Carbon materials, known for their lightweight properties, are widely utilized in electromagnetic wave absorption applications. In order to overcome the limitations of effective absorption of electromagnetic waves by a single carbon material, magnetic cobalt nanoparticles were incorporated into a carbon network derived from flour, resulting in the development of Co/C nanocomposites with a porous structure through fermentation. The research results show that the electromagnetic wave absorbing material prepared in this way has advantages such as being thin (1.80 mm), lightweight, having a wide effective absorption frequency range (8.07 GHz), and high electromagnetic wave absorption capacity (-61.6 dB).The electromagnetic wave absorption capability of the material originates from the multi-level interfaces in Co/C nanocomposites, the porous carbon structure formed during flour fermentation, and the dielectric relaxation generated during the polarization process. The excellent electromagnetic wave absorption performance is mainly attributed to impedance matching and attenuation factor optimization. The presence of a small amount of amorphous carbon in the carbon network reduces the condensation and oxidation of magnetic cobalt nanoparticles, thus enhancing the impedance matching. By adjusting the Co/C ratio inside the nanocomposites, the impedance matching of the Co/C nanocomposites is improved, and the absorption capacity of the CO /C nanocomposites is improved. This article reports the method of determining the ideal content of absorbent in flour based composites and the principle of optimizing the electromagnetic wave absorption capacity of nanocomposites.
Research Article
Ultra-lightweight carbon nanocomposites as microwave absorber with high absorbing performance derived from flour
https://doi.org/10.21203/rs.3.rs-5138548/v1
This work is licensed under a CC BY 4.0 License
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microwave absorption
dielectric loss
impedance matching
porous carbon
With the development and progress of modern electronic science and technology, particularly the development of 5G technology, the density of base stations is continuously increasing, leading to growing concerns about the impact of electromagnetic radiation on health. Meanwhile, electromagnetic waves can interfere with certain production activities, particularly those involving precision instruments [1–6]. Consequently, addressing the issue of electromagnetic interference is an unavoidable and significant challenge in contemporary society, demanding urgent resolution. This also promotes the generation of absorbing materials and the development and progress of absorbing materials [7–10].
In principle, when a beam of electromagnetic wave is incident on the surface of an object, its direction is usually reflected on the surface of the object, and loss and transmission occur inside the material. The loss mechanisms in materials for electromagnetic wave attenuation typically include interference loss, magnetic loss, and dielectric loss. The material that can make the electromagnetic wave enter the material and be effectively attenuated is called the absorbing material. The principle of absorption is based on the material's consumption of the incident wave's energy in the form of other types of energy loss or interference cancellation. The performance of decay is determined by the electromagnetic properties of the material itself, that is, the two parameters: dielectric constant and permeability. The real parts of these parameters represent the storage capacities of the material for electric and magnetic energy, respectively, while the imaginary parts describe their loss capacities for electromagnetic performance [50]. An excellent absorbing material should consume as much energy as possible into the interior of the material to reduce the reflection and transmission on the surface [11–12].
In order to meet the requirements of light weight, most of us use carbon materials as the main composite material source of wave absorbing materials [15]. Compared with ceramic materials and metal materials, the density of carbon materials is very low, so the application of carbon materials can reduce the weight of the material and greatly solve the problem of light weight. In addition, we can further solve the problem of light weight by constructing multi-stage holes. As we all know, the classification of material holes is divided into large pores, mesoporous pores and micropores. The construction of multi-stage holes can not only reduce the density of absorbing materials to further solve the problem of light weight of materials, but also enable electromagnetic waves to be reflected several times after entering the material, thus attenuating the loss. This can then improve the absorbing properties of the material [51].
However, only the use of biomass porous carbon materials is far from enough, pure carbon materials only have dielectric loss and almost no magnetic loss, which also limits the improvement of its wave absorption performance. However, the density of a single magnetic metal material is too high and the mass is too large, which affects the light weight of the material and is not conducive to its practical application. Therefore, in order to optimize the wave absorption performance of the material, we usually use porous carbon materials and other kinds of materials to build composite materials, which can not only reduce the weight of the material, but also meet the requirements of the material's excellent wave absorption performance. Through research, we found that the absorbent materials constructed by magnetic metals such as Fe, Co and Ni are excellent absorbent materials with advantages such as high saturation magnetization, high snoek limit and strong magnetic loss [19].
