A porous SiCN nw C / Si3N4 composite wave-absorbing ceramic and preparation method thereof

Abstract

The application relates to a porous SiCN nw composite wave-absorbing ceramic and a preparation method thereof, and belongs to the field of ceramic-based wave-absorbing materials. The porous SiCN nw composite wave-absorbing ceramic comprises a porous Si3N4 ceramic matrix, a carbon layer uniformly wrapped on the pore wall surface of the porous Si3N4 ceramic matrix, and SiCN nanowires uniformly grown in the pores of the porous Si3N4 ceramic matrix; wherein the total mass of the carbon layer and the SiCN nanowires is 1-10 wt% of the porous Si3N4 ceramic matrix. By introducing the carbon layer and the SiCN nanowires with different structures into the pore structure of the Si3N4 matrix ceramic, the two components of the wave-absorbing phase can synergistically exert the advantages of each component, optimize the impedance matching degree of the material, and broaden the absorption bandwidth of the material. Moreover, the process is simple, the proportion of the two wave-absorbing phases can be regulated by changing the heat treatment temperature, the impedance matching degree of the material is optimized, and the prepared porous SiCN nw composite ceramic has good wave-absorbing performance.

Description

Technical Field

[0001] This invention relates to a porous SiCN nw / C / Si3N4 composite microwave absorbing ceramics and their preparation methods belong to the field of ceramic-based microwave absorbing materials. Background Technology

[0002] The effectiveness of microwave absorbing materials in electromagnetic interference, electromagnetic pollution control, and stealth technology for military equipment has attracted widespread attention from researchers. These materials can convert electromagnetic energy into heat energy, thereby reducing the harm caused by secondary transmission of electromagnetic waves. Advances in science and technology have placed higher demands on microwave absorbing materials, such as thinness, light weight, wide absorption bandwidth, and high absorption intensity. Ceramic-based microwave absorbing materials have advantages such as low density, tunable dielectric properties, and high mechanical strength, and have broad application prospects in the field of microwave absorption.

[0003] SiCN ceramics possess strong chemical stability, high-temperature resistance, and low dielectric loss, making them an effective absorbing phase material for enhancing the microwave absorption performance of Si3N4 ceramics. Combining different absorbing and transmitting phases to synergistically address various electromagnetic wave loss mechanisms is an effective strategy for improving absorption performance. Porous Si3N4 ceramics exhibit a favorable through-pore structure and low dielectric constant, which facilitates microstructure design and impedance matching optimization. Through appropriate compositional control and microstructure design, the microwave absorption performance of Si3N4-based ceramics can be improved.

[0004] Patent CN108329037B discloses a method for preparing SiC / Si3N4 composite microwave absorbing ceramics. This method involves impregnating a porous Si3N4 ceramic matrix prepared by pressureless sintering into polysilazane and then pyrolyzing it to prepare silicon carbide nanotube composite porous silicon nitride ceramics. However, the microwave absorbing ceramics prepared by this method have a narrow effective absorption bandwidth below -10dB, with a minimum reflection loss of approximately -17.8dB, indicating that the microwave absorption performance needs further improvement. Patent CN115745627A discloses a SiCN ceramic microwave absorbing agent and its preparation method. This involves synthesizing ZIF-67, crosslinking it with a polysilazane precursor, and then pyrolyzing it under nitrogen, helium, and argon atmospheres to obtain the ZIF-67 / SiCN ceramic microwave absorbing agent. The introduction of ZIF-67 effectively lowers the crystallization temperature of polymer-converted ceramics. While promoting the in-situ formation of nanocrystalline phases such as silicon carbide, silicon nitride, and crystalline carbon within the ceramic's internal microstructure, it also forms other high-dielectric nanophases such as metal silicides, creating a multi-phase hierarchical microstructure within the ceramic. However, the cost of preparing SiCN using polysilazane or polysiloxane as precursors is high, making it unsuitable for industrial production. Patent CN116676554A discloses a high-temperature resistant mullite / SiCN composite microwave absorbing coating and its preparation method. This method uses mullite / SiCN composite powder as raw material and employs a plasma spraying process to spray the composite powder onto a substrate. Compressed air is used to cool the substrate during the spraying process, resulting in a mullite / SiCN composite microwave absorbing coating with a thickness greater than or equal to 0.4 mm on the substrate. However, the coating-type microwave absorbing material cannot bear mechanical stress and adds additional weight to the substrate. Furthermore, relying solely on SiCN absorbing phase material and a simple powder mixing process results in a simple material composition and structure, leading to poor absorption performance. The reflection loss value in the 8-18 GHz range is higher than -10 dB.

