Rare Metal Materials and EngineeringVolume 42, Issue 11, November 2013Online English edition of the Chinese language journal

Cite this article as: Rare Metal Materials and Engineering, 2013, 42(11): 2237-2241.

Received date: November 5, 2012Foundation item: NSFC(51074060)Corresponding author: Li Yungang, Ph. D., Key Laboratory of the Ministry of Education for Modern Metallurgy Technology, College of Metallurgy & Energy, Hebei United University, Tangshan 063009, P. R. China, Tel: 0086-315-2592156, E-mail: lyg@heuu.edu.cn

Copyright © 2013 Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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ARTICLE

Electrochemical Behavior of Tungsten in the Electrochemical Behavior of Tungsten in the Electrochemical Behavior of Tungsten in the Electrochemical Behavior of Tungsten in the (NaCl(NaCl(NaCl(NaCl----KClKClKClKCl----NaFNaFNaFNaF----NaNaNaNa2222WOWOWOWO4444) Molten Salt) Molten Salt) Molten Salt) Molten SaltLi Jie1,2, Li Yungang2, Liu Limin2, Cai Zongying2, Zhang Xinyu1, Liu Riping 1

1 Key Laboratory of Metastable Materials Science & Technology, Yanshan University, Qinhuangdao 066004, China; 2 Key Laboratory ofthe Ministry of Education for Modern Metallurgy Technology, Hebei United University, Tangshan 063009, China

Abstract: The electrochemical reaction mechanism and the electrocrystallization process of tungsten in the NaCl-KCl-NaF-Na2WO4 molten salt were investigated at 973 K (700 °C) by means of cyclic voltammetry, chronopotentiometry and chronoamperometry technique. The results show that the electrochemical reaction process of tungsten is a quasi-reversible process mix-controlled by ion diffusion rate and electron transport rate; the electrocrystallization process of tungsten is an instantaneous hemispheroid three-dimensional nucleation process; the tungsten ion diffusion coefficient is 2.543×10-5cm·s-1 at the experimental condition of XNaCl:XKCl:XNaF: XNa2WO4= 2:2:1:0.01.

Key words: NaCl-KCl-NaF-Na2WO4; tungsten; electrochemical reduction mechanism; electrocrystallization process

Tungsten is a hard steel-grey metal with a very high melting point (3,410 °C), high intension and outstanding chemical stability, and widely applied in the fields such as material for plasma-facing components in fusion devices, tungsten coating on the semiconductor substrate for MEMS, coating of heating pipe, heat shield materialand rotating anode target material for high-power X-ray machine and so on [1-5]. Tungsten has an important strategic position, so the efforts to prepare metaltungsten film are made by various means. Among them, electrodeposition is one of the important techniques for the economical and low cost. As known, the metaltungsten film can not be prepared by electrolysis in water solution, while the electrodeposition of tungsten in molten salts is one of the most widely acceptedtechniques and many reports are available on the deposition of tungsten by electrodeposition in molten salts [6-9].

Electrodeposition of tungsten in the NaLiB2O4-NaLi WO4-WO3 melt was researched by G. L. Davis et al. in 1956[10]. Further, the experiment was improved and optimized by F. X. Mcawley et al. using NaBO2-Na2WO4-LiBO2-Li2WO4-WO3 melt[11]. Electrodeposition of tungsten in fluoride system was studied in detail by S.

Sendderoff et al.[12] and M. Broc et al.[13]. Respectively. P G Dudley et al. researched tungsten electrodeposition inchloride system and obtained the deposit of tungsten on the cathode[14]. However, the electrolysis of a simple molten system containing an oxide and an alkali metal halides mixture and the theoretical research on electrochemical process have not been studied, which can give a better understanding of the influence of process parameters on the morphology of the final product.

The aim of this study is to obtain the tungsten film in the NaCl-KCl-NaF-Na2WO4 melt. The electrochemical reduction and the nucleation mechanism of W (VI) in the molten salt were explored.

1 1 1 1 Experiment Experiment Experiment Experiment

All the reagents in the experiment were analytic grade reagents. The mixture of NaCl:KCl:NaF = 2:2:1 in molarratio and Na2WO4 (0.01 in molar ratio) was dried under a vacuum for more than 4 h at 200 °C to remove residual water thoroughly and then grinded in agate mortar for experiments.

A three-electrode cell was used for the electrochemical experiment. A tungsten wire with 1 mm diameter was used as the working electrode; the platinum wire with 0.5 mm

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diameter was selected as the reference electrode, and anothertungsten wire with 1 mm diameter served as a auxiliary electrode. To obtain exact experimental results, all electrodessurfaces were polished mechanically to a mirror finish before each measurement. Before each measurement, the platinum wire and another tungsten wire were washed ultrasonically in deionized water carefully. In this article, the measurement or setting of the electrode potential was relative to the potential of Pt reference electrode.

