Graphene quantum dot modified alumina composite ceramic based on surface chemical bonding and preparation method thereof

Abstract

The application belongs to the field of new materials, in particular, the field of advanced ceramic materials, and particularly relates to a graphene quantum dot modified alumina composite ceramic based on surface chemical bonding and a preparation method thereof. Graphene quantum dots are used as reinforcing agents, and a reinforcing agent-matrix bidirectional surface functionalization technology is combined to obtain high-performance composite ceramics with uniform dispersion of quantum dots in an alumina matrix and strong interface bonding. The chemical reaction is mild and controllable, no extreme harsh conditions or expensive reagents are used, and the application has high repeatability and stability, thereby providing a feasible technical route for large-scale production of high-performance GQD / Al2O3 composite ceramics.

Description

Technical Field

[0001] This invention belongs to the field of new materials, particularly advanced ceramic materials, and specifically relates to a graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding and its preparation method. Background Technology

[0002] With the ever-increasing demands of modern industry for material performance, advanced structural ceramics, represented by alumina (Al2O3), have been widely used in aerospace, precision machinery, biomedicine, and electronic packaging due to their high strength, wear resistance, high temperature resistance, and excellent chemical stability. However, the inherent high brittleness and low fracture toughness of alumina ceramics are fatal weaknesses that limit their reliability and breadth of application as key structural components. Therefore, developing effective toughening technologies while maintaining their original advantages is a core issue driving the development of alumina ceramics.

[0003] To overcome the brittleness of alumina ceramics, introducing nanoscale second-phase reinforcing agents into the ceramic matrix is ​​an effective technical approach. Currently, common nano-reinforcing agents include zero-dimensional hard particles (such as SiC and ZrO2) and one-dimensional nanomaterials (such as CNTs), which improve the toughness of ceramics through mechanisms such as crack deflection, bridging, and grain refinement. However, regardless of the reinforcing agent chosen, traditional nanocomposite ceramic preparation techniques generally face two major insurmountable technical bottlenecks: first, the severe agglomeration problem of nano-reinforcing agents; and second, the weak interfacial bonding between the reinforcing agent and the matrix. These two bottlenecks mean that the actual performance of many nanocomposite ceramics falls far short of theoretical expectations.

[0004] Faced with the limitations of traditional reinforcing agents and their processing techniques, there is an urgent need in this field to find a novel reinforcing agent that combines excellent intrinsic properties with good process adaptability. In recent years, graphene quantum dots (GQDs), as an emerging zero-dimensional carbon nanomaterial with a unique sheet-like morphology, have shown great potential as an ideal ceramic reinforcing and toughening agent, with comprehensive advantages far exceeding those of existing reinforcing agents. Compared with traditional zero-dimensional spherical particles (such as SiC), GQDs not only inherit the ultra-high theoretical mechanical properties of graphene, but their unique nanoscale sheet-like configuration is also more efficient in inducing crack deflection and bridging. Compared with one-dimensional carbon nanotubes (CNTs) that are prone to physical entanglement, GQDs fundamentally avoid the problem of entanglement and agglomeration, and the edges of their sp² carbon skeleton have high chemical reactivity, giving them unparalleled potential for surface chemical modification. Compared with large-sized two-dimensional graphene nanosheets (GNPs), the nanoscale size of GQDs can more effectively pin and refine the matrix grains and achieve isotropic reinforcement effects. The advantages of GQD provide unprecedented possibilities for constructing strong interfacial bonds through chemical means and solving the dispersion problem from the source. However, existing technologies lack precise preparation methods that can first activate its chemical modification potential (such as introducing carboxyl groups) and then further utilize this functional group to achieve uniform dispersion and strong interfacial bonding in the ceramic matrix. As a result, its full potential cannot be synergistically realized, and the toughening effect cannot be fully achieved. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding and its preparation method. Graphene quantum dots are used as a reinforcing agent, combined with a two-way surface functionalization technique between the reinforcing agent and the matrix, to obtain a high-performance composite ceramic with uniform dispersion and strong interfacial bonding of quantum dots in the alumina matrix. Furthermore, the chemical reaction is mild and controllable, without the use of extremely harsh conditions or expensive reagents, exhibiting high repeatability and stability. This provides a practical and feasible technical route for the large-scale production of high-performance GQD / Al2O3 composite ceramics.

[0006] The main technical problems to be solved by this invention are as follows: 1. Improve the fracture toughness of alumina-based composite ceramics while ensuring strength; 2. Solve the problem of GQD self-aggregation due to high surface energy and π-π stacking effect, and the difficulty of uniform mixing with Al2O3 powder due to differences in physical properties; 3. To solve the problem of weak interfacial bonding between GQD and Al2O3 matrix due to lack of chemical bonding, which leads to ineffective load transfer and hinders crack propagation.

