Raman spectroscopic study of a substituted poly(phosphazene)

Summary Polyphosphazenes present a range of unique and fascinating physical properties, including thermotropic behaviour, which are highly dependent both on the type of substitution at the P atom in the inorganic backbone, and the thermal history of the material. Recent technological advances have allowed us to study this group of polymers using vibrational spectroscopy. Fourier transform (FT) Raman spectra have been recorded from samples of poly[bis(4-methoxy-phenoxy)phosphazene] with different thermal histories. With the aid of spectral subtraction methods, differences observed between the spectra have been related to the crystalline and mesophase content of the materials.     

正交结构与黄铜矿结构LiGaSe_2的制备及结构表征(英文)

利用水平单温区管式电阻炉,合成了LiGaSe2多晶材料,并用X射线衍射(X-ray diffraction,XRD)和Raman光谱对其结构进行了表征。XRD谱表明:LiGaSe2多晶材料具有正交结构或黄铜矿结构,2种结构的多晶材料都是高纯的且结晶性良好。在合成过程中,最后阶段的恒温温度及降温速率将决定LiGaSe2的物相。通过Raman谱分析,找到2种类闪锌矿振动峰,其中低频峰与Ga—Se键的晶格振动有关,而高频峰与Li—Se键的振动有关。2种结构LiGaSe2中Li—Se键和Ga—Se键力常数存在微小差异,导致2种Raman谱谱峰发生移动。     

影响炭质中间相形成和发展的因素——Ⅰ.原料和热缩聚条件

炭质中间相是制备炭材料,尤其是高附加值炭材料的优质前驱体。它在高性能炭纤维、超高功率电极用针状焦、锂离子二次电池负极、超高比表面积活性炭及催化剂载体、泡沫炭等领域得到了较好的应用。综述了原料和热缩聚条件对炭质中间相形成和发展的影响,这些影响因素包括原料的种类、热缩聚温度、反应压力和保温时间等。通过讨论指出,由于炭质中间相涉及的原料种类多样,热缩聚制备过程中涉及的反应、物相复杂,因此,在制备特定性能的产品时,应针对产品的特定性能要求来选择原料、设定热缩聚条件,以制得具有最佳结构和性能的产品。     

A NEXAFS determination of the electronic structure of carbons for lithium-ion cells

We have applied the near-edge X-ray absorption fine structure (NEXAFS) technique to investigate the electronic and structural properties of specialty carbonaceous materials and anode electrodes made from the synthesized carbons. The carbon K-edge features of the carbonaceous materials were characterized by two relatively sharp resonances at 286 and 293 eV and a broad peak centered at 305 eV. A peak at 288 eV is related to the C–H* interaction and the relative intensity of this peak is higher for those samples prepared with pyrene as the carbon precursor. Oxygen and Li K-edges were also determined. The electrochemical performance of the carbon anodes is compared and related to the electronic and structural features of the NEXAFS spectra.     

E.s.r. study of vanadyl complexes in various carbonaceous materials at high temperature

E.s.r. spectra of vanadyl complexes in various carbonaceous materials were measured in a range between room temperature and 435 °C. The e.s.r. spectrum of vanadyl complex doped in Taching reduced crude was a slow tumbling pattern at temperatures between 60 °C and 225 °C. By the rapid freezing technique, it became clear that the vanadyl complex in Taching residue was oriented to the magnetic field in its temperature range. Further, activation energies for rotations of vanadyl complexes in those carbonaceous materials were estimated from Arrhenius plots of the correlation times. Those carbonaceous materials were classified into two groups according to values of the activation energies. This classification was consistent with that based on mesophase textures of the cokes made from them.     

Quantitative determination of anisotropic domain size in mesophase pitch

The optical texture of the carbonaceous mesophase is related to the nature of the pitch precursor, the mesophase viscosity and mode of preparation. The anisotropic domain size, represented by the distance between extinction lines, has been a useful parameter for characterizing mesophase pitches. A procedure for quantitative determination of mesophase domain size, utilizing annealing techniques and image analysis, is described and applied to several different mesophase pitch systems.     

