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Synthesis of a bimodal porous Cu with nanopores on the inner surface of Gasar pores: Influences of preparation conditions
Applied Surface Science 360 (2016) 148–156
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Synthesis of a bimodal porous Cu with nanopores on the inner surface of Gasar pores: Influences of preparation conditions Ming Du a,1 , Hua-wei Zhang a,b,1 , Yan-xiang Li a,b,∗ , Yuan Liu a,b , Xiang Chen a,b , Yun He a a b
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Key Laboratory for Advanced Materials Processing Technology, MOE, PR China
a r t i c l e
i n f o
Article history: Received 22 August 2015 Received in revised form 2 November 2015 Accepted 3 November 2015 Available online 10 November 2015 Keywords: Bimodal porous structure Gasar Dealloying
a b s t r a c t A bimodal porous Cu with regular Gasar micrometer or millimeter pores and random dealloying nanometer pores was fabricated by chemical corrosion of Cu–Zn alloy layers on the inner surface of Gasar Cu pores. In order to accomplish this object, a two-step corrosion method was conducted, including the selective corrosion of Zn element from Cu–Zn alloy layers by NaOH solution, and the removal of the surface oxide obtained in the first step to expose the bottom nanoporous Cu films by HCl solution. Influences of preparation conditions, such as HCl solution concentration and processing time, NaOH solution dealloying time, Gasar pore diameter, and the distance from Gasar pore openings on the microstructure of the resulting nanoporous Cu films were discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Gasar is a metal–gas eutectic unidirectional solidification process, which has been applied in preparing porous materials with regularly directional pores of micrometer or millimeter scales [1,2]. Both elementary substances, such as Cu [3,4], Mg [5,6], Al [7,8] and Si [9], and alloys, such as Cu–Mn [10,11], have been fabricated into the unimodal porous structure using the Gasar technique. Owing to their anisotropic mechanical properties, good permeability and lightweight originating from the oriented pores, Gasar structures have become promising candidates for being used as heat sinks and biomaterials recently [12–15]. Dealloying is a popular process to fabricate random nanoporous metals by a selective corrosion of the most electrochemical active element in alloys [16]. Recently, this corrosion method has been applied in numerous of binary alloy systems, such as Au–Ag [17], Au–Cu [18], Pt–Cu [19], Ag–Zn [20], Ni–Mn [21], Cu–Mn [22], Cu–Al [23] and Cu–Zn [24], to fabricate nanoporous metals which have attracted much interest for applications in catalysis, sensing, photonics, energy storage, etc. due to their perfect electrical conductivity, enhanced physicochemical activity and high surface area [25–27].
∗ Corresponding author at: School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (Y.-x. Li). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.apsusc.2015.11.033 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Bimodal micro/nanoporous structures comprised of micrometer or millimeter pores and nanometer pores are considered to be potential candidates in fields of surface-enhanced Raman scattering [28], electrodes [29] and fuel cells [30], because of their outstanding permeability and high surface area. So far, several processes have been reported to fabricate this kind of porous materials [31–33]. Among them, the combined technique of Gasar and dealloying has been proposed to fabricate a kind of bimodal porous Cu with regular big and random small pores [34,35]. The feasible route includes two steps: (1) fabricating Gasar Cu–Mn alloys; and (2) chemical or electrochemical dealloying. After these steps, the resultant small dealloying pores penetrate big Gasar pore walls. Thus, the bimodal porous Cu possesses not only a good mass transport property originating from Gasar pores but also high surface area inheriting from nanopores. Here, another method is proposed to prepare an allotrope of this kind of bimodal porous Cu, which includes four steps: (1) fabrication of Gasar Cu; (2) deposition of an active metal on the inner surface of Gasar Cu pores; (3) annealing treatment to form alloy layers in situ; and (4) selective corrosion of the active element to prepare nanoporous structures by a dealloying method. The resulting nanopores only distributed on the inner surface of Gasar pores can increase the surface area of the Gasar Cu substrate as well as keep the structure integrality of the Gasar Cu substrate furthest. In addition, the typical structure of nanopores distributed on the surface of a metal substrate has an application advantage in fields where use its surface features, such as sensors [36] and surface-enhanced boiling heat transfers [37]. For the bimodal porous Cu with nanopores on the inner surface of Gasar Cu
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pores, it can be used in some potential fields, such as sensors and heat sinks, on the basis of the good directionality and permeability of Gasar pores and the high surface area of nanopores. Taken heat sinks for example, it has been widely reported [12,14,38,39] that heat sinks fabricated by Gasar Cu possesses excellent heat dissipation property because of Gasar pores providing flowing channels for coolant and high heat conductivity of Cu matrix. In addition, according to many reports [40–42], a rough pore wall with high surface area can enhance the heat dissipation performance of heat sinks, especially when the pore diameter is micrometer range. Therefore, the nanopores can not only coarsen the inner surface of Gasar pores, but also increase the surface area of Gasar Cu, which means that heat sinks fabricated by the bimodal porous Cu will possess more outstanding heat dissipation performance than that of Gasar Cu heat sinks. Because the standard equilibrium potential of Zn2+ /Zn is much lower than that of Cu2+ /Cu, the active Zn component in Cu–Zn alloys can be selectively corroded, while the noble Cu remains and diffuses to form nanoporous structures [43]. Therefore, Zn element is chosen as the transitional metal to fabricate the bimodal porous Cu in this work. A two-step corrosion method is employed to fabricate nanopores on the inner surface of Gasar pores, including that Zn element is selectively corroded by NaOH solution, and then the surface oxide obtained in the first step is removed by HCl solution. Our previous work [44] has justified that the established process route is feasible to fabricate the bimodal porous Cu, but does not address influences of corrosion technological conditions and Gasar structural parameters on the microstructure of the obtaining nanopores on the inner surface of Gasar pores. However, these conditions are the key factors in tuning the microstructure of the obtained nanopores. Therefore, on the basis of the fixed corrosion route, preparation conditions are studied and optimized further here. Particularly, for the first step of NaOH solution selective corrosion, how dealloying time, Gasar pore diameter, and the distance from Gasar pore openings impact on the microstructure of the obtained nanoporous Cu films is studied respectively. For the second step of HCl solution treatment, the HCl solution concentration and processing time are optimized to remove the surface oxide and simultaneously to avoid the coarsening of the nanoporous structure.
2. Experimental procedures Gasar Cu samples with average pore diameter of 400 m and 1.5 mm were prepared by the unidirectional solidification Gasar process [45]. Half-cubic specimens with the width of 10 mm, the length of 10 mm, and the height of 5 mm were cut from the Gasar ingot by an electric sparking cutting machine. Prior to the deposition, Gasar Cu specimens were degreased in HCl solution for 2 min with ultrasonic washing, and then cleaned in acetone, alcohol and deionized water, orderly. After the cleaning, Zn coatings were deposited at the temperature of about 70 ◦ C using a material system which contains 5 M NaOH solution and 3 g Zn powders. Gasar Cu, NaOH solution and Zn powders constituted a kind of a copper–zinc cell, resulting in that Zn coatings were deposited on the inner surface of Gasar pores. Ultrasonic vibration was employed to discharge the air and the generated gas from Gasar pores and to drive sufficient solution inflowing timely. After rinsed extensively with deionized water and dried in a vacuum drying oven, specimens with Zn coatings were annealed at 410 ◦ C for 0.5 h in an Ar-filled tube furnace to obtain Cu–Zn alloy layers. The depositing and alloying processes have been discussed in detail in our previous report [46]. The tempered specimens were then immerged into the corrosion solution (1.5 M NaOH solution) at the ambient temperature
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for a period between 3 h and 24 h. Electromagnetic stirring was proposed to drive the solution flowing into oriented Gasar pores throughout the dealloying process, which could assist in the dealloying of Cu–Zn alloy layers on the inner surface of Gasar pores. After the first corrosion process, specimens were immerged into HCl solution with varying concentrations and processing times in order to remove the surface oxide. Specimens after HCl solution treatment were placed in a vacuum drying oven to avoid the oxidation of the high free energy nanoporous surfaces, waiting for the following microstructural characterization. Phase constitution of the Gasar Cu substrate, the precursory alloy layer, and the as-dealloyed specimen before and after HCl solution treatment was determined by X-ray diffraction (XRD) analysis on a D8-Advance diffraction meter (Cu K␣ radiation, = 0.154 nm) at a scanning rate of 4◦ /min. Microstructure and element composition of the precursory Cu–Zn alloy layers and the nanoporous Cu films obtained after the two-step corrosion method was observed using field emission scanning electron microscopy (FESEM) (Zeiss MERLIN-VP-COMPACT) equipped with energydispersive X-ray spectroscope (EDX) analyzer (Oxford INCA).
