A sintering aid, a preparation method and application thereof, and a preparation method of hexagonal boron nitride-based composite ceramic

Abstract

The present application relates to the technical field of composite ceramics, and particularly relates to a sintering aid, a preparation method and application thereof, and a preparation method of hexagonal boron nitride-based composite ceramics.The present application provides a sintering aid, which comprises Ca2MgSi2O7 and CaMgSiO4; the mass ratio of the Ca2MgSi2O7 and the CaMgSiO4 is (65-75):(25-35). The sintering aid has a relatively low melting point (the melting points are 1450 DEG C and 1390 DEG C respectively), so that a liquid phase is formed in the subsequent sintering process, the atomic migration is accelerated, and thus the sintering temperature is reduced and the ceramic densification process is accelerated. After cooling, the sintering aid can also serve as a binder to enhance the bonding strength of the composite ceramics, and thus the mechanical properties of the composite ceramics are improved.

Description

Technical Field

[0001] This invention relates to the field of composite ceramics technology, and in particular to a sintering aid, its preparation method and application, and a method for preparing hexagonal boron nitride-based composite ceramics. Background Technology

[0002] Hexagonal boron nitride (h-BN) and its composites are widely used in plasma jet furnaces, crucibles, boats, high-frequency electrical insulation materials, integrated circuit heat sinks, and ion rocket nozzles due to their excellent thermal shock resistance, corrosion resistance, electrical insulation, and good mechanical properties. However, the stable lamellar structure of h-BN makes the ceramics difficult to sinter, which is the main reason for the high cost of h-BN and its composites. Therefore, hot pressing sintering is currently commonly used to prepare h-BN and its composite ceramics. This method can significantly improve the density of ceramics, but hot pressing sintering has disadvantages such as long process flow, high energy consumption, and a large amount of waste. These problems limit the further development of h-BN ceramic industrialization. Therefore, developing a pressureless sintering process for h-BN composite ceramics would help solve the various problems that urgently need to be solved in the industrialization process.

[0003] In recent years, the exploration of pressureless sintering processes for h-BN ceramics has been a research hotspot. Generally speaking, these processes fall into three categories: first, improving the size of h-BN sheets; second, reaction sintering; and third, adding oxide sintering aids. However, the first two methods still require a certain pressure, such as 0.2 MPa N2 gas pressure for firing. This pressure places higher demands on the equipment, which to some extent limits the development of related industries. Summary of the Invention

[0004] The purpose of this invention is to provide a sintering aid, its preparation method and application, and a method for preparing hexagonal boron nitride-based composite ceramics. The sintering aid enables pressureless sintering of the hexagonal boron nitride-based composite ceramics and improves their mechanical properties.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a sintering aid comprising Ca2MgSi2O7 and CaMgSiO4;

[0007] The mass ratio of Ca2MgSi2O7 to CaMgSiO4 is (65-75):(25-35).

[0008] This invention also provides a method for preparing the sintering aid described in the above technical solution, comprising the following steps:

[0009] Calcium carbonate, magnesium oxide, and silicon dioxide are mixed and then ball-milled and calcined sequentially to obtain the sintering aid.

[0010] Preferably, the molar ratio of calcium carbonate, magnesium oxide and silicon dioxide is (1-4):1:(1-4).

[0011] Preferably, the ball milling is a wet ball milling process;

[0012] The ball milling medium for the wet ball milling is anhydrous ethanol, and the ratio of the mixture obtained by mixing to anhydrous ethanol is (10-50) g: (200-500) mL.

[0013] The wet ball milling speed is 100-400 r / min, and the time is 1-24 h.

[0014] Preferably, the calcination temperature is 1200–1500℃, and the holding time is 1–10 h.

[0015] The present invention also provides the application of the sintering aid described in the above technical solution or the sintering aid prepared by the preparation method described in the above technical solution in the preparation of hexagonal boron nitride-based composite ceramics.

[0016] This invention also provides a method for preparing hexagonal boron nitride-based composite ceramics, comprising the following steps:

[0017] Hexagonal boron nitride and sintering aid are mixed, ball-milled, and then mixed with binder. The mixture is then pressed and sintered sequentially to obtain the hexagonal boron nitride-based composite ceramic.

