CN114772606B

CN114772606B - Carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption and preparation method thereof

Info

Publication number: CN114772606B
Application number: CN202210228024.7A
Authority: CN
Inventors: 阮丽萍; 邱梓洋; 史争争
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2022-03-10
Filing date: 2022-03-10
Publication date: 2023-09-19
Anticipated expiration: 2042-03-10
Also published as: CN114772606A

Abstract

The application provides a preparation method of a carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption, which is characterized in that tetraethyl orthosilicate is dripped into ethanol-water-ammonia water mixed solution, resorcinol and formaldehyde are added after full stirring, the reaction is carried out for 6-36 h under stirring at 20-35 ℃, reaction products are separated, washed and dried, and finally the reaction products are fully carbonized under the vacuum condition at 700-800 ℃. The core-shell composite nano material prepared by the method is nano particles with a core-shell structure, wherein the nano particles are composed of a core, an intermediate layer wrapping the core and a shell wrapping the intermediate layer, the main component of the core is silicon dioxide, the intermediate layer is a heterojunction layer composed of silicon dioxide and carbon, and the main component of the shell is carbon. The application can effectively simplify the synthesis process and increase the industrial applicability of the preparation process on the basis of ensuring the excellent electromagnetic wave absorption performance of the composite nano material.

Description

Carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption and preparation method thereof

Technical Field

The application belongs to the field of electromagnetic wave absorbing materials, and relates to a core-shell composite nano material for electromagnetic wave absorption and a preparation method thereof.

Background

With the rapid development and wide use of wireless communication devices, the damage of electromagnetic pollution to electronic devices and human bodies is also becoming serious. Electromagnetic wave absorbing materials are therefore becoming increasingly interesting to researchers, and such materials can absorb electromagnetic waves and convert them into thermal or other forms of energy. According to electromagnetic wave absorption mechanism, the performance of the absorption material is mainly determined by the good impedance matching of the material itself and the electromagnetic attenuation capability consisting of polarization loss, conduction loss and magnetic loss.

The carbon material with the nano structure is used as a typical electromagnetic wave absorbing material, and has the advantages of low density, wide source, adjustable chemical property, good electronic conductivity and the like. Carbon nanocomposites have also been widely studied in order to improve the impedance matching of materials and enhance their electromagnetic wave absorption capacity, and there have been reports of preparing carbon-based composites using magnetic substances, metal oxides, metal sulfides, semiconductor materials, and the like in combination with carbon materials. The core-shell structure is commonly used for carbon-based composite materials due to the advantages of abundant interfacial polarization, good chemical uniformity, limiting effect and the like, for example, liang et al prepare SiC@C core-shell structure nano particles, wang et al synthesize SiC/rGO core-shell structure materials, and the core-shell structure is proved to be beneficial to improving the electromagnetic wave absorption performance of the materials.

However, the industrial application of the carbon-based core-shell electromagnetic wave absorbing material still has some challenges, mainly because the materials have complex design, large synthesis process difficulty and long synthesis time consumption, and the multi-layer core-shell structure often needs multi-step cladding operation, and the preparation process often accompanies harsh reaction conditions such as high temperature, high pressure and the like. The synthesis of carbon-based core-shell materials reported in the prior literature generally requires a day or even longer, and excessively long synthesis time increases the synthesis cost of the materials. In order to enhance the industrial applicability of the electromagnetic wave absorbing material, the development of a simple and efficient synthesis process is a key problem which needs to be solved in the preparation of the high-performance carbon-based core-shell type wave absorbing material.

Yu et al disclose the synthesis of core-shell carbon-silicon particles using a one-pot process with silica primary particles as templates, in the absence of surfactants, followed by etching to produce mesoporous hollow carbon. Ji et al prove on this basis that the mesoporous carbon hollow carbon can reach the wave absorbing performance of-50.9 dB under the filling rate of 20%. However, this method requires the use of highly corrosive hydrofluoric acid or sodium hydroxide in removing the silica template, and has limited its application in practical production for safety reasons. Therefore, if a method with simpler synthetic route, shorter synthetic time and higher safety in the synthetic process can be developed to prepare the carbon-based core-shell material with excellent electromagnetic wave absorption performance, the method has positive effects on promoting the practical application of the carbon-based core-shell electromagnetic wave absorption material.

Disclosure of Invention

The application aims to overcome the defects of the prior art and provide a carbon-silicon dioxide core-shell composite nanomaterial for electromagnetic wave absorption and a preparation method thereof, so that the synthesis process of the composite nanomaterial is effectively simplified on the basis of ensuring the excellent electromagnetic wave absorption performance of the composite nanomaterial, and the industrial applicability of the preparation process is increased.

In order to achieve the above purpose, the application adopts the following technical scheme:

the composite nano material is nano particles with a core-shell structure, which are composed of an inner core, an intermediate layer wrapping the inner core and an outer shell wrapping the intermediate layer, wherein the main component of the inner core is silicon dioxide, the intermediate layer is a heterojunction layer composed of silicon dioxide and carbon, and the main component of the outer shell is carbon; the nanocomposite has a pore structure therein.

In the above technical scheme of the carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption, the particle size of the composite nanomaterial is preferably 150-240 nm. Further preferably, the silica core in the composite nanomaterial is spherical, the thickness of the intermediate layer is 20-55 nm, and the thickness of the shell is 10-25 nm.

In the technical scheme of the carbon-silicon dioxide core-shell composite nanomaterial for electromagnetic wave absorption, the phase composition of carbon in the composite nanomaterial is graphitized carbon and disordered carbon, graphitized carbon is used as a main component, and the ratio of the intensity of a D band to the intensity of a G band in the Raman spectrum of the composite material is 0.77-0.82.

In the technical scheme of the carbon-silicon dioxide core-shell composite nanomaterial for electromagnetic wave absorption, the carbon of the composite nanomaterial also contains oxygen-containing polar functional groups including hydroxyl and carbonyl.

