A frequency selective absorbing material based on CrFe-MOFs and a preparation method thereof

Abstract

The present invention proposes a frequency selective absorbing material based on CrFe-MOFs and a preparation method thereof, and relates to the field of preparation of absorbing materials. The frequency selective absorbing material is used for band absorbing in the microwave band. The method comprises: preparing a CrFe-MOFs precursor containing a magnetic metal and a non-magnetic metal by a hydrothermal method in a programmed temperature manner; then placing the CrFe-MOFs precursor in a tubular furnace for calcination treatment to obtain a CrFe / C nanocomposite material as the frequency selective absorbing material. The frequency selective absorbing material prepared by the present invention has the characteristics of high porosity and large specific surface area in structure; at the same time, it overcomes the shortcomings of a single magnetic metal material having only magnetic loss and high density, and a single non-magnetic metal having a low dielectric constant.

Description

Technical Field

[0001] The invention belongs to the field of preparation of wave absorbing materials, and in particular relates to a frequency selective wave absorbing material and a preparation method thereof. Background Art

[0002] With the explosive development of electronic science and technology, the increasingly serious electromagnetic pollution problems in radiation and interference have received great attention. These pollutions not only hinder the normal operation of electronic equipment, but also lead to the deterioration of cell cancer and hormone abnormalities, affecting human health. In order to regulate these negative effects, microwave absorbing materials are indispensable in electromagnetic wave technology.

[0003] In terms of the attenuation mechanism of electromagnetic waves, electromagnetic wave absorption materials can generally be divided into conductive materials, dielectric materials and magnetic materials. Typical conductive absorption materials are mainly carbon materials, such as graphene and carbon nanotubes, which have obvious advantages in low density and high stability. However, impedance mismatch hinders the further application of carbon materials. Inorganic ceramics and semiconductors are representatives of dielectric materials, which have significant advantages in thermal stability. However, low attenuation capacity limits the enhancement of absorption intensity. Magnetic metals and ferrites are widely used to prepare thinner electromagnetic wave absorbing materials due to their high saturation magnetization and high magnetic permeability at high frequencies. However, high density and inherent impedance mismatch as their inevitable shortcomings limit their further application. Therefore, in order to overcome the shortcomings of single-component electromagnetic wave absorbing materials, the design and fusion of multiple components has become an inevitable choice. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes a frequency selective absorbing material based on CrFe-MOFs and a preparation method thereof. The present invention uses magnetic and non-magnetic metal salts as raw materials to prepare the frequency selective absorbing material, which overcomes the problem that a single magnetic metal material only has magnetic loss and high density, and a single non-magnetic metal has a low dielectric constant.

[0005] The first aspect of the present invention discloses a method for preparing a frequency selective absorbing material based on CrFe-MOFs. The preparation method comprises:

[0006] Step S1, preparing a bimetallic salt solution containing a magnetic metal and a non-magnetic metal and an organic ligand solution respectively.

[0007] Step S2, adding the organic ligand solution to the bimetallic salt solution to obtain a mixed solution of the bimetallic salt solution and the organic ligand solution, transferring the obtained mixed solution to a reactor with a polytetrafluoroethylene substrate, moving the reactor to an oven for heating, and using a programmed temperature method to heat the mixed solution to prepare a CrFe-MOFs suspension.

[0008] Step S3, placing the CrFe-MOFs suspension in a centrifuge for centrifugal treatment to obtain a CrFe-MOFs precipitate after removing the supernatant, and placing the CrFe-MOFs precipitate in a vacuum drying oven for drying to obtain CrFe-MOFs powder as a bimetallic MOFs precursor.

[0009] Step S4, placing the CrFe-MOFs precursor in a tube furnace for calcination treatment to obtain a CrFe / C nanocomposite material as the frequency selective absorbing material.

[0010] According to the preparation method of the first aspect of the present invention, in step S1:

[0011] The double metal salt solution uses iron salt as a magnetic metal and chromium salt as a non-magnetic metal as raw materials and deionized water as a solvent.

[0012] The organic ligand solution uses dicarboxylic acid as a raw material and deionized water as a solvent.

