Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Machinable Al2O3/BN composite ceramics with strong mechanical properties

Al₂O₃/BN composite ceramics with nano-sized BN dispersions ranging from 0 to 30 vol.% were successfully fabricated by hot-pressing α-Al₂O₃ powders with turbostratic BN (t-BN) coating, which was prepared through chemical processes using boric acid and urea. SEM observations revealed that the nano-sized hexagonal BN (h-BN) particulates were homogeneously dispersed within Al₂O₃ grains as well as at grain boundaries. Vickers hardness of materials decreased with an increase in BN content. The fracture toughness was improved but the fracture strength had a small decrease, in comparison to Al₂O₃ monolithic ceramics. The nanocomposite ceramics with BN content more than 20 vol.% exhibited excellent machinability, which could be drilled using conventional hard metal alloy drills. Drilling rates and normal forces demonstrate the ease of machining of these materials. The preliminary information on the relationship between microstructures and properties are provided. The mechanism of material removal is also discussed.     

The role of the surface roughness on the integrity of thermally generated oxide scales. Application to the Al2O3/MA956 system

This work deals with the effect of the metal roughness on the integrity of thermally generated oxide scales. For illustrative purposes, experimental evidence is shown for the alumina forming MA 956 alloy. The experimental results reveal that scale spallation occurs more readily in scales with rough surfaces than in scales with smooth surfaces, preferentially at the crests of the scale profile. In order to explain this feature, the effect of the roughness of both the gas/scale and scale/metal interfaces on the thermal stress distribution was analyzed by the finite element method. This analysis shows that with increasing roughness a gradient of compression stresses develops in the scale, being the maximum value of stresses located near the gas/scale interface. In general, the higher the roughness the higher the difference between the maximum and minimum values of the stresses. However, the average value of the stress distribution through the scale thickness decreases with increasing surface roughness. The effect of a planar gas/scale and a rough scale/metal interfaces was also modelled. In this case, the stress gradient in the scale was found to monotonically increase with increasing roughness although in a lower extension than when a rough gas/scale interface was also considered. On the basis of the experimental results and the stress distribution analyses a sequence of the scale failure during the cooling stage are proposed for both cases. It is concluded that the stress component that is normal to the interface and the shear stress play a key role on the scale integrity.     

Preparation of ZnAl2O4-based microwave dielectric ceramics and GPS antenna by aqueous gelcasting

A novel forming method-aqueous gelcasting was used to prepare 90 wt.% (0.75ZnAl₂O₄-0.25TiO₂)-10 wt.% MgTiO₃ (ZTM) microwave dielectric ceramics and GPS antenna. The effects of aqueous gelcasting and dry pressing on the phase compositions, microstructures and microwave dielectric properties of ZTM ceramics were investigated. The samples’ cracking problem happening in the process of drying and binder removal was successfully overcome. The phase compositions are completely the same no matter what forming method is adopted, but the ZTM ceramics prepared by aqueous gelcasting are denser than that prepared by dry pressing. Fewer pores and more uniform microstructures are observed in the ZTM ceramics prepared by aqueous gelcasting. Therefore, much better microwave dielectric properties are obtained in the ZTM ceramics prepared by aqueous gelcasting. Finally, a GPS antenna was successfully fabricated by aqueous gelcasting using the ZTM material, which meets the requirements of GPS application.     

Preparation and characterization of Al2O3-Ti3SiC2 composites and its functionally graded materials

Alumina/titanium silicon carbide (Al₂O₃-Ti₃SiC₂) composites and its functionally graded materials (FGMs) were fabricated by a powder metallurgy processes and their microstructure and properties were investigated, respectively. The experimental results showed that the Vickers hardness of composites decreased with increasing Ti₃SiC₂ content while the fracture toughness and strength exhibited the opposite trend. Minimum Vickers hardness (4 GPa), maximum strength (598 MPa) and maximum toughness (11.24 MPa m^1/2) were reached in the pure Ti₃SiC₂ material. Strength and hardness of FGMs were evaluated. Observation using an scanning electron microscope (SEM) indicated that the presence of Ti₃SiC₂ of FGMs inhibited the growth of alumina grains through a pinning mechanism. The study shows that the combination of the layered Ti₃SiC₂ structure and the fine alumina grains can result in a Al₂O₃-Ti₃SiC₂ composites possessing a high toughness and low Vickers hardness without a sacrifice in the strength.     

Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE₂O₃, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y₂O₃, was used as sintering additive of hot-pressed Si₃N₄ ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 °C, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 °C, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol⁻¹, corresponding to the heat of solution of the Si₃N₄ in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y₂O₃-Al₂O₃-doped-Si₃N₄ ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the β-Si₃N₄ grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE₂O₃ can be a cheap alternative in the fabrication of Si₃N₄ ceramics, resulting in promising mechanical properties.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Al2O3颗粒形貌对注凝成型ZrO2/Al2O3陶瓷性能的影响

采用3种不同形貌的Al₂O₃原料对注凝成型制备ZrO₂/Al₂O₃（ZTA）陶瓷工艺中悬浮体的流变性能进行了研究。以低毒的单体N,N-二甲基丙烯酰胺（DMAA）制备了ZrO₂/Al₂O₃坯体和陶瓷。讨论了3种不同形貌的Al₂O₃原浆料的分散剂用量、球磨时间和固含量对浆料流变性的影响。Al₂O₃粉体呈扁平状有利于降低浆料的黏度,Al₂O₃粉体呈棒状对生坯强度的提高有利。制得的3种ZrO₂/Al₂O₃坯体颗粒间结合紧密,抗弯强度分别达到21.45,19.87,25.90 MPa。Al₂O₃粉体呈颗粒状有利于最终陶瓷力学性能的提高,陶瓷的抗弯强度及断裂韧性分别为680 MPa和7.49 MPa·m^1/2,453.1 MPa和6.8 MPa·m^1/2,549.4 MPa和6.34 MPa·m^1/2。     

Bond strength and microstructure investigation of Al2O3/Al/Al2O3 joints with surface modification of alumina by titanium

Joints of Al₂O₃/Al/Al₂O₃ are formed by liquid-state bonding of alumina substrates covered with thin titanium film of 800 nm thickness using an Al interlayer of 30 or 300 μm at 973 K under a vacuum of 0.2 mPa for 5 min and an applied pressure of 0.01 MPa. The bond strength of the joints is examined by a four-point bend testing at room temperature coupled with optical, scanning and transmission electron microscopy. Results show that: (i) bonding occurs due to the formation of a reactive interface on the metal side of the joint with the presence of Al₃Ti precipitates (ii) a decrease in Al layer thickness leads to stronger Al₂O₃/Al/Al₂O₃ bonds accompanied by a change of both the distribution of reaction products (Al₃Ti) in the region of the interface and the failure surface characteristics.     

CoAl2O4/Al2O3纳米复合陶瓷的结构与表征

利用溶胶-凝胶方法制备了的CoAl2O4/Al2O3纳米复合陶瓷,并用X-ray分析(XRD)、红外光谱(IR)和扫描电镜分析(SEM)对其结构进行了分析.结果表明,随Al2O3含量增多,CoAl2O4尖晶石相从CoAl2O4/Al2O3凝胶中的析晶温度升高.SEM形貌也说明,随Al2O3含量增多,CoAl2O4/Al2O3陶瓷中形成一定的层状结构.     

Fabrication of porous Al2O3-MgAl2O4 ceramics using combustion-synthesized powders containing in situ produced pore-forming agents

Novel pore-forming agents as well as other starting materials were produced in situ via a solution combustion process and were used to fabricate porous Al₂O₃-MgAl₂O₄ ceramics. The relative fuel-to-oxidant ratios (φ_e) of 1.06, 1.28, 1.49 and 1.70 were achieved by varying the amount of starch in the precursors. Effects of φ_e on the weight percent of the pore-forming agents in the powders, open porosity and Vickers hardness of the as-prepared porous ceramics were investigated. Experimental results revealed that the weight percent of the pore-forming agents in the powders increased significantly_, while porosity of the as-prepared ceramics first increased and then decreased as φ_e rose from 1.06 to 1.70. Meanwhile, pore size distribution became far narrower and the Vickers hardness of the porous ceramics increased as φ_e increased.     

Al2O3基石油加氢脱硫催化剂研究现状与进展

综述了以三氧化二铝(Al_2O_3)为载体,负载Mo、Ni单一组元和Mo-Ni-W等复合组元,用于炼制石油加氢脱硫的催化剂的国内外研究现状与进展。全面总结了传统的浸渍法、混捏法、共沉淀法、离子交换法以及新型的微波-超声波法等催化剂合成方法,对Al_2O_3基催化剂的加氢脱硫性能进行了讨论,着重对催化剂合成过程中载体、活性组分、助剂(P、F、B等)和pH值对催化剂性能的影响进行了概述,在此基础上总结了Al_2O_3基加氢脱硫催化剂的不足之处,并展望了此类催化剂的发展方向与研究前景。     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Combustion synthesis of ferromagnetic Al2O3-based cermets in thermal explosion mode

The ferromagnetic Al₂O₃-based cermets with different ratios of Co and Co–50Ni alloys were successfully prepared by combustion synthesis in thermal explosion (TE) mode. The reaction process, microstructure, and magnetic property of cermets were investigated. The relative density of cermets can be over 95% via uniaxial loading at the time of ignition when the cermets are hot and ductile. In Al₂O₃–Co cermets, β-Co and α-Co co-exist at room temperature with average size of less than 10 μm and disperse homogeneously in the matrix, while in Al₂O₃–(Co–50Ni) cermets, the network-like Co–50Ni alloy can infiltrate into the boundary gaps of Al₂O₃ particles. The ferromagnetic Co and Co–50Ni alloys are responsible of the magnetic properties of Al₂O₃-based cermets. The saturation magnetization strongly depends on the magnetic characteristics and ratios of ferromagnetic phases. Al₂O₃–(Co–50Ni) cermets have soft magnetic properties with high magnetic susceptibility and low coercive force.     

