Thickness-controllable coating of SiO₂ on Co microspheres with tunable electromagnetic properties and enhanced oxidation resistance

Abstract

A modified Stöber method was utilized for coating SiO₂ on Co microspheres with tunable thickness as a filler for electromagnetic absorbing coatings with enhanced oxidation resistance. Co microspheres with diameters of 1.5–3.5 μm were prepared using an aqueous-reduction process, and Co@SiO₂ core–shell microspheres with different shell thicknesses were subsequently fabricated by a modified Stöber method using tetraethyl orthosilicate (TEOS) as a Si source. The phase, morphology, and structure of composite microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA). Both ε and μ of Co@SiO₂ microspheres increase with the increasing filling ratio. No strong eddy current effect induced by local agglomeration was observed as the presence of a SiO₂ shell protects the Co particles from agglomeration and the filling ratio is up to 45 vol%. Due to the presence of the SiO₂ shell, the core–shell Co@SiO₂ composite microspheres exhibit better antioxidation capability than that of pure Co microspheres. The oxidation temperature of Co@SiO₂ is up to 720 °C, much higher than that of Co microspheres (380 °C). The effects of SiO₂ shell thicknesses and annealing treatment on microstructure evolution and on EM parameters of Co@SiO₂ composites were also investigated.

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1. Introduction

Recently, many studies have been focused on designing and preparing high-efficiency microwave absorbers to eliminate unwanted electromagnetic reflections, which is becoming increasingly crucial for wireless communication, as well as for radar stealth technology. Ferromagnetic metal/alloy particles are promising candidates for electromagnetic absorbing (EMA) fillers as they possess high electromagnetic loss efficiency.^1,2 The excellent performance of ferromagnetic metal/alloy particles, however, has not been fully excavated when they are filled in a matrix to fabricate EMA coatings. First, the negative effect of eddy current can be evident since the aggregation or overlap between particles is usually inevitable in this series of materials. In this case, the permittivity may increase, whereas the permeability decreases dramatically, leading to deteriorated permittivity/permeability matching.³ Second, the oxidation or corrosion of ferromagnetic metal/alloy particles are likely to occur when used as fillers in EMA coatings, particularly when the fabrication and application are conducted under high temperature and corrosive environment. These problems become serious when high filling ratio is applied,^4–10 which then blocks the efforts towards EMA coating with high filling ratio. To avoid the negative eddy current effect and decay, filling ratio is currently limited to be below 20 vol% in most EMA coatings using ferromagnetic metal/alloy particles as fillers. The intrinsic superiority of fillers' is hereafter dilutes off. To suppress these negative effects is thus crucial for developing high performances EMA coatings.

Considering the requirement for isolating and protecting ferromagnetic particles from aggregation and decaying, coating ferromagnetic metal/alloy particles with an inert shell to construct composite particles with a core–shell structure has been proposed to solve the above mentioned issues.^7,11,12 The various types of shell materials used include carbon materials, polymers, SnO₂, SiO₂, TiO₂, BaTiO₃, as well as graphene composites.^13–17 Zhao et al. synthesized Ni@TiO₂ and Ni@SiO₂ composite microspheres via a solvothermal method. The EMA properties of the related coatings were significantly enhanced. Furthermore, SiO₂-coated Ni core–shell composite microspheres exhibit superior oxidation resistance.¹⁸ In our previous study, SiO₂ coated Co particles were prepared via a sol–gel process, protecting Co cores from oxidation and corrosion.¹⁹ Tang coated FeNi₃ nanoparticles with silica shells, which effectively reduced the eddy current effect in the high frequency band.²⁰ Che prepared CoNi@air@TiO₂ yolk–shell microspheres by a sol–gel/solvothermal route,²¹ which is helpful to suppress the eddy current effect. These core–shell materials exhibited enhanced EMA performance and improved the oxidation/corrosion resistance as the shell isolated the ferromagnetic metal/alloy particles from aggregation and decaying.

