Transcript スライド 1 - University of Tokyo

Niobium Powder Production
in Molten Salt
by Electrochemical Pulverization
Boyan Yuan* and Toru H. Okabe**
*: Graduate Student,
Department of Materials Engineering,
University of Tokyo
**: Associate Professor,
Institute of Industrial Science,
University of Tokyo
1
Fancj
May 20-28,2005
Wuhu,Anhui,China
Niobium and Tantalum
Table Comparison of Nb with Ta
Nb
Atomic number
VB 41
Crystal structure
bcc
Melting point
2468 ˚C
8.56 g/cm3
Density
Dielectric constant 41
of pentoxide
Reserves
4,400,000 ton Nb
Annual
world productivity 23,000 ton Nb
Ta
VB 73
bcc
2980 ˚C
16.65 g/cm3
27
Price
~ 50 $/kg
~ 700 $/kg
Major
applications
Microalloy
element
for steel
Solid
electrolytic
capacitor
Aluminothermic
Commercial
production process reduction
(ATR)
Nb/FeNb bar
Product form
Development
Next
generation
capacitors
43,000 ton Ta
2,300 ton Ta
Sodiothermic
reduction
(Hunter)
Ta powder
Higher
performance
capacitor
Nb, a potential substitute of Ta
for next generation capacitors
2
Hunter process
K2TaF7(l) + 5 Na(l) → Ta(s) + 5 NaF(l) + 2 KF(l)
Diluent
Stirrer
K2TaF7 and Na feeding ports
Reactor
Molten halides diluent
Electric furnace
Tantalum powder deposits
Figure Schematic illustration of the Hunter process.
Features
◎ Well controlled powder purity and morphology
× Batch type process
× Time and labor consuming reduction process
followed by mechanical and hydrometallurgical
separation operations
× Large amount of fluorides wastes
3
Direct reduction processes of oxide
A
(a)
e-
Current monitor
External circuit
Nb2O5 powder
Molten CaCl2
Ca-Ni-Ag liquid alloy
Nb2O5 powder
(b)
Porous Ta plate
Ar gas nozzle
Okabe, et al.
1999
Nb2O5+ 10 e- →
2 Nb + O25 Ca →
5 Ca2+ + 10 eH.C.Starck,
2001
Nb2O5 +5 Mg →
2 Nb + 5 MgO
Mg vapor reductant
Mg chips
(c)
e-
External power source Fray, et al.
2002
Graphite anode
Nb O + 10 e- →
2
Nb2O5 pellet
CaCl2-NaCl molten salt
(d)
5
Nb+ 5 O2C + x O2- →
COx + 2x e-
Preform (Nb2O5 + flux) Okabe, et al.
2003
Stainless steel mesh
Nb2O5 +5 Mg →
2 Nb + 5 MgO
Mg vapor reductant
Mg (or Mg alloy) chips
4
Objectives of this study
To develop a new, low cost, high quality
niobium powder production process for
capacitor or other electronic applications.
Essential process features:
◎ fine and homogeneous niobium powder
have to be obtained.
◎ purity and morphology of the niobium
powder have to be controlled.
〇 the process is required to be low cost
and efficient,
◎ (semi-) continuous,
〇 environmentally sound.
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Electrochemical Pulverization (EP)
Anodic:
→
Nb (bulk, s)
Cathodic: n Dy3+(l) + n e→
Redox
Nbn+ (l) + n Dy2+(l) →
reaction in
molten salt:
Overall Reaction : Nb (bulk, s) →
Nbn+ (l) + n en Dy2+ (l)
Nb (powder, s) + n Dy3+(l)
Nb (powder, s)
e-
e-
Nbn+(l)
Nb(s)
Dy2+(l)
Dy3+(l)
Nb rod feed (anode)
Mg-Ag liquid alloy (cathode)
Molten salt containing Dy2+
Nb powder
Figure Schematic illustration of the configuration of EP.
Features:
◎ Fine powder production by
homogeneous ionic redox reaction.
◎ Purity and morphology can be controlled by:
dissolution speed of Nb bulk,
concentration of Dy2+ ions reductant,
temperature.
◎ Low cost:
cheap ATR Nb ingot feed can be used.
〇 Environmentally sound:
reductant is not consumed,
molten salt can be reused.
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Thermodynamic analysis
(a) Nb-Dy-Cl system
T = 1000 K
Cl2(g)
log pCl2
NbCl5 (l, in salt)
NbCl4 (l, in salt)
NbCl3 (l, in salt)
NbCl2 (l, in salt)
DyCl3
(l, in salt)
-40
a
Nb (s)
b
-60
-60
Dy (s)
-60
-40
-40
-20
-20
0
(b)
In molten salt
Dy2+
Dy3+
eb
Nb
Cl(NbCl2)
Nb2+
a e-
Figure (a) Three dimensional chemical potential diagram for the
Nb-Dy-Cl system at 1000 K. (b) A mechanism for
niobium powder production using Dy3+ /Dy2+
equilibrium in molten salt.
Nb ions reduction using Dy3+/Dy2+ equilibrium
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is thermodynamically feasible
Experimental procedure
(a) Flowchart of Experimental procedure
NaCl
KCl
MgCl2
Molten salt preparation
NaCl-KCl-MgCl2 molten salt
Dy, Ag
Dy2+ addition
into molten salt
CV analysis
Dy + Ag + MgCl2
→DyCl2 + Ag-Mg
NaCl-KCl-MgCl2-DyCl2 molten salt
Nb rod
CV analysis
Producing Nbn+ ions by anodic dissolution Galvanostatic
technique
n+
2+
Reduction of Nb
by Dy
in molten salt
Nb powder with salt
CV analysis
Leaching
Nb powder
XRD, SEM
Particle size
XRF, ICP-AES
analysis
(b) Experimental conditions for EP of ATR-Nb
Exp. Dy add.