The temperature of the resistance furnace was controlled by an artificial intellective controller (Model: AI-808p) and temperature measurement was carried out by the platinum-platinum rhodium thermocouple (Model: S) with an accuracy of ±1 °C. The electrochemical reduction mechanism and electrocrystallization process of tungsten in NaCl-KCl-NaF-Na2WO4 system were performed by an electrochemical measurement workstation (Model: IM6eX, Germany).

All electrochemical experiments were carried out in a sealed stainless-steel container in dry argon atmosphere. The electrolytic crucible was made of high purity graphite, positioned in the stainless steel outer vessel, then heated from the room temperature to the working temperature of 973 K (700 °C). Keeping of 973 K (700 °C) for 20 min, the cathode and anode tungsten were put into the electrolytes for pre-electrolysis under 50 mA·cm-2. When the tungsten presented on cathode, the electrodes were took out, and then the working electrode, auxiliary electrode and reference electrode were put into the electrolytes for testing in an electrochemical measurement workstation.

2 2 2 2 ElectrocElectrocElectrocElectrochemical Reduction Mechanism of hemical Reduction Mechanism of hemical Reduction Mechanism of hemical Reduction Mechanism of TungstenTungstenTungstenTungsten

2.1 Cyclic VoltammetryFig.1 shows the voltammograms of the tungsten

electrode in the NaCl-KCl-NaF-Na2WO4 (XNaCl:XKCl:XNaF:X

42WONa =2:2:1:0.01) system at 973 K (700 °C). The correlation data obtained from Fig.1 are shown in Table 1.

In the voltammograms, only one cathodic current peak at0.064 V (vs Pt) is observed. It suggests that W ion should be deposited around this potential and the reduction of Wion is a single-step charge transfer. A constant electric potential electrolysis on Cu electrode is carried out at 0.064V for 10 min. It is found that the black film materialon the cathode is tungsten which is proved by energy spectrum (Fig.2).

The study of the voltammetric curves (Fig.1 and Table1)recorded at different potential scanning rates clearly shows that the values of Epc and Epa are related to the potential scanning rates. With increasing in scanning rate, the values of Epc and Epa shift toward the negative or positive potential separately. Electric current function (ipc/v1/2) is also related to the scanning rate v, and decreased with increasing of

Fig.1 Cyclic voltammograms at various potential scan rates inNaCl-KCl-NaF-Na2WO4 system at 973 K (700 °C)

Fig.2 EDS spectrum of the cathode product on Cu electrode at 0.064 V at 973 K (700 °C)

scanning rate but they do not show a strict linear relation(Fig.3). It is suggested that the reduction process corresponding with the cathodic reduction peak is a simple electrode process.

According to the above analysis, it is seen that the electrode process corresponding to the cathodic reduction peak is a quasi-reversible process, meaning that the electrochemical reaction process of tungsten in this system is mix-controlled by ion tungsten diffusion rate and electron transport rate.

After quenching, the NaCl-KCl-NaF-Na2WO4 molten saltfor the experiment was characterized by X-ray diffraction. Fig.4 shows that the main existence form of W ion is W6+ in the system, which is reduced to tungsten.2.2 Chronopotentiometry

The NaCl-KCl-NaF-Na2WO4 molten salt system was investigated using chronoamperometry at 973 K (700 °C) (Fig.5). The change of electric potential on curve O~A can be explained as ohmic polarization of the solution on the electrode surface for a sudden constant current applied to the system. As a result of W being deposited on the cathodeunder a constant current polarization, the electric potential

2 4 6 8 10Energy/keV

1500

1000

500

0

Intensity

/cps

W

W

WWCu

O

Cu

1.0 0.8 0.6 0.4 0.2 0.0 -0.2

0.08

0.06

0.04

0.02

0.00

-0.02

-0.04

I/A

E/V(vs,Pt)

1- 300 mV/s2- 350 mV/s3- 400 mV/s4- 500 mV/sRE : PtT=700 oC

SWE=0.071 cm2

1234

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Table 1 Major parameters in cyclic voltammetric curvesNo. v/mV·s-1 Ipc/A ipc/A·cm-2 Epc/V Epc/2/V Epc�Epc/2 ipc/v1/2

1 300 –0.0438 –0.617 0.072 0.160 –0.088 –0.03562 350 –0.0490 –0.689 0.064 0.150 –0.086 –0.03683 400 –0.0548 –0.772 0.055 0.140 –0.085 –0.03864 500 –0.0646 –0.910 0.045 0.120 –0.075 –0.0407

