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

and Perspectives Sophie Carenco,†,‡,§,∥,⊥ David Portehault,*,†,‡,§ Ced́ric Boissier̀e,†,‡,§ Nicolas Meźailles, and Cleḿent Sanchez*,†,‡,§ †Chimie de la Matier̀e Condenseé de Paris, UPMC Univ Paris 06, UMR 7574, Colleg̀e de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡Chimie de la Matier̀e Condenseé de Paris, CNRS, UMR 77574, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Chimie de la Matier̀e Condenseé de Paris, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Laboratory Heteroelements and Coordination, Chemistry Department, Ecole Polytechnique, CNRS-UMR 7653, Palaiseau, France

Nanoscaled metal borides and phosphides: recent developments and perspectives

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.

/pdf/on-the-phase-identity-and-its-thermal-evolution-of-lead-free-85q9xvljk1.pdf

On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Bismuth sodium titanate (BNT)-derived materials have seen a flurry of research interest in recent years because of the existence of extended strain under applied electric fields, surpassing that of lead zirconate titanate (PZT), the most commonly used piezoelectric. The underlying physical and chemical mechanisms responsible for such extraordinary strain levels in BNT are still poorly understood, as is the nature of the successive phase transitions. A comprehensive explanation is proposed here, combining the short-range chemical and structural sensitivity of in situ Raman spectroscopy (under an applied electric field and temperature) with macroscopic electrical measurements. The results presented clarify the causes for the extended strain, as well as the peculiar temperature-dependent properties encountered in this system. The underlying cause is determined to be mediated by the complex-like bonding of the octahedra at the center of the perovskite: a loss of hybridization of the 6s2 bismuth lone pair interacting with the oxygen p-orbitals occurs, which triggers both the field-induced phase transition and the loss of macroscopic ferroelectric order at the depolarization temperature.

Lone-Pair-Induced Covalency as the Cause of Temperature- and Field-Induced Instabilities in Bismuth Sodium Titanate

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi1/2Na1/2TiO3–0.06BaTiO3–0.02K1/2Na1/2NbO3 was investigated using in situ high-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting from a0a0c+ to a−a−a− and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.

/pdf/field-induced-phase-transition-in-bi1-2na1-2tio3-based-lead-dgukm2rprl.pdf

Field-induced phase transition in Bi1/2Na1/2TiO3- based lead-free piezoelectric ceramics

The structural changes of hydrothermally synthesized cubic Li 2 TiO 3 were studied in-situ during heating and cooling at the neutron powder diffractometer SPODI at FRM II, Garching. The initially cubic (F?M) phase shows a slight Li deficit, but becomes stoichiometric with increasing temperature. It transforms to a monoclinic (C2/c) structure above 300 °C and back to the same cubic (Fm3m) structure at 1155 °C. A two-phase region is observed between 300 and 500 °C, indicating a first order character of the transformation. Below 1000 °C, during the first heating, the monoclinic Li 2 TiO 3 phase is disordered due to stacking faults. Between 1160 and 1140°C, the cubic phase becomes monoclinic again upon cooling and stays well ordered down to room temperature. Anomalies in the thermal expansion of the monoclinic modification occur at around 600 °C for both, heating and cooling. These are assigned to strains building up during the first phase transformation upon heating and the confinement in the compacted ceramic sample during cooling, respectively.

Temperature-dependent structural transformations of hydrothermally synthesized cubic Li2TiO3 studied by in-situ neutron diffraction

Because of the very small scattering power of the light element, the crystal structures of metal borides that contain heavy metal atoms are difficult to determine unambiguously from X-ray diffraction data only. Using neutron diffraction methods and applying them to isotopically enriched 11B boride powders, the crystal structures of ReB2, RuB2, and OsB2 were re-determined and analysed with respect to the boron atom arrangement. In accordance with the findings from X-ray diffraction experiments, the structures exhibit corrugated boron atom layers of conjugated six-rings, either seat-like (ReB2) or boat-like (RuB2, OsB2). ReB2 crystallises in the hexagonal crystal system, space group P63/mmc (no. 194, a = 290.05(1) pm, c = 747.72(1) pm); OsB2 and RuB2 are isostructural and crystallise orthorhombically, space group Pmmn (no. 59, a = 464.479(5) pm, b = 286.515(3) pm, c = 404.560(6) pm (RuB2); a = 468.408(5) pm, b = 287.255(3) pm, c = 407.693(6) pm (OsB2)). Boron–boron distances vary between 181.7 and 189.9 pm. For RuB2 and OsB2, shortest metal-boron distances range from 217.2 to 217.3 pm, indicating a covalent interaction between metal and boron, compared to 222.7 pm for ReB2. Metal–metal distances are between 286.5 pm and 302.2 pm. All three compounds have been described as very hard or incompressible materials.

/pdf/crystal-structures-of-the-metal-diborides-reb2-rub2-and-osb2-n929hpt5jl.pdf

Crystal structures of the metal diborides ReB2, RuB2, and OsB2 from neutron powder diffraction

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Field-induced phase transition in Bi1/2Na1/2TiO3- based lead-free piezoelectric ceramics

Temperature-dependent structural transformations of hydrothermally synthesized cubic Li2TiO3 studied by in-situ neutron diffraction

Crystal structures of the metal diborides ReB2, RuB2, and OsB2 from neutron powder diffraction