3. Results and discussion Fig. 1 shows schematic diagrams of the route for fabricating the bimodal porous Cu. It can be seen that the route covers broadly four steps: fabricating Gasar Cu (Fig. 1a), depositing Zn coatings (Fig. 1b), annealing (Fig. 1c) and dealloying (Fig. 1d). Consequently, the bimodal porous structure with nanopores only distributed on the inner surface of Gasar pores is formed, whose detailed structure is represented in Fig. 1(e). Fig. 2 shows FESEM images of the inner surface of Gasar pores (a) after depositing (b) and annealing (c). It can be seen from Fig. 2(a) that the initial inner surface of Gasar pores is dense. After depositing Zn coatings, the surface presents a porous structure, which is shown in Fig. 2(b). However, as can be seen from Fig. 2(c), the surface becomes compact and uniform again because of the densification and alloying promoted by the annealing treatment. EDX result indicates that the Zn content in the obtained alloy layers is about 34 at.%. The phase constitution of the original Gasar Cu specimens and the alloy layers is detected by XRD analysis, and the patterns are shown in Fig. 3. It can be seen from Fig. 3(a) that the phase constitution of Gasar Cu is the single-phase Cu (pattern 1) (JCPDS file no. 04-0836). However, after the depositing and the annealing treatments, minor peaks occur and can be indexed to ␣Cu–Zn phase (pattern 2) (JCPDS file no.25-0322). Fig. 3(b) and c shows XRD diffraction peaks locating at the 2 of 41.5–42.8◦ (b) and 48.4–49.8◦ (c). It can be clearly seen that the peak intensity changes in these ranges, visibly proving that the detected phase of the alloy layers obtained after depositing and alloying is ␣Cu–Zn. Cu–Zn alloy layers have been prepared on the inner surface of Gasar pores uniformly using depositing and annealing processes. In the following, the alloys are tried to be dealloyed by chemical corrosion with electromagnetic stirring. Fig. 4(a) shows the microstructure of the alloy layers on the inner surface of Gasar pores just after the NaOH solution dealloying for 12 h. It can be seen that the obtained surface is covered by nanoparticles, and EDX analysis indicates that they are rich in oxygen element. The phase constitution of the specimen after the NaOH solution dealloying for 12 h is detected by XRD analysis. The obtained pattern is shown in Fig. 4(b). Comparing with Fig. 3, the diffraction peaks of ␣Cu–Zn have disappeared and been replaced by Cu2 O (JCPDS file no. 35-1091). This indicates that the oxidation occurs during the dealloying process. Combined with many previous reports [47–49], the fact can be ascribed to the oxidation of the obtained nanoporous Cu films during the dealloying process. In order to remove the
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Fig. 1. Schematic diagrams of the proposed strategy for fabricating the bimodal porous Cu: fabricating Gasar Cu (a); depositing (b); annealing treatment (c); dealloying (d), and the detailed structural representation of bimodal porous Cu (e).
Fig. 2. Morphology of the inner surface of Gasar pores (a) after depositing with Zn coatings (b) and after annealing treatment (c).
oxide and further to expose the bottom nanoporous Cu films, the surface is treated by HCl solution. However, it has been reported [50,51] that HCl solution can potentially bring in the coarsening of nanopores because of the high activity of nanoporous structures. Therefore, in order to remove the surface oxide without coarsening
the bottom nanopores as far as possible, the HCl solution treatment technology needs to be optimized. Here, influences of two parameters of HCl solution concentration and processing time on the microstructure of the nanoporous Cu films are investigated. Fig. 5 shows the microstructure of the NaOH solution dealloyed inner
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Fig. 3. XRD patterns of Gasar Cu (pattern 1) and Gasar Cu with alloy layers (pattern 2) (a); the diffraction peaks at the 2 of 41.5–42.8◦ (b) and 48.4–49.8◦ (c).
Fig. 4. FESEM images of the inner surface of Gasar pores with average diameter of 400 m after the NaOH solution dealloying for 12 h (a); XRD pattern of the NaOH solution dealloyed specimen (b).
surface of Gasar pores for 12 h after being treated by HCl solution with different concentrations (a: 0.1 M; b: 1 M; c: 2 M; d: 5 M; e: 10 M) for 2 min. It can be seen that, with the concentration increase, the size of surface nanoparticles gradually decreases. When the concentration increases to 2 M, the nanoparticle almost disappears, and the surface exhibits a nanoporous structure. After this point, the ligament size of the nanoporous structure is increased with the concentration of HCl solution increase because of the coarsening effect of high concentration of HCl solution. Thus, it can be inferred that 2 M is an optimal point under the range of the adopted concentration of HCl solution. Besides of the concentration, the processing time is another important factor resulting in the coarsening of nanopores. Fig. 6 shows the microstructure of the NaOH solution dealloyed inner surface of Gasar pores for 12 h after being treated by 2 M HCl solution for varying times (a: 10 s; b: 30 s; c: 2 min; d: 5 min). For the time of 10 s, some nanoparticles can still be observed from the surface (Fig. 6a). However, when the time reaches to 30 s (Fig. 6b), the nanoporous structure emerges, and EDX result shows that the oxygen content is only 1 at.%. Beyond this point, the
coarsening of the nanoporous structure also takes place (Fig. 6c and d). It can be believed therefore, the optimal processing time is about 30 s. In addition, the XRD pattern of the NaOH solution dealloyed Gasar Cu specimen for 12 h after being treated by 2 M HCl solution for 30 s is shown in Fig. 7. It can be seen that the previous ␣Cu–Zn and Cu2 O diffraction peaks have disappeared, and the detected phase is only Cu (JCPDS file no. 04-0836), indicating that Zn element has been selectively corroded by the NaOH solution dealloying and the surface oxide has been removed fully by the optimal HCl solution treatment parameters: the concentration of 2 M and the processing time of 30 s. In the following discussion, all specimens after the NaOH solution dealloying are treated by these optimized parameters. The average Gasar pore diameter of 400 m is employed to study the microstructure of the obtained nanopores at different positions of Gasar pores. Fig. 8 shows FESEM images of the inner surface of Gasar pores after dealloying for 3 h. It can be seen that a small number of pores are formed at the distance of 0.5 mm from Gasar pore openings, which is shown in Fig. 8(a). Fig. 8(b) shows the
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Fig. 5. FESEM images of the NaOH solution dealloyed inner surface of Gasar pores with average diameter of 400 m after being treated by HCl solution with different concentrations: 0.1 M (a); 1 M (b); 2 M (c); 5 M (d); 10 M (e).
