Preparation method and application of porous silicon nitride ceramic with adjustable porosity and pore orientation

Abstract

The invention relates to the technical field of ceramic material preparation, in particular to a preparation method of porous silicon nitride ceramic with adjustable porosity and pore orientation, which comprises the following steps: S1, preparing raw materials; s2, preparing beta-phase silicon nitride whiskers; s3, preparing a polypyrrole coated beta-phase silicon nitride and iron oxide compound; s4, preparing slurry; s5, carrying out tape casting; s6, drying; s7, degreasing is conducted; and S8, sintering to obtain a porous silicon nitride ceramic finished product. According to the porous silicon nitride ceramic, directional arrangement of the pore-forming agent or the silicon nitride particles is induced by applying an external magnetic field, so that control over pore orientation is achieved, the material has excellent wave-transparent performance, normal work of a guided missile seeker antenna can be guaranteed in a severe environment, and the service life of the guided missile seeker antenna is prolonged. Due to the characteristics of high strength and low density, the radome is excellent in performance when bearing various mechanical loads in high-speed flight, and meanwhile, the radome plays a very important role in protecting the radome from possible impact and vibration damage in flight.

Description

Technical Field

[0001] The present application relates to the technical field of ceramic material preparation, and in particular to a preparation method and application of porous silicon nitride ceramics with adjustable porosity and pore orientation. Background Art

[0002] With the development of high-speed aircraft, the performance requirements of antenna cover materials are constantly increasing. Porous silicon nitride ceramics, as a antenna cover material with excellent comprehensive performance, mainly reduce the dielectric constant and improve the wave transmission performance by increasing the porosity, which also brings about the problem of decreased mechanical properties.

[0003] Therefore, it is necessary to develop a preparation method and application for improving the mechanical properties of porous silicon nitride ceramics by grain orientation. Summary of the Invention

[0004] Based on this, the purpose of this application is to provide a preparation method of porous silicon nitride ceramics with adjustable porosity and pore orientation using tape casting, so as to achieve effective regulation of the pore structure parameters and pore orientation of porous silicon nitride ceramics and meet the special requirements of material performance in different application scenarios.

[0005] The technical solution of this application is as follows:

[0006] A method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation comprises the following steps:

[0007] Step S1: Raw material preparation

[0008] High-purity silicon nitride powder is selected as the matrix material, and suitable organic monomers and sintering aids are prepared.

[0009] Step S2: Preparation of β-phase silicon nitride whiskers

[0010] Silicon nitride powder and a sintering aid are added to a mixed solution of an organic solvent and a dispersant in a predetermined proportion, and ball-milled in a ball mill for 6-18 hours to obtain a slurry that is dried and sieved; the slurry is calcined in a gas pressure furnace at 1700°C-1850°C to obtain long rod-shaped silicon nitride whiskers; and the long rod-shaped β-phase silicon nitride whiskers of high purity are obtained through pickling and removal of impurities; specifically, the purity of the long rod-shaped silicon nitride whiskers is ≥98%.

[0011] The mechanism of action in step S2 is a dissolution-reprecipitation mechanism. Under high temperatures (typically >1500°C) and the presence of a liquid phase (a molten silicate formed by a sintering aid), the surface of the α-Si3N4 particles partially dissolves in the liquid phase, forming a solution rich in Si, N, and O. The dissolved Si-NO units recrystallize on the surface of the β-Si3N4 crystal nuclei, forming the β phase. The β phase grains grow along a specific direction (such as the c-axis) into long columnar or needle-like structures.

[0012] Step S3: Preparation of polypyrrole-coated β-phase silicon nitride and iron oxide composite

[0013] Long rod-shaped β-phase silicon nitride whiskers, organic monomers, oxidants and sintering aids are added to 100 ml of distilled water in a predetermined ratio and mechanically stirred for 3-6 hours. The resulting product is washed with water until neutral and dried to obtain a polypyrrole-coated β-phase silicon nitride and iron oxide composite, which can be deflected in a magnetic field. The oxidant of the present application is Fe-containing 3+ of compounds.

[0014] The mechanism of action of step S3: In the presence of an oxidant (such as FeCl3), organic monomer (such as pyrrole) molecules undergo oxidative polymerization to form polypyrrole. In this process, the oxidant provides an oxidizing environment, which prompts the pyrrole monomer to form a polymer. β-phase silicon nitride whiskers and oxides serve as the substrate, and polypyrrole is gradually adsorbed and wrapped on their surfaces during the formation process to form a composite coating structure. Sintering aids can assist the bonding of polypyrrole to the substrate during the coating process, ensuring more uniform coating and reducing the difficulty of subsequent material sintering, which is beneficial to avoid phase change of β-phase silicon nitride whiskers due to high temperature (such as transformation into a phase).

