Preparation method of high-strength foamed ceramic under freeze-thaw cycles

Abstract

The application provides a preparation method of high-strength foamed ceramics under freeze-thaw cycles, and belongs to the technical field of foamed ceramic preparation. First, raw materials are finely ground and dried to be configured into first ceramic powder, further finely ground after adding fluxing agent to obtain second ceramic powder, then foamed material obtained by mixing gel and water is sprayed on the second ceramic powder to obtain third ceramic powder, and pore-forming agent is added to the third ceramic powder, and the pre-sintering material is obtained after fine grinding, then the pre-sintering material is pre-sintered and degassed, and then poured into a mold for cycle freezing and thawing, and finally sintered under a nitrogen atmosphere to obtain high-strength foamed ceramics. The method makes the foamed ceramics have good performance after freeze-thaw cycle treatment.

Description

Technical Field

[0001] This invention relates to the field of foamed ceramics preparation technology, and in particular to a method for preparing high-strength foamed ceramics under freeze-thaw cycles. Background Technology

[0002] In ceramic forming methods, in addition to traditional colloidal forming methods such as slip casting, tape casting, and injection molding, new colloidal forming methods, such as gel casting and direct solidification casting, have also been developed. These methods have good particle packing uniformity and fewer defects, making them more suitable for preparing complex-shaped components with high reliability, and are particularly suitable for near-net-shape forming of ceramic composite materials.

[0003] Gel casting utilizes the concept of monomer reactions to form a three-dimensional entangled state-space macromolecular network, fixing ceramic particles in the desired shape. Compared with other molding methods, gel casting has advantages such as short curing time, high green body strength, low processing cost, and high yield, making it more suitable for preparing ceramics with complex shapes. However, the most commonly used acrylamide-N,N′-methylenebisacrylamide (AM-MBAMM) gel system is highly toxic, limiting its further application. Therefore, in recent years, many researchers have focused on developing low-toxicity or non-toxic gel systems.

[0004] Direct solidification casting (DCC), as another common colloidal forming method for near-net-shape ceramic components, has attracted considerable attention to date. It utilizes the instability of suspensions, and by shifting their pH to the isoelectric point (IEP) or increasing their ionic strength, it can be used to obtain uniform green bodies. However, the low strength of wet-set samples at around 10 kPa should be quite considerable, and the long solidification time and cumbersome operating conditions limit the large-scale production of ceramic components.

[0005] Polyvinyl alcohol (PVA) is a biocompatible, non-toxic, water-soluble polymer. It is a semi-crystalline polymer with good chemical and thermal stability, and has been widely used in medical, cosmetic, and packaging fields. Interestingly, repeated freezing and thawing of PVA can produce high-strength PVA gels, likely due to intramolecular and intermolecular hydrogen bonds.

[0006] Based on this, this invention systematically studied the effects of PVA gel on the composition, pore structure, and properties of slag-foamed ceramics under different freeze-thaw cycles and freezing times, using coal-fired slag as the main raw material. It revealed the formation mechanism and influencing factors of the pore structure of the foamed ceramics, clarified the compression failure behavior and reinforcement mechanism of the foamed ceramics, and proposed the design principles and preparation methods of high-strength foamed ceramics. Summary of the Invention

[0007] This invention provides a method for preparing high-strength foamed ceramics under freeze-thaw cycles. This method can reduce the occurrence of uneven foaming, improve the freeze-thaw resistance of foamed ceramics, and enhance their thermal insulation properties.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0009] A method for preparing high-strength foamed ceramics under freeze-thaw cycles includes the following steps:

[0010] S1. The raw materials are ball-milled and dried separately, and then prepared into the first ceramic powder according to the proportion;

[0011] S2. Transfer the first ceramic powder into a ball mill, add the flux sodium tetraborate and metal wire, and stir in the ball mill to obtain the second ceramic powder;

[0012] S3. Spray the foaming material evenly into the second ceramic powder to obtain the third ceramic powder;

[0013] S4. Transfer the third ceramic powder into a ball mill, add pore-forming agent powder, and stir in a mixer to obtain pre-fired material;

[0014] S5. Place the pre-fired material into the mold for pre-firing, and then stir vigorously to remove gas;

[0015] S6. Pour the degassed mixture from step S5 into a mold for cyclic freeze-thaw cycles.

