Now showing items 12860-12879 of 13918

    • Theory of triangular lattice quasi-one-dimensional charge-transfer solids

      Clay, R. T.; Gomes, N.; Mazumdar, S.; Univ Arizona, Dept Phys (AMER PHYSICAL SOC, 2019-09-27)
      Recent investigations of the magnetic properties and the discovery of superconductivity in quasi-onedimensional triangular lattice organic charge-transfer solids have indicated the severe limitations of the effective 1/2-filled band Hubbard model for these and related systems. We present computational studies within the 1/4-filled band Hubbard model for these highly anisotropic systems. Individual organic monomer molecules, and not their dimers, constitute the sites of the Hamiltonian within our theory. We find enhancement of the longrange component of superconducting pairing correlations by the Hubbard repulsive interaction U for band parameters corresponding to kappa-(BEDT-TTF)(2)CF3SO3, which is superconducting under moderate pressure. We find significantly weaker superconducting pairing at realistic values of U in kappa-(BEDT-TTF)(2)B(CN)(4), and we ascribe the experimentally observed transition to a spin-gapped insulator to the formation of a paired-electron crystal. We make the testable prediction that the spin gap will be accompanied by charge ordering and period doubling in two lattice directions. The weaker tendency to superconductivity in kappa-(BEDT-TTF)(2)B(CN)(4) compared to kappa-(BEDT-TTF)(2)CF3SO3 is consistent with the more one-dimensional character of the former. Pressure-induced superconductivity is, however, conceivable. The overall results support a valence bond theory of superconductivity we have proposed recently.
    • “There’s Always Going to Be Uncertainty”: Exploring Undergraduate Student Parents’ Sources of Uncertainty and Related Management Practices

      Scharp, Kristina M.; Cooper, R. Amanda; Worwood, Jared V.; Dorrance Hall, Elizabeth; Univ Arizona (SAGE PUBLICATIONS INC, 2020-03-01)
      Framed by uncertainty management theory, the present study explores the uncertainty issues and management practices of undergraduate student parents. Results from 40 narrative interviews reveal seven sources of uncertainty, eight management practices, and two uncertainty trade-offs. Findings reveal that having two interrelated identities (student and parent) not only exacerbate some uncertainties but create completely new ones. This intersectionality also holds implications for management practices. Theoretical implications and practical applications are discussed.
    • Thermal alteration of labile elements in carbonaceous chondrites

      Springmann, Alessondra; Lauretta, Dante S.; Klaue, Bjoern; Goreva, Yulia S.; Blum, Joel D.; Andronikov, Alexandre; Stekloff, Jordan K.; Univ Arizona, Lunar & Planetary Lab (Elsevier, 2019-05-19)
      Carbonaceous chondrite meteorites are some of the oldest Solar System planetary materials available for study. The CI group has bulk abundances of elements similar to those of the solar photosphere. Of particular interest in carbonaceous chondrite compositions are labile elements, which vaporize and mobilize efficiently during post-accretionary parent-body heating events. Thus, they can record low-temperature alteration events throughout asteroid evolution. However, the precise nature of labile-element mobilization in planetary materials is unknown. Here we characterize the thermally induced movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous chondrites by conducting experimental simulations of volatile-element mobilization during thermal metamorphism. This process results in appreciable loss of some elements at temperatures as low as 500 K. This work builds on previous laboratory heating experiments on primitive meteorites and shows the sensitivity of chondrite compositions to excursions in temperature. Elements such as S and Hg have the most active response to temperature across different meteorite groups. Labile element mobilization in primitive meteorites is essential for quantifying elemental fractionation that occurred on asteroids early in Solar System history. This work is relevant to maintaining a pristine sample from asteroid (101955) Bennu from the OSIRIS-REx mission and constraining the past orbital history of Bennu. Additionally, we discuss thermal effects on surface processes of near-Earth asteroids, including the thermal history of “rock comets” such as (3200) Phaethon. This work is also critical for constraining the concentrations of contaminants in vaporized water extracted from asteroid regolith as part of future in situ resource utilization for sustained robotic and human space exploration.
    • Thermal balance and comparison of gas and dust properties of dense clumps in the Hi-GAL survey