[0013] According to the preparation method of the first aspect of the present invention, in step S1:

[0014] The bimetallic salt solution uses chromium nitrate nonahydrate and ferric chloride hexahydrate as raw materials and deionized water as solvent.

[0015] The organic ligand solution uses terephthalic acid as a raw material and deionized water as a solvent.

[0016] According to the preparation method of the first aspect of the present invention, the organic ligand solution also includes triethylamine, an organic base, as an auxiliary raw material.

[0017] Wherein, the molar ratio of the triethylamine to the terephthalic acid is 1:50.

[0018] According to the preparation method of the first aspect of the present invention, the mass of the chromium nitrate nonahydrate in the bimetallic salt solution is 0.75 to 0.85 g, the mass of the ferric chloride hexahydrate is 0.45 to 0.57 g, and the solvent is 15 to 25 ml of deionized water.

[0019] The mass of the terephthalic acid in the organic ligand solution is 0.25 to 0.35 grams, and the solvent is 15 to 25 milliliters of deionized water.

[0020] According to the preparation method of the first aspect of the present invention, the molar ratio of the chromium nitrate nonahydrate to the ferric chloride hexahydrate is 1:1.

[0021] According to the preparation method of the first aspect of the present invention, in step S2:

[0022] Before the organic ligand solution is added dropwise to the bimetallic salt solution, the bimetallic salt solution is placed on a magnetic stirrer for stirring. When the stirring rate reaches 390 to 420 rpm, the organic ligand solution is started to be added dropwise to the bimetallic salt solution, and the stirring rate is maintained at 390 to 420 rpm until the organic ligand solution is completely added to the bimetallic salt solution. Stirring is continued for 5 to 10 minutes to fully mix the bimetallic salt solution and the organic ligand solution.

[0023] According to the preparation method of the first aspect of the present invention, in step S2, the mixed solution is heated by a programmed temperature method, the temperature is programmed to 170-200°C, and the solution is heated for 16-24 hours; wherein the rate of programmed temperature increase and temperature decrease is 0.5-1°C / min.

[0024] According to the preparation method of the first aspect of the present invention, in the step S3, the centrifugal rate of the centrifugal treatment is 7500 to 8500 rpm, and the treatment time of the centrifugal treatment is 8 to 12 minutes.

[0025] According to the preparation method of the first aspect of the present invention, in the step S3, the drying temperature of the vacuum drying oven is 50 to 70 degrees Celsius, and the treatment time of the drying treatment is 10 to 16 hours.

[0026] According to the preparation method of the first aspect of the present invention, in the step S4, the heating rate of the tube furnace is 4 to 6 degrees Celsius per minute, the calcination temperature of the calcination treatment is 450 to 650 degrees Celsius, and the treatment time of the calcination treatment is 100 to 140 minutes.

[0027] The second aspect of the present invention discloses a frequency selective absorbing material prepared based on the preparation method provided by the first aspect of the present invention, wherein the frequency selective absorbing material is used for frequency band absorbing in the microwave band.

[0028] In summary, the solution proposed by the present invention has the following technical effects:

[0029] The present invention designs the composition of bimetallic MOFs materials to improve the wave absorbing performance of the materials after calcination. The present invention adopts a combination of magnetic metal and non-magnetic metal to prepare frequency selective wave absorbing materials, thus overcoming the disadvantages of single magnetic metal wave absorbing materials with high magnetic loss and density.

[0030] In addition, the CrFe / C frequency selective absorbing material prepared by the present invention retains the original carbon skeleton structure of CrFe-MOFs, has a unique structure with high porosity and large specific surface area, and increases the complexity of the composite material morphology. High porosity is conducive to optimizing impedance matching, and large specific surface area can promote the scattering attenuation of electromagnetic waves; at the same time, high porosity and large specific surface area can greatly increase the conductive polarization of the CrFe bimetallic and the dielectric polarization of the interface between the CrFe bimetallic and the carbon, which is conducive to further enhancing the dielectric loss capacity by inducing surface polarization and polarization relaxation. Due to the combination of the CrFe bimetallic and the carbon material, the strong dielectric loss caused by the conductive loss and surface polarization, and the magnetic loss caused by the ferromagnetic resonance and eddy current loss, the absorption performance of the CrFe / C frequency selective absorbing material prepared by the present invention for electromagnetic waves has been significantly improved.

