Ferroelectromagnetic characteristic of Na-doped 0.75BiFeO3–0.25BaTiO3 multiferroic ceramics

BiFeO₃-based materials are expected to have both ferroelectricity and ferromagnetism simultaneously. In this study, effects of Na-doping (0.5, 1.0, 3.0, and 5.0 mol%) on ferromagnetic and ferroelectric properties of 0.75BiFeO₃–0.25BaTiO₃ ceramics which have been fabricated by the solid state reaction technique are studied. The effects of Na-doped 0.75BiFeO₃–0.25BaTiO₃ ceramics on the crystal structure, and magnetic and electrical properties were investigated and discussed. Rhombohedrally distorted 0.75BiFeO₃–0.25BaTiO₃ showed weak ferromagnetic and ferroelectric properties. In addition, ferroelectric and ferromagnetic properties of 0.75BiFeO₃–0.25BaTiO₃ have been controlled by Na doping, and the maximum values of magnetization and polarization were observed at 5.0 mol%.     

Hardening behavior and highly enhanced mechanical quality factor in (K0.5Na0.5)NbO3–based ceramics

This paper relates the microstructure, crystallographic structure, ferroelectric, and piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN) ceramics modified with 0.38 mol% K_5.4Cu_1.3Ta₁₀O₂₉ (KCT) and different amounts of CuO. Results revealed that the addition of KCT and CuO were effective in enhancing the sinterability of KNN. The internal bias field (E_ib) increased from 0.3 kV/mm to 0.58 kV/mm at 0.5 mol% CuO–added KNN+KCT ceramics. The increase of E_ib corresponds very well with the observed increase of the mechanical quality factor (Q_m) from 112 to 2665 for 0.5 mol% CuO. Besides, addition of 0.5 mol% CuO to KNN+KCT resulted in a large increase of the EPR signal, which is related to the increased amount Cu²⁺ and a corresponding increase of the concentration of defect dipoles. This result is in good agreement with the increased E_ib and the resulting hardening behavior.     

Microstructure and electrical properties of Mn/Y codoped Ba0.67Sr0.33TiO3 ceramics

Pure and Mn/Y codoped Ba_0.67Sr_0.33TiO₃ (BST) ceramics were fabricated via the citrate–nitrate combustion technique, and the microstructure and electrical properties of BST ceramics were mainly investigated. The Mn/Y codoping concentration has a strong influence on the microstructure and electrical properties of BST ceramics. All BST ceramics possess a pure polycrystalline structure. The density, dielectric loss, leakage current, and ferroelectric properties are improved by codoping 0.5 mol% Mn and 1.0 mol% Y to BST. The relative density of 0.5 mol% Mn/1.0 mol% Y-codoped BST (BST0510) ceramics reaches 97.5% of the theoretical value. BST0510 ceramics have the lowest dielectric loss (tanδ < 0.0073 at 1 kHz) among all BST ceramics. BST0510 ceramics also demonstrate a low leakage current density (1.23 × 10^?7 A/cm²) at an applied field of 10 kV/cm, and excellent ferroelectric properties with a remanent polarization of 2P_r = 15.327 μC/cm² and a coercive field of 2E_c = 3.456 kV/cm. Therefore, the Mn and Y with optimum content help improve the electrical properties of BST materials.     

Effect of annealing temperature on the multiferroic properties of 0.7BiFeO3–0.3Bi0.5Na0.5TiO3 solid solution prepared by sol–gel method

Room-temperature multiferroic 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics have been prepared by the sol–gel method. We have discussed the annealing temperature dependence of the multiferroic properties. The samples are annealed at 1023, 1123, 1223 and 1323 K for 3 h, respectively. X-ray diffraction patterns identify that all samples are pure. Scanning electron micrographs present the increasing grain size with higher annealing temperature. Magnetic, ferroelectric and dielectric properties are enhanced obviously with the increase in annealing temperature. The coexistence of ferroelectric and ferromagnetic properties is also proved at room temperature. In addition, it reveals that the optimal annealing temperature accompanied with favorable multiferroic properties of 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics is near 1223 K.     

