopted for ferrite-based chip inductors after electroplating. Eq. (4) shows that the outdiffusion of hydrogen is independent of the content of oxygen in ambient atmosphere, so an inert atmosphere can be used for the heat-treatment to prevent oxidization of Ni and Sn(Pb) layers. Now we can more clearly understand the short-circuit failure of ferrite-based chip inductors in electroplating. The resistivity of the ferrites must have been dramatically decreased by hydrogen generated in electroplating so metals can be deposited on the relatively more conducting ferrite surface. Presently, we have tested and found that a discontinuous electroplating, namely electroplating divided into some sections with suitable intervals between them, is helpful to suppress this kind of failure. The intervals are adopted for the inductors to recover their resistivity, preventing a continuous decrease in resistivity during electroplating and thus decreasing the short-circuit failure. Many of us have the experience that our electronic products, such as mobile phones, are damaged by an accidental presence of water. This study reminds us that on such occasions we should first remove the power source to minimize the time of undesirable hydrogen-charging that occurs to the ferrite inductors inside. The spontaneous recovery of ferrite inductors becomes more and more difficult with increasing time of electrochemical hydrogen charging. Of course, it is best to increase ferrites’ resistance to hydrogen and this should be a criterion for the further development of ferrite inductors.
1639
4. Conclusions Hydrogen generated by the electrolysis of water greatly degrades the properties of Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors in that the inductance and the quality factor of the chip inductors are greatly deceased at high frequencies. These changes result from the reduction of Fe3+ to Fe2+ in the ferrite by atomic hydrogen, which exists as an interstitial ion in the lattice. Hydrogen is metastable in the ferrite lattice and the degraded properties can be mostly recovered through an aging of 4 h at 250 8C in N2. Hydrogen-induced degradation and its recovery behavior are important for ferrites-based chip inductors in electroplating and in operation.
Acknowledgements This work has been supported by the Postdoctoral Research Fellowship Scheme and the Centre for Smart Materials of the Hong Kong Polytechnic University.
References [1] C.Y. Tsay, K.S. Liu, T.F. Lin, I.N. Lin, J. Magn. Magn. Mater. 209 (2000) 189. [2] S.F. Wang, Y.R. Wang, T.C.K. Yang, C.F. Chen, C.A. Lu, C.Y. Huang, J. Magn. Magn. Mater. 220 (2000) 129. [3] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285. [4] J.L. Cao, X.H. Wang, L. Zhang, L.T. Li, Mater. Lett. 57 (2002) 386. [5] A. Moore, Electron. Prod. 18 (2) (1989) 25. [6] J.D. Baniecki, J.S. Cross, M. Tsukada, J. Watanabe, Appl. Phys. Lett. 81 (2002) 3837. [7] W.P. Chen, L.T. Li, Y. Wang, Z.L. Gui, J. Mater. Res. 13 (1998) 1110. [8] W.P. Chen, Y. Wang, J.Y. Dai, S.G. Lu, X.X. Wang, F.P. Lee, H.L.W. Chan, C.L. Choy, Appl. Phys. Lett. 84 (2004) 103. [9] J.L. Cao, L.T. Li, L. Zhang, J. Electrochem. Soc. 149 (2002) J89. [10] B.P. Rao, K.H. Rao, J. Mater. Sci. 32 (1997) 6049. [11] S. Aggarwal, S.R. Perusse, C.J. Kerr, R. Ramesh, J.T. Evans Jr., L. Boyer, G. Velasquez, Appl. Phys. Lett. 76 (2000) 918. [12] P. Baldi, M.P. De Micheli, K. El Hadi, S. Nouh, A.C. Cino, P. Aschieri, D.B. Ostrowsky, Opt. Eng. 37 (1998) 1193.
