Report on Malcolm F. Nicol Graduate Scholar Research Activities 2013-2014
Daniel Mast Advisor: Paul Forster
In December of 1951 electricity was first produced using a nuclear fission. The United States operates 99 commercial nuclear power plants that now produce 19.7 % of our annual electricity, 805.3 billion kilowatt-hours. During the fission process that is used to create the heat necessary for electrical energy generation there are a number of fission products produced, including the radioelement technetium. There are no stable isotope of technetium, meaning that all isotope of technetium are radioactive. The most prevalent isotope is 99mTc, used in nuclear medicine and its daughter, 99Tc which has a half-life of 210,000 years. 99Tc is a major fission product which amounts to approximately 6 grams of 99Tc produced daily per reactor and is also a long lived isotope. Given the large amounts of technetium produced in power plants it is imperative that the fundamental chemistry and physics of this material is understood.
During this time period technetium chemistry was explored for the first time as reasonable quantities first became available to researchers. The chemistry of technetium remains relatively unexplored compared to rhenium. At the same time that technetium was first explored in the laboratory, a device called a diamond anvil cell was being developed for generating high pressure. The field of high pressure physics quickly grew using the diamond anvil cell to study material under high pressures. One the most fundamental measurements in the field is to determine the relationship between pressure and volume by means of an equation of state which allows us to define a bulk modulus or compressibility for a material. The equation of state was determined for a majority of the elements during the earlier years with only a few exceptions left unknown. These elements mostly hazardous substances that were difficult to handle and short-lived radioelements that sufficient quantities are not available. Technetium is one of these elements that is hazardous due to the radioactive nature and presents additional safety precautions in order to handle. Due to the scarcity and radioactivity, technetium chemistry and physics under extreme conditions has remained minimally explored with 6 experimental papers covering the topics of superconductivity of technetium metal and alloys at pressure, the effects of pressure on radioactive decay, and the electrical resistivity at pressure.
While a Malcolm F. Nicol Graduate Scholar in 2013-2014 I joined the Radiochemistry group and High Pressure Science and Engineering Center, HiPSEC, at UNLV. My research focus is the structural behavior of technetium and technetium oxides under non-ambient conditions. The principle project during my first year was to measure the equation of state of technetium metal in order to build a foundation for conducting diamond anvil cell experiments in collaboration between the Radiochemistry group, HiPSEC, and the High Pressure-Collaborative Access Team, HP-CAT, at the Advanced Photon Source, APS.
In planning the measurement for an isothermal compression at room temperature of technetium metal there were a few obstacles to overcome before being allowed to conduct the experiment. Containment of the radioactive sample and loading a suitable pressure transmitting medium. The pressure transmitting medium that was used in these experiments could not be loaded using a high pressure gas loading system as is typically used in equation of state experiments because the system that is available for our use is not compatible with containing radioactive material. Given our desire for a hydrostatic medium and our limitations we chose to use a mixture of 4:1 methanol-ethanol. The design of containment during the experiment is highly dependent on the style of DAC chosen. A Mao-Bell type piston-cylinder DAC that was developed by the High Pressure Physics group at Lawrence Livermore National Laboratory suited our purpose. With limited access to the sample chamber through the DAC body the addition of windows to either end of the DAC outside of the diamonds allowed for three layers of containment to be maintained throughout the lifetime of the experiment. After a few attempts, we successfully compressed the sample up to 67 GPa and collected powder X-ray diffraction patterns over the entire pressure range. The pressure-volume data was fit using the 3rd-order Vinet equation of state where the zero pressure bulk moduli was fixed to 288 GPa as determined by ultrasonic measurements. The refined values are V′o = 14.320(2) B′o = 5.9(2). This result agrees with periodic trends in bulk moduli values calculated by DFT. Knowing that this measurement was conducted under nonhydrostatic conditions it can be assumed that the bulk moduli is smaller than reported here. If technetium is very sensitive the nonhydrostatic environment and this difference is large, it could suggest that DFT is incorrectly modeling the behavior of technetium. This is important because technetium has a half filled d-shell and lies at either a maxima or minima in the periodic trend. Theoretical calculations predict that the 4d elements behave more similarly to the 5d transition metals in which case technetium should have the largest bulk moduli value of the 4d transition metals.
Future work on this project will include pursuing an experimental set-up that allows for gas loading and compression under better hydrostatic conditions. This would allow for more reliable data at higher pressures.
Mast et al. Journal of Physics and Chemistry of Solids 95 (2016) 6–11