[0015] Specifically, during the chemical polymerization process, pyrrole monomers first lose electrons under the action of an oxidant, forming cationic radicals. These radicals then collide and combine to form a dicationic dipyrrole containing two cationic radicals. The dicationic dipyrrole undergoes disproportionation in the system to form an electrically neutral dipyrrole. The electrically neutral dipyrrole then combines with the cationic radicals in the system to form a trimerized polypyrrole. This process repeats itself, ultimately yielding a long polypyrrole chain.

[0016] Step S4: Slurry preparation

[0017] High-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, sintering aid and plasticizer are added to a mixed solution of organic solvent and dispersant in a predetermined proportion, and ball milled in a ball mill at a speed of 300RPM for 24 hours to prepare a uniform and stable ceramic slurry.

[0018] The plasticizer is DBP (dibutyl phthalate), which can improve the ductility of the green body, adjust the drying stress, promote degreasing and decomposition, and assist the bonding between particles, laying the foundation for subsequent sintering.

[0019] During the slurry preparation process, the porosity and structure of the porous silicon nitride ceramics are preliminarily designed by controlling factors such as monomers and sintering aids. The composite can be deflected in a magnetic field. During the sintering process, the coated organic matter is burned off, and the space originally occupied by polypyrrole forms pores (the porosity can be controlled by controlling the amount of pyrrole added); the long rod-shaped β-phase silicon nitride can be used as a template to obtain porous silicon nitride ceramics with controllable pore structure and orientation. Specifically, pyrrole is used as a pore-forming agent in this scheme, and its addition amount will affect the structure of the reaction product during sintering. When the amount of pyrrole added is 3% of the mass of the powder (the mass of the powder is silicon nitride powder, composite and sintering aid), the porosity of the porous silicon nitride ceramic obtained is 10%; when the amount of pyrrole added is 5% of the mass of the powder, the porosity is 20%; when the amount of pyrrole added is 10% of the mass of the powder, the porosity is 36%.

[0020] Step S5: tape casting

[0021] The prepared ceramic slurry is poured into a casting machine. During the casting process, an external magnetic field is applied to induce the directional arrangement of the ceramic slurry (specifically, the deflectable particles in the ceramic slurry, such as the pore-forming agent pyrrole, silicon nitride particles, composites, etc.). The particles of the ceramic slurry are oriented according to the direction of the magnetic field, thereby achieving control of the pore orientation.

[0022] By applying a circular magnetic field around the cast film with a magnetic field strength of 6T or greater, deflectable particles, such as the composite, are distributed perpendicular to the magnetic field. This application uses a polypyrrole and iron oxide composite to coat β-phase silicon nitride, giving it the ability to deflect in a magnetic field. By adjusting the amount of a pore-forming agent capable of deflecting in a magnetic field, this application produces porous ceramics with adjustable pore orientation. The resulting porous ceramics are stronger, with a maximum flexural strength of 400 MPa.

[0023] Step S6: Drying

[0024] The green body film after tape casting is placed in a drying oven for drying treatment, and the drying temperature is controlled at 50-80° C. to remove the organic solvent in the green body film to obtain a dry porous silicon nitride ceramic green body.

[0025] Step S7: Degreasing

[0026] The dried green body was placed in a degreasing furnace for degreasing. The degreasing temperature was set at 800°C, the heating rate was 0.5°C / min, and the temperature was kept at 800°C for 2 hours.

[0027] Step S8: Sintering

[0028] The degreased green body is placed in a high-temperature sintering furnace for sintering to obtain a porous silicon nitride ceramic product. The sintering atmosphere is a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen, the sintering temperature is 1800°C, the sintering time is 3 hours, and the nitrogen pressure is 4MPa.

[0029] Furthermore, the organic monomer is one or two of pyrrole and nitrogen-nitrogen methylene bisacrylamide.

[0030] The organic monomer is pyrrole, which has the following advantages:

[0031] (1) The chemical oxidation polymerization of pyrrole to produce polypyrrole has the advantages of good product quality, easy control of parameters, and mild reaction conditions;

[0032] (2) During the synthesis of polypyrrole, Fe 3+ It can act as both an oxidant and a magnetic source, making polypyrrole magnetic. This magnetized polypyrrole will exhibit a magnetic response in a magnetic field, resulting in deflection. At the same time, the sintering aid ferrosoferric oxide is coated on the polypyrrole and β-phase silicon nitride coating, which can also deflect in a magnetic field.

[0033] (3) The coated polypyrrole, as a polymer, forms pores during the sintering process, and porous oriented porous silicon nitride ceramics can be obtained; at the same time, the porosity of the porous silicon nitride ceramics can be controlled by adjusting the amount of organic monomer added.

