A toughened alumina ceramic and a method of making the same

Abstract

This invention discloses a toughened alumina ceramic and its preparation method, comprising the following components by weight: 2-6 parts clay, 20-30 parts nano-aluminum powder, 70-90 parts alumina powder, 8-12 parts concentrated sulfuric acid, 8-12 parts microcrystalline cellulose, 3-4 parts sintering aid, 3-4 parts quartz, 3-4 parts calcite, 12-16 parts graphene, and 5-7 parts binder. This invention relates to the field of alumina ceramic technology. A pressing molding method is used to uniformly add nanofibers to the ceramic material. The nanofibers, acting as a toughening phase in the matrix, significantly enhance the fracture toughness and relative strength of the composite material. During fracture, the fiber-reinforced ceramic exhibits increased surface energy through fiber pull-out, bridging, debonding, and fracture, as well as crack micronization, bending, and deflection, thereby improving toughness.

Description

Technical Field

[0001] This invention relates to the field of alumina ceramics technology, specifically to a toughened alumina ceramic and its preparation method. Background Technology

[0002] Alumina ceramics possess excellent mechanical and electrical properties and are inexpensive to manufacture, making them one of the most widely used advanced ceramic materials. High-purity submicron-grade alumina ultrafine powder has been used to prepare alumina precision ceramics with flexural strengths reaching 700–1000 MPa. However, the fracture toughness of this material is very low, typically only 3–4 MPa·m1 / 2. Therefore, improving the fracture toughness of alumina ceramics has always been a goal and a focus of research for ceramic materials scientists.

[0003] There are generally four methods for toughening ceramics: dispersion toughening, phase transformation toughening, fiber and whisker toughening, and the recently developed carbon nanotube toughening. Dispersion toughening has very limited effects; while phase transformation toughening of zirconia ceramics can increase the fracture toughness of the material by 2-3 times, the effect decreases sharply with increasing temperature; carbon nanotubes are prone to structural damage during mass production, dispersion, and subsequent sintering, and their synthesis process is complex and expensive, limiting their application. The significant toughening effect and good high-temperature performance make fiber and whisker toughening the preferred method and a key research focus for ceramic toughening.

[0004] Due to their excellent mechanical properties and the toughening effects they produce, such as crack deflection, crack bridging, and whisker pull-out, the room temperature and high temperature flexural strength, fracture toughness, thermal shock resistance, wear resistance, and creep properties of ceramic composites toughened by fibers or whiskers are improved to varying degrees.

[0005] There are various molding methods for alumina ceramic products, including dry pressing, slip casting, extrusion, cold isostatic pressing, injection molding, tape casting, hot pressing, and hot isostatic pressing. In recent years, domestic and international manufacturers have also developed molding technologies such as pressure filtration molding, direct solidification molding, gel casting, centrifugal slip casting, and solid freeform molding. Different product shapes, sizes, complex designs, and precision requirements necessitate different molding methods.

[0006] Dry pressing: Dry pressing technology for alumina ceramics is limited to objects with simple shapes and inner wall thicknesses exceeding 1mm, and length-to-diameter ratios not exceeding 4:1. Forming methods include uniaxial or biaxial processes. Presses are available in hydraulic and mechanical types, and can be semi-automatic or fully automatic. The maximum press pressure is 200 MPa. Production capacity can reach 15-50 pieces per minute. Due to the uniform stroke pressure of hydraulic presses, the height of pressed parts may vary when there are differences in powder filling. Slip casting: Slip casting is the earliest forming method used for alumina ceramics. It is low-cost and easy to form large-sized, complex-shaped parts due to the use of plaster molds. The key to slip casting is the preparation of the alumina slurry. Water is usually used as the flux medium, with the addition of a desizing agent and binder. After thorough grinding and degassing, the slurry is poured into a plaster mold.

[0007] Existing ceramic composite materials lack fracture toughness and relative strength, resulting in relatively poor mechanical properties and easy cracking and fracture on the ceramic surface, which affects the quality of the ceramic. Summary of the Invention

[0008] To solve the above-mentioned technical problems, the present invention provides a toughened alumina ceramic, comprising the following components by weight: 2-6 parts clay, 15-35 parts nano aluminum powder, 65-95 parts alumina powder, 6-14 parts concentrated sulfuric acid, 6-14 parts microcrystalline cellulose, 2-5 parts sintering aid, 2-5 parts quartz, 2-5 parts calcite, 12-16 parts graphene, and 5-7 parts binder. The method for preparing the alumina powder includes the following steps: Step A1: Select aluminum salt as the aluminum source and recrystallize the aluminum salt to improve its purity; Step A2: Weigh the purified aluminum salt and calculate the required amount of precipitant (ammonia) according to the stoichiometric ratio of the chemical reaction. Step A3: Use deionized water as the solvent and control the conductivity of the water to be below 1 μS / cm; Step A4: Select sodium dodecylbenzenesulfonate, accurately weigh the required amount, and prepare a solution with a concentration of 0.01-0.05 mol / L using deionized water for later use; Step A5: Under stirring conditions, slowly add the aluminum salt solution dropwise to the precipitant solution, monitor the pH, and after the addition is complete, continue stirring for 1-2 hours to allow the reaction to proceed fully and obtain aluminum hydroxide precipitate; Then, an aging process is carried out, and the reaction system is allowed to stand and age for 6-12 hours, with the aging temperature controlled at 30-40℃, to obtain aluminum hydroxide precursor. Step A6: The aluminum hydroxide precursor is washed by multiple centrifugal washing. After washing, solid-liquid separation is performed using a vacuum filtration device. A microporous filter membrane with a pore size of 0.1-0.2 μm is selected to obtain pure aluminum hydroxide precursor. Step A7: Transfer the washed aluminum hydroxide precursor to a high-pressure reactor, add deionized water at a solid-liquid ratio of 1:10-1:20 g / mL, and add the previously prepared organic additive solution. After sealing the reactor, place it in a heating furnace for hydrothermal reaction. Step A8: After the hydrothermal reaction is completed, allow the reactor to cool naturally to room temperature. After cooling, perform centrifugation again to separate the plate-shaped crystalline alumina powder. Dry the separated plate-shaped alumina powder in a vacuum drying oven. Grind the dried product using an agate mortar to obtain the final plate-shaped crystalline alumina powder.

