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Elimination of droplet rebound off soluble substrate in metal droplet deposition
Materials Letters 216 (2018) 232–235
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Elimination of droplet rebound off soluble substrate in metal droplet deposition Hao Yi a, Lehua Qi a,⇑, Jun Luo a, Yongan Guo b, Shaolin Li b, Ni Li c,⇑ a
School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China c Department of Mechanical Engineering, California State University, Los Angeles 90032, USA b
a r t i c l e
i n f o
Article history: Received 16 December 2017 Received in revised form 18 January 2018 Accepted 22 January 2018
Keywords: 3D printing Interfaces Intermetallic alloys and compounds Al droplets Soluble core
a b s t r a c t This letter presents a method and its mechanism of using Ag coating to inhibit rebound of Al droplets over soluble substrates. Experiments show that Ag coating can eliminate droplet rebound, which makes it possible to print complex parts with high-quality inner surface. Further investigation reveals that Ag coating suppresses the rebound of Al droplets through strong chemical interaction between liquid Al and Ag layer, which results in fast Ag dissolution in liquid Al and formation of a dendritic network of Al-Ag intermetallic compounds (IMCs) on their interface. XRD and EDS observation indicates that the IMCs caused by Ag atom diffusion and precipitation are mainly composed of Ag2Al intermetallics and a-Al solid solutions. It is found that the thickness of the IMCs layer increases as the substrate temperature is raised. The temperature increase of the substrate will slow down the droplet cooling process, which allows more thorough chemical reaction. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Droplet-based 3D printing is of great interesting in industrial manufacture [1], and is very promising for fabrication of complex thin-wall microwave devices such as waveguide tubes since it can print a shell by utilizing only several layers of droplets [2]. Because metal droplets are naturally of scalloped shapes [3], the convention droplet-based 3D printing method cannot produce thin-wall tubes with high-quality inner surface that can meet the requirement of electromagnetic transmission [4]. Inspired by the conventional casting process, the cavity surface quality of the printed parts could be improved if soluble cores are used as support materials. However, due to the poor wettability between the metal droplets and the soluble cores (e.g., gypsum and ceramic), the metal droplets rebound from the soluble core surface after impact. Hence, deposition of metal droplets onto such soluble cores remains a challenge. It is known that metal materials are usually with good wettability with metals, therefore, metal interlayers could be applied to improve the wettability between the molten droplets and substrate [5]. This letter presents a method of coating Ag, which is widely used as the inner surface coating materials in microwave
devices, on the soluble cores to prevent Al droplets from rebounding. Moreover, the mechanism of this method to suppress the rebound of Al droplets is investigated. 2. Experimental procedure Fig. 1(a) shows the schematic of experimental apparatus. Details of the experimental setup have been described in our previous work [6]. Pure Al (99.999%) was heated to 1023 K in a graphite crucible. Uniform droplets with a diameter of 800 lm were deposited on an Ag-coated soluble gypsum. The Ag coating was fabricated by the combination of silver slurry and screenprinting processes. The surface of the coating was smooth without blistering, cracking or detachment, and its thickness was 50 lm. The microstructural morphologies and crystalline structure of the samples were examined by scanning electron microscope (SEM, VEGA3, TESCAN) with energy dispersive spectrometer (EDS) and X-ray diffraction (XRD, X’Pert PRO MPD, PANalytical), respectively. The dynamic images of metal droplet impact on solid surfaces were captured by high-speed CCD camera (MotionbBLITZ Cube 1). 3. Results and discussion
⇑ Corresponding authors. E-mail addresses: [email protected] (L. Qi), [email protected] (N. Li). https://doi.org/10.1016/j.matlet.2018.01.127 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Fig. 1(b) and (c) show the initial dynamic behavior of a typical Al droplet impact on soluble gypsum and Ag-coated soluble
H. Yi et al. / Materials Letters 216 (2018) 232–235
233
Fig. 1. (a) Schematic diagram of metal droplet generation and deposition; initial dynamic behavior of Al droplet impact on (b) soluble gypsum and (c) Ag-coated soluble gypsum.
