Multiwavelength Raman microspectroscopy for rapid prediction of soot oxidation reactivity

Multiwavelength Raman microspectroscopy (MWRM) analysis for characterization of soot structure and reactivity was developed. This new method is based on the dispersive character of carbon D mode in Raman spectra (i.e., red shift and increase in intensity at higher excitation wavelength, λ(0)). The approach was proven by investigating various diesel soot samples and related carbonaceous materials at different λ(0) (785, 633, 532, and 514 nm). In order to compare the behavior of the D mode for various samples and to derive a single parameter characterizing the soot structure, the difference of integrals for pairs of spectra collected at different λ(0) was calculated. MWRM analysis revealed substantial differences in the structural ordering which decreases from graphite, over Printex XE2 and various diesel soot samples, to spark discharge soot. To obtain the relation between structure and reactivity of soot, MWRM analysis was combined with temperature-programmed oxidation (TPO). TPO allowed us to characterize the oxidation behavior of soot in terms of the maximum emission (CO + CO(2)) temperature and reactivity index. The latter was calculated by introducing the reactivity limits: spark discharge soot containing a large amount of disorder represents the upper limit, whereas the lower limit is given by graphite powder with high structural order. The comparison of MWRM (viz., the observed Raman difference integrals) and TPO data revealed a linear correlation between soot structure and oxidation reactivity. Thus, we demonstrated for the first time the potential of MWRM for a robust and rapid prediction of diesel soot reactivity based on the structure-reactivity correlation.     

Nanostructure and reactivity of carbon particles from co-pyrolysis of biodiesel surrogate methyl octanoate blended with n-butanol

This paper presents nanostructure and reactivity of carbon particles (soot) from co-pyrolysis of biodiesel surrogate methyl octanoate blended with n-butanol at temperatures from 1023 K to 1223 K in a quartz tube flow reactor, as n-butanol concentration increases from 0 to 50% (mole fraction) in the mixed fuels. The soot structure was characterized by low and high resolution transmission electron spectroscopy (LRTEM and HRTEM) and X-ray diffraction (XRD). The soot reactivity was investigated by thermogravimetric analyzer (TGA). When the pyrolysis temperature increased, as well as the rise of n-butanol addition, soot presented the higher degree of graphitization with the larger fringe length and lower fringe tortuosity. TGA results revealed that the higher amount addition of n-butanol can induce the formations of less reactive soot, and soot yielded at lower temperatures was more reactive than that at high temperatures. In addition, the high correlations between soot reactivity with its nanostructure were given in details.     

Direct observation of atomic scale graphitic layer growth

The demand for better understanding of the mechanism of soot formation is driven by the negative environmental and health impact brought about by the burning of fossil fuels. While soot particles accumulate most of their mass from surface reactions, the mechanism for surface growth has so far been characterized primarily by measurements of the kinetics. Here we provide atomic-scale scanning tunneling microscope images of carbon growth by chemistry similar to that of importance in soot formation. At a temperature of 625 K, exposure of the surface of highly ordered pyrolytic graphite to 1 Langmuir of acetylene leads to the formation of both graphitic and amorphous carbonaceous material at the edges of nanoscale pits. Given the similarity of the electronic structure at these graphite defect sites to that of soot material growing in flames at higher temperatures, the present studies shed light on the mechanism for soot growth. These experiments also suggest that healing of defect sites in graphene nanostructures, which are of considerable interest as novel electronic devices, should be possible at modest surface temperatures by exposure of defected graphene to unsaturated hydrocarbons.     

Dielectric properties of PMMA/Soot nanocomposites

Dielectric analysis (DEA) of relaxation behavior in poly(methyl methacrylate) (PMMA) soot nanocomposites is described herein. The soot, an inexpensive material, consists of carbon nanotubes, amorphous and graphitic carbon and metal particles. Results are compared to earlier studies on PMMA/multi-walled nanotube (MWNT) composites and PMMA/single-walled nanotube (SWNT) composites. The beta relaxation process appeared to be unaffected by the presence of the soot, as was noted earlier in nanotube composites. The gamma relaxation region in PMMA, normally dielectrically inactive, was "awakened" in the PMMA/soot composite. This occurrence is consistent with previously published data on nanotube composites. The dielectric permittivity, s', increased with soot content. The sample with 1% soot exhibited a permittivity (at 100 Hz and 25 degrees C) of 7.3 as compared to 5.1 for neat PMMA. Soot increased the dielectric strength, deltaE, of the composites. The 1% soot sample exhibited a dielectric strength of 6.38, while the neat PMMA had a value of 2.95 at 40 degrees C. The symmetric broadening term (alpha) was slightly higher for the 1% composite at temperatures near the secondary relaxation and near the primary relaxation, but all samples deviated from symmetrical semi-circular behavior (alpha = 1). The impact of the soot filler is seen more clearly in dielectric properties than in mechanical properties studies conducted earlier.     