In addition to the material selection for core/shell of the composite particles, the thickness, as well as the microstructure of shell, is also a vital factor that influences the EM properties of composite particles. Except the insulating and protecting properties, the dielectric relaxation, ferromagnetic resonance as well as the cross-particles exchange couple are all related to the composition, solidity and thickness of the shell. It is thus important to tailor the microstructures of composite particles for designing EMA materials with desired properties.

In the current study, we use a two-step process to prepare Co@SiO₂ composites and investigate the microstructure and EM properties, aiming to explore the feasibility to develop highly-filled EMA coatings that can serve at a high temperature and corrosive environment. The study mainly focuses on the influences of microstructure, including the shell thicknesses and the phase composition of Co core, on EM properties. The influence of filling ratio and the oxidation resistance was also evaluated. Our work might shed insight on the design of novel microwave absorbers with high performance.

2. Experimental

2.1 Synthesis of Co@SiO₂ core–shell microspheres

Co particles were prepared through an aqueous-reduction process similar to that developed in our previous study.²² Co@SiO₂ composite particles were subsequently prepared using a modified Stöber method.²³ Briefly, the obtained Co microspheres (5.0 g) were first added into a mixture of ethanol (400 mL) and deionized water (100 mL). Then, 10 mL of aqueous ammonia solution (28 wt%) was added by stirring. Finally, 0.5 mL of tetraethyl orthosilicate (TEOS) was added. The mixture was stirred at room temperature for 5 h for sol–gel reaction to occur. The prepared Co@SiO₂ microspheres were filtered, washed three times with ethanol and deionized water before drying at 60 °C for 24 h. Some of the Co@SiO₂ microspheres that were coated one time were annealed in H₂ atmosphere at 500, 600 and 800 °C for 120 min for microstructure tailoring.

2.2 Characterization

The morphology of the particles was observed using a scanning electron microscope (SEM, FEI Quanta 200F). The compositions were analyzed using energy dispersive X-ray spectroscopy (EDS). The phase composition of the specimen was characterized via X-ray diffraction (XRD, Rigaku D/max-rB, CuK_α). The saturation magnetization (M_s) and coercivity (H_c) of Co@SiO₂ microspheres of various conditions were measured on a vibrating sample magnetometer (VSM, Lakeshore7300) at room temperature. The complex permeability and complex permittivity of paraffin-matrixed specimens were measured on a vector network analyzer (VNA, Agilent N5230A) in 2–18 GHz range. Co@SiO₂ of various conditions and paraffin were mixed thoroughly after heated to 80 °C and then pressed into a mould to prepare VNA specimens. The fabricated coaxial cylindrical specimens are coaxial with outer diameter of 7 mm, inner diameter of 3.04 mm, and thickness of about 3 mm. The oxidation resistance of Co@SiO₂ particles was evaluated through TG-DSC analysis exposed to air in a thermoanalyzer (SDT Q600) from room temperature to 1000 °C.

3. Results and discussion

3.1 Phase structure

The phase identification and structure analysis of the prepared particles were carried out via XRD analysis. All diffraction peaks shown in Fig. 1(a) can be well indexed to close-packed hexagonal phase (hcp-Co) cobalt, referring to standard JCPDS card number 05-0727. The low intensity of peaks reveals that the as-prepared Co particles are of low crystallinity. The XRD analysis also suggests that some of the Co is amorphous. XRD peaks corresponding to crystalline SiO₂ cannot be observed in the XRD patterns, indicating that the prepared SiO₂ shell is of amorphous state, as shown in Fig. 1(b). XRD patterns of Co@SiO₂ particles annealed at different temperatures for microstructure tailoring are shown in Fig. 1(c)–(e). The SiO₂ shell retains the amorphous structure regardless the annealing performed; however, evident microstructure evolution is observed in Co. Bulk Co possesses a hcp structure at room temperature and would transform to a fcc structure.^24,25 Diffraction peaks corresponding to hcp cobalt and fcc cobalt co-exist in the XRD patterns when particles were annealed at 500 and 600 °C, as shown in Fig. 1(c) and (d), indicating that Co particles of these conditions are a mixture of hcp cobalt and fcc cobalt. The relative content of fcc cobalt increases as the annealing temperature increases, indicating that the hcp–fcc transition accelerates as the temperature increases. The hcp cobalt was completely converted to fcc cobalt after being annealed at 800 °C for 2 hours, as shown in Fig. 1(e). Moreover, the amorphous Co observed in the as-prepared Co microspheres could be completely converted to crystalline Co. The crystal structure transformation of Co in the Co@SiO₂ composite was similar to that observed in Co particles.²² Based on the above mentioned observations, it is clear that the presence of SiO₂ shell does not affect the crystal structure of Co core during the annealing.

