CN114773082A

CN114773082A - Silicon nitride ceramic radome with symmetrical continuous gradient structure and preparation method thereof

Info

Publication number: CN114773082A
Application number: CN202210368224.2A
Authority: CN
Inventors: 叶昉; 成来飞; 赵凯; 李明星; 张聪琳; 崔雪峰; 付志强; 张立同
Original assignee: Northwestern Polytechnical University
Current assignee: Northwestern Polytechnical University
Priority date: 2022-03-30
Filing date: 2022-03-30
Publication date: 2022-07-22
Anticipated expiration: 2042-03-30
Also published as: CN114773082B

Abstract

The invention relates to a silicon nitride ceramic antenna housing with a symmetrical continuous gradient structure and a preparation method thereof, and the method comprises the steps of (1) adopting electromagnetic simulation software CST to carry out electromagnetic performance simulation and broadband wave-transparent performance optimization of the antenna housing; (2) preparing a silicon nitride-based ceramic porous core layer by adopting a gel casting-precursor impregnation cracking method; (3) preparing a silicon nitride nanowire on the surface of the core layer by adopting a carbothermic nitridation method/a silicon powder nitridation method/a catalytic cracking method; (4) performing surface layer densification on the antenna housing with the surface containing the nano wires by adopting a chemical vapor deposition/permeation method; (5) and (4) performing precision machining on the antenna housing. The technical scheme provided by the invention is a combined process, and can realize the synergistic improvement of the broadband wave-transmitting, high-temperature bearing and environmental erosion resistance of the high-performance antenna housing and the near-net-size integrated molding of the large-scale complex-profile continuous gradient structure antenna housing, thereby solving the problem of difficult molding of the current gradient structure antenna housing and realizing the continuous change of the antenna housing structure and the performance gradient.

Description

Silicon nitride ceramic antenna housing with symmetrical continuous gradient structure and preparation method

Technical Field

The invention belongs to a high-temperature wave-transparent nitride ceramic antenna housing and a preparation method thereof, relates to a silicon nitride ceramic antenna housing with a symmetrical continuous gradient structure and a preparation method thereof, and particularly relates to a symmetrical continuous gradient structure multi-scale design of the silicon nitride ceramic antenna housing and an integrated near-net-size molding preparation method thereof.

Background

The antenna housing is positioned at the nose of an aircraft cabin, has the function of protecting the normal and effective work of an internal radar antenna, and is a structural member of the aircraft and an important component of a guidance system. The antenna housing of the high-speed aircraft has a harsh service environment, and is not only required to bear mechanical stress caused by acceleration and high temperature generated by pneumatic heating in the flight process of the aircraft, but also required to be subjected to erosion of raindrops and collision of particles in the atmosphere, and simultaneously used as a channel for transmitting electromagnetic waves and also required to ensure normal transmission of signals. The antenna housing protects the normal operation of systems such as aircraft communication, telemetering measurement, guidance, detonation and the like in such a severe environment, is a structure/functional component integrating high performance requirements such as heat resistance, wave transmission, bearing, weather resistance and the like, and correspondingly, the material for the antenna housing also meets the multifunctional integration requirement. Silicon nitride ceramic materials are recognized as one of the most suitable candidate materials for high-speed aircraft antenna radomes due to excellent high temperature resistance, corrosion resistance and mechanical properties, and moderate dielectric constant.

The wave-transmitting performance of the antenna housing is closely related to the structure of the housing wall. The cover wall structure of a typical broadband wave-transparent radome is divided into three types: thin-walled structures, sandwich structures, and gradient structures. The wave-transmitting performance of the thin-wall structure radome is excellent, but the synergy of broadband wave-transmitting and mechanical properties is difficult to realize due to the thinner radome wall thickness; the sandwich structure can realize broadband wave transmission by designing material systems and thicknesses of different layers, and is successfully applied to the low-Mach-number missile radome at present. However, because the difference of thermophysical properties of different layer materials is large, when the structure is applied to a high-Mach-number missile antenna cover, the problem of thermal mismatch of each layer is easy to occur, so that the use requirement of high temperature resistance and wide-frequency wave transmission synergy cannot be met; the gradient structure can be regarded as formed by combining a plurality of layers of dielectric materials according to the gradient dielectric property change sequence, has dielectric designability, can meet the requirement of broadband wave transmission, has excellent thermal stress relaxation property, can prevent the material from being broken due to interlayer thermal stress when used in a high-temperature environment, has better high temperature resistance and thermal shock resistance, and meets the use requirement of a high-Mach-number missile radome. At present, researches on silicon nitride ceramic antenna housing with gradient structure are reported, and the technology needs to be developed to meet the use requirements of high temperature resistance, wide frequency wave transmission and load bearing.

The preparation process of the gradient structure radome mainly comprises a variable density blank body co-sintering process, a multilayer bonding process and a surface coating process at present. Among them, Verzemnieks et al (VERZEMNIEKS J, SIMPSON FH. silicon nitride adhesives with controlled multiple properties regions; US5103239[ P)]1992-04-07) regulating and controlling the ceramic density by controlling the content of the pore-forming agent, and preparing the ceramic with the density of 0.9-1.8 g/cm by adopting a green body co-sintering method³The silicon nitride ceramic radome with the gradient structure; boeing company (KOETJE EL, SIMPSON FH, SCHORSCH JF. broadband high temperature radiation apparatus APPARATUS; US4677443[ P ]]19870630) adopting a pore canal forming method, and controlling the material density by controlling the material porosity to prepare the silicon nitride ceramic radome with the gradient structure. The key of the process lies in the integrated molding of the variable-density blank, the process is difficult to control, and the volume shrinkage in the sintering process ensures that the radome is formedPrecise control of the profile is difficult to achieve.

Goto et al and Leggett et al (Goto T, FUJI a, KAWAI c. radome; US 6091375P. 2000-07-18; Leggett h. ceramic boudband radome; US 4358772P. 1982-11-09) produced dense coatings on the surface of a porous silicon nitride ceramic core, respectively, to form a gradient structure silicon nitride ceramic radome. But the interlayer thermophysical properties are difficult to match due to the lack of an effective continuous transition layer between the coating layer and the core layer.

Compared with the foreign countries, the research on the broadband wave-transmitting radome is carried out later in China. Yan Fang Qiang et al (Yan Fang Qiang. design and preparation of sandwich structure radome material and its broadband wave-transmitting performance [ D ]]Wuhan, doctor's academic paper of Wuhan university of science and technology, 2007) ZrP with a gradient structure is prepared by adopting a phosphate bonding technology₂O₇/SiO₂/ZrP₂O₇System A Sandwich Structure and SiO₂/Si₃N₄-BN-SiO₂(n)/SiO₂The interlayer structure ceramic flat plate material of the system B adopts a layered preparation bonding method, has the problem of low interlayer bonding strength, is mainly suitable for preparing flat plate structure parts, and is difficult to prepare the radome with a complex profile structure.

To sum up, the preparation of the gradient-structure broadband wave-transparent radome is realized abroad, but the problems of high difficulty in control of the preparation process, difficult control of the size shrinkage of the radome, mismatching of thermal and physical properties among gradient structure units and the like exist in the existing process method, the research on the manufacturing technology of the continuous gradient-structure silicon nitride broadband wave-transparent radome is not developed in China, the material-level research is developed only on a sandwich-structure ceramic flat plate, the preparation process of layered manufacturing and bonding is adopted, and the integrated continuous gradient structure cannot be realized. Therefore, it is urgently needed to develop a novel silicon nitride material with a continuous gradient structure and an integrated forming process of the radome thereof, and the requirements of the radome of the hypersonic aircraft on high temperature resistance, broadband wave transmission, bearing and the like are met.

Disclosure of Invention

Technical problem to be solved

Aiming at the problems that the research on the silicon nitride ceramic antenna housing with the domestic gradient structure is blank, the preparation process of the silicon nitride ceramic antenna housing with the international gradient structure cannot realize integrated precise forming, the thermophysical properties among gradient structure units are not matched, and the cooperation of wave transmission and mechanical properties is difficult, the invention provides a multiscale structure design method of a silicon nitride material with the continuous gradient structure and an integrated near-net-size forming preparation method of the antenna housing. The invention can realize the cooperative promotion of the broadband wave-transmitting, high-temperature bearing and environmental erosion resistance of the high-performance antenna housing and the near-net-size manufacture of the large-scale complex-profile continuous gradient structure antenna housing.

Technical scheme

The invention provides a preparation method of a silicon nitride ceramic radome with a symmetrical continuous gradient structure. Firstly, developing the structural design of a silicon nitride ceramic broadband wave-transparent cover wall with a symmetrical continuous gradient structure; then, a silicon nitride whisker radome preform is molded through a gel casting process, a nitride matrix is prepared in a preform pore by adopting a precursor impregnation cracking method (PIP method), then, silicon nitride nanowires are grown on the radome surface by adopting a carbothermic reduction nitridation method, a silicon powder nitridation method and a catalytic cracking method, then, a silicon nitride coating is prepared by adopting a low-pressure chemical vapor infiltration/chemical vapor deposition method (LPCVI/CVD method), and finally, the symmetrical continuous gradient structure silicon nitride ceramic radome is prepared through finish machining. The antenna housing has the characteristic of a symmetrical continuous gradient structure, the porosity is gradually changed from the center to the surface along the wall thickness direction, the dielectric property is gradually changed, the mechanical property is gradually changed, the whole material has pure components, high temperature stability, the thermal physical properties of all phases are matched, and the physical/chemical compatibility is excellent. The preparation method can realize the integrated molding and near-net-size preparation of the silicon nitride ceramic antenna housing with the symmetrical continuous gradient structure, the gradient structure is in continuous transition, the layering characteristic is avoided, the volume change and the profile deformation of a blank body are almost avoided in the preparation process, the process controllability is good, the manufacturing precision is high, and the period is short.

