One-step synthesis of prolate spheroidal-shaped carbon produced by the thermolysis of octene under its autogenic pressure

The dependence of the macroscopic shape of pure carbon on the precursor structure is observed using Reactions under Autogenic Pressure at Elevated Temperatures (RAPET) for the thermal dissociation of several precursors, including stearic acid, oleic acid, linoleic acid, methyl 3 butenoate, methyl butyrate, octadecane, octadecene, octane, octene and acrolein. The precursors are dissociated under their autogenic pressure developed at 700 °C to create a range of pure carbon microstructures. Prolate spheroidal-shaped carbon (PSSHC) is prepared by heating octene, among others, at 700 °C in a closed cell in a one-step process. The dimensions of the carbon bodies were 3–5 μm for the polar diameter and 6–8 μm for the equatorial diameter. Obtaining the PSSHC from octene is in contrast to a previous work, which required a long hydrocarbon chain and the presence of oxygen for the formation of PSSHC upon thermolysis under identical autogenic pressure. The products of the RAPET reaction of octene showed a strong ESR signal, resulting from nonbonding dangling electrons on the carbon surface. However, treating the carbons with a beam of hydrogen atoms has almost completely eliminated the ESR signal. The question why the thermal dissociation of some precursors yield PSSHC, while other precursors yield other morphologies, is discussed.     

On the nature of simultaneous formation of nano- and micron-size diamond fractions under pressure–temperature-induced transformations of binary mixtures of hydrocarbon and fluorocarbon compounds

Based on comparative studies of pressure–temperature-induced transformations of naphthalene, octafluoronaphthalene, and their binary mixtures, the nature of formation of nano- and micron-size diamond fractions in the products of transformations of hydrocarbons and fluorocarbons has been revealed. It was found that the main reason for the massive formation of nano-size diamonds is the specifics of carbonization of fluorocarbon compounds under pressure. In this particular process, micron-size particles of graphite are formed simultaneously with a significant amount of closed two- to five-layered carbon nanoparticles of 5–15-nm size, acting as precursors for the formation of nano-size diamond fractions. The obtained results open up a new avenue for the metal catalyst-free synthesis of nano/micron-size fractions of pure and doped diamonds.     

The role of microstructure on high-temperature oxidation behavior of hafnium carbide

Microstructure development of the products formed upon oxidation of hafnium carbide (HfC_x, x = 0.65, 0.81, or 0.94) at 1300°C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO₂C_x that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase-separated region vary from cubic to monoclinic as grains coarsen from ca. 2–20 nm, respectively. Immediately adjacent to the phase-separated region is carbon-free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high-temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO₂C_x product.     

Synthesis of ferromagnetic nanopowders from iron pentacarbonyl in capacitively coupled RF plasma

Ferromagnetic nanopowders (NP), synthesized with magnetically decoupled cores, may represent interesting raw materials for their further conversion into nanostructured magnetic bulk materials, with properties suitable for their utilization in low and high-frequency applications. Capacitively coupled RF discharges at a pressure of 100-250 Pa, were used for the synthesis of binary iron/carbon NP from iron pentacarbonyl (Fe(CO)₅) vapours, diluted with argon. The properties of the obtained NP products were investigated by TEM, XRD, TGA, FTIR and magnetization measurements. Diamagnetic NP, with ≈4% iron content, were obtained at lower pressures (100-130 Pa), whereas increasing the pressure to 250 Pa resulted in the synthesis of ≈40% iron content NP that demonstrated ferromagnetic properties (M_s ≈ 50 emu/gr). The first type of these NP is formed of carbon/iron rods, being 10-50 nm long and several nm wide, covered by graphitic layers and integrated into an amorphous carbon matrix. The second NP type, ferromagnetic NP, consists of isolated hexagonal shaped 20-60 nm crystals of Fe₇C₃ or Fe₂C₃ carbides, again embedded in an amorphous carbon matrix. Factors for favoring the formation of the ferromagnetic carbide NP at the higher pressure are discussed. These factors include: the lower temperatures presumed in the reaction zone, the shifting of the chemical equilibrium from non-bound Fe and its oxides towards the state of carburization, and the decreasing rate of removal of the Fe oxidation products towards the reactor walls.     

Chemical vapor infiltration of carbon fiber felt: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 °C and methane pressures from 5 to 30 kPa to confirm the inside-outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside-outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 °C and up to 13.5 kPa at 1095 °C, leading to bulk densities above 1.9 g/cm³. A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 °C. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures.     

The dependence of the oriented growth of carbon filaments on the intensity of a magnetic field

Carbon filaments have been obtained by the thermal decomposition of mesitylene in the presence of a strong magnetic field. The shape dependence of the pristine carbon filaments on increasing the magnetic field is demonstrated. The proposed mechanism for filament growth involves the formation of a colloid-like system during dehydrogenation and dissociation of mesitylene. The imposed orientation of graphitic clusters (basic structural units) in the strong magnetic-field forces those located at the curved surfaces forming the tips of the filaments to expose their reactive edges, facilitating the observed preferential growth of filaments.     

