How to obtain a reliable structural characterization of polished graphitized carbons by Raman microspectroscopy

A series of graphitized carbon materials, produced by the pyrolysis of an anthracene‐based coke at temperatures ranging from 1600 to 2900 °C, were studied by Raman microspectroscopy to assess the applicability of this technique to the particular case of polished carbon materials. The polishing process was shown to change significantly the first‐order Raman spectra (D band intensity increase) and therefore to induce unacceptable errors in the characterization of the intrinsic structure of these materials. The deconvolution of Raman spectra, related to the unpolished graphitized carbons at varying temperatures, highlighted a linear relationship between the intensity ratio I_D/I_G and the G band width. Thus, as the latter appears to be insensitive to the polishing, we highly recommend using it for a reliable assessment of the intrinsic structural disorder of polished carbon materials. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat capacity of the white pine biocarbon preform and the related biocarbon/copper composite

This paper reports on measurements in the 80–300-K temperature interval of the heat capacity at constant pressure C _p (T) of high-porosity amorphous white pine carbon preforms (biocarbon) prepared by pyrolysis (carbonization) at T _carb = 1000 and 2400°C in an argon flow. The dependences C _p (T) for biocarbon/copper composites based on the carbon preforms obtained have also been determined. It is shown that the mixture rule holds for the composites, i.e., that C _p (T) of the composite is a sum of the heat capacities of the constituent materials taken in the corresponding ratios. Phonon mean free paths for the white pine carbon preforms prepared at T _carb = 1000 and 2400°C have been calculated and used to estimate the size of the nanocrystallites contributing to formation of the carbon frameworks of these preforms.     

Synthesis of carbon encapsulated nanocrystals of WP by reacting W(CO)<Subscript>6</Subscript> with triphenylphosphine at elevated temperature under autogenic pressure

The thermal decomposition of a reaction mixture containing W(CO)₆ and PPh₃ yielded carbon encapsulated nanocrystals of tungsten monophosphide (WP). This reaction was carried out using the RAPET technique (reaction under autogenic pressure at elevated temperature) at 850°C under inert (N₂) atmosphere. Carbon nanostructures of ∼100–300 nm encapsulated several 30 nm WP nanocrystallites, as evidenced by HRTEM (high resolution transmission electron microscopy). The reaction mechanism for the formation of WP encapsulated in carbon nanostructures is suggested on the basis of morphological, structural, and surface characterization data.     

Thermal stability of second generation carbosilane dendrimers with peripheral ammonia groups

Spinel nickel zinc ferrite nanowires were successfully prepared in mesoporous silica SBA-15 as a host matrix. The powder was annealed at a range of temperatures (500–900 °C) with heating rate 0.5 °C/min. The required NiZnFe₂O₄ phase was obtained at 700 °C. The specific surface area S_BET data revealed that the surface area of the mesoporous silica after annealing was decreased from 821 to 90 m²/g which indicated that the spinal ferrite fills the channels of mesoporous materials. The one-dimensional spinel nanostructures were characterized by X-ray diffraction, infrared spectroscopy, vibrating sample magnetometer, and transmission electron microscopy before and after a selective removal of the silica template in aqueous solution of NaOH or HF. The presence of SBA-15 lowers the formation temperature of nickel zinc ferrite nanowires compared to the corresponding bulk material. The magnetic properties revealed a high saturation magnetization level (~43 emu/g) for the Ni–Zn nanowires at 900 °C.     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

Microstructure and oxidation behaviour of SiC fibre-reinforced Si3N4-AlN-Al2O3-Y2O3 matrix composites

A series of SiC fibre-reinforced Si₃N₄-AlN-Al₂O₃-Y₂O₃ matrix composites with different matrix compositions are fabricated by slurry infiltration followed by hot pressing at 1600°C for 30 min. The diffusion of yttrium and aluminium into fibres is apparent during the high temperature processing. All the as-processed composites show fracture with fibre pull-out. After heat treatment in air at 1000°C for 60 min, composites with minimal Y₂O₃ and Al₂O₃ in the matrix composition demonstrate the fracture behaviour with most extensive fibre pull-out. Composites with the highest aluminium and yttrium oxide content form an yttrium–aluminium–garnet phase and an aluminosilicate glassy phase. The latter phase provides an oxygen diffusion path, resulting in the removal of the carbon-rich interphase by oxidation. This results in catastrophic fracture without fibre pull-out after heat treatment of the composite in air.     

