Gelling behavior of MgAl2O4 slurries with a common dispersant

MgAl₂O₄ transparent ceramics were shaped by a commonly used polyacrylic acid (PAA), which acted as both dispersant and gelling agent. The spinel slurries were prepared by ball-milling MgAl₂O₄ powder, PAA, and water in an attrition mill. The gelling of slurries happened at room temperature in air atmosphere without any other organic additive. The gelling mechanism was the formation of chelates between Mg²⁺ and carboxyl groups (-COO⁻) of PAA. The frequency-based testing method was applied to investigate the gelling process of the as-prepared slurry. In addition, a novel in situ characterization method based on a modified indentation testing was invented to better understand the strengthening of the wet green body with time and to guide when demolding could be carried out. After sintering, transparent MgAl₂O₄ ceramics with high in-line transmittance were resulted.     

Eco-friendly tape casting of borosilicate glass/Al2O3 sheets for LTCC applications

This work reports the innovative development of a borosilicate glass/Al₂O₃ tape for LTCC applications using an eco-friendly aqueous tape casting slurry. Polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) were the respective dispersants, while carboxymethyl cellulose (CMC) and styrene acrylic emulsion (SA) were the respective binders. The results showed that PVP was more suitable than PAA as the dispersant for the aqueous casting slurry, and that 1.5 wt% PVP would achieve well dispersion of CABS glass/Al₂O₃ powder in the aqueous slurry. Moreover, a small amount of 2.0 wt% CMC binder could yield smooth CABS glass/Al₂O₃ tapes crack free. A high-quality CABS glass/Al₂O₃ tape with a smooth surface was made from an aqueous slurry containing 1.5 wt% PVP dispersant, 2.0 wt% CMC binder, and 2.0 wt% PEG-400 plasticizer. The density, tensile strength, and surface roughness of the green tape were 2.05 g/cm³, 0.87 MPa, and 148 nm, respectively. The resulting CABS glass/Al₂O₃ composites sintered at 875 °C exhibited a bulk density of 3.14 g/cm³, a dielectric constant of 8.09, a dielectric loss of 1.0 × 10^?3, a flexural strength of 213 MPa, a thermal expansion coefficient of 5.30 ppm/^°C, and a thermal conductivity of 3.2 W m^?1 K^?1, thus demonstrating its broad prospects in LTCC applications.     

基于平均飞行航迹的CCO减噪效用研究

近年来，通过优化飞行程序降低机场飞机噪声影响成为机场环境保护的重要研究方向。文章首先建立了基于飞机“噪声-功率-距离”数据的噪声计算模型，介绍了平均飞行航迹以及连续爬升运行(Continuous Climb Opera-tion, CCO)离场程序的相关理论，最后以大型国际机场为实例，使用飞机平均飞行航迹进行噪声预测，运用综合噪声模型计算出噪声影响面积并绘制噪声影响等值线图，比较了CCO离场相对常规的标准仪表离场(Standard Instru-ment Departure, SID)的降噪效果。结果表明，CCO离场程序可有效降低机场噪声影响，在高噪声级影响区域的降噪效果更佳。     

Volatilization of fluorine in a CaO–SiO2–CaF2-based system with the substitution of Na2O with K2O

Cuspidine-based systems are used to control the crystallization in mold fluxes, which is enabled by CaF₂ additions. However, excess CaF₂ increases the corrosion of casting machines. Therefore, Na₂O and K₂O are added to the mold flux system to ensure an optimized crystallization and lubrication ability of the flux with the CaF₂ content. This study investigated the effect of substituting Na₂O with K₂O on the volatilization of fluorine in a CaO–SiO₂–CaF₂-based slag system at high temperatures. The substitution of Na₂O with K₂O was performed at 5 mol% intervals. The volatilization was observed by thermogravimetric analysis under several isothermal conditions. The mass loss was measured at a heating rate of 5, 10, and 20 K/min. As the temperature increased, the volatilization of the mixed samples increased. The activation energy was calculated using the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods. A kinetic analysis of the volatilization of fluorine was conducted using the calculated parameters and several known kinetic models. Consequently, the volatilization of the Na-rich sample was controlled by chemical reactions and that of the K-rich sample was identified to be controlled by a phase-boundary-controlled reaction. These results suggest that the addition of mixed alkali oxide promote the volatilization during the early stages of the reaction. From the post-experimental composition analyses, the remaining Na and K in the samples suggested a different mechanism for the Na and K volatilization. The volatilization of Na increased with time, whereas K volatilized easily during the beginning of the reaction.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

汽车加油口双色注射模设计

分析了汽车加油口塑件结构,确定了注射成型方案、进胶方式,并介绍了通过滑块与开模动作配合实现塑件脱模的过程。     

Achieving fabrication of highly transparent Y2O3 ceramics via air pre-sintering by deionization treatment of suspension

An easy albeit quite effective deionization suspension treatment was adopted to alleviate the detrimental effects related to the hydrolysis of Y₂O₃ in an aqueous medium. Fabrication of highly transparent Y₂O₃ ceramics with a fine grain size via air pre-sintering and post–hot isostatic pressing (HIP) treatment without using any sintering additive was achieved using the treated suspensions. The hydrolysis issue of Y₂O₃ powder in an aqueous medium was effectively alleviated by using deionization treatment, and a well-dispersed suspension with a low concentration of dissolved Y³⁺ species was obtained. The dispersed suspensions were consolidated by the centrifugal casting method, and the green bodies derived from the suspension of 35.0 vol% solid loading showed an improved homogeneity with a relative density of 52.1%. Fully dense Y₂O₃ transparent ceramic with high transparency was obtained by pre-sintering consolidated green compacts at a low temperature of 1400°C for 16 h in air followed by a post-HIP treatment at 1550°C for 2 h under 200 MPa pressure. The sample had a fine average grain size of 690 nm. The in-line transmittance of the sample reached 83.3% and 81.8% at 1100 nm and 800 nm, respectively, very close to the theoretical values of Y₂O₃.     

