Cyclic electric field response of morphotropic Bi1/2Na1/2TiO3-BaTiO3 piezoceramics

In this study, the evolution of field induced mechanisms in lead-free piezoelectric ceramics (1-x)Bi1/2Na1/2TiO3-xBaTiO(3) with x = 0.06 and 0.07 was investigated by transmission electron microscopy, neutron, and X-ray diffraction. Preliminary investigations revealed a strong degradation of macroscopic electromechanical properties within the first 100 bipolar electric cycles. Therefore, this structural investigation focuses on a comparative diffraction study of freshly prepared, poled, and fatigued specimens. Transmission electron microscopy and neutron diffraction of the initial specimens reveal the coexistence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively. In situ electric field X-ray diffraction reveals a pronounced field induced phase transition from a pseudocubic state to a phase composition of significantly distorted phases upon poling with an external electric field of 4 kV/mm. Although the structures of the two compositions are pseudocubic and almost indistinguishable in the unpoled virgin state, the electric field response shows significant differences depending on composition. For both compositions, the application of an electric field results in a field induced phase transition in the direction of the minority phase. Electric cycling has an opposite effect on the phase composition and results in a decreased phase fraction of the minority phase in the fatigued remanent state at 0 kV/mm. (C) 2015 AIP Publishing LLCopen

Effect of uniaxial stress on ferroelectric behavior of (Bi1/2Na1/2)TiO3-based lead-free piezoelectric ceramics

Prior studies have shown that a field-induced ferroelectricity in ceramics with general chemical formula (1-x-y) (Bi1/2 Na1/2) TiO3 -x BaTiO3 -y (K0.5 Na0.5) NbO3 and a very low remanent strain can produce very large piezoelectric strains. Here we show that both the longitudinal and transverse strains gradually change with applied electric fields even during the transition from the nonferroelectric to the ferroelectric state, in contrast to known Pb-containing antiferroelectrics. Hence, the volume change and, in turn, the phase transition can be affected using uniaxial compressive stresses, and the effect on ferroelectricity can thus be assessed. It is found that the 0.94 (Bi1/2 Na1/2) TiO3 -0.05 BaTiO3 -0.01 (K0.5 Na0.5) NbO3 ceramic (largely ferroelectric), with a rhombohedral R3c symmetry, displays large ferroelectric domains, significant ferroelastic deformation, and large remanent electrical polarizations even at a 250 MPa compressive stress. In comparison, the 0.91 (Bi1/2 Na1/2) TiO3 -0.07 BaTiO3 -0.02 (K0.5 Na0.5) NbO3 ceramic (largely nonferroelectric) possesses characteristics of a relaxor ferroelectric ceramic, including a pseudocubic structure, limited ferroelastic deformation, and low remanent polarization. The results are discussed with respect of the proposed antiferroelectric nature of the nonferroelectric state.open291

МНОГОФАЗНО-ОДНОФАЗНыЕ РЕВЕРСИВНыЕ ЭЛЕКТРОМАШИННО-ВЕНТИЛЬНыЕ ПРЕОБРАЗОВАТЕЛИ БЕСКОНТАКТНыХ МАШИН ДВОЙНОГО ПИТАНИЯ

Розглянуто процеси в багатофазно-однофазних реверсивних електромашинно-вентильних перетворю- вачах безконтактних машин подвійного живлення. Рассмотрены процессы в многофазно-однофазных реверсивных электромашинно-вентильных преобра- зователях бесконтактных машин двойного питания

Point defect segregation and its role in the detrimental nature of Frank partials in Cu(In,Ga)Se2 thin-film absorbers

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material Cu(In,Ga)Se₂ was studied by aberration-corrected scanning transmission electron microscopy in combination with electron energy-loss spectroscopy and calculations based on density-functional theory. We find that Cu accumulation occurs outside of the dislocation cores bounding the stacking fault due to strain-induced preferential formation of Cu‾²In, which can be considered a harmful hole trap in Cu(In,Ga)Se₂. In the core region of the cation-containing α-core, Cu is found in excess. The calculations reveal that this is because Cu on In-sites is lowering the energy of this dislocation core. Within the Se-containing β-core, in contrast, only a small excess of Cu is observed, which is explained by the fact that Cu¡ⁿ and Cu¡ are the preferred defects inside this core, but their formation energies are positive. The decoration of both cores induces deep defect states, which enhance nonradiative recombination. Thus, the annihilation of Frank loops during the Cu(In,Ga)Se₂ growth is essential in order to obtain absorbers with high conversion efficiencies

Elemental redistributions at structural defects in Cu(In,Ga)Se₂ thin films for solar cells

The microstructural evolution of Cu(In,Ga)Se2 absorber layers during a three-stage-type co-evaporation process was studied to elucidate the effect of a Cu-rich stage on the formation of extended structural defects. Defect densities for two Cu-poor samples, one interrupted before and one after this crucial Cu-rich composition stage, were investigated by scanning transmission electron microscopy (STEM) imaging. The structure and chemical nature of individual defects were investigated by aberration-corrected high-resolution STEM in combination with electron energy-loss spectroscopy on the atomic-scale. In spite of the different defect densities between the two samples, most of the individual defects exhibited similar chemistry. In particular, the elemental distributions of atomic columns at {112} twin planes, which are very frequent in Cu(In,Ga)Se2 thin films, were found to be the same as in the defect-free grain interiors. In contrast, within grain boundaries, dislocation cores, and other structurally more complex defects, elemental redistributions of Cu and In were observed