[0007] To address this, the inventors pre-modified and targeted the chemical properties of GQD and Al2O3 powders, endowing them with opposite surface charges. Then, they utilized electrostatic attraction to drive their precise and robust bonding in the liquid phase, ultimately obtaining an alumina-based composite ceramic with an ideal microstructure and excellent mechanical properties. More specifically, the innovation lies in: 1. Creatively apply GQD to improve the mechanical properties of ceramics: Utilize its unique characteristics of "zero-dimensional size + two-dimensional configuration + strong interface bonding" to synergistically activate multiple efficient toughening mechanisms within a single system.

[0008] 2. Two-way surface functionalization strategy: Abandoning the traditional physical blending approach, this study creatively employs a two-way chemical pretreatment of the reinforcing agent GQD and the matrix Al2O3. Specifically: ① Controllable surface carboxylation of GQD is performed, enabling it to be stably dispersed in water and carry a negative charge. ② Surface amination of Al2O3 is performed, utilizing a silane coupling agent as a "molecular bridge" to enable its surface to carry a positive charge.

[0009] 3. pH-Controlled Electrostatic Self-Assembly Technology: This technique creatively applies the principle of electrostatic self-assembly to the precision composite of high-strength, chemically inert oxide ceramic powder and carbon nanotube reinforcement for the first time. Abandoning the traditional approach that relies on the material's own zeta potential, this technique, based on a bidirectional surface functionalization strategy, further constructs a strong, directional electrostatic attraction between the two components by precisely controlling the pH value of the mixed system (6.0-7.0), thereby achieving in-situ, site-specific, and uniform anchoring of GQD on the surface of Al2O3 particles. This "design-driven" self-assembly method represents a significant breakthrough in solving the two major challenges of agglomeration and interfacial bonding in ceramic nanocomposites at the molecular level.

[0010] Based on the above ideas, the specific technical solution of the present invention is as follows: A graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding, wherein the graphene quantum dot content is 0.05-2.0 wt%, and the balance is alumina.

[0011] In this composite ceramic, graphene quantum dots are bonded to the surface or grain boundaries of the alumina matrix grains via a silane coupling agent as a chemical bridge, in the form of covalent or ionic bonds. Compared to unmodified alumina ceramics prepared using the same process, its fracture toughness is increased by at least 30% without reducing strength. More preferably, the graphene quantum dot content is 0.1-1.0 wt%.

[0012] Furthermore, the relative density of the aforementioned ceramic samples can reach over 99.0%, preferably over 99.5%, and in the most preferred embodiment, it can reach 99.8%. Through three-point bending tests, its flexural strength can reach over 450 MPa, with a maximum of over 490 MPa. Its fracture toughness, determined using the single-sided notched beam (SENB) method, is 4.5 MPa·m. 1 / 2 The maximum pressure can reach 4.8 MPa·m 1 / 2 Compared to pure alumina ceramics prepared using the same process (flexural strength 350 MPa, SENB fracture toughness 3.2 MPa·m), 1 / 2 Without reducing strength, its fracture toughness can be improved by more than 40%, preferably more than 50%. Scanning electron microscopy (SEM) was used to observe the fracture morphology, which showed typical intragranular and intergranular fracture modes, and pull-out of GQD at grain boundaries was observed.

[0013] The aforementioned composite ceramics, employing a two-way surface functionalization and electrostatic self-assembly strategy, can improve the fracture toughness of alumina ceramics from 3.2 MPa·m. 1 / 2 Increased to 5.9 MPa·m 1 / 2 The increase is as high as 84%, which is a huge breakthrough that is difficult to achieve through simple physical blending in existing technologies. In addition, in terms of sintering performance, the composite powder prepared by this invention can achieve a density of 99.2% at a relatively low temperature of 1300℃, which significantly reduces energy consumption compared to the sintering temperature of 1500-1600℃ usually required for traditional alumina, and shows excellent low-temperature sintering activity.

[0014] The inventors further provided a method for preparing the above-mentioned graphene quantum dot modified alumina composite ceramic, including the following steps: (1) Preparation of carboxylated GQD (GQD-COOH) The original GQD powder was dispersed in concentrated nitric acid and refluxed at 90-110℃ for 8-16 hours to introduce a large number of carboxyl functional groups at its edge active sites. After dilution, the reaction product was titrated with 1.0-3.0 M NaOH solution to neutralize it to pH 7.0-8.0, thus converting it into a highly water-dispersible sodium carboxylate (GQD-COONa).

