Journal of Mining and Metallurgy, 39 (1‡2) B (2003) 149 - 166.
ELECTRODEPOSITION OF RHENIUM
FROM CHLORIDE MELTS - ELECTROCHEMICAL
NATURE, STRUCTURE AND APPLIED ASPECTS
O. N. Vinogradov-Zhabrov, L. M. Minchenko,
N. O. Esina and A. A. Pankratov
Institute of High-Temperature Electrochemistry, Ural Branch of the Russian Academy of
Sciences, 20 S. Kovalevskaya St., Ekaterinburg, Russia, 620219
(Received 12 January 2003; accepted 12 March 2003)
Abstract
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
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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.
Keywords: rhenium, electrodeposition, molten salts, structure, kinetics, electroplating
1. 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 0C), the largest strength among all metals at temperatures above 1500 0C, 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.
2. 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.
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Electrodeposition of rhenium from chloride melts .....
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/cm2 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.
3. Results and discussion
3.1. Precipitation of textured deposits
When electrolysis was performed in the KCl-NaCl-ReCl4 melt in a quartz electrolyzer [3] over the temperature interval of 680 to 970 0C at the cathodic current density from
5 to 200 mA/cm2, 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).
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Figure 1. Surface morphology of rhenium deposit with the texture (1120),
magnification - 1600; j=0.1 A/cm2, t=830 °C.
Table 1. Microhardness of electrolytic deposits of rhenium, kg/mm2
Deposition temperature,°C Texture
700
800
900
(1010)
(1120)
(101L)
Cross-microsections
800-900
300-450
300-320
Longitudinal microsections
800-900
300-450
550
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 (O2 , H2O , CO2) 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.
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Electrodeposition of rhenium from chloride melts .....
3.2. 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-ReCl4
melt (CRe+4 = 7 wt. %) at t=750 0C and t=850 0C 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:
Zr + 2O2- → ZrO2 ↓
(1)
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 cm3 of oxygen were added to
the electrolyte (100 g). When extra 20 cm3 of oxygen were added, a few bubbles
appeared on the cathode at i=0.1-0.2 A/cm2. A fine metal powder was detected on the
melt surface. Bubbles appeared on the cathode already at ic=0.05 A/cm2 after further 20
cm3 of oxygen were passed through the melt. The size of the bubbles increased slowly
as current density increased up to 0.10 A/cm2. At ic=0.20 A/cm2 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 ic=0.20 A/cm2.
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:
2Me2ReCl6 +O → ReOCl3 + 4MeCl + ReCl5
2ReO3+ + 6e- → ReO2 + Re
(2)
(3)
where Me is an alkali metal.
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ReO2 was implanted in the deposit at low temperatures and decomposed at high temperatures according to the reaction:
7ReO2 → Re + Re2O7 (bubbles)
(4)
Figure 2. Surface of rhenium deposit with bubbles (oxygen addition in the melt).
Magnification - 200.
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 0C), 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.
3.3. 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
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Electrodeposition of rhenium from chloride melts .....
virtually at once even near the liquidus line (∼800 0C) 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 0C.
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: NaCl → NaCl (50 % M)KCl (50 % M) → CsCl (52 % M)-NaCl (48 % M) → KCl → CsCl (65 % M)-NaCl (35
% M) → CsCl (85 % M)-NaCl (15 % M) → CsCl (95 % M)-NaCl (5 % M)
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.
Figure 3. Temperature dependence of the texture transition:
in argon, ♦- (10 10), n - (11 2 0), 5 - (0001);
- - - - in air, × - (10 1 0), * - (11 2 0), • - (0001).
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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 0C 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 0C and 820 0C 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).
3.4. 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 cm3) 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
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Electrodeposition of rhenium from chloride melts .....
Figure 4. Polarization curve of Re depending on temperature. KCl, polycrystalline
electrode.(1-pure conditions;2-addition of oxygen;3- incomplete conditioning)
Figure 5. Polarization curve of Re depending on the oxygen addition in the melt. KCl,
polycrystalline electrode.(1-pure conditions;2-addition of oxygen;
3-incomplete conditioning)
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Figure 6. Polarization curve of Re on: 1- polycrystalline and 2- monocrystalline
electrodes, KCl.
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 i0 and α⋅Z. The exchange current density
increased on the polycrystalline surface from 0.02 A/cm2 at t=790 0C to 0.05 A/cm2 at
t=830 0C and 0.100 A/cm2 at t=890 0C. The i0 value dropped to 0.02 A/cm2 at t=830 0C
when oxygen was added to the melt. If oxygen-containing impurities were not removed
completely, i0 was 0.036 A/cm2. After the melt was conditioned, the exchange current
density returned to the value before oxygen was added. The i0 value on the single-crystal
surface of the electrode was much smaller (i0=0.011 A/cm2 at t=760 0C and i0=0.022
A/cm2 at t=850 0C) 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 ϕ = ln (1 − t τ ) coordinates
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Electrodeposition of rhenium from chloride melts .....
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 cm2/s at t=790 0C and
D=3.5⋅10−5 cm2/s at t=840 0C. 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].
Figure 7. Polarization curve of Re depending on the electrolyte composition,
t=780 °C, monocrystalline electrode:
1. CsCl(95 % M)-NaCl(5 % M);
2. CsCl(90 % M)-NaCl(10 % M);
3. CsCl(85 % M)-NaCl(15 % M);
4. CsCl(80 % M)-NaCl(20 % M);
5. CsCl.
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3.5. 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).
Figure 8. Rhenium needles on graphite,
magnification - 1300
Figure 9. The top of a rhenium needle,
magnification - 4000.
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
106 needles/cm2. The needle deposits were characterized by a “high perfection” of the
(0001) texture, ST=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
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Electrodeposition of rhenium from chloride melts .....
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].
Figure 10. Rhenium needles growing from
a single center, magnification 400.
Figure 11. Rhenium needles on the poly
crystalline rhenium foil,
magnification - 800.
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 cm2. 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
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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].
3.6. 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 0.02-0.10 A/cm2. 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
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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.
Figure 12. Surface morphology of epitaxial deposits on single crystal rhenium
substrates with different orientations: a-(0001), b-(1120), c-(1010),
magnification - 2000.
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/mm2 that testifies to the
high purity of deposited rhenium.
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Figure 13. Cross-sectional micrographs of rhenium deposit on the polycrystalline
rhenium substrate with (1010) orientation, magnification - 400.
Figure 14. Micrographs of rhenium deposits fractures: (a) monocrystalline rhenium
deposit, magnification - 800; (b) polycrystalline rhenium deposit,
magnification - 800.
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
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Electrodeposition of rhenium from chloride melts .....
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.
References
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