The Field-Dependence of the Dielectric Constant during the Anodic Oxidation of Tantalum, Niobium, and Tungsten

Ord, J. L.; Hopper, M. A.; Wang, W. P.

doi:10.1149/1.2404226

Cited by 42 publications

(25 citation statements)

References 14 publications

(42 reference statements)

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“…A variable density of the film material is compatible with evidence that the density of anodic films on niobium, tantalum and tungsten is linearly dependent upon the field (Ord et al 1972). The fields during growth at 0.1 and 10rnAcm-2 differ by about lo%, leading to the possibility of significantly increased density of inner layer material formed at higher current.…”

Section: 4supporting

confidence: 76%

“…Stepped-current and stepped-field experiments have a long history in the study of anodic films on relatively pure metals, particularly tantalum (Dewald 1957, Young 1961, Smith and Young 1982, niobium (Young 1956) and aluminium (Goad and Dignam 1972a,b). During the growth of the film, the current or voltage is stepped to a new value, with monitoring of the consequential response of the system.…”

mentioning

confidence: 99%

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History effects in anodic oxidation of Al-W alloys

Iglesias-Rubianes

Skeldon

Thompson

et al. 2001

Philosophical Magazine A

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Section: 4supporting

confidence: 76%

mentioning

confidence: 99%

History effects in anodic oxidation of Al-W alloys

Iglesias-Rubianes

Skeldon

Thompson

et al. 2001

Philosophical Magazine A

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“…In our original study of electrostrictive effects in valve metals (13), we used an a-c technique %o study the field dependence of the low-frequency dielectric constant. More recently (4,14,15) we have used opencircuit transient analysis for this purpose.…”

Section: Resultsmentioning

confidence: 99%

Optical Anisotropy and Electrostriction in the Anodic Oxide of Niobium

Matthews

Ord

Wang

1983

J. Electrochem. Soc.

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The anodic oxidation of niobium in sulfuric acid electrolyte is studied using a self-nulling ellipsometer to follow the optical changes that occur when the field in the oxide film is switched. The oxide is optically anisotropic with the anodizing field applied, and the degree of anisotropy, the ratio between index changes parallel and transverse to the field, is found to equal 2.2 by a procedure that requires consistency from the analysis of field-switching transients for a wide range of oxide film thicknesses. The film thickness increases and its refractive index values decrease when the field is applied. The dependences are not the parabolic dependences predicted by the theory of electrostriction, and the significance of this finding is discussed. Although the galvanostatic charge:discharge transients that are used to study the changes in the lowfrequency dielectric constant exhibit significant hysteresis, they show that the decrease in dielectric constant with increasing field observed at optical frequencies occurs also at low frequencies. The results are compared with the dependence of dielectric constant on field deduced from open-circuit transient analysis, and the significance of the discrepancy is discussed. It is concluded that the possibility that the oxide film has a significantly polar structure remains an open question.The anodic oxides of most valve metals appear to be optically anisotropic during anodization. The uniaxial negative anisotropy disappears when the field is removed from tantalum (1) and niobium (2-4), but substantial anisotropy remains at zero field for vanadium (5) and molybdenum (6). The associated changes in the low-frequency dielectric constant are in the same direction as at optical frequency, and the fractional changes are considerably larger at low frequency. The thickness of the oxide film is found to increase when the field is applied, and the change is of about the magnitude r.equired by the Clausius-Mossotti relation to account for the dielectric constant changes.The changes in film thickness and dielectric constant are thought to be due to electrostriction, the universal second-order effect which couples strain and field in nonpolar materials. In the theory of electrostriction, the dielectric constant depend~ on strain in much the same way ,as in the Clausius-Mossotti relation, and the sign of the electrostrictive coefficient determines both how the dielectric constant depends on the strain and how the strain depends on the field. When we compare the experimental results with the theory, we see that it is possible to fit the strain data by choosing a sign for the electrostrictive coefficient.that is opposite to the sign in the Clausius-Mossotti relation, but neither sign can fit the dielectric constant data. If the dielectric constant data are valid, the theory can be shown to violate energy conservation (7), and it must either be modified to make the dielectric constant an explicit function of the field, or the oxide films must be assumed to be polar. Since it is the behav...

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“…This ox value is quite larger than that measured during the growth ͑ ox = 42-44͒ under a high electric field, 2,14,38 and the difference in ox has been attributed to the presence of the large electrostrictive effect, as reported in Ref. 39. According to Ord et al, 39 the dependence of the dielectric constant from the electric field, F el , can be written as…”

Section: Samplementioning

confidence: 99%

Physicochemical Characterization of Thermally Aged Anodic Films on Magnetron-Sputtered Niobium

Mantia

Santamaria

Quarto

et al. 2010

J. Electrochem. Soc.

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The influence of thermal aging, at intermediate temperature ͑1 h at 250°C͒ and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron-sputtered niobium metal, has been studied. A detailed physicochemical characterization of the a-Nb 2 O 5 /0.5 M H 2 SO 4 electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well as by differential admittance ͑DA͒ measurements. A change in the optical bandgap ͑3.45 eV͒ of niobia film has been observed after aging ͑3.30 eV͒ at 250°C in air for 1 h. A cathodic shift ͑0.15-0.2 V͒ in the flatband potential of the junction has been observed. The frequency dependence of DA data agrees with expectations of the theory of amorphous semiconductor Schottky barrier. The fitting of both components of DA allowed to get information on the distribution of the electronic density of states as a function of energy and distance from the metal oxide interface. The DA measurements evidenced for vacuum-treated niobia film an insulating to semiconductor transition. These findings can help to explain the large changes in the measured values of capacitance, after aging, and the larger leakage current observed in niobia electrolytic capacitors.

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The Field-Dependence of the Dielectric Constant during the Anodic Oxidation of Tantalum, Niobium, and Tungsten

Cited by 42 publications

References 14 publications

History effects in anodic oxidation of Al-W alloys

History effects in anodic oxidation of Al-W alloys

Optical Anisotropy and Electrostriction in the Anodic Oxide of Niobium

Physicochemical Characterization of Thermally Aged Anodic Films on Magnetron-Sputtered Niobium

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