Electrocaloric properties of high dielectric constant ferroelectric ceramics

In the last two decades, the electrocaloric (EC) effect which is associated to the temperature (θ) dependence of the macroscopic polarization P(E, θ) under electric field E has been spasmodically studied in ferroelectric materials in order to find an alternative to the classical refrigeratory devices using freon. Basically, large electrocaloric temperature variation ΔT originates from electric field-induced phase transition at the Curie temperature, but temperature changes of the sample are difficult to measure and depend on the experimental conditions. In this paper, the electrocaloric effect has been quantified directly and precisely by measuring the thermal energy exchanged under isothermal conditions using a modified Differential Scanning Calorimetry (DSC) apparatus. The DSC technique allowed to compare the EC properties of high-dielectric-constant (ɛ) ceramics in the vicinity of ferroelectric–paraelectric phase transition. The measurements were also simulated starting from polarization versus electric field hysteresis loops for different temperatures. It is shown excellent agreement between simulations and direct DSC measurements, except in a limited temperature range where the hysteresis of the polarization versus temperature is high.     

Structure,dielectric, electrostrictive and electrocaloric properties of environmentally friendly Bi-substituted BCZT ferroelectric ceramics

In this study, environmentally friendly Bi-substituted [(Ba_0.85Ca_0.15)_1-3x/2Bi_x](Zr_0.1Ti_0.9)O₃ (BCZT-xBi) ferroelectric ceramics with x = 0–0.08 were prepared using a solid-state sintering method. The structures, dielectric, electrostrictive and electrocaloric properties of the Bi-substituted BCZT ceramics were thoroughly investigated. With an increase in the Bi³⁺ content, the temperature corresponding to the maximum permittivity (T_m) decreased monotonously. Meanwhile, the broadening of the dielectric peaks and the increase in the relaxation coefficient of the ceramics transformed their typical ferroelectric-to-paraelectric phase transition to diffuse phase transition (DPT). This was further confirmed by the trends shown by the ferroelectric properties of the ceramics. The current peak intensity in current-electric field (I-E) curves decreased with an increase in x and finally became constant, indicating that domain reversal disappeared gradually. High electrostrictive coefficient Q₃₃ values of 0.0307, 0.0299 and 0.0223 m⁴/C² were obtained for the ceramics with x = 0.02, 0.04 and 0.06 respectively. The Q₃₃ values of the ceramics indicated their temperature-insensitive nature over the temperature range of 30–120 °C. Although the maximum value of the electrocaloric adiabatic temperature change (ΔT) (0.91 K) was achieved at x = 0.02, the thermally stability of ΔT is improved with an increase in x from 0.02 to 0.06. The results indicated that x = 0.06 improved the thermal stability of the electrostrictive and electrocaloric performance. By increasing the driving field strength, better electrostrictive and electrocaloric responses could be achieved.     

Effect of grain size on piezoelectric,ferroelectric and dielectric properties of PMN-PT ceramics

Grain size has significant effects on the electromechanical response of piezoceramics. Since the sintering temperature and holding duration affect the grain size and correspondingly their effective piezoelectric response, these issues should be appropriately addressed in order to properly design smart and intelligent systems. In this paper, PMN-PT piezoceramics with different grain sizes are synthesized by using conventional solid state processing and two-stage sintering method. Grain size effects on the hysteresis, fatigue and creep response are analyzed. Dielectric, ferroelectric and piezoelectric material properties of PMN-PT ceramics are investigated. The piezoelectric coefficient and remnant polarization exhibit diverse grain size effects depending on the sintering temperature and holding duration. The relative dielectric permittivity, piezoelectric coupling constant and dielectric constant show maximum values of 4104.2, 559 pC/N 562.291 pC/N and 0.705 for the microstructure with average grain size of 3.8?μm. The myriad effects of grain size on piezoelectric response are reported in details.     

