Electrochemical behaviors of Mg₂₊ and B₃₊ deposition in fluoride molten salts

SHI Zhong-ning, LI Min, LI Lan-lan, GAO Bing-liang, HU Xian-wei, WANG Zhao-wen

School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China

Received 31 August 2010; accepted 19 January 2011

Abstract: By using cyclic and linear sweep voltammetry, the electrochemical deposition behaviors of Mg²⁺ and B³⁺ in fluorides molten salts of KF-MgF₂ and KF-KBF₄ at 880 °C were investigated, respectively. The results show that the electrochemical reduction of Mg²⁺ is a one-step reaction as Mg²⁺+2e^-→Mg in KF-1%MgF₂ molten salt, and the electrochemical reduction of B³⁺ is also a one-step reaction as B³⁺+3e^-→B in KF-KBF₄ (1%, 2% KBF₄) molten salts. Both the cathodic reduction reactions of Mg²⁺ and B³⁺ are controlled by diffusion process. The diffusion coefficients of Mg^{2+ ­}in KF-MgF₂ molten salts and B³⁺ in KF-KBF₄ molten salts are 6.8×10^{-7 cm2}/s and 7.85×10^{-7 cm2}/s, respectively. Moreover, the electrochemical synthesis of MgB₂ by co-deposition of Mg and B was carried out in the KF-MgF₂-KBF₄ (molar ratio of 6:1:2) molten salt at 750 °C. The X-ray diffraction analysis indicates that MgB₂ can be deposited on graphite cathode in the KF-MgF₂-KBF₄ molten salt at 750 °C.

Key words: magnesium diboride; electro-deposition; fluoride molten salts; diffusion coefficient

1 Introduction

MgB₂, which is a kind of superconductive material with simple structure [1], has attracted much attention in the fundamental physics and engineering application. There are many methods such as pulse laser deposition, chemical vapor deposition (CVD), DC magnetron sputtering, to prepare high quality alkali metal boride [2] and alkaline-earth metal boride [3]. By electro-deposition, magnesium and boron will co-deposit on the cathode, and then react spontaneously with each other by the reaction:

Mg+2B=MgB₂                               (1)

Recently, YANG et al [4], HIROSHI et al [5] and KENJI and HIDEKI [6] successfully fabricated MgB₂ by electro-deposition in chloride-based molten salts. However, the evolving of Cl₂ from anode will reduce the concentration of chlorine ion in the electrolyte which consists of B₂O₃ or MgB₂O₄ as solute, and make the MgB₂ not uniform and costly. If using the fluoride electrolyte with B₂O₃, MgB₂O₄ or MgO as solute for MgB₂ electrochemical deposition, the anode product is O₂ (with inert anode) or CO₂ (with carbon anode). Thus the concentration of fluorine ion will keep stable during electro-deposition, which will make the electro- deposition process more easily be controlled.

Although several articles have reported the electrochemical behavior of  Mg²⁺ in ionic liquid [7] and B³⁺ in the fluoride molten salts [8-9], the electrochemical behaviors of MgB₂ in KF-MgF₂-KBF₄ molten salt systems are not involved. Therefore, the aim of this study is to investigate the reduction mechanism of Mg²⁺ and B³⁺ in KF-MgF₂ and KF-KBF₄ molten salts.

2 Experimental

Electrochemical research was conducted in a three-electrode system with tungsten wire (d0.48 mm, ≥99.95%) as work electrode, platinum wire (d2.4 mm, 99.99%) as reference electrode and graphite rod as counter electrode. A high pure graphite crucible with holding capacity of 70 g electrolyte, which has a specification of 45mm in diameter and 80 mm in depth with a sidewall thinness of 5 mm, was used. The whole apparatus was protected in argon gas. The used chemicals of KF, MgF₂ and KBF₄ are anhydrous and analytically pure reagents, respectively. Cyclic voltammetry and linear sweep voltammograms were performed with an AUTO Lab 30 potentiostat instrument.

3 Results and discussion

3.1 Electrochemical behavior of Mg²⁺ in KF-MgF₂

Cyclic voltammograms were carried out on a tungsten wire working electrode in a KF-1%MgF₂ molten salt system at 880 °C. The cyclic voltammograms under different scanning rates of 0.4, 0.6 and 0.8 V/s are shown in Fig. 1. It is clear that magnesium deposition occurs from -1.7 V (Point A) to -1.9 V (Point B), while potassium deposition occurs from -2.0 V (vs Pt, Point C) to -2.5 V (Point D) versus the quasi-reference electrode. During the reverse sweep, the oxidation peaks of magnesium and potassium run at -2.15 V (Point E) and -1.6 V (Point F), respectively.

