Development of a TEM to study in situ structural and chemical changes at an atomic level during gas-solid interactions at elevated temperatures.

A Philips (Eindhoven, The Netherlands) 430 (300 keV) high resolution transmission electron microscope has been modified for in situ study of gas-solid interactions at elevated temperatures. This microscope can be best described as a synthesis, processing, and characterization laboratory for nano-size materials. A differentially pumped environmental cell (E-cell), capable of handling up to 20 torr of gas pressure, is fitted in the objective lens pole-piece gap. Single-tilt or double-tilt heating holders can be used to heat the samples up to 1,300 degrees C and 850 degrees C, respectively. The system can handle any non-corrosive gases such as H2, O2, N2, NH3, CO, water vapor. Electron diffraction patterns are used to elucidate the reaction path and to identify stable and/or metastable phases formed. Time, temperature, and pressure resolved electron diffraction patterns can also be used to estimate the thermodynamical conditions for the onset of a reaction and the stability range of different phases observed during the process can also be determined. The high resolution imaging capabilities enable elucidation of the basic structural mechanisms involved at near atomic level. The TV rate camera/video recording system is used to measure the reaction rates (kinetics of the reaction). A post projector energy filter (Gatan Imaging Filter, GIF) is attached at the bottom of the microscope in order to filter the inelastic scattering from the gases/thick samples as well as to obtain energy filtered images (chemical maps). The GIF can also be used as a parallel electron energy loss spectrometer (PEELS) to obtain changes in the sample composition during the reactions. The changes in the near-edge structures of PEELS spectrum is used to monitor changes in bonding and/or chemical environment elements during reaction. The chemical maps obtained can also be used to identify preferred regions of gas reactions, e.g., grain boundaries or surfaces, etc. Various modifications of the microscope are described in detail, with suitable examples showing the performance.