Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO₂ bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti³⁺ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO₂ specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛ_r) and quality factor (Q × f) of SPS-TiO₂ re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛ_r ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO₂ specimens prepared using SPS were discussed in terms of grain size variation and Ti⁴⁺ reduction.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Microwave dielectric properties and microstructures of Nb2O5-Zn0.95Mg0.05TiO3 + 0.25TiO2 ceramics with Bi2O3 addition

The effects of Bi₂O₃ addition on the microwave dielectric properties and the microstructures of Nb₂O₅-Zn_0.95Mg_0.05TiO₃ + 0.25TiO₂ (Nb-ZMT′) ceramics prepared by conventional solid-state routes have been investigated. The results of X-ray diffraction (XRD) indicate the presence of four crystalline phases, ZnTiO₃, TiO₂, Bi₂Ti₂O₇, and (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ in the sintered ceramics, depending upon the amount of Bi₂O₃ addition. In addition, in order to confirm the existence of (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ phase in the samples, the microstructure of Nb-ZMT′ ceramic with 5 wt.% B₂O₃ addition was analyzed by using a transmission electron micrograph. The dielectric constant of Nb-ZMT′ samples was higher than ZMT′ ceramics. The Nb-ZMT′ ceramic with 5 wt.% Bi₂O₃ addition exhibits the optimum dielectric properties: Q × f = 12,000 GHz, ?_r = 30, and τ_f = ?12 ppm/°C. Unlike the ZMT′ ceramic sintered at 900 °C, the Nb-ZMT′ ceramics show higher Q value and dielectric constant. Moreover, there is no Zn₂TiO₄ existence at 960 °C sintering. To understand the co-sinterability between silver electrodes and the Nb-ZMT′ dielectrics, the multilayer samples are prepared by multilayer thick film processing. The co-sinterability (900 °C) between silver electrode and Nb-ZMT′ dielectric are well compatible, because there are no cracks, delaminations, and deformations in multilayer specimens.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

Low sintering temperature of (Zn0.65Mg0.35)TiO3–xCaTiO3-based dielectric with controlled temperature coefficient

(Zn_0.65Mg_0.35)TiO₃–xCaTiO₃-based dielectric ceramics sintered at low temperature thanks to ZnO–B₂O₃ glass phase addition are investigated. The effects of such additions on the dilatometric curves, the microstructure, the phase composition and the dielectric properties have been carefully examined. It is shown that the sintering temperature is significantly lowered to 930 °C by the addition of 2 wt.% of ZnO–B₂O₃ glass phase. The temperature coefficient of permittivity (τ_?) could be controlled by varying the CaTiO₃ content and lead to near zero τ_? value. As an optimal composition, (Zn_0.65Mg_0.35)TiO₃ + 7%CaTiO₃, co-sinterable with silver electrodes at 930 °C, exhibits at 1 MHz, a relative permittivity of ?_r = 21, a temperature coefficient of the permittivity τ_? of ?4 ppm/°C and low dielectric losses (tan(δ) < 10^?3). These interesting properties make this system promising to manufacture Ag-based electrodes multilayer dielectric devices.     

Microwave dielectric properties of Mg3(VO4)2–xBa3(VO4)2 ceramics for LTCC with near zero temperature coefficient of resonant frequency

The Mg₃(VO₄)₂–xBa₃(VO₄)₂ ceramics have been investigated to obtain a low-temperature co-fired ceramic (LTCC). The highest quality factor (Q_f) of approximately 114,000 GHz was obtained when the ceramic with x = 0.2 was sintered at 950 °C for 5 h in air. The temperature coefficient of resonant frequency (τ_f) of the ceramics sintered at 1025 °C varied from −90 to 60 ppm/°C as the amount of xBa₃(VO₄)₂ increased, and was a near zero value in the sample obtained at x = 0.5 where the dielectric constant (ɛ_r) and the Q_f values were approximately 12 and 55,000 GHz, respectively. In order to reduce the sintering temperatures of Mg₃(VO₄)₂–xBa₃(VO₄)₂ ceramics, the effects of Li₂CO₃ addition as a sintering aid on the microwave dielectric properties of Mg₃(VO₄)₂–0.5Ba₃(VO₄)₂ ceramics were also characterized in this study. The Li₂CO₃ addition was effective in reducing the sintering temperature without detrimental effects on the Q_f values of the ceramics. One result: the microwave dielectric properties of Mg₃(VO₄)₂–0.5Ba₃(VO₄)₂ with 0.0625 wt%-doped Li₂CO₃ ceramic, which was sintered at 950 °C for 5 h in air, has a ɛ_r value of 13, a Q_f value of 74,000 GHz, and a τ_f value of −6 ppm/°C.     

