BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives

Recently, our group synthesized perovskite-type oxide ABO3 (A = Sr, Ba; B = Ti, Zr) with high crystallinity simply by leaving a powder mixture of A(OH)2•nH2O and BO2 hydrous gel at room temperature to 323 K for 3–10 days. In this study, perovskite solid solution (Ba, Sr)TiO3 and Ba(Zr, Ti)O3 were synthesized to investigate the reaction mechanism in this synthesis method in detail. The perovskite solid solution (Ba, Sr)TiO3 was obtained by using barium hydroxide, strontium hydroxide and TiO2 hydrous gel as the staring materials. On the other hand, in the case of using barium hydroxide and the mixture of TiO2 hydrous gel and ZrO2 hydrous gel as the starting materials, a mixed phase of BaTiO3 and BaZrO3 was obtained. B-site-substituted perovskite solid solution Ba(Zr0.45Ti0.55)O3 was obtained by using barium hydroxide and (Zr0.45Ti0.55)O2 hydrous gel as the starting materials. From the above, it was clarified that in this synthetic method, the Sr and Ba ions diffuse into the water part of hydrous gel and then slowly react with Ti(OH)4 to form a perovskite lattice.

Poling treatment of field application below Curie point of ferroelectric ceramics is termed “normal poling” and a dc field application above Curie point is termed “high poling”. When a high poling is performed for BaTiO3 or (Sr, Ba)TiO3, a space-charge polarization (P2 type) in the direction of high poling field and an internal bias field E-i with the direction opposite to high poling field are generated. When a ferroelectric sample such as BaTiO3 and Pb(Zr, Ti)O3 are aged at temperatures below Tc after normal poling, another space-charge polarization (-P2 type) in the direction opposite to that of the normal poling and the internal bias field Ei with the same direction as that of normal poling field are generated. In this paper, two kinds of space-charge models in ferroelectric ceramics and related phenomena such as space-charge-stabilized effects on the electromechanical strength, the pseudo-pyro-electric current of high-poled (Sr, Ba) TiO3 and asymmetric polarization reversal caused by the -P2 effects will be reviewed and discussed.

The Pb(Zr, Ti)O3 and PbTiO3 precursor films formed on Pt/TiOx/SiO2/Si substrate below 400°C by metal-organic chemical-vapor-deposition (MOCVD) method were treated hydrothermally in Ba(OH)2-Pb(OH)2 mixture alkaline solution at 240°C and successfully transformed into (Pb, Ba)(Zr,Ti)O3 and (Pb, Ba)TiO3 ferroelectric thin film. This low temperature preparation method is compatible with the ULSI process becouase thermal damage to the circuit is avoidable. XRD χ − 2θ reciprocal space mapping measurements, shows that orientation of the thin film treated hydrothermally is influenced much by crystallite states of MOCVD precursor film. When the PbTiO3 precursor film is pre-crystallized having small (001) peak in XRD, the (Pb, Ba)TiO3 thin film has clear (001) orientation after hydrothermal treatment. The (Pb, Ba)TiO3 thin film treated at 240°C for 3 h in 0.2 M Ba(OH)2 − 0.1 M Pb(OH)2 solution has a hysteresis loop with good shape, and the polarization at zero electric field of the treated thin film is 8 μC/cm2.

Influence of In doping on the structure, stability and electrical conduction behavior of Ba(Ce,Ti)O3 solid solution

Thin films of (Pb,La)(Zr,Ti)O3, (Sr,Ba)TiO3, Ba2NaNb5O15, BaTiO3, LiNbO3, KNbO3, KTiOPO4, beta-BaB2O4, and LiB3O5, some of them highly oriented, have been deposited on various substrates by using rf magnetron sputtering and/or solution coating methods. All the films have the correct crystal structure and stoichiometry. Electro-optic effects and second harmonic generation in these films have been measured and will be presented. The potential applications of these films for integrated optics will be discussed.

