Synthesis of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction

Ammonium ferric citrate (AFC) was used as a single-source molecular precursor to prepare Fe/Fe₃C nanoparticles encapsulated in nitrogen-doped carbon by pyrolysis in Ar atmosphere followed by acid-leaching. Comparative studies, using citric acid and ferric citrate as the precursors, indicated that the ammonia and ferric ion in AFC and the pyrolysis temperature affected the composition of iron species and the properties of carbon in AFC-derived materials. Above the pyrolysis temperature of 600 °C, the iron species were Fe/Fe₃C, and the carbon had a hollow graphitic nanoshell structure in AFC-derived materials. The specific surface area and content of nitrogen element decreased with increasing pyrolysis temperature. The AFC-derived material pyrolyzed at 600 °C had the optimal graphitization degree, specific surface area (489 m² g⁻¹) and content of nitrogen (1.8 wt.%), thus resulted in the greatest activity for oxygen reduction reaction among the AFC-derived materials pyrolyzed at different temperatures. The AFC-derived material pyrolyzed at 600 °C exhibited improved methanol-resistance ability compared with Pt/C catalyst.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

Sulphur transformation and removal for Western Kentucky coals

Inhibition isotherms were measured for Western Kentucky No.9 coal. Crushed and sieved coal (?25 + 140 U.S. mesh) was fluidized in 10-g batches in a 22-mm i.d. quartz reactor up to a temperature of 870 °C. The release of hydrogen sulphide during heatup under nitrogen and at the run temperature (usually 1–2 h) under the same gas (pyrolysis), hydrogen, or hydrogen/hydrogen sulphide mixtures was followed by gas chromatography. The residue or char was analysed for pyritic, organic, sulphide, sulphate, and total sulphur. Inhibition isotherms, which are pseudo-equilibria between sulphur in the char and gaseous hydrogen sulphide, were measured at 600 and 870 °C. At the lower temperature the isotherm was found to be independent of the hydrogen sulphide concentration in the gas stream and the char sulphur content remained constant at 2.6%. At 870 °C the sulphur content of the char was greater than that of the original coal when gas mixtures of 1, 3, and 6% hydrogen sulphide in hydrogen were used, indicating the necessity of maintaining low hydrogen sulphide concentration for sulphur removal. In pure hydrogen, sulphur removal increased continuously from 47% at 600 °C to 84% at 870 °C. For pyrolysis under nitrogen, sulphur removal was 40% at 600 °C and increased to 59% at 740 °C. No further removal occurred above this temperature up to 870 °C. In addition to the inhibition isotherms, sulphur-form transformation diagrams were constructed for coal treated with nitrogen, hydrogen, and hydrogen/hydrogen sulphide mixtures. Pyritic sulphur, which comprised 40% of the sulphur in the original coal, was completely converted to ferrous sulphide at 600 °C in hydrogen and 740 °C in nitrogen. At 870 °C the sulphur content of the char produced under hydrogen was 1.1% made up of 48.4% ferrous sulphide, 43.4% organic sulphur, and 8.2% sulphate.     

Conversion behaviour and resulting mechanical properties of polysilazane-based coatings

Polymer-derived ceramics exhibit a convenient route for the processing of low-dimensional ceramics like coatings or fibres. In previous investigations unfilled and composite coatings have been developed using ammonolysed bis(dichloromethylsilyl)ethane (ABSE) or perhydropolysilazane (PHPS) as precursors and BN, ZrO₂ or glass particles as filler materials. The coating systems provide excellent corrosion and oxidation resistance to underlying metals. This paper reports on the effect of the precursor system and the pyrolysis parameters on the conversion behaviour, shrinkage and mechanical properties, including hardness and Young's modulus, of ABSE- and PHPS-based coatings. Therefore the crosslinking and pyrolysis behaviour as well as the mechanical properties of the coatings were investigated up to pyrolysis temperatures of 1000 °C in nitrogen and in air by ATR-IR, SEM, profilometry and nanoindentation measurements. The coatings pyrolysed at 1000 °C in nitrogen, have hardness values of 13 GPa and Young's moduli up to 155 GPa.     