[0005] The key to solving the above problems lies in selecting low-cost raw materials, designing different components, and developing a simple preparation process. Summary of the Invention

[0006] To address the shortcomings of current composite ceramics, such as complex preparation, high cost, poor absorption capacity, and narrow absorption bandwidth, this invention provides a porous SiCN... nw / C / Si3N4 composite microwave absorbing ceramics and their preparation method. By introducing two different absorbing phases—a carbon layer and SiCN nanowires—into the porous structure of a Si3N4 matrix ceramic, the two components synergistically leverage the advantages of each, optimizing the impedance matching and broadening the absorption bandwidth. Furthermore, the process described in this invention is simple, and the impedance matching can be optimized by adjusting the ratio of the two absorbing phases through heat treatment. The resulting porous SiCN... nw / C / Si3N4 composite ceramics have good microwave absorption properties.

[0007] In a first aspect, the present invention provides a porous SiCN nw The / C / Si3N4 composite microwave absorbing ceramic comprises a porous Si3N4 ceramic matrix, a microwave absorbing phase carbon layer uniformly wrapped around the pore wall surface of the porous Si3N4 ceramic matrix, and microwave absorbing phase SiCN nanowires uniformly grown in the pores of the porous Si3N4 ceramic matrix; wherein the total mass of the carbon layer and SiCN nanowires is 1 to 10 wt% of the porous Si3N4 ceramic matrix.

[0008] Preferably, the mass ratio of the carbon layer to the SiCN nanowire is 1:0.1 to 1:10.

[0009] Preferably, there is a heterogeneous interface structure between the microwave absorbing carbon layer and the Si3N4 ceramic matrix.

[0010] Preferably, the porous Si3N4 ceramic matrix has a porosity of 30-75% and a pore size of 5-50 μm; the carbon layer has a thickness of 10-1000 nm; and the SiCN nanowires have a diameter of 30-500 nm and a length of 5-100 μm.

[0011] Preferably, the porous SiCN nw When the thickness of the C / Si3N4 composite absorbing ceramic is 2.0 to 4.0 mm, the effective absorption bandwidth is 1.89 to 6.30 GHz, and the minimum reflection loss is -14.31 to -61.24 dB.

[0012] Secondly, the present invention provides a porous SiCN nw The preparation method of / C / Si3N4 composite microwave absorbing ceramics includes the following steps:

[0013] Si3N4 powder, sintering aid, pore-forming agent and solvent are ball-milled and mixed to obtain ceramic slurry;

[0014] The ceramic slurry is dried and sieved to obtain a mixed powder.

[0015] The mixed powders were prepared into ceramic green bodies;

[0016] The ceramic green body is then debonded and sintered under air pressure to obtain a porous Si3N4 ceramic matrix;

[0017] A porous Si3N4 ceramic matrix was vacuum-immersed in a phenolic resin solution, followed by heat treatment to obtain porous SiCN. nw / C / Si3N4 composite microwave absorbing ceramic.

[0018] Preferably, the mass ratio of the microwave-absorbing carbon layer and the microwave-absorbing SiCN nanowires is controlled by adjusting the heat treatment temperature; the total mass of the microwave-absorbing carbon layer and the microwave-absorbing SiCN nanowires is controlled by adjusting the phenolic resin content.

[0019] Preferably, the mass ratio of Si3N4 powder to sintering aid is (90-98):(2-10); more preferably, the sintering aid is one or more of lutetium oxide, alumina, and yttrium oxide; the mass ratio of the total mass of Si3N4 powder and sintering aid to the mass of pore-forming agent is 1:(0.1-0.7); more preferably, the pore-forming agent is one or more of PMMA microspheres, starch, and polyvinyl alcohol.

[0020] Preferably, the debinding temperature is 400–600°C, the debinding time is 2–4 hours, and the debinding atmosphere is air; the gas pressure sintering temperature is 1600–1800°C, the gas pressure sintering time is 2–4 hours, the gas pressure sintering atmosphere is nitrogen, and the gas pressure sintering pressure is 0.1–5 MPa.

[0021] Preferably, the vacuum impregnation time is 5 to 60 minutes; more preferably, the phenolic resin in the phenolic resin solution has a mass fraction of 5 to 50%; more preferably, the solvent of the phenolic resin solution is ethanol; the heat treatment temperature is 1200 to 1600°C, and the heat treatment time is 1 to 5 hours; more preferably, the heat treatment temperature is 1350 to 1450°C.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Phenolic resin is used as the microwave absorbing phase raw material, and it is transformed into two microwave absorbing phase materials. That is, after vacuum impregnation and heat treatment, the phenolic resin is distributed in the form of SiCN nanowires and carbon layers in a porous Si3N4 ceramic matrix as the microwave absorbing phase.

