Received 11 June 2015; accepted 26 October 2015; published 29 October 2015

ABSTRACT

Influence of terms electrolysis and concentration of electrolysis components on the microstructure obtaining films of the chalcogenides rhenium has been studied. Appearance and structure electrolytical depositions of chalcogenides rhenium, obtaining by electrochemical method, depend from electrolysis conditions. In the process of studying of electrolysis conditions has determinated that for obtaining qualitative depositions of rhenium chalcogenides most suitable temperature is 75˚C - 80˚C. Microscopic studying of surface of catholic deposition of Re-Se showed that on the cathode had been obtained dense fine-crystalline black coverings. More qualitative depositions have been obtained for low current density. Increasing if current density adducts to gradual deterioration of structure cathode depositions. It has been determinated that specific influence on the process of deposition Re-S alloys has changing of electrolyte pH and current density. At low pH values, the colloidal sulfur formed in electrolyte has adsorbed on the cathode and bearing to the deposits quality. Microscopic studying of surface of cathode depositions Re-Te₂ showed that on the cathode at current density 2 mA/sm² had been obtained dense grey fine crystalline deposits of ReTe₂.

Keywords:

Electrochemistry, Rhenium Chalcogenides, Microstructure, Electrodeposition, Alloys Films

1. Introduction

Metallic rhenium and it’s alloys have unique physico-chemical properties, due to winch find wide application in the most important fields of modern technique [1] -[10] . Rhenium alloys with metals of platinum group can be used for production of electric contacts having high operational properties. Distinctive property of rhenium is its resistance to electro-corrosion that prevents burning of contacts.

Further is opening a new field for application of rhenium-oil refining industry, in which rhenium in combination with platinum is used as catalyst in oil reforming processes for production of high octane benzenes [3] . Thus the application of catalysts of mentioned type provided new quality jump in oil refinery. Thin coatings of rhenium disulphide were offered as catalyst for alcohols dehydrogenization process. Mostly interested are new reports about works in the field of application of rhenium and its alloys for needs of air- and rocket industry. Not less interesting and important may be considered the application of rhenium in nuclear. We also have been studied semi-conducting coatings of chalcogenides rhenium from different electrolytes [7] -[13] .

For obtaining semi-conducting of Re-Se alloy, we investigated joint electrodepositing of rhenium and selenium from alkaline [7] and sulfate [10] [11] electrolytes technique [7] [11] . Re-Te alloy, we proposed and chloride-sulfate [11] electrolytes. For obtaining semi-conducting alloy Re-Te in a form of thin films was found optimal regime and electrolyte. For obtaining thin films of rhenium chalcogenides in the work was proposed the electrolyte and optimal regime for producing alloys. An electrolyte and optimal regime to get rhenium-tellurium thin films in chloride-borate electrolyte [8] for obtaining alloys Re-Te was proposed. Using potentiostatic, temperature-kinetic, voltammetric methods were investigated kinetics and mechanism of separate and co-electro- deposition of rhenium and tellurium from chloride-borate electrolyte. Alloys of rhenium and sulfur are using as a photosensitive material in semi-conducting technique in a form of coatings. Work was widely investigated co-electrodeposition of rhenium and sulfured from sulfate and thiosulfate electrolyte [9] . Using the method of voltammetry, a process of electrodeposition of rhenium sulfide films at platinum and titanium cathodes was investigated. The rhenium alloys have found wide application in radio technique, electronics, semi-conducting industry and other fields of modern technique. In this connection, undoubted practical and theoretical interest has the electrochemical deposition of rhenium.

The physical-mechanical properties of electrolytic deposits are essential characteristics of coatings quality and determine their applicability to a particular area of technology. Properties of the electrolytic deposits are determined by the presence of inclusions in these deposits various foreign particles, as well as by the degree of irreversibility of the electrode process in the reduction of the ions of metals. This circumstance is due to the fact that the reducible ions, passing through an electric double layer having a large electric field intensity, become higher speed, which he immediately loses upon entering the crystal lattice, it leads to the formation of non-equilibrium lattice with changes of its parameters.