Control of diameter distribution of single-walled carbon nanotubes using the zeolite-CCVD method at atmospheric pressure

Single-walled carbon nanotubes (SWCNTs) have been synthesized on zeolite powder with Fe/Co catalysts by a catalytic chemical alcohol-vapor deposition (CCAVD). We have first used a cold wall reactor at the atmospheric pressure, the system having been modified for the zeolite-CCAVD specifications by the use of radio-frequency heating. The G/D ratio (∼25), estimated by analysis of Raman spectroscopy, obtained here is equivalent to that by the conventional CCAVD method under reduced pressure, indicating the high purity of the present specimen. The estimated diameter distributions of the SWCNTs obtained at synthesis temperatures of 900, 1000 °C and constant ethanol temperature of 0 °C are 0.9-1.8 and 1.2-2.2 nm, respectively, whereas that of synthesized at synthesis temperature of 900 °C and ethanol temperatures of 40 °C ranges form 0.8 to 1.4 nm. The diameter distribution shifts towards larger diameters as the synthesis temperature is increased and the carbon supply rate (ethanol temperature) decreases, from which we suggest a selective growth model due to a competition between deposition and etching of carbon atoms.     

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.     

A simple route for the synthesis of coral-like accretion of hollow carbon microspheres with thin walls

Coral-like accretion of hollow carbon microspheres with thin walls has been successfully synthesized through a carbon dissolution-precipitation process, where the decomposition of cyclohexane on nickel powder, at 550 °C in a sealed system, and is followed by rapid cooling. The final product is tens of microns in size with smooth and uniformly thin shell thickness of approximately 3-4 nm. XRD, FESEM, EDX, TEM, HRTEM and Raman spectra were used to characterize the products. The effects of reaction temperature, type of carbon source and cooling process on the formation of the final products have been studied, and a possible formation mechanism for the coral-like product has been proposed. This coral-like accretion of large capacity hollow carbon microspheres has potential applications as a catalyst carrier, in storage or protection of unstable organic materials and microreactors.     

Synthesis and characteristics of carbon encapsulated magnetic nanoparticles produced by a hydrothermal reaction

A novel process to synthesize carbon encapsulated magnetic nanoparticles was developed by heating an aqueous glucose solution containing Fe@Au (Au coated Fe nanoparticles) or Ni nanoparticles at 160–180 °C for 2 h. In comparison with traditional methods, such a hydrothermal approach is not only simple but also able to provide functional groups such as –OH on the surface of carbon sphere. Only pure Fe nanoparticles did not favor the formation of carbon encapsulated magnetic nanoparticles due to the oxidation of Fe nanoparticles by H₂O during the reaction and their surfaces had to be coated by an Au shell in advance. The results of TEM, HRTEM, XRD, XPS and vibration sample magnetometer characterization show that uniform carbon spheres containing some embedded Fe@Au nanoparticles with a saturation of 14.6 emu/g are obtained and the size of a typical product is 200 nm. Carbon encapsulated Ni nanoparticles have been successfully prepared in the same way.     

Nanosized carbon forms in the processes of pressure-temperature-induced transformations of hydrocarbons

The products of thermal conversions of naphthalene, anthracene, pentacene, perylene, and coronene at 8 GPa in the temperature range up to 1300 °C have been studied by scanning electron and high-resolution transmission electron microscopies. As a result, it has been established that various nanometer-sized carbon species (spherical and coalesced two-core onion-like carbon particles, faceted polyhedral particles, graphitic ribbons, graphitic folds, and nanocrystalline diamonds) are present in the conversion products together with micron-sized crystallites of graphite and diamond.     

Toxicity of the pyrolysis and combustion products of poly(vinyl chlorides): A literature assessment

Poly(vinyl chlorides) (PVC) constitute a major class of synthetic plastics, Many surveys of the voluminous literature have been performed. This report reviews the literature published in English from 1969 through 1984 and endeavors to be more interpretive than comprehensive. PVC compounds, in general, are among the more fire resistant common organic polymers, natural or synthetic. The major products of thermal decomposition include hydrogen chloride, benzene and unsaturated hydrocarbons. In the presence of oxygen, carbon monoxide, carbon dioxide and water are included among the common combustion products. The main toxic products from PVC fires are hydrogen chloride (a sensory and pulmonary irritant) and carbon monoxide (an asphyxiant). The LC₅₀ value calculated for a series of natural and synthetic materials thermally decomposed according to the NBS toxicity test method ranged from 0.045 to 57 mg l^?1 in the flaming mode and from 0.045 to > 40 mg l^?1 in the non-flaming mode. The LC₅₀ results for a PVC resin decomposed under the same conditions were 17 mg l^?1 in the flaming mode and 20 mg l^?1 in the non-flaming mode. These results indicate that PVC decomposition products are not extremely toxic when compared with those from other common building materials. When the combustion toxicity (based on their HCI content) of PVC materials in compared with pure HCI experiments, it appears that much of the post-exposure toxicity can be explained by the HCI that is generated.     