[0024] 2. By adjusting the heat treatment temperature, the relative content of SiCN nanowires and carbon layers can be effectively controlled, thereby optimizing the impedance matching of the ceramic and enhancing electromagnetic wave absorption.

[0025] 3. The carbon layer distributed on the pore walls of Si3N4 has numerous heterogeneous interface structures with Si3N4, which promotes charge accumulation at the interface, thereby forming interfacial polarization loss electromagnetic waves. Moreover, the SiCN nanowires inside the pores lengthen the electron transport path, which is beneficial for enhancing the conductivity loss of electromagnetic waves. Attached Figure Description

[0026] Figure 1 The porous SiCN prepared in Examples 1-4 of this invention nwSEM images and elemental distribution diagrams of / C / Si3N4 composite ceramics; wherein, (a), (b), (c) and (d) are SEM images of Example 1, (e), (f), (g) and (h) are SEM images of Example 2, (i) and (j) are SEM images of Example 3, and (k), (l) and (m) are SEM images and elemental distribution diagrams of Example 4;

[0027] Figure 2 The porous SiCN prepared in Examples 1-4 of this invention nw Raman spectra of C / Si3N4 composite ceramics;

[0028] Figure 3 The porous SiCN prepared in Example 3 of this invention nw Real part curve of dielectric constant of C / Si3N4 composite ceramic;

[0029] Figure 4 The porous SiCN prepared in Example 3 of this invention nw Imaginary part curve of dielectric constant of C / Si3N4 composite ceramic;

[0030] Figure 5 The porous SiCN prepared in Example 3 of this invention nw Loss tangent curve of / C / Si3N4 composite ceramic;

[0031] Figure 6 The porous SiCN prepared in Example 3 of this invention nw 2D microwave absorption performance diagram of / C / Si3N4 composite ceramic;

[0032] Figure 7 The porous SiC prepared in Comparative Example 2 of this invention nw SEM images and elemental distribution diagrams of the / C / Si3N4 composite ceramics; where (a) and (b) are SEM images of Comparative Example 2, and (c) is the elemental distribution diagram of Comparative Example 2. Detailed Implementation

[0033] The present invention is further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0034] The porous SiCN of the present invention nw / C / Si3N4 composite microwave absorbing ceramics include a porous Si3N4 ceramic matrix, a microwave-absorbing carbon layer uniformly wrapped around the pore walls of the porous Si3N4 ceramic matrix, and microwave-absorbing SiCN nanowires uniformly grown in the pores of the porous Si3N4 ceramic matrix. The heterogeneous interface between the carbon layer and the Si3N4 ceramic matrix promotes charge accumulation, forming interfacial polarization losses. The three-dimensional conductive network structure formed by the SiCN nanowires extends the electron transport path, thereby consuming electromagnetic energy. Compared to porous Si3N4 ceramic matrices containing only a single microwave-absorbing carbon layer or SiCN nanowires, porous SiCN... nw / C / Si3N4 composite absorbing ceramics can synergistically utilize multiple loss mechanisms such as interface polarization loss, defect dipole loss, and conductivity loss to achieve efficient attenuation of electromagnetic waves.

[0035] The total mass of the carbon layer and SiCN nanowires is 1–10 wt% of the porous Si3N4 ceramic matrix. Preferably, the total mass of the carbon layer and SiCN nanowires is 2–3 wt% of the porous Si3N4 ceramic matrix. If the total mass of the carbon layer and SiCN nanowires is too low relative to the total mass of the porous Si3N4 ceramic matrix, the material will have insufficient electromagnetic wave attenuation capability and a narrow absorption bandwidth. If the total mass of the carbon layer and SiCN nanowires is too high relative to the total mass of the porous Si3N4 ceramic matrix, electromagnetic waves will be reflected at the material surface, resulting in impedance mismatch.

[0036] The mass ratio of the carbon layer to the SiCN nanowires can be 1:0.1 to 1:10. Preferably, the mass ratio of the carbon layer to the SiCN nanowires can be 1:0.1 to 1:1. A lower mass ratio of the carbon layer to the SiCN nanowires will result in a lower dielectric constant and poor absorption performance in the composite ceramic with SiCN nanowires as the main absorbing phase. A higher mass ratio of the carbon layer to the SiCN nanowires will result in a higher dielectric constant and poor absorption performance in the composite ceramic with the carbon layer as the main absorbing phase.