Fig. 2. (a) SEM image of a typically deposited droplet after removing the core; backscattered electron scanning micrograph of (b) the Al-Ag interface and (c) enlarged view of IMCs; elemental mapping of (d) Ag and (e) Al; (f) XRD of the cross-section of droplet-coating; (g) the microstructures of different precipitated phase within IMCs; (h) enlarged view of the selected section in (g).
gypsum, respectively. The temperature of the substrate is 300 K. It indicates that Al droplet rebounds after impacts on the soluble gypsum at time (t + 2) ms. When a droplet deposits on the Agcoated soluble gypsum, recoil and oscillation occur instead of rebounding from time (t + 2) to (t + 3) ms.
To understand the above observed phenomena, the interface behavior between Al droplet and Ag coating was studied. Fig. 2 (a) shows the side-view of a typically deposited Al droplet after removing the core. The brighter layer (50 lm) at the bottom of the droplet shows that the Ag coating is tightly bonded to the
234
H. Yi et al. / Materials Letters 216 (2018) 232–235
Fig. 3. Backscattered electron scanning micrograph of the Al-Ag interface with different substrate temperatures of (a) 300 K, (b) 473 K and (c) 673 K.
metal droplets. Fig. 2(b) shows the backscattered electron scanning micrograph of the droplet-coating interface in Fig. 2(a). Clearly visible in Fig. 2(b) is the IMCs layer formed between the Al droplet and Ag coating. The microstructure of this IMCs layer shows a very fine white dendritic network, which seems to be two-phase. Because the bright zone is more enriched with heavier atoms than the dark zone, the bright and dark zone are Ag-rich and Al-rich region, respectively. EDS and XRD analyses were adopted to further determine the composition, element distribution and migration within the interface. As shown in Fig. 2(c)–(e), EDS surface scan images of the AlAg interface demonstrate that Ag is precipitated as a dendritic network structure in Al matrix, which further verifies that strong reactions occurred. XRD of the cross-section of droplet-coating shown in Fig. 2(f) indicates that the interface is mainly composed of Ag, Al and Ag2Al intermetallics. Fig. 2(g)–(h) show the enlarged microstructures of IMCs, and it is visible that the interfacial region is mainly composed of three kinds of microstructure zones, which are the bright zones (dendritic structures), the grey zones (needlelike structures) and the dark zones, respectively. Table 1 shows the chemical compositions of these three zones which are represented by point A, B and C. According to the above EDS and XRD test results, combining with the Al-Ag binary diagram, it can be concluded that the bright zones and grey zones are mainly composed of Ag2Al intermetallics, the dark zones are mainly composed of aAl solid solutions. The formation of these IMCs indicates the good wetting and bonding between the Al droplet and Ag coating, which helps pin the droplet to the substrate and suppresses the rebound of Al droplets [7,8]. The formation of this dendritic network IMCs layer is due to the liquid-solid reaction between the Al and Ag, which is similar to the wetting reaction process between molten solder droplet and pad [7–9]. During the transient contact, Ag was quickly dissolved into Al droplet. Al-Ag intermetallics were precipitated on reaching the solubility limit during the solidification. Moreover, because the impact, spread and solidification process of Al droplet is nonisothermal, a vertical temperature gradient exists between the droplet and substrate [10]. The supply of solute atoms depends on the heat flow and the diffusion field, and the growth of crystal is faster in the direction of heat and mass transfer. Therefore, the IMCs layer tends to grow vertically. Because the cooling rate of Al droplet in
our experiment is 103–104 K/s, Ag2Al precipitation is restrained and presents such white network microstructure, which is often formed by rapid solidification. This result is similar to the previous study [11] conducted in a rapidly solidified condition (104 K/s). It can be assumed that the primary Ag2Al precipitates would grow much coarser with the decreasing of cooling rate. To achieve good bonding between neighboring droplets and obtain high precise metal traces, substrate temperature needs to be adjusted to different requirements. Therefore, it is essential to understand the effect of substrate temperature on the interface behavior between the Al droplet and Ag coating. To this end, we conducted three sets of experiments with the preheating substrate temperature of 300 K, 473 K and 673 K, respectively. Comparing Fig. 3(a)–(c), it is visible that the thicknesses of the IMCs layer increases (from 60 lm to 80 lm) with the increase of preheating temperature. Besides, the dendritic network becomes denser and thicker. The interface temperature and solidification time of Al droplet will both increase with the increase of substrate temperature, which means the dissolution rate and the diffusion time of Ag to Al droplet will increase. Therefore, the thicknesses of the IMCs layer increases and the IMCs precipitates become much denser and thicker.
