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At least 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Grain-Size-Limited Mobility in Methylammonium Lead Iodide Perovskite Thin Films

We report a systematic study of the gigahertz-frequency charge carrier mobility found in methylammonium lead iodide perovskite films as a function of average grain size using time-resolved microwave conductivity and a single processing chemistry. Our measurements are in good agreement with the Kubo formula for the AC mobility of charges confined within finite grains, suggesting (1) that the surface grains imaged via scanning electron microscopy are representative of the true electronic domain size and not substantially subdivided by twinning or other defects not visible by microscopy and (2) that the time scale of diffusive transport across grain boundaries is much slower than the period of the microwave field in this measurement (-100 ps). The intrinsic (infinite grain size) minimum mobility extracted form the model is 29 +/- 6 cm2 V-1 s-1 at the probe frequency (8.9 GHz).

36 MATERIALS SCIENCE↗
DOE OSTI2012

Methanol Decomposition over Palladium Particles Supported on Silica: Role of Particle Size and Co-Feeding Carbon Dioxide on the Catalytic Properties

Monodisperse palladium particles of six distinct and controlled sizes between 4-16 nm were synthesized in a one-pot polyol process by varying the molar ratios of the two palladium precursors used, which contained palladium in different oxidation states. This difference permitted size control by regulation of the nucleation rate because low oxidation state metals ions nucleate quickly relative to high oxidation state ions. After immobilization of the Pd particles on silica by mild sonication, the catalysts were characterized by X-ray absorption spectroscopy and applied toward catalytic methanol decomposition. This reaction was determined as structure sensitive with the intrinsic activity (turnover frequency) increasing with increasing particle size. Moreover, observed catalytic deactivation was linked to product (carbon monoxide) poisoning. Co-feeding carbon dioxide caused the activity and the amount of deactivation to decrease substantially. A reaction mechanism based on the formation of the {pi}-bond between carbon and oxygen as the rate-limiting step is in agreement with antipathetic structure sensitivity and product poisoning by carbon monoxide.

10 SYNTHETIC FUELS↗
DOE OSTI2018

Universality in size-driven evolution towards bulk polarizability of metals

The properties and characteristics of materials on the subnano/nano scale are very different from those of their bulk counterparts. The evolution of materials properties with size is the holy grail of nanoscience. An intriguing question then is: Can one predict what type of material (metal, semiconductor or insulator) an unidentified element will be, when in bulk quantities, solely from the properties it exhibits over a limited range of the subnano/nano size-regime? We demonstrate here that for nominally metallic elements ( i.e. , elements that are metals in bulk quantities) the answer to this question is “yes”, and the very identity of the element also can be established. Most importantly, we show that the phenomenon of size-induced transition to metallicity, as gauged by polarizability, is characterized by features and trends that are universal for all metals. Finally, combining numerical simulation data with an analytical model we introduce a universal constant and derive equations that express the universality explicitly.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Sizing and Location Selection of Medium‐Voltage Back‐to‐Back Converters for DER‐Dominated Distribution Systems

Medium‐voltage back‐to‐back (MVB2B) converters can connect two distribution systems and quantifiably transfer power between them. This function can enable the MVB2B converter to exchange distributed energy resource (DER)‐generated power between two systems and bring significant value to enhancing distribution system DER adoption. Our previous work analysed and demonstrated the value MVB2B converter can bring to DER integration. As continuous work, this paper presents a methodology that helps address the MVB2B converter sizing and location selection problem in distribution systems with high DER penetrations. The proposed methodology aims to address three critical problems for MVB2B converter implementation in the real world: (1) which distribution systems are better to be connected, (2) what converter size is appropriate for connecting the distribution systems, and (3) where the optimal connection points are in the systems for connecting the MVB2B converter. The proposed methodology has been demonstrated by case studies that include various scenarios involving distribution systems with different dominated load types and high photovoltaic penetrations. The results demonstrate that selecting the optimal converter size based on net revenue and time of return considerations leads to a balance between maximizing energy savings and minimizing financial payback periods. Furthermore, feeder pair selection based on load profile standard deviation effectively identifies systems that derive the greatest value from MVB2B integration. Finally, an optimized connection point selection approach using a voltage load sensitivity matrix ensures minimal system impact while facilitating efficient power exchange. These findings provide practical insights for the real‐world deployment of MVB2B converters to enhance DER hosting capacity and improve grid resilience.

