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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2015

International scientific collaboration: the Radiation Measurements Cross Calibration (RMCC) project.

Detecting the presence of radioactive sources, preventing the illicit use of radiological materials, supporting arms control treaties, responding to accidental radiation releases, and disposing of radioactive sources safely and securely are common concerns in the Middle East. The Radiation Measurements Cross Calibration (RMCC) project aims to improve radiation measurement capabilities across the region and establish common standards. The RMCC project has been an ongoing initiative for the last twelve years. Its goal is to build core competencies in radioanalysis in the Middle East by facilitating the exchange of expertise and fostering dialogue to improve methods and strengthen a growing network. This year the Middle East Scientific Institute for Security (MESIS) in Amman, Jordan assumed leadership of the RMCC.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Observations of GW170817 by DESGW and the DECam GW-EM Collaboration

On August 17, 2017 LIGO/Virgo detected a binary neutron star via gravitational waves. We observed 70 sq-degrees in the LIGO/Virgo spatial localization with the DECam on the 4m Blanco telescope covering 80% of the nal map. Our group independently discovered an optical counterpart in NGC 4993. We searched our entire imaged region: the object in NGC 4993 was the only viable candidate. Our observations of NGC4993 show complicated morphology but simple star formation history. Our x-ray and radio observations indicate an o-axis jet as afterglow. Our high-cadence optical and infrared spectra show a source that must be described by at least two components, one of which is dominated by the r-process nucleosynthesis elements characteristic of a kilonova. Our modeling of the light curve demonstrates such a model in which 0.05 M of material is ejected from the system. Finally, we discuss the first standard siren measurement of H0.

Annis, James↗
DOE OSTI2018

Collaborative atomic-scale design, analysis, and nanofabrication for record breaking, single-crystal Zn(x)Cd(1-x)Te solar cell arrays

Cadmium telluride (CdTe) based photovoltaics provide a viable solar energy solution because of their short payback time, low manufacturing cost, and high theoretical efficiency ~29%. However, the highest record laboratory efficiency is only 22.1% and long-term performance degradation is ~1 % per year which hinders greater economic viability. Higher open circuit voltage (Voc) and fill factor (FF) are needed to increase the efficiency of the solar cell. Cells with record energy conversion efficiencies report short current (Jsc) values close to their theoretical maximum. However, Voc and FF in these devices remain well below their anticipated optimum. These electrical parameters depend on the quality of the various layers in the solar cell. In particular they depend on the crystalline quality including the defect and impurity concentrations. Therefore understanding how defects and impurities form and evolve in the material layers is important to design devices with higher efficiency and stability. The aim of this project is to create a novel path around the fundamental barriers to achieve high open-circuit voltages (VOC ≥ 1.0 V) in CdTe/CdS solar cells. The idea of the project was to combine nanopatterned crystal growth and alloy compositional grading (with concomitant lattice parameter and energy band gap grading) to reduce the defect density and dramatically improve spatial uniformity of ZnxCd(1-x)Te/CdS cells. The lattice mismatch between ZnTe and CdS is 3.9% compared to a much larger 9.2% between CdTe and CdS. Importantly, when crystal growth is contained to the nanoscale, the strain in each layer is reduced by a factor of two due to strain partitioning and 3D strain relief mechanisms that come into play. This reduces the strain in the ZnTe from 3.9% to 1.95% which is within the range of metamorphic crystal growth. In preliminary studies, molecular dynamics simulation of ZnTe/CdS islands predicted dislocation-free material when the patterning was reduced to ~80 nm in diameter. The energy band gap of ZnxCd(1-x)Te is also graded in conjunction with alloy composition. ZnTe has an energy band gap of 2.3 eV. Finally, an added benefit of patterned growth is that it enables direct one-to-one study of the relationships between fabrication, microstructure and performance of individual crystal grains within the array. Experimental characterization techniques such as atomic probe tomography (APT) and transmission electron microscopy (TEM) have elucidated much about the microstructure of the layers. However, the techniques are destructive, costly and have limitations on the information they convey. For example, TEM gives information with atomic scale resolution but on 2-dimensional substrates. On the other hand, APT gives 3-dimensional information but is not able to resolve atomic scale structure. Alternatively, Molecular Dynamics (MD) simulations offer a solution to rapidly generate time-resolved, 3-dimensional, atomic scale structures of crystal growth. Direct MD simulations of microstructures of Cu-doped II-VI compound multilayers can impact the research on CdTe/CdS solar cells. Such MD simulations require an interatomic potential database that includes at least four elements Cu, Cd, Te, and S. The Stillinger-Weber (SW) potential is the most widely used interatomic potential for semiconductors. However, while SW potentials have been developed for Ga-In-As-Se-Te, Al-Ga-In-P-As-Sb, Cd-Te-Zn-As-Si, In-Ga-N systems, no potentials existed to simulate Cd-Zn-Hg-Se-S-Te-Cu. The goal of this effort was to fill the knowledge gap by developing a Zn-Cd-Hg-S-Se-Te-Cu SW potential database to enable MD simulations of microstructures of Cu-doped CdTe/CdS solar cells. The new potential was then applied, via MD crystal growth simulations, to study the growth mechanisms and atomic structures of the ZnTe/Cu/CdTe/CdS multilayers under various conditions. The objective of the project was to inform the solar cell community on crystal growth and defect formation at the interface between CdTe-based epilayers grown on CdS substrates. MD simulations offered a unique opportunity to analyze the 3-dimensional crystal growth evolution of Cu doped ZnxCd(1-x)TeyS(1-y) alloys with atomic scale resolution under a variety of conditions. The effect of substrate orientation and polycrystallinity on crystal growth was studied. Substrates with grain boundaries were created and the growth evolution on these substrates were analyzed. Another focus was to elucidate the effect of dopant incorporation on the nature of polycrystallinity crystal growth. Dopants available through the Stillinger-Weber potential developed in this project included Cu, Zn, S, Se, Hg and native vacancies.

14 SOLAR ENERGY↗