Environmental Scanning Electron Microscope Chamber Research Articles

Microstructural insight into the nonlinear swelling of argillaceous rocks

Argillaceous rocks are chosen as possible host rocks for underground radioactive nuclear waste disposal. These rocks exhibit complex coupled thermo–hydro–chemo-mechanical behavior, the description of which would strongly benefit from an improved experimental insight on micro-scale. In this work we present some recent observations of the evolution of these rocks upon swelling on the scale of their composite microstructure, essentially made of a clay matrix with embedded grains of calcite and quartz with sizes ranging from a few to several hundreds of micrometers. The micro-scale experimental investigation was based on the combination of high definition and high resolution imaging in an environmental scanning electron microscope (ESEM) and digital image correlation techniques. Samples were held at a constant temperature of 2°C while the vapor pressure in the ESEM chamber was varied from a few to several hundreds of Pascals, generating a relative humidity (RH) ranging from about 10% up to 99%. Results on micro-scale showed strongly heterogeneous deformation fields, which result from complex hydromechanical interactions between different components of argillaceous rocks. The swelling of argillaceous rocks is moderate at low RH but becomes significant at high RH. The observations demonstrated that the nonlinearity is related not only to the micro-cracking upon wetting, but also to the nonlinear swelling of the clay matrix itself that is governed by different mechanisms.

Development of an Integrated Thermocouple for the Accurate Sample Temperature Measurement During High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) Experiments.

We have developed two integrated thermocouple (TC) crucible systems that allow precise measurement of sample temperature when using a furnace associated with an environmental scanning electron microscope (ESEM). Sample temperatures measured with these systems are precise (±5°C) and reliable. The TC crucible systems allow working with solids and liquids (silicate melts or ionic liquids), independent of the gas composition and pressure. These sample holder designs will allow end users to perform experiments at high temperature in the ESEM chamber with high precision control of the sample temperature.

In SituStudy of Live Specimens in an Environmental Scanning Electron Microscope

In this paper we introduce new methodology for the observation of living biological samples in an environmental scanning electron microscope (ESEM). The methodology is based on an unconventional initiation procedure for ESEM chamber pumping, free from purge-flood cycles, and on the ability to control thermodynamic processes close to the sample. The gradual and gentle change of the working environment from air to water vapor enables the study of not only living samples in dynamic in situ experiments and their manifestation of life (sample walking) but also its experimentally stimulated physiological reactions. Moreover, Monte Carlo simulations of primary electron beam energy losses in a water layer on the sample surface were studied; consequently, the influence of the water thickness on radiation, temperature, or chemical damage of the sample was considered.

Novel Method for Controlled Wetting of Materials in the Environmental Scanning Electron Microscope

Environmental scanning electron microscopy has been extensively used for studying the wetting properties of different materials. For some types of investigation, however, the traditional ways of conducting in situ dynamic wetting experiments do not offer sufficient control over the wetting process. Here, we present a novel method for controlled wetting of materials in the environmental scanning electron microscope (ESEM). It offers improved control of the point of interaction between the water and the specimen and renders it more accessible for imaging. It also enables the study of water transport through a material by direct imaging. The method is based on the use of a piezo-driven nanomanipulator to bring a specimen in contact with a water reservoir in the ESEM chamber. The water reservoir is established by local condensation on a Peltier-cooled surface. A fixture was designed to make the experimental setup compatible with the standard Peltier cooling stage of the microscope. The developed technique was successfully applied to individual cellulose fibers, and the absorption and transport of water by individual cellulose fibers were imaged.

In situ ESEM investigation of the initial stages of corrosion phenomena on zinc and magnesium surfaces

This article investigates the initial stages of corrosion phenomena on zinc and magnesium promoted by chlorine contamination by means of an environmental scanning electron microscope (ESEM). In particular, the corrosion of the substrates was promoted in situ in the ESEM chamber by modifying the relative humidity, thus promoting the condensation of water on the metal surface. The metal surfaces were previously contaminated with sodium chlorides. The corrosion morphologies obtained in the ESEM chamber were compared to the degradation promoted by exposing zinc and magnesium samples in the humidostatic chamber.

Swelling–shrinkage measurements of bentonite using coupled environmental scanning electron microscopy and digital image analysis

The swelling clays have been proposed as engineered barriers in geological disposal systems for waste because these materials are assumed to build a better impermeable zone around wastes by swelling. However, the swelling potential of soils is also considered a prevalent cause of damage to buildings and constructions. For these reasons, it is fundamental to investigate the physicochemical and mechanical behavior of swelling clays. In the current study, the swelling–shrinkage potential (aggregates scale) was estimated using an environmental scanning electron microscope (ESEM) coupled with a digital image analysis (DIA) program (Visilog). In fact, the isolated aggregates of raw and cation-exchanged bentonite were directly observed at different relative humidities in an ESEM chamber. Then the “Visilog” software was used to estimate the percent augmentation of the aggregate surface as a function of time and as a function of relative humidity. This estimation allows for the calculation of the swelling–shrinkage potential (%) of bentonite. Finally, a kinetic model of first order was tested to fit the kinetic experimental data of swelling–shrinkage potential. The results show that ESEM–DIA coupling can be a powerful method of estimating the swelling–shrinkage potential of expansive clays. In addition, the exponential models fit well with the kinetic experimental data.

