Preparation method of an iron-aluminum-based hierarchical porous material

Abstract

The present invention relates to the technical field of alloy materials, and provides a preparation method of an iron-aluminum-based hierarchical pore material, comprising the following steps: mixing iron powder, aluminum powder and molten paraffin and stirring, then adding magnesium nitride powder and stirring evenly, pulverizing after the molten paraffin cools and solidifies, then putting it into a mold for molding by pressing, and then subjecting the formed green compact to vacuum sintering to obtain the iron-aluminum-based hierarchical pore material; the particle size of the magnesium nitride powder is larger than that of the iron powder and the aluminum powder. By using the method of wrapping with a paraffin mixed solution, the present invention enables the iron powder particles and the aluminum powder particles to wrap the magnesium nitride particles. During the sintering process, a series of intermetallic compounds are formed between iron and aluminum, while magnesium nitride can sublimate and decompose through the voids in the intermetallic compounds, thereby forming an eggshell-like structure with an intermetallic compound of iron and aluminum as the outer shell and a hollow interior at the micro level, which can further increase the porosity of the porous material and reduce the material weight at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy materials, and particularly relates to a preparation method of an iron-aluminum-based hierarchical porous material. Background Art

[0002] During the operation of an automobile, the exhaust pipe of the automobile contains a large amount of high-temperature dust-containing, sulfur-containing and corrosive gases. The iron-aluminum intermetallic compound has excellent high-temperature oxidation and anti-sulfidation properties and can adapt to the extreme working environment in the automobile exhaust pipe. At the same time, the iron-aluminum intermetallic compound has good ductility and welding properties at room temperature and can adapt to the production and assembly of the filter material for the automobile exhaust pipe. Therefore, the porous material based on the iron-aluminum intermetallic compound can meet the requirements of the filter material for the automobile exhaust pipe for filtration separation and mechanical properties under high-temperature extreme conditions.

[0003] The preparation methods of the porous iron-aluminum intermetallic compound material mainly include reaction synthesis method, high-temperature self-propagating synthesis method and pore-forming agent method. The reaction synthesis method has a long preparation period, high energy consumption, and the open porosity of the prepared porous material is not high, generally about 50%. The open porosity of the porous material prepared by the high-temperature self-propagating synthesis method is about 60%. In addition, the reaction of this method is relatively violent, difficult to control, and the prepared porous material is prone to cracks and deformation. The open porosity of the porous iron-aluminum intermetallic compound material prepared by the pore-forming agent method is about 50%, and there is no significant advantage compared with the above two methods. In addition, this method cannot ensure the complete removal of the pore-forming agent, and the residue of the pore-forming agent may affect the performance of the porous material.

[0004] In the existing method (CN112853139A) for preparing an iron-aluminum intermetallic compound porous material by sintering iron, aluminum and magnesium ternary metal powders, magnesium elements are prone to react with iron and aluminum elements, resulting in a decrease in the performance of the porous material. Therefore, a preparation method of an iron-aluminum-based hierarchical porous material with a higher porosity and better filtration efficiency is needed. Summary of the Invention

[0005] In view of this, the present invention provides a preparation method of an iron-aluminum-based hierarchical porous material with a higher porosity and better filtration efficiency.

[0006] The technical solution of the present invention is realized as follows: The present invention provides a preparation method of an iron-aluminum-based hierarchical porous material, including the following steps:

[0007] Mix and stir iron powder, aluminum powder and molten paraffin, then add magnesium nitride powder and stir evenly. After the molten paraffin cools and solidifies, it is crushed, then put into a mold and molded by pressing. Subsequently, the formed green compact is subjected to vacuum sintering to obtain the iron-aluminum-based hierarchical porous material; the particle size of the magnesium nitride powder is larger than that of the iron powder and the aluminum powder.

[0008] On the basis of the above technical solutions, preferably, the molding by pressing is carried out by traditional pressing molding or cold isostatic pressing, and the pressure for pressing and forming is less than 50 Mpa.

[0009] On the basis of the above technical solutions, preferably, the degree of vacuum for vacuum sintering is less than 0.01 Pa.

[0010] The present invention uses iron powder and aluminum powder to wrap magnesium nitride particles, and completes the formation of eggshell hollow pores through diffusion / reaction pore formation during sintering and vacuum sublimation and high-temperature decomposition of magnesium nitride during sintering, thereby further increasing the porosity of the porous iron-aluminum intermetallic compound material, reducing the weight of the porous material, and obtaining a porous iron-aluminum intermetallic compound material with high porosity and multi-level pores.

[0011] The present invention synergistically uses the following three approaches and one structure to increase the porosity of the porous iron-aluminum intermetallic compound material:

[0012] Utilize the asymmetric diffusion of iron and aluminum elements in the solid-solid state and solid-liquid state to consume aluminum for pore formation;

[0013] The vacuum sublimation of magnesium nitride. In a high-temperature vacuum environment, magnesium nitride will undergo sublimation behavior to achieve gas-phase removal of magnesium nitride for pore formation;

[0014] The high-temperature decomposition of magnesium nitride. In a high-temperature vacuum environment, magnesium nitride will decompose into magnesium vapor, nitrogen groups and nitrogen gas. The nitrogen groups and nitrogen gas can escape in a vacuum environment while the magnesium vapor will adhere to the furnace wall or condense and deposit in the low-temperature area of the furnace tube, achieving cracking and removal of magnesium nitride for pore formation;

[0015] Utilize the pore structure formed after the sublimation and decomposition of magnesium nitride particles wrapped by iron and aluminum powders.