Production of nanoTiC–graphite composites using Ti-doped self-sintering carbon mesophase powder

This work reports the synthesis of nanoTiC–graphite composites using mesophase pitch containing titanium as TiC or TiO₂ nanoparticles. NanoTiC–graphite composites have been prepared using Ti-doped self-sintering mesophase powders as starting materials without using any binders or a metal carbide-carbon mixing stage. The effect of manufacture variables on the graphite compacts properties was studied. Graphites were characterised using XRD and Raman spectroscopy, SEM and TEM, as well as by their mechanical, electrical and thermal properties. The presence of TiC promotes graphitisation producing materials with larger crystal sizes. The kind of titanium source and mesophase content of the starting pitch affects to the final properties. Mesophase pitch with higher amount of mesophase content produces graphites with higher degree of graphitisation. The incorporation of TiC nanoparticles to the graphites composites improved thermal conductivity more than four times, and mechanical properties are not significantly modified by the presence of TiC.     

Spherical natural graphite coated by a thick layer of carbonaceous mesophase for use as an anode material in lithium ion batteries

A method for coating a thick layer of carbonaceous mesophase was developed to treat spherical natural graphite (SNG) for use as anodes in lithium ion batteries. The carbonaceous mesophase layer was fabricated by heat treatment of a mixture of SNG and coal tar pitch. The thickness of the carbonaceous mesophase on the surface of the SNG was approximately 2.5 μm, which is effective for enhancing the strength of the carbonaceous mesophase shell and for allowing the shell to maintain good integrity at a high anode density (1.6 g cm⁻³). The mesophase layer increased the initial columbic efficiency from approximately 90% to 95%, dramatically improved the capacity retention and reduced the irreversible capacity by greatly decreasing the SNG surface area. The initial efficiency, cycle life and rate capability for the SNG anode covered by a thick mesophase layer gave comparable results as the mesocarbon microbeads (MCMB) anode, while the SNG anode reversible capacity of 341 mAh g⁻¹ was higher than that of MCMB, 319 mAh g⁻¹. Electrochemical measurements showed that SNG particles coated by a thick carbonaceous mesophase layer are strong candidates for use as possible anode materials in high energy density lithium ion batteries.     

Flow of molten mesophase pitch

The rheological behavior of molten mesophase pitch has become a subject of major scientific curiosity as well as of technological importance. With the exception of a few recent studies, the early experimental techniques were not entirely adequate for precise rheological characterization of such a complex fluid as molten mesophase pitch. A reasonably extensive literature, therefore, does not seem to exist on mesophase pitch rheology. The factors which could contribute to the difficulties of accurately measuring the apparent viscosity of carbonaceous anisotropic materials are discussed herein. A Couette-flow instrument has been modified for measuring the apparent viscosity of mesophase pitch, circumventing most of the experimental difficulties. The reliability of this apparatus has been tested by cross checking the apparent viscosity measurements with the values measured from two other rheometers, Instron Capillary and Seiscor/Han, for a number of fluids. Changes in rheological properties that occur during the transformation of a commercial pitch into a totally anisotropic pitch have now been studied by an improved technique on the modified Couette apparatus. The rheology of two mesophase pitches, made from model compounds, have also been investigated for comparison.     

影响炭质中间相形成和发展的因素——I．原料和热缩聚条件

炭质中间相是制备炭材料，尤其是高附加值炭材料的优质前驱体。它在高性能炭纤维、超高功率电极用针状焦、锂离子二次电池负极、超高比表面积活性炭及催化剂载体、泡沫炭等领域得到了较好的应用。综述了原料和热缩聚条件对炭质中间相形成和发展的影响，这些影响因素包括原料的种类、热缩聚温度、反应压力和保温时间等。通过讨论指出，由于炭质中间相涉及的原料种类多样，热缩聚制备过程中涉及的反应、物相复杂，因此，在制备特定性能的产品时，应针对产品的特定性能要求来选择原料、设定热缩聚条件，以制得具有最佳结构和性能的产品。     

