Enhancement of the dielectric permittivity and magnetic properties of Dy substituted strontium titanate ceramics

Structural, dielectric and magnetic properties of dense Dy-substituted strontium titanate ceramics are investigated. In the Sr_1-1.5xDy_xTiO₃ system, incorporation of Dy onto the Sr site is confirmed by a linear decrease of the lattice parameter up to x = 0.05. Dielectric spectroscopy analysis of Sr_1-1.5xDy_xTiO₃ ceramics reveals four relaxations. Two relaxations observed below 55 K are attributed to dipoles formed by off-centre displacement of Dy³⁺ ions on the Sr sites. Other two dielectric relaxations found at higher temperatures are attributed to the oxygen vacancy related mechanisms. As result, very high dielectric permittivity of ～33500 at 28 K and of ～9600 around room temperature at moderate dissipation factor of ～0.02 are obtained for Sr_0.985Dy_0.01TiO₃ ceramics, making it a promising material for capacitor electronic applications. Paramagnetic behaviour observed for Sr_1-1.5xDy_xTiO₃ as well as for Sr_1-xDy_2xTi_1-xO₃ ceramics indicates impossibility to induce a magnetic order and hence magnetoelectric coupling in strontium titanate by Dy substitution.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

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.     

Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics

SiO₂-added barium strontium titanate ceramics Ba_0.4Sr_0.6TiO₃-xwt%SiO₂ (x=0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO₂ additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO₂ additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO₂ additive led to improve the breakdown strength, further increase of SiO₂ deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm³ and energy storage efficiency of 79% at 134 kV/cm.     

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.     

Effects of Sr-doping on the giant magnetocaloric effect of EuTiO3

The magnetic properties and magnetocaloric effect of Eu_1−xSr_xTiO₃ (x=0–0.1) compounds are investigated. With slight Sr-doping, the ferromagnetic (FM) coupling significantly increased and FM exchange is dominant in the delicate balance. A giant reversible magnetocaloric effect (MCE) and large refrigerant capacity (RC) for Eu_1−xSr_xTiO₃ compounds were observed. The values of −ΔS_M^max are evaluated to be around 10 J/kg K under a magnetic field change of 1 T and 21 J/kg K under a magnetic field change of 2 T, respectively. But, the values of RC are increased with the more Eu in EuTiO₃ to be substituted by Sr. Therefore, the giant reversible MCE and large RC make the Eu_1−xSr_xTiO₃ compound a good candidate for magnetic refrigerant working at low-temperature and low-field.     

Impedance spectroscopic studies of sol–gel derived barium strontium titanate thin films

In the present work electroceramic thin films of barium strontium titanate (Ba_1?xSr_xTiO₃ – BST) were deposited on stainless steel substrates by sol–gel technique. Homogeneous Ba_0.6Sr_0.4TiO₃ thin films as well as spatially inhomogeneous BST thin films exhibiting artificial gradients in composition normal to the growth surface were deposited. Both up- and down-graded BST films were fabricated by depositing successive layers with Sr mole fraction x ranging from x = 0.5 to x = 0.3. In the present study the tool of impedance spectroscopy has been used to study the dielectric properties of BST thin films at room temperature. To analyze the impedance spectroscopy data the Nyquist (Z″ vs. Z′) plots as well as the simultaneous representation of the imaginary part of impedance and electrical modulus (Z″, M″) vs. frequency were used. Experimental data were fitted using the CNLS fitting method. Agreement between experimental and simulated data was established. The data indicated that the thin film samples fabricated can be represented by an equivalent circuit with two relaxation frequencies.     

