Improvement of dielectric thermal stability of BST ferroelectric material for tunable applications

100(1 − y)wt%(Ba_0.6Sr_0.4)(Ti_0.8Zr_0.2)O₃-100y wt%MgO (BSTZ20-MgO, y = 0.4, 0.5, 0.6, 0.7) composite ceramics were synthesized via a solid-state reaction route, aiming at improving the dielectric thermal stability of BST for tunable applications. Excellent dielectric thermal stability (low temperature coefficient of permittivity value) was gained in this system. According to our experiment, diffuse phase transition (DPT) induced by Zr ion substitution, ferroelectric-dilution effect of MgO phase and grain size refinement were considered as contributions to this good dielectric thermal stability.     

Microstructure and dielectric relaxor behavior of Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3–BaBi4Ti4O15 ceramics by tape casting

Plate-like BaBi₄Ti₄O₁₅ powders were used to fabricate 0.952[Ba(Zr_0.2Ti_0.8)O₃–(Ba_0.7Ca_0.3)TiO₃]–0.048BaBi₄Ti₄O₁₅(abbr. BZCT-BBT) ceramics by tape casting. The microstructure and dielectric relaxor behaviors of BZCT-BBT ceramics were investigated. BZCT-BBT ceramics can be sintered well at 1,100?°C and mainly consisted of tetragonal perovskite phase and BaBi₄Ti₄O₁₅ (abbr. BBT) phase. The lattice constants decrease as the sintering temperature increases due to substitution of Bi³⁺ for the A-site atoms of the perovskite structure. There is no obvious difference between the structure in the perpendicular and parallel directions, however, an evident difference of dielectric properties in the two directions is observed. Comparing with Ba(Zr_0.2Ti_0.8)O₃–(Ba_0.7Ca_0.3)TiO₃(abbr. BZCT) ceramics, BZCT-BBT ceramics show obvious relaxor characteristics which are evidenced by the degree of diffuseness γ calculated using the modified Curie–Weiss law. Meanwhile, the addition of BBT decreases T_m, which results from the decrease of grain size. The reduction of ε_m is mainly caused by phase structure deviation from the coexisting rhombohedral and tetragonal structure to single tetragonal.     

Dielectric energy storage densities in Ba1-xSrxTi1-yZryO3 ceramics

This paper presents measurements of the Weiss temperature, the Curie constant, the spontaneous polarization and the stored dielectric energy density of Ba_1-xSr_xTi_1-yZr_yO₃ ceramics for compositions with 0≤x≤1 and 0≤y≤0.5. The first three of these dielectric parameters are used to predict the density of energy storage that can be obtained when ceramics of these compositions are used as capacitor dielectrics. These predictions are compared with direct measurements of energy densities which show that for moderate electric field strengths (-50 kV mm ^-1) the stored dielectric energy densities were the largest for ceramics with x>0.7 and y?～0.2. © 2000 Kluwer Academic Publishers     

Microstructure and electrical properties of Ba0.7Sr0.3(Ti1 − xZrx)O3 thin films prepared on copper foils with sol-gel method

Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0, 0.1, 0.2) (BSZT) thin films have been prepared on copper foils using sol-gel method. The films were annealed in an atmosphere with low oxygen pressure so that the substrate oxidation was avoided and the formation of the perovskite phase was allowed. The X-ray diffraction results show a stable polycrystalline perovskite phase, with the diffraction peaks of the BSZT films shifting toward the smaller 2θ with increasing Zr content. Scanning electron microscopy images show that the grain size of the BSZT thin films decreases with increasing Zr content. High resolution transmission electron microscopy shows the clear lattice and domain structure in the film. The dielectric peaks of the BSZT thin films broaden with increasing Zr content. Leakage current density of Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0.1) thin film is the lowest over the whole applied voltage.     

Slip casting of sol–gel-synthesized barium strontium zirconium titanate ceramics

The sol–gel method was used to synthesize two different Ba_0.75Sr_0.25Ti_0.95Zr_0.05O₃ powders: one of high purity and the other of low purity. These two sol–gel-synthesized powders show two distinct particle sizes and surface areas. The slip casting method was applied to these two sol–gel powders followed by a pressureless sintering, which shows large differences in sintered density and grain size for the pressureless sintered disks. Neutron powder diffraction shows a transition to the nonpolar cubic Pm–3m space group at higher temperatures for both materials. Pair distribution function analysis was used to examine the local displacements of the Ti⁴⁺ and Zr⁴⁺ cations. The dielectric constant, loss tangent, and bias were measured on these two materials.     

