Silicon nitride substrate with ultrahigh thermal conductivity and preparation method thereof

Abstract

The invention provides a silicon nitride substrate with ultrahigh thermal conductivity and a preparation method thereof.The preparation method comprises the steps that superfine high-purity silicon nitride powder is adopted and sequentially subjected to isostatic cool pressing, first hot pressing sintering, cutting, second hot pressing sintering and slicing, and the silicon nitride substrate with the ultrahigh thermal conductivity is obtained. The density of the silicon nitride substrate is better and is closer to the theoretical density of silicon nitride, the heat-conducting property and mechanical property of the silicon nitride substrate are further ensured, and the heat conductivity of the silicon nitride substrate is larger than or equal to 100 W / (m.K).

Description

Technical Field

[0001] The present invention relates to the technical field of silicon nitride material preparation, and in particular to an ultra-high thermal conductivity silicon nitride substrate and a preparation method thereof. Background Art

[0002] Silicon nitride substrate is a high-performance semiconductor material with good thermal conductivity, electrical insulation, high temperature resistance and chemical stability. Therefore, it is widely used in heat dissipation devices in the fields of electronics, aerospace and automobiles. CN115703683 A discloses a high-strength, high-thermal-conductivity, large-size silicon nitride ceramic and its preparation method, using submicron low-oxygen silicon nitride powder as raw material, adding oxygen-free nitride and fluoride as composite sintering aids, mixing powder, cold isostatic pressing or vibration pressing, hot pressing sintering, cooling annealing, to obtain a silicon nitride ceramic block blank with a bending strength of more than 800MPa, low oxygen content, less grain boundary glass phase, and thermal conductivity of more than 80W / (m·K).

[0003] However, the existing methods for preparing silicon nitride substrates have the following main problems: first, the preparation yield is low, less than 36% on average, mainly because the silicon nitride sheets are prone to cracks and warping during the sintering process; second, the thermal conductivity of the obtained silicon nitride substrate is affected by its impurities, process and oxygen content. The current average thermal conductivity in the industry is about 90W / (m·K), and the thermal conductivity and mechanical properties of the silicon nitride substrate are poor.

[0004] Therefore, how to improve the yield rate, thermal conductivity and mechanical properties while ensuring the quality of silicon nitride is an important issue facing the current silicon nitride substrate preparation technology. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides an ultra-high thermal conductivity silicon nitride substrate and a preparation method thereof. The preparation method adopts ultra-fine high-purity silicon nitride powder, and sequentially undergoes cold isostatic pressing, first hot pressing sintering, cutting, second hot pressing sintering, and slicing to obtain an ultra-high thermal conductivity silicon nitride substrate. By adopting the method of "first sintering as a whole and then cutting into slices", the density of the silicon nitride substrate is better and closer to the theoretical density of silicon nitride, thereby ensuring the thermal conductivity and mechanical properties of the silicon nitride substrate. The thermal conductivity of the silicon nitride substrate is ≥100W / (m·K).

[0006] To achieve this object, the present invention adopts the following technical solutions:

[0007] One of the purposes of the present invention is to provide a method for preparing an ultra-high thermal conductivity silicon nitride substrate, the preparation method comprising the following steps:

[0008] Step S1: preparing ultrafine high-purity silicon nitride powder, and obtaining silicon nitride bricks by cold isostatic pressing;

[0009] Step S2: performing a first hot pressing and sintering on the silicon nitride brick to obtain a first silicon nitride blank;

[0010] Step S3: cutting the first silicon nitride blank and trimming the whole to a specified size;

[0011] Step S4: performing a second hot pressing sintering on the cut first silicon nitride blank to obtain a second silicon nitride blank;

[0012] Step S5: Slicing the second silicon nitride blank to obtain an ultra-high thermal conductivity silicon nitride substrate.

