Microwave dielectric properties of Sr2Ce2Ti5O16 ceramics

Sr₂Ce₂Ti₅O₁₆ dielectric ceramics were prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics were investigated by X-ray diffraction and scanning electron microscopic methods. The Sr₂Ce₂Ti₅O₁₆ has a psuedocubic structure. It has ɛ_r of 113, unloaded quality factor (Q_u × f) of 8000 GHz and temperature coefficient of resonant frequency of 306 ppm/°C. The effects of various dopants on the structure, microstructure and microwave dielectric properties of the material have been investigated. It is found that addition of small amount of dopants such as PbO, Al₂O₃, Nd₂O₃, MoO₃, CeO₂, La₂O₃, Fe₂O₃ and NiO improve the microwave dielectric properties of Sr₂Ce₂Ti₅O₁₆.     

Microwave dielectric properties of Ba3La2Ti2Nb2−xTaxO15 ceramics

High dielectric constant and low loss ceramics in the system Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ (x = 0–2) have been prepared by conventional solid-state ceramic route. Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 49.8 to 45.1, quality factor Q_u × f from 22,000 to 31,040 GHz and temperature variation of resonant frequency from + 6.9 to − 13.4 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Effect of La2O3 doping on the tunable and dielectric properties of BST/MgO composite for microwave tunable application

Undoped and La₂O₃doped (0.5, 1.0, 2.0, 3.0 wt.%) Ba_0.55Sr_0.45TiO₃/MgO composites were prepared by traditional ceramic processing and their structural, surface morphological, tunable properties and their dielectric properties at low frequency and microwave frequency were systemically examined. The result shows that La₂O₃ dopant has a strong effect on the average grain size. The La₂O₃ doped samples have lower temperature coefficient of capacitance than the undoped. The 0.5 wt.% La₂O₃ doped sample has a little higher tunability than the undoped and the tunability of other doping concentration samples is lower as compared to the undoped. The addition of La₂O₃ decreases the dielectric constant and increases quality factor (Q × f) at microwave frequency. The 0.5 wt.% La₂O₃ doped samples have the best properties among these samples and have a higher tunability, lower dielectric constant and lower dielectric loss tangent at microwave frequency and these properties are very beneficial to the development of the tunable devices application.     

Synthesis and microwave dielectric properties of Ba4Ti3P2O15 ceramics

Present work introduces a new kind of microwave dielectric ceramic, Ba₄Ti₃P₂O₁₅. Ba₄Ti₃P₂O₁₅ ceramic can be prepared by solid state reaction method and be well densified after being sintered at above 1175 °C for 4 h in air. All the XRD patterns can be fully indexed as single-phase structure. The best microwave dielectric properties can be obtained in ceramic sintered at 1200 °C for 4 h with permittivity about 20.7, Q × f about 42,210 GHz and TCF about 37 ppm °C^?1. Measurements of the microwave dielectric properties of Ba₄Ti₃P₂O₁₅ ceramic revealed the existence of a maximum in the temperature dependence of the dielectric loss because of the defect dipoles relaxation.     

Structure,chemical stability and electrical conductivity of perovskite La0.6Sr0.4M0.3Fe0.7O3−δ (M = Co,Ti) oxides

The perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) powders have been synthesized by the citrate gel method. The structural and chemical stability of the La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. The electrical conductivities of the sintered La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) ceramics were measured. The results demonstrate the chemical stability in H₂/helium (He) atmosphere of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ oxide is improved significantly compared to that of the La_0.6Sr_0.4Co_0.3Fe_0.7O_3?δ oxide. The incorporation of Ti^3+/4+ ions in the perovskite structure can significantly stabilize the neighboring oxygen octahedral due to the stronger bonding strength, leading to the enhanced structural and chemical stability of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ. In addition, the perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides possess much higher chemical stability in CO₂/He atmosphere than that of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ oxide, in which the perovskite structure is destroyed completely in a flowing CO₂-containing atmosphere.     

