CN113087531B

CN113087531B - High-strength high-thermal-conductivity silicon nitride ceramic substrate and preparation method and application thereof

Info

Publication number: CN113087531B
Application number: CN202110361923.XA
Authority: CN
Inventors: 伍尚华; 吕东霖; 杨平; 李建斌
Original assignee: Guangdong University of Technology
Current assignee: Guangdong University of Technology
Priority date: 2021-04-02
Filing date: 2021-04-02
Publication date: 2022-06-14
Anticipated expiration: 2041-04-02
Also published as: CN113087531A

Abstract

The embodiment of the invention discloses a high-strength high-thermal-conductivity silicon nitride ceramic substrate and a preparation method and application thereof, and relates to the technical field of ceramic substrate preparation. The preparation method comprises the following steps: carrying out ball milling treatment on the dispersing agent and the solvent to obtain a dispersing agent solution; sequentially adding a sintering aid and silicon nitride ceramic powder into the dispersant solution under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain a stable suspension; adding a plasticizer into the stable suspension, adding an adhesive into the stable suspension for multiple times under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain casting slurry; defoaming the casting slurry; preparing the casting slurry into a green sheet through a casting process; stacking the green sheets to obtain an initial blank body; and (3) preparing the initial blank into ceramic through a cold isostatic pressing process, a glue discharging process and a sintering process in sequence. The method provided by the embodiment of the invention can be used for preparing the silicon nitride ceramic substrate with high strength and high thermal conductivity, and the silicon nitride ceramic substrate has small strength anisotropy.

Description

High-strength high-thermal-conductivity silicon nitride ceramic substrate and preparation method and application thereof

Technical Field

The invention relates to the technical field of ceramic substrate preparation, in particular to a high-strength high-thermal-conductivity silicon nitride ceramic substrate and a preparation method and application thereof.

Background

Energy has become an indispensable important support in the social development process as an important material basis for human social life and production. The energy widely used at present is mainly fossil energy. As the fossil energy generates pollution gas in the using process, the pollution gas is harmful to the environment; furthermore, fossil energy, as a non-renewable resource, cannot permanently provide energy to humans. Therefore, a new alternative energy source is urgently needed to be found. Electric energy is concerned by people because of the characteristics of cleanness, high efficiency, reproducibility and the like, and is widely applied to daily life and industrial production. The Insulated Gate Bipolar Transistor (IGBT) is used as a core device for electric energy conversion and transmission, and is widely applied in the fields of smart power grids, high-speed trains, new energy automobiles and the like. With the continuous promotion of the use demand, the IGBT gradually develops towards the aspects of higher power density, smaller product size, more integrated function and the like. This causes the IGBT to generate more heat during operation. If the redundant heat in the IGBT can not be dissipated in time, the heat can be accumulated in the IGBT, the service performance of the IGBT is reduced, and the service life of the IGBT is damaged. The packaging technology is used as a key link in an IGBT industrial chain, so that the IGBT can be isolated from the outside, and mechanical support protection and electrical insulation effects are provided; the IGBT also has the function of dissipating the excessive heat inside the IGBT. Ceramic materials have been commonly used as package substrate materials in the field of IGBTs due to their good thermal conductivity, excellent insulation properties and mechanical properties. In order to meet the development direction and the use requirement of the IGBT, the ceramic substrate material needs to have higher thermal conductivity and greater mechanical strength, so that the ceramic substrate field mainly composed of the conventional ceramic material faces huge challenges. Therefore, there is a need to develop a novel ceramic substrate material having both excellent thermal conductivity and mechanical properties.

