Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg1/3Nb2/3O3), which crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, ...

Relaxing with relaxors: a review of relaxor ferroelectrics

A century of diligent R&D has resulted in a wide range of ceramic dielectrics and processing technologies. The technology used to manufacture an MLCC (multilayer ceramic capacitors) that costs pennies was unimaginable 30 years ago. The present trends of enhanced mobility, connectivity, and reliability in consumer, industrial, and military electronics will continue to drive future innovations in ceramic capacitor technology. In addition, power electronics applications are an emerging market in which ceramic capacitors will play an increasing role through improved breakdown strength, enhanced dielectric stability in harsh environments, and innovative packaging. The investment made by the US government to develop high energy density and high temperature capacitor technology will also contribute to the advancement of dielectric materials technology for pulse and power electronic capacitors.

A brief introduction to ceramic capacitors

The structural and dielectric properties of (1−x)BaTiO3–xBiScO3 (x=0–0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi2O3 and BiScO3 substitutions in a BaTiO3 solid solution. The solubility limit for BiScO3 into the BaTiO3 perovskite structure was determined to be about x=0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x=0.05–0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO3 to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO3. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel–Fulcher model, which yielded an activation energy of 0.2–0.3 eV. The attempt characteristic frequency was 1013 Hz and the freezing temperature, Tf, ranged from −177° to −93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO3–BaTiO3 solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.

Weakly Coupled Relaxor Behavior of BaTiO3–BiScO3 Ceramics

The solid solutions (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3, (x)PbTiO3-(1−x)Bi(Zn1∕2Zn1∕2)O3, and (x)PbTiO3-(1−x)Bi(Zn1∕2Sn1∕2)O3, have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Unlike most PbTiO3-based solid solutions, these systems show enhanced tetragonality with substitution for PbTiO3. In particular, the (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3 system exhibits a high c∕a ratio of 1.11 for x=0.60. Accordingly, the Curie temperature (TC) also increases, exceeding 700°C at the same composition. It is proposed that PbTiO3-Bi(B)O3-type solid solutions which contain only highly polarizable cations on the B site are likely to exhibit similar enhancements in tetragonality and TC.

Enhanced tetragonality in (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3 and related solid solution systems

Experimental search for high-temperature ferroelectric perovskites is a challenging task due to the vast chemical space and lack of predictive guidelines. Here, we demonstrate a two-step machine learning approach to guide experiments in search of xBi
 $$[ {{\mathrm{Me}}_y' {\mathrm{Me}}_{(1 - y)}'' } ]$$
 O3–(1 − x)PbTiO3-based perovskites with high ferroelectric Curie temperature. These involve classification learning to screen for compositions in the perovskite structures, and regression coupled to active learning to identify promising perovskites for synthesis and feedback. The problem is challenging because the search space is vast, spanning ~61,500 compositions and only 167 are experimentally studied. Furthermore, not every composition can be synthesized in the perovskite phase. In this work, we predict x, y, Me′, and Me″ such that the resulting compositions have both high Curie temperature and form in the perovskite structure. Outcomes from both successful and failed experiments then iteratively refine the machine learning models via an active learning loop. Our approach finds six perovskites out of ten compositions synthesized, including three previously unexplored {Me′Me″} pairs, with 0.2Bi(Fe0.12Co0.88)O3–0.8PbTiO3 showing the highest measured Curie temperature of 898 K among them. Experimental search for high-temperature ferroelectric perovskites is challenging due to the vast chemical space and lack of predictive guidelines. Here the authors demonstrate a two-step machine learning approach to sequentially guide experiments in search of promising perovskites with high ferroelectric Curie temperature.

/pdf/experimental-search-for-high-temperature-ferroelectric-46h8gdh6ku.pdf

Experimental search for high-temperature ferroelectric perovskites guided by two-step machine learning

Extending the investigations on Bi-based perovskite solid solutions for high-temperature piezoelectric ceramics, this paper considers the binary solid-solution system (1−x)Bi(Ni1∕2Ti1∕2)O3–xPbTiO3 [(1−x)BNT–xPT] for 0.39⩽x⩽1.00. High-density polycrystalline ceramics were fabricated using conventional solid-state processing methods. These ceramics are then taken for structural and electrical properties and differential scanning calorimetry measurements. A morphotropic phase boundary (MPB) was found at the composition 0.51BNT–0.49PT, with a corresponding paraelectric-ferroelectric phase transition TC≈400°C. The electrical poled ceramics demonstrated piezoelectric d33 coefficients≈260pC∕N at room temperature for the MPB BNT–PT compositions. Experimental data are also given for the influence of MnO2 doping to the BNT–PT system close to the MPB compositions, noting an improvement in dielectric losses but a reduction in d33≈180pC∕N. Tricritical behavior is also identified in the tetragonal phase field, with a T...

