- Email: [email protected]
Preparation of ultrafine tungsten wire via electrochemical method in an ionic liquid
Fusion Engineering and Design 88 (2013) 23–27
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Preparation of ultrafine tungsten wire via electrochemical method in an ionic liquid Xueqi Gao, Wencheng Hu ∗ , Yushu Gao State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Chengdu, 610054, PR China
h i g h l i g h t s
The method of electrochemical corrosion is used to prepare ultra-fine tungsten wire less than 10 m in diameter. Ionic liquid as a non-aqueous electrolyte was used in electrochemical corrosion experiments. The situation of anode polarization was different from the usual situation. Diameter of tungsten wire has been cut down to 8.5 m uniformly under the optimized electric potential.
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 27 September 2012 Accepted 28 September 2012 Available online 25 October 2012 Key words: Electrochemical corrosion Ionic liquid electrolyte Ultrafine tungsten wire Electrochemical behaviors
a b s t r a c t Ultrafine tungsten wire less than 10 m in diameter is often used as wire array load applied in Inertial Confinement Fusion (ICF) physical experiments. In order to obtain a higher yield of X-ray, both initial radius and line quality of metal wire were required to be of high quality simultaneously. This paper has studied the electrochemical method to corrode tungsten wires uniformly in an ionic liquid electrolyte containing 1 wt% sodium hydroxide. A three electrode system composed of a tungsten anode electrode, a stainless steel cathode and a saturated calomel electrode as a reference electrode, was used in the electrochemical experiments. Liner sweep voltammetry (LSV) and Tafel experiments were used to investigate the electrochemical behaviors of tungsten wires in ionic liquid and aqueous solution. Based on scanning electron microscope (SEM) observation, the morphologies of tungsten wire surface with uniform corrosion under different applied voltages have been demonstrated. X-ray diffraction (XRD) methods were employed to track the evolution of the crystal structure before and after corrosions, and there is an obvious difference in peak intensities. The ultrafine tungsten wire with a uniform diameter of 8.5 m was obtained under the optimized electric potential (2.5 V) applied for decreasing diameter at 30 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In a Z-pinch experiment, a magnetic pressure is created by the azimuthal magnetic field associated with the axial flow of current through a cylindrically symmetric plasma that accelerates the plasma radially inward at high velocities [1]. X-rays are produced when the imploding plasma stagnates on the cylindrical axis of symmetry. In recent years, remarkable progress has been achieved in fast Z pinches, where effective conversion of the kinetic energy of imploding plasma into soft X-ray pulses has been achieved [2]. The key factor in this progress has been the use of cylindrical arrays of a large number (∼400) of fine (∼10 m) metallic wires as a Z-pinch load [2]. Since the late of 1990s, X-ray output power and energy have been greatly improved in Z-pinch physics experiment due to
∗ Corresponding author. E-mail address: [email protected] (W. Hu). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.09.022
the using of metal wires with micrometer-sized diameter applied in preparing cylindrical ICF capsule [1,3]. Tungsten is a metal with an extremely high hardness properties, high melting point, and high corrosion resistance. Hence, tungsten wire is one of most common choice applied in Z-pinch physical experiments [4]. For obtaining a higher yield of X-ray to convert electricity energy stored in the accelerator to implosion kinetic energy to the maximum, it was found in the study of Zpinch that both initial radius and line quality of metal wire were of high quality simultaneous. As the main load wire, tungsten filament was required to be less than 10 m in diameter, which is called ultrafine tungsten. Tungsten wires purchased on the market in general are prepared by a drawing machine. The limitations of technologies on mold holes manufacturing, tungsten tensile strength, and running accuracy of drawing machine lead to the production of ultrafine tungsten process quite complex so that drawing process has much difficulty for volume production. Generally, ultrafine tungsten wires, which are below 10 m in diameter, are obtained by
24
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
chemical corrosion and electrochemical corrosion. During chemical corrosion process, it is required strictly to control the reaction rate, and it is also difficult to guarantee the consistency and concentricity of wire diameter. Therefore, chemical corrosion is rarely used in actual production. Electrochemical corrosion, as an alternative choice, is applied to produce ultrafine tungsten wires. In 2005, Motoki et al. [5] used a 5 mol L−1 solution of KOH as an electrolytic solution to electropolish a 0.5 mm diameter tungsten wire, and the result showed that the polished tungsten wire has a tip radius on the order of 100 nm. In 2008, Bernal and Ávila [6] suggested that appropriate voltages for carrying out the electrochemical reaction are 2.5 V and 3 V for 2 mm and 3 mm depth of immersion in a 2 mol−1 solution of KOH, and they also found that the current was higher for an immersion of 3 mm, which implied the shorter fabrication time with the increasing immersion depth. In 2009, the DC electropolishing process was used by Kulakov et al. [7] to corrode tungsten wires in a 2 mol−1 solution of KOH. Reaction mainly occurred in the vicinity of the electrolyte–air interface in order to create two separate wire parts with nanosharp tips. All of the above study groups focused their interest on the corrosion of tungsten wire in alkaline solution to produce tungsten nano-cusps and the discussion of their properties [5–7]. However, few previous reports considered about decreasing tungsten wire in diameter uniformly by electrochemical corrosion. In the present work, tungsten wires as anodes were decreased in diameter uniformly by electrochemical method in an ionic liquid containing ethylene glycol and choline chloride with a mole ratio of 2:1 and some additives. SEM analysis was also investigated the effect on the result of diameter decreasing under different voltage conditions.
