1,894 research outputs found
Nanoscale Heat Transfer from Magnetic Nanoparticles and Ferritin in an Alternating Magnetic Field
Recent suggestions of nanoscale heat confinement on the surface of synthetic and biogenic magnetic nanoparticles during heating by radio frequency-alternating magnetic fields have generated intense interest because of the potential utility of this phenomenon for noninvasive control of biomolecular and cellular function. However, such confinement would represent a significant departure from the classical heat transfer theory. Here, we report an experimental investigation of nanoscale heat confinement on the surface of several types of iron oxide nanoparticles commonly used in biological research, using an all-optical method devoid of the potential artifacts present in previous studies. By simultaneously measuring the fluorescence of distinct thermochromic dyes attached to the particle surface or dissolved in the surrounding fluid during radio frequency magnetic stimulation, we found no measurable difference between the nanoparticle surface temperature and that of the surrounding fluid for three distinct nanoparticle types. Furthermore, the metalloprotein ferritin produced no temperature increase on the protein surface nor in the surrounding fluid. Experiments mimicking the designs of previous studies revealed potential sources of the artifacts. These findings inform the use of magnetic nanoparticle hyperthermia in engineered cellular and molecular systems
Nanoscale Heat Transfer from Magnetic Nanoparticles and Ferritin in an Alternating Magnetic Field
Recent suggestions of nanoscale heat confinement on the surface of synthetic and biogenic magnetic nanoparticles during heating by radio frequency-alternating magnetic fields have generated intense interest because of the potential utility of this phenomenon for noninvasive control of biomolecular and cellular function. However, such confinement would represent a significant departure from the classical heat transfer theory. Here, we report an experimental investigation of nanoscale heat confinement on the surface of several types of iron oxide nanoparticles commonly used in biological research, using an all-optical method devoid of the potential artifacts present in previous studies. By simultaneously measuring the fluorescence of distinct thermochromic dyes attached to the particle surface or dissolved in the surrounding fluid during radio frequency magnetic stimulation, we found no measurable difference between the nanoparticle surface temperature and that of the surrounding fluid for three distinct nanoparticle types. Furthermore, the metalloprotein ferritin produced no temperature increase on the protein surface nor in the surrounding fluid. Experiments mimicking the designs of previous studies revealed potential sources of the artifacts. These findings inform the use of magnetic nanoparticle hyperthermia in engineered cellular and molecular systems
Dislocation strain as the mechanism of phonon scattering at grain boundaries
Thermal conductivities of polycrystalline thermoelectric materials are satisfactorily calculated by replacing the commonly used Casimir model (freqeuncy-independent) with grain boundary dislocation strain model (frequency-dependent) of Klemens. It is demonstrated that the grain boundaries are better described as a collection of dislocations rather than perfectly scattering interfaces
Two-Dimensional Axisymmetric Collapse of Thermally Unstable Primordial Clouds
We have performed two-dimensional hydrodynamic simulations of the collapse of
isolated axisymmetric clouds condensing via radiative cooling in a primordial
background gas. In order to study the development of the so-called
``shape-instability'', we have considered two types of axisymmetric clouds,
oblate and prolate clouds of various sizes and with axial ratios of . We find that the degree of oblateness or
prolateness is enhanced during the initial cooling phase. But it can be
reversed later, if the initial contrast in cooling times between the cloud gas
and the background gas is much greater than one. In such cases an oblate cloud
collapses to a structure composed of an outer thin disk and a central prolate
component. A prolate cloud, on the other hand, becomes a thin cigar-shape
structure with a central dense oblate component. The reversal of shape in the
central part of the cooled clouds is due to supersonic motions either along the
disk plane in the case of oblate clouds or along the symmetry axis in the case
of prolate clouds. For a background gas of K and n_h=0.1
\cm3 in a protogalactic halo environment, the mean density of the cloud gas
that has cooled to K increases to or so, in our simulations
where nonequilibrium cooling is adopted and the background gas cools too. The
spherical Jeans mass of such gas is estimated to be about M_J \sim
5\times10^{7}\Msun. In order for cloud mass to exceed the Jeans mass and at
the same time in order for the thermal instability to operate, the initial
cloud size should be around where is the
cooling length.Comment: 31 pages including 12 figures (reduced resolution), to appear in The
Astrophysical Journal (v584 n2 ApJ February 20, 2003 issue). Pdf with full
resolution figures can be downloaded from
ftp://canopus.chungnam.ac.kr/ryu/ryu.pd
Observation of First-Order Metal-Insulator Transition without Structural Phase Transition in VO_2
An abrupt first-order metal-insulator transition (MIT) without structural
phase transition is first observed by current-voltage measurements and
micro-Raman scattering experiments, when a DC electric field is applied to a
Mott insulator VO_2 based two-terminal device. An abrupt current jump is
measured at a critical electric field. The Raman-shift frequency and the
bandwidth of the most predominant Raman-active A_g mode, excited by the
electric field, do not change through the abrupt MIT, while, they, excited by
temperature, pronouncedly soften and damp (structural MIT), respectively. This
structural MIT is found to occur secondarily.Comment: 4 pages, 4 figure
High thermoelectric performance in (Bi_(0.25)Sb_(0.75)_2 Te_3 due to band convergence and improved by carrier concentration control
Bi_2Te_3 has been recognized as an important cooling material for thermoelectric applications. Yet its thermoelectric performance could still be improved. Here we propose a band engineering strategy by optimizing the converging valence bands of Bi_2Te_3 and Sb_2Te_3 in the (Bi_(1−x)Sb_x)_2Te_3 system when x = 0.75. Band convergence successfully explains the sharp increase in density-of-states effective mass yet relatively constant mobility and optical band gap measurement. This band convergence picture guides the carrier concentration tuning for optimum thermoelectric performance. To synthesize homogeneous textured and optimally doped (Bi0.25Sb0.75)2Te3, excess Te was chosen as the dopant. Uniform control of the optimized thermoelectric composition was achieved by zone-melting which utilizes separate solidus and liquidus compositions to obtain zT = 1.05 (at 300 K) without nanostructuring
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