Probing the local environment of two-dimensional ordered vacancy structures in Ga2SeTe2 via aberration-corrected electron microscopy

There has been considerable interest in chalcogenide alloys with high concentrations of native vacancies that lead to properties desirable for thermoelectric and phase-change materials. Recently, vacancy ordering has been identified as the mechanism for metal-insulator transitions observed in GeSb2Te4 and an unexpectedly low thermal conductivity in Ga2Te3. Here, we report the direct observation of vacancy ordering in Ga2SeTe2 utilizing aberration-corrected electron microscopy. Images reveal a cation-anion dumbbell inversion associated with the accommodation of vacancy ordering across the entire crystal. The result is a striking example of the interplay between native defects and local structure.Comment: 9 pages, 5 figure

Measurements with Pinhole and Coded Aperture Gamma-Ray Imaging Systems

From a safeguards perspective, gamma-ray imaging has the potential to reduce manpower and cost for effectively locating and monitoring special nuclear material. The purpose of this project was to investigate the performance of pinhole and coded aperture gamma-ray imaging systems at Oak Ridge National Laboratory (ORNL). With the aid of the European Commission Joint Research Centre (JRC), radiometric data will be combined with scans from a three-dimensional design information verification (3D-DIV) system. Measurements were performed at the ORNL Safeguards Laboratory using sources that model holdup in radiological facilities. They showed that for situations with moderate amounts of solid or dense U sources, the coded aperture was able to predict source location and geometry within ~7% of actual values while the pinhole gave a broad representation of source distribution

Effect of stoichiometric vacancies on the structure and properties of the Ga2SeTe2 compound semiconductor

Ga2SeTe2 belongs to a family of materials with large intrinsic vacancy concentrations that are being actively studied due to their potential for diverse applications that include thermoelectrics and phase-change memory. In this article, the Ga2SeTe2 structure is investigated via synchrotron x-ray diffraction, electron microscopy, and x-ray absorption experiments. Diffraction and microscopy measurements showed that the extent of vacancy ordering in Ga2SeTe2 is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga2SeTe2 (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga2SeTe2 material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga2SeTe2 band gap redshifts by ~0.05 eV as the vacancies order and accompanied by gains in charge carrier mobility. The results serve as an encouraging example of altering material properties via intrinsic structural rearrangement as opposed to extrinsic means such as doping

Effect of stoichiometric vacancies on the structure and properties of the Ga2SeTe2 compound semiconductor

Ga2SeTe2 belongs to a family of materials with large intrinsic vacancy concentrations that are being actively studied due to their potential for diverse applications that include thermoelectrics and phase-change memory. In this article, the Ga2SeTe2 structure is investigated via synchrotron x-ray diffraction, electron microscopy, and x-ray absorption experiments. Diffraction and microscopy measurements showed that the extent of vacancy ordering in Ga2SeTe2 is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga2SeTe2 (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga2SeTe2 material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga2SeTe2 band gap redshifts by ~0.05 eV as the vacancies order and accompanied by gains in charge carrier mobility. The results serve as an encouraging example of altering material properties via intrinsic structural rearrangement as opposed to extrinsic means such as doping

Carburization Kinetics of Zircalloy-4 and Its Implication for Small Modular Reactor Performance

Carburization of cladding materials has long been a concern for the nuclear industry and has led to the restricted use of high-thermal conductivity fuels such as uranium carbides. With the rise of small modular reactors (SMRs) that frequently implement a graphite core-block, carburization of reactor components is once more in the foreground as a potential failure mechanism. To ensure commercial viability for SMRs, neutron-friendly cladding materials such as Zr-based alloys are required. In this work, the carburization kinetics of Zircaloy-4 (Zry-4), for the temperature range 1073–1673 K (covering typical operating temperatures and off-normal scenarios) are established. The following Arrhenius relationship for the parabolic constant describing ZrC growth is derived: Kp (in μm2/s) = 609.35 exp(−1.505 × 105/RT)). Overall, the ZrC growth is sluggish below 1473 K which is within the operational temperature range of SMRs. In all cases the ZrC that forms from solid state reaction is hypo-stoichiometric, as confirmed through XRD. The hardness and elastic modulus of carburized Zry-4 are also examined and it is shown that despite the formation of a ZrC layer, C ingress in the Zry-4 bulk does not impact the mechanical response after carburization at 1073 K and 1473 K for 96 h