[0009] A method for preparing toughened alumina ceramic includes the following steps: Step 1: Preparation of toughening material: Prepare a reaction vessel, inject concentrated sulfuric acid and microcrystalline cellulose into the reaction vessel for mixing, and complete the preparation of nanocellulose. Step 2: Ceramic powder preparation: Nano aluminum powder and high-purity alumina powder are used as initial raw materials, mixed evenly, and the mixed slurry is dried and ground to obtain ceramic powder; Step 3: Mix the ceramic powder and sintering aid, and use distilled water as the ball milling medium to perform mixing ball milling to obtain alumina slurry; Step 4: Add nanocellulose to alumina slurry and ball mill to disperse it evenly to obtain a mixed slurry; Step 5: Mix the slurry with clay, quartz, graphene, calcite and binder to obtain a mixed blank; Step 6: Granulate, press, dry, and sinter the mixed raw material to obtain toughened alumina ceramic.

[0010] The alumina is alumina powder with a purity of ≥99.9%; The average particle size D50 of the alumina is 0.3–0.4 μm; The magnesium oxide is magnesium oxide powder of analytical purity or higher; The average particle size D50 of the magnesium oxide is 0.2–0.4 μm; The zirconium oxide is zirconium oxide powder with an analytical purity of not less than that of analytical grade; The average particle size D50 of the zirconium oxide is 0.1–0.3 μm.

[0011] Preferably, the method for preparing the nanocellulose includes the following steps: Step 1: Prepare a measuring cup, add 10 parts by weight of microcrystalline cellulose into the reactor, and then take out 300-400ml of 45-65% concentrated sulfuric acid and add it into the reactor. Use an external heating device to heat the entire reactor. Step 2: Control the heating time within 30-50 minutes. After the color changes, add an appropriate amount of water directly into the reactor and use an external centrifuge to stir and settle the mixture. Step 3: The precipitate produced after stirring is screened in batches, with each batch consisting of 60ml. The precipitates of the same proportion are screened to extract nanocellulose.

[0012] Preferably, the sintering aid includes at least one of calcium oxide, magnesium oxide, silicon dioxide, lanthanum oxide, and titanium dioxide.

[0013] Preferably, the sintering aid is a mixture of titanium dioxide and magnesium oxide, wherein the mass ratio of titanium dioxide to magnesium oxide in the mixture is 1 to 3:1.

[0014] Preferably, the sintering aid is prepared by the following method: titanium dioxide and magnesium oxide are mixed in a certain mass ratio, zirconium oxide is used as the milling ball, the speed of the mill is controlled at 700-900 r / min, the milling time is controlled at 4-8 h, and the mixture is obtained by dry milling.

[0015] Preferably, the pressing and molding in step six is ​​performed using an external pressing and molding device, and the pressing time and pressing weight are effectively controlled. The pressing time is controlled at 2-5 minutes, and the pressing weight is set at 5-10 N.

[0016] The pressing process preferably includes one of roll forming, isostatic pressing, and axial pressing; the pressing pressure is preferably 100-500 MPa, more preferably 200-300 MPa; the pressing time is preferably 1-5 min, more preferably 2-3 min.

[0017] Preferably, the drying in step six refers to drying the pressed blank and cutting the dried blank into segments, and drying multiple sets of blanks with different segments in batches. When drying the blank, the drying temperature is set at 50-80℃ and the drying time is set at 8-12h.

[0018] The process of obtaining nanofiber modified ceramic green bodies by segmented drying of the formed green bodies is as follows: the green bodies are dried in segments at 40-60℃ for 10-12 hours; kept at 60-80℃ for 10-12 hours; and kept at 80-100℃ for 22-24 hours.

[0019] Preferably, in step six, sintering refers to placing the segmented dried blank in a sintering furnace, controlling the sintering temperature at 1400-1600℃, and controlling the sintering time at 5-7 minutes to prepare a toughened alumina ceramic blank.

[0020] The sintering temperature is preferably 1200–1500℃, more preferably 1300–1400℃; the sintering time is preferably 30–120 min, more preferably 60–100 min, and even more preferably 80–90 min. In this invention, the sintering can improve the density of the pressed preform, thereby improving the mechanical properties of the composite material.

[0021] The sintering is preferably performed using vacuum sintering; the vacuum degree of the vacuum sintering is preferably not higher than 3 × 110 Pa.