Wideband dielectric spectra from the kilohertz to terahertz range are discussed for BaTiO₃-based ferroelectrics. Ceramics of BaTiO₃ (BT), Ba(0.6)Sr(0.4)TiO₃ (BST-0.6), and BaZr(0.25)Ti(0.75)O₃ (BZT-0.25) were selected as normal ferroelectrics, ferroelectrics with diffuse phase transition (DPT ferroelectrics), and relaxor ferroelectrics, respectively. The variation of ionic polarization in both BT and BST-0.6 ceramics with temperature could be explained by the softening of the soft phonon mode. In BZT-0.25, a permittivity anomaly at the dielectric maximum temperature (T(m)) at low frequencies is not attributed to the softening of the soft phonon mode, but originates from the permittivity derived from the dipole polarization (ϵ(dipole)). Relaxor behavior in BZT-0.25 is derived from the increase in the depression of ϵ(dipole) on cooling across the T(m) with increasing frequency. In dipole polarization, BT, BST- 0.6, and BZT-0.25 all exhibited a similar tendency of ϵ(dipole) above the Curie temperature (T(c)) and T(m). However, behavior of ϵ(dipole) below the T(c) can be explained by the ferroelectric domains in BT, whereas the variation of ϵ(dipole) below the Tm could be explained by growth process of polar nanoregions (PNRs) in BST-0.6 and BZT-0.25. Regarding this, BT can be distinguished from BST-0.6 and BZT-0.25. Relaxation in BT could be interpreted as successive change in polarization mechanism from normal ferroelectrics to relaxor ferroelectrics via DPT ferroelectrics.

Experimental data confirming the presence of dielectric dispersion over the meter-wave range in the ferroelectric phase of BaTiO3 and (Ba, Sr)TiO3, Pb(Zr, Ti)O3 solid solutions are presented. The effect of initial polarization, temperature and crystallite grain size on dispersion curves is investigated. Considerations about the probable nature of the observed dispersion are given.

The microstructures of two layered films consisting of 2% Mn doped (Ba,Sr)TiO3 (Mn:BST) and 2% Mn doped Ba (Zr, Ti)O3 (Mn:BZT) layers on a MgO (001) substrate deposited by pulsed laser deposition were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution TEM and nanoindentation. The films contained four alternating layers of Mn:BST and Mn:BZT either the former or the latter as the first layer. The film deposited with Mn:BST as the first layer has a rough surface, an epitaxial continuous layer structure for the first two layers next to the interface and columnar structures for the top two layers. The first layer has a single oriented crystal structure with an in-plane lattice mismatch of ∼−5.70% and an atomically sharp interface with respect to the substrate. The second layer exhibited twin-induced domain structures. The columnar structures in the top two layers have an average width of ∼70 nm and are tightly attached to their adjacent grains without leaving any gaps or pores. The film deposited with Mn:BZT as the first layer has a relatively smooth surface and discrete needle-like nanopillars through the four layers from the interface to the top surface of the film. The first Mn:BZT layer consists of an epitaxial structure and its associated twin-coupled domains by sharing their {111} and {110} planes. The epitaxial Mn:BZT has an in-plane lattice mismatch of −3.34% with respect to MgO. The second, third and fourth layers have textured structures with {111} twin domains. The film deposited with Mn:BST as the first layer exhibited a higher hardness and modulus than the film with Mn:BZT as the first layer. The effects of the deposition sequence on the microstructure and mechanical properties are discussed.

BaTiO3-(Bi0.5Na0.5)TiO3 (BTBNT) solid solution ceramics with the Curie temperature higher than 150°C were prepared, which were promising for X9R MLCCs application. (Bi0.5Na0.5)TiO3 (BNT) was first synthesized by the conventional solid state reaction and then it was mixed with BaTiO3 (BT) with increasing BNT content from 0 to 12 mol%. BaTiO3-(Bi0.5Na0.5TiO3 solid solutions were obtained after calcining at 1100°C. The structural and dielectric properties of BTBNT and Nb-doped BTBNT ceramics were investigated.

