Phase relations and microwave dielectric properties of vanadium modified Ba5Nb4O15 ceramics

Microwave dielectric ceramics of Ba₅Nb_4−xV_xO₁₅ (x = 0-1) were prepared by a solid-state reaction method. Vanadium substitution can markedly lower the sintering temperature of Ba₅Nb₄O₁₅ from 1450 to 1100 °C. The X-ray powder diffraction analysis reveals the multiphase nature of this system. A hexagonal-to-orthorhombic phase transition was also observed for the BaNb₂O₆ secondary phase. The microwave dielectric properties, such as τ_f, ε_r and Q × f value, decreased with increasing vanadium content for samples sintered at 1100 °C. There was an apparent increase in τ_f and Q × f value for samples (x ≥ 0.5) sintered at 1200 °C due to the hexagonal-to-orthorhombic phase transition of the BaNb₂O₆ phase. These results suggested that the microwave dielectric properties of multiphase ceramics strongly depended on the phase compositions and the phase transitions.     

Dielectric properties and high-temperature dielectric relaxation of Ba3Ti4Nb4O21 ceramic

A₃B₈O₂₁ hexagonal perovskite-like Ba₃Ti₄Nb₄O₂₁ ceramics were synthesized by a conventional solid-state reaction technique, which exhibit high dielectric constant at room temperature. A high-temperature dielectric relaxation was observed in frequency range from 40 Hz to 1 MHz above the Curie temperature. The real and imaginary parts of the impedance (Z′ and Z″) as functions of frequency indicate the presence of two relaxation processes at high temperatures, which are attributed to grain and grain boundary responses. The conductivities of both grain and grain boundary obey the Arrhenius law with activation energies of 1.04 eV and 1.27 eV, respectively. The dielectric relaxation processes are related to the oxygen vacancies inside the ceramic, indicating that the Q × f value of Ba₃Nb₄Ti₄O₂₁ might be improved via eliminating oxygen vacancies.     

Microwave dielectric properties of Mg1.8Ti1.1O4 ceramics

Low-loss Mg_1.8Ti_1.1O₄ ceramics were prepared by the conventional solid-state route and their microwave dielectric properties were investigated for the first time. The forming of tetragonal-structured Mg_1.8Ti_1.1O₄ main phase associated with a second phase MgTiO₃ were confirmed by the X-ray diffraction patterns. However, the presence of the second phase would cause no significant variance in the dielectric properties of the specimen because the second phase properties are very similar to that of the main phase. A fine combination of microwave dielectric properties (?_r ∼ 15.74, Q × f ∼ 141,000 GHz at 10.57 GHz, τ_f ∼ − 52.4 ppm/°C) was achieved for Mg_1.8Ti_1.1O₄ ceramics sintered at 1450 °C for 4 h.     

Preparation, structure and dielectric properties of Ba4LaMNb3O15 (M = Ti, Sn ) ceramics

Two new cation-deficient hexagonal perovskites Ba₄LaMNb₃O₁₅ (M = Ti, Sn) ceramics were prepared by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The Ba₄LaTiNb₃O₁₅ has high dielectric constant of 52, high quality factors (Q) 3500 (at 4.472 GHz), and temperature variation of resonant frequency (τ_f) +93 ppm °C⁻¹ at room temperature; Ba₄LaSnNb₃O₁₅ has dielectric constant of 39 with high Q value of 2510 (at 5.924 GHz), and τ_f −29 ppm °C⁻¹.     

Microwave dielectric properties of Mg4Al2Ti9O25 ceramics

Phase-singular Mg₄Al₂Ti₉O₂₅ ceramics with the pseudobrookite structure suitable for microwave devices such as antenna substrate have been prepared by gel-carbonate method with the dielectric permittivity of 24.7 (at 2-8 GHz), Q-values > 30,000 and temperature coefficients in permittivity (TCK) of less than + 17 ppm K^− 1. The dielectric characteristics are accountable in terms of ordering in the cation sub-lattice.     

