Amorphous anodically formed oxides

The average capacitance and specific resistivity of the barrier aluminium oxide films are determined to be 430. .. 470nF/cm and 1.3. .. 2.4 10 " Qcm, respectively. By using the anodisation factor of 1.2 nm/V for the films formed at low formation voltage, dielectric constants of 5.8. .. 6.4 are calculated from the measured capacitance values. The comparatively low dielectric constant is in agreement with the formation of an amorphous anodic aluminium oxide film as discussed above rather than a crystalline structure for which a higher dielec-... 

Before discussing derivatlzation of anodic iridium oxide films (AIROFs), we will briefly review what is known about this material. The AIROF has been extensively studied as an electro-chromic and electrocatalyst ( l-5), however its exact composition and structure are still not known. It is amorphous, low density film which can be formed on Ir by potential cycling in aqueous acids. It exists in two oxidation states a reduced, colorless form containing Ir + and an oxidized, blue-black form containing... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

A completely novel approach to technical electrolysis for anodic oxygen evolution from alkaline solution is the use of amorphous metals, i.e. chilled melts of nickel/cobalt mixtures whose crystallization is prevented by the addition of refractory metals like Ti, Zr, B, Mo, Hf, and P (46-51). For this type of material, enhanced catalytic activity in heterogeneous catalysis of gas-phase reactions has been observed (51). These amorphous metals are shown to be more corrosion resistant than the respective crystallized alloys, and the oxides being formed at their surfaces often exhibit a higher catalytic activity than those formed on ordered alloys, as shown by Kreysa (52-54). 

The investigation of anodic oxide on various metals shows that at first usually amorphous structures are formed with a dense coverage of the terraces with grains, which change to nano-crystallites with time. The extent and the rate of this change depend on the system under study. This crystallization occurs for Cr within hours [127], whereas Cu keeps the amorphous grain structure for a very few minutes only and develops a well-ordered, faceted, crystalline layer covering the whole electrode surface [128, 129], In the next section, the details of the structure of layers formed on Cu are discussed, followed by a summary of some other more reactive metals like Ni and Cr. 

Cupric oxide, CuO.—This oxide is obtained as a black, amorphous powder by igniting cupric hydroxide, carbonate, or nitrate.7 It is also formed on copper anodes in electrolytic oxidation.8 The amorphous oxide can be converted into lustrous, cubic tetrahedra by heating with potassium hydroxide,9 the crystalline variety being also produced by ignition to redness in a platinum crucible of a small amount of cuprous chloride.10... 

Galvanostatic electrosynthesis of an Y-Ba-Cu oxide precursor was accomplished [231]. The incorporation of yttrium into the oxide from the solution did not take place, so a YCU4 alloy was used as the substrate. In a saturated Ba(OH)2 solution, and also in 7.2 m KOH with Ba + additives, a loose black layer of amorphous deposit was formed on the anode surface. The layer spontaneously detached and fell to the bottom of the cell. Being annealed in air, it was transformed into a mixture of the YBCO, CuO, and Ba2Cu03 phases. 

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