Creation of thin oxide coatings and oxide nanopowders by anodic oxidation of metals in molten salts

Article abstract of DOI:10.1134/S0036023608040098

We studied the feasibility of synthesizing ultrafine oxide powders by anodic oxidation of metals, such as zirconium and tantalum, in chloride + nitrate melts at temperatures above 830K. We showed that, varying the electrolyte composition, oxidation temper

Full text of DOI:10.1134/S0036023608040098

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 4, pp. 539–544. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © L.A. Elshina, V.Ya. Kudyakov, V.B. Malkov, N.G. Molchanova, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 4, pp. 594–600.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Creation of Thin Oxide Coatings and Oxide Nanopowders
by Anodic Oxidation of Metals in Molten Salts
L. A. Elshina, V. Ya. Kudyakov, V. B. Malkov, and N. G. Molchanova
Institute of High-Temperature Electrochemistry, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Received December 15, 2006
Abstract—We studied the feasibility of synthesizing ultrafine oxide powders by anodic oxidation of metals,
such as zirconium and tantalum, in chloride + nitrate melts at temperatures above 830 K. We showed that, vary-
ing the electrolyte composition, oxidation temperature, and anodic current density, one obtains either compact
protective coatings on the specified metals or oxide powders with particle sizes of 50 to 200 nm.
DOI: 10.1134/S0036023608040098
Manufacturing of thin oxide films on metals and ties and for a high demand in zirconia (the major com-
oxide powders with ultrafine grain sizes and associated ponent for the synthesis of the electrolyte for solid
high specific surface areas is an important problem of oxide fuel cells) and in tantalum pentoxide (for manu-
materials science. Several processes exist for manufac- facturing condensers with high specific capacities).
turing such powders with set grain sizes, including Iodide zirconium (99.99%) and tantalum (99.00%)
electrochemical ones. However, the implementation of were used in oxidation. Zirconium and tantalum sam-
these processes either is difficult or requires high power ples were in the form of cylinders with surface areas of
2
supply or additional stages (the preparation of precursors
and the separation of final products). Several studies con-
cerned the manufacture of oxide nanopowders from mol-
ten salts [1, 2]. But the processes described there can
only be implemented in an electrical discharge; more-
over, they only produce very small amounts of oxide
nanopowders or require some complicated preliminary
operations, such as oxychloride synthesis.
~
3.5 cm . The electrolyte was prepared from chemi-
cally pure salts. Experiments were carried out in a
three-electrode high-temperature quartz cell with an
encapsulated silver/silver chloride reference electrode,
which was connected to a working electrode through a
porous asbestos screen; a platinum wire was an auxiliary
electrode. A fused and finely ground cesium chloride +
sodium chloride eutectic mixture with 0.1–30 wt %
In this context, a good alternative to the aforemen- sodium nitrate (chemically pure grade) was placed into
tioned methods is the direct one-pot synthesis of thin a quartz tube. Anodic polarization in the potentiostatic
oxide films and oxide nanopowders by reacting metals mode was carried out using a PI-50-1 potentiostat.
that have high oxygen affinities with chloride melts Potentials were changed in 10-mV steps for 10−30 min
doped with an oxygen-containing anion, in particular, until they acquired a constant current value.
nitrate anion.
After operations in melt were performed, a cooled
This work studies whether it is feasible to prepare salt melt was dissolved in distilled water; the precipitate
tantalum pentoxide and zirconia with particle sizes was filtered and dried. The filtrate was analyzed on an
within 70 nm and to create thin oxide films by oxidation Optima 4300 DV spectrometer. The surfaces of zirco-
of zirconium and tantalum in cesium chloride + sodium nium and tantalum samples after oxidation and the
chloride eutectic melts containing 0.1 to 30 wt % powders isolated from solution were studied with a Jeol
sodium nitrate under an argon atmosphere. Metal oxi- SM 5900 LV scanning electron microscope, a Camebax
dation in oxygen-containing aqueous solutions or salt electron probe microanalyzer, and a DRON-3 X-ray
melts remains one of the simplest and efficient methods diffraction setup.
for synthesizing oxides on the surfaces of various metal
products and billets. Multiple studies in this field dem-
onstrated that protection against atmospheric corrosion
RESULTS AND DISCUSSION
was provided by surface passivation and high-tempera-
ture oxidation.
Oxide synthesis is now carried out in two routes: to
manufacture ultrathin films and ultrafine powders. Zir-
conium and tantalum metal samples are coated with
oxide layers even during currentless exposure to
cesium chloride + sodium chloride eutectic melts con-
EXPERIMENTAL
We chose to study zirconium and tantalum for their taining 10 wt % NaNO
. The higher the nitrate ion con-
3
rather high oxygen affinities and high thermal stabili- centration in the melt, the more rapidly metal corrosion
5
39

