Unit Cell Structure Research Articles

Enhancing the Terahertz Absorption Spectrum Based on the Low Refractive Index All-Dielectric Metasurface

We presented an angle-multiplexing low refractive index dielectric metasurface for terahertz absorption spectrum enhancement of lactose with high Q resonance properties. The unit cell structure of the acrylonitrile butadiene styrene (ABS) square structure is designed and optimized. The resonant peak shifts of the dielectric metasurface are analyzed by changing the incident angle of the terahertz wave. Then the α-lactose films with different thicknesses are added to investigate the enhancement effect. The resonant peak amplitude of the angle-multiplexing low refractive index dielectric metasurface absorption spectrum changes greatly with the absorption spectrum of the samples. The results illustrate that the enhanced absorption spectrum formed by the linked envelope can be up to 45 times stronger than that without the unit cell structures and also exhibits a high-quality factor. The proposed dielectric metasurface provides great potential in enhancing the terahertz absorption spectrum.

An FSS structure with a band‐stop performance for UWB applications

AbstractIn this paper, a frequency selective surface (FSS) structure with a band‐stop performance for ultra‐wideband (UWB) applications is introduced. The unit cell of the FSS structure consists of a combination of a square loop, a ring loop, and a cross dipole with four gaps. The unit cell has 0.115λg × 0.115λg size where λg is the wavelength in the substrate corresponding to the minimum frequency (2.79 GHz) of the operating band. The bandwidth of the FSS design is from 2.79 to 10.81 GHz frequencies (at normal incidence and |S21| < −3 dB criterion) which covers all UWB bands. The introduced FSS structure has angular stability up to 80° incident angles both TM and TE polarizations due to the small size of the unit cell. The highest resonance frequency deviation is 0.08% at 80° and this value is quite low. The proposed FSS structure was fabricated and the results obtained through simulations were supported by the measurement results.

Multiscale topology optimization of electromagnetic metamaterials using a high-contrast homogenization method

This study proposes a multiscale topology optimization method for electromagnetic metamaterials using a level set-based topology optimization method that incorporates a high-contrast homogenization method. The high-contrast homogenization method can express wave propagation behavior in metamaterials for various frequencies. It can also capture unusual properties caused by local resonances, which cannot be estimated by conventional homogenization approaches. We formulated multiscale topology optimization problems where objective functions are defined by the macroscopic wave propagation behavior, and microstructures forming a metamaterial are set as design variables. Sensitivity analysis was conducted based on the concepts of shape and topological derivatives. As numerical examples, we offer optimized designs of metamaterials composed of multiple unit cell structures working as a demultiplexer based on negative permeability. The mechanism of the obtained metamaterials is discussed based on homogenized coefficients.

Sound transmission of active elastic wave metamaterial with double locally resonant substructures surrounded by external mean flow

The elastic wave metamaterial with active feedback control is presented to discuss its dynamic effective parameter and sound transmission properties. Both studies are initiated from the local resonance with the active feedback control and dynamic structural response. The acoustic-structure coupling of the metamaterial with resonators and orthogonal stiffeners are analyzed with effects of the external mean flow. Different analytical methods are used to derive the effective mass density and sound transmission loss (STL) of the elastic wave metamaterial. The effective density is obtained by the dynamic effective medium method firstly. And the second one applies the spatial harmonic expansion method with the principle of virtual work for the STL based on the Bloch–Floquet theorem and Poisson summations. The effective mass density shows frequency-dependent properties and appears positive and negative values in a certain frequency region simultaneously. This research also illustrates both characteristics can be tuned by the active feedback control with the acceleration and displacement feedback control actions. Numerical results of coupled structures show that the changes of transmission characteristics in the certain frequency region are consistent with the effective mass density properties of uncoupled unit cell structure. In addition, influences of the lattice constants, incident angles and Mach number on the STL are illustrated and discussed.

Two-Dimensional Distributed Transmission-Line Models for Broadband Full-Tensor Anisotropic Acoustic Metamaterials Based on Transformation Acoustics

We propose the design method for broadband acoustic metamaterials based on the concept of transformation acoustics. Two-dimensional distributed transmission-line (TL) models for full-tensor anisotropic electromagnetic metamaterials are applied to full-tensor anisotropic acoustic metamaterials and the design formulas are shown to uniquely determine the structural parameters of the unit cells. Two-dimensional acoustic waveguide unit cell structures for realizing the TL models are proposed and an acoustic carpet cloak and an acoustic illusion medium are designed according to the introduced theory. The complex sound pressure distributions in the acoustic waveguides of the unit cells are calculated by full-wave simulations to verify the validity of the proposed method, and the broadband operations of the designed carpet cloak and illusion medium are confirmed from the results.

