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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 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Development of high Fe content squeeze cast 2A16 wrought Al alloys with enhanced mechanical properties at room and elevated temperatures

Highlights: • Applied pressure improved tensile properties at both room and elevated temperatures. • Experimental alloys meet the requirement of safety-critical component in automotive industry. • Increasing value of mechanical properties at elevated temperatures resulting from pressure decreases - Abstract: In this study, the evolution of iron-rich intermetallics and their effects on room and elevated-temperature tensile properties of gravity die cast and squeeze cast 2A16 wrought Al alloys with high Fe content of 0.5% after T7 heat treatment were investigated by various methods. Compared to gravity die-cast alloys, squeeze cast 2A16 wrought Al alloys showed superior tensile properties at both room and elevated temperatures, especially in terms of elongation. This can be attributed to refinement in grain size and iron-rich intermetallics, which decreased porosity and raised dispersoids in α(Al) matrix. However, the increase in mechanical properties at high temperature resulting from applied pressure was lower than that obtained at room temperature. The results were attributed to the refinement in grain size and the weakness of grain boundary at elevated temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The magnetic properties of Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.81}M{sub 0.09}Ti{sub 0.1}O{sub 3} solid solutions (M = Co, Mn, Sc, Al)

Graphical abstract: In Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.9}Ti{sub 0.1}O{sub 3} solid solution, both magnetic ions (Co{sup 3+}, Mn{sup 3+}) and nonmagnetic ions (Sc{sup 3+}, Al{sup 3+}) were chosen to partially replace Fe{sup 3+}. Their magnetic properties were carefully investigated. Figures show M-H loops at RT of Co{sup 3+} doped sample (left) and Sc{sup 3+} substituted solid solution (right). Highlights: {yields} The electrical resistivity of BiFeO{sub 3} is enhanced by forming solid solution with BaTiO{sub 3}. {yields} We replace Fe{sup 3+} by both magnetic ions Co{sup 3+}, Mn{sup 4+} and nonmagnetic ions Sc{sup 3+}, Al{sup 3+} to investigate the magnetic properties. {yields} Structure-magnetic property relationship has been established by Rietveld refinement. -- Abstract: Solid solutions with general formula Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.81}M{sub 0.09}Ti{sub 0.1}O{sub 3} (M = Co, Mn, Sc, Al) together with parental Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.9}Ti{sub 0.1}O{sub 3} were prepared by the traditional solid state reaction method. Their structural, room temperature magnetic, and dielectric properties were investigated. X-ray diffraction analysis indicated that all samples maintained original R3c space group. M-H hysteresis loop of Co{sup 3+} doped sample saturated at an applied field of 1 T with spontaneous magnetization of 1.735 emu/g, while Mn{sup 4+} substitution enhanced the magnetization of Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.9}Ti{sub 0.1}O{sub 3} less strongly; addition of Sc{sup 3+} helped decrease magnetic coercive field while Al{sup 3+} modified sample exhibited paramagnetic M-H hysteresis loop. Differential scanning calorimetry was applied to determine the Neel temperature (T{sub N}) and the T{sub N} for undoped, Co{sup 3+}, Mn{sup 4+}, Sc{sup 3+}, Al{sup 3+} doped solid solutions were 318.1, 324.3, 335.7, 293.9 and 295.8 respectively. Sc{sup 3+} substitution had little influence on the dielectric properties of Bi{sub 0.9}Ba{sub 0.1}Fe{sub 0.9}Ti{sub 0.1}O{sub 3} while Al{sup 3+} doping improved its dielectric constant. In contrast, Co{sup 3+}, Mn{sup 4+} doped samples showed decreased permittivity but inhibited tan {delta} at frequencies larger than 30 kHz.

