Microscopic and Spectroscopic Characterization of Stacking‐Sequence Disordered SiC

Nanometric silicon carbide (SiC) powder (~5 nm) with a stacking‐sequence disordered structure (SD‐SiC), synthesized from elemental powders of Si and C, was investigated by microscopic and several spectroscopic methods. The structure of SD‐SiC was characterized by transmission electron microscopy (TEM), ¹³C, and ²⁹Si‐NMR, and by infrared (IR), Raman, and X‐ray photoelectron spectroscopy (XPS) methods. TEM characterizations showed relatively large deviations of the lattice parameters in the as‐synthesized SiC, indicative of the presence of stacking‐sequence disorder. IR analysis showed a weaker Si‐C bond in the SD‐SiC than in the 3C‐SiC. XPS determinations showed that C and Si in SD‐SiC are similar to those in 3C‐SiC. Broader peaks of ²⁹Si and ¹³C MAS‐NMR also indicate that the structure of SD‐SiC is different from that of 3C‐SiC. Raman spectroscopy exhibited activities for the crystalline polytypes and the amorphous of SiC but lack of them for the SD‐SiC. The inactivity of Raman spectroscopy for the SD‐SiC along with large deviation of the lattice constant and the extremely broad X‐ray diffraction peaks would indicate that SD‐SiC is a possible intermediate state between conventional polytype SiC and amorphous SiC, that is, a possible new type of SiC.     

Investigation of the damage behavior in CVD SiC irradiated with 70 keV He ions by NEXAFS,Raman and TEM

Chemical vapor deposited (CVD) SiC was irradiated with 70 keV He ions at room temperature. The damage behavior was investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM). NEXAFS spectra at the Si K-edge display the obvious decrease in intensity of crystalline peaks and near disappearance of the peak at 1852 eV, suggesting an increase in crystalline disorder resulting from an increased number of Si vacancies caused by irradiation. Raman spectra show the decomposition of crystalline Si-C bonds and the formation of homonuclear (Si–Si and CC) bonds during irradiation. TEM results show the transition from slight disorder to full amorphization with increasing dose. The dose to amorphization (DTA) is estimated to be about 1 dpa. It is also found that high density of stacking faults in CVD SiC may contribute to the enhancement of amorphization resistance compared to single crystal β-SiC.     

Irradiation effects on Amosic-3 silicon carbide composites by Si ions implantation

SiC_f/SiC composites were irradiated to over 100 dpa with 300 keV Si ions at 300 ℃. Here, electron microscopy and Raman spectroscopy were utilized to study the microstructural evolution. The Raman spectra of fiber and matrix showed that the crystal structure was seriously damaged. TEM images revealed that the fiber underwent grain nucleation and growth in lower fluence region, accompanied by an amorphous layer near the damage peak area. Also, the matrix went through recrystallization, and the columnar grains turned into equiaxial ones. Moreover, stacking faults and massive amorphous islands were observed in high resolution TEM images. Following irradiation at 300 ℃, the matrix swelled, but the fiber and interphase shrunken along the axis. And, more remarkably, the hardness of fiber and matrix decreased to different extents, a result that was explained by the generation of amorphous islands and breakdown of covalent bonds, and recrystallization might be responsible for this.     

Microstructure and texture of polycrystalline 3C–SiC thick films characterized via EBSD

Cubic silicon carbide (3C–SiC) is an excellent protective film on graphite and has been fabricated via laser chemical vapor deposition (LCVD) with an extremely high deposition rate by our research group. To understand comprehensively its growth behavior, the polycrystalline 3C–SiC thick films with the preferred orientation of <111> and <110> were characterized by diverse measurements, especially electron back-scatter diffraction (EBSD). Along the growth direction of the <110>-oriented 3C–SiC, the microstructure changed from equiaxed grains to elongated grains with <111> orientation, and eventually the <110>-oriented columnar grains. The stacking faults in the <110>-oriented 3C–SiC could be marked as <11–20>-oriented 6H–SiC. On the other hand, in the <111>-oriented 3C–SiC films, the microstructure changed from mainly equiaxed grains to large columnar grains. The high-density stacking faults in <111>-oriented 3C–SiC films may lead to the identification of the nominal 2H, 4H and 6H polytypes by Raman spectra and EBSD. The (0001) planes of 2H-, 4H–SiC are perpendicular to the growth direction, while that of 6H–SiC are parallel.     