Fig.3 Relationship between ipc and v1/2

Fig.4 XRD pattern of the NaCl-KCl-NaF-Na2WO4 molten salt system

Fig.5 Chronopotentiometry curve of the NaCl-KCl-NaF-Na2WO4 molten salt system

increases to the characteristic value (corresponding to location B on the curve) of this reducing reaction. With the reduction of electrical activity W6+ and the precipitation of W on the cathode, the concentration ratio of the matters in oxidation state to the matters in reduction state changes continuously with the electrolysis time. When the concentration ratio approaches 1, the electric potential caused by a certain amount of electron in unit time changes slowly, as shown on curve B~C. When the diffusion rate of electrical activity W6+ on the electrode surface is not as fast as the electrode reaction rate, the concentration polarization is caused and increases greatly the electric potential as shown on curve C~D. Finally, the concentration of the reactant on the electrode surface decreases to zero (corresponding to location D on the curve), which indicates a new ion electrochemical reduction process to occur on the electrode. Here, the electric potential changes rapidly.

According to Sand equation[15,16], the relation between the transition time and the diffusion coefficient D of electrical activity ion in the solution can be derived by:

2

2 2 20

4 iDn F C

τ∗

⋅ ⋅=

⋅ ⋅ ⋅ (1)

where C0* is the concentration of Na2WO4, n is reacting

electron number, F is Faraday constant.Taking the chronopotentiometry curve of �I=500 mA as

an example, Table 2 lists a part of data extracted from Fig.5 and diffusion coefficient D of electrical activity W6+ is obtained out by equation (1).

According to the characteristic equation of E~t curveunder constant current electrolysis, E vs ln ( 1/2-t1/2) presents a linear relation when the electrode process is reversible and the product is insoluble or irreversible. But the electric potential E and ln( 1/2-t1/2) obtained from Fig.5 and Table 2are not a strict linear relation as shown in Fig.6, from which it can be inferred that the electrode process of W in the NaCl-KCl-NaF-Na2WO4 system is a quasi-reversible process, which is the same as that measured by cyclic voltammetry.2.3 Electrocrystallization Process of Tungsten

The electrocrystallization process of tungsten in the NaCl-KCl-NaF-Na2WO4 molten salt system was investigated using chronoamperometry at 973 K (700 °C)(Fig.7). In the system, the change of current-time transient curves indicates the e lec t rocrys ta l l i za t ion nuc lea t ion and growth characteristics of tungsten on the cathode. At first, the double layer of tungsten electrode is charged and the first nuclei of W is formed, so the current increases to a high value in a

17 18 19 20 21 22 23-0.6

-0.7

-0.8

-0.9

i pc/A·cm

-2

v1/2/(mV·s-1)1/2

0 20 40 60 80 100-1000

01000200030004000500060007000

��NaCl��KCl��NaF��Na

2W

2O7

��Na2WO

4

�

�

�

�

�

�

�

�

Intensity

/cps

2θ /(o)

�

�

�

�

�

0.0 0.5 1.0 1.5 2.0 2.51.65

1.70

1.75

1.80

1.85

1.90

1.95�I=500 mA700�CE: PtSWE=0.047 cm

2

E/V

t/s

A

BC

D

o

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Table 2 Data from Chronopotentionmetry curve of NaCl-KCl-NaF-Na2WO4 molten salt system

I/A i/A·cm-2 /s D/�10-5 cm2·s-1 D /cm2·s-1

0.50 5.32 2.0 2.7310.45 4.78 2.3 2.5350.40 4.26 2.7 2.364

2.543×10-5

Table 3 I, t data from curve b in NaCl-KCl-NaF-Na2WO4 systemt/s 0.0352 0.0654 0.0956 0.1258 0.1560 0.1862I/A 0.0662 0.0629 0.0601 0.0577 0.0557 0.0539

t1/2/s1/2 0.1876 0.2557 0.3092 0.3547 0.395 0.4315t3/2/s3/2 0.0066 0.0167 0.0296 0.0446 0.0616 0.0803

Fig.6 Relationship between E and ln( 1/2-t1/2)

Fig.7 Chronoamperometry curves of the NaCl-KCl-NaF-Na2WO4 molten salt system

very short time. Subsequently, the current decreases because of the concentration polarization, which is the result of the number of W6+ electro-migrated to the active position could not meet the nucleation rate of W. secondly, due to the growth of W crystal nucleuses formed on the cathode and the deficiency of active ions in the diffusion layer, the current decreases slowly and droves to stabilization.

Taking curve b in Fig.7 as an example, data of current Iand time t from curve b are presented in Table 3. Curves of I~t1/2and I~t3/2 are shown in Fig.8.

