N. Spyrellis

Electrochemical synthesis of semiconducting CdSe thin films

Abstract Semiconducting CdSe thin films were synthesized by cathodic electrodeposition from an aqueous acid solution. Alloy formation occurs when the growth rate is limited by selenium ion mass transport and when the cadmium is underpotentially deposited. The composition and the physical properties of the asgrown layers are very sensitive to the selenium concentration of the solution and to the applied potential. CdSe films always have a zinc blende structure (instead of the normal wurtzite structure) with a strong (111) preferred orientation. It is shown that the best characteristics (stoichiometry, crystallinity, band gap width, etc.) are obtained with low selenium concentrations and in a narrow potential range. By using these appropriate preparation conditions, it will be possible to reduce the chemical and physical post-treatments generally applied to these kinds of semiconducting thin films in order to increase their solar energy conversion performances in solid or liquid junction cells.

Structure and properties of CdSe and CdSexTe1–x electrolytic deposits on Ni and Ti cathodes: influence of the acidic bath pH

Abstract The structural, morphological and photoelectrochemical features of coherent CdSe and CdSexTe1−x thin film semiconductors, prepared by cathodic electrodeposition from an acid sulfate solution, were investigated. In particular, the effect of deposition potential on the properties of layers grown onto Ti and Ni substrates, in connection with the electrolyte acidity within the bath pH interval [2.2, 1.6], was studied. As verified by X-ray diffraction (XRD), scanning electron microscopy (SEM), reflectance and photoelectrochemical cell (PEC) tests, the preparation method renders compact semiconductive films, presenting a remarkably intense in the case of CdSe/Ni(111) cubic structure, without any post-thermal treatment. The passive surface of Ti is associated with weaker texture; however, deposits on Ti generally exhibit better photoelectrochemical behavior, under similar preparation conditions.

Electrodeposition of Ni/SiC composites by pulse electrolysis : Electrodeposition in Electronics

SUMMARY Nickel matrix composite coatings containing micron- and nano-sized SiC particles were prepared in order to study the interdependence of the SiC particles embedding and the deposits ‘mechanical behaviour. SiC particles of two different sizes, namely 1 μm and 20 nm, were codeposited with nickel from Watts solutions under pulse current conditions. It has been observed that the embedding of SiC particles in the nickel matrix and the pulse current application result in deposits with more uniform particle distribution and better surface morphology than those obtained under direct current conditions. The study of the composite deposits revealed that the microhardness is not only increased by the presence and the reduced size of the particles, but also influenced by the current conditions, i.e. duty cycle and pulse frequency. Moreover, microhardness of the deposits can be further ameliorated by specific thermal treatment.

Influence of heat treatment on structure and properties of electrodeposited CdSe of Cd(Te, Se) semiconducting coatings

Abstract CdSe and mixed CdSe and CdTe semiconducting thin films, prepared by cathodic electrodeposition from an acid sulphate solution, containing selenium and tellurium oxides in Various amounts, were submitted to a to a thermal treatment at temperatures ranging between 400 and 520 °C. The crystal structure, composition, band-gap width and photoelectrochemical response of the annealed materials was investigated. It was found that all materials, rich in selenium, change their structure from cubic (zinc blende) to hexagonal (wurtzite), when annealed within the above mentioned region of temperatures. In many cases, an improvement of their semiconducting properties has been also confirmed.

Influence of pulse reversed current technique on the crystalline orientation and surface morphology of nickel electrodeposits

Abstract The reversed current (RC) technique perturbs the nickel electrocrystallization process by modification of the adsorption-desorption phenomena occuring on the cathodic area and causes a radical change of the structural characteristics and properties of the deposits. The purpose of this work is to study the modifications occurring in the crystalline orientation and surface morphology of nickel deposits, prepared from an organic-free Watts bath, by application of the RC technique. The texture analysis of the deposits was realized by X-ray diffractometry and the surface morphology was studied by scanning electron microscopy techniques. It was observed that variation of the RC parameters results in a total modification of the texture and surface morphology of the deposited metal. The roughness of the metallic surface was also examined and conditions for the preparation of bright and smooth deposits were determined.

Hexagonal cadmium chalcogenide thin films prepared by electrodeposition from near-boiling aqueous solutions

Thin, n-type, CdSe and CdSexTe1−x semiconductive films were prepared by cathodic electrodeposition onto titanium electrodes. An electrochemical cell was specially designed in order to perform electrodeposition in a near-boiling aqueous-ethyleneglycol bath at a temperature of approximately 110°C. The composition of the as-grown films, their crystal structure, morphology and band-gap width were studied as a function of the deposition potential and chalcogen ion concentration. It is shown that high temperatures have a positive effect on the crystal quality and the photoresponse stability of cadmium chalcogenide thin films even by employing electrolytes rather concentrated in selenous acid. Under specific conditions, a small shift in deposition potential brings about a complete phase transformation of the CdSe layers. In this manner, the described method enables the preparation of hexagonal CdSe deposits.

