E. Vallés

Influence of the bath composition and the pH on the induced cobalt-molybdenum electrodeposition

Abstract Experimental electrodeposition parameters (pH, cobalt(II), citrate and molybdate concentrations) were varied in order to analyse their influence on induced cobalt–molybdenum codeposition. Voltammetry was the main technique used. In Co–Mo electrodeposition an initial molybdenum oxide layer is formed, over which the alloy is deposited. The process depends on the nature of the species in solution. At quasi-neutral pH, CoCit − is the main Co(II) species when there is sufficient complexing agent. Thus, cobalt is deposited as a result of the reduction of CoCit − , and molybdenum is reduced from MoO 4 2− to MoO 2 . This intermediate oxide evolves to molybdenum via the formation of [MoO 2 –CoCit − ] ads . In acidic media, molybdate complexes with citrate, so molybdenum oxides are formed from H r MoO 4 Cit [5− r ] . On the other hand, cobalt is deposited mainly by CoHCit reduction. CoCit − /CoHCit and molybdenum oxides are reduced simultaneously, so cobalt nuclei are always needed to induce the codeposition.

CHARACTERISATION OF ZINC+COBALT ALLOY PHASES OBTAINED BY ELECTRODEPOSITION

Abstract The anomalous codeposition of Zn+Co alloys on vitreous carbon, copper and nickel substrates in a chloride bath was studied. The results indicate that the substrate influences both the initial electrodeposition stages and the alloy formation. For the same electrodeposition conditions, vitreous carbon and copper substrates favour the formation of deposits richer in zinc than those obtained over a nickel substrate. In the stripping analysis of deposits obtained during anomalous codeposition, up to three major oxidation peaks were observed, the two corresponding to the more negative potentials are attributable to the zinc oxidation in the alloy and the more positive one to the oxidation of the remaining porous cobalt matrix of the alloy. Under our experimental conditions, two kinds of deposits have been analysed using X-ray diffraction (XRD). XRD analysis showed that deposits with very low cobalt percentage (less than 3%) and a hexagonal platelet structure had a distorted hcp zinc η-phase. Polyhedral-grained deposits with between 4 and 10% cobalt were made up of quasi-pure zinc and a Zn+Co γ-phase of bcc structure. A correlation between the stripping curve and the type of deposit has been found.

Nickel electrodeposition on different metallic substrates

Abstract The influence of different metallic substrates on nickel electrodeposition was studied using platinum, nickel and iron electrodes. For the deposition process, the general electrochemical behaviour of these electrodes was similar to that observed on vitreous carbon. However, specific surface characteristics modified the formation of nuclei and the proportion of hydrogenated forms at short deposition times. For final deposits, differences due to grain size were determined but no preferred orientations were observed.

Electrodeposited cobaltmolybdenum magnetic materials

Abstract Cobaltmolybdenum (CoMo) induced electrodeposition has been studied from a sulphate+citrate bath on carbon electrodes, under conditions near to neutral pH. Crack-free homogeneous deposits with a low percentage of molybdenum can be easily obtained from low molybdate concentrations applying low deposition potentials or current densities. The formation of this kind of deposit is related to Q ox / Q red around 1 in the voltammetric/stripping experiments. Moreover, percentages of molybdenum up to 60% can be obtained from high molybdate concentrations but, in this case, the deposits show cracks. The formation of these cracked deposits can be predicted from the observed distortions in the j – t and E – t deposition transients. Coatings with a partially amorphous structure, or with a crystalline structure of nanometric crystal size, are obtained. The magnetisation results show that the saturation magnetisation gradually decreases when the percentage of molybdenum increases in the deposit. Simultaneously, a clear decrease of the coercitivity is observed from the lowest percentage of molybdenum with respect to the value of pure cobalt coatings.

Electrodeposited Co-Ni alloys for MEMS

The electrodeposition process has been studied and optimized in order to obtain homogeneous Co-Ni deposits on Si/SiO2/Ti/Ni substrates. The electrochemical and magnetic characterization of the deposited layers shows the features of a soft magnetic material, i.e. high saturation magnetization (1.2 T) and low coercivity, which may be varied as a function of the electrodeposition parameters. Good adherence, brilliant aspect, smooth surfaces and a constant deposition rate have been obtained in all the processed samples. The compatibility of the Co-Ni plating process with the main microelectromechanical systems (MEMS) fabrication techniques, surface and bulk, has been evaluated and these results widely demonstrate the versatility of this magnetic layer in MEMS production. Due to the high selectivity and homogeneity of the deposition process, it has been possible to pattern the deposits with a definition down to 3 μm and an aspect ratio of 4.6 has been achieved. A novel method to liberate the patterned structures by sacrificial etching of the seed layer has been developed. Uniform and homogeneous deposits of Co-Ni alloy have also been obtained in devices fabricated by bulk technologies. For these devices an anisotropic wet etching procedure in tetramethyl ammonium hydroxide (TMAH) has been optimized to release the magnetic layer. Low stressed free-standing structures have been obtained by both surface and bulk methods and are presented in this paper.