Therefore, in this study, we use a certain proportion of cobalt acetate aqueous solution mixed with flour to prepare Co/C nanocomposites, so that magnetic Co nanoparticles can be evenly distributed in the carbon matrix network constructed by flour. In addition, in the experiment, NaCl was added to improve the mechanical properties of the material, and the method of yeast fermentation at 35℃ was improved to make the material have a porous structure, increase the multiple reflection loss of electromagnetic waves in the interior, and thus improve the absorption performance of the material. The Co/C nanocomposite has the advantages of light weight, thin thickness, wide effective absorption frequency, large absorption percentage of microwave and so on. Among them, the presence of carbon network is greatly conducive to preventing the agglomeration of cobalt particles and suppressing the leakage current in the cross-linked cobalt network [20–21].
2.1. Sample preparation
The chemicals selected for the experiment were cobalt acetate tetrahydrate (CH3COO)2Co▪4H2O and NaCl, which were purchased from Sinopsin Group. The carbon material in the experiment was derived from the flour of Golden Arana brand, the yeast was highly active dry yeast, and the manufacturer was Angel Yeast Co., LTD. (suitable fermentation temperature was 30–40℃). All the drugs were directly used without any treatment.
Two parts of flour (160 g each) were mixed with 80 g of water and divided into two groups A and B. Among them, 2 g of NaCl was added to group A and no NaCl was added to group B, and the flour was fully and evenly mixed with water and NaCl through continuous rubbing. For group C, D and E, (CH3COO)2Co▪4H2O was mixed with 80 g of water according to 0.08 mol/L, 0.15 mol/L and 0.20 mol/L.
Part of the mixture was added with yeast, and the mixture was fully mixed with flour. The steps of group A-E were repeated. The mixed dough was put into the oven for aerobic fermentation for 3h, and the oven temperature was set at 36.5℃.After the fermentation is complete, cut the fermented dough and the first five groups of dough into small pieces, seal them into packaging bags, and freeze them in the refrigerator. After freezing for 24 h, we use a freeze-drying machine to put small pieces of dough into the freeze-drying machine to vacuum freeze dry for 48 h to remove the moisture in it.
After freeze-drying, we use a tube furnace to carbonize the freeze-dried material at high temperatures. In order to explore the influence of different carbonization temperatures on the electromagnetic wave absorption performance of the materials, we divided the materials into two groups, respectively at 600℃ and 800℃, in the atmosphere of nitrogen. Among them, limited to the safe operation requirements of the tube furnace, the heating rate shall not exceed 5℃ per minute, and the heat preservation shall be carried out for 3 h.
After the high temperature carbonization is completed, we vacuum seal the material to prevent the reduced magnetic cobalt metal from reacting with oxygen in the air, thus reducing the wave absorption performance of the material.
Through Figure. 1, we can clearly understand the preparation process of Co/C nanocomposites and the porous structure produced by yeast fermentation.
2.2. Characterization
The microstructure of the composites was studied by field emission scanning electron microscopy (FESEM, Hitachi SU-70, Tokyo, Japan) and electron energy spectrum (EDS). The crystal structure of the sample was characterized by X-ray diffraction (XRD, Rigaku D max 2500 VB) in the range of 2θ (10°~ 90°). The Raman spectra of incident radiation at 532.05 nm were obtained by a microscopic Raman spectrometer (Jobin Yvon HR800, France). X-ray photo-electron spectroscopy (XPS) was obtained on a Thermo Fisher ESCALAB 250Xi spectrometer with an Al Ka X-ray source (1486.6 eV). The Co/C powder was evenly dispersed in paraffin wax with different mass ratios, and the annular sample with an inner diameter of 3.01 mm and an outer diameter of 6.99 mm was compressed. The dielectric constant and permeability were obtained by vector network analyzer (VNA, Agilent N522A) in the transmission/reflection mode in the frequency range of 2–18 GHz. Finally, we use the following formula to calculate the reflection loss (RL).
$${{\text{Z}}_{{\text{in}}}}{\text{=}}{{\text{Z}}_0}{\left( {{\mu _{\text{r}}}/{\varepsilon _{\text{r}}}} \right)^{1/2}}\tanh \left\{ {j\left( {2\pi f{d \mathord{\left/ {\vphantom {d c}} \right. \kern-0pt} c}} \right){{\left( {{\mu _{\text{r}}}{\varepsilon _{\text{r}}}} \right)}^{1/2}}} \right\}$$
1
$${\text{RL(dB)=20lg}}\left| {{{({Z_{in}} - {Z_0})} \mathord{\left/ {\vphantom {{({Z_{in}} - {Z_0})} {({Z_{in}}+{Z_0})}}} \right. \kern-0pt} {({Z_{in}}+{Z_0})}}} \right|$$
2
where j is the imaginary number, f is the electromagnetic wave frequency, d is the thickness of the paraffin composite material, and c is the speed of light in vacuum. [52]
As we all know, the structure of the material, especially the microstructure of the material and the composition of the material, determines the specific application properties and functions of the material. In order to explore the relationship between the properties and functions of the electromagnetic wave absorbing material and its microstructure and composition, we use X-ray diffraction (XRD) and Raman spectroscopy to analyze the specific microstructure and structure of the absorbing material, so as to explain why the absorption performance can be improved by constructing the microstructure of the material. And why the Co/C nanocomposites constructed in this way have excellent electromagnetic wave absorption performance [20–24].
