A Review on Metal-Organic Framework-Derived Porous Carbon-Based Novel Microwave Absorption Materials

The development of microwave absorption materials(MAMs) is a considerable important topic because our living space is crowed with electromagnetic wave which threatens human’s health.And MAMs are also used in radar stealth for protecting the weapons from being detected.Many nanomaterials were studied as MAMs,but not all of them have the satisfactory performance.Recently,metal-organic frameworks(MOFs) have attracted tremendous attention owing to their tunable chemical structures,diverse properties,large specific surface area and uniform pore distribution.MOF can transform to porous carbon(PC) which is decorated with metal species at appropriate pyrolysis temperature.However,the loss mechanism of pure MOF-derived PC is often relatively simple.In order to further improve the MA performance,the MOFs coupled with other loss materials are a widely studied method.In this review,we summarize the theories of MA,the progress of different MOF-derived PC-based MAMs,tunable chemical structures incorporated with dielectric loss or magnetic loss materials.The different MA performance and mechanisms are discussed in detail.Finally,the shortcomings,challenges and perspectives of MOF-derived PC-based MAMs are also presented.We hope this review could provide a new insight to design and fabricate MOF-derived PC-based MAMs with better fundamental understanding and practical application.

State of the Art and Prospects in Metal-Organic Framework-Derived Microwave Absorption Materials

Microwave has been widely used in many fields,including communication,medical treatment and military industry;however,the corresponding generated radiations have been novel hazardous sources of pollution threating human’s daily life.Therefore,designing high-performance microwave absorption materials(MAMs)has become an indispensable requirement.Recently,metal-organic frameworks(MOFs)have been considered as one of the most ideal precursor candidates of MAMs because of their tunable structure,high porosity and large specific surface area.Usually,MOF-derived MAMs exhibit excellent electrical conductivity,good magnetism and sufficient defects and interfaces,providing obvious merits in both impedance matching and microwave loss.In this review,the recent research progresses on MOF-derived MAMs were profoundly reviewed,including the categories of MOFs and MOF composites precursors,design principles,preparation methods and the relationship between mechanisms of microwave absorption and microstructures of MAMs.Finally,the current challenges and prospects for future opportunities of MOF-derived MAMs are also discussed.

Study on Performance and Mechanism of Oil Absorption Materials

Both the commonly used and the PHBV based oil absorption materials were studied and the absorption mechanism was analyzed. The results show that the oil pick up ratios and the absorption rates of molded PHBV are almost the same as that of oil absorption polypropylene felt. In addition, the oil keeping ability of molded PHBV is superior to the latter. So the PHBV is a valuable and bio degradable oil absorption material.

Application of Air-cooled Blast Furnace Slag Aggregates as Replacement of Natural Aggregates in Cement-based Materials：A Study on Water Absorption Property

The influence of air-cooled blast furnace slag aggregates as replacement of natural aggregates on the water absorption of concrete and mortar was studied, and the mechanism was analyzed. The interface between aggregate and matrix in concrete was analyzed by using a micro-hardness tester, a laser confocal microscope and a scanning electron microscope with backscattered electron image mode. The pore structure of mortar matrixes under different curing conditions was investigated by mercury intrusion porosimetry. The results showed that when natural aggregates were replaced with air-cooled blast furnace slag aggregates in mortar or concrete, the content of the capillary pore in the mortar matrix was reduced and the interfacial structure between aggregate and matrix was improved, resulting in the lower water absorption of mortar or concrete. Compared to the concrete made with crushed limestone and natural river sand, the initial absorption coefficient, the secondary absorption coefficient and the water absorption capacity through the surface for 7 d of the concrete made from crushed air-cooled blast furnace slag and air-cooled blast furnace slag sand were reduced by 48.9%, 52.8%, and 46.5%, respectively.

Progress in MXene-based materials for microwave absorption

Due to the rapid development of radar technology,the demand for absorbing stealth materials is increas-ing,and ultra-broadband absorption(effective absorption bandwidth>8 GHz)has become an inevitable requirement.As a new type of two-dimensional material,MXene material possesses the characteristics of excellent wave absorbing material due to its easy preparation,easy modulation of defects and sur-face functional groups,and high conductivity.This work summarizes the absorbing theory and research progress on MXene-based absorbing materials in recent years,including pure MXene absorbing materials and MXene-based magnetic or dielectric composite materials with multiple losses.Some shortcomings and research directions of MXene-based materials were pointed out.Currently,research on MXene-based absorbent materials is thriving and in a state of vigorous development.Excellent absorbent materials have been reported,but their shortcomings are also apparent.The factors that affect the performance of MXene-based absorbent materials are complex,and the absorption mechanism is relatively simple.Further systematic and detailed research is needed to clarify these influencing mechanisms,broaden the absorption bandwidth,and reduce the matching thickness to meet practical usage requirements in the future.

