Electrical properties of CaBi4Ti4O15–Bi4Ti3O12 piezoelectric ceramics

CaBi₄Ti₄O₁₅–Bi₄Ti₃O₁₂ (CBT–BIT) ceramics were synthesized using a solid state reaction method. The X-ray diffraction (XRD) analysis revealed the existence of bismuth layered perovskite phase with orthorhombic crystal structure. High-resolution transmission electron microscopy (HRTEM) confirmed the alternate arrangement of CBT part and BIT part along c axis in the intergrowth structure. CBT–BIT ceramics showed excellent thermal stability of the dielectric loss (tan δ), but the relaxation of dielectric loss in the 100 Hz to 1 MHz frequency range had been observed. Meanwhile, an enhanced piezoelectric constant (d₃₃) value of 15 pC/N was observed without degradation even the temperature up to 650 °C. The dc resistivity (ρ_dc) of CBT–BIT performed a high value of 5.68×10¹⁴ (Ω cm) at room temperature (RT). In addition, the ρ_dc values of CBT–BIT within the temperature range of 100–450 °C were close to those of CBT and kept almost one hundred times higher than those of BIT.     

Structural and electric properties of polycrystalline Bi1−xErxFeO3 ceramics

Polycrystalline Bi_1?xEr_xFeO₃ ceramics were synthesized by the solid state reaction method followed by rapid liquid phase sintering. The effects of Er substitution on the structure, morphology and electrical properties of the BiFeO₃ multiferroic ceramics were investigated. X-ray diffraction and Raman studies reveal that the structure of BiFeO₃ is changed from rhombohedral to orthorhombic in the Er concentration range of 0.10–0.15, and the impurity phases decrease both due to Er substitution. The X-ray photoelectron spectroscopy shows that Fe²⁺ could be suppressed by Er substitution. The SEM investigations suggest that the Er substitution could significantly reduce the grain sizes and increase the density of the samples. The leakage current is found to be decreased with increasing Er concentration. The dielectric and ferroelectric measurements show that dielectric constant, dielectric loss and ferroelectric properties are strongly dependent on the Er concentration. Er substitution can significantly improve the dielectric constant and remnant polarization, and decrease the dielectric loss by reducing the leakage current.     

High- and low-field dielectric responses and ferroelectric properties of (Bi0.5Na0.5)Zr1−xTixO3 ceramics

Bismuth sodium zirconate titanate (Bi_0.5Na_0.5)Zr_1?xTi_xO₃ with (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ceramics was fabricated by a conventional sintering technique at 850–1000 °C for 2 h. From X-ray diffraction study, three regions of different phases were observed in the ceramic system; i.e., orthorhombic phase region (0≤x≤0.2), mixed-phase region (0.3≤x≤0.4), and rhombohedral phase region (0.5≤x≤0.6). It was observed that the phase evolution from orthorhombic to rhombohedral symmetry resulted in a noticeable increase of the dielectric properties. The results from the high- and low-field dielectric responses indicated that the dielectric properties of both BNZ and BNZT ceramics were dominantly attributed to the reversible contribution. It was also noticed that grain size showed only partial influence on the increase of low-field dielectric constant in Ti-rich BNZT ceramic.     

Structural,dielectric and ferroelectric properties of Ba1−x(Bi0.5Na0.5)xTiO3 ceramics

Lead free Ba_1?x(Bi_0.5Na_0.5)_xTiO₃ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) ferroelectric ceramics were synthesized by conventional solid state reaction technique. Sintering was done at 1200 °C for 2 h in air atmosphere. The final products have tetragonal symmetry with decreasing c/a ratio confirmed by X-ray diffraction analysis. The grain size varies between 300 nm to 1000 nm for x=0 to 0.1. With increase in Bi_0.5Na_0.5TiO₃ [BNT] content, the room temperature permittivity decreases whereas the Curie temperature (T_c) increases and its highest value was found to be 155 °C for 10 mol% of BNT addition. The ceramics show stable and low dielectric loss characteristics. The remnant polarization (P_r) and the coercive field (E_c) increases monotonously with increase in BNT content. The highest value of 2P_r (=17 μC/cm²) and 2E_c (=22 Kv/cm) was obtained for x=10 mol% BNT addition.     

Synthesis,structural and electrical properties of Bi1−xDyxFeO3 multiferroic ceramics

Polycrystalline ceramic samples of dysprosium (Dy³⁺) doped bismuth ferrite of general formula Bi_1?xDy_xFeO₃ (x=0.00, 0.01, 0.05 and 0.1) have been prepared by standard solid state reaction method. Powder X-ray diffraction (XRD) analysis reveals that all the samples crystallize in the rhombohedral structure with noncentrosymmetric R3c space group. The refined lattice parameters decrease with the increase of Dy concentration within the same structure symmetry. The bond lengths among atoms for all the compounds were calculated by the Rietveld analysis. The frequency and temperature dependent dielectric constants (real and imaginary parts) have been measured. The real part of dielectric constant reveals that the Neel temperature decreases with the increase of Dy-substitution down to ～200 °C for 10% substitution to the Bi site.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Phase structure,piezoelectric, ferroelectric,and electric-field-induced strain properties of Nb-modified 0.8Bi0.5Na0.5TiO3−0.2Sr0.85Bi0.1TiO3 ceramics