[0018] The debinding sintering is pressureless sintering;

[0019] The sintering aid is the sintering aid described in the above technical solution or the sintering aid prepared by the preparation method described in the above technical solution.

[0020] Preferably, the mass ratio of the hexagonal boron nitride to the sintering aid is 100:(29-31).

[0021] Preferably, the adhesive comprises paraffin wax;

[0022] The mass ratio of the hexagonal boron nitride to the binder is 100:(2-10).

[0023] Preferably, the debinding sintering includes debinding and sintering performed sequentially;

[0024] The temperature for discharging the adhesive is 300–700℃, and the heat preservation time is 1–12 hours.

[0025] The sintering temperature is 1500–2000℃, and the holding time is 1–12 hours.

[0026] This invention provides a sintering aid comprising Ca2MgSi2O7 and CaMgSiO4, wherein the mass ratio of Ca2MgSi2O7 to CaMgSiO4 is (65-75):(25-35). The sintering aid of this invention has low melting points (1450℃ and 1390℃, respectively), thus forming a liquid phase during the subsequent sintering process, accelerating atomic migration, thereby lowering the sintering temperature and accelerating the ceramic densification process. After cooling, the sintering aid can also act as a binder to enhance the bonding strength of the composite ceramic, thereby improving the mechanical properties of the composite ceramic.

[0027] This invention also provides a method for preparing hexagonal boron nitride-based composite ceramics, comprising the following steps: mixing hexagonal boron nitride and a sintering aid, ball milling, mixing with a binder, and then sequentially pressing and debinding sintering to obtain the hexagonal boron nitride-based composite ceramic; the debinding sintering is pressureless sintering. The preparation method of this invention is simple, low-cost, and highly reproducible, meeting all the requirements for preparing h-BN-based composite ceramics; the obtained ceramic phase is stable and meets the requirements for high-temperature resistant irregular-shaped h-BN-based composite ceramic parts. Attached Figure Description

[0028] Figure 1 The XRD pattern of the sintering aid described in Example 1;

[0029] Figure 2 This is a SEM image of the hexagonal boron nitride-based composite ceramic described in Example 2. Detailed Implementation

[0030] This invention provides a sintering aid comprising Ca2MgSi2O7 and CaMgSiO4;

[0031] The mass ratio of Ca2MgSi2O7 to CaMgSiO4 is (65-75):(25-35).

[0032] In this invention, the mass ratio of Ca2MgSi2O7 to CaMgSiO4 is (65-75):(25-35), preferably (67-73):(27-33), and more preferably (69-71):(29-31).

[0033] In this invention, the Ca2MgSi2O7 and CaMgSiO4 serve to form a liquid phase during sintering, accelerate atomic migration, thereby reducing the sintering temperature, accelerating the ceramic densification process, and acting as a binder to enhance the bonding strength of the composite ceramic, thereby improving the mechanical properties of the composite ceramic.

[0034] This invention also provides a method for preparing the sintering aid described in the above technical solution, comprising the following steps:

[0035] Calcium carbonate, magnesium oxide, and silicon dioxide are mixed and then ball-milled and calcined sequentially to obtain the sintering aid.

[0036] In this invention, unless otherwise specified, all raw materials used in the preparation are commercially available products well known to those skilled in the art.

[0037] In this invention, the purity of the calcium carbonate, magnesium oxide, and silicon dioxide is preferably greater than 99.5%.

[0038] In this invention, the molar ratio of calcium carbonate, magnesium oxide, and silicon dioxide is preferably (1-4):1:(1-4), more preferably (1.5-3.5):1:(1.5-3.5), and most preferably (2-3):1:(2-3). This invention does not impose any special limitations on the mixing process; any process well-known to those skilled in the art can be used.

[0039] In this invention, the ball milling is preferably wet ball milling; the ball milling medium for the wet ball milling is preferably anhydrous ethanol; the ratio of the mixture obtained by mixing to anhydrous ethanol is preferably (10-50) g: (200-500) mL, more preferably (15-45) g: (250-450) mL, and most preferably (20-30) g: (300-400) mL.

