Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO₄ ceramics were investigated as a function of H₃BO₃ and/or Bi₂O₃ content and sintering temperature. For a single addition of H₃BO₃ (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H₃BO₃, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H₃BO₃ content of 0.5 wt.% Bi₂O₃–yH₃BO₃ (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO₄ ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi₂O₃–H₃BO₃ content; however, Q × f value increased up to 9 wt.% H₃BO₃ of 0.5 wt.% Bi₂O₃–yH₃BO₃, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi₂O₃–H₃BO₃ content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi₂O₃–9 wt.% H₃BO₃ sintered at 850 °C for 30 min.     

Structure and dielectric properties of novel low temperature co-fired Bi2O3-RE2O3-MoO3 (RE=Pr,Nd, Sm,and Yb) based microwave ceramics

Novel monoclinic Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based low temperature co-fired ceramics (LTCC) systems with high sintering density and low microwave dielectric loss are synthesized by conventional solid state reaction technique. The structure and dielectric properties of Bi₂O₃-xRE₂O₃-yMoO₃ ceramics are investigated. Dense BiNdMoO₆ ceramics sintered at 900 °C for 8 h in air have a low dielectric constant ε_r=~7.5, a high quality factor Q×f=~ 24, 800 GHz at 7.0 GHz, and τ_f=~−16 ppm/̊C. Especially, good chemical compatibility of BiNdMoO₆ with Ag electrodes is represented as well. In contrast, BiSmMoO₆ ceramics sintered at 1000 °C for 8 h show enhanced Q×f=~43, 700 GHz at 7.8 GHz with ε_r=~8.5 and τ_f=~−27 ppm/°C. Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based ceramics could be considered as promising microwave ceramics for LTCC applications.     

Effect of Bi2O3 addition on the sintering and microstructural development of gadolinia-doped ceria ceramics

The effect of small amounts (0.2–2.0 wt.%) of bismuth oxide on the sintering behavior and microstructural development of Ce_0.9Gd_0.1O_1.95 (GDC) submicronized powders has been studied using XRD for the lattice parameter measurements, the constant heating rate (CHR) method in air to monitor the shrinkage kinetics of powder compacts, and scanning electron microscopy (SEM) to study the microstructure of the sintered samples. Sintering of GDC compacts was significantly improved by adding small amounts of Bi₂O₃ (≤2.0 wt.%), and samples of doped-GDC sintered at 1200–1400 °C for 2–4 h were dense bodies (98–99.5% of theoretical density). Measurements showed that the addition of Bi₂O₃ could reduce the sintering temperature by about 250–300 °C lower than that for undoped-GDC samples. A liquid phase-assisting mechanism was assumed as the main cause for the enhancement of the densification process. The average grain size of doped-GDC sintered samples grew with the increasing of Bi₂O₃ addition up to 1.0 wt.%, and then decreased indicating a poor wetting properties of the formed liquid phase.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Mechanical and optical properties of spark plasma sintered transparent Y2O3 ceramics

Commercial Y₂O₃ nanopowder was used to fabricate transparent Y₂O₃ ceramics by spark plasma sintering under the pressure of 100 MPa for 20 min with the heating rate of 100 °C/min. The microstructures, mechanical and optical properties of the Y₂O₃ ceramics sintered at different temperatures were investigated in detail. Densification occurred up to a sintering temperature of 1500 °C, and above 1500 °C, rapid grain growth and pore growth occurred. The highest relative density of 99.58% and the minimum average grain size of 0.58±0.11 µm were obtained at 1500 °C. The flexural strength, hardness and fracture toughness of the optimal spark plasma sintered Y₂O₃ ceramic were 122 MPa, 7.60 GPa and 2.06 MPa.m^1/2, respectively. The Y₂O₃ ceramic sintered at 1500 °C had the in-line transmission of about 11–54% and 80% in the wavelength range of 400–800 nm and 3–5 µm, respectively.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

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.     