      Merello, M; Molinari, S; Rygl, K L J; Evans, N J; Elia, D; Schisano, E; Traficante, A; Shirley, Y; Svoboda, B; Goldsmith, P F; et al. (OXFORD UNIV PRESS, 2019-03)
      We present a comparative study of physical properties derived from gas and dust emission in a sample of 1068 dense Galactic clumps. The sources are selected from the cross-match of the Herschel Infrared Galactic Plane Survey with 16 catalogues of NH3 line emission in its lowest inversion (1,1) and (2,2) transitions. The sample covers a large range in masses and bolometric luminosities, with surface densities above Sigma = 0.1 g cm(-2) and with low virial parameters alpha < 1. The comparison between dust and gas properties shows an overall agreement between T-kin and T-dust at volumetric densities n greater than or similar to 1.2 x 10(4)cm(-3), and a median fractional abundance chi (NH3) = 1.46 x 10(-8). While the protostellar clumps in the sample have small differences between T-kin and T-dust, prestellar clumps have a median ratio T-kin/T-dust = 1.24, suggesting that these sources are thermally decoupled. A correlation is found between the evolutionary tracer L/M and the parameters T-kin/T-dust and chi(NH3) in prestellar sources and protostellar clumps with L/M < 1 L circle dot M circle dot-1 . In addition, a weak correlation is found between non-thermal velocity dispersion and the L/M parameter, possibly indicating an increase of turbulence with protostellar evolution in the interior of clumps. Finally, different processes are discussed to explain the differences between gas and dust temperatures in prestellar candidates, and the origin of non-thermal motions observed in the clumps.
    • Thermal broadening of bottomonia: Lattice nonrelativistic QCD with extended operators

      Larsen, Rasmus; Meinel, Stefan; Mukherjee, Swagato; Petreczky, Peter; Univ Arizona, Dept Phys (AMER PHYSICAL SOC, 2019-10-21)
      We present lattice nonrelativistic QCD calculations of bottomonium correlation functions at temperatures T similar or equal to 150-350 MeV. The correlation functions were computed using extended bottomonium operators and on background gauge-field configurations for 2 + 1-flavor QCD having physical kaon and nearly physical pion masses. We analyzed these correlation functions based on simple theoretically motivated parametrizations of the corresponding spectral functions. The results of our analyses are compatible with significant in-medium thermal broadening of the ground state S- and P-wave bottomonia.
    • Thermal conductivity of random polycrystalline BC3 nanosheets: A step towards realistic simulation of 2D structures

      Fooladpanjeh, Sasan; Yousefi, Farrokh; Molaei, Fatemeh; Dehaghani, Maryam Zarghami; Sajadi, S. Mohammad; Abida, Otman; Habibzadeh, Sajjad; Mashhadzadeh, Amin Hamed; Saeb, Mohammad Reza; Mining and Geological Engineering Department, The University of Arizona (Elsevier BV, 2021-06)
      Boron carbide nanosheets (BC3NSs) are semiconductors possessing non-zero bandgap. Nevertheless, there is no estimation of their thermal conductivity for practical circumstances, mainly because of difficulties in simulation of random polycrystalline structures. In the real physics world, BC3NS with perfect monocrystalline is rare, for the nature produces structures with disordered grain regions. Therefore, it is of crucial importance to capture a more realistic picture of thermal conductivity of these nanosheets. Polycrystalline BC3NS (PCBC3NSs are herein simulated by Molecular Dynamics simulation to take their thermal conductivity fingerprint applying ΔT of 40 K. A series of PCBC3NSs were evaluated for thermal conductivity varying the number of grains (3, 5, and 10). The effect of grain rotation was also modeled in terms of Kapitza thermal resistance per grain, varying the rotation angle (θ/2 = 14.5, 16, 19, and 25°). Overall, a non-linear temperature variation was observed for PCBC3NS, particularly by increasing grain number, possibly because of more phonon scattering (shorter phonon relaxation time) arising from more structural defects. By contrast, the heat current passing across the slab decreased. The thermal conductivity of nanosheet dwindled from 149 W m−1 K−1 for monocrystalline BC3NS to the values of 129.67, 121.32, 115.04, and 102.78 W m−1 K−1 for PCBC3NSs having 2, 3, 5, and 10 grains, respectively. The increase of the grain̛s rotation angle (randomness) from 14.5° to 16°, 19° and 25° led to a rise in Kapitza thermal resistance from 2⨯10−10 m2 K·W−1 to the values of 2.3⨯ 10−10, 2.9⨯10−10, and 4.7⨯ 10−10 m2 K·W−1, respectively. Thus, natural 2D structure would facilitate phonon scattering rate at the grain boundaries, which limits heat transfer across polycrystalline nanosheets. © 2021 Elsevier Inc.
    • Thermal disruption of soil bacterial assemblages decreases diversity and assemblage similarity

