Microwave-based preparation and characterization of Fe- cored carbon nanocapsules with novel stability and super electromagnetic wave absorption performance

Microwave-metal discharge was proposed as a facile methodology to prepare unique Fe-cored carbon nanocapsules (Fe@CNCs) with high purity, novel stability and extraordinary electromagnetic wave (EMW) absorption performance. The effect of microwave power, irradiation time and cyclohexane/ferrocene ratio on the production * Corresponding author. ** Corresponding author. *** Corresponding author. E-mail address: wwenlong@sdu.edu.cn (W. Wang), sunj7@sdu.edu.cn (J. Sun), Cheng-Gong.Sun@nottingham.ac.uk (C. Sun). of Fe@CNCs was examined and the properties of the nanocapsules, such as their Fe content, phase, yield, degree of graphitization and associated microstructures were investigated in detail. It was found that the prepared Fe@CNCs, which can easily be separated from the reaction system, displayed exceedingly high electromagnetic wave (EMW) absorption performance over the 2-18 GHz range. At the minimal reflection loss (RL) values over -10 dB, the EMW absorption bandwidth can reach up to 13.8 GHz with an absorber thickness of 1.5-5 mm. In addition, novel thermo-oxidative stability and super anti-corrosion property were also obtained for the Fe@CNCs as no signs of any corrosion or oxidative degradation loss were observed from the accelerated degradation tests in air and acid at temperatures up to 420 °C. The exceedingly high EMW absorption performance coupled with the superior anti-degradation and anticorrosion properties of the prepared nanocomposite microcapsules highlights the novel capability of microwave-metal discharge in synthesizing advanced metal-cored nanocarbon microcapsules with promising application potentials in diverse fields, such as but not limited to microwave absorption, EM shielding and advanced separations etc.


Introduction
Nanocapsules are a class of nanocomposites with a unique core/shell structure with size ranging from a few nanometers to hundreds of nanometers. Due to their special optical, electrical, magnetic and catalytic properties [1,2] and their promising application potentials in electromagnetic wave (EMW) absorption [3][4][5], battery technology [6][7][8], catalyst [9,10], biomedical [11] and electromagnetic (EM) shielding [12], how to develop metal-cored nanocapsules with robust performances have attracted intensive research activities over recent years. Nanocapsules with appropriate materials as being the shell can effectively protect the metal core from getting oxidized thus to improve their stability in hostile environment [13,14]. A number of nanocapsules have been investigated, with the shell structures fabricated ranging from carbon materials [15,16], boron nitride [17], conductive polymers (e.g. phosphates) [18] to metal or semiconductor oxides [19,20]. Among them, carbon shell materials are considered ideal owing to their proper dielectric properties, low cost, easy access and good environmental stability. As a result, the carbon-based soft magnetic metal nanocapsules, which are often referred to as metal-cored carbon nanocapsules (M@CNCs) usually with Fe, Co, Ni and their alloys as the core, become a research of intensive interest because the exchange coupling effect at the heterogeneous interface can induce the aforementioned desirable properties that cannot be possibly achieved otherwise.
One of the novel properties of M@CNCs is their unparalleled performance in EMW absorption over single-substance materials owing to the unique core-shell structure.
Magnetic metal nanoparticles have the characteristics of high saturation magnetization and shape anisotropy. Compared with micron magnetic metal particles, M@CNCs show higher magnetic permeability and magnetic loss in wide frequency ranges of gigahertz [21,22]. As their characteristic particle sizes are 1~100 nm, lower than their skin depth in the EMW band, the restriction of eddy current effect can be furthermore avoided [23]. However, magnetic nanometals alone cannot achieve effective absorption of EMW due to the limited achievable dielectric losses. Nevertheless, the existence of carbon shell can effectively solve this problem, thanks to their excellent conductivity and dielectric properties. In addition, the increase of electrical resistivity can also effectively improve the attenuation constant. It is the cross-coupling effects among magnetic loss and dielectric loss as well as the larger attenuation constant that jointly contribute to the enhanced EMW absorption performance of the M@CNCs.
