Xiangyun Deng

Phase transition and high dielectric constant of bulk dense nanograin barium titanate ceramics

Barium titanate (BTO) ceramics with an average grain size of 8nm were fabricated and studied. The temperature dependent Raman spectra revealed successive transitions from rhombohedral to orthorhombic, tetragonal, and cubic as the temperature increased from 87to673K. Local piezoresponse force measurements indicated that the ceramics had switchable polarization at room temperature. Dielectric measurements showed a broad ferroelectric-to-paraelectric phase transition with a maximum permittivity of 1800 at 390K. All these results suggest that ferroelectricity could remain in BTO ceramics with grain size as small as 8nm in diameter.

Ferroelectric properties of nanocrystalline barium titanate ceramics

Dense nanocrystalline BaTiO3 ceramics with the average grain size of 20nm obtained by spark plasma sintering were investigated. The dielectric data show a ferroelectric to paraelectric phase transition with a maximum permittivity of ≈930 at 115°C and at 1kHz. The polarization-reversal characteristics and the local ferroelectric switching behavior were measured; the typical piezoelectric hysteresis loops were recorded. The present results provide experimental evidence, indicating that if a critical grain size exists for ferroelectricity it is less than 20nm for polycrystalline BaTiO3 ceramics.

Characterization of (100)-oriented BiScO3-PbTiO3 thin films synthesized by a modified sol-gel method

The ferroelectric and piezoelectric properties of (100)-oriented BiScO3–PbTiO3 (BSPT) thin films were investigated. (100)-oriented 0.34BiScO3–0.66PbTiO3 thin films were synthesized on Pt(111)∕Ti∕SiO2∕Si(111) substrates using a modified sol-gel method. Saturated polarization hysteresis loops were observed, showing a remanent polarization of 33μC∕cm2. The dielectric constant and loss for the thin films were 1200 and 5%, respectively. The Curie temperature appeared at 435°C, over 80°C higher than that of the Pb(Zr,Ti)O3 (PZT) thin films. The local effective piezoelectric coefficient d33* was approximately 70pm∕V, which was comparable to that of the highly (100)-oriented PZT thin films.

Effect of crystallization process on the ferroelectric properties of sol-gel derived BiScO3–PbTiO3 thin films

The dielectric and ferroelectric properties of sol-gel derived BiScO3–PbTiO3 thin films prepared by two different kinds of crystallization processes were studied. The film prepared by a single crystallization had a higher remanent polarization and dielectric constant than the one prepared by a multiple crystallization. The remanent polarization and dielectric constant were 50μC∕cm2 and 1609 for the single crystallization sample, and 40μC∕cm2 and 1429 for the multiple crystallization one. The investigation of the Rayleigh law revealed that the film prepared by a multiple crystallization, processed more irreversible domain wall motion contribution and less reversible contribution with respect to the one by a single crystallization. This was attributed to the layer-to-layer interfaces generated during the multiple crystallization processing, which acted as energy barrier and had a pinning effect on the domain wall motion, resulting more irreversible domain wall motion.

Observation of ferroelectric domain patterns in nanocrystalline BaTiO3 ceramics

Scanning nonlinear dielectric microscopy measurements revealed temperature-dependent variations in contrast, which was attributed to domain rearrangements in 20nm BaTiO3 ceramics. The experiment result showed the presence of a-c 90° nanoferroelectric domains in 20nm BaTiO3 ceramics from 25to290°C; furthermore the piezoresponse hysteresis loops showed that the 20nm BaTiO3 ceramics were switchable and ferroelectricity was retained at the high temperature of 290°C, demonstrating the existence of nanoferroelectric domains and the ferroelectric phase still retained above Curie temperature in 20nm BaTiO3 ceramics, which confirmed the diffused phase transition character in nanograin BaTiO3 ceramics.