High-performance Microwave Research Articles

Lightweight and High-Performance Microwave Absorbing Carbon Derived from Wasted bamboo

With the rapid advancements in communication, broadcasting, and statellite navigation technologies, the issue of electromagnetic wave pollution has emerged as a pressing research focus. Electromagnetic wave (EMW) absorbing materials provide a promising and practical solution to address this challenge. This paper presents a novel approach for fabricating electromagnetic wave absorbing material using in situ impregnation and pyrolysis of bamboo's inherent porous structure. By optimizing dielectric loss and integrating multiple magnetic iron phases, the material's impedance matching and magnetic loss characteristics were significantly enhanced, resulting in efficient electromagnetic wave absorption. Through adjusting the concentration of the precursor iron solution, the material attained a maximum reflection loss of -61.2dB and an bandwidth of 4.982GHz. Simulation results further validate the superior electromagnetic wave absorption performance of the modified material. In conclusion, this study introduces an eco-friendly electromagnetic wave absorbing composite, presenting a new pathway for the development of next-generation electromagnetic wave absorbing materials.

Synergistic dielectric regulation strategy of one-dimensional MoO2/Mo2C/C heterogeneous nanowires for electromagnetic wave absorption

Dielectric-dielectric nanocomposites with excellent synergistic effects are considered as a prospective avenue for the development of high-performance microwave absorbers. In this work, one-dimensional MoO2/Mo2C/C heterogeneous nanowires were synthesized via a straightforward co-precipitation and in situ pyrolysis process. The components were modulated by adjusting the reduction temperatures to achieve tunable electromagnetic parameters thereby optimizing the dielectric loss. The results showed that the optimized MoO2/Mo2C/C composite exhibited a minimum reflection loss of -50.7 dB at an ultra-thin thickness of 1.8 mm and an effective absorption bandwidth of 6 GHz at 2.3 mm when the calcination temperature was 700 °C. Based on the studies of electromagnetic parameters and radar cross section simulation outcomes, both the interwoven one-dimensional structure and synergistic effects between the components have endowed the material with good impedance matching and introduced various loss mechanisms such as conductivity loss, multiple polarization relaxation, and multiple reflection/scattering. This work presented a viable strategy for the preparation of one-dimensional MoO2-based dielectric microwave absorption materials.

Controlled preparation of lightweight, resilient helical carbon fibers for high-performance microwave absorption and oil-water separation

Controlled preparation of lightweight, resilient helical carbon fibers for high-performance microwave absorption and oil-water separation

Preparation of MWCNT/Fe3O4/Si3N4 ternary composite material and microwave absorption properties

MWCNT/Fe3O4/Si3N4 composite was synthesized via hydrothermal synthesis. The morphology and structure of MWCNT/Fe3O4/Si3N4 were investigated by means of SEM, XPS, XRD, and FT-IR. Their electromagnetic parameters were measured by a vector network analyzer. The microwave attenuation properties of as-synthesized composite could be modulated by regulating the feed ratio of MWCNT, Fe3O4, and Si3N4. According to the different amounts of modified nano Fe3O4 added, they are labelled as S1-S3. When the feed ratio is 3:15:20, the maximum reflection loss value of the sample is −23.3 dB at 14.4 GHz, the corresponding thickness is 2.5 mm, and the corresponding effective absorption bandwidth is 5.36 GHz. Moreover, the test results of the thermogravimetric analyzer show that the composite material has excellent heat resistance. Our research results provide a reference value for the development of excellent heat resistance and high-performance MWCNT-based microwave absorption composite materials.

Magnetic isocyanate-based polyimide composite foam for efficient microwave absorption

Developing and fabricating high-performance microwave absorption materials with excellent comprehensive properties becomes an urgent necessity. In this work, a facile magnetic isocyanate-based polyimide foam with strong interfacial interaction is fabricated by vacuum-impregnating carbon nanotube (CNT)/anisotropic iron flake polyamide acid (PAA)-suspension on the surface of the skeleton. The successful loading of conductive CNT and anisotropic iron flake can facilitate the optimization of impedance matching and the generation of multiple loss mechanisms in foam, endowing it with an efficient microwave absorption performance. More importantly, self-enhancement effect of PAA as the precursor of polyimide significantly reinforces the interfacial interaction between foam and CNT/anisotropic iron flake, due to the similar molecular structure with the isocyanate-based polyimide. The strong interfacial interaction combined with their intrinsic properties further contributes to the improvement of stability and durability, such as high/low-temperature, corrosion, and flame resistance. Therefore, such excellent comprehensive performance makes it possible to become a promising defense material to be applied in harsh marine environments.

