A natural microcrystalline graphite-based SiC / C composite microwave absorbing material and its preparation method

Abstract

The present invention provides a natural microcrystalline graphite-based SiC / C composite microwave absorbing material and a preparation method thereof. The preparation method includes crushing a natural microcrystalline graphite ore to obtain graphite powder with a required particle size; the ash content of the natural microcrystalline graphite ore is 15% to 25%; loading the graphite powder into a crucible, first evacuating, then introducing a protective gas, heating to a temperature of 1350°C to 1600°C, and holding for 2 h to 10 h, and then cooling to obtain the natural microcrystalline graphite-based SiC / C composite microwave absorbing material. For the natural microcrystalline graphite-based SiC / C composite microwave absorbing material, the space impedance matching coefficient |Z in / Z0|≈1, the reflection loss ≤ -20 dB, and the effective absorption bandwidth ≥ 2 GHz.

Description

Technical Field

[0001] The present invention belongs to the field of microwave absorbing materials, and particularly relates to a natural microcrystalline graphite-based SiC / C composite microwave absorbing material and a preparation method thereof. Background Art

[0002] The development and wide application of electromagnetic technologies have provided great convenience for human life, but at the same time have also brought some potential negative impacts or even hazards. High-intensity electromagnetic waves can affect human health; electromagnetic interference can affect the normal operation of highly sensitive electronic devices; in the military field, radar detection technology poses a threat to the safety of military aircraft and other weaponry. Therefore, to weaken electromagnetic radiation and electromagnetic interference, the development of electromagnetic wave absorbing materials with thin thickness, low density, strong absorption, high loss, wide absorption band, and strong practicability has important practical requirements in the civilian, commercial, and military fields.

[0003] Graphite has long been used to fill the sandwich of aircraft skins as a microwave absorbing material to absorb radar waves. As a typical representative of electric loss type microwave absorbing materials, it remains a research hotspot to this day. Although graphite has advantages such as light weight, large specific surface area, and high electrical conductivity, it also has deficiencies such as large dielectric constant and non-magnetic properties. When directly applied, the electromagnetic wave absorption performance is often poor due to the low level of spatial impedance matching. Therefore, people often perform surface modification or doping modification on graphite materials and compound them with other loss materials to construct high-performance lightweight carbon-based composite microwave absorbing materials with different microstructures and multiple loss mechanisms. As a semiconductor material, silicon carbide (SiC) has excellent dielectric properties and high-frequency microwave absorbing characteristics, and occupies an important position in the field of microwave absorbing materials.

[0004] In recent years, researchers have mainly focused on modifying graphite to obtain graphene oxide, nano-graphite, or plating metals on modified graphite flakes, and then compounding them with other types of microwave absorbing materials to prepare composites with stronger microwave absorbing performance. Among them, nano-silicon carbide / carbon-based composites have great potential in improving microwave absorbing performance and have become one of the current important basic research directions. However, the currently reported preparation methods of silicon carbide nano-materials at home and abroad mainly include chemical vapor deposition method, carbothermal reduction method, sol-gel method, template method, and electrospinning method. No matter which method is used, there are problems such as complex preparation process, high cost, and difficulty in industrialization.

[0005] Natural microcrystalline graphite is a kind of graphite mineral resource mainly formed by the contact metamorphism of anthracite. Its crystals are fine (usually less than 1 μm), and each particle is composed of many microcrystals stacked disorderly. Natural microcrystalline graphite ore usually contains 15% - 50% of other mineral impurities, and the mineral impurities are diffusely distributed among the microcrystalline graphite crystal particles. The mass proportion of quartz (SiO2) component in the impurities is about 40%. For a long time, the industrial community believes that to achieve the high-value utilization of natural microcrystalline graphite, it is necessary to carry out purification treatment first. And the most difficult part to handle in the purification of natural microcrystalline graphite is the SiO2 impurity component. To effectively remove this impurity component, methods such as hydrofluoric acid method or high-temperature alkali fusion acid leaching method must be used. No matter which method is used, there are problems such as harsh production equipment, high cost, and serious environmental pollution.

