Transparent alon ceramic and method of making the same

Abstract

The present application relates to a kind of transparent AlON ceramic and its preparation method.The preparation method of the transparent AlON ceramic includes: Al2O3 and AlN powder are mixed in thermosetting phenolic resin to obtain slurry, after drying, high-temperature pyrolysis, form the Al2O3-AlN-C composite powder of the granular surface surface-coated pyrolytic carbon, then carbon thermal reduction, obtain high-purity AlON powder;The composite oxide sintering aid containing boron and rare earth element is introduced into the high-purity AlON powder by deposition coating or mechanical mixing process, to obtain AlON ceramic sintering powder;AlON ceramic sintering powder is shaped into green body, then is buried in BN-AlON buried material containing boron oxide or aluminum borate and is pressureless sintered, to obtain the transparent AlON ceramic.

Description

Technical Field

[0001] This invention belongs to the field of ceramic material preparation technology, specifically relating to a transparent AlON ceramic and its preparation method. Background Technology

[0002] Transparent AlON (aluminum oxynitride solid solution) ceramics possess excellent mechanical and optical properties, making them promising candidates for applications in modern defense, such as transparent armor, high-speed missile radomes, and optoelectronic windows. However, due to thermodynamic limitations, the synthesis temperature of AlON powder is relatively high, typically around 1750℃. The aggregation and growth of Al₂O₃ and AlN raw material particles during synthesis results in coarse AlON powder particles. These factors, coupled with the strong covalent bond characteristics of AlON itself, make the sintering of transparent AlON quite difficult.

[0003] Although the introduction of rare earth oxide sintering aids (such as yttrium oxide and lanthanum oxide) can promote sintering to some extent, the high temperature of the liquid phase within the material system during sintering (>1700℃) means that solid-state sintering remains the dominant process, limiting the effectiveness of the sintering aids. For these reasons, the pressureless sintering conditions for transparent AlON ceramics are currently extremely demanding, with sintering temperatures typically reaching 1950–2000℃ and sintering times as long as 20–30 hours. This results in excessive grain growth within the ceramic body, severely impacting the material's mechanical properties and application reliability. How to effectively reduce the sintering temperature of transparent AlON and optimize its mechanical and optical properties has become a pressing problem to be solved in the development of transparent AlON ceramics. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing transparent AlON ceramics by low-temperature pressureless sintering, in order to meet the application requirements of transparent AlON ceramics.

[0005] In a first aspect, the present invention provides a method for preparing transparent AlON ceramics, comprising:

[0006] Al2O3 and AlN powders are mixed in thermosetting phenolic resin to obtain a slurry. After drying and high-temperature pyrolysis, Al2O3-AlN-C composite powder with pyrolytic carbon coated on the particle surface is formed. Then, carbothermal reduction is performed to obtain high-purity AlON powder.

[0007] The high-purity AlON powder is introduced with a composite oxide sintering aid containing boron and rare earth elements through precipitation coating or mechanical mixing process to obtain AlON ceramic sintered powder.

[0008] AlON ceramic sintering powder is formed into a green body, and then embedded in BN-AlON embedding material containing boron oxide or aluminum borate for pressureless sintering to obtain the transparent AlON ceramic.

[0009] Preferably, the mass ratio of the raw materials Al2O3, AlN and phenolic resin is 80-87:12-16:2-4.

[0010] Preferably, the high-temperature pyrolysis temperature is 800–1000°C and the time is 1–2 hours.

[0011] Preferably, the carbothermic reduction temperature is 1680–1720°C, and the time is 1–3 hours.

[0012] Preferably, the boron-containing composite oxide sintering aid is a rare earth oxide / boron oxide, a rare earth borate / boron oxide, or a rare earth oxide / rare earth borate.

[0013] The rare earth oxide is at least one of yttrium oxide, lanthanum oxide, or scandium oxide, and the rare earth borate is at least one of yttrium borate, lanthanum borate, or scandium borate.

