Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.

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Domain wall nanoelectronics

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

We present a critical review that encompasses the fundamentals and state-of-the-art knowledge of barium titanate-based piezoelectrics. First, the essential crystallography, thermodynamic relations, and concepts necessary to understand piezoelectricity and ferroelectricity in barium titanate are discussed. Strategies to optimize piezoelectric properties through microstructure control and chemical modification are also introduced. Thereafter, we systematically review the synthesis, microstructure, and phase diagrams of barium titanate-based piezoelectrics and provide a detailed compilation of their functional and mechanical properties. The most salient materials treated include the (Ba,Ca)(Zr,Ti)O3, (Ba,Ca)(Sn,Ti)O3, and (Ba,Ca)(Hf,Ti)O3 solid solution systems. The technological relevance of barium titanate-based piezoelectrics is also discussed and some potential market indicators are outlined. Finally, perspectives on productive lines of future research and promising areas for the applications of these ma...

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

Grain Boundary Complexions

Barium titanate precursors with Ba/Ti ratios ranging from 2:1 to 1:9 were prepared by controlled hydrolysis of mixed barium and titanium species in an alcohol medium. Details of the synthesis and characterization of the resultant products are given. Amorphous powders precipitated by hydrolysis from ethanol solutions of barium and titanium alkoxides crystallize to single-or two-phase 1:2 and 1:5 compounds at ∼700°C. These compounds transform at higher temperatures to other known crystalline phases, the 1:5 phase being maintained metastably to ∼1100°C.

Alkoxide Precursor Synthesis and Characterization of Phases in the Barium-Titanium Oxide System

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al2O3 at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al2O3, AI2O3 with anorthite, AI2O3 with anorthite and MgO, and Al2O3 with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al2O3.

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

The primary goal of sintering research is the controlled manipulation of microstructure. Out of the entire range of microstructures which are theoretically possible, each material system will be able to achieve only a subset of them, depending on the intrinsic material properties. Within these material constraints, the aim is to produce microstructures which enhance specific properties. Our understanding of the relationships among materials processing, microstructure, and properties is just beginning to emerge, and is producing unexpected results. For example, in a recent study of toughness in Al2O3 by Bennison and Lawn, microstructures with platy grains and a bimodal grain size distribution in undoped Al2O3 exhibited a greater resistance to crack propagation than did the more uniform microstructures in MgO-doped Al2O3 [1]. As a result of this emerging understanding, the focus of sintering science is changing from the modification of microstructures in incremental ways for correspondingly incremental improvement in properties to more effectual manipulation of microstructures to optimize properties. However, the production of the optimum microstructure will be dependent on both the material and the application and may require radically different processing routes for different materials. In this review paper, we have examined the research in sintering science over the past five years which has advanced the goal of microstructure manipulation.

Sintering of Ceramics

The stress due to thermal expansion anisotropy in polycrystalline Al2O3 was measured. The broadening of spectroscopic R lines (692 and 693 nm, due to Cr3+ impurities) was used to measure the stresses (at 77 K) in samples with grain sizes of 50 to 150 μm that had been cooled, from 2150 K, at constant rates from 0.1 to 100 K/min. The maximum stress was found to vary from 80 to 100 MPa, depending on the thermal history of the sample. The results are compared to the predictions of a model based on stress relaxation by diffusional creep and are in good agreement for the dependence on cooling rate. No effect due to grain-size changes was observed due to the limited range of grain sizes accessible in this study.

Measurement of Stress Due to Thermal Expansion Anisotropy in Al2O3

Dielectric permittivity and x-ray diffraction measurements were used to identify a region of phase coexistence between the rhombohedral and tetragonal phases near the morphotropic phase boundary in (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-BCT). This phase coexistence prevails over a considerable composition and temperature range and is bounded by single rhombohedral or tetragonal phases. The maximum piezoelectric response measured in terms of maximum strain divided by maximum electric field, Smax/Emax, is extraordinarily high, with the largest value of 1310 pm/V for x = 0.45. Electrical poling induces ferroelastic domain textures in both the rhombohedral and tetragonal phases simultaneously, which increases the piezoelectric performance significantly. The stability of that ferroelastic texture is limited by the phase transition at the morphotropic phase boundary, suggesting coupling between both coexisting phases and limiting potential applications. The results were confirmed using in situ temperature...

John E. Blendell

Papers

Alkoxide Precursor Synthesis and Characterization of Phases in the Barium-Titanium Oxide System

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

Sintering of Ceramics

Measurement of Stress Due to Thermal Expansion Anisotropy in Al2O3

Phase coexistence and ferroelastic texture in high strain (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 piezoceramics