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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Collective Dispersal Leads to Variance in Fitness and Maintains Offspring Size Variation within Marine Populations

Variance in fitness is well known to influence the outcome of evolution but is rarely considered in the theory of marine reproductive strategies. In coastal environments, turbulent mesoscale eddies can collect larvae into packets, resulting in collective dispersal. Larvae in packets return to the coast or are lost offshore in groups, producing variance in fitness. Using a Markov process to calculate fixation probabilities for competing phenotypes, we examine the evolution of offspring size and spawning duration in species with benthic adults and pelagic offspring. The offspring size that provides mothers with the highest mean fitness also generates the greatest variance in fitness, but pairwise invasion plots show that bet-hedging strategies are not evolutionarily stable; maximizing expected fitness correctly predicts the unique evolutionarily stable strategy. Nonetheless, fixation can take a long time. We find that selection to increase spawning duration as a risk avoidance strategy to reduce the negative impacts of stochastic recruitment success can allow multiple offspring sizes to coexist in a population for extended periods. This has two important consequences for offspring size: (1) coexistence occurs over a broader range of sizes and is longer when spawning duration is longer because longer spawning durations reduce variation in fitness and increase the time to fixation, and (2) longer spawning durations can compensate for having a nonoptimal size and even allow less optimal sizes to reach fixation. Collective dispersal and longer spawning durations could effectively maintain offspring size variation even in the absence of good and bad years or locations. Finally, empirical comparisons of offspring size would therefore not always reflect environment-specific differences in the optimal size.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Importance of finite-size corrections for accurate ab initio modeling of carrier capture at semiconductor defects: A case study of substitutional C N in GaN

In ab initio studies of carrier-capture processes in defective semiconductor materials, the single-effective-mode formalism and the static-coupling approximation have become the predominant theoretical approaches for determining carrier-capture coefficients. The single-mode formalism relies on accurate nonequilibrium defect energies obtained from density-functional theory (DFT), where required inputs are a series of configurationally displaced, defect-containing supercells obtained using an interpolative ansatz, and where the DFT outputs are corresponding total energies that have traditionally been postprocessed using a long-established ground-state formulation of finite-size corrections and defect-formation energies. This formulation remains commonly used even though the defects that form a configuration-coordinate (CC) diagram typically exist as structures that are displaced from the ground state. To remedy this inconsistency, Kumagai has recently proposed novel methods for implementing finite-size corrections specifically intended for DFT calculations of the defect energies used to construct CC diagrams and implement the single-mode formalism [Y. Kumagai, Phys. Rev. B 107, L220101 (2023)]. Kumagai's approach builds on the latest finite-size-correction methods introduced to describe vertical charge-state transitions for charge-localizing point defects in semiconductors and insulators [T. Gake et al., Phys. Rev. B 101, 020102 (2020); S. Falletta et al., Phys. Rev. B 102, 041115 (2020)]. The newly identified finite-size artifact treated in these studies is the polarization charge induced on a configurationally frozen defect and its subsequent interaction with a vertical transition in charge state. In this work, we evaluate Kumagai's proposed methodology by applying it in a high-precision DFT study of carrier capture by substitutional C N in GaN, a well-characterized and technologically relevant defect and material. We have rigorously calculated C N defect energies across various supercell sizes for each defect configuration and charge state on the hole-capture CC diagram of C N (𝑞=−1), enabling a direct comparison of the slopes of the defect energies versus inverse cell size with those predicted by Kumagai. The most consequential prediction of Kumagai's method is that these slopes distinctly vary as the square of the linear-interpolation parameter used to construct the nonequilibrium defect configurations. Our results quantitatively support this prediction. Moreover, with these new finite-size corrections and multiple-cell-size DFT calculations in place, we find that the classical energy barrier for hole capture by C N (𝑞=−1) in GaN decreases to 0.092–0.127 eV. This finding confirms the recent ≈ 0.1 eV prediction of Reshchikov based on the weak temperature dependence for hole capture observed in photoluminescence experiments [M. A. Reshchikov, J. Appl. Phys. 129, 121101 (2021)]. These results stand in stark contrast to previously calculated barriers of 0.486 and 0.73 eV, which also used the single-mode formalism but were obtained by instead using ground-state-based finite-size corrections. Our reduced classical barrier for capture increases the temperature-dependent hole-capture coefficient of a C N (𝑞=−1) defect by more than two to four orders of magnitude for temperatures of 100–600 K, compared to the previous 0.486 eV results. While other defects may not be as dramatically affected as here, we suggest that incorporating proper finite-size corrections for the vertical-transition-like states embedded within CC diagrams is an essential, yet previously unrecognized, component of accurate modeling of carrier-capture when using the single-effective-mode formalism.

