Nano-additives and their effects on the microwave absorptions and mechanical properties of the composite materials

. The review addresses the effect of various carbon and iron-based percentage nano-additives on both electromagnetic (EM) wave and mechanical properties of composite materials. It also assessed the in ﬂ uence of particle and ﬁ ber size along with the manufacturing process, on mechanical properties (tensile strength and ﬂ exural strength), fracture behaviors (fracture toughness) and electromagnetic properties (re ﬂ ection loss). Reviewing the selection of nanomaterials for a particular frequency band and application, as well as their impacts on bulk materials in relation to loading, were overviewed. As per this review, adding those iron and carbon-based additives in ﬂ uence positively for both electromagnetic and mechanical properties. Furthermore, review organized natural based ﬁ ber and ﬁ ller-based composites along with ﬁ llers for the production of green strong radar materials. The review also showed, how highest and smaller percentage of iron-based ﬁ llers affected for microwave absorption and mechanical properties. Mainly, the optimized use of nano particles percentage for both mechanical and electromagnetic wave to produce strong radar materials were overlooked. Finally, these papers give a quick hint on how these nano particles manufacturing methods and particle size affect the mechanical properties and micro wave absorption of composite materials.


Introduction
Microwave absorbing materials (MAMs), mostly called radar absorbing materials, are opening the way and new technology advancement for interesting applications in Electromagnetic (EM) wave pollution prevention and defense security. Because these materials provide an advanced alternative to traditional reflection principles for essentially eliminating EM radiation [1]. However challenges occurred to design the microwave absorbing materials one of the challenges for both academia and industry is to develop wideband microwave absorbing and shielding (MA&S) structures that are economical, lightweight, strong, and have unusual electromagnetic and mechanical properties [2]. Stealthy aircraft are less observable (or ideally unseen) to the detection devices like; radar, acoustic sensors, laser, infrared (IR) or near infra-red (NIR) [3]. Mainly stealthy vehicles are made of radar absorbing materials (RAM) in parts of it and the proper handling of shapes in designing, the hull and superstructure [4],which mainly allows significant reduction in the amount of energy reflected to enemy radars [5]. Likewise most stealth ships are designed by geometry or shaping and painting, due to some complexity only a few stealthy ships are constructed by applying RAM stealthy technique [6].
The applications of RAM in different sectors of aerospace and aeronautics (particularly aircraft and spacecraft) can be expressed by their durability, high strength, mobility, and radar invisibility. Those materials are also used in nuclear physics, which is for protection from nuclear EM pulses (NEMP), and shields in particle accelerators. RAMs are also used in the mitigation of human exposure to direct or indirect protection approaches against various EM signals (EM sources), electronic device development, from mobile phones to radios, which affect human and animal life. There for EM, signal shielding is critical for protecting vehicles, electronic devices and human health [7,8].
The main objective of this review is to compile the influence of nano particles on both mechanical and electromagnetic properties of composites for strong radar materials. Loading of these nano particles are critical and identifying the appropriate concentration is challenging. Some nano loading may be good for electromagnetic properties however, not for mechanical properties, and some loadings are good for both. This review mainly assesses the optimal concentrations of nano loadings for both electromagnetic and mechanical property increments of composites. Furthermore, the manufacturing method, wt% size and shape of carbon nano particle, and iron powder loading will be addressed for both mechanical and electromagnetic properties.
2 Effect of carbon, iron-based additives and green materials on microwave absorption 2.1 Effect of carbon-based additives on micro wave absorption Carbon-matrix composite containing continuous carbon fibers was discovered to be a good EMI shielding material due to high dielectric properties. Carbon has attractive electrical properties due to its semiconductor properties since it allows a little amount of charge to pass through it [32]. The current state of research on various carbon nanomaterials, including 0D nanocarbon having spherical shapes, 1D carbon nanotubes, 2D carbon platelets, and 3D porous carbons, with their composites like ceramics, magnetic materials, metal sulfides, and conductive polymer is still undergoing research to get excellent electrical and mechanical properties [33]. The most known carbonbased materials used for electromagnetic wave absorption are nano carbon, carbon-filled polymers, hybrid composites, carbon or graphene scaffolds, and carbon nanotubes [34]. The wide spectrum of radar absorbing fillers are carbon-based materials, like carbon black, carbon fiber, single walled & MWCNTs, graphene, reduced graphene oxides, and so on [3].