[0013] The preparation method of the present invention adopts ultrafine high-purity silicon nitride powder, and sequentially undergoes cold isostatic pressing, first hot pressing sintering, cutting, second hot pressing sintering, and slicing to obtain an ultra-high thermal conductivity silicon nitride substrate. By adopting the method of "first sintering as a whole and then cutting into slices", the density of the silicon nitride substrate is better and closer to the theoretical density of silicon nitride, thereby ensuring the thermal conductivity and mechanical properties of the silicon nitride substrate. The thermal conductivity of the silicon nitride substrate is ≥100W / (m·K).

[0014] It should be noted that, in the preparation method of the present invention, step S3 is to cut the first silicon nitride blank and trim it as a whole to a specified size for repairing and leveling its six surfaces.

[0015] As a preferred technical solution of the present invention, in step S1, the method for preparing the ultrafine high-purity silicon nitride powder comprises:

[0016] High-purity silicon nitride powder with a purity of ≥99.9995% is mixed with a solvent and ball-milled to obtain a suspension with a particle size D50 less than 500nm; the suspension is evenly mixed with ultrapure water to form a slurry, and spray-dried to obtain ultrafine high-purity silicon nitride powder.

[0017] As a preferred technical solution of the present invention, the solvent includes any one of ethanol, n-butanol, acetone or butanone, or a combination of at least two of them. Typical but non-limiting examples of the combination include: a combination of ethanol and n-butanol, a combination of ethanol and acetone, or a combination of acetone and butanone, etc.

[0018] Preferably, the mass ratio of the high-purity silicon nitride powder with a purity of ≥99.9995% to the solvent is 1:(1-3), such as 1:1, 1:1.5, 1:2, 1:2.5 or 1:3.

[0019] Preferably, the grinding medium used in the ball mill is silicon nitride grinding ball, and the mass ratio of the grinding medium to the high-purity silicon nitride powder with a purity of ≥99.9995% is 4:(1-3), such as 4:1, 4:1.5, 4:2, 4:2.5 or 4:3.

[0020] Preferably, the ball milling time is 5-12 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h.

[0021] As a preferred technical solution of the present invention, the volume ratio of the suspension to ultrapure water is 1:(0.8-1.2), for example, 1:0.8, 1:0.9, 1:1.0, 1:1.1 or 1:1.2.

[0022] Preferably, the spray drying slurry injection speed is 10-50 L / min, such as 10 L / min, 15 L / min, 20 L / min, 25 L / min, 30 L / min, 35 L / min, 40 L / min, 45 L / min or 50 L / min, etc., the hot air temperature is 220-330 ° C, such as 220 ° C, 240 ° C, 260 ° C, 280 ° C, 300 ° C, 310 ° C or 330 ° C, etc., the hot air speed is 0.8-1.8 m / s, such as 0.8 m / s, 1.0 m / s, 1.2 m / s, 1.4 m / s, 1.6 m / s or 1.8 m / s, etc., and the maximum air volume of the fan is 6500-7500 m 3 / h, for example 6500m 3 / h、6700m 3 / h、6900m 3 / h、7000m 3 / h、7100m 3 / h、7300m 3 / h or 7500m 3 / h, etc.

[0023] As a preferred technical solution of the present invention, in step S1, the cold isostatic pressing pressure is 180-280 MPa, for example, 180 MPa, 200 MPa, 220 MPa, 240 MPa, 260 MPa or 280 MPa, and the holding time is 20-50 min, for example, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min or 50 min, etc.

[0024] As a preferred technical solution of the present invention, in step S2, the atmosphere in the furnace of the first hot pressing sintering is nitrogen and / or ammonia.

[0025] It should be noted that the preparation method of the present invention uses ultrafine high-purity silicon nitride powder as raw material, and does not use other sintering aids, thereby reducing the oxygen content in the raw material as much as possible and ensuring the purity of the silicon nitride substrate; during the hot pressing sintering process, if the atmosphere in the furnace contains ammonia, the ammonia will decompose into nitrogen and hydrogen, and the reducing effect of hydrogen can reduce the oxygen content of the silicon nitride substrate, thereby improving the thermal conductivity and mechanical properties of the silicon nitride substrate; if the atmosphere in the furnace is a combination gas of nitrogen and ammonia, the volume ratio of the two is preferably 3:1.