A Triple-Band Reflective Polarization Conversion Metasurface with High Polarization Conversion Ratio for Ism and X-Band Applications.

A compact and triple-band polarization converting reflective type metasurface (PCRM) with a high polarization conversion ratio (PCR) is proposed for strategic wireless antenna-integrated applications. The unit cell of the metasurface is composed of S- and G-shaped patches separated with a parasitic gap and the grounded via is connected to the full ground plane. The unit cell is etched on an FR4 substrate (dielectric constant, εr = 4.4, loss tangent, tan δ = 0.02), with compact dimensions of 10 mm3 × 10 mm3 × 1.6 mm3. This structure provides a resonance at 5.2 (ISM), 6.9, and 8.05 GHz (X-band) frequencies. The designed unit cell structure is studied for Transverse Electric (TE)/Transverse Magnetic (TM) incident waves and their responses to the various incident angles. The corresponding PCR is calculated, which shows 92% in the lower frequency band (5.2 GHz), 93% in the second frequency band (6.9 GHz), and 94% in the high-frequency band (8.05 GHz). The total efficiency of the structure shows 83.2%, 62.95%, and 64.6% at the respective resonance bands. A prototype of the proposed PCRM with 3 × 3 unit cells is fabricated to validate the simulated results. The experimental data agrees with the simulation results. The compactness, triple-band operation with a high PCR value of more than 92% makes use of the designed metasurface in wireless antenna-integrated applications at ISM and X-bands.

Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently.

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

Design and Robustness Evaluation of Valley Topological Elastic Wave Propagation in a Thin Plate with Phononic Structure

Based on the concept of band topology in phonon dispersion, we designed a topological phononic crystal in a thin plate for developing an efficient elastic waveguide. Despite that various topological phononic structures have been actively proposed, a quantitative design strategy of the phononic band and its robustness assessment in an elastic regime are still missing, hampering the realization of topological acoustic devices. We adopted a snowflake-like structure for the crystal unit cell and determined the optimal structure that exhibited the topological phase transition of the planar phononic crystal by changing the unit cell structure. The bandgap width could be adjusted by varying the length of the snow-side branch, and a topological phase transition occurred in the unit cell structure with threefold rotational symmetry. Elastic waveguides based on edge modes appearing at interfaces between crystals with different band topologies were designed, and their transmission efficiencies were evaluated numerically and experimentally. The results demonstrate the robustness of the elastic wave propagation in thin plates. Moreover, we experimentally estimated the backscattering length, which measures the robustness of the topologically protected propagating states against structural inhomogeneities. The results quantitatively indicated that degradation of the immunization against the backscattering occurs predominantly at the corners in the waveguides, indicating that the edge mode observed is a relatively weak topological state.

Multi-octave metasurface-based refractory superabsorber enhanced by a tapered unit-cell structure

An ultra-broadband metasurface-based perfect absorber is proposed based on a periodic array of truncated cone-shaped text {TiO}_2 surrounded by TiN/text {TiO}_2 conical rings. Due to the refractory materials involved in the metasurface, the given structure can keep its structural stability at high temperatures. The proposed structure can achieve a broadband spectrum of 4.3 µm at normal incidence spanning in the range of 0.2–4.5 µm with the absorption higher than 90% and the average absorption around 94.71%. The absorption can be tuned through the angle of the cone. By optimizing geometrical parameters, a super absorption is triggered in the range of 0.2–3.25 µm with the absorption higher than 97.40% and substantially average absorption over 99%. In this regard, the proposed structure can gather more than 99% of the full spectrum of solar radiation. Furthermore, the absorption of the designed structure is almost insensitive to the launching angle up to 50^circ for TE polarization, while it has a weak dependence on the incident angle for TM polarization. The proposed structure can be a promising candidate for thermal energy harvesting and solar absorption applications.