      Weiser, Michael D.; Ning, Daliang; Buzzard, Vanessa; Michaletz, Sean T.; He, Zhili; Enquist, Brian J.; Waide, Robert B.; Zhou, Jizhong; Kaspari, Michael; Univ Arizona, Dept Ecol & Evolutionary Biol (WILEY, 2019-02-12)
      The metabolic theory of ecology assumes that rates of selection and adaptation for organisms are functions of temperature. Niche theory predicts that strong selection pressure should simplify assemblages as species are extirpated and taxa pre-adapted for the new environment thrive. Here, we use dosed mesocosms to test the prediction that higher temperatures decrease species richness and increase assemblage similarity more and faster than lower temperatures. We incubated two temperate forest soil types at constant temperatures from 10 degrees to 35 degrees, destructively sampling mesocosms at 30, 180, and 440 d. We quantified taxonomic richness and assemblage similarity of soil bacteria using 16S rRNA gene amplicons. As predicted, mesocosms at higher temperatures lost more taxa than those at lower temperature. Contrary to predictions, the simplified assemblages at higher temperatures became less similar to each other over time. After 440 d of incubation, the number of taxa lost was a linear function of the difference between treatment temperature and site mean annual temperature, while assemblage similarity decreased as an accelerating function of this temperature difference.
    • Thermal Emission in the Southwest Clump of VY CMa

      Gordon, Michael S.; Jones, Terry J.; Humphreys, Roberta M.; Ertel, Steve; Hinz, Philip M.; Hoffmann, William F.; Univ Arizona, Dept Astron, Steward Observ (IOP PUBLISHING LTD, 2019-02)
      We present high spatial resolution LBTI/NOMIC 9-12 mu m images of VY CMa and its massive outflow feature, the Southwest (SW) Clump. Combined with high-resolution imaging from the Hubble Space Telescope (0.4-1 mu m) and LBT/LMIRCam (1-5 mu m), we isolate the spectral energy distribution (SED) of the clump from the star itself. Using radiative-transfer code DUSTY, we model both the scattered light from VY CMa and the thermal emission from the dust in the clump to estimate the optical depth, mass, and temperature of the SW Clump. The SW Clump is optically thick at 8.9 mu m with a brightness temperature of similar to 200 K. With a dust chemistry of equal parts silicates and metallic iron, as well as assumptions on grain size distribution, we estimate a dust mass of 5.4 x 10(-5) M-circle dot. For a gas-to-dust ratio of 100, this implies a total mass of 5.4 x 10(-3) M-circle dot. Compared to the typical mass-loss rate of VY CMa, the SW Clump represents an extreme, localized mass-loss event from less than or similar to 300 yr ago.
    • Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

      Molaro, J. L.; Hergenrother, C. W.; Chesley, S. R.; Walsh, K. J.; Hanna, R. D.; Haberle, C. W.; Schwartz, S. R.; Ballouz, R-L; Bottke, W. F.; Campins, H. J.; et al. (AMER GEOPHYSICAL UNION, 2020-08)
      Many boulders on (101955) Bennu, a near-Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue-driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from similar to 1 mm to 10 cm and disaggregated particles have ejection speeds up to similar to 2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.
    • Thermal imaging shows submarine groundwater discharge plumes associated with ancient settlements on rapa nui (Easter Island, Chile)

      DiNapoli, R.J.; Lipo, C.P.; de Smet, T.S.; Hunt, T.L.; Honors College, University of Arizona; School of Anthropology, University of Arizona (MDPI AG, 2021)
      Submarine groundwater discharge (SGD) is an important component of many coastal environments and hydrologic processes, providing sources of nutrients to marine ecosystems, and potentially, an important source of fresh water for human populations. Here, we use a combination of unpiloted aerial systems (UAS) thermal infrared (TIR) imaging and salinity measurements to characterize SGD on the remote East Polynesian island of Rapa Nui (Easter Island, Chile). Previous research has shown that coastal freshwater seeps are abundant on Rapa Nui and strongly associated with the locations of ancient settlement sites. We currently lack, however, information on the differential magnitude or quality of these sources of fresh water. Our UAS‐based TIR results from four locations on Rapa Nui suggest that locations of variably‐sized SGD plumes are associated with many ancient settlement sites on the island and that these water sources are resilient to drought events. These findings support previous work indicating that ancient Rapa Nui communities responded to the inherent and climate‐induced hydrological challenges of the island by focusing on these abundant and resilient freshwater sources. Our results highlight the efficacy of using UASbased TIR for detecting relatively small SGD locations and provide key insights on the potential uses of these water sources for past and current Rapa Nui communities. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    • Thermal Infrared Imaging of MWC 758 with the Large Binocular Telescope: Planetary-driven Spiral Arms?