Curing temperature effect on smokeless fuel briquettes prepared with molasses and H3PO4

Chars from the co-carbonisation of a low-rank coal and olive stones have been used to prepare environmental acceptable smokeless fuel briquettes. The blend was a mixture of char, molasses and H₃PO₄. This acid was added to favour the polymerisation of the binder. The effect of the curing temperature on the physico-chemical features of the briquettes was studied by Fourier Transform Infrared Spectroscopy and Temperature Programmed Decomposition followed by Mass Spectrometry. The presence of H₃PO₄ as well as the curing process at 200 °C of temperature, contribute to the formation of carboxylic acids which lead to the production of briquettes with adequate mechanical properties.     

Recycling of the Products Obtained in the Pyrolysis of Fibre-Glass Polyester SMC

An SMC (Sheeting Moulding Compound) of fibre-glass and orthophthalic polyester has been pyrolysed in a 3·5 dm³ autoclave for 30 min in a nitrogen atmosphere at 300, 400, 500, 600 and 700°C. Gas yields of 8–13 weight%, liquid yields of 9–16 weight% and solid residues of 72–82 weight% were obtained. The suitability of the pyrolysis process for recycling SMC is discussed. The characteristics, compositions and possible ways of reusing the liquid and gaseous fractions are presented. The solid pyrolysis residue has been recycled in another thermoset composite (Bulk Moulding Compound, BMC) and its mechanical properties have been compared with those of virgin BMC. The main conclusion is that pyrolysis can be an appropriate method for recycling thermoset polymeric composites such as SMC. The pyrolysis gas fraction can be sufficient to provide the energy requirements of the process plant. The pyrolysis liquids have high gross calorific values (36·8 MJ kg⁻¹) and are non-polluting liquid fuels; about 40 weight% of such liquids could be used as petrols, and the remaining 60% could be mixed with fuel oils. The solid pyrolysis residue can be recycled in BMC with no detrimental effect on the BMC mechanical properties. Concerning temperature, it has been concluded that 400–500°C are the most suitable temperatures for recycling SMC by pyrolysis. © 1997 SCI.     

Effect of pyrolysis temperature on the mechanical evolution of SiCf/SiC composites fabricated by PIP

Three types of SiC_f/SiC composites with a four-step three-dimensional SiC fibre preform and pyrocarbon interface fabricated via precursor infiltration and pyrolysis at 1100 °C, 1300 °C, and 1500 °C were heat-treated at 1300 °C under argon atmosphere for 50 h. The effects of the pyrolysis temperature on the microstructural and mechanical properties of the SiC_f/SiC composites were studied. With an increase in the pyrolysis temperature, the SiC crystallite size of the as-fabricated composites increased from 3.4 to 6.4 nm, and the flexural strength decreased from 742 ± 45 to 467 ± 38 MPa. After heat treatment, all the samples exhibited lower mechanical properties, accompanied by grain growth, mass loss, and the formation of open pores. The degree of mechanical degradation decreased with an increase in the pyrolysis temperature. The composites fabricated at 1500 °C exhibited the highest property retention rates with 90% flexural strength and 98% flexural modulus retained. The mechanism of the mechanical evolution after heat treatment was revealed, which suggested that the thermal stability of the mechanical properties is enhanced by the high crystallinity of the SiC matrix after pyrolysis at higher temperatures.     

Thermally modified acrylic polymers as sorbents

Acrylic fibres were modified with ethylene diamine and pyrolysed at temperatures up to 600°C. According to IR spectroscopy, polymers pyrolysed at 300°C contain nitrile and other groups which are present in the initial polymer. The specific surface area is decreased by increasing the pyrolysis temperature. The pore size distribution reveals mainly the presence of macropores. Acrylic polymers and those pyrolysed at 300°C sorb more methylene blue and alizarin yellow from aqueous solution than a non-polar commercial sorbent of high specific surface area. The experimental results for dye sorption were treated theoretically using a diffusion-limited sorption method. The polymers can be ranked by the parameters of this method or their sorption ability. The dye sorption is little affected by the pores of the polymers and is mainly affected by the interactions between the functional groups of the polymers and those of the dyes.     

