Environmental High Resolution Electron Microscopy in Materials Science

Abstract

An environmental cell high resolution electron microscope (EHREM) has been developed for in-situ studies of dynamic gas molecule-solid interactions on the atomic scale. It allows access to metastable intermediate phases of materials, to both surface and bulk structural changes which are often interrelated, to reaction mechanisms for probing materials performance in reaction environments and to sequences of reversible microstructural and chemical development associated with their operation. Materials transported through air can be restored or recreated and samples damaged, e.g. by dehydration, by the usual vacuum environment in a conventional electron microscope can be preserved. A Philips CM30 high resolution transmission EM (HRTEM)/Scanning transmission EM (STEM) system has been extensively modified in our laboratory to add an environmental cell in the EM column which provides facilities for in-situ gas-solid reaction studies in controlled atmospheres of gas or vapor at pressures of 0–50 mbar, instead of the regular TEM high vacuum environment. The integrated new environmental cell capability is combined with the original 0.23 nanometer (nm) TEM lattice resolution with access to interatomic spacings, STEM imaging (bright field/annular dark field), electron diffraction and chemical microanalyses. Regular sample holders are used and include hot stages to >1000 °C. The learnings from well designed dynamic in-situ EM studies of reaction sequences between gases or vapors and solids can be considerable. Examples of in-situ applications include direct studies of dynamic reactions with chemically stabilized silica based ceramics, the formation of active vanadyl pyrophopsphate catalysts from their hemihydrate precursors for vapor phase oxidation of hydrocarbons and carbon nanostructures.