Doing linear regression to the data in Fig.8, linear

regression equations (2) and (3)are got. R2 of the equations (2) is 0.9998, that means a linear relation between I and t1/2. R2

of the equations (3) is 0.9528, that means a nonlinear relation between I and t3/2.

2/10.050790.07579 tI −= R2=0.9998 (2)2/30.161130.06585 tI −= R2=0.9528 (3)

According to hemispheroid three-dimensional nucleation theory[17], when the electrocrystallization process is controlled by the diffusion of metallic atoms on electrode surface and is progressive nucleation growth, the relation of Ito t is represented by equation (4):

( )1/ 2

3 / 2 3 / 20 0

2 23 n

MI nF K N DC tρ

=

(4)

where n, F, Kn, N0, D, M.�,C0, are reacting electron number,

Fig.8 Relationship between I~t1/2 (a) and I~t3/2 (b) of curve b in Fig.7

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08 CE: Pt700 �SWE

=0.047 cm2

I/A

t/s

800 mV

700 mV

650 mV

600 mV

abcd

0.00 0.02 0.04 0.06 0.080.052

0.056

0.060

0.064

0.068b

Actual curve Fitting curve

I/A

t3/2/s3/2

0.0 -0.5 -1.0 -1.5 -2.0

1.826

1.828

1.830

1.832

1.834

1.836

1.838

E/mV(vs P

t)

ln��1/2-t1/2�

0.15 0.20 0.25 0.30 0.35 0.40 0.450.052

0.056

0.060

0.064

0.068a Actual curve

Fitting curve

I/A

t1/2/s1/2

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Faraday constant, nucleation rate constant, maximum crystal nucleus density, diffusion coefficient of ion, atomic mass of electrodeposit, density of electrodeposit and concentration of ion, respectively.

When the electrocrystallization process is instantaneousnucleation growth, the relation of I to t is represented byequation (5):

( )1/ 2

3/ 2 1/ 20 02 MI nF N DC t

ρ

=

(5)

It is quite obvious that the electrocrystallization process of tungsten in the NaCl-KCl-NaF-Na2WO4 system is an instantaneous hemispheroid three-dimensional nucleation process.

3333 ConclusionConclusionConclusionConclusionssss

1) The electrochemical reaction process of tungsten inthe NaCl-KCl-NaF-Na2WO4 molten salt system is a quasi-reversible process mix-controlled by W6+ diffusion rate and electron transport rate.

2) At 973 K (700°C), the diffusion coefficient of W ion in the NaCl-KCl-NaF-Na2WO4 system is 2.543×10-5cm·s-1 when XNaCl:XKCl:XNaF:X Na2WO4= 2:2:1:0.01.

3) The electrocrystallization process of tungsten in theNaCl-KCl-NaF-Na2WO4 system is an instantaneous hemispheroid three-dimensional nucleation process.

ReferencesReferencesReferencesReferences

1 Lavigne S B, Moreau C, Jacques R G S. Journal of Thermal

Sp ray Technology[J], 1995, 4: 2612 Dhote A M, Ogale S B. Applied Physics Letters[J], 1994, 64:

28093 Varacalle D J, Lundberg L B, Jacox M G et al. Surface and

Coatings Technology[J], 1993, 61: 794 Philipps V. Journal of Nuclear Materials [J], 2011, 415: S25 Maier H, Koetterl S, Krieger K et al. Journal of Nuclear

Materials [J], 1998, 258-263: 9216 Yand Qiqin. Electrochemistry [J], 1997, 3:117( in Chinese)7 Senderoff S, Mellors G W. Science [J], 1976, 153: 15758 White S H, Twardoch V M. Journal of Applied Electroche-

mistry[J], 1987, 17: 2259 Ma Ruixin, Li Guoxun. Materials Protection [J], 1999, 32: 4

(in Chinese)10 Davis G L, Gentry C H R. Metallurgia [J], 1956, 53: 311 Mcawley F X , Kenahan C B. J Metals [J], 1964, 17: 9212 Mellors G W, Senderoff S. Belg Patent, No.658463[P]. 196513 Broc M et a1. Molten Salt Electrolysis in Metal Production

[M]. London: Institution of Mining and Metallurgy, 197714 Dudley P G et a1. Proceeding of 2nd International Symposium

on Molten Salts[C]. Pittsburgh: The Electrochemical Society, 1981

15 Bermejo M R et al. Electrochimica Acta [J], 2008, 53: 510616 Bard A J, Faulkner L R. Electrochemical Methods:

Fundamental and Applications [M]. New York: Wiley, 200117 Allongue P, Souteyrand E. Journal of Electroanalytical

Chemistry and Interfacial Electrochemistry [J], 1990, 286:217