SPEAKERS:
Dr Renaud Podor and Dr Johann Ravaux, ICSM (Institut de Chimie Séparative de Marcoule)
SYNOPSIS:
The characteristic advantage of Environmental Scanning Electron Microscopy (ESEM) is the possibility of working with a gaseous environment in the specimen chamber (up to 2700Pa water vapour or 750Pa of any other gas). Besides, several specific stages can be implemented in the microscope chamber (Peltier stage, high temperature furnace), transforming the usual SEM chamber into a “microlab” as it allows performing real time experiments and to directly observe reactions at the micrometre or even nanometre scale. Specific strategies must be developed according to the scientist requirements in order to record images which will allow further determination of kinetic and/or thermodynamic data. After the general presentation of the HT-ESEM technique, the results of three different outstanding and recent studies using HT-ESEM performed at high temperature will be reported.
Development of self-healing composites for SOFC: A new family of self-healing vitreous composites have been recently designed by the Unité de Catalyse et de Chimie du Solide (UMR8181 – Lille) to prevent the leakage of reactant gases through seals in solid oxide fuel cells (SOFC). A specific procedure has been developed to characterize the self-healing property by HT-ESEM in the application temperature range1. The chemical and physical mechanisms controlling the healing of the material have been identified, and the influence of several environmental parameters (temperature, nature and pressure of gas, composition of the active particles) on this property has been determined. The healing property is obtained by the spreading of molten boron oxide, formed by the oxidation at 700°C of boron particles. The images recorded by HT-ESEM have been completed with X-Ray nano-tomograms. X‐ray nano‐tomography showed that crack healing obeys to this mechanism even in the bulk of the sample, where oxygen diffusion may be limited. Furthermore, the particle oxidation is not complete after a healing cycle, meaning that several cycles can be conducted with the same composite. (Coll. UCCS Lille – F.O. Méar, L. Montagne)
Direct observation of ceramics sintering: Sintering and grain growth processes occurring during ceramic elaboration are generally studied using global methods. The HT-ESEM was used to record images at the same location of a sample at high temperature, for long durations, in order to follow the morphological modifications of a unique grain population during sintering processes. The image series have been computed to determine the grain growth laws of CeO2 notably2 and to measure original kinetic parameters such as the rate of grain boundary displacement or the rate of grain elimination3. A specific strategy has been developed to study specifically the first stage of sintering on a 2 or 3 nanospheres system. This approach is totally new and offers great opportunities to achieve new experimental data. (Coll. ICSM Marcoule – N. Clavier, N. Dacheux)
One step elaboration of superalloy coatings: a new concept of thermal barrier has been developed in the framework of the Particoat european project. The formation of the bondcoat and of the topcoat is obtained in a single step elaboration process using Al microspheres as a precursor. During the heat treatment, the metallic microspheres act as an Al reservoir for the formation of the bondcoat and they allow the formation of hollow alumina spheres that form the topcoat. This layer is mechanically stabilized by sintering and it forms a thermal barrier. The study of the hollow sphere formation has been performed by HT-ESEM 4. (Coll. LASIE La Rochelle – F. Pedraza)
References
1. D. Coillot et al. Adv. Funct. Mater. (2010) 20, 4371–4374
2. R. Podor et al. J. Am. Ceram. Soc. (2012) 95(11), 3683-3690
3. R. Podor et al. J. Eur. Ceram. Soc. (2012) 32, 353-362
4. F. Pedraza & R. Podor, Submitted to J. Am. Ceram. Soc.