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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 OSTI2021

Optical properties of differing nanolayered structures of divalent europium doped barium fluoride thin films synthesized by pulsed laser deposition

Optically-active thin films are employed in a variety of applications, such as LEDs and photovoltaics, due to their ability to act as up- or down-photon energy converters. Their performance depends critically on their composition and structure; thus, the use of novel synthesis techniques that allow for their control at the nanoscale level can result in improved efficiency and practicality. Here, layered thin films consisting of Eu 2+ - doped BaF 2 nanocrystalline layers separated by amorphous Al 2 O 3 were synthesized via sequential pulsed laser deposition using three separate targets for the different components; this synthesis technique provides precise control of layer thickness at the nanoscale along with dopant distribution within the film. Cross-sectional transmission electron microscopy analysis verified the desired nano-layering. Post-deposition heat treatments in a nitrogen atmosphere resulted in samples exhibiting steady emission with a broad peak ranging from 400 to 600 nm and a shoulder at 410 nm. The CIE 1931 chromaticity coordinates are x = 0.26-0.29 and y = 0.32-0.35 and vary as a function of the sample configuration. Because the chromaticity coordinates are close to those of a pure white light (x = 0.33, y = 0.33), these films demonstrate advantageous properties for applications with UV-pumped white light LEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Virus Disinfection in Water by Biogenic Silver Immobilized in Polyvinylidene Fluoride Membranes

The development of innovative water disinfection strategies is of utmost importance to prevent outbreaks of waterborne diseases related to poor treatment of (drinking) water. Recently, the association of silver nanoparticles with the bacterial cell surface of Lactobacillus fermentum (referred to as biogenic silver or bio-Ag{sup 0}) has been reported to exhibit antiviral properties. The microscale bacterial carrier matrix serves as a scaffold for Ag{sup 0} particles, preventing aggregation during encapsulation. In this study, bio-Ag{sup 0} was immobilized in different microporous PVDF membranes using two different pre-treatments of bio-Ag{sup 0} and the immersion-precipitation method. Inactivation of UZ1 bacteriophages using these membranes was successfully demonstrated and was most probably related to the slow release of Ag{sup +} from the membranes. At least a 3.4 log decrease of viruses was achieved by application of a membrane containing 2500 mg bio-Ag{sup 0}{sub powder} m{sup -2} in a submerged plate membrane reactor operated at a flux of 3.1 L m{sup -2} h{sup -1}. Upon startup, the silver concentration in the effluent initially increased to 271 {mu}g L{sub -1} but after filtration of 31 L m{sup -2}, the concentration approached the drinking water limit (= 100 {mu}g L{sup -1}). A virus decline of more than 3 log was achieved at a membrane flux of 75 L m{sup -2} h{sup -1}, showing the potential of this membrane technology for water disinfection on small scale.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

Optical properties of Eu{sup 2+} doped antipervoskite fluoride single crystals

Single crystals of pure and Eu{sup 2+} doped LiBaF{sub 3} have been grown from melt by using a vertical Bridgman-Stockbarger method. Absorption and luminescence spectra for pure and rare-earth-doped LiBaF{sub 3} were studied. At ambient conditions the photoluminescence spectra consisted of sharp lines peaked at {approx}359 nm attributed to the {sup 6}P7/2{yields}{sup 8}S7/2 transitions in the 4f{sub 7} electronic configuration of Eu{sup 2+} and a broad band extending between 370 and 450 nm attributed to Eu{sup 2+} trapped exciton recombination. The effect of {sup 60}Co gamma irradiation has also been investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Optical characterization of femtosecond laser induced active channel waveguides in lithium fluoride crystals

We successfully realized broad-band light-emitting color center waveguides buried in LiF crystals by using femtosecond laser pulses. The characterization of the waveguides was performed by optical microscopy, photoluminescence spectra, loss measurements and near-field profiling. The experimental results show that the direct-writing fabrication process induces low-index contrast active channel waveguides: their wavelength-dependent refractive index changes, estimated from 10{sup −3} to 10{sup −4} depending on the writing conditions, allow supporting few modes at visible and near-infrared wavelengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of sensitizer ions tunable-distribution in fluoride nanoparticles for efficient accretive three-center energy transfer

Cooperative upconversion luminescence of Yb³⁺-Yb³⁺ couples and three-center energy transfer mechanisms have been deeply investigated in Yb³⁺ doped and Yb³⁺-Tb³⁺ co-doped β-PbF₂ nanoparticles. As sensitizer ions, the distribution of Yb³⁺ ions, which is a key factor that affects the cooperative upconversion luminescence and three-center energy transfer processes, can be tuned by the structure of nanoparticles. Based on the three-center distributions in tetragonal PbYb{sub x}Tb{sub 1–x}F₅ nanoparticles, two different energy transfer models, Cooperative Energy Transfer (CET) and Accretive Energy Transfer (AET) mechanisms were established. Especially, AET model is observed and verified in this work for the first time. Experimental results obtained from photoluminescence spectroscopy study are in agreement with the theoretical calculations by applying rate equations in these models, strongly supporting the proposed three-center energy transfer mechanisms. The sensitization between Yb³⁺ ions only existing in AET process can greatly improve the energy transfer rates, further to enhance the quantum efficiency. The results that the calculated luminescence quantum efficiency in AET quantum cutting process is much higher than that in CET process (134% and 104%, respectively), can benefit for further increasing the conversion efficiency of c-Si solar cells.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electrons

Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density–electroluminescence–voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Preparation and characterization of nanocomposite polymer electrolytes poly(vinylidone fluoride)/nanoclay

Polymer electrolytes are defined as semi solid electrolytes used as separator in lithium ion battery. Separator used as medium for transfer ions and to prevent electrical short circuits in battery cells. To obtain the optimal battery performance, separator with high porosity and electrolyte uptake is required. This can reduce the resistance in the transfer of ions between cathode and anode. The main objective of this work is to investigate the impact of different solvent (Dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone (NMP) and dimethyl formamide (DMF)), pore forming agent poly(vinylpyrolidone) (PVP) and nanoclay as filler in addition of membrane using phase inversion method on the morphology, porosity, electrolyte uptake and degree of crystallinity. The membrane was prepared by the phase inversion method by adding PVP and Nanoclay using different solvents. The phase inversion method was prepared by dissolving Nanoclay and PVP in solvent for 1-2 hours, and then add the PVDF with stirring for 4 hours at 60°C. The membranes were characterized by porosity test, electrolyte uptake test, scanning electron microscope (SEM), and X-ray diffraction (XRD). The results showed that DMAc as solvent gives the highest value of porosity and electrolyte uptake. The addition of nanoclay and PVP enlarge the size of the pores and reduce the degree of crystallinity. So, the usage of DMAc as solvent is better than NMP or DMF.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Hydrothermally synthesized barium fluoride nanocubes for thermoluminescence dosimetry

In this work, we report a hydrothermally synthesized Dy doped BaF{sub 2} (BaF{sub 2}:Dy) nanocubes and its Thermoluminescence studies. The synthesized BaF{sub 2}:Dy samples was found to posses FCC structure and having average size ~ 60-70 nm, as revealed through X-Ray Diffraction. Cubical morphology having size ~90 nm was observed from TEM analysis. The {sup 60}Co γ- ray irradiated BaF{sub 2}:Dy TL dosimetric experiments shows a pre-dominant single glow peak at 153 °C, indicating a single level trap present as a metastable state. Furthermore, BaF{sub 2}:Dy nanophosphor shows a sharp linear response from 10 Gy to 3 kGy, thus it can be applicable as a gamma dosimeter.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Influence of induced colour centres on the frequency – angular spectrum of a light bullet of mid-IR radiation in lithium fluoride

The influence of the occurrence of a structure consisting of long-lived colour centres, formed in an LiF crystal upon filamentation of femtosecond mid-IR radiation, on the supercontinuum characteristics is investigated. With an increase in the number of incident pulses, the length and transverse size of the structure of colour centres induced in LiF increase, and the supercontinuum spectrum in the short-wavelength region is markedly transformed due to the occurrence of the waveguide propagation regime, absorption, and scattering of radiation from the newly formed structure of colour centres. Under these conditions, the intensity of the anti-Stokes wing decreases by two orders of magnitude after several tens of pulses. Spectral components arise in the visible range, the angular divergence of which increases with increasing wavelength. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Synthesis of Nonstoichiometric Samarium Fluoride SmF{sub 2+x}

The products of interaction of SmF{sub 3} with metallic Sm{sup 0}, Si, and molecular hydrogen in quartz, graphite, and molybdenum containers have been investigated by X-ray diffraction. It is shown that reduction occurs with the formation of nonstoichiometric cubic SmF{sub 2+x} (sp. gr. Fm3̅m, unit-cell parameter a = 5.8517(2) Å) with a content x = 0.108 only in the case of synthesis of SmF{sub 3} with metallic Sm{sup 0} in a quartz ampoule. The transformation of the SmF{sub 2+x} phase composition in air at room temperature has been studied for the first time. It is found that SmF{sub 2.108} passes into the superstructural phase Sm{sub 13}F{sub 32–δ} (sp. gr. R3̅) with the unit-cell parameters a = 14.7354(4) Å and c = 10.1156(4) Å for 60 days. The use of Sm{sup 0}, Si, and H{sub 2} as reducing agents for SmF{sub 3} is inefficient because of the weak tendency of the latter to reduction: the process occurs also with a low yield of the Sm{sub 13}F{sub 32–δ} phase and of the α-SmF{sub 3}-based phase as impurity in the case of direct reduction by hydrogen.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

