Dimensionality determined microwave absorption properties in ferrite/bio-carbon composites

Composition and structural design play a very influential role in the microwave absorption (MA) manipulation of ferrite/carbon composites. Here, by carefully choosing the dimensionality of the bio-carbon materials, the interfacial geometries and MA properties of ferrite/bio-carbon composites have been controlled effectively. The one dimensional (1D), two dimensional (2D), and three dimensional (3D) biomass-based carbon materials decorated with ZnFe₂O₄ (ZFO) particles were obtained respectively from carbon fibers (1D), tree leaves (2D), wheat straw (2D), peanut shell (3D) and orange peel (3D) by a simple two-step synthesis method. With increasing the bio-carbon's dimensionality from 1D, 2D to 3D, the ferrite/carbon composite's MA properties are promoted and the minimum reflection loss is enhanced from −9 dB to −45 dB. By changing the ZFO/3D-bio-carbon samples' thickness, a broad absorption range from 4 to 18 GHz can be covered. Moreover, the effective absorption bandwidth for ZFO/3D-bio-carbon can be modified up to 7.1 GHz, which covers the whole Ku band. These observations identified the important roles of the ferrite/carbon interface and dimensionality of carbon materials and provided an effective and low-cost route to design microwave absorption materials based on biomass-industrial waste composites.     

Geometry effect on membrane absorption for CO2 capture. Part I: A hybrid modeling approach

Membrane absorption (MA) has a great prospect for CO₂ capture. In MA modeling, conventional one-dimensional (1D)- and two-dimensional (2D)-models make simplification of membrane contactor (MC) geometry. Geometry simplification allows an easy process modeling and numerical solution, however, is only reasonable for particular MCs. Here, efforts are underway to quantify the geometry effect on the MA-CO₂ performance. First, we proposed a rigorous 3D model without geometry simplification for simulating the MA-CO₂ process in real MCs and then validated it with experimental data. More importantly, we highlighted a preferable hybrid model in which two correction factors were introduced to a 2D model to make the simulation results approximately equal to the 3D simulation values. The correction factors were correlated with dimensionless fluid dynamic parameters for characterizing the geometry effects on flowing fluids. Such hybrid modeling contributes to characterizing the influence of geometry on the MA-CO₂ performance and improving computation accuracy-efficiency combinations.     

Enhancement on high-temperature microwave absorption properties of TiB2–MgO composites with multi-interfacial effects

TiB₂–MgO microwave absorbing materials with TiB₂ as the absorber, MgO as the matrix are prepared by spark plasma sintering (SPS). The influences of commercial TiB₂ content and sintering temperature on dielectric and microwave absorption (MA) properties are studied. Besides, to optimize the MA performance, TiB₂–MgO composite containing TiB₂ synthesized by the carbonthermal process is prepared. Meanwhile, its high-temperature dielectric and MA properties are investigated. Indeed, both the commercial TiB₂ content and sintering temperature play key roles in dielectric and MA properties, as they reaching 8 wt% and 1400 °C, the composite presents the optimal MA performance. For composite with synthetic TiB₂ as the absorber, the temperature has a positive effect on dielectric and MA properties. The enhanced high-temperature MA properties with minimum reflection loss (RL_min) of ?52.11 dB at 1.6 mm under 500 °C and effective absorption bandwidth (EAB, RL < ?5 dB) of 4.2 GHz at 1.4–1.6 mm under 800 °C are obtained, which is mainly attributed to the temperature-dependent interfacial polarization compared to the temperature-insensitive conductivity. The excellent mechanical properties (flexural strength = 212.48 MPa), thin absorbing layer (d < 2 mm), enhanced thermal stability and high-temperature MA properties indicate that the TiB₂–MgO composites can be considered as new candidates for high-temperature structure microwave absorbing materials.     

