Synthesis and characterization of neodymium oxide nanoparticles

The nanometric precursors of neodymium oxide of various morphology from fibrous to well-dispersed spheroidal were prepared via a solvothermal reaction routes. The precursors and their thermal evolution to neodymium oxide phase were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that the reaction parameters, kind of solvent as well as neodymium salt used played a key role for the product formation of desired morphology and structure. Similarly, kind of neodymium oxide precursor determined the morphology and the crystal structure (haxagonal or cubic) of the final oxide. The potential application of Nd₂O₃ precursors prepared by solvothermal method as convenient material for preparation of homogeneous thin coatings on planar substrates is shown.     

Effects of ternary alloying on mechano-synthesis and nano-crystal stability of an iron-silicon alloy

This paper reports the effects of ternary alloying additions Al, Cu and Nb to Fe₇₅Si₂₅ in high energy ball milling to produce nano-crystalline alloy powder, and its microstructural stability during subsequent high temperature annealing. It is shown that all additions generally retard grain growth up to some temperature. Nb appears to amorphise the alloy. The binary base alloy and Al containing alloy forms the DO₃ ordered structure at high temperatures accompanied by rapid grain growth. The Cu and Nb containing alloys precipitate Cu and Nb₂Fe at high temperatures but do not become ordered.     

In situ nanocrystalline Fe–Si coating by mechanical alloying

A new approach for deposition of in situ nanocrystalline Fe–Si alloy coating on mild steel substrate by mechanical milling has been proposed. The thickness of nanocrystalline coating was a function of milling time and speed. Milling speed of 200 rpm was the optimum condition for development of uniform, hard, adherent and dense 200–300 μm thick nanocrystalline coating. A possible mechanism, consisting of three steps like repeated impact, cold welding and delamination, has been proposed for the formation of coating. These coatings have resulted in the increase of the hardness to almost double the value before coating.     

Flame synthesis of carbon nanotubes with high density on stainless steel mesh

Multi-walled carbon nanotubes grew directly on wires of stainless steel mesh in controllable methane diffusion flames. The formation and morphology of carbon nanotubes were dependent on gas composition of the flames. On pre-etched mesh wires with HCl, high density of carbon nanotubes were synthesized with uniform outer diameters of 60 nm and large inner diameters of 50 nm. With the high yield of carbon nanotubes, less carbon impurities were formed in the process. A mechanistic model was proposed in detail to suggest the formation of catalyst directly on bulk surface and explain the whole process of carbon nanotubes growth in this study.     

Surface functionalization of multi-walled carbon nanotubes via electron reduction of benzophenone by potassium metal

The covalent sidewall functionalization of multi-walled carbon nanotubes (MWCNTs) via the electron reduction of benzophenone by potassium metal is reported. Fourier transform infrared spectroscopy (FTIR) results show that diphenylcarbinol (DPC) groups were successfully grafted to the MWCNTs sidewalls after 10 days of reaction time. Raman and UV–vis spectroscopies reveal the presence of covalent sidewall functionalization. The percentage of residues for DPC-MWCNTs was found to be lower than that for pristine MWCNTs, which indicates the existence of functional groups on the sidewalls of DPC-MWCNTs. It is shown that the sidewall of the DPC-MWCNTs was covered by non-uniform layer of DPC, as observed by transmission electron microscopy (TEM). Results from Raman spectroscopy, FTIR, TGA, UV–vis spectroscopy and TEM confirm that the functionalization of the covalent sidewalls of MWCNTs was successfully performed by this method.     

Temperature-dependant non-catalytic growth of ultraviolet-emitting ZnO nanostructures on silicon substrate by thermal evaporation process

Needle-like nanowires, nanorods, and nanosheets containing nanowires of ZnO have been synthesized on silicon substrate by the thermal evaporation of metallic zinc powder in the presence of oxygen without the use of any catalyst or additives. It was observed that a particular type of ZnO nanostructure can be obtained in a specific temperature zone and morphology can be well controlled simply by adjusting the substrate temperature. Detailed structural analysis revealed that the formed ZnO nanostructures are single-crystalline with wurtzite hexagonal phase, grown along the [0 0 0 1] direction in preference. Raman scattering and room-temperature photoluminescence spectra showed the good crystallinity with hexagonal wurtzite phase and excellent optical properties, respectively for all the deposited ZnO nanostructures.     

Small-angle X-ray scattering from phase-separating amorphous metallic alloys undergoing nanocrystallization

We performed a combined small- and wide-angle X-ray scattering (SAXS/WAXS) study of nanocrystallization during annealing of binary Al₉₂Sm₈ and Al₉₁Gd₉ melt-spun glassy alloys. In course of the transformation an interference maximum develops in the SAXS region while WAXS spectra show formation of fcc-Al nanocrystals in the glassy phase. In order to determine the origin of the SAXS maximum the theoretical SAXS intensity and distance distribution function were calculated considering two structural models. The first model represented a single nanocrystal with a solute layer on its surface and the second one a phase-separated alloy with spatially correlated compositional fluctuations. The results of the calculations are compared with the experimental data. It is demonstrated that only the model representing correlated fluctuations reasonably reproduces the experimental results. We conclude that nano-scale glassy phase separation occurs in the investigated alloys and the nanocrystals form inside the Al-rich amorphous regions.     