[0014] Preferably, the total content of the boron-containing and rare earth element composite oxide sintering aid in the AlON ceramic sintering powder is controlled to be 0.4-0.8 wt%; the total content of rare earth oxides in the boron-containing and rare earth element composite oxide sintering aid does not exceed 0.5 wt%, and the total content of boron oxide does not exceed 0.4 wt%.

[0015] Preferably, when boron oxide is added to the embedding material, the content ratio of BN, AlON and B2O3 is 10-20 wt% BN, 79.8-89.4 wt% AlON and 0.2-0.6 wt% B2O3; when aluminum borate is added to the embedding material, the content ratio of BN, AlON and AlBO3 is 10-20 wt% BN, 79.5-88.5 wt% AlON and 0.5-1.5 wt% AlBO3.

[0016] Preferably, the pressureless sintering temperature is 1820-1860℃, the sintering time is 5-10h, and the sintering atmosphere is nitrogen.

[0017] Secondly, the present invention provides a transparent AlON ceramic obtained by the above preparation method, wherein the transparent AlON ceramic has a linear transmittance of 81-86% in the 300-2000nm wavelength range, a three-point bending strength of 300-350MPa, and a Vickers hardness of 17-18GPa.

[0018] Beneficial effects

[0019] This invention uses AlN / Al2O3 composite powder with carbon coating on the surface as raw material to synthesize AlON through carbothermic-nitriding reaction. Compared with AlON synthesized by carbothermic-nitriding reaction using Al2O3 with carbon coating on the surface as raw material, the raw material powder has less carbon content, which can effectively inhibit the high-temperature volatilization of Al2O3 during AlON synthesis and reduce the reaction damage to the heat insulation material inside the furnace.

[0020] Compared with the conventional AlN / Al2O3 solid-phase reaction synthesis of AlON, the presence of carbon coating layers on the surface of Al2O3 and AlN raw material particles can significantly inhibit the aggregation and growth of Al2O3 and AlN particles, improve the AlON synthesis reaction kinetics, and reduce the synthesis temperature and particle size of AlON powder.

[0021] The AlON powder synthesis method of this invention can achieve the synthesis of high-purity AlON powder at around 1700℃. The powder synthesis temperature is reduced by more than 50℃ compared with conventional methods. The resulting AlON powder particles are smaller and have higher sintering activity. Using the above powder as raw material, combined with the use of efficient composite oxide sintering aids and special embedding materials in this invention, pressureless sintering preparation of high-transparency AlON ceramics can be achieved at 1820-1860℃. The sintering temperature of the material is reduced by more than 80℃ compared with conventional technology, and the material has excellent mechanical and optical properties. Attached Figure Description

[0022] Figure 1 The XRD pattern of the high-purity AlON powder in Example 1;

[0023] Figure 2 This is a scanning electron microscope image of the high-purity AlON powder in Example 1 after ball milling for 15 hours;

[0024] Figure 3 A digital photograph of the transparent AlON ceramic sample prepared in Example 1;

[0025] Figure 4 The optical linear transmittance curve of the 2 mm thick transparent AlON ceramic sample prepared in Example 1;

[0026] Figure 5 A digital photograph of the transparent AlON ceramic sample prepared in Example 2;

[0027] Figure 6 The image shows the linear optical transmittance curve of the 2mm thick transparent AlON ceramic sample prepared in Example 2. Detailed Implementation

[0028] The present invention is further illustrated by the embodiments described below. It should be understood that the embodiments described below are for illustrative purposes only and are not intended to limit the present invention.

[0029] This invention provides a method for preparing transparent AlON ceramics by pressureless sintering, involving low-temperature synthesis of high-purity AlON powder, design of efficient sintering aid composition, and design of embedded material composition. The following exemplarily illustrates the preparation method of transparent AlON ceramics provided by this invention, which includes the following steps.