4. Conclusion In summary, we propose a novel method of using Ag coating to prevent Al droplets bouncing off soluble cores. The mechanism of this method on inhibiting the rebound of Al droplets lies in the strong chemical interaction between liquid Al and Ag layer, which results in fast Ag dissolution in liquid Al and formation of a dendritic network of Al-Ag intermetallic compounds (IMCs) on their interface. XRD and EDS observation reveals the composition of the interfacial zone are mainly Ag2Al intermetallics and a-Al solid solutions. It is found that the thickness of the IMCs layer increases as the substrate temperature is raised. The temperature increase of the substrate will slow down the droplet cooling process, which allows more thorough chemical reaction. This investigation lays the foundation of using droplet-based 3D printing to manufacture tube parts with smooth cavity surface.
Acknowledgements Table 1 Results of chemical analysis of three points in Fig. 2(h). Element
Al (%)
Ag (%)
A B C
35.19 61.49 80.68
64.81 38.51 19.32
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 51675436), Key Research and Development Plan of Shaanxi Province (2017ZDXM-GY-110), Science and Technology Fund Project, Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201704).
H. Yi et al. / Materials Letters 216 (2018) 232–235
References [1] Z. Luo, X. Wang, L. Wang, D. Sun, Z. Li, Drop-on-demand electromagnetic printing of metallic droplets, Mater. Lett. 188 (2017) 184–187. [2] Y.P. Chao, L.H. Qi, Y. Xiao, J. Luo, J.M. Zhou, Manufacturing of micro thin-walled metal parts by micro-droplet deposition, J. Mater. Process. Technol. 212 (2) (2012) 484–491. [3] J. Luo, W. Wang, W. Xiong, H. Shen, L. Qi, Formation of uniform metal traces using alternate droplet printing, Int. J. Mach. Tools Manuf. 122 (Suppl. C) (2017) 47–54. [4] C. Garcia, R. Rumpf, H. Tsang, J. Barton, Effects of extreme surface roughness on 3D printed horn antenna, Electron. Lett. 49 (12) (2013) 734–736. [5] Y.J. Ko, J. Yoon, J. Lee, J.H. Han, Effects of Cu interlayer on the wettability of aluminum on carbon, J. Alloys Compd. 574 (2013) 526–531. [6] H. Yi, L. Qi, J. Luo, Y. Jiang, W. Deng, Pinhole formation from liquid metal microdroplets impact on solid surfaces, Appl. Phys. Lett. 108 (4) (2016) 041601.
235
[7] F.-Q. Li, C.-Q. Wang, Influence of interfacial reaction between molten SnAgCu solder droplet and Au/Ni/Cu pad on IMC evolution, Trans. Non-Ferrous Met. Soc. f China 16 (1) (2006) 18–22. [8] M. van Bellegema, H. Ankersmitb, R. van Langhc, W. Weid, Solder on silver: historical usage and the problem of fretting, Metal 04, in: Proceedings of the International Conference on Metals Conservation, 4–8 October 2004, National Museum of Australia, Canberra, Australia, , 2004, pp. 104–113, ill. [9] L. Zang, Z. Yuan, H. Zhao, X. Zhang, Wettability of molten Sn–Bi–Cu solder on Cu substrate, Mater. Lett. 63 (23) (2009) 2067–2069. [10] H. Li, P. Wang, L. Qi, H. Zuo, S. Zhong, X. Hou, 3D numerical simulation of successive deposition of uniform molten Al droplets on a moving substrate and experimental validation, Comput. Mater. Sci. 65 (2012) 291–301. [11] I. Yamauchi, T. Kajiwara, T. Mase, M. Saraoka, Formation of highly saturated Al–Ag precursor by rapid solidification for skeletal silver synthesis, J. Alloys Compd. 336 (1) (2002) 206–212.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Elimination of droplet rebound off soluble substrate in metal droplet deposition Hao Yi a, Lehua Qi a,⇑, Jun Luo a, Yongan Guo b, Shaolin Li b, Ni Li c,⇑ a
School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China c Department of Mechanical Engineering, California State University, Los Angeles 90032, USA b
a r t i c l e
i n f o
Article history: Received 16 December 2017 Received in revised form 18 January 2018 Accepted 22 January 2018
Keywords: 3D printing Interfaces Intermetallic alloys and compounds Al droplets Soluble core
