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Mesoporous carbon from SiO2-C nanocomposite prepared with industrial raw materials: Synthesis and characterization
Abstract
A SiO2-C composite (SC) was prepared via sol-gel method using industrial raw materials: commercial partially hydrolysed tetraethyl orthosilicate as an ethoxylated silica precursor (ESP) and phenolic-formaldehyde resin as a carbon precursor. The synthesized composite, constituted by a silica network cross-linked with a carbon network, resulted in a material with a sharp pore size distribution (pore size diameter ~ 32 nm) and a high specific surface area (~350 m²/g). In a subsequent step, it was possible to isolate each network: Carbon (C) and Silica (S). In this work, the composite synthesis, and the isolation paths to obtain the individual structures are described. In addition, the composite and the isolated structures were mineralogically and structurally characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (laser of 785 nm = 1.58 eV) (RS), nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA). The textural characterization was performed by nitrogen adsorption (SBET), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM-EDS). The three materials exhibited properties suitable for applications such as catalysis, immobilization of bioactive molecules and dyes, drug delivery, among others. In particular, the carbonaceous network had a pseudo graphene structure and microdomains of high activity, which would enhance its use as a low-cost material in new green technologies for wastewater treatment and drinking water purification.
... Two activated carbons were studied. The ceramic-derived carbon, CeDC, is a carbon produced from a SiO 2 -C composite prepared via sol-gel method according to Benito et al. [17]. Briefly, the composite was prepared mixing commercial partially hydrolyzed tetraethyl orthosilicate, ethyl alcohol and phenolic-formaldehyde resin. ...
... The diffractogram of CeDC (Fig. 5a) presented the typical bands of an amorphous carbon: 25.6°2θ (d002) and 43.6-43.7°2θ (d100 and d101), indicating the presence of a non-crystalline pseudographitic structure [17]. CoAC reveals (Fig. 5a), in addition to a non-crystalline subgraphitic structure, diffraction peaks characteristic of silicon oxide (SiO 2 , Reference code: 01-077-1060). ...
Emerging pollutants, including pharmaceuticals and personal care products, have been detected in surface and groundwaters. The adsorption of paracetamol and ibuprofen, two widespread drugs, has been studied in aqueous medium, using a ceramic-derived carbon (CeDC) and a commercial activated carbon (CoAC). CeDC yielded a BET surface area of 895 m 2 g −1 , a bimodal pore size distribution (13.2 and 35 nm) and a total pore volume of 1.99 cm 3 g −1. CoAC had an approximate surface area of 1000 m 2 g −1 , a homogeneous pore size distribution and a total pore volume of 0.42 cm 3 g −1. Kinetic and equilibrium tests were carried out in batch systems to study the materials' sorption performances. The intraparticle diffusion model best fitted the experimental kinetic data. The maximum ibuprofen sorption capacities were 120 mg g −1 and 133 mg g −1 for CoAC and CeDC, respectively, whereas no major differences on the maximum paracetamol sorption capacities (qm) were observed among the sorbents (150-159 mg g −1). Therefore, CeDC, synthesized easily from a ceramic composite, improved time and sorption capacity of paracetamol and ibuprofen compared to the commercial activated carbon, indicating the potential of the developed carbon as an emerging pollutant sorbent material.
In this study we investigated how traditional mercury intrusion porosimetry (MIP), constant-rate-controlled mercury porosimetry (CMP), Low-field NMR spectral analysis (LFNMR), and micro focus computerized tomography (μCT) compare in revealing the pore size distribution (PSD) characteristics of coals. The comparison was made using the same source samples throughout. Two limitations of mercury porosimetry are addressed. First, the high-pressure intrusion by mercury may either deform or destroy the coal sample and eventually induce suspect value of coal porosity, thus correction of pore structure compressibility must be made in analyzing lignite or other coals that with very open structure. Second, pore shielding effects can induce high uncertainty of MIP results, in particular when clusters of smaller pores occur in isolated domains in a continuous network of larger pores. This can result in temporary mercury entrapment during the extrusion process and result in inaccurate estimations of PSD. Another pore shielding effect is due to isolated clusters of large pores in a continuous network of smaller pores. Her mercury is prone to be trapped permanently. This effect can induce inaccurate estimations of the total pore volume. CMP is an effective method that can provide much more detailed PSD information of macropores, however it is deficient in analyzing mesopores of coals. After comparison with the results by μCT and other traditional methods, it was found that LFNMR is an efficient tool for nondestructively quantifying the PSD of coal.