According to the finding of Tang et al. [35] carbon black/reduced graphene oxide (CB/RGO) composites had higher absorption intensity and a wider effective absorption bandwidth, when compared to CB particles. The maximum absorption was as high as 47.5 dB with a modest filler content of 7 wt%, and the effective absorption bandwidth (reflection loss À10 dB) reaches 5.92 GHz, encompassing 37% of the whole measured bandwidth at a thickness of just 2.2 mm. Another researcher Ling et al. [36] did on the effect of CB on micro wave absorption of polymer. From the researchers result, the appropriate loadings from 0 to 10 wt%; 7 wt% of carbon black showed maximum reflection loss (RL) and achieves 24.02 dB (99.6% absorption) at 9.0 GHz with 2.75 mm thickness.
Nano particle loading and thickness of the materials are factors on micro wave absorption of materials. For example, CNT films and silicone rubber (SR) plate composite showed better microwave absorption for small CNT loading (0.5%),with a minimum reflection loss (RLmin) of À26.55 dB for CNT films/SR composite [38]. Multi-walled carbon nanotubes (MWCNTs) and different ceramic particles were also used as absorbers in microwave absorbent composites and greatly increased the absorption produced by adding ceramic particles [39]. For stealth applications, wideband dielectric absorbers based on carbon fiber and carbon black powder were developed, prepared, and electromagnetically tested by Baskey et al. [40]. The result showed that, for 2 mm thickness, the minimum absorption capacity was 97% throughout the frequency range of the X-band. Another interesting Reprint with the permission from [37]. Copyright (2020) American Chemical Society. research was done by Che et al. [41] on influence of three commercially available multi-walled CNTs, Baytubes C150P (Bayer Material-Science AG, Germany), Nanocyl NC7000 (Nanocyl S.A., Belgium) and MWCNT-VAST (VAST, Vietnam) (MWCNT) materials on the micro-wave absorption of epoxy composites and was made via ballmilling nanotube dispersion in the resin matrix. The highest microwave absorption was found in Nanocyl NC7000, which had a CNT content of only 0.25 wt% and the maximum reflection loss peak 18.4 dB was observed at 8.8 GHz and 16.5 dB at 10.3 GHz.
From Figure 2, the resonant absorption peak was significantly off-centered with respect to frequency, and the variation in PANI content greatly affects its amplitude. When compared to pure milled carbon fiber (MCF)/epoxy nanocomposites, the RL performances of MCF/PANI/ epoxy nano composites were significantly improved. As the PANI concentration increased from MCF + PANI (1:0) to MCF + PANI (1:4), the RL value within the studied frequency range varied from À7.01 to À8.7 dB. PANI (1 wt%) + MCF (4 wt%) was found to have the greatest RL value of À29.8 dB at 8.6 GHz central frequency. For 1.1 GHz width band, 10 dB RL (more than 90% absorption) was also observed with 2.5 mm thickness. Figure 3b illustrates the absorption characteristics of the composites (2.3 mm thickness) when the absorbent filler was swapped out for porous carbon fibers. The porous carbon fibers have outstanding microwave absorption properties, which are readily apparent. At 9.7 GHz, the composite with porous carbon fibers with 6 wt% and a thickness of 2.3 mm exhibits the lowest reflection loss which was À32 dB. With similar content of porous carbon fibers increase by 19.8 dB by varying the thickness to 3 mm. It is believed that the dielectric-type absorption and the interference of multi-reflected microwaves enhanced microwave absorption for porous carbon fibers [43].
To explore both mechanical and electromagnetic properties, a MWCNT modified polyaniline epoxy in glass fiber reinforced polymer laminates using the wet-layup vacuum bagging method was produced by Jelmy et al. [44].
Based on the result, MWCNT/PANI modified epoxy with 1.5 wt% and 2 wt% has the lowest reflection loss at 3GHz. À16 dB was recorded for the laminate with 0.5 wt% MWCNT. Adding 2 wt% MWCNT also decrease the RL to À30.1dB.
Carbon black (CB) was mixed into epoxy resin composites containing hardener, aceton, and resin acceptor to create wave-absorbing coatings by Mehdizadeh et al. [45] and the absorption bandwidth of the double-layer sample with CB ratio (7 wt% + 10 wt%) in each layer reaches À32.1 GHz.