In the technical scheme of the carbon-silicon dioxide core-shell composite nanomaterial for electromagnetic wave absorption, the specific surface area of the composite nanomaterial is 100-150 m ² /g。

The application also provides a preparation method of the carbon-silicon dioxide core-shell composite nanomaterial for electromagnetic wave absorption, which comprises the following steps:

dripping tetraethyl orthosilicate into an ethanol-water-ammonia water mixed solution, stirring for 15-30 min, adding resorcinol and formaldehyde into the obtained reaction solution, stirring and reacting for 6-36 h at 20-35 ℃, separating out a reaction product, washing, drying, and fully carbonizing at 700-800 ℃ under vacuum condition to obtain the carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption;

the ethanol-water-ammonia water mixed solution is formed by mixing ethanol, water and ammonia water according to the volume ratio of (60-80): (5-15): (2-4); the volume ratio of the tetraethyl orthosilicate to the ethanol-water-ammonia water mixed solution is 1 (160-170).

In the technical scheme of the preparation method, the water adopted in the preparation of the ethanol-water-ammonia water mixed solution is deionized water, distilled water or ultrapure water.

In the technical scheme of the preparation method, resorcinol is preferably added according to the proportion of 0.3-0.5 g of resorcinol added into each 1mL of tetraethyl orthosilicate; preferably, formaldehyde is added in a ratio of tetraethyl orthosilicate to formaldehyde of 1 (0.4-0.7) by volume.

In the technical scheme of the preparation method, the stirring reaction is preferably carried out at 20-35 ℃ for 6-24 hours, more preferably at 20-35 ℃ for 6-18 hours, still more preferably at 20-35 ℃ for 6-12 hours, still more preferably at 20-35 ℃ for 6-10 hours.

In the technical scheme of the preparation method, the carbonization time under the vacuum condition of 700-800 ℃ is preferably 0.8-1.2 h.

In the technical scheme of the preparation method, the adopted ammonia water is ammonia water with the concentration of 25-28 wt%, and the adopted formaldehyde is formaldehyde with the concentration of 37-40 wt%.

In the application, the carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption is mainly formed by the following steps: in the process of stirring reaction for 6-36 h at 20-35 ℃, tetraethyl orthosilicate is hydrolyzed at a relatively fast speed, the reaction system mainly takes the rapid nucleation and growth of silicon dioxide as the main materials, resorcinol and formaldehyde are slowly polymerized in alkaline solution to form precursor phenolic resin of carbon components, and during the period, the growth speed of the silicon dioxide is obviously higher than the formation speed of the phenolic resin, so that a core taking the silicon dioxide as a main component is formed, and a very small amount of phenolic resin is deposited in the core; as the reaction proceeds, the rate of hydrolysis of tetraethyl orthosilicate gradually decreases, while the rate of polymerization between resorcinol and formaldehyde gradually increases, during which the silica particles formed by hydrolysis co-deposit with the precursor phenolic resin of the carbon component formed by the polymerization, forming an intermediate shell layer containing both silica and phenolic resin; as the reaction proceeds, the rate of polymerization between resorcinol and formaldehyde exceeds the rate of hydrolysis of tetraethyl orthosilicate, during which the reaction forms an outer shell layer based on phenolic resin with very little silica. After carbonization operation, the phenolic resin is converted into a carbon component, so that the nano particle with a core-shell structure, which is formed by the inner core, the middle layer wrapping the inner core and the outer shell wrapping the middle layer, is formed. Since the whole preparation process is continuously carried out by adopting a one-pot method, the core inevitably contains a very small amount of carbon components after carbonization, the shell inevitably contains a very small amount of silicon dioxide components, and the phenolic resin after carbonization can leave a small amount of oxygen-containing polar functional groups including hydroxyl groups and carbonyl groups, so that the main component of the core is considered to be silicon dioxide, the middle layer is a heterojunction layer composed of silicon dioxide and carbon, and the main component of the shell is carbon.

The application characterizes the electromagnetic wave absorption capacity of the composite nano material through experiments, and discovers that the composite nano material has good impedance matching and excellent electromagnetic wave attenuation capacity. In particular, the composite nanomaterial prepared at a reaction time of 6 hours shows particularly excellent electromagnetic wave absorption capacity, RL at an absorber thickness of 3.8mm and a frequency of 6.1GHz _min With a value of-56.28 dB in the absorberThe maximum effective absorption bandwidth occurs at 5.15GHz at a thickness of 1.8mm.

The inventor of the application makes experimental investigation on the mechanism of the composite nano material for absorbing electromagnetic waves in the research process, and discovers that the composite nano material has excellent electromagnetic wave absorption performance mainly for the following reasons: first, when graphitized carbon (sp 2) is present in an alternating electric field, a microcurrent is formed between the carbon layer network, which facilitates electron transport. And disordered carbon (sp 3) can act as a resistor to convert electrical energy into thermal energy, resulting in conduction losses. Second, the carbon formed by carbonization of phenolic resin polymers contains a large number of defects and residual oxygen-containing functional groups, such as hydroxyl and carbonyl groups, which have been confirmed by infrared and XPS spectra, that act as polarization centers, resulting in an even polarization that enhances the dielectric loss capability of the material. Finally, the composite nanoparticle has a carbon layer shell and a carbon-silicon intermediate layer, with a large number of carbon-silicon heterogeneous interfaces that enhance the Maxwell-Wagner effect and relaxation-related interfacial polarization as part of dielectric loss.