Fig. 6. FESEM images of the NaOH solution dealloyed inner surface of Gasar pores with average diameter of 400 m after being treated by 2 M HCl solution for 10 s (a); 30 s (b); 2 min (c); 5 min (d).
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Fig. 7. XRD pattern of the NaOH solution dealloyed Gasar Cu specimens with average pore diameter of 400 m after being treated by 2 M HCl solution for 30 s.
FESEM image of the inner surface of Gasar pores after dealloying at the distance of 2.5 mm from Gasar pore openings. This position does not show porous structures at this moment. It indicates that the dealloying rate in the deeper location of Gasar pores is slower than that in the shallower place. Meanwhile, the residual content of Zn element detected by EDX analysis increases from 20 at.% to 30 at.% with distance from Gasar pore openings rising from 0.5 mm to 2.5 mm. Fig. 9 shows FESEM images of the as-dealloyed inner surface for 6 h at different positions of Gasar pores along the axial direction. At the distance of 0.5 mm from Gasar pore openings, a uniform nanoporous structure is formed, which can be observed from Fig. 9(a). With the distance raising up to 2.5 mm, the uniform nanoporous structure also appears, which is shown in Fig. 9(b). EDX result shows that the nanoporous film consists of Cu and Zn elements, and whose contents are confirmed to be 97 at.% and 3 at.%, respectively. As the dealloying time increases from 3 h to 6 h, Zn element is dissolved further but not completely at the same position of Gasar pores. When the dealloying time increases to 12 h, the microstructure of the inner surface of Gasar pores at different locations along the axial direction is presented in Fig. 10. For the position at the distance of 0.5 mm from Gasar pore openings, the nanoporous structure is coarsened by the dealloying time increasing from 6 h (Fig. 9a) to 12 h (Fig. 10a) because of the further diffusion of Cu atoms. With the distance up to 2.5 mm, the uniform nanoporous structures still exists, which is shown in Fig. 10(b). However, their ligament size is obviously larger than that shown in Fig. 9(b). EDX result shows
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that all of Zn element has been removed from the precursory alloy layers. Fig. 11 displays FESEM images of nanoporous Cu at different distances from Gasar pore openings for the dealloying time of 24 h. The previous EDX result has justified that all of Zn element has been removed after dealloying for 12 h. With the extension of dealloying time, the microstructure of nanoporous Cu films at both of the positions is varied, which is shown in Fig. 11(a) and (b) respectively. By contrast with Fig. 10, the nanoporous films obtained for the dealloying time of 24 h are coarsened correspondingly. By comparing with Figs. 8–11, two facts can be summarized: (1) the ligament size of the obtained nanoporous Cu decreases with the increase of distance from Gasar pore openings; (2) the ligament size on any positions of Gasar pores increases with dealloying time increase. In the following, these facts will be discussed respectively. First, the influence of the distance from Gasar pore openings on the ligament size of the obtaining nanoporous structure. As can be seen from Figs. 8–11, a uniform nanoporous Cu can be gotten at distances of 0.5 mm and 2.5 mm from Gasar pore openings for dealloying times of 12 h and 24 h. Therefore, both of the dealloying times are chosen to analyze the influence of the distance from Gasar pore openings on the obtained nanoporous Cu. Quantitative measurement of average ligament size is conducted in light of the insets shown in Figs. 10 and 11. Statistical results represent that, with the distance from Gasar pore openings rising up from 0.5 mm to 2.5 mm, the ligament size is separately turned from 300 ± 20 nm to 150 ± 25 nm for the dealloying time of 12 h, and from 500 ± 10 nm to 280 ± 20 nm for the dealloying time of 24 h. In order to dealloy the Cu–Zn alloy layers on the inner surface of Gasar pores quickly and efficiently, electromagnetic stirring is applied to drive corrosion solution to flow through the elongated pores. Assuming that Gasar pores are cylindrical passages, owing to the constant traffic volume of fluid at the inlet and outlet, the average flow rate at any cross-sections of Gasar pores is same. And the flow rate is in direct proportion to the Gasar pore diameter when the electromagnetic stirring rate is constant. In addition, during the process of fluid flowing throughout the elongated pores, the concentration of OH− gradually decreases along the flow direction because the alloy layer near to the flow entrance consumes part of corrosion solution. Influenced by this, the dealloying rate will be decreased with the increase of distance from Gasar pore openings. Therefore, the distributing heterogeneity of nanopores on the inner surface of Gasar pores with average diameter of 400 m can be ascribed to the gradual reduction of corrosion solution content along the Gasar pore axial direction. Thus, the distributing uniformity of nanopores along the Gasar pore axial direction is expected to be improved by the plenty of supplement of corrosion solution resulting from the increase of Gasar pore diameter. Fig. 12 shows FESEM images of the inner surface of Gasar pores with average diameter of 1.5 mm at the distance of 0.5 mm and 2.5 mm from Gasar pore openings after dealloying for 3 h. It can be seen that
Fig. 8. FESEM images and corresponding insets of the inner surface of the Gasar pores with average diameter of 400 m after dealloying for 3 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
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Fig. 9. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 400 m after dealloying for 6 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 10. FESEM images and corresponding insets of the inner surface of Gasar pore with average diameter of 400 m after dealloying for 12 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 11. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 400 m after dealloying for 24 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 12. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 1.5 mm after dealloying for 3 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
similar and uniform nanoporous structures are formed at different positions of Gasar pores. The improved distribution uniformity of nanopores along the Gasar pore axial direction can be explained by the following two aspects. First, the refresh rate of corrosion solution is increased by the quickening flow velocity resulted from the increase of Gasar pore diameter. Thus, the action time of solution with concentration gradient will be shortened giving rise to the
improvement of distributing uniformity of nanopores. Second, the large Gasar pore diameter can provide excess corrosion solution for the dealloying of Cu–Zn alloy layers because the volume of Gasar pores with diameter of 1.5 mm is about 14 times greater than that of Gasar pore size of 400 m. Correspondingly, the concentration gradient between the inlet and outlet is reduced. Thus, the difference of microstructure along the flow direction will be weakened. It
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is believed therefore, the nanopores distributing uniformity along the flowing direction can be improved by the increase of Gasar pore diameter. By the way, for the Gasar pore diameter of 1.5 mm, the complete dealloying of the alloy layer only needs 3 h. This indicates that big Gasar pore diameter can bring in fast dealloying rate, resulting in the shortening of the dealloying time. Second, the influence of the dealloying time on the microstructure of the obtained nanoporous Cu. Taken the position at the distance of 2.5 mm from Gasar pore openings where possesses the minimum dealloying rate for example, the evolution of nanopores as a function of the dealloying time is discussed. From Fig. 8(b), it can be seen that few of independent pores scattered on the inner surface of Gasar pores. With the dealloying time increase to 6 h and further to 12 h, a uniform porous structure is formed (Figs. 9b and 10b) respectively. Also, a nanoporous structure is observed when the dealloying time is 24 h (Fig. 11b), and the ligament size increases from 150 nm ± 25 nm to 280 nm ± 20 nm as the dealloying time rises up from 12 h to 24 h. The conventional coarsening theory agrees that the ligament size exhibits significant time dependence and increases with the dealloying time increase because of the further diffusion of the noble metal and the dissolution of less noble element [51,52]. This fact can also be represented by a typical equation [53] shown in the following equation (Eq. (1)): n ddealloying = C · t · Ds
(1)
where ddealloying is the ligament size of nanopores; Ds is the coefficient of surface diffusion of Cu at a constant temperature; C is a constant; t is the dealloying time; n is the coarsening exponent. It indicates that the ligament size is proportional to the dealloying time when other parameters are constant. Therefore, nanopores on the inner surface of Gasar pores also accords with the abovementioned changing law between the ligament size of nanopores and the dealloying time.