Calculated X-ray Diffraction Data for Diamond Polytypes

Calculated X-ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterized.     

Structure,microstructure and disorder in low temperature chemical vapor deposited SiC coatings

Chemical vapor deposited SiC coatings were investigated at different scales by X-Ray diffraction, Raman microspectroscopy and transmission electron microscopy. They were prepared under specific conditions explaining the various (micro)structures obtained. The deposits all have a columnar morphology with a preferential orientation and a faulted cubic structure. They differ in how disorder is incorporated in the structure. Fine XRD analyses and stacking fault density assessment by TEM revealed the one-dimensionally-disordered (ODD) polytype in the < 111 > textured coatings. The frequency and spatial distribution of stacking faults vary and sometimes locally generate periodic alpha sequences. A specific type of disorder was also identified where {111} planes are arranged parallel to the growth direction within the columns. These disorders, more energetic than stacking faults, induce multiple and particularly large Raman modes. Crystal distortions, such as dislocations, are localized at the ODD domain boundaries, which are frequently interrupted as they extend during the growth.     

Optical and vibrational studies on single walled carbon nanotubes/short oligo‐para‐methoxy‐toluene composite

In a solution state, an oligophenylene prepared by electrochemical oxidation of para‐methoxy‐toluene exhibited luminescence in the violet‐to‐blue region. However, in a solid state, a clear red shift was observed as a signature of a supramolecular π‐stacking interaction between oligomer chains. In a de‐doped state, by chemical reduction, a fine structure in the form of two peaks was observed and luminescence intensity was strongly improved. When mixed with single walled carbon nanotubes (SWNTs), a quenching in the luminescence intensity accompanied by a slight blue shift of the PL peak of the nanocomposite was observed. This implies the shortening of the oligomer's effective π‐conjugation length caused by the added amount of SWNTs. A charge transfer in the photo‐excited state was also noted. The interaction taking place between the two materials was supported by optical infrared absorption and Raman scattering measurements and then a functionalizing mechanism was proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Thermal Expansion and Thermal Expansion Anisotropy of SiC Polytypes

The principal axial coefficients of thermal expansion for the (3C), (4H), and (6H) polytypes of SiC are considered to identify the structural role of the stacking layer sequence as it affects the thermal expansion. A general equation based on the fractions of cubic and hexagonal layer stacking is developed that expresses the principal axial thermal expansion coefficients of all of the SiC polytypes. It is then applied to address the thermal expansion anisotropy of the noncubic SiC structures.     

Microstructure/Properties Relations of Advanced Materials Computer Simulations of Diffraction Effects due to Stacking Faults in -SiC: I, Simulation Results

X-ray diffraction (XRD) patterns from nominally β-SiC specimens often differ from those expected for the cubic crystal structure. These differences include the presence of additional peaks, enhanced background intensities, peak broadening, changes in relative peak heights, and shifts in peak positions. It has long been recognized that they are due to the presence of stacking faults, and models relating the experimental observations to stacking fault population have continued to evolve. The presence and relative magnitude of these features vary among different β-SiC specimens. In this work, computer simulations were used to show that the variations are closely related to differences in the type and spatial distribution of stacking faults in each specimen. In these simulations, stacking sequences were generated using a selectively activated 1-D Ising model with a Boltzmann-type probability function for specifying errors, which allows a wide variety of fault configurations to be generated. Direct correlations between different features in the XRD data to the underlying fault population are demonstrated, which are discussed in this paper. It is also shown that this computer model is general, in the sense that many of the models presented in prior work can be interpreted as limiting cases of it.     

