Open-pored peanut-shaped perovskite NdFeO3 microwave absorbing powder material and its preparation method

Abstract

Aiming at the deficiencies in the morphology and structure of NdFeO3 microwave-absorbing fine powders, the structure and morphology of the materials are improved through temperature processes. An open-pored peanut-shaped perovskite-type NdFeO3 microwave-absorbing fine powder material is invented and designed. The preparation method is as follows: Weigh neodymium(III) nitrate hexahydrate and iron(III) nitrate nonahydrate in a certain proportion, and then add citric acid monohydrate and ammonia water. The NdFeO3 microwave-absorbing fine powder is prepared by the sol-gel method. By controlling the pH, sintering temperature, and sintering time, peanut-pored perovskite-type NdFeO3 microwave-absorbing fine powder is prepared. It has good microwave absorption performance in the 15 - 18 GHz microwave band, and the particles are generally in the shape of neat and regular rice grains, providing the possibility for coating the material surface in the later stage to prepare directional reflection components. At the same time, by controlling the drying temperature, the process operation is made easy.

Description

Technical Field

[0001] The present invention relates to a perovskite-type ferrite microwave absorption material, specifically a perovskite-type NdFeO3 microwave absorption powder material with an open-pored peanut-like structure, belonging to the technical field of microwave absorption materials.

Background Art

[0002] In this era of rapid technological development, materials are the most important pillar for the development of modern human civilization. Every aspect of life is inseparable from them and is closely related to them. The field of materials has long penetrated into all aspects of people's lives, bringing many conveniences, but at the same time, it has also brought serious electromagnetic pollution. Research shows that electromagnetic radiation has a great impact on people's bodies. Long-term exposure to a radiation environment can cause various adverse symptoms in people's bodies, such as headaches, fatigue, memory loss, vision loss, chest tightness, and neurological disorders. The electromagnetic waves radiated by electronic devices can interfere with each other, resulting in a decline or even failure of the performance of the electronic devices that are working. To solve this problem, many scientific researchers hope to study a microwave absorption material. As an effective functional material for suppressing electromagnetic radiation, microwave absorption materials play an increasingly important role in military and civilian fields and have become a research hotspot in various countries.

[0003] Among all microwave absorption materials, ferrite microwave absorption materials are relatively mature in research. There are mainly three types of ferrites: spinel-type ferrites and garnet-type ferrites with a cubic crystal system, and perovskite-type ferrites with a hexagonal crystal system. Among these three types, perovskite-type and spinel-type ferrites are more studied and applied in microwave absorption materials. Yao Qingrong et al. disclosed a perovskite-structured ferrite microwave absorption material and its preparation method (Patent No.: 2021104702893). The molecular formula of the perovskite-structured ferrite microwave absorption material is NdFe 1-x Ni3O3, (x = 0, 0.1, 0.2, 0.3). The preparation method uses neodymium nitrate hexahydrate, iron nitrate nonahydrate, and nickel nitrate hexahydrate as raw materials, dissolves them stoichiometrically in deionized water, adds an appropriate amount of citric acid monohydrate, drops ammonia water to adjust the pH to an alkaline environment, and places it in a water bath for heating and stirring to obtain a uniform wet gel; the obtained wet gel is dried in a forced-air drying oven and then pre-calcined to obtain a precursor. The obtained precursor powder is granulated with PVA, ground evenly, and molded into a cylindrical block by die pressing.

[0004] However, for the perovskite-structured ferrite microwave absorption material and its preparation method disclosed by Yao Qingrong et al. above, it mainly considers improving the microwave absorption performance of the material from the perspective of composition, and does not analyze the influence of the morphology and structure of the prepared material from the process, and then proposes to improve the microwave performance of the material.

Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention provides a Nd-Fe-based ferrite microwave absorbing fine powder material with a relatively large magnetic permeability, a relatively thin matching thickness (millimeter level), and good microwave absorption performance in the 15-18 GHz band, and a preparation method thereof. Morphologically, the porous alloy fine powder is beneficial to microwave absorption; structurally, the cavity is beneficial to the absorption of incident microwaves. Therefore, in the process of the present invention, a perovskite-type NdFeO3 microwave absorbing fine powder material with an open-pored peanut shape is prepared to achieve the purpose of improving microwave absorption. The microscopic morphology of the sample powder presents neatly arranged cavities, which provides the possibility for coating the surface of the material in the later stage to prepare a directional reflection component.