Compared with the prior art, the technical scheme provided by the application has the following beneficial technical effects:

1. the application provides a carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption, which is nano particles with a core-shell structure, wherein the nano particles are composed of a core, an intermediate layer wrapping the core and a shell wrapping the intermediate layer, the main component of the core is silicon dioxide, the intermediate layer is a heterojunction layer composed of silicon dioxide and carbon, and the main component of the shell is carbon. On one hand, the phase composition graphitized carbon of the carbon in the composite nano material is mainly and simultaneously contains disordered carbon, when the graphitized carbon exists in an alternating electric field, micro-current is formed between carbon layer networks, so that electron transmission is facilitated, and the disordered carbon can serve as a resistor to convert electric energy into heat energy, so that conduction loss is generated; on the other hand, carbon formed by carbonizing the phenolic resin polymer contains a large number of defects and residual oxygen-containing functional groups which can be used as polarization centers, and the generated dipoles can enhance the dielectric loss capacity of the material; more importantly, the composite nanoparticle has a carbon shell and a carbon-silicon interlayer, which contain a large number of carbon-silicon heterogeneous interfaces, which can enhance the Maxwell-Wagner effect and interface polarization associated with relaxation as part of dielectric loss. Through the combined action of multiple factors such as structure, component composition, phase composition and the like, the composite nano material has good electromagnetic wave absorption performance.

2. Experiments prove that the composite nano material provided by the application has good impedance matching and excellent electromagnetic wave attenuation capability. In particular, the composite nanomaterial with a rough profile prepared at a reaction time of 6 hours shows particularly excellent electromagnetic wave absorption ability, RL at an absorber thickness of 3.8mm and a frequency of 6.1GHz _min The value is-56.28 dB, and the maximum effective absorption bandwidth is 5.15GHz (12.48-17.63 GHz) when the thickness of the absorber is 1.8mm.

3. The application also provides a one-pot synthesis method of the carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption, which has the advantages of simple process, mild condition and short synthesis time, the time for forming silicon dioxide by hydrolysis and forming a carbon precursor by polymerization can be even shortened to 6 hours, and the composite nano material with excellent electromagnetic wave absorption performance can be prepared through the subsequent carbonization process for about 1 hour. The method has good industrial applicability, can solve the problems of complex design, large synthesis process difficulty, long synthesis time consumption, complex operation, severe reaction conditions such as high temperature, high pressure and the like required in the preparation process, the use of high-corrosiveness reagents and the like of the existing carbon-based core-shell electromagnetic wave absorbing material in industrial production and application, and can generate positive pushing effect on the practical application of the carbon-based core-shell electromagnetic wave absorbing material.

Drawings

Fig. 1 is a TEM image and particle size distribution diagram of S0.5, S1.5 and S3.

Fig. 2 shows SEM and TEM images of S6 and S36 in (a) to (c) and (d) to (f), respectively, and shows element distribution diagrams of S6 and S36 in (g) to (k) and (l) to (p) of fig. 2.

Fig. 3 (a) and (b) are diameter distribution diagrams of the core and the nanoparticle of S6, respectively, and fig. 3 (c) and (d) are diameter distribution diagrams of the core and the nanoparticle of S36, respectively.

Fig. 4 is a TEM image and a diameter distribution diagram of silica particles obtained after calcination of S0.5, S1.5, S6 and S36 in an air atmosphere.

FIG. 5 is a graph of the thermal weight curves of S0.5, S1.5, S3, S6, S9, S12, S18, S24 and S36 ((a) graph), the weight distribution of the carbon and silica components under different reaction time conditions ((b) graph), and the growth rate of the silica and carbon components under different growth time conditions ((c) graph).

Fig. 6 is an XRD spectrum of S6 and S36.

Fig. 7 (a) is a raman spectrum of S6, S12, S18, S24, and S36, and fig. 7 (b) is an infrared spectrum of S6 and S36.

Fig. 8 (a) is a full spectrum of XPS spectra of S6 and S36, and fig. 8 (b) is a C1S spectrum of S6 and S36.

Fig. 9 is a low temperature nitrogen adsorption-desorption isothermal plot of S6 and S36.

FIG. 10 is a graph (a) - (d) showing the real part, imaginary part, dielectric loss tangent and calculated RL of the complex dielectric constants of S1.5, S3, S6, S12, S18, S24 and S36 _min A reflection loss curve in which the reflection loss curve is located.

Fig. 11 (a) to (f) are three-dimensional images of theoretical calculation RL values of S3, S6, S12, S18, S24, and S36, respectively.

Fig. 12 (a) shows the effective absorption bandwidth of the samples S6 of different thicknesses in the frequency range of 2 to 18GHz, and fig. 12 (b) shows the attenuation constants of S1.5, S3, S6, S12, S18, S24 and S36 in the frequency range of 2 to 18 GHz.

Fig. 13 (a) and (b) are complex permittivity values of solid silica and solid carbon nanoparticles, respectively, and fig. 13 (c) and (d) are three-dimensional images of theoretical RL values of solid silica and solid carbon nanoparticles, respectively.

Fig. 14 is a three-dimensional image of theoretical calculated RL values at different filling rates.

Fig. 15 is a TEM image of SP24.

Fig. 16 is a reflection loss curve where the minimum value of reflection loss of SP6, SP12, SP18 and SP24 is located.

FIG. 17 is a graph of sample at different frequenciesThe impedance matching value Z (z= |z) _in /Z ₀ I), wherein the (a) to (e) graphs represent samples S6, S12, S18, S24, and S36, and the (f) to (i) graphs represent samples SP6, SP12, SP18, and SP24.

Detailed Description

The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption and the preparation method thereof provided by the present application are further described below by way of examples, which are only some embodiments of the present application, but not all embodiments. Based on the above summary of the application, all embodiments obtained by a person of ordinary skill in the art without making any inventive effort are within the scope of the present application.

The information on the chemical reagents used in the following examples and comparative examples is shown in Table 1, and the information on the instruments and equipment used is shown in Table 2.

