Quantification of extreme thermal gradients during in situ transmission electron microscope heating experiments

Studies on materials affected by large thermal gradients and rapid thermal cycling are an area of increasing interest, driving the need for real time observations of microstructural evoultion under transient thermal conditions. However, current in situ transmission electron microscope (TEM) heating stages introduce uniform temperature distributions across the material during heating experiments. Here, a methodology is described to generate thermal gradients across a TEM specimen by modifying a commercially available MEMS-based heating stage. It was found that a specimen placed next to the metallic heater, over a window, cut by FIB milling, does not disrupt the overall thermal stability of the device. Infrared thermal imaging (IRTI) experiments were performed on unmodified and modified heating devices, to measure thermal gradients across the device. The mean temperature measured within the central viewing area of the unmodified device was 3–5% lower than the setpoint temperature. Using IRTI data, at setpoint temperatures ranging from 900 to 1,300°C, thermal gradients at the edge of the modified window were calculated to be in the range of 0.6 × 10⁶ to 7.0 × 10⁶°C/m. Additionally, the Ag nanocube sublimation approach was used, to measure the local temperature across a FIB-cut Si lamella at high spatial resolution inside the TEM, and demonstrate “proof of concept” of the modified MEMS device. The thermal gradient across the Si lamella, measured using the latter approach was found to be 6.3 × 10⁶°C/m, at a setpoint temperature of 1,000°C. Finally, the applicability of this approach and choice of experimental parameters are critically discussed.     

High‐temperature oxidation of nickel and chromium studied with an in‐situ environmental scanning electron microscope

Investigations of the morphology of metal oxide scales formed at high temperatures in oxidative environments are usually undertaken after exposure of the samples is completed. In this study, an environmental scanning electron microscope (ESEM) was used as a tool for the in‐situ observation of oxide scale formation. Pure nickel and chromium samples were oxidized at a temperature of 973 K in either pure oxygen or water vapour at a pressure of 667 Pa. The evolution of an oxide scale was followed in‐situ for up to 3 h. The morphology of the developing oxide scales was found to be a function of the metal substrate and the gaseous species. The growth mechanisms of the different metal oxide scales are reviewed and related to the analysed in‐situ images. Emphasis is placed on the relationship between oxidation mechanism and scale morphology. Nickel is seen to oxidise by outward diffusion of nickel probably on oxide grain boundaries when exposed to oxygen. Water vapour changes the scale morphology and a duplex‐type scale arises due to preferential overgrowth. The scale which develops due to chromium oxidation in oxygen is a fine‐grained, thin, and dense layer. In contrast, water vapour leads to whisker growth on chromium and an open, felt‐like structure forms. The applicability of the ESEM to the study of such systems is demonstrated, and its limitations are outlined. The results are encouraging examples of the possibilities which the in‐situ ESEM technique possesses.     

Automated jet polishing of transmission electron microscope specimens for in situ straining

A jet-polishing technique has been developed for use in the preparation of microtensile specimens for HVEM examination. The technique requires the use of a pair of Teflon sheet inserts with rectangular openings in a conventional specimen holder. When inserts with optimum opening dimensions are used, specimens having elliptical holes close to the center of the gauge section are produced with large electron-transparent areas at both ends of the long axis. Annealed metal specimens, such as brass or aluminum, prepared by this method are stronger, and can be handled more easily, than those prepared by conventional methods. An advantage of the technique is that reproducible electropolishing conditions and the automatic detection of perforation by a photocell can be used in the normal way.     

High-temperature electron backscatter diffraction and scanning electron microscopy imaging techniques: in-situ investigations of dynamic processes

In-situ heating experiments have been conducted at temperatures of approximately 1200 K utilising a new design of scanning electron microscope, the CamScan X500. The X500 has been designed to optimise the potential for electron backscatter diffraction (EBSD) analysis with concomitant in-situ heating experimentation. Features of the new design include an inclined field emission gun (FEG) column, which affords the EBSD geometrical requirement of a high (typically 160 degrees) angle between the incoming electron beam and specimen surface, but avoids complications in heating-stage design and operation by maintaining it in a horizontal orientation. Our studies have found that secondary electron and orientation contrast imaging has been possible for a variety of specimen materials up to a temperature of at least 900 degrees C, without significant degradation of imaging quality. Electron backscatter diffraction patterns have been acquired at temperatures of at least 900 degrees C and are of sufficient quality to allow automated data collection. Automated EBSD maps have been produced at temperatures between 200 degrees C and 700 degrees C in aluminium, brass, nickel, steel, quartz, and calcite, and even at temperatures >890 degrees C in pure titanium. The combination of scanning electron microscope imaging techniques and EBSD analysis with high-temperature in-situ experiments is a powerful tool for the observation of dynamic crystallographic and microstructural processes in metals, semiconductor materials, and ceramics.     