[0037] The porous Si3N4 ceramic matrix has a porosity of 30–75% and a pore size of 5–50 μm. For example, the porous Si3N4 ceramic matrix has a porosity of 64.48% and a pore size of 15–25 μm. The porous Si3N4 ceramic matrix is ​​composed of rod-shaped silicon nitride. The thickness of the carbon layer is 10–1000 nm. Furthermore, the SiCN nanowires have a diameter of 30–500 nm and a length of 5–100 μm.

[0038] The porous SiCN nw When the thickness of the / C / Si3N4 composite absorbing ceramic is 2.0–4.0 mm, the effective absorption bandwidth is 1.89–6.30 GHz, and the lowest reflection loss is -14.31–-61.24 dB. In some embodiments, the porous SiCN... nwWhen the thickness of the / C / Si3N4 composite absorbing ceramic is 2.63 mm, the effective absorption bandwidth is 6.30 GHz. In some technical solutions, the porous SiCN... nw When the thickness of the C / Si3N4 composite absorbing ceramic is 2.77 mm, the lowest reflection loss value is -61.24 dB.

[0039] The porous SiCN of the present invention nw The preparation method of / C / Si3N4 composite microwave absorbing ceramics involves using porous Si3N4 ceramic as a matrix, impregnating it with a phenolic resin solution, and then heat-treating it to prepare porous SiCN / Si3N4 composite microwave absorbing ceramics. nw / C / Si3N4 composite microwave absorbing ceramic. The following is an example illustration.

[0040] Si3N4 powder, sintering aid, pore-forming agent and solvent are ball-milled and mixed to obtain ceramic slurry.

[0041] The mass ratio of Si3N4 powder to sintering aid is (90-98):(2-10). For example, the mass ratio of Si3N4 powder to sintering aid is 96:4. Sintering aids include, but are not limited to, lutetium oxide, alumina, and yttrium oxide. Preferably, the sintering aid is lutetium oxide.

[0042] The total mass ratio of the Si3N4 powder and sintering aid to the pore-forming agent is 1:(0.1-0.7). Preferably, the total mass ratio of the Si3N4 powder and sintering aid to the pore-forming agent is 1:0.33. The pore-forming agent includes, but is not limited to, PMMA microspheres, starch, polyvinyl alcohol, etc. Preferably, the pore-forming agent is PMMA microspheres. The particle size of the pore-forming agent can be 10-50 μm. As an example, PMMA microspheres with a particle size of 30 μm are used as the pore-forming agent.

[0043] The solvent includes, but is not limited to, anhydrous ethanol. The total mass ratio of the Si3N4 powder, sintering aid, and pore-forming agent to the solvent is 1:(0.5-3). Preferably, the total mass ratio of the Si3N4 powder, sintering aid, and pore-forming agent to ethanol is 1:1.

[0044] The ceramic slurry is dried and sieved to obtain a mixed powder. The ball milling speed can be 200-400 rpm, and the ball milling time can be 4-12 hours. The ball milling media can be ethanol. The drying temperature can be 50-80℃, and the time can be 12-24 hours. The sieving can be done through a 60-120 mesh sieve. Drying can be done in an oven.

[0045] The mixed powder is prepared into ceramic green bodies. Forming methods include, but are not limited to, uniaxial dry pressing. Uniaxial dry pressing is simple to operate and easy to mechanize. For example, the forming pressure can be 5–50 MPa.

[0046] The ceramic green body is then debonded and sintered under pressure to obtain the porous Si3N4 ceramic matrix. The debonding temperature is 400–600°C, and the time is 2–4 hours. The debonding atmosphere is air, which can oxidize and remove the pore-forming agent. As an example, the debonding process includes: first heating to 250°C at a rate of 3–5°C / min, then heating to 600°C at a rate of 1–3°C / min and holding for 2–4 hours. Debonding can be carried out in a muffle furnace.

[0047] The gas pressure sintering temperature is 1600–1800℃, the sintering time is 2–4 hours, the sintering atmosphere is nitrogen, and the sintering pressure is 0.1–5 MPa. Gas pressure sintering under a certain pressure nitrogen atmosphere can suppress the decomposition of Si3N4 ceramics during sintering. As an example, the parameters for gas pressure sintering include: first raising the temperature to 1200℃ at a rate of 5–10℃ / min, then raising it to 1600–1800℃ at a rate of 2–4℃ / min and holding it for 2–4 hours. For example, the gas pressure sintering steps are: first raising the temperature to 1200℃ at a rate of 7℃ / min, then raising it to 1670℃ at a rate of 3℃ / min and holding it for 3 hours, with a nitrogen atmosphere and a pressure of 0.3 MPa. Sintering can be carried out in a gas pressure sintering furnace.