[0034] When preparing polypyrrole-coated β-phase silicon nitride and iron oxide composites, the organic monomer nitrogen methylene bisacrylamide (N,N'-methylene bisacrylamide) usually acts as a crosslinking agent, and its specific functions are as follows:

[0035] (1) Promotes cross-linking of polypyrrole networks: The synthesis of polypyrrole (PPy) is usually achieved through the oxidative polymerization of pyrrole monomers. Nitrogen-methylenebisacrylamide contains two acrylamide groups, which can undergo cross-linking reactions with pyrrole monomers or polymerized polypyrrole chains to form a three-dimensional network structure. This can enhance the mechanical strength and stability of the polypyrrole coating and prevent the coating from falling off during preparation or use.

[0036] (2) Regulating the coating structure: As a crosslinking agent, it can adjust the degree of polymerization and network density of polypyrrole, affecting the porosity, conductivity and other properties of the coating. For example, an appropriate degree of crosslinking can optimize the interfacial bonding between polypyrrole and β-phase silicon nitride and iron oxide, thereby improving the overall performance of the composite (such as electromagnetic properties and catalytic activity).

[0037] (3) Assisting uniform dispersion of the composite: In the composite system, the introduction of the crosslinker may promote the uniform dispersion of β-phase silicon nitride and iron oxide particles through intermolecular interactions (such as hydrogen bonding and π-π stacking), reduce agglomeration, and make polypyrrole more evenly coated on the particle surface to form a stable core-shell or composite structure.

[0038] Preferably, the organic monomer of the present application is a mixture of pyrrole and nitrogen-nitrogen methylene bisacrylamide in a mass ratio of 1:2. The mixture of pyrrole and nitrogen-nitrogen methylene bisacrylamide (MBA) as organic monomers exhibits significant synergistic effects (the complementary effect of "linear polymerization-crosslinked network"). Pyrrole provides a conductive skeleton, while MBA strengthens the structure and regulates the morphology through cross-linking, and synergistically improves interfacial compatibility, ultimately optimizing the mechanical properties, conductivity, and functional characteristics of the composite, as embodied in the following aspects:

[0039] (1) Synergistic effect in polymerization reaction: initiation and cross-linking are carried out simultaneously: pyrrole is a monomer of polypyrrole, and in the presence of oxidants (such as Fe 3+ ) or oxidative polymerization under electrochemical conditions to form linear or branched polypyrrole chains; while MBA, as a cross-linking agent, the bisacrylamide groups in its molecules can undergo free radical copolymerization with the double bonds and active sites in the pyrrole monomer or polypyrrole chain, forming covalent cross-links between the linear chains and generating a three-dimensional network structure. When the two act together, polymerization and cross-linking proceed simultaneously, avoiding the problems of too short segments and loose structures that may occur in the polymerization of pure pyrrole. Regulating the polymerization rate and product morphology: The addition of MBA can regulate the kinetic process of pyrrole polymerization. For example, the double bond activity of MBA is relatively high, and it may act as a "node" in the polymerization reaction, slowing down the excessive growth of linear chains and promoting intermolecular cross-linking, thereby controlling the particle size, porosity and morphology of polypyrrole (such as the transformation from amorphous to a porous network). This regulation helps to optimize the specific surface area and conductive pathways of the product.

[0040] (2) Synergistic optimization of product structure and performance: Enhanced mechanical strength and stability: The mechanical properties of pure polypyrrole are poor (easy to crack), while the network structure formed by MBA cross-linking can significantly improve the toughness and deformation resistance of the material. For example, when the cross-linked polypyrrole coating is loaded with particles such as β-phase silicon nitride and iron oxide, it can be more firmly attached to the surface of the particles, reducing the shedding or cracking of the coating. Synergistic improvement of conductivity and functionality: Polypyrrole itself has good conductivity, but the formation of a cross-linked network may synergistically optimize performance in the following ways: a. The cross-linked network can shorten the charge transfer distance between polypyrrole chains and form a more continuous conductive path; b. If the composite contains magnetic particles such as iron oxide, the MBA cross-linked polypyrrole network can bind to the particle surface through π-π stacking or hydrogen bonding, enhancing interfacial charge transfer. At the same time, the introduction of magnetic particles may form a "conductive-magnetic" synergistic effect with the polypyrrole network (such as improving electromagnetic loss capacity).

[0041] (3) Synergistic improvement of interface bonding and dispersibility: Promote uniform dispersion of inorganic particles: The amide group (-CONH2) of MBA can form hydrogen bonds with the hydroxyl groups (-OH) on the surface of β-phase silicon nitride and iron oxide, or be adsorbed on the surface of particles through electrostatic interaction, reducing particle agglomeration; at the same time, pyrrole uses MBA as a crosslinking point during the polymerization process and grows around the particle surface to form a "particle-crosslinked polypyrrole" composite structure, making the inorganic particles more evenly dispersed in the network and improving the integrity of the composite. Optimize interface compatibility: The presence of the crosslinked network can reduce the interfacial tension between polypyrrole and inorganic particles, reduce interface defects (such as voids and stress concentration), and thus enhance the interface bonding force. For example, in electromagnetic functional composite materials, good interface bonding can promote multiple reflections and losses of electromagnetic waves between the polypyrrole network and inorganic particles, thereby improving the absorption performance.