Electrical properties of amorphous SiCxNyHz-ceramics derived from polyvinylsilazane

Electrical properties of monolithic, amorphous SiC_xN_yH_z-ceramics derived from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisilazane via polymer pyrolysis were investigated from room temperature to 400 °C using impedance spectroscopy. Depending on the pyrolysis temperature T_p, the d.c. conductivity varies up to 8 orders of magnitude. The temperature dependence of samples pyrolysed at low temperatures (T_p=700–1200 °C) follows a Mott law, whereas samples pyrolysed at high temperature (T_p=1400 °C) show an Arrhenius dependence. Structural changes during pyrolysis were characterized by solid state magic angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy and X-ray diffractometry. NMR and especially Raman measurements indicate the formation of sp²-carbon atoms, which rearrange towards graphitic-like domains with increasing pyrolysis temperature. This observation can explain the related increase of the d.c.-conductivity.     

Microstructural and mechanical characterization of sol gel-derived Si–O–C glasses

The mechanical properties of three silicon oxycarbide glasses pyrolysed under inert (Ar) atmosphere were studied as a function of the pyrolysis temperature. The silicon oxycarbide glasses were prepared from various alkyl substituted alkoxysilanes such as HSi(OEt)₃ and HMeSi(OEt)₂ in different ratios by using the sol-gel method. The Si–O–C-glasses obtained were respectively: (i) silicon oxycarbide network with excess carbon, (ii) stoichometric SiC_xO_2(1−x) where x=0.30 and (iii) silicon oxycarbide matrix with an excess of Si. Si–C bonds introduced in the starting silica gel network can be partially retained in the final glass after pyrolysis under inert atmosphere. After pyrolysis at temperatures between 600–1500 °C, the presence of tetracoordinated C atoms in the silica network results in an improvement of mechanical properties and thermal stability compared with silica glass. By using elemental analysis, density, SEM, BET and XRD (combined with Rietveld-analysis), the glass characterization was performed. Flexural strength (MOR), elastic modulus (E) and Vickers hardness (H_V) were measured and will be discussed in terms of glass composition and microstructure.     

An Experimental Study on Binderless Briquetting of Low‐Rank Coals

The potential of binderless briquetting as a means of transforming low‐rank coals into low moisture high grade solid fuel products has been studied. Using two dried low‐rank coals, binderless briquettes with high mechanical strength have been successfully produced through mechanical compression. An increase in heating value was achieved as a result of moisture reduction in the briquettes compared to as‐received coals. The residue moisture content in the briquettes had a predominant effect on briquetting characteristics and there existed an optimum moisture content for the maximum briquettes strength. The chemical structure and wettability of binderless briquettes were analyzed using FTIR and contact angle measurement. The results showed that hydrophobicity and chemical structure significantly affected the briquette properties.     

Use of pyrolysed oil shale as filler in poly(ethylene‐co‐vinyl alcohol)

The pyrolysed oil shale (POS) obtained from the pyrolysis of bituminous rock was used as filler in poly(ethylene‐co‐vinyl alcohol) (EVAL). The effects of vinyl alcohol content in the EVAL and the particle size of pyrolysed oil shale in the mechanical properties were investigated. The EVAL was prepared by hydrolysis of poly(ethylene‐co‐vinyl acetate) (EVA) with 8 and 18 wt % of vinyl alcohol content. The composites were prepared in a rotor mixer at 180°C with concentration of pyrolysed oil shale up to 5 wt %. Stress–strain plots of compression‐molded composites showed a synergic behavior in the mechanical properties for low concentrations (1–5 wt %) of POS in all particle sizes and EVAL used. Such behavior indicates a close packing and strong interactions between the inorganic filler and the polymer. Increasing of the vinyl alcohol content of EVAL improved the compatibility between the polymer and filler, but decreasing the POS particle size had no effect on the properties. The modulus and the ultimate tensile strength also increased in all concentrations of POS for both EVAL. Mechanical properties and dynamic mechanical analysis also demonstrated the compatibility between EVAL and POS. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1658–1665, 2004     

