CN114772606A

CN114772606A - Carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption and preparation method thereof

Info

Publication number: CN114772606A
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: 2022-07-22
Anticipated expiration: 2042-03-10
Also published as: CN114772606B

Abstract

The invention provides a preparation method of a carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption, which comprises the steps of dropwise adding tetraethyl orthosilicate into an ethanol-water-ammonia water mixed solution, fully stirring, adding resorcinol and formaldehyde, stirring at 20-35 ℃ for reaction for 6-36 hours, separating out a reaction product, washing, drying, and finally fully carbonizing at 700-800 ℃ under a vacuum condition to obtain the carbon-silicon dioxide core-shell composite nano material. The core-shell composite nano material prepared by the method is a nano particle with a core-shell structure, which consists of an inner core, an intermediate layer wrapping the inner core and a shell wrapping the outer layer of the intermediate layer, wherein the main component of the inner core is silicon dioxide, the intermediate layer is a heterojunction layer consisting of silicon dioxide and carbon, and the main component of the shell is carbon. The invention can effectively simplify the synthesis process of the composite nano material on the basis of ensuring the excellent electromagnetic wave absorption performance of the composite nano material and increase the industrial applicability of the preparation process.

Description

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

Technical Field

The invention belongs to the field of electromagnetic wave absorption 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 harm of electromagnetic pollution to electronic devices and human bodies is becoming more serious. Therefore, electromagnetic wave absorbing materials are receiving more and more attention from researchers, and such materials can absorb electromagnetic waves and convert them into heat energy or other forms of energy. According to the 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 taken as a typical electromagnetic wave absorption material and has the advantages of low density, wide source, adjustable chemical property, good electronic conductivity and the like. In order to improve impedance matching of materials and enhance electromagnetic wave absorption capability of the materials, carbon nanocomposites have also been extensively studied, and the preparation of carbon-based composites using magnetic substances, metal oxides, metal sulfides, semiconductor materials, and the like in combination with carbon materials has been reported. The core-shell structure is commonly used for carbon-based composite materials due to the advantages of rich interface polarization, good chemical uniformity, limiting effect and the like, for example, Liang et al prepares SiC @ C core-shell structure nanoparticles, Wang et al synthesizes SiC/rGO core-shell structure materials, and proves that the core-shell structure is 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 design of the material is complex, the synthesis process is difficult, the synthesis time is long, the multilayer core-shell structure often needs multi-step coating operation, and the preparation process is often accompanied with harsh reaction conditions such as high temperature and high pressure. The synthesis of the carbon-based core-shell material reported in the existing literature usually requires one day or more, and the synthesis cost of the material is increased due to the excessively long synthesis time. 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 urgently in the preparation of the high-performance carbon-based core-shell wave absorbing material.

Yu et al disclose that core-shell type carbon-silicon particles are synthesized using silica primary particles as a template using a one-pot method in the absence of a surfactant, and then mesoporous hollow carbon is prepared by etching. Ji et al have demonstrated that the mesoporous carbon hollow carbon can achieve a wave-absorbing performance of-50.9 dB at a filling rate of 20%. However, this method requires highly corrosive hydrofluoric acid or sodium hydroxide to be used when removing the silica template, and its application in practical production is limited for safety reasons. Therefore, if a method with a simpler synthetic route, shorter synthetic time and higher safety of the synthetic process can be developed to prepare the carbon-based core-shell material with excellent electromagnetic wave absorption performance, the method will have a positive effect on promoting the practical application of the carbon-based core-shell material for electromagnetic wave absorption.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption and a preparation method thereof, so as to effectively simplify the synthesis process of the composite nanomaterial and increase the industrial applicability of the preparation process on the basis of ensuring that the composite nanomaterial has excellent electromagnetic wave absorption performance.

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

a carbon-silicon dioxide nuclear shell composite nano material for electromagnetic wave absorption is a nano particle with a nuclear shell structure, which consists of an inner core, an intermediate layer wrapping the inner core and a shell wrapping the intermediate layer, wherein the inner core mainly comprises silicon dioxide, the intermediate layer is a heterojunction layer consisting of silicon dioxide and carbon, and the shell mainly comprises carbon; the nano composite material has a pore structure.

In the 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 inner core of the silicon dioxide in the composite nano material is spherical, the thickness of the middle layer is 20-55 nm, and the thickness of the shell is 10-25 nm.

