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Electrochemical deposition of Al–Mg alloys on tungsten wires from AlCl3–NaCl–KCl melts
Fusion Engineering and Design 103 (2016) 8–12
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Electrochemical deposition of Al–Mg alloys on tungsten wires from AlCl3 –NaCl–KCl melts Yaling Li, Peng Zhao, Yinhai Dai, Mengqi Yao, Haibo Gan, Wencheng Hu ∗ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
h i g h l i g h t s • • • •
The method of electrochemical deposition is used to prepare Al–Mg alloys on tungsten wires. AlCl3 –NaCl–KCl melts as a non-aqueous electrolyte is used in electrochemical deposition. The effects of deposition voltage and molten salt temperature on the surface morphology and magnesium content of the Al–Mg deposits are studied. Al–Mg alloys with 9.14 at.% Mg are obtained.
a r t i c l e
i n f o
Article history: Received 6 June 2015 Received in revised form 30 November 2015 Accepted 30 November 2015 Keywords: Electrodeposition Al–Mg alloys Tungsten wires Molten salts Wire arrays Inertial confinement fusion
a b s t r a c t This paper studies the electrochemical method that obtains the Al–Mg alloys on tungsten wires from AlCl3 –NaCl–KCl melts containing a mass fraction of 3% MgCl2 . Electrochemical experiments are performed with a three electrode system. Linear sweep voltammetry is used to determine the electrodeposition potential of Al–Mg alloys in molten salts. X-ray diffraction is employed to examine the crystallographic structure of Al–Mg alloy electrodeposits. Results show that the Al–Mg alloy coating consists of an Al (Mg) solid solution and the amorphous phase. The effects of the electrodeposition potential and temperature on the morphology of Al–Mg electrodeposits are demonstrated by scanning electron microscopy. © 2015 Published by Elsevier B.V.
1. Introduction As reported, the target chamber load is the key part in the Zpinch driven inertial confinement fusion (ICF), which determines the rate of the converting laser light to the X-ray conversion [1,2]. A wide variety of cylindrical Z-pinch loads, including thin annular foils, gas puffs, low-density foams, and low-number wire arrays, have been used in early experiments. A dramatic breakthrough in X-ray power was achieved in 1995 [3], and tungsten wire has been widely applied in Z-pinch physical experiments [4,5]. Al–Mg alloys are excellent lightweight metallic materials because of their attractive properties, such as high volumetric heat content, high specific strength, high specific stiffness, low density, and strong corrosion resistance [6]. Therefore, Al–Mg alloy wire arrays are also one of the most common materials applied in Z-pinch physical experiments. As previously reported [7–9], a
∗ Corresponding author. Tel.: +86 28 83201171; fax: +86 28 83202550. E-mail address: [email protected] (W. Hu). http://dx.doi.org/10.1016/j.fusengdes.2015.11.053 0920-3796/© 2015 Published by Elsevier B.V.
mixture of aluminum and magnesium is observed to maximize the radiated kilovolt X-ray yield at 50 kJ, which is 1.5 times higher than that obtained with pure aluminum [7]. The mixed Al–W array load can emit equivalent X-ray pulses as the W array load, but the intensity of the X rays pulses is lower than that of pure Al wire array [10]. Magnesium is more active than aluminum. Al–Mg alloys have the advantages of excellent electrical conductivity and thermal conductivity. A higher yield of X rays can be obtained by eletrodepositing Al–Mg alloys onto tungsten wire surface as wire arrays load. The reduction potentials of Al and Mg (−1.706 V for Al and −2.375 V for Mg) are more negative than that of water for hydrolysis [11]. Therefore, aqueous electrolytes can-not be used for the electrodeposition of Al–Mg alloys. Al, Mg, and their alloys have been obtained from non-aqueous electrolytes [12–15]. The electrodeposition of Al–Mg alloys from an organic solvent has limitations, such as poor stability, high toxicity, and heavy pollution. The molten salt system is relatively simple, economical, and environment friendly. In particular, inorganic molten salts can dissolve various metal salts, which are ideal for various metal electrodeposition process.
Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
As previously reported in 2000 [16–22], the Mg atomic ratio during the electrodeposition of Al–Mg alloys was less than 2.2 at.% from the acidic AlCl3 –EMIC melts containing MgCl2 [16]. In 2010, the aluminum-alloyed coating on AZ91D magnesium alloys was obtained from their molten salts containing AlCl3 at lower temperatures. Previous results showed that aluminum-alloyed coating consists of Mg2 Al3 and Mg17 Al12 intermetallic compounds [17]. The Al–Mg alloys were also electrodeposited by an organometallicbased electrolyte with Na[AlEt4 ] + 2 AlEt3 + 3.3 toluene as the electrolyte [19], this previous study was only able to obtain a maximum of approximately 24 wt.% Mg in the alloys. In the present paper, Al–Mg alloys on tungsten wires are obtained from AlCl3 –NaCl–KCl melts (by a Al:Na:K mole ratio of 61:26:13) containing a mass fraction of 3% MgCl2 . SEM analysis also investigated the effect of the electrodeposition potential, temperature on the morphology of Al–Mg electrodeposits. 2. Experimental 2.1. Preparation and purification of melts All chemicals are of analytical-grade. Magnesium chloride and aluminum chloride must be used in a dry environment because of their easy moisture absorption. The aluminum chloride regent bottle is wax sealed. The fine powder of sodium chloride and potassium chloride is heated for 8 h at 300 ◦ C in the muffle furnace to remove moisture [23]. Magnesium chloride is placed in a drying oven at 80 ◦ C for 24 h. A mixture of AlCl3 –NaCl–KCl melts is obtained with a Al:Na:K mole ratio of 61:26:13 [19]. Magnesium chloride at a mass (total mass of AlCl3 –NaCl–KCl powder) fraction of 3% is added into the AlCl3 –NaCl–KCl powder. After weighing, the mixed powder is gently shaken to mix it evenly. The glass beaker containing the mixed powder is placed in an argon atmosphere for 20 min.
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When the oil bath reached the required temperature (T = 180, 200, and 220 ◦ C), the glass beaker is immersed into the bath. The power was melted under the argon atmosphere until the mixture turned into a brown liquid. 2.2. Electrochemical apparatus and electrodes Electrochemical experiments are performed using Metrohm Autolab B.V. electrochemical workstation (PGSTAT302N, Netherlands) with a three electrode system. The working electrode is tungsten wire (99.95%). The counter electrode is a platinum wire electrode (99.99%). The reference electrode is a silver electrode. First, the W electrode is successively polished by SiC emery paper with grits of 500, 1000, and 2000 to obtain a smooth surface before electrochemical measurements. After fully cleaned in an ultrasonic bath for 5 min and dried by thin cool air, the W electrode is immersed in the solution (NaOH, 25 g L−1 ; Na2 CO3 , 50 g L−1 ; Na3 PO4 , 40 g L−1 ) for 10–15 min to remove oil stains and tungsten oxide in an ultrasonic cleaner. Then, Ag reference electrode is immersed in 0.1 mol L−1 HNO3 for a few seconds to remove the oxide. Both the W electrode and the Ag electrode are cleaned with deionized water and ethanol. The platinum wire electrode is cleaned with laboratory detergent and deionized water. Finally, after drying in clean cool air, the three electrodes are directly immersed into the molten salts. Data are collected by the electrochemical workstation. 2.3. Electrodeposition of Al–Mg alloys The substrates are tungsten (99.95%) wires. A cylinder-shaped aluminum foil electrode is used to surround the tungsten electrode, as shown in Fig. 1. The tungsten wires are cleaned as described in Section 2.2. However, the tungsten wires do not need to be mechanically polished by SiC emery paper. The aluminum foil electrode is polished by SiC emery paper of different grits to remove Table 1 Electrodeposition parameters used in this study. Deposit #
E (V)
T (◦ C)
1 2 3 4 5
−1.20 −1.30 −1.40 −1.30 −1.30
200 200 200 180 220
E = deposition potential, T = temperature.
Fig. 1. A schematic diagram of experimental set-up: (1) air inlet; (2) air outlet; (3) rubber stopper; (4) glass beaker; (5) numerical show constant temperature oilbathing; (6) aluminum foil; (7) oil; (8) PTFE; (9) tungsten wires; (10) molten salt.
