Jiwei Zhai

Dielectric properties and relaxor behavior of rare-earth (La, Sm, Eu, Dy, Y) substituted barium zirconium titanate ceramics

Based on the Ti-vacancy defect compensation model, (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 (Ln=La,Sm,Eu,Dy,Y) ceramics have been fabricated via the conventional solid-state reaction method. The microstructures, dielectric properties, and ferroelectric relaxor behavior of (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 ceramics have been investigated. The results indicate that rare-earth ions with various ionic radii enter the unit cell to substitute for A-site Ba2+ ions and inhibit the grain growth. The typical ferroelectric relaxor behavior is induced due to the rare-earth ions substitution. The diffuseness of the phase transition and the degree of ferroelectric relaxor behavior are enhanced, the TC is remarkably shifted to lower temperature, and the tunability is suppressed with the increase of x value and substituted ionic radius for (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 (x=0.005–0.04, Ln=La,Sm,Eu,Dy,Y) ceramics. Tunable ferroelectric materials with moderate dielectric constant and low dielectric loss can be obtained by manipulating the doping am...

Dielectric tunable properties of low dielectric constant Ba0.5Sr0.5TiO3–Mg2TiO4 microwave composite ceramics

Ba0.5Sr0.5TiO3–Mg2TiO4 composite ceramics are fabricated via the conventional solid-state reaction method. The microstructures, dielectric tunability, and microwave properties of composite ceramics are investigated. The dielectric constant is tailored from 335 to 35 by manipulating the addition of Mg2TiO4 from 50% to 80% weight ratio and the tunability is 10.8% measured at 10kHz for the 80% Mg2TiO4 addition. The composite ceramics with high Q value (>200) at L band are useful for potential tunable microwave device applications in the wireless communication system.

Improving the dielectric constant and energy density of poly(vinylidene fluoride) composites induced by surface-modified SrTiO3 nanofibers by polyvinylpyrrolidone

Compared to the spherical ceramic fillers, the one-dimensional ceramic fillers are more effective in enhancing the dielectric constant of the composites at low concentration because the depolarization factor is strongly dependent on the aspect ratio of the fillers. Moreover, their smaller specific surface can help to reduce the surface energy and thus prevent the nanofillers from agglomerating in the polymer matrix. The ceramic–polymer composites consisting of SrTiO3 nanofibers (ST NF) with a large aspect ratio prepared via electrospinning and modified by polyvinylpyrrolidone (PVP) as fillers and poly(vinylidene fluoride) (PVDF) as matrix have been fabricated by a solution casting method. The composites exhibit enhanced dielectric constant and reduced loss tangents at a low volume fraction of surface-modified ST NF. The composite containing 2.5 vol% of PVP modified ST NF exhibits an energy density as high as 6.8 J cm−3 at 3800 kV cm−1, which is more than double of the pure PVDF of 2.8 J cm−3 at 4000 kV cm−1. Moreover, the efficiency of the composites with 2.5 vol% PVP modified ST NF is higher than 85% at electric fields below 1000 kV cm−1 and still higher than 60% at an electric field of 3800 kV cm−1. Such significant enhancement is closely related to combined effects of the surface modification, large aspect ratio and paraelectric behavior of the ST NF.

Dielectric nonlinear characteristics of Ba(Zr0.35Ti0.65)O3 thin films grown by a sol-gel process

Ba(Zr0.35Ti0.65)O3 (BZT) thin films were deposited via a sol-gel process on Pt-coated silicon substrates. The BZT films were in the perovskite phase and had polycrystalline structure. Temperature-dependent dielectric measurements revealed that the thin films have relaxor behavior and diffuse phase transition characteristics. The tunability K of the dielectric constant (at 600 kV/cm) is about 40% in the temperature range of 179–293 K. The improved temperature stability from this BZT thin film is beneficial to applications requiring a wide range of operating temperatures, thereby eliminating the need for environmental controls. Although the K value is not extraordinarily large compared to (Ba,Sr)TiO3, the low dielectric constant of BZT is attractive for microwave frequency applications. This provides an additional possibility in balancing K and dielectric constant through materials engineering for optimum device performance.