Preparation of high performance microwave absorbing materials based on waste peanut shells

Abstract The emergence of advanced wave-absorbing materials has led to a growing interest in biomass-derived porous carbon due to its abundant availability, low density, and eco-friendly nature. This study utilized peanut shell biomass waste to produce porous carbon material (KPS) through a two-step carbonation-activation process. We investigated the influence of varying ratios of carbonized peanut shells (CPS) to potassium hydroxide (KOH) activator on the pore structure and morphology of KPS. The comprehensive analysis of the electromagnetic parameters revealed that KPS sample achieved a remarkable reflection loss of −42.38 dB and an effective bandwidth of 2.58 GHz with the thickness of 2.5 mm when the quality ratio of CPS to activator KOH was 1:3 and the carbonation temperature was 600 °C. Notably, the KPS material demonstrated substantial specific surface area of 1037.589 m2 g−1 and complex pore architecture, facilitating the development of conductive network and promoting multiple reflections of electromagnetic waves. This research highlighted the potential of efficiently utilizing recyclable resources to create microwave-absorbing materials that are not only simple to produce but also environmentally sustainable.

Tunable magnetic properties and microwave absorbing properties of (Nd1-xYx)2Fe17N3-δ

Magnetic microwave absorbing materials of rare earth-transition metal (R-T) intermetallic compounds have the advantage of high absorbing performance, low matching thickness, and rich tunability. In this work, we report the magnetic and microwave absorbing properties of R-T intermetallic compounds (Nd1-xYx)2Fe17N3-δ (x = 0 ∼ 1). The crystalline structure of these compounds transitions from Th2Zn17-type rhombohedral (for x < 0.6) to Th2Ni17-type hexagonal (for x > 0.8) as the composition varies, attributed to the reduction in lattice volume. The saturation magnetization (Ms) of the (Nd1-xYx)2Fe17N3-δ remains almost unchanged as x changes. However, the out-of-plane anisotropy field decreases significantly from 111kOe to 25kOe while x increases. The high-frequency magnetic permeability of the material is also significantly elevated and thus leads to a significant enhancement in microwave absorbing performance. A 5.8 GHz maximum effective absorption bandwidth (EAB, RL < -10 dB) could be achieved within a thickness of only 1.1 mm for Y2Fe17N3-δ(x = 1)/paraffin composite primarily due to its heavy magnetic loss. Furthermore, the minimum reflection loss (RL) reaches −49 dB at 6.6 GHz with a thickness of 2.2 mm. The tunable magnetic properties and excellent microwave absorbing performances of (Nd1-xYx)2Fe17N3-δ make it a strong contender for applications in high-performance microwave absorbing materials.

One-step solvothermal synthesis of hollow Fe3O4/single walled carbon nanohorns composites with excellent microwave absorption properties

With the increasingly serious electromagnetic radiation and pollution, high-performance microwave absorption materials are urgently needed. In this work, the hollow Fe3O4/single-walled carbon nanohorns (SWCNHs) composites have been synthesized through one-step solvothermal method. The compositions, morphologies, microstructures, and microwave absorption performance of the hollow Fe3O4/SWCNHs composite have been comprehensively investigated. Benefitting from the unique hollow spherical structure and synergistic effects of dielectric loss and magnetic loss, the as-obtained hollow Fe3O4/SWCNHs composite exhibits an optimum reflection loss of −46.9 dB at 16.8 GHz with a matching thickness of 1.5 mm, and a broad effective absorption bandwidth of 7.21 GHz ranging from 10.79 to 18 GHz with a thickness of 2.0 mm, suggesting that the hollow Fe3O4/SWCNHs composite can be used for high-efficiency microwave absorption.

A Developed Approach for Synthesizing Novel Fe3O4/FeO/BaCl2 Composites with Broadband and High-Efficiency Microwave Absorption Performance.

