Preparation of dense 0.9Al2O3−0.1TiO2 ceramics with highly improved microwave dielectric properties by a noncontaminated DCC method

In this paper, dense 0.9Al₂O₃ ??0.1TiO₂ ceramics with highly improved microwave dielectric properties were prepared by a noncontaminated direct coagulation casting (DCC) method. The suspension was destabilized and coagulated by consumption of the dispersant without introducing impurity ions. The effect of dispersant content, pH value and solid loading on the rheological properties of 0.9Al₂O₃ ??0.1TiO₂ suspension was investigated. It was found that 0.9Al₂O₃ ??0.1TiO₂ suspension with a high solid loading of 50?vol% and low viscosity of 0.7?Pa?s could be prepared by adding 0.5?wt% TMAOH at the pH in the range of 10–12. The suspension was coagulated by adding 2?vol% GDA when it was treated at 60 ~ 80?°C for 40 ~ 60?min. Compared with dry pressing method, more homogeneous and denser microstructure could be obtained in 0.9A1₂O₃ ??0.1TiO₂ ceramics prepared by DCC via dispersant reaction which were sintered at 1550?°C for 3?h and annealed at 1100?°C for 5?h. The Al₂TiO₅ second phase in 0.9A1₂O₃ ??0.1TiO₂ ceramics prepared by DCC via dispersant reaction could be eliminated more easily by annealing treatment. After annealing treatment, only Al₂O₃ and TiO₂ phases could be detected. Therefore, higher density and much better microwave dielectric properties with ρ?=? 3.81?±?0.02?g/cm³, ε_r =?12.17?±?0.02, Q ×?f =?25,637?±?749?GHz, τ_f =?13.12?±?1.62?ppm/°C were obtained by DCC via dispersant reaction, and the Q ×?f value almost improved by 25%. Without introducing impurity ions, it provides a new insight into preparing complex shaped function ceramics with high properties.     

Multiferroic ceramics in BaO–Y2O3–Fe2O3–Nb2O5 system

Multiferroic ceramics in BaO–Y₂O₃–Fe₂O₃–Nb₂O₅ system were synthesized and their dielectric, ferroelectric and magnetic properties were evaluated. XRD results showed that the ceramic composite consists of a major phase of tetragonal tungsten bronze structured Ba₂YFeNb₄O₁₅, and minor phases of monoclinic YNbO₄ and hexagonal Ba₃Fe₂Nb₆O₂₁. Three dielectric relaxations were observed in the temperature range from 125 to 575 K. The relaxor dielectric behavior in the temperature range from 125 to 350 K was attributed to the random occupation of Fe³⁺ and Nb⁵⁺ ions at B site of the tungsten bronze structure. The electrode polarization and the inhomogeneous structure contributed to the high-temperature and middle-temperature dielectric relaxations, respectively. Both the ferroelectric hysteresis loop and the magnetic hysteresis loop were measured, which suggested that the synthesized ceramic composite was a promising candidate of multiferroics.     

Crystallographic characterization of silicon nitride ceramics sintered with Y2O3–Al2O3 or E2O3–Al2O3 additions

Silicon nitride ceramics were sintered using Y₂O₃–Al₂O₃ or E₂O₃–Al₂O₃ (E₂O₃ denotes a mixed oxide of Y₂O₃ and rare-earth oxides) as sintering additives. The intergranular phases formed after sintering was investigated using high-resolution X-ray diffraction (HRXRD). The use of synchrotron radiation enabled high angular resolution and a high signal to background ratio. Besides the appearance of β-Si₃N₄ phase the intergranular phases Y₃Al₅O₁₂ (YAG) and Y₂SiO₅ were identified in both samples. The refinement of the structural parameters by the Rietveld method indicated similar crystalline structure of β-Si₃N₄ for both systems used as sintering additive. On the other hand, the intergranular phases Y₃Al₅O₁₂ and Y₂SiO₅ shown a decrease of the lattice parameters, when E₂O₃ was used as additive, indicating the formation of solid solutions of E₃Al₅O₁₂ and E₂SiO₅, respectively.     

