Tuning of electrical properties of xBi(Zn0.5Ti0.5)O3-(1-x) (Ba0.5Sr0.5)TiO3 ceramics by composition design and bandgap engineering

Herein, the xBi(Zn_0.5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ (x = 0.05, 0.10, 0.15, 0.20) novel negative temperature coefficient (NTC) ceramic materials were fabricated by solid-state method. X-ray diffraction revealed that xBi(Zn_0·5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ successfully formed solid solution. The UV–vis diffuse spectra of the samples indicate that the band gap increases with the increasing Bi(Zn_0·5Ti_0.5)O₃ content. The resistance temperature curve showed that with the increase of Bi(Zn_0·5Ti_0.5)O₃ content, the resistivity ρ of the ceramics at 400 °C increased from 5.96 × 10⁶ to 2.67 × 10⁷ Ω cm, as well as an increase in the B_400/800 from 12374.6 to 13469.1 K. The enhanced resistivity is attributed to the increased band gap and reduced carrier pairs caused by the Bi(Zn_0.5Ti_0.5)O₃ modification. The impedance data indicates that the conduction process is activated by thermal. The ceramic samples exhibit the excellent NTC characteristics over a range of 400 °C–1000 °C. Hence, the xBi(Zn_0.5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ ceramics have the potential to become high temperature NTC ceramics that can operate in a wide temperature range.     

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg_0·5Ti_0.5)³⁺ for Al³⁺ lowered the sintering temperature and improved the Q × f value of MgAl₂O₄ ceramic. The MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with ε_r = 9.1, Q × f = 98,000 GHz, τ_f = −61.36 ppm/°C. The sintering temperature of MgAl_1·94(Mg_0·5Ti_0.5)_0.06O₄ microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl₂O₄ ceramics.     

Enhanced energy-storage properties and dielectric temperature stability of (Bi0·5Na0.5)0.84Sr0·16Ti1-x(Y0·5Nb0.5)xO3 lead-free ceramics

A series of lead-free (Bi_0·5Na_0.5)_0.84Sr_0·16Ti_1-x(Y_0·5Nb_0.5)_xO₃ (abbreviated as BNST-100xYN) relaxor ferroelectric ceramics were prepared by solid state reaction sintering. The micro morphology, dielectric properties, and energy storage properties of the ceramics with increasing doping content were systematically studied, and their conductive mechanism was also studied. The perovskite structure was not significantly changed with the addition of (Y_0·5Nb_0.5)⁴⁺ complex ions, but it led to a certain amount of flake grains appear and element precipitation with increasing composition. And the larger dielectric breakdown strength (DBS) and lower remanent polarization (P_r) were attained with the recoverable energy storage density (W_rec) of ~1.0433 J/cm³ for x = 0.04 composition. In addition, it showed outstanding dielectric temperature stability and cycle stability. These results indicated that BNST-4YN ceramics are an excellent candidate for energy storage device and temperature-stable dielectric equipment.     

High energy storage efficiency of NBT-SBT lead-free ferroelectric ceramics

Ceramic-based dielectrics have been widely used in pulsed power capacitors owing to their good mechanical and thermal properties. Bi_0.5Na_0.5TiO₃-based (NBT-based) solid solutions exhibit relatively high polarization, which is considered as a promising dielectric energy storage material. However, the high remnant polarization and low energy efficiency limit their application in dielectric capacitors. Herein, a typical relaxor ferroelectric Sr_0·7Bi_0·2TiO₃ (SBT) was introduced into the NBT system to strengthen the overall relaxor behavior, resulting in reduced remnant polarization. We prepared (1-x)NBT-xSBT (x = 0.35, 0.45, 0.55, and 0.65) ceramics by the conventional solid-phase reaction method and further investigated their microstructures, dielectric and energy storage properties. With the increase of SBT content, the size of the grains and the maximum dielectric constant gradually decreased, simultaneously. Furthermore, the dielectric shoulder corresponding to the maximum dielectric constant shifted to a lower temperature, indicating that the enhancement of polarization dynamics was a consequence of the domain refinement. As a result, the optimum property was identified in the 0.45NBT-0.55SBT sample with a high recoverable energy density of 1.34 J/cm³ and an outstanding energy efficiency of 96% at a low electric field of 100 kV/cm.     