Designing high-performance microwave absorbing materials that are thin and exhibit strong absorption capabilities across a wide frequency range is critical for mitigating electromagnetic pollution through a simple, highly adaptable, and cost-effective approach. However, achieving these three targets remains a significant challenge. In this research a simple approach suitable for large-scale production of microwave absorbing materials, namely, Fe3O4/FeO/BaCl2 composites, is proposed, which includes the processes of chemical coprecipitation and calcination. The above approach can adjust the mass ratio of Fe3O4/FeO while prompt the formation of BaCl2 with mesoporous structure on the surface of Fe3O4/FeO, meeting the need for desirable microwave absorbing performance. Subsequently, the impacts of varying mass ratios of the Fe3O4/FeO/BaCl2 composites on microstructures, magnetic properties, and microwave absorption properties were examined. Based on this investigation, a mass ratio close to 3.5:5.5:1 was determined to be optimal. At this ratio, the Fe3O4/FeO/BaCl2 composites realize an effective absorption bandwidth of 6.70 GHz at only 1.16 mm thickness, covering the whole Ku-band, and the maximum reflection loss can be close to -46.8 dB at 1.4 mm. The robust microwave absorption performance of Fe3O4/FeO/BaCl2 composites can be attributed to heterostructured multi-interface structural design, the comprehensive effects of multiple reflections and dielectric/magnetic losses induced by BaCl2 with mesoporous structure as well as the aggregated Fe3O4/FeO particles. This work may offer insights into designing and preparing effective microwave absorption materials.

The microwave absorption performance of CF coated with MnO2 nanowires grown by simple hydrothermal method

Microwave absorber can effectively reduce the influence of electromagnetic interference. MnO2 nanowires were deposited on the CF surface by adopting a simple one-step hydrothermal method to fabricate MnO2/CF composites in order to obtain a high-performance microwave absorber. Its microwave absorption (MA) performance and microstructure could be manipulated by adjusting the coating amount. The introduction of MnO2 makes composites possess good MA performance by improving impedance matching. The minimum reflection loss (RLmin) of the MnO2/CF composite is −42.9 dB (1.2 mm, 17.6 GHz), and its effective absorption bandwidth (EAB) reaches 3.84 GHz (1.6 mm). Moreover, its effective absorption band Δƒ (0.5–4 mm thickness) reaches 11.6 GHz. It shows that the MnO2/CF composite could be regarded as an ideal microwave absorber for high frequency.

Fabrication of hollow Fe3O4 microspheres encapsulated by single-walled carbon nanohorns for high-performance microwave absorption

Due to the increasingly serious electromagnetic radiation and pollution problems, it is urgent to develop high-performance electromagnetic wave absorbing materials. Herein, the hollow Fe3O4 microspheres encapsulated by single-walled carbon nanohorns (H-Fe3O4@SWCNHs) have been synthesized via facile solvothermal approach. The obtained H-Fe3O4@SWCNHs composite exhibits a unique hollow core-shell structure and superior microwave absorption performance. When the matching thickness is 2.0 mm, the composite shows a minimum reflection loss value of −42.2 dB at 13.6 GHz and a maximum effective absorption broadband value of 6.2 GHz covering from 11.8 to 18 GHz. The excellent microwave absorption performance of the H-Fe3O4@SWCNHs composite is mainly attributed to the unique hollow core-shell structure, optimal impedance matching, and synergistic effects of dielectric loss and magnetic loss. Therefore, the present work provides a new kind of ideal microwave absorbing material with light weight, thin thickness, wide bandwidth and strong absorption capability for high-performance microwave absorption.

High-performance honeycomb porous microwave absorbers: Gelatin-based carbon nanotube/nickel nanowire aerogels

Biomass-derived carbon absorbent materials have received significant attention due to their renewability, diverse sources, and ease of preparation. In this work, a honeycomb porous electromagnetic wave absorber with excellent performance was successfully prepared by blending gelatin and multi-walled carbon nanotubes (MWCNTs), followed by freeze-drying. Metallic nickel nanowires (NiNWs) were then introduced as a second phase using an in-situ growth method. The minimum reflection loss (RL) of the aerogel containing 8.06 wt% MWCNTs and 3.36 wt% NiNWs was −36.1 dB at a thickness of 3.1 mm, with the effective absorption bandwidth (EAB) covering the entire X-band. Additionally, the aerogel exhibited a very low density (36 mg/cm3) and remarkable strength, capable of supporting over 1658 times its weight for more than 24 h without deformation. These properties make it highly valuable for commercial use and provide new inspiration for the future development of biomass carbon-based electromagnetic absorbing materials.