The structure information and crystallization conditions of the Co/C nanocomposites were obtained by X-ray diffraction and Raman spectroscopy. As shown in Figure. 2. (a), the diffraction peak of Co metal can be obviously observed, but because the diffraction peak of Co metal is too strong, it is difficult for us to clearly identify the diffraction peak of carbon element. Therefore, in order to analyze the form of carbon, we conducted a separate analysis of XRD of pure carbon powder. For the analysis of pure carbon, we found that pure carbon has a wide peak at 2θ = 26°, while graphite carbon has no characteristic peak at 2θ = 44°, indicating that the main form of carbon in Co/C nanocomposites is amorphous carbon, and this amorphous carbon is generated by the pyrolysis of flour in the material. In the process of XRD analysis of Co/C nanocomposites, we found that the three strongest peaks of Co element are 2θ = 44.2°, 51.52° and 75.85°, and these three peaks represent the diffraction peaks of the (111), (200) and (220) crystal faces of cubic Co, respectively. The Co here is produced by the thermal decomposition reaction of CoCO3 and the thermal reduction reaction caused by C.
The detailed carbon composition of Co/C nanocomposites in this experiment was studied by Raman spectroscopy. Raman spectral analysis in Figure. 2. (b) shows that pure flour and Co/C nanocomposites have two peaks, namely peak D at 1358 cm− 1 and peak G at 1589 cm− 1. As we all know, the G-band usually appears near 1300 cm− 1, which is caused by the in-plane vibration of sp2 carbon atoms. The degree of graphitization in carbon materials is commonly assessed through the ID/IG intensity ratio, representing the ratio of the D peak to the G peak. In Raman spectroscopy, the D peak arises from the stretching vibrations of C-C and C = C bonds, indicating structural defects and the level of amorphousness, whereas the G peak is generated by the stretching vibrations of sp2 hybridized carbon atoms. As the graphitization degree increases in carbon materials, the intensity of the G band peak amplifies, indicating a reduction in lattice defects and leading to a decrease in the ID/IG ratio [25–26].
From Figure. 2. (b), it is evident that the ID/IG ratio of the Co/C nanocomposite material synthesized in the experiment ranges from 0.8365 to 0.9363. Specifically, the Co-2 sample, with a Co element content of 0.15 mol/L, exhibits the highest intensity ratio between the D and G bands. This suggests a lower degree of graphitization in Co-2 compared to Co-1 and Co-3, indicating a predominant presence of crystalline carbon within the material. Despite the crystalline carbon's adverse effect on material conductivity, it boosts impedance matching, thereby contributing to the superior absorption performance of Co-2. Notably, the ID/IG values of the Co/C nanocomposite material and carbon powder in this study surpass those of previously prepared materials. The crystalline carbon content in the material serves as a protective barrier for the internal Co metal nanoparticles against oxidation, preserving the magnetic properties essential for efficient electromagnetic wave absorption. Additionally, the presence of a limited amount of amorphous carbon in Co-2 results in an ID/IG ratio of 0.9363, aligning with the findings of the XRD analysis conducted earlier.
We use XPS to analyze the elemental composition of Co/C nanocomposites, and the proportion of different elements and their existing forms in the material. XPS analysis of Co/C nanocomposites is shown in Fig. 3. In Figure. 3. (a), there are obvious peaks in Co, O and C, indicating that the main components of the material are C, O and Co.
Through Figure. 3. (b), the peak of carbon atom is divided into three peaks, which are around 284.49 eV, 284.93 eV and 285.05 eV respectively, indicating C = C, C-C and C-O-C [33, 34]. Among them, C-C, C = C, although the two forms of carbon occupy the majority, but do not account for the whole proportion, and through practical analysis, we also found that there is still a small part of carbon in the form of C-O-C (2.98%), which can interact with electromagnetic and enhance the absorption performance of materials [35–40]. This is also another piece of evidence that the Co/C nanocomposites contain amorphous carbon, consistent with previous XRD analysis.