36 MATERIALS SCIENCE↗
DOE OSTI2012

A comparative study on the optical limiting properties of different nano spinel ferrites with Z-scan technique

Highlights: ► First report in which the optical limiting properties of five different nano spinel ferrites are compared. ► The obtained nonlinearity fits to a two-photon like absorption process. ► Except for NiFe{sub 2}O{sub 4}, the observed nonlinearity has contributions from excited state absorption. ► A size dependent optical limiting response is obtained. ► Among the investigated ferrites, ZnFe{sub 2}O{sub 4} is found to be a better candidate for the optical limiting applications. -- Abstract: We report the optical limiting properties of five different spinel ferrites, NiFe{sub 2}O{sub 4}, Ni{sub 0.5}Zn{sub 0.5}Fe{sub 2}O{sub 4}, ZnFe{sub 2}O{sub 4}, Ni{sub 0.5}Co{sub 0.5}Fe{sub 2}O{sub 4}, and CoFe{sub 2}O{sub 4} with an average particle grain size of 8 nm. The optical limiting properties are investigated using the open aperture Z-scan technique. The obtained nonlinearity fits to a two-photon like absorption process. Except for NiFe{sub 2}O{sub 4}, the observed nonlinearity has contributions from excited state absorption. The optical limiting response is also studied against particle size and the nonlinearity is found to increase with increasing particle size within the range of our investigations. On comparing the optical limiting properties, ZnFe{sub 2}O{sub 4} is found to be a better candidate for the optical limiting applications. To the best of our knowledge, this is the first report where the optical limiting properties of spinel ferrites are compared.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Tuning of crystal phase and luminescence properties of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} phosphors

Graphical abstract: The optical properties of Dy{sup 3+} were found to be sensitive to the crystal phase of the as-synthesized Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} phosphors, especially the distinct features of the charge transfer (CT) transition. The mechanism involved was discussed in detail based on the experiments and theoretic calculations. Highlights: ► The crystal phase of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} was tuned by changing the pH value of the solution in hydrothermal system. ► The optical properties of Dy{sup 3+} were found to be sensitive to the crystal phase of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+}. ► The mechanisms of different crystal phases affected on the luminescence properties of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} were discussed. -- Abstract: The orthorhombic and monoclinic Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} were successfully synthesized by a hydrothermal process with a subsequent annealing treatment at 800 °C for 4 h. The crystal phase of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} was controlled as a function of the pH value of the solution. The crystallization and microstructures of the samples were characterized by Powder X-ray diffraction (XRD) and scanning electron micrograph (SEM). Furthermore, the optical properties were investigated by the diffuse reflection, excitation and emission spectra. The mechanisms of different crystal phases affected on the luminescence properties of Gd{sub 2}(MoO{sub 4}){sub 3}:Dy{sup 3+} were discussed. The electric dipole–dipole interaction between Dy{sup 3+} ions was identified as the main mechanism for the concentration quenching of the two structures. Finally, the chromatic natures of all the samples were analyzed in detail. The results indicate that the orthorhombic phosphor Gd{sub 1.84}(MoO{sub 4}){sub 3}:Dy{sub 0.16}{sup 3+} can be considered as a suitable candidate for white light emitting diodes (W-LEDs).

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electrical property and characterization of nano-SnO{sub 2}/wollastonite composite materials

Graphical abstract: Resistivity as a function of different factors: hydrolysis temperature and time. Highlights: ► We have synthesized nano-tin oxide deposited on the surface of wollastonite. ► The antistatic properties were measured by rubber sheeter and four-point probe (FPP) sheet resistance measurement. ► The results showed that the nano-SnO{sub 2}/wollastonite composite materials showed better antistatic properties. ► The surface of wollastonite fiber was coated with a layer of 10–15 nm thickness of tin oxide grains. - Abstract: Nano-tin oxide was deposited on the surface of wollastonite using the mixed solution including stannic chloride pentahydrate precursor and wollastonite by a hydrolysis precipitation process. The antistatic properties of the wollastonite materials under different calcined conditions and composite materials (nano-SnO{sub 2}/wollastonite, SW) were measured by rubber sheeter and four-point probe (FPP) sheet resistance measurement. Effects of hydrolysis temperature and time, calcination temperature and time, pH value and nano-SnO{sub 2} coating amount on the resistivity of SW powders were studied, and the optimum experimental conditions were obtained. The microstructure and surface properties of wollastonite, precipitate and SW were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), specific surface area analyzer (BET), thermogravimetry (TG), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier translation infrared spectroscopy (FTIR) respectively. The results showed that the nano-SnO{sub 2}/wollastonite composite materials under optimum preparation conditions showed better antistatic properties, the resistivity of which was reduced from 1.068 × 10{sup 4} Ω cm to 2.533 × 10{sup 3} Ω cm. From TG and XRD analysis, the possible mechanism for coating of SnO{sub 2} nanoparticles on the surface of wollastonite was proposed. The infrared spectrum indicated that there were a large number of the hydroxyl groups on the surface of wollastonite. This is beneficial to the heterogeneous nucleation reaction. Through morphology, EDS and XPS analysis, the surface of wollastonite fiber was coated with a layer of 10–15 nm thickness of tin oxide grains the distribution of which was uniform.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ferroelectric, dielectric and piezoelectric properties of Sr{sub 0.6}(BiNa){sub 0.2}Bi{sub 2}Nb{sub 2}O{sub 9} ceramics