Silicon nitride is used as an important structural ceramic material, has excellent mechanical property and thermal shock resistance (the silicon nitride is heated to more than 1000 ℃ in the air and then rapidly quenched and rapidly heated without fragmentation), and research results show that the theoretical thermal conductivity of the single crystal silicon nitride at room temperature can reach 450W/m.K. Therefore, silicon nitride ceramics are considered as one of the ideal materials for preparing high-performance ceramic substrates. The thermal conductivity of the actually prepared silicon nitride is far lower than the theoretical thermal conductivity of the single crystal silicon nitride due to phonon scattering caused by lattice oxygen, impurity atoms, dislocations, vacancies, grain boundaries, residual second phases and other factors. In order to obtain high-thermal-conductivity silicon nitride ceramics, long-time high-temperature heat preservation is generally utilized, and the silicon nitride crystal lattice is purified and the oxygen content of the crystal lattice is reduced through the faster diffusion rate of oxygen at high temperature. In addition, the long-time high-temperature heat preservation can also promote the grain growth and reduce the obstruction effect of the relative heat conductivity of the grain boundary. However, according to the Hall-Petch principle, the crystal grain size is too large, which causes rapid deterioration of the mechanical properties of the silicon nitride ceramic. It is known that the high thermal conductivity and the high strength of silicon nitride are contradictory properties, and it is difficult to achieve both the high thermal conductivity and the high strength of the silicon nitride ceramic.

On the other hand, since the β -silicon nitride crystal grains are long and rod-shaped, there is anisotropy in mechanical properties and thermal conductivity, and it is known that the arrangement direction of the β -silicon nitride crystal grains will also affect the mechanical properties and thermal conductivity of silicon nitride. The prior methods for preparing crystal grains with certain orientation comprise tape casting, strong magnetic field auxiliary molding, hot-pressing flowing and the like. The silicon nitride product with high grain orientation can be obtained by strong magnetic domain auxiliary forming, but because the strong magnetic domain of about 10T is required to be assisted, the equipment price and the use cost are high, and the large-scale popularization and application are difficult; the hot press flow method also allows for higher grain oriented samples, but only one sample can be fired at a time and is of a single shape. In addition, the samples prepared by the hot-pressing flow sintering and the strong magnetic field auxiliary forming method need to be cut into sheets, the processing difficulty and the cost are high, and the method is not suitable for industrial popularization and application.

Based on the above discussion, the preparation of high-performance silicon nitride ceramic substrates currently faces two main problems: firstly, the high strength and the high thermal conductivity of the silicon nitride are difficult to be considered simultaneously, and secondly, the performance of the textured fired product has anisotropy.

Disclosure of Invention

The technical problem to be solved by the embodiment of the invention is how to prepare the silicon nitride substrate which has high strength and heat conductivity and isotropic strength.

In order to solve the above problems, the present invention proposes the following technical solutions:

in a first aspect, an embodiment of the present invention provides a method for preparing a high-strength high-thermal conductivity silicon nitride ceramic substrate, including:

s1, performing ball milling treatment on the dispersing agent and the solvent to obtain a dispersing agent solution; sequentially adding a sintering aid and silicon nitride ceramic powder into the dispersant solution under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain a stable suspension;

s2, adding a plasticizer into the stable suspension, adding an adhesive into the stable suspension for multiple times under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain casting slurry;

s3, defoaming the casting slurry;

s4, preparing the casting slurry into a green sheet through a casting process, wherein the mass ratio of the silicon nitride raw material powder, the sintering aid, the solvent, the dispersant, the binder and the plasticizer is 100 (5-12): (40-100): (1-3.5): (6-10): (6-15); the green sheet comprises a reinforced green sheet and a heat-conducting green sheet; the mass ratio of the alpha-silicon nitride to the beta-silicon nitride whisker in the silicon nitride raw material powder of the reinforced green sheet is (97-99): (1-3); the mass ratio of the alpha-silicon nitride to the beta-silicon nitride seed crystal in the silicon nitride raw material powder of the heat conduction green sheet is (95-97): (3-5);

s5, stacking the green sheets to obtain an initial green body, wherein the initial green body comprises a heat conduction unit and reinforcing units arranged on two sides of the heat conduction unit, the heat conduction unit comprises a heat conduction green sheet, the reinforcing units are a plurality of reinforcing green sheets stacked mutually, and the casting directions of the reinforcing green sheets of the reinforcing units form an angle with each other;

and S6, preparing the initial blank into ceramic through a cold isostatic pressing process, a glue discharging process and a sintering process in sequence.

