ELECTROCHEMICAL PULVERIZATION OF BULK METAL FOR
PRODUCING FINE NIOBIUM POWDER
Boyan Yuan1 and Toru H. Okabe2
1: Department of Materials Engineering, The University of Tokyo
2: Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan
The electrochemical pulverization (EP) of bulk niobium was investigated with the
purpose of developing a new process for producing fine niobium powder for use in electronic
devices. A niobium rod (anode) produced by aluminothermic reduction (ATR-Nb) was
immersed in NaCl-KCl-MgCl2-DyCl2 molten salt at 1000 K, and it was electrochemically
dissolved in the molten salt from the anode. The dissolved niobium ions were reduced in situ
by Dy2+ ion in the molten salt, and fine niobium powder was successfully obtained. The Dy2+
ion in the molten salt, which acts as the reductant, was regenerated either by electrochemical
reactions on the cathode or by magnesiothermic reduction. Niobium powder with an average
particle size of approximately 1 m and a narrow particle size distribution (D10 = 1.0 m, D50
= 1.1 m, D90 = 1.2 m) was obtained under a specific condition. The electrochemical
properties of the molten salt were studied, and the behavior of impurities during the
electrochemical pulverization was analyzed. The optimum parameters for producing
high-purity powder with a controlled morphology are currently under investigation.
1 Background
The recent trend of increasing miniaturization of electronic devices has accelerated the
demand for high-performance tantalum (Ta) capacitors. Fine, high-purity Ta powder is
essential for such capacitors. Currently, it is produced by the sodiothermic reduction of
potassium heptafluorotantalate (K2TaF7); this process is known as the Hunter process as
illustrated in Fig. 1 (a). Limited resources and high cost of Ta powder have motivated
significant attempts to commercialize new inexpensive capacitors that use niobium (Nb)
instead of Ta. In recent years, research on Nb powder production processes, particularly direct
oxide reduction processes, have attracted considerable interest (Fig. 1 (b-d)) for the
development of Nb capacitors. However, only a few of these processes have gained practical
industrial significance. Considering this situation, a new electrochemical pulverization (EP)
technique of bulk Nb (Fig. 1 (e)) was developed with the objective of developing a new
production process for Nb powder [1]. This process was proved to be feasible for producing
fine and homogeneous powder.
(a)
Stirrer
Liquid Na
feeding port
K2TaF7 powder
feeding port
(b)
Current
Nb2O5 monitor
(cathode)
A
e-
(c)
Graphite
(anode)
Cl2(-COx) (g)
e-
[Ca2+]
Na (l)
Ta (s) [NaF]
[K2TaF7]
[O2-]
Ta Reactor Electric
powder
furnace
(d)
Nb2O5 (s,l)
Nb (s)
Mg (g)
MgO (s,l)
C
[Cl-]
([O2-])
[Ca2+]
Nb2O5
powder
Ca-X liquid alloy
CaCl2 (-CaO) molten salt
Mg shot Mg vapor
Ar gas
(e)
Nb2O5
pellet
(cathode)
Graphite
(anode)
e-
e-
COx (g)
C
[O2-]
Nb rod
(anode)
e-
Molten salt
containing
Dy2+ ion
[Nbn+]
[Dy2+]
Nb (s)
[Dy3+]
e-
Mg-Ag
liquid alloy
(cathode)
CaCl2-NaCl molten salt
Mild steel reactor
Nb powder
Figure 1 Comparison of the new electrochemical
pulverization (EP) technique with the Hunter process
and other methods.
(a) Sodiothermic reduction of K2TaF7
(the Hunter process);
(b) Electronically mediated reaction/
molten salt electrolysis (EMR/MSE);
(c) Magnesium vapor reduction;
(d) Electrochemical de-oxidation;
(e) Electrochemical pulverization (EP), (this study).