Graphical abstract: The oxygen vacancy was confirmed by the left figure and the role of oxygen vacancy on the piezoelectric properties was discussed. - Highlights: • The d{sub 33}, ρ and T{sub c} were found to be 22 pC/N, ρ ∼ 10{sup 6} Ω cm and 586 °C. • The electromechanical properties: k{sub p} ∼ 5.0%, k{sub t} ∼ 8.7% and Q{sub m} ∼ 651. • The oxygen vacancy is responsible for electrical properties at high temperature. - Abstract: Aurivillius-type ceramic, Sr{sub 0.6}(BiNa){sub 0.2}Bi{sub 2}Nb{sub 2}O{sub 9} (SBNBN), was synthesized by using conventional solid-state processing. Phase structure and microstructural morphology were confirmed by X-ray diffraction analyses (XRD) and the scanning electron microscopy (SEM). Dielectric, piezoelectric and electromechanical properties of the SBNBN ceramic were investigated in detail. Curie temperature (T{sub c}), piezoelectric coefficient (d{sub 33}), electromechanical coupling coefficient k{sub p}, k{sub t} and quality factor Q{sub m} of the SBNBN ceramic were found to be 586.5 °C, 22 pC/N, 5.0%, 8.7% and 651, respectively. In addition, the reasons for varieties of the resistivity and dielectric properties at high temperature were also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Piezoelectric and ferroelectric properties of lead-free niobium-rich potassium lithium tantalate niobate single crystals

Graphical abstract: - Highlights: • Lead-free K{sub 0.95}Li{sub 0.05}Ta{sub 1−x}Nb{sub x}O{sub 3} single crystals were grown using the top-seeded melt growth method. • The piezoelectric and ferroelectric properties of as-grown crystals were systematically investigated. • The piezoelectric properties are very attractive, e.g. for x = 0.60 composition, k{sub t} ≈ 70%, k{sub 31} ≈ 70%, k{sub 33} ≈ 77%, d{sub 31} ≈ 230 pC/N, d{sub 33} ≈ 600 pC/N. • The coercive fields of P–E hysteresis loops are quite small, about or less than 1 kV/mm. - Abstract: Lead-free potassium lithium tantalate niobate single crystals with the composition of K{sub 0.95}Li{sub 0.05}Ta{sub 1−x}Nb{sub x}O{sub 3} (abbreviated as KLTN, x = 0.51, 0.60, 0.69, 0.78) were grown using the top-seeded melt growth method. Their piezoelectric and ferroelectric properties in as-grown crystals have been systematically investigated. The phase transitions and Curie temperatures were determined from dielectric and pyroelectric measurements. Piezoelectric coefficients and electromechanical coupling factors in thickness mode, length-extensional mode and longitudinal mode were obtained. The piezoelectric properties are very attractive, e.g. for x = 0.60 composition, k{sub t} ≈ 70%, k{sub 31} ≈ 70%, k{sub 33} ≈ 77%, d{sub 31} ≈ 230 pC/N, d{sub 33} ≈ 600 pC/N are comparable to the lead-based PZT composition. The polarization versus electric field hysteresis loops show saturated shapes. In short, lead-free niobium-rich KLTN system possesses comparable properties to those in important lead-based piezoelectric material nowadays.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Linking constituent phase properties to ductility and edge stretchability of two DP 980 steels