2. Experimental procedures Analytical-grade choline chloride (ChCl) and ethylene glycol (EG) were purchased from Chengdu Changzheng Chemical Corporation. Ethanol with a purity over 99.7% were from Chengdu Kelong Chemical Reagent Factory. Both analytical-grade Sodium hydroxide (NaOH) and calcium chloride anhydrous (CaCl2 ) were also from Kelong Chemical Reagent Factory. The ionic liquid electrolytes were prepared from a mixture of choline chloride and ethylene glycol with a mole ratio of ChCl:EG = 1:2 at 70 ◦ C for 0.5 h [8], and then the mixture was magnetically stirred to form a homogeneous colorless liquid. Before electrochemical experiments, the ionic liquid was treated under a one-tenth atmospheric pressure at 100 ◦ C for 8 h. Further purification of ethanol was a series of treatment involved adding CaCl2 to ethanol, ultrasonic oscillation for 10 min, and then placed the mixture still for 24 h, followed by filtration. Finally, non-aqueous electrolyte solution was obtained from a uniform mixture of ionic liquids with 1 wt% ethanol solution of sodium hydroxide as a kind of additive. Prior to the electrochemical experiment, tungsten wires were immersed in a 3 wt% NaOH solution to remove the oil stains and tungsten oxide in an ultrasonic cleaner, and then washed with water and ethanol in turn. After dried in atmosphere, the tungsten wire was placed in the electrolyte solution as an anode as shown in Fig. 1. The top part was connected to the positive of the power supply, and the opposite part was attached by a small PTFE ring to keep the tungsten electrode vertical. In this system, a cylinder-shaped steel counter electrode was used to surround the tungsten electrode, and a saturated calomel electrode as a reference electrode was applied during electrochemical experiment. In order to reduce tungsten diameter uniformly, we used constant voltage electrolysis corrosion at 30 ◦ C. Electrochemical behaviors of tungsten surface were characterized by Tafel and LSV
1
(-)
(+)
5
6
2 4 3
8 7
Fig. 1. A schematic diagram of experimental set up: (1) power supply, (2) glass electrolytic cell, (3) electrolytic solution + additives, (4) counter electrode, (5) working electrode, (6) reference electrode (SCE), (7) magnet and (8) a PTFE ring.
tests recorded in an electrochemical workstation (CHI660D). XRD experiments were performed in a Philips X’pert X-ray diffractometer with Ni-filtered Cu K␣ radiation and a 0.15406 nm wavelength operated at 40 kV and 40 mA. A field-emission scanning electron microscopy (FE-SEM, Inspect F Co.) was used to observe surface morphology of the corroded tungsten at 20 kV. As a supplement, the compositions of the tungsten surface were characterized by an EDAX Co. energy-dispersive X-ray spectroscopy (EDS) instrument. 3. Analysis and discussion 3.1. Electrochemistry of the tungsten electrode in the ILs electrolytes Fig. 2 shows a relationship curve of the corrosion voltage of tungsten wire with current value in ionic liquid. A tungsten wire working electrode, a steel counter electrode, and a SCE reference electrode were assembled to form a three-electrode system in electrochemical experiment. The applied potential is scanned from 0 V to 4 V to track the electrochemical reaction. From Fig. 2, there is no passivation interval observed, indicating a new mechanism of tungsten dissolved. As the applied voltage increases, the corrosion of tungsten atoms is transformed from a state of active dissolution to a state of rapid corrosion directly. Tungsten atoms on the surface
Fig. 2. curve of tungsten anodic polarization was performed in the ionic liquids at 30 ◦ C, which sweep voltage was from 0 V to 4 V.