[0026] Preferably, in step S2, the pressure of the first hot pressing sintering is 8-10 MPa, for example, 8 MPa, 8.3 MPa, 8.5 MPa, 8.8 MPa, 9 MPa, 9.3 MPa, 9.5 MPa, 9.8 MPa or 10 MPa.

[0027] Preferably, in step S2, the target temperature of the first hot pressing sintering is 1450-1550°C, for example, 1450°C, 1470°C, 1480°C, 1500°C, 1510°C, 1530°C or 1550°C.

[0028] As a preferred technical solution of the present invention, in step S2, the first hot pressing sintering adopts a step-by-step heating, including: starting from room temperature, the heating rate is 6-12°C / min, for example, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min or 12°C / min, etc., rising to 750-850°C, for example, 750°C, 760°C, 780°C, 800°C, 810°C, 830°C or 850°C, etc., and keeping warm for 1-3h, for example, 1h, 1.5h, 2h, 2.5h or 3h, etc., the heating rate is 4-8°C / min, for example, 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min, 6°C / min, 6.5°C / min, 7°C / min, 7.5°C / min or 8°C / min, etc., rising to 1 150-1250°C, for example, 1150°C, 1170°C, 1180°C, 1200°C, 1210°C, 1230°C or 1250°C, etc., keep warm for 2-4h, for example, 2h, 2.5h, 3h, 3.5h or 4h, etc., the heating rate is 2-6°C / min, for example, 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4. The temperature is raised to 1450-1550°C, such as 1450°C, 1470°C, 1480°C, 1500°C, 1510°C, 1530°C or 1550°C, etc., and kept for 4-8h, such as 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h or 8h, and then cooled with the furnace.

[0029] As a preferred technical solution of the present invention, in step S4, the atmosphere in the furnace for the second hot pressing sintering is nitrogen and / or ammonia.

[0030] It should be noted that the preparation method described in the present invention uses ultrafine high-purity silicon nitride powder as raw material, and does not use other sintering aids, thereby reducing the oxygen content in the raw material as much as possible and ensuring the purity of the silicon nitride substrate; during the hot pressing sintering process, if the atmosphere in the furnace contains ammonia, the ammonia will decompose into nitrogen and hydrogen. The reducing effect of hydrogen can reduce the oxygen content of the silicon nitride substrate, thereby improving the thermal conductivity and mechanical properties of the silicon nitride substrate.

[0031] Preferably, in step S4, the pressure of the second hot pressing sintering is 10-15 MPa, for example, 10 MPa, 11 MPa, 12 MPa, 13 MPa, 14 MPa or 15 MPa.

[0032] Preferably, in step S4, the target temperature of the second hot pressing sintering is 1800-1900°C, for example, 1800°C, 1810°C, 1820°C, 1830°C, 1840°C, 1850°C, 1860°C, 1870°C, 1880°C, 1890°C or 1900°C.

[0033] As a preferred technical solution of the present invention, in step S4, the second hot pressing sintering adopts a step-by-step heating, including: starting from room temperature, the heating rate is 6-12°C / min, such as 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min or 12°C / min, etc., rising to 1150-1250°C, such as 1150°C, 1170°C, 1180°C, 1200°C, 1210°C, 1230°C or 1250°C, etc., keeping warm for 1-3h, such as 1h, 1.5h, 2h, 2.5h or 3h, etc., the heating rate is 3-6°C / min, such as 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min or 6°C / min, etc., rising to 1450-1 550℃, such as 1450℃, 1470℃, 1480℃, 1500℃, 1510℃, 1530℃ or 1550℃, etc., keep warm for 2-4h, such as 2h, 2.5h, 3h, 3.5h or 4h, etc., the heating rate is 1-4℃ / min, such as 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min in or 4°C / min, etc., raised to 1800-1900°C, such as 1800°C, 1810°C, 1820°C, 1830°C, 1840°C, 1850°C, 1860°C, 1870°C, 1880°C, 1890°C or 1900°C, etc., and kept warm for 24-48h, such as 24h, 28h, 32h, 36h, 40h, 44h or 48h, etc., and then cooled along with the furnace.

