The influence of grain boundary impedances on the p-type conductivity of undoped BaTiO3 ceramics

Impedance spectroscopy is used to deconvolute the dc conductivity (σ) of undoped BaTiO₃ ceramics (∼95% of the theoretical X-ray density) into bulk (σ_b) and grain boundary (σ_gb) components at two oxygen partial pressures, P_O2 ∼10 Pa (N₂) and ∼0.21 MPa (air). At 900°C, σ∼σ_b in both atmospheres, however, at lower temperatures Z¹ plots are dominated by the grain boundary component and σ∼σ_gb. The temperature of the switch from σ∼σ_b to σ∼σ_gb is different in the two atmospheres and occurs at ∼850°C in air and ∼650°C in N₂. Isothermal plots of log σ_b vs log P_O2 in the temperature range 450–900°C show the expected oxygen partial pressure dependence with a gradient of +1/4. In contrast, σ_gb is relatively insensitive to P_O2 and log σ_gb vs log P_O2 plots have gradients < +1/4 with values as low as ∼+1/14.0. In general, isothermal log σ vs log pO₂ plots have gradients <+1/4 as σ is dominated by the grain boundary component. This may explain the wide range of gradients (∼1/4–1/9) reported in the literature for isothermal dc conductivity measurements on polycrystalline BaTiO₃ in the p-type regime.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Relaxor behaviour and nonlinear dielectric properties of lead-free BZT–BZN composite ceramics

To improve the dielectric performance of Ba(Zr_0.2Ti_0.8)O₃-based ceramics, (1-x) Ba(Zr_0.2Ti_0.8)O₃-xBi(Zn_2/3Nb_1/3)O₃ composite ceramics (as short as (1-x) BZT20-xBZN) were synthesised using a solid-state reaction. The effects of Bi(Zn_2/3Nb_1/3)O₃ on the microstructure and dielectric properties were studied. Stable cubic symmetry and polar nanoregions (PNRs)-related first-order Raman modes were observed as the amount of BZN increased. The 0.94BZT20–0.06BZN sample exhibited improved dielectric constants with a higher degree of relaxor behaviour (γ = 1.91). BZN-dependent dielectric nonlinearity was observed under an applied electric field. The evolution of the relaxor behaviour and dielectric nonlinearity in the (1-x) BZT20-xBZN composite ceramics were related to PNRs variations. The largest figure of merit (~65) was obtained for the 0.94BZT20–0.06BZN ceramics, while the dielectric tunability was 75%.     

Microstructure and electrical performance of Sm2O3-doped BCTSG lead-free ceramics

This work aims to upgrade the comprehensive electrical properties of BaTiO₃-based ceramics (1-x)[(Ba_0.94Ca_0.06)(Ti_0.92Sn_0.08)]-xSm₂O₃-0.06 mol% GeO₂ [abbreviated as (1-x)BCTS-xSm-0.06G]. First, piezoceramics were synthesized via a conventional solid-state method. Next, the phase structures, surface topographies, ferroelectric domains were evaluated using XRD, XRD Rietveld refinements, SEM, OP-PFM, TEM. The results demonstrate that all the ceramics possess an orthorhombic-tetragonal (O-T) phase when at room temperature. Significantly, optimized piezoelectricity is gained at x = 0.03 mol% (piezoelectric constant of d₃₃ = 630 ± 20 pC/N and planar electromechanical coupling factor of k_p = 61%). The ferroelectric domains of ceramics were examined using OP-PFM and TEM, which ulteriorly indicate that the favorable piezoelectricity is attributed to the coexistence of the O-T phase boundary and the subdued energy density of the domain wall. Furthermore, all the ceramics are confirmed to be relaxor ferroelectrics, with the relaxor degree increasing with the increasing content of Sm₂O₃.     

Construction of multi-domain coexistence enhanced piezoelectric properties of Bi0.5Na0.5TiO3-based thin films

Although the multi-phase coexistence makes Bi_0.5Na_0.5TiO₃-based piezoelectric thin films possess stronger piezoelectric properties and more spacious application prospects in electronic devices, the domain reversal mechanism of Bi_0.5Na_0.5TiO₃-based thin films cannot be accurately understood due to the size effect. In this study, the relationship between domain structure and piezoelectric properties of the (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xBi(Fe_0.95Mn_0.03Ti_0.02)O₃ thin films are studied by using visualization technology PFM, structure and electrical properties characterizations. The results show that the addition of Bi(Fe_0.95Mn_0.03Ti_0.02)O₃ creates a long-range ordered/short-range disordered nanodomain coexisting structure. This kind of coexisting domain structure can realize the long-range reversal driven by disordered nanodomains under the external electric field, reduce the potential barrier and the hysteresis, and significantly enhance the piezoelectric properties of the thin films. Under the same conditions, the piezoelectric properties of the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ thin films are enhanced nearly 2.3 times. This provides a reference for exploring the physical mechanism of high performance lead-free piezoelectric thin films.     

