Ammonium Pentaborate Research Articles

Influence of silicon on the growth of barrier-type anodic films on titanium

Abstract Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti–Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ∼60 V on the Ti–6 at.% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at.% silicon. The crystalline oxide, nucleated at a depth of ∼40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.

High Voltage Pulse Anodization of a NiTi Shape Memory Alloy

In an attempt to improve the corrosion resistance of a NiTi shape memory alloy, thick oxide films were fabricated on the surface by an anodic oxidation process. High-voltage square shape pulse anodization was carried out in various electrolytes over the whole range of pH values from sulfuric acid, acetate buffer, ammonium pentaborate, sodium diphospho heptoxide, sodium aluminate, and potassium hydroxide. Microscopic observation showed that the films are porous or tile structured in nature and the structural analyses by X-ray diffraction and Raman spectroscopy demonstrated their amorphous nature. From the chemical analyses, the oxide films were determined to consist mainly of titanium as the metal component, whereas the nickel content of the oxide films depends on the pH value of the electrolyte as well as the amount of anions incorporated from the electrolyte. The application of an asymmetrical rectangle waveform was effective in increasing the density of the films. An electrochemical technique was used for screening the corrosion resistance of the anodized films and demonstrating the improvement in the resistance against localized corrosion of NiTi.

Fast migration of fluoride ions in growing anodic titanium oxide

The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12 at% silicon alloy, where film growth proceeds at nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50 V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55 V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O 2− ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.

Field crystallization of anodic niobia

Influences of electrolyte, pre-thermal treatment and substrate composition have been examined to elucidate the mechanism of field crystallization of anodic niobia formed on magnetron-sputtered niobium. The field crystallization occurs during anodizing at 100 V in 0.1 mol dm −3 ammonium pentaborate electrolyte at 333 K, with the crystalline oxide growing more rapidly than the amorphous oxide, resulting in petal-like defects. The nucleation of crystalline oxide is accelerated by pre-thermal treatment of the niobium at 523 K in air, while vacuum treatment hinders nucleation. Notably field-crystallization is also absent in 0.1 mol dm −3 phosphoric acid electrolyte or when anodizing Nb–10at.%N and Nb–29at.%W alloys in the ammonium pentaborate electrolyte. The behaviour is explained by the role of the air-formed oxide in providing nucleation sites for field crystallization at about 25% of the thickness of the subsequently formed anodic film, the location being due to the growth mechanism of the anodic oxide and the nature of crystal nuclei. Incorporation of tungsten, nitrogen and phosphorus species to this depth suppresses the field crystallization. However, boron species occupy a relatively shallow layer and are unable to affect the nucleation sites.

Field Crystallization of Anodic Niobia on Nb-O Substrates

Solid-solution Nb-O films, containing up to 50 at% oxygen, prepared by magnetron sputtering were used to investigate the influence of the oxygen on field crystallization during anodizing at 100 V in 0.1 mol dm-3 ammonium pentaborate electrolyte at 333 K. The findings reveal that field crystallization is hindered dramatically by addition of 20 at% oxygen to the substrate, while no crystallization occurs for an Nb-50 at% O substrate. Prior thermal treatment accelerates field crystallization of niobium, but not of the Nb-50 at% O substrate. The thermal treatment is considered to promote generation of pre-cursor sites for crystal nucleation. However, sufficient oxygen in the substrate may restrict pre- cursor development and/or reduce the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth.

Suppression of Field Crystallization of Anodic Niobia by Oxygen

Solid-solution films containing up to 50 atom % oxygen, prepared by magnetron sputtering, were used to investigate the influence of the oxygen on field crystallization during anodizing at in ammonium pentaborate electrolyte at . The findings reveal that field crystallization is hindered dramatically by addition of 20 atom % oxygen to the substrate, while no crystallization occurs for a atom % O substrate. Prior thermal treatment accelerates field crystallization of niobium, but not the atom % O substrate. The thermal treatment is considered to promote generation of precursor sites for crystal nucleation. However, sufficient oxygen in the substrate may restrict precursor development and/or reduce the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth.

Anodic oxidation and dielectric behaviour of aluminium–niobium alloys

The anodizing behaviour of sputtering-deposited Al–Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb 2O 5 and Al 2O 3 in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb 5+, Al 3+ and O 2− species, with cation transport numbers, in the range 0.31–0.35, and formation ratios, in the range 1.35–1.64 nm V −1, intermediate between those of anodic alumina and anodic niobia. Al 3+ ions migrate slightly faster than Nb 5+ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb 2O 5 into anodic alumina increases the dielectric constant from about 9 to the range 11–22 for the investigated alloys.

