Investigation of hygrothermal aging phenomena in model composites by Raman spectroscopy

Abstract

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal aging in model composites based on an epoxy resin containing a single filament of a reinforcing fibre. Three fibre types, namely PBO, M5 and Twaron, were investigated using two specimen configurations - the diffusion slab (DS) and the double fibre pullout (DFPO) geometry. Variables such as the fibre type, surface treatment in the case of Twaron fibres, and the penetrant type (water vapour and liquid phase), were screened by establishing development of the Raman strain profiles ?(x, t) and the water uptake M(t) with time. Experimental data were successfully modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Relative water uptakes determined gravimetrically were reduced to time dependencies controlled by the specimen geometry, diffusion coefficient of the matrix D and the equilibrium uptake of water, M(∞). Using simple micromechanical models, the Raman strain profiles were rationalised in terms of the midfibre strain ?(x=0, t) and the dimensionless parameter n, which controls the stress transfer along the interface. The fibre strain energy G(t) was also obtained from the area under the Raman profile. At longer exposure times and for some fibres, debonding was observed and the time dependence of the propagation of the debond length L _d was quantified. Generic conclusions on composite behaviour were drawn by eliminating the exposure time. The swelling coefficient β was derived from the linear dependence of midfibre strain on the water uptake and a change in the fibre strain energy was related to the debond propagation.     

Investigation of the initial growth of ultrananocrystalline diamond films by multiwavelength Raman spectroscopy

The initial growth phase of ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) has been investigated by scanning electron microscopy, atomic force microscopy and especially Raman spectroscopy. As due to resonance effects Raman spectra of carbon materials strongly depend on the excitation wavelength, a multiwavelength analysis has been performed with λ_exc ranging from the UV region (325 nm) over the visible range (488 and 514 nm) to the IR region (785 nm). In addition, a set of measurements has been performed with a confocal Raman microscope, i.e. depth resolved, with a wavelength of 532 nm. The samples investigated were deposited with constant parameters, the deposition time being the only parameter varied, resulting in film thicknesses from 100 to 500 nm. It turned out that the diamond fraction and also the grain boundary material do not vary during that stage whereas there are slight but distinct changes of the nature of the amorphous matrix which reflect, among others, in a shift of the graphite-related G band to higher wavenumbers and in an increase of the ratio of D and G bands with increasing film thickness. These changes are discussed in terms of the above mentioned resonance effects; the major changes are a transition of hydrogen containing sp² chains to hydrogen-free condensed sp² rings when the material is no longer in the surface region of the films but becomes incorporated within the film bulk.     

Investigation of fatigue-type processes in polycrystalline diamond tools using Raman spectroscopy

Polycrystalline diamond (PCD) cylindrical tool-bits used in oil well drilling are susceptible to fracture due to the hostile environment of randomly occurring impact loads to which they are subjected. The fact that the tool-bits fail after repeated use suggests the possibility of fatigue type processes in PCD. The study of stress fields on the surface of the PCD thus becomes crucial in the quest to have extended lives for these tool-bits. Since the diamond Raman peak reveals both the nature and magnitude of the stress present in the material, this technique can be employed as a non-destructive measurement tool to investigate these stress fields. Raman stress measurements at room temperature were carried out using a 36 point mapping array in area close to the size of the PCD samples. The mapping points provided histograms of the magnitude and nature of these small individually stressed regions showing a general compressive stress for the lower numbers of fatigue cycles which deteriorates to a high proportion of tensile regions. The data are also illustrated by 2-D surface maps as an alternative mode of presentation again confirming the change from surface stresses being dominantly compressive to dominantly tensile with exposure to the higher numbers of fatigue cycles. Whereas a general compressive stress is desirable in the PCD layer as it inhibits the propagation of cracks, on the contrary tensile stresses facilitate the formation of cracks ultimately leading to catastrophic failure of the tool-bits.     

