Timothy P. Comyn

Electric-field-induced phase switching in the lead free piezoelectric potassium sodium bismuth titanate

High-energy synchrotron radiation has been used to demonstrate an electric-field-induced phase transformation in the ceramic xK1/2Bi1/2TiO3–(1−x)Na1/2Bi1/2TiO3, x=0.2. Application of an electric field ≥2u2002kVu2009mm−1 leads to a transformation from single phase rhombohedral to a mixture of tetragonal and rhombohedral phases. The texturing of the tetragonal phase increases with field strength. The electric-field-induced transformation suggests an excursion into the morphotropic phase boundary in this system. The compositions x=0.1 and 0.3 reveal rhombohedral and tetragonal symmetry, respectively, both before and after the application of an electric field.

High-temperature (1−x)BiSc1∕2Fe1∕2O3-xPbTiO3 piezoelectric ceramics

Perovskite-structured ceramics in the (1−x)BiSc1∕2Fe1∕2O3-xPbTiO3 (BSF-PT) system were fabricated for x⩾0.45 in which a morphotropic phase boundary (MPB) occurred between rhombohedral and tetragonal phases at x≈0.50(TC=440°C). Samples close to the MPB gave piezoelectric and electromechanical coupling coefficients of ∼300pC∕N and 0.5, respectively. The low cost of BSF-PT in comparison to (1−x)BiScO3-xPbTiO3 (BS-PT) coupled with its high TC and usable piezoelectric properties suggests future commercial exploitation.

Investigation of high Curie temperature (1−x)BiSc1−yFeyO3–xPbTiO3 piezoelectric ceramics

Ceramics around the morphotropic phase boundary (MPB) in the (1−x)BiSc1−yFeyO3–xPbTiO3 solid solution were fabricated. For y=0.5, ceramics were single phase, and piezoelectric coefficients (d33) and electromechanical coupling coefficients (kp) for MPB compositions were 300 pC/N and 0.49, respectively; a level of piezoelectric activity similar to that of hard, lead zirconate titanate compositions but with TC∼60u2009°C higher at ∼440u2009°C. For ceramics with y≥0.7, dielectric measurements in combination with diffraction contrast transmission electron microscopy revealed the existence of two ferroelectric phases for most PbTiO3 contents studied. The presence of two ferroelectric phases was associated with a decrease in piezoelectric activity and although raw materials costs for y=0.7 and 0.8 with respect to y=0 were significantly lower (less Sc2O3) and TC greater (∼500u2009°C), d33 (∼100u2002pC/N) and kp (0.18) were too low to be commercially useful for actuator applications.

Piezoelectric materials for high temperature transducers and actuators

Piezoelectric sensors and actuators are a mature technology, commonplace amongst a plethora of industrial fields including automotive, maritime and non-destructive testing. However the environments that these devices are required to serve in are becoming more demanding, with temperatures being driven higher to increase efficiencies and reduce shut-downs. Materials to survive these temperatures have been the focus of many research groups over the last decade, but there still remains no standard for the measurement of piezoelectric materials at high temperature. This is required to effectively determine comparable Figures of Merit into which devices can be successfully designed. As part of a recent European effort to establish metrological techniques for high temperature evaluation of electro-mechanical properties, we present here a review of the most promising high temperature polycrystalline materials. Where their properties allow operation above that of the ubiquitous commercial material lead zirconate titanate, as well as work done to modify a promising high temperature system, for use as a material standard.

Shift in Morphotropic Phase Boundary in La-Doped BiFeO3–PbTiO3 Piezoceramics

Polycrystalline BiFeO3–PbTiO3 (BF–PT) powders with mixed tetragonal and rhombohedral crystal structure (morphotropic phase boundary: MPB) were modified with lanthanum to provide a wide variation in tetragonal distortion. X-ray diffraction from both the powder and the corresponding bulk ceramic demonstrated that the MPB in the bulk is shifted from 1–5 mol % towards the tetragonal PT as compared to the powder. This shift was correlated with the degree of tetragonal distortion as quantified by c/a ratio.

