Microscopic study of modified polyacrylonitrile fibres

Conclusions Microscopic studies have been made on polyacrylonitrile fibres which have been modified with solutions of CdSO₄, ethylenediamine, and a mixture of these solutions.Treatment with an ethylenediamine solution aids in a significant change in structure of the fibre surface.Treatment with a mixture of CdSO₄ and ethylenediamine solutions leads to development of a structural nonuniformity in the fibre and the twisting of the fibre on drying in the free state.Translated from Khimicheskie Volokna, No. 2, pp. 33–34, March–April, 1989.     

Energy nonuniformity in the surface of carbon fibres and other graphitic carbon materials

Conclusions -- A method is proposed for the mathematical treatment of experimental data on the adsorption of hydroxoaquo complexes of iron (III) ion, which makes it possible to determine the energy and chemical uniformity of a surface. For a number of carbon fibres, distribution functions for the active centers of the surface with respect to change in free energy of sorption have been obtained.-- The described method, in combination with the results of other physico-chemical studies, will make it possible to obtain information about the chemical and energy nonuniformity of the surface of carbon fibres and of other graphitic carbon materials, and may become the basis for predicting the technical activity of composite materials and catalysts based on carbon fibres.St. Petersburg Technological Institute. Translated from Khimicheskie Volokna, No. 5, pp. 46–48, September–October, 1992.     

A study of pitch-based precursors for general purpose carbon fibres

The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone-60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature.     

A study of pyrolysis of phenolic resin reinforced with carbon fibres and oxidized pan fibres

Pyrolysis of composites prepared from a phenol-formaldehyde resin and carbon fibres (CFRC) or oxidized PAN fibres (OFRC) has been investigated up to 1270 K. Two temperature regions can be distinguished in the pyrolysis of the CFRC: below 770 K the carbon fibres hold the composite structure resisting the tendency of the resin to produce composite shrinkage; above 770 K the CFRC volume and porosity abruptly increase due to a weakening of fibre/resin adhesion. During the pyrolysis of the OFRC a chemical reaction at the fibre/resin interface apparently takes place, beginning at 570 K and producing important changes in the material at 870–970 K. Somewhat smaller weight losses and markedly lesser shrinkage of the pyrolysed composite with respect to its separately treated components have been found. The fibres and the resin coalesce into a product having relatively high density and low porosity, the latter mainly consisting of narrow pores. Hydrogen content of the product is lower than that of the separately treated constituents. The reflections of both the resin and the oxidized PAN fibres are seen only in the X-ray patterns of the OFRC treated at low temperature, but above 670 K only the reflection due to the fibres appears. The interlayer spacing of the fibres in the composite is greater than in the fibres alone treated under the same conditions. According to the IR spectra, the CN groups and aromatic CH groups disappear from the composite at lower temperature than from its components treated separately.The existence of the reaction between the resin and the oxidized PAN fibres, leading to the destruction of the fibre materials, means that the co-carbonization product at 1270 K is not a composite and will not have the necessary mechanical properties. Therefore, this study must continue to look for fibres and matrices which are compatible during co-carbonization to 1270 K.     

Magnetic properties of PAN-based carbon fibres modified with magnetite nanoparticles

The magnetic properties of PAN-based carbon fibres modified with magnetite nanoparticles are presented and analyzed. PAN fibres and PAN-based carbon fibres modified with different amounts of magnetite are characterized by the use of magnetization measurements and Mössbauer spectroscopy techniques. The investigations revealed that magnetite (Fe₃O₄) decomposed into Fe_α, Fe_γ and cementite (Fe₃C). Precise analysis of the phase’s contents for different carbon fibres has been carried out in relation to the initial magnetite compound. Analysis of the Mössbauer spectra allowed the determination of the phase contents in fibres with different initial magnetite concentrations. Partial transformation of magnetite into γ-Fe induces catalytic carbonization and formation of a highly crystalline carbon matrix at 1000 °C. The apparent crystallite size in carbon fibres containing 30% magnetite was almost seven times higher than that found in the pure carbon fibres.     

