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Conducting polymer cathodes for high-frequency operable electrolytic niobium capacitors
SYilTIIETII[ IMETBLS ELSEVIER
Synthetic Metals 74 (1995) 165-170
Conducting polymer cathodes for high-frequency operable electrolytic niobium capacitors Poopathy Kathirgamanathan *, Seenivasagam Ravichandran Unitfor Speciality Electronic Polymers, School of Electrical Electronic and Information Engineering, South Bank University, 103 Borough Road, London SE10AA, UK Received 25 April 1995; accepted 9 June 1995
Abstract
A capacitor based on niobium pentoxide (anodized niobium) has been successfully produced with conducting polyaniline and polypyrrole as the cathodes, which performs well up to 800 kHz. This has been compared with a commercialtantalum pentoxide capacitor with manganese dioxide as cathode, which performs well only up to 100 kHz. Leakage current, equivalent series resistance and impedance are reported for both devices. Keywords: Polymer cathodes; Capacitors; Niobium
1. Introduction
Capacitors are used in almost all electronic equipment. Currently commercially available capacitors are made of metallized polymer films [1], metal oxide (e.g. A1203, Ta2Os) and ceramics (BaTiO3, lead titanium zirconate) [ I ]. Amongst the medium-value capacitors, tantalum-based capacitors are very popular. A typical metal oxide capacitor (also known as an electrolytic capacitor) is produced by the anodization of the metal (e.g. Ta) in an acidic bath (e.g. 0.01% HaPO4) for a pre-determined period of time. The anodized tantalum electrode (i.e. Ta2Os/Ta) is then coated with MnU(NO3) 2 and heated in a furnace in the presence of oxygen to give a coating of conducting manganese dioxide (or = 0.1 S cm- ' ) onto Ta/Ta205. The MnO2-coated electrode is then coated with carbon paint, followed by silver paint to form a capacitor. The manganese dioxide not only provides the electrical contact but also provides the protection for the dielectric because of its self-healing properties (i.e. any non-anodized area of the Ta electrode is oxidized to Ta205 by MnO2). The performance of a capacitor is determined by three parameters: leakage current density, capacitance versus frequency (f) characteristics and equivalent series resistance (ESR) versus f. A good capacitor should have a low leakage current (below 1/zA cm-2), constant capacitance over the frequency range of interest and low ESR (below 1 1"1cm -2 at 100 kHz). * Corresponding author. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
SSD10379-6779(95)03363-0
Capacitors using manganese dioxide as cathodes have three major disadvantages. First the capacitors are subject to severe thermal shock owing to high temperature (typically 400-600 °C) required for thermal decomposition of manganese(II) nitrate. This enhances the failure rate of such capacitors in equipment after the assembly stage. Secondly, the capacitance of the capacitor is not constant beyond 10 kHz. Thirdly, the relatively low conductivity of MnO2 imparts too high an ESR. This also limits the operation of the capacitor above 10 kHz. There is an increasing demand for capacitors that are capable of operating up to 100 kHz and preferably up to 1 MHz, because modern equipment is expected to transport signals at faster rates than ever before. Organic conducting polymers and conducting adducts are good candidates to replace MnO2 as they (i) have higher conductivity ( 1-1000 S cm- 1) than MnO2 [ 2,3 ], (ii) have constant capacitance versus frequency characteristics up to 1 MHz [4], and (iii) can be deposited at or near room temperature [5-11 ]. N-(n-Butyl)isoquinolinium TCNQ salts [ 8], polypyrrole and polyanilines [5,6,8-11 ] have been used as a replacement for MnO2 on aluminium oxide and tantalum pentoxide capacitors. In this paper, we report the fabrication of niobium pentoxide (Nb2Os)-based electrolytic capacitors with polyaniline/polypyrrole as cathode materials and the electrical characterization of the resulting capacitor.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (I 995) 165-170
166
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10O.00
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-
G"
r.