Facile combustion synthesis and photoluminescence properties of Ce3+ doped Sr2La8(SiO4)6O2 phosphors

The Sr₂La₈(SiO₄)₆O₂:Ce³⁺ powder phosphor with apatite structure has been successfully synthesized via a facile route of sol-combustion technique. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectroscopy were used to characterize the as-prepared samples. Sr₂La₈(SiO₄)₆O₂:Ce³⁺ phosphor was composed of particles with average sizes range about 300 nm. The phosphor exhibited an absorption ranging from 220 to 390 nm in ultraviolet range and a broad blue-violet emission band peaked at 403 nm with a CIE coordinates of (0.167, 0.028). The concentration quenching mechanism was also determined to be dipole–dipole interaction.     

Effects of Pb2+ doping on La4Ti9O24 ceramics

The kinetic structural evolution of the Pb²⁺-doped La₄Ti₉O₂₄ ceramics was investigated. Using electron diffraction and Rietveld analysis of the X-ray powder diffraction patterns, we show that the increase in Pb²⁺ doping results in the structural transition from La₄Ti₉O₂₄ to a La_2/3TiO₃-type phase (Ibmm, No. 74). Further kinetic studies of Pb²⁺ diffusion into La₄Ti₉O₂₄ ceramics suggest that the La₄Ti₉O₂₄–La_2/3TiO₃ phase transition requires an activation energy of 607 ± 60 kJ/mol.     

Synthesis and luminescence properties of red long-lasting phosphor Y2O2S:Eu3+, Mg2+, Ti4+ nanoparticles

We report an effective method to synthesize Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ nanoparticles. Tube-like Y(OH)₃ were firstly synthesized by hydrothermal method to serve as the precursor. Nanocrystalline long-lasting phosphor Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ was obtained by calcinating the precursor with co-activators and S powder. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM and TEM observation reveals that the precursor synthesized via a hydrothermal routine has tube-like structure and the final phosphor reveals a hexagonal shape. The fine nanoparticles which have the particle size ranging from 30 to 50 nm show uniform size and well-dispersed distribution. From the spectrum, the main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). After irradiation by 325 nm for 10 min, the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ long-lasting phosphor shows very bright red afterglow and the longest could last for more than 1 h even after the irradiation source had been removed. It is considered that the long-lasting phosphorescence is due to the contribution from the electron traps with suitable trap depth.     

High permittivity and low dielectric loss of Na0.5Bi0.5−xLaxCu3Ti4O12 ceramics

The phase structure, microstructure and dielectric properties of Na_0.5Bi_0.5?xLa_xCu₃Ti₄O₁₂ (NBLCTO) ceramics were investigated. La³⁺ substitution had a great influence on the phase structure and dielectric properties. The results showed that the pure phase could be more easily obtained when substituting La³⁺ for Bi³⁺. Under the same processing condition (970 °C for 7.5 h) and measuring condition (10 kHz around room temperature), NBLCTO ceramics with x = 0.10 possessed the highest permittivity (1.02 × 10⁴) and lowest dielectric loss (0.022). The obtained NBLCTO ceramics with x = 0.10 also had good frequency stability and good temperature stability (?1.87% to +3.27%) from ?60 °C to 120 °C at both 1 and 10 kHz. Complex impedance results revealed that the grain resistance R_g was 7.18 Ω cm and the grain boundary resistance R_gb was 1.19 × 10⁶ Ω cm.     

Ba2La2TiNb2O12: A new microwave dielectric of A4B3O12-type cation-deficient perovskites

A new A₄B₃O₁₂-type cation-deficient perovskite Ba₂La₂TiNb₂O₁₂ was prepared by the conventional solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The compound crystallizes in the trigonal system with unit cell parameter a = 5.6726(3) Å, c = 27.740(2) Å, V = 773.04(9) ų and Z = 3. The microwave dielectric properties of the ceramic were studied using a network analyzer, and it shows a high dielectric constant of 42.70, a high quality factor with Q × f of 31,130 GHz, and a small negative τ_f of − 4.2 ppm °C^− 1.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Microstructure and photoluminescent properties of Sr2SiO4:Eu3+ phosphors with various NH4Cl flux concentrations