[0031] In addition, the present invention utilizes a simple high-temperature calcination method to partially destroy the original carbon skeleton structure of CrFe-MOFs, realize the agglomeration of some magnetic metal and non-magnetic metal particles, and obtain an absorbing material with both low cost and strong frequency selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 is a flow chart of a method for preparing a frequency selective absorbing material according to an embodiment of the present invention;

[0034] Figure 2a-2d The SEM images of the specific example 1 and the comparative examples 1-3 according to the embodiment of the present invention;

[0035] Figure 3 TEM image of specific example 1 according to an embodiment of the present invention;

[0036] Figure 4 XRD test diagrams of specific example 1 and comparative examples 1-2 according to an embodiment of the present invention;

[0037] Figure 5a-5c It is a comparison chart of reflection loss curves of specific example 1 according to an embodiment of the present invention and comparative examples 1-2 in the microwave frequency band of 2 GHZ to 18 GHZ. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0039] The first aspect of the present invention discloses a method for preparing a frequency selective absorbing material based on CrFe-MOFs. Figure 1 FIG. 1 is a flow chart of a method for preparing a frequency selective absorbing material based on CrFe-MOFs according to an embodiment of the present invention. Figure 1 As shown, the preparation method comprises:

[0040] Step S1, preparing a bimetallic salt solution containing a magnetic metal and a non-magnetic metal and an organic ligand solution respectively.

[0041] The present invention uses magnetic and non-magnetic metal salt solutions as raw materials to prepare frequency selective wave absorbing materials, which overcomes the problems that a single magnetic metal material only has magnetic loss and high density, and a single non-magnetic metal has a low dielectric constant. At the same time, the present invention combines different metal centers, which not only increases the interface polarization loss and multiple reflection loss, but also the synergistic effect of magnetic-dielectric loss and good impedance matching can improve the composite material's electromagnetic wave absorption performance.

[0042] Step S2, adding the organic ligand solution to the bimetallic salt solution to obtain a mixed solution of the bimetallic salt solution and the organic ligand solution, transferring the obtained mixed solution to a reactor with a polytetrafluoroethylene substrate, moving the reactor to an oven for heating, and using a programmed temperature method to heat the mixed solution to prepare a CrFe-MOFs suspension.

[0043] The reaction rate of the precursor can be controlled by using a programmed temperature increase method, ensuring that each part of the mixed solution is heated evenly and has a consistent boiling point distribution range, thereby making the structure and performance of the MOFs of the present invention more stable.

[0044] Step S3, placing the CrFe-MOFs suspension in a centrifuge for centrifugal treatment to obtain a CrFe-MOFs precipitate after removing the supernatant, and placing the CrFe-MOFs precipitate in a vacuum drying oven for drying to obtain CrFe-MOFs powder as a bimetallic MOFs precursor.

[0045] Step S4, placing the CrFe-MOFs precursor in a tube furnace for calcination treatment to obtain a CrFe / C nanocomposite material as the frequency selective absorbing material.

[0046] The CrFe-MOFs precursor is pyrolyzed at high temperature during the calcination process, and the original carbon skeleton structure is gradually destroyed, so that a small amount of metal ions are agglomerated and converted into a porous carbon nanomaterial modified with CrFe metal, which further increases the specific surface area and porosity of the frequency selective absorbing material, which is beneficial to optimize impedance matching and promote the scattering attenuation of electromagnetic waves.

[0047] In step S1, a double metal salt solution containing a magnetic metal and a non-magnetic metal and an organic ligand solution are prepared respectively.

[0048] In some embodiments, the double metal salt solution uses iron salt as a magnetic metal and chromium salt as a non-magnetic metal as raw materials, and deionized water as a solvent.

[0049] The organic ligand solution uses dicarboxylic acid as a raw material and deionized water as a solvent.