Disclination structures of carbonaceous mesophase spherules

The permissible disclination structures of carbonaceous mesophase spherules with tangential boundary conditions were analysed theoretically. Besides the well-known Brooks-Taylor spherules, we observed during experiment the equilibrium carbonaceous mesophase spherules with symmetric configurations of 5 and 6 disclinations which are in close agreement with theoretical results. These spherules have been found to be formed and stabilized in coalescence of two or more mesophase spherules with a Brooks-Taylor structure. A thorough study has been made of the dynamics of the coalescence of carbonaceous mesophase spherules and disclination reactions in them, including the annihilation of disclinations of different sign. The rate of change of distance r between the disclination nuclei was shown to be r² = a (t−t_c), where t_c is the time of disclination annihilation. It is known that the bulk carbonaceous mesophase contains primarily wedge disclinations of strength . We have found that the disclinations of greater strengths, namely, and +2 may also exist. The effect of small impurity particles and gaseous bubbles on the stabilization of these possible but energetically less favorable disclinations in the carbonaceous mesophase is discussed.     

Carbon-boron-nitrogen alloys from borazarene-derived mesophase pitches

Synthetic routes to new C-B-N-containing mesophase materials from borazarene-type precursors have been explored. Such mesophases have the potential for forming C/BN alloys which could be processed in analogous ways to carbonaceous mesophases, e.g. into cokes, fibres and composites. Carbon-boron-nitrogen-based mesophase pitches have been generated at ambient pressure by pyrolysis of borazarenes in the presence of AlCl₃. The AlCl₃ is essential for mesophase formation in these systems. Pyrolyses of 10-chloro-9-aza-10-boraphenanthrene and 2,2′-bis(dichloroborylamino)biphenyl have both yielded black, optically anisotropic pitches. These precursors form C/BN ceramic alloys which are turbostratic following heat treatment to 1000 °C. The alloys show a clear advantage in terms of their oxidation resistance, compared with that of carbon derived from naphthalene mesophase pitch, despite contamination with AlCl₃, which probably catalyses their oxidation.     

Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm⁻¹, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm⁻¹ was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm⁻¹. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm⁻¹, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.     

Carbon fibres from mesophase pitch

Union Carbide Corporation has recently developed a process for preparing high-modulus carbon fibres from mesophase pitch. These high-modulus carbon fibres can be prepared from low-cost carbonaceous pitches which have been converted to a liquid crystal state. The liquid crystal or mesophase state results when a proper size distribution of planar aromatic molecules is produced by dehydrogenative condensation reactions. Mesophase pitches can be melt-spun into fibres possessing a high degree of axial preferred orientation which can be preserved and enhanced through carbonizing and graphitizing. Mesophase pitch-derived carbon and graphite fibres can have several transverse structures and are capable of achieving various combinations of high strength and high modulus. Their electronic properties are extremely sensitive to structure and further confirm the graphitizability of carbon fibres derived from mesophase pitch. A number of strength-limiting defects have been identified and their incidence reduced. The number of applications of carbon fibres continues to increase. The unique structure and properties of mesophase pitch-based carbon fibres make them particularly suitable for those applications requiring high stiffness, high electrical conductivity, high thermal conductivity, and low coefficient of thermal expansion.     

Comparative Study of Vibrational Characteristics of Fluorozirconate and Fluorohafnate Glasses

The vibrutional characteristics of fluorozirconate and fluorohafnate glasses have been determined from ir reflectivity and Raman scattering. Fluorohafnate-based glasses possess lower ir oscillator strengths than fluorozirconate glasses, and their fundamental absorption peaks are shifted to a lower frequency. The dominant Raman peaks, however, are nearly identical for both glasses.     

Microscopic investigation of carbonaceous deposits formed in a solvent-treated coal reactor

The mode of occurrence and optical characteristics of the carbonaceous deposits from a 3 t/d process development unit solvent-treated coal reactor were investigated by optical microscopy to increase understanding of the formation and development of reactor blockage materials such as mesophase and semicoke. The solid deposits showed a layered texture consisting of mesophase matrix as a binder, particles of non-plastic coal, pyrolytic carbon and mineral matter. The layered appearance of the deposits was due to variations in the proportions of the microscopic constituents and pores. The variations in reflectance values of the carbonaceous deposits from the centre of the reactor to the wall provided evidence of the thermal alteration after deposition. Based on these results, the formation and sedimentation of carbonaceous deposits and subsequent secondary alteration processes are discussed.     