In this paper, effect of NH₄Cl flux concentrations (0, 1, 2, 3 and 4 wt%) on the crystal structure, morphology and photoluminescent properties of Sr₂SiO₄:Eu³⁺ phosphors synthesized by a microwave sintering at 1200 °C for 1 h was investigated and discussed. X-ray powder diffraction analysis showed the crystal structure was not affected and the pure Sr₂SiO₄ phase was formed without second phase or phases of starting materials when adding with the NH₄Cl flux. The SEM images indicated that increase of the NH₄Cl flux enlarged the particle size of the phosphor particles. The photo-luminescence results showed the addition of 1 wt% NH₄Cl flux much improved the emission intensity at λ_em of 617 nm as the excitation spectrum at λ_ex of 395 nm. The decay times of Sr₂SiO₄:Eu³⁺ phosphors with different NH₄Cl flux concentrations were obtained around 2.47–2.52 ms.     

Formation and electrical characterization of Ti-modified Sr0.3Ba0.7Nb2O6 ceramic system

Titanium modified Sr_0.3Ba_0.7Nb₂O₆ ceramic system has been studied in a wide range of compositions. As the sintering temperature exceeds the 1250 °C, the substitution of niobium by titanium induces liquid phase formation, which enhances the densities of the samples with compositions in the monophasic range. X-ray diffraction analysis shows a linear titanium incorporation into the Sr_0.3Ba_0.7Nb_2−yTi_yO_6−y/2 system up to a solubility limit 0.07 < y_m < 0.1, which it is also confirmed by the ferroelectric–paraelectric temperature transition. Both, the diffuse character of the system and the dielectric constant at room temperature increase as the Ti content increase. The dielectric constant (ɛ_r = 420) of the sample with a titanium content of 0.07 are two times higher than the reported for the SBN (30/70) system.     

Low-temperature sintering and microwave dielectric properties of CaTi1−x(Fe0.5Nb0.5)xO3 ceramics with B2O3 addition

CaTi_1−x(Fe_0.5Nb_0.5)_xO₃ (0 ≤ x ≤ 1) dielectrics were synthesized via the solid state reaction route and structure analysis was performed together with the dielectric characterization. The substitution of Ti⁴⁺ by Fe³⁺/Nb⁵⁺ and developed phase were studied by X-ray diffraction. The dielectric constant and temperature coefficient of resonant frequency decrease rapidly with an increase of x. The influence of 1–5 wt.% B₂O₃ as a sintering additive investigated at CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ solid solutions. The dielectric properties were found to strongly depend on the sintering conditions and contents of B₂O₃ additions. ɛ_r = 52.3, Q × f_o = 2930 GHz and T_f = 13 ppm/°C were obtained for CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ specimen 3 wt.% B₂O₃ sintered at 900 °C for 2 h.     

Electrical conductivity of LaxSr2−xFe1−yRuyO4±δ

Materials of the K₂NiF₄ structure type have been prepared and the electrical conductivity in air determined for a number of compositions in the La_xSr_2−xFe_1−yRu_yO_4±δ solid solution series including three with Ru substituted for Fe on the B site: La_0.2Sr_1.8Fe_0.6Ru_0.4O_4±δ, La_0.4Sr_1.6Fe_0.7Ru_0.3O_4±δ, and La_0.6Sr_1.4Fe_0.8Ru_0.2O_4±δ. Overall the total conductivity values measured were lower than expected for the Ru-doped materials, with a peak conductivity of ≈2 S cm⁻¹ at 700 °C. In the undoped La_xSr_2−xFeO_4±δ materials, a significant jump in conductivity was observed between the x = 0.7 and 0.8 compositions and was related to the bonding in the materials and the Fe³⁺/Fe⁴⁺ redox behaviour. In all materials, the conduction behaviour was found to follow a semi-conducting trend.     