Crystallization and relaxation dynamics of glass-forming liquids at the Kauzmann temperature

If the entropy extrapolation of supercooled liquids (SCL) suggested by Kauzmann was correct, then they would have the same entropy as their stable crystalline phase at a certain low temperature, below the laboratory glass transition (T_g), known as the Kauzmann temperature (T_K). Extrapolating even further, the liquid entropy would be null at a temperature above absolute zero, violating the Third Law of Thermodynamics and constituting a paradox. Several possibilities have been proposed over the past 70 years to solve this paradox with different degrees of success. Our objective here is to access liquid dynamics at deep supercoolings to test the so-called crystallization solution to the paradox. By comparing the relaxation and crystallization kinetics determined above T_g and extrapolated down to T_K, a possible solution would be that the crystallization time is shorter than the relaxation time, which would mean that a SCL cannot reach the T_K. In this case, the liquid stability limit or kinetic spinodal temperature (T_ks) should be higher than T_K. We tested two fragile glass-forming liquids (diopside and wollastonite) and two strong liquids (silica and germania). For the fragile substances, T_ks ? T_K, hence such a supercooled liquid cannot exist at T_K, and the entropy crisis is averted. On the other hand, the results for the strong liquids were inconclusive. We hope the findings of this work encourage researchers to further investigate the liquid dynamics of different strong glass-forming systems at deep supercoolings.     

Elementoorganic dendrimer characterization by Raman spectroscopy

The FT-Raman (10-3500 cm⁻¹) spectra of ten generations of the elementoorganic dendrimers have been recorded and analyzed for the first time. Their spectral pattern is determined by the ratio N_t/N_r (N_t—number of terminal groups, N_r—number of repeating units). This ratio trends to m_r−1 (m_r—branching functionality of repeating unit), and becomes constant, when the generation number is higher than 3-5. Experimental Raman spectra of generations higher than 4 are very close similar, according theoretical approach. The dependence of line full width at half height in Raman spectra on generation number is established. The low frequency Raman spectra in R(ν) representation are used to investigate the librations of terminal groups. The possibility appears to separate the lines assigned to the core, repeated units and terminal groups of dendrimers by difference spectroscopy method. From the difference Raman spectra of dendrimers it follows that for the generations higher than six steric congestion disturb the conformations of terminal groups. FT-Raman spectroscopy provides unique detailed information about the structure of the technologically relevant materials, which could not be obtained before with any other technique.     

Disorder in ball-milled graphite revealed by Raman spectroscopy

High-energy ball milling has been used to produce nanocrystalline and amorphous structures in nature graphite, but Raman spectroscopy analysis shows that the structure of the amorphous phase produced by ball milling is less disordered than the amorphous structures in ion-beam bombarded graphite and sputtered amorphous carbon. The band intensity ratio (I_D/I_G) increases first and then drops to 1.1 after 40 h of milling treatment, and remains constant during extended milling up to 100 h. Electron microscopy observation and surface area measurement show that the formation of some nanosized clusters during the extended milling period might be responsible for the less disordered structure in the milled graphite.     

Characterization of the anisotropy of pyrolytic carbon by Raman spectroscopy

The anisotropy of pyrolytic carbon is one of the most important properties for the development of TRISO (tristructural isotropic) coated fuel particles. Polarized Raman spectroscopy has been used to measure the anisotropy of pyrolytic carbon coatings ranging from high (highly oriented pyrolytic graphite) to low texture samples. Values obtained were correlated to the texture measured from the orientation angle (OA) of selected area electron diffraction patterns. The nodal behavior of the Raman intensity signals in graphite has been used to observe changes in the first order bands when the laser and scattered signal were polarized parallel or perpendicular to the graphene layers. Texture observed by Raman spectroscopy was largely affected by the orientation of the graphene layers inside growth features. Discrepancies in the values of texture measured between Raman spectroscopy and the OA was due to the difference of spatial resolution of each technique, 0.2 μm for OA and 2 μm for Raman spectroscopy. Both polarized Raman spectroscopy and nano-indentation were used to characterize the inner pyrolytic carbon in coated fuel particles before and after SiC deposition at 1500 °C.     