Tunable microwave absorption features in bi-layer absorber based on mesoporous CuS micro-particle with 3D hierarchical structure and nanosphere like NiCo2O4

There has been a growing demand for materials with superior absorption capabilities, such as strong absorbing capacity, thin thickness, and light weight, to solve challenges related to EM radiation pollution. While the majority of the research is focused on optimizing material compositions, component microstructure and absorber structure are also critical factors for improving microwave absorption performance. In this research, we show how the microstructure of components and absorber design may increase dissipation features. Solvothermal and hydrothermal methods were utilized for synthesizing mesoporous CuS micro-particles with a 3D hierarchical structure as a dielectric component and nanospheres like NiCo₂O₄ as magnetic components respectively. The formation of pure phases with the mentioned microstructures was confirmed via XRD, FTIR, UV–Vis, XPS, VSM, FESEM and BET analysis. According to VNA results, the minimum reflection loss can be achieved to ?33 dB at 11 GHz with a total thickness of 2 mm in which each layer thickness was considered 1 mm (CuS placed at top layer and NiCo₂O₄ placed at bottom layer). The RL values of bilayer absorber were strongly affected by both the microstructure of the components and tuning the thickness and arrangement of each layer. We offer a potential technique for enhancing microwave dissipation performance by combining the synergistic effects between the microstructure, thickness and arrangement of layers in a bilayer absorber.     

Recent advances in the development OF Fe3O4-BASED microwave absorbing materials

Electromagnetic pollution has become a serious concern with the immense utilization of wireless information technologies and this has aroused huge interest in the area of microwave absorption. To solve this issue, fabrication of advanced, novel and superior microwave absorbing materials (MAM) with high electromagnetic wave absorption, wide absorption bandwidth, lightweight and cost-efficient are highly required. To date, magnetite (Fe₃O₄) is being thoroughly investigated as MAM, due to its exceptional dual electromagnetic properties (permittivity and permeability), proper saturation magnetization and high Curie temperature. However, large density and impedance mismatch are some of the limiting factors that hinder its microwave absorption performance (MAP). To circumvent these challenges, reduction of size to the nanoscale, fabrication of hierarchical nanostructures and/or conjugation with other lossy materials have been extensively explored as viable solutions to optimize the MAP of Fe₃O₄. In this review, the progressive research in the fabrication of Fe₃O₄ based nanocomposites as MAM is discussed. The factors influencing the MAP of these absorptive materials are likewise discussed in detail. Conclusively, some challenges, limitations, and future prospects in the development of Fe₃O₄ based MAM are put forth.     

Facile fabrication of Nd2O2S/C nanocomposite with enhanced microwave absorption induced by defects

The derivatives of metal-organic frameworks (MOFs) in which various metal and/or metal oxides are uniformly distributed in porous carbon materials have attracted considerable attention owing to their special characteristics such as large specific surface area, adjustable pore structure, highly ordered pores, and uniform metal sites. Presently, the electromagnetic absorption property of MOF derivatives is primarily enhanced by changing their metal ions, and it is still a challenge to improve the loss ability of MOF-derived absorbers by modifying the organic ligands of MOFs. In this work, rhombic Nd₂O₂S/C nanocomposites with excellent microwave absorption (MA) performance are synthesized via in-situ pyrolysis of (Nd[TDA][CH₃COO][H₂O])_n (H₂TDA: 2,5-thiophenedioic acid). The influence of pyrolysis temperature on the MA performance of the composites is examined. The Nd₂O₂S/C-800 composite exhibits a minimum reflection loss of –52.3 dB at 2.56 mm thickness, which is ascribed to the synergistic effect of multiple loss mechanisms, including polarization, conduction loss, and so forth. Overall, this paper provides a useful method to synthesize high-performance MA materials by utilizing MOFs with rare-earth ions connected by sulfur-containing organic ligand.     

