High-temperature Environmental Scanning Electron Research Articles

Revealing the kinetics of non-metallic inclusion reactions in steel using in-situ high temperature environmental scanning electron microscopy

The kinetics of solid-state reactions involving non-metallic inclusions (NMIs) in steel is a topic of growing interest due to the need to understand the effects of heat treatments on NMI composition and stability. In this study, a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) is utilized to conduct in-situ observations of various types of NMIs, including calcium aluminate complex, CaS, MgAl2O4-CaS, MnS, and TiN, in steels at temperatures of 800 °C and 900 °C for different holding times. The observed morphological variations in the NMIs indicate a decrease in the fractions of CaS and MnS, promoting void formation and ferrite–austenite phase transformation. To estimate the NMI reactions with the residual atmosphere in the sample chamber during experiments, HT-ESEM-driven thermodynamic calculations have been performed using FactSage 8.2 software. These calculations provide valuable insights into the interpretation of the experimental data, facilitating a better understanding of the complex NMI reactions in steel.

Effect of the process atmosphere on glass foam synthesis: A high-temperature environmental scanning electron microscopy (HT-ESEM) study

The effect of the process atmosphere composition on the foam formation of cathode-ray tube (CRT) glass containing graphite and MnO2 was studied using in situ environmental scanning electron microscopy at high temperature (HT-ESEM). When compared to He+4% H2, O2 or air, water steam facilitates glass grain sintering. This is probably due to the formation of hydroxyl groups at the glass grain surface that locally decrease the glass viscosity. We have shown that increasing the steam pressure from 50 Pa to 750 Pa decreases both sintering and foaming onset temperatures by approximately 100 °C, favouring the formation of closed pores in viscous glass. At high temperature, the presence of water steam or oxygen promotes foam formation, while the presence of a reducing atmosphere (He+4%H2) limits glass melt foaming. A synergetic effect of O2 and H2O on the onset temperature of glass sintering and foam formation is evidenced.

Crystallization mechanism of BaO-CaO-Al2O3-SiO2 (BCAS) glass thin-films

Crystallization pathways of BaOCaO-Al2O3-SiO2 (BCAS) glass were determined as a function of time and temperature, using a set of adapted and in-situ characterization techniques such as differential thermal analysis, high-temperature X-ray diffraction, nuclear magnetic resonance and high-temperature environmental scanning electron microscopy. Glass systems are a melted quenched bulk form and a deposited thin-film form (Pulsed Laser Deposition technique was used to obtain a coating on Si-substrate with a thickness up to 100 nm). The composition of the crystallized phases depends on the geometry of the glass, and of the applied geometrical constraints. These constraints control the volume fraction of crystallized glass melt as a function of time and temperature, and thus modify the reactional pathways.

Early stages of UO2+x sintering by in situ high-temperature environmental scanning electron microscopy

Early stages of various uranium oxides sintering were monitored in situ through High Temperature Environmental Scanning Electron Microscopy. Systems composed of two UO2+x microspheres were heated up to 900−1200 °C under PO2 ranging from 10−10 to 25 Pa. The oxide phases stabilized were assessed through thermodynamic calculations and powder X-ray diffraction. In all the conditions tested, the formation and the development of a neck was evidenced and image processing led to quantitative data describing the morphological changes.The evolution of the sintering degree was fitted using an exponential law and allowed the evaluation of the activation energy. When PO2 led to stabilize hyper-stoichiometric UO2+x oxides, the values obtained increased with x, typically in the 200−400 kJ mol−1 range. Under air atmosphere, the stabilization of U3O8 led to EA = 260 ± 40 kJ mol−1. Finally, the main diffusion mechanism driving neck formation was found to be volume diffusion, independently from the oxide stoichiometry.

Oxidation as an Early Stage in the Multistep Thermal Decomposition of Uranium(IV) Oxalate into U3O8.

The thermal decomposition of actinide oxalates is greatly dependent on the oxidation state of the cation, the gas involved, and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C2O4)2·6H2O can be viewed as a peculiar case, as its sensibility toward oxidation leads to a specific series of reactions when heating under an oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO3, we show here an extended characterization of the different intermediates through a combination of X-ray diffraction, vibrational spectroscopies and X-ray absorption near-edge spectroscopy. In this frame, uranium oxidation was found to occur at low temperature (200 °C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350 °C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to substoichiometric UO3-δ after heating at 550 °C. Finally, U3O8 was obtained as the final stable phase after heating above 660 °C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through high-temperature environmental scanning electron microscopy observations: decomposition led to a 20% reduction in the size of the aggregates, while uranium oxidation clearly promoted growth within the agglomerates.

