“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

Effect of using different precursors on electrospinning of CaCu3Ti4O12

In this study, the rheological behavior of electrospinning solutions containing different copper and calcium salts (Cu(NO₃)₂·3H₂O, CuCl₂, Ca(NO₃)₂·4H₂O and CaCl₂) were investigated. To find out the suitable electrospinning solution for producing the high purity CaCu₃Ti₄O₁₂ nanofibers, solutions containing different copper and calcium salts were prepared and CaCu₃Ti₄O₁₂ fibers with different morphological and size were produced. The results showed that the nature of the metals complexes in the ceramic solutions had an obvious effect on the rheological behavior of the electrospinning solutions. FTIR spectras of the electrospinning solutions demonstrated that the interaction between the metal ions and carbonyl groups in the polyvinylpyrrolidone unit occurred and the polyvinylpyrrolidone chains underwent conformational variations. Intensity of the interaction between the metal ions and polymer chains in chloride salts solutions is more than nitrate salts solutions in order to the viscosities of chloride solutions that are more than nitrate solutions. So, thinner high purity polycrystalline CaCu₃Ti₄O₁₂ nanofibers with diameters ranging <200 nm were successfully synthesized by selecting a novel solution containing copper and calcium nitrates after sintering at 900 °C for 4 h.     

Preparation and properties of C/C−ZrB2−SiC composites by high-solid-loading slurry impregnation and polymer infiltration and pyrolysis (PIP)

Ultrahigh-temperature ceramics were added to C/C composites to meet their application requirement in a high-temperature oxidizing environment. C/C−ZrB₂−SiC composites were fabricated by high-solid-loading slurry impregnation with polymer infiltration and pyrolysis. The dispersion and rheological behavior of ZrB₂ slurry and the microstructural, mechanical, and ablation properties of the C/C−ZrB₂−SiC composites were investigated. Results indicated that a well-dispersed and low-viscosity ZrB₂ slurry was obtained using 0.40 wt.% polyethyleneimine as a dispersant at pH 5. Ceramics were uniformly distributed in the short-cut fiber layer and needle-punched area. The flexural strength of the C/C−ZrB₂−SiC composites was 309.30 MPa. The composites exhibited satisfactory ablation resistance under the oxyacetylene flame of 2500 °C, and the mass and linear ablation rates were 0.40 mg/s and 0.91 μm/s, respectively. A continuous and compact ZrO₂ layer, which could effectively reduce the diffusion rate of oxygen and protect the composites from being ablated, was formed.     

Microstructural evolution,mechanical and thermal properties of TiC-ZrC-Cr3C2 composites

Dense TiC-ZrC-Cr₃C₂ composites with various TiC content from 19.6 mol% to 78.4 mol% have been fabricated by hot-pressing sintering at 1950 °C using 2.0 mol% Cr₃C₂ as sintering aid. The effect of TiC content on the microstructure, mechanical and thermal properties of TiC-ZrC-Cr₃C₂ composites are investigated systematically. The single (Zr, Ti, Cr)C solid solution is obtained when TiC content is 19.6 mol%, while with increasing TiC content, the composites begin to consist of Zr-rich (Zr, Ti)C solid solution and Ti-rich (Ti, Zr, Cr)C solid solution phase. SEM and EDS analysis confirm that Cr element is not favorable to diffuse into ZrC lattice to form (Zr, Cr)C solid solution. Flexural strength and Vickers hardness increase gradually with increasing TiC content, but fracture toughness does not improve significantly. Fracture toughness are in the range of 3.34–4.01 MPa∙m^1/2 for all composites, and the optimum value reaches 4.01 MPa·m^1/2 with 49.0 mol% TiC. Experimental results of the thermal expansion coefficient reveal that the addition of TiC raises the thermal expansivity of TiC-ZrC-Cr₃C₂ composites. Noticeably, the thermal conductivities of TiC-ZrC-Cr₃C₂ composites show a decrement trend with increasing TiC content, not as theoretical predicting by the rule of mixtures. For instance, the thermal conductivity at 25 °C ranges from 18.0 W/m∙K for 8Z2T2C composite down to 10.6 W/m∙K for 2Z8T2C composite.     

