Influence of low concentration MgCo2(VO4)2 addition on microwave dielectric properties of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO_3?x MgCo₂(VO₄)₂ ceramics with x = 0, 0.5, 1.0, 2.0, and 5.0 wt% was fabricated via conventional solid-state reaction process. The effects of such additives on the structure, dielectric and tunability properties were systemically investigated. A small number of secondary phase identified as Ba₃(VO₄)₂ appeared in Ba_0.6Sr_0.4TiO_3?x MgCo₂(VO₄)₂ ceramics when x is more than 5.0 wt%. With increasing of MgCo₂(VO₄)₂ content, the peak values of permittivity gradually decreased and shifted to low temperature. The Ba_0.60Sr_0.40TiO₃ added with 0.5 wt% MgCo₂(VO₄)₂ possesses a dielectric constant of 2763, Q value of 267 at ～1 GHz and tunability of 35.9% under dc electric field of 30 kV/cm at 10 kHz.     

Improved dielectric properties of Ba0.5Sr0.5TiO3–ZnAl2O4 composite ceramics using double sintering

Ba_0.5Sr_0.5TiO₃–ZnAl₂O₄ composite ceramics were prepared by double sintering and conventional sintering. The results show that the double sintering can effectively reduce the ion diffusion between Ba_0.5Sr_0.5TiO₃ and ZnAl₂O₄ phases. The double sintered samples exhibit higher density and more uniform grain size distribution than the conventional sintered samples. The dielectric permittivity of double sintered samples is lower than that of conventional sintered samples. Impedance spectrum analysis shows that the oxygen vacancy content and grain boundary resistance of the double sintered samples is lower than that of the conventional sintered samples, which indicates that the Q value of the double sintered samples is higher than that of the conventional sintered samples. The optimum dielectric tunability and Q value of double sintered 60 wt%Ba_0.5Sr_0.5TiO₃-40 wt%ZnAl₂O₄ sample are 23.4% at 30 kV/cm and 276 at 2.257 GHz, respectively. Therefore, double sintering is a strategy that can effectively adjust the dielectric tunability and Q value of BST-ZA composite ceramics.     

Combustion-synthesized ferrites and ferroelectrics for microwave applications

The individual phases of Ni_{1 ? x} Cd _x Fe₂O₄ (x = 0.2, 0.4, 0.6) ferrite and Ba_0.8Sr_0.2TiO₃ ferroelectric (BST phase) were successfully prepared by autocombustion route. The oscillatory behavior of transmittance for the Ni-Cd ferrite was observed at about 0.44 at 8.2 GHz. The absorption study depicts the hopping phenomena of microwaves through the Ni-Cd phase. The dependence of microwave conductivity on ferrite content was discussed. The dielectric permittivity of Ni-Cd ferrites varied between 10 and 30. Compared to the Ni-Cd phase, the transmittance of the Ba_0.8Sr_0.2TiO₃ phase was found to be low. The dip in reflection loss for Ba_0.8Sr_0.2TiO₃ is equivalent to the minimum reflection or the maximum absorption of the microwave power for BST phase. The maximum microwave conductivity for Ba_0.8Sr_0.2TiO₃ was found to be about 0.459 S/cm. The high microwave permittivity for Ba_0.8Sr_0.2TiO₃ had a value of about 120.21.     

Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Effect of strontium substitution on the structure and dielectric properties of unfilled tungsten bronze Ba4-xSrxSmFe0.5Nb9.5O30 ceramics

The effects of Sr²⁺ substitution for Ba²⁺ on phase structure, microstructure, dielectric and electric properties for Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2, 3 and 4) ceramics were systematically researched. X-ray diffraction patterns show that Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2 and 3) ceramics are tetragonal tungsten bronze compound with a space group of P4bm, while the sample for x = 4 is an orthorhombic structure compound. The result can be corroborated by the analysis of Raman spectroscopy. As the Sr²⁺ contents increase from 0 to 3, the full width at half maximum of Raman lines of all samples increase gradually, indicating that the degree of lattice distortion increase. All tetragonal tungsten bronze ceramics exhibited a broad permittivity peaks, accompanied by frequency dispersion, indicating all samples are relaxor. The electrical properties of BSSFN ceramics were further studied by complex impedance spectroscopy. XPS spectrum shows that Fe²⁺ and Fe³⁺ coexist in Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ ceramics, and their proportion varies with the concentration of Sr²⁺.     