Switching of morphotropic phase boundary and large strain response in lead-free ternary (Bi0.5Na0.5)TiO3–(K0.5Bi0.5)TiO3–(K0.5Na0.5)NbO3 system

In this work, we report the phase diagram of lead-free ternary (1 - y)[(1 - x)(Bi0.5Na0.5) TiO3-x (Bi0.5K0.5)TiO3]-y(K0.5Na0.5)NbO3 (BNT-BKT-KNN) system and study the switching characteristics of the morphotropic phase boundary (MPB). The addition of KNN intrinsically changes the structural nature of the system with the shift of MPB from MPB(I) between ferroelectric rhombohedral and ferroelectric tetragonal phases to MPB(II) between ferroelectric rhombohedral and relaxor pseudocubic phases. As the MPB(I) switches to MPB(II), large piezoelectric response with d(33) similar to 150pC/N that obtained for BNT-0.20BKT near MPB(I) almost disappears. Instead, a significant jump of electric-filed-induced strain S up to 0.32%-0.46% (S-max/E-max = 400-575 pm/V) is achieved near MPB(II) due to the shift of the ferroelectric-relaxor transition temperature TF-R down to room temperature. In this study, giant strain similar to 0.46% occurs in a very narrow region in the BNT-BKT-KNN system with x = 0.20, y = 0.01, which lies on an underlying tricritical triple point of a rhombohedral (R), tetragonal (T), and pseudocubic (Pc) phases. In-situ high-energy X-ray scattering experiments with external electric field reveal an initial electric-field-induced distortion from the Pc structure for the MPB(II) compositions, while those with single R phase shows no such distortion, which suggests that the large strain achieved near the MPB(II) is likely to be induced by the electric-field-induced structure distortion due to its relative instability structure. We believe that the discovery of a compositional line in the pseudo-ternary system, where the strain response is consistently derivable, should be useful for designing high-performance piezoelectric materials in other BNT-based systems by searching MPBs

Excellent energy density of polymer nanocomposites containing BaTiO3@Al2O3 nanofibers induced by moderate interfacial area

Inorganic/polymer nanocomposites, using one-dimensional (1D) core–shell structure BaTiO3@Al2O3 nanofibers (BT@Al2O3 nfs) as fillers and poly(vinylidene fluoride) (PVDF) as the polymer matrix, have been prepared. The core–shell structure BT@Al2O3 nfs have been synthesized via coaxial electrospinning. The breakdown strength (Eb) and discharged energy density of the nanocomposites can be significantly improved by creating an insulating Al2O3 shell layer with moderate dielectric constant on the surfaces of BT nanofibers to form a moderate interfacial area. The Al2O3 shell layer could effectively confine the mobility of charge carriers, which reduces energy loss by reducing the Maxwell–Wagner–Sillars (MWS) interfacial polarization and space charge polarization between the fillers and the polymer matrix. As a result, the nanocomposite films filled with 5 vol% BT@Al2O3 nfs exhibit a excellent discharge energy density of 12.18 J cm−3 at 400 MV m−1, which is ≈254% over bare PVDF (4.8 J cm−3 at 350 MV m−1) and ≈1015% greater than the biaxially oriented polypropylenes (BOPP) (≈1.2 J cm−3 at 640 MV m−1). The work here indicates that this promising state-of-the-art method of preparing high energy density nanocomposites can be used in the next generation of dielectric capacitors.