According to Table 1, combining two nano additives (dielectric or magnetic) improves the composite materials' electromagnetic characteristics more than using only one. But it's also crucial to pick the right combination of these ingredients. For batter absorption, the bulk material must also be properly taken into account. For instance, the MWCNT Nano additive displayed several characteristics, including variations in concentration.
Length of fibers, composite layup and size of nano additives affect the micro wave absorption properties of composite materials. For example, Breiss et al. [48] studied the effect of carbon fiber length on carbon fiber composites for microwave absorption. Short carbon fiber with 3 mm length showed losses for higher frequencies (about 15 GHz), however, long carbon fibers with 12 mm length showed the highest dielectric losses for low frequencies (below 4 GHz). In this study, 3 mm, 6 mm and 12 mm carbon fiber was used with 0.5% concentration. Assal et al. [49] also studied carbon fiber lengths which length from 1 mm to 12 mm with 3 mm range on dielectric properties. The research discovered that long fibers showed higher dielectric properties than short fibers that range lower than À50 dB for frequencies ranging between 8 GHz and 18 GHz. But this amazing di electric property was obtained because of using pyramidal absorber and it had advantage of both material and shape of absorbers. The reason why these properties obtain was due to advantage of shape and nano particle used. Zhou et al. [50] also studied the effect of composite layup configuration on micro wave absorption of carbon fiber reinforced composites. The unidirectional CFRP laminate shifts from good conductor to lossy dielectric materials when the ply angle varies from 0°to 90°. The fiber orientation highly affects the microwave response of multidirectional CFRP laminates. The impedance of the laminate will be substantially lower than that of air if the parallel alignment is used in the head-most layer(s). The laminate consequently tends to reflect the microwave. The multidirectional CFRP laminate can be converted to microwave absorbers when the cross-ply is placed distant from the microwave-radiating surface. Singh et al. [51] did nano composites using CB nano particle and epoxy which has particle sizes of 15, 25, 55, and 65 nm. 25 nm particles showed significant EM absorption capability at 9.5 GHz. The effect of profile was also studied by Bizhani et al. [52] and the result showed, nano additive with triangular profile provided the lowest reflection over the widest bandwidth.

Effect of green carbon-based materials on micro wave absorption
Natural based material used as filler or base material for electromagnetic wave absorption due to their high carbon content. Due to its high carbon content, charcoal improved microwave absorption of dried bananas as well as other agricultural byproducts like coconut byproducts, rice husk, sugarcane bagasse (SCB) and corn which are potential for microwave because of their, low density, easy availability, and green for environment. As corn charcoal has 83% carbon content, it is the best with regard to microwave absorption performance and is promising for production of green radar material [53]. Simulation analysis which were done by Pattanayak et al. [54] showed, that the absorption rate can vary by varying the dimensions of the sample. The coconut fiber, coir and charcoal composites have a huge potential in serving as an alternative material in fabricating eco-friendly microwave absorbers. Different agricultural residues are assed for micro wave absorption techniques in [32] and a futuristic vision of these microwave-absorbing materials as a feasible alternative for serving as green microwave absorber. The reason behind this fact is due to high presence of carbon in these materials. Agricultural wastes are used to make pyramidal absorbers in a very efficient and effective way, with very few chemicals used. The performance of banana leaves has been substantially improved and used for microwave absorber, because of high carbon content material [53]. A single-layer microwave absorber based on rice husk Ash/CNT composites was investigated by Lee et al. [55], adding CNTs to rice husk ash (RHA) increased the minimum absorption from À8 dB to À15 dB for a bandwidth of 10.8 GHz to 12.8 GHz. Demands for lower energy consumption and lower environmental impact are pushing the development of natural fiber-reinforced composites (NFRCs) in a variety of industries. Natural fiber has various advantages over synthetic fiber, including biodegradability, low cost, light weight, superior life-cycle performance, along with satisfactory mechanical qualities [56]. The electromagnetic shielding properties are promising and need more investigation to commercialize as a shielding material. Some natural fiber composites were reported with their cellulose content ((C 6 H 10 O 5 ) n); Kenaf with 53.14%, sisal with 68%, Cissus Quadrangularis Stem with 82.73%, jute fiber 72% and red banana with 72.9% [57]. The percentage of carbon directly affects the microwave absorption of natural fiber based composite materials.