In Figure. 3. (c), the n1s spectrum is divided into three N types, namely graphite N (400.6-400.8 eV), pyridine N (399.3 ~ 399.6 eV) and oxide N (404.3 ~ 404.5 eV). It is not difficult to see from the data that the proportion of graphite nitrogen is the largest in N 1s, while the proportion of oxidation state N is the smallest, indicating that most nitrogen is successfully combined into the carbon lattice structure. The Co 2p spectrum is divided into four peaks, and the peaks at 781.66 eV and 786.87 eV show the presence of metal Co in the form of 2p 3/2 and 2p 1/2. And it can be seen that due to the existence and protection of amorphous carbon, the oxidation degree of Co is also low. XPS analysis of oxygen elements in Co/C nanocomposites is shown in Figure S1.
In the experiment, we also used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to analyze the specific microstructure of the prepared Co/C nanocomposites and the specific influence of the microstructure on the absorption properties of the materials.
The Co/C nanocomposite material synthesized through this method demonstrates a unique honeycomb-like structure, as shown in Figure. 4.(d), with numerous observable pore structures distributed throughout. This porous architecture serves a dual purpose: decreasing material density by incorporating hollow pores to satisfy lightweight material specifications, and augmenting the reflection of electromagnetic waves at multiple interfaces within the material, consequently enhancing its electromagnetic wave absorption capabilities.
Analysis of Figure. 4. (d) and Figure. 4. (e) reveals that the pore size ranges from 13 to 17 µm. Upon further magnification of the pore structure, small particles are observed on the pore walls and material surface. Figure. 4. (f) at a magnification of 12000 times shows cobalt metal particles on the inner walls of the pores. Comparison of images between Figure. 4 and Figure. S5. (a, b) demonstrates that post-yeast fermentation, the material exhibits increased porous structures with a more pronounced honeycomb pattern. Through reflection loss (RL) analysis, it is apparent that the porous structure significantly enhances the material's electromagnetic wave absorption performance, in line with experimental principles. This underscores the optimization of material performance through the improved scattering of electromagnetic waves within the material.
By examining Figures. 4. (a, b, c), we can effectively compare the impact of varying levels of Co salt on the density of magnetic cobalt nano metal adhered to the inner walls of material pores. Figure. 4. (a) illustrates the blank carbon sample, devoid of added acetic acid cobalt, in contrast to samples exhibiting a substantial amount of cobalt metal attachment on the inner walls. The absence of magnetic metal in the blank carbon leads to a deficiency in magnetic retention, thereby resulting in inferior absorbent performance compared to samples with the addition of metal. Figures. 4. (b) and 4. (c) depict scenarios with insufficient and excessive metal content, respectively.
Figure. 4. (g) depicts the TEM image of the honeycomb structure wall surface. The different colors suggest that the dark regions correspond to the material's pore structure, given the honeycomb-like internal structure. Upon detailed scrutiny, it is evident that higher magnification accentuates the dark areas, revealing clearer pore structures as depicted in Figure. 4. (h), affirming the successful construction of a multi-level pore structure. Furthermore, under increased magnification, well-defined carbon lattices with a diameter of 3.801 nm are visible.
Energy-dispersive X-ray spectroscopy (EDS) analysis in Figure. S5. (c) reveals the presence of carbon, cobalt, and oxygen in this material portion, demonstrating effective incorporation of cobalt, a magnetic metal, into the carbon network. In terms of oxygen content, a minor fraction persists in the precursor during the carbide process, with a mass fraction of 4.83%, indicating a low concentration. This finding suggests that carbon provides reliable protection for the magnetic metal, while the cobalt maintains minimal oxidation levels.
The dielectric constant and the real and imaginary part of the permeability of the material are measured by the vector network analyzer, and are converted into reflection loss (RL) by the formula above. The figure below shows the 3D reflection loss curve at different thicknesses and frequencies. Among them, three-dimensional curves of reflection loss (RL) for different Co/C content ratios of fermented samples can be obtained by Figure. S2- S4.
As we all know, when the reflection loss (RL)= -10 dB, the absorption ratio of the material to the incident electromagnetic wave can reach 90%; When the reflection loss is -20 dB, the absorption ratio of the material reaches 99% [53]. However, in practical applications, this gap has little impact on the actual performance of the material, so we consider the effective absorption of the material for electromagnetic waves by specifying the part of the material in RL < -10 dB, that is, the part of the electromagnetic wave absorption ratio of 90%. By analyzing the microwave absorption performance of Co/C nanocomposites carbonized at 600°C, it is evident from Figure S9 that the reflection loss (RL) value of the Co/C nanocomposites exceeds − 10 dB. In contrast to the samples carbonized at 800°C with equivalent Co content, those carbonized at 600°C demonstrate minimal microwave absorption capability. This disparity arises due to the incomplete carbonization at the lower temperature, resulting in inadequate crystalline carbon formation, suboptimal impedance matching, and insufficient catalytic properties of the carbon, hindering the reduction of magnetic cobalt. Consequently, the diminished reduction of magnetic cobalt curtails its involvement in electromagnetic wave absorption, thereby impairing the microwave absorption performance of the material. Therefore, when analyzing the content of the absorption agent with the best wave absorption performance, we focus on the analysis of carbonized samples at 800 degrees Celsius.