[0030] (1) Preparation of high-purity AlON powder. Using citric acid as a dispersant, nano-Al2O3 and submicron AlN powders with a specific composition ratio were uniformly dispersed and mixed in an ethanol solution of thermosetting phenolic resin to obtain a slurry; after vacuum drying, a composite precursor of Al2O3 and AlN particles coated with phenolic resin was obtained; then, it was pyrolyzed at high temperature in an inert atmosphere to form Al2O3-AlN-C composite powder with pyrolyzed carbon coated on the particle surface; then, it was carbothermally reduced in nitrogen to obtain the high-purity AlON powder.

[0031] In some embodiments, the particle size of the nano-Al₂O₃ can be 20-50 nm, and the particle size of the submicron AlN can be 0.3-0.6 μm. Controlling the particle size of the raw material powder can effectively increase the AlON synthesis reaction rate while avoiding AlN hydrolysis.

[0032] In some embodiments, the mass ratio of the raw materials Al2O3, AlN, and phenolic resin can be 80-87:12-16:2-4 to ensure the purity of the subsequently synthesized AlON powder. The amount of citric acid dispersant can be 1-2 wt% of the total mass of Al2O3 and AlN.

[0033] The vacuum drying temperature is 60-80℃, and the time is 10-24h.

[0034] The inert gas can be argon; the high-temperature pyrolysis in the inert atmosphere can be at a temperature of 800–1000°C for 1–2 hours. The pyrolysis carbon from phenolic resin inhibits the aggregation and growth of Al₂O₃ and AlN particles, thereby improving the AlON synthesis reaction kinetics and reducing the synthesis temperature and particle size of AlON powder.

[0035] The carbothermic reduction in nitrogen can be carried out at a temperature of 1680–1720 °C for 1–3 hours. If the synthesis temperature is too low or the synthesis time is too short, the AlON powder synthesis may be insufficient, resulting in impurity phases. Conversely, if the synthesis temperature is too high or the synthesis time is too long, the AlON powder particles may grow excessively.

[0036] Preferably, after carbothermic reduction, the composite powder can be oxidized in air at 650-700°C for 1-2 hours to remove any residual carbon that may be present in the powder.

[0037] The high-purity AlON powder has a particle size of less than 1.5 μm and a purity of more than 95%.

[0038] (2) Preparation of AlON ceramic sintered powder. The high-purity AlON powder obtained in step (1) is ball-milled, and then a composite oxide sintering aid containing boron and rare earth elements is introduced into the high-purity AlON powder through precipitation coating or mechanical mixing process to obtain AlON ceramic sintered powder.

[0039] In some embodiments, the ball milling speed can be 250-300 rpm, and the milling time can be 15-30 hours. The particle size of the high-purity AlON powder after ball milling can be controlled to be 0.3-0.6 μm. Ball milling can significantly reduce the particle size and agglomeration of the powder, and improve the sintering properties of the powder.

[0040] The precipitation coating process can be as follows: a mixed solution of rare earth nitrate and boric acid of appropriate concentration is added dropwise to the AlON slurry, and then ammonia gas is introduced to cause the rare earth ions and boric acid to undergo a precipitation reaction, thereby achieving uniform coating of sintering aids on the surface of AlON particles.

[0041] The mechanical mixing process can be: wet ball milling or wet mechanical stirring mixing of AlON powder and sintering aid.

[0042] The boron- and rare earth element-containing composite oxide sintering aid can be a rare earth oxide / boron oxide, a rare earth borate / boron oxide, or a rare earth oxide / rare earth borate. The rare earth oxide can be at least one of yttrium oxide (Y₂O₃), lanthanum oxide (La₂O₃), or scandium oxide (Sc₂O₃); the rare earth borate can be at least one of yttrium borate (YBO₃), lanthanum borate (LaBO₃), or scandium borate (ScBO₃).

[0043] In some embodiments, the total content of the boron- and rare earth element composite oxide sintering aid in the AlON ceramic sintering powder can be controlled to be 0.4–0.8 wt%; preferably, the total content of rare earth oxides in the boron- and rare earth element composite oxide sintering aid does not exceed 0.5 wt%, and the total content of boron oxide does not exceed 0.4 wt%. If the content of the sintering aid is too low, the sintering aid effect will be insufficient, causing the sintering temperature of the material to rise; conversely, if the content of the sintering aid is too high, it is easy to introduce impurity phases into the material, affecting the transparency of the material.