The results from DSC and thermogravimetric analysis of gels produced from a mixture of tetraethoxysilane (TEOS) and octyltriethoxysilane
(OtEOS) both with and without immobilized Ru(II) tris(4,7-diphenyl-1,10-phenanthroline) dichloride as well as DSC and TG data for films deposited by deep-or spin-coating from
the same gels, are reported. The initial products are characterized by elemental analysis, IR, solid state NMR and mass spectroscopy.
Elemental analysis and IR spectroscopy are applied for identification of some of the intermediates obtained after heating
at different temperatures. The final products are characterized by X-ray diffractometry. A hypothesis for the thermodecomposition
processes taking place is proposed. The results reported contribute to elucidation of the properties as well as the temperature
intervals in which the studied microcomposites could be used as sensing components of oxygen sensors.
Graphene sheets that are now routinely obtained by the exfoliation/reduction of graphite oxide exhibit Raman spectra unlike traditional graphene systems. The general attributes of the Raman spectra of these ‘wrinkled graphene’ are first reaffirmed by evaluating the spectra of samples prepared by seven different exfoliation-reduction methods. These graphene sheets exhibit highly broadened D and G Raman bands and in addition, have a modulated bump in place of the conventional 2D (G′) band. It is shown that the high wavenumber ‘bump’ can be resolved into the conventional 2D band and several defect activated peaks such as G*, D+D′ and 2D′. The broad G band could also be deconvoluted into the actual G band and the D′ band, thereby attributing the broadening in the G band to the presence of this defect activated band. Two additional modes, named as D* at 1190 cm-1 and D** at ∼1500 cm-1 could be identified. These peculiar features in the Raman spectrum of ‘graphene’ are attributed to the highly disordered and wrinkled (defective) morphology of the sheets. The affect of defects are further augmented due to the finite crystallite size of these graphene sheets. The dispersion in the band positions and peak intensities with respect to the laser energy are also demonstrated.
The hydrolysis and polymerization of a non-catalyzed silica sol was investigated by Fourier Transform Infrared Spectroscopy (FT-IR) and deconvolution of the infrared spectra. The hydrolysis was followed by the 1168 and 812 cm bands which have been observed that decrease continously with the reaction time, and they dissapear showing the complete hydrolysis. The bands located at 1200 and 1147 cm are assigned to polymerization of Si-OH groups forming Si-O-Si bonds in cyclic or linear structures respectively. Both bands increase with time and are present simultaneously in the spectra showing that both kind of cross-linking of Si-OH groups are taking place in the sol to form the gel. In the gel the presence of Si-OH groups and Si-O free broken bonds have been detected by means of the bonds located at 960 and 920 cm respectively.
To establish guidelines for the development of a scientific basis for catalyst preparation is perhaps a very ambitious goal. One would re required first to answer the following rhetorical questions: what are the properties which determine the performance of a catalytic material; how can these properties be introduced, developed, and/or improved during preparation? The answer to these questions involves a comprehensive discussion of the theories of catalysis, which is beyond the scope of this review. The authors will attempt, instead, to provide a rationale for each reader to answer these questions on the basis of his/her own interests. They start the discussion by describing the fundamental steps in producing bulk catalysts and/or catalyst supports. The fundamental processes involved are those derived from traditional three-dimensional chemistry. The topic areas will include single-component and multicomponent metal oxides. Unsupported metallic catalysts are formed by transformations involving physical or chemical processes, and the preparation methods for this class of materials will be discussed next. Attention will then turn to the preparation of supported catalytic materials. The main topics to be discussed will be those related to the interaction between the support and the active phase when they are put together to generate the catalyst. In this approach, the authors exploit the virtually unexplored field of surface, or two-dimensional, physical chemistry. The materials considered include dispersed metals and alloys and composite oxides. 366 refs.
Amorphous and nanocrystalline carbon films possess special chemical and physical properties such as high chemical inertness, diamond-like properties, and favorable tribological proprieties. The materials usually consist of graphite and diamond microstructures and thus possess properties that lie between the two. Amorphous and nanocrystalline carbon films can exist in different kinds of matrices and are usually doped with a large amount of hydrogen. Thus, carbon films can be classified as polymer-like, diamond-like, or graphite-like based on the main binding framework. In order to characterize the structure, either direct bonding characterization methods or the indirect bonding characterization methods are employed. Examples of techniques utilized to identify the chemical bonds and microstructure of amorphous and nanocrystalline carbon films include optical characterization methods such as Raman spectroscopy, Ultra-violet (UV) Raman spectroscopy, and infrared spectroscopy, electron spectroscopic and microscopic methods such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, transmission electron microscopy, and electron energy loss spectroscopy, surface morphology characterization techniques such as scanning probe microscopy (SPM) as well as other characterization methods such as X-ray reflectivity and nuclear magnetic resonance. In this review, the structures of various types of amorphous carbon films and common characterization techniques are described.