Effects of composition and crystallite size on the accuracy of the Rietveld method in determining lattice parameters of polytypes in multiphase SiC ceramics

The accuracy of the Rietveld method in determining the lattice parameters of polytypes in multiphase SiC ceramics was investigated as a function of the polytype composition and of the polytype crystallite size, using as benchmarks two sets of standard X-ray diffraction (XRD) patterns obtained by computer simulation. It was found that the Rietveld method is remarkably accurate, but also that the measurement of lattice parameters is sensitive to the polytype composition and to the polytype crystallite size. In particular, the accuracy of Rietveld-calculated lattice parameters for a given polytype not only decreases with increasing number of polytypes in the SiC ceramics, but also depends on which other polytypes are present. These two findings are interpreted based on the severity of the corresponding peak overlap phenomenon in the XRD patterns. In addition, the accuracy of the Rietveld analyses also decreases with decreasing crystallite size of the polytypes, which is due to the progressive loss of definition in the location of the peak maxima and to the greater severity of the peak overlap, both resulting from the corresponding peak broadening. This identification of the confidence limits of the Rietveld-calculated lattice parameters for a wide range of microstructural features may have important implications for the future characterization of SiC ceramics by XRD.     

Computer Simulations of Diffraction Effects due to Stacking Faults in β-SiC: II, Experimental Verification

Earlier results from computer simulation studies suggest a correlation between the spatial distribution of stacking errors in the β-SiC structure and features observed in XRD patterns of the material. Reported here are experimental results obtained from two types of nominally β-SiC specimens, which yield distinct XRD data. These samples were analyzed using high-resolution transmission electron microscopy (HRTEM) and the stacking error distribution was directly determined. The HRTEM results compare well to those deduced by matching the XRD data with simulated spectra, confirming the hypothesis that the XRD data are indicative not only of the presence and density of stacking errors, but also that it can yield information regarding their distribution. Moreover, the stacking error population in both specimens is related to their synthesis conditions, and is similar to the relation developed by others to explain the formation of the corresponding polytypes.     

Microstructural Characterization of SiC–SiC Joint by Raman Spectroscopy

Analysis of the SiC–SiC joint and its brazing mixture has been performed using a Raman spectroscopy microprobe technique. A careful mapping of the sample clearly shows the spatial distribution of the chemical species close to and within the joint. A different distribution of the 4H and 6H α-SiC polytypes, grown during the brazing process, was observed inside the joint. Furthermore, identification of the bands related to the Nowotny phase was also possible.     

Investigation of Ti2AlC formation mechanism through carbon and TiAl diffusional reaction

Ti₂AlC formation mechanism with carbon and TiAl alloy as reactants at 1000–1200 °C have been investigated through Raman spectrometer and transmission electron microscope (TEM). Reaction products TiC_x and Ti₂AlC are confirmed. Full width at half height (FWHH) of Ti₂AlC Raman peak is used to characterize crystallinity, which indicates better perfection of Ti₂AlC grains near TiAl area and with higher reaction temperature. Variations of TiC_x microstructure, especially stacking faults(SF) and nanotwins, are observed by TEM along reaction layer. Ti₂AlC lattice uniformity of different reaction temperature is also confirmed by TEM, which is consistent with Raman result. TiC_x is believed first formed through reaction between TiAl and C. With help of carbon vacancy, Al-doped TiC_x SF and nanotwin can be formed under hot pressing, which becomes the basal frame of Ti₂AlC.     

Transmission electron microscopy study of extended defect evolution and amorphization in SiC under Si ion irradiation

The damage induced in 3C-SiC epilayers on a silicon wafer by 2.3-MeV Si ion irradiation for fluences of 10¹⁴, 10¹⁵, and 10¹⁶ cm⁻², was studied by conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The evolution of extended defects and lattice disorder is followed in both the 3C-SiC film and Si substrate as a function of ion fluence, with reference to previous FTIR spectroscopy data. The likelihood of athermal unfaulting of native stacking faults by point defect migration to the native stacking faults is discussed in relation to damage recovery. Threshold energy densities and irradiation doses for dislocation loop formation and amorphous phase transformation are deduced from the damage depth profile by nuclear collisions. The role of electronic excitations on the damage recovery at high fluence is also addressed for both SiC and Si.     