Effects of CuO nanoparticles addition on properties of PMNT ceramics

This research investigates the role of CuO nanoparticles addition on properties of 0.9PMN–0.1PT (PMNT) ceramics. Phase purity, density, microstructure, dielectric and ferroelectric properties of the ceramics were investigated. The density of the ceramics reaches a maximum when 0.5 wt% of CuO is added into the ceramics while grain size of the ceramics tends to increase with the increase in CuO content. ε_max of the ceramics tends to increase with increasing CuO concentration. The improvement of P_r of the ceramics is observed in the ceramic incorporated with over 0.1 wt% of CuO.     

Investigation on ferromagnetic and ferroelectric properties of (La,K)-doped BiFeO3–BaTiO3 solid solution

Multiferroic materials of BiFeO₃–BaTiO₃ solid solution have been fabricated in order to improve ferromagnetic and ferroelectric properties. The effects of La (1 mol%) and K (varied from 0.5–5 mol%) doped 0.75BiFeO₃–0.25BaTiO₃ on phase formation, ferromagnetic and ferroelectric properties have been investigated and discussed. The rhombohedral perovskite phase of specimens was characterized by XRD technique. Fracture morphology reveals the grain growth characteristics with increasing K content. (La, K)-doped 0.75BiFeO₃–0.25BaTiO₃ with La=1 mol% and K=3 mol% exhibits the highest remnant polarization and remnant magnetization.     

Influence of cobalt and sintering temperature on structure and electrical properties of BaZr0.05Ti0.95O3 ceramics

Pure and Co-modified BaZr_0.05Ti_0.95O₃ ceramics were fabricated by the traditional solid state reaction technique. The influence of cobalt and sintering temperature on structure, dielectric, ferroelectric properties and diffuse phase transition of BZT ceramics were investigated systematically. 1300 °C was the optimal sintering temperature for BZT ceramics. The solid solubility limit of Co ions in BZT matrix was determined to be 0.4 mol%. The introduction of a moderate amount of Co ions was believed to benefit the microstructure development and make the grain size more uniform. Compared with undoped counterparts, 0.4 mol% Co-modified ceramics showed equivalent ferroelectric properties with a high remnant polarization (P_r=9.6 μC/cm²) and a low coercive field (E_c=0.21 kV/mm). Besides these, a relative high dielectric coefficient (ε_r=2030) and a low dielectric loss (tan δ=1.85%) were also obtained on this composition. The degree of diffuse phase transition was enhanced by the addition of Co ions. The related mechanism of the diffused phase transition behavior was discussed.     

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.     

Enhancement of Qm in CuO-doped compositionally optimized Li/Ta-modified (Na,K)NbO3 lead-free piezoceramics

The effects of CuO on electrical properties of compositionally optimized Li/Ta-modified (Na,K)NbO₃ (LT-KNN) piezoceramics were studied to develop LT-KNN with high mechanical quality factor Q_m and low dielectric loss tg δ as well as relatively high piezoelectric constant d₃₃ for actuator applications. Both “hard” and “soft” effects were observed due to CuO addition, and the discussion related to high Li doping amount was presented to explain this phenomenon. By increasing CuO addition, the d₃₃ gradually decreases while the Q_m increases significantly. Particularly, tg δ at 1 kHz can be reduced from ～2.7% for LT-KNN ceramics to ～0.8% for CuO-added LT-KNN ceramics. When the CuO addition is lower than 1 mol%, both high Q_m (～323) and relatively high d₃₃ (>180 pC/N) can be obtained, in addition to a relatively high dielectric constant ?_r ～ 500.     

Large enhancement of piezoelectric properties in Mn-modified SrBi4Ti4O15 and its thermal stabilities at elevated temperatures

Manganese-modified strontium bismuth titanate (SrBi₄Ti₄O₁₅, SBT) ceramic oxides were synthesized by substituting a small amount of manganese ions into the Ti⁴⁺ sites using conventional solid-state reaction. The resultant Mn-modified SBT (SBT-Mn) exhibits better piezoelectric properties in comparison to unmodified SBT. The piezoelectric properties of Mn-modified SBT is optimized with only 4 mol% Mn substitution. SBT-4Mn exhibits a large piezoelectric constant (d₃₃=30 pC/N), approximately twice the value of unmodified SBT (d₃₃=13 pC/N), while its Curie temperature, T_c, remains almost unchanged at ~530 °C. The temperature-dependent electrical impedance and electromechanical coupling factors k_p and k_t reveal that the SBT-4Mn exhibits thermally stable electromechanical coupling characteristics up to 300 °C but electromechanical coupling factors deteriorate significantly at a higher temperature due to increased conductivity. Our work suggested that Mn-modified SBT ceramics are promising materials for high temperature piezoelectric applications.     