High-Q microwave dielectrics in low-temperature sintered (Zn1−xNix)3Nb2O8 ceramics

The effects of Ni substitution for Zn on microwave dielectric properties of (Zn_1−xNi_x)₃Nb₂O₈ (x = 0.02–0.08) ceramics were investigated in this study. The XRD patterns of the sintered samples reveal single-phase formation with a monoclinic structure. The tremendous improvement of Q × f value can be achieved by a small level of Ni substitution (x = 0.05). The τ_f value was found to decrease with a decreasing A-site bond valence. In addition, B₂O₃ and CuO were used as a sintering aid to lower the sintering temperature from 1180 to 900 °C. Excellent microwave dielectric properties (ɛ_r ∼ 20.7, Q × f ∼ 98,000 GHz and τ_f ∼ −85.2 ppm/°C) and a chemical compatibility with Ag electrodes can be obtained for 4 wt% B₂O₃–CuO doped (Zn_0.95Ni_0.05)₃Nb₂O₈ ceramics sintered at 930 °C for 2 h. This constitutes a very promising material for LTCC applications.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The B₂O₃ added Ba(Zn_1/3Nb_2/3)O₃ (BBZN) ceramic was sintered at 900 °C. BaB₄O₇, BaB₂O₄, and BaNb₂O₆ second phases were found in the BBZN ceramic. Since BaB₄O₇ and BaB₂O₄ second phases have an eutectic temperature around 900 °C, they might exist as the liquid phase during sintering at 900 °C and assist the densification of the BZN ceramics. Microwave dielectric properties of dielectric constant (ɛ_r) = 32, Q × f = 3500 GHz, and temperature coefficient of resonance frequency (τ_f) = 20 ppm/°C were obtained for the BZN with 5.0 mol% B₂O₃ sintered at 900 °C for 2 h. The BBZN ceramics were not sintered below 900 °C and the microwave dielectric properties of the BBZN ceramics sintered at 900 °C were very low. However, when CuO was added, BBZN ceramic was well sintered even at 875 °C. The liquid phase related to the BaCu(B₂O₅) second phase could be responsible for the decrease of sintering temperature. Good microwave dielectric properties of ɛ_r = 36, Q × f = 19,000 GHz and τ_f = 21 ppm/°C can be obtained for CuO doped BBZN ceramics sintered at 875 °C for 2 h.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Low-sintering ceramic materials based on Bi2O3–ZnO–Nb2O5 compounds

In this work, sintering behaviour of Bi₂O₃–ZnO–Nb₂O₅ compounds was investigated in order to develop LTCC materials with suitable microwave properties. Structure, dielectric properties and sintering were studied for ceramic dielectrics based on the system: Bi₂ZnNb₂O₉ with the pyrochlore structure and ZnNb₂O₆ with a columbite one. The work was carried out over a wide range of initial components concentration. Ceramic samples of these materials were prepared by the mixed oxide technique. The effect of adding glass to the materials have been discussed. The sintering behaviour, dielectric permittivity, quality factor and crystal structures have been characterized for ceramic samples depending on compositions. Low-temperature co-firable ceramic material with ɛ ∼ 30, τ_ɛ = 0 and Q × f = 3500 GHz based on the above system was synthesized.     

Low temperature sintering of the Ba2Ti9O20 ceramics using B2O3/CuO and BaCu(B2O5) additives

The effects of B₂O₃/CuO and BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of Ba₂Ti₉O₂₀ ceramics were investigated. The B₂O₃ added Ba₂Ti₉O₂₀ ceramics were not able to be sintered below 1000 °C. However, when both CuO and B₂O₃ were added, they were sintered below 900 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the Ba₂Ti₉O₂₀ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the Ba₂Ti₉O₂₀ ceramics. Good microwave dielectric properties of Qxf = 16,000 GHz, ɛ_r = 36.0 and τ_f = 9.11 ppm/°C were obtained for the Ba₂Ti₉O₂₀ ceramics containing 10.0 mol% BaCu(B₂O₅) sintered at 875 °C for 2 h.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics

B₂O₃ (25.0 mol%) was added to Zn_2?xSiO_4?x ceramics (0.0 ≤ x ≤ 0.5) to decrease the sintering temperature. Specimens with 0.0 ≤ x ≤ 0.3 sintered at 900 °C were well sintered with a high density due to the formation of a B₂O₃ or B₂O₃–SiO₂ liquid phase. The Q × f value of the Zn₂SiO₄ ceramic was relatively low, 32,000 GHz, most likely due to the presence of a ZnO second phase. A maximum Q × f value of 70,000 GHz was obtained for the specimens with x = 0.2–0.3, and their ?_r and τ_f values were approximately 6.0 and ?21.9 ppm/°C, respectively. Ag metal did not interact with the 25.0 mol% B₂O₃-added Zn_1.8SiO_3.8 ceramic, indicating that Zn_2?xSiO_4?x ceramics containing B₂O₃ are a good candidate materials for low temperature co-fired ceramic devices.     

