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<jats:p>In recent years, there is a strong interest in thermal cloaking at the nanoscale, which has been achieved by using graphene and crystalline silicon films to build the nanoscale thermal cloak according to the classical macroscopic thermal cloak model. Silicon carbide, as a representative of the third-generation semiconductor material, has splendid properties, such as the high thermal conductivity and the high wear resistance. Therefore, in the present study, we build a nanoscale thermal cloak based on silicon carbide. The cloaking performance and the perturbation of the functional area to the external temperature filed are analyzed by the ratio of thermal cloaking and the response temperature, respectively. It is demonstrated that silicon carbide can also be used to build the nanoscale thermal cloak. Besides, we explore the influence of inner and outer radius on cloaking performance. Finally, the potential mechanism of the designed nanoscale thermal cloak is investigated by calculating and analyzing the phonon density of states (PDOS) and mode participation rate (MPR) within the structure. We find that the main reason for the decrease in the thermal conductivity of the functional area is phonon localization. This study extends the preparation method of nanoscale thermal cloaks and can provide a reference for the development of other nanoscale devices.</jats:p>

<jats:p>The characteristic clogging structures of granular spheres blocking three-dimensional granular flow through hopper outlet are analyzed based on packing structures reconstructed using magnetic resonance imaging techniques. Spheres in clogging structures are arranged in a way with typical features of load-bearing, such as more contacting bonds close to the horizontal plane and more mutually-stabilized contact configurations than packing structures away from the orifice. The requirement of load-bearing inevitably leads to the cooperativity of clogging structures with a correlation length of several particle diameters. This correlation length being comparable with the orifice diameter suggests that a clogging structure is composed of several mutually-stabilized structural motifs to span the orifice perimeter, instead of a collection of independent individual spheres to cover the whole orifice area. Accordingly, we propose a simple geometric model to explain the unexpected linear dependence of the average size of three-dimensional clogging structures on orifice diameter.</jats:p>

