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Pouring position as the input heat source has great influence on the temperature field evolution. In this study, the Flow3D simulation software was applied to investigate the influence of pouring methods (with fixed or moving pouring channel) on AZ91 Mg alloy horizontal centrifugal casting (HCC) process. The simulation results show that the moving pouring channel method can effectively increase the cooling rate and formability of casting pipe. The casting experiment shows that an AZ91 Mg alloy casting pipe with homogeneous microstructure and clear contour was obtained by the moving pouring channel method, and the grain size of the casting pipe is significantly decreased. Meanwhile, serious macro-segregation appeared in the AZ91 casting pipe by the fixed pouring channel HCC process. Compared with the fixed pouring channel, the moving pouring channel can remarkably improve the ultimate tensile strength and elongation of the AZ91 HCC pipe from 142.2 MPa to 201.5 MPa and 6.2% to 6.7%, respectively . [ABSTRACT FROM AUTHOR]

We propose a multi-layer structure for concealing an electromagnetic sensing system (a sensor is wrapped with a transparent protective layer), using single-negative (SNG) materials whose material parameters are completely independent of those of the host matrix as well as the concealed system. The numerical results show that only three different kinds of SNG materials are sufficient to yield the cloaking effect even in the presence of weak loss. This may significantly facilitate the experimental realization of a well-performing sensor-cloaking device. [ABSTRACT FROM AUTHOR]

An improved and highly efficient synthetic approach to multibenzoylated nucleosides bearing free 5'-hydroxyl groups is described here. By employing t-butyldimethylsilyl (TBDMS) rather than the more commonly used dimethoxytrityl (DMTr) as a temporary 5'-OH protecting group of the starting nucleoside, this methodology provides the expected products in nearly quantitative yields, thereby substantially reducing the cost and effort of synthesis. [ABSTRACT FROM AUTHOR]

Phase modulation has scarcely been mentioned in diffusive physical systems because the diffusion process does not carry the momentum like waves. Recently, non-Hermitian physics provides a unique perspective for understanding diffusion and shows prospects in thermal phase regulation, exemplified by the discovery of anti-parity-time (APT) symmetry in diffusive systems. However, precise control of thermal phase remains elusive hitherto and can hardly be realized, due to the phase oscillations. Here we construct the PT-symmetric diffusive systems to achieve the complete suppression of thermal phase oscillation. The real coupling of diffusive fields is readily established through a strong convective background, and the decay-rate detuning is enabled by thermal metamaterial design. We observe the phase transition of PT symmetry breaking with the symmetry-determined amplitude and phase regulation of coupled temperature fields. Our work shows the existence of PT symmetry in dissipative energy exchanges and provides unique approaches for harnessing the mass transfer of particles, wave dynamics in strongly scattering systems, and thermal conduction. [ABSTRACT FROM AUTHOR]

The article describes the syntheses of stable tetravalent phosphonium enolate zwitterions through simple one-pot, three-component coupling reactions between tertiary phosphines, alkynoates and aldehydes. The isolation and X-ray crystallographic characterization of dipolar intermediates of zwitterions, establishing the tetravalent nature of their phosphorus atoms are also described. The structures of zwitterions might explain their instability and high reactivity in the reactions.

Focuses on the discovery of a 2-methylene-but-3-enoate synthon and its phosphine-catalyzed [4 + 2] annulation with aldimines. Provision of substituted tetrahydropyridine derivatives; Presence of functionalized tetrahydropyridines and piperidines in biologically active natural products; Reactivity and regioselectivity of the Diels-Alder reaction.

We theoretically and experimentally demonstrate the unidirectional negative refraction in an acoustic metafluid prism. The acoustic metafluid is designed to be uniaxial and highly anisotropic in inertia. Since the directions of phase and group velocities do not coincide in the anisotropic medium, it is possible to realize the unidirectional negative refraction by the broken symmetry of phase velocity vectors with respect to the acoustic axis. Being a distinct case of the asymmetric sound propagation, the unidirectional negative refracting effect could be potentially significant in various areas such as underwater sensing, and medical ultrasonics. [ABSTRACT FROM AUTHOR]

The article focuses on efficient nonreciprocal mode transitions in spatiotemporally modulated acoustic metamaterials. Topics include the mechanism relies on the coupling between an ultrathin membrane and external biasing electromagnetic fields, realizing programmable dynamic control of the acoustic impedance over a motionless and noiseless platform, and the fast and flexible impedance modulation of our metamaterial imparts an effective unidirectional momentum in space-time.