[0006] The technical solution to implement this scheme is: a perovskite-type NdFeO3 microwave absorbing fine powder material with an open-pored peanut shape, and its preparation method includes the following steps:

[0007] (1) According to the atomic ratio of Nd and Fe in the NdFeO3 compound, weigh the corresponding proportions of neodymium nitrate hexahydrate and iron nitrate nonahydrate, pour them into an appropriate amount of deionized water in sequence and mix, then add an appropriate amount of citric acid monohydrate, and stir until the solute is completely dissolved to obtain a mixed solution. The molar ratio of the nitrate ion to citric acid monohydrate is: 1 ≤ n(C6H8O7·H2O):n(NO3-) ≤ 1.2;

[0008] (2) Add ammonia water to the mixed solution to adjust the pH = 8;

[0009] (3) Put it into a thermostatic heating magnetic stirrer with a collecting heat function, set the temperature at 90 °C, and stir constantly until the water is evaporated to obtain a viscous gel;

[0010] (4) Place the gel in a drying oven and dry it at 180-200 °C to obtain a dry gel;

[0011] (5) Grind the obtained dry gel into powder, put it into a crucible and place it in a muffle furnace for sintering to obtain microwave absorbing fine powder. The sintering temperature is 700 °C, and the sintering time is 2-8 h.

[0012] Preferably, in step (4), the gel is placed in a drying oven and dried at 200 °C.

[0013] Preferably, in step (5) during the sintering process in the muffle furnace, the sintering time is 8 h.

[0014] Preferably, the microwave absorbing fine powder obtained in step (5) has relatively perfect grain growth, presents an open-pored peanut shape on the surface, and the holes are regularly arranged.

[0015] The ferrite microwave absorbing fine powder material and its process prepared by the present invention have the following advantages:

[0016] 1. The morphology of the prepared material presents a porous structure. The porous structure is conducive to the entry of microwaves into the pores, thereby enhancing the microwave absorption ability. Moreover, the particles are generally in a neat and regular peanut shape, which provides the possibility for coating the material surface in the later stage to prepare directional reflection components;

[0017] 2. Ammonia water is added, and the optimized pH = 8, which can promote the gel to be more fluffy, making the gel contain a large number of pore structures. The gel containing a large number of pore structures will promote the full dispersion of metal ions, which is beneficial to sintering to obtain a ferrite with accurate metering;

[0018] 3. The gel is placed in an oven and dried at 180 - 200 °C to obtain a dry gel, which will not cake. When it is transferred to large-scale industrial production in the later stage, the transfer operation is relatively convenient;

[0019] 4. It has good microwave absorption characteristics in the microwave band of 15 - 18 GHz, and the wave absorption performance is good.

Description of the Drawings

[0020] Figure 1 : XRD patterns of samples sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h;

[0021] Figure 2 : SEM images of NdFeO3 samples obtained by sintering at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h, where: Figure 2 -a is 500 °C, Figure 2 -b is 600 °C, Figure 2 -c is 700 °C, Figure 2 -d is 800 °C, Figure 2 -e is 900 °C;

[0022] Figure 3 : Electromagnetic parameter diagrams of NdFeO3 samples obtained by sintering at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h in the range of 2 - 18 GHz, where: Figure 3 -a, 3-b, 3-c, 3-d are the curves of ε′, ε″, μ′, μ″ varying with frequency respectively;

[0023] Figure 4 : Relationship curves of the tangent of the loss angle of NdFe03 obtained by sintering at different sintering temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h varying with frequency, where: Figure 4 -a, Figure 4 -b, Figure 4 -c, Figure 4 -d, Figure 4-e are the relationship curve diagrams of the loss tangent value varying with frequency under the conditions of 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C respectively;

[0024] Figure 5 : Relationship diagram of reflectance and frequency of NdFeO3 samples sintered for 8 h at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C);

[0025] Figure 6 : XRD patterns of NdFeO3 samples prepared at different sintering times (2 h, 8 h, 4 h, 20 h, 26 h);

[0026] Figure 7 : SEM images of NdFeO3 samples prepared at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h), where: Figure 7 -a, 7-b, 7-c, 7-d, 7-e are the SEM images of the samples corresponding to 2 h, 8 h, 14 h, 20 h, and 26 h respectively;

[0027] Figure 8 : Electromagnetic parameter diagrams of NdFeO3 samples prepared at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h) in the range of 2 - 18 GHz, where: Figure 8 -a, 8-b, 8-c, 8-d are the curve diagrams of ε′, ε″, μ′, and μ″ varying with frequency;

[0028] Figure 9 : Relationship curves of the loss tangent value of NdFe03 prepared at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h) varying with frequency, where: Figure 9 -a, Figure 9 -b, Figure 9 -c, Figure 9 -d, Figure 9 -e are the relationship curve diagrams of the loss tangent value varying with frequency under the conditions of 2 h, 8 h, 14 h, 20 h, and 26 h respectively;

[0029] Figure 10 : Relationship diagram of reflectance and frequency of NdFeO3 samples prepared at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h).

Specific Embodiments

[0030] The following further describes the present invention in detail with specific embodiments, which do not constitute any limitation to the present invention.