<jats:p>We study the characteristics of temperature fluctuation in two-dimensional turbulent Rayleigh–Bénard convection in a square cavity by direct numerical simulations. The Rayleigh number range is 1 × 10<jats:sup>8</jats:sup> ≤ <jats:italic>Ra</jats:italic> ≤ 1 × 10<jats:sup>13</jats:sup>, and the Prandtl number is selected as <jats:italic>Pr</jats:italic> = 0.7 and <jats:italic>Pr</jats:italic> = 4.3. It is found that the temperature fluctuation profiles with respect to <jats:italic>Ra</jats:italic> exhibit two different distribution patterns. In the thermal boundary layer, the normalized fluctuation <jats:italic>θ</jats:italic> <jats:sub>rms</jats:sub>/<jats:italic>θ</jats:italic> <jats:sub>rms,max</jats:sub> is independent of <jats:italic>Ra</jats:italic> and a power law relation is identified, <jats:italic>i.e.</jats:italic>, <jats:italic>θ</jats:italic> <jats:sub>rms</jats:sub>/<jats:italic>θ</jats:italic> <jats:sub>rms,max</jats:sub>∼ (<jats:italic>z</jats:italic> / <jats:italic>δ</jats:italic>)<jats:sup>0.99 ± 0.01</jats:sup>, where <jats:italic>z</jats:italic> / <jats:italic>δ</jats:italic> is a dimensionless distance to the boundary (<jats:italic>δ</jats:italic> is the thickness of thermal boundary layer). Out of the boundary layer, when <jats:italic>Ra</jats:italic> ≤ 5 × 10<jats:sup>9</jats:sup>, the profiles of <jats:italic>θ</jats:italic> <jats:sub>rms</jats:sub>/<jats:italic>θ</jats:italic> <jats:sub>rms,max</jats:sub> descend, then ascend, and finally drop dramatically as <jats:italic>z</jats:italic>/<jats:italic>δ</jats:italic> increases. While for <jats:italic>Ra</jats:italic> ≥ 1 × 10<jats:sup>10</jats:sup>, the profiles continuously decrease and finally overlap with each other. The two different characteristics of temperature fluctuations are closely related to the formation of stable large-scale circulations and corner rolls. Besides, there is a critical value of <jats:italic>Ra</jats:italic> indicating the transition, beyond which the fluctuation 〈 <jats:italic>θ</jats:italic> <jats:sub>rms</jats:sub>〉<jats:sub> <jats:italic>V</jats:italic> </jats:sub> has a power law dependence on <jats:italic>Ra</jats:italic>, given by 〈 <jats:italic>θ</jats:italic> <jats:sub>rms</jats:sub>〉<jats:sub> <jats:italic>V</jats:italic> </jats:sub> ∼ <jats:italic>Ra</jats:italic> <jats:sup>−0.14 ± 0.01</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Pipe-like confinements are ubiquitously encountered by microswimmers. Here we systematically study the ratio of the speeds of a force- and torque-free microswimmer swimming in the center of a cylindrical pipe to its speed in an unbounded fluid (speed ratio). Inspired by <jats:italic>E. coli</jats:italic>, the model swimmer consists of a cylindrical head and a double-helical tail connected to the head by a rotating virtual motor. The numerical simulation shows that depending on swimmer geometry, confinements can enhance or hinder the swimming speed, which is verified by Reynolds number matched experiments. We further developed a reduced model. The model shows that the swimmer with a moderately long, slender head and a moderately long tail experiences the greatest speed enhancement, whereas the theoretical speed ratio has no upper limit. The properties of the virtual motor also affect the speed ratio, namely, the constant-frequency motor generates a greater speed ratio compared to the constant-torque motor.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Instantaneous and precise velocity sensing is a critical part of research on detonation mechanism and flow evolution. This paper presents a novel multi-projection tunable diode laser absorption spectroscopy solution, to provide a real-time and reliable measurement of velocity distribution in detonation exhaust flow with obvious nonuniformity. Relations are established between overlapped spectrums along probing beams and Gauss velocity distribution phantom according to the frequency shifts and tiny variations in components of light-of-sight absorbance profiles at low frequencies analyzed by the fast Fourier transform. With simulated optical measurement using H<jats:sub>2</jats:sub>O feature at 7185.6 cm<jats:sup>−1</jats:sup> carried out on a phantom generated using a simulation of two-phase detonation by a two-fluid model, this method demonstrates a satisfying performance on recovery of velocity distribution profiles in supersonic flow even with a noise equivalent absorbance up to 2 × 10<jats:sup>−3</jats:sup>. This method is applied to the analysis of rapidly decreasing velocity during a complete working cycle in the external flow field of an air-gasoline detonation tube operating at 25 Hz, and results show the velocity in the core flow field would be much larger than the arithmetic average from traditional tunable diode laser doppler velocimetry. This proposed velocity distribution sensor would reconstruct nonuniform velocity distribution of high-speed flow in low cost and simple operations, which broadens the possibility for applications in research on the formation and propagation of external flow filed of detonation tube.</jats:p>

<jats:p>Currently, wire bonding is the most popular first-level interconnection technology used between the die and package terminals, but even with its long-term and excessive usage, the mechanism of wire bonding has not been completely evaluated. Therefore, fundamental research is still needed. In this study, the mechanism of microweld formation and breakage during Cu–Cu wire bonding was investigated by using molecular dynamics simulation. The contact model for the nanoindentation process between the wire and substrate was developed to simulate the contact process of the Cu wire and Cu substrate. Elastic contact and plastic instability were investigated through the loading and unloading processes. Moreover,the evolution of the indentation morphology and distributions of the atomic stress were also investigated. It was shown that the loading and unloading curves do not coincide, and the unloading curve exhibited hysteresis. For the substrate, in the loading process, the main force changed from attractive to repulsive. The maximum von Mises stress increased and shifted from the center toward the edge of the contact area. During the unloading process, the main force changed from repulsive to attractive. The Mises stress reduced first and then increased. Stress concentration occurs around dislocations inthe middle area of the Cu wire.</jats:p>