Hydrogen-induced degradation in NiCuZn ferrite-based multilayer chip inductors W.P. Chena,b,T, J.Q. Qib, Y. Wangb, X.H. Wangc, Z.W. Mac, L.T. Lic, H.L.W. Chanb a Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, PR China c Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
b
Received 26 August 2004; received in revised form 17 January 2005; accepted 19 January 2005 Available online 2 March 2005
Abstract Hydrogen-induced degradation in Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors was studied through an electrochemical hydrogen charging method, in which the silver electrodes of the inductors were made a cathode in 0.01 M NaOH solution to evolve hydrogen by the electrolysis of water. After the treatment, the inductance and the quality factor of the inductors at high frequencies were dramatically decreased. The degradation showed a little spontaneous recovery at room temperature and could be mostly recovered through a heat-treatment of 4 h at 250 8C in N2. It is proposed that hydrogen generated by the electrolysis of water is incorporated into the ferrite lattice and exists as an interstitial proton after reducing Fe3+ to Fe2+. The stability of hydrogen in ferrites decreased with increasing temperature and its outdiffusion resulted in the recovery. Hydrogen-induced degradation is important to ferrite-based chip inductors in fabrication and in operation. D 2005 Elsevier B.V. All rights reserved. PACS: 72.10.d; 75.50.Gg; 78.30.Hv Keywords: Ferrite; Hydrogen; Inductor; Degradation
1. Introduction Ferrites are one of the most important materials for multilayer ceramic chip inductors due to their high permeability in the RF frequency region, high electrical resistivity and high stability [1]. Ferrite chip inductors are applied in large amounts in various electronic circuits and they have greatly benefited the miniaturization of many latest electronic products, including mobile phones, notebook computers, and video cameras. Now extensive studies are being conducted to further improve their properties and to lower their sintering temperatures [2,3]. With the wide application of ferrite chip inductors, the influence of hydrogen on ferrites has also attracted more and more attention. Cao et al. recently reported that in electroplating hydrogen leads to a serious decrease in T Corresponding author. Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China. Tel.: +86 27 6131 2396; fax: +86 27 6875 2569. E-mail address: [email protected] (W.P. Chen). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.01.031
the inductance of NiCuZn ferrite-based multilayer chip inductors, and sometimes also causes a short-circuit failure as electroplating occurs on the ferrite substrate surface beyond the silver termination electrodes [4]. Electroplating is widely adopted to form surface-mountable three-layer termination electrodes (Ag/Ni/Sn–Pb) for various chip components [5]. Some other researchers have shown that water vapor can have a strong impact on the properties of electronic devices through the electrolysis of water [6], in which the reaction of hydrogen should be a key factor. This is especially important for ferrites chip inductors because they have no polymer coating to prevent the condensation of water on them. Obviously, the influence of hydrogen is important to ferrite-based chip inductors and should be studied in depth.
2. Experimental procedure A group of Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors were used in the present inves-
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
3. Results and discussion Fig. 1 shows a representative SEM micrograph taken on a fractured surface of a chip inductor. It is clear that the inductor has a very dense microstructure. For comparison, some inductors had been immersed in the NaOH solution at 20 8C for 100 h with no hydrogen charging. No discernible changes occurred in the properties of the samples after the immersion, indicating that the dilute solution did not influence the samples through permeation, which was in agreement with the microstructural observation. In contrast, even a very short period of electrochemical hydrogen charging resulted in great decreases in inductance and in quality factor of the inductors at high frequencies. Some representative results obtained for a chip inductor are shown in Fig. 2. A DC voltage of 3 V had been applied between its silver
Fig. 1. SEM micrograph taken for a fractured surface of a multilayer chip inductor.
Inductance ( µH )
10
Quality Factor
tigation. The inductors were in the size of 2.11.30.8 mm3 with two silver termination electrodes. Their fabrication and sintering had been reported in detail in another paper [4]. Some of the chip inductors were placed in a 0.01 M NaOH solution and DC voltages were applied between the silver electrodes of the chip inductors and a counter Pt electrode in the solution. The silver electrodes of the samples acted as cathode and the Pt electrode acted as anode. The applied DC voltages led to the electrolysis of water with hydrogen deposited on the silver electrodes of the samples and this treatment is referred to hereafter as electrochemical hydrogen charging [7,8]. The DC voltages were removed after some designated periods of time and the samples were taken out, cleaned with de-ionized water and dried. An Agilent 4294 A impedance analyzer was used to measure the frequency spectra of inductance and quality factor of the chip inductors. Microstructural analyses were conducted on a scanning electron microscope STEROSCAN 440.