The thermal and mechanical properties of a polyurethane/multi-walled carbon nanotube composite

A polyurethane/multi-walled carbon nanotube elastomer composite was synthesized. The microstructure of the composite was examined by field-emission scanning electron microscopy and transmission electron microscopy. The thermal and mechanical properties of the composite were characterized by dynamic mechanical thermal analysis, thermogravimetric analysis and tensile testing. The chemical linkage of carbon nanotubes with polyurethane matrix was confirmed by Fourier transform infrared spectra. The study on the structure of the composite showed that carbon nanotubes could be dispersed in the polymer matrix well apart from a few of clusters. The results from thermal analysis indicated that the glass transition temperature of the composite was increased by about 10 °C and its thermal stability was obviously improved, in comparison with pure polyurethane. The investigation on the mechanical properties showed that the modulus and tensile strength could be obviously increased by adding 2 wt% (by weight) CNT to the matrix.     

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.     

Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

The polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high crystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to ∼1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5-40 μm single crystals. The p,T values of diamond formation here obtained are significantly lower than those previously known for direct graphite-diamond transformation.     

Intergranular amorphous films formed by DC electric field in pure zirconia

It is difficult to obtain pure ZrO₂ sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO₂ can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO₂ flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.     

Thermisches Hydrocracken von Kohlenwasserstoffen

Thermal hydrocracking of hydrocarbons . Thermal hydrocracking or hydropyrolysis processes are currently under development in various modifications for upgrading of heavy oils. The present investigation examines thermal hydrocracking of pure model hydrocarbons. Typical petroleum hydrocarbons, i.e., alkanes, naphthenes, and aromatics were employed. At elevated conversions, alkanes and naphthenes are transformed predominantly into C₁-through C₃-alkanes. Ethane is the main product. In alkylaromatics the side chain is cracked exclusively up to high conversions. Therefore, dealkylated aromatics can be obtained in high yields as intermediate products.     

Gaseous Products formed by γ-Irradiation of Poly(l,4-Butylene Terephthalate), Poly(Ethy1ene Terephthalate) and Poly(Ethy1ene 2,6-Naphthalenedicarboxylate)

Summary A comparative investigation of gaseous products formed by γ-irradiation of poly( 1,4-butylene terephthalate), PBT; poly(ethylene terephthalate), PET; and poly(ethylene 2,6-naphthalene-dicarboxylate), PEN has been performed. These polyesters were irradiated under vacuum at doses from 0.5 to 4 MGy. The gases formed were analyzed by gas chromatography – mass spectrometry, and their radiation-chemical yields (G) were calculated. Hydrogen, carbon monoxide and carbon dioxide, as the main products, and methane of low quantity were found after radiolysis of all polyesters studied. Other hydrocarbons such as ethane, propane, propene and n-butane were also detected in gases mixture resulted from irradiation of PBT. Traces of ethane were also registered in the radiolysis products of PET. Irradiation of PEN did not lead to formation of hydrocarbons other than methane. The total G values and compositions of the formed gaseous mixtures depended on chemical structure of the irradiated polyesters. Mechanism of gaseous products formation has been discussed. Received: 17 October 2002/Revised version: 5 February 2003/Accepted: 6 February 2003 RID="*" Correspondence to Roustam Aliev     

Interaction of a phosphorus‐based FR,a nanoclay and PA6. Part 2 interaction of the complete PA6 polymer nanocomposites

Polyamide 6 (PA6) is modified with a nanoclay (NC), Cloisite 30B and/or a flame retardant (FR), OP1311. The thermal decomposition of pure PA6 and PA6 nanocomposites is done by thermogravimetric analysis (TGA). The decomposition products from TGA in nitrogen and air are analysed online by Fourier transform infrared (FTIR) spectroscopy in order to examine the time/temperature‐dependent thermal degradation processes and monitor the evolved gases online. The profiles of the evolved gases are compared with ε‐caprolactam spectra, which are the main species in the gas phase. Results show that the addition of the fire retardant decreases the degradation temperature, whereas the incorporation of NC (PA6+NC) contributes to increased residual mass and char formation. The evolved gases from TGA‐FTIR in nitrogen from pure PA6 and (PA6+NC) are hydrocarbons, carbon dioxide, water, ε‐caprolactam and ammonia. The (PA6+FR) and (PA6+NC+FR) evolve the same volatiles with an additional phosphorus‐containing species, namely diethylphosphinic acid. The thermo‐oxidative degradation of all these composites in air yields carbon monoxide with an increased production of carbon dioxide, water and hydrogen cyanide. Another important result is that the hydrogen cyanide does not increase when the phosphinate FR is used. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational Character of the Hydrogen-Oxidation Process Initiated by a Pulsed Electron Beam

Results of an experimental study of kinetics of hydrogen oxidation in a stoichiometric mixture with oxygen under initiation of ignition by a pulsed electron beam in a closed steel reactor are described. It is shown that the process has an oscillatory character and resembles the regime of cold flames in oxidation of hydrocarbons. Possible mechanisms of self-induced oscillations of the hydrogen-oxidation process are analyzed. It is shown that pressure oscillations in the reactor cannot be related to formation of standing acoustic waves. The most probable reason is the nonisothermal character of the chain process and violation of conditions of chain development with increasing temperature.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 18–21, July–August, 2005.