High-temperature acoustic properties of porous titanium fiber metal materials

The high-temperature acoustic absorption performance of porous titanium fiber material was investigated in terms of sample thickness, porosity, temperature, air-cavity thickness and double-layer structure arrangement. The effects on absorption coefficient were systematically assessed. The results show that the sound absorption performance is improved by increasing the sample porosity and/or thickness, and/or increasing the air-cavity thickness. Meanwhile, increasing the temperature gives better acoustic absorption performance in the low frequency range but also lowers the performance in the high frequency range, while double-layer structure enables better acoustic absorption performance.

Why Sustainable Porous Carbon Should be Further Explored as Radar-Absorbing Material? A Comparative Study with Different Nanostructured Carbons

Radar Absorbing Materials(RAM)are a class of composites that can attenuate incident electromagnetic waves to avoid radar detection.Most carbon allotropes that have the potential to be used as RAM are either carbon nano-tubes(CNTs),graphene,carbon black(CB)and ultimately,sustainable porous carbon(SPC).Here,black wattle bark waste(following tannin extraction)was used as a sustainable source to produce SPC made from biomass waste.It was characterized and used as afiller for a silicone rubber matrix to produce aflexible RAM.The elec-tromagnetic performance of this composite was compared with composites made with commercial CB and CNT through reflection loss(RL),where-10 dB is equivalent to 90%of attenuation.These composites were evaluated in single-layer,double-layer,and as radar absorbing structures(RAS)with the aim of improving their effective absorption bandwidth(EAB)performances and a reduction in costs.The CNT composite presented a RL of-26.85 dB at 10.89 GHz and an EAB of 2.6 GHz with a 1.9 mm thickness,while the double-layer structures using CNT and SPC provided a RL of-19.74 dB at 10.75 GHz and an EAB of 2.51 GHz.Furthermore,the double-layer structures are~42%cheaper than the composite using only CNT since less material is used.Finally,the largest EAB was achieved with a RAS using SPC,reaching~2.8 GHz and a RL of-49.09 dB at 10.4 GHz.Summarizing,SPC made of black wattle bark waste can be a competitive,alternative material for use as RAM and RAS since it is cheaper,sustainable,and suitable for daily life uses such as absorbers for anechoic chambers,sensors,and elec-tromagnetic interference shields for electronics,wallets,vehicles,and others.

A Combined Shape and Topology Optimization Based on Isogeometric Boundary Element Method for 3D Acoustics

A combined shape and topology optimization algorithm based on isogeometric boundary element method for 3D acoustics is developed in this study.The key treatment involves using adjoint variable method in shape sensitivity analysis with respect to non-uniform rational basis splines control points,and in topology sensitivity analysis with respect to the artificial densities of sound absorption material.OpenMP tool in Fortran code is adopted to improve the efficiency of analysis.To consider the features and efficiencies of the two types of optimization methods,this study adopts a combined iteration scheme for the optimization process to investigate the simultaneous change of geometry shape and distribution of material to achieve better noise control.Numerical examples,such as sound barrier,simple tank,and BeTSSi submarine,are performed to validate the advantage of combined optimization in noise reduction,and to demonstrate the potential of the proposed method for engineering problems.

Mechanical Properties and Microstructure of Al_(2)O_(3)/SiC Composite Ceramics for Solar Heat Absorber

Al_(2)O_(3)/SiC composite ceramics were prepared fromα-Al_(2)O_(3) and SiC by a pressureless sinter method in this study.The effect of SiC contents on the mechanic properties,phase compositions and microstructure is studied.Experimental results show that the vickers hardness,wear resistance and thermal conductivity of the samples increase with the increase in the SiC content,and the hardness of the sample reaches 16.22 GPa,and thermal conductivity of the sample reaches 25.41 W/(m.K)at room temperature when the SiC content is 20 wt%(B5)and the sintering temperature is at 1640℃.Higher hardness means higher scour resistance,and it indicates that the B5 material is expected to be used for the solar heat absorber of third generation solar thermal generation.The results indicate the mechanism of improving mechanical properties of Al_(2)O_(3)/SiC composite ceramics:SiC plays a role in grain refinement that the grain of SiC inhibits the grain growth of Al_(2)O_(3),while the addition of SiC changes the fracture mode from the intergranular to the intergranular-transgranular.

