- Email: [email protected]
Three-dimensional nanoporous copper with high surface area by dealloying Mg–Cu–Y metallic glasses
Materials Letters 76 (2012) 96–99
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Three-dimensional nanoporous copper with high surface area by dealloying Mg–Cu–Y metallic glasses Xuekun Luo, Ran Li ⁎, Zengqian Liu, Lu Huang, Minjie Shi, Tao Xu, Tao Zhang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 28 December 2011 Accepted 4 February 2012 Available online 16 February 2012 Keywords: Amorphous materials Microstructure Porous materials Thermal properties
a b s t r a c t Uniform three-dimensional nanoporous copper (NPC) with open pores was synthesized by chemically dealloying Mg–Cu–Y metallic glasses in H2SO4 aqueous solution. The NPC dealloyed from Mg50Cu40Y10 glassy alloy exhibits high Brunauer–Emmett–Teller (BET) surface area (12.27 m2/g), which is attributed to the formation of interconnective nanopores with size of 30–60 nm and accumulative nanoparticles with size of 3–8 nm on the surface of ligaments. The results reveal that the main forming mechanism of current NPC is fast dissolution of Mg and Y elements in the glassy alloys coupled with nucleation and growth of Cu clusters via the alloy/solution interface diffusion of Cu atoms. The NPC with large specific surface area and proper thermal conductivity is expected to be low-temperature heat exchanger materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous metals with high surface area-to-volume ratio and low density have attracted great attention in many technological applications including catalysis, actuators and fuel cells [1–3]. A variety of chemical and physical methods have been introduced to synthesize porous metals [4,5]. Among them, chemical or electrochemical dealloying, which refers to the selective dissolution of one or more components out of an alloy [6], is superior in the fabrication of threedimensional nanoporous metals with open pores owing to its high reactivity of some alloying elements and controllability of chemical reactions [7]. This method has been successfully adopted in the fabrication of nanoporous noble metals in different alloy systems including Cu–Pt [8], Ag–Au [9,10] and Ag–Au–Pt [11]. However, wide industrial applications of nanoporous noble metals are strictly limited by their high cost. In comparison with the noble materials, nanoporous copper (NPC) is one of the attractive inexpensive nanoporous materials with rosy prospects for wide application in both laboratories [7] and industries [12]. Many crystalline alloys [7,12–14] have been selected so far to synthesize NPC. It is generally recognized that NPC with ideal three-dimensional bicontinuous structure can be obtained from single-phase solid solution. However, the types of such available solid solutions are limited in possible choices of phase diagrams. Superior to crystalline solid solution, metallic glass is regarded as chemically homogeneous single phase while containing a greater concentration of different solute elements due to their unique disordered structure. Moreover, thousands of metallic glasses with different component
⁎ Corresponding authors. Tel.: + 86 10 82316192; fax: + 86 10 82339705. E-mail addresses: [email protected] (R. Li), [email protected] (T. Zhang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.028
elements and relatively wide composition ranges in various alloy systems offer abundant candidates for fabricating nanoporous metals. In addition, metallic glasses are free from defects such as grain boundaries, dislocations and segregations, which offer homogeneous structure that is beneficial for dealloying. Therefore, the unique metallic glass is expected to be an ideal choice for the formation of a uniform nanoporosity throughout the precursor. So far nanoporous platinum [15], palladium [16] and gold [16] have been synthesized by electrochemically dealloying from corresponding metallic glasses. However, less attention has been paid to the synthesis of monolithic nanoporous copper through chemical dealloying of metallic glasses [17]. In this study, three-dimensional NPC with high surface area was fabricated from chemical dealloying of Mg–Cu–Y metallic glasses. 2. Experimental methods Metallic glass ribbons with nominal compositions of Mg60Cu30Y10 and Mg50Cu40Y10 (at.%) were produced by single roller melt-spinning method. The ribbons obtained are 20–40 μm thick and 1–3 mm wide. Dealloying of the melt-spun ribbons was conducted in a deaerated 0.04 M H2SO4 aqueous solution at room temperature for about 1.5 h. Free corrosion reaction of dealloying proceeded until no bubble emerged. After dealloying, the as-dealloyed samples were rinsed with distilled water and dehydrated alcohol, and kept in a vacuum chamber not exceeding 24 h to avoid oxidation. The X-ray diffractograms of the as-spun and as-dealloyed ribbons were obtained using the copper Kα wavelength (Bruker AXS D8). Microstructures and morphologies of the as-dealloyed ribbons were checked by electron microscopy (SEM) (Hitachi S4800 and CamScan Appollo 300) coupled with an energydispersive X-ray analyzer (EDX) and transmission electron microscopy (TEM) (JEM-2100F) operated at 200 keV. The size of a ligament,
X. Luo et al. / Materials Letters 76 (2012) 96–99
97
distribution were evaluated using a NOVA4200e Nitrogen adsorption– desorption experimental instrument at 77 K. Electrical resistivity was measured as a function of temperature using a physical property measurement system (PPMS) by a standard four-probe technique.