[0016] S7. The mixture after the freeze-thaw cycle in step S6 is fired under a nitrogen atmosphere to obtain high-strength foamed ceramic.

[0017] The raw materials in step S1 are lithium mica tailings, ceramic polishing waste, fly ash, and borax. The mass ratio of lithium mica tailings, ceramic polishing waste, fly ash, and borax is 16-20:32-40:16-20:14-26.

[0018] In step S1, the ball milling speed is 150 r / min-210 r / min;

[0019] The ball milling time for the lithium mica tailings in the raw materials is 12-24 hours; the ball milling time for ceramic polishing waste is 8-15 hours; the ball milling time for fly ash is 6-10 hours; and the ball milling time for borax is 6-8 hours.

[0020] The drying temperature is 90-120℃, and the drying time is 6-12 hours.

[0021] In step S2, the amount of sodium tetraborate used is 14%-26% of the mass of the first ceramic powder, and the amount of metal wire used is 30%-50% of the mass of the first ceramic powder; the ball mill speed is 45r / min-70r / min, the ball milling and stirring time is not less than 1h, and the particle size of the obtained second ceramic powder is less than 75μm.

[0022] The diameter of the metal wire is ≤1mm, the length of the metal wire is 0.5-1.0cm, and the melting point of the metal wire is not lower than 1200℃.

[0023] In step S3, the foaming material is obtained by mixing PVA gel and water at a mass ratio of 7-15:100, and the amount of foaming material used is 7%-15% of the mass of the second ceramic powder.

[0024] In step S4, the pore-forming agent is Si3N4, the average particle size of the pore-forming agent is 20-60μm, the amount of pore-forming agent is 16-21% of the mass of the third ceramic powder, and the stirring time in the mixer is not less than 12h.

[0025] In step S5, the pre-fired material is fired under a nitrogen atmosphere with a nitrogen pressure of 0.05–0.2 MPa, a firing temperature of 900–1000 °C, and a sintering time of 20–40 min. This avoids incomplete reduction of metal oxides in the ceramic and prevents oxidation of the ceramic material surface, thus avoiding the generation of impurities.

[0026] During the strong stirring and degassing process in step S5, the rotation speed is 210 r / min-340 r / min.

[0027] The freeze-thaw cycle in step S6 specifically involves freezing at -10 to -15°C for 20 to 30 hours, then thawing at room temperature for 20 to 30 hours, and then drying in an oven at 70 to 110°C for 12 to 24 hours to achieve one freeze-thaw cycle.

[0028] In step S6, the number of freeze-thaw cycles shall not be less than 2.

[0029] In step S7, the firing temperature is 900–1000℃ and the firing time is 30–90 min.

[0030] Preferably, the drying temperature in step S1 is 100℃~120℃, and the drying time is 12~24h.

[0031] Preferably, in step S2, the first ceramic powder, metal wire, and flux sodium tetraborate are ball-milled in a ball mill for 4 hours. The added metal wire has a melting point of not less than 1200℃ to ensure that it has a certain strength while foaming at high temperature, and the diameter of the added metal strip does not exceed 1 mm and the length does not exceed 1 cm. Metals such as copper, iron, aluminum, and titanium can be selected.

[0032] Preferably, the degree of polymerization of the foaming material in step S3 is not less than 1700.

[0033] Preferably, the diameter of the pore-forming agent powder in step S4 is 20-60 micrometers, so as to ensure that the metal strip can participate well in the foaming process during foaming, thereby improving thermal conductivity and reducing the fluidity of the liquid phase crystal; the pore-forming agent powder adopts silicon nitride hollow spheres.

[0034] Preferably, the pre-firing temperature in step S5 is 900-950°C.

[0035] In step S5, the fired product is subjected to vacuum treatment in a vacuum furnace for a degassing time of not less than 10 minutes to avoid the generation of bubbles during the firing process, thereby ensuring the density and quality of the foamed ceramic after firing.

[0036] Preferably, in step S7, the nitrogen atmosphere pressure is not less than 0.5 MPa.

[0037] In this invention, by adding metal wires, the thermal conductivity and catalytic effect of the metal strips can be used to better sinter the ceramic powder together, thereby improving the strength and heat resistance of the foamed ceramics. At the same time, it promotes the formation of the pore structure of the foamed ceramics, which to a certain extent improves the thermal insulation performance of the foamed ceramics.