Bulk and surface chemical functionalities of type III PAN-based carbon fibres

PAN-based carbon fibres carbonised at relatively low temperature, i.e. type III carbon fibres, were submitted to heat treatment at 2300 °C (GR) or oxidation in nitric acid. The samples were characterised by XPS, FTIR, wetting measurements, gas adsorption, elemental analysis and acid/base titration. While oxidation only slightly affects the nitrogen concentration, it produces an appreciable change in the nature of the chemical functions, namely the conversion of pyridine-type nitrogen and quaternary nitrogen into aliphatic functions. Oxidation treatment modifies all the material constituting the fibre, the oxygen concentration being about 1.5 times higher at the fibre external surface compared with the whole material. Three components (531.2, 532.6 and 533.8 eV) are clearly identified in the oxygen XPS peak, allowing a comparison to be made between the whole material and the external surface regarding chemical species. The acidic groups are mainly carboxyl. Fibres submitted to extensive oxidation also show a high basicity, attributed mainly to calcium carboxylate. Although the acidic and basic groups present in the whole material can be titrated with aqueous solutions, the fibres develop only a very small surface area and no microporosity as determined by krypton adsorption. The material may be viewed as a sponge, collapsed when dry but able to swell in water and developing a high cation-exchange capacity.     

Experimental study of the thermophysical properties of carbon fibres

A number of thermophysical properties of carbon fibres was investigated. The measurement method, experimental setup, and experimental data obtained at 300–400 K are described. The temperature dependence of the thermal conductivity of carbon fibres is explained based on the phonon model of heat transfer. __________ Translated from Khimicheskie Volokna, No. 4, pp. 16–19, July–August, 2007.     

Development of porosity and surface heterogeneity upon air activation of graphitized carbon black

One graphitized carbon black (Vulcan 3G) and three samples prepared by its activation in air at 978K have been used as supports for platinum catalysts. Surface characteristics of the four supports have been studied by physical adsorption of N₂ at 77 and 90K. Surface area increases smoothly with burn-off from 62 m² g^?1 (original V3G) to 121 m²g^?1 (52.8% burn-off sample). Porosity and surface heterogeneity develop upon activation but all samples have a large degree of surface homogeneity and low porosity.     

Influence of surface properties of carbon fibres on the adsorption of catalase

PAN-based carbon fibres obtained by carbonisation at relatively low temperature and submitted to high-temperature treatment and oxidation in wet conditions provided a range of materials differing according to surface composition and hydrophobicity. The adsorption of catalase by these fibres was studied by XPS and by activity measurements in the liquid and in the adsorbed state. Three typical situations are featured: limited adsorbed amount in the case of low hydrophobicity; significant adsorption and preservation of an appreciable enzyme activity for samples of moderate hydrophobicity; strong adsorption with deactivation of the enzyme in the case of high hydrophobicity.     

A study of electrochemically treated PAN based carbon fibres by IGC and XPS

Inverse gas chromatography (IGC) has been used with conventional and novel probes in conjunction with X-ray photoelectron spectroscopy (XPS) to assess the surface chemistry of a range of aged polyacrylonitrile carbon fibres. Using conventional probes the thermodynamic surface properties were investigated at 100 °C. The dispersive component of the surface free energy, , was determined and ranged from 73.1 mJ m⁻² to 92.2 mJ m⁻². Unconventional probes, chosen as analogues of the functionalities present in a potential matrix material, were also eluted through the IGC columns. Surface compositions and peak fitting of C 1s spectra from XPS analysis helped identify groups on the fibre surface. Using these results, mechanisms of interactions occurring at the interface of a carbon fibre reinforced polymer composite have been proposed. Hydrogen bonding and stronger acid-base interactions are the main mechanisms of interactions. Although, the carbon fibres are aged, oxygen and nitrogen functionalities introduced from surface treatment have remained active.     

Preparation of modified active carbon from brown coal by ammoxidation

The technology of obtaining active carbon enriched in nitrogen from brown coal is described. The effect of ammoxidation by a mixture of ammonia and air at the ratio 1:3 at 300 and 350 °C, at each stage of the active carbon production has been tested. The amount of nitrogen introduced into the active carbon has been proved to depend on the stage at which ammoxidation was performed. Carbonisation and activation with steam of the samples enriched in nitrogen have been found to lead to a significant decrease in its content and to cause an increase in the stability of the nitrogen groups. The ammoxidation of the active carbon has decreased their surface area, while the ammoxidation and high temperature of activation favour the formation of surface oxide groups of basic character.     