[0040] In this invention, the rotational speed of the wet ball mill is preferably 100–400 r / min, more preferably 150–350 r / min, and most preferably 200–300 r / min; the time is preferably 1–24 h, more preferably 5–20 h, and most preferably 10–15 h. In this invention, the ball milling is preferably carried out in a polytetrafluoroethylene (PTFE) ball mill jar.

[0041] In this invention, the purpose of ball milling is to mix the calcium carbonate, magnesium oxide and silicon dioxide evenly.

[0042] After ball milling, the present invention preferably includes drying, grinding, and sieving in sequence. The drying process is not particularly limited in the present invention; any process well-known to those skilled in the art can be used. The grinding process is not particularly limited in the present invention; any process well-known to those skilled in the art can be used, ensuring uniform grinding in the alumina mortar. In the present invention, sieving is preferably performed using a standard sieve with a mesh size of 40-200.

[0043] In this invention, the calcination temperature is preferably 1200-1500℃, more preferably 1250-1450℃, and most preferably 1300-1400℃; the holding time is preferably 1-10h, more preferably 2-8h, and most preferably 4-6h.

[0044] In this invention, the calcination is used to synthesize Ca2MgSi2O7 and CaMgSiO4.

[0045] This invention also provides the application of the sintering aid described in the above technical solution and the sintering aid prepared by the preparation method described in the above technical solution in the preparation of hexagonal boron nitride-based composite ceramics.

[0046] This invention also provides a method for preparing hexagonal boron nitride-based composite ceramics, comprising the following steps:

[0047] Hexagonal boron nitride and sintering aid are mixed, ball-milled, and then mixed with binder. The mixture is then pressed and sintered sequentially to obtain the hexagonal boron nitride-based composite ceramic.

[0048] The debinding sintering is pressureless sintering;

[0049] The sintering aid is the sintering aid described in the above technical solution or the sintering aid prepared by the preparation method described in the above technical solution.

[0050] In this invention, the purity of the hexagonal boron nitride is preferably >99.9%. The particle size of the hexagonal boron nitride is preferably 2-30 μm.

[0051] In this invention, the mass ratio of the hexagonal boron nitride to the sintering aid is preferably 100:(29-31), and more preferably 100:30.

[0052] In this invention, the ball milling is preferably wet ball milling; the ball milling medium for the wet ball milling is preferably anhydrous ethanol; the ratio of the mixture obtained by mixing to anhydrous ethanol is preferably (10-50) g: (200-500) mL, more preferably (15-45) g: (250-450) mL, and most preferably (20-30) g: (300-400) mL.

[0053] In this invention, the rotational speed of the wet ball mill is preferably 100–400 r / min, more preferably 150–350 r / min, and most preferably 200–300 r / min; the time is preferably 1–12 h, more preferably 3–10 h, and most preferably 6–8 h. In this invention, the ball milling is preferably carried out in a polytetrafluoroethylene (PTFE) ball mill jar.

[0054] In this invention, the purpose of ball milling is to uniformly mix the hexagonal boron nitride and sintering aids.

[0055] After ball milling, the present invention preferably further includes sequentially filtering, drying, pulverizing, and sieving the slurry obtained from ball milling; the filtration is preferably performed using a 40-200 mesh filter; the drying temperature is preferably 50-100℃, more preferably 60-90℃, and most preferably 70-80℃; the drying time is preferably 1-12 hours, more preferably 3-10 hours, and most preferably 4-6 hours; in the present invention, the drying is preferably carried out in a forced-air drying oven. The present invention does not impose any special limitations on the pulverizing process; a process well-known to those skilled in the art can be used in an alumina mortar. In the present invention, the sieving is preferably performed using a 40-200 mesh standard sieve.

[0056] In this invention, the binder preferably includes one or more of paraffin wax, beeswax, polypropylene, and polyvinyl alcohol, more preferably paraffin wax. When the binder is two or more of the above-mentioned specific selections, this invention does not impose any special limitation on the ratio of the above-mentioned specific substances, and they can be mixed in any ratio. In this invention, the particle size of the binder is preferably 200-800 μm, more preferably 300-700 μm, and most preferably 400-500 μm.

