Electrode reactions on synthetic diamond and cubic boron nitride in ionic melts

Article abstract of DOI:10.1023/A:1020346109600

The behavior and corrosion resistance of synthetic diamond and cubic boron nitride in chloride, chloride-oxide, and oxide salt melts at 750-900°C were studied by potentiometric and voltammetric methods.

Full text of DOI:10.1023/A:1020346109600

Russian Journal of Applied Chemistry, Vol. 75, No. 5, 2002, pp. 733 739. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 5,
002, pp. 752 758.
Original Russian Text Copyright
2
2002 by Novoselova, Gab.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrode Reactions on Synthetic Diamond and Cubic Boron
1
Nitride in Ionic Melts
I. A. Novoselova and A. I. Gab
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,
Kiev, Ukraine
Received November 13, 2001
Abstract The behavior and corrosion resistance of synthetic diamond and cubic boron nitride in chloride,
chloride oxide, and oxide salt melts at 750 900 C were studied by potentiometric and voltammetric methods.
Considerable recent attention of electrochemists
has been focused on diamond as a new electrode
material whose behavior in aggresive media is charac-
terized by unique chemical, physical, and electro-
chemical features [1, 2]. All the studies were carried
out in aqueous and organic electrolytes, mainly with
film diamond and diamond-like electrodes doped and
modified with various dopants (B, N, Pt). No such
researches were made in molten media because of dif-
ficulties in designing a diamond electrode suitable for
the operation in salt melts at high temperatures. Fur-
thermore, diamond materials were used as inert elec-
trodes for the implementation of processes that are
difficult to realize on usual electrodes such as plati-
num or glassy carbon (for example, electrolytic reduc-
tion of nitrate ions to ammonia [3]).
EXPERIMENTAL
In this work we used dielectric powders of single
crystals of AS15 and ASS diamonds (grain size
50/500, 315/250, and 160/125 m) and of KR
6
cubic boron nitride c-BN (grain size 200/160 and
1
60/125 m), and also compact c-BN pelletized in2
disks of diameter 6.45 mm and thickness 0.66 mm.
Synthetic diamonds were synthesized from the sys-
tem Ni Mn C under static conditions and purified to
remove the residual melt-solvent by acid methods
(
boiling with K Cr O H SO ). Cubic boron nitride
2 2 7 2 4
of KR grade was obtained from hexagonal boron ni-
tride at high pressures and temperatures in the field
of thermodynamic stability of the cubic modification
using magnesium as a process stimulant. To isolate
the cubic modification of boron nitride from the syn-
thesis products, we treated them with dilute solutions
of hydrochloric acid and with a mixture of molten
potassium and sodium hydroxides. The content of the
cubic modification in thus isolated c-BN crystals
was 99% [9].
This work is concerned with deposition of high-
melting metals on grains of dielectrics. Plating of
grains of superhard materials by high-melting metals
is one of the most radical ways for improving the per-
formance of abrasive tools [4]. This problem was first
solved for diamond [5]. To explain the appearance of
the electrode function of dielectric diamonds in salt
melts, the concept of surface conductivity arising from
redox reactions on the diamond ionic melt interfaces
was offered [6, 7]. Having extended the range of sup-
ports, we were able to metal-plate cubic boron nitride,
which is the closest structural analog of diamond. In
was also shown [8] that these two dielectrics exhibit
a similar electrochemical behavior in salt melts. This
work summarizes earlier data and new experimental
data on the electrochemical behavior of diamond and
cubic boron nitride (c-BN).
Pelletized compact c-BN was obtained by solid-
phase polymorphic transition of hexagonal graphite-
like form of boron nitride at high pressures (about
8
.5 GPa) and temperatures (1573 2273 K).
The impurity content of the samples under study is
given in Table 1.
We determined the dc resistivity of natural dia-
mond and compact c-BN at 1023 K with an R5083
bridge. The resulting values are given below:
Material Natural diamond AS15 (500/400)
c-BN
7.2 108 2.7 106 [10] 4.7 10
2
,
m
1
2
Reported at the X Kola Seminar Electrochemistry of Rare
Metals, Apatity, December 4 7, 2000.
