Band Gap Characteristics Research Articles

Analytical study on the flexural wave band gaps of arbitrary periodic stiffened plates by using beam-plate coupling theory

In this paper, the flexural wave band gap characteristics of periodic stiffened plates are studied by using beam-plate coupling theory. A theoretical model of arbitrary periodic stiffened plate is established considering the coupling effect of Timoshenko beam and Kirchhoff plate. The flexural wave band structures of periodic stiffened plate are calculated by using plane wave expansion method and Bloch theorem. Numerical calculation and experimental tests were conducted to verify the proposed theoretical model. In addition, the parametric studies were performed to examine the effects of geometric parameters on both bandgap and vibration reduction characteristics of periodic stiffened plate. Results show that periodic stiffened plate can yield significant flexural wave band gaps and vibration isolation performance in a specific frequency range, which shows a good agreement with numerical simulation and experimental results. The geometrical parameters of stiffened plates have a significant influence on adjusting low-frequency flexural wave band gaps. These research findings offer a new technical approach for low frequency vibration and sound optimization of stiffened structures.

Flexoelectric effect on bandgap properties of periodic bi-directional-graded curved nanoshells

With the miniaturization of devices, the size effect of structures becomes increasingly apparent and it becomes crucial to consider flexoelectricity in MEMS/NEMS. Given that functionally graded materials (FGMs) can generate strain gradients that enhance flexoelectricity, it is both novel and crucial to investigate the synergistic effects of FGMs and flexoelectricity on bandgap characteristics. Understanding these interactions is essential for advancing materials science and could lead to significant innovations in nanotechnology. In this work, a theorectical model for the periodic flexoelectric doubly-curved nanoshells with bi-directional functionally graded(BDFG) and porosity considered are constructed. As the governing equations for longitudinal FG problems are more difficult to be solved analytically, their precise analysis is more challenging. Thus, an improved transfer matrix method based on state-space is proposed innovatively to obtain the complex band structures for longitudinal FG problems. Subsequently, the influences of the flexoelectric effect, strain gradient effect, BDFG indices, and porosity distributions on bandgaps are systematically discussed. The results indicate that the flexoelectric effect is non-negligible at small scales, which generally widens the bandgaps, with higher frequency ranges and larger attenuation capacity. Bandgaps are sensitive to the variation of functionally graded indices. Particularly, flexoelectricity brings about the appearance and disappearance of some bandgaps during the variation of FG index in the thickness direction n3, and complicates the bandgaps variation. Besides, the porosity distribution patterns and porosity coefficients enrich the regulation of bandgaps. Our investigation offers valuable insights into the application of flexoelectric electrical components combined with BDFG in MEMS/NEMS to the wave propagation domain.

Study on band-gap characteristics and formation mechanism of four-ligament chiral elastic metamaterials

Abstract Chiral elastic metamaterials, owing to their exceptional properties distinct from conventional materials and their superior mechanical performance, exhibit significant potential for applications in vibration reduction, noise suppression, energy absorption, and cushioning. To address the challenge of low-frequency vibration control, this paper proposes a dual-component chiral elastic metamaterial structure with four ligament elements. The study explores the bandgap characteristics and elastic wave propagation behavior of this structure within the 1000 Hz frequency range. By analyzing the vibration modes of the unit cell and calculating the group and phase velocities of elastic waves, the physical mechanism underlying bandgap formation is elucidated. The results demonstrate that the proposed four-ligament chiral elastic metamaterial exhibits excellent bandgap properties, with the bandgap covering more than 80.4% of the frequency range below 1000 Hz. This highlights its capability for low-frequency elastic wave control and offers a theoretical reference for the design of novel vibration reduction and noise suppression structures, as well as for low-frequency elastic wave regulation.