[0057] In this invention, the mass ratio of the hexagonal boron nitride to the binder is preferably 100:(2-10), more preferably 100:(4-7), and most preferably 100:5.

[0058] In this invention, the binder serves to adhere the ceramic powder, mix it uniformly, increase its fluidity, allow it to be better pressed and molded, and can be removed from the sample during the debinding stage while maintaining the sample's intact shape.

[0059] In this invention, the mixing temperature is preferably 130–200°C, more preferably 140–190°C, and most preferably 150–180°C; the mixing time is preferably 10–30 min, more preferably 15–25 min, and most preferably 20 min. In this invention, the mixing is preferably carried out under stirring conditions. This invention does not impose any special limitations on the stirring conditions; conditions well known to those skilled in the art can be used.

[0060] After the mixing is completed, the present invention preferably includes sequential cooling and sieving; the present invention does not have any special limitations on the cooling process, and any process well known to those skilled in the art can be used. In the present invention, the sieving is preferably performed using a standard sieve with a mesh size of 40 to 200.

[0061] In this invention, the pressing pressure is preferably 10-20t, more preferably 12-18t, and most preferably 14-16t; the pressing time is preferably 3-10min, more preferably 4-7min, and most preferably 5min. In this invention, the pressing is preferably performed using a 30T manual hydraulic press for unidirectional pressurization.

[0062] In this invention, the adhesive removal and sintering preferably includes sequential adhesive removal and sintering; the adhesive removal temperature is preferably 300–700°C, more preferably 350–650°C, and most preferably 400–500°C; the holding time is preferably 1–12 hours, more preferably 3–10 hours, and most preferably 4–6 hours. In this invention, the adhesive removal is preferably carried out in an air atmosphere. The purpose of the adhesive removal is to remove the binder.

[0063] In this invention, the sintering temperature is preferably 1500–2000℃, more preferably 1600–1900℃, and most preferably 1700–1800℃; the holding time is preferably 1–12 h, more preferably 2–9 h, and most preferably 4–6 h. In this invention, the sintering is preferably carried out under the condition of an inert gas flow, and the inert gas flow rate is preferably 20–80 mL / min, more preferably 30–70 mL / min, and most preferably 40–60 mL / min.

[0064] In this invention, the sintering process densifies the hexagonal boron nitride powder to form a dense polycrystalline sintered body.

[0065] This invention prepares h-BN-based composite ceramics by adding sintering aids using a pressureless sintering method. During high-temperature sintering, the sintering aids form a liquid phase, accelerating atomic migration, thereby lowering the sintering temperature and accelerating the ceramic densification process. After cooling, the sintering aids also act as binders to enhance the bonding strength of the boron nitride sheets. Therefore, the h-BN-based composite ceramics prepared by this invention have superior performance and can basically meet the mechanical property requirements of h-BN-based composite ceramics in actual production and application. Furthermore, this invention uses inexpensive CaCO3, MgO, and SiO2 as sintering aid raw materials, which are inexpensive and reduce the cost of ceramic preparation. The process is simple, the equipment cost is low, and the repeatability is good, meeting all the requirements for the preparation of h-BN composite ceramic irregularly shaped parts. The resulting ceramic phase is stable and meets the requirements for high-temperature resistant h-BN composite ceramic irregularly shaped parts.

[0066] The following detailed descriptions, in conjunction with embodiments, illustrate the sintering aids, their preparation methods, applications, and the preparation method of hexagonal boron nitride-based composite ceramics provided by this invention. However, these descriptions should not be construed as limiting the scope of protection of this invention.

[0067] Example 1

[0068] 22.26g of CaCO3 with a purity greater than 99.5%, 4.49g of MgO with a purity greater than 99.5%, and 13.37g of SiO2 with a purity greater than 99.5% were mixed and added to 401mL of anhydrous ethanol in a 500mL polytetrafluoroethylene ball mill jar. The mixture was ball milled at 300r / min for 12h, filtered, dried, and finally ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. The mixture was then kept at 1400℃ for 4h to obtain a sintering aid (comprising Ca2MgSi2O7 and CaMgSiO4, with a mass ratio of 69:31).