The samples were prepared in the Institute of Superhard
Materials, National Academy of Science of Ukraine.
1
070-4272/02/7505-0733$27.00 2002 MAIK Nauka/Interperiodica

7
34
Table 1. Impurity content of diamond and cubic boron nitride
Contents of impurities, wt %
NOVOSELOVA, GAB
Sample
Si
Fe
Mg
Al
Ca
Ti
AC15 (315/250)
KP (200/160)
Blende c-BN
0.05 0.1
0.09
0.02
0.03 0.08
0.02
0.002
0.003 0.005
0.8
0.002 0.003
0.01
0.01 0.02
0.05
0.01
0.004
0.002
0.05
Contents of impurities, wt %
Sample
C
O
B
Ni
Mn
Co
AC15 (315/250)
KP (200/160)
Blende c-BN
0.033 0.040
0.90 2.20
0.80 1.75
0.001 0.002
0.3
0.04
0.20
0.08
The resistivities of diamond and cubic boron ni-
tride at the experimental temperatures allow them to
be classed with dielectric materials.
periment, we controlled the weight and appearance of
powders and made their X-ray phase analysis using
a DRON 3.0 X-ray diffractometer.
Potentiometric and voltammetric mesurements
were carried out in a three-electrode electrochemical
To deposit coatings, we used nickel grids and
current leads to a dielectric material powder and tung-
cell with a PI-50-1.1 potentiostat and a PR-8 program- sten or molybdenum rods of diameter 10 mm, which
mer. Voltammograms were recorded on an LKD 4-003
device at scanning rates from 0.02 to 0.2 V s . The
served as anodes. Metal-plated grains were separated
from the remaining electrolyte by decanting after
30-min boiling in distilled water.
1
electrode potentials were recorded on Shch68003 and
F4800 high-resistance voltmeters. The experiments
were carried out in an electric furnace at 973 1173 K.
The working electrode in a three-electrode cell was
a platinum-grid container filled with a dielectric
powder and immersed in a melt with the aid of a pla-
tinum wire. This electrode was called a combined
platinum dielectric electrode (CPDE), as it contains
two phases: platinum as a conductive phase and dia-
mond or c-BN powder as a dielectric phase. Further
studies have shown that both phases participate in
electrochemical processes.
The thermogravimetric studies were carried out
on a Q-1500D derivatograph, and atomic absorption
analysis, on a PYE UNICAM SP 9 atomic absorption
spectrophotometer. The X-ray emission spectra were
taken on an RSM-500 X-ray ultrawave spectrometer
with oil-free evacuation. The ESR spectra were re-
corded on an SE/X 2547 (Radiopan corporation) ra-
diospectrometer of 3-cm range at room temperature.3
To estimate the chemical resistance of the diamond
and c-BN, we have carried out thermodynamic analy-
sis of possible reactions of these substances with
oxygen-containing compounds of Group IV VI ele-
ments [12]. On this basis, we constructed series of
such compounds in the order of increasing oxidizing
power toward the dielectrics under study (at 1000 K).
For diamond, such a series is as follows: Na CO ,
We used a lead electrode Pb NaCl KCl PbCl in
2
chloride and chloride oxide melts as a reference elec-
trode [11]. A platinum or glassy carbon crucible
served as a container for a melt and simultaneously
as an anode.
2
3
The graphite electrode was a rod of diameter 5 mm
made from spectrally pure graphite. Its upper part,
which extended from a melt, was covered with a
porcelain tube. A platinum wire served as a current
lead. A pellet of cubic boron nitride was immersed
in a melt with the aid of a platinum wire.
Na O, Na WO , Na MoO , Na W O , CO , WO ,
2
2
4
2
4
2
2
7
2
3
Na SO , Na CrO , Na Mo O , MoO , SO , and
2
4
2
4
2
2
7
3
3
CrO3.
3
The X-ray emission spectra were taken in the Institute of
Materials Science Problems, National Academy of Sciences
of Ukraine. The ESR spectra were taken in the Institute of
Superhard Materials, National Academy of Sciences of
Ukraine.
