Search

Tien riverbank in Cai Be district, Tien Giang province has quite a complex geological structure and human activities, so the riverbanks are frequently instability. The article refers to the instability of the Tien riverbank flowing through Cai Be district, Tien Giang Province. Based on the collected documents, analyzing remote sensing images, and field investigations, two types of riverbank instability with varying severity were identified. The main causes of instability are river flow erosion, influence of waves, geological factors, hydraulic pressure, sand depletion, and human activities. The selection of countermeasures against riverbank erosion in the study area should consider the soft soil ground structure along the riverbank, and the impact of horizontal waves casued by boat activities. Regarding the countermeasures for stability of riverbank, the method of pre-stressed piles exhibit a good solution against riverbank erosion. The research results are the basis for proposing effective prevention solutions to serve sustainable development along this river.

Wind power has been widely utilized and has become a safe and renewable energy source in many countries. In Vietnam, many wind power farms have been built in coastal provinces such as Soc Trang, Bac Lieu, Ninh Thuan, Binh Thuan, Ca Mau, Vung Tau, and the Central Highlands. For onshore wind power farms, the dynamic loads generated from wind and the operation of the turbine will act on soil ground and affect the long-term strength and liquefaction potential of soil, especially for sandy soil. Thus, it is necessary to investigate the potential for liquefaction of the sandy soil layer in wind power farms. In this study, the liquefaction potential of sand distributed in onshore wind power farms in Soc Trang province, Vietnam was evaluated using the undrained cyclic triaxial test. The sand specimens were remolded using dry pluviation and moist tamping methods to simulate the field density. The analysis of test results showed that the very fine sand with a loose state distributed at the depth of less than or equal to 11.5 m could be liquefied under the testing conditions. While the fine sand specimens with a dense to very dense state distributed deeper than 30 m were not liquefied under testing conditions. However, for these specimens, the pore water ratio could be increased by 11.3 to 18.6% and this increase can lead to a decrease in soil strength.

Metamaterials, recognized as advanced artificial materials endowed with distinctive properties, have found diverse applications in everyday life, military endeavors, and scientific research. Starting from monolayer metamaterials, multilayer ones are increasingly researched, especially in the field of electromagnetic wave absorption. In this article, we propose a multilayer metamaterial-absorber (MA) structure comprising two resonant layers crafted with copper and FR-4 dielectric. The presented multilayer MA structure exhibited an absorption greater than 90% in a frequency range from 4.84 to 5.02 GHz, with two maximum absorption peaks at 4.89 and 4.97 GHz. The bandwidth of the multilayer MA surpassed that of the individual single-layer MAs, with extension fractions reaching 360% and 257%, respectively. Through the simulation and calculation, the field distribution and equivalent circuit model elucidated that both individual magnetic resonances and their interplay contribute significantly to the absorption behavior of the multilayer MA. The absorption of the proposed multilayer MA structure was also investigated for the oblique incidence in the transverse electric (TE) and transverse magnetic (TM) modes. In the TE mode, the absorption intensity of two maximum peaks was maintained at over 93% up to an incident angle of 40 degrees and dropped to below 80% at an incident angle of 60 degrees. In the TM mode, the absorption was more stable and not significantly affected by the incident angle, ranging from 0 to 60 degrees. An absorption greater than 97% was observed when the incident angle increased from 0 to 60 degrees in the TM mode. Additionally, the approach in our work was further demonstrated by adding more resonant layers, making 3- and 4-layer structures. The results indicated that the absorption bandwidths of the 3- and 4-layer structures increased by 16% and 33%, respectively, compared to the bilayer structure. Furthermore, we analyzed the thermal distribution within the MA to understand the dissipation of absorbed electromagnetic energy. This research offers valuable insight into the augmented MA through a multilayer structure, presenting the implications for microwave applications like electromagnetic shielding, as well as in the design of MAs for terahertz devices and technologies, including emission and thermal imaging. These findings contribute to the advancement of knowledge in enhancing the absorption capabilities across various frequency ranges, expanding the potential applications of metamaterials.

