Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Introduction: There is considerable interest in the use of three-dimensional porous scaffolds for tissue regeneration. The presence of an interconnected framework of pores with large surface area facilitates the formation of extracellular matrix and permits cellular ingrowth into implanted structures. For scaffolds to be useful for tissue regeneration, they must maintain good dimensional stability during the lifetime of the implant. While the initial scaffold architecture is often well characterized, a systematic study of the influence of incubation on the scaffold architecture is critical to ensure that the scaffolds retain their interconnected network of pores during their useful lifetime. Herein, we report on the evaluation of the architecture of polyarylate scaffolds and their stability under in vitro conditions using scanning electron microscopy (SEM).

The polymers used in this study were selected from a library of degradable polyarylates. This library is the first reported combinatorial library of biodegradable condensation polymers.

As a crystal gets smaller, diffraction spots stay visible for larger deviations from the Bragg condition. The upper limit of such deviations is connected to the threshold for getting lattice fringes in TEM images. This in turn allows one to quantify the probability of seeing cross lattice fringes along a certain zone axis. In this abstract we examine a simple semi-empirical model for the probability of detecting (001) zone cross-fringes of a spherical crystal of cubic lattice.

The upper limit for the deviation of crystal orientation from the exact Bragg condition, without losing cross fringes down a given zone, is expressed as the maximum half-angle θ1 between the zone and the electron beam. The solid angle σ subtended by a cone with this half-angle is proportional to the probability px that a randomly-oriented crystal will show the cross-fringes associated with that zone. A schematic, illustrating the principle used to calculate the probability of seeing cross-fringes, is given in Figure 1.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Most scanning electron microscopy is performed at low magnification; applications utilising the large depth of field nature of the SEM image rather than the high resolution aspect. Some environmental SEMs have a particular limitation in that the field of view is restricted by a pressure limiting aperture (PLA) at the beam entry point of the specimen chamber. With the original ElectroScan design, the E-3 model ESEM utilised a 500 urn aperture which gave a very limited field of view (∼550um diameter at a 10mm working distance [WD]). An increase of aperture size to ∼lmm provided an improved but still unsatisfactory field of view. The simplest option to increase the field of view in an ESEM was noted to be a movement of the pressure and field, limiting aperture back towards the scan coils1. This approach increased the field of view to ∼2mm, at a 10mm WD. A commercial low magnification device extended this concept and indicated the attainment of conventional fields of view.

We report here a specific type of edge strength anisotropy observed in field emission scanning electron microscope (FESEM) images. The images show weaker edge gradients in the scanning direction and hence these edges frequently go undetected. Direct application of edge detection algorithms to images with nondistinct edges, such as powder particles, show strong bias to edges perpendicular to the scanning direction. Edge orientation polarograms obtained from these images always show strong fictitious particle orientation in the scanning direction. In this work, we discuss an edge-sharpening algorithm that corrects for this bias and results in relatively more accurate and consistent edge orientation information.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Development of high-density longitudinal magnetic recording media (used for computer hard disks) with good noise performance and high thermal stability requires optimization of both alloy composition and processing methods. In CoCr(PtTa) thin films, intergranular Cr segregation is responsible for decoupling the magnetic exchange between the small ferromagnetic grains. The corresponding Cr depletion within the grains affects the “bulk” magnetic anisotropy. However, the nanoscale structural and chemical details that are needed for modeling and for guiding material development are not well understood. Energy-filtered transmission electron microscopy (EFTEM) has been used to characterize a series of CoCrTa/Cr, CoCrPt/Cr, and CoCrPtTa/Cr media sputtered under various processing conditions, in order to understand their structure-property-processing relationships.

Processing typical for hard-disk media was used: 30 or 60-nm films of a Cr underlayer followed by a Co alloy were d.c. magnetron sputtered onto a NiP-plated Al substrate pre-heated to 250°C.

Accurate descriptions of dispersed metals are of central importance in optimizing the activity, selec-tivity and stability of a diverse set of catalysts and chemistries. Improved methods for quantifying metal dispersion, alloying and support interactions help form mechanistic understandings of catalyst function at the atomic scale. In this paper we discuss ideal capabilities of a transmission electron microscope for catalysis, and give an update on the use of a next-generation field-emission TEM/STEM for structural and analytical studies of supported metal catalysts.

Characterizing nanometer-sized metal particles requires high performance imaging, high sensitivity analysis, and minimal disruption of the physical state of the particles. No single microscope combines all key performance factors with ease of use. Schottky field-emitter instruments provide high current-density probes with excellent stage stability (nm drifts in > 10 min. observed), < 0.7 eV energy reso-lution for EELS, and a turbo-pumped vacuum system for minimal carbon contamination while using nanometer probes and nanoamps of current. Scanning TEM performance is also key.

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Application of an appropriate ceramic surface coating to mechanical components such as bearings and gears can provide longer life and increased performance reliability. Metal-containing hydrocarbon (Me-C:H) coatings possess high hardness, together with low friction and low wear rate. They have also been suggested to adhere better to metallic substrates. This combination of attractive mechanical/tribological properties makes Me-C:H coatings potentially useful for surface modification of a wide range of mechanical components.