Fig. 1 XRD patterns of Co particles (a), Co@SiO₂ samples (b), and Co@SiO₂ composites annealed in H₂ atmosphere for 120 min (c) 500 °C; (d) 600 °C; (e) 800 °C.

3.2 Morphology

Fig. 2 shows the SEM images of the Co and Co@SiO₂ particles. As shown in Fig. 2(a), raw Co particles are spheres with diameters of 1–3.5 μm and the surface is quite smooth. The morphology of the Co@SiO₂ composite microspheres is shown in Fig. 2(b). It can be seen that the spherical shape is well remained, whereas the originally smooth surfaces of the Co microspheres became rough after SiO₂ coating. The SiO₂ shell is dense with no observed cracks or pores. The island like clusters on the shell surface is related to the forming process of SiO₂ in the Stöber process.^26,27 The observation indicated that the SiO₂ shell was successfully coated onto the surface of Co microspheres.

Fig. 2 SEM images of as-prepared Co particles (a), Co@SiO₂ samples (b and c), (d) EDS, and (e–g) elemental mapping of Co, Si and O of a single Co@SiO₂ composite microsphere.

EDS analysis was performed and the results are shown in Fig. 2(c)–(g). The EDS profile of an individual Co@SiO₂ microsphere validates the existence of Co, Si and O. The elemental maps reveal that Co, Si and O distribute uniformly within the core–shell structures, as shown in Fig. 2(d) and (e). In particular, the annular dark spot observed in Fig. 2(e) suggests that Co concentrates in the core region, whereas the Si and O in the shell regions. Based on the EDS survey, we can infer that the unique core–shell structured with Co cores and SiO₂ shells were formed through the sol–gel process.

SEM observation was performed to examine the effects of annealing on the morphology of Co@SiO₂; the SEM images of the annealed products are shown in Fig. 3. Compared with the as-prepared product, the annealed products remain spherical morphology as no apparent changes in the size can be observed, as shown in Fig. 3. In our previous study, the particles' morphology cannot be retained and occurrence of migration and fusion cross over particles leads to merging during high temperature annealing.²² This reveals that the introduced SiO₂ layer can effectively separate and isolate Co microspheres from contacting to each other, which can suppress the cross-particle diffusion and aggregation.

Fig. 3 SEM images of Co@SiO₂ samples annealed in an H₂ atmosphere for 120 min at different temperatures: (a) 500 °C; (b) 600 °C; (c) 800 °C.

Stöber process was repeated for 1, 3 and 5 times to explore the method to fabricate a SiO₂ shell with different thicknesses. Fig. 4 shows the SEM and TEM images of the Co@SiO₂ samples with different shell thicknesses. The magnified SEM images indicate that the roughness of the spherical surface increases with increasing coating times from one to five. To compare with the as-prepared particles, no apparent change in microsphere except for the increase of surface roughness, as shown in Fig. 4(a) and (b). Some composite microspheres were partially broken when coated the three times, as shown in Fig. 4(d) and (e). The phenomenon becomes much more common when coating times further increase to five as some fragment shape up between particles, as seen in Fig. 4(g) and (h). The energy dispersive X-ray spectroscopy (EDS) as well as elemental maps of a single broken Co@SiO₂ composite microsphere coated for three times confirmed (Fig. S1, ESI†) that the peeling layer was SiO₂. The core–shell structure of the composite particles was further examined by TEM. The TEM images in Fig. 4(c), (f) and (i) show that the morphology of products is consistent with SEM observation. The TEM images show that the composite microspheres possess a unique core–shell structure, with a dark sphere encapsulated in a gray shell. The thicknesses of SiO₂ shell are observed to increase from around 22 nm to 64 nm, then to 98 nm, as the coating times increase from 1 to 3 and 5. The shell thickness increases for around 22 nm for each time of coating. Based on the observations, we can conclude that the core–shell composite is composed of a Co core and an outer SiO₂ shell. Moreover, it is clear that changing coating times is an effective way to tailor the thickness of SiO₂ shell.