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.     

Property optimization of Ba0.4Sr0.6TiO3–BaMoO4 composite ceramics for tunable microwave applications

(1 ? x)Ba_0.4Sr_0.6TiO₃–xBaMoO₄ ceramics with x = 5, 10, 20, 30, 40 and 60 wt% were prepared by traditional solid-state reaction method. Two crystalline phases, a cubic perovskite structure Ba_0.4Sr_0.6TiO₃ (BST) and a tetragonal scheelite structure BaMoO₄ (BM) were obtained by XRD analysis. The microwave dielectric properties of Ba_0.4Sr_0.6TiO₃–BaMoO₄ composite ceramics were investigated systematically. The results show that the composite ceramics exhibited promising microwave properties. The dielectric constant can be adjusted in the range from 900 to 78, while maintaining relatively high tunability from 27.3% to 12.8% under a direct current electric field of 60 kV/cm and Q values from 619 to 67 in the gigahertz frequency region.     

Preparation of La1?x Sr x CoO3 Oxide Electrode Material and its Influence on the Structure and Dielectric Performance of the Supported Ba1?x Sr x TiO3 Thin Films

Using the sol–gel method, La_1−x Sr _x CoO₃ (LSCO) electrode films were first fabricated on the Si (100) substrates, followed by the growth of Ba_1−x Sr _x TiO₃ (BST) thin films on the LSCO electrode film. The crystal structure and surface morphology of these films were characterized by XRD and SEM. The effects of Sr-doping and annealing temperature on the structure and electric resistivity of the LSCO films and the dielectric properties of the BST films were studied. Results show that the La_0.5Sr_0.5CoO₃ electrode annealed at 750 °C has the lowest electric resistivity, 1.1 × 10⁻³Ω cm. The relative permittivity of the La_0.5Sr_0.5CoO₃-supported BST films first increases and then decreases with Sr-doping. The relative permittivity of the BST film decreases while the dielectric loss increases with frequency. Among the studied BST films, Ba_0.5Sr_0.5TiO₃ has the largest relative permittivity and the smallest dielectric loss (95 and 0.1, respectively) when the frequency is 1 kHz.     

Dielectric and Magnetic properties of (1 − x)BiFeO3–xBa0.8Sr0.2TiO3 ceramics

The polycrystalline samples of (1 ? x)BiFeO₃–xBa_0.8Sr_0.2TiO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4 and x = 1) were prepared by the conventional solid state reaction method. The effect of substitution in BiFeO₃ by Ba_0.8Sr_0.2TiO₃ on the structural, dielectric and magnetic properties was investigated. X-ray diffraction study showed that these compounds crystallized at room temperature in the rhombohedral distorted perovskite structure for x ≤ 0.3 and in cubic one for x = 0.4. As Ba_0.8Sr_0.2TiO₃ content increases, the dielectric permittivity increases. This work suggests also that the Ba_0.8Sr_0.2TiO₃ substitution can enhance the magnetic response at room temperature. A remanent magnetization M_r and a coercive magnetic field H_C of about 0.971 emu/g and 2.616 kOe, respectively were obtained in specimen with composition x = 0.1 at room temperature.     

