This work investigates improving dielectric resonator antenna (DRA) radiation bandwidth through the integration of linearly graded permittivity profiles. Using Characteristic mode analysis (CMA), it is shown that such profiles broaden modal distribution, consequently extending the bandwidth of the dominant resonant mode. The proposed antenna design is enabled by additive manufacturing techniques, which allow precise control over material composition and geometry. Experimental results show a significant improvement in both modal and impedance bandwidth from 9.1% to 24,1%, leading to a more stable gain greater than 5 dBi across the operating frequency range.This study enhances the bandwidth of (DRAs by introducing a linearly graded permittivity profile. CMA reveals that this approach broadens the modal distribution and extends the dominant resonant mode bandwidth. Enabled by additive manufacturing for precise material and geometric control, the antenna achieves a bandwidth increase from 9.1% to 24.1% with a stable gain above 5 dBi.

Glass foam composites have been fabricated and characterized for electromagnetic wave absorption application. The cullet based on recycling glass waste was used as a raw material for the manufacture of foams. Carbonic fillers of rubber powder and carbon fibers were utilized with different amounts to test the absorption efficiency of electromagnetic waves. The composites show a relatively homogeneous porous structure. The density of the produced composites varies between 213 and 347 kg/m3. Observation by scanning electron microscope shows significant degradation of rubber powders unlike carbon fibers. Gravimetric thermal analysis reveals a mass loss of 62.06

This study introduces a periodic structure composed of silicone foam reinforced with carbon fibers, designed as an electromagnetic absorber for microwave frequencies. This absorber, assimilated to an organic metamaterial, is fabricated using two simple methods. Firstly, the "particle-template" technique employs a sacrificial filler to create a composite with a low density of 0.33 g.cm⁻. Secondly, simple periodic structuring is achieved manually using scissors to form the metamaterial. The resulting absorber prototype, with a total thickness of 16 mm, demonstrates broadband absorption performance, maintaining a reflection coefficient (S11) below −10 dB from 4 to 18 GHz for both normal (0°) and oblique (30°) incidences of electromagnetic waves. These results validate the potential of this structured organic absorber.

This study reports a simple fabrication technique for creating a flexible, organic, and microwave -absorbing material using carbon fiber -loaded silicone foam. The "particle -template" elaboration method, using sugar crystals as sacrificial particles, is conducted. The material ' s density is adjusted by varying the sugar crystal concentration while ensuring effective dispersion of carbon fibers in the silicone foam matrix. The dielectric characterization of a composite with a density of 0.33 g.cm(-3) and loaded with 2 wt.% of 12 mm carbon fibers results in a permittivity ranging from 7 to 1.1 and a dielectric loss ranging from 1.9 to 0.7 across the frequency range of 2-18 GHz. These properties are used in a parametric study to optimize the overall dimensions of a structured absorber to achieve wideband absorption performance. The resulting prototype, with a total thickness of 16 mm, demonstrates promising performance by exhibiting broadband absorption (4-18 GHz), as predicted by simulation.

Bulk SrTa 2 O 6 ceramics were produced via solid-state reactive sintering, characterized by x-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy, and their dielectric properties determined via resonant cavity in the 5G frequency domain. Two polymorphs were obtained. Orthogonal beta phase ceramics, obtaining much lower relative densities, achieved, at -5 GHz, an effective permittivity ( epsilon eff ) of -22, dielectric losses (tan delta ) around 1 x 10 -2 and temperature coefficient of resonant frequency ( tau f ) of around 250 ppm/ degrees . Tetragonal tungsten bronze beta ' phase ceramics achieved relative densities of -87 %, and at -2.8 GHz a higher ' epsilon eff ' between 79 and 85, lower 'tan delta ' around 5 x 10 -3 , and a ' tau f ' of about 300 ppm/ degrees at 90 degrees C. Both phases are found to be Sr -deficient, which may have caused a liquid -phase sintering in the powder bed configuration used for these ceramics. The performance of a prototype dielectric resonator antenna incorporating a beta ' ceramic was demonstrated to validate the measured dielectric properties.

