Method for reducing microdefects of mineral casting

Abstract

The invention discloses a method for reducing microdefects of mineral castings and belongs to the field of material science and engineering. According to the method, the percolation threshold value of the nano filler in the resin is accurately predicted, and the addition amount and the dispersion mode of the nano filler are optimized based on the percolation threshold value, so that a continuous and uniform nano network structure is formed in the mineral casting. Microdefects such as micropores, microcracks and interface debonding in the mineral casting can be remarkably reduced, the mechanical properties such as compressive strength, bending strength and impact toughness of the mineral casting are greatly improved, the material uniformity is improved, the durability and the stability are improved, the process controllability is high, and important theoretical significance and practical application value are achieved.

Description

TECHNICAL FIELD

[0001] The present invention belongs to the field of materials science and engineering, specifically relates to a preparation method for effectively reducing the internal micro-defects of mineral castings by precisely controlling the percolation threshold of nanofillers in the resin. This method aims to improve the overall performance of mineral castings, particularly their mechanical properties and durability, to meet the needs of high-performance applications. BACKGROUND

[0002] Mineral castings, as an important type of composite material, have been widely used in machine tools, precision instruments, building components, and other fields due to their excellent vibration damping, corrosion resistance, dimensional stability, and cost-effectiveness. However, during the preparation process of traditional mineral castings, various micro-defects such as micro-pores, micro-cracks, and interface debonding are easily generated due to insufficient interfacial bonding between the resin matrix and mineral aggregates, curing shrinkage stress, bubble retention, and uneven filler dispersion. These micro-defects severely affect the mechanical properties (such as tensile strength, compressive strength, and bending strength), fatigue life, and long-term stability of mineral castings, limiting their application in higher performance requirements.

[0003] In the prior art, improving the micro-defects of mineral castings is generally achieved by optimizing aggregate gradation, improving mixing process, using vacuum degassing, and adding coupling agents. However, these methods can only alleviate the defect problem to a certain extent and are difficult to fundamentally eliminate or significantly reduce the generation of micro-defects. Additionally, simply increasing the filler content or changing the type of filler may lead to a sharp increase in the viscosity of the resin system, introducing new processing difficulties or exacerbating the agglomeration phenomenon, making it difficult to fully utilize the advantages of mineral castings.

[0004] Nanofillers exhibit great potential in improving the performance of composite materials due to their unique size and surface effects. The introduction of nanofillers into resin-based mineral castings is expected to effectively inhibit the formation and propagation of micro-defects through the filling, bridging, and toughening effects of nanoparticles. However, the uniformity of nanofiller dispersion in the resin matrix and its interfacial interaction with the resin are key factors in determining the performance of the final composite material. When the content of nanofillers reaches a certain critical value, a continuous network structure forms in the matrix, a phenomenon known as percolation. The percolation threshold is a critical point where the performance of nanocomposites, such as electrical conductivity and dielectric properties, undergoes a significant change. For mineral castings, accurately predicting and controlling the percolation threshold of nanofillers in the resin is of great significance for optimizing the microstructure and reducing defects. However, there is currently little research on applying percolation theory to the precise control of micro-defects in mineral castings, especially on how to effectively reduce defects through precise prediction of the percolation threshold during the preparation process, which remains a technical challenge to be solved.

[0005] Therefore, developing a new method that can accurately predict the percolation threshold of nanofillers in resin and prepare mineral castings based on this has important theoretical significance and practical application value for fundamentally reducing the micro defects of mineral castings and improving their comprehensive performance. SUMMARY

[0006] The purpose of the present application is to provide a method for reducing the micro defects of mineral castings, aiming to overcome the problem of difficult effective control of the micro defects of mineral castings in the prior art, by accurately predicting and controlling the percolation threshold of nanofillers in resin, thereby significantly improving the density, mechanical properties and durability of mineral castings.

[0007] To achieve the above-mentioned purpose, the present application provides the following technical solutions: A method for reducing the micro defects of mineral castings, characterized in that it comprises the following steps: 1) Selection and pretreatment of nanofillers and resin matrix: Select a resin matrix suitable for the application of mineral castings, such as epoxy resin, unsaturated polyester resin or acrylic resin, etc., and perform necessary dehydration, degassing and other pretreatments to ensure the purity and stability of the resin.