[0016] On the basis of the above technical solutions, preferably, the iron powder and aluminum powder are proportioned according to the atomic percentage ratio of Fe 1-x -Al x where x is 22.5% - 76.5%.

[0017] It can be seen from the Fe-Al binary phase diagram that when the atomic percentage of aluminum element is 22.5% - 76.5%, a series of iron-aluminum intermetallic compounds can be formed with iron element, such as FeAl, FeAl2, FeAl3, Fe2Al5 and Fe3Al, etc. Therefore, the initial atomic ratio of the two elements in the Fe-Al system is Fe 1-x -Al x where x is the atomic percentage, and the value range of x is 22.5% - 76.5%.

[0018] Based on the above technical solutions, preferably, the mass ratio of the iron-aluminum powder mixture to the molten paraffin is 2:1 to 6:1, and the mass ratio of the magnesium nitride powder to the iron-aluminum powder mixture is 2:5 to 4:5.

[0019] Mix the proportioned iron and aluminum powder with the paraffin solution according to a mass ratio of 2:1 to 6:1 and stir. Weigh the magnesium nitride powder particles and the iron and aluminum powder in the paraffin solution according to a mass ratio of 2:5 to 4:5 and mix them into the paraffin solution and stir. After the paraffin solution cools, crush it to obtain structural particles with magnesium nitride powder particles coated with iron and aluminum metal particles. Then put the coated structural particles into a sintering mold for pressing and perform subsequent sintering.

[0020] Based on the above technical solutions, preferably, the particle sizes of the iron powder and the aluminum powder are 1 to 100 μm, and the particle size of the magnesium nitride powder is 500 to 1000 μm.

[0021] Based on the above technical solutions, preferably, the vacuum sintering method includes the following steps:

[0022] S1, first heat up to a temperature above the boiling point of paraffin and hold the reaction at this temperature for more than 1 h;

[0023] S2, then continue to heat up to the temperature for solid-state diffusion of iron and aluminum and hold the reaction at this temperature for 0.5 h to 5 h;

[0024] S3, then continue to heat up to a temperature above the melting point of aluminum but not exceeding the temperature at which magnesium nitride starts to sublime, and hold the reaction at this temperature for 0.5 h to 3 h;

[0025] S4, then continue to heat up to the temperature for sublimation and decomposition of magnesium nitride and hold the reaction at this temperature for 1 to 6 h.

[0026] Based on the above technical solutions, preferably, in step S1, heat up at a heating rate of 1 °C / min to 5 °C / min to 400 to 500 °C to completely volatilize the paraffin and initially stabilize the iron and aluminum shell structures through diffusion between the iron and aluminum particles;

[0027] Based on the above technical solutions, preferably, in step S2, heat up at a heating rate of 1 °C / min to 5 °C / min to 500 to 660 °C. Specifically, different temperature points can be selected in this solid-state diffusion and reaction temperature range for holding operations for different times according to the different ratios of the two components of iron and aluminum in the Fe-Al-Mg3N2 ternary system.

[0028] Based on the above technical solutions, preferably, in step S3, the temperature is raised at a heating rate of 1 °C / min to 5 °C / min to 660 to 700 °C, so that the remaining aluminum in the solid state is completely converted into liquid aluminum. The liquid aluminum quickly wraps around the iron particles and reacts with them to form a series of iron-aluminum intermetallic compounds. This process ensures that there is no elemental aluminum in the sintered blank. While further increasing the porosity, it avoids the reaction of elemental aluminum with the magnesium vapor, nitrogen groups, and nitrogen gas generated by the decomposition of magnesium nitride, thereby avoiding affecting the composition and performance of the prepared hierarchical porous iron-aluminum intermetallic compound material. At the same time, the iron-aluminum intermetallic compounds formed during this process will wrap around the magnesium nitride particles like an eggshell.