A tunable frequency selective surface integrated high isolation MIMO antenna for THz applications

A novel tunable triple stop band frequency selective surface (FSS) is designed and integrated with the multiple inputs multiple outputs (MIMO) antenna as an isolation structure to verify its validity. The proposed FSS renders excellent isolation characteristics for the reported MIMO system which is verified from the ECC and DG plot. In FSS, the graphene materials are attached at the end of three stubs to realize tuning over the respective bands. The center frequency of the attained triple bands shows an obvious shift from 2.2 to 2.4 THz, 3.15 to 3.5 THz, and 4.6 to 4.9 THz on increasing the chemical potential of graphene from 0 to 1.5 eV. The tuning range can be enhanced by further increasing the applied potential and hence its service might extend to multi or wideband MIMO systems. The proposed FSS unit cell achieves miniaturization to the level of 0.17 λ o. The validity of the proposed FSS is tested with a linear array MIMO antenna. The designed MIMO antenna consists of two linear array antennas separated by 40 μm with each linear array is made up of three individual antenna elements. The introduction of FSS between the linear arrays provides the minimum isolation of 30 dB. The presented highly isolated linear array MIMO antenna realizes maximum efficiency and gain of 83%, 12.2 dBi respectively. The FSS unit cell structure is modeled and their electromagnetic characteristics are simulated by the 3D electromagnetic simulator CST Studio Suite. The auto meshing technology and frequency domain solver are used to acquire S-parameter. This wearable antenna is a significant component in tracking and monitoring systems including the miniaturized multiband antennas exhibiting stable radiation characteristics during bending, stretching, and crumpling conditions to suit military applications. Nowadays, wearable antennas are being designed for military applications by perfectly matching the industry requirements such as thin, low profile, lightweight, low maintenance, robust, conformal, easy integration into clothing without affecting the movement of the soldier.

Description of crystal defect properties in BCC Cr with extended Finnis–Sinclair potential

PurposeThe purpose of this study is to propose extended potentials and investigate the applicability of extended Finnis–Sinclair (FS) potential to Cr with the unit cell structure of body-centered cubic (BCC Cr).Design/methodology/approachThe parameters of each potential are determined by fitting the elastic constants, cohesive energy and mono-vacancy formation energy. Furthermore, the ability of the extended FS potential to describe the crystal defect properties is tested. Finally, the applicability of reproducing the thermal properties of Cr is discussed.FindingsThe internal relationship between physical properties and potential function is revealed. The mathematical relationship between physical properties and potential function is derived in detail. The extended FS potential performs well in reproducing physical properties of BCC Cr, such as elastic constants, cohesive energy, surface energy and the properties of vacancy et al. Moreover, good agreement is obtained with the experimental data for predicting the melting point, specific heat and coefficient of thermal expansion.Originality/valueIn this study, new extended potentials are proposed. The extended FS potential is able to reproduce the physical and thermal properties of BCC Cr. Therefore, the new extended potential can be used to describe the crystal defect properties of BCC Cr.

Dielectric Studies of Tin Oxide and Tin Dioxide Added with Barium Titanate

In the present work, tin oxide (SnO) and tin dioxide (SnO2) are added to barium titanate in various weight percentages (0%, 2%, 4%, and 6%) and sintered at 800° C for 4 h. The primary peak angle shifts to the lower angle, indicating changes in unit cell structure. The dielectric constant reduces initially for 2% SnO and SnO2, but then increases, showing that tin accumulates in oxides form, resulting in an increase in polarization. The dielectric constant of SnO2 is found to be greater than that of SnO, illustrating the role of the oxygen ion.

In-depth investigations of size and occupancies in cobalt ferrite nanoparticles by joint Rietveld refinements of X-ray and neutron powder diffraction data.

Powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) have been used to investigate the crystal structure of CoFe2O4 nanoparticles prepared via different hydro-thermal synthesis routes, with particular attention given to accurately determining the spinel inversion degrees. The study is divided into four parts. In the first part, the investigations focus on the influence of using different diffraction pattern combinations (NPD, Cu-source PXRD and Co-source PXRD) for the structural modelling. It is found that combining PXRD data from a Co source with NPD data offers a robust structural model. The second part of the study evaluates the reproducibility of the employed multipattern Rietveld refinement procedure using different data sets collected on the same sample, as well as on equivalently prepared samples. The refinement procedure gives reproducible results and reveals that the synthesis method is likewise reproducible since only minor differences are noted between the samples. The third part focuses on the structural consequences of (i) the employed heating rate (achieved using three different hydro-thermal reactor types) and (ii) changing the cobalt salt in the precursors [aqueous salt solutions of Co(CH3COOH)2, Co(NO3)2 and CoCl2] in the synthesis. It is found that increasing the heating rate causes a change in the crystal structure (unit cell and crystallite sizes) while the Co/Fe occupancy and magnetic parameters remain similar in all cases. Also, changing the type of cobalt salt does not alter the final crystal/magnetic structure of the CoFe2O4 nanoparticles. The last part of this study is a consideration of the chemicals and parameters used in the synthesis of the different samples. All the presented samples exhibit a similar crystal and magnetic structure, with only minor deviations. It is also evident that the refinement method used played a key role in the description of the sample.

