化学气相沉积法制备碳纳米管过程中NH3对其微观结构和石墨化程度的影响

以二茂铁为催化剂前驱体,C₂H₂为碳源,在NH₃和N₂气氛中在Al₂O₃多孔基体上化学气相沉积制备出了一致取向的碳纳米管阵列.采用拉曼散射、扫描电子显微镜、高分辨电子显微镜、X射线衍射和元素分析等技术对产物进行表征.实验结果表明,以NH₃为载气时,所制备的碳纳米管为竹节形结构,并且有一定量的N掺杂在其中,其质量含量大约在3.46%,碳纳米管的石墨层间距在0.384 nm左右,实验产物中有Fe₃C和Fe₂N或Fe₃N的生成,NH₃在催化剂表面的吸附导致催化剂的表面性质的改变以及N的掺杂使生成的碳纳米管结构有一些缺陷,石墨化程度有所降低.碳纳米管的管径随着反应温度的增加而增大.     

Co—Mo／MgO作催化剂CVD法制备碳纳米管

研究了催化剂的制备方法对合成碳纳米管的影响,分别采用溶胶凝胶法、浸渍法和燃烧法制备了Co-Mo／MgO催化剂,并以乙炔为碳源,Ar气为保护气,在750-950℃常压下生长碳纳米管。采用TEM对所得产物进行了表征,结果表明,对于Co-Mo／MgO体系,相对浸渍法和燃烧法,溶胶凝胶法是很好的选择,可以得到数量与质量均较好的碳纳米管,并讨论了溶胶凝胶法制备碳纳米管的过程中工艺参数的择优。     

Carbon nanotubes prepared in situ in a cellular ceramic by the gelcasting-foam method

The synthesis of carbon nanotubes from an oxide solid solution foam is reported for the first time. A foam of Mg_0.9Co_0.1Al₂O₄ solid solution is prepared by the gelcasting-foam method using notably mono- and di-functional acrylate monomers. Using a surfactant in the alkylpolyglucoside family allows to prevent structure changes in the foam before the onset of polymerisation. The wet ceramic foam is dried in air, producing a foam with an open porosity. Total porosity is equal to about 98% and the diameter of the pores is in the range 25–300 μm, about 90% being smaller than 200 μm. The Mg_0.9Co_0.1Al₂O₄ foam is reduced in H₂–CH₄ atmosphere, giving rise to a CNTs–Co–MgAl₂O₄ composite foam. Using the foam instead of the corresponding powder allows a four-fold increase in the production of carbon nanotubes, more than 95% of which have only 1 (70%) or 2 walls.     

Synthesis of SiC whiskers via catalytic reaction method in self-bonded SiC composites

Catalyzed by in-situ formed Fe nanoparticles (NPs), 3C–SiC whiskers were prepared from expanded graphite and Si powders after firing at 1573 K for 3 h in Argon. The density functional theory calculations revealed that Fe catalysts facilitated the formation of SiC nucleus and the epitaxial growth of SiC whiskers via reducing the bonding strength in CC dimer as well as Si–O and C–O bonds. Moreover, using SiC, expanded graphite and silicon powders as starting materials Fe-catalyzed self-bonded SiC composites were fabricated. Lots of SiC whiskers were synthesized in the as-prepared composites, leading to remarkable enhancements in high temperature mechanical behavior, oxidation resistance and cryolite resistance of the self-bonded SiC composites.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

Properties of PMMA/Carbon nanotubes nanocomposites prepared by “grafting through” method

A number of batch polymerizations were performed to study the effect of multi‐walled carbon nanotubes (MWCNTs) on the properties of PMMA/MWCNTs nanocomposites. To improve the dispersion of nanotubes in PMMA matrix, MWCNTs were functionalized with methacrylate groups via a four‐step modification process and the modified nanoparticles were used to synthesize the nanocomposites. The prepared samples were characterized by Raman spectroscopy, thermogravimetric analysis, dynamic mechanical thermal analysis, differential scanning calorimetry, gel permeation chromatography, UV–visible, and TEM techniques. According to the results, modified nanotubes improved thermal and mechanical properties better than the pristine MWCNTs. The main improvement in the mechanical and thermophysical properties was achieved for the nanocomposite containing 0.5 wt% of MWCNTs. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Performance optimization of complicated structural SiC/Si composite ceramics prepared by selective laser sintering