Disorder‐induced symmetry lowering in Ba(Y1/2Nb1/2)O3 ceramics probed by Raman spectroscopy

This work investigates the evolution of the crystal structure of microwave‐hydrothermal synthesized Ba(Y_1/2Nb_1/2)O₃ powders as a function of firing temperature by Raman spectroscopy. The samples were produced at 200 °C and fired at temperatures ranging from 600 to 1600 °C. Raman spectra were obtained at room temperature for all samples and the results showed that materials fired at 1600 °C exhibited tetragonal (I4/m or ) structure, whereas those fired at lower temperatures exhibited the triclinic (P1 or C_i¹) structure. The results were compared with those observed for ceramics obtained by conventional solid‐state methods. It is believed that the lowering of the symmetry verified in materials fired below 1600 °C is a consequence of the local disorder of Y⁺³ and Nb⁺⁵ ions in octahedral sites. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

Raman studies on nanocomposite silicon carbonitride thin film deposited by r.f. magnetron sputtering at different substrate temperatures

Raman studies of nanocomposite SiCN thin film by sputtering showed that with increase of substrate temperature from room temperature to 500 °C, a transition from mostly sp² graphitic phase to sp³ carbon took place, which was observed from the variation of I_D/I_G ratio and the peak shifts. This process resulted in the growth of C₃N₄ and Si₃N₄ crystallites in the amorphous matrix, which led to increase in hardness (H) and modulus (E) obtained through nanoindentation. However, at a higher temperature of 600 °C, again an increase of sp² C concentration in the film was observed but the H and E values showed a decrease due to increased growth of the graphitic carbon phase. The whole process got reflected in a modified four‐stage Ferrari–Robertson model of Raman spectroscopy. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of high pressure and temperature on carbon isotope compositions of some acyclic alkanes and preservation of organic matter

The effects of high pressure and temperature on carbon isotopic compositions of acyclic alkanes and the stability of the acyclic alkanes were experimentally investigated. The pyrolysis of lignite with water in a closed system was conducted at 400–700°C and 1–3 GPa. The carbon isotope data, variations of peak carbon and evident odd–even predominance of acyclic alkanes indicated that: (1) the high pressure retarded the maturation of organic matter and destruction of hydrocarbons, (2) n-C₁₂₊ hydrocarbons from biogenic sources could be preserved in the cool slab subducted into the upper mantle, and (3) some organic compounds might preserve the carbon isotope signals inherited from biogenic sources. The results favor tracing the origins of organic matter in mantle rocks and extraterrestrial organic matter in meteorites and the process of deep carbon cycle.     

Features of electrical properties of BE-C(Fe) biocarbons carbonized in the presence of an Fe-containing catalyst

The effect of partial graphitization on electrical and galvanomagnetic properties of BE-C(Fe) biomorphic carbons produced by beech wood carbonization at temperatures of 850–1600°C in the presence of an iron-containing catalyst is studied. The use of an Fe catalyst at Т _carb ≥ 1000°C leads to the formation of nanoscale graphite-phase inclusions; its total volume and nanocrystallite sizes increase with Т _carb. The data on the carrier concentration and mobility are obtained. It was shown that partially graphitized BE-C(Fe) carbons with Т _carb ≥ 1000°C in the conductivity type and magnetoresistance features relate to highly disordered metal systems whose conductivity can be described taking into account the contribution of quantum corrections, mainly the correction caused by the electron–electron interaction. It is shown that nonmonotonic dependences of the Hall constant R on the magnetic field are characteristic of BE-C(Fe) samples with 1000 ≤ Т _carb < 1600°C, which is most probably caused by the contribution of various carrier groups, i.e., electrons and holes. In BE-C(Fe) samples with Т _carb = 1600°C, the Hall coefficient corresponds to the metal state, which is associated with conducting medium homogenization resulting from the formation of a significant graphite phase volume.     

Structure of the amorphous phase of pyrolisates of lanthanum diphthalocyanine according to X-ray scattering data

Data on X-ray diffraction in lanthanum diphthalocyanine pyrolysates synthesized at temperatures of 800–1800°С demonstrate the formation of an amorphous carbon phase with embedded lanthanum atoms. Low-temperature pyrolysis (800–900°С) creates layered carbon structures. Due to annealing at 1000°С, carbon integrates into globules whose number of atoms is m ~ 100. Such structures with gyration radii of R _g ~ 0.4–0.5 nm on the order of the precursor molecule size are synthesized in the temperature range of 1000–1800°С, and are stable in terms of size and mass. In this case, their density approaches that of graphite.     

