High-purity low-density boron nitride crucible, preparation method and application thereof

Abstract

This application provides a high-purity, low-density boron nitride crucible, its preparation method, and its application, relating to the field of crucible technology. The boron nitride crucible preparation method of this application includes the following steps: pulverizing and drying boron nitride raw materials to obtain boron nitride powder; mixing boron nitride powder, boric acid powder, and melamine powder evenly, baking at 300℃-400℃ for 4-8 hours, and then isostatically pressing into a blank; under flowing nitrogen protection, heating the blank to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, and then further heating at a heating rate of 0.2℃ / min-0.5℃ / min to... The temperature is initially set at 550℃-600℃, then increased to 1150℃-1250℃ at a rate of 3℃ / min-5℃ / min, followed by a further increase to 1850℃-1900℃ at a rate of 5℃ / min-10℃ / min. The temperature is held for 1-3 hours and then cooled. After processing into plates and frames of different sizes, the temperature is further increased to 1950℃-2000℃ under flowing nitrogen protection, held for 20-50 hours, cooled, and assembled into boron nitride crucibles. The boron nitride crucibles obtained in this application have high purity and low density, and can be used to prepare silicon nitride substrates; the preparation cost is low.

Description

Technical Field

[0001] This application relates to the field of crucible technology, and in particular to a high-purity, low-density boron nitride crucible, its preparation method, and its application. Background Technology

[0002] In recent years, with the booming development of the electric vehicle industry, the demand for high-voltage IGBT (Insulated Gate Bipolar Transistor) power chips has increased significantly. The academic and industrial communities recognize that silicon nitride ceramic substrates for automotive power chip packaging are the best choice. Although the thermal conductivity of silicon nitride substrates is lower than that of aluminum nitride ceramic substrates, silicon nitride has high strength, can be made thinner, and after copper plating, it has high bonding strength with the copper layer and long thermal cycle life; its overall performance is far superior to that of aluminum nitride substrates.

[0003] In the sintering process of silicon nitride substrates, crucibles made of high-purity (extremely low oxygen content) low-density boron nitride ceramics are typically required. However, boron nitride ceramics are difficult to sinter and generally require hot pressing. High-purity boron nitride is even more difficult to sinter due to its low oxygen content; at the same time, large-size hot-pressed products place higher demands on the hot-pressing molds, resulting in high processing costs for high-purity low-density boron nitride products. Therefore, the low-cost preparation of high-purity low-density boron nitride crucibles is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this application is to provide a high-purity, low-density boron nitride crucible, its preparation method, and its application, so as to improve the purity of the boron nitride crucible, reduce its density, and eliminate cracks; and reduce the preparation cost. The specific technical solution is as follows:

[0005] The first aspect of this application provides a method for preparing a boron nitride crucible, comprising the steps of:

[0006] S1: The boron nitride raw material is pulverized and dried to obtain boron nitride powder;

[0007] S2: Mix the boron nitride powder, boric acid powder and melamine powder evenly, bake at 300℃-400℃ for 4h-8h, and then isostatically press to form a blank.

[0008] S3: Under the protection of flowing nitrogen, the billet is heated to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heated to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heated to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heated to 1850℃-1900℃ at a heating rate of 5℃ / min-10℃ / min, held at this temperature for 1h-3h, and then cooled.

[0009] S4: After processing the cooled blank in S3 into plates and frames of different sizes, under the protection of flowing nitrogen, heat it to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heat it to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heat it to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heat it to 1950℃-2000℃ at a heating rate of 10℃ / min-15℃ / min. Hold it at this temperature for 20h-50h. During the holding period, vacuum is drawn and nitrogen is introduced multiple times. Then cool it to room temperature.

[0010] S5: Assemble the cooled plates and frames from S4 into a crucible to obtain the boron nitride crucible.

[0011] In some embodiments of this application, in S1, the particle size of the boron nitride powder is 33μm-125μm; when the boron nitride powder is washed with water, the conductivity of the filtered water is less than 20μS / cm.

[0012] In some embodiments of this application, in S1, the method for preparing the boron nitride powder includes: [the following steps are described in the original text, but the translation is incomplete and requires further context.] 3 -2.05g / cm 3 The boron nitride raw material was pulverized and sieved to collect powder with a particle size of 33μm-125μm; then it was soaked in hydrochloric acid to remove metal impurities, and then washed with water multiple times until the conductivity of the filtered water was less than 20μS / cm; then it was dried to obtain the boron nitride powder.

