Effects of Al2O3 addition on the sintering behavior and microwave dielectric properties of CaSiO3 ceramics

The effects of Al₂O₃ addition on the densification, structure and microwave dielectric properties of CaSiO₃ ceramics have been investigated. The Al₂O₃ addition results in the presence of two distinct phases, e.g. Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which can restrict the growth of CaSiO₃ grains by surrounding their boundaries and also improve the bulk density of CaSiO₃-Al₂O₃ ceramics. However, excessive addition (≥2 wt%) of Al₂O₃ undermines the microwave dielectric properties of the title ceramics since the derived phases of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈ have poor quality factor. The optimum amount of Al₂O₃ addition is found to be 1 wt%, and the derived CaSiO₃-Al₂O₃ ceramic sintered at 1250 °C presents improved microwave dielectric properties of ?_r = 6.66 and Q × f = 24,626 GHz, which is much better than those of pure CaSiO₃ ceramic sintered at 1340 °C (Q × f = 13,109 GHz).     

Sintering mechanism and dielectric properties of LiTaO3 matrix composites with the addition of Al2O3 particles

Highly densified Al₂O₃/LiTaO₃ (ALT) ceramic composites were fabricated by hot-pressing in a nitrogen atmosphere. The addition of Al₂O₃ particles could significantly improve the densification of LiTaO₃. Sintering mechanism of the LiTaO₃ ceramic incorporated with Al₂O₃ particles is proposed. Dielectric constant of 5 vol.% Al₂O₃/LiTaO₃ (5ALT) composite ceramic was slightly increased in the range from 30 kHz to 10⁶ Hz, but the dielectric loss was lowered in the whole range from 10³ Hz to 10⁶ Hz. Piezoelectric constant (d₃₃) of the 5ALT ceramic composite is about 50% of that of LiTaO₃ single crystal.     

Effects of CaSiO3 addition on sintering behavior and microwave dielectric properties of Al2O3 ceramics

The effects of CaSiO₃ addition on the sintering behavior and microwave dielectric properties of Al₂O₃ ceramics have been investigated. The addition of CaSiO₃ into Al₂O₃ ceramics resulted in the emergence of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which acting as liquid sintering aids can effectively lower the sintering temperature of Al₂O₃ ceramic. The Q × f value of Al₂O₃-CaSiO₃ ceramics decreased with the CaSiO₃ addition increasing because of the lower Q × f value of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Compared with the pure CaSiO₃ ceramic, the Al₂O₃-CaSiO₃ ceramic with 20 wt% CaSiO₃ addition possessed good dielectric properties of ?_r = 9.36 and Q × f = 13,678 GHz at the similar sintering temperature.     

Relationship between fracture toughness characteristics and morphology of sintered Al2O3 ceramics

The influence of sintering temperature and soaking time on fracture toughness of Al₂O₃ ceramics has been investigated. The samples were prepared by solid state sintering at 1500, 1600 and 1700 °C for different soaking time periods. The fracture toughness of the sintered samples was determined by inducing cracks using Vickers indentation technique. Microstructural investigations on fracture surfaces obtained by three point bend test mode were made and correlated with fracture toughness. Crack deflection in the samples sintered at 1500 and 1600 °C for which ranges of fracture toughness are 5.2–5.4 and 5.0–5.6 MPa m^1/2 respectively, are found. The samples sintered at 1700 °C have lower fracture toughness ranging between 4.6 and 5.0 MPa m^1/2. These samples have larger grains and transgranular fracture mode is predominant. The crack deflection has further been revealed by SEM and AFM observations on fracture surface and fracture surface roughness respectively.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Crack propagation and stress distribution in binary and ternary directionally solidified eutectic ceramics

The development of directionally solidified eutectic (DSE) ceramics for gas turbine applications necessitates improving their strength and toughness. The early stage of crack propagation is investigated in either binary (Al₂O₃/Y₃Al₅O₁₂, Al₂O₃/GdAlO₃ and Al₂O₃/Er₃Al₅O₁₂) or ternary (Al₂O₃/Y₃Al₅O₁₂/ZrO₂, Al₂O₃/GdAlO₃/ZrO₂ and Al₂O₃/Er₃Al₅O₁₂/ZrO₂) DSE ceramics. Post-mortem scanning electron microscopy (SEM) examination of biaxial flexure induced cracks revealed crack deflection and branching in the various phases and in the phase boundaries. These observations are correlated to analytical and finite element (FE) internal stress calculations, FE determination of the axial shear stress component in the interfaces in the vicinity of the specimen surface (free-edge effect) and FE calculations of the stress distribution resulting from an applied loading. Results from ruby (Cr³⁺) fluorescence piezo-spectroscopy measurements are analyzed, taking into account the hydrostatic and plane stress hypotheses. Moreover, transmission electron microscopy (TEM) examinations have confirmed the role of interfaces in the crack nucleation and propagation modes.     