Influence of sintering temperature on energy storage properties of BaTiO3–(Sr1−1.5xBix) TiO3 ceramics

The effect of sintering temperature on microstructure, dielectric properties and energy storage properties of BaTiO₃–(Sr_1?1.5xBi_x)TiO₃ (x = 0.09) (BT–SBT) ceramics was investigated. The sintering temperature has pronounced influence on the grain size, shrinkage, and dielectric properties of the BT–SBT ceramics. With increasing sintering temperature, the dielectric constant increases largely. However, the increasing tendency of the dielectric breakdown strength (BDS) is less noticeable but become more evident with the consideration of Weibull modulus. For the BT-SBT ceramics, the unreleased energy density decreases and the electric field stability of the energy storage efficiency enhances with the increase of sintering temperature.     

Microwave dielectric properties of the (BaxSr1−x)TiO3 thin films on alumina substrate

Barium strontium titanate, (Ba_xSr_1?x)TiO₃ (BST) thin films have been prepared on alumina substrate by sol–gel technique. The X-ray patterns analysis indicated that the thin films are perovskite and polycrystalline structure. The interdigital electrode with 140 nm thickness Au/Ti was fabricated on the film with the finger length of 80 μm, width of 10 μm and gaps of 5 μm. The temperature dependence of dielectric constant of the BST thin films in the range from ?50 °C to 50 °C was measured at 1 MHz. The dielectric properties of the BST thin films were measured by HP 8510C vector network analyzer from 50 MHz to 20 GHz.     

Investigation of Ba1−xSrxTiO3 ceramics prepared from powders synthesized by the modified Pechini route

Ba_1−xSr_xTiO₃ (x = 0, 0.20, 0.25, 0.30 and 0.35) nanopowders were prepared by Pechini method from titanium isopropoxide, barium and strontium carbonates using citric acid as a chelating agent and ethylene glycol as an esterification agent. X-ray diffraction data show the formation of (Ba,Sr)TiO₃ solid solutions, free from secondary phases as BaCO₃ or Ti-rich oxides, when the polymeric precursors were calcined in air at 850 °C for 2 h. Ceramic pellets with relative density of 85–93% were obtained after sintering at 1350 °C for 3 h. High values of the dielectric constants (of ∼1500–12,000), low losses at the room temperature and a shift of the ferro-para phase transition temperature in the range of 7–127 °C with x decreasing were found. Lower values of the Curie constant for higher x indicate the increase of the chemical and electrical local heterogeneity degree.     

Continuous hydrothermal synthesis of Ca1−xSrxTiO3 solid-solution nanoparticles using a T-type micromixer

In this work, a continuous hydrothermal synthesis method in supercritical water was applied to environmentally benign production of Ca_1−xSr_xTiO₃ (x = 0.0–1.0) solid-solution nanoparticles as key materials in conducting, electric, magnetic, and photocatalytic fields. A T-type micromixer (330 μm id) was introduced for rapid heating of stating solutions of Ca(NO₃)₂, Sr(NO₃)₂, and TiO₂ sol using turbulent flow mixing with preheated NaOH aqueous solution and also for exact control of reaction temperature. At 673 K and 30 MPa for 5.0 s mean residence time, Ca_1−xSr_xTiO₃ solid-solution nanoparticles having crystallite diameters of around 20 nm with monomodal diameter distributions were obtained without byproducts and production of CaTiO₃ and SrTiO₃ separately over the whole composition range. Effects of NaOH molality, Ca and Sr compositions in the starting solutions, and mean residence time on the reaction were examined. The results showed that TiO₂ sol dissolution and Ca_1−xSr_xTiO₃ precipitation were almost finished within 0.25 s mean residence time, and after that Ostwald ripening proceeded.     

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

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

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.     