dielectric properties↗
DOE OSTI2019

Quasi‐static confined uniaxial compaction of granular alumina and boron carbide observing the particle size effects

Abstract The quasi‐static confined uniaxial compaction of granular alumina and boron carbide was studied, and the effect of triaxial stress on the materials as a function of increasing particle size was observed. The average particle sizes studied for granular alumina were 170 ± 63, 230 ± 55, 330 ± 67, and 450 ± 83 µm. The average particle size studied for granular boron carbide were 170 ± 40, 190 ± 34, 320 ± 59, and 470 ± 90 µm. The material response at hydrostatic pressure as a function of porosity, the bulk modulus as a function of hydrostatic pressure, and the transmission ratio as a function of applied load was evaluated for increasing particle size. For alumina, the increase in particle size resulted in an increase in strength for a fixed porosity, the bulk modulus of this material did not show clear particle size‐dependent trends, and the transmission ratio increased with increase in particle size. Conversely, for granular boron carbide, the hydrostatic pressure‐porosity curve shifted to the right with increasing particle size, the change in bulk modulus increased with increasing particle size, and no clear particle size‐dependent trends were observed when looking at the transmission ratio during the experiment. Post‐experiment scanning electron microscopy revealed that alumina powder fragmented from elongated shapes to block‐like structures, while boron carbide powder appeared more circular before the experiments and fragmented into smaller comminuted pieces. This paper discusses the implication of the work in the context of the limited experimental data in the field and the modeling of granular advanced ceramics behavior.

Nicewicz, Piotr↗
DOE OSTI2014

SU-D-BRE-03: Dosimetric Impact of In-Air Spot Size Variations for Commissioning a Room-Matched Beam Model for Pencil Beam Scanning Proton Therapy

Purpose: To quantitatively evaluate dosimetric consequence of spot size variations and validate beam-matching criteria for commissioning a pencil beam model for multiple treatment rooms. Methods: A planning study was first conducted by simulating spot size variations to systematically evaluate dosimetric impact of spot size variations in selected cases, which was used to establish the in-air spot size tolerance for beam matching specifications. A beam model in treatment planning system was created using in-air spot profiles acquired in one treatment room. These spot profiles were also acquired from another treatment room for assessing the actual spot size variations between the two treatment rooms. We created twenty five test plans with targets of different sizes at different depths, and performed dose measurement along the entrance, proximal and distal target regions. The absolute doses at those locations were measured using ionization chambers at both treatment rooms, and were compared against the calculated doses by the beam model. Fifteen additional patient plans were also measured and included in our validation. Results: The beam model is relatively insensitive to spot size variations. With an average of less than 15% measured in-air spot size variations between two treatment rooms, the average dose difference was −0.15% with a standard deviation of 0.40% for 55 measurement points within target region; but the differences increased to 1.4%±1.1% in the entrance regions, which are more affected by in-air spot size variations. Overall, our single-room based beam model in the treatment planning system agreed with measurements in both rooms < 0.5% within the target region. For fifteen patient cases, the agreement was within 1%. Conclusion: We have demonstrated that dosimetrically equivalent machines can be established when in-air spot size variations are within 15% between the two treatment rooms.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

SU-E-T-196: Comparative Analysis of Surface Dose Measurements Using MOSFET Detector and Dose Predicted by Eclipse - AAA with Varying Dose Calculation Grid Size