[0022] This invention provides a toughened alumina ceramic and its preparation method. It has the following beneficial effects: 1. This invention improves the mechanical properties of ceramics by adding nanofibers. The nanofibers are uniformly added to the ceramic material by compression molding. The nanofibers serve as a toughening phase of the matrix, which significantly enhances the fracture toughness and relative strength of the composite material. 2. The nanofibers used in this invention have a small fiber diameter, and their specific surface area is generally 1-2 orders of magnitude larger than that of micron-sized fibers. For fiber-reinforced ceramics, the fracture process of ceramic materials is essentially a process of increasing surface energy. When fiber-reinforced ceramics fracture, the surface energy increment during fracture is increased through fiber pull-out, bridging, debonding and fracture, as well as crack miniaturization, bending and deflection, thereby improving toughness. 3. The presence of nanofibers during the high-temperature sintering process of this invention provides a large interface area and size effect as a reinforcing phase, which can generate a stable crystalline phase, effectively hinder the occurrence of phase transformation reaction at high temperature, improve the stability of fiber-reinforced ceramics, and improve their toughness and strength after high-temperature sintering. 4. The alumina ceramic composite material provided by the present invention uses alumina as the matrix. By adjusting the content, graphene and zirconium oxide can play a synergistic toughening effect, thereby improving the toughness of the alumina ceramic material. Detailed Implementation

[0023] The following detailed description of specific embodiments further illustrates the present invention. The embodiments of the present invention are given for illustrative and descriptive purposes only and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and design various embodiments with various modifications suitable for a particular purpose.

[0024] Example 1: A toughened alumina ceramic, comprising the following components by weight: 4 parts clay, 25 parts nano aluminum powder, 80 parts alumina powder, 10 parts concentrated sulfuric acid, 10 parts microcrystalline cellulose, 3.5 parts sintering aid, 3.5 parts quartz, 3.5 parts calcite, 14 parts graphene, and 6 parts binder. The method for preparing the alumina powder includes the following steps: 1. Aluminum source selection and purification High-purity aluminum salts are selected as the aluminum source, such as analytical grade aluminum nitrate or aluminum sulfate. The aluminum salts are then recrystallized to further improve their purity. Taking aluminum nitrate as an example, it is dissolved in an appropriate amount of deionized water, heated until completely dissolved, and then slowly cooled to allow crystallization. The crystallized product is collected by filtration. This process is repeated 2-3 times to remove impurity ions. Accurately weigh the purified aluminum salt, accurately calculate the required amount of precipitant (such as ammonia) according to the stoichiometric ratio of the chemical reaction, and prepare a slightly excess of precipitant to take into account the fine adjustment of the pH value in the subsequent reaction system.

[0025] 2. Solvent and Additive Preparation Deionized water was used as a solvent, and the conductivity of the water was controlled to be below 1 μS / cm in order to reduce the influence of impurities in the water on the reaction. Prepare organic additives, such as polyethylene glycol (PEG) or sodium dodecylbenzenesulfonate (SDBS). For PEG, choose a molecular weight in the range of 2000-6000, accurately weigh a certain amount according to experimental requirements, and dissolve it in a small amount of deionized water to prepare a solution for later use. For sodium dodecylbenzenesulfonate, accurately weigh an appropriate amount and prepare a solution with a concentration of 0.01-0.05 mol / L using deionized water.

[0026] 3. Precursor preparation stage 4. Precipitation reaction operation Under stirring conditions, the aluminum salt solution is slowly added dropwise to the precipitant solution, while the pH value of the reaction system is monitored in real time using a precision pH meter. By coordinating the dropwise and stirring speeds, the pH value of the reaction system is maintained within a stable range (e.g., for ammonia precipitants, the pH value is stabilized at 9-10). The stirring speed is initially set at 200-300 rpm, and can be appropriately reduced to 100-150 rpm in the later stages of the dropwise addition to avoid generating excessive bubbles. After the addition is complete, continue stirring for 1-2 hours to ensure the reaction proceeds fully and aluminum hydroxide precipitate is obtained. Then, an aging treatment is performed by allowing the reaction system to stand for 6-12 hours at a temperature controlled at 30-40℃, which helps the growth and stabilization of the precipitate particles.

[0027] 5. Precursor washing and separation The aluminum hydroxide precursor was washed using multiple centrifugal washes. Each centrifugation was performed at 3000-5000 rpm for 10-15 minutes. The amount of deionized water used was 3-5 times the mass of the precursor, and the washing was repeated 3-5 times to ensure the removal of residual impurity ions from the precursor. After washing, solid-liquid separation is performed using a vacuum filtration device. A microporous filter membrane with a pore size of 0.1-0.2 μm is selected to ensure good filtration effect and obtain a pure aluminum hydroxide precursor filter cake.

[0028] 6. Reactor Pretreatment The high-pressure reactor should be cleaned and dried. First, rinse the inner wall of the reactor with deionized water 3-5 times, then dry it in an oven at 80-100℃ for 2-3 hours. Apply a thin, even layer of polytetrafluoroethylene (PTFE) coating to the inner wall of the reactor to prevent the product from adhering to the reactor wall during the reaction.

[0029] Hydrothermal reaction parameter control The washed aluminum hydroxide precursor was transferred to a high-pressure reactor. Deionized water was added at a solid-liquid ratio (precursor mass to water volume ratio) of 1:10-1:20 (g / mL), along with the previously prepared organic additive solution. The reactor was sealed and placed in a heating furnace for hydrothermal reaction. The reaction temperature was set at 150-180℃, and the reaction pressure was automatically adjusted based on the relationship between the saturated vapor pressure of water and the reaction temperature. The reaction time was 12-24 hours. During the heating process, the heating rate was controlled at 2-3℃ / min to avoid excessively rapid temperature changes affecting the crystal form of the product.