50.00 -
a,
%****%°
.
0.00
/
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(Time / minutes) Fig. 1. Leakage current measurement. Current vs. time plots at 16 V through a resistance of 11 kl~ for niobium capacitor produced by anodization at (12]) 100 V and ( O ) 150 V in 0.01% phosphoric acid at 80 °C.
Fig. 2. Scanning electron micrograph of niobium anodized at 50 V for 3 h in 0.01% phosphoric acid.
Fig. 3. Scanning electron micrograph of de-doped polyaniline-coated anodized niobium.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
2. Experimental A tinned copper wire was attached to a niobium foil (20 × 20 × 0.031 mm) with silver paint. The tinned copper wire was then mounted into a glass tube and sealed with Araldite. The niobium electrode was polished with alumina powder (10 /zm), washed with deionized water and then anodized at various potentials 50, 100, 150 and 200 V for 3 h. On completion of anodization, the electrode was washed with deionized water, acetone and dried in air. Scanning electron microscopy (SEM) of the anodized surface shows porous (0.1-5/zm) uneven structure. It was also established by SEM that the thickness of Nb205 was 210___ 10 nm. Anodized niobium electrodes were then immersed into an N-methyl pyrrolidone solution containing deprotonated polyaniline (2% mass/vol.) under ultrasonication for 15 min. The electrode was then drip dried and vacuum dried at 80 °C for 12 h. De-doped polyaniline (o-~ 10-6-10 -8 S cm -1) has to be proton doped to enhance its conductivity. Thus, the electrode coated with de-doped polyaniline was dipped in 1 Mp-toluenesulfonic acid for 1 h, drip dried and vacuum dried at 80 °C for l 2 h. The now conducting surface can be used as an electrode for further electrodeposition with much more highly conducting polymers. However, an electrode has to be attached to the conducting surface as the NbzO5 is insulating and the previously attached tinned wire cannot be used as a contact. Thus, a tinned copper wire was attached on one top corner of the electrode and immersed in a cell containing pyrrole (0.5 M), sodium p-toluenesulfonate (0.5 M) with a concentric counter electrode made of nickel foil (100 × 10 mm). Electropolymerization was carried out at 1 mA cm -2 for 1 h. On completion of the electrolysis, the electrode was
Fig. 4. Scanningelectron micrographof doped polyaniline-coatedanodized niobium,
167
washed with deionized water, rinsed with acetone and then dried in vacuum at 80 °C for 12 h. Nb/Nb2OJpolyaniline/ polypyrrole electrodes were then dipped in graphite paint for 20 s, drip dried and then vacuum dried at 80 °C for 12 h. A further coating with silver paint was made to facilitate connection of a tinned copper wire. The leakage current was measured at 16 V with a load of either I or 11 kl]. The capacitance, ESR and impedance were measured using a Hewlett-Packard impedance analyser (4192A) either at 0 or 2.2 V bias up to 1 MHz.
3. Results and discussion Surface resistances, as measured by the four-probe conductivity cell, for de-doped polyaniline, doped polyaniline and electrodeposited polypyrrole coatings were 3.3 × 10 3, 3.3 × 104 and 6.8 × 10 3 ['~, respectively. The decrease in surface resistance is expected as the de-doped polyaniline is doped and more highly conducting polypyrrole is electrodeposited. Fig. 1 shows the charging current versus time plot for two niobium capacitors (one anodized at 100 V, the other at 150 V) through a I 1 kl) load. The leakage current 30/zA cm -e for the niobium capacitor anodized at 150 V is much lower than that of the capacitor anodized at 100 V which is 83/zA c m - 2. This is expected in view of the larger amount of electricity passed in the former case, resulting in a thicker Nb205 coating. On ageing the capacitors for seven days the leakage current dropped to 0.1 /zA cm -2 for the former and 15/zA cm-2 for the latter. On reducing the external load to 1 kl),
Fig. 5. Scanningelectron micrographof electrochemicallydeposited polypyrrole onto doped polyaniline/anodizedniobium.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
168 5.0
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.................................................................................