A review on the optical characterization of V2O5 micro-nanostructures

Bulk V₂O₅ is a diamagnetic semiconductor with a band gap (E_g) of about 2.3 eV, which is based on the ionic configuration with filled O2p and unoccupied V3d orbitals. However, the band edge absorption and photoluminescence (PL) peak positions of low-dimensional V₂O₅ materials do not coincide and are distributed over wide ranges of 0.75–3.49 eV and 0.73–3.3 eV, respectively. This review summarizes the fabrication processes, structure, and optical characterization of V₂O₅ micro-nanostructures, including 0D, 1D, 2D, and 3D morphologies. The wide ranges of band edge absorption and broad PL of V₂O₅ micro-nanostructures are clarified in terms of factors such as the morphology, synthesis method, growth conditions, crystal size, micro-nano size, phase transition, and measurement conditions. The relations among the separation, diffusion, recombination, and degradation of the electron-hole pairs in V₂O₅ micro-nanostructures are also discussed. Fundamental understanding of the optical characteristics plays a key role in V₂O₅ micro-nano device applications. The review also demonstrates the role of V₂O₅ micro-nanostructures and other materials (MOs) in V₂O₅/OMs heterostructures for slowing down recombination, prolonging lifetime, improving electron-hole separation, and increasing photocurrent to enhance the photocatalytic activity.     

First-principles Studies of Band Gap Engineering in Ferroelectric Oxides

In the last decade there has been a resurgence of activity in the field of photoferroelectrics with a focus on the studies of oxides. Ferroelectric (FE) materials have a spontaneous switchable polarization that can lead to interesting light-matter interactions such as the bulk photovoltaic effect. While a wide range of compositions has been explored for ferroelectrics, until recently, FE oxides were considered to be capable of absorbing UV light only. Playing a key role in the advance of the field, first-principles calculations have guided the experimental materials discovery efforts that have identified a wide variety of visible-light-absorbing oxides. First-principles research efforts have also achieved considerable progress in revealing the interplay between the composition, structure and light absorption properties in these materials, and it is now understood that even small changes in the local environment can lead to large changes in band gap character and magnitude. Here, we survey the first-principles efforts following different band-gap engineering strategies and the advances in the computational methods that have been driven by these studies.     

SiCnw@SiC foam prepared based on vapor-solid mechanism for efficient microwave absorption

A SiC_nw@SiC foam with highly efficient microwave absorption (MA) performance was successfully synthesized based on Vapor-Solid (V–S) growth mechanism. SiC nanowire (SiC_nw) and SiC foam skeleton efficiently form a double network coupling structure, which gives additional interface polarization and dielectric loss for the SiC foam, significantly enhancing the MA capacity of the foam. In this study, the SiC_nw@SiC foam has a minimum reflection loss (RL_min) of ?86.31 dB and an effective absorption bandwidth (EAB) of 12.55 GHz in room-temperature environment. However, the MA performance of SiC_nw@SiC foam decreases with increasing temperature, which may be due to the thickening of the SiO₂ layer in the SiC_nw at high temperature. At 600 °C, it has no effective absorption bandwidth, while at 1000 °C, the EAB and RL_min were 0.6 GHz and ?13.04 dB, respectively. As the temperature reaches 1000 °C, the defects in the material further increase, leading to a recovery in the MA performance.     

Preparation and properties of MgAl2O4 based ceramics reinforced with rod-like microcrystallines by co-doping Sm2O3 and La2O3

A rod-like microcrystalline reinforced MgAl₂O₄ (MA) based ceramic with high strength and good thermal shock resistance has been successfully prepared by co-doping Sm₂O₃ and La₂O₃ via a single-stage solid-state reaction sintering (SRS) process. The effects of addition amounts of Sm₂O₃ and La₂O₃ on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density, high temperature compressive strength and thermal shock resistance of the MA based ceramics have been investigated. The results showed that MA, SmAlO₃, La₁₀Al₄O₂₁, Sm_4.67(SiO₄)₃O and La_4.67(SiO₄)₃O phases could be found in the ceramics after co-doping additives. The new formed rare earth compounds could prevent the growth of MA grains leading to a densification microstructure of the MA based ceramics. The Sm_4.67(SiO₄)₃O and La_4.67(SiO₄)₃O were formed by the reaction between additives and impurities of raw materials, which presented as rod-like microcrystallines to effectively clean the impurities in the grain boundaries of the MA based ceramics. Accordingly, the sintering properties, high temperature compressive strength and thermal shock resistance of the MA based ceramics were improved markedly.     