[0015] The equation for the above reaction is as follows: Oxidation reaction: GQD-H + ​​HNO3 → GQD-COOH + NO2↑ + H2O Neutralization reaction: GQD-COOH + NaOH → GQD-COONa + H2O; To thoroughly remove inorganic salt byproducts (such as NaNO3) and impurities, the crude GQD-COONa product was transferred to a dialysis bag with a molecular weight cutoff of 1000 Da, and then dialyzed continuously in deionized water for 48-72 hours until the external water conductivity was below 5 µS / cm. Finally, the purified product was freeze-dried under vacuum to obtain a fluffy, high-purity GQD-COONa powder.

[0016] The original GQD powder used above can be selected from commercially available products that meet the following technical characteristics or can be prepared by methods known in the art: its average particle size is less than 10 nm, it has a good graphitized carbon structure, and the carbon atoms on its surface or edge have chemical activity that can be oxidized.

[0017] Preferably, the mass-to-volume ratio of the original GQD powder to concentrated nitric acid is (0.5-2.0) g: 100 mL, and the concentration of the concentrated nitric acid is 68 wt%; the reaction is carried out under reflux at 100°C for 12 hours; and the solution is titrated with 2.0 M NaOH solution to neutralize to pH 7.5.

[0018] This step provides an ideal functionalized precursor for the subsequent construction of GQD / Al2O3 composites with strong interfacial bonding through electrostatic self-assembly.

[0019] (2) Preparation of substrate surface functionalization (Al2O3-NH2) 1) Activation with silane coupling agent: In an ethanol / water solution with a volume ratio of 95:5, adjust the pH to 4.5±0.2 with glacial acetic acid, add 1.0-2.0% of the silane coupling agent by weight of Al2O3 powder, and stir at room temperature for 30-60 minutes. The silane coupling agent can be selected from aminosilane coupling agents having the general formula NH2-R-Si-(OR')3, wherein R is an alkylene group and R' is a methyl or ethyl group; more preferably, silane coupling agent KH-550 is preferred, and the corresponding chemical reaction equation is as follows: NH2-(CH2)3-Si-(OC2H2)3 + 3H2O ---(H + (Catalysis) --> NH2-(CH2)3-Si-(OH)3 +3CH3CH2OH.

[0020] 2) Surface grafting: Add pre-dried Al2O3 powder, ultrasonically disperse for 15-30 minutes, and then heat and react at 65-75℃ for 2-4 hours; the pre-drying refers to drying the Al2O3 powder at 120℃ for more than 12 hours; the purity of the Al2O3 powder is greater than 99.9%.

[0021] The chemical reaction equation is as follows: Al2O3-OH + HO-Si-(CH2)3-NH2 --(heating)--> Al2O3-O-Si-(CH2)3-NH2+H2O; 3) Purification and refining: The reaction product is repeatedly centrifuged and washed 3-5 times with anhydrous ethanol, and then vacuum dried at 60-80℃ for more than 24 hours to obtain Al2O3-NH2 powder.

[0022] Silane coupling agents are used as molecular bridges. Under weak acidity, their silane ends hydrolyze into highly reactive -Si-OH, which preferentially reacts with -Al-OH on the Al2O3 surface to form stable Al-O-Si covalent bonds, thereby "grafting" reactive amino groups (-NH2) onto the Al2O3 surface.

[0023] Compared with existing technologies, this invention is the first to "design and combine" the two completely different surface functionalization strategies mentioned above, applying them respectively to both ends of the reinforcement and matrix of the ceramic composite material, creating a precursor system with "positive-negative" charge characteristics that can be used for subsequent electrostatic self-assembly. Existing technologies either only modify the reinforcement before physical blending or only modify the matrix; there has never been a report of such a "two-way, synergistic" chemical pretreatment of the ceramic reinforcement-matrix to achieve a specific self-assembly purpose. Therefore, the above-mentioned approach constitutes the main difference and progress between this application and existing technologies.

[0024] (3) Preparation of stabilized dispersion of reinforcing agent (GQD monodisperse) The GQD-COONa powder obtained in step (1) was dispersed in deionized water at a concentration of 0.5-2.0 mg / mL. The mixture was then treated in an ice-water bath using a probe-type ultrasonic processor in pulse mode (working for 2 seconds and pausing for 3 seconds) for 30-60 minutes to obtain a GQD dispersion. The ultrasonic power is 200-600W, corresponding to a power density of 5-20 W / mL; The chemical equation for the ionization reaction is as follows: GQD-COONa (solid) --(soluble in H2O)--> GQD-COO - + Na + .

[0025] (4) Targeted connection and drying Electrostatic self-assembly: Al2O3-NH2 powder is slowly added to the GQD dispersion obtained in step (3). The pH of the system is precisely controlled within the range of 6.0-7.0. The pH is monitored and controlled at 6.0-7.0 throughout the process. The reaction is carried out at room temperature for 2-6 hours under this pH.