Improvement of dielectric loss of (Ba,Sr)(Ti,Zr)O3 ferroelectrics for tunable devices

(Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ (0.05 ≤ x ≤ 0.3) ferroelectric materials have cubic perovskite structure and show paraelectric properties at room temperature. Curie point shifted to a negative value as increasing Zr content in (Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ system. When Zr substituted 0.1 mol, the dielectric constant, dielectric loss, tunability, Curie point and FOM were 4500, 0.0005, 63%, −1.6 °C and 1260, respectively. This composition shows excellent microwave dielectric properties than those of (Ba_0.6Sr_0.4)TiO₃ ferroelectrics, which are limelight materials for tunable devices such as varactors, phase shifters and frequency agile filters, etc.     

The characterization of mixed titanate Ba1−xSrxTiO3 phase formation from oxalate coprecipitated precursor

Mixed titanates Ba_1−xSr_xTiO₃ were synthesized via calcination of oxalate coprecipitated precursors. On heating there were three thermal event occurred: T<250°C corresponds to the evaporation of trapped water and dehydration, T=250°C–450°C corresponds to the decomposition of oxalate and the formation of an intermediate phase where the composition is close to [Ba_1−xSr_x]₂Ti₂O₅.CO₃ and T=600°C–700°C corresponds to the carbonate decomposition and the formation of Ba_1−xSr_xTiO₃ phase. Powders calcined at T=700°C for 2 h are single phase, have grain size ranges of 0.2–2 μm and elongated morphology. Rietveld refinements of the XRD data showed single phase of perovskite structure in which their tetragonality decreased with increasing concentration of Sr²⁺ incorporated in Ba²⁺ site. The transition temperature showed strong correlation with the tetragonality.     

Ba0.4Sr0.6TiO3/MgO Composites with Enhanced Energy Storage Density and Low Dielectric Loss for Solid-State Pulse-Forming Line

(100−x) wt% Ba_0.4Sr_0.6TiO₃−x wt% MgO composites (10≤x≤30) were prepared using Ba_0.4Sr_0.6TiO₃ powder and nanosized MgO powder (∼60 nm) by a solid-state reaction. The energy storage density and dielectric loss were investigated for the purpose of a potential application in solid-state pulse-forming line. The results show that Ba_0.4Sr_0.6TiO₃/MgO composites exhibit a notably enhanced energy density and low dielectric loss, compared with pure Ba_0.4Sr_0.6TiO₃. The enhancement of the energy density is attributed to the notable increase in breakdown strength of the composites and the improvement of dielectric constant stability with regard to electric field. In the case of x=30, the samples exhibited a breakdown strength of 33.1 kV/mm, an energy density of 1.14 J/cm³, a moderate dielectric constant of 270, and a low dielectric loss of 4 × 10⁻⁴.     

Tunable BaxSr1-xTiO3 nanoparticles induced high energy storage density in layer-structured asymmetric polymer-based nanocomposites

In order to solve the problem of low charging and discharging energy density of dielectric capacitors, the structure design of layered polymer matrix composites is carried out in this paper. Ba_0.7Sr_0.3TiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.9Sr_0.1TiO₃ nanoparticles were successfully prepared by the oxalate coprecipitation method. The surface of Ba_xSr_1-xTiO₃ was successfully coated with dopamine, which promoted the dispersion of the polymer matrix of the ceramic powder. Monolayer Ba_xSr_1-xTiO₃/PVDF composites containing Ba_xSr_1-xTiO₃ with different Ba/Sr ratios were successfully prepared by the casting method. Three-layer asymmetric composites with different fillers were successfully prepared by layer-by-layer casting. The phase and microstructure of the as-prepared materials were analyzed by XRD and SEM. The dielectric, electrical conductivity, ferroelectric and energy storage properties of the composites were tested. The effects and laws of the design of the three-layer asymmetric structure on the dielectric properties and energy storage properties of the layered composites are mainly studied. When the structure of the three-layer asymmetric composite is 1-2-3, the breakdown field strength reaches 330 kV/mm, the discharge energy density reaches 8.51 J/cm³, and the charge-discharge efficiency is 67%. This work demonstrates that layered composites with asymmetric properties can facilitate the development of electrical energy storage.     

