1,777 research outputs found
Nonequilibrium phonon mean free paths in anharmonic chains
Harnessing the power of low-dimensional materials in thermal applications
calls for a solid understanding of the anomalous thermal properties of such
systems. We analyze thermal conduction in one-dimensional systems by
determining the frequency-dependent phonon mean free paths (MFPs) for an
anharmonic chain, delivering insight into the diverging thermal conductivity
observed in computer simulations. In our approach, the MFPs are extracted from
the length-dependence of the spectral heat current obtained from nonequilibrium
molecular dynamics simulations. At low frequencies, the results reveal a
power-law dependence of the MFPs on frequency, in agreement with the diverging
conductivity and the recently determined equilibrium MFPs. At higher
frequencies, however, the nonequilibrium MFPs consistently exceed the
equilibrium MFPs, highlighting the differences between the two quantities.
Exerting pressure on the chain is shown to suppress the mean free paths and to
generate a weaker divergence of MFPs at low frequencies. The results deliver
important insight into anomalous thermal conduction in low-dimensional systems
and also reveal differences between the MFPs obtained from equilibrium and
nonequilibrium simulations.Comment: 8 pages, 7 figures, minor changes to v
Role of anharmonic phonon scattering in the spectrally decomposed thermal conductance at planar interfaces
Detailed understanding of vibrational heat transfer mechanisms between solids
is essential for the efficient thermal engineering and control of
nanomaterials. We investigate the frequency dependence of anharmonic scattering
and interfacial thermal conduction between two acoustically mismatched solids
in planar contact by calculating the spectral decomposition of the heat current
flowing through an interface between two materials. The calculations are based
on analyzing the correlations of atomic vibrations using the data extracted
from non-equilibrium molecular dynamics simulations. Inelastic effects arising
from anharmonic interactions are shown to significantly facilitate heat
transfer between two mass-mismatched face-centered cubic lattices even at
frequencies exceeding the cut-off frequency of the heavier material due to (i)
enhanced dissipation of evanescent vibrational modes and (ii)
frequency-doubling and frequency-halving three-phonon energy transfer processes
at the interface. The results provide substantial insight into interfacial
energy transfer mechanisms especially at high temperatures, where inelastic
effects become important and other computational methods are ineffective.Comment: minor changes to v
Magnetic nanocomposites at microwave frequencies
Most conventional magnetic materials used in the electronic devices are
ferrites, which are composed of micrometer-size grains. But ferrites have small
saturation magnetization, therefore the performance at GHz frequencies is
rather poor. That is why functionalized nanocomposites comprising magnetic
nanoparticles (e.g. Fe, Co) with dimensions ranging from a few nm to 100 nm,
and embedded in dielectric matrices (e.g. silicon oxide, aluminium oxide) have
a significant potential for the electronics industry. When the size of the
nanoparticles is smaller than the critical size for multidomain formation,
these nanocomposites can be regarded as an ensemble of particles in
single-domain states and the losses (due for example to eddy currents) are
expected to be relatively small. Here we review the theory of magnetism in such
materials, and we present a novel measurement method used for the
characterization of the electromagnetic properties of composites with
nanomagnetic insertions. We also present a few experimental results obtained on
composites consisting of iron nanoparticles in a dielectric matrix.Comment: 20 pages, 10 figures, 5 table
Static friction measurements on steel against uncoated and coated cast iron
Static friction is a phenomenon we may mainly consider as related to frictional joints within staticmechanics. The step from static friction to tribological phenomena is, however, rather short, since at theonset of sliding in a mechanical contact, the static friction determines the initial resistance against motion.Static friction furthermore plays a role in contacts subjected to traction and fretting. Although being aphenomenon of short duration, the tribological phenomena during the transition from static friction to slidingfriction may be of great importance for the operational life of the contact surfaces, particularly if theprocedure is repeated for a sufficient number of times.The present paper describes the principles of static friction measurements, details of the employed staticfriction tribometer and the results of measurements with unlubricated and lubricated sliding couplesconsisting of steel against uncoated and coated cast iron
Milloin epäillä skitsofreniaa?
Skitsofrenia ilmenee laaja-alaisena tunteiden, havaitsemisen, motivaation ja käyttäytymisen säätelyn häiriönä. Skitsofrenian kehittymisen ajatellaan liittyvän keskushermoston kehittymis- ja kypsymisprosessiin, johon liittyy usein häiriöitä kognitiivisissa toiminnoissa. Potilaat ovat myös herkkiä erilaisille ympäristön kuormitustekijöille. Varhainen psykoosioireiston tunnistaminen ja hoidon aloittaminen parantavat ennustetta. Suuressa psykoosiriskissä olevien nuorten tunnistaminen ja hoito voivat estää skitsofrenian puhkeamisen. Diagnostinen selvittely ja hoidon aloitus on usein perusteltua toteuttaa erikoissairaanhoidossa, kun kyseessä on uusi skitsofreniapotilas.Peer reviewe
Maallikkohypnoosiin liittynyt akuutti skitsofreenistyyppinen psykoosi
Vaikka hypnoosia pidetään varsin haitattomana hoitomuotona, kirjallisuudessa on kuvattu sen varomattomaan käyttöön liittyneitä komplikaatioita, kuten dissosiatiivisia ja lyhytkestoisia psykoottisia tiloja. Yli 50-vuotiaan aiemmin terveen ja työelämässä menestyneen naisen akuutti skitsofreenistyyppinen psykoottinen häiriö alkoi varomattomasti käytetyn maallikkohypnoosin yhteydessä. Potilas kokeili sellaista ensimmäistä kertaa, ja hänen kokemansa hallusinaatiot ja harhaluulot liittyivät yksinomaan hypnoosiin. Potilaan oireistoon liittyi vaarallisia piirteitä, kuten harhaääniä, jotka kehottivat häntä riskikäyttäytymiseen liikenteessä. Lyhyen sairaalahoidon jälkeen potilas voi seurannassa hyvin. Hypnoosia tulisi käyttää vain valvotusti terveydenhuollossa.Peer reviewe
- …