Xia et al. [58] produced Hybrid composites of natural fiber mats, aluminum sheets, and epoxy resin using the closed-mold process. Micro wave absorption and mechanical properties were also tested. The samples were neat hemp fiber mats, hemp and aluminum sheet within, to see the effect of natural fibers in electromagnetic shielding. The presence of Al in the material increases the reflection and decreases absorption. Micro wave absorption in electromagnetic shielding increased when the natural fiber reinforced composite increased. Xia et al. [59] also applied Cu film magnetron sputtering in kenaf fiber composites for electromagnetic shielding function. After 0.5 h of treatment, the composite surface developed a coppery color and increased from 2.8 dB to 23.8 dB with 0.5 Hr Cu sputtering in Kenaf fiber composites, it is almost 99.5799% micro wave absorption. While adding the magnetron sputtering time from 0.5 h to 3 h, the specimen color didn't change significantly, however, the shielding increased from 23.8 dB to 48.3 dB. Another interesting research was done by Ding et al. [60] on electromagnetic shielding properties of kenaf fiber-based composites with various amounts of iron oxide impregnation for frequency range of 9-11 GHz. Ferric and ferrous ion solution was introduced by fiber magnetization. Those ion solutions absorbed the fibers uniformly after 14 h of immersion and the iron oxide nanoparticles formed inside the fibers' micro-pore structures. By adding iron oxide particles in to kenaf fibers, the shielding efficiency of kenaf composites was improved. Optimum shielding efficiency of kenaf fibers was seen with 18% Fe content. Electromagnetic, mechanical characterization and fractography of iron reinforced jute fiber composites were studied by Kala et al. [61]. The first sample was 20:35:45, the second 20:40:40, the third 20:50:30 and the fourth sample was 20:55:25 percentage of jute, iron and epoxy respectively. The iron powder was mixed manually with epoxy resin for 30 min in distinct proportions as required in the composite and followed by ultrasonic mixing for 30 min. With the ratio of jute:Iron:epoxy 20:50:30 was the best microwave absorber around an average of 98.1% which has a thickness of 6 mm and wide band. The thickness of these composites also really matters.
For microwave absorption, Ni/C porous fibers were produced utilizing a simple in situ template approach with jute fiber as the raw material. Jute fibers were submerged in an aqueous solution of Ni (NO 3 ) 2 at a certain concentration. The wet jute fiber was extracted with tweezers and dried following soaking for 24 h to generate Ni/C porous fibers. Under the same circumstances, the jute fiber was annealed. The produced Ni/C porous fibers displayed outstanding microwave absorption capability with thickness (1.5-3.5 mm) to get porous carbon fiber, which was named PCF [62]. Figure 4 shows clearly for PCF with 2 mm thickness, shows the maximum reflection at 13 GHz and 1.5 mm thickness at 18 GHz. For this type of material, the thin material is more effective for higher frequency. For Ni/ C-0.2, the maximum reflection was recorded with 3mm thickness and was À44 dB at a frequency of 8 GHz.
For green fiber-based composites, the influence of manufacturing technique is oversimplified, and its impact on electromagnetic characteristics was not compared with that of the same samples to determine the ideal manufacturing tetchiness. However, future research will concentrate on this. In accordance with Table 2, magnetron sputtering offers excellent potential for electromagnetic absorption. When the electromagnetic and mechanical properties are in concern, manual mixing or mechanical mixing is also beneficial.

Effect of iron based additives on microwave absorption
Gultom et al. [63] used Natural zeolites which contains 80.3% SiO 2 , 14.19% Al 2 O 3 , 0.91% Fe 2 O 3 and ferric oxide as fillers in the microwave absorptive polyurethane composites. Ferriteoxide particles have excellent microwave absorption properties, however, the ratio of zeolite to ferric-oxide influenced the optimal microwave absorption. Fe ion-doped SnO 2 / (MWCNTs) composites were produced and their microwave characteristics was investigated by Bayat et al. [23], and 48.8% Fe ion shows a minimum (RL) of À44.5 at 15.44 GHz with a thickness of 1.5 mm. For Ku band, the lowest RL, À10 dB was recorded at 4.5 GHz. Hussein et al. [64] also discussed the microwave absorption and shielding capabilities of (MWCNT) composites chemically modified with metal or metal alloy oxides and put in a polyurethane matrix. As a consequence, all metals, especially CoFe at 20% concentration increased the reflection loss from 10% to 20%. Zhang et al. [65] also provided another assessment on the microwave absorption capabilities of Fe@carbon fiber, with the research finding that a composite loaded with 30% carbon fibers@Fe got as the best microwave absorption. Ben 3 Glass fiber reinforced epoxy  NPs with CFs (Parallel) showed less than À10 dB from 14 GHz to 18 GHz and the minimum recorded was À24 dB @17.9 GHz for a plate thickness of 4.10 mm to 4.48 mm. Table 3 demonstrates that SnO2/MWCNT composites with 48.8% iron concentration and (GOCIPs) with increased GO concentration to 2% exhibit extremely favorable microwave absorption. And it is evident that carbon-based composites require relatively little concentration, in contrast to composites with iron-based additives, which require greater concentration to have outstanding electromagnetic properties. However, because of parameter inconsistency, it is challenging to draw a judgment using such forms.