According to the data in Figure. 5, it can be clearly seen that under a certain material thickness and a certain frequency, these four samples all have effective absorption of electromagnetic waves, but the maximum reflection loss, effective absorption bandwidth and material thickness of different samples are different. According to the actual measurement data, we can find that the absorption performance of the fermented sample is often better than that of the unfermented material at the same content, and the thickness of the material reaching the maximum reflection loss value is also thinner, and the effective absorption bandwidth is also better, which is very in line with our design intention of thin, light, wide and strong absorbing materials. At the same time, this also shows that the porous structure generated by the fermentation of the material increases the number of electromagnetic waves reflected inside the material and increases the attenuation of the material to electromagnetic waves, thus enhancing the wave absorption performance.
For the unfermented sample, the minimum RL of the Co-0 sample is -59.8dB at the thickness of 3.61 mm, and the maximum reflection loss bandwidth is 6.1 GHz at the thickness of 2.20 mm. Co-1 has a minimum RL of -56.2dB at a thickness of 1.75 mm and a maximum effective absorption bandwidth of 7.9 GHz at a thickness of 2.11 mm. When the thickness of Co-2 is 1.80 mm, the minimum RL is -61.6 dB, and when the thickness is 2.15 mm, the widest effective absorption bandwidth is 8.07 GHz, and the overall absorption performance of the material under this content ratio is the best. When the thickness of Co-3 is 1.81 mm, the minimum RL value is -62.4 dB. When the thickness of Co-3 is 2.25 mm, the maximum effective absorption bandwidth is 6.3 GHz [38]. It is not difficult to see from the experimental data that the maximum reflection loss of Co-1, Co-2 and Co-3 is better than that of Co-0 samples, and the maximum reflection loss of Co-3 samples is the best. The widest effective absorption bandwidth of Co-1, Co-2 and Co-3 is also enhanced compared with that of Co-0, and the thickness of the widest absorption bandwidth of Co-1 and Co-2 is also reduced. Therefore, it is not difficult to conclude that adding magnetic metals to carbon materials can increase the wave absorbing properties of the materials, and with the increase of magnetic metal content, the wave absorbing properties of the materials will be enhanced [54].
Since the maximum value of reflection loss of such samples is close, we can also find that when the maximum loss is reached, the sample thickness of Co-1, Co-2 and Co-3 is thinner than that of Co-0. Therefore, we can conclude that the addition of magnetic cobalt nano-metal can reduce the thickness of the material during effective wave absorption [41].
However, the maximum reflection loss of Co-3 is relatively better than that of Co-1 and Co-2, but both the thickness and the effective absorption bandwidth are relatively poor, so we can know that although the addition of magnetic cobalt nanoparticles can enhance the absorption performance of carbon materials, and can make the material achieve excellent absorption performance at a lower thickness, However, the addition of too much magnetic metal will also have a negative impact on the absorption properties of carbon materials. In addition, according to Figure. S11, we can obtain the dependence of frequency and reflection loss under different thicknesses of blank carbon (a) and Co-2 sample (b).
Numerous semicircles observed in the Cole-Cole curve of the sample suggest the presence of multiple polarization relaxation processes. The linear segment in the Cole-Cole diagram signifies conduction losses, with the length of the line correlating with the level of conductivity. Notably, the linear length in Figure S10 (c) of the Cole-Cole plot is extended, indicating enhanced conductivity. It exemplifies that variations in the Co/C ratio can modify conductivity and escalate conduction losses.
In order to evaluate the wave-absorbing capacity of the Co/C nanocomposites prepared in this experiment, Table 1 lists the reflection loss, maximum reflection loss thickness, and widest effective wave-absorbing frequency of the Co/C nanocomposites in this and other studies. It is not difficult to see from the comparison in the table that, compared with other Co/C nanocomposites, the Co/C nanocomposites prepared in this experiment have thinner thickness, wider bandwidth and stronger electromagnetic wave absorption capacity, indicating that the nanocomposites prepared in this study have good electromagnetic wave absorption performance.