Fig. 1 Cyclic voltammograms in KF-1%MgF₂ molten salt system at 880 °C

It is confirmed that the reduction peak at Point B (-1.9 V, vs Pt) is associated with the electrochemical reduction of Mg²⁺. As seen in Fig. 1, the change in the scanning rate range of 0.4-0.8 V/s does not make the reduction potential shift in the voltammograms. According to Refs. [10-11], the number of exchanged electrons can be deduced from the cyclic voltammograms. For the magnesium reduction peak, the plot of potential φ vs ln[I/(I_PC-I)] (I_PC is the reduction peak current, and I is the current) is linear, as seen in  Fig. 2. From the slope (RT/nF) of linear equation in Fig. 2, the exchanged electron number of Mg²⁺ reduction is approximately equal to 2 (n≈2). So, it can be concluded that two electrons are exchanged in this reduction. That is to say, the electrochemical reduction of Mg²⁺ proceeds as follows: Mg²⁺+2e^-→Mg. It is a single step for the reaction of Mg²⁺ reduction.

Furthermore, the relationship between the reduction peak current (I_PC) and the square root of the scanning rate (v^1/2) is plotted in Fig. 3. From Fig. 3, it is concluded that the reduction peak current (I_PC) is linearly related with

Fig. 2 ln[I/(I_PC-I)] as function of potential (φ) in KF-1%MgF₂ system

Fig. 3 Plot of peak current (I_PC) vs square root of scanning rate (v^1/2) in KF-1%MgF₂ system at 880 °C

I_PC/v^1/2=-31 mA?s^1/2/V^1/2                        (3)

Taking all parameters (n=2, S=0.226 cm², c=4.29×10^-4 mol/cm³, F=96 487 C/mol, R=8.314 J/(K·mol), T=1 153 K) into Eq. (2), and combining   Eqs. (2) and (3), we can obtain the diffusion coefficient of magnesium cation D(Mg²⁺) under the experiments condition. The calculated D(Mg²⁺) is 6.8×10^{-7 cm2}/s. This value is less than (7.2-9.9)×10^{-6 cm2}/s in MgCl₂-CaCl₂-NaCl chloride system [12] and 5.3×10^{-6 cm2}/s in chloride-fluoride MgCl₂-MgF₂ system [13]. This difference is caused by the different operating temperatures and different molten salt systems.

3.2 Electrode behavior of B³⁺ in KF-KBF₄ molten salts

Figure 4 shows the linear sweep voltammograms of B³⁺ on tungsten electrode in KF-1%KBF₄ and KF- 2%KBF₄ molten salts at 880 °C. Only one reduction peak (Point B) in the voltammograms is found, which indicates that the reduction of B³⁺ occurs in a one-step reaction. It is clear that boron deposition occurs from near -0.8 V (vs Pt, Point A) to -1.25 V (vs Pt, Point B). The reduction peak of boron in the cathode is at -1.25 V (vs Pt, Point B).

Fig. 4 Liner sweep voltammograms at 880 °C with different scanning rates: (a) KF-1% KBF₄; (b) KF-2% KBF₄

From Fig. 4, it is observed that the change in the scanning rate does not make the reduction potential shift in the voltammograms. The number of exchanged electrons of boron reduction can be deduced with the same method of Mg²⁺. The relationship between the potential (φ) and ln[I/(I_PC-I)] are plotted in Fig. 5. By calculating the slope (RT/nF) of these linear equations, the average number of exchanged electrons for B³⁺ reduction reaction (existing as BF₄^- complex anion actually [14]) is approximately equal to 3. Therefore, it can be concluded that three electrons are exchanged in this reduction. That is to say, the electrochemical reduction of B³⁺ proceeds as follows: B³⁺+3e^-→B.

Fig. 5 ln[I/(I_PC-I)] as function of potential (φ) in KF-KBF₄ system

Moreover, the relationship between the reduction peak current (I_PC) and the square root of the scanning rate(v^1/2) is also plotted, as shown in Fig. 6.

Fig. 6 Plot of peak current (I_Pc) vs square root of scanning rate (v^1/2) in KF-KBF₄ system at 880 °C

From Fig. 6, we can conclude that the B³⁺ reduction peak current (I_PC) is also linearly related with the square root of scanning rate (v^1/2) like Mg²⁺. So, the electrochemical reduction process is also controlled by the diffusion of boron cation in the solution. The value of I_PC/v^1/2 is as follows.

1) In KF-1%KBF₄ molten salt,

I_PC/v^1/2=-61.03 mA?s^1/2/V^1/2                     (4)

2) In KF-2%KBF₄ molten salt,

I_PC/v^1/2=-118.40 mA?s^1/2/V^1/2                    (5)

Taking all the parameters into Eq. (2), the diffusion coefficient of boron cation at 880 °C can be calculated as follows:

1) In KF-1%KBF₄ molten salt, D(B³⁺)=7.9×10^{-7 cm2}/s;

2) In KF-2%KBF₄ molten salt: D(B³⁺)=7.8×10^{-7 cm2}/s.

These diffusion coefficient values are more than 4.5×10^{-10 cm2}/s on steel cathode at 800 °C and 1.13×10^{-10 cm2}/s on molybdenum cathode at 830 °C in KF-LiF-KBF₄ molten fluorides [15-16]. The difference is caused by the different substrates, temperatures and molten salt systems.

The concentration of B³⁺ does not much affect the diffusion coefficient of B³⁺. Therefore, the corresponding diffusion coefficient of B³⁺ can be treated using an average value as 7.85×10^{-7 cm2}/s.