[0029] Based on the above technical solutions, preferably, in step S4, the temperature is raised at a heating rate of 1 °C / min to 10 °C / min to 700 to 1100 °C. Specifically, multiple holding temperatures can be selected within this temperature range for holding for different times in this step, and the specific situation can be determined according to the content of magnesium nitride in the Fe-Al-Mg3N2 ternary system. For example, in the preferred embodiment 1 of the present invention, only one temperature point is selected for holding operation in the temperature range of magnesium nitride vacuum sublimation and high-temperature decomposition. This temperature point must be higher than 800 °C, and the sublimation and decomposition behaviors of magnesium nitride occur simultaneously at this stage. In the preferred embodiments 2 and 3 of the present invention, two temperature points are selected for holding operation. The first temperature point should be in the range of 700 to 800 °C, and mainly the sublimation behavior of magnesium nitride occurs; the second temperature point should be in the range of 800 to 1100 °C, and the behavior occurring at this temperature point depends on the holding time at the first temperature point. If the holding time at the first temperature point is not long enough and the magnesium nitride in the ternary system is not completely sublimated into the furnace tube, then the sublimation and decomposition behaviors of magnesium nitride will occur simultaneously at this temperature point; if the holding time at the first temperature point is long enough and the magnesium nitride in the ternary system is completely sublimated into the furnace tube, then only the decomposition behavior of magnesium nitride will occur at this temperature point. At the same time, during this process, due to the sublimation and decomposition of magnesium nitride, a hollow hierarchical porous material with an iron-aluminum intermetallic compound as the outer shell is gradually formed, thereby further increasing the porosity of the material.

[0030] On the basis of the above technical solutions, preferably, it further includes heating to 900 - 1100°C at a heating rate of 1°C / min to 10°C / min and holding for 1 - 2 h. Whether this step needs to be implemented should be determined according to the selection of the temperature point and holding time point in step S4 and the composition of the prepared iron-aluminum intermetallic compound. If the holding time at a temperature in the range of 900 - 1100°C has exceeded 2 h in step S4, this step can be omitted. If the holding operation in the above temperature range has not been completed, continue to heat to 900 - 1100°C at a heating rate of 1°C / min to 10°C / min and hold for 1 - 2 h. In this temperature range, high-temperature phase transformation will occur to generate the desired iron-aluminum intermetallic compound and achieve the homogenization of composition and structure. The finally obtained hierarchical pore iron-aluminum intermetallic compound material has an open porosity of more than 60%, a total porosity of more than 65%, and a pore size range of 1 - 300 μm. Specifically, in preferred embodiment 1 and preferred embodiment 2 of the present invention, the holding operation in this step was not implemented; in preferred embodiment 3 of the present invention, the holding operation in this step was implemented.

[0031] The principle of the present invention is as follows: Using paraffin mixed with iron and aluminum powder particles to wrap larger-sized magnesium nitride powder particles. When the temperature is between 500 - 660°C, the lower boiling point of paraffin is used for evaporation and the solid-state diffusion and reaction between iron and aluminum to obtain a ternary system with only iron and aluminum coating magnesium nitride particles. In the ternary Fe-Al-Mg3N2 system, below 700°C, no chemical reaction occurs between magnesium nitride and the two components of iron and aluminum. This stage is mainly dominated by the diffusion and reaction between the two components of iron and aluminum. As is well known, the melting point of aluminum is about 660°C. When the temperature of the ternary system rises to near the melting point of aluminum (between 660 - 700°C), the remaining aluminum in the solid state will all form liquid aluminum to wrap the iron particles and quickly react with the surrounding iron particles to generate a series of iron-aluminum intermetallic compounds. After the reaction in the above stage, the elemental aluminum in the ternary system disappears, and only iron-aluminum intermetallic compounds exist in the system. At the same time, the space originally occupied by the aluminum powder particles forms voids.

[0032] Secondly, the present invention also relates to the pore formation by vacuum sublimation and high-temperature decomposition of magnesium nitride, which further improves the porosity of the porous iron-aluminum intermetallic compound material. It is reported that magnesium nitride begins to sublime at around 700°C. When the temperature of the ternary system rises above 700°C, magnesium nitride will sublime in a high-vacuum environment, realizing pore formation by gas-phase dealloying. While removing magnesium nitride, the porosity is further increased. In addition, magnesium nitride will decompose above 800°C, generating magnesium vapor, nitrogen groups and nitrogen gas. In a high-vacuum environment, the nitrogen groups and nitrogen gas can escape from the furnace tube, while the magnesium vapor will move towards the high-vacuum area in the furnace and deposit on the low-temperature area of the inner wall of the furnace tube, thus ensuring that the magnesium vapor does not crystallize and deposit on the sample surface. When the temperature of the ternary system rises above 800°C, magnesium nitride will decompose in a high-vacuum environment, realizing pore formation by cracking and removing magnesium nitride. While further removing magnesium nitride, the porosity is continuously increased. Through the two removal methods of vacuum sublimation and high-temperature decomposition, the removal of magnesium nitride can be fully ensured, avoiding the residue of the pore-forming agent and its influence on the performance of the prepared hierarchical porous iron-aluminum intermetallic compound material. Among them, the magnesium gas generated by decomposition can react with the residual oxygen in the furnace tube environment to remove oxygen, thereby reducing the oxidation degree of the material and improving the material performance; the nitrogen gas generated by decomposition can be used as a protective atmosphere, thus further purifying the furnace tube environment.

[0033] In addition, by using the method of wrapping with a paraffin mixed solution in the present invention, iron powder particles and aluminum powder particles are used to wrap magnesium nitride particles. During the sintering process, a series of intermetallic compounds are formed between iron and aluminum, and magnesium nitride can sublime and decompose through the voids in the intermetallic compounds, thereby forming an eggshell-like structure with an iron-aluminum intermetallic compound as the outer shell and a hollow interior at the micro level, which can further increase the porosity of the porous material and reduce the material weight at the same time.