[0006] However, the disadvantages of natural microcrystalline graphite that are traditionally considered, such as fine crystal grains and diffusely distributed impurities among graphite crystals, are actually its unique advantages from another perspective. Using appropriate natural microcrystalline graphite as raw material and through appropriate process treatment, nano-silicon carbide / graphite composite materials can be in-situ synthesized, which have great potential in the field of developing nano-silicon carbide / graphite composite absorbing materials with low cost, low energy consumption, simple process, and scalable production and coexisting multiple loss mechanisms, and can open up new ways for broadening the application fields of microcrystalline graphite and improving the added value of products. Summary of the Invention

[0007] The purpose of the present invention is to provide a natural microcrystalline graphite-based SiC / C composite absorbing material with simple process, low cost, and excellent performance and its preparation method.

[0008] A preparation method of a natural microcrystalline graphite-based SiC / C composite absorbing material includes the following steps:

[0009] 1) Crushing the natural microcrystalline graphite raw ore to obtain graphite powder with the required particle size; the ash content of the natural microcrystalline graphite raw ore is 15% - 25%;

[0010] 2) Loading the graphite powder in step 1) into a crucible, first evacuating, then introducing a protective gas, heating to 1350°C - 1600°C and holding for 2h - 10h, and cooling to obtain a natural microcrystalline graphite-based SiC / C composite absorbing material;

[0011] For the natural microcrystalline graphite-based SiC / C composite absorbing material, the space impedance matching coefficient |Z in / Z0|≈1, the reflection loss ≤ -20 dB, and the effective absorption bandwidth ≥ 2 GHz.

[0012] Preferably, the crushing is one or several of ball milling, Raymond grinding, jet milling, or ring-roll milling.

[0013] Preferably, the ash content is 15% to 25%.

[0014] Preferably, the particle size of the graphite powder is less than or equal to 75 μm.

[0015] Preferably, the particle size D 50 of the graphite powder is less than or equal to 5 μm.

[0016] Preferably, the crucible is a graphite crucible, a carbon crucible or an alumina crucible.

[0017] Preferably, the protective gas is one or more of nitrogen, argon and helium.

[0018] Preferably, it is heated to 1450 °C to 1500 °C.

[0019] Preferably, the SiC is mainly β-SiC.

[0020] A natural microcrystalline graphite-based SiC / C composite absorbing material is prepared by the above preparation method.

[0021] Preferably, in the natural microcrystalline graphite-based SiC / C composite absorbing material, SiC is uniformly dispersed in the graphite in the form of nano-curves.

[0022] The design of the absorbing material mainly involves two aspects: the impedance matching characteristics on the material surface and the attenuation characteristics inside the material. The impedance matching characteristic requires that the incident electromagnetic wave enters the material interior as much as possible to reduce the reflection on the dielectric surface. At this time, it is required that the input impedance at the interface matches the free space impedance, that is, try to satisfy |Z in / Z0| = 1. The absorption of electromagnetic waves by graphite is mainly due to conduction loss. Although the loss intensity is large, the too high conductivity of the absorbing material is extremely likely to cause strong reflection of electromagnetic waves at the interface between the free space and the material, resulting in impedance mismatch and thus reducing its absorbing performance; silicon carbide is mainly dielectric loss. The presence of silicon carbide can significantly reduce the conductivity of the graphite material, avoid the reflection of electromagnetic waves, and improve the impedance matching performance of the material. Therefore, the appropriate content of graphite and silicon carbide is the key to ensuring good space impedance matching of the absorbing material. In the present invention, when the ash content of the natural microcrystalline graphite ore is 15% to 25% and the heating temperature is 1450 °C to 1500 °C, the prepared absorbing material has the best space impedance matching performance. The size and shape of the material can directly affect the absorbing characteristics of the material. As the size of the material decreases, the specific surface area of the material will increase, the active chemical sites will increase, and the interactions such as phase dielectrics, mismatched electron pairs, eddy current losses and resonance losses will increase. The smaller size of the material will also increase effects such as interface polarization and multiple reflections, thereby increasing the absorption ability of the material to electromagnetic waves.

[0023] In the present invention, when the original microcrystalline graphite ore is crushed to less than 200 mesh (75 μm), the prepared wave-absorbing material can obtain good wave-absorbing performance. When the particle size (D 50 ) is less than or equal to 5 μm, the usage amount of the wave-absorbing material can be significantly reduced, and good wave-absorbing effect can be obtained even when it is relatively thin. C and SiO2 start an in-situ reaction at a high temperature of 1350 °C to generate β-SiC. With the increase of temperature, the reaction rate accelerates. However, at a high temperature above 1500 °C, the β-SiC grains will grow and transform into α-SiC, and the β-SiC with good dielectric properties is the one. Therefore, in order to improve the wave-absorbing performance of the product, it is necessary to control the β-SiC grain size and the generation amount of α-SiC. In the present invention, the main product generated at a temperature of 1450 °C to 1500 °C is β-SiC, and the generation rate is fast and the grains are small. For the natural microcrystalline graphite-based SiC / C composite wave-absorbing material, its spatial impedance matching coefficient |Z in / Z0|≈1, the reflection loss ≤ -20 dB, and the effective absorption bandwidth ≥ 2 GHz.