[0048] A porous Si3N4 ceramic matrix was vacuum-immersed in a phenolic resin solution, followed by heat treatment to obtain the porous SiCN. nw / C / Si3N4 composite microwave absorbing ceramic.

[0049] The solvent for the phenolic resin solution needs to allow the phenolic resin to dissolve, and the solvent should be easily volatile during heat treatment without leaving impurities. The solvent includes, but is not limited to, organic solvents such as ethanol, acetone, and methanol.

[0050] The phenolic resin solution contains 5-50% phenolic resin by mass. The concentration of the phenolic resin solution can regulate the total amount of the absorbing phase in the porous Si3N4 ceramic matrix, thereby optimizing the material's dielectric constant. Preferably, the phenolic resin solution contains 10-25% phenolic resin by mass. More preferably, the phenolic resin solution contains 15-25% phenolic resin by mass. The vacuum impregnation time can be 5-60 minutes. Sufficient vacuum impregnation ensures that the phenolic resin solution completely fills the porous Si3N4 ceramic matrix, which is beneficial for the uniform distribution of the absorbing phase in the matrix. For example, the vacuum impregnation time is 30 minutes.

[0051] The heat treatment temperature is 1200–1600°C. Preferably, the heat treatment temperature is 1350–1450°C. At lower heat treatment temperatures, such as below 1300°C, the phenolic resin is only converted into a carbon layer. At higher heat treatment temperatures, such as above 1500°C, the phenolic resin and the porous Si3N4 ceramic matrix react to almost completely convert into SiCN nanowires. If the heat treatment temperature continues to rise, for example, to 1800°C, the Si3N4 ceramic matrix begins to decompose. Therefore, the preferred heat treatment temperature of this invention is 1350–1450°C, within which the phenolic resin is converted into both a carbon layer and SiCN nanowires, both microwave-absorbing phases. Moreover, the higher the heat treatment temperature within this range, the higher the ratio of SiCN nanowires to carbon layer. The heat treatment can be performed in a tube furnace.

[0052] As an example, the heat treatment process includes: first heating to 500-800°C at a heating rate of 1-5°C / min, then heating to 1200-1600°C at a rate of 2-7°C / min and holding at that temperature for 1-5 hours. Preferably, the temperature is first heated to 500-800°C at a heating rate of 1-5°C / min, then heated to 1350-1450°C at a rate of 2-7°C / min.

[0053] Argon can be used as the heat treatment atmosphere. Nitrogen cannot be used because a nitrogen atmosphere will inhibit the reaction between the carbon layer and the porous Si3N4 ceramic matrix to form SiCN nanowires.

[0054] The method described in this invention uses low-cost raw materials, has a simple preparation process, and requires minimal equipment. Furthermore, by impregnating a porous Si3N4 ceramic matrix in a phenolic resin solution and then heat-treating it, two different absorbing phases—SiCN nanowires and a carbon layer—can be generated within the porous structure. Moreover, by changing the heat treatment temperature, the ratio of the two absorbing phases can be controlled, thereby optimizing the material's impedance matching and exhibiting highly efficient absorption characteristics.

[0055] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0056] Example 1: Preparation of porous C / Si3N4 composite microwave absorbing ceramics

[0057] Step (1): Si3N4 powder, lutetium oxide, PMMA, and ethanol were ball-milled and mixed. The particle size of the pore-forming agent PMMA was 30 μm. The mass ratio of Si3N4 powder to lutetium oxide was 96:4. The mass ratio of the total mass of Si3N4 powder and lutetium oxide to the mass of the pore-forming agent was 1:0.33. The mass ratio of the total mass of Si3N4 powder, lutetium oxide, and the pore-forming agent to the mass of ethanol was 1:1. The ball milling time was 4 hours. The ball-milled ceramic slurry was dried in a 60℃ oven for 24 hours and passed through a 60-mesh sieve to obtain the mixed powder.

[0058] Step (2): Ceramic green bodies were prepared using uniaxial dry pressing. The mixed powder was heated to 250°C in a muffle furnace at 3°C / min, and then heated to 600°C at 2°C / min and held for 2 hours to remove the pore-forming agent PMMA. Subsequently, sintering was carried out in a gas pressure sintering furnace, first heated to 1200°C at 7°C / min, and then heated to 1670°C at 3°C / min and held for 3 hours. The sintering atmosphere was nitrogen and the pressure was 0.3 MPa, resulting in porous Si3N4 ceramics.