Despite the superiorities, the practical applications of M@CNCs still faces major challenges or performance hurdles such as the lack of scalable production methodologies and the associated issues of product purity. Following the arc discharge method that was first used by Rouff et al, a variety of other preparation methods have been investigated, such as chemical vapor deposition (CVD), high temperature carbonization, pyrolysis, hydrothermal and explosion methodologies [1,[24][25][26][27][28]. Among these technologies, CVD appears to be the best available methodology at present but suffers from major drawbacks, such as the product purity and separation problems.
Other technologies currently under development also face their own disadvantages. For instance, pyrolysis method benefits from process simplicity but the selection of appropriate precursors is the major challenge yet with high energy consumption and long preparation cycle whereas the complicated explosion methodology is almost impossible to operate and control with confidence. In addition to the shortcomings of preparation methodologies, the performance of the product also faced some problems.
For example, carbon-encapsulated iron hybrids and graphite-coated FeNi nanoparticles synthesized by hydrothermal and arc-discharge methods are stable only in air and at temperatures below 180 ℃ and 240 ℃ respectively, highlighting their relatively poor thermo-oxidative stability [9,29]. In addition, the absorption bandwidth for a given absorber thickness of the M@CNCs prepared by arc-discharge plasma method with RL values exceeding -10 dB is narrow [30,31], which limits the potentials of their applications. Therefore, more practical or effective technologies have to be developed.
Microwave-metal (MW-M) discharge can potentially serve as a novel approach to fabricate high-performance M@CNCs, due to its capabilities in facilitating multiple effects to take place simultaneously, such as strong selective heating, photo-catalytic and plasma effects [32]. When metal with tips or sharp edges were exposed to the microwave field, intense discharge phenomena may occur. Our previous investigations have demonstrated the novel application potentials of microwave-metal discharge in eliminating toxic volatile organic emissions and associated mechanisms [33,34]. It was then interestingly found that the decomposition of carbonaceous compounds was often accompanied by the formation of dense carbon fragments. The transient local high temperature with localized pressure in the discharge area, which can reach up to 3000 ℃ [35,36], provide desirable conditions to facilitate the formation of density carbons with high degrees of graphitization.
Herein, the potential of using MW-M discharge as an approach to prepare iron-cored nanocapsules (Fe@CNCs) was for the first time explored, and the properties of the material were characterized using a variety of advanced characterization tools. To demonstrate the application potentials, the performance of this material for microwave absorption and EM shielding was evaluated with outstanding results.

Materials and sample preparation
All chemicals used, typically including cyclohexane, ethanol and concentrated nitric acid were mainly purchased from Aladdin Industrial Corporation (ferrocene) and Sinopharm Chemical Reagent Co. Ltd. (including cyclohexane, ethanol and concentrated nitric acid). The concentrated nitric acid was diluted to 3 mol/L by deionized water, as acid lotion. Nickel wires (1 mm in diameter and 3-6 mm in length) obtained from Shanghai Shen Long High Temperature Line were used to induce microwave discharges in a self-designed industrial microwave oven (300-4000 W, 2.45 GHz). High purity argon (99.999%) acted as a protective gas.
A purpose-built quartz reactor with high-purity quartz glass, which can provide the required microwave transparency, was used to synthesize the Fe@CNPs with microwave-metal discharge via a dissolution-precipitation mechanism. In brief, to prepare the Fe@CNPs, cyclohexane and ferrocene were first mixed at a pre-determined weight ratio (cyclohexane: ferrocene = 5:1, 10:1, 15:1, 20:1, 25:1 and 30:1) in a quartz tube and then the nickel wires (2 g) were added into the aqueous mixture. The selection of the ferrocene/cyclohexane ratio is based on two important factors, namely the solubility of ferrocene in cyclohexane and the minimal volume required to facilitate the materials synthesis for the given experimental conditions. The nickel wires with tips are used to induce the desirable discharge under microwave irradiation and the amount of nickel used is determined by the required level of MW-metal discharge required for a given volume of the reactants. The mixture was then subjected to ultrasonic dispersion for 10 min before it was positioned in a quartz reactor. The reactor containing the mixture was then placed in the microwave oven and purged thoroughly with argon at 200 mL/min to create an oxygen-free environment. Then, the mixture was exposed to microwave irradiation at different power output levels (800, 1000, 1200, 1400, 1600, Before determining the EM parameters (permittivity and permeability), the Fe@CNCs (30 wt%) and the paraffin mixture were first processed into a toroidal sample (7 mm outer diameter and 3.04 mm inner diameter). A vector network analyzer (Agilent PNA-N5244A) was then used to measure the EMW absorption performance of the sample at a frequency of 2-18 GHz.

