Partial carbonization of aramid fibers

Heat treatment of aramid fiber was conducted in the temperature range 300–710°C nominally for 10 and 30 s in both static air and flowing nitrogen atmosphere. Crystallinity, crystal orientation, and crystallite size were determined using x-ray diffraction. Fibers with a skin–core structure were produced at intermediate temperatures, as revealed by scanning electron microscopy of fibers after partial dissolution of the fiber in 95–98% sulfuric acid. The skin, which forms in both nitrogen and air, is amorphous and brittle. It is insoluble in sulfuric acid, suggesting it is a cross-linked polymer. Formation of the skin may be facilitated by the removal of an aggressive chemical species that forms during heat treatment. The species may diffuse out of the outer layer of the fiber, allowing it to cross-link. The molecular weight of the dissolved core, analyzed using intrinsic viscosity, decreases with increasing heat treatment temperature. The tenacity, modulus, elongation-to-break, and toughness of fibers with a skin–core structure decrease with heat treatment and the fiber loses its fibrillar character. Mechanical property reductions are greater in air than nitrogen. X-ray data are also consistent with the notion that oxygen assists attack of crystals at high temperatures. Scanning electron microscopy shows that fibers have become skin–core composites with quite different mechanical properties between the two regions. A fiber failure mechanism is proposed. © 1994 John Wiley & Sons, Inc.     

In-situ fracture investigations of MgLi-carbon fibre composite materials by AES and data analysis by means of factor and cluster analysis

Uncoated and 300 nm pyrocarbon coated polyacrylonitril (PAN) based T300 type carbon fibres were pressure infiltrated with molten Mg8Li and Mg12Li alloys at 903 K and 963 K, respectively, and at contact times of 4 s and 30 s. The fracture surfaces were characterized by in-situ Auger electron spectroscopy (AES) to investigate the reason for differences in mechanical properties, which were obtained by tensile strength experiments. Pressure increase occurred during the fracture of all the MgLi samples under ultra high vacuum conditions in the analyzer chamber of the AES apparatus, indicating porosity probably due to poor wetting of the fibres during the infiltration process. As visible in the scanning electron micrographs, the pyrocarbon coated carbon fibres were less attacked by the molten MgLi than the uncoated fibres. The damage seemed to increase, the higher the Li contents, the higher the contact time and the higher the temperature were chosen, whereas pyrocarbon coating may retard this affection. Characteristic objects of the sample fracture surfaces (fibre fracture surface, fibre jacket surface, fibre impression surface and matrix) were investigated by laterally high resolution Auger electron spectroscopy. Oxygen was found to be distributed all over the fracture surfaces of the as-fractured samples, and Mg/Li were identified in a chemically bound form. On all the characteristic objects carbide (Li₂C₂) formation has been detected. Factor analysis was performed to exctract the principal components from the whole set of differentiated Auger electron spectra in the Mg/Li, C and O spectral regions, respectively. Cluster analysis of the weighting factors (the factor loadings) of the most important principal components lead to a separation of the data set into two main groups due to the presence or absence of a strong carbide C (KVV) peak as result of Ar⁺ sputter removal of the covering oxide layer; no separation of the objects concerning the reaction conditions was found. Received: 18 June 1996 / Revised: 26 July 1996 / Accepted: 30 July 1996     

High-temperature phase transitions in the La<Subscript>0.25</Subscript>Sr<Subscript>0.75</Subscript>FeO<Subscript>3−δ</Subscript> solid solution with a perovskite structure

Phase transformations in a sample from the series La_1?x Sr _x FeO_3?δ (x = 0.75) with cubic symmetry and perovskite type structure were studied by high-temperature X-ray diffraction. In air, the solid solution was stable to the synthesis temperature (1200°C), despite the loss of some part of oxygen. Heating in vacuum led to a two-phase state at 900°C (or even at 600°C when a small amount of palmitic acid was added to the sample); the unit cell parameter changed abruptly. The amount of the new phase increased with temperature, and at 1000°C, the sample, as well as the starting phase, was monophase perovskite with cubic symmetry; i.e., an isosymmetric phase transition occurred, which was accompanied by an abrupt change in the cell volume. Cooling in vacuum to ～800°C led to the ordering of the oxygen vacancies and the formation of a Grenier phase type structure. When the sample was heated again in vacuum, a reversible phase transition occurred from the Grenier phase type structure to a perovskite structure (again at ～800°C).     

