The successful fabrication and implementation of graphene-reinforced aluminum (Al) matrix composites (AMCs) have been obstructed by the undesirable graphene-Al reactions during their casting or inability of complex part manufacturing through powder metallurgy techniques. The emergence of the laser powder bed fusion (L-PBF) process with extremely short melt duration and almost no limitations in terms of the manufacturing of intricate features has renewed the interests for fabrication of graphene-reinforced AMCs. In this study, the influence of graphene incorporation into AlSi12 on L-PBF processability and defect formation is studied. The specific heat capacity, coefficient of thermal expansion, thermal diffusivity and thermal conductivity of composites were compared to those of the monolithic AlSi12 alloy. Microstructure-thermal properties relationship was studied through transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron backscatter diffraction (EBSD), electron dispersive spectroscopy (EDS) and Raman spectroscopy. This study provides valuable insights into (i) the chance of survival of graphene, (ii) possibility of graphene changing into other forms of carbon, and (iii) graphene-Al reactions during the L-PBF process. It was found that most of the graphene/graphite particles transformed into Al4C3. 4 C 3 . Among the survived carbon material, it appears they are more disordered than the initial graphene/graphite, though highly ordered ones with almost no defects were also detected. Thermal expansion measurements showed that the coefficient of thermal expansion decreased from 27.5x10-6/& ring;C x10-6/& ring;C for AlSi12 to 25.3x10-6/& ring;C x10-6/& ring;C for AlSi12-0.25 Gr and 25.5x10-6/& ring;C x10-6/& ring;C for AlSi12-0.5 Gr. Regarding thermal conductivity, in the case of AlSi12-0.5 Gr, it either matched or was lower than that of pure AlSi12 within the tested temperature range. In contrast, AlSi12-0.25 Gr exhibited higher thermal conductivity than AlSi12 in the temperature range of 150-350 & ring;C.

This month, I am going to talk about a different spectroscopy that rivals Raman in its usefulness. A-TEEM stands for Absorption, Transmission, Excitation/Emission Matrix Fluorescence. While it does not give direct information on the structure of a molecule, its value lies in its ability to detect subtle changes in structure due to changes in conformation of large organic molecules or changes in interactions with other molecules. While this is not a technology with which I have been directly involved, I have been impressed with its potential and thought this column would be a good place to share my thoughts.

Poly(3-hydrxybutyrate-co-3-hydroxyhexanoate) (PHBHx) is a biopolymer that is produced and degraded by microbes. Because of the potential to replace polymers derived from petrochemicals with these materials, there is a high level of expectation for its commercial uses if its physical and chemical properties can be understood and controlled. Among other things these properties are determined by the polymer's morphology - that is its crystallinity, and orientation of both crystalline and amorphous phases. The focus on the Raman characteristics of the crystalline phase enables elucidation of the characteristics of the polymer experiencing dynamic crystallization under various conditions. In this article we will start by reviewing the changes in the Raman spectrum from an amorphous to a crystalline material in an isothermal crystallization study. In that study a correlation field splitting between a CH stretching band that interacts with the carbonyl group on the opposite chain in the unit cell was identified. Then we will show the polarized Raman spectra of single crystals which enable an explanation of the residual amorphous material seen in the spectra of single crystals. Using the information from the single crystal measurements we can then study the Raman behavior of spherulites and confirm the model that proposes an explanation for the appearance of rings in the polarized light microscope (PLM) images of some spherulites. The polarized Raman studies confirm that the crystal ribbons that grow along the radii are twisting about the growth direction. The two-dimensional correlation spectroscopy (2D-COS) analysis of the polarized spectra of spherulites suggest the presence of strain that has been proposed to induce the twisting.

By now, it is well known that Raman spectroscopy provides information on the chemical composition of materials, and that said information can be made available in real time–that is, during a reaction. While many types of reactions can be monitored, one of the most important types is polymerization. Polymerization reactions typically involve the loss of a carbon double bond as the chain length is increased, and because the signals from π electrons are strong, the ability to follow this reaction until the end is quite good. However, setting up a polymerization experiment for demonstration purposes can be hazardous. Here, we will show the chemical and spectral changes that occur during the cure of a commercial epoxy.