Densification behavior and electrical properties of CuO-doped Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ceramics

CuO modified Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) ternary relaxor based ferroelectrics with the composition near the morphotropic phase boundary were synthesized by two-step columbite precursor method. The introduction of CuO significantly improved the sinterability of PIN–PMN–PT ceramics, resulting in the full densification of samples at lower sintering temperatures. It also profoundly modified the crystal structure and fracture mode of the ceramics. Properly increasing CuO content led to the disappearance of rhombohedral-tetragonal phase transition, remarkably improved the Curie temperature (T_c), and made the ceramics more relaxorlike. The ternary ceramics doped with 0.25 wt% CuO possessed optimum piezoelectric properties (d₃₃=584 pC/N, d₃₃^*=948 pC/N, and k_p=0.68), high ferroelectric properties (E_c=9.9 kV/cm, and P_r=33.1 μC/cm²), low dielectric loss (tan δ=0.9%), and wider temperature usage range (T_c=225 °C). The obtained properties are much higher than those of previously reported PIN–PMN–PT based ceramics, indicating that CuO doped PIN–PMN–PT is a promising candidate for electromechanical applications with high performance and wide temperature/electric field usage ranges.     

High-temperature dielectric properties and microwave absorption abilities of Bi1−xMgxFeO3 nanoparticles

BiFeO₃ (BFO) multiferroic nanoparticles have attracted increasing attention owing to the coexistence of ferroelectric and ferromagnetic properties. In this work, Bi_1−xMg_xFeO₃ (x = 0.05, 0.1, 0.15) multiferroic nanoparticles were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption performance in the temperature range of 323–723 K at X-band were investigated. The qualified bandwidth (absorption intensity < −10 dB) of the Mg-doped BFO materials covers the whole X-band at 673 K, suggesting promising candidates as high-temperature electromagnetic absorbers.     

Low temperature sintering of lead-free Bi0.5(Na0.82K0.18)0.5TiO3 piezoelectric ceramics by co-doping with CuO and Nb2O5

Effect of excess CuO additive on the sintering behavior and piezoelectric properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ ceramics was investigated. The addition of small amount of excess CuO as low as 1 mol% was quite effective to lower the sintering temperature (T_s) of BNKT ceramics down to 975 °C while their piezoelectric properties were degraded by Cu doping. However, the electric field-induced strain was markedly enhanced by further addition of Nb₂O₅ with CuO without elevating T_s. The normalized strain S_max/E_max of 427 pm/V was obtained with a specimen sintered with 0.02 mol CuO and 0.03 mol Nb₂O₅ in excess.     

Stabilization of the relaxor phase by adding CuO in lead-free (Bi1/2Na1/2)TiO3‒SrTiO3‒BiFeO3 ceramics

We investigated microstructures, crystal structures, and electrical properties of high temperature sintered (1‒x)BiNaTiO₃‒xSrTiO₃‒2BiFeO₃ (BNST100x‒2BF, x = 0.20 and 0.22) ceramics without added CuO and compared these results with low temperature sintered BNST100x‒2BF ceramics with 1 mol% of added CuO. The sintering temperature and average grain size of BNST100x‒2BF ceramics decreased after addition of CuO. We found that changes in the stabilized relaxor phase with decreasing sintering temperature resulted in changes in the temperature dependent dielectric properties and provided the enhanced electromechanical strain properties in BNST100x‒2BF ceramics. Therefore, we suggest that the nonergodicity to ergodicity transition is able to induce by changes in sintering temperature with added CuO and this result is important to tailor the electrical properties of BNT‒based lead‒free ceramics in proceeding with practical application.     

Magnetic and ferroelectric domain structures in BaTiO3–(Ni0.5Zn0.5)Fe2O4 multiferroic ceramics

BaTiO₃–(Ni_0.5Zn_0.5)Fe₂O₄ composites prepared by co-precipitation were investigated. The macroscopic magnetic properties derived from the magnetic phase (low coercivity, almost no M(H) hysteretic behavior and high permeability) are preserved in the composite. The dielectric properties are strongly influenced by interface phenomena (Maxwell-Wagner), due to the local electrical inhomogeneity. At low frequencies, the composites present thermally activated conductivity and relaxation, while at 1 MHz permittivity of around 500 and tan δ < 8% is obtained at room temperature. The multiferroic character was demonstrated at nanoscale by the presence of the magnetic and ferroelectric domain structure in the same region. Imprint polarisation in the regions corresponding to the ferroelectric phase is found, as result of an internal electrical field created at the interfaces between the (Ni,Zn)-ferrite and BaTiO₃ regions.     