Microwave dielectric properties of (Ca0.8Sr0.2)(SnxTi1−x)O3 ceramics

In this paper, we study the behavior of the B-site behavior with the incorporation of Sn⁴⁺ ion in (Ca_0.8Sr_0.2)TiO₃ ceramics. An excess of Sn⁴⁺ resulted in the formation of a secondary phase of CaSnO₃ and SrSnO₃ affecting the microwave dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics. The dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics were improved because of the solid solution of Sn⁴⁺ substitution in the B-site. The temperature coefficient of resonant frequency (τ_f) of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics also improved with increasing Sn content.     

Grain growth,densification and electrical properties of lead-free piezoelectric ceramics from nanocrystalline (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 powder by sol–gel technique

The sintering behavior revealed in the sintering processes of the conventional and a two-step process and electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics from the nanocrystalline powders synthesized by a sol–gel technique were systematically studied. It was found that the sintering process of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics made from nanocrystalline powders was significantly improved, the sintering temperature was reduced markedly from 1,540 to 1,280 °C, as well as a high relative density (>97 %) was obtained in the conventional sintering. Under the two-step sintering conditions, the full densification and the most suppression of grain growth was achieved simultaneously. The (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics from nanocrystalline powders sintered by the two-step sintering technique (sintered at T ₁ of 1,300 °C for 1 min and T ₂ of 1,150 °C for 20 h) exhibited the optimum average grain size of 700 nm and a high relative density of 98 %. The electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics were greatly influenced by the grain size and phase structure formed under the both sintering conditions, with sintering temperature and grain size increased, the electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics, which made from nanocrystalline powders, shows an enhancing trend: d ₃₃ ~100 pC/N, k _p ~53.3 % for the specimen sintered at 1,300 °C for 1 min and 1,150 °C for 20 h, d ₃₃ ~310 pC/N, k _p ~53.3 % for the specimen sintered at 1,350 °C for 2 h respectively.     

Effect of Sr substitution on structural,dielectric, magnetic and magnetoelectric properties of rapid liquid sintered BiFe0.8Ti0.2O3 ceramics

Structural, dielectric, magnetic and magnetoelectric properties of polycrystalline Sr doped BiFe_0.8Ti_0.2O₃ ceramics [Bi_1?xSr_x(Fe_0.8Ti_0.2)O₃; x = 0.05, 0.10 and 0.15)] were studied. All the samples were prepared by rapid liquid phase sintering method. Rietveld refinement of X-ray diffraction patterns of all samples confirmed that the samples crystallize in a rhombohedral structure and showed change in Fe–O–Fe bond angle and Fe–O bond length which in turn enhanced magnetization from 0.33 to 0.73 emu/g with the increase in Sr concentration from x = 0.05 to 0.15. The dielectric constant and dielectric loss were observed to increase with the increase in temperature from 30 to 500 °C. An anomalous peak has been observed in dielectric constant versus temperature plot around 300 °C for all the samples, which is close to the magnetic transition temperature of BiFeO₃. The composition-dependent magnetic properties with the expected Fe²⁺/Fe³⁺ ratio fluctuations were correlated by X-ray photoelectron spectroscopy. Magnetic and electric hysteresis loops showed a systematic increase in magnetization and polarization as a result of Sr doping in BiFe_0.8Ti_0.2O₃ ceramics. The value of magnetocapacitance at 10 kHz was observed as 0.95, 1.23 and 1.73 for x = 0.05, 0.10, 0.15 respectively in Bi_1?xSr_xFe_0.80Ti_0.20O₃ ceramics.     

