Speaker
Description
APC Nb3Sn wires are made using an internal oxidation process to create ZrO2 or HfO2 nanoparticles during diffusional growth of Nb3Sn. These particles introduce a second pinning mechanism, as well as refine the grain size, strengthening the grain-boundary pinning above that in non-oxidized Nb3Sn. The two mechanisms taken together increase the flux pinning force Fp, and also shift the peak pinning force to higher field, increasing the Jc at high fields while suppressing the magnetization at low fields. The nanoparticles form at the interface between the Nb3Sn and the Nb alloy, and these particles get trapped as the interface progresses. The particles then grow over the remaining heat treatment time via diffusion of the solutes through the Nb3Sn. The size and size distribution of the nanoparticles has been characterized through image analysis of transmission electron micrographs. A phase field model has been created which uses known thermodynamic and kinetic properties of the component materials to model the nucleation and growth process. Combining the model with microscopy allows evaluation of the effect of heat treatment temperature and explanation of the different nanoparticle sizes seen when the oxidizing element is Hf vs. Zr. This model will aid in the optimization of strand design and heat treatment for high field magnets.