Fracture in metal–ceramic composites

This research focuses on fracture mechanisms in metal–ceramic composites. Two co-continuous composites, Cu/Al 2O 3 and Al/Al 2O 3 and a metal–matrix composite Al/SiC were studied. It was found that each composite displayed a different fracture mechanism. The crack propagation inside the metal–matrix composite was dominated by the Al matrix characteristics. However, crack propagation inside both the co-continuous composites was influenced by their microstructure, thermal residual stresses and contiguity. This unique fracture characteristic of co-continuous composites has been elucidated in the present research by experimentation as well as computational modeling. In situ three-point bend tests were performed inside an environmental scanning electron microscope chamber to observe crack growth at the microstructural scale. Finite element modeling was performed by using global–local approach to simulate crack propagation and understand the effects of the microstructure and thermal residual stresses. It was shown that the crack propagated inside the metallic phase and at the interface for the Cu/Al 2O 3 composite due to a high level of tensile thermal stresses inside the metallic phase, as well as due to low contiguity of ceramic phase. However, in the case of Al/Al 2O 3 composite, the crack propagated inside the ceramic due to significantly smaller thermal stresses inside the metallic phase as well as higher contiguity of ceramic phase.

Dynamic Characterisation of Industrial Textiles Using an Environmental Scanning Electron Microscope

The fundamental understanding of individual fibers and fiber assemblies has driven innovations in industrial textiles. The use of new tools is of importance in this innovation process. The use of Environmental Scanning Electron Microscope (ESEM) for characterization of industrial textiles under dynamic conditions has been explored in this study. We have developed different techniques for examining and imaging industrial textiles under varying conditions. The dynamic experiments of wetting, absorption, loading deformation, and thermal bonding of industrial textiles were performed in the Philips XL 30 FEG-ESEM. ESEM provides new approaches to dynamic studies of industrial textiles. The relative humidity in the ESEM chamber was raised up to 100% by controlling the specimen's temperature and the chamber pressure to produce the condition for dynamic wetting and absorption tests. The tensile testing was carried out on a tensile stage that was placed in the ESEM chamber. The thermal bonding process was imaged on a hot stage in the ESEM. Studies under dynamic conditions have given new insight into the kinetics of structure formation, rearrangement, and breakdown that are important for processing and product development of industrial textiles.

Solid Gold Nanostructures Fabricated by Electron Beam Deposition

Direct writing with gold by electron beam deposition is a method for rapid fabrication of electrically conducting nanostructures. An environmental scanning electron microscope (ESEM) equipped with a source of the precursor gas dimethylacetylacetonate gold(III) was used to fabricate nanoscale tips and bridges. Transmission electron microscopy was used to study how the composition of these structures was affected when the background gas in the ESEM chamber and the electron beam parameters were varied. The nanostructures were layered composites of up to three different materials each characterized by a certain range of gold/carbon ratios. Above a certain threshold of ESEM chamber water vapor pressure and a certain threshold of electron beam current, the deposited tips contained a solid polycrystalline gold core. The deposition technique was used to fabricate free-standing nanowires and to solder free-standing carbon nanotubes to gold electrodes as well as to other carbon nanotubes.

Dynamic Wetting of Fibers Observed in an Environmental Scanning Electron Microscope

Wetting is an important characteristic of textile materials in many applications. Micro scopic observations of dynamic wetting of untreated and oxygen plasma treated polypro pylene fibers on the environmental scanning electron microscope (ESEM) are explored in this study. The relative humidity in the ESEM chamber is raised to 100% by controlling the specimen's temperature and the chamber pressure to produce water condensation on the fiber surfaces. Dynamic wetting, that is, the formation, spreading, and coalescence of water droplets on fiber surfaces, is observed and recorded in real time. Contact angles are measured from the ESEM images, which reveal the dynamic wetting of textile fibers on a micron level.

Detection of the Fracture Path under Tensile Loads through in situ Tests in an ESEM Chamber

Summary In situ tensile tests parallel to the grain were carried out in an Environmental Scanning Electron Microscope (ESEM) chamber on Norway spruce (Picea abies [L.] Karst.) samples. The ESEM-mode combined with a cooling device allowed examination of the specimens at a moisture content of 12% with unsputtered surfaces. By recording load-displacement curves and observing crack propagation simultaneously, a detailed image of fracture progress and tissue interaction could be described. Since these experiments required a sufficient specimen size and geometry, focus was concentrated on the methodology.