Fabrication and ferroelectric/dielectric properties of La-doped PMN-PT ceramics with high optical transmittance

Relaxor ferroelectric 0.75(Pb_1–3x/2La_x)(Mg_1/3Nb_2/3)O₃-0.25(Pb_1–3x/2La_x)TiO₃ (La³⁺:PMN-PT x/75/25, where x=2.8, 3.0, 3.5, and 4.0 mol% of La³⁺) transparent ceramics were fabricated by the combination of oxygen atmosphere pressureless sintering and hot-pressing sintering process. The optical transmittances of above four ceramics are higher than 60% at the wavelength of 500–900 nm. La³⁺:PMN-PT 3.0/75/25 exhibits the highest transparency around 70% at 900 nm which is very close to the theoretical transmittance 71%. Each of the four ceramics exhibits the pure perovskite phases. They show fully dense microstructures and their relative densities are higher than 99.8%. The ferroelectric and dielectric measurements indicate that these four ceramics exhibit relaxation characteristics. With increasing La³⁺ content, (200) peak in XRD patterns shifts to higher angles and the average grain size increases, while the temperature Tε_max corresponding to the maximum ε_r, the remanent polarizations P_r and coercive fields E_c decrease gradually.     

Study of structural,dielectric, ferroelectric and magnetic properties of vanadium doped BCT ceramics

In this paper, the structural, microstructural, dielectric, ferroelectric and magnetic properties of vanadium-doped Barium Calcium Titanate (BCT) ceramics have been reported. All the ceramic samples were prepared by the conventionalsolid-state reaction method. By Rietveld refinement of X-Ray diffraction patterns, we confirm the pervoskite structure of all the prepared samples. A decrease has been observed in lattice parameters (a and c) and lattice volume on the substitution of Ti⁴⁺ ions by vanadium ions in BCT lattice. SEM micrographs revealed change in microstructure from irregular, large grain porous structure to a regular small and compact structure. The dielectric constant data above T_c has been fitted with both Curie-Weiss Law and Power Law. The dielectric behavior of the presently studied system is more accurately explained by Power law. Feeble ferromagnetism has also been observed with vanadium substitution in BCT ceramics.     

Effect of Fe substitution on the piezoelectric,dielectric and ferroelectric properties of PNZST ceramics

Fe-doped (Pb_0.99Nb_0.02)[(Zr_0.70Sn_0.30)_0.52Ti_0.48]_0.98O₃ (PNZST) ceramics were prepared via conventional solid state reaction method, and the effect of Fe doping on their structural and electrical properties was investigated in detail. Results showed that Fe³⁺ cations could dissolve into readily the B-sites of perovskite structure for the PNZST ceramics with the less amount of Fe content (≤0.8 wt%), resulting in the full densification after sintered at 1300 °C. Meanwhile, Fe doping caused a structure transform from the tetragonal to the rhombohedral. The better electric properties for PNZST ceramic with 0.6 wt% Fe content were obtained, i.e. piezoelectric constant d₃₃=380 pC/N, electromechanical coupling factor k_p=0.57, mechanical quality factor Q_m=225, dielectric constant ε_r=1190, loss tangent tan δ=0.007 and curie temperature T_c=318 °C.     

Structural,thermal, dielectric and ferroelectric properties of K0.5Bi0.5TiO3 ceramics

Dense K_0.5Bi_0.5TiO₃ (KBT) lead-free ceramics were prepared by conventional solid reaction route. Their temperature behavior (up to 600 °C) was investigated by X-ray diffraction, DSC, dielectric spectroscopy and electric field-polarization technique. The first temperature dependent Raman scattering studies were also performed. X-ray and Raman scattering results show that samples exhibit a single perovskite structure with cubic symmetry at temperatures higher than approximately 400 °C and with coexistence of the cubic and tetragonal phases below this temperature. Two structural phase transitions between tetragonal phases in temperature range 200–225 °C and between tetragonal and cubic ones near 400 °C are observed. The content of the tetragonal phase increases with decreasing temperature and at room temperature it reaches more than 70%. Temperature- dependent P-E loops and pyroelectric data revealed a polar behavior in KBT up to about 400 °C, which means that the intermediate phase (～270–380 °C) is rather ferroelectric than antiferroelectric.     