Highly dispersed and confined Ni/MMO catalyst synthesized in a rotating packed bed for hydrogenation of maleic anhydride

Catalytic hydrogenation of maleic anhydride (MA) into succinic anhydride (SA) is one of the most important transformations in synthetic organic chemistry. Herein, we firstly synthesized well-dispersed nickel particles confined by mixed metal oxides (Ni/MMO) derived from in situ transformation of Ni-Al hydrotalcite in a rotating packed bed (RPB) to catalyze this process. A series of Ni/MMO catalysts (63 wt%–89 wt% Ni) were effectively fabricated and the structure–activity relationship was established. Results showed that a Ni/MMO catalyst (82 wt% Ni) with substantial surface defect sites and the highest Ni surface area among the prepared Ni/MMO catalysts, demonstrates the highest activity with ~100% MA conversion and ~100% selectivity to SA under 25°C within 77 min. This is, to our knowledge, the highest conversion and selectivity under room temperature to date. Moreover, the Ni/MMO catalyst prepared by RPB has higher specific surface area and Ni surface area, therefore possessing a higher hydrogenation rate compared to that by stirred tank reactor (1.69 vs. 1.36, 10⁻³·mol_MA/g_cat/min). These results will provide an attractive option of the catalysts for MA hydrogenation, and a novel strategy for synthesizing nickel catalyst derived from Ni-Al hydrotalcite.     

具有自清洁与自修复双重特性的纤维用涂层的制备与研究

为了解决纤维用涂层在长期使用过程中污渍吸附和开裂磨损等技术问题,首先合成末端带有呋喃基团的硅烷改性预聚体,然后制备末端带有呋喃基团的改性纳米 TiO₂,最后将硅烷改性预聚体、改性纳米 TiO₂以及 4,4′-亚甲基双（ N-苯基马来酰亚胺）（BMI）进行 DA（Diels-Alder）反应得到树脂 /纳米填料一体化复合涂层,并对涂层的结构和性能以及自清洁、自修复效果进行研究。结果表明：涂层损伤能够在不影响疏水性能的前提下得到修复,同时纤维涂层具有很强的光降解污染物的功能,使该涂层具备自清洁和自修复双重特性,提高了纤维用涂层的使用寿命。     

Influences of SiC infiltration and coating on compressive mechanical behaviours of 2D C/SiC composites up to 1600 °C at wide-ranging strain rates

The influences of the SiC infiltration and coating on the compressive mechanical behaviours of 2D C/SiC composites were determined up to 1600 °C at 0.001 and 1000/s strain rates in argon and air. In addition, the failure mechanisms responsible for the compressive mechanical behaviours were elucidated through in-situ observation and micro-analysis-based methods. The 2D C/SiC composite compressive strength was highly sensitive to temperature, loading rate, and oxidation, and was enhanced by the change in the thermal residual stress and decreased by oxidation. In argon, because of the extra infiltrated SiC matrix, SiC treated 2D C/SiC specimens exhibited higher compressive strengths and lower strain rate sensitivity factors than SiC untreated 2D C/SiC specimens. The SiC coating effectively improved the oxidation resistance of the 2D C/SiC composites in air, regardless of the temperature, strain rate, and oxidative damage-which depends on SiC coating, strain rate, and temperature.     

Low-temperature diffusion bonding of Ti3Si(Al)C2 ceramic with Au interlayer

Herein, a reliable diffusion bonding of Ti₃Si(Al)C₂ ceramic is achieved by applying Au foil as an interlayer at 650 °C for 30 min with an axial pressure of 20 MPa. This novel method significantly decreases the bonding temperature, which is about 150 °C lower than the lowest bonding temperature from current research to the best of our knowledge. Maximum shear strength of 58 MPa is achieved at 650 °C among the bonding temperature range of 600 °C~800 °C. The microstructure evolution mechanism and the relationship between microstructure and mechanical property are discussed. The facile mutual diffusion of Au with de-intercalated Al and Si from Ti₃Si(Al)C₂ is considered critical in achieving sound interfacial bonding.     

Polymer derived ZrO2 reinforced SiC-ZrB2 polycrystalline fiber

Most polycrystalline SiC-based fibers were prepared at a sintering temperature higher than 1600 °C. In this work, a ZrO₂ reinforced SiC-ZrB₂ polycrystalline fiber was prepared at 1400 °C via the polymer-derived ceramic method from a new Zr- and B-containing polycarbosilane. The morphology and microstructure of the polycrystalline nanocomposite fiber were studied using XRD, XPS, EDS, SEM, and TEM. The results showed that t-ZrO₂ was formed at relatively lower temperature (<1000 °C). The most interesting result is that the polycrystalline nanocomposite SiC-ZrB₂ was generated after heat-treatment at 1400 °C, producing an excellent ZrO₂ reinforced SiC-ZrB₂ polycrystalline fiber. The present study provides a novel strategy for the fabrication of SiC-based polycrystalline fiber at a relatively low temperature.