改性碳纳米管在硅胶中的应用研究

本文研究了改性碳纳米管(以下简称CNTs)和复配CNTs对硅胶混炼胶的硫化特性、硫化胶的常规物理机械性能、DIN磨耗、电阻率以及导热系数等的影响。结果表明,随改性CNTs用量的增加,混炼胶的硫化程度升高,t90逐渐增大;硫化胶硬度增大,撕裂强度逐渐上升;拉伸强度基本不变,100%定伸应力逐渐增大,扯断伸长率逐渐下降,DIN磨耗基本不变;表面电阻率、体积电阻率均减小,变化较小;热扩散率和导热系数提高。加入复配CNTs的性能变化规律与上述结论基本相同,但表面电阻率和体积电阻率变化明显。     

Electrochemical oxidation of carbon fibres in aqueous solutions and analysis of the surface oxides

Carbon fibres with notably different surface oxides can be prepared by varying the electrochemical oxidation conditions. Through correlation of voltammetric analysis and mass spectroscopy, interpretation of the reduction peaks of the surface oxides on the basis of their potential and width is possible. Narrow voltammetric reduction peaks at strongly negative potential are indicative of the predominance of –COOH type groups, while wide peaks at more positive potential are indicative of the presence of an excess of type groups. A quantitative determination of the surface acidic groups (–COOH and) is achieved by combination of Ag⁺ ion exchange and esterification of groups with 3,5-dinitrobenzoyl chloride. All results are confirmed by the independent method of –COOH group determination via their electrocatalytic behaviour in the reduction of azobenzene in methanol. The amounts of the –COOH and groups formed depend notably on the conditions of the electrochemical oxidation of the carbon fibres. The ratio between –COOH and groups coincides in all cases with that obtained by the mass spectrosopic data.     

Screen-printed carbon electrode modified on its surface with amorphous carbon nitride thin film: Electrochemical and morphological study

The surface of a screen-printed carbon electrode (SPCE) was modified by using amorphous carbon nitride (a-CN_x) thin film deposited by reactive magnetron sputtering. Scanning electron microscopy and photoelectron spectroscopy measurements were used to characterise respectively the morphology and the chemical structure of the a-CN_x modified electrodes. The incorporation of nitrogen in the amorphous carbon network was demonstrated by X ray photoelectron spectroscopy. The a-CN_x layers were deposited on both carbon screen-printed electrode (SPCE) and silicon (Si) substrates. A comparative study showed that the nature of substrate, i.e. SPCE and Si, has a significant effect on both the surface morphology of deposited a-CN_x film and their electrochemical properties. The improvement of the electrochemical reactivity of SPCE after a-CN_x film deposition was highlighted both by comparing the shapes of voltammograms and calculating the apparent heterogeneous electron transfer rate constant.     

Structure of carbon fibres

Conclusions -- Carbon fibres have been examined as carbon band oriented amorphous-crystalline polymers.-- Characteristic regions in the formation of the structure of carbon fibres have been established, plus the interconnection between structural parameters.-- The components of preferential orientation of carbon fibres have been marked out, and a systematization of orientation in the transverse section of fibres has been presented.-- The character of change in axial component of texture with change in treatment temperature has been given, plus the connection between it and the elasticity and strength of fibres.-- A characteristic has been proposed for evaluating the activity of the surface of carbon fibres.Combustible Minerals Institute. Translated from Khimicheskie Volokna, No. 3, pp. 26–29, May–June, 1991.     

Chemical bonds in carbon fibres

In reporting the results of an examination of the mechanism of formation of chemical bonds in carbon fibres, the following conclusions can be drawn: — the existing concepts concerning curved hybrid bonds and σ-, π-bonds do not explain their compatibility with the high thermal stability of carbon fibres; — the concept of formation of chemical bonds in carbon fibres because of the gravitational effect of atoms with energy corresponding to the energy of chemical bond formation due to the high value of the gravitational constant of 1.6·10⁻²⁸ cm³/g·sec²) caused by the high density of the substance in the atom of 10¹²–10¹³g/cm³, is substantiated; — the gravitational constant is calculated with the equation of the dynamics of orbital motion for the potential energy; the values obtained are in satisfactory agreement with the results of the calculation based on tensiometric and spectroscopic data. __________ Translated from Khimicheskie Volokna, No. 6, pp. 41–44, November–December, 2006.