5
40
ELSHINA et al.
E, V
within 2 h. A rise in the oxidation temperature shifts the
tantalum potential toward positive values for all sodium
nitrate additions. Interestingly, the acquisition of the
corrosion potential at 970 K and 1.0 wt % NaNO3
occurs in steps because of the poor adhesion of tanta-
lum oxide flakes to the metal substrate: flakes at times
0
.4
.2
0
0
5
0
100 150 200 250 300 350 400 fall into the melt (Fig. 1). At higher or lower sodium nitrite
τ, min concentrations in chloride melts, the potential of oxidized
–
0.2
0.4
1
tantalum is acquired more smoothly, without jumps, and
reaches constant values more rapidly. The increasing
sodium nitrate concentrations in the salt melt shift the cor-
rosion potentials of the tantalum electrode into the anodic
range (Table 1). There are indications of several potential
2
3
–
Fig. 1. Tantalum corrosion potentials vs. time in NaCl +
CsCl + NaNO3 melts containing (1) 0.10, (2) 10, and
plateaus, which systematically fall between the potential
values for 0.1 and 10.0 wt % NaNO3.
(3) 1 wt % NaNO at T = 970 K.
3
Anodic polarization curves for tantalum are passiva-
tion curves, with a passivation range of 300−400 mV and
2
i, mÄ / cm
2
low passivation current densities (0.03–0.30 mA/cm )
1
4
(Fig. 2).
1
2
3
1
2
The surface passivation of tantalum by solid corro-
sion products occurs even when the melt contains
.1 wt % NaNO , producing thin (up to 1 µm thick)
1
0
8
6
4
2
0
0
3
black layers of tantalum pentoxide. These films are
very dense and keep tantalum from further dissolution.
An increase in the sodium nitrate concentration leads to
white loose deposits of tantalum oxide and sodium tan-
talate. Nitrate ions can reduce to various oxidation
states (0, +2, or +4) in the reaction of tantalum with
sodium (or cesium) nitrate, yielding tantalum oxide or
tantalates. However, reactions with nitrogen or nitrogen
oxide evolution are more likely; nitrogen dioxide
almost is not expelled under steady-state conditions dur-
ing exposure of tantalum and zirconium to chloride +
nitrate melts.
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
E, V
Fig. 2. Anodic polarization curves on tantalum in CsCl +
NaCl + 10 wt % NaNO melts at í = (1) 870, (2) 810,
and (3) 970 K.
3
We used X-ray powder diffraction and electron
probe microanalysis to characterize the solid products
potentials are acquired. We relate this tendency to the
appearance of oxide layers on the metal surface. For
of tantalum corrosion in NaCl + CsCl +NaNO melts.
3
We showed that the anodic polarization of tantalum at
example, the tantalum corrosion potential in CsCl + 810 K in melts containing NaNO produces β-Ta O ;
3
2
5
NaCl melts containing 0.1–1.0 wt % NaNO was when the oxidation temperature rises to 970 K, sodium
3
acquired within 5 h; in the melt containing 10.0 wt %, tantalate (Na
TaO ) layers are formed. Ultrafine pow-
3
2
Table 1. Tantalum corrosion parameters in CsCl + NaCl + NaNO melts
3
icorr, A/cm2
V, g/(cm h)
2
ipass, mA/cm2
NaNO wt %
T, K
Ecorr, V
3
0
1
.1
810
–0.407
–0.455
–0.263
–0.087
0.420
0.322
0.430
0.434
0.211
4.6 × 10–5
8.7 × 10–5
6.3 × 10–5
3.1 × 10–5
0.7 × 10–5
2.3 × 10–5
0.5 × 10–5
1.3 × 10–5
11 × 10–5
1.2 × 10–3
2.33 × 10–3
0.9 × 10–3
2.0 × 10–3
1.7 × 10–3
2.7 × 10–3
0.49 × 10–2
1 × 10–2
0.28
0.4
8
9
70
70
0.24
0.17
0.10
0.23
0.03
0.16
0.9
.0
810
8
9
70
70
1
0.0
810
8
9
70
70
0.14
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