The further technical scheme is that D50 of silicon nitride in the silicon nitride raw material powder is 0.5-3 um.

The further technical scheme is that the sintering aid comprises an alkali metal compound and a rare earth compound, wherein the alkali metal compound is MgO and MgF₂、Al₂O₃And CaO, the rare earth compound is Y₂0₃、Yb₂O₃、CeO、Dy₂O₃、YF₃And YbF₃At least one of (1).

The further technical proposal is that the solvent is at least one of ethanol, isopropanol, n-butanol, ethyl acetate, butyl acetate, butanone and cyclohexanone; the dispersant is at least one of triethyl phosphate, triolein, castor oil and triethanolamine.

The further technical proposal is that the binder is at least one of polyvinyl butyral and methyl acrylate; the plasticizer is at least one of phthalate, polyethylene glycol and glycerol.

The further technical scheme is that the step S1 includes: adding a dispersing agent and a solvent into a nylon ball milling tank, putting the nylon ball milling tank into a planetary ball mill, and carrying out ball milling for 1-2h at the rotating speed of 200-300r/min, so that the dispersing agent is uniformly dispersed in the solvent in advance to obtain a dispersing agent solution; adding a sintering aid into the nylon ball milling tank and continuing to ball mill for 1-2h at the rotating speed of 200-; adding silicon nitride ceramic powder into a nylon ball milling tank, and continuing ball milling for 4-12h at the rotating speed of 200-300r/min to obtain stable suspension;

step S2 includes: adding a plasticizer into a nylon ball milling tank, and ball milling for 0.5-1h at the rotating speed of 200-300 r/min; controlling the rotating speed of the star ball mill to be 200-; after the addition of the binder is finished, continuing ball milling for 4-24h to obtain casting slurry; wherein the addition interval of the adhesive is 0.5-1 h.

The further technical scheme is that the technological parameters of the defoaming treatment comprise: the vacuum degree is minus 0.1Mpa, and the defoaming time is 15-30 min; the technological parameters of the cold isostatic pressing process comprise: the cold isostatic pressing temperature is 60-100 ℃, and the pressurizing pressure is 100 and 200 MPa; the technological parameters of the glue discharging process comprise: the glue discharging temperature is 450 ℃ and 600 ℃; the technological parameters of the sintering process comprise: the sintering temperature is 1750 ℃ and 1900 ℃, and the sintering time is 2-10 h; the technological parameters of the tape casting process comprise: the height of the scraper is 0.2-0.8mm, and the casting speed is 10-2000 mm/min; the temperature of the three sections of the drying area of the casting machine is 30-60 ℃.

The further technical scheme is that for the reinforced green sheet, the adopted casting forming process comprises the following process parameters: the height of the scraper is 0.2-0.25um, and the casting speed is 1500-;

for the heat-conducting green sheet, the adopted casting process parameters comprise: the height of the scraper is 0.35-0.5mm, and the casting speed is 20-40 mm/min.

In a second aspect, an embodiment of the present invention provides a high-strength high thermal conductivity silicon nitride ceramic substrate, which is prepared by the method in the first aspect.

In a third aspect, embodiments of the present invention provide a use of the high-strength and high-thermal-conductivity silicon nitride ceramic substrate according to the second aspect in an electronic device substrate.

Compared with the prior art, the embodiment of the invention can achieve the following technical effects:

in the embodiment of the invention, long rod-shaped beta-silicon nitride whiskers with a large length-diameter ratio are added into silicon nitride original powder, casting is carried out at a lower scraper height and a higher casting speed, so that a two-dimensional textured silicon nitride casting green body with a higher orientation is obtained, the green body is cut into a certain shape, pieces are stacked at a certain angle difference, interlayer long rod-shaped crystal grains can be crossed, a similar reinforced cement structure is constructed, the strength is improved, and meanwhile, the anisotropy of the strength in all directions is reduced.