A silicon nitride ceramic antenna housing with a symmetrical continuous gradient structure is characterized in that the antenna housing has the characteristic of a symmetrical continuous gradient structure and is of an integrally molded and near-net-size structure; the antenna cover has the advantages that the porosity is gradually changed from the center to the surface along the wall thickness direction, the dielectric property is gradually changed, and the mechanical property is gradually changed.

A method for preparing the silicon nitride ceramic radome with the symmetrical continuous gradient structure is characterized by comprising the following steps:

step 1: the symmetrical continuous gradient structure of the antenna housing is taken as a symmetrical three-stage structure, namely an upper surface layer, a lower surface layer and an inner core layer; simulating the wave transmission rate of the radome by adopting electromagnetic simulation software CST, establishing the relation between the thickness, the dielectric constant and the loss tangent parameters of the core layer and the surface layer and the wave transmission rate of silicon nitride, and optimizing the parameters to obtain a structural scheme with the optimal comprehensive performance;

1. the optimization method of the dielectric constant and the dielectric loss of the core layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁Surface layer material dielectric constant ε₁The dielectric loss tan delta of the skin material₁And core layer material thickness d₂Then dielectric constant epsilon of the core layer material in CST software₂Dielectric loss tan delta₂The optimization is carried out, and the dielectric constant and the dielectric loss range of the core layer material meeting the requirement of high wave-transmitting rate of silicon nitride are taken as the optimization principle;

2. the optimization method of the dielectric constant and the dielectric loss of the surface layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁And core layer material thickness d₂Then according to the preferred core material dielectric constant ε₂And dielectric loss tan delta₂Dielectric constant ε of surface layer material in CST software₁Dielectric loss tan delta₁Optimization is carried out, and the dielectric constant and the dielectric loss range of the surface layer material meeting the requirement of high wave-transmitting rate of silicon nitride are taken as the optimization principle;

3. the optimization method of the thicknesses of the surface layer material and the core layer material comprises the following steps: dielectric constant epsilon of surface layer material and core layer material which are preferably calculated according to CST₁、ε₂And dielectric loss tan delta₁、tanδ₂Optimizing the thickness of the surface layer material and the core layer material in CST software, and determining the surface layer material meeting the requirement of high wave-transmitting rate of silicon nitrideAnd a core layer material thickness range;

step 2, preparing a silicon nitride ceramic radome core layer with a symmetrical continuous gradient structure: the method comprises the steps of gel casting to form a prefabricated body, drying and glue-removing the prefabricated body and preparing a nitride matrix with a certain content in the prefabricated body;

designing a mold for forming the radome according to the core layer thickness optimized in the step 1;

gel casting of the preform: deionized water H as solvent₂O, dispersant ammonium polyacrylate PAA-NH₄TMAH serving as a pH regulator, TMAH serving as a wetting agent, 400 serving as polyethylene glycol, AM serving as an organic monomer, MBAM serving as a crosslinking agent, and Si serving as silicon nitride whiskers₃N_4wFiller Si₃N₄Ball-milling the powder and the BN nanosheets to obtain silicon nitride whisker slurry; adding initiator ammonium persulfate APS, and vacuum stirring to remove bubbles; injecting the slurry into a mold, vibrating for defoaming, and keeping the temperature at a preset temperature to realize crosslinking, curing and molding;

drying and glue removing treatment of the prefabricated body; taking out the antenna housing inner cavity mold, embedding the antenna housing inner cavity, and drying in a constant temperature and humidity environment to completely volatilize water in the prefabricated body; taking the prefabricated body out of the mold, and then carrying out glue removal and carbon removal treatment at a slow heating rate in an aerobic environment to obtain an isotropic porous silicon nitride whisker prefabricated body with a uniform structure;

preparing a matrix containing nitride in the prefabricated body: preparing a nitride matrix in the prefabricated body by adopting a precursor impregnation cracking process matched with the special micron-sized pore structure of the whisker prefabricated body so as to complete the manufacturing of the porous core layer;

mixing xylene solvent with polyborosilazane PSNB, polysilazane PSN and polyborosilazane PBN as ceramic precursors, stirring, vacuum pressure impregnating to uniformly distribute the precursor solution in the pores of the preform, placing the preform impregnated with the precursor in an atmosphere furnace, and heating to N₂Under the atmosphere, respectively preserving heat at the solvent volatilization temperature, the precursor crosslinking and curing temperature and the precursor cracking temperature to obtain a polynary nitride substrate with different proportions of Si, B and N elements;

step 3, preparing silicon nitride nanowires on the surface layer of the silicon nitride ceramic radome with the symmetrical continuous gradient structure: preparing silicon nitride nanowires with different morphologies and orientations on the surface of the core layer by respectively adopting a carbothermic reduction nitridation method, a silicon powder nitridation method and a catalytic cracking method;

step 4, preparing a compact silicon nitride coating on the surface layer of the silicon nitride ceramic radome with the symmetrical continuous gradient structure: placing the radome with the surface containing the nanowires prepared in the step 3 in a silicon nitride deposition furnace for surface layer densification to obtain a silicon nitride ceramic radome with a symmetrical continuous gradient structure;

the specific implementation method of the silicon nitride coating deposition process comprises the following steps: by using silicon tetrachloride SiCl₄NH, ammonia gas₃The deposition temperature is 800-1200 ℃, the system pressure is 2-5 kPa, and the deposition time is 120-300 h; by adjusting the deposition process parameters, silicon nitride coatings with different microstructures and thicknesses are prepared.

And (5) for the silicon nitride ceramic radome with the symmetrical continuous gradient structure obtained in the step (4), precisely processing the radome according to the thickness of the core layer and the surface layer of the radome optimized in the step (1), and then cleaning and drying the radome so that the dimensional precision and the surface roughness of the radome meet the design requirements.

In the step 2, the concrete implementation method of the gel casting preform comprises the following steps: adding 20-35 wt.% of deionized water, 0.1-0.5 wt.% of ammonium polyacrylate, 1-3 wt.% of tetramethylammonium hydroxide and 1-1.5 wt.% of polyethylene glycol 400 into a ball milling tank, uniformly mixing, and adding 40-55 wt.% of silicon nitride whisker and 5-10 wt.% of filler Si for multiple times₃N₄Ball-milling and dispersing the powder and the BN nanosheet for 2-6 h; continuously adding 5-8 wt.% of acrylamide AM and 0.3-0.5 wt.% of N, N-methylene bisacrylamide MBAM into the slurry, and performing ball milling dispersion for 0.5-4 hours; after uniformly mixing, adding 0.3-0.45 wt.% of ammonium persulfate APS serving as an initiator into the slurry, removing bubbles in vacuum for 5-10 min, and pouring along the inner wall of a mold after uniformly stirring for injection molding so as to avoid generating bubbles; then shaking to remove bubbles for 5-10 min, and keeping the slurry at the temperature of 60-90 ℃ in a constant temperature box to finish crosslinking and curing of the slurry。

In the step 2, the specific implementation method of the drying and glue removing treatment of the preform comprises the following steps: after the prefabricated body which is well formed by crosslinking and curing is subjected to internal mold stripping, the tap density is 0.5-1.5 g/cm³The boron nitride powder, the silicon nitride powder or the alumina powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and the filling is uniform; placing the cavity, and the prefabricated body and the powder in the cavity in an environment with the temperature of 10-25 ℃ and the humidity of 50-90%, and drying for 48-96 hours; and then demolding the completely dried prefabricated body, placing the completely dried prefabricated body in a high-temperature furnace, raising the temperature to 500-700 ℃ at a heating rate of 0.5-2 ℃/min in an aerobic atmosphere, preserving the temperature for 4-10 hours, and then cooling the prefabricated body to room temperature along with the furnace.

