For ultrasonic transducers, piezoelectric ceramics offer a range of dielectric constants (K/spl sim/1000-5000), large piezoelectric coefficients (d/sub ij//spl sim/200-700 pC/N), and high electromechanical coupling (k/sub t//spl sime/50%, k/sub 33//spl sime/75%). For several decades, the material of choice has been polycrystalline ceramics based on the solid solution Pb(Zr/sub 1-x/B/sub 2x/)O/sub 3/ (PZT), compositionally engineered near the morphotropic phase boundary (MPB). The search for alternative MPB systems has led researchers to revisit relaxor-based materials with the general formula, Pb(B/sub 1/,B/sub 2/)O/sub 3/ (B/sub 1/:Zn/sup 2+/, Mg/sup 2+/, Sc/sup 3+/, Ni/sup 2+/..., B/sub 2/:Nb/sup 5+/ Ta/sup 5+/...). There are some claims of superior dielectric and piezoelectric performance compared to that of PZT materials. However, when the properties are examined relative to transition temperature (T/sub 3/), these differences are not significant. In the single crystal form, however, Relaxor-PT materials, represented by Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZN-PT), Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMN-PT) have been found to exhibit longitudinal coupling coefficients (k/sub 33/)>90%, thickness coupling (k/sub t/)>83%, dielectric constants ranging from 1000 to 5000 with low dielectric loss 2000 pC/N, the later promising for high energy density actuators. For single crystal piezoelectrics to become the next generation material of ultrasonic transducers, further investigation in crystal growth, device fabrication and testing are required.

Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers

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

Environment-friendly lead-free piezoelectric ceramics have been studied extensively in the past decade with great progress particularly in systems based on a niobate perovskite compound formulated as (K, Na)NbO3 (abbreviated as KNN). A comprehensive review on the latest development of KNN-based piezoelectric ceramics is presented in this article, including the phase structure, property enhancement approaches, and sintering processes as well as the status of some promising applications. The phase structure of KNN was reexamined and associated with the effect of chemical modification on its tetragonal-to-orthorhombic transition. Then, a special focus is placed on the temperature dependence of piezoelectric properties of KNN-based ceramics, followed by reviewing the recent approaches devoted to the temperature-stability enhancement. The processing fundamentals related to the sintering of KNN-based ceramics are also presented with an emphasis on compositional and microstructural control. Finally, this review introduces several industrial attempts of traditional piezoceramic products using KNN-based ceramics and the studies on some promising application in authors' laboratory.

(K, Na)NbO3-Based Lead-Free Piezoceramics: Fundamental Aspects, Processing Technologies, and Remaining Challenges

Semiconducting BaTiO 3 can be prepared by substituting small amounts of ions of higher valency for Ba or Ti ions. At higher concentrations the foreign ions are compensated by metal ion vacancies. Polycrystalline samples prepared in air show an enormous increase in resistivity above the ferro-electric Curie point. According to Heywang this is a result of surface barriers which are very sensitive to the value of dielectric constant. This theory is extended with a ferro-electric effect.

Some aspects of semiconducting barium titanate

https://dr.ntu.edu.sg/bitstream/10356/101824/1/A%20comprehensive%20review%20on%20the%20progress%20of%20lead%20zirconate-based%20antiferroelectric%20materials.pdf

A comprehensive review on the progress of lead zirconate-based antiferroelectric materials

Thermal and x‐ray diffraction measurements on compositions from the system NaxK1−xNbO3 have revealed the presence of phase boundaries at approximately 825, 675, and 525 mole × NaNbO3 at 25°C, in addition to one very near the NaNbO3 end member The optimum piezoelectric properties in this system are apparently associated with compositions near the latter boundary Calorimetric determinations of the enthalpy and entropy changes associated with the ferroelectric→ferroelectric and ferroelectric→paraelectric transformations in the system clearly revealed the presence of the phase boundaries while measurements of the transformation temperatures alone did not readily reveal the boundaries Two different superlattice perovskite phases were observed in the system, one existing in the region ∼098≥x≥0825 and another in the region 0825≥x≥0675 No multiple‐cell structures were observed in the system at NaNbO3 concentrations below about 70 mole%