Phase diagrams of polyethylene blends by Raman spectroscopy

Summary The Raman internal mode region analysis of polyethylene (PE) is extended to studies of blends of different PEs so as to obtain a phase diagram showing the magnitude of crystalline (P_c), amorphous (P_a) and interfacial (P_b) phases at different compositions. The non-cocrystallizable HDPE/LDPE blends shows additivity in phase structure with blend composition. Enlarged share of P_b is seen for compatible blends of HDPE/LLDPE and especially for HDPE with VLDPE of low crystallinity. This method offers an easy access to direct observing compatibility of different PEs on a molecular level. Supported by Chinese Petrochemical Corporation (SINOPEC)     

Raman spectroscopy in lipid analysis

In the last decade, Raman spectroscopy has gained in maturity as a valuable technique for lipid analysis. The technique is non‐destructive, does not need sample presentation thereby allowing use in in situ and on‐line analysis, and is relatively cost effective. Recent studies have demonstrated the potential of Raman spectroscopy for determining and quantifying different parameters and properties of lipids, and for following oxidative deterioration.     

Quantitative characterization of orientation in PET fibres by Raman spectroscopy

Attempts to obtain quantitative measures of orientation on fibres of diameter ～ 10μm by the method used earlier on films were unsuccessful. This is believed to be due to polarization scrambling caused by reflection and refraction at the surface of the fibre. It is shown that by immersing the fibres in a liquid of refractive index equal to the mean of those of the fibre the method can be used successfully.     

Investigation of optical properties of Bi12GeO20 sillenite crystals by spectroscopic ellipsometry and Raman spectroscopy

Bi₁₂GeO₂₀ (BGO) compound is one of the fascinating members of sillenites group due to its outstanding photorefractive and photocatalytic characteristics. The present paper aims at investigating optical properties of BGO crystals by means of spectroscopic ellipsometry and Raman spectroscopy measurements. Bi₁₂GeO₂₀ single crystals grown by Czochralski method were structurally characterized by X-ray diffraction (XRD) experiments and the analyses showed that studied crystals have cubic crystalline structure. Raman spectrum exhibited 15 peaks associated with A, E and F modes. Spectroscopic ellipsometry measurement data achieved in the energy region between 1.2 and 6.2 eV were used in the air/sample optical model to get knowledge about complex pseudodielectric constant, pseudorefractive index, pseudoextinction and absorption coefficients of the crystals. Spectral change of real and imaginary part of complex pseudodielectric constant were discussed in detail. Band gap energy of Bi₁₂GeO₂₀ single crystals was calculated to be 3.18 eV using absorption coefficient dependency on photon energy. Critical point energies at which photons are strongly absorbed were determined by utilizing the second energy derivative spectra of components of complex pseudodielectric function. Fitting of both spectra resulted in the presence of four interband transitions with energies of 3.49, 4.11, 4.67 and 5.51 eV.     

Raman spectroscopy in the analysis of fire gases

Raman scattering is a possible technique for analysing gas mixtures. In the work here described Raman scattering was used for detection of gases extracted from different model fires, where wood, polymethylmetachrylate (PMMA) and polystyrene were used as test materials. Raman spectra of gas samples from differently ventilated model fires are presented as well as the variation of O², CO² and CO concentrations as a function of time with an effective time constant of less than 5 s. The sensitivity of the experimental set-up was estimated to be about 1000 ppm, but suggestions are given how to reach a detection limit of about 1 ppm. The feasibility of the technique and various ways of improving it are briefly discussed.     

Composition profiling of inhomogeneous SiGe nanostructures by Raman spectroscopy

In this work, we present an experimental procedure to measure the composition distribution within inhomogeneous SiGe nanostructures. The method is based on the Raman spectra of the nanostructures, quantitatively analyzed through the knowledge of the scattering efficiency of SiGe as a function of composition and excitation wavelength. The accuracy of the method and its limitations are evidenced through the analysis of a multilayer and of self-assembled islands.     