Wang et al. [74] used in situ synthesis, densification, and magnetron sputtering methods for a natural fiberbased sandwich-structured magnetic composite. The iron oxide's particle size in the core layer of the magnetic composites was 135 mm, and the total iron oxide content was 18.7 wt%. The interlayer surface had an iron oxide particle size of 492 mm, and the total iron oxide content was 26.1 wt%. 0.8 mm thick composite insulated about 99.2% of the electromagnetic radiation. Carbon fibers of three various lengths (80,150, and 350 mm), two different fiber densities (1000 and 2000 fibers/mm 2 ), along with two different quantities of Fe 3 O 4 nanoparticles were used to study the effect (0.5 and 1 wt%) on EM properties. Composite with micro carbon fibers of 350 mm length and a density of 2000 fibers/mm 2 with 1 wt.% of Fe 3 O 4 nanoparticles had the highest total SE value [75].
Carbonyl iron particle composite with graphene oxide exhibited the lowest reflection loss but in medium frequency in the other hand, MWCNT composite with iron particle shows minimum reflection loss for relatively higher frequency as shown in Figure 5. Glass fiber with MWCNT shows its RL min with minimum frequency. From this graph it is clear that for increased frequency using both iron and carbon-based additives in the same time is effective. The composites for Figure 5 are selected with the best performance and concertation.

Effect of carbon and iron-based additives on mechanical behavior of composite materials
While targeting to improve the microwave absorption capability of materials using additive nanofillers, it is worthwhile to examine the effect of inclusion of nanofillers on the mechanical behavior of the reference/pristine materials to identify the best concertation that could enhance or reserve the reference performances, i.e., strength, fracture toughness, and stiffness. In this regard, there has been a limited researches that have used the same sample for both mechanical and microwave absorption studies; however, various bulk materials with these nano additions, i.e., carbon-based and iron-based actives, are being reviewed to correlate the effect of these additives in microwave absorption without affecting the mechanical  and fracture behaviors for the production of strong radar material. Carbon nanotubes also increased damage tolerance for radar absorbing structure and prevent material damage [76]. Flame retardant self-healing, shielding and fiber modification are some applications of nanotechnology for fiber-reinforced polymer composites [77]. The impact of nano-scale materials (carbon nanotubes and oxide nanoparticles) on the mechanical properties of nanocomposites with various metallic and polymeric matrices, was also studied by Fathi et al. [78] and concluded that mechanical characteristics like fracture toughness, hardness, tensile and flexural strength, was improved. Different carbon and metallic fillers introduced by researchers. For example; Abu-Okail et al. [79] introduced graphene and alumina nanoplatelets on carbon/glass and studied the mechanical characteristics. The results showed good properties in hardness, tensile strength and bending strength.

Effect of carbon-based nano particles on mechanical behavior of composite material
Research on nano-carbon materials, including fullerenes, CNTs, CNFs, and graphene, has been a focus of nanoscience over the past two decades and was used in a variety of applications [80]. Jelmy et al. [44] did the addition of conducting fillers to epoxy matrix reinforced by glass fiber and improved the mechanical properties, because of the high aspect ratio and highly elastic behavior of CNTs during loading and the strong interfacial bonding between epoxy and PANi-coated MWCNTs. Failure strain, hardness, strength, and modulus were also determined by Nezhad et al. [81] by using graphite carbon nanoparticles (CNPs) implanted in a low viscosity epoxy resin with a CNP weight percentage (wt%) between 1 and 5%. The integration of 1 and 2 wt% of CNP increased the With 05 wt% C, the highest on-axis ultimate tensile strength was achieved, when compared to clean glass fiber reinforced composites, a gain of 18-79% was made. Carbon nano tubes have demonstrated their usefulness in improving the mechanical and water absorption capabilities of bamboo and ramie fiber reinforced polymer composites [83]. Rasoolpoor et al. [84] and Naghizadeh et al. [85] studied low and high velocities impact performance, respectively. Adding carboxyl-modified multi-walled carbon nanotubes (COOH-MWCNTs) to the sandwich panels with three different weight percentages of 0.3, 0.5, and 1 wt% of COOH-MWCNTs was done. Furthermore, increasing the COOH-MWCNT composition increased high and low velocity impact response of composite materials.