This study presents the dielectric properties of a barium titanate–gadolinium ferrite composite material, obtained through a solid-state reaction method. The aim of this research was to create a composite material with enhanced dielectric properties compared to each individual component, and to investigate the electrical properties of the composites, using impedance spectroscopy. The structural and morphologic properties were analyzed using X-ray diffraction and scanning electron microscopy, respectively. Impedance spectroscopy measurements were performed over a wide frequency range (100–0.1 GHz) and temperature (45–170 °C) to evaluate the electrical behavior of the material. The dielectric relaxations were analyzed using the Havriliak–Negami function, and the key electrical parameters such as relaxation frequency, dielectric strength, and electrical conductivity were extracted. Several relaxation processes were identified, which depend on the mixture of the initial titanate and ferrite materials, and a correlation between structural, morphologic, and electrical properties was exposed. The sample with the highest dielectric constant was the 25 wt% gadolinium ferrite composite, with ε′ close to 240 and loss tangent values below 0.1, affording it the more appropriate composition for energy storage devices such as lead-free dielectric capacitors.

This study explores the use of additive manufacturing to create ceramic dielectric resonators with controlled permittivity through the incorporation of air inclusions. Cylindrical samples of alumina and zirconia were fabricated with varying air volume fractions. Results showed that increasing air inclusions up to 95.5% reduced the permittivity from 9.4 to 1.3 in alumina and from 32.7 to 6.6 in zirconia. The effectiveness of these materials was demonstrated in antenna applications.

This work focuses on the realization of ceramic dielectric resonators with porous and complex geometry using additive manufacturing to control their effective permittivity. Several cylindrical alumina and zirconia samples integrating different volume fractions of air inclusions were developed and characterized. By incorporating up to 95.5 vol% of inclusions, we reduced the effective permittivity from 9.4 to 1.3 for alumina and from 32.7 to 7.8 for zirconia. The effectiveness of these samples in antenna applications was also demonstrated.

This work investigates the dielectric properties of barium titanate/gadolinium ferrite ceramic composites, with different concentrations of each material. Our objective was to increase the storage ability of this material, finding a compromise between high permittivity and low dielectric losses. A two-step sintering procedure was used in the preparation of the composites to attain the desired results. Their morphological, structural and electrical properties were tested using scanning electron microscopy, X-Ray powder diffraction and impedance spectroscopy, respectively. Dielectric characterizations were performed on the frequency band of 100 Hz-1 MHz and for different temperatures (180-380 K). The best compromise between barium titanate and gadolinium ferrite in the composition was calculated in order to obtain a potential material for electrical energy storage. The sample with 25% gadolinium ferrite presented the best results. The dielectric constant reached values of the order of 2000, at 1 kHz and 340 K. It was also important not to have very high losses, and this was confirmed by the calculated loss tangent.

This work deals with the use of periodic structures, composed of silicone foam loaded with long carbon fibers (12 mm), for the elaboration of microwave absorber. This absorber, which can be considered as an organic metamaterial absorber, is made using two simple manufacturing concepts: a very simple elaboration method of the flexible silicone foam composite and a simple periodic structuring of the absorber. The proposed absorber presents low density (0.33 g.cm -3 ) and broadband absorption performance (reflectivity below −10 dB) between 4 and 18 GHz, for normal (0°) and oblique (30°) incidences of electromagnetic waves, with a thickness of only 16 mm.