14 SOLAR ENERGY↗
DOE OSTI2012

Well-isolated L1{sub 0} FePt-SiN{sub x}-C nanocomposite films with large coercivity and small grain size

FePt-SiN{sub x}-C films on TiN/CrRu/glass substrate with large coercivity, (001) texture, and small isolated grains were obtained by co-sputtering FePt, Si{sub 3}N{sub 4}, and C targets at 380 deg. C. It was found that when C was doped into the FePt-SiN{sub x} films, the out-of-plane coercivity increased while the small in-plane coercivity remained unchanged. Grain size decreased and grain size distribution became more uniform with increasing the C doping concentration. The x-ray photoelectron spectroscopy (XPS) depth profile showed a uniform depth distribution of Si in the FePt layer. The Si2p XPS spectrum implied the existence of Fe-Si bonds, indicating that SiN{sub x} was located at the FePt grain boundaries and was stable against diffusion to the surface, thus favoring grain isolation. Well-isolated FePt (001) granular films with coercivity higher than 21.5 kOe and an average grain size of 5.6 nm were obtained by doping 40 vol. % of SiN{sub x} and 20 vol. % of C.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of a polynomial arbitrary dust size distribution on dust acoustic solitons

The investigation of dust-acoustic solitons when dust grains are size-distributed and ions adiabatically heated is conducted. The influence of an arbitrary dust size-distribution described by a polynomial function on the properties of dust acoustic waves is investigated. An energy-like integral equation involving Sagdeev potential is derived. The solitary solutions are shown to undergo a transformation into cnoidal ones under some physical conditions. The dust size-distribution can significantly affect both lower and upper critical Mach numbers for both solitons and cnoidal solutions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Size-dependent emission efficiency and luminescence characteristics of YBO{sub 3}:Tb{sup 3+} nanocrystals under vacuum ultraviolet excitations

The luminous efficiency and lifetime of plasma display panels (PDPs) are directly related to the performance of phosphors used in PDPs, thus higher efficiency, higher stability against high temperature processes, and a long lifetime along with good color chromaticity against vacuum-ultraviolet radiation are major concerns in selecting suitable phosphors for PDPs. In the same pursuit, well crystallized pure hexagonal phase YBO{sub 3}:Tb{sup 3+} nanocrystals were prepared using co-precipitation method. The prepared YBO{sub 3}:Tb{sup 3+} nanocrystals showed bright green luminescence, color chromaticity (0.21, 0.61), and could be assigned to {sup 5}D{sub 4}{yields}{sup 7}F{sub j} transitions (j = 2-6) due to electric dipole-dipole interaction of Tb{sup 3+} ions. The vacuum ultraviolet photoluminescence spectroscopy of the prepared YBO{sub 3}:Tb{sup 3+} nanocrystals showed size dependent nonlinear luminescence enhancement with relatively shorter life time as compared to the commercial bulk YBO{sub 3}:Tb{sup 3+} phosphor. The quenching concentration of Tb{sup 3+} doping for ({sup 5}D{sub 4}{yields}{sup 7}F{sub 5}) transition was found significantly enhanced with the decrease in particle size suggesting it an ideal green phosphor for plasma display panels. The possible explanation for size-dependent emission efficiency and observed luminescence characteristics were proposed via charge transfer process and lowering of the structural symmetry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

The effect of a dust size distribution on electrostatic sheaths in unmagnetized dusty plasmas

In this work, the structure of plasma sheaths in presence of dust particles with different sizes is investigated numerically in a multifluid framework, where the dust size distribution is modeled by Gauss' law. For this, we have established a 1D, stationary, unmagnetized, and weakly collisional electronegative dusty plasma sheath model. The electrons and negative ions are considered in a local thermodynamic equilibrium, therefore, described by a Boltzmann distribution. On the other hand, positive ions and dust grains are described by fluid equations. The charging process is described by the orbit motion limited model. It is shown that taking into account dust grains with different sizes reduces considerably the sheath thickness. The behavior of dust surface potential is not affected, but the dust charge number is reduced, as well as the electrostatic force. It results in a decrease of layered structure. The presence of negative ions makes the structure of the electrostatic potential more oscillatory. The other physical parameters are also analyzed and discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Grain size softening effect in Al{sub 62.5}Cu{sub 25}Fe{sub 12.5} nanoquasicrystals

