A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

/pdf/the-cosmological-constant-4ljm1nl1cq.pdf

The Cosmological Constant

Relaxor ferroelectrics were discovered almost 50 years ago among the complex oxides with perovskite structure. In recent years this field of research has experienced a revival of interest. In this paper we review the progress achieved. We consider the crystal structure including quenched compositional disorder and polar nanoregions (PNR), the phase transitions including compositional order-disorder transition, transition to nonergodic (probably spherical cluster glass) state and to ferroelectric phase. We discuss the lattice dynamics and the peculiar (especially dielectric) relaxation in relaxors. Modern theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented.

Recent progress in relaxor ferroelectrics with perovskite structure

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

This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

In addition to x-ray diffraction, Raman spectrum measurements provide direct evidence of the tetragonal and orthorhombic phases coexistence in lead-free ceramics (1−x)K0.5Na0.5NbO3 (KNN)–xLiTaO3 when x equals 5mol%. This is caused by the phase transition temperature between tetragonal and orthorhombic decreasing to around room temperature due to the Li and Ta doping in KNN, and not by constituting a region of morphotropic phase boundary as presented by most published papers. The results indicate that although this kind of ceramic displays good properties, it needs further study to verify if it is suitable to be used in a varying temperature environment.

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

In this paper, we report a new entropy-stabilized fluorite oxide, formed by solid-state reacting the equimolar mixture of CeO2, ZrO2, HfO2, TiO2 and SnO2 at 1500 °C. We demonstrated that the oxide is truly entropy-stabilized by showing that the oxide was transferred to a multiphase state when annealed at lower temperatures, and the transition between the low-temperature multiphase and high-temperature single-phase states is reversible. Room-temperature thermal conductivity of the fluorite oxide was measured to be 1.28 Wm−1 K−1. The value is only half of that for 7 wt% yttria-stabilized zirconia, suggesting the material could be useful for thermal-insulation applications.

A five-component entropy-stabilized fluorite oxide

Lead-free K1−xNaxNbO3 ceramics with x=048–054 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on phase structures and electrical properties The results suggest that a typical morphotropic phase boundary exists at x=052–0525, separating the monoclinic and orthorhombic phases The sample with the composition near x=052 shows the maximum values of the piezoelectric constant (d33=160 pC/N) and the electromechanical coupling coefficient (kt=47%) The results provide a helpful guidance to consider the optimal ratio of K to Na for designing and developing new (K,Na)NbO3-based ceramics

Morphotropic phase boundary and electrical properties of K1−xNaxNbO3 lead-free ceramics

The temperature sensitivity of enhanced piezoelectricity in (K,Na)NbO3 perovskite-based lead-free piezoelectrics is one of the technical bottlenecks impeding practical applications. This work found a recipe that possesses a piezoelectric strain coefficient d33* as high as 600 pm V−1 over a wide temperature range up to 100 °C, which is additionally insensitive against the frequency of an applied electrical field. Electrical property measurement and nanostructure observation revealed that the high performance combining high piezoelectricity and excellent stability benefits from a diffused phase transition (DPT), which is closely associated with the transition from ferroelectric to relaxor behavior induced by the local structural disorder due to the complicated compositional modification. The high-resolution transmission electron microscopy images of Moire fringes imply the existence of polar nanoregions (PNRs) that are responsible for the relaxor-like electrical behavior in the present materials. The findings and understanding obtained in this work should be valuable for pushing lead-free piezoelectric ceramics towards practical applications.

High-performance lead-free piezoelectrics with local structural heterogeneity

A hierarchical micro-/nanoscale domain structure is revealed in MC phase of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal near the morphotropic phase boundary by combination of analytical electron microscopy and polarized light microscopy. The hierarchical domain structure is a self-assembling of nanodomains with tetragonal structure, submicrodomains appearing as monoclinic, and microdomains exhibiting the same monoclinic state due to an average effect. The outstanding piezoelectric properties may result from the cooperative response of the hierarchical micro-/nanoscale domain structure.

Xiaowen Zhang

Papers

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

A five-component entropy-stabilized fluorite oxide

Morphotropic phase boundary and electrical properties of K1−xNaxNbO3 lead-free ceramics

High-performance lead-free piezoelectrics with local structural heterogeneity

Hierarchical micro-/nanoscale domain structure in Mc phase of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystal