Preparation and Microwave-Absorbing Property of Solid-Waste-Derived Ceramic Foam

Abstract

Recently, electromagnetic wave (EMW)-absorbing materials have obtained increasing attention for both military and civil applications. This study adopted the powder sintering method and the concept of recycled wastes in fabricating functional ceramic foam (CF). Firstly, a ceramic green body composed of pulverized granite residues, waste glass, and a foaming agent was sintered. The influence of the sintering temperature and SiC addition on CF was investigated, and then surface graphitization post-treatment of CF was performed as well. The truly enhanced compressive strength and EMW-absorbing property of surface graphitization ceramic foam (SG-CF) with a homogeneous porous structure was realized in the present work, which is promising as a candidate in EMW absorption systems.

1. Introduction

In recent years, electromagnetic wave (EMW)-absorbing materials have aroused extensive attention for military and civil fields with the applications of modernized instruments, such as high-voltage substations, radio broadcast stations, high-frequency medical devices, and radio communication equipment, triggering increasingly serious electromagnetic radiation pollution and electromagnetic interference, which not only threaten the national information security and the normal utilization of electronic equipment but also damage human health [1,2,3]. Thereby, development of EMW-absorbing materials has become a vital topic for protecting people’s health and devices from external harm as well as national defense security.

Porous materials have been widely employed in EMW absorption systems as porous structures can control a relative dielectric constant and play an important role in EMW-absorbing properties by adjusting electromagnetic parameters and impedance matching [4,5,6]. The currently fashionable ceramic/glass foams prepared by the powder sintering method present the comprehensive features of high porosity, light weight, high strength, excellent heat insulating and preservation, and sound insulation [7,8,9]. These can be manufactured by employing waste species, such as discarded glass cullet [10], waste cathode ray tubes (CRT) [11], fly ash [12], red mud [13], municipal solid waste slag [14], blast furnace slag [15], silicon slag [16], granite scraps [17], sand sludge [18], iron tailing [19], and germanium tailing [20]. Thus, such ceramic/glass foams can be developed into the industrial production of cheap and lightweight EMW-absorbing materials.

Until now, extensive studies on the improvement the EMW absorption properties of ceramic/glass foams by adding certain materials (zinc, graphite, carbon fiber, C, etc.) into the green body have been reported. For instance, Chen et al. [21] fabricated foam glass by sintering a mixture of raw pure foam glass material and zinc powder, indicating that such foam could absorb EMW depending on the content of the zinc filler in the glass matrix. A report from Li et al. [22] revealed the effects of a graphite additive on the dielectric properties and microwave absorption performance of zinc-containing foam glass, showing that changing the graphite additive was an effective way to design the dielectric and microwave absorption properties of samples. Benhaoua et al. [23] determined the influence of carbon fibers addition on the dielectric properties of foam glass and demonstrated that carbon fibers increased the radiofrequency absorptive powers waves, thereby expanding the novel application of EMW attenuation in lower operating frequencies for mobile phones and Wi-Fi. A study by Benzerga et al. [24] dealt with the microwave properties of glass foams from glass industrial waste with different foaming agents (C, SiC, AlN), showing that the electromagnetic properties of glass foams were primarily determined by the apparent density and nature of the foaming agent. Particularly, foams with carbon as a foaming agent presented a high dielectric loss and could be used as a green electromagnetic absorbent in building industry. Similarly, Laur et al. [25] investigated the dielectric properties of glass foams from glass cullet and foaming agents (carbon or a combination of carbon, TiO₂, MnO₂, and AlN), implying that carbon-based glass foams showed a great potential as microwave absorbers with a shielding effectiveness of 20 dB/cm at 12 GHz. However, the dispersity of additive components in a green body is uncertain and uncontrollable, which thus has a negative impact on the stability of the microwave-absorbing properties of the resultant foams.