[0031] Example 1

[0032] 1. Set experimental drugs:

[0033] The following are the drugs used in the examples. The experimental drugs are shown in Table 1:

[0034] Table 1. Experimental Drugs

[0035]

[0036]

[0037] 2. Equipment and Instruments:

[0038] The following are the equipment and instruments used in the examples. The equipment and instruments are shown in Table 2:

[0039] Table 2. Equipment and Instruments

[0040]

[0041] 3. Method Steps:

[0042] 3.1 Sample Preparation:

[0043] (1) According to the Nd and Fe atomic ratios of the NdFeO3 compound, weigh the corresponding proportions of neodymium nitrate hexahydrate and iron nitrate nonahydrate, pour them into an appropriate amount of deionized water in sequence and mix, then add an appropriate amount of citric acid monohydrate, and stir until the solute is completely dissolved to obtain a mixed solution. Control the molar ratio of nitrate ions to citric acid monohydrate. The molar ratio of nitrate ions to citric acid monohydrate is: 1 ≤ n(C6H8O7·H2O):n(NO 3- ) ≤ 1.2;

[0044] (2) Add ammonia water (concentration 25%) to the mixed solution to adjust the pH to 8, and the solution changes from reddish-brown to green;

[0045] (3) Place it in a thermostatic heating magnetic stirrer with a heat collector, set the temperature to 90 °C and stir constantly until the water is evaporated to obtain a viscous gel;

[0046] (4) Place the viscous gel in an oven and dry it at 200 °C to obtain a dry gel;

[0047] (5) Grind the obtained dry gel into powder, put it into a crucible and place it in a muffle furnace for sintering to obtain microwave absorbing fine powder, and then take 5 equal parts and sinter them at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) respectively, and the sintering time is 8 h.

[0048] 3.2 Preparation of Coaxial Specimens:

[0049] When using a vector network analyzer to test the electromagnetic performance parameters of a sample, the powder sample needs to be made into a coaxial specimen. The sample to be tested is required to be a solid block. Since the powder is difficult to be directly pressed into a block, an adhesive is needed to make the sample into a block. Therefore, paraffin is used as an adhesive. The preparation process is as follows:

[0050] (1) Weigh an appropriate amount of paraffin wax solids, pour it into a clean small beaker, place the beaker on the asbestos net, and heat it with an alcohol lamp;

[0051] (2) Weigh a certain amount of microwave absorbing micropowder according to the ratio of microwave absorbing micropowder to paraffin wax = 4:1 (mass ratio), pour it into the heated and melted paraffin liquid, stir until it is uniform, stop heating and wait for the paraffin wax to solidify;

[0052] (3) Slowly fill the solidified sample into a clean mold and press it with a pressure of 5 MPa to form a coaxial sample with an outer diameter of 7 mm and an inner diameter of 3 mm, and a thickness of about 3.5 mm.

[0053] (4) When preparing the coaxial sample, please pay attention to the following: First, mix the sample powder and paraffin wax evenly; second, fill the mixed sample into the mold and press it thoroughly to ensure that the sample is air-free and flat after pressing.

[0054] 3.3 Vector Network Analyzer (VNA) Test:

[0055] Vector network analyzer (VNA) is an instrument used to test the wave absorption performance and electromagnetic parameters of materials. The prepared coaxial sample is placed in the instrument and the test data ε′, μ′, ε″, μ″ are obtained. By calculating the tanδ value (tanδ=tanδ E +tanδ M where tanδ E =ε″ / ε′ represents the dielectric loss tangent, tanδ M =μ″ / μ′ represents the tangent value of the magnetic conductivity loss angle) to reflect the loss capacity of the absorbing material to electromagnetic waves. The larger the value, the stronger the material's loss capacity to electromagnetic waves. The calculated results can be analyzed to determine the quality of the material's absorbing performance. At the same time, the reflectivity R of the sample is calculated according to the following formula.

[0056] Where: ε γ , μ γ and d are the relative dielectric constant, relative magnetic permeability and thickness of the absorbing material, respectively, f is the frequency of the electromagnetic wave, c is the propagation speed of the electromagnetic wave in free space, and j is the imaginary unit.

[0057] 3.4X-ray diffractometer (XRD) test

[0058] The test procedure is to grind the sample thoroughly, then use a medicine spoon to scrape out a small amount of the sample and place it in the concave-convex groove of the glass slide. Then, gently spread the sample into a smooth surface with a coverslip for easy testing. In this way, the test sample is ready. The next step is to put it into the instrument for testing, obtain the peak diagram, and analyze the data.

[0059] 3.5 Scanning Electron Microscope (SEM) Test

[0060] Scanning Electron Microscope (SEM, Scanning Electron Microscopy) is a new type of electron optical instrument. It has the characteristics of simple sample preparation, wide adjustable magnification range, high image resolution, etc. It can analyze the tissue morphology, dispersion of the sample, as well as the composition contrast and size of the sample. The steps before the test are to clean a certain amount of the sample to be tested with alcohol, then bond the sample to the sample holder with special tape, perform sputtering for conductivity treatment, load it onto the machine, and observe. Observe the morphology, size, distribution of the sample particles, etc.