CREATION OF THIN OXIDE COATINGS AND OXIDE NANOPOWDERS
541
2
00 nm in size) exist; their fraction is not high. The tan-
talum oxide crystals prepared by anodic oxidation in
chloride + nitrate melts are well faceted and have near-
cubic shapes.
Inasmuch as tantalum oxide has a mixed conductiv-
ity, including oxygen-ion conductivity, the through oxi-
dation of metal can be carried out under current to yield
pure tantalum oxide (Fig. 4). The through tantalum oxi-
dation is also made possible by the discontinuity of the
oxide layer generated in an excess sodium nitrate; the
layer consists of rather large oxide flakes, is defective,
and cannot inhibit further penetration of oxygen to the
metal.
0
.2 µm
The zirconium corrosion potentials in CsCl + NaCl +
(
‡)
1
wt % NaNO melts also shift toward positive values
3
as temperature rises, which is characteristic of an indif-
ferent electrode. As the sodium nitrate concentration in
the melt increases to 4.0–10.0 wt %, the potential shifts
in the negative direction, as for a second-type electrode.
Figure 5 shows the zirconium potential as a function
of oxidation temperature. When the chloride melt is
doped by 1.0 wt % NaNO , the potential shifts in the
3
positive direction with rising temperature because of
the deposition of black dense zirconia layers with good
adhesion to the metal substrate. The protective proper-
ties of these layers improve with rising deposition tem-
perature. The corrosion potential is determined only by
the activity of adsorbate oxygen on the metal surface at
the oxygen partial pressure, which is controlled by the
thermolysis of sodium nitrate; i.e., zirconium behaves
as an indifferent electrode, which serves as an electron
conductor and does not participate in the electrochemi-
0
.1 µm
(
b)
Fig. 3. Electron micrographs of a crystalline nanopowder
after filtration from a cooled salt mass NaCl + CsCl +
cal process. Addition of 4.0 or 10.0 wt % to NaNO the
NaNO : (a) Ta O and (b) NaTaO .
3
3
2
5
3
chloride melt shifts the corrosion potential toward neg-
ative values, probably because of the deposition of
loose white oxide layers not adhering to the metal sub-
strate. The oxide electrode potential is determined by
dered tantalum pentoxide falls into the melt (Fig. 3).
The average particle size of powdered tantalum pentox-
ide is 50–100 nm. However, coarser crystals (up to the metal activity in the oxide phase [3].
Ecorr, V
–
–
–
–
–
–
–
0.6
0.8
1.0
1.2
1.4
1.6
E = 0.003T – 3.8273
1
2
3
E = –0.0008T – 0.6261
E = –0.0029T + 1.0831
1.8
8
00 820 840 860 880 900 920 940 960 980 1000
T, K
5
µm
Fig. 5. Zirconium corrosion potential vs. temperature in
CsCl + NaCl melts containing (1) 1, (2) 4, and (3) 10 wt %
NaNO3.
Fig. 4. Electron micrograph of a tantalum sample fully oxi-
dized at 970 K.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