Effects of liquid phases on densification of TiO2-doped Al2O3–ZrO2 composite ceramics

This study investigated the densification behaviors and microstructural evolution of Al₂O₃–ZrO₂ (3Y) composite ceramics doped with four different amounts of TiO₂ (0, 1, 4, and 8 wt%; denoted as 0T, 1T, 4T, and 8T, respectively) to clarify the effect of TiO₂ dopants on densification. The shrinkage rate during densification increased with the increase in the amount of TiO₂. The development of grain boundary feature was also examined. The undoped ceramic showed clean grain boundaries. Thin liquid grain boundary phases were observed in 1T, whereas large liquid phases were found on the grain boundary and at the junction pockets in 4T and 8T. The results were discussed in terms of the relationship between densification and grain boundary feature.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

Electrical and dielectric characteristics of erbium-added ZnO–V2O5-based varistor ceramics

This paper focuses on that the erbium-added ZnO–V₂O₅-based ceramics are attained at a sintering temperature as low as 875 °C. The effect of Er₂O₃ addition on microstructure, electrical properties, and dielectric characteristics has been investigated. Increasing the amount of Er₂O₃ slightly increased the densities of sintered pellets in the range of 5.52–5.59 g/cm³. The increase in the amount of Er₂O₃ increased the breakdown field from 6991 to 7408 V/cm up to 0.1 mol%, whereas a further addition decreased it. The sample added with 0.1 mol% Er₂O₃ exhibited the highest nonlinear coefficient (α=55) and the sample added with 0.25 mol% Er₂O₃ exhibited the lowest nonlinear coefficient (α=14). The donor concentration increased from 2.92×10¹⁷ to 8.48×10¹⁷ cm⁻³ with an increase in the amount of Er₂O₃.     

Twinning and charge compensation in Nb2O5–doped SnO2–CoO ceramics exhibiting promising varistor characteristics

We investigated the effects of dual doping of SnO₂ varistor ceramics with 1 mol% CoO and different amounts of Nb₂O₅ (0.1–2 mol%) on the formation of twin boundaries, microstructure development and electrical properties. Nb₂O₅ addition shifts densification to higher temperatures (up to 1430 °C), producing microstructures composed of twinned SnO₂ grains. Already 0.1 mol% Nb₂O₅ triggers a three-fold increase in growth rate via the diffusion induced grain boundary mobility (DIGM). At 0.5 mol% of Nb₂O₅ chemical equilibrium is achieved and SnO₂ grains undergo normal grain growth. Electron back-scatter diffraction (EBSD) has shown that the prevailing type of twins is {101}. Cyclic twins are common. High-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) image analysis revealed non-uniform segregation of Nb along the twin boundaries, indicating that they are not directly triggered by Nb₂O₅, but are a result of yet unexplained sequence of topotaxial replacement reactions. Energy dispersive spectroscopy (EDS) has shown that by dual doping of SnO₂ with CoO and Nb₂O₅ the amount of Co dissolved in SnO₂ grains is always ~4x lower compared to the amount of incorporated Nb and propose the following mechanism of tin out-diffusion: $6Sn{\left(IV\right)}_{Sn(IV)}^{\times }?Sn{\left(II\right)}_{Sn(IV)}^{\text{'}\text{'}}+Co{\left(II\right)}_{Sn(IV)}^{\text{'}\text{'}}+4Nb{\left(V\right)}_{Sn(IV)}^{?}$. Optimal electrical properties were achieved at 1 mol% Nb₂O₅ addition displaying high nonlinearity (α=50), moderate break-down voltage (571±12 V/mm) and low leakage current (I_L = 4.2 µA). The addition of 2 mol% of Nb₂O₅ has an inhibiting effect on densification and SnO₂ grain growth, resulting in a collapse of nonlinearity and increase of leakage current.     

Bi2O3对K2O–B2O3–SrO–Al2O3–Nb2O5–SiO2玻璃陶瓷介电储能性能的影响

随着电力电子系统的不断发展,高功率脉冲电容器的需求增多。电介质电容器因具有放电功率大、充放电速度快及性能稳定等优点,在电力系统、电子器件、脉冲电源等方面发挥着重要作用,广泛应用于民用领域及军事领域。通过熔融压延制备玻璃基体,采用可控结晶工艺研究了不同含量的Bi₂O₃ (x=0.0%、1.0%、2.0%、4.0%,摩尔分数)对K₂O–B₂O₃–Sr O–Al₂O₃–Nb₂O₅–SiO₂玻璃陶瓷物相演化、微观结构、介电和储能性能的影响。在该玻璃陶瓷中,KSr₂Nb₅O₁₅为主要析出晶相,当Bi₂O₃的加入量为x=2.0%(摩尔分数)时,热处理温度为950℃时,玻璃陶瓷样品的储能密度最大可达到1.27 J/cm³,室温下介电常数可达342,是热处...     