İşlevsel Geçişli Gözenekli Yapıların Tasarımı ve Eklemeli Üretilen Parçalar Arasındaki Uyumluluğun Araştırılması

In this study, deviations for the porosity level of the Ti-6Al-4V functionally graded porous structures for three different cell structures were investigated. For this purpose, functionally graded porous structures are designed and produced by selective laser melting (SLM). It is also aimed to investigate the effects of unit cell structure, unit cell size, and column (strut) thickness on the porosity deviation level. The specimens were scanned at micro-computed tomography (micro-CT) to determine the structure dimensions after production. According to the results obtained from micro-CT, an average increase of 150-300 μm was observed on the column thicknesses of all functionally graded porous structures. It has been observed that the horizontal columns of cubic and octagonal structures have sagging due to metal melting during production. It has been determined that the porosity of the manufactured parts was deviated between 5.71%-10.54% for cubic, 8.59%-12.39% for octahedroid, and 13%-16.49% for diamond structures compared to the design values.

Development of spatially variant photonic crystals to control light in the near-infrared spectrum

Spatially Variant Photonic Crystals (SVPCs) have shown the ability to control the propagation and direction of light in the near-infrared spectrum. Using a novel approach for simplified modeling and fabrication techniques, we designed unique, spatially-varied, unit-cell structures to develop photonic crystals that maintain self-collimation and direction of light for desired beam tuning applications. The finite-difference time-domain technique is used to predict the self-collimation and beam-bending capabilities of our SVPCs. These SVPC designs and the simulation results are verified in laboratory testing. The experimental evidence shows that two-dimensional SVPCs can achieve self-collimation and direct light through sharp bends. The simplicity and quality of these designs show their potential for widespread implementation in modern devices. These SVPCs will serve as a unique solution to optical systems for optical computing, multiplexing, data transfer, and more.

Abnormal thermal quenching and optical performance of new niobate perovskite Sr3LaNb3O12:Sm3+ phosphors for w-LEDs

Novel orange-red Sr3LaNb3O12 (SLNO):xSm3+ (0.01 ≤ x ≤ 0.03) phosphors are successfully prepared using the traditional solid-state method. The feasibility that the SLNO compounds can act as an appropriate host has been effectively demonstrated by unit cell structure and bandgap structure. The phase purity, particle size distribution, and elemental composition of SLNO:Sm3+ phosphors are studied. The luminescence properties, concentration-quenching mechanism, color purity, thermostability, CIE chromaticity coordinates, and correlated color temperature (CCT) of the obtained SLNO:Sm3+ products are also discussed. Under 408 nm excitation, SLNO:0.10Sm3+ sample emits a bright orange-red light at 598 nm owing to the 4G5/2 to 6H7/2 transition of Sm3+. The concentration-quenching mechanism is found to be dipole-dipole interaction by the calculated Rc value (17.12 Å) and Dexter's theory. Impressively, the color purity of the phosphors is high and stable (all reach 99.9%), which is rarely affected by the Sm3+ doping concentration. It is worth noting that the obtained phosphors exhibit an abnormal thermal quenching (ATQ) behavior, increasing by 16.12% from 300 to 380 K (SLNO:0.10Sm3+). The CIE x and y chromaticity coordinates vary slightly with the change of temperature, indicating a good thermostability. The mechanism of ATQ has been studied as the lattice defects, and the thermal activation energy is calculated to be 0.51 eV. Ultimately, a white-light-emitting diode (w-LED) with a high color rendering index (Ra) of 88 and a CIE chromaticity coordinate of (0.333, 0.333) has been prepared. The results above reveal the potential of SLNO:Sm3+ to be widely used in w-LEDs.