[0061] 4. Data Analysis

[0062] Influence of Different Sintering Temperatures on the Structure, Morphology and Microwave Absorbing Properties of NdFeO3

[0063] Figure 1 XRD patterns of NdFeO3 samples sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h were obtained. When the sintering temperature reached 500 °C, there was only a certain wide peak at a diffraction angle of 30°, and no obvious crystal diffraction peaks appeared in other ranges, indicating that the crystal form of the product obtained at this temperature was not completely transformed. When the sintering temperature was 600 °C, 700 °C, 800 °C, 900 °C, crystal diffraction peaks of NdFeO3 appeared, indicating that the crystal form was transformed to obtain perovskite-type ferrite of NdFeO3. No other impurity peaks appeared in the pattern, indicating that the obtained sample was pure perovskite-type NdFeO3 ferrite. As the temperature increased from 600 °C to 900 °C, except for a slight shift in the position of the peak, no other peaks appeared, indicating that after the temperature reached 600 °C, the temperature change had little effect on the crystal structure change of the reaction product.

[0064] Figure 2SEM images of NdFeO3 samples sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h, magnified 10,000 times, show that the surface of the sample sintered at 500 °C has some shallow grooves after magnification; as the sintering temperature rises to 600 °C, a partial peanut-like prototype begins to appear on the sample surface; when the temperature reaches 700 °C, a large number of peanut-like structures appear, and the surface of the peanut-like structures is porous, and the peanut-like structure particles are relatively fluffy; when the temperature reaches 800 °C, the pores on the surface of the peanut-like structure close, but there are still gaps between the particles and they are relatively fluffy; when the temperature reaches 900 °C, not only does the peanut-like porous structure disappear, but the particles become relatively close. The powder presents a porous structure, and the entry of microwaves into the pores is beneficial to microwave absorption, thereby improving the microwave absorption ability of the material. Moreover, the particles are generally in good shape and are neatly and regularly rice-grain-shaped, providing the possibility for coating the material surface in the later stage to prepare directional reflection components. Therefore, when the temperature reaches 700 °C, the structure formed is most beneficial to microwave absorption and is also conducive to the later modification to prepare specific microwave components.

[0065] Figure 3 Electromagnetic parameter diagrams of NdFeO3 samples sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h in the range of 2 - 18 GHz, where Figure 3 -a is a curve graph of the real part of the complex permittivity ε′ varying with frequency. It can be seen from the graph that in the low and medium frequency range of 2 - 14 GHz, the ε′ of each sintered sample changes relatively gently; in the high frequency range of 14 - 18 GHz, the change is relatively large and has a relatively large peak. The ε′ of the sample sintered at 500 °C has a peak of 0.33 at 16.08 GHz, the ε′ of the sample sintered at 600 °C has a peak of 0.95 at 16.40 GHz, the ε′ of the sample sintered at 700 °C has a peak of 0.79 at 15.84 GHz, and no obvious peak appears in the ε′ of the ferrites sintered at 800 °C and 900 °C.

[0066] From Figure 3From the curve of -b, it can be seen that the imaginary part ε″ of the complex permittivity of the ferrite sintered at 500 °C changes relatively gently in the range of 2 - 12 GHz. In the range of 12 - 15.2 GHz, it increases rapidly with the increase of frequency and reaches a peak value of 4.74 at 15.2 GHz, and then decreases rapidly with the increase of frequency. The imaginary part ε″ of the complex permittivity of the ferrite sintered at 600 °C changes relatively gently in the range of 2 - 14 GHz, and the change range is relatively large in the range of 14 - 18 GHz, where there are two peak values of 2.46 and 2.99 at 15.76 Hz and 16.96 Hz respectively. The imaginary part ε″ of the complex permittivity of the ferrite sintered at 700 °C also changes relatively gently in the range of 2 - 14 GHz. In the range of 14 - 16.4 GHz, it increases rapidly with the increase of frequency and reaches a peak value of 4.8 at 16.4 GHz, and then decreases rapidly with the increase of frequency. The change of the imaginary part ε″ of the complex permittivity of the ferrite sintered at 800 °C and 900 °C with frequency has little effect. Therefore, it can be preliminarily judged that the reflectivity of the ferrite sintered at 700 °C may be better.

[0067] From Figure 3 the curve of -c, we can see that the real part μ′ of the complex permeability of the ferrite sintered at 800 °C and 900 °C basically maintains a small change and fluctuation around 1, while the real part μ′ of the complex permeability of the ferrite sintered at 500 °C, 600 °C, and 700 °C also maintains a small change and fluctuation around 1 in the range of 2 - 14 GHz, but the change and fluctuation are relatively large in the high-frequency band. The sample sintered at 500 °C has a peak value of 2.14 at 16.64 GHz, the sample sintered at 600 °C has a peak value of 1.95 at 16.16 GHz, and the sample sintered at 700 °C has a peak value of 2.16 at 15.52 GHz.

