Microtexture and magnetoresistance of glass-like carbons

Microtextures of glass-like carbons commercially available were investigated through scanning electron microscopy and measurement of their magnetoresistance. A field-emission electron gun type SEM, which is characterized by a specimen chamber equipped in its objective lens, was operated with the acceleration voltage of 2 kV and high fidelity observations were thereby achieved with a resolving power of 2 nm. All the specimens exhibit granular microtextures with the mean grain diameter spanning 6–13 nm. The mean grain diameter correlates to the mean crystallite size L_a(110) determined by the X-ray powder diffractometry. The density of each sample calculated by using the mean diameter and the X-ray parameters reproduces well its catalog value. Negative magnetoresistance was observed just for the specimens whose average grain sizes are not smaller than 7.3 nm. The crystallite size _La(110) of the specimen whose grain size shows the minimum value for appearance of negative magnetoresistance is estimated to be 2.7 nm. The absolute value of negative magnetoresistance under a constant magnetic field increases with increasing the average grain size.     

Electrical resistivity and magnetoresistance of carbon/graphite fibers intercalated with nitric acid and arsenic pentafluoride

A series of high performance, experimental carbon/graphite fibers was intercalated and examined with respect to their metallic conductivity behavior by resistivity and magnetoresistance versus temperature measurements. One fiber was a polyacrylonitrile (PAN)-type precursor and three were pitch base precursors. All four types showed substantially similar behavior in the pristine state with respect to room temperature resistivity and the sign and magnitude of the temperature coefficient of resistivity. After intercalation with either nitric acid or nitric acid followed by AsF₅, the PAN-based fibers displayed a resistivity versus temperature behavior qualitatively similar to their pristine counterparts but displaced to lower resistivity. On the other hand, the pitch fibers with the same intercalation treatment exhibited metallic behavior (a positive temperature coefficient of electrical resistivity and a small magnetoresistance). These manifestations of metallic behavior are usually indicative of some three dimensional graphite structure in the carbon fibers.     

Characterisation of the microstructure and deformation of high modulus cellulose fibres

This paper reports on the microstructure and deformation of one type of high modulus cellulose fibre characterised using the techniques of Raman spectroscopy and synchrotron X-ray diffraction and it compares this fibre to a lower modulus conventional viscose fibre. The crystallinity of the fibres has been determined using X-ray diffraction. The orientation parameter has been determined by measuring the width of the (200) equatorial reflections for each fibre using microfocus synchrotron radiation and it has also been shown that the crystal orientation parameter varies from the skin to core of the fibres and is different for each type. Mechanical properties of the fibres are reported and it is shown that the high modulus cellulose fibres have very different stress-strain behaviour to the viscose fibres. Finally, it is shown for the high modulus fibre that the 1414, 1260, 1095 and 895 cm⁻¹ Raman bands shift under the application of tensile deformation towards a lower wavenumber with the 1095 cm⁻¹ band giving information about the backbone chain stretching of the cellulose. The viscose fibres show less significant shifts in this peak. The crystal modulus of the high modulus cellulose fibre has also been determined by calculating the change in the c-spacing upon the application of tensile deformation to individual cellulose monofilaments. This change in the c-spacing is determined from the change in position of the (002) meridional reflection giving a crystal modulus of 77 GPa. This value is a little low compared to other published data, and reasons for this are discussed. The shear modulus between crystallites is also calculated and compared to previously published data.     

Restructuring butterfat through blending and chemical interesterification. 2. Microstructure and polymorphism

Blending of butterfat with canola oil and subsequent chemical interesterification modified the crystal morphology and X-ray diffraction patterns of butterfat, 90∶10 (w/w), and 80∶20 (w/w) blends of butterfat-canola oil. The morphology of 50∶50 (w/w) was also greatly influenced by interesterification. Polarized light microscopy revealed that addition of canola oil led to gradual aggregation of the crystal structure. Scanning electron microscopy revealed all samples to be mixtures of defined crystalline regions and amorphous areas with greater amorphism as oil content increased. Most samples revealed segregation of solid from liquid. Confocal laser scanning microscopy of butterfat revealed complex aggregated structures that were composed of outwardly radiating filaments from a central nucleus. X-ray diffraction analysis revealed a predominance of β′ and a small proportion of β crystals for all samples examined except interesterified butterfat, which consisted solely of β′ crystals.     

