4,961 research outputs found
Experimental investigation on synthesis, characterization, stability, thermo-physical properties and rheological behavior of MWCNTs-kapok seed oil based nanofluid
Several researchers devoted their efforts for the thermal conductivity enhancement of Carbon Nanotubes (CNTs) based nanofluids as CNTs have excellent thermal properties. However, limited research is reported on the detailed thermo-physical properties of CNTs and oil based nanofluids. In this work, the one-step method synthesis of a new MWCNTs-Kapok seed oil based nanofluid at constant nanoparticle concentration (0.1 wt./wt.) is reported. The nanofluid is characterized by FESEM, FTIR, visual stability analysis and thermophysical properties are experimentally measured. The viscosity found in the range of (0.049–10.101¿Pa·s), the thermal conductivity of (0.165–0.207¿W/m·K) and enhancement of thermal conductivity (6.1538%) were observed. Moreover, the viscosity decreases, and thermal conductivity increases with an increase in temperature. The experimentally obtained data are found in agreement with existing models and modified correlations. The rheological behavior showed that nanofluid is non-Newtonian in nature and exhibiting shear thinning or pseudo plastic behavior.Preprin
Spin Gating of Mesoscopic Devices
Inefficient screening of electric fields in nanoconductors makes electric
manipulation of electronic transport in nanodevices possible. Accordingly,
electrostatic (charge) gating is routinely used to affect and control the
Coulomb electrostatics and quantum interference in modern nanodevices. Besides
their charge, another (quantum mechanical) property of electrons - their spin -
is at the heart of modern spintronics, a term implying that a number of
magnetic and electrical properties of small systems are simultaneously
harvested for device applications. In this review the possibility to achieve
"spin gating" of mesoscopic devices, i.e. the possibility of an external spin
control of the electronic properties of nanodevices is discussed. Rather than
the Coulomb interaction, which is responsible for electric-charge gating, we
consider two other mechanisms for spin gating. These are on the one hand the
magnetic exchange interaction in magnetic devices and on the other hand the
spin-orbit coupling ("Rashba effect"), which is prominent in low dimensional
conductors. A number of different phenomena demonstrating the spin gating
phenomenon will be discussed, including the spintro-mechanics of magnetic
shuttling, Rashba spin splitting, and spin-gated weak superconductivity.Comment: Submitted to a special issue of "Synthetic Metals" to appear in March
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Thermal conductivity and rheology behavior of aqueous nanofluids containing alumina and carbon nanotubes
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this study, thermal conductivity and rheology behavior of aqueous alumina and multi-walled carbon nanotube (MWCNT) nanofluids were measured and compared with several analytical models. Both thermal conductivity and viscosity of the two nanofluids increase with increasing volume fraction. The experimental thermal conductivity data for the two nanofluids are located near the lower Hashin-Shtrikman bound and far away from the upper Hashin-Shtrikman bound. Therefore there is still enough room for thermal conductivity enhancement. Further conductivity enhancement of the nanofluids can be achieved by manipulating particle or agglomeration distribution and morphology. The structure-property relationship was checked for the nanofluids. Possible agglomeration size and interfacial thermal resistance were obtained and partially validated. Based on the Chen et al. model, a revised model was developed by incorporating the effects of interfacial thermal resistance into the Hamilton-Crosser model. The revised model can accurately reproduce the experimental data based on the agglomeration size extracted from the rheology analysis. In addition, thermal conductivity change of the alumina/water nanofluid with elapsed time was also investigated. The average thermal conductivity decreases with elapsed time. Besides, thermal conductivity measurements were conducted for nanofluid mixtures of alumina/water and MWCNT/water nanofluids
Symmetry-Adapted Phonon Analysis of Nanotubes
The characteristics of phonons, i.e. linearized normal modes of vibration,
provide important insights into many aspects of crystals, e.g. stability and
thermodynamics. In this paper, we use the Objective Structures framework to
make concrete analogies between crystalline phonons and normal modes of
vibration in non-crystalline but highly symmetric nanostructures. Our strategy
is to use an intermediate linear transformation from real-space to an
intermediate space in which the Hessian matrix of second derivatives is
block-circulant. The block-circulant nature of the Hessian enables us to then
follow the procedure to obtain phonons in crystals: namely, we use the Discrete
Fourier Transform from this intermediate space to obtain a block-diagonal
matrix that is readily diagonalizable. We formulate this for general Objective
Structures and then apply it to study carbon nanotubes of various chiralities
that are subjected to axial elongation and torsional deformation. We compare
the phonon spectra computed in the Objective Framework with spectra computed
for armchair and zigzag nanotubes. We also demonstrate the approach by
computing the Density of States. In addition to the computational efficiency
afforded by Objective Structures in providing the transformations to
almost-diagonalize the Hessian, the framework provides an important conceptual
simplification to interpret the phonon curves.Comment: To appear in J. Mech. Phys. Solid
Ground-state cooling of a carbon nanomechanical resonator by spin-polarized current
We study the nonequilibrium steady state of a mechanical resonator in the
quantum regime realized by a suspended carbon nanotube quantum dot contacted by
two ferromagnets. Because of the spin-orbit interaction and/or an external
magnetic field gradient, the spin on the dot couples directly to the flexural
eigenmodes. Accordingly, the nanomechanical motion induces inelastic spin flips
of the tunneling electrons. A spin-polarized current at finite bias voltage
causes either heating or active cooling of the mechanical modes. We show that
maximal cooling is achieved at resonant transport when the energy splitting
between two dot levels of opposite spin equals the vibrational frequency. Even
for weak electron-resonator coupling and moderate polarizations we can achieve
ground-state cooling with a temperature of the leads, for instance, of
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