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Electrochemistry of Deposition of Rare Earth Metals and Their Alloys in

Molten Salts: An overview of molten-salt studies at Zhongshan University

 

Yang Qiqin

School of Chem. and Chem. Eng., Zhongshan University, Guangzhou 510275,

P. R. China

e-mail: cesyqq@zsulink.zsu.edu.cn

 

The rare earth(RE) metals and their alloys have many functional

performances and have been widely applied in magnetic, optical,

superconducting etc. materials. We have studied the electroreduction of

La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+,

Yb3+, Lu3+, Y3+ and electrolytic preparation of the rare earth alloys

with Fe, Co, Ni, Cu, Mg in high temperature molten salts (723-1123 K,

NaCl-KCl, KCl, LiCl-KCl, NaF-CaF2 ) since 1980. Recently, we develop the

research of electrodeposition rare earth-iron group alloy films in low

temperature molten salts (373-398 K, urea-NaBr-KBr, urea-NaCl,

urea-acetamide-NaBr).

1. Electroreduction of rare earth metallic ions in chloride melts

The electrode processes were studied by cyclic voltammetry,

convolution voltammetry, choronopotentiometry, potentiostatic

electrolysis and open circuit potential-time curve after electrolysis,

EDAX and XRD, Most of RE3+ reduced on Mo or W electrode are reversible

in one step. Reduction of Sm3+, Eu3+, Tm3+, Yb3+ is stepwise, i.e.

RE3+e=RE2+, RE2++2e=RE. The diffusion coefficients (D) of RE3+ in melt

were determined as10-6~10-5 cm2 s-1 and the diffusion activation

energies were calculated as about 40 kJ mol-1. When RE3+ are reduced on

Fe, Co, Ni, Cu electrodes, rare earth alloys formed first and then rare

earth metals are deposited.

2. Cathodic alloying of RE3+ reduced on Fe, Co, Ni, Cu electrodes

The rate determining step of cathodic alloying is the deposited RE

atoms diffuse into the cathode. The D of RE atoms in the alloy phases

were determined as 10-11~10-10 cm2 s-1 and the diffusion activation

energies were calculated as about 100 kJ mol-1 at 973-1193 K. The

formation free energies of intermetallic compounds were determined, eg

?Gf0,YCu = -63.1 kJ mol-1 (973 K). Many surface layers of rare earth

alloy were obtained by cathodic alloying.

3. Electrolytic preparation of rare earth alloys in high temperature

molten salts

Most alloys were prepared by consumable cathode in NaCl-KCl melt.

The RE contents of the alloys attain 90 wt%. Both current

efficiencies(CE) and rare earth recoveries of alloys of La, Pr with Fe,

Co, Ni, Cu reach 85%-95%. For preparation of heavier rare earth alloys,

it is preferable to use fluoride melt, eg Y-Ba-Cu electrodeposited in

NaF-CaF2-YF3-Y2O3 melt (CE:~80%). Y?Mg obtained in YCl3-MgCl2-KCl by

electrolytic codeposition has higher Y content compared with that

obtained by liquid cathode, the former is~70wt%, the later is ~30 wt%.

4. Electroreduction of Fe2+, Co2+, Ni2+ and inductive codeposition

with RE3+ in urea melt

The electric conductivities of molten urea-alkali metal halides at

different temperatures were determined. The conductivities of these

melts are 15~20 mS cm-1 at 373 K.

Reduction of Fe2+, Co2+, Ni2+ to Fe, Co, Ni (M) is irreversible in

one step. The transfer coefficients of M2++2e=M and D of M2+ in melt

were determined as 0.3~0.5 and10-6 cm2 s-1.

The rare earth can be inductively codeposited with iron group metals

to form alloys. Over twenty RE-M alloy films were obtained. The RE

contents in the deposited films changed with cathode potential, current

density, molar ratio of RE3+/M2+ in melt and reach 90wt%. The deposited

films present an amorphous structure determined by XRD. The

crystallizing process was monitored by DSC. The crystallizing

temperature of Er-Co film was determined as 743 K.