2,377 research outputs found
Investigation of the shape transferability of nanoscale multi-tip diamond tools in the diamond turning of nanostructures
In this article, the shape transferability of using nanoscale multi-tip diamond tools in the diamond turning for scale-up manufacturing of nanostructures has been demonstrated. Atomistic multi-tip diamond tool models were built with different tool geometries in terms of the difference in the tip cross-sectional shape, tip angle, and the feature of tool tip configuration, to determine their effect on the applied forces and the machined nano-groove geometries. The quality of machined nanostructures was characterized by the thickness of the deformed layers and the dimensional accuracy achieved. Simulation results show that diamond turning using nanoscale multi-tip tools offers tremendous shape transferability in machining nanostructures. Both periodic and non-periodic nano-grooves with different cross-sectional shapes can be successfully fabricated using the multi-tip tools. A hypothesis of minimum designed ratio of tool tip distance to tip base width (L/Wf) of the nanoscale multi-tip diamond tool for the high precision machining of nanostructures was proposed based on the analytical study of the quality of the nanostructures fabricated using different types of the multi-tip tools. Nanometric cutting trials using nanoscale multi-tip diamond tools (different in L/Wf) fabricated by focused ion beam (FIB) were then conducted to verify the hypothesis. The investigations done in this work imply the potential of using the nanoscale multi-tip diamond tool for the deterministic fabrication of period and non-periodic nanostructures, which opens up the feasibility of using the process as a versatile manufacturing technique in nanotechnology
Variation of Critical Crystallization Pressure for the Formation of Square Ice in Graphene Nanocapillaries
Two-dimensional square ice in graphene nanocapillaries at room temperature is
a fascinating phenomenon and has been confirmed experimentally. Instead of
temperature for bulk ice, the high van der Waals pressure becomes an
all-important factor to induce the formation of square ice and needs to be
studied further. By all-atom molecular dynamics simulations of water confined
between two parallel graphene sheets, which are changed in size (the length and
the width of the graphene sheets) over a wide range, we find that the critical
crystallization pressure for the formation of square ice in nanocapillary
strongly depends on the size of the graphene sheet. The critical
crystallization pressure slowly decreases as the graphene size increases,
converging to approximately macroscopic crystallization pressure. The
unfreezable threshold for graphene size is obtained by estimating the actual
pressure and it is difficult to form the square ice spontaneously in practice
when the graphene sheet is smaller than the threshold. Moreover, the critical
crystallization pressure fluctuates when the graphene size is minuscule, and
the range of oscillation narrows as the sheet size increases, converging to the
macroscopic behavior of a single critical icing pressure for large sheets. The
graphene size also affects the stability and crystallization time of square
ice.Comment: 11 pages, 6 figure
From Rayleigh-B\'enard convection to porous-media convection: how porosity affects heat transfer and flow structure
We perform a numerical study of the heat transfer and flow structure of
Rayleigh-B\'enard (RB) convection in (in most cases regular) porous media,
which are comprised of circular, solid obstacles located on a square lattice.
This study is focused on the role of porosity in the flow properties
during the transition process from the traditional RB convection with
(so no obstacles included) to Darcy-type porous-media convection with
approaching 0. Simulations are carried out in a cell with unity aspect ratio,
for the Rayleigh number from to and varying porosities
, at a fixed Prandtl number , and we restrict ourselves to the
two dimensional case. For fixed , the Nusselt number is found to vary
non-monotonously as a function of ; namely, with decreasing , it
first increases, before it decreases for approaching 0. The
non-monotonous behaviour of originates from two competing effects of
the porous structure on the heat transfer. On the one hand, the flow coherence
is enhanced in the porous media, which is beneficial for the heat transfer. On
the other hand, the convection is slowed down by the enhanced resistance due to
the porous structure, leading to heat transfer reduction. For fixed ,
depending on , two different heat transfer regimes are identified, with
different effective power-law behaviours of vs , namely, a steep one
for low when viscosity dominates, and the standard classical one for large
. The scaling crossover occurs when the thermal boundary layer thickness
and the pore scale are comparable. The influences of the porous structure on
the temperature and velocity fluctuations, convective heat flux, and energy
dissipation rates are analysed, further demonstrating the competing effects of
the porous structure to enhance or reduce the heat transfer
Diagnosis and treatment of pericallosal artery aneurysms
Objective
Pericallosal artery aneurysms are not common clinically. The microsurgery and endovascular therapy are surgically challenging operations. The objective of the study is to summarize their clinical symptoms and optimal treatment strategies of pericallosal artery aneurysms.
