A low-firing microwave dielectric material in Li2O-ZnO-Nb2O5 system

LiZnNbO₄ ceramic was fabricated by the conventional solid state reaction method and its microwave dielectric properties were reported for the first time. The phase structure, microstructure, and sintering behavior were also investigated. The LiZnNbO₄ ceramic could be well densified at around 950 °C and demonstrated high performance microwave dielectric properties with a low relative permittivity ~ 14.6, a high quality factor (resonant frequency/dielectric loss) ~ 47, 200 GHz (at 8.7 GHz), and a negative temperature coefficient of resonant frequency approzmiately −64.5 ppm/°C. The LiZnNbO₄ ceramic is chemically compatible with Ag electrode material at its sintering temperature. It can be a promising microwave dielectric material for low-temperature co-fired ceramic technology.     

Ln2Mo3O12 (Ln = La, Nd): A novel group of low loss microwave dielectric ceramics with low sintering temperature

Ln₂Mo₃O₁₂ (Ln = La, Nd) ceramics with a defect scheelite related structure were prepared via a solid state reaction method. The La₂Mo₃O₁₂ ceramics sintered at 930 °C for 2 h exhibited a low dielectric permittivity of 10.1, a high quality factor (Qf value) of 60,000 GHz and a temperature coefficient of −80 ppm/°C at 12.7 GHz. The Nd₂Mo₃O₁₂ ceramics sintered at 945 °C for 2 h possessed a dielectric permittivity of 8.2, a Qf value of 80,000 GHz, and a temperature coefficient of −60 ppm/°C at 15.8 GHz. This group of ceramics could be good candidates for microwave substrate applications.     

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.     

Temperature stable high-Q microwave dielectric ceramics in (1 − x)BaTi4O9-xBaZn2Ti4O11 system

The microwave dielectric properties of (1 − x)BaTi₄O₉-xBaZn₂Ti₄O₁₁ ceramics were investigated by solid-state reaction technique for obtaining high-Q dielectric ceramics in BaO-ZnO-TiO₂ system. And they were strongly determined by the chemical composition. As x was increased from 0.05 to 0.50, BaZn₂Ti₄O₁₁ phase formed more and more. Therefore, the ε_r decreased from 37.3 to 32.8 and the Q × f values first raised from 45,300 GHz to 60,600 GHz (x = 0.30) and then started to decline to 58,700 GHz (x = 0.40), and the τ_f values varied gradually from 12 ppm/°C to − 13 ppm/°C. 0.7BaTi₄O₉-0.3BaZn₂Ti₄O₁₁ ceramics sintered at 1240 °C for 3 h had excellent comprehensive microwave dielectric properties: ε_r = 34.2, Q × f = 60,600 GHz and τ_f = − 2 ppm/°C.     

A new low-loss microwave dielectric using (Ca0.8Sr0.2)TiO3-doped MgTiO3 ceramics

The microwave dielectric properties and the microstructures of the (1-x)MgTiO₃-x(Ca_0.8Sr_0.2)TiO₃ ceramic system prepared by the conventional solid-state route were investigated. (Ca_0.8Sr_0.2)TiO₃ was employed as a τ_f compensator and was added to MgTiO₃ to achieve a temperature-stable material. Ilmenite-structured MgTiO₃ and perovskite-structured (Ca_0.8Sr_0.2)TiO₃ were coexisted and the two-phase system was confirmed by the X-ray diffraction patterns and the energy-dispersive X-ray analysis. Although the ε_r of the specimen could be boosted by increasing amount of (Ca_0.8Sr_0.2)TiO₃, it would instead render a decrease in the Q × f. The τ_f value is strongly correlated to the compositions and can be controlled through the existing phases. In fact, τ_f could be adjusted to a near-zero value by mixing 94 mole% MgTiO₃ and 6 mole% (Ca_0.8Sr_0.2)TiO₃. A dielectric constant (ε_r) of 21.42, a high Q × f value of 83,700 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of − 1.8 ppm/°C were obtained for 0.94MgTiO₃-0.06(Ca_0.8Sr_0.2)TiO₃ sintered at 1300 °C for 4 h. It is proposed as a low-loss and low-cost dielectric material for microwave and millimeter wave applications.     