3. Results and discussion
Fig. 1. XRD patterns of melt-spun glassy ribbons and the corresponding NPC obtained through chemical dealloying in 0.04 M H2SO4 aqueous solution at room temperature for Mg60Cu30Y10 and Mg50Cu40Y10 alloys.
regarded as cylinder, was measured from the diameter of randomly selected ligaments at their center. And the average ligament sizes were determined manually by identifying and averaging a minimum of 50 ligaments. Brunauer–Emmett–Teller (BET) surface area and pore size
XRD patterns (Fig. 1) of melt-spun Mg60Cu30Y10 and Mg50Cu40Y10 ribbons exhibit broad diffraction halos without any distinguishable crystalline Bragg peak, indicating their amorphous structure. After dealloying in the 0.04 M H2SO4 aqueous solution at room temperature for about 1.5 h, only a face centered cubic (f.c.c.) Cu phase can be indentified. Mg and Y elements were preferentially dealloyed from the original alloys and crystallization occurs in the survived Cu ribbons during the dealloying process. Fig. 2 shows microstructures of the as-dealloyed Mg60Cu30Y10 and Mg50Cu40Y10 ribbons. Both of the samples exhibit uniform threedimensional porous structure with open pores and bicontinuous interpenetrating ligaments throughout the whole ribbons. The ligament size lies within 60–80 nm for the sample of dealloyed Mg60Cu30Y10 and 80–100 nm for that of dealloyed Mg50Cu40Y10. Moreover the interconnective nanopore sizes of both NPC are 30–60 nm. The absolute dissolution of Mg and Y was further confirmed by the EDX analysis (inset of Fig. 2d) in which only peaks of Cu was identified. Combined with the XRD results (Fig. 1), it indicates
Fig. 2. Microstructural images of NPC dealloyed from Mg60Cu30Y10 (a, c and e) and Mg50Cu40Y10 (b, d and f) ribbons. (c and d) plane views; (a, b, e, and f) section views.
98
X. Luo et al. / Materials Letters 76 (2012) 96–99
three-dimensional NPC can be synthesized by dealloying of the ternary Mg–Cu–Y metallic glasses. The surface area is crucial for nanoporous copper especially in various applications of heat exchange [18], water–gas catalysis reaction [12] etc. In this study, BET surface area of the obtained NPC was evaluated through nitrogen adsorption–desorption measurement. The present NPC samples with a pore size in the range 30 to 60 nm both exhibit high surface area, i.e. 10.77 m 2/g for the one dealloyed from Mg60Cu30Y10 and 12.27 m 2/g for the other from Mg50Cu40Y10, respectively. The surface area of current NPC samples is comparable with that of Pt–nanoporous gold (Pt-NPG) (pore size of 3.8–4.0 nm) [11] and five times larger than those of nanoporous gold (pore size of ~ 75 nm) dealloyed from Ag70Au30 [19].