Fig. 2. LSV of the AlCl3 –NaCl–KCl melts at the scan rate 5 mV s−1 at 200 ◦ C. (The red and black lines are the LSV curves of the electrolytes without MgCl2 and with a mass fraction of 3% MgCl2 , respectively.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
Fig. 3. Magnification (×5000) of surface morphologies of Al–Mg alloy electrodeposits on tungsten wires: (a) −1.20 V; (b) −1.30 V; (c) −1.40 V. The corresponding EDS analysis of electrodeposits is shown in Fig. 4d, e and f, respectively.
the oxide, cleaned in an ultrasonic bath for 5 min, and immersed in a diluted hydrochloric acid solution for 60 s. After being completely rinsed with deionized water and dried with clean cool air, the cathode and anode are immediately transferred into the glass beaker for immediate use. Prior to electrodeposition of Al–Mg alloy on tungsten wires, the pre-electrodeposition is performed using a stainless steel sheet as a cathode at 70 mA cm−2 for 60 s to remove the oxide layer of aluminum foil. After the electrodeposition experiment, the resulting electrodeposit is thoroughly cleaned by ethanol and deionized water in the ultrasonic cleaner and dried with cool air. A new tungsten wire electrode is used for each deposition. The electrodeposition potentials at −1.20 V, −1.30 V, and −1.40 V are used to investigate the effect of the potential on the Al–Mg deposition. The effect of the temperature is studied at 180, 200 and 220 ◦ C, respectively, while maintaining the electrodeposited time
at approximately 40 min. Table 1 presents the parameters for the experiments in this study.
2.4. Characterization of coating The surface morphologies and nanostructures of Al–Mg alloy coatings are observed by an field emission scanning electron microscopy (FE-SEM) system (Inspect F, FEI Co., U.S.) at 20 kV, coupled with an energy dispersive spectroscopy (EDS) instrument (Edax, Inc. Instruments Co., U.S.), which is used to analyze the element composition of the electrodeposits. The X-ray diffraction (XRD) patterns of the electrodeposits on the tungsten plate, which are cleaned as tungsten wires, are described in Section 2.2 and determined by an X-ray diffractometer (D-Max-␥ type A, Rigaku Co., Japan) with Ni-filtered Cu K␣ radiation and a 0.15406 nm
Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
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wavelength operated at 40 kV and 40 mA. Data are obtained from 2Â of 10◦ to 90◦ at a scan rate of 2◦ per step. 3. Results and discussion 3.1. Electrochemical results The linear sweep voltammetry (LSV) of the AlCl3 –NaCl–KCl melts at 200 ◦ C is shown in Fig. 2. The red and black lines are the LSV curves of the electrolytes without MgCl2 and with a mass fraction of 3% MgCl2 , respectively. The potential window ranges from 0 V to −1.80 V at a sweep rate is 5 mV s−1 . When the mole ratio of Al in Al:Na:K is higher than 50% [23], the complex ion Al2 Cl7 − can be produced by the change of AlCl4 − in the AlCl3 –NaCl–KCl melts. Therefore, peaks C1 and C3 , located at −0.90 V and −0.80 V, are ascribed to the electrodeposition of Al reduced from Al2 Cl7 − . The current significantly increases at the potential below −1.2 V (peak C2 ) [24,25]. The increase in the current in this region is possibly generated by the electrodeposition of Al–Mg alloys [26]. 3.2. Effect of constant potential on the morphology and composition of electrodeposits The constant electrodeposition potential of the Al–Mg alloys is conducted on the tungsten wires from the molten salts at 200 ◦ C. The electrodeposition time for each sample is 20 min. The effect of constant potential is investigated at −1.20 V, −1.30 V, and −1.40 V (Table 1). The applied potential significantly affected the morphology of the electrodeposits. As shown in Fig. 3a, the layer of Al–Mg alloys obtained at 180 ◦ C is a very thin. Fig. 3b shows that relatively uniform particles (0.5–1.5 m in size) are electrodeposited on tungsten wires to form an alloy layer. Fig. 3c shows uneven large-size particles with rough surfaces. The appearance of the electrodeposits obtained at −1.30 V is obviously smoother and less granular
Fig. 4. The X-ray diffraction pattern of the Al–Mg electrodeposits: (a) tungsten substrate; (b) −1.20 V; (c) −1.30 V; (d) −1.40 V.