Two DP 980 steels were characterized with multiple methods to determine its microstructure characteristics, tensile properties and edge stretchability. The first (DP980-B) is a commercial grade DP980 steel produced by AK Steel, while the second (DP980-T) is a modification of the first one by a tempering process. Both contain two individual phases: ferrite and martensite with body centered cubic (BCC) and tetragonal (BCT) crystal structure respectively. ASTM standard tensile tests were performed for both materials, revealing that DP980-B has higher work hardening and larger total elongation than DP980-T. Standard hole piercing and subsequent hole expansion and extrusion tests were also performed. The critical hole expansion ratio (HER) of hole pierced sheet at different nominal clearances, on the other hand, show an opposite trend: the HER of DP980-T almost doubles that of DP980-B. This indicates the tensile ductility and edge stretchability are determined by different mechanisms due to different deformation modes. The ductility of sheet metal during uniaxial tension is determined by deformation instability, i.e. necking, due to less constraint at the region of deformation and fracture will occur quickly after necking. The contribution of post-necking deformation to the total elongation is insignificant for the high strength grades such as DP980 steels. The ductility during hole-expansion, however, is mainly determined by material fracture behavior that is dependent on phase property disparity and material intrinsic fracture parameters. With the individual phase properties for both DP steels determined by in situ tensile tests under high energy X-ray diffraction, the results show that DP980-T steel has lower property disparities between the two phases than DP980-B steel, due to the tempering process. This explains why the DP980-T steel has higher HER than DP980-B steel: lower phase disparity will lead to less local deformation during loading. The higher work hardening rate for DP980-B steel contributes to its higher uniform elongation compared with DP980-T steel based on the maximum load condition of deformation instability. An integrated finite element simulation framework for studying hole expansion is also presented here, based on the calculated individual phase properties from the combined high energy x-ray diffraction (HEXRD) and elastic plastic self-consistent modeling. The simulation results correlate well with the experimental results on the HER difference between the two materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Porous architecture and thermal properties of thermal barrier coatings deposited by suspension plasma spray

Besides the intrinsic low heat transfer capability of material, the thermal insulation property of thermal barrier coatings (TBCs) also relies on their microstructures. For better understanding the relationship between process parameters, porous architecture, and thermal properties of coatings, YSZ coatings were firstly manufactured by suspension plasma spray (SPS). Afterwards their total porosities were characterized by using the technique of X-ray transmission, the nano/submicro pores in those coatings were detected with Ultra-Small Angle X-ray Scattering (USAXS), and the thermal properties of coatings were measured using the laser flash method. The results indicated that: i) the porous architecture of SPS coatings can be tailored by adapting process parameters. ii) increasing total porosity is an effective means for reducing the heat transfer capability of SPS coatings. iii) nano/submicro pores have higher influence sensitivity on thermal properties than that of larger pores. Increasing the content of nano-submicro pores is therefore more effective for improving the thermal insulation property of SPS coatings. The potential of SPS technique for fabricating TBCs was discussed as well.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Minimal Impacts of Microplastics on Soil Physical Properties under Environmentally Relevant Concentrations

Agricultural soils are a major reservoir of microplastics, and concerns have arisen about the impacts of microplastics on soil properties and functioning. In this report we measured the physical properties of a silt loam in response to the incorporation of polyester fibers and polypropylene granules over a wide range of concentrations. We further elucidated the underlying mechanisms through determining the role of microplastic shape and the baseline effects from the amendment of soil particles. The incorporation of microplastics into soil tended to increase contact angle and saturated hydraulic conductivity, decrease bulk density and water holding capacity, while did not affect aggregate stability. Polyester fibers affected soil physical properties more profoundly than polypropylene granules, due to the vastly different shape of fibers from that of soil particles. However, changes in soil properties were gradual, and significant changes did not occur until a high concentration of microplastics was reached (i.e., 0.5% w/w for polyester fibers and 2% w/w for polypropylene granules). Currently, microplastic concentrations in soils not heavily polluted with plastics are far below these concentrations, and results from this study suggest that microplastics at environmentally relevant concentrations have no significant effects on soil physical properties.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Round Robin Measurements of Molten Salt Properties for LiF-NaF-KF (FLiNaK) and NaCl-KCl Mixtures