The de Haas-van Alphen effect in nanostructures of cadmium fluoride

Measurements of the field and temperature dependences of static magnetic susceptibility demonstrate de Haas-van Alphen oscillations at high temperatures and low magnetic fields in sandwich nanostructures, which are represented by an ultranarrow p-type CdF{sub 2} quantum well confined by {delta} barriers heavily doped with boron on the surface of an n-type CdF{sub 2} crystal. The temperature dependences of the de Haasvan Alphen oscillation amplitudes indicate a small value of the effective mass of two-dimensional holes, as a result of which, the strong field assumption, {mu}B Much-Greater-Than 1, is fulfilled at high temperatures. It is for the first time that a periodic variation in the de Haas-van Alphen oscillation frequency is detected and is accompanied by a diamagnetic response as temperature is increased. This phenomenon manifests itself as synchronous temperature oscillations of the density and effective mass of two-dimensional holes as a result of the mesoscopic properties of {delta} barriers.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of erbium fluoride doping on the photoluminescence of SiO{sub x} films

The photoluminescence of SiO{sub x} films deposited on c-Si wafers by the thermal evaporation of SiO in a vacuum and, for the first time, doped with ErF{sub 3} by coevaporation is studied. It is shown that, like undoped SiO{sub x} films, the unannealed SiO{sub x}:ErF{sub 3} films passivate the surface of the Si wafers and, thus, increase their edge photoluminescence intensity almost fivefold. A similar increase is observed after annealing of the doped films in air at 750 Degree-Sign C. Doping with ErF{sub 3} suppresses the photoluminescence of Si nanoclusters, if the films have been subjected to high-temperature annealing (at 750 Degree-Sign C). In this case, the PL intensity of the band with a peak at {approx}890 nm decreases as well. The {approx}890 nm band is observed for the first time and, due to its features, is attributed to transitions in SiO{sub x} matrix defects. The experimentally observed effect of ErF{sub 3} doping on SiO{sub x} film photoluminescence is interpreted. An intense photoluminescence signal from Er{sup 3+} ions in the nearinfrared spectral region (the {sup 4}I{sub 11/2} {yields} {sup 4}I{sub 15/2} and {sup 4}I{sub 13/2} {yields} {sup 4}I{sub 15/2} transitions) is observed in the SiO{sub x}:ErF{sub 3} films annealed in air at 750 Degree-Sign C. This finding shows that 1.54 {mu}m luminescent emitters, which are currently in popular demand, can be produced by a simple low-cost method.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Anodic processes in the chemical and electrochemical etching of Si crystals in acid-fluoride solutions: Pore formation mechanism

The interaction of heavily doped p- and n-type Si crystals with hydrofluoric acid in the dark with and without contact with metals having greatly differing work functions (Ag and Pd) is studied. The dependences of the dissolution rates of Si crystals in HF solutions that contain oxidizing agents with different redox potentials (FeCl{sub 3}, V{sub 2}O{sub 5} and CrO{sub 3}) on the type and level of silicon doping are determined. Analysis of the experimental data suggests that valence-band holes in silicon are not directly involved in the anodic reactions of silicon oxidation and dissolution and their generation in crystals does not limit the rate of these processes. It is also shown that the character and rate of the chemical process leading to silicon dissolution in HF-containing electrolytes are determined by the interfacial potential attained at the semiconductor–electrolyte interface. The mechanism of electrochemical pore formation in silicon crystals is discussed in terms of selfconsistent cooperative reactions of nucleophilic substitution between chemisorbed fluorine anions and coordination- saturated silicon atoms in the crystal subsurface layer. A specific feature of these reactions for silicon crystals is that vacant nonbonding d{sup 2}sp{sup 3} orbitals of Si atoms, associated with sixfold degenerate states corresponding to the Δ valley of the conduction band, are involved in the formation of intermediate complexes. According to the suggested model, the pore-formation process spontaneously develops in local regions of the interface under the action of the interfacial potential in the adsorption layer and occurs as a result of the detachment of (SiF{sub 2}){sub n} polymer chains from the crystal. Just this process leads to the preferential propagation of pores along the <100> crystallographic directions. The thermodynamic aspects of pore nucleation and the effect of the potential drop across the interface, conduction type, and free-carrier concentration in the crystal on the pore size and structure are discussed. The concepts developed in the study can consistently account for experimental facts characterizing the etching of silicon crystals with various electrical parameters under various conditions providing the anodic polarization of crystals in HF-containing solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Optical Method of Measuring the Temperature of Fluoride Crystals Activated by Yb{sup 3+} and Tm{sup 3+} Ions

The fluorescence intensities ratio (FIR) method is applied for measuring the temperature inside the luminescent LiY{sub 0.8}Yb{sub 0.2}F{sub 4}:Tm{sup 3+} crystal. The obtained results are used for monitoring of the sample temperature in the experiment when the exciting radiation waist is shifted by the focusing lens inside the sample. It is shown that a considerable change in the luminescence intensity of the LiY{sub 0.8}Yb{sub 0.2}F{sub 4}:Tm{sup 3+} crystal (0.2%) is not accompanied by a significant change in the temperature. This verifies the hypothesis about the appearance of amplified spontaneous luminescence (ASL) of Yb{sup 3+} ions during intense laser pumping in these crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