Reduced graphene oxide-ferrite microcomposites based on petung bamboo (dendrocalamus asper) charcoal and iron sand as lightweight microwave absorbing materials

Reduced graphene oxide-ferrite (rGO/Fe3O4) microcomposites as lightweight microwave absorbing materials has been successfully synthesized from petung bamboo and iron sand by mechanical mixing method at different rGO content (0:1, 1:0, 1:1, 1:2, 1:3, 2:1 and 3:1 wt%). Reduced graphene oxide as dielectric material was synthesized from petung bamboo charcoal using carbonization method and ferrite as magnetic material was synthesized from iron sand using extraction-milling method. Structural characterization by x-ray Fluorosence, Fourier Transform Infrared, Vibrating Sample Magnetometry, x-ray Diffraction, Scanning Electron Microscopy proved that micrometer sized reduced graphene oxide ferrite in the microcomposites was responsible for the ferromagnetic behavior of the composites. The reflection loss of microcomposites measured in the microwave frequency range of 8–12 GHz using a Vector Network Analyzer. The results showed that at the micro scale, rGO had a higher absorption power with maximum reflection loss (RL m ) value of −21.81 dB at matching frequency (f m ) 10.85 GHz compared with Fe3O4 (RL m value of −9.25 dB at f m = 10.60 GHz) at thickness of 1 mm. The rGO/Fe3O4 (2:1 wt%) microcomposites microwave absorber shows the optimum absorption with maximum reflection loss value of −16.51 dB at matching frequency (f m ) 10.74 GHz at thickness of 1 mm. The use of natural materials and the controlled rGO/Fe3O4 microcomposites structure with simple synthesis methods, which the urgent need for developing high performance lightweight microwave abrsorbing materials and are environmentally friendly.


Introduction
Lightweight microwave absorbing materials (LMAM) working on the 8-12 GHz (X-band) frequency has and is attracting a lot of attention to applications in the fields of industrial, civil, shielding, health and military technology [1][2][3][4]. LMAM is usually developed based on magnetic materials, but is currently being developed from dielectric materials namely reduced graphene oxide (rGO). The mechanism of absorption of electromagnetic waves in LMAM involves magnetic and electrical properties through energy transfer. Electromagnetic waves received by magnetic materials will be used to change the orientation of the magnetic dipole moment, while the electromagnetic waves that come on the dielectric material are used to change the orientation of the electric dipole moment. Changes in the orientation of magnetic and electric dipole moments take place every time causing vibrations between molecules so that there is an energy conversion of electromagnetic waves into heat energy [5]. Ideal LMAM has optimal absorption power, absorbed frequency range (bandwidth) wide, low density and thin thickness [6,7].
Reduced garphene Oxide is a two dimensional carbon atom that is hybridized sp 2 and bonded together in a hexagonal lattice [8]. Since it was discovered in 2004, rGO has been widely explored because of its physics properties such as high electrical conductivity, large surface area, electron mobility and mechanical properties [9][10][11]. However, graphene also has good microwave absorption properties [12][13][14]. As rGO lacks magnetic properties, the absorption rate of radar waves can potentially be increased by adding magnetic material. Fe 3 O 4 known as black iron oxide or ferrous ferrite has the strongest magnetic properties among other oxides iron, so magnetite is widely used in various fields [15]. Fe 3 O 4 has a high saturation magnetization and magnetic permeability so that it is likely to be applied as a catalyst, energy storage, magnetic data storage, ferofluid or as a radar wave absorbent material [16,17].
LMAM based on reduced graphene oxide ferrite have been and is being developed in recent years. Many efforts have been done to obtain the lightweight radar absorption materials base on composite structure. The synergistic effect of dielectrics and magnetics particles, microwave absorption can be enhanced with appropriate proportion. Microwave absorption of rGO/Fe 3 O 4 microcomposite is influenced by particle size, composition and thickness. Generally, the absorbers are synthesized using a complex methods, such as microwave hydrothermal, mechanochemistry, combustion, co-precipitation methods, etc and its made from commercially-expensive, high production costs [18].
In the present paper, a novel approach combining simple methods (mechanichal dry-mixing) and cheap raw material as an alternative substitute for commercial ingredients to synthesize rGO/Fe 3 O 4 microcomposites exhibiting good microwave absorbing performances and lightweight. The raw materials are rGO as a dielectric material synthesized from petung bamboo (dendrocalamus asper) charcoal using the carbonization method and Fe 3 O 4 as a magnetic material synthesized based iron sand using the extraction milling method.

