A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.

Bulk samples of MgB 2 were prepared with 5, 10, and 15 wt % Y 2 O 3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ϳ3-5 nm YB 4 nanoparticles embedded within MgB 2 grains of ϳ400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density J C was observed, most notably for 10% doping. At 4.2 K, the lower bound J C value was ϳ2 ϫ10 5 A cm Ϫ2 at 2 T. At 20 K, the corresponding value was ϳ8ϫ10 4 A cm Ϫ2 . Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1506184͔In slightly more than one year after the discovery of superconductivity in magnesium diboride, there is now a wide body of evidence indicating that MgB 2 does not contain intrinsic obstacles to current flow between grains, unlike the high-temperature superconducting cuprates. Evidence for strongly coupled grains has been found even in randomly aligned, porous, and impure samples, 1,2 suggesting that dense forms of MgB 2 will be attractive in high-current applications at 20-30 K and perhaps 4.2 K. So far, however, bulk samples have demonstrated modest values of the irreversibility field 0 H*(T) reaching about 4 T at 20 K and 8 T at 4.2 K.3 For comparison, established low-temperature superconductors, e.g., NbTi ͑10 T͒ and Nb 3 Sn ͑20 T͒, have significantly higher irreversibility fields at 4.2 K, while Bi 2 Sr 2 Ca 2 Cu 3 O 10 ͑3 T͒ is becoming established at 20 K. 4 MgB 2 tape results are somewhat more promising, with 0 H* values of above 5 at 20 K, 5-8 where partial orientation of crystallites parallel to the field is playing a role. Since the irreversibility field is the practical limit to magnet applications, it is desirable to make 0 H* values as high as possible.A central question is how to further increase the irreversibility field in addition to introducing crystallographic texture. Alloying additions, such as atomic substitution for Mg or B or added interstitial atoms, increase electron scattering and decrease the coherence length, producing higher upper critical and irreversibility fields.9,10 Adding nanometer-scale defects can produce similar effects. For example, proton irradiation studies showed that 0 H* increased significantly from ϳ3.5 to ϳ6 T at 20 K with only moderate damage, corresponding to atomic displacements of a few %, due to either vacancies or interstitials.11 Mechanical processing also produces structural defects, and similar increases in the irreversibility field have been reported. 6,8,12 These increases were steeper than the concomitant reductions in the critical temperature T c , suggesting it is viable to improve the accessible field range without sacrificing other superconducting properties too much.To explore more practical and scaleable routes to defect incorporation in bulk MgB 2 , the present study explores chemical and nanostructural changes via addition of nanoparticles. Coherently ordered Mg-B-O precipitates are known to ...

Oxygen diffusion and surface exchange coefficients have been measured on polycrystalline samples of the double perovskite oxide PrBaCo2O5+δ by the isotope exchange depth profile method, using a time-of-flight SIMS instrument. The measured diffusion coefficients show an activation energy of 1.02 eV, as compared to 0.89 eV for the surface exchange coefficients in the temperature range from 300 to 670 °C. Inhomogeneity was observed in the distribution of the oxygen-18 isotopic fraction from grain to grain in the ceramic samples, which was attributed to anisotropy in the diffusion and exchange of oxygen. By the use of a novel combination of electron back scattered diffraction measurements, time-of-flight, and focused ion beam SIMS, this anisotropy was confirmed by in-depth analysis of single grains of known orientation in a ceramic sample exchanged at 300 °C. Diffusion was shown to be faster in a grain oriented with the surface normal close to 100 and 010 (ab-plane oriented) than a grain with a surface normal close to 001 (c-axis oriented). The magnitude of this anisotropy is estimated to be close to a factor of 4, but this is only a lower bound due to experimental limitations. These findings are consistent with recent molecular dynamic simulations of this material where anisotropy in the oxygen transport was predicted.

No abstract

ZnO nano-rods were grown on polycrystalline Zn foil by cathodic electrodeposition in an aqueous zinc chloride/calcium chloride solution at 80 • C. Variations in the solution concentration and substrate surface preparation were explored to shed light on the nucleation of the nano-rods. It was found that the nano-rod diameter increased with increasing solution concentration. Rolling striations and native ZnO on the surface of the Zn appeared to enhance nucleation and allowed more highly aligned, dense structures to be grown. By using low solution concentrations (5.0 × 10 −4 M ZnCl 2 ) and non-electropolished Zn substrates, well faceted, hexagonal nano-rod structures of dimension ∼80 nm diameter and >1 µm length were obtained. X-ray studies showed the samples to be highly aligned but containing a Zn-oxychloride impurity phase. Annealing caused the impurity phase to disappear and resulted in the films having a sharp photoluminescence double peak at 380/396 nm.

Articles you may be interested inPhase structure, microstructure, and electrical properties of bismuth modified potassium-sodium niobium leadfree ceramics

No abstract

No abstract