Purpose: Skin dose can be the limiting factor and fairly common reason to interrupt the treatment, especially for treating head-and-neck with Intensity-modulated-radiation-therapy(IMRT) or Volumetrically-modulated - arc-therapy (VMAT) and breast with tangentially-directed-beams. Aim of this study was to investigate accuracy of near-surface dose predicted by Eclipse treatment-planning-system (TPS) using Anisotropic-Analytic Algorithm (AAA)with varying calculation grid-size and comparing with metal-oxide-semiconductor-field-effect-transistors(MOSFETs)measurements for a range of clinical-conditions (open-field,dynamic-wedge, physical-wedge, IMRT,VMAT). Methods: QUASAR™-Body-Phantom was used in this study with oval curved-surfaces to mimic breast, chest wall and head-and-neck sites.A CT-scan was obtained with five radio-opaque markers(ROM) placed on the surface of phantom to mimic the range of incident angles for measurements and dose prediction using 2mm slice thickness.At each ROM, small structure(1mmx2mm) were contoured to obtain mean-doses from TPS.Calculations were performed for open-field,dynamic-wedge,physical-wedge,IMRT and VMAT using Varian-21EX,6&15MV photons using twogrid-sizes:2.5mm and 1mm.Calibration checks were performed to ensure that MOSFETs response were within ±5%.Surface-doses were measured at five locations and compared with TPS calculations. Results: For 6MV: 2.5mm grid-size,mean calculated doses(MCD)were higher by 10%(±7.6),10%(±7.6),20%(±8.5),40%(±7.5),30%(±6.9) and for 1mm grid-size MCD were higher by 0%(±5.7),0%(±4.2),0%(±5.5),1.2%(±5.0),1.1% (±7.8) for open-field,dynamic-wedge,physical-wedge,IMRT,VMAT respectively.For 15MV: 2.5mm grid-size,MCD were higher by 30%(±14.6),30%(±14.6),30%(±14.0),40%(±11.0),30%(±3.5)and for 1mm grid-size MCD were higher by 10% (±10.6), 10%(±9.8),10%(±8.0),30%(±7.8),10%(±3.8) for open-field, dynamic-wedge, physical-wedge, IMRT, VMAT respectively.For 6MV, 86% and 56% of all measured values agreed better than ±20% for 1mm and 2.5mm grid-sizes respectively. For 18MV, 56% and 18% of all measured-values agreed better than ±20% for 1mm and 2.5mm grid-sizes respectively. Conclusion: Reliable Skin-dose calculations by TPS can be very difficult due to steep dose-gradient and inaccurate beam-modelling in buildup region.Our results showed that Eclipse over-estimates surface-dose.Impact of grid-size is also significant,surface-dose increased up to 40% from 1mm to 2.5mm,however, 1mm calculated-values closely agrees with measurements. Due to large uncertnities in skin-dose predictions from TPS, outmost caution must be exercised when skin dose is evaluated,a sufficiently smaller grid-size(1mm)can improve the accuracy and MOSFETs can be used for verification.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-454: Impact of Air Gap On PDDs of 6 MV Photon Beam for Various Field Sizes in Inhomogeneous Phantoms

Purpose: To investigate how the shape of air gap and its size will impact the percent depth doses (PDDs) of a 6MV photon beam for various field sizes. Methods: Two in-house phantoms were manufactured containing rectangular (phantom A) and circular (phantom B) air gaps. Both phantoms A and B were composed of same top layer (solid-water; 30×30cm{sup 2},5cm thickness) and bottom layer (solid-water; 30×30cm {sup 2},8cm thickness), but middle layer was varied to observe air gap effects and scatter contribution to the measurement point. In phantom A, a rectangular shaped air gap (30×30cm{sup 2},7cm thickness) was created by placing Styrofoam blocks between top and bottom layers of the phantom. In phantom B, middle layer was replaced by “inhomogenous block”, composed of acrylic plate (30×30cm{sup 2}, 4cm thickness) followed by PVC(30×30cm{sup 2},3cm thickness). Additionally, circular air gap was created by drilling a hole (diameter=2.8cm, length=7cm) at the center of “inhomogenous block”. In both phantoms, measurement readings were obtained at 13cm depth (i.e., 1cm after air gap) and depth of maximum dose(6MV energy; 100 MUs; field sizes ranged from 3×3cm{sup 2} to 10×10cm{sup 2}). The PDDs at 13cm depth were compared in both phantoms. Results: The measurements in both phantoms A and B showed an almost linear increase in PDDs with increasing field size, especially for smaller field sizes (from 3×3 to 7×7cm{sup 2}). For each field size, the PDD in phantom A was smaller compared to the one in phantom B. The difference in PDDs between two phantoms decreased with an increase in field size as the PDD difference decreased from 9.0% to 6.4%. Conclusion: The shape and size of air gap affect the PDD measurements in secondary build-up region as 6 MV primary beam traverses through the center of air gap. The scatter contribution due to increase in field size was more noticeable for field sizes ≤7×7cm{sup 2}.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Automated Identification of Characteristic Droplet Size Distributions in Stratocumulus Clouds Utilizing a Data Clustering Algorithm

