Novel SiC synthesis method applicable to SiC solid phase sintering

Most of the present production processes of SiC sintered bodies require some powder mixing using a mechanical milling process (ball milling, and so on). In this case, relatively long hours are required, and there is the problem of contamination during the preparation process. To avoid these problems, we developed a new process for obtaining a self-sinterable, stoichiometric SiC powder, whose precursor material is water-soluble; the precursor material was synthesized from aqueous silica and citric acid containing a small amount of aluminum compound. In order to obtain the stoichiometric SiC composition, the above aqueous precursor material was adequately cured in air (200°C-400°C); subsequently carbonization reaction (~800°C) in nitrogen atmosphere, carbothermal reduction (~1600°C) in argon atmosphere, and pressureless sintering (~1900°C) were performed. Among these processes, the curing process (cross-linking process) is very important for obtaining the equivalent composition (silica and carbon) for the subsequent carbothermal reduction. In this study, the adequate curing temperature and suitable preparation condition for the carbothermal reduction were investigated for the production of stoichiometric self-sinterable SiC powder. The pressureless sintered body achieved using the obtained SiC powder demonstrated a desirable trans-crystalline fracture behavior.     

Synthesis and characterization of [Zn(acetate)2(amine)x] compounds (x=1 or 2) and their use as precursors to ZnO

As an obvious candidate for a p-type dopant in ZnO, nitrogen remains elusive in this role. Nitrogen containing precursors are a potential means to incorporate nitrogen during MOCVD growth. One class of nitrogen-containing precursors are zinc acetate amines, yet, they have received little attention. The synthesis and single crystal X-ray structure of [Zn(acetate)₂(en)], and the synthesis of [Zn(acetate)₂(en)₂], [Zn(acetate)₂(benzylamine)₂], [Zn(acetate)₂(butylamine)₂], [Zn(acetate)₂(NH₃)₂], and [Zn(acetate)₂(tris)₂], where en=ethylenediamine and tris=(tris[hydroxymethyl]aminomethane) are reported. The compounds were characterized by thermogravimetric analysis and pyrolyzed in air and inert gas to yield ZnO. These compounds are useful single source precursors to ZnO bulk powders by alkali precipitation and ZnO thin films by spray pyrolysis. The amine bound to the zinc influences the ZnO crystal size and shape and acts as a nitrogen donor for preparing nitrogen-doped ZnO during alkali precipitation. Thin films of ZnO prepared by spray pyrolysis using the precursors had a (100) preferred orientation and measured n-type to intrinsic conductivity.     

Occurrence and formation potential of N-nitrosodimethylamine in ground water and river water in Tokyo

N-nitrosodimethylamine (NDMA), a disinfection byproduct of water and wastewater treatment processes, is a potent carcinogen. We investigated its occurrence and the potential for its formation by chlorination (NDMA-FP_2Cl) and by chloramination (NDMA-FP_2NHCl) in ground water and river water in Tokyo. To characterize NDMA precursors, we revealed their molecular weight distributions in ground water and river water. We collected 23 ground water and 18 river water samples and analyzed NDMA by liquid chromatography-tandem mass spectrometry. NDMA-FP_2Cl was evaluated by chlorinating water samples with free chlorine for 24 h at pH 7.0 while residual free chlorine was kept at 1.0-2.0 mgCl₂/L. NDMA-FP_2NHCl was evaluated by dosing water samples with monochloramine at 140 mgCl₂/L for 10 days at pH 6.8. NDMA precursors and dissolved organic carbon (DOC) were fractionated by filtration through 30-, 3-, and 0.5 kDa membranes. NDMA concentrations were <0.5-5.2 ng/L (median: 0.9 ng/L) in ground water and <0.5-3.4 ng/L (2.2 ng/L) in river water. NDMA concentrations in ground water were slightly lower than or comparable to those in river water. Concentrations of NDMA-FP_2Cl were not much higher than concentrations of NDMA except in samples containing high concentrations of NH₃ and NDMA precursors. The increased NDMA was possibly caused by reactions between NDMA precursors and monochloramine unintentionally formed by the reaction between free chlorine and NH₃ in the samples. NDMA precursors ranged from 4 to 84 ng-NDMA eq./L in ground water and from 11 to 185 ng-NDMA eq./L in river water. Those in ground water were significantly lower than those in river water, suggesting that NDMA precursors were biodegraded, adsorbed, or volatilized during infiltration. The molecular weight of NDMA precursors in river water was dominant in the <0.5 kDa fraction, followed by 0.5-3 kDa. However, their distribution was inconsistent in ground water: one was dominant in the <0.5 kDa fraction, and the other in 0.5-3 kDa. Molecular weight distributions of NDMA precursors were very different from those of DOC. This is the first study to reveal the widespread occurrence and characterization of NDMA precursors in ground water.     