4. Conclusions (1) A bimodal porous structure with random nanopores only distributed on the inner surface of regular Gasar micrometer/millimeter-pores is fabricated by a two-step corrosion method: NaOH solution selectively corroding Zn element from Cu–Zn alloy layers and HCl solution removing the surface oxide obtained in the first step. (2) In order to remove the surface oxide obtained in the NaOH solution dealloying process and simultaneously to avoid the coarsening of nanopores, the HCl solution concentration and processing time are optimized to be 2 M and 30 s respectively. (3) Nanopores can be fabricated on the whole inner surface of Gasar Cu specimens with average pore diameter of 400 m for the dealloying time of over 6 h after HCl solution treatment. However, nanopores distribute unevenly along the Gasar pore axial direction. The fundamental reason for the nonuniformity is the slow refresh rate of corrosion solution and the high concentration gradient along the axial direction of Gasar pores. (4) The distributing uniformity of nanopores along the Gasar pore axial direction can be improved by the increase of average Gasar pore diameter because of the reduction of concentration gradient and the increased volume of corrosion solution participating in the dealloying process. (5) The ligament size of nanoporous Cu on any positions of Gasar pores is increased with the dealloying time increase. (6) The bimodal porous Cu possesses good permeability (Gasar pores), as well as high surface area (nanopores). This can not only enhance the usability of Gasar Cu substrate in verified fields, such as heat sinks and filers, but also explore new
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application prospects in some functional fields, such as electrodes, catalysis and sensors. Acknowledgements This work was supported by National Natural Science Foundations (Nos. 51371104, 51101092 and 51271096), International Science and Technology Cooperation Program of China (No. 2013DFR50330) and Program for New Century Excellent Talents in University-China (NCET-12-0310). References [1] Y. Liu, Y.X. Li, R.F. Liu, R. Zhou, Y.H. Jiang, Z.H. Li, Theoretical analysis on effect of transference velocity on structure of porous metals fabricated by continuous casting Gasar process, Acta Metall. Sin. 46 (2010) 129–134. [2] H.W. Zhang, Y.X. Li, Y. Liu, The critical processing conditions for directional solidification of solid/gas eutectics, Acta Metall. Sin. 43 (2007) 589–594. [3] H.W. Zhang, Y.X. Li, Y. Liu, Evaluation of porosity in lotus-type porous Cu fabricated with Gasar process, Acta Metall. Sin. 42 (2006) 1165–1170. [4] S. Yamamura, H. Shiota, K. Murakami, H. Nakajima, Evaluation of porosity in porous copper fabricated by unidirectional solidification under pressurized hydrogen, Mater. Sci. Eng. A 318 (2001) 137–143. [5] H.W. Zhang, Y.X. Li, Y. Liu, Gas pressure condition for obtaining uniform lotus-type porous structure by Gasar process, Acta Metall. Sin. 42 (2006) 1171–1176. [6] Y. Liu, Y.X. Li, H.W. Zhang, J. Wan, Effect of Gasar processing parameters on structure of lotus-type porous magnesium, Rare Metal Mater. Eng. 34 (2005) 1128–1130. [7] T. Ide, Y. Iio, H. Nakajima, Fabrication of porous aluminum with directional pores through continuous casting technique, Metall. Mater. Trans. A 43 (2012) 5140–5152. [8] H.W. Zhang, Y.X. Li, Y. Liu, Fabricating porous aluminium with directional solidification of Al–H system, Acta Metall. Sin. 43 (2007) 11–16. [9] Q.Q. Yang, Y. Liu, Y.X. Li, Pore structure of unidirectional solidified porous silicon, Trans. Nonferrous Met. Soc. China 24 (2014) 3517–3523. [10] G.R. Jiang, Y.X. Li, Y. Liu, Influence of solidification mode on pore structure of directionally solidified porous Cu–Mn alloy, Trans. Nonferrous Met. Soc. China 21 (2011) 88–95. [11] G.R. Jiang, Y.X. Li, Y. Liu, Experimental study on the pore structure of directionally solidified porous Cu–Mn alloy, Metall. Mater. Trans. A 41 (2010) 3405–3411. [12] L.T. Chen, H.W. Zhang, Y. Liu, Y.X. Li, Experimental research on heat transfer performance of directionally solidified porous copper heat sink, Acta Metall. Sin. 48 (2012) 329–333. [13] S.K. Hyun, K. Murakami, H. Nakajima, Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification, Mater. Sci. Eng. A 299 (2001) 241–248. [14] H.W. Zhang, L.T. Chen, Y. Liu, Y.X. Li, Experimental study on heat transfer performance of lotus-type porous copper heat sink, Int. J. Heat Mass Transfer 56 (2013) 172–180. [15] X.N. Gu, W.R. Zhou, Y.F. Zheng, Y. Liu, Y.X. Li, Degradation and cytotoxicity of lotus-type porous pure magnesium as potential tissue engineering scaffold material, Mater. Lett. 64 (2010) 1871–1874. [16] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Evolution of nanoporosity in dealloying, Nature 410 (2001) 450–453. [17] Y. Ding, Y. Kim, J. Erlebacher, Nanoporous gold leaf: “ancient technology”, Adv. Mater. 16 (2004) 1897–1900. [18] R. Morrish, K. Dorame, A.J. Muscat, Formation of nanoporous Au by dealloying AuCu thin films in HNO3 , Scripta Mater. 64 (2011) 856–859. [19] D.V. Pugh, A. Dursun, S.G. Corcoran, Electrochemical and morphological characterization of Pt–Cu dealloying, J. Electrochem. Soc. 152 (2005) B455–B459. [20] C. Zhang, J.Z. Sun, J.L. Xu, X.G. Wang, H. Ji, C.C. Zhao, Z.H. Zhang, Formation and microstructure of nanoporous silver by dealloying rapidly solidified Zn–Ag alloys, Electrochim. Acta 63 (2012) 302–311. [21] M. Hakamada, M. Mabuchi, Preparation of nanoporous Ni and Ni–Cu by dealloying of rolled Ni–Mn and Ni–Cu–Mn alloys, J. Alloys Compd. 485 (2009) 583–587. [22] J.R. Hayes, A.M. Hodge, J. Biener, A.V. Hamza, Monolithic nanoporous copper by dealloying Mn–Cu, J. Mater. Res. 21 (2006) 2611–2616. [23] Z. Qi, C.C. Zhao, X.G. Wang, J.K. Lin, W. Shao, Z.H. Zhang, X.F. Bian, Formation and characterization of monolithic nanoporous copper by chemical dealloying of Al–Cu alloys, J. Phys. Chem. C 113 (2009) 6694–6698. [24] Y.W. Lin, C.C. Tai, I. Sun, Electrochemical preparation of porous copper surfaces in zinc chloride-1-ethyl-3-methyl imidazolium chloride ionic liquid, J. Electrochem. Soc. 154 (2007) D316–D321. [25] H.J. Qiu, F.X. Zou, Fabrication of stratified nanoporous gold for enhanced biosensing, Biosens. Bioelectron. 35 (2012) 349–354. [26] Y. Yu, L. Gu, X.Y. Lang, C.B. Zhu, T. Fujita, M.W. Chen, J. Maier, Li storage in 3D nanoporous Au-supported nanocrystalline tin, Adv. Mater. 23 (2011) 2443–2447.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of a bimodal porous Cu with nanopores on the inner surface of Gasar pores: Influences of preparation conditions Ming Du a,1 , Hua-wei Zhang a,b,1 , Yan-xiang Li a,b,∗ , Yuan Liu a,b , Xiang Chen a,b , Yun He a a b
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Key Laboratory for Advanced Materials Processing Technology, MOE, PR China
a r t i c l e
i n f o
Article history: Received 22 August 2015 Received in revised form 2 November 2015 Accepted 3 November 2015 Available online 10 November 2015 Keywords: Bimodal porous structure Gasar Dealloying
a b s t r a c t A bimodal porous Cu with regular Gasar micrometer or millimeter pores and random dealloying nanometer pores was fabricated by chemical corrosion of Cu–Zn alloy layers on the inner surface of Gasar Cu pores. In order to accomplish this object, a two-step corrosion method was conducted, including the selective corrosion of Zn element from Cu–Zn alloy layers by NaOH solution, and the removal of the surface oxide obtained in the first step to expose the bottom nanoporous Cu films by HCl solution. Influences of preparation conditions, such as HCl solution concentration and processing time, NaOH solution dealloying time, Gasar pore diameter, and the distance from Gasar pore openings on the microstructure of the resulting nanoporous Cu films were discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Gasar is a metal–gas eutectic unidirectional solidification process, which has been applied in preparing porous materials with regularly directional pores of micrometer or millimeter scales [1,2]. Both elementary substances, such as Cu [3,4], Mg [5,6], Al [7,8] and Si [9], and alloys, such as Cu–Mn [10,11], have been fabricated into the unimodal porous structure using the Gasar technique. Owing to their anisotropic mechanical properties, good permeability and lightweight originating from the oriented pores, Gasar structures have become promising candidates for being used as heat sinks and biomaterials recently [12–15]. Dealloying is a popular process to fabricate random nanoporous metals by a selective corrosion of the most electrochemical active element in alloys [16]. Recently, this corrosion method has been applied in numerous of binary alloy systems, such as Au–Ag [17], Au–Cu [18], Pt–Cu [19], Ag–Zn [20], Ni–Mn [21], Cu–Mn [22], Cu–Al [23] and Cu–Zn [24], to fabricate nanoporous metals which have attracted much interest for applications in catalysis, sensing, photonics, energy storage, etc. due to their perfect electrical conductivity, enhanced physicochemical activity and high surface area [25–27].