Abstract In this article, we investigated the efficiency of a magnetic resonant wireless power transfer (MR-WPT) in conducting medium and found out an optimal frequency for designing the system. In conducting environment, the eddy current loss is generated by the high-frequency alternating currents in the coils. It is manifested by increased radiation resistance of resonator coil leads to decrease the quality factor (Q-factor), which reduces the wireless power transfer (WPT) efficiency in conducting medium. The Q-factor of the resonator coil strongly depending on the conductivity, frequency, and thickness of conducting block. Two MR-WPT systems operating at 10.0 MHz and 20.0 MHz are implemented to study the effect of conducting medium on efficiency. The achieved results indicated that the 20.0 MHz system has higher efficiency at a conductivity smaller than 6.0 S/m. However, at the larger conductivity, the 10.0 MHz system is more efficient. The results provide a method to determine the optimal frequency of a WPT system operating in the conducting medium with various conductivities and thickness blocks. This method can be used to design MR-WPT systems in numerous situations, such as autonomous underwater vehicles and medical implants.

This study presents a tri-layer broadband metamaterial absorber that operates in the GHz range. The absorber was composed of a polyhedral iron-cobalt alloy/graphite nanosheet material arranged in a flat sheet with two punched-in rings for the top layer, a continuous FR-4 layer at the middle, and a continuous copper layer at the bottom. For the normal incidence of the electromagnetic wave, the proposed absorber demonstrated an exceptional broadband absorption in a frequency range of 7.9–14.6 GHz, revealing an absorption exceeding 90%. The absorption magnitude remains to be above 90% in a frequency range of 8–11.1 GHz for transverse-electric-polarized waves at incident angles up to 55°. For both the transverse-magnetic- and electric-polarized waves, the absorption exceeds 90% in a frequency range of 9.5–14.6 GHz. The physical mechanism behind the absorption properties is analyzed thoroughly through the electric and magnetic field distributions. The obtained results could contribute potentially to the development of microwave applications based on metamaterial absorbers, such as radar-stealth technology, electromagnetic shielding for health safety, and reduced electromagnetic interferences for high-performance communications and electronic devices.

In this work, we demonstrated a tunable metamaterial perfect absorber (MPA) with broadband absorption by tuning the different states of flexible sandwiched structures (graphene conductive ink/rubber/metallic layers). The broadband absorption spectrum was tuned mechanically by changing the concave-up/down states of flexible hemispherical unit-cells. When the unit-cell was concave-up, our proposed MPAs played as a broadband absorber with a fractal bandwidth (FBW) of 107% (since an absorption over 90% ranges from 5.28 to 17.6 GHz) at the normal incidence; at the same time, this broadband absorption feature could remain well at large incident angles up to 40 deg. and regardless of polarization of the incoming electromagnetic waves. In the case of the concave-down state, a narrow FBW of only 6.8% was noticed. These results could promote great application potential, such as regarding advanced stealth devices, advances in the biomedical and the communication fields, and more.

In this paper, we present a flexible magnetic metamaterial structure for enhancing the efficiency of wireless power transfer (WPT) systems operating at 13.56 MHz. The metasurface between transmitter (Tx) and receiver (Rx) coils of the WPT system is constructed of a 3 × 5 metamaterial unit cell array with a total size of 150 × 300 mm2. Most metamaterial structures integrated into WPT systems are in planar configurations with a rigid substrate, which limits practical applications. The proposed metasurface is fabricated on an FR-4 substrate with a thin thickness of 0.2 mm; therefore, it can be bent with radii greater than 80 mm. A defect cavity is formed in the non-homogeneous metasurface by controlling the resonant frequency of the unit cell with an external capacitor. Simulation and measurement results show that the efficiency of the WPT system is significantly enhanced with metasurfaces. The performance of the WPT system can also be optimized with suitable bend profiles of metasurfaces. This proposed flexible metasurface could be widely applied to WPT systems, especially asymmetric, bendable, or wearable WPT systems.

The structural, optical, and photocatalytic properties of TiO2 and Ag–TiO2 thin films grown by a hydrothermal method were studied by using an x-ray diffractometer, scanning electron microscope, energy-dispersive x ray, and UV–vis spectroscopy. The results indicated that all films were of single-phase and the Ag presence enhanced catalytic and water-splitting performances in the visible light region. In particular, the methylene blue solution was disintegrated up to 99% after 12 and 8 h for TiO2 and Ag–TiO2 films, respectively. Meanwhile, the splitting efficiency increased from 0.3% for TiO2 to 1% for Ag–TiO2. These performances demonstrate the great potential of Ag–TiO2 films in photocatalytic and water-splitting applications.