1637
5
as -sintered Hydrogen -charged 10 min 20 min 30 min
0
10
1
0.1 10
3
10
4
10
5
10
6
10
7
Frequency ( Hz ) Fig. 2. Frequency spectra of inductance and quality factor of a chip inductor before and after electrochemical hydrogen charging.
termination electrodes and the anode in the solution. One can see that after 30 min electrochemical hydrogen charging, the inductance and the quality factor were decreased at 10 MHz by more than 20 and 100 times, respectively, while they almost remained unchanged at low frequencies. It is well known that chip inductors usually work at high frequencies so the hydrogen charging had greatly degraded the properties of the chip inductor. Obviously, the changes in the hydrogen-charged chip inductors should have resulted from the reaction of hydrogen. Through X-ray photoelectron spectroscopy (XPS) analysis, Cao et al. showed that Fe3+ in Ba3Co2Fe24O41-based ferrites was partially reduced to Fe2+ after electroplating. They proposed the reduction was due to atomic hydrogen generated in electroplating [9]. It is reasonable to assume that hydrogen had induced a similar reduction to the samples in this study, which can be expressed as: H2 O þ e YOH þ Hads ;
ð1Þ
Hads þ Hads YH2 ;
ð2Þ
Hads þ FeFe YH!i þ FeV Fe;
ð3Þ
where Hads represents an adsorbed hydrogen atom, H!i represents an ionized hydrogen in an interstitial site and FeFeV represents a divalent iron in the lattice site of a trivalent iron. The formation of interstitial hydrogen in oxides has recently been proven through FTIR analysis on rutile single crystals [8]. It is well known that the appearance of Fe2+ leads to electrical conduction through hopping of electrons between Fe2+ and Fe3+ in the octahedral sites of spinel lattice [10]. So the resistance of hydrogen-charged inductors must have been greatly decreased and the quality factor was thus greatly decreased. A systematic investigation on the properties of hydrogen-charged ferrites still needs to be conducted.
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
Quality Factor
Inductance ( µH )
10
as-sintered
10
Quality Factor
It has been demonstrated that the hydrogen-induced degradation of chip inductors in electroplating can be eliminated through a heat-treatment of 2 h at 500 8C in air [4,9]. In practice, however, such a treatment cannot be adopted as the layers of Ni and Sn(Pb) will be oxidized and the solderability will be seriously affected. For hydrogen-degraded TiO2 capacitors, we have observed a spontaneous recovery through aging at room temperature [8]. It is necessary to have an in-depth investigation on the stability of hydrogen in ferrites. For the hydrogen-charged chip inductor of Fig. 2, its frequency spectra of inductance and quality factor did show some spontaneous recovery at room temperature. The inductance and quality factor at high frequencies increased with increasing aging time, as shown in Fig. 3. It can be seen that after an aging of 283 h at 20 8C in air, the inductance at high frequencies has recovered quite well, while the quality factor at high frequencies was still much lower than its as-sintered value. Fig. 4 shows the relationship between the quality factor at 10 MHz and the aging time. At the early stage of aging, the quality factor increased very quickly with time. When the aging time was over 10 h, however, the recovery speed was gradually slowed down. It would take a very long period of time to further increase the quality factor through aging at 20 8C. To speed up the recovery process, we had subsequently placed the chip inductor in a chamber that was kept at 130 8C in air and the results are shown in Figs. 3 and 4, respectively. The quality factor was indeed further increased quickly. The aging at 130 8C was quite similar to that at 20 8C, namely, the recovery was fast at the early stage of aging, but became slower as aging time increased. We had finally heated the chip inductor in a tube furnace with flowing N2 at 250 8C for 4 h. N2 was helpful to avoid the oxidation of Ni and Sn(Pb) layers on the termination electrodes. Both the inductance and the quality factor were further increased, though aging had
O
250 C in N 2
Quality Factor
1638
O
1.5h 20 C in air O 7.5h 20 C in air O 283h 20 C in air O 44h 130 C in air O 4h 250 C in N 2
0
10
1
0.1 10
3
10
4
10
5
10
6
10
7
Frequency ( Hz ) Fig. 3. Frequency spectra of inductance and quality factor of a chip inductor with aging at different temperatures after 30 min of electrochemical hydrogen charging.