| Materials | Content-mass ratio (%) | RLmin | Matching thickness (mm) | Frequency range (GHz) | d(RL<-10dB) | Ref. | |
|---|---|---|---|---|---|---|---|
| Co/C | 30 | -52.42 dB | 1.93 mm | 2.9 GHz | 2.00 mm | [16] | |
| Co/C | 50 | -56.9 dB | 1.92 mm | 2.7 GHz | 2.00 mm | [28] | |
| Co/C | 30 | -18.0 dB | 3.20 mm | 3.6 GHz | 2.00 mm | [29] | |
| CoNi/C | 35 | -46.0 dB | 2.00 mm | 5.5 GHz | 2.00 mm | [30] | |
| Co/C | 20 | -56.0 dB | 3.50 mm | 3.6 GHz | 2.30 mm | [31] | |
| Co/CNTs | 20 | -36.5 dB | 4.10 mm | 2.9 GHZ | 1.80 mm | [32] | |
| Co/C | 17.5 | -62.4 dB | 1.81 mm | 6.3 GHz | 2.25 mm | This work | |
| Co/C | 15 | -61.6 dB | 1.80 mm | 8.07 GHz | 2.15 mm | This work | |
As we all know, if a material wants to have excellent absorption capacity for electromagnetic waves, it not only needs to effectively lose electromagnetic waves entering the material, but also needs to make electromagnetic waves can greatly enter the material, reduce electromagnetic waves on the surface of the material reflection and other behaviors, and impedance matching is an important parameter for this performance evaluation. Theoretically, the impedance ratio (Zin/Z0) = 1 can indicate that electromagnetic waves can enter the interior of the material well, but this situation is difficult to achieve in the application of actual materials, and the closer the impedance ratio (Zin/Z0) is to 1, the better the impedance matching performance of the wave absorbing material, so in this experiment, We take the impedance ratio Zin/Z0 = 0.8–1.2 as the effective impedance ratio of the material, and the impedance matching under this impedance ratio is considered to be the effective impedance matching [55].
Zin/Z0 is calculated from formula (1) above. From the image in Figure. 6, it is not difficult to see that we have obtained the three-dimensional impedance matching scale diagram at various frequencies and thickens for the sample with the best reflection loss, that is, the fermented 0.15 mol/L sample.
By studying the impedance matching images of different Co/C content ratios at 0.15 mol/L concentration, the relationship between material impedance matching and magnetic Co metal content was analyzed. In the experiment, images with material volume fraction of 13%, 14%, 15% and 17% were selected and named as samples a, b, c and d respectively. Through the analysis of the images of samples (a), (b) and (c), the impedance matching performance of 14% and 15% Co/C content ratio is significantly better than that of 13% Co/C content ratio, which proves that the addition of magnetic metal Co improves the impedance matching function of the material, and in this range, the higher the Co metal content is, the better the impedance matching performance is. The better the impedance matching performance.
Through the comparison of the four figures (a), (b), (c) and (d) in Figure. 6, it is not difficult to see that with the gradual increase of the Co/C content ratio of the sample, the effective impedance matching of the material first gradually increases and then decreases. By increasing the proportion of magnetic Co metal, we will also find that after the proportion of Co/C content increases to a certain range, there is almost no effective impedance matching range for the material. Therefore, we draw a conclusion that the impedance matching of the material will increase with the increase of the proportion of magnetic metal content in a certain range, and an optimal impedance matching can be obtained in this range. However, in the case of excessive content of magnetic metal particles, the presence of magnetic metal will make the impedance matching performance of the material worse [39–41]. Figure. S6-S7 shows the relationship between reflection loss and impedance ratio of unfermented sample and frequency.
Dielectric constant (ε) and permeability (µ) are two basic electromagnetic parameters that affect the interaction between absorbing materials and electromagnetic waves, and they affect the impedance matching and attenuation performance of the materials for electromagnetic waves [57]. We studied the electromagnetic parameters of the material by studying the real and imaginary parts of the dielectric constant and permeability of the material, so as to determine the influence of the Co/C ratio of the material on the absorption properties of the material [58].
As can be seen from Figure. 7, when the Co/C content is 14% and 15% (volume fraction), the real part (ε ') of the dielectric constant decreases with the increase of the Co/C ratio, which is different from our normal conclusion. The reason for this phenomenon is that in the testing process, we usually use paraffin and Co/C samples to build composite materials, because their mass is determined, so the increase of Co/C ratio will increase the total mass of the composite material, thereby reducing the volume fraction of the paraffin composite sample. The higher the concentration of Co element, the smaller the volume fraction of paraffin complex, so the reduction of sample volume fraction generally leads to the reduction of dielectric constant. The dielectric constant of the material will maintain a relatively stable state between 6 GHz and 11 GHz, but in the range of 11 GHz to 15 GHz, due to the phenomenon of dielectric relaxation, the real part of the dielectric constant will significantly reduce, and the virtual part of the dielectric constant will also significantly increase, which can be clearly obtained from the image.