[0013] In some embodiments of this application, in S2, the mass ratio of the boron nitride powder, the boric acid powder, and the melamine powder is (12-76):10:(4-6).

[0014] In some embodiments of this application, in S2, the mixing is a dry mixing.

[0015] In some embodiments of this application, in step S2, the blank is a cubic or cuboid blank with an edge length of 150mm-600mm, and the molding density of the blank is 1.80g / cm³. 3 -1.90g / cm 3 .

[0016] In some embodiments of this application, in step S3, the density of the cooled billet is 1.55 g / cm³. 3 -1.65g / cm 3 .

[0017] In some embodiments of this application, in step S5, the oxygen content of the boron nitride crucible is less than or equal to 0.1%.

[0018] The second aspect of this application provides a boron nitride crucible prepared according to the method described in the first aspect of this application.

[0019] The third aspect of this application provides the use of the boron nitride crucible described in the second aspect of this application in the preparation of silicon nitride substrates.

[0020] The beneficial effects of this application are as follows: This application provides a method for preparing a boron nitride crucible, which involves mixing boron nitride powder, boric acid powder and melamine powder, followed by baking and high-temperature sintering. The resulting boron nitride crucible has high purity (oxygen content less than or equal to 0.1%) and low density, and can be used to prepare silicon nitride substrates. The preparation method is simple and has low production cost.

[0021] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0022] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0023] The first aspect of this application provides a method for preparing a boron nitride crucible, comprising the steps of:

[0024] S1: The boron nitride raw material is pulverized and dried to obtain boron nitride powder;

[0025] S2: Mix the boron nitride powder, boric acid powder and melamine powder evenly, bake at 300℃-400℃ for 4h-8h, and then isostatically press to form a blank.

[0026] S3: Under the protection of flowing nitrogen, the billet is heated to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heated to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heated to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heated to 1850℃-1900℃ at a heating rate of 5℃ / min-10℃ / min, held at this temperature for 1h-3h, and then cooled.

[0027] S4: After processing the cooled blank in S3 into plates and frames of different sizes, under the protection of flowing nitrogen, heat it to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heat it to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heat it to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heat it to 1950℃-2000℃ at a heating rate of 10℃ / min-15℃ / min. Hold it at this temperature for 20h-50h. During the holding period, vacuum is drawn and nitrogen is introduced multiple times. Then cool it to room temperature.

[0028] S5: Assemble the cooled plates and frames from S4 into a crucible to obtain the boron nitride crucible.

[0029] In step S2 of this application, the baking temperature can be 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or any combination of two values ​​within this range; the baking time can be 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, or any combination of two values ​​within this range. Controlling the baking temperature and baking time of the mixed powder within the above ranges in this application is beneficial for dehydrating boric acid into boron oxide, while the boron oxide forms a reaction intermediate with the slightly decomposed melamine, which is beneficial for subsequent molding and prevents cracking of the green body in step S4.

[0030] In S2 of this application, after the boron nitride powder, boric acid powder, and melamine powder are mixed evenly, the mixed powder can be placed in a stainless steel pan for baking; before isostatic pressing into a green body, the slightly agglomerated product can be broken up. This application uses boric acid powder and melamine powder, which can react boron oxide with melamine in the range of 900℃-1100℃ to synthesize boron nitride at low temperature. The boron nitride synthesized at low temperature has a lower degree of crystallinity, finer particles, and higher sintering activity; at the same time, this part of the reaction process occurs between the large boron nitride particles, and the low-temperature boron nitride synthesized in situ is produced in the gaps between the large boron nitride particles and appropriately connects these large particles to increase the overall strength of the green body. Meanwhile, the discharge of reaction waste gases such as carbon monoxide and nitrogen can maintain a certain porosity of the green body.