[0050] In some embodiments, the bimetallic salt solution uses chromium nitrate nonahydrate and ferric chloride hexahydrate as raw materials and deionized water as solvent.

[0051] The organic ligand solution uses terephthalic acid as a raw material and deionized water as a solvent.

[0052] In some embodiments, the organic ligand solution further comprises triethylamine, an organic base, as an auxiliary raw material. The triethylamine is used to promote the dissolution of terephthalic acid in water.

[0053] Wherein, the molar ratio of the triethylamine to the terephthalic acid is 1:50.

[0054] In some embodiments, the mass of the chromium nitrate nonahydrate in the bimetallic salt solution is 0.75 to 0.85 g, the mass of the ferric chloride hexahydrate is 0.45 to 0.57 g, and the solvent is 15 to 25 ml of deionized water.

[0055] The mass of the terephthalic acid in the organic ligand solution is 0.25 to 0.35 grams, and the solvent is 15 to 25 milliliters of deionized water.

[0056] In some embodiments, the molar ratio of the chromium nitrate nonahydrate to the ferric chloride hexahydrate is 1:1.

[0057] In the preparation process of bimetallic absorbers, the different ratios of magnetic metals and non-magnetic metals will directly affect the grain size, morphology and reflection loss performance of bimetallic absorbers. The grain morphology of Cr-MOFs without iron doping is a regular polyhedral structure. When doped with Fe, the grain morphology changes. x Fe y / C presents a regular cylindrical structure. As the proportion of iron increases, the grain size increases. When the ratio of chromium to iron is not equal to 1, the reflection loss of the bimetallic absorbing material decreases.

[0058] In step S2, the organic ligand solution is added to the bimetallic salt solution to obtain a mixed solution of the bimetallic salt solution and the organic ligand solution, the obtained mixed solution is transferred to a reactor with a polytetrafluoroethylene substrate, the reactor is moved to an oven for heating, and the mixed solution is heated by a programmed temperature method to prepare a CrFe-MOFs suspension.

[0059] In some embodiments, in step S2, before the organic ligand solution is added dropwise to the bimetallic salt solution, the bimetallic salt solution is placed on a magnetic stirrer for stirring, and when the stirring rate reaches 390 to 420 rpm, the organic ligand solution is started to be added dropwise to the bimetallic salt solution, and the stirring rate is maintained at 390 to 420 rpm until the organic ligand solution is completely added to the bimetallic salt solution, and stirring is continued for 5 to 10 minutes to fully mix the bimetallic salt solution and the organic ligand solution.

[0060] The present invention fully mixes the bimetallic salt solution and the organic ligand solution by means of a magnetic stirrer to ensure smooth progress of the subsequent hydrothermal reaction process.

[0061] In some embodiments, in step S2, the mixed solution is heated by a programmed temperature rising method, the temperature is programmed to 170-200°C, and the mixture is heated for 16-24 hours; wherein the rate of programmed temperature rising and cooling is 0.5-1°C / min.

[0062] When the CrFe-MOFs precursor is prepared by the hydrothermal method, the mixed solution is first heated to a target temperature of 170-200°C at a relatively low heating rate of 0.5-1°C / min to ensure that all parts of the mixed solution are heated evenly. During this process, the solubility of terephthalic acid in water gradually increases, and it reacts with Cr salt and Fe salt to generate a small amount of CrFe-MOFs nuclei; as the heating time increases, the CrFe-MOFs nuclei grow from small to large into CrFe-MOFs crystals, and the number of CrFe-MOFs crystals increases until the reaction is completely completed; then the temperature is lowered at a rate of 0.5-1°C / min, and the relatively low rate can avoid the agglomeration of CrFe-MOFs and form large-sized grains.

[0063] In step S3, the CrFe-MOFs suspension is placed in a centrifuge for centrifugal treatment to obtain a CrFe-MOFs precipitate after removing the supernatant, and the CrFe-MOFs precipitate is placed in a vacuum drying oven for drying to obtain CrFe-MOFs powder as a bimetallic MOFs precursor.