Ultralight Cellulose-Derived Carbon Nanofibers from Freeze-Drying Emulsion Towards Superior Microwave Absorption

Carbon nanofibers (CNFs) are usually prepared by the carbonization of cellulose aerogels obtained from freeze-drying. However, cellulose with low concentration (below 1 wt%) is required to maintain the good porosity of the aerogels due to the strong hydrogen bonding between the cellulose molecules. In order to address this problem, here, ultralight cellulose-derived CNFs have been fabricated by freeze-drying cyclohexane (CHE)/cellulose nanofiber emulsions and carbonization. Field emission scanning electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are used to characterize the resulting CNFs. It is found that the CNFs consist of three-dimensional carbon networks, whose microstructure is easily adjusted by changing the CHE ratio (from 0 to 25 vol%) in the emulsions. The CNFs with high porosity are attributed to the fact that CHE as the oil phase can effectively weaken the hydrogen bonding and reduce the aggregation of the cellulose nanofibers. Carbon lattice defects and residual oxygen-containing functional groups are regarded as polarization centers, leading to the enhancement of dielectric loss. The conductive carbon networks also improve the conductive loss. All these factors improve the microwave absorption performance of the CNFs. So, the produced CNFs exhibit a superior electromagnetic wave performance with a minimum reflection loss of −42.18 dB and effective absorption bandwidth up to 4.9 GHz at 2 mm with a filling ratio of 2 wt%. This work provides a simple, low-cost, and sustainable synthesis route for CNFs used for ultralight high-performance microwave absorption materials.

Thickness-controlled HsGDY/N-doped double-shell hollow carbon tubes for broadband high-performance microwave absorption

With the increasing deterioration of the electromagnetic environment, it is important to develop new and efficient electromagnetic wave-absorbing materials. In this work, bilayer hollow HsGDY/NC nanotubes with controllable thickness are constructed by introducing high conductivity polydopamine (PDA) and hydrogen-substituted graphdiyne (HsGDY) followed by the etching and carbonization. Subsequently, the effects of the composition, coating order, and structural characteristics of nanotubes on the electromagnetic parameters are thoroughly investigated, thereby analyzing the microwave absorption mechanism. The impedance matching of HsGDY/NC is optimized by changing the thickness of NC layer, while utilizing unique multi-heterogeneous interfaces and hierarchical conductive structures that enhance the interface polarization and conductive loss. As a result, HsGDY@NC-3 displays the optimal absorbing performance with an effective absorption bandwidth (EAB) of 7.8 GHz (10.1–17.9 GHz) and a minimum reflection loss (RLmin) of −47.18 dB at the filler content of only 11 %. This work broadens the application scopes of HsGDY and provides a novel insight into the design of lightweight, high-efficiency, and wide-frequency microwave absorbers, which is expected to be a potentially effective microwave-absorbing material.

CO2 plasma inducing steel-slag with active sites for efficient growth of carbon nanotubes for the high-performance microwave absorption

As a solid waste generated in the metallurgical process, the highly efficient catalysis of steel-slag for carbon nanotube (CNT) growth remains a huge challenge. Herein, a CO2 plasma-inducing approach for modifying the steel-slag catalyst with rich active sites was developed for CNT growth in the chemical vapor deposition (CVD) process. Furthermore, the high-energy CO2 plasma refines the particles on the steel-slag surface, reducing the particle size on the surface of steel-slag effectively increasing the active sites of the catalyst. In addition, the thermal effect generated by the gas molecule ionization process under CO2 increases the background temperature of the steel-slag, and the interaction between the oxygen-active substances produced and the iron oxide on the surface of steel-slag undergoes an oxidation reaction upon full contact with high-energy particles, which greatly improves the catalytic efficiency of the steel-slag as a catalyst for CNT, and increases the catalytic efficiency by 50 % compared with the non-plasma modified steel-slag as a catalyst for catalyzing CNTs. In addition, catalytically grown CNTs encapsulate magnetic steel-slag catalysts to form a three-dimensional CNT network structure, with high-performance electromagnetic wave absorption. The composite absorbing material with a thickness of only 3.55 mm has a minimum absorption rate of −64.08 dB for electromagnetic waves at a frequency of 7.9 GHz. The dielectric and the magnetic loss of steel-slag lead to enhanced electromagnetic wave absorption. Therefore, this study provides an inexpensive and environmentally friendly idea for the design of high-efficiency CNT growth and solid waste catalyst modification synthesized with high-performance absorbers.

