Modification of NdNbO4 microwave dielectric ceramic by Bi substitutions

In the present work, Bi³⁺ was used to substitute for Nd³⁺ in the NdNbO₄ ceramic and pure fergusonite solid solution was formed within 20 mol. % substitutions. Microwave dielectric permittivity of the (Nd_1-xBi_x)NbO₄ (x ≤ 0.2) ceramics increased linearly with x value due to the larger ionic polarizability of Bi³⁺ than Nd³⁺. Excellent microwave dielectric properties with a permittivity (ε_r) ~22.5, a Qf (Q = quality factor, f = resonant frequency) ~50 000 GHz, and a TCF ~−9 ppm/°C were obtained in the (Nd_0.9Bi_0.1)NbO₄ ceramic. This method might work in other fergusonite-type rare-earth ortho-niobates.     

Impedance spectroscopy of Na0.5Bi4.50+xTi4Oy (x = -0.02, 0, 0.02) ceramics with excellent dielectric properties

The electric and dielectric properties of Na_0.5Bi_4.50+xTi₄O_y (x = −0.02, 0, 0.02) prepared by conventional mixed oxide route have been investigated by impedance spectroscopy (IS) over a wide temperature range. Single-phase bismuth layer-structured perovskite patterns were observed through X-ray diffraction of the three samples Na_0.5Bi_4.5Ti₄O₁₅, Na_0.5Bi_4.48Ti₄O_y, and Na_0.5Bi_4.52Ti₄O_y. The results show that the relative permittivity (εr) increases with the increase in temperature and reaches its maximum at about 675℃. With the continuous increase in temperature, the permittivity decreases gradually. Both relative permittivity and dielectric loss show great stability at the low-temperature zone. The ceramic of x = 0.02 with Ea of 1.09 eV has the maximum oxygen ionic transport number between 600 and 800℃ for all samples. And at this time, it has the maximum electrical conductivity. All the results indicated that Na_0.5Bi_4.50+xTi₄O_y (x = −0.02, 0, 0.02) ceramics were promising base materials for high-temperature capacitor because of their high dielectric properties.     

Dielectric and Piezoelectric Properties of (1−x)K0.5Bi0.5TiO3–xBa(Ti0.8Zr0.2)O3 Ceramics

The properties of relaxor ceramics in the compositional series (1?x)K_0.5Bi_0.5TiO₃–xBa(Ti_0.8Zr_0.2)O₃ have been investigated. Values of T_m, the temperature of maximum relative permittivity, decreased from 380°C at x = 0.0 to below room temperature for x > 0.7. Compositions x = 0.1 and 0.2 were piezoelectric and ferroelectric. The maximum value of d₃₃ piezoelectric charge coefficient, 130 pC/N, and strain, 0.14%, occurred at x = 0.1. Piezoelectric properties of x = 0.1 were retained after thermal cycling from room temperature to 220°C, consistent with results from high‐temperature X‐ray diffraction indicating a transition to single‐phase cubic at ~300°C.     

Structural,dielectric and transport properties of samarium-doped cobaltites

Cobaltites have gained considerable importance in the field of material science, due to their valuable dielectric, biological and magnetic properties. This work highlights the importance of samarium (Sm) doping to enhance the structural and dielectric properties of bismuth calcium cobaltite. All samples were synthesized using the co-precipitation technique and were sintered at 750 °C for 2 h. X-Ray Diffraction (XRD) analyses, as well as Fourier-transform infrared spectroscopy, are used to realize the monoclinic structure of all samples. In addition, XRD analysis was used to determine the lattice strains, lattice constant, crystallite size, the volume of a unit cell, and dislocation density. This analysis shows that Sm doping increases the dislocation density and lowers the average crystallite size. The AC conductivity, (σ_ac), dielectric constant (ε), and loss tangent tan (δ) for all samples were measured at various fixed frequencies and in the variable frequency range (100Hz – 3 MHz). The obtained results are also discussed using the non-linear modified Debye function and Jonscher's power law. The composition Bi₂Ca_1.95Sm_0.05CoO₆ proves to be the better dielectric material at 350 °C–600 °C as it achieves a high value for dielectric constant while maintaining the lower dielectric loss than that of other compositions. Bi₂Ca_1.95Sm_0.05CoO₆ acquires a higher value for σ_ac than the previously reported σ_ac values of Ca_2-xY_xCo₂O₅. These qualities make the aforementioned specimen highly preferable for energy applications.     