Quantitative phase analysis of multicomponent mixtures using X-ray powder diffraction data has been approached with a modified version of the Rietveld computer program of Wiles & Young [J. Appl. Cryst. (1981), 14, 149–151]. This new method does not require measurement of calibration data nor the use of an internal standard; however, the approximate crystal structure of each phase of interest in a mixture is necessary. The use of an internal standard will allow the determination of total amorphous phase content in a mixture. Analysis of synthetic mixtures yielded high-precision results, with errors generally less than 1.0% absolute. Since this technique fits the complete diffraction pattern, it is less susceptible to primary extinction effects and minor amounts of preferred orientation. Additional benefits of this technique over traditional quantitative analysis methods include the determination of precise cell parameters and approximate chemical compositions, and the potential for the correction of preferred orientation and microabsorption effects.
Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
The preparation of glass and ceramic systems from metal-organic compounds permits the mixing of the constituents at the molecular level. These mixes form clear glasses or sinter to dense bodies at temperatures considerably lower than the equivalent compositions prepared by classical methods. A significant recent development in this field has been the preparation of monolithic glass and ceramic material without the need of melting or high temperature sintering. Glass-forming reactions, previously achievable only by thermal means, can take place by chemical polymerization at room temperatures. Structural studies indicate that these materials are indeed amorphous or glassy in naturE. Thus, the often used description of glass as a “supercooled liquid” is, in the literal sense, inapplicable for these materials.
Two different methods were used for preparation of novolac phenolic resin nanohybrids containing carbon nanotube and silica/siloxane moieties. Oxidized carbon nanotube was modified with furfuryl alcohol and 3-(trimethoxysilyl)propyl methacrylate (MPS) to obtain CNTFASi. Carbon nanotube xerogel (CNTFAX) was then obtained by incorporation of CNTFASi into silica/siloxane network by using tetraethyl orthosilicate (TEOS). The first nanohybrid was prepared by addition of CNTFAX into the novolac phenolic resin matrix. Incorporation of (3-glycidyloxypropyl) trimethoxysilane-modified novolac phenolic resin, CNTFASi, and TEOS into silica/siloxane network results in the second nanohybrid. These two types of nanohybrids were compared from the viewpoint of thermal properties. Successful modification of carbon nanotube and novolac phenolic resin was proved by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis results. Molar ratios of furfuryl alcohol in CNTFA and MPS in CNTFASi are 597 and 108 μmol g⁻¹, respectively. Formation of silica/siloxane xerogel network in CNTFAX was evaluated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that nanotubes with smooth surface are incorporated into the spongy silica/siloxane matrix. Char residue of CR is decreased from 58.2 to 50.6% by the addition of only 4 mass% of CNTFAX. Finally, char residue of 71.1% for the nanohybrid of modified novolac resin with 4 mass% of CNTFASi shows 12.8% increase in char residue with respect to the neat cured resin.
Formation of silicon carbide in the Acheson process was studied using a mass transfer model which has been developed in this study. The century old Acheson process is still used for the mass production of silicon carbide. A heat resistance furnace is used in the Acheson process which uses sand and petroleum coke as major raw materials. It is a highly energy intensive process. No mass transfer model is available for this process. Therefore, a mass transfer model has been developed to study the mass transfer aspects of the process along with heat transfer. The reaction kinetics of silicon carbide formation has been taken from the literature. It has been shown that reaction kinetics has a reasonable influence on the process efficiency. The effect of various parameters on the process such as total gas pressure, presence of silicon carbide in the initial charge, etc. has been studied. A graphical user interface has also been developed for the Acheson process to make the computer code user friendly.
The metal-based catalysts have been playing a major role in various industrial processes, whereas carbon based nanomaterials have recently been demonstrated to be promising metal-free alternatives for low cost catalytic processes. The doping of nitrogen in carbon and the electrocatalytic properties of the nitrogen doped carbon for oxygen reduction reactions are investigated in this article. We propose a simple method for doping nitrogen in various carbon materials like carbon black and ketjen black, and comparatively studied their physical and electrochemical characteristics. Raman and XPS analyses show significant peak shifts of their characteristic peaks caused by nitrogen doping on the surface of carbon materials. N-doped carbons show higher surface area in Brunauer, Emmett and Teller surface area analysis and more porous structure than undoped carbons in scanning electron microscope images. The N-doped sample was also ball milled to study the surface properties. All samples, i.e. undoped, N-doped and N-doped ball-milled carbons were compared for electrocatalytic activity by cyclic voltammetry and oxygen reduction reaction measurements. Nitrogen doped carbons exhibit better electrocatalytic activity with high mass activity and positive shift of peak potential than undoped samples with electron transfer number of 3.6 close to that of commercial Pt/C.