[0064] In some embodiments, in step S3, the centrifugal treatment has a centrifugal rate of 7500 to 8500 rpm, and the treatment time of the centrifugal treatment is 8 to 12 minutes.

[0065] In some embodiments, in step S3, the drying temperature of the vacuum drying oven is 50 to 70 degrees Celsius, and the drying time is 10 to 16 hours.

[0066] In step S4, the CrFe-MOFs precursor is placed in a tube furnace for calcination to obtain a CrFe / C nanocomposite material as the frequency selective absorbing material.

[0067] In some embodiments, in step S4, the heating rate of the tube furnace is 4 to 6 degrees Celsius per minute, the calcination temperature of the calcination treatment is 450 to 650 degrees Celsius, and the treatment time of the calcination treatment is 100 to 140 minutes.

[0068] By controlling the calcination temperature, the electromagnetic parameters can be adjusted and the impedance matching can be optimized to obtain electromagnetic wave absorbing materials with better performance.

[0069] When the calcination temperature is between 450 and 650 degrees Celsius, a small amount of the original CrFe-MOFs precursor skeleton structure is destroyed, and the CrFe metal ions are converted into CrFe metal elements; at the same time, the organic components lose H, O and part of C in the form of water and organic vapor at high temperatures, and are converted into a more condensed carbon network. Within the calcination temperature and calcination time range of the present invention, the high porosity and large specific surface area structural characteristics of the CrFe-MOFs precursor itself can be transferred to the formed porous CrFe / C composite material to a large extent, and the porous CrFe / C composite material retains the overall structural characteristics of CrFe-MOFs, thereby forming high porosity and large specific surface area.

[0070] When the calcination temperature is lower than 450 degrees Celsius, the carbonization of CrFe-MOFs is incomplete, and the absorbing ability of the frequency selective absorbing material is poor. When the calcination temperature is higher than 650 degrees Celsius, the C in CrFe-MOFs is completely oxidized and the metal nanocrystals are completely gasified, and the absorbing ability of the CrFe / C frequency selective absorbing material is poor.

[0071] The second aspect of the present invention discloses a frequency selective absorbing material based on CrFe-MOFs, wherein the frequency selective absorbing material is used for frequency band absorbing in the microwave band.

[0072] Example 1: Preparation of CrFe / C frequency selective absorbing material

[0073] 0.8 g of chromium nitrate nonahydrate and 0.541 g of ferric chloride hexahydrate were weighed and dispersed in 20 ml of deionized water to prepare a bimetallic salt solution; 0.332 g of terephthalic acid was weighed and dispersed in 20 ml of deionized water to prepare an organic ligand solution.

[0074] The bimetallic salt solution was placed on a magnetic stirrer and stirred at a rate of 400 rpm. At the same time, the organic ligand solution was added dropwise to the bimetallic salt solution until the organic ligand solution was completely added to the bimetallic salt solution. The stirring was continued for 5 minutes.

[0075] The obtained mixed solution was transferred to a reactor with a polytetrafluoroethylene substrate, and the reactor was moved to a programmed temperature oven for heating. The heating time was 18 hours, the heating temperature was 180°C, and the programmed heating and cooling rates were both 0.5°C / min.

[0076] After the hydrothermal reaction is completed, the obtained CrFe-MOFs suspension is placed in a centrifuge for centrifugal treatment. The centrifugal time is set to 10 minutes and the centrifugal speed is set to 8000 rpm.

[0077] The supernatant of the solution obtained after centrifugation is removed to obtain a CrFe-MOFs precipitate, and the obtained CrFe-MOFs precipitate is placed in a vacuum drying oven for drying to obtain a CrFe-MOFs precursor, wherein the drying temperature is 60 degrees Celsius and the drying time is 12 hours.

[0078] The CrFe-MOFs precursor was placed in a tube furnace for calcination to obtain the CrFe / C frequency selective wave absorbing material, wherein the tube furnace heating rate was 5 degrees Celsius per minute, the calcination temperature was 500 degrees Celsius, and the calcination time was 120 minutes.