Field-insensitive giant dynamic piezoelectric response and its structural origin in (Ba,Ca)(Ti,Zr)O3 tetragonal-orthorhombic phase-boundary ceramics

BaTiO₃ (BT)-based ceramics usually exhibit superior quasi-static piezoelectric response but relatively low electrostrain, which limits their actuator applications. In this study, lead-free (Ba_0.835+xCa_0.165-x)(Ti_0.91Zr_0.09)O₃ (x = 0–0.06) (Ba_xCTZ) ceramics with the compositions close to the tetragonal (T)-rich side of orthorhombic (O)-T polymorphic phase boundary (PPB) were reported to exhibit a field insensitive giant dynamic piezoelectric response (d₃₃* >1050 pm/V) over a wide electric field range up to 2 kV/mm, resulting in the large strain value of ∼0.21 %. Detailed structural investigations combined with various electrical properties measurements reveal that the superior dynamic piezoelectric response is attributed to the combination of piezoelectric effect and domain switching behavior due to the chemical modulated O-T PPB, and the field induced partially irreversible T-O phase transition. The results demonstrate that the studied compositions have great potential for applications of lead-free actuator piezoceramics.     

Piezoelectric and thermophysical performances of La3+ and Ir4+ co-doped Ba0.95Ca0.05Ti0.94Zr0.06O3 ceramics

Rare earth element-doped piezoelectric ceramics exhibit excellent electrical properties. Platinum group elements can render them catalytically active and conductive. In this study, lead-free Ba_0.95Ca_0.05Ti_0.94Zr_0.06O₃ piezoelectric ceramics doped with 0.12% lanthanum and iridium (varying contents, x) were sintered at 1260 °C. The thermal expansion coefficients and fracture strengths of these ceramics were affected by their strains and structural defects. With an increase in x, the diffuse phase transition weakened first and then accelerated (with further increase in x (> 0.75%)). The best ferroelectric properties were obtained at 400 ms and 25 kV/cm. The effects of the phase, structural defects, and internal stress on the electrical properties of the ceramics were systematically investigated. Relations of electrical properties, mechanical strength, and thermophysical performances were originally researched. In addition, the electrical properties (piezoelectric constants, ∼270 pC/N; mechanical quality factors, ∼70) and thermophysical performances (thermal expansion coefficient, ∼1.2 × 10⁻⁵ K⁻¹) of the ceramics were found to be suitable for practical applications.     

Low dielectric loss,colossal permittivity,and high breakdown electric field in Al-doped Y2/3Cu3Ti4O12 ceramics

The miniaturization and high capacitance of electronic components are driving the development of high-performance electronic ceramic materials. In this work, we design a new strategy to achieve satisfactory dielectric properties with low loss, colossal permittivity, and a high breakdown electric field (E_b) in Al-doped Y_2/3Cu₃Ti₄O₁₂ (YCTO) ceramics prepared by a solid-phase synthesis method. The dielectric loss decreased with Al doping in the YCTO. The dielectric constant and the E_b were improved upon Al doping. With Al doping levels of 0.03 and 0.05, Y_2/3Al_0.03Cu_2.97Ti₄O₁₂ and Y_2/3Al_0.05Cu_2.95Ti₄O₁₂ ceramics displayed, respectively, a suppressed loss tangent of about 0.028 and 0.031, a high dielectric constant of approximately 9540 and 11792, and an E_b of approximately 4.32 and 4.54 kV/cm. The improved dielectric properties of the produced ceramics were closely linked to enhanced grain boundaries resistance. This study explores the physical mechanism behind the high performance of the YCTO-based ceramics, and also provides theoretical support for the application of devices comprising YCTO and related materials.     