Effects of PbO and SrO contents on crystallization and dielectric properties of PbO–SrO–Na2O–Nb2O5–SiO2 glass–ceramics system

PbO–SrO–Na₂O–Nb₂O₅–SiO₂ glass–ceramics were prepared via roll-quenching followed by controlled crystallization from 700 °C to 900 °C. The effects of PbO and SrO contents on crystallization and dielectric properties were investigated. The results show that Pb₂Nb₂O₇, Sr₂Nb₂O₇ and their solid solutions crystallize at 700 °C, NaNbO₃ is the primary phase at 800 °C, Pb₂Nb₂O₇ disappears and PbNb₂O₆ forms at 900 °C. The dielectric properties of the glass–ceramics formed through controlled crystallization has a strong dependence on the phase compositions that were developed during heat treatment. The highest dielectric constants (∼600) are found in samples with 6.0 mol% SrO annealed at 900 °C for 3 h. The dielectric–temperature characteristics of the samples show stability over the range from −60 °C to 180 °C, except the sample without SrO heated at 900 °C.     

Densification and microwave dielectric properties of CaO–B2O3–SiO2 system glass–ceramics

In order to relieve the narrow processing window and poor material compatibility in practical applications as well as understand the microwave dielectric properties, investigation on the formulations of CaO/B₂O₃/SiO₂ glasses on their structure, thermal properties, and microwave properties were performed in this study. Six glasses with different molar ratios of CaO/B₂O₃/SiO₂ (designed as CBS-3, CBS-5, CBS-7, CBS-8, CBS-9, and CBS-10) were prepared and pulverized. Results indicate that most softening points of glasses are ranging from 680 to 710 °C. They were sintered at different temperatures to reach maximum densification. Among various glass formulations, CBS-9 glass–ceramic containing the largest amount of SiO₂ has the lowest CTE. The dielectric constants can be divided into two groups including around 4–5 and 7–8, and the dielectric losses (tan δ) are all below 0.005 in the frequency of ≈10 GHz. The dielectric constants and dielectric losses are generally frequency dependent. For CBS-9 glass–ceramic, the dielectric constant and dielectric loss at 4.7 and 18.6 GHz are 4.13 and 0.0018, and 4.20 and 0.0063, respectively.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

Study of AC electrical properties of V2O5–P2O5–B2O3–Dy2O3 glasses

The AC conductivity of glass samples of composition 60V₂O₅–5P₂O₅–(35−x)B₂O₃–xDy₂O₃, 0.4≤x≤1.2 has been analyzed. The samples were prepared by the usual melt-quench technique. The prepared compounds were analyzed by X-ray diffraction (XRD) and thermo gravimetric–differential thermal analysis (TG/DTA). The activation energies were evaluated using glass transition temperature (T_g) and peak temperature of crystallization (T_c) from TG/DTA. The dependence of activation energy on composition was discussed. The electrical conductance and capacitance were measured over a frequency range of 20 Hz to 1 MHz and a temperature range of 303–473 K; these reveal semiconducting features based predominantly on an ionic mechanism. The dielectric and complex-impedance response of the sample is discussed. The relaxation time was found to increase with increasing temperature. Jonscher's universal power law is applied to discuss the conductivity. The electrode polarization was found to be negligible and confirmed from electrical modulus.     

Low-temperature preparation and microwave dielectric properties of ZBS glass–Al2O3 composites

The preparation and microwave dielectric properties of ZnAl₂O₄-based glass–ceramic composites were investigated. Using zinc borosilicate (ZBS) glass and Al₂O₃, glass–ZnAl₂O₄ composites with high quality factor was successfully prepared at temperatures below 950 °C. The linear shrinkage for 50 vol% ZBS glass–ZnAl₂O₄ composite showed a steep increase up to 650 °C and a plateau between 700 °C and 950 °C, implying that one-stage densification process occurred. The crystallization of ZnAl₂O₄ was observed above 700 °C and an insufficient densification occurred due to the consumption of the glass. As the sintering temperature increased, the quality factor (Q × f₀) showed an increase with an S-type curve whereas the dielectric constant was almost constant. The formation of ZnAl₂O₄ might correspond to the increase of Q × f₀; a high value of 17,757 GHz (1415 at 12.6 GHz) was obtained for the specimen sintered at 900 °C.     

Structural,dielectric and piezoelectric properties of xBiFeO3–(1−x)BaTi0.9Zr0.1O3 ceramics

Effect of BiFeO₃ (BFO) content on the microstructure and electrical properties of BaTi_0.9Zr_0.1O₃ (BTZ) ceramics prepared by the solid-state reaction technique was investigated. X-ray diffraction analyses show that BFO diffused into the lattice of BTZ to form a solid solution with perovskite structure. The relative density of the BTZ ceramics is increased by the introduction of BFO. The dielectric study reveals that the dielectric constant and the average dielectric loss of the solid solution decreased simultaneously with an increase in BFO content. The materials undergo a diffuse type ferroelectric phase transition. The diffusivity increases with increase in BFO contents in the studied composition range. On the other hand, the piezoelectric coefficient and electromechanical coupling coefficient decrease simultaneously with increasing the BFO content, whereas the mechanical quality factor increases gradually. The structure–property relationship and the mechanism associated with the change of the electrical properties are discussed intensively.     