In addition, the dielectric relaxation phenomenon of the material is also improved by the porous structure we previously designed, and in this way, the absorption properties of the material are also enhanced. It is worth noting that the material we constructed this time is different from the traditional material, that is, the virtual part of the permeability of the material (µ ") appears negative.
Figure. 7. (c) shows the tangent diagram of dielectric loss of the prepared nanocomposite material. There is a strong peak near 12 GHz, indicating the existence of strong dielectric loss, which can correspond to a strong electromagnetic absorption band. Figure. 7. (c, d) shows the real and imaginary parts of the permeability at different frequencies. In the range of 2 ~ 10 GHz, the permeability increases with the increase of the proportion of Co/C content, which is caused by the increase of the number of magnetic Co nanometal particles in the Co/C nanocomposites. At the frequency of 10 ~ 18 GHz, there is an obvious loss peak on the virtual part of the permeability (µ ") curve, which is caused by magnetic resonance, which clearly explains the existence of natural resonance loss.
In fact, some of the negative electromagnetic parameters of the material have also appeared in some electromagnetic metamaterials, and the negative permeability phenomenon in this experiment also shows that the Co/C nanocomposite constructed in this project has the potential to act as a metamaterial, and many previous works have also appeared such a situation.
For electromagnetic metamaterials, many previous works have also been studied, such as negative dielectric materials. Negative dielectric materials can be periodic or aperiodic structures, and the realization of negative permittivity can be realized through dielectric resonance or plasma oscillation, so we can also divide negative dielectric materials into resonant and equidiscrete dielectric materials according to mechanism [59].
As the real part of the dielectric constant of the wave absorbing material is relatively small, in order to achieve better impedance matching, so that the electromagnetic wave can better enter the inside of the material, the experimental negative dielectric material can also be applied to the wave absorbing material accordingly. Moreover, since the dielectric property of the negative dielectric material is often affected or even determined by the percolation theory, we can also extend it to the wave absorbing material. As analyzed above, before the Co/C ratio of Co/C nanocomposites reaches a certain value, it has no obvious influence on the wave absorption performance of the material. When the ratio increases to a certain value, the wave absorption performance of the material, especially the reflection loss value, will have a large leap and reach a maximum value [41–43].
Figure. 8 shows the tangent diagram of dielectric loss and magnetic loss of Co/C nanocomposites at different frequencies. It is not difficult to see from Figure.8 that although the material with a content of 13 vol% has the highest magnetic loss, it is found in the test and analysis that its actual wave absorption performance is poor, which is because the wave absorption performance of a material is affected. The combined action of dielectric loss and magnetic loss is required. In Figure. 8. (a), we find that the dielectric loss of 13 vol% is too poor, while the dielectric loss of 17 vol% is optimal, which is also consistent with our previous analysis of the wave absorption performance of the material. The image of dielectric loss and magnetic loss tangent value of blank sample, that is, blank carbon, as well as the relationship between the real and imaginary parts of dielectric constant and permeability and frequency, can be obtained from Figure. S12-S13.
A good performance absorbing material not only needs a good impedance match, because a good impedance match means that the material can let electromagnetic waves into the interior, and an excellent attenuation factor can attenuate electromagnetic waves well. Attenuation factor (α) is calculated by formula (3). The Figure.9 shows the relationship between the frequency of the four samples and the attenuation factor. The attenuation factor of pure carbon sample is lower than that of 0.08 mol/L sample, and the absorption performance of 0.08 mol/L sample is stronger. This is because the 0.08 mol/L sample not only has a better impedance matching than the pure carbon sample, but also a better attenuation factor, which is consistent with the fact that attenuation factor and impedance matching together determine the absorption capacity [11].
However, as can be seen from Fig. 9, the attenuation factor of 0.15 mol/L sample is better than that of 0.08 mol/L sample, resulting in stronger attenuation and loss ability of the material to electromagnetic waves, while the attenuation factor of 0.20 mol/L sample is worse than that of 0.15 mol/L, which is also the same as the previous analysis, and it is not the higher the proportion of magnetic metal, the better.
The wave absorption performance of 0.08 mol/L and 0.15 mol/L samples is better than that of pure carbon, so it can be concluded that the higher the impedance matching is not the better, because the attenuation factor is reduced when the high impedance matching, resulting in the electromagnetic wave could not enter the material well, thus reducing the wave absorption performance. However, if the attenuation coefficient is too low, the loss of the incident electromagnetic wave will be reduced, and the absorption performance of the material will be reduced. Therefore, we need to choose the appropriate attenuation factor size and find the appropriate impedance matching and attenuation factor coordination range. The attenuation factor (α) can be expressed as:
$$\alpha {\text{=}}\frac{{\sqrt 2 \pi f}}{c} \times \sqrt {\left( {\mu ^{\prime\prime}\upvarepsilon ^{\prime\prime} - \mu ^{\prime}\upvarepsilon ^{\prime}} \right)+\sqrt {{{\left( {\mu ^{\prime\prime}\upvarepsilon ^{\prime\prime} - \mu ^{\prime}\upvarepsilon ^{\prime}} \right)}^2}+{{\left( {\mu ^{\prime}\upvarepsilon ^{\prime\prime}+\mu ^{\prime\prime}\upvarepsilon ^{\prime}} \right)}^2}} }$$
3
Where µ 'and µ "are the real and imaginary parts of the permeability, and ε' and ε" are the real and imaginary parts of the dielectric constant.