[0079] Comparative Example 1: Preparation of Cr2Fe1 / C composite material

[0080] The difference from Example 1 is that the mass of chromium nitrate nonahydrate used is 1.6 g.

[0081] Comparative Example 2: Preparation of Cr1Fe2 / C composite material

[0082] The difference from Example 1 is that the mass of ferric chloride hexahydrate used is 1.082 g.

[0083] Comparative Example 3: Preparation of Cr / C composite material

[0084] The difference from Example 1 is that ferric chloride hexahydrate is not used.

[0085] Results Analysis

[0086] Figure 2a-2d The scanning electron microscope photos of Example 1 and Comparative Examples 1-3 are shown in sequence. Figure 2a As shown in the figure, when the Cr-Fe elements are mixed in a 1:1 molar ratio, the grain diameter of the CrFe / C absorbing material is about 100nm; the grain morphology of the Cr / C composite material without Fe element doping is a regular polyhedral structure ( Figure 2d ). When doped with magnetic metal Fe, the morphology of the composite material grains changes to a regular cylindrical structure. Figure 2b As shown in the scanning electron microscope photograph of Comparative Example 1, when the Cr-Fe elements are mixed in a molar ratio of 2:1, the grain structure is a regular cylindrical structure with a grain diameter of about 150nm. Figure 2c As shown in the scanning electron microscope photograph of Comparative Example 2, when the Cr-Fe elements are mixed in a molar ratio of 1:2, as the Fe element increases, the diameter of the grain cylinder increases to about 2 um.

[0087] Figure 3 It is a transmission electron microscope image according to Example 1 of the present invention. It further confirms that the grain diameter of the CrFe / C absorbing material is about 100 nm.

[0088] Figure 4 The XRD test diagrams of Example 1 and Comparative Examples 1-2 according to the present invention illustrate that when the molar ratio of Cr-Fe is 1:1, the crystal phase of the CrFe / C absorbing material is the most complete and the diffraction peaks of each element are the strongest.

[0089] Figure 5a-5c This is a comparison chart of reflection loss curves in the microwave frequency range of 2-18 GHz according to Example 1 of the present invention and Comparative Examples 1-2. Figure 5a It can be seen that in Example 1, the effective absorption frequency band is about 15-18 GHz, the effective absorption bandwidth is about 3 GHz, and when the thickness of the absorbing material is 1.94 mm, the CrFe / C absorbing material reaches a maximum reflection loss of -54.47 dB at 16.6 GHz. Figure 5b and 5c It can be seen that Cr x Fe y The maximum reflection loss performance of the absorbing material with a Cr / C ratio decreases with the increase of different Cr-Fe component ratios, further indicating that the absorbing material has the best performance when the Cr-Fe molar ratio is 1:1.

[0090] Example 2: Preparation of CrFe / C frequency selective absorbing material

[0091] 0.75 g of chromium nitrate nonahydrate and 0.507 g of ferric chloride hexahydrate were weighed and dispersed in 20 ml of deionized water to prepare a bimetallic salt solution; 0.311 g of terephthalic acid was weighed and dispersed in 20 ml of deionized water to prepare an organic ligand solution.

[0092] The bimetallic salt solution was placed on a magnetic stirrer and stirred at a rate of 390 rpm. At the same time, the organic ligand solution was added dropwise to the bimetallic salt solution until the organic ligand solution was completely added to the bimetallic salt solution. Stirring was continued for 10 minutes.

[0093] The obtained mixed solution was transferred to a reactor with a polytetrafluoroethylene substrate, and the reactor was moved to a programmed temperature oven for heating. The heating time was 24 hours, the heating temperature was 170°C, and the programmed heating and cooling rates were both 1°C / min.

[0094] After the hydrothermal reaction is completed, the obtained CrFe-MOFs suspension is placed in a centrifuge for centrifugal treatment. The centrifugal time is set to 8 minutes and the centrifugal speed is set to 8500 rpm.

[0095] The supernatant of the solution obtained after centrifugation is removed to obtain a CrFe-MOFs precipitate, and the obtained CrFe-MOFs precipitate is placed in a vacuum drying oven for drying to obtain a CrFe-MOFs precursor, wherein the drying temperature is 50 degrees Celsius and the drying time is 16 hours.