X-Ray Absorption Spectroscopy Study of the Incorporated Copper Species in Anodic Alumina Films Formed on an Al-2 wt % Cu Alloy

Barrier anodic films, formed on an Al-2 wt % Cu binary alloy at a current density of in ammonium pentaborate electrolyte, have been examined by analytical transmission electron microscopy, which revealed the presence of copper species across the amorphous film section, indicating incorporation and outward mobility under the electric field. From the position of the absorption edge, X-ray absorption spectroscopy revealed conclusively the presence of Cu(II) species within the alumina film. Analysis of the extended X-ray absorption fine structure showed further that each copper atom in the film is surrounded by oxygen atoms at a mean distance of , but with little long-range order.

Anodic behaviour of a model second phase: Al–20at.%Mg–20at.%Cu

The anodic behaviour of sputtering-deposited Al–20at.%Mg–20at.%Cu alloy is investigated during anodizing and potentiodynamic polarization treatments using transmission electron microscopy, X-ray photoelectron spectroscopy and medium energy ion scattering. The composition of the alloy is close to that of the S-phase in 2024 aluminium alloy. The anodizing behaviour in both 0.1 M ammonium pentaborate and 0.1 M sodium hydroxide electrolytes follows the behaviour of more dilute, solid-solution, aluminium alloys, with enrichment of copper developing in the alloy during the growth of an alumina-based initial oxide containing incorporated magnesium species. Oxygen gas is generated following sufficient enrichment of copper for its oxidation to proceed and hence, for copper species to enter the oxide film. The generation of oxygen gas causes extensive damage to the film, which limits the voltage to relatively low values. Potentiodynamic polarization in 0.1 M sodium hydroxide electrolyte revealed mainly passive behaviour following an initial period of corrosion during which the passive film is developed. In this initial period, copper enriches in the alloy, beneath an oxide film containing aluminium and magnesium species. The magnesium species migrate faster through the film than the aluminium species and form a surface layer of MgO/Mg(OH) 2, which protects against losses of aluminium species to solution and permits the establishment of the passive film. The steady open-circuit potential of the passivated alloy in the hydroxide solution is about −550 mV (SCE), compared with about −1940 mV (SCE) for aluminium.

Anodic oxidation of InAlAs

The anodic oxidation of InAlAs is investigated by transmission electron microscopy, Rutherford backscattering spectroscopy and medium energy ion scattering in order to elucidate the mechanism of oxide growth. For this purpose, anodizing was carried out at 5 mA cm −2 in 0.1 M sodium tungstate and 0.1 M ammonium pentaborate electrolytes at 293 K, which results in relatively efficient film growth over an initial voltage increment of about 70 V. In this period, an amorphous oxide develops with a formation ratio of 2.0 ± 0.2 nm V −1. The film consists mainly of an outer layer of In 2O 3 and an inner layer of units of In 2O 3, Al 2O 3 and As 2O 3, the former representing about 14% of the film. There is suggestion of a fine, intermediate layer containing units of In 2O 3 and Al 2O 3 only. The layering correlates with the bond energies of the cation and oxygen species in the oxide and hence their relative migration rates. Further, for films formed in tungstate electrolyte, tungsten species are incorporated into the outer 40% of the film. Bubbles of oxygen gas are present in the film, probably developed within the In 2O 3 layer. At higher voltages, the film undergoes breakdown, with resulting major changes in the film morphology.

Incorporation of oxide nanoparticles into barrier-type alumina film via anodic oxidation combined with electrophoretic deposition

The incorporation of metal oxide nanoparticles into a barrier-type anodic alumina film on an Al substrate was investigated using a one-step anodic oxidation combined with electrophoretic deposition. Basically, the electrolysis was carried out in an oxide nanoparticle dispersed ammonium pentaborate solution using an Al substrate as the anode. As a result, the TiO2 and SiO2 nanoparticles with a negative surface charge could be introduced into the alumina film grown on the Al anode, and the film thickness increased by the dispersion of the nanoparticles inside the alumina layer. The measurement of the capacitance and corrosion potential demonstrated that the present method was valid for enhancing the characteristics of the barrier-type alumina film.