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

In the technical scheme of the carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption, 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 nano material for electromagnetic wave absorption, the specific surface area of the composite nano material is 100-150 m²/g。

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

dropwise adding tetraethyl orthosilicate into an ethanol-water-ammonia water mixed solution, stirring for 15-30 min, adding m-dihydroxybenzene and formaldehyde into the obtained reaction solution, stirring and reacting for 6-36 h at 20-35 ℃, separating a reaction product, washing, drying, and finally fully carbonizing at 700-800 ℃ under a 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 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 every 1mL of tetraethyl orthosilicate; the formaldehyde is preferably added according to the volume ratio of tetraethyl orthosilicate to formaldehyde of 1 (0.4-0.7).

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

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 25-28 wt% of ammonia water, and the adopted formaldehyde is 37-40 wt% of formaldehyde.

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

The application shows that the ability of the composite nano material to absorb electromagnetic waves is represented through experiments, and the composite nano material is foundHas good impedance matching and excellent electromagnetic wave attenuation capability. In particular, the composite nanomaterial prepared when the reaction time is 6 hours shows particularly excellent electromagnetic wave absorption capacity, RL at an absorber thickness of 3.8mm and a frequency of 6.1GHz_minThe value is-56.28 dB, and the maximum effective absorption bandwidth of 5.15GHz occurs at an absorber thickness of 1.8 mm.

The inventor of the present application has conducted experimental research on the mechanism of the composite nanomaterial for absorbing electromagnetic waves in the research process, and found that the main reasons why the composite nanomaterial has excellent electromagnetic wave absorption performance are as follows: first, graphitized carbon (sp2) forms microcurrents between the carbon layer network in the presence of an alternating electric field, which facilitates electron transport. While disordered carbon (sp3) can act as a resistor, converting electrical energy into heat energy, creating conduction losses. Secondly, the carbon formed by the carbonization of the phenolic polymer contains a large number of defects and residual oxygen-containing functional groups, such as hydroxyl and carbonyl groups, which have been confirmed by infrared spectroscopy and XPS spectroscopy, and these defects and oxygen-containing functional groups will act as polarization centers, and the resulting dipole polarization can enhance the dielectric loss capability of the material. Finally, the composite nanoparticle possesses a carbon layer shell and a carbon-silicon intermediate layer with a large number of carbon-silicon heterointerfaces as part of the dielectric loss, which enhance the Maxwell-Wagner effect and relaxation-related interfacial polarization.

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

1. the carbon-silicon dioxide core-shell composite nano material for electromagnetic wave absorption provided by the invention is a nano particle with a core-shell structure, which consists of an inner core, an intermediate layer wrapping the inner core and a shell wrapping the intermediate layer, wherein the inner core mainly comprises silicon dioxide, the intermediate layer is a heterojunction layer consisting of silicon dioxide and carbon, and the shell mainly comprises carbon. On one hand, the phase composition graphitized carbon in the composite nanometer material is the main component and contains disordered carbon, when the graphitized carbon exists in an alternating electric field, microcurrent is formed among carbon layer networks, so that the electronic transmission is facilitated, and the disordered carbon can be used as a resistor to convert electric energy into heat energy, so that the 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 dipole polarization can enhance the dielectric loss capacity of the material; more importantly, the composite nanoparticle possesses a carbon shell and a carbon-silicon interlayer that contain a large number of carbon-silicon heterointerfaces that enhance the Maxwell-Wagner effect and relaxation-related interfacial polarization as part of the dielectric loss. Through the combined action of multiple factors such as structure, component composition, phase composition and the like, the composite nano material provided by the invention has good electromagnetic wave absorption performance.

2. Experiments prove that the composite nano material provided by the invention has good impedance matching and excellent electromagnetic wave attenuation capability. In particular, the composite nanomaterial having a coarse outer shape prepared when the reaction time is 6 hours shows particularly excellent electromagnetic wave absorption ability, RL at an absorber thickness of 3.8mm and a frequency of 6.1GHz_minThe 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 1.8 mm.

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

Drawings

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

The images (a) to (c) and (d) to (f) in fig. 2 are SEM and TEM images of S6 and S36, respectively, and the distributions (g) to (k) and (l) to (p) in fig. 2 are element distributions of S6 and S36.