[0069] At a mass ratio of 1:0.29, 20g of h-BN powder with a purity greater than 99.9% and a size of 8-10μm was mixed with the sintering aid (the total mass of the h-BN powder and sintering aid was 25.8g), and added to a ball mill jar. This mixture, along with 258mL of anhydrous ethanol, was then placed in a 500mL polytetrafluoroethylene ball mill jar and ball-milled continuously at 300r / min for 12 hours. The homogeneous slurry was then filtered through a 100-mesh filter and placed at 80°C. The mixture was dried in a forced-air drying oven at ℃ for 8 hours. The dried mixed powder was then ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. Paraffin particles with a particle size of about 500μm were added as a binder. The mixture was heated and stirred at 150℃ for 20 minutes to mix evenly. After the powder cooled, it was passed through an 80-mesh standard sieve again. The resulting mixed powder was then pressed into thin sheet-like green bodies with a size of 40mm*40mm*2mm by a 30T manual hydraulic press under a 10t unidirectional pressure mode for 5 minutes.

[0070] The thin sheet-like green body was kept in air at 500°C for 4 hours to remove paraffin components, and then sintered at 1800°C in an atmosphere with a N2 flow rate of 30 mL / min for 4 hours to obtain a hexagonal boron nitride-based composite ceramic (h-BN+29wt.%(2CaO-MgO-2SiO2) composite ceramic), the apparent density of which was 1.54 g / cm³. 3 Its flexural strength is 17.2 MPa;

[0071] Figure 1 The XRD pattern of the sintering aid is shown below. Figure 1 It can be seen that the XRD diffraction peaks of the sintering aid synthesized from 2CaO-MgO-2SiO2 correspond to the diffraction peaks of Ca2MgSi2O7 and CaMgSiO4, indicating that it is a mixture of Ca2MgSi2O7 and CaMgSiO4.

[0072] Example 2

[0073] 22.26g of CaCO3 with a purity greater than 99.5%, 4.49g of MgO with a purity greater than 99.5%, and 13.37g of SiO2 with a purity greater than 99.5% were mixed and added to 401mL of anhydrous ethanol in a 500mL polytetrafluoroethylene ball mill jar. The mixture was ball milled at 300r / min for 12h, filtered, dried, and finally ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. The mixture was then kept at 1400℃ for 4h to obtain a sintering aid (comprising Ca2MgSi2O7 and CaMgSiO4, with a mass ratio of 69:31).

[0074] At a mass ratio of 1:0.3, 20g of h-BN powder with a purity greater than 99.9% and a size of 8-10μm was mixed with the sintering aid (the total mass of the h-BN powder and sintering aid was 26.0g), and added to a ball mill jar. This mixture, along with 260mL of anhydrous ethanol, was then placed in a 500mL polytetrafluoroethylene ball mill jar and ball-milled continuously at 300r / min for 12 hours. The homogeneous slurry was then filtered through a 100-mesh filter and placed at 80℃. The mixture was dried in a forced-air drying oven for 8 hours. The dried mixed powder was then ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. Paraffin particles with a particle size of about 500μm were added as a binder. The mixture was heated and stirred at 150℃ for 20 minutes to mix evenly. After the powder cooled, it was passed through an 80-mesh standard sieve again. The resulting mixed powder was then pressed into thin sheet-like green bodies with a size of 40mm*40mm*2mm by a 30T manual hydraulic press under a 10t unidirectional pressure mode for 5 minutes.

[0075] The thin sheet-like green body was kept in air at 500°C for 4 hours to remove paraffin components, and then sintered at 1800°C in an atmosphere with a N2 flow rate of 30 mL / min for 4 hours to obtain a hexagonal boron nitride-based composite ceramic (h-BN+30wt.%(2CaO-MgO-2SiO2) composite ceramic). The apparent density of the hexagonal boron nitride-based composite ceramic was 1.58 g / cm³. 3 Its flexural strength is 18.6 MPa;

[0076] Figure 2 Here is a SEM image of the hexagonal boron nitride-based composite ceramic, from... Figure 2 It is known that the hexagonal boron nitride-based composite ceramic is mainly composed of stacked h-BN sheets. The h-BN sheets are of uneven size and arranged in a disordered state. There are gaps between different h-BN sheets, which hinders the densification of the hexagonal boron nitride-based composite ceramic.