Analysis on the influence factors of the acoustic absorption performance of porous fiber materials

Porous fiber materials are the most widely used acoustic absorption materials at present,and they have excellent acoustic absorption performance.This paper uses the finite element method to explore the factors affecting the acoustic absorption performance of porous fiber materials,including flow resistance,thickness of the porous fiber material,incidence angle,and back cavity thickness.Due to the complex acoustic absorption mechanism of porous fiber materials,an equivalent fluid model is used to simulate the acoustic absorption properties of the porous fiber materials.The correlation of acoustic absorption performance and the model of the back cavity was analyzed.An impedance tube test was implemented to verify the simulation results.

Review and Perspective of Tailorable Metal-Organic Framework for Enhancing Microwave Absorption

Metal-organic frameworks(MOFs)and their pyrolytic derivates,displaying diverse chemical compositions and microstructures,pro-vide the infinite potential for preparing high-performance microwave absorption materials(MAMs)and have attracted extensive at-tention.In this review,we systematically reviewed the recent progress of MOF-based MAMs,including three types of MOF-based MAMs(MOF-derived metal/carbon nanocomposites,MOF-based hybridization materials and conductive MOF).Besides that,the microwave absorption properties and their related physical and chemical appearance were also analyzed.On behalf of synergistic effects between microstructures,dielectric components and magnetic response,the MOF-based MAMs show excellent microwave absorption performance,which is superior to that of traditional single metal,metal alloy and pure carbon-based MAMs.Further-more,the novel conductive MOF with tunable electrical conductivity shows great potential in MAMs due to the fact that it can dra-matically simplify the synthesis process.

Infrared Emissivity of La0.8Sr0.2MnO3 with Three Different Structures

La_（0.8）Sr_（0.2）MnO_3 samples with rhombohedral, orthohombic and monoclinic structures were prepared by solid state reaction, sol-gel and co-precipitation methods, respectively. Lattice parameters, grain size, morphology, infrared absorption and emissivity of samples were investigated. The results indicated that the average crystallite size calculated from XRD result and particle size of orthohombic sample were smaller than those of the other two samples, and honeycomb shape grains were observed in orthohombic sample. Due to lower crystal symmetry, Mn-O stretching vibration peaks of the three samples shifted to higher infrared wavenumber. According to the theory of wave optics and Kirchhoff law, bigger rhombohedral sample showed higher emissivity than monoclinic one. However, due to the honeycomb structure of orthohombic sample, repeated reflection and scattering led to the increase of absorption, and orthohombic sample exhibited the highest emissivity.

Achieving strong microwave absorption capability and wide absorption bandwidth through a combination of high entropy rare earth silicide carbides/rare earth oxides

Developing electromagnetic(EM) wave absorbing materials with low reflection coefficient and optimal operating frequency band is urgently needed on account of the increasingly serious EM pollution. However, the applications of common EM absorbing materials are encumbered by poor high-temperature stability, poor oxidation resistance, narrow absorption bandwidth or high density. Herein, the strong EM absorption capability and wide efficient absorption bandwidth of high entropy ceramics are reported for the first time, which are designed by a combination of the novel high entropy(HE) rare earth silicide carbides/rare earth oxides(RE3 Si2 C2/RE2 O3). Three HE powders, i.e., HERSC-1(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2),HERSC-2 HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3) and HERSC-3(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3), are synthesized. Although HERSC-1 exhibits a limited absorption effect(the minimum reflection loss(RLmin) is-11.6 d B at 3.4 mm) and a relatively narrow effective absorption bandwidth(EAB) of 1.7 GHz, the optimal absorption RLminvalue and EAB of HERSC-2 and HERSC-3 are-40.7 d B(at 2.9 mm), 3.4 GHz and-50.9 d B(at 2.0 mm), 4.5 GHz,respectively, demonstrating strong microwave absorption capability and wide absorption bandwidth.Considering the better stability, low density and strong EM absorption effect, HE ceramics are promising as a new type of EM absorbing materials.

Review of wave mechanics theory for microwave absorption by film

The film and the material have been confused in current microwave absorption theory.This confusion has led to the establishment of the wrong theory of impedance matching and the wrong absorption mechanism.Progress on this subject has been reviewed in this work,and it shows that the wave mechanics theory which was proposed recently in the field of microwave absorption can be used to explain all aspects of the behavior of microwave absorption in film.Important aspects of the theory have been reviewed in detail involving the different absorption properties between film and material,the inverse relationship between frequency and film thickness that is related to the quarter-wavelength theory.