5
42
lni [A/cm ]
ELSHINA et al.
2
The zirconium ion yield into the melt decreases sub-
1
2
3
–
–
–
–
–
–
–
–
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
stantially (about twofold) as the oxidation temperature
in the CsCl + NaCl + 1.0 wt % NaNO rises from 813
3
to 973 K; this agrees with our suggestion that a dense
protective film is formed at high temperatures on the
zirconium surface. This is also confirmed by gravimet-
ric analysis, according to which the zirconium corro-
sion rate decreases by one order of magnitude in
response to a 160-K rise in the oxidation temperature.
The altered morphology (loosening) of the oxide film in
response to the increase in the sodium nitrate concen-
tration to 4.0 wt % indicates that the zirconium corro-
sion rate increases considerably (by one to two orders
of magnitude), as made evident by gravimetric and
chemical analyses. The oxide film formed in an excess
of oxygen is not dense and does not keep nitrate ions
from approaching the metal surface and zirconium
under the oxide film from dissolving. The same is
0
0.2
0.4
0.6
0.8
1
E, V
Fig. 6. Anodic polarization curves on zirconium in CsCl +
NaCl eutectic melts containing (1) 1, (2) 4, and 3) 10 wt %
NaNO at 903 K.
3
Chemical analysis does not show tantalum ions in the observed for addition of 10.0 wt % NaNO to the melt
3
salt melt; all tantalum is firmly bound into the oxide film. (Table 2).
At the same time, considerable amounts of nitrate ions are
There are short anodic dissolution segments on the
determined; e.g., in a melt containing 5 g NaNO at 870 K,
3
zirconium polarization curves; starting with potentials
there is 0.110 g nitrate ions. The absence of tantalum ions
in the salt melt is due to the oxide film on its surface; this
was verified by X-ray powder diffraction and X-ray pho- rate of a zirconium anode in the CsCl + NaCl + 1.0 wt %
toelectron spectroscopy.
of +0.5 V, a constant current density equal to
2
3
.10−3.45 mA/cm is acquired (Fig. 6). The corrosion
NaNO eutectic melt calculated from anodic polariza-
3
tion curves decreases as the oxidation temperature rises
from 813 to 973 K in agreement with gravimetric and
chemical analysis data (Table 3). A further rise in tem-
Zirconium ion yields are rather considerable (up to
.6 mg/(h cm )), probably because of an insufficient
adhesion of the zirconia film to the metal substrate. perature to 1023–1073 K gives a dramatic rise to the
Sodium or cesium zirconate formation was not corrosion rate; we assign this rise to the appearance of
observed because we recorded X-ray powder diffrac- pitting on the zirconium anode surface.
2
0
tion patterns after samples were washed with distilled
It is only at 903 and 973 K that the zirconium anodic
water. This fact is explained as follows: the stability of
polarization curve in the CsCl + NaCl + 10.0 wt %
zirconates decreases in the order Li ZrO –Na ZrO –
2
3
2
3
NaNO is a passivation curve without an active dissolu-
3
K Zr O [4].
2
3
7
tion segment and with the passivation current density
Table 2. Comparison of the results of chemical and gravi- Table 3. Corrosion and electrochemical parameters of zir-
metric analyses
conium in CsCl + NaCl + NaNO melts
3
–
2
NaNO3
icorr, A/cm2 Vcorr, g/(cm h)
2
Zr4+
concentra-
tion, %
NO
Vcorr
,
T, K
Ecorr, V
NaNO3
T, K
wt %
2
wt %
concentra-
tion, %
g/(cm h)
5.815 × 10–3
5.37 × 10–3
6.14 × 10–3
0.47 × 10–3
1
813
–1.376
–1.279
–0.784
–1.217
–1.087
–1.288
–1.341
–1.467
–1.250
–1.206
–1.433
–1.673
9
9
03
73
34 × 10–3
4.5 × 10–3
1
813
0.03545
0.021
0.018
–
0.6575
1.08
0.87
0.53
1.54
1.96
1.66
1.51
5.355 × 10–3
9
9
03
73
–7.5 × 10–4
1023
1073
813
16.3 × 10–3 –0.82 × 10–3
16.5 × 10–3
0.585 × 10–3
1.02 × 10–3
1.56 × 10–3
0.35 × 10–2
0.82 × 10–2
5.1 × 10–4
4
4
0
813
03
813
5.9 × 10–3
9
03
973
813
2.1 × 10–3 0.795 × 10–3
9
0.25
3.8 × 10–2
7.26 × 10–4
4.077 × 10–4
3.12 × 10–4
1.87 × 10–2
4.18 × 10–2
0.86 × 10–2
4.2 × 10–2
10
1
0.43
8
9
9
63
03
73
8
9
63
03
0.061
0.061
1.3 × 10–2
4.2 × 10–3
2.17 × 10–4 –0.226 × 10–2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