Results and discussion
To investigate the porosity and the BET surface area of the Fe-cored nanocapsules, the N2 adsorption-desorption experiment was carried out. The N2 adsorption-desorption isotherm and pore size distribution curve are shown in Fig. 1. The isotherm is a typical type IV isotherm with a H3 type hysteresis according the to the classification of IUPAC [37], being indicative of the slit-like mesopores that are created by the aggregates of platy particles namely the Fe-cored capsular nanocarbon particles. The nanocapsules has a specific surface area of 56.8 m 2 /g and pore volume of 0.19 cm 3 /g, with a single modal pore size distribution centred at about 10 nm. The BET surface area of the nano-capsules prepared with MW discharge is considerbly larger than those of the same type of the reported nanocapsules prepared by other methodologies, such as GN-pFe3O4@ZnO (22.2 m 2 /g) [3], Fe x @CS (18.0 m 2 /g) and Fe 0 /Fe3C@CS (42.3 m 2 /g) [9]. The existence of porous structure can significantly improve the interfacial polarization effect, which can further enhance the dielectric loss in the high frequency range, thus affecting the EMW absorption performance of the nanocomposites.

Total solid and Fe@CNCs yield
To better define the yield of the product, in this work, the total solid is the weight of the raw product in each batch and the Fe@CNCs yield is defined as the mass ratio of the raw product after and before acid washing treatment.  [32], which contributes to the decomposition of organometallic compounds and the carbonization of amorphous carbon. However, when the microwave power is too high, violent discharges will lead to the evaporation of raw materials. At this point, the greater the power, the faster the evaporation, resulting in a decline in yield. The variation regularity of total solid under different microwave irradiation time was shown in Fig. 2b, that the weight increases first with the increase of microwave irradiation time, and it tends to be stable after 4 min. This is because a relatively short irradiation time cannot provide sufficient energy for the carbonization process and a large amount of ferrocene was carried by the evaporated solution.  Fig. 2c shows that the total solid is positively correlated with the content of ferrocene in the raw material, which means that the total solid is mainly determined by the ferrocene content. It is known that the addition amount of ferrocene of the six samples is 1.8, 0.9, 0.6, 0.45, 0.36 and 0.3 g, the mass ratio of total solid/ferrocene can be calculated as 0.23, 0.24, 0.33, 0.31, 0.25 and 0.20, respectively. It can be seen that the amount of nano-carbon particles formed as a fraction of the absolute amount of ferrocene used remains relatively constant or appears to be irrespective of the concentration of ferrocene in cyclohexane, suggesting that ferrocene, as opposed to the solvent of cyclohexane used, is primarily responsible for the formation and deposition of the nano-carbon particles. Indeed, it was found that most of the cyclohexane can be condensed out and recycled in the downstream process. The yields of nanocarbon particles were calculated to be more than 20%, which is significantly higher than those of other methods, such as microwave arcing process (~15%) [14] and explosive method (10-15%) [26]. In addition, all carbon particles showed similar content of Fe@CNCs, which was averaged at 86.2 % (as shown in Fig. 2d), confirming the exceedingly high efficiency of the formation of the ferromagnetic encapsulated nanoparticles. Compared with the carbon arc discharge (8-25%) [38], the Fe@CNCs yield obtained by this methodology has obvious advantages, indicating a high process efficiency. Both high yield and short preparation cycle have laid a solid foundation for the efficient application of MW-M discharge. Based on the above data, the formation mechanism of Fe@CNCs can be speculated. That is, hot-spot effect and plasma effect generated by MW-M discharge induce the micro-discharge of Fe in the ferrocene, which causes the collapse of cyclopentadiene on both sides of the Fe atom. Due to the desirable environmental conditions and the catalytic effect of iron [39][40][41], cyclopentadiene coated around the Fe atom can be thoroughly dehydrogenated and carbonized and eventually lead to the formation of the integrated graphite-like structures.  Where g is the graphitization degree, d002 is the interlayer spacing along the c axis of graphite.