[0042] Furthermore, in step S1, the high-purity silicon nitride powder has a D50 of 0.5-0.7 μm, a purity of >99%, an α-Si3N4 content of >95 wt%, and an oxygen content of <1.5%.

[0043] The high-purity silicon nitride powder of this application can ensure that the pore size of the porous silicon nitride ceramic is controllable. If silicon nitride powder with multi-phase particle size distribution is used, the pore size of the porous silicon nitride ceramic will be uncontrollable, affecting the product effect of this application.

[0044] Furthermore, the sintering aid is at least one of nanometers, iron oxide, and ferrosoferric oxide to promote the sintering of silicon nitride.

[0045] Nano-MgO can react with silicon dioxide SiO2 on the surface of silicon nitride to form magnesium silicate liquid phase, which reduces the sintering temperature and promotes the diffusion of materials between particles, thereby improving the sintering density.

[0046] Iron oxide (Fe2O3): It can form a low melting point eutectic phase with SiO2, produce a liquid phase during the sintering process, fill the gaps between particles, and promote sintering densification through ion diffusion.

[0047] Preferably, the sintering aid is ferroferric oxide. This oxide can serve as both a sintering aid and a magnetically responsive medium. Fe₃O₄ is a typical ferromagnetic material with spontaneous magnetization at room temperature. Its magnetic moment can be oriented in an external magnetic field. When the Fe₃O₄ particles in the composite are subjected to a uniform magnetic field H, each particle generates a magnetic moment μ, which aligns the particles along the magnetic field and causes the polypyrrole and β-phase silicon nitride composite to undergo regular deflection in the magnetic field. In this solution, ferroferric oxide simultaneously functions as a sintering aid and a magnetically responsive medium.

[0048] Furthermore, the organic solvent is at least one of ethanol and butanone, and the dispersant is at least one of modified fish oil, Xinnuo LD 1137-50, and styrene maleic acid copolymer solution.

[0049] Preferably, the organic solvent is a mixture of ethanol and butanone in a mass ratio of 1:1 or 2:1.

[0050] Ethanol and butanone can dissolve some components of the system (such as dispersants and solutes) and evenly disperse solid particles in the solvent, forming a stable suspension or solution and preventing particle agglomeration. By adjusting the ratio of organic solvents, the viscosity of the system can be controlled, making it easier to apply in processes such as coating and spraying. As a medium, they help the various components (such as powders and additives) mix thoroughly and ensure system uniformity.

[0051] The function of dispersants such as modified fish oil, Sino LD 1137-50, and styrene maleic acid copolymer solution is to prevent particle agglomeration: dispersants adsorb on the surface of solid particles to form a charge repulsion layer or steric hindrance effect, preventing the particles from aggregating with each other and keeping them highly dispersed in the solvent. Improve stability: Reduce particle sedimentation or flocculation to ensure that the system remains uniform during storage and use, avoiding performance fluctuations. Improve process performance: Uniformly dispersed particles can improve the uniformity and quality of subsequent processing (such as sintering and film formation). For example, in the dispersion of ceramic powders, it can improve the density and mechanical properties of the sintered body.

[0052] Furthermore, in step S2, the mass ratio of silicon nitride powder to sintering aid is 8-10:1-2, preferably 9:1. The amount of dispersant added is 0.8-2% of the mass of the powder (the sum of silicon nitride powder and sintering aid). The amount of organic solvent added is sufficient to ensure uniform mixing of the powder and dispersant. Preferably, the amount of organic solvent used is 1.5-2 times the mass of the powder.

[0053] Furthermore, in step S3, the mass ratio of the β-phase silicon nitride whiskers, the organic monomer, the oxidant and the sintering aid is 1-3:2-12:6-8:0.5-1.5.

[0054] Among them, the organic monomer is pyrrole, the oxidant is FeCl3, and the sintering aid is Fe3O4, among which ferrosoferric oxide also provides the ability of the composite to deflect in a magnetic field.

[0055] Furthermore, in step S4, the mass ratio of high-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, sintering aid, binder and plasticizer is 55-90:8-45:8-12:8-12:1-3.

[0056] In step S4, the amount of organic solvent added is 25-45% of the mass of the powder; the amount of dispersant added is 0.8-2% of the mass of the powder, and the solid content of the ceramic slurry is controlled to be 65-75wt%.

[0057] Furthermore, the step S4 is specifically as follows:

[0058] High-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, and sintering aid are added to an organic solvent (a mixed solution of alcohol and butanone), followed by addition of dispersant-modified fish oil and plasticizer DBP, and ball milling is performed using a ball mill to prepare a powder slurry;

[0059] The binder was added to an organic solvent (a mixed solution of alcohol and butanone), and placed in an oven (60°C) until the binder was completely dissolved to prepare a colloidal solution;

[0060] The colloidal solution is added into the powder slurry and the castable ceramic slurry is prepared by ball milling.

