Real time dynamics of ferroelectric nanodomains
(Physical Review Letters, 107, 187601-1-5 (2011)., Applied Physics Letters, 100, 172902-1-3 (2012).)
Ferroelectric crystals such as PZT (Pb(Zr,Ti)O3) and PMN-PT (Pb(Mg1/3Nb2/3)O3-PbTiO3) exhibits very high piezoelectricity and are used for various piezoelectric devices. Since these piezoelectric materials are composed of ferroelectric nanodomains, response of the nanodomains to external electric fields should be observed to clarify the piezoelectricity mechanisms.
With the use of in-situ transmission electron microscopy, for the first time, we observed the nanodomain dynamics in real time. As seen in the movie below, nadomains exhibit reversible response on the application/release of electric field, which accords with the ahysteric strain-electric field curve characteristic of this material. It is suggested that the domain effects can contribute to a part of piezoelectricity of this material.
Monoclinic nanodomain in morphotropic phase boundary ferroelectrics
(Applied Physics Letters, 104, 082905-1-4 (2014).)
Morphotropic phase boundary (MPB) is a bounary regime in between rhombohedran and tetragonal phases in crystal phase diagram. Very high piezoelectricity is obtained in ferroelectrics that posesses MPB, and the piezoelectricity is attributed to monoclinic crystal phase. On the other hand, the material is composed of nanoscale domains that complicates the understanding of true crystal structure at the single domain level.
To determine the crystal structure of nanodomain, we have carried out dedicated electron diffraction experiment for this material. Splitting of diffraction spot found in this material proves that the nanodomain is of monoclinic phase. Based on this result and the domain dynamics, we propose that domain effect can contribute to about 0.2% of piezoelectric strain with ahysteric E-field-strain curve.
Atomic structure of zinc oxide (ZnO) grain boundary and the role on electrical property
(Physics Review B, 87, 140101 (2013)., Physics Review B, 80, 094114 (2009)., Physics Review Letters, 97, 106802 (2006). JACerS, 90, 337-357 (2007). etc.)
Grain boundary (GB) and interface can largely influence material properties. For a well known case, GB in zinc oxide (ZnO) ceramics bring about nonlinear current-voltage characteristics that are used for varistor devices. In order to clarifty the origin of electrical property, we have studied several ZnO GBs.
Combining electron microscopy and theoretical calculations, we have determined the atomic arrangements in the ZnO GBs. We further revealed that rare-earth dopant such as praseodymium localizes in the GBs and play a role to create zinc vacancies that leads to the nonlinear current-voltage characteristics.
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