This document deals with the characterization of porous materials having pore
widths in the macropore range of 50 nm to 500 μm. In recent years, the development of
advanced adsorbents and catalysts (e.g., monoliths having hierarchical pore networks) has
brought about a renewed interest in macropore structures. Mercury intrusion–extrusion
porosimetry is a well-established method, which is at present the most widely used for determining
the macropore size distribution. However, because of the reservations raised by the
use of mercury, it is now evident that the principles involved in the application of mercury
porosimetry require reappraisal and that alternative methods are worth being listed and evaluated.
The reliability of mercury porosimetry is discussed in the first part of the report along
with the conditions required for its safe use. Other procedures for macropore size analysis,
which are critically examined, include the intrusion of other non-wetting liquids and certain
wetting liquids, capillary condensation, liquid permeation, imaging, and image analysis. The
statistical reconstruction of porous materials and the use of macroporous reference materials
(RMs) are also examined. Finally, the future of macropore analysis is discussed.
The pyrolysis of carbon/phenolic composites is a critical step in the manufacture of high temperature carbon/carbon components. Understanding the reaction kinetics of the pyrolysis reactions is essential for advancement in the processing of carbon/carbon materials. To increase the understanding of the reaction kinetics, Fourier transform infrared spectroscopy (FTIR) and gas product evolution analysis have been used to investigate the pyrolysis reactions of a carbon/phenolic composite system. Published mechanisms for pyrolysis of neat phenolic resin are evaluated to clarify discrepancies that exist between mechanisms and determine their applicability to a composite system. A mechanism for the pyrolysis of carbon/phenolic composites is proposed, which describes the resin pyrolysis reaction as occurring in three major reaction regions: formation of additional crosslinks, breaking of the crosslinks, and stripping of the aromatic rings.
Nuclear magnetic resonance (NMR) has been long recognized as a powerful tool for the chemical and physical investigation of both organic and inorganic substances. If in the more remote past high-resolution NMR studies were almost entirely confined to liquid substances or solutions, the advent of high-resolution solid-state methods in the 1970s made solid-state NMR an equally widespread and useful spectroscopic technique [1,2]. As such, NMR has been used for the study of many forms of carbon-based materials, including graphite, diamond, fullerenes, coals, pitches, cokes, and chars. By far the most commonly used approach in these studies involves the recording of¹H and¹³C NMR spectra, obviously due to the large contents of these dominant elements in carbon materials. But other nuclei such as⁷Li,¹⁴N,¹⁷O,²³Na,²⁹Si,³¹P, and¹²⁹Xe, among others, have been used as local probes of chemical and structural features in specific investigations as well.
Written by distinguished researchers, the long-running Chemistry and Physics of Carbon series provides a comprehensive and critical overview of carbon materials in terms of molecular structure, intermolecular relationships, bulk and surface properties, and their behavior in current and emerging applications. Volume 31 not only retains the high-quality content and reputation of previous volumes, but also complements them with reliable and timely coverage of the latest advances in the field. Maintaining the high level established by its predecessors, this book contains a prestigious and authoritative series of review chapters covering both chemistry and physics of carbon. The book examines properties and behavior of carbon materials ranging from coal to graphite, from activated carbons, chars, cokes, and carbon blacks to carbon fibers, fullerenes, nanotubes, and graphene. It complements previous volumes in the series by presenting updated information on ‘disordered’ carbons, a complex field that impacts nearly all aspects of carbon materials research. It includes a chapter on novel methods of characterization of carbon materials using ever more powerful techniques, as well as a chapter on the use of carbon materials in photocatalysis, a fast-moving and potentially exciting application. Emphasizing key experimental results and practical aspects, as well as important theoretical issues in every chapter, Volume 31 is a vital resource for those engaged in developing new technologies in a wide range of applicability of traditional and novel carbon materials from drug delivery to energy storage and conversion.
The Raman spectra of crystalline graphite, graphite damaged by ion-etching, and structurally disordered pyrolytic and glassy carbons were examined as a function of excitation wavelength λ over the range 488.0 to 647.1 nm. The bands located at 1360, 2720 and 2950 cm 1 with λ = 488.0 nm undergo a progressive red-shift as λ increases, while the positions of the other major bands (1580 and 1620 cm 1) are invariant. The significance of these observations is briefly discussed.