      Wagner, Kevin; Stone, Jordan M.; Spalding, Eckhart; Apai, Daniel; Dong, Ruobing; Ertel, Steve; Leisenring, Jarron; Webster, Ryan; Univ Arizona, Steward Observ; Univ Arizona, Lunar & Planetary Lab; et al. (IOP PUBLISHING LTD, 2019-08-28)
      Theoretical studies suggest that a giant planet around the young star MWC 758 could be responsible for driving the spiral features in its circumstellar disk. Here, we present a deep imaging campaign with the Large Binocular Telescope with the primary goal of imaging the predicted planet. We present images of the disk in two epochs in the L' filter (3.8 mu m) and a third epoch in the M' filter (4.8 mu m). The two prominent spiral arms are detected in each observation, which constitute the first images of the disk at and the deepest yet in L' (Delta L' = 12.1 exterior to the disk at 5 sigma significance). We report the detection of an S/N similar to 3.9 source near the end of the Southern arm, and, from the source's detection at a consistent position and brightness during multiple epochs, we establish a similar to 90% confidence-level that the source is of astrophysical origin. We discuss the possibilities that this feature may be (a) an unresolved disk feature, and (b) a giant planet responsible for the spiral arms, with several arguments pointing in favor of the latter scenario. We present additional detection limits on companions exterior to the spiral arms, which suggest that a less than or similar to 4 M-Jup planet exterior to the spiral arms could have escaped detection. Finally, we do not detect the companion candidate interior to the spiral arms reported recently by Reggiani et al., although forward modeling suggests that such a source would have likely been detected.
    • Thermal Properties of Bayfol® HX200 Photopolymer

      Blanche, Pierre-Alexandre; Mahamat, Adoum H.; Buoye, Emmanuel; College of Optical Sciences, University of Arizona (MDPI AG, 2020-12-02)
      Bayfol® HX200 photopolymer is a holographic recording material used in a variety of applications such as a holographic combiner for a heads-up display and augmented reality, dispersive grating for spectrometers, and notch filters for Raman spectroscopy. For these systems, the thermal properties of the holographic material are extremely important to consider since temperature can affect the diffraction efficiency of the hologram as well as its spectral bandwidth and diffraction angle. These thermal variations are a consequence of the distance and geometry change of the diffraction Bragg planes recorded inside the material. Because temperatures can vary by a large margin in industrial applications (e.g., automotive industry standards require withstanding temperature up to 125◦C), it is also essential to know at which temperature the material starts to be affected by permanent damage if the temperature is raised too high. Using thermogravimetric analysis, as well as spectral measurement on samples with and without hologram, we measured that the Bayfol® HX200 material does not suffer from any permanent thermal degradation below 160◦C. From that point, a further increase in temperature induces a decrease in transmission throughout the entire visible region of the spectrum, leading to a reduced transmission for an original 82% down to 27% (including Fresnel reflection). We measured the refractive index change over the temperature range from 24◦C to 100◦C. Linear interpolation give a slope 4.5 × 10−4 K−1 for unexposed film, with the extrapolated refractive index at 0◦C equal to n0 = 1.51. This refractive index change decreases to 3 × 10−4 K−1 when the material is fully cured with UV light, with a 0◦C refractive index equal to n0 = 1.495. Spectral properties of a reflection hologram recorded at 532 nm was measured from 23◦C to 171◦C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches 171◦C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of 384 × 10−6 K−1 by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    • Thermal state and evolving geodynamic regimes of the Meso- to Neoarchean North China Craton