than that of the samples obtained at −1.20 V and −1.40 V. The corresponding EDS data of the electrodeposits are shown in Fig. 3d–f. The EDS results reveal that these electrodeposits are elemental Al and Mg with a small amount of oxygen, which is attributed to the exposure of electrodeposits to air. The samples at −1.20 V and −1.40 V contain 2.55 at.% and 7.68 at.% Mg, respectively, whereas the electrodeposits at −1.30 V contain more than 9 at.% Mg. X-ray diffraction experiments are used to examine the crystal structure of the Al–Mg alloy electrodeposits. Fig. 4 shows the XRD patterns of the electrodeposits obtained at different electrodeposition potentials. The four main reflections peaks at 40.264◦ , 58.274◦ , 73.195◦ , and 87.021◦ correspond to the (1 1 0), (2 0 0), (2 1 1), and (2 2 0) crystalline faces, respectively, which are related to tungsten
Fig. 5. Magnification (×5000) of surface morphologies and EDS data of Al–Mg alloys electrodeposits obtained at −1.30 V: (a and c) 180 ◦ C; (b and d) 220 ◦ C.
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Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
(PDF #04-0806). In the X rays diffraction patterns of the alloyed metal, the primary change is the increased electrodeposition potential, which leads to the disappearance of the face-centered cubic of the Al (Mg) solid solution reflections peak; the development of a broad reflection centered from 20◦ to 30◦ indicates the presence of an amorphous phase of the alloyed metal [27]. The electrodeposits at −1.20 V contain less than 3 at.% Mg (Fig. 3d) and have diffraction patterns where the face-centered cubic Al peaks shift to higher angles, thereby suggesting that the alloying of Mg with Al leads to the formation of an Al (Mg) solid solution [28]. Fig. 4b indicates that this electrodeposit has two phases: the face-centered cubic phase and the amorphous phase [29]. As the electrodeposition potential is increased, the peak intensity of the face-centered cubic phase in the alloy decreases, whereas that of the amorphous phase increases (Fig. 4c–d). Fig. 4c shows that the electrodeposit containing more than 9 at.% Mg (Fig. 3e) is completely amorphous. 3.3. Effect of temperature on electrodeposit morphology The effect of temperature on the Al–Mg electrodeposit is also investigated with AlCl3 –NaCl–KCl (by a Al:Na:K mole ratio of 61:26:13), which contains a mass fraction of 3% MgCl2 . The electrodeposition parameters at three different electrodeposition temperatures are shown in Table 1 (deposits 2, 4 and 5). The SEM images of the Al–Mg alloy electrodeposits obtained at various temperatures are shown in Figs. 3b and 5a and b. The grain size of the electrodeposit gradually increases when the electrodeposition temperature changes from 180 ◦ C to 220 ◦ C. However, the appearance and structure of the electrodeposits remain the same. The grain size of the electrodeposit is influenced by the competition between the nucleation rate and the growth rate of nuclei [26,30]. With the higher nucleation rate during electrodeposition, the appearance and size of the electrodeposit particles are improved. The overpotential is decreased and the metal ion mobility is increased with increasing electrodeposition temperature. Lower overpotential can decrease the nucleation rate. Therefore, the growth rate of nuclei is positively related to the electrodeposition temperature. The EDS data (Figs. 3e and 5c and d) reveal that the amount of Mg in the electrodeposits is increased with electrodeposition temperature increasing from 180 ◦ C to 200 ◦ C, whereas the amount of Mg of electrodeposits at 220 ◦ C is lower. 4. Conclusion Al-Mg alloy coatings on tungsten wires are obtained by electrochemical deposition from AlCl3 –NaCl–KCl melts containing a mass fraction of 3% MgCl2 . Al–Mg alloys with 9.14 at.% Mg are obtained, thereby indicating the strong possibility to create Al–Mg alloy coating on tungsten wire. The effects of deposition voltage and molten salt temperature on the surface morphologies and magnesium content of the Al–Mg deposits are also investigated. As the deposition voltage is increased from −1.20 V to −1.40 V, the grain size of the Al–Mg electrodeposits is gradually increased. The amount of magnesium in the electrodeposits first increases and then decreases. The same trend is observed when increasing the temperature from 180 ◦ C to 220 ◦ C. Therefore, the optimized electrolysis process of the Al–Mg alloy coating of tungsten wire is an electrodeposition potential of −1.30 V at 200 ◦ C. A higher yield of X rays can be obtained by eletrodepositing the Al–Mg alloys onto the surface of tungsten wires, which can be applied in Z-pinch driven ICF experiments. References [1] C. Cerjan, P.T. Springer, S. Sepke, Integrated diagnostic analysis of inertial confinement fusion capsule performancea, Phys. Plasmas 20 (5) (2013) 056319.