[0026] If the reaction time is insufficient (e.g., less than 2 hours), the adsorption of GQD on the Al2O3 surface may not reach saturation equilibrium, resulting in insufficient coverage and uneven distribution, thus affecting the final toughening effect. If the reaction time is too long (e.g., more than 6 hours), it will not bring significant performance improvement, but will unnecessarily increase the production cycle and cost. Therefore, 2-6 hours is the optimal time window to balance reaction saturation and process economy. The above-mentioned reaction time is sufficient for the system to reach adsorption saturation equilibrium, thereby maximizing the coverage of GQD on the Al2O3 surface, eliminating the need for complex endpoint monitoring and reducing process difficulty.

[0027] The weight of GQD in the GQD dispersion accounts for 0.05-2.0 wt% of the total weight of the entire mixture (Al2O3 and pure GQD).

[0028] The chemical reaction equations for the above reactions are as follows: Amine protonation: Al2O3-...-NH2+ H2O ⇌ Al2O3-...-NH3 + + OH - Electrostatic bonding: Al2O3-...-NH3 + + - OOC-GQD → Al2O3-...-NH3 + - OOC-GQD; Purification and drying: Centrifuge and wash the slurry after reaction 1-2 times. Pre-freeze the resulting paste precipitate at -40℃ or below for 12-24 hours, and then perform vacuum freeze drying. The vacuum degree of vacuum drying is <10 Pa, and the time is 48-72 hours.

[0029] The above steps result in a large number of amino groups on the Al2O3 surface being protonated into positively charged -NH3 groups within the "golden window" of pH 6.5 ± 0.5. + The GQD surface, on the other hand, is a negatively charged -COO- - The strong electrostatic attraction between the two drives GQD to be uniformly and firmly adsorbed onto the surface of Al2O3 particles. Freeze-drying, through the sublimation of water, avoids the hard agglomeration caused by capillary forces in conventional drying, perfectly maintaining the high dispersion of the powder.

[0030] The obtained composite powder appears macroscopically as a uniformly colored, loosely textured solid. Verification revealed that the powder exhibits excellent redispersibility in water, forming a stable suspension after brief sonication without significant rapid sedimentation. This demonstrates that the unique electrostatic self-assembly and freeze-drying process of this invention fundamentally inhibits the formation of hard agglomerates, allowing graphene quantum dots and alumina particles to maintain a highly uniform composite state.

[0031] (5) Pressing and sintering densification Preform forming: The composite powder is preformed by uniaxial molding at 50-150 MPa, then vacuum sealed, and cold isostatic pressing (CIP) is performed at 200-400 MPa to obtain the green preform.

[0032] Spark plasma sintering (SPS): The green blank is placed in a graphite mold and heated to 1300-1400℃ at a rate of 100-200℃ / min under vacuum (<5.0 Pa) and axial pressure of 50-80 MPa, and held for 5-10 minutes. After cooling in the furnace, the surface graphite layer is removed by grinding to obtain the composite ceramic.

[0033] Preferably, the specific polishing process is as follows: First, use metallographic sandpaper of progressively finer grades (e.g., 200# → 800# → 2000#) to polish the surface, removing the graphite contamination layer and processing marks. Then, use a polishing cloth containing diamond polishing paste with different particle sizes (e.g., 9μm → 3μm → 1μm) for fine polishing. The polishing process continues until the sample surface exhibits the inherent, uniform color of the ceramic material (usually white or off-white for alumina ceramics), and under different lighting angles, no visible black graphite residue spots or streaks are observed, and the surface exhibits a uniform and smooth texture. At this point, the polishing is considered complete.

[0034] The spark plasma sintering (SPS) process used in this invention differs significantly from conventional pressureless sintering processes in that it utilizes pulsed high current to achieve an extremely high heating rate and a lower sintering temperature. This "rapid and low-temperature" characteristic is crucial for the composite ceramic of this invention: it not only effectively inhibits the growth of alumina matrix grains, resulting in a fine-grained structure, but more importantly, it completes the densification of the ceramic before significant thermal decomposition of the GQD, thereby preserving the structural integrity of the GQD within the matrix to the maximum extent. The final material obtained is a high-performance composite ceramic with high density, a fine-grained structure (average grain size <500nm), and good structural integrity and high dispersion of the internal GQD reinforcing phase.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A significant breakthrough was achieved in improving the fracture toughness of alumina-based composite ceramics while maintaining strength: GQD was innovatively introduced into the alumina matrix, and for the first time, its unique advantages of "zero-dimensional size + two-dimensional configuration" were fully utilized through strong interfacial chemical bonding. For example, the introduction of 0.5 wt% GQD increased the fracture toughness of the composite ceramic from 3.2 MPa•m for Al2O3. 1 / 2 Increased to 5.9 MPa•m 1 / 2The increase was as high as 84%. This is because chemically bonded GQDs can act as efficient "pinning points" at grain boundaries and within grains, effectively inducing crack deflection and bridging of nanoparticles, thus achieving a huge breakthrough in performance.