[0068] From Figure 3 the curve of -d, it can be seen that the μ″ value of the samples sintered at 800 °C and 900 °C is little affected by frequency and basically does not change in the range of 2 - 18 GHz. The μ″ value of the samples sintered at 500 °C, 600 °C, and 700 °C is also little affected by frequency in the low-frequency band, but the influence changes greatly in the high-frequency band. In the high-frequency range of 14 - 18 GHz, the μ″ values of the samples sintered at 500 °C, 600 °C, and 700 °C show a relatively large peak value. Specifically, the peak value of μ″ of the sample sintered at 500 °C is at 16.24 GHz, and its value is 2.154. The peak value of μ″ of the sample sintered at 600 °C is at 16.32 GHz, and its value is 2.51. The peak value of μ″ of the sample sintered at 700 °C is at 15.76 GHz, and its μ′ value is 2.56.

[0069] Figure 4 The loss tangent value (tanδ) of the NdFeO3 samples sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h E= ε″ / ε′, tanδ M = μ″ / μ′) versus frequency curves. From Figure 4 -a, it can be seen that the variation trends of the dielectric loss tangent value and the magnetic loss tangent value of the ferrite sintered at 500 °C with frequency are consistent, that is, in the low and medium frequency ranges of 2 - 12 GHz, the dielectric loss tangent value and the magnetic loss tangent value change little with the change of frequency. In the high frequency range of 12 - 16.08 GHz, they gradually increase. The dielectric loss tangent value reaches a peak of 9.14 at 16.08 GHz, and the maximum magnetic loss tangent value reaches a peak of 5.36 at 16.08 GHz. After that, with the increase of frequency, both losses gradually decrease.

[0070] From Figure 4 -b, it can be seen that for the ferrite sintered at 600 °C, the dielectric loss tanδ E and the magnetic loss tanδ M show similar variation trends with the influence of frequency. In the low and medium frequency ranges of 2 - 14 GHz, the change is relatively gentle. In the high frequency range, the change with frequency is relatively large, and there are strong loss peaks in the range of 16 - 17 GHz. Among them, the peak value of the dielectric loss tanδ E is 2.3 at 16.4 GHz, and the peak value of the magnetic loss tanδ M is 2.46 at 16.48 GHz.

[0071] From Figure 4 -c, it can be seen that for the ferrite sintered at 700 °C, the dielectric loss tanδ E slowly decreases with the increase of frequency in the range of 2 - 15 GHz. In the range of 15 - 15.92 GHz, with the increase of frequency, its dielectric loss increases rapidly. The dielectric loss peak value reaches 3.09 at 15.92 GHz, and then with the increase of frequency, its dielectric loss rapidly decreases to about 0.02. The magnetic loss tangent value of the ferrite and the corresponding dielectric loss tangent change trends are similar. In the range of 2 - 14 GHz, the influence of frequency change is small. In the frequency range of 15 - 17 GHz, there is a loss peak, and its loss peak value is 3.69, and the position of the loss peak is at 15.84 GHz.

[0072] From Figure 4 -d, it can be seen that for the ferrite sintered at 800 °C, in the ranges of 2 - 10.96 GHz and 12.3 - 13.2 GHz, the magnetic loss tangent value is dominant. In other ranges, the dielectric loss tangent value is dominant. From the overall variation trend, in the range of 2 - 16 GHz, both tangent values fluctuate with the increase of frequency. In the high frequency range of 16 - 18 GHz, both increase with the increase of frequency.

[0073] From Figure 4-e It can be seen that as the frequency changes, the variation ranges of its dielectric loss and magnetic loss tangent values are relatively large. In the range of 2 - 8 GHz, the magnetic loss tangent value is much larger than the dielectric loss tangent value, while in the range of 15 - 17 GHz, the dielectric loss tangent value is much larger than the magnetic loss tangent value. Figure 4 -e is the relationship curve between the loss tangent value of the ferrite sintered at 900 °C and the frequency. It can be seen from the figure that in the range of 2 - 16 GHz, both the magnetic loss tangent value and the dielectric loss tangent value increase with the increase of frequency. Especially in the range of 15 - 16 GHz, the increasing rate is relatively large, and a strong peak is formed near 16 GHz, and then it rapidly decreases.

[0074] Figure 5 The NdFeO3 samples were sintered at different temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) for 8 h. When the thickness d of the microwave absorbing coating is 2.5 mm, the relationship diagram between the reflectivity and the frequency is shown. It can be seen from the figure that the reflectivity of each sample changes relatively gently with the increase of frequency in the low - frequency band. In the high - frequency band, the ferrites sintered at 500 °C, 600 °C, and 700 °C have good absorption performance. The minimum reflectivity of the 500 °C sample is - 24.17 dB at 16.16 GHz, the minimum reflectivity of the 600 °C sample is - 25.47 dB at 16.40 GHz, and the minimum reflectivity of the 700 °C sample is - 29.53 dB at 15.84 GHz and the reflectivity is less than - 10 dB in the range of 15.12 - 16.24 GHz. From the temperature change of 500 °C, 600 °C, and 700 °C, as the temperature increases, the intensity of the absorption peak gradually increases. However, when the temperature rises to 800 °C and 900 °C, their microwave absorption performance deteriorates sharply and there is basically no obvious absorption in the frequency range of 2 - 18 GHz. Therefore, the sample sintered at 700 °C has good microwave absorption performance. This is because when the temperature reaches 700 °C, a large number of peanut - like structures are generated in the particles. At the same time, the surface of the peanut - like structure is porous, and the peanut - like structure particles are relatively loose. After the microwave enters the micropores, it is strongly absorbed, thus improving the microwave absorption performance. However, when the temperature rises to 800 °C and 900 °C, the pores on the surface of the peanut - like structure gradually close, and the microwave cannot enter the particle pores, resulting in a rapid deterioration of the microwave performance.

