ELECTRODEPOSITION OF RHENIUM FROM CHLORIDE MELTS-ELECTROCHEMICAL NATURE , STRUCTURE AND APPLIED ASPECTS

Processes involved in the electrodeposition of rhenium from chloride melts have been studied over the temperature interval from 680 to 970 0C at a cathodic current density of 5 to 250 mA/cm2. It has been found that rhenium is deposited in the form of continuous layers. In addition to that the growth of deposits as separate single-crystal needles has also been noticed. Continuous layers had axial growth textures. The crystallographic direction of the textures is due to electrolysis conditions, such as concentration of oxygen-containing impurities, temperature, melt composition and cathodic current density. When the concentration of oxygen-containing impurities in the melt decreased, electrolysis temperature increased, the average radius of the supporting electrolyte cations became smaller, or cathodic current density diminished, the direction of the growth textures was changing as follows: (1010) → (1120) → (101L) → (0001) → (0001)needles. The microhardness of the deposits in this series is 900 to 250 kg/mm2. The growth of deposits on textured rhenium substrates and single crystals having different orientations, including bent substrates, was studied. It has been found that the epitaxial growth is virtually unlimited in depth if the orientation of the substrate coincides with the growth texture under given conditions. If the substrate orientation deviated from the growth texture, the epitaxial growth was nearly absent. Kinetic parameters were measured using the galvanostatic method. The exchange current density was determined over the interval of (0.01-0.1) A/cm2 depending on the concentration of oxygen-containing impurities, cation composition, type of the surface and its condition. The parameter α⋅Z, which was estimated by two methods, was equal to 2.1-3.1. The diffusion vinogradov.indd 7/3/03, 2:50 PM 1 O. N. Vinogradov Zhabrov et al J. Min. Met. 39 (1 ‡ 2) B (2003) 150 coefficient of rhenium ions has been found to be 2.8⋅10−5 cm2/s at 790 0C and 3.5⋅10−5 cm2/s at 840 0C. Galvanoplastic production of rhenium products, such as crucibles, ampoules, foils, wire, and intricate articles, was performed.


Introduction
Rhenium is a scattered rare metal.It is found in small quantities as an accompanying element in molybdenum and copper ores.This fact determines its large cost.However, metallic rhenium has been efficiently used in engineering due to its unique properties, such as high melting point (3170 0 C), the largest strength among all metals at temperatures above 1500 0 C, chemical resistance to hydrogen, nitrogen, water vapor, carbon and hydrocarbons at elevated temperatures and a large electron work function.Rhenium is used as a wear-resistant constructional material in, for example, rocket production.Single crystals of refractory chalcogenides and oxides are grown in rhenium crucibles.Rhenium wire and foil are used in electronics and mass spectrometry as emitters and cathodes.
However, due to high melting point, large oxygen sensitivity (hot brittleness) and high workhardening rate make the standard metallurgical stages and machining operations very difficult.Therefore, methods that allow producing articles with minimal subsequent machining or without it are perspective.
A method of electrodeposition of rhenium from chloride melts, which was developed at the Institute of High-Temperature Electrochemistry (Ural Branch of the RAS) [1,2], is among those methods.By our method it is possible to deposit continuous layers from several microns to several millimeters thick, and also make intricate articles not requiring futher machining.
Rhenium has the hexagonal closely packed (hcp) lattice and anisotropic properties in different crystallographic directions.One of the goals of the study was to analyze the conditions necessary for the preparation of continuous well-textured deposits of rhenium having different orientations.This would provide articles with desired operating properties.

Experimental
Experiments were performed in hermetic quartz and metallic electrolyzers mainly in glassy-carbon or pyrographite crucibles under argon atmosphere.Rhenium rods or rhenium deposited on the walls beforehand were used as the anode.Annealed graphite bars, glassy-carbon plates, rhenium single crystals, or polycrystalline rhenium foil served as the cathode.
Components of the supporting electrolyte (salts) were dried in vacuum.The required concentration of rhenium ions was obtained by chlorination of rhenium in the melt using a special technique.Rhenium ion content of the electrolyte was 1-7-wt.%.