[0036] 2. Highly uniform dispersion of the nano-reinforcing agent was achieved, effectively alleviating the agglomeration defect problem: Through innovative bidirectional surface functionalization and electrostatic self-assembly strategies, GQD was uniformly and firmly anchored on the surface of each Al2O3 particle in the form of individual particles. This excellent dispersion state is most powerfully reflected in the superior performance of the final ceramic product: for example, even with a relatively high GQD content of 0.5 wt%, the composite ceramic still achieved a relative density of up to 99.5% and a strength of 5.9 MPa•m. 1 / 2 The fracture toughness is excellent. In the field of nanocomposite ceramics, the highly uniform dispersion of the reinforcement and the significant reduction of agglomerates, which serve as defect sources, are key prerequisites for achieving a synergistic improvement in high density and high performance. Therefore, the excellent macroscopic performance data obtained in this invention fully demonstrate that its microscopic dispersion state has reached a high level that is difficult to achieve with existing technologies.

[0037] 3. A robust interface bond was constructed, providing a theoretical guarantee for efficient toughening: GQD and the Al2O3 matrix are bonded through "Al2O3-O-Si-(...)-NH3". +- The OOC-GQD chemical bonds bridge the gap, forming a strong interfacial bond. This strong interface can effectively transfer loads and dissipate a large amount of energy during crack propagation through mechanisms such as pull-out and bridging. The obvious crack bridging phenomenon observed in the attached figure is a direct manifestation of this strong interfacial effect.

[0038] 4. The process flow is clear and controllable, with promising prospects for industrial application: The process parameters for each stage proposed in this invention are clearly defined, and the chemical reactions are all carried out under mild conditions (such as atmospheric pressure and a water bath at <100℃), without using extremely harsh conditions (such as ultra-high pressure or highly corrosive atmospheres) or expensive reagents (such as precious metal catalysts). This characteristic of "mild and controllable chemical reactions" endows this technology with high repeatability and stability, providing a practical and feasible technical route for the large-scale production of high-performance GQD / Al2O3 composite ceramics. Attached Figure Description

[0039] Figure 1 This is a flowchart of the preparation method of the graphene quantum dot modified alumina composite ceramic described in this application; Figure 2 This is a schematic diagram of the fracture microstructure of the composite ceramic prepared in Example 1. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, which will enable those skilled in the art to have a more comprehensive understanding of the invention, but will not limit the invention in any way. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. The GQD powder used in the following embodiments can be selected from commercially available products prepared by small molecule carbon source pyrolysis that meet the technical characteristics described in the present invention (average particle size <10nm, surface can be oxidized), such as products provided by companies like XFNANO (Nanjing Xianfeng Nano) and Xiandao Nano (Suzhou).

[0041] As a preferred embodiment, the original GQD powder can also be prepared by the following methods, with specific examples as follows: 1. Hydrothermal carbonization: Weigh 2.0 g of citric acid and dissolve it in 20 mL of deionized water. Transfer the solution to a hydrothermal reactor, seal it, and react it at 200℃ for 6 hours to obtain a dark brown carbonized product.

[0042] 2. Alkalization and purification: After the product cools, adjust the pH to 9.0 with 1.0 M NaOH solution. Dialyze the resulting crude dispersion of carbonized product to deionized water using a dialysis bag with a molecular weight cutoff of 1000 Da for 72 hours until the external water conductivity is below 5 µS / cm.

[0043] 3. Drying into powder: Finally, the purified GQD colloidal dispersion is pre-frozen at -50℃ for 16 hours, and then freeze-dried under vacuum (<10 Pa, 48 hours) to obtain about 1.1 g of original GQD powder.

[0044] Al2O3 powder can be selected from high-purity alumina products on the market that meet the technical characteristics described in this invention (purity >99.9%, D50=200-500nm), such as products provided by companies like Sinocera Materials (Shandong) or Sumitomo Chemical (AKP series).

[0045] Example 1: A graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding, the specific steps of its preparation method are as follows: Step 1: Preparation of GQD-COOH: Weigh 1.0 g of GQD powder (with an average particle size of 2-8 nm and good graphitization crystallinity), add it to 100 mL of concentrated nitric acid (68%), and reflux at 100 °C for 12 hours. After cooling, dilute in 500 mL of ice water, and adjust the pH to 7.5 with 2.0 M NaOH solution. Collect the product by centrifugation, and dialyze it against deionized water using a 1000 Da dialysis bag for 3 days, changing the external deionized water regularly and monitoring it with a conductivity meter until the conductivity of the external water stabilizes below 5 µS / cm. Freeze-dry the purified colloidal dispersion to obtain 0.95 g of brownish-yellow GQD-COONa powder.