Flexural wave bandgap characteristics analysis of periodic double-span beam based on spectral geometry method:a unified formulation

Flexural wave bandgap characteristics analysis of periodic double-span beam based on spectral geometry method:a unified formulation

Coaxial composite resonator for vibration damping: Bandgap characteristics and experimental research

Coaxial composite resonator for vibration damping: Bandgap characteristics and experimental research

Ultra-wide low-frequency bandgap characteristics of auxeticity-based composite resonator for elastic wave manipulation and machine learning-based inverse structural design

Although auxetic metamaterials are generally light in weight, strong in compression and superior in energy absorption, less research has focused on their low-frequency bandgap realization and inverse structural design. Moreover, broadening the width of bandgap of auxetic metamaterials keep challenging as well in practical low-frequency applications. To broaden their applications, this work provides a new auxeticity-based composite resonator design by filling hard engineering material into the soft auxetic structure perforated by orthogonally-arrayed peanut-shaped holes to achieve ultra-wide low-frequency bandgap. The soft material is silicone rubber and the hard material may be concrete, steel or plumbum. Firstly, the wave propagation mechanics of the present composite structure is studied and then the bandgap characteristics are thoroughly investigated by parameter analysis. On this basis, a machine learning (ML) method combining the standard back-propagation neural network and genetic algorithm is proposed for bandgap prediction and inverse structural design. Results indicate that the present composite structure performs well in generating ultra-wide low-frequency bandgap, i.e. [11.35, 24.07] Hz, and the relative bandgap width reaches 71.76 %. Moreover, the present ML method is effective in the forward prediction of bandgap and the inverse structure design fitting the specific bandgap requirement. For the created optimal designs, the ML method and the finite element simulation can give similar results, and the maximum relative error between them is just about 5 %, which verifies the accuracy of the ML design method. This work provides a new way to realize the structural design of auxeticity-based acoustic metamaterials with low-frequency and broad bandgap.

Thermal Regulation of the Acoustic Bandgap in Pentamode Metamaterials

This study used the finite element method to investigate the acoustic bandgap (ABG) characteristics of three-dimensional pentamode metamaterial (PM) structures under the thermal environment, and a method for controlling the PM ABG based on external temperature variation is also proposed. The results indicate that the complete acoustic bandgap can be obtained for a PM in the thermal environment, which makes the PM combine the bandgap characteristics of phononic crystals. More than that, the bandwidth and locations of ABGs can be effectively manipulated by controlling the temperature. Considering the softening effect of thermal stresses, the ABG gradually moves to lower frequencies as the temperature increases. Based on this, different degrees of ABG tunability can be achieved by changing the thermal environment to propagate or suppress acoustic waves of different frequencies. This work provides the possibility for PMs to realize intelligent regulation of the bandgap.

First-principles calculations of the effect of vacancy defects on the optoelectronic and mechanical properties of the double perovskite Cs2NaInCl6

Abstract Cs2NaInCl6 have become a hot research topic in perovskite optoelectronic materials. However, there is no research on the vacancy properties of Cs2NaInCl6 at present. Herein，using density functional theory-based first-principles calculations, the impact of Cs, Na, In, and Cl single vacancies on the microstructure, electronic, optical, and mechanical properties of the double perovskite Cs2NaInCl6 were investigated. The research shows that the presence of vacancy defects leads to distortion of the Cs2NaInCl6 double perovskite structure and an increase in the unit cell volume. Cs vacancy (VCs) and Na vacancy (VNa) cause an increased band gap for Cs2NaInCl6, by 2.92eV and 3.1eV respectively, without changing the characteristics of the initial direct band gap; VIn and VCl cause the intrinsic semiconductor Cs2NaInCl6 to transform into p-type and n-type semiconductors, respectively, altering the conductivity of Cs2NaInCl6. In terms of optical properties, different vacancy defect systems exhibit different absorption abilities in the visible and ultraviolet light ranges. The defect system overall improves light absorption in the low energy region, and as the incident photon energy increases, the material exhibits the best optical performance at around 15eV. In addition, it is found that VCs, VNa, and VIn caused blue shift in the absorption edge of Cs2NaInCl6, while VCl caused red shift in the absorption edge. In terms of mechanical properties, defects cause varying degrees of reduction in Young's modulus (E), shear modulus (G), and bulk modulus (B) of Cs2NaInCl6, which to some extent alters the mechanical properties of Cs2NaInCl6. These research results provide theoretical guidance for understanding the influence of vacancies on the properties of Cs2NaInCl6 double perovskite and optimizing the performance of perovskite optoelectronic devices, and they also enhance its applications in photovoltaic cells, optoelectronic sensors, and other optoelectronic coupling devices.