The effect of crab shell with a carbon content of 24.53% and CaO content of 71.42% [86] particles with 0 wt%, 2 wt %, 4 wt%, and 6 wt% on sisal based hybrid composites was explored by Soundhar et al. [87]. In comparison to other combinations, the addition of four weight percentages of crab shell particles improved the tensile strength by 50% and flexural strength by 38% significantly. Further addition with 6%wt decreased relative to 4% however, it slightly increased relative to pristine.
Rehman et al. [88] also did on influence of microcrystalline cellulose (MCC) particles on jute fiber reinforce epoxy composite, which has a carbon content of 66.62 % [89] with loading (0%, 1.5%, 3%, 5%, 7%, 9%, and 11%). The result showed adding 3%, 4%, 5% was the best and adding higher percentage led to decrease the mechanical properties due to agglomeration and higher percentage induced damage. It's worth noting that adding up to 7% MCC improved the tensile strength and modulus by 48 and 25.6%, respectively.
Bhowmik et al. [90] did experiments on the effect of (MWCNT) on the mechanical behavior and damage of woven carbon fiber reinforced epoxy composites. Both MWCNT/epoxy nanocomposites and (CNT) reinforced laminated open hole composites with various MWCNT weight ratios underwent tensile tests. For MWCNT dosage of 0.5, 1.0, and 1.5 wt%, respectively, the tensile modulus of CNT/epoxy nanocomposite was increased by 15.0%, 37.86%, and 22.86%. Tensile strength was raised by 19.76%, 25.78%, and 6.70%, whereas the comparable improvements in tensile modulus for woven composites were 3.45%, 10.25%, and 1.53%. Due to MWCNT agglomeration, the improvement of mechanical properties was reduced for 1.5 wt% MWCNT laminates. Another interesting research was done by Ismail et al. [91] on addition of MWCNT in amounts of 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt% for flax hybrid bio-mechanical composites which may use in aircraft structural applications. According to the experimental findings, MWCNTs increased the composite flexural strength by 2.8-50%. Nor et al. [92] also investigated the effects of MWCNT on bamboo/glass fiber hybrid composites. The presence and quantity of CNTs had an impact on the hybrid composites' tensile strength and modulus. The addition of CNTs (0.1 wt%) improved tensile strength of pure epoxy by 16 Rajmohan et al. [95] also did hybrid banana-glass fiber reinforced composites and the effect of CNT on tensile, compression and flexural strength. Adding of CNTs with 0.5% and 1% increased the tensile strength from 196 (N/mm 2 ) to 238 (N/mm 2 ) and 278 (N/mm 2 ) respectively. The compressive strength also increased from 108 (N/mm 2 ) to 122 (N/mm 2 ) and 142 (N/mm 2 ). Table 4 shows a representative summary of on carbon-based additives on mechanical properties of composite material and shows the optimum concertation for best additive percentage.

