Development of Precision TEM Holder Assemblies for Use in Extreme Environments

This dissertation focuses on the development of "in-situ" environmental - transmission electron microscope (TEM) specimen holder assemblies. To date, experimentation with TEM has essentially been two-dimensional and static. In-situ experiments are presently not possible for analyzing complex hot deformation, dynamic recovery and dynamic recrystallization processes because of limitations in the specimen holder design. By developing technology that allows in- situ TEM, it will be possible to assess phenomena such as materials microstructural changes throughout heating process rather than using traditional post mortem techniques. The ability to conduct environmental in -situ experiments is also important because many biological applications currently analyze catalyst samples under static, post-reaction conditions that do not adequately represent reaction dynamics. The lack of an ability to perform in -situ analysis has delayed the fundamental understanding of dynamic catalytic surfaces, complex structural changes, and reaction mechanisms that evolve during processes such as oxidation-reduction. In this respect, this dissertation present novel holder technology that enables in- situ experiments for dynamically capturing microstructure and texture changes in specimens. Such analyses are not capable at the present time and will lead to breakthrough technologies for evaluating mechanisms of microstructural evolution and chemical process reactions. Environmental instability has been shown to be the limiting factor in holder design when attempting to obtain TEM atomic resolution. In this dissertation, two of the most common environmental problems associated with TEM resolution have been analyzed, air pressure and temperature fluctuations. The present approach to solving these limitations has included extensive experimental and numerical analyses. Based on these analyses, we have designed and optimized two heated TEM holders that overcome the current technical problems associated with performing in situ experiments. These novel holder designs are dynamically stable and accurate for image processing, are capable of elevating the sample temperature to 2000 K, allow the specimens to be subject to diverse environments, and have minimum drift in sample position over time. Our initial work utilized the finite element method to dynamically and thermally analyze presently available TEM holder assemblies. From these simulations, the dynamic and thermal behavior of the holders were obtained. This information proved to be critical for establishing the limitations of the present TEM technology. It also allowed creation of optimization objectives for designing holders for in-situ TEM experiments. Utilizing the numerical results of the current TEM holders as a basis, two novel different holder technologies were designed and numerically analyzed. The first holder utilized existing resistive heating technology and was found to be significantly better than existing holders with respect to its dynamic stability, uniformity in heating, and minimization of stresses induced in the sample. The new design also had less thermal drift and significantly lower heat loss from the furnace than prior holder designs. The second holder design incorporated a localized laser diode to heat the sample to 2000 K. Such technology has never been applied to TEM technology, making it completely novel to the field. Based on the numerical simulations performed, the laser-diode holder significantly outperformed all of the other heater specimen holder designs analyzed in this dissertation. In addition, laser heating of the sample was found to overcome important restrictions associated with small pole piece gaps that are required in resistive heating holders. Overcoming these restrictions will allow the TEM holder to be used in completely new application of tomography. Finally, in addition to the finite element simulations, Direct Simulation Monte Carlo (DSMC) analyses were performed to determine the performance of the holder in different gas environments. These analyses allowed for optimizing the holder design when specific molecular densities or molecule impingement rates were required to induce a chemical reaction. Through the DSMC and finite element analyses, the new specimen holders were demonstrated to be greatly improved over existing holder technology and will allow a new level of materials analysis. Both the new resistive heater and laser diode designs will significantly advance TEM technology by allowing atomic level and chemical reaction information to be obtained dynamically, something which is not possible today.