Grain boundary pinning phase

Mishra emphasizes the fact that the amorphous phase found at the grain boundary plays a very important role in the pinning. This can be inferred, e.g., from the low coercivity found in materials with relatively low Nd-content, for which one would expect no or only a small quantity of the intergranular amorphous phase to be present. 

Pinning in A15 superconductors occurs at grain boundaries. This was demonstrated qualitatively by Pulver s measurements on VsSi single crystals [% where the A15 critical current densities were lower than those of polycrystalline material by a factor of 10. It is assumed that second-phase material precipitated at the grain boundaries was responsible for pinning at these sites. 

It is assumed that a polycrystalline solid contains randomly dispersed fine second-phase particles, also known as fine inclusions or precipitates, which are insoluble and immobile in the system. When a grain boundary encounters a fine particle during its movement driven due to its curvature, it will be pinned by the particle, while the rest portion of the grain boundary continues to move. The grain boundary will break away only as the rest portion has moved for a sufiftciently far distance. Therefore, if the grain boundary encounters a sufiicient number of particles, it could be entirely pinned, i.e., the motion of the grain boundary is completely inhibited. This scenario can be described by using two models. 

The critical driving force may also be increased by finely dispersed second-phase particles. If dislocations at a grain boundary are pinned by second-phase particles, excess energy is needed for atom movement from the boundary. In this case the atom flux J from the grain boundary can be... 

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