Chemical solution deposited thin films of titanium,chromium, zirconium,and tin oxides

TiO₂, Cr₂O₃, ZrO₂, and SnO₂ films with thicknesses of ～10–100 nm were produced via dipping into solution and subsequent annealing in air. The films were studied by the methods of scanning electron microscopy, elemental X-ray spectral analysis, optical spectroscopy, and X-ray diffraction. The electrical conductivity of the films in air and in a vacuum was measured. The adhesion of most of the films to the substrate was found to be high. A crystalline structure was observed for films thicker than 10 nm. The films have a specific surface resistance of 10⁸–10¹² Ω in air and 10⁹–10¹⁴ Ω in a vacuum. The films are promising as coatings for various purposes, including the development of structures of the core–shell type.     

The Proportion of Fe‐NX,N Doping Species and Fe3C to Oxygen Catalytic Activity in Core‐Shell Fe‐N/C Electrocatalyst

A bifunctional oxygen electrocatalyst composed of iron carbide (Fe₃C) nanoparticles encapsulated by nitrogen doped carbon sheets is reported. X‐ray photoelectron spectroscopy and X‐ray absorption near edge structure revealed the presence of several kinds of active sites (Fe?N_x sites, N doping sites) and the modulated electron structure of nitrogen doped carbon sheets. Fe₃C@N‐CSs shows excellent oxygen evolution and oxygen reduction catalytic activity owing to the modulated electron structure by encapsulated Fe₃C core via biphasic interfaces electron interaction, which can lower the free energy of intermediate, strengthen the bonding strength and enhance conductivity. Meanwhile, the contribution of the Fe?N_x sites, N doping sites and the effect of Fe₃C core for the electrocatalytic oxygen reaction is originally revealed. The Fe₃C@N‐CSs air electrode‐based zinc‐air battery demonstrates a high open circuit potential of 1.47 V, superior charge‐discharge performance and long lifetime, which outperforms the noble metal‐based zinc‐air battery.     

碱性燃料电池Co-N/C复合催化剂的电化学性能

碳黑经过酸处理后再加入醋酸钴经氨气900 ℃热处理后, 以其制备的气体扩散电极在6 mol•L^―1 KOH溶液中对氧还原反应(ORR)的电催化性能得到大大提高. XRD物相分析表明: 碳粉中加入醋酸钴经氨气热处理生成了氮化钴(Co_5.47N). 通过极化曲线和交流阻抗方法对制备的气体扩散电极在空气中的性能进行了研究. 室温时在－0.2 V (vs. Hg/HgO)电位下, 未经处理的碳电极对氧还原基本没有电流产生; 用酸处理后的碳电极在空气中的电流密度提高到57 mA•cm^―2; 而Co-N/C复合电极在同样条件下电流密度可达170 mA•cm^―2, 交流阻抗显示氮化物的生成减小了氧还原反应的阻抗, 增强了对氧还原反应的电催化作用.     

Thermal oxidation of polyacetylene

The thermal oxidation of undoped trans-polyacetylene powder in dry air has been studied and the principal features of the mechanism have been developed. Thermogravimetric and differential thermal analysis reveal an exothermic process that first leads to a weight increase, followed by precipitous weight loss above 240°C due to formation of volatile oxidation products. Isothermal weight gain studies between 25 and 142°C show first-order kinetics below 90°C with a rate constant of 3.10^?7 s^?1 at 25°C and an apparent activation energy of 16 kcal/mol. A weight gain of more than 40% has been observed at 25°C after 2000 h of exposure to air. A change in first-order kinetics occurs at temperatures above 90°C. Identification of solid oxidation products with photoacoustic infrared spectroscopy reveals that oxygen intercalates into the polymer structure in large concentrations, similar to other electron acceptors. However, oxidative attack on the polymer backbone occurs simultaneously. At elevated temperatures or for long-term oxygen exposure, the concentration of dopant oxygen decreases, probably by intramolecular regrouping of hydrogen atoms, resulting in the formation of hydroxyl groups and enhanced polymer degradation. This mechanism is consistent with the finding of others that the conductivity of polyacetylene upon oxygen exposure increases initially before decreasing significantly with continued exposure, especially at elevated temperatures.     

