Low-vacuum Scanning Electron Microscopy Research Articles

The microstructural location of impurities in ice

The microstructural locations of impurities in natural ice have been of interest for over a century, but only in recent years have efforts been made to determine their location. Here, we outline the few measurements that have been performed. The focus is on a technique that we developed, which has also been subsequently used by others, that involves the controlled sublimation of uncoated ice in a low-vacuum scanning electron microscope. This process leaves behind concentrated impurities, in both the grain boundaries and the grain interiors, which are then identified using energy dispersive X-ray microanalysis. An overview of observations on meteoric ice from both Greenland and Antarctica; on accretion ice from Vostok Station, Antarctica; and on temperate pond and river ice is presented. PACS No.: 61.72-y

Microstructural study of asphaltene precipitated with methylene chloride and n-hexane ☆

This work presents a study of the microstructure, molecular structure and elemental composition of asphaltene precipitated from vacuum residue using a solvent mixture. The infrared spectroscopy (FTIR) results show vibration bands of functional groups such as aromatic C–H (4 adj H and 1 adj H) and sulfoxide (C 2SO). Low vacuum scanning electron microscopy (LV-SEM) reveals a highly porous and a smooth surface asphaltene. C, S and V are the main elements identified by energy dispersive X-ray spectroscopy (EDS) analysis, though some variations in concentration were observed between the porous and the smooth surface asphaltene. Transmission electron microscopy (TEM) analysis shows that asphaltenes are constituted by nanometric particles (micelles) of ∼50 nm in diameter. These particles conform agglomerates (flocs) from ∼350 to ∼550 nm in size, some of them in layer arrangement with a tendency to graphitize. These results reveal important information about flocculation processes of the asphaltene.

Preparation of cellulose acetate films with controlled porosity

AbstractPorous polymeric films were obtained by immersing a solution of cellulose acetate and high molecular weight poly(acrylic acid) into an ice-water mixture. The films, the porosity of which can be controlled (reversibly) by immersion into hot water for varying periods of time, were characterized by IR spectroscopy and lowvacuum scanning electron microscopy.

Interpretation of secondary electron images obtained using a low vacuum SEM

Charging of insulators in a variable pressure environment was investigated in the context of secondary electron (SE) image formation. Sample charging and ionized gas molecules present in a low vacuum specimen chamber can give rise to SE image contrast. “Charge-induced” SE contrast reflects lateral variations in the charge state of a sample caused by electron irradiation during and prior to image acquisition. This contrast corresponds to SE emission current alterations produced by sub-surface charge deposited by the electron beam. “Ion-induced” contrast results from spatial inhomogeneities in the extent of SE signal inhibition caused by ions in the gaseous environment of a low vacuum scanning electron microscope (SEM). The inhomogeneities are caused by ion focusing onto regions of a sample that correspond to local minima in the magnitude of the surface potential (generated by sub-surface trapped charge), or topographic asperities. The two types of contrast exhibit characteristic dependencies on microscope operating parameters such as scan speed, beam current, gas pressure, detector bias and working distance. These dependencies, explained in terms of the behavior of the gaseous environment and sample charging, can serve as a basis for a correct interpretation of SE images obtained using a low vacuum SEM.

Observation of sulfate crystallites in Vostok accretion ice

Controlled sublimation in a low-vacuum scanning electron microscope has been used to reveal unusual crystalline features in the triple junctions of accretion ice from the Antarctic ice sheet above Lake Vostok. Energy dispersive X-ray microanalysis showed that the features were composed mostly of magnesium, sulfur and oxygen, although small amounts of sodium and calcium were also occasionally observed. Synchrotron X-ray topography showed that the ice grains contained dislocation densities ranging from ∼1×10 7 to >1×10 10 m −2.

Electric fields produced by electron irradiation of insulators in a low vacuum environment

We report on the properties of electric fields generated as a result of electron irradiation of dielectrics in a low vacuum scanning electron microscope. Individual field components produced by (i) ionized gas molecules located outside the sample surface and (ii) subsurface trapped charge were detected by measurements of changes in (i) primary electron landing energy and (ii) secondary electron (SE) emission current, respectively. The results provide experimental evidence for a recently proposed model of field-enhanced SE emission from electron irradiated insulators in a low vacuum environment [Toth et al., J. Appl. Phys. 91, 4479 (2002)]. Errors introduced into x-ray microanalysis by the electric fields generated by ionized gas molecules can be alleviated by minimizing the steady state ion concentration by the provision of efficient ion neutralization routes. It is demonstrated how this can be achieved using simple sample–electrode geometries.