Further, in order to achieve a thermal conductivity, another β -silicon nitride seed crystal having a small length to diameter ratio is prepared and cast at a higher blade height and a slower casting speed, thereby reducing the orientation of crystal grains to obtain a higher thermal conductivity in the thickness direction thereof. Alternatively, a different formulation of the promoter (e.g., a fluoride promoter with less oxygen incorporated) may be provided as compared to the cross-stacked layers.

One significant feature of the present invention that distinguishes it from the prior art is its difference in microstructure between layers. The microcosmic structure of the crystal is approximately composed of three different crystal grain structures, in two reinforcing unit layers containing whiskers with a large length-diameter ratio, because the reinforcing unit layers are staggered by a certain angle when being stacked, the section of a sample parallel to the casting direction can observe an upper layer and a lower layer, one layer has more long rod-shaped crystal grains, and the other layer has less long rod-shaped crystal grains; for the heat conducting unit interlayer existing for heat conduction, the proportion of the long rod-shaped crystal grains is between the two layers, and the layer is thicker.

In summary, the silicon nitride ceramic substrate with high strength and high thermal conductivity can be prepared by the method provided by the embodiment of the invention, and the anisotropy of the strength of the silicon nitride ceramic substrate is small.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a schematic interface diagram of an initial blank according to an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a stacking method of an initial blank according to an embodiment of the present invention;

fig. 3 is a schematic diagram of a stacking method in the prior art.

Detailed Description

The technical solutions in the embodiments will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, wherein like reference numerals represent like elements in the drawings. It is apparent that the embodiments to be described below are only a part of the embodiments of the present invention, and not all of them. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It is also to be understood that the terminology used in the description of the embodiments of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments of the invention. As used in the description of embodiments of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The embodiment of the invention provides a preparation method of a high-strength high-thermal-conductivity silicon nitride ceramic substrate, which comprises the following steps:

s1, performing ball milling treatment on the dispersing agent and the solvent to obtain a dispersing agent solution; and sequentially adding a sintering aid and silicon nitride ceramic powder into the dispersant solution under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain a stable suspension.

In specific implementation, a dispersing agent and a solvent are added into a nylon ball milling tank, the nylon ball milling tank is placed into a planetary ball mill to be ball milled for 1-2 hours at the rotating speed of 200-300r/min, and the dispersing agent is uniformly dispersed in the solvent in advance to obtain a dispersing agent solution; adding a sintering aid into the nylon ball milling tank and continuing to ball mill for 1-2h at the rotating speed of 200-300 r/min; adding silicon nitride ceramic powder into a nylon ball milling tank, and continuing ball milling for 4-12h at the rotating speed of 200-300r/min to obtain stable suspension.

And S2, adding a plasticizer into the stable suspension, adding a bonding agent into the stable suspension for multiple times under the condition of keeping the ball milling state, and continuously carrying out ball milling to obtain casting slurry.

In the specific implementation, the plasticizer is added into a nylon ball milling tank, and ball milling is carried out for 0.5-1h at the rotating speed of 200-; controlling the rotating speed of the star ball mill to be 200-; after the addition of the binder is finished, continuing ball milling for 4-24h to obtain casting slurry; wherein the addition interval of the adhesive is 0.5-1 h.

And S3, defoaming the casting slurry.

In specific implementation, the technological parameters of the defoaming treatment include: the vacuum degree is minus 0.1Mpa, and the defoaming time is 15-30 min.

S4, preparing the casting slurry into a green sheet through a casting forming process.

In the concrete implementation, in the steps S1-S4, the mass ratio of the silicon nitride raw material powder, the sintering aid, the solvent, the dispersant, the binder and the plasticizer is 100 (5-12): (40-100): (1-3.5): (6-10): (6-15).