[0059] Step (3): Vacuum immersion of porous Si3N4 ceramic in a phenolic resin solution. The phenolic resin solution has a mass fraction of 20% and the solvent is alcohol. The vacuum immersion time is 30 minutes. The porous Si3N4 ceramic immersed in the phenolic resin solution is placed in a tube furnace, and the heat treatment atmosphere is argon. The temperature is first raised to 700℃ at a rate of 1℃ / min, then raised to 1200℃ at a rate of 3℃ / min and held for 2 hours to obtain porous C / Si3N4 composite microwave absorbing ceramic.

[0060] The microstructure of the composite absorbing ceramic prepared in Example 1 is as follows: Figure 1 As shown in (ad), it can be seen that after heat treatment at 1200℃, the phenolic resin is distributed in the form of a carbon layer on the pore walls of the porous Si3N4 ceramic. The carbon layer is relatively thick between the rod-shaped silicon nitride. At this time, the porous Si3N4 ceramic matrix contains only one microwave absorbing phase material, the carbon layer. Figure 2 The Raman spectra results showed that at 1595 cm⁻¹ -1 and 1350cm -1 Two distinct peaks are observed nearby, belonging to the G and D bands of the carbon-based material, confirming the presence of abundant carbon in the porous Si3N4 ceramic matrix. Table 1 shows that the composite absorbing ceramic has an effective absorption bandwidth of 0 GHz and a minimum reflection loss of -8.91 dB, indicating poor absorption performance.

[0061] Example 2:

[0062] It is basically the same as Example 1, except that the heat treatment temperature in step (3) is 1300℃.

[0063] The microstructure of the composite absorbing ceramic prepared in Example 2 is as follows: Figure 1 As shown in (eh), it can be seen that after heat treatment at 1300℃, the phenolic resin is still distributed in the form of a carbon layer on the pore walls of the porous Si3N4 ceramic, which is similar to the microstructure of the composite ceramic in Example 1. At this time, the porous Si3N4 ceramic matrix contains only a carbon layer as a microwave absorbing phase. Figure 2 The Raman spectra results showed that at 1595 cm⁻¹ -1 and 1350cm -1 The presence of two distinct peaks nearby further confirms the presence of a large amount of carbon in the porous Si3N4 ceramic matrix. Table 1 shows that the composite absorbing ceramic has an effective absorption bandwidth of 0.53 GHz and a minimum reflection loss of -10.50 dB, indicating poor absorption performance.

[0064] Example 3:

[0065] It is basically the same as Example 1, except that the heat treatment temperature in step (3) is 1400℃.

[0066] The porous SiCN prepared in Example 3 nw The microstructure of / C / Si3N4 ceramics is as follows: Figure 1 As shown in (i) and (j), carbon generated by the carbonization of phenolic resin is distributed between the rod-shaped silicon nitride on the pore walls, and nanowires are also generated in the ceramic pore structure. Figure 2 The Raman spectra results showed that at 1595 cm⁻¹ -1 and 1350cm -1 The presence of two distinct peaks nearby confirms that the carbon in the porous Si3N4 ceramic matrix was not completely converted into SiCN nanowires. At this point, the porous Si3N4 ceramic matrix contains both carbon and SiCN nanowires, two microwave-absorbing phases.

[0067] In addition, the porous Si3N4 ceramic obtained in step (2) has a mass of 0.7928 g, and the porous SiCN obtained in step (3) nw The mass of the / C / Si3N4 ceramic is 0.8127g. Based on the change in mass before and after, it can be calculated that the total mass of the carbon layer and SiCN nanowires in Example 3 is approximately 2.51wt% of the porous Si3N4 ceramic matrix. Subsequently, the porous SiCN nanowires obtained in step (3) are... nw The / C / Si3N4 ceramic was held at 600℃ in a muffle furnace for 2 hours to oxidize and remove the microwave-absorbing carbon layer in the ceramic. The mass of the oxidized ceramic was 0.8062g. Based on the change in mass before and after oxidation, it can be calculated that the mass ratio of the carbon layer to the SiCN nanowires in this Example 3 is approximately 1:0.485.

[0068] Figure 3 The real part of the dielectric constant is shown to be 4.7–8.3. Figure 4The imaginary part of the dielectric constant is shown to be 1.7–4.8. Figure 5 The displayed loss tangent is 0.35–0.60. Figure 6 The reflection loss results show that the prepared composite ceramic has an effective absorption bandwidth of 6.30 GHz when the thickness is 2.63 mm and a minimum reflection loss of -61.24 dB when the thickness is 2.77 mm, exhibiting efficient wave absorption characteristics.

[0069] Example 4:

[0070] It is basically the same as Example 1, except that the heat treatment temperature in step (3) is 1500℃.

