A preparation method of a silicon nitride ceramic sheet with ultra-high thermal conductivity

Abstract

The invention discloses a method for preparing ultra-high thermal conductivity silicon nitride ceramic sheets, and relates to the technical field of silicon nitride ceramic sheets. The method comprises the following steps: Step 1: ball-milling and degassing the raw materials to make a slurry and tape-casting to obtain a green body; Step 2: stacking the green body with boron nitride powder to debind and obtain a laminated green body; Step 3: sintering the laminated green body under a protective atmosphere to obtain an ultra-high thermal conductivity silicon nitride ceramic sheet. Advantages: The present invention controls the amount of magnesium oxide added and matches the addition of seed crystals to reduce the addition of additional oxygen content while controlling the growth of grains, thereby improving the thermal conductivity and flexural strength of the obtained silicon nitride ceramic sheet. At the same time, the binder part introduces modified polysilazane, which improves the quality of the silicon nitride ceramic sheet while avoiding the generation of impurities during high-temperature sintering, which is in line with practical applications.

Description

Technical Field

[0001] The invention relates to the technical field of silicon nitride ceramic sheets, and in particular to a method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet. Background Art

[0002] With the rapid development of my country's high-speed rail, aerospace, military industry and other fields, the demand for high-power power electronic devices will increase in the future. In order to adapt to more complex and harsh application conditions, high-power power electronic devices are developing towards high temperature, high frequency, low power consumption, intelligence, modularization and systematization, which poses a severe challenge to the heat dissipation of the entire electronic device. The function of the substrate in the power device is to absorb the heat generated by the chip and transfer it to the heat sink to achieve heat exchange with the outside world. Therefore, the preparation of high thermal conductivity substrate materials has become the key to the research and development of high-power module electronic products. However, the actual thermal conductivity of silicon nitride ceramics is still far lower than the theoretical thermal conductivity value, and some high thermal conductivity silicon nitride ceramics are still in the laboratory stage.

[0003] At present, the factors that affect the thermal conductivity of silicon nitride ceramics include lattice oxygen, crystal phase, grain boundary phase, etc. Among them, the increase in silicon nitride grain size is conducive to improving thermal conductivity, but when the grain size reaches a critical value, the increase in grain size has no obvious effect on improving the thermal conductivity of silicon nitride. Therefore, it is no longer possible to continuously improve thermal conductivity by only promoting abnormal growth of silicon nitride grains through high-temperature sintering and long-term heat preservation, and other more effective ways must be explored. The lattice oxygen content in silicon nitride will reduce the thermal conductivity of silicon nitride. Part of the oxygen content in the lattice oxygen comes from oxide-based sintering aids. The addition of magnesium oxide is to promote the densification and grain growth of silicon nitride and provide an environment for grain growth, but too much will affect the thermal conductivity of silicon nitride. The current main method is to replace oxide additives with non-oxide additives, but it is still in the research and development stage and cannot be mass-produced.

[0004] Therefore, in order to solve the above problems, a method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet is prepared. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet to solve the problems raised in the prior art.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet comprises the following steps:

[0008] Step 1: ball milling and degassing the raw materials to make slurry and tape casting to obtain green body;

[0009] Step 2: stacking the green body with boron nitride powder to remove binder, so as to obtain a laminated green body;

[0010] Step 3: Sinter the laminated green body under a protective atmosphere to obtain an ultra-high thermal conductivity silicon nitride ceramic chip.

[0011] More preferably: The raw materials of the ultra-high thermal conductivity silicon nitride ceramic chip include the following components: by weight, 60-75 parts of silicon powder, 30-75 parts of solvent, 0.25-2.5 parts of dispersant, 3-30 parts of binder, 10-30 parts of plasticizer, 1.5-8 parts of yttrium oxide, 1.5-6 parts of magnesium oxide, and 1-3 parts of β-Si3N4 crystal seeds.