Direct observation by in situ transmission electron microscopy of the behaviour of IF‐MoS2 nanoparticles during sliding tests: influence of the crystal structure

Direct observation of the behaviour of individual inorganic fullerenes (IF)‐MoS₂ nanoparticles in a sliding interface is essential for the understanding of the influence of the intrinsic characteristics of the nanoparticles on their lubrication mechanisms, when they are used as additives in lubricant oil. In this work, in situ transmission electron microscopy sliding tests were performed on two different types of MoS₂ nanoparticles synthesised by two different methods. It is shown that the IF‐MoS₂ nanoparticles having perfect structure with a high crystalline order and without defects are able to roll and to slide under the combined effect of pressure and shear stress, whereas the IF‐MoS₂ nanoparticles containing many defects exfoliate immediately in the same conditions to deliver MoS₂ layers covering the mating surfaces. A link between these results, the lubrication mechanisms of the nanoparticles and their tribological properties at the macro‐scale was established, proving that the lubrication mechanisms of fullerenes depend on their intrinsic characteristics. Copyright © 2013 John Wiley & Sons, Ltd.     

Cryogenic‐temperature electron microscopy direct imaging of carbon nanotubes and graphene solutions in superacids

Cryogenic electron microscopy (cryo‐EM) is a powerful tool for imaging liquid and semiliquid systems. While cryogenic transmission electron microscopy (cryo‐TEM) is a standard technique in many fields, cryogenic scanning electron microscopy (cryo‐SEM) is still not that widely used and is far less developed. The vast majority of systems under investigation by cryo‐EM involve either water or organic components. In this paper, we introduce the use of novel cryo‐TEM and cryo‐SEM specimen preparation and imaging methodologies, suitable for highly acidic and very reactive systems. Both preserve the native nanostructure in the system, while not harming the expensive equipment or the user. We present examples of direct imaging of single‐walled, multiwalled carbon nanotubes and graphene, dissolved in chlorosulfonic acid and oleum. Moreover, we demonstrate the ability of these new cryo‐TEM and cryo‐SEM methodologies to follow phase transitions in carbon nanotube (CNT)/superacid systems, starting from dilute solutions up to the concentrated nematic liquid‐crystalline CNT phases, used as the ‘dope’ for all‐carbon‐fibre spinning. Originally developed for direct imaging of CNTs and graphene dissolution and self‐assembly in superacids, these methodologies can be implemented for a variety of highly acidic systems, paving a way for a new field of nonaqueous cryogenic electron microscopy.     

Cross‐section metal sample preparations for transmission electron microscopy by electro‐deposition and electropolishing

A cross‐section sample preparation technique is described for transmission electron microscopy studies of metallic materials. The technique uses jet electro‐polishing for the final perforation. Examples are provided of using this technique for copper‐support/copper‐films/copper‐support multilayer structures, grown by electro‐deposition. The samples prepared by our current technique are compared with the ones made by ion‐milling. The technique is also applicable to materials which are susceptible to ion beam and thermal damages. Microsc. Res. Tech. 76:476–480, 2013. © 2013 Wiley Periodicals, Inc.     

Measurement technique for the incident electron current in secondary electron detectors and its application in scanning electron microscopes.

A measurement technique for incident electron current in secondary electron (SE) detectors, especially the Everhart-Thornley (ET) detector, based on signal-to-noise ratio (SNR), which uses the histogram of a digital scanning electron microscope (SEM) image, is described. In this technique, primary electrons are directly incident on the ET detector. This technique for measuring the correlation between incident electron current and SNR is applicable to the other SE detectors. This correlation was applied to estimate the efficiency of the ET detector itself, to evaluate SEM image quality, and to measure the geometric SE collection efficiency and the SE yield. It was found that the geometric SE collection efficiency at each of the upper and lower detectors of a Hitachi S-4500 SEM was greater than 0.78 at all working distances.     