The ultrastructural control of materials through sol-gel processes offers significant promise for the achievement of reliable performance in glass, glass-ceramics, ceramics and composites. This will be attainable only if the fundamental chemistry of the sol-gel process is understood. Several examples based on this approach will be presented for optical glass, structural ceramics and electromagnetic materials. New concepts, such as ceramic or glass molecular composites and optically active gels, will be discussed. It will be shown how these new concepts are derived from an understanding of polymer chemistry and chemical reactivity. Advanced sol-gel basic research directions and their prospects will be discussed.
Verification of reliability of mercury intrusion porosimetry results for natural soils by confronting a volume of pores filled with water at particular pF with volumes of corresponding pores determined by mercury porosimetry/particle density was discussed. It was stated that a positive value of the difference between the mentioned volumes constitutes the sufficient evidence of sample destruction during drying and/or mercury intrusion, while the negative value constitutes the necessary condition to state that there are no such changes. Some of our preliminary results and literature data were considered from this point of view. The comparison of respective volumes has led to the conclusion that one can assume the lack of sample structure changes during air- or oven-drying/mercury intrusion only in the case of sands.
Phenolic/silica ceramers were prepared by the sol-gel method. Carbon fiber reinforced phenolic/silica ceramer composites with high thermal resistance were fabricated. Tetraethyl orthosilicate (TEOS) was used as a monomer for sol-gel system. Different ratios of the sol-gel solutoins and phenolic resins were adopted and the resulting ceramers were used as matrices for carbon fiber reinforced composites. The mechanical and thermal properties of the fabricated composites were studied. The results show that the incorporation of inorganic ingredients into the phenolic resins will increase the thermal resistance of the fabricated composites but not affect the flexural strength of the carbon fiber reinforced phenolic/silica ceramer composites up to 60 wt%. The morphologies of the ceramer matrices were examined by SEM. SiO2 particles from at the gaps between fibers for higher inorganic contents.
A Raman spectroscopic study has been made of the surface layer structure of ion implanted carbon materials. The substrates used are two types of graphitic materials: highly oriented pyrolytic graphite (HOPG) sheets, and glass-like carbon (GLC) sheets Argon- or nitrogen-ion implantations into the substrates were performed at an energy of 150 keV. The surface layer structure has been investigated by means of laser Raman spectroscopy. The Raman spectra for HOPG have a single peak corresponding to graphite structure, and those for GLC have two peaks corresponding to graphite and long-range disordered graphite structures. It is found that the spectra for both ion implanted HOPG and GLC consist of four principal peaks. Two well-known peaks belong to graphite and long-range disordered graphite structures. The origin of two other new peaks will be considered to be as follows: One is corresponding to an amorphous-like structure with C=C bonding, and the other is corresponding to a kind of hydrogenated-like carbon with chains of alternate C=C and C-C bonds. The ratio of the amorphous-like structure among four structures increases with the increase in the fluence or with the decrease in the target temperature during ion implantation.
A review of the work involving the preparation, characterization and catalytic applications of aerogels and cryogels is given for the period elapsing from 1991 up to now. Environmental catalytic aerogel supports for high-temperature combustion as well as aerogel catalysts for water treatment are described in more detail because of the importance of the need to protect the purity of air and water by catalytic means.
A survey over the role of chemistry in sol-gel processing is given. The basic chemistry of the sol-gel process is complex due to the different reactivities of the network forming and the network modifying components and the wide variety of reaction parameters. Despite the important progress in the investigations of the mechanisms of hydrolysis and condensation, a direct relation of reaction parameters to material properties is still very difficult.
The fate of 0.5 M tetraethyl orthosilicate (TEOS) in HNO3 solutions (pH 2−3) was followed by “in situ” ATR−FTIR spectroscopy. ATR spectra with excellent signal-to-noise ratios were obtained with collection times of 2 min. A qualitative examination of spectra measured at different reaction times showed that TEOS was first converted into ethanol and Si(OH)4, and then amorphous SiO2 was precipitated from the solution. The TEOS hydrolysis rate increased with decreasing pH of the nitric solution. In addition, under reaction conditions that make the system homogeneous (stable micelles, solution, or stable suspension), the use of standards enabled measurement of concentrations of the reactant products: EtOH, Si(OH)4, and SiO2. Calibration curves for EtOH in water and in methanol, for TEOS in ethanol, and for SiO2 in water fit equations of the form y = bx, with R2 values between 0.999 and 0.99. The mass balance on the acidified TEOS/water systems was good: initial concentrations of 0.56 F of TEOS resulted in [EtOH] = 2.12 M and [SiO2] = 0.53 M.
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.