In the step 2, the specific implementation method for preparing the nitride matrix in the preform comprises the following steps: adding 0-50 wt.% of PSNB precursor, 0-50 wt.% of PSN precursor, 0-50 wt.% of PBN precursor and 50-90 wt.% of solvent xylene into a sealed tank, preparing an impregnation solution, sealing to prevent oxidation, and magnetically stirring for 10-100 min to uniformly mix the solution; firstly, placing the prefabricated body after the glue discharging treatment in vacuum of-0.1 to-0.08 MPa, maintaining the pressure for 20 to 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state, and immersing for 30-60 min; then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of nitrogen or argon being 0.8-2.0 MPa, and soaking for 30-60 min under pressure; placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking: at 30-60 sccm N₂Heating to 50-130 ℃ at a flow rate of 1-10 ℃/min, and preserving heat for 1-4 h to complete solvent xylene volatilization; then, the flow rate is controlled at 30-60 sccm N₂Heating to 170-300 ℃ at a flow rate of 1-10 ℃/min, and preserving heat for 2-6 h to complete precursor crosslinking and curing; then, the reaction solution is added into the reaction solution at 10-150 sccm NH₃Heating to 800-1000 ℃ at a rate of 0.2-5 ℃/min and preserving heat for 2-6 h to complete precursor cracking; finally, the flow rate is controlled at 30-60 sccm N₂Heating to 1300-1500 ℃ at a rate of 1-10 ℃/min and preserving heat for 1-4 h, and carrying out heat treatment on the multi-element nitride substrate obtained by cracking to ensure that the multi-element nitride substrate has higher stability and moisture absorption resistance; in the above immersion cracking process, the precursor for PIP is used next timeThe solution needs to be prepared according to the characteristics of porosity, mechanics and dielectric properties of the PIP rear core layer at the last time, and the impregnation cracking process needs to be repeated for 2-5 times according to different requirements of application scenes on the properties of the core layer.

And 3, when the carbothermic nitridation method or the silicon powder nitridation method is adopted, the quasi-oriented growth nanowire is prepared on the surface layer through the combination of slurry preparation and brushing and the subsequent corresponding high-temperature reaction process.

And 3, preparing the randomly oriented nanowires on the surface layer by adopting a catalytic cracking method and combining a vacuum impregnation and high-temperature cracking process.

Advantageous effects

The invention provides a silicon nitride ceramic radome with a symmetrical continuous gradient structure and a preparation method thereof, which comprises the steps of (1) adopting electromagnetic simulation software CST to carry out electromagnetic performance simulation and broadband wave-transparent performance optimization of the radome; (2) preparing a silicon nitride-based ceramic porous core layer by adopting a gel casting-precursor impregnation cracking method; (3) preparing a silicon nitride nanowire on the surface of the core layer by adopting a carbothermic nitridation method/a silicon powder nitridation method/a catalytic cracking method; (4) performing surface layer densification on the antenna housing with the surface containing the nano wires by adopting a chemical vapor deposition/permeation method; (5) and (4) precisely processing the antenna housing. The technical scheme provided by the invention is a combined process, and can realize the synergistic improvement of the broadband wave-transmitting, high-temperature bearing and environmental erosion resistance of the high-performance antenna housing and the near-net-size integrated molding of the large-scale complex-profile continuous gradient structure antenna housing, thereby solving the problem of difficult molding of the current gradient structure antenna housing and realizing the continuous change of the antenna housing structure and the performance gradient.

The invention has the beneficial effects that:

(1) the near-net-size integrated molding of the large-size complex-profile continuous gradient structure silicon nitride ceramic radome can be realized by adopting a gelcasting-precursor impregnation cracking- (catalysis) chemical vapor infiltration/deposition combined process. A new technical system is innovated and developed internationally, the problem that the existing gradient structure antenna housing is difficult to form is effectively solved, and the design target of continuous gradient change of the wall structure and the performance of the antenna housing cover is really realized.

(2) The silicon nitride crystal whisker with a single crystal structure is used as a core layer reinforcement, so that the core layer has excellent high temperature resistance and mechanical property. The nitride matrix with a certain content is introduced into the core layer, so that the core layer can be further reinforced, and the dielectric property of the core layer can be regulated and controlled, so that the design requirement of CST simulation on the dielectric property of the core layer is met.

(3) The in-situ growth of the silicon nitride nanowire can realize the tight connection between the core layer and the surface silicon nitride coating, and the pinning effect is realized. In the transition region where the crystal whisker in the core layer is overlapped with the nanowire on the surface of the core layer, the mechanical property is further improved due to the combined action of the crystal whisker-nanowire two-phase micro-nano reinforcement. The nano-wires on the surface of the core layer have a substrate induction effect on the deposition of the subsequent silicon nitride coating, and the silicon nitride coating can be rapidly and uniformly prepared.

(4) When the chemical vapor infiltration/deposition process is adopted to prepare the surface silicon nitride coating, the control of the deposition rate, the deposition thickness and the infiltration depth of the coating can be realized by optimizing process parameters. The silicon nitride can be controllably infiltrated in the core layer, so that the core layer whisker and the surface layer nanowire are well wrapped, the good combination of the core layer whisker and the surface layer nanowire is facilitated, and a continuous gradient structure is realized. The silicon nitride coating is uniform and compact and has higher nominal modulus, and the overall mechanical property level of the silicon nitride ceramic with the gradient structure can be improved. The hole sealing effect of the silicon nitride coating on the core layer can obviously improve the environmental corrosion resistance of the material. Meanwhile, the introduction of the silicon nitride coating does not have any adverse effect on the wave-transmitting performance of the material.

Drawings

FIG. 1 is a process flow diagram of the process of the present invention.

Fig. 2 is an SEM photograph of the prepared symmetrical continuous gradient structure silicon nitride ceramic radome core layer.

Fig. 3 is an SEM photograph of the silicon nitride nanowires on the surface layer of the prepared silicon nitride ceramic radome with the symmetrical continuous gradient structure.

(a) SEM photograph of quasi-oriented silicon nitride nanowire; (b) SEM photograph of randomly oriented silicon nitride nanowire

Fig. 4 is an SEM photograph of the prepared symmetrical continuous gradient structure silicon nitride ceramic radome wall structure.

Fig. 5 is a macroscopic photograph of the prepared silicon nitride ceramic radome with the symmetrical continuous gradient structure.

Detailed Description

The invention will now be further described with reference to the following examples, and the accompanying drawings:

the invention is characterized by comprising the following steps:

1. electromagnetic performance simulation and broadband wave-transparent performance optimization of antenna housing

Aiming at the use requirements of the antenna housing, firstly, electromagnetic performance simulation and broadband wave-transmitting performance optimization of the symmetrical continuous gradient structure silicon nitride ceramic antenna housing are carried out. For convenience of simulation calculation, the symmetrical continuous gradient structure is simplified into a symmetrical three-level structure, namely an upper surface layer, a lower surface layer and an inner core layer.

Electromagnetic simulation software CST is adopted to simulate the wave transmission rate of the radome, the relation between the thickness, the dielectric constant, the loss tangent and other parameters of the core layer and the surface layer and the wave transmission rate of the silicon nitride is established, and the structural scheme with the best comprehensive performance is obtained by optimizing the parameters.

In the step 1, the method for optimizing the dielectric constant and the dielectric loss of the core layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁The dielectric constant of the material of the surface layer₁The dielectric loss tan delta of the skin material₁And core layer material thickness d₂Then the dielectric constant epsilon of the core layer material is measured in CST software₂Dielectric loss tan delta₂And preferably, determining the dielectric constant and the dielectric loss range of the core layer material meeting the requirement of high wave-transparent rate of silicon nitride.

In the step 1, the method for optimizing the dielectric constant and the dielectric loss of the surface layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁And core layer material thickness d₂Then according to the preferred dielectric constant epsilon of the core material₂And dielectric loss tan delta₂Dielectric constant ε of surface layer material in CST software₁Dielectric loss tan delta₁Optimization is carried out, and a table meeting the requirement of high wave-transparent rate of silicon nitride is determinedLayer material dielectric constant and dielectric loss range.

In the step 1, the method for optimizing the thicknesses of the surface layer material and the core layer material comprises the following steps: dielectric constant epsilon of surface layer material and core layer material which are preferably calculated according to CST₁、ε₂And dielectric loss tan delta₁、tanδ₂And optimizing the thicknesses of the surface layer material and the core layer material in CST software, and determining the thickness ranges of the surface layer material and the core layer material meeting the requirement of high wave-transmitting rate of silicon nitride.

2. Preparation of silicon nitride ceramic radome core layer with symmetrical continuous gradient structure

The preparation process of the core layer comprises gel casting of a preform, drying and glue removal of the preform and preparation of a nitride matrix with a certain content in the preform.

Firstly, a whisker preform is formed. And designing a mold required by forming the radome according to the optimized thickness of the core layer in the first step. Adding solvent deionized water (H) into a ball milling tank₂O), dispersant ammonium polyacrylate (PAA-NH)₄) The pH regulator is tetramethylammonium hydroxide (TMAH), a wetting agent polyethylene glycol 400(PEG-400), an organic monomer Acrylamide (AM), a cross-linking agent N, N-Methylene Bisacrylamide (MBAM), and silicon nitride whiskers (Si)₃N_4w) Filler Si₃N₄And ball-milling the powder, the BN nanosheet and a proper amount of ball milling beads to obtain silicon nitride whisker slurry. When the slurry is prepared, the control of the aperture size and the open porosity of the preform can be realized by regulating and controlling the volume ratio of the silicon nitride crystal whiskers to the filler and the particle size of the filler. And (3) after the slurry is uniformly mixed, adding an initiator Ammonium Persulfate (APS), injecting the mixture into a mould after vacuum stirring and defoaming, and insulating the mixture at a preset temperature to realize crosslinking, curing and forming after oscillating and defoaming.