Thermal and X‐Ray Diffraction Studies of the NaNbO3–KNbO3 System

The effect of additions of 0.0015 to 0.0030 mole fraction of rare earth oxides on the d-c resistivity of sintered barium titanate was investigated. The substitution may be represented by (X/sub 2/O/sub 3/)M (BaTiO/sub 3/)/sub 1-M/, where X is the rare earth. The rare earths samarium, gadolinium, and holmium were introduced singly into the titanate, and the resistivity was measured as a function of temperature from -170 deg to +330 deg C. An anomalous increase near the tetragonal -- cubic transition temperature at 120 deg C occurred which in some cases amounted to an increase in the resistivity of 4000 times the value in the tetragonal phase. The thermoelectric power of the material changed sign at the Curie temperature. The tetragonal phase exhibited n-type behavior whereas the cubic phase was p-type. The rhombohedral and orthorhombic phases exhibited conduction activation energies of the order of 0.2 ev whereas that in the tetragonal phase was approximately 0.1 ev. (auth)

Investigation of Rare‐Earth Doped Barium Titanate

A high-temperature X-ray diffraction study of a high-purity PbZrO3 specimen at 25° to 236°C is reported. The symmetry of the perovskite sub-cell of PbZrO3 was determined unambiguously by observing the splitting of the line groups N= 12 and N= 16 when the diffraction pattern was indexed on the basis of the perovskite cell being a unit cell for the structure. Diffractometer studies were conducted of the N= 12 line group using Fe Kα radiation and of the N= 16 line group using Cu Kα radiation. The ferroelectric phase of PbZrO3 possessed a rhombohedral sub-cell with the angle α being acute. Stable two-phase mixtures and considerable thermal hysteresis were observed at the antiferroelectric α ferroelectric transition temperature. A very weak “extra” diffraction line was observed in the diffraction patterns of the ferroelectric and paraelectric phases, indicating that the unit cells of these structures may be multiples of the perovskite subcell.

High‐Temperature Phase Transitions in PbZrO3

Dielectric measurements and X-ray diffraction studies were made in the system BaTiO3-BaHfO3 for polycrystalline specimens containing from 0 to 30 mole % BaHfO3. The maximum in the dielectric constant for each composition in creased as the BaHfO3 concentration was in creased to 16 mole % and then decreased with further BaHfO3 additions. Room-temperature lattice constants and hysteresis loops were meas ured for all compositions. A partial solid-state phase diagram is suggested for the solid solution area of the system. The phase diagram and the dielectric behavior of the system up to 16 mole % BaHfO3 are explained on the basis of Devonshire's thermodynamic theory. The ferroelectric-paraelectric transition in the composition containing 16 mole % BaHfO3 was essentially of second order and occurred between a ferroelectric rhombohedral phase and a paraelectric cubic phase.

Dielectric and Structural Investigations of the System BaTiO3‐BaHfO3

Extensive solid solution was observed in the system Pb(Sc1/2/,Nb1/2,)1-x,Tix,O3. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO3−PbTiO3. The compound Pb(Sc1/2Nb1/2)O3 exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc1/2Nb1/2)0.575Ti0.4250O3. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.

Victor J. Tennery

Papers

Thermal and X‐Ray Diffraction Studies of the NaNbO3–KNbO3 System

Investigation of Rare‐Earth Doped Barium Titanate

High‐Temperature Phase Transitions in PbZrO3

Dielectric and Structural Investigations of the System BaTiO3‐BaHfO3

Ferroelectric and Structural Properties of the Pb(Sc1/2Nb1/2)1-x,Tix,O3 System