ALBERT

Description: Broadband sound energy enhancement is essential in practical scenarios, such as acoustic positioning and acoustic communication. In this paper, a dual anisotropic metamaterial composed of an inner Mie resonator and an outer acoustic grating is proposed, aiming to achieve enhanced broadband monopole emission and acoustic energy harvesting (AEH) via the coupling of the first and second monopole resonances. Considering thermo-viscous dissipation, numerical simulations and experimental results demonstrate that the dual anisotropic metamaterial can realize omnidirectional enhanced broadband monopole emission at 795 Hz–1511 Hz, the maximum sound pressure level (SPL) gain is 16.4 dB and the SPL gain fluctuation is 3 dB. Furthermore, simulation results reveal that the broadband AEH can be achieved by the dual anisotropic metamaterial, the fluctuation of the SPL gain at 794 Hz–1537 Hz is 3 dB and the maximum is 14.7 dB. Based on the results, the dual anisotropic metamaterial is expected to show significant potentials in acoustic positioning and acoustic communication.

Description: Co0.5Ni0.5Nd0.02Fe1.98O4 (CoNiNdFO) nanospinel ferrites (NSFs) with and without plant extracts (cardamom seeds, date fruits, flaxseed, tragacanth gum, lavender seeds, and moringa) were prepared using a sol-gel approach. The impact of utilizing different plant extracts on the structural, morphological, and magnetic features of the final products is discussed. Room temperature (300 K) magnetization findings illustrate superparamagnetic (SPM) characteristics for the products synthesized using the cardamom and moringa plant extracts mediated sol-gel approach, whereas the remaining products display ferromagnetic characteristics. On the other hand, lower temperature (10 K) magnetization curves indicate that all the prepared samples exhibit ferromagnetic (FM) behavior. It was found that the magnetic parameters vary greatly with the synthesis approach, plant extract solution, distribution of cations, morphology, and size of NSFs, spin canting, etc. The obtained results indicate that the different synthesis approaches employed in the present investigation are simple, inexpensive, and eco-friendly, and could lead to obtaining interesting magnetic properties. The biological applications of these NSFs were examined. The impact of NSFs with and without cardamom and moringa extracts on human colorectal carcinoma cells (HCT-116) and cervical cancer cells (HeLa) was assessed by checking cell viability using 3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and 4′,6-diamidino-2-phenylindole (DAPI) staining methods. The treatments of NSFs with and without cardamom and moringa extracts decreased cancer cell viability. We calculated the inhibitory concentration (IC50) values for NSFs with and without cardamom and moringa extracts. The treatment of NSFs with and without cardamom and moringa extracts on HCT-116 cells and HeLa cells induced a significant decrease in cancer viability, as revealed by MTT assay. The treatment of NSFs with and without cardamom and moringa extracts caused a noteworthy decrease in colon cancer cells as the number of DAPI stained cells was found to be less in the treated cells. The evaluation of the anti-staphylococcal activity of NSFs with and without cardamom and moringa extracts via the colony-forming unit (CFU) method showed growth inhibition of S. aureus. It was observed that the nanoparticles synthesized via green synthesis had a marked cell count reduction, proving the microbial toxicity of nanoparticles to S. aureus.

Description: Two-dimensional (2D) materials, due to their unique electronic, optical and structural properties, have attracted extensive attention of researchers in the world. However, most of 2D materials have low optical absorption efficiencies in the visible and near-infrared regimes, which leads to the weak light–matter interaction and limits their further applications in optoelectronic devices. Thus, enhancing the light–matter interaction of various 2D materials in the visible and near-infrared regimes, has been a key topic for many optoelectronic equipment and related applications. In this topical review, we summarized the recent developments of the 2D materials-based optical absorbers in the visible and near infrared regimes, focusing mainly on the methods and relevant physical mechanisms of several typical perfect absorbers, such as narrowband perfect absorbers, dual-band perfect absorbers, and broadband perfect absorbers. Finally, several prospective research directions from our perspectives are presented at the end.

Description: The anisotropic droplet formulation is generalized from hydrophilic to superhydrophobic surfaces. An experimental method to calibrate the ellipsoidal droplet volume on both hydrophilic and hydrophobic surfaces is presented. A broad range of contact angles (CAs) is produced on the copper and stainless-steel surfaces using femtosecond laser patterning. The effects of line spacing between the laser scanning on the formation of anisotropic CAs are discussed. The comparative study of the evolution of anisotropic CAs and droplet’s spreading dynamics are studied on both surfaces. According to the triple contact line (TCL) theory, CAs are determined by the TCL between droplet and surface rather than the contact area. We presented the mathematical formalism and the experimental validity of the TCL theory on ellipsoidal droplets over a broad range of CAs, from as low as 37°–172°. This work experimentally validated the TCL theory over a broad range of CAs with good confidence.

Description: MnBi 2 n Te 3 n + 1 (MBT) is the first intrinsic magnetic topological insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n   ⩾   4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compounds, including MnBi8Te13 and Sb doped MnBi 2 n Te 3 n + 1 ( n = 2 , 3 ) using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the saturation fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare ‘double-peak’ behavior observed in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.