Synthesis of Ti2(InxAl1-x)C (x = 0–1) solid solutions with high-purity and their properties

Herein, high-purity Ti₂(In_xAl_1-x)C (x = 0–1) solid solutions were successfully synthesized. The crystal structure and actual composition of solid solutions were confirmed using XRD, SEM, and TEM analyses, and their formation mechanism was revealed by thermal analysis. On the In-rich side (x ≥ 0.5), primary Ti₂InC first formed and then acted as a crystalline seed for the subsequent solid solutions, resulting in a cluster-like morphology. The lattice constants of Ti₂(In_xAl_1-x)C were found to well follow Vegard’s law. The examined properties of Ti₂(In_xAl_1-x)C also greatly depended on their A-site compositions. Ti₂AlC exhibited the highest hardness and elastic moduli, while the best corrosion resistance was achieved at Ti₂InC, and all Ti₂(In_xAl_1-x)C displayed active dissolution in 0.5 M HCl solution. Thus, adjusting the In/Al ratio at A-site can yield a desired set of performances, which provides a good example for regulating the performance of MAX phases via A-site solid solution strategy.     

Bi0·5Na0·5TiO3–Sr0.85Bi0·1TiO3 ceramics with high energy storage properties and extremely fast discharge speed via regulating relaxation temperature

Bi_0·5Na_0·5TiO₃-based relaxor ferroelectric ceramics have recently gained increasing attention due to their outstanding energy storage properties. However, the trade-off between the recoverable energy storage density/efficiency and discharge rate resulted from the hysteresis of domain switching process, severely limits their applications. Herein, a strategy realizing synergistic excellent energy storage properties and fast discharge rate is proposed through regulating relaxation temperature. The relaxation temperature was decreased to below room temperature by introducing Sr_0·85Bi_0·1TiO₃ into Bi_0·5Na_0·5TiO₃ [(1-x)Bi_0·5Na_0·5TiO₃-xSr_0.85Bi_0·1TiO₃, x = 0.5–0.7)], enabling the small size and weak correlation polar nanoregions (PNRs) with relatively high polarization. The trade-off was overcome by reducing the hysteresis of electrical switching of weak correlation PNRs. Thus, large recoverable energy storage density of 2.32 J/cm³ and high efficiency of 80.1% (250 kV/cm) were achieved simultaneously for x = 0.7 ceramics. Meanwhile, extremely rapid discharge rate (<30 ns) and remarkable power density of 63.7 MW/cm³, which were superior to the previously reported lead-free ceramics were realized. Besides, the 0.3BNT-0.7SBT ceramics also possess good thermal stability over 25 °C–115 °C at 100 kV/cm and good frequency stability (5–100 Hz). These properties make the 0.3BNT-0.7SBT ceramic an ideal candidate for energy storage applications.     

Influence of annealing environments on the conduction behaviour of KNN-based ceramics

The electrical conduction behaviour of the lead-free (1-x)K_0·5Na_0·5Nb_0·95Sb_0·05O₃-xBi_0.5Na_0·41K_0·09ZrO₃ (x = 0.05) ceramics annealed in air, oxygen, and argon environments at various temperatures was investigated. Post-sintering annealing in different atmospheres affected the temperature of dielectric phase transition and electrical conductivity values significantly. The ceramic sample annealed in an oxygen environment showed the lowest conductivity for grain (σ_g ～ 1.22 × 10^?5 S/m) and grain boundary (σ_gb ～ 1.56 × 10^?6 S/m) regions at 250 °C which is due to the reduction in oxygen vacancy defects. The activation energy E_a for DC conduction in this sample obeyed the Arrhenius relation and was found to be ～ 0.99 ± 0.03 eV. An additional anomaly observed at T ～ 250 °C dielectrics vs. temperature plots for the as-sintered and argon-annealed samples is ascribed to defect relaxation, which was inconspicuous in the samples annealed in air and oxygen. In addition, the in-situ impedance measurement was performed to analyze the impact of argon atmosphere on the electrical conductivity of the O₂-annealed samples with high-temperature electrode curing.     

Crystal structure and microwave dielectric properties of (Ba1−αSrα)6−3xSm8+2xTi18O54 solid solutions

The tungstenbronze-type-like (Ba_1−αSr_α)_6−3xR_8+2xTi₁₈O₅₄ (R = rare earth) solid solutions have been studied. Microwave dielectric properties of these solid solutions link to the substitution of large cations such as Ba ions by small Sr ions. In these solid solutions for x = 0, the quality factor (Q·f) exibits extremely low due to the inner strain resulting by the occupation of large Ba ions at relatively small A1-sites. However, dielectric constant (ɛ_r) of this composition show a high value. In this paper, improvement of quality factor (Q·f) by substituting small Sr ions for Ba ions in (Ba_1−αSr_α)_6−3xSm_8+2xTi₁₈O₅₄ solid solutions, where x = 0, is reported. In addition, the relationship between crystal structure and microwave dielectric properties is discussed from the viewpoint of structural formula and occupational state of large cations such as Ba, Sr and Sm in A1-sites.     