The green sheet manufactured through the steps S1-S4 includes a reinforcing green sheet and a heat conductive green sheet. The mass ratio of the alpha-silicon nitride to the beta-silicon nitride whisker in the silicon nitride raw material powder of the reinforced green sheet is (97-99): (1-3). The mass ratio of the alpha-silicon nitride to the beta-silicon nitride seed crystal in the silicon nitride raw material powder of the heat conduction green sheet is (95-97): (3-5).

Further, the process parameters of the tape casting process include: the height of the scraper is 0.2-0.8mm, and the casting speed is 10-2000 mm/min; the temperature of the three sections of the drying area of the casting machine is 30-60 ℃.

Optionally, for the reinforced green sheet, the process parameters of the tape casting process adopted by the reinforced green sheet include: the height of the scraper is 0.2-0.25um, and the casting speed is 1500-.

S5, obtain initial idiosome after piling the embryo piece, initial idiosome includes the heat conduction unit and locates the enhancement unit of heat conduction unit both sides, the heat conduction unit includes heat conduction embryo piece, strengthen a plurality of reinforcing embryo pieces that pile up each other of unit, strengthen a plurality of reinforcing embryo pieces' S of unit curtain coating direction each other and become the angle.

In specific implementation, the heat-conductive green sheet and the reinforced green sheet are prepared through steps S1-S4, respectively, and cut into a predetermined shape (e.g., circular, square, etc.); then stacking a plurality of reinforced green sheets into a reinforced unit; and adding a heat-conducting green sheet (heat-conducting unit) between the two reinforcing units to obtain an initial blank.

The technological parameters of the cold isostatic pressing process in the specific implementation comprise: the cold isostatic pressing temperature is 60-100 ℃, and the pressurizing pressure is 100-. The technological parameters of the glue discharging process comprise: the glue discharging temperature is 450-600 ℃. The technological parameters of the sintering process comprise: the sintering temperature is 1750 ℃ and 1900 ℃, and the sintering time is 2-10 h.

Further, D50 of silicon nitride in the silicon nitride raw material powder is 0.5-3 um.

The sintering aid comprises an alkali metal compound and a rare earth compound, wherein the alkali metal compound is MgO or MgF₂、Al₂O₃And CaO, the rare earth compound is Y₂0₃、Yb₂O₃、CeO、Dy₂O₃、YF₃And YbF₃At least one of (1).

The solvent is at least one of ethanol, isopropanol, n-butanol, ethyl acetate, butyl acetate, butanone and cyclohexanone.

The dispersant is at least one of triethyl phosphate, triolein, castor oil and triethanolamine.

The binder is at least one of polyvinyl butyral and methyl acrylate; the plasticizer is at least one of phthalate, polyethylene glycol and glycerol.

The embodiment of the invention provides a high-strength high-thermal-conductivity silicon nitride ceramic substrate which is prepared by the method provided by the embodiment.

The embodiment of the invention provides application of the high-strength high-thermal-conductivity silicon nitride ceramic substrate in an electronic device substrate. Such as a substrate for an IGBT.

Referring to fig. 1-2, in an embodiment, the initial blank body includes a reinforced green sheet a, a reinforced green sheet C, a heat-conducting green sheet B, a reinforced green sheet a, and a reinforced green sheet C stacked in sequence from top to bottom, wherein the casting directions of the reinforced green sheet a and the reinforced green sheet C are perpendicular to each other.

Referring to fig. 3, fig. 3 is a schematic view of a conventional stacking manner. In FIGS. 2 to 3, the direction of the arrow indicates the casting direction.

