Lead has recently been expelled from many commercial applications and materials (for example, from solder, glass and pottery glaze) owing to concerns regarding its toxicity. Lead zirconium titanate (PZT) ceramics are high-performance piezoelectric materials, which are widely used in sensors, actuators and other electronic devices; they contain more than 60 weight per cent lead. Although there has been a concerted effort to develop lead-free piezoelectric ceramics, no effective alternative to PZT has yet been found. Here we report a lead-free piezoelectric ceramic with an electric-field-induced strain comparable to typical actuator-grade PZT. We achieved this through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution, and the development of a processing route leading to highly textured polycrystals. The ceramic exhibits a piezoelectric constant d33 (the induced charge per unit force applied in the same direction) of above 300 picocoulombs per newton (pC N(-1)), and texturing the material leads to a peak d33 of 416 pC N(-1). The textured material also exhibits temperature-independent field-induced strain characteristics.

Lead-free piezoceramics

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

Ferroelectric ceramics were born in the early 1940s with the discovery of the phenomenon of ferroelectricity as the source of the unusually high dielectric constant in ceramic barium titanate capacitors. Since that time, they have been the heart and soul of several multibillion dollar industries, ranging from high-dielectric-constant capacitors to later developments in piezoelectric transducers, positive temperature coefficient devices, and electrooptic light valves. Materials based on two compositional systems, barium titanate and lead zirconate titanate, have dominated the field throughout their history. The more recent developments in the field of ferroelectric ceramics, such as medical ultrasonic composites, high-displacement piezoelectric actuators (Moonies, RAINBOWS), photostrictors, and thin and thick films for piezoelectric and integrated-circuit applications have served to keep the industry young amidst its growing maturity. Various ceramic formulations, their form (bulk, films), fabrication, function (properties), and future are described in relation to their ferroelectric nature and specific areas of application.

/pdf/ferroelectric-ceramics-history-and-technology-40j9enkrwl.pdf

Ferroelectric ceramics : History and technology

Electrical actuators were made from films of dielectric elastomers (such as silicones) coated on both sides with compliant electrode material. When voltage was applied, the resulting electrostatic forces compressed the film in thickness and expanded it in area, producing strains up to 30 to 40%. It is now shown that prestraining the film further improves the performance of these devices. Actuated strains up to 117% were demonstrated with silicone elastomers, and up to 215% with acrylic elastomers using biaxially and uniaxially prestrained films. The strain, pressure, and response time of silicone exceeded those of natural muscle; specific energy densities greatly exceeded those of other field-actuated materials. Because the actuation mechanism is faster than in other high-strain electroactive polymers, this technology may be suitable for diverse applications.

http://science.sciencemag.org/content/sci/287/5454/836.full.pdf

High-Speed Electrically Actuated Elastomers with Strain Greater Than 100%

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

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Growth and characterization of high temperature La3Nb0.5Ga5.3Al0.2O14 (LNGA) and La3Ta0.5Ga5.3Al0.2O14 (LTGA) piezoelectric single crystals

A new tellurate, BaTiTe3O9, was found in the BaO-TiO2-TeO2 ternary system Using conventional solid state reaction, the new single phase compound, BaTiTe3O9 phase was synthesized and sintered over the very low temperature range of only 625-700°C In addition, different calcinations routes, including the effect of TiTe3O8 precursor in the phase formation, were examined The dielectric properties of BaTiTe3O9 ceramics were determined in the microwave region as well as at low frequencies The relative dielectric permittivity (er) and the temperature coefficient of permittivity (TCe) of BaTiTe3O9 at 1 MHz were measured as 30 and 790 ppm/°C, respectively The dielectric properties were maintained up to microwave region without degradation The dielectric permittivity and Q×f product obtained were 29 and 1,700 GHz at 76 GHz, respectively The temperature coefficient of resonance frequency (TCF) showed a large negative value of -372 ppm/°C

Synthesis of BaTiTe3O9 ceramics for LTCC application and its dielectric properties

Large-size and high-quality ReCa4O(BO3)3 (ReCOB, Re = rare earth) single crystals were grown by the Czochralski pulling method. In this work, the electrical properties were investigated at room temperature and elevated temperature for YCa4O(BO3)3 (YCOB). The dielectric permittivity, piezoelectric strain coefficient, and electromechanical coupling were found to be on the order of 11, 6.5 pC/N, and 12.5%, respectively, with a high piezoelectric voltage coefficient around 0.067 Vm/N. The electrical resistivity of YCOB was found to be 2 times 108 Ohmmiddotcm at 800degC, with Q values of > 4,500 at 950degC. The frequency/temperature coefficient of YCOB was found to be -75 to -85ppm/K in the temperature range of 30 to 950degC, depending on the crystal orientations. Together with their temperature-independent properties, ReCOB crystals are promising candidates for sensing applications at elevated temperatures.

High temperature piezoelectric single crystal GdCa4O(BO 3)3 for sensor application

A new ternary system (1-x)BiScO3–x{(1-y)PbTiO3–y(Ba0.294Sr0.706)TiO3} was studied in relation to morphotropic phase boundaries (MPB). Analogous to modified PZT ceramics, barium and strontium were introduced to the recently developed MPB (1-x)BiScO3–xPbTiO3 for x>0.60, in order to investigate Curie temperature and piezoelectric property variations. With increasing barium and strontium substitution up to y=0.3, the MPB line moved from x=0.64 to 0.70 and d33 and kp gradually decreased. X-ray diffraction phase analysis correlated the variations in piezoelectric properties showing a maximum in the compositions near rhombohedral–tetragonal phase boundary. Grain sizes were around 2 µm for all compositions and tetragonality decreased with the downward shifting of Tc in the range of 216–371°C. The frequency dependence of dielectric properties about the Curie maximum shows a dispersion in both e' and tan δ. This relaxation behavior becomes more pronounced with increasing (Ba,Sr)TiO3 content, and high Curie maximum temperatures Tmax's are found compared to lead based perovskites with complex B-sites.

https://iopscience.iop.org/article/10.1143/JJAP.42.5181/pdf

Thomas R. Shrout

Papers

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Growth and characterization of high temperature La3Nb0.5Ga5.3Al0.2O14 (LNGA) and La3Ta0.5Ga5.3Al0.2O14 (LTGA) piezoelectric single crystals

Synthesis of BaTiTe3O9 ceramics for LTCC application and its dielectric properties

High temperature piezoelectric single crystal GdCa4O(BO 3)3 for sensor application

Piezoelectric Properties in the Perovskite BiScO3–PbTiO3–(Ba,Sr)TiO3 Ternary System