Search

The three key properties required for microwave dielectric ceramics are suitable dielectric constant (εr), high quality factor (Q × f), and near‐zero temperature coefficient of resonant frequency (τf). Due to the intricate coupling relationship among these three properties, regulating one often leads to the deterioration of the remaining two. In this study, the synergetic regulation of dielectric properties of Ca0.7Nd0.2TiO3 was investigated to reduce τf to near‐zero while maintaining its high εr by A‐ and B‐site substitutions. To avoid the seesaw effect and improve the efficiency of composition optimization, machine learning method was introduced in this study. First, the models for predicting dielectric properties were fitted based on a small amount of high‐quality data gathered via a uniform experimental design. Then the dielectric properties of 1037 compositions of (Ca0.7Nd0.2)1−x(Li0.5Nd0.5)xTi1−y(Mg1/3Nb2/3)yO3 were predicted, from which 69 microwave dielectric ceramics with near‐zero τf and adjustable εr of 95–125 were quickly picked out, and 5 of them were proved by experiments with very high prediction accuracy. This work was finished within a period of 1 month, which proves the obvious acceleration of composition designing process of microwave dielectric ceramics with the aid of machine learning. [ABSTRACT FROM AUTHOR]

Through the avenue of adjusting the thermal properties of BaO–MgO–Al2O3–SiO2 (BMAS) glass-ceramic by changing the MgO/BaO ratio, a BMAS glass-ceramic with the MgO/BaO ratio of 1:2 has been obtained, which shows satisfactory softening and crystalline behavior. Then, this BMAS glass-ceramic is used to lower the sintering temperature of Al2O3 ceramic. After systematic research on the sintering and physical properties of BMAS /Al2O3 composite with different glass-ceramic content, we find that when the content of BMAS glass-ceramic is 50%, the composite presents adorable comprehensive properties: dielectric constant of 5.40 @ ~10 GHz, dielectric loss of 1.91 × 10− 3 @ ~10 GHz and bending strength of 143 MPa, meeting the requirement of LTCC substrate material for microwave application. [ABSTRACT FROM AUTHOR]

Low-temperature sinterable and low-loss composites consisting of La2O3–B2O3–ZnO glass-ceramics and Al2O3 filler have been investigated. The phase composition of LBZ/Al2O3 composites is closely related to the sintering temperature. When the sintering temperature was 850 °C, the main phases included LaBO3 and La(BO2)3 precipitated from LBZ glass-ceramics, ZnAl2O4 and LaAl2.03(B4O10)O0.54 phases generated by interfacial reactions, and Al2O3 filler. The high content of crystalline phases in the composites indicated the low content of residual glassy phases, which was related to the low loss of the composites at high frequencies, especially at microwave frequencies. The initial sintering temperature and the temperature corresponding to the fastest shrinkage speed of LBZ/40A and LBZ/50A composites were related to the content of LBZ glass-ceramics, and all the composites could be densified at low temperatures. Optimum properties were exhibited for the composite of LBZ glass-ceramic with 50 wt% Al2O3 sintered at 900 °C with a permittivity εr = 7.3 and a dielectric loss tan δ = 1.62 × 10−3 (at 12.1 GHz). The low εr and tanδ coupled with a low sintering temperature, suggesting that the LBZ glass-ceramic/Al2O3 composites had potential to meet the requirements of LTCC substrate materials for high frequencies. [ABSTRACT FROM AUTHOR]

In this paper, the effects of glass-ceramics sintering aid, La2O3–B2O3–CaO–P2O5 (LBCP), on the sinterability, microstructure, and microwave dielectric properties of 0.5(Ca 0. 7 Nd 0. 2) TiO3–0.5(Li 0. 5 Nd 0. 5) TiO3 (CNT–LNT) ceramic have been investigated. The results indicated that LBCP glass-ceramics has good wettability to CNT–LNT (contact angle at 980 ∘ C is 31.9 ∘ ), and it can be used as an effective sintering aid to reduce the sintering temperature of CNT–LNT ceramic from 1300 ∘ C to 980 ∘ C. LBCP glass-ceramics did not change the main crystal phase (perovskite structure) of the sample, but a small amount of LaBO3 and LaPO4 phases was precipitated. Since the LaBO3 and LaPO4 phases are low-loss phases, it is believed that the crystal phases can improve the dielectric properties of the sample, especially the dielectric loss. The samples with 10 wt.% of glass additive sintered for 4 h at 980 ∘ C exhibit the optimized properties: a high dielectric constant of 80.8 and a Q × f value of 2031 GHz. The high r and Q × f value, coupled with a relatively low sintering temperature, suggest that the optimized compositions have the potential to be used in microwave low-temperature co-fired ceramics applications. [ABSTRACT FROM AUTHOR]

