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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 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Linking Carbon and Boron-Nitride Nanotubes: Heterojunction Energetics and Band Gap Tuning

We investigate the energetics of forming heteronanotubes, which are combinations of pure carbon nanotube (CNT) segments and boron-nitride nanotube (BNNT)segments. Our density functional theory calculations predict that the adverse impacts of heterojunctions on the nanotube stability can be minimized if the CNTand/or the BNNT building block segments are sufficiently large along the axial direction (corresponding to circular junctions). As such, carbon-boron-nitride heteronanotubes can be thermodynamically competitive in stability, as compared to pure CNTs and BNNTs of similar geometry, and this is in good agreement with previous experimental observations. In addition, we find that the highest occupied crystal orbital/lowest unoccupied crystal orbital (HOCO-LUCO) gap of carbon-boron-nitride heteronanotubes can be significantly tuned by modifying the CNT and BNNT combinations, the tube chirality, or the junction geometry (i.e., circular or linear).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Tuning the Optical Properties of Mesoporous TiO2 Films by Nanoscale Engineering

Introducing mesoscale pores into spincoated titanium dioxide films, prepared by spincoating different sol-gel precursor solutions on silicon substrates and subsequent annealing at 350 C, 400 C or 450 C, respectively, affects several optical properties of the material. The change in refractive index observed for different mesoporous anatase films directly correlates with changes in pore size, but is also in a more complex manner influenced by the film thickness and the density of pores within the films. Additionally, the band gap of the films is blueshifted by the stress the introduction of pores exerts on the inorganic matrix. The differently sized pores were templated by Pluronic{reg_sign} block copolymers in the solgel solutions and tuned by employing different annealing temperatures for the film preparation. This study focused on elucidating the effect different templating materials (F127 and P123) have on the pore size of the final mesoporous titania film, and on understanding the relation of varying polymer concentration (taking P123 as an example) in the sol-gel solution to the pore concentration and size in the resultant titania film. Titania thin film samples or corresponding titanium dioxide powders were characterized by X-ray diffraction, nitrogen adsorption, ellipsometery, UV/Vis spectrometry and other techniques to understand the interplay between mesoporosity and optical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Tuning Structural and Mechanical Properties of Two-Dimensional Molecular Crystals: The Roles of Carbon Side Chains

A key requirement for the future applicability of molecular electronics devices is a resilience of their properties to mechanical deformation. At present, however, there is no fundamental understanding of the origins of mechanical properties of molecular films. Here we use quinacridone, which possesses flexible carbon side chains, as a model molecular system to address this issue. Eight molecular configurations with different molecular coverage are identified by scanning tunneling microscopy. Theoretical calculations reveal quantitatively the roles of different molecule-molecule and molecule-substrate interactions and predict the observed sequence of configurations. Remarkably, we find that a single Young's modulus applies for all configurations, the magnitude of which is controlled by side chain length, suggesting a versatile avenue for tuning not only the physical and chemical properties of molecular films but also their elastic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Symmetry tuning via controlled crossed-beam energy transfer on the National Ignition Facility

The Hohlraum energetics experimental campaign started in the summer of 2009 on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)]. These experiments showed good coupling of the laser energy into the targets [N. Meezan et al., Phys. Plasmas 17, 056304 (2010)]. They have also demonstrated controlled crossed-beam energy transfer between laser beams as an efficient and robust tool to tune the implosion symmetry of ignition capsules, as predicted by earlier calculations [P. Michel et al., Phys. Rev. Lett. 102, 025004 (2009)]. A new linear model calculating crossed-beam energy transfer between cones of beams on the NIF has been developed. The model has been applied to the subscale Hohlraum targets shot during the National Ignition Campaign in 2009. A good agreement can be found between the calculations and the experiments when the impaired propagation of the laser beams due to backscatter is accounted for.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Influence of frequency tuning and double-frequency heating on ions extracted from an electron cyclotron resonance ion source

The electromagnetic field within the plasma chamber of an electron cyclotron resonance ion source (ECRIS) and the properties of the plasma waves affect the plasma properties and ion beam production. We have experimentally investigated the ''frequency tuning effect'' and ''double frequency heating'' on the CAPRICE ECRIS device. A traveling wave tube amplifier, two microwave sweep generators, and a dedicated experimental set-up were used to carry out experiments in the 12.5-16.5 GHz frequency range. During the frequency sweeps the evolution of the intensity and shape of the extracted argon beam were measured together with the microwave reflection coefficient. A range of different ion source parameter settings was used. Here we describe these experiments and the resultant improved understanding of these operational modes of the ECR ion source.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators

A density perturbation in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 {+-} 3.6%, divergence of 4 {+-} 0.8 mrad, and charge of 6 {+-} 1.8 pC.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Low repetition rate and broad frequency tuning from a grating-coupled passively mode-locked quantum dot laser