Thick cobalt coatings obtained by electrodeposition

Thick cobalt coatings (10–40 μm) with a range of morphology and structure were obtained by electrodeposition on both vitreous carbon and copper electrodes. There is a direct relation between the morphology, structure and magnetic properties of cobalt deposits. A chloride medium at pH 4 and low deposition rates favoured the formation of black, ridge-like deposits of hexagonal close packed (h.c.p.) structure with mixed (100) + (110) preferred orientations. In CoCl2 at pH 4 at current densities in excess of −80 mA cm−2 and in CoSO4, dull grey deposits of h.c.p. (110) structure formed. Sulfate + citrate and chloride + citrate baths at pH 1.5 and very negative current densities promoted the formation of metallic grey deposits with face centred cubic (f.c.c.) structure. Constant saturation magnetization (Ms) was obtained for cobalt deposits independently of their structure (Ms = 160 emu g−1) although the coercive field (Hc) of the material varied: h.c.p. (100) > h.c.p. (110) > f.c.c.

Nano- and micrometric approaches to cobalt electrodeposition on carbon substrates

Abstract The first stages of electrodeposition of cobalt on different carbon substrates are studied at different [Cl−]/[Co(II)] ratios by Tapping mode atomic force microscopy, scanning electron microscopy and electrochemical analysis. Two charge deposition ranges are considered, ensuring that nano- and micrometric levels are attained. At the nanometric scale, a gradual increase in the number of nuclei over time is observed but, below a critical value, aggregation of the nuclei occurs. A high chloride concentration in solution favours the nucleation, promoting flat nuclei and star-shaped deposits. The species responsible for this behaviour are proposed to be chloro-cobalt complexes that adsorb on the crystallites, hindering the growth in some directions.

Detection and characterization of molybdenum oxides formed during the initial stages of cobalt-molybdenum electrodeposition

The initial stages of cobalt–molybdenum electrodeposition on a vitreous carbon electrode were studied to obtain information about the mechanism of cobalt–molybdenum induced codeposition. Solutions containing cobalt sulphate, sodium molybdate and sodium citrate at pH 6.6 were used. A first step in the mechanism of alloy deposition is proposed. This step takes into account the formation of molybdenum(IV) oxides over which Co–Mo alloy may be only deposited if sufficient potential is applied. Co–Mo electrodeposition occurs through an early stage involving low reduction current, related to the formation of molybdenum oxides, followed by a later stage in which the reduction current suddenly increases, corresponding to alloy codeposition. When a low potential is applied, a continuous coloured molybdenum oxide film is formed on the electrode and Co–Mo is not deposited. To induce the alloy deposition on the ‘oxide film’ it is necessary to apply more negative potentials than a threshold value, which depends on the composition of the electrolytic bath. By increasing molybdate concentration in solution, the ‘threshold potential’ shifts to more negative values. Intermediate molybdenum oxides were characterized using scanning electron microscopy (SEM), compositional analysis, Raman measurements and Auger and X-ray photoelectron spectroscopies.

Electrodeposition of zinc-nickel alloy coatings: influence of a phenolic derivative

The electroplating of Zn−Ni alloy films from a chloride bath has been studied under different plating conditions, both in the absence and presence of a phenolic derivative. Under the conditions examined, the electrodeposition of the alloys belonged to the anomalous type. The morphology and composition of the deposits varied with current density, temperature, bath composition and additive concentration. The results show that the additive modifies the structure and surface topography of the deposits to a large extent and produces smoother deposits. The corrosion resistance of the alloys has been analyzed by means of salt-spray tests.

Electrodeposition of zinc + cobalt alloys: inhibitory effect of zinc with convection and pH of solution

The influence of pH and agitation on the electrodeposition of zinc + cobalt alloys from aqueous chloride solutions was studied on vitreous carbon electrodes paying special attention to the initial stages of the electrodeposition process. Several Zn(II)Co(II) ratios in the solution were used, keeping the total concentration of metallic ions at 0.1 mol dm−3 and chloride ion at 1 mol dm−3. The presence of zinc in the bath always inhibited cobalt deposition, leading to anomalous co-deposition. For Zn(II)Col(II) ratios lower than 1 and stationary conditions, inhibition disappeared after a certain time, leading to normal co-deposition. For a fixed Zn(II)Co(II) ratio, the end of the inhibition was favoured when the pH of solution was increased. With rotating disk electrodes (RDE), co-deposition remained anomalous. Moreover, under conditions of normal deposition, the system could be returned to anomalous behaviour by convection using the RDE. Thus, the end of the anomalous codeposition was directly related to the depletion of Zn(II) in the environment of the electrode.