1070-4272/02/7505-0733$27.00C2002 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 75, No. 5, 2002, pp. 733!739. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 5,
2002, pp. 752!758.
Original Russian Text Copyright + 2002 by Novoselova, Gab.
AND CORROSION PROTECTION OF METALS
Electrode Reactions on Synthetic Diamond and Cubic Boron
Nitride in Ionic Melts
I. A. Novoselova and A. I. Gab
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,
Received November 13, 2001
Abstract-The behavior and corrosion resistance of synthetic diamond and cubic boron nitride in chloride,
chloride3oxide, and oxide salt melts at 7503900oC 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 ).
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 . This problem was first
solved for diamond . 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 diamond3ionic 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  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).
Reported at the X Kola Seminar [Electrochemistry of Rare
Metals,] Apatity, December 437, 2000.
In this work we used dielectric powders of single
crystals of AS15 and ASS diamonds (grain size
650/500, 315/250, and 160/125 mm) and of KR
cubic boron nitride c-BN (grain size 200/160 and
160/125 mm), and also compact c-BN pelletized in
disks of diameter 6.45 mm and thickness 0.66 mm.
Synthetic diamonds were synthesized from the sys-
tem Ni3Mn3C under static conditions and purified to
remove the residual melt-solvent by acid methods
(boiling with K
). Cubic boron nitride
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% .
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 (157332273 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
r, W m 7.2010
The samples were prepared in the Institute of Superhard
Materials, National Academy of Science of Ukraine.