Quantitative appraisal of the interfacial anchoring state of polyaromatic hydrocarbons during the formation of C/C composites

The preferred anchoring state for polycyclic aromatic hydrocarbons (PAHs) pyrolyzed onto carbon substrates has been studied by semi-quantitative methods, examining their interfacial lattice arrangements. The samples were prepared by decomposing petroleum pitch inside carbon nanotubes resulting in a variety of crystallinities forming one-dimensional C/C composites. Studies indicate that the preferred anchoring state of PAH molecules depends on the nature of the substrate. Accordingly the PAHs in mesophase pitch should exhibit a face-on orientation on the carbonaceous substrates, including a graphite sheet, glassy carbon, and pyrolytic carbon. However, it showed that the anchoring state (face-on or edge-on) of PAH units can be altered even on carbonaceous substrates. The results demonstrated that in C/C composites the anchoring state is predominantly determined by the relative degree of crystallinity of the pitch/carbon substrate, and can be semi-quantitatively estimated using the I_D/I_G ratio from Raman spectra. Face-on anchoring is preferred when the I_D/I_G ratio of substrate is smaller (higher crystallinity) than that of the pyrolyzed precursor, whereas edge-on anchoring is favored when it is larger. Such an estimation method is useful in tailoring microstructures in the fabrication of C/C composites using PAH precursors.     

Enhanced Fluorescence and Local Vibrational Mode in Near‐White‐Light‐Emitting ZnO:Mg Nanorods System

We report exciton and phonon properties of ZnO:Mg nanorods of different Mg doping concentration. X‐ray diffraction studies (XRD) confirm the growth of wurtzite phase ZnO nanostructures. XRD reveals doping‐induced shift in peaks and formation of secondary phase related to Mg. Optical properties of the prepared nanorods are investigated by using UV‐Visible absorption and photoluminescence spectroscopic techniques. Optical absorption studies show strong free excitonic absorption of ZnO and extra absorption bands related to the defect centers of the secondary phase (MgO) formed after Mg doping. Photoluminescence studies show sharp band in UV region and defects‐related broad band emission in the visible range. Gaussian‐fitted photoluminescence spectra show that the emission is composed of free exciton recombination and its longitudinal optical (LO) phonon replica. In addition, Mg‐related local vibrational mode observed in Raman and FTIR spectra after Mg doping, indicates the incorporation of Mg into the lattice positions of wurtzite ZnO.     

Inhomogeneity study of alanine-doped TGS crystal using micro-probe raman spectroscopy

Micro-probe Raman spectroscopy is used for studying positional inhomogeneity in alanine-doped TGS crystals. 20 data points are measured along the c- and a-axes. Several known Raman peaks are observed unchanged, but most other peaks are found to be changed remarkably. The major difference in spectra are found to be related to SO₄ vibrations. The vibrational frequencies are found to be lowered near the seed position. The most unstable spectra are detected in the vicinity of the seed region.     

采用Raman光谱研究硅烷接枝LLDPE的结晶结构

采用傅里叶变换红外光谱(FTIR)和Raman光谱表征了硅烷接枝线型低密度聚乙烯(LLDPE)的结晶结构。用FTIR和Raman光谱上硅烷特征峰与LLDPE亚甲基峰的强度比,可相对比较硅烷接枝LLDPE的接枝率,随着硅烷用量的增加,由FTIR计算的吸光比从0.96增至2.62,由Raman光谱计算的强度比从11.53增至14．00,两者的变化趋势相同。Raman光谱分析还表明,随着硅烷用量的增加,硅烷接枝聚乙烯的结晶相质量分数由57．58％降至46．99％,非晶相质量分数由11．58％升至16．52％,中间相质量分数从30．84％升至36．48％。