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Electrochemical deposition of Al–Mg alloys on tungsten wires from AlCl3 –NaCl–KCl melts Yaling Li, Peng Zhao, Yinhai Dai, Mengqi Yao, Haibo Gan, Wencheng Hu ∗ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
h i g h l i g h t s • • • •
The method of electrochemical deposition is used to prepare Al–Mg alloys on tungsten wires. AlCl3 –NaCl–KCl melts as a non-aqueous electrolyte is used in electrochemical deposition. The effects of deposition voltage and molten salt temperature on the surface morphology and magnesium content of the Al–Mg deposits are studied. Al–Mg alloys with 9.14 at.% Mg are obtained.
a r t i c l e
i n f o
Article history: Received 6 June 2015 Received in revised form 30 November 2015 Accepted 30 November 2015 Keywords: Electrodeposition Al–Mg alloys Tungsten wires Molten salts Wire arrays Inertial confinement fusion
a b s t r a c t This paper studies the electrochemical method that obtains the Al–Mg alloys on tungsten wires from AlCl3 –NaCl–KCl melts containing a mass fraction of 3% MgCl2 . Electrochemical experiments are performed with a three electrode system. Linear sweep voltammetry is used to determine the electrodeposition potential of Al–Mg alloys in molten salts. X-ray diffraction is employed to examine the crystallographic structure of Al–Mg alloy electrodeposits. Results show that the Al–Mg alloy coating consists of an Al (Mg) solid solution and the amorphous phase. The effects of the electrodeposition potential and temperature on the morphology of Al–Mg electrodeposits are demonstrated by scanning electron microscopy. © 2015 Published by Elsevier B.V.
1. Introduction As reported, the target chamber load is the key part in the Zpinch driven inertial confinement fusion (ICF), which determines the rate of the converting laser light to the X-ray conversion [1,2]. A wide variety of cylindrical Z-pinch loads, including thin annular foils, gas puffs, low-density foams, and low-number wire arrays, have been used in early experiments. A dramatic breakthrough in X-ray power was achieved in 1995 [3], and tungsten wire has been widely applied in Z-pinch physical experiments [4,5]. Al–Mg alloys are excellent lightweight metallic materials because of their attractive properties, such as high volumetric heat content, high specific strength, high specific stiffness, low density, and strong corrosion resistance [6]. Therefore, Al–Mg alloy wire arrays are also one of the most common materials applied in Z-pinch physical experiments. As previously reported [7–9], a
∗ Corresponding author. Tel.: +86 28 83201171; fax: +86 28 83202550. E-mail address: [email protected] (W. Hu). http://dx.doi.org/10.1016/j.fusengdes.2015.11.053 0920-3796/© 2015 Published by Elsevier B.V.
mixture of aluminum and magnesium is observed to maximize the radiated kilovolt X-ray yield at 50 kJ, which is 1.5 times higher than that obtained with pure aluminum [7]. The mixed Al–W array load can emit equivalent X-ray pulses as the W array load, but the intensity of the X rays pulses is lower than that of pure Al wire array [10]. Magnesium is more active than aluminum. Al–Mg alloys have the advantages of excellent electrical conductivity and thermal conductivity. A higher yield of X rays can be obtained by eletrodepositing Al–Mg alloys onto tungsten wire surface as wire arrays load. The reduction potentials of Al and Mg (−1.706 V for Al and −2.375 V for Mg) are more negative than that of water for hydrolysis [11]. Therefore, aqueous electrolytes can-not be used for the electrodeposition of Al–Mg alloys. Al, Mg, and their alloys have been obtained from non-aqueous electrolytes [12–15]. The electrodeposition of Al–Mg alloys from an organic solvent has limitations, such as poor stability, high toxicity, and heavy pollution. The molten salt system is relatively simple, economical, and environment friendly. In particular, inorganic molten salts can dissolve various metal salts, which are ideal for various metal electrodeposition process.
Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
As previously reported in 2000 [16–22], the Mg atomic ratio during the electrodeposition of Al–Mg alloys was less than 2.2 at.% from the acidic AlCl3 –EMIC melts containing MgCl2 [16]. In 2010, the aluminum-alloyed coating on AZ91D magnesium alloys was obtained from their molten salts containing AlCl3 at lower temperatures. Previous results showed that aluminum-alloyed coating consists of Mg2 Al3 and Mg17 Al12 intermetallic compounds [17]. The Al–Mg alloys were also electrodeposited by an organometallicbased electrolyte with Na[AlEt4 ] + 2 AlEt3 + 3.3 toluene as the electrolyte [19], this previous study was only able to obtain a maximum of approximately 24 wt.% Mg in the alloys. In the present paper, Al–Mg alloys on tungsten wires are obtained from AlCl3 –NaCl–KCl melts (by a Al:Na:K mole ratio of 61:26:13) containing a mass fraction of 3% MgCl2 . SEM analysis also investigated the effect of the electrodeposition potential, temperature on the morphology of Al–Mg electrodeposits. 2. Experimental 2.1. Preparation and purification of melts All chemicals are of analytical-grade. Magnesium chloride and aluminum chloride must be used in a dry environment because of their easy moisture absorption. The aluminum chloride regent bottle is wax sealed. The fine powder of sodium chloride and potassium chloride is heated for 8 h at 300 ◦ C in the muffle furnace to remove moisture [23]. Magnesium chloride is placed in a drying oven at 80 ◦ C for 24 h. A mixture of AlCl3 –NaCl–KCl melts is obtained with a Al:Na:K mole ratio of 61:26:13 [19]. Magnesium chloride at a mass (total mass of AlCl3 –NaCl–KCl powder) fraction of 3% is added into the AlCl3 –NaCl–KCl powder. After weighing, the mixed powder is gently shaken to mix it evenly. The glass beaker containing the mixed powder is placed in an argon atmosphere for 20 min.
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When the oil bath reached the required temperature (T = 180, 200, and 220 ◦ C), the glass beaker is immersed into the bath. The power was melted under the argon atmosphere until the mixture turned into a brown liquid. 2.2. Electrochemical apparatus and electrodes Electrochemical experiments are performed using Metrohm Autolab B.V. electrochemical workstation (PGSTAT302N, Netherlands) with a three electrode system. The working electrode is tungsten wire (99.95%). The counter electrode is a platinum wire electrode (99.99%). The reference electrode is a silver electrode. First, the W electrode is successively polished by SiC emery paper with grits of 500, 1000, and 2000 to obtain a smooth surface before electrochemical measurements. After fully cleaned in an ultrasonic bath for 5 min and dried by thin cool air, the W electrode is immersed in the solution (NaOH, 25 g L−1 ; Na2 CO3 , 50 g L−1 ; Na3 PO4 , 40 g L−1 ) for 10–15 min to remove oil stains and tungsten oxide in an ultrasonic cleaner. Then, Ag reference electrode is immersed in 0.1 mol L−1 HNO3 for a few seconds to remove the oxide. Both the W electrode and the Ag electrode are cleaned with deionized water and ethanol. The platinum wire electrode is cleaned with laboratory detergent and deionized water. Finally, after drying in clean cool air, the three electrodes are directly immersed into the molten salts. Data are collected by the electrochemical workstation. 2.3. Electrodeposition of Al–Mg alloys The substrates are tungsten (99.95%) wires. A cylinder-shaped aluminum foil electrode is used to surround the tungsten electrode, as shown in Fig. 1. The tungsten wires are cleaned as described in Section 2.2. However, the tungsten wires do not need to be mechanically polished by SiC emery paper. The aluminum foil electrode is polished by SiC emery paper of different grits to remove Table 1 Electrodeposition parameters used in this study. Deposit #
E (V)
T (◦ C)
1 2 3 4 5
−1.20 −1.30 −1.40 −1.30 −1.30
200 200 200 180 220
E = deposition potential, T = temperature.
Fig. 1. A schematic diagram of experimental set-up: (1) air inlet; (2) air outlet; (3) rubber stopper; (4) glass beaker; (5) numerical show constant temperature oilbathing; (6) aluminum foil; (7) oil; (8) PTFE; (9) tungsten wires; (10) molten salt.
