Numerical analysis of heating by a current pulse of a niobium nitride membrane in its longitudinal section

A numerical calculation of the evolution of the temperature distribution in the longitudinal section of a niobium nitride membrane when it is heated by an electric current pulse is performed. Mathematical modeling was carried out on the basis of a two-dimensional initial-boundary value problem for an inhomogeneous heat equation. In the initial boundary value problem, it was taken into account that current and potential contacts to the membrane serve simultaneously as contacts for heat removal. The case was considered for the third from the left and the first from the right initial-boundary value problem. Analysis of the numerical solution showed that effective heat removal from the membrane can be provided by current-carrying and potential clamping contacts made, for example, of beryllium bronze. This makes it possible to study the current-voltage characteristics of superconducting membranes near the critical temperature of the transition to the superconducting state by currents close to the critical density without significant heating.

Water-based lubrication of niobium nitride

Water-based lubrication has attracted wide attention as an oil-free lubrication method owing to its greener and cleaner lubrication means. However, due to operating in the water environment, most moving parts would inevitably suffer from abrasion, rusting, and aging problems. Developing a novel solid-water composite system with ultra-low friction and wear will open new possibilities for innovative lubrication material research and development. Here, we first revealed the water-based lubrication behavior of a high-hardness niobium nitride coating (NbN). In a three-phase contact environment (water, air, and NbN), oxidation and hydrolytic reactions of NbN result in the formation of “colloidal solutions”, containing Nb2O5 colloidal particles between the tribo-pairs. Utilizing the double electric layer repulsion and weak shear action of the “colloidal solution”, NbN achieves ultra-low friction and wear; the corresponding values are as low as 0.058 and 1.79 × 10−10 mm3·N−1·m−1, respectively. In addition, other VB transition metal nitrides (VB TMNs) exhibit the same low friction feature as NbN in the three-phase contact environment; the friction coefficients are even lower than those in an oil-based environment. The water-based lubrication of VB TMNs provides a new reliable scheme for optimizing solid-water composite lubrication systems without additives and is expected to be applied in environments with high humidity or insufficient water coverage.

MEASURING AMPLITUDE AND FREQUENCY CHARACTERISTICS OF KINETIC INDUCTANCE IN SUPERCONDUCTING FILMS

This research was performed using thin superconducting strips in the shape of tightly packed meanders made of niobium nitride (NbN) ultrathin superconducting strips about 100 nm wide, 5 nm thick and up to 100 µm long. The structures revealed the effect of high kinetic inductance at temperatures lower than the critical temperature. Consideration was given to the temperature dependence of LC-circuit resonant frequency where a highlyinductive superconducting meander was used as a source of L inductance and а chip capacitor as a source of C capacitance. Experimental data point to the fact that kinetic inductance depends on temperature, since there was a shift in the resonant frequency at temperatures lower than the critical one relative to the normal state. This is indicative to the fact that in such systems impedance is changed due to growing kinetic inductance during the transition to the superconducting state. This effect is of relevance both in applied and basic research. When using different superconducting detectors, kinetic inductance can exert a distinct effect on their response rate. In basic research developments, where the on-chip implementation of charge (current) stabilization is required, high frequency impedance of connecting circuits is very useful.