Imaging of minerals, ores and related products to determine mineral characteristics

Imaging is a fundamental requirement when characterizing minerals, ores and related products. Images are used for presentation, texture analysis, characterization, image analysis, identification, etc. The image mode that is utilized is dependent on the application, mineral properties and available instruments. Different instruments produce images that display different features. In order of increasing complexity they are: hand specimens, a non-polarizing microscope, polarizing optical microscope, scanning electron microscope (SEM), low vacuum SEM, microprobe, secondary ion mass spectrometer, staining, cathodoluminescence, tomography and others. Image analysis uses images produced with either an optical microscope or a SEM, but the SEM images are superior in this application.

Three-dimensional helical coiling structures and band patterns of hydrous metaphase chromosomes observed by low vacuum scanning electron microscopy.

Helical coiling structures and band patterns of hydrous metaphase chromosomes were documented three-dimensionally by low vacuum scanning electron microscopy (SEM). Fixed or unfixed isolated Chinese hamster metaphase chromosomes were stained with platinum blue (Pt blue) and observed in the backscattered electron mode for low vacuum SEM without any hypotonic treatment or drying processes. Fibrous structures were shown both in the fixed and unfixed hydrous chromosomes; helical chromatid coils and their subcoils were clarified especially in the fixed chromosomes having contrasting alternative bands of light and darkness, while the translucent perichromosomal matrix and compact fibrous structures were recognized in the unfixed chromosomes. The helical coils were more clearly represented in a loosened chromatid of metaphase chromosomes. Treatment with a tris-HCl buffer solution and Pt blue staining in a hydrous condition successfully produced banding patterns similar to G-bands on metaphase chromosomes. These banded chromosomes observed by low vacuum SEM were also analyzed stereoscopically by field emission SEM after critical point drying. These findings indicate that: 1) native or unfixed chromosomes maintain the compact arrangement of high-order helical structures covered with the peri-chromosomal matrix; 2) helical coiling appearances of chromatids frequently observed in previous papers might be caused by loosening of the final level of the high-order structure of the metaphase chromosome; and 3) banding patterns might be produced by the rearrangement or reorganization of chromatin fibers at the 30 nm fiber level after the extraction of some chromosomal components including the peri- or intra-chromosomal materials during the banding procedure.

On the role of electron-ion recombination in low vacuum scanning electron microscopy.

Here we demonstrate the effects of electron-ion recombination on imaging signals utilized in low vacuum scanning electron microscopes (SEMs). The presented results show that, under normal operating conditions, recombination of ionized gas molecules with secondary electrons (SEs) suppresses a significant fraction of emitted electrons. If the ion flux (and hence the spatial dependence of the SE-ion recombination rate) is laterally inhomogeneous across the imaged region of a specimen, contrast in SE images can be influenced and in some cases (under conditions of high detector field strength and long ionic mean free path) dominated by variations in the recombination rate. Consequently, SE images of features such as topographic asperities can exhibit edge-darkening, leading to inversion of some topographic contrast. Recognition of the extent and nature of electron-ion recombination is required for a correct understanding of processes occurring in variable pressure SEMs and, subsequently, for models of image formation.

Consequences of positive ions upon imaging in low vacuum scanning electron microscopy.

The effects caused by an excess quantity of ionized gas molecules within the low vacuum, variable pressure and environmental scanning electron microscope (ESEM) are described with reference to mechanisms by which they can influence imaging conditions. These effects can include specimen charging, recombination and development of space charge. They are demonstrated for three different classes of sample: (1) an electrically grounded conductor, (2) an electrically floating conductor, and (3) an electrical insulator. A new device is presented that will aid excess charge removal within the ESEM and help correct for some of these effects, thereby dramatically improving imaging over a wide range of operating conditions and samples. The mechanism of image enhancement is demonstrated with reference to the three classes of sample described above.

Some Morphologic Peculiarities of the Tectorial Membrane by Low Vacuum Scanning Electron Microscope and Scanning Probe Microscope

Department of Otorhinolaryngology, School of Medicine, Nihon-University Editor's Note: A substantial amount of high-quality research in otology and related fields is conducted in Japan. In many cases, publications describing from these efforts appear only in the Japanese language. In an effort to provide a broader forum for these important contributions, Otology & Neurotology is pleased to republish selected abstracts from Otology Japan. Gratitude is extended to Kimitaka Kaga, M.D., Editor-in-Chief of Otology Japan, for choosing an excellent selection of articles from the 2001 edition.