Time Series Extended Finite‐State Machine‐Based Relevance Vector Machine Multi‐Fault Prediction

Fault prediction means to detect faults that can occur in the future. While most studies focus on predicting one fault at a time, multi‐fault prediction is more practical for industrial processes as multiple faults can cause much more damage than a single one. A time series extended finite‐state machine (TS‐EFSM)‐based relevance vector machine (RVM) approach is proposed for multi‐fault prediction. Time lags and correlation coefficients between the process variables and process states are determined. Then, a variable and a state dependence diagram based on the correlation coefficients is established with the EFSM. Furthermore, the RVM is applied to identify parameters for the sake of better prediction accuracy and shorter testing times. With the prediction parameters, faults can be predicted using the aforementioned TS‐EFSM state transitions.     

Experimental and numerical study of distortion in flat,L‐shaped,and U‐shaped carbon fiber–epoxy composite parts

In this study, flat composite panels were fabricated to find the effect of different manufacturing parameters, including stacking sequence, part thickness, and tooling material, on distortion of carbon fiber‐epoxy composite parts. L‐shaped and U‐shaped panels were also made to investigate the effect of stacking sequence on spring‐in angle and warpage of the curved panels. Results showed that distortion of the flat panels caused by asymmetry in the stacking sequence was an order of magnitude greater than distortion of the panels with an unbalanced stacking sequence; whereas in the curved panels, the panel with an asymmetric stacking sequence showed the least spring‐in angle, and the largest angle was observed in the symmetric panel. MSC Marc was used to predict distortion of the panels, and the simulation results were compared with the experimental results for several stacking sequences of the flat and the L‐shaped panels. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40439.     

Raman Spectroscopy of Laser‐Shocked Nitrobenzene

A technique for in situ Raman spectroscopy of laser‐shocked nitrobenzene was developed. Raman spectra of shocked nitrobenzene are obtained up to ∼3.5 GPa, and peak shifts at particular frequencies are observed. The shifts are plotted as a function of density and compared with the data under isothermal compression. Both data provide the same results. This indicates that the numbers of peak shifts of nitrobenzene depend solely on material density.     

Gas phase condensation of few-layer graphene with rotational stacking faults in an electric-arc

We report the generation and properties of few-layer graphene (FLG) in an external magnetic field modulated DC carbon arc in different non-reactive buffer gases. The effects of buffer gases on the anode erosion rate and the cathode deposit (CD) formation rate have been investigated during the synthesis of FLG. The constituents of the as-synthesized CDs were investigated using transmission electron microscopy, selected area electron diffraction, Raman spectroscopy, scanning tunneling microscopy and X-ray diffraction analysis. A plausible growth mechanism of such FLG is predicted. The results indicate that, under a parametrically optimized condition, an electric-arc of this kind can efficiently introduce rotational stacking faults to the as-synthesized flat FLG, which exhibits superior electron transport property than bulk graphite. A guideline for controlling the number of layers of such FLG has also been suggested.     

Microemulsion copolymerization of styrene‐methyl methacrylate followed on line by low‐resolution Raman spectroscopy

In this work, the kinetics of microemulsion copolymerization of styrene (St) and methyl methacrylate (MMA) at 60°C using dodecyltrimethylammonium bromide (DTAB) as the surfactant was followed on line by low‐resolution Raman spectroscopy. Raman spectra were taken every 30 s. Because the spectrophotometer has low resolution, the C?C peak overlaps with the St aromatic ring stretch vibration and curve deconvolution was necessary. Overall conversion obtained by Raman spectroscopy agrees with that obtained by gravimetry. Copolymer composition was determined by following the area changes of the C? O? C bending mode peak of MMA. Compositions determined by this method agree with those measured by NMR spectroscopy. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers