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.     

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.     

Photocatalytic properties of TiO2-P2O5 glass ceramics

Binary TiO₂-P₂O₅ glasses with 69 mol% and 76 mol% TiO₂ were prepared and converted into glass ceramics by heat-treatments. XRD measurements show that the main crystalline phases precipitated in the glass ceramics are anatase-type TiO₂ crystals or (TiO)₂P₂O₇ crystals, depending on the concentration of titanium constituent. Photocatalytic activities of the glass ceramics were evaluated by the decomposition of methylene blue (MB) and measuring the water contact angle. It is found that the glass ceramics containing anatase crystals exhibit both photocatalytic oxidation activity and highly photo-induced hydrophilicity under UV irradiation with intensity of 1.0 mW/cm².     

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.     

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.     

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.     

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.     

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.     

Phase evolution and dielectric properties of MgTi2O5 ceramic sintered with lithium borosilicate glass

Phase evolution, densification, and dielectric properties of MgTi₂O₅ dielectric ceramic, sintered with lithium borosilicate (LBS) glass, were studied. Reaction between LBS glass and MgTi₂O₅ was significant in forming secondary phases such as TiO₂ and (Mg,Ti)₂(BO₃)O. The glass addition was not necessarily deleterious to the dielectric properties due to the formation of TiO₂: permittivity increased and temperature coefficient of resonance frequency could be tuned to zero with the addition of LBS glass, although the inevitable glass-induced decrease of quality factor was not retarded by the formation of TiO₂. The sintered specimen with 10 wt% LBS fired at 950 °C for 2 h showed permittivity of 19.3, quality factor of 6800 GHz, and τ_f of −16 ppm/°C.     

Structural and impedance properties of KBa2V5O15 ceramics

The polycrystalline sample of KBa₂V₅O₁₅ ceramics was prepared by a mixed oxide method at low temperature (i.e., at 560 °C). The formation of the compound was confirmed using an X-ray diffraction technique at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 323 °C, and exhibits diffuse phase transition. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of both grain (>10³ Hz) and the grain boundary (<10³ Hz) effects in the material. Studies of electrical conductivity over a wide temperature range suggest that the compound exhibits the negative temperature coefficient of resistance behavior. The ac conductivity spectrum was found to obey Jonscher's universal power law.     

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.     

Microwave dielectric properties and microstructures of CuO- and ZnO-doped LaAlO3 ceramics

The microwave dielectric properties and the microstructures of 0.25 wt.% CuO-doped LaAlO₃ ceramics with ZnO additions have been investigated. The sintered LaAlO₃ ceramics are characterized by X-ray diffraction spectra and scanning electron microscopy (SEM). Tremendous reduction in sintering temperature can be achieved with the addition of sintering aids CuO and ZnO. The ceramic samples show that dielectric constants (ε_r) of 22−24 and Q×f values of 33,000−57,000 (at 9.7 GHz) can be obtained at low sintering temperatures 1340−1460°C. The temperature coefficient of resonant frequency varies from −24 to −48 ppm/°C. At the level of 0.25 wt.% CuO and 1 wt.% ZnO additions, LaAlO₃ ceramics possesses a dielectric constant (ε_r) of 23.4, a Q×f value of 57,000 (at 9.7 GHz) and a τ_f value of −38 ppm/°C at 1400°C for 2 h.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

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.     

Modified tunable dielectric properties by addition of MgO on BaZr0.25Ti0.75O3 ceramics

Phase composition, microstructure and tunable dielectric properties of (1 − x)BaZr_0.25Ti_0.75O₃-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg²⁺ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below −100 °C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant α_(T) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr_0.25Ti_0.75O₃-0.4MgO sample at 10 kHz and room temperature.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Crystallographic and dielectric properties of barium-substituted Pb(Mg1/3Ta2/3)O3 ceramics

Ceramic powders of (Ba,Pb)Pb(Mg_1/3Ta_2/3)O₃ were prepared via a B-site precursor route. Crystal symmetries and lattice parameters were determined. Monophasic perovskite was developed after the two-step reaction process, in which the lattice parameters showed linear changes in the entire composition range. Dielectric responses of the ceramics with compositional and frequency changes were investigated. The results were also compared with the (Ba,Pb)(Zn_1/3Ta_2/3)O₃ data.     

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.     

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.