Effects of Y2O3 addition on microwave dielectric characteristics of Al2O3 ceramics

Microwave dielectric characteristics of alumina ceramics with yttria addition were investigated. The sintering temperature was lowered, and the dielectric constant (ε_r) did not remarkably change by adding yttria. The microwave dielectric loss (tan δ) increased from 8.4 × 10^− 5 to 2.2 × 10^− 4, due to the presence of Al₅Y₃O₁₂ secondary phase. The grain size had significant effects on the dielectric loss, and there was an optimum grain size where the dielectric loss reached the minimum.     

The transformation of Eu→Eu in SiO2-Al2O3-NaF-YF3: Eu glasses and glass ceramics

Transparent glasses and glass ceramics of SiO₂-Al₂O₃-NaF-YF₃: Eu³⁺ containing the YF₃ nano-crystals were prepared in air atmosphere and their spectroscopic properties are presented. The blue emission peaked at 433 nm shown in the spectra indicate the existence of Eu²⁺.The transformation of Eu³⁺→Eu²⁺ is related with the existence of F⁻, which is the key trigger for the transformation in air atmosphere. In addition, the emission intensities of Eu³⁺ and Eu²⁺ were also related with the concentrations of Eu³⁺. The emission intensities increased heavily with the increasing dopant of EuF₃ in both glasses and glass ceramics_. When the doped concentration of EuF₃ was higher than 0.5 mol.%, the concentration quenching effect occurred.     

Photoacoustic thermal characterization of Al2O3–Ag ceramic nanocomposites

Laser-induced nondestructive photoacoustic (PA) technique has been employed to determine the thermal diffusivity of nanometal (Ag) dispersed ceramic alumina matrix sintered at different temperatures. The thermal diffusivity values are evaluated by knowing the transition frequency from the amplitude spectrum of PA signal using the one-dimensional heat flow model of Rosencwaig and Gersho. Analysis of the data shows that heat transport and hence the thermal diffusivity value is greatly affected by the influence of incorporation of foreign atom. It is also seen that sintering temperature affects the thermal diffusivity value in a substantial manner. The results are interpreted in terms of variation in porosity and carrier-assisted heat transport mechanism in nanometal dispersed ceramics.     

Effect of Pb(Fe1/2Nb1/2)O3 modification on dielectric and piezoelectric properties of Pb(Mg1/3Nb2/3)O3-PbZr0.52Ti0.48O3 ceramics

10 mol% Pb(Fe_1/2Nb_1/2)O₃ (PFN) modified Pb(Mg_1/3Nb_2/3)O₃-PbZr_0.52Ti_0.48O₃ (PMN-PZT) relaxor ferroelectric ceramics with compositions of (0.9 − x)PMN-0.1PFN-xPZT (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared. X-ray diffraction investigations indicated that as-prepared ceramics were of pure perovskite phase and the sample with composition of x = 0.8 was close to morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase. Dielectric properties of the as-prepared ceramics were measured, and the Curie temperature (T_c) increased sharply with increasing PZT content and could be higher than 300 °C around morphotropic phase boundary (MPB) area. At 1 kHz, the sample with composition of x = 0.1 had the largest room temperature dielectric constant ?_r = 3519 and maximum dielectric constant ?_m = 20,475 at T_m, while the sample with composition of x = 0.3 possessed the maximum dielectric relaxor factor of γ = 1.94. The largest d₃₃ = 318 pC/N could be obtained from as-prepared ceramics at x = 0.9. The maximum remnant polarization (P_r = 28.3 μC/cm²) was obtained from as-prepared ceramics at x = 0.4.     

Preparation of polycrystalline BaTi2O5 ferroelectric ceramics

Polycrystalline BaTi₂O₅ (BT₂) was prepared by solid state sintering in air using BaCO₃ and TiO₂ as starting materials. A rod-like BT₂ sintered bodies in a single phase were obtained above 1025 °C by adding 5 wt.% B₂O₃. The densification of BT₂ sintered bodies decreased with increasing sintering temperature. The maximum permittivity of BT₂ sintered body was 640 at the Curie temperature (T_c) of 461 °C and the frequency of 100 kHz. The addition of B₂O₃ was effective to obtain BT₂ in a single phase at low temperatures with elongated microstructure and to improve the permittivity, indicating the formation of preferred orientation at low temperature was related with liquid-phase sintering mechanism.