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
25
Fig. 3. curve of tungsten anodic polarization in aqueous solution at 30 ◦ C, which sweep voltage was from −0.5 V to 4 V.
are dissolved spontaneously in an activity state when the applied potential below 2.5 V (Fig. 2A to B), and the electrochemical reaction is mainly controlled by the thermodynamic factors. At this voltage region, tungsten electrode dissolved slower than tungsten ion diffused in solution. However, the dissolution of the surface atoms will be mainly affected by kinetic factors and tungsten atoms are dissolved rapidly when the voltage was kept above 2.5 V (Fig. 2B to C). At the applied potential about 2.5 V, the polishing layer is formed by high concentration salt containing tungsten cations and electrolyte anions between tungsten electrode surface and electrolyte solution. Actually, the etching rate and diffusion rate of tungsten is almost equal each other. Therefore, anode reaction is described exactly with the following reaction equation: anode : W(s) + 8OH− → WO4 2− + 4H2 O + 6e−
(1)
As a comparative experiment, the case of tungsten anodic polarization in a NaCl–NaOH–NaNO3 solution with a mass fraction of 3.5%, 1% and 1%, was carried out in the LSV experiment, and the result was shown in Fig. 3. The result is very perfect compatible with many previous studies [9,10] about polarization behavior of other metals in electrochemical experiment. In Fig. 3, there were three typical regions, containing a spontaneous reaction region, a passivation plateau, and pit generation for higher potential followed by oxygen evolution, for high anodic potentials. To gather further evidence, a potentiodynamic polarization scan was performed. The overpotentials were scanned at 0.01 V/s from negative (cathodic) to positive (anodic). The polarization situations of tungsten anodes in ionic liquid and in aqueous solution were confirmed in the Tafel experiments illustrated in Fig. 4a and b, respectively. Observed from the curves, it is not difficult to find that no obvious passivation plateau appears in the curve of Fig. 4a, which can be seen at the voltage region from −0.2 V to 0.1 V in Fig. 4b.
Fig. 4. (a and b) Tefal curve (a) of tungsten anode was performed in the ionic liquids, which sweep voltage was from 0 V to 4 V, and curve (b) was performed in aqueous solution at the range from −0.5 V to 4 V of sweep voltage.
3.2. SEM analysis The SEM image of tungsten wire without corrosion, whose diameter is about 12.8 m, was showed in Fig. 5. Scratches generated in the drawing process are clearly visible on the surface of tungsten wire. Fig. 6 displays three typical SEM images of tungsten wire corroded at different voltages in the ionic liquid. In the voltage region below 2.5 V (Fig. 2A to B), tungsten surface is in a state of active dissolution due to tungsten electrode dissolved slower than tungsten ion diffused in solution so that tungsten wire
Fig. 5. image of the raw materials: tungsten wire before electrochemical corrosion.
26
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
Fig. 7. EDS analysis of the tungsten surface performed at 20 kV: y-axis (KCnt = 1000’s of counts) represents the number of counts per channel; and the x-axis represents X-ray energy.
2.5 V could be the optimal potential for corrosion of tungsten wire applied in decreasing diameter uniformly. Fig. 6c shows the typical image of tungsten wire with pits, which was corroded at the applied potential 3.0 V (Fig. 2B to C). The pits are inevitable due to a lager of gas generated with the increasing voltage. 3.3. EDS analysis To further confirm the purpose of no tungsten oxide generated during the process of corrosion in the ionic liquid, EDS was performed to detect the composition of the tungsten surface and the result was shown in Fig. 7. The strong signal of W was shown in the EDS pattern, which clearly revealed the composition of the sample surface as pure tungsten with 100% concentration. This EDS result is agreement with the experimental result of the electrochemical. 3.4. XRD analysis X-ray diffraction experiments were used to examine the crystal structure of tungsten wires before and after corrosion. Fig. 8 shows XRD patterns of tungsten wires before and after corrosion. It is obvious that only the diffraction peaks of tungsten crystal were observed, indicating no tungsten oxide or other by-products formed in electrochemical experiment. In Fig. 8, five diffraction peaks with lattice spacings d = 2.24, 1.58, 1.29, 1.11 and 1.00 were
Fig. 6. (a–c) Images of tungsten wires of three specimens corroded at different voltages: (a) 2.2 V; (b) 2.5 V, and (c) 3.0 V.
is corroded to decrease diameter resulting in a not smooth surface, which scratches similar with non-corroded surface are still found in Fig. 6a. With applied voltage increasing, concentration polarization is caused rapidly on the surface of tungsten electrode at about 2.5 V (Fig. 2B). The etching rate of tungsten atoms and diffusion rate of tungsten ion are almost equal each other, leading to a uniform surface that is critical for decreasing uniformly in diameter. Fig. 6b shows uniform tungsten wire with a diameter of 8.5 m. The smooth and flat surface without scratches or pits confirms that
Fig. 8. XRD patterns of tungsten wires before and after corrosion.
