P. Skeldon

Crystallization of anodic titania on titanium and its alloys

Crystallization of amorphous anodic films grown at constant current density on sputteringdeposited titanium, and Ti–Si and Ti–Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35–38% of the film thickness. Oxide zones, of size � 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at � 40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy. 2003 Elsevier Science Ltd. All rights reserved.

Effects of Alloying Elements in Anodizing of Aluminium

SummaryThe anodic oxidation of aluminium alloys is reviewed and discussed with reference to recent results of the authors on the anodizing of model binary aluminium alloys. Attention is given primarily to the oxidation of alloying elements at the alloy/film interface during the formation of barrier-type anodic films. However, the findings are also considered to be applicable to the formation of porous anodic films. The enrichments of alloy layers of about 1–5 nm thickness is revealed to be a relatively common occurrence following anodizing of aluminium alloys. The enrichments, present in the alloy just beneath the anodic film, are a direct consequence of the formation of the anodic film on the alloy. The influence of alloy composition on the enrichments of the alloying element within the alloy is highlighted, and correlated with the Gibbs free energy per equivalent for formation of the alloying element oxide. The development of enriched alloy layers is not confined to anodizing and is found following othe...

A Model for the Incorporation of Electrolyte Species into Anodic Alumina

A semiquantitative model is presented for the incorporation of species derived from electrolyte anions into amorphous barrier-type films formed on aluminum in aqueous electrolytes at ambient temperature. The model relates the film compositions to the concentrations of adsorbed electrolyte anions which form the double-layer charge at the film surface. During film growth, the adsorbed anions are incorporated into the film, either directly or following transformation to a new form, as so-called electrolyte species. The incorporated electrolyte species present in the film can have positive, negative, or effectively no charge, and hence electrolyte species can be immobile, migrate inward, or migrate outward within the film under the electric field. The concentration of electrolyte species in the film depends upon the type and concentration of the adsorbed anions, the direction of migration of the electrolyte species in the film, and the faradaic efficiency of film growth. The validity of the model has been assessed by comparing the predicted and experimental compositions of films formed in a wide range of electrolytes. For reasonable selection of the type of adsorbed anion, the measured concentrations of electrolyte species in films, determined by Rutherford backscattering spectroscopy and nuclear reaction analysis, are typically about 70% of the predicted values, which is a satisfactory level of agreement given uncertainties in precise values of model parameters

Anodic oxidation of aluminium

Abstract Key aspects of anodic film growth on aluminium at ambient temperatures in aqueous electrolytes, encompassing barrier- and porous-type films, are described. By direct observation of film sections, incorporating inert marker layers and tracers, in the transmission electron microscope and appropriate analysis, the locations of solid-film growth and electrolyte anion effects can be determined precisely. Thus, during barrier-film formation, at high current efficiency, Al3+ ion egress and O2-/OH− ingress proceed across the pre-existing air-formed film to develop solid material at the film/electrolyte and metal/film interfaces respectively. With decrease in current efficiency, the former contribution declines through a mechanism of direct ejection of Al3+ ions at the film/electrolyte interface. At a critical current density, all outwardly mobile Al3+ ions are lost to the electrolyte. Concerning anions of the forming electrolyte, such species (or more strictly their transformation products) may be mobile...

XPS studies of MoS2 formation from ammonium tetrathiomolybdate solutions

Abstract Preparation of MoS 2 by acidification of ammonium tetrathiomolybdate solution has been studied by X-ray photoelectron spectroscopy (XPS). The precipitates formed from the solution are mainly composed of MoS 3 , the Mo 3d chemical shift of which is approximately 4.2eV. Heat treatments of the above product at 450 and 850°C, in a vacuum of 1.33 × 10 −4 Pa, lead largely to formation of MoS 2 , with typical chemical shifts of abouth 1.2eV. The MoS 2 formed consists of nano-sized, crystalline particles of hexagonal (2H-type) structure. The validity of MoS 2 formation by this route is confirmed by comparative study, namely the decomposition of ATT solids under the same respective heat treatment conditions. Apart from these main aspects, origins of minor XPS peaks are also considered.