0.8Bi_0.5Na_0.5Ti_(1-x)Nb_xO₃−0.2Sr_0.85Bi_0.1TiO₃ (BNT-SBT-xNb, x = 0.00, 0.01, 0.02, and 0.03) piezoelectric ceramics were prepared by traditional solid state reaction and the influence of Nb substitution on the phase structure, ferroelectric, piezoelectric, and electric-field-induced strain properties in BNT-SBT ceramics were studied. XRD results exhibited that Nb⁵⁺ ions could fully diffuse into BNT-SBT structure to form a solid solution when x = 0.01. P-E loops and S-E curves suggested that the ferroelectric phase transformed to ergodic relaxor state (FE-to-ER) with the increasing the amount of Nb additive, indicating the ferroelectric long-ranged order was disturbed by the excess of Nb. With increasing Nb doping, phase transition temperature from normal ferroelectric to ergodic relaxor (short for T_F-R) could be reduced from 120 °C to 40 °C. Furthermore, for sample with x = 0.01, the normalized strain d₃₃^* got a maximum value ~571 pm/V due to the phase transition from ergodic relaxor to ferroelectric (ER-to-FE) under electric field.     

Mixed-valent structure,dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics

Single-phase Ca_1−3x/2Tb_xCu₃Ti_4−xTb_xO₁₂ (0.025≤ x≤0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 µm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu²⁺/Cu⁺, Ti⁴⁺/Ti³⁺, and Tb³⁺/Tb⁴⁺ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm⁻¹ was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (ε_r'_RT =3590‒5200) decreased relative to CCTO (ε_r'_RT =10240) at f =1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ=0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (E_gb) and semi-conductive grains (E_g) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb⁴⁺ at the Ti site, which resulted in a decrease in the IBLC effect.     

Morphotropic phase boundary and electrical properties of lead-free (K0.5Bi0.5)TiO3–Bi(Ni0.5Ti0.5)O3 relaxor ferroelectric ceramics

Lead-free relaxor ferroelectric ceramics (1?x)(K_0.5Bi_0.5)TiO₃–xBi(Ni_0.5Ti_0.5)O₃ were prepared by a conventional solid-state route, the phase transition behavior and corresponding electrical properties were investigated. A typical morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified to be in the range of 0.05<x<0.07 where the optimum piezoelectric and electromechanical properties of d₃₃=126 pC/N and k_P=18% were achieved. Most importantly, a high Curie temperature ~320 °C, around which the material shows a typical relaxor ferroelectric behavior characterized by the presence of diffuse phase transition and frequency dispersion, was obtained in MPB compositions, significantly higher than those of some existing MPB lead-free titanate systems. These results demonstrate a tremendous potential of the studied system for device applications.     

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO₃-xCaHfO₃ was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO₃ (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06 ≤ x ≤ 0.08. Both large piezoelectric coefficient (d₃₃～400 pC/N), inverse piezoelectric coefficient (d₃₃^*～547 pm/V) and planar electromechanical coupling factor (k_p～58.2%) can be achieved for the composition with x = 0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.     

A-site strontium doping effects on structure,magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics

Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi_1−xSr_x)FeO_3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr²⁺ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi_1−xSr_x)FeO_3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi_1−xSr_x)FeO_3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.     

Ferroelectric properties of Pb(Zr0.52Ti0.48)O3–Bi3.25La0.75Ti3O12 ceramics

The ceramic samples of compound (1 ? x)Pb(Zr_0.52Ti_0.48)O₃–xBi_3.25La_0.75Ti₃O₁₂ (when x = 0, 0.03, 0.05, 0.07, 0.10, 0.15 and 0.20) were prepared by a solid-state mixed oxide method. X-ray diffraction analysis showed that complete solid solutions occurred for all compositions. Perovskite phase with tetragonal crystal structure and corresponding lattice distortion was observed. Scanning electron micrographs of sample surfaces showed equiaxed grains for all compositions. Ferroelectric measurements revealed that the addition of small amount of BLT (x = 0.03) showed high remanent polarization (～33.5 μC cm^?2) and low coercive field (～2.74 kV mm^?1). Further increasing BLT content could maintain ferroelectric properties of PZT–BLT ceramics. Based on this study, ferroelectric properties of this PZT–BLT ceramic system can be improved for being further used in ferroelectric memory applications.     

Phase formation,dielectric and ferroelectric properties of CaxBa1−xNb2O6 ceramics

Lead-free Ca_xBa_1?xNb₂O₆ (CBN, 0.20≤x≤0.35) ceramics were prepared by the conventional solid state method. Effects of Ca content on the phase formation, microstructure, dielectric and ferroelectric properties for the prepared CBN ceramics were systematically studied. XRD results showed that pure CBN phase with tungsten bronze structure could be obtained from the solid solutions of BaNb₂O₆ and CaNb₂O₆ in all ceramics. Higher Ca contents favored the occurrence of grains with anisometric morphology, but no abnormal grain growth could be found in all compositions. With the increase of x, Curie temperature T_c shifted downward, whereas the maximum dielectric constant ε_m increased initially and then decreased. All the ceramics showed an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics according to the modified Curie–Weiss law. Normal ferroelectric hysteresis loops were observed in all compositions. Both remnant polarization P_r and coercive field E_c increased initially and then decreased with the increase of x. The ceramics with homogeneous microstructure, high density and better properties were obtained at x=0.28 with ε_m=2998, T_c=234 °C, P_r=3.98 μC cm^?2 and E_c=14.03 kV cm^?1.     