Rapid preparation of novel MgNH4PO4·H2O porous scaffolds via 3D-printing combined with a hydrothermal-process-assisted post treatment

Novel dittmarite (MgNH₄PO₄·H₂O) 3D porous scaffolds were firstly fabricated via 3D-printing combined with a hydrothermal-process-assisted post treatment. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to characterize the phases, morphologies, and element compositions of the 3D porous structures. The hydrothermal process played the key role in the formation of dittmarite phase. The porosity, compressive strength, and in vitro degradation of the dittmarite scaffolds were studied in detail. In addition, the cytotoxicity on MC3T3-E1 osteoblast cells and cell adhesion were evaluated and the results showed that the dittmarite porous scaffold possessed superior cytocompatibility and could support MC3T3-E1 cellular attachment. The research indicated that dittmarite porous scaffolds had a wide application prospect in bone tissue repair fields.     

Computation of surface energy and surface segregation phenomena of perfluorinated copolymers and blends – A molecular modeling approach

In this paper, we studied the surface properties and surface segregation phenomena of perflourinated copolymers and blends using molecular mechanics (MM) and molecular dynamics (MD) simulation in the NVT ensemble. The importance of functional group, 1H, 1H-dihydroperfluorohendecyl methacrylate (F10MA) and their surface preference over polymer backbone segments viz., methyl methacrylate (MMA) has been investigated. We have shown that degree of blockiness and change in chain architecture have significant effects on surface energy values. Surface energy differences between MMA and F10MA segments have been asserted by introducing a surface critical parameter, χ_s. Computations have been carried out to obtain bulk properties like cohesive energy density (CED) and solubility parameter (δ) by performing MM and MD simulations. Surface energies of MMA/F10MA blends have been computed by bulk pressure–volume–temperature (PVT) properties. Molecular dynamics simulation using NPT ensemble has been used to obtain specific volume as a function of temperature for different compositions of MMA/F10MA blends. From these results and using the equation of state approaches, thermal expansion coefficient has been obtained to calculate PVT parameters. These surface energy values compare well with the surface energy data calculated by the Zisman equation. Finally, the surface-enrichment behavior of F10MA components in the blend has been examined.     

Glycine-assisted solution combustion synthesis of NiCo2O4 electromagnetic wave absorber with wide absorption bandwidth

Design of high-performance electromagnetic (EM) wave absorbing materials has been regarded as an effective solution to excessive EM wave interference problem. As a promising candidate, NiCo₂O₄ absorbers have attracted enormous research attentions. However, currently reported morphology-manipulation synthetic methods of NiCo₂O₄ absorbers are time-consuming and require high energy consumption, which inhibit their practical applications. Herein, a more facile and cost-effective solution combustion synthesis was utilized to fabricate NiCo₂O₄ materials. The absorber prepared by using glycine as fuel displayed the best EM wave absorption performance. Impressively, ultra wide absorption bandwidth of 7.44 GHz from 10.56 GHz to 18 GHz could be achieved with relatively thin thickness of 2.1 mm NiCo₂O₄ sample fabricated in this work displayed the widest effective absorption bandwidth (EAB) among reported NiCo₂O₄-based EM wave absorbing materials so far. In view of its simple and low-cost synthetic process and excellent EM wave dissipation capacity, NiCo₂O₄ samples in this work showed great feasibility as practical absorber. In addition, our findings may also provide new sight for facile preparation of other high-performance EM wave absorbers by solution combustion synthesis instead of complex morphology-manipulation routes.     

Two-step preparation of fly ash-based composites for microwave absorption

To develop a novel utilization avenue for fly ash (FA), the Co-loaded FA (CoFA) was constructed utilizing FA as raw material to acquire the enhanced microwave absorption (MA) performance. In this study, the CoFA composites were fabricated by a two-step method, including the construction of FA-based ceramic matrix and a subsequent loading of magnetic components. The results of XRD, SEM, and elemental mapping images revealed that Co particles generated from the carbothermal reduction were well dispersed over the interior and surface of matrix. Compared with pure FA, the as-prepared CoFA composites demonstrated the impressive MA performances, which were attributed to the good impedance matching, conduction loss, and interfacial polarization effect between the matrix and Co. When the annealing temperature kept at 700°C, the minimum reflection loss (RL_min) of as-prepared CoFA700 reached up to −40.5 dB and the broad absorption band was measured to be 4.7 GHz with a thickness of 2.0 mm, which was superior to pure FA. Our strategy might provide a new direction to the fabrication of high-efficient MA materials derived from FA.     

