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.     

Low-temperature firing and microwave dielectric properties of ZnO-Nb2O5-TiO2-SnO2 ceramics with CuO-V2O5

The effects of CuO-V₂O₅ addition on the sintering temperature and microwave dielectric properties of ZnO-Nb₂O₅-TiO₂-SnO₂ were investigated. The CuO-V₂O₅ addition lowered the sintering temperature of ZnO-Nb₂O₅-TiO₂-SnO₂ ceramics effectively from 1150 to 860 °C due to the liquid-phase effect of Cu₂V₂O₇ and Cu₃(VO₄)₂, as observed by XRD. The microwave dielectric properties were found to strongly correlate with the sintering temperature and the amount of CuO-V₂O₅ addition. The maximum Qf values decreased with increasing CuO-V₂O₅ content, due to the formation of the second phase, Cu₃(VO₄)₂ and CuNbO₃. Zero τ_f value can be obtained by properly adjusting the sintering temperature. At 860 °C, ZnO-Nb₂O₅-TiO₂-SnO₂ ceramics with 1.5 wt.% CuO-V₂O₅ gave excellent microwave dielectric properties: ?_r = 42.3, Qf = 9000 GHz and τ_f = 8 ppm/°C.     

Low-temperature sintering of temperature-stable LaNbO4 microwave dielectric ceramics

We demonstrate the correlation between sintering behavior and microstructural observations in low-temperature sintered, LaNbO₄ microwave ceramics. Small CuO additions to LaNbO₄ significantly lowered the sintering temperature from 1250 to 950 °C. To elucidate the sintering mechanism, the internal microstructure of the sample manipulated by a focused ion beam (FIB) was investigated using transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). LaNbO₄ with 3 wt% CuO sintered at 950 °C for 2 h possessed the following excellent microwave dielectric properties: a quality factor (Qxf) of 49,000 GHz, relative dielectric constant (?_r) of 19.5, and temperature coefficient of resonant frequency (τ_f) of 1 ppm/°C. The ferroelastic phase transformation was also investigated using in situ X-ray diffraction (XRD) to explain the variation of τ_f in low-temperature sintered LaNbO₄ as a function of CuO content.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Preparation of polycrystalline BaTi2O5 by pressureless sintering

Polycrystalline BaTi₂O₅ (BT₂) was prepared by pressureless sintering in air using BaCO₃ and TiO₂ as starting materials. XRD results of the calcined powder showed BaCO₃ and TiO₂ reacted completely after being calcined above 950 °C, showing a mixture of BaTiO₃ (BT), BT₂, BaTi₄O₉ and Ba₄Ti₁₃O₃₀. A small amount of ZrO₂ (less than 0.1 wt%) was effective to prepare BT₂ in a single phase and the second phase of BT and B₆T₁₇ increased with ZrO₂ content. BT₂ sintered body in a single phase was obtained at 1175-1300 °C when ZrO₂ content was 0.08 wt%. The maximum permittivity of BT₂ sintered body was 340 at the Curie temperature (T_c) of 463 °C and the frequency of 100 kHz.     

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.     

Synthesis and microwave dielectric properties of Sr3Zn1−xMgxNb2O9 phases

Oxides of the type, Sr₃Zn_1−xMg_xNb₂O₉ (0≤x≤1) have been obtained by the ceramic method. These oxides crystallize in the hexagonal cell corresponding to ordered triple perovskites. Sintered disks show nearly frequency-independent dielectric constant for all the compositions. Compositions sintered at 1425°C yield dielectric constant of 20-22 at ∼6 GHz, with quality factor ranging from 1300 to 1500. Sr₃Zn_0.5Mg_0.5Nb₂O₉ shows a very low temperature coefficient of resonant frequency (τ_f) of +4 ppm/°C.     

Low temperature sintering and microwave dielectric properties of SmAlO3 ceramics

The effects of sintering aids on the microstructures and microwave dielectric properties of SmAlO₃ ceramics were investigated. CuO and ZnO were selected as sintering aids to lower the sintering temperature of SmAlO₃ ceramics. With the additions, the sintering temperature of SmAlO₃ can be effectively reduced from 1650 to 1430°C. The crystalline phase exhibited no phase differences at low addition level while Sm₄Al₂O₉ appeared as a second phase as the doping level was over 0.5 wt.%. In spite of the additions, the dielectric constants showed no significant change and ranged 19-21. However, the quality factor Q×f was strongly dependent upon the type and amount of additions. The Q×f values of 51,000 and 41,000 GHz could be obtained at 1430°C with 0.25 wt.% CuO and ZnO additions, respectively. The temperature coefficients depended on the additions and varied from −40 to −65 ppm/°C. Results of X-ray diffractions, EDS analysis and scanning electron microscopy were also presented.     

Crystal structure and microwave dielectric properties of (1 − x)NdAlO3-xCaTiO3 ceramics

Crystal structure and microwave dielectric properties of (1−x)NdAlO₃-xCaTiO₃ ceramics have been investigated. Crystal structure of the specimens changed with the composition. Rhombohedral structure was found for the specimens with x≤0.1. When 0.3≤x≤0.7, the specimens had the tetragonal structure and it changed to the orthorhombic structure as x exceeded 0.7. Two types of the second phases were observed in (1−x)NdAlO₃-xCaTiO₃ ceramics. For the specimens with x≤0.5, Nd₄Al₂O₉ phase was observed and Al-rich phase was found in the specimens with x≥0.7. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) increased with the increase of x. The Q×f value of the specimen increased with x and exhibited the maximum value when x=0.5. The microwave dielectric properties of Q×f=45,000 GHz, ε_r=45 and τ_f=−1.5 ppm/°C were obtained for 0.3NdAlO₃-0.7CaTiO₃ ceramics.     