[0034] The second object of the present invention is to provide an ultra-high thermal conductivity silicon nitride substrate, which is prepared by the preparation method described in the first object; preferably, the thickness of the silicon nitride sheet obtained by slicing is 0.35-0.4 mm, and then it is polished to obtain an ultra-high thermal conductivity silicon nitride substrate.

[0035] Compared with the prior art solutions, the present invention has at least the following beneficial effects:

[0036] (1) The preparation method of the present invention uses ultrafine high-purity silicon nitride powder, and sequentially undergoes cold isostatic pressing, first hot pressing sintering, cutting, second hot pressing sintering, and slicing to obtain an ultra-high thermal conductivity silicon nitride substrate. By adopting the method of "first sintering as a whole and then cutting into slices", the density of the silicon nitride substrate is better and closer to the theoretical density of silicon nitride, thereby ensuring the thermal conductivity and mechanical properties of the silicon nitride substrate. The thermal conductivity of the silicon nitride substrate is ≥100W / (m·K);

[0037] (2) The preparation method of the present invention uses ultrafine high-purity silicon nitride powder as raw material and does not use other sintering aids, thereby reducing the oxygen content in the raw material as much as possible and ensuring the purity of the silicon nitride substrate. During the hot pressing sintering process, if the atmosphere in the furnace contains ammonia, the ammonia will decompose into nitrogen and hydrogen. The reducing effect of hydrogen can reduce the oxygen content of the silicon nitride substrate, thereby improving the thermal conductivity and mechanical properties of the silicon nitride substrate. DETAILED DESCRIPTION

[0038] For the convenience of understanding the present invention, the present invention lists the following embodiments. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0039] The material models of the embodiments of the present invention are: silicon nitride powder, particle size 2-5 μm, purity 99.9995%; ethanol, n-butanol, acetone, butanone, analytical grade, domestic chemical plant; spray drying equipment, produced in Changzhou; sintering equipment, produced in Hunan.

[0040] Example 1

[0041] This embodiment provides a method for preparing an ultra-high thermal conductivity silicon nitride substrate, the preparation method comprising the following steps:

[0042] Step S1: high-purity silicon nitride powder (purity ≥ 99.9995%) is placed in a ball mill, ethanol is added at a solid-liquid mass ratio of 1:1.8, silicon nitride grinding balls are added at a mass ratio of grinding medium to high-purity silicon nitride powder of 4:1, and ball milling is performed at a ball mill speed of 300r / min and a ball milling time of 5h to obtain a suspension with a particle size D50 less than 500nm; the suspension is uniformly mixed with ultrapure water at a volume ratio of 1:1 to form a slurry, and spray drying is performed using a spray drying device, and the slurry injection speed is controlled to be 15L / min, the hot air temperature is 230°C, the hot air speed is 0.8m / s, and the maximum air volume of the fan is 7000m 3 / h, to obtain ultrafine high-purity silicon nitride powder;

[0043] The ultrafine high-purity silicon nitride powder is loaded into the mold and cold isostatically pressed using a cold isostatic pressing device. The cold isostatic pressing pressure is controlled to be 180 MPa and the pressure is maintained for 30 minutes to form silicon nitride bricks.

[0044] Step S2: placing the silicon nitride brick in a hot pressing sintering furnace for the first hot pressing sintering, the atmosphere in the furnace is nitrogen, and the pressure is 8MPa; the first hot pressing sintering adopts step-by-step heating, starting from room temperature, the heating rate is 11°C / min, rising to 800°C, and keeping at 800°C for 1h; the heating rate is 8°C / min, rising to 1200°C, keeping at 800°C for 2h; the heating rate is 4°C / min, rising to 1500°C, and keeping at 4h; then cooling with the furnace to obtain the first silicon nitride blank;

[0045] Step S3: placing the first silicon nitride blank on a cutting platform, repairing and leveling the six surfaces thereof, and trimming the entire blank to a specified size; the cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.45 μm;