[0012] More preferably: The parameters for debinding are set as follows: heating to 500-900 °C at a heating rate of 2-5 °C / min and holding for 4-24 h;

[0013] The parameters for sintering are set as follows: the air pressure is 0.1-10 MPa, heating to 1650 °C - 1900 °C at a heating rate of 1-5 °C / min, and holding for 18-48 h.

[0014] More preferably: The dispersant includes one or more of castor oil, fish oil, and glycerol ester; the plasticizer includes one or more of dioctyl phthalate and dibutyl phthalate; the solvent includes one or both of isopropyl alcohol and ethyl acetate.

[0015] In a further embodiment, the solvent is isopropyl alcohol and ethyl acetate with a mass ratio of 1:(1-2.5).

[0016] More preferably: The binder is polyvinyl butyral.

[0017] More preferably: The binder is polyvinyl butyral and modified polysilazane with a mass ratio of 2-2.2:1-1.8.

[0018] More preferably: The preparation process of the modified polysilazane is as follows:

[0019] S1: Under a protective atmosphere, dissolve vinyl polysilazane in xylene, add thiosalicylic acid and azobisisobutyronitrile, and heat to 50-60 °C and react for 3-4 h to obtain intermediate A;

[0020] S2: Mix intermediate A, melamine, 1,3-dicyclohexylcarbodiimide, N-hydroxysuccinimide, and ethylene glycol, heat to 70-80 °C, and react for 4-5 h to obtain the modified polysilazane.

[0021] More preferably: The intermediate A comprises the following components: by weight, 20-30 parts of vinyl polysilazane, 100-120 parts of xylene, 15-20 parts of thiosalicylic acid, 1-2 parts of azobisisobutyronitrile; the modified polysilazane comprises the following components: by weight, 10-12 parts of intermediate A, 5-8 parts of melamine, 0.5-1 part of 1,3-dicyclohexylcarbodiimide, 0.5-1 part of N-hydroxysuccinimide, 60-80 parts of ethylene glycol.

[0022] More preferably: As the grain size of Si3N4 increases, the distribution of the intergranular phase will change, and the number of thin layers of the intergranular phase between adjacent two grains will decrease. Within a certain size range, the increase of the grains can improve the thermal conductivity of Si3N4. By adjusting the addition amount of magnesium oxide, the growth of the grains can be controlled. However, too much magnesium oxide will increase the content of lattice oxygen in silicon nitride, thus affecting the thermal conductivity of silicon nitride.

[0023] Therefore, in the present invention, by controlling the addition amount of magnesium oxide and combining with the addition of seed crystals, while reducing the addition of extra oxygen content, the growth of the grains is controlled, so that the thermal conductivity of the obtained silicon nitride ceramic sheet is improved. At the same time, during the high-temperature sintering process of the silicon nitride ceramic sheet, it is easy to be oxidized, and the oxidized substances will penetrate into the surface of the ceramic sheet and form new impurity phases at high temperature, affecting the thermal conductivity. Therefore, in the present invention, modified polysilazane is introduced into the binder part. On the one hand, the modified polysilazane is mixed with the silicon nitride powder, which helps to form a casting slurry with a certain colloidal viscosity, and can spread into a uniform thin film on the film belt during the casting process to form a ceramic sheet with a uniform thickness; on the other hand, the modified polysilazane can decompose into reducing gases under high-temperature conditions to prevent the generation of impurity phases. Specifically as follows:

[0024] (1) Adding an appropriate amount of β-Si3N4 seed crystals with larger size to the Si3N4 raw material can promote the rapid dissolution and precipitation of fine particles on the β-Si3N4 seed crystals during the sintering process, improve the driving force for grain growth, and promote the growth of β-Si3N4 grains. The growth of the grains is beneficial to the gradual extrusion of the grain boundary phase into the polycrystalline junction, reducing the distribution of the grain boundary phase in the lattice, thereby improving the thermal conductivity;