In this work, ceramic foam (CF) was fabricated by sintering a mixture of granite residues, waste glass, and a foaming agent. The effect of the sintering temperature and SiC addition content on the pore structure of CF was investigated. Then, surface graphitization post-treatment of CF was performed to enhance the microwave-absorbing properties, which is easier than adding materials to a green body and has been rarely reported.

2. Experimental Procedure

2.1. Materials

Granite scrap (GS) was sampled from rock cutting and polishing, and waste glass (WG) was gathered from waste sheet glass. Both the chemical and mineral compositions of GS and WG are listed in Table 1 and Figure 1, respectively. The major crystalline phases of GS are quartz (SiO₂, PDF No. 46-1045), albite (Na(Si₃Al)O₈, PDF No. 72-1246), and anorthite (CaAl₂Si₂O₈, PDF No. 12-0301). WG is entirely composed of the amorphous glass phase.

2.2. Sample Processing

The elaboration process of CF is illustrated in Figure 2. Firstly GS, WG, and SiC were thoroughly mixed and grinded to produce a homogeneous batch (Figure 2a). When the above mixture reached a temperature close to the softening temperature of the sample, SiC released gas by the redox reaction, resulting in expansion (Figure 2b). After a sufficient reaction time, the entirety of the SiC was consumed, resulting in an inflated porous structure (Figure 2c). After surface graphitization, the graphitized layer, wrapped around the surface of the light ceramic matrix, acted as the EMW absorbent. As such, surface graphitization ceramic foam (SG-CF) was obtained, as shown in Figure 2d.

More specifically, the synthesis parameters of CF were as follows. First, 70 wt.% GS and 29 wt.% WG with 0–3.0 wt.% SiC powder (α-SiC, average particle size of 15 μm, purity of 99.5%) were mixed and then homogenized with a planetary ball mill for 2 h. Afterwards, the uniform mixtures with ethyl alcohol in a 10:2 mass ratio was pressed into a φ 60 mm × 20 mm cylinder using a pressure of 18 MPa. This green body was sintered in air at different sintering temperatures from 1140 °C to 1230 °C in an electrical laboratory sintering furnace. The sample was heated from room temperature to the required sintering temperature at a rate of 5 °C/min, held at the maximum temperature for 30 min, and cooled to room temperature at a natural cooling rate.

Surface graphitization post-treatment of CF was performed under stirring in a 500 mL beaker. The obtained CF, with a size of 22 × 10 × 5 mm³, was cut, and graphite was mixed in at a 3:1 mass ratio. Then, a solution of sodium silicate (Na₂O·nSiO₂, liquid, n = 3.0) as a bonding agent was poured into the mixture at a liquid-to-solid ratio of 10 mL/g. The coated samples were dried in an oven at 60 °C for 12 h to eventually obtain SG-CF.

2.3. Characterization Techniques

The compressive strength of the sample was measured by a universal electronic tester (CMT5504, Jinan, China), and five repeated tests were performed to obtain the final result. The macrostructure was observed by a digital camera, and morphology observation of the sample was taken on an SEM (QUANTA 250 FEG, Waltham, MA, USA). Image-Pro Plus software was used to determine the pore size distribution and the average pore size.

The EMW-absorbing properties were analyzed by Network Analyzer (Agilent N5234A, Santa Clara, CA, USA) equipped with an amplifier and a scattering parameter (S-parameter) test set in the x-band region. Rectangular samples with dimensions of 22.4 mm × 10.2 mm × 10.0 mm were fixed into the wave guide sample holder. At least 5 specimens were prepared for CF and SG-CF to finally obtain the average result.