Interfacial ultramorphology evaluation of resin luting cements to dentin: A correlative scanning electron microscopy and transmission electron microscopy analysis

The objective of this study was to analyze the dentin‐resin cements interfacial ultramorphologies using two different methods: scanning (SEM) and transmission electron microscopy (TEM). Four commercial products were evaluated: two conventional cementing system (RelyX ARC/Adper? Scotchbond? Multi‐Purpose Plus, 3M ESPE and Clearfil Esthetic Cement/DC Bond, Kuraray) and two self‐adhesive resin cements (RelyX Unicem, 3M ESPE and Clearfil SA Cement, Kuraray). Prepolymerized resin disks (Sinfony, 3M ESPE) were cemented on oclusal dentin surfaces of 24 third human molars, simulating the indirect restorations. After 24 h, teeth were sectioned into 0.9‐mm thick slabs and processed for microscopy analyses (SEM or TEM/ n = 3). Qualitative characterization of dentin‐resin cement interface was performed. Hybrid layer formation with long and dense resin tags was observed only for RelyX ARC cementing system. Clearfil Esthetic Cement/DC Bond system revealed few and short resin tags formation, whereas no hybridization and resin tags were detected for self‐adhesive resin cements. Some interfacial regions exhibited that the self‐adhesive resin cements were not bonded to dentin, presenting bubbles or voids at the interfaces. In conclusion, TEM and SEM bonding interface analyses showed ultramorphological variations among resin cements, which are directly related to dental bonding strategies used for each resin cement tested. Microsc. Res. Tech. 76:1234–1239, 2013. © 2013 Wiley Periodicals, Inc.     

Study of the surfactant role in latex–aerogel systems by scanning transmission electron microscopy on aqueous suspensions

For insulation applications, boards thinner than 2 cm are under design with specific thermal conductivities lower than 15 mW m^?1 K^?1. This requires binding slightly hydrophobic aerogels which are highly nanoporous granular materials. To reach this step and ensure insulation board durability at the building scale, it is compulsory to design, characterise and analyse the microstructure at the nanoscale. It is indeed necessary to understand how the solid material is formed from a liquid suspension. This issue is addressed in this paper through wet‐STEM experiments carried out in an Environmental Scanning Electron Microscope (ESEM). Latex–surfactant binary blends and latex–surfactant–aerogel ternary systems are studied, with two different surfactants of very different chemical structures. Image analysis is used to distinguish the different components and get quantitative morphological parameters which describe the sample architecture. The evolution of such morphological parameters during water evaporation permits a good understanding of the role of the surfactant.     

In situ TEM studies of micron‐sized all‐solid‐state fluoride ion batteries: Preparation,prospects, and challenges

Trustworthy preparation and contacting of micron‐sized batteries is an essential task to enable reliable in situ TEM studies during electrochemical biasing. Some of the challenges and solutions for the preparation of all‐solid‐state batteries for in situ TEM electrochemical studies are discussed using an optimized focused ion beam (FIB) approach. In particular redeposition, resistivity, porosity of the electrodes/electrolyte and leakage current are addressed. Overcoming these challenges, an all‐solid‐state fluoride ion battery has been prepared as a model system for in situ TEM electrochemical biasing studies and first results on a Bi/La_0.9Ba_0.1F_2.9 half‐cell are presented. Microsc. Res. Tech. 79:615–624, 2016. © 2016 Wiley Periodicals, Inc.     

A real‐time image dynamic range compensation for scanning electron microscope imaging system

This paper presents the development and implementation of a real‐time dynamic range compensation system for scanning electron microscope (SEM) imaging applications. Compared with conventional automatic brightness contrast compensators that are based on the average image or pixel intensity level, the proposed system utilizes histogram‐profiling techniques to compensate continuously the dynamic range of the processed video signal. The algorithms are implemented in software with a frame grabber card forming the front‐end video capture element. The proposed technique yields better image compensation compared with conventional methods.     

Disclosure in 3D of slit-membrane as well as -strands, and en-face basal lamina in situ of renal glomerulus of normal rats in embedment-free section transmission electron microscopy

With higher contrast and transparency due to the absence of epon and stereo-viewing effect due to thicker sections than conventional electron microscopy as methodological advantages, the renal glomerular slits were re-examined in embedment-free section electron microscopy. In addition to clear demonstration of strands bridging the slits in forms of ladders with highly irregular intervals and various extension-directions and length, this study disclosed clearly for the first time in the "section" TEM thin sheets which partially spanned the slit together with the strand-ladders. No strands were found to align in forms of typical zippers in normal kidney. Furthermore, en-face ultrastructure of the basal lamina in situ was clearly demonstrated in superimposed sites of the endothelial fenestrae with the slits.     

In situ observations of unusual dislocation mechanisms in the intermetallic alloy Ti3Al

In situ straining experiments in a transmission electron microscope have been carried out on a Ti₃Al intermetallic alloy, with the aim of determining the microscopic mechanisms controlling glide in prism, basal and pyramidal planes. Five different antiphase boundary energies have been measured and compared with the corresponding densities of incorrect first nearest neighbour atoms. The determination of a tension–compression asymmetry in pyramidal slip, and the detailed analysis of the complex microscopic mechanisms involved illustrate the efficiency of in situ experiments to solve complex problems in plasticity. A comparison between the properties of the different slip systems shows that they are controlled by different microscopic mechanisms, none of them being of covalent origin.     