石墨表层对四面体非晶炭膜中受激电子的石墨建序化作用

对渗气阴极真空电弧法制备的四而体非晶炭(ta-c)膜实施氧等离子体刻蚀,消除其表面石墨层后,发现:原沉积膜中ta-C石墨表层的消除会影响其受激电子的石墨建序化.应用发射电子能耗谱,表面增强拉曼光谱和表面敏化X光吸收光谱等测量方法,测定了其表层的淌除(程度).样品的氧等离子体刻蚀阻迟了受激电子的石墨化作用,可能归因于多相成核过程中石墨晶核的缺失之故.     

Nanostructure and reactivity of nascent carbon particles from 2,5-dimethylfuran/n-heptane swirling inverse diffusion flames

Carbon particles (Soot) have been the one of primary pollutants inevitably with the combustion of fossil fuels. A better understanding on incipient soot was useful to build the models of soot formation and even control soot emissions. The present work focuses on the nanostructure and oxidation reactivity of nascent carbon particles (soot) formed from 2,5-dimethylfuran (DMF)/n-heptane non-swirling and swirling inverse diffusion flames (IDFs). The nascent soot samples were derived from three different fuels: 100% n-heptane, 50% n-heptane/50% DMF and 100% DMF. In addition, the effects of swirling combustion and collection time on characteristics of nascent soot were investigated in detail using the high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that nascent soot from pure n-heptane flames presented the film-like morphology and nanostructure of amorphous nature, while the nearly primary particles with more well-organized nanostructure were found in pure DMF soot. Swirling combustion increased the entire flame front areas and diameters due to the existence of tangential velocity at the outlet of burner. Moreover, swirling combustion could enhance the mixability of the oxidizer stream and fuel stream, leading to more young soot within the fuel stream being oxidized in the inception stage. However, the collection time on soot characteristics exhibited quite negligible impacts in comparison to the swirling effects.     

Laser-induced incandescence measurements of soot in turbulent pool fires

We present what we believe to be the first application of the laser-induced incandescence (LII) technique to large-scale fire testing. The construction of an LII instrument for fire measurements is presented in detail. Soot volume fraction imaging from 2?m diameter pool fires burning blended toluene/methanol liquid fuels is demonstrated along with a detailed report of measurement uncertainty in the challenging pool fire environment. Our LII instrument relies upon remotely located laser, optical, and detection systems and the insertion of water-cooled, fiber-bundle-coupled collection optics into the fire plume. Calibration of the instrument was performed using an ethylene/air laminar diffusion flame produced by a Santoro-type burner, which allowed for the extraction of absolute soot volume fractions from the LII images. Single-laser-shot two-dimensional images of the soot layer structure are presented with very high volumetric spatial resolution of the order of 10(-5)?cm3. Probability density functions of the soot volume fraction fluctuations are constructed from the large LII image ensembles. The results illustrate a highly intermittent soot fluctuation field with potentially large macroscale soot structures and clipped soot probability densities.     

Loose-fit graphitic encapsulation of silicon nanowire for one-dimensional Si anode design

Silicon nanowires(SiNWs) encapsulated with graphene-like carbon sheath(GS) having a void space in between(SiNW@V@GS) are demonstrated for the improved electrochemical performance of Si anode in lithium ion battery. The Si NW@V@GS structure was synthesized by a scalable fabrication method including four successive reactions: metal-catalyzed CVD growth of Si NWs, controlled thermal oxidation, and deposition of the graphitic layer, to form Si NW@SiO_2@GS and additional chemical etching of sacrificial SiO_2 layer between Si NWs and carbon sheath. During the synthetic process, the thickness of the void spacing was controlled by adjusting the oxidation-dependent process. The well-controlled void space and crystalline graphitic carbon sheath of the SiNW@V@GS structure enable good reversible capacity of1444 m Ahg~(-1) and cycling stability of 85% over 150 cycles.     

Fractionation of carbon-based nanomaterials by anion-exchange HPLC

A sample containing carbon nanoparticles was obtained starting with the soot generated during combustion of inexpensive paraffin oil in a flame. The complexity of the sample, however, required fractionation to isolate its components. Anion-exchange high-performance liquid chromatography (AE-HPLC) was used for the analysis and collection of soot-derived carbon nanoparticles. The fractionated species were monitored by ultraviolet (UV) absorption and laser-induced photoluminescence detection, providing the chromatographic UV absorption and emission profiles of the separated sample. Chromatographic fractionation allowed for bulk measurements of electronic properties for individual fractions and further analysis via transmission electron microscopy (TEM). TEM of fractionated species showed a predominant size of about 4-5 nm diameter particulates. A general trend between photoluminescence and elution time was observed; the later eluting species in the chromatogram exhibited photoluminescence at longer wavelengths than the early eluting species. The AE-HPLC approach can have an immediate impact on the analysis and fractionation of various other nanomaterials, demonstrated here by analyzing samples containing graphitic oxide nanoparticles.     