The effect of pH and of catalysts on the course of the hydrolysis and condensation of tetraethoxysilane (TEOS) in water—ethanol solution was studied with the aid of chromatography, potentiometry and gelation tests. Strong acids (HCl, HClO4, HNO3, H2SO4, p-toluenesulphonic acid), weak acids (Cl3CCOOH, (COOH)2, ClCH2COOH, CH3COOH, HCOOH) and LiOH were used as catalysts. The rate of hydrolysis depended on the pH of the solution and not on the chemical structure of the catalyst. The hydrolysis was both acid and base catalysed and its rate was at a minimum at pH 7.0.The rate of condensation of the reaction products of the hydrolysis of TEOS (water—ethanol solutions of ethoxyhydroxysiloxanes) was at a minimum at a pH of about 2.0. The condensation was both acid and base catalysed and was markedly accelerated by both HF and H3PO4.
The model and theoretical understanding of the Raman spectra in disordered and amorphous carbon are given. The nature of the G and D vibration modes in graphite is analyzed in terms of the resonant excitation of π states and the long-range polarizability of π bonding. Visible Raman data on disordered, amorphous, and diamondlike carbon are classified in a three-stage model to show the factors that control the position, intensity, and widths of the G and D peaks. It is shown that the visible Raman spectra depend formally on the configuration of the sp2 sites in sp2-bonded clusters. In cases where the sp2 clustering is controlled by the sp3 fraction, such as in as-deposited tetrahedral amorphous carbon (ta-C) or hydrogenated amorphous carbon (a-C:H) films, the visible Raman parameters can be used to derive the sp3 fraction.
The peak near 1150 cm-1 in the visible Raman spectra of poor quality chemical-vapor-deposited diamond is often used as the signature of nanocrystalline diamond. We argue that this peak should not be assigned to nanocrystalline diamond or other sp3-bonded phases. Its wave number disperses with excitation energy, its intensity decreases with increasing excitation energy, and it is always accompanied by another peak near 1450 cm-1, which acts similarly. This behavior is that expected for sp2-bonded configurations, with their smaller band gap. The peaks are assigned to transpolyacetylene segments at grain boundaries and surfaces.
Raman spectra are reported from single crystals of graphite and other graphite materials. Single crystals of graphite show one single line at 1575 cm−1. For the other materials like stress‐annealed pyrolitic graphite, commercial graphites, activated charcoal, lampblack, and vitreous carbon another line is detected at 1355 cm−1. The Raman intensity of this band is inversely proportional to the crystallite size and is caused by a breakdown of the k‐selection rule. The intensity of this band allows an estimate of the crystallite size in the surface layer of any carbon sample. Two in‐plane force constants are calculated from the frequencies.
Phenolic resin/silica hybrid ceramers were prepared through sol–gel technology. Differential scanning calorimetry and thermogravimetric analysis methods were utilized to study the thermal properties of the fabricated hybrid ceramers. The results showed that the heat resistance of the ceramers was slightly higher than that of the phenolic resin. The hydrogen bonding occurring inside the hybrid ceramers was investigated by Fourier transform infrared. The results showed that the intermolecular hydrogen bonding between the phenolic resin and the silica was stronger than the intramolecular hydrogen bonding between the phenolic resin molecules themselves. Furthermore, the hybrid ceramers were utilized to fabricate carbon-fiber-reinforced composites. The fabricated ceramer composites possessed better flexural strength and flexural modulus than that fabricated from neat phenolic resin. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1699–1706, 2000
13C high-resolution solid-state nuclear magnetic resonance (NMR) was employed to study carbon materials prepared through the thermal decomposition of four different organic precursors (rice hulls, endocarp of babassu coconut, peat, and PVC). For heat treatment temperatures (HTTs) above about 600°C, all materials presented 13C NMR spectra composed of a unique resonance line associated with carbon atoms in aromatic planes. With increasing HTT, a continuous broadening of this resonance and a diamagnetic shift in its central frequency were verified for all samples. The evolution of the magnitude and anisotropy of the magnetic susceptibility of the heat-treated carbon samples with HTT explains well these findings. It is shown that these results are better understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite, for which the magnetic susceptibility effect is also present and is much more pronounced.
Novel phenolic/silica hybrid ceramers were synthesized by the sol-gel process. FTIR and 29 Si NMR were used to characterize the structure of the hybrids. The results revealed that Q 4 , Q 3 andT 3 are the major microstructures. That is, network structures were formed. SEM, TEM and Si mapping revealed that the hybrids were nanocomposites. The thermal properties were investigated by thermogravimetric analysis (TGA). The char yields of the hybrids increased with TEOS content. T d5 (the degradation temperature at 5% weight loss) of the hybrid that contained 20 wt.% TEOS was 290 C. The T d5 of the hybrid rose to 312 C as the TEOS content was increased to 80 wt.%. TEOS inorganic components enhance the thermal stability of the hybrids. The limiting oxygen index (L.O.I.) and the UL-94 test revealed that the hybrid ceramer possesses excellent flame retardance.