Sintering,microstructural development,and electrical properties of gadolinia-doped ceria electrolyte with bismuth oxide as a sintering aid

A considerable reduction (≥250 °C) in the sintering temperature, enhancement of the sintering density, and a slight improvement of the electrical properties, can be achieved by using bismuth oxide in the range of 0.2 to 2 wt.%, as a sintering aid for gadolinia-doped ceria (GDC) ceramic electrolytes. Dilatometric experiment (CHR) and SEM observations indicate that a liquid phase-assisting sintering mechanism contributes to the improvement in sintering density for bismuth oxide concentrations exceeding 0.5 wt.%. The addition of small amount of Bi₂O₃ ≤0.5 wt.% also results in the achievement of highly dense ceramic bodies (≥99% of theoretical density) after sintering at 1200 °C for 4 h, which indicates that the addition of Bi₂O₃ to gadolinia-doped ceria promoted the sintering process by a cooperating volume diffusion-liquid phase-assisting mechanism. Based on the lattice constant data, the solid solubility limit of Bi₂O₃ in gadolinia-ceria is, probably, lower than 1.0 wt.%. Grain size also increased with increasing Bi₂O₃ content up to 0.5 wt.% and then it decreased with further addition of Bi₂O₃. The addition of the smaller amounts of bismuth oxide, i.e., ≤1.0 wt.% Bi₂O₃ slightly enhanced the total ionic electrical conductivity of the gadolinia-doped ceria electrolyte. The sintering temperature strongly influenced the electrical conductivity of the doped-GDC ceramics. The best sample was that containing 1.0 wt.% Bi₂O₃ sintered at 1400 °C for 2 h which had an ionic electrical conductivity of 4 S m⁻¹ at 700 °C, and an activation energy of 0.58 eV for the oxide-ion conduction process in air.     

Synthesis and microwave dielectric properties of Bi2Ge3O9 ceramics for application as advanced ceramic substrate

During the synthesis of Bi₂Ge₃O₉ ceramics using Bi₂O₃ + 3GeO₂ powders, the Bi₄Ge₃O₁₂ phase was formed at low temperature (≤800 °C). Bi₄Ge₃O₁₂ preferentially adopted GeO₂-excess phase, and this phase was consistently present in the sintered Bi₂Ge₃O₉ ceramic as a secondary phase. Therefore, Bi₄Ge₃O₁₂ powder was first calcined and subsequently reacted with GeO₂ powder to obtain the pure Bi₂Ge₃O₉ ceramic through the following reaction: 1/2Bi₄Ge₃O₁₂ + 3/2GeO₂ → Bi₂Ge₃O₉. Formation of the Bi₂Ge₃O₉ phase was initiated at temperature of 850 °C. The pure Bi₂Ge₃O₉ ceramic sintered at 875 °C for 8 h had a dense microstructure with an average grain size of 2.7 μm. Furthermore, the pure Bi₂Ge₃O₉ ceramic exhibited promising microwave dielectric properties for the advanced ceramic substrate: ε_r = 9.7, Q × f = 48,573 GHz and τ_f = −29.5 ppm/°C.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Pb(Zr0.95Ti0.05)O3 powders and porous ceramics prepared by one-step pyrolysis process using non-aqueous Pechini method

Using non-aqueous Pechini method, Pb(Zr_0.95Ti_0.05)O₃ powders were prepared at low temperature by one-step pyrolysis process. The polymeric gels and powders were characterized using a range of techniques, such as DTG, XRD, SEM, Raman spectroscopy, and laser particle size distribution. The perovskite phase was formed at about 350–400 °C and some oxocarbonate impurities can be detected in all samples after calcining at 400–850 °C by one-step pyrolysis process. Phase pure and porous Pb(Zr_0.95Ti_0.05)O₃ ceramics were obtained without pore formers from the powders by one-step pyrolysis process at 500 °C for 4 h. The relative densities were 87%, 91% and 94% for the ceramics sintered at 1100, 1150 and 1200 °C for 2 h, respectively. The porous ceramics sintered at 1200 °C for 2 h have homogeneously dispersed pores and fine-grain structures with an individual grain size of 0.7–2 μm.     

Micro-structure of ITO ceramics sintered at different temperatures and its effect on the properties of deposited ITO films

Structural characterizations of two ITO ceramics that were respectively sintered at 1560 °C and 1600 °C were focused on and the results indicate that the lower sintering temperature is good for ITO ceramics to have the triangle fine grains, larger elemental concentration gradients of indium and tin and more content of In₄Sn₃O₁₂ phase which displays the stronger grain orientation growth along the crystallographic direction of [0-11]. ITO films with 100 nm thickness deposited at 25 °C–230 °C were used to investigate the effect of micro-structure on the film properties. Grain orientation growth of In₄Sn₃O₁₂ phase is conductive to form ITO films of columnar structure. Otherwise, uniform micro-structure and higher solubility of SnO₂ in In₂O₃ main phase contribute to deposit ITO films of higher sheet resistance, less thickness uniformity and higher transmittance at 25 °C, smaller etching angle and lower etching rate at 230 °C.     