Abstract Droplet-level interactions in clouds are often parameterized by a modified gamma fitted to a “global” droplet size distribution. Do “local” droplet size distributions of relevance to microphysical processes look like these average distributions? This paper describes an algorithm to search and classify characteristic size distributions within a cloud. The approach combines hypothesis testing, specifically, the Kolmogorov–Smirnov (KS) test, and a widely used class of machine learning algorithms for identifying clusters of samples with similar properties: density-based spatial clustering of applications with noise (DBSCAN) is used as the specific example for illustration. The two-sample KS test does not presume any specific distribution, is parameter free, and avoids biases from binning. Importantly, the number of clusters is not an input parameter of the DBSCAN-type algorithms but is independently determined in an unsupervised fashion. As implemented, it works on an abstract space from the KS test results, and hence spatial correlation is not required for a cluster. The method is explored using data obtained from the Holographic Detector for Clouds (HOLODEC) deployed during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign. The algorithm identifies evidence of the existence of clusters of nearly identical local size distributions. It is found that cloud segments have as few as one and as many as seven characteristic size distributions. To validate the algorithm’s robustness, it is tested on a synthetic dataset and successfully identifies the predefined distributions at plausible noise levels. The algorithm is general and is expected to be useful in other applications, such as remote sensing of cloud and rain properties. Significance Statement A typical cloud can have billions of drops spread over tens or hundreds of kilometers in space. Keeping track of the sizes, positions, and interactions of all of these droplets is impractical, and, as such, information about the relative abundance of large and small drops is typically quantified with a “size distribution.” Droplets in a cloud interact locally, however, so this work is motivated by the question of whether the cloud droplet size distribution is different in different parts of a cloud. A new method, based on hypothesis testing and machine learning, determines how many different size distributions are contained in a given cloud. This is important because the size distribution describes processes such as cloud droplet growth and light transmission through clouds.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Lead and Arsenic Bioaccessibility and Speciation as a Function of Soil Particle Size

Bioavailability research of soil metals has advanced considerably from default values to validated in vitro bioaccessibility (IVBA) assays for site‐specific risk assessment. Previously, USEPA determined that the soil‐size fraction representative of dermal adherence and consequent soil ingestion was <250 μm. This size fraction was widely used in testing efforts for both in vivo and in vitro experiments. However, recent studies indicate the <150‐μm size fraction better represents the particle size that adheres to skin for potential ingestion. At issue is the relevance of validated in vivo and in vitro methods developed with <250 μm moving to the <150‐μm fraction. The objectives of this study were to investigate <250‐μm versus <150‐μm particle size and particle size groups for evaluating lead (Pb) and arsenic (As) IVBA and speciation. Soils with different properties were homogenized, oven dried, and sieved: <250 to > 150, <150 to >75, <75 to >38, and <38 μm. Sieved versus ground subsamples of <250‐μm and <150‐μm bulk soils were also used for IVBA and synchrotron‐based Pb and As speciation. Although we observed an increase in total and IVBA‐extractable Pb and As with decreased soil particle size, changes in %IVBA of Pb and As (dependent on the ratio extractable:total) remained consistent in all of the tested soils. No significant changes in Pb and As speciation were observed across the soil fractions. The results suggest that using the more relevant <150‐μm fraction will not undermine currently validated IVBA protocols in future bioavailability studies. Core Ideas Identified critical knowledge gaps regarding the <250‐μm vs. <150‐μm particle size used for IVBA. Wider implementation of the <150‐μm size fraction maintains past validation with <‐250 μm fraction. Regardless of increase in total and extractable Pb and As, %IVBA results remained consistent. Using <150‐μm particle size for IVBA requires much higher volumes/masses of soils. Synchrotron‐based speciation is significant in bioavailability studies to understand the fate and transport of contaminants.

