Zhaohui Zhou

NiFe2O4 nanoparticles formed in situ in silica matrix by mechanical activation

Nanocrystalline nickel ferrite (NiFe2O4) particles were successfully synthesized in situ in an amorphous silica matrix by mechanical activation at room temperature. Phase development in the amorphous precursors, derived via a modified sol–gel synthesis route, with increasing mechanical activation time was studied in detail by employing transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. NiFe2O4 nanoparticles of 8.05 nm in mean particle size with a standard deviation of 1.24 nm, which were well dispersed in the silica matrix, were realized by 30 h of mechanical activation. The phase formation of nanocrystalline NiFe2O4 particles involves the nucleation of Fe3O4 in amorphous silica at the initial stage of mechanical activation, followed by the growth of nickel ferrite by incorporation of Ni2+ caions into Fe3O4. Their magnetic anisotropy, surface spin disorder, and cation distribution are investigated by considering both the strain imposed by silica matrix and the buffer effect durin...

Ferroelectric and electrical behavior of (Na0.5Bi0.5)TiO3 thin films

Sodium bismuth titanate (Na0.5Bi0.5)TiO3 (NBT) of perovskite structure is among the best known lead-free piezoelectric∕ferroelectric that promises a number of applications in sensors and actuators. However, NBT in thin film form has not been properly investigated, although NBT in bulk ceramic form has been widely studied. In this letter, we report the growth of polycrystalline NBT thin films by radio-frequency magnetron sputtering and their ferroelectric behavior. The NBT thin films exhibit a well-defined hysteresis loop, with a remanent polarization of 11.9μC∕cm2 and coercive field of 37.9kV∕cm when measured at room temperature. There is a steady decrease of dielectric constant in the range of 650–470 over the frequency range of 10–105Hz. A change in the controlling mechanism of electrical behavior from the grain interior to the grain boundary is observed for the NBT thin film with increasing temperature. Hopping of oxygen vacancies trapped at the grain boundaries is responsible for the high dielectric l...

Giant strain in PbZr0.2Ti0.8O3 nanowires

The authors report a giant reversible piezoelectric strain of ∼4.2% achieved with single-crystalline PbZr0.2Ti0.8O3 nanowires of ∼70nm in diameter, which is nearly 300% greater than the nominal values reported for ceramic perovskite single crystals. The giant strain arises from the reversible switching of ferroelastic domains. In the nanowires, there is a competition between surface tension, which forces in-plane polarization, and applied electric field, which stabilizes out-of-plane polarization, through the entire voltage cycle, the result of which is the reversible movement of ferroelastic domains.

Heterolayered lead zirconate titanate thin films of giant polarization

Heterolayered Pb(Zr1−xTix)O3 (PZT) films consisting of alternating PbZr0.8Ti0.2O3 and PbZr0.2Ti0.8O3 layers, which were successfully synthesized via a multistep sol-gel route, demonstrate giant polarizations. They show (001)/(100) preferred orientation, the degree of which is strongly affected by both the combination sequence and the number of alternating layers as well as the processing temperature. There occurs a strong interaction between the layers of the two different compositions, leading to the preferred orientations. Both the ferroelectric and dielectric properties are dependent on the number of alternating layers. The six-heterolayered PZT film shows a giant remnant polarization of as high as 71.9μC∕cm2 and a dielectric permittivity of 905, which can easily double those of the conventional polycrystalline PZT films. Studies of the polarization behaviors under subswitching fields suggest a domain wall pinning mechanism. The giant polarization observed at high electric field can be accounted for by...

Leakage current and charge carriers in (Na0.5Bi0.5)TiO3 thin film

A sodium bismuth titanate (Na0.5Bi0.5)TiO3 (NBT) thin film of perovskite structure, synthesized by radio-frequency magnetron sputtering, exhibited a remanent polarization of 11.9 µC cm−2 and a coercive field of 37.9 kV cm−1 at room temperature. It however showed a rather high leakage current density of ~6 × 10−5 A cm−2 at an applied electric field of 100 kV cm−1. There occurs a change in the controlling mechanism of the electrical behaviours of the NBT thin film from grain interiors to grain boundaries with increasing temperature. The ac conductivity obeys the Jonscher relation. The activation energies for dc conductivity and hopping frequency of the charge carriers are calculated to be 0.92 eV and 1.00 eV, respectively, suggesting oxygen vacancies are the most likely charge carriers at high temperatures. Hopping of oxygen vacancies trapped at the grain boundaries and excitation of polarons in the grain interior are responsible for the relatively high dielectric loss and high dc conductivity. The contribution of hopping charge carriers to the dielectric response is demonstrated by the frequency dispersion observed for the relative permittivity in the low frequency region.

