Preparation and Characterization of Barium Titanate ...

Journal

J. Am. Ceram. Soc., 81 [9] 2429–42 (1998)

Preparation and Characterization of Barium Titanate Electrolytic Capacitors from Porous Titanium Anodes Sridhar Venigalla,*,† Robert E. Chodelka,* and James H. Adair*,‡ Department of Materials Science and Engineering, University of Florida, Gainesville, Florida

BaTiO3 is widely used as the dielectric in ceramic chip capacitors and multilayer capacitors, because of its high dielectric constant and ferroelectric properties. Multilayer capacitors provide fairly high capacitance per unit volume (volumetric efficiency); however, processing difficulties in the preparation of ultrathin layers limit further enhancement. Tantalum solid electrolytic capacitors, on the other hand, provide very high volumetric efficiencies, because of the large surface area of the sintered, porous tantalum anode on which the dielectric Ta2O5 is electrochemically deposited. Recent developments in electrochemical methods to deposit BaTiO3 on titanium substrates provide an opportunity to fabricate barium titanate electrolytic capacitors using sintered, porous titanium anodes. The high dielectric constant of BaTiO3 and the high surface area of the sintered, porous anode provide a good combination to achieve larger volumetric efficiencies. Current work involves the fabrication and characterization of barium titanate electrolytic capacitors. Effects of electrochemical processing parameters on the formation of BaTiO3 on the surface of sintered titanium anodes are described. Influence of the purity of titanium powder, the porosity of the sintered anode, and the post-deposition heat treatment on the dielectric properties of the fabricated capacitors is discussed. Complete penetration of the electrolyte solution and a thin uniform coating of TaTiO3 over the entire titanium surface was achieved using high-porosity (35%–40% of theoretical density) sintered titanium anodes. Samples treated for 8 h in 0.5M Ba(OH)2ⴢ8H2O electrolyte solutions at 100°C with an applied cell voltage of 12 V show the formation of a dense, uniform BaTiO3 coating on the surface of the titanium anode. High-purity, chloride-free titanium powder provides smaller dissipation factors at low frequencies. Heat treatment at 400°C significantly increases the capacitance at all frequencies, whereas the heat treatment lowers the dissipation factors at low frequencies. Calculated volumetric efficiencies are comparable to those typically obtained for tantalum solid electrolytic capacitors but are not as high as expected for barium titanate electrolytic capacitors. Penetration of the colloidal-carbon (external) electrode was limited to a depth of ∼300 µm, which might have caused the lower volumetric efficiencies.

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volume at a low cost. Electrolytic capacitors are deservedly popular in bypass, blocking, and power-supply filter applications and for motor-starting purposes. The dielectric material in an electrolytic capacitor consists of an anodically formed oxide of the anode material, which serves as the positive electrode of the capacitor. The most-used metals are aluminum and tantalum, and the anodic oxides are ␥-Al2O3 and Ta2O5, respectively.1,2 The effective dielectric constant of pure ␥-Al2O3 is 8.4 and that of Ta2O5 is 28. In certain commercial-grade capacitors, the dielectric constant can be slightly lower for Al2O3 and appreciably lower for Ta2O5, because of impurities. Electrolytic capacitors are produced in two basic styles: (i) the sintered anode or pellet style, where either a wet or dry electrolyte is used only for tantalum electrolytic capacitors,3 and (ii) the foil style, which uses a wet, dry, or paste electrolyte and also includes aluminum electrolytics. The sintered tantalum anode electrolytic capacitor is fundamentally different from any other electrolytic in that it contains no liquids; rather, it consists solely of stable, inorganic, nonvolatile materials. This condition results in important advantages, which include a small volume, absence of the need for a hermetic seal, flexible shape, superior temperature characteristics, a relatively low dielectric loss factor (0.1–0.5), and indefinitely long shelf life. However, the main disadvantage of this type of electrolytic capacitor is its low maximum recommended dc operating potential (5–100 V). However, this limitation has not proved to be a serious problem in most applications, especially in transistor circuits, wherein the principal consumption of electrolytic capacitors lies. Fabrication of a sintered solid tantalum anode electrolytic capacitor involves attaching a dense tantalum wire to the porous tantalum body (∼60% dense) either by embedding it in the porous block during pressing or by subsequent welding. A layer of dielectric Ta2O5 is formed on the tantalum surfaces electrochemically by making tantalum the anode in an electrolytic bath that contains phosphates or borates. After the formation of an oxide layer of desired thickness, a layer of semiconductor is deposited over the entire dielectric surface. MnO2, which is formed via the pyrolysis of Mn(NO3)2, is used for this purpose.3,4 The development of barium titanate electrolytic capacitors based on the technology previously described for the solid tantalum electrolytic capacitors will have significant impact on the applicability of these capacitors. The higher dielectric constant of BaTiO3 (∼3000 for pure, monolithic ceramic) can tremendously improve the volumetric efficiency of these capacitors and open a wide range of new applications, where very large capacitances are required at low operating voltages. Recent advances in methods to electrochemically synthesize BaTiO3 thin films on titanium substrates at low temperatures make it feasible to prepare barium titanate electrolytic capacitors. The deposition or growth of sparingly soluble singlecomponent oxides on metal surfaces is a well-established technique.5 The process of anodization is achieved via anodic polarization of the metal surface to form the resistive metal-oxide coating. The composition of the coating is dependent on the composition of the metal and the solution in which the anodization is performed, with cationic and anionic species incor-