The structure perfection, the texture of deposited rhenium layers and surface morphology were examined by X-ray diffraction, metallographic and opticogoniometric methods using DRON-3 and DRON-3M diffractometers, a Neophot-32 optical microscope, and a SEM "Camebax".The microstructure of the deposits and their microhardness were determined on parts of the test samples.The cross-and longitudinal micro-sections were prepared by mechanical grinding on abrasive paper and diamond pastes, which was followed by chemical etching.
Polarization curves were recorded in galvanostatic regime on a PI-50 potentiostat.An S9-8 digital storage oscilloscope was used to measure the ϕ-t curves.The current density (i) was 5 to 250 mA/cm 2 at a pulse length of 0.1 s.To preserve the electrode surface, each cathodic pulse was followed by an anodic pulse having the same magnitude and duration.Initial potentials at the electrode were checked by means of a digital voltmeter.They changed by not more than 2 mV after polarization at a large current density.

Precipitation of textured deposits
When electrolysis was performed in the KCl-NaCl-ReCl 4 melt in a quartz electrolyzer [3] over the temperature interval of 680 to 970 0 C at the cathodic current density from 5 to 200 mA/cm 2 , rhenium precipitated on the cathode as continuous deposits, whose structure depended on the electrolysis conditions.The current efficiency was close to 100 %, if one supposes the Re(IV) + 4 e -= Re cathodic process.A similar current efficiency was achieved in other melts.
First deposits, precipitated at low temperatures and high current densities, had a fine crystalline structure and the (1010) texture.The surface was formed by facets inclined at an angle of 60 degrees to the deposit plane.
Subsequent deposits, obtained by repeating electrolysis at the same current density and temperature, had a different texture.The (1010) texture was preserved at the lowest temperature and the highest current density.The rhenium layers had the (1120) texture after the temperature was increased.Perfection of the texture improved progressively with increasing thickness of the deposit.The surface was formed by facets inclined at an angle of 31-40 degrees to the deposit plane.The facets were mirror-smooth.They formed tetrahedral and, much more frequently, stellated pyramids with a six order symmetry axis (Fig. 1).
As the temperature increased, the (1120) texture of the deposits changed to (101L), where L varied between 3 and ∞.This corresponded to the (0001) texture.The deposits had a fine crystalline structure and their microhardness decreased (see Table 1).
The process, during which the quality of deposits improved in a series of electrolyses performed under similar conditions and, in our case, textures were changed, was referred to as "conditioning" of the melt.By this we mean the removal of impurities affecting the quality of deposits.
Since brown-yellow sublimates were deposited in the upper part of the cell (in the cold zone) and initial reagents and metallic rhenium were pure enough, we suggested that oxygen-containing compounds (O 2 , H 2 O , CO 2 ) were the so-called "harmful impurity".Uncontrolled quantities of those compounds were added when assembling and loading the electrolyzer.When the electrolyte was heated and melted, they formed rhenium oxycompounds, which, first, sublimated from the melt owing to their high volatility and, second, co-deposited together with rhenium because the oxycations deposition potentials are close to the deposition potential of the rhenium cation.Special experiments were performed to verify this assumption.

Effect of impurities of rhenium oxycompounds in the melt on the properties of electrolytic deposits
The experiments were performed to determine the effect of oxygen on the structure.Rhenium deposits with (1120) and (0001) textures were obtained in the KCl-NaCl-ReCl 4 melt (C Re+4 = 7 wt.%) at t=750 0 C and t=850 0 C respectively.An addition to that, 0.06 wt.% potassium perchlorate, which corresponded to 0.03 wt.% oxygen, was introduced into the melt through a lock.After this, electrodeposition was realized.Textures of the growing deposits changed to (1010) and (1120) respectively.The deposits had surface blisters that were due to bubbles of different pores formed during electrolysis.Equivalent quantity of zirconium was added to remove oxygen by the following reaction: Rhenium was deposited again at the same temperatures and current densities.The textures of the deposits did not change compared to those obtained before potassium perchlorate was added.
A special quartz cell and a furnace with optical ports were made to watch the electrolysis process.The cell had two compartments.The first compartment was intended for the "conditioning" of the melt mentioned above.After maximum purity of the melt was achieved, argon forced it to the second compartment through a quartz filter.The growth of deposits under different conditions was watched in the second compartment.