And then, taking out the mold of the inner cavity of the antenna housing, embedding the inner cavity of the antenna housing, and drying for a long time in a constant temperature and humidity environment to ensure that the moisture in the prefabricated body is completely volatilized. The embedding method is favorable for realizing near net size forming, and the filler in the prefabricated body is a key factor for reducing shrinkage in the drying process. And taking the completely dried preform out of the mold, and then carrying out glue removal and carbon removal treatment at a slow temperature rising rate in an aerobic environment, thereby obtaining the porous silicon nitride crystal whisker preform with isotropy and uniform structure.

And then, preparing a nitride matrix in the prefabricated body by adopting a precursor impregnation cracking process (PIP process) matched with the special micron-sized pore structure of the whisker prefabricated body so as to complete the manufacture of the porous core layer. Mixing a solvent xylene with a ceramic precursor Polyborosilazane (PSNB), a Polysilazane (PSN) and a Polyborosilazane (PBN) in proportion, uniformly stirring, impregnating under vacuum pressure to uniformly distribute a precursor solution in pores of a preform, placing the preform impregnated with the precursor in an atmosphere furnace, and heating in the atmosphere furnace under N₂And respectively preserving the heat at the solvent volatilization temperature, the precursor crosslinking and curing temperature and the precursor cracking temperature under the atmosphere to obtain the multi-element nitride matrix with different proportions of Si, B and N elements. The multi-element nitride matrix has the advantages of strong designability of components, widening the adjustment and control space of the dielectric property of the core layer, excellent thermal stability and improvement on the use temperature of materials. And preparing precursor solutions with different concentrations according to the pore opening rate change of the preform in the PIP process, and performing the next circulation PIP process. And tracking the change of the mechanical and dielectric properties of the core layer under different precursor components, precursor concentration and PIP times, and using the measured dielectric parameters as basic data for CST simulation. And the mechanical and dielectric properties of the core layer are cooperatively regulated and controlled by controlling the precursor proportion and the PIP frequency, so that the porous core layer with the optimized thickness is finally obtained.

In the step 2, the concrete implementation method of the gel casting preform comprises the following steps: adding 20-35 wt.% of deionized water, 0.1-0.5 wt.% of ammonium polyacrylate, 1-3 wt.% of tetramethylammonium hydroxide and 1-1.5 wt.% of polyethylene glycol 400 into a ball milling tank, uniformly mixing, and adding 40-55 wt.% of silicon nitride whisker and 5-10 wt.% of filler Si for multiple times₃N₄And ball-milling and dispersing the powder and the BN nanosheet for 2-6 h. Continuously adding 5-8 wt.% of Acrylamide (AM) and 0.3-0.5 wt.% of N, N-Methylene Bisacrylamide (MBAM) into the slurry, and performing ball milling dispersion for 0.5-4 h. After uniformly mixing, adding 0.3-0.45 wt.% of Ammonium Persulfate (APS) serving as an initiator into the slurry, removing bubbles in vacuum for 5-10 min, uniformly stirring, and pouring along the inner wall of the moldInjection molding is performed to avoid the generation of bubbles. And then oscillating to remove bubbles for 5-10 min, and keeping the temperature of the slurry in a constant temperature box at 60-90 ℃ to finish crosslinking and curing of the slurry.

In the step 2, the specific implementation method of the drying and glue removing treatment of the preform comprises the following steps: after the prefabricated body which is well formed by crosslinking and curing is subjected to internal mold removal, the tap density is 0.5-1.5 g/cm³The boron nitride powder, the silicon nitride powder or the alumina powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and the filling is uniform in vibration. And (3) drying the cavity, the prefabricated body and the powder in the cavity for 48-96 hours in an environment with the temperature of 10-25 ℃ and the humidity of 50-90%. And then demolding the completely dried prefabricated body, placing the completely dried prefabricated body in a high-temperature furnace, raising the temperature to 500-700 ℃ at a heating rate of 0.5-2 ℃/min in an aerobic atmosphere, preserving the temperature for 4-10 hours, and then cooling the prefabricated body to room temperature along with the furnace.

In the step 2, the specific implementation method for preparing the nitride matrix in the preform comprises the following steps: adding 0-50 wt.% PSNB precursor, 0-50 wt.% PSN precursor, 0-50 wt.% PBN precursor and 50-90 wt.% solvent xylene into a sealed tank, preparing an impregnation solution, sealing to prevent oxidation, and magnetically stirring for 10-100 min to uniformly mix. Firstly, placing the prefabricated body after the glue discharging treatment in vacuum of-0.1 to-0.08 MPa, maintaining the pressure for 20 to 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state, and immersing for 30-60 min; and then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of 0.8-2.0 MPa of nitrogen or argon, and soaking for 30-60 min under pressure. Placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking: at 30-60 sccm N₂And under the flow rate, heating to 50-130 ℃ at the speed of 1-10 ℃/min, and preserving the temperature for 1-4 h to finish the volatilization of the solvent xylene. Then, the flow rate is controlled at 30-60 sccm N₂And under the flow rate, heating to 170-300 ℃ at a speed of 1-10 ℃/min, and preserving the temperature for 2-6 h to finish the precursor crosslinking and curing. Then, the flow rate is adjusted to 10-150 sccm NH₃And under the flow, heating to 800-1000 ℃ at the speed of 0.2-5 ℃/min, and preserving the temperature for 2-6 h to finish precursor cracking. Finally, the flow rate is controlled at 30-60 sccm N₂Heating to 1300-1500 ℃ at a rate of 1-10 ℃/min and preserving heat for 1-4 h under the condition of flow rate, and cracking to obtain moreThe nitride substrate is heat treated to provide higher stability and moisture absorption resistance. In the above dipping and cracking process, a precursor solution for next PIP is prepared according to the characteristics of porosity, mechanics and dielectric properties of a core layer after last PIP, and the dipping and cracking process needs to be repeated for 2-5 times according to different requirements of application scenes on the performance of the core layer.

3. Preparation of silicon nitride nanowire on surface layer of silicon nitride ceramic radome with symmetrical continuous gradient structure

According to different requirements of application scenes on the mechanical properties of the surface layer, one of three processes of a carbothermic nitridation method, a silicon powder nitridation method and a catalytic cracking method is selected to prepare the silicon nitride nanowires with different appearances and orientations on the surface of the core layer (namely the surface layer of the silicon nitride ceramic with the symmetrical continuous gradient structure). For the carbothermic nitridation method and the silicon powder nitridation method, the quasi-oriented growth nanowire is prepared on the surface layer through slurry preparation and brushing combined with the subsequent corresponding high-temperature reaction process; for the catalytic cracking method, the randomly oriented nanowires are prepared on the surface layer by combining vacuum impregnation with a high-temperature cracking process.

When the silicon nitride nanowire is prepared by adopting a carbothermic nitridation method, firstly slurry is prepared according to a proportion, solvents of Absolute ethyl alcohol (Absolute ethanol) and butanone (MEK), a dispersant of triethyl phosphate (TEP), a binder of polyvinyl butyral (PVB), a plasticizer of glycerol (Glycerin) and dioctyl phthalate (DOP), and reactants of SiO in a ball milling tank are added₂And C powder and catalyst NiCl₂·6H₂O, nucleating agent silicon nitride crystal whisker and proper amount of ball milling beads, and uniformly brushing the mixture on the surface of the core layer after ball milling and mixing. And then placing the silicon nitride nanowire in a high-temperature atmosphere furnace, and fully reacting at a preset heating rate and a preset heat preservation temperature to realize the uniform growth of the silicon nitride nanowire on the surface of the core layer. The thickness of the reaction layer and the length-diameter ratio of the silicon nitride nanowire are regulated and controlled by controlling the brushing times. The silicon nitride whiskers added into the slurry can provide nucleation sites for the growth of the nanowires and promote the growth of the nanowires. The orientation of the crystal whisker can be adjusted by controlling the brushing mode, and the growth direction of the nano wire is influenced.

When the silicon nitride nanowire is prepared by adopting a silicon powder nitriding method, the silicon nitride nanowire is thermally reduced with carbonThe original nitriding method has similar processes. Firstly, replacing reactants in the slurry with Si powder, preparing the slurry according to a proportion, ball-milling and mixing the slurry, and then uniformly brushing the mixture on the surface of a core layer. Then it was placed in a high temperature atmosphere furnace at N₂And carrying out nitridation reaction under the atmosphere to obtain the silicon nitride nanowire.

When the silicon nitride nanowire is prepared by adopting a catalytic cracking method, firstly, the PSN of a precursor, a solvent xylene and a catalyst ferrocene are uniformly mixed in proportion, and the mixture is introduced into the surface pore of the core layer by vacuum impregnation. Then it is placed in a high temperature atmosphere furnace at N₂And (3) respectively keeping the temperature at the solvent volatilization temperature, the precursor solidification temperature and the nanowire growth temperature at a preset temperature rise rate in the atmosphere, and completing the preparation of the silicon nitride nanowire. By controlling the gas flow and the partial pressure thereof, gaseous intermediate products generated by decomposing the precursor can be gathered on the surface, and the nano wire can grow in a localized manner on the surface of the core layer without growing in the core layer under the action of the catalyst, so that the silicon nitride nano wire with random orientation and large length-diameter ratio is prepared.