Phase and structure characterization
The results intuitively show that the graphitization degree of the iron-cored nanocarbon particles can reach up to about 70%. The sample prepared from using a cyclohexane/ferrocene ratio of 15 (A1800-4-15) has a higher graphitization degree, followed by the sample A1800-4-5. This suggest that the amount of ferrocene available is vital for the formation of the well-graphited carbon shells. In addition, it is noteworthy that the presence of cementite Fe3C, which was detected in appreciable quantities, may suggest that this compound may serve as an intermediate for the formation of the graphite layers [39,43]. Since Fe3C usually can only be formed at extremely high temperatures, this highlights the capability of microwave-metal discharge in creating the desirable localized high temperatures and associated pressures for the formation of quality metal-cored nanocarbon particles. To further characterize the degree of graphitization of the metal-cored nanocarbon particles, which is an important factor that determines the quality of the materials for advanced applications (e.g. as an EMW absorbing material [44]), Raman spectroscopy was used to reveal further information. Fig. 4 shows the Raman spectra of the Fe@CNCs prepared under different conditions. All the samples present two typical characteristic peaks, which are often respectively referred to as the D band (1333 cm -1 ) that is related to the disorder of the carbon material and the G band (1586 cm -1 ) that is used to indicate the crystalline degree of the hexagonal lattices of carbon materials [45].
As a result, the intensity ratio of G and D band (IG/ID) is often used to characterize the graphitization degree of the carbon material, with samples having a higher graphitization degree giving rise to greater IG/ID of the ratio [38]. The Raman results show that the IG/ID value of all the samples is significantly higher than 1.2 and with the sample prepared with a microwave power output level of 1800W and irradiation time of 4 min having the highest IG/ID ratio of 1.81, which are significantly higher than those of the metal-cored CNCs prepared from using other methodologies, such as hightemperature carbonization [44], carbon arc discharge [38] and thermal plasma torch method [46].
It is well known that iron core has catalytic effect on the formation and growth of graphitized nanocarbon shells [39], but clearly the catalytic activity of iron alone cannot fully account for the significantly higher degree of graphitization obtained for the Fe@CNCs prepared from using the microwave-metal discharge methodology. It can be seen that for a given duration time of microwave discharge, the graphitization degree of Fe@CNCs first increased with increasing the power output up to 1800W, followed by a decrease with a further increase in the output level used. It is believed that the excessive power output may lead to excessive rate of carbon formation and thus affect the crystallization or alignment of the carbon on the surface of the iron core, giving rise to greater defects of the carbon shells formed. Similar trend was also observed with respect to the effect of microwave-metal discharge time (e.g. Sample A1800-2-15, A1800-4-15 and A1800-5-15). For a given power output level, it was found that the use of longer irradiation time led to the formation of larger sizes of Fe-cored nanocarbon capsules but at a cost of reduced degree of graphitization, due to the entrapment of amorphous carbon in the capsule structures during the process of particle deposition or agglomeration. clusters containing a typical core-shell structure, with nanocarbon tubes also observed in appreciable quantities, due to the catalytic effect of metallic iron [47] and the high temperatures induced by the MV discharges. The sizes of the nano-capsules or their clusters vary typically from 30 to 100 nm. The occurrence of nano-capsule clusters or agglomerates is clearly attributable to the strong cohesion or even electromagnetic interaction between the Fe-cored CNCs of nanoparticles as to be discussed later.