#
wDy /g w /g
ms
Molten salt
Temp.Current
T/K i/A
1000 2
1000
A
30.5
Composition (mol%)
1296 NaCl-36KCl-9MgCl2-1DyCl2
B
50.1
1049 NaCl-36KCl-8MgCl2-2DyCl2
2 8
Cyclic voltammogram of Nb
(a) Experimental setup
Ar gas inlet/outlet
Thermocouple
Heating element
Nb wire working electrode
Ni quasi-reference electrode
Graphite counter electrode
Silver shots
Ceramics insulator
(a) Cyclic voltammogram of Nb in
NaCl-36 mol%KCl-10 mol%MgCl2 molten salt
Current density,
j / A·cm-2
Mg2+/Mg Nb/Nb2+ Nb2+/Nb3+
0 0.5
-1.1
15
10
5
C
A'
B
0
-5
-10
-15
-2
A
1
0
-1
Potential, E / V vs. Ni quasi-ref
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CV before and after Dy2+ addition
Dy lump
(a) Exp.
apparatus
Ag shot
Stainless steel holder
Mg-Ag liquid alloy RE
Graphite WE
Mg2+/ Mg RE
Graphite CE
Current density,
j / A·cm-2
2
A'
D
Cl-/Cl2
1
(b) CV before
Dy addition
0
Mg2+/Mg
-1
A
NaCl-36KCl-10MgCl2
-2
Mg2+/Mg Dy3+/Dy2+
Current density,
j / A·cm-2
2
A'
E
D
1
(c) CV after
Dy addition
0
-1
-2
E'
A
25.15 g Dy add.
50.14 g Dy add.
0
1
2
3
Potential, E / V vs. Mg-Ag liquid alloy
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EP of Nb in Dy2+ containing molten salt
(a) Experimental apparatus for EP of Nb rod anode
Electrochemical interface
Thermocouple
Molten salt containing Dy2+
Nb rod anode
Mg-Ag liquid alloy cathode
Stainless steel Nb
powder collecting dish
Potential, E / V
vs. Mg-Ag liq. alloy
(b) Chronopotentiomatric curve of Nb anode (i = 2 A)
E = 1.9 V
2.0
1.5
ENb3+/Nb2+
1.0
ENb2+/Nb
0.5
EDy3+/Dy2+
0.0
EMg2+/Mg
0
1
2
t / ks
3
4
Anodic current efficiency (Nb →Nbn+ + n e-)
n=2, 58%
n=3, 87%
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Appearances before and after EP
(a) Nb anode before and after EP
Before
After EP
Initial
Nb rod
dissolved
Nb rod
10 mm
10 mm
10 mm
Immersed
part of
Nb rod
Nb rod was
dissolved
(b) Stainless steel holder of liquid alloy cathode after EP
Cathode
current lead
Supporting rod
for cathode
Stainless steel
holder
10 mm
No Nb
deposition
on cathode
was observed
(c) Nb deposits in powder collecting dish after EP
Supporting rod for
collecting dish
Nb powder
was obtained in
Nb powder with salt
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collecting dish
10 mm
Collecting dish
XRD and XRF analysis
(a) XRD pattern of the Nb powder obtained by EP.
Intensity, I (a.u.)
: Nb JCPDS #34-0370
20
60
40
80
Angle, 2q (degree)
100
(b) XRF results of the Nb powder obtained by EP.
Exp.
#
Nb
A
97.92
B
92.68
Concentration of element i, Ci (mass%)
Fe
Cr
Ni
Ag
Mg
W
Yield
Ta
0.12 0.01 0.14 <0.01 0.06 0.70 0.05 92%
2.93 0.90 0.23 <0.01 0.44 1.04 <0.01 98%
Presently, niobium powder with purity of
98 mass% was obtained.
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SEM and particle size analysis
2 mm
Frequency, F (%)
(a) Exp. A: in NaCl-36 mol%KCl-9 mol% MgCl2-1 mol% DyCl2
10
D10 = 1.0 mm
D50 = 1.8 mm
D90 = 3.0 mm
8
6
4
2
0
0.1
1
10
100
Particle size, d (mm)
2 mm
Frequency, F (%)
(b) Exp. B: in NaCl-36 mol%KCl-8 mol% MgCl2-2 mol% DyCl2
10
8
6
D10 = 0.2 mm
D50 = 0.5 mm
D90 = 0.9 mm
4
2
0
0.1
1
10
100
Particle size, d (mm)
Figure SEM image and particle size distribution profile of
the Nb powder obtained by EP technique.
Fine and homogeneous Nb powder
was obtained
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Summary and future work
Summary:
The electrochemical pulverization technique of
bulk niobium in molten NaCl-KCl-MgCl2 salt
containing Dy2+ ions was demonstrated to be
effective in producing fine and homogeneous
niobium powder.
Nb2+
Dy2+
Mg
Nb
Dy3+
Mg2+
Future work:
• Development of powder purity and
morphology controlling techniques.
• Improvement of current efficiency.
Development of the electrochemical pulverization
technique to be applied to the production
technology of niobium powder for
next generation high performance capacitors.
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