a b s t r a c t This letter presents a method and its mechanism of using Ag coating to inhibit rebound of Al droplets over soluble substrates. Experiments show that Ag coating can eliminate droplet rebound, which makes it possible to print complex parts with high-quality inner surface. Further investigation reveals that Ag coating suppresses the rebound of Al droplets through strong chemical interaction between liquid Al and Ag layer, which results in fast Ag dissolution in liquid Al and formation of a dendritic network of Al-Ag intermetallic compounds (IMCs) on their interface. XRD and EDS observation indicates that the IMCs caused by Ag atom diffusion and precipitation are mainly composed of Ag2Al intermetallics and a-Al solid solutions. It is found that the thickness of the IMCs layer increases as the substrate temperature is raised. The temperature increase of the substrate will slow down the droplet cooling process, which allows more thorough chemical reaction. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Droplet-based 3D printing is of great interesting in industrial manufacture [1], and is very promising for fabrication of complex thin-wall microwave devices such as waveguide tubes since it can print a shell by utilizing only several layers of droplets [2]. Because metal droplets are naturally of scalloped shapes [3], the convention droplet-based 3D printing method cannot produce thin-wall tubes with high-quality inner surface that can meet the requirement of electromagnetic transmission [4]. Inspired by the conventional casting process, the cavity surface quality of the printed parts could be improved if soluble cores are used as support materials. However, due to the poor wettability between the metal droplets and the soluble cores (e.g., gypsum and ceramic), the metal droplets rebound from the soluble core surface after impact. Hence, deposition of metal droplets onto such soluble cores remains a challenge. It is known that metal materials are usually with good wettability with metals, therefore, metal interlayers could be applied to improve the wettability between the molten droplets and substrate [5]. This letter presents a method of coating Ag, which is widely used as the inner surface coating materials in microwave
devices, on the soluble cores to prevent Al droplets from rebounding. Moreover, the mechanism of this method to suppress the rebound of Al droplets is investigated. 2. Experimental procedure Fig. 1(a) shows the schematic of experimental apparatus. Details of the experimental setup have been described in our previous work [6]. Pure Al (99.999%) was heated to 1023 K in a graphite crucible. Uniform droplets with a diameter of 800 lm were deposited on an Ag-coated soluble gypsum. The Ag coating was fabricated by the combination of silver slurry and screenprinting processes. The surface of the coating was smooth without blistering, cracking or detachment, and its thickness was 50 lm. The microstructural morphologies and crystalline structure of the samples were examined by scanning electron microscope (SEM, VEGA3, TESCAN) with energy dispersive spectrometer (EDS) and X-ray diffraction (XRD, X’Pert PRO MPD, PANalytical), respectively. The dynamic images of metal droplet impact on solid surfaces were captured by high-speed CCD camera (MotionbBLITZ Cube 1). 3. Results and discussion
⇑ Corresponding authors. E-mail addresses: [email protected] (L. Qi), [email protected] (N. Li). https://doi.org/10.1016/j.matlet.2018.01.127 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Fig. 1(b) and (c) show the initial dynamic behavior of a typical Al droplet impact on soluble gypsum and Ag-coated soluble
H. Yi et al. / Materials Letters 216 (2018) 232–235
233
Fig. 1. (a) Schematic diagram of metal droplet generation and deposition; initial dynamic behavior of Al droplet impact on (b) soluble gypsum and (c) Ag-coated soluble gypsum.