[0046] Step 2: Preparation of Al2O3-NH2: In a mixed solvent of 95 mL ethanol and 5 mL water, the pH was adjusted to 4.5 with glacial acetic acid, and 0.3 g of KH-550 was added. The mixture was stirred and activated at room temperature for 45 minutes. 20 g of pre-dried high-purity Al2O3 powder (D50 = 300 nm) was added, and the mixture was ultrasonically dispersed in a water bath for 20 minutes in an ultrasonic cleaning tank with a power of 100-300 W and a frequency of 40 kHz. The mixture was then reacted at 70 °C for 3 hours. The product was washed four times by centrifugation with ethanol and vacuum dried at 70 °C for 24 hours to obtain 20.25 g of Al2O3-NH2 powder.

[0047] Step 3: Preparation of composite powder (GQD content 0.1wt%): Weigh 20mg of GQD-COONa powder and disperse it in 20mL of water. Under an ice-water bath, use a probe-type ultrasonic processor (power set to 300-500W) in a pulse mode with 2 seconds of operation followed by 3 seconds of pause for 30 minutes. Add the above 20g of Al2O3-NH2 powder, and adjust the pH to 6.5 with 0.1M hydrochloric acid while stirring. Monitor the pH throughout this process using a calibrated pH meter. React at room temperature for 4 hours at this pH. Centrifuge and wash twice, pre-freeze at -50℃ for 12 hours, and then freeze-dry under vacuum for 48 hours to obtain approximately 20g of highly dispersed composite powder.

[0048] Step 4: Preparation of composite ceramics: The above composite powder was molded at 100 MPa and then subjected to cold isostatic pressing at 300 MPa. The green blank was placed in a spark plasma sintering apparatus and heated to 1350℃ at a rate of 100℃ / min under vacuum and 60 MPa pressure, and held for 5 minutes. After furnace cooling, it was polished to obtain a dense GQD / Al2O3 composite ceramic sample.

[0049] Performance test results: The relative density of the obtained ceramic sample reached 99.8%. Its flexural strength was 490 MPa through a three-point bending test. Its fracture toughness was determined using the single-sided notched beam (SENB) method, yielding a result of 4.8 MPa·m.1 / 2 Compared to the pure alumina ceramic obtained in Comparative Example 1 (flexural strength 350 MPa, SENB fracture toughness 3.2 MPa·m), 1 / 2 The performance is significantly improved. A schematic diagram of the fracture surface microstructure is shown below. Figure 2 As shown, typical intragranular and intergranular fracture modes are visible, and pull-out of GQD at grain boundaries is observed.

[0050] Example 2: A graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding, the specific steps of its preparation method are as follows: Steps 1 & 2: The preparation methods of GQD-COONa powder and Al2O3-NH2 powder are exactly the same as those in Example 1.

[0051] Step 3: Preparation of composite powder (GQD content 0.5wt%): Weigh 100mg of GQD-COONa powder and disperse it in 50mL of water. Under an ice-water bath, use a probe-type ultrasonic processor (power set to 300-500W) in a pulse mode with 2 seconds of operation followed by 3 seconds of pause for 30 minutes. Then add 20g of Al2O3-NH2 powder (ensuring the theoretical GQD content is approximately 0.5wt%). Adjust the pH to 6.8 with 0.1M hydrochloric acid while stirring. Monitor the pH throughout the process using a calibrated pH meter. React at room temperature for 4 hours at this pH. Subsequent purification and drying steps are the same as in Example 1. TEM observation shows that GQD remains uniformly adsorbed on the surface of Al2O3 particles without significant agglomeration.

[0052] Step 4: Preparation of composite ceramics: The pressing process is the same as in Example 1. During sintering, to promote atomic diffusion of higher GQD content at the interface, the SPS sintering parameters were adjusted as follows: under vacuum and 70 MPa pressure, the temperature was increased to 1400℃ at a rate of 100℃ / min and held for 5 minutes.

[0053] Performance test results: The obtained ceramic sample had a relative density of 99.5%. Its flexural strength reached 550 MPa, and its fracture toughness reached 5.9 MPa•m. 1 / 2 Compared to the sample in Example 1, its toughness was significantly improved. This indicates that the preparation method of the present invention can maintain excellent dispersibility and bring about a more significant toughening effect even at high GQD content. SEM revealed more obvious crack bridging and crack deflection phenomena.