No visible changes were detected in the deposit after 5 cm 3 of oxygen were added to the electrolyte (100 g).When extra 20 cm 3 of oxygen were added, a few bubbles appeared on the cathode at i=0.1-0.2A/cm 2 .A fine metal powder was detected on the melt surface.Bubbles appeared on the cathode already at i c =0.05 A/cm 2 after further 20 cm 3 of oxygen were passed through the melt.The size of the bubbles increased slowly as current density increased up to 0.10 A/cm 2 .At i c =0.20 A/cm 2 new bubbles appeared in addition to the growing bubbles (Fig. 2).
As one might expect, the concentration of oxygen-containing impurities in the melt decreased when it was held for a long time without electrolysis, but argon was bubbled continuously through the melt.A few bubbles appeared only at i c =0.20 A/cm 2 .
In our opinion, the interaction of the rhenium electrolyte with oxygen and the electrochemical behavior of the product formed can be described by the following reactions: where Me is an alkali metal.
ReO 2 was implanted in the deposit at low temperatures and decomposed at high temperatures according to the reaction: These assumptions were confirmed by the following facts.First, stressed cracking porous deposits were obtained in the melt with large oxygen content at low temperatures.Second, the temperature (above 750 0 C), at which bubbles (blisters) appeared in the deposits, coincided with the temperature of rhenium dioxide decomposition by reaction (4).
The oxygen concentrations, which determined the appearance of bubbles, coincided irrespectively of whether they were introduced by potassium perchlorate or by the gas.

Effect of the cation composition on the texture of electrodeposited rhenium
The experiments demonstrated that the observed quantity of sublimates changed with time depending on the cation composition of the electrolyte.Brown sublimates (rhenium oxychlorides) were deposited onto the electrolyzer walls in the cold zone (at the top) almost immediately after the NaCl-based electrolyte was melted.However, the conditioning time of the melt was short.Deposits with the (0001) texture were formed virtually at once even near the liquidus line (∼800 0 C) of the electrolyte.The quantity of sublimates was low in the CsCl (85 % M)-NaCl (15 % M) melt.However, deposits with the (0001) texture appeared in the conditioned melt only at temperatures above 950 0 C. To elucidate the effect of the cation composition on the temperature transition of textures under similar experimental conditions, alkali-chlorides-based electrolytes and their mixtures were tested.Most of the experiments were performed in an argon atmosphere, while some experiments were carried out in air.The gas pipe, which supplied argon to the electrolyzer, was detached from the pipe union and the gas chamber of the electrolyzer was vented to air.
The temperature of the texture transformation from (1010) to ( 1120) and ( 1120) to (0001) increased with increasing the cation or the average radius of cations in the salt mixtures in the supporting electrolyte in the following series: The obtained texture map is shown in the "average radius of the cation-temperature" coordinates in Fig. 3.The types of deposit textures are designated with different symbols of experimental points.The increase in the texture transition temperatures with increasing the average radius of a cation in the salt mixtures in the supporting electrolyte can be explained by smaller volatility of oxygen-containing impurities depending on the complex-forming capacity of the solvent salt.The larger the average cation radius in the supporting electrolyte is, the stronger oxychlorides are retained in the melt.An analogous dependence of the transition between textures was obtained in air.The only exception was that transition limits in air were 100-200 0 C higher than those in argon.
CsCl has cations of the largest radius among the electrolytes studied.One might think that the temperature of transition between textures should be largest in this melt.However, this electrolyte was an exception.Transitions from (1010) to (1120) and from (1120) to (0001) textures occurred at temperatures of about 720 0 C and 820 0 C respectively.Probably, cesium has a larger affinity for oxygen in the melt than other cations do and therefore rhenium oxycations are reduced on the cathode to a lesser degree (at more negative potentials).

Kinetic parameters of rhenium electrodeposition
As it was shown above, the quality of rhenium deposits is determined by several interrelated factors, which probably showed up in kinetic parameters of the process.For this reason, the kinetics of rhenium electrodeposition was analyzed as function of temperature, concentration of rhenium oxychlorides in the melt, cation composition, as well as type and condition of the surface [4].The experiments were performed in a glassy-carbon container.