Using the technique of inductively coupled plasma (ICP) assisted vapor deposition[1], we have synthesized Ti-containing hydrocarbon (Ti-C:H) coatings with a wide range of Ti compositions[2]. Coating mechanical properties such as modulus and hardness have been measured by the technique of nanoindentation and correlated to Ti and hydrogen compositions[2,3].

We have performed detailed microstructural examination of Ti-C:H coatings by transmission electron microscopy (TEM), Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and X-ray Absorption Near Edge Structure (XANES) spectroscopy.

A variety of porous materials are used in a number of tissue-engineering applications such as cell transplantation and drug delivery. They provide a temporary scaffolding for a large cell mass, however they must have a fully interconnected pore structure. One such material is sodium alginate, a naturally derived polysaccharide which is isolated from seaweed. Sodium alginate can de dissolved in water to form a highly viscous solution that can be cross-linked in the presence of divalent cations (e.g., Ca2+) to form what is known a as hydrogels. We have developed a method for the fabrication of porous beads of sodium alginate and have attempted to measure the pore size, density and distribution in the beads. Characterization of the pore size and distribution is necessary to determine if the material is appropriate to promote cellular in-growth. The characterization procedures have been limited to conventional scanning electron microscopy (SEM) and mercury intrusion porosimetry. There has been some concern as to the potential introduction of artifacts (spatial distortion) during dehydration and sample preparation during these characterizations.

Each astrocyte forms an extensive network of fine processes within the surrounding neural tissue, interacting extensively with neighboring neurons and blood vessels. Fine glial processes surround synapses and probably modulate synaptic transmission. Glial endfeet on capillaries are responsible for transport of ions and metabolites and possibly control blood flow. Alterations in these fine structures may be of significance in brain function and disease. Glial fibrillary acidic protein (GFAP) is an intermediate filament found in astrocytes of the central nervous system. GFAP is commonly found in the perikarya and processes of protoplasmic and fibrous type astrocytes. Immunohistochemical labeling of GFAP is extensively used as a means of determining the location and shape of astrocytes. However, its labeling pattern varies with brain region (e.g. cortex vs. hippocampus), with cell state (natural vs. reactive astrocytes), and with the specific α- GFAP antibody used. Furthermore, Golgi-stained or dye-filled astrocytes show numerous small appendages or vellate structures that conform to the surrounding tissue and do not stain for GFAP.

Understanding cytoskeletal dynamics in living tissue is prerequisite to understanding mechanisms of injury, mechanotransduction, and mechanical signaling. Real-time visualization is now possible using transfection with plasmids that encode fluorescent cytoskeletal proteins. Using this approach with the muscle-specific intermediate filament protein desmin, we found that a green fluorescent protein–desmin chimeric protein was unevenly distributed throughout the muscle fiber, resulting in some image areas that were saturated as well as others that lacked any signal. Our goal was to analyze the muscle fiber cytoskeletal network quantitatively in an unbiased fashion. To objectively select areas of the muscle fiber that are suitable for analysis, we devised a method that provides objective classification of regions of images of striated cytoskeletal structures into “usable” and “unusable” categories. This method consists of a combination of spatial analysis of the image using Fourier methods along with a boosted neural network that “decides” on the quality of the image based on previous training. We trained the neural network using the expert opinion of three scientists familiar with these types of images. We found that this method was over 300 times faster than manual classification and that it permitted objective and accurate classification of image regions.

Two-phase α2+γ TiAl alloys with microalloying additions, Fig. 1, are of interest due to the high strength-to-weight ratio they can provide in automotive and aircraft applications. In boron-doped α2+γTiAl containing Cr, Nb, and W, the B levels were found to be significantly depleted below the nominal alloy content in both the α2 andγ phases. The boron solubilities in the γ and α2 phases were 0.011 ± 0.005 at. % B and 0.003 ± 0.005 at. % B, respectively in Ti-47% Al-2% Cr-1.8% Nb-0.2% W-0.15 % B that was aged for 2 h at 900°C (base alloy). The majority of the B was in a variety of borides including TiB, TiB2 and a Cr-enriched (Ti,Cr)2B precipitate. With the exception of the smaller (< 50 nm thick) Cr-enriched (Ti,Cr)2B precipitates, Fig. 2, most of the borides were larger than ∼100 nm. A significant proportion of the microalloying additions is in these borides, Table 1.

This work evaluates the use of light microscopes (LMs) as a tool for interlaminar fracture of polymer composite investigation with the aid of correlative fractography. Correlative fractography consists of an association of the extended depth of focus (EDF) method, based on reflected LM, with scanning electron microscopy (SEM) to evaluate interlaminar fractures. The use of these combined techniques is exemplified here for the mode I fracture of carbon–epoxy plain-weave reinforced composite. The EDF-LM is a digital image-processing method that consists of the extraction of in-focus pixels for each x-y coordinate in an image from a stack of Z-ordered digital pictures from an LM, resulting in a fully focused picture and a height elevation map for each stack. SEM is the most used tool for the identification of fracture mechanisms in a qualitative approach, with the combined advantages of a large focus depth and fine lateral resolution. However, LMs, with EDF software, may bypass the restriction on focus depth and present enough lateral resolution at low magnification. Finally, correlative fractography can provide the general comprehension of fracture processes, with the benefits of the association of different resolution scales and contrast modes.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009