Effects of complex substitution of La and Nd for Bi on the microwave dielectric properties of BiNbO4 ceramics

La₂O₃ and Nd₂O₃ were used to substitute Bi₂O₃ and the effects of complex substitution on the sintering behavior and the microwave dielectric properties of BiNbO₄ ceramics were studied. With 0.5 wt.% CuO-V₂O₅ mixtures addition, all of the Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics could be densified below 920 °C. The triclinic phases are identified in Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics with x=0.01 sintered at 820 °C and the triclinic intensities increase with increasing the x value and sintering temperature. The saturated bulk density slightly decreases from 7.17 to 7.13 g/cm³ and the ε_r value from 44.24 to 42.76 with increasing x from 0 to 0.07 for Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics. The saturated Q×f value is between 10,300 and 12,400 GHz depending on the x value. The τ_f values of dense Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics decrease from 28.32 to 12.79 ppm/°C with x varying from 0 to 0.01 and remain almost unchanged with further increasing x.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

Role of chromium ion valence states in ZnO-As2O3-Sb2O3 glass system by means of spectroscopic and dielectric studies

ZnO-Sb₂O₃-As₂O₃ transparent glasses containing small concentrations of chromium ions (introduced as Cr₂O₃) ranging from 0 to 0.2 mol% is prepared. A number of studies viz., XRD, SEM, DTA, optical absorption, FT-IR, Raman, ESR spectra, magnetic susceptibility and dielectric properties (constant ?′, loss tan δ, ac. conductivity σ_ac over a wide range of frequency and temperature as well as dielectric breakdown strength at room temperature) of these glasses have been carried out as a function of chromium ion concentration. The results have been analysed in the light of different oxidation states of chromium ions. The analyses indicates that when the concentration of chromium ions is low, these ions mostly exist in Cr⁶⁺ and Cr⁵⁺ states, occupy network forming positions with CrO₄²⁻ and CrO₄³⁻ structural units respectively and increase the rigidity of the glass network. When the concentration of chromium ions is gradually increased, these ions seem to be existing mostly in Cr³⁺ state.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.     

Effect of B2O3-Bi2O3-SiO2-ZnO glass on the sintering and microwave dielectric properties of 0.83ZnAl2O4-0.17TiO2

The 0.83ZnAl₂O₄-0.17TiO₂ (ZAT) ceramics were synthesized by solid state ceramic route. The effect of 27B₂O₃-35Bi₂O₃-6SiO₂-32ZnO (BBSZ) glass on the microwave dielectric properties of ZAT was investigated. The crystal structure and the microstructure of the ceramic-glass composites were studied by X-ray diffraction and scanning electron microscopic techniques. The low frequency dielectric loss was measured at 1 MHz. The dielectric properties of the sintered samples were measured in the microwave frequency range by the resonance method. Addition of 0.2 wt% of BBSZ improved the dielectric properties with quality factor (Q_u × f) > 120,000 GHz, temperature coefficient of resonant frequency (τ_f) = −7.3 ppm/°C and dielectric constant (?_r) = 11.7. Addition of 10 wt% of BBSZ lowered the sintering temperature to about 950 °C with Q_u × f > 10,000 GHz, ?_r = 10 and τ_f = −23 ppm/°C. The reactivity of 10 wt% BBSZ added ZAT with silver was also studied. The results show that ZAT doped with suitable amount of BBSZ glass is a possible material for low-temperature co-fired ceramic (LTCC) application.     

Structure, dielectric and ferroelectric anisotropy of Sr2−xCaxBi4Ti5O18 ceramics

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?_R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Structural, spectroscopic and dielectric investigations on Ba8Zn(Nb6−xSbx)O24 microwave ceramics

Ba₈Zn(Nb_6−xSb_x)O₂₄ (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.4) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered in the range 1400-1425 °C. The structure of the system was analyzed by X-ray diffraction, Fourier transform infrared and Raman spectroscopic methods. The theoretical and experimental densities were calculated. The microstructure of the sintered pellets was analyzed using scanning electron microscopy. The low frequency dielectric properties were studied in the frequency range 50 Hz-2 MHz. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The τ_f values of the samples reduced considerably with the increase in Sb concentration. The materials have intense emission lines in the visible region. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Enhanced characteristics of Ba5SmTi3Nb7O30 ferroelectric nanocrystalline ceramic prepared by mechanical activation process: A comparative study

In the present work, structural, dielectric, ferroelectric, piezoelectric and pyroelectric properties of tungsten-bronze structured Ba₅SmTi₃Nb₇O₃₀ ceramics, synthesized by mechanical activation and conventional solid-state reaction processes, have been studied with an objective of comparing the properties of the specimens synthesized by the two processes. X-ray diffraction analysis shows the formation of the compound in an orthorhombic structure by both the processes. Scanning electron microscopy reveals the formation of a fine granular microstructure in the specimen synthesized by mechanical activation process compared to the specimen synthesized by conventional solid-state method. The specimen prepared using mechanical activation process exhibits improved electrical properties with higher dielectric constant, lower dielectric loss and higher resistivity. The effective remanent polarization () value, d₃₃ value and pyroelectric coefficients are also observed to be higher in the sample synthesized by the mechanical activation process.     

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.