The chemicals in use were of chemically pure and
analytically pure grades. Before and after each ex-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 5 2002

ELECTRODE REACTIONS ON SYNTHETIC DIAMOND
735
0
Table 2. Gibbs energies
G
of reactions of cubic boron nitride with oxygen-containing compounds of Group VI
T
elements
0
G , kJ, at indicated T, K
T
Reaction
9
00 K
1000 K
1100 K
1200 K
8
8
8
8
8
8
BN + 4Na WO = 8NaBO2 + 3N2 + 2W N
333.66
473.08
379.91
601.12
293.59
454.59
362.37
499.82
428.94
648.10
341.79
501.58
380.43
518.90
477.73
694.75
381.23
549.11
401.36
540.99
526.43
741.03
414.36
589.48
2
4
2
BN + 4Na MoO = 8NaBO2 + 3N2 + 2Mo N
2
4
2
BN + 4WO3 = 4B O + 2W N + 3N
2
3
2
2
BN + 4MoO3 = 4B O + 2Mo N + 3N
2
3
2
2
BN + 2Na W O = 4NaBO2 + 2W N + 3N2 + 2B O3
2
2
7
2
2
BN + 2Na Mo O = 4NaBO2 + 2Mo N + 3N2 + 2B O3
2
2
7
2
2
Table 3. Results of corrosion tests of AS-15 (315/250) diamonds in Na WO4 melts at 1173 K
2
10 , g m h 1
1
2
m, %
V
10 , g m 2 h 1
1
m, %
V
10 , g m h 1
1
2
m, %
V
Addition to melt,
0 mol %
at indicated test duration, h
2.0
2
1
.0
5.0
1
1.49
2.47
.39
7.24
7.84
13.01
2.22
2.33
3.62
5.92
6.14
9.53
5.23
4.78
9.75
5.51
4.13
10.27
WO3
MoO3
The series for cubic boron nitride is Na W O ,
tests. Changes in the weight and phase composition
of the powders under study in the system NaCl KCl
(1 : 1) were detected neither in air nor in argon atmos-
phere. Addition of sodium tungstate (or molybdate)
in amounts of up to 50 mol % into this melt results in
minor weight losses m (up to 1% in 2 h). The bub-
2
2 7
Na WO , CO , Na CO , WO , Na MoO , Na SO ,
2
2
4
2
2
2
3
2
3
2
3
4
2
3
4
Na Mo O , MoO , Na CrO , SO , and CrO .
7
3
4
It is evident that the mildest oxidizing agents for
diamond and c-BN are tungstate, ditungstate, sodium
carbonate, and carbon dioxide, i.e., on the one hand,
redox processes in melts containing these compounds
can occur on the surface of the dielectric materials
under study, and, on the other hand, the corrosion of
these latter should not be very strong.
bling of CO through the NaCl KCl melt (1 : 1) does
2
not change the appearance and weight of the dielectric
powders either.
Weight losses m of both diamond and c-BN were
detected in melts of sodium tungstate and oxide mix-
tures based on it (Table 3). No changes in the phase
compositions of the powders under study were found
in the corrosion tests.
It is necessary to note that, according to our ther-
modynamic analysis, the nitrides Mo N and W N of
2
2
high-melting metals can be formed in the reactions of
c-BN with Na MO , Na M O , and MO (M = Mo,
2
4
2
2
7
3
W) along with the high-melting metals and their
dioxides (Table 2). However, our thermogravimeric
study of the behavior of cubic boron nitride in the sys-
tem Na WO WO has not revealed changes in the
It follows from Table 3 that the weight loss of dia-
monds increases with addition of 20% of WO or
3
MoO into an Na WO melt. The corrosion rate V de-
3
2
4
creases as the time of keeping the sample in the melts
increases.
2
4
3
weight and phase composition of c-BN, and by the
data of the atomic absorption analysis this melt con-
tains no boron. Therefore, the occurrence of the reac-
tions listed in Table 2, despite their high negative
Gibbs energies, was not proved experimentally.