TEM observation was conducted on the present NPC samples to investigate the origin of the high surface area. As shown in Fig. 3, the nanoporous structure with pores in sizes of 30–60 nm was further verified by the bright-field TEM images. In addition, high resolution transmission electron microscopy (HRTEM) images show that many ultrafine nanoparticles with sizes of 3–8 nm accumulate on the ligament surface (shown by arrows in Fig. 3c-f). Therefore, according to the above observations, the large surface area of NPC is provided by both the interconnective nanopores and the ultrafine nanoparticles accumulated on the surface of ligaments. The typical selected area electron diffraction (SAED) pattern of ligaments consists of polycrystalline rings, corresponding to f.c.c. (111)Cu, (200)Cu and (220)Cu reflections (inset of Fig. 3a). The large difference of lattice structure between NPC
Fig. 3. TEM images of nanoporous structure of the NPC dealloyed from Mg60Cu30Y10 (a, c and e) and Mg50Cu40Y10 (b, d and f) with ultrafine nanoparticles (indicated by black arrows) accumulated on ligaments (c, d, e and f). The inset in (a) is a typical selected area electron diffraction (SAED) pattern of the NPC sample.
X. Luo et al. / Materials Letters 76 (2012) 96–99
99
Table 1 Physical parameters of our nanoporous copper dealloyed from Mg50Cu40Y10 metallic glass and some previous low-temperature heat exchanger materials (SPBP: sintered platinum black powder; SSC: sintered submicrometer copper; SSP: sintered silver powder; NPG: nanoporous gold from Ag70Au30 crystalline alloy; NPC*: nanoporous copper from Mg50Cu40Y10 metallic glass). Type
Packing fraction (%)
Surface area (m2/g)
Resistance (4.2 K) (μΩ·cm)
Resistance (300 K) (μΩ•cm)
Thermal conductivity (4.2 K) (×10− 8 W/K)
Thermal conductivity (300 K) (×10− 8 W/K)
References
SPBP SSC SSP NPG NPC*
33 46–47 40 31 32
13.3 2.2–2.9 2.1 2.14 12.26
— 3.6–8.4 6.1 57 44.4
230 12–21 — 77 54.5
— 1.225–2.858 1.687 0.181 0.232
3.196 35.000–61.250 — 9.545 13.486
[21] [22] [23] [19,24,25] This work
and metallic glass indicates that the formation mechanism of our NPC is different from that of the previous nanoporous gold dealloying from Ag–Au alloys [9]. It is well known that alloy composition has a significant influence upon the dealloying process and formation of nanoporous structure [14,16]. The evolution of porosity during chemical dealloying of Mg–Cu–Y metallic glass is qualitatively described in the following way: Consider a surface of a metallic glass as an original condition. As immersing the alloy in the acid solution, chemical reactions occur at the solid/liquid interface. Because Mg and Y have more negative standard electrode potential than that of Cu (− 2.36 V (SHE) for Mg 2+/Mg, −2.37 V (SHE) for Y 3+/Y and +0.34 V (SHE) for Cu 2+/Cu), they are continuously fast dissolved into electrolyte and become ions. The loss of less noble Mg and Y elements leads to a high concentration of Cu atoms at the alloy/solution interface. The Cu atoms are driven to agglomerate into clusters due to local energy fluctuation [6]. A large quantity of Cu clusters nucleates and grows extending along the low-energy-level lattice planes to form polycrystalline ligaments of NPC by solid/solution interface diffusion of Cu atoms [20]. In addition, immature Cu clusters are absorbed on the surface of ligaments and slowly grow into the nanoparticles (as shown in Fig. 3 by arrows). Therefore, the formation mechanism of the NPC dealloyed from the Mg-based metallic glasses is mainly controlled by fast dissolution of less noble atoms (Mg and Y atoms) and nucleation and growth of Cu clusters through the solid/solution interface diffusion of Cu atoms. The dealloying mechanism implicates that metallic glasses consisting of active dealloyed element(s) and more noble element(s) show wide compositional options suitable for synthesizing and tailoring the multiply nanoporous structure. One of the most attractive applications of NPC is to be used as heat exchanger material at low temperatures. Materials with high thermal conductivity and large surface area are ideal candidates for heat exchanger at low temperature [18]. Thermal conductivity of the cur rent NPC is derived from Wiedemann–Franz relation κ ¼ LT ρ , where