Fig. 4 SEM and TEM images of Co@SiO₂ particles with different coating times: (a–c) one; (d–f) three; (g–i) five.

3.3 Magnetic properties

The static magnetic properties of Co@SiO₂ composite microspheres of different conditions were measured at room temperature using VSM, and the results are shown in Fig. 5. The magnetic hysteresis loop illustrates that all samples exhibit ferromagnetic properties. Hysteresis loops in Fig. 5(a) indicate that the M_s of Co microspheres is 157.9 emu g⁻¹, slightly lower than that of bulk cobalt (168 emu g⁻¹).²⁵ The decrease of M_s for Co microspheres is mainly attributed to the low crystallinity^28,29 and existence of impurities.³⁰ Co@SiO₂ spheres exhibit lower M_s compared with pristine Co particles. M_s gradually decreases with the increase of SiO₂ shell' thickness from 153.2 to 149.3 and 132.5 emu g⁻¹, as shown in Fig. 5(a). The decrease is due to the additional mass of nonmagnetic SiO₂ layer.¹⁸

Fig. 5 Magnetic properties of (a) as-prepared Co and Co@SiO₂ particles, (b) Co@SiO₂ samples annealed at different temperatures for 120 min in H₂ atmosphere.

Comparison between samples of different hydrogen-thermal treatment temperatures suggests that M_s of annealed products increased obviously. M_s increased to 160 emu g⁻¹ after the hydrogen heat treatment at a high temperature, as shown in Fig. 5(b) and Table 1. The enhancement of M_s for annealed Co@SiO₂ composite microspheres is primarily ascribed to the improved crystallinity as well as reduced defects during the hydrogen-thermal treatment process.

Table 1 The magnetic properties of Co and Co@SiO₂ particles with different annealing temperatures

Samples	Hc (Oe)	Ms (emu g^−1)
Co	124.0	157.9
Co–SiO2 (one time)	112.2	153.2
Co–SiO2(three times)	98.3	149.3
Co–SiO2 (five times)	89.3	132.5
500 °C	83.6	160.6
600 °C	56.3	160.9
800 °C	73.3	159.4

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It is well known that coercivity (H_c) is a vital parameter for assessing magnetic properties, and a high-frequency resonance of the absorbent may be achieved with a high H_c.^15,31 In the current research, pristine Co spheres exhibit higher coercivity (124.0 Oe) compared with bulk cobalt (10 Oe).³² After coating the Co spheres with an SiO₂ layer, a decreased H_c was observed. H_c gradually decrease from 112.2 Oe to 98.3 Oe then to 89.3 Oe with the increase of SiO₂ layer thickness, as shown in Table 1. It considerably decreases to around 83.6 Oe and 56.3 Oe, after hydrogen-thermal treatment at 500 °C and 600 °C. However, the decrease did not continue further, an increase of H_c was observed when annealed at 800 °C.

The coercivity of magnetic material is related to the magnetocrystalline anisotropy as well as defects in the materials.^33,34 The high H_c of as-prepared Co spheres may be related to the high density of defects that developed during the preparation. The decreased H_c during annealing is mainly attributed to elimination of defects. The slight increase of coercivity observed after annealed at 800 °C may be related to the conversion from amorphous to crystalline Co as amorphous Co usually possess lower coercivity. The Co@SiO₂ composite microspheres with high saturation magnetization and coercivity should possess better microwave absorbing ability.