The two graphs (a) and (b) of fig. 3 are the diameter distribution graphs of the core and the nanoparticle of S6, respectively, and the two graphs (c) and (d) of fig. 3 are the diameter distribution graphs 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 thermogravimetric plot of S0.5, S1.5, S3, S6, S9, S12, S18, S24 and S36 (plot (a)), the weight distribution of the carbon and silicon dioxide components under different reaction time conditions (plot (b)), and the growth rate of the silicon dioxide and carbon components under different growth time conditions (plot (c)).

Figure 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.

The (a) diagram of fig. 8 is XPS spectrum full spectrum of S6 and S36, and the (b) diagram of fig. 8 is C1S spectrum of S6 and S36.

Fig. 9 is the low temperature nitrogen adsorption-desorption isotherm curves of S6 and S36.

FIGS. 10 (a) to (d) are graphs in which real parts, imaginary parts, dielectric loss tangent values of complex permittivities of S1.5, S3, S6, S12, S18, S24 and S36 and RL calculated_minThe reflection loss curve.

The diagrams (a) to (f) of fig. 11 are three-dimensional images of theoretically calculated RL values of S3, S6, S12, S18, S24, and S36, respectively.

FIG. 12 (a) is a graph showing effective absorption bandwidths of the samples S6 with different thicknesses in the frequency range of 2 to 18GHz, and FIG. 12 (b) is a graph showing attenuation constants of S1.5, S3, S6, S12, S18, S24 and S36 in the frequency range of 2 to 18 GHz.

The graphs (a) and (b) of fig. 13 are complex dielectric constant values of solid silica and solid carbon nanoparticles, respectively, and the graphs (c) and (d) of fig. 13 are three-dimensional images of theoretical RL values of solid silica and solid carbon nanoparticles, respectively.

FIG. 14 is a three-dimensional image of theoretically calculated RL values at different fill ratios.

Fig. 15 is a TEM image of SP 24.

Fig. 16 is a reflection loss curve on which the reflection loss minimum values of SP6, SP12, SP18, and SP24 are located.

Fig. 17 shows the impedance matching value Z (Z ═ Z) of the sample at different frequencies_in/Z₀|) 2D contour plots, in which plots (a) to (e) represent samples S6, S12, S18, S24 and S36, and plots (f) to (i) represent samples SP6, SP12, SP18 and SP 24.

Detailed Description

The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption and the preparation method thereof provided by the present invention are further illustrated by the following examples, which are only a part of embodiments of the present invention, but not all embodiments. Based on the above disclosure of the present invention, those skilled in the art can implement all the embodiments without creative efforts, which fall within the protection scope of the present invention.

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

Table 1 chemical agent information

TABLE 2 Instrument and Equipment information

Example 1

In this example, a carbon-silica core-shell composite nanomaterial (SiO) for electromagnetic wave absorption was prepared₂@SiO₂/Carbon @ Carbon), the procedure is as follows:

adding tetraethyl orthosilicate (TEOS) dropwise into the ethanol-water-ammonia water mixed solution, stirring for 15min, and reactingAdding resorcinol and 37 wt% formaldehyde into the solution, stirring at 30 deg.C for 6h, centrifuging to obtain reaction product, washing with deionized water and ethanol for three times, vacuum drying at 60 deg.C for 12h, carbonizing in tubular furnace at 700 deg.C for 1h to obtain final product SiO₂@SiO₂/Carbon @ Carbon and designated as S6.

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

Example 2

To compare the differences in the products produced at different reaction times, different products were produced at different reaction times in this example.

The operation of this example is essentially the same as example 1, except that the reaction times at 30 ℃ with stirring are 0.5, 1.5, 3, 9, 12, 18, 24 and 36h, respectively, and the products obtained are designated S0.5, S1.5, S3, S9, S12, S18, S24 and S36, respectively.

Comparative example 1

In this comparative example, SiO was prepared₂@SiO₂/Carbon。

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

Comparative example 2

In this comparative example, solid SiO was prepared₂And (3) nanoparticles.

Use of

Method for preparing SiO₂And (3) nanoparticles. 100mL of ethanol and 8mL of aqueous ammonia (concentration25 wt%) was added to the flask, and then a mixed solution of 8mL of TEOS and 20mL of ethanol was added to the beaker, and the reaction was stirred at 40 ℃ for 6 hours. Centrifuging to collect the product, washing with water and ethanol for 3 times, and vacuum drying to obtain solid SiO₂And (3) nanoparticles.