Identification of the combustion behaviour of lignite char between 350 and 900 °C

The combustion behaviour of four different lignite samples pyrolysed at 800 and 1000 °C has been studied. The reaction order and the activation energy of the char combustion reaction can be easily determined from the time to reach 50% conversion. Under 550 °C the reaction order of lignite samples are 0.7 and the activation energies are 27.4–27.9 kcal gmole^?1. Above 550 °C the measurement of the intrinsic reaction rate is limited by the film diffusion resistance. The pyrolysis temperature affects the reactivity significantly. The reactivity of char pyrolysed at 800 °C is 3.5 times that of char pyrolysed at 1000 °C.     

Effect of ZrC content on the properties of biomorphic C–ZrC–SiC composites prepared using hybrid precursors of novel organometallic zirconium polymer and polycarbosilane

A novel organometallic zirconium polymer was synthesized through the copolycondensation using n-butyllithium, 1,4-diethynylbenzene, phenylacetylene and zirconium tetrachloride as raw materials. Then biomorphic C–ZrC–SiC composites were fabricated from corn stover templates by precursor infiltration and pyrolysis process using hybrid polymeric precursors containing the organometallic zirconium polymer and polycarbosilane. The microstructure, mechanical properties and oxidation resistance of the composites were investigated. With ZrC content increasing, the mechanical properties of the composites were enhanced due to dispersion strengthening and grain fining of the homogeneously dispersed ZrC nanoparticles. The oxidation behavior of C–SiC–ZrC indicated that the oxidation resistance of the composite was reduced at 1000 °C but improved at 1500 °C with the increase of ZrC content. The improved oxidation resistance was mainly attributed to a proper ZrC content, the formation of ZrSiO₄ layer on the surface of the composite, and its matrix microstructure characterized by a nano-sized dispersion of ZrC–SiC phases.     

Effectiveness of gases in desulphurization of coal

The effect of air, steam and hydrogen on the desulphurization of 10 U.S. high-volatile bituminous coals was investigated. Air treatment was most effective at 450 °C where an average of 38% total sulphur, comprising 51% of the inorganic sulphur and 20% of the organic sulphur, was removed. With steam at 600 °C, 61% of the total sulphur, 87% of inorganic and 25% of organic was lost. Hydrogen was not effective below 850 °C, but at 900 °C 86% of the total sulphur was dispelled, i.e. 94% of the inorganic and 76% of the organic sulphur. Without oxidative pretreatment the sulphur was much more difficult to remove; after oxidative pretreatment at 300 °C for 10 min followed by treatment with hydrogen at 900 °C, as much sulphur was removed in 4 min as in 60 min without the pretreatment. With raw coal, heating under nitrogen ‘cooked-in’ or fixed some of the sulphur making it more difficult to remove with hydrogen; whereas following oxidative pretreatment, heating for up to 1 h did not lessen the reduction of sulphur with hydrogen. For temperature-swelling coals with large quantities of organic sulphur, heating at 300 °C in air followed by reduction with hydrogen at 900 °C appears to permit rapid discharge (3–10 min) of the organic as well as the inorganic sulphur, to produce a smokeless product with a CV (per unit of product) similar to the fuel value of the untreated coal.     