In the embodiment of the invention, long rod-shaped beta-silicon nitride whiskers with a large length-diameter ratio (such as a rod-shaped quadrangle in figure 2) are added into silicon nitride raw powder, casting is carried out at a lower scraper height and a higher casting speed, so that a two-dimensional textured silicon nitride casting green body with a higher orientation is obtained, the green body is cut into a certain shape, pieces are stacked at a certain angle difference, interlayer long rod-shaped crystal grains can be crossed, a similar reinforced cement structure is constructed, the strength is improved, and the anisotropy of the strength in all directions is reduced.

Further, in order to achieve a thermal conductivity, another β -silicon nitride seed crystal having a relatively small long diameter ratio is prepared and cast at a higher blade height and a slower casting speed (e.g., the thermally conductive green sheet B in fig. 1 and 2), thereby reducing the orientation of crystal grains to obtain a higher thermal conductivity in the thickness direction thereof. Alternatively, a different formulation of the promoter (e.g., a fluoride promoter with less oxygen incorporated) may be provided as compared to the cross-stacked layers.

One significant feature of the present invention that is distinct from the prior art is its inter-layer microstructural variability. The microcosmic structure of the crystal is approximately composed of three layers of different crystal grain structures, in two layers of strengthening unit layers containing whiskers with large length-diameter ratio, because the strengthening unit layers are staggered by a certain angle when being stacked, one layer has more long rod-shaped crystal grains and the other layer has less long rod-shaped crystal grains in the upper layer and the lower layer which can be observed on a section of a sample parallel to the casting direction; for the heat conducting unit interlayer existing for heat conduction, the proportion of the long rod-shaped crystal grains is between the two layers, and the layer is thicker.

To better illustrate the technical solution of the present invention, specific examples are now provided as follows:

the formulations used in the examples are mainly the following four, the specific material composition of which is shown in table 1 below.

TABLE 1 formulation constitution Table

The preparation method of the high-strength high-thermal-conductivity silicon nitride ceramic substrate comprises the following overall process flow:

a. adding a dispersing agent and a solvent into a nylon ball milling tank, putting the nylon ball milling tank into a planetary ball mill, carrying out ball milling for 1h at the rotating speed of 300r/min, adding a sintering aid into the nylon ball milling tank, continuing ball milling for 1h at the rotating speed of 300r/min, finally adding silicon nitride ceramic powder into the nylon ball milling tank, continuing ball milling for 6h at the rotating speed of 300r/min, and obtaining a stable suspension;

b. and (b) adding a plasticizer into the stable suspension obtained in the step a, and carrying out ball milling for 1h at the rotating speed of 300 r/min. And adding the adhesive twice, wherein the time interval of each time is 30min, the rotating speed is still controlled at 300r/min, and after all the adhesive is added, continuing ball milling for 12h to obtain the casting slurry.

c. And c, defoaming the casting slurry obtained in the step b for 15min under the vacuum degree of about-0.1 MPa, thereby preparing the final silicon nitride casting slurry.

d. And c, casting the final silicon nitride casting slurry obtained in the step c by using a casting machine, wherein the height of a scraper is set to be 0.2-0.8mm, the casting speed is set to be 10-2000mm/min, the temperature of three sections of a drying area of the casting machine is set to be 30-60 ℃, and finally the dried casting film is peeled off from the PET film to obtain a green sheet.

e. Cutting the green sheet into circles, squares or other desired shapes; stacking a plurality of reinforced green sheets to form a reinforced unit, wherein the reinforced green sheets are crossed at 90 degrees or other angles along the casting direction during stacking; and adding heat-conducting green sheets between the reinforcing units to obtain an initial blank. The heat-conducting green sheet is formulated differently from the reinforced green sheet, for example, conventional MgO-Y may be used for the reinforcing layer₂O₃The adjuvant formulation allows the use of YF with a lower oxygen content for the thermally conductive layer₃-MgSiN₂And (5) preparing an auxiliary agent formula.

f. And (3) carrying out warm pressing at 80 ℃ and cold isostatic pressing at 200MPa on the stacked initial blank, then carrying out glue removal at 600 ℃ and sintering at 1850 ℃ for 4 hours under normal pressure in a nitrogen atmosphere to obtain a sintered product.

