Niobium Powder Production in Molten Salt by Electrochemical Pulverization

1. Fancj May 20-28,2005 Wuhu,Anhui,China 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

2. 41 27 Dielectric constant of pentoxide 8.56 g/cm3 16.65 g/cm3 Density Niobium and Tantalum Table Comparison of Nb with Ta Nb Ta Atomic number VB 73 VB 41 Crystal structure bcc bcc Melting point 2468 ˚C 2980 ˚C Reserves 4,400,000 ton Nb 43,000 ton Ta Annual world productivity 23,000 ton Nb 2,300 ton Ta Price ~ 50 $/kg ~ 700 $/kg Microalloy element for steel Solid electrolytic capacitor Major applications Aluminothermic reduction (ATR) Commercial production process Sodiothermic reduction (Hunter) Ta powder Nb/FeNb bar Product form Development Next generation capacitors Higher performance capacitor Nb, a potential substitute of Ta for next generation capacitors

3. K2TaF7(l) + 5 Na(l)→ Ta(s) + 5 NaF(l) + 2 KF(l) Diluent Hunter process 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

4. Okabe, et al. 2003 (d) Preform (Nb2O5 + flux) Stainless steel mesh Nb2O5 +5 Mg → 2 Nb + 5 MgO Mg vapor reductant Mg (or Mg alloy) chips A Current monitor Okabe, et al. 1999 (a) e- External circuit Nb2O5 powder Nb2O5+ 10 e- → 2 Nb + O2- Molten CaCl2 5 Ca → 5 Ca2+ + 10 e- Ca-Ni-Ag liquid alloy (c) External power source Fray, et al. 2002 Graphite anode Nb2O5 + 10 e- → Nb+ 5 O2- Nb2O5 pellet C + x O2-→ COx + 2x e- CaCl2-NaCl molten salt Direct reduction processes of oxide e- H.C.Starck, 2001 Nb2O5 powder (b) Porous Ta plate Nb2O5 +5 Mg → 2 Nb + 5 MgO Ar gas nozzle Mg vapor reductant Mg chips

5. 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.

6. Electrochemical Pulverization (EP) Anodic: Nb (bulk, s) → Nbn+ (l)+ n e- Cathodic: n Dy3+(l) + n e-→ n Dy2+ (l) Redox Nbn+ (l) + n Dy2+(l) → Nb (powder, s) + n Dy3+(l) reaction in molten salt: Overall Reaction : Nb (bulk, s) → Nb (powder, s) e- Nb rod feed (anode) e- Nbn+(l) Dy2+(l) Mg-Ag liquid alloy (cathode) Nb(s) Dy3+(l) 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 morphologycan 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.

7. a a b b Thermodynamic analysis (a) Nb-Dy-Cl system T = 1000 K Cl2(g) NbCl5 (l, in salt) NbCl4 (l, in salt) DyCl3 (l, in salt) NbCl3 (l,in salt) NbCl2 (l, in salt) DyCl2 (l, in salt) log pCl2 -40 Nb (s) Dy (s) -60 -60 -60 -40 -40 -20 -20 log aNb log aDy 0 (b) In molten salt Dy2+ Dy3+ Cl- (NbCl2) e- Nb2+ Nb 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 is thermodynamically feasible

8. NaCl KCl MgCl2 Producing Nbn+ ions by anodic dissolution Reduction of Nbn+ by Dy2+ in molten salt Experimental procedure (a) Flowchart of Experimental procedure Molten salt preparation NaCl-KCl-MgCl2 molten salt CV analysis Dy, Ag Dy2+ addition into molten salt Dy + Ag + MgCl2 →DyCl2 + Ag-Mg NaCl-KCl-MgCl2-DyCl2 molten salt CV analysis Galvanostatic technique Nb rod CV analysis Nb powder with salt Leaching Nb powder XRD, SEM Particle size XRF, ICP-AES analysis (b) Experimental conditions for EP of ATR-Nb Molten salt Exp. # Dy add. Temp. Current wDy /g wms/g Composition (mol%) T / K i / A A 30.5 NaCl-36KCl-9MgCl2-1DyCl2 1000 2 1296 1000 2 B 1049 NaCl-36KCl-8MgCl2-2DyCl2 50.1

9. 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 Mg2+/Mg Nb/Nb2+ Nb2+/Nb3+ 0 -1.1 0.5 15 C A' 10 B 5 Current density, j / A·cm-2 0 -5 -10 A -15 1 -2 0 -1 Potential, E / V vs. Ni quasi-ref

10. Exp. • apparatus (b) CV before Dy addition (c) CV after Dy addition 25.15 g Dy add. 50.14 g Dy add. CV before and after Dy2+ addition Dy lump Ag shot Stainless steel holder Mg-Ag liquid alloy RE Graphite WE Mg2+/ Mg RE Graphite CE 2 A' D Cl-/Cl2 1 Current density, j / A·cm-2 0 Mg2+/Mg -1 A NaCl-36KCl-10MgCl2 -2 Dy3+/Dy2+ Mg2+/Mg 2 E A' D 1 Current density, j / A·cm-2 0 E' -1 A -2 0 1 2 3 Potential, E / V vs. Mg-Ag liquid alloy

11. 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 (i = 2 A) (b) Chronopotentiomatric curve of Nb anode E = 1.9 V 2.0 ENb3+/Nb2+ 1.5 Potential, E / V vs. Mg-Ag liq. alloy ENb2+/Nb 1.0 0.5 EDy3+/Dy2+ EMg2+/Mg 0.0 0 1 4 2 3 t / ks Anodic current efficiency (Nb →Nbn+ + n e-) n=2, 58% n=3, 87%

12. Appearances before and after EP 10 mm 10 mm 10 mm (a) Nb anode before and after EP Before After EP Initial Nb rod dissolved Nb rod Nb rod was dissolved Immersed part of Nb rod 10 mm (b) Stainless steel holder of liquid alloy cathode after EP Cathode current lead No Nb deposition on cathode was observed Supporting rod for cathode Stainless steel holder (c) Nb deposits in powder collecting dish after EP Supporting rod for collecting dish Nb powder was obtained in collecting dish Nb powder with salt Collecting dish 10 mm

13. : Nb JCPDS #34-0370 20 100 60 40 80 Concentration of element i,Ci (mass%) Yield Exp. # Nb Ag Ni Mg Fe Cr W Ta A 0.05 0.01 0.12 0.14 0.06 92% 0.70 97.92 <0.01 <0.01 0.23 <0.01 98% 0.44 1.04 92.68 0.90 2.93 B XRD and XRF analysis (a) XRD pattern of the Nb powder obtained by EP. Intensity, I (a.u.) Angle, 2q (degree) (b) XRF results of the Nb powder obtained by EP. Presently, niobium powder with purity of 98 mass% was obtained.

14. (a) Exp. A: in NaCl-36 mol%KCl-9 mol% MgCl2-1 mol% DyCl2 10 8 D10 = 1.0 mm D50 = 1.8 mm D90 = 3.0 mm 6 Frequency, F(%) 4 2 1 1 10 10 100 100 0.1 0.1 0 2 mm Particle size, d (mm) (b) Exp. B: in NaCl-36 mol%KCl-8 mol% MgCl2-2 mol% DyCl2 10 D10 = 0.2 mm D50 = 0.5 mm D90 = 0.9 mm 8 6 Frequency, F(%) 4 2 2 mm 0 Particle size, d (mm) SEM and particle size analysis Figure SEM image and particle size distribution profile of the Nb powder obtained by EP technique. Fine and homogeneous Nb powder was obtained

15. Summary: Future work: Summary and future work 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+ • 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.