Enhanced dielectric,thermal stability,and energy storage properties in compositionally engineered lead-free ceramics at morphotropic phase boundary

Structural inhomogeneity at morphotropic phase boundary (MPB) offers a novel paradigm to explore and modulate the physical properties of dielectric materials to design next-generation multifunctional devices. In this work, two lead free materials at MPB; Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) and (Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNTBT), are combined together to synthesize polycrystalline composite samples of (1-x) BCZT-xBNTBT with x = 0.0, 0.25, 0.50, 0.75, and 1.0. Structural investigations using XRD show the coexistence of double phases for pristine BCZT (tetragonal (P4mm) + rhombohedral (R3m)), and for pristine BNTBT (tetragonal (P4bm) + rhombohedral (R3c)). However, all the doped samples with x = 0.25, 0.5, and 0.75 display a coexistence of triple phases with P4mm, P4bm, and R3c symmetries. Detailed dielectric study reveals a normal ferroelectric to macroscopic ergodic relaxor crossover for samples with x = 0.25 and 0.75. Intriguingly, sample x = 0.25 displays a coexistence of high dielectric constant (4050), ultralow dielectric loss (≤0.02), high temperature thermal stability of permittivity (variation ≤ ±15%) in a temperature range 135 °C–450 °C, large recoverable energy density (W_rec = 423 mJ/cm³) with ultrahigh energy storage efficiency (η = 95.4%) at low applied electric field - 23 kV/cm. Nevertheless, at similar applied field strength, the obtained values of W_rec and η exceed most of the selected lead-free energy storage materials. This work may pave a new path to design superior high-temperature dielectrics, through intermixing of MPBs, for energy storage applications.     

Dielectric performance of pyrochlore-type Bi2MgNb2-xTaxO9 ceramics: The effects of tantalum doping

The solid solutions based on the pyrochlore-type system Bi₂MgNb_2-xTa_xO₉ were formed in the compositional range х = 0–2.0 (Bi_1·6Mg_0·8Nb_1.6-tTa_tO_7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi₂MgNb_2-xTa_xO₉ (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi_1·6Mg_0·8Ta_1·6O_7.2). The increase in the Ta⁵⁺ concentration led to the decrease in the dielectric permeability from 104 (Bi_1·6Mg_0·8Nb_1·6O_7.2) to 20 (Bi_1·6Mg_0·8Ta_1·6O_7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.     

Colossal permittivity of (Gd + Nb) co-doped TiO2 ceramics induced by interface effects and defect cluster

Material with high dielectric constant plays an important role in energy storage elements. (Gd + Nb) co-doped TiO₂ (GNTO) ceramics with giant dielectric permittivity (>10⁴), low dielectric loss, good temperature and frequency stability in broad range of 30–150 °C and 10²–10⁶ Hz have been systematically characterized. Especially, a low dielectric loss of 0.027 and a giant dielectric permittivity of 5.63 × 10⁴ at 1 kHz are attained for the composition with x = 0.01. Results of complex impedance spectroscopy, I–V curve and frequency dependent dielectric constant under DC bias indicate that internal barrier layer capacitance (IBLC) effect, electrode effect and electron-pinned defect-dipole (EPDD) effect contribute to the colossal permittivity (CP) property simultaneously.     

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.     

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Perovskite solid solution ceramics of (1 ? x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (BT–BMN) (x = 0.05–0.2) were synthesized by solid‐state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT‐based ceramics. X‐ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor‐like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature‐stable permittivity plateau (from ?50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 10¹³ Ω cm at x = 0.15, and an energy density of about 1.13 J/cm³ is achieved in the sample of x = 0.1.     

Thermally Stable BaTiO3‐Bi(Mg2/3Nb1/3)O3 Solid Solution with High Relative Permittivity in a Broad Temperature Usage Range

A combined X‐ray diffraction (XRD), Raman spectra, X‐ray photoelectron spectroscopy, Scanning electron microscopy, and dielectric characterization of (1–x)BaTiO₃?xBi(Mg_2/3Nb_1/3)O₃ ceramic system were investigated for compositions of 0 ≤ x ≤ 0.2. Single‐phase perovskite‐type XRD patterns were observed for all compositions. A systematically structural change from tetragonal to pseudocubic symmetry occurred at 0.04 < x < 0.06, which agrees well with the analysis of Raman spectra. Dielectric measurements indicated that the crossover from a classic ferroelectric to relaxor ferroelectric occurred at x ≥ 0.04. Compared with other compositions, the temperature independence of relative permittivity at T > T_m significantly ameliorated at x = 0.1: near‐stable temperature coefficient of higher relative permittivity (~6800 ± 15%) and the corresponding loss tanδ ≤ 0.09 over a more broader temperature range of 25°C–240°C (1 kHz), which indicates that this ceramic is a promising dielectric material for elevated temperature dielectrics.     