[0025] (2) Vinyl polysilazane reacts through a click reaction to obtain intermediate A, and then grafts melamine groups through an amidation reaction to obtain modified polysilazane. During the preparation process of the silicon nitride ceramic sheet, the modified polysilazane and polyvinyl butyral act as binders together, and they work together to effectively improve the quality of the silicon nitride ceramic sheet. At the same time, the grafted melamine groups of the modified polysilazane can act as reducing agents to generate reducing gases under high-temperature conditions, avoiding the generation of impurity phases during the high-temperature sintering process and reducing the thermal conductivity. Description of the Drawings

[0026] Figure 1 This is a SEM image of the silicon nitride ceramic sheet obtained in Example 1 of the present invention;

[0027] Figure 2 This is a SEM image of the silicon nitride ceramic sheet obtained in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] It should be noted that the purchase manufacturers of all raw materials involved in the present invention are not subject to any special restrictions and are exemplified as follows: in the following embodiments, vinyl polysilazane CAS is 503590-70-3, and the brand is Lana White; 1,3-dicyclohexylcarbodiimide CAS is 538-75-0, and the manufacturer is Qianyan Chemical Technology (Wuhan) Co., Ltd.; N-hydroxysuccinimide CAS is 6066-82-6, and the manufacturer is Shanghai MacLean Biochemical Technology Co., Ltd.

[0030] Embodiment 1: This embodiment provides an ultra-high thermal conductivity silicon nitride ceramic sheet, and the specific preparation method is as follows:

[0031] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 20 parts of dioctyl phthalate, 3.5 parts of yttrium oxide, 2.5 parts of magnesium oxide and 2 parts of β-Si3N4 seed crystals are ball-milled and degassed to make a slurry, and then tape-casted to obtain a green body;

[0032] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder and the temperature is raised to 500°C at a rate of 2°C / min, and the temperature is kept for 4 hours to perform binder removal to obtain a laminated green body;

[0033] Step 3: Sinter the laminated green body in a nitrogen atmosphere, raise the temperature to 1850°C at a heating rate of 5°C / min at a pressure of 10 MPa, and keep it for 45 hours to obtain an ultra-high thermal conductivity silicon nitride ceramic sheet.

[0034] Embodiment 2: This embodiment provides an ultra-high thermal conductivity silicon nitride ceramic sheet, and the specific preparation method is as follows:

[0035] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:2.5), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 10 parts of modified polysilazane, 20 parts of dioctyl phthalate, 5 parts of yttrium oxide, 3 parts of magnesium oxide and 2 parts of β-Si3N4 seed crystals are ball-milled, degassed to make a slurry and tape-cast to obtain a green body;

[0036] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 5°C / min to 900°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0037] Step 3: Sintering the laminated green body in a nitrogen atmosphere, heating the temperature to 1850°C at a rate of 5°C / min at a pressure of 10 MPa, and keeping the temperature for 45 hours to obtain an ultra-high thermal conductivity silicon nitride ceramic sheet;

[0038] Wherein, the preparation process of modified polysilazane is:

[0039] S1: Under a protective atmosphere, 30 parts of vinyl polysilazane were dissolved in 100 parts of xylene, 15 parts of thiosalicylic acid and 1 part of azobisisobutyronitrile were added and mixed, the temperature was raised to 60°C, and the reaction was carried out for 3 hours to obtain intermediate A;

[0040] S2: 10 parts of intermediate A, 6 parts of melamine, 0.5 parts of 1,3-dicyclohexylcarbodiimide, 0.5 parts of N-hydroxysuccinimide and 80 parts of ethylene glycol were mixed, the temperature was raised to 80° C., and the reaction was performed for 5 hours to obtain modified polysilazane.