~-
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20 L o g (F/Hz) Fig. 6. C a p a c i t a n c e vs. f r e q u e n c y characteristics o f ( [ 3 ) a N b / N b 2 O s / p o l y a n i l i n e / p o l y p y r r o l e / C / A g capacitor and ( ~ ) a c o m m e r c i a l T a / T a 2 O J M n O 2 / C / A g capacitor.
the leakage current remained the same for the capacitor anodized at 150 V, whereas, for the one anodized at 100 V, the leakage current increased to 150/xA cm -2. This indicates that the higher anodization voltage gives pore-free oxide coatings. Commercial Ta/Ta205/MnO2 capacitors typically have a leakage current 1.5-15/xA cm -2 and our niobium capacitor compares favourably. Fig. 2 shows the morphology of the niobium electrode anodized at 50 V for 3 h in 0.01% H3PO4 indicating a nonuniform deposit of Nb2Os. The surface structure of de-doped polyaniline coating on Nb/Nb:O5 is shown in Fig. 3. The de-doped polyaniline deposit shows a porous structure (pore size 1-4/zm) with cucumber-shaped nodules. On doping the de-doped polyaniline-coated Nb/Nb2Os, the surface becomes smoother and pore free (Fig. 4). Upon electrochemical deposition of polypyrrole onto the Nb/Nb2OJdopedpolyaniline, the surface becomes globular (see Fig. 5). Typical capacitance values from 102 Hz to 1 MHz for Nb/ Nb2OJdoped-polyaniline/polypyrrole/carbon / silver are shown in Fig. 6. The capacitance at 120 Hz at a bias voltage of 2.2 V was 660 nF. The theoretically expected capacitance
can be calculated as 675 nF from the thickness of Nb205 (210 nm in this case) taking the relative permittivity as 40 for Nb2Os. This indicates that there is no electrical short-circuiting between the niobium electrode and the doped polyaniline coating. Commercially available Ta/Ta2OJMnO2/C/Ag capacitors show oscillations above 100 kHz and are found to be unstable above 10 kHz, whereas the niobium-based capacitors with conducting polymer cathodes can be taken up to 800 kHz. Equivalent series resistances for Nb/NbzOJdoped-polyaniline/polypyrrole/C/Ag (hereinafter called cap Nb) capacitors at 100 kHz were typically 1.0 5:0.5 f / c m - 2, which is comparable to that of commercial tantalum capacitors. Normalized equivalent series resistance of cap Nb and commercial tantalum and alumina capacitors, where MnO2 is used as the cathode material, are shown in Fig. 7. It is clear from Fig. 7 that the ESR is the lowest for the conducting polymer-based capacitors. Normalized impedance versus frequency plots (Fig. 8) indicate that cap Nb has comparable impedance to that of tantalum capacitors.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
169
1.00"
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~ 0.50-
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0.25 -
0.00
Log (F/Hz) Fig. 7. Ratio of effective series resistanceat any frequency f to effectiveseries resistanceat I00 Hz vs. frequency of (El) a pyrrole/C/Ag capacitor and (O) a commercial Ta/Ta2OJMnO2/C/Ag capacitor.
Nb/Nb2OJpolyaniline/poly-
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Log (F/Hz) Fig. 8. Ratio of impedance at any frequencyfto impedance at frequency 100 Hz vs. frequency of (E]) a Nb/Nb~OJpolyaniline/polypyrrole/C/Ag capacitor and ( ~ ) a commercial Ta/Ta2OJMnO2/C/Ag capacitor.
170
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
4. Conclusions It has been demonstrated that a niobium-based capacitor, with an inherently conducting polymer cathode, produces superior capacitors to those utilizing MnO2 cathodes.