Synthesis of reduced graphene oxide-ferrite microcomposites
The rGO dielectric material was synthesized from petung bamboo (dendrocalamus asper) charcoal using the carbonization method. Petung bamboo is chosen at the base hard and high density with around 3-4 years of age. The synthesis process begins by burning petung bamboo to form charcoal. Petung bamboo charcoal formed is then mashed using mortar and filtered. The carbonization process in the furnace at a temperature of 600°C for 45 min. Washing the samples is done using an ultrasonic cleaner for 2 h to produce rGO with high purity. Fe 3 O 4 as magnetic material was synthesized from iron sand powder with stages the selection, sifting and separation of magnetic particles from non-magnetic particles using a permanent magnetic separator. Magnetic separation is carried out repeatedly 10 times and was washed using distilled water and dried in room temperature with a repeatition cycle of 10 times to obtain high purity of Fe 3 O 4 powder.
Then milling is done on rGO and Fe 3 O 4 using a planetary ball mill with a speed of 100 rpm in 2 h to form particle size in micro order. So, the rGO and Fe 3 O 4 micro powders which have been synthesized from petung bamboo and iron sand are formed into microcomposites by dry mixing and then compressing by mechanical. Samples as-prepared with a mass composition of rGO/Fe 3 O 4 (0:1, 1:0, 1:1, 1:2, 1:3, 2:1 and 3:1 wt%) at thickness 1, 1.5 and 2 mm.

Characterization
The elemental content of Fe 3 O 4 were identified using x-ray Fluorosence (XRF), PW4030/45B model. Analysis of rGO, Fe 3 O 4 and rGO/Fe 3 O 4 phase were characterized using x-ray Diffractometer (XRD), Philips x-pert model multi purpose diffractometer system 40 kV and 30 watt of Cu Kα. The morphology and particles size of rGO/Fe 3 O 4 were identified using Scanning Electron Microscopy (SEM), Phenom ProX model. The functional bond groups of rGO were observed using Fourier Transform Infrared (FTIR), Shimadzu 8400S model and electrics conductivity were measured using the LCR Meter model two point probe. Magnetics samples were measured using Vibrating Sample Magnetometer (VSM), Oxford model at room temperature. The measured samples for microwave absorption measurements were prepared by mixing the products and the parafin wax (50 wt% : 50 wt%) to get a uniform composites and then compressing them into a toroidal sample ( mm and mm 3 7 in out with thickness of 1 mm, 1.5 mm and 2 mm. The electromagnetic parameters including complex effective permitivity and permeability for the toroidal samples were measured by Vector Network Analyzer (VNA), Advantest R-3770 model at the frequency range of 8-12 GHz.

Results and discussion
3.1. Elemental content and functional group analysis Iron sand as a raw material for making magnetic powder Fe 3 O 4 as a result of selection, filtering, separation (extraction) and washing is carried out to test the elemental content. To increase the purity of Fe content by minimizing the content of other elements, extraction can be done using strong magnets with more repetition intensity.
Testing the elemental content of iron sand powder using the XRF (table 1). The composition of the elements of iron sand powder where the elemental content of Fe is quite high around 85.45% and other elements such as Ti, Si, Al etc The composition is quite low, which is less than 7.5%. This shows the potential for the presence of magnetic Fe 3 O 4 from iron sand is quite high and is worthy of being a raw material for Fe 3 O 4 magnetic particles naturally where the magnetite phase can be observed from phase testing using XRD.
Reduced graphene oxide as a dielectric material is synthesized from petung bamboo charcoal using the carbonization at 600°C temperature. The formation of rGO and confirmation of rGO in rGO/Fe 3 O 4 microcomposites characterization of molecular structures using FTIR showed in figure 1. The rGO Fourier Transform Infra-Red spectrum of carbonized petung bamboo charcoal (figure 1(a)), shows there is a double bond C=C which is the main carbon bond that forms molecule struture of reduced graphene oxide in a sp 2 hybridized state [19]. The peaks are located at the same wavelength of 1535 cm −1 can be indicated to the aromatic C=C stretching group, which shows the rGO molecule structure formed in both samples. The C-O strain bond is included in the phenol functional group and appears at wave number 1103 cm −1 in rGO and rGO/Fe 3 O 4 . rGO/Fe 3 O 4 (1:1 wt%) microcomposite, Fe-O function group appears at wave number 582 cm −1 which shows the formation of Fe 3 O 4 structure [20]. The success of rGO/Fe 3 O 4 microcomposite formation is indicated by the absence of functional group bonds between Fe-C [21,22].