[0034] Based on the above technical solutions, preferably, the iron-aluminum-based hierarchical porous material has a hollow structure, an open porosity of 60% - 62%, and a total porosity of 65% - 70%.

[0035] The preparation method of an iron-aluminum-based hierarchical porous material of the present invention has the following beneficial effects compared with the prior art:

[0036] The present invention makes full use of the eggshell-like hollow structure of the iron-aluminum intermetallic compound left by the sublimation and decomposition of magnesium nitride. With the structural feature of pores within pores, it can further increase the porosity while meeting the filtration requirements, and reduce the weight to achieve lightweight.

[0037] At a stage below the decomposition temperature of magnesium nitride, the diffusion / reaction of iron and aluminum elements is fully utilized to create pores. The through-holes formed between iron powder particles at this stage will serve as the sublimation channels for magnesium nitride and the escape channels for the decomposition products of magnesium nitride during the high-temperature stage. Additionally, aluminum is fully consumed at this stage to prevent the remaining elemental aluminum from reacting with the decomposition products of magnesium nitride and affecting the composition and properties of the hierarchical pore iron-aluminum intermetallic compound material.

[0038] The present invention fully utilizes the stable chemical properties of magnesium nitride, as well as the properties of magnesium nitride being easy to sublimate and decompose. Through two methods of vacuum sublimation and high-temperature decomposition, the residue of magnesium nitride in the ternary Fe-Al-Mg3N2 system is avoided, which is beneficial for the preparation of a pure binary hierarchical pore iron-aluminum intermetallic compound material with high porosity.

[0039] The present invention utilizes the characteristic that magnesium has a high saturated vapor pressure at high temperatures. During the sintering process, it avoids reacting with iron and aluminum during the diffusion / reaction stage of iron and aluminum, avoids the crystallization and deposition of magnesium generated from the decomposition of magnesium nitride on the surface of the sample, and avoids the residue of magnesium elements and their influence on the composition and properties of the hierarchical pore iron-aluminum intermetallic compound material.

[0040] The present invention fully utilizes the reaction of the magnesium gas generated from the decomposition of magnesium nitride with the residual oxygen in the furnace tube and fully utilizes the nitrogen gas generated from the decomposition of magnesium nitride as a protective gas, further purifying the sample preparation environment in the furnace tube and further reducing the degree of oxidation during sample preparation.

[0041] The present invention fully utilizes a composite material formed by a series of intermetallic compounds formed by iron and aluminum and elemental iron. This composite material has both the elastoplasticity of metal materials and the heat resistance and corrosion resistance of ceramic materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. 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.

[0043] Figure 1 Schematic diagram of the eggshell structure composed of iron, aluminum, and magnesium nitride before firing for Example 1;

[0044] Figure 2 Schematic diagram of the cross-sectional structure of the iron-aluminum-based hierarchical pore material obtained in Example 1;

[0045] Figure 3 Scanning electron microscope photograph of the cross-section of the iron-aluminum-based hierarchical pore material obtained in Example 1.

[0046] In the figure: 1 is a magnesium nitride powder particle; 2 is a mixed shell of iron powder and aluminum powder particles; 3 is a cavity left after sublimation and decomposition of magnesium nitride particles; 4 is an iron-aluminum intermetallic compound; 5 is a cavity left after complete diffusion reaction of aluminum particles. Specific Embodiment

[0047] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

[0048] Example 1

[0049] Example 1 of the present invention provides a method for preparing an iron-aluminum-based hierarchical porous material, which specifically includes the following steps:

[0050] (1) A structure of Fe-Al binary powder-coated powder particles is prepared by paraffin coating. The particle size of magnesium nitride powder is 500 μm, and the particle sizes of iron powder and aluminum powder are both 30 μm. In order to obtain a hierarchical porous iron-aluminum intermetallic compound material with a composition of FeAl, the powder mass is proportioned according to the atomic ratio of 50% Fe - 50% Al, that is, the atomic ratio of iron to aluminum is 1:1. The proportioned iron and aluminum powders are mixed and stirred with paraffin solution according to a mass ratio of 7:2. The magnesium nitride powder particles are weighed and mixed into the paraffin solution containing iron and aluminum powders according to a mass ratio of 3:5 and stirred. After the paraffin solution cools, it is pulverized to obtain structural particles with iron and aluminum metal particles coating magnesium nitride powder particles. Subsequently, the coated structural particles are placed in a sintering mold and molded under a pressure of 15 MPa.

[0051] (2) The pressed green compact is placed in a tube furnace for vacuum sintering. It is heated to 450 °C at a heating rate of 5 °C / min and held for 1 h to completely volatilize the paraffin and preliminarily stabilize the iron and aluminum shell structure through diffusion between iron and aluminum particles.

