Cu Activated Sintering Mechanism Of Mgb₂ At Low Temperature And Its Superconductivity

The synthesis of MgB₂ with high critical current density is the highlight of superconducting field. Low-temperature sintering developed in recent years is a potentially effective method for obtaining improved J_c. However, the reaction between magnesium and boron at low temperature takes a very long time to form the complete MgB₂ phase as the result of the low diffusion rate of atoms in the solid-state. Hence, to search a convenient and effective way in improving the sintering efficiency together with increasing J_c is very urgent. Based on these backgrounds, the activated sintering mechanism as well as doping mechanism in MgB₂ prepared at low temperature was explored in present work. Accordingly, the techniques that both autoxidation treatment of short-time milled original powders and effective C doping were introduced into the Cu-activated sintering process of MgB₂ at low temperature. As a result, excellent J_c was obtained in the synthesized samples, which basically meet the practical need of MgB₂ superconductors.Combined with in-situ X-ray diffraction and thermal analysis, the sintering process of Mg-B system from low temperature to high temperature was investigated. Then the kinetic mechanism of MgB₂ phase formation at low temperature was clarified according to the isothermal kinetics calculation. The result indicated that the reaction between Mg and B during the low-temperature sintering is controlled by varied mechanisms. At initial stage, the activated Mg and B atoms are limited and the reaction rate is mainly determined by the phase boundary reaction mechanism. As the reaction prolonging and the synthesized MgB₂ layer increasing, the diffusion-limited mechanism gradually becomes dominant. The corresponding activation energy is also decreased firstly and then increased again.The influence of different metals (Cu, Sn and Ag) on the sintering efficiency of MgB₂ at low temperature was systemically studied. Accordingly, the metal activated sintering mechanism of MgB₂ at low temperature was clarified and also the criteria for selecting appropriate activator were proposed. It was found that during the sintering process of Mg-Cu-B system, Cu firstly reacted with Mg forming Mg-Cu liquid, which could provide high transport for the diffusion of Mg atoms into B and thus accelerate the MgB₂ phase formation. The effect of Cu addition on the sintering process was proved to follow the activated sintering mechanism. Besides, Ag and Sn also formed local liquid at low temperature and thus improved the sintering efficiency of MgB₂ obviously. However, the enhancement in the sintering efficiency resulting from Ag and Sn addition is not as significant as Cu addition. This phenomenon can be explained well on the basis of activated sintering mechanism. The minor Cu addition is finally determined to be the most suitable activator according to above results.The effect of Cu activator on the microstructure and superconductive properties of MgB₂ sintered at both low temperature and high temperature were investigated in detail. Results showed that the Cu activator could reduce the amount of MgO impurity in the prepared MgB₂ samples dramatically. During the low-temperature sintering process, the formation of local Mg-Cu liquid could wrap the Mg particles and thus protect them from the oxidation caused by the oxide impurity in the starting powders and the gaseous O; During the high-temperature sintering process, the Cu addition could decrease the vapor pressure of Mg liquid and thus reduce the volatilization of Mg liquid, which could depress the reaction between gaseous Mg and O₂ impurity in the protective Ar gas forming MgO impurity. It is also found that the lamellar MgB₂ grains could be synthesized by Cu activated sintering at low temperature. It is proposed that the Mg-Cu liquid environment is the critical condition, and the lamellar MgB₂ grains was formed at the liquid-solid interface between Mg-Cu liquid and B following the two dimensional nucleation and growth mechanism. According to the analysis on the temperature dependence of resistivity, the lamellar MgB₂ grains possess much better grain connectivity than the typical morphology of MgB₂ grains synthesized by the traditional solid-state sintering.In order to further optimize J_c in the MgB₂ synthesized by Cu activated sintering at low temperature, the C doping mechanism in the carbon-based chemicals added MgB₂ at low temperature was discussed and then the criteria for judging validity of C doping during the Cu activated sintering process of MgB₂ were proposed. The results indicated that following factors should be considered when discussing the validity of C doping at low temperature: (â…°) carbon-based chemicals ought to release C before or simultaneously with the formation of the MgB₂ phase. (â…±) The released C should be active enough to guarantee that it could enter the MgB₂ crystal lattice and replace B atoms. Accordingly, the validity of C doping resulting from nano SiC and glucose (C₆H₁₂O₆) at low temperature were verified: Nano SiC could release related high active C via reaction between Mg and SiC at low temperature and then introduce C doping effectively while C6H12O6 could not release sufficiently high active C through its decomposition and failed to bring about C doping. The carbon-based chemical which will introduce C doping effectively into MgB₂ sample prepared by Cu activated sintering at low temperature must meet two criteria as follows: (â…°) It should not react with Cu during the low temperature sintering process, and therefore not destroy its effect of activated sintering. (â…±) On the other hand, Cu should improve or at least not inhibit C doping caused by this carbon-based chemical addition. In view of above, nano SiC was selected as appropriate dopant. Consequently, the corresponding SiC doped MgB₂ samples activated sintered at low temperature exhibit excellent J_c.Besides C doping, ball milling treatment of original powders was also applied to further increase J_c in MgB₂ samples prepared by Cu activated sintering at low temperature. It is observed that the slight oxidation of short-time milled original powders led to the increase of J_c at high field. Based on the investigation of this phenomenon, the autoxidation technique of milled original powders was developed and then introduced into the Cu activated sintering process of MgB₂ at low temperature. On one hand, autoxidation treatment of milled original powders could improve Cu activated sintering and result in higher density. Moreover, some lamellar MgB₂ grains were observed in the Cu-activated sintered sample after autoxidation treatment of milled original powders. Both of factors could improve the connectivity between MgB₂ grains; On the other hand, autoxidation treatment of milled original powders could generate lots of homogenously distributed MgO nanoimpurities as well as refine MgB₂ grains in the sintered sample, which both significantly increase the number of flux pinning centers. Hence, the J_c in the MgB₂ sample prepared by combining autoxidation treatment of milled original powders with Cu activated sintering at low temperature was dramatically improved.