TABLE 1 chemical reagent information

Table 2 instrument and device information

Example 1

In this example, a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption (SiO ₂ @SiO ₂ Carbon@carbon), as follows:

dripping tetraethyl orthosilicate (TEOS) into ethanol-water-ammonia water mixed solution, stirring for 15min, adding resorcinol and formaldehyde with concentration of 37wt% into the obtained reaction solution, stirring at 30 ℃ for reaction for 6h, centrifugally separating to obtain a reaction product, washing the reaction product with deionized water and ethanol for three times respectively, vacuum drying at 60 ℃ for 12h, carbonizing the dried reaction product in a tube furnace at 700 ℃ for 1h under vacuum condition to obtain a final product SiO ₂ @SiO ₂ Carbon@carbon, and designated S6.

The ethanol-water-ammonia water mixed solution is formed by mixing ethanol, deionized water and 25wt% ammonia water according to the volume ratio of 70:10:3, and the volume ratio of TEOS to the ethanol-water-ammonia water mixed solution is 1:166; resorcinol is added in a proportion of 0.4g resorcinol per 1mL TEOS; formaldehyde was added in a ratio of TEOS to formaldehyde of 1:0.56 by volume.

Example 2

To compare the differences in the products prepared at different reaction times, the present example prepared different products at different reaction times.

The operation of this example was substantially the same as in example 1, except that the stirring reaction time at 30℃was 0.5, 1.5, 3, 9, 12, 18, 24 and 36 hours, respectively, and the prepared products were designated as S0.5, S1.5, S3, S9, S12, S18, S24 and S36, respectively.

Comparative example 1

In this comparative example, siO was prepared ₂ @SiO ₂ /Carbon。

The operation of this comparative example was substantially the same as in example 1, except that Tetrapropoxysilane (TPOS) was used as a silicon source instead of tetraethyl orthosilicate (TEOS), the stirring reaction times at 30℃were controlled to be 6, 12, 18 and 24 hours, respectively, and the prepared products were designated as SP6, SP12, SP18 and SP24, respectively.

Comparative example 2

In this comparative example, solid SiO was prepared ₂ And (3) nanoparticles.

UsingPreparation of SiO by the method ₂ And (3) nanoparticles. 100mL of ethanol and 8mL of aqueous ammonia (25 wt% concentration) were added to the flask, and then a mixed solution of 8mL of TEOS and 20mL of ethanol was added to the flask, and the mixture was stirred at 40℃for reaction for 6 hours. Centrifuging, collecting the obtained product, washing with water and ethanol for 3 times, and vacuum drying to obtain solid SiO ₂ And (3) nanoparticles.

Comparative example 3

In this comparative example, solid carbon nanoparticles were prepared.

The operation of this comparative example was substantially the same as in example 1, except that TEOS was not added.

Example 3

In this example, siO was discussed in conjunction with experimental data ₂ @SiO ₂ Growth process of/carbon@carbon.

SiO ₂ @SiO ₂ The formation of carbon@carbon begins with the hydrolysis of TEOS, then resorcinol and formaldehyde are polymerized in alkaline solution, after a certain time of reaction, the reaction product is separated, washed and dried, and the dried reaction product is carbonized under the vacuum condition of 700 ℃.

Fig. 1 is a TEM image and a particle diameter distribution chart of S0.5, S1.5, and S3, wherein (a 1) (a 2) is a TEM image and a particle diameter distribution chart of S0.5, (b 1) (b 2) is a TEM image and a particle diameter distribution chart of S1.5, and (c 1) (c 2) is a TEM image and a particle diameter distribution chart of S3. Fig. 2 (a) to (c) and (d) to (f) are SEM and TEM images of S6 and S36, respectively, and fig. 2 (g) to (k) and (l) to (p) are element distribution diagrams of S6 and S36, respectively. The average diameters of S0.5, S1.5, S3, S6 and S36 were counted, and the results are shown as the average diameters of the samples before calcination in Table 3.

TABLE 3 average diameter of samples before and after calcination

As is clear from FIGS. 1 to 2 and Table 3, when the reaction time was 0.5h, spherical nanoparticles having an average diameter of 85.5nm were formed by carbonization. When the reaction time was 1.5 hours, the average diameter of the nanoparticles gradually increased to 97.8nm. When the reaction time was increased to 3h, the average diameter of the nanoparticles was increased to 118.7nm, compared to S1.5, and a rough outer surface appeared. The TEM image of fig. 2 shows that when the reaction time is 6h, a core-shell structure has been significantly produced, the surface of S6 and the core-shell boundary are relatively irregular, while the surface of S36 is smoother, and a more pronounced core-shell boundary occurs.

The distribution of the Core diameter and Particle diameter of S6 and S36 was counted, and as a result, as shown in fig. 3, two graphs (a) and (b) of fig. 3 are respectively diameter distribution diagrams of the Core and the nanoparticle of S6, and two graphs (c) and (d) of fig. 3 are respectively diameter distribution diagrams of the Core and the nanoparticle of S36. The average diameter of the S6 nano-particles is 157.9nm, and the average diameter of the inner core is 136.7nm. The average diameter of the S36 nano-particles is 237.9nm, and the average diameter of the inner core is 194.6nm.

As can be seen from the element distribution diagram of fig. 2, the C element is uniformly distributed throughout the nanoparticle of S6, while the Si and O elements are mainly distributed in the inner core of the particle, and a small amount of Si element is present in the outer shell of the nanoparticle. The above results indicate that when the reaction time is 6h, a core-shell structure has been formed. The elemental profile of S36 is substantially identical to S6, except that there is little Si element in the shell of S36.