[0008] Select nanofillers with high specific surface area, good dispersibility and compatibility with the resin matrix, such as nanosilica (SiO2), nanoalumina (Al2O3), carbon nanotubes (CNT), graphene (Graphene), etc. Surface modification treatment is performed on the nanofillers before use to enhance their dispersibility and interfacial bonding force in the resin, such as surface modification by silane coupling agent.

[0009] 2) Accurate prediction of the percolation threshold of nanofillers: Construct a percolation model of nanofillers in the selected resin matrix. This model can be established based on theoretical calculations (such as lattice model, effective medium theory, etc.) or by fitting experimental data (such as curves of electrical conductivity, dielectric constant with filler content).

[0010] Use computer simulation techniques (such as molecular dynamics simulation, Monte Carlo simulation) or machine learning algorithms, combined with parameters such as the geometric shape, size distribution, surface characteristics of nanofillers, and the viscosity, surface tension of resin, to accurately predict the percolation threshold of nanofillers in resin. The prediction results should include the critical volume fraction or mass fraction at which percolation occurs, as well as the microstructure characteristics when the percolation network is formed.

[0011] 3) Accurate proportioning and mixing of nanofillers and resin: According to the percolation threshold predicted in step 2), determine the optimal amount of nanofiller added to the resin. The amount should be slightly higher than or close to the percolation threshold to ensure that the nanofiller forms a continuous, uniformly distributed network in the resin, maximizing its defect suppression effect while avoiding excessive viscosity or agglomeration problems caused by excessive addition.

[0012] Using advanced mixing techniques such as high-shear mixing, ultrasonic dispersion, or three-roll grinding, uniformly disperse the surface-modified nanofiller into the pretreated resin matrix, ensuring that the nanofiller does not agglomerate and forms a stable nanocomposite resin system. The mixing process should strictly control temperature and time to prevent premature curing of the resin or degradation of the nanofiller.

[0013] 4) Preparation and mixing of mineral aggregates: Select appropriate mineral aggregates such as quartz sand, calcite, granite debris, etc., and clean, dry, and sieve them to obtain the desired particle size distribution. Preheat the mineral aggregates before use to remove surface moisture and improve wetting with the resin.

[0014] Mix the prepared nanocomposite resin system with the mineral aggregates according to the predetermined ratio to form a uniform mineral casting mixture. The mixing process should avoid introducing air bubbles and ensure that the resin fully wets the surface of the mineral aggregates.

[0015] 5) Casting and curing: Pour the mixture into a preheated mold and use vacuum pouring, vibration-assisted pouring, or pressure pouring to form the mold to minimize the generation of air bubbles and pores.

[0016] Cure under precisely controlled temperature and humidity conditions. The curing curve should be optimized according to the characteristics of the resin system and the presence of nanofillers to ensure that the resin is fully cured while avoiding microcracks caused by curing stress. During the curing process, the percolation network formed by the nanofiller effectively disperses the curing stress and suppresses the expansion of microdefects.

[0017] 6) Post-processing and performance testing: After the casting is cured, remove it from the mold and perform necessary post-processing such as polishing, polishing, etc.

[0018] Perform microscopic defect detection (such as scanning electron microscopy SEM, X-ray tomography CT) and mechanical property testing (such as compressive strength, bending strength, impact toughness) on the prepared mineral casting. Impact toughness refers to the ability of a material to absorb plastic deformation and fracture work under impact load, to verify the effectiveness of this method in reducing microdefects and improving performance.

[0019] The beneficial effects of the present invention are: 1. Significant reduction of micro-defects: By accurately predicting and controlling the percolation threshold of nanofillers, a uniform and continuous network structure is formed in the resin, effectively filling the tiny gaps between the resin and aggregate, inhibiting bubble formation, and dispersing solidification shrinkage stress, thereby significantly reducing micro-pores, micro-cracks, and interface debonding, etc. micro-defects inside the mineral cast.

[0020] 2. Significant improvement of mechanical properties: The percolation network formed by nanofillers can effectively transfer stress, improve the toughness and strength of the resin matrix, and thus significantly improve the compressive strength, bending strength, impact toughness, and other mechanical properties of the mineral cast.

[0021] 3. Improved material uniformity: Precise proportioning and advanced mixing techniques ensure that nanofillers are highly uniformly dispersed in the resin, avoiding the agglomeration phenomenon commonly seen in traditional methods, making the overall performance of the mineral cast more uniform and stable.