[0046] Step S4: placing the repaired and leveled first silicon nitride blank in a hot pressing sintering furnace for a second hot pressing sintering, the atmosphere in the furnace is nitrogen, and the pressure is 10 MPa; the second hot pressing sintering adopts a step-by-step heating, starting from room temperature, the heating rate is 8°C / min, rising to 1200°C, and keeping warm for 1 hour; the heating rate is 6°C / min, rising to 1500°C, and keeping warm for 2 hours; the heating rate is 4°C / min, rising to 1850°C, and keeping warm for 24 hours; then cooling with the furnace to obtain a second silicon nitride blank;

[0047] Step S5: Take out the sintered second silicon nitride blank, and cut the silicon nitride block in the wide direction by wire cutting equipment. The cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.15 μm. Cut it into silicon nitride sheets with a thickness of 0.37 mm to obtain an ultra-high thermal conductivity silicon nitride substrate.

[0048] Example 2

[0049] This embodiment provides a method for preparing an ultra-high thermal conductivity silicon nitride substrate, the preparation method comprising the following steps:

[0050] Step S1: high-purity silicon nitride powder (purity ≥ 99.9995%) is placed in a ball mill, ethanol is added at a solid-liquid mass ratio of 1:2, silicon nitride grinding balls are added at a mass ratio of grinding medium to high-purity silicon nitride powder of 2:1, and ball milling is performed at a ball mill speed of 300 r / min and a ball milling time of 7 h to obtain a suspension with a particle size D50 less than 500 nm; the suspension is uniformly mixed with ultrapure water at a volume ratio of 1:1 to form a slurry, and spray drying is performed using a spray drying device, and the slurry injection speed is controlled to be 25 L / min, the hot air temperature is 270°C, the hot air speed is 1.2 m / s, and the maximum air volume of the fan is 7000 m 3 / h, to obtain ultrafine high-purity silicon nitride powder;

[0051] The ultrafine high-purity silicon nitride powder is loaded into the mold and cold isostatically pressed using a cold isostatic pressing device. The cold isostatic pressing pressure is controlled to 220 MPa and maintained for 30 minutes to form silicon nitride bricks.

[0052] Step S2: placing the silicon nitride brick in a hot pressing sintering furnace for the first hot pressing sintering, the atmosphere in the furnace is ammonia and the pressure is 8MPa; the first hot pressing sintering adopts step-by-step heating, starting from room temperature, the heating rate is 10℃ / min, rising to 800℃, and keeping at 800℃ for 2h; the heating rate is 8℃ / min, rising to 1200℃, keeping at 800℃ for 3h; the heating rate is 3℃ / min, rising to 1500℃, keeping at 6h; then cooling with the furnace to obtain the first silicon nitride blank;

[0053] Step S3: placing the first silicon nitride blank on a cutting platform, repairing and leveling the six surfaces thereof, and trimming the entire blank to a specified size; the cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.45 μm;

[0054] Step S4: placing the repaired and leveled first silicon nitride blank in a hot pressing sintering furnace for a second hot pressing sintering, the atmosphere in the furnace is nitrogen, and the pressure is 12MPa; the second hot pressing sintering adopts a step-by-step heating, starting from room temperature, the heating rate is 8°C / min, rising to 1200°C, and keeping warm for 1h; the heating rate is 4°C / min, rising to 1500°C, and keeping warm for 2h; the heating rate is 2°C / min, rising to 1850°C, and keeping warm for 36h; then cooling with the furnace to obtain a second silicon nitride blank;

[0055] Step S5: Take out the sintered second silicon nitride blank, and cut the silicon nitride block in the wide direction by wire cutting equipment. The cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.15 μm; cut into 0.37 mm thick silicon nitride sheets to obtain ultra-high thermal conductivity silicon nitride substrates.