[0041] Embodiment 3: This embodiment provides an ultra-high thermal conductivity silicon nitride ceramic sheet, and the specific preparation method is as follows:

[0042] Step 1: 70 parts of silicon powder, 75 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol ester, 20 parts of polyvinyl butyral, 10 parts of modified polysilazane, 20 parts of dioctyl phthalate, 6 parts of yttrium oxide, 5 parts of magnesium oxide and 2 parts of β-Si3N4 seed crystals are ball-milled and degassed to prepare slurry and tape-cast to obtain a green body;

[0043] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 2°C / min to 600°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0044] Step 3: Sintering the laminated green body in a nitrogen atmosphere, heating the temperature to 1850°C at a rate of 5°C / min at a pressure of 10 MPa, and keeping the temperature for 45 hours to obtain an ultra-high thermal conductivity silicon nitride ceramic sheet;

[0045] Wherein, the preparation process of modified polysilazane is:

[0046] S1: Under a protective atmosphere, 30 parts of vinyl polysilazane were dissolved in 100 parts of xylene, 15 parts of thiosalicylic acid and 1 part of azobisisobutyronitrile were added and mixed, the temperature was raised to 60°C, and the reaction was carried out for 3 hours to obtain intermediate A;

[0047] S2: 10 parts of intermediate A, 6 parts of melamine, 0.5 parts of 1,3-dicyclohexylcarbodiimide, 0.5 parts of N-hydroxysuccinimide and 80 parts of ethylene glycol were mixed, the temperature was raised to 80° C., and the reaction was performed for 5 hours to obtain modified polysilazane.

[0048] Comparative Example 1: Adding an excess of β-Si3N4 seed crystals, the rest is the same as Example 1, specifically as follows:

[0049] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 20 parts of dioctyl phthalate, 3.5 parts of yttrium oxide, 2.5 parts of magnesium oxide and 4 parts of β-Si3N4 seed crystals are ball-milled and degassed to make a slurry, and then tape-casted to obtain a green body;

[0050] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 5°C / min to 900°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0051] Step 3: Sinter the laminated green body in a nitrogen atmosphere, raise the temperature to 1850°C at a heating rate of 5°C / min at a pressure of 10 MPa, and keep the temperature for 45 hours.

[0052] Comparative Example 2: No β-Si3N4 seed crystals were added, and the rest was the same as Example 1, specifically as follows:

[0053] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 20 parts of dioctyl phthalate, 3.5 parts of yttrium oxide, and 2.5 parts of magnesium oxide are ball-milled and degassed to prepare a slurry, and then tape-casted to obtain a green body;

[0054] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 5°C / min to 900°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0055] Step 3: Sinter the laminated green body in a nitrogen atmosphere, raise the temperature to 1850°C at a heating rate of 5°C / min at a pressure of 10 MPa, and keep the temperature for 45 hours.

[0056] Comparative Example 3: Excessive magnesium oxide was added, and the rest was the same as in Example 1, specifically as follows:

[0057] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 20 parts of dioctyl phthalate, 3.5 parts of yttrium oxide, 8 parts of magnesium oxide and 2 parts of β-Si3N4 seed crystals are ball-milled and degassed to make a slurry, and then tape-casted to obtain a green body;

[0058] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 5°C / min to 900°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0059] Step 3: Sinter the laminated green body in a nitrogen atmosphere, raise the temperature to 1850°C at a heating rate of 5°C / min at a pressure of 10 MPa, and keep the temperature for 45 hours.

[0060] Comparative Example 4: No magnesium oxide was added, and the rest was the same as Example 1, specifically as follows:

[0061] Step 1: 65 parts of silicon powder, 65 parts of solvent isopropanol / ethyl acetate (the mass ratio of the two is 1:1), 1.5 parts of glycerol, 20 parts of polyvinyl butyral, 20 parts of dioctyl phthalate, 3.5 parts of yttrium oxide and 2 parts of β-Si3N4 seed crystals are ball-milled and degassed to make a slurry, and then tape-casted to obtain a green body;

[0062] Step 2: In a nitrogen atmosphere, the green body is stacked with boron nitride powder at a heating rate of 5°C / min to 900°C, and the temperature is kept for 24 hours to perform binder removal to obtain a laminated green body;

[0063] Step 3: Sinter the laminated green body in a nitrogen atmosphere, raise the temperature to 1850°C at a heating rate of 5°C / min at a pressure of 10 MPa, and keep the temperature for 45 hours.