Combining reaction-bonded (RB) process and selective laser sintering (SLS) method is an effective approach to prepare ceramic components with complex shapes. The purpose of this paper is to find efficient ways to improve the performance of SiC/Si composites prepared by SLS/RB technologies. Effects of epoxy resin binder on the performance and microstructure of preforms and sintered bodies were studied first. Then, based on the above results, graphite with low reactivity was used as an alternative slow-release carbon source to promote sintering densification process and improve the carbon density of preforms. When the added amount of graphite increased, clusters of nanometer-sized SiC grains formed and the silicon phase transformed into pieces, which reduced the content and dimension of silicon phase. Furthermore, by applying a two-step sintering method, the residual silicon content of SiC/Si composites decreased further and the flexural strength at high temperatures increased.     

Interfacial structure and stability of a co-continuous SiC/Al composite prepared by vacuum-pressure infiltration

The interfacial structure of a SiC_3D/Al composite prepared by vacuum-pressure infiltration was investigated to determine whether interfacial reactions occur and specific orientation relationships exist between SiC and Al. The resulting SiC/Al interfaces were continuous and free of any precipitates and voids. There is no consensus for the orientation relationship between 6H-SiC and Al. In addition, the mismatch between the two phases at the interface was accommodated by mismatch dislocation. The stability of the SiC_3D/Al composite after annealing at 873 K for 10 h was determined. No Al₄C₃ precipitated at the interface, although grain coarsening of Al occurred. Compared with the concentration gradients at the initial interfaces, those obtained at the interface after annealing were still steep, indicating that concurrent interdiffusion occurred between SiC and Al in a very narrow region near the interfaces. Theoretical calculations suggest similarly the chemical and structural stability of the SiC/Al interface during the annealing treatment.     

预制体浸渗法制备苎麻纤维织物SiC/Cu复合材料

以苎麻纤维织物SiC陶瓷为生物模板,通过腐蚀、敏化、活化、施镀等工艺镀金属镍,后经高温真空环境熔渗金属Cu,制备出了苎麻纤维织物SiC/Cu复合材料。通过物理性能、显微组织等测试分析,研究了镀镍时间对样品的各项性能和微观形貌的影响。结果表明,镀镍时间超过30 min,随镀镍时间增加,苎麻纤维织物SiC预制体的密度增加、开口气孔率降低、体积密度变化不大;苎麻纤维织物SiC陶瓷独特有序的生物结构和金属镍层的存在共同改善了金属Cu的熔渗效果。     

预制体浸渗法制备苎麻纤维织物SiC/Cu复合材料

以苎麻纤维织物SiC陶瓷为生物模板,通过腐蚀、敏化、活化、施镀等工艺镀金属镍,后经高温真空环境熔渗金属Cu,制备出了苎麻纤维织物SiC/Cu复合材料。通过物理性能、显微组织等测试分析,研究了镀镍时间对样品的各项性能和微观形貌的影响。结果表明,镀镍时间超过30 min,随镀镍时间增加,苎麻纤维织物SiC预制体的密度增加、开口气孔率降低、体积密度变化不大;苎麻纤维织物SiC陶瓷独特有序的生物结构和金属镍层的存在共同改善了金属Cu的熔渗效果。     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.     

Deformation processes of ultrahigh porous multiwalled carbon nanotubes/polycarbonate composite fibers prepared by electrospinning

Mechanical deformation processes of electrospun composite fibers based on polycarbonate with multiwalled carbon nanotubes (MWCNTs) were investigated by in situ tensile tests under transmission electron microscope (TEM) depending on morphology. Using chloroform as solvent and optimizing process conditions, uniform nanoporous composite fibers were generated by electrospinning process. TEM images indicate that the MWCNTs were embedded in the fibers as individual elements, highly aligned parallel to one another along the fiber axis, which makes the mechanical load transfer from the polymer matrix to the MWCNT more favorable. Due to the slippage of individual MWCNTs within the fibers the strain at break of composite fibers is significantly enhanced. In addition, the nanopores on the fiber surface provide the effective sites for stress concentration for the plastic deformation to form nanonecking of fibers under tensile load. Combination of these unique features makes the electrospun composite fibers extremely strong and tough. The results from present work may provide a feasible consideration of such electrospun composite fibers for use as the reinforcing elements in a polymer based composite of a new kind.     