Microwave dielectric characteristics of Li2(Mg0.94M0.06)Ti3O8 (M=Zn,Co, and Mn) ceramics

Li₂(Mg_0.94M_0.06)Ti₃O₈ (M=Zn, Co, and Mn) ceramics were synthesized by the conventional solid-state reaction route. The effect of M (Zn, Co, and Mn) substitution on the structure, microstructure and microwave dielectric properties of Li₂(Mg_0.94M_0.06)Ti₃O₈ has been investigated. The XRD patterns of sintered samples revealed the single-phase formation with spinel structure. With the increase in ionic radius of M, the Qf value decrease is attributed to the decrease of packing fraction and grain size. The Li₂(Mg_0.94Zn_0.06)Ti₃O₈ ceramic sintered at 1075 °C for 4 h showed the best microwave dielectric properties with a dielectric constant of 27.1, a Qf value of 44 800 GHz, and a temperature coefficient of resonant frequency of (+)1.9 ppm/°C.     

High piezoelectricity in CuO-modified Ba(Ti0.90Sn0.10)O3 lead-free ceramics with modulated phase structure

High piezoelectricity was achieved in Ba(Ti_0.90Sn_0.10)O₃ lead-free ceramics by optimizing CuO addition and sintering temperature. The phase structure of 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramic is coexisting rhombohedral and tetragonal phases as sintered at 1300 °C. The coexistence of rhombohedral, tetragonal and orthorhombic phases appears in 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramics as sintered at 1350–1450 °C, which leads to highly enhanced d₃₃ up to 650pC/N. This work demonstrates that high piezoelectric property (d₃₃ = 650pC/N) can be obtained in BaTiO₃-based lead-free piezoceramics with a simple composition modification by modulating phase structures, which also indicates that Ba(Ti,Sn)O₃ is a promising candidate to replace the lead-based piezoceramics.     

Enhanced energy-storage performance of 0.94NBT-0.06BT thin films induced by a Pb0.8La0.1Ca0.1Ti0.975O3 seed layer

0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ ferroelectric thin films with and without the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer were deposited on platinum-buffered silicon substrates by using Sol–Gel process. The influence of Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer and annealing temperatures on the microstructures, ferroelectric properties and energy-storage performances of the as-prepared films were investigated in details. The low annealing temperature and Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer could improve the values of electric break-down field strength and P_max-P_r, which play a vital role for high recoverable energy-storage density. Owing to the high electric break-down field strength value of 3310 kV/cm, a large recoverable energy density of W=17.2 J/cm³ and a high energy efficiency of η=74.3% were obtained for the 0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ thin film with the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer, which was annealed at 450 °C.     

Tailoring Curie temperature and dielectric properties by changing the doping sites of Y ions in (Ba,Ca)(Zr,Ti)O3 ceramics

Doping rare earth element significantly influences electrical properties of BaTiO₃-based ceramics. In this work, Y-doped (Ba_0.95Ca_0.05)(Zr_0.19Ti_0.81)O₃ (BCZT) ceramics are investigated by comparing the microstructure, dielectric and conduction properties of BCZT with different doping-sites of Y ions, A-site, B-site and A/B-site, respectively. Y ions are successfully incorporated into the BCZT lattice, irrespective of the doping sites. A-site doping has marginal effect on the grain size and Curie Temperature, however, B-site doping leads to a decreased grain size and Curie temperature with doping concentration. For A/B-site doping sample, the grain size and Curie temperature are dependent on doping concentration. Furthermore, the T_c regulation mechanism is discussed based on the average size effect (<R>), variance effect (σ²), the local electric field and tolerance factor (t) theories. This work provides a potential strategy for adjusting T_c and improving diele_ctric performance by tuning the doping sites and selecting appropriate rare earth elements with suitable size.     

The role of diffusion behavior on the formation and evolution of the core-shell structure in BaTiO3-based ceramics

In this work, the influence of starting particle size and sintering conditions on the microstructures and dielectric properties of BaTiO₃-based ceramics coated with 0.3Bi(Zn_1/2Ti_1/2)O₃-0.7BaTiO₃ were investigated to reveal the core-shell structure by using high resolution transmission electron microscopy technique coupled with energy-dispersive spectrometer analysis. The ion-diffusion behavior plays a critical role in the formation and evolution of the core-shell structure and, therefore, significantly influences the dielectric properties. When using starting powders containing BaTiO₃ particles larger than 100 nm in size and sintering for shorter dwelling times (0.5-2.0 hours), a core-shell structure could be formed and retained owing to the limited diffusion behavior, enabling BaTiO₃-based ceramics to meet the X8R specification for multilayer ceramic capacitors applications at high temperatures. However, when using 80 nm BaTiO₃ nanopowders and further extending the dwelling time to 6.0 hours, more driving energy was provided to prompt ion diffusion, which led to the compositional inhomogeneity becoming homogenized.     