Charge-Up Scanning Electron Microscopy Images of Intergranular Phases in Semiconducting BaTiO3 Positive Temperature Coefficient of Resistivity Ceramics

A charge-up image was observed in the intergranular phases of semiconducting BaTiO₃ positive temperature coefficient of resistivity (PTCR) ceramics when the ceramic specimens were heated to the Curie temperature in a scanning electron microscope. Experiments showed that this image was caused by intense secondary electron emission localized in the phases. This charge-up state seemed to be closely related to the PTCR mechanism.     

Local order and electrical properties of boron carbonitride films

Thin films of carbon-boron-nitrogen alloys were prepared by chemical vapor deposition from gaseous mixtures of C₂H₂, BCl₃, NH₃, and H₂. Their turbostratic graphitic structure was investigated by electron probe micro analysis (EPMA), X-ray diffraction (XRD), Raman spectroscopy, and nuclear magnetic resonance. Those compounds were shown to be solid solutions with compositions varying in a continuous domain bounded by carbon, boron nitride, and BC₃. Measurements of the electrical transport properties (conductivity and Hall coefficient) were performed from 295 down to 4.2 K and in the presence of magnetic fields up to 5 T. The behavior of these solid solutions can be interpreted in terms of a metal-non metal transition, when the carbon content decreases. Heat-treatment of these compounds between 1400 and 2000 °C involves strongly modified transport properties. The resistivity of all materials is lowered. But the main property change is that of the magnetoresistance.     

Hydrogen plasma etching of pyromeric carbon films

We report on a study of the etching behavior of amorphous carbon films prepared by the thermal conversion of polyfurfuryl alcohol in a hydrogen plasma. It was demonstrated that the etch rates were sensitive to the heat treatment temperature of the pyrolysis process. Etch rates increased with the gas pressure and the substrate temperature. Scanning electron microscopy revealed regions of anomalously enhanced etching and the formation of mosaic “riverlike” patterns. The enhanced etching is believed to be associated with the internal stress of carbon films.     

TEM and electron tomography studies of carbon nanospheres for lithium secondary batteries

The structure of carbon nanospheres of 100-200 nm diameter, which showed superior high-speed charge-discharge behavior as the negative electrode in a lithium ion battery, was investigated with XRD, SEM and TEM with an electron tomography attachment. Observation of carbon 0 0 2 lattice images, as well as electron diffraction patterns, illustrated that heterogeneous microtexture was formed as the polyhedronization of the particle proceeded with heat-treatment. The outside region of the particle heat-treated at 2800 °C has stacking structure of aromatic layers with some distribution of d₀₀₂, while the center region consisted of non-graphitic. Structure defects seemed to be concentrated along the ridgelines of the polyhedronized particles after heat-treatment. The electron tomography technique clarified the morphology of the graphitized particles, although the images should be understood with other crystallographic measurements. A slice image computed in the 3D-reconstruction process showed the inner texture of the graphitized particles more clearly than the conventional TEM bright-field image.     

铝、铝硅溶胶及其粉末的合成

以硝酸铝为铝源、硅溶胶为硅源、柠檬酸为稳定剂,采用溶胶凝胶法制备出了稳定的铝溶胶及铝硅溶胶,分析了硝酸铝与柠檬酸的摩尔比(N/C比)、反应温度、保温时间对铝溶胶形成的影响,确定N/C比3:1、100℃及保温1h是铝溶胶较优的工艺参数。XRD分析表明,经1200℃煅烧2h后,铝凝胶粉转变成了α-Al2O3晶相,铝硅凝胶粉末经1200℃煅烧2h后,样品主要为正交莫来石结构,SEM显示所得粉末为无规则形态。     

Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide

Synthesis of nanoporous carbide-derived carbon, CDC, by extraction of titanium and silicon from Ti₃SiC₂ by chlorine is discussed in this work. Thermodynamic simulations using a Gibbs free energy minimization program provided general guidelines to the experimental design. Raman spectroscopy, X-ray diffraction, and electron microscopy studies showed that the structure of CDC depends on the chlorination temperature. The low temperature synthesis resulted in an amorphous CDC structure. Noticeable graphite formation starts above 800 °C and well ordered graphite ribbons of 1-3 nm in thickness form at 1200 °C. The macroscopic volume and shape of Ti₃SiC₂ preform were preserved during the transformation. However, the chlorination resulted in the formation of cracks between the former grains of the polycrystalline Ti₃SiC₂ preform. These cracks are believed to be caused by a contraction in the direction perpendicular to the basal planes of Ti₃SiC₂. The synthesized nanoporous carbon demonstrated excellent sorption properties. Energy dispersive X-ray spectroscopy studies showed that Ti₃SiC₂ material chlorinated at 400 °C is capable of trapping over 40 wt.% of Cl₂.     

Highly stable polyimide composite films based on 1,2,4-triazole ring reinforced with multi-walled carbon nanotubes: Study on thermal,mechanical, and morphological properties

A novel diamine bearing aromatic pendant triazole ring, namely, 3,5-diamino-N-(1H-[1,2,4]triazol-3-yl)-benzamide, was successfully synthesized. The prepared diamine and a commercial dianhydride were reacted in situ in the presence of carboxylated multi-walled carbon nanotubes (MWCNTs) with stirring to give a homogeneous MWCNT/poly(amic acid) mixture which was then heated under a heating program to give a series of MWCNT/polyimide (PI) composites with different proportions of MWCNT (5, 10, and 15 wt%). The composite films were tested for different properties including spectral, morphological, thermal, and mechanical properties. Scanning and transmission electron microscopy revealed the modified MWCNTs were well dispersed in the PI matrix while the structure of the polymer and the MWCNTs structure were stable in the preparation process. The thermal stability of the films containing MWCNTs was improved as the MWCNT content increased from 5 to 15 wt% due to the improved interfacial interaction between the PI matrix and surface-modified MWCNTs. Tensile tests on the composites showed an increase in the elastic modulus and the yield strength, and decrease in the failure strain.     

Preparation of vapor-grown carbon fibers from deoiled asphalt

Vapor-grown carbon fibers (VGCFs) with high-purity have been successfully prepared from the thermal cracking of deoiled asphalt (DOA) with ferrocene as catalyst by chemical vapor deposition (CVD) in argon atmosphere and characterized systematically by field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. Results showed that VGCFs with a diameter of 150-200 nm and a maximum length of 10-40 μm can be obtained.     

Identification of carbon forms and other phases in automotive brake composites using multiple analytical techniques

One of the most important parameters characterizing the performance of brake materials is their formulation. Therefore, the precise analysis of brake materials is needed both to investigate formulation-performance relationship, and to maintain quality control. A typical brake composite contains more than ten phases (components), either crystalline or amorphous. Combinations of X-ray diffraction (XRD), light microscopy (LM), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) are used to identify the phases present. However, the combination of these methods is insufficient for precise resolution of all phases in brake composites. The large number of phases can cause the overlapping of peaks in the XRD pattern, making them difficult to identify. This problem has been overcome by using density gradient centrifugation (DGC) to separate the powdered sample into density fractions that are easier to identify. The results of this study show that a combination of applied techniques can be effective in identifying brake formulation. Because carbon materials are significant components of brake composites, special attention was given to the discrimination of these components such as coke, graphite, and aramid fibers.     