High dielectric tunability of ferroelectric (Ba1−x,Srx)(Zr0.1,Ti0.9)O3 ceramics

(Ba_1?x,Sr_x)(Zr_0.1,Ti_0.9)O₃ (BSZT) ceramics with x = 0, 0.05, 0.15, 0.25, 0.35 and 0.45 were prepared by conventional solid state reaction method. The structural characterization with X-ray diffraction and scanning electron microscopy indicate a monotonical drop in lattice constants and grain size with the increase of Sr concentration. Consequently, the Curie temperature and remnant polarization of the ceramics exhibit a strong compositional dependence. A linear relationship between the Curie temperature and Sr concentration is revealed. At x = 0.45, the BSZT ceramics show substantially high tunability of over 55 % under 20 kV/cm dc electric field with very low dielectric loss value of 0.0025 at room temperature, suggesting the BSZT ceramics could be a promising alternative to traditional (Ba,Sr) TiO₃ ferroelectrics for developing high frequency tunable dielectric devices.     

Microwave dielectric properties of (1 − y)Nd(1−2x/3)Bax(Mg0.5Sn0.5)O3–yCa0.8Sr0.2TiO3 ceramic

The (1 ? y)Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little Nd₂Sn₂O₇ as the second phase. An apparent density of 6.89 g/cm³, a dielectric constant (ε _r ) of 19.1, a quality factor (Q × f) of 212,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?68 ppm/°C were obtained when the Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,550 °C for 4 h. The temperature coefficient of resonant frequency (τ _f ) increased from ?68 to +55 ppm/°C as y increased from 0 to 0.7 when the (1 ? y)Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were sintered at 1,600 °C for 4 h. 0.4Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–0.6 Ca_0.8Sr_0.2TiO₃ ceramic that was sintered at 1,600 °C for 4 h had a τ _f of ?7 ppm/°C.     

Influence of sintering and annealing temperature on grain boundary barrier layer capacitors in a modified reduction-reoxidation method

To produce grain boundary barrier layer capacitors with a simpler fabrication process and more-stable dielectric characteristics, a modified reduction-reoxidation method was developed. Nb₂O₅-doped (Ba_0.8 Sr_0.2)(Ti_0.9 Zr_0.1)O₃ (BSTZ) was calcined at 1170 °C for complete formation of ABO₃ phases. After calcination CuO was added to BSTZ as a liquid-phase promoter and insulating boundary layer material. The ceramics were sintered in a reducing atmosphere, and then the fired samples were annealed in air to reoxidize the reduced CuO to form insulating layers. The dielectric constant of the fabricated capacitors was decreased for higher annealing temperature, longer annealing time and smaller grain size. The loss tangent of the fabricated capacitors was increased for BSTZ with more CuO added, and was almost unchanged with annealing temperature, annealing time and grain size because of the existence of an insulating boundary layer material (CuO).     

Enhanced dielectric and tunable properties of barium strontium titanate thin films through introducing Nd(Zn1/2Ti1/2)O3 and adjusting Ba/Sr

Ba_0.6Sr_0.4TiO₃ (BST) and 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba _x Sr_1?x TiO₃ (NZT–BST) thin films with x = 0.6, 0.7, 0.75, and 0.8 were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method. The structures, surface morphology, dielectric, and ferroelectric properties, and thermal stability of BST and NZT–BST thin films were investigated as a function of NZT and Ba content. It was found that introducing NZT into BST decreased significantly dielectric loss, however, along with the tunability. On this basis, increasing Ba/Sr in NZT–BST thin films led to the simultaneous increase of dielectric constant and tunability of thin films. As a result, optimized dielectric and tunable properties were obtained for 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba_0.7Sr_0.3TiO₃ thin film with the highest FOM value of 43.22. It awakens us that, for reducing dielectric loss, introducing a certain amount of low permittivity oxides or non-ferroelectrics like NZT into weak ferroelectric perovskite tunable materials, not into paraelectric perovskite tunable materials, may obtain more excellent dielectric and tunable performances.     

Synthesis and characterization of sol–gel derived (Ba,Ca)(Ti,Zr)O<Subscript>3</Subscript> nanoparticles

A citrate precursor method was employed to synthesize lead-free perovskite 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT) crystallites. Powders and gels were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectra, thermal analysis and transmission electron microscopy. It was indicated that BZT-0.5BCT transparent gel can be obtained via the chelation of citric acid with metal ions. Gels transformed into crystalline powders with single-phase perovskite structure when heat-treated above 650 °C, significantly lower than that in a solid-state reaction method. The primary particle size of the powders increased from 30 to 60 nm as the decomposition temperature was raised from 600 to 750 °C.     