Karna, Ranju R.↗
DOE OSTI2019

Quiescent Galaxy Size and Spectroscopic Evolution: Combining HSC Imaging and Hectospec Spectroscopy

We explore the relations between size, stellar mass and average stellar population age (indicated by D n 4000 indices) for a sample of ~ 11000 intermediate-redshift galaxies from the SHELS spectroscopic survey (Geller et al. 2014) augmented by high-resolution Subaru Telescope Hyper Suprime-Cam imaging. In the redshift interval 0.1 < z < 0:6, star forming galaxies are on average larger than their quiescent counterparts. The mass-complete sample of ~ 3500 M * > 10 10 M ⊙ quiescent galaxies shows that the average size of a 1011M quiescent galaxy increases by . 25% from z ~ 0.6 to z ~ 0.1. This growth rate is a function of stellar mass: the most massive (M * > 10 11 M ⊙ ) galaxies grow significantly more slowly in size than an order of magnitude less massive quiescent systems that grow by 70% in the 0.1 . z . 0.3 redshift interval. For M * < 10 11 M ⊙ galaxies age and size are anti-correlated at fixed mass; more massive quiescent systems show no significant trend in size with average stellar population age. The evolution in absolute and fractional abundances of quiescent systems at intermediate red- shift are also a function of galaxy stellar mass. The suite of evolutionary trends suggests that galaxies more massive than ~ 10 11 M have mostly assembled their mass by z ~ 0.6. Quiescent galaxies with lower stellar masses show more complex evolution that is characterized by a combination of individual quiescent galaxy size growth (through mergers) and an increase in the size of newly quenched galaxies joining the population at later times (progenitor bias). The low-mass population (M * ~ 10 10 M ⊙ ) grows predominantly as a result of progenitor bias. For more massive (M * ~ 5 x 10 10 M ⊙ ) quiescent galaxies, (predominantly minor) mergers and progenitor bias make more comparable contributions to the size growth. At intermediate redshift quiescent size growth is mass-dependent; the most massive (M * > 10 11 M ⊙ ) galaxies experience the least rapid increase in size from z ~ 0.6 to z ~ 0.1.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2008

A New Bond Albedo for Performing Orbital Debris Brightness to Size Transformations

We have developed a technique for estimating the intrinsic size distribution of orbital debris objects via optical measurements alone. The process is predicated on the empirically observed power-law size distribution of debris (as indicated by radar RCS measurements) and the log-normal probability distribution of optical albedos as ascertained from phase (Lambertian) and range-corrected telescopic brightness measurements. Since the observed distribution of optical brightness is the product integral of the size distribution of the parent [debris] population with the albedo probability distribution, it is a straightforward matter to transform a given distribution of optical brightness back to a size distribution by the appropriate choice of a single albedo value. This is true because the integration of a powerlaw with a log-normal distribution (Fredholm Integral of the First Kind) yields a Gaussian-blurred power-law distribution with identical power-law exponent. Application of a single albedo to this distribution recovers a simple power-law [in size] which is linearly offset from the original distribution by a constant whose value depends on the choice of the albedo. Significantly, there exists a unique Bond albedo which, when applied to an observed brightness distribution, yields zero offset and therefore recovers the original size distribution. For physically realistic powerlaws of negative slope, the proper choice of albedo recovers the parent size distribution by compensating for the observational bias caused by the large number of small objects that appear anomalously large (bright) - and thereby skew the small population upward by rising above the detection threshold - and the lower number of large objects that appear anomalously small (dim). Based on this comprehensive analysis, a value of 0.13 should be applied to all orbital debris albedo-based brightness-to-size transformations regardless of data source. Its prima fascia genesis, derived and constructed from the current RCS to size conversion methodology (SiBAM Size-Based Estimation Model) and optical data reduction standards, assures consistency in application with the prior canonical value of 0.1. Herein we present the empirical and mathematical arguments for this approach and by example apply it to a comprehensive set of photometric data acquired via NASA's Liquid Mirror Telescopes during the 2000-2001 observing season.

Mulrooney, Mark K.↗
NASA NTRS2006

Observations of Precipitation Size and Fall Speed Characteristics within Coexisting Rain and Wet Snow

