聚碳硅烷纤维在环己烯气氛中的不熔化处理

将聚碳硅烷(PCS)纤维在环己烯气氛中进行化学气相交联不熔化处理,其氧含量比空气不熔化处理大大降低,组成和结构也发生了变化,气体副产物中存在环己烷和小分子硅烷.在环己烯气氛中,随着温度的升高,PCS分子的Si-H键的反应程度逐渐提高,纤维的凝胶含量逐渐增大.环己烯受热引发PCS分子中的Si-H和Si-CH3键断裂生成Si自由基和Si-CH2自由基,促进PCS分子间形成Si-CH2-Si交联结构;同时,环己烷作为侧基引入到PCS分子结构中,使纤维的碳含量随之增高.随着反应温度的升高,部分环己烷侧基和少量小分子硅烷会从PCS分子主链脱出.     

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state²⁹Si and¹³C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of -SiC and -SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.     

三氯环硼氮烷改性聚碳硅烷的合成及陶瓷转化

为研究三氯环硼氮烷(TCB)对聚碳硅烷(PCS)性能及陶瓷转化的影响,将一定量的TCB加入PCS中制备TCB改性PCS聚合物,分析了TCB与PCS的反应性及TCB用量对改性PCS结构、陶瓷收率、可加工性和SiC产物微结构的影响,并采用红外光谱、热重、X射线衍射等测试技术对相应产物进行表征.结果表明,PCS中的Si-H键可部分地与TCB中的-Cl反应生成HCl;随着TCB质量分数的增加,PCS中Si—H键相对于Si-CH3的浓度比呈下降趋势.TCB的加入可显著提高PCS的陶瓷收率,TCB质量分数大于8%时,陶瓷收率质量分数约增加10%.TCB质量分数为5%～8%时,可在提高PCS收率的同时使其保持较好的可加工性能,TCB质量分数大于8%时,可加工性能变差.B、N的引入对SiC的微结构有影响:在氩气保护下,经1000℃热处理时,TCB的加入促进SiC晶粒的生长;经1400℃热处理时,TCB的加入抑制SiC晶粒的长大.     

低氧含量液态超支化聚碳硅烷的合成与陶瓷化研究

以甲醇对氯甲基三氯硅烷(Cl3SiCH2Cl)进行烷氧化反应,然后经过格氏偶联反应和还原反应,制备了低氧含量液态超支化聚碳硅烷(HBPCS)。通过凝胶渗透色谱法(GPC)、核磁共振(NMR)以及元素分析对由此制备的HBPCS进行表征。结果表明,提高烷氧化比例,可以有效抑制溶剂四氢呋喃开环的副反应,降低先驱体氧含量。通过热失重分析(TGA)和X射线衍射(XRD)分别对HBPCS的热性能及相应陶瓷在高温下的结晶行为进行研究。     

Preparation of a hyperbranched polycarbosilane precursor to SiC ceramics following an efficient room-temperature cross-linking process

Room-temperature cross-linking of a hyperbranched polycarbosilane (HBPCS) with divinylbenzene (DVB) in the presence of the cyclohexanone peroxide–cobaltous naphthenate (CHP–CN) initiator system was studied. According to the Fourier transform infrared spectroscopy (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) results, the cross-linking reaction occurred via the vinyl polymerization. The GPC analysis confirmed the molecular weight of the cross-linked HBPCS significantly increased. Thermal behaviors of cross-linked HBPCS and original HBPCS were investigated by thermal gravimetric analysis-differential thermal analysis (TGA–DTA). The TGA results indicated that the ceramic yield of HBPCS remarkably increased by the cross-linking treatment. For the HBPCS/10 wt% DVB system, the maximum of reaction degree of HBPCS was obtained, which might be responsible for the highest ceramic yield of 70.1 wt% at 1000 °C. However, the ceramic yield of the non-crosslinked HBPCS was only 45 wt% at 1000 °C. The evolution of crystal structure of SiC as a function of pyrolysis temperature was traced by means of X-ray diffraction (XRD) and FT-IR. With the pyrolysis temperature increasing, the β-SiC peaks became sharper and the grain size also grew larger. As the DVB content increased, the intensity of β-SiC peaks significantly reduced, indicating smaller β-SiC grain size.     

Raman study of SiC fibres made from polycarbosilane

We have examined the evolution of Raman spectra of SiC fibres through structural and compositional transformations caused by heat treatment. The SiC fibre was made from polycarbosilane. Raman spectra of the SiC fibre indicate that it consists of (i) amorphous or microcrystalline SiC, (ii) carbon microcrystals, and (iii) silicon oxide. The amount of microcrystalline carbon in the fibre increases with heat treatment temperature up to 1400° C, and it decreases abruptly in those fibres heat treated above 1500° C. The tensile strength of the fibre drops virtually to zero after the heat treatment at 1500° C. Carbon microcrystals are precipitated from the Si-C random network with excess carbon, and they are distributed uniformly in the fibre. These carbon particles suppress the growth of SiC crystals. It is shown that the carbon microcrystals play an important role in maintaining the high mechanical strength of the SiC fibre.     