Description: The generation of a large cold plasma jet while maintaining the reproducibility and homogeneity of the discharge is one of the major challenges encountered by the plasma community to efficiently apply this technology in the industry. Here, we report on the discharge in a recently developed device called the plasma candle (PC), wherein a stable plasma jet with a diameter of 20 mm can be generated at atmospheric pressure and temperature. Unlike the discharge morphology previously reported for conventional plasma jet devices, the unique configuration of PC device resulted in distinctive discharge patterns. Homogenous discharge was generated in the electrode gap and followed by a swirling discharge toward the tube nozzle. Fast photography and electrical measurements revealed that filament propagation and its morphology form the visually observable swirl discharge. Detailed analysis indicated that residual helium metastable species (Hem) and their penning ionization play an essential role in the discharge mode and its transition, which was verified by changing the feeding gas and the frequency of the applied voltage. For instance, it is found that only filamentary discharge was observed along the entire tube at frequencies less than 3 kHz, at which the time between consecutive discharges was long enough for Hem decay. Consequently, the homogenous discharge pattern was recovered by increasing the pre-ionization levels by adding a trace of impurities (N2, O2 or H2O) to the feeding gas. However, the level of these impurities must be carefully adjusted to achieve a homogenous discharge without negatively affecting the jet properties. A trivial change in the gas impurity, in the range of adsorption and desorption of water from the gas tubing, is sufficient to cause a noticeable change and instability in the discharge mode. This finding is critical to predicting the production of reactive species and plasma-surface interaction for different applications.

Description: Although many studies have been done and advanced progress has been made in understanding partial discharge (PD) behavior in the void, this is not the case firinception of PD, especially its stochastic nature. The statistical behaviors of PD inception voltage (PDIV) and inception time delay (PDTD) inside the void were investigated in this study through repeated tests to observe the stochastic nature of PD inception. The results show that the PDIV and PDTD of the void are highly dispersed and obey Weibull and exponential distributions, respectively. The significant dispersion of PDIV can be attributed to the statistical time delay of PD inception. The lengthy inception delay is attributable to a lack of free electrons. The exponential distribution of PDTD indicates that free-electron generation is completely random; further, the stochastic nature of void PD inception is determined by the supply of free electrons. The test method (voltage rise rate, test time, and test time interval), void parameters (size, material, and surface condition), and background radiation determine PD inception by affecting the volume ionization or surface-emission process providing free electrons. Enhanced background ionization or significant increase in test voltage and test time allow for the effective detection of void defects during PD tests. This work contributes to an empirical understanding of the physical process of PD inception in voids and improving existing PD testing technologies.

Description: There is a widely observed phenomenon in the microwave absorption field that an absorber always exhibits good oblique incidence absorption capacity if it has high performance at normal incidence. However, if a certain angle is exceeded, this kind of effective absorptive capacity will no longer be maintained. Besides, an absorber performs differently for incident transverse electric (TE) and transverse magnetic (TM) waves: for the TE case, the absorber can no longer obtain effective absorption; for the TM case, another efficient absorption region was observed at higher frequencies even when the incident angle exceeded 80°. These phenomena are widely found in the literature, which demonstrates that they are caused by physical laws rather than material properties. To demonstrate the underlying reason, in this study, the common spherical carbonyl iron-polyurethane composite absorbers were fabricated as a typical example. Their absorbing performance was investigated via both simulation and experiment. All the phenomena mentioned above were observed, studied in detail by employing the multiple reflection model, and explained quantitatively. Further, along with establishing the underlying mechanism of electromagnetic wave transmission in the absorber, two formulas were deduced to predict: (a) the maximum incident angle for efficient absorption of the TE polarized wave; and (b) the required absorber thickness for obtaining efficient absorption for a large incident angle of the TM polarized wave.

Description: This study presents an analysis of a three-dimensional unsteady two-temperature simulation of atmospheric pressure direct current electric arc inside a commercial cascaded-anode plasma spray torch; it coupled the arc model with the torch electrodes and used an open-source computational fluid dynamics software (code_saturne). The previously published models of plasma spray torch either deal with conventional plasma torches or assume local thermodynamic equilibrium in cascaded-anode plasma torches. The paper presents the computation of the two-temperature argon plasma properties, compares two enthalpy formulations that differ in association of the ionization part of enthalpy and finally demonstrates the influence of the radiation heat loss data by comparingthe results for two different literature sources. It is the first to compare different enthalpy formulations in the context of plasma torch and discuss the differences in terms of the enthalpy gains and losses. It also explains why an unphysical simulation artifact of electron temperature lower than the heavy species temperature can occur in simulated plasma flow. The solution, then, consists in associating the ionization part of enthalpy to electrons and selecting the appropriate source of the data of radiation heat loss. However, negligible thermal non-equilibrium persists even in the hot core of electric arc, which ensures that the heavy species are heated up by collisions with electrons. The flexibility of the open-source software allows all the necessary modifications and adjustments to achieve satisfactory simulation results. Thus, the paper could be considered as a manual for development of a plasma spray torch model.

Description: In this work, we present a self-energy model based on the complex absorbing potential (CAP) method to calculate the transmission function through an extended molecule using scattering theory. Once the CAP mimics an infinite environment at the ends of a finite system, it can be used as a model for self-energy with a low computational cost. Moreover, the matrixes required for the transport calculation can be obtained from an ab initio calculation of some extended molecules in a single step using an adjustable model, thus taking into account changes in the electronic structure of the system. This approach was applied to study electron transport across a biphenyl molecular system for different torsion angles under an external applied electric field. The results obtained are in good agreement with the available theoretical and experimental results in the literature and provide an efficient approach, with a low computational cost method, for the interpretation of electrical transport at the molecular level.