Inverse Hall-Petch (IHP) behavior in nano-quasicrystalline Al{sub 62.5}Cu{sub 25}Fe{sub 12.5} is reported. Powders with varying grain sizes were produced by mechanical milling of spray-formed quasicrystals. The hardness of the milled powders increased with decreasing grain size down to about 40 nm and decreased with further refinement, demonstrating the IHP behavior. This critical grain size was found to be larger compared to other metallic nanocrystalline alloys. This IHP behaviour has been attributed to the structural complexity in quasicrystals and to thermally activated shearing events of atoms at the grain boundaries.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Effect of grain size on charge and magnetic ordering in Bi{sub 0.67}Ca{sub 0.33}MnO{sub 3}

In the present work we report the electron spin resonance(ESR) and magnetic measurements to understand the effect of grain size on the charge ordering (CO), orbital ordering (OO), long range anti ferromagnetic (AFM) ordering in Bi{sub 0.67}Ca{sub 0.33}MnO{sub 3} manganite synthesized by sol-gel method. The temperature dependence of linewidth (ΔH) of the ESR signal and magnetization have been used to get information about magnetic interactions. The CO is suppressed and AFM ordering remains unaffected with increase in grain size of the sample. The competition between FM and AFM correlations is strongly dependent on the grain size of the sample.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Impact of temperature on zinc oxide particle size by using sol-gel process

Zinc oxide (ZnO) nanoparticles were prepared and synthesized via sol-gel method, by using citric acid as a precursor. The impact of annealing on the particle size was investigated. Based on the results from the Thermogravimetric Analysis (TGA), three different annealing temperature which is 500, 600 and 700 °C were chosen followed by the characterization of the ZnO nanoparticle by using Powder X-Ray Diffraction (PXRD), Transmission Electron Microscopy (TEM) and Field Emission Scanning Electron Microscopy (FESEM). Results showed that the crystallite size estimated from PXRD increased with the annealing temperature which was hexagonal structure for ZnO. TEM further revealed the same tendency which the Zn NPs size also increased with the annealing temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetoresistance of polycrystalline gadolinium with varying grain size

In this paper, we report a study of evolution of low field magnetoresistance (MR) of Gadolinium as the grain size in the sample is changed from few microns (∼4 μm) to the nanoscopic regime (∼35 nm). The low field MR has a clear effect on varying grain size. In large grain sample (few μm), the magnetic domains are controlled by local anisotropy field determined mainly by the magnetocrystalline anisotropy. The low field MR clearly reflects the temperature dependence of the magnetocrystalline anisotropy. For decreasing gain size, the contribution of spin disorder at the grain boundary increases and enhances the local anisotropy field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of grain size on transition temperature of thermochromic VO{sub 2}

Vanadium(IV) oxide (VO{sub 2}) is a unique material that undergoes a reversible phase transformation around 68 °C. The material could potentially be used as an energy-efficient coating for windows since its reflectance in the infrared (IR) increases significantly more than in the visible region. Currently, VO{sub 2} is limited by a transition temperature (τ{sub c}) that is too high, luminous transmittance that is too low or both. In this study, a transition temperature of 45 °C is achieved for a reactively sputtered, undoped film by restricting grain size to approximately 30 nm. It is concluded that a higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces τ{sub c}. Similarly, a higher density of grain boundaries may reduce the hysteresis width (difference between transition temperatures in heating and cooling). Also in this study, a new set of optical performance metrics is proposed in which the solar spectrum is divided into the ultraviolet (UV), visible and near infrared (NIR) regions. This approach is more closely aligned with the goals of limiting UV, allowing luminous and modulating NIR transmission. Using these metrics, the optical properties of the low-τ{sub c} sample were: 2% UV transmittance, 47% luminous transmittance, and 23% NIR modulation (decrease from 43 to 33%). This study demonstrates that the grain size of VO{sub 2} should be viewed as an important parameter for controlling the transition temperature of the material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Size effect on multiferroic TbMn{sub 2}O{sub 5} nanorods