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
located at 40.26◦ , 58.274◦ , 73.19◦ , 87.02◦ and 100.65◦ , corresponding to the characteristic diffraction peaks ((1 1 0), (2 0 0), (2 1 1), (2 2 0), and (3 1 0)) of the face-centered cubic W texture (PDF#040806). After calculation, lattice constants a = 3.615 A˚ was obtained. Although the two samples are similar to some extent in crystalline structure, significant differences still exist obviously. The peak intensities shown in the pattern b were relatively higher than those in the pattern a, especially the diffraction peak of (1 1 0) crystal faces. There are two possibilities to explain this result. Firstly, part of crystal faces disappeared during the preparation of tungsten wires by the drawing method, resulting in a decreasing in intensity of diffraction peaks. Another is probably related to lattice energy. Compared pattern b with pattern a in Fig. 8, intensities of all crystal faces actually increased in some degree. Obviously, (1 1 0) crystal faces certainly increased much more than others. According to the principles of thermodynamics, surface atoms in (1 1 0) crystal faces with low lattice energy were dissolved slower than those with a relatively high lattice energy, which leads to an accumulation of many crystal faces with a high lattice energy during the process of electrochemical reaction. 4. Conclusion Ultrafine tungsten wire was obtained by the method of electrochemical corrosion in ionic liquid electrolyte composed of choline chloride and ethylene glycol, and additive. From the SEM images, ultrafine tungsten wire with an 8.47 m diameter is observed, indicating that there is a strong possibility to create ultrafine tungsten wire with smaller diameter by the electrochemical method. It is shown that the electrochemical behavior of tungsten anode is different from the sample in the aqueous system and no passivation plateaus were observed in the LSV and the Tafel experiments. There
27
were no oxidation and dissolution processes of tungsten during the electrochemical reaction in the ionic liquid, however, those phenomena will occur in aqueous solutions. From the result of electrochemical experiments, the optimal applied potential for uniformly decreasing in diameter of tungsten wire is 2.5 V. The XRD patterns reveal that surface atoms with low lattice energy are dissolved slower than those with high lattice energies. References [1] M. Keith Matzen, M.A. Sweeney, R.G. Adams, J.R. Asay, J.E. Bailey, G.R. Bennett, et al., Pulsed-power-driven high energy density physics and inertial confinement fusion research, Physics of Plasmas 12 (2005) 1–16. [2] S.V. Lebedev, F.N. Beg, S.N. Bland, J.P. Chittenden, A.E. Dangor, M.G. Haines, et al., X-ray backlighting of wire array Z-pinch implosions using X-pinch, Review of Scientific Instruments 72 (2001) 671–673. [3] N. Singer, Another dramatic climb toward fusion conditions for Sandia Z accelerator [EB/OL] [2005–06–03], http://www.sandia.gov/media/z290.htm [4] M.E. Cuneo, E.M. Waisman, S.V. Lebedev, J.P. Chittenden, W.A. Stygar, G.A. Chandler, et al., Characteristics and scaling of tungsten-wire-array Z-pinch implosion dynamics at 20 MA, Physical Review E 71 (2005) 1–43. [5] T. Motoki, W. Gao, S. Kiyono, T. Ono, A nanoindentation instrument for mechanical property measurement of 3D micro/nanostructured surfaces, Measurement Science and Technology 17 (2006) 495–499. [6] R. Bernal, A. Ávila, Reproducible fabrication of scanning tunneling microscope tips, Revista de Ingeniería. 27 (2008) 43–48. [7] M. Kulakov, I. Luzinov, K.G. Kornev, Capillary and surface effects in the formation of nanosharp tungsten tips by electropolishing, Langmuir 25 (2009) 4462–4468. [8] A.P. Abbott, G. Capper, B.G. Swain, D.A. Wheeler, Electropolishing of stainless steel in an ionic liquid, Transactions of the Institution of Metal Finishing 83 (2005) 51–53. [9] M. Yu, J.L. Yi, J.H. Liu, S.M. Li, G.L. Wu, L. Wu, Effect of electropolishing on electrochemical behaviours of titanium alloy Ti–10V–2Fe–3Al, Journal of Wuhan University of Technology – Materials Science Edition 26 (2011) 469–477. [10] B. Dua, I.I. Suni, Mechanistic studies of Cu electropolishing in phosphoric acid electrolytes, Journal of the Electrochemical Society 151 (2004) 375–378.