Effect of the Nb2O5 content on electrical properties of lead-free BaTiO3–Bi0.5Na0.5TiO3 ceramics

(1−x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (BT–BNT) ceramics were prepared by the solid-state reaction method. With an increase of BNT content, both the Curie temperature and the room temperature resistivity increased. At 1 mol% BNT addition, the sample was not semiconducting, due to Bi₂O₃ volatilization resulting from the decomposition of pre-calcined BNT during sintering. Appropriate extra Nb₂O₅ doping in the raw materials could offset Bi₂O₃ volatilization and neutralize the redundant acceptor Na⁺ ions. When the extra Nb₂O₅ content was 0.6 mg, the sample room-temperature resistivity was 6.3×10³ Ω cm, with the Curie point about 135 °C and a high PTC effect of ∼3 orders of magnitude.     

Low temperature sintering and microwave dielectric properties of Li2ZnTi3O8 ceramics doped with ZnO–La2O3–B2O3 glass

Li₂ZnTi₃O₈ ceramics doped with ZnO–La₂O₃–B₂O₃ glass were prepared by the conventional solid-state ceramic route. The effects of the ZnO–La₂O₃–B₂O₃ glass on the sintering temperature, phase composition, microstructure and microwave dielectric properties of Li₂ZnTi₃O₈ ceramics were investigated. The addition of ZLB glass can reduce the sintering temperature of Li₂ZnTi₃O₈ ceramic from 1075 °C to 925 °C without obvious degradation of the microwave dielectric properties. Only a single phase Li₂ZnTi₃O₈ with cubic spinel structure is formed in Li₂ZnTi₃O₈ ceramic with ZLB addition sintered at 925 °C. Typically, 1.0 wt% ZLB-doped Li₂ZnTi₃O₈ ceramic sintered at 925 °C can reach a maximum relative density of 95.8% and exhibits good microwave dielectric properties of ε_r=24.34, Q×f=41,360 GHz and τ_f=−13.4 ppm/°C. Moreover, this material is compatible with Ag electrode, which makes it a promising candidate for LTCC application.     

含Nb2O5和La2O3系统玻璃形成区

用新的玻璃形成区探索方法研究了La     

Synthesis and properties of core–shell structured BaCe0.9Y0.1O2.95:BaZr0.9Y0.1O2.95

The perovskite proton conductor BaZr_0.9Y_0.1O_2.95 (BZY10) shows better chemical stability but lower conductivity than BaCe_0.9Y_0.1O_2.95 (BCY10). In this paper we attempted to synthesize BCY10:BZY10 core–shell materials in which BCY10 particles prepared by solid reaction were wrapped by a sol–gel deposited thin layer of BZY10 with ZnO as sintering aid to improve the sinterability of the materials. The effects of the BCY10/BZY10 ratios on the phase purity, microstructure, chemical stability and electrical conductivity of the samples were characterized by XRD, TEM, SEM, TGA and electrochemical impedance spectroscopy. A dense core–shell structure was formed after being sintered at 1300 °C for 10 h. The core–shell samples displayed improved stability against CO₂ and water vapor at high temperature. With BCY10/BZY10 ratio varying from 9:1 to 7:3, the core–shell samples became more stable, and the total conductivities decreased.     

Influence of Al2O3 whisker concentration on flexural strength of Al2O3(w)–ZrO2 (TZ-3Y) composite

On investigating the possibility of using alumina whisker reinforced 3 mol% Yttria stabilized Tetragonal Zirconia (TZ-3Y) composite for bioceramic applications, presented here is the influence of varying whisker concentration (2, 5, 10, 15 and 20 wt%) on flexural strength of the composite. Whiskers of hydrothermally synthesized Ammonium Aluminum Carbonate Hydroxide (AACH) were used for composite synthesis. These whiskers transformed in situ into alumina during sintering. It was found that with addition of alumina whiskers, strength was increased and reached a maximum value of 1212±60 MPa and 1325±65 MPa, in pure and 1 wt% CTAB added samples respectively, at a concentration of 10 wt% Al₂O₃ whiskers. The strength values of the synthesized composite can compete well with commercially available materials for dental applications.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.