[0052] (3) It is heated to 600 °C at a rate of 5 °C / min and held for 5 h; since the intrinsic diffusion coefficient of aluminum is much larger than that of iron, aluminum atoms will continuously diffuse into the surrounding iron particles at this stage; the aluminum atoms diffusing into the iron particles will first dissolve in the iron. When the aluminum atoms reach the solubility limit in the iron matrix, aluminum reacts with iron (aluminum) to form a series of intermetallic compounds. A large number of vacancies will be formed around the aluminum particles at this stage, and the aggregation of vacancies forms pores.

[0053] (4) Continue to heat at a heating rate of 1 °C / min to 660 °C and hold for 3 h. At this time, the remaining solid aluminum in step (3) will all turn into liquid, and the liquid aluminum will quickly wrap the surrounding iron particles. At this time, the intrinsic diffusion coefficient of iron is greater than that of aluminum, and a large amount of iron particles diffuse into the aluminum solution. Kirkendall voids are generated inside the iron particles, and aluminum reacts with the surrounding iron particles and is completely consumed. At this stage, all the positions originally occupied by aluminum particles become pores, and the porosity further increases. At the same time, the formed iron-aluminum intermetallic compound will wrap the magnesium nitride particles.

[0054] (5) Continue to heat at a heating rate of 5 °C / min to 1000 °C and hold for 1 h. At this stage, the sublimation and decomposition behaviors of magnesium nitride will occur simultaneously. Part of the magnesium nitride in the Fe-Al-Mg3N2 ternary system will sublimate into the furnace tube along the open pores formed in steps (3) and (4). In addition, since this temperature has exceeded the starting decomposition temperature of magnesium nitride, the magnesium nitride contained in the green compact and the magnesium nitride sublimated into the furnace tube will eventually decompose into magnesium vapor, nitrogen groups, and nitrogen gas. The nitrogen groups and nitrogen gas will escape in a vacuum environment, while the magnesium vapor will deposit at both ends of the furnace tube. This stage can ensure the complete removal of magnesium nitride, so pores will be formed at the positions originally occupied by magnesium nitride particles, and the porosity of the green compact further increases. In addition, this stage can also complete the high-temperature phase transformation and achieve the homogenization of composition and structure; finally, an eggshell-type hierarchical porous iron-aluminum intermetallic compound material with a composition of FeAl, an open porosity of 60%, and a total porosity of 65% is obtained.

[0055] Figure 1 The schematic diagram of the eggshell structure composed of iron, aluminum, and magnesium nitride before firing in this embodiment is shown.

[0056] Figure 2 The schematic cross-sectional structure diagram of the iron-aluminum-based hierarchical porous material prepared in this embodiment is shown. It can be seen from the figure that the prepared iron-aluminum-based hierarchical material contains a large number of pores, mainly the pores left after the sublimation and decomposition of magnesium nitride with a relatively large particle size, as well as the pores left after the diffusion and reaction of iron and aluminum.

[0057] Figure 3 The scanning electron microscope photograph of the interface of the iron-aluminum-based hierarchical porous material prepared in this embodiment is shown. It can be seen from the figure that the prepared iron-aluminum-based hierarchical porous material has a large number of pores.

[0058] Example 2

[0059] The embodiment 2 of the present invention provides a preparation method of an iron-aluminum-based hierarchical porous material, which specifically includes the following steps:

[0060] (1)The structure of powder particles with Fe-Al binary powder coated by paraffin was prepared. The particle size of magnesium nitride powder was 800 μm, and the particle sizes of iron powder and aluminum powder were both 60 μm. In order to obtain a hierarchical porous iron-aluminum intermetallic compound material with the composition of FeAl3, the powder mass was proportioned according to the atomic ratio of 23.5%Fe - 76.5%Al, that is, the atomic ratio of iron to aluminum was 23.5:76.5. The prepared iron and aluminum powders were mixed and stirred with the paraffin solution according to the mass ratio of 2:1. The magnesium nitride powder particles were weighed and mixed into the paraffin solution containing iron and aluminum powders according to the mass ratio of 2:5 and stirred. After the paraffin solution cooled, it was crushed to obtain structural particles with iron and aluminum metal particles coating magnesium nitride powder particles. Subsequently, the coated structural particles were put into a sintering mold and molded under a pressure of 15 MPa.

[0061] (2)The compacted green body was put into a tube furnace for vacuum sintering. It was heated to 400 °C at a heating rate of 1 °C / min and held for 1 h to completely volatilize the paraffin and preliminarily stabilize the iron and aluminum shell structure through the diffusion between iron and aluminum particles.

[0062] (3)It was heated to 500 °C at a rate of 1 °C / min and held for 0.5 h. Since the intrinsic diffusion coefficient of aluminum is much larger than that of iron, aluminum atoms will continuously diffuse into the surrounding iron particles at this stage. The aluminum atoms diffusing into the iron particles will first dissolve in the iron. When the aluminum atoms reach the solubility limit in the iron matrix, aluminum reacts with iron (aluminum) to form a series of intermetallic compounds. A large number of vacancies will be formed around the aluminum particles at this stage, and the aggregation of vacancies forms pores.