[0031] In S3 of this application, the billet is heated to 350℃-400℃ (T1) at a heating rate of 3℃ / min-5℃ / min (V1), then to 550℃-600℃ (T2) at a heating rate of 0.2℃ / min-0.5℃ / min (V2), then to 1150℃-1250℃ (T3) at a heating rate of 3℃ / min-5℃ / min (V3), and finally to 1850℃-1900℃ (T4) at a heating rate of 5℃ / min-10℃ / min (V4), and held at that temperature for 1h-3h. The heating rate V1 can be... The heating rate V2 can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two values ​​within that range; the temperature T1 can be 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or any two values ​​within that range; the heating rate V2 can be 0.2℃ / min, 0.25℃ / min, 0.3℃ / min, 0.35℃ / min, 0.4℃ / min, 0.45℃ / min, 0.5℃ / min, or any two values ​​within that range; the temperature T2 can be 5℃ / min. The temperature range is 50℃, 560℃, 570℃, 580℃, 590℃, 600℃, or any two values ​​within this range; the heating rate V3 can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two values ​​within this range; the temperature T3 can be 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, or any two values ​​within this range; the heating rate V4 can be 5... The heating rate can be 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min, 8℃ / min, 8.5℃ / min, 9℃ / min, 9.5℃ / min, 10℃ / min, or any two values ​​within this range; the holding temperature after heating can be 1850℃, 1860℃, 1870℃, 1880℃, 1890℃, 1900℃, or any two values ​​within this range; the holding time can be 1h, 1.5h, 2h, 2.5h, 3h, or any two values ​​within this range. Controlling the heating rate, holding temperature, and holding time within the above ranges in this application is beneficial for reducing the forming density of the green body, reducing the oxygen content, and preventing cracks in the green body.

[0032] In S3 of this application, the isostatically pressed blank can be placed in a graphite crucible and then placed in a graphite heating vacuum furnace for heating and sintering. The cooling after the holding period can be furnace cooling.

[0033] In S4 of this application, the temperature is increased to 350℃-400℃ (T10) at a heating rate of 3℃ / min-5℃ / min (V10), then increased to 550℃-600℃ (T11) at a heating rate of 0.2℃ / min-0.5℃ / min (V11), followed by increasing to 1150℃-1250℃ (T12) at a heating rate of 3℃ / min-5℃ / min (V12), and then increased to 1950℃-2000℃ (T13) at a heating rate of 10℃ / min-15℃ / min (V13), and held at that temperature for 20h-50h; wherein, the heating rate V10 can be... The heating rate can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two values ​​within this range; the temperature T10 can be 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or any two values ​​within this range; the heating rate V11 can be 0.2℃ / min, 0.25℃ / min, 0.3℃ / min, 0.35℃ / min, 0.4℃ / min, 0.45℃ / min, 0.5℃ / min, or any two values ​​within this range; the temperature T11 can be 550℃, 56℃ / min, 370℃, 380℃, 390℃, 400℃, or any two values ​​within this range. The temperature range is 0℃, 570℃, 580℃, 590℃, 600℃, or any two values ​​within this range; the heating rate V12 can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two values ​​within this range; the temperature T12 can be 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, or any two values ​​within this range; the heating rate V13 can be 10℃ / min, 10.5℃ / min, or any two values ​​within this range. The heating rate can be 11℃ / min, 11.5℃ / min, 12℃ / min, 12.5℃ / min, 13℃ / min, 13.5℃ / min, 14℃ / min, 14.5℃ / min, 15℃ / min, or any two values ​​within this range; the holding temperature after heating can be 1950℃, 1960℃, 1970℃, 1980℃, 1990℃, 2000℃, or any two values ​​within this range; the holding time can be 20h, 25h, 30h, 35h, 40h, 45h, 50h, or any two values ​​within this range. Controlling the heating rate, holding temperature, and holding time within the above ranges in this application helps reduce the density of the plate and frame assembly into the crucible, reduces the oxygen content of the crucible, and prevents the crucible from cracking.

[0034] In S4 of this application, the cooled billet can be processed into plates and frames of different sizes and specifications according to actual needs, and finally assembled into a crucible. The plates and frames can be placed in a graphite crucible and then placed in a graphite heating vacuum furnace for heating and sintering. The cooling after the holding time can be furnace cooling.

[0035] In S4 of this application, there is no particular limitation on the number of times vacuuming and nitrogen filling are performed during the heat preservation period, as long as the purpose of this invention can be achieved. For example, vacuuming and nitrogen filling can be performed 2, 3, 4, 5, 6, 7, 8, 9, or 10 times.