Blends of polyamide1010 with unmodified and maleic‐anhydride‐grafted high‐impact polystyrene

The crystallization behaviors, dynamic mechanical properties, tensile, and morphology features of polyamide1010 (PA1010) blends with the high‐impact polystyrene (HIPS) were examined at a wide composition range. Both unmodified and maleic‐anhydride‐(MA)‐grafted HIPS (HIPS‐g‐MA) were used. It was found that the domain size of HIPS‐g‐MA was much smaller than that of HIPS at the same compositions in the blends. The mechanical performances of PA1010–HIPS‐g‐MA blends were enhanced much more than that of PA1010–HIPS blends. The crystallization temperature of PA1010 shifted towards higher temperature as HIPS‐g‐MA increased from 20 to 50% in the blends. For the blends with a dispersed PA phase (≤35 wt %), the T_c of PA1010 shifted towards lower temperature, from 178 to 83°C. An additional transition was detected at a temperature located between the T_g's of PA1010 and PS. It was associated with the interphase relaxation peak. Its intensity increased with increasing content of PA1010, and the maximum occurred at the composition of PA1010–HIPS‐g‐MA 80/20. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 857–865, 1999     

Two new aluminophosphates, IST-1 and IST-2: First examples of a dual templating role of water and methylamine in generating microporous structures

This study is aimed at exploring the ability of very small sized N-bearing molecules to generate and stabilize microporous aluminophosphates. Two new AlPO₄-n materials, called IST-1 and IST-2, have been obtained in aqueous media using, as main template, methylamine (MA), directly added, or generated in situ from methylformamide (MF) degradation. While IST-1 topology proved to be novel, IST-2 appears structurally related to AlPO₄-53(A). The obtained materials were characterized by powder XRD, TG/DSC, SEM and solid-state NMR. Tetraalkylammonium (TEA) cations were used as potential co-templates but only MA and water were found incorporated in the pore volumes of both structures, which argues for their true templating role. In IST-1, ¹³C solid-state NMR studies showed that half of MA species, presumably protonated, is H-bonded to framework oxygens while the other half surprisingly bonds directly to framework Al atoms. ¹³C NMR showed that only protonated MA occurs in IST-2 channels. TEA⁺ cations definitely do not play any specific template role. They indirectly favor the crystallization of IST-1 or IST-2 devoid from other crystalline or amorphous side phases, by interacting with part of the Al and P in solution and forming soluble [AlPO₄(OH)]–[TEA,HMA] complexes, substantially modifying the compositions of gels precursors to each phase during nucleation and/or growth steps. While both IST-1 and IST-2 crystallize from gels of similar initial compositions, it was demonstrated that the new MA/T ratio (T = Al or P) obtained after in situ complexation was the key parameter that specifically governs the crystallization of each phase.     

Ni-MOF/Ti3C2Tx derived multidimensional hierarchical Ni/TiO2/C nanocomposites with lightweight and efficient microwave absorption

Benefiting from its large specific surface area, abundant defects and functional groups, two-dimensional (2D) laminated Ti₃C₂T_x MXene is a kind of electromagnetic wave (EMW) absorber with great potential. However, the impedance mismatch caused by the excessive conductivity, inappropriate permittivity and lack of magnetic loss seriously hinders the application of MXene to EMW absorption. Herein, multidimensional hierarchical Ni/TiO₂/C nanocomposites composed of three-dimensional (3D) hydrangea-like Ni/C microspheres and well-arranged 2D carbon sheets embedded with TiO₂ nanoparticles were successfully fabricated from a Ni-based trimellitic acid framework (Ni-BTC) and Ti₃C₂T_x MXene via facile in-situ solvothermal assembly and annealing processes. As expected, excellent EMW absorption properties were obtained only by changing the annealing temperature. The minimum reflection loss (RL_min) value of -45.6 dB and the effective absorption bandwidth (EAB) of 3.40 GHz (14.6–18.0 GHz) with a layer thickness of only 1.5 mm is obtained by annealing the sample at 700 °C. The outstanding ternary multilayer structure and the optimization of magnetoelectric synergy in impedance matching jointly create its remarkable EMW absorption performance. This work is expected to provide a simple and effective method to design MXene-based EMW absorbing materials possessing high absorption intensity, light weight, wide EAB and thin thickness.     