Fig. 2. LSV of the AlCl3 –NaCl–KCl melts at the scan rate 5 mV s−1 at 200 ◦ C. (The red and black lines are the LSV curves of the electrolytes without MgCl2 and with a mass fraction of 3% MgCl2 , respectively.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Magnification (×5000) of surface morphologies of Al–Mg alloy electrodeposits on tungsten wires: (a) −1.20 V; (b) −1.30 V; (c) −1.40 V. The corresponding EDS analysis of electrodeposits is shown in Fig. 4d, e and f, respectively.
the oxide, cleaned in an ultrasonic bath for 5 min, and immersed in a diluted hydrochloric acid solution for 60 s. After being completely rinsed with deionized water and dried with clean cool air, the cathode and anode are immediately transferred into the glass beaker for immediate use. Prior to electrodeposition of Al–Mg alloy on tungsten wires, the pre-electrodeposition is performed using a stainless steel sheet as a cathode at 70 mA cm−2 for 60 s to remove the oxide layer of aluminum foil. After the electrodeposition experiment, the resulting electrodeposit is thoroughly cleaned by ethanol and deionized water in the ultrasonic cleaner and dried with cool air. A new tungsten wire electrode is used for each deposition. The electrodeposition potentials at −1.20 V, −1.30 V, and −1.40 V are used to investigate the effect of the potential on the Al–Mg deposition. The effect of the temperature is studied at 180, 200 and 220 ◦ C, respectively, while maintaining the electrodeposited time
at approximately 40 min. Table 1 presents the parameters for the experiments in this study.
2.4. Characterization of coating The surface morphologies and nanostructures of Al–Mg alloy coatings are observed by an field emission scanning electron microscopy (FE-SEM) system (Inspect F, FEI Co., U.S.) at 20 kV, coupled with an energy dispersive spectroscopy (EDS) instrument (Edax, Inc. Instruments Co., U.S.), which is used to analyze the element composition of the electrodeposits. The X-ray diffraction (XRD) patterns of the electrodeposits on the tungsten plate, which are cleaned as tungsten wires, are described in Section 2.2 and determined by an X-ray diffractometer (D-Max-␥ type A, Rigaku Co., Japan) with Ni-filtered Cu K␣ radiation and a 0.15406 nm
Y. Li et al. / Fusion Engineering and Design 103 (2016) 8–12
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wavelength operated at 40 kV and 40 mA. Data are obtained from 2Â of 10◦ to 90◦ at a scan rate of 2◦ per step. 3. Results and discussion 3.1. Electrochemical results The linear sweep voltammetry (LSV) of the AlCl3 –NaCl–KCl melts at 200 ◦ C is shown in Fig. 2. The red and black lines are the LSV curves of the electrolytes without MgCl2 and with a mass fraction of 3% MgCl2 , respectively. The potential window ranges from 0 V to −1.80 V at a sweep rate is 5 mV s−1 . When the mole ratio of Al in Al:Na:K is higher than 50% [23], the complex ion Al2 Cl7 − can be produced by the change of AlCl4 − in the AlCl3 –NaCl–KCl melts. Therefore, peaks C1 and C3 , located at −0.90 V and −0.80 V, are ascribed to the electrodeposition of Al reduced from Al2 Cl7 − . The current significantly increases at the potential below −1.2 V (peak C2 ) [24,25]. The increase in the current in this region is possibly generated by the electrodeposition of Al–Mg alloys [26]. 3.2. Effect of constant potential on the morphology and composition of electrodeposits The constant electrodeposition potential of the Al–Mg alloys is conducted on the tungsten wires from the molten salts at 200 ◦ C. The electrodeposition time for each sample is 20 min. The effect of constant potential is investigated at −1.20 V, −1.30 V, and −1.40 V (Table 1). The applied potential significantly affected the morphology of the electrodeposits. As shown in Fig. 3a, the layer of Al–Mg alloys obtained at 180 ◦ C is a very thin. Fig. 3b shows that relatively uniform particles (0.5–1.5 m in size) are electrodeposited on tungsten wires to form an alloy layer. Fig. 3c shows uneven large-size particles with rough surfaces. The appearance of the electrodeposits obtained at −1.30 V is obviously smoother and less granular
Fig. 4. The X-ray diffraction pattern of the Al–Mg electrodeposits: (a) tungsten substrate; (b) −1.20 V; (c) −1.30 V; (d) −1.40 V.