Low temperature sintering and microwave dielectric properties of ZnTiNb2O8 ceramics with BaCu(B2O5) additions

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈ ceramics with BaCu(B₂O₅) additions prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The pure ZnTiNb₂O₈ ceramic shows a high sintering temperature of about 1250 °C. However, it was found that the addition of BaCu(B₂O₅) lowered the sintering temperature of ZnTiNb₂O₈ ceramics from above 1250 °C to 950 °C due to the BCB liquid-phase. The results showed that the microwave dielectric properties were strongly dependent on densification, crystalline phases and grain size. Addition of 3 wt% BCB in ZnTiNb₂O₈ ceramics sintered at 950 °C afforded excellent dielectric properties of ?_r = 32.56, Q × f = 20,100 GHz (f = 5.128 GHz) and τ_f = −64.87 ppm/°C. These represent very promising candidates for LTCC dielectric materials.     

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.     

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.     

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.     

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.     

A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

Dielectric properties of (1 − x)(Mg0.95Zn0.05)TiO3-x(Na0.5Nd0.5)TiO3 ceramic system at microwave frequencies

Ceramics in the system (1 − x)(Mg_0.95Zn_0.05)TiO₃-x(Na_0.5Nd_0.5)TiO₃ were prepared by the conventional mixed oxide route. It shows a two-phase system of an ilmenite structured (Mg_0.95Zn_0.05)TiO₃ and a perovskite structured (Na_0.5Nd_0.5)TiO₃, which were confirmed by XRD and EDX. In addition, (Mg_0.95Zn_0.05)Ti₂O₅ was identified as a second phase. It was also responsible for a rapid drop in the Q × f value. The temperature coefficient of resonant frequency was a function of compositional ratio. Specimen with x = 0.16 possessed an excellent combination of microwave dielectric properties: ε_r ~ 24.27, Q × f ~ 82,000 GHz (at 9 GHz) and τ_f ~ 0 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.     

Dielectric anomaly in LiCa2V5O15 ceramics

A polycrystalline sample of LiCa₂V₅O₁₅ (LCV) was prepared using a mixed oxide method at low temperature (i.e., at 630 °C). X-ray structural analysis shows the single-phase formation of the compound in the orthorhombic crystal system at room temperature. A study on the surface morphology of the compound showed uniform grain distribution on the surface and in the bulk of the sample with less porosity. A dielectric anomaly suggests that the compound has a transition temperature at 274 °C. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), of the compound was found to be 0.67 eV at 10 kHz. The nature of the variation of conductivity and value of activation energy in different regions, suggest that the conduction process is of mixed type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies).     

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.     

Structural and dielectric studies of Li2SiO3 ceramic

Lithium metasilicate (Li₂SiO₃) ceramic was prepared via solid-state reaction technique. X-ray diffraction pattern showed that Li₂SiO₃ ceramic is orthorhombic. Microstructural analysis by field emission scanning electron microscopy (FE-SEM) shows that the compound has well defined grains separated by grain boundaries. Dielectric studies of the compound shows a strong frequency dispersion of permittivity in the low frequency region followed by a nearly frequency independent behavior in the high frequency region. The dielectric permittivity and dielectric loss at 5 MHz are 25.66 and 0.033 at room temperature. The activation energy (E_a) of the sample calculated from the plot of ac conductivity versus inverse of absolute temperature was found to be less than 1 eV. The smaller activation energy of the compound within moderate temperature range suggests the presence of singly ionized oxygen vacancies in the conduction process.     

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.     

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 discharge properties of bulk Na2O-BaO-PbO-Nb2O5-SiO2 glass-ceramic dielectrics through electrode structure design

For pulsed power applications, bulk Na₂O-BaO-PbO-Nb₂O₅-SiO₂ glass-ceramic dielectrics in considerable size (about 150 × 60 × 4 mm³) were prepared via controlled-crystallization route. The discharge properties (mainly the discharge speed dI/dt and the duration time τ) of the dielectrics were investigated. The results show that the capability of charge or energy storage in the bulk dielectrics is relatively lower than that of anticipation based on smaller specimen, due to a serious drop of breakdown strength (BDS) of the bulk dielectrics. Several electrode structures were designed and applied in the measurement circuit to enhance the BDS of the bulk dielectrics. Among which, a bowl-like electrode structure was proved to reduce electrode edge effect most effectively in the BDS measurement and result in the improved dI/dt and τ.     

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.