Acknowledgements The Royal Society of Great Britain and AVX Ltd. (Devon) are thanked for providing an equipment grant and some of the starting materials, respectively. The authors also thank Professor Bryan Bridge for his encouragement, and Mr I. Bishop for a stimulating discussion.
References [ 1 ] A.J. Moulson and J. Herbert, Electroceramics, Chapman and Hall, London, 1992. [2] M.R. Bryce, A. Chissel, P. Kathirgamanathan, D. Parker and N.R.M. Smith, J. Chem. Soc., Chem. Commun., (1987)466. [3] P. Kathirgamanthan, S.A. Mucklejohn and D.R. Rosseinsky, J. Chem. Soc., Chem. Commun., (1979) 86. [4] P. Kathirgamanathan, M.A. Mazid and D.R. Rosseinsky, J. Chem. Soc., Perking Trans. 2, (1982) 593. [ 5 ] S.D. Ross, US Patent No. 5 223 002 ( 1993 ). [6] Y.S. Park, K. Yamamoto, S. Takeoka and E. Tsuchida, Bull. Chem. Soc. Jpn., 65 (1992) 1860. [7] S. Niwa and H. Inoie, UKPatentApplic. No. GB 2113 916A (1983). [8] Y. Kudrh, S. Tsuchiya, T. Kojima, M. Fukuyama and S. Yoshimura, Synth. Met., 41-43 (1991) 1133. [9] L.H.M. Krings, E.E. Havinga, J.J.T.M. Donkers and F.T.A. Vork, Synth. Met., 54 (1993) 453. [ 10 ] S.D. Ross, US Patent No. 5 223 002 (1993). [11] F. Takei, H. Shirroto and N. Sawatari, Abstr. No. 188, Electrochem. Soc. Conf., New Orleans, LA, USA, Oct. 1993.
Synthetic Metals 74 (1995) 165-170
Conducting polymer cathodes for high-frequency operable electrolytic niobium capacitors Poopathy Kathirgamanathan *, Seenivasagam Ravichandran Unitfor Speciality Electronic Polymers, School of Electrical Electronic and Information Engineering, South Bank University, 103 Borough Road, London SE10AA, UK Received 25 April 1995; accepted 9 June 1995
Abstract
A capacitor based on niobium pentoxide (anodized niobium) has been successfully produced with conducting polyaniline and polypyrrole as the cathodes, which performs well up to 800 kHz. This has been compared with a commercialtantalum pentoxide capacitor with manganese dioxide as cathode, which performs well only up to 100 kHz. Leakage current, equivalent series resistance and impedance are reported for both devices. Keywords: Polymer cathodes; Capacitors; Niobium
1. Introduction
Capacitors are used in almost all electronic equipment. Currently commercially available capacitors are made of metallized polymer films [1], metal oxide (e.g. A1203, Ta2Os) and ceramics (BaTiO3, lead titanium zirconate) [ I ]. Amongst the medium-value capacitors, tantalum-based capacitors are very popular. A typical metal oxide capacitor (also known as an electrolytic capacitor) is produced by the anodization of the metal (e.g. Ta) in an acidic bath (e.g. 0.01% HaPO4) for a pre-determined period of time. The anodized tantalum electrode (i.e. Ta2Os/Ta) is then coated with MnU(NO3) 2 and heated in a furnace in the presence of oxygen to give a coating of conducting manganese dioxide (or = 0.1 S cm- ' ) onto Ta/Ta205. The MnO2-coated electrode is then coated with carbon paint, followed by silver paint to form a capacitor. The manganese dioxide not only provides the electrical contact but also provides the protection for the dielectric because of its self-healing properties (i.e. any non-anodized area of the Ta electrode is oxidized to Ta205 by MnO2). The performance of a capacitor is determined by three parameters: leakage current density, capacitance versus frequency (f) characteristics and equivalent series resistance (ESR) versus f. A good capacitor should have a low leakage current (below 1/zA cm-2), constant capacitance over the frequency range of interest and low ESR (below 1 1"1cm -2 at 100 kHz). * Corresponding author. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
SSD10379-6779(95)03363-0