[0024] Advantages of the present invention:

[0025] 1. In the present invention, natural microcrystalline graphite is used as the raw material, and a graphite / nano-silicon carbide composite wave-absorbing material is generated through an in-situ carbothermal reduction reaction. Compared with the existing production technology, it has significant advantages such as simple production process, low energy consumption, low cost, and scalable production;

[0026] 2. In the present invention, natural microcrystalline graphite is used as the raw material. In the obtained graphite / nano-silicon carbide composite wave-absorbing material, the silicon carbide nano-curves are uniformly dispersed in the graphite, which is beneficial to improving the homogeneity and stability of the wave-absorbing performance of the material;

[0027] 3. In the present invention, the raw material is crushed to a particle size (D 50 ) less than or equal to 5 μm by a jet mill or a ring-roll mill, which is beneficial to increasing effects such as interfacial polarization and multiple reflections of the product, thereby increasing the absorption ability of electromagnetic waves and significantly reducing the material usage amount;

[0028] 4. In the present invention, by controlling the reaction temperature, the crystal form and grain size of the generated silicon carbide can be controlled;

[0029] 5. In the present invention, natural microcrystalline graphite is used as the raw material, which can broaden the application field of natural microcrystalline graphite and significantly increase its added value. Description of the Drawings

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0031] Figure 1 Transmission electron microscope (left figure) and energy spectrum analysis diagram (right figure) of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 1;

[0032] Figure 2 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 1 by the coaxial ring method;

[0033] Figure 3 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 2 by the coaxial ring method;

[0034] Figure 4 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 3 by the coaxial ring method;

[0035] Figure 5 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 4 by the coaxial ring method;

[0036] Figure 6 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 5 by the coaxial ring method;

[0037] Figure 7 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 6 by the coaxial ring method;

[0038] Figure 8 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 7 by the coaxial ring method;

[0039] Figure 9 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 8 by the coaxial ring method;

[0040] Figure 10 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Example 9 by the coaxial ring method;

[0041] Figure 11 Test result diagram of the natural microcrystalline graphite-based SiC / C composite wave-absorbing material in Comparative Example 1 by the coaxial ring method;

[0042] Figure 12Test result diagram of the coaxial ring method for the natural microcrystalline graphite-based SiC / C composite absorbing material in Comparative Example 2;

[0043] Figure 13 Test result diagram of the coaxial ring method for the natural microcrystalline graphite-based SiC / C composite absorbing material in Comparative Example 3;

[0044] Figure 14 Test result diagram of the coaxial ring method for the natural microcrystalline graphite-based SiC / C composite absorbing material in Comparative Example 4; Specific implementation manners

[0045] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above objects, features, and advantages of the present invention more obvious and understandable, the specific implementation manners of the present invention will be further described below.

[0046] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0047] A natural microcrystalline graphite-based SiC / C composite absorbing material and a preparation method thereof provided by the present invention will be described in detail below through specific embodiments for the preparation method of the present invention.

[0048] Example 1:

[0049] In this embodiment, the ash content of the natural microcrystalline graphite raw ore is 18.8%; the SiO2 content is 8.4%; the fixed carbon content is 78.2%; the production process of this embodiment is carried out according to the following steps:

[0050] Step 1: Crush the dried natural microcrystalline graphite raw ore to a particle size (D 50 ) of 1.6 μm;

[0051] Step 2: Load the graphite powder into a graphite crucible and then into a high-temperature furnace. First, evacuate, then introduce nitrogen, heat to 1500 °C and hold for 4 h, stop introducing nitrogen after naturally cooling to 400 °C, and continue to cool to 100 °C and then take out of the furnace.

[0052] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this embodiment, wherein the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 17.1 nm. The analysis results of the transmission electron microscope and energy spectrum of the sample are shown in Figure 1 .