Plasma Treatment Technologies for GaN Electronics

Nowadays, the third-generation semiconductor led by GaN has brought great changes to the semiconductor industry. Utilizing its characteristics of a wide bandgap, high breakdown Electric field, and high electron mobility, GaN material is widely applied in areas such as 5G communication and electric vehicles to improve energy conservation and reduce emissions. However, with the progress in the development of GaN electronics, surface and interface defects have become a main problem that limits the further promotion of their performance and stability, increasing leakage current and causing degradation in breakdown voltage. Thus, to reduce the damage, Plasma treatment technologies are introduced in the fabrication process of GaN electronics. Up to now, designs like the high-resistivity p-GaN cap Layer, passivating termination, and surface recovery process have been established via Plasma treatment, reaching the goals of normally-off transistors, diodes with high breakdown voltage and high-reliability GaN electronics, etc. In this article, hydrogen, fluorine, oxygen, and nitrogen Plasma treatment technologies will be discussed, and their application in GaN electronics will be reviewed and compared.

Analysis of Influence of Excitation Source Direction on Sound Transmission Loss Simulation Based on Alloy Steel Phononic Crystal

As a type of locally resonant phononic crystal, alloy steel phononic crystals have achieved notable advancements in vibration and noise reduction, particularly in the realm of low-frequency noise. Their exceptional band gap characteristics enable the efficient reduction of vibration and noise at low frequencies. However, the conventional transmission loss (TL) simulation of finite structures remains the benchmark for plate structure TL experiments. In this context, the TL in the XY-direction of phononic crystal plate structures has been thoroughly investigated and analyzed. Given the complexity of sound wave incident directions in practical applications, the conventional TL simulation of finite structures often diverges from reality. Taking tungsten steel phononic crystals as an example, this paper introduces a novel finite element method (FEM) simulation approach for analyzing the TL of alloy steel phononic crystal plates. By setting the Z-direction as the excitation source, the tungsten steel phononic crystal plate exhibits distinct responses compared to excitation in the XY-direction. By combining energy band diagrams and modes, the impact of various excitation source directions on the TL simulations is analyzed. It is observed that the tungsten steel phononic crystal plate exhibits a more pronounced energy response under longitudinal excitation. The TL map excited in the Z-direction lacks the flat region present in the XY-direction TL map. Notably, the maximum TL in the Z-direction is 131.5 dB, which is significantly lower than the maximum TL of 298 dB in the XY-direction, with a more regular peak distribution. This indicates that the TL of alloy steel phononic crystals in the XY-direction is closely related to the acoustic wave propagation characteristics within the plate, whereas the TL in the Z-direction aligns more closely with practical sound insulation and noise reduction engineering applications. Therefore, future research on alloy steel phononic crystal plates should not be confined to the TL in the XY-direction. Further investigation and analysis of the TL in the Z-direction are necessary. This will provide a novel theoretical foundation and methodological guidance for future research on alloy steel phononic crystals, enhancing the completeness and systematicness of studies on alloy steel phononic crystal plates. Simultaneously, it will advance the engineering application of alloy steel phononic crystal plates.

Bandgap characteristics of four component periodic pile barriers in passive vibration isolation