[0075] Example 2

[0076] The influence of different sintering times on the structure, morphology and microwave absorption performance of NdFeO3

[0077] Referring to Example 1, the difference between this example and Example 1 is that the high - temperature sintering temperature in the muffle furnace is 700 °C, and the sintering times are 2 h, 8 h, 14 h, 20 h, and 26 h respectively.

[0078] Figure 6XRD patterns of NdFeO3 samples prepared at 700 °C with different sintering times (2 h, 8 h, 4 h, 20 h, 26 h). It can be seen from the patterns that only under the condition of sintering for 2 h, the peak intensity of the obtained pattern is weak, and the morphologies of the other patterns are similar, and all correspond to the standard peaks of the NdFeO3 alloy. It is concluded by analysis that the holding time of sintering has little effect on perovskite-type NdFeO3 and does not cause much change in crystal form.

[0079] Figure 7 SEM images of samples sintered at 700 °C with different sintering times (2 h, 8 h, 14 h, 20 h, 26 h). It can be seen from the figures that the samples prepared at each sintering time show a fluffy and porous structure inside and a neatly arranged peanut-like structure on the outside.

[0080] Figure 8 Electromagnetic parameter diagrams of NdFeO3 samples obtained at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h) in the range of 2 - 18 GHz Figure 8 -a is the curve of the real part of the complex permittivity varying with frequency. Generally, the curve of the real part of the complex permittivity changes gently in the range of 2 - 14 GHz, and its real part value of the complex permittivity remains around 5.5 - 7. It fluctuates greatly in the range of 14 - 18 GHz and there is a peak valley in this interval. When the sintering time is 2 h, in the range of 14 - 15.6 GHz, as the frequency increases, the real part value of its complex permittivity decreases rapidly, reaches the lowest value of 2.826 at 15.6 GHz, then as the frequency increases, the real part value of its complex permittivity increases rapidly again, reaches the maximum value of 7.147 at 16.56 GHz, and then as the frequency continues to increase, the real part value of its complex permittivity decreases again to 5.876 at 18 GHz; in the range of 14 - 18 GHz, as the frequency increases, the real part values of the complex permittivity of the samples with sintering times of 8 h, 14 h, 20 h, and 26 h all show a trend of first decreasing and then increasing. The real part value of the complex permittivity of the sample with a sintering time of 8 h has a lowest value of 0.79 at 15.84 GHz; the sample with a sintering time of 14 h has a lowest value of 0.83 at 17.52 GHz; the real part value of the complex permittivity of the sample with a sintering time of 20 h has a lowest value of 1.32 at 16.56 GHz; when the sintering time increases to 26 h, the lowest value of its real part value of the complex permittivity is 1.38, and the corresponding frequency is 16.88 GHz.

[0081] Figure 8-b is the curve of the imaginary part of the complex permittivity varying with frequency. Generally speaking, in the range of 2 - 14 GHz, the imaginary part values of the complex permittivity of each sample are relatively small and fluctuate little with the change of frequency. For the samples with sintering times of 2 h and 8 h, the imaginary part values of the complex permittivity gradually decrease with the increase of frequency, while for the samples with sintering times of 14 h, 20 h, and 26 h, the imaginary part values of the complex permittivity gradually increase with the increase of frequency. At 12.3 GHz, the imaginary part values of the complex permittivity of each sample approach equality. In the range of 14 - 18 GHz, the imaginary part values of the complex permittivity of each sample first increase and then decrease with the change of frequency, and there is a maximum peak value. The peak values of the samples with sintering times of 2 h, 8 h, and 14 h are relatively large, which are 5.433 at 15.84 GHz, 4.8 at 16.4 GHz, and 4.377 at 17.2 GHz respectively. While the peak values of the samples with sintering times of 20 h and 26 h are relatively small, which are 2.604 at 16.8 GHz and 2.399 at 16.8 GHz respectively.