I. Introduction capacitors are the first to be considered when large blocks of capacitance are needed in electrical circuits. No other capacitors offer such a large capacitance per unit

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LECTROLYTIC

N. J. Dudney—contributing editor

Manuscript No. 191895. Received April 8, 1996; approved September 6, 1997. Supported by the Department of Materials Science and Engineering, University of Florida. *Member, American Ceramic Society. † Now with Cabot Performance Materials, Boyertown, PA 19512. ‡ Now at Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802.

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Journal of the American Ceramic Society—Venigalla et al.

porated into the structure of the oxide. Many investigators6–11 reported the synthesis of perovskite (ABO3) complex oxides, such as BaTiO3, SrTiO3, and CaTiO3, and their solid solutions on titanium substrates using hydrothermal–electrochemical methods at temperatures in the range of 55°–300°C. This process involves the anodization of a titanium substrate in an electrolyte that contains the A-site ion (Ba2+, Sr2+, and Ca2+) at elevated temperatures and pressures. The thickness, microstructural uniformity, and crystallinity of these films are dependent on various processing parameters, such as temperature, reaction time, electrolyte chemistry, atmosphere, applied potential, and current density. Electrochemical synthesis of BaTiO3 thin films on titanium substrates, as well as barium titanate electrolytic capacitors at temperatures as low as 55°C, using open reaction vessels, have been reported.9–11 The use of highly alkaline electrolyte solutions (pH > 14) has been identified as the key to the synthesis of BaTiO3 at low temperatures. To minimize dielectric losses in the films, the use of a nonalkaline electrolyte (tetraethylammonium hydroxide, TEAOH) to adjust solution pH has been recommended. 10 A lowtemperature (200°C) heat treatment of the synthesized films was reported to have increased the dielectric constant through a phase transition to ferroelectric tetragonal BaTiO3.10 Based on the state of the art for synthesizing BaTiO3 via anodic oxidation of titanium in barium-containing electrolytes and the technology developed for the fabrication of the solid tantalum electrolytic capacitors, the current work attempts to develop barium titanate electrolytic capacitors, as schematically shown in Fig. 1. Issues in the preliminary report on the preparation of barium titanate electrolytic capacitors have indicated that impregnation of the porous titanium body with electrolyte solution, BaCO3 contamination from handling and reaction with the atmosphere, and the electrolytic salt have a role in the performance of the capacitor.11 The current work is

Fig. 1. Schematic cross section of the proposed barium titanate electrolytic capacitor.

Vol. 81, No. 9

focused on improving the properties of the BaTiO3 electrolytic capacitors through careful understanding and control of the processing conditions. This report also includes characterization of the microstructure and dielectric properties of the developed barium titanate electrolytic capacitors. II.

Materials and Methods

(1) Preparation of Sintered, Porous Titanium Anodes The processing steps involved in the preparation of barium titanate electrolytic capacitors are described in Fig. 2. Two grades of titanium powder were used in this study, to evaluate the effect of their chemical purity on dielectric properties of the barium titanate electrolytic capacitors. A 99.4%-pure, −100 mesh titanium powder (AESAR, Johnson Matthey, Ward Hill, MA) was obtained and sieved to obtain a narrow size distribution (−200/+270 mesh, average particle size of 50 ␮m). A higher-grade (99.6% pure, particle diameter of