Ceramic Ba0.6Sr0.4TiO3 (BST-0.6) samples were studied in the broad spectral range of 106–1014 Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectric dispersion mechanism in the paraelectric phase was shown to be of strongly anharmonic soft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectric transition was shown to consist of two coupled overdamped THz excitations, which show classical features of a coupled soft and central mode, known from many ferroelectric crystals with a dynamics near the displacive and order–disorder crossover. Similar behaviour has been recently revealed and theoretically simulated in pure BaTiO3 (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101 167402). Also for the BST system, this feature was confirmed by the theory based on molecular dynamics simulations with an effective first-principles Hamiltonian. In all the ferroelectric phases, additional relaxation dispersion appeared in the GHz range, assigned to ferroelectric domain-wall dynamics. The microwave losses were analysed from the point of view of applications. The paraelectric losses above 1 GHz are comparable with those in single crystals and appear to be of intrinsic multi-phonon origin. The ceramic BST system is therefore well suited for applications in the whole microwave range.
“Multiglass” materials with simultaneous occurrence of two different glassy states extend the frame of conventional multiferroicity, which is devoted to crystalline materials with coexisting uniform long-range electric and magnetic ordering. The concept applies to Sr0.98Mn0.02TiO3 ceramics, where A-site substituted Mn2+ ions are at the origin of both a polar and a spin cluster glass. Spin freezing is initiated below the dipolar glass temperature, T-g (e) a parts per thousand 38 K, which is seemingly indicated by a divergence of the nonlinear susceptibility, chi(3). Below T-g (m) a parts per thousand 34 K both glass phases are independently verified by memory and rejuvenation effects. Biquadratic interaction of the Mn2+ spins with ferroelectric correlations of their off-center pseudospins in the incipient ferroelectric host crystal SrTiO3 explains the high spin glass temperature and comparably strong third-order magnetoelectric coupling between the polar and the magnetic degrees of freedom. Preliminary results on the related compound K0.97Mn0.03TaO3 favorably comply with the magnetoelectric multiglass concept.
TiTe3O8 ceramics and films can be well sintered below 800 °C and consequently have the potential to be used for low-temperature co-fired ceramics. TiTe3O8 thick films were fabricated by electrophoreticdeposition on platinized Si substrates using TiTe3O8 powders synthesized by a conventional solid-state reaction. The permittivity of TiTe3O8 films is ∼54, with loss tan δ of ∼0.009, measured at 100 kHz. The temperature coefficients of permittivity of TiTe3O8 films and ceramics between 35 and 200 °C are +78 and −100 ppm °C−1, respectively.
Morphology modification of ferroelectric materials at the nanoscale is an important aspect for further development of ferroelectrics. In this work, the synthesis of nanoporous barium titanate prepared under the assistance of Pluronic PE 10300 block copolymer is reported. Nanopores reside inside the BaTiO3 crystallites that exhibit a specific BET surface area around 70 m2/g and nanopore volume fraction of 44.7%. X-ray diffraction of nanoporous BaTiO3 indicates an average cubic structure while Raman spectra suggest local structural distortions of the lattice. With Landauer-Bruggeman effective medium approximation (LB-EMA), the dielectric constant (at 1 MHz) of nanoporous BaTiO3 and solid BaTiO3 (excluding all porosity) was estimated as 112 and 706, respectively, implying an effective way to manipulate the dielectric constant of materials by introducing nanopores.
The piezoelectric and pyroelectric properties of PVDF mainly depend on its β-phase. This work reports the study by scanning force microscopy in a piezoresponse mode of the variations in the topological morphology and piezoelectric surface response of β-PVDF prepared by the main preparation methods of this phase. Clear differences in the poled region distribution and size, as well as in the local piezoactivity, have been identified. The piezoelectric activity at a mesoscale reflects the semicrystalline phase of the polymer.
Single-crystalline magnetoelectric (ME) antiferromagnetic Cr2O3 exchange coupled to a ferromagnetic multilayer (Pt/Co/Pt)n, n>=1, represents a multiphase multiferroic material with sophisticated multifunctional properties. They comprise the possibility of switching the exchange bias (EB) of the ferromagnetic hysteresis loop via the linear ME effect of Cr2O3, and to design MERAM and logic cells operating at room temperature. Quadratic and cubic ME effects – promoted by soft-mode quantum fluctuations – are observed at lowest order in ceramic Sr0.98Mn0.02TiO3 at low temperatures. Dipolar and spin glass orders occur simultaneously on the Mn2+ subsystem and form a multiglass by analogy with conventional multiferroics.