[0082] Figure 8 -c is the curve of the real part of the complex permeability μ′ versus frequency. Generally speaking, in the range of 2 - 12 GHz, the real part values of the complex permeability of each sample are approximately equal and about 1. Starting from 12 GHz, for the ferrites sintered for 2 h and 8 h, they first increase, then decrease, and then increase again. The sample sintered for 2 h has the highest peak value of 1.316 at 14.96 GHz and the valley value of 0.541 at 15.92 GHz; the sample sintered for 8 h has the highest peak value of 2.16 at 15.52 GHz and the valley value of 0.597 at 16.08 GHz; for the ferrite sintered for 14 h, with the increase of frequency, in the range of 12 - 16 GHz, the real part value of the complex permeability increases slightly, and in the range of 16 - 18 GHz, the change amplitude of the real part value of the complex permeability is large, with the valley value of 0.3845 continuously appearing at 17.44 GHz and the maximum peak value of 2.038 at 17.84 GHz; for the ferrite sintered for 20 h, in the range of 12 - 18 GHz, it first increases and then decreases with the increase of frequency, and the maximum peak value of 1.994 appears at 16.32 GHz; for the ferrite sintered for 26 h, there are 2 peak values in the range of 12 - 18 GHz, which are 1.4645 at 16.56 GHz and 1.701 at 17.04 GHz respectively.

[0083] Figure 8-d is the curve of the imaginary part μ″ of the complex permeability varying with frequency. Overall, the imaginary part μ″ of the complex permeability of each sample varies relatively gently in the range of 2 - 14 GHz, and shows a trend of first increasing and then decreasing in the range of 14 - 18 GHz. Among them, the samples of 2h, 8h, 14h, 20h, and 26h are respectively at 15.52 GHz, 15.76 GHz, 17.6 GHz, 16.56 GHz, and 16.8 GHz, and the corresponding peak values are 0.756, 2.56, 1.97, 2.4035, and 2.047 respectively.

[0084] Figure 9 It is the relationship curve of the tangent of the loss angle of NdFe03 prepared at different sintering times (2h, 8h, 14h, 20h, 26h) varying with frequency. It can be seen from the figure that the variation trends of the dielectric loss tangent value and the magnetic loss tangent value of the samples are consistent and most of the values overlap. The variation trends of the samples with different sintering times are also similar, that is, in the range of 2 - 14 GHz, the tangent of the loss angle is small and changes relatively gently with the increase of frequency. Especially for the samples with sintering times of 14h, 20h, and 26h, the tangent of the loss angle is close to 0 in the range of 2 - 14 GHz. For each sample in the range of 14 - 18 GHz, both its dielectric loss tangent value and magnetic loss tangent value increase first and then decrease with the increase of frequency, and strong peaks appear. The peak value of the dielectric loss tangent of the sample with a sintering time of 2h is 1.57 at 15.6 GHz, and the peak value of the magnetic loss tangent is 1.1677 at 15.76 GHz.

[0085] The peak value of the dielectric loss tangent of the sample with a sintering time of 8h is 3.09 at 15.92 GHz, and the peak value of the magnetic loss tangent is 3.69 at 15.84 GHz. The peak values of the dielectric loss tangent and the magnetic loss tangent of the sample with a sintering time of 14h are both at the point of 17.52 GHz, which are 4.2169 and 4.6278 respectively. The peak values of the dielectric loss tangent and the magnetic loss tangent of the sample with a sintering time of 20h are also both at the point of 16.56 GHz, which are 1.5496 and 1.7747 respectively. The peak values of the dielectric loss tangent and the magnetic loss tangent of the sample with a sintering time of 26h are also at the same frequency point. The frequency of this point is 16.8 GHz, and the dielectric loss tangent and the magnetic loss tangent are 1.7876 and 1.7893 respectively.

[0086] Figure 10Curves of reflectance versus frequency for NdFeO3 samples prepared at different sintering times (2 h, 8 h, 14 h, 20 h, 26 h). It can be seen from the figure that the trends of all samples with frequency change are consistent, that is, in the low-frequency band, the reflectance decreases gently with the increase of frequency, but the reflectance values are still generally high. However, there are obvious and relatively strong sharp valleys in the medium and high-frequency bands. The sample sintered for 2 h has two valleys. The first valley appears at 13.28 GHz with a valley value of -16.0 dB, and the second valley exists at 15.2 GHz with a valley value of -30.27 dB. The sample sintered for 8 h shows a small fluctuation in the range of 12 - 14 GHz and then a valley with an intensity of -29.53 dB appears at 15.84 GHz. At the same time, its reflectance is lower than that of other samples in the range of 2 - 14 GHz. The sample obtained when the sintering time is 14 h has two smaller valleys and one larger valley at frequencies of 13.2 GHz, 16.56 GHz, and 17.68 GHz respectively, and the corresponding valley values are -6.87 dB, 11.73 dB, and -26.35 dB respectively. The sample sintered for 20 h shows absorption peaks at 13.2 GHz and 16.64 GHz respectively, with valley values of -10.2 dB and -34.46 dB. After the sintering time is increased to 26 h, the sample has a weak absorption peak at 13.2 GHz with a valley value of -9.16 dB, and two strong absorption peaks at frequencies of 16.72 GHz and 17.12 GHz, which are -32.36 dB and -27.1 dB respectively. From the perspective of the strongest absorption peak, when the sintering time increases from 8 h to 14 h, the frequency corresponding to the valley peak moves to the high frequency, while in the range of 14 h - 26 h, the frequency corresponding to the valley peak moves to the low frequency with the increase of temperature. Generally speaking, the sample sintered for 8 h has better microwave absorption performance compared with other temperatures.