In the step 3, the concrete implementation method of the slurry preparation and brushing process in the carbothermal reduction nitridation method and the silicon powder nitridation method comprises the following steps: adding 15-30 wt.% of ethanol, 15-30 wt.% of butanone, 1-3 wt.% of triethyl phosphate (TEP), 1-3 wt.% of glycerol, 1-3 wt.% of dioctyl phthalate, 1.5-3.5 wt.% of polyvinyl butyral (PVB), 10-40 wt.% of silicon nitride whiskers and 5-25 wt.% of SiO into a ball milling tank₂And 1 to 10 wt.% of C (SiO)₂And C is only used in a carbothermic nitridation method) or 5-40 wt.% of Si powder (the Si powder is only used in a silicon powder nitridation method), and a proper amount of ball milling beads, and ball milling is carried out for 2-12 h to uniformly mix the materials. Transferring the slurry with uniform ball milling into a clean beaker, dipping the slurry with a special brush head, coating the surface of the core layer along the height direction of the radome, after the slurry is dried, coating the slurry along the circumferential direction of the radome, repeating the process for 2-10 times to obtain a coating layer with a certain thickness, and then drying the coating layer for 1-6 hours in an environment with the temperature of 10-25 ℃ and the humidity of 50-90%.

In the step 3, the specific implementation method of the silicon nitride nanowire growth process in the carbothermic reduction nitridation method comprises: placing the core layer with the surface coated with the slurry in a high-temperature atmosphere furnace at 30-100 sccm N₂And raising the temperature to 500-700 ℃ at a heating rate of 0.5-2 ℃/min under the flow rate, and preserving the temperature for 1-6 hours to completely convert organic matters in the brushing slurry into C. Then, the flow rate is controlled at 30-80 sccm N₂And (3) raising the temperature to 1350-1550 ℃ at the temperature rise rate of 2-10 ℃/min under the flow, and preserving the temperature for 2-6 h to finish the carbothermic reduction nitridation reaction, thereby realizing the growth of the silicon nitride nanowire. And finally, cooling to room temperature along with the furnace.

In the step 3, a specific implementation method of the silicon nitride nanowire growth process in the silicon powder nitridation method comprises the following steps: placing the core layer with the surface coated with the slurry in a high-temperature atmosphere furnace at 30-100 sccm N₂Raising the temperature to 200 ℃ at a temperature raising rate of 0.5-2 ℃/min, and then raising the temperature to 30-100 sccm NH₃Raising the temperature to 500-700 ℃ at a heating rate of 0.5-2 ℃/min under the flow rate, and preserving the temperature for 1-6 hours to completely convert organic matters in the brushing slurry into C and remove the C by reacting with ammonia gas. Then, the flow rate is 50-150 sccm N₂And (3) raising the temperature to 1350-1550 ℃ at the temperature rise rate of 2-10 ℃/min under the flow rate, and preserving the heat for 2-6 h to finish the silicon powder nitridation reaction so as to realize the growth of the silicon nitride nanowire. And finally, cooling to room temperature along with the furnace.

In the step 3, the specific implementation method for preparing the silicon nitride nanowire by the catalytic cracking method comprises the following steps: firstly, 50-90 wt.% of solvent xylene, 7-45 wt.% of PSN precursor and 0.1-5 wt.% of catalyst ferrocene are added into a sealed tank, and after a precursor solution is prepared, the mixture is sealed to prevent oxidation, and is magnetically stirred for 10-100 min to be uniformly mixed. And then placing the core layer in vacuum of-0.1 to-0.08 MPa for 20 to 40min, and exhausting air in the core layer. And then, soaking the precursor in the prepared precursor solution in a vacuum state for 30-60 min. And then placing the core layer soaked with the precursor solution in a high-temperature atmosphere furnace for curing and cracking: at 30-60 sccm N₂And under the flow rate, heating to 50-130 ℃ at the speed of 1-10 ℃/min, and preserving the temperature for 1-4 h to finish the volatilization of the solvent xylene. Then, the flow rate is controlled at 30-60 sccm N₂And (3) heating to 170-300 ℃ at a flow rate of 1-10 ℃/min, and preserving the temperature for 2-6 h to complete precursor crosslinking and curing. Then 50-200 sccm N₂At a flow rate of 1-10 ℃/min, heating to 1300 to EPreserving the heat for 1-4 h at 1500 ℃, so that the precursor is converted into the nanowire under the action of the catalyst. And finally, cooling to room temperature along with the furnace.

4. Preparation of compact silicon nitride coating on surface layer of silicon nitride ceramic radome with symmetrical continuous gradient structure

And (4) placing the radome with the surface containing the nanowires prepared in the step (3) in a silicon nitride deposition furnace for surface layer densification to obtain a dense shell with the optimized thickness. The specific implementation method of the silicon nitride coating deposition process comprises the following steps: by using silicon tetrachloride (SiCl)₄) Ammonia (NH)₃) The deposition temperature is 800-1200 ℃ as a precursor gas source, the system pressure is 2-5 kPa, and the deposition time is 120-300 h. According to different requirements of application scenes on the mechanical properties of the surface layer, silicon nitride coatings with different microstructures and thicknesses are prepared by adjusting deposition process parameters.

5. Precision machining of silicon nitride ceramic radome with symmetrical continuous gradient structure

And D, precisely processing the radome according to the thickness of the radome core layer and the surface layer optimized in the step I, and then cleaning and drying to enable the size precision and the surface roughness of the radome to meet the design requirements.

The specific embodiment is as follows:

example 1

Step (1) electromagnetic performance simulation and broadband wave-transparent performance optimization of antenna housing

The thickness d of the surface layer material can be obtained by optimizing various parameters of the material through CST software₁1mm, dielectric constant ε of the surface layer material₁<6.00, dielectric loss tan. delta₁<0.001, thickness d of core layer Material₂4mm, dielectric constant ε of the core material₂<3.5 dielectric loss tan delta₂<At 0.01, the wave-transparent rate of the silicon nitride ceramic with the gradient structure is more than 80 percent in the frequency range of 0-13.5 GHz.

Step (2) preparation of silicon nitride ceramic radome core layer with symmetrical continuous gradient structure

25 wt.% deionized water (H) was added to the ball mill pot₂O), 0.3 wt.% ammonium polyacrylate (PAA-NH)₄) 2 wt.% tetramethylammonium hydroxide (TMAH), 15 wt.% of polyethylene glycol 400(PEG-400), and after uniformly mixing, 55 wt.% of silicon nitride whiskers (Si) are added in multiple times₃N_4w) 10 wt.% Si₃N₄And (5) performing ball milling and dispersing on the powder for 4 hours. 5.5 wt.% of Acrylamide (AM) and 0.4 wt.% of N, N-methylenebisacrylamide (MBAM) were continuously added to the slurry, and ball milling and dispersion were performed for 1 hour. After uniformly mixing, adding 0.3 wt.% Ammonium Persulfate (APS) serving as an initiator into the slurry, removing bubbles for 10min in vacuum, uniformly stirring, and pouring along the inner wall of a mold for injection molding to avoid generating bubbles; and then shaking to remove bubbles for 10min, and keeping the temperature of the slurry in a thermostat at 70 ℃ to finish crosslinking and curing of the slurry.

Demoulding the cross-linked cured and molded prefabricated body, and adopting the tap density of 1g/cm³The alumina powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and is filled uniformly by vibration. And (3) drying the die cavity, the prefabricated body and the powder in the die cavity for 96 hours in an environment with the temperature of 25 ℃ and the humidity of 50 percent. And then placing the completely dried preform in a high-temperature furnace, raising the temperature to 500 ℃ at a heating rate of 0.5 ℃/min in an aerobic atmosphere, preserving the temperature for 10 hours, and then cooling the preform to room temperature along with the furnace.

30 wt.% of PSNB precursor, 30 wt.% of PBN precursor and 40 wt.% of solvent xylene are added into a sealed tank, after an impregnation solution is prepared, the mixture is sealed to prevent oxidation, and the mixture is magnetically stirred for 30min to be uniformly mixed. Firstly, placing the prefabricated body after the glue discharging treatment under the vacuum of-0.09 MPa, maintaining the pressure for 40min, and discharging the air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state for 40 min; and then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of nitrogen or argon being 1.0MPa, and soaking for 40min under pressure. Placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking: at 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by 50sccm N₂Heating to 200 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to complete the cross-linking and curing of the precursor. Then at 100sccm NH₃Heating to 900 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 4h to finish precursor cracking. Finally, the flow rate is 50sccm N₂At a flow rate of 2 DEG CHeating to 1500 ℃ in min, and preserving heat for 3 h. The above dip cracking process was repeated 3 times, wherein the dip solution ratio in the second furnace was 20 wt.% PSNB precursor, 20 wt.% PSN precursor, 20 wt.% PBN precursor and 40 wt.% solvent xylene. The immersion solution in the third furnace was formulated with 30 wt.% PSNB precursor, 30 wt.% PSN precursor, and 40 wt.% solvent xylene.