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Preparation of ultrafine tungsten wire via electrochemical method in an ionic liquid Xueqi Gao, Wencheng Hu ∗ , Yushu Gao State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Chengdu, 610054, PR China
h i g h l i g h t s
The method of electrochemical corrosion is used to prepare ultra-fine tungsten wire less than 10 m in diameter. Ionic liquid as a non-aqueous electrolyte was used in electrochemical corrosion experiments. The situation of anode polarization was different from the usual situation. Diameter of tungsten wire has been cut down to 8.5 m uniformly under the optimized electric potential.
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 27 September 2012 Accepted 28 September 2012 Available online 25 October 2012 Key words: Electrochemical corrosion Ionic liquid electrolyte Ultrafine tungsten wire Electrochemical behaviors
a b s t r a c t Ultrafine tungsten wire less than 10 m in diameter is often used as wire array load applied in Inertial Confinement Fusion (ICF) physical experiments. In order to obtain a higher yield of X-ray, both initial radius and line quality of metal wire were required to be of high quality simultaneously. This paper has studied the electrochemical method to corrode tungsten wires uniformly in an ionic liquid electrolyte containing 1 wt% sodium hydroxide. A three electrode system composed of a tungsten anode electrode, a stainless steel cathode and a saturated calomel electrode as a reference electrode, was used in the electrochemical experiments. Liner sweep voltammetry (LSV) and Tafel experiments were used to investigate the electrochemical behaviors of tungsten wires in ionic liquid and aqueous solution. Based on scanning electron microscope (SEM) observation, the morphologies of tungsten wire surface with uniform corrosion under different applied voltages have been demonstrated. X-ray diffraction (XRD) methods were employed to track the evolution of the crystal structure before and after corrosions, and there is an obvious difference in peak intensities. The ultrafine tungsten wire with a uniform diameter of 8.5 m was obtained under the optimized electric potential (2.5 V) applied for decreasing diameter at 30 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In a Z-pinch experiment, a magnetic pressure is created by the azimuthal magnetic field associated with the axial flow of current through a cylindrically symmetric plasma that accelerates the plasma radially inward at high velocities [1]. X-rays are produced when the imploding plasma stagnates on the cylindrical axis of symmetry. In recent years, remarkable progress has been achieved in fast Z pinches, where effective conversion of the kinetic energy of imploding plasma into soft X-ray pulses has been achieved [2]. The key factor in this progress has been the use of cylindrical arrays of a large number (∼400) of fine (∼10 m) metallic wires as a Z-pinch load [2]. Since the late of 1990s, X-ray output power and energy have been greatly improved in Z-pinch physics experiment due to
∗ Corresponding author. E-mail address: [email protected] (W. Hu). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.09.022
the using of metal wires with micrometer-sized diameter applied in preparing cylindrical ICF capsule [1,3]. Tungsten is a metal with an extremely high hardness properties, high melting point, and high corrosion resistance. Hence, tungsten wire is one of most common choice applied in Z-pinch physical experiments [4]. For obtaining a higher yield of X-ray to convert electricity energy stored in the accelerator to implosion kinetic energy to the maximum, it was found in the study of Zpinch that both initial radius and line quality of metal wire were of high quality simultaneous. As the main load wire, tungsten filament was required to be less than 10 m in diameter, which is called ultrafine tungsten. Tungsten wires purchased on the market in general are prepared by a drawing machine. The limitations of technologies on mold holes manufacturing, tungsten tensile strength, and running accuracy of drawing machine lead to the production of ultrafine tungsten process quite complex so that drawing process has much difficulty for volume production. Generally, ultrafine tungsten wires, which are below 10 m in diameter, are obtained by
24
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
chemical corrosion and electrochemical corrosion. During chemical corrosion process, it is required strictly to control the reaction rate, and it is also difficult to guarantee the consistency and concentricity of wire diameter. Therefore, chemical corrosion is rarely used in actual production. Electrochemical corrosion, as an alternative choice, is applied to produce ultrafine tungsten wires. In 2005, Motoki et al. [5] used a 5 mol L−1 solution of KOH as an electrolytic solution to electropolish a 0.5 mm diameter tungsten wire, and the result showed that the polished tungsten wire has a tip radius on the order of 100 nm. In 2008, Bernal and Ávila [6] suggested that appropriate voltages for carrying out the electrochemical reaction are 2.5 V and 3 V for 2 mm and 3 mm depth of immersion in a 2 mol−1 solution of KOH, and they also found that the current was higher for an immersion of 3 mm, which implied the shorter fabrication time with the increasing immersion depth. In 2009, the DC electropolishing process was used by Kulakov et al. [7] to corrode tungsten wires in a 2 mol−1 solution of KOH. Reaction mainly occurred in the vicinity of the electrolyte–air interface in order to create two separate wire parts with nanosharp tips. All of the above study groups focused their interest on the corrosion of tungsten wire in alkaline solution to produce tungsten nano-cusps and the discussion of their properties [5–7]. However, few previous reports considered about decreasing tungsten wire in diameter uniformly by electrochemical corrosion. In the present work, tungsten wires as anodes were decreased in diameter uniformly by electrochemical method in an ionic liquid containing ethylene glycol and choline chloride with a mole ratio of 2:1 and some additives. SEM analysis was also investigated the effect on the result of diameter decreasing under different voltage conditions.
