Ceramics International 30 (2004) 1575–1579 Origin of self-aligned nano-domains in MgB 2 S. Li a,∗ , T.H. Yip a , C.Q. Sun b , S. Widjaja c , M.H. Liang a a School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore b School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore c Corning Incorporated, New York, NY, USA Received 26 November 2003; received in revised form 9 December 2003; accepted 22 December 2003 Available online 6 May 2004 Abstract Random arrangement of B atoms in parent amorphous phase leads to a number of atomic defects, such as dislocations, formed in the reaction product of Mg and B. During the crystallization, dislocation walls form from random arrays of dislocations. Stress at the end of dislocation wall segments attract the surrounding edge dislocations, which are then incorporated and result in wall growth, forming small angle boundaries to connect well-ordered nano-domains. To minimize the energy of the system, the dislocations migrate to interdomain boundaries surrounding the nano-domains. These dislocation rearrangements result in rotation of adjacent nano-domains form a contiguous crystal. By continuing this subgrain rotation process on neighboring nano-domains, large (2 1 1) nano-domains can be aligned as observed by high-resolution transmission electron microscopy (HRTEM). It is demonstrated that the (2 1 1) plane may have the minimum surface energy in MgB 2 and the (2 1 1) zone is the favored orientation for crystal growth. © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Nano-domains; Crystal growth 1. Introduction The discovery of superconductivity at 39 K in MgB 2 has initiated enormous scientific interest in order to understand and develop this material to better exploit its high intrinsic performance for magnetic and electronic applications [1,2]. The strongly linked current flow measured from polycrys- talline MgB 2 shows that this superconductor class is not compromised by weak-link problem and the supercurrent density in MgB 2 is controlled predominantly by flux pinning rather than by the grain boundary connectivity [3]. In the bulk material, the critical current density (J c ) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can “pin” the quantized magnetic vortices that permeate the material. It suggested that the lack of natural defects is responsible for the rapid decline of critical current density (J c ) with increasing field strength [4]. In order to improve the high field performance of MgB 2 , a number of techniques, such as proton irradiation, addition of hetero-nanoparticles, fabrication of fine-grained ∗ Corresponding author. Fax: +65-6790-4931. E-mail address: assxli@ntu.edu.sg (S. Li). thin films, and oxygen substitution in boron, etc., have been used to induce crystal defects by atomic displacement, inclusions, grain boundaries, and lattice distortion, respec- tively [5–8]. Recently, a significant flux pinning enhance- ment in a mixture of MgB 2 and SiC has been reported and an effective vortex pinning source—semi-crystalline defect wells in self-aligned nano-structured MgB 2 was discovered in this material [9,10]. However, the mechanism of the self-aligned nano-structured MgB 2 is still not clear. In this work, we report an investigation in the origin of self-aligned nano-domians in MgB 2 . 2. Experimental procedure MgB 2 samples were prepared by pressing a mixture of Mg 99% purity and amorphous B 99% purity powders in the stoichiometeric ratio of Mg:B = 1:2 with addition of 10 wt.% SiC powder into the pellets. The pellets of di- mensions 10 mm × 2 mm were sealed in a Fe tube and subsequently sintered at 700–900 ◦ C for 1 h in flowing high purity Ar atmosphere, followed by furnace cooling to produce MgB 2 (80%) with impurities of MgO (13%) and Mg 2 Si (7%) as determined by quantitative X-ray diffraction 0272-8842/$30.00 © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2003.12.196