[0096] The CrFe-MOFs precursor was placed in a tube furnace for calcination to obtain the CrFe / C frequency selective wave absorbing material, wherein the tube furnace heating rate was 6 degrees Celsius per minute, the calcination temperature was 650 degrees Celsius, and the calcination time was 100 minutes.

[0097] Example 3: Preparation of CrFe / C frequency selective absorbing material

[0098] 0.84 g of chromium nitrate nonahydrate and 0.570 g of ferric chloride hexahydrate were weighed and dispersed in 20 ml of deionized water to prepare a bimetallic salt solution; 0.350 g of terephthalic acid was weighed and dispersed in 20 ml of deionized water to prepare an organic ligand solution.

[0099] The bimetallic salt solution was placed on a magnetic stirrer and stirred at a rate of 420 rpm. At the same time, the organic ligand solution was added dropwise to the bimetallic salt solution until the organic ligand solution was completely added to the bimetallic salt solution. Stirring was continued for 8 minutes.

[0100] The obtained mixed solution was transferred to a reactor with a polytetrafluoroethylene substrate, and the reactor was moved to a programmed temperature oven for heating. The heating time was 16 hours, the heating temperature was 200°C, and the programmed heating and cooling rates were both 0.8°C / min.

[0101] After the hydrothermal reaction, the obtained CrFe-MOFs suspension was placed in a centrifuge for centrifugal treatment. The centrifugal time was set to 12 minutes and the centrifugal speed was set to 7500 rpm.

[0102] The supernatant of the solution obtained after centrifugation is removed to obtain a CrFe-MOFs precipitate, and the obtained CrFe-MOFs precipitate is placed in a vacuum drying oven for drying to obtain a CrFe-MOFs precursor, wherein the drying temperature is 70 degrees Celsius and the drying time is 10 hours.

[0103] The CrFe-MOFs precursor was placed in a tube furnace for calcination to obtain the CrFe / C frequency selective wave absorbing material, wherein the tube furnace heating rate was 4 degrees Celsius per minute, the calcination temperature was 450 degrees Celsius, and the calcination time was 140 minutes.

[0104] Example 4: Preparation of CrFe / C frequency selective absorbing material

[0105] The difference from Example 1 is that the prepared organic ligand solution contains triethylamine, and the molar ratio of the triethylamine to the phthalic acid is 1:50.

[0106] In summary, the present invention designs the composition of bimetallic MOFs materials to improve their absorbing performance as absorbing materials after calcination. The present invention adopts a combination of magnetic metal and non-magnetic metal to prepare frequency selective absorbing materials, overcoming the shortcomings of single magnetic metal absorbing materials with high magnetic loss and density.

[0107] In addition, the CrFe / C frequency selective absorbing material prepared by the present invention retains the original carbon skeleton structure of CrFe-MOFs, has a unique structure with high porosity and large specific surface area, and increases the complexity of the composite material morphology. High porosity is conducive to optimizing impedance matching, and large specific surface area can promote the scattering attenuation of electromagnetic waves; at the same time, high porosity and large specific surface area can greatly increase the conductive polarization of the CrFe bimetallic and the dielectric polarization of the interface between the CrFe bimetallic and the carbon, which is conducive to further enhancing the dielectric loss capacity by inducing surface polarization and polarization relaxation. Due to the combination of the CrFe bimetallic and the carbon material, the strong dielectric loss caused by the conductive loss and surface polarization, and the magnetic loss caused by the ferromagnetic resonance and eddy current loss, the absorption performance of the CrFe / C frequency selective absorbing material prepared by the present invention for electromagnetic waves has been significantly improved.

[0108] In addition, the present invention utilizes a simple high-temperature calcination method to partially destroy the original carbon skeleton structure of CrFe-MOFs, realize the agglomeration of some magnetic metal and non-magnetic metal particles, and obtain an absorbing material with both low cost and strong frequency selectivity.