[0030] 7. Cooling and Separation After the hydrothermal reaction is complete, the reactor is allowed to cool naturally to room temperature. During the cooling process, the internal pressure of the reactor is gradually reduced to avoid rapid pressure drop that could damage the product structure. After cooling, centrifugation is performed again, using the same parameters as during precursor washing. The solid product obtained is the initially prepared plate-like crystalline alumina powder.

[0031] 8. Drying and grinding The separated plate-shaped alumina powder was dried in a vacuum drying oven. The drying temperature was set at 60-80℃, the vacuum degree was controlled at -0.08-0.1MPa, and the drying time was 8-12 hours. The dried product was then ground using an agate mortar, taking care to avoid introducing impurities during the grinding process, to obtain the final plate-shaped crystalline alumina powder product. The particle size distribution and microstructure of the product were analyzed using a laser particle size analyzer and a scanning electron microscope to evaluate the product quality.

[0032] A method for preparing toughened alumina ceramic includes the following steps: Step 1: Preparation of toughening material: Prepare a reaction vessel, inject concentrated sulfuric acid and microcrystalline cellulose into the reaction vessel for mixing, and complete the preparation of nanocellulose. Step 2: Ceramic powder preparation: Nano aluminum powder and high-purity alumina powder are used as initial raw materials, mixed evenly, and the mixed slurry is dried and ground to obtain ceramic powder; Step 3: Mix the ceramic powder and sintering aid, and use distilled water as the ball milling medium to perform mixing ball milling to obtain alumina slurry; Step 4: Add nanocellulose to alumina slurry and ball mill to disperse it evenly to obtain a mixed slurry; Step 5: Mix the slurry with clay, quartz, graphene, calcite and binder to obtain a mixed blank; Step 6: Granulate, press, dry, and sinter the mixed raw material to obtain toughened alumina ceramic.

[0033] In the grinding process of this invention, it is preferably carried out in a ball mill, with a preferred milling speed of 400-500 r / min; the preferred milling time is 1-1.5 h, more preferably 1.2-1.3 h. This invention does not have a specific limitation on the type of ball mill; any ball mill well-known to those skilled in the art can be used. This invention achieves uniform dispersion of raw materials by controlling the milling speed and time.

[0034] The sintering aids include at least one of calcium oxide, magnesium oxide, silicon dioxide, lanthanum oxide, and titanium dioxide.

[0035] The sintering aid is a mixture of titanium dioxide and magnesium oxide, with a mass ratio of titanium dioxide to magnesium oxide of 1 to 3:1.

[0036] The sintering aid was prepared by mixing titanium dioxide and magnesium oxide in a certain mass ratio, using zirconium oxide as the milling ball, controlling the speed of the mill to be 700-900 r / min, and controlling the milling time to be 4-8 h, and then performing dry milling.

[0037] In step six, the pressing and molding process is carried out using an external pressing and molding device. The pressing time and pressing weight are effectively controlled. The pressing time is controlled at 2-5 minutes, and the pressing weight is set at 5-10 N.

[0038] The drying process in step six refers to drying the pressed and shaped blanks, cutting the dried blanks into segments, and drying multiple sets of blanks with different segments in batches. When drying the blanks, the drying temperature is set at 50-80℃ and the drying time is set at 8-12 hours.

[0039] The sintering in step six refers to placing the segmented dried green body into a sintering furnace, controlling the sintering temperature at 1400-1600℃, and controlling the sintering time at 5-7 minutes to prepare a toughened alumina ceramic green body.

[0040] Example 2: A toughened alumina ceramic, comprising the following components by weight: 4 parts clay, 25 parts nano aluminum powder, 80 parts alumina powder, 10 parts concentrated sulfuric acid, 10 parts microcrystalline cellulose, 3.5 parts sintering aid, 3.5 parts quartz, 3.5 parts calcite, 14 parts graphene, and 6 parts binder. A method for preparing toughened alumina ceramic includes the following steps: Step 1: Preparation of toughening material: Prepare a reaction vessel, inject concentrated sulfuric acid and microcrystalline cellulose into the reaction vessel for mixing, and complete the preparation of nanocellulose. Step 2: Ceramic powder preparation: Nano aluminum powder and high-purity alumina powder are used as initial raw materials, mixed evenly, and the mixed slurry is dried and ground to obtain ceramic powder; Step 3: Mix the ceramic powder and sintering aid, and use distilled water as the ball milling medium to perform mixing ball milling to obtain alumina slurry; Step 4: Add nanocellulose to alumina slurry and ball mill to disperse it evenly to obtain a mixed slurry; Step 5: Mix the slurry with clay, quartz, graphene, calcite and binder to obtain a mixed blank; Step 6: Granulate, press, dry, and sinter the mixed raw material to obtain toughened alumina ceramic.

[0041] In the grinding process of this invention, it is preferably carried out in a ball mill, with a preferred milling speed of 400-500 r / min; the preferred milling time is 1-1.5 h, more preferably 1.2-1.3 h. This invention does not have a specific limitation on the type of ball mill; any ball mill well-known to those skilled in the art can be used. This invention achieves uniform dispersion of raw materials by controlling the milling speed and time.

