Alloy composition dependence of formation of porous Ni prepared by rapid solidification and chemical dealloying
Introduction
It has been known that nanoporous metals can be self-assembled by dealloying or selective dissolution of a less noble component from a binary alloy [1], [2], [3]. Besides touching on fundamental issues of alloy thermodynamics and corrosion science [4], [5] and providing a means for obtaining high surface area granular metal catalysts such as Raney nickel [6], the process offers the potential to produce monolithic metal bodies with a nanoscale pore structure. Raney Ni with a sponge-like structure is a well-known catalyst used in several hydrogenation processes [7].
Rapid solidification (RS) was first introduced by Duwez et al. [8], and melt spinning is the most commonly used technique to achieve rapid solidification and to produce long and continuous ribbons [9]. Rapid solidification processing involves exceptionally high cooling rates (104 to 108 K s−1) during solidification from the molten state [10]. The levels of undercooling achievable at such high cooling rates lead to significant and often potentially beneficial modifications to rapidly solidified microstructures compared with those produced under conventional conditions. These include the refinement of the as-solidified microstructure, the refinement of the scale of segregation, the increase in the equilibrium solid solubilities of solute elements and the formation of novel metastable crystalline and amorphous phases [11]. Therefore, rapid solidification has had a substantial impact on our fundamental understanding of materials synthesis by solidification, as well as on our ability to develop materials for technological applications.
Raney Ni catalyst precursors, with the Al–50 wt.% Ni alloy as a representative, are conventionally solidified naturally. Al, in the form of intermetallic compounds or eutectic, is leached by alkali, and then porous catalysts with surface areas as high as ∼100 m2 g−1 are obtained [12]. Incorporation of the rapid solidification technique to the preparation of the Raney-type alloys is expected to retain the merits of the rapidly quenched alloys, while circumventing the drawback of low surface area [13]. Recently, attention has been paid to novel skeletal Ni catalysts (designated as RQ Ni) prepared by alkali dealloying of rapidly quenched Al–Ni alloys [13], [14], [15], [16]. Lei et al. [14] reported that melt quenching and pretreatment with hydrogen can effectively enhance the catalytic activity of Raney Ni. The RQ Ni catalyst exhibited higher selectivity in hydrogenation of 2-ethylanthraquinone to “active quinones” than that of the Raney Ni catalyst prepared from a conventional alloy [15]. More notably, the composition, texture, structure, and consequently activity and selectivity of the RQ Ni catalyst deviate markedly from the conventional Raney Ni [13], [16].
To date, however, less attention has been paid to the influence of alloy composition on the microstructure of rapidly solidified Al–Ni alloys and on the formation of porous Ni catalysts by dealloying. The present work aims to investigate the alloy composition dependence of formation of porous Ni prepared by rapid solidification and chemical dealloying.
Section snippets
Experimental
In this work, the Al–Ni alloys with nominal compositions of Al–20, 25 and 31.5 at.% Ni were prepared from pure Al (99.9 wt.%) and pure Ni (99.9 wt.%). The charges were melted in a quartz crucible by high frequency induction heating and were cast into ingots in an iron chill mould. The ingots obtained are rod-like with a diameter of 10 mm. Using a single roller melt spinning apparatus, the pre-alloyed Al–Ni ingots were remelted in a quartz tube by high-frequency induction heating and then rapidly
Results and discussion
Fig. 1 shows the microstructures of three Al–Ni alloy ingots solidified under conventional conditions. It is obvious that alloy composition has a significant influence on the microstructures of the Al–Ni alloy ingots. For the Al–20 at.% Ni alloy, columnar dendrites can be observed in the microstructure, as shown in Fig. 1a. In local areas, the trunks of the dendrites are parallel to each other. Fig. 1b clearly shows the microstructure of the alloy in detail at a higher magnification. It can be
Conclusions
Rapid solidification and alloy composition have a significant influence on the phase constitution and microstructure of Al–Ni alloys. The Al–Ni alloys solidified under conventional conditions are composed of α-Al, NiAl3 and Ni2Al3. The amount of the α-Al phase decreases and that of the Ni2Al3 phase increases with increasing Ni contents from 20 to 31.5 at.%. The melt spun Al–20 at.% Ni alloy consists of α-Al, NiAl3 and Ni2Al3, while the melt spun Al–25 and 31.5 at.% Ni alloys comprise NiAl3 and Ni2
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 50701028) and Excellent Middle-age and Young Scientists Research Award Foundation of Shandong Province (No. 2007BS04024). The SEM experiments from Ruhr University Bochum (Bochum, Germany) are also acknowledged. Z.H. Zhang gives his thanks for the financial support from DFG-SFB 459 for guest visit. We thank Dr. Wenhua Zhang for his help of N2 adsorption/desorption experiments.
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Present address: Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuang Road, Haidian District, Beijing 100083, PR China.