The TEM image (see Fig. 5b) reveals that the carbon shell of Fe-cored nano-capsules has a distinctive layered structure with an interval spacing of around ~0.34 nm, which is very close to the theoretical layer spacing of the graphite (0.3354), highlighting the high degree of graphitization of the carbon shells. Additionally, the thickness of the crystalline carbon shell was obtained to be more than 10 nm (as shown in Fig. 5b and Fig. 5d), which is higher than virtually all previous reported metal-cored carbon capsules prepared by high temperature carbonization, arc discharge and explosion methods [21,48,49]. It is also evident from the SEM/TEM images that the Fe metal particles are well seeded into the core of the carbon lattice with no evident structural defects, highlighting the novel material integrity and potential super stability of the Fe- However, in this work, the onion-like carbon shell was rarely found, indicating that the shell of Fe@CNCs prepared by MW-M discharge have a high integrity.

TGA characterization and thermo-oxidative stability of Fe@CNCs
Thermal gravimetric analysis (TGA) was used to examine the relative composition and the thermos-oxidative stability of the Fe@CNCs capsules prepared under different conditions [51]. Fig. 6 shows the proximate analyses for the Fe@CNCs samples. As can be seen, the iron content in the form of iron oxides varied from 17.5 wt% to 22.0 wt%, with those prepared at higher MW output levels and longer discharge times generally having higher iron or lower carbon contents. The material balance of iron from the preparation can be expressed as follows: Iron in ferrocene = Iron removed by acid-washing + Iron coated in carbon shells + Iron loss. Based on the mass conservation law, the effective utilization rate of iron in this preparation was calculated to be between 20-25% at a power level of 1800 W and an irradiation time of 4 mins. However, it is interesting to note that the sample prepared at the highest ferrocene concentration (sample A1800-4-5) was found to have the lowest iron (17.5 wt%) or highest carbon content (82.5 wt%), and this may suggest that the carbon contained in the ferrocene is preferentially deposited out at a loss of iron or some of the ferrocene-contained iron is not encapsulated to form the iron-cored nanocarbon capsules. By and large, the proximate analyses show that a cyclohexane/ferrocene mass ratio of 15 appears to be the optimal for the efficient formation of the Fe-cored capsules. Compared to other carbon-coated nanocapsules prepared from using other methodologies [5,52], the carbon contents or the thicknesses of the carbon shells of the Fe-cored capsules generated from the MW-M discharge are considerably higher, highlighting again the novel capability of MW discharge in fabricating high quality of metal-core capsules. In order to investigate the anti-corrosion performance and thermo-oxidative stability of the Fe-cored nanocapsules, the sample A1800-4-5 was first subjected to an acid washing treatment in dilute nitric acid under ambient and vigorous stirring conditions for variable duration times, before they are subjected to the burn-out tests in air with TGA, which were used as an accelerated testing of the thermo-oxidative stability of the capsule samples. Fig. 7 shows the accelerated thermo-oxidative stability testing results of the samples before and after the acid treatments. The slight weight gain observed at temperatures up to 420 ℃ for the sample A1800-4-5 before the acid treatment are believed to result from the oxidation of trace quantities of free or partly encapsulated iron [9] whereas the following slight weight loss obtained from 420 to 450 ℃ was due to the burnout of the amorphous carbons formed, respectively. This confirms that the formation of amorphous carbon and non or partly encapsulated iron appears to be closely related, and this phenomenon seems to occur mainly at high ferrocene concentrations used under the MW-M discharge conditions.