Original oxide ceramics, based on strontium and tantalum, are investigated for antenna applications. First, it concerns perovskite ceramics (Sr 2 Ta 2 O 7 ) 100-x (La 2 Ti 2 O 7 ) x , and, secondly, the SrTa 2 O 6 composition, belonging to the tetragonal tungsten bronze (TTB) compounds. Their synthesis is conducted by a solid-state reaction route and results in dense ceramics exhibiting moderate permittivity (∼ 80) and low losses (≤ 5.10 -3 ), that have been integrated in miniature dielectric resonator antenna (DRA) prototypes. Achieved ferroelectric perovskite ceramic based antennas present measured total efficiencies of 79% and 84%, at resonant frequencies ‘F R ’ of 5.80 GHz and 4.85 GHz, respectively, whereas the pure dielectric TTB ceramic DRA achieves an efficiency of 91% at 4.01 GHz.

In this paper, we propose a new and original absorbing material based on silicone foam loaded with long carbon fibers (12 mm). This flexible composite is structured in a periodic way, like "metamaterials". The originality of the proposed absorbing material resides, on the one hand, in the association of two simple fabrication concepts (a simple production of the silicone foam and a simple periodic structuring of this foam) and, on the other hand, in the use of dielectric lossy material to make periodic broadband absorbing structures. The idea here is to replace standard metamaterial absorbers, based on structured metallic film (often copper) deposited on a dielectric substrate, by organic metamaterial. Thus, a flexible absorber of 16 mm thickness, with the shape of a "chessboard" and having a broadband absorption bandwidth with the reflectivity below −10 dB in the frequency range of 4-18 GHz is proposed and implemented. Moreover, we also obtain a good absorption response by varying the incident angle up to 30°.

Roughness characterizes the quality of the machined surfaces, mainly determined by the geometric tolerances and induced by the different factors involved in the cutting process. The aim of this paper is to study the influence of cutting parameters (cutting speed (Vc), feed rate (f), tool nose radius (Rc) and cutting depth (ap)), on the cylinder surface roughness, in order to value the friction coefficient (μ) and the wear rate (W) of the contact surfaces of steel-steel pair: 42CrMo4-20MnCr5. Full factorial design (DOE) of 9 tests is used, to develop theoretical model of the roughness of machined metal parts on turning. Hence, the influence of the cylinder roughness on the friction and wear behavior of the steel-steel pair contact is studied, using a pin-cylinder tribometer. The results revealed that the friction coefficient and wear rate, increases progressively by varying the surface roughness (cutting parameters). The processing and observations of the results were recorded using a profilometer, an optical microscope, a scanning electron microscopy (SEM) followed by an analysis in energy dispersive spectroscopy (EDS). However, the surface of the pin is plowed, cracked and plastically deformed, thus inducing more loss of material by adhesion and oxidation of the formed particles on the cylinder, which bring about a material transfer and formation of a metal oxide layer.

In this paper, ferroelectric ceramics with (Sr2Ta2O7)(100)(-x)(La2Ti2O7)(x) (STLTO) compositions have been investigated and their dielectric properties have been characterized in wide frequency band (from few kHz to few GHz); their integration in Dielectric Resonator Antennas (DRA) was conducted. The dense STLTO ceramics have been obtained by high temperature sintering of powders synthetized by solid state chemistry route. STLTO crystalline cell parameters and volume vary linearly as a function of the chemical composition (x) thus demonstrating an ideal solid solution domain for 0 <= x <= 3. Dielectric characterizations highlight that the permittivity and the dielectric loss vary according to the composition (x) and that the lowest losses are obtained for x< 1.65 compositions. The latter corresponds to the transition between the ferroelectric and paraelectric compositions of the STLTO material at room temperature. A low profile DRA structure was realized using a cylindrical paraelectric STLTO resonator (with x = 0) with a permittivity of 83 and losses tan delta = 5 x 10(-3)@3.3 GHz. The DRA prototype was simulated, produced and tested. It exhibits a hybrid HEM11 delta mode, with a resonant frequency at 5.80 GHz, a 4.9% bandwidth and a gain of 6.4 dB. These features confirm the potential of the paraelectric STLTO compositions in compact antennas radiating at frequencies below 6 GHz. (C) 2021 Elsevier B.V. All rights reserved.