Carbon foams prepared by supercritical foaming method

Multifunctional carbon foams with mean pore size smaller than 200 μm were prepared by supercritical foaming, in which toluene and mesophase pitch were used as the supercritical agent and carbonaceous precursor, respectively. The supercritical foaming behaviors and nucleation mechanisms were analyzed. According to the composition of mesophase pitch and micrographs of resultant carbon foams, the interface between light components and quinoline insoluble components is considered as the nucleation site.     

Carbon nanotubes/poly(ε-caprolactone) composite vapour sensors

The influence of carbon nanotube (CNT) grafting with poly(ε-caprolactone) (PCL) on vapour sensing properties has been investigated for a series of Conductive Polymer Composite (CPC) transducers developed by layer by layer spray from PCL-CNT solutions. Grafting of ε-caprolactone on the CNT surface through in situ ring opening polymerisation was demonstrated by nuclear magnetic resonance after solvent extraction of ungrafted chains. Atomic force microscopy observations allowed an evaluation of CNT coating and dispersion level. Chemo-electrical properties of CPC sensors exposed to different vapours: water, methanol, toluene, tetrahydrofuran and chloroform have been analysed in terms of signal sensitivity, selectivity, reproducibility and stability. An increase of sensor response amplitude was observed when using PCL grafted CNT (PCL-g-CNT) composites in formulations.     

Lead-free BLTO/NMFO magnetoelectric composite films prepared by the sol-gel method

The lead-free ferroelectric films of Bi_4?xLa_xTi₃O₁₂(BLTO) and ferromagnetic films of Ni_1?xMn_xFe₂O₄(NMFO) were prepared on Pt/Ti/SiO₂/Si substrate by means of the sol-gel and spin-coating method. The lead-free magnetoelectric composite films with the structure of Bi_3.4La_0.6Ti₃O₁₂/Ni_0.7Mn_0.3Fe₂O₄/substrate (BN) and Ni_0.7Mn_0.3Fe₂O₄/Bi_3.4La_0.6Ti₃O₁₂/ substrate (NB) were also deposited on Pt/Ti/SiO₂/Si substrate. The X-ray diffraction results show that two composite films possess BLTO and NMFO phases without any intermediate phase. The SEM images show that two composite films exhibit layered structure, clear interface and no transition layer between BLTO and NMFO films. Two composite films exhibit both good ferromagnetic and ferroelectric properties, as well as magnetoelectric coupling effect. The deposition sequence of ferroelectric and ferromagnetic films in the composite films has significant influence on the ferroelectric, ferromagnetic and magnetoelectric coupling properties of the composite films. The values of magnetoelectric voltage coefficient of the BN composite films are higher than those of the NB composite films at any fixed H_bias.     

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50?nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400?µm, and the outer two layers are about 200?µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773?K for 361?h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80?mm) after erosion at 1873?K for 130?h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.     

Morphology of composite particles of single wall carbon nanotubes/biodegradable polyhydroxyalkanoates prepared by spray drying

Spray drying was investigated as a strategy for producing single wall carbon nanotube (SWCNT)/polymer composites. The spray-drying method produced SWCNT/poly(3-hydroxybutyrate) and SWCNT/poly(3-hydroxyoctanoate) composite particles in which the SWCNTs have been trapped in a well-dispersed state throughout the polymer matrix. Increasing SWCNT content in the composite led to a change in particle morphology from spherical and smooth to rosette shape with angular distortions. The technique shows potential for bulk carbon composite fabrication.     

Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process

Multi-walled carbon nanotubes have been produced with high yield on an iron supported catalyst by catalytic chemical vapor deposition in a fluidized bed reactor. The choice of such a technique allows to reach high selectivity towards the desired material. A remarkable feature of this process is the huge bed expansion observed during the nanotubes growth that affects the fluidization regime due to the evolution of the apparent density of the composite powder. The catalytic powder, the composite material and the purified nanotubes have been analyzed by SEM, TEM and BET nitrogen adsorption.