碳化硅纳米晶须生长和显微结构

采用两步生长生在碳化硅纳米晶须，首先是二氧化硅与硅反应生成一氧化硅，再与碳纳米管先驱体反应生成β－ＳｉＣ纳米晶须，其直径为３～３５ｎｍ，长度为２～２０μｍ，用高分辨透射电镜研究晶须形貌，显微结构，讨论了碳化硅纳米晶须生长机制。     

一维钛纳米材料的SEM和TEM形貌研究

在10mol/L氢氧化钠溶液中,以溶胶状纳米TiO_2为钛源,分别于150℃和180℃采用水热法合成了钛纳米晶须和钛纳米线;以金红石型纳米TiO_2为原料在150℃水热合成了钛纳米管。用扫描隧道显微镜(SEM)、电子透射电镜(TEM)对产物进行了表征,结果表明钛酸钠纳米晶须扫描形貌为球形颗粒状,透射形貌为直径1～3nm针尖状,钛纳米线扫描图呈面条状,长度为40μm,宽度为40～200hm,透射观察为实心多层结构,间距为0.8nm。钛纳米管的长度达2μm,管径为10nm。观察发现在纳米晶须中存在向纳米线过渡的中间相,而纳米带中存在钛纳米片,结合相关机理对其形貌结构进行了解释。     

氧化还原法制备钨晶须及其生长机理研究

研究了钨晶须的制备工艺,并分析了其生长机理.采用X射线衍射、扫描电镜、能谱分析、透射电镜对制备的钨晶须进行物相、形貌、成分、微观结构的分析和表征.研究表明:钨晶须长度大致在1～10μm,直径在1μm以下,部分达到纳米级;晶须为单晶bcc结构,生长方向为110;钨晶须形成过程为钨粉及其氧化产物与水汽反应生成气相水合物WO2(OH)2,遇氢气还原后形核并沉积,进而定向生长为晶须结构,钨晶须的生成遵循VS机理.     

碳化硅纳米纤维的溶胶-凝胶法合成

以酚醛树脂和正硅酸乙酯为碳源和硅源,硝酸镧作为生长助剂,通过溶胶-凝胶和碳热还原制备了碳化硅纳米纤维.采用X射线衍射、扫描电子显微镜和透射电子显微镜对所制备的碳化硅纳米纤维进行表征.结果表明,通过此方法所制备的碳化硅纳米纤维的长度可达100μm以上,直径为40～50nm,反应遵循V-L-S机理.     

二次烧结法制备纳米TiO_2晶须的性能研究

以二氧化钛(TiO_2)纳米球状颗粒为原料,经过2次高温烧结处理制得TiO_2晶须,通过XRD、XPS和SEM技术对其性能结构进行表征。结果表明,通过二次烧结法制得的TiO_2晶须的晶型结构为金红石型TiO_2,晶须结构稳定、尺寸均匀,直径280~340nm,长度1.95~3.75μm。     

球磨退火法制备氮化硼纳米管及其生长机理研究

以氧化硼(B2O3)为主要原料,氨气为反应气体,采用球磨退火法制备了氮化硼纳米管(BNNTs)。采用XRD、SEM、TEM等手段对产物进行了表征分析。结果表明,合成的BNNTs尺寸较为均匀,瓷舟内的BNNTs直径20~50nm,长度2~20μm;陶瓷基片上的BNNTs直径10~20nm,长度2~20μm。研究了BNNTs生成的反应方式及其生长过程,B2O3与氨气在高温下反应是通过气-气反应和气-固反应两种反应方式进行的,BNNTs是由氮化硼纳米片(BNNSs)卷曲连接而形成的。     

水热合成氧化铁纳米结构及机理分析

利用硝酸铁与油酸钠反应所得的化合物在高温水热条件下分解制备多种氧化铁纳米结构.研究了水热反应温度、反应时间和热处理工艺对于产物结构的影响,并且探讨了产生各种纳米结构的机理.在高温较短反应时间下,可以制得直径为15nm,长度为3μm的纳米线结构,延长反应时间至25h,得到边长为15nm的氧化铁四方颗粒.将含有羟基氧化铁相的氧化铁纳米线在不同温度下进行热处理,得到了直径为15nm,长度为1μm的氧化铁纳米线和直径为1～3μm的氧化铁微米球.     