Structure and properties of Bi(Zn½Ti½)O3-modified Pb(Zr,Ti)O3 piezo-/ferroelectric ceramics around the ternary morphotropic phase boundary

To develop high-performance piezo-/ferroelectric materials, Bi(Zn_½Ti_½)O₃–PbZrO₃–PbTiO₃ (BZT–PZ–PT) ternary solid solution with compositions around the morphotropic phase boundary (MPB) is synthesized by solid-state reaction. The crystal structure and electric properties are investigated systematically by X-ray powder diffraction (XRD), dielectric spectroscopy, and ferroelectric and piezoelectric measurements. On the basis of the results of the XRD, dielectric and ferroelectric measurements, the pseudo-binary phase diagram of the yBi(Zn_½Ti_½)O₃–(1 − y)[(1 − x)PbZrO₃–xPbTiO₃] system has been constructed for three series, namely, y = 0.05, 0.10, and 0.15. It is found that the introduction of BZT into the PZT system makes the paraelectric to ferroelectric phase transition more diffuse, brings the MPB to a lower PT content, and enlarges the MPB region. The best properties with an improved dielectric constant ε' = 1248, and a large remnant polarization P_r = 33 μC/cm², as well as a relatively high T_C = 286°C, and a high coercive field E_c = 23 kV/cm was achieved in the y = 0.15 series with MPB composition x = 0.425, making it a promising material for high-power piezoelectric applications.     

Phase transition behavior and electrical properties of lead-free (Bi0.5K0.5)TiO3–LiNbO3 relaxor ferroelectric ceramics

(1?x)(Bi_0.5K_0.5)TiO₃–xLiNbO₃ ((1?x)BKT–xLN) lead-free relaxor ferroelectric ceramics were prepared by a conventional solid-state route and their phase transition behavior and the corresponding electrical properties were investigated. A morphotropic phase boundary separating rhombohedral and tetragonal phases was identified in the composition range of 0.015<x<0.03, where the improved electrical properties of piezoelectric constant d₃₃=75 pC/N and electromechanical coupling factor k_p=0.18 were obtained. Moreover, all samples show typical relaxor behavior characterized by the presence of diffuse phase transition and frequency dispersion. It was found that the dielectric relaxation behavior of BKT ceramics can be obviously enhanced with the addition of LN. In addition, the effect of the LN addition on the ferroelectric properties was also investigated by measuring polarization versus electric field hysteresis loops.     

Structural evolution and ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-xSrTiO3 solid solutions

Bi_0.5Na_0.5TiO₃ (BNT)-based dielectric ceramics have received a lot of attention due to the increased demand for pulse ceramic capacitors. However, comprehensive study on the relationship between their internal phase structure, dielectric characteristics, and ferroelectric properties is still lacking. The phase evolution and its impact on dielectric and ferroelectric properties of an important BNT-based solid solution, Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ (x = 0, 0.1, 0.2, 0.3 and 0.4), were investigated systematically in this work using structural, dielectric, and ferroelectric characterization techniques. X-ray diffraction indicated the coexistence of rhombohedral and tetragonal phases. The frequency- and temperature-dielectric characterization was then used to derive the characteristic temperatures T_B, T_m, T_s, and T_d, and a phase diagram was developed. Furthermore, the temperature-dependent current against electric field curves and polarization versus electric field loops were used to derive the characteristic temperatures connected to high electric field features. This study not only explains the phase evolution of the Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ solid solution, but it also correlates microscopic domains and polar nanoregions to macroscopic dielectric and ferroelectric properties.     

Infrared luminescence in Er3+:Yb3Al5O12 bulk ceramics prepared by sol–gel method

Erbium doped and pure ytterbium aluminium garnet (YbAG, Yb₃Al₅O₁₂) bulk ceramics were successfully prepared by a chelating sol–gel route based on the polyesterification of ethylenediaminetetraacetic acid (EDTA) with triethanolamine (TEA). The gel decomposition and phase formation in precursor powders were studied using XRD and TG/DTA. Amorphous precursor was directly converted to YbAG phase after calcinations at 800 °C. The influence of intermediate grinding on microstructure and luminescent properties was investigated. The discrete luminescence bands of the ⁴I_13/2 → ⁴I_15/2 transition were observed in the infrared emission spectra of erbium doped samples. The lifetime of luminescence at 1530 nm was 2.82 ms and 1.82 ms for the doped samples. This may be attributed to the different efficiency of surface recombination channel, caused by different grain size distribution. Prepared samples are suitable as a standard for photoluminescence measuring of Er-doped YbAG thin films.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.