[0053] Tested by the coaxial ring method, when the content is 20% and the thickness is 1.6 mm, the lowest reflection loss (RL) = -43 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 1.055, and the effective absorption bandwidth (EAB) = 4.6 GHz (13.4 GHz to 18.0 GHz); when the thickness is 1.7 mm, the widest effective absorption bandwidth (EAB) = 5.2 GHz (12.3 GHz to 17.5 GHz), the corresponding reflection loss (RL) = -28 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 0.978.

[0054] Example 2:

[0055] The content of this example is compared with that of Example 1, and the only difference is that the heating temperature is 1450 °C, and the others are the same as in Example 1.

[0056] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 15.9 nm.

[0057] Tested by the coaxial ring method, when the content is 20% and the thickness is 1.5 mm, the lowest reflection loss (RL) = -34 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 0.976, and the effective absorption bandwidth (EAB) = 4.0 GHz (14.0 GHz to 18.0 GHz).

[0058] Example 3

[0059] The content of this example is compared with that of Example 1, and the only differences are that the ash content of the natural microcrystalline graphite ore is 15.7%; the SiO2 content is 6.8%; the fixed carbon content is 81.1%, and it is pulverized to a particle size (D 50 ) of 1.9 μm, and the others are the same as in Example 1.

[0060] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 16.8 nm.

[0061] Tested by the coaxial ring method, when the content is 25% and the thickness is 1.5 mm, the lowest reflection loss (RL) = -36 dB, and the corresponding impedance matching coefficient |Z in| / Z0| = 1.023, effective absorption bandwidth (EAB) = 4.3 GHz (13.7 GHz to 18.0 GHz); when the thickness is 1.6 mm, the widest effective absorption bandwidth (EAB) = 4.7 GHz (12.7 GHz to 17.4 GHz), corresponding reflection loss (RL) = -26 dB, corresponding impedance matching coefficient |Z in / Z0| = 1.011.

[0062] Example 4

[0063] The content of this example is compared with that of Example 1, and the difference is only that the ash content of the natural microcrystalline graphite raw ore is 23.5%; the SiO2 content is 10.2%; the fixed carbon content is 73.1%, and it is crushed to a particle size (D 50 ) of 2.1 μm, and the others are the same as in Example 1.

[0064] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 17.6 nm.

[0065] Tested by the coaxial ring method, when the content is 25% and the thickness is 1.6 mm, the lowest reflection loss (RL) = -48 dB, corresponding impedance matching coefficient |Z in / Z0| = 1.085, effective absorption bandwidth (EAB) = 4.6 GHz (12.8 GHz to 17.4 GHz); when the thickness is 2.0 mm, the effective absorption bandwidth (EAB) = 3.8 GHz (10.0 GHz to 13.8 GHz), corresponding reflection loss (RL) = -24 dB, corresponding impedance matching coefficient |Z in / Z0| = 0.957.

[0066] Example 5

[0067] The content of this example is compared with that of Example 1, and the difference is only that the natural microcrystalline graphite raw ore is crushed by a Raymond mill to pass through a 200-mesh (75 μm) sieve, and the others are the same as in Example 1.

[0068] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 22.4 nm.

[0069] Tested by the coaxial ring method, when the content is 40% and the thickness is 2.4 mm, the lowest reflection loss (RL) = -40 dB, corresponding impedance matching coefficient |Z in|Z / Z0| = 1.030, effective absorption bandwidth (EAB) = 3.0 GHz (7.9 GHz to 10.9 GHz); when the thickness is 2.5 mm, there is an optimal impedance matching coefficient |Z in |Z / Z0| = 1.011, the corresponding widest effective absorption bandwidth (EAB) = 3.0 GHz (7.5 GHz to 10.5 GHz), and the corresponding reflection loss (RL) = -28 dB.

[0070] Example 6

[0071] The content of this example is compared with that of Example 1. The only difference is that the natural microcrystalline graphite ore is pulverized by a Raymond mill to pass through a 200-mesh (75 μm) sieve, and the heat preservation time is 10 h. Others are the same as in Example 1.

[0072] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in the form of nano-curved and dot-like structures, and its average grain size (thickness) is about 23.7 nm.

[0073] Tested by the coaxial ring method, when the content is 40% and the thickness is 2.4 mm, there is a minimum reflection loss (RL) = -52 dB, and the corresponding impedance matching coefficient |Z in |Z / Z0| = 1.032, effective absorption bandwidth (EAB) = 2.4 GHz (7.7 GHz to 10.1 GHz); when the thickness is 2.5 mm, there is an optimal impedance matching coefficient |Z in |Z / Z0| = 1.000, the corresponding widest effective absorption bandwidth (EAB) = 2.4 GHz (7.2 GHz to 9.6 GHz), and the corresponding reflection loss (RL) = -37 dB.