Growth and optical characterizations of (Ba0.32Sr0.68)5Nb4O15 crystal

(Ba_0.32Sr_0.68)₅Nb₄O₁₅ crystal with sizes of Ø 17 × 35 mm was grown successfully by Czochralski technique method. The thermal anisotropy was discussed. The principal coefficients of thermal expansion along (100), (010), (001) directions were precisely measured to be 1.308 × 10^− 5, 1.288 × 10^− 5, 1.478 × 10^− 5 K^− 1, respectively. Its optical transparency range has been measured and found to span from 323 to 5500 nm. The bands present in the IR spectra were identified and assigned to the corresponding vibration modes of NbO₆ anions.     

Crystal structures of KBa2Nb5O15 and LaK2Nb5O15 with the TTB-type structure

Single crystals of two niobates, KBa₂Nb₅O₁₅ and LaK₂Nb₅O₁₅, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4bm (#100) for KBa₂Nb₅O₁₅ and the centrosymmetrical one P4/mbm (#127) for LaK₂Nb₅O₁₅), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa₂Nb₅O₁₅ (LaK₂Nb₅O₁₅) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R₁ = 0.040 (0.047) and wR₂=0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa₂Nb₅O₁₅ were distributed statistically at two crystallographic sites and K and La atoms in LaK₂Nb₅O₁₅ were ordered.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Crystallographic properties of four-fold grain K3Li2Nb5O15 thin films

The structural properties of a potassium lithium niobate (KLN; K₃Li₂Nb₅O₁₅) thin film deposited by rf-magnetron sputtering on a Pt/Ti/SiO₂/Si(100) substrate were investigated. The crystalline structures of the Pt under layer and KLN thin films were examined using θ-2θ, θ-rocking, and mesh scan X-ray diffraction (XRD). The XRD results revealed that the Pt under layer was a strong (111) direction orientated poly crystal. Unlike the Pt under layer film, the KLN(001) peak was found to consist of two separate peaks, one with a broad full width half maximum (FWHM) and the other with a narrow FWHM, indicating that the KLN film had a single crystalline structure. The surface and cross-section morphology were investigated using a scanning electron microscope (SEM). Accordingly, from the results of the SEM and XRD experiments, it was concluded that the KLN film was composed of small single crystals, which had a four-fold symmetry morphology with a c-axis normal to the substrate.     

Effects of Ca and Zr substitution upon structure and properties of Ba5LaTi3Ta7O30 low-loss dielectric ceramics

Effects of Ca and Zr substitution upon the dielectric properties of Ba₅LaTi₃Ta₇O₃₀ ceramics were investigated together with the structural characterization. All the samples of Ba₅La(Zr_xTi_1−x)₃Ta₇O₃₀ formed a filled tungsten-bronze structures, whereas the solid solution limit was determined as x=0.25 in (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀. Beyond this limit secondary phase of CaTa₂O₆ was detected and it would become the major phase for the Ca-rich compositions. The temperature coefficient of dielectric constant was improved with increasing Zr content while the dielectric constant decreased and the low dielectric loss varied little (in the order of 10⁻⁴). In the case of (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀, small temperature coefficient of dielectric constant could be obtained with increasing Ca content while the dielectric constant decreased significantly, and a small amount substitution of Ca for Ba induced decrease in dielectric loss.     

Structure, dielectric and magnetic properties of Ba6FeNb9O30 tungsten bronze ceramics

Ba₆FeNb₉O₃₀ ceramics were synthesized by a standard solid-state sintering process. X-ray powder diffraction (XRD) refinements were carried out to analyze the crystal structure, and the dielectric, ferroelectric and magnetic properties were investigated and discussed. The tetragonal tungsten bronze structure in space group P4bm was determined with the lattice parameters: a = 12.597(6) ?, b = 12.597(6) ?, c = 3.989(1) ?. An extremely high dielectric constant was indicated at higher temperatures, and it dropped quickly when the sample was cooled down through a critical temperature, and this critical temperature showed strong frequency dependence. This dielectric relaxation was more obviously observed in the dielectric loss curve. The nonlinear magnetic hysteresis curve was observed in the present ceramics at 5 K, which was related to the magnetic ions (Fe³⁺) in tungsten bronze structure.     