[0071] The porous SiCN prepared in Example 4 nw The microstructure of / C / Si3N4 ceramics is as follows: Figure 1 As shown in (k) and (l), it can be seen that as the heat treatment temperature increases, the content of nanowires in the pore structure increases, which means that the vast majority of carbon participates in the reaction to generate nanowires. Figure 1 The elemental distribution diagram (m) shows that the main elemental composition of the nanowires is Si, N, and C. Figure 2 The Raman spectra results showed that at 1595 cm⁻¹ -1 and 1350cm -1 The G-band and D-band peak intensities are significantly weakened. At this point, the porous Si3N4 ceramic matrix is ​​almost entirely composed of SiCN nanowire absorbing phase material. The absorption performance results in Table 1 show that the effective absorption bandwidth of the composite absorbing ceramic is 0 GHz, and the lowest reflection loss is -7.16 dB, indicating poor absorption performance.

[0072] Example 5:

[0073] As described in Example 1, the difference is that in step (3), the mass fraction of phenolic resin in the phenolic resin solution is 10%, and the heat treatment temperature is 1400℃.

[0074] Example 6:

[0075] As described in Example 1, the difference is that in step (3), the mass fraction of phenolic resin in the phenolic resin solution is 15%, and the heat treatment temperature is 1400℃.

[0076] Example 7:

[0077] As described in Example 1, the difference is that in step (3), the mass fraction of phenolic resin in the phenolic resin solution is 25%, and the heat treatment temperature is 1400℃.

[0078] Comparative Example 1

[0079] The porous Si3N4 ceramic matrix prepared in Example 1 was selected as Comparative Example 1.

[0080] The absorption performance results in Table 1 show that the effective absorption bandwidth of the porous Si3N4 ceramic matrix is ​​0 GHz, and the lowest reflection loss value is only -0.88 dB, exhibiting wave transmission rather than wave absorption characteristics.

[0081] Table 1 lists the porous SiCN prepared according to the present invention. nw Preparation and properties of C / Si3N4 composite microwave absorbing ceramics:

[0082]

[0083] Comparative Example 2

[0084] Porous SiC nw Preparation of / C / Si3N4 composite microwave absorbing ceramics

[0085] Step (1): Si3N4 powder, lutetium oxide, PMMA, and ethanol were ball-milled and mixed. The particle size of the pore-forming agent PMMA was 30 μm. The mass ratio of Si3N4 powder to lutetium oxide was 96:4. The mass ratio of the total mass of Si3N4 powder and lutetium oxide to the mass of the pore-forming agent was 1:0.33. The mass ratio of the total mass of Si3N4 powder, lutetium oxide, and the pore-forming agent to the mass of ethanol was 1:1. The ball milling time was 4 hours. The ball-milled ceramic slurry was dried in a 60℃ oven for 24 hours and passed through a 60-mesh sieve to obtain the mixed powder.

[0086] Step (2): Ceramic green bodies were prepared using uniaxial dry pressing. The mixed powder was heated to 250°C in a muffle furnace at 3°C / min, and then heated to 600°C at 2°C / min and held for 2 hours to remove the pore-forming agent PMMA. Subsequently, sintering was carried out in a gas pressure sintering furnace, first heated to 1200°C at 7°C / min, and then heated to 1670°C at 3°C / min and held for 3 hours. The sintering atmosphere was nitrogen and the pressure was 0.3 MPa, resulting in porous silicon nitride ceramics.

[0087] Step (3): The porous Si3N4 ceramic is pre-oxidized in air at 1000℃ for 2 hours. The porous Si3N4 ceramic undergoes partial oxidation to generate SiO2.

[0088] Step (4): Vacuum immersion of pre-oxidized porous Si3N4 ceramic in a phenolic resin solution. The phenolic resin solution has a mass fraction of 20%, and the solvent is alcohol. The vacuum immersion time is 30 minutes. The porous Si3N4 ceramic immersed in the phenolic resin solution is placed in a tube furnace, and the heat treatment atmosphere is argon. The temperature is first increased to 700℃ at a rate of 1℃ / min, then increased to 1500℃ at a rate of 3℃ / min and held for 2 hours to obtain porous SiC. nw / C / Si3N4 composite microwave absorbing ceramic.

[0089] The microstructure of the porous ceramic prepared in Comparative Example 2 is as follows: Figure 7 As shown, after pre-oxidation treatment, nanowires also grow in the porous Si3N4 ceramic matrix impregnated with phenolic resin solution within the pore structure. Unlike Examples 3 and 4, the elemental composition of the nanowires is mainly Si and C, indicating that SiC nanowires, rather than SiCN nanowires, are formed in the pores. That is, the oxidation product SiO2 undergoes a carbothermic reduction reaction with carbon to generate SiC nanowires. Because SiC nanowires have a high dielectric constant, the conversion of carbon into SiC nanowires cannot optimize the impedance matching of the material, resulting in impedance mismatch and poor microwave absorption performance.