Phase diagram and structure-property relationships in the lead-free piezoelectric system: Na 0.5 K 0.5 NbO 3 -LiTaO 3

A phase-diagram for the Na_0.5K_0.5NbO₃-LiTaO₃ solid solution series (NKN-LT) is presented for compositions ≤10 mol% LT, based on the combined results of temperature-variable X-ray powder diffraction and dielectric measurements. In addition to the reported orthorhombic and tetragonal polymorphs of NKN-LT, a monoclinic phase is revealed. Changes to electrical properties as a function of LT substitution are correlated to phase content. Increasing the LT content from 5 to 7 mol% LT led to improved temperature stability of piezoelectric properties because of the avoidance of the monoclinic-tetragonal polymorphic phase transition during thermal cycling (at >;25°C). For 7 mol% LT samples: d₃₃ = 200 pC/N; T_c = 440°C; ε_r = 550 and tan δ = 0.02 (at 20°C). Modification of this composition by solid solution with BiScO₃ led to a decrease in d₃₃ values. Transmission electron microscopy of a sample of 0.95[0.93 NKN-0.07LT]-0.05BiScO₃ indicated a core-shell grain structure which led to temperature-stable dielectric properties.

Observation of a time-dependent structural phase transition in potassium sodium bismuth titanate

High-energy synchrotron diffraction has been employed to demonstrate a time-dependent structural phase transition in 0.2K1/2Bi1/2TiO3–0.8Na1/2Bi1/2TiO3. It is demonstrated that the rapid increase in electric field (≥0.25u2002kVu2009mm−1u2009s−1) induces a transformation from rhombohedral to mixed phase rhombohedral and tetragonal symmetry. When the electric field is applied slowly (<0.25u2002kVu2009mm−1u2009s−1), no transformation occurs, and rhombohedral symmetry is maintained. The extent of transformation increases with the rate of increase in electric field. High-speed capture of diffraction images (7 frames per second) has been used to show the evolution of the phase transformation as a function of time.

Electron backscatter diffraction mapping of herringbone domain structures in tetragonal piezoelectrics

Herringbone domain structures have been mapped using electron backscatter diffraction in two tetragonal piezoelectrics: lead zirconate titanate [Pb(Zr,Ti)O3] and bismuth ferrite–lead titanate [(PbTi)0.5(BiFe)0.5O3]. Analysis of the domain misorientations across the band junctions shows that the structures correspond very well to crystallographic models. High resolution mapping with a 20nm step size allowed the crystal rotation across one of these band junctions in lead zirconate titanate to be studied in detail and allowed an improved estimation of the peak strain at the junction, of 0.56GPa. The significance of this for crack nucleation and propagation in such materials is discussed.

Tailoring the structure and piezoelectric properties of BiFeO3-(K0.5Bi0.5)TiO3–PbTiO3 ceramics for high temperature applications

There is a growing requirement for piezoelectric materials and systems which can operate in extreme environments, for example, oil & gas, and aerospace. Here, we present the high temperature BiFeO3-K0.5Bi0.5TiO3-PbTiO3 (BF-KBT-PT) polycrystalline perovskite system. X-ray diffraction, impedance analysis, and Berlincourt measurements reveal a large region of phase coexistence, which can be tailored to optimise performance; Tc and the tetragonal spontaneous strain correlate strongly with the PbTiO3 concentration. The highest temperature composition has a d33 of 140 pmV−1 with a Tcu2009=u2009542u2009°C, occupying previously unchartered territory on the classical d33–TC plot.

Strain-relief by single dislocation loops in calcite crystals grown on self-assembled monolayers

Most of our knowledge of dislocation-mediated stress relaxation during epitaxial crystal growth comes from the study of inorganic heterostructures. Here we use Bragg coherent diffraction imaging to investigate a contrasting system, the epitaxial growth of calcite (CaCO3) crystals on organic self-assembled monolayers, where these are widely used as a model for biomineralization processes. The calcite crystals are imaged to simultaneously visualize the crystal morphology and internal strain fields. Our data reveal that each crystal possesses a single dislocation loop that occupies a common position in every crystal. The loops exhibit entirely different geometries to misfit dislocations generated in conventional epitaxial thin films and are suggested to form in response to the stress field, arising from interfacial defects and the nanoscale roughness of the substrate. This work provides unique insight into how self-assembled monolayers control the growth of inorganic crystals and demonstrates important differences as compared with inorganic substrates.