摘要 :
The crucial parameter in the analysis of impact events is the impact velocity v_i. In case of inertial impactors v_i was assumed to be 85% of the average gas jet velocity, following the work of Marple. Numerical analysis of the im...
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The crucial parameter in the analysis of impact events is the impact velocity v_i. In case of inertial impactors v_i was assumed to be 85% of the average gas jet velocity, following the work of Marple. Numerical analysis of the impact process in low pressure impactors shows that this assumption is inappropriate and leads to overestimation of v_i,- near the inset of particle deposition, while, v_i is underestimated in the regime of high impact velocities. In this paper the whole process of nanoparticle acceleration and impact in low pressure impactors is investigated numerically. In order to assure correct numerical procedures, the employed methods are thoroughly validated by comparison with experimental results. Finally, a new analytical model for the calculation of v_i- on the basis of similarity theory is proposed that is independent of the impactor geometry and particle properties and holds well for the whole incompressible region. The model allows to perform defined collision experiments in low pressure impactors regarding impact velocity, without need of demanding numerical effort that is often beyond the scope of experimental studies. The model replaces the old rule of thumb and allows a quantitative re-evaluation of existing experimental data, e.g. on nanoparticle agglomerate fragmentation.
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A new method of investigating the probability of adhesion of nanoparticles upon impact on a surface in a Single Stage Low Pressure Impactor is presented in this article. It allows the determination of the critical velocity for nan...
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A new method of investigating the probability of adhesion of nanoparticles upon impact on a surface in a Single Stage Low Pressure Impactor is presented in this article. It allows the determination of the critical velocity for nanoparticle rebound, v_(cr), for the first time. First measurements with dense particles of sodium chloride and silver in the size-range between 10 nm and 100 nm are presented. Independently of the material, two regimes are identified. For particles larger than a critical size of 40 nm in case of silver and 20 nm in case of NaCl, the determined values of v_(cr) are small, typically below 0.5 m/s, indicating low energy dissipation during collision and mainly elastic contact. For particles smaller than the respective critical size, v_(cr) increases sharply with decreasing particle size, implying a transition to plastic deformation. The dependence of v_(cr) on the mode of deformation is shown for structurally weakened porous NaCl particles. The values of v_(cr) in the plastic regime are in comparable magnitude to micron-size particles.
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Impact fragmentation can be used to disperse nanoparticle-agglomerates. While the fragmentation of openly structured (fractal dimension Df?展开
Impact fragmentation can be used to disperse nanoparticle-agglomerates. While the fragmentation of openly structured (fractal dimension Df?收起
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The synthesis and some physical properties of a new quasi-one-dimensional tetracyanidoplatinate, Cs_4[Pt(CN)_4](CF_3SO_3)_2 (CsCP(OTf)) are reported and described in comparison to the well-known K_2[Pt (CN)_4]Br_(0.30)·3.2H_2O (K...
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The synthesis and some physical properties of a new quasi-one-dimensional tetracyanidoplatinate, Cs_4[Pt(CN)_4](CF_3SO_3)_2 (CsCP(OTf)) are reported and described in comparison to the well-known K_2[Pt (CN)_4]Br_(0.30)·3.2H_2O (KCP). Single-crystal X-ray diffraction reveals Pt-Pt spacings to be greater than those of KCP by 5% longitudinal and 38% transverse, but much shorter than comparable spacings in other non-partially oxidized platinates. Anomalies are observed between temperatures 100 K and 200 K: (1) Longitudinal DC conductivity is two orders of magnitude higher and is non-monotonic with temperature, showing a minimum at around 170 K. (2) Nuclear magnetic resonance (NMR) longitudinal relaxation time T_1 is at least three orders of magnitude higher than that of KCP, and is also non-monotonic with temperature, showing a sharp peak at around 120 K. Since X-ray diffraction reveals no structural transition at 120 K, these suggest a possible lattice freezing or stiffening at around 120 K.
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Based on direct measurements of the critical velocity for nanoparticle rebound, v_(cr), the possibility of thermal rebound is revisited. The thermal velocity, v_(therm), for Ag and NaCl particles in the size range between 10 nm an...
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Based on direct measurements of the critical velocity for nanoparticle rebound, v_(cr), the possibility of thermal rebound is revisited. The thermal velocity, v_(therm), for Ag and NaCl particles in the size range between 10 nm and 100 nm is calculated and compared to the respective value of v_(cr). Thermal rebound is impossible for dense spherical silver particles and porous NaCl particles. For dense NaCl particles v_(therm) exceeds the measured values for v_(cr) for particles between 20 nm and 60 nm in diameter, proving the possibility of thermal rebound in a size-range much larger than expected. This result is in disagreement with the work of other researchers that did not observe thermal rebound in this size-range for NaCl particles. As these studies were performed under ambient pressure, while the direct measurement of v_(cr) was performed at reduced pressure around 10 mbar, thermal rebound must be pressure dependent for nanoparticles. This conclusion is confirmed with a simple model calculation, showing that at ambient pressure gas friction contributes significantly to the particle energy loss before the particle leaves a surface completely. This necessitates a redefinition of the term 'critical velocity'.
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Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional s-wav...
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Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional s-wave superconductors put hard constraints on these experiments. Significantly larger gaps and higher transition temperatures in cuprate superconductors might be an attractive alternative to considerably relax these constraints, but it is not clear whether the proximity effect would be effective in heterostructures involving cuprates and topological insulators. Here, we present angle-resolved photoemission studies of thin Bi_2Se_3 films grown in situ on optimally doped Bi_2Sr_2CaCu_2O_(8+δ) substrates that show the absence of proximity-induced gaps on the surfaces of Bi_2Se_3 films as thin as a 1.5 quintuple layer. These results suggest that the superconducting proximity effect between a cuprate superconductor and a topological insulator is strongly suppressed, likely due to a very short coherence length along the c axis, incompatible crystal and pairing symmetries at the interface, small size of the topological surface state's Fermi surface, and adverse effects of a strong spin-orbit coupling in the topological material.
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