Low temperature synthesis of Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nanopowders by solution based auto combustion method

Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT–BCT) nanopowders were synthesized by citrate–nitrate auto combustion method using TiO₂ as a source of titanium. The formation mechanism, phase evolution and particle size were investigated using thermal analysis, Fourier transform infra-red spectroscopy, X-ray diffraction, Raman spectroscopy and transmission electron microscope. The molar ratio of metal, citric acid and ethylene diamine tetraacetic acid was kept at 1:1.5:0.1 and pH ~ 9, to get a uniform and complete combustion. Perovskite BZT–BCT was formed in the as-burnt state (280 °C), though calcination at 700 °C is required to remove a small amount of impurities, which is significantly lower than the solid state reaction method. The particle size of BZT–BCT powder was in the range of 40–70 nm.     

Microstructure and microwave dielectric properties of xSm(Mg0.5Ti0.5)O3–(1 − x)Ca0.8Sr0.2TiO3 ceramics

xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ (x = 0.50–0.95) ceramics are prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties are investigated as a function of the x-value and sintering temperature. The single phase solid solutions were obtained throughout the studied compositional range. The variation of bulk density and dielectric properties are related with the x-value. Increasing sintering temperature can effectively promote the densification and dielectric properties of xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ ceramic system. With the content of Sm(Mg_0.5Ti_0.5)O₃ increasing, the temperature coefficient of resonant frequency τ _f value decreased, and a near-zero τ _f could be obtained for the samples with x = 0.80. The optimal microwave dielectric properties with a dielectric constant ε _r of 30.1, Q × f of 115,000 GHz (at 8.0 GHz), and τ _f of 8.9 ppm/°C were obtained for 0.80Sm(Mg_0.5Ti_0.5)O₃–0.20Ca_0.8Sr_0.2TiO₃ sintered at 1,550 °C for 3 h, which showed high density and well-developed grain growth.     

Dielectric and magnetoelectric properties of (Ni,Cu)Fe2O4 + [(Ba,Pb)(Ti,Zr)]O3 composites

The observation of magnetoelectric coupling is reported in (x) Ni_0.8Cu_0.2Fe₂O₄ + (1 − x) Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ composites synthesized by normal solid state reaction method. The phase formation was confirmed by X-ray diffraction technique and no intermediate or impurity phase is present in all the composites. The dielectric constant (ε′) and dielectric loss (tan δ) were measured as a function of frequency and temperature. The dielectric constant was found to be enhanced with increase in ferrite content in the composite. It was also revealed that magnetoelectric output decreases with increase in ferrite content in the composite due to leakage current. The maximum voltage coefficient of 0.411 mV/cm/Oe was observed in 15% Ni_0.8Cu_0.2Fe₂O₄ + 85% Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ composite.     

Effects of zirconium on the structural and dielectric properties of (Ba,Sr)TiO3 solid solution

The effects of zirconium on (Ba, Sr)TiO₃ solid solution are studied by preparing (Ba_0.75Sr_0.25) (Zr_y Ti_1-y)O₃ (BSZT) ceramics with ranging from 0 to 0.15. With increased incorporation of zirconium the lattice parameters increase with decreasingc/a ratio, while a linear relationship is found between (a ² c)^1/3 values and the content of zirconium. The Curie temperature (T _c) is also lowered linearly with increasing . The Curie temperature of the (Ba_1-xSr_x (Zr_yTi_1-y)O₃ system can be estimated byT _c = 125 – 270x – 350_y (°C). Observations of microstructures also show that a small amount of zirconium promotes the grain growth, while a higher content will then inhibit it. Dielectric properties of BSZT dielectrics and BSZT-based boundary layer (BL) capacitors are also correlated to have similar temperature dependence, where the maximum dielectric constant at room temperature is found aty = 0.1.     

