先驱体法合成氮化硼研究进展

综述了无氧有机先驱体法合成氮化硼的研究进展 ,系统介绍了由硼烷、硼吖嗪、卤化硼合成氮化硼的工艺条件 ,及由这些化合物制备聚合物先驱体的合成途径及其陶瓷转化 ,概述了先驱体法待研究的问题 .     

Synthesis and characterization of amorphous SiO2 nanowires derived from a polymeric precursor

Bulk quantity and long-scale amorphous SiO2 nanowires have been prepared simply by direct pyrolysis of an iron-modified perhydropolysilazane (Fe-PHPS) at 1300 degrees C in the flowing Ar atmosphere. Scanning electronic microscope (SEM) study shows that most SiO2 nanowires have smooth surface and they consist of straight and smoothly curved parts, with diameters in the range of 50 to 200 nm and lengths up to 200 microm. Beside this normal form of SiO2 nanowires via the typical vapor-liquid-solid (VLS) mechanism, four other different helical forms of nanowires, named spring-shaped nanowires, twinborn nanospring, fishbone-shaped nanowires and braided-like helical SiO, nanowires, are also observed. The formation of these nanowires is proposed to be the modulated contact angle anisotropy (CAA) mechanism based on the VLS mechanism.     

液态SiBCN前驱体转化陶瓷的抗氧化性能及机制

前驱体转化法制备的SiBCN陶瓷具有优异的耐高温抗氧化性能,有望作为高温热结构材料应用于航空航天领域。本文主要对前驱体转化法制备的SiBCN陶瓷在1 200、1 400℃下的抗氧化性能进行研究。采用XPS对陶瓷氧化前后化学键结合方式进行表征,分析了氧化前后化学结构的变化;采用XRD和SEM对陶瓷氧化前后表面相组成、微观形貌和截面氧化层进行分析,并通过氧化层厚度随时间变化对高温抗氧化动力学进行了研究。结果表明:SiBCN陶瓷经高温氧化后在表面形成了致密的氧化膜,氧化层主要以无定形态存在,且与基体结合紧密;陶瓷的高温氧化速率受氧扩散控制,其在1 200、1 400℃下的氧化动力学常数分别为0.0224 μm2/h和0.1045 μm2/h,小于SiC陶瓷的0.0449 μm2/h和0.1288 μm2/h。由于SiBCN陶瓷形成的BN （C）结构以及高温氧化后形成的SiO_xN_y致密氧化膜降低了氧气在氧化层中的扩散速率以及反应活性,使得SiBCN陶瓷具有比SiC陶瓷更加优异的高温抗氧化性能。      

Synthesis of silicon oxynitride from a polymeric precursor

Previous studies suggest that silicon oxynitride can be synthesized by pyrolysing polymers which are produced by the copolymerization of cyclic polysiloxanes and cyclic polysilazanes. The cyclic polysiloxanes required are [CH₃Si(H)O]_m with m=4, 5 and 6, and these can be produced by the hydrolysis of dichloromethylsilane. In the present work, a significantly improved combined yield of these cyclic polysiloxanes is reported. Three main reasons are attributed to this achievement: an improved hydrolysis procedure; optimization of the initial acidity of the reaction medium; and changes in the total reaction time. Thus a mixture containing > 95 mol% cyclic tetramer, pentamer and hexamer has been prepared and this represents a 20 mol% improvement over previously reported results. Investigations of the preparation and characterization of the cyclic polysilazanes, copolymers and silicon oxynitride will be described in Parts II–IV, respectively.     

Synthesis of silicon oxynitride from a polymeric precursor

Proton nuclear magnetic resonance (¹H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and gas chromatography (GC) have been used to study the products of ammonolysis of dichloromethylsilane in diethyl ether at 0 °C. Results indicate that the major products are trimethylcyclotrisilazane and tetramethylcyclotetrasilazane. Hydrolysis of the cyclosilazanes has also been qualitatively investigated. A simple and useful method for the identification of cyclosilazanes has been developed using the ¹H NMR technique.     

Synthesis of silicon oxynitride from a polymeric precursor

In the present work, homopolymerization and copolymerization of methylcyclosiloxanes and methylcyclosilazanes are investigated. Homopolymers and copolymers produced are characterized using ¹H NMR and Fourier transform infrared spectroscopy. It is found that the reactivity of methylcyclosiloxanes is much lower than that of methylcyclosilazanes. The structures of the homopolymers and copolymers produced are discussed and it is suggested that the latter consists of a block structure containing siloxanes and silazanes in which the siloxane units are the smaller.     