Structural features of the formation of La1?xCaxFeO3?δ (0 ≤x ≤ 0.7) hetero valent solid solutions

A synthesis method with the use of polymer-salt compositions (calcination temperature 800°C) provides the preparation of various solid solutions of a La_1−x Ca _x FeO_3−δ series in the 0≤ x≤ 0.7 range, which belong to the perovskite structure type. A morphotropic phase transition occurs from the orthorhombic perovskite modification (0≤ x ≤ 0.4) to the cubic one (0.5 ≤ x≤ 0.7). A growing number of microdistortions in the perovskite structure and the formation of a microblock structure in the morphotropic phase transition region are observed with increasing degree of calcium substitution for lanthanum. Calcination of solid solutions with x = 0.6 and 0.7 at temperatures above 1000°C in the air or under conditions of reduced oxygen partial pressure (laboratory vacuum of 10⁻³ Torr) results in the formation of a nanostructured state with coherently grown blocks of perovskite and Grenier phase, which is due to irreversible oxygen loss.     

An oxyluminescence investigation of the auto-oxidation of nylon 66

The kinetics of the thermal oxidation of stabilised and unstabilised nylon 66 fibres and films have been studied by photon counting oxyluminescence methods from 50°C to 150°C. The activation energies for initiation (E₁), propagation (E₃) and termination (E₅) over this temperature range are: E₁ = 16 kcal mol^?1, E₃ = 17·5 kcal mol^?1 and E₅ ≈ 12 kcal mol^?1. The extent of orientation of the polymer does not change the nature of the oxyluminescence curve or E₃ and E₅ above 110°C.Significant losses of critical mechanical properties of the fibres occur in the induction period at 100°C and non-stationary kinetics are described to enable this region to be studied by oxyluminescence. The oxidation rate in the induction period and the limiting rate region in air is one-third the rate in oxygen at atmospheric pressure. Non-stationary methods show that alkyl radical reactions are competitive with alkyl peroxy radical formation in air over the temperature range 100°C to 140°C. This affects the course of the oxidation reaction and the stabiliser efficiency and explains the observation of unsaturated oxidation products by phosphorescence spectroscopy.     

Influence of the heat treatment conditions on the formation of CuFe2O4 from mechanical milled precursors oxides

Stoichiometric mixture of CuO and ??-Fe₂O₃ milled in air up to 30?h was subjected to different heat treatments. The evolution of the heat treated milled powders was investigated by X-ray diffraction (XRD). The CuFe₂O₄ was partially obtained by milling, the material consisting in a mixture of phases. By applying different heat treatments in air and in vacuum, for 2?C6?h, in 500?C800?°C temperature range the phases composition of the milled samples is changed. A heat treatment at 500?°C in vacuum favours the formation of delafossite (CuFeO₂) and tenorite (CuO) phases. If the same heat treatment is made in air, the CuFe₂O₄ phase formation with a cubic structure is favoured. Differential scanning calorimetry (DSC) investigation realised in Ar atmosphere revealed two large exothermic peaks. The first one is associated with the formation of the delafossite and tenorite phases and the second one with the formation of CuFe₂O₄. The XRD patterns of the samples subjected to the DSC measurements present maxima corresponding to the delafossite and cuprospinel (CuFe₂O₄) phases. For the heat treatment at 600?°C in air the phases present in the sample are the same as for the annealing performed at 500?°C: CuFe₂O₄, ??-Fe₂O₃ and CuO. The heat treatment in air at 800?°C leads to the complete reaction between the different phases and the formation of CuFe₂O₄ phase in whole the sample volume. The CuFe₂O₄ ferrite crystallises after this heat treatment in two crystal systems: cubic and tetragonal.     