Electrical conductivity and nonstoichiometry in doped Sr3Ti2O7

A series of doped Ruddlesden–Popper phases, of general formula Sr₃Ti_2-xM_xO_7-δ (M=Al, Ga, Co), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O₂)> 10⁻⁵ atm, followed by a p(O₂)-independent electrolytic regime, and n-type electronic conductivity at very low p(O₂). The electrolytic regime exhibits activation energies in the range 1·7–1·8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O₂). Furthermore, an apparent ionic regime at intermediate p(O₂) is observed, characterized by high conductivity (> 10⁻² S/cm at 700°C) and low activation energy (0·6 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.     

Giant electro-strain and enhanced energy storage performance of (Y0.5Ta0.5)4+ co-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 lead-free ceramics

0.94(Bi_0.5Na_0.5)(Y_0.5Ta_0.5)_xTi_1-xO₃-0.06BaTiO₃ lead-free piezoelectric ceramics were prepared by a conventional solid-state reaction method to study their excellent electro-strain properties and energy storage characteristics systematically. All ceramics exhibited a dense surface morphology. The introduction of (Y_0.5Ta_0.5)⁴⁺ complex ions destroyed the long-range ferroelectric order, which reduced the T_F-R to the operating temperature, resulting in an easier field-induced transition between relaxor and ferroelectric phase. Therefore, for x = 0.01 component attained unipolar strain of 0.37% under 52 kV/cm, and the corresponding normalized strain d₃₃* was 708 pm/V. Besides, the destruction of the ferroelectric phase also induced a pinched hysteresis loop and the maximum storage density of 1.215 J/cm³ with the efficiency of 68.7% obtained at 98 kV/cm for BNYT30 ceramics. These all demonstrated that the doping of complex ions (Y_0.5Ta_0.5)⁴⁺ made the BNT-BT an outstanding candidate for actuators and energy storage devices.     

Effects of Ga content on the structure and electrical performances of 0.69BiFe1-xGaxO3-0.31BaTiO3 lead-free ceramics

Lead-free 0.69BiFe_1-xGa_xO₃-0.31BaTiO₃ (x, 0–0.06) piezoceramics were synthesized via traditional sintering techniques. The phase structure, dielectric, piezoelectric and ferroelectric performances of the ceramics were studied systematically. The results revealed that all the samples locate near MPB of rhombohedral (R)-pseudocubic (pC) phase coexistence, and that Ga doping has distinct influences on the R/pC phase content ratio. An appropriate content of Ga doping favors densification and grains growth of the ceramics during sintering. With the increment of Ga content, the Curie temperature of the samples shifts towards lower temperature owing to increased tolerance factor t of the perovskites, and enhanced diffuse phase transition behavior was observed. In addition, both the piezoelectric and ferroelectric property are sensitive to the concentration of Ga doping. Significantly, the excellent piezoelectric coefficient d₃₃ up to 206 pC/N along with strong remanent polarization P_r of 25 μC/cm² are obtained in 0.69BiFe_0.985Ga_0.015O₃-0.31BaTiO₃ materials which would be a promising substitute for the conventional lead zirconate titanate system ceramics.     

Structural modification and evaluation of dielectric and ferromagnetic properties of Ce-modified BiFeO3–BaTiO3 ceramics

An investigation on Rare earth constituent Ce incorporated BiFeO₃–BaTiO₃ ceramics has been focused in the present study. The ceramic samples of (Bi₀.₇Ba_0.3)_1-xCe_x(Fe₀.₇Ti_0.3)O₃ (x = 0–0.12) were formulated adopting the cost-effective solid-state sintering method. The influence of aliovalent Ce ions on the structural, microstructural, dielectric, ferromagnetic, and optical properties of BiFeO₃–BaTiO₃ was evaluated in this paper. The coexistence of the Tetragonal and the Rhombohedral phases was established by the Rietveld refinement process. The refined crystallographic parameters showed maximum cell volume (V_cell) and the highest percentage of the Rhombohedral phase for x = 0.06; and consequently, the ceramic exhibited the topmost dielectric constant of 946 at x = 0.06. The scanning electron microscopy of the samples revealed the manifestation of polygonal grain morphology. Besides, remarkably improved ferromagnetic properties were evinced for Ce doped ceramics. The magnitude of saturation (M_s) and remnant (M_r) magnetizations were boosted from 0 emu/g and 0.0019 emu/g to 0.9186 emu/g and 0.3745 emu/g respectively with increasing x from 0 to 0.12. Additionally, the optical band gaps of all the samples were evaluated and found to be in the range of 2.941–3.077 eV.     