∗ Corresponding author at: School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (Y.-x. Li). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.apsusc.2015.11.033 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Bimodal micro/nanoporous structures comprised of micrometer or millimeter pores and nanometer pores are considered to be potential candidates in fields of surface-enhanced Raman scattering [28], electrodes [29] and fuel cells [30], because of their outstanding permeability and high surface area. So far, several processes have been reported to fabricate this kind of porous materials [31–33]. Among them, the combined technique of Gasar and dealloying has been proposed to fabricate a kind of bimodal porous Cu with regular big and random small pores [34,35]. The feasible route includes two steps: (1) fabricating Gasar Cu–Mn alloys; and (2) chemical or electrochemical dealloying. After these steps, the resultant small dealloying pores penetrate big Gasar pore walls. Thus, the bimodal porous Cu possesses not only a good mass transport property originating from Gasar pores but also high surface area inheriting from nanopores. Here, another method is proposed to prepare an allotrope of this kind of bimodal porous Cu, which includes four steps: (1) fabrication of Gasar Cu; (2) deposition of an active metal on the inner surface of Gasar Cu pores; (3) annealing treatment to form alloy layers in situ; and (4) selective corrosion of the active element to prepare nanoporous structures by a dealloying method. The resulting nanopores only distributed on the inner surface of Gasar pores can increase the surface area of the Gasar Cu substrate as well as keep the structure integrality of the Gasar Cu substrate furthest. In addition, the typical structure of nanopores distributed on the surface of a metal substrate has an application advantage in fields where use its surface features, such as sensors [36] and surface-enhanced boiling heat transfers [37]. For the bimodal porous Cu with nanopores on the inner surface of Gasar Cu
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pores, it can be used in some potential fields, such as sensors and heat sinks, on the basis of the good directionality and permeability of Gasar pores and the high surface area of nanopores. Taken heat sinks for example, it has been widely reported [12,14,38,39] that heat sinks fabricated by Gasar Cu possesses excellent heat dissipation property because of Gasar pores providing flowing channels for coolant and high heat conductivity of Cu matrix. In addition, according to many reports [40–42], a rough pore wall with high surface area can enhance the heat dissipation performance of heat sinks, especially when the pore diameter is micrometer range. Therefore, the nanopores can not only coarsen the inner surface of Gasar pores, but also increase the surface area of Gasar Cu, which means that heat sinks fabricated by the bimodal porous Cu will possess more outstanding heat dissipation performance than that of Gasar Cu heat sinks. Because the standard equilibrium potential of Zn2+ /Zn is much lower than that of Cu2+ /Cu, the active Zn component in Cu–Zn alloys can be selectively corroded, while the noble Cu remains and diffuses to form nanoporous structures [43]. Therefore, Zn element is chosen as the transitional metal to fabricate the bimodal porous Cu in this work. A two-step corrosion method is employed to fabricate nanopores on the inner surface of Gasar pores, including that Zn element is selectively corroded by NaOH solution, and then the surface oxide obtained in the first step is removed by HCl solution. Our previous work [44] has justified that the established process route is feasible to fabricate the bimodal porous Cu, but does not address influences of corrosion technological conditions and Gasar structural parameters on the microstructure of the obtaining nanopores on the inner surface of Gasar pores. However, these conditions are the key factors in tuning the microstructure of the obtained nanopores. Therefore, on the basis of the fixed corrosion route, preparation conditions are studied and optimized further here. Particularly, for the first step of NaOH solution selective corrosion, how dealloying time, Gasar pore diameter, and the distance from Gasar pore openings impact on the microstructure of the obtained nanoporous Cu films is studied respectively. For the second step of HCl solution treatment, the HCl solution concentration and processing time are optimized to remove the surface oxide and simultaneously to avoid the coarsening of the nanoporous structure.
2. Experimental procedures Gasar Cu samples with average pore diameter of 400 m and 1.5 mm were prepared by the unidirectional solidification Gasar process [45]. Half-cubic specimens with the width of 10 mm, the length of 10 mm, and the height of 5 mm were cut from the Gasar ingot by an electric sparking cutting machine. Prior to the deposition, Gasar Cu specimens were degreased in HCl solution for 2 min with ultrasonic washing, and then cleaned in acetone, alcohol and deionized water, orderly. After the cleaning, Zn coatings were deposited at the temperature of about 70 ◦ C using a material system which contains 5 M NaOH solution and 3 g Zn powders. Gasar Cu, NaOH solution and Zn powders constituted a kind of a copper–zinc cell, resulting in that Zn coatings were deposited on the inner surface of Gasar pores. Ultrasonic vibration was employed to discharge the air and the generated gas from Gasar pores and to drive sufficient solution inflowing timely. After rinsed extensively with deionized water and dried in a vacuum drying oven, specimens with Zn coatings were annealed at 410 ◦ C for 0.5 h in an Ar-filled tube furnace to obtain Cu–Zn alloy layers. The depositing and alloying processes have been discussed in detail in our previous report [46]. The tempered specimens were then immerged into the corrosion solution (1.5 M NaOH solution) at the ambient temperature
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for a period between 3 h and 24 h. Electromagnetic stirring was proposed to drive the solution flowing into oriented Gasar pores throughout the dealloying process, which could assist in the dealloying of Cu–Zn alloy layers on the inner surface of Gasar pores. After the first corrosion process, specimens were immerged into HCl solution with varying concentrations and processing times in order to remove the surface oxide. Specimens after HCl solution treatment were placed in a vacuum drying oven to avoid the oxidation of the high free energy nanoporous surfaces, waiting for the following microstructural characterization. Phase constitution of the Gasar Cu substrate, the precursory alloy layer, and the as-dealloyed specimen before and after HCl solution treatment was determined by X-ray diffraction (XRD) analysis on a D8-Advance diffraction meter (Cu K␣ radiation, = 0.154 nm) at a scanning rate of 4◦ /min. Microstructure and element composition of the precursory Cu–Zn alloy layers and the nanoporous Cu films obtained after the two-step corrosion method was observed using field emission scanning electron microscopy (FESEM) (Zeiss MERLIN-VP-COMPACT) equipped with energydispersive X-ray spectroscope (EDX) analyzer (Oxford INCA).