Fabrication,microstructures, and properties of copper matrix composites reinforced by molybdenum-coated carbon nanotubes

Multiwalled carbon nanotubes(CNTs) were coated by a molybdenum layer using carbonyl thermal decomposition process with a precursor of molybdenum hexacarbonyl.The Mo-coated CNTs(Mo-CNTs) were added into copper powders to fabricate Mo-CNT/Cu composites by means of mechanical milling followed by spark plasma sintering.The Mo-CNTs were uniform dispersion in the Cu matrix when their contents were 2.5 vol.%-7.5 vol.%,while some Mo-CNT clusters were clearly observed at additions of 10.0 vol.%-15.0 vol.% Mo-CNTs in the mixture.The mechanical,electrical,and thermal properties of the Mo-CNT/Cu composites were characterized,and the results showed that the tensile strength and hardness were 2.0 and 2.2 times higher than those of CNT-free specimens,respectively.Moreover,the Mo-CNT/Cu composites exhibited an enhanced thermal conductivity but inferior electrical conductivity compared with sintered pure Cu.The uncoated CNT/Cu composites were fabricated by the similar processes,and the measured tensile strength,hardness,thermal conductivity,and electrical conductivity of the CNT/Cu composites were lower than those of the Mo-CNT/Cu composites.     

Aligned multi-walled carbon nanotubes on different substrates by floating catalyst chemical vapor deposition: Critical effects of buffer layer

Aligned multi-walled carbon nanotubes (MWCNTs) were synthesized by floating catalyst chemical vapor deposition on two types of substrates, with emphasis on the effects of an aluminum buffer layer. It has been revealed that the presence of the aluminum buffer layer on insulating/semiconducting silicon oxide/silicon wafer resulted in a higher growth rate, narrower diameter distribution, neater morphology and improved crystalline quality of MWCNTs. When an aluminum buffer layer is deposited on electrically conductive carbon paper, high yield CNTs can be achieved while no CNTs can be observed without this buffer layer. Morphology, structure and chemical states of the products were examined by field-emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The generation of alumina from the aluminum buffer layer is thought to play an important role in promoting the nanotube growth. Detailed growth mechanism of MWCNTs was also discussed.     

In situ synthesis and photoluminescence of SiC nanowires by microwave-assisted pyrolysis of methane

A simple method was used to synthesize SiC nanowires directly on quartz plate by microwave pyrolysis of methane. The samples have been characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy techniques. The results showed that the nanowires are of cubic β-SiC structure with stacking faults. The diameters of SiC nanowires are in the range of 50–200 nm and their lengths up to several tens of microns. The effect of the reaction gas concentration and the reaction temperature on the morphologies of SiC nanowires was also investigated. Two broad photoluminescence peaks at the center wavelength of about 309 and 417 nm were observed at room temperature.     

Phase transformations in the B2 phase of Co-rich Co-Al binary alloys

Phase transformations in the β (B2) phase of Co-21 and -23 at.% Al alloys were examined using transmission electron microscopy, energy dispersive X-ray spectroscopy and differential scanning calorimetry. The microstructures obtained from as-quenched specimens were found to be strongly affected by the quenching condition. While relatively thick sheet-specimens with a lower quenching rate showed bainitic plate precipitates with a fcc structure, a martensite-like structure was observed by optical microscopy in relatively thin specimens with a higher quenching rate. Regardless of the quenching condition, a spinodal-like microstructure composed of A2 and B2 phases was also detected and the A2 phase changed to a metastable hcp phase during further aging.     

电子束辐照对多壁碳纳米管的影响（英文）

在室温下采用透射电子显微镜中汇聚的电子束辐照多壁碳纳米管。结果表明,在能量为100 keV的电子束辐照下除了碳纳米管管壁有一些弯曲外没有其他结构被破坏;当电子能量增加到200 keV时,纳米管有明显的损伤,可以观察到纳米管的无定型化、纳米管外壁的凹坑和缺口。200 keV的电子束辐照还能形成碳洋葱和2根多壁纳米管的焊接。多壁碳纳米管的离位阀能为83~110 keV。能量超过阀能的电子束可以很轻易地损伤纳米管而低于阀能的电子束则很难损坏纳米管,其损伤机理为溅射和原子离位。     

Modified multi-walled carbon nanotubes with nano-europium oxide

In order to improve optical property of the multi-walled carbon nanotubes （MWNTs）, MWNTs were decorated with europium oxide （Eu2O3） nanoparticles by using co-deposition method. The MWNTs/Eu2O3 composites were examined by XRD, scanning electron microscopy, transmission electron microscopy, and VUV-Vis Luminescence spectroscopy and citric acid （CA） molecules were introduced onto the surface of MWNTs. The results show that there are many oxygenated functional groups on the surface of the MWNTs after the treatment of mixture acid, such as carboxy, hydroxl, carbony and amidocyanogen. The results of electron microscopy illuminate that the MWNTs are coated by nano-europium oxide after annealed at 750℃. The MWNTs/Eu2O3 composite emits much strong red light at about 610 nm under UV excitation.     