      Sun, G.; Liu, S.; Cawood, P.A.; Tang, M.; van Hunen, J.; Gao, L.; Hu, Y.; Hu, F.; Department of Geosciences, University of Arizona (Nature Research, 2021)
      Constraining thickness and geothermal gradient of Archean continental crust are crucial to understanding geodynamic regimes of the early Earth. Archean crust-sourced tonalitic–trondhjemitic–granodioritic gneisses are ideal lithologies for reconstructing the thermal state of early continental crust. Integrating experimental results with petrochemical data from the Eastern Block of the North China Craton allows us to establish temporal–spatial variations in thickness, geothermal gradient and basal heat flow across the block, which we relate to cooling mantle potential temperature and resultant changing geodynamic regimes from vertical tectonics in the late Mesoarchean (~2.9 Ga) to plate tectonics with hot subduction in the early to late Neoarchean (~2.7–2.5 Ga). Here, we show the transition to a plate tectonic regime plays an important role in the rapid cooling of the mantle, and thickening and strengthening of the lithosphere, which in turn prompted stabilization of the cratonic lithosphere at the end of the Archean. © 2021, The Author(s).
    • Thermal studies of individual Si/Ge heterojunctions — The influence of the alloy layer on the heterojunction

      Wang, Sien; Xu, Dongchao; Gurunathan, Ramya; Snyder, G. Jeffrey; Hao, Qing; Univ Arizona, Dept Aerosp & Mech Engn (ELSEVIER, 2020-03-02)
      Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials. The phonon scattering by an interface can dramatically suppress the thermal transport, which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices. In this aspect, existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces. In this work, planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (R-K) that is further compared with predictions from existing simulations and analytical models. The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation. Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure. It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance (R-K). Some oxygen impurities may also help to increase R-K due to the formation of glassy structures. Following this, R-K reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction. (C) 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V.
    • Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm —Comparison between Different Pore-Drilling Techniques

      Hao, Qing; Xu, Dongchao; Zhao, Hongbo; Xiao, Yue; Medina, Fabian Javier; Univ Arizona, Aerosp & Mech Engn (NATURE PUBLISHING GROUP, 2018-06-13)
      In recent years, nanoporous Si films have been widely studied for thermoelectric applications due to the low cost and earth abundance of Si. Despite many encouraging results, inconsistency still exists among experimental and theoretical studies of reduced lattice thermal conductivity for varied nanoporous patterns. In addition, divergence can also be found among reported data, due to the difference in sample preparation and measurement setups. In this work, systematic measurements are carried out on nanoporous Si thin films with pore pitches on the order of 100 nm, where pores are drilled either by dry etching or a focused ion beam. In addition to thermal conductivity measurements, the specific heat of the nanoporous films is simultaneously measured and agrees with the estimation using bulk values, indicating a negligible change in the phonon dispersion. Without considering coherent phonon transport, the measured thermal conductivity values agree with predictions by frequency-dependent phonon Monte Carlo simulations assuming diffusive pore-edge phonon scattering. In Monte Carlo simulations, an expanded effective pore diameter is used to account for the amorphization and oxidation on real pore edges.
    • Thermally Activated Delayed Fluorescence Properties of Trioxoazatriangulene Derivatives Modified with Electron Donating Groups

      Tsuchiya, Youichi; Ishikawa, Yuma; Lee, Sang‐Hoon; Chen, Xian‐Kai; Brédas, Jean‐Luc; Nakanotani, Hajime; Adachi, Chihaya; Department of Chemistry and Biochemistry, The University of Arizona (Wiley-VCH Verlag, 2021-03-03)
      With the aim of achieving high-performance thermally activated delayed fluorescence, a series of trioxoazatriangulene derivatives are systematically developed by modifying the donor substituents. The emission colors are shifted from green to greenish-yellow and to yellow with rather broad spectral widths of 70–95 nm by introducing carbazole, triphenylamine, or diphenylamine donor units, indicating that each emission originates from a charge-transfer transition. On the other hand, the trioxoazatriangulene modified with three diphenylamines shows orange emission with a narrow emission spectrum (45 nm), suggesting that the transition mainly originates from a multiple resonance effect. © 2021 Wiley-VCH GmbH
    • Thermally Activated Delayed Fluorescence Sensitization for Highly Efficient Blue Fluorescent Emitters