A simple methodology to fabricate ultra-porous and lightweight epoxy foam composites loaded with carbon fibers is presented. These materials are proposed for microwave absorbing application. The elaborated composite density can be controlled via proper proportioning of carbon fiber/solvent/surfactant/epoxy components constituting the absorber composites. The influence of the weight percentage of these different components, on the composite structure, is studied. The density of the composites can be reduced to 0.05g.cm(-3) while the density of the neat epoxy foam is about 0.12g.cm(-3). Dielectric characterization of composites elaborated with low weight percentages of carbon fibers (from 0.0125 wt.% to 0.075 wt.%) is conducted using free space method. The simulation of a multilayer absorber is conducted and predicts a promising absorption performance (reflection coefficient <-10 dB) for the wide studied frequency band; this result was confirmed by the measurement of a multilayer absorber prototype. Finally, these performances are compared to commercial absorber materials having same dimensions.

In this paper, we propose a multi-resonance MM design which will be used for the realization of an ultra-wideband hybrid microwave absorber. The metamaterial absorber consists of a symmetrical structure, called V-shape, with different scales of coupled resonators and the hybrid absorber is carried out by the association of this metamaterial to a lossy dielectric layer, made of epoxy foam composite loaded with low weight percentage (0.075 wt.%) of 12 mm length carbon fibers. The simulation and measurement results of the hybrid material of 16.2 mm thickness show an absorption bandwidth between 2.6 GHz and 18 GHz for both normal and oblique incidences.

A miniature frequency-tunable slot-loop antenna (0.128λ 0 × 0.077λ 0 ) based on ferroelectric K 0.5 Na 0.5 NbO 3 (KNN) interdigitated varactors is investigated at Ku-band. KNN material has been selected among the ferroelectric oxide family because of its competitive properties for microwave applications. Its high permittivity value (ϵ r ≈ 360 at 10 GHz) and its wide frequency tunability under an external static electric field E bias make it a promising candidate for frequency-agile antennas and circuits at microwaves. The antenna prototype has been designed and elaborated on MgO substrate (ϵ r = 9.8 and tanδ ≈ 10 -4 at 10 GHz). Fabrication details including laser microetching process of the ferroelectric layer are also described. The center frequency of the slot-loop antenna can therefore be tuned monotonously from 15.22 to 15.97 GHz under E bias = 88 kV/cm, leading to a frequency tunability rate equal to 4.6%. These results demonstrate for the first time the ability of the KNN material to be used for miniature tunable antennas, in comparison with Ba x Sr 1-x TiO 3 , currently considered as the most relevant material for such applications.

This article presents a strategy for designing optimal microwave planar multilayer absorbers based on epoxy foam composites loaded with carbon fibers of 12 mm length. Firstly, the impedance gradient principle (gradual loaded composites) was adopted to realize two multilayer absorbers, of 125 mm thickness, using slightly loaded composites (0.0125 wt% < CFs < 0.075 wt%) and relative highly loaded composites (0 wt% < CFs < 0.4 wt%), respectively. The simulation of these absorbers shows that composites with very low CF rates are sufficient to achieve a very close absorption performance and bandwidth to that of the commercial absorber, in the entire studied frequency range (0.75–18 GHz). Secondly, the genetic algorithm optimizer is used to achieve a multilayer absorber that presents the best compromise between absorption performance and thickness. Different CF-loaded composites and layer thicknesses are therefore tested; a multilayer absorber with a total thickness of 98 mm is then proposed. This absorber shows a better reflection coefficient and a better compromise (absorption/total thickness) than that of the commercial absorber, while presenting a reduction of 22% in thickness. The presented simulation and measurement results confirm that a judicious choice of the composition and the thickness of each layer is necessary to optimize the absorption performance of a planar multilayer absorber. This paper also shows the advantage of using an optimizer to improve the absorption performance while reducing the total thickness of the absorber. Graphical abstract