3. Results and Discussion

3.1. The Effect of the Sintering Temperature on CF

The macroscopic morphology, pore structure, and pore size distribution of CF sintered at various temperatures from 1140 °C to 1230 °C are presented in Figure 3 and Figure 4. It seems obvious that as the expansion degree of CF increased, the pore size distribution changed significantly, and the average pore size gradually increased with the rise in sintering temperature. It can be inferred that the formation of CF with a porous structure could be divided into two processes with the rising sintering temperature. The first one was the sintering densification process for the pore walls. In this process, the densification degree of the pore wall increased with more generation of the liquid phase and silicate crystals. In the other process, the size and quantity of pores increased with more gas releasing from the foaming agent. The existence of gas could exert gas pressure on the pore wall, which resisted sample densification. With the rising sintering temperature, the gas pressure inside the pore increased, accompanied by the increased liquid phase volume and decreased liquid phase viscosity, causing the gradual growth of pores. At 1140 °C, the sample present a minor expansion with small-sized pores (~0.6 mm). At 1170–1200 °C, a uniform pore structure with different pore sizes could be indicated in samples with a major expansion. Huge pores (5~10 mm) could be observed in the sample sintered at 1230 °C, which was due to the reduced viscosity of the liquid phase in the sample at 1230 °C causing small pores to combine into large pores, which is harmful to the mechanical strength of CF. In a word, a homogeneous porous structure can be seen in the sample sintered at 1170–1200 °C, which is derived from the good balance between the viscosity of the liquid phase and gas pressure at a high temperature. Compared to 1200 °C, the samples prepared at 1170 °C exhibited a narrower pore size distribution range, with a more balanced number of pores within each distribution interval. This indicates that the samples prepared at this temperature have a more uniform pore structure, making this temperature more suitable for preparing FC.

3.2. The Effect of the Foaming Agent on CF Content

In previous experiments, the optimal sintering temperature was determined to be 1170 °C. As shown in Figure 5, the macroscopic morphology and pore structure of samples with different SiC amounts sintered at 1170 °C were measured, and the pore distribution of samples are presented in Figure 6.

For the sample with no SiC addition, the sample was dense and had a high density. In order to obtain a sample with a porous structure, the addition of SiC was necessary. Indeed, different sizes and quantities of pores appear in the samples with different SiC amounts. With an SiC addition up to 1.0%, the sample presented with small-sized pores (2~4 mm) accompanied by a homogeneous pore structure. The samples with a 2–3% SiC addition showed an nonuniform pore morphology, and the pore size increased significantly compared to the sample with 1% SiC. This trend illustrates that a moderate addition of SiC (1 wt%) in CF was favorable for the foaming of CF, while an excessive SiC addition caused large pores in the samples. Gas is released from the reaction of SiC with O₂, which is then fixed in a liquid phase to form the pores in the sample. With an increased SiC addition, the gas production increased and pressure increased, causing multiple pores to combine into large connected pores.

3.3. Surface Treatment on CF

Based on the above analysis, the pore structure of CF can be optimized by regulating the doping of SiC and the sintering process. It has been determined that adding 1 wt% of SiC and sintering at 1170 °C can yield a uniform porous structure and good overall performance. In order to further obtain functional CF, the CF prepared under the aforementioned conditions was subjected to surface graphitization post-treatment. Figure 7 presents the comparison of compressive strength of the CF and SG-CF specimens. The compressive strength of CF had a value of 1.12 MPa, while the SG-CF sample, with a value of 3.20 MPa, was significantly strengthened compared to CF by surface graphitization post-treatment. The enhancement of SG-CF was probably due to the solidified sodium silicate bonding agent wrapping the matrix surface. Hence, such a bonding agent can not only localize the graphite on the surface but also improve the overall strength of SG-CF. From the insert, the prepared CF exhibited a porous structure composed of closed pores of 1.5~2.0 mm size and small closed pores (about 200~400 μm) in the pore wall. After surface graphitization, the internal porous structure still remained intact, and the graphitized surface layer wrapping the porous matrix can be assumed to provide the EMW-absorbing property. Accordingly, the test results are given in Figure 8.