IFSS, TG, FT-IR spectra of impregnated sugar palm (Arenga pinnata) fibres and mechanical properties of their composites

This study aimed to investigate the effect of resin impregnation on the interfacial shear strength (IFSS), thermogravimetric (TG) and fourier transform infrared (FT-IR) of sugar palm (Arenga pinnata) fibres. In addition, the effect of resin impregnation on the mechanical properties of sugar palm fibre reinforced unsaturated polyester (UP) composites was also studied. The fibres were impregnated with UP via vacuum resin impregnation process at a pressure of 600 mmHg for 5 min. Composites of 10, 20, 30, 40 and 50 % fibre loadings were fabricated and tested for tensile and flexural properties. It was observed that the impregnation process caused the fibres to be enclosed by UP resin and this gave a strong influence to the increase of its interfacial bonding by the increase of its IFSS from single fibre pull-out test. It was also observed with TG and FT-IR spectra that the impregnated fibre had lower moisture uptake than the control and there was no significant increase in thermal stability of the impregnated fibre. The sequence of fibre decomposition started from the evaporation of moisture, hemicelluloses, cellulose, lignin and finally ash content and the presence of these components were proven by FT-IR spectra. For the composite specimens, due to the high interfacial bonding of the impregnated fibre and the matrix, the impregnated composites showed consistently higher tensile strength, tensile modulus, elongation at break, flexural strength, flexural modulus and toughness than the control samples. It was also observed that 30 % fibre loading gave optimum properties.     

Electron spin resonance study of poly-3,3-bis-(chloromethyl)oxetane irradiated with ultraviolet light

When poly-3,3-bis(chloromethyl)oxetane has been irradiated at ?196°C in a nitrogen atmosphere with ultraviolet light, a triplet spectrum is observed. After warming the sample, both a doublet and a singlet ESR spectra are observed. These spectra are attributed to and ? CH₂? O, respectively. The formation mechanism of these free radicals is discussed. It is concluded that the main process of radical formation is the dissociation of chemical bonds from the excited state of the polymer produced through the energy absorption by irregular groups acting as sensitizers. In the presence of oxygen, the radical yield at ?196°C is greater than that in nitrogen atmosphere. This is attributed to the extra absorption of light by the charge transfer complexes of polymers with oxygen molecules. It is also proposed that participation of a charge transfer complex in photooxidation of ether is important in the primary radical formation step. When a polymer sample irradiated in vacuum with ultraviolet light is treated at ?78°C for a few minutes in the presence of air, peroxy radicals form. This shows that oxygen molecules diffuse very easily into this polymer, even at this low temperature.     

Surface and bulk properties of severely fluorinated carbon fibres

The development of ultra-inert composites using fluorinated carbon fibres as the reinforcement requires fluorinated carbon fibres with a durable surface composition. Here we report the effect of direct fluorination using an F₂/N₂ mixture at 653 K on the surface and bulk properties of two types of high strength carbon fibres. These were treated up to a surface fluorine content of ∼64 at.% and a bulk fluorine content of ∼15 mass%. A colour change was observed after fluorination caused by the changes in the graphitic band structure of the carbon fibres by the introduction of carbon sp³ hybridisation. The tensile strength and Young's modulus decrease after fluorination by up to 33 and 22%, respectively. XRD shows marginal changes in the interlayer distance but the crystallite size increases. Changes in the electrical conductivity of the fluorinated carbon fibres indicate that the modification is confined to the near surface volume. Predominantly covalent C-F bonds are formed as shown by X-ray photoelectron spectroscopy (XPS) and measured zeta (ζ)-potentials. Hence the fluorinated fibres are hydrophobic and have low surface tensions. This and the large increase in fibre surface area, as determined by nitrogen adsorption, is expected to facilitate interfacial interaction between fluorinated carbon fibres and fluoropolymers.     