Comparative example 3

In this comparative example, solid carbon nanoparticles were prepared.

The operation of this comparative example is essentially the same as example 1, except that TEOS is not added.

Example 3

In this example, SiO was investigated in combination with experimental data₂@SiO₂Growth Process of/Carbon @ Carbon.

SiO₂@SiO₂The 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 and washed and dried, and then the dried reaction product is carbonized under the vacuum condition of 700 ℃.

Fig. 1 is a TEM image and a particle size distribution diagram of S0.5, S1.5 and S3, in which the (a1) (a2) diagram is a TEM image and a particle size distribution diagram of S0.5, (b1) (b2) diagram is a TEM image and a particle size distribution diagram of S1.5, and (c1) (c2) diagram is a TEM image and a particle size distribution diagram 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 elemental maps of S6 and S36, respectively. Statistics were made on the average diameters of S0.5, S1.5, S3, S6, and S36, and the results are shown as the average diameters of the samples before calcination in table 3.

TABLE 3 average diameter of sample before and after calcination

As can be seen 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 1.5h, the average diameter of the nanoparticles gradually increased to 97.8 nm. When the reaction time was increased to 3h, the average diameter of the nanoparticles 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 was 6h, a core-shell structure had clearly developed, the surface and core-shell boundaries of S6 were relatively irregular, while the surface of S36 was smoother and more distinct core-shell boundaries appeared.

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

As can be seen from the elemental distribution diagram of fig. 2, the C element is uniformly distributed throughout the nanoparticle of S6, whereas the Si and O elements are mainly distributed in the core of the particle, and a small amount of the Si element is present in the shell of the nanoparticle. The above results show that when the reaction time is 6h, a core-shell structure has been formed. The elemental profile of S36 substantially coincided with S6, except that there was almost no Si element in the shell of S36.

To further study the internal structure of the nanoparticles, S0.5, S1.5, S6, and S36 were calcined in an air atmosphere to remove carbon therefrom, 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 TEM images of S0.5, S1.5, S6 and S36 in fig. 4 are shown in (a1) to (d1) respectively, and the particle size distribution diagrams of S0.5, S1.5, S6 and S36 in fig. 4 are shown in (a2) to (d2) respectively. As can be seen from FIG. 4, the inner core obtained after calcination was 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, all at about 89 nm. S6 and S36 showed a large number of fragments around the core after calcination, and comparing TEM images and mean diameter data before and after calcination, it was found that the diameters of S6 and S36 after calcination were far from the size of the black core in fig. 2, indicating that the dark black core in fig. 2 is not completely a silica particle, and there should be a silica core in the center of the dark black part. The dark black "core" portion of fig. 2 should therefore be composed of an initially reacted silica core of about 89nm average diameter and a carbon-silica heterojunction layer encapsulating the core.

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

After the silica core is formed, the size of the core is substantially unchanged and the thicknesses of the intermediate layer and the shell are gradually increased as the reaction time is prolonged. Similarly, in combination with the particle diameter data of S1.5 before and after calcination, S1.5 was found to have 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 had been completely formed when the reaction time was 1.5 hours. The carbon-silica heterojunction layer then becomes the main framework structure as resorcinol polymerizes with formaldehyde. Therefore, the large amount of fragments of S6 and S36 appearing on the surface after calcination should be derived from the residual silicon dioxide of the carbon-silicon dioxide heterojunction layer after calcination for carbon removal.

