312 research outputs found
Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7
Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a
combination of structural, magnetic, and electrical and thermal transport
studies. Analysis of synchrotron X-ray and neutron diffraction patterns
suggests some site disorder on the A-site in the pyrochlore sublattice: Ru
substitutes on the Nd-site up to 7.0(3)%, regardless of the different
preparative conditions explored. Intrinsic magnetic and electrical transport
properties have been measured. Ru 4d spins order antiferromagnetically at 143 K
as seen both in susceptibility and specific heat, and there is a corresponding
change in the electrical resistivity behaviour. A second antiferromagnetic
ordering transition seen below 10 K is attributed to ordering of Nd 4f spins.
Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be
independent of the Ru-antisite disorder on the Nd site. The electrical
properties of Nd2Ru2O7 are presented in the light of data published on all
A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi3+
ions on the A-site play in driving metallic behaviour. High-temperature
thermoelectric properties have also been measured. When considered in the
context of known thermoelectric materials with useful figures-of-merit, it is
clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents
it from being an effective thermoelectric. A method for screening candidate
thermoelectrics is suggested.Comment: 19 pages, 10 figure
Development of a Risk Framework for Industry 4.0 in the Context of Sustainability for Established Manufacturers
The concept of “Industry 4.0” is expected to bring a multitude of benefits for industrial value creation. However, the associated risks hamper its implementation and lack a comprehensive overview. In response, the paper proposes a framework of risks in the context of Industry 4.0 that is related to the Triple Bottom Line of sustainability. The framework is developed from a literature review, as well as from 14 in-depth expert interviews. With respect to economic risks, the risks that are associated with high or false investments are outlined, as well as the threatened business models and increased competition from new market entrants. From an ecological perspective, the increased waste and energy consumption, as well as possible ecological risks related to the concept “lot size one”, are described. From a social perspective, the job losses, risks associated with organizational transformation, and employee requalification, as well as internal resistance, are among the aspects that are considered. Additionally, risks can be associated with technical risks, e.g., technical integration, information technology (IT)-related risks such as data security, and legal and political risks, such as for instance unsolved legal clarity in terms of data possession. Conclusively, the paper discusses the framework with the extant literature, proposes managerial and theoretical implications, and suggests avenues for future research
Superheavy dark matter and ultrahigh energy cosmic rays
The phase of inflationary expansion in the early universe produces superheavy
relics in a mass window between 10^{12} GeV and 10^{14} GeV. Decay or
annihilation of these superheavy relics can explain the observed ultrahigh
energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff. We emphasize that
the pattern of cosmic ray arrival directions with energies beyond 20 EeV will
decide between the different proposals for the origin of ultrahigh energy
cosmic rays.Comment: Based on an invited talk given by RD at Theory Canada 1, Vancouver,
June 2-5, 200
On The Injection Spectrum of Ultrahigh Energy Cosmic Rays in the Top-Down Scenario
We analyze the uncertainties involved in obtaining the injection spectra of
UHECR particles in the top-down scenario of their origin. We show that the
DGLAP evolution of fragmentation functions (FF) to (mass of the X
particle) from their initial values at low is subject to considerable
uncertainties. We therefore argue that, for x\lsim 0.1 (the region of
interest for most large values of interest, being the
scaled energy variable), the FF obtained from DGLAP evolution is no more
reliable than that provided, for example, by a simple Gaussian form (in the
variable ) obtained under the Modified Leading Log Approximation
(MLLA). Additionally, we find that for x\gsim0.1, the evolution in of
the singlet FF, which determines the injection spectrum, is ``minimal'' -- the
singlet FF changes by barely a factor of 2 after evolving it over 14
orders of magnitude in . We, therefore, argue that as long as the
measurement of the UHECR spectrum above \sim10^{20}\ev is going to remain
uncertain by a factor of 2 or larger, it is good enough for most practical
purposes to directly use any one of the available initial parametrisations of
the FFs in the region x\gsim0.1 based on low energy data even without
evolving them to the requisite value.Comment: Minor changes, added a reference, version to appear in Phys. Rev.
Pulsar kicks from neutrino oscillations
Neutrino oscillations in a core-collapse supernova may be responsible for the
observed rapid motions of pulsars. Given the present bounds on the neutrino
masses, the pulsar kicks require a sterile neutrino with mass 2-20 keV and a
small mixing with active neutrinos. The same particle can be the cosmological
dark matter. Its existence can be confirmed the by the X-ray telescopes if they
detect a 1-10 keV photon line from the decays of the relic sterile neutrinos.