Melting behaviour of gelspun/drawn polyolefins

The melting behaviour of gelspun/drawn UHMW-PE and UHMW-PP fibres was investigated. Unconstrained UHMW-PE and UHMW-PP fibres melt at 142°C and 170°C, respectively. Upon constraining, by holding the fibres at a fixed length or by embedding the fibres in a matrix, an increase in the melting temperature of both fibres is observed. In the case of UHMW-PE fibres a solid-solid phase transition in polyethylene at 155°C from the orthorombic to the hexagonal crystal structure occurs. Above 155°C, the fibres can not sustain any load. This solid-solid phase transition at 155°C sets an upper limit to both the maximum curing and continuous use temperature of PE-fibre reinforced composites. In gelspun/drawn UHMW-PP, such a detrimental solid-solid phase transition is absent, and therefore the increase in melting temperature can be utilized effectively. For example, heating of UHMW-PP fibres for 30 minutes at 200°C does hardly affect the room temperature Young's modulus and tensile strength if the fibre is constrained during heating.     

NiSO_4改性对聚丙烯腈原丝及其碳纤维结构与性能的影响

碳纤维具有高比强度、高比模量、导电、耐热、自润滑等优异的综合性能,在纤维增强复合材料中得到了广泛的应用.可制备碳纤维的前驱体有人造丝、沥青、聚丙烯腈纤维、木质素、聚乙烯纤维、聚苯并噻唑(PBO)纤维等.但大多数高强碳纤维目前仍然是由聚丙烯腈纤维制备的,同时,许多工作都集中在更进一步提高碳纤维的机械性能.特别是在我国, 碳纤维质量与某些发达国家相比,还有较大的差距,急需解决的问题就是如何尽快研制出高力学性能的碳纤维.采用氨基硅氧烷、脂肪族羧酸[1]、CuCl[2]、KMnO 4[3]、CoCl2[4]等有机或无机化学试剂对聚丙烯腈原丝进行化学处理, 以改进最终碳纤维的结构与性能是一种有效的方法.国内在这方面的研究还很少.文献[1 ～4]中所采用的方法都是利用商业聚丙烯腈原丝在碳化前进行洗油、浸渍、洗涤烘干处理 ,增加了碳纤维制备的工序,同时,原丝损伤较大,在连续生产中难以适用.我们在原丝连续制备的同时采用NiSO4溶液浸渍处理聚丙烯腈纤维,本文主要研究了采用NiSO4浸渍改性后聚丙烯腈原丝及其碳纤维的结构与性能.研究表明,采用NiSO4在线浸渍改性聚丙烯腈原丝,生产工艺简单,且能有效地改进最终碳纤维的结构与性能.     

The effect of different atmospheres on structural changes of titanate nanotubes during heating

Thermal stability of titanate nanotubes (Ti-NT) was studied in temperature range from room temperature up to 1000 °C in three different atmospheres—in air, vacuum and helium—and differences were observed. In air, vacuum and helium, the release of adsorbed and crystal water was detected in temperatures up to 200 °C. Transformation of the initial structure to anatase form of TiO₂ was found independently of the atmosphere used. But in air the transformation to anatase occurred at lower temperature. The difference between these atmospheres was approximately 100 °C. However, only in air, the transformation to sodium hexatitanate and rutile form of TiO₂ occurred at higher temperatures. In vacuum, only the anatase phase was detected up to 800 °C. In helium, the transformation to rutile was observed but not to sodium hexatitanate. The changes of the initial structure of Ti-NT during the elevation of temperature were studied by combination of in situ powder X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis and mass spectroscopy.     

Photodegradation of aramids. II. Irradiation in air

A series of isomeric fully aromatic polyamides (aramids) were photodegraded in the presence of oxygen. Films and fibers of these aramids gave carboxylic acids as the major products when measured by infrared spectroscopy and potentiometric titration. These acids probably resulted from the oxygen interception of the radicals generated by photocleavage of the amide bonds. In contrast to results found upon irradiation in the absence of oxygen, carboxylic acid formation was accompanied by a rapid loss in molecular weight, and a decrease in useful mechanical properties. Quantum yields for carboxylic acid formation were ≤5.5 × 10^?5 mole/einstein and decreased along the aramid series roughly in agreement with increases in T_g. The photo-Fries rearrangement product was observed in aramid fibers irradiated in air, whereas no rearrangement product was seen in films irradiated in air.     