Structural,dielectric, and piezoelectric properties of Ce-doped Bi7Ti4.2Ta0.3W0.5O21 intergrowth ferroelectric ceramics

The Bi_7-xCe_xTi_4.2Ta_0.3W_0.5O₂₁ (BTTW-BITT-xCe, x = 0.05, 0.10, 0.15, 0.20) ceramics were studied as potential materials for high-temperature applications. The microstructure, dielectric, piezoelectric and ferroelectric properties of Ce doped BTTW-BITT samples were analyzed in detail. The results indicated that an appropriate amount of Ce ion doping could inhibit the growth of grains, suppress the relaxation peak, reduce high-temperature dielectric losses, and greatly improve the piezoelectric activities. The optimal ceramics was obtained at x = 0.15, which possessed a maximum piezoelectric constant of d₃₃ = 23.4 pC/N, a high Curie temperature of 713 °C, a loss value of 6% at 500 °C, and a favorable thermal stability of d₃₃ = 21.1 pC/N (90% of the initial value) at 500 °C. This result indicates that BTTW-BITT-0.15Ce has great potential for applications in the high temperature fields. In addition, XPS results showed that there were two Ce valences states, Ce³⁺ and Ce⁴⁺present in the BTTW-BITT-xCe ceramics.     

Dielectric temperature stability of Nb-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

In this study, the phase structure, microstructure and dielectric properties of Bi_0.5(Na_0.78K_0.22)_0.5(Ti_1-xNb_x)O₃ lead-free ceramics prepared by traditional solid phase sintering method were studied. The second phase pyrochlore bismuth titanate (Bi₂Ti₂O₇) was produced in the system after introduction of Nb⁵⁺. The dielectric constant of the sample (x = 0.03) sintered at 1130 °C at room temperature reached a maximum of 1841, and the dielectric loss was 0.045 minimum. It had been found that the K⁺ and Nb⁵⁺ co-doped Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics exhibited outstanding dielectric-temperature stability within 100–400 °C with T_cc ≤±15%. Result of this research provides a valuable reference for application of BNT based capacitors in high temperature field.     

Effects of cerium on structures and electrical properties of (Nb,Ta) modified Bi4Ti3O12 piezoelectric ceramics

Bi₄Ti₃O₁₂ high-temperature piezoelectric ceramics composed of 0.03 mol (Nb, Ta)⁵⁺ substituting B site and x mol CeO₂ (x = 0–0.05, abbreviated as BCTNT100x) substituting A site were synthesized by the conventional solid-state reaction method. The effects of Ce additive on the structures and electrical properties of resulting Bi₄Ti₃O₁₂-based ceramics were systematically investigated. In-situ temperature-dependent X-ray diffraction (XRD) confirmed that the phase structure of BCTNT100x ceramics change from orthorhombic structure to tetragonal structure as temperature increased. The ceramics at Ce content x = 0.03 illustrated optimal performances with superior piezoelectric constant (d₃₃ = 36.5 pC/N), high Curie temperature (T_C = 649 °C), and large remanent polarization (2P_r = 21.6 μC/cm²). BCTNT3 ceramics also possessed high d₃₃ of 32.5 pC/N at an annealing temperature of 600°C, with electrical resistivity preserved at 10⁶ Ω cm at 500 °C. These results demonstrate that BCTNT100x ceramics can be used as high-temperature piezoelectric devices.     

Sol–Gel Synthesis and Characterization of Na0.5Bi0.5TiO3–NaTaO3Thin Films

Thin films with the composition 70 mol% Na_0.5Bi_0.5TiO₃ + 30 mol% NaTaO₃ were prepared by sol–gel synthesis and spin coating. The influence of the annealing temperature on the microstructural development and its further influence on the dielectric properties in the low‐ (kHz–MHz) and microwave‐frequency (15 GHz) ranges were investigated. In the low‐frequency range we observed that with an increasing annealing temperature from 550°C to 650°C the average grain size increased from 90 to 170 nm, which led to an increase in the dielectric permittivity from 130 to 240. The temperature‐stable dielectric properties were measured for thin films annealed at 650°C in the temperature range between ?25°C and 150°C. The thin films deposited on corundum substrates had a lower average grain size than those on Si/SiO₂/TiO₂/Pt substrates. The highest average grain size of 130 nm was obtained for a thin film annealed at 600°C, which displayed a dielectric permittivity of 130, measured at 15 GHz.     