[0063] (4)Continue to heat it to 670 °C at a heating rate of 5 °C / min and hold for 1 h. At this time, the remaining solid aluminum in step (3) will all become liquid, and the liquid aluminum will quickly wrap the iron particles and react with the surrounding iron particles and be completely consumed. At this stage, all the positions originally occupied by the aluminum particles become pores, and the porosity is further increased. At the same time, the formed iron-aluminum intermetallic compound will wrap the magnesium nitride particles.

[0064] (5)Continue to heat it to 700 °C at a heating rate of 5 °C / min and hold for 2 h. At this stage, the magnesium nitride in the green body begins to sublime, and the magnesium nitride sublimes along the open pores formed in steps (3) and (4) into the furnace tube. Pores will be formed at the positions originally occupied by the magnesium nitride particles at this stage, and the porosity of the green body is further increased.

[0065] (6) Continue to heat up to 1050 °C at a heating rate of 10 °C / min and hold for 1.5 h. In this stage, magnesium nitride will decompose to form magnesium vapor, nitrogen groups and nitrogen gas. The nitrogen groups and nitrogen gas will escape in a vacuum environment, while the magnesium vapor will deposit at both ends of the furnace tube. This stage can further remove magnesium nitride and ensure that magnesium nitride does not remain in the green compact. While increasing the porosity in this stage, it can also achieve the homogenization of composition and structure. Finally, an eggshell-like hierarchical porous iron-aluminum intermetallic compound material with a composition of FeAl3, an open porosity of 60%, and a total porosity of 70% is obtained.

[0066] Example 3

[0067] Example 3 of the present invention provides a preparation method of an iron-aluminum-based hierarchical porous material, which specifically includes the following steps:

[0068] (1) A structure in which Fe-Al binary powder wraps powder particles is prepared by paraffin coating. The particle size of magnesium nitride powder is 1000 μm, and the particle sizes of iron powder and aluminum powder are both 100 μm. In order to obtain a hierarchical porous iron-aluminum intermetallic compound material with a composition of Fe3Al, the powder mass is proportioned according to the atomic ratio of 77.5% Fe - 22.5% Al, that is, the atomic ratio of iron to aluminum is 77.5:22.5. The proportioned iron and aluminum powders are mixed with paraffin solution according to a mass ratio of 6:1 and stirred. The magnesium nitride powder particles are weighed and mixed into the paraffin solution containing iron and aluminum powders according to a mass ratio of 4:5 and stirred. After the paraffin solution cools, it is crushed to obtain structural particles with iron and aluminum metal particles coating magnesium nitride powder particles. Subsequently, the coated structural particles are put into a sintering mold and molded under a pressure of 15 MPa.

[0069] (2) The green compact formed by pressing is put into a tube furnace for vacuum sintering. It is heated up to 500 °C at a heating rate of 3 °C / min and held for 1 h to completely volatilize the paraffin and preliminarily stabilize the iron and aluminum shell structures through the diffusion between iron and aluminum particles.

[0070] (3) The green compact formed by pressing is put into a tube furnace for vacuum sintering. First, it is heated up to 660 °C at a rate of 5 °C / min and held for 1.5 h. Since the intrinsic diffusion coefficient of aluminum is much larger than that of iron, aluminum atoms will continuously diffuse into the surrounding iron particles in this stage. The aluminum atoms diffusing into the iron particles will first dissolve into the iron. When the aluminum atoms reach the solubility limit in the iron matrix, aluminum reacts with iron (aluminum) to form a series of intermetallic compounds. A large number of vacancies will be formed around the aluminum particles in this stage, and the aggregation of vacancies forms pores.

[0071] (4) Continue to heat at a heating rate of 3 °C / min to 700 °C and hold for 1.5 h. At this time, the remaining aluminum in step (3) will all become liquid, and the liquid aluminum will quickly react with the iron particles around it and be completely consumed. In this stage, all the positions originally occupied by the aluminum particles will become pores, and the porosity will be further increased. At the same time, the formed iron-aluminum intermetallic compound will wrap the magnesium nitride particles.

[0072] (5) Continue to heat at a heating rate of 1 °C / min to 750 °C and hold for 3 h. In this stage, the magnesium nitride in the green compact begins to sublime, and the magnesium nitride sublimes along the open pores formed in step (3) and (4) into the furnace tube. In this stage, pores will be formed at the positions occupied by the magnesium nitride particles, and the porosity of the green compact will be further increased.

[0073] (6) Continue to heat at a heating rate of 5 °C / min to 850 °C and hold for 2 h. In this stage, the magnesium nitride will decompose to form magnesium vapor, nitrogen groups and nitrogen gas. The nitrogen groups and nitrogen gas will escape in a vacuum environment, while the magnesium vapor will be deposited at both ends of the furnace tube. This stage can achieve further removal of magnesium nitride to ensure that magnesium nitride will not remain in the green compact. In addition, this stage can also further increase the porosity of the hierarchical porous iron-aluminum intermetallic compound material.