Synthesis of silicon oxynitride from a polymeric precursor

In this final part of our investigation, the copolymers prepared (reported in Part III) have been pyrolysed to yield silicon oxynitride. Thermogravimetric studies performed on several copolymers in nitrogen, ammonia/nitrogen mixtures, air and oxygen are discussed in the present paper. One copolymer was used for further investigation and its pyrolysis conditions (variation of pyrolysis atmosphere according to temperature) were optimized to produce the maximum yield of white amorphous residue which was crystallized by heating to 1600°C to produce silicon oxynitride powder. The powder particles were equiaxed in shape and in the size range 50–150 nm but in agglomerates of size 0.3–0.1 m. Crystallization at higher temperatures resulted in a mixture of a silicon nitride-silicon oxynitride.     

Synthesis and magnetic characterization of MnAs nanoparticles via nanoparticle conversion

We report on the synthesis of ferromagnetic manganese arsenide (MnAs) nanoparticles via the conversion of primary Mn particles which are generated in an aerosol process in a spark discharge generator. After sintering and size selection in an aerosol setup, the particles are deposited on GaAs(100)B and Si(111) substrates. Subsequent conversion to MnAs particles takes place in an annealing process under a hydrogen atmosphere with an arsine background pressure. The magnetic properties are studied using a SQUID magnetometer. The annealed MnAs particles exhibit hexagonal facets and show anisotropic magnetic behaviour on GaAs(100)B substrates, whereas on Si(111) they remain spherical and show isotropic magnetic behaviour. Scanning transmission electron microscopy studies are used to confirm the conversion from Mn to MnAs.     

Synthesis and characterization of polyvinylsilazane as a precursor for Si3N4 based ceramic materials

This investigation focused on the synthesis and characterization of a polyvinylsilazane (PVSZ) polymer. This material shows promise as a precursor for silicon nitride/silicon carbide based ceramic materials. The polymer was synthesized via the ammonolysis of vinyltrichlorosilane (VTS) in tetrahydrofuran (THF). The polymer was characterized using Fourier Transform Infrared (FT-IR) spectroscopy, ¹H/¹³C nuclear magnetic resonance (NMR) spectroscopy, and gel permeation chromatography (GPC). This polymer was found to have a molecular weight of around 880 g/mol, as determined by GPC. Thermal cross-linking, up to 1500 °C in an inert atmosphere, produced a ceramic material with a char yield of around 85%. The gas by-products from the partial decomposition of the polymer to a ceramic material were characterized using residual gas analysis (RGA). The crystallite phases of the ceramic char, pyrolyzed in different atmospheres, were determined by X-ray powder diffraction (XRD). Lastly, the ability of the polymer to form a free standing monolithic structure as well as the ability to be drawn into fibers was evaluated.     

Synthesis, characterization and gas sensing property of hydroxyapatite ceramic

Hydroxyapatite (HAp) biomaterial ceramic was synthesized by three different processing routes viz. wet chemical process, microwave irradiation process, and hydrothermal technique. The synthesized ceramic powders were characterized by SEM, XRD, FTIR and XPS techniques. The dielectric measurements were carried out as a function of frequency at room temperature and the preliminary study on CO gas sensing property of hydroxyapatite was investigated. The XRD pattern of the hydroxyapatite biomaterial revealed that hydroxyapatite ceramic has hexagonal structure. The average crystallite size was found to be in the range 31–54 nm. Absorption bands corresponding to phosphate and hydroxyl functional groups, which are characteristic of hydroxyapatite, were confirmed by FTIR. The dielectric constant was found to vary in the range 9–13 at room temperature. Hydroxyapatite can be used as CO gas sensor at an optimum temperature near 125°C. X-ray photoelectron spectroscopic studies showed the Ca/P ratio of 1.63 for the HAp sample prepared by chemical process. The microwave irradiation technique yielded calcium rich HAp whereas calcium deficient HAp was obtained by hydrothermal method.     

Synthesis of SiC ceramic fibers from nuclear reactor irradiated polycarbosilane ceramic precursor fibers

Polycarbosilane (PCS) ceramic precursor fibers are irradiated in a nuclear reactor and pyrolyzed under inert atmosphere. Bridge structure of Si–CH₂–Si is formed in the irradiated products by the rupture of Si–H bonds and succeeding cross-linking. When irradiated at the neutron fluence of 2.2 × 10¹⁷ cm⁻² under N₂ atmosphere, the gel content and ceramic yield at 1,273 K of PCS fibers are up to 80% and 94.3%, respectively, and their pyrolysis products are still fibrous, which illuminates that the infusibility of PCS fibers has been achieved. FT-IR spectra indicate that the chemical structure of pyrolysis products is very similar to that of pure SiC, while X-ray diffraction curves suggest that β-SiC microcrystals are formed in the fibers, and their mean grain size is about 7.5 nm. The oxygen content (1.69–3.77 wt%) is much lower than that of conventional SiC fibers by oxidation curing method (about 15 wt%). Tensile strength of the SiC fibers is up to 2.72 GPa, which demonstrates that their mechanical properties are excellent. After heat-treated at 1,673 K in air for an hour or at 1,873 K under Ar gas atmosphere for 0.5 h, their external appearance is still undamaged and dense, and their tensile strength decreases to a small extent, which verifies that heat resistance of the SiC fibers is eximious.     