3. Results and discussion Fig. 1 shows schematic diagrams of the route for fabricating the bimodal porous Cu. It can be seen that the route covers broadly four steps: fabricating Gasar Cu (Fig. 1a), depositing Zn coatings (Fig. 1b), annealing (Fig. 1c) and dealloying (Fig. 1d). Consequently, the bimodal porous structure with nanopores only distributed on the inner surface of Gasar pores is formed, whose detailed structure is represented in Fig. 1(e). Fig. 2 shows FESEM images of the inner surface of Gasar pores (a) after depositing (b) and annealing (c). It can be seen from Fig. 2(a) that the initial inner surface of Gasar pores is dense. After depositing Zn coatings, the surface presents a porous structure, which is shown in Fig. 2(b). However, as can be seen from Fig. 2(c), the surface becomes compact and uniform again because of the densification and alloying promoted by the annealing treatment. EDX result indicates that the Zn content in the obtained alloy layers is about 34 at.%. The phase constitution of the original Gasar Cu specimens and the alloy layers is detected by XRD analysis, and the patterns are shown in Fig. 3. It can be seen from Fig. 3(a) that the phase constitution of Gasar Cu is the single-phase Cu (pattern 1) (JCPDS file no. 04-0836). However, after the depositing and the annealing treatments, minor peaks occur and can be indexed to ␣Cu–Zn phase (pattern 2) (JCPDS file no.25-0322). Fig. 3(b) and c shows XRD diffraction peaks locating at the 2 of 41.5–42.8◦ (b) and 48.4–49.8◦ (c). It can be clearly seen that the peak intensity changes in these ranges, visibly proving that the detected phase of the alloy layers obtained after depositing and alloying is ␣Cu–Zn. Cu–Zn alloy layers have been prepared on the inner surface of Gasar pores uniformly using depositing and annealing processes. In the following, the alloys are tried to be dealloyed by chemical corrosion with electromagnetic stirring. Fig. 4(a) shows the microstructure of the alloy layers on the inner surface of Gasar pores just after the NaOH solution dealloying for 12 h. It can be seen that the obtained surface is covered by nanoparticles, and EDX analysis indicates that they are rich in oxygen element. The phase constitution of the specimen after the NaOH solution dealloying for 12 h is detected by XRD analysis. The obtained pattern is shown in Fig. 4(b). Comparing with Fig. 3, the diffraction peaks of ␣Cu–Zn have disappeared and been replaced by Cu2 O (JCPDS file no. 35-1091). This indicates that the oxidation occurs during the dealloying process. Combined with many previous reports [47–49], the fact can be ascribed to the oxidation of the obtained nanoporous Cu films during the dealloying process. In order to remove the
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Fig. 1. Schematic diagrams of the proposed strategy for fabricating the bimodal porous Cu: fabricating Gasar Cu (a); depositing (b); annealing treatment (c); dealloying (d), and the detailed structural representation of bimodal porous Cu (e).
Fig. 2. Morphology of the inner surface of Gasar pores (a) after depositing with Zn coatings (b) and after annealing treatment (c).
oxide and further to expose the bottom nanoporous Cu films, the surface is treated by HCl solution. However, it has been reported [50,51] that HCl solution can potentially bring in the coarsening of nanopores because of the high activity of nanoporous structures. Therefore, in order to remove the surface oxide without coarsening
the bottom nanopores as far as possible, the HCl solution treatment technology needs to be optimized. Here, influences of two parameters of HCl solution concentration and processing time on the microstructure of the nanoporous Cu films are investigated. Fig. 5 shows the microstructure of the NaOH solution dealloyed inner
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Fig. 3. XRD patterns of Gasar Cu (pattern 1) and Gasar Cu with alloy layers (pattern 2) (a); the diffraction peaks at the 2 of 41.5–42.8◦ (b) and 48.4–49.8◦ (c).
Fig. 4. FESEM images of the inner surface of Gasar pores with average diameter of 400 m after the NaOH solution dealloying for 12 h (a); XRD pattern of the NaOH solution dealloyed specimen (b).
surface of Gasar pores for 12 h after being treated by HCl solution with different concentrations (a: 0.1 M; b: 1 M; c: 2 M; d: 5 M; e: 10 M) for 2 min. It can be seen that, with the concentration increase, the size of surface nanoparticles gradually decreases. When the concentration increases to 2 M, the nanoparticle almost disappears, and the surface exhibits a nanoporous structure. After this point, the ligament size of the nanoporous structure is increased with the concentration of HCl solution increase because of the coarsening effect of high concentration of HCl solution. Thus, it can be inferred that 2 M is an optimal point under the range of the adopted concentration of HCl solution. Besides of the concentration, the processing time is another important factor resulting in the coarsening of nanopores. Fig. 6 shows the microstructure of the NaOH solution dealloyed inner surface of Gasar pores for 12 h after being treated by 2 M HCl solution for varying times (a: 10 s; b: 30 s; c: 2 min; d: 5 min). For the time of 10 s, some nanoparticles can still be observed from the surface (Fig. 6a). However, when the time reaches to 30 s (Fig. 6b), the nanoporous structure emerges, and EDX result shows that the oxygen content is only 1 at.%. Beyond this point, the
coarsening of the nanoporous structure also takes place (Fig. 6c and d). It can be believed therefore, the optimal processing time is about 30 s. In addition, the XRD pattern of the NaOH solution dealloyed Gasar Cu specimen for 12 h after being treated by 2 M HCl solution for 30 s is shown in Fig. 7. It can be seen that the previous ␣Cu–Zn and Cu2 O diffraction peaks have disappeared, and the detected phase is only Cu (JCPDS file no. 04-0836), indicating that Zn element has been selectively corroded by the NaOH solution dealloying and the surface oxide has been removed fully by the optimal HCl solution treatment parameters: the concentration of 2 M and the processing time of 30 s. In the following discussion, all specimens after the NaOH solution dealloying are treated by these optimized parameters. The average Gasar pore diameter of 400 m is employed to study the microstructure of the obtained nanopores at different positions of Gasar pores. Fig. 8 shows FESEM images of the inner surface of Gasar pores after dealloying for 3 h. It can be seen that a small number of pores are formed at the distance of 0.5 mm from Gasar pore openings, which is shown in Fig. 8(a). Fig. 8(b) shows the
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Fig. 5. FESEM images of the NaOH solution dealloyed inner surface of Gasar pores with average diameter of 400 m after being treated by HCl solution with different concentrations: 0.1 M (a); 1 M (b); 2 M (c); 5 M (d); 10 M (e).