To investigate the growth kinetics of the 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 (a) of fig. 5. As can be seen from the graph (a) of fig. 5, the weights of S0.5 and S1.5 hardly changed during the thermogravimetric test, indicating that the main product generated in the first 1.5h of the reaction process is silicon dioxide with almost no accumulation of carbon precursor. And the weight of S3, S6, S9, S12, S18, S24 and S36 all dropped sharply between 500 and 700 ℃, indicating that after 1.5h 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 rough surface of S3 should be caused by carbon formed after carbonization of the precursor of carbon deposited on the particle surface, whereas 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 the initial stage to about 6 hours dropped sharply from 73.9mg/h to 5.88mg/h and then remained at a very low growth rate until 24 hours. The growth rate of carbon shows a trend that the carbon gradually increases in the first 6h of reaction and then decreases with the lapse of reaction time, and reaches the highest average growth rate of about 9.45mg/h within 3-6 h. It is noted that within 6h of reaction, 98.3 wt% of the silica and 51.2 wt% of the carbon had completed deposition. After 6 hours of reaction, the growth rate of silicon dioxide is only about 0.1mg/h, while the growth rate of carbon is much faster, and the growth rate in the stage of 6-24 hours of reaction is 3.9-1.4 mg/h. This indicates that the deposition rate of carbon is at least 10 times higher than that of silicon dioxide when the reaction time is in the range of 6 to 24 hours. In 24-36 h of the final stage of the reaction, the growth rates of the silicon dioxide and the carbon are reduced to the degree close to zero, which indicates that the growth of the particles is basically completed when the reaction time is 24 h.

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

Based on the above, the SiO of the present invention₂@SiO₂The dynamic growth process of/Carbon @ Carbon is as follows: in the first 1.5h of the reaction process, because tetraethyl orthosilicate is hydrolyzed at a relatively fast speed, the reaction system is mainly silicon dioxideMainly the rapid nucleation and growth of the crystal. During this time, the growth rate of silicon dioxide is significantly greater than that of the precursor of carbon (phenolic resin), so SiO₂@SiO₂The main component of the core of/Carbon @ Carbon is silica. As the reaction proceeds, the hydrolysis rate of tetraethyl orthosilicate decreases rapidly, and the polymerization rate of carbon precursors increases gradually. After about 1.5h of reaction, the silica particles formed by hydrolysis and the precursor of the carbon formed by polymerization co-deposit. As the reaction proceeds, the formation rate of the precursor of carbon gradually becomes greater than the growth rate of silicon dioxide. In particular, after 6h of reaction, the deposition rate of the precursor of carbon is significantly increased compared to the deposition rate of silicon dioxide, which results in the appearance of a crust structure. After carbonization, an inner core containing silica as a main component, an intermediate layer containing silica and carbon as main components, and an outer shell containing carbon as a main component were formed.

Example 4

In this example, for SiO₂@SiO₂Characterization was performed on/Carbon @ Carbon.

The phase composition and structural state of the carbons in S6 and S36 were studied by X-ray diffraction (XRD) and raman spectroscopy. As shown in fig. 6, S6 and S36 have similar characteristic peaks of XRD diffraction. The broad diffraction peak located around 2 θ of 21.5 ° is attributed to the characteristic mixed peak of amorphous silicon dioxide and the characteristic plane (002) of graphitized carbon. The peak appearing at 43.3 ° 2 θ is associated with the characteristic plane (101) of the graphitized carbon. Raman spectroscopy shown in FIG. 7 (a) shows that S6, S12, S18, S24 and S36 are all at 1330cm^-1And 1590cm^-1The left and right show two prominent peaks corresponding to the D band and G band, respectively, of the carbon material. D-band is associated with carbon atoms sp3 hybridized disordered carbon and lattice defects of carbon atoms; the G band is caused by tensile vibration of sp2 bonds due to the presence of graphitized carbon. The strength ratio of the D band to the G band represents the degree of graphitization of the carbon material. The intensity ratios (ID/IG ratios) of the D band to the G band 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 graphitization degrees. The proper degree of graphitization facilitates electron transport and enhancement of dielectric loss.

S6 and S36 were characterized by fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) detection. The IR spectrum of S6 was similar to that of S36 and was 471cm in FIG. 7 (b)^-1Is caused by bending vibration of O-Si-O and stretching vibration of hydroxyl group, and is located at 3446cm^-1The peak of (A) is tensile vibration of hydroxyl group, and is located at 1111cm^-1And 801cm^-1Two distinct peaks with SiO₂The asymmetry and the symmetric tensile vibration of the medium Si-O-Si are related and are located at 1626cm^-1The energy bands on the left and right are related to the stretching vibration of the C ═ O group, 2346cm^-1The nearby band corresponds to the stretching vibration of the C-O group. The XPS spectrum of S6 and S36 is shown in fig. 8 (a), the C1S spectrum of S6 and S36 is shown in fig. 8 (b), in which the peaks of 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, associated with C-C, C-O and C ═ O bonds. The XPS spectra of S36 and S6 are highly similar. These oxygen-containing polar functional groups such as hydroxyl groups and carbonyl groups remain in the carbon after carbonization of the phenol resin, and the oxygen-containing polar functional groups including hydroxyl groups and carbonyl groups can induce dielectric polarization as polarization centers, thereby contributing to absorption of electromagnetic waves by the material.