C/C复合材料石墨化度P1模型的表征及测定

C/C复合材料的石墨化度反映了材料中碳结构与理想石墨晶体结构的接近程度,并且是影响其性能的一个重要结构参数,石墨化度P1是一种能够比较准确地表示碳材料石墨程度结构的参数,本文利用P1的基本原理,编制了相应的计算程序,通过X光衍射（XRD）分析,计算了不同热处理温度C／C复合材料石墨化度P1,结果表明它可以较好的表征C／C复合材料的石墨化程度,并进一步讨论了热处理温度（HTT）与石墨化度的关系。     

Steam locomotive soot and the formation of thaumasite in shotcrete

After approximately 40 years the sprayed concrete lining in the Koblenz railway tunnel started to break off from its stonework support. The old masonry showed relicts of a soot layer from the ages of steam locomotive services. Locally small amounts of low mineralised water were running through the tunnel lining. Due to the finding of ettringite at the rear surface of the shotcrete together with soot it was assumed that sulfur from the locally present soot layer was the cause for the shotcrete detachment.

Detailed investigations showed the formation of thaumasite at the concrete interface to its support and up to 15 mm inside the concrete. Thaumasite could be detected filling microcracks and veins parallel to the shotcrete surface. The amount of secondary sulfate minerals and the extent of sulfate uptake are orders of magnitude larger to what could be expected if the sulfur of the soot layer would be present as sulfate.

It is assumed that the finding of thaumasite in cracks and veins is caused by long term interaction with a low mineralised groundwater and is leading toward a complete alteration of the concrete into a mushy material.     

Plasmabromierung von graphitischen Materialien

Plasma bromination of graphitic materials The high‐yield and high‐selective plasma chemical bromination of polyolefin surfaces was transferred to graphitic materials. Graphene‐like surfaces of highly‐oriented pyrolytic graphite (HOPG), natural graphite, multi‐walled carbon nanotubes(MWCNT) and carbon fibres were exposed to bromine plasma. This bromination proceeds at polyolefin surfaces as radical substitution by hydrogen abstraction and bromine attachment or as nucleophilic substitution. However, bromination of graphitic structures picks up sp³ carbon atoms occurring as structural defects or more probably proceeds as electrophilic addition onto the substituted aromatic double bonds. In this process the planar sp² carbon‐carbon bonds are transferred to the tetrahedral sp³ C‐C bonds with non‐conducting structure. Using the bromine plasma maximal bromination yields varied, were dependent on the type of carbon material and ranged from 10–50% Br/C. Using bromoform as Br‐precursor and accepting layer deposition about 70% Br/C were found. Subsequently, different amines were grafted wet‐ or gas phase chemically by nucleophilic substitution. The grafting yields amounted 1–10 molecules per 100 carbon atoms, which was lower than for grafting yields on brominated polyolefin surfaces ranging between 1–22 molecules per 100 carbon atoms. After grafting more or less all non‐grafted Br‐atoms were also diminished, that indicated a partial reconstruction of the planar sp² graphitic structure.     

Multiwall and bamboo-like carbon nanotube growth by CVD using a semimetal as a catalyst

Multiwall carbon nanotubes have been synthesized on silicon substrates via atmospheric pressure chemical vapour deposition technique using bismuth as a catalyst. Field emission scanning electron microscopy analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy analysis shows spaghetti-like multiwall carbon nanotubes and with a bamboo-like structure obtained using the Bi catalyst. The quality, in terms of the graphitic crystallinity of the as grown carbon nanotubes, was analyzed by Raman analysis. The study shows that the catalyst, namely bismuth strongly influences the growth density and graphitic crystallinity of the grown carbon nanotubes.     

Comments on “spectral albedos of mid-latitude snowpacks”

Choudhury (1982) has found that small amounts of soot added to a thin snow layer could increase the snowpack albedo in his computer model. This result is physically unrealistic; the effect of adding soot to snow is to darken it, not to brighten it. An explanation for the error in Choudhury's model is that in his single-scattering computation he has apparently exaggerated the extinction cross-section of soot by eight orders of magnitude.     