It’s necessary to note some interesting capabilities opening when the joint discharge of metal ions takes place meaning a possibility of obtaining coatings with properties that cannot be created in other ways. So, by the electrolytic deposition is possible to obtain alloys that by crystallization from a molten state cannot be obtained.

Alloys obtained by electrolysis, are nonequilibrium and differ sharply in structure from the alloys produced by thermic method. Therefore, the equilibrium state diagram cannot mechanically transferred to an alloy obtained by electrolysis as depending on the method of producing alloys differ both in structure and in properties. Deposits obtained electrolytically contain both metal inclusions and a large number of non-metallic inclusions, namely hydroxides, oxides, hydrogen, halogens, surfactants and others. With it are linked their special physical-mechanical properties.

In this work, we represent [7] -[12] carried out the main regularities for mutual electrodeposition of thin semiconducting films of the rhenium chalcodenides from different electrolytes, also some data about microstructure rhenium chalcogenides obtaining by electrochemical methods were represented. Some of these data obtaining in the results carried out researches have got scientific interest. We think it is necessary to involve in the chamber, these data confirming implemented research.

2. Methods of Experiment

X-ray studies of cathode deposits were carried out by automatic recording of roentgenograms on the tape at rotation speed of the sample 2˚C per minute on the X-ray machine DRON-4.

The structure of the surface and the cross-sections of cathode deposits of Re-X (X = S, Se, Te) was studied using a scanning electron microscope Testa Bs-301. Before polishing of the transverse sections of deposits they were coated by epoxy resin and after solidification of powders they were polished by chrome paste and etched with a solution of iodine in ethanol. A thickness of films was measured by interference method using MII-4 with standard error of 0.4 microns. Phase analysis of the deposits with low content of rhenium chalcogenides was also performed on the P-5827M potentiostat getting the polarization curves of the anodic dissolution of the alloy. Adhesion of cathode deposits with the cathode surface was tested by scratching and bending of the electrode at an angle of 90 degrees, the porosity of the coating was determined by applying filter paper.

Proceeding from the data it should be noted that thin films of rhenium chalcogenides possess rather low values of temperature coefficient and electroresistance what allows to apply such films in electronic technics.

3. Results and Their Discussion

The possibility of the obtaining of electrolytic coatings with a various desired properties contributed to the expansion of their application in industry.

Was investigated the microstructure of the cathode films of rhenium chalcogenides Re-X (X = S, Se, Te), obtained by the electrochemical method from various electrolytes. Was studied the influence of electrolysis conditions and the concentration of electrolyte components on the microstructure of produced rhenium chalcogenides films. The appearance and structure of the electrolytic deposits of rhenium chalcogenides obtained by the electrochemical method, depends on the conditions of electrolysis.

3.1. Re-Se

In the process of studying the electrolysis conditions was found that for obtaining of high-quality alloys Re-Se, most suitable temperature is 75˚C - 80˚C. Microscopic study of the surface of the cathode deposits Re-Se showed that the on the cathode are obtaining dense microcrystalline black coloured coatings. Particularly strong influence on the quality of the deposits has a current density. Higher-quality deposits are obtaining at low current densities. Increasing the current density leads to a gradual deterioration of the structure of the cathode deposits. As can be seen from Figure 1, at low current densities (1 - 5 mA/cm²) on the cathode are obtaining glossy fine-grained deposits having thickness of 5 microns. At a current density higher than 10 mA/cm² at the cathode are formed rather friable black deposits easily sleeping off from the metallic substrate.

