A novel combination of precursor pyrolysis assisted sintering and rapid sintering for construction of multi-composition coatings to improve ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites

A double-layer coating composed of MoSi₂–SiO₂–SiC/ZrB₂–MoSi₂–SiC was designed and successfully constructed by a novel combination of precursor pyrolysis assisted sintering and rapid sintering to improve the ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites (CSs). The ZrB₂–MoSi₂–SiC inner layer coating was in relatively uniform distribution in the zone of 0–3 mm from the surface of CSs through the slurry/precursor infiltration in vacuum and SiOC precursor pyrolysis assisted sintering, which played a predominant role in improving oxidation and ablation resistance and maintaining the morphology of CSs. The MoSi₂–SiO₂–SiC outer layer coating was prepared by the spray and rapid sintering to further protect CSs from high-temperature oxidation. The ablation resistance of CSs coated with double-layer coating was evaluated by an oxygen-acetylene ablation test under the temperature of 1600–1800 °C with different ablation time of 1000 and 1500 s. The results revealed that the mass recession rates increased with the rise of ablation temperature and extension of ablation time, ranging from 0.47 g/(m²·s) to 0.98 g/(m²·s) at 1600–1800 °C for 1000 s and from 0.72 g/(m²·s) to 0.86 g/(m²·s) for 1000–1500 s at 1700 °C, while the linear recession rates showed negative values at 1700 °C due to the formation of oxides, such as SiO₂ and ZrO₂. The ablation mechanism of the double-layer coating was analyzed and found that a SiO₂–ZrO₂–Mo_4.8Si₃C_0.6 oxidation protection barrier would be formed during the ablation process to prevent the oxygen diffusion into the interior CSs, and this study provided a novel and effective way to fabricate high-temperature oxidation protective and ablation resistant coating.     

The investigation of gold/zirconia as a photocatalyst for the direct synthesis of imines from alcohols and aniline

Au/ZrO₂ nanoparticles have been widely used as photocatalysts in various organic syntheses because of their localized surface plasmon resonance (LSPR) effects. In our work, Au/ZrO₂ has been synthesized by a solution method and it was used as a heterogeneous catalyst in the synthesis of imines from alcohols and aniline with irradiation by visible light. The reaction occurred in two steps: step 1 was the aerobic oxidation of the alcohols and step 2 was the nucleophilic addition of aniline. Dimethyl sulfoxide (DMSO) was used as a solvent in the reaction. The selectivity in the synthesis of imines over 3 wt% Au/ZrO₂ (with mean particle size of 5 nm) was high (over 90%) with irradiation by visible light at room temperature, and an obvious difference in the conversion was observed between the reactions with light irradiation and those without light. The intensity and wavelength of the light strongly affected the reaction. The Au/ZrO₂ could be used at least 5 times. A reaction mechanism was proposed based on the experimental results. The results indicate that the reaction of alcohols and aniline using Au/ZrO₂ as the photocatalyst can proceed under mild conditions. Furthermore, this process is environmentally friendly and green.     

An adaptive Kriging method with double sampling criteria applied to hydrogen preparation case

Kriging model has been widely used to approximate expensive black-box problems in many engineering design fields. How to choose an appropriate sampling strategy to produce new expensive updated points is crucial. For this purpose, an adaptive Kriging method with double sampling criteria (AKM-DSC) is proposed. During every iteration of it, maximum curvature criterion and maximum variance criterion based on Kriging model are respectively optimized by trust region (TR) strategy to produce two candidate points. And then, a new screening method is used to determine final expensive-evaluation points from the two candidates. The proposed method is compared with the two typical Kriging modeling methods. The comparison results of seventeen benchmark functions verify that the proposed method can generate higher accuracy Kriging model. Finally, a hydrogen preparation case illustrates the engineering application value of the AKM-DSC method.     

镍离子掺杂TiO2涂层的制备及亲水性研究

目的 研究不同含量的镍离子掺杂对TiO2纳米涂层亲水性能的影响.方法 采用溶胶-凝胶法制备不同含量Ni2+掺杂TiO2纳米复合溶胶,通过浸渍提拉法在载玻片上成膜,经过热处理后,得到不同含量Ni2+掺杂TiO2复合涂层.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)、紫外-可见光分光光度计(UV-Vis)、傅里叶变换红外光谱仪(FT-IR)和表面接触角仪,对掺杂不同浓度Ni2+后TiO2的晶型、涂层微观形貌、光吸收性能、结构组成和涂层亲水性等进行表征分析,从而确定最佳Ni2+浓度的复合涂层材料配方.结果 制备的TiO2主要由锐钛矿相和少量金红石相组成,Ni掺杂抑制了锐钛矿相向金红石相的转变.随着Ni含量的增加,TiO2晶粒尺寸逐渐降低.适量掺杂Ni2+制备的TiO2纳米涂层表面形貌光滑,粒子分布致密均匀.掺杂使吸收波长阈值向长波方向偏移,禁带宽度减小,在一定程度上提高了TiO2涂层的亲水性.当Ni2+掺杂质量分数为1.5％时,TiO2涂层亲水性最佳.结论 采用溶胶-凝胶法实现了镍离子掺杂TiO2的改性,掺杂Ni2+后,Ni2+/TiO2复合涂层的亲水性能明显提高.     