[0036] This application prepares a boron nitride crucible with high purity (oxygen content less than or equal to 0.1%) and low density by mixing boron nitride powder, boric acid powder and melamine powder, followed by baking and high-temperature sintering. It can be used to prepare silicon nitride substrates. The preparation method is simple and the production cost is low.

[0037] In some embodiments of this application, in S1, the particle size of the boron nitride powder is 33μm-125μm; when the boron nitride powder is washed with water, the conductivity of the filtered water is less than 20μS / cm.

[0038] In S1 of this application, the particle size of the boron nitride powder is controlled to be 33μm-125μm. This coarser portion of the boron nitride powder mainly acts as a skeleton similar to aggregate in concrete, forming more and larger gas channels. This facilitates the smooth discharge of melamine decomposition gases and prevents excessive internal stress from causing cracking of the green body. The boron nitride powder used in this application needs to have metallic impurities such as iron removed. The purpose of water washing is to remove soluble impurities. The higher the conductivity of the filtered water, the higher the impurities in the filtered water. The conductivity of pure water is generally below 3μS / cm, and the conductivity of the filtered water is controlled below 20μS / cm, indicating that the acid ions introduced in the previous pickling process and the related impurity ions dissolved by hydrochloric acid have been reduced to a certain level.

[0039] In some embodiments of this application, in S1, the method for preparing the boron nitride powder includes: [the following steps are described in the original text, but the translation is incomplete and requires further context.] 3 -2.05g / cm 3The boron nitride raw material is pulverized and sieved to collect powder with a particle size of 33μm-125μm. Then, it is soaked in hydrochloric acid to remove metallic impurities, followed by multiple washes with water until the conductivity of the filtered water is less than 20μS / cm. After drying, the boron nitride powder is obtained. The boron nitride raw material used in this application can be leftover scraps or hot-pressed blanks from conventional hot-pressing boron nitride processing, pulverized using a pulverizer. The collected powder with a particle size of 33μm-125μm can be selected from powder that passes through a 120-mesh sieve and passes through a 625-mesh sieve. This application does not have a particular limitation on the concentration of hydrochloric acid, as long as it can remove metallic impurities such as iron; for example, a 15wt%-30wt% hydrochloric acid solution is acceptable. This application also does not have a particular limitation on the number of water washes, as long as the conductivity of the filtered water is less than 20μS / cm.

[0040] In some embodiments of this application, in S2, the mass ratio of the boron nitride powder, the boric acid powder, and the melamine powder is (12-76):10:(4-6). The mass ratio of boron nitride powder, boric acid powder, and melamine powder can be 12:10:4, 20:10:4, 30:10:4, 40:10:4, 50:10:4, 60:10:4, 70:10:4, 76:10:4, 12:10:5, 20:10:5, 30:10:5, 40:10:5, 50:10:5, 60:10:5, 70:10:5, 76:10:5, 12:10:6, 20:10:6, 30:10:6, 35:10:6, 40:10:6, 50:10:6, 60:10:6, 70:10:6, 76:10:6, or any range of any two of these ratios. This application controls the mass ratio of boron nitride powder, boric acid powder, and melamine powder within the aforementioned range. The synergistic effect of these three components improves the overall strength of the green body while ensuring a certain porosity. When only boron nitride powder and boric acid powder are mixed, it is equivalent to adding boron oxide to boron nitride. While the low-temperature melting of boron oxide can act as a binder for sintering, introducing excessive oxygen reduces the product's high-temperature resistance. Furthermore, the volatilization of boron nitride at high temperatures can easily cause crucible cracking, and the volatilized boron oxide can affect the sintering of the silicon nitride substrate. Therefore, boric acid powder cannot be directly added as a binder. When only boron nitride powder and melamine powder are mixed, the boron nitride is only mixed with melamine. Melamine decomposes completely at high temperatures, and without the presence of boron oxide, low-temperature boron nitride cannot be produced in situ. Therefore, simply mixing melamine powder into boron nitride powder cannot produce a boron nitride crucible.

[0041] In some embodiments of this application, the mixing in S2 is a dry mixing process. This application does not particularly limit the apparatus used for dry mixing, as long as it can achieve the inventive objective of this application; for example, a V-shaped barrel or a twin-screw conical barrel may be used.