Microwave dielectric properties of LaMgAl11O19

The suitable choice of a substrate material is one of the aims to be fulfilled in high speed microwave technology. LaMgAl₁₁O₁₉ oxide ceramic material, which belongs to the magnetoplumbite family, has been reported earlier as a potential candidate for such applications. This material has been prepared by conventional solid-state ceramic route. The structure has been studied by X-ray diffraction and characterized at microwave frequencies. The effect of dopant and glass addition on the microwave dielectric properties of this material has also been investigated. LaMgAl₁₁O₁₉ has relatively low dielectric constant (ε_r=14), low dielectric loss or high quality factor (Q_u×f>28,000 GHz at 7 GHz) and small temperature variation of resonant frequency (τ_f=−12 ppm/°C) at room temperature (300 K). These properties make LaMgAl₁₁O₁₉ as a good substrate material and as a dielectric resonator to be used in microwave devices operating at relatively high frequencies.     

Microwave dielectric properties and microstructures of MgTa2O6 ceramics with CuO addition

The effect of CuO addition on the microstructures and the microwave dielectric properties of MgTa₂O₆ ceramics has been investigated. It is found that low level-doping of CuO (up to 1 wt.%) can significantly improve the density of the specimens and their microwave dielectric properties. Tremendous sintering temperature reduction can be achieved due to the liquid phase effect of CuO addition observed by scanning electronic microscopy (SEM). The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. With 0.5 wt.% CuO addition, MgTa₂O₆ ceramic can be sintered at 1400 °C and possesses a dielectric constant (_r) of 28, a Q × f value of 58000 GHz and a temperature coefficient of resonant frequency (τ_f) of 18 ppm/°C.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Dy掺杂BaTiO3陶瓷的制备及其介电性能

采用溶胶凝胶法制备了掺杂不同量Dy2O3(掺杂摩尔分数分别为0.001,0.002,0.003,0.005,0.007)的BaTiO3陶瓷,并对其介电性能的变化进行了研究.结果表明Dy2O3掺杂使BaTiO3陶瓷的电阻率明显降低,当添加量为0.005mol时,电阻率最小,为4.19×108Ω·m.Dy2O3掺杂使BaTiO3陶瓷的介电性能在不同掺杂量和不同频率下发生了明显变化,掺杂量为0.001mol、0.002mol时,BaTiO3陶瓷的介电特性和频率特性得到明显改善,在频率为1000Hz时介电性能相对较好.Dy2O3掺杂使BaTiO3陶瓷的介电温谱有所展宽,且Curie温度有所降低,交流电导随着温度的升高而增大,并在Curie点附近达到最高.     

Microwave dielectric and elastic properties of glass-added Ba6−3xR8+2xTi18O54 (x = 2/3)

Microwave dielectric properties of Ba_6−3xSm_8+2xTi₁₈O₅₄ (x = 2/3) [BST] ceramics with the addition of 0–3 wt.% of various glasses have been studied. It has been found that the addition of 0.5 wt.% of the glasses decreases the sintering temperature by about 150 °C. In general, addition of 0.5 wt.% of Zn, Mg and Pb-based glasses deteriorate the quality factor, whereas aluminum and barium borosilicates do not decrease it considerably. The quality factor and dielectric constant decrease with increasing amount of glass. The temperature coefficient of resonant frequency shifts towards positive or negative depending on the composition of the glass. A glass–ceramic composite with a dielectric constant 64, Q × f nearly 8500 GHz and near to zero τ_f could be obtained at a sintering temperature of 1175 °C when 3–4 wt.% Al₂O₃–B₂O₃–SiO₂ glass was added to BST ceramic. The Young's modulus decreases with increasing amount of glass, irrespective of the composition of glass.     

Thickness-ratio-dependent dielectric properties of Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 bilayered thin films

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN)/Ba_0.5Sr_0.5TiO₃ (BST) thin films were prepared on Pt/Ti-coated sapphire substrates by radio frequency magnetron sputtering. The specific relationship between the dielectric properties and the thickness ratio of the BZN thickness to the BST thickness was investigated. The presence of BZN films effectively reduced the dielectric loss of the thin films. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films.