[0054] Example 3: A graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding, the specific steps of which are as follows: Steps 1, 2 & 3: The preparation methods of GQD-COONa powder, Al2O3-NH2 powder and composite powder with 0.1wt% GQD content are exactly the same as those in Example 1.

[0055] Step 4: Preparation of composite ceramics (low-temperature sintering): The pressing process is the same as in Example 1. During sintering, a lower sintering temperature was used to verify the excellent activity of the powder. The SPS sintering parameters were adjusted as follows: under vacuum and 80 MPa pressure, the temperature was increased to 1300°C at a rate of 100°C / min, and the holding time was extended to 10 minutes.

[0056] Performance test results: Even at a sintering temperature 50°C lower than that of Example 1, the resulting ceramic sample still achieved a relative density of 99.2%, demonstrating excellent low-temperature sintering performance. Its flexural strength was 465 MPa, and its fracture toughness was 5.5 MPa·m. 1 / 2 .

[0057] Comparative Example 1: Preparation of Alumina Ceramics Weigh 20g of pre-dried high-purity Al2O3 powder, identical to that in Example 1, without any surface functionalization treatment. The powder was directly subjected to compact forming (100MPa molding, 300MPa cold isostatic pressing) and spark plasma sintering (vacuum, 60MPa pressure, heating to 1350℃ at 100℃ / min, holding for 5 minutes). After furnace cooling, the sample was polished to obtain a pure alumina ceramic sample.

[0058] The product was tested and found to have a relative density of 99.9%, a flexural strength of 350 MPa, and a fracture toughness of 3.2 MPa·m. 1 / 2 .

[0059] The comparative results of the examples and comparative examples show that the highly dispersible composite powder prepared by the present invention has extremely high sintering activity and can prepare high-performance dense ceramics with lower energy consumption, which has significant potential for industrial application.

[0060] This invention includes, but is not limited to, the above embodiments. Any equivalent substitutions or partial improvements made under the spirit and principles of this invention shall be considered to be within the scope of protection of this invention.

Claims

1. A graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding, characterized in that, The graphene quantum dot content is 0.05-2.0 wt%, with the balance being aluminum oxide; The specific steps of its preparation method are as follows: (1) Preparation of carboxylated GQD The raw GQD powder was dispersed in concentrated nitric acid and refluxed at 90-110℃ for 8-16 hours. After dilution, the reaction product was titrated with 1.0-3.0 M NaOH solution to neutralize it to pH 7.0-8.0, converting it into a highly water-dispersible sodium carboxylate. The crude GQD-COONa product was transferred to a dialysis bag with a molecular weight cutoff of 1000 Da and then continuously dialyzed in deionized water until the external water conductivity was below 5 µS / cm. Finally, the purified product was freeze-dried under vacuum to obtain a fluffy, high-purity GQD-COONa powder. (2) Preparation of substrate surface functionalization 1) Activation with silane coupling agent: In an ethanol / water solution with a volume ratio of 95:5, adjust the pH to 4.5±0.2 with glacial acetic acid, add 1.0-2.0% of the silane coupling agent by weight of Al2O3 powder, and stir at room temperature for 30-60 minutes. 2) Surface grafting: Add pre-dried Al2O3 powder, ultrasonically disperse for 15-30 minutes, and then heat at 65-75℃ for 2-4 hours. 3) Purification and refining: The reaction product is repeatedly centrifuged and washed with anhydrous ethanol 3-5 times, and then vacuum dried at 60-80℃ for more than 24 hours to obtain Al2O3-NH2 powder; (3) Preparation of stable dispersion of reinforcing agent The GQD-COONa powder obtained in step (1) was dispersed in deionized water at a concentration of 0.5-2.0 mg / mL and treated with an ultrasonic processor in an ice-water bath for 30-60 minutes to obtain a GQD dispersion. (4) Targeted connection and drying Electrostatic self-assembly: Al2O3-NH2 powder is slowly added to the GQD dispersion obtained in step (3), and the pH of the system is controlled in the range of 6.0-7.

0. The pH is monitored and controlled in the range of 6.0-7.0 throughout the process. The reaction is carried out at room temperature for 2-6 hours under this pH. Purification and drying: The slurry after reaction is centrifuged and washed 1-2 times. The resulting paste precipitate is then freeze-dried under vacuum to obtain composite powder. (5) Pressing and sintering densification Compactor forming: The composite powder is preformed by uniaxial molding at 50-150 MPa, then vacuum sealed, and cold isostatic pressing at 200-400 MPa to obtain the green blank. Spark plasma sintering: The green blank is placed in a graphite mold and heated to 1300-1400℃ at a rate of 100-200℃ / min under vacuum and axial pressure of 50-80 MPa, and held for 5-10 minutes. Composite ceramics can be obtained by grinding and removing the surface graphite layer after the furnace cools down.