Ground-based measurements of particle size and fall speed distributions using a Particle Size and Velocity (PARSIVEL) disdrometer are compa red among samples obtained in mixed precipitation (rain and wet snow) and rain in the Oregon Cascade Mountains and in dry snow in the Rock y Mountains of Colorado. Coexisting rain and snow particles are distinguished using a classification method based on their size and fall sp eed properties. The bimodal distribution of the particles' joint fall speed-size characteristics at air temperatures from 0.5 to 0 C suggests that wet-snow particles quickly make a transition to rain once mel ting has progressed sufficiently. As air temperatures increase to 1.5 C, the reduction in the number of very large aggregates with a diame ter > 10 mm coincides with the appearance of rain particles larger than 6 mm. In this setting. very large raindrops appear to be the result of aggregates melting with minimal breakup rather than formation by c oalescence. In contrast to dry snow and rain, the fall speed for wet snow has a much weaker correlation between increasing size and increasing fall speed. Wet snow has a larger standard deviation of fall spee d (120%-230% relative to dry snow) for a given particle size. The ave rage fall speed for observed wet-snow particles with a diameter great er than or equal to 2.4 mm is 2 m/s with a standard deviation of 0.8 m/s. The large standard deviation is likely related to the coexistence of particles of similar physical size with different percentages of melting. These results suggest that different particle sizes are not required for aggregation since wet-snow particles of the same size can have different fall speeds. Given the large standard deviation of fa ll speeds in wet snow, the collision efficiency for wet snow is likely larger than that of dry snow. For particle sizes between 1 and 10 mm in diameter within mixed precipitation, rain constituted I % of the particles by volume within the isothermal layer at 0 C and 4% of the particles by volume for the region just below the isothermal layer where air temperatures rise from 0" to 0.5"C. As air temperatures increa sed above 0.5 C, the relative proportions of rain versus snow particl es shift dramatically and raindrops become dominant. The value of 0.5 C for the sharp transition in volume fraction from snow to rain is sl ightly lower than the range from 1 .l to 1.7 C often used in hydrolog ical models.

Yuter, Sandra E.↗
DOE OSTI2016

Gap Size Uncertainty Quantification in Advanced Gas Reactor TRISO Fuel Irradiation Experiments

The Advanced Gas Reactor (AGR)-3/4 experiment is the combination of the third and fourth tests conducted within the tristructural isotropic fuel development and qualification research program. The AGR-3/4 test consists of twelve independent capsules containing a fuel stack in the center surrounded by three graphite cylinders and shrouded by a stainless steel shell. This capsule design enables temperature control of both the fuel and the graphite rings by varying the neon/helium gas mixture flowing through the four resulting gaps. Knowledge of fuel and graphite temperatures is crucial for establishing the functional relationship between fission product release and irradiation thermal conditions. These temperatures are predicted for each capsule using the commercial finite-element heat transfer code ABAQUS. Uncertainty quantification reveals that the gap size uncertainties are among the dominant factors contributing to predicted temperature uncertainty due to high input sensitivity and uncertainty. Gap size uncertainty originates from the fact that all gap sizes vary with time due to dimensional changes of the fuel compacts and three graphite rings caused by extended exposure to high temperatures and fast neutron irradiation. Gap sizes are estimated using as-fabricated dimensional measurements at the start of irradiation and post irradiation examination dimensional measurements at the end of irradiation. Uncertainties in these measurements provide a basis for quantifying gap size uncertainty. However, lack of gap size measurements during irradiation and lack of knowledge about the dimension change rates lead to gap size modeling assumptions, which could increase gap size uncertainty. In addition, the dimensional measurements are performed at room temperature, and must be corrected to account for thermal expansion of the materials at high irradiation temperatures. Uncertainty in the thermal expansion coefficients for the graphite materials used in the AGR-3/4 capsules also increases gap size uncertainty. This study focuses on analysis of modeling assumptions and uncertainty sources to evaluate their impacts on the gap size uncertainty.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Development of size reduction equations for calculating power input for grinding pine wood chips using hammer mill

Size reduction is an unavoidable operation for preparing biomass for biofuels and bioproduct conversion. Yet, there is considerable uncertainty in power input requirement and the uniformity of ground biomass. Considerable gains are possible if the required power input for a size reduction ratio is estimated accurately. In this research three well-known mechanistic equations attributed to Rittinger, Kick, and Bond available for predicting energy input for grinding pine wood chips were tested against experimental grinding data. Prior to testing, samples of pine wood chips were conditioned to 11.7% wb, moisture content. The wood chips were successively ground in a hammer mill using screen sizes of 25.4 mm, 10 mm, 6.4 mm, and 3.2 mm. The input power and the flow of material into the grinder were recorded continuously. The recorded power input vs. mean particle size showed that the Rittinger equation had the best fit to the experimental data. The ground particle sizes were 4 to 7 times smaller than the size of installed screen. Geometric mean size of particles were calculated using two methods (1) Tyler sieves and using particle size analysis and (2) Sauter mean diameter calculated from the ratio of volume to surface that were estimated from measured length and width. The two mean diameters agreed well, pointing to the fact that either mechanical sieving or particle imaging can be used to characterize particle size. In conclusion, specific energy input to the hammer mill increased from 1.4 kWh t –1 (5.2 J g –1 ) for large 25.1-mm screen to 25 kWh t –1 (90.4 J g –1 ) for small 3.2-mm screen.