2. Experimental procedures Analytical-grade choline chloride (ChCl) and ethylene glycol (EG) were purchased from Chengdu Changzheng Chemical Corporation. Ethanol with a purity over 99.7% were from Chengdu Kelong Chemical Reagent Factory. Both analytical-grade Sodium hydroxide (NaOH) and calcium chloride anhydrous (CaCl2 ) were also from Kelong Chemical Reagent Factory. The ionic liquid electrolytes were prepared from a mixture of choline chloride and ethylene glycol with a mole ratio of ChCl:EG = 1:2 at 70 ◦ C for 0.5 h [8], and then the mixture was magnetically stirred to form a homogeneous colorless liquid. Before electrochemical experiments, the ionic liquid was treated under a one-tenth atmospheric pressure at 100 ◦ C for 8 h. Further purification of ethanol was a series of treatment involved adding CaCl2 to ethanol, ultrasonic oscillation for 10 min, and then placed the mixture still for 24 h, followed by filtration. Finally, non-aqueous electrolyte solution was obtained from a uniform mixture of ionic liquids with 1 wt% ethanol solution of sodium hydroxide as a kind of additive. Prior to the electrochemical experiment, tungsten wires were immersed in a 3 wt% NaOH solution to remove the oil stains and tungsten oxide in an ultrasonic cleaner, and then washed with water and ethanol in turn. After dried in atmosphere, the tungsten wire was placed in the electrolyte solution as an anode as shown in Fig. 1. The top part was connected to the positive of the power supply, and the opposite part was attached by a small PTFE ring to keep the tungsten electrode vertical. In this system, a cylinder-shaped steel counter electrode was used to surround the tungsten electrode, and a saturated calomel electrode as a reference electrode was applied during electrochemical experiment. In order to reduce tungsten diameter uniformly, we used constant voltage electrolysis corrosion at 30 ◦ C. Electrochemical behaviors of tungsten surface were characterized by Tafel and LSV
1
(-)
(+)
5
6
2 4 3
8 7
Fig. 1. A schematic diagram of experimental set up: (1) power supply, (2) glass electrolytic cell, (3) electrolytic solution + additives, (4) counter electrode, (5) working electrode, (6) reference electrode (SCE), (7) magnet and (8) a PTFE ring.
tests recorded in an electrochemical workstation (CHI660D). XRD experiments were performed in a Philips X’pert X-ray diffractometer with Ni-filtered Cu K␣ radiation and a 0.15406 nm wavelength operated at 40 kV and 40 mA. A field-emission scanning electron microscopy (FE-SEM, Inspect F Co.) was used to observe surface morphology of the corroded tungsten at 20 kV. As a supplement, the compositions of the tungsten surface were characterized by an EDAX Co. energy-dispersive X-ray spectroscopy (EDS) instrument. 3. Analysis and discussion 3.1. Electrochemistry of the tungsten electrode in the ILs electrolytes Fig. 2 shows a relationship curve of the corrosion voltage of tungsten wire with current value in ionic liquid. A tungsten wire working electrode, a steel counter electrode, and a SCE reference electrode were assembled to form a three-electrode system in electrochemical experiment. The applied potential is scanned from 0 V to 4 V to track the electrochemical reaction. From Fig. 2, there is no passivation interval observed, indicating a new mechanism of tungsten dissolved. As the applied voltage increases, the corrosion of tungsten atoms is transformed from a state of active dissolution to a state of rapid corrosion directly. Tungsten atoms on the surface
Fig. 2. curve of tungsten anodic polarization was performed in the ionic liquids at 30 ◦ C, which sweep voltage was from 0 V to 4 V.
