Significant improvement of J_c and flux pinning force of MgB₂ by doping NTE-ZrW₂O₈ particles

Although introducing second phase particles as additional pinning centers is an effective method to improve the current carrying capacity of MgB₂, the thermal strain caused by second phase particles in MgB₂ and their impact on flux pinning have not received much attention. In this paper, flux pinning behavior of the thermal strain induced by the second phase particles in the MgB₂ bulk materials was studied by doping ZrW₂O₈ particles which have negative thermal expansion (NTE) characteristics. Due to the significant difference in thermal expansion characters between ZrW₂O₈ and MgB₂, drastic thermal strain was induced in the lattice of MgB₂ by doping the NTE-ZrW₂O₈ particles. These strains work as additional flux pinning centers and significantly enhance the irreversibility field, H_irr, and critical current density, J_c, of the MgB₂. Taking J_c as an example, at 4.2 K and 5 T, its J_c value reaches 4.1 × 10⁴ A/cm², which is a 105 % performance improvement compared to the 2.0 × 10⁴ A/cm² of the pure MgB₂ sample W-0; at 20 K and 2 T, its J_c also reaches 1.3 × 10⁵ A/cm², which is 1.78 times of the 7.3 × 10⁴ A/cm² for the pure MgB₂ sample under the same conditions. It is interesting that doping ZrW₂O₈ does not significantly change the scaling behavior of the pinning force, indicating that the lattice strain work like surficial pinning center, while the point defect pinning center initiated by ZrW₂O₈ particles themselves may only contribute to the high field region, causing the peak in the pinning force scaling curve to shift towards higher fields.