Capacitors using manganese dioxide as cathodes have three major disadvantages. First the capacitors are subject to severe thermal shock owing to high temperature (typically 400-600 °C) required for thermal decomposition of manganese(II) nitrate. This enhances the failure rate of such capacitors in equipment after the assembly stage. Secondly, the capacitance of the capacitor is not constant beyond 10 kHz. Thirdly, the relatively low conductivity of MnO2 imparts too high an ESR. This also limits the operation of the capacitor above 10 kHz. There is an increasing demand for capacitors that are capable of operating up to 100 kHz and preferably up to 1 MHz, because modern equipment is expected to transport signals at faster rates than ever before. Organic conducting polymers and conducting adducts are good candidates to replace MnO2 as they (i) have higher conductivity ( 1-1000 S cm- 1) than MnO2 [ 2,3 ], (ii) have constant capacitance versus frequency characteristics up to 1 MHz [4], and (iii) can be deposited at or near room temperature [5-11 ]. N-(n-Butyl)isoquinolinium TCNQ salts [ 8], polypyrrole and polyanilines [5,6,8-11 ] have been used as a replacement for MnO2 on aluminium oxide and tantalum pentoxide capacitors. In this paper, we report the fabrication of niobium pentoxide (Nb2Os)-based electrolytic capacitors with polyaniline/polypyrrole as cathode materials and the electrical characterization of the resulting capacitor.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (I 995) 165-170
166
15o.oo t
10O.00
b._
-
G"
r.
50.00 -
a,
%****%°
.
0.00
/
o
~
(Time / minutes) Fig. 1. Leakage current measurement. Current vs. time plots at 16 V through a resistance of 11 kl~ for niobium capacitor produced by anodization at (12]) 100 V and ( O ) 150 V in 0.01% phosphoric acid at 80 °C.
Fig. 2. Scanning electron micrograph of niobium anodized at 50 V for 3 h in 0.01% phosphoric acid.
Fig. 3. Scanning electron micrograph of de-doped polyaniline-coated anodized niobium.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
2. Experimental A tinned copper wire was attached to a niobium foil (20 × 20 × 0.031 mm) with silver paint. The tinned copper wire was then mounted into a glass tube and sealed with Araldite. The niobium electrode was polished with alumina powder (10 /zm), washed with deionized water and then anodized at various potentials 50, 100, 150 and 200 V for 3 h. On completion of anodization, the electrode was washed with deionized water, acetone and dried in air. Scanning electron microscopy (SEM) of the anodized surface shows porous (0.1-5/zm) uneven structure. It was also established by SEM that the thickness of Nb205 was 210___ 10 nm. Anodized niobium electrodes were then immersed into an N-methyl pyrrolidone solution containing deprotonated polyaniline (2% mass/vol.) under ultrasonication for 15 min. The electrode was then drip dried and vacuum dried at 80 °C for 12 h. De-doped polyaniline (o-~ 10-6-10 -8 S cm -1) has to be proton doped to enhance its conductivity. Thus, the electrode coated with de-doped polyaniline was dipped in 1 Mp-toluenesulfonic acid for 1 h, drip dried and vacuum dried at 80 °C for l 2 h. The now conducting surface can be used as an electrode for further electrodeposition with much more highly conducting polymers. However, an electrode has to be attached to the conducting surface as the NbzO5 is insulating and the previously attached tinned wire cannot be used as a contact. Thus, a tinned copper wire was attached on one top corner of the electrode and immersed in a cell containing pyrrole (0.5 M), sodium p-toluenesulfonate (0.5 M) with a concentric counter electrode made of nickel foil (100 × 10 mm). Electropolymerization was carried out at 1 mA cm -2 for 1 h. On completion of the electrolysis, the electrode was
Fig. 4. Scanningelectron micrographof doped polyaniline-coatedanodized niobium,
167
washed with deionized water, rinsed with acetone and then dried in vacuum at 80 °C for 12 h. Nb/Nb2OJpolyaniline/ polypyrrole electrodes were then dipped in graphite paint for 20 s, drip dried and then vacuum dried at 80 °C for 12 h. A further coating with silver paint was made to facilitate connection of a tinned copper wire. The leakage current was measured at 16 V with a load of either I or 11 kl]. The capacitance, ESR and impedance were measured using a Hewlett-Packard impedance analyser (4192A) either at 0 or 2.2 V bias up to 1 MHz.
