hui li, jinglong liang*, shanshan xie, ramana g. reddy and

Hui Li, Jinglong Liang*, Shanshan Xie, Ramana G. Reddy and Lanqing Wang

Electrochemical and Phase Analysis of Si(IV) on FeElectrode in Molten NaCl-NaF-KCl-SiO2 System

https://doi.org/10.1515/htmp-2017-0113Received August 16, 2017; accepted January 20, 2018

Abstract: The electrochemical behaviour of Si(IV) ions onFe electrode in the NaCl-NaF-KCl-SiO2 molten salt system(0.177 mol·L−1 SiO2) at 1,103 K was studied by using cyclicvoltammetry, square wave voltammetry, chronoamperome-try, and chronopotentiometry．Experiments conductedusing the first two methods indicated that the reduction ofSi(IV) occurred in two steps: Si(IV) → Si(II) → Si(0).Furthermore, the electrochemical reaction is a quasi-reversible process, which is controlled by both the iondiffusion and electron transport rates. The electrochemicalcrystallization of Si was found to be a transient, the three-dimensional nucleation process. The cyclic voltammetrycurves indicate that the diffusion coefficient of Si(IV) is1.16 × 10–5 cm2·s−1. The phases formed on the surface of thedeposit were analysed by scanning electronmicroscopy andX-ray diffractometer. The results show that Fe and Si haveformed intermetallic compounds Fe3Si, FeSi, and Fe5Si3.

Keywords: cyclic voltammetry, square wave voltammetry,chronoamperometry, chronopotentiometry, compoundsystems

Introduction

Silicon steel is an important soft magnetic alloy used in theelectric power and electronic industries and military appli-cations, mostly for various kinds of motors, generators, and

transformers. Research has shown that increasing the con-tent of silicon could improve the properties of silicon steel.Especially, excellent performance could be achieved at 6.5wt% Si: the magnetostrictive rate is almost zero, and themaximum permeability and resistivity both reach the high-est values [1]. Typically, this material is prepared by silicondepositionmethods, such as electron beam physical vapourdeposition [2], laser cladding [3], hot dip [4], andmolten saltelectrodeposition [5]. Since the 1990s, electrodeposition inmolten salt has attracted considerable attention [6–8].Many researchers reported successful electrodeposition ofSi(IV) from fluoride melts such as NaF-KF-Na2SiF6 [9] andLiF-KF-K2SiF6 [10], and the chloride–fluoride melts of NaCl-KCl-NaF-K2SiF6 [11]. In addition, the electrodeposition of Sifrom molten BaF2-CaF2-SiO2 at 1,200°C has also beendemonstrated [12]. But the mechanism of Si(IV) in the mol-ten salt system of NaCl-NaF-KCl-SiO2 has not been reported.

The electrochemical reduction and nucleation mechan-ism of Si(IV) in molten salt NaCl-NaF-KCl-SiO2 were studiedby electrochemical method. The electrochemical behaviourof Si(IV) on a Fe electrode in the molten salt system ofNaCl-NaF-KCl-SiO2 (cSiO2 = 0.177 mol·L−1) with various elec-trochemical methods [13–15] were investigated. The phasesformed on the surface of the deposit were analysed byscanning electron microscopy and X-ray diffractometry. Itprovides a theoretical basis for the preparation of functionalmaterials for silicon steel.

Experimental process

KCl, NaCl, NaF, and SiO2 (analytical grade) with the massratio of NaCl : KCl : NaF= 1 : 1: 2 were mixed thoroughly,and the content of SiO2 was 2 wt%. The mixed chemicalswere dried in a DZF-6020 (Bo Xun Industrial, Shanghai)vacuum drying furnace for 8 h at 473 K, and then werecooled to room temperature.

The experiments with molten salts were performed inan argon atmosphere with a 3KL10·BYL tubular resistancefurnace (Yunjie Electric Furnace Factory, Baotou) and aP4000 electrochemical workstation (Princeton, USA). Allelectrode surfaces were polished mechanically to a mirrorfinish before measurements. In the three-electrode systemused for the electrochemical tests, an iron wire (1mm