Fatigue behavior of heterostructured Pb(Zr,Ti)O3∕(Bi,Nd)4Ti3O12 ferroelectric thin films

Heterolayered Pb(Zr0.52Ti0.48)O3∕(Bi3.15Nd0.85)Ti3O12 (PZT/BNT) thin films were synthesized via a route of combining sol-gel and rf sputtering. A fatigue anomaly is observed for the heterolayered PZT/BNT thin films, where a switchable polarization peak, which is more than five times higher than that of the virgin state, occurs upon polarization switching for 108–109cycles. Interestingly it shifts towards smaller numbers of switching cycles at elevated temperatures. Both the aging and dielectric behavior suggest that the fatigue anomaly is related to the defects accumulated at the interfaces in the heterolayers.

Cluster glass structure in nanohybrids of nonstoichiometric zinc ferrite in silica matrix

A cluster glass structure has been observed to occur in nanohybrids consisting of zinc ferrite in an amorphous silica matrix. Such a unique cluster glass structure has been supported by studies of x-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and magnetic characterization. By increasing the Fe/Zn ratio in silica gel to 10, crystallites of zinc ferrite of partially inverted structure were nucleated and grown in amorphous iron oxide pockets dispersed in the silica network. The zinc ferrite crystallites, together with the confinement of silica matrix, suppress nucleation and crystallization of α-Fe2O3 to a temperature above 900 °C, preserving the cluster glass structure. The resulting cluster glass structure consists of amorphous iron oxide pockets of ∼10 nm in dimensions together with zinc ferrite crystallites of ∼3 nm in sizes, which are uniformly dispersed in the amorphous silica glass matrix.

Ferroelectric and conductivity behavior of multilayered PbZr0.52Ti0.48O3∕Pb(Mg1∕3Ta2∕3)0.7Ti0.3O3∕PbZr0.52Ti0.48O3 thin films

Ferroelectric and impedance behavior of sandwich-structured PbZr0.52Ti0.48O3∕Pb(Mg1∕3Ta2∕3)0.7Ti0.3O3∕PbZr0.52Ti0.48O3 thin films was studied as a function of temperature (23–300°C) and frequency (0.1–104Hz). A change in the controlling mechanism of the electrical behavior from grain interior to grain boundary occurred in the temperature range studied. A low-frequency dielectric relaxation was observed in the temperature range of 200–300°C, the activation energy of which was calculated to be 0.90eV. This suggests that oxygen vacancies are the most likely charge carriers at high temperatures. The change in fatigue behavior of the sandwich-structured thin film with temperature can be accounted for by the increased mobility of oxygen vacancies at elevated temperatures. Frequency dependent conductivities were analyzed with an augmented Jonscher relation. The activation energies for dc conductivity and hopping frequency were calculated to be 0.90 and 0.89eV, respectively.

DESIGN ISSUES OF MULTILAYER PIEZOELECTRIC BIOSENSORS

One of the work kind modes of the multilayer piezoelectric biosensors is to detect changes in resonant frequency resulted from binding and hybridization of biomolecules. In this work, mechanical design issues of multilayer piezoelectric biosensor (consisting of Si, SiO2, PZT, Pt electrode and biolayer) with high sensitivity are investigated. Piezoelectric effect matrices are included in the finite element simulation model. Harmonic response analysis is employed to determine the geometry parameters of the multilayer biosensor and predict the piezoelectrical signals subjected to base vibration. The relationships between the dimensions of the multilayer and the resonance frequency, mass sensitivity, piezoelectrical response are studied. At the same time, the thermal mismatch effects on the mechanical and electrical characteristics of the multilayer biosensor are also investigated. It is shown that the frequency of the sensor increase by the thermal mismatch bending, while it is small in the application temperature range of the biosensors. Simulation results also showed that the multilayer piezoelectric biosensor can have a greatly enhanced minimum detectable mass density (MDMD) of 0.05 ng/cm2, as compared to 10 ng/cm2 from the quartz crystal microbalance (QCM).