Textural modifications in nickel electrodeposition under pulse reserved current

Abstract Nickel electrodeposits consists of crystallites whose orientation, microstructure and size depend on the electrolytic conditions. Study of the mechanism of nickel electrocrystallization has indicated that the oriented growth observed is determined by the specific inhibition exerted by the chemical species which exist or are formed on the metal-electrolyte interface (catholyte) and which are selectively absorbed on the continually renewed metal surface. The pulse reversed current (PRC) technique was used to perturb the electrocrystallization process and hence to change the texture and the structural characteristics of nickel electrodeposits. The strucrtural changes that occur in deposit orientation under PRC conditions were studied. Nickel electrodeposition was realized from a Watts-type bath on a rotated cathode, and the electrolysis parameters (current density, rotation velocity, electrolyte pH and temperature) were chosen in such a way that using direct current they led to the preparation of perfectly [100], [211], [210], [110] A and [110] B textured deposits. The deposit texture was determined by X-ray diffractometry and the results are summarized in texture diagrams given as a function of PRC parameters: pulse frequency and duty cycle. It was proved that the texture is directly related to PRC parameters, which provoke strong perturbation in the crystal growth process. This phenomenon is reinforced when both the pulse frequency and the duty cycle are small. The PRC technique affects the nickel electrocrystallization process according to a mechanism analogous to the action of organic additives under direct current conditions by intervening in the catholyte composition and hence in the adsorption-desorption phenomena occuring at the metal-solution interface during the dissolution and the deposition time.

On a thermodynamic description of Se(IV) electroreduction and CdSe electrolytic formation on Ni, Ti and Pt cathodes in acidic aqueous solution

Abstract A discussion on tetravalent Se electroreduction and CdSe cathodic electrodeposition in terms of thermodynamics, in association with an investigation on the voltammetric behavior of Pt, Ni and Ti working electrodes in a high temperature (85 °C) acidic aqueous bath, is presented. The underpotential co-deposition of Cd with Se is described within the frame of a known electrochemical model of compound formation. The data presented serve as an effective basis for studying the electroreduction of Se(IV) as well as for determining the relevance of theoretical predictions to experimental findings on a binary compound electrodeposition process.

Electrocrystallization of CdSe upon various substrates. Structural arrangement and photoelectrochemical performance

Abstract The growth of cadmium chalcogenide thin film semiconductors, in the context of a typical electrolytic method of formation with an aqueous bath, is to a large extent determined by the deposition substrate, together with the potential, for a given electrolyte composition and temperature. The effect of various substrates ([100]-oriented Ni, commercially pure Ni, Ti) and procedures (Ni electropolishing, Ti anodization, double-step deposition, etc.) on the structural arrangement and the resulting photoelectrochemical (PE) behavior of cathodically electroplated CdSe layers is presented. The outcome of the preparation process is analyzed in terms of structural–optical properties relation. As verified, some microcrystalline, porous samples give higher PE efficiencies than larger-grained ones, owing to the increased contact area with the PE redox electrolyte and possibly the establishment of a particular charge transfer mechanism in the solid. The latter is associated with the existence of a nanostructure.

Nickel matrix composite coatings : Application in textile machinery and evaluation of cotton products quality

SUMMARY Coatings applied to several mechanical parts involved in the cotton textile industry are expected to improve production efficiency and to decrease the relevant costs. The quality of cotton products is one of the most crucial parameters in the development of textile industry and is significantly associated with the mechanical and tribological properties of the coated machinery parts. For this purpose, nickel matrix coatings containing silicon carbide (SiC) micro-particles, prepared under pulse plating conditions were applied on several parts. Surface morphology, preferred orientation of the crystals, incorporated SiC percentage and mechanical properties of these coatings were investigated. Moreover, the influence of the surface coating on the cotton products quality was studied by the means of a prototypal method, which determines the extent of wear on a typical cotton fabric, under strictly controlled conditions. It has been proved that Ni/SiC coatings, prepared under very well defined conditions (pulse current density, frequency, duty cycle, etc), present relatively low roughness, high microhardness, low sliding friction coefficient between the coating and typical cotton products and reduced wear against cotton fabrics or yarns.