To further investigate the internal structure of the nanoparticles, S0.5, S1.5, S6 and S36 were calcined in an air atmosphere to remove carbon therein, and finally pure silica particles were obtained. The TEM images and the diameter distribution diagrams of the silica particles obtained after calcination are shown in fig. 4, the (a 1) to (d 1) diagrams of fig. 4 are TEM images of S0.5, S1.5, S6 and S36, respectively, after calcination, and the (a 2) to (d 2) diagrams of fig. 4 are particle diameter distribution diagrams of S0.5, S1.5, S6 and S36, respectively, after calcination. As can be seen from FIG. 4, the core obtained after calcination is spherical. As shown in Table 3, the average diameter of S0.5 after calcination was 85.1nm, while the average diameters of S1.5, S6 and S36 after calcination were substantially the same and were all around 89 nm. After calcination, S6 and S36 appear as a large number of fragments around the core, and comparing TEM images before and after calcination with average diameter data, it was found that the diameter of S6 and S36 after calcination is far different from the size of the black core in FIG. 2, indicating that the deep black core in FIG. 2 is not entirely silica particles, and that there should be a silica core in the center of the deep black portion. Therefore, the deep black "core" portion of FIG. 2 should be composed of an inner core of silica having an average diameter of about 89nm initially formed by the reaction and a carbon-silica heterojunction layer surrounding the inner core.

Thus, S6 nanoparticles with an average diameter of 157.9nm are divided into three parts from the core to the shell: an inner core composed of silica having an average diameter of 89nm, an intermediate layer composed of silica and carbon having an average thickness of 23.9nm, and an outer shell composed mainly of carbon having an average thickness of 10.6 nm. Similarly, S36 nanoparticles with an average diameter of 237.9nm are also divided into three parts from the core to the shell: an inner core composed of silica having an average diameter of 89nm, an intermediate layer composed of silica and carbon having an average thickness of 52.8nm, and an outer shell composed mainly of carbon having an average thickness of 21.6 nm.

After the silica core is formed, the size of the core is substantially unchanged with the extension of the reaction time, while the thickness of the intermediate layer and the shell is gradually increased. Similarly, in combination with the particle diameter data of S1.5 before and after calcination, it was found that S1.5 had a silica core with an average diameter of about 89nm and a carbon-silica heterojunction layer with an average thickness of about 4.5nm, indicating that the silica core was fully formed when the reaction time was 1.5 h. The carbon-silica heterojunction layer then becomes the primary framework structure as resorcinol and formaldehyde polymerize. Thus, the large amount of fragments that appear at the post-calcination surfaces of S6 and S36 should be from the silica that remains after calcination of the carbon-silica heterojunction layer.

To study the growth kinetics of nanoparticles, thermogravimetric curves of S0.5, S1.5, S3, S6, S9, S12, S18, S24 and S36 were tested in an air atmosphere, as shown in fig. 5 (a). As can be seen from fig. 5 (a), there was little change in the weight of S0.5 and S1.5 during the thermogravimetric test, indicating that the main product formed during the first 1.5h of the reaction process was silica with little accumulation of carbon precursor. While the weight of S3, S6, S9, S12, S18, S24 and S36 all decreased dramatically between 500 and 700 c, indicating that after 1.5 hours of the reaction process, the carbon precursor began to deposit on the silica core. This is also reflected in the TEM image of fig. 1, where the roughened surface of S3 should be caused by carbon formed after carbonization of the precursor of carbon deposited on the particle surface, whereas the TEM images of S0.5 and S1.5 show that the surfaces of both are relatively smooth.

The cumulative amount and average growth rate of carbon and silica at different reaction times were calculated based on thermogravimetric data, and the results are shown in fig. 5 (b) (c) and table 4. The average growth rate of silica from 73.9mg/h drops sharply to 5.88mg/h from the initial stage to about 6h, and then remains at a very low growth rate up to 24h. The growth rate of carbon showed a tendency to gradually increase during the first 6 hours of the reaction and then decrease with the lapse of the reaction time, and reached the highest average growth rate of about 9.45mg/h in a period of 3 to 6 hours. Notably, 98.3wt% silica and 51.2wt% carbon had been deposited within 6 hours of reaction. After 6 hours of reaction, the growth rate of silica was only about 0.1mg/h, while the growth rate of carbon was much faster, with growth rates of 3.9 to 1.4mg/h during the 6 to 24 hour period. This means that the carbon deposition rate is at least 10 times higher than that of silica when the reaction time is in the range of 6 to 24 hours. The growth rate of both silica and carbon decreased to near zero during the final 24-36 h period of the reaction, indicating that particle growth was substantially complete at 24h reaction time.

TABLE 4 average growth rate of carbon and silica over different time periods

Based on the above, it can be seen that the SiO of the present application ₂ @SiO ₂ The dynamic growth process of/carbon@carbon is as follows: during the first 1.5h of the reaction process, the reaction system mainly comprises the rapid nucleation and growth of silicon dioxide due to the hydrolysis reaction of tetraethyl orthosilicate which occurs at a relatively rapid speed. During this time, the growth rate of the silica is significantly greater than that of the carbon precursor (phenolic resin), so that SiO ₂ @SiO ₂ The main component of the core of the/carbon@carbon is silica. The hydrolysis rate of tetraethyl orthosilicate decreases rapidly as the reaction proceeds, while the polymerization rate of the carbon precursor increases gradually. After about 1.5h of reaction, the hydrolyzed formThe resulting silica particles are co-deposited with a precursor of polymerized carbon. As the reaction proceeds, the rate of formation of the carbon precursor is progressively greater than the growth rate of the silica. In particular, after 6 hours of reaction, the deposition rate of the carbon precursor was significantly increased compared to that of silicon dioxide, which resulted in the appearance of the crust structure. After carbonization, an inner core mainly composed of silica, an intermediate layer mainly composed of silica and carbon, and an outer shell mainly composed of carbon are formed.

Example 4

In this embodiment, for SiO ₂ @SiO ₂ The characterization is carried out by means of a/carbon@carbon.