Temperature‐Stable Relative Permittivity from −70°C to 500°C in (Ba0.8Ca0.2)TiO3–Bi(Mg0.5Ti0.5)O3–NaNbO3 Ceramics

Temperature‐stable relaxor dielectrics have been developed in the solid solution system: 0.45Ba_0.8Ca_0.2TiO₃–(0.55 ? x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃. Ceramics of composition x = 0 have a relative permittivity ?_r = 950 ± 15% over a wide temperature range from +70°C to 600°C. Modification with NaNbO₃ at x = 0.2 decreases the lower limiting temperature to ?70°C, but also decreases relative permittivity such that ?_r ~ 600 ± 15% over the temperature range ?70°C to 500°C. For composition x = 0.3, the low‐temperature dispersion in loss tangent, tan δ, (at 1 kHz) shifts to lower temperature, giving tan δ values ≤0.02 across the temperature range ?60°C to 300°C in combination with ?_r ~ 550 ± 15%. Values of dc resistivity for all samples are of the order of 10¹⁰ Ω m at 250°C and 10⁷ Ω m at 400°C.     

Effect of SrTiO3 concentration and sintering temperature on microstructure and dielectric constant of Ba1−xSrxTiO3

The Ba_1−xSr_xTiO₃ materials have received increased attention as one of the most important materials for electroceramic components, such as high dielectric ceramic capacitors, tunable phase shifters and PTCR. In this paper, the effect of SrTiO₃ concentration and sintering temperature on the microstructure and dielectric constant of Ba_1−xSr_xTiO₃ materials at the Curie temperature have been investigated. When Ba_1−xSr_xTiO₃ materials were sintered at 1350 °C, the peak value of the dielectric constant, ϵ_max, monotonically decreased with increasing SrTiO₃ concentration. At the sintering temperature of 1400 °C the dielectric constant maximum at the T_C increased with an increase in the x value, reaching the highest value at around x=0.4 and then decreased. As sintering temperature increased to 1450 °C, ϵ_max increased with increasing SrTiO₃ concentration up to x=0.6. The dielectric properties of Ba_1−xSr_xTiO₃ materials were discussed in terms of SrTiO₃ concentration and microstructure.     

Microstructure and dielectric properties of Ba0.6Sr0.4TiO3-MgAl2O4 composite ceramics

(1 − x)Ba_0.6Sr_0.4TiO₃-xMgAl₂O₄(x = 25, 30, 35 and 40 wt%) composite ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and tunability of the composites have been investigated. The XRD patterns analysis reveals two crystalline phases, a cubic perovskite structure Ba_0.6Sr_0.4TiO₃ (BST) and a spinel structure MgAl₂O₄ (MA). SEM observations show that the BST grains slightly dwindle and agglomerate with increasing amounts of MA. A dielectric peak with very strong frequency dispersion is observed at higher MA content, and the Curie temperature shifts to a higher temperature with increasing MA content. The ceramic sample with 30 wt% MA has the optimized properties: the dielectric constant is 1503, the dielectric loss is 0.003 at 10 kHz and 25 °C, the tunability is 23.63% under a dc electric field of 1.0 kV/mm, which is suitable for ferroelectric phase shifter.     

Microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics

The densification, microstructural evolution and microwave dielectric properties of (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba_1−ySr_y)WO₄ phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics is increased by raising the Sr²⁺ content, which significantly lowers the sintering temperature. The microstructure of the (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic sintered at 1400 °C results in the best microwave dielectric properties, including ε_r of 20.6, Q×f of 152,600 GHz and τ_f of +24.0 ppm/°C.     