09 BIOMASS FUELS↗
DOE OSTI2021

Improvement of interfacial adhesion of unidirectional textile grade carbon fiber (TCF) with unsized, epoxy and urethane sizing reinforced in thermoset urethane composites

This work considers a unique wide tow (450k filaments) form of low-cost carbon fiber intended for non-aerospace applications. TCF is currently produced in epoxy and urethane sizing, and there is a need to understand its resulting composite properties. In this work, the interfacial adhesion of sized TCF reinforced in thermoset urethane (TSU) composites are examined through the surface, thermal and mechanical characterization techniques. Atomic force microscopy (AFM) results showed an increase in surface roughness for urethane (276%) and epoxy sized (78%) versus unsized TCF. XPS results showed 531% increase in O in epoxy sized TCF and 250% N content in urethane sized TCF compared to unsized TCF. The surface energy of epoxy and urethane sized TCF is enhanced by 78% and 96%, respectively compared to unsized TCF. The storage modulus showed improvement for urethane (23%) and epoxy (21%) sized than unsized TCF-TSU composites. The flexural, interlaminar shear strength (ILSS), and impact properties of urethane sized TCF increase by 24%, 50%, and 273%, respectively, than unsized TCF. The results demonstrate that the surface and thermal properties correlate with the mechanical properties of TCF-TSU composites and sizing enhances the wettability of the composites.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Particle Size on the Basic and Time-Dependent Rheological Behaviors of Cemented Paste Backfill

The rheological properties of cemented paste backfill (CPB) significantly influence the material’s transportability, and these properties are strongly affected by the material’s particle size. This paper mainly focuses on the influence of particles size on the basic and time-dependent rheological behaviors of CPB. The rheology tests and microstructure analysis were conducted using multiple CPB samples with different solid contents, cement contents, and particle sizes. Results show that the CPB samples with finer particle sizes have a lower apparent maximum packing density compared to the CPB samples with larger particle sizes. Fresh CPB mixture samples exhibit shear thinning characteristics which are more pronounced in samples with larger particle sizes and higher volume fraction to apparent maximum packing density (ϕ/ϕ{sub m}) ratios. When the ϕ/ϕ{sub m} ratio is lower than 0.875, the samples with finer particle sizes display a higher yield stress but less time-dependent rheological behavior than coarser samples. In all CPB samples that contained cement, the shear stress decreases first and then increases over the shearing time when samples are sheared at a constant shear rate. In the CPB samples without cement, samples with finer particle sizes show lower apparent viscosities, and it is observed in the sample with the finest particles that the shear stress only decreases over the shearing time. Moreover, it is found that the number of hydration products and the rate of particle sedimentation are both higher in samples with larger particle sizes, which result in these samples having higher apparent viscosities.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Disk radii and grain sizes in Herschel-resolved debris disks

The radii of debris disks and the sizes of their dust grains are important tracers of the planetesimal formation mechanisms and physical processes operating in these systems. Here we use a representative sample of 34 debris disks resolved in various Herschel Space Observatory (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA) programs to constrain the disk radii and the size distribution of their dust. While we modeled disks with both warm and cold components, and identified warm inner disks around about two-thirds of the stars, we focus our analysis only on the cold outer disks, i.e., Kuiper-belt analogs. We derive the disk radii from the resolved images and find a large dispersion for host stars of any spectral class, but no significant trend with the stellar luminosity. This argues against ice lines as a dominant player in setting the debris disk sizes, since the ice line location varies with the luminosity of the central star. Fixing the disk radii to those inferred from the resolved images, we model the spectral energy distribution to determine the dust temperature and the grain size distribution for each target. While the dust temperature systematically increases toward earlier spectral types, the ratio of the dust temperature to the blackbody temperature at the disk radius decreases with the stellar luminosity. This is explained by a clear trend of typical sizes increasing toward more luminous stars. The typical grain sizes are compared to the radiation pressure blowout limit s {sub blow} that is proportional to the stellar luminosity-to-mass ratio and thus also increases toward earlier spectral classes. The grain sizes in the disks of G- to A-stars are inferred to be several times s {sub blow} at all stellar luminosities, in agreement with collisional models of debris disks. The sizes, measured in the units of s {sub blow}, appear to decrease with the luminosity, which may be suggestive of the disk's stirring level increasing toward earlier-type stars. The dust opacity index β ranges between zero and two, and the size distribution index q varies between three and five for all the disks in the sample.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE EFFECTS OF GRAIN SIZE AND TEMPERATURE DISTRIBUTIONS ON THE FORMATION OF INTERSTELLAR ICE MANTLES