Step (3) preparation of silicon nitride ceramic radome surface silicon nitride nanowires with symmetrical continuous gradient structures

20 wt.% ethanol, 20 wt.% Methyl Ethyl Ketone (MEK), 3 wt.% triethyl phosphate (TEP), 3 wt.% glycerol, 3 wt.% dioctyl phthalate (DOP), 3 wt.% polyvinyl butyral (PVB), 20 wt.% silicon nitride whiskers (Si) were added to the ball mill pot₃N_4w) 20 wt.% SiO₂8 wt.% of C and a proper amount of ball milling beads, and ball milling for 8 hours to mix the components evenly. And transferring the slurry uniformly subjected to ball milling into a clean beaker, dipping the slurry by using a special brush head, coating the surface of the core layer along the height direction of the radome, drying, coating the core layer along the circumferential direction of the radome, repeating the process for 4 times to obtain a coating layer with a certain thickness, and drying for 4 hours in an environment with the temperature of 25 ℃ and the humidity of 50%.

Placing the core layer with the surface coated with the slurry in a high-temperature atmosphere furnace at 60sccm N₂Raising the temperature to 700 ℃ at the temperature rise rate of 2 ℃/min under the flow rate, and preserving the temperature for 4 hours to completely convert organic matters in the brushing slurry into C. Followed by 60sccm N₂And (3) raising the temperature to 1500 ℃ at the temperature rise rate of 5 ℃/min under the flow rate, and preserving the temperature for 4h to finish the carbothermic reduction nitridation reaction so as to realize the growth of the silicon nitride nanowire. And finally, cooling to room temperature along with the furnace.

Step (4) preparation of compact silicon nitride coating on surface layer of silicon nitride ceramic radome with symmetrical continuous gradient structure

Deposition of dense silicon nitride on the surface layer: placing the antenna housing with the surface containing the nano-wire in a silicon nitride deposition furnace by adopting silicon tetrachloride (SiCl)₄) Ammonia (NH)₃) Depositing for 200h at 1000 deg.C and 2kPa as precursor gas source to obtain silicon nitride ceramic antenna with symmetrical continuous gradient structureAnd (4) a cover.

Step (5) precision machining of silicon nitride ceramic radome with symmetrical continuous gradient structure

The antenna housing is precisely processed, and then is cleaned and dried, so that the dimensional precision and the surface roughness of the antenna housing meet the design requirements.

The antenna housing prepared in the embodiment is subjected to sampling test, the bending strength is 216.52MPa, the dielectric constant is 4.67, the dielectric loss is 0.038, and the wave transmittance in a frequency band of 1-14 GHz is more than 80%.

Example 2

The thickness d of the surface layer material can be obtained by optimizing various parameters of the material through CST software₁1mm, the dielectric constant epsilon of the skin material₁<6.00, dielectric loss tan. delta₁<0.001, thickness d of core layer Material₂4mm, dielectric constant ε of the core material₂<3.5 dielectric loss tan delta₂<At 0.01, the wave-transparent rate of the silicon nitride ceramic with the gradient structure is more than 80 percent in the frequency range of 0-13.5 GHz.

25 wt.% deionized water (H) was added to the ball mill pot₂O), 0.3 wt.% ammonium polyacrylate (PAA-NH)₄) 2 wt.% of tetramethylammonium hydroxide (TMAH) and 1.5 wt.% of polyethylene glycol 400(PEG-400), and after uniformly mixing, 55 wt.% of silicon nitride whiskers (Si) are added in multiple times₃N_4w) 10 wt.% of BN nano-sheet, and performing ball milling dispersion for 4 hours. 5.5 wt.% of Acrylamide (AM) and 0.4 wt.% of N, N-methylenebisacrylamide (MBAM) were continuously added to the slurry, and ball milling and dispersion were performed for 1 hour. After uniformly mixing, adding 0.3 wt.% of Ammonium Persulfate (APS) serving as an initiator into the slurry, removing bubbles in vacuum for 10min, and pouring along the inner wall of a mold for injection molding after uniformly stirring to avoid generating bubbles; and then shaking to remove bubbles for 10min, and keeping the temperature of the slurry in a thermostat at 70 ℃ to finish crosslinking and curing of the slurry.

Demoulding the cross-linked and cured prefabricated body, and adopting the tap density of 1g/cm³The silicon nitride powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and is uniformly filled in a vibration mode. And (3) drying the die cavity, the prefabricated body and the powder in the die cavity for 96 hours in an environment with the temperature of 25 ℃ and the humidity of 50%. And then placing the completely dried preform in a high-temperature furnace, heating to 500 ℃ at the heating rate of 0.5 ℃/min in the presence of oxygen, preserving the temperature for 10 hours, and then cooling to room temperature along with the furnace.

30 wt.% of PSNB precursor, 30 wt.% of PBN precursor and 40 wt.% of solvent xylene are added into a sealed tank, after an impregnation solution is prepared, the mixture is sealed to prevent oxidation, and the mixture is magnetically stirred for 30min to be uniformly mixed. Firstly, placing the prefabricated body after the glue removing treatment in vacuum with the vacuum degree of-0.09 MPa, maintaining the pressure for 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state for 40 min; and then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of 1.0MPa of nitrogen or argon, and carrying out pressure soaking for 40 min. Placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking, and performing pyrolysis treatment at 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by a reaction at 50sccm N₂Heating to 200 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to complete the cross-linking and curing of the precursor. Then at 100sccm NH₃Heating to 900 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 4h to finish precursor cracking. Finally at 50sccm N₂Heating to 1500 ℃ at the flow rate of 2 ℃/min, and preserving the heat for 3 h. The above dip cracking process was repeated 3 times, wherein the dip solution ratio in the second furnace was 20 wt.% PSNB precursor, 20 wt.% PSN precursor, 20 wt.% PBN precursor and 40 wt.% solvent xylene. The immersion solution in the third furnace was formulated with 30 wt.% PSNB precursor, 30 wt.% PSN precursor, and 40 wt.% solvent xylene.

20 wt.% ethanol, 20 wt.% Methyl Ethyl Ketone (MEK), 3 wt.% triethyl phosphate (TEP), 3 wt.% glycerol, 3 wt.% dioctyl phthalate (DOP), 3 wt.% polyvinyl alcoholButyraldehyde (PVB), 20 wt.% silicon nitride whiskers (Si)₃N_4w) 28 wt.% of Si and a proper amount of ball milling beads, and ball milling is carried out for 8 hours to ensure that the materials are uniformly mixed. And transferring the slurry with uniform ball milling into a clean beaker, dipping the slurry with a special brush head, coating the surface of the core layer along the height direction of the radome, after the slurry is dried, coating the surface of the core layer along the circumferential direction of the radome, repeating the process for 4 times to obtain a coating layer with a certain thickness, and then drying the coating layer for 4 hours in an environment with the temperature of 25 ℃ and the humidity of 50%.

Placing the core layer with the surface coated with the sizing agent in a high-temperature atmosphere furnace at 60sccm N₂Raising the temperature to 700 ℃ at the temperature rise rate of 2 ℃/min under the flow rate, and preserving the temperature for 4 hours to completely convert organic matters in the brushing slurry into C. Followed by 60sccm N₂And (3) raising the temperature to 1500 ℃ at the temperature rise rate of 5 ℃/min under the flow rate, and preserving the heat for 4h to finish the silicon powder nitridation reaction so as to realize the growth of the silicon nitride nanowire. And finally, cooling to room temperature along with the furnace.

Placing the antenna housing core layer with the surface containing the nano wire in a silicon nitride deposition furnace by adopting silicon tetrachloride (SiCl)₄) Ammonia (NH)₃) And as a precursor gas source, depositing for 180 hours under the conditions that the deposition temperature is 1100 ℃ and the deposition pressure is 2kPa, and performing surface densification. And obtaining the silicon nitride ceramic radome with the symmetrical continuous gradient structure.

Step (5) precision machining of silicon nitride ceramic antenna housing with symmetrical continuous gradient structure

The antenna housing is precisely processed, and then is cleaned and dried, so that the size precision and the surface roughness of the antenna housing meet the design requirements.

Example 3

The thickness d of the surface layer material can be obtained by optimizing various parameters of the material through CST software₁1mm, the dielectric constant epsilon of the skin material₁<6.00, dielectric loss tan. delta₁<0.001, thickness d of core layer material₂4mm, medium of core layer materialElectric constant epsilon₂<3.5 dielectric loss tan delta₂<At 0.01, the wave-transparent rate of the silicon nitride ceramic with the gradient structure is more than 80 percent in the frequency range of 0-13.5 GHz.

25 wt.% deionized water (H) was added to the ball mill pot₂O), 0.3 wt.% ammonium polyacrylate (PAA-NH)₄) 2 wt.% tetramethylammonium hydroxide (TMAH) and 1.5 wt.% polyethylene glycol 400(PEG-400), and after uniformly mixing, 55 wt.% silicon nitride whisker (Si) was added in several portions₃N_4w) 10 wt.% Si₃N₄And (5) performing ball milling and dispersing on the powder for 4 hours. 5.5 wt.% of Acrylamide (AM) and 0.4 wt.% of N, N-methylenebisacrylamide (MBAM) were continuously added to the slurry, and ball milling and dispersion were performed for 1 hour. After uniformly mixing, adding 0.3 wt.% Ammonium Persulfate (APS) serving as an initiator into the slurry, removing bubbles for 10min in vacuum, uniformly stirring, and pouring along the inner wall of a mold for injection molding to avoid generating bubbles; and then shaking to remove bubbles for 10min, and keeping the temperature of the slurry in a thermostat at 70 ℃ to finish crosslinking and curing of the slurry.

