Studies of Compounds for Superconductivity

A number of metal carbides, borides, nitrides, and a hydride, in the form of powders, have been examined for superconductivity down to 1.8^∘{}K using a magnetic method. The substances tested were TiC, VC, ZrC, TaC, WC, ZrB₂, NbB₂, TaB₂, WB, MoB, TiBₓ, ThB₂, LaN, CeN, NbN, and LaH2.45. All specimens were characterized by x-ray diffraction methods and, in most instances, by chemical analysis. Of these substances only NbN gave evidence for superconductivity. The results for NbN are in general agreement with the work of others. The results for TiC and VC are in agreement with those found by Meissner et al., using an electrical resistance method, while those for TaB₂, NbB₂, WC, and WB are in agreement with observations made by Matthias and Hulm using a magnetic method. On the other hand, ZrC, TaC, WC, and "zirconium boride" had previously been observed to exhibit superconductivity above 1.8^∘{}K by the electrical resistance method. The results for WC and MoB have recently been confirmed by Matthias and Hulm. The substances ThB₂, LaN, CeN, and LaH2.45 had not previously been examined for superconductivity.Particle size measurements were made on the carbide and boride powders. The particles were large enough that magnetic field penetration effects should have caused no difficulty, if a penetration depth of 1×{}10^-5 cm, observed for pure metal, is assumed to apply to the present compounds.The failure to observe superconductivity in the substances ZrC, TaC, WC, and MoB in the present study is discussed in terms of a postulated physical distribution of superconducting impurities of undetermined composition which are assumed to account for the superconductivity observed in these compounds by other investigators.