FIG. 9 is the low temperature nitrogen adsorption-desorption isotherms of S6 and S36, in which S-6h and S-36h represent samples S6 and S36, respectively, from which it can be seen that irregular pores exist in S6 and S36. From S6 to S36, the extension of the reaction time increased the relative amount of amorphous phenolic resin polymer, and the specific surface area of the sample was from 104.17m²The/g is increased to 149.71m²(ii) in terms of/g. However, as the reaction time increased, the pore volume of the nanoparticles was from 0.210cm³The/g is reduced to 0.155cm³And/g, which shows that in the growth process of the nano particles, partial pore channels are filled by subsequent growth products, so that the structure of the nano particles is more compact.

Example 5

In this example, SiO was tested₂@SiO₂The wave absorbing performance of/Carbon @ Carbon.

1. Electromagnetic parameters

On one hand, the electromagnetic wave absorption material with excellent performance needs to have good impedance matching so that more electromagnetic waves can enter 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 an electromagnetic wave absorbing material are closely related to its own complex permittivity and complex permeability. The carbon material belongs to a dielectric loss type wave-absorbing material, the real part epsilon 'of the dielectric constant of the material represents the storage capacity of the material to electromagnetic wave energy, the imaginary part epsilon' corresponds to the loss capacity of the electromagnetic wave energy, and the magnetic loss is negligible. The dielectric loss tangent (tan δ ═ e "/e') generally represents the dielectric loss capability of the electromagnetic wave absorbing material. Materials having excellent absorption ability in the frequency range of 2 to 18GHz generally have wide practical applicability, and therefore the electromagnetic wave absorption properties of S1.5, S3, S6, S12, S18, S24 and S36 are tested here in this frequency range.

The sample and paraffin wax are uniformly mixed according to the mass ratio of 1:1, and the mixture is made into a ring shape with the outer diameter of 7mm and the inner diameter of 3mm, and then the electromagnetic parameter measurement is carried out. Measuring complex relative dielectric constant (epsilon) of a sample in a frequency range of 2-18 GHz by using a vector network analyzer by adopting a coaxial line method_rEpsilon' -i epsilon ") and permeability (mu)_r＝μ'-iμ”)。

Real, imaginary, dielectric loss tangent values of complex permittivities of S1.5, S3, S6, S12, S18, S24 and S36 and calculated RL_minThe reflection loss curves are shown in fig. 10 (a) to (d). As can be seen from fig. 10, the complex permittivity (including real and imaginary parts) of S1.5 is almost zero in the test frequency range because the reaction time is too short to generate a carbon silicon heterojunction layer on the surface of the silica core particle, while the complex permittivity value of pure silica is small. When the reaction time reaches more than 3h, a carbon-silicon heterojunction layer is formed due to the accumulation of carbon, and the complex dielectric constant is obviously increased. This is consistent with thermogravimetric analysis. It is noted that the complex dielectric constants of S3, S6, S12, S18, S24 and S36 show a decreasing trend between 2 GHz and 18GHz, which is associated with the dipoles in the carbon materialThe frequency of polarization in the gigahertz range is dependent on the hysteresis response. The dielectric loss tangent of S6 was highest in all samples in the frequency range of 2-18 GHz, indicating that S6 has the strongest dielectric loss capability.

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. Based on the complex permittivity and complex permeability data collected, the RL value for each sample was calculated for a given absorber thickness by the following equation.

In formulae (1) to (2), Z_inRepresenting the input impedance of the absorber, Z₀Is the impedance of free space, f is the frequency of the electromagnetic wave, d is the thickness of the sample, c is the velocity of the electromagnetic wave in vacuum,. epsilon._r(ε_rE ∈ "-e') and μ_r(μ_rμ "- μ') correspond to the complex permittivity and complex permeability of the absorber, respectively.

Generally, more than 90% of the electromagnetic energy can be absorbed when the RL value is less than-10 dB. The effective absorption bandwidth is the frequency range covered by all RL values below-10 dB at a certain thickness of the absorbent.