Iron–cobalt nanocrystalline alloy supported on a cubic mesostructured silica matrix: FeCo/SBA-16 porous nanocomposites

A series of novel nanocomposites constituted of FeCo nanoparticles dispersed in an ordered cubic Im3m mesoporous silica matrix (SBA-16) have been successfully synthesized using the wet impregnation method. SBA-16, prepared using the non-ionic Pluronic 127 triblock copolymer as a structure-directing agent, is an excellent support for catalytic nanoparticles because of its peculiar three-dimensional cage-like structure, high surface area, thick walls, and high thermal stability. Low-angle X-ray diffraction, N₂ physisorption, and transmission electron microscopy analyses show that after metal loading, calcination at 500 °C, and reduction in H₂ flux at 800 °C, the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. FeCo alloy nanoparticles with spherical shape and narrow size distribution (4–8 nm) are homogeneoulsy distributed throughout the matrix and they seem in a large extent to be allocated inside the pores.     

High temperature Raman spectroscopy studies of carbon nanowalls

High temperature Raman experiments were carried out on carbon nanowalls (CNWs). The intensity of the defect‐induced D mode decreased significantly after the sample was heated in air ambient. The Raman intensity ratio of D mode and G mode, I_D/I_G, changed from 2.3 at room temperature to 1.95 after the sample was heated to 600 °C. This change was attributed to the removal of surface amorphous carbon by oxidation. In contrast to I_D/I_G, the intensity ratio of the D′ mode and the G mode, I_D′/I_G, did not change much after heating, indicating that the surface amorphous carbon and surface impurity do not contribute as much to the intensity of the D′ mode. The dominant contributor to the D′ mode could be the intrinsic defects. Copyright © 2007 John Wiley & Sons, Ltd.     

Probing structural stability of double-walled carbon nanotubes at high non-hydrostatic pressure by Raman spectroscopy

Theoretical calculations predict that the collapse pressure for double-walled carbon nanotubes (DWCNTs) is proportional to 1/R ³, where R is the effective or average radius of a DWCNT. In order to address the problem of CNT stability at high pressure and stress, we performed a resonance Raman study of DWCNTs dispersed in sodium cholate using 532 and 633 nm laser excitation. Raman spectra of the recovered samples show minor versus irreversible changes with increasing I _D/I _G ratio after exposure to high non-hydrostatic pressure of 23 and 35 GPa, respectively. The system exhibits nearly 70% pressure hysteresis in radial breathing vibrational mode signals recovery on pressure release which is twice that predicted by theory.     

Characterization of nitrogen and fluorine co-doped titania photocatalyst: Effect of temperature on microstructure and surface activity properties

Nitrogen and fluorine co-doped titania photocatalyst samples to be used for air purification were prepared by spray pyrolysis using a mixed solution of TiCl₄ and NH₄F. Droplets of the mixed solution formed by nebulizer passed through a ceramic tube furnace under a suction of an aspirator and a titania-based powder was formed at temperatures in the range from 700 to 1000 °C. The resulting nanopowders were characterized by electron microscopy, X-ray diffraction, temperature programmed desorption of NH₃, methods of thermal analysis, particle size, surface area and porosity determination by nitrogen adsorption. Morphology and surface activity of the samples prepared at various conditions were compared. The thermal behavior of the samples characterized by TG, DTA and ETA under air heating conditions is discussed considering the differences in samples preparation. A high photocatalytic activity for acetaldehyde decomposition in a visible region of spectrum depended on the spray pyrolysis temperature and can be ascribed to a synergetic effect of nitrogen and fluorine doping.     

Thermopower of beech wood biocarbon

This paper reports on measurements of the thermopower S of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures (T _carb). The thermopower S has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves S(T _carb) have revealed a noticeable increase of S for T _carb < 1000°C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for T _carb ∼ 1000°C they undergo a phase transition of the insulator-(at T _carb < 1000°C)-metal-(at T _carb > 1000°C) type. The existence of this transition is attested also by the character of the temperature dependences S(T) of beech wood biocarbon samples prepared at T _carb above and below 1000°C.     

Controlled SnO2 nanocrystal growth in SiO2–SnO2 glass‐ceramic monoliths

Crack‐free (100–x) SiO₂–x SnO₂ glass‐ceramic monoliths have been prepared by the sol–gel method obtaining for the first time SnO₂ concentrations of 20% with annealing at 1100 °C. Heat‐treatment resulted in the formation and growth of SnO₂ nanocrystals within the silica matrices. Combined use of Fourier transform–Raman spectroscopy and in situ high‐temperature X‐Ray diffraction shows that SnO₂ particles begin to crystallize in the cassiterite‐type phase at 80 °C and that their average apparent size remains around 7 nm, even after annealing at 1100 °C. Nanocrystal sizes and size distributions determined by low‐wavenumber Raman are in good agreement with those obtained from transmission electron microscopy measurements. Results indicate that the formation and the growth of SnO₂ nanocrystals impose a residual porosity in the silica matrix. Copyright © 2011 John Wiley & Sons, Ltd.