[0061] This step involves pre-dissolving the binder PVB in an organic solvent until it is completely dissolved, and then mixing it with the powder slurry in a planetary ball mill in step S4 to produce a casting slurry. This ensures that the PVB is fully dissolved, preventing agglomeration, and helps improve the slurry mixing efficiency and stability, ensuring consistent adhesion to the powder particles, thereby improving the strength of the subsequent green body and the density of the sintered body.

[0062] PVB (polyvinyl butyral) can form a uniform and stable slurry when mixed with dispersants and powders in the casting process. It plays a bonding role and has good bonding force on ceramic powders, additives and composites. It has the functions of improving green body strength, providing flexibility to prevent brittle cracking, regulating slurry fluidity and assisting in powder dispersion and stability.

[0063] The invention discloses an application of porous silicon nitride ceramics with adjustable porosity and pore orientation, which is made by the preparation method and is applied to the cover body of a missile antenna cover.

[0064] A porous silicon nitride ceramic with adjustable porosity and pore orientation is produced by the preparation method.

[0065] The porous silicon nitride ceramics of the present invention achieve control of pore orientation by inducing directional arrangement of pore-forming agents or silicon nitride particles by applying an external magnetic field. The material has excellent wave transmission performance (dielectric constant 3-5), which can ensure the normal operation of missile seeker antennas in harsh environments. Its high strength (flexural strength can reach up to 400MPa-500MPa, while conventional porous silicon nitride ceramics are 200-300MPa) and low density (density 1.8g / cm 3 -2.4g / cm 3) characteristics enable it to perform well in withstanding various mechanical loads in high-speed flight, and also play a very important role in protecting the radome from impact and vibration damage that may be encountered during flight.

[0066] Compared with the prior art, this application has the following advantages and positive effects:

[0067] 1. Effective control of pore orientation: The use of polypyrrole to coat iron oxide and β-phase silicon nitride can cause the long rod-shaped β-phase silicon nitride to be deflected in a magnetic field; the effect of the magnetic field during the tape casting process can make the whiskers arranged in an orderly manner in a predetermined direction, giving the porous silicon nitride ceramic anisotropic physical and mechanical properties.

[0068] 2. The porous orientation is perpendicular to the magnetic field plane. By adjusting the pore direction, the flexural strength of the porous silicon nitride ceramic can be effectively improved (the pore orientation is perpendicular to the force direction). The flexural strength can reach above 400 MPa and up to 500 MPa. Under the same porosity, the flexural strength is 20-50% higher than that of existing porous silicon nitride ceramics.

[0069] 3. The tape casting process can realize large-area and thin-thickness forming of ceramic green bodies, and can produce porous silicon nitride ceramic green bodies with uniform thickness, smooth surface, controllable pore structure and orientation, which is conducive to industrial large-scale production and subsequent processing.

[0070] 4. The porosity is adjustable. By adjusting the amount of organic monomer in the powder, the porosity can be controlled within 10-40%, which is beneficial for preparing silicon nitride ceramic materials with different densities. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0072] Figure 1 This is the XRD pattern of the β-phase silicon nitride of the present application.

[0073] Figure 2 This is the SEM image of the β-phase silicon nitride of this application.

[0074] Figure 3 This is a schematic diagram of the direction of the external magnetic field of this application.

[0075] Figure 4 This is an SEM image of the porous silicon nitride ceramic prepared in Example 1 of the present application.

[0076] Figure 5 This is an SEM image of the porous silicon nitride ceramic prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0077] Below in conjunction with embodiment and example, further elaborate the application.It should be understood that these examples are only used to illustrate the application and are not used to limit the scope of the application.In addition, it should be understood that after reading the content taught in this application, those skilled in the art can make various changes or modifications to the application, and these equivalent forms also fall within the protection scope of the claims appended hereto.

[0078] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0079] The term "and / or" as used herein includes any one of two or more related listed items, and also includes any and all combinations of the related listed items, wherein any and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items.

[0080] In this application, "further" and the like are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of this application.

[0081] In this application, the terms "first," "second," "third," and "fourth," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor as implicitly indicating the importance or quantity of the technical features indicated. Furthermore, the terms "first," "second," "third," and "fourth," etc., are used only for non-exhaustive enumeration and description purposes and should not constitute closed-ended limitations on quantity.

[0082] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0083] In this application, when referring to a numerical interval (i.e., a numerical range), unless otherwise specified, the optional numerical distribution within the above numerical interval is considered continuous and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every numerical value between the two numerical endpoints. Unless otherwise specified, when a numerical interval refers only to integers within the numerical interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all subranges included therein.

[0084] The temperature parameters in this application, unless otherwise specified, allow both constant temperature treatment and temperature fluctuation within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range of the instrument control.

[0085] In this application, weight can be μg, mg, g, kg and other mass units commonly known in the field of medicine, health products or food.

