碳热还原制备不同形貌的碳化硅纳米线

以酚醛树脂为碳源，正硅酸乙酯为硅源，硝酸镧和表面活性剂为调控剂，通过溶胶-凝胶和碳热还原反应制备了不同形貌的碳化硅纳米线。采用X射线衍射、扫描电子显微镜、透射电子显微镜和X射线散射能谱对所制备的碳化硅纳米线进行表征。结果表明，通过此方法所制备的碳化硅纳米线均为立方结构的β-SiC，分别具有直线状、竹节状和链珠状、分枝状等不同形貌。金属催化剂和表面活性剂对碳化硅纳米线的结构和形貌变化有重要影响。     

In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000–1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiN_xC_y nanocrystals (x + y = 1) are homogeneously formed “in situ” in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si₃N₄, TiN_xC_y (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.     

耐超高温SiC(Al)纤维先驱体——聚铝碳硅烷纤维的研究

以聚硅碳硅烷(PSCS)与乙酰丙酮铝(Al(AcAc)3)为原料,在常压高温条件下反应制备出聚铝碳硅烷(PACS),经过熔融纺丝制备了PACS纤维.应用GPC、IR、XPS、29Si-NMR、27Al-NMR、TG、SEM、元素分析和增重等一系列分析,分别对PACS纤维的微观组成、结构以及性能进行了分析.研究结果表明,以原料质量配比为6∶100(Al(AcAc)3∶PSCS)合成的PACS化学式为SiC2.0H7.5O0.13Al0.018,数均分子量为1700左右,最适宜制备PACS纤维;PACS纤维中主要存在SiC4、SiC3H等结构,同时存在Si—O—Al键;在氮气气氛中,PACS纤维的陶瓷产率达到52%左右;预氧化处理,PACS纤维中Si—H键与空气中的氧反应形成Si—O—Si交联结构,较聚碳硅烷(PCS)纤维易于氧化,经过预氧化的PACS纤维陶瓷产率达到80%左右,是制备耐超高温SiC(Al)陶瓷纤维的合适纤维;用预氧化PACS纤维制备的SiC(OAl)纤维和SiC(Al)纤维抗拉强度高,耐高温性能好.     

6H-SiC/3C-SiC/6H-SiC量子阱结构制备及其发光特性

利用固源分子束外延(SSMBE)生长技术, 在1350K的衬底温度下, 通过改变Si束流强度, 在6H-SiC(0001)面上外延生长6H-SiC/3C-SiC/6H-SiC量子阱结构薄膜, 并用反射高能电子衍射(RHEED)与光致发光(PL)谱对生长的薄膜的晶型和发光特性进行表征. RHEED 结果显示生长的薄膜为6H-SiC/3C-SiC/6H-SiC量子阱结构薄膜. 室温下He-Gd激光激发的光致发光(PL)谱显示, 薄膜在480-600 nm范围内存在衬底未观察到的较强发光. 拟合得到的发光峰与依据量子阱结构模型计算出的发光位置较为一致. 由此表明, 该强发光带可能是6H-SiC/3C-SiC/6H-SiC量子阱结构的发光.     

聚锆碳硅烷陶瓷先驱体的制备与表征

为了提高SiC陶瓷纤维的综合性能,利用聚二甲基硅烷(PDMS)热解制得的液相产物聚硅碳硅烷(PSCS)与乙酰丙酮锆(Zr(AcAc)4)反应,制备了含锆SiC陶瓷纤维的先驱体聚锆碳硅烷(PZCS).选用液相PSCS作为反应原料,可使锆元素在先驱体中分布更加均匀,并能防止Zr(AcAc)4在反应过程中升华.实验合成的PZCS化学式为SiC1.94HxO0.066Zr0.0104,数均分子量Mn=200~400,再成型性良好.反应机理研究表明,反应过程中存在PSCS裂解重排反应,Si—H键在反应中显示出很高的活性,PZCS分子量的增加是PSCS形成的Si—H键与Zr(AcAc)4的配位基发生交联反应的结果.利用PZCS制备的Si—Zr—C—O陶瓷纤维平均强度2.6GPa,平均直径11μm,性能优异.     