Cup-stacked carbon nanotubes hybridized Si3N4/Si3N4 composite ceramics for high-effciency microwave absorption with excellent thermal stability

Hybridization between carbon nanotubes (CNTs) and Si₃N₄ is a promising strategy for developing high-temperature microwave absorption (MA) materials for military application. Toward long-life services, it's important to achieve strong MA at a filler loading as low as possible on account of antioxidant protection against CNTs wastage. Herein, cup-stacked CNTs (CSCNTs) have been prepared in porous Si₃N₄ ceramics by chemical vapor deposition (CVD) and then CVD Si₃N₄ has been coated on them, forming CSCNT-Si₃N₄/Si₃N₄ composite ceramics. Results show that CSCNTs possess abundant exposed atomic edges on the outer surface and in the inner channel. Such unique defects not only benefit the impedance match but also cause considerable conductive loss, which helps CSCNT-Si₃N₄/Si₃N₄ with a filler content of only 0.79 wt% to achieve an effective absorption bandwidth (EAB) of 3.74 GHz in the X band at a thickness of 3.5 mm coupled with a minimum reflection loss of ?43.3 dB and an EAB covering the entire Ku band at a thickness of 2.25 mm. The ultralow filler loading generates a high efficiency of CVD Si₃N₄ in protecting CSCNTs against high-temperature oxidation, leading to a steady MA performance for CSCNT-Si₃N₄/Si₃N₄ during 23–1200 °C thermal shock tests in air.     

Water absorption behavior of different types of organophilic montmorillonite‐filled polyamide 6/polypropylene nanocomposites

The water absorption behavior of different types of organophilic montmorillonite (OMMT)‐filled polyamide 6/polypropylene nanocomposites with and without compatibilizers (maleated PP or PP‐g‐MA and maleated styrene‐ethylene/butylene‐styrene or SEBS‐g‐MA) was evaluated. Four different types of OMMT, i.e., dodecylamine‐modified MMT (D‐MMT), 12 aminolauric acid‐modified MMT (A‐MMT), stearylamine‐modified MMT (S‐MMT), and commercial organo‐MMT (C‐MMT) were used as reinforcement. The water absorption response of the nanocomposites was studied and analyzed by tensile test and morphology assessment by scanning electron microscopy (SEM). The kinetics of water absorption of the nanocomposites conforms to Fick's law. The M_m and D are dependent on the types of OMMT and compatibilizers. The equilibrium water content and diffusivity of PA6/PP blend were increased by the addition of OMMT but decreased in the presence of compatibilizers. On water absorption, both strength and stiffness of the nanocomposites were drastically decreased, but the ductility was remarkably increased. Both PP‐g‐MA and SEBS‐g‐MA played an effective role as compatibilizers for the nanocomposites. This was manifested by their higher retention ability in strength and stiffness (in the wet and re‐dried states), reduced the equilibrium water content, and diffusivity of the nanocomposites. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

A new simple procedure to calculate monomer reactivity ratios by using on-line 1H NMR kinetic experiments: Copolymerization system with greater difference between the monomer reactivity ratios

Free radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) in solution of benzene-d₆ using benzoyl peroxide (BPO) as the initiator was studied with on-line ¹H NMR kinetic experiments at 60 °C. It was observed that composition drifts in the comonomer mixture with reaction progress is significant. Hence, the monomer reactivity ratios of VA/MA system could be calculated by the data collected only from one sample via on-line following the comonomer mixture and copolymer compositions at different reaction time intervals up to medium overall monomer conversions. The results were in good agreement with the literature data reported for this system. The good fitting between theoretical and experimental changes in the comonomer mixture compositions as a function of reaction progress was observed, indicating the accuracy of the monomer reactivity ratios calculated by the new procedure presented here.