Investigation of the Performance of Ti6Al4V Lattice Structures Designed for Biomedical Implants Using the Finite Element Method

The development of medical implants is an ongoing process pursued by many studies in the biomedical field. The focus is on enhancing the structure of the implants to improve their biomechanical properties, thus reducing the imperfections for the patient and increasing the lifespan of the prosthesis. The purpose of this study was to investigate the effects of different lattice structures under laboratory conditions and in a numerical manner to choose the best unit cell design, able to generate a structure as close to that of human bone as possible. Four types of unit cell were designed using the ANSYS software and investigated through comparison between the results of laboratory compression tests and those of the finite element simulation. Three samples of each unit cell type were 3D printed, using direct metal laser sintering technology, and tested according to the ISO standards. Ti6Al4V was selected as the material for the samples. Stress–strain characteristics were determined, and the effective Young’s modulus was calculated. Detailed comparative analysis was conducted between the laboratory and the numerical results. The average Young’s modulus values were 11 GPa, 9 GPa, and 8 GPa for the Octahedral lattice type, both the 3D lattice infill type and the double-pyramid lattice and face diagonals type, and the double-pyramid lattice with cross type, respectively. The deviation between the lab results and the simulated ones was up to 10%. Our results show how each type of unit cell structure is suitable for each specific type of human bone.

A multiband frequency reconfigurable and bifunctional metasurface

ABSTRACT A multiband, frequency reconfigurable, and bifunctional metasurface for simultaneous full-space control of Electromagnetic (EM) waves is proposed for wearable applications. The unit-cell structure contains concentric rectangular rings and empty substrate-integrated waveguides (SIWs) for miniaturisation. The miniaturised dimension of the Polydimethylsiloxane (PDMS) substrate is 18 × 18 × 1 mm3. Two varactor diodes are integrated for frequency switching operations by configuring the structure using bias voltage (forward/reverse) through the ground via the proposed surface resonates at 3.5,5.8,7.6 (On),8.1 (Off) GHz frequencies, which provide simultaneous bi-functionalities: artificial Magnetic Conductor (AMC) reflector/absorber in reflection and transmission space of operations, respectively. In reflection space, it acts as an AMC reflector by providing in-phase reflection with a radiation efficiency of >84% and unidirectional E-field patterns. However, the magnitude of absorbance is more than 96% in transmission, thereby simultaneously controlling full-space EM wave in normal incidence. The obtained simultaneous metasurface functionalities are investigated for different oblique angles of the transverse electric (TE) and transverse magnetic (TM) incident wave. The proposed surface is fabricated and experimentally validated. The features of the designed bifunctional metasurface provide a strategic platform for potential multifunctional wearable devices.

A simple green synthesis of pure and sterling silver nanoparticles via pulsed laser ablation in deionized water: characterization and comparison

Pure silver (Ag) and its alloy nanoparticles (NPs) with intense and tunable SPR bands in the visible region are widely exploited for biosensors, information storage, and solar energy systems. Pure Ag and Sterling silver (Ag92.5Cu7.5) NPs were synthesized by the laser ablation method in deionized water using a pulsed Nd:YAG laser. The prepared NPs were characterized and compared for their structural, morphological, and optical properties. The x-ray diffraction (XRD) and selected area electron diffraction (SAED) results revealed that the NPs have polycrystalline nature with five lattice directions. The diffraction peak positions for Ag92.5Cu7.5 NPs exhibited an average redshift of 0.1 ̊ compared to pure Ag NPs due to the presence of copper atoms in the composite crystal unit cell structure. The formation of spherical NPs with an average size of 9.1 nm and 8.4 nm for Ag and Ag92.5Cu7.5 NPs was confirmed using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). It was found that the concentration of synthesized Ag92.5Cu7.5 alloy NPs was considerably higher than that of pure Ag NPs. Going from pure to alloy silver NPs, the wavelength of surface plasmonic resonance (SPR) peak shifted from 400 nm to 395 nm. The UV–vis absorption spectra at different aging times revealed that pure Ag colloidal solution is relatively stable. Both colloidal solutions exhibited a similar pattern of photoluminescence (PL) emission spectra with peaks in the blue region.

Superparamagnetic and Perfect-Paramagnetic Zinc Ferrite Quantum Dots from Microwave-Assisted Tunable Synthesis.

The properties of quantum dot (QD)-size material depend directly upon its unit cell structure. Spinel zinc ferrite QD powder is produced via a one-pot microwave-assisted hydrothermal synthesis for just 5 min. Varying initial pH values of the preparation sol from 6 to 12 enlarges the Zn/Fe atomic ratio (by ca. 10%), unit cell volume (by ca. 0.5%), particle size (3.5–4.5 nm), and degree of inversion. This leads to a change in the magnetic behavior of the QD-size zinc ferrite from a superparamagnetic to a perfect-paramagnetic type. This novel finding points that the significant changes in the inherent structural parameters of spinel ZnFe2O4 QDs (Zn/Fe ratio and degree of inversion) induced by the systematic pH change of the preparation sol are exclusively responsible for the observed unique magnetic behavior instead of mere QD (single domain) nanosizes.