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A novel technique for modifying RE2BaCuO5 (211, RE = rare earth elements such as Nd, Sm, Gd, Y etc) particles and tuning their size, distribution and content in REBa2Cu3O7-δ bulk superconductors was demonstrated. Varied ultrasonic power was employed to produce fine-sized, individual, sharp-edged 211 phase particles along with mechanically damaged surfaces. The size and content of the 211 phase of infiltration growth-processed YBa2Cu3O7-δ bulk samples were gradually decreased. The effective grain refinement enhanced the critical current density by a factor of two. A maximum trapped field of 0.48 T at 77 K was obtained for a sample with 20 mm diameter by treating the 211 phase particles for 60 min with 300 W energy. [ABSTRACT FROM AUTHOR]