3.3 Electro-deposition of MgB₂ in KF-MgF₂-KBF₄ system

In the above experiments, the operating temperatures were 880 °C in both KF-1%MgF₂ and KF-1%KBF₄ molten salts system. However, in an eutectic MgF₂-KF-KBF₄ system, the electro-deposition could be carried out near 750 °C, which is lower than the melting point of MgB₂ (800 °C). According to the Gibbs free energy of Eq. (1), the value of ?G^Θ_1023K is significantly negative, so MgB₂ coating could be deposited at a suitable deposition potential.

An electro-deposition of MgB₂ experiment was conducted in a close system of argon environment at 750 °C. The electrolyte consisted of MgF₂, KF and KBF₄ with a molar ratio of 1:6:2. The graphite crucible (d75 mm×80 mm, sidewall thickness of 7.5 mm) acted as an anode, and the cathode was high cylinder pure graphite (d10 mm×50 mm). The current density was 0.35 A/cm². At the beginning of the electro-deposition process, pre- electrolysis was performed for 10 min.

During the course of electro-deposition, the cell voltage fluctuated in a range of 3.6-3.9 V, as seen in Fig. 7. After experiment, the cathode surface was analyzed with XRD, and MgB₂ was found, as seen in Fig. 8.

According to the ionic structure of KF-MgF₂-KBF₄ molten slat, the electrode reactions are given as follows:

Anodic reaction,

8F^--8e^-=4F₂                                (6)

Cathodic reaction,

Mg²⁺+2B³⁺+8e^-=MgB₂                       (7)

The overall reaction is

MgF₂+2KBF₄=2KF+4F₂+MgB₂                (8)

Using graphitic anode, the anodic gas of fluorine will react with carbon at 750 °C:

2F₂+C=CF₄                                 (9)

3F₂+2C=C₂F₆                              (10)

Fig. 7 Relationship between cell voltage and time during MgB₂ electrochemical deposition

Fig. 8 XRD pattern of electro deposited product from KF-MgF₂-KBF₄ system

Therefore, the following expression could describe this process:

MgF₂+KBF₄+C→F₂+CF₄ (or C₂F₆)+KF+MgB₂       (11)

When using oxides MgO and B₂O₃, CO₂ (with carbon anode) or oxygen (with inert anode) could be evolved, which is similar to the Hall-Heroult aluminium reduction process [15]. This will provide an environmental friendly, low cost and high efficiency process for MgB₂ synthesis.

4 Conclusions

1) Both electrochemical reduction processes of Mg²⁺ in the KF-MgF₂ and B³⁺ in the KF-KBF₄ molten salts are governed by the diffusion process. The diffusion coefficient of Mg²⁺ in KF-1%MgF_2­­ is 6.8×10^{-7 cm2}/s, while the diffusion coefficient of B³⁺ in KF-KBF₄ is 7.85×10^{-7 cm2}/s at 880 °C.

2) The deposition processes of Mg²⁺ in the KF-MgF₂ molten salt and B³⁺ in the KF-KBF₄ molten salt are one-step reaction process at 880 °C.

3) MgB₂ could be fabricated by electro-deposition on graphite electrode in the KF-MgF₂-KBF₄ molten salt at 750 °C. The overall reaction may be: MgF₂+KBF₄+ C→F₂+CF₄(or C₂F₆)+KF+MgB₂.

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氟化物熔盐体系中Mg²⁺和B³⁺的电化学沉积行为

石忠宁，李 敏，李兰兰，高炳亮，胡宪伟，王兆文

东北大学 材料与冶金学院，沈阳 110004

摘  要：采用循环伏安和线性扫描伏安技术，研究在880 °C下Mg²⁺和B³⁺分别在KF-MgF₂和KF-KBF₄氟化物熔盐体系中的电化学沉积行为。结果表明：Mg²⁺在KF-1％MgF₂熔盐体系中的还原是一步两电子反应，Mg²⁺+2e^-→Mg；B³⁺在KF-KBF₄熔盐体系中的还原是一步三电子反应，B³⁺+3e^-→B。Mg²⁺和B³⁺的还原过程受扩散控制，Mg²⁺在KF-MgF₂ 体系中的扩散系数为6.8×10^{-7 cm2}/s，B³⁺在KF-KBF₄熔盐体系中的扩散系数为7.85×10^{-7 cm2}/s。在温度为750 °C、摩尔比为6:1:2的KF-MgF₂-KBF₄熔盐体系中进行电沉积制备MgB₂。X射线衍射分析结果表明，在石墨阴极的表面得到了MgB₂。

关键词：二硼化镁；电沉积；氟化物熔盐；扩散系数

(Edited by YANG Hua)

Foundation item: Project (50804010) supported by the National Natural Science Foundation of China; Project (2007CB210305) supported by the National Basic Research Program of China

Corresponding author: SHI Zhong-ning; Tel: +86-24-83686464; E-mail: znshi@163.com

DOI: 10.1016/S1003-6326(11)60910-2