Passively mode-locked quantum dot lasers with a grating-coupled external cavity arrangement are investigated. A broad repetition-rate tuning range of fundamental mode-locking from 2 GHz to a record-low frequency of 79.3 MHz is achieved with selecting the wavelength at 1.28 μm. A narrow RF linewidth of ∼25 Hz and an intrinsic linewidth as low as 0.15 Hz are also obtained.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication

This paper introduces a nanopipette combined with a quartz tuning fork-atomic force microscope system (nanopipette/QTF-AFM), and describes experimental and theoretical investigations of the nanoscale materials used. The system offers several advantages over conventional cantilever-based AFM and QTF-AFM systems, including simple control of the quality factor based on the contact position of the QTF, easy variation of the effective tip diameter, electrical detection, on-demand delivery and patterning of various solutions, and in situ surface characterization after patterning. This tool enables nanoscale liquid delivery and nanofabrication processes without damaging the apex of the tip in various environments, and also offers force spectroscopy and microscopy capabilities.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A resonant sensor composed of quartz double ended tuning fork and silicon substrate for digital acceleration measurement

Presented in this paper is a micro-resonant acceleration sensor based on the frequency shift of quartz double ended tuning fork (DETF). The structure is silicon substrate having a proof mass supported by two parallel flexure hinges as doubly sustained cantilever, with a resonating DETF located between the hinges. The acceleration normal to the chip plane induces an axial stress in the DETF beam and, in turn, a proportional shift in the beam resonant frequency. Substrate is manufactured by single-crystal silicon for stable mechanical properties and batch-fabrication processes. Electrodes on the four surfaces of DETF beam excite anti-phase vibration model, to balance inner stress and torque and imply a high quality factor. The sensor is simply packaged and operates unsealed in atmosphere for measurements. The tested natural frequency is 36.9 kHz and the sensitivity is 21 Hz/g on a nominally ±100 g device, which is in good agreement with analytical calculation and finite element simulation. The output frequency drifting is less than 0.5 Hz (0.0014% of steady output) within 1 h. The nonlinearity is 0.0019%FS and hysteresis is 0.0026%FS. The testing results confirm the feasibility of combining quartz DETF and silicon substrate to achieve a micro-resonant sensor based on simple processing for digital acceleration measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Tuning the local temperature during feedback controlled electromigration in gold nanowires

Feedback controlled electromigration (FCE) in metallic nanowires has been widely used for various purposes. However, the control of the local temperature during FCE remains a challenge. Here, we report that the environment temperature can be used as a knob to tune the local temperature during FCE in gold nanowires. FCE was performed in gold nanowires at various environment temperatures ranging from 4.2 K to 300 K. We find that the dissipated power normalized by the cross section area of the nano constriction is linearly proportional to the environment temperature. Interestingly, the estimated local maximum temperature parabolically depends on the environment temperature. A minimum in the local temperature can be reached if an appropriate environment temperature is chosen. Our findings are well supported by the finite element simulation. Moreover, the data indicates the coupling between FCE triggering current density and local temperature.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Resonance frequency-retuned quartz tuning fork as a force sensor for noncontact atomic force microscopy

Based on a two-prong type quartz tuning fork, a force sensor with a high Q factor, which we call a retuned fork sensor, was developed for non-contact atomic force microscopy (nc-AFM) with atomic resolution. By cutting a small notch and attaching an AFM tip to one prong, its resonance frequency can be retuned to that of the other intact prong. In balancing the two prongs in this manner, a high Q factor (>50 000 in ultrahigh vacuum) is obtained for the sensor. An atomic resolution image of the Si(111)-7 × 7 surface was demonstrated using an nc-AFM with the sensor. The dependence of the Q factor on resonance frequency of the sensor and the long-range force between tip and sample were measured and analyzed in view of the various dissipation channels. Dissipation in the signal detection circuit turned out to be mainly limited by the total Q factor of the nc-AFM system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Charge tuning in [111] grown GaAs droplet quantum dots

We demonstrate charge tuning in strain free GaAs/AlGaAs quantum dots (QDs) grown by droplet epitaxy on a GaAs(111)A substrate. Application of a bias voltage allows the controlled charging of the QDs from −3|e| to +2|e|. The resulting changes in QD emission energy and exciton fine-structure are recorded in micro-photoluminescence experiments at T = 4 K. We uncover the existence of excited valence and conduction states, in addition to the s-shell-like ground state. We record a second series of emission lines about 25 meV above the charged exciton emission coming from excited charged excitons. For these excited interband transitions, a negative diamagnetic shift of large amplitude is uncovered in longitudinal magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Radiative heat transfer in anisotropic many-body systems: Tuning and enhancement