The fabrication of ferroelectric thin films on flexible metal foils or polymeric substrates is of particular interest for embedded capacitors integrated into Printed Circuit Boards (PCB). In this work, BaxSr1-xTiO3 (x = 1, 0.9, 0.7, 05, 0.3, 0.1, and 0) polycrystalline thin films were fabricated at low temperature (120 °C) and for a short time (1 h) by a hydrothermal process on commercial flexible polymeric Kapton films. Microstructural analysis showed that dense BaTiO3 thin films comparable to those derived from long time hydrothermal processes were achieved with the present synthesis conditions. With increasing strontium content, BaxSr1-xTiO3 film thickness decreased and grain size increased. For x = 0, i.e., SrTiO3, the films displayed a monolayer of coarse grains. The variation of the film morphology with the composition suggests a faster nucleation and reaction kinetics for BaTiO3 than SrTiO3. Electrical characterization showed high capacitance densities and acceptable dielectric loss for the films after postannealing in oxygen. Existence of polar domains in the hydrothermally synthesized BaTiO3 thin films was proved by piezo-response force microscopy (PFM). All the used processes were compatible with commercial Kapton films, showing the technological value of this work for the deposition of large area capacitors onto flexible printed circuits.
An alternative approach to tailor the temperature coefficient of permittivity (TCεr) of high Q dielectric BaO–Re2O3–TiO2 (Re: rare earth elements) thick films is presented. 10- to 80-µm-thick BaNd2Ti5O14 (BNT) films are fabricated by electrophoretic deposition on Pt foils under different processing conditions. Observed anisotropic grain growth is facilitated by constrained sintering. The increase of the sintering temperature increases markedly the aspect ratio of the grains, decreases the dielectric permittivity and TCεr changes from −114 to +12 ppm °C−1. By controlling the sintering temperature, near-zero TCεr, high Q thick films can be fabricated with 45 < εr < 70. These findings are of technological relevance since they demonstrate that control of substrate constraint and sintering conditions can be used to control grain anisotropy and thus microwave properties of the BaO–Re2O3–TiO2. The thick films facilitate scaling to small device sizes for high frequency operation. Similar observations are expected in other microwave systems thus opening further technological opportunities.
Lead zirconium titanate (PZT) sol–gel solutions were prepared based on distilled lead acetate precursor solutions. A detailed analysis of the distillation effect on the lead precursor and the final PZT solution were carried out by Infrared and Raman techniques. It was found that the increase in the number of distillation steps experienced by the lead precursor solutions removes the constitutional water and increases the lead acetate–2-methoxyethanol interconnectivity; thus improving stability and avoiding the aging effect of the resulting PZT solutions. The thermal decomposition process of the PZT solutions was analyzed based on the thermogravimetric (TG) and differential thermogravimetric analysis (DTA) measurements. It was found that as the number of distillation steps in the lead precursor solutions increases, the decomposition rate increases and the formationtemperature of pure perovskite PZT films decreases. X-ray diffraction (XRD) technique was used to study the film phase formation. A pure perovskite phase at 500 °C was found by the XRD analysis after the second distillation step. Scanning electron microscope technique was used to carry out the microstructural analysis. Dense microstructure was found in all analyzed films and an incipient columnar grain growth was revealed in PZT films prepared based on lead precursor solution with more than three distillation steps. The dependence of the dielectric, ferroelectric and piezoelectric properties on the number of distillation steps was revealed and a correlation between the distillation process, film microstructure properties and electrical performance was established.
The effect of nonstoichiometry on the densification of SrTiO3 ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO2 and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.
The characterization of sol–gel derived morphotropic phase boundary (MPB) 0.37BiScO3–0.63PbTiO3 (BSPT) thin films, deposited on IrO2/TiO2/SiO2/Si and Pt/TiO2/SiO2/Si substrates, was performed to identify the influence of the IrO2 electrodes on the film’s microstructure and electrical properties. Though the ferroelectric behaviour of both films is similar, with remanent polarization values of 26 µC cm−2 and 23 µC cm−2, respectively, for IrO2 and Pt bottom electroded films, the leakage current density (JL) at room temperature markedly decreases from > 10−6 Acm−2 for BSPT on Pt to the order of ≤ 10−8 Acm−2 for BSPT on IrO2, under the maximum voltage of 4 V ([similar]80 kV cm−1). The formation of an interface layer between the film and the electrode, as observed by Rutherford Backscattering Spectroscopy, is proposed to have a space charge sink effect and, as a consequence, to account for the improvement of the leakage current behaviour of the IrO2 electroded films.