A composite visible-light photocatalyst for hydrogen production

A composite photocatalyst, Pd–TiO_2−xN_x–WO₃, was synthesized by the template method and characterized by energy dispersive X-ray microanalysis (EDX), X-ray diffraction (XRD), UV–vis spectrometer, and scanning electron microscope (SEM). The results of EDX analysis reveals that the molecular formula of the composite photocatalyst can be expressed as Pd–TiO_1.72N_0.28–WO₃. The UV–vis absorption spectrum indicates that the absorption edge of the catalyst red-shifts to around 600 nm. Under the irradiation with ultraviolet and visible light, the catalyst showed good performance for photocatalytic hydrogen production with a Na₂S/Na₂SO₃ system as the sacrificial agent.     

微波辐射催化合成乙酸异戊酯

以乙酸与异戊醇为原料,对甲苯磺酸为催化剂,微波辐射催化合成了乙酸异戊酯,优化了合成条件。结果表明,n(异戊醇)∶n(冰醋酸)=1∶1.45,催化剂用量2.10 g,反应温度100℃,反应时间5 min,微波功率440 W,此时产品收率达78.3%。     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Synthesis of SiC precursors by a two-step sol–gel process and their conversion to SiC powders

A two-step sol–gel processing was developed to synthesize phenolic resin–SiO₂ hybrid gels as SiC precursors, with tetraethoxysilane (TEOS) and novolac phenolic resin being the starting materials, and oxalic acid (OA) and hexamethylenetetramine (HMTA) being the catalysts. At the first step TEOS was prehydrolyzed under the catalysis of OA. At the second step HMTA was added to facilitate gelation. The influences of the molar ratio of OA/TEOS and prehydrolysis time on the sol–gel reaction were investigated. There existed an optimum OA/TEOS ratio where prehydrolysis time needed to form transparent gels was the shortest. The increase of temperature could accelerate sol–gel reaction. The dried hybrid gels were yellowish transparent glassy solids, with uniform microstructure composed of nanometer-sized particles. The conversion of the gels to silicon carbide powders was complete when heated at 1650°C for 30 min in vacuum. The oxygen and free carbon were 0.43 and 0.50 wt%, respectively, in the powder produced from the gel prepared with starting resin/TEOS being 0.143 g/ml.     

Amination of ethanolamine over cobalt modified H-ZSM-5 catalysts

The amination of ethanolamine (EA) with ammonia proceeded over a cobalt modified H-ZSM-5 under rather moderate conditions in a fixed-bed reactor. These catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, and infra-red spectroscopy. The reaction mechanism was also discussed, and Co⁰ is believed to be the active site of the hydrogenation/dehydrogenation process and the acid sites of H-ZSM-5 is thought to be the active site of the amination process. Thus, the synergetic effect of the cobalt and the acidic sites of H-ZSM-5 is necessary for the amination of ethanolamine.     

Characterization of Al2O3 composites with Mo particulates,II. Densification and mechanical properties

Three processes were employed to mix MoO₃ or Mo powder with submicrometric Al₂O₃ powder in the preparation of the Mo/Al₂O₃ precursors. The first two processes employed a dissolution route which dissolved MoO₃ powder in ammonia solution, followed by spray-drying or hot-plate-drying to obtain ammonium molybdate/Al₂O₃ granules. Then, the granulated powders were reduced in H₂ atmosphere and hot-pressed to obtain Mo/Al₂O₃ composites. The third process involved mechanically mixing metallic Mo particles with Al₂O₃. The composites normally can achieve 99% relative density after hot-pressing. Microstructural results of hot-pressed composites reveal that intergranular Mo grains of micrometer size dispersed at the grain boundaries of dense Al₂O₃ matrix, and some fine grain Mo grains of nanometric size were entrapped within the Al₂O₃ grains in the composite. The four-point bending strength and the toughness of the dense composites increases 26% and 32% respectively, with the addition of Mo content, but are controlled by the uniformity of the microstructure of the composites. The hardness of the composites basically obeys the prediction of the rule of mixture. The strengthening and toughening mechanisms of the composites with respect to the microstructure are analyzed.