Structure and property investigation of a Bi-based perovskite solid solution: (1−x)Bi(Ni1∕2Ti1∕2)O3–xPbTiO3

The ferroelectric transition temperature (Tc) behavior of perovskite solid solutions based on PbTiO3–Bi(Me′Me″)O3 (Me′=Fe3+, Zn2+, Sc3+, In3+, Mg2+, Ni2+, etc., and Me″=Ti4+, Nb5+, W6+) was considered. Trends in the Tc compositional dependence near the PbTiO3 end member could be described with a geometrical polynomial expression. Three main cases were observed: Case 1, a continued increase in transition temperature above the end member PbTiO3 (495°C); case 2, an increase and then decrease of the transition temperature; and case 3, a continuous decrease in the transition temperature with Bi(Me′Me″)O3 additions. It was noted that for all case 2 examples the enhancement of ΔTc=Tc(max)−Tc(PT) increased as the distribution of B-site ionic radii increased. A correlation was therefore proposed between the maximum enhancement in transition temperature and the spread of tolerance factor (Δt) and∕or variance in B-site ionic radius (σ2). Finally, it was proposed that these observations are consistent with random-fie...

Classification of transition temperature behavior in ferroelectric PbTiO3-Bi(Me'Me )O3 solid solutions

A new compositional family of relaxor ferroelectrics was investigated based on the high-temperature Bi(Me)O 3 -PbTiO 3 ferroelectric perovskite family. Compositions were fabricated near an estimated morphotropic phase boundary (MPB) of the xBiScO 3-y Pb(Mg 1/3 Nb 2/3 )O 3 -zPbTi03 (xBS-yPMN-zPT) ternary system exhibiting high-temperature relaxor properties of T max ∼250°-350°C and e max ∼10000-24000 at 1 kHz. Analysis of the low-field a.c. permittivity by a Vogel-Fulcher type dependence enabled key parameters of activation energy, E A , and freezing temperature, T f , to be determined. The remanent polarization was studied over a broad temperature range and was observed to show classical ferroelectric square loop hysteresis behavior at temperatures T T max , the deviation temperature, To, was obtained from Curie-Weiss analysis and found to be ∼600°C. A comparison of characteristic electrical properties was made between the high-temperature perovskite relaxors and the classical complex lead relaxor compound, Pb(Mg 1/3 Nb 2/3 )O 3 (PMN).

Dielectric Characteristics of Perovskite-Structured High-Temperature Relaxor Ferroelectrics: The BiScO3–Pb(Mg1/3Nb2/3)O3–PbTiO3 Ternary System

The industrial and scientific communities have expressed the need for sensing and actuation over a broad temperature range. This review presents high temperature piezoelectric materials that are commercially available and those that are under development. Key materials, in order of increasing Curie temperature (T/sub C/), are Pb(Zr,TiO)/sub 3/ (PZT), PbTiO/sub 3/, (Pb,Ba)Nb/sub 2/O/sub 6/, Na/sub 0.5/Bi/sub 4.5/Ti/sub 4/O/sub 15/, and LiNbO/sub 3/. The maximum operation temperature is limited by T/sub C/ and dielectric loss combined with the level of electrical resistivity. With increased T/sub C/ also comes the expense of reduced piezoelectric coefficient (d), being further reduced in non-morphotropic phase boundary (MPB) systems. Recently new high T/sub C/ systems with MPBs analogous to PZT have been developed. Predicted by a perovskite crystal structure tolerance factor relationship, compositions based on Bi(Me)O/sub 3/-PbTiO/sub 3/, where Me=Sc/sup +3/,(Mg/sup +2/,Ti/sup +4/), etc., exhibit piezoelectric activity compared to PZT, with T/sub C/s greater than 100/spl deg/C higher, making them promising candidates for high temperature applications.

High performance, high temperature perovskite piezoelectrics

The dielectric and piezoelectric properties for bismuth-based perovskite system (1−x)Bi(Ni2∕3Nb1∕3)O3–xPbTiO3 (BNN–PT100x) with x=06–095 were investigated High-density polycrystalline ceramics were fabricated using a conventional solid-state reaction method Morphotropic phase boundary (MPB) at x=0625–065 composition was observed by x-ray-diffraction measurements, separating rhombohedral and tetragonal phases Analogous to Pb(ZrTi)O3 ceramic, enhanced dielectric and piezoelectric activities were associated with the composition near the MPB The dielectric constant (K33T) and piezoelectric coefficient (d33) for BNN–PT65 composition were found to be 1100 and 140pC∕N, respectively, with a Curie temperature (Tc) around 273 °C The addition of manganese (Mn) resulted in lowering the dielectric loss and increasing the mechanical quality factor Q when compared with the pure counterpart The coercive field was found to increase to 307kV∕cm when BNN-PT65 was modified with magnesium (Mg) These results demons

Investigation of bismuth-based perovskite system: (1−x)Bi(Ni2∕3Nb1∕3)O3–xPbTiO3

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