Doped spiral alumina nanowires

Nien-Fang Wu, Hung-Jen Chen, Yu-Lun Chueh, Su-Jien Lin, Li-Jen Chou, Wen-Kuang Hsu
2005 Chemical Communications  
Spiral alumina nanowires, doped with Cr and Si, are directly and reliably produced in bulk quantities via annealing of high entropy alloys. Various oxide nanowires have recently become important in the fabrication of nano-scale devices, for example Ga 2 O 3 , MgO, In 2 O 3 , SiO 2 , and ZnO nanowires and/or nanobelt structures have been successfully generated and tested. 1 Researchers continue to explore other oxides materials, e.g. the alumina nanowires, which exhibit good photo-luminescence
more » ... rformance. 1 The alumina nanowires are normally made via transition metal catalysis. 1-3 Interestingly, the direct heating of SiO 2 and Al mixture can also produce the alumina nanowires doped with Si. 4 When the concentration of Si-dopants increases the alumina nanowires become amorphous. 5 In this Communication, we report a simple route to bulk yields of doped spiral alumina nanowires, which has been accidentally discovered during the annealing of high entropy alloys. Studies of traditional alloys have recently focused on the multi-element system, so-called the high entropy alloys (HEAs). 6 HEAs, usually made from an equimolar mixture of more than five different elements, have been tested and have realized a higher degree of hardness, resistivity, thermal stability and anti-corrosion than the common binary and ternary systems. 6 However, atoms with differing radii are difficult to pack simultaneously within a unit cell, thus, the HEAs often exhibit a quasi-crystalline structure and their configurational entropy (DS conf 5 R ln n, R: gas constant, n: the number of elements) is 2.2R-2.7R, ca. three times greater than the normal alloy systems (,1.1R). Our experiments were initially focused on the enhancement of HEA crystallinity via the annealing process, so the atoms are allowed to rearrange via Frenkel and Schottky defects. Unexpectedly, the spiral nanowires, grown on the HEA surfaces, are present. Our routine experiments focus on the Ti-Si-Ni-Zr-Cr-Al-Fe system and experimental procedures are as follows. An equimolar mixture of granular Ti, Si, Ni, Zr, Cr, Al, and Fe (purity, 99.9%), corresponding to 5% y 35% at% for each element in bulk, was firstly melted by an induction furnace in a vacuum. The resultant bulk alloy was then ground into a powder form (dark grey, ca. 300-400 mesh) via a ball-milling system. The annealing of alloy powder (2 g, loaded in an alumina container) was carried out in a ceramic tube (55 mm in diameter and 1200 mm in length) placed in a horizontal electrical furnace. The alumina tube has been vacuated to 10 22 Torr, followed by the introduction of a 100 sccm Ar-H 2 flow (9 : 1, 100 sccm). The furnace temperature was first raised to 950 uC for 1 h, and further increased to 1350 uC for 1 h. The furnace was then allowed to cool to room temperature. We find that the annealing results in a sintered layer formation on the HEA surfaces and the removal of the sintered layer exposes brown-yellow powders, which are collected for electron microscopy and other related analyses. SEM images show nanowire coverage of the HEA matrix ( Fig. 1a and b ) and the close-up image reveals that the nanowires * Fig. 1 (a) A SEM image of As-made sample.(b) a close-up SEM image from the nanowire coverage. Insert: a close-up SEM image at the HEA surface.(c) SEM image of an individual nanowires, which clearly shows a spiral surface morphology.(d) HRTEM image of a spiral nanowire. Square: a highlighted HRTEM image from the Fig. 1d. (e) Selected electron diffraction of a nanowire.(f) an EDX profile of nanowire. COMMUNICATION | ChemComm 204 | Chem. Commun.,
doi:10.1039/b413700k pmid:15724186 fatcat:qzyhkgbienhf7jyd6cz6afuym4