Preparation and superconducting properties of ultrafine powders and sintered compacts of NbC and NbN

Abstract

We have produced ultrafine powders of refractory metals with the nominal compositions NbC and NbN by conversion of a Nb-bearing spray-dried powder in hydrogen-methane and ammonia atmospheres at temperatures as low as 800 and 700 °C, respectively. The resulting powders consist of 5–10-μm balls subdivided into 10–100-nm microcrystallites. We have investigated some of the factors which affect the microcrystallite size and the superconducting transition temperatures of the powders and have observed the changes of morphology which occur during two stages of sintering. In the more heavily sintered compacts of NbN, the finer scale of division into microcrystallites was lost, leaving a compact of approximately 70% density with a characteristic scale for crystallites and voids of 1–5 μm. At 4.2 K the best NbN sample had Hc2≳120 kG, and Jc≳103 A/cm2 at all fields up to 110 kG. These Jc values are generally twice those of the best previously reported sintered compacts of niobium carbonitride materials. It is possible that similar techniques can be applied to the production of other refractory metals such as tungsten carbide.