珠江源水中THMFP的分子质量分布及其特征研究

以珠江广州段水源水为研究对象，采用超滤分离方法将其中的有机物分成分子质量不同的几种形态，考察了各分子质量组分在DOC、UV、THMFP等方面的差异。结果表明：①分子质量在3～10ku间的有机物是珠江广州段源水中最主要的THMs前驱物；②分子质量最小（〈1ku）的组分，尽管SUVA值最低，THMs生成能力（THMFP／DOC）却最强；③经超滤分离有机物量的损失很少，但紫外吸收明显减弱，THMs生成量也随之减少。     

前驱体结构对多环芳烃选择性开环Pt/Beta催化剂的影响

以海绵铂为原料合成出［Pt（NH₃）₆］Cl₄络合物,采用热重分析（TG）、扫描电镜-能谱（SEM-EDS）、紫外-可见分光光度计（UV-Vis）、质谱（MS）、X射线光电子能谱（XPS）等手段确定了［Pt（NH₃）₆］Cl₄的结构组成;以H₂PtCl₆、Pt（NH₃）₄Cl₂和［Pt（NH₃）₆］Cl₄为前驱体,采用等体积浸渍法制得Pt/Beta催化剂,采用X射线衍射（XRD）、X射线荧光光谱（XRF）、氨程序升温脱附（NH₃-TPD）、氢氧滴定（H₂-O₂）、透射电镜（TEM）、氢气程序升温脱附（H₂-TPD）等表征了Pt/Beta催化剂的物化性质,并考察了Pt/Beta催化剂的多环芳烃选择性开环性能。结果表明,［Pt（NH₃）₆］Cl₄络合物具有更高的“抗自还原”能力,可从前驱体结构上降低铂氨前驱体受热分解时的自还原现象。前驱体结构对铂纳米颗粒的几何尺寸及分布有较大影响,一方面络合物的价态显著影响前驱体与分子筛间的静电作用,进而影响铂纳米颗粒的落位与尺寸;另一方面络合物的空间结构影响前驱体在分子筛微孔中的分布,影响铂纳米颗粒的Ostwald熟化速率。前驱体结构可调变Pt/Beta催化剂的双功能匹配关系,显著影响Pt/Beta催化剂转化甲基萘的活性、稳定性,采用［Pt（NH₃）₆］Cl₄前驱体制备的Pt/Beta催化剂具有更优的活性及长周期稳定性。     

C/C–SiC composites derived from single-source poly(silylene–acetylene) precursors

Carbon-fiber-reinforced carbon–silicon carbide (C/C–SiC) composites were prepared by impregnating carbon fibers with ethynylphenyl-terminated poly(silylene–acetylene) (EPTSA) as a single-source precursor with subsequent hot pressing and pyrolysis. The structural evolution, crystallization behavior, and graphitization of bulk C–SiC ceramics, as well as their mechanical properties and ablation behavior, were investigated. The EPTSA precursor starts to transform into inorganic SiC ceramic materials at 800°C, which is characterized by an amorphous structure with weight loss, shrinkage, and densification between 800 and 1000°C. The formation of SiC crystals inhibited the growth of the graphitic structure between 1000 and 1200°C. As the temperature was raised, both graphite and SiC crystals continued to grow, and the crystalline forms became more complete. The carbon-fiber cloth (T300CF)-reinforced C–SiC composite (T300CF/C–SiC) prepared using polymer infiltration and pyrolysis (PIP) exhibited excellent mechanical properties. After five PIP cycles, the flexural strength, flexural modulus, and interlaminar shear strength of the T300CF/C–SiC composite reached 169 MPa, 32.5 GPa, and 9.38 MPa, respectively. In addition, the chopped-carbon-fiber-reinforced C–SiC composite fabricated using the PIP process demonstrated good oxyacetylene-torch ablation properties.     

Loss and formation of malodorous volatile sulfhydryl compounds during wine storage

Volatile sulfur compounds (VSCs), particularly low molecular weight sulfhydryls like hydrogen sulfide (H₂S) and methanethiol (MeSH), are often observed in wines with sulfurous off-aromas. Recent work has shown both H₂S and MeSH can increase up to a few µM (> 40 µg/L) during anoxic storage, but the identity of the latent sources of these sulfhydryls is still disputed. This review critically evaluates the latent precursors and pathways likely to be responsible for the loss and formation of these sulfhydryls during wine storage based on the existing enology literature as well as studies from food chemistry, geochemistry, biochemistry, and synthetic chemistry. We propose that three precursor classes have sufficient concentration and metastability to serve as latent sulfhydryl precursors in wine: 1) transition metal-sulfhydryl complexes, particularly those formed following Cu(II) addition, which are released under anoxic conditions through an unknown mechanism; 2) asymmetric disulfides, polysulfanes, and (di)organopolysulfanes formed through transition-metal mediated oxidation (e.g., Cu(II)) of sulfhydryls or pesticide degradation, and released through sulfitolysis, metal-catalyzed thiol-disulfide exchange or related reactions; 3) S-alkylthioacetates, primarily formed during fermentation, and releasable hydrolytically. Some evidence also exists for S-amino acids serving as precursors. Based on these findings, we propose a “decision tree” approach to choosing appropriate strategies for managing wines with sulfurous off-aromas.