The overall steps between formulations 1-4 are similar, but the effect is slightly different in the details of steps d, e depending on the formulation, where formulation 1 is to obtain a reinforcement layer (reinforcement unit) formulation with higher grain orientation, so the doctor blade is lower, the casting speed is faster, the solution is thinner to allow stronger shear stress of the slurry under the doctor blade to obtain higher grain orientation, while formulations 2, 3, 4 are the opposite to a high thermal conductivity interlayer (thermal conductive unit) formulation. Specific differences are shown in table 2 below.

TABLE 2 preparation method difference table for recipes 1-4

The green sheets prepared based on the above recipes 1 to 4 provided 8 examples of preparing silicon nitride ceramic substrates, the differences of each example being shown in table 3 below.

Table 3. examples 1-8 stacking differences the test results for examples 1-8 are shown in table 4 below. Wherein, 1) the density of the silicon nitride is measured according to an Archimedes drainage method and then is divided by the theoretical density under the formula of the auxiliary agent to obtain the density;

2) the thermal conductivity is measured by the laser flash method, and then the thermal diffusion coefficient of the silicon nitride is measured by the formula k ═ alpha · ρ · C_pCalculated where α is the thermal diffusion coefficient of the sample, ρ is the density of the sample, C_pIs the isobaric heat capacity (0.68J/K.g).

3) The test specimens for flexural strength were processed according to American standard (1.5X 2X 25mm), placed in a universal testing machine and the maximum load P before fracture was measured at a loading rate of 0.05mm/min, and then according to the following formula

σ_f＝3PL/2bh²And obtaining the intensity value.

TABLE 4 test results table

Analysis of results

As can be seen from examples 1, 5, 6, 7 and 8 in table 1, when the crystal grains are oriented to a certain degree, the strength has a more significant anisotropy, and there is a certain limitation in practical application as a substrate, and the anisotropy of the sample after cross-stacking has a more significant decrease. As examples 2 and 3 with low grain orientation, thermal conductivity could reach 82.76(W/mK) at the highest, and strength anisotropy was not significant but was also low. And samples with certain grain orientation are crossed and stacked to form reinforcing units, and after a heat conduction reinforcing layer with higher heat conduction and lower grain orientation degree is added between the reinforcing units, both the heat conduction and the strength can obtain better balance (73.35W/mK and 837/854MPa respectively), and the anisotropy of the strength is also reduced. According to examples 4 and 8, it is known that too much seed crystal of the high thermal conductive layer is not favorable for obtaining a dense body, and thus affects thermal conductivity and strength. Further, in comparison with examples 5 and 7, it is found that the strength does not change much when there are more reinforcing layers, but the thermal conductivity is significantly decreased, which means that the number of reinforcing units is not necessarily too large.

In the above embodiments, the descriptions of the respective embodiments have respective emphasis, and for parts that are not described in detail in a certain embodiment, reference may be made to related descriptions of other embodiments.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, while the invention has been described with respect to the above-described embodiments, it will be understood that the invention is not limited thereto but may be embodied with various modifications and changes.

While the invention has been described with reference to specific embodiments, the scope of the invention is not limited thereto, and various equivalent modifications and substitutions may be easily made by those skilled in the art within the technical scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A preparation method of a high-strength high-thermal-conductivity silicon nitride ceramic substrate is characterized by comprising the following steps:

s3, defoaming the casting slurry;

s4, preparing the casting slurry into a green sheet through a casting process, wherein the mass ratio of the silicon nitride ceramic powder, the sintering aid, the solvent, the dispersant, the adhesive and the plasticizer is 100 (5-12): (40-100): (1-3.5): (6-10): (6-15); the green sheet comprises a reinforced green sheet and a heat-conducting green sheet; the mass ratio of the alpha-silicon nitride to the beta-silicon nitride whisker in the silicon nitride ceramic powder of the reinforced green sheet is (97-99): (1-3); the mass ratio of the alpha-silicon nitride to the beta-silicon nitride seed crystal in the silicon nitride ceramic powder of the heat-conducting green sheet is (95-97): (3-5);