The effect of the size of TbMn{sub 2}O{sub 5} on their magnetoelectric properties is studied using a field emission transmission electron microscope, ac magnetic susceptibility, relative dielectric constant, specific heat capacity, and x-ray diffraction at various temperatures. A bulk sample and nanorods with two axial lengths, 〈L{sub C}〉, are investigated. Nanorods with 〈L{sub C}〉 of 216 nm and 64 nm crystallize with orthorhombic Pbam symmetry as the bulk material. The axial direction of the two sizes of nanorod is parallel to the c axis of the crystal. No particular radial direction is observed. The critical size for the magnetic ordering of TbMn{sub 2}O{sub 5} is found to be between 〈L{sub C}〉 = 64 nm and 216 nm. Experiments on specific heat capacity verified these results. No similar phenomenon concerning the ferroelectric property is observed. At T = 30 K and 35 K, the 〈L{sub C}〉 = 216 nm sample yields a charge-ordering-like x-ray diffraction peak, which may be associated with the magnetically induced charge ordering superlattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of grain size on optical properties of Sr{sub 2}CeO{sub 4} nanocrystals

The absorption, excitation, and emission spectra of the Sr{sub 2}CeO{sub 4} nanocrystals prepared by the modified sol–gel method were investigated. The impact of the average grain size of Sr{sub 2}CeO{sub 4} nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr{sub 2}CeO{sub 4} nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang–Rhys factors. The grain size dependence of optical parameters of Sr{sub 2}CeO{sub 4} nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

The magnetized sheath of a dusty plasma with grains size distribution

The structure of a plasma sheath in the presence of dust grains size distribution (DGSD) is investigated in the multi-fluid framework. It is shown that effect of the dust grains with different sizes on the sheath structure is a collective behavior. The spatial distributions of electric potential, the electron and ion densities and velocities, and the dust grains surface potential are strongly affected by DGSD. The dynamics of dust grains with different sizes in the sheath depend on not only DGSD but also their radius. By comparison of the sheath structure, it is found that under the same expected value of DGSD condition, the sheath length is longer in the case of lognormal distribution than that in the case of uniform distribution. In two cases of normal and lognormal distributions, the sheath length is almost equal for the small variance of DGSD, and then the difference of sheath length increases gradually with increase in the variance.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Grain size tuning of nanostructured Cu{sub 2}O films through vapour phase supersaturation control and their characterization for practical applications

A strategy for creating nanostructured films is the alignment of nanoparticles into ordered superstructures as living organisms synthesize biomaterials with superior physical properties using nanoparticle building blocks. We synthesized nanostructured films of Cu{sub 2}O of variable grain size by establishing the condition of supersaturation for creation of nanoparticles of copper which deposited as nanograined films and which was then oxidized. This technique has the advantage of being compatible with conventional vacuum processes for electronic device fabrication. The Cu{sub 2}O film samples consisted of a secondary structure of spherical particles of almost uniform size, each particle being an agglomerate of primary nanocrystals. Fractal analysis of the AFM images of the samples is carried out for studying the aggregation mechanism. Grain size tuning of the nanostructured Cu{sub 2}O films has been studied using XRD, and micro-Raman and photoluminescence spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Grain size effect on yield strength of titanium alloy implanted with aluminum ions

The paper presents a transmission electron microscopy (TEM) study of the microstructure and phase state of commercially pure titanium VT1-0 implanted by aluminum ions. This study has been carried out before and after the ion implantation for different grain size, i.e. 0.3 µm (ultra-fine grain condition), 1.5 µm (fine grain condition), and 17 µm (polycrystalline condition). This paper presents details of calculations and analysis of strength components of the yield stress. It is shown that the ion implantation results in a considerable hardening of the entire thickness of the implanted layer in the both grain types. The grain size has, however, a different effect on the yield stress. So, both before and after the ion implantation, the increase of the grain size leads to the decrease of the alloy hardening. Thus, hardening in ultra-fine and fine grain alloys increased by four times, while in polycrystalline alloy it increased by over six times.

36 MATERIALS SCIENCE↗