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
25
Fig. 3. curve of tungsten anodic polarization in aqueous solution at 30 ◦ C, which sweep voltage was from −0.5 V to 4 V.
are dissolved spontaneously in an activity state when the applied potential below 2.5 V (Fig. 2A to B), and the electrochemical reaction is mainly controlled by the thermodynamic factors. At this voltage region, tungsten electrode dissolved slower than tungsten ion diffused in solution. However, the dissolution of the surface atoms will be mainly affected by kinetic factors and tungsten atoms are dissolved rapidly when the voltage was kept above 2.5 V (Fig. 2B to C). At the applied potential about 2.5 V, the polishing layer is formed by high concentration salt containing tungsten cations and electrolyte anions between tungsten electrode surface and electrolyte solution. Actually, the etching rate and diffusion rate of tungsten is almost equal each other. Therefore, anode reaction is described exactly with the following reaction equation: anode : W(s) + 8OH− → WO4 2− + 4H2 O + 6e−
(1)
As a comparative experiment, the case of tungsten anodic polarization in a NaCl–NaOH–NaNO3 solution with a mass fraction of 3.5%, 1% and 1%, was carried out in the LSV experiment, and the result was shown in Fig. 3. The result is very perfect compatible with many previous studies [9,10] about polarization behavior of other metals in electrochemical experiment. In Fig. 3, there were three typical regions, containing a spontaneous reaction region, a passivation plateau, and pit generation for higher potential followed by oxygen evolution, for high anodic potentials. To gather further evidence, a potentiodynamic polarization scan was performed. The overpotentials were scanned at 0.01 V/s from negative (cathodic) to positive (anodic). The polarization situations of tungsten anodes in ionic liquid and in aqueous solution were confirmed in the Tafel experiments illustrated in Fig. 4a and b, respectively. Observed from the curves, it is not difficult to find that no obvious passivation plateau appears in the curve of Fig. 4a, which can be seen at the voltage region from −0.2 V to 0.1 V in Fig. 4b.
Fig. 4. (a and b) Tefal curve (a) of tungsten anode was performed in the ionic liquids, which sweep voltage was from 0 V to 4 V, and curve (b) was performed in aqueous solution at the range from −0.5 V to 4 V of sweep voltage.
3.2. SEM analysis The SEM image of tungsten wire without corrosion, whose diameter is about 12.8 m, was showed in Fig. 5. Scratches generated in the drawing process are clearly visible on the surface of tungsten wire. Fig. 6 displays three typical SEM images of tungsten wire corroded at different voltages in the ionic liquid. In the voltage region below 2.5 V (Fig. 2A to B), tungsten surface is in a state of active dissolution due to tungsten electrode dissolved slower than tungsten ion diffused in solution so that tungsten wire
Fig. 5. image of the raw materials: tungsten wire before electrochemical corrosion.
26
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
Fig. 7. EDS analysis of the tungsten surface performed at 20 kV: y-axis (KCnt = 1000’s of counts) represents the number of counts per channel; and the x-axis represents X-ray energy.
2.5 V could be the optimal potential for corrosion of tungsten wire applied in decreasing diameter uniformly. Fig. 6c shows the typical image of tungsten wire with pits, which was corroded at the applied potential 3.0 V (Fig. 2B to C). The pits are inevitable due to a lager of gas generated with the increasing voltage. 3.3. EDS analysis To further confirm the purpose of no tungsten oxide generated during the process of corrosion in the ionic liquid, EDS was performed to detect the composition of the tungsten surface and the result was shown in Fig. 7. The strong signal of W was shown in the EDS pattern, which clearly revealed the composition of the sample surface as pure tungsten with 100% concentration. This EDS result is agreement with the experimental result of the electrochemical. 3.4. XRD analysis X-ray diffraction experiments were used to examine the crystal structure of tungsten wires before and after corrosion. Fig. 8 shows XRD patterns of tungsten wires before and after corrosion. It is obvious that only the diffraction peaks of tungsten crystal were observed, indicating no tungsten oxide or other by-products formed in electrochemical experiment. In Fig. 8, five diffraction peaks with lattice spacings d = 2.24, 1.58, 1.29, 1.11 and 1.00 were
Fig. 6. (a–c) Images of tungsten wires of three specimens corroded at different voltages: (a) 2.2 V; (b) 2.5 V, and (c) 3.0 V.
is corroded to decrease diameter resulting in a not smooth surface, which scratches similar with non-corroded surface are still found in Fig. 6a. With applied voltage increasing, concentration polarization is caused rapidly on the surface of tungsten electrode at about 2.5 V (Fig. 2B). The etching rate of tungsten atoms and diffusion rate of tungsten ion are almost equal each other, leading to a uniform surface that is critical for decreasing uniformly in diameter. Fig. 6b shows uniform tungsten wire with a diameter of 8.5 m. The smooth and flat surface without scratches or pits confirms that
Fig. 8. XRD patterns of tungsten wires before and after corrosion.