Multilayer polymer films are engineered to accommodate a variety of requirements where polymer layers are fused together, with each polymer selected for a specific purpose. Specific analysis approaches can troubleshoot a defective film or allow us to reverse-engineer a film of unknown composition. Since the inception of Raman microscopy in the mid-1970s, it has been argued that a well designed confocal Raman microscope can analyze the composition of a multilayer film non-destructively. This is a powerful capability because it has the potential to provide information with optical resolution (better than 1 μm) below sample surfaces non-destructively. However, because standard microscope optics do not maintain focus inside of a material with a finite index of refraction, and the errors become greater with increasing numerical aperture and increasing depth, we wanted to determine the quality of the information achieved by comparing a depth profile with a two dimensional map of a cross-section. In this article, we show a cross-sectional map of a film compared to a depth profile to evaluate the quality of depth profile measurements.

Silicon carbide (SiC) is a wide bandgap semiconductor that is being developed for use in high-power integrated circuits because its large electronic indirect bandgap enables it to carry high currents without overheating. SiC is actually an interesting material. Its cubic phase has the same tetrahedral structure as silicon (Si), diamond, and germanium (Ge), with the important difference being that the two atoms in the unit cell are different. However, of greater interest is that there are many related polymorphs in which differences in the stacking order of the cubic (111) planes of SiC bilayers (that can be visualized as rotations around the [111] axis of the cubic cell), result in hexagonal and rhombohedral phases—many of them depending on the periodicity of the repeating planes. The reason that so many phases are possible is that the nearest neighbors are always the same, which means that the nearest neighbor interactions are almost the same in all structures. This is academically quite interesting, but it also has important implications in growing single crystals or films on which devices can be built—there can be local islands or defects that would interfere with a device’s performance, in part because the bandgaps of different polymorphs are different, but also because defects will interfere with electron flow. Because there is a different Raman signature for each of the polymorphs as well as the contaminants, Raman microscopy is an ideal tool for analyzing the structure of these materials as well as identifying possible contaminants that would also interfere with performance. In addition, there are characteristics in the Raman signature that reveal doping levels when n > 2x1016, thus providing an additional source of information for qualifying materials for integrated circuits.

Raman spectra of bioplastic poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx at 13.8 % Hx) were recorded between -250 cm- 1 and 3200 cm- 1 during isothermal crystallization at 250C after quenching from the melt in liquid nitrogen. At room temperature the crystallization proceeds slowly, so spectra were recorded over a 14-hour period. While there are spectral changes throughout the spectrum, the focus was on interpretable bands known to be sensitive to crystalline form. These bands included the carbonyl band that sharpens and shifts, a pair of bands on the high energy side of the carbon-hydrogen stretch, and a low frequency band that we assign to the molecular phonon in the crystal unit cell. After appropriate pre-processing of the spectra, they were further analyzed by 2D-COS (two-dimensional correlation spectroscopy) that provides determination of the order in which the polymer functional regions assemble into the crystalline state. According to this analysis one of the methyl CH's interacts with the carbonyl bond to produce a line at 3000 cm- 1. Following that, multiple changes appear in the carbonyl region, the strong CH band at 2930 cm- 1 of the crystalline phase grows, then the 80 cm- 1 phonon band, and the splitting of the methyl CH only appears after the phonon. From this sequence one can derive a picture of how the polymer unit locks into the crystal form. This can be of interest to commercialization of the materials because mechanical properties are intimately controlled by the crystallinity of the material. By understanding how the crystallization process proceeds, it can be engineered to be "fit for purpose" for a polymer targeted for a specific use.

Current benchtop instruments with edge filters providing Raman spectra down to 50 cm-1 can be used to study the crystallization of PHBHx (polyhydroxybutyrate-hexanoate) without the complexity of larger instruments. By collecting the averaged signal from spherulites, the effects of orientation can be averaged out, enabling the use of multivariate techniques to compare samples. These polymers are being commercialized because they are created by fermentation and are biodegradable, making them an ideal replacement for petroleum-derived polymers that do not biodegrade. In addition, it is understood how to control the polymer’s physical properties by controlling the molecular weight and the percent of sidechains which, in turn, controls the crystallinity. To compare spectra of different samples, it is necessary to remove the effects of polymer chain orientation, which averaging the spectrum of a spherulite will enable.