[0042] The preparation method of nanocellulose in step three includes the following steps: Step 1: Prepare a measuring cup, add 10 parts by weight of microcrystalline cellulose into the reactor, and then take out 300-400ml of 45-65% concentrated sulfuric acid and add it into the reactor. Use an external heating device to heat the entire reactor. To further explain, concentrated sulfuric acid is slowly added dropwise to the reaction vessel while stirring continuously at a speed of 100-150 r / min. Step 2: Control the heating time within 30-50 minutes. After the color changes, add an appropriate amount of water directly into the reactor and use an external centrifuge to stir and settle the mixture. To further explain, an intelligent temperature control system is used to heat the reactor, with the heating temperature set at 55-60℃. When the reactants change color after heating, the heating is stopped, and an appropriate amount of water (the ratio of water added is 5 times the volume of concentrated sulfuric acid) is added through the reactor's automatic feeding system. Then, the high-speed stirring device is started, and the stirring speed is increased to 500-600 r / min. The stirring time is maintained for 10-15 minutes to ensure that the reaction products are fully dispersed in the water. After stirring, immediately switch to centrifugal sedimentation mode, control the centrifugation speed at 400-450 r / min, and set the centrifugation time to 15-20 min; Step 3: The precipitate produced after stirring is screened in batches, with each batch consisting of 60ml. The precipitates of the same proportion are screened to extract nanocellulose. To further explain, after centrifugation, the precipitate is transferred to an automatic screening device. This device is designed with multiple layers of screens with different pore sizes, arranged from largest to smallest, based on the particle size distribution characteristics of nanocellulose. The top layer of screens has a pore size of 0.5 μm, which gradually decreases to the bottom layer with a pore size of 0.05 μm. During the screening process, the precipitate trapped by each layer of screen is collected and measured separately. Each 60ml is a metering unit. Nanocellulose that meets the particle size requirements is collected and combined as toughening material for subsequent preparation processes. The centrifugal speed inside the external centrifuge device is controlled at 350 r / min, the heating temperature is controlled at 50℃, and the proportion of water added is 5 times that of concentrated sulfuric acid liquid.

[0043] The sintering aids include at least one of calcium oxide, magnesium oxide, silicon dioxide, lanthanum oxide, and titanium dioxide.

[0044] The sintering aid is a mixture of titanium dioxide and magnesium oxide, with a mass ratio of titanium dioxide to magnesium oxide of 1 to 3:1.

[0045] The sintering aid was prepared by mixing titanium dioxide and magnesium oxide in a certain mass ratio, using zirconium oxide as the milling ball, controlling the speed of the mill to be 700-900 r / min, and controlling the milling time to be 4-8 h, and then performing dry milling.

[0046] In step six, the pressing and molding process is carried out using an external pressing and molding device. The pressing time and pressing weight are effectively controlled. The pressing time is controlled at 2-5 minutes, and the pressing weight is set at 5-10 N.

[0047] The drying process in step six refers to drying the pressed and shaped blanks, cutting the dried blanks into segments, and drying multiple sets of blanks with different segments in batches. When drying the blanks, the drying temperature is set at 50-80℃ and the drying time is set at 8-12 hours.

[0048] The sintering in step six refers to placing the segmented dried green body into a sintering furnace, controlling the sintering temperature at 1400-1600℃, and controlling the sintering time at 5-7 minutes to prepare a toughened alumina ceramic green body.

[0049] Example 3: A toughened alumina ceramic, comprising the following components by weight: 4 parts clay, 25 parts nano aluminum powder, 80 parts alumina powder, 10 parts concentrated sulfuric acid, 10 parts microcrystalline cellulose, 3.5 parts sintering aid, 3.5 parts quartz, 3.5 parts calcite, 14 parts graphene, and 6 parts binder. The method for preparing the alumina powder includes the following steps: 1. Aluminum source selection and purification High-purity aluminum salts are selected as the aluminum source, such as analytical grade aluminum nitrate or aluminum sulfate. The aluminum salts are then recrystallized to further improve their purity. Taking aluminum nitrate as an example, it is dissolved in an appropriate amount of deionized water, heated until completely dissolved, and then slowly cooled to allow crystallization. The crystallized product is collected by filtration. This process is repeated 2-3 times to remove impurity ions. Accurately weigh the purified aluminum salt, accurately calculate the required amount of precipitant (such as ammonia) according to the stoichiometric ratio of the chemical reaction, and prepare a slightly excess of precipitant to take into account the fine adjustment of the pH value in the subsequent reaction system.

[0050] 2. Solvent and Additive Preparation Deionized water was used as a solvent, and the conductivity of the water was controlled to be below 1 μS / cm in order to reduce the influence of impurities in the water on the reaction. Prepare organic additives, such as polyethylene glycol (PEG) or sodium dodecylbenzenesulfonate (SDBS). For PEG, choose a molecular weight in the range of 2000-6000, accurately weigh a certain amount according to experimental requirements, and dissolve it in a small amount of deionized water to prepare a solution for later use. For sodium dodecylbenzenesulfonate, accurately weigh an appropriate amount and prepare a solution with a concentration of 0.01-0.05 mol / L using deionized water.

[0051] 3. Precursor preparation stage 4. Precipitation reaction operation Under stirring conditions, the aluminum salt solution is slowly added dropwise to the precipitant solution, while the pH value of the reaction system is monitored in real time using a precision pH meter. By coordinating the dropwise and stirring speeds, the pH value of the reaction system is maintained within a stable range (e.g., for ammonia precipitants, the pH value is stabilized at 9-10). The stirring speed is initially set at 200-300 rpm, and can be appropriately reduced to 100-150 rpm in the later stages of the dropwise addition to avoid generating excessive bubbles. After the addition is complete, continue stirring for 1-2 hours to ensure the reaction proceeds fully and aluminum hydroxide precipitate is obtained. Then, an aging treatment is performed by allowing the reaction system to stand for 6-12 hours at a temperature controlled at 30-40℃, which helps the growth and stabilization of the precipitate particles.