[0038] By adding flux, during the high-temperature foaming of foamed ceramics, borax pentahydrate can decompose to form Na2O and [BO3] triangles. The resulting sodium oxide can induce the decomposition of Si-O bonds, and [BO3] can insert into [[SiO]4] tetrahedra to break the glass structure, thereby reducing the glass softening temperature and allowing sintering at low temperatures.

[0039] A spray-applied polyvinyl alcohol (PVA) solution is used to uniformly adhere to the ceramic powder surface, forming a uniform coating. This improves the adhesion between ceramic particles, promoting the formation and sintering of the chloroplast. Secondly, it wets and encapsulates the ceramic particles, aiding in particle dispersion and uniform distribution, thus promoting the formation of a uniform chloroplast and preventing uneven porosity during sintering. Thirdly, it reduces the surface tension of the liquid, allowing for better coverage and wetting of the ceramic powder surface during spraying, improving the flowability and dispersibility of the ceramic particles while reducing porosity and voids between particles. Finally, it allows the ceramic powder and PVA solution to be uniformly mixed, forming a slurry with good plasticity. This helps improve the plasticity and shapeability of the chloroplast during the forming process, making it easier to mold into the desired shape.

[0040] By adding pore-forming agents, tiny bubbles can be formed, thereby achieving closed pores in ceramics and creating a porous structure.

[0041] The above technical solution has at least the following advantages compared with the existing technology:

[0042] The above scheme incorporates metal strips that enhance thermal conductivity and catalytic effect during foaming, as well as the strength and thermal insulation properties of the foamed ceramic. The added flux lowers the required foaming temperature, allowing foaming to occur at lower temperatures and reducing energy loss during foaming. Spraying the polyvinyl alcohol solution improves the dispersion, wetting, and adhesion of the ceramic powder, thereby enhancing the molding performance and sintering quality of the ceramic. Finally, the added pore-forming agent ensures the uniformity of pores generated during foaming, improving the various properties of the foamed ceramic. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a process flow diagram of a method for preparing high-strength foamed ceramics under freeze-thaw cycles according to the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0046] This invention provides a method for preparing high-strength foamed ceramics under freeze-thaw cycles.

[0047] like Figure 1 As shown, the method includes the following steps:

[0048] S1. The raw materials are ball-milled and dried separately, and then prepared into the first ceramic powder according to the proportion;

[0049] S2. Transfer the first ceramic powder into a ball mill, add the flux sodium tetraborate and metal wire, and stir in the ball mill to obtain the second ceramic powder;

[0050] S3. Spray the foaming material evenly into the second ceramic powder to obtain the third ceramic powder;

[0051] S4. Transfer the third ceramic powder into a ball mill, add pore-forming agent powder, and stir in a mixer to obtain pre-fired material;

[0052] S5. Place the pre-fired material into the mold for pre-firing, and then stir vigorously to remove gas;

[0053] S6. Pour the degassed mixture from step S5 into a mold for cyclic freeze-thaw cycles.

[0054] S7. The mixture after the freeze-thaw cycle in step S6 is fired under a nitrogen atmosphere to obtain high-strength foamed ceramic.

[0055] The following description, in conjunction with specific embodiments, illustrates this point.

