| Nb3Al superconductor has excellent strain tolerance and high critical current density, there are different synthesis processes about Nb3Al that has been studied, and a commercial process was being researched. Because of its high sythsis temperature and some deviations from stoichiometry at low temperature, many difficulties exist in its synthesis process. Powder metallurgy and mechanical alloying have been used to sythsis Nb3Al at low temperature, and we studied the phase formation mechanism and superconducting properties, especially researched the reversible phase transition between the supersaturated solid solution Nb(Al)ss and Al 5-Nb3Al in mechanical alloying process.We proposed two-step solid-state diffusion process to sythsis pure Nb3Al in powder metallurgy process. The first step can get low-temperature stable phase Nb2Al, NbAl3 and unreacted Nb by the Nb/Al composite powder at 900℃, in the second step Nb2Al and NbAl3 reacted with Nb at a high temperature of 1100℃ and finally Nb3Al was formed. But the lattice parameter of Nb3Al was too large, and its superconducting critical transition temperature (Tc) was lower than the stoichiometric Nb3Al. These revealed that Nb3Al belong Nb-rich state. So the Nb/Al initial ratio, diffusion temperature and the partial pressure of the first step was optimized to increase the content of Al in Nb3AL But the study found that the high Al content led to generate Nb2Al, and too much Nb2Al would hinder the formation of Nb3Al, thus increasing the initial ratio of Al and the partial pressure was not effective to get the stoichio metric Nb3AL The improvement of diffusion temperature was advantageous to its superconducting properties, and the maximum Tc of Nb3Al was 13K in this process.In mechanical alloying process, Nb and Al (the molar ratio of Al≥25at%) formed the supersaturated solid solution Nb(Al)ss by high energy ball milling, Nb3Al was obtained by additional annealing at 900℃, but there was still a small amount of Nb2AL The maximum Tc was 14.9K, witch still below the Tc of the stoichio metric Nb3AL Because Nb3Al, Nb2Al and NbAl3 are brittle, they can be crushed easily by high energy ball milling, reducing the powder particles, promote Nb2Al to be reacted. It was indicated that Nb3Al can reform Nb(Al)ss, Nb3Al was formed with additional annealing treatment, but its superconducting transition temperature had a sharp decrease, it may be due to stacked the increased stacking faults and destroyed long-range order in Nb3AL It indicated that there was a reversible phase transition between Nb(Al)ss and Nb3Al by mechanical alloying.High temperature is an important factor for improving stoichiometry and superconducting properties of Nb3AL The samples were synthesized at 1500℃ by mechanically alloying and powder metallurgy respectively. The former generated Nb2Al and a little Nb3Al, this may be related to the stability of Nb3Al, Nb2Al, NbAl3 at high temperature, but its superconducting property was poor. Significantly, Tc of the later was 16.8K.In order to decrease the sythsis temperature of Nb3Al and improve stoichiometry by mechanically alloyed sample, Ag, Cu, and B was doped in the Al site of Nb3AL Experiments showed that Nb3Al doped by Ag and Cu respectively resulted in decreased Tc, on the one hand because stoichiometry was not improved after doping, on the other hand because the dopant amount of Ag and Cu was limited, the contact between Ag/Cu and the superconducting phase at grain can produce proximity effect, leading to decreases of Tc. When a small amount of B was doped in Nb3Al, its Tc was increased slightly, and its Tc decreased drastically with increased dopant... |
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