Mechanical Properties of Regenerated Cellulose Fibres for Composites

Summary: A broad variety of regenerated cellulose fibres was subjected to single fibre tensile tests in order to determine the modulus of elasticity, tensile strength, and failure strain. The results were compared to glass fibres and flax fibres, which are considered the most important technical and natural fibres, respectively. With regard to their modulus of elasticity and tensile strength, regenerated cellulose fibres showed clearly lower values than glass fibres, even when their low density was taken into account. The average modulus of elasticity and tensile strength of regenerated cellulose fibres was also lower than the values measured for flax fibres, but when variability was considered, both fibres performed similarly. In terms of interfacial shear strength with polypropylene, lyocell fibres performed significantly less well than sized glass fibre and ramie fibre. The most important difference between regenerated cellulose fibres and both glass and flax fibres is their high failure strain and thus high work to fracture. The high work to fracture of regenerated cellulose fibres makes them particularly useful for composite applications where high fracture toughness is required.     

Moisture Removal from Natural Jute Fibre by Plasma Drying Process

Low temperature plasma process is an effective alternative method compared to the conventional vacuum drying method for removing moisture. Plasma drying removes the moisture from fibres faster and to a lower level than conventional methods. It also improves the surface properties of the fibres. The jute fibre was treated with inert gas argon plasma without damaging the fibre. The OES was used to monitor the moisture desorbed from the fibre during processing. The XRD results revealed a change in the macromolecular structure as well as the crystallinity of the treated fibre. The FTIR and TGA provided the evidence of moisture removal from the fibres. It was found that the plasma treated fibres contain less than 1.8% (wt.) moisture which is a promising result when compared with conventional drying processes.The average tensile strength of the plasma treated fibres increased by 12.5% compared with those treated with the conventional vacuum dry process.     

Multi‐instrument characterization of poly(divinylbenzene) microspheres for use in liquid chromatography: as received,air oxidized,carbonized, and acid treated

We report a multi‐instrument characterization of the carbon particles in carbon/polymer/nanodiamond core‐shell materials used for high‐performance liquid chromatography. These particles are prepared by the carbonization/pyrolysis of poly(divinylbenzene) (PDVB) microspheres. Scanning electron microscopy showed that the particles (4.9 µm initially) decreased in size after air oxidation (to 4.4 µm) and again after carbonization (down to 3.5 µm) but remained highly spherical. Brunauer–Emmett–Teller measurements showed low surface areas initially (as received: 1.6 m²/g, after air oxidation: 2.6 m²/g) but high values after carbonization (445 m²/g). Fourier transform infrared spectroscopy revealed the changes in the functional groups after air oxidation (C = O and C–O stretches appear), carbonization (carbon‐oxygen containing moieties disappear), and acid treatment (reintroduction of carbon‐oxygen containing moieties). X‐ray photoelectron spectroscopy (XPS) and elemental analysis revealed the surface and bulk oxygen contents before and after treatments. By XPS, the atom percent oxygen for the as received, air oxidized, carbonized, and acid treated particles are 8.7, 16.6, 3.7, and 13.8, respectively, and by elemental analysis, the percent oxygen in the materials is 0.6, 8.1, 0.9, 16.9, respectively. A principal components analysis of time‐of‐flight secondary ion mass spectrometry data identified ions that were enhanced in the different materials, where almost 90% of the variation in the analyzed peak areas was captured by two principle components. X‐ray diffraction and Raman spectroscopy suggested that the carbonized PDVB was disordered. Thermogravimetric analysis showed significant differences between the differently treated PDVB microspheres. This work applies directly to a commercial product and the process for preparing it. Copyright © 2015 John Wiley & Sons, Ltd.