In addition, one may be able to detect gravity waves from a pulsar being
accelerated by neutrinos in the event of a nearby supernova.Comment: invited review article to appear in Int. J. Mod. Phys. (21 pages, 6
figures
Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb Half-Heusler Compounds
The 18-electron ternary intermetallic systems TiNiSn and TiCoSb are promising for applications as high-temperature thermoelectrics and comprise earth-abundant, and relatively nontoxic elements. Heusler and half-Heusler compounds are usually prepared by conventional solid state methods involving arc-melting and annealing at high temperatures for an extended period of time. Here, we report an energy-saving preparation route using a domestic microwave oven, reducing the reaction time significantly from more than a week to one minute. A microwave susceptor material rapidly heats the elemental starting materials inside an evacuated quartz tube resulting in near single phase compounds. The initial preparation is followed by a densification step involving hot-pressing, which reduces the amount of secondary phases, as verified by synchrotron X-ray diffraction, leading to the desired half-Heusler compounds, demonstrating that hot-pressing should be treated as part of the preparative process. For TiNiSn, high thermoelectric power factors of 2 mW/mK^2 at temperatures in the 700 to 800 K range, and zT values of around 0.4 are found, with the microwave-prepared sample displaying somewhat superior properties to conventionally prepared half-Heuslers due to lower thermal conductivity. The TiCoSb sample shows a lower thermoelectric figure of merit when prepared using microwave methods because of a metallic second phase
Pulsar Kicks and Spin Tilts in the Close Double Neutron Stars PSR J0737-3039, PSR B1534+12 and PSR B1913+16
In view of the recent measurement of the scintillation velocity for PSR
J0737-3039, we examine the complete set of constraints imposed on the pulsar B
natal kicks (magnitude and orientation) and predict the most favorable pulsar
kick velocity and spin tilt for both isotropic and polar kicks. Our analysis
takes into account both currently unknown parameters: the orientation of the
orbital plane on the sky (Omega) and the radial component of the systemic
velocity (V_r). Assuming that the system's peculiar velocity is entirely due to
the second supernova explosion, we find that the system may have crossed the
Galactic plane multiple times since the birth of the second neutron star and
that the post-supernova peculiar velocity could have been as high as 1200km/s.
We also confirm the absolute lower and upper limits on the physical parameters
derived in our earlier study. For specific combinations of the two unknown
parameters Omega and V_r, however, we find much tighter constraints on the
pre-supernova binary configuration and natal kicks imparted to pulsar B, as
well as on the age of system. Once Omega is measured in the coming year, it
will be straightforward to use the results presented here to further constrain
the natal kicks and the spin-tilt predictions. We complete our comprehensive
study and derive similar constraints and spin-tilt predictions for PSR
B1534+12, where the only free parameter is V_r. Lastly, for PSR B1913+16, we
update the progenitor and kick constraints using the measured pulsar spin tilt
and allowing for Roche-lobe overflow from the progenitor of the pulsar
companion.Comment: Replaced Fig. 16 with corrected version. See ApJ 616, p. 414 for
high-resolution figures and notes added in proo
Constraints on the Ultra High Energy Photon flux using inclined showers from the Haverah Park array
We describe a method to analyse inclined air showers produced by ultra high
energy cosmic rays using an analytical description of the muon densities. We
report the results obtained using data from inclined events
(60^{\circ}<\theta<80^{\circ}) recorded by the Haverah Park shower detector for
energies above 10^19 eV. Using mass independent knowledge of the UHECR spectrum
obtained from vertical air shower measurements and comparing the expected
horizontal shower rate to the reported measurements we show that above 10^19 eV
less than 48 % of the primary cosmic rays can be photons at the 95 % confidence
level and above 4 X 10^19 eV less than 50 % of the cosmic rays can be photonic
at the same confidence level. These limits place important constraints on some
models of the origin of ultra high-energy cosmic rays.Comment: 45 pages, 25 figure
Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From Observations to Modeling
Código de proyecto: Isotope Network for Tropical Ecosystem Studies (ISONet). Producción relacionada con el Observatorio del Agua y Cambio Global (OACG).Tropical regions cover approximately 36% of the Earth’s landmass. These regions are home to 40% of the world’s population, which is projected to increase to over 50% by 2030 under a remarkable climate variability scenario often exacerbated by El Niño Southern Oscillation (ENSO) and other climate teleconnections. In the tropics, ecohydrological conditions are typically under the influence of complex land-ocean-atmosphere interactions that produce a dynamic cycling of mass and energy reflected in a clear partition of water fluxes. Here, we present a review of 7 years of a concerted and continuous water stable isotope monitoring across Costa Rica, including key insights learned, main methodological advances and limitations (both in experimental designs and data analysis), potential data gaps, and future research opportunities with a humid tropical perspective. The uniqueness of the geographic location of Costa Rica within the mountainous Central America Isthmus, receiving moisture inputs from the Caribbean Sea (windward) and the Pacific Ocean (complex leeward topography), and experiencing strong ENSO events, poses a clear advantage for the use of isotopic variations to underpin key drivers in ecohydrological responses. In a sequential approach, isotopic variations are analyzed from moisture transport, rainfall generation, and groundwater/surface connectivity to Bayesian and rainfall-runoff modeling. The overarching goal of this review is to provide a robust humid tropical example with a progressive escalation from common water isotope observations to more complex modeling outputs and applications to enhance water resource management in the tropics.UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI)UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de FísicaUCR::Vicerrectoría de Docencia::Ciencias Sociales::Facultad de Ciencias Sociales::Escuela de Geografí
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