Development of input output relationships for self-healing Al2O3/SiC ceramic composites with Y2O3 additive using design of experiments

Al₂O₃/SiC ceramic composites with Y₂O₃ as an additive, was synthesized using the Taguchi method of design of experiments, so as to develop statistically sound input output relationships. The proportion of SiC was varied from 12 to 21 vol.% whereas that of Y₂O₃ was varied from 2.5 to 4 vol.%. The composites were sintered at 1500 °C for a soaking time period of 12 h in an air atmosphere. Cracks were induced on the composite surface using a Vickers indenter with a load varying between 20 and 40 kg. Fractographical analyses have been carried out using optical and/or scanning electron microscopy to investigate the surface crack propagation behavior. Thermal aging at 1300 °C in the time range of 0.5-12.5 h was applied to find optimal conditions for healing of the pre-cracked samples. The output parameters such as crack length, healed crack length, hardness and fracture toughness of the samples were correlated with appropriate inputs such as contents of SiC and Y₂O₃, crack-healing temperature, healing time, compaction pressure, indentation load using statistical analysis. Further, the extent of influence, exerted by pertinent input parameters on output parameters, was also identified.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Densification, microstructural evolution and microwave dielectric properties of fluxed sintered 12R-Ba(Ti0.5Mn0.5)O3 ceramics

Doped hexagonal BaTiO₃ (h-BaTiO₃) ceramics have recently been identified as potential candidates for use in microwave dielectric resonators. However, similar to other common microwave ceramics, doped h-BaTiO₃ ceramics require a sintering temperature higher than 1400 °C. In this study, the effects of Bi₂O₃ and Li₂CO₃ on the densification, microstructural evolution and microwave properties of hexagonal 12R-Ba(Ti_0.5Mn_0.5)O₃ ceramics were examined. Results indicate that Bi₂O₃ and Li₂CO₃ are able to effectively reduce the sintering temperature of 12R-Ba(Ti₀₅Mn_0.5)O₃ ceramics through liquid phase sintering while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 12R-Ba(Ti_0.5Mn_0.5)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 1200 °C (?_r: 36.0, Qf_r: 6779 GHz, and τ_f: 25.3 ppm/°C), and 5 wt% Li₂CO₃ sintered at 1200 °C (?_r: 28.1, Qf_r: 5304 GHz, and τ_f: 35.3 ppm/°C).     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Preparation and microwave dielectric properties of 3ZnO·B2O3 ceramics with low sintering temperature

The preparation and dielectric properties of 3ZnO·B₂O₃ ceramics were investigated. Dense 3ZnO·B₂O₃ ceramics were obtained as sintered in the temperature range from 950 to 1000 °C for 3 h. The X-ray diffraction showed that the obtained ceramics were of a monoclinic 3ZnO·B₂O₃ structure. The ceramic specimens fired at 955 °C for 1 h exhibited excellent microwave dielectric properties: ?_r ∼ 6.9, Q × f ∼ 20,647 GHz (@6.35 GHz), and τ_f ∼ −80 ppm/°C. The dependences of relative density, ?_r, and Q × f of ceramics sintered at 955 °C on sintering soaking time showed that they all reached their plateaus as the soaking time was up to 60 min. Meanwhile, 3ZnO·B₂O₃ ceramics had no reaction with silver during cofiring, indicating it is a potential candidate for low-temperature cofired ceramic (LTCC) substrate.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Chemical zoning of calcium aluminoferrite formed during melt crystallization in CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₂(Al_xFe_1−x)₂O₅, with x<0.7 (ferrite), Ca₂SiO₄ (belite), Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇), were crystallized out of a complete melt during cooling at 8.3 °C/min. Upon cooling to 1370 °C, both the crystals of ferrite with x=0.41 and belite would start to nucleate from the melt. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.7. At ambient temperature, the ferrite crystals had a zonal structure, the x value of which successively increased from the cores toward the rims. The value of 0.45 was confirmed for the cores by EPMA. The chemical formula of the rims was determined to be Ca_2.03[Al_1.27Fe_0.68Si_0.02]_Σ1.97O₅ (x=0.65). As the crystallization of ferrite and belite proceeded, the coexisting melt would become progressively enriched in the aluminate components. After the termination of the ferrite crystallization, the C₃A and belite would immediately crystallize out of the melt, followed by the nucleation of C₁₂A₇. The C₁₂A₇ accommodated about 2.1 mass% Fe₂O₃ in the chemical formula Ca_12.03[Al_13.61Fe_0.37]_Σ13.98O₃₃, being free from the other foreign oxides (SiO₂ and P₂O₅).     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Preparation of nanocrystalline Sr3Al2O6 powders via citric acid precursor

Sr₃Al₂O₆ was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr₃Al₂O₆ were investigated. Increasing the CA/M promoted the formation of Sr₃Al₂O₆. Single-phase and well-crystallized Sr₃Al₂O₆ was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr₃Al₂O₆ nanoparticles with a diameter of about 50-70 nm were obtained.