Demoulding the cross-linked cured and molded prefabricated body, and adopting the tap density of 1g/cm³The alumina powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and is filled uniformly by vibration. And (3) drying the die cavity, the prefabricated body and the powder in the die cavity for 96 hours in an environment with the temperature of 25 ℃ and the humidity of 50%. And then placing the completely dried preform in a high-temperature furnace, heating to 500 ℃ at the heating rate of 0.5 ℃/min in the presence of oxygen, preserving the temperature for 10 hours, and then cooling to room temperature along with the furnace.

30 wt.% of PSNB precursor, 30 wt.% of PBN precursor and 40 wt.% of solvent xylene are added into a sealed tank, after an impregnation solution is prepared, the mixture is sealed to prevent oxidation, and the mixture is magnetically stirred for 30min to be uniformly mixed. Firstly, placing the prefabricated body after the glue removing treatment in vacuum with the vacuum degree of-0.09 MPa, maintaining the pressure for 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state for 40 min; then the prepared precursor solution and the prefabricated body immersed in the precursor solution are placed in nitrogen or argon gas pressure of 1.0MPand a, soaking under pressure for 40 min. Placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking, and performing pyrolysis treatment at 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by 50sccm N₂Heating to 200 ℃ at the flow rate of 2 ℃/min, and preserving heat for 3h to finish precursor crosslinking and curing. Then at 100sccm NH₃Heating to 900 ℃ at the flow rate of 2 ℃/min, and preserving the temperature for 4h to finish precursor cracking. Finally, the flow rate is 50sccm N₂At the flow rate, the temperature is raised to 1500 ℃ at the speed of 2 ℃/min and is kept for 3 h. The above dip cracking process was repeated 3 times, wherein the dip solution ratio in the second furnace was 20 wt.% PSNB precursor, 20 wt.% PSN precursor, 20 wt.% PBN precursor and 40 wt.% solvent xylene. The immersion solution in the third furnace was formulated with 30 wt.% PSNB precursor, 30 wt.% PSN precursor, and 40 wt.% solvent xylene.

Adding 60 wt.% of solvent xylene, 38 wt.% of PSN precursor and 2 wt.% of catalyst ferrocene into a sealed tank, preparing a precursor solution, sealing to prevent oxidation, and magnetically stirring for 60min to uniformly mix the materials. Firstly, placing the core layer shield body material with certain density in vacuum with the vacuum degree of-0.08 MPa, maintaining the pressure for 40min, and exhausting air in the core layer shield body material; then, the precursor solution was immersed in the prepared precursor solution under vacuum for 40 min. The core layer cover body dipped with the precursor solution is placed in a high-temperature atmosphere furnace for solidification and cracking, and the temperature is 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by 50sccm N₂Heating to 300 ℃ at the flow rate of 2 ℃/min, and preserving the heat for 3h to complete the cross-linking and curing of the precursor. Then at 100sccm N₂And (3) heating to 1350 ℃ at the flow rate of 5 ℃/min, and preserving the heat for 4h to finish the catalytic cracking reaction of the PSN precursor, thereby realizing the growth of the silicon nitride nanowire. And finally, cooling to room temperature along with the furnace.

Surface layer dense silicon nitride deposition: placing the antenna housing with the surface containing the nano-wire in a silicon nitride deposition furnace by adopting silicon tetrachloride (SiCl)₄) Ammonia (NH)₃) And (3) as a precursor gas source, depositing for 300 hours under the conditions that the deposition temperature is 900 ℃ and the deposition pressure is 2kPa to prepare the silicon nitride ceramic radome with the symmetrical continuous gradient structure.

Example 4

Optimizing each parameter of the material by CST software to obtain the thickness d of the surface layer material₁1mm, dielectric constant ε of the surface layer material₁<6.00, dielectric loss tan. delta₁<0.001, thickness d of core layer Material₂4mm, dielectric constant ε of the core material₂<3.5 dielectric loss tan delta₂<At 0.01, the wave-transmitting rate of the silicon nitride ceramic with the gradient structure is more than 80 percent in the frequency range of 0-13.5 GHz.

Will crosslinkDemoulding the cured and molded prefabricated body, and adopting the tap density of 1g/cm³The alumina powder is filled in the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and is uniformly filled in a vibration mode. And (3) drying the die cavity, the prefabricated body and the powder in the die cavity for 96 hours in an environment with the temperature of 25 ℃ and the humidity of 50 percent. And then placing the completely dried preform in a high-temperature furnace, heating to 500 ℃ at the heating rate of 0.5 ℃/min in the presence of oxygen, preserving the temperature for 10 hours, and then cooling to room temperature along with the furnace.

30 wt.% of PSNB precursor, 30 wt.% of PBN precursor and 40 wt.% of solvent xylene are added into a sealed tank, after an impregnation solution is prepared, the mixture is sealed to prevent oxidation, and the mixture is magnetically stirred for 30min to be uniformly mixed. Firstly, placing the prefabricated body after the glue removing treatment in vacuum with the vacuum degree of-0.09 MPa, maintaining the pressure for 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution under a vacuum state for 40 min; and then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of 1.0MPa of nitrogen or argon, and carrying out pressure soaking for 40 min. Placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking, and performing pyrolysis treatment at 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by a reaction at 50sccm N₂Heating to 200 ℃ at the flow rate of 2 ℃/min, and preserving heat for 3h to finish precursor crosslinking and curing. Then at 100sccm NH₃Heating to 900 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 4h to finish precursor cracking. Finally, the flow rate is 50sccm N₂At the flow rate, the temperature is raised to 1500 ℃ at the speed of 2 ℃/min and is kept for 3 h. The above dip cracking process was repeated 3 times, wherein the dip solution ratio in the second furnace was 20 wt.% PSNB precursor, 20 wt.% PSN precursor, 20 wt.% PBN precursor, and 40 wt.% solvent xylene. The immersion solution in the third furnace was formulated with 30 wt.% PSNB precursor, 30 wt.% PSN precursor, and 40 wt.% solvent xylene.

60 wt.% of solvent xylene, 38 wt.% of PSN precursor and 2 wt.% of catalyst ferrocene are added into a sealed tank to prepare the precursorAfter the solution was sealed to prevent oxidation, it was magnetically stirred for 60min to mix well. Firstly, placing the core layer shield body material with certain density in vacuum with the vacuum degree of-0.08 MPa, maintaining the pressure for 40min, and exhausting air in the core layer shield body material; then, the precursor solution was immersed in the prepared precursor solution under vacuum for 40 min. Placing the core layer cover body dipped with the precursor solution in a high-temperature atmosphere furnace for curing and cracking, and performing cracking at 50sccm N₂Heating to 100 ℃ at the flow rate of 2 ℃/min, and keeping the temperature for 3h to finish the volatilization of the solvent xylene. Followed by 50sccm N₂Heating to 300 ℃ at the flow rate of 2 ℃/min, and preserving the heat for 3h to complete the cross-linking and curing of the precursor. Then at 100sccm N₂And under the flow, heating to 1350 ℃ at the speed of 5 ℃/min, and preserving heat for 4h to finish the catalytic cracking reaction of the PSN precursor and realize the growth of the silicon nitride nanowire. The precursor is made to grow one-dimensional nanometer line under the action of catalyst to complete the preparation of nanometer line. And finally, cooling to room temperature along with the furnace.

Deposition of dense silicon nitride on the surface layer: placing the antenna housing with the surface containing the nano-wire in a silicon nitride deposition furnace by adopting silicon tetrachloride (SiCl)₄) Ammonia (NH)₃) And (3) as a precursor gas source, depositing for 300 hours under the conditions that the deposition temperature is 1000 ℃ and the deposition pressure is 2kPa to prepare the silicon nitride ceramic radome with the symmetrical continuous gradient structure.

Claims

1. A silicon nitride ceramic antenna housing with a symmetrical continuous gradient structure is characterized in that the antenna housing has the characteristic of a symmetrical continuous gradient structure and is of an integrally molded and near-net-size structure; the antenna cover has the advantages that the porosity is gradually changed from the center to the surface along the wall thickness direction, the dielectric property is gradually changed, and the mechanical property is gradually changed.