Structural analysis of polycrystalline graphene systems by Raman spectroscopy

A theoretical model supported by experimental results explains the dependence of the Raman scattering signal on the evolution of structural parameters along the amorphization trajectory of polycrystalline graphene systems. Four parameters rule the scattering efficiencies, two structural and two related to the scattering dynamics. With the crystallite sizes previously defined from X-ray diffraction and microscopy experiments, the three other parameters (the average grain boundaries width, the phonon coherence length, and the electron coherence length) are extracted from the Raman data with the geometrical model proposed here. The broadly used intensity ratio between the C–C stretching (G band) and the defect-induced (D band) modes should be used to measure samples with crystallite sizes larger than the phonon coherence length, which is found equal to 32 nm. The Raman linewidth of the G band is more appropriate to characterize the crystallite sizes below the phonon coherence length, down to the average grain boundaries width, which is found to be 2.8 nm. “Ready-to-use” equations to determine the crystallite dimensions based on the Raman spectroscopy data are given.     

Quantitative assessment of carbon nanotube dispersions by Raman spectroscopy

Aqueous dispersions of single wall carbon nanotubes (C-SWNTs), prepared using different dispersing agents, have been analysed by Raman spectroscopy. Normalising the spectra with respect to the area of the water O-H stretching transition eliminates the effects of photon scattering and absorption on the way through the dispersion, and the dispersions can be assessed quantitatively by comparison of the areas of the carbon nanotube G-band. The normalised G-band areas show linear concentration dependence according to Beer’s law. The influences of different dispersing agents and excitation wavelengths are discussed and the results are compared to the commonly used UV-Visible spectroscopic analysis. The method presented here is semi-quantitative and it is proposed to use the most effective dispersing agent found in this study, sodium dodecylbenzene sulfonate (SDBS), as a benchmark for future dispersion experiments.     

Damage of polymers studied by micro-Fourier Transform Raman spectroscopy

Summary The potential of the micro-Fourier Transform Raman tool in examining specific localized regions in polymeric materials with some degree of fluorescence when analyzed by conventional Raman spectroscopy is examined. Analysis of characteristic bands of the vibrational spectra obtained in a small area damaged by a visible and NIR laser beam in commercial Polyethylene Terephthalate (PET) shows a different conformer ratio than that observed in a non irradiated zone.     

Characterization of doped single-wall carbon nanotubes by Raman spectroscopy

Single-wall carbon nanotubes were grown by thermal chemical vapor deposition using either boron- or nitrogen-containing feedstocks or both. Carrier doping was evidenced by hardenings of the G band in Raman spectra, and the estimated carrier concentration reached ∼0.4%. In the G′ and D band spectra, a doping-induced component was observed at the high- or low-energy side of the original one. However, the appearance of the new component did not always coincide with the carrier doping. The doped SWCNTs often show radial breathing mode peaks in the off-resonance region, indicating a defect-induced modification of absorption spectrum.     

Low frequency excitations in amorphous polycarbonate studied by Raman spectroscopy

Low frequency excitations in amorphous polycarbonate-bisphenol A have been studied by Iaser Raman spectroscopy in the frequency region $\text{5}\text{cm}{}^{\mathrm{?1}}\text{< Δ}\text{v}\text{?}\text{< 200}\text{cm}{}^{\mathrm{?1}}$. Depolarized spectra were recorded at temperatures between ambient and 85 K, the reduced intensity spectra show no temperature dependence. Two bands are resolved in the reduced intensity spectrum: a strong band around 96 cm^?1 and a weak band around 45 cm^?1; these are attributed to the effects of longitudinal and transverse phonon waves originating in backbone motion. The low frequency Raman intensities provide information on the density of state g(ω) from which the specific heat, C_v, of polycarbonate has been calculated. This is found to vary with temperature in a manner similar to the calorimetrically measured C_v in the low temperature range ～1 K < T < 4 K.