Based on the local resonance theory, the vibration isolation performance of four-component periodic pile barriers consisting of a matrix, an outer pipe, an inner pipe, and a pile core was investigated using the finite element method (FEM). This configuration is contrasted with traditional three-component periodic pile barriers, which are composed of a matrix, a pipe, and a pile core. The four-component systems demonstrate significantly broader bandgaps compared to three-component systems. To validate the efficiency of the FEM, model test on pile barriers were conducted, facilitating a comparison between the FEM and experimental observations of bandgap characteristics. Additionally, the effects of the outer pipe’s density, elastic modulus, and thickness on the complete bandgap characteristics were comprehensively studied within the four-component structure. It is found that the broadest vibration isolation band gaps occur under hexagonal arrangement pattern. In square and hexagonal lattice configurations, the density, elastic modulus, and thickness of the outer pipe pile’s pile have predominantly influenced the upper bound frequency (UBF). There has been a positive correlation between the elastic modulus and the UBF, while the density and thickness have shown inverse relationships. Moreover, there exists a elastic modulus threshold, marking a transition in the displacement mode of the UBF, from the outer irreducible Brillouin zone (M or X) to the inner zone (Γ). Once the threshold is exceeded, both the vibration displacement mode and the width of bandgap remain substantially unchanged. The results obtained in the present paper are very useful for the design and application of periodic pile barriers in ambient vibration reduction.

Numerical investigation of bandgap characteristics in integrated metallic metafilters for nonlinear guided wave applications

Guided waves propagating in nonlinear media, featuring second harmonic generation, represent a promising avenue for early-stage damage detection due to their high sensitivity and long-range propagation capabilities. However, nonlinear ultrasonic measurements are hindered by nonlinearities induced by the experimental system, necessitating careful calibrations that have restricted their application to laboratory settings. While several phononic crystal and metamaterial designs have been devised to enhance nonlinear-based ultrasonic testing, most are tailored for suppressing second harmonics within a frequency range of 100–300 kHz, primarily utilizing low-frequency excitation. In this paper, we propose a metallic ring-shaped metafilter designed to explore high-order bandgaps. To fully understand the bandgap characteristics, we begin by analyzing mode shapes, providing insights into the underlying wave mechanics. The efficacy of the designed filter is subsequently assessed through 3D time step elastodynamic simulations. In addition, this study underscores the significance of parameters such as the number of rings employed in the filter, signal duration, and bandgap width in optimizing its performance. Furthermore, the observed mode conversion phenomena from S0 to A0 guided wave modes underscore the filter’s capacity to influence guided wave propagation. The defect localization technique, based on the time difference of arrival of second-order wave modes, accurately predicts the defect location with an error margin of less than 0.2%. The present investigation showcases advancements in the sensitivity of nonlinear-based guided wave testing for characterizing microstructural changes, promising substantial potential for detecting incipient damage in practical structural health monitoring applications.

Investigation of structural, electronic, optical, and photocatalytic properties of new double perovskites Cs2InSbX6 (X = F, Cl) under strain effects

Inorganic halide perovskites, such as Cs₂InSbX₆ (X = F, Cl), have shown potential for next-generation solar cells and photocatalysis due to their light-capturing properties. However, concerns about stability and toxicity in lead-based perovskites drive the need for eco-friendly alternatives like antimony-based compounds. This study employed Density Functional Theory (DFT) calculations to explore how mechanical strain affects the electronic and optical properties of Cs₂InSbX₆. Without strain, Cs₂InSbF₆ and Cs₂InSbCl₆ exhibit bandgaps of 2.06 eV and 1.20 eV, respectively. We investigated how strain ranging from −5% to +5 % influences these materials' performance in optoelectronic and photocatalytic applications. Notably, negative strain enhances absorption and quantum efficiency for both compounds, particularly Cs₂InSbCl₆, which shows superior absorption in the visible spectrum. Our findings suggest that Cs₂InSbCl₆, with a tunable bandgap and favorable optical characteristics under strain, is a promising candidate for environmentally friendly applications, including solar cells and photocatalysis.

Influence of growth temperature on the structural and optical characteristics of PbZnS thin films

Ternary thin films have garnered significant attention due to their adjustable band gap characteristics, making them suitable for many applications. In this research, we examined the structural and optical characteristics of PbZnS thin films fabricated onto glass substrates employing the spray pyrolysis techniuqe. The films were fabricated at different temperatures ranging from 300 to 400°C. X-ray diffraction (XRD) analysis indicated that all films displayed a cubic structure with a preferred orientation along the (200) plane. From the analysis of XRD peaks, it was observed that the crystal structure initially improved with increasing temperature but then began to degrade. These films demonstrated strong absorption in the range of 350–1600 nm. The optical bandgap (Eg) values, calculated based on the relationship between the absorption coefficient and photon energy, showed slight variations around 2 eV. Notably, the band gap decreased with growth temperatures up to 350°C, then increased at higher temperatures. These fluctuations are attributed to factors such as thermal expansion, strain, defects, surface/interface effects, and changes in doping or composition.