Observation of wetting behavior of polypropylene microfibers by environmental scanning electron microscope

The wetting behavior of polypropylene meltblown microfibers by water and oil was investigated using an environmental scanning electron microscope (ESEM). Water droplets were formed on the fiber surfaces by controlling temperature and pressure in the ESEM chamber and oil drops were added onto the fibers using a microinjector mounted on the ESEM chamber. The contact angles of water and oil were measured from the ESEM micrographs.

The role of induced contrast in images obtained using the environmental scanning electron microscope

Generation of contrast in images obtained using the environmental scanning electron microscope (ESEM) is explained by interpretation of images acquired using the gaseous secondary electron detector (GSED), ion current, and the Everhart-Thornley detector. We present a previously unreported contrast component in GSED and ion current images attributed to signal induction by changes in the concentration of positive ions in the ESEM chamber during image acquisition. Changes in positive ion concentration are caused by changes in electron emission from the sample during image acquisition and by a discrepancy between the drift velocities of negative and positive charge carriers in the imaging gas. The proposed signal generation mechanism is used to explain contrast reversal in images produced using the GSED and ion current signals and accounts for discrepancies in contrast observed, under some conditions, in these types of images. Combined with existing models of signal generation in the ESEM, the proposed model provides a basis for correct interpretation of ESEM images.

A new correction method for high‐resolution energy‐dispersive x‐ray analyses in the environmental scanning electron microscope

AbstractSpurious x‐ray signals, which previously prevented high‐resolution energy‐dispersive x‐ray analysis (EDS) in the environmental scanning electron microscope (ESEM), can be corrected using a simple method presented here. As the primary electron beam travels through the gas in the ESEM chamber, a significant fraction of the primary electrons is scattered during collisions with gas molecules. These scattered electrons form a broad skirt that surrounds the primary electron beam as it impacts the sample. The correction method assumes that changes in the width of the electron skirt with pressure are less important than changes in the skirt intensity; this method works as follows: The influence of the gas on the overall x‐ray data is determined by acquiring EDS spectra at two pressures. Subtracting the two spectra provides us with a difference spectrum which is then used to correct the original data, using extrapolation, back to the x‐ray spectrum expected under high‐vacuum conditions. Low‐noise data are required to resolve small spectral peaks; however, the principle should apply equally to x‐ray maps and even to low‐magnification images.

Dynamic imaging of structural changes in silver catalysts by environmental scanning electron microscopy.

Polycrystalline silver catalysts are used extensively for the partial oxidation of methanol to formaldehyde, which is then primarily incorporated in the synthesis process for adhesives and resins. In order to maximize formaldehyde production it is essential to gain a comprehensive understanding of the complex microstructural changes which occur in the catalyst during reaction conditions. However, conventional electron microscopic techniques are incapable of imaging catalysts at high temperatures and in the presence of a gaseous atmosphere. Therefore, an environmental scanning electron microscope (ESEM) has been used to image polycrystalline silver catalysts during simulated industrial conditions. The most dramatic effect of heating various catalysts to 700 degrees C in the ESEM chamber was the formation of "pinholes" in the silver surface. These "pinholes" occur at specific temperatures and are inherently associated with the catalytic process, resulting from near-surface explosions caused by subsurface hydroxyl recombination. Of particular interest was the nature and location of the holes, which preferentially occur in the vicinity of surface defects such as platelets and edge structures. To the best of our knowledge, this study represents the first time that the progress of a catalytic reaction has been observed under in situ conditions by scanning electron microscopy.

Electron behavior in the gaseous environment of the ESEM chamber

Measurements of electrons striking the sample in the Environmental Scanning Electron Microscope (ESEM) are needed to begin to understand the effect of the presence of the gas on analytical measurements. Accurate beam current is important to x-ray microanalysis and it is typically measured with a faraday cup. A faraday cup (Figure 1) was constructed from a carbon block embedded in non-conductive epoxy with a 45 micrometer bore platinum aperture over the hole. Currents were measured with an electrometer and recorded as instrument parameters were varied.Instrument parameters investigated included working distance, chamber pressure, condenser percentage, and accelerating voltage. The conditions studied were low vacuum with gaseous secondary electron detector (GSED) voltage on; low vacuum with GSED voltage off; and high vacuum (GSED off). The base conditions were 30 kV, 667 Pa (5 Torr) water vapor, 100,000x magnification with the beam centered inside aperture, GSED voltage at 370 VDC, condenser at 50%, and working distance at 19.5 mm. All modifications of instrument parameters were made from these conditions.