One pot conversion of furfural to 2-methylfuran in the presence of PtCo bimetallic catalyst

Biomass-derived furfural (FAL) is the platform chemical for synthesis of various value-added chemicals and fuels. One of the FAL-derived chemicals, i.e., 2-methylfuran (2-MF), is the potential biofuel due to its attractive chemical and physical properties. Various methods are reported for conversion of FAL to 2-MF which are operated at high temperature and high H₂ pressure. In present work, one pot catalytic method was developed in batch mode process for conversion of FAL to 2-MF. Reactions are carried out in the presence of PtCo/C bimetallic catalyst under 0.5–1 MPa H₂ pressure. Monometallic and bimetallic catalysts with different Pt and Co loading were prepared by wet impregnation method, and catalysts were characterized by transmission electron microscopy, BET surface area, X-ray photoelectron spectroscopy, X-ray diffraction and inductive coupled plasma atomic emission spectroscopy techniques. 59% 2-MF yield was achieved at 180 °C and lower (0.5 MPa) H₂ pressure.     

聚碳硅烷先驱体转化法制备SiC涂层研究

以自合成的聚二甲基硅烷(PDMS)为原料,常压裂解合成聚碳硅烷(PCS),通过FT-IR分析PCS的结构,用GPC测定其分子量及分布,用熔点分析仪测定其熔点.在此基础上,采用聚碳硅烷先驱体转化法在石墨基体上制备SiC涂层,通过X射线衍射对涂层进行晶相分析,用扫描电镜分析涂层表面和横断面的形貌.结果表明,在石墨基体上形成了明显的β-SiC晶相,可以获得均匀、致密的SiC涂层,其厚度可通过涂层次数的改变进行调节,单次涂层最大厚度大约为2μm.     

Non-passivating polymeric structures in electrochemical conversion of phenol in the presence of NaCl

The formation of non-passivating polymeric structures was investigated during electrochemical conversion of phenol using carbon electrodes and NaCl as electrolyte. The influence of initial phenol concentration, current density and reaction temperature on phenol conversion and polymer morphology was studied by FTIR and STM, while the fate of intermediate compounds was analyzed by GC/MS. Unlike previous work, non-passivating solid polymer was produced at high voltage and current density values in the presence of NaCl. The most orderly polymer formed at 912 mg l(-1) initial phenol concentration, current density 32.9 mA cm(-2), NaCl concentration 120 g l(-1) and temperature 25 degrees C. Higher operational parameters yielded disorderly formed aggregates of the polymer in decreasing surface density on STM images. Along with the polymer, only toxic mono-, di- and tri-chlorophenols were formed as intermediate compounds during the electrochemical conversion, which eventually were polymerized and/or oxidized to final products. FTIR analysis and enlarged STM image implied the repeating phenol units in the polymer structure. The results may lead to appropriate techniques for the removal of phenol from wastewater in the form of a solid polymer.     

Model-based process optimization in the presence of parameter uncertainty

In model-based process optimization one uses a mathematical model to optimize a certain criterion, for example the product yield of a chemical process. Models often contain parameters that have to be estimated from data. Typically, a point estimate (e.g. the least squares estimate) is used to fix the model for the optimization stage. However, parameter estimates are uncertain due to incomplete and noisy data. In this article, it is shown how parameter uncertainty can be taken into account in process optimization. To quantify the uncertainty, Markov Chain Monte Carlo (MCMC) sampling, an emerging standard approach in Bayesian estimation, is used. In the Bayesian approach, the solution to the parameter estimation problem is given as a distribution, and the optimization criteria are functions of that distribution. The formulation and implementation of the optimization is studied, and numerical examples are used to show that parameter uncertainty can have a large effect in optimization results.     

The process of electrochemical deposition of zinc in the presence of boron-doped detonation nanodiamonds

The influence of detonation of nanodiamonds doped with boron during the detonation synthesis (DND-boron) on the process of electrochemical deposition of zinc from a zincate electrolytic solution is investigated. It is shown that the throwing power (the coating uniformity) increases 2to 4-fold depending on the DND–boron concentration, the electrolytic conductivity remains unchanged, the corrosion resistance (as measured by the corrosion currents) of the Zn–DND-boron coating grows 2.6 times when tested in the 3% NaCl solution and 3 times in the climatic chamber.     