This article describes the preparation of novolac-type phenolic resin/silica hybrid organic-inorganic nanocomposite, with a sol- gel process. The coupling agent was used to improve the interface between the organic and inorganic phases. The effect of the structure of the nanocomposite on its physical and chemical properties is dis- cussed. The coupling agent reacts with the resin to form covalent bonds. The structure of the modified hybrid nanocomposites was identified with a Fourier transform infrared spectroscope. The silica network was characterized by nuclear magnetic resonance imaging (29Si NMR). Results revealed that Q4 (tetrasubstituted) and T3 (trisubstituted) are the dominant microstructures. The size of the silica in the phenolic resin was characterized with a scanning electron microscope. The size of the particles of inorganic silica in the modified system was less than 100 nm. The nanocomposite exhibited good transparency. Moreover, the thermal and mechanical properties exhibited significant improvement. The modified hybrid composite exhibited favorable thermal properties. The temperature at which a weight loss of 5% occurred increased from 281 to 350 °C. The flexural strength increased by 6 -30%. The limiting oxygen index of the nanocom- posite reached 37, and the Underwriters Laboratory test was 94V-0. Consequently, these materials possess excellent flame-retardant properties. © 2003 Wiley Periodicals,
Thermodynamic and kinetic conditions for the formation of SiC whiskers are established. The mechanism of their nucleation and growth are studied and, on this basis, the magnitude of the thermally activated barrier is determined from the rate of reduction data. The microstructures of whiskers are analysed and the role of interfacial tension between the nuclei and impurities, and the metallic iron catalyst is studied in relation to the formation of SiC whiskers. A possible reason for polytypism in SiC whiskers is also proposed.
Molecular separation using membranes is widely considered as an energy-efficient alternative for conventional industrial separation
techniques. For the preparation of such membranes sol-gel technology is highly suitable. Using sol-gel techniques thin (50–100
nm) amorphous nanoporous layers having pore sizes in the micropore (<2 nm) or fine mesoporous (<5 nm) region can be prepared
on a porous substrate. These layered porous systems, usually in tubular form, can be used for nanofiltration, pervaporation
and gas separation applications. The application window is dependent on the material properties, such as the pore size and
pore size distribution, the interfacial properties of the pores, and the defect density. The success of this technology in
actual industrial applications strongly depends on reproducible large scale production of the sol-gel membranes and on a sufficient
stability of the membranes with respect to flux and selectivity. In addition the production cost of the full membrane system
is an important aspect. Here, we will focus on the more chemical aspects in the membrane preparation. Main topics are synthesis
and properties of the sols, the preparation of microporous thin films, and the search for membrane materials that have a high
hydrothermal stability.
Basic research on the formation of monolithic gel as the precursor of oxide glass has been done concerning the hydrolysis and gelling of silicon tetramethoxide and the properties of dry gel. Disks of dried gel were prepared without fracture by adding the necessary amount of water for hydrolysis into a methanol solution of silicon tetramethoxide, followed by drying the formed gel while keeping the vapour pressure of the methanol under control. The dried gel was porous with micropores ranging from 15 to 100 in diameter. It had a specific surface area of more than 300 m2 g–1 and about 30% porosity. Monolithic gel was also prepared by using dilute ammonia water for hydrolysis. The time taken to gel became shorter with increasing ammonium ion concentration, but resulted in a gel of coarser structure. Dehydration polymerization of 60 to 75% occurred at room temperature. The polymerization was more pronounced in gels made with higher ammonium concentrations, but the amount of unreacted ethoxy group was also larger in these gels.
The porosity (i.e., pore volume, pore size, and surface area) of ceramic materials prepared by sol-gel processing depends on the size and structure of primary particles or polymers formed by condensation reactions, the organization of these structures, often by aggregation, to form a gel, and the collapse of the gel by drying. This paper reviews these ideas in the context of the formation of thin films suitable for inorganic membranes and introduces a number of specific strategies designed to control pore sizes in the range appropriate for gas separation: (1) aggregation of fractals; (2) management of capillary pressure, (3) control of condensation rate, and (4) the use of organic or microporous templates in composite thin film structures. These strategies are contrasted with the more traditional particle packing approach to preparing controlled porosity materials.