[0052] 5. Precursor washing and separation The aluminum hydroxide precursor was washed using multiple centrifugal washes. Each centrifugation was performed at 3000-5000 rpm for 10-15 minutes. The amount of deionized water used was 3-5 times the mass of the precursor, and the washing was repeated 3-5 times to ensure the removal of residual impurity ions from the precursor. After washing, solid-liquid separation is performed using a vacuum filtration device. A microporous filter membrane with a pore size of 0.1-0.2 μm is selected to ensure good filtration effect and obtain a pure aluminum hydroxide precursor filter cake.

[0053] 6. Reactor Pretreatment The high-pressure reactor should be cleaned and dried. First, rinse the inner wall of the reactor with deionized water 3-5 times, then dry it in an oven at 80-100℃ for 2-3 hours. Apply a thin, even layer of polytetrafluoroethylene (PTFE) coating to the inner wall of the reactor to prevent the product from adhering to the reactor wall during the reaction.

[0054] Hydrothermal reaction parameter control The washed aluminum hydroxide precursor was transferred to a high-pressure reactor. Deionized water was added at a solid-liquid ratio (precursor mass to water volume ratio) of 1:10-1:20 (g / mL), along with the previously prepared organic additive solution. The reactor was sealed and placed in a heating furnace for hydrothermal reaction. The reaction temperature was set at 150-180℃, and the reaction pressure was automatically adjusted based on the relationship between the saturated vapor pressure of water and the reaction temperature. The reaction time was 12-24 hours. During the heating process, the heating rate was controlled at 2-3℃ / min to avoid excessively rapid temperature changes affecting the crystal form of the product.

[0055] 7. Cooling and Separation After the hydrothermal reaction is complete, the reactor is allowed to cool naturally to room temperature. During the cooling process, the internal pressure of the reactor is gradually reduced to avoid rapid pressure drop that could damage the product structure. After cooling, centrifugation is performed again, using the same parameters as during precursor washing. The solid product obtained is the initially prepared plate-like crystalline alumina powder.

[0056] 8. Drying and grinding The separated plate-shaped alumina powder was dried in a vacuum drying oven. The drying temperature was set at 60-80℃, the vacuum degree was controlled at -0.08-0.1MPa, and the drying time was 8-12 hours. The dried product was then ground using an agate mortar, taking care to avoid introducing impurities during the grinding process, to obtain the final plate-shaped crystalline alumina powder product. The particle size distribution and microstructure of the product were analyzed using a laser particle size analyzer and a scanning electron microscope to evaluate the product quality.

[0057] A method for preparing toughened alumina ceramic includes the following steps: Step 1: Preparation of toughening material: Prepare a reaction vessel, inject concentrated sulfuric acid and microcrystalline cellulose into the reaction vessel for mixing, and complete the preparation of nanocellulose. Step 2: Ceramic powder preparation: Nano aluminum powder and high-purity alumina powder are used as initial raw materials, mixed evenly, and the mixed slurry is dried and ground to obtain ceramic powder; Step 3: Mix the ceramic powder and sintering aid, and use distilled water as the ball milling medium to perform mixing ball milling to obtain alumina slurry; Step 4: Add nanocellulose to alumina slurry and ball mill to disperse it evenly to obtain a mixed slurry; Step 5: Mix the slurry with clay, quartz, graphene, calcite and binder to obtain a mixed blank; Step 6: Granulate, press, dry, and sinter the mixed raw material to obtain toughened alumina ceramic.

[0058] In the grinding process of this invention, it is preferably carried out in a ball mill, with a preferred milling speed of 400-500 r / min; the preferred milling time is 1-1.5 h, more preferably 1.2-1.3 h. This invention does not have a specific limitation on the type of ball mill; any ball mill well-known to those skilled in the art can be used. This invention achieves uniform dispersion of raw materials by controlling the milling speed and time.

[0059] The preparation method of nanocellulose in step three includes the following steps: Step 1: Prepare a measuring cup, add 10 parts by weight of microcrystalline cellulose into the reactor, and then take out 300-400ml of 45-65% concentrated sulfuric acid and add it into the reactor. Use an external heating device to heat the entire reactor. To further explain, concentrated sulfuric acid is slowly added dropwise to the reaction vessel while stirring continuously at a speed of 100-150 r / min. Step 2: Control the heating time within 30-50 minutes. After the color changes, add an appropriate amount of water directly into the reactor and use an external centrifuge to stir and settle the mixture. To further explain, an intelligent temperature control system is used to heat the reactor, with the heating temperature set at 55-60℃. When the reactants change color after heating, the heating is stopped, and an appropriate amount of water (the ratio of water added is 5 times the volume of concentrated sulfuric acid) is added through the reactor's automatic feeding system. Then, the high-speed stirring device is started, and the stirring speed is increased to 500-600 r / min. The stirring time is maintained for 10-15 minutes to ensure that the reaction products are fully dispersed in the water. After stirring, immediately switch to centrifugal sedimentation mode, control the centrifugation speed at 400-450 r / min, and set the centrifugation time to 15-20 min; Step 3: The precipitate produced after stirring is screened in batches, with each batch consisting of 60ml. The precipitates of the same proportion are screened to extract nanocellulose. To further explain, after centrifugation, the precipitate is transferred to an automatic screening device. This device is designed with multiple layers of screens with different pore sizes, arranged from largest to smallest, based on the particle size distribution characteristics of nanocellulose. The top layer of screens has a pore size of 0.5 μm, which gradually decreases to the bottom layer with a pore size of 0.05 μm. During the screening process, the precipitate trapped by each layer of screen is collected and measured separately. Each 60ml is a metering unit. Nanocellulose that meets the particle size requirements is collected and combined as toughening material for subsequent preparation processes. The centrifugal speed inside the external centrifuge device is controlled at 350 r / min, the heating temperature is controlled at 50℃, and the proportion of water added is 5 times that of concentrated sulfuric acid liquid.