The sintering behavior and dielectric properties of La2O3–B2O3–CaO–P2O5 (LBCP) glass and La2O3–B2O3–CaO–P2O5 glass/Al2O3 (LBCP/A) composites have been investigated. The results show that the major crystalline phase of LBCP glass–ceramics sintered at 625 °C is low-temperature LaBO3 phase, and it transits to high-temperature LaBO3 phase with the sintering temperature up to 850 °C. With the increase of La/B ratio in LBCP glass, the crystallization tendency of LaBO3 phase increases. LBCP/A composites could be well sintered at 850 °C for 2 h. The bulk density of LBCP/A composites increases with the glass content rising and then decreases owing to foaming. The main crystalline phases of LBCP/A composites are LaBO3, CaAl2B2O7, and Al2O3. Dielectric properties are related to the glass composition and content, which determines the density of the composites. The optimized dielectric properties of LBCP/A composites are dielectric constant of 7.108 and dielectric loss of 1.024 × 10–3 in 50LBCP3/50A, which nicely meet the requirements of LTCC substrate materials. [ABSTRACT FROM AUTHOR]

An interesting attempt to develop low temperature sintering glass-ceramic/ceramic composite based on La 2 O 3 -B 2 O 3 -CaO (LBC) glass-ceramic and LaBO 3 ceramic, which was reported to be the main crystalline phase precipitated from La 2 O 3 -B 2 O 3 -based glass-ceramics, has been taken. The sintering behavior, phase evolution, microstructures and dielectric properties of LBC/LaBO 3 composites have been studied. The densification of LBC/LaBO 3 composite is achieved by partially reactive sintering. The LaBO 3 filler is directly involved in the sintering process of glass/ceramic composite as additional liquid phase provider at high sintering temperature, and it will suppress the formation of other crystalline phases so that the produced LBC/LaBO 3 composites exhibit unusual simple phase structures, which is beneficial to regulate the performance of composites. LBC/LaBO 3 composite with 50 wt% LaBO 3 sintered at 950 ºC for 2 h has a dielectric constant ε r = 10.12, a dielectric loss tan δ = 1.82 × 10―3, a Q × f = 9312 GHz (at 16.95 GHz), and shows good chemical compatibility while co-firing with Ag electrode. This indicates that LBC glass/LaBO 3 ceramic composites have a potential to meet the requirements of microwave LTCC applications. [ABSTRACT FROM AUTHOR]

The microstructure and dielectric properties of LaMnO3 (LM) doped BaTiO3 (BT) ceramics have been investigated. By adding LM, (1–x)BT-xLM (x = 0–0.5) ceramics keep in a single-phase solid-solution state, but gradually transform from tetragonal structure to pseudocubic one, and much denser microstructure is obtained. As for dielectric properties, when x ≤ 0.1, the Curie temperature of BT-LM shifts to a lower temperature and the permittivity maximum is lowered with the increase of LM content, which are attributed to the introduction of La2O3 and the degradation of tetragonality, respectively. When x > 0.1, BT–LM presents a typical relaxation behavior. Colossal permittivity appears as x reaches 0.4, which is related to large number of giant electron-pinned defect-dipoles ( Mn 4 + · 2 e - V O · · ) and existence of grain-boundary segregation. Detailed analyses show that 0.5BT–0.5LM possesses the highest permittivity maximum (41,683) and widest temperature range (– 42 to 300 °C) with permittivity larger than pure BT at 100 kHz. Therefore, 0.5BT–0.5LM can be a very promising candidate as colossal permittivity material. [ABSTRACT FROM AUTHOR]