[0042] In some embodiments of this application, in step S2, the blank is a cubic or cuboid blank with an edge length of 150mm-600mm, and the molding density of the blank is 1.80g / cm³. 3 -1.90g / cm 3 .

[0043] The cubic or cuboid blank with an edge length of 150mm-600mm as described in this application refers to a cubic blank with all edges of equal length, and the edge length is 150mm-600mm, such as a cubic blank with an edge length of 400mm; or a cuboid blank with edges of not completely equal length, and the edge lengths (length, width, and height) are 150mm-600mm respectively, that is, the length, width, and height are not completely the same, such as a cuboid blank with a length of 500mm, a width of 400mm, and a height of 300mm.

[0044] In this application S2, the edge length of the blank can be 150mm, 200mm, 250mm, 300mm, 350mm, 400mm, 450mm, 500mm, 550mm, 600mm, or any two values ​​within this range. By controlling the edge length of the blank within the above range, this application makes it easier to process it into plates and frames of different sizes and specifications, and assemble them into crucibles that meet the size requirements.

[0045] In S2 of this application, the molding density of the blank can be 1.80 g / cm³. 3 1.81 g / cm 3 1.82g / cm 3 1.83g / cm 3 1.84 g / cm 3 1.85g / cm 3 1.86 g / cm 3 1.87 g / cm 3 1.88g / cm 3 1.89 g / cm 3 1.90g / cm 3 Or it can be the range formed by any two values ​​in between.

[0046] In some embodiments of this application, in step S3, the density of the cooled billet is 1.55 g / cm³. 3 -1.65g / cm 3 In S3 of this application, the density of the cooled billet can be 1.55 g / cm³. 3 1.56g / cm 3 1.57g / cm 3 1.58g / cm 3 1.59g / cm 31.60g / cm 3 1.61 g / cm 3 1.62g / cm 3 1.63g / cm 3 1.64 g / cm 3 1.65g / cm 3 Or it can be the range formed by any two values ​​in between.

[0047] In some embodiments of this application, in step S5, the oxygen content of the boron nitride crucible is less than or equal to 0.1%. In step S5 of this application, the oxygen content of the boron nitride crucible can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or a range of any two values ​​within this range. In this application, boron nitride itself is formed by the reaction of boron oxide with a nitrogen source. Both the surface of the boron nitride particles and the interior of the crystal lattice contain some oxygen. Heating in the 1950℃-2000℃ range in this application can promote the eventual release of oxygen contained in the blank as boron oxide gas. However, when the oxygen content is very low, such as 0.1%, further release is very difficult and costly. When the total oxygen content of the boron nitride crucible is less than 0.1%, the released boron oxide at high temperatures is very weak and will not affect the effect of sintering silicon nitride.

[0048] The densities of the boron nitride raw materials, billets, cooled billets, and boron nitride crucibles involved in this application can all be determined using commonly used testing methods in the field, and there are no particular limitations here.

[0049] The second aspect of this application provides a boron nitride crucible prepared according to the method described in the first aspect of this application. The boron nitride crucible prepared in this application has high purity (oxygen content less than or equal to 0.1%), low density, and is free of cracks, and can be used to prepare silicon nitride substrates.

[0050] The third aspect of this application provides the use of the boron nitride crucible described in the second aspect of this application in the preparation of silicon nitride substrates.

[0051] The basic process for sintering silicon nitride substrates involves casting a slurry into a paper-like blank, drying it, and then layering thin blanks separated by boron nitride separating powder. These layers are then placed on a high-purity, low-density boron nitride (BN) plate, and another BN plate is placed on top. Several layers of silicon nitride ceramic blanks are then sandwiched between two BN plates and placed in a debinding furnace to remove the binder. The organic binder decomposes and vaporizes, exiting through the micropores inside the BN ceramic. Air can also diffuse into the silicon nitride blank through the pores inside the BN ceramic to oxidize any residual carbon. The debinded blanks, along with the BN plates, are placed in a crucible made of high-purity, low-density boron nitride ceramic and stacked in a pressure sintering furnace for high-temperature sintering. The high-purity, low-density boron nitride crucibles used in the stacking process contain tiny vents, allowing for the relatively slow escape of internal air during vacuum sintering. This also ensures that nitrogen gas can uniformly enter the crucible through the micropores during pressure sintering, maintaining a small pressure difference between the inside and outside of the crucible and preventing strong airflow that could negatively impact the internal green body (which has extremely poor strength after binder removal). Furthermore, during the sintering shrinkage of the silicon nitride ceramic substrate, the rigid boron nitride platen ensures the flatness of the substrate without being too heavy and causing excessive friction that could affect sintering shrinkage; therefore, a low-density boron nitride crucible is also required. At high temperatures, to prevent the volatilization of boron oxide from the boron nitride ceramic from affecting the sintering of the silicon nitride ceramic, the boron nitride ceramic must have high purity, i.e., extremely low oxygen content. The boron nitride crucible prepared in this application has high purity (oxygen content less than or equal to 0.1%) and low density, making it suitable for use in the silicon nitride substrate fabrication process.