This study investigates the possibility of using dry mechanical mixing techniques to prepare ideal composite powders for L-PBF applications. For this purpose, two AlSi12-reinforced composite powders with 0.5 and 1 wt% graphene were prepared by ball milling and regular mixing processes. The powder attributes (size, size distribution and shape), behaviors (absorptivity, packing density, and flowability), relative humidity, and O/C/H content of the composite powders were quantitatively assessed after mixing and compared to those of monolithic AlSi12. The attachment of the graphene particles and their evolutions during each mixing technique were qualitatively investigated. The quality of the graphene powder before and after mixing was studied by Raman spectroscopy, and the nature of the carbonaceous constituent in the composite powders was identified based on the Raman peak positions, shapes and relative intensities. The results of this study prove that only through ball milling ideal composite powders for the L-PBF applications can be achieved.

The formation of spherulites in polymers is a well-known phenomenon; when the polymer is crystallized by cooling from the melt, crystal lamellae grow out from a nucleation site in a spherical pattern. If the material is annealed on a planar surface, and viewed between crossed polarizers in a microscope, a Maltese cross with a banding pattern is observed. Where the crystals grow in a direction not parallel to the polarizers, the sample lights up. Often banding of the lit regions is observed, and is believed to be due to rotations of the crystal lamellae around the growth direction. Because it is well known that polarized Raman spectra respond to crystal orientation, we thought it would be interesting to try to document the relationship between the banding behavior and Raman polarization/orientation behavior. In this column I will show results of such an investigation of spherulites of poly(hydroxybutyate-co-hydroxyhexanoate) (PHBHx) with varying composition.

In a follow-up to my February 2020 column, I started a more systematic study of extractables and leachables. Following a suggestion from Mark Witkowski of the FDA, I looked at three sets of centrifuge vials that were exposed to the following liquids in an effort to evaluate the potential of Raman microscopy to identify compounds exiting in polymers under particular conditions: saline, phosphate buffer, water, saline treatment at 100 0C, phosphate buffer treatment at 100 0C, water treatment at 100 0C, ethanol, chloroform, pH 5, and pH 9. Although all containers were made of polypropylene (PP), they didn’t behave similarly. Compounds that were extracted from PP vials from different manufacturers were not always the same. Although the number of spectral types that are recorded is large, this article focuses on a few whose interpretation is interesting. The goal was to figure out when it makes sense to employ Raman microscopy for such identification. The characteristics considered were ease of sample preparation, the minimum quantity of material amenable to analysis, and the quality of the identification.

In the last 30 to 40 years, various new types of carbon materials have been engineered for multiple industrial uses. It is now well-known that the Raman spectrum is sensitive to the structure, even though the spectrum is rather uncomplicated. Because Raman spectroscopy now has a reputation for providing good information, potential users of Raman equipment can request information on the quality of their sample. However, they are often not able to define clearly what they mean by “quality.” If they are growing diamond films, they may or may not want interstitial sp2 carbon to glue polycrystalline diamond together. If they are growing hard diamond-like carbon (DLC) films, they may want to correlate the spectral characteristics with physical characteristics of the film. In this column, I explain how the Raman characteristics can aid in characterization of carbon materials.

In this month’s column, I review the band assignments of a protein spectrum, pointing out why it can be useful to know the band assignments when attempting to use the Raman spectra to understand the functionality of proteins. In fact, the American Chemical Society (ACS) has just issued a virtual issue of Journal of Physical Chemistry entitled “Protein Crowding and Stability,” discussing how the protein conformation impacts its biological functionality. I address how the Raman spectra can help with the determination of protein structure.

