N.A. Pangarov

The crystal orientation of electrodeposited metals

Abstract Calculations have been made for the work of formation of the two-dimensional nucleus of the metals crystallizing in cubic face-centred and cubic body-centred lattices. It is shown that the crystal orientation of thick deposits depends upon this work. The crystallites of electrodedeposited metal have preferred orientation, when the work of formation of the two-dimensional nucleus for some net plane is smaller than that for the other net planes. It is shown that the crystal orientation depends on the supersaturation, i.e. the overvoltage during the deposition of the metal ions. The results obtained are in very good agreement with the experimental data.

Preferred orientation of electrodeposited iron crystallites

Abstract The work of formation of two-dimensional nuclei with a b.c.c. lattice is theoretically calculated, taking into consideration the forces of interaction of up to fourth neighbours. The work of formation is plotted as a function of supersaturation. The orientation axes of iron electrodeposits have been studied experimentally at various overvoltage values. Orientations along the [310] axis were obtained for the first time. The effect of potassium iodide upon overvoltage and crystal orientation was also investigated. The experimental results are in agreement with the theory of preferred orientation of crystallises in electrodeposited metals, developed on the basis of calculations of the work necessary for the formation of two-dimensional nuclei at various supersaturations.

Thermodynamics of electrochemical phase formation and underpotential metal deposition

Abstract A new thermodynamic theory of the electrochemical phase formation and the underpotential deposition (UPD) of metals was developed. The half-crystal position of the two-dimensional crystal phase is introduced in order to characterise the electrochemical potential of the solid phase. It is shown that this half-crystal position depends on the specific adsorption of foreign species. New results are obtained for the equilibrium electrode potentials of underpotentially deposited metal layers onto crystalline substrates with different surface structures. Relations are derived for the adatom activity upon different substrates and its dependence on the supporting electrolyte and the specific adsorption. The obtained results explain quite well the position of the peaks in the voltammograms as well as their shape. The problem of the Gibbs free energy of nucleation and its dependence on the overvoltage of nucleation are comprehensively treated. A clear thermodynamic difference is shown between this overvoltage and the total value of the overvoltage. The influence of the supporting electrolyte and the specific adsorption on the UPD is explained. The condition for the transition from underpotential to overpotential deposition is shown. The obtained results are in very good agreement with all known experimental facts concerning the UPD of metals and lead to original conclusions which could be proved experimentally.

The orientation of silver nuclei on a platinum substrate

Abstract The initial stages of silver electrocrystallization on a platinum electrode have been investigated. A pulse method was used, in which an electron pulse-generator was combined with a pulse potentiostat. A single, square potentiostatic pulse, with height and duration chosen in a way that single crystal nuclei were formed, was superposed on an initial potential at the beginning of the experiment. The crystals grew further on account of the potential initially set, and their growth was continuously followed microscopically. The crystal orientations obtained in the various overvoltage regions, are: for the orientation axis [111], 20–50 mV; for [100], 60–100 mV; for [110], 120–170 mV; for [113], 170–200 mV; for [210], over 200 mV. The experimentally obtained orientation axes are in full agreement with those theoretically calculated for metals with a f.c.c. lattice.

On some relationships between the structure and brightness of oriented electrodeposited nickel coatings

Abstract The nature and perfection of texture as well as the change in the amount of twinning of galvanic nickel prepared in the presence of brighteners have been investigated by X-ray texture analysis. A correlation was found between the structure characteristic of the coatings and their reflectivity. On the basis of the experimental data obtained and using some simple models, an approach is suggested for explaining the mechanism of formation of bright oriented nickel coatings. The dominant effect of the surface crystallographic structure on the brightness of the coatings is stressed, as well as its relationship to the bulk structural features.

Double-layer capacitance measurements by a pulse potentiostatic method☆

Abstract The possibilities are considered of applying the pulse potentiostatic method in measuring double-layer capacitance at the metal/solution interface, for the practically perfectly polarizable mercury surface (with no electrochemical reaction taking place) and for solid metal electrodes, where it is not always possible to eliminate the effect of the faradaic current. Short square potential pulses of small amplitude (10 mV) are superimposed over a preliminary set electrode potential, maintained potentiostatically. The i / f ( t ) curves obtained are then investigated. The quantity of electricity needed for charging the double layer is determined by graphical integration of Q = . The value C = 16·7 ± 1·0 μF/cm 2 has been obtained for the double-layer capacitance of mercury in 0·1 N HCl, and C = 17·6 ± 2·0 μF/cm 2 (visible surface) for smooth platinum in 1 N H 2 SO 4 . Measurements of double-layer capacitance of electrodeposited β-cobalt showed that if etching is performed in the anodic region and then the measurements in the cathodic region are repeated, the double layer capacitance is increased about 2·5 times. Applying the pulse potentiostatic method, one may use the data on double-layer capacitance to estimate the real reaction surface, when electrochemical investigations are carried out on large metal surfaces.

Twinning processes in silver electrocrystallization

Abstract A pulse potentiostatic method has been used for the study of the percentage change in twinned crystals to the total crystal number, as a function of overvoltage and pulse duration. The results show that the twinning processes depend on the amplitude and the length of the pulse. The work needed for breaking the bond between two first neighbours in the silver crystal lattice is calculated on the basis of the data. The obtained value coincides with that calculated by an independent method.

Preferred orientation of nickel crystallites deposited from oxygen-free solution☆

The influence of cd, temperature and pH of the solution upon the preferred orientation of crystallises in nickel electrodeposits from a solution of 280 g/l NiSO4.7H2O with or without H3BO3 have been studied by X-ray diffraction and electron-microscopic methods. Nickel was deposited galvanostaticaly in circulated oxygen-free solution. With increasing overvoltage, the texture axis changes in the sequence [100] + [111] → [100] → [100] + [110] → [110]. The results are in agreement with the theory of two-dimensional nuclei.

Preferred orientation of electrodeposited nickel crystallites at high temperatures

Abstract The structure of nickel deposited from a solution of 2·5 M nickel chloride at pH 3 and at 260°C has been investigated. X-ray diffraction methods showed that the nickel crystallites have [111] orientation axis. Electron microscopic studies showed that the deposits have a gross crystalline structure with the (111) plane parallel to the substrate. The results are in full agreement with the theory of preferred orientation developed by calculating the work of formation of the two-dimensional nuclei of the equilibrium planes.

Electronographic investigation of the degree of preferred orientation of nickel electrodeposits

Abstract The influence of a polished platinum substrate upon the degree of orientation of [110]-oriented nickel electrodeposits has been investigated electronographically, in relation to layer thickness and current density. It has been shown that the influence of the substrate is restricted to only very thin layers of electrodeposited nickel. This influence decreases with the increase of current density, or with the decrease of size of the nickel crystallites. The obtained experimental data are in good agreement with the two-dimensional theory of preferred orientation, when the epitaxial influence of the polycrystalline substrate is taken into consideration.