<jats:p>Most amorphous carbon (a-C) applications require films with ultra-thin thicknesses; however, the electronic structure and opto-electronic characteristics of such films remain unclear so far. To address this issue, we developed a theoretical model based on the density functional theory and molecular dynamic simulations, in order to calculate the electronic structure and opto-electronic characteristics of the ultra-thin a-C films at different densities and temperatures. Temperature was found to have a weak influence over the resulting electronic structure and opto-electronic characteristics, whereas density had a significant influence on these aspects. The volume fraction of sp<jats:sup>3</jats:sup> bonding increased with density, whereas that of sp<jats:sup>2</jats:sup> bonding initially increased, reached a peak value of 2.52 g/cm<jats:sup>3</jats:sup>, and then decreased rapidly. Moreover, the extinction coefficients of the ultra-thin a-C films were found to be density-sensitive in the long-wavelength regime. This implies that switching the volume ratio of sp<jats:sup>2</jats:sup> to sp<jats:sup>3</jats:sup> bonding can effectively alter the transmittances of ultra-thin a-C films, and this can serve as a novel approach toward photonic memory applications. Nevertheless, the electrical resistivity of the ultra-thin a-C films appeared independent of temperature. This implicitly indicates that the electrical switching behavior of a-C films previously utilized for non-volatile storage applications is likely due to an electrically induced effect and not a purely thermal consequence.</jats:p>

<jats:p>Structural stability in terms of the decomposition temperature in LiMn<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> was systematically investigated by a series of high-temperature and high-pressure experiments. LiMn<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> was found to have structural stability up to 5 GPa at room temperature. Under ambient pressure, the compound decomposed at 1300 °C. The decomposition temperature decreased with increasing pressure, yielding more complex decomposed products, Below the decomposition temperature, the crystal structure of LiMn<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> varied with pressure. The presented results in this study offer new insights into the thermal and pressure stability of LiMn<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> materials as a cathode for lithium-ion batteries that can operate under extreme conditions. Therefore, these findings may serve as a useful guide for future work for improving lithium-ion batteries.</jats:p>

<jats:p>Heavy elements (<jats:italic>X</jats:italic> = Ta/W/Re) play an important role in the performance of superalloys, which enhance the strength, anti-oxidation, creep resistance, and anti-corrosiveness of alloy materials in a high-temperature environment. In the present research, the heavy element doping effects in FCC-Ni(<jats:italic>γ</jats:italic>) and Ni<jats:sub>3</jats:sub>Al(<jats:italic>γ</jats:italic>′) systems are investigated in terms of their thermodynamic and mechanical properties, as well as electronic structures. The lattice constant, bulk modulus, elastic constant, and dopant formation energy in non-spin, spin polarized, and spin–orbit coupling (SOC) calculations are compared. The results show that the SOC effects are important in accurate electronic structure calculations for alloys with heavy elements. We find that including spin for both <jats:italic>γ</jats:italic> and <jats:italic>γ</jats:italic>′ phases is necessary and sufficient for most cases, but the dopant formation energy is sensitive to different spin effects, for instance, in the absence of SOC, even spin-polarized calculations give 1% to 9% variance in the dopant formation energy in our model. Electronic structures calculations indicate that spin polarization causes a split in the metal d states, and SOC introduces a variance in the spin-up and spin-down states of the d states of heavy metals and reduces the magnetic moment of the system.</jats:p>

<jats:p>Divalent metal clusters have received great attention due to the interesting size-induced nonmetal-to-metal transition and fascinating properties dependent on cluster size, shape, and doping. In this work, the combination of the CALYPSO code and density functional theory (DFT) optimization is employed to explore the structural properties of neutral and anionic Mg<jats:sub> <jats:italic>n</jats:italic> + 1</jats:sub> and SrMg<jats:sub> <jats:italic>n</jats:italic> </jats:sub> (<jats:italic>n</jats:italic> = 2–12) clusters. The results exhibit that as the atomic number of Mg increases, Sr atoms are more likely to replace Mg atoms located in the skeleton convex cap. By analyzing the binding energy, second-order energy difference and the charge transfer, it can be found the SrMg<jats:sub>9</jats:sub> cluster with tower framework presents outstanding stability in a studied size range. Further, bonding characteristic analysis reveals that the stability of SrMg<jats:sub>9</jats:sub> can be improved due to the strong s–p interaction among the atomic orbitals of Sr and Mg atoms.</jats:p>