Impact of oxygen vacancies on the structural, electronic, and optical properties of Cu<i>2</i>O and nitrogen-doped Cu<sub>2</sub>O for optoelectronic applications: a first-principles study

This study utilizes Density Functional Theory (DFT) within the Quantum ESPRESSO framework to explore the effects of oxygen vacancies and nitrogen doping on the electronic structure and optical properties of cuprous oxide (Cu2O). Pristine Cu2O, an intrinsic p-type semiconductor, typically exhibits a direct band gap. Introducing an oxygen vacancy in the absence of a dopant broadens the band structure, leading to both direct and indirect band gap characteristics. In contrast, nitrogen doping transforms the band gap into a direct one, significantly reducing it from 1.97 eV to 1.09 eV, which deviates from previous findings due to potential variations in the initial state of the Cu2O host crystal. Furthermore, spin-polarization effects indicate spin-dependent behaviour within the material. The study also examines the absorption properties of pristine, defective, and nitrogen-doped Cu2O systems using the complex dielectric function. The results show distinct absorption peaks and transitions, with nitrogen-doped Cu2O exhibiting strong absorption in the visible light range, underscoring its potential for solar cell applications.

Stochastic modeling of periodic beams under uncertain boundary conditions and environmental fluctuations

Periodic structures have been attracting a great deal of academic and industrial interest lately, due to their distinctive vibration and wave propagation behavior, which can be explored for the development of innovative solutions to structural dynamics and vibroacoustic problems. Although such a potential has been demonstrated in a large number of studies, the investigation of detrimental effects, which can be present in practical applications, is still necessary. This paper reports investigations on the combined influence of uncertainties affecting ambient temperature — which alters material properties and induces stress-stiffening due to constrained thermal dilatation — and boundary conditions (BCs) on the bandgap characteristics of periodic beams. The space-dependent temperature fluctuations are represented as a one-dimensional stationary Gaussian random field, discretized using the Karhunen-Loève expansion, while non-ideal BCs, represented as springs, are modeled as discrete random variables. Sampling-based stochastic analyses of the central frequency and bandwidth of the beam’s attenuation bands are performed using Monte Carlo simulations. The results demonstrate that the variability in the attenuation band features is influenced not only by the coefficients of variation (CVs) of the input random quantities, but also by the correlation length of the random temperature fluctuations. Numerical simulations reveal that the bandgap central frequency is primarily affected by the temperature random field, while the BCs govern the bandwidth. Although low CV and standard deviation values are obtained for the dispersion of the bandgap features, reliability analyses indicate that some designs exhibit low reliability. Increased variability in both the bandgap central frequency and bandwidth is observed for greater temperature correlation lengths and CVs. The contributions of the study include the proposal of a comprehensive stochastic modeling procedure duly accounting for relevant random influences, and evidencing that those influences can be significant, requiring consideration in the design of robust periodic structures.

The mercurial rise in research of halide perovskites: what´s next

AbstractPerovskites are of high potential in the ongoing academic research, due to their distinctive electrical properties and crystalline structures. Halide perovskites show high light emissive properties and panchromatic light absorption across the visible spectrum. The exceptional electrical characteristics, such as their long carrier lifespan, high diffusion length, and charge carrier mobility, allow the electric charges to be transported and collected effectively. Furthermore, by tuning the cations and anions composition, perovskite’s opto-electrical properties can be altered. Moreover, dimension reduction affects their band gap and intrinsic features to induce higher structural stability but at the cost of the quantum confinement effect. Owing to their exceptional properties, halide perovskites are being researched in energy-related and semiconducting applications, hold high promise and the future looks bright. But challenges remain, and the larger question is what needs to be done to make them more stable.