Fig. 6. FESEM images of the NaOH solution dealloyed inner surface of Gasar pores with average diameter of 400 m after being treated by 2 M HCl solution for 10 s (a); 30 s (b); 2 min (c); 5 min (d).
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Fig. 7. XRD pattern of the NaOH solution dealloyed Gasar Cu specimens with average pore diameter of 400 m after being treated by 2 M HCl solution for 30 s.
FESEM image of the inner surface of Gasar pores after dealloying at the distance of 2.5 mm from Gasar pore openings. This position does not show porous structures at this moment. It indicates that the dealloying rate in the deeper location of Gasar pores is slower than that in the shallower place. Meanwhile, the residual content of Zn element detected by EDX analysis increases from 20 at.% to 30 at.% with distance from Gasar pore openings rising from 0.5 mm to 2.5 mm. Fig. 9 shows FESEM images of the as-dealloyed inner surface for 6 h at different positions of Gasar pores along the axial direction. At the distance of 0.5 mm from Gasar pore openings, a uniform nanoporous structure is formed, which can be observed from Fig. 9(a). With the distance raising up to 2.5 mm, the uniform nanoporous structure also appears, which is shown in Fig. 9(b). EDX result shows that the nanoporous film consists of Cu and Zn elements, and whose contents are confirmed to be 97 at.% and 3 at.%, respectively. As the dealloying time increases from 3 h to 6 h, Zn element is dissolved further but not completely at the same position of Gasar pores. When the dealloying time increases to 12 h, the microstructure of the inner surface of Gasar pores at different locations along the axial direction is presented in Fig. 10. For the position at the distance of 0.5 mm from Gasar pore openings, the nanoporous structure is coarsened by the dealloying time increasing from 6 h (Fig. 9a) to 12 h (Fig. 10a) because of the further diffusion of Cu atoms. With the distance up to 2.5 mm, the uniform nanoporous structures still exists, which is shown in Fig. 10(b). However, their ligament size is obviously larger than that shown in Fig. 9(b). EDX result shows
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that all of Zn element has been removed from the precursory alloy layers. Fig. 11 displays FESEM images of nanoporous Cu at different distances from Gasar pore openings for the dealloying time of 24 h. The previous EDX result has justified that all of Zn element has been removed after dealloying for 12 h. With the extension of dealloying time, the microstructure of nanoporous Cu films at both of the positions is varied, which is shown in Fig. 11(a) and (b) respectively. By contrast with Fig. 10, the nanoporous films obtained for the dealloying time of 24 h are coarsened correspondingly. By comparing with Figs. 8–11, two facts can be summarized: (1) the ligament size of the obtained nanoporous Cu decreases with the increase of distance from Gasar pore openings; (2) the ligament size on any positions of Gasar pores increases with dealloying time increase. In the following, these facts will be discussed respectively. First, the influence of the distance from Gasar pore openings on the ligament size of the obtaining nanoporous structure. As can be seen from Figs. 8–11, a uniform nanoporous Cu can be gotten at distances of 0.5 mm and 2.5 mm from Gasar pore openings for dealloying times of 12 h and 24 h. Therefore, both of the dealloying times are chosen to analyze the influence of the distance from Gasar pore openings on the obtained nanoporous Cu. Quantitative measurement of average ligament size is conducted in light of the insets shown in Figs. 10 and 11. Statistical results represent that, with the distance from Gasar pore openings rising up from 0.5 mm to 2.5 mm, the ligament size is separately turned from 300 ± 20 nm to 150 ± 25 nm for the dealloying time of 12 h, and from 500 ± 10 nm to 280 ± 20 nm for the dealloying time of 24 h. In order to dealloy the Cu–Zn alloy layers on the inner surface of Gasar pores quickly and efficiently, electromagnetic stirring is applied to drive corrosion solution to flow through the elongated pores. Assuming that Gasar pores are cylindrical passages, owing to the constant traffic volume of fluid at the inlet and outlet, the average flow rate at any cross-sections of Gasar pores is same. And the flow rate is in direct proportion to the Gasar pore diameter when the electromagnetic stirring rate is constant. In addition, during the process of fluid flowing throughout the elongated pores, the concentration of OH− gradually decreases along the flow direction because the alloy layer near to the flow entrance consumes part of corrosion solution. Influenced by this, the dealloying rate will be decreased with the increase of distance from Gasar pore openings. Therefore, the distributing heterogeneity of nanopores on the inner surface of Gasar pores with average diameter of 400 m can be ascribed to the gradual reduction of corrosion solution content along the Gasar pore axial direction. Thus, the distributing uniformity of nanopores along the Gasar pore axial direction is expected to be improved by the plenty of supplement of corrosion solution resulting from the increase of Gasar pore diameter. Fig. 12 shows FESEM images of the inner surface of Gasar pores with average diameter of 1.5 mm at the distance of 0.5 mm and 2.5 mm from Gasar pore openings after dealloying for 3 h. It can be seen that
Fig. 8. FESEM images and corresponding insets of the inner surface of the Gasar pores with average diameter of 400 m after dealloying for 3 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
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Fig. 9. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 400 m after dealloying for 6 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 10. FESEM images and corresponding insets of the inner surface of Gasar pore with average diameter of 400 m after dealloying for 12 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 11. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 400 m after dealloying for 24 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
Fig. 12. FESEM images and corresponding insets of the inner surface of Gasar pores with average diameter of 1.5 mm after dealloying for 3 h at distances of 0.5 mm (a) and 2.5 mm (b) from Gasar pore openings.
similar and uniform nanoporous structures are formed at different positions of Gasar pores. The improved distribution uniformity of nanopores along the Gasar pore axial direction can be explained by the following two aspects. First, the refresh rate of corrosion solution is increased by the quickening flow velocity resulted from the increase of Gasar pore diameter. Thus, the action time of solution with concentration gradient will be shortened giving rise to the
improvement of distributing uniformity of nanopores. Second, the large Gasar pore diameter can provide excess corrosion solution for the dealloying of Cu–Zn alloy layers because the volume of Gasar pores with diameter of 1.5 mm is about 14 times greater than that of Gasar pore size of 400 m. Correspondingly, the concentration gradient between the inlet and outlet is reduced. Thus, the difference of microstructure along the flow direction will be weakened. It
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is believed therefore, the nanopores distributing uniformity along the flowing direction can be improved by the increase of Gasar pore diameter. By the way, for the Gasar pore diameter of 1.5 mm, the complete dealloying of the alloy layer only needs 3 h. This indicates that big Gasar pore diameter can bring in fast dealloying rate, resulting in the shortening of the dealloying time. Second, the influence of the dealloying time on the microstructure of the obtained nanoporous Cu. Taken the position at the distance of 2.5 mm from Gasar pore openings where possesses the minimum dealloying rate for example, the evolution of nanopores as a function of the dealloying time is discussed. From Fig. 8(b), it can be seen that few of independent pores scattered on the inner surface of Gasar pores. With the dealloying time increase to 6 h and further to 12 h, a uniform porous structure is formed (Figs. 9b and 10b) respectively. Also, a nanoporous structure is observed when the dealloying time is 24 h (Fig. 11b), and the ligament size increases from 150 nm ± 25 nm to 280 nm ± 20 nm as the dealloying time rises up from 12 h to 24 h. The conventional coarsening theory agrees that the ligament size exhibits significant time dependence and increases with the dealloying time increase because of the further diffusion of the noble metal and the dissolution of less noble element [51,52]. This fact can also be represented by a typical equation [53] shown in the following equation (Eq. (1)): n ddealloying = C · t · Ds
(1)
where ddealloying is the ligament size of nanopores; Ds is the coefficient of surface diffusion of Cu at a constant temperature; C is a constant; t is the dealloying time; n is the coarsening exponent. It indicates that the ligament size is proportional to the dealloying time when other parameters are constant. Therefore, nanopores on the inner surface of Gasar pores also accords with the abovementioned changing law between the ligament size of nanopores and the dealloying time.