1
2 Electrochemical pulverization of a Nb rod for producing fine Nb powder
The experimental apparatus for the EP technique is shown in Fig. 2 (a). Cyclic
voltammetry (CV) measurement was conducted using the electrodes, e.g., the glassy carbon
electrodes, which were immersed in the preliminary molten salt (NaCl-32 mol%KCl-11
mol%MgCl2). After the CV measurement, a stainless steel holder containing Dy metal and Ag
shot was immersed into the molten salt, and Dy2+ ion was generated in situ according to the
following reaction: MgCl2 + Dy + Ag → Mg-Ag + DyCl2, and was supplied to the
preliminary molten salt. After confirming the Dy2+ ion generation and Mg-Ag liquid alloy
synthesis by CV analysis, a constant current of 2 A was applied between the Nb rod (anode)
and Mg-Ag alloy (cathode) to feed Nbn+ ions into the molten salt. Cyclic voltammograms of
the molten salt containing Dy2+ ion (NaCl-32 mol%KCl-10 mol%MgCl2-1 mol%DyCl2),
shown in Fig. 3, indicate that the peak couple A/A’ is due to the reactions of deposition and
dissolution of Mg, peak B is caused by the reaction of chlorine evolution, and the redox peak
couples C/C’ and D/D’ can be attributed to the redox reactions of Dyn+ ions because these
peaks are not detected by CV using the same electrodes in the molten salt before Dy2+ ion
addition. The electrochemically dissolved Nbn+ ions were reduced in situ by the Dy2+ ion in
the molten salt, and fine Nb powder was recovered in the collecting dish (see Fig. 2 (b)). Pure
Nb powder was obtained after removing the salt by leaching with acid followed by rinsing
with water and acetone.
0.8
e-
A
Electrochemical interface
Thermocouple
Ar gas inlet
Stainless steel tube current lead
Rubber plug
Glassy carbon electrode (WE, or CE)
0.4
0
D’
C’
-0.4
A’
-0.8
Nb rod (anode)
A
0.4
D
C
0
-0.4
A’
C’ D’
-0.8
-0.4 -0.2 0 0.2 0.4
Potential, E / V vs.Mg-Ag liquid alloy
0
1
2
Potential, E / V vs. Mg-Ag liquid alloy
Ni electrode (RE)
3
Figure 3 Cyclic votammograms of glassy carbon
electrode in the molten salt: NaCl-32 mol%KCl-10
mol%MgCl2-1 mol%DyCl2. Counter electrode:
graphite. Scanning rate: 20 mV / s.
Mg-Ag liquid alloy (RE, or cathode)
➾
Dissolved part of Nb rod
Molten salt:
NaCl-32 mol%KCl-10 mol%MgCl2-1 mol%DyCl2
B
D
C
Current, i / A‧cm-2
Ni wire potential lead
Current, i / A‧cm-2
e-
➾
➾
(a)
(a)
Stainless steel powder collecting dish
Ceramic insulator
(b)
Figure 2 (a) Schematic illustration of the experimental apparatus for
carrying out the electrochemical pulverization (EP) technique.
(b) Representative image of the powder collecting dish after experiment.
(c)
Frequency, F (%)
10 mm
10
8
6
4
2
0
0.1
Intensity, I (a.u.)
(b)
1
10
Particle size, d / m
100
80
60
40
20
0
100
Cumulative
percentage, C (%)
1 m
Niobium powder deposit with salt
:
Fig. 4 shows the scanning electron micrograph,
particle size distribution profile and X-ray diffraction
pattern of the Nb powder obtained in this study. Nb
20
40
60
80
100
Angle, 2 (deg.)
powder with an average particle size of approximately
Figure 4 Characterization results of the niobium
powder produced by the electrochemical
1 m with a narrow particle size distribution (D 10 =
pulverization (EP) technique. (a) XRD pattern,
(b) SEM image, (c) particle size distribution profile.
1.0 m, D50 = 1.1 m, D90 = 1.2 m) was successfully
obtained using the electrochemical method. These results demonstrate that the
electrochemical pulverization technique is effective in producing fine and homogeneous Nb
powder directly from bulk metal.
Nb JCPDS #34-0370
Reference
[1] Boyan Yuan and Toru H. Okabe: Proceedings of First Asian and Ninth China-Japan Bilateral
Conference on Molten Salt Chemistry and Technology, Wuhu, Anhui, China, (2005), 129–132. 2