κ is the thermal conductivity, electrical resistivity (ρ) at low temperature T, and L is the Lorentz constant 2.45 × 10 − 8 ΩWK − 2. The electrical resistivity (ρ) at 300 and 4.2 K were measured using a physical property measurement system (PPMS) by a standard fourprobe technique. Parameters of the present NPC samples and those of the reported low-temperature heat exchanger materials are summarized in Table 1 for comparison [19,21–25]. The thermal conductivity of the present NPC is 0.232 × 10 − 8 W/K at 4.2 K and 13.486 × 10 − 8 W/K at 300 K. Considering the large surface area and high thermal condctivity the NPC dealloyed from Mg–Cu–Y metallic
glasses is expected to be a potential candidate as low-temperature heat exchanger material. 4. Conclusions Uniform three-dimensional NPC with open pores was synthesized by chemically dealloying ternary Mg–Cu–Y metallic glasses. The obtained NPC samples exhibit high surface areas, which are attributed to the unique nanoporous microstructure with nanoparticles accumulated on the ligaments. Metallic glasses are attractive candidates as fabrication precursors for nanoporous materials. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant Nos. 51071008 and 51131002) and Program for New Century Excellent Talents in University (NCET). References [1] Bond GC, Thompson DT. Catal Rev 1999;41:319–88. [2] Weissmuller J, Viswanath RN, Kramer D, Zimmer P, Wuerschum R, Gleiter H. Science 2003;300:312–5. [3] Joo SH, Choi SJ, Kwa KJ, Liu Z. Nature 2001;412:169–72. [4] Kijima T, Yoshimura T, Uota M, Ikeda T, Fujikawa D, Mouri S, et al. Angew Chem 2003;116:230–4. [5] Yao B, Fleming D, Morris MA, Lawrence SE. Chem Mater 2004;16:4851–5. [6] Erlebacher J. J Electrochem Soc 2004;151:C614–26. [7] Chen LY, Yu JS, Fujita T, Chen MW. Adv Funct Mater 2009;19:1221–6. [8] Pugh DV, Dursun A, Corcoran SG. J Electrochem Soc 2005;152:B455–9. [9] Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. [10] Qian LH, Chen MW. Appl Phys Lett 2007;91:083105. [11] Snyder J, Asanithi P, Dalton AB, Erlebacher J. Adv Mater 2008;20:4883–6. [12] Smith AJ, Trimm DL. Annu Rev Mater Res 2005;35:127–42. [13] Lu HB, Li Y, Wang FH. Scr Mater 2007;56:165–8. [14] Zhang ZH, Wang Y, Qi Z, Zhang WH, Qin JY, Frenzel J. J Phys Chem 2009;C113: 12629–36. [15] Thorp JC, Sieradzki K, Tang L, Crozier PA, Misra A, Nastasi M, et al. Appl Phys Lett 2006;88:033110. [16] Yu JS, Ding Y, Xu CX, Inoue A, Sakurai T, Chen MW. Chem Mater 2008;20:4548–50. [17] Aburada T, Fitz-Gerald JM, Scully JR. Corros Sci 2011;53:1627–32. [18] Guenault AM, Keith V, Kennedy CJ, Miller IE, Pickett GR. Nature 1983;302:695–6. [19] Tulimieri DJ, Yoon J, Chan MHW. Phys Rev Lett 1999;82:121–4. [20] Zhang Q, Zhang ZH. Phys Chem Chem Phys 2010;12:1453–72. [21] Roach PR, Takano Y, Hilleke RO, Vrtis ML, Jin D, Sarma BK. Cryog 1986;26:319–21. [22] Rogacki K, Kubota M, Syskakis EG, Mueller RM, Pobell FJ. Low Temp Phys 1985;59: 397–412. [23] Busch PA, Cheston SP, Greywall DS. Cryog 1984;24:445–7. [24] Ertenberg RW, Andraka B, Takano Y. Physica B 2000;284–288:2022–3. [25] Yoon J, Chan MHW. Phys Rev Lett 1997;78:4801–4.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Three-dimensional nanoporous copper with high surface area by dealloying Mg–Cu–Y metallic glasses Xuekun Luo, Ran Li ⁎, Zengqian Liu, Lu Huang, Minjie Shi, Tao Xu, Tao Zhang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 28 December 2011 Accepted 4 February 2012 Available online 16 February 2012 Keywords: Amorphous materials Microstructure Porous materials Thermal properties
a b s t r a c t Uniform three-dimensional nanoporous copper (NPC) with open pores was synthesized by chemically dealloying Mg–Cu–Y metallic glasses in H2SO4 aqueous solution. The NPC dealloyed from Mg50Cu40Y10 glassy alloy exhibits high Brunauer–Emmett–Teller (BET) surface area (12.27 m2/g), which is attributed to the formation of interconnective nanopores with size of 30–60 nm and accumulative nanoparticles with size of 3–8 nm on the surface of ligaments. The results reveal that the main forming mechanism of current NPC is fast dissolution of Mg and Y elements in the glassy alloys coupled with nucleation and growth of Cu clusters via the alloy/solution interface diffusion of Cu atoms. The NPC with large