3. Results and discussion Surface resistances, as measured by the four-probe conductivity cell, for de-doped polyaniline, doped polyaniline and electrodeposited polypyrrole coatings were 3.3 × 10 3, 3.3 × 104 and 6.8 × 10 3 ['~, respectively. The decrease in surface resistance is expected as the de-doped polyaniline is doped and more highly conducting polypyrrole is electrodeposited. Fig. 1 shows the charging current versus time plot for two niobium capacitors (one anodized at 100 V, the other at 150 V) through a I 1 kl) load. The leakage current 30/zA cm -e for the niobium capacitor anodized at 150 V is much lower than that of the capacitor anodized at 100 V which is 83/zA c m - 2. This is expected in view of the larger amount of electricity passed in the former case, resulting in a thicker Nb205 coating. On ageing the capacitors for seven days the leakage current dropped to 0.1 /zA cm -2 for the former and 15/zA cm-2 for the latter. On reducing the external load to 1 kl),
Fig. 5. Scanningelectron micrographof electrochemicallydeposited polypyrrole onto doped polyaniline/anodizedniobium.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
168 5.0
/ /
/
i
... 4.0
.................................................................................
~-
........................."¢,........................................................ O ..................¢- ................¢- ..............~ ..........~-----~.............
o
.d
3.0
20 L o g (F/Hz) Fig. 6. C a p a c i t a n c e vs. f r e q u e n c y characteristics o f ( [ 3 ) a N b / N b 2 O s / p o l y a n i l i n e / p o l y p y r r o l e / C / A g capacitor and ( ~ ) a c o m m e r c i a l T a / T a 2 O J M n O 2 / C / A g capacitor.
the leakage current remained the same for the capacitor anodized at 150 V, whereas, for the one anodized at 100 V, the leakage current increased to 150/xA cm -2. This indicates that the higher anodization voltage gives pore-free oxide coatings. Commercial Ta/Ta205/MnO2 capacitors typically have a leakage current 1.5-15/xA cm -2 and our niobium capacitor compares favourably. Fig. 2 shows the morphology of the niobium electrode anodized at 50 V for 3 h in 0.01% H3PO4 indicating a nonuniform deposit of Nb2Os. The surface structure of de-doped polyaniline coating on Nb/Nb:O5 is shown in Fig. 3. The de-doped polyaniline deposit shows a porous structure (pore size 1-4/zm) with cucumber-shaped nodules. On doping the de-doped polyaniline-coated Nb/Nb2Os, the surface becomes smoother and pore free (Fig. 4). Upon electrochemical deposition of polypyrrole onto the Nb/Nb2OJdopedpolyaniline, the surface becomes globular (see Fig. 5). Typical capacitance values from 102 Hz to 1 MHz for Nb/ Nb2OJdoped-polyaniline/polypyrrole/carbon / silver are shown in Fig. 6. The capacitance at 120 Hz at a bias voltage of 2.2 V was 660 nF. The theoretically expected capacitance
can be calculated as 675 nF from the thickness of Nb205 (210 nm in this case) taking the relative permittivity as 40 for Nb2Os. This indicates that there is no electrical short-circuiting between the niobium electrode and the doped polyaniline coating. Commercially available Ta/Ta2OJMnO2/C/Ag capacitors show oscillations above 100 kHz and are found to be unstable above 10 kHz, whereas the niobium-based capacitors with conducting polymer cathodes can be taken up to 800 kHz. Equivalent series resistances for Nb/NbzOJdoped-polyaniline/polypyrrole/C/Ag (hereinafter called cap Nb) capacitors at 100 kHz were typically 1.0 5:0.5 f / c m - 2, which is comparable to that of commercial tantalum capacitors. Normalized equivalent series resistance of cap Nb and commercial tantalum and alumina capacitors, where MnO2 is used as the cathode material, are shown in Fig. 7. It is clear from Fig. 7 that the ESR is the lowest for the conducting polymer-based capacitors. Normalized impedance versus frequency plots (Fig. 8) indicate that cap Nb has comparable impedance to that of tantalum capacitors.