New glass-ceramic/ceramic composites based on lead-free low-softening La2O3-B2O3-Al2O3 (LBA) glass-ceramics and Al2O3 fillers were prepared. The phase compositions, microstructure, and dielectric properties of the composites were investigated. LaBO3, La(BO2)3 and LaAl2.03(B4O10)O0.54 including Al2O3 fillers were developed in the LBA glass-ceramics/alumina composites sintered at 875 °C, but the relative amounts of the phases varied greatly with different glass-ceramic compositions and were considered to be determined by the ratio of La2O3 to B2O3. The onset sintering temperatures of green composites were ~ 670 °C and the sintering shrinkage of the samples was mainly determined by the low-softening glass-ceramics. Optimum properties were exhibited for the composite of 25 La2O3-65 B2O3-10 Al2O3 glass-ceramic with 40 wt% Al2O3 sintered at 900 °C with a permittivity εr = 7.91, a dielectric loss tan δ = 2.12 × 10−3 (at 8.1 GHz). The low εr and tan δ coupled with a low sintering temperature, suggested that the LBA glass-ceramic/Al2O3 composites were potential candidates for LTCC applications. [ABSTRACT FROM AUTHOR]

A novel MnTiNb 2 O 8 -based microwave dielectric ceramic was synthesized by solid-state route and its microwave dielectric properties were investigated. The effects of excess Mn2+ on the microstructure and microwave dielectric properties of Mn x TiNb 2 O 8 (x = 1.00–1.20) were investigated. The X-ray diffraction and scanning electron microscopy results showed that MnTiNb 2 O 8 was the dominant crystalline phase in all samples, but TiO 2 , as a second phase, was consistently present. The complex impedance spectroscopy and X-ray photoelectron spectroscopy analysis suggested that the addition of a small amount of Mn could inhibit the formation of oxygen vacancies and hinder the reduction of Ti4+. The study found that by adding a small amount of Mn, the quality factor (Q × f) of ceramics can be significantly improved without affecting the dielectric constant (ε r), thereby greatly optimizing the dielectric properties of ceramics. Subsequently, the temperature coefficient of resonant frequency (τ f) of the ceramic was successfully adjusted to near zero by adding a small amount of Zr4+ instead of Ti4+. Among all Mn x Ti 1- y Zr y Nb 2 O 8 (x = 1.00–1.20, 0 ≤ y ≤ 0.28) ceramics, samples with x = 1.05 and y = 0.1 sintered at 1200 °C showed the best microwave dielectric properties: ε r = 32.9, Q × f = 11,427 GHz, and τ f = 0 ppm/°C. [ABSTRACT FROM AUTHOR]

Lanthanum borate-based glass-ceramic has recently received considerable attention in the field of high-frequency communications as a sintering aid, due to the low sintering temperature and the precipitation of crystalline phases with excellent dielectric and mechanical properties. Importantly, the densification and crystallization behaviors of glass-ceramic are not only determined by its composition, but also affected by various processing parameters, such as the initial particle size, which is rarely explored for optimizing the dielectric and mechanical properties of lanthanum borate-based glass-ceramic/ceramic composites. Herein, La 2 O 3 –B 2 O 3 –CaO–P 2 O 5 –SiO 2 (LBCPS) glass-ceramics with a series of initial particle sizes are achieved by ball milling method through controlling the milling time. Employing LBCPS glass-ceramics with different particle sizes as the sintering aid, various LBCPS/Al 2 O 3 composites are fabricated, and the dielectric and mechanical properties of which are further investigated. Notably, the LBCPS/Al 2 O 3 composite with a moderate glass-ceramic particle size of 2.7 μm exhibits the excellent comprehensive performance with a low dielectric loss of 8.6 × 10−4 at 9.8 GHz and a high flexural strength of 199 MPa. Moreover, the influencing mechanisms of the glass-ceramic particle size on the crystallization, microstructure, and densification of the LBCPS/Al 2 O 3 composite have also been systematically explored and proposed. This research work provides an intriguing strategy for optimizing the dielectric and mechanical properties of the glass-ceramic/ceramic as the high-performance low-temperature co-fired ceramic (LTCC) substrate. [ABSTRACT FROM AUTHOR]