[0064] Detection experiment:

[0065] (1) The silicon nitride ceramic sheets obtained in Example 1 and Comparative Examples 1-5 were tested for three-point bending strength using a universal testing machine, with a span of 30 mm and a pressing speed of 0.5 mm / min. The critical load when the sample broke was determined and the bending strength was calculated as follows:

[0066]

[0067] Among them, F represents the breaking load, that is, the maximum force that the material can withstand in the bending test; L represents the span, that is, the distance between the support points; b represents the width of the specimen; d represents the thickness of the specimen.

[0068] (2) The silicon nitride ceramic sheets obtained in Example 1 and Comparative Examples 1-5 were subjected to laser flash spectroscopy to measure the thermal diffusion coefficient and calculate the thermal conductivity in the following manner:

[0069] λ = ρ × Cp × α

[0070] Wherein, λ is the thermal conductivity; α is the thermal diffusivity; ρ is the density; Cp is the specific heat capacity.

[0071] The obtained data are shown in the following table:

[0072]

[0073] Table 1

[0074] Conclusion: In Examples 2-3, by controlling the addition amount of magnesium oxide and combining with the addition of seeds, while reducing the addition of extra oxygen content, the growth of grains was controlled, and the thermal conductivity and flexural strength of the obtained silicon nitride ceramic chips were improved. At the same time, the binder partially introduced modified polysilazane, which improved the quality of the silicon nitride ceramic chips while avoiding the generation of impurity phases during the high-temperature sintering process, meeting the actual application requirements.

[0075] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet, characterized in that: The following steps are involved: Step 1: ball milling and degassing the raw materials to make slurry and tape casting to obtain green body; Step 2: stacking the green body with boron nitride powder to remove binder, so as to obtain a laminated green body; Step 3: Sintering the laminated green body under a protective atmosphere to obtain an ultra-high thermal conductivity silicon nitride ceramic sheet; The raw materials of the ultra-high thermal conductivity silicon nitride ceramic sheet include the following components: by weight, 60-75 parts of silicon powder, 30-75 parts of solvent, 0.25-2.5 parts of dispersant, 3-30 parts of binder, 10-30 parts of plasticizer, 1.5-8 parts of yttrium oxide, 1.5-6 parts of magnesium oxide, and 1-3 parts of β-Si3N4 seed crystals; The binder is polyvinyl butyral and modified polysilazane in a mass ratio of 2-2.2:0.8-1; The preparation process of the modified polysilazane is: S1: Under a protective atmosphere, vinyl polysilazane is dissolved in xylene, thiosalicylic acid and azobisisobutyronitrile are added and mixed, the mixture is heated to 50-60°C, and the reaction is carried out for 3-4 hours to obtain intermediate A; S2: Mix the intermediate A, melamine, 1,3-dicyclohexylcarbodiimide, N-hydroxysuccinimide and ethylene glycol, raise the temperature to 70-80° C., and react for 4-5 hours to obtain modified polysilazane; The intermediate A comprises the following components: by weight, 20-30 parts of vinyl polysilazane, 100-120 parts of xylene, 15-20 parts of thiosalicylic acid, and 1-2 parts of azobisisobutyronitrile; the modified polysilazane comprises the following components: by weight, 10-12 parts of intermediate A, 5-8 parts of melamine, 0.5-1 part of 1,3-dicyclohexylcarbodiimide, 0.5-1 part of N-hydroxysuccinimide, and 60-80 parts of ethylene glycol.

2. The method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet according to claim 1, characterized in that: The debinding parameters are set as follows: heating to 500-900°C at a heating rate of 2-5°C / min and keeping the temperature for 4-24h; The sintering parameters are set as follows: gas pressure of 0.1-10 MPa, heating to 1650° C.-1900° C. at a heating rate of 1-5° C. / min, and heat preservation for 18-48 hours.

3. The method for preparing an ultra-high thermal conductivity silicon nitride ceramic sheet according to claim 1, characterized in that: The dispersant includes one or more of castor oil, fish oil, and glyceride; the plasticizer includes one or more of dioctyl phthalate and dibutyl phthalate; and the solvent includes one or two of isopropanol and ethyl acetate.

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