Synthesis of carbon-coated iron nanoparticles by detonation technique

Carbon-coated iron nanoparticles were synthesized by detonating a mixture of ferrocene, naphthalene and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in an explosion vessel under low vacuum conditions (8.1 kPa). The RDX functioned as an energy source for the decomposition of ferrocene and naphthalene. The carbon-coated iron nanoparticles were formed as soot-like deposits on the inner surface of the reactor, which were characterized by XRD, TEM, HRTEM, Raman spectroscopy and vibrating sample magnetometer. And a portion of the detonation soot was treated with hydrochloric acid. The product was carbon-coated nanoparticles in perfect core-shell structures with graphitic shells and bcc-Fe cores. The detonation technique offers an energy-saving route to the synthesis of carbon-coated nanomaterials.     

Submicron-sized boron carbide particles encapsulated in turbostratic graphite prepared by laser fragmentation in liquid medium

Submicron-sized B4C spherical particles were obtained by laser fragmentation of large B4C particles dispersed in ethyl acetate. The irradiated surface of large B4C raw particles was heated and melted by laser energy absorption. B4C droplets were then cooled down, and finally B4C spherical particles were obtained. Moreover, each B4C particle obtained was encapsulated in a graphitic layer that is useful for medical functionalization of particles. Thus, obtained B4C particles encapsulated in graphitic layer may have potential uses in boron neutron capture therapy.     

Improved adhesion of ultra-hard carbon films on cobalt–chromium orthopaedic implant alloy

While interfacial graphite formation and subsequent poor film adhesion is commonly reported for chemical vapor deposited hard carbon films on cobalt-based materials, we find the presence of O₂ in the feedgas mixture to be useful in achieving adhesion on a CoCrMo alloy. Nucleation studies of surface structure before formation of fully coalesced hard carbon films reveal that O₂ feedgas helps mask the catalytic effect of cobalt with carbon through early formation of chromium oxides and carbides. The chromium oxides, in particular, act as a diffusion barrier to cobalt, minimizing its migration to the surface where it would otherwise interact deleteriously with carbon to form graphite. When O₂ is not used, graphitic soot forms and films delaminate readily upon cooling to room temperature. Continuous 1 μm-thick nanostructured carbon films grown with O₂ remain adhered with measured hardness of 60 GPa and show stable, non-catastrophic circumferential micro-cracks near the edges of indent craters made using Rockwell indentation.     

Oxidation Protective Multilayer CVD SiC Coatings Modified by a Graphitic B-C Interlayer for 3D C/SiC Composite

A layered graphitic CVD B-C coating was introduced between two CVD SiC coating layers. Microstructure and chemical characterization of the CVD B-C and the hybrid SiC/B-C/SiC multilayer coating was performed using SEM, EDS, XPS and XRD. Oxidation protection ability of the coating for the C/SiC composite was studied using a thermogravimetric analyzer (TGA) in the isothermal mode and by measuring residual flexural strength. The layered graphitic CVD B-C coating middle layer reduced the maximum crack width in the CVD SiC coating. The hybrid SiC/B-C/SiC multilayer coating provided a better oxidation protection for C/SiC composite than a three layer CVD SiC coating due to coating crack control and sealing effects at temperatures up to 1,300°C for 900 min.     

SiC／Ni3Al界面固相反应区的相组成与显微结构

使用光学显微镜、扫描电子显微镜和能谱仪等对1000℃×5h加热处理的SiC／Ni3Al界面固相反应区显微结构、相组成以及反应区中元素分布等进行观察、分析和测试。研究表明,SiC／Ni3Al界面固相反应形成Ni2Si、石墨态碳沉积物和Ni5.4AlSi2。SiC／Ni3Al界面固相反应形成两层结构的反应区,其厚度大约是16μm。其中,靠近SiC侧的反应区由Ni2Si,Ni5．4AlSi2和分布在其中的颗粒状的石墨颗粒构成,而靠近Ni3Al侧的反应区则由Ni2Si和Ni5．4AlSi2构成。     

Effect of fibre surface morphology on stability of C/TiBx coatings in Ti–6Al–4V/σ (SM1240) fibre metal matrix composite

Abstract

Sigmafibres (SM1240) produced by a chemical vapour deposition process using a 15 μm tungsten wire corefor SiC deposition have a duplex coating of graphitic carbon and TiBx. Nodules present on the fibre surface are attributed to the deposition of the carbon coating over soot particles present on the substrate. Both the carbon and TiB_x coatings were stable in vacuum or air at temperatures up to 973 K. The nodules werefound to be sites of preferential attack by the titanium alloy matrix. The average number of nodules per fibre decreased more rapidly when the specimens were heated in air than in vacuum. It is suggested that the nodules may reduce the stability temperature of the coatings.

MST/2028