热蒸发法碳化硅纳米晶须阵列的合成与表征

在1600℃不同真空度下, 采用热蒸发硅的方法, 在石墨基板和聚丙烯腈(PAN)炭纤维两种碳源基体原位生长具有一定取向的碳化硅纳米晶须——垂直于石墨片表面森林状和试管刷状碳化硅纳米晶须阵列。通过X射线衍射及场发射扫描电镜, 发现晶须为3C-SiC, 直径约100nm, 长度约50μm。炭纤维表面的产物顶端多为针尖状, 而石墨片表面的产物多为六方棱柱状。因其纳米尺寸效应, 在380nm波长的光激发下, 所制晶须在波长为468nm 附近出现光致发光峰。透射电镜、 多点衍射电子衍射图表明, 所制得的3C-SiC晶须为单晶, 其生长方向为3C-SiC的[111]方向。基于反应过程中硅熔体与碳源分离的事实, 讨论了3C-SiC晶须阵列生长的气固反应机理。      

掺B对制备纳米SiC的影响

以正硅酸乙酯和葡萄糖分别作Si源和C源,草酸和硼酸分别作催化剂,采用溶胶-凝胶法制备SiC前驱体,并采用碳热还原法制备纳米SiC。采用XRD、SEM对样品的物相和形貌进行表征。结果表明,用草酸作催化剂制备的前驱体在1550℃制备的SiC是颗粒与晶须的混合体,颗粒粒径为30～50nm,晶须长度为1～3μm,晶须直径为60～100nm;用硼酸作催化剂制备的前驱体其碳热还原温度显著降低,在1400℃就能制备出SiC颗粒与晶须的混合体,颗粒粒径为20～30nm,由于B加入后的抑制作用,SiC晶须的含量明显减少。     

单晶银纳米带的合成与机理分析

采用甲苯作为还原剂,水热条件下还原生成宽度为20nm左右,长度1～2μm单晶银纳米带.甲苯与银离子形成的配位聚合物的线状结构导致了银纳米结构的取向生长,是形成银纳米带的关键因素.     

Continuous synthesis of silicon carbide whiskers

A two-step reaction scheme has been employed for the synthesis of SiC whiskers at 1450 °C under an argon or hydrogen flow. First, SiO vapour was generated via the carbothermal reduction of silica in a controlled manner. Second, the generated SiO vapour was reacted with carbon-carrying vapours such as CO and CH₄, which resulted in the growth of SiC whiskers on a substrate away from the batch. A higher growth rate was observed in the hydrogen atmosphere due to the formation of CH₄ which provides a more favourable reaction route. By the use of thermodynamic calculations, the preferred reaction routes have been selected for an efficient synthesis of SiC whiskers, and a continuous reactor has been designed. The system consists of a boat-train loaded with the silica-carbon mixture and iron-coated graphite substrate above it in an alumina-tube reactor. By pushing the boat-train into the hot zone at a fixed speed, SiO vapour is constantly generated. High-quality SiC whiskers have been grown on the substrate with diameters of 1–3 m. The yield was about 30% based on the silicon input as SiO₂ and silicon output as SiC whiskers. This demonstrates the feasibility of continuous production of high-quality SiC whiskers which does not require additional processes such as purification and classification.     

A simple method for the synthesis of silicon carbide nanorods

SiC nanorods were synthesized by a reaction at a temperature of 1200 degrees C, under an argon gas atmosphere, from silicon and amorphous carbon powders mixed by ball milling. The reaction product, which contain SiC nanorods and nanoparticles, has been characterized by high-resolution transmission electron microscopy, X-ray diffraction, and micro-Raman spectroscopy. The synthesized nanorods are more than 1 micron long with a mean diameter of about 10-30 nm. The nanorods possess a well-defined crystalline structure with a thin layer of amorphous SiO2 on the surface. Raman shifts of SiC nanorods and the role of structural defects are discussed.     

SiC nanorods prepared from SiO and activated carbon

SiC nanorods with 20–100 nm diameter and 10–100 m length were synthesized by reaction between SiO and amorphous activated carbon (AAC) at 1380°C. Microstructural characterization of the SiC nanorods was carried out by high resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The SiC nanorods grow on either a chain or from facets of SiC nanoparticles. They are usually straight and preferentially orientated along the [111] direction. Branching phenomenon exists for these nanorods. Typical SiC nanorod tip was analyzed by HRTEM image and EDS analysis. Based on an experimental analysis, a formation mechanism is proposed to explain the microstructural characterization of the SiC nanorods.     

Direct growth of core-shell SiC-SiO(2) nanowires and field emission characteristics

A simple, direct synthesis method was used to grow core-shell SiC-SiO(2) nanowires by heating NiO-catalysed silicon substrates. A carbothermal reduction of WO(3) provided a reductive environment and carbon source to synthesize crystalline SiC nanowires covered with SiO(2) sheaths at the growth temperature of 1000-1100?°C. Transmission electron microscopy showed that the SiC core was 15-25?nm in diameter and the SiO(2) shell layer was an average of 20?nm in thickness. The thickness of the SiO(2) shell layer could be controlled using hydrofluoric acid (HF) etching. Field emission results of core-shell SiC-SiO(2) and bare SiC nanowires showed that the SiC nanowires coated with an optimum SiO(2) thickness (10?nm) have a higher field emission current than the bare SiC nanowires.     