[0044] By utilizing the low liquid phase temperature characteristics of composite oxide sintering aids containing boron and rare earth elements and their high-temperature solid solubility in AlON, the sinterability of sintered powders can be effectively improved and the sintering temperature of materials can be reduced.

[0045] (3) Preparation of transparent AlON ceramic. The AlON ceramic sintering powder obtained in step (2) is shaped into a green body with a diameter of 100-120 mm and a thickness of 7-8 mm. Then, the green body is embedded in BN-AlON embedding material containing boron oxide (B2O3) or aluminum borate (AlBO3) for pressureless sintering to obtain the transparent AlON ceramic.

[0046] By utilizing the high-temperature volatilization of boron in the embedding material, the loss of boron from the sintering aids inside the material during sintering is effectively compensated, ensuring low-temperature sintering of the material. Otherwise, if the embedding material does not contain boron oxide (B2O3) or aluminum borate (AlBO3), the sintering temperature of the material will rise due to the volatilization of boron oxide sintering aids inside the green body.

[0047] In some embodiments, when boron oxide (B2O3) is added to the embedding material, the content ratio of BN, AlON and B2O3 can be 10-20 wt% BN, 79.8-89.4 wt% AlON and 0.2-0.6 wt% B2O3; when aluminum borate (AlBO3) is added to the embedding material, the content ratio of BN, AlON and AlBO3 can be 10-20 wt% BN, 79.5-88.5 wt% AlON and 0.5-1.5 wt% AlBO3.

[0048] In some embodiments, the pressureless sintering temperature can be 1820-1860℃, the sintering time can be 5-10 hours, and the sintering atmosphere is nitrogen. If the sintering temperature is too high or the sintering holding time is too long, excessive grain growth can easily occur, leading to a decrease in the mechanical properties of the material. Conversely, if the sintering temperature is too low or the sintering holding time is too short, insufficient sintering density of the material can easily occur, resulting in a decrease in optical properties.

[0049] In the preparation process disclosed in this invention, high-purity AlON powder can be synthesized at around 1700℃, which is more than 50℃ lower than the conventional method. Furthermore, the resulting AlON powder particles are smaller and have higher sintering activity. Using the above powder as raw material, combined with the use of the efficient composite oxide sintering aid and special embedding material in this invention, pressureless sintering preparation of high-transparency AlON ceramics can be achieved at 1820-1860℃. The sintering temperature is more than 80℃ lower than conventional techniques, and the material exhibits excellent mechanical and optical properties.

[0050] The transparent AlON ceramics obtained by the preparation method provided by this invention have a linear transmittance of 81-86% in the 300-2000nm wavelength range, a three-point bending strength of 300-350MPa, and a Vickers hardness of 17-18GPa.

[0051] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention fall within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0052] Example 1

[0053] (1) Preparation of high-purity AlON powder.

[0054] 3g of thermosetting phenolic resin, 86g of γ-Al₂O₃ with a particle size of 30nm, 12.5g of AlN with a particle size of 0.5μm, and 1.5g of citric acid were added to 300ml of anhydrous ethanol and ball-milled for 4 hours to ensure uniform mixing of the components and obtain a slurry. The obtained slurry was then dried at 65℃ for 24h and pyrolyzed at 900℃ under an argon atmosphere for 2h to obtain Al₂O₃-AlN-C composite powder with pyrolytic carbon coated on the particle surface.

[0055] The obtained Al2O3-AlN-C composite powder was subjected to a carbothermic reduction reaction at 1700℃ in nitrogen for 2 hours, then oxidized in air at 700℃ for 1 hour to remove any residual carbon that might be present in the powder, and further ball-milled for 15 hours to obtain the high-purity AlON powder.

[0056] (2) Preparation of AlON ceramic sintered powder.