X. Gao et al. / Fusion Engineering and Design 88 (2013) 23–27
located at 40.26◦ , 58.274◦ , 73.19◦ , 87.02◦ and 100.65◦ , corresponding to the characteristic diffraction peaks ((1 1 0), (2 0 0), (2 1 1), (2 2 0), and (3 1 0)) of the face-centered cubic W texture (PDF#040806). After calculation, lattice constants a = 3.615 A˚ was obtained. Although the two samples are similar to some extent in crystalline structure, significant differences still exist obviously. The peak intensities shown in the pattern b were relatively higher than those in the pattern a, especially the diffraction peak of (1 1 0) crystal faces. There are two possibilities to explain this result. Firstly, part of crystal faces disappeared during the preparation of tungsten wires by the drawing method, resulting in a decreasing in intensity of diffraction peaks. Another is probably related to lattice energy. Compared pattern b with pattern a in Fig. 8, intensities of all crystal faces actually increased in some degree. Obviously, (1 1 0) crystal faces certainly increased much more than others. According to the principles of thermodynamics, surface atoms in (1 1 0) crystal faces with low lattice energy were dissolved slower than those with a relatively high lattice energy, which leads to an accumulation of many crystal faces with a high lattice energy during the process of electrochemical reaction. 4. Conclusion Ultrafine tungsten wire was obtained by the method of electrochemical corrosion in ionic liquid electrolyte composed of choline chloride and ethylene glycol, and additive. From the SEM images, ultrafine tungsten wire with an 8.47 m diameter is observed, indicating that there is a strong possibility to create ultrafine tungsten wire with smaller diameter by the electrochemical method. It is shown that the electrochemical behavior of tungsten anode is different from the sample in the aqueous system and no passivation plateaus were observed in the LSV and the Tafel experiments. There
27
were no oxidation and dissolution processes of tungsten during the electrochemical reaction in the ionic liquid, however, those phenomena will occur in aqueous solutions. From the result of electrochemical experiments, the optimal applied potential for uniformly decreasing in diameter of tungsten wire is 2.5 V. The XRD patterns reveal that surface atoms with low lattice energy are dissolved slower than those with high lattice energies. References [1] M. Keith Matzen, M.A. Sweeney, R.G. Adams, J.R. Asay, J.E. Bailey, G.R. Bennett, et al., Pulsed-power-driven high energy density physics and inertial confinement fusion research, Physics of Plasmas 12 (2005) 1–16. [2] S.V. Lebedev, F.N. Beg, S.N. Bland, J.P. Chittenden, A.E. Dangor, M.G. Haines, et al., X-ray backlighting of wire array Z-pinch implosions using X-pinch, Review of Scientific Instruments 72 (2001) 671–673. [3] N. Singer, Another dramatic climb toward fusion conditions for Sandia Z accelerator [EB/OL] [2005–06–03], http://www.sandia.gov/media/z290.htm [4] M.E. Cuneo, E.M. Waisman, S.V. Lebedev, J.P. Chittenden, W.A. Stygar, G.A. Chandler, et al., Characteristics and scaling of tungsten-wire-array Z-pinch implosion dynamics at 20 MA, Physical Review E 71 (2005) 1–43. [5] T. Motoki, W. Gao, S. Kiyono, T. Ono, A nanoindentation instrument for mechanical property measurement of 3D micro/nanostructured surfaces, Measurement Science and Technology 17 (2006) 495–499. [6] R. Bernal, A. Ávila, Reproducible fabrication of scanning tunneling microscope tips, Revista de Ingeniería. 27 (2008) 43–48. [7] M. Kulakov, I. Luzinov, K.G. Kornev, Capillary and surface effects in the formation of nanosharp tungsten tips by electropolishing, Langmuir 25 (2009) 4462–4468. [8] A.P. Abbott, G. Capper, B.G. Swain, D.A. Wheeler, Electropolishing of stainless steel in an ionic liquid, Transactions of the Institution of Metal Finishing 83 (2005) 51–53. [9] M. Yu, J.L. Yi, J.H. Liu, S.M. Li, G.L. Wu, L. Wu, Effect of electropolishing on electrochemical behaviours of titanium alloy Ti–10V–2Fe–3Al, Journal of Wuhan University of Technology – Materials Science Edition 26 (2011) 469–477. [10] B. Dua, I.I. Suni, Mechanistic studies of Cu electropolishing in phosphoric acid electrolytes, Journal of the Electrochemical Society 151 (2004) 375–378.