Microstructure and thermoelectric properties of La0.1Dy0.1SrxTiO3 ceramics

La_0.1Dy_0.1Sr_xTiO₃ (x=0.80, 0.78, 0.75, 0.70) powders were synthesized via a sol-gel method, followed by sintering at 1550 °C in a reducing atmosphere of 5 vol% hydrogen in nitrogen. The microstructure and thermoelectric properties of the Sr-deficient La and Dy co-doped SrTiO₃ were investigated. The result of XRD revealed that La_0.1Dy_0.1Sr_xTiO₃ consisted of SrTiO₃ with a cubic crystal structure as the main phase and of a small amount of Dy₂Ti₂O₇ as the second phase. All the Sr-deficient samples exhibited a step-like microstructure. As the nominal Sr deficient content increased, the electrical conductivity of the Sr-deficient La_0.1Dy_0.1Sr_xTiO₃ ceramics enhanced due to the increasing Sr and oxygen vacancies, the absolute value of the Seebeck coefficient increased a little, and the thermal conductivity decreased to ~3.0 W m⁻¹ K⁻¹, leading to a high ZT value of 0.19 for La_0.1Dy_0.1Sr_0.75TiO₃ at 500 °C.     

Effect of La doping on tunable behavior of sol–gel-derived PST thin film

Pb_xSr_1−xTiO₃ and (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin films were prepared on ITO/glass substrate by sol–gel technique. The crystalline phase structures of thin films were investigated by XRD. The dielectric properties were measured by LCZ meter. Results show that the perovskite phase was stable in Pb_xSr_1−xTiO₃ thin film. Its lattice constant was found to increase with the increase of x. When Pb/Sr ratio was about 50/50, the transformation of the perovskite phase between cubic and tetragonal took place at room temperature. To ensure better tunability, Pb_0.4Sr_0.6TiO₃ thin film was selected in both paraelectric state (cubic structure) and near the phase transformation point between cubic and tetragonal. The tunability of more than 80% and figure of merit (FOM) of 14.17 were obtained. (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ was also prepared as La-doped thin film. Its dielectric constant was decreased below x = 0.4 and then increased above x = 0.4 with the increase in x. The inharmonic coefficient depicting ability of tunability of (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film kept constant due to the changeable shape of the crystal cell under electric field at a fixed temperature below x = 0.4, which is in good agreement with Johnson's theory. And the coefficient reveals a distinctive variable because of easily changeable shape under different bias above x = 0.4. The tunability of about 70% and FOM of about 7 were obtained in the (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Microwave dielectric properties of tungstenbronze type like (Ba1−αSrα)6−3xR8+2xTi18O54 (R = Sm,Nd) solid solutions

Tungstenbronze type like Ba_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) dielectric ceramics reveal high quality factor Q·f as well as high dielectric constant ɛ_r. We have investigated the effect of Sr substitution for Ba ions on the microwave dielectric properties of the compounds. (Ba_1−αSr_α)_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) ceramics were prepared in the composition ranges of x = 0–0.2 and α = 0–0.312 and the microwave dielectric properties were investigated. (Ba_1−αSr_α)_6−3xSm_8+2xTi₁₈O₅₄, where x = 0.1 and α = 0.298, and (Ba_1−αSr_α)_6−3xNd_8+2xTi₁₈O₅₄, where x = 0.2, α = 0.296 revealed remarkably higher Q·f value among the solid solutions, indicating that Q·f increased with substituting Sr ions into Ba ions at the rhombic A1-site. This fact suggests that relaxation of local distortions at the A1-sites is closely related to improvement of Q·f.     

Dielectric and piezoelectric properties of (1−x)(Bi,Na)TiO3–x(Bi,K)TiO3 lead-free ceramics for piezoelectric energy harvesters

The piezoelectric and dielectric properties of the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0.12, 0.14, 0.18, 0.20 and 0.30) lead-free ceramics were investigated. Specimens were prepared by the conventional mixed oxide method and sintered at 1170 °C in air. Scanning electron microscopy indicated that increasing x from 0.12 to 0.30 causes a decrease in the grain size. The (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ ceramics shows a homogeneous microstructure and excellent dielectric and piezoelectric properties. Specimens with optimum composition showed a piezoelectric charge constant d₃₃ of 166 pC/N, an electromechanical coupling factor k_p of 0.5, a dielectric constant ε_r of 1591.32 at 1 kHz and generated power output of 37.49 nW/cm².     

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.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.