*Corresponding author: Jinglong Liang, Key Laboratory of Ministryof Education for Modern Metallurgy Technology, College ofMetallurgy and Energy, North China University of Science andTechnology, Tangshan 063210, China, E-mail: [email protected] Li: E-mail: [email protected], Shanshan Xie:E-mail: [email protected], Key Laboratory of Ministry ofEducation for Modern Metallurgy Technology, College of Metallurgyand Energy, North China University of Science and Technology,Tangshan 063210, ChinaRamana G. Reddy, Department of Metallurgical and MaterialsEngineering, The University of Alabama, Tuscaloosa, AL 35487, USA,E-mail: [email protected] Wang, Key Laboratory of Ministry of Education for ModernMetallurgy Technology, College of Metallurgy and Energy, NorthChina University of Science and Technology, Tangshan 063210,China, E-mail: [email protected]

High Temp. Mater. Proc. 2018; 37(9-10): 921–928

diameter) and two platinum wires (0.5mm in diameter)were used as the working, reference, and auxiliary electro-des, respectively. Figure 1 shows the equipment of electro-chemical process. A zirconia crucible filled with the mixedsalt was placed in the furnace, heated to 1,103 K andmaintained at this temperature for about 3.5 h. Then, thethree electrodes were inserted into the molten salt forelectrochemical measurements. The electrodeposition wascarried out at a constant potential of −1.7 V for 1 h . Afterdeposition, the samples were analysed by an S-4800 scan-ning electron microscope (Hitachi, Japan) and a Noran 7 X-ray diffractometer (Thermo Fisher, Waltham, MA, USA).

Results and discussion

Cyclic voltammetry

Figure 2 shows the cyclic voltammograms in the moltensalt at 1,103 K with and without SiO2. In the absence of

SiO2 (Curve a), there are no reduction or oxidation peaksin the voltage range between −2.5 and −1 V. Hence, noredox reactions occur in the base molten salt of NaCl-NaF-KCl. In the presence of 0.177 mol·L−1 SiO2 under thesame conditions, there are two redox peaks A’/A and B’/B; Curve b contains two obvious oxidation peaks A´ andB´ at −0.2 and −1.72 V vs. Pt, reduction peaks A and B at−0.8 and −1.7 V vs. Pt, respectively, which indicate that Si(IV) is reduced on the Fe electrode in two steps. This isconsistent with the findings of G et al. [16].

Square wave voltammetry

In the square wave voltammetry curve of NaCl-NaF-KCl-SiO2 on the Fe electrode (Figure 3), there are also tworeduction peaks A and B at −0.8 and −1.7 V vs. Pt,respectively; in accordance with the two steps of Si(IV)reduction on the electrode. According to the literature[17], the half-width of a peak (W1/2) at low frequencies islinked to the number of exchanged electrons (n):

W1=2 = 3.52 RT=nF (1)

where T is the experimental temperature (1,103 K), R isthe gas constant, and F is Faraday’s constant. The W1/2

value found for peak A is about 0.18 V. Using eq. (1), thecalculated n is 1.9 for peak A. Since this value is close to2, the corresponding reaction of Si(IV) on the Fe electrodecan be expressed as follows:

Si IVð Þ+ 2e ! Si IIð Þ (2)

The value of W1/2 for peak B is about 0.21 V, leading ton= 1.6 (which is also close to 2). Therefore, the corre-sponding electrochemical reaction on the Fe electrode is:

Figure 1: The equipment of electrochemical process.

-3.0 -2.4 -1.8 -1.2 -0.6 0.0 0.6 1.2 1.8

-0.6

-0.3

0.0

0.3

0.6

0.9

I /A

E /V

ab

A

B

B'

A'

Figure 2: Cyclic voltammograms of NaCl-NaF-KCl melts (a) beforeand (b) after the addition of 0.177 mol·L−1 SiO2 at 1,103 K on Feelectrode. Scan rate: 0.9 V·s−1.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

/A

/V

A

B

Figure 3: Square wave voltammogram on a Fe electrode in NaCl-NaF-KCl-SiO2 (cSiO2=0.177 mol·L−1) melt at 1,103 K. Pulse height: 180mV, potential step: 72 mV, and frequency: 20 Hz.

922 H. Li et al.: Electrochemical and Phase Analysis

Si IIð Þ+ 2e ! Si (3)

In summary, the above analysis shows that the reductionof Si(IV) on the Fe electrode occurs in two steps (i. e. Si(IV) → Si(II) → Si(0)), which is consistent with the con-clusion from cyclic voltammetry.

Chronopotentiometry

The electrochemical reduction of Si(IV) on Fe electrodein the molten salt was further examined with chronopo-tentiometry. Figure 4 displays the constant-current tran-sient curves. Again, there are two obvious steps between−0.3 and −0.9 A. When the current exceeds −0.5 A, thereappears a step A which is consistent with the reductionpeak A at −0.8 V in Figures 2 and 3. In this step, Si(IV) isreduced to Si(II) on the Fe electrode. When the currentincreases to −0.9 A, another step B appears that isconsistent with the reduction peak B at −1.7 V in

Figures 2 and 3. This step reduces Si(II) to Si(0) on theFe electrode.