Orientation and phase mapping in the transmission electron microscope using precession‐assisted diffraction spot recognition: state‐of‐the‐art results

A recently developed technique based on the transmission electron microscope, which makes use of electron beam precession together with spot diffraction pattern recognition now offers the possibility to acquire reliable orientation/phase maps with a spatial resolution down to 2 nm on a field emission gun transmission electron microscope. The technique may be described as precession‐assisted crystal orientation mapping in the transmission electron microscope, precession‐assisted crystal orientation mapping technique–transmission electron microscope, also known by its product name, ASTAR, and consists in scanning the precessed electron beam in nanoprobe mode over the specimen area, thus producing a collection of precession electron diffraction spot patterns, to be thereafter indexed automatically through template matching. We present a review on several application examples relative to the characterization of microstructure/microtexture of nanocrystalline metals, ceramics, nanoparticles, minerals and organics. The strengths and limitations of the technique are also discussed using several application examples.     

Multi‐walled carbon nanotubes decorated by platinum catalyst nanoparticles—Examination and microanalysis using scanning and transmission electron microscopies

Carbon nanotubes (CNTs) decorated with platinum (Pt) nanoparticles (NPs) have been characterized using a cold field‐emission scanning electron microscope (SEM) and a high resolution field‐emission transmission electron microscope (TEM). With this particular composite material, the complementary nature of the two instruments was demonstrated. Although the long CNTs were found to be mostly bent and defective in some parts, the nucleation of Pt occurred randomly and uniformly covered the CNTs. The NPs displayed a large variation in size, were sometimes defective with twins and stacking faults, and were found to be faceted with the presence of surface steps. The shape and size of the NPs and the presence of defects may have significant consequences on the activity of the Pt catalyst material. Also, thin layers of platinum oxide were identified on the surface of some NPs.     

Challenges of microtome‐based serial block‐face scanning electron microscopy in neuroscience

Serial block‐face scanning electron microscopy (SBEM) is becoming increasingly popular for a wide range of applications in many disciplines from biology to material sciences. This review focuses on applications for circuit reconstruction in neuroscience, which is one of the major driving forces advancing SBEM. Neuronal circuit reconstruction poses exceptional challenges to volume EM in terms of resolution, field of view, acquisition time and sample preparation. Mapping the connections between neurons in the brain is crucial for understanding information flow and information processing in the brain. However, information on the connectivity between hundreds or even thousands of neurons densely packed in neuronal microcircuits is still largely missing. Volume EM techniques such as serial section TEM, automated tape‐collecting ultramicrotome, focused ion‐beam scanning electron microscopy and SBEM (microtome serial block‐face scanning electron microscopy) are the techniques that provide sufficient resolution to resolve ultrastructural details such as synapses and provides sufficient field of view for dense reconstruction of neuronal circuits. While volume EM techniques are advancing, they are generating large data sets on the terabyte scale that require new image processing workflows and analysis tools. In this review, we present the recent advances in SBEM for circuit reconstruction in neuroscience and an overview of existing image processing and analysis pipelines.     

A global investigation into in situ nanoindentation experiments on zirconia: from the sample geometry optimization to the stress nanolocalization using convergent beam electron diffraction

Nanoindentation experiments inside a transmission electron microscope are of much interest to characterize specific phenomena occuring in materials, like for instance dislocation movements or phase transformations. The key points of these experiments are (i) the sample preparation and the optimization of its geometry to obtain reliable results and (ii) the choice of the transmission electron microscope observation mode, which will condition the type of information which can be deduced from the experiment. In this paper, we will focus on these two key points in the case of nanoindentation of zirconia, which is a ceramic material well known to be sensitive to stress because it can undergo a phase transformation. In this case, the information sought is the stress localization at the nanometre scale and in real time. As far as the sample preparation is concerned, one major drawback of nanoindentation inside a transmission electron microscope is indeed a possible bending of the sample occurring during compression, which is detrimental to the experiment interpretation (the stress is not uniaxial anymore). In this paper, several sample preparation techniques have been used and compared to optimize the geometry of the sample to avoid bending. The results obtained on sample preparation can be useful for the preparation of ceramics samples but can also give interesting clues and experimental approaches to optimize the preparation of other kinds of materials. The second part of this paper is devoted to the second key point, which is the determination of the stress localization associated to the deformation phenomena observed by nanoindentation experiments. In this paper, the use of convergent beam electron diffraction has been investigated and this technique could have been successfully coupled to nanoindentation experiments. Coupled nanoindentation experiments and convergent beam electron diffraction analyses have finally been applied to characterize the phase transformation of zirconia.