[0057] 0.17g Y(NO3)3·6H2O, 0.033g La(NO3)3·6H2O, 0.49g Sc(NO3)3·6H2O, and 0.5g H3BO3 were dissolved in 50ml of anhydrous ethanol to form a rare earth nitrate / boric acid mixed solution. Using 1.5g citric acid as a dispersant, 100g of AlON powder prepared in step (1) was dispersed in 200ml of anhydrous ethanol by ball milling to form a stable AlON slurry. While stirring and titrating, the above rare earth nitrate / boric acid mixed solution was slowly added to the AlON slurry. Stirring was continued for 15 minutes in an ammonia atmosphere to fully convert the rare earth nitrates and boric acids in the slurry into rare earth hydroxides and ammonium borate precipitates, which were uniformly coated on the surface of the AlON particles.

[0058] The above slurry was dried at 65°C for 20 hours and calcined in air at 700°C for 2 hours to completely convert the rare earth hydroxides and ammonium borate into rare earth oxides and boron oxide, yielding AlON ceramic sintered powder containing a composite sintering aid of rare earth oxides and boron oxide. The contents of Y₂O₃, La₂O₃, Sc₂O₃, and B₂O₃ were 0.1 wt%, 0.025 wt%, 0.2 wt%, and 0.28 wt%, respectively.

[0059] (3) Preparation of transparent AlON ceramics.

[0060] The AlON ceramic sintered powder obtained in step (2) was cold isostatically pressed under a pressure of 200 MPa to obtain an AlON green body. Then, it was embedded in a BN-AlON embedding material containing B2O3 and sintered without pressure at 1840℃ for 6 hours. The composition of the BN-AlON embedding material used for sintering was 0.3 wt% B2O3, 15 wt% BN, and 84.7 wt% AlON. The sintering process was carried out in a high-temperature graphite furnace under a nitrogen atmosphere, resulting in transparent AlON ceramic. Measurements showed that the three-point flexural strength of the transparent AlON ceramic sample reached 350 MPa, and the Vickers hardness reached 18 GPa.

[0061] Figure 1 The image shows the XRD pattern of the high-purity AlON powder in Example 1. As can be seen from the image, the XRD pattern of the AlON powder synthesized at 1700℃ shows almost no diffraction peaks for impurity phases such as Al2O3 and AlN, indicating that the powder has very high purity.

[0062] Figure 2 The image shows a scanning electron microscope (SEM) image of the high-purity AlON powder from Example 1 after ball milling for 15 hours. As can be seen from the image, the average particle size of the AlON powder is approximately 0.5 μm, and the particles are uniform in size and well-dispersed, which lays a good foundation for the subsequent sintering preparation of transparent AlON ceramics.

[0063] Figure 3 This is a digital photograph of the transparent AlON ceramic sample prepared in Example 1. As can be seen from the image, the material exhibits good optical transparency; text is clearly visible beneath a 5mm thick sample.

[0064] Figure 4 The figure shows the optical linear transmittance curve of the 2 mm thick transparent AlON ceramic sample prepared in Example 1. As can be seen from the figure, the linear transmittance of the material reaches 82.6% in the 300 nm ultraviolet band and 86.0% in the 600 nm visible light band, and 86.1% in the 2000 nm infrared band. This indicates that the material exhibits high linear transmittance from the ultraviolet to the infrared bands, fully demonstrating its excellent optical performance.

[0065] Example 2

[0066] (1) Preparation of high-purity AlON powder. Refer to Example 1.

[0067] (2) Preparation of AlON ceramic sintered powder.

[0068] Using high-purity alumina balls as the ball milling medium and 1.5g citric acid as the dispersant, 100g of AlON powder prepared in step (1) was uniformly mixed with 0.26g YBO3, 0.03g LaBO3 and 0.2g B2O3 in 200ml anhydrous ethanol through ball milling to form a stable AlON slurry.

[0069] The AlON slurry was dried at 65°C for 20 hours and calcined in air at 700°C for 2 hours to obtain AlON ceramic sintering powder containing a composite oxide sintering aid of boron and rare earth elements.