specific surface area and proper thermal conductivity is expected to be low-temperature heat exchanger materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous metals with high surface area-to-volume ratio and low density have attracted great attention in many technological applications including catalysis, actuators and fuel cells [1–3]. A variety of chemical and physical methods have been introduced to synthesize porous metals [4,5]. Among them, chemical or electrochemical dealloying, which refers to the selective dissolution of one or more components out of an alloy [6], is superior in the fabrication of threedimensional nanoporous metals with open pores owing to its high reactivity of some alloying elements and controllability of chemical reactions [7]. This method has been successfully adopted in the fabrication of nanoporous noble metals in different alloy systems including Cu–Pt [8], Ag–Au [9,10] and Ag–Au–Pt [11]. However, wide industrial applications of nanoporous noble metals are strictly limited by their high cost. In comparison with the noble materials, nanoporous copper (NPC) is one of the attractive inexpensive nanoporous materials with rosy prospects for wide application in both laboratories [7] and industries [12]. Many crystalline alloys [7,12–14] have been selected so far to synthesize NPC. It is generally recognized that NPC with ideal three-dimensional bicontinuous structure can be obtained from single-phase solid solution. However, the types of such available solid solutions are limited in possible choices of phase diagrams. Superior to crystalline solid solution, metallic glass is regarded as chemically homogeneous single phase while containing a greater concentration of different solute elements due to their unique disordered structure. Moreover, thousands of metallic glasses with different component
⁎ Corresponding authors. Tel.: + 86 10 82316192; fax: + 86 10 82339705. E-mail addresses: [email protected] (R. Li), [email protected] (T. Zhang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.028
elements and relatively wide composition ranges in various alloy systems offer abundant candidates for fabricating nanoporous metals. In addition, metallic glasses are free from defects such as grain boundaries, dislocations and segregations, which offer homogeneous structure that is beneficial for dealloying. Therefore, the unique metallic glass is expected to be an ideal choice for the formation of a uniform nanoporosity throughout the precursor. So far nanoporous platinum [15], palladium [16] and gold [16] have been synthesized by electrochemically dealloying from corresponding metallic glasses. However, less attention has been paid to the synthesis of monolithic nanoporous copper through chemical dealloying of metallic glasses [17]. In this study, three-dimensional NPC with high surface area was fabricated from chemical dealloying of Mg–Cu–Y metallic glasses. 2. Experimental methods Metallic glass ribbons with nominal compositions of Mg60Cu30Y10 and Mg50Cu40Y10 (at.%) were produced by single roller melt-spinning method. The ribbons obtained are 20–40 μm thick and 1–3 mm wide. Dealloying of the melt-spun ribbons was conducted in a deaerated 0.04 M H2SO4 aqueous solution at room temperature for about 1.5 h. Free corrosion reaction of dealloying proceeded until no bubble emerged. After dealloying, the as-dealloyed samples were rinsed with distilled water and dehydrated alcohol, and kept in a vacuum chamber not exceeding 24 h to avoid oxidation. The X-ray diffractograms of the as-spun and as-dealloyed ribbons were obtained using the copper Kα wavelength (Bruker AXS D8). Microstructures and morphologies of the as-dealloyed ribbons were checked by electron microscopy (SEM) (Hitachi S4800 and CamScan Appollo 300) coupled with an energydispersive X-ray analyzer (EDX) and transmission electron microscopy (TEM) (JEM-2100F) operated at 200 keV. The size of a ligament,
X. Luo et al. / Materials Letters 76 (2012) 96–99
97
distribution were evaluated using a NOVA4200e Nitrogen adsorption– desorption experimental instrument at 77 K. Electrical resistivity was measured as a function of temperature using a physical property measurement system (PPMS) by a standard four-probe technique.
