1,909 research outputs found
Tribochemistry of graphene on iron and its possible role in lubrication of steel
Recent tribological experiments revealed that graphene is able to lubricate
macroscale steel-on-steel sliding contacts very effectively both in dry and
humid conditions. This effect has been attributed to a mechanical action of
graphene related to its load-carrying capacity. Here we provide further insight
into the functionality of graphene as lubricant by analysing its tribochemical
action. By means of first principles calculations we show that graphene binds
strongly to native iron surfaces highly reducing their surface energy. Thanks
to a passivating effect, the metal surfaces coated by graphene become almost
inert and present very low adhesion and shear strength when mated in a sliding
contact. We generalize the result by establishing a connection between the
tribological and the electronic properties of interfaces, which is relevant to
understand the fundamental nature of frictional forces.Comment: 19 pages, 6 figure
Insigths into the tribochemistry of silicon-doped carbon based films by ab initio analysis of water/surface interactions
Diamond and diamond-like carbon (DLC) are used as coating materials for
numerous applications, ranging from biomedicine to tribology. Recently, it has
been shown that the hydrophilicity of the carbon films can be enhanced by
silicon doping, which highly improves their biocompatibility and frictional
performances. Despite the relevance of these properties for applications, a
microscopic understanding on the effects of silicon is still lacking. Here we
apply ab initio calculations to study the interaction of water molecules with
Si-incorporated C(001) surfaces. We find that the presence of Si dopants
considerably increases the energy gain for water chemisorption and decreases
the energy barrier for water dissociation by more than 50%. We provide a
physical rational for the phenomenon by analysing the electronic charge
displacements occuring upon adsorption. We also show that once hydroxylated,
the surface is able to bind further water molecules much strongly than the
clean surface via hydrogen-bond networks. This two-step process is consistent
with and can explain the enhanced hydrophilic character observed in
carbon-based films doped by silicon
A multiwavelength view of BL Lacs neutrino candidates
The discovery of high-energy astrophysical neutrinos by IceCube kicked off a
new line of research to identify the electromagnetic counterparts producing
these neutrinos. Among the extragalactic sources, active galactic nuclei (AGN),
and in particular Blazars, are promising candidate neutrino emitters. Their
structure, with a relativistic jet pointing to the Earth, offers a natural
accelerator of particles and for this reason a perfect birthplace of high
energy neutrinos. A good characterisation of the spectral energy distribution
(SED) of these sources can improve the understanding of the physical
composition of the source and the emission processes involved. Starting from
our previous works in which we assumed a correlation between the -ray
and the neutrino flux of the BL Lacs of the 2FHL catalogue (detected by Fermi
above 50GeV), we select those BL Lac in spatial correlation with the IceCube
events. We obtain a sample of 7 sources and we start an observational campaign
to have a better characterisation of the synchrotron peak. During the analysis
of the data a new source has been added because of its position inside the
angular uncertainty of a muon track event detected by IceCube. This source,
namely TXS0506+056, was in a high-state during the neutrino event and we will
consider it as benchmark to check the proprieties of the other sources of the
sample during the related neutrino detection.
We obtain a better characterisation of the SED for the sources of our sample.
A prospective extreme Blazar, a very peculiar low synchrotron peak (LSP) source
with a large separation of the two peaks and a \textit{twin} of TXS0506+056
come up. We also provide the -ray light curve to check the trend of the
sources around the neutrino detection but no clears patterns are in common
among the sources.Comment: 10 pages, 4 figures, accepted to MNRA
Neutrino emission from BL Lac objects: the role of radiatively inefficient accretion flows
The origin of the astrophysical high-energy neutrinos discovered by IceCube
is currently a major mystery. The recent detection of IceCube-170922A, a
300 TeV neutrino potentially correlated with the flaring -ray
source TXS 0506+056, directs attention toward BL Lac objects (BL Lacs), the
subclass of blazars with weak emission lines. While high-energy neutrinos can
be produced via photohadronic interactions between protons accelerated in their
jets and ambient low-energy photons, the density of the latter in such objects
had generally been thought to be too low for efficient neutrino emission. Here
we consider the role of radiatively inefficient accretion flows (RIAFs), which
can plausibly exist in the nuclei of BL Lacs, as the source of target photons
for neutrino production. Based on simple model prescriptions for the spectra of
RIAFs at different accretion rates, we find that they can be sufficienly
intense to allow appreciable neutrino emission for the class of
low-synchrotron-peak BL Lacs such as TXS 0506+056. In constrast, for
high-synchrotron-peak BL Lacs including Mkn 421 and Mkn 501, the contribution
of RIAFs is subdominant and their neutrino production efficiency can remain
low, consistent with their non-detection by IceCube to date.Comment: 5 pages, 4 figures, accepted to MNRAS as Lette
Extrasolar and Bl Lac observations at OARPAF
This paper reports the work done at the Osservatorio Astronomico Regionale Parco Antola comune di Fascia (OARPAF), as a part of my master thesis. The paper is divided in three parts: the first part describes the instrumentation at the OARPAF; the second part shows the characterization of the instrumentation, from the pointing model of the telescope to the conception and the realization of the software for data analysis; the third part illustrates the results of the photometric calibration
High-energy neutrinos from FR0 radio-galaxies?