Preparation and characterization of silicon carbide fibers from activated carbon fibers

Silicon carbide fibers were prepared by the reaction between activated carbon fibers and silicon monoxide generated from a mixture of silicon and silicon dioxide at temperatures from 1200 to 1300°C in an inert atmosphere of argon. The reaction was completed at temperatures as low as 1200°C, which means that activated carbon fibers had a high reactivity. The resulting sample maintained the original morphology of the starting material, which was an advantage because of the difficulty in post shaping silicon carbide, and led to a silicon carbide fiber with high specific surface area. The resulting samples were characterized by powder X-ray diffraction, thermal gravimetric analysis, and by nitrogen adsorption measurements at 77.4 K to obtain surface area and pore size distributions. The morphology of the resulting sample was observed by scanning electron microscopy and the electronic structure was investigated by Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy techniques.     

Synthesis of carbon nanotubes on pulse plated Ni nanocrystalline substrate in ethanol flames

A novel process for synthesizing carbon nanotubes (CNTs) in ethanol flames is described. The CNTs grow on a nanocrystalline Ni layer which was electro-deposited on a Ni substrate using periodic reverse (PR) pulse plating. The grain size of the plating and CNT morphology were revealed using XRD, SEM, TEM and Raman spectroscopy. It was found that the quality of the plating and the corresponding CNTs were related to two plating parameters: output pulse frequency (f) and duty cycle (r). The growth mechanism of CNTs in this process is discussed.     

A multi-criteria decision analysis on injection moulding of polymeric microcellular nanocomposite foams containing multi-walled carbon nanotubes

In this paper, polyamide 6 (PA6)/multi-walled carbon nanotubes (MWCNTs) nanocomposites were foamed using an injection moulding machine. Morphological properties were characterised using X-ray diffraction and scanning electron microscopy (SEM). The samples were produced based on the Taguchi L₁₆ orthogonal array in different processing conditions and variant content of MWCNT. Specific tensile strength (STS) and Rockwell hardness, mean cell size and density were investigated and considered as different criteria for selecting the best sample. Criteria weighting and alternative ranking were performed using analytical hierarchy process and two methods of technique for order of preference by similarity to ideal solution and multi-objective optimisation on the basis of ratio analysis, respectively. The results of the X-ray diffraction test showed 0.85, 0.94 and 1?Å increase in the distance between MWCNT’s walls of nanocomposites containing 0.5, 1 and 1.5?wt-% of MWCNTs, respectively, and SEM test results indicated that an appropriate microcellular structure was achieved. Mechanical properties results show that STS and Rockwell hardness of samples were improved about 147 and 17% by adding 1 and 1.5?wt-% of MWCNTs, respectively. Also, the results of multi-criteria decision-making methods revealed that the alternative number 12 (A-12) is the best alternative. The processing conditions of this sample were: 1?wt-% of MWCNT, holding pressure (HP) time of 4?s and HP of 100?MPa.     

A comparative study on the magnetic properties of MFe12O19 and MAlFe11O19 (M = Sr, Ba and Pb) hexaferrites with different morphologies

M-type nano hexaferrites MFe₁₂O₁₉ and MAlFe₁₁O₁₉ (M = Sr, Ba and Pb) have been prepared by the sol-gel method to investigate the shielding effect of inorganic ions KCl, KBr and KI on the phase growth of ferrites. FTIR frequency bands in the range 560-580 cm⁻¹ and 430-470 cm⁻¹corresponds to the formation of tetrahedral and octahedral clusters of metal oxides in ferrites, respectively. X-ray powder diffractographs do not show any peaks for the as obtained samples showing the amorphous nature of the samples, however regular peaks for M-type structure have been obtained for all the annealed samples. There is negligible small change in the lattice parameters ‘a’ and ‘c’ with substitution of the hexagonal ferrites with aluminium. Magnetic measurements showed that the coercivity (Hc) values of all the samples with KCl and KBr enhance due to KCl and KBr to act as deactivators. However, the coercivity value decreases with KI as it oxidise to I₂ during annealing. The saturation magnetization of the hexaferrites decreases with Al³⁺ ion substitution for Fe³⁺ ion due to preferential occupancy of aluminium in octahedral sites.