Inter-relationship between structure and dielectric properties of crystalline anodic zirconia

Crystalline anodic oxide films have been formed on sputter-deposited zirconium and Zr–Ti alloys in 0.1 mol dm −3 ammonium pentaborate electrolyte, with their structure, composition, dielectric properties and growth behaviour examined using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and electrochemical impedance spectroscopy. The films formed on zirconium consist of a high-temperature stable ZrO 2 phase (cubic or tetragonal) as well as monoclinic ZrO 2, with the latter predominant with increasing formation voltage. The incorporation of titanium species into the films stabilizes the high-temperature phase up to high voltages. Associated with the change in the crystalline structure with the incorporation of titanium species, the permittivities of the oxides increase, in agreement with the higher permittivity of bulk cubic and tetragonal ZrO 2 containing stabilizing cations compared with that of bulk monoclinic ZrO 2. The transport number of cations during crystalline oxide growth is about 0.05, irrespective of titanium incorporation.

Behaviour of zinc in electropolished and etched Al–Zn alloys and effect on corrosion potential

The enrichments of zinc developed in binary, solid solution Al–0.3at.%Zn, Al–0.4at.%Zn and Al–1at.%Zn alloys by electropolishing and alkaline etching are examined using Rutherford backscattering spectroscopy and medium energy ion scattering with additional interest in how such enrichments affect the corrosion potentials of the alloys. During alkaline etching in 0.1 M sodium hydroxide solution, significant enrichments of zinc arise in the alloy, similar to that achieved by an anodizing treatment. However, enrichment is unusually low following electropolishing in perchloric acid solution. Contrary to the effect of enriched copper in Al–Cu alloys, zinc enrichment has minor influence on the corrosion potentials of etched alloys in ammonium pentaborate solution, which remain roughly within ±100 mV of those of non-enriched alloys.

Oxidation-resistant multilayer coatings using an anodic alumina layer as a diffusion barrier on γ-TiAl substrates

Diffusion barrier layers of anodic alumina, introduced between an oxidation-resistant Al–Nb–Cr alloy and γ-TiAl substrate, have been evaluated using scanning electron microscopy and electron probe microanalysis. Diffusion of titanium to the Al–Nb–Cr coating is obvious when the Al–Nb–Cr alloy coated directly on the TiAl substrate is oxidized at 900 °C for 168 h. The anodic alumina layer has been introduced by sputter-depositing aluminum and subsequent anodizing of the aluminum layer at a constant current density in 0.01 mol dm −3 ammonium pentaborate electrolyte at 298 K. Then, the Al–Nb–Cr alloy is further sputter-deposited on the anodized specimens. It is clearly demonstrated that the thin anodic alumina layer, less than 500 nm thick, suppresses effectively the inter-diffusion between the oxidation-resistant alloy coating and the TiAl substrate, particularly when a thin aluminum layer is remained beneath the anodic alumina. Although microcracks are generated and the substrate is oxidized through the cracked regions for the coatings that contain the metallic aluminum layer, the oxidation of the substrate through the cracks is prevented by pre-oxidation treatment at 800 °C.

Effect of Copper Enrichment on the Electrochemical Potential of Binary Al-Cu Alloys

Using Al-Cu alloys, containing between 0.1 and 26 atom % Cu, deposited by magnetron sputtering and etching in sodium hydroxide solution, enrichments of copper have been developed selectively in the alloys. Rutherford backscattering spectroscopy and medium energy ion scattering quantified the enrichments and their locations just beneath the alumina-based oxides remaining from the etching. In some cases, the enrichment was sufficient for oxidation of copper to take place; in other cases, it was not, so that only aluminum was oxidized, with copper being confined to the alloy, enriching in an alloy layer about 2 nm thick. The potentials of the etched alloys in 0.1 M ammonium pentaborate solution at 293 K increased as the copper content of the enriched alloy layer increased. The enrichment of copper in the alloy beneath an alumina-based film free of copper species, i.e., without requirement for oxidation of copper and incorporation of copper species into the oxide, was sufficient to generate increases in potential of similar magnitude to those found for specimens in which oxidation of copper had taken place. The potential increased by ∼410 mV with an increase in the level of enrichment to Cu atoms © 2003 The Electrochemical Society. All rights reserved.

Effect of impurities on crystal growth rate of ammonium pentaborate

The effect of sodium chloride, borax and boric acid of different concentrations on the growth rate of ammonium pentaborate octahydrate crystals (APBO) was measured and was found to depend on supersaturation in a fluidized bed crystallizer. The presence of impurities in APBO solution increases the growth rate compared with growth from pure solution. It was found that the presence of sodium chloride, borax and boric acid decreases the reaction rate constant k r , while it increases the mass-transfer coefficient, K, of APBO crystals. In pure aqueous solution, the crystal growth rate of APBO is mainly controlled by diffusion. However, both diffusion and integration steps affect the growth rate of APBO crystals in the presence of sodium chloride, borax and boric acid. The mass-transfer coefficient, K, reaction rate constant, k r and reaction order, r were calculated from general mass-transfer equation by using genetic algorithm method making no assumption.