In situ surface imaging: High temperature environmental SEM study of the surface changes during heat treatment of an Al[sbnd]Si coated boron steel

AlSi coated boron steels have been of interest for many years because of their superior mechanical properties and their excellent oxidation resistance. These high strength steels are particularly used in hot stamping processes during which the AlSi coating undergoes multiple microstructural transformations. In this study, morphology and structure transformations are investigated using an original approach. Most of the time, the coating evolution is studied in cross section, by ex situ characterization of its different layers after heat treatment. In this work, the evolution of the surface is explored for the first time using in situ high-temperature Environmental Scanning Electron Microscopy (HT-ESEM). The austenitization step reproduced in the ESEM chamber allows a precise description of several surface changes occurring, depending on the temperature range. Among them, the main change in morphology is occurring between 650 and 800 °C and is linked to different reactions between aluminium and bi- and ternary Fe-Al-Si phases. The heating rate is also pointed out as a key parameter that affects the surface morphology. In fact, with low heating rates the surface is mostly composed of hexagonal and rectangular elements coming from τ5 (Fe2Al8Si) structuration, while needle-shaped FeAl3 structures are found for higher heating rates. In addition, it is observed that the heating rate also affects the surface roughness depending on the surface morphology. Finally, the origin of the presence of micrometrics pores appearing between 800 and 900 °C at the coating surface is discussed. Our hypothesis supported by ex situ characterization is that they result from formation of a crystallized oxide layer, probably close to α-Al2O3 structure.

Oxidation of UC: An in situ high temperature environmental scanning electron microscopy study

In situ HT-ESEM oxidation of sintered UC fragments revealed the morphological changes occurring during the transformation between UC to UO2 and UO2 to U3O8 at 723–848 K and in an atmosphere of 10–100 Pa O2. Two main oxidation pathways were revealed. Oxidation at 723 K in atmospheres ≤25 Pa O2 showed the transformation from UC to UO2+x, as confirmed by post mortem HRTEM analysis. This oxidation pathway was comprised of three steps: (i) an induction period, where only surface UC particles oxidised, (ii) a sample area expansion accompanied by crack formation and propagation, (iii) a stabilisation of the total crack length inferring that crack propagation had stopped. Samples oxidised under 50 Pa O2 at 723 K and at 773–848 K for 10–100 Pa O2 showed an “explosive” oxidation pathway: (i) sample area expansion occurred as soon as oxygen was inserted into the chamber and crack propagation and crack length followed an exponential law; (ii) cracks propagated as a network and the oxide layer fragmented, (iii) an “explosion” occurred causing a popcorn-like transformation, typical for oxidation from UO2 to U3O8. HRTEM characterisation revealed U3O8 preferentially grow in the [001] direction. The explosive growth, triggered by ignition of UC, proceeded as a self-propagating high-temperature synthesis reaction, with a propagation speed of 150–500 ± 50 μm/s.

Increased chemical reactivity of single-walled carbon nanotubes on oxide substrates: In situ imaging and effect of electron and laser irradiations

We studied the oxygen etching of individual single-walled carbon nanotubes on silicon oxide substrates using atomic force microscopy and high-temperature environmental scanning electron microscopy. Our in situ observations show that carbon nanotubes are not progressively etched from their ends, as frequently assumed, but disappear segment by segment. Atomic force microscopy, before and after oxidation, reveals that the oxidation of carbon nanotubes on substrates proceeds through a local cutting that is followed by a rapid etching of the disconnected nanotube segment. Unexpectedly, semiconducting nanotubes appear more reactive under these conditions than metallic ones. We also show that exposure to electron and laser beams locally increases the chemical reactivity of carbon nanotubes on such substrates. These results are rationalized by considering the effect of substrate-trapped charges on the nanotube density of states close to the Fermi level, which is impacted by the substrate type and the exposure to electron and laser beams.

