9 research outputs found
Corrected Navier-Stokes equations for compressible flows
For gas flows, the Navier-Stokes (NS) equations are established by
mathematically expressing conservations of mass, momentum and energy. The
advantage of the NS equations over the Euler equations is that the NS equations
have taken into account the viscous stress caused by the thermal motion of
molecules. The viscous stress arises from applying Isaac Newton's second law to
fluid motion, together with the assumption that the stress is proportional to
the gradient of velocity1. Thus, the assumption is the only empirical element
in the NS equations, and this is actually the reason why the NS equations
perform poorly under special circumstances. For example, the NS equations
cannot describe rarefied gas flows and shock structure. This work proposed a
correction to the NS equations with an argument that the viscous stress is
proportional to the gradient of momentum when the flow is under compression,
with zero additional empirical parameters. For the first time, the NS equations
have been capable of accurately solving shock structure and rarefied gas flows.
In addition, even for perfect gas, the accuracy of the prediction of heat flux
rate is greatly improved. The corrected NS equations can readily be used to
improve the accuracy in the computation of flows with density variations which
is common in nature.Comment: 13 pages, 7 figure
Lipid droplets: a classic organelle with new outfits
Lipid droplets are depots of neutral lipids that exist virtually in any kind of cell. Recent studies have revealed that the lipid droplet is not a mere lipid blob, but a major contributor not only to lipid homeostasis but also to diverse cellular functions. Because of the unique structure as well as the functional importance in relation to obesity, steatosis, and other prevailing diseases, the lipid droplet is now reborn as a brand new organelle, attracting interests from researchers of many disciplines
Organics- and Surfactant-Free Molten Salt Medium Controlled Synthesis of Pt‑M (M = Cu and Pd) Bi- and Trimetallic Nanocubes and Nanosheets
We
present a novel synthetic strategy for the shape-controlled
synthesis of Pt-based alloy nanoparticles (NPs) in inorganic molten
salt without using any organic surfactants or capping agents. Graphene
oxide (GO) was chosen as the stabilizer in the inorganic molten salt
synthetic strategy, due to the existence of various oxygen-containing
functional groups on its surface, which adsorb, anchor, and stabilize
the metal ions or NPs. After GO was added in molten salt, the PtPd
nanosheets were formed on its surface in H<sub>2</sub> atmosphere.
In addition, when KI was chosen as the shape-inducing agent to selectively
adsorb on and fully protect the (100) facets of alloys, PtPd nanocubes
with core–shell structure, PtCu nanohemicubes, and PtPdCu nanocubes
were prepared successfully on the surface of GO in molten salt. Introduction
of GO as the stabilizer in molten salt proves a new approach in synthesis
of Pt-based nanocrystals with controlled morphologies
Synergistic Effect Induced High Photothermal Performance of Au Nanorod@Cu<sub>7</sub>S<sub>4</sub> Yolk–Shell Nanooctahedron Particles
Au nanorod (NR) which has strong
LSPR (longitudinal surface plasmon
resonance) effect in near-infrared (NIR) region was introduced into
the Cu<sub>7</sub>S<sub>4</sub> hollow NPs to form Au NR@Cu<sub>7</sub>S<sub>4</sub> yolk–shell structured nanoparticles (YSNPs)
for improving the photothermal property of NPs. The optimum photothermal
conversion efficiency of the as-prepared YSNPs is 68.6%. The hybrid
YSNPs had the highest photothermal property compared with the equivalent
used Au NR and pure Cu<sub>7</sub>S<sub>4</sub> because of the synergistic
effect of metal and semiconductor. In this case, the synergistic effect
in YSNPs was discussed by tuning sizes of the YSNPs and the thickness
of Cu<sub>7</sub>S<sub>4</sub> shell. The experimental results demonstrated
that the NIR photoabsorption and the photothermal conversion performance
of Au NR@Cu<sub>7</sub>S<sub>4</sub> YSNPs were much dependent on
the geometric change of YSNPs, since the electrical field interaction
between inner Au NR core and outer Cu<sub>7</sub>S<sub>4</sub> shell
is closely effected by the distance of two materials and thickness
of out-shell, as confirmed by the 3D finite-difference time domain
simulation (FDTD) theory simulation. Moreover, we proved that the
hollow yolk–shell structure of the YSNPs also endowed the NPs
with a large potential in drug delivery
Structural and Electronic Stabilization of PtNi Concave Octahedral Nanoparticles by P Doping for Oxygen Reduction Reaction in Alkaline Electrolytes
The
enhancement in the catalytic activity of PtM (transition metals,
TMs) alloy nanoparticles (NPs) results from the electronic structure
of Pt being modified by the TM. However, the oxidation of the TM would
lead to the electronegativity difference between Pt and TM being much
lowered, which induces a decrease in the number of electrons transferred
from the TM to Pt, resulting in excessive oxygenated species accumulating
on the surface of Pt, thus deteriorating their performance. In this
work, the oxygen reduction reaction (ORR) performance of PtNi (Pt<sub>68</sub>Ni<sub>32</sub>) concave octahedral NPs (CONPs) in alkaline
electrolytes is much improved by doping small amounts of phosphorus.
The P-doped PtNi CONPs (P-PtNi) show about 2 and 10 times enhancement
for ORR compared to PtNi and commercial Pt/C catalysts. The high-angle
annular dark-field scanning transmission electron microscopy and energy-dispersive
X-ray spectroscopy mapping characterizations reveal that the P dopant
uniformly distributes throughout the CONPs, Pt mainly locates at the
edges and corners, whereas Ni situates at the center, forming a P-doped
Pt-frame@Ni quasi-core–shell CONP. The X-ray photoelectron
spectroscopy spectra indicate that the P dopant obviously increases
the electron density of Pt compared with that of PtNi NPs, which contributes
to the stabilization of the electronic structure of PtNi CONPs, thus
restraining the excessive HO<sub>2</sub><sup>–</sup> species
produced on the catalysts, which endow them with a high catalytic
performance in the ORR. In addition, the P attached to the Ni sites
in the PtNi NPs partially prevents the Ni atoms being oxidized by
the external O species, which is conducive to the structural and electrochemical
stability of the PtNi NPs during the ORR. The present results provide
a new insight into the development of ORR catalysts with low utilization
of Pt