Study on phase structures and compositions,microstructures, and dielectric characteristics of (1-x)NdGaO3-xBi0.5Na0.5TiO3 microwave ceramic systems

(1–x)NdGaO₃-xBi_0.5Na_0.5TiO₃ [(1–x)NdGaO₃-xBNT, 0.05 ≤ x ≤ 0.7)] ceramic systems were fabricated using a conventional solid-state reaction, and their phase structures, microstructures, and microwave dielectric characteristics were systematically investigated. The XRD patterns results showed an orthorhombic perovskite structure as the main phase with a Pbnm space group at x = 0.05. In a range of x = 0.1–0.3, the main Nd₃Ga₅O₁₂ cubic structure phases (Ia-3d space group) had been formed due to the increase in the cation vacancies resulting from the Na and Bi ion volatilization at the higher densification sintering temperatures (1400–1450 °C). As (Na_0.5Bi_0.5)²⁺ ions increased and the sintering temperatures decreased, the orthorhombic perovskite-type Pnma space group solid solutions (the main phases) were detected at x = 0.4 and 0.5, accompanied with a certain amount of second phase Bi₂O₃ in the samples at x = 0.6 and 0.7. The results of Raman spectroscopy analysis, EDS data analysis, and surface morphologies observations were basically in keeping well with the XRD analysis results, wherein the Raman-active modes also implied that the crystal structure of the main phase actually had a tendency to change the perovskite structure when x = 0.3. The ε_r value increased gradually as the x value increased from 0.05 to 0.5 because of the increase in the ionic polarizability, and increased slowly at x = 0.6, then decreased slightly as the x value further increased to 0.7 due to the formation of the second phase. The Q × f values of the (1-x)NdGaO₃-xBNT (x = 0.05–0.7) ceramic systems were strongly influenced by the densification, lattice defects, and phase composition and content. The τ_f values could be well predicted by replacing Na, Bi and Ti ions with suitable substitutions for these ceramic systems. As a result, the new temperature-stable ceramics with optimal dielectric properties of ε_r ~43.1, Q × f ~6700 GHz (at 5.85 GHz), and τ_f ~ −24.5 ppm/°C and ε_r ~ 44.5, Q × f ~ 5600 GHz (6.12 GHz), and τ_f ~ +2.4 ppm/°C were obtained in the (1-x)NdGaO₃-xBNT composite series at x = 0.5 and x = 0.6 sintered at 1320 °C and 1250 °C for 4 h, respectively.     

Microwave dielectric properties of BaO–Ta2O5–TiO2 system

Microwave dielectric properties of the BaO–Ta₂O₅–TiO₂ system were investigated by the solid-state reaction method. It was recognized that the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions have the higher Q · f value in comparison with the Ba₈(Ta_4 − xNb_x)Ti₃O₂₄ solid solutions. The limit of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions was approximately x = 0.75; the lattice parameter c of the solid solutions, which is related to the change in the B(1)O₆ octahedron, was significantly increased in the composition range from 0 to 0.75. The Q · f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions are remarkably improved by the Sn substitution for Ti; the highest Q · f value of 59,100 GHz is obtained at x = 0.75. Moreover, the ɛ_r and τ_f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions at x = 0.75 were 25.6 and 30.3 ppm/°C, respectively.     

Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

The high nano-domain improves the piezoelectric properties of KNN lead-free piezo-ceramics

Due to their high Curie temperature and large dielectric constant, potassium sodium niobate-based lead-free piezo-ceramics (KNN) are regarded as one of the most hopeful piezo-ceramics candidate materials. Herein, (1-x) (K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃ - x (Ba_0.5Sr_0.25)ZrO₃ [abbreviated as (1-x) KNNS - x BSZ, x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05] lead-free piezo-ceramics are prepared through chemical doping using the traditional solid phase method. The phase structure, domain structure, and microstructure of KNN ceramics have been thoroughly examined. Doping of BZS causes the formation of R-O-T phase boundaries and increases the proportion of polar nano-domains within the crystals, thus increasing the rate of motion of the domain walls and making the domains more easily deflected. The piezoelectric and dielectric properties of the material are improved simultaneously. When x = 0.04, the piezoelectric properties of ceramics reach the optimal value (d₃₃ = 351 pC/N, T_C = 305 °C, K_p = 43% and ε_r = 41267). This work offers a fresh concept for enhancing the overall performance of lead-free piezo-ceramics and aids in understanding the nature of doping modification of lead-free piezo-ceramics.     