In general, the TGA results indicate that all Fe-cored capsules demonstrate exceedingly high thermo-oxidative stability at temperatures up to 420 ℃ and no evident effect of the strongly aggressive acid treatment was observed on the stability. This highlights the robust anti-corrosion performance and novel thermo-oxidative stability of Fe@CNCs prepared via the MW discharge methodology, which are significantly higher than those fabricated with other methodologies, such as hydrothermal, arc-discharge and CVD method [14,53], where the Fe or other metal-cored capsules can only afford temperatures generally well below 240 ℃. To further investigate the thermos-oxidative stability of the Fe-cored nanocapsules prepared, the sample A1800-4-5 was selected for an annealing treatment in air for 2 h at a pressure of -0.05 MPa and temperature of 550 ℃. Fig. 8a shows the Raman spectra for the A1800-4-5 sample before and after the annealing treatment. It can be seen that the IG/ID value of the sample after the annealing treatment decreased from 1.57 to 1.25, but it is still considerably higher than most of the other Fe@CNCs reported with other methodologies [11,14], being indicative of the novel integrity of the Fe/carbon shell structures. The XRD pattern for the annealed sample (Fig. 8b) shows that the Fe3C species present in the capsule structures have been transformed into Fe3O4 after the annealing treatment. Compared with the CNCs with Fe/Fe3C as the core, the capsules with Fe/Fe3O4-cored exhibit better electrochemical performance. More importantly, the detection of crystal lattice of Fe3O4 as can be clearly seen from the inset in Fig. 8b indicates that the thermal annealing treatment contributes to the further development of the desirable crystalline structures, which helps to enhance the metal-cored capsules'  saturation magnetization [54]. It is worth mentioning that the chemical composition of the residual solids remains the same i.e. C, α-Fe and Fe3C after thermal degradation in N2 atmosphere. The high resolution TEM images shown in Fig. 9 show that after the thermal annealing treatment, the size and carbon layer thickness of the capsule samples increased significantly, indicating a better anti-corrosion performance. It was also found from the TEM images that the degree of structural disorder and fusion between the small nanoparticles is increased (Fig. 9b), which is consistent with the observed decease in the IG/ID value shown in Raman spectrum in Fig. 8a. It is noteworthy that a remarkable recovery of higher than 95% was achieved from the nitric acid washing of the thermally annealed sample, being suggestive of the novel thermo-oxidative stability of the Fe@CNCs in extreme environments, for example, as stealth material on the surface of high speed flying objects [55].

Performance assessment of the Fe-cored nanocapsules prepared as magnetic and EMW absorbing materials
The magnetic hysteresis loop obtained from a magnetometer at room temperature, as shown in Fig. 10, is used to characterize the magnetic absorption characteristics of the synthesized nanocapsules. It can be seen that the Fe-cored nanocapsules exhibit strong ferromagnetic behavior and the magnetization reaches saturation at 10 kOe. The saturation magnetization (MS) value of the sample is 23.1 emu g -1 , which is equivalent approximately to 11% of the bulk iron contained [56]. It is known that the MS of the Fe@CNCs is mainly determined by the total Fe content [38], so the value can be further improved by increasing the amount of encapsulated iron content in the structure. Based on this, it can be determined that the sample A1800-4-15 will exhibit better magnetic performance than the sample A1800-4-5 because of its higher iron content.
It is also interesting to note that the Fe@CNC capsules show extraordinary magnetic properties as demonstrated in a simple experiment where it was found that all the Fe@CNCs particles dispersed in an aqueous suspension can be quickly separated from the mixture by using a common magnet, changing the colour of the aqueous suspension from black to colorless as shown in the inset of Fig. 10. In particular, the advantage of easy separation of the products is unattainable by CVD method, and it can be used as recyclable catalyst for chemical reactions enhancement. It is known that the EMW absorption performance is mainly determined by the dielectric tangent loss (tan δe = ε″ / ε′) and the magnetic tangent loss (tan δm = μ″ / μ′), as shown in Fig. 11. Among them, ε′, ε″, μ′ and μ″ are the real and imaginary part of complex permittivity and complex permeability, respectively. It should be noted that the drastic fluctuations of these two curves are caused by the relaxation process [29].