Effect of iron-based additives for mechanical behavior of composite materials
Nanotechnology has inspired a lot of interest in numerous fields of research for the last two decades, especially for the development of nanoscale materials which made using a variety of approaches, such as physical or chemical procedures [99]. For example Bafakeeh et al. [100] did research using Al 2 O 3 , and Wang et al. [101] used silver Piffer et al. [105] discussed how adding iron ore tailings (IOTs) to polypropylene and maleic anhydride grafted polypropylene (MAPP) composites affects their mechanical, thermal, and morphological qualities. The modulus of elasticity increased with IOT content for both types of polymers, whereas the impact and ultimate tensile strength of the composite dropped. Arun Prakash et al. [106] also did another interesting research on the influence of Iron (III) oxide particles between 0.5 and 1.0 vol.% on E-glass fiber as received and siliconized glass fiber reinforced epoxy composite. The highest tensile strength was achieved by 0.5 vol.% dosage of iron (III) oxide particles with treated glass fiber and 800 nm, compared to 1 vol.%, 100 nm, and 200 nm particle sizes. This concentration with a particle size of 200 nm produced the highest recorded value for flexural strength. Dinesh et al. [107] used a magnetic stirrer to mix quantitative epoxy resin with 5 and 10% unsaturated polyester (UP) to make E-glass Fibre/Iron (III) oxide composite material. A fixed amount of UP/ Epoxy blend (10-90 vol%) was combined with 0.5 and 1.0 vol% silane treated iron (III)oxide. In addition, 40% silanetreated e-glass fibre was installed, along with a particlestrengthened UP/epoxy blended system. The tensile strength was increased from 114 Mpa to 126 Mpa in 100/10/40/0.5 and 100/10/40/1.0 as epoxy/UP/Si-fibre/ Si-iron (III)oxide vol% composition. From pristine type the tensile strength recorded was 73 Mpa. However, for 0.5-10 wt%, it decreased the flexural strength of this composite. Increased in Si-iron (III) oxide increased the inter laminar shear strength to 16 Mpa from 14 Mpa. Iron filing in the concentration of 5 wt%, 10 wt% and 15 wt% was used in [108] to reinforce polystyrene based resin (PBR). The young's modulus (N/mm 2 ) was increased from 335.72 ± 0.63 to 484.41 ± 0.96 for 5 wt%, to 630.15 ± 1.47 for 10 wt% and 1131.80 ± 1.84 for 15 wt%. Adding iron powder to natural fiber reinforced composite and E-glass fiber reinforced composite decreased the tensile strength and flexural strength as per [61,109]. For example in [109] with the addition of 0.5 vol.% and 1.0 vol% Fe 2 O 3 , the tensile strength of E-glass fiber reinforced composite reduced from 135 Mpa to 128 Mpa and 114 Mpa, respectively, and the hardness increased from 84 to 88 for both samples. Julyes Jaisingh et al. [110] also investigated Iron (III) oxide nano particles effect on the mechanical properties of Kevlar Fiber Reinforced Epoxy Nanocomposites. Tensile strength of the silane modified iron (III) oxide nano particles reinforced composite sheets was significantly higher than that of the base composite (without iron (III) oxide). There was an increase in strength at minimal loading (less than 1.5 wt%), however, when the loading has increased by more than 1.5%, the rate of decline begins. Unfortunately, when the concentration was increased above 2.5 wt%, the strength was lower than that of pristine Kevlar Fiber Reinforced composites.
Shah et al. [72] [111]. Yadav et al. [112] also showed Iron oxide platelets and graphene oxide (GO) showed powerful synergistic effect on strengthening the chitosan matrix. As 0.5% Fe 3 O 4 and 1% GO are added to the composite, the tensile strength and Young's modulus increase by around 28% and 74%, respectively, when compared to chitosan.
To prepare cellulose nano fibers reinforced polyhydroxybutyrate(PHB)/Polycaprolactone(PCL)/Fe 3 O 4 shape memory polymer composites, 3D printing technology was used to combine PHB, nano iron oxide (Fe3O4, 99.5%, 20 nm), and cellulose nanofiber dispersions (CNFs,1.1 wt%) in [113]. Fe 3 O 4 and CNFs were studied for their effects on characteristics such as thermomechanical, static and mechanical behaviors. PHB/PCL (80:20) composites containing 10% Fe 3 O 4 and 0.5 wt% CNFs had the best overall mechanical and magneto-responsive shape-memory capabilities, according to the findings. The polymer matrix used was high density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS), with metal Fe powder in 5, 10, and 15 vol% to produce iron filled polymer composites in [114]. The hardness of HDPE was raised more than that of polypropylene (PP), and polystyrene when Fe powder was added. PS-Fe composites outperformed HDPE-Fe and PP-Fe composites in terms of yield, tensile strength, and modulus of elasticity. In all matrixes, increasing the Fe concentration from 5% wt% to 10% wt% and 15% wt% decreased the yield and tensile strength. Reis et al. [115] also examined the fracture properties of aluminum oxide (Al 2 O 3 ) and iron oxide (Fe 2 O 3 ) nanoparticles distributed in epoxy matrices at weight fractions of 3%, 5%, 7%, and 10%. The inclusion of both Al 2 O 3 and Fe 2 O 3 nanoparticles into the epoxy matrix enhanced the modulus of elasticity, fracture toughness, and fracture energy. 3.0 wt% content showed the greatest results as a toughening for both nanoparticles. Amorphous shape Fe 52 Cr 15 Mo 26 C 3 B 1 Y 3 particle was used to reinforce pure aluminum (Al) matrix, by X. Zhou, et al. [116]. The Fe-based amorphous powder was fully mixed with pure Al powder at 5%, 10%, 15%, and 20%. When the reinforcement content was 15%, the maximum tensile strength increased by 154% higher than that of pure Al. Sun et al. [117] also tested the mechanical properties of an epoxy nano composite (ENC) reinforced with Fe 2 O 3 nano particles (NPs). 4 wt% modified Fe 2 O 3 NPs increased the tensile strength by 50.2% and fracture toughness by 106%, based on a single-edge notch bend test. Water absorption was improved as iron oxide nanoparticles added to medium density fiberboard which produced from urea-formaldehyde resin and poplar wood fibers [118]. Table 5 shows a representative summary of on iron-based additives on mechanical properties of composite material to show the optimum concertation for best additive percentage.