2. A method for preparing a silicon nitride ceramic radome with a symmetrical continuous gradient structure according to claim 1, which is characterized by comprising the following steps:

step 1: the symmetrical continuous gradient structure of the antenna housing is taken as a symmetrical three-stage structure, namely an upper surface layer, a lower surface layer and an inner core layer; simulating the wave transmission rate of the antenna housing by adopting electromagnetic simulation software CST, establishing the relation between the thickness, the dielectric constant and the loss tangent parameters of the core layer and the surface layer and the wave transmission rate of silicon nitride, and optimizing the parameters to obtain a structural scheme with optimal comprehensive performance;

1. the optimization method of the dielectric constant and the dielectric loss of the core layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁The dielectric constant of the material of the surface layer₁The dielectric loss tan delta of the skin material₁And core layer material thickness d₂Then the dielectric constant epsilon of the core layer material is measured in CST software₂Dielectric loss tan delta₂The optimization is carried out, and the dielectric constant and the dielectric loss range of the core layer material meeting the requirement of high wave-transmitting rate of silicon nitride are taken as the optimization principle;

2. the optimization method of the dielectric constant and the dielectric loss of the surface layer material comprises the following steps: first, the basic parameters of the material are determined, including the thickness d of the surface layer material₁And core layer material thickness d₂Then according to the preferred core material dielectric constant ε₂And dielectric loss tan delta₂Dielectric constant ε of surface layer material in CST software₁Dielectric loss tan delta₁Optimization is carried out, and the dielectric constant and the dielectric loss range of a surface layer material meeting the requirement of high wave-transmitting rate of silicon nitride are taken as the optimization principle;

3. the optimization method of the thicknesses of the surface layer material and the core layer material comprises the following steps: dielectric constant epsilon of surface layer material and core layer material which are preferably calculated according to CST₁、ε₂And dielectric loss tan delta₁、tanδ₂Optimizing the thicknesses of the surface layer material and the core layer material in CST software, and determining the thickness ranges of the surface layer material and the core layer material meeting the requirement of high wave-transmitting rate of silicon nitride;

step 2, preparing a silicon nitride ceramic radome core layer with a symmetrical continuous gradient structure: the method comprises the steps of gel casting to form a prefabricated body, drying and glue discharging of the prefabricated body and preparation of a nitride matrix with a certain content in the prefabricated body;

gel casting of the preform: deionized water H as solvent₂O, dispersant ammonium polyacrylate PAA-NH₄TMAH serving as a pH regulator, PEG 400 serving as a wetting agent, acrylamide AM serving as an organic monomer, MBAM serving as a crosslinking agent and Si serving as silicon nitride whiskers₃N_4wFiller Si₃N₄Ball-milling the powder and the BN nanosheet to obtain silicon nitride whisker slurry; adding initiator ammonium persulfate APS, and vacuum stirring to remove bubbles; injecting the slurry into a mold, vibrating for defoaming, and keeping the temperature at a preset temperature to realize crosslinking, curing and molding;

drying and glue removing treatment of the prefabricated body; taking out the mold of the inner cavity of the radome, embedding the inner cavity of the radome, and drying the mold in a constant temperature and humidity environment to completely volatilize water in the prefabricated body; taking out the prefabricated body from the mold, and then carrying out glue removal and carbon removal treatment at a slow temperature rise rate in an aerobic environment to obtain an isotropic porous silicon nitride whisker prefabricated body with a uniform structure;

preparing a matrix containing nitride in a prefabricated body: preparing a nitride matrix in the prefabricated body by adopting a precursor impregnation cracking process matched with a special micron-sized pore structure of the whisker prefabricated body so as to finish the manufacturing of the porous core layer;

mixing xylene solvent with polyborosilazane PSNB, polysilazane PSN and polyborosilazane PBN as ceramic precursors, stirring, vacuum pressure impregnating to uniformly distribute the precursor solution in the pores of the preform, placing the preform impregnated with the precursor in an atmosphere furnace, and heating to N₂Respectively preserving the heat at the solvent volatilization temperature, the precursor crosslinking and curing temperature and the precursor cracking temperature under the atmosphere to obtain a multi-element nitride matrix with different proportions of Si, B and N elements;

step 4, preparing a compact silicon nitride coating on the surface layer of the silicon nitride ceramic radome with the symmetrical continuous gradient structure: placing the radome with the surface containing the nanowires prepared in the step (3) in a silicon nitride deposition furnace for surface layer densification to obtain a silicon nitride ceramic radome with a symmetrical continuous gradient structure;

the specific implementation method of the silicon nitride coating deposition process comprises the following steps: by using silicon tetrachloride SiCl₄NH-ammonia gas₃The deposition temperature is 800-1200 ℃, the system pressure is 2-5 kPa, and the deposition time is 120-300 h; by adjusting the deposition process parameters, silicon nitride coatings with different microstructures and thicknesses are prepared.

3. The method of claim 2, wherein: and (5) for the silicon nitride ceramic radome with the symmetrical continuous gradient structure obtained in the step (4), precisely processing the radome according to the thickness of the core layer and the surface layer of the radome optimized in the step (1), and then cleaning and drying the radome so that the dimensional precision and the surface roughness of the radome meet the design requirements.

4. The method of claim 2, wherein: in the step 2, the concrete implementation method of the gel casting preform comprises the following steps: adding 20-35 wt.% of deionized water, 0.1-0.5 wt.% of ammonium polyacrylate, 1-3 wt.% of tetramethylammonium hydroxide and 1-1.5 wt.% of polyethylene glycol 400 into a ball milling tank, uniformly mixing, and adding 40-55 wt.% of silicon nitride whisker and 5-10 wt.% of filler Si for multiple times₃N₄Ball-milling and dispersing the powder and the BN nanosheet for 2-6 h; continuously adding 5-8 wt.% of acrylamide AM and 0.3-0.5 wt.% of N, N-methylene bisacrylamide MBAM into the slurry, and performing ball milling dispersion for 0.5-4 hours; after uniformly mixing, adding 0.3-0.45 wt.% ammonium persulfate APS as an initiator into the slurry, removing bubbles for 5-10 min in vacuum, and pouring along the inner wall of a mold for injection molding after uniformly stirring to avoid generating bubbles; then oscillating to remove bubbles for 5-10 min to ensure that the slurry is constant at 60-90 DEG CAnd preserving heat in the incubator to complete crosslinking and curing of the slurry.

5. The method of claim 2, wherein: in the step 2, the specific implementation method of the drying and glue removing treatment of the preform comprises the following steps: after the prefabricated body which is well formed by crosslinking and curing is subjected to internal mold removal, the tap density is 0.5-1.5 g/cm³The boron nitride powder, the silicon nitride powder or the alumina powder fills the inner cavity of the prefabricated body and the gap between the prefabricated body and the cavity, and the filling is uniform; placing the cavity and the prefabricated body and powder in the cavity in an environment with the temperature of 10-25 ℃ and the humidity of 50-90%, and drying for 48-96 hours; and then demolding the completely dried prefabricated body, placing the prefabricated body in a high-temperature furnace, raising the temperature to 500-700 ℃ at a heating rate of 0.5-2 ℃/min in an aerobic atmosphere, preserving the temperature for 4-10 hours, and then cooling the prefabricated body to room temperature along with the furnace.

6. The method of claim 2, wherein: in the step 2, the specific implementation method for preparing the nitride matrix in the preform comprises the following steps: adding 0-50 wt.% of PSNB precursor, 0-50 wt.% of PSN precursor, 0-50 wt.% of PBN precursor and 50-90 wt.% of solvent xylene into a sealed tank, preparing an impregnation solution, sealing to prevent oxidation, and magnetically stirring for 10-100 min to uniformly mix the solution; firstly, placing the prefabricated body after the glue discharging treatment in vacuum of-0.1 to-0.08 MPa, maintaining the pressure for 20 to 40min, and discharging air in the prefabricated body; then immersing the precursor in a prepared precursor solution in a vacuum state, and immersing for 30-60 min; then placing the prepared precursor solution and the prefabricated body soaked in the precursor solution in an environment with the pressure of nitrogen or argon being 0.8-2.0 MPa, and soaking for 30-60 min under pressure; placing the preform dipped with the precursor solution in an atmosphere furnace for curing and cracking: at 30-60 sccm N₂Heating to 50-130 ℃ at a flow rate of 1-10 ℃/min, and preserving heat for 1-4 h to complete solvent xylene volatilization; then, the flow rate is controlled at 30-60 sccm N₂Heating to 170-300 ℃ at a flow rate of 1-10 ℃/min, and preserving heat for 2-6 h to complete precursor crosslinking and curing; then, the reaction solution is added into the reaction solution at 10-150 sccm NH₃Heating to 800-1000 ℃ at a rate of 0.2-5 ℃/min and preserving heat for 2-6 h to complete precursor cracking; most preferablyThen 30-60 sccm N₂Heating to 1300-1500 ℃ at a rate of 1-10 ℃/min and preserving heat for 1-4 h, and carrying out heat treatment on the multi-element nitride substrate obtained by cracking to ensure that the multi-element nitride substrate has higher stability and moisture absorption resistance; in the above impregnation cracking process, the precursor solution for next PIP is prepared according to the characteristics of porosity, mechanics and dielectric properties of the core layer after last PIP, and the impregnation cracking process needs to be repeated for 2-5 times according to different requirements of application scenes on the performance of the core layer.

7. The method of claim 2, wherein: and 3, when the carbothermic nitridation method or the silicon powder nitridation method is adopted, the quasi-oriented growth nanowire is prepared on the surface layer through the combination of slurry preparation and brushing and the subsequent corresponding high-temperature reaction process.

8. The method of claim 2, wherein: and 3, preparing the randomly oriented nanowires on the surface layer by adopting a catalytic cracking method and combining a vacuum impregnation process with a high-temperature cracking process.