Initial stages of plasma electrolytic oxidation of titanium

The initial stages of oxide growth on titanium are examined in a recently developed commercial alkaline pyrophosphate/aluminate electrolyte of interest for plasma electrolytic oxidation of light metal alloys. Constant current anodizing was employed, with resultant films examined by scanning and transmission electron microscopies and Rutherford backscattering spectroscopy. The initial film is relatively uniform and composed of TiO2, with low concentrations of aluminium and phosphorus species incorporated from the electrolyte. With increase in voltage the film breaks down locally, and regions of original and modified film develop simultaneously, with the latter occupying more of the surface as the voltage rises. Porous regions due to dielectric breakdown also become increasingly evident. At � 240 V, sparking commences, and the surface reveals extensive, relatively uniform porosity, with the coating now containing much enhanced concentrations of aluminium and phosphorus species compared with the coating at lower voltages. The films develop at low efficiency due to generation of oxygen. The oxygen is produced within the original film material and at sites of dielectric breakdown. The former type of film develops a two-layered morphology, with an outer layer of amorphous TiO2 and an inner layer with numerous fine and course cavities. The cavities are due to the generation of oxygen that may be associated with the formation of anatase in the inner layer. 2003 Elsevier Ltd. All rights reserved.

Ionic transport in amorphous anodic titania stabilised by incorporation of silicon species

Incorporation of silicon species from an alloy substrate into anodic titania is shown to stabilise the structure of the film, facilitating investigation of the ionic transport processes in amorphous titania grown at high efficiency. Thus, an amorphous anodic film developed on a sputtering-deposited Ti–6 at.%Si alloy formed to 100 V in phosphoric acid electrolyte in contrast to a partially crystalline film developed on relatively pure titanium at <20 V. Silicon species, which are immobile and act as marker species in the growing film, are present in the inner 58% of the film thickness. Evidently, the film material forms simultaneously at the film/electrolyte and alloy/film interfaces by co-operative transport of cations and anions, as is usual in amorphous anodic oxides. The phosphate anions incorporated from the electrolyte migrate inward at 0.34 times the rate of O2− ions and hence are present in the outer 62% of the film thickness.

Copper enrichment in Al-Cu alloys due to electropolishing and anodic oxidation

The average thickness and composition of the copper-enriched alloy layer that is present at the alloy/oxide interface during anodic oxidation of an eleetropolished Al-0.9 al.% Cu alloy at a constant current density of 50 A m−2 have been determined by Rutherford backscattering spectroscopy and transmission electron microscopy. The copper-enriched layer, of about 2 nm thickness, has an average composition of Al-40 at.% Cu and contains about 5.4×1015 Cu atoms cm−2. The average composition and thickness of the layer do not change significantly during anodizing from 10 to 200 V- The essentially steady-state, copper-enriched layer is established mainly by the prior electropolishing of the alloy. As a consequence of the pre-enrichment of copper, both aluminium and copper atoms are oxidized immediately at the alloy/film interface on subsequent anodizing. Owing to the importance of copper enrichment in Al-Cu alloys to the oxidation of copper atoms, alloy pre-trealment has an important role in determining the initial oxidation behaviour.

Influence of Water Content on the Growth of Anodic TiO2 Nanotubes in Fluoride-Containing Ethylene Glycol Electrolytes

The effect of water additions, from 0 to 50 vol %, to an ammonium fluoride/ethylene glycol electrolyte on the composition and morphology of titania-based nanotubes was investigated by scanning and transmission electron microscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. Further to the presence of units of TiO 2 , the films contained fluorine, carbon, nitrogen, and probably hydrogen species derived from the electrolyte. The compositions of the films appeared to be relatively independent of the water content of the electrolyte. Following a small increase in efficiency of film growth for additions between 0 and 1 vol % water, the efficiency decreased from about 25 to about 10% between water contents of 1 and 25 vol %, respectively, possibly due to a combination of loss of titanium species to the electrolyte and evolution of oxygen. The densities of the nanotube layers were estimated to be in the range of 1.25―1.75 g cm ―3 , with an average value of 1.47 g cm ―3 .

Nanoscale enrichments of substrate elements in the growth of thin oxide films

The enrichment of alloying elements as a consequence of formation of amorphous anodic films on relatively dilute, metastable binary aluminium alloys, prepared by magnetron sputtering, has been determined quantitatively employing Rutherford backscattering spectroscopy for a range of alloying elements. The measurements relate to the enrichments that must be developed in order for oxidation of the alloying element at the alloy/film interface to proceed. The enrichments, occurring in a thin layer of alloy of 1–5 nm thickness immediately beneath the anodic film, can be correlated with the Gibbs free energy per equivalent for formation of the alloying element oxides relative to that of alumina. The enrichment increases progressively, approximately linearly, for alloying elements associated with oxides of increasingly higher Gibbs free energy per equivalent, with no enrichment for alloying elements associated with oxides of lower Gibbs free energy per equivalent. A thermodynamic approach per se is insufficient to explain the enrichment phenomenon completely.