A similar effect of current density on the appearance and structure of electrolytic deposits ReSe₂ can be explained as follows: at a current density below the limit, delivery of ions to the surface of the electrode is not limited by diffusion, and related crystallization rate is satisfactory. However, with achieving a zone of limiting current density the delivery of ions to the cathode surface is hampering affecting the crystallization rate. Due to the reduction of the number of ions discharging at the cathode, the electrode surface is partially passivated, crystallization occurs at the active sectors.

As the result are forming spongy and rather friable deposits in the form of randomly oriented microcrystalline aggregates, weakly bound between themselves and with the surface of the cathode.

The same effect on deposits quality has rise of the temperature of the electrolyte, as this also increases the area of current densities at which high-quality deposits are forming.

Photomicrograph of cross section of a platinum cathode with the deposit having the composition 54% Re + 46% Se showed that deposit consists of single phase.

For production of thin coatings of alloy Re-Se from acidic electrolyte was used an electrolyte of the following composition (mol/l): 0.05NH₄ReO₄ + 0.05SeO₂ + 2.0H₂SO₄ at a current density of 4 mA/cm² [10] .

Both in the alkaline and acidic electrolytes at the cathode are obtaining fine-grained deposits of alloy of Re-Se with different composition and with changing of the conditions of electrolysis changes a quality of deposits.

Thus, with increasing of the current density from 1 to 4 mA/cm² on the cathode are forming deposits of alloy Re-Se 10 microns thick, black-gray colored.

With increasing of current density above 5 mA/cm² the quality of cathode deposits deteriorates, on the cathode form friable, black deposits of alloy Re-Se. There were analyzed the content and morphology of thin layers ReSe₂, electrodeposited on the platinum electrode. From the X-ray phaseous analysis it follows the film consist from 54% of Re and 46% of Se (according to the mass). The diagram of X-ray phaseous analysis of the film ReSe₂ is presented in Table 1.

We have established that the compound ReSe₂ is crystallized in triclinic syngony with the parameters of the lattice: a = 6.7275 Å; b = 6.6065 Å; c = 6.7196 Å (Table 1).

3.2. Re-S

This work is devoted to research of microstructure of a cathode precipitation of Re-S obtained by electrodeposition from sulfate thioureous solutions, containing (mol/l): 2.2 × 10⁻³ NH₄ReO₄; (NH₂)CS₂ 1.0 × 10⁻³, H₂SO₄ 1.23 × 10⁻³ [9] .

Study of influence of electrolysis conditions on a microstructure of obtained films confirmed the assumption of the mechanism of joint sedimentation of rhenium with sulfur.

It is established that change of electrolyte рН and current density has special influence on process of sedimentation. At low pН values the colloidal sulfur formed in electrolyte has adsorbed on the cathode and bearing to the precipitation quality.

During researches it was established that depending on conditions, at the low density of current the multiphase precipitation were of dark color with brown, red and blue fragments, with yellow crystal dissemination .

Apparently, at the low current density the insignificant quantities of Re₂S₇ (brown color), ReO₂ (red color), Re₂S₅ and S elementary (yellow color) are the products of electroreducing. Formation of rhenium oxides and Re₂S₇ can be explained to that at the low density of current delivery of perrenate ions to a surface of the cathode and their discharge is complicated.

Availability of crystal sulfur, obviously is connected with that wish at the low density of current the potential of sulfur reducing to sulfide ion isn’t reached.

At increase of current density to its limit value, character of a surface of films sharply changes. At the current density of 30 mA/cm² (at optimum conditions) the film looks like a single-phase uniform micro crystallographic deposit of rhenium disulfide. In these conditions the speed of the discharge of ReO₄ isnt limited by diffusion and the crystallization speed related with it is satisfactorily. At further increase in density of current the precipitates remains single-phase, but in this case they characterized by friability.

The microphotography of a cross section of the platinum cathode with a deposit of the following composition, wt %: Re = 74.4; S = 25.6 showed that the deposit consist of one phase (Figure 2) and its structure corresponds to ReS₂ compound.