[0056] according to Figure 1 The process shown involves ball milling lepidolite tailings, ceramic polishing waste, fly ash, and borax for 4–8 hours each, followed by drying at 110°C for 6–12 hours. These are then mixed in a mass ratio of 16:35:18:20. The mixed ceramic powder is transferred to a ball mill, and 40% by weight of metal wire and 20% by weight of sodium tetraborate pentahydrate (with a diameter not exceeding 1 mm and a length of 1 cm) are added. The mixture is then stirred in the ball mill for at least 4 hours. Polyvinyl alcohol (PVA) gel and water are uniformly mixed in a 7:100 ratio to form a foaming material. This solution is then uniformly sprayed onto the ceramic powder and stirred. The resulting pre-fired material is added to pore-forming silicon nitride hollow spheres and transferred to a ball mill for further stirring. The pre-fired material is then transferred to a mold and heated at 900–950°C. Pre-fire the sample under a nitrogen atmosphere of 0.05–0.2 MPa for 20–40 min; transfer the sinter to a ball mill for vigorous stirring; transfer the stirred sinter to a vacuum furnace for vacuum treatment, with a degassing time of not less than 10 min; pour the degassed sinter into a mold; transfer the mold to a freezer at -10 to -15℃ for 24 h; thaw the frozen sample at room temperature for 20 h; transfer the thawed sample to an oven at 70–100℃ for 24 h; repeat the freeze-thaw-dry cycle; transfer the sample after two freeze-thaw cycles to a mold and fire it at 950–100℃ under a nitrogen atmosphere of 0.5–1 MPa for 65 min to obtain a porosity of 73.37%–75.78% and a bulk density of 1.53–2.13 g / cm³. 3 The flexural strength is 62.1–166.3 MPa, and the crack toughness is 1.78–3.54 MPa m. 1 / 2 High-strength foamed ceramic with a dielectric constant of 3.74–4.76 at 25℃ and 10GHz, a thermal conductivity of 2.19–2.41 W / (m·K), and a water absorption rate exceeding 4%.

[0057] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing high-strength foamed ceramics under freeze-thaw cycles, characterized by, The method comprises the following steps: S1, the raw materials are respectively ball milled and dried, and then proportioned into first ceramic powder; S2, the first ceramic powder is transferred into a ball mill, fluxing agent sodium tetraborate and metal wire are added, and the ball mill is stirred to obtain second ceramic powder; S3, the foaming material is uniformly sprayed into the second ceramic powder to obtain third ceramic powder; S4, the third ceramic powder is transferred into a ball mill, and pore forming agent powder is added, and the mixture is stirred in the ball mill to obtain pre-sintering material; S5, the pre-sintering material is placed into a mold for pre-sintering, and then strong stirring is performed to remove gas; S6, the mixture after gas removal in step S5 is poured into a mold for cyclic freezing and thawing; S7, the mixture after cyclic freezing and thawing in step S6 is sintered in a nitrogen atmosphere to obtain high-strength foamed ceramic; In step S1, the raw materials are lithium mica tailings, ceramic polishing waste, fly ash and borax, and the mass ratio of lithium mica tailings, ceramic polishing waste, fly ash and borax is 16-20:32-40:16-20:14-26; In step S5, the pre-sintering material is sintered in a nitrogen atmosphere, the nitrogen pressure is 0.05-0.2 MPa, the sintering temperature is 900-1000℃, and the sintering time is 20-40 min; In step S6, the cyclic freezing and thawing is specifically as follows: freezing at a temperature of-10 to-15℃ for 20-30h, then thawing at room temperature for 20-30h, and then drying in an oven at 70-110℃ for 12-24h to realize one cycle of freezing and thawing; The number of cycles of freezing and thawing in step S6 is not less than 2.

2. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S1, the ball milling speed is 150r / min-210r / min; In the raw materials, the ball milling time of lithium mica tailings is 12-24h, the ball milling time of ceramic polishing waste is 8-15h, the ball milling time of fly ash is 6-10h, and the ball milling time of borax is 6-8h; the drying temperature is 90-120℃, and the drying time is 6-12h.

3. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S2, the amount of sodium tetraborate is 14%-26% of the mass of the first ceramic powder, and the amount of metal wire is 30%-50% of the mass of the first ceramic powder; the ball mill speed is 45r / min-70r / min, the ball milling time in the ball mill is not less than 1h, and the particle size of the obtained second ceramic powder is less than 75μm; The diameter of the metal wire is ≤1mm, the length of the metal wire is 0.5-1.0cm, and the melting point of the metal wire is not less than 1200℃.

4. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S3, the foaming material is obtained by mixing PVA gel and water at a mass ratio of 7-15:100, and the amount of the foaming material is 7%-15% of the mass of the second ceramic powder.

5. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S4, the pore forming agent is Si3N4, the average particle size of the pore forming agent is 20-60μm, the amount of the pore forming agent is 16-21% of the mass of the third ceramic powder, and the stirring time in the stirrer is not less than 12h.

6. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S5, when strong stirring is performed to remove gas, the speed is 210r / min-340r / min.

7. The method for preparing high-strength foamed ceramic under freeze-thaw cycles according to claim 1, characterized in that, In step S7, the sintering temperature is 900-1000℃, and the sintering time is 30-90min.

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