纳米氧化钛的制备、表征及应用研究

介绍了应用钛酸四丁酯和草酸作原料，在有机溶剂乙醇中形成混合前驱体的方法制备了粒径在70－90nm之间的二氧化钛，探讨了煅烧温度对二氧化钛晶型和晶粒大小的影响。扫描电镜观察粒子的形貌，大小及其粒径分布，利用光照对二氧化钛的甲基橙悬浮液的变色作用，考察了二氧化钛的光催化性能。     

Synthesis and characterization of Cu-doped polymeric carbon nitride

Polymeric carbon nitride doped with copper through a solid-state reaction was characterized by several techniques, among them are UV-visible spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, etc. The material is a semiconductor with a wide band gap of 2.74 eV. Sites of both Cu(I) and Cu(II) were detected, apparently only coordinated by the polymer. The material comprises crumpled nanosheets, and is substantially an amorphous layered material with a prevalent 2D structure with low inter-planar interactions, as shown by X-ray diffractometry and TeraHertz spectroscopy. Photo-oxidation of benzyl alcohol was used to probe the active sites of the material, comparing them with the non-doped material. The higher activity and selectivity toward salicylic alcohol of the non-doped material can be due to both a more localized electron transfer and a longer lifetime of the hole–electron pair. Cu-CN favored the oxidation of hydroxymethyl group. Therefore, the presence of copper can favor different reaction pathways with respect to the non-doped material.     

含锆耐高温陶瓷化先驱体的合成和性能研究

以丁基锂、三氯乙烯、苯乙炔和四氯化锆为原料,合成了一种耐高温的含锆陶瓷化先驱体(简称PZA)。采用FT-IR和GPC对树脂结构进行表征,通过TGA和XRD对聚合物耐热性和陶瓷化转化进行研究。结果表明,PZA树脂具有优良的耐热性能和陶瓷化转化性能,N2气氛下Td10(质量损失10%的温度)达600℃,1 000℃下的质量保留率为61.9%,1 450℃下完全热解为Zr C。     

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.     

Synthesis of S-doped graphene by liquid precursor

Doping is a common and effective approach to tailor semiconductor properties. Here, we demonstrate the growth of large-area sulfur (S)-doped graphene sheets on copper substrate via the chemical vapor deposition technique by using liquid organics (hexane in the presence of S) as the precursor. We found that S could be doped into graphene's lattice and mainly formed linear nanodomains, which was proved by elemental analysis, high resolution transmission microscopy and Raman spectra. Measurements on S-doped graphene field-effect transistors (G-FETs) revealed that S-doped graphene exhibited lower conductivity and distinctive p-type semiconductor properties compared with those of pristine graphene. Our approach has produced a new member in the family of graphene based materials and is promising for producing graphene based devices for multiple applications.     

具有优异纺丝性的陶瓷先驱体含铝聚碳硅烷的合成及表征

选择适当分子量的低分子量聚碳硅烷与Al(AcAc)3在自制常压高温合成装置中合成了含铝碳化硅纤维的先驱体--聚铝碳硅烷(polyaluminocarbosilane,PACS).并对PACS进行了软化点测试、傅立叶红外光谱(FT-IR)分析、凝胶渗透色谱(GPC)测试、元素分析以及可纺性研究.由中等分子量聚碳硅烷为原料合成出的软化点为194.8～220.1,Si-H键含量为0.857,数均分子量为2353,分子量分布呈"双峰"分布的PACS,经熔融纺丝得到了直径为5μm、表面光滑、直径均匀的原丝,表现出了优异的纺丝性能.     

Synthesis of TaC nanopowders by liquid precursor route

TaC nanopowders were synthesized by a liquid precursor route. The synthesis procedures include (1) preparation of tantalum ethylate solution by sufficient mixing TaCl₅ powders with ethanol, (2) adding activated carbon nanopowders to the solution, (3) dispersing activated carbon nanopowders in the solution evenly to form a liquid precursor by an ultrasonic equipment, (4) brushing the liquid precursor on an alumina crucible and blowing the liquid precursor to dry, (5) heat treatment in a graphitization furnace in a vacuum, and (6) brushing the alumina crucible to obtain products. X-ray diffraction patterns indicated that the products were cubic TaC. The formation temperature (1300 °C) of TaC in this process is lower than that (1700 °C) in the conventional method. The reaction time of forming TaC in this process is half an hour. The observed size of TaC powders by TEM was smaller than 50 nm. The specific surface area of the powders by a surface area analyzer was 28.399 m²/g.