3.4 Electromagnetic properties

The electromagnetic parameters (complex permittivity and complex permeability) of the wax-matrixes specimens containing composites particles were measured in the 2–18 GHz band. It can be observed from Fig. 6(a) that ε′ of pristine Co microspheres remain almost constant over the frequency range except for very weak fluctuation behavior, whereas the ε′′ increases with the increasing frequency. Moreover, a peak is observed at around 14 GHz, indicating a mild dielectric relaxation at the band. After coating Co with an SiO₂ layer, the complex permittivity of Co@SiO₂ particles have no obvious change all through the 2–18 GHz band, either in ε′ or ε′′, demonstrating that the presence of SiO₂ shell, and the increase thicknesses did not significantly influence the permittivity. The permittivity of Co@SiO₂ composite particles mainly derive from the free charge polarization on the conductor/insulator interfaces that generate dielectric relaxation. The conductivity and interface areas dominate the relaxation intensity, and thus the permittivity of composites. In the Co@SiO₂ core–shell structured products, the formation of SiO₂ layers and the increasing thickness have slight effect on the above factors, leading to slight variation of dielectric relaxation or dielectric resonance behavior.

Fig. 6 Electromagnetic properties of the Co@SiO₂/paraffin composite materials with different coating times: (a) complex permittivity; (b) complex permeability.

Fig. 6(b) shows the μ′ and μ′′ of all Co@SiO₂ composite microspheres in the frequency range of 2–18 GHz. It can be seen that the μ′ and μ′′ slightly decrease with the increasing thickness of SiO₂. The natural resonance of all samples occurs at below 2 GHz, and the presence of SiO₂ shell would not influence the natural resonance behavior. In addition, no obvious eddy current effect is observed in all the samples. The permeability of ferromagnetic particles mainly depends on the M_s and the eddy current effect. The eddy current effect was quite weak since the bare Co particles prepared through solution chemistry process possess high resistivity. In addition, the bare as well as the clad particles are very fine and well dispersed, which is not favorable to induce local eddy current by particles aggregation. Thus, the decrease of the permeability is attributed to the variations of M_s, which is ascribed to the introduction of non-ferromagnetic SiO₂ shells.

In conclusion, the introduction as well as the thickness of SiO₂ shell has no significant influence on the electromagnetic properties of Co particles, which is beneficial for the preparation and design of the stealth coatings.

Fig. 7 depicts the complex permittivity and permeability of the Co@SiO₂ microspheres with different filling ratios. As seen in Fig. 7(a), the ε′ enhances obviously with the increase of filling ratio. For example, the ε′ of Co@SiO₂ microspheres increases from 4.1 to 7.2 and 10.9 at 8 GHz, when the filling ratio increases from 15 vol% to 25 vol% and 45 vol%, respectively. The dielectric relaxation is relatively weak, and the relaxation frequency remains at about 14 GHz. In the present study, the enhancement in permittivity is attributed to the enhanced surface charge polarization. The relaxation intensity is related to the total interface areas of conductor/isolator. The increased filling ratio yields more interface and thus contributes to enhanced dielectric relaxation. On the other hand, the filling ratio has little effect on the relaxation frequency as the relaxation frequency almost remains unchanged when filling ratio increases.

Fig. 7 Electromagnetic properties of the Co@SiO₂/paraffin composite materials with different volume of Co@SiO₂ particles: (a) complex permittivity; (b) complex permeability.

The complex permeability of Co@SiO₂ samples, for both μ′ and μ′′, obviously increases with the increasing filling ratio as presented in Fig. 7(b), indicating that permeability of the composite materials is highly dependent on the content of Co@SiO₂ microspheres. Moreover, the frequency of natural ferromagnetic resonance is found to shift to a higher frequency when the filling ratio increases, suggesting that the filling ratio has significant influence on the frequency and intensity of natural resonance. In addition, when the filling ratio is up to 45 vol%, no strong eddy current effect, induced by local agglomeration, is observed as the presence of SiO₂ shell protects the Co particles from agglomeration.