In-Situ Surface Analysis of SOFC Cathode Degradation Using High Temperature Environmental Scanning Electron Microscopy

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

In-Situ Surface Analysis of SOFC Cathode Degradation Using High Temperature Environmental Scanning Electron Microscopy

In this study the surface properties of LSCF have been studied in in-situ from room temperature to 1000oC using a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) under a number of different atmospheres. Dense, polished La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF) was observed during thermal annealing under a vacuum, pure oxygen, pure water and humid air atmospheres. The effect each atmosphere had on the surface morphology has been characterized in real time and analysed chemically using EDX. Significant differences in the surface behaviour were seen under each atmosphere. The results observed in the HT-ESEM were complimentary to in-operando surface exchange measurements and have enabled a more detailed understanding of SOFC cathode decomposition.

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.

Approach of phase transformations and densification in nanostructured and (Bi 2 O 3 , ZnO, TiO 2 ) doped silica ceramics

The low temperature sintering of silica is studied under the influence of sintering aids and nanosized powders using X-Ray Diffractometry (XRD) and high-temperature environmental scanning electron microscopy analyses (HT-ESEM). Two particular aids were chosen to conduct this study, [Bi2O3–ZnO]eutectic and titania. We report a lowering of the crystallization temperature when the former compound is introduced in the silica while a raise is observed when the latter is used. Moreover, the amorphous silica crystallization into cristobalite inhibits drastically the kinetics of densification of silica-based materials. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Mechanism of RuO2 Crystallization in Borosilicate Glass: An Original in Situ ESEM Approach

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO(2) particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochemical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO(2) particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds formed by reactions at high temperature between a simplified RuO(2)-NaNO(3) precursor and a sodium borosilicate glass (SiO(2)-B(2)O(3)-Na(2)O). The key parameter in the formation of acicular or polyhedral RuO(2) crystals is the chemistry of the ruthenium compound under oxidized conditions (Ru(IV), Ru(V)). The precipitation of needle-shaped RuO(2) crystals in the melt might be associated with the formation of an intermediate Ru compound (Na(3)Ru(V)O(4)) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO(2) crystals in the melt followed by an Ostwald ripening mechanism.

Temperature induced transition from hexagonal to circular pits in graphite oxidation by O2

We report on an in-situ monitoring of graphite oxidation using a high temperature environmental scanning electron microscope. A morphological transition is clearly identified around 1040 K between hexagonal pits at low temperatures and circular pits at high temperatures, with apparently no change in the kinetic law. A kinetic Monte Carlo model allows rationalizing these findings in terms of the competitive oxidation of armchair and zig-zag edge sites and provides an estimate of the rate laws associated to these two events. Extended to three dimensions, the model also explains the “in-depth” transition between the stepwise hexagons and the hemispheres observed by atomic force microscopy.

Characterization of self-healing glassy composites by high-temperature environmental scanning electron microscopy (HT-ESEM)

In situ high-temperature healing of cracks in composites made of glass and vanadium boride (VB) particles was observed using an environmental scanning electron microscope equipped with a high-temperature chamber (HT-ESEM). HT-ESEM is an adequate tool for studying the self-healing property of these materials. The change in crack length as a function of redox atmospheric conditions is reported. No self-healing behaviour was observed under reducing conditions, while a complete and rapid healing of the cracks was measured under oxidizing conditions. HT-ESEM image analyses enabled the monitoring of the healing effect. The self-healing mechanism was identified as a consequence of the VB active particles oxidation and subsequent pouring of fluid oxides into the cracks. These innovative composites offer an interesting potential in the domain of solid oxide fuel cell sealants.

Characterisation of microstructural evolutions in refractory castables by in situ high temperature ESEM

This paper deals with the characterisation of the microstructural evolutions during the first heating of two commercial refractory castables by dilatometry and by in situ high temperature environmental scanning electron microscopy (HT-ESEM) up to 1300°C. Investigations have been made both on the castables and on their respective matrix. Effects of microstructural changes have been observed on dilatometric results. Four temperature domains could be defined, in which shrinkage is occurring. During in situ HT-ESEM observations, important microstructural changes could be seen in these four temperature domains. They deal first with microcrack initiation due to dehydration mechanisms. Microcrack opening then increases when increasing the sample temperature. An acceleration of these microcrack-opening mechanisms was observed when sintering starts in the 800–950°C temperature range. Between 1000 and 1100°C, liquid phases appear in the material and liquid phase sintering starts to occur. An increase of the liquid phase amount and of liquid phase sintering effects are observed when increasing temperature up to 1300°C. Crystal growth was identified as occurring at this temperature too. Above 1100°C, a competition between shrinkage mechanisms and expansion ones take place. The balance between these two types of mechanisms is not the same for the two studied refractory castables.