The microstructure and critical currents of superconducting YBa2Cu3O7-Y2BaCuO5 composites with small CeO2 additions prepared using a directional solidification procedure have been investigated. The small CeO2 additions cause the decomposition of the Y2BaCuO5 phase leading to the formation of Y2O3 and BaCeO3. Experimental evidence is given for the subsequent nucleation of the Y2BaCuO5 phase on the Y2O3 particles. This new decomposition-nucleation mechanism leads to textured YBa2Cu3O7 having enhanced critical currents. An addition of 0.3 wt % CeO2 can increase Jabc by a factor of 2 in samples having similar concentration of Y2BaCuO5 precipitates. Critical currents above 105 A/cm2 at 77 K and zero field are obtained by this method.
The superconducting properties of directionally solidified YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 composites, with critical currents above 10 5 A/cm 2 at 77 K and zero magnetic field, are reported in a wide variety of samples having very different contents of Y 2 BaCuO 5 ͑211 phase͒ precipitates with different particle size and magnetic fields up to 22 T. The field, temperature, and composition dependence of the critical currents allow us to identify interfacial pinning by 211 precipitates as a very effective pinning mechanism. Nevertheless, single vortex interfacial pinning has a dominant role only in a narrow region below about 1 T and temperatures 40 KрTр80 K. In this region the system shows a behavior very akin to that observed in ion irradiated single crystals and described as correlated disorder. On lowering the temperature the thermal wandering of the vortex from secondary weak pinning centers decreases leading to a new single vortex pinning regime extending up to very high magnetic fields where a mixture of strong and weak pinning centers are active. On the other hand, increasing the magnetic field, the characteristic footprints of the small bundle and large bundle regimes are identified at intermediate temperatures. The former is found below the H-T line given by the maximum of the macroscopic pinning force while the latter is observed above this line up to the irreversibility line. Finally, collecting all these results together a magnetic phase diagram of the mixed state of the YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 textured composites is proposed.
Finding lead-free ceramics with good piezoelectric properties is nowadays one of the most important challenges in materials science. The (K, Na, Li)(Nb, Ta, Sb)O3 system is one of the most promising candidates as a lead-free ceramic for transducer applications and is currently the object of important research work. In this paper, (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 was prepared by a conventional ceramic processing route. For this composition, orthorhombic-to-tetragonal phase transition was observed at temperatures very close to room temperature. As a consequence, good room temperature electromechanical properties were observed, displaying good thermal stability. We show that the most important contribution to dielectric, piezoelectric and elastic response comes from extrinsic effects, as was observed in other perovskite based materials. Nonlinearities in electromechanical properties induced by high electric field or mechanical stress were studied. Non-linear dielectric response was found to be less important than for soft PZT ceramics and was analysed within the Rayleigh framework. The results reveal that the non-linear response at room temperature in this material is mainly due to the irreversible wall domain movement.
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