A 3D-Printed Bi-Material Bragg-Based Reflectarray Antenna.

This paper presents a 3D-printed fully dielectric bi-material reflectarray with bandgap characteristics for multi-band applications. To achieve bandgap characteristics, a "1D Bragg reflector" unit cell is used. The latter is a layered structure characterized by a spatial distribution of refractive index that varies periodically along one dimension. By appropriately selecting the dimensions, the bandgap can be shifted to cover the desired frequency bands. To validate this bandgap characteristic, a (121.5 mm × 121.5 mm) with an f/D ratio of 0.5 reflectarray was fabricated. The measured gain at 27 GHz is 27.22 dBi, equivalent to an aperture efficiency of 35.05%, demonstrating good agreement between simulated and measured performances within the frequency range of 26-30 GHz. Additionally, the transparency of the reflectarray was verified by measuring the transmission coefficient, which exhibited a high level of transparency of 0.32 dB at 39 GHz. These features make the proposed reflectarray a good candidate for multi-band frequency applications.

Investigating the structural, electronic, optical and thermodynamic properties of ATiO3 (A = Ba, Ca, Ra) for low-cost energy applications

The structural, optoelectronic, and thermodynamic characteristics of ATiO3 (A = Ba, Ca, Ra) perovskites have comprehensively been investigation first-principles based DFT calculations. The GGA method was employed to handle exchange and correlation potentials. The analysis of the E-V plots indicates that RaTiO3 exhibits the highest level of stability in comparison to BaTiO3 and CaTiO3. The direct band-gap characteristic of the ATiO3 (A = Ba, Ca, Ra) perovskites is apparent in the band structure plots. The band gaps for BaTiO3, CaTiO3, and RaTiO3 are 2.48, 3.15, and 1.91 eV, respectively. The analyzed materials exhibit the highest level of photon absorption in the near UV area, as demonstrated in the ε2(ω) plots. The refractive index n(ω) values reveal that ATiO3 (A = Ba, Ca, Ra) are active optical materials as their n(ω) values are between 1 and 2. These perovskite oxides have optical features that make them promising prospects for anti-reflecting coatings over entire energy span as these materials exhibits a very low reflection (∼20 %) of incident photons. The thermodynamic properties of ATiO3 (A = Ba, Ca, Ra) are assessed to determine their dynamical stability and suitability for thermal applications. The results of the investigation demonstrate that these perovskite oxides have the potential to be used in optoelectronic devices.

Realization of topological Bragg and locally resonant interface states in one-dimensional metamaterial beam-resonator-foundation system

Abstract In this study, the one-dimensional (1D) metamaterial beam-foundation system is innovatively improved into a metamaterial beam-resonator-foundation system by inserting resonators into the elastic foundation for ultra-low frequency vibration attenuation and enhanced topological energy trapping. Abundant band gap characteristics are obtained including quasi-static band gap starting from 0 Hz, Bragg scattering band gaps (BSBGs), and local resonance band gaps (LRBGs). Five band folding points are obtained through the band folding mechanism which can be opened by tuning inner and outer resonance parameters. However, only three band folding induced band gaps support mode inversion and Zak phase transition, including one BSBG and two LRBGs. The topological inversion in LRBGs is rarely reported in the 1D mechanical system, which can induce topological locally resonant interface states. The underlying physical mechanism of the topological phase transition in LRBG is revealed, which results from the topological inversion band gap transition from an initial BSBG to a LRBG with resonance parameters changes. Different from conventional 1D topological metamaterials that merely utilize local resonance to lower the band frequency and achieve subwavelength topological states in BSBGs, the topological interface states in LRBGs can localize wave energy to fewer unit cells near the interface, exhibiting enhanced energy localization capacity. The topologically protected interface states are validated with defective cases, demonstrating the potential of topological metamaterials for robust energy harvesting. This study provides new insights into the topological theory of 1D mechanical systems and contributes to the development and implementation of multi-functional devices integrating vibration attenuation and energy trapping.