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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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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Enhanced Microwave Absorption Properties of Doped M-Type Barium Hexagonal Ferrites in Ka-band Frequencies

The present study reports the microwave absorption characteristics of La{sup 3+}-Na{sup +} substituted barium hexaferrites with chemical composition: Ba{sub (1−2x)}La{sub x}Na{sub x}Fe{sub 10}CoTiO{sub 19} and Ba{sub (1−2x)}La{sub x}Na{sub x}Fe{sub 10}Co{sub 0.5}TiMn{sub 0.5}O{sub 19} (0.00 ≤ x ≤ 0.25) respectively in Ka-band frequencies. The electromagnetic characteristics such as complex permittivity, complex permeability, and microwave absorption properties were studied using Vector Network Analyzer in 26.5–40.0 GHz frequency band. Dielectric properties showed almost constant variation with increase in frequency. Magnetic permeability spectra observed wavy variation with frequency. Comparing the electromagnetic properties of two series, it is observed that Co–Ti–Mn series observes higher values of electromagnetic properties in comparison to Co–Ti series. For composition x = 0.15 of Ba{sub (1−2x)}La{sub x}Na{sub x}Fe{sub 10}Co{sub 0.5}TiMn{sub 0.5}O{sub 19} series, −55.33 dB reflection loss at 30 GHz frequency with −10 dB absorption bandwidth around 3.51 GHz has been observed. Thus, these materials can be employed as effective absorbers for Ka-band microwave absorbers.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Designing a high sensitivity hexagonal nano-cavity photonic crystal resonator for the purpose of seawater salinity sensing

In this paper a sensing platform based on nano-cavity photonic crystal resonator (NC-PCR) is proposed for seawater salinity concentration detection. It has been demonstrated that the proposed NC-PCR has high sensitivity and selectivity for various concentrations of seawater salinity. The sensitivity is computed to be as high as 544 nm/RIU (nm per refractive index unit) and the quality factor (Q) is 3860. Also it has been shown that the figure of merit for the proposed structure calculated to be as high as ~ 1.234 × 10{sup 3} RIU{sup −1} which is very appropriate for sensing applications. Moreover, the minimum detection limit for the presented structure is obtained as 8.1 × 10{sup −5} RIU. The finite-difference time-domain method is employed to investigate the transmission characteristics of the device for a verity of refractive indices. It should also be mentioned that our design is comparatively easy to fabricate and certainly the advantages of our design justify its usage in sensing applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Effect of Aluminum Addition on the Evolution of Microstructure, Crystallographic Texture and Mechanical Properties of Single Phase Hexagonal Close Packed Mg-Li Alloys

In the present investigation, an effort has been made to understand the effect of aluminum addition to α Mg-Li alloys. The corresponding composition Mg-4Li-xAl (x = 0, 2, 4 and 6 wt.%) alloys have been prepared by stir casting route under an argon environment. Extrusion was carried out at 300 °C with the extrusion ratio of 15:1. Significant grain refinement was observed after extrusion. X-ray diffraction-based investigation of the cast and extruded alloys showed the presence of intermetallic compounds such as Mg{sub 17}Al{sub 12} and AlLi in the Al-rich alloys namely, Mg-4Li-xAl (x = 4 and 6 wt.%). These precipitates were also present in the extruded plus annealed samples, indicating the stability of the precipitates at high temperature. The bulk x-ray texture measurement revealed a crystallographic texture where the c-axis of the h.c.p crystals was perpendicular to the extrusion direction (ED) for extruded sample. A texture transition was observed on annealing. The c-axis was oriented parallel to the ED. Mechanical properties of the cast, extruded and extruded plus annealed material illustrate that the addition of Al led to enhancement in hardness, yield strength and ultimate tensile strength.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spectroscopic signatures of edge states in hexagonal boron nitride

Here, we use Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on an aberration-corrected scanning transmission electron microscope to investigate the local electronic states of boron atoms at different edge structures in monolayer and bilayer h-BN. We find that edges with bonding unsaturated sp2 boron atoms have a unique spectroscopic signature with a prominent pre-peak at ~ 190.2 eV in the B K-edge fine structure. First-principles calculations reveal that the observed pre-peak arises from excitations to the in-plane lowest-energy empty sp 2 boron dangling bonds at the B-terminated edge. This spectroscopic signature can serve as a fingerprint to explore new edge structures in h-BN.

edge structure↗
DOE OSTI2020

Towards defect engineering in hexagonal MoS 2 nanosheets for tuning hydrogen evolution and nitrogen reduction reactions

Combined computational and experimental approaches were used to evaluate defective 2H-MoS 2 nanosheets for their activity and selectivity for hydrogen evolution reaction (HER) and nitrogen reduction reaction (NRR). Density functional theory calculations were used to understand the relationship between HER and NRR activity on the ideal basal MoS 2 plane, seven grain-boundaries, ten single-/few-atom vacancies and anti-sites, and zigzag and armchair edge sites. The results confirm that 2H-MoS 2 should contain several defects with high activity for HER: armchair and zigzag edges, VS vacancy, MoS 2 anti-site, and S-S and Mo-Mo grain boundaries. Considering Gibbs free energy change for all the steps in the NRR mechanism and kinetic barriers for a key NRR step, we have found that activation of perspective NRR selective sites in 2H-MoS 2 , namely V MoS6 and clusters of S-vacancies, would require large overpotential, conditions at which HER dominates. The DFT conclusions are supported by the electrochemical studies of NRR activity and selectivity under aqueous conditions, which show an increase in NRR activity but a decrease in Faradaic efficiency as applied cell potential becomes more negative. The results of this work therefore highlight the challenges in activating natural 2H-MoS 2 for NRR, which would require additional material engineering or reaction condition optimization as a way to suppress HER, decrease the NRR overpotential or preferentially both.

2H-MoS2↗