Phase transitions, relaxor behavior, and large strain response in LiNbO3-modified Bi0.5(Na0.80K0.20)0.5TiO3 lead-free piezoceramics

The effect of LiNbO3 (LN) addition on the ferroelectric behavior and piezoelectric properties of Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT20) lead-free piezoceramics were systematically investigated. Results showed that the LN substitution into BNKT20 induced a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to relaxor pseudocubic phases, which is accompanied by the significant disruption of ferroelectric order and with the shift of the ferroelectric-relaxor transition temperature TF-R down to room temperature. Accordingly, a large accompanying normalized strain of ∼0.38% (corresponding to a large signal d33* of ∼475 pm/V) were obtained in BNKT20 with 2.5 mol. %LN addition near the phase boundary. Temperature-dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation. Moreover, an attractive property for application as hi...

Significantly enhanced dielectric property in PVDF nanocomposites flexible films through a small loading of surface-hydroxylated Ba0.6Sr0.4TiO3 nanotubes

Polymer composite flexible films with high dielectric constant are highly desirable in electronic and electrical industry. Higher loading of the ceramic fillers is usually needed in order to realize high dielectric constant. However, such composites exhibit low breakdown strength and poor mechanical and processing properties. In this work, by incorporating high aspect ratio surface-hydroxylated Ba0.6Sr0.4TiO3 nanotubes (BST NT) prepared via electrospinning into a polyvinylidene-fluoride (PVDF) matrix, PVDF nanocomposite flexible films with high dielectric constant have been successfully obtained. The nanocomposite containing 10 vol% BST NT-OH has a dielectric constant of 48.2 at 1 kHz, which is 6.1 times higher than that of the pure PVDF (7.9). The dielectric properties of the composites are closely related to the combined effects of the surface modification, large aspect ratio, high surface area and paraelectric polarization behavior of the BST NT.

Surface-modified Ba(Zr0.3Ti0.7)O3 nanofibers by polyvinylpyrrolidone filler for poly(vinylidene fluoride) composites with enhanced dielectric constant and energy storage density

Ferroelectric-relaxor behavior of Ba(Zr0.3Ti0.7)O3 nanofibers (BZT NF) with a large aspect ratio were prepared via electrospinning and surface modified by PVP as dielectric fillers. The nanocomposite flexible films based on surface modified BZT NF and polyvinylidene fluoride (PVDF) were fabricated via a solution casting. The results show that the surface-modified BZT NF fillers are highly dispersed and well integrated in the PVDF nanocomposites. The nanocomposites exhibit enhanced dielectric constant and reduced loss tangents at a low volume fraction of surface-modified BZT NF. The polymer nanocomposites maintain a relatively high breakdown strength, which is favorable for enhancing energy storage density in the nanocomposites. The nanocomposite containing of 2.5 vol. % of PVP modified BZT NF exhibits energy density as high as 6.3 J/cm3 at 3800 kV/cm, which is more than doubled that of the pure PVDF of 2.8 J/cm3 at 4000 kV/cm. Such significant enhancement could be attributed to the combined effects of the surface modification and large aspect ratio of the BZT NF. This work may provide a route for using the surface modified ferroelectric-relaxor behavior of ceramic nanofibers to enhance the dielectric energy density in ceramic-polymer nanocomposites.

Phase transitional behavior and electric field-induced large strain in alkali niobate-modified Bi0.5(Na0.80K0.20)0.5TiO3 lead-free piezoceramics

The effect of (KyNa1-y)NbO3 (KyNN) addition on the ferroelectric stability and consequent changes in the electrical properties of lead-free Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT20) piezoceramics were systematically studied. Results showed that the KyNN substitution into BNKT20 induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long-range ferroelectric order, and correspondingly adjusts the ferroelectric-relaxor transition point TF-R to room temperature. Accordingly, a large electric-field-induced strain of 0.33%–0.46% (Smax/Emax = 413–575 pm/V), which is derived from a reversible field-induced ergodic relaxor to ferroelectric phase transformation, was obtained in KyNN-modified compositions near the phase boundary. Moreover, a relationship between the position of the ferroelectric-relaxor phase boundary and the tolerance factor t of the end member KyNN was found in BNKT20–xKyNN system, which is expected to provide...