than that of the samples obtained at −1.20 V and −1.40 V. The corresponding EDS data of the electrodeposits are shown in Fig. 3d–f. The EDS results reveal that these electrodeposits are elemental Al and Mg with a small amount of oxygen, which is attributed to the exposure of electrodeposits to air. The samples at −1.20 V and −1.40 V contain 2.55 at.% and 7.68 at.% Mg, respectively, whereas the electrodeposits at −1.30 V contain more than 9 at.% Mg. X-ray diffraction experiments are used to examine the crystal structure of the Al–Mg alloy electrodeposits. Fig. 4 shows the XRD patterns of the electrodeposits obtained at different electrodeposition potentials. The four main reflections peaks at 40.264◦ , 58.274◦ , 73.195◦ , and 87.021◦ correspond to the (1 1 0), (2 0 0), (2 1 1), and (2 2 0) crystalline faces, respectively, which are related to tungsten
Fig. 5. Magnification (×5000) of surface morphologies and EDS data of Al–Mg alloys electrodeposits obtained at −1.30 V: (a and c) 180 ◦ C; (b and d) 220 ◦ C.
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(PDF #04-0806). In the X rays diffraction patterns of the alloyed metal, the primary change is the increased electrodeposition potential, which leads to the disappearance of the face-centered cubic of the Al (Mg) solid solution reflections peak; the development of a broad reflection centered from 20◦ to 30◦ indicates the presence of an amorphous phase of the alloyed metal [27]. The electrodeposits at −1.20 V contain less than 3 at.% Mg (Fig. 3d) and have diffraction patterns where the face-centered cubic Al peaks shift to higher angles, thereby suggesting that the alloying of Mg with Al leads to the formation of an Al (Mg) solid solution [28]. Fig. 4b indicates that this electrodeposit has two phases: the face-centered cubic phase and the amorphous phase [29]. As the electrodeposition potential is increased, the peak intensity of the face-centered cubic phase in the alloy decreases, whereas that of the amorphous phase increases (Fig. 4c–d). Fig. 4c shows that the electrodeposit containing more than 9 at.% Mg (Fig. 3e) is completely amorphous. 3.3. Effect of temperature on electrodeposit morphology The effect of temperature on the Al–Mg electrodeposit is also investigated with AlCl3 –NaCl–KCl (by a Al:Na:K mole ratio of 61:26:13), which contains a mass fraction of 3% MgCl2 . The electrodeposition parameters at three different electrodeposition temperatures are shown in Table 1 (deposits 2, 4 and 5). The SEM images of the Al–Mg alloy electrodeposits obtained at various temperatures are shown in Figs. 3b and 5a and b. The grain size of the electrodeposit gradually increases when the electrodeposition temperature changes from 180 ◦ C to 220 ◦ C. However, the appearance and structure of the electrodeposits remain the same. The grain size of the electrodeposit is influenced by the competition between the nucleation rate and the growth rate of nuclei [26,30]. With the higher nucleation rate during electrodeposition, the appearance and size of the electrodeposit particles are improved. The overpotential is decreased and the metal ion mobility is increased with increasing electrodeposition temperature. Lower overpotential can decrease the nucleation rate. Therefore, the growth rate of nuclei is positively related to the electrodeposition temperature. The EDS data (Figs. 3e and 5c and d) reveal that the amount of Mg in the electrodeposits is increased with electrodeposition temperature increasing from 180 ◦ C to 200 ◦ C, whereas the amount of Mg of electrodeposits at 220 ◦ C is lower. 4. Conclusion Al-Mg alloy coatings on tungsten wires are obtained by electrochemical deposition from AlCl3 –NaCl–KCl melts containing a mass fraction of 3% MgCl2 . Al–Mg alloys with 9.14 at.% Mg are obtained, thereby indicating the strong possibility to create Al–Mg alloy coating on tungsten wire. The effects of deposition voltage and molten salt temperature on the surface morphologies and magnesium content of the Al–Mg deposits are also investigated. As the deposition voltage is increased from −1.20 V to −1.40 V, the grain size of the Al–Mg electrodeposits is gradually increased. The amount of magnesium in the electrodeposits first increases and then decreases. The same trend is observed when increasing the temperature from 180 ◦ C to 220 ◦ C. Therefore, the optimized electrolysis process of the Al–Mg alloy coating of tungsten wire is an electrodeposition potential of −1.30 V at 200 ◦ C. A higher yield of X rays can be obtained by eletrodepositing the Al–Mg alloys onto the surface of tungsten wires, which can be applied in Z-pinch driven ICF experiments. References [1] C. Cerjan, P.T. Springer, S. Sepke, Integrated diagnostic analysis of inertial confinement fusion capsule performancea, Phys. Plasmas 20 (5) (2013) 056319.
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