[0109] The beneficial technical effects of the present invention include: (1) the frequency selective absorbing material of the present invention can act efficiently in the microwave frequency band; (2) the composite material prepared by the present invention has a high electromagnetic loss capability and can be used as a radar stealth material; (3) the composite material prepared by the present invention has a simple preparation process, a wide range of raw materials and a low price, and is easy to carry out large-scale industrial production.

[0110] Please note that the technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above embodiments only express several implementation methods of the present application, and their descriptions are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, without departing from the concept of the present application, several variations and improvements can be made, which all belong to the scope of protection of the present application. Therefore, the scope of protection of the patent in this application shall be based on the attached claims.

Claims

1. A method for preparing a frequency selective absorbing material based on CrFe-MOFs, characterized in that: The preparation method comprises: Step S1, preparing a bimetallic salt solution containing a magnetic metal and a non-magnetic metal and an organic ligand solution respectively; The bimetallic salt solution uses chromium nitrate nonahydrate and ferric chloride hexahydrate as raw materials and deionized water as solvent; the molar ratio of the chromium nitrate nonahydrate to the ferric chloride hexahydrate is 1:1; The organic ligand solution uses terephthalic acid as a raw material and deionized water as a solvent; Step S2, adding the organic ligand solution to the bimetallic salt solution to obtain a mixed solution of the bimetallic salt solution and the organic ligand solution, transferring the obtained mixed solution to a reactor with a polytetrafluoroethylene substrate, moving the reactor to an oven for heating, and heating the mixed solution by a programmed temperature method to prepare a CrFe-MOFs suspension; Step S3, placing the CrFe-MOFs suspension in a centrifuge for centrifugal treatment to obtain a CrFe-MOFs precipitate after removing the supernatant, and placing the CrFe-MOFs precipitate in a vacuum drying oven for drying to obtain a CrFe-MOFs powder as a CrFe-MOFs precursor; Step S4, placing the CrFe-MOFs precursor in a tube furnace for calcination treatment to obtain a CrFe / C nanocomposite material as the frequency selective absorbing material.

2. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: The mass of the chromium nitrate nonahydrate in the bimetallic salt solution is 0.75 to 0.85 g, the mass of the ferric chloride hexahydrate is 0.45 to 0.57 g, and the solvent is: 15 to 25 ml of deionized water; The mass of the terephthalic acid in the organic ligand solution is 0.25 to 0.35 grams, and the solvent is 15 to 25 milliliters of deionized water.

3. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: In step S2: Before the organic ligand solution is added dropwise to the bimetallic salt solution, the bimetallic salt solution is placed on a magnetic stirrer for stirring. When the stirring rate reaches 390 to 420 rpm, the organic ligand solution is started to be added dropwise to the bimetallic salt solution, and the stirring rate is maintained at 390 to 420 rpm until the organic ligand solution is completely added to the bimetallic salt solution. Stirring is continued for 5 to 10 minutes to fully mix the bimetallic salt solution and the organic ligand solution.

4. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: In the step S2, the mixed solution is heated by a programmed temperature rising method, the temperature is programmed to 170-200° C., and the solution is heated for 16-24 hours; wherein the rate of programmed temperature rising and cooling is 0.5-1° C. / min.

5. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: In step S3, the centrifugal treatment has a centrifugal rate of 7500 to 8500 rpm, and a treatment time of the centrifugal treatment is 8 to 12 minutes.

6. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: In step S3, the drying temperature of the vacuum drying oven is 50 to 70 degrees Celsius, and the drying time is 10 to 16 hours.

7. The method for preparing a frequency selective absorbing material based on CrFe-MOFs according to claim 1, characterized in that: In the step S4, the heating rate of the tube furnace is 4 to 6 degrees Celsius per minute, the calcination temperature of the calcination treatment is 450 to 650 degrees Celsius, and the treatment time of the calcination treatment is 100 to 140 minutes.

8. A frequency selective absorbing material prepared by the method for preparing a frequency selective absorbing material based on CrFe-MOFs according to any one of claims 1 to 7, characterized in that: The frequency selective absorbing material is used for absorbing waves in the microwave band.

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