[0086] Example 1

[0087] This embodiment provides a method for preparing a porous silicon nitride ceramic with adjustable porosity and pore orientation according to the present application, comprising the following steps:

[0088] The main raw material is silicon nitride powder with a D50 of 0.6μm, a purity of >99%, an average particle size of 0.5μm, an α-Si3N4 content of >95wt%, and an oxygen content of 1.35%.

[0089] Sintering aid nano magnesium oxide: Xiamen Gold Tungsten, purity ≥99.9%, 50nm.

[0090] Whisker Preparation: 288g of silicon nitride powder and 12g of magnesium oxide (a sintering aid) were added to a nylon ball mill. Anhydrous ethanol was added as a dispersion medium, and silicon nitride balls were added for ball milling. The mass ratio of raw powder to anhydrous ethanol to silicon nitride balls was 1:1:2. The mill was placed in a planetary ball mill and milled at a speed of 300 rpm for 12 hours to thoroughly mix the powders. After ball milling, the powders were dried in an oven at 50-70°C. The dried powders were screened through a 100-mesh sieve. The mixed powders were evenly spread on the bottom of a graphite crucible to avoid close accumulation, which would cause sintering and agglomeration and affect subsequent washing. The powders and crucible were placed in a sintering furnace for pressureless sintering. The temperature was raised from 10°C to 1800°C, held at this temperature, and then cooled to room temperature in the furnace to obtain sintered β-phase silicon nitride whiskers. The resulting whiskers were acid-washed, dried, and sieved to obtain high-purity β-phase silicon nitride whiskers.

[0091] Preparation of the composite: 80 g of β-phase silicon nitride whiskers, 400 g of organic monomer pyrrole, 280 g of oxidant FeCl3, and 40 g of ferrosoferric oxide were added to 100 ml of distilled water and mechanically stirred for 4 hours to completely coat the whiskers. The resulting product was washed with water until neutral, dried, and passed through a 100-mesh sieve to obtain a polypyrrole-coated β-phase silicon nitride and iron oxide composite.

[0092] Prepare the slurry by adding 328g of α-phase silicon nitride, 32g of the coating obtained above, and 40g of magnesium oxide to a mixture of 150g of ethanol and 150g of butanone. Also add 40g of modified fish oil and 8g of DBP (plasticizer). Mill the mixture with 1000g of silicon nitride grinding balls at 300 RPM for 24h.

[0093] Add 24g of polyvinyl butyral (PVB) to a mixed solution of 15g of alcohol and 15g of butanone, and place in an oven (60°C) until the colloid is completely dissolved;

[0094] 40 g of the above colloidal solution was added to the powder slurry and ball milled at 300 RPM for 24 h to obtain a castable slurry.

[0095] The slurry is cast at a scraper height of 0.5 mm and a film speed of 45 cm / min. During the casting process, a magnetic field perpendicular to the casting direction is applied to deflect the β-phase silicon nitride composite. The schematic diagram of the direction of the applied magnetic field is shown in FIG. Figure 3 As shown, the magnetic field strength is 6T around the B direction.

[0096] The obtained cast film was subjected to warm isostatic pressing to obtain a 1 cm thick silicon nitride green sheet; the temperature was increased to 800°C at a rate of 0.5°C / min, kept warm for 2 hours, and then degreased in a gas pressure furnace, and sintered at 1850°C and 4 MPa nitrogen pressure for 2 hours to obtain porous silicon nitride ceramics.

[0097] In this embodiment, the amount of organic monomer pyrrole added is 4% of the mass of the powder (the mass of the powder is silicon nitride powder, compound and sintering aid), and the porosity is adjusted to 10%. The flexural strength of the porous silicon nitride ceramics prepared by the above method is up to 500 MPa, which is 20-51.5% higher than that of traditional porous silicon nitride ceramics (the flexural strength of existing porous silicon nitride ceramics with a porosity of 10% is generally 330-400 MPa), which greatly broadens its application scenarios.

[0098] Example 2

[0099] This embodiment provides a method for preparing a porous silicon nitride ceramic with adjustable porosity and pore orientation. The steps are basically the same as those in Example 1, except that:

[0100] The sintering aid of this embodiment is ferrosoferric oxide; when preparing the whiskers, anhydrous ethanol is added as the dispersion medium; the organic monomer is a mixture of pyrrole and nitrogen-nitrogen methylene bisacrylamide in a mass ratio of 1:2; when preparing the slurry, the dispersant is Xinnuo LD 1137-50.

[0101] In this embodiment, the amount of organic monomer added is 17% of the mass of the powder (the mass of the powder is silicon nitride powder, compound and sintering aid), the porosity is adjusted to 36%, and the flexural strength of the porous silicon nitride ceramic prepared by the above method is 260MPa, which is 30-52.9% higher than the flexural strength of traditional porous silicon nitride ceramics (the flexural strength of existing porous silicon nitride ceramics with a porosity of 10% is generally 170-200MPa), which greatly broadens its application scenarios. The rest of the content of this embodiment is the same as that of Example 1 and will not be repeated here.