[0090] The above embodiments are possible implementations of the present invention, but the implementation of the present invention is not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and are included within the protection scope of the present invention.

Claims

1. A porous SiCN nw / C / Si3N4 composite microwave absorbing ceramic, characterized in that... The porous SiCN nw The / C / Si3N4 composite microwave absorbing ceramic comprises a porous Si3N4 ceramic matrix, a microwave absorbing carbon layer uniformly coated on the pore walls of the porous Si3N4 ceramic matrix, and microwave absorbing SiCN nanowires uniformly grown in the pores of the porous Si3N4 ceramic matrix; the total mass of the carbon layer and SiCN nanowires is 2wt%~3wt% of the porous Si3N4 ceramic matrix; the mass ratio of the carbon layer to the SiCN nanowires is 1:0.1~1:1; the porous SiCN... nw The preparation method of / C / Si3N4 composite microwave absorbing ceramics includes the following steps: Si3N4 powder, sintering aid, pore-forming agent, and solvent are ball-milled and mixed to obtain a ceramic slurry; the ceramic slurry is dried and sieved to obtain a mixed powder; the mixed powder is prepared into a ceramic green body; the ceramic green body is then debonded and pressure sintered to obtain a porous Si3N4 ceramic matrix; the porous Si3N4 ceramic matrix is ​​vacuum-immersed in a phenolic resin solution, and then heat-treated to obtain porous SiCN. nw / C / Si3N4 composite microwave absorbing ceramic; the heat treatment atmosphere is argon; the heat treatment temperature is 1350~1450℃, and the heat treatment time is 1~5 hours; the mass fraction of phenolic resin in the phenolic resin solution is 15%~25%.

2. The porous SiCN according to claim 1 nw / C / Si3N4 composite microwave absorbing ceramic, characterized in that... The porous Si3N4 ceramic matrix has a porosity of 30%~75% and a pore size of 5~50 μm; the carbon layer has a thickness of 10~1000 nm; and the SiCN nanowires have a diameter of 30~500 nm and a length of 5~100 μm.

3. The porous SiCN according to claim 1 nw / C / Si3N4 composite microwave absorbing ceramic, characterized in that... The porous SiCN nw When the thickness of the C / Si3N4 composite absorbing ceramic is 2.0~4.0 mm, the effective absorption bandwidth is 1.89~6.30 GHz and the minimum reflection loss is -14.31~-61.24 dB.

4. The porous SiCN according to any one of claims 1 to 3 nw The preparation method of / C / Si3N4 composite microwave absorbing ceramic is characterized by, Includes the following steps: Si3N4 powder, sintering aid, pore-forming agent, and solvent were ball-milled and mixed to obtain a ceramic slurry. The ceramic slurry was dried and sieved to obtain a mixed powder. The mixed powder was prepared into a ceramic green body. The ceramic green body was then debonded and sintered under air pressure to obtain a porous Si3N4 ceramic matrix. The porous Si3N4 ceramic matrix was vacuum-immersed in a phenolic resin solution and then heat-treated to obtain porous SiCN. nw / C / Si3N4 composite microwave absorbing ceramic; the heat treatment atmosphere is argon; the heat treatment temperature is 1350~1450℃, and the heat treatment time is 1~5 hours; the mass fraction of phenolic resin in the phenolic resin solution is 15%~25%.

5. The preparation method according to claim 4, characterized in that, The mass ratio of Si3N4 powder to sintering aid is (90~98):(2~10); the sintering aid is one or more of lutetium oxide, alumina, and yttrium oxide; the mass ratio of the total mass of Si3N4 powder and sintering aid to the mass of pore-forming agent is 1:(0.1~0.7).

6. The preparation method according to claim 4, characterized in that, The pore-forming agent is one or more of PMMA microspheres, starch, and polyvinyl alcohol.

7. The preparation method according to claim 4, characterized in that, The de-adhesion temperature is 400~600℃, the de-adhesion time is 2~4 hours, and the de-adhesion atmosphere is air; The gas pressure sintering temperature is 1600~1800℃, the gas pressure sintering time is 2~4 hours, the gas pressure sintering atmosphere is nitrogen, and the gas pressure sintering pressure is 0.1~5 MPa.

8. The preparation method according to claim 4, characterized in that, Vacuum immersion time is 5 to 60 minutes.

9. The preparation method according to claim 4, characterized in that, The solvent for the phenolic resin solution is ethanol.

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