[0074] (7) Continue to heat at a heating rate of 8 °C / min to 1100 °C and hold for 1 h. This stage mainly completes the high-temperature phase transformation to achieve the homogenization of composition and structure. Finally, a hierarchical porous iron-aluminum intermetallic compound material in the form of an eggshell with a composition of Fe3Al, an open porosity of 62%, and a total porosity of 70% is obtained.

[0075] Comparative Example 1

[0076] In Comparative Example 1, the atomic ratio of iron to aluminum is 9:1 compared with Example 1, and it specifically includes the following steps:

[0077] (1) A structure of Fe-Al binary powder-coated powder particles is prepared by paraffin coating, where the particle size of the magnesium nitride powder is 500 μm, and the particle sizes of the iron powder and aluminum powder are both 30 μm. According to the atomic ratio of 90% Fe - 10% Al, that is, the atomic ratio of iron to aluminum is 9:1, the prepared iron and aluminum powder bodies are mixed and stirred with the paraffin solution at a mass ratio of 7:2. The magnesium nitride powder particles are weighed and mixed into the paraffin solution containing the iron and aluminum powder bodies at a mass ratio of 3:5 and stirred. After the paraffin solution cools, it is pulverized to obtain structural particles with iron and aluminum metal particles coating the magnesium nitride powder particles. Subsequently, the coated structural particles are placed in a sintering mold and molded under a pressure of 15 MPa.

[0078] (2) The green compact obtained by molding is placed in a tubular furnace for vacuum sintering, heated to 450 °C at a heating rate of 5 °C / min, and held for 1 h.

[0079] (3) Heat it up to 600 °C at a rate of 5 °C / min and hold for 5 h.

[0080] (4) Continue to heat it up to 660 °C at a heating rate of 1 °C / min and hold for 3 h.

[0081] (5) Continue to heat it up to 1000 °C at a heating rate of 5 °C / min and hold for 1 h.

[0082] Finally, a porous material with a composition of Fe3Al and Fe, an open porosity of 33%, and a total porosity of 46% is obtained.

[0083] In Comparative Example 1, the atomic ratio of iron to aluminum is significantly biased towards iron (9:1). On the one hand, it results in insufficient aluminum atoms, unable to effectively participate in all diffusion and reactions, with fewer vacancies formed, and the quantity and scale of pore generation being significantly restricted, ultimately leading to a decrease in the overall porosity of the material. On the other hand, when the iron content is too high and the aluminum content is too low, iron particles occupy the main component, weakening their diffusion effect, and the liquid aluminum is insufficient to effectively promote the large-scale formation of cavities, thus reducing the porosity. Part of the iron exists in the form of free elemental state, which not only reduces the degree of homogenization of the material but also reduces the effective diffusion process involved in pore formation. At the same time, the connection strength between incompletely formed intermetallic compounds is small, resulting in cavity collapse and destruction of the "egg shell type" hierarchical pore structure, thereby affecting the porosity.

[0084] Comparative Example 2

[0085] Compared with Example 1, in Comparative Example 2, the atomic ratio of iron to aluminum is 1:9, and the preparation method of the iron-aluminum-based porous material specifically includes the following steps:

[0086] (1) A structure in which Fe-Al binary powder wraps powder particles is prepared by paraffin coating. The particle size of magnesium nitride powder is 500 μm, and the particle sizes of iron powder and aluminum powder are both 30 μm. The powder mass is proportioned according to the atomic ratio of 10% Fe - 90% Al, that is, the atomic ratio of iron to aluminum is 1:9. The proportioned iron and aluminum powders are mixed and stirred with paraffin solution at a mass ratio of 7:2. The magnesium nitride powder particles are weighed and mixed into the paraffin solution containing iron and aluminum powders at a mass ratio of 3:5 and stirred. After the paraffin solution cools, it is crushed to obtain structural particles with iron and aluminum metal particles coating magnesium nitride powder particles. Subsequently, the coated structural particles are put into a sintering mold and molded under a pressure of 15 MPa.

[0087] (2) Put the compacted green body into a tubular furnace for vacuum sintering. Heat it up to 450 °C at a rate of 5 °C / min and hold for 1 h.

[0088] (3)Heat it up to 600 °C at a rate of 5 °C / min and hold for 5 h.

[0089] (4)Continue to heat it up to 660 °C at a heating rate of 1 °C / min and hold for 3 h.

[0090] (5)Continue to heat it up to 1000 °C at a heating rate of 5 °C / min and hold for 1 h.

[0091] Finally, a porous material with a composition of FeAl3 and Al, an open porosity of 28%, and a total porosity of 42% is obtained.

[0092] In Comparative Example 2, when aluminum is overly abundant, the diffusion process tends to saturate, resulting in a decrease in the diffusion rate and a reduction in the generation of vacancies around aluminum particles. At the same time, since the holding temperature exceeds the melting point of aluminum, aluminum will rapidly fill the remaining voids in a liquid form, which directly reduces the number and size of pores. In addition, due to the low iron content, the process of aluminum solid solution and reaction with iron to form intermetallic compounds during diffusion is restricted, which further reduces the generation of pores. Moreover, when the iron content is too low, the number of iron particles available for diffusion is significantly reduced, and correspondingly, the Kirkendall voids inside the iron particles decrease, greatly affecting the overall number and connectivity of pores in the material.

