Fabrication And Properties Of Glycine-doped Polycrystalline MgB₂ Bulks

MgB₂ with the superconductivity below 39 K has been applied to superconducting transformers, windings, and particle accelerators after the development for more than ten years. Current carrying capability is a metric deemed in general to evaluate the performance of the MgB₂ products; however, the development of the undoped MgB₂ remains limited by the sensitivity of critical current density（Jc） to increasing applied magnetic field. By yielding an enhancement of critical current density especially at high field, chemical doping enables MgB₂ to meet higher demand in practical applications, and within the dopants? family, carbon–containing organic compounds definitely attract the most attention. The original motivation for carbon doping was based on its potential to donate its additional valence electron（compared to boron） to the σ conduction band of MgB₂, gaining the carrier density, but so far, a dopant with elements more than C, H, and O has been barely involved. In view of this, glycine（C2H5NO2, abbr. Gly）, an amino acid with amidogen and carboxyl groups, is selected to be the object dopant for polycrystalline MgB₂ bulks in this study. Techniques including ball milling, solution coating, low–temperature sintering, and two–step sintering were implemented after the kinetics computation of the Mg–B solid–solid reaction, to make C–substitution sites homogeneous, and to introduce strong pinning effects other than the lattice distortion. By means of microstructure observation and phase identification on the electron microscopes and the X–ray diffractometer, we focused on the size and location of the impurities, especially MgO in the MgB₂ samples, and their influence on the Jc performance, which is measured by the physical properties measurement system. All these efforts are aimed at building a relationship between the microstructure and the superconducting properties of bulk MgB₂, and the research contents are listed below as well as the obtained conclusions.The optimal doping concentration of glycine ought to be figured out to balance the pinning effects and the grain connectivity in the MgB₂ samples, and 2–8 wt.% glycine were added into the MgB₂ for starters. More than releasing free C atoms to replace the B atoms in MgB₂ lattice, this amino acid, glycine, prevents the MgO from over growth by producing CO2 before the formation of MgB₂. Plus other merits like grain refinement and degrading the crystallinity of MgB₂, the critical current density at 20 K is enhanced over the entire field. From the viewpoint of kinetics, first order reaction, rather than second–order Avrami–Erofeev mechanism in the undoped system, is the best interpretation for the Mg–B solid–solid reaction in the Gly–doped MgB₂ system, and it means the growth is controlled by interface. Specifically, the pre–generated MgO happened to be the nucleation site for MgB₂, and the reaction rate depends merely on the concentration of the reactants. Glycine is therefore a promising dopant for MgB₂, and the optimal doping concentration is 3 wt.%.The ball milling technique was used to treat the original B powder for the fabrication of MgB₂. Mg2 Si impurities, which functioned as effective pinning centers, inhibited grain growth, and thereby increased the grain boundary area density, were introduced into the Gly–doped MgB₂ system by the ball milling process. The ball milling technique improved the limited C–substitution level in Gly–doped MgB₂ samples and upgraded the Jc under high field as well as the irreversible magnetic field of the samples experienced appropriate milling time, but samples underwent long milling time contained large quantities of MgO and Mg2 Si impurities in return, suffering a decrease in the critical current density.To make the gaseous products homogenous, glycine solution was used to handle the original Mg and B powders, so that the reaction between Mg and CO2 could proceed as completely as possible, and as what we had expected, MgB₂ samples with excellent critical current density were successfully prepared by solution coating technique. More than the advantages brought by glycine doping, the coating technique further encouraged the C substitution sites and nano–sized MgO particles to distribute homogeneously in the sample, which realized in excellent grain connectivity and strong pinning effects. This sort of combination, on the contrary to the crossover effect that raising the high–field Jc is always accompanied by a Jc decrease at low field, just meets the suggestion of avoiding this phenomenon by researchers, and enhanced the Jc over the entire field.Cu addition solved the problem that Mg–B–glycine system primarily generated MgO phase rather than MgB₂ during the low–temperature sintering process. The MgB₂ sample containing Cu and glycine sintered at 600 °C for 15 h showed the highest performance of critical current density, which is basically due to the pinning effects of MgCu2（¹0 nm） and MgO（²0 nm）. By illustrating the whole sintering process and the precipitation behavior, it is believed that MgCu2, generated from the Mg–Cu（l） formed at 485 °C and precipitated along the steps of MgB₂ grains, had included in the MgB₂ grains before it grew into undesirable bulks, together with the MgO formed from the product of glycine before Mg–B reaction.The presence of the small–sized MgB₄ impurities in bulk MgB₂ prepared by two–step sintering has been observed to function as effective pinning centers and generated significant lattice distortion in the MgB₂ lattice to yield improved Jc performance over the entire applied field. The lattice distortion mainly derived from the coherent or semi–coherent relation between MgB₄ and the matrix, but the relation was destroyed to become incoherent when the second–step sintering temperature reaches 1000 oC, which was relative to the decrease of Jc. In view of this, the sequence of sintering at 750 oC, and then 900 oC, was applied to the Gly–doped MgB₂ sample. Although MgB₄ particles failed to become effective pinning centers, they improved the upper critical field as impurity scattering centers, and the obtained upper critical field values for undoped and Gly–doped samples are 11.9 T, 14.2 T, and 14.3 T, respectively.Combined with the effects of histidine, another dopant with a higher carbon–containing content and a lower melting point, on the critical current density of MgB₂, we summed up the requirements for an amino acid to be an effective dopant. The selected one is expected to be capable of low–temperature decomposition for CO2, while the decomposition temperature is as close as the onset temperature of the Mg–B solid–solid reaction. A rich content of carbon in the amino acid is conductive to a higher C–doping level.