Electrocaloric effect and pyroelectric energy harvesting of (0.94 − x)Na0.5Bi0.5TiO3-0.06BaTiO3-xSrTiO3 ceramics

Ceramics with the composition (0.94 − x)Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–xSrTiO₃ (NBBSTx) where x = 0.10, 0.15, 0.20, and 0.25 were synthesized by a conventional solid-state reaction method to investigate their electrocaloric effect (ECE) and pyroelectric energy harvesting (PEH) properties. The ferroelectric, dielectric, and pyroelectric properties of the prepared ceramics were measured and discussed. It is found that the strontium titanate (ST) content and bias field greatly affect the ferroelectric–relaxor transition. Increasing ST content lowers the depolarization temperature of the ceramics, and both the ECE and PEH behavior of the ceramics strongly depend on their ST content because of the composition-induced decrease of the ferroelectric–relaxor transition temperature. The present investigation demonstrates that the ECE and PEH properties of NBBSTx ceramics can be tuned by introducing ST. Furthermore, a high PEH density of 425 kJ/m³ is obtained for NBBST0.20, which is much higher than those of conventional Pb-based ferroelectrics.     

Strong photoluminescence and good electrical properties in Eu-modified SrBi2Nb2O9 multifunctional ceramics

Bismuth layer-structured ferroelectric (BLSFs) ceramics of Sr_1−xEu_x Bi₂Nb₂O₉ (SBT-xEu, x=0.000, 0.002, 0.004, 0.006) were prepared by a conventional solid-state reaction method. All the samples have a bismuth oxide layered structure with a dense microstructure. The ferroelectric, piezoelectric, dielectric and optical properties of the ceramics were investigated. After Eu³⁺ doping, samples show a bright red photoluminescence upon blue light excitation of the 400–500 nm. Upon the excitation of 465 nm light, the materials have two intense emission bands peaking around 593 nm (yellow) and 616 nm (red). Meanwhile, good electrical properties with large piezoelectric constant d₃₃ of 14 pC/N and large remnant polarization 2P_r of 11.97 μC/cm² are obtained at x=0.006. Moreover, this material has a high Curie temperature (T_c=429 °C) and high resistivity, which makes the material resistant to thermal depolarization up to its Curie temperature. This feature indicates that the SBN-xEu ceramics have a latent use in high temperature applications.     

Lead-free Bi1/2(Na0.82K0.18)1/2TiO3 relaxor ferroelectrics with temperature insensitive electrostrictive coefficient

The electric field-induced strain of Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) ceramics modified with BaZrO₃ (BZ) was investigated as a function of composition and temperature. Unmodified BNKT ceramics revealed a typical ferroelectric butterfly-shaped bipolar S–E loop at room temperature, whose normalized strain (S_max/E_max) showed a significant temperature coefficient of 0.38 pm/V/K. As the BZ content increased in the solid solution up to 5 mol%, the ferroelectric BNKT gradually transformed to a relaxor. Finally, 5 mol% BZ-modified BNKT ceramics showed a typical electrostrictive behavior with a thermally stable electrostrictive coefficient (Q₃₃) of 0.025 m⁴/C², which is comparable to that of Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics that have been primarily used as Pb-based electrostrictive materials.     

Effect of WO3 doping on dielectric and ferroelectric properties of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics

WO₃(0–6 mol%)-doped 0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃ lead-free ceramics were synthesized by conventional solid-state reaction. The effect of WO₃ addition on the structure and electrical properties were investigated. The result revealed that a small amount of WO₃ (≤1 mol%) can diffuse into the lattice and does not significantly affect the phase structure, however, more addition will result in distortion and enlargement of the unit cells. The maximum permittivity temperature (T_m) is suppressed dramatically as the dopant increasing, while the depolarization temperature (T_d) fall to the minimum with 1 mol% WO₃ additive. The remanent polarization (P_r) was enhanced and coercive field (E_c) was reduced by doping with WO₃. The strain shows the largest value for 1 mol% doped sample, which is due to a field-induced antiferroelectric–ferroelectric phase transition.     

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.