Generally, the use of metallic reinforcements to improve mechanical performance in metal matrix composites is also futuristic. For Al matrices, it was discovered that Fe, Ni, Ta, and metallic glasses, which are primarily used as metallic reinforcement, result in a change of mechanical properties that are somewhat better than ceramic reinforced AMCs however, lower than the unreinforced Al matrix [121].

Conclusion and future look
The review reported in this paper was to address the particle loading in radar absorbing materials. With limited information available that investigated both EM and mechanical properties of nano-modified composite materials, the authors attempted to highlight relatively optimal carbon and iron nano-particles for producing strong radar materials.
The following conclusions are drawn from the review conducted: -Both iron and carbon-based additives increase the radar absorbing and some mechanical properties of composite materials. However, it is very important to have the optimum concentration and must be studied for selecting the weight percentage to compensate both properties. -Glass fiber reinforced composite is not effective for radar absorbing material, even if low content CNT is applied as its frequency range for RL < À10 dB is very low. -For the sandwich structure made of Al and natural fiber reinforced composites, adding natural fiber plate can increase the electromagnetic wave absorption and the mechanical properties of the total composite. -Addition of Carbon Black and carbon nano fiber in moderate percentage from i.e. 5-10% of loading in the matrix increases electromagnetic wave absorption, however for different types of CNT, smaller percentage loading shows good radar material properties recommended to apply less than 3%. Lower concentrations of these CNTs are good for lower frequencies and higher percentage in CB and CF can be applied for higher frequencies. The particle size and shape of these carbonbased nano additives also affect the electromagnetic properties and smaller size is preferred. In addition, the fiber length, angle of orientation also affects the electromagnetic properties as the impedance of the laminate is substantially lower than that of air for parallel alignment. Green carbon-based particles like corn, charcoal and natural fibers are also promising additives if different manufacturing techniques and dielectric materials are applied to these materials. Thickness of micro wave absorbing material is also one determining factor so that finding the preferred thickness is needed to be chosen for a specific working frequency. Adding CNTs are also promising in natural based carbon composites to produce green microwave absorbers. -Adding di-electric additives in radar absorbing material also increases the electromagnetic absorbing properties, among these materials iron particles is the most common.
Higher loading of iron particles shows good electromagnetic absorption, i.e., more than 20% of total volume. -Smaller amount of particle loading is recommended for improving mechanical properties and seen in both iron and carbon-based additives relative to electromagnetic absorbing applications. -Applying both iron and CNT can produce a thin absorbing material. Multiwall carbon nanotube (MWCNT) composites functionalized with metal or metal alloy oxides and embedded with polymer matrix also show good electromagnetic absorbing properties. -These nano particles may affect the bulk material negatively for both mechanical and electromagnetic properties unless, it is applied in proper conditions. These avoid agglomerations, maintain electrical, thermal properties and charge flow of materials are critical.
Generally, adding nanoparticles to composite materials typically improves its electromagnetic wave absorption, fracture toughness and mechanical properties, if proper manufacturing technique, loadings, size, and kind of nanomaterials are used. To better comprehend the properties of mechanical and electromagnetic waves, future research should concentrate on optimum concertation and mathematical correlations.
For mechanical qualities, a higher concentration of Nano addition is linked to agglomeration.
However, there is a lack of clarity on elector magnetic absorption, necessitating more research material.
Additionally, a combined understanding of physics and chemistry is required to understand the relationship between concentration and electromagnetic characteristics.