A general formalism for calculating the radiative heat transfer in many body systems with anisotropic component is presented. Our scheme extends the theory of radiative heat transfer in isotropic many body systems to anisotropic cases. In addition, the radiative heating of the particles by the thermal bath is taken into account in our formula. It is shown that the radiative heat exchange (HE) between anisotropic particles and their radiative cooling/heating (RCH) could be enhanced several order of magnitude than that of isotropic particles. Furthermore, we demonstrate that both the HE and RCH can be tuned dramatically by particles relative orientation in many body systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Continuous tuning of W-doped VO{sub 2} optical properties for terahertz analog applications

Vanadium dioxide (VO{sub 2}), with its characteristic metal-insulator phase transition, is a prospective active candidate to realize tunable optical devices operating at terahertz (THz) frequencies. However, the abrupt phase transition restricts its practical use in analog-like continuous applications. Incorporation of tungsten is a feasible approach to alter the phase transition properties of thin VO{sub 2} films. We show that amplitude THz modulation depth of ∼65%, characteristic phase transition temperature of ∼40 °C, and tuning range larger than 35 °C can be achieved with W-doped VO{sub 2} films grown on sapphire substrates. W-doped VO{sub 2} films can also be used to suppress Fabry-Perot resonances at THz frequencies but at temperatures much lower than that observed for undoped VO{sub 2} films. The gradual phase transition temperature window allows for precise control of the W-doped VO{sub 2} optical properties for future analog based THz devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Tuning thermal conductivity in homoepitaxial SrTiO{sub 3} films via defects

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO{sub 3} films deposited by reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cation stoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity (k{sub 33}), with the greatest decrease in films of the same composition observed for films containing planar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivity can be modified by as much as 80%—from 11.5 W m{sup −1}K{sup −1} for stoichiometric homoepitaxial SrTiO{sub 3} to 2 W m{sup −1}K{sup −1} for strontium-rich homoepitaxial Sr{sub 1+δ}TiO{sub x} films—by incorporating (SrO){sub 2} Ruddlesden-Popper planar defects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Quartz enhanced photoacoustic H{sub 2}S gas sensor based on a fiber-amplifier source and a custom tuning fork with large prong spacing

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplified laser source and a custom quartz tuning fork (QTF) with its two prongs spaced ∼800 μm apart, is reported. The sensor employs an acoustic micro-resonator (AmR) which is assembled in an “on-beam” QEPAS configuration. Both length and vertical position of the AmR are optimized in terms of signal-to-noise ratio, significantly improving the QEPAS detection sensitivity by a factor of ∼40, compared to the case of a sensor using a bare custom QTF. The fiber-amplifier-enhanced QEPAS sensor is applied to H{sub 2}S trace gas detection, reaching a sensitivity of ∼890 ppb at 1 s integration time, similar to those obtained with a power-enhanced QEPAS sensor equipped with a standard QTF, but with the advantages of easy optical alignment, simple installation, and long-term stability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Band gap tuning and orbital mediated electron–phonon coupling in HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1)

We report on the evidenced orbital mediated electron–phonon coupling and band gap tuning in HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1) compounds. From the room temperature Raman scattering, it is apparent that the electron-phonon coupling is sensitive to the presence of both the Fe and Cr at the B-site. Essentially, an A{sub g} like local oxygen breathing mode is activated due to the charge transfer between Fe{sup 3+} and Cr{sup 3+} at around 670 cm{sup −1}, this observation is explained on the basis of Franck-Condon mechanism. Optical absorption studies infer that there exists a direct band gap in the HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1) compounds. Decrease in band gap until x = 0.5 is ascribed to the broadening of the oxygen p-orbitals as a result of the induced spin disorder due to Fe{sup 3+} and Cr{sup 3+} at B-site. In contrast, the increase in band gap above x = 0.5 is explained on the basis of the reduction in the available unoccupied d-orbitals of Fe{sup 3+} at the conduction band. We believe that above results would be helpful for the development of the optoelectronic devices based on the ortho-ferrites.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Tuning the critical solution temperature of polymers by copolymerization

We study statistical copolymerization effects on the upper critical solution temperature (CST) of generic homopolymers by means of coarse-grained Langevin dynamics computer simulations and mean-field theory. Our systematic investigation reveals that the CST can change monotonically or non-monotonically with copolymerization, as observed in experimental studies, depending on the degree of non-additivity of the monomer (A-B) cross-interactions. The simulation findings are confirmed and qualitatively explained by a combination of a two-component Flory-de Gennes model for polymer collapse and a simple thermodynamic expansion approach. Our findings provide some rationale behind the effects of copolymerization and may be helpful for tuning CST behavior of polymers in soft material design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