The phase composition and structural state of the carbon in S6 and S36 were studied by X-ray diffraction (XRD) and raman spectroscopy. As shown in fig. 6, S6 and S36 have similar XRD diffraction characteristic peaks. The broad diffraction peak around 2θ=21.5° is due to the characteristic mixed peak of amorphous silica and the characteristic plane (002) of graphitized carbon. The peak appearing at 2θ=43.3° is related to the characteristic plane (101) of graphitized carbon. The Raman spectrum shown in FIG. 7 (a) shows that S6, S12, S18, S24 and S36 are at 1330cm ^-1 And 1590cm ^-1 Two prominent peaks are shown on the left and right, corresponding to the D and G bands of carbon material, respectively. Band D is associated with disordered carbon hybridized by carbon sp3 and lattice defects of carbon atoms; the G-band is caused by stretching vibrations of sp2 bonds of graphitized carbon. The intensity ratio of D-band to G-band represents the graphitization degree of the carbon material. The intensity ratios (ID/IG ratios) of the D bands to the G bands of S6, S12, S18, S24, and S36 were 0.77, 0.82, 0.78, and 0.79, respectively. These similar ID/IG ratios indicate that they have similar degrees of graphitization. A suitable degree of graphitization is advantageous for electron transport and dielectric loss enhancement.

S6 and S36 were characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) detection. As shown in FIG. 7 (b), S6 and S36 have similar infrared spectra, at 471cm ^-1 The peak of (C) is caused by flexural vibration of O-Si-O and tensile vibration of hydroxyl group, and is located at 3446cm ^-1 The peak of (C) is the stretching vibration of the hydroxyl group, which is located at 1111cm ^-1 And 801cm ^-1 Is characterized by two distinct peaks and SiO ₂ The asymmetry of Si-O-Si is related to the symmetrical stretching vibration and is located at 1626cm ^-1 The energy band of the left and right is related to the stretching vibration of the C=O group, 2346cm ^-1 The nearby energy bands correspond to the stretching vibration of the c—o group. The full spectrum of XPS spectra of S6 and S36 is shown in figure 8 (a), the C1S spectrum of S6 and S36 is shown in figure 8 (b), wherein 284eV, 532eV, 153eV and 103eV are characteristic peaks of C1S, O1S, si 2S and Si2p respectively, and the C1S spectrum of S6 shows three main peaks at 284.6eV, 285.4eV and 288.2eV respectively related to C-C, C-O and C=O bonds. S36 is highly similar to XPS spectra of S6. The hydroxyl and carbonyl oxygen-containing polar functional groups remain in carbon after carbonization of the phenolic resin, and the oxygen-containing polar functional groups including the hydroxyl and carbonyl groups can be used as polarization centers to induce dielectric polarization, thereby facilitating the absorption of electromagnetic waves by the material.

FIG. 9 is a low temperature nitrogen adsorption-desorption isothermal curve of S6 and S36, wherein S-6h and S-36h represent samples S6 and S36, respectively, and irregular voids are found in S6 and S36. From S6 to S36, the relative content of the amorphous phenolic resin polymer is increased by prolonging the reaction time, so that the specific surface area of the sample is 104.17m ² Increase/g to 149.71m ² And/g. However, as the reaction time was prolonged, the pore volume of the nanoparticles was from 0.210cm ³ The/g is reduced to 0.155cm ³ And/g, showing that in the growth process of the nano-particles, partial pore channels are filled with subsequent growth products, so that the structure of the nano-particles is more compact.

Example 5

In this example, siO was tested ₂ @SiO ₂ Wave absorbing properties of/carbon@carbon.

1. Electromagnetic parameters

On the one hand, excellent impedance matching is required for the electromagnetic wave absorbing material with excellent performance so that more electromagnetic waves can enter the interior of the material; on the other hand, it is required to have good attenuation characteristics in order to consume the incoming electromagnetic waves to the maximum.

The attenuation characteristics of electromagnetic wave absorbing materials are closely related to the complex permittivity and complex permeability of the electromagnetic wave absorbing materials themselves. The carbon material belongs to a dielectric loss type wave absorbing material, the real part epsilon 'of the dielectric constant of the carbon material represents the storage capacity of the material to electromagnetic wave energy, the imaginary part epsilon' corresponds to the electromagnetic wave energy loss capacity, and the magnetic loss is negligible. The dielectric loss tangent (tan δ=epsilon "/epsilon') generally represents the dielectric loss capacity of an electromagnetic wave absorbing material. Materials having excellent absorption capacity in the frequency range of 2 to 18GHz generally have wide practical applicability, and thus electromagnetic wave absorption properties of S1.5, S3, S6, S12, S18, S24 and S36 are tested here in this frequency range.

Uniformly mixing the sample and paraffin in a mass ratio of 1:1, preparing a ring shape with an outer diameter of 7mm and an inner diameter of 3mm, and measuring electromagnetic parameters. Measuring complex relative permittivity (. Epsilon.) of sample in 2-18 GHz frequency range by coaxial line method using vector network analyzer _r =ε' -iε ") and permeability (μ) _r ＝μ'-iμ”)。

S1.5, S3, S6, S12, S18, S24 and S36, and calculated RL _min The reflection loss curves are shown in fig. 10 (a) to (d), respectively. As can be seen from fig. 10, the complex dielectric constant (including the real part and the imaginary part) of S1.5 is almost zero in the test frequency range, because the reaction time is too short, the carbon-silicon heterojunction layer cannot be formed on the surface of the silicon dioxide core particle, and the complex dielectric constant value of pure silicon dioxide is very small. When the reaction time reaches more than 3 hours, a carbon-silicon heterojunction layer is formed due to carbon accumulation, and the complex dielectric constant is obviously increased. This is consistent with thermogravimetric analysis. Notably, the complex dielectric constants of S3, S6, S12, S18, S24, and S36 exhibit a decreasing trend between 2 and 18GHz, which is related to the hysteresis response of dipole polarization in carbon materials with frequency variation in the gigahertz range. Of all samples, the dielectric loss tangent of S6 was highest in the frequency range of 2-18 GHz, indicating that S6 has the strongest dielectric loss.