On pyrolysis of a phenol-formaldehyde resin at temperatures up to 700°C there was no change in the infra-red spectrum below 300°C but above this temperature the hydroxyl absorption began to decrease. This and other changes indicated the formation of diphenyl ether type linkages between benzene nuclei with the elimination of water. Above 400°C the changes which occurred in the spectra suggested that dibenzyl ether structures decompose to benzylphenyl ethers and that xanthene or diphenylene oxide type oxygen groups are probably formed. Some methylene bridges decompose, forming carbonyl or methyl groups, which reach a maximum content at 500–550°C Above 500°C the diphenyl ether type linkages decompose rapidly and poly-substitution of benzene nuclei increases. From 600°C onwards the aromatic hydrogen also begins to be eliminated and the spectra become structureless, perhaps because of the condensation of aromatic nuclei. The oxygen left at this stage may be present as inner-ring oxygen, as in diphenylene oxide.
A brief account of the methods generally employed for the synthesis of metal alkoxides has been presented. Successful elucidation of the structural features of simple alkoxides on the basis of a coordination model involving alkoxy bridges has prompted a detailed study of a variety of bimetallic alkoxides during the last two decades followed by the extension of the work to a novel series of ter- and tetra-metallic alkoxides during the last two years. Most of these polymetallic alkoxides (including derivatives of strongly electropositive alkali, alkaline earth metals and lanthanons) are stable to heat and can be volatilized unchanged.After listing the typical properties and reactions of these alkoxy derivatives, their hydrolytic reactions are briefly discussed. The uses of metallic alkoxides for ceramic materials and the potential applications of bimetallic alkoxides in this direction are discussed.
The weight of a phase in a mixture is proportional to the product of the scale factor, as derived in a multi-component Rietveld analysis of the powder diffraction pattern, with the mass and volume of the unit cell. If all phases are identified and crystalline, the weight fraction W of phase p is given by (Figure presented.) where S, Z, M and V are, respectively, the Rietveld scale factor, the number of formula units per unit cell, the mass of the formula unit and the unit-cell volume. This is the basis of a method providing accurate phase analyses without the need for standards or for laborious experimental calibration procedures. The method is demonstrated by measurements on binary mixtures of rutile, corundum, silicon and quartz.
A "teardown" method to create large mesotunnels (approximately 9 nm) on the pore walls of ordered mesoporous silicas is demonstrated by digesting the organic constituents from polymer-silicate nanocomposites. The ordered mesostructured polymer-silicate composites were first obtained via the evaporation-induced triconstituent co-assembly method by using a low-molecular-weight phenolic resin (resols) as an organic precursor; prehydrolyzed TEOS as an inorganic precursor, and triblock copolymer F127 as a template. All of organic components including F127 and phenolic resins are removed by the microwave digestion (MWD) method from mesostructured polymer-silica composites. While the removal of triblock copolymer F127 generates main pore channels, the phenolic resins can also be torn down from the pore walls, yielding mesotunnels between the channels. The resulting silica products exhibit ordered 2-D hexagonal mesostructure, large pore volume (up to 1.92 cm(3)/g), and very large pore size (up to 22.9 nm), which is even larger than their mesostructural cell parameter (14.2 nm). TEM images confirm the existence of mesotunnels on the silica pore walls. FT-IR and (29)Si solid-state NMR results reveal that these silica products have a large number of silanol groups.
A rigorous theory for chemical shifts in crystal is developed. In this formalism, it is clearly seen that the shielding tensor is divided into a demagnetizing term and a microscopic term. The former is caused by the demagnetizing field and is proportional to the bulk susceptibility. The latter is caused by a periodic current, which is, in the atomic limit, reduced to the ordinary expression for the shielding tensor by Ramsey. A new expression for the shielding tensor for the on-site approximation is derived based on the rigorous theory. The present theory is applied to graphite, and it is shown that the temperature-dependent part of the observed shift is due to the demagnetizing term.
Diamond and diamondlike thin films produced by various chemical-vapor-deposition processes have been examined using Raman spectroscopy. These films exhibit features in the Raman spectra, suggesting that they are composites of crystalline and amorphous diamond and graphitic structures. The components of this composite structure that contribute to the Raman scattering are discussed in terms of sp2- and sp3-bonded structures. The use of Raman spectroscopy as a technique for estimating the sp2-to-sp3 bonding ratio is considered. Powder composites of BN-diamond and graphite-diamond have been studied as a means of modeling the films, and a simple theoretical model of the Raman scattering from these samples is proposed. From these results it is shown that it is necessary to make assumptions about the domain size of the graphitic sp2 regions. It is found that the Raman scattering associated with sp2 bonding in the films is much stronger than that from single-crystalline or microcrystalline graphite structures. Shifts of the vibrational modes are also observed. The optical and vibrational properties of the sp2 component in the films implies a different atomic microstructure. A model of the sp2-bonding configurations in the films is proposed which may account for the observed features in the Raman spectra.
Synthesis of texturally biphasic mesoporous carbon-silica composites and carbons
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