2. The graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding according to claim 1, characterized in that, The graphene quantum dot content is 0.1-1.0 wt%.

3. The preparation method of graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding as described in claim 1, characterized in that, The specific steps are as follows: (1) Preparation of carboxylated GQD The raw GQD powder was dispersed in concentrated nitric acid and refluxed at 90-110℃ for 8-16 hours. After dilution, the reaction product was titrated with 1.0-3.0 M NaOH solution to neutralize it to pH 7.0-8.0, converting it into a highly water-dispersible sodium carboxylate. The crude GQD-COONa product was transferred to a dialysis bag with a molecular weight cutoff of 1000 Da and then continuously dialyzed in deionized water until the external water conductivity was below 5 µS / cm. Finally, the purified product was freeze-dried under vacuum to obtain a fluffy, high-purity GQD-COONa powder. (2) Preparation of substrate surface functionalization 1) Activation with silane coupling agent: In an ethanol / water solution with a volume ratio of 95:5, adjust the pH to 4.5±0.2 with glacial acetic acid, add 1.0-2.0% of the silane coupling agent by weight of Al2O3 powder, and stir at room temperature for 30-60 minutes. 2) Surface grafting: Add pre-dried Al2O3 powder, ultrasonically disperse for 15-30 minutes, and then heat at 65-75℃ for 2-4 hours. 3) Purification and refining: The reaction product is repeatedly centrifuged and washed with anhydrous ethanol 3-5 times, and then vacuum dried at 60-80℃ for more than 24 hours to obtain Al2O3-NH2 powder; (3) Preparation of stable dispersion of reinforcing agent The GQD-COONa powder obtained in step (1) was dispersed in deionized water at a concentration of 0.5-2.0 mg / mL and treated with an ultrasonic processor in an ice-water bath for 30-60 minutes to obtain a GQD dispersion. (4) Targeted connection and drying Electrostatic self-assembly: Al2O3-NH2 powder is slowly added to the GQD dispersion obtained in step (3), and the pH of the system is controlled in the range of 6.0-7.

0. The pH is monitored and controlled in the range of 6.0-7.0 throughout the process. The reaction is carried out at room temperature for 2-6 hours under this pH. Purification and drying: The slurry after reaction is centrifuged and washed 1-2 times. The resulting paste precipitate is then freeze-dried under vacuum to obtain composite powder. (5) Pressing and sintering densification Compactor forming: The composite powder is preformed by uniaxial molding at 50-150 MPa, then vacuum sealed, and cold isostatic pressing at 200-400 MPa to obtain the green blank. Spark plasma sintering: The green blank is placed in a graphite mold and heated to 1300-1400℃ at a rate of 100-200℃ / min under vacuum and axial pressure of 50-80 MPa, and held for 5-10 minutes. Composite ceramics can be obtained by grinding and removing the surface graphite layer after the furnace cools down.

4. The method for preparing graphene quantum dot-modified alumina composite ceramics based on surface chemical bonding according to claim 3, characterized in that, In step (1), the mass-to-volume ratio of the original GQD powder to concentrated nitric acid is (0.5-2.0) g:100 mL, and the concentration of the concentrated nitric acid is 68 wt%. The reaction is refluxed at 100°C for 12 hours. The solution is then titrated with 1-3 M NaOH solution to neutralize the pH to 7.

5.

5. The preparation method of graphene quantum dot-modified alumina composite ceramic based on surface chemical bonding according to claim 3, characterized in that, The silane coupling agent in step (2) is selected from aminosilane coupling agents with the general formula NH2-R-Si-(OR')3, wherein R is alkylene and R' is methyl or ethyl; the purity of Al2O3 powder is greater than 99.9%.

6. The method for preparing graphene quantum dot-modified alumina composite ceramics based on surface chemical bonding according to claim 3 or 5, characterized in that, The silane coupling agent in step (2) is selected from KH-550; the pre-drying is the drying of Al2O3 powder at 120°C for more than 12 hours.

7. The method for preparing graphene quantum dot-modified alumina composite ceramics based on surface chemical bonding according to claim 3, characterized in that, In step (3), the ultrasonic power is 200-600W, corresponding to a power density of 5-20 W / mL.

8. The method for preparing graphene quantum dot-modified alumina composite ceramics based on surface chemical bonding according to claim 3, characterized in that, In step (4), the obtained paste precipitate is pre-frozen at -40°C or below for 12-24 hours and then subjected to vacuum freeze drying with a vacuum degree of <10 Pa for 48-72 hours.

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