2. Reflection loss

The electromagnetic wave absorption performance of the material can be evaluated by a reflection loss value (RL) calculated based on the transmission line theory. From the acquired complex permittivity and complex permeability data, RL values for each sample were calculated at a given absorber thickness by the following formula.

In the formulae (1) to (2), Z _in Represents the input impedance of the absorbent, Z ₀ Is the impedance of free space, f is the frequency of the electromagnetic wave, d is the sample thickness, c is the velocity of the electromagnetic wave in vacuum, ε _r (ε _r =ε "- ε') and μ _r (μ _r The =μ "- μ') corresponds to the complex permittivity and complex permeability of the absorber, respectively.

Typically, more than 90% of the electromagnetic wave energy is absorbed when the RL value is below-10 dB. The frequency range covered by the absorber with all RL values below-10 dB at a certain specific thickness is the effective absorption bandwidth.

The graphs (a) - (f) of FIG. 11 are three-dimensional images of theoretical calculated RL values of S3, S6, S12, S18, S24 and S36, respectively, with S1.5 having little ability to absorb electromagnetic waves, and S3' S RL _min The value was-26.6 dB, corresponding to a thickness of 1.8mm for the absorber. Notably, when the reaction time is prolonged to 6h, the absorption performance of the material is greatly improved, and when the thickness of S6 is 3.8mm, RL _min Is-56.3 dB; and S6 shows the widest effective bandwidth of 5.15GHz (12.48-17.63 GHz). RL of S12 and S18 _min The values were-48.2 dB and-51.0 dB, respectively. S24 has the strongest absorption peak, RL _min The value is-60.4 dB, and the thickness of the corresponding absorbent is 1.5mm; RL of S36 _min The value was-58.7 dB, with a corresponding thickness of 2.4mm. The above data indicate that nanoparticles having formed core-shell structures exhibit excellent electromagnetic wave absorption properties when the reaction time is 6h or more.

Further, the effective absorption bandwidths of the samples S6 of different thicknesses in the frequency range of 2 to 18GHz are as shown in fig. 12 (a), and by changing the thickness of S6, the effective absorption bandwidths of S6 almost cover C, X and Ku bands (4.76 to 18 GHz). S6 has the shortest synthesis time, relatively excellent electromagnetic wave absorption performance and is very beneficial to industrial application.

To evaluate the attenuation ability of a material for an incident electromagnetic wave, we calculate attenuation constants according to the attenuation value formula (3), the attenuation constants of S1.5, S3, S6, S12, S18, S24 and S36 in the frequency range of 2 to 18GHz are shown in the graph (b) of fig. 12.

In general, a larger value of the attenuation constant α means that the material has a stronger electromagnetic wave attenuation capability, and as shown in fig. 12 (b), S1.5 has a minimum value of α, indicating that the attenuation capability is weaker. Starting from S3, a significant increase in damping capacity occurs. When the reaction time reaches 6h or more, the attenuation constant value of the material further rises and is more similar to the corresponding wave band, and the similar and efficient electromagnetic wave attenuation capability is shown. Fig. 13 (a) and (b) are two graphs of complex permittivity values of the solid silica prepared in comparative example 2 and the solid carbon nanoparticle prepared in comparative example 3, respectively, and fig. 13 (c) and (d) are three-dimensional images of theoretical RL values of the solid silica and the solid carbon nanoparticle, respectively. As can be seen from fig. 13, the solid silica silicon spheres have almost no absorption performance compared to S6; RL of solid carbon nanoparticles _min The value is only-20.7 dB at 12.9GHz and the thickness is 6.0mm, which is far different from the wave absorbing capability of S6.

The content of the absorber in the paraffin matrix significantly affects the performance of the electromagnetic wave absorbing material, and in order to examine the effect of the material filling rate on the electromagnetic wave absorbing performance, the electromagnetic wave absorbing performance of S24 at filling rates of 10wt%, 30wt%, 50wt% and 70wt% was tested, and the results are shown in fig. 14. At a filling rate of 10wt%, the electromagnetic wave absorption performance of the sample is poor, RL _min The value was-23.51 dB. As the filling rate increases, the electromagnetic wave absorption performance of the sample is significantly improved. At filling rates of 30wt% and 50wt%, RL _min Respectively-47.18 dB and60.38dB. However, at a fill level of 70wt%, RL _min Only-15.57 dB due to the impedance mismatch caused by the too high fill rate. Thus, the optimum filling ratio of the material in paraffin wax is around 50 wt%.

Example 6

In this example, siO prepared in comparative example 1 ₂ @SiO ₂ Carbon@Carbon and SiO prepared in comparative example 1 ₂ @SiO ₂ Wave absorbing properties of the Carbon.

Comparative example 1A SiO was prepared in the same manner using TPOS having a slower hydrolysis rate than TEOS as a silicon source ₂ @SiO ₂ In the Carbon, a TEM image of SP24 is shown in FIG. 15, and FIG. 15 shows that SP24 is also a core-shell structure. With reference to the method in example 5, the RL of SP6, SP12, SP18 and SP24 was tested _min The values, the minimum value of the reflection loss, and the reflection loss curve are shown in FIG. 16, their RL _min The values were-20.7 dB, -37.4dB, -47.9dB and-45.1 dB in that order.

In the reaction process of comparative example 1, silica formed by slow hydrolysis of phenolic resin and TPOS is continuously co-deposited on the surface of the silica core, so that the carbon-silica coexisting layer is continuously thickened, which is beneficial to improving the electromagnetic wave absorption performance of the material. Thus, RL of the product in FIG. 16 _min The reaction time is prolonged and is continuously increased until the reaction time reaches 18h, and the reaction time is not obviously increased.