Computational models of interstellar gas-grain chemistry have historically adopted a single dust-grain size of 0.1 micron, assumed to be representative of the size distribution present in the interstellar medium. Here, we investigate the effects of a broad grain-size distribution on the chemistry of dust-grain surfaces and the subsequent build-up of molecular ices on the grains, using a three-phase gas-grain chemical model of a quiescent dark cloud. We include an explicit treatment of the grain temperatures, governed both by the visual extinction of the cloud and the size of each individual grain-size population. We find that the temperature difference plays a significant role in determining the total bulk ice composition across the grain-size distribution, while the effects of geometrical differences between size populations appear marginal. We also consider collapse from a diffuse to a dark cloud, allowing dust temperatures to fall. Under the initial diffuse conditions, small grains are too warm to promote grain-mantle build-up, with most ices forming on the mid-sized grains. As collapse proceeds, the more abundant, smallest grains cool and become the dominant ice carriers; the large population of small grains means that this ice is distributed across many grains, with perhaps no more than 40 monolayers of ice each (versus several hundred assuming a single grain size). This effect may be important for the subsequent processing and desorption of the ice during the hot-core phase of star formation, exposing a significant proportion of the ice to the gas phase, increasing the importance of ice-surface chemistry and surface–gas interactions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2015

Turbulent Concentration of mm-Size Particles in the Protoplanetary Nebula: Scale-Dependent Cascades

The initial accretion of primitive bodies (here, asteroids in particular) from freely-floating nebula particles remains problematic. Traditional growth-by-sticking models encounter a formidable "meter-size barrier" (or even a mm-to-cm-size barrier) in turbulent nebulae, making the preconditions for so-called "streaming instabilities" difficult to achieve even for so-called "lucky" particles. Even if growth by sticking could somehow breach the meter size barrier, turbulent nebulae present further obstacles through the 1-10km size range. On the other hand, nonturbulent nebulae form large asteroids too quickly to explain long spreads in formation times, or the dearth of melted asteroids. Theoretical understanding of nebula turbulence is itself in flux; recent models of MRI (magnetically-driven) turbulence favor low-or- no-turbulence environments, but purely hydrodynamic turbulence is making a comeback, with two recently discovered mechanisms generating robust turbulence which do not rely on magnetic fields at all. An important clue regarding planetesimal formation is an apparent 100km diameter peak in the pre-depletion, pre-erosion mass distribution of asteroids; scenarios leading directly from independent nebula particulates to large objects of this size, which avoid the problematic m-km size range, could be called "leapfrog" scenarios. The leapfrog scenario we have studied in detail involves formation of dense clumps of aerodynamically selected, typically mm-size particles in turbulence, which can under certain conditions shrink inexorably on 100-1000 orbit timescales and form 10-100km diameter sandpile planetesimals. There is evidence that at least the ordinary chondrite parent bodies were initially composed entirely of a homogeneous mix of such particles. Thus, while they are arcane, turbulent concentration models acting directly on chondrule size particles are worthy of deeper study. The typical sizes of planetesimals and the rate of their formation can be estimated using a statistical model with properties inferred from large numerical simulations of turbulence. Nebula turbulence is described by its Reynolds number Re = (L/eta)(exp 4/3), where L = H alpha(exp 1/2) is the largest eddy scale, H is the nebula gas vertical scale height, alpha the turbulent viscosity parameter, and eta is the Kolmogorov or smallest scale in turbulence (typically about 1km), with eddy turnover time t(sub eta). In the nebula, Re is far larger than any numerical simulation can handle, so some physical arguments are needed to extend the results of numerical simulations to nebula conditions. In this paper, we report new physics to be incorporated into our statistical models.

mm-size particles↗