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
169
1.00"
0.75
\ ,v
~ 0.50-
' "\ "\\.
0.25 -
0.00
Log (F/Hz) Fig. 7. Ratio of effective series resistanceat any frequency f to effectiveseries resistanceat I00 Hz vs. frequency of (El) a pyrrole/C/Ag capacitor and (O) a commercial Ta/Ta2OJMnO2/C/Ag capacitor.
Nb/Nb2OJpolyaniline/poly-
0,03 -
0.02-
\
\
\
0.01 '
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i
v
i@
i
~
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~ v
Log (F/Hz) Fig. 8. Ratio of impedance at any frequencyfto impedance at frequency 100 Hz vs. frequency of (E]) a Nb/Nb~OJpolyaniline/polypyrrole/C/Ag capacitor and ( ~ ) a commercial Ta/Ta2OJMnO2/C/Ag capacitor.
170
P. Kathirgamanathan, S. Ravichandran / Synthetic Metals 74 (1995) 165-170
4. Conclusions It has been demonstrated that a niobium-based capacitor, with an inherently conducting polymer cathode, produces superior capacitors to those utilizing MnO2 cathodes.
Acknowledgements The Royal Society of Great Britain and AVX Ltd. (Devon) are thanked for providing an equipment grant and some of the starting materials, respectively. The authors also thank Professor Bryan Bridge for his encouragement, and Mr I. Bishop for a stimulating discussion.
References [ 1 ] A.J. Moulson and J. Herbert, Electroceramics, Chapman and Hall, London, 1992. [2] M.R. Bryce, A. Chissel, P. Kathirgamanathan, D. Parker and N.R.M. Smith, J. Chem. Soc., Chem. Commun., (1987)466. [3] P. Kathirgamanthan, S.A. Mucklejohn and D.R. Rosseinsky, J. Chem. Soc., Chem. Commun., (1979) 86. [4] P. Kathirgamanathan, M.A. Mazid and D.R. Rosseinsky, J. Chem. Soc., Perking Trans. 2, (1982) 593. [ 5 ] S.D. Ross, US Patent No. 5 223 002 ( 1993 ). [6] Y.S. Park, K. Yamamoto, S. Takeoka and E. Tsuchida, Bull. Chem. Soc. Jpn., 65 (1992) 1860. [7] S. Niwa and H. Inoie, UKPatentApplic. No. GB 2113 916A (1983). [8] Y. Kudrh, S. Tsuchiya, T. Kojima, M. Fukuyama and S. Yoshimura, Synth. Met., 41-43 (1991) 1133. [9] L.H.M. Krings, E.E. Havinga, J.J.T.M. Donkers and F.T.A. Vork, Synth. Met., 54 (1993) 453. [ 10 ] S.D. Ross, US Patent No. 5 223 002 (1993). [11] F. Takei, H. Shirroto and N. Sawatari, Abstr. No. 188, Electrochem. Soc. Conf., New Orleans, LA, USA, Oct. 1993.