3. Results and discussion
Fig. 1. XRD patterns of melt-spun glassy ribbons and the corresponding NPC obtained through chemical dealloying in 0.04 M H2SO4 aqueous solution at room temperature for Mg60Cu30Y10 and Mg50Cu40Y10 alloys.
regarded as cylinder, was measured from the diameter of randomly selected ligaments at their center. And the average ligament sizes were determined manually by identifying and averaging a minimum of 50 ligaments. Brunauer–Emmett–Teller (BET) surface area and pore size
XRD patterns (Fig. 1) of melt-spun Mg60Cu30Y10 and Mg50Cu40Y10 ribbons exhibit broad diffraction halos without any distinguishable crystalline Bragg peak, indicating their amorphous structure. After dealloying in the 0.04 M H2SO4 aqueous solution at room temperature for about 1.5 h, only a face centered cubic (f.c.c.) Cu phase can be indentified. Mg and Y elements were preferentially dealloyed from the original alloys and crystallization occurs in the survived Cu ribbons during the dealloying process. Fig. 2 shows microstructures of the as-dealloyed Mg60Cu30Y10 and Mg50Cu40Y10 ribbons. Both of the samples exhibit uniform threedimensional porous structure with open pores and bicontinuous interpenetrating ligaments throughout the whole ribbons. The ligament size lies within 60–80 nm for the sample of dealloyed Mg60Cu30Y10 and 80–100 nm for that of dealloyed Mg50Cu40Y10. Moreover the interconnective nanopore sizes of both NPC are 30–60 nm. The absolute dissolution of Mg and Y was further confirmed by the EDX analysis (inset of Fig. 2d) in which only peaks of Cu was identified. Combined with the XRD results (Fig. 1), it indicates
Fig. 2. Microstructural images of NPC dealloyed from Mg60Cu30Y10 (a, c and e) and Mg50Cu40Y10 (b, d and f) ribbons. (c and d) plane views; (a, b, e, and f) section views.
98
X. Luo et al. / Materials Letters 76 (2012) 96–99
three-dimensional NPC can be synthesized by dealloying of the ternary Mg–Cu–Y metallic glasses. The surface area is crucial for nanoporous copper especially in various applications of heat exchange [18], water–gas catalysis reaction [12] etc. In this study, BET surface area of the obtained NPC was evaluated through nitrogen adsorption–desorption measurement. The present NPC samples with a pore size in the range 30 to 60 nm both exhibit high surface area, i.e. 10.77 m 2/g for the one dealloyed from Mg60Cu30Y10 and 12.27 m 2/g for the other from Mg50Cu40Y10, respectively. The surface area of current NPC samples is comparable with that of Pt–nanoporous gold (Pt-NPG) (pore size of 3.8–4.0 nm) [11] and five times larger than those of nanoporous gold (pore size of ~ 75 nm) dealloyed from Ag70Au30 [19].