Enhancement of corrosion resistance of electrocodeposited Ni–SiC composites by magnetic field

The corrosion behavior and surface morphology of Ni–SiC composite coatings produced by electrodeposition with the aid of magnetic field were studied. The results of the electrochemical analysis including polarization resistance and potentiodynamic polarization curves showed that a magnetic field of 0.1 T could significantly improve the corrosion resistance of the composite. The electrochemical impedance spectra revealed that a passive layer was formed on the surface of the Ni–SiC coating with the magnetic field. The microstructures of electrodeposited Ni–SiC composite coatings were also examined. More SiC particles were found to be incorporated into the coating with the presence of magnetic field, which was considered to be one of the reasons for the enhancement of corrosion resistance as SiC particles were reported to be corrosion inhibitors. Contribution to special issue “Magnetic field effects in Electrochemistry”     

不同Al含量聚铝碳硅烷纤维空气不熔化及氧含量控制研究

利用低分子量聚碳硅烷(L-PCS)与乙酰丙酮铝间的热聚合反应,通过调节原料配比和反应条件,合成了不同Al含量,且具有良好可纺性的聚铝碳硅烷(PACS).研究了PACS的分子量分布和分子结构,PACS纤维空气不熔化特点并与PCS纤维进行比较.结果表明,[Al(Ac Ac)3]以-Al(Ac Ac)2悬挂和-Al(Ac Ac)-桥联两种方式接入L-PCS分子结构,PACS分子量呈双峰分布.PACS中Si—H含量和反应活性随Al含量增加而下降,氧化生成的Si—OH难以进一步形成Si—O—Si交联结构.这导致Al含量越高,凝胶点温度越高,凝胶含量随温度升高增加缓慢,同时引入多余氧.通过预氧化与高温处理相结合的方法,将不熔化纤维中氧含量控制在11 wt%以下.高温处理过程中发生自交联:Si—OH间脱水生成Si—O—Si;Si—H与Si—OH或Si—CH3脱氢生成Si—CH2—Si.     

掺铒a-SixC1-x-H薄膜的发光特性

用等离子体增强化学气相沉积（PECVD）的方法，以固定的氢气（H2）流量和不同的硅烷（SiH4）和甲烷（CH4）流量比沉积了一系列的氢化非晶SiC（a-Si，C1-x-H）膜。用这种宽带隙的a-SixC1-x-H材料作为掺铒的基体材料，通过离子注入的方法得到掺铒的a-SixC1-x-H（a-SixC1-x-H：Er）膜。注入以后的样品经过不同温度的退火。用X射线光电子能谱（XPS）、红外吸收光谱（IR）、拉曼散射谱（Raman）等技术研究不同的SiH4／CH4流量比和退火温度对a-SixC1-x-H：Er发光强度的影响。结果表明，高温退火引起了膜中C的分凝，对饵的发光是不利的。通过低温和室温下铒发光强度的比较，表明这种材料具有较弱的温度猝灭效应。     

聚铝碳硅烷纤维预氧化过程组成结构演变的研究

采用热重-差热分析、元素分析、扫描电子显微镜、凝胶渗透色谱、红外光谱和核磁共振等手段, 研究了聚铝碳硅烷(PACS)纤维预氧化过程中组成、结构演变的规律和反应机理. 结果表明, 空气中PACS纤维从210 ℃左右开始与氧发生放热反应; 随着预氧化温度的升高, 纤维的氧含量逐渐增加, 凝胶含量在氧增重为6～8 wt%时急剧增加, 纤维表面出现细小的微裂纹. 预氧化初期, 主要是Si—H键与氧的反应, 生成Si—O—Si键, 纤维的数均分子量急剧增加, 形成交联结构; 预氧化中期, Si—H键继续反应, Si—O—Si结构明显增多, 同时Si—CH₃和Si—H与氧反应, 生成少量的Si—O—C结构; 预氧化后期, 纤维完全交联, 纤维中存在SiC₄, SiC₃H, Si—O—Si和少量的Si—O—C结构.     

Silicon carbide nanotubes and nanotubular fibers: Synthesis, stability, structure, and classification

Published data on silicon carbide nanotubes (SiC-NT) are analyzed. According to theoretical calculations, single-layer SiC-NTs do not dissociate, but they have not yet been detected experimentally. According to the experimental data, metastable SiC-NTs with walls consisting of several layers and nanotube fibers were produced. The optimized structure of single-layer SiC-NTs was calculated by the RHF/6-31G quantum-chemical method. The possibility of obtaining SiC-NTs by gas-phase chemical deposition from methyltrichlorosilane in the temperature range of 800–1000 °C was investigated. Nanofibers and polygrained SiC nanotubes were obtained, but ordinary layer SiC nanotubes were not detected. To remove the inconsistencies it was first proposed to classify the nanotubes according to the structure of their walls, separating all the SiC-NTs into three types: 1) ordinary layer nanotubes with rolled layers, similar to carbon nanotubes; 2) polynanocrystalline nanotubular fibers or nanotubes with walls consisting of linked differently oriented nanograins; 3) monocrystalline synthetic nanotubes with ideal crystalline walls. It was concluded that the ordinary SiC-NTs of the first type are unstable with the exception of one-or two-layer nanotubes; stable SiC-NTs of the first type and SiC-NTs of the third type have not yet been discovered; only nanotubular fibers of the second type were obtained experimentally. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 3–13, January–February, 2006.