Fabrication of a micro-porous Ti–Zr alloy by electroless reduction with a calcium reductant for electrolytic capacitor applications
Introduction
Anodic oxide films formed on valve metals such as aluminum, tantalum, and niobium, play an important role as dielectric films in electrolytic capacitors, and the electric capacitance, C, is expressed by the following equation:where ε0 is the vacuum permittivity, ε is the specific dielectric constant of the oxide film, S is the surface area of the electrode, and δ is the thickness of the oxide film. Recent developments for capacitor applications require a large ε-value of the oxide film, and titanium dioxide (TiO2) has significant advantage over other oxides because ε-value of TiO2 (ε = 40–135) is larger than those of the commonly used aluminum oxide (Al2O3, ε = 9.8) and tantalum oxide (Ta2O5, ε = 27.6) [1], [2], [3], [4], [5], [6], [7]. Growth of the oxide film on titanium by anodizing, however, involves an amorphous-to-crystalline transition at low voltage, and this crystallization causes the formation of electron conductive pathways through the oxide film, enabling oxygen gas evolution on the crystalline oxides during anodizing. Therefore, titanium and its oxide are seldom applied to electrode materials for electrolytic capacitor applications.
Recently, Habazaki et al. investigated the anodizing of titanium–zirconium, molybdenum, aluminum, tungsten, and silicon alloys formed by DC magnetron sputtering [8], [9], [10], [11], [12], [13]. From these investigations, they reported the formation of high capacitance composite oxide films without oxygen gas evolution during the anodizing of a Ti–62.5 at% Zr alloy [10], [12], [14]. The structure and composition of the oxide film were examined by X-ray diffraction and transmission electron microscopy, and the film formed on the alloy had nanocrystals of a monoclinic ZrO2 phase in an amorphous matrix. The capacitance of the oxide (1.8 mF/m2) was much higher than that of aluminum (0.7 mF/m2) and tantalum (1.4 mF/m2) formed under the same anodizing conditions; thus, this Ti–Zr alloy and its oxide have the potential to be electrode materials for electrolytic capacitors with high capacitance.
Titanium and zirconium are produced industrially by the Kroll process [15], [16], [17], [18]. This process consists of three steps of operation: conversion from oxide to tetrachloride, subsequent reduction of tetrachloride to metal by liquid magnesium, and molten electrolysis of the byproduct, magnesium chloride. Sponge-like structures of metallic titanium and zirconium with low oxygen content can be commercially obtained by the Kroll process. Consequently, at least the following six processes are needed to fabricate a Ti–Zr alloy for novel electrolytic capacitor applications: (a) chlorination of oxides, (b) magnesium reduction, (c) crushing, (d) arc melting for alloying, (e) sintering, and (f) anodizing for oxide film formation. The Kroll process, however, is a high-cost batch-type production method and requires several days for the production of these metals. Therefore, a simpler technique for the production Ti–Zr alloy, without the Kroll process, must be developed to fabricate the novel Ti–Zr capacitor [19], [20], [21], [22], [23], [24], [25].
In the present investigation, the authors report a new production process for a Ti–Zr alloy with a large surface area for Ti–Zr electrolytic capacitors. In the process, titanium and zirconium oxides are reduced at high temperature to a Ti–Zr alloy by electroless reduction with a calcium reductant in calcium chloride molten salt. Metallic calcium can easily reduce oxide directly into metal, and many researchers have already reported the production of metals by electroless reduction with a calcium reductant [16], [17], [18], [23], [24]. Calcium chloride molten salt acts as a solvent and has a high solubility for metallic calcium and the calcium oxide formed by the electroless reduction. Direct production of the Ti–Zr alloy by calcium electroless reduction simplifies the fabrication process and reduce the price of the novel Ti–Zr capacitor because chlorination, magnesium reduction, crushing, and arc melting are no longer needed in the production process. In addition, the reduction of micro-particles of TiO2 and ZrO2 (starting materials) is expected to form Ti–Zr micro-particles or micro-porous structures with high surface area. The aim of this investigation is to study the viability of using electroless reduction with calcium to produce well-defined Ti–Zr alloy microstructures directly from mixed oxides.
Section snippets
Experimental
Commercially available titanium and zirconium oxide powders (99.9 wt%, Furuuchi Chemical Co., Japan) were used as the starting materials. The oxides, which had a composition of TiO2–70 at%ZrO2, were mixed by wet ball-milling with zirconia balls in ethanol at 180 rpm for 2 h [10], [12]. After milling, the starting materials and anhydrous calcium chloride (99.0 wt%, Kojundo Chemical Laboratory Co., Japan), which was used as a molten salt, were dried in a vacuum oven at 473 K for several hours.
The
Reduction of pure titanium and zirconium oxides
The electroless reduction of zirconium oxide powder with calcium as the reductant in CaCl2 molten salt at 1173 K was performed to elucidate the details of the electroless reduction behavior. Fig. 2 shows X-ray diffraction patterns obtained from electrochemically reduced zirconium oxides after electroless reduction for 1 h. Here, the amount of calcium reductant was adjusted to (a) e = 100%, (b) 150%, or (c) 200%. A composite oxide of calcium/zirconium (calcium zirconate, CaZrO3) and a lower
Conclusions
In this investigation, titanium dioxide and zirconium oxide were reduced by a calcium reductant in calcium chloride molten salt at 1173 K. The effects of the electroless reduction time and the amount of calcium reductant were investigated to optimize the reduction conditions. A pure metallic zirconium was obtained by electroless reduction for 1 h with 50–100% excess of calcium reductant over the theoretical amount; this reduction occurred via lower and composite oxides. Electroless reduction of
Acknowledgements
The authors would like to thank Mr. Nobuyuki Miyazaki (Hokkaido University) for his assistance with the EPMA analysis. This work was financially supported by the Japan Society for the Promotion of Science (JSPS) “KAKENHI”.
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