Nonstoichiometric effect on dielectric and large-signal electromechanical properties of environmentally friendly BNT-6BT ferroelectric ceramics

Although the nonstoichiometric influence on the small-signal dielectric and piezoelectric properties of (Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectrics has been studied extensively over the past decade, the features of large-signal electric field-induced strain (electrostrain), which are of particular importance to actuator devices, have not been thoroughly investigated. In this study, we used the solid-state reaction method to manufacture nonstoichiometric 0.94(Bi_0.5+xNa_0.5?x)TiO₃-0.06BT (BNTx-6BT) ceramics, where x = 0.0–0.05, and investigated the nonstoichiometric effect on the dielectric and large-signal electromechanical properties, with special emphasis on the electrostrain properties. Our results suggest that the room-temperature phase structures of BNTx-6BT ceramics changed from a regular ferroelectric phase to a relaxor ferroelectric phase as the Bi/Na ratio increased from stoichiometric 50/50 to nonstoichiometric 55/45 owing to the nonstoichiometric effect on the long-range ferroelectric order. In the x = 0.02 nonstoichiometric composition, an ultrahigh and electrostrictive-type electrostrain of 0.53% was identified. Compared to their stoichiometric counterparts, nonstoichiometric compositions have stronger temperature stability during polarization, resulting in good temperature stability of the electrostrain. Our findings not only reveal the nonstoichiometric effect on the phase evolution and its impact on the dielectric and large-signal electromechanical properties of BNTx-6BT ceramics but also provide a new method for tailoring the large-signal electrostrain properties of BNT-based ceramics.     

Effect of La and Ni substitution on structure,dielectric and ferroelectric properties of BiFeO3 ceramics

In this communication, we present the results on Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples processed by solid-state reaction route in order to study crystalline and electronic structure, dielectric and ferroelectric properties. The best refinement was achieved by choosing rhombohedral structure (R3c) for BiFeO₃ and Bi_0.9La_0.1FeO₃ samples. Whereas, the XRD pattern of BiFe_0.95Ni_0.05O₃ and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ samples were refined by choosing rhombohedral (R3c) and cubic (I23) structure. Raman scattering measurement infers nine Raman active phonon modes for all the as prepared samples. The substitution of Ni ion at Fe-site in BiFeO₃ essentially changes the modes position i.e. all the modes are observed to shift to lower wave number. Dielectric constant (ε′) and dielectric loss (tan δ) as a function of frequency have been investigated and they decreases with increasing frequency of the applied alternating field and become constant at high frequencies. This feature is a characteristic of Maxwell Wagner type of interfacial polarization. The remnant polarization (P_r) for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.08, 0.11, 0.69 μC/cm², respectively and the value of coercive field for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.53, 0.67, 0.68 kV/cm, respectively. X-ray absorption spectroscopy (XAS) experiments at Fe L₂,₃ and O K-edges are performed to investigate the electronic structure of well-characterized Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples. The presence of reasonable ferroelectric polarization at room temperature in Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ ceramics makes it suitable for technological applications.     

Thermal stability of dielectric and energy storage performances of Ca-substituted BNTZ ferroelectric ceramics

In this study, we synthesized [Ca_x(Bi₀.₅Na_0.5)_1?x](Ti₀₈₅Zr_0.15)O₃ (Ca-substituted BNTZ) ferroelectric ceramics with x = 0–0.15 using a solid-state reaction technique. The structural evolution of Ca-substituted BNTZ was revealed by X-ray diffraction combined with Rietveld crystal structure refinement. A pseudocubic structure with P4bm symmetry is suggested for all Ca-substituted BNTZ samples. Temperature-dependent dielectric properties show a clear and broad dielectric peak of approximately 340 °C. The dielectric peak becomes even wider, and the thermal stability of the permittivity is dramatically improved when x gradually increases. In the x = 0.10 composition, the permittivity at 25–450 °C varies between +5% and ?14.5%. A recoverable energy storage density (W_rec) of 2.79 J/cm³ with an energy storage efficiency (η) of 76% was achieved in the x = 0.07 composition, which suggests superior properties over other BNT-based systems. In addition, the compositions of x = 0.07, 0.10 and 0.15 show excellent thermal stability of W_rec and η. This work proves that the thermal stability of dielectric and energy storage performances in BNT-based ferroelectric ceramics can be achieved by introducing ions without contributing to the polarization.     