The melt was conditioned until deposits with the (101L) texture were obtained.After this, polarization curves were recorded at different conditions on a polycrystalline electrode (Fig. 4).
Controlled volumes of oxygen (1, 3, and 10 cm 3 ) were bubbled through the melt.Polarization was measured after each bubbling cycle.Polarization increased as the volume of oxygen grew.Then the melt was conditioned.Purity of the electrolyte was judged from texture characteristics of the deposits.Polarization returned to its initial value (Fig. 5) as oxygen-containing impurities were removed from the melt.
Subsequently, polarization was measured using a single-crystal electrode.In this case, polarization was much larger than the one measured on the polycrystalline electrode at similar values of current density, which were calculated for the geometric surface of the electrode (Fig. 6).
If a polycrystalline layer of rhenium was deposited on a single-crystal surface of the electrode, polarization curves were identical to those recorded earlier at the polycrystalline electrode.When the quantity of electricity used for the anodic dissolution of part of the deposited layer was smaller than in the cathodic pulse, the electrode surface was activated and polarization decreased.As the anodic dissolution of the electrode was continued further until the surface became monocrystalline again, polarization increased and regained its initial value corresponding to that of a single crystal.This dependence of polarization on the surface state implies that the crystallization overpotential dominates the total overvoltage, which is due to a limited number of growth sites.
The overvoltage-current dependencies were processed in η=f(i) and η=f(ln i) coordinates, which were used to estimate i 0 and α⋅Z.The exchange current density increased on the polycrystalline surface from 0.02 A/cm 2 at t=790 0 C to 0.05 A/cm 2 at t=830 0 C and 0.100 A/cm 2 at t=890 0 C. The i 0 value dropped to 0.02 A/cm 2 at t=830 0 C when oxygen was added to the melt.If oxygen-containing impurities were not removed completely, i 0 was 0.036 A/cm 2 .After the melt was conditioned, the exchange current density returned to the value before oxygen was added.The i 0 value on the single-crystal surface of the electrode was much smaller (i 0 =0.011A/cm 2 at t=760 0 C and i 0 =0.022A/cm 2 at t=850 0 C) than its counterpart measured on the polycrystalline surface.
The α⋅Z value equal to 2.5-3.1 was found from the slope of cathodic branches of th e Tafel curves.Processing chronopotentiometric curves in ( ) coordinates yielded similar α⋅Z values equal to 2.1-3.1.The diffusion coefficients of rhenium ions were calculated from the same graphs, appearing to be D=2.8⋅10−5 cm 2 /s at t=790 0 C and D=3.5⋅10 −5 cm 2 /s at t=840 0 C.These values are in a good agreement with the data of [5].Polarization was measured in melts based on CsCl and CsCl-NaCl mixture containing 5, 10, 15, and 20 wt.% NaCl.Measurements were done first in the CsClbased electrolyte and then the required amounts of NaCl were added to the electrolyte without opening the cell.Polarization decreased as the average cation radius in the supporting electrolyte of the CsCl-NaCl system decreased.Polarization of the electrolyte with the largest cation radius (CsCl) was less than polarization of the CsCl(95 % M) -NaCl(5 % M) electrolyte with a smaller average radius of cations (Fig. 7).These findings agree well with our ideas about the dependence of texture temperature transition on the average cation radius in the supporting electrolyte.The asymmetry of the anodic and cathodic branches and the dependence of polarization on the origin of the electrode surface confirm the assumption that the stage of transition at a limited number of growth sites is slow [6].

Precipitation of needle deposits
Deposits having an unusual structure [7] were obtained in particularly clean conditions including the use of an all-metal electrolyzer and a glassy-carbon container, elimination of rubber components (in the gas line too), and purification of argon with a high-temperature solid-electrolyte pump.The samples resembled black velvet in their external appearance.Microscopy analysis showed that the deposits comprise a dense brush of acicular crystals (Fig. 8, 9).Cross-section of rhenium deposit on a graphite substrate had a continuous layer 1-5 µm thick with needles growing from the layer.Depending on electrolysis conditions, the needles were one micrometer to tens of micrometers thick, while their number was about 10 6 needles/cm 2 .The needle deposits were characterized by a "high perfection" of the (0001) texture, S T =80.