The diffusion coefficient of Si(IV) ions in the moltensalt can be calculated by the Sand equation using thecurrent curve of −0.6 A [18]:

I τ1=2 = nFSC0D1=2π1=2=2 (4)

where I is the current (A), τ is the transition time (s), S is theelectrode surface area (cm2), C0 is the solute concentration(mol·m−3), and D is the diffusion coefficient (cm2·s−1). Theestimated value of DSi(IV) is 7.9 × 10

−3 cm2·s−1.

Chronoamperometry

Figure 5 displays the potentiostatic transient curvesobtained using chronoamperometry at 1,103 K for theNaCl-NaF-KCl-SiO2 (cSiO2 = 0.177 mol·L−1) molten salt sys-tem. In Figure 5(a), there is a distinct step A between thepotentials of −0.7 and −0.8 V. Because silicon was reducedcontinuously in the molten salt, there was imbalanced Si(IV) concentration near the cathode after applying the vol-tage, causing the current step. This is in agreement with thereduction peak A at −0.8 V observed in Figures 2 and 3, i. e.Si(IV) + 2 e→ Si(II). In Figure 5(b), there is another obviousstep B between −1.6 and −1.7 V, indicating the presence ofanother electron transfer. This is consistent with the reduc-tion peak B at −1.7 V in Figures 2 and 3. Namely, Si(II)accepts two electrons and is reduced to Si(0).

Instant change of current with time corresponds to theelectrochemical crystallite nucleation and growth of sili-con on the cathode. At the beginning of this instant crys-tallization process, nuclei are formed at the active sites onthe substrate surface. According to the principle of ther-modynamics, the nuclei are independent of each other,and can grow to the critical radius in a process controlled

0 2 4 6 8 10 12 14

-2.0

-1.5

-1.0

-0.5

/V

/s

-0.9A -0.8A -0.7A -0.6A -0.5A -0.4A -0.3A

B

A

Figure 4: Chronopotentiometry results at different current intensitieson a Fe electrode in NaCl-NaF-KCl-SiO2 (cSiO2=0.177 mol·L−1) melt at1103 K.

0 2 4 6 8 10 12

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

/A

/s

-0.5V-0.6V-0.7V-0.8V

A

(a)

0 2 4 6 8 10 12-0.41

-0.40

-0.39

-0.38

-0.37

-0.36

/A

/s

B

-1.4V-1.5V-1.6V-1.7V

(b)

Figure 5: Chronoamperometry results at different voltage on a Fe electrode in NaCl-NaF-KCl-SiO2 (cSiO2=0.177 mol·L−1) melt at 1,103 K. (a)Peak A, shown in the voltage range of −0.5 to −0.8 V, and (b) peak B in the voltage range of −1.4 to −1.7 V.

H. Li et al.: Electrochemical and Phase Analysis 923

by hemispherical diffusion [19]. After forming the firstnuclei, the current increases to an extremum maximumvalue quickly, mainly due to the electrical double layerand the continuous reduction of Si(IV) ions on the cathodeto form Si nuclei. When the concentration of Si(IV) ionscannot match the Si nucleation rate, a concentration polar-ization occurs on the cathode surface and the current isreduced. The crystal nuclei of Si then grow without form-ing new ones, so the current tends to be stable [20, 21].

According to the three-dimensional nucleationmechanism [20, 22], when the electrical crystallizationprocess is controlled by the diffusion of the electroactiveions and the gain and loss of the electron, and it is con-tinuous nucleation growth, the relationship between I andt can be represented by eq. (5). On the other hand, if theprocess is instantaneous growth of the formed nuclei, therelation between I and t can be expressed by eq. (6):

I =23nFπKnN0 2DC0ð Þ23 M

ρ

� �12

t32 (5)

I = nFπN0 2DC0ð Þ32 Mρ

� �12

t12 (6)

where Kn is the nucleation rate constant, N0 is the max-imum nuclei density, M is the atomic weight of the elec-trodeposited species, and ρ is electrodeposition density.According to the chronoamperometry data at the poten-tial −0.8 V, a relationship between I–t3/2 and I–t1/2 wasobtained. Figures 6(a) and 6(b) show the correspondingcorrelation.