Sintering,microstructure and electricity properties of ITO targets with Bi2O3–Nb2O5 addition

High density and low electrical resistivity ITO targets were prepared by normal pressure sintering in oxygen with Bi₂O₃–Nb₂O₅ addition. The relative density, microstructure and electrical properties of the ITO targets can be adjusted by changing the sintering temperature (1350 °C~1550 °C) and the content of Bi₂O₃–Nb₂O₅. The results show that the sintering temperature of ITO targets with Bi₂O₃–Nb₂O₅ decreased from 1550 °C to 1450 °C, and the maximum relative density (99.6%) and the lowest electrical resistivity (1.78×10⁻⁴ Ω cm) were reached when the sintering temperature was 1450 °C with 5 wt% Bi₂O₃–Nb₂O₅. The carrier concentration increased as the increase of the contents of Bi₂O₃–Nb₂O₅ and sintering temperature. The mobility first increased, and then decreased above 1450 °C as the sintering temperature increased.     

Silver cofirable (Ca0.9, Mg0.1)SiO3 microwave dielectric ceramics with Li2CO3–Bi2O3 additive

The effects of Li₂CO₃–Bi₂O₃ (LB) additive on the microstructure, phase formation, microwave dielectric properties and applicability for low-temperature co-fired ceramics (LTCC) technology of (Ca_0.9Mg_0.1)SiO₃ (CMS) ceramics were investigated. The sintering temperature of the CMS ceramics was reduced from 1290 °C to 890 °C by the addition of LB. Secondary phases SiO₂ and Bi₄(SiO₄)₃ were detected when LB content was less than 9 wt%. Low melting point liquid phases were formed when LB content was 11–14 wt%. The Q_f value initially increased with the addition of LB and attained the maximum value for the 9 wt% LB-doped CMS ceramic. When the LB content exceeded 9 wt%, the Q_f value decreased because of the presence of liquid phase and abnormal growth of grains. ?_r of 6.92, Q_f of 27,600 GHz and τ_f of ?43.6 ppm/°C were obtained for 9 wt% LB-doped CMS ceramics sintered at 890 °C for 2 h. Also the ceramics can be well co-fired with Ag electrode.     

Microstructure evolution during air bonding of Al2O3 to Al2O3 joints using bismuth–borate–zinc glass

Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi₂O₃–40B₂O₃–20ZnO (mol.%). The wettability of the glass on the Al₂O₃ substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi₂₄B₂O₃₉ was detected to be the product in the joints sealed at 530–580 °C, while ZnAl₂O₄ was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al₂O₃ substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl₂O₄ increased with increasing holding time.     

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.     

Temperature–stable dielectrics based on Cu–doped Bi2Mg2/3Nb4/3O7 pyrochlore ceramics for LTCC

Temperature–stable dielectrics based on Cu–doped Bi₂Mg_2/3Nb_4/3O₇ pyrochlore ceramics were prepared by conventional solid–state reaction. Microstructure analysis indicates that all of the specimen maintain the cubic pyrochlore phase, a fluorite–like phase of Bi₃NbO₇ and a Bi₅Nb₃O₁₅ formed for Cu doping. The dielectric constant is dominated by densification of samples and secondary phases, while the dielectric loss is related by the secondary phases, grain boundaries, and leakage current characteristics. The (1-x)BMN - xCuO(x = 0.1 mol%) ceramic sintered at 925 °C shows excellent dielectric properties with dielectric constant of ~184.06, dielectric loss of ~0.0017 and near zero τ_ε (?20 ppm/°C) is obtained at sintering temperature of 925 °C, which could be a promising candidate for LTCC.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Dielectric and magnetic properties of BiFeO3 ceramics prepared by hydrothermal synthesis

Single-phase BiFeO₃ powders were prepared at a temperature of 200 °C by a hydrothermal synthesis. BiFeO₃ ceramics were prepared with the powders by a conventional ceramic process. The BiFeO₃ ceramics with no impurity phase were prepared at the sintering temperature of 650–800 °C. The dense microstructure was observed in the BiFeO₃ ceramics sintered at a temperature of 700 °C and higher. BiFeO₃ ceramics show linear M–H curves in low H, which are antiferromagnetic behaviors. The dielectric dispersion was observed at the frequency range of 10 kHz to 1 MHz in the BiFeO₃ ceramic sintered at 700 °C or lower. The dielectric constant and loss of the BiFeO₃ ceramics sintered at 750 °C or higher were about 85 and 0.4 at 100 kHz, respectively.