Chemical reactions occurring in the melts under
study can be described by the following equations:
Na WO4 + 3C
W + 3CO + Na O,
2
2
We studied corrosion of diamond and c-BN in
halide, halide oxide, and oxide melts during long-run
G = 176.22 kJ;
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 5 2002

736
NOVOSELOVA, GAB
mental data on the chemical behavior of diamonds in
E, V
oxide melts qualitatively give the same results and
demonstrate that the redox reactions on the interface
give rise to an electrochemical equilibrium and to
the appearance of an intrinsic electrolytic potential.
The potentiometric study of the behavior of com-
bined diamond and c-BN electrodes in chloride oxide
systems was carried out by measuring emfs of the fol-
lowing circuits:
Pt,
dielectric
NaCl KCl
(1 : 1)
NaCl KCl(1 : 1)
2.5 mol % PbCl2
Pb,
(1)
c, mol %
Fig. 1. Potential E as a function of concentration c of
1, 2, 6 9) Na WO and (3 5) Na MoO , as measured in
(
2
4
2
4
an NaCl KCl (1 : 1) melt on various electrodes. Electrode:
1 3) combined diamond AS15 (160/125), (2 4) combined
Pt,
NaCl KCl(1 : 1)
NaCl KCl(1 : 1)
Pb,
(
dielectric xNa WO (Na MoO ) 2.5 mol % PbCl2
2
4
2
4
diamond AS15 (315/250), (5, 6) platinum, (7) combined
c-BN KR (250/200), (8) combined c-BN KR (160/125),
and (9) compact c-BN.
(2)
Pt,
dielectric
NaCl KCl(1 : 1)
CO (bubling)
NaCl KCl(1 : 1)
2.5 mol % PbCl2
Pb,
3)
2
2
Na W O + 3C
2W + 3CO2 + 2Na WO ,
2 4
2
2
7
(
G = 84.66 kJ;
Na Mo O + 3C 2Mo + 3CO + 2Na MoO ,
2 2 7
where dielectric is diamond or c-BN.
2
2
2
4
We found that the potential of combined diamond
and c-BN electrodes in a pure chloride melt NaCl
KCl(1 : 1) coincides with the potential of platinum.
Addition of Na WO (or Na MoO ) into the melt
G = 263.73 kJ.
2
4
2
4
A high-melting metal formed on the surface of
diamonds apparently dissolves in oxide melts.
results in the displacement of potentials of the com-
bined electrodes to the negative region (Fig. 1). The
compact c-BN electrode behaves similarly to the
combined c-BN electrode. In this case, the following
relation was observed: the finer the grains of a dielec-
tric material, the greater the deviations of its potential
from the platinum potential. The potentials of all the
combined electrodes under study became more nega-
tive than the potentials of platinum, which served as
a metal container.
Using X-ray emission spectroscopy and electron
spin resonance (ESR), we have studied how the treat-
ment of diamonds in various melts [equimolar mixture
NaCl KCl, equimolar mixture NaCl KCl CO (pres-
2
sure 10 atm), Na WO , and Na WO xWO (MoO )]
2
4
2
4
3
3
at 1023 K affects the surface texture of the diamonds.
The ESR experiments have not revealed any conduc-
tive (metallic) films on the surface of diamonds after
their 3-h keeping in the above melts in the absence of
current. We have shown by X-ray emission spectros-
copy that the treatment with melts results in a rupture
of bond between the surface carbon atoms and atoms
of other elements adsorbed on the diamond surface
during synthesis and subsequent removal of the re-
sidual melt-solvents from the diamonds. In this case,
recombination of C C bonds, reconstruction of the
diamond surface, and filling of the surface carbon
atoms with electrons can take place. Ihm et al. [13],
using the self-consistent pseudopotential method, cal-
culated the electronic structure of (111) diamonds and
drew on this basis a rather interesting conclusion:
an ideal surface of diamonds should have a metallic
conductivity.
Bubbling CO through an NaCl KCl (1 : 1) melt
2
also results in displacement of potentials of graphite,
platinum, and combined platinum dielectric electrodes
to the negative region, the potential of a combined
diamond electrode approaching the potential of the
graphite electrode [7]. When the gas feeding is ter-
minated, the initial potentials of all the electrodes are
restored.