s5, stacking the green sheets to obtain an initial green body, wherein the initial green body comprises a heat conduction unit and reinforcing units arranged on two sides of the heat conduction unit, the heat conduction unit comprises heat conduction green sheets, the reinforcing unit on each side is two reinforcing green sheets stacked mutually, and the casting directions of the two reinforcing green sheets form an angle with each other;

s6, preparing the initial blank into ceramic through a cold isostatic pressing process, a glue discharging process and a sintering process in sequence; the technological parameters of the defoaming treatment comprise: the vacuum degree is minus 0.1MPa, and the defoaming time is 15-30 min; the technological parameters of the cold isostatic pressing process comprise: the cold isostatic pressing temperature is 60-100 ℃, and the pressurizing pressure is 100-200 MPa; the technological parameters of the glue discharging process comprise: the glue discharging temperature is 450 ℃ and 600 ℃; the technological parameters of the sintering process comprise: the sintering temperature is 1750 ℃ and 1900 ℃, and the sintering time is 2-10 h; the temperature of a third section of a drying zone of the casting machine is 30-60 ℃; for the reinforced green sheet, the casting process adopted by the reinforced green sheet has the following process parameters: the height of the scraper is 0.2-0.25mm, and the casting speed is 1500-; for the heat-conducting green sheet, the technological parameters of the casting process adopted by the heat-conducting green sheet comprise: the height of the scraper is 0.35-0.5mm, and the casting speed is 20-40 mm/min.

2. The method of claim 1, wherein D50 of silicon nitride in the silicon nitride ceramic powder is 0.5-3 um.

3. The method as claimed in claim 1, wherein the sintering aid comprises an alkaline earth metal compound and a rare earth compound, wherein the alkaline earth metal compound is MgO or MgF₂、Al₂O₃And CaO, the rare earth compound is Y₂O₃、Yb₂O₃、Dy₂O₃、YF₃And YbF₃At least one of (1).

4. The method as claimed in claim 1, wherein the solvent is at least one of ethanol, isopropanol, n-butanol, ethyl acetate, butyl acetate, butanone, and cyclohexanone; the dispersant is at least one of triethyl phosphate, triolein, castor oil and triethanolamine.

5. The method as claimed in claim 1, wherein the adhesive is at least one of polyvinyl butyral and methyl acrylate; the plasticizer is at least one of phthalate, polyethylene glycol and glycerol.

6. The method as claimed in claim 1, wherein step S1 includes: adding a dispersing agent and a solvent into a nylon ball milling tank, putting the nylon ball milling tank into a planetary ball mill, and carrying out ball milling for 1-2h at the rotating speed of 200-300r/min, so that the dispersing agent is uniformly dispersed in the solvent in advance to obtain a dispersing agent solution; adding a sintering aid into the nylon ball milling tank and continuing to ball mill for 1-2h at the rotating speed of 200-; adding silicon nitride ceramic powder into a nylon ball milling tank, and continuing ball milling for 4-12h at the rotating speed of 200-300r/min to obtain stable suspension;

step S2 includes: adding a plasticizer into a nylon ball milling tank, and carrying out ball milling for 0.5-1h at the rotating speed of 200-300 r/min; controlling the rotating speed of the star ball mill to be 200-; after the adhesive is added, continuing to perform ball milling for 4-24h to obtain casting slurry; wherein the addition interval of the adhesive is 0.5-1 h.

7. A high-strength high-thermal-conductivity silicon nitride ceramic substrate prepared by the method of any one of claims 1 to 6.

8. Use of a high strength, high thermal conductivity silicon nitride ceramic substrate according to claim 7 in an electronic device substrate.