In order to investigate the effect of annealed temperature on the EM wave absorption of Co@SiO₂ composite, the EM parameters were also measured in the frequency range from 2 to 18 GHz. Fig. 8 shows the complex permittivity and permeability of annealed Co@SiO₂ composite. When the annealing temperature was below 600 °C, ε′ gradually increases with the increase of annealing temperature; however, the ε′′ almost remained unchanged. When the annealing temperature increased to 800 °C, the Co@SiO₂ composite spheres present great change in ε′ and ε′′. A peak on the curve of ε′′ in 6.2 GHz is observed, illustrating an intense dielectric relaxation, as shown in Fig. 8(a). In this study, the conductivity of Co@SiO₂ composite particles increased after annealing in H₂, leading to the increase of permittivity. Nevertheless, the significant increase in conductivity and the emergence of dielectric relaxation resulted in the highest permittivity when the particles annealed at 800 °C.

Fig. 8 Electromagnetic properties of the Co/paraffin composite materials as-prepared Co, Co@SiO₂ and Co@SiO₂ annealed at different temperatures for 120 min in H₂ atmosphere with 15 vol% of Co. (a) Complex permittivity; (b) complex permeability.

The effects of annealing temperature on the complex permeability are shown in Fig. 8(b). After annealed at different temperatures, the permeability of Co@SiO₂ composite particles does not change obviously. The permeability of the Co@SiO₂ composite particles is related to the M_s. Since the variation in M_s is very slight, the permeability remains unchanged.

3.5 Oxidation resistance

TG analysis was performed in open air to evaluate the anti-oxidation capacity of Co and Co@SiO₂ particles with different shell thicknesses and the results are shown in Fig. 9. The oxidation in uncoated Co microspheres initiates at around 380 °C, as inferred from Fig. 9. The oxidation temperature increases to about 550 °C after SiO₂ coating, indicating that the oxidation resistance of Co@SiO₂ composites is superior to that of Co particles. The oxidation resistance is further enhanced as shell thickness increases. The oxidation temperature increases from 550 °C to around 650 °C and 720 °C when the shell thickness increases from 22 nm to 64 nm and 98 nm, respectively. Increasing shell thickness is found to be an effective way to improve the protecting properties of SiO₂ shells.

Fig. 9 TG patterns of as-prepared Co and Co@SiO₂ composite microspheres with different coating times.

4. Conclusions

In summary, core–shell Co@SiO₂ composite microspheres have been successfully prepared via a two-step process, and it is feasible to tailor the thickness of SiO₂ layer by changing the coating times. In a typical process, increase of 22 nm in the shell thickness can be obtained via one time of coating. The presence of SiO₂ layers of various thicknesses does not influence the permittivity and permeability of Co particles evidently. The ε and μ of Co@SiO₂ composite materials increase as the filling ratio increases. The eddy current effect is not observed in specimens with filling ratio high to 45 vol%, indicating the isolating effect from SiO₂ shells. The oxidation temperature considerably increases after Co particles are cladded by SiO₂ shells. The protecting properties can be further improved via increasing the shell thickness. Shell with thickness of 98 nm can protect the Co particles from oxidation temperature below 720 °C. The introduction of SiO₂ shell is proved as an effective way to retain the morphology of Co particles. It can also protect Co particles from oxidation and agglomeration. The results reveal that the Co@SiO₂ composites obtained in the current study exhibit merits of high oxidation resistance and high dispersibility and are attractive candidates for the new types of electromagnetic wave absorptive materials in high temperature and high filling ratio applications.

Acknowledgements

This study was financially supported by the Natural Science Foundation of China (Grant no. 51201048), the Ph.D. Programs Foundation of Ministry of Education of China (Grant no. 20112302120021) and the SAST Foundation.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24519f

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