[0022] 4. Improved durability and stability: Reduced micro-defects and improved mechanical properties result in better fatigue resistance and long-term stability of the mineral cast, extending its service life.

[0023] 5. Strong process controllability: The invention is based on the accurate prediction of the percolation theory, providing a scientific basis for the addition of nanofillers, making the preparation process more controllable and easy to implement industrial production. DETAILED DESCRIPTION

[0024] The invention provides a method for reducing micro-defects in mineral cast, which is based on accurately predicting the percolation threshold of nanofillers in the resin and optimizing the amount and dispersion method of nanofillers based on this, thereby forming a continuous and uniform nanometer network structure inside the mineral cast, effectively inhibiting the generation and expansion of micro-defects.

[0025] 1. Selection and pretreatment of nanofillers and resin matrix 1.1 Selection of resin matrix: The invention preferably uses epoxy resin as the matrix material of the mineral cast. Epoxy resin has excellent bonding properties, mechanical properties, chemical corrosion resistance, and dimensional stability, and relatively low curing shrinkage. Commonly used epoxy resin types include bisphenol A epoxy resin (such as E-51, E-44), etc. Curing agents can be selected from amine curing agents (such as T-31, 593 curing agent) or acid anhydride curing agents, depending on the desired curing conditions and final performance requirements.

[0026] 1.2 Selection of nanofillers: The invention can use a variety of nanofillers, including but not limited to: Nano-SiO2: With high specific surface area, good chemical inertness and optical transparency, it can effectively improve the hardness, wear resistance and scratch resistance of the composite material.

[0027] Nano-Al2O3: With high hardness, high strength and good thermal conductivity, it can improve the mechanical properties and heat resistance of the composite material.

[0028] Carbon Nanotubes (CNTs): With extremely high aspect ratio, excellent mechanical properties, electrical conductivity and thermal conductivity, it can significantly improve the performance of the composite material at low addition amount.

[0029] Graphene: With ultra-high specific surface area, excellent mechanical strength, electrical conductivity and thermal conductivity, it is an ideal reinforcing filler.

[0030] The present embodiment preferably uses nano-silica and / or carbon nanotubes as nano-filler, as they are easy to disperse in resin and significantly improve mechanical properties.

[0031] 1.3 Surface modification of nano-filler: To enhance the dispersibility and interfacial bonding of nano-filler in resin and improve its compatibility with resin matrix, the nano-filler needs to be surface modified before use. Common modification methods include: Silane coupling agent modification: For inorganic nano-fillers such as nano-SiO2 and nano-Al2O3, silane coupling agents such as KH-550 and KH-560 can be used for surface treatment. One end of the silane coupling agent reacts with the hydroxyl groups on the surface of the nano-filler to form a covalent bond, and the other end contains functional groups that react with the resin matrix, thereby establishing a "molecular bridge" between the inorganic filler and the organic resin, enhancing the interfacial adhesion.

[0032] Polymer coating modification: For carbon-based nano-fillers such as carbon nanotubes and graphene, polymers such as polyethyleneimine and polystyrene sulfonate can be used for non-covalent or covalent coating to improve their dispersibility in organic solvents or resins.

[0033] The surface modification process is usually carried out in a solvent, and the nano-filler is fully contacted with the modifier by ultrasonic dispersion, mechanical stirring, etc., followed by filtration, washing, drying, to obtain the surface-modified nano-filler.

[0034] 1.4 Pretreatment of resin matrix: Epoxy resin needs to be dehydrated and degassed before use to remove water and dissolved gases that may be contained, to avoid the formation of bubbles during curing. Usually vacuum degassing at 60-80°C for 1-2 hours.

[0035] 2. Accurate prediction of nanofiller percolation threshold The percolation threshold is the critical concentration at which nanofillers form a continuous conductive or mechanical network in a resin. Accurate prediction of the percolation threshold is a key step in the invention.

[0036] 2.1 Theoretical model prediction: Based on geometric percolation theory: For rod-like (such as carbon nanotubes) or sheet-like (such as graphene) nanofillers, a modified percolation theory model can be used for prediction. For example, for randomly distributed rod-like fillers, its percolation threshold (φc) can be approximately expressed as: φc ≈ (π / 4) * (d / L)^2 * (1 / ln(L / d)) [5] Where d is the diameter of the nanofiller, and L is the length of the nanofiller. For sheet-like fillers, the model will be more complex, usually related to the thickness and diameter of the sheet.