Phase and magnetics properties analysis
Characterization of phase and crystalline structure of ferrite, reduced graphene oxide and reduced graphene oxide-ferrite microcomposites were carried out by using x-ray Diffractometer shown figure 2. The rGO synthesized from petung bamboo charcoal using the carbonization method has an amorphous structure with two wide angles at .93 q = has a crystal plane (002) [24], which shows reduced graphene oxide can formed at temperatures of 600°C. Identification of Fe 3 O 4 microparticle phase synthesized was carried out qualitatively using Match! Software from test data using the XRD. The processed products showed that Fe 3 O 4 microparticles had a single Face Centered Cubic crystal structure. The series of the diffraction peaks at  2(b)). This shows that the rGO/Fe 3 O 4 structure is confirmed in the form of a composite structure.    respectively. The hysteresis loops shows a decrease with increasing rGO content due to the non-magnetism of the rGO. rGO is a dielectric material with an electrical conductivity of 2.16 × 10 -7 S m −1 (based on LCR Meter measurements), which causes a weak magnetic dipole but increasing electric dipole content [25]. The addition of rGO into Fe 3 O 4 can reduce magnetic dipole interactions between adjacent magnetic particles [26,27], thereby reducing the inductive effect of the external magnetic field (H). In this study, magnetic material can be classified in soft magnets (super paramagnetic) because it has a low coercivity field (H c ) and produces a small (thin) curve area.   In order to determine the absorption properties of microwaves of the as-synthesized samples, reflection loss of samples were measured by using VNA advantest R-3770 in 8-12 GHz frequency at room temperature. The microwave absorbing properties of the rGO, Fe 3 O 4 and rGO/Fe 3 O 4 microcomposites defined by maximum reflection loss (RL m ) are given by theory of the absorbing wall and based on the transmit-line theory [28], the maximum reflection loss are calculated by equation: where Z in is the input impedance of the absorber, c is the velocity of electromagnetic (EM) waves in free space, ε r is the complex permittivity, μ r is the complex permeability, f is the microwave frequency and d is the absorber thickness. There are two ways to increase absorption of microwaves, namely (1) the material impedance value must be the same as the free space impedance so that microwaves can be absorbed maximally and produce a zero reflex state, (2) incoming electromagnetic waves must be absorbed with maximum intensity when the waves hit the absorbent materials. The common features of the microwave behavior of dielectric and magnetic materials are resonance, relaxation and precession. For magnetic loss, the resonance phenomenon can happen when the frequency of the microwave field applied at the right angles to the static field is the same as the precession frequency. Absorption of an electromagnetic wave by magnetic materials as ferrites (Fe 3 O 4 ) is also considered to occuring via relaxation induced by the domain wall movement or by a natural spin resonance mechanism depends highly on the magnetic anisotropy and thus on the structure and shape of magnetic particles. As for dielectric materials as reduced graphene oxide (rGO), when microwaves penetrate and propagate through the material, translational motions of free or bound charges such as electrons or ions are induced by the internal field generated within the affected volume, it rotates the dipoles. Elastic, inertial and frictional forces resist these induced motions and this causes energy losses.
The mechanism of absorption of electromagnetic waves in rGO/Fe 3 O 4 microcomposites involves magnetic and electrical properties through energy transfer. Electromagnetic waves received by magnetic materials will be used to change the orientation of the magnetic dipole moment, while the electromagnetic waves that come on the dielectric material are used to change the orientation of the electric dipole moment. Changes in the orientation of magnetic and electric dipole moments take place every time causing vibrations between molecules so that there is an energy conversion of electromagnetic waves into heat energy. Additional loss can also occur via molecular polarization phenomena, such as dipole rotation, space charge relaxation and hopping of confined charges. The absorption power of microwaves is indicated by the value of maximum reflection loss (RL m ).
The maximum reflection loss of rGO and Fe 3 O 4 are shown figures 5. While rGO/Fe 3 O 4 microcomposites, each of which is synthesized from petung bamboo charcoal and iron sand were measured using VNA are shown in figures 6-8 for each different tickness and figure 6. Negative reflection loss states the magnitude of the radar wave that is not reflected. The greater the value of negative reflection loss, the better a radar wave absorbent material. The wave absorption properties are strongly influenced by dielectrics loss and magnetics loss. Dielectrics loss is obtained from dielectric materials such as reduced graphene oxide in this case from petung bamboo charcoal, while magnetic loss is obtained from magnetic materials such as Fe 3 O 4 , in this case from iron sand. Based on the measurement of maximum reflection loss on all thickness variations the rGO reflectivity values are all below −10 dB, while the Fe 3 O 4 reflectivity values are all above −10 dB (table 2). In the case of microparticles, rGO has a maximum reflection loss higher than Fe 3 O 4 which shows the contribution of dielectric loss to the absorption of microwaves is very high, on the contrary the contribution of magnetic loss is not dominant [29].
The dielectric performance commonly depends on ion polarization, electron polarization, electric dipolar polarization and interfacial polarization occurring in the materials [30].   These is because an electric dipole of rGO is easier to absorb radar waves than a magnetic dipole of Fe 3 O 4 . The dielectric loss values of rGO/Fe 3 O 4 are greater than those of the magnetic loss, which illustrates dielectric loss of rGO makes a major contribution to the electromagnetic loss. Furthermore, the core-shell structure between rGO and Fe 3 O 4 can cause additional interfacial polarization and space charge polarization and thereby increase electromagnetic wave attenuate. According to table 3, the increasing thickness of RAM microcomposites based dielectric and magnetic from natural will be decreasing matching frequency. As a radar wave absorber the rGO/Fe 3 O 4 microcomposites show that competitive results compared to rGO/Fe 3 O 4 nanocomposites from commercial materials, coprecipitation and hydrothermal methods which are expensive and more complicated [18,22].

Conclussions
Reduced graphene oxide-ferrite microcomposite as a lightweight microwave absorbing material can be made from inexpensive natural base materials by a simple method. The rGO and Fe 3

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.