O
250 C in N2 1 O
20 C in air
O
130 C in air
0 10 2 0 30 4 0 50
Aging Time ( h ) 1 O
20 C in air
0
10
20
30
40
50
60
70
80
90
100
Aging Time ( h ) Fig. 4. Quality factor (10 MHz) vs. aging time for a hydrogen-charged chip inductor with three stages of aging: 283 h at 20 8C in air, 44 h at 130 8C in air and 4 h at 250 8C in N2. Inset: Quality factor (10 MHz) vs. aging time for another hydrogen-charged chip inductor with two stages of aging: 20 h at 20 8C in air and 4 h at 250 8C in N2.
been conducted at 20 8C and 130 8C for long periods of time. To verify whether the recovery achieved through aging at low temperatures makes any contribution to the final recovery, we had studied the recovery behavior of another chip inductor, which had been hydrogen charged under the same condition, or with the application of a 3 V DC voltage for 30 min. It was stored at 20 8C in air for 20 h and then heat-treated at 250 8C for 4 h in N2. As shown in the inset of Fig. 3, its final quality factor was almost the same as that of the previous sample, though its recovery obtained at lower temperatures was much smaller. It indicates that the aging at the highest temperature, including the duration time and the temperature, decides the degree of recovery. For some hydrogen-degraded oxides, it has been found that the degradation is recovered when hydrogen is driven out or diffused out from the oxides [8,11]. We believe that the recovery observed in this study is by the same mechanism, which can be expressed as: 2H!i þ 2FeV Fe Y2FeFe þ H2 :
5
10
ð4Þ
The outdiffusion of hydrogen results in the conversion of Fe2+ to Fe3+ and the various properties can thus be recovered. In proton-exchanged waveguides of LiNbO3 and LiTaO3, two states of hydrogen have been identified that correspond to a sharp absorption peak at 3510 cm1 and a broad absorption band centered at 3280 cm1 in the infrared absorption spectra [12], respectively. The peak of 3510 cm1 is due to the stretch mode of OUH bond, indicating that its hydrogen forms a bond with oxygen in the lattices and it is indeed very stable in the lattices. On the other hand, hydrogen corresponding to the broad band of 3280 cm1 is found to be only metastable in the lattices. Recently, metastable hydrogen corresponding to an absorption peak at 3280 cm1 was also observed in
W.P. Chen et al. / Materials Letters 59 (2005) 1636–1639
electrochemically hydrogen-charged rutile single crystals and it was proposed that this kind of hydrogen is in an interstitial site and it is only metastable in the lattices as it does not form a bond with oxygen in the lattices [8]. Electrochemically charged hydrogen must be in the same metastable state in the inductors as it is in rutile, so the degraded inductors started to recover at room temperature. The stability of hydrogen in ferrites seems to decrease greatly with increasing temperature, and hydrogen-induced degradation can be more quickly recovered through a mild heat-treatment. In practice, a mild heat-treatment is always adopted for ferrite-based chip inductors after electroplating. Eq. (4) shows that the outdiffusion of hydrogen is independent of the content of oxygen in ambient atmosphere, so an inert atmosphere can be used for the heat-treatment to prevent oxidization of Ni and Sn(Pb) layers. Now we can more clearly understand the short-circuit failure of ferrite-based chip inductors in electroplating. The resistivity of the ferrites must have been dramatically decreased by hydrogen generated in electroplating so metals can be deposited on the relatively more conducting ferrite surface. Presently, we have tested and found that a discontinuous electroplating, namely electroplating divided into some sections with suitable intervals between them, is helpful to suppress this kind of failure. The intervals are adopted for the inductors to recover their resistivity, preventing a continuous decrease in resistivity during electroplating and thus decreasing the short-circuit failure. Many of us have the experience that our electronic products, such as mobile phones, are damaged by an accidental presence of water. This study reminds us that on such occasions we should first remove the power source to minimize the time of undesirable hydrogen-charging that occurs to the ferrite inductors inside. The spontaneous recovery of ferrite inductors becomes more and more difficult with increasing time of electrochemical hydrogen charging. Of course, it is best to increase ferrites’ resistance to hydrogen and this should be a criterion for the further development of ferrite inductors.
1639
4. Conclusions Hydrogen generated by the electrolysis of water greatly degrades the properties of Ni0.38Cu0.12Zn0.50Fe2O4-based multilayer ceramic chip inductors in that the inductance and the quality factor of the chip inductors are greatly deceased at high frequencies. These changes result from the reduction of Fe3+ to Fe2+ in the ferrite by atomic hydrogen, which exists as an interstitial ion in the lattice. Hydrogen is metastable in the ferrite lattice and the degraded properties can be mostly recovered through an aging of 4 h at 250 8C in N2. Hydrogen-induced degradation and its recovery behavior are important for ferrites-based chip inductors in electroplating and in operation.
Acknowledgements This work has been supported by the Postdoctoral Research Fellowship Scheme and the Centre for Smart Materials of the Hong Kong Polytechnic University.
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