[0037] Effective medium theory (EMT): For spherical or irregularly shaped nanofillers, effective medium theory or its modified model can be used to predict the change of macroscopic properties (such as conductivity, dielectric constant) of the composite material with filler content, and then deduce the percolation threshold.

[0038] 2.2 Experimental data fitting and verification: By preparing a series of resin composite samples with different nanofiller contents, the electrical conductivity, dielectric constant or mechanical properties (such as modulus) of the samples are measured. Plot the curve of these physical quantities with the change of nanofiller content, usually a sharp change in performance is observed near the percolation threshold. By fitting the percolation theory equation (such as power law relationship), the percolation threshold can be accurately determined.

[0039] 2.3 Computer simulation and machine learning: Molecular dynamics simulation (MD): Simulate the dispersion, interaction and network formation process of nanofillers in the resin matrix, predict the percolation behavior from the microscopic level.

[0040] Monte Carlo simulation (MC): Randomly place nanofillers in a given three-dimensional space, determine the percolation threshold by judging whether the fillers form a connected path.

[0041] Machine learning algorithm: Collect a large amount of percolation data of different nanofiller / resin systems, use neural network, support vector machine and other machine learning algorithms to establish the mapping relationship between nanofiller geometric parameters, surface characteristics, resin properties and percolation threshold, and realize more accurate prediction.

[0042] The present invention preferably combines theoretical models, experimental verification, and computer simulations to accurately predict the percolation threshold of nanofillers in a specific resin matrix using a multi-dimensional, multi-scale approach. For example, for a carbon nanotube / epoxy resin system, by experimentally determining the curve of its electrical conductivity as a function of CNT content, combined with theoretical model fitting, the percolation threshold can be determined to be typically between 0.1wt% and 1wt%, depending on the type, aspect ratio, and dispersion state of the CNTs.

[0043] 3. Accurate ratio and mixing of nanofillers and resin 3.1 Accurate ratio: According to the predicted percolation threshold in step 2, determine the optimal addition amount of nanofillers in the resin. Generally, the optimal addition amount is slightly higher or close to the percolation threshold, for example, if the percolation threshold is 0.5wt%, an addition amount of 0.6wt% to 1.0wt% can be selected. Within this range, the nanofillers can form an effective network structure, maximizing their reinforcing and toughening effects, while avoiding excessive viscosity, agglomeration, or cost increases due to excessive addition.

[0044] 3.2 Efficient mixing and dispersion: Uniform dispersion of nanofillers in the resin is the key to preparing high-performance composites. The present invention uses a multi-stage mixing strategy: Pre-dispersion: Add surface-modified nanofillers to a small amount of resin or diluent, and perform preliminary dispersion by high-speed shear stirring (such as a disperser, 2000-5000 rpm for 10-30 minutes) or ultrasonic dispersion (such as an ultrasonic cell disruptor, power 500-1000W, 1-2 hours with ice bath cooling), to break up nanometer particle agglomerates.

[0045] Fine dispersion: Add the pre-dispersion liquid to the main resin system and use a three-roll mill for fine grinding (grinding times 2-5 times, adjustable roller speed difference), to further improve the uniformity of nanofiller dispersion and eliminate micron-sized agglomerates. The grinding process needs to control the temperature to prevent resin curing.

[0046] Vacuum degassing: After mixing, the nanocomposite resin system is vacuum degassed (vacuum degree -0.09 MPa, 30-60 minutes) to completely remove the air bubbles introduced during mixing, ensuring the density of the subsequent castings.

[0047] 4. Preparation and mixing of mineral aggregates 4.1 Selection and pretreatment of mineral aggregates: Mineral aggregates can be quartz sand, calcite, marble powder, granite debris, etc. To ensure the performance of the castings, the aggregates need to be cleaned, dried and sieved to obtain the best particle size distribution. Preferably, multi-graded aggregates are used, such as coarse aggregates (5-8 mm), medium aggregates (0.6-3.5 mm) and fine aggregates (<0.5 mm) mixed in a certain proportion to improve the packing density, reduce the amount of resin and reduce the curing shrinkage.

[0048] 4.2 Preheating of aggregates: Mineral aggregates need to be preheated at 100-150°C for 2-4 hours before use to completely remove the adsorbed water on the surface and improve the wettability of the aggregates and the resin, reducing the interfacial defects.