FIGS. 11 (a) - (f) are three-dimensional images of theoretically calculated RL values of S3, S6, S12, S18, S24 and S36, respectively, S1.5 has little ability to absorb electromagnetic waves, and the RL of S3_minThe value was-26.6 dB, corresponding to a thickness of 1.8mm for the absorber. It is worth noting that 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 is adopted_minIs-56.3 dB; and S6 shows the widest effective bandwidth of 5.15GHz (12.48-17.63 GHz). RL of S12 and S18_minThe values are-48.2 dB and-51.0 dB, respectively. S24 has the strongestAbsorption peak of (3), RL_minThe value is-60.4 dB, corresponding to an absorber thickness of 1.5 mm; RL of S36_minThe value is-58.7 dB, corresponding to a thickness of 2.4 mm. The above data indicate that nanoparticles having formed a core-shell structure exhibit excellent electromagnetic wave absorption properties when the reaction time is 6 hours or more.

In addition, the effective absorption bandwidth of the sample S6 with different thicknesses in the frequency range of 2 to 18GHz is shown in the graph (a) of FIG. 12, and the effective absorption bandwidth of S6 almost covers C, X and Ku bands (4.76 to 18GHz) by changing the thickness of S6. The synthesis time of S6 is shortest, and the S6 has relatively excellent electromagnetic wave absorption performance, and is very beneficial to industrial application.

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

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

The content of the absorbent in the paraffin matrix can obviously influence the electromagnetic wavePerformance of the absorbing material, in order to examine the influence of the filling rate of the material on the electromagnetic wave absorbing performance, the electromagnetic wave absorbing performance was tested at the filling rates of 10 wt%, 30 wt%, 50 wt% and 70 wt% of S24, and the results are shown in fig. 14. At a filling rate of 10 wt%, the electromagnetic wave absorption property of the sample was poor, RL_minThe value is-23.51 dB. With the increase of the filling rate, the electromagnetic wave absorption performance of the sample is obviously improved. RL at fill ratios of 30 wt.% and 50 wt.%_minRespectively-47.18 dB and-60.38 dB. However, at a fill level of 70 wt%, RL_minOnly-15.57 dB because of impedance mismatch due to too high a fill factor. Therefore, the optimum filling ratio of the material in paraffin wax is about 50 wt%.

Example 6

In this example, SiO prepared in comparative example 1₂@SiO₂/Carbon @ Carbon with SiO prepared in comparative example 1₂@SiO₂The wave absorbing performance of/Carbon.

Comparative example 1 TPOS having a slower hydrolysis rate than TEOS was used as a silicon source, and SiO was prepared in the same manner₂@SiO₂[ Carbon ], TEM image of SP24 is shown in FIG. 15, and FIG. 15 shows that SP24 also has a core-shell structure. RL of SP6, SP12, SP18 and SP24 was tested by reference to the method in example 5_minThe reflection loss curves for the values, reflection loss minimum, are shown in FIG. 16, their RL_minThe values are-20.7 dB, -37.4dB, -47.9dB and-45.1 dB in sequence.

In the reaction process of comparative example 1, the phenolic resin and the silica formed by slow hydrolysis of TPOS were continuously co-deposited on the surface of the silica core, so that the carbon-silica coexistence layer was continuously thickened, which is advantageous for improving the electromagnetic wave absorption performance of the material. RL of the product in FIG. 16, therefore_minThe reaction time is increased continuously, and the reaction time is not obviously increased until reaching 18 h.

However, in comparison to examples 1 and 2, comparative example 1 uses TPOS as the silicon source to prepare SiO₂@SiO₂The effective absorption bandwidth of/Carbon is narrower, and the absorption performance of the electromagnetic wave below 14GHz is weaker. To further divide intoFor analysis reasons, we calculated the impedance matching value (| Z) of the product in each frequency region under different thicknesses_in/Z₀| and plotted as a two-dimensional contour plot, the results are shown in fig. 17, fig. 17 is the impedance matching value Z (Z ═ Z) of the sample at different frequencies_in/Z₀|) 2D contour plots, in which plots (a) to (e) represent samples S6, S12, S18, S24, and S36, respectively, and plots (f) to (i) represent samples SP6, SP12, SP18, and SP24, respectively. Generally, an impedance matching value between 0.8 and 1.2 means that the absorber with the corresponding thickness is well matched in impedance at the frequency.