With the aim of clearing out the crystalloid structure of the obtained thin coverings ReS₂, there was carried out their X-ray graphical investigation.

According to RPA it was established, the ReS₂ complexes are crystallized in a triclinc syngony with the parameters of elementary cell: a = 6.455Å, b = 6.363 Å, с = 6.4 (Table 2).

3.3. Re-Te

Was studied the micro-structure of Re-Te alloy, obtained from chloride-sulfates electrolyte with complexation.

Was studied effect of current density from 1 to 10 mA/cm² to appearance and micro-structure of cathode deposits.

Microscopic study of the surface of the cathode deposits Re-Te showed that on the cathode at a current density of 2 mA/cm² are obtaining dense fine-grained coatings ReTe₂.

As it can be seen from Figures 3(a)-(c), with an increase in current density the structure of the cathode deposits gradually deteriorating. In this case, at a current density of 8 mA/cm² at the cathode are obtaining rather friable coatings easily falling off from metallic surface.

In order to obtain the thin, smooth coatings of ReTe₂ alloy, the electrolysis should be carried out at a current density of 2 mA/cm² at temperature of 75˚C from the electrolyte with the composition (mol/l): 0.03NH₄ReO₄ + 0.03TeO₂ + 1.5HCl + 1.5H₂SO₄ + 0.01(NH₄)₂SO₄ [13] .

In this case at the cathode are obtaining compounds of single-phase coating ReTe₂, wherein the rhenium content is 42%, cross-sectional micrographs of a platinum electrode with the composition showed 42% and 58% rhenium tellurium that the precipitates consist of one phase (Figure 3(d)).

It was found that finely crystalline, glittering coatings with a composition of ReTe₂ (42 wt% Re) and a thickness of up to 5 mk formed from the electrolyte containing (M) 0.03NH₄ReO₄ + 0.03TeO₂ + 1.5HCl + 1.5H₂SO₄ + 0.01(NH₄)₂SO₄ (electrolysis time 30 min, current density 1.2 A/dm², 0.25 - 0.20 V, s.c.e.). According to X-ray phase analysis data, ReTe₂ crystallized as an orthorhombic compound with unit cell parameters a = 1.301 nm, b = 1.307 nm, and с = 1.428 nm. Figure 4 shows the X-ray spectra of the ReTe₂ alloy on a platinum electrode. Table 3 lists the interplanar distances and intensities for the ReTe₂ compound obtained from the chloride-sul- fate electrolyte.

In the conclusion that the co-deposition of rhenium with tellurium occurs with minor depolarization. The magnitude of depolarization depends on the energy liberated during the alloy formation.

4. Conclusions

1) The process of studying of electrolysis conditions has determinated that for obtaining qualitative depositions of rhenium chalcogenides, most suitable temperature is 75˚C - 80˚C.

2) Microscopic studying of surface of catholic deposition of Re-Se showed that on the cathode had been obtained dense fine-crystalline black coverings. More qualitative depositions have been obtained for low current density. Increasing if current density adducts to gradual deterioration of structure cathode depositions. It has been determinated that specific influence on the process of deposition of Re-S alloys has changing of electrolyte pH and current density.

3) Microscopic studying of surface of cathode depositions Re-Te₂ showed that on the cathode at current density 2 mA/sm² had been obtained dense grey fine crystalline deposits of ReTe₂.

Acknowledgements

This work was supported by the Science Development Foundatoin under the Prezident of the Republic of Azerbaijan (Grant No. EIF-2013-9(15)-46/19/4).

Cite this paper

E. A.Salakhova,D. B.Tagiyev,K. F.Ibrahimova,P. E.Kalantarova, (2015) The Investigation of Microstructure and the X-Ray Phase Analysis of Re-X Alloys (X = S, Se, Te). Journal of Materials Science and Chemical Engineering,03,1-8. doi: 10.4236/msce.2015.310001

References