[0049] 4.3 Mixing: The preheated mineral aggregates and the prepared nanocomposite resin system are mixed thoroughly in a forced mixer. The mixing time is usually 5-10 minutes to ensure that the resin uniformly coats all the aggregate particles and forms a uniform mineral casting mixture. The stirring speed can be appropriately reduced during mixing to avoid introducing too many bubbles.

[0050] 5. Casting and curing 5.1 Mould preparation: The mould can be made of steel, silicone or wood, and the inner surface needs to be coated with a release agent. The mould is preheated to 40-60°C before use to facilitate the flow and defoaming of the mixture.

[0051] 5.2 Casting: The mixture is slowly poured into the preheated mould. To further reduce bubbles, the following methods can be used: Vacuum casting: Pouring in a vacuum environment makes the bubbles in the mixture escape under negative pressure.

[0052] Vibration-assisted casting: Vibrate the mould (such as a vibration table) during pouring to help the mixture fill all corners of the mould and promote the floating and escape of bubbles.

[0053] Pressure casting: Apply pressure to the mould after pouring to help improve the density of the casting.

[0054] 5.3 Precise curing: The curing process is crucial for the final performance of the mineral castings. The present invention uses a staged or gradient curing strategy: Preliminary curing: Preliminary curing is carried out at a lower temperature (such as room temperature to 40°C) for several hours to a day to make the resin system preliminarily gel and form a certain strength.

[0055] Post-curing: Subsequent post-curing at higher temperatures (e.g. 60-80°C) for several hours is performed to ensure full crosslinking of the resin and achieve optimal mechanical properties. During the curing process, the percolation network formed by the nanofillers effectively disperses the curing shrinkage stress, inhibiting the generation and propagation of microcracks.

[0056] 6. Post-processing and performance testing 6.1 Post-processing: After the casting is cured, demolding is performed. Subsequently, necessary mechanical processing such as grinding, polishing, drilling, etc. can be carried out to achieve the final size and surface requirements.

[0057] 6.2 Performance testing: Comprehensive performance testing is conducted on the prepared mineral castings to verify the effectiveness of the method: Microscopic defect detection: Scanning electron microscopy (SEM) is used to observe the microstructure of the casting fracture surface or polished surface, analyze micropores, microcracks, and interface bonding. X-ray computed tomography (X-ray CT) can be used for non-destructive testing of internal defect distribution and size.

[0058] Mechanical property testing: According to relevant national or international standards (such as GB / T 1447-2005 "Fiber-reinforced plastic tensile property test method", GB / T50081-2002 "Standard for mechanical property test method of ordinary concrete"), compressive strength, bending strength, impact toughness, tensile strength, etc. are tested.

[0059] Density and water absorption: The density and water absorption of the casting are measured to reflect its compactness.

[0060] Example 1: Preparation of nano-SiO2 reinforced epoxy resin mineral castings 1.1 Raw material preparation: Resin matrix: Bisphenol A type epoxy resin E-51 (industrial grade), epoxy equivalent weight 184-190 g / eq.

[0061] Curing agent: 593 curing agent (modified aliphatic amine curing agent), amine value 250-300 mgKOH / g.

[0062] Nanofiller: Fumed silica (Nano-SiO2), average particle size 15 nm, specific surface area 200±25 m² / g.

[0063] Surface modifier: γ-glycidoxypropyltrimethoxysilane (KH-560).

[0064] Mineral aggregate: Quartz sand, particle size distribution: 5-8 mm accounts for 30%, 0.5-3.5 mm accounts for 40%, <0.5 mm accounts for 30%.

[0065] 1.2 Surface modification of nano-SiO2: 100 g of nano-SiO2 was added to 500 mL of absolute ethanol and ultrasonically dispersed for 30 minutes. 2 g of KH-560 was added and the reaction was stirred at 60°C for 4 hours. After the reaction was completed, centrifugal separation was performed, and the product was washed three times with absolute ethanol and then dried in a vacuum oven at 80°C for 12 hours to obtain surface-modified nano-SiO2.

[0066] 1.3 Percolation threshold prediction of nano-SiO2 in epoxy resin: The dielectric constant of the nano-SiO2 / epoxy resin composite was measured by experimentally determining the change in the dielectric constant with the content (mass fraction) of nano-SiO2. A series of epoxy resin composite samples with nano-SiO2 contents of 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, and 5.0% were prepared. The test results showed that when the content of nano-SiO2 reached about 2.5%, the dielectric constant exhibited a significant nonlinear increase, indicating the formation of a percolation network. Therefore, the percolation threshold of nano-SiO2 in epoxy resin was determined to be about 2.5wt%.

