Abstract
The dependence of the superconducting critical temperature Tc on composition has been investigated for A15-type Nb3Al compounds. The Tc and the composition of the samples have been determined by the specific heat measurement and exactly by the electron probe microanalysis (EPMA) measurements, respectively. The Tc is determined, on the other hand, by the magnetic flux transmission ratio through the sample. The Tc dependence on the Al composition determined by the specific heat measurement is the same as that by the magnetic flux transmission measurement. The determination of Tc by the magnetic flux transmission give the macroscopic Tc of the samples where the composition and its distribution are certified by the EPMA measurements.
The superconducting critical temperature Tc and the lattice parameter are investigated for Nb3(Al, Ge) compounds, whose Ge atoms go into the Al-site, in comparison with those of Nb3Al compounds. The prepared samples Nb1−x(Al, Ge)x have the composition between 0.22≤x≤0.26. The Tc is evaluated by the magnetic flux transmission ratio. The samples are characterized by the powder X-ray diffraction, and the lattice parameter is evaluated quantitatively by the Rietveld method. The Tc of Nb3(Al, Ge) compounds shows above 18.0 K. The Tc=18 K of Nb3(Al, Ge) compounds, which is equal to that of the stoichiometric composition of Nb3Al, is strongly related to the Nb-ordering for the Nb-site. The ordering for the (Al, Ge)-site and the balance of among Nb, Al and Ge atoms in addition to the decrease of the lattice parameter is important to increase the Tc above 18 K. The solute region of A15-type Nb3(Al, Ge) is larger than that of A15-type Nb3Al at the side of the (Al, Ge)-rich composition.