[0074] Example 7

[0075] The content of this example is compared with that of Example 1. The only difference is that the natural microcrystalline graphite ore is pulverized by a Raymond mill to pass through a 200-mesh (75 μm) sieve, and the heating temperature is 1600 °C. Others are the same as in Example 1.

[0076] The natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, in which the generated silicon carbide is mainly in the form of nano-curved and dot-like structures, and its average grain size (thickness) is about 25.2 nm.

[0077] Tested by the coaxial ring method, when the content is 40% and the thickness is 2.9 mm, there is a minimum reflection loss (RL) = -50 dB, and the corresponding impedance matching coefficient |Z in |Z / Z0| = 0.998, effective absorption bandwidth (EAB) = 2.0 GHz (6.7 GHz to 8.7 GHz).

[0078] Example 8

[0079] The content of this example is different from that of Example 1 only in that the natural microcrystalline graphite ore is crushed by a Raymond mill to pass through a 200-mesh (75 μm) sieve, and the heating temperature is 1350 °C, and the others are the same as in Example 1.

[0080] For the natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 20.7 nm.

[0081] Tested by the coaxial ring method, when the content is 40% and the thickness is 3.0 mm, the reflection loss (RL) = -34 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 0.975, and the effective absorption bandwidth (EAB) = 2.2 GHz (6.0 GHz to 8.2 GHz).

[0082] Example 9

[0083] The content of this example is different from that of Example 1 only in that the natural microcrystalline graphite ore is crushed by a Raymond mill to pass through a 200-mesh (75 μm) sieve, and the heat preservation time is 2 h, and the others are the same as in Example 1.

[0084] For the natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this example, the generated silicon carbide is mainly in a nano-curved structure, and its average grain size (thickness) is about 22.4 nm.

[0085] Tested by the coaxial ring method, when the content is 40% and the thickness is 3.0 mm, the lowest reflection loss (RL) = -28 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 1.027, and the effective absorption bandwidth (EAB) = 2.1 GHz (6.0 GHz to 8.1 GHz).

[0086] Comparative Example 1

[0087] The content of this comparative example is different from that of Example 1 only in that the heating temperature is 1250 °C, and the others are the same as in Example 1.

[0088] In the sample prepared in this comparative example, through X-ray diffraction analysis, no silicon carbide is generated, indicating that under the conditions of 1250 °C heat preservation for 4 h, the graphite matrix and SiO2 cannot undergo a carbothermal reduction reaction to generate silicon carbide.

[0089] Tested by the coaxial ring method, when the content is 25% and the thickness is 1.5 mm, there is a lowest reflection loss (RL) = -17 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 0.814, and the effective absorption bandwidth (EAB) = 4.1 GHz (13.9 GHz to 18.0 GHz).

[0090] Although the sample prepared in this comparative example has a relatively wide effective absorption bandwidth (EAB), reaching 4.1 GHz, its spatial impedance matching performance is poor, and the |Z in / Z0| result is only 0.814, far less than 1, indicating that the conductivity of the material is too high and a large amount of electromagnetic waves will be reflected, so it has no practical value.

[0091] Comparative Example 2

[0092] The content of this comparative example is the same as that of Example 1, except that the heating temperature is 1650 °C, and the others are the same as in Example 1.

[0093] For the natural microcrystalline graphite-based SiC / C composite absorbing material prepared in this comparative example, the generated silicon carbide is mainly columnar and dot-like structures, and its average grain size (thickness) is about 31.4 nm, and it contains a relatively high α-SiC component, indicating that at a temperature of 1650 °C, the generated β-SiC grains grow and most of them are transformed into α-SiC, resulting in a significant decrease in the effective concentration of β-SiC in the material.

[0094] Tested by the coaxial ring method, when the content is 20% and the thickness is 2.0 mm, the minimum reflection loss (RL) = -22 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 1.351, and the effective absorption bandwidth (EAB) = 5.9 GHz (11.8 GHz to 17.7 GHz). Although the effective absorption bandwidth (EAB) is relatively wide, its spatial impedance matching performance is poor and it has no practical value; when the thickness is 5.0 mm, the minimum reflection loss (RL) = -39 dB, and the corresponding impedance matching coefficient |Z in / Z0| = 1.069, and the effective absorption bandwidth (EAB) = 1.5 GHz (4.1 GHz to 5.6 GHz). Although the minimum reflection loss and the impedance matching coefficient are good, the effective absorption bandwidth is narrow and the material is thick, not meeting the requirements of "thin thickness, light weight, wide range, and strong absorption" of the absorbing material, so it has no practical value.