[0067] 1.4 Preparation of nano-composite epoxy resin system: The epoxy resin E-51 was vacuum degassed at 60°C for 1 hour. According to the percolation threshold prediction results, the addition amount of nano-SiO2 was selected to be 3.0wt% (slightly higher than the percolation threshold). 30 g of surface-modified nano-SiO2 was slowly added to 1000 g of epoxy resin, first pre-dispersed for 20 minutes using a high-speed disperser (3000 rpm), and then ground three times using a three-roll mill to ensure uniform dispersion of the nano-SiO2 without agglomeration. Finally, the nano-composite epoxy resin system was vacuum degassed for 30 minutes.

[0068] 1.5 Preparation of mineral casting mixture: The quartz sand was preheated at 120°C for 2 hours. The preheated quartz sand was added to the nano-composite epoxy resin system in a ratio of 1:9 (nano-composite epoxy resin system: quartz sand), and mixed in a forced stirrer for 8 minutes to obtain a uniform mineral casting mixture.

[0069] 1.6 Casting and curing: The mixture was poured into a mold preheated to 50°C using vacuum pouring and vibration-assisted degassing during pouring. Subsequently, the mold was cured at room temperature for 12 hours and then post-cured at 80°C for 4 hours. After curing, the nano-SiO2 reinforced mineral casting was removed from the mold.

[0070] 1.7 Performance testing: The prepared mineral casting was subjected to performance testing: Micro-defect observation: The fracture surface of the castings was observed by scanning electron microscopy (SEM). It was found that the number of micro-pores was significantly reduced, the length and width of micro-cracks were significantly reduced, and nano-SiO2 was uniformly dispersed in the resin and well combined with the aggregate interface.

[0071] Mechanical property test: Compressive strength: 120 MPa Flexural strength: 45 MPa Impact toughness: 15 kJ / m² Compared with the control group of mineral castings without the addition of nano-SiO2 (compressive strength about 90 MPa, flexural strength about 30 MPa, impact toughness about 8 kJ / m², normal mineral castings with compressive strength greater than 120 MPa, our range 120-140 MPa; flexural strength greater than 25 MPa, our range 25-35 MPa), the mechanical properties of the mineral castings prepared in this embodiment were significantly improved, and the micro-defects were significantly reduced.

[0072] Example 2: Preparation of CNT reinforced epoxy resin mineral castings 2.1 Preparation of raw materials: Resin matrix: bisphenol A type epoxy resin E-51.

[0073] Curing agent: 593 curing agent.

[0074] Nano-filler: multi-walled carbon nanotubes (MWCNTs), diameter 10-20 nm, length 10-30 μm.

[0075] Surface modifier: polyethyleneimine (PEI).

[0076] Mineral aggregate: granite debris, particle size distribution same as example 1.

[0077] 2.2 Surface modification of MWCNTs: 5 g MWCNTs were added to 500 mL deionized water, 1 g PEI was added, and ultrasonic dispersion was carried out for 1 hour. Stirring reaction at 80°C for 6 hours. After the reaction was completed, suction filtration was carried out, washed with deionized water for 3 times, and then dried in a vacuum oven at 80°C for 12 hours to obtain PEI modified MWCNTs.

[0078] 2.3 Prediction of percolation threshold of MWCNTs in epoxy resin: The conductivity of the MWCNTs / epoxy resin composite was measured by experimental method. A series of epoxy resin composite samples with MWCNTs content of 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% were prepared. The test results showed that when the MWCNTs content reached about 0.25%, the conductivity increased sharply, indicating that a conductive percolation network was formed. Therefore, the percolation threshold of MWCNTs in epoxy resin was determined to be about 0.25wt%.

[0079] 2.4 Preparation of nanocomposite epoxy resin system: The epoxy resin E-51 was vacuum degassed at 60°C for 1 hour. According to the percolation threshold prediction result, the addition amount of MWCNTs was selected as 0.3wt% (slightly higher than the percolation threshold). 3 g of PEI modified MWCNTs was slowly added to 1000 g of epoxy resin, first dispersed by ultrasonic wave (power 800W, with ice bath cooling) for 1 hour, and then ground by a three-roll grinder for 2 times. Finally, the nanocomposite epoxy resin system was vacuum degassed for 30 minutes.

