The Residual Resistance Ratio “RRR” is the ratio of resistance at 300K (room temperature) to the resistance at 10K. Lower values of RRR indicate a greater concentration of imperfections. The resistance drops abruptly to zero if materials enter the superconducting state at a critical temperature “Tc”. Niobium (Nb) possesses a rather high temperature of superconductive transition Tc = 9.2K.
The value of RRR is an indication of the purity and the low-temperature thermal conductivity of the niobium, and is often used as a material specification in commerce. Pure niobium in its superconducting state is used for high-quality-factor resonant cavities for particle accelerators, synchrotron light sources, and neutron sources.
The interstitial impurities oxygen (O), nitrogen (N), carbon (C) and hydrogen (H) have major influence on the thermal conductivity of niobium. By rising of the thermal conductivity the thermal breakdown happens at higher field level. The RRR gives information about the total impurity content and allows a rough estimation of the thermal conductivity. Also important is the non-homogeneous distribution of impurities (O, N, C and H) in Nb.
The superconducting properties are influenced considerably by the purity of the material and the production technique. The interstitial impurities C, O, N and H damages most of the cavity performances. The total impurities content can be determined by measuring the specific RRR. The commercial niobium for cavities is produced by several melting cycles in the electron-beam (EB) furnace. During the last decade, the RRR of niobium ingots (produced in a weight up to one metric ton), are improved to 300 from 30 through better melting practices. Cavities made from these niobium ingots and sheets normally achieve a range of accelerating field Eacc=13-17 MV/m.
Higher RRR is desirable for improving the cavities performance. Currently the solid state gettering is used to further increase RRR. The quality control of niobium for superconducting cavities production pins down to three factors: purity (H<5ppm, C, O & N <10ppm, normally) , workability and surface quality (Grain size ASTM 6-8). Typically niobium is nearly 100% recristallized and absent of anisotropy.