Structure and properties of low temperature sintered ZnTa2O6 microwave dielectric ceramics

ZnTa₂O₆ microwave dielectric ceramics have been prepared using ZnTa₂O₆ nano-powders synthesized by sol–gel processing in this study. The crystal structure and microstructure of the ZnTa₂O₆ powders and ceramics were characterized by XRD and SEM techniques. ZnTa₂O₆ ceramics can be densified at a lower sintering temperature of 1200 °C. Microwave dielectric properties show that both of Q × f and ?_r values are lower than those of ceramics prepared by solid state route, and the τ_f values do not show different from that of solid state route. ZnTa₂O₆ ceramics sintered at 1200 °C exhibit good microwave dielectric properties: Q × f = 50,600 GHz, ?_r = 35.12 and τ_f = 9.69 ppm/°C.     

PZT-nickel ferrite and PZT-cobalt ferrite comparative study: Structural,dielectric, ferroelectric and magnetic properties of composite ceramics

Comparative study of different PZT-based composite materials ((x)PbZr_0.52Ti_0.48O₃ + (1-x)CoFe₂O₄ and (x)PbZr_0.52Ti_0.48O₃ +(1-x)Ni_0.7Zn_0.3Fe₂O₄ (x = 0.8 and 0.9)) is presented in the frame of structural, dielectric, ferroelectric and magnetic properties. PZT and NZF/CF powders were synthesized by auto combustion technique. The composites were synthesized by mixing the appropriate amount of individual phases using conventional sintering. XRD data indicated the formation of well crystallized structure of PZT and NZF/CF, without the presence of undesirable phases. SEM micrographs revealed a uniform grain distribution of both, ferroelectric and ferromagnetic phases. Non-saturated hysteresis loops are evident in all samples due to the existence of non-ferroelectric ferrite phase. All the samples exhibit typical ferromagnetic hysteresis loop, indicating the presence of the order magnetic structure. Dielectric investigations revealed that ferrites are the main source of charge carriers, which must be of electronic origin. The activation energy of effective electrical resistivity is heavily influenced by the ferroelectric phase.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

Preparation and dielectric properties of polymer-derived SiCTi ceramics

SiCTi ceramics were prepared by a polymer-derived-ceramic route, with allylhydridopolycarbosilane (AHPCS) and bis(cyclopentadienyl) titanium dichloride (Cp₂TiCl₂) as starting materials. The cross-linking and ceramization of the AHPCS/Cp₂TiCl₂ hybrid precursors were characterized by means of FT IR, NMR, TGA and EDS. The results indicate that the cross-linking of hybrid precursors was significantly catalyzed by using Cp₂TiCl₂ as a catalyst, which might be responsible for a high ceramic yield of 80.8% at 1200 °C. The polymer-to-ceramic conversion was completed at 900 °C to give an amorphous ceramic. The chemical composition of the final ceramics could be tailored by the weight ratio of Cp₂TiCl₂ to AHPCS in feed. The microstructure and dielectric properties of final SiCTi ceramics were investigated by means of XRD, Raman spectroscopy and vector network analyzer. The results indicate that the 1600 °C SiCTi ceramics are composed of amorphous SiCTi, SiC crystal, TiC crystal and graphite. The dielectric loss of SiCTi is up to 0.34, which is 6 times higher than that of SiC (0.058), indicating that the SiCTi ceramics are promising wave-absorbing materials.     

Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering

Structural, dielectric and ferroelectric properties of polycrystalline BaTiO₃ (BTO) ceramics prepared with hybrid sintering i.e., microwave assisted radiant heating (MARH) are reported. It is observed that the permittivity (ε) and true switched ferroelectric charge density (Q_SW) of BTO ceramics can be enhanced by employing MARH. An enhancement of 58% in ε and 17% in Q_SW is observed for the BTO sample prepared with 30% microwave power applied during MARH as compared to the conventional radiant heating. The results are explained in terms of microstructure resulting from the microwave assisted sintering.     