[0087] Example 3

[0088] Referring to Example 2, the difference between this example and Example 2 is that: the sintering time is set to 8 h, and the pH of the solution is adjusted to 6, 6.5, 7, 7.5, 8, 8.5, 9 with ammonia water. The xerogels dried in the drying oven were observed, and the observation results are shown in Table 3.

[0089] Table 3. Gels Prepared at Different pH Values

[0090] Serial number PH Gel state Gel pores 1 6 The colloid is generally fluffy The specific surface area and pore volume are generally large 2 6.5 The colloid is generally fluffy The specific surface area and pore volume are generally large 3 7 The colloid is generally fluffy The specific surface area and pore volume are generally large 4 7.5 The colloid is relatively fluffy The specific surface area and pore volume are generally large 5 8 The colloid is very fluffy The specific surface area and pore volume are very large 6 8.5 The colloid is generally fluffy The specific surface area and pore volume are generally large 7 9 The colloid is generally fluffy The specific surface area and pore volume are relatively small

[0091] As shown in Table 3, when ammonia water is added to adjust the pH to 8, the solution is alkaline, and a certain amount of ammonia molecules are dissolved in the solution. During the drying process of step (4), the ammonia molecules escape, which promotes the gel to be more fluffy, so that the gel contains a large number of pore structures. The gel containing a large number of pore structures will promote the full dispersion of metal ions, which is conducive to sintering to obtain accurately measured ferrite. When the pH continues to increase, in an alkaline environment, iron ions are easily complexed and dissolved by ammonia water. During the evaporation process, they quickly aggregate to produce a denser sol. The gel has small pores, which is not conducive to the preparation of porous materials. Therefore, pH = 8 is most suitable.

[0092] Example 4

[0093] Referring to Example 3, the difference between this example and Example 3 is that: the pH of the solution is adjusted to 8 by ammonia water, the viscous gel is placed in a drying oven at different temperatures (160°C, 170°C, 180°C, 190°C, 200°C, 210°C) for drying, and the dried dry gel is observed. The observation results are shown in Table 4.

[0094] Table 4. Gels prepared at different drying temperatures

[0095] Serial number Temperature / °C Gel change state 1 160 It forms a block and is difficult to take out, and the color is cyan 2 170 It forms a block and is difficult to take out, and the color is cyan 3 180 It does not form a block, becomes powdery and is easy to take out, and the color is cyan 4 190 It does not form a block, becomes powdery and is easy to take out, and the color is cyan 5 200 It does not form a block, becomes powdery and is easy to take out, and the color is cyan 6 210 It does not form a block, becomes powdery and is easy to take out, and the color is yellow

[0096] As shown in Table 4, when the temperature is 160-170°C, the citrate is not completely evaporated and removed, the gel is agglomerated into blocks and adheres to the wall of the container, which is not easy to remove and causes inconvenience in operation. When the temperature is as high as 210°C, the gel turns yellow due to the high temperature. Yellow is mainly the color of NdFeO3 crystals, indicating that part of the sol has undergone crystal transformation at 210°C, which is not conducive to the subsequent high-temperature sintering to prepare single NdFeO3 crystals. High temperature is conducive to the evaporation and removal of citrate. At the same time, considering that the gel should not form a crystal structure, it is most appropriate to adjust the temperature to 200°C.

[0097] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A perovskite-type NdFeO3 microwave absorbing fine powder material in the shape of an open-pored peanut, characterized in that, The preparation method of this material comprises the following steps: (1) According to the atomic ratio of Nd and Fe in the NdFeO3 compound, weigh the corresponding proportions of neodymium(III) nitrate hexahydrate and iron(III) nitrate nonahydrate, pour them into an appropriate amount of deionized water in sequence and mix, then add citric acid monohydrate, and stir until the solutes are completely dissolved to obtain a mixed solution. The molar ratio of nitrate ions to citric acid monohydrate is: 1 ≤ n(C6H8O7·H2O) : n(NO3⁻) ≤ 1.2; (2) Add ammonia water to the mixed solution to adjust the pH to 8; (3) Place it in a thermostatic heating magnetic stirrer with heat collection, set the temperature at 90 °C and stir constantly until the water is evaporated to obtain a viscous gel; (4) Place the gel in a drying oven and dry it at 200 °C to obtain a dry gel; (5) Grind the obtained dry gel into powder, put it into a crucible and then place it in a muffle furnace for sintering to obtain microwave absorbing fine powder. The sintering temperature is 700 °C and the sintering time is 8 h. The obtained microwave absorbing fine powder has relatively perfect crystal grain growth, presents an open pore peanut shape on the surface and the pores are arranged regularly.

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