Enhanced microwave absorption properties of Zn-substituted SrW-type hexaferrite composites in the Ku-band

Enhanced microwave absorption properties were successfully achievable from SrFe_2-xZn_xFe₁₆O₂₇ (SrFe_2-xZn_xW; x = 0.0, 0.5, 1.0, and 2.0) hexaferrite filler-epoxy resin matrix composites. The composite samples were fabricated with the filler volume fractions (V_f) of 30, 50, 70, and 90%. Compared with fully Zn-substituted SrZn₂W composite (x = 2.0), unsubstituted and partially Zn-substituted SrFe_2-xZn_xW (x = 0.0, 0.5, and 1.0) composites exhibited much higher real and imaginary parts of complex permittivity (ε_r), which is attributable to higher electron hopping between Fe²⁺ and Fe³⁺ ions, and also slightly higher real and imaginary parts of complex permeability (μ_r) due to higher saturation magnetization (M_s). Among all samples, a 2.8 mm-thick SrFe_1·5Zn_0·5W (x = 0.5) composite with the V_f of 90% exhibited the most appropriate for application in the region of 3.4–3.8 GHz, having the minimum reflection loss (RL_min) of ?46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38–3.81 GHz) below ?10 dB, while a 2.15 mm-thick SrFeZnW (x = 1.0) composite with the V_f of 70% showed the most appropriate for application in the region of 5.9–7.1 GHz, possessing the RL_min value of ?23.7 dB at 6.6 GHz with the bandwidth of 1.38 GHz (5.85–7.23 GHz) below ?10 dB. Consequently, partially Zn-substituted SrW-type hexaferrites are very promising microwave absorbers for 5G mobile communications in the Ku band (0.5–18 GHz).     

Room temperature constructing rhombohedral-tetragonal phase boundary in novel (Bi,Na)(Zr,Ti)O3 modified (K,Na)(Nb,Sb)O3 ceramics: Phase structure,defect and piezoelectric performance

Lead-free (1-x)(K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃-x(Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃ ceramics (abbreviated as (1-x)KNNS-xBNZT, x = 0, 0.01, 0.02, 0.03, 0.035 and 0.04) were synthesized by the solid-state method, and the dependence of phase evolution, microstructure, oxygen vacancy defect and electrical properties on compositions were carefully investigated. All ceramics had a pure perovskite structure and a dense microstructure. The phase transition temperatures (T_R-O and T_O-T) of the ceramics were adjusted by adding BNZT, and the rhombohedral-tetragonal (R-T) phase coexistence boundary was successfully constructed at room temperature when x = 0.03, the excellent piezoelectric performance (d₃₃ ～ 323 pC/N, k_p ～ 0.372) and high Curie temperature (T_C ～ 276 °C) have been achieved at this time. The grain size of the ceramics showed a strong difference on x content, and the maximum relative density value of 95.42% was obtained. The domain structure characterized by PFM confirmed that the ceramics possess small-sized nano-domains and complex domains at x = 0.03, which are the origin of enhanced piezoelectric properties. Moreover, the oxygen vacancy defect that can pin the domain walls was increased with the addition of (Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃. As a result, the doping with BNZT can significantly affect the phase structure and electrical properties of the ceramics, indicating that the (1-x)KNNS-xBNZT ceramics system with a R-T phase boundary is a promising lead-free piezoelectric material.     

Ionic/electronic mixed conduction relations in perovskite-type oxides by defect structure

Perovskite-type oxides La_1-aA_aM_1-bB_bO_3-x with A=Sr²⁺, Ln³⁺, Ce⁴⁺ and M=Fe, Co, Ga; B=Co, Fe, Mg were prepared in the concentration range a=0·1 to 1 mol and b=0·1 to 0·5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of pO₂ in the range 10⁵>pO₂>10⁻¹⁴ Pa and temperature (500 to 1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III-perovskites Sr(Co,Fe)O_3-x in their stabilized form are excellent mixed conductors (maximum 500 S cm⁻¹ at 400°C) and have up to 2 orders of magnitude higher oxygen ionic conductivity than the preferred III–III-perovskite La(Sr)Mn(Co)O_3-x. The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O_3-x, was increased by doping with 0·1 mol Co. By applying higher Co or Fe doping concentrations the lanthanum gallate, becomes a mixed conductor.     