Preparation and microwave dielectric properties of cristobalite ceramics

Dense SiO₂ ceramics with cristobalite phase were prepared by the solid state sintering route, and the microwave dielectric properties were evaluated. The dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) of the pure cristobalite ceramics showed little dependence on the sintering temperature. While, the Qf value increased significantly with increasing the sintering temperature, and it was due to the increasing grain size. The optimized microwave dielectric properties with very low ?_r of 3.81, high Qf value of 80,400 GHz and low τ_f of ?16.1 ppm/°C were obtained for the cristobalite ceramics sintered at 1650 °C for 3 h. It was indicated that cristobalite ceramic was a promising candidate as a low-dielectric-constant microwave material for applications in microwave substrates.     

Structure dependence of microwave dielectric properties in Zn2-xSiO4-x-xCuO ceramics

In this work, the Zn_2-xSiO_4-x-xCuO (x = 0, 0.04, 0.08, 0.12, 0.16 and 0.20) ceramics were synthesized through solid state reaction. The dependence of microwave dielectric properties on the structure was investigated through X-ray diffraction (XRD) with Rietveld refinements, Scanning electron microscope (SEM) and Raman spectra. The melting of CuO can reduce the densification temperature of Zn_2-xSiO_4-x ceramics. In comparison with x = 0, the x = 0.08 ceramics were densified at 1150℃ and the excellent microwave dielectric properties with low dielectric constant (ε_r = 6.01), high quality factor (Qf = 105 500 GHz) and τ_f = ?28 ppm/°C, were obtained. The ε_r, Qf and τ_f value are dominated by covalency of Si-O bond and secondary phase, crystallinity and lattice energy, respectively. This provides a theoretical basis to further adjust the microwave dielectric property (especially τ_f value) from the structural point of view.     

Effects of defect dipoles on tunable dielectric response in relaxor ferroelectric ceramics

Tunable dielectric materials have drawn much attention due to their wide applications including capacitors and microwave tunable devices. Ferroelectrics materials have special spontaneous polarization which can be reversibly switched by an external electric field. Therefore, tunable dielectric constant can be easily achieved in ferroelectrics. However, the study of nonlinear dielectric response induced by defect dipoles is rarely concerned. Here, we report the effects of defect dipoles on tunable dielectric response under alternative current (AC) and direct current (DC) electric field in defect dipoles introduced Pb(Lu_1/2Nb_1/2)O₃–PbTiO₃ ceramics. A modified Rayleigh model is proposed to successfully characterize dielectric nonlinearity and reveals the interaction between domain walls and defect dipoles. The defect dipoles had more sensitive effect on dielectric response under AC field than that of defect dipoles-free samples. The drop of intrinsic dielectric contribution under AC field results from the detriment effect of defect dipoles. The irreversible contribution is altered by the movements of defect dipoles under AC field, subsequently inducing the nonlinearity of dielectric response. Samples with defect dipoles have larger tunable scope of dielectric properties than that of defect dipoles-free samples. The present work discovers the potential of application of defect dipoles-tuned dielectric response ferroelectrics in devices which requires both high AC and DC biases, and help to better understand the complex dielectric response of ferroelectrics.     

Low-frequency dielectric properties of SrLaAlO4 ceramics

SrLaAlO₄ ceramic samples were prepared via solid state reaction method. The low-frequency (20–10⁷ Hz) dielectric properties were investigated in the temperature range from room temperature to 700 °C. It was found that SrLaAlO₄ shows intrinsic dielectric behavior with a dielectric constant of 13 in the temperature range below ~300 °C. In the temperature range from 300 °C to 560 °C, the bulk dielectric contribution due to oxygen-vacancy-related polarons dominates the dielectric properties of the samples. However, the dielectric properties are controlled by sample/electrode contacts when the temperature is risen to above 560 °C. Our results indicate that the bulk effect instead of interfacial effect is the main contribution to dielectric loss in the lower temperature range.