The potentials of combined platinum diamond
electrodes (CPDE) in oxide melts appreciably differ
from the potential of platinum. They are sensitive to
the variations of acid base properties of melts. When
acid components (WO and MoO ) are added, the
3
3
potential of a combined dielectric electrode is shifted
to the positive region, and basic additions (Na CO )
Thus, our thermodynamic analysis and experi-
2
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 5 2002

ELECTRODE REACTIONS ON SYNTHETIC DIAMOND
737
Table 4. Effect of MoO concentration on steady-state potentials E of electrodes in an Na WO melt at 1173 K
3
2
4
E, V, at indicated MoO3 concentration, mol %
Electrode
0
2
4
8
12
16
20
Platinum
0.60
0.97
0.98
1.24
(1.3 1.5)
0.19
0.79
0.74
0.87
1.00
0.16
0.73
.0.71
0.79
0.94
0.06
0.62
0.63
0.65
0.85
0.04
0.62
0.60
0.03
0.61
0.61
0.02
0.56
0.60
CPDE ASS (630/500)
CPDE AS15 (315/250)
Graphite
Molybdenum
0.80
0.76
0.73
*
Potentials are given relative to the reference electrode Na WO 0.2 mol % WO O , Pt.
2 4 3 2
shift it to the negative region of potentials [6]. The
results of our potentiometric study of the behavior of
combined platinum diamond, graphite, and metallic
In this work we have studied the CPDE behavior at
750 C by voltammetry in the following systems:
(Pt and Mo) electrodes in an oxide melt at varied con-
NaCl KCl(1 : 1) 0.5 M Na WO 0.05 0.15 M NaPO ,
2
4
3
centration of molybdenum oxide are given in Table 4.
(4)
Table 4 shows that the CPDE potential in pure
melt of sodium tungstate is more negative that the
potential of the platinum electrode approximately by
NaCl KCl(1 : 1) 0.5 M Na MoO 0.05 0.20 M NaPO .
2
4
3
(5)
400 mV, and this difference increases to 600 mV
when molybdenum oxide is added. At the same time,
the CPDE potential is more positive by 200 mV than
the potential of the graphite electrode in a tungstate
melt, but these potentials become almost equal as
molybdenum oxide is added. The potential of the
molybdenum electrode in all experiments was more
negative than the CPDE potential.
These systems were developed for the electrolytic
deposition of tungsten, molybdenum, and their car-
bides on metallic electrodes [14].
The cyclic I E curves obtained on a platinum elec-
trode and CPDE are given in Fig. 2. The presence of
current maxima in the curves is characteristic of the
processes with a mixed kinetics when the rates of the
charge transfer and mass transfer of ions are com-
parable. This is confirmed by the dependence of
the peak potentials on the potential scanning rate.
It follows from Table 5 that diamond powder AS15
(315/250) behaves as an electroconductive constituent
of an electrode: the cathode current peak Ip(c) in-
creases as compared to the platinum electrode, and its
We can assume that the potential-determining proc-
esses on graphite and combined platinum diamond
electrodes are similar and can be described by the fol-
lowing electrode reaction:
C + O2
C Oads + 2e.
We can write the following expression for the poten-
tial of this electrode:
RT [C Oads
]
2.3RT
2F
I, A
E = E0
+
ln
= E
0
+
pO2 ,
*
2
2
F
[O ]
RT
0
0
where E = E +
ln[C Oads].
*
2
F
Correspondingly, the equation of the electrode
reaction and the potential of the combined c-BN elec-
trode can be presented as [6]
E, V
BN + 2O2
Fig. 2. Voltammograms obtained on (1) platinum electrode
and (2) CPDE in the melt NaCl KCl 0.5 M Na MoO
BO2 + 1/2N2 + 3e,
2
4
1
/2
1
RT [BO ][N ]
0.15 M NaPO at 1023 K. Polarization rate 0.1 V s .
2
2
3
E = E0
+
ln
.
(
I) Current and (E) potential.