Fig. 2. (a) SEM image of a typically deposited droplet after removing the core; backscattered electron scanning micrograph of (b) the Al-Ag interface and (c) enlarged view of IMCs; elemental mapping of (d) Ag and (e) Al; (f) XRD of the cross-section of droplet-coating; (g) the microstructures of different precipitated phase within IMCs; (h) enlarged view of the selected section in (g).
gypsum, respectively. The temperature of the substrate is 300 K. It indicates that Al droplet rebounds after impacts on the soluble gypsum at time (t + 2) ms. When a droplet deposits on the Agcoated soluble gypsum, recoil and oscillation occur instead of rebounding from time (t + 2) to (t + 3) ms.
To understand the above observed phenomena, the interface behavior between Al droplet and Ag coating was studied. Fig. 2 (a) shows the side-view of a typically deposited Al droplet after removing the core. The brighter layer (50 lm) at the bottom of the droplet shows that the Ag coating is tightly bonded to the
234
H. Yi et al. / Materials Letters 216 (2018) 232–235
Fig. 3. Backscattered electron scanning micrograph of the Al-Ag interface with different substrate temperatures of (a) 300 K, (b) 473 K and (c) 673 K.
metal droplets. Fig. 2(b) shows the backscattered electron scanning micrograph of the droplet-coating interface in Fig. 2(a). Clearly visible in Fig. 2(b) is the IMCs layer formed between the Al droplet and Ag coating. The microstructure of this IMCs layer shows a very fine white dendritic network, which seems to be two-phase. Because the bright zone is more enriched with heavier atoms than the dark zone, the bright and dark zone are Ag-rich and Al-rich region, respectively. EDS and XRD analyses were adopted to further determine the composition, element distribution and migration within the interface. As shown in Fig. 2(c)–(e), EDS surface scan images of the AlAg interface demonstrate that Ag is precipitated as a dendritic network structure in Al matrix, which further verifies that strong reactions occurred. XRD of the cross-section of droplet-coating shown in Fig. 2(f) indicates that the interface is mainly composed of Ag, Al and Ag2Al intermetallics. Fig. 2(g)–(h) show the enlarged microstructures of IMCs, and it is visible that the interfacial region is mainly composed of three kinds of microstructure zones, which are the bright zones (dendritic structures), the grey zones (needlelike structures) and the dark zones, respectively. Table 1 shows the chemical compositions of these three zones which are represented by point A, B and C. According to the above EDS and XRD test results, combining with the Al-Ag binary diagram, it can be concluded that the bright zones and grey zones are mainly composed of Ag2Al intermetallics, the dark zones are mainly composed of aAl solid solutions. The formation of these IMCs indicates the good wetting and bonding between the Al droplet and Ag coating, which helps pin the droplet to the substrate and suppresses the rebound of Al droplets [7,8]. The formation of this dendritic network IMCs layer is due to the liquid-solid reaction between the Al and Ag, which is similar to the wetting reaction process between molten solder droplet and pad [7–9]. During the transient contact, Ag was quickly dissolved into Al droplet. Al-Ag intermetallics were precipitated on reaching the solubility limit during the solidification. Moreover, because the impact, spread and solidification process of Al droplet is nonisothermal, a vertical temperature gradient exists between the droplet and substrate [10]. The supply of solute atoms depends on the heat flow and the diffusion field, and the growth of crystal is faster in the direction of heat and mass transfer. Therefore, the IMCs layer tends to grow vertically. Because the cooling rate of Al droplet in
our experiment is 103–104 K/s, Ag2Al precipitation is restrained and presents such white network microstructure, which is often formed by rapid solidification. This result is similar to the previous study [11] conducted in a rapidly solidified condition (104 K/s). It can be assumed that the primary Ag2Al precipitates would grow much coarser with the decreasing of cooling rate. To achieve good bonding between neighboring droplets and obtain high precise metal traces, substrate temperature needs to be adjusted to different requirements. Therefore, it is essential to understand the effect of substrate temperature on the interface behavior between the Al droplet and Ag coating. To this end, we conducted three sets of experiments with the preheating substrate temperature of 300 K, 473 K and 673 K, respectively. Comparing Fig. 3(a)–(c), it is visible that the thicknesses of the IMCs layer increases (from 60 lm to 80 lm) with the increase of preheating temperature. Besides, the dendritic network becomes denser and thicker. The interface temperature and solidification time of Al droplet will both increase with the increase of substrate temperature, which means the dissolution rate and the diffusion time of Ag to Al droplet will increase. Therefore, the thicknesses of the IMCs layer increases and the IMCs precipitates become much denser and thicker.