TEM observation was conducted on the present NPC samples to investigate the origin of the high surface area. As shown in Fig. 3, the nanoporous structure with pores in sizes of 30–60 nm was further verified by the bright-field TEM images. In addition, high resolution transmission electron microscopy (HRTEM) images show that many ultrafine nanoparticles with sizes of 3–8 nm accumulate on the ligament surface (shown by arrows in Fig. 3c-f). Therefore, according to the above observations, the large surface area of NPC is provided by both the interconnective nanopores and the ultrafine nanoparticles accumulated on the surface of ligaments. The typical selected area electron diffraction (SAED) pattern of ligaments consists of polycrystalline rings, corresponding to f.c.c. (111)Cu, (200)Cu and (220)Cu reflections (inset of Fig. 3a). The large difference of lattice structure between NPC
Fig. 3. TEM images of nanoporous structure of the NPC dealloyed from Mg60Cu30Y10 (a, c and e) and Mg50Cu40Y10 (b, d and f) with ultrafine nanoparticles (indicated by black arrows) accumulated on ligaments (c, d, e and f). The inset in (a) is a typical selected area electron diffraction (SAED) pattern of the NPC sample.
X. Luo et al. / Materials Letters 76 (2012) 96–99
99
Table 1 Physical parameters of our nanoporous copper dealloyed from Mg50Cu40Y10 metallic glass and some previous low-temperature heat exchanger materials (SPBP: sintered platinum black powder; SSC: sintered submicrometer copper; SSP: sintered silver powder; NPG: nanoporous gold from Ag70Au30 crystalline alloy; NPC*: nanoporous copper from Mg50Cu40Y10 metallic glass). Type
Packing fraction (%)
Surface area (m2/g)
Resistance (4.2 K) (μΩ·cm)
Resistance (300 K) (μΩ•cm)
Thermal conductivity (4.2 K) (×10− 8 W/K)
Thermal conductivity (300 K) (×10− 8 W/K)
References
SPBP SSC SSP NPG NPC*
33 46–47 40 31 32
13.3 2.2–2.9 2.1 2.14 12.26
— 3.6–8.4 6.1 57 44.4
230 12–21 — 77 54.5
— 1.225–2.858 1.687 0.181 0.232
3.196 35.000–61.250 — 9.545 13.486
[21] [22] [23] [19,24,25] This work
and metallic glass indicates that the formation mechanism of our NPC is different from that of the previous nanoporous gold dealloying from Ag–Au alloys [9]. It is well known that alloy composition has a significant influence upon the dealloying process and formation of nanoporous structure [14,16]. The evolution of porosity during chemical dealloying of Mg–Cu–Y metallic glass is qualitatively described in the following way: Consider a surface of a metallic glass as an original condition. As immersing the alloy in the acid solution, chemical reactions occur at the solid/liquid interface. Because Mg and Y have more negative standard electrode potential than that of Cu (− 2.36 V (SHE) for Mg 2+/Mg, −2.37 V (SHE) for Y 3+/Y and +0.34 V (SHE) for Cu 2+/Cu), they are continuously fast dissolved into electrolyte and become ions. The loss of less noble Mg and Y elements leads to a high concentration of Cu atoms at the alloy/solution interface. The Cu atoms are driven to agglomerate into clusters due to local energy fluctuation [6]. A large quantity of Cu clusters nucleates and grows extending along the low-energy-level lattice planes to form polycrystalline ligaments of NPC by solid/solution interface diffusion of Cu atoms [20]. In addition, immature Cu clusters are absorbed on the surface of ligaments and slowly grow into the nanoparticles (as shown in Fig. 3 by arrows). Therefore, the formation mechanism of the NPC dealloyed from the Mg-based metallic glasses is mainly controlled by fast dissolution of less noble atoms (Mg and Y atoms) and nucleation and growth of Cu clusters through the solid/solution interface diffusion of Cu atoms. The