2
3
F
[BN][O ]
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 5 2002

738
NOVOSELOVA, GAB
Table 5. Characteristics of cyclic voltammograms obtained on a platinum electrode and on CPDE in the system NaCl
KCl 0.05 M Na MoO 0.15M NaPO
2
4
3
Ep(c)
Ep/2(c)
Ep(a)
Ep(c) Ep(a)
Ep/2(c) Ep(c)
Electrode
Ip(c), A
n
V
Platinum
CPDE
0.215
0.320
1.68
1.90
1.28
1.35
0.36
0.08
1.32
1.82
0.40
0.55
0.408
0.298
Table 6. Coating thickness h and degree of diamond
plating m by carbides of high-melting metals at various
electrolysis durations
The degree of irreversibility of the electrode proc-
ess on a diamond electrode is much greater.
Knowing the specific surface area of diamond
powders in use, we determined the fraction of the sur-
face participating in the electrode process. We found
that this fraction increases with increasing NaPO3
concentration from 0 to 18.10% for system (4) and
from 3.30 to 13.00% for system (5).
m, wt % h,
m
m, wt % h,
m
,
min
W C
Mo C
2
2
3
6
9
0
0
0
0.38
10.56
12.40
0.01
0.30
0.40
0.63
7.62
8.00
0.035
0.44
0.46
The voltammetric study of the CPDE behavior in
systems (4) and (5) allowed us to determine the op-
timal conditions for the electrodeposition of carbides
of high-melting metals (W C, Mo C) on diamond.
potentials are strongly stretched along the potential
axis. More positive peak potentials of anode current
Ep(a), as compared to the platinum electrode, seem to
result from a chemical reaction between a generated
high-melting metal and diamond. Therefore, the prod-
uct formed in the cathode cycle on CPDE (a carbide
phase) is more difficult to dissolve than a high-melt-
ing metal formed on platinum. Apparent charge-
2
2
Electrolyte composition (mol %): NaCl 45.00 47.30,
KCl 45.00 47.30, Na MoO4 (or Na WO ) 5.00
2
2
4
8
1
.00, and NaPO 0.40 2.00. Current density i = (7
3 c
0) 10 3 A cm , temperature 750 800 C. The
2
optimal electrolysis duration, as follows from Table 6,
is 60 min.
Continuous solid coatings of molybdenum carbide
transfer coefficients n for the cathode process, cal- on cubic boron nitride were obtained from the sys-
culated by the equation for irreversible charge transfer tem NaCl LiF(1 : 1) 1.3 mol % Na MoO 1 mol %
2
4
[15], are given in Table 5.
Na CO .
2 3
When this system is used for plating c-BN with
molybdenum carbide, the degree of plating depends
on the cathode current density and the electrolysis
Ep/2(c)
Ep(c) = 1.857 RT/ n F.
i , A cm 2
m, wt %
time (Fig. 3). At T = 1173 K, = 90 min, and i =
c
c
2
0
.02 A cm , we obtained a solid Mo C coating on
2
c-BN with the thickness h 4.80 m.
Continuous and well adhering to the support
coatings of high-melting metals (Mo, W) were ob-
tained on c-BN from the following systems: NaCl
LiF(1 : 1) 2.5 mol % Na MoO at T = 1173 K, i =
2
4
c
2
0
1
.02 A cm ,
= 45 min, m = 14.6 wt %, h =
.02 m; Na WO 1 mol % WO at T = 1173 K, i =
2
4
3
c
,
min
2
0
.20 0.30 A cm , = 90 min, m = 220 wt %, h =
Fig. 3. Degree of c-BN plating m with molybdenum
7.80 m.
carbide as a function of (1) electrolysis duration and
2) cathode current density i measured in the melt NaCl
The optimal systems and conditions for the deposi-
tion of coatings of high-melting metals and their car-
bides on diamond and c-BN are given in Table 7.