Flexible and ultrathin wide-band metamaterial absorbers are suggested and demonstrated in the microwave-frequency range. By using resonators of different sizes and conductive fibers on metallic-pattern layer, the total thickness of metamaterial absorber is reduced to be only 1/349 with respect to the operating wavelength at 0.97 GHz. We present the absorption mechanism in terms of the impedance matching with the free space, the distributions of surface current and the three-dimensional distributions for power loss. In simulation, the absorption was over 97% at 0.97–6.12 GHz, and the corresponding experimental absorption band over 97% was 0.87–6.11 GHz. Furthermore, the dielectric substrate of metamaterial absorbers was replaced with flexible substrate in order to have the flexibility and the broadband absorption properties. The absorption band is expanded and the high-absorption performance maintains at the same time. The total thickness of metamaterial absorber comes to be only 1/5194 of the operating wavelength at 0.75 GHz. Our work is expected to contribute to the flexible microwave/electronic devices in the near future.

We demonstrated a multi-band plasmonic metamaterial absorber (MA), based on the near-field coupled resonators. In addition to the individual resonances of resonators in the proposed structure, which were split-ring resonator (SRR) and cross-shape structures, another resonance was also excited owing to the coupling of resonators, revealing a triple-band absorption. Furthermore, to control the absorption behavior, on the top of the SRRs, the identical SRRs made of graphene ink were pasted. By increasing the resistance of graphene ink, the coupling strength was weakened, changing the triple-band absorption to a dual-band one. Our work might be useful as the controllable devices, based on graphene-integrated plasmonic MA, such as filters, detectors and energy harvesters.

We demonstrate a dual-band, polarization-insensitive, ultrathin and flexible metamaterial absorber (MA), based on high-order magnetic resonance. By exploiting a flexible polyimide substrate, the thickness of MA came to be 1/148 of the working wavelength. The absorption performance of the proposed structure was investigated for both planar and bending models. In the case of the planar model, a single peak was achieved at a frequency of 4.3 GHz, with an absorption of 98%. Furthermore, additional high-order absorption peaks were obtained by the bending structure on a cylindrical surface, while the fundamental peak with a high absorption was maintained well. Our work might be useful for the realization and the development of future devices, such as emitters, detectors, sensors, and energy converters.

In this work, we proposed a flexible broadband metamaterial perfect absorber (FBMPA) by exploiting a pasted conductive-graphene ink on a polyimide substrate. For the flat FBMPA, an absorption over 90% was found to cover a wide frequency range (from 7.88 to 18.01 GHz). The high-absorption feature was polarization-insensitive and regarded as stable with respect to the oblique incidence up to 30 degrees of electromagnetic wave. The high absorption was maintained well even when the absorber was wrapped. That is, the FBMPA was attached to cylindrical surfaces (with the varying radius from 4 to 50 cm). For both flat and curved states, the absorption mechanism was explained by the perfect impedance matching and the dielectric loss of the proposed absorber. Our work provides the groundwork for the commercialization of future meta-devices such as sensors, optical filters/switchers, photodetectors, and energy converters.

A metamaterial (MM), mimicking electromagnetically-induced transparency (EIT) in the GHz regime, was demonstrated numerically and experimentally by exploiting the near-field coupling of asymmetric split-ring and cut-wire resonators. By moving the resonators towards each other, the original resonance dip was transformed to a multi-band EIT. The phenomenon was explained clearly through the excitation of bright and dark modes. The dispersion characteristic of the proposed MM was also investigated, which showed a strongly-dispersive behavior, leading to a high group index and a time delay of the MM. Our work is expected to contribute a simple way to develop the potential devices based on the multi-band EIT effect.

In this work, the possibility of a switchable metamaterial absorber is proposed to control absorption bandwidth in the WiMAX/LTE (worldwide interoperability for microwave access/long term evolution) band, by taking advantage of the low cost and myriad structural configurations afforded by water-based metamaterials. By exploiting truncated cone-type resonators, the fractional bandwidth of 27.6% of absorption spectrum can be adjusted flexibly to be 7.4% of the narrow-band absorption depending on the volume of injected water, in both simulation and experiment at room temperature. In particular, this control method can be applied stably for different temperature of injected water. We describe a dynamic mechanism for broadband MA, as well as a principle for controlling the absorption characteristics utilizing a combination of magnetic resonance and perfect impedance matching. These results are a stepping-stone towards the realization of smart electronics integrated with multi-functional metamaterials in military, biomedical, communication and other fields.