For instance, the tan δm is always smaller than the tan δe in the frequency range from 2 to 18 GHz, and the tan δm changes slightly around zero. Compared with other nanocapsules [11,21], the material has a high dielectric loss, which can improve not only the conductivity of the material but also the attenuation constant. In order to have an explicit understanding of the EMW absorption performance of the Fe-cored nanocapsules, the reflection loss (RL) was calculated via testing complex permeability and permittivity at different frequency based on the transmission line theory as follows [31,57]: Where 0 is the input impedance of free space (usually 0 get 1), in is the input impedance of the absorber, and are the complex permittivity and complex permeability, respectively, is the frequency of the EMW, is the light velocity in the free space and is the thickness of the absorber. Fig. 12 shows the RL values of the prepared Fe@CNCs, and it can be seen that the thickness of the packed absorber varies from 1.0 mm to 5.0 mm. When the RL values is higher than -10 dB, it means that more than 90% of the EMW will be absorbed and when this value is decreased to -20 dB, the absorption ratio can reach 99%. It is clear that the sample exhibits a great EMW absorption performance and with the RL value being close to as low as -22.5 dB at 10.2 GHz for a thickness of 2.5 mm. The minimal RL values of the sample are less than -18 dB for a thickness of 1.5-5 nm, which is even smaller than that of graphene (-11 dB) [37]. This indicates that the material has a better EMW absorption performance than pure graphite. It is interesting that the strongest RL value was found to be in the favourable high frequency region, and this compares to typical magnetic metal materials which their absorptive properties are only observed in the low frequency region. This is due to the high resistivity of the graphite shell, which can effectively reduce the effect of the eddy currents on the magnetic loss at high frequency [29,58]. In addition, as shown in the figure, with the RL values being less than -10 dB, the corresponding absorption bandwidth can reach up to13.8 GHz (4.2-18 GHz). Compared with other core-shell nanocomposites such as CoNi@SiO2@TiO2 [59], CoxFey@C [60], (Fe, Ni)/C NPs [4] and (Fe)C NPs [30] , the Fe@CNCs prepared from using MW discharge display desirable absorption behavior over a significantly wider range of EMW bandwidth. It is also found that for a RL value being as low as -10 dB, the matching thickness is only 1.5 mm, which is significantly thinner than the reported thickness for other materials, which ranged mostly from 2 to 4 mm [3,21,61]. The results suggest that to achieve the same level of EMW absorption, the required coating thickness of Fe@CNCs will be much thinner than those of the aforementioned other metal-cored nanocomposites. Calculations also demonstrate that the added weight of Fe@CNCs in the absorber is only 30 wt%, which is also significantly lower than those of other previously reported materials, which are usually more than 40 wt% [21] . Based on the above data, it is believed that the Fe@CNCs prepared from microwave discharge is a super EMW absorbing material over a wide-range of bandwidth, outperforming virtually all the previously reported materials prepared from using other approaches.

Conclusions
The use of MW-M discharge as novel facile technology for the fast production of high purity Fe@CNCs has been explored with great promise. Using ferrocene dissolved in cyclohexane as the precursor material, the results demonstrate that Fe-cored nanocarbon capsules with exceedingly high material integrity were generated, with the highly graphitized carbon shells being almost exclusively derived from the precursor ferrocene, giving rise to the virtually full recovery of the solvent of cyclohexane used, and this highlights the novel capability of MW-M discharge in selectively targeting the ferrocene precursor material for the formation of F@CNCs particles. It was also found that the Fe-cored nanocapsules have super anti-corrosion performance and novel thermo-oxidative stability, outperforming virtually all previously reported M@CNCs prepared from using other methodologies. Thermal annealing treatment in air was found to lead to the desirable transformation of Fe3C to Fe3O4, which further helps to improve the electrochemical properties of the material. EMW absorption tests reveal that with a thin layer of the carbon shells having extraordinary anti-corrosion and thermos-oxidative stability, the Fe@CNCs particles show exceedingly high absorption of EMW across a wide-range of EMW bandwidth. The results from the exploratory investigation, which appears to be the first of its kind to the best of our knowledge, augurs extremely well for the use of MW-M discharge as a novel approach for synthesizing novel functionalized materials for targeted applications such as the EMW absorbing materials, high-performance electrode materials and easy-to-separate catalysts.