[0052] Example

[0053] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0054] <Test of Oxygen Content in Boron Nitride Crucible>

[0055] The total oxygen content of a sample is measured using an oxygen-nitrogen analyzer, including the following steps: Take approximately 0.2g of sample, wrap it in nickel foil, and place it in a small graphite crucible. After evacuation, fill the crucible with argon gas. Heat the small graphite crucible with electricity to melt the nickel foil, and further raise the temperature (up to 3000℃). With the help of molten nickel, the sample reacts with the graphite at high temperature. All the oxygen elements react with the graphite to form carbon monoxide, which is then oxidized into carbon dioxide. The amount of carbon dioxide produced is then measured by infrared absorption, thereby calculating the oxygen content in the sample.

[0056] Example 1

[0057] S1: A substance with a density of 2.0 g / cm³ 3The boron nitride raw material (from Fujian Serite Technology Co., Ltd.) was pulverized and sieved using a pulverizer, and powder with a particle size of 33μm-125μm was collected, which was selected from the 120-mesh sieve and the 625-mesh sieve. Then, it was soaked in hydrochloric acid to remove metal impurities, and then washed multiple times with deionized water until the conductivity of the filtered water was less than 20μS / cm. After drying, the boron nitride powder was obtained.

[0058] S2: 44 parts by weight of boron nitride powder, 10 parts by weight of boric acid powder, and 5.1 parts by weight of melamine powder are dry-mixed until homogeneous. The mixture is then placed in a stainless steel pan and baked at 350℃ for 6 hours. Slightly agglomerated products are then broken up and isostatically pressed into cubic blanks with an edge length of 400 mm and a molding density of 1.80 g / cm³. 3 .

[0059] S3: The above isostatically pressed cubic blank was placed in a graphite crucible and then placed in a graphite heating vacuum furnace. Under the protection of flowing nitrogen, the temperature was increased to 380℃ (T1) at a heating rate of 4℃ / min (V1), then increased to 570℃ (T2) at a heating rate of 0.35℃ / min (V2), then increased to 1200℃ (T3) at a heating rate of 4℃ / min (V3), and finally increased to 1880℃ (T4) at a heating rate of 8℃ / min (V4). The temperature was held for 2 hours, and then cooled in the furnace to obtain a density of 1.55 g / cm³. 3 The cooled blank.

[0060] S4: After the cooled blank in S3 is processed into plates and frames of different sizes, it is placed in a graphite crucible and heated to 380℃ (T10) at a heating rate of 4℃ / min (V10) under the protection of flowing nitrogen. Then it is heated to 570℃ (T11) at a heating rate of 0.35℃ / min (V11), then to 1200℃ (T12) at a heating rate of 4℃ / min (V12), and then to 1980℃ (T13) at a heating rate of 12℃ / min (V13). The temperature is held for 35 hours. During the holding period, the vacuum is evacuated and nitrogen is circulated three times. Then it is cooled to room temperature.

[0061] S5: Assemble the cooled plates and frames from S4 into a crucible to obtain the boron nitride crucible.

[0062] The oxygen content of the boron nitride crucible was determined to be 0.06% and its density to be 1.54 g / cm³ using the method described above. 3 .

[0063] Example 2-15

[0064] Except for adjusting the relevant preparation parameters as shown in Table 1 and the density as shown in Table 2, everything else is the same as in Example 1.