(
c
LiF(3 : 1) 1.3 mol % Na MoO 1 mol % Na CO at
1
2
4
2
3
173 K.
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ELECTRODE REACTIONS ON SYNTHETIC DIAMOND
739
Table 7. Optimal conditions for the deposition of coatings of high-melting metals and their carbides with thickness h on
dielectric materials
i ,
A cm
,
h,
m
c
Dielectric Coating
System
T, K
2
min
Diamond
c-BN
Mo C
NaCl KCl (1 : 1) 5 mol % Na MoO 0.75 mol % NaPO
1073
1073
1173
1173
1123
0.075
0.010
0.020
0.020
0.220
60
60
90
45
90
0.46
0.40
4.80
1.02
7.80
2
2
4
3
W C
NaCl KCl (1 : 1) 6 mol % Na WO 0.4 mol % NaPO
2
2 4 3
Mo C
NaCl LiF (3 : 1) 1.3 mol % Na MoO 1 mol % Na CO
2 4 2
2
3
Mo
W
NaCl LiF(3 : 1) 2.5 mol % Na MoO4
2
Na WO 1 mol % WO
2
4
3
CONCLUSIONS
ma, A., J. Electroanal. Chem., 1993, vol. 347, nos. 1
, pp. 409 415.
2
(
1) Potentials of combined dielectric electrodes
4
5
. Paika i metallizatsiya sverkhtverdykh instrumental’-
nykh materialov (Soldering and Metal Plating of
Superhard Tool Materials), Naidich, Yu.V., Ed., Kiev:
Naukova Dumka, 1977.
(
diamond and c-BN) in pure chloride melts coincide
with the potential of platinum. In this case, no weight
and phase changes take place in the dielectrics.
. Shapoval, V.I., Kushkhov, Kh.B., and Malyshev, V.V.,
Poroshk. Metall., 1986, no. 7, pp. 43 45.
(2) When weak oxidizing agents (Na WO ,
2 4
Na MoO , and CO ) are added to a chloride melt,
2
4
2
minor weight losses (up to 1 wt %) of diamond and
c-BN are observed. The potential of a combined plati-
num-dielectric electrode becomes more negative than
the potential of platinum, the deviations of the poten-
tials depending on the grain size. Electroplated coat-
ings of tungsten and molybdenum carbides up to
6. Kushkhov, Kh.B., Shapoval, V.I., and Barabosh-
kin, A.N., Dokl. Akad. Nauk SSSR, 1990, vol. 312,
no. 6, pp. 1405 1408.
7
. Kushkhov, Kh.B., Novoselova, I.A., and Shapo-
val, V.I., Elektrokhimiya, 1993, vol. 29, no. 7,
pp. 904 906.
0
.50 m thick can be obtained from halide oxide
8
. Malyshev, V.V., Novoselova, I.A., Gab, A.I., and
Shapoval, V.I., Poverkhnost’, 1997, no. 9, pp. 70 74.
electrolytes.
(3) Both diamond and c-BN have their interinsic
9. Bogatyreva, G.P., Zusmanov, E.R., Kotova, N.V.,
potentials in purely oxide melts based on sodium
tungstate. These potentials are more negative than the
potential of platinum. The potential of a combined
platinum dielectric electrode depends on acid base
properties of a melt. The corrosion of the dielectrics
is determined by the concentration of acid additions
and corresponds to the series of oxidizing powers con-
structed on the basis of the thermodynamic analysis.
et al., Poverkhnost’, 1997, no. 10, pp. 99 108.
1
0. Aleshin, V.G., Smekhnov, A.A., Bogatyreva, G.P.,
and Kruk, V.B., Khimiya poverkhnosti almaza (Chem-
istry of the Diamond Surface), Kiev: Naukova Dum-
ka, 1990.
1
1
1. Alabyshev, A.F., Lantratov, M.F., and Morachev-
skii, A.G., Reference Electrodes for Fused Salts,
Washington: Sigma, 1965.
2. Malyshev, V.V., Novoselova, I.A., Gab, A.I., et al.,
Teor. Osn. Khim. Tekhnol., 2000, vol. 34, no. 4,
pp. 435 448.
ACKNOWLEDGMENTS
This work was financially supported by the Scien-
tific and Technical Center of Ukraine (project
no. 1622).
1
1
3. Ihm, J., Louie, S.G., and Cohen, M.L., Phys. Rev.
Bull., 1978, no. 2, vol. 17, pp. 769 775.
4. Novoselova, I.A., High-Temperature Electrochemical
Synthesis of Molybdenum and Tungsten Carbides
under Excess Pressure of Carbon Dioxide, Cand. Sci.
Dissertation, Kiev, 1988.
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