[0093] At high temperatures, magnesium nitride particles will sublime and decompose, and the positions originally occupied by magnesium nitride particles will turn into pores. However, this process requires the formation of a large number of open channels to provide a path for the sublimation of magnesium nitride. In Comparative Example 2, the iron content is low, and the material as a whole lacks sufficient intermetallic compound skeletons and structural supports. The generated pore size and connectivity are insufficient, and the path for forming pores is blocked, resulting in incomplete removal of magnesium nitride and thus reducing the porosity.

[0094] In addition, in Comparative Example 2, the excess of aluminum and the scarcity of iron result in insufficient and unevenly distributed intermetallic compounds. When the intermetallic compounds are few, they cannot provide a relatively uniform "eggshell-type" hierarchical pore structure like in Example 1. Excessive aluminum is prone to form a thin film-like liquid covering at high temperatures, and this liquid aluminum covering may fill the existing pores and cause the collapse of some pores, further reducing the porosity.

[0095] Comparative Example 3

[0096] The preparation method of the iron-aluminum-based porous material in Comparative Example 3 specifically includes the following steps:

[0097] (1) Select three powders, namely elemental iron powder, elemental aluminum powder, and magnesium nitride powder, as the original powders, all with a particle size of 30 μm. In order to obtain a porous iron-aluminum intermetallic compound material with a composition of FeAl, according to the atomic ratio of ((50%Fe - 50%Al)-30%Mg3N2), that is, the atomic ratio of iron to aluminum is 1:1, and the ratio of the total number of iron and aluminum to the magnesium nitride molecule is 7:3, calculate the weights of the three powders and weigh them. Then, mix the powders in a vacuum mixing tank for 8 hours. Finally, press the uniformly mixed ternary powder into a mold at a pressure of 15 MPa.

[0098] (2) Place the pressed green compact into a tube furnace for vacuum sintering. First, heat it at a rate of 5 °C / min to 600 °C and hold for 5 h.

[0099] (3) Continue to heat it at a rate of 1 °C / min to 660 °C and hold for 3 h.

[0100] (4) Continue to heat it at a rate of 5 °C / min to 1000 °C and hold for 1 h, finally obtaining a porous iron-aluminum intermetallic compound material with a composition of FeAl, an open porosity of 50%, and a total porosity of 60%.

[0101] The above is only the preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing an iron-aluminum multi-level porous material, characterized in that: The following steps are involved: The iron powder, aluminum powder and molten paraffin are mixed and stirred, and then magnesium nitride powder is added and stirred evenly, and the molten paraffin is crushed after cooling and solidifying, and then put into a mold for compression molding, and then the molded green compact is vacuum sintered to obtain an iron-aluminum multi-level porous material; The iron-aluminum multi-level porous material is an eggshell-like hollow structure with holes in holes; The mass ratio of the iron-aluminum powder mixture to the molten paraffin is 2-6:1, and the mass ratio of the magnesium nitride powder to the iron-aluminum powder mixture is 2-4:5; The iron powder and aluminum powder are Fe 1-x -Al x The ingredients are prepared according to the atomic percentage ratio, where x is 22.5%~76.5%; The particle size of the iron powder and aluminum powder is 1-100 μm, and the particle size of the magnesium nitride powder is 500-1000 μm; The vacuum sintering method comprises the following steps: S1, first raise the temperature to a temperature above the boiling point of paraffin, and keep the temperature at this temperature for more than 1 hour; S2, then continue to heat up to the temperature of solid diffusion of iron and aluminum, and keep the reaction at this temperature for 0.5h~5h; S3, then continue to raise the temperature to above the melting point of aluminum, but not exceeding the temperature at which magnesium nitride begins to sublime, and keep the reaction at this temperature for 0.5h~3h; S4, then continue to raise the temperature to the sublimation and decomposition temperature of magnesium nitride, and keep the reaction at this temperature for 1 to 6 hours.

2. The method for preparing an iron-aluminum multi-level porous material according to claim 1, characterized in that: In step S1, the temperature is increased to 400-500°C at a heating rate of 1°C / min-5°C / min.

3. The method for preparing an iron-aluminum multi-level porous material according to claim 1, characterized in that: In step S2, the temperature is increased to 500-660°C at a heating rate of 1°C / min-5°C / min.

4. The method for preparing an iron-aluminum multi-level porous material according to claim 1, characterized in that: In step S3, the temperature is increased to 660-700°C at a heating rate of 1°C / min-5°C / min.

5. The method for preparing an iron-aluminum multi-level porous material according to claim 1, characterized in that: In step S4, the temperature is increased to 700-1100°C at a heating rate of 1°C / min-10°C / min.

6. The method for preparing an iron-aluminum multi-level porous material according to claim 1, characterized in that: The open porosity of the iron-aluminum multi-level porous material is 60%-62%, and the total porosity is 65%-70%.

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