Imaging of uncoated snow crystals using a low-vacuum scanning electron microscope

Imaging of uncoated snow crystals using a low-vacuum scanning electron microscope

Use of oolong tea extract staining of soft-tissue specimens in low-vacuum scanning electron microscope with a cooling stage.

For direct observation of the surface structures of soft-tissue specimens, we examined rat tracheal tissue in a low-vacuum scanning electron microscope (SEM) equipped with a cooling stage. In specimens fixed with glutaraldehyde and osmium tetroxide, back-scattered electron images of the surface structure could not be clearly observed in the low-vacuum SEM because of the disruption of fine structures and a low signal-to-noise (S/N) ratio. Processing of the specimens in 70% ethanol resulted in marked shrinkage, in contrast to results when processing in 30% ethanol. To overcome these problems, the trachea was initially fixed in 2.5% glutaraldehyde (0.1 M phosphate buffer pH 7.4), treated with a mixture of 0.2% oolong tea extract (OTE) and 2.5% glutaraldehyde, and postfixed in 1% osmium tetroxide. The sample was immersed in 30% ethanol and examined in a chilled SEM at -10 degrees C. The luminal coutour of the tracheal epithelial cells was clearly observed because of the decrease in shrinkage. Cilia of ciliated cells and microvilli of nonciliated cells were also clearly observed. These specimens also showed a high S/N ratio, thus allowing the observation of samples without the need for complete dehydration, critical-point drying, or metal coating. This OTE-incorporated conductive staining method is simple and rapid, and should prove to be highly useful for rapid SEM analyses of biological specimens.

Characterization of biological stains on external concrete walls and influence of concrete as underlying material

Investigations were carried out on concrete walls stained by biological growth. Pieces of this material were removed down and observed using optical microscopy, low-vacuum scanning electron microscopy (LVSEM) and normal SEM. The results show that biological stains are due to two different kinds of microscopic algae, Chlorophyceae and Cyanophyceae, whose presence depends on the amount of moisture on the concrete wall. Accelerated laboratory tests of biological growth on mortar samples that were performed show that algal developments increase with the porosity of the underlying material. Thus, it seems that the use of dense, high-performance mortars can slow down or even inhibit microorganism growth.

Observation of impurities in ice.

This paper presents a new method for determining the microstructural location of impurities in polycrystalline ice, which involves allowing the ice to sublimate under vacuum and then identifying the concentrated impurities using energy dispersive X-ray microanalysis in a low-vacuum scanning electron microscope. The method allows the presence of impurities in both the grain boundaries and the lattice of natural polycrystalline ice to be observed. Using this technique on natural ice, filaments consisting chiefly of NaCl were observed in some grain boundaries, small amounts of sulfur and chlorine were found in the grain interiors of the ice, and large concentrations of sulfur were observed in inclusions.

Using a Polycapillary Optic as a Spatial Filter to Improve Micro Xray Analysis in Low-Vacuum and Environmental SEM Systems

Abstract An inevitable consequence of the presence of the gas in the sample chamber of a low-vacuum scanning electron microscope (LV-SEM) and environmental SEM (ESEM) is the electron beam broadening due to the scattering in the gas. The electron broadening has a large impact on x-ray analysis because the fluorescent characteristic x rays generated far from the center of the electron probe form a high background, which reduces the detection sensitivity of x-ray analysis and degrades the x-ray image contrast. We report in this paper of using a polycapillary focusing x-ray optic between the sample and the energy-dispersive spectrometer as a spatial filter to filter out unwanted x-rays generated far from the specimen. As a result, the x-ray image contrast and the detection sensitivity of the system were notably improved. A polycapillary focusing optic collects a large solid angle of x rays from an x-ray source of small area at its input focus, guide them through the curved channels by multiple external total reflections, and focus them to the output focus.

Effects of Gaseous ion Concentration on Secondary Electron Contrast of Dielectric Materials Imaged in the ESEM

An increasing body of experimental evidence indicates that the flux and distribution of gaseous ions significantly affects secondary electron contrast of dielectric materials imaged in low vacuum and environmental scanning electron microscopes. Unusual contrast has been identified in some insulating materials, and attributed to charging effects (1-3). However, as these contrast mechanisms have not been reported in conventional microscopes, it would seem that the ions play an important role in making the contrast manifest. The effects of ions can include the creation of space charge in the gas above the specimen, influencing the charge state of the specimen surface, and electron-ion recombination (possibly leading to energy filtering of emitted secondary electrons). Space charge and surface potential can influence the gain realised in the gas amplification cascade by altering the detector field (4). However, surface potential, and recombination affect the number of electrons injected into the amplification cascade, to give rise to contrast variations in the final image.