Simultaneously achieving high energy-storage density and power density under low electric fields in NBT-based ceramics

Aiming at the problem that power density and energy density are difficult to obtain simultaneously under low field, a novel composition (1-x)Na_0·5Bi_0·5TiO₃-xBaZn_1/3Ta_2/3O₃((1-x)NBT-xBZT) was designed and fabricated via solid-state methods. With the addition of BZT, the crystal lattice, structural symmetry, grain size, and dense degree were all increased proved by XRD, Raman, and Archimedes drainage method et al. Because of the enhancement of relaxor behavior, the x=0.10 sample displayed a high permittivity ε_r of 2871±15% and a low dielectric loss tan δ ≤ 0.025 in the wide temperature range of 60–400 ^oC. This ceramic also showed maximum recoverable energy density W_d (2.07 J/cm³) with high efficiency η (71.5%) under a low field of 150 kV/cm. Moreover, pulse discharge testing proved that this ceramic possessed both a significant discharge energy density W_D (0.96 J/cm³) and a record high power density P_D (108.54 MW/cm³). This work provided a promising material for high power and energy applications.     

Phase,microstructure and ferroelectric properties of new complex-structured ferroelectric ceramics in the PZT–SBN system

This study aimed to fabricate and characterize new complex-structured ceramics with formula (1-x)Pb(Zr_0.52Ti_0.48)O₃–xSrBi₂Nb₂O₉ or (1-x)PZT–xSBN (where x=0, 0.1, 0.3 and 0.5 weight fraction). The ceramics were prepared by a solid-state mixed-oxide method and sintered at temperatures between 1000 and 1250 °C. Optimum sintering temperature for this system was found to be 1050 °C for 3 h dwell time. X-ray diffraction patterns of (1-x)PZT–xSBN powders showed peak intensities of two-phase mixture corresponding to the relative amount of each phase as a result of SBN addition. Microstructure of (1-x)PZT–xSBN ceramics showed a variation in grain shape and grain size. The small addition of SBN (x=0.1) was also found to improve ferroelectric properties of pure PZT ceramic.     

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.     

Microstructure induced ultra-high energy storage density coupled with rapid discharge properties in lead-free Ba0.9Ca0.1Ti0.9Zr0.1O3–SrNb2O6 ceramics

Novel lead-free (1-x)Ba_0·9Ca_0·1Ti_0·9Zr_0·1O₃-xSrNb₂O₆ ceramics were synthesized via a two-step high energy ball milling process. The evolution of microstructural properties, phase transformation, and energy storage characteristics was comprehensively investigated to assess the applicability of material in multi-layered ceramic capacitors. The substitution of SrNb₂O₆ (SNO) in Ba_0·9Ca_0·1Ti_0·9Zr_0·1O₃ (BTCZ) has resulted in substantial improvement in materials density along with a small increase in the grain size of the synthesized ceramic. A thorough microstructural investigation indicates an excellent dispersibility and compatibility between BTCZ and SNO phases. With an increase in SNO substitution, a transition from typical ferroelectric to relaxor ferroelectric has been observed, which has led to a significantly slimmer ferroelectric loop along with frequency dispersive dielectric properties. The optimized composition (i.e., x = 0.10) exhibits an ultra-high recoverable energy density of 2.68 J/cm³ along with a moderately high energy efficiency of 83.4%. Further, SNO substituted samples have also shown an enhancement in breakdown strength. The improvement in energy storage performance and breakdown strength of SNO substituted BTCZ composites are mainly attributed to relatively homogeneous grain morphology, optimum grain size, microstructural density, and improved grain boundary interface.     

Ultra-high quality factor and low dielectric constant of (Zn0.5Ti0.5)3+ co-substituted MgAl2O4 ceramic

In this study, MgAl₂O₄-based ceramics with high quality factor (Qf) and low dielectric constant (ε_r ≤ 10) were obtained by fabricating MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ (x = 0–0.5) ceramics via conventional solid-state reaction method. Excellent microwave dielectric properties were achieved for samples at x = 0.5 and sintered at 1550 °C, i.e., ε_r = 9.86, Qf = 263 900 GHz (five times better than that for x = 0 sample) and τ_f = ?92 ppm/°C. The X-ray diffraction (XRD) patterns displayed characteristic peaks of MgAl₂O₄ with spinel structure. MgTi₂O₅ and MgTiO₃ were considered as secondary phases. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and relative density analysis indicated that ultra-high Qf values were dominated by dense microstructure, secondary phase and cation vacancies; whereas ε_r values were mainly affected by secondary phase and ionic polarizability. MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics with excellent microwave dielectric properties have potential application in millimeter-wave communication, dielectric filters, dielectric antennas and resonators.