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The past two decades have seen a great enhancement of piezoelectric coefficients (d33) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant‐graded ceramic exhibits an outstanding d33 of 508 pC/N and a very large piezoelectric strain (Suni, of 0.18%). More importantly, an excellent temperature stability (d33 variation within 13% over the temperature range of 25–150 °C) is achieved, which is superior to that of most state‐of‐art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell–Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high‐performance piezoelectric materials with desirable temperature reliability. [ABSTRACT FROM AUTHOR]

Environmentally friendly BiFeO3 capacitors have great potential for applications in pulsed-discharge and power conditioning electronic systems because of their excellent intensity of spontaneous polarization (Ps). Herein, (0.7−x)BiFeO3-0.3BaTiO3-xNaTaO3 + 0.3 wt% MnO2 (abbreviated as BF-BT-xNT) multilayer ceramic capacitors (MLCCs) were designed and prepared to improve the energy storage performance via enhancing ion bonding and dielectric relaxation. The BDS of BF-BT-xNT MLCCs from 500 kV cm−1 at x = 0.05 increases greatly to 800 kV cm−1 at x = 0.15 due to the increase of the band gap and resistance, and the decrease of dielectric permittivity. An excellent energy storage density Urec = 9.1 J cm−3 and efficiency η > 80% were obtained since ultrahigh BDS (780 kV cm−1) and low Pr value (2.1 μC cm−2 at measured electric field 780 kV cm−1) were achieved simultaneously in BF-BT-xNT multilayer capacitors at x = 0.12. This work provides a strategy for improving energy storage properties of BiFeO3, which is via enhancing ionic bonding and relaxor behavior to achieve high BDS, low Pr and large Pmax, simultaneously. [ABSTRACT FROM AUTHOR]

The enhanced piezoelectric and ferroelectric properties of the tetragonal 0.67BiFeO3–0.33BaTiO3 (BF–33BT) ceramics without MnO2 doping or other dopants via tailoring sintering temperature (TS) and dwell time (td) are reported. All ceramics crystallized in a single tetragonal phase, independent of the adopted TS and td. As the TS or td is increased, the BF–33BT ceramics show an increase in grain size and in relative density, while TS > 980 °C or td > 6 h would result in more oxygen vacancies caused by Bi3+ evaporation and/or reduction of Fe3+ to Fe2+, along with a reduced relative density and an increased current leakage density. Because of the increased grain size, the higher relative density, and relatively less oxygen vacancies, high d33 = 151 pC/N and Pr = 25.9 μC/cm2 were achieved in tetragonal BF–33BT ceramics with TS = 980 °C and td = 6 h. In addition, large unipolar strain of 0.146% and d 33 ∗ =326 pm/V (40 kV/cm), as well as high TC = 427 °C were also obtained in the BF–33BT ceramics with TS = 980 °C and td = 6 h. Our studies indicate that the grain size, the relative density, and the oxygen vacancies play important roles for the piezoelectric properties of BF–33BT ceramics. [ABSTRACT FROM AUTHOR]

Antiferroelectric materials are attractive for energy storage applications and are becoming increasingly important for power electronics. Lead-free silver niobate (AgNbO3) and sodium niobate (NaNbO3) antiferroelectric ceramics have attracted intensive interest as promising candidates for environmentally friendly energy storage products. This review provides the fundamental background of antiferroelectricity with an introduction to the definition of antiferroelectricity, historical research evolution of antiferroelectric materials, and some advanced techniques for structural characterization. Meanwhile, recent progress on lead-free antiferroelectric ceramics, represented by AgNbO3 and NaNbO3, is highlighted in terms of their crystal structures, phase transitions and potential dielectric energy storage applications. Specifically, the origin of the enhanced energy storage performance is discussed from a scientific point of view. The modification approaches are then summarized for the development of new strategies to further improve the energy storage performance. This article concludes with a discussion of the remaining challenges and opportunities for further development of lead-free antiferroelectrics. [ABSTRACT FROM AUTHOR]

Anatase titania-coated bismuth ferrite nanocomposites (BiFeO3/TiO2) have been fabricated via a hydrothermal approach combined with a hydrolysis precipitation processing. Analysis of the microstructure and phase composition reveals that a core-shell BiFeO3/TiO2 structure can be formed, which results in a significant redshift in the UV-vis absorption spectra as compared to a simple mechanical mixture of BiFeO3–TiO2 nanopowders. The core-shell structured BiFeO3/TiO2 nanocomposites exhibit higher photocatalytic activity for photodegradation of Congo red under visible-light (λ>400 nm) irradiation, which should be attributed to the enhancement of the quantum efficiency by separating the electrons and holes effectively. The obtained BiFeO3/TiO2 nanocomposites can be used as potential visible-light driven photocatalysts. [ABSTRACT FROM AUTHOR]

Coarse (about 1 μm) and fine (about 100 nm) Bi2Te3 particles were synthesized by mechanical alloying for different times, respectively. Their mixtures at different ratios were consolidated by spark plasma sintering to produce nano-/microstructured composites with the same chemical compositions. The Seebeck coefficient was significantly enhanced by addition of fine particles through the potential barrier scattering. The electrical resistivity increased rapidly when the weight fraction of fine particles exceeded about 60%, probably due to an effect similar to percolation phenomenon. However, the thermal conductivity was reduced almost linearly with increasing fraction of fine particles. Because of the combined effects of the potential barrier scattering, nonlinear change in electrical property and linear change in thermal transport property, a peak value of dimensionless figure of merit (ZT) was achieved for the sample containing 60% fine particles. This study demonstrated the possibility to enhance ZT value by introducing nonuniform microstructures even without changing chemical compositions. [ABSTRACT FROM AUTHOR]

(Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 (BNT-BKT) ceramics as a promising candidate for lead-free piezoelectric ceramics were studied with a special emphasis on the compositional dependence of piezoelectric and ferroelectric properties. One mole percent excess Bi was added to compensate for the volatilization of Bi3+ ions during sintering, which was found to be effective in improving the piezoelectric properties of the resultant BNT-BKT ceramics. The piezoelectric, ferroelectric, and dielectric properties of the Bi-compensated BNT-BKT ceramics were investigated and discussed in relation to the morphotropic phase boundary that is close to the composition range of 20–24 mol % BKT. The maximum piezoelectric constant d33 (207 pC/N) and electromechanical coupling factor kp (35%) were obtained at the compositions of 23% and 20%, respectively. [ABSTRACT FROM AUTHOR]

CoSb3-xTex(x=0.05-0.3) skutterudite polycrystals with an average grain size of 160 nm were fabricated by mechanical alloying combined with spark plasma sintering. The variation of lattice parameter with Te content indicates that the solution limit of Te was x=0.15, above which the impurity phases of Te, CoTe2, and CoSb2 appeared, and the matrix cracked above 500 °C. All samples behaved as degenerate semiconductors. The forbidden energy gap was estimated to be 0.047 eV from the temperature corresponding to the occurrence of intrinsic excitation, which is in good agreement with Singh’s theoretical calculation (0.05 eV) [D. J. Singh and W. E. Pickett, Phys. Rev. B 50, 11235 (1994)]. The CoSb2.85Te0.15 sample had the highest power factor and the lowest thermal conductivity, resulting in the highest thermoelectric figure of merit, ZT=0.93 at 547 °C. The role of Te substitution in enhancing thermoelectric properties is discussed in relation to the bipolar diffusion mechanism. [ABSTRACT FROM AUTHOR]