Microstructures and electrical properties of lead zinc niobate–lead titanate–lead zirconate ceramics using microwave sintering

Electrical properties and microstructural characteristics of (1 − x)(0.94PbZn_1/3Nb_2/3O₃ + 0.06BaTiO₃) + xPbZr_yTi_1−yO₃ (PZN–PZ–PT) ceramics, sintered by microwave heating, were investigated using electron microscopy, energy-dispersive spectroscopy (EDS) and electrical property measurement. Experimental results imply that the microwave-sintered (MW) samples with x = 0.5 and y = 0.52 (1150 °C, 10 min) possess higher dielectric constant than the conventionally sintered (CS) specimens (1150 °C, 2 h). Microstructural investigations show that ZnO precipitated on the surfaces of specimens during a thermal process, implying that ZnO diffusion may have influenced the distribution of phases in a specimen due to an eutectic reaction of PbO and ZnO. TEM–EDS investigations show that the CS specimens exhibit pronounced elemental segregation of PbO and ZnO at the grain boundaries, but it is much less significant for MW samples. The results imply that microwave sintering not only enhances material densification markedly, but also reduces the PbO/ZnO segregation and amorphous intergranular layers effectively, and thus improve the electrical properties of PZN–PZ–PT ceramics.     

Influence of TiO2 additive on sintering temperature and microwave dielectric properties of Mg0.90Ni0.1SiO3 ceramics

(1-x)Mg_0.90Ni_0.1SiO₃-xTiO₂ (x = 0, 0.01, 0.03, 0.05) ceramics were successfully formed by the conventional solid-state methods and characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and their microstructure and microwave dielectric properties systematically investigated. It was observed that when TiO₂ content increased from 0 to 5 wt%, the Q_uf_o of the sample decreased from 118,702 GHz to 101,307 GHz and increases the τ_f value from −10 ppm/°C to +3.14 ppm/°C accompanied by a notable lowering in the sintering temperature (125 °C). A good combination of microwave dielectric properties (ε_r ∼ 8.29, Q_uf_o ∼ 101,307 GHz and τ_f ∼ −2.98 ppm/°C) were achieved for Mg_0.90Ni_0.1SiO₃ containing 3 wt% of TiO₂ sintered at 1300 °C for 9 h which make this material of possible interest for millimeter wave applications.     

Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.     

Dielectric and piezoelectric properties of low temperature synthesized iso-valent modified BT ceramics

Ba_(1?x)Y_xTiO₃ (where Y = Ca, Mg and Sr, x = 0.02, 0.04, 0.06 and 0.08) ferroelectric ceramic samples were synthesized in single perovskite phase by modified solid state reaction (MSSR) route. For single perovskite phase formation and dense grain morphology, 900 °C and 1300 °C were optimized as calcination and sintering temperatures. With the increase of substitutions% of Ca in BCT ceramic samples, the position of T_c increases slightly, whereas with the increase of Mg and Sr substitution% in BMT and BST systems, the position of T_c decreased but remains above RT. Decreased processing temperature with low temperature coefficient of capacitance made BCT ceramic samples useful for dielectric applications. Symmetric nature of the S–E loops indicated the increase of piezoelectric nature with the increase of Ca substitution% in BCT system.     

Bismuth titanates candidates for high permittivity LTCC

Bi₂O₃–TiO₂ composites are known to possess attractive microwave dielectric properties. However, producing LTCC analogues with equally promising dielectric properties is problematic. Here, we show that judicious choice of both TiO₂ starting powders and dopants can produce composites with excellent properties. Three TiO₂ powders were evaluated: 1 μm-anatase, 1 μm-rutile and a nanosized (30 nm) mixture of 75–25 anatase-rutile. The best dielectric properties were obtained by using uncalcined nanosized anatase/rutile with Bi₂O₃ powder. By doping this Bi₂O₃–TiO₂ powder mixture with 0.112 wt.% CuO dielectric properties of Q × f = 9000 GHz, ɛ_r = 80 and τ_f = 0 ppm/K (at 300 K) were obtained at a sintering temperature of 915 °C.     

Influence of sintering conditions on piezoelectric properties of KNbO3 ceramics

A homogeneous KNbO₃ (KN) phase was formed in specimens that were sintered at 1020 °C and 1040 °C, without formation of the K₂O-deficient secondary phase, indicating that the amount of evaporation of K₂O during sintering was very small. However, the KN liquid phase was formed during sintering and assisted the densification of the KN ceramics. A dense microstructure was developed in the specimen sintered at 1020 °C for 6 h and abnormal grain growth occurred in this specimen. A similar microstructure was observed in the specimens sintered at 1040 °C for 1.0 h. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The KN ceramics sintered at 1020 °C for 6 h, which showed a large relative density that was 95% of the theoretical density, exhibited promising electrical properties: ɛ^T₃₃/ɛ_o of 540, d₃₃ of 109 pC/N, k_p of 0.29, and Q_m of 197.