Linear regression analysis of the data in Figure 6 leadsto eqs. (7) and (8). The R2 values indicate that eq. (7)describes the results better [23].

I = 0.17623 −0.11088t1=2 R2 = 0.9941 (7)

I = 0.13307−0.1057t3=2 R2 = 0.9892 (8)

According to the fitted eq. (8), it can be inferred that Sideposition in the NaCl-NaF-KCl-SiO2 molten salt system isconsistent with the model of hemispherical three-dimen-sional nucleation followed by instantaneous growth.Based on the chronoamperometry data at the potentialof −0.8 V, the diffusion coefficient was calculated by theCornell equation [16].

I = − nFAD1=2C0π − 1=2t − 1=2 (9)

The calculated value of DSi(IV) is 9.06 × 10–5 cm2·s−1.

Calculation of diffusion coefficient

Figure 7 shows the cyclic voltammetry curves at various scanrates (0.5–0.9 V·s−1). These curves are consistent with Curveb in Figure 2, showing two reduction peaks A and B at −0.8and −1.7 V vs. Pt, respectively. The peaks correspond to thedeposition and dissolution of Si. The cathode and anodicpeak potentials (Epc, Epa) of peaks A and B move toward thenegative direction with increasing scan rate v.

0.60 0.62 0.64 0.66 0.68-0.137

-0.136

-0.135

-0.134

-0.133

-0.132

/A

1/2/s1/2

Actual resultsFitting curve

(a)

0.20 0.22 0.24 0.26 0.28 0.30 0.32-0.137

-0.136

-0.135

-0.134

-0.133

-0.132

/A

3/2/s3/2

Actual resultsFitting curve

(b)

Figure 6: Curves of (a) I–t1/2 and (b) I–t3/2 on a Fe electrode in NaCl-NaF-KCl-SiO2 (cSiO2=0.177 mol·L−1) melt at 1,103 K.

-2.4 -1.6 -0.8 0.0 0.8 1.6 2.4

-0.6

-0.3

0.0

0.3

0.6

0.9

/A

/V

0.5V/s 0.6V/s 0.7V/s 0.8V/s 0.9V/s

A

B

B'

A'

Figure 7: Cyclic voltammograms of NaCl-NaF-KCl-SiO2 (cSiO2=0.177mol·L−1) melt on the Fe electrode at 1,103 K under various scanrates.


According to the cyclic voltammogram at a sweep rate of0.9 V·s−1, the relation between Ipc and v1/2 (shown inFigure 8) is nonlinear, indicating that the cathodic pro-cess corresponding to peaks A and B meets the criterionof the quasi-reversible electrode process. That means thatthe electrode reactions of Si(IV)→Si(II)→Si(0) are con-trolled by the diffusion of the electroactive ions, and thegain and loss of electronic. The diffusion coefficient of Si(IV) ions can be expressed by the Berzins–Delahay equa-tion [9, 24]:

Ipc =0.61AC0 nFð Þ3=2 Dv=RTð Þ1=2 (10)

where v is the potential scan rate (V·s−1). At the rate of−0.9 V·s−1, the value of Ipc at peak A is 0.291 A, and theestimated value of DSi(IV) is 1.16 × 10–5 cm2·s−1.

Surface composition analysis

Based on the results of electrochemical analysis, 1.2%silicon steel sheet was used as the substrate for the

electrodeposition experiments. Figures 9 and 10 are theenergy-dispersive X-ray spectroscopy (EDS) mappingresults of the silicon steel sheet substrate before andafter Si deposition in the NaCl-KCl-NaF-SiO2 moltensalt system, respectively. The Si content in the deposi-tion diffusion layer was increased, and Si diffused intothe substrate. This indicates that the deposition of Si at1,103 K is accompanied by diffusion, forming a Fe–Sidiffusion layer. The surface scans also showed that Si isuniformly distributed in the Fe–Si alloy before and afterthe deposition.

Phase analysis by x-ray diffraction

The X-ray diffraction (XRD) patterns of sediment coating(Figure 11) show that the intermetallic compounds Fe3Si,FeSi, and Fe5Si3 are present on the surface of the depositedlayer. According to our calculation with the HSC thermo-dynamic software [5] and the Fe–Si alloy phase diagram inFigure 12 [25]. In the range of the temperature of solid

Figure 9: EDS mapping result of the 1.2% Si silicon steel matrix.