4. Conclusions (1) A bimodal porous structure with random nanopores only distributed on the inner surface of regular Gasar micrometer/millimeter-pores is fabricated by a two-step corrosion method: NaOH solution selectively corroding Zn element from Cu–Zn alloy layers and HCl solution removing the surface oxide obtained in the first step. (2) In order to remove the surface oxide obtained in the NaOH solution dealloying process and simultaneously to avoid the coarsening of nanopores, the HCl solution concentration and processing time are optimized to be 2 M and 30 s respectively. (3) Nanopores can be fabricated on the whole inner surface of Gasar Cu specimens with average pore diameter of 400 m for the dealloying time of over 6 h after HCl solution treatment. However, nanopores distribute unevenly along the Gasar pore axial direction. The fundamental reason for the nonuniformity is the slow refresh rate of corrosion solution and the high concentration gradient along the axial direction of Gasar pores. (4) The distributing uniformity of nanopores along the Gasar pore axial direction can be improved by the increase of average Gasar pore diameter because of the reduction of concentration gradient and the increased volume of corrosion solution participating in the dealloying process. (5) The ligament size of nanoporous Cu on any positions of Gasar pores is increased with the dealloying time increase. (6) The bimodal porous Cu possesses good permeability (Gasar pores), as well as high surface area (nanopores). This can not only enhance the usability of Gasar Cu substrate in verified fields, such as heat sinks and filers, but also explore new
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application prospects in some functional fields, such as electrodes, catalysis and sensors. Acknowledgements This work was supported by National Natural Science Foundations (Nos. 51371104, 51101092 and 51271096), International Science and Technology Cooperation Program of China (No. 2013DFR50330) and Program for New Century Excellent Talents in University-China (NCET-12-0310). References [1] Y. Liu, Y.X. Li, R.F. Liu, R. Zhou, Y.H. Jiang, Z.H. Li, Theoretical analysis on effect of transference velocity on structure of porous metals fabricated by continuous casting Gasar process, Acta Metall. Sin. 46 (2010) 129–134. [2] H.W. Zhang, Y.X. Li, Y. Liu, The critical processing conditions for directional solidification of solid/gas eutectics, Acta Metall. Sin. 43 (2007) 589–594. [3] H.W. Zhang, Y.X. Li, Y. Liu, Evaluation of porosity in lotus-type porous Cu fabricated with Gasar process, Acta Metall. Sin. 42 (2006) 1165–1170. [4] S. Yamamura, H. Shiota, K. Murakami, H. Nakajima, Evaluation of porosity in porous copper fabricated by unidirectional solidification under pressurized hydrogen, Mater. Sci. Eng. A 318 (2001) 137–143. [5] H.W. Zhang, Y.X. Li, Y. Liu, Gas pressure condition for obtaining uniform lotus-type porous structure by Gasar process, Acta Metall. Sin. 42 (2006) 1171–1176. [6] Y. Liu, Y.X. Li, H.W. Zhang, J. Wan, Effect of Gasar processing parameters on structure of lotus-type porous magnesium, Rare Metal Mater. Eng. 34 (2005) 1128–1130. [7] T. Ide, Y. Iio, H. Nakajima, Fabrication of porous aluminum with directional pores through continuous casting technique, Metall. Mater. Trans. A 43 (2012) 5140–5152. [8] H.W. Zhang, Y.X. Li, Y. Liu, Fabricating porous aluminium with directional solidification of Al–H system, Acta Metall. Sin. 43 (2007) 11–16. [9] Q.Q. Yang, Y. Liu, Y.X. Li, Pore structure of unidirectional solidified porous silicon, Trans. Nonferrous Met. Soc. China 24 (2014) 3517–3523. [10] G.R. Jiang, Y.X. Li, Y. Liu, Influence of solidification mode on pore structure of directionally solidified porous Cu–Mn alloy, Trans. Nonferrous Met. Soc. China 21 (2011) 88–95. [11] G.R. Jiang, Y.X. Li, Y. Liu, Experimental study on the pore structure of directionally solidified porous Cu–Mn alloy, Metall. Mater. Trans. A 41 (2010) 3405–3411. [12] L.T. Chen, H.W. Zhang, Y. Liu, Y.X. Li, Experimental research on heat transfer performance of directionally solidified porous copper heat sink, Acta Metall. Sin. 48 (2012) 329–333. [13] S.K. Hyun, K. Murakami, H. Nakajima, Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification, Mater. Sci. Eng. A 299 (2001) 241–248. [14] H.W. Zhang, L.T. Chen, Y. Liu, Y.X. Li, Experimental study on heat transfer performance of lotus-type porous copper heat sink, Int. J. Heat Mass Transfer 56 (2013) 172–180. [15] X.N. Gu, W.R. Zhou, Y.F. Zheng, Y. Liu, Y.X. Li, Degradation and cytotoxicity of lotus-type porous pure magnesium as potential tissue engineering scaffold material, Mater. Lett. 64 (2010) 1871–1874. [16] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Evolution of nanoporosity in dealloying, Nature 410 (2001) 450–453. [17] Y. Ding, Y. Kim, J. Erlebacher, Nanoporous gold leaf: “ancient technology”, Adv. Mater. 16 (2004) 1897–1900. [18] R. Morrish, K. Dorame, A.J. Muscat, Formation of nanoporous Au by dealloying AuCu thin films in HNO3 , Scripta Mater. 64 (2011) 856–859. [19] D.V. Pugh, A. Dursun, S.G. Corcoran, Electrochemical and morphological characterization of Pt–Cu dealloying, J. Electrochem. Soc. 152 (2005) B455–B459. [20] C. Zhang, J.Z. Sun, J.L. Xu, X.G. Wang, H. Ji, C.C. Zhao, Z.H. Zhang, Formation and microstructure of nanoporous silver by dealloying rapidly solidified Zn–Ag alloys, Electrochim. Acta 63 (2012) 302–311. [21] M. Hakamada, M. Mabuchi, Preparation of nanoporous Ni and Ni–Cu by dealloying of rolled Ni–Mn and Ni–Cu–Mn alloys, J. Alloys Compd. 485 (2009) 583–587. [22] J.R. Hayes, A.M. Hodge, J. Biener, A.V. Hamza, Monolithic nanoporous copper by dealloying Mn–Cu, J. Mater. Res. 21 (2006) 2611–2616. [23] Z. Qi, C.C. Zhao, X.G. Wang, J.K. Lin, W. Shao, Z.H. Zhang, X.F. Bian, Formation and characterization of monolithic nanoporous copper by chemical dealloying of Al–Cu alloys, J. Phys. Chem. C 113 (2009) 6694–6698. [24] Y.W. Lin, C.C. Tai, I. Sun, Electrochemical preparation of porous copper surfaces in zinc chloride-1-ethyl-3-methyl imidazolium chloride ionic liquid, J. Electrochem. Soc. 154 (2007) D316–D321. [25] H.J. Qiu, F.X. Zou, Fabrication of stratified nanoporous gold for enhanced biosensing, Biosens. Bioelectron. 35 (2012) 349–354. [26] Y. Yu, L. Gu, X.Y. Lang, C.B. Zhu, T. Fujita, M.W. Chen, J. Maier, Li storage in 3D nanoporous Au-supported nanocrystalline tin, Adv. Mater. 23 (2011) 2443–2447.
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