[0060] The sintering aids include at least one of calcium oxide, magnesium oxide, silicon dioxide, lanthanum oxide, and titanium dioxide.

[0061] The sintering aid is a mixture of titanium dioxide and magnesium oxide, with a mass ratio of titanium dioxide to magnesium oxide of 1 to 3:1.

[0062] The sintering aid was prepared by mixing titanium dioxide and magnesium oxide in a certain mass ratio, using zirconium oxide as the milling ball, controlling the speed of the mill to be 700-900 r / min, and controlling the milling time to be 4-8 h, and then performing dry milling.

[0063] In step six, the pressing and molding process is carried out using an external pressing and molding device. The pressing time and pressing weight are effectively controlled. The pressing time is controlled at 2-5 minutes, and the pressing weight is set at 5-10 N.

[0064] The drying process in step six refers to drying the pressed and shaped blanks, cutting the dried blanks into segments, and drying multiple sets of blanks with different segments in batches. When drying the blanks, the drying temperature is set at 50-80℃ and the drying time is set at 8-12 hours.

[0065] The sintering in step six refers to placing the segmented dried green body into a sintering furnace, controlling the sintering temperature at 1400-1600℃, and controlling the sintering time at 5-7 minutes to prepare a toughened alumina ceramic green body.

[0066] Comparative Example 1: Weigh ceramic powder (alumina ceramic, quartz, calcite), use distilled water as the ball milling medium, mix the powder in a planetary ball mill for 2 hours, and dry it to obtain a mixed powder. Repeat the preparation process of Example 4 to obtain alumina ceramic.

[0067] According to the test results, in Comparative Example 1, the ceramic bending strength was 285.51 MPa and the fracture toughness was 4.35 MPa·m1 / 2. The ceramic bending strength in Example 1 decreased by 14.37% and the ceramic fracture toughness increased by 72.87%.

[0068] In Comparative Example 2, 70g of distilled water was weighed, stirred for 10 minutes, and then the pH value was adjusted to 12 with ammonia water to obtain a premixed solution; the preparation process of Example 2 was repeated for the rest to obtain alumina ceramics.

[0069] According to the test results, in Comparative Example 2, the ceramic bending strength was 295.99 MPa and the fracture toughness was 3.91 MPa·m1 / 2. In Example 2, the ceramic bending strength decreased by 6.71% and the ceramic fracture toughness increased by 39.39%.

[0070] Test method: 1. Grain morphology and size: Grain morphology images were taken using a scanning electron microscope, and then the limiting / average grain size and porosity were calculated using ImageJ software on the images. 2. Bending strength: The substrate sample was laser-cut into rectangular standard parts with l = 40 mm and b = 24 mm. The thickness h of each sample was measured using a micrometer. The test sample was placed on an electronic universal testing machine with a span L = 30 mm, a roller diameter d = 3 mm, and a loading rate v = 0.5 mm / min. The critical bending force F of the substrate sample was measured. The sample strength data was obtained by using the strength calculation formula δ = 3FL / 22bh. 3. Fracture toughness: The fracture toughness of the material was measured using a WDW-type electronic universal testing machine via the single-sided notched beam method (SENB method), with the loading rate set at 0.05 mm / min. The calculation formula is: KIC = Y³PLa / 2bw, from which the fracture toughness of the material can be obtained. In the formula, P is the load at which the specimen breaks, N; L is the span between supports, mm; a is the depth of the specimen notch, mm; b is the width of the specimen cross section, mm; and w is the height of the specimen cross section, mm. 4. Thermal conductivity: According to the specific scheme in the national standard GB / T5598-2015, the sample is cut into a circular piece with a diameter of d = 10 mm. The sample and a standard sample of the same thickness are tested with a laser scintillation thermal diffusivity tester to obtain the thermal diffusivity α and specific heat Cp of the sample. The bulk density of the sample is tested by Archimedes method. The thermal conductivity of the sample is obtained by the thermal conductivity formula λ = α·Cp·ρ.

[0071] Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art and related fields based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described and explained in the present invention, unless otherwise specified or limited, shall be implemented according to conventional means in the art.