When the temperature was high and the current density was small, the needles represented elongated hexahedral prisms with a pyramidal top and without side branches.The top is formed of planes inclined to the crystal axis at an angle of 57.79 degrees, which corresponds to the (1122) faces.Lateral planes correspond to (1120) faces.
When the temperature was low and the current density was high, lateral faces were fuzzy and there was not a clear transition between lateral face and the face forming the angle top.The top became more acute.In some cases, probably under transient conditions, when an impurity was co-deposited and passivated in the top, needle 160 spherulite was formed.It consisted of similar needles growing from a single center, namely the top of the primary needle (Fig. 10).
During electrodeposition on a rhenium foil with the (0001) texture, needles formed laths inclined at different angles to the substrate plane.Needles were mutually parallel in each lath (Fig. 11).Considering the layout macrogeometry of the laths, one may think that (0001) planes formed at the stage of the continuous deposit are situated at the base of the laths.This hypothesis is confirmed by the fact that during electrodeposition on a rhenium substrate with a high perfection of the (0001) texture all needles are almost perpendicular to the substrate plane.Growth conditions of the continuous layer were disturbed and transition to the needle deposit occurred due to the lack of electrochemically active surface components, namely oxycations, all other factors -electrolysis temperature and current density -being equal [7].
Needle structures appeared in experiments on large-scale laboratory electrolysis with electrolyte mass of 15-20 kg and surface area of the cathodes (matrices) being hundreds of cm 2 .During long-term (6 days) experiments at large currents a needle structure appeared in the KCl-NaCl eutectic melt at a temperature near to the liquidus line.This is obviously connected with large surface areas of the melt and the cathode, which facilitates evaporation of oxychlorides and increases the number of co-deposited initial impurities.A special solid was added to the melt to stabilize the growth of continuous layers, by generating the required quantity of oxygen during electrolysis.The threshold concentration of oxygen in the melt, which was necessary for the growth of a continuous Figure 11.Rhenium needles on the poly crystalline rhenium foil, magnification -800.
layer, was estimated from depletion of the addition.It proved to be less than 10 −7 wt.%.Obviously, the actual value is even smaller, because some generated oxychlorides were sublimated in the cold zone.Unfortunately, maximum purity could not be achieved in kinetic measurements and, therefore, it was impossible to determine current exchange density values under these conditions.However, analysis of evidence adduced above shows that they were still larger than those obtained during the growth of the (101L) textures.Therefore, the changeover of the deposit growth from the continuous mechanism to the "dendritic" one presents an excellent illustration of Baraboshkin's idea about criteria for stable growth of a flat front [8].

Epitaxial growth of rhenium on own single-crystal substrates with different orientations
It was shown earlier that epitaxial layers of molybdenum and tungsten could be produced on monocrystalline molybdenum substrates with different orientations during electrocrystallization from oxide and halide melts [8][9].In our opinion, it was interesting to investigate the influence of the electrodeposited metal symmetry and its physicochemical properties on the epitaxial growth mechanism.For this purpose rhenium metal with the hcp lattice was chosen.The experiments were carried out in a molten mixture of cesium and rhenium chlorides at 870 °C in inert atmosphere.The range of cathodic current densities was 0.02-0.10A/cm 2 .The straight and bent monocrystalline rhenium plates with orientations (1010), (1120), (0001) were used as cathode materials.It was established that rhenium deposits on compact graphite substrate had usually the (101L) growth texture, with 3 ≤ L ≤ 5.In this case, the epitaxial growth onto the rhenium monocrystalline substrate with (0001) orientation occurred up to the end of electrolysis, but the epitaxial growth on the substrates with (1120) and (1010) orientations was broken up and the formation of the (101L) growth texture started.If the growth texture of rhenium deposits on the graphite was missing, the epitaxial growth on the rhenium substrates with orientations (0001) and (1120) took place.In this case, the epitaxial growth on the plate with orientation (1010) was broken up and the formation of polycrystalline deposit was observed.The same regularity was revealed for the bent single crystal substrates with orientations (0001), ( 1120) and (1010), and for all curvature radii.
