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Capacitance enhancement of anodic oxide films on zirconium by adding silicon is reported here with correlation to the phase transformation of the oxide. The anodic oxide film formed on zirconium consists mainly of monoclinic ZrO 2 , which changes to tetragonal ZrO 2 phase on the Zr-5.5 atom % Si. Further increase in the silicon contents to 10 and 16 atom % results in the formation of amorphous oxide up to 30 V, above which two-layered films, comprising an outer crystalline tetragonal-phase oxide layer and an inner amorphous layer, are developed. The relative thickness of the outer crystalline layer to the total film thickness increases with formation voltage. The highest capacitance of the anodic oxide films is obtained on the Zr-10 atom % Si. The changes in capacitance, permittivity and formation ratio of anodic oxide films with alloy composition are discussed with phase transformation and growth process of anodic oxides. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3503592͔ All rights reserved.Manuscript submitted August 9, 2010; revised manuscript received September 23, 2010. Published October 22, 2010 ZrO 2 is an important inorganic material for high-temperature applications, due to many useful properties such as high strength and stability at high temperatures, oxide ion conductivity at elevated temperatures and radiation-resistant properties. These features make ZrO 2 attractive for oxygen sensors, solid oxide fuel cells, and nuclear materials. Moreover, in recent years, zirconia-based materials, including ZrO 2 -SiO 2 , have been proposed as a promising high-gate dielectric material for metal-oxide-semiconductor ͑MOS͒ transistors. 8,9 Anodizing is the most simple and easiest method to form dielectric oxides, and this has been extensively used to form dielectric oxide films on aluminum and tantalum for electrolytic capacitor applications. Tantalum electrolytic capacitors are widely used in electronics industry, but due to limitation of natural resources of tantalum and for high demand of increasing capacitance, alternative abundant materials that form oxides with higher permittivity, such as niobium and titanium, have been studied. [10][11][12][13][14][15][16] Zirconium forms usually crystalline anodic oxide films, in contrast to the formation of amorphous anodic oxides on a range of valve metals, including aluminum, bismuth, niobium, tantalum and tungsten. 17 The crystalline structure of anodic zirconium oxide films is influenced by surface treatments prior to anodizing and electrolyte for anodizing. When magnetron-sputtered zirconium is anodized in ammonium pentaborate electrolyte without pretreatment of the deposited surface, crystalline oxide films, consisting mainly of monoclinic ZrO 2 , are developed. 9 The monoclinic phase of ZrO 2 is thermodynamically the most stable at ambient temperature, but amorphous and high-temperature stable phases of cubic or tetragonal ZrO 2 are also often found in the anodic oxide films formed on chemically polished zirconium, particularly at low formation voltage...

Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm −3 ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.

Anodic oxide films with nanocrystalline tetragonal ZrO 2 precipitated in an amorphous oxide matrix were formed on Zr-Si and Zr-Al alloys and had significantly enhanced capacitance in comparison with those formed on zirconium metal. The capacitance enhancement was associated with the formation of a high-temperature stable tetragonal ZrO 2 phase with high relative permittivity as well as increased ionic resistivity, which reduces the thickness of anodic oxide films at a certain formation voltage. However, 2 there is a general empirical trend that single-phase materials with higher permittivity have lower ionic resistivity. This study presents a novel material design based on a nanocrystalline-amorphous composite anodic oxide film for capacitor applications.

Zr-Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm -3 ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO 2 with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO 2 phase, are formed on the Zr-Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films becomes thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr-11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.