dealloying mechanism implicates that metallic glasses consisting of active dealloyed element(s) and more noble element(s) show wide compositional options suitable for synthesizing and tailoring the multiply nanoporous structure. One of the most attractive applications of NPC is to be used as heat exchanger material at low temperatures. Materials with high thermal conductivity and large surface area are ideal candidates for heat exchanger at low temperature [18]. Thermal conductivity of the cur rent NPC is derived from Wiedemann–Franz relation κ ¼ LT ρ , where κ is the thermal conductivity, electrical resistivity (ρ) at low temperature T, and L is the Lorentz constant 2.45 × 10 − 8 ΩWK − 2. The electrical resistivity (ρ) at 300 and 4.2 K were measured using a physical property measurement system (PPMS) by a standard fourprobe technique. Parameters of the present NPC samples and those of the reported low-temperature heat exchanger materials are summarized in Table 1 for comparison [19,21–25]. The thermal conductivity of the present NPC is 0.232 × 10 − 8 W/K at 4.2 K and 13.486 × 10 − 8 W/K at 300 K. Considering the large surface area and high thermal condctivity the NPC dealloyed from Mg–Cu–Y metallic
glasses is expected to be a potential candidate as low-temperature heat exchanger material. 4. Conclusions Uniform three-dimensional NPC with open pores was synthesized by chemically dealloying ternary Mg–Cu–Y metallic glasses. The obtained NPC samples exhibit high surface areas, which are attributed to the unique nanoporous microstructure with nanoparticles accumulated on the ligaments. Metallic glasses are attractive candidates as fabrication precursors for nanoporous materials. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant Nos. 51071008 and 51131002) and Program for New Century Excellent Talents in University (NCET). References [1] Bond GC, Thompson DT. Catal Rev 1999;41:319–88. [2] Weissmuller J, Viswanath RN, Kramer D, Zimmer P, Wuerschum R, Gleiter H. Science 2003;300:312–5. [3] Joo SH, Choi SJ, Kwa KJ, Liu Z. Nature 2001;412:169–72. [4] Kijima T, Yoshimura T, Uota M, Ikeda T, Fujikawa D, Mouri S, et al. Angew Chem 2003;116:230–4. [5] Yao B, Fleming D, Morris MA, Lawrence SE. Chem Mater 2004;16:4851–5. [6] Erlebacher J. J Electrochem Soc 2004;151:C614–26. [7] Chen LY, Yu JS, Fujita T, Chen MW. Adv Funct Mater 2009;19:1221–6. [8] Pugh DV, Dursun A, Corcoran SG. J Electrochem Soc 2005;152:B455–9. [9] Erlebacher J, Aziz MJ, Karma A, Dimitrov N, Sieradzki K. Nature 2001;410:450–3. [10] Qian LH, Chen MW. Appl Phys Lett 2007;91:083105. [11] Snyder J, Asanithi P, Dalton AB, Erlebacher J. Adv Mater 2008;20:4883–6. [12] Smith AJ, Trimm DL. Annu Rev Mater Res 2005;35:127–42. [13] Lu HB, Li Y, Wang FH. Scr Mater 2007;56:165–8. [14] Zhang ZH, Wang Y, Qi Z, Zhang WH, Qin JY, Frenzel J. J Phys Chem 2009;C113: 12629–36. [15] Thorp JC, Sieradzki K, Tang L, Crozier PA, Misra A, Nastasi M, et al. Appl Phys Lett 2006;88:033110. [16] Yu JS, Ding Y, Xu CX, Inoue A, Sakurai T, Chen MW. Chem Mater 2008;20:4548–50. [17] Aburada T, Fitz-Gerald JM, Scully JR. Corros Sci 2011;53:1627–32. [18] Guenault AM, Keith V, Kennedy CJ, Miller IE, Pickett GR. Nature 1983;302:695–6. [19] Tulimieri DJ, Yoon J, Chan MHW. Phys Rev Lett 1999;82:121–4. [20] Zhang Q, Zhang ZH. Phys Chem Chem Phys 2010;12:1453–72. [21] Roach PR, Takano Y, Hilleke RO, Vrtis ML, Jin D, Sarma BK. Cryog 1986;26:319–21. [22] Rogacki K, Kubota M, Syskakis EG, Mueller RM, Pobell FJ. Low Temp Phys 1985;59: 397–412. [23] Busch PA, Cheston SP, Greywall DS. Cryog 1984;24:445–7. [24] Ertenberg RW, Andraka B, Takano Y. Physica B 2000;284–288:2022–3. [25] Yoon J, Chan MHW. Phys Rev Lett 1997;78:4801–4.