[0065] Comparative Examples 1-6

[0066] Except for adjusting the relevant preparation parameters as shown in Table 1 and the density as shown in Table 2, everything else is the same as in Example 1.

[0067] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0068] Table 1

[0069]

[0070] Note: " / " indicates that it does not exist.

[0071] Table 2

[0072]

[0073] The results above show that the amounts of boron nitride powder, boric acid powder, and melamine powder controlled in Examples 1 to 15 of this application are within the scope of this application. Simultaneously, the baking temperature in S2, baking time in S2, heating program in S3, holding time in S3, heating program in S4, and holding time in S4 are all controlled within the scope of this application. The resulting boron nitride crucibles all have a low density (1.54 g / cm³). 3 -1.64g / cm 3 Meanwhile, the oxygen content was less than or equal to 0.1%, and there were no cracks; however, the heating programs in S3 of Comparative Examples 3 to 4 and in S4 of Comparative Examples 5 to 6 were not within the scope of this application, and the density of the prepared boron nitride crucibles was too low (less than or equal to 1.31 g / cm³). 3 This resulted in boron nitride crucibles having excessively low strength; furthermore, the boron nitride crucibles exhibited varying degrees of visible cracking, failing to meet the requirements for preparing silicon nitride substrates. Additionally, in Comparative Example 1, boron nitride was only mixed with melamine, failing to produce a boron nitride crucible; and in Comparative Example 2, only boron nitride powder and boric acid powder were mixed, resulting in boron nitride crucibles with clearly visible cracks.

[0074] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a boron nitride crucible, characterized in that, Including the following steps: S1: The boron nitride raw material is pulverized and dried to obtain boron nitride powder; wherein the particle size of the boron nitride powder is 33μm-125μm; when the boron nitride powder is washed with water, the conductivity of the filtered water is less than 20μS / cm; S2: The boron nitride powder, boric acid powder and melamine powder are mixed evenly, baked at 300℃-400℃ for 4h-8h, and then isostatically pressed into a blank; wherein, the mass ratio of the boron nitride powder, the boric acid powder and the melamine powder is (12-76):10:(4-6). S3: Under the protection of flowing nitrogen, the billet is heated to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heated to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heated to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heated to 1850℃-1900℃ at a heating rate of 5℃ / min-10℃ / min, held at this temperature for 1h-3h, and then cooled. S4: After processing the cooled blank in S3 into plates and frames of different sizes, under the protection of flowing nitrogen, heat it to 350℃-400℃ at a heating rate of 3℃ / min-5℃ / min, then heat it to 550℃-600℃ at a heating rate of 0.2℃ / min-0.5℃ / min, then heat it to 1150℃-1250℃ at a heating rate of 3℃ / min-5℃ / min, and then heat it to 1950℃-2000℃ at a heating rate of 10℃ / min-15℃ / min. Hold it at this temperature for 20h-50h. During the holding period, vacuum is drawn and nitrogen is introduced multiple times. Then cool it to room temperature. S5: Assemble the cooled plates and frames from S4 into a crucible to obtain the boron nitride crucible.

2. The preparation method according to claim 1, characterized in that, In S1, the method for preparing the boron nitride powder includes: [the following steps are described in the original text, but the translation is incomplete and requires further context.] 3 -2.05g / cm 3 The boron nitride raw material was pulverized and sieved to collect powder with a particle size of 33μm-125μm; then it was soaked in hydrochloric acid to remove metal impurities, and then washed with water multiple times until the conductivity of the filtered water was less than 20μS / cm; then it was dried to obtain the boron nitride powder.

3. The preparation method according to claim 1, characterized in that, In S2, the mixing is a dry mixing process.

4. The preparation method according to claim 1, characterized in that, In S2, the blank is a cubic or cuboid blank with an edge length of 150mm-600mm, and the molding density of the blank is 1.80g / cm³. 3 -1.90g / cm 3 .

5. The preparation method according to claim 1, characterized in that, In S3, the density of the cooled billet is 1.55 g / cm³. 3 -1.65g / cm 3 .

6. The preparation method according to claim 1, characterized in that, In S5, the oxygen content of the boron nitride crucible is less than or equal to 0.1%.

7. A boron nitride crucible prepared by the method according to any one of claims 1-6.

8. The application of the boron nitride crucible according to claim 7 in the preparation of silicon nitride substrates.

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