Adsorptive Properties of Fish Scales of Oreochromis Niloticus (Mojarra Tilapia) for Metallic Ion Removal from Waste Water

The present work presents the experimental study of the adsorptive properties for the removal of Cu2+ , Pb2+, Co2+, and Ni2+ metallic ions from water of thermally pretreated fish scales from a very common and abundant source in Mexico, the Oreachromis Niloticus fish, known as “Mojarra Tilapia”. To determine the influence and importance that the two main fractions (the organic or protein fraction and the inorganic fraction mainly composed of hydroxyapatite) of this material play in this adsorptive capacity, the original pretreated fish scales are subjected to an acid demineralization and a basic deproteinization treatment to modify the organic/inorganic matter ratios present, and their materials are characterized in terms of their adsorption ability, especially in the removal of the Cu2+ ion. The adsorption isotherms for the thermally pretreated fish scales show that they have a maximum Cu2+ adsorption capacity of 58 mg of Cu2+ /g of adsorbent with an ion selectivity removal in the order Cu2+ > Pb2+ > Co2+ > Ni2+. When the pure organic and inorganic parts of the fish scales are used in the adsorption experiments, the inorganic part has a 75% higher removal capacity than the organic fraction. The adsorption experiments using fish scales with different organic or inorganic fractions show a nonadditive, synergistic effect on the equilibrium amount of metallic ion adsorbed, which is independent of the inorganic content between 30 and 90 wt % of this inorganic fraction. Low-vacuum scanning electron microscopy and elemental analysis reveal that the ion-exchange reaction is the main mechanism for the metallic ion adsorption by fish scales.

PRELIMINARY STUDIES OF ASPHALTENE AGGREGATES BY LOW VACUUM SCANNING ELECTRON MICROSCOPY

A study of vacuum residue and asphaltenes from a Mexican oil refinery was performed using low vacuum scanning electron microscopy (LV-SEM) to determine the morphology of aggregates, and energy dispersive spectroscopy (EDS) to get the elemental composition of them. Irregular shaped particles were observed on vacuum residue, with a size range from 10 to 30 μm, that emerged from the oily fraction under the action of the electron beam. Those particles were sulphur and silicon rich. We also found some particles with important concentrations of Mg, Cl, K, and Fe. Asphaltenes precipitated with different n-alkane solvents showed slight differences in morphology. Fragile particles of several sizes were analyzed (230 × 130 μm to 730 × 240 μm); most of them presented a smooth face side by side with a rough one. Higher magnification of the smooth faces showed some randomly distributed pores. Rough facets tended to show larger pores. Some inclusions were also found; the sizes of those ranged from 7 to 25 μm. EDS revealed that sulphur and silicon are more abundant in the inclusions. Some Fe and Mg rich particles were also found. All the samples presented small amounts of V, and some of them Zn and Sn, whose concentration depends on the solvent used.

Compositional and morphological imaging of CO2 laser irradiated human teeth by low vacuum SEM, confocal laser scanning microscopy and atomic force microscopy.

Enamel and dentin of human teeth irradiated by CO2 laser were investigated by confocal laser scanning microscopy (CLSM), low vacuum scanning electron microscopy (WET-SEM) and atomic force microscopy (AFM). Optical tomographic imaging by CLSM, compositional imaging based on atomic number effect of reflected electrons by WET-SEM, high resolution observation of surface morphology by AFM were done for both the irradiated and non-irradiated area of the same specimen throughout. The crystals of about 50 microm length and the bright spots were observed by CLSM at the bottom of the cavity induced by laser irradiation. They turned out from the observation by WET-SEM as the acicular crystals with the cross section of an irregularly hexagonal shape situated parallel and perpendicular, respectively, to the inner surface of the cavity. The thickness of the thermally deteriorated zone of the cavity was about 25 microm. The crystals unidirectionally grown up to the size of several hundreds nm were also observed by AFM, while the apatite crystallites of 50-150 nm were recognized all over in non-irradiated area. All the results suggest that after instantaneous melting at the surface of teeth by CO2 laser shot the crystals of calcium phosphate were recrystalized and grown to a large size. The compositional imaging in addition to morphological observation was useful to obtain the information of the change in materials induced by laser irradiation.