The sources responsible for the emission of high-energy ( 100 TeV)
neutrinos detected by IceCube are still unknown. Among the possible candidates,
active galactic nuclei with relativistic jets are often examined, since the
outflowing plasma seems to offer the ideal environment to accelerate the
required parent high-energy cosmic rays. The non-detection of single point
sources or -- almost equivalently -- the absence, in the IceCube events, of
multiplets originating from the same sky position, constrains the cosmic
density and the neutrino output of these sources, pointing to a numerous
population of faint sources. Here we explore the possibility that FR0
radiogalaxies, the population of compact sources recently identified in large
radio and optical surveys and representing the bulk of radio-loud AGN
population, can represent suitable candidates for neutrino emission. Modeling
the spectral energy distribution of a FR0 radiogalaxy recently associated to a
-ray source detected by the Large Area Telescope onboard Fermi, we
derive the physical parameters of its jet, in particular the power carried by
it. We consider the possible mechanisms of neutrino production, concluding that
reactions in the jet between protons and ambient radiation is too
inefficient to sustain the required output. We propose an alternative scenario,
in which protons, accelerated in the jet, escape from it and diffuse in the
host galaxy, producing neutrinos as a result of scattering with the
interstellar gas, in strict analogy with the processes taking place in
star-forming galaxies.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Belladrilus (Santomesia) auka n. sp. (Oligochaeta, Ocnerodrilidae) from Argentina
From Cordoba Province, Argentina the Ocnerodrilidae Belladrilus (Santomesia) auka n. sp. is described by dissections and serial histological sections. The species is well characterised by the tumid T-shaped male porophore in XVll with an anterior half-moon depression and a posterior cross thickening. The seminal grooves are convergent and thin in the anterior half, straight and wider in the posterior half.
Key words: Oligochaeta, Ocnerodrilidae, Cordoba, Argentina, Anatomy, Taxonomy.From Cordoba Province, Argentina the Ocnerodrilidae Belladrilus (Santomesia) auka n. sp. is described by dissections and serial histological sections. The species is well characterised by the tumid T-shaped male porophore in XVll with an anterior half-moon depression and a posterior cross thickening. The seminal grooves are convergent and thin in the anterior half, straight and wider in the posterior half.
Key words: Oligochaeta, Ocnerodrilidae, Cordoba, Argentina, Anatomy, Taxonomy.From Cordoba Province, Argentina the Ocnerodrilidae Belladrilus (Santomesia) auka n. sp. is described by dissections and serial histological sections. The species is well characterised by the tumid T-shaped male porophore in XVll with an anterior half-moon depression and a posterior cross thickening. The seminal grooves are convergent and thin in the anterior half, straight and wider in the posterior half.
Key words: Oligochaeta, Ocnerodrilidae, Cordoba, Argentina, Anatomy, Taxonomy
Ab initio insights into the interaction mechanisms between H, HO, and O molecules with diamond surfaces
Diamond displays outstanding chemical, physical, and tribological properties,
making it attractive for numerous applications ranging from biomedicine to
tribology. However, the reaction of the materials with molecules present in the
air, such as oxygen, hydrogen, and water, could significantly change the
electronic and tribological properties of the films. In this study, we
performed several density functional theory calculations to construct a
database for the adsorption energies and dissociation barriers of these
molecules on the most relevant diamond surfaces, including C(111), C(001), and
C(110). The adsorption configurations, reaction paths, activation energies, and
their influence on the structure of diamond surfaces are discussed. The results
indicate that there is a strong correlation between adsorption energy and
surface energy. Moreover, we found that the dissociation processes of oxygen
molecules on these diamond surfaces can significantly alter the surface
morphology and may affect the tribological properties of diamond films. These
findings can help to advance the development and optimization of devices and
antiwear coatings based on diamond
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