[0056] Example 3

[0057] This embodiment provides a method for preparing an ultra-high thermal conductivity silicon nitride substrate, the preparation method comprising the following steps:

[0058] Step S1: high-purity silicon nitride powder (purity ≥ 99.9995%) is placed in a ball mill, ethanol is added at a solid-liquid mass ratio of 1:2.3, silicon nitride grinding balls are added at a mass ratio of grinding medium to high-purity silicon nitride powder of 2:1, and ball milling is performed at a ball mill speed of 300 r / min and a ball milling time of 9 h to obtain a suspension with a particle size D50 less than 500 nm; the suspension is uniformly mixed with ultrapure water at a volume ratio of 1:1 to form a slurry, and spray drying is performed using a spray drying device, and the slurry injection speed is controlled to be 40 L / min, the hot air temperature is 300°C, the hot air speed is 1.5 m / s, and the maximum air volume of the fan is 7000 m 3 / h, to obtain ultrafine high-purity silicon nitride powder;

[0059] The ultrafine high-purity silicon nitride powder is loaded into the mold and cold isostatically pressed using a cold isostatic pressing device. The cold isostatic pressing pressure is controlled to 270 MPa and maintained for 30 minutes to form silicon nitride bricks.

[0060] Step S2: placing the silicon nitride brick in a hot pressing sintering furnace for the first hot pressing sintering, the atmosphere in the furnace is ammonia and the pressure is 10MPa; the first hot pressing sintering adopts step-by-step heating, starting from room temperature, the heating rate is 8°C / min, rising to 800°C, and keeping at 800°C for 2h; the heating rate is 6°C / min, rising to 1200°C, keeping at 3h; the heating rate is 2°C / min, rising to 1500°C, and keeping at 10h; then cooling with the furnace to obtain the first silicon nitride blank;

[0061] Step S3: placing the first silicon nitride blank on a cutting platform, repairing and leveling the six surfaces thereof, and trimming the entire blank to a specified size; the cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.45 μm;

[0062] Step S4: placing the repaired and leveled first silicon nitride blank in a hot pressing sintering furnace for a second hot pressing sintering, wherein the atmosphere in the furnace is a combination of nitrogen and ammonia gas, with a mass ratio of 3:1 and a pressure of 15 MPa; the second hot pressing sintering adopts a step-by-step heating, starting from room temperature, the heating rate is 6°C / min, rising to 1200°C, and keeping warm for 1 hour; the heating rate is 3°C / min, rising to 1500°C, and keeping warm for 2 hours; the heating rate is 1.5°C / min, rising to 1850°C, and keeping warm for 48 hours; then cooling with the furnace to obtain a second silicon nitride blank;

[0063] Step S5: Take out the sintered second silicon nitride blank, and cut the silicon nitride block in the wide direction by wire cutting equipment. The cutting wire is a tungsten-based diamond-plated diamond wire with a wire diameter of 0.15 μm. Cut it into silicon nitride sheets with a thickness of 0.37 mm to obtain an ultra-high thermal conductivity silicon nitride substrate.

[0064] Example 4

[0065] This embodiment provides a method for preparing a silicon nitride substrate with ultra-high thermal conductivity. Compared with Embodiment 1, the only difference is that the atmosphere in the furnace of the first hot pressing sintering in step S2 is replaced with ammonia.

[0066] Example 5

[0067] This embodiment provides a method for preparing a silicon nitride substrate with ultra-high thermal conductivity. Compared with Embodiment 1, the only difference is that the target temperature of the first hot pressing sintering in step S2 is reduced to 1400°C.

[0068] Example 6

[0069] This embodiment provides a method for preparing a silicon nitride substrate with ultra-high thermal conductivity. Compared with Embodiment 1, the only difference is that the target temperature of the first hot pressing sintering in step S2 is increased to 1600°C.

[0070] Example 7

[0071] This embodiment provides a method for preparing an ultra-high thermal conductivity silicon nitride substrate. Compared with Embodiment 1, the only difference is that the target temperature of the second hot pressing sintering in step S4 is reduced to 1750°C.