The analysis of the attenuation factor in Figure. S8 reveals that, despite the identical Co/C ratio content in the Co-0, Co-1, Co-2, and Co-3 samples depicted in Fig. 9, the attenuation factor of the samples carbonized at 600°C is significantly lower than that of those carbonized at 800°C. This observation aligns with the previous analysis of reflection loss (RL). The lower carbonization temperature of 600°C leads to incomplete carbonization, resulting in inadequate formation of crystalline carbon, poor impedance matching, and insufficient catalysis of crystalline carbon for the reduction of magnetic cobalt metal. Consequently, this hinders the reduction of a considerable amount of magnetic cobalt metal, thus limiting the involvement of Co metal in electromagnetic wave absorption and consequently impacting the material's microwave absorption performance.
C0 is often used to represent the contribution of eddy current loss to electromagnetic wave magnetic loss, [43–46] expressed by Eq. (5):
It is generally believed that in the presence of eddy current loss, C0 remains constant with frequency change. However, it can be seen from Figure. 10 that the C0 value changes significantly with the increase of frequency and shows a decreasing trend, indicating that eddy current loss does not exist in the Co/C nanocomposites prepared in this work. Therefore, the magnetic loss of the composite is mainly natural resonance loss, and the representative hysteresis loss is too weak in the GHz frequency range. Although the absorption capacity of electromagnetic wave is the result of the joint action of dielectric loss and magnetic loss, the experimental results of this work show that the influence of dielectric loss on the absorption performance of materials plays a dominant role in this experiment. In the analysis of this work, we need to focus on dielectric loss, and the influence of magnetic loss is relatively small [28]. Therefore, when the peak of the imaginary part of the dielectric constant (ε ") and the permeability (µ ") is inconsistent, attention should be paid to the effect of ε "on the absorption ability.
The electromagnetic wave absorption mechanism of porous Co/C nanocomposites is illustrated in Figure. 11. Proper impedance matching causes incident electromagnetic waves to scatter and reflect across numerous interfaces. Magnetic cobalt metal particles adhered to the pore walls amplify the material's magnetic loss for electromagnetic waves, facilitating wave absorption and thermal conversion. Additionally, the distinctive porous network structure of the material augments reflection and aids in further attenuating electromagnetic waves.
A method for preparing porous Co/C nanocomposites is presented in this paper. The presence of amorphous carbon inhibits the aggregation of Co nanoparticles, resulting in a good impedance matching. The presence of crystalline carbon inhibits the oxidation of magnetic Co metal particles in the material. By changing the Co/C ratio, we determined the best ratio of Co/C content in Co/C nanocomposites, so that it has a good impedance matching performance. Under the condition of thin thickness, light weight and wide absorption band, excellent reflection loss (RL < -10 dB) is obtained. Its excellent absorption performance is due to the loss of resonance and dielectric relaxation of cobalt nanoparticles in the multi-layer carbon structure. Moreover, the construction of porous carbon after flour fermentation also increases the attenuation of the material to the incident electromagnetic wave. This study provides a new paradigm for determining the optimal absorbent content, which will play an important role in practical applications in the electromagnetic field.
In addition, we will continue to study the influence and specific control of the percolation theory in negative dielectric materials and the regulation of negative dielectric constant on the wave absorption properties when some electromagnetic parameters are negative, and we will continue to study and explore the design and construction of negative dielectric metamaterials in the later stage.
Competing interests:
The authors declare no competing interests.
Funding
The authors received financial support for this work from the National Natural Science Foundation of China (52101176).
Author Contribution
All authors have made contributions to this study work. Yiming Zhong, Yunchen Long, Yongxin Li and Peitao Xie wrote the manuscript. Yiming Zhong, Yunchen Long, Yinuo Sun, Jiachen Qin, Yongxin Li, Gemeng Liang, Jinshuo Zou and Peitao Xie all made contributions to experiment, data collection, and data analysis. Peitao Xie gave financial support for this work. All authors read and approved the final manuscript.
Data availability statement
Data can be made available on request to the corresponding authors.
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No competing interests reported.