59,350 research outputs found
Cryomilling as environmentally friendly synthesis route to prepare nanomaterials
The milling of materials at cryogenic temperature has gained importance both in academic as well as the industrial community in the last two decades, primarily because of significant advantages this technique as compared to milling at room temperature; environmental friendly nature, cost-effectiveness, rapid grain refinement, less contamination, and large scale production capability of various nanomaterials. Scientifically, milling at cryo-temperature
exhibits several distinct material related phenomena; suppression of recovery and recrystallization, predominant fractures over cold welding, significantly low oxidation, and
contamination, leading to rapid grain refinement. Cryomilling has extensively been used to obtain finer scale powder of spices for the preservation of aroma, medicines for effective
dissolution, or amorphization. It has been considered an environmentally friendly process as it utilizes benign liquid nitrogen or argon without discharging any toxic entity to the
environment, making the process attractive and sustainable. The present review is intended to provide various scientific as well as technological aspects of cryomilling, environmental impact, and future direction
Classical Langevin dynamics of a charged particle moving on a sphere and diamagnetism: A surprise
It is generally known that the orbital diamagnetism of a classical system of
charged particles in thermal equilibrium is identically zero -- the Bohr-van
Leeuwen theorem. Physically, this null result derives from the exact
cancellation of the orbital diamagnetic moment associated with the complete
cyclotron orbits of the charged particles by the paramagnetic moment subtended
by the incomplete orbits skipping the boundary in the opposite sense. Motivated
by this crucial, but subtle role of the boundary, we have simulated here the
case of a finite but \emph{unbounded} system, namely that of a charged particle
moving on the surface of a sphere in the presence of an externally applied
uniform magnetic field. Following a real space-time approach based on the
classical Langevin equation, we have computed the orbital magnetic moment which
now indeed turns out to be non-zero, and has the diamagnetic sign. To the best
of our knowledge, this is the first report of the possibility of finite
classical diamagnetism in principle, and it is due to the avoided cancellation.Comment: Accepted for publication in EP
Femtosecond carrier dynamics and saturable absorption in graphene suspensions
Nonlinear optical properties and carrier relaxation dynamics in graphene,
suspended in three different solvents, are investigated using femtosecond (80
fs pulses) Z-scan and degenerate pumpprobe spectroscopy at 790 nm. The results
demonstrate saturable absorption property of graphene with a nonlinear
absorption coefficient, , of ~2 to 9x10^-8 cm/W. Two distinct time scales
associated with the relaxation of photoexcited carriers, a fast one in the
range of 130-330 fs (related to carrier-carrier scattering) followed by a
slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are
observed.Comment: 3 pages, 2 figures, 2 table
Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction
Among various parent compounds of iron pnictide superconductors, EuFe2As2
stands out due to the presence of both spin density wave of Fe and
antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal
neutron diffraction studies have been carried out to determine the magnetic
structure of this compound and to investigate the coupling of two magnetic
sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190
K and 19 K, respectively. The ordering of Fe2+ moments is associated with the
wave vector k = (1,0,1) and it takes place at the same temperature as the
tetragonal to orthorhombic structural phase transition, which indicates the
strong coupling between structural and magnetic components. The ordering of Eu
moment is associated with the wave vector k = (0,0,1). While both Fe and Eu
spins are aligned along the long a axis as experimentally determined, our
studies suggest a weak coupling between the Fe and Eu magnetism.Comment: 7 pages, 7 figure
Ferromagnetism and the Effect of Free Charge Carriers on Electric Polarization in Y_2NiMnO_6 Double Perovskite
The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81
K. The ferromagnetic order at low temperature is confirmed by the saturation
value of magnetization (M_s) and also, validated by the refined ordered
magnetic moment values extracted from neutron powder diffraction data at 10 K.
This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The
cation-ordered P 21/n nuclear structure is revealed by neutron powder
diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric
constant and equivalent circuit analysis of impedance data takes into account
the bulk contribution to total dielectric constant. This reveals an anomaly
which coincides with the ferromagnetic transition temperature (T_c).
Pyrocurrent measurements register a current flow with onset near Tc and a peak
at 57 K that shifts with temperature ramp rate. The extrinsic nature of the
observed pyrocurrent is established by employing a special protocol
measurement. It is realized that the origin is due to re-orientation of
electric dipoles created by the free charge carriers and not by spontaneous
electric polarization at variance with recently reported magnetism-driven
ferroelectricity in this materialComment: Published in Physical Review
Effect of simultaneous application of field and pressure on magnetic transitions in LaCaMnO
We study combined effect of hydrostatic pressure and magnetic field on the
magnetization of LaCaMnO. We do not observe any
significant effect of pressure on the paramagnetic to ferromagnetic transition.
However, pressure asymmetrically affects the thermal hysteresis across the
ferro-antiferromagnetic first-order transition, which has strong field
dependence. Though the supercooling (T*) and superheating (T**) temperatures
decrease and the value of magnetization at 5K (M) increases with
pressure, T* and M shows abrupt changes in tiny pressure of 0.68kbar.
These anomalies enhance with field. In 7Tesla field, transition to
antiferromagnetic phase disappears in 0.68kbar and M show significant
increase. Thereafter, increase in pressure up to 10kbar has no noticeable
effect on the magnetization
The ionizing sources of luminous compact HII regions in the RCW106 and RCW122 clouds
Given the rarity of young O star candidates, compact HII regions embedded in
dense molecular cores continue to serve as potential sites to peer into the
details of high-mass star formation. To uncover the ionizing sources of the
most luminous and compact HII regions embedded in the RCW106 and RCW122 giant
molecular clouds, known to be relatively nearby (2-4 kpc) and isolated, thus
providing an opportunity to examine spatial scales of a few hundred to a
thousand AU in size. High spatial resolution (0.3"), mid-infrared spectra
(R=350), including the fine structure lines [ArIII] and [NeII], were obtained
for four luminous compact HII regions, embedded inside the dense cores within
the RCW106 and RCW122 molecular cloud complexes. At this resolution, these
targets reveal point-like sources surrounded by nebulosity of different
morphologies, uncovering details at spatial dimensions of <1000AU. The
point-like sources display [ArIII] and [NeII] lines - the ratios of which are
used to estimate the temperature of the embedded sources. The derived
temperatures are indicative of mid-late O type objects for all the sources with
[ArIII] emission. Previously known characteristics of these targets from the
literature, including evidence of disk or accretion suggest that the identified
sources may grow more to become early-type O stars by the end of the star
formation process
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