We recorded the Raman spectra of Nodax (TM) (PHBHx-copolymer of hydroxybutyrate) biopolymer class with varying amounts of comonomer hydroxyhexanoate (Hx). The properties of Nodax is controlled by the amount of Hx added. Engineering the polymer for specific applications requires understanding the effects of Hx on the physical and chemical properties, and vibrational spectra provide a window on this information. Inspection of the structure of PHBHx indicates that the addition of Hx only replaces the methyl group with a propyl group on the hydroxyhexanoate segments of the polymer chains. To utilize the vibrational spectra to aid in the prediction of properties, the bands associated with these extra groups should be identified. The spectra shown here were acquired from polymers dissolved in deuterated chloroform; the carbon-hydrogen band of the solvent that would normally overlap with the spectrum of the polymer is replaced by a carbon-deuterium (CD) stretch in a region in which nothing else occurs. In solution, the spectra of the polymers exhibit the noncrystalline disordered phase and can be used to evaluate the contributions of the various functional groups.

Depth profiles of a laminar material have been measured on a confocal Raman microscope with a selection of microscope objectives. It is well known that the quality of the profiles will depend on the quality of the focus, which requires matching the objective's engineered properties with the properties of the material, particularly the effects of the indices of refraction. Depth profile comparisons performed with a series of objectives will illustrate how better matching improves the spatial resolution and chemical speciation in the depth profile.

By now, it is well known that Raman spectroscopy has the potential to be used for a variety of analytical applications, and that the hurdles to using the technique 30–40 years ago have been largely overcome. However, a remaining impediment to exploiting the technology is that very few analysts have the ability to know what they are looking at. Even if there is experience in infrared (IR) spectroscopy, which looks at similar molecular vibrations, making the Raman measurements and interpreting the results will be quite different. There are databases and searching programs that can aid the analyst, but using these resources effectively also requires a learning curve. What I am going to try to do in this article is provide some advice for dealing with those situations where the searching program does not provide a definitive result.

The study of vibrational spectra of linear alkanes has been used in the past to model the structure of polyethylenes. In particular, from these studies it has become clear that, in polyethylene, the longitudinal acoustic mode (LAM) frequency is inversely related to the length of polymer units between folds (lamellae), which is determined by sample history. This relationship had been developed by studying a series of linear alkanes. Spectra of samples of (C44H90), (C20D42), (C24D50), and (C36D74) are reported here. Although full spectra were recorded, we were more interested in the behavior in the low frequency region. Predictions for the LAM modes were in the range between 50 and 120 cm(-1), but we recorded spectra down to 4 cm(-1). In addition to the LAM modes, we observed sharp bands, some quite strong. An extensive literature search enabled assignment of these bands to librational modes (hindered rotations in the lattice). The ability to make this type of very low frequency measurement reliably can be used to predict physical properties of polymers and the crystalline polymorph of active pharmaceutical ingredients (API's), which has implications for bioavailability, shelf-life, stability, and protection of intellectual property (IP) for pharmaceutical manufacturers.

Any product used for medical purposes has to follow the adage "do no harm." In the current work, we assess if and how Raman microscopy, in combination with X-ray fluorescence (XRF), can aid in the characterization of material leached from several implantable devices. The results show both the presence of metal oxides and organic compounds. In one case, two crystalline forms of an identical oxide existed simultaneously, and we suggest that such information may be useful in characterizing the oxidation of the metal. One of the samples was tinged pink, and its spectrum was consistent with a resonance Raman (RR) spectrum of a pigment. Because information on the source of these samples was not known, it is not possible to perform a complete characterization, but we can suggest ways that these results can be used in the future when a more complete study can be done.

Chandrasekhara Venkata Raman (1888–1970) reported the light scattering phenomenon that has become known as the Raman effect in 1928 (Raman and Krishnan 1928). This followed theoretical predictions by Smekal (1923) that such a phenomenon should exist. Raman initially used sunlight, and then the light from a mercury lamp, to excite the spectrum presumably produced when a photon of light lost a small amount of its energy to a molecular vibration.

The Raman spectrum of water measured on a fixed grating near-infrared (NIR) spectrograph with a dual wavelength laser enables recording the signal in both the fingerprint and the OH stretching region. This article will show spectra recorded between 5 degrees C and 80 degrees C, and treated with both band-fitting and the 2D-COS algorithm. The results will show the complementarity of the two data treatments in providing different insights to the spectral behavior.