This chapter presents a study in which an amorphous-to crystalline transition of anodic niobia is examined by using magnetron-sputtered niobium and its alloy substrates, with particular attentions ...paid to the preexisting surface oxide layer as a nucleation site of crystalline oxide and incorporation of foreign species hindering its nucleation. Niobium and Nb-N films were prepared by magnetron sputtering on to glass, silicon and aluminum substrates. Aluminum substrates were electropolished and subsequently anodized in 0.1 mol dm-3 ammonium pentaborate electrolyte to 200 V to provide flat surfaces. The niobium and niobium alloy films prepared were anodized at a constant current density of generally 50 A m-2 to selected voltages with and without current decay in stirred 0.1 mol dm-3 ammonium pentaborate and 0.1 mol dm-3 phosphoric acid electrolytes at 60º C. Platinum sheet was used as a counter electrode. Amorphous-to-crystalline transition of anodic niobia formed on the sputter-deposited niobium films, which are free from preferable sites, such as inclusions and surface roughness, of the transition, initiated at approximately outer 30% of the film thickness where air-formed oxide is located. The growth of the crystalline oxide to the metal/film interface is rapid and the phosphorus species incorporated from the phosphoric acid electrolyte suppress effectively the amorphous-to-crystalline transition. Similarly, the foreign species incorporated from the substrate, such as nitrogen, silicon, and tungsten species suppress the crystalline oxide formation.
Binary Co–Mo and Co–Al, and ternary Co–Mo–Al alloy electrodes for hydrogen evolution were prepared by using sputter deposition on a nickel substrate with good adhesion. The hydrogen evolution ...reaction (HER) of these alloy electrodes in the deaerated 1 kmol·m−3 NaOH at 303 K is enhanced significantly compared with pure cobalt electrode. The effective surface area of these alloy electrodes estimated from ac impedance technique is largely increased by leaching of aluminum into alkaline solution. The electrocatalytic activity sequence is Co–Mo–Al>Co–Mo>Co–Al. The maximum activity is obtained for Co–10Mo–58Al alloy with a hydrogen overpotential as low as 110 mV at 103 A·m−2. The enhancement of HER is attributed to both the synergistic effect of alloy constituents and the large effective surface area.
The addition of 30 at% or more tantalum to Ni–5P alloy greatly enhances the corrosion resistance in 12 M HCl at 30°C. The corrosion rate of amorphous Ni–40Ta–5P alloy is about three orders of ...magnitude lower than that of Ni–5P alloy. The addition of sufficient amounts of tantalum to Ni–5P alloys leads to an ennoblement of the open circuit potential and to spontaneous passivation. The addition of a large amount of tantalum to the Ni–Ta alloy contained a small amount of phosphorus, such as 5 at%, enhances the formation of the tantalum enriched passive film which prevents oxidation of phosphorus.
Sputter deposition technique has been applied to the preparation of amorphous alloy precursors of catalysts on a fine oxide powder, in order to overcome the low surface area of the catalysts prepared ...from melt-spun amorphous alloy precursors. Amorphous Ni–Ta–Pd alloys have been sputter-deposited onto the γ-alumina powder with a high surface area, and then pre-oxidized at 1023 K in a 0.5% NO atmosphere. During the preoxidation, the amorphous alloys have been converted to palladium catalysts supported on the NiTa2O6 double oxide. The BET areas of the catalysts thus prepared (sputter-deposited catalysts) are approximately 50 times higher than those prepared from a melt-spun Ni–40Ta–1Pd alloy precursor (melt-spun catalyst). The catalytic activity of the sputter-deposited catalysts for NO decomposition becomes about twice as high as that of the melt-spun catalyst. Furthermore, the selectivity of nitrogen formation is also improved compared with the melt-spun catalyst. Accordingly, the application of sputtering technique is quite suited for preparing amorphous alloy catalyst precursors with a high surface area.
Amorphous Ni–40Ta–Pd alloys are known as the precursor of the catalysts for the decomposition of nitrogen monoxide. Since the catalytic activity appears after oxidation, the relationship between the ...activity and structure was investigated after oxidation in air at 750°C for different periods of time. The structure developed by oxidation was characterized with special interest in the dispersion state of palladium phase in the matrix oxides by means of SEM/EDX, EPMA and TEM/EDX. At the early stage of the oxidation, NiO, Ta2O5 and PdO are first formed. The oxides consist of two layers; comprising an outer NiO layer and an inner layer containing Ni, Ta and Pd ions. The outer NiO is developed by the outward diffusion of nickel ion during oxidation. Prolonged oxidation leads to the development of porosity of the specimen and to the formation of double NiTa2O6 oxide by the reaction of NiO with Ta2O5 in the inner layer. The inner layer formed after the complete oxidation for the amorphous Ni–40Ta–Pd alloy has sponge-like morphology, and consists of PdO, NiO and NiTa2O6. The addition of palladium prevents the diffusion of nickel to the surface, leading to the decrease in the thickness of the surface NiO layer and to the decrease in the grain size of matrix oxides. No change in the supporting oxides was observed but PdO is decomposed to Pd after the decomposition of NO gas. Palladium particles of 50–100 nm in diameter were dispersed homogeneously in the matrix oxide. The high catalytic activity for the decomposition of NO gas is attributable to the palladium dispersed on the matrix oxides with fine grain size and porous structure developed by the oxidation of amorphous Ni–Ta–Pd alloys.
The effect of phosphorus content on the structure and corrosion resistance of electrodeposited Ni-P alloys was investigated. The phosphorus content of the plated film decreased with increasing ...current density and decreasing H3PO3 content in plating electrolyte. X-ray diffraction revealed that the deposits containing 16.7-25.6at% P are amorphous, while 8.7 at% P alloy consists of the mixture of amorphous matrix with fcc crystals. The amorphous Ni-P alloy deposits (≥16.7at% P) possess excellent corrosion resistance compared with a Ni-8.7 P deposit containing crystalline fcc phase and a pure Ni deposit. The optical micrographs and EPMA analysis of the cross section of amorphous Ni-P deposits showed that the deposits were composed of finely spaced lamellar structure consisting of alternately low and high phosphorus content layers.
AlxSi1-xOn films exhibit a drastic change of proton conductivity across the film by reducing its thickness to less than 100 nm. The temperature- and humidity-dependence of conductivity of the sub-100 ...nm films is quite different from those of the thicker films. Furthermore, in the former thickness range, the value of conductivity markedly increases with reducing the film thickness, and its thickness dependence follows a power law with a fixed index of -2.1. This size-scaling effect can be explained by the percolation conductivity model that the probability for percolating of the conductive moiety in AlxSi1-xOn films increases with decreasing the thickness.
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