The key role of hydrogen in the growth of SiC/SiO(2) nanocables

SiC/SiO(2) nanocables, consisting of a crystalline SiC core surrounded by an amorphous silica shell, have been grown by thermal chemical vapour deposition (CVD) at 950?°C on Ni-covered silicon substrates. The addition of methane to a 375?Torr hydrogen atmosphere, after heating the substrate in argon, leads to the growth of the SiC/SiO(2) nanocables, by the carbothermal reduction of silicon oxide as the initial stage. The growth mechanism follows the model previously proposed by us for a reducing medium. From the results obtained, several effects of hydrogen on the deposition process have been established: (a)?reduction of the nickel nucleation sites, thus favouring the formation of SiC from the initial stage; (b)?oxygen removal in the medium hindering the oxidative effect over the SiO and C species, thus promoting the nanocable growth, and (c)?increase of the SiO concentration in the neighbourhood of the active nucleation sites. In addition, it is important to mention that SiC/SiO(2) nanocables, following the already proposed model, are obtained uniquely in a narrow hydrogen pressure range. At?high hydrogen pressure, the unexpected formation of silica nanowires together with the SiC/SiO(2) nanocables has been detected.     

Beta-MnO2/SiO2 core-shell nanorods: synthesis and dielectric properties

Large-scale beta-MnO2/SiO2 core-shell nanorods were synthesized by hydrolysis process. The product was characterized by XRD, EDS, SEM and TEM. The thickness of the SiO2 shell layer is about 3 nm approximately 5 nm, which can be tuned by changing the amount of tetraethyl orthosilicate (TEOS) and the reaction time. The dielectric properties of the synthesized core-shell nanorods at the temperature range from 373 K to 773 K in X-band were investigated in detail and the mechanism of the dielectric response was discussed. The dielectric loss of the SiO2-coated MnO2 nanorods at 773 K was about twice than that at 373 K. The high dielectric loss is mainly attributed to the interfacial polarization and the electromagnetic impedance match between the SiO2 shell layer and MnO2 core layer. The quantitative formula between the permittivity of beta-MnO2/SiO2 core-shell nanorods and the thickness of the SiO2 shell is established, which can be used to tune the dielectric properties of the core-shell nanorods through controlling the thickness of the SiO2 shell layer.     

铁诱导低温SiC薄膜的合成

采用热丝化学气相沉积法，以铁作为催化剂，在较低的衬底温度合成纳米SiC薄膜，铁粒子是在400Pa氢气的气氛中，通过用脉冲激光烧蚀铁靶5min引入的。用扫描电镜和拉曼谱对样品进行了分析。扫描电镜观察到了直径为10-30nm，长度短于1μm的无序SiC棒，拉曼谱中的横向生子模式的红移表明生长方向的限制效应，所有这些说明Fe粒子的大小将影响到SiC棒的生长。     

Synthesis,microstructure and photoluminescence of well-aligned ZnO nanorods on Si substrate

Well-aligned zinc oxide (ZnO) nanorods were densely grown on Si substrate using ZnO thin-film seed layer without any catalysts and/ or additives by a simple solid–vapour phase thermal sublimation technique. The growth mechanism can be interpreted as self-catalyst of zinc particles based on vapour–solid (VS) mechanism. High-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern confirmed that the single-crystalline growth of the nanorods were preferentially along c-axis of hexagonal crystal system. High-crystal quality ZnO nanorods with strong near band edge emission centred at 380 nm can be achieved on Si substrate by the introduction of sufficient oxygen during the nanorod growth processing.     

Interface evolution in the platelet-like SiC@C and SiC@SiO<Subscript>2</Subscript> monocrystal nanocapsules

Carbon-coated SiC@C nanocapsules (NCs) with a hexagonal platelet-like morphology were fabricated by a simple direct current (DC) arc-discharge plasma method.The SiC@C NCs were monocrystalline,120-150 nm in size,and approximately 50 nm thick.The formation of the as-prepared SiC@C NCs included nucleation of truncated octahedral SiC seeds and subsequent anisotropic growth of the seeds into hexagonal nanoplatelets in a carbon-rich atmosphere.The disordered carbon layers on the SiC@C NCs were converted into SiO2 shells of SiC@SiO2 NCs by heat treatment at 650 ℃ in air,during which the shape and inherent characteristics of the crystalline SiC core were obtained.The interface evolution from carbon to SiO2 shells endowed the SiC@SiO2 NCs with enhanced photocatalytic activity due to the hydrophilic and transparent nature of the SiO2 shell,as well as to the photosensitive SiC nanocrystals.The band gap of the nanostructured SiC core was determined to be 2.70 eV.The SiC@SiO2 NCs degraded approximately 95％ of methylene blue in 160 min under visible light irradiation.