The surface morphology of rhenium deposits was also investigated.The pyramids faced with the plane (1013) were formed in the case of the (101L) growth texture on the surface of polycrystalline deposits.The growth pits in the form of the reverse 6-12 sided pyramids faced with the (1011) or (2025) planes appeared during epitaxial growth on the monocrystalline rhenium straight and bent substrates with (0001) orientation (Fig. 12a).The reverse 12-sided pyramids had obtusion on the tops.The reverse 4-sided pyramids faced with the (1011) planes were observed on the rhenium monocrystalline substrate with (1120) orientation during the epitaxial growth (Fig. 12b).The layer by layer epitaxial growth was resulted in the growth steps morphology on the rhenium substrate with (1010) orientation (Fig. 12c).Surface morphology of the deposits on the straight and bent substrates was identical.It was established that epitaxial growth proceeded layer by layer by a two-dimensional mechanism.The main morphological features of single crystal rhenium deposits were the growth pits with symmetry corresponding to the symmetry of monocrystalline substrates or the growth steps.
The cross-section parts of rhenium deposits on the graphite and on the straight and bent rhenium substrates of different orientations were investigated.The boundary between rhenium deposit and substrate was absent at epitaxial growth on the monocrystalline rhenium foil.In the case of deposition without epitaxial growth, the sharp boundary between the deposited rhenium and substrate was observed (Fig. 13).
We investigated the fractures of poly-and monocrystalline rhenium deposits [11].It was found that the character of their fractures are different.In the case of the monocrystalline deposits, we can observe the pits relief of the fractures typical of sliding fractures (Fig. 14a).The mixed character of destruction -brittle intercrystalline fracture with gliding parts of fracture and small parts of brittle breakage is typical of polycrystalline rhenium deposits (Fig. 14b).The deposits had the columnar structure with the grain size 20-30µm.
The microhardness of the epitaxial monocrystalline rhenium deposits with (0001) orientation and deposits with the (0001) growth texture was measured on the crosssection parts.The microhardness changes from 240 to 270 kg/mm 2 that testifies to the high purity of deposited rhenium.

Conclusions
Considering the long-term research and practical work with rhenium, it is possible to state that this element is unique not only in its physical-chemical properties, but also as material that allowed grasping the beauty of the electrochemical nature thanks to the hcp  lattice, its energetics, and nearly equal values of electroreduction potentials of rhenium ions and oxycations in chloride melts.It should also be noted that chemical purity of the deposited metal was 99.99-99.999% under optimal conditions when deposits were analyzed for 30-40 impurity elements.This is due to a considerable electropositivity of rhenium.
The fundamental studies provided technologies for the fabrication of various products: -Crucibles and ampoules 100 mm in diameter and 150 mm in height with walls of 2 mm thickness (maximum dimensions) were made.Single crystals of chalcogenide and oxide refractory compounds were grown in these vessels.-Devices for growing shaped single crystals (crucibles, molds fixing covers, and molds representing a pair of "tubes" of a preset shape with a gap of 0.5 to 1.0 mm) were fabricated.They were used to grow high quality shaped single-crystal articles from refractory oxide compounds.
-Intricate articles -casings of liquid-propellant low-thrust rocket engines with walls of 1.0 to 1.5 mm thickness -were made.-About 1.5 kg of rhenium foil 20 to 40 µm thick was prepared.The foil was and, probably, is being used in mass spectrometers at dozens of research and industrial institutions.-Tens of meters of rhenium wire (100 to 200 µm in diameter) were produced by continuous drawing in a molten salt.Testing this wire in gas mass spectrometers demonstrated its considerable advantage over wire prepared by the standard method.Electrolytic wire preserves its shape at temperatures up to 2000 °C.Therefore it is unnecessary to calibrate mass spectrometers during repeated analyses.-Components for heat isolation were produced.They consisted of three tubes 10 to 6 mm in diameter and 50 mm long with 100 µm thick walls.The tubes were assembled with a gap and their butt ends were electric arc welded.They demonstrated durability and vacuum tightness during repeated on-off cycles of the engines.
-Ion detectors for mass spectrometers were made.They represented rhenium plates with a needle structure.The ion detectors have the property of a blackbody, i.e., they can replace Faraday cups, which are used in the instruments.