[0102] Example 3

[0103] This embodiment provides a method for preparing a porous silicon nitride ceramic with adjustable porosity and pore orientation. The steps are basically the same as those in Example 1, except that:

[0104] The sintering aid of this embodiment is iron oxide; when preparing the whiskers, anhydrous ethanol and butanone are added as dispersion media; the organic monomer is pyrrole; and when preparing the slurry, the dispersant is modified fish oil.

[0105] In this embodiment, the amount of organic monomer pyrrole added is 9% of the mass of the powder (the mass of the powder is silicon nitride powder, compound and sintering aid), and the porosity is adjusted to 20%. The flexural strength of the porous silicon nitride ceramic prepared by the above method is 350MPa, which is 20.7-48.9% higher than the flexural strength of traditional porous silicon nitride ceramics (the flexural strength of existing porous silicon nitride ceramics with a porosity of 10% is generally 235-290MPa), which greatly broadens its application scenarios. The rest of the content of this embodiment is the same as that of Example 1 and will not be repeated here.

[0106] Example 4

[0107] This embodiment provides a method for preparing a porous silicon nitride ceramic with adjustable porosity and pore orientation. The steps are basically the same as those in Example 1, except that:

[0108] The sintering aid of this embodiment is ferrosoferric oxide; when preparing whiskers, anhydrous ethanol is added as a dispersion medium; the organic monomer is a mixture of pyrrole and nitrogen-nitrogen methylene bisacrylamide in a mass ratio of 1:2; when preparing the slurry, the dispersant is a styrene-maleic acid copolymer solution.

[0109] In this embodiment, the amount of organic monomer pyrrole added is 20.5% of the mass of the powder (the mass of the powder is silicon nitride powder, compound and sintering aid), and the porosity is adjusted to 40%. The flexural strength of the porous silicon nitride ceramic prepared by the above method is 200MPa, which is 25-53.8% higher than the flexural strength of traditional porous silicon nitride ceramics (the flexural strength of existing porous silicon nitride ceramics with a porosity of 10% is generally 130-160MPa), which greatly broadens its application scenarios. The rest of the content of this embodiment is the same as that of Example 1 and will not be repeated here.

[0110] The characteristic indicators of the porous silicon nitride ceramics prepared in Examples 1-4 of the present application are as follows:

[0111]

[0112] Figure 1 and Figure 2 The diffraction peaks are all β-phase, and there is no intercrystalline phase, indicating that the prepared silicon nitride whiskers are of high purity; the β-Si3N4 whiskers are long rod-shaped, with a length of 3-7μm, a width of 0.5-1μm, and an aspect ratio of 5-8. The whiskers prepared by this method have a more uniform morphology and fewer broken whiskers. Figure 3 It can be seen that the deflectable particles such as the composite are distributed perpendicular to the direction of the magnetic field. Figure 4 This is the SEM image of the slurry in the casting state (no magnetic field is added at this time). It can be seen that the deflectable particles such as the composite are parallel to the casting direction plane; Figure 5 This is a SEM image of porous silicon nitride ceramics. Under the influence of a magnetic field perpendicular to the casting direction, the deflectable particles of the composite are perpendicular to the casting direction. Figure 4 、 Figure 5 It can be seen that the long rod-shaped β-phase silicon nitride composite is basically deflected in the direction perpendicular to the magnetic field.

[0113] The porous silicon nitride ceramics prepared in this application have the following advantages: First, by utilizing the effect of the magnetic field during the tape casting process, the whiskers can be arranged in an orderly manner in a predetermined direction, effectively controlling the pore orientation, and giving the porous silicon nitride ceramics anisotropic physical and mechanical properties. Second, by adjusting the pore direction, the flexural strength of the porous silicon nitride ceramic can be effectively improved (the pore orientation is perpendicular to the force direction), and the flexural strength can reach more than 400MPa, and up to 500MPa; under the same porosity, the flexural strength is increased by 20-50% compared with the existing porous silicon nitride ceramics. Third, the tape casting process can realize large-area and thin-thickness molding of the ceramic body, and can produce porous silicon nitride ceramic bodies with uniform thickness, smooth surface, and controllable pore structure and orientation, which is conducive to industrial large-scale production and subsequent processing. Fourth, the porosity is adjustable. By adjusting the amount of organic monomer in the powder, the porosity can be controlled at 10-40%, which is conducive to the preparation of silicon nitride ceramic materials with different densities.

[0114] All documents mentioned in this application are cited as references in this application, just as each document is cited as reference separately. Unless they conflict with the application purpose and / or technical solution of this application, the cited documents involved in this application are cited in their entirety and for all purposes. When cited documents are involved in this application, the definitions of relevant technical features, terms, nouns, phrases, etc. in the cited documents are also cited. When cited documents are involved in this application, the examples and preferred embodiments of the cited relevant technical features may also be incorporated into this application as references, but are limited to the ability to implement this application. It should be understood that when the cited content conflicts with the description in this application, the present application shall prevail or be adaptively amended according to the description in this application.