The development, operation, and regulation of nuclear reactors that utilize molten salts as fuel or as heat transfer media require knowledge of the thermal properties of the salt systems and quantification of the corresponding uncertainties. Knowledge of molten salt properties is also necessary for applications in material synthesis, processing, separations, solar thermal power generation, and energy storage. A round robin was conducted with national laboratory and university participants from twenty-one laboratories in five countries to compare property measurements, to better understand uncertainties, and to identify possible best practices. Two salt mixtures, each from a common batch, were distributed to participants for evaluation: equimolar NaCl-KCl and 45.0LiF-13.7NaF-41.3KF mol % (FLiNaK). Measurements were performed to determine the major constituent composition, oxygen content, density, thermal expansivity, melting point, and thermal conductivity. Error analysis was performed on each measurement for uncertainty quantification for each type of property that was explored. Finally, the resulting discussion of the methodologies used in this work is meant to lay the groundwork for the development of standard methods and reference materials for future high-temperature property measurements on halide melts.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Blending Education and Polymer Science: Semiautomated Creation of a Thermodynamic Property Database

Structured databases of chemical and physical properties play a central role in the everyday research activities of scientists and engineers. In materials science, researchers and engineers turn to these databases to quickly query, compare, and aggregate various properties, thereby allowing for the development or application of new materials. The vast majority of these databases have been generated manually, through decades of labor-intensive harvesting of information from the literature, yet while there are many examples of commonly used databases, a significant number of important properties remain locked within the tables, figures, and text of publications. The question addressed in our work is whether and to what extent the process of data collection can be automated. Students of the physical sciences and engineering are often confronted with the challenge of finding and applying property data from the literature, and a central aspect of their education is to develop the critical skills needed to identify such data and discern their meaning or validity. To address shortcomings associated with automated information extraction while simultaneously preparing the next generation of scientists for their future endeavors, we developed a novel course-based approach in which students develop skills in polymer chemistry and physics and apply their knowledge by assisting with the semiautomated creation of a thermodynamic property database.

Collaborative/Cooperative Learning↗
DOE OSTI2026

Ultrasonic-Assisted Extrusion Processing for Enhancing Physical Properties of High-Density Polyethylene by Flow-Induced Crystallization

The evolution of crystallinity resulting from stress imposed on a melt, known as flow-induced crystallinity, can strongly influence the mechanical and physical properties of semicrystalline polymers. This study investigates shear-induced crystallization by applying an ultrasonic field to the melt flow as it passes through dies with various geometries. A custom-built sonication die is employed for controlling the dynamic temperature and shear environment, resulting in molecular alignment and potential for flow-induced crystallization. Application of both conventional and ultrasonic shear rates at the equilibrium melt temperature of high-density polyethylene (HDPE) was investigated to accelerate crystallinity and manipulate the crystal morphology across the film in pursuit of improved mechanical and gas barrier properties without the need for additives or other polymer layers. The relationships among ultrasonic-assisted extrusion processing, polymer structure, and performance were analyzed using wide- and small-angle X-ray scattering (WAXS and SAXS), tensile testing, and oxygen transmission rate (OTR) analysis. Multiple linear regression models were implemented to predict the correlation among HDPE structure, process, and properties. Structural analysis revealed that both conventional and ultrasonic shear rates had the most significant influence on lamellar spacing and redistribution of rigid and soft amorphous fractions within the crystalline domains, ultimately dictating the mechanical and physical properties of the films. The goal is to explore the potential of the ultrasonic-assisted high crystallinity monolayer that can replace some of the functionality of complex, heterogeneous multilayer packaging with a single-material film having enhanced oxygen barrier properties.

crystallinity↗
DOE OSTI2024

Bayesian blacksmithing: discovering thermomechanical properties and deformation mechanisms in high-entropy refractory alloys

Finding alloys with specific design properties is challenging due to the large number of possible compositions and the complex interactions between elements. This study introduces a multi-objective Bayesian optimization approach guiding molecular dynamics simulations for discovering high-performance refractory alloys with both targeted intrinsic static thermomechanical properties and also deformation mechanisms occurring during dynamic loading. The objective functions are aiming for excellent thermomechanical stability via a high bulk modulus, a low thermal expansion, a high heat capacity, and for a resilient deformation mechanism maximizing the retention of the BCC phase after shock loading. Contrasting two optimization procedures, we show that the Pareto-optimal solutions are confined to a small performance space when the property objectives display a cooperative relationship. Conversely, the Pareto front is much broader in the performance space when these properties have antagonistic relationships. Density functional theory simulations validate these findings and unveil underlying atomic-bond changes driving property improvements.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Regulating hydrogel mechanical properties with an electric field

