Brett Guisti
High-temperature x-ray diffraction experiments have long been of interest for the characterization of materials with phase transitions and other structural variations above room temperature. Examples include cubic zirconia transitioning from cubic to tetragonal below 500°C, β-Alumina gradually moving from an ordered to a disordered state as temperature increases, and superionic conductors like Bi4V2O11. With a more complete understanding of the microscopic structural transitions, the origins of macroscopic properties can be identified.
The present work describes the development and construction of a novel micro-furnace which will be used initially at the new single-crystal diffraction facility at Brigham Young University. The unit is also portable and can be utilized at other locations such as the Advanced Photon Source in Chicago. The entire heating apparatus can be mounted on a standard goniometer, thus avoiding the need for special equipment and reduced sample mobility. No enclosure is used; and because the heat transfer is by contact instead of convection, temperature instability is minimal (± 2° C). Temperature control is maintained through a custom circuit that uses feedback from a thermocouple to regulate the current supplied to the heating wire. The furnace can reach ~1100° C or more before the heating element fails.
The major design concepts and innovations that make our microfurnace an exceptional new research tool, include combining the heating element, temperature sensor, and crystal mount into a single physical structure, making these structures disposable, achieving crystal temperatures in excess of 1000° C, and having an unobstructed x-ray beam.
Though not yet meeting all of the initial performance goals, a novel micro-furnace for single crystal x-ray diffraction experiments was successfully developed. One of the main improvements over previous models was the combination of the temperature measuring device, the heating element, and the crystal mount into a single physical entity. A process of TIG welding heater and thermocouple wires into a beaded junction was perfected. The addition of a STM tip to elevate the crystal gave the x-ray diffractometer an almost completely unobstructed experimental path to collect data of the heated sample. A simple resistor/capacitor combination as a low-pass filter effectively allowed the DC thermocouple voltage signal to pass while filtering out AC contamination from the heating element current. A custom-built control unit succeeded in achieving a high level of temperature control, despite the heater being in an open environment. Vespel® proved to be a robust plastic suited perfectly for the temperature and machining demands of the furnace structure.
Future improvements include other types of process control. A phase-control SCR device, as opposed to the present on/off relay would reduce the fluctuations in the position of the crystal due to thermal expansions and contractions of the heater wires. This would have the added benefit of prolonging element lifetime. Another method of fixing the crystal to the end of the STM tip needs to be explored, as the adhesive method of platinum paint proved insufficient when the sample was repositioned while at high temperatures.
This research project fulfilled my graduation requirement for the Department of Physics and Astronomy of completing a senior thesis, a copy of which can be found in the department library. Due to these remaining obstacles and a lack of time to overcome them during my undergraduate career, the results of this research are not suitable for publication as had been hoped. Presentations were made at various stages of the project including the 2004 Spring Research Conference of the BYU College of Physical and Mathematical Sciences and the 2004 APS Four Corners Meeting in Albuquerque, NM.