Cladding inner surface of steel tubes with Al foils by ball attrition and heat treatment

A method used to clad the inner surface of steel tubes with Al foils has been developed: Al foils were welded onto the inner surface of steel tubes by ball attrition in planetary rotation, followed by annealing at 660 °C for 20 min. SEM, EDS and XRD methods were used to analyze the microstructure, the composition and the phases of the cladding layer. Studies on the interface microstructure indicated that the bonding strength between Al foil and substrate is highly related to the rotation speed, diameter of alloy balls and treatment time. A high extent of plastic deformation increases the interfacial bonding strength. But severe plastic deformation may break Al foils and separate the foils from the inner surface. Subsequent annealing leads to the formation of multilayers of intermetallic compound and alloy phases through diffusion of Al, Fe, and other elements. This technique provides a simple but quite efficient way to improve the properties of inner surface of steel tubes.     

Effects of Hf additions on high-temperature mechanical properties of a directionally solidified NiAl/Cr(Mo) eutectic alloy

In order to improve the high-temperature mechanical properties of NiAl/Cr(Mo) alloys, the effects of Hf additions on microstructure and mechanical properties were systemically examined. Two directionally solidified alloys with composition of Ni-32Al-28Cr-(6−x)Mo-xHf (x = 0.2 and 0.5 at.%, respectively), named as 0.2Hf and 0.5Hf hereafter, were prepared. The Hf additions disturbed the cellular structure. The 0.2Hf alloy consisted of dendritic structure, while the 0.5Hf alloy had an intercellular structure. In the 0.5Hf alloy, the Ni₂AlHf and Ni₁₆Hf₆Si₇ precipitates were also confirmed. The high-temperature strength and brittle-to-ductile transition temperature (BDTT) increased with increasing of Hf additions, due to the different strengthening mechanism. In contrast, the ductility and creep resistance decreased with increasing of Hf because of the disturbance of cellular structure.     

Effect of the addition of In on the microstructural formation of Sn–Ag–Zn lead-free solder

The effect of addition of In, up to 1 wt.%, on the formation of intermetallic compounds (IMCs) in the solidified Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. As observed by microstructural analysis, the typical structure of Sn–Ag–Zn solder is composed of β-Sn phase and mixed granules of Ag₃Sn and AgZn IMCs. After alloying with In, it evolves into a mixture of randomly distributed rods and granules of Ag₃Sn and AgZn. Clearly, the addition of In into the explored Sn–Ag–Zn solder promotes the formation of rod-like IMCs for the reason that the growth competition of the Ag₃Sn and AgZn IMCs was destroyed by the selective adsorption of In atoms on a certain preferable crystalline planes of the separated IMCs. The change in the morphology of the formed IMCs leads to a great difference in the mechanical performances, for example, the measured microhardness of the investigated solders evolves from 16.95 HV to 21.35 HV with the increase of In content.     

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.     

A simple catalyst-free route for large-scale synthesis of SiC nanowires

A large number of SiC nanowires were fabricated by a simple catalyst-free method using silicon powders and expandable graphite as raw materials. Digital camera, X-ray diffractometer, Fourier transform infrared spectrometer, field-emission scanning electron microscopy and transmission electron microscopy demonstrate that a large number of loose products were obtained in graphite crucible. The products are composed of single crystalline 3C-SiC nanowires with lengths up to several tens of micrometers and diameters of 20-60 nm. The vapor-solid mechanism was proposed to interpret the growth procedure of SiC nanowires. The expandable graphite as carbon source can provide enough growth space for nanowires, which is helpful to improve the yield of SiC nanowires. The simple method provides a promising candidate for industrial fabrication of SiC nanowires.     

On the amorphous and nanocrystalline Zr–Cu and Zr–Ti co-sputtered thin films

In the current study, we examined and compared the mixing and vitrification behavior of the Zr–Cu and Zr–Ti binary systems in the form of co-sputtered thin films with or without post-annealing. The co-sputtered Zr–Cu films are all amorphous under various co-sputtering conditions, suggesting the high vitrification tendency. The amorphous Zr–Cu thin film will start to crystallize into nano-crystalline Zr₂Cu and Zr₇Cu₁₀ phases upon long exposure at temperatures above 350 °C. On the other hand, it is difficult to form amorphous film with the Zr–Ti system, except at a low sputtering power of 30–50 W. The low powers enable the co-sputtered Zr–Ti thin film to exhibit the diffuse hump in the X-ray diffraction. Examination by high resolution transmission electron microscopy reveals numerous fine nano-crystalline phases around 2 nm in the amorphous matrix. Upon exposure at 700 °C, the Zr–Ti films transform into crystalline hexagonal close-packed α and body-centered cubic β phases.     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.