[0095] Comparative Example 3

[0096] The content of this comparative example is the same as that of Example 1, except that the ash content of the natural microcrystalline graphite raw ore is 34.4%; the SiO2 content is 15.2%; the fixed carbon content is 62.0%, and the others are the same as in Example 1.

[0097] The natural microcrystalline graphite-based SiC / C composite microwave absorbing material prepared in this comparative example, in which the generated silicon carbide is mainly columnar and punctate in structure, and its average grain size (thickness) is about 40.1 nm, indicating that too high SiO2 content in the raw materials will significantly increase the contact area of the generated silicon carbide, resulting in the crystal merging and growing, and the effective concentration is significantly reduced.

[0098] Tested by the coaxial ring method, when the content is 20% and the thickness is 2.0 mm, the effective absorption bandwidth (EAB) = 4.8 GHz (12.6 GHz to 17.4 GHz), and the corresponding impedance matching coefficient |Z in / Z0| = 1.431, and the reflection loss (RL) = -22 dB. Although the absorption bandwidth is relatively wide, the space impedance matching coefficient is much greater than 1, so it has no practical value.

[0099] Comparative Example 4

[0100] The content of this comparative example is compared with that of Example 1, and the difference is only that the ash content of the natural microcrystalline graphite ore is 9.8%; the SiO2 content is 3.8%; the fixed carbon content is 87.3%, and the others are the same as in Example 1.

[0101] The natural microcrystalline graphite-based SiC / C composite microwave absorbing material prepared in this comparative example, in which the generated silicon carbide is mainly nano-curved in structure, and its average grain size (thickness) is about 16.8 nm.

[0102] Tested by the coaxial ring method, it has the best microwave absorbing performance when the content is 20% and the thickness is 4.5 mm, and the space impedance matching coefficient |Z in / Z0| = 1.000, the reflection loss (RL) = -43 dB, and the effective absorption bandwidth (EAB) = 1.5 GHz (4.5 GHz to 6.0 GHz). Although the space impedance matching and reflection loss performances are better, due to the low effective silicon carbide content, the absorption bandwidth of the product is relatively narrow, and the material is relatively thick, which does not meet the requirements of microwave absorbing materials for "thin thickness, light weight, wide range, and strong absorption", so it has no practical value.

[0103] The above has made a detailed description of the embodiments of the present invention, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principle and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material, characterized in that: The following steps are involved: 1) crushing natural microcrystalline graphite ore to obtain graphite powder of required particle size; the ash content of the natural microcrystalline graphite ore is 15% to 25%, wherein the SiO2 content is 6.8%, 8.4% or 10.2%; 2) putting the graphite powder in step 1) into a crucible, first evacuating the crucible, then introducing a protective gas, heating to a temperature of 1350° C. to 1600° C., and keeping the temperature for 2 h to 10 h, and cooling to obtain a natural microcrystalline graphite-based SiC / C composite absorbing material; The natural microcrystalline graphite-based SiC / C composite absorbing material has a spatial impedance matching coefficient |Z in / Z0|≈1, reflection loss ≤-20dB, effective absorption bandwidth ≥2GHz.

2. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 1, characterized in that: The pulverization is one or more of ball milling, Raymond milling, air flow milling or roller milling, and the particle size of the graphite powder is less than or equal to 75 μm.

3. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to any one of claims 1 or 2, characterized in that: The particle size D of the graphite powder 50 Less than or equal to 5μm.

4. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 1, characterized in that: The crucible is a graphite crucible, a carbon crucible or an alumina crucible.

5. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 1, characterized in that: The protective gas is one or more of nitrogen, argon and helium.

6. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 1, characterized in that: The temperature is 1450°C to 1500°C.

7. The method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 1, characterized in that: The SiC is β-SiC.

8. A natural microcrystalline graphite-based SiC / C composite absorbing material, characterized in that: The material is prepared according to the method for preparing a natural microcrystalline graphite-based SiC / C composite absorbing material according to any one of claims 1 to 7.

9. A natural microcrystalline graphite-based SiC / C composite absorbing material according to claim 8, characterized in that: SiC is uniformly dispersed in graphite in the form of nano-curves.

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