Effect of Mn:Mg ratio on sintering behavior and microwave dielectric properties of (Mn,Mg)V2O6 ceramics at ultra-low sintering temperature

Ultra-low-firing-temperature ceramics (Mn_1−xMg_x)V₂O₆ (x = 0–1) were prepared using the conventional solid-state reaction method. The effects of the Mn:Mg ratio on the crystal structure and microwave dielectric properties of the prepared ceramics were systematically investigated. The results indicated that an appropriate Mn:Mg ratio effectively improves the dielectric properties of the compounds. Specimens with x = 0.01 and x = 0.93 sintered at 630 °C exhibited the following microwave dielectric properties: ε_r = 12.4 and 9.8, high Q×f = 57,000 and 21,000 GHz, and τ_f = –15 and −24 ppm/°C, respectively. This suggests that the (Mn_0.99Mg_0.01)V₂O₆ ceramic is a potential material for ULTCC applications.     

Microstructures and Microwave Dielectric Properties of Bi2O3‐Deficient Bi12SiO20 Ceramics

The Microstructure and microwave dielectric properties of Bi₂O₃‐deficient Bi₁₂SiO₂₀ ceramics were investigated. A small amount of unreacted Bi₂O₃ phase melted during sintering at 825°C and assisted with densification and grain growth in all samples. The melted Bi₂O₃ reacted with remnant SiO₂ during cooling to form a Bi₄Si₃O₁₂ secondary phase. The nominal composition of Bi_11.8SiO_19.7 ceramics sintered at 825°C for 4 h had a high relative density of 97% of the theoretical density, and good microwave dielectric properties: ε_r = 39, Q × f = 74 000 GHz, and τ_f = ?14.1 ppm/°C. Moreover, this ceramic did not react with Ag at 825°C.     

Effects of Nb2O5 Doping on the Microwave Dielectric Properties and Microstructures of Bi2Mo2O9 Ceramics

This study investigated the effects of the addition of Nb₂O₅ and sintering temperature on the properties of Bi₂Mo₂O₉ ceramics. The ceramics were sintered in air at temperatures ranging from 620°C to 680°C. The addition of small amounts of Nb₂O₅ as a dopant significantly affected the crystalline phase and the microwave dielectric properties of the Bi₂Mo₂O₉ ceramics. The secondary phase, γ‐Bi₂MoO₆, was observed when Nb₂O₅ was added. However, unlike the Bi₂Mo₂O₉ ceramic without Nb₂O₅ sintered above 645°C, the ceramics with 3 mol% Nb₂O₅ contained no γ‐Bi₂MoO₆ when sintered at 660°C. The Q × f value and τ_f of the Bi₂Mo₂O₉ ceramics were improved by Nb₂O₅ doping. The Bi₂Mo₂O₉ ceramics doped with 2 mol% Nb₂O₅ exhibited the best microwave dielectric properties, with a permittivity of 36.5, a Q × f value (f = resonant frequency, Q = 1/dielectric loss at f) of 14100 GHz and τ_f of +5.5 ppm/°C after sintering at 620°C.     

Structure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of BiFeO3‐BaTiO3‐Bi0.5Na0.5TiO3 Lead‐Free Multiferroic Ceramics

Lead‐free multiferroic ceramics of BiFeO₃‐BaTiO₃‐Bi_0.5Na_0.5TiO₃ have been prepared by a conventional ceramic technique. The microstructure, multiferroic, and piezoelectric properties of the ceramics have been studied. The ceramics sintered at 1000°C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x = 0.02. After the addition of Bi_0.5Na_0.5TiO₃, two dielectric anomalies are observed at high temperatures (T_m ~ 510°C–570°C and T₂ ~ 720°C). The phase transition around T_m becomes wider gradually with increasing x. The ferroelectricity, piezoelectricity, and ferromagnetism of the ceramics are significantly improved after the addition of Bi_0.5Na_0.5TiO₃. High resistivity (~1.3 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 27.4 μC/cm²), good piezoelectricity (d₃₃ =140 pC/N, k_p = 31.4%), and weak magnetic properties (M_r =0.19 emu/g) are observed.     

Sintering Mechanism and Microwave Dielectric Properties of Bi12TiO20 Ceramics

A homogeneous Bi₁₂TiO₂₀ phase was developed in a specimen that was calcined at 700°C without the formation of a secondary phase. A small amount of the Bi₁₂TiO₂₀ phase melted during sintering and assisted the densification of the specimen. The Bi₂O₃ and Bi₈TiO₁₄ secondary phases were found in all specimens. All the specimens that were sintered at temperatures ≥775°C exhibited high relative densities above 98% of the theoretical density. The Q × f value of the Bi₁₂TiO₂₀ ceramics was influenced by the grain size. The Bi₁₂TiO₂₀ ceramics sintered at 800°C for 5 h showed promising microwave dielectric properties of ε_r = 41, Q × f = 10 400 GHz, and τ_f = ?10.8 ppm/°C.