[0115] The various technical features of the above-mentioned implementation modes and examples can be combined in any appropriate manner. In order to make the description concise, not all possible combinations of the various technical features in the above-mentioned implementation modes and examples are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of the description in this specification.

[0116] The embodiments described above only express several implementation methods of the present application, but they should not be understood as limiting the scope of the patent application. It should be pointed out that, for those of ordinary skill in the art, without departing from the concept of the present application, several variations and improvements can be made, which all fall within the scope of protection of the present application. In addition, it should be understood that after reading the above-mentioned teaching content of the present application, those skilled in the art can make various changes or modifications to the present application, and the equivalent forms obtained also fall within the scope of protection of the present application. It should also be understood that the technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments on the basis of the technical solutions provided in the present application are all within the scope of protection of the claims attached to the present application. Therefore, the scope of protection of the patent application of the present application shall be based on the attached claims, and the description can be used to interpret the content of the claims.

Claims

1. A method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation, characterized in that: The steps include: Step S1: Raw material preparation Select high-purity silicon nitride powder as the matrix material; Step S2: Preparation of β-phase silicon nitride whiskers Add silicon nitride powder and sintering aid in a predetermined ratio to a mixed solution of an organic solvent and a dispersant, and use a ball mill to mix and grind the mixture to obtain a slurry, drying and sieving it; and calcining it in a gas pressure furnace at 1700°C-1850°C; Obtaining long rod-shaped silicon nitride whiskers; and obtaining high-purity β-phase silicon nitride whiskers through pickling and removal of impurity phases; Step S3: Preparation of polypyrrole-coated β-phase silicon nitride and iron oxide composite β-phase silicon nitride whiskers, organic monomers, oxidants and sintering aids are added to water in a predetermined ratio and stirred for 3-6 hours. The resulting product is washed with water until neutral and dried to obtain a polypyrrole-coated β-phase silicon nitride and iron oxide composite. Step S4: Slurry preparation High-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, sintering aid, binder and plasticizer are added to a mixed solution of organic solvent and dispersant in a predetermined proportion, and ball milling is performed using a ball mill to prepare a uniform and stable ceramic slurry. Step S5: tape casting The prepared ceramic slurry is poured into a tape casting machine. During the tape casting process, an external magnetic field is applied to induce the directional arrangement of the ceramic slurry, thereby achieving control of the pore orientation. Step S6: Drying The green body film after tape casting is placed in a drying oven for drying to remove the organic solvent in the green body film to obtain a dry porous silicon nitride ceramic green body; Step S7: Degreasing The dried green body is placed in a degreasing furnace for degreasing; Step S8: Sintering The degreased green body is placed in a high-temperature sintering furnace for sintering to obtain a porous silicon nitride ceramic product.

2. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: The organic monomer is one or two of pyrrole and nitrogen-nitrogen methylene bisacrylamide.

3. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: In step S1, the high-purity silicon nitride powder has a D50 of 0.5-0.7 μm, a purity of >99%, an α-Si3N4 content of >95 wt%, and an oxygen content of <1.5%.

4. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: The sintering aid is at least one of nanometers, iron oxide, and ferrosoferric oxide.

5. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: The organic solvent is at least one of ethanol and butanone, and the dispersant is at least one of modified fish oil, Xinnuo LD1137-50, and styrene maleic acid copolymer solution.

6. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: In step S2, the mass ratio of silicon nitride powder to sintering aid is 8-10:1-2, and the added amount of dispersant is 0.8-2% of the mass of the powder; the added amount of the organic solvent is sufficient to ensure that the powder and dispersant are evenly mixed.

7. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: In step S3, the mass ratio of the β-phase silicon nitride whiskers, the organic monomer, the oxidant and the sintering aid is 1-3:2-12:6-8:0.5-1.

5.

8. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: In step S4, the mass ratio of high-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, sintering aid, binder and plasticizer is 55-90:8-45:8-12:8-12:1-3.

9. The method for preparing porous silicon nitride ceramics with adjustable porosity and pore orientation according to claim 1, characterized in that: The step S4 is specifically as follows: High-purity silicon nitride powder, polypyrrole-coated β-phase silicon nitride and iron oxide composite, and sintering aid are added to an organic solvent, and then a dispersant and a plasticizer are added, and ball milling is performed using a ball mill to prepare a powder slurry; Adding the binder to the organic solvent and placing it in an oven until the binder is completely dissolved to prepare a colloidal solution; The colloidal solution is added into the powder slurry and the castable ceramic slurry is prepared by ball milling.

10. An application of porous silicon nitride ceramics with adjustable porosity and pore orientation, characterized in that: The cover body is made by the preparation method and is applied to a missile antenna cover.

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