Tuning of shear thickening behavior and elastic strength of polyvinylidene fluoride via doping of ZnO-graphene

ZnO rice like nonarchitects are grafted on the graphene carbon core via a rapid microwave synthesis route. The prepared grafted systems are characterized via XRD, SEM, RAMAN, and XPS to examined the structural and morphological parameters. Zinc oxide grafted graphene sheets (ZnO-G) are further doped in β-phase of polyvinylidene fluoride (PVDF) to prepare the polymer nanocomposites (PNCs) via mixed solvent approach (THF/DMF). β-phase confirmation of PVDF PNCs is done by FTIR studies. It is observed that ZnO-G filler enhances the β-phase content in the PNCs. Non-doped PVDF and PNCs are further studied for rheological behavior under the shear rate of 1–100 s⁻¹. Doping of ZnO-G dopant to the PVDF matrix changes its discontinuous shear thickening (DST) behavior to continues shear thickening behavior (CST). Hydrocluster formation and their interaction with the dopant could be the reason for this striking DST to CST behavioral change. Strain amplitude sweep (10⁻³% -10%) oscillatory test reveals that the PNCs shows extended linear viscoelastic region with high elastic modulus and lower viscous modulus. Effective shear thickening behavior and strong elastic strength of these PNCs present their candidature for various fields including mechanical and soft body armor applications.     

Anisotropic strength and fracture resistance of epoxy-ceramic composite materials produced by ultrasound freeze-casting

The anisotropic mechanical properties of ultrasound freeze cast epoxy-ceramic composite materials were studied by measuring flexural strength and fracture resistance curves (R-curves) using both unnotched and notched three-point beam bending experiments, respectively, cut in three different orientations relative to the directional freezing axis. Three ultrasound frequencies of 0.699, 1.39 and 2.097 MHz were used in order to introduce different length scales into the microstructure, with 0 MHz used as the control samples. For all cases, the composites showed higher strength and fracture resistance when the crack plane cut across the direction of ice growth (denoted as the YX orientation). The anisotropic properties were more evident for the material produced without ultrasound (0 MHz) where the flexural strength was approximately 160% higher for the YX orientation compared to two orthogonal orientations. Most of the fracture resistance increase was found to occur within a crack extension, Δa, of ～0.5 mm. Comparing the fracture resistance at Δa = 0.5 mm for the highly anisotropic 0 MHz samples showed that the YX orientation was approximately 86% tougher than the two orthogonal orientations. When the ultrasound operation frequencies of 0.699, 1.39 and 2.097 MHz were applied, the amount of anisotropy in the strength and fracture resistance gradually decreased as the operating frequency increased. The high strength and fracture resistance for the YX orientation was attributed to the alignment of the ceramic particles along the freeze front direction creating a barrier for crack propagation. Ultrasound modifies the material microstructure, introducing relatively dense ceramic layers perpendicular to the freezing front direction that act as an additional, orthogonal barrier to crack propagation. The addition of the denser layers acts to improve the mechanical properties in the weaker orientations and reduce the overall anisotropy.     

Effects of particle size distribution and sintering temperature on properties of alumina mold material prepared by stereolithography

Alumina mold materials prepared by stereolithography usually have considerable sintering shrinkage, and their properties related to casting have been rarely studied. In this study, alumina molds materials were prepared by stereolithography, and the effects of particle size distribution and sintering temperature on the properties of the materials were investigated. Results show that the viscosity of the slurries decreases as the fraction of fine powder increases, and the particle size distribution affects the curing behaviors slightly. Sintering shrinkage increases as the fraction of fine powder or the sintering temperature increases. Although lower sintering shrinkage can be achieved by sintering at 1350 °C or 1450 °C, the mold materials sintered at lower temperatures would continue to shrink under the service temperature of 1550 °C, and thus 1550 °C is determined as the optimal sintering temperature. As the fraction of fine powder increases, the creep resistance first increases and then decreases, and specimens prepared with 0.1 fraction of fine powder exhibit the best creep resistance with the droop distance of 4.44 ± 0.45 mm. Specimens prepared with 0.1 fraction of fine powder and sintered at 1550 °C exhibit linear shrinkage of 6.36% along the X/Y direction and 11.39% along the Z direction, and have a flexural strength of 78.15 ± 3.50 MPa and porosity of 30.12 ± 0.08%. The resulting material possesses relatively low sintering shrinkage, proper mechanical strength, porosity and high-temperature properties that meet the requirements for casting purposes.