Effects of non-stoichiometric defects on the dielectric properties of (Ba0.5Sr0.5)xTiO3-ZnGa2O4 composite ceramics

In order to explore the effects of non-stoichiometric defects on the dielectric properties of composite ceramics, 70 wt% (Ba_0.5Sr_0.5)_xTiO₃-30 wt%ZnGa₂O₄ ((BS)_xT50-ZG, x = (Ba + Sr)/Ti = 0.99, 1.00, 1.01 and 1.05) composite ceramics were fabricated by the traditional sintering technique. The association between structure and dielectric properties has been studied. The results show that the distortion of the crystal lattice brought by the partial Schottky defects, namely [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^×, induces a decrease in the Curie temperature of (BS)_xT50-ZG composite ceramics. The orientation of the elastic dipoles brought by oxygen vacancies causes the pinning of the domain walls, which reduces the dielectric loss at low frequencies. Tunability is related to the dipole polarization caused by [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^× defect complexes. In addition, compared with the composite ceramic with x = 1, the Q values of the composite ceramics with x < 1 and x > 1 decreases due to the deterioration of the microstructure homogeneity and the enhancement of the disorder of the B-site cations in (BS)_xT50.     

Microstructure and microwave-frequency electromagnetic properties of Ni0.4Zn0.6Fe2O4/Ba0.6Sr0.4TiO3 composites

(x)Ni_0.4Zn_0.6Fe₂O₄+(1−x)Ba_0.6Sr_0.4TiO₃ composite ceramics with x=0.6, 0.7, 0.8, 0.9 and 1 were synthesized by solid state reaction method. The high dense composites have only two phases, i.e., Ni_0.4Zn_0.6Fe₂O₄ and Ba_0.6Sr_0.4TiO₃. The permittivity ε′ of the composites decreases slightly with the frequency increasing from 3 MHz to 1 GHz. The permittivity ε′′ of the composites also shows a little increase with frequency in the 3 MHz–1 GHz range. The permeability displays a relaxation resonance within the 3 MHz–1 GHz frequency range. The permeability μ′ increases while the cut-off frequency decreases with the Ni_0.4Zn_0.6Fe₂O₄ concentration, obeying the Snoek's law μ_if_r=constant. The permittivity ε′ of the composites decreases with Ni_0.4Zn_0.6Fe₂O₄ concentration. The composites have a relatively higher ε′ than the pure Ni_0.4Zn_0.6Fe₂O₄ at 1–10 GHz. In the frequency range of 1–10 GHz, the magnetic permeability μ′ reaches its maximum and μ′′ shows a minimum for the composite with x=0.6 in all ceramics. The permeability μ′ of the composites decreases with dc magnetic field at 1–10 GHz. The permeability shows a domain wall resonance, and the resonance frequency shifts to high frequency with the dc magnetic field. The permittivity was also influenced by the dc magnetic field due to a magnetodielectric effect.     

Dielectric tunable properties of high dielectric breakdown Ba0.5Sr0.5TiO3–Zn2P2O7 composite ceramics

Dielectric properties of Ba_0.5Sr_0.5TiO₃–xZn₂P₂O₇ (x = 1, 3, 5, 10, 15 wt%) composite ceramics, which were prepared by solid-state reaction process, were intensively investigated. The results showed that the Curie temperature (T_c) of composites gradually shifted to lower temperature (?140 °C) with increasing the content of Zn₂P₂O₇, and the dielectric constant were tuned effectively from 2020 to 107, while maintaining a relatively high tunability. Zn₂P₂O₇ additions remarkably inhibited the grain growth of Ba_0.5Sr_0.5TiO₃ phases, and improved the breakdown strength of samples up to 385 kV/cm. The sample with x = 10 wt% exhibited good dielectric properties (?_r = 290, tg δ = 0.0006, T = 20.5%, BDS = 297 kV/cm). Meanwhile Zn₂P₂O₇ addition also made the T_c far away from the room temperature, which reduced the sensitivity of the dielectric constant to temperature change and simultaneously improved the stability of materials.