Large electric field induced strain of Bi(Mg1/2Ti1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics textured by Template Grain Growth

<001> oriented xBi(Mg_1/2Ti_1/2)O₃-(0.7-x)Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (BMT-PMN-PT) textured ceramics are successfully fabricated by the template grain growth method using BaTiO₃ platelets as template. BMT-PMN-PT textured ceramics with different BMT contents are studied in terms of crystal structure, microstructures, dielectric and ferroelectric properties, and electric field induced strain. The as-fabricated BMT-PMN-PT textured ceramics were found to have a strong orientation along <001> direction. The frequency dispersion of dielectric constant of BMT-PMN-PT textured ceramics increases gradually and its relaxability becomes stronger with increasing BMT content. A large electric-field induced strain (0.42 % at 4 kV/mm) is obtained in 0.25BMT-0.45PMN-0.3PT textured ceramics with Lotgering factor 0.94, which is about 83 % enhancement than that of the randomly oriented ceramics (0.23 % at 4 kV/mm). The strain of 0.25BMT-0.45PMN-0.3PT textured ceramics have a relatively high thermal stability, with a slight decrease from 0.42 % to 0.28 % in the temperature range of 20−100 °C. Our research suggests that 0.25BMT-0.45PMN-0.3PT textured ceramics have a greatly potential for actuator devices applications owing to its advantages of large electric field induced strain response.     

Effects of BiMO3 on dielectric,ferroelectric, and piezoelectric properties of perovskite lead‐free piezoelectric BaTiO3–(Bi0.5Na0.5)TiO3 ceramics

New lead‐free piezoelectric ceramics of 0.9BaTiO₃–(0.1?x)(Bi_0.5Na_0.5)TiO₃–xBiMO₃, M=Al and Ga, where x=0.00‐0.10, were fabricated by the solid‐state reaction technique. The effect of BiMO₃ contents on the perovskite structure, phase transition, and dielectric, ferroelectric, and piezoelectric properties was investigated. X‐ray diffraction patterns showed that the ceramics exhibit a monophasic perovskite phase up to x=0.06, suggesting stabilized perovskite structures with B‐site aliovalent substitutions. Compositional‐dependent phase transitions were observed from tetragonal to pseudo‐cubic phase with increasing BiMO₃ amounts. Al³⁺ ions were found to stabilize the transition temperature of the ceramics, while significantly decreasing transition temperature, and a change in the dielectric peak were found with an increasing amount of Ga³⁺. Regarding Al³⁺ substitution, the remanent polarization (P_r) values were found to decrease slightly with the Al³⁺ amount. With regard to Ga³⁺ substitution, P_r values decreased with the Ga³⁺ amount up to 0.06 and then increased slightly. The ceramics became softer with a higher degree of substitution according to the lower coercive field (E_c), when compared with 0.9BaTiO₃–0.1(Bi_0.5Na_0.5)TiO₃ ceramics. Ceramics with a lower degree of substitution and tetragonal phase showed butterfly strain loops that correlated with normal ferroelectric behavior.     

Diffuse phase transition and ferroelectric study of (Ba,Bi)(Ti,Cr)O3 ceramics

Lead-free perovskite type (Ba_0.94Bi_0.06)(Ti_0.94Cr_0.06)O₃ (BBTC) ceramics have been prepared by the conventional mixed oxide method. The XRD results showed that BBTC ceramics have single phase tetragonal symmetry with space group P4mm. Dielectric studies exhibited a diffuse phase transition characterized by a strong temperature and frequency dispersion of permittivity. The quantitative characterization based on empirical parameters (ΔT_m, γ, ΔT_relax, and ΔT_dif) confirmed its relaxor nature. The origin of relax ferroelectric behavior is caused by the polar nanoclusters, which were arose due to the heterovalent substitutions of Bi³⁺ and Cr³⁺ at Ba²⁺ and Ti⁴⁺ sites. The P–E loops obtained at the temperature quite above T_m supported the diffuse phase transition behavior of the samples.