[0072] Example 8

[0073] This embodiment provides a method for preparing a silicon nitride substrate with ultra-high thermal conductivity. Compared with Embodiment 1, the only difference is that the target temperature of the second hot pressing sintering in step S4 is increased to 1950°C.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing a silicon nitride substrate. Compared with Example 1, the only difference is that: Step S3 and Step S4 are omitted, that is, the first hot pressing sintering is performed by stepwise heating to 1850° C.;

[0076] The adjusted step S2' is specifically as follows: placing the silicon nitride brick in a hot pressing sintering furnace for the first hot pressing sintering, the atmosphere in the furnace is nitrogen, and the pressure is 8MPa; the first hot pressing sintering adopts a step-by-step temperature increase, starting from room temperature, the heating rate is 11°C / min, rising to 800°C, and keeping at 800°C for 1h; the heating rate is 8°C / min, rising to 1200°C, and keeping at 800°C for 2h; the heating rate is 4°C / min, rising to 1500°C, and keeping at 1500°C for 4h; the heating rate is 4°C / min, rising to 1850°C, and keeping at 1850°C for 24h; then cooling with the furnace to obtain the first silicon nitride blank;

[0077] Adjusted step S3': take out the first silicon nitride blank after sintering, place it on a cutting platform for cutting, repair and level its six surfaces, and trim the whole to a specified size. The cutting wire adopts a tungsten-based diamond-plated diamond wire with a wire diameter of 0.45 μm; then, use a wire cutting device to cut the silicon nitride block in the direction of its wide surface. The cutting wire adopts a tungsten-based diamond-plated diamond wire with a wire diameter of 0.15 μm; cut it into a silicon nitride sheet with a thickness of 0.37 mm, and obtain an ultra-high thermal conductivity silicon nitride substrate.

[0078] The silicon nitride substrates prepared in the above-mentioned embodiment and comparative example were tested for thermal conductivity, bending strength and fracture toughness, respectively. The specific test results are shown in Table 1.

[0079] Table 1

[0080]

[0081]

[0082] From Table 1, we can see the following points:

[0083] (1) The preparation method of the present invention uses ultrafine high-purity silicon nitride powder, and sequentially undergoes cold isostatic pressing, first hot pressing sintering, cutting, second hot pressing sintering, and slicing to obtain an ultra-high thermal conductivity silicon nitride substrate. By adopting the method of "first sintering as a whole and then cutting into slices", the density of the silicon nitride substrate is better and closer to the theoretical density of silicon nitride, thereby ensuring the thermal conductivity and mechanical properties of the silicon nitride substrate. The thermal conductivity of the silicon nitride substrate is ≥100 W / (m·K), and the yield rate is above 70%, preferably above 75%;

[0084] (2) Comparing Example 1 with Example 4, since Example 4 replaces the furnace atmosphere of the first hot pressing sintering in step S2 with ammonia, the ammonia will decompose into nitrogen and hydrogen. The reducing effect of hydrogen can reduce the oxygen content of the silicon nitride substrate, thereby improving the thermal conductivity and mechanical properties of the silicon nitride substrate.

[0085] (3) Comparing Example 1 with Examples 5 and 6, since Example 5 lowers the target temperature of the first hot pressing sintering in step S2 to 1400°C, the density of the first silicon nitride blank does not meet the target requirement, resulting in a decrease in the thermal conductivity and mechanical properties of the silicon nitride substrate, and the yield rate also drops to 71%; although Example 6 raises the target temperature of the first hot pressing sintering in step S2 to 1600°C, the thermal conductivity and mechanical properties of the silicon nitride substrate do not change much, the yield rate does not increase, and the energy consumption is increased instead;

[0086] (4) Comparing Example 1 with Examples 7 and 8, since Example 7 lowers the target temperature of the second hot pressing sintering in step S4 to 1750°C, the density of the silicon nitride substrate is reduced, resulting in a decrease in the thermal conductivity and mechanical properties of the silicon nitride substrate, and the yield rate is also reduced to 72%; although Example 8 raises the target temperature of the first hot pressing sintering in step S2 to 1950°C, the thermal conductivity and mechanical properties of the silicon nitride substrate do not change much, and the yield rate is only increased to 76%, which will increase energy consumption;

[0087] (5) Comparing Example 1 with Comparative Example 1, since Step S3 and Step S4 are omitted in Comparative Example 1, that is, the first hot pressing sintering is performed by stepwise heating to 1850° C., on the one hand, the surface of the first silicon nitride blank to be cut increases, resulting in waste; on the other hand, the density of the first silicon nitride blank is uneven, and the thermal conductivity and mechanical properties of the silicon nitride substrate cut into thin sheets are significantly reduced, and the yield rate is reduced to 53%.