Effects of La-doping on microstructure, dielectric and piezoelectric properties of Ba0.85Ca0.15Ti0.90Zr0.10O3 lead-free ceramics

Lead-free (Ba_0.85Ca_0.15)_1?xLa_2/3xTi_0.90Zr_0.10O₃ + 1 mol% MnO₂ ceramics have been prepared an ordinary sintering technique and the effects of La-doping on the microstructure, dielectric and piezoelectric properties of Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ were studied. All the ceramics possess a pure perovskite structure, indicating that La ions are incorporated into Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ lattices to form a lead-free solid solution. The ceramics are transformed from coexistence of orthorhombic and tetragonal phases to pseudocubic phase with the doping level of La increasing. After the doping of La, grain growth is inhibited, ferroelectric-paraelectric phase transition temperature (T _C) is decreased and the degree of diffuse phase transition is increased. The ferroelectricity of the ceramics is weakened after the addition of La. Unlike donor-doped lead zirconate titanate ceramics, the piezoelectric properties of the ceramics are degraded after the partial substitution of La³⁺ for (Ba_0.85Ca_0.15)²⁺ because of the weakness or disappearance of coexistence of orthorhombic and tetragonal phases near room temperature. The (Ba_0.85Ca_0.15)_1?xLa_2/3xTi_0.90Zr_0.10O₃ + 1 mol% MnO₂ ceramic with x = 0 exhibit the optimum piezoelectric properties: d ₃₃ = 277 pC/N and k _p = 30.3 %, respectively.     

Effects of Metallic Spacer in Layered Superconducting Sr2(Mg y Ti1−y )O3FeAs

The highly two-dimensional superconducting sys tem Sr₂(Mg _y Ti_1?y )O₃FeAs, recently synthesized in the range of 0.2≤y≤0.5, shows an Mg concentration-dependent T _c . Reducing the Mg concentration from y=0.5 leads to a sudden increase in T _c , with a maximum T _c ≈40 K at y=0.2. Using first principles calculations, the unsynthesized stoichiometric y=0 and the substoichiometric y=0.5 compounds have been investigated. For the 50 % Mg-doped phase (y=0.5), Sr₂(Mg _y Ti_1?y )O₃ layers are completely insulating spacers between FeAs layers, leading to the fermiology such as that found for other Fe pnictides. At y=0, representing a phase with metallic Sr₂TiO₃ layers, the Γ-centered Fe-derived Fermi surfaces (FSs) considerably shrink or disappear. Instead, three Γ-centered Ti FSs appear, and in particular two of them have similar size, like in MgB₂. Interestingly, FSs have very low Fermi velocity in large fractions: the lowest being 0.6×10⁶ cm/s. Furthermore, our fixed spin moment calculations suggest the possibility of magnetic ordering, with magnetic Ti and nearly nonmagnetic Fe ions. These results indicate a crucial role of Sr₂(Mg _y Ti_1?y )O₃ layers in this superconductivity.     

Effects of cobalt and sintering temperature on electrical properties of Ba0.98Ca0.02Zr0.02Ti0.98O3 lead-free ceramics

Lead-free (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃-xmol% (x = 0–1.6) cobalt ceramics (BCZT-xCo) have been fabricated by the traditional solid-state reaction technique and the effects of Co and sintering temperature on ferroelectric, dielectric and piezoelectric properties of (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃ lead-free ceramics have been studied systematically. The orthorhombic–tetragonal (T _O–T) transition shift towards lower temperature with increasing Co addition, while Curie temperature (T _c) remained at relatively high value of 107 °C. And the Main piezoelectric parameters are optimized at x = 0.8 mol% with a high piezoelectric coefficient (d ₃₃ = 330 pC/N), a planar mode electromechanical coupling factor (k _p = 46.7 %), a high dielectric constant (ε _r = 2,675) and a low dielectric loss (tanδ = 0.90 %) at 1kHZ. Besides these, high remnant polarization (P _r) and low coercive field (E _c) of 11.5 μC/cm², 0.31 kV/cm are also obtained at (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃-0.8 mol%Co lead-free ceramics. Furthermore, greatly enhanced temperature stability of the piezoelectric properties was obtained in the temperature range from 20 to 90 °C. The above results indicate that BCZT-Co ceramics are promising lead-free materials for practical applications.