Influences of structure and composition on growth of anodic oxide films on TiZr alloys

Sputter-deposited TiZr alloys of various compositions have been anodized at a constant current density in 0.1 mol dm −3 ammonium pentaborate electrolyte at 298 K. The resultant anodic films have been characterized using transmission electron microscopy (TEM), glow discharge optical emission spectroscopy (GD-OES), Rutherford backscattering spectroscopy and capacitance measurements to elucidate the influence of alloy composition on their structure, growth behavior and dielectric properties. The Ti–10.5 at.% Zr alloy reveals oxygen formation within the film above ∼40 V, associated with crystallization of the anodic oxide to anatase. In contrast, amorphous anodic films, with a thin outer layer consisting of zirconium-free TiO 2, grow up to about 250 V on Ti–23.0 at.% Zr and Ti–42.0 at.% Zr alloys at high current efficiency. Further increase in zirconium content to 62.5 at.% results in the formation of a film containing nanocrystals of monoclinic ZrO 2 phase in an amorphous matrix. The crystalline oxide is formed predominantly in the inner region of the film that is developed by anion migration inward. An anodic film composed of monoclinic ZrO 2 throughout the film thickness is formed on the zirconium. The influences of structure and composition of the anodic films on their growth behavior and dielectric properties are discussed and compared with findings for AlTa alloys, which form amorphous oxides over the entire composition range.

Oxygen generation in anodized Ta–Cu alloys

Comparison is made of the influences of copper in the development of amorphous anodic films on Ta-Cu alloys, containing up to 57 at.%Cu, in ammonium pentaborate electrolyte at 293 K. The various films are based on tantala, with low concentrations of copper, relative to those of the alloys, since copper species migrate through the films more rapidly than Ta5+ ions and are lost to the electrolyte on reaching the film surface. Of particular interest, film growth on Ta-1.5at.% Cu alloy at 1, 0.1 and 0.01 mA cm−2 proceeds with generation of oxygen, which is contained in nanoscale bubbles within the oxide. Bubbles are more numerous as the current density decreases. In the case of Ta-12at.% Cu and Ta-57at.%Cu alloys, the films are flawed extensively by more numerous oxygen bubbles, which are present mainly in the outer parts of the anodic films; oxygen production and film damage due to the bursting of bubbles limit the anodizing voltage to relatively low values. It is suggested that copper species modify the electron levels, and possibly structure, of the tantala-based films, thereby facilitating oxygen generation within the bulk film by oxidation of O2- ions of the oxide.

Influence of silicon species on the electric properties of anodic niobia

The influence of incorporation of silicon species on the electric properties of anodic niobia, formed in 0.1 mol dm −3 ammonium pentaborate electrolyte, has been examined using sputter-deposited Nb–Si alloys containing 5 and 17 at.% silicon. The potential dependence of the capacitance of anodic niobia, originating from its n-type semiconducting properties, becomes less significant by incorporation of silicon species. In addition, the leakage current decreases with increasing silicon content in the alloy. The thermal stability of the anodic niobia is also enhanced by silicon species; the capacitance and leakage current, which increase significantly for niobium, are little influenced by annealing up to 523 K. The silicon species are incorporated in the inner 72% of the film thickness, as a consequence of immobility of the species in growing anodic niobia. The immobility of silicon species is associated with a strong Si 4+O bond, which may also contribute to the reduction of leakage current.

Anodic oxidation of Ta/Fe alloys

The behaviour of iron during anodizing of sputter-deposited Ta/Fe alloys in ammonium pentaborate electrolyte has been examined by transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. Anodic films on Ta/1.5 at.% Fe, Ta/3 at.% Fe and Ta/7 at.% Fe alloys are amorphous and featureless and develop at high current efficiency with respective formation ratios of 1.67, 1.60 and 1.55 nm V −1. Anodic oxidation of the alloys proceeds without significant enrichment of iron in the alloy in the vicinity of the alloy/film interface and without oxygen generation during film growth, unlike the behaviour of Al/Fe alloys containing similar concentrations of iron. The higher migration rate of iron species relative to that of tantalum ions leads to the formation of an outer iron-rich layer at the film surface.