Characteristics of tantalum electrolytic capacitors using soluble polypyrrole electrolyte
Introduction
A solid tantalum electrolytic capacitor using a tantalum thin film was developed by Balke in 1929 [1] and a capacitor using MnO2 as a solid electrolyte was developed by Haring and Taylor in 1965 [2]. Capacitance, loss tangent, and impedance characteristics of tantalum electrolytic capacitors are influenced not only by the property of carbon or silver paste but also by the type of electrolyte and the degree of homogeneity of the coating inside tantalum oxide pores [3], [4], [5], [6].
MnO2 has been widely used as a conventional electrolyte due to its high thermal stability and reliability. However, it has been difficult to achieve large capacitance and low impedance with this material because of its low electrical conductivity (<10−1 S/cm) [7], [8]. It has been realized that tantalum electrolytic capacitors prepared by electrochemical coating of conducting polymer electrolyte (polyaniline, polypyrrole (Ppy)) showed low impedance and high capacitance at a high frequency region [9], [10]. However, this method has several disadvantages such as uneven electrolyte thickness, complicated manufacturing process and consumption of electric energy during the synthesis and coating of the electrolytes.
Recently, several groups reported tantalum electrolytic capacitors adopting chemically synthesized soluble polyaniline as an electrolyte [11], [12], [13], [14]. This soluble polymer electrolyte makes the process easy, simple, and economical. However, the device did not show satisfactory values for the loss tangent and capacitance characteristics, which is attributed to the difficulty in making homogeneous coating inside the tantalum oxide pores. In order to achieve homogeneous coating, several conditions such as concentration of the electrolyte, the number of dipping, the use of additives, were controlled. Among these, the control of the concentration of electrolyte and the number of dipping have a limit to improve the affinity between the electrolyte and the Ta2O5 (tantalum oxide) due to the property of the electrolyte itself. So the use of additives to improve the device performance may have not only scientific but also technological importance.
In this work, various additives (surfactants, coupling agents) were added to soluble Ppy solutions in various organic solvents to prepare electrolyte solutions that can enhance the homogeneity of the Ppy film on the surface of tantalum oxide. Tantalum electrolytic capacitors were fabricated with these solutions and the effects of the additives on their electrical performances were examined.
Section snippets
Preparation of polypyrrole electrolyte
Pyrrole (Tokyo Chemical Industry Co.) was vacuum distilled prior to use. Pyrrole (0.4 mol) and a dopant (0.15 mol, di(2-ethylhexyl)sulfosuccinate sodium salt (DEHSNa) or butylnaphthalenesulfonate sodium salt (BNSNa), Tokyo Chemical Industry Co.) were mixed in distilled water (900 ml in a 1 l beaker) with magnetic stirring and the resulting mixture was cooled to 0 °C. A solution of ammonium peroxydisulfate, (NH4)2S2O8, (0.10 mol) in 100 ml distilled water was cooled to 0 °C and added to the mixture in ∼2
Results and discussion
In order to build a tantalum electrolytic capacitor having high capacitance and low loss tangent, the electrolyte should have good affinity to tantalum oxide so that the surface of the pores in tantalum oxide can be uniformly coated [11]. From this point of view, protic solvents seem to be appropriate candidates since they can interact with tantalum oxide strongly through hydrogen bonds [11].
In this study, several surfactants and coupling agents were added to Ppy electrolyte solutions in order
Conclusion
Polypyrroe powder (Ppy-DEHS, Ppy-BNS) was dissolved in various solvents (DMF, chloroform, trifluoroacetic acid, etc.) to prepare the polymer electrolyte. Tantalum electrolytic capacitors with a small size (D4 size) and high capacitance were fabricated by dip coating in these electrolyte solutions. A device prepared with Ppy-DEHS electrolyte dissolved in a protic solvent, trifluoroacetic acid, showed significantly high capacitance and low tan δ. Addition of either a surfactant (N
Acknowledgements
This work was supported by Korea Research Foundation Grant (KRF-2002-005-C00013).
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