Claims

1. A method for preparing toughened alumina ceramic, characterized in that: It comprises the following components by weight: 2-6 parts clay, 15-35 parts nano aluminum powder, 65-95 parts alumina powder, 6-14 parts concentrated sulfuric acid, 6-14 parts microcrystalline cellulose, 2-5 parts sintering aid, 2-5 parts quartz, 2-5 parts calcite, 12-16 parts graphene, and 5-7 parts binder. The method for preparing the alumina powder includes the following steps: Step A1: Select aluminum salt as the aluminum source and recrystallize the aluminum salt to improve its purity; Step A2: Weigh the purified aluminum salt and calculate the amount of precipitant required according to the stoichiometric ratio of the chemical reaction. The precipitant is ammonia. Step A3: Use deionized water as the solvent and control the conductivity of the water to be below 1 μS / cm; Step A4: Select sodium dodecylbenzenesulfonate, accurately weigh the required amount, and prepare a solution with a concentration of 0.01-0.05 mol / L using deionized water for later use; Step A5: Under stirring conditions, slowly add the aluminum salt solution dropwise to the precipitant solution, monitor the pH, and after the addition is complete, continue stirring for 1-2 hours to allow the reaction to proceed fully and obtain aluminum hydroxide precipitate; Then, an aging process is carried out, and the reaction system is allowed to stand and age for 6-12 hours, with the aging temperature controlled at 30-40℃, to obtain aluminum hydroxide precursor. Step A6: Wash the aluminum hydroxide precursor by multiple centrifugation washes. Each centrifugation speed is set at 3000-5000 rpm, and the centrifugation time is 10-15 minutes. The amount of deionized water used for washing is 3-5 times the mass of the precursor, and the washing is performed 3-5 times. After washing, solid-liquid separation is performed using a vacuum filtration device. A microporous filter membrane with a pore size of 0.1-0.2 μm is selected to obtain pure aluminum hydroxide precursor. Step A7: Transfer the washed aluminum hydroxide precursor to a high-pressure reactor, add deionized water at a solid-liquid ratio of 1:10-1:20 g / mL, and add the previously prepared organic additive solution. After sealing the reactor, place it in a heating furnace for hydrothermal reaction. Set the reaction temperature to 150-180℃, and automatically adjust the reaction pressure according to the relationship between the saturated vapor pressure of water and the reaction temperature. The reaction time is 12-24 hours. During the heating process, control the heating rate at 2-3℃ / min. Step A8: After the hydrothermal reaction is completed, allow the reactor to cool naturally to room temperature. After cooling, perform centrifugation again with the same centrifugation parameters as during precursor washing. Separate the plate-shaped alumina powder. Dry the separated plate-shaped alumina powder in a vacuum drying oven at a temperature of 60-80℃ and a vacuum of -0.08-0.1MPa for 8-12 hours. Grind the dried product using an agate mortar to obtain the final plate-shaped alumina powder. The preparation method of toughened alumina ceramics includes the following steps: Step 1: Preparation of toughening material: Prepare a reaction vessel, inject concentrated sulfuric acid and microcrystalline cellulose into the reaction vessel for mixing, and complete the preparation of nanocellulose. Step 2: Ceramic powder preparation: Nano aluminum powder and high-purity alumina powder are used as initial raw materials, mixed evenly, and the mixed slurry is dried and ground to obtain ceramic powder; Step 3: Mix the ceramic powder and sintering aid, and use distilled water as the ball milling medium to perform mixing ball milling to obtain alumina slurry; Step 4: Add nanocellulose to alumina slurry and ball mill to disperse it evenly to obtain a mixed slurry; Step 5: Mix the slurry with clay, quartz, graphene, calcite and binder to obtain a mixed blank; Step 6: Granulate, press, dry, and sinter the mixed raw material to obtain toughened alumina ceramic; The method for preparing the nanocellulose includes the following steps: Step 1: Prepare a measuring cup to measure and add 10 parts by weight of microcrystalline cellulose into the reactor. Then, take out 300-400ml of 45-65% concentrated sulfuric acid and add it into the reactor. Use an external heating device to heat the entire reactor. Step 2: Control the heating time within 30-50 minutes. After the color changes, add an appropriate amount of water directly into the reactor and use an external centrifuge to stir and settle the mixture. Step 3: The precipitate produced after stirring is screened in batches, with each batch consisting of 60ml. The precipitates of the same proportion are screened to extract nanocellulose.

2. The method for preparing toughened alumina ceramic according to claim 1, characterized in that: The sintering aid includes at least one of calcium oxide, magnesium oxide, silicon dioxide, lanthanum oxide, and titanium dioxide.

3. The method for preparing toughened alumina ceramic according to claim 1, characterized in that: The sintering aid is a mixture of titanium dioxide and magnesium oxide, wherein the mass ratio of titanium dioxide to magnesium oxide in the mixture is 1 to 3:

1.

4. The method for preparing toughened alumina ceramic according to claim 3, characterized in that: The sintering aid is prepared by mixing titanium dioxide and magnesium oxide in a certain mass ratio, using zirconium oxide as the milling ball, controlling the speed of the mill to be 700-900 r / min, and controlling the milling time to be 4-8 h, and then performing dry milling.

5. The method for preparing toughened alumina ceramic according to claim 1, characterized in that: In step six, the pressing and molding process is carried out using an external pressing and molding device. The pressing time is controlled at 2-5 minutes, and the pressing weight is set at 5-10 N.

6. The method for preparing toughened alumina ceramic according to claim 1, characterized in that: The drying process in step six refers to drying the pressed blank and cutting the dried blank into segments. Multiple sets of blanks with different segments are dried in batches. When drying the blank, the drying temperature is set at 50-80℃ and the drying time is set at 8-12 hours.

7. The method for preparing toughened alumina ceramic according to claim 1, characterized in that: The sintering in step six refers to placing the segmented dried blank into a sintering furnace, controlling the sintering temperature at 1400-1600℃, and controlling the sintering time at 5-7 minutes to prepare a toughened alumina ceramic blank.