The sample with a sintering temperature of 1170 °C and an SiC content of 1 wt% was tested for its EMW absorption performance. Figure 8a,b shows the complex permittivity spectra, with real (ε’) and imaginary (ε”) parts, for the CF and SG-CF samples in the frequency range of 8.2–12.4 GHz. Both ε’ and ε” of SG-CF were increased compared to the CF sample. The ε’ value increased from ~1.5 for CF to ~7.6 for SG-CF, and the ε” spectra showed a similar variation from ~0 for CF to ~3.2 for SG-CF. Figure 8c depicts the frequency dependence of the samples’ reflection loss. It is observed that surface graphitization decreased the reflection loss of the samples at both low and high frequencies. The reflectivity of CF was around 0 dB, indicating a poor EMW absorption property. However, the SG-CF specimen achieved a reflectivity of less than −12 dB in the whole X-band, and a minimum reflectivity value of −14.5 dB was obtained at 12.4 GHz for SG-CF.

Previously, Chen et al. [21] investigated the effect of zinc powder on the microwave-absorbing properties of foam glasses and revealed that the optimum sample, with a thickness of 2.2 mm, achieved a reflection loss below −10 dB in the range of 11.3–12.4 GHz, and the minimum value was −15.6 dB at both 12.0 and 12.4 GHz. Li et al. [22] confirmed that zinc-containing foam glass with a 2.0 wt.% graphite additive achieved a reflection loss of less than −10 dB in the whole X-band, with a thickness ranging from 2.3 to 3.5 mm and a minimum reflectivity of −23.8 dB at 9.3 GHz. From what has been discussed above, the microwave absorption property of SG-CF was evenly matched with that of the reported foams with additional materials, except those with underdeveloped minimum reflectivity. This indicates that surface graphitization can effectively enhance the microwave-absorbing property of ceramic foam, which is simple and easy to operate, and can avoid the uneven dispersion of additives in the green body. Based on the above encouraging permittivity and reflection loss, we can conclude that such a foam after surface graphitization can be used as a protective and absorbent material in areas with high EMW.

4. Conclusions

Ceramic foam (CF) with a uniform porous structure was obtained by sintering a mixture of granite residues, waste glass, and a 1 wt% SiC foaming agent at 1170–1200 °C. The post-treatment surface graphitization of the ceramic foam (SG-CF) sample had a higher compressive strength, with a value of 3.20 MPa, than CF alone (1.12 MPa). The microwave properties revealed that increased ε’ and ε”, as well as decreased reflectivity of SG-CF, were obtained. This suggests that such surface graphitization is a promising method for EMW shielding.

Author Contributions

Conceptualization, M.M. and C.J.; Methodology, Z.W.; Formal analysis, X.L.; Investigation, M.M.; Data curation, X.L.; Writing—original draft, Z.W.; Writing—review & editing, C.J. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is contained within the article.

Acknowledgments

This work was supported by the Science and Technology Project of University of Jinan (XBS2203).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. XRD pattern of (a) GS and (b)WG.

Figure 2. Schematic illustration of SG-CF sample. (a) Green body, (b) Expansion process, (c) Ceramic foam, (d) Surface graphitization ceramic foam.

Figure 3. Macroscopic morphology and pore structure of CF at different sintering temperatures.

Figure 4. Pore size distribution of CF at different sintering temperatures.

Figure 5. Macroscopic morphology and pore structure of CF with different SiC additions.

Figure 6. Pore size distribution of CF with different SiC additions.

Figure 7. Comparison for compressive strength of CF and SG-CF. The inserts are the macro- and micro-structure of the specimen.

Figure 8. (a) Real part and (b) imaginary part of permittivity and (c) reflectivity of CF and SG-CF samples.

Table 1

Chemical compositions of GS and WG (wt. %).

Raw Materials	SiO2	Al2O3	CaO	MgO	Fe2O3	Na2O	K2O	LOI
Granite scrap (GS)	66.13	13.30	3.75	3.05	3.04	1.83	1.32	7.58
Waste glass (WG)	65.51	1.59	8.05	1.98	0.26	17.59	4.55	0.47

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