      Abroshan, Hadi; Zhang, Yadong; Zhang, Xiaoqing; Fuentes-Hernandez, Canek; Barlow, Stephen; Coropceanu, Veaceslav; Marder, Seth; Kippelen, Bernard; Brédas, Jean-Luc; Univ Arizona, Dept Chem & Biochem (John Wiley & Sons, Inc., 2020-09)
      Hyperfluorescence is emerging as a powerful strategy to develop optoelectronic devices with high‐color purity and enhanced stability. It requires appropriate integration of a sensitizer displaying efficient thermally activated delayed fluorescence (TADF) and an emitter displaying strong, narrow‐band fluorescence. Here, through a joint computational and experimental approach, an unprecedented, end‐to‐end systems level description of the electronic and optical processes that take place in a hyperfluorescent emissive layer composed of a TADF sensitizer, 2,5‐bis(2,6‐di(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (4CzDPO), and a fluorescent emitter, 2,5,8,11‐tetra‐tert‐butylperylene (TBPe) is provided. The photophysical properties measurement of the emissive layer is combined with the computational determination of the electronic properties, film morphology, and excitation transfer phenomena. The Förster resonance energy transfer rates from 4CzDPO to TBPe are on the order of 1011 s−1, considerably higher than the radiative and nonradiative recombination rates for 4CzDPO. These features ensure nearly complete energy transfer to TBPe, leading to a five‐fold increase in the photoluminescence quantum yields in the 4CzDPO:TBPe system in comparison to neat films of 4CzDPO. This approach highlights the factors that can provide efficient energy transfer from TADF molecules to fluorescent emitters, suppress energy transfer among TADF molecules, and avoid the need for a host material within the emissive layer.
    • Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks

      Evans, Austin M.; Giri, Ashutosh; Sangwan, Vinod K.; Xun, Sangni; Bartnof, Matthew; Torres-Castanedo, Carlos G.; Balch, Halleh B.; Rahn, Matthew S.; Bradshaw, Nathan P.; Vitaku, Edon; et al. (Nature Research, 2021-03-18)
      As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m−1 K−1) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.
    • Thermally Stimulated Currents in Nanocrystalline Titania

      Bruzzi, Mara; Mori, Riccardo; Baldi, Andrea; Carnevale, Ennio; Cavallaro, Alessandro; Scaringella, Monica; Univ Arizona, LBT Observ (MDPI AG, 2018-01-05)
      A thorough study on the distribution of defect-related active energy levels has been performed on nanocrystalline TiO2. Films have been deposited on thick-alumina printed circuit boards equipped with electrical contacts, heater and temperature sensors, to carry out a detailed thermally stimulated currents analysis on a wide temperature range (5-630 K), in view to evidence contributions from shallow to deep energy levels within the gap. Data have been processed by numerically modelling electrical transport. The model considers both free and hopping contribution to conduction, a density of states characterized by an exponential tail of localized states below the conduction band and the convolution of standard Thermally Stimulated Currents (TSC) emissions with gaussian distributions to take into account the variability in energy due to local perturbations in the highly disordered network. Results show that in the low temperature range, up to 200 K, hopping within the exponential band tail represents the main contribution to electrical conduction. Above room temperature, electrical conduction is dominated by free carriers contribution and by emissions from deep energy levels, with a defect density ranging within 10(14)-10(18) cm(-3), associated with physio- and chemi-sorbed water vapour, OH groups and to oxygen vacancies.
    • Thermo-mechanical strain rate-dependent behavior of shape memory alloys as vibration dampers and comparison to conventional dampers

      Gur, S.; Mishra, S. K.; Frantziskonis, G. N.; Univ Arizona, Dept Civil Engn & Engn Mech (SAGE PUBLICATIONS LTD, 2015-05-31)
      A study on shape memory alloy materials as vibration dampers is reported. An important component is the strain rate-dependent and temperature-dependent constitutive behavior of shape memory alloy, which can significantly change its energy dissipation capacity under cyclic loading. The constitutive model used accounts for the thermo-mechanical strain rate-dependent behavior and phase transformation. With increasing structural flexibility, the hysteretic loop size of shape memory alloy dampers increases due to increasing strain rates, thus further decreasing the response of the structure to cyclic excitation. The structure examined is a beam, and its behavior with shape memory alloy dampers is compared to the same beam with conventional dampers. Parametric studies reveal the superior performance of the shape memory alloy over the conventional dampers even at the resonance frequency of the beam-damper system. An important behavior of the shape memory alloy dampers is discovered, in that they absorb energy from the fundamental and higher vibration modes. In contrast, the conventional dampers transfer energy to higher modes. For the same beam control, the stiffness requirement for the shape memory alloy dampers is significantly less than that of the conventional dampers. Response quantities of interest show improved performance of the shape memory alloy over the conventional dampers under varying excitation intensity, frequency, temperature, and strain rate.