Hai Wen

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.

Epitaxial growth of sol-gel derived BiScO3-PbTiO3 thin film on Nb-doped SrTiO3 single crystal substrate

BiScO3–PbTiO3 (BSPT) thin film was grown on electrical conductive Nb-doped SrTiO3 (100) (Nb-STO) single crystal substrate by a sol-gel method. The x-ray diffraction and high resolution transmission electron microscopy proved the epitaxial growth relation between the BSPT thin film and Nb-STO substrate. The [001]-epitaxial BSPT thin film exhibited considerable high remanent polarization of 74μC∕cm2 and effective piezoelectric coefficient d33* of 130pm∕V, which were much higher than those of the (100)-oriented BSPT thin film grown on traditional silicon substrate.

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.

Dielectric behaviour of BaTiO3-based ceramic multilayer capacitors under high dc bias field

The dielectric behaviour of X7R-MLCCs with different active layers under high dc bias field (up to 6.25 MV m−1) has been investigated in the whole measurement temperature range from 213 to 453 K. The results show that as the dc bias field increases, the dielectric permittivity in the whole measurement range and the frequency dispersion at lower temperature are both suppressed, while dielectric loss initially decreases till the temperature reaches about 423 K and subsequently remains invariable. These results are attributed to the response of the core-shell structure in grains to the applied dc bias field. Furthermore, the dielectric permittivity of X7R-MLCCs with more dielectric layers is more significantly affected because of a more powerful pinning effect of the residual stress between the dielectric layer and the Ni electrode on the migration of the domain walls and the influence of created interfaces on dielectric properties of the dielectric materials close to the interface. The Curie peaks for two samples shift towards higher temperature at a rate 0.55 × 10−5 K m V−1 in the presence of the dc bias field. It is more significant that an interesting field-induced transition from paraelectric phase to ferroelectric phase in the shell part was found at about 323 K and Curie temperature.

Mechanical properties of sol-gel derived BiScO3–PbTiO3 thin films by nanoindentation

The mechanical properties of sol-gel derived BiScO3–PbTiO3 (BSPT) thin films annealed at different temperatures were investigated by nanoindentation. The effects of annealing temperature on the microstructures and mechanical properties were discussed. As the annealing temperature increased from 550to800°C, the (100) orientation degree of the BSPT thin films increased from a Lotgering factor [J. Inorg. Nucl. Chem. 9, 113 (1959)] of 0.47 to 0.67, while the grain size increased from 22to54nm. Due to the grain size effect, hardness for the BSPT thin films dropped from 7.8to6.7GPa. By fitting the experiment results with a Hall-Petch equation, a probable lattice friction stress of 4.69GPa and a Hall-Petch constant of 15.35GPanm1∕2 were obtained for the BSPT thin films. Because of the combined effect of grain size and film orientation, Young’s modulus did not show a clear trend with the annealing temperature. The highest Young’s modulus was 132GPa, appearing in the sample annealed at 550°C.

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.

Fabrication and properties of epitaxial growth BiScO3–PbTiO3 thin film via a hydrothermal method

BiScO3–PbTiO3 thin film was hydrothermally deposited on Nb-doped SrTiO3 [100] single crystal substrate at 180°C with a large thickness of 3.1μm. The x-ray diffraction and high resolution transmission electron microscopy confirmed the epitaxial growth relationship between the BSPT thin film and Nb-STO substrate. Single-crystal-like polarization hysteresis loops were observed, with the remanent polarization of 30μC∕cm2. Piezoelectric force microscopy revealed a single +c domain structure of the film, while exceptional linear piezoelectric response was observed with the effective piezoelectric constant d33* of 65pm∕V. The temperature dependence of the effective d33* indicated that this film exhibited a high temperature stability.

Effect of milling process on the core-shell structures and dielectric properties of fine-grained BaTiO3-based X7R ceramic materials

Abstract Fine-grained BaTiO 3 -based X7R ceramic materials were prepared and the effects of milling process on the core-shell structures and dielectric properties were investigated using scanning electron microscope, transmission electron microscope, and energy dispersive spectroscopy (EDS). As the milling time extends, the dielectric constant of the ceramics increases, whereas the temperature coefficient of capacitance at 125°C drops quickly. The changes in dielectric properties are considered relevant to the microstructure evolution caused by the milling process. Defects on the surface of BaTiO 3 particles increase because of the effects of milling process, which will make it easier for additives to diffuse into the interior grains. As the milling time increases, the shell region gets thicker and the core region gets smaller; however, EDS results show that the chemical inhomogeneity between grain core and grain shell becomes weaker.

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.