Construction of a 3D spider web structure with spherical Ho2O3 wrapped by carbon nanofibers for high-efficiency microwave absorption and corrosion protection

Reasonable composition control and microstructure design are the effective ways to realize multi-functional high-performance absorbing materials. In this work, a three-dimensional (3D) spider web structure was designed by the electrospinning and high temperature carbonization technology, where Ho2O3 spheres were wrapped by carbon nanofibers. Herein, the spherical Ho2O3 were surrounded by layers of carbon nanofibers (spider silk), which were like the preys inside the spider's web. This unique bionic structure working in conjunction with the tuned components enables the as-prepared composites with improved impedance matching and enhanced loss capacity. The superior electromagnetic wave absorption performance was obtained as the minimum reflection loss of −61.37 dB at 2.90 mm and the maximum effective absorption bandwidth of 6.88 GHz (11.12–18 GHz) at 2.64 mm. At a thickness of 3.44 mm, the effective absorption bandwidth is 4.80 GHz (8–12.80 GHz), covering the entire X-band. At the same time, the composite soaked in the corrosive medium for 7 days owns a low corrosion current density (10−4 ∼10−6 A cm−2) and a high polarization resistance (105∼107 Ω), which exhibits the obvious anticorrosion ability. This work designs a specific spider web structure with high-performance microwave absorption and corrosion protection.

Architectural Design of a Multichannel Porous Carbon Fiber for Efficient Electromagnetic Microwave Absorption.

An ingenious microstructure of electromagnetic microwave absorption materials is crucial to achieve strong absorption and a broad bandwidth. Herein, one-dimensional (1D) carbon fibers with implantation of zero-dimensional (0D) ZIF-8-derived carbon frameworks and construction of a three-dimensional (3D) microcosmic multichannel porous structure are fabricated by electro-blown spinning, solvent-thermal reaction, and high-temperature pyrolysis techniques. The 1D carbon fiber skeleton with a multichannel structure provides a direct axial conductive pathway for charge transport, which plays an important role in dielectric loss. The 0D surface carbon frameworks offer plenty of heterogeneous interfaces to trigger intensive interfacial polarization loss and act as dihedral angles for microwave scattering. The 3D microcosmic multichannel pores can not only generate multiple reflections as much as possible to dissipate electromagnetic microwave energy but also supply huge interior cavities to improve impedance matching. Thanks to the synergistic effect of a strong electrically conductive pathway for enhancing the conductive loss, a plenteous heterogeneous interface for triggering intensive interfacial polarization loss, microcosmic multichannel pores for generating multiple reflections and improving impedance matching, and N and O atom doping for inducing dipole polarization, the optimal sample with an ingenious microstructure delivers an excellent absorption performance of a minimum reflection loss of -35.5 dB at a thickness of 5.0 mm and an effective absorption bandwidth of 6.72 GHz (10.96-17.68 GHz) at a thickness of 2.0 mm. Such a well-designed multichannel porous carbon fiber may pave the way for the exploitation of high-performance microwave absorbing materials.

Resolving the Role of Configurational Entropy in the Optimization of Mechanical, Thermal, and Microwave Dielectric Properties of SrLa(Al0.50-xGaxZn0.125Mg0.125Ti0.25)O4 Ceramics.