Preparation,characterization and dielectric properties of Ba3−xSrxM4Ti4O21 (M = Nb,Ta and 0 ≤ x ≤ 3) ceramics

Ba_3−xSr_xM₄Ti₄O₂₁ (M = Nb, Ta and 0 ≤ x ≤ 3) ceramics have been prepared through solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and Scanning Electron Microscopic studies. In the present study, both Ba₃Nb₄Ti₄O₂₁ and Ba₃Ta₄Ti₄O₂₁ are obtained as single phase compositions whereas strontium rich compositions exhibit multiphase in nature. Energy dispersive X-ray analysis has been performed to identify the nature of additional phases present in the strontium rich compositions. The dielectric constant, dielectric loss and temperature variation of dielectric constant of the well sintered ceramic specimens have been studied in the low frequency region (< 13 MHz) using an impedance analyzer. The present study reveals that high dielectric compositions can be realized by Sr-substitution in the BaO–TiO₂–Nb₂O₅/Ta₂O₅ ternary systems.     

Modified giant dielectric properties of samarium doped CaCu3Ti4O12 ceramics

Effects of Sm³⁺ substitution on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The grain size of CaCu₃Ti₄O₁₂ ceramics was greatly decreased by doping with Sm³⁺, resulting from the ability of Sm³⁺ to inhibit the grain growth rate. This result can cause a decrease in the dielectric constant (?′) and loss tangent (tan δ) of CaCu₃Ti₄O₁₂ ceramics. Interestingly, high dielectric permittivity (?′ ～ 10,863) and low loss tangent (tan δ ～ 0.043 at 20 °C and 1 kHz) were observed in the Ca_0.925Sm_0.05Cu₃Ti₄O₁₂ ceramic. Nonlinear electrical properties of CaCu₃Ti₄O₁₂ ceramics were modified by doping with Sm³⁺. The dielectric relaxation behavior of Sm-doped CaCu₃Ti₄O₁₂ ceramics can be well ascribed based on the internal barrier layer capacitor model of Schottky barriers at the grain boundaries.     

Dielectric properties of lanthanum substituted barium titanate microwave ceramics

In this study, we report the high dielectric constant lanthanum substituted barium titanate ceramic material for its possible applications at microwave frequencies. The microwave dielectric characterization of Ba_6 − 3xLa_8 + 2xTi₁₈O₅₄ solid solutions with 0.0 ≤ x ≤ 0.7 prepared by conventional mixed oxide route method has been carried out. The lattice parameters were obtained from the X-ray diffraction patterns. It was observed that lattice parameters increased with respect to an increase in the ‘La’ content. The crystal symmetry investigated was orthorhombic with space group of Pbam. From the evaluation of microwave dielectric properties of lanthanum doped barium titanate ceramics, it was observed that a maximum value of dielectric constant (ε′) = 157 and a minimum tangent loss (tanδ) = 0.0572 was obtained. The minimum value of a.c. conductivity (σ_a.c.) was observed to be 1.76e − 07 S/m.     

Luminescence of Sm2+ in strontium haloborates

The luminescence properties of Sm²⁺ in Sr₂B₅O₉R (R=Cl, Br) have been studied and compared with those in SrB₄O₇. In the range from 80 to 300 K the emission of Sm²⁺ in Sr₂B₅O₉R is predominantly due to the 4f⁵5d → 4f⁶ transition, while SrB₄O₇ : Sm²⁺ shows a very efficient emission of the 4f⁶ → 4f⁶ type. Differences in the luminescence of Sm²⁺ in the strontium haloborates and tetraborate are discussed.     

Dielectric and piezoelectric studies of Fe doped PLZT ceramics

Polycrystalline samples of Pb_0.9(La_1 − zFe_z)_0.1(Zr_0.65Ti_0.35)_0.975O₃ [referred as PLFZT] (where z = 0.0, 0.3, 0.5 and 0.7) have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis suggests the formation of single-phase compounds with tetragonal structure. The dielectric studies of the compounds as a function of temperature (from room temperature (RT) to 600 K) at a frequency 10 kHz show that the compounds undergo a phase transition of diffuse type. Diffusivity studies of the phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ_dc) as a function of temperature (RT to 600 K) at a constant biasing field dc 10 V/cm suggests that the compounds exhibit negative temperature coefficient of resistance (NTCR). The piezoelectric parameters as planar coupling coefficient, frequency constant and mechanical quality factor have been presented.