4. Conclusion In summary, we propose a novel method of using Ag coating to prevent Al droplets bouncing off soluble cores. The mechanism of this method on inhibiting the rebound of Al droplets lies in the strong chemical interaction between liquid Al and Ag layer, which results in fast Ag dissolution in liquid Al and formation of a dendritic network of Al-Ag intermetallic compounds (IMCs) on their interface. XRD and EDS observation reveals the composition of the interfacial zone are mainly Ag2Al intermetallics and a-Al solid solutions. It is found that the thickness of the IMCs layer increases as the substrate temperature is raised. The temperature increase of the substrate will slow down the droplet cooling process, which allows more thorough chemical reaction. This investigation lays the foundation of using droplet-based 3D printing to manufacture tube parts with smooth cavity surface.
Acknowledgements Table 1 Results of chemical analysis of three points in Fig. 2(h). Element
Al (%)
Ag (%)
A B C
35.19 61.49 80.68
64.81 38.51 19.32
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 51675436), Key Research and Development Plan of Shaanxi Province (2017ZDXM-GY-110), Science and Technology Fund Project, Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201704).
H. Yi et al. / Materials Letters 216 (2018) 232–235
References [1] Z. Luo, X. Wang, L. Wang, D. Sun, Z. Li, Drop-on-demand electromagnetic printing of metallic droplets, Mater. Lett. 188 (2017) 184–187. [2] Y.P. Chao, L.H. Qi, Y. Xiao, J. Luo, J.M. Zhou, Manufacturing of micro thin-walled metal parts by micro-droplet deposition, J. Mater. Process. Technol. 212 (2) (2012) 484–491. [3] J. Luo, W. Wang, W. Xiong, H. Shen, L. Qi, Formation of uniform metal traces using alternate droplet printing, Int. J. Mach. Tools Manuf. 122 (Suppl. C) (2017) 47–54. [4] C. Garcia, R. Rumpf, H. Tsang, J. Barton, Effects of extreme surface roughness on 3D printed horn antenna, Electron. Lett. 49 (12) (2013) 734–736. [5] Y.J. Ko, J. Yoon, J. Lee, J.H. Han, Effects of Cu interlayer on the wettability of aluminum on carbon, J. Alloys Compd. 574 (2013) 526–531. [6] H. Yi, L. Qi, J. Luo, Y. Jiang, W. Deng, Pinhole formation from liquid metal microdroplets impact on solid surfaces, Appl. Phys. Lett. 108 (4) (2016) 041601.
235
[7] F.-Q. Li, C.-Q. Wang, Influence of interfacial reaction between molten SnAgCu solder droplet and Au/Ni/Cu pad on IMC evolution, Trans. Non-Ferrous Met. Soc. f China 16 (1) (2006) 18–22. [8] M. van Bellegema, H. Ankersmitb, R. van Langhc, W. Weid, Solder on silver: historical usage and the problem of fretting, Metal 04, in: Proceedings of the International Conference on Metals Conservation, 4–8 October 2004, National Museum of Australia, Canberra, Australia, , 2004, pp. 104–113, ill. [9] L. Zang, Z. Yuan, H. Zhao, X. Zhang, Wettability of molten Sn–Bi–Cu solder on Cu substrate, Mater. Lett. 63 (23) (2009) 2067–2069. [10] H. Li, P. Wang, L. Qi, H. Zuo, S. Zhong, X. Hou, 3D numerical simulation of successive deposition of uniform molten Al droplets on a moving substrate and experimental validation, Comput. Mater. Sci. 65 (2012) 291–301. [11] I. Yamauchi, T. Kajiwara, T. Mase, M. Saraoka, Formation of highly saturated Al–Ag precursor by rapid solidification for skeletal silver synthesis, J. Alloys Compd. 336 (1) (2002) 206–212.