Boron carbideltitanium diboride composites with 20 and 40 vol% particulate TiB, and various amounts of free carbon were investigated with respect to microcrack toughening. In agreement with previous work, the mere addition of TiB, was found to raise the toughness from 2.2 MPaml" up to 3.0 and 3.5 MPam'", respectively. A further and very significant increase of composite toughness up to 6.0 MPam'" was discovered upon the incorporation of free carbon. SEM and TEM observations reveal that this toughening is associated with microcracking at B,C-TiB, phase boundaries. Microcracking is triggered by thin carbon interlayers, which are located at hetero interfaces and supply a weak fracture path.
The effect of excess iron-doping in piezoelectric lead-zirconate-titanate (PZT) on the microstructure was investigated by means of transmission electron microscopy in conjunction with electron diffraction and energy dispersive x-ray spectroscopy analysis. Electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the site of the iron functional centers. A crystalline secondary magnetic phase formed upon sintering, magnetoplumbite (PbFe12O19), has been unequivocally identified. Furthermore, the phase assemblage observed in the Fe3+-doped PZT 60/40 sample could be correlated with an exchange-coupled resonance in the EPR spectra.
Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed “rotopolarization”. These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.
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