Controlled pyrolysis of maleic hydrazide

Maleic hydrazide (MH) has been pyrolysed at 600°C in our controlled pyrolysis apparatus. During pyrolysis volatile compounds distil out ahead of the moving heated zone and are not extensively repyrolysed, while those substances which are absorbed by pyrolysand or are relatively nonvolatile are repyrolysed. Also, reactive intermediates can react with one another or with unpyrolysed material. Pyrolysis products determined include CO₂ (24 wt%), CO (2 wt%), HCN (3 wt%), NH₃ (9 wt%) and N₂ (3 wt%). The gases identified account for 41 % of the maleic hydrazide pyrolysed. An additional 50% of the material pyrolysed remains as a black solid having the approximate composition C₁₅H₁₅N₅O₂. Approximately 1 % of the maleic hydrazide is unchanged. Thus, the fate of 92% of the maleic hydrazide can be accounted for. A trace amount of hydrazine could be detected. The results obtained using controlled pyrolysis are compared with results obtained by us and others using different pyrolytic conditions.     

Study on char reinforcing of different inorganic fillers for polyethylene composites

Ceramifiable polymer composite is an excellent method for enhancing the flame retardancy of the materials. In case of fire, ceramifiable polymer composites are converted into a ceramic barrier that can impede heat transfer, segregate combustible material from oxygen and prevent the polymer matrix from pyrolysis. The ceramic fillers added to the composites play a key role in the ceramic forming process of the composites. Here, the effects of three types of Si-based mineral fillers on the ceramifiable and mechanical properties of polyethylene composites were investigated. Wollastonite (CaSiO₃), silica powder (SiO₂), or pyrophyllite (Al₂Si₄O₁₀(OH)₂) was incorporated into low-density polyethylene with glass powder to prepare the composite materials. The composites were subjected to sintering at various temperatures (600, 800, and 1000 °C). The ceramic residues formed were characterized by flexural strength, apparent porosity, linear shrinkage, and thermogravimetric measurements. All these three fillers greatly enhanced the ceramifiable properties of the polyethylene composites and the mechanical properties of the ceramic residues. X-ray diffraction analysis and scanning electron microscopy reveal that the filler molecules were firmly connected by the molten glass while the pores in the ceramic residues were released at elevated temperatures, leading to a dense ceramic structure with excellent mechanical properties.     

Alternating electrical conductivity of polyphenylenes and their pyrolysis residues after doping

o,m,p-Polyphenylenes were prepared by oxidation-cationic polymerization of biphenyl and then pyrolyzed up to 800°C. The o,m,p-polyphenylenes as well as their pyrolysis residues were doped with anhydrous FeCl₃ from their solution in acetonitrile. The real part of the alternating electrical conductivity σ′ of the undoped and doped materials was determined at room temperature. Decreasing σ′-values of pyrolyzed residues at 600°C without doping were observed because of the large weight losses during the pyrolysis leading to chain scission reactions within the polymer. At higher pyrolysis temperatures (i.e. 700°C, 800°C) the σ′-values are increased due to the formation of closed aromatic systems in the polymer. The doped materials have higher σ′-values in relation to the corresponding specimens without doping, with the exception of the pyrolysis residues at 800°C. The dependence of doping effect and time was also determined. The transition of the organic polymer to the pyropolymers can be followed and characterized by the measurement of alternating electrical conductivity.     

Spray pyrolysis synthesis of mesoporous TiO<Subscript>2</Subscript> microspheres and their post modification for improved photocatalytic activity

Mesoporous TiO₂ microspheres were prepared by spray pyrolysis for photocatalysis. Post modification of TiO₂ by heat treatment was performed to optimize its photocatalytic performance. First, spherical TiO₂ particles with mesoporous structure were synthesized at pyrolysis temperatures of 500, 600, and 700 °C. After characterization by XRD, SEM, and N₂ adsorption, a sample prepared at 500 °C was found to possess desirable properties for photocatalytic performance through post-modification. In methylene blue degradation, mesoporous TiO₂ microspheres synthesized at 500 °C outperformed other microspheres. Furthermore, samples obtained by spray pyrolysis at 500 °C were calcined at various temperatures as a post-modification process. The sample calcined at 350 °C showed improved photocatalytic activity due to optimal anatase crystallinity and surface area.