22 23 24 25 26 27 28 29 30 31

-35

-30

-25

-20

-15

pc/ (

mc.A

-2)

v1/2/(mV.s-1)1/2

(a)

22 23 24 25 26 27 28 29 30 31

-75

-70

-65

-60

-55

-50

-45

-40

pc/(

mc.A

-2)

1/2/(mV.s-1)1/2

(b)

Figure 8: Cathodic peak currents at different potential scan rates on a Fe electrode in NaCl-NaF-KCl-SiO2 (cSiO2=0.177 mol·L−1) melt at 1,103K. (a) Ipc vs. v

1/2, corresponding to Si(IV)→Si(II), (b) Ipc vs. v1/2, corresponding to Si(II)→Si(0).


fusion, the region of α phase expands to 13wt%Si. With theincrease of Si content, the transition temperature of α–γincreases, and the transition temperature of γ–δ decreases.As the silicon content and temperature change, a closedtwo-phase region formed between α phase and γ phase.The α1 phase is DO3 ordered based on Fe3Si, and the α2phase is the B2 ordered phase of Fe3Si. Under 540°C, α2phase eutectoid decomposes into disordered α-Fe(Si) solidsolution (bcc phase) and α1 phase. Fe and Si couldundergo seven possible reactions at 1,003–1,103 K toform different intermetallic compounds:

Fe + Si!FeSi ΔGΘ =−18.63344−7.03993E− 4T + 1.23473E− 6

(11)

Figure 10: EDS mapping result of the silicon steel sample after deposition.

Figure 11: XRD patterns on the surface of the deposited layer.

Figure 12: The Fe–Si phase diagram.


Fe + Si ! FeSi Að Þ ΔGΘ = − 18.27835 + 0.01501T

(12)

Fe + 2Si ! FeSi2 ΔGΘ = − 20.0155 + 0.0036T (13)

Fe + 2.33Si ! FeSi2.33 ΔGΘ = − 14.15724 + 4.88636E − 4T

(14)

Fe + 2.43Si ! FeSi2.43

ΔGΘ = − 15.39918 −0.00235E− 4T + 3.59895E − 6T2

(15)

3Fe + Si ! Fe3Si

ΔGΘ = − 22.25087 − 954256E − 4T− 6.75641E − 6T2(16)

5Fe + 3Si ! Fe5Si3

ΔGΘ = − 57.28483−0.00841E− 6T + 3.32005E− 6T2

(17)

The composition of the Fe–Si system is calculated bythe phase diagram as shown in Figure 13 at 1103 K. Withincreasing Si content, the content of Fe in the ferrosiliconsystem is gradually reduced. Furthermore, the first inter-metallic compound to form is Fe3Si. When the amount ofsilicon reaches 0.1 and 0.75 kmol, Fe5Si3 and FeSi areformed, respectively. The content of Fe3Si first increasesand then decreases with the silicon content, reaching apeak value at 0.85 kmol. The same trend occurs for Fe5Si3with a peak concentration at 2.0 kmol Si. In contrast, thecontent of FeSi monotonously increases with the Si con-centration. Figure 10 shows that Fe3Si, FeSi, and Fe5Si3are all formed in the deposited coating, and the surface Sicontent should be at least 0.75 kmol or 11%, where non-negligible amounts of Fe5Si3 start to form according toFigure 13 [25].

Conclusions

The electrochemical reduction of Si on Fe electrode in theNaCl-NaF-KCl-SiO2 molten salt system was studied byvarious electrochemical analytic methods. The followingconclusions can be drawn:(1) The electrochemical reduction of Si(IV) occurs in two

steps of two-electron transfer: Si(IV)→Si(II)→Si(0).(2) The electrode reaction of Si(IV) is a quasi-reversible

process, which is controlled by both the ion diffu-sion rate and electron transport rate.

(3) The electrical crystallisation of Si in the NaCl-NaF-KCl-SiO2 system agrees with the three-dimensionalnucleation and instantaneous growth mechanisms.

(4) According to the surface scans, silicon is distributeduniformly in the Fe–Si alloy. The XRD spectra con-firmed the formation of intermetallic compoundsFe3Si, FeSi, and Fe5Si3.

According to all of the results, the two-step mechan-ism and the diffusion constants would be the new find-ings which have not been reported in this or similarsystems.

Funding: The National Natural Science Foundation ofChina: 51401075, 51674120. Natural Science Foundationof Hebei Province: E2016209163. Hebei ProvincialDepartment of Education: BJ2017050

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0.0 0.5 1.0 1.5 2.0 2.50.0

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hui li, jinglong liang*, shanshan xie, ramana g. reddy and

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