[0070] (3) Preparation of transparent AlON ceramics.

[0071] The AlON ceramic sintered powder prepared in step (2) was cold isostatically pressed under a pressure of 200 MPa to obtain an AlON green body; then it was embedded in a BN-AlON embedding material containing AlBO3 and sintered without pressure at 1860℃ for 6 h. The composition ratio of the BN-AlON embedding material used for sintering was 0.6 wt% AlBO3, 15 wt% BN and 84.4 wt% AlON. The sintering process was carried out in a high-temperature graphite furnace under a nitrogen atmosphere to obtain transparent AlON ceramic.

[0072] Figure 5 This is a digital photograph of the transparent AlON ceramic sample prepared in Example 2. As can be seen from the image, the material has good transparency, and the text beneath the 3mm thick sample is clearly visible.

[0073] Figure 6 The figure shows the optical linear transmittance curve of the 2 mm thick transparent AlON ceramic sample prepared in Example 2. As can be seen from the figure, the material exhibits excellent optical properties, with linear transmittances of 81.6%, 85.1%, and 86.2% in the 300 nm, 600 nm, and 2000 nm wavelength bands, respectively, demonstrating good linear transmittance from the ultraviolet to the mid-infrared bands.

[0074] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for preparing transparent AlON ceramics, characterized in that, include: Al2O3 and AlN powders are mixed in thermosetting phenolic resin to obtain a slurry. After drying and high-temperature pyrolysis, Al2O3-AlN-C composite powder with pyrolytic carbon coated on the particle surface is formed. Then, carbothermic reduction is carried out in nitrogen to obtain high-purity AlON powder. The high-purity AlON powder is introduced with a composite oxide sintering aid containing boron and rare earth elements through precipitation coating or mechanical mixing process to obtain AlON ceramic sintered powder. AlON ceramic sintering powder is formed into a green body, and then embedded in BN-AlON embedding material containing boron oxide or aluminum borate for pressureless sintering to obtain the transparent AlON ceramic.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the raw materials Al2O3, AlN and phenolic resin is 80-87:12-16:2-4.

3. The preparation method according to claim 1, characterized in that, The high-temperature pyrolysis is performed at a temperature of 800–1000℃ for 1–2 hours.

4. The preparation method according to claim 1, characterized in that, The carbothermal reduction is carried out at a temperature of 1680–1720°C for 1–3 hours.

5. The preparation method according to claim 1, characterized in that, The boron-containing and rare earth element composite oxide sintering aid is a rare earth oxide / boron oxide, a rare earth borate / boron oxide, or a rare earth oxide / rare earth borate. The rare earth oxide is at least one of yttrium oxide, lanthanum oxide, or scandium oxide, and the rare earth borate is at least one of yttrium borate, lanthanum borate, or scandium borate.

6. The preparation method according to claim 1, characterized in that, The total content of the boron- and rare earth element composite oxide sintering aid in the AlON ceramic sintering powder is controlled at 0.4-0.8 wt%; the total content of rare earth oxides in the boron- and rare earth element composite oxide sintering aid does not exceed 0.5 wt%, and the total content of boron oxide does not exceed 0.4 wt%.

7. The preparation method according to claim 1, characterized in that, When boron oxide is added to the embedding material, the content ratio of BN, AlON and B2O3 is 10-20wt% BN, 79.8-89.4wt% AlON and 0.2-0.6wt% B2O3; when aluminum borate is added to the embedding material, the content ratio of BN, AlON and AlBO3 is 10-20wt% BN, 79.5-88.5wt% AlON and 0.5-1.5wt% AlBO3.

8. The preparation method according to claim 1, characterized in that, The pressureless sintering temperature is 1820-1860℃, the sintering time is 5-10h, and the sintering atmosphere is nitrogen.

9. A transparent AlON ceramic obtained by the preparation method according to claim 1, characterized in that, The transparent AlON ceramic has a linear transmittance of 81-86% in the 300-2000nm wavelength range; a three-point bending strength of 300-350MPa; and a Vickers hardness of 17-18GPa.

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