[0088] The present invention illustrates the detailed process equipment and process flow of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed process equipment and process flow, that is, it does not mean that the present invention must rely on the above-mentioned detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing an ultra-high thermal conductivity silicon nitride substrate, characterized in that: The preparation method comprises the following steps: Step S1: preparing ultrafine high-purity silicon nitride powder, and obtaining silicon nitride bricks by cold isostatic pressing; Step S2: performing a first hot pressing and sintering on the silicon nitride brick to obtain a first silicon nitride blank; Step S3: cutting the first silicon nitride blank and trimming the whole to a specified size; Step S4: performing a second hot pressing sintering on the cut first silicon nitride blank to obtain a second silicon nitride blank; Step S5: Slicing the second silicon nitride blank to obtain an ultra-high thermal conductivity silicon nitride substrate.

2. The preparation method according to claim 1, characterized in that: In step S1, the method for preparing the ultrafine high-purity silicon nitride powder comprises: High-purity silicon nitride powder with a purity of ≥99.9995% is mixed with a solvent and ball-milled to obtain a suspension with a particle size D50 less than 500nm; the suspension is evenly mixed with ultrapure water to form a slurry, and spray-dried to obtain ultrafine high-purity silicon nitride powder.

3. The preparation method according to claim 2, characterized in that: The solvent includes any one of ethanol, n-butanol, acetone or butanone, or a combination of at least two thereof; Preferably, the mass ratio of the high-purity silicon nitride powder with a purity of ≥99.9995% to the solvent is 1:(1-3); Preferably, the grinding medium used in the ball mill is silicon nitride grinding ball, and the mass ratio of the grinding medium to the high-purity silicon nitride powder with a purity of ≥99.9995% is 4:(1-3); Preferably, the ball milling time is 5-12 hours.

4. The preparation method according to claim 2 or 3, characterized in that: The volume ratio of the suspension to ultrapure water is 1:(0.8-1.2); Preferably, the spray drying slurry injection speed is 10-50 L / min, the hot air temperature is 220-330°C, the hot air speed is 0.8-1.8 m / s, and the maximum air volume of the fan is 6500-7500 m 3 / h.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step S1, the cold isostatic pressing pressure is 180-280 MPa, and the holding time is 20-50 min.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step S2, the atmosphere in the furnace for the first hot pressing sintering is nitrogen and / or ammonia; Preferably, in step S2, the pressure of the first hot pressing sintering is 8-10 MPa; Preferably, in step S2, the target temperature of the first hot pressing sintering is 1450-1550°C.

7. The preparation method according to claim 6, characterized in that: In step S2, the first hot pressing sintering adopts a step-by-step temperature increase, including: starting from room temperature, the temperature is increased at a rate of 6-12°C / min to 750-850°C, and kept warm for 1-3h; the temperature is increased at a rate of 4-8°C / min to 1150-1250°C, and kept warm for 2-4h; the temperature is increased at a rate of 2-6°C / min to 1450-1550°C, and kept warm for 4-8h; and then the temperature is reduced along with the furnace.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step S4, the atmosphere in the furnace for the second hot pressing sintering is nitrogen and / or ammonia; Preferably, in step S4, the pressure of the second hot pressing sintering is 10-15 MPa; Preferably, in step S4, the target temperature of the second hot pressing sintering is 1800-1900°C.

9. The preparation method according to claim 8, characterized in that: In step S4, the second hot pressing sintering adopts a step-by-step heating method, including: starting from room temperature, heating at a rate of 6-12°C / min, heating to 1150-1250°C, and keeping warm for 1-3h; heating at a rate of 3-6°C / min, heating to 1450-1550°C, and keeping warm for 2-4h; heating at a rate of 1-4°C / min, heating to 1800-1900°C, and keeping warm for 24-48h; and then cooling down with the furnace.

10. An ultra-high thermal conductivity silicon nitride substrate, characterized in that: The preparation method is described in any one of claims 1 to 9.

Images