Methods
Nine cases of pericallosal artery aneurysms detected by digital subtraction angiography (DSA) were reviewed. The clinical manifestation, brain imaging characteristics, and optimal treatment methods were summarized.
Results
Patients with spontaneous aneurysm had good clinical outcomes after endovascular coiling or microsurgical clipping treatment. There were no any neurological function deficits in five patients. One patient suffered from permanent neurological function deficits. Patients with traumatic aneurysm pericallosal had relatively poor outcomes, including two patients showing disturbed consciousness and the paralysis of the lower limbs with slow recovery, and one patient was dead after the surgery.
Conclusion
Spontaneous subarachnoid hemorrhage and interhemispheric fissure hematoma suggest spontaneously pericallosal aneurysm, while traumatic corpus callosum hematoma as well the accompanying embryo of intraventricular hemorrhage suggest traumatic pericallosal aneurysm. Endovascular embolization is the primary surgical treatment for pericallosal aneurysm, while patients with pericallosal aneurysm are not suitable for surgical treatment. Microsurgical clipping treatment may be a choice. However, both of these treatment strategies have high risk
Prediction of Van Hove singularity systems in ternary borides
A computational search for stable structures among both and
phases of ternary ATB4 borides (A= Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or
4d transition elements) has been performed. We found that -ATB4
compounds with A=Mg, Ca, Al, and T=V, Cr, Mn, Fe, Ni, and Co form a family of
structurally stable or almost stable materials. These systems are metallic in
non-magnetic states and characterized by the formation of the localized
molecular-like state of 3d transition metal atom dimers, which leads to the
appearance of numerous Van Hove singularities (VHS) in the electronic spectrum.
The closeness of these VHS to the Fermi level can be easily tuned by electron
doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these VHS
led to magnetic instabilities and new magnetic ground states with a weakly
metallic or semiconducting nature. The magnetic ground states in these systems
appear as an analog of the spin glass state. Experimental attempts to produce
MgFeB4 and associated challenges are discussed, and promising directions for
further synthetic studies are formulated.Comment: 9 figure
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Revealing of the Activation Pathway and Cathode Electrolyte Interphase Evolution of Li-Rich 0.5Li2MnO3·0.5LiNi0.3Co0.3Mn0.4O2 Cathode by in Situ Electrochemical Quartz Crystal Microbalance.
The first-cycle behavior of layered Li-rich oxides, including Li2MnO3 activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li2MnO3 activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi2MnO3·(1- x) LiNi0.3Co0.3Mn0.4O2 ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structures can significantly affect the activation pathway of Li1.2Ni0.12Co0.12Mn0.56O2, leading to an extra-high capacity. It is demonstrated that Li2MnO3 activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li2MnO3 governed by chemical decomposition (Li2O evolution). CEI evolution is closely related to Li+ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries
Mechanical performance of 22SiMn2TiB steel welded with low-transformation-temperature filler wire and stainless steel filler wire
TX-80 low-transformation-temperature (LTT) welding wire was used to replace the traditional ER 307Si welding wire to realize the connection of 22SiMn2TiB armor steel in manual overlay welding. The previously existing issues, such as welding cracks, large welding deformation, and severe welding residual stress, were solved to ensure good strength and ductility requirements. In particular, with the same welding conditions, TX-80 LTT wire eliminates welding cracks. It reduces the welding deformation no matter the base pretreatment of pre-setting angle or no pre-setting angle. By comparison, it was found that the microstructure at the TX-80 weld is mainly composed of martensite and a small amount of retained austenite. In contrast, the microstructure of the ER 307Si weld consists of a large amount of austenite and a small amount of skeleton-like ferrite. The variation trend of residual stress and microhardness from the weld to the base were investigated and compared with the mechanical properties of base materials. The TX-80 and the ER 307Si tensile samples elongation is 6.76% and 6.01%, while the ultimate tensile strengths are 877 and 667Â MPa, respectively. The average impact toughness at room temperature of the ER 307Si weld is 143.9Â J/cm2, much higher than that of the TX-80 weld, which is only 36.7Â J/cm2. The relationship between impact and tensile properties with microstructure species and distribution was established. In addition, the fracture surface of the tensile and the impact samples was observed and analyzed. Deeper dimples, fewer pores, larger radiation zone, and shear lips of TX-80 samples indicate better tensile ductility and worse impact toughness than those of ER 307Si weld.</p
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