Bi_xSb_1-x alloys are the best n-type materials for thermoelectric and magneto-thermoelectric cooling at temperatures below 200 K [1]. It was shown that Bi_0.9Sb_0.1 reveals unique properties and is being investigated as a novel quantum material [2]. Nanostructured Bi_xSb_1-x materials draw attention because they have different properties due to quantum effects [3]. Many different methods can be employed to prepare nanomaterials. One promising method that provides control over nanocrystal formation is low-temperature annealing of amorphous metallic alloys [4,5]. Therefore, we used this method for preparation of nanostructured Bi_xSb_1-x alloys, which was not applied for these alloys earlier.

We used the spinning method for obtaining of amorphous Bi_xSb_1-x alloys. In this experiment, an alloy with composition Bi_0.9Sb_0.1 was prepared by melting and mixing components in a quartz crucible of a Czochralski apparatus at a temperature near 700 K for 2 h at 0.4 atm in high purity Ar. Bi of 99.9999% purity and Sb of 99.9999% purity were used as source materials. After crystallization, a Bi_0.9Sb_0.1 alloy polycrystal with weight 100 g was obtained in pieces smaller than 7 mm. These pieces were placed into a quartz ampoule with 8–20 mm inner diameter and 115 mm length, which had an external resistance heater. High purity Ar at a gauge pressure of 0.1 atm was passed through the ampoule to prevent alloy oxidization. The melt temperature during spinning was controlled from 750 K to 920 K. After the alloy melted, and a steady state established, the melt was poured out onto a cold copper plate with 300 mm diameter, rotating at 1100 rpm. The amorphous alloy formed as a film with 20 μm thickness, 2–5 mm width, and 10–25 mm length. The alloy samples were annealed at temperatures above 80 °C for 1 h in a special cylindrical furnace with less than 1 K/cm temperature gradient in the axial and radial directions. A cross section of the films was prepared by breaking the samples in liquid nitrogen. The film microstructure was studied using a scanning electron microscope “Quanta 600 H”.

 The Bi_0.9Sb_0.1 films had many microcrystals with sizes 1–5 μm after spinning process with the melt temperature less than 800 K. Amorphous films were cooled from 920 K melt temperature by spinning. However, amorphous films had a few microcracks perpendicular to their surface under these conditions. The microcracks could appear at the moment of breaking in liquid nitrogen. Nanocrystals were detected in amorphous Bi_0.9Sb_0.1 films only after annealing at 150 °C. A small amount of individual nanocrystals with dimensions from 20 to 80 nm were sparsely located in a central part of the films. Vice versa, a large amount of nanocrystals with dimensions 40–100 nm formed densely at the film surface. From this study, we have shown the potential of using low temperature annealing of amorphous Bi_xSb_1-x alloys for formation of nanocrystals.

References

[1] G.N. Kozhemyakin, J. Crystal Growth, 257, 237 (2003).

[2] D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava, M. Z. Hasan, Nature, 452, 970 (2008).

[3] O. Rabin, Y.M. Lin, M.S. Dresselhaus, Appl. Phys. Lett. 79, 81 (2001).

[4] K. Lu, J.T. Wang, W.D. Wei, Scrip. Metallurgica Mater., 25, 619 (1991).

[5] T. Cheng, Nanostruct. Mater., 1, 19 (1992). 

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