CREATION OF THIN OXIDE COATINGS AND OXIDE NANOPOWDERS
543
1
0 µm
5 µm
Fig. 7. Zirconium anode surface after 5-h exposure to a
Fig. 8. Zirconium anode surface after 5-h exposure to a
CsCl + NaCl eutectic + 1 % NaNO melt at 810 K (black
CsCl + NaCl eutectic + 10 % NaNO melt at 870 K (white
3
3
oxide).
oxide).
2
equal to 0.55 and 0.17 mA/cm , respectively; the extent Large amounts of grayish brown NO gas are expelled
2
of the passivation plateau is 300 mV at 903 K and more during an experiment.
than 1000 mV at 973 K. Anodic oxidation in the melt
Removal of nitrite ions from the near-electrode
containing 4.0 or 10.0 wt % NaNO at 973 K consider-
3
layer enhances the appearance of dense, well adhering
anodic oxide layers on the zirconium surface. Overlay-
ably shifts the potentials acquired after polarization
toward positive values (200–300 mV). This can be due
ing of the oxide film continues during the entire anodic
to the structural alterations that occur in the oxide at
polarization period.
this temperature.
Although the anodic polarization curves on zirco-
Zirconium oxidation yields oxide films of various nium are not typical passivation curves, the passivation
colors, from black to white, with various corrosion currents established under all conditions are comparable
properties.
to the values obtained during zirconium polarization in
carbonate melts [5] but are one order of magnitude
smaller than the zirconium passivation currents in chlo-
ride + peroxide melts [6]. However, there is a common
tendency for zirconium oxidation in chloride + nitrate
and chloride + hydroxide melts: passivation currents
decrease as the oxidation temperature increases to 1073–
Varied properties of oxide films are related to their
deviation from the ZrO stoichiometry. A white oxide
2
film strictly corresponds to zirconium dioxide in its
chemical composition; it loses protective properties
during exposure to a greater degree than a black (oxy-
gen-deficient) film. From this and because of the spe-
cifics of the anion-deficient structure of the oxide, a
1123 K; we assign this tendency to the formation of
denser oxide layers with good adhesion. Zirconium oxi-
dation in carbonate melts shows an opposite tendency.
metastable ZrO phase has a higher corrosion resis-
2
tance. This oxide phase is nonstoichiometric zirconia.
This phase coats the metal surface with a dense layer,
insulating it from direct contact with the electrolyte,
but does not interrupt the electrochemical reaction. An
oxide phase with an anion-deficient structure and
compositionally close to zirconium dioxide is a semi-
Vigorous evolution of grayish brown nitrogen dioxide
accompanies the anodic polarization of zirconium,
Because the oxide layers deposited during the currentless
exposure of zirconium to CsCl + NaCl + 4.0 (10.0) wt %
NaNO melts are loose and do not hinder the diffusion
3
2–
conductor, which has ionic and electronic conductivi- of oxygen ion O through the oxide film, the zirconia
ties [4].
formed on the metal surface can reach considerable
thicknesses.
Oxide layers capable of adsorbing oxygen from the
X-ray powder diffraction shows that the black layers
gas space of the cell are formed on a zirconium elec-
trode while it is exposed to a CsCl + NaCl + NaNO3
melt until acquiring a steady state. During anodic polar-
ization nitrite and nitrate ions discharge on the zirco-
nium electrode coated with an oxide film:
(
about 1 µm thick) generated by the reaction of zirco-
nium with CsCl + NaCl eutectic + 1.0 wt % NaNO3
melts are mixtures of tetragonal and monoclinic ZrO2
(
Fig. 7). Their black color indicates a possible off-sto-
ichiometry (oxygen deficit) of zirconia. The white lay-
ers formed in CsCl + NaCl eutectic melts doped with
–
2
NO – e
NO2,
1
2
4.0 (10.0) wt % NaNO consist of pure baddeleyite (m-
3
–
3
NO – e
NO + --O .
2
2
ZrO ) and have thicknesses of 0.07–0.12 µm (Fig. 8).
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

5
44
ELSHINA et al.
Thus, the oxide film that appears in the initial
moment of tantalum and zirconium oxidation in chlo-
ride + nitrate melts retards the process but does not
interrupt ion exchange at the metal–melt interface. In
the progress of oxide formation as the corrosion
becomes a steady-state process, the oxidation rate is
controlled by oxygen ion diffusion from the film sur-
face into the bulk of the metal. Tantalum and zirconium
oxidation yields oxide films of various colors (ranging
from black to white), which have different corrosion
properties.
The thinnest zirconia and tantalum pentoxide films,
which have high protective properties, were obtained
during anodic polarization of the metals. Considerable
amounts of oxide powders with grain sizes from 50 to
0
.2 µm
(
‡)
2
00 nm can pile up in the melt.
To summarize, the oxidation of tantalum and zirco-
nium in a molten eutectic mixture of cesium and
sodium chlorides containing 0.1 to 30.0 wt % NaNO is
3
efficient for manufacturing thin films and nanopowders
of tantalum pentoxide, sodium tantalate, and zirconia.
REFERENCES
1
2
3
4
. P. Afanasiev and C. Geantet, Coord. Chem. Rev., Nos.
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1
. E. G. Gillan and R. B. Kaner, J. Mater. Chem. 11, 1951
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0
.2 µm
(
b)
. M. V. Smirnov, Electrode Potentials in Molten Chlorides
(Nauka, Moscow, 1973) [in Russian].
Fig. 9. Electron micrographs of zirconia powders filtered
off a chloride + nitrate salt melt: (a) faceted crystals and (b)
a column of large crystals.
. B. G. Parfenov, V. V. Gerasimov, and G. I. Venediktova,
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(
Fig. 9). Zirconia grain sizes average 70–100 nm. Com-
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. I. N. Ozeryanaya, I. V. Zauzolkov, N. D. Shamanova, and
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008