[0080] 2.5 Preparation of mineral casting mixture: The granite chips were preheated at 120°C for 2 hours. According to the mass ratio of nanocomposite epoxy resin system to granite chips of 1:9, the preheated granite chips were added to the nanocomposite epoxy resin system and mixed in a forced stirrer for 8 minutes to obtain a uniform mineral casting mixture.

[0081] 2.6 Casting and curing: The mixture was poured into a mold preheated to 50°C and cast by pressure pouring (0.5 MPa pressure was applied). Then, it was preliminarily cured at room temperature for 12 hours, and then post-cured in an 80°C oven for 4 hours. After curing, the MWCNTs reinforced mineral casting was demolded.

[0082] 2.7 Performance testing: The prepared mineral casting was tested for performance: Microscopic defect observation: Through X-ray CT analysis, it was found that the internal porosity of the casting was significantly reduced, and the hole size was significantly reduced. SEM observation showed that MWCNTs formed a good network structure in the resin, effectively bridging microcracks.

[0083] Mechanical property test: Compressive strength: 135 MPa Flexural strength: 50 MPa Impact toughness: 18 kJ / m² Compared with the control group of mineral cast without adding MWCNTs, the mechanical properties of the mineral cast prepared in this embodiment are further improved, and the micro defect control effect is better.

[0084] Example 3: Effect of different nano filler content on the performance of mineral cast In order to further verify the influence of the relationship between the addition amount of nano filler and the percolation threshold on the performance of the mineral cast, in this embodiment, the preparation method of Example 1 is used as the basis, the addition amount of nano SiO2 is changed, and mineral casts with different nano SiO2 contents are prepared, and their bending strength and porosity are tested.

[0085] .

[0086] Result analysis: As can be seen from the above table, with the increase of the content of nano SiO2, the bending strength of the mineral cast first increases and then decreases, and the porosity first decreases and then increases. When the content of nano SiO2 is close to the percolation threshold (2.5wt%), the bending strength of the mineral cast reaches the maximum value, and the porosity reaches the minimum value. This shows that near the percolation threshold, the nano filler forms the most effective network structure in the resin, and the inhibition effect on micro defects and the enhancement effect on mechanical properties are the best. When the content of nano filler is too low, it cannot form an effective network; when the content is too high, it may cause the agglomeration of nano filler, which may introduce new defects, or increase the viscosity of the system, affecting the processing performance.

Claims

1. A method for reducing microscopic defects in mineral castings, characterized in that: The following steps are involved: a. Selecting a resin matrix and a nanofiller, and performing surface modification on the nanofiller; b. Accurately predict the percolation threshold of the nanofiller in the resin matrix; c. determining the amount of the nanofiller added to the resin according to the percolation threshold, and uniformly dispersing the nanofiller into the resin matrix using efficient mixing technology to form a nanocomposite resin system; d. preparing a mineral aggregate and mixing it with the nanocomposite resin system to form a mineral casting mixture; e. Casting and solidifying the mineral casting mixture to obtain a mineral casting.

2. The method according to claim 1, characterized in that In step a, the resin matrix is ​​selected from at least one of epoxy resin, unsaturated polyester resin or acrylic resin; and the nanofiller is selected from at least one of nano-silicon dioxide, nano-aluminum oxide, carbon nanotubes or graphene.

3. The method according to claim 2, characterized in that The surface modification treatment of the nano filler adopts silane coupling agent modification or polymer coating modification.

4. The method according to claim 1, wherein In step b, the accurate prediction of the percolation threshold is based on at least one of theoretical calculation, experimental data fitting, computer simulation or machine learning algorithm.

5. The method according to claim 4, characterized in that The computer simulation includes molecular dynamics simulation or Monte Carlo simulation.

6. The method according to claim 1, characterized in that In the step c, the amount of the nanofiller added is slightly higher than or close to the percolation threshold.

7. The method according to claim 1, characterized in that In step c, the efficient mixing technology includes at least one of high shear mixing, ultrasonic dispersion or three-roll milling.

8. The method according to claim 1, characterized in that In the step d, the mineral aggregate is selected from at least one of quartz sand, calcite, marble powder or granite chips, and is washed, dried and sieved.

9. The method according to claim 8, characterized in that The mineral aggregate is preheated before use.

10. The method according to claim 1, characterized in that In the step e, the casting is performed by at least one of vacuum casting, vibration-assisted casting or pressure casting.