The effect of Zr4+ substitution for Ti4+ on the phase evolution and microwave dielectric properties of (Cu 1/3 Nb 2/3) 0.25 Ti 0.75- x Zr x O 2 (0 = x ≤ 0.35) ceramics prepared via conventional solid-state method has been investigated. With the increase of Zr substitution, the ceramics exhibit a gradual phase transition from a tetragonal rutile phase to an orthorhombic srilankite phase, where the synthesis of the srilankite phase at atmospheric pressure and temperature below 1200 °C in this study is reported for the first time. The crystal structure is tetragonal rutile type for the unsubstituted composition, orthorhombic srilankite type for the compositions with x > 0.30, and the compositions in between is a two-phase region with a mixture of rutile and srilankite. The Zr substitution has a great influence on the microstructure of the ceramics. In particular, for compositions where x > 0.1, the grain boundaries are surrounded by elongated powdery particles. The microwave dielectric properties of the ceramics are closely related to the phase transition process, in which Q × f is significantly improved and τ f decreases almost linearly with the increase of the content of srilankite phase. Optimum microwave dielectric properties with a permittivity of 35.1, a Q × f value of 7270 GHz, and a near-zero τ f of 9.0 ppm/°C are obtained in the ceramics with x = 0.3 sintered at 1150 °C for 4 h, which provides a new option for microwave devices requiring medium-permittivity dielectric ceramics. [ABSTRACT FROM AUTHOR]

Abstract: The effects of Bi4B2O9 addition on the sintering behaviour, microstructure and microwave dielectric properties of 0.25CaTiO3–0.75(Li1/2Nd1/2)TiO3 ceramics have been investigated. The addition of Bi4B2O9 effectively reduced the sintering temperature to 1200°C. The secondary phase was observed for the 7.5wt% Bi4B2O9-doped sample sintered at 1300°C. The dense microstructure was developed and the grain size was increased with the creasing of Bi4B2O9 content. Bi4B2O9 addition significantly affected the microwave dielectric properties. The value was increased by the Bi4B2O9 addition due to increased grain size and the temperature coefficient of resonant frequency () can be tailored by the Bi4B2O9 addition as well. In particular, the excellent microwave dielectric properties of , =2830GHz and =−7.8ppm/°C could be obtained for the 10wt% Bi4B2O9-doped sample sintered at 1200°C. [Copyright &y& Elsevier]

TiO 2 modified CaCu 3 Ti 4 O 12 (CCTO) has been synthesized via a facile sol–gel precipitation process followed by solid state sintering. The influence of the precipitated TiO 2 crystallites on the microstructure, phase composition, and dielectric properties of CCTO has been investigated. The morphology and chemistry of grain boundaries in CCTO ceramics can be significantly tailored by the initial TiO 2 crystallites, which results in enhanced barrier layer capacitor structures. The formation mechanism of barrier layers based on the instability of Cu ions has been discussed. A new bimodal brick layer mechanism has been proposed to explain the effect of TiO 2 modification. The optimized TiO 2 modified CCTO sample exhibits stable dielectric permittivity which is twice as high as the unmodified one, and shows relatively low dielectric loss over the frequency range from 10 2 to 10 5 Hz. Such enhancement is deemed to arise from the modified and bimodal internal barrier layer capacitor effect. [ABSTRACT FROM AUTHOR]

The preparation, sintering behaviour, and dielectric properties of low loss LaBO3 ceramics have been investigated. Single-phase LaBO3 powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ∼30μm) were obtained by sintering at 1300°C in air. The electrical conductivity of LaBO3 follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62eV and 0.90eV, respectively. The LaBO3 ceramics sintered at 1300°C exhibit excellent microwave dielectric properties with a relative permittivity, ɛ r ∼11.8, a quality factor, Q × f 0 value ∼76,869GHz (at ∼15GHz), and a negative temperature coefficient of resonant frequency τ f ∼−52ppm/°C. [ABSTRACT FROM AUTHOR]