Approaches for preparing 〈1 1 1〉-textured Bi0.5Na0.5TiO3-based ceramics by hetero-templated grain growth

Lead lanthanum zirconium titanate (PLZT), or (Pb,La)(Zr,Ti)O3 solid solutions, ferroelectric ceramics are being studied for electro-optical applications, as well as for high-speed electrophoretic printing. Thin films of a related material, lead zirconium titanate, or Pb(Zr,Ti)O3 solid solutions (PZT), are used for non-volatile memories and actuators.1 Proper composition and compositional homogeneity of ferroelectric ceramics are essential for application, however the fabrication process may produce materials with element concentrations and thus physical characteristics that may differ from expectation. Therefore, a PLZT reference material for determining the composition of these ceramics would be useful. It will also be useful for quantitative microanalysis; it is stable under the point beam of the electron microprobe (EPMA) at excitation voltages up to 25 kV and probably higher at currents of more than 100 nA.A specimen of PLZT prepared by the Ceramics Department of the Jozef Stefan Institute in Ljubljana, Slovenia was studied with the electron microprobe to determine the composition and the extent of heterogeneity.

The theoretical backgrounds and some experimental data on the electronic properties of polycrystalline Pb(Zr, Ti)O3 solid solutions are summarized. These substances occur in the perovskite structure, and so their electronic band structure is presumed to be analogous to other perovskite compounds. Their forbidden gap energy has been determined to be about 3.30–3.56 eV on the basis of optical, photoacoustical, and photoelectrical measurements. Many local levels and traps exist in the forbidden gap because of the polycrystalline character and heterovalent doping. Their actual configurations and states strongly influence the optical, photoelectrical, piezoelectrical, and other physical properties of Pb(Zr, Ti)O3 solid solutions.

Chemical deposition methods include a wide spectrum of techniques ranging from chemical solution deposition (CSD) to chemical vapor deposition (CVD) which all have their advantages and drawbacks. The complementary aspects of the methods are demonstrated and film growth and film properties are discussed for examples from different application fields, including high k-dielectrics for highly integrated capacitors, e.g. (Ba, Sr)TiO3, and ferroelectrics, e.g. Pb(Zr, Ti)O3 and (Pb, Ba)TiO3.

The dependence of concentration of synthesized BaZr x Ti(1–x)O3 solid solutions on the type of original mixture of BaTiO3 + ZrO2 or BaCO3 + TiO2 + ZrO2 powders was established. The influence of particle sizes of ZrO2 (microor nanopowders) on the concentration was revealed.

The sequence of phase transitions due to octahedral tilting across theSr(Zr,Ti)O3 solid solution series has been investigated by resonant ultrasoundspectroscopy at high and low temperatures using ceramic samples. The elasticbehaviour associated with phase transitions as a function of composition inSr(Zr,Ti)O3 at room temperature is proposed to be analogous to that as a function of temperature inSrZrO3, with the transition at SrZr0.57Ti0.43O3, at SrZr0.35Ti0.65O3, and at SrZr0.05Ti0.95O3. Changes in elastic constants and acoustic dissipation with temperature have beenanalysed for samples across the compositional range. The intermediate phases,I4/mcm and what isassumed to be Imma, appear to have stability fields across the full compositional range and both show largedissipation effects, most probably due to twin wall mobility. In contrast, the Zr-richPnma phase, which should contain transformation twin walls, is an unexpectedly stiff andnon-dissipating material, similar to the high temperature and/or Ti-rich phase. In the case of Pnma, this is attributed to coupling between the two order parameters, which could impederelaxation responses to an applied stress. The structure is a classically stiff cubic perovskite and no transformation-related dissipationprocesses are expected.