The demand for high-performance microwave dielectric ceramics has surged with the proliferation of fifth-generation (5G) communication networks. In this work, SrLa(Al0.50-xGaxZn0.125Mg0.125Ti0.25)O4 (x = 0-0.20) ceramics were designed by leveraging the unique properties of SrLaAlO4 ceramics and high-entropy engineering. The effects of configurational entropy (Sconf = 1.23R - 1.54R) on the mechanical, thermal, and microwave dielectric properties of SrLa(Al0.50-xGaxZn0.125Mg0.125Ti0.25)O4 ceramics were investigated. X-ray diffractometer and transmission electron microscope analyses confirmed that each composition belonged to the tetragonal structure with a space group of I4/mmm. Significant improvements in Vickers hardness were observed with increasing Sconf, reaching 8.05 GPa at Sconf = 1.54R compared to 5.64 GPa in SrLaAlO4 ceramics. Additionally, the increasing entropy showed great potential in reducing the thermal expansion coefficient (CTE) from 12.32 to 11.49 ppm/°C. The optimal quality factor (Q × f) of 98,000 GHz was achieved at Sconf = 1.37R, attributed to the optimization of intrinsic lattice energy and infrared-damped modes. The temperature coefficient of resonant frequency (τf) was successfully modified toward zero due to entropy-driven CTE and structural modifications. Excellent microwave dielectric properties with εr = 22.5, Q × f = 98,000 GHz, and τf = -2.0 ppm/°C were obtained at Sconf = 1.37R. This work highlights the potential of entropy-engineering in developing high-performance microwave dielectric ceramics, offering a promising pathway for the advancement of 5G communication components.

Self-Templating Engineering of Hollow N-Doped Carbon Microspheres Anchored with Ternary FeCoNi Alloys for Low-Frequency Microwave Absorption.

Rational design and precision fabrication of magnetic-dielectric composites have significant application potential for microwave absorption in the low-frequency range of 2-8GHz. However, the composition and structure engineering of these composites in regulating their magnetic-dielectric balance to achieve high-performance low-frequency microwave absorption remains challenging. Herein, a self-templating engineering strategy is proposed to fabricate hollow N-doped carbon microspheres anchored with ternary FeCoNi alloys. The high-temperature pyrolysis of FeCoNi alloy precursors creates core-shell FeCoNi alloy-graphitic carbon nano-units that are confined in carbon shells. Moreover, the anchored FeCoNi alloys play a critical role in maintaining hollow structural stability. In conjunction with the additional contribution of multiple heterogeneous interfaces, graphitization, and N doping to the regulation of electromagnetic parameters, hollow FeCoNi@NCMs exhibit a minimum reflection loss (RLmin) of -53.5dB and an effective absorption bandwidth (EAB) of 2.48GHz in the low-frequency range of 2-8GHz. Furthermore, a filler loading of 20wt% can also be used to achieve a broader EAB of 5.34GHz with a matching thickness of 1.7mm. In brief, this work opens up new avenues for the self-templating engineering of magnetic-dielectric composites for low-frequency microwave absorption.

Multi-dimensional, multi-interface Fe3C/carbon sheets/carbon tubes nanoarchitecture towards high-performance microwave absorption

The serious problem of electromagnetic (EM) interference brings growing demand for developing high-performance electromagnetic shielding materials. In this study, three-dimensional (3D) Fe3C/carbon/carbon tubes (Fe3C/C/CT) absorber with multi-heterointerface was prepared by a facile self-propagating followed by chemical vapor deposition (CVD) strategy. Initially, Fe3O4 nanoparticles were anchored onto carbon nanosheets through the self-propagating process. After different etching times with diluted acid, CTs with diverse dimensions and quantities were catalyzed by the modified Fe3O4 catalysts during the CVD approach and grown from the carbon sheets. Such a special nanoarchitecture, which consists of carbon sheets and carbon tubes, acts as the source of dielectric loss. Meanwhile, Fe3C nanoparticles provide magnetic loss, and the abundant interfaces facilitate synergistic electromagnetic loss. Moreover, large numbers of heterointerfaces from the unique multidimensional nanoarchitecture significantly enhance the impedance matching and produce abundant dipole polarization. Ultimately, with etching time of 30 min, the Fe3C/C/CT-30 achieves excellent electromagnetic wave (EMW) absorption property with a minimum reflection loss of −46.4 dB at 10.56 GHz with a thickness of 3.0 mm. When the thickness is 3.57 mm, the effective absorption bandwidth (EAB) can extend up to 4.48 GHz. Moreover, the practical application of the absorbers was evaluated using radar cross-section (RCS) simulation, indicating that Fe3C/C/CT-30 achieves a maximum RCS reduction value of 58 dBm2. The multi-dimensional and multi-heterointerface absorber presented in this work might offer valuable insights for optimizing electromagnetic absorbers.