Single-crystal superconductors of the general formula (LRE)-Ba-Cu-O (light rare earth, LRE = Nd, Sm, Eu and Gd) have considerable potential for engineering applications because of their ability to trap magnetic fields significantly higher than those achievable with permanent magnets. But the lack of a process by which these materials can be fabricated reliably and economically in the form of large single grains has severely hindered their development. We report a practical processing method for the fabrication in air of single-crystal (RE)BCO. The technique is economical and offers considerable freedom in terms of the processing parameters and reproducibility in growth of oriented single grains. The process is based primarily on the development of a new type of generic seed crystal that can effectively promote the epitaxial nucleation of any (RE)BCO system, and secondly on suppressing the formation of RE-Ba solid solution in a controlled manner within large grains processed in air.

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A variety of multiseeding techniques have been investigated over the past 20 yr in an attempt to enlarge bulk (RE)BCO superconducting samples fabricated by the top-seeded melt growth (TSMG) process for practical applications. Unfortunately, these studies have failed to establish whether technically useful values of trapped field can be achieved in multiseeded bulk samples. In this work specially designed, 0°-0°and 45°-45°bridge seeds of different lengths have been employed to produce improved alignment of the seeds during the TSMG process. The ability of these bridge-seeded samples to trap magnetic field, which is the key superconducting property for practical applications of bulk (RE)BCO, is compared for the samples seeded using 0°-0°and 45°-45°bridge seeds of different lengths. The grain boundaries produced by these bridge seeds are analyzed in detail, and the similarities and differences between the two bridge-seeding processes are discussed.

We fabricate nanosized superconducting YBa(2)Cu(3)O(7-δ) (Y-123) and nonsuperconducting Y(2)BaCuO(5) (Y-211) powders using carbon nanotubes as template. The mean particle size of Y-123 and Y-211 is 12 and 30 nm, respectively. The superconducting transition temperature of the Y-123 nanopowder is 90.9 K, similar to that of commercial, micrometer-scale powders fabricated by conventional processing. The elimination of carbon and the formation of a high purity superconducting phase both on the micro- and macroscale is confirmed by Raman spectroscopy and X-ray diffraction. We also demonstrate improvement in the superconducting properties of YBCO single grain bulk samples fabricated using the nanosize Y-211 powder, both in terms of trapped field and critical current density. The former reaches 553 mT at 77 K, with a ∼20% improvement compared to samples fabricated from commercial powders. Thus, our processing method is an effective source of pinning centers in single grain superconductors.

In-situ Al-MgAl 2 O 4 metal matrix composite was successfully manufactured using SiO 2 with the aid of ultrasonication. MgAl 2 O 4 particles and their clusters were identified at grain boundaries and interdendritic regions within the grain envelopes. The composite showed 2-5 fold of grain size reduction with respect to the reference alloy cast at similar conditions. The composite has shown 10% increase in yield stress and 15% increase in UTS while maintaining the ductility similar to reference alloy. CTE mismatch strengthening and grain boundary strengthening are suggested to be influencing in the improvement in mechanical properties.

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

Single domains up to 2.5 cm in diameter of YBa2Cu3O7−δ (YBCO) (Y-123 + 30 mol% Y-211 + 0.1 wt% Pt) doped with varying Zn content have been fabricated by melt processing. The Zn2+ ions substitute onto the in-plane Cu sites [Cu(2)] in the superconducting YBCO phase matrix, resulting in a well-known decrease in Tc. Despite this decrease, a significant increase in Jc is observed for very small amounts of Zn doping, even under relatively high applied magnetic fields. Here, we report the effect of varying the Zn content on the structural and physical properties of melt-processed YBCO samples for doping levels of Zn between 0 and 0.6%.

MgO and MgAl2O4 are believed to be effective heterogeneous nuclei for Al based alloys due to their small lattice misfits with Al. However, there is a strong evidence to suggest that liquid Al react with MgO and MgAl2O4 phases but the heterogeneous nucleation behavior of such phases is rarely discussed. In order to identify the nucleation mechanism of Al, under the interference of the chemical reaction, the heterogeneous nucleation process is systematically investigated through thermal analysis and high resolution transmission electron microscopy (HRTEM). The observed multi-nucleation interfaces (Al/MgO, Al/MgAl2O4 and Al/Al2O3) and scattered experimental undercooling data indicate an independent multi-phase nucleation process in these systems.