The study for exotic topological effects of sound has attracted uprising interests in fundamental physics and practical applications. Based on the concept of valley pseudospin, we demonstrate the topological valley transport of plate-mode waves in a homogenous thin plate with periodic stubbed surface, where a deterministic two-fold Dirac degeneracy is form by two plate modes. We show that the topological property can be controlled by the height of stubs deposited on the plate. By adjusting the relative heights of adjacent stubs, the valley vortex chirality and band inversion are induced, giving rise to a phononic analog of valley Hall phase transition. We further numerically demonstrate the valley states of plate-mode waves with robust topological protection. Our results provide a new route to design unconventional elastic topological insulators and will significantly broaden its practical application in the engineering field. [ABSTRACT FROM AUTHOR]

Anomalous Floquet topological insulators (AFIs) for airborne sound have recently been realized in experiments. However, the implemented version suffers from significant loss and narrowband due to thermal viscosity and dispersive coupling strength between unit-cells. Here, we propose a solution for realizing low-loss and broadband acoustic AFI. We show that the loss after passing through one unit-cell can be less than 2% for the topological edge states. It is also theoretically unveiled that in the frequency range of nearly unitary coupling (~0.97 from 4.8 kHz to 7.0 kHz in our case), around 84% corresponds to topological bands. Our proposal may promote the application of large-dimension acoustic topological devices. [ABSTRACT FROM AUTHOR]

We have constructed an interface which separates two different phononic crystals (PCs) with respectively effective negative density and negative bulk modulus through band inversion. Besides the eigenstates in weak localization stemming from the sign flipping of imaginary acoustic impedances at the interface, we observed an unusual type of strongly localized states at the band-inverting contact after a periodic lattice dislocation is purposely introduced. From the layered multiple scattering theory, we have uncovered that the underlying physics for these unique interface states in the hetero-structured PC are due to nontrivial constructive interferences of high-ordered Mie-scattered acoustic waves from the mismatched cylinders. The intriguing features include interface resonances of enormous quality factors (∼3 × 104) and chiral field patterns along the dislocation line. We envision potential applications of the work in slow sound trapping, notch filtering, and nonlinearity strengthening, etc. [ABSTRACT FROM AUTHOR]

We demonstrate the transient parity-time (PT) symmetry in electronics. It is revealed by equivalent circuit transformation according to the switching states of electronic systems. With the phasor method and Laplace transformation, we derive the hidden PT-symmetric Hamiltonian in the switching oscillation, which are characterized by free oscillation modes. Both spectral and dynamic properties of the PT electronic structure demonstrate the phase transition with eigenmode orthogonality. Importantly, the observed transient PT symmetry enables exceptional-point-induced optimal switching oscillation suppression, which shows the significance of PT symmetry in electronic systems with temporary responses. Our work paves the way for breakthroughs in the PT symmetry theory and has essential applications such as anti-interference in switch-mode electronics. [ABSTRACT FROM AUTHOR]

The generation of acoustic acceleration beams is a research hotspot that has attracted increasing interest recently. In this article, we propose to generate controllable acoustic acceleration beams by using a flexible active surface with homogeneous phase and amplitude distributions. Thus, the active surface does not necessarily require very complex electronic control systems. Analytically, we show that the beam trajectories can be related to the shape of flexible active surfaces via the technique of inverse Legendre transformation. Consequently, we can directly create diffraction-suppressed acoustic beams along curve trajectories in space. We numerically and experimentally demonstrate that the acoustic beam projected from a bent active surface propagates along the predesigned path, which is in good agreement with theoretical prediction. We envision that the active soft acoustic surface may have versatile applications in the fields of medical treatment, particle manipulation and acoustic communication. [ABSTRACT FROM AUTHOR]

Stimulated adiabatic passage utilizes radiation pulses to efficiently and selectively transfer population between quantum states, via an intermediate state that is normally decaying. In this Letter, we propose the analog of stimulated adiabatic passage in an acoustic system. It is realized with cavities that correlate through adiabatically time-varying couplings, where the cavities and time-varying couplings mimic discrete states and radiation pulses, respectively. With appropriate arrangements of coupling actions, an acoustic wave can be efficiently transferred from the initial excited cavity to the target cavity in the forward direction, immune to the intermediate dark cavity. On the other hand, for the backward propagation, the acoustic energy is perfectly localized in the intermediate dark cavity and completely dissipated. We analytically, numerically, and experimentally demonstrate such unidirectional sound localization and unveil the essential role of zero-eigenvalue eigenstates in the adiabatic passage process. [ABSTRACT FROM AUTHOR]