As can be seen from FIG. 17, it is shown that₂@SiO₂[ Carbon @ Carbon comparison, SiO₂@SiO₂the/Carbon has more serious impedance mismatch regions (red part in the figure), indicating that a large amount of electromagnetic waves are in SiO₂@SiO₂the/Carbon sample surface is reflected rather than absorbed. The slow hydrolysis of TPOS forms a thicker carbon-silicon heterojunction layer as a shell, while the fast hydrolysis of TEOS tends to form a carbon-silicon heterojunction intermediate layer and an outermost carbon shell, and the reflection loss graph and impedance matching graph show that SiO₂@SiO₂the/Carbon @ Carbon is more favorable for absorption and impedance matching of electromagnetic waves. The above results indicate that an appropriate heterojunction layer and carbon shell play an important role for the silicon carbon material to absorb electromagnetic waves.

Example 7

dropping TEOS into the ethanol-water-ammonia water mixed solution, stirring for 30min, adding resorcinol and formaldehyde with the concentration of 37 wt% into the obtained reaction solution, stirring and reacting for 10h at 20 ℃, performing centrifugal separation to obtain a reaction product, washing the reaction product with deionized water and ethanol for three times respectively, then performing vacuum drying for 12h at 60 ℃, putting the dried reaction product into a tubular furnace, and carbonizing for 1.2h at 700 ℃ under the vacuum condition to obtain the final product SiO₂@SiO₂/Carbon@Carbon。

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

Example 8

dropping TEOS into the ethanol-water-ammonia water mixed solution, stirring for 20min, adding resorcinol and formaldehyde with the concentration of 37 wt% into the obtained reaction solution, stirring and reacting for 8h at 35 ℃, performing centrifugal separation to obtain a reaction product, washing the reaction product with deionized water and ethanol for three times respectively, then performing vacuum drying for 12h at 60 ℃, putting the dried reaction product into a tubular furnace, carbonizing for 0.8h at 800 ℃ under the vacuum condition, and obtaining a final product SiO₂@SiO₂/Carbon@Carbon。

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

Claims

1. A carbon-silicon dioxide nuclear shell composite nano material for electromagnetic wave absorption is characterized in that the composite nano material is a nano particle with a nuclear shell structure, which is composed of an inner core, an intermediate layer wrapping the inner core and a shell wrapping the outer layer of 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 shell is carbon; the nanocomposite has a pore structure therein.

2. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 1, wherein the particle size of the composite nanomaterial is 150 to 240 nm.

3. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 2, wherein in the composite nanomaterial, the silica core is spherical, the thickness of the middle layer is 20-55 nm, and the thickness of the shell is 10-25 nm.

4. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to any one of claims 1 to 3, wherein the phase composition of carbon in the composite nanomaterial is graphitized carbon and disordered carbon, the graphitized carbon is the main component, and the ratio of the intensity of the D band to the intensity of the G band in the Raman spectrum of the composite material is 0.77-0.82.

5. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to any one of claims 1 to 3, characterized in that the carbon of the composite nanomaterial further contains oxygen-containing polar functional groups including hydroxyl groups and carbonyl groups.

6. The carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to any one of claims 1 to 3, wherein the specific surface area of the composite nanomaterial is 100-150 m²/g。

7. A method for preparing the carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to any one of claims 1 to 6, comprising the steps of:

dropwise adding 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 a reaction product, washing, drying, and finally fully carbonizing at 700-800 ℃ under a vacuum condition to obtain the carbon-silicon dioxide core-shell composite nanomaterial 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 tetraethyl orthosilicate to ethanol-water-ammonia water mixed solution is 1 (160-170).

8. The method for preparing a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 7, wherein resorcinol is added in a ratio of 0.3-0.5 g resorcinol to 1mL tetraethyl orthosilicate; adding formaldehyde according to the volume ratio of tetraethyl orthosilicate to formaldehyde of 1 (0.4-0.7).

9. The method for preparing a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to claim 7, wherein the reaction is carried out at 20-35 ℃ for 6-24 hours with stirring.

10. The method for preparing a carbon-silica core-shell composite nanomaterial for electromagnetic wave absorption according to any one of claims 7 to 9, wherein the carbonization time under vacuum conditions at 700 to 800 ℃ is 0.8 to 1.2 hours.