[0077] Example 3

[0078] 22.26g of CaCO3 with a purity greater than 99.5%, 4.49g of MgO with a purity greater than 99.5%, and 13.37g of SiO2 with a purity greater than 99.5% were mixed and added to 401mL of anhydrous ethanol in a 500mL polytetrafluoroethylene ball mill jar. The mixture was ball milled at 300r / min for 12h, filtered, dried, and finally ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. The mixture was then kept at 1400℃ for 4h to obtain a sintering aid (comprising Ca2MgSi2O7 and CaMgSiO4, with a mass ratio of 69:31).

[0079] At a mass ratio of 1:0.31, 20g of material with a purity greater than 99.9% and a size of 8–10 μm was used. h-BN powder is mixed with the sintering aid (the total mass of h-BN powder and sintering aid is 26.2g) and added to a ball mill jar. This mixture, along with 262mL of anhydrous ethanol, is then placed in a 500mL polytetrafluoroethylene ball mill jar and continuously ball-milled at 300r / min for 12 hours. The resulting slurry, after being ball-milled and mixed evenly, is filtered through a 100-mesh filter and dried in a forced-air drying oven at 80℃ for 8 hours. The dried powder is then ground in an Al2O3 mortar and passed through an 80-mesh standard sieve. Paraffin particles with a particle size of approximately 500μm are added as a binder, and the mixture is heated and stirred at 150℃ for 20 minutes until homogeneous. After cooling, the powder is passed through an 80-mesh standard sieve again. The resulting powder is then pressed for 5 minutes using a 30T manual hydraulic press under a 10t unidirectional pressure mode to form thin sheet-like green bodies with dimensions of 40mm*40mm*2mm.

[0080] The thin sheet-like green body was kept in air at 500°C for 4 hours to remove paraffin components, and then sintered at 1800°C in an atmosphere with a N2 flow rate of 30 mL / min for 4 hours to obtain a hexagonal boron nitride-based composite ceramic (h-BN+31wt.%(2CaO-MgO-2SiO2) composite ceramic), the apparent density of which was 1.56 g / cm³. 3 Its flexural strength is 18.0 MPa.

[0081] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing a hexagonal boron nitride-based composite ceramic, characterized by, Includes the following steps: Hexagonal boron nitride and sintering aid are mixed, ball-milled, and then mixed with binder. The mixture is then pressed and sintered sequentially to obtain the hexagonal boron nitride-based composite ceramic. The debinding sintering is pressureless sintering; the debinding sintering includes debinding and sintering performed sequentially. The temperature for discharging the adhesive is 300~700℃, and the heat preservation time is 1~12h; The sintering temperature is 1500~1800℃, and the holding time is 1~12h; The sintering aids include Ca2MgSi2O7 and CaMgSiO4; The mass ratio of Ca2MgSi2O7 to CaMgSiO4 is 69:

31.

2. The preparation method according to claim 1, characterized in that, The preparation method of the sintering aid includes the following steps: Calcium carbonate, magnesium oxide and silicon dioxide are mixed and then ball-milled and calcined sequentially to obtain the sintering aid. The molar ratio of calcium carbonate, magnesium oxide and silicon dioxide is (1~4):1:(1~4).

3. The preparation method according to claim 2, characterized in that, The ball milling method is wet ball milling; The ball milling medium for the wet ball milling is anhydrous ethanol, and the ratio of the mixture obtained by mixing to anhydrous ethanol is (10~50) g: (200~500) mL. The wet ball milling speed is 100~400 r / min, and the time is 1~24 h.

4. The preparation method according to claim 2, characterized in that, The calcination temperature is 1200~1500℃, and the holding time is 1~10h.

5. The preparation method according to claim 1, characterized in that, The mass ratio of the hexagonal boron nitride to the sintering aid is 100:(29~31).

6. The preparation method according to claim 1, characterized in that, The adhesive includes paraffin wax; The mass ratio of the hexagonal boron nitride to the binder is 100:(2~10).

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