However, in comparison with examples 1 and 2, comparative example 1 uses TPOS as the SiO prepared as the silicon source ₂ @SiO ₂ The effective absorption bandwidth of/Carbon is narrower and the electromagnetic wave absorption performance below 14GHz is weaker. For further analysis reasons we calculated the impedance match values (|z) for each frequency region of the product at different thicknesses _in /Z ₀ I) and plotted as a two-dimensional contour plot, the result is shown in fig. 17, fig. 17 being the impedance matching value Z (z= |z) of the sample at different frequencies _in /Z ₀ I), wherein the (a) to (e) graphs represent samples S6, S12, S18, S24, and S36, respectively, and the (f) to (i) graphs represent samples SP6, SP12, SP18, and SP24, respectively. Typically an impedance match value between 0.8 and 1.2 means at this frequencyThe lower corresponding thickness of the absorber has good impedance matching.

As can be seen from FIG. 17, siO is contained in the composition ₂ @SiO ₂ Compared with carbon@carbon, siO ₂ @SiO ₂ Carbons have more severe impedance mismatch areas (red part of the figure), indicating that a large number of electromagnetic waves are in SiO ₂ @SiO ₂ the/Carbon sample surface is reflected rather than absorbed. Slow hydrolysis of TPOS forms a thicker carbon-silicon heterojunction layer as the shell, while fast hydrolysis of TEOS tends to form a carbon-silicon heterojunction intermediate layer and an outermost carbon shell, reflection loss and impedance matching graphs show SiO ₂ @SiO ₂ The carbon@carbon is more beneficial to the absorption and impedance matching of electromagnetic waves. The above results indicate that the proper heterojunction layer and carbon shell play an important role in absorbing electromagnetic waves by the silicon-carbon material.

Example 7

dripping TEOS into ethanol-water-ammonia water mixed solution, stirring for 30min, adding resorcinol and formaldehyde with concentration of 37wt% into the obtained reaction solution, stirring at 20 ℃ for reaction for 10h, centrifugally separating to obtain a reaction product, washing the reaction product with deionized water and ethanol for three times respectively, vacuum drying at 60 ℃ for 12h, and carbonizing the dried reaction product in a tube furnace at 700 ℃ for 1.2h under vacuum condition to obtain the final product SiO ₂ @SiO ₂ /Carbon@Carbon。

The ethanol-water-ammonia water mixed solution is formed by mixing ethanol, deionized water and 25wt% ammonia water according to the volume ratio of 60:5:2, and the volume ratio of TEOS to the ethanol-water-ammonia water mixed solution is 1:160; resorcinol is added in a proportion of 0.3g resorcinol per 1mL TEOS; formaldehyde was added in a ratio of TEOS to formaldehyde of 1:0.4 by volume.

Example 8

In this example, a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption (SiO ₂ @SiO ₂ /Carbon@CCarbon), steps are as follows:

dripping TEOS into ethanol-water-ammonia water mixed solution, stirring for 20min, adding resorcinol and formaldehyde with concentration of 37wt% into the obtained reaction solution, stirring and reacting for 8h at 35 ℃, centrifugally separating to obtain a reaction product, washing the reaction product with deionized water and ethanol for three times respectively, vacuum drying at 60 ℃ for 12h, and carbonizing the dried reaction product in a tube furnace at 800 ℃ for 0.8h under vacuum condition to obtain the final product SiO ₂ @SiO ₂ /Carbon@Carbon。

The ethanol-water-ammonia water mixed solution is formed by mixing ethanol, deionized water and 25wt% ammonia water according to the volume ratio of 80:15:4, and the volume ratio of TEOS to the ethanol-water-ammonia water mixed solution is 1:170; resorcinol is added in a proportion of 0.5g resorcinol per 1mL TEOS; formaldehyde was added in a ratio of TEOS to formaldehyde of 1:0.7 by volume.

Claims

1. The carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption is characterized in that the composite nano material is nano particles with a core-shell structure, wherein the nano particles are composed of a core, an intermediate layer wrapping the core and a shell wrapping the intermediate layer, the main component of the core is silicon dioxide, the intermediate layer is a heterojunction layer composed of silicon dioxide and carbon, and the main component of the shell is carbon; the composite nano material has a pore structure; the particle size of the composite nano material is 150-240 nm; in the composite nanomaterial, a silicon dioxide inner core is spherical, the thickness of an intermediate layer is 20-55 nm, and the thickness of an outer shell is 10-25 nm; the phase composition of carbon in the composite nano material is graphitized carbon and disordered carbon, graphitized carbon is used as a main component, and the strength ratio of D band to G band in the Raman spectrum of the composite nano material is 0.77-0.82;

the preparation method of the composite nano material comprises the following steps:

dripping tetraethyl orthosilicate into an ethanol-water-ammonia water mixed solution, stirring for 15-30 min, adding resorcinol and formaldehyde into the obtained reaction solution, stirring for reaction for 6-12 h at 20-35 ℃, separating out a reaction product, washing, drying, and carbonizing for 0.8-1.2 h under a vacuum condition at 700-800 ℃ to obtain the carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption;

the ethanol-water-ammonia water mixed solution is formed by mixing ethanol, water and ammonia water according to the volume ratio of (60-80): (5-15): (2-4); the volume ratio of the tetraethyl orthosilicate to the ethanol-water-ammonia water mixed solution is 1 (160-170); adding resorcinol according to the proportion of adding 0.3-0.5 g of resorcinol into each 1mL of tetraethyl orthosilicate; and adding formaldehyde according to the volume ratio of tetraethyl orthosilicate to formaldehyde of 1 (0.4-0.7).

2. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 1, wherein the carbon of the composite nanomaterial further contains an oxygen-containing polar functional group including a hydroxyl group and a carbonyl group.

3. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 1, characterized in that the specific surface area of the composite nanomaterial is 100-150 m ² /g。