Osteoarthritic process modifies expression response to NiTi alloy presence

Nickel-titanium alloy (nitinol, NiTi) is a biomaterial with unique thermal shape memory, superelasticity and high damping properties. Therefore NiTi has been used in medical applications. In this in vitro study, the effect of NiTi alloy (with two surface modifications – helium and hydrogen) on gene expression profile of selected interleukins (IL-1β, IL-6 and IL-8) and matrix metalloproteinases (MMP-1 and MMP-2) in human physiological osteoblasts and human osteoarthritic osteoblasts was examined to respond to a question of the different behavior of bone tissue in the implantation of metallic materials in the presence of cells affected by the osteoarthritic process. The cells were cultivated in contact with NiTi and with or without LPS (bacterial lipolysaccharide). Changes in expression of target genes were calculated by 2^-ΔΔCt method. An increased gene expression of IL-1β in osteoarthritic osteoblasts, with even higher expression in cells collected directly from the metal surface was observed. In case of physiological osteoblasts, the change in expression was detected after LPS treatment in cells surrounding the disc. Higher expression levels of IL-8 were observed in osteoarthritic osteoblasts after NiTi treatment in contact with alloy, and in physiological osteoblasts without relation to location in combination of NiTi and LPS. IL-6 was slightly increased in physiological osteoblastes after application of LPS. MMP-1 expression level was obviously significantly higher in osteoarthritic osteoblasts with differences regarding the metal surface and location. MMP-2 expression was decreased in both cell lines after LPS treatment. In conclusion, results of present study show that the NiTi alloy and the treatment by LPS, especially repeated doses of LPS, change the gene expression of selected ILs and MMPs in human osteoblast cell cultures. Some of the changes were depicted solely to osteoarthritic osteoblasts.     

聚碳硅烷纤维氧化交联机理的研究

对聚碳硅烷(PCS)原丝在不同氧化交联温度区间生成的逸出产物进行红外、核磁和GC-MAS分析,并结合交联丝的红外分析,推测氧化交联的机理。结果表明,PCS的氧化交联主要是其Si—H氧化生成Si—OH,后者进而彼此缩合生成Si—O—Si交联结构;氧化交联温度高于150℃时,其部分Si—CH3也开始氧化生成Si—OH并进而交联;同时,在氧化交联过程还发生PCS侧链的热裂解,所形成小分子也通过Si—OH彼此结合,形成较大分子,且其分子量随交联温度的提高而提高。因此,要及时排除氧化交联过程废气,以免逸出产物黏附在纤维表面而导致粘结。     

聚碳硅烷先驱体法制备连续SiC自由膜研究

以聚碳硅烷(PCS)为先驱体,通过自行设计的喷膜板熔融纺出连续PCS自由薄膜,并对其进行氧化交联与高温裂解烧结可制得连续SiC自由薄膜.用扫描电镜分析薄膜表面和横断面的形貌,通过红外光谱(FTIR)分析氧化交联后薄膜的结构,通过拉曼光谱(Raman spectroscopy)、X射线衍射(XRD)与场发射高分辨透射电子显微镜(HRTEM)对薄膜进行微观分析.结果表明,熔融纺膜与聚碳硅烷先驱体法相结合可制得均匀、致密的连续β-SiC自由薄膜,其厚度可通过调节喷膜板的喷膜口尺寸大小和纺膜速度进行控制,薄膜的厚度大约在10～30μm.     

聚碳硅烷的挤出胀大与Y型SiC纤维的异形度

研究了聚碳硅烷(PCS)熔体在Y型喷丝孔中的挤出胀大机理、膨胀模型以及膨胀比对Y型碳化硅纤维异形度的影响.结果表明:采用圆形喷丝孔时PCS的膨胀比为1.02～1.15,Y型喷丝孔时膨胀比为1.02～1.43,在与圆形喷丝孔成形条件相当的情况下,Y型喷丝孔的膨胀比比圆形孔的高出约15%,Y型口模周边的剪切应力分布不均、剪切应力内外差异和区域差异是造成其膨胀比增大的根本原因.PCS熔体温度降低或软化点提高,其出口挤出胀大效应增大.由于模口膨胀的影响,Y型PCS纤维的异形度比喷丝孔异形度下降10%～20%.     

Synthesis and properties of ceramic fibers from polycarbosilane/polymethylphenylsiloxane polymer blends

We synthesized ceramic fibers based on silicon carbide (SiC) from polymer blends of polycarbosilane (PCS) and polymethylphenylsiloxane (PMPhS) by melt-spinning and radiation curing. PMPhS was compatible with PCS up to 30 mass%, and formed a transparent melt at temperatures higher than 513 K. The softening point was also lowered by adding PMPhS and 15 mass% of PMPhS to PCS was the most suitable condition for obtaining thin fibers with an average diameter of 14.4 μm. Due to the lowered softening point of the PCS–PMPhS fibers, γ-ray curing in air was adopted. The ceramic yield of the cured fiber was 85.5% after pyrolysis at 1273 K. In spite of the small diameter, the resulting tensile strength at 1273 K was rather limited at 0.78 GPa. Blooming of the PMPhS component during pyrolysis may have caused surface defects. After high-temperature pyrolysis at 1673–1773 K, a porous nanocrystalline SiC fiber with a unique microstructure was obtained with surface area of 70–150 m²/g. When the fiber was pyrolyzed at the same temperature under a highly reductive atmosphere, wire bundle-shaped fibers were obtained by gas evolution and reactions.