Stimuli-responsive polymeric materials have attracted significant attention due to their ability to change properties in response to various external stimuli. Using an electric field as the stimulus is of particular interest as it possesses the potential for seamless integration of materials with electronic systems. While many materials with electric field responsive actuation have an associated mechanical property change, it is beneficial to develop materials that exhibit mechanical property changes without accompanying significant shape deformation. To address this challenge, here we designed a semi-interpenetrating polymer network (semi-IPN) hydrogel system containing both polyelectrolytes and salt ions, which enables electric field induced changes in mechanical properties while minimizing actuation. We first successfully verified the viability of our design by removing salt ions through a diffusion-only method where we witnessed the stiffness increased to 4.5 times the initial value while still being highly deformable. After this, we applied an electric field to transport the salt ions out of the hydrogel, as shown by both Raman spectroscopy and scanning electron microscopy. We were able to show a time-dependent stiffness increase, the maximum of which was 5 times the original stiffness. We quantified ion transport and water-splitting in the hydrogel by both experiments and simulations. Following this, we showed functional system reversibility by reversing the direction of the current to reinject salt ions into the semi-IPN hydrogel and reducing its stiffness to below the initial value. It's worth noting that our simulations enable us to understand the governing mechanisms behind ion generation and salt transport that leads to mechanical property changes. Finally, we were able to fabricate a spatially variable stiffness haptic interface with our hydrogel, with demonstrated reversibility and cyclability. This research can possibly find applications in soft robotics and haptics and also inspire the development of bio-compatible electronics related devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Comprehensive study of the influence of different environments on degradation processes in F8BT: Correlating optoelectronic properties with Raman measurements

There is a growing interest in conjugated polymers from both industrial and academic points of views. The reasons are their tunable optoelectronic properties, ease of production, and excellent mechanical properties. However, the ease with which their optoelectronic properties are tunable make devices based on them prone to fast degradation and therefore, short life time. The issue of degradation of organic based optoelectronic devices is the topic of many ongoing researches. However, much less attention is given to degradation processes of the individual components of the devices and their dependence on the environmental conditions. In this work, we report on the degradation of a film of a polyfluorene block copolymer F8BT that is used in a variety of optoelectronic devices under different environments: Sun exposure, heating, and UV exposure in inert and ambient conditions. Degradation was observed in most of the optoelectronic properties of the film. Topographic measurements did not show observable changes of the film morphology following degradation. However, Raman spectroscopy measurements show changes that indicate degradation in one of the building blocks of the copolymer that is associated with electron's conduction. The absolute value of the correlation coefficient between the decrease in the Raman signal and the decrease in the optoelectronic properties is larger than 0.95 under sun exposure it is larger than 0.8 under all other ambient exposures and smaller than 0.65 under inert conditions. These results support the assumption that Oxygen, not necessarily through photo-oxidation, and also water play an important role in the degradation process and indicate the part of the polymer that is most susceptible to degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Double quantum dot in a quantum dash: Optical properties

We study the optical properties of highly elongated, highly flattened quantum dot structures, also referred to as quantum dashes, characterized by the presence of two trapping centers located along the structure. Such a system can exhibit some of the properties characteristic for double quantum dots. We show that sub- and super-radiant states can form for certain quantum dash geometries, which is manifested by a pronounced transfer of intensity between spectral lines, accompanied by the appearance of strong electron-hole correlations. We also compare exciton absorption spectra and polarization properties of a system with a single and double trapping center and show how the geometry of multiple trapping centers influences the optical properties of the system. We show that for a broad range of trapping geometries the relative absorption intensity of the ground state is larger than that of the lowest excited states, contrary to the quantum dash systems characterized by a single trapping center. Thus, optical properties of these structures are determined by fine details of their morphology.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electrical properties of dislocations in III-Nitrides

Research on GaN, AlN, InN (III-N) and their alloys is achieving new heights due their high potential applications in photonics and electronics. III-N semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain many threading dislocations (TDs). While their structural properties have been widely investigated, their electrical characteristics and their role in the transport properties of the devices are still debated. In the present contribution we will show conductive AFM studies of TDs in GaN and Al/In GaN ternary alloys to evidence the role of strain, different surface polarity and composition on their electrical properties. Local I-V curves measured at TDs allowed us to clarify their role in the macroscopic electrical properties (leakage current, mobilities) of III-N based devices. Samples obtained by different growers (AIXTRON, III-V Lab) were studied. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