A low-density alumina hollow sphere microporous foam ceramic new material and a preparation method thereof

Abstract

This invention discloses a method for preparing a novel low-density alumina hollow sphere microporous foam ceramic material and the material itself, belonging to the field of refractory materials technology. The preparation method includes: firstly, weighing alumina powder, alumina hollow spheres, silica powder, and silica sol, adding a dispersant and water, and then dispersing with ultrasonic assistance combined with high-speed shearing to obtain a uniform slurry; subsequently, adding a foaming agent to the slurry and mechanically stirring to foam it, then adding a coagulant and a pore-forming agent and stirring to mix evenly, obtaining a foamed slurry; then injecting the foamed slurry into a mold and allowing it to solidify into a green body, followed by drying the green body; finally, placing the dried green body in a high-temperature kiln for sintering and holding at that temperature, and cooling it with the furnace to obtain the novel low-density alumina hollow sphere microporous foam ceramic material. The material prepared by this invention has a high degree of lightweight, a reasonable pore structure, and excellent high-temperature insulation and structural stability, solving the problems of high density, simple pore structure, and poor high-temperature service performance of traditional alumina hollow sphere bricks.

Description

Technical Field

[0001] This invention belongs to the field of refractory materials technology, specifically a new low-density alumina hollow sphere microporous foam ceramic material and its preparation method. Background Technology

[0002] The rapid development of high-temperature industries has placed higher demands on energy conservation and consumption reduction in kilns, making lightweight insulation materials a research hotspot. Currently, under high-temperature (≥1600℃) conditions, the lightweight insulation materials widely used in industry mainly include alumina hollow sphere bricks and alumina fiber products. Among them, alumina hollow sphere bricks are widely used as the hot-face lining material of high-temperature kilns due to their high service temperature, erosion resistance, and good volume stability. This type of material is usually prepared by using pre-fabricated alumina hollow spheres as aggregate, supplemented with alumina micro powder and binder, through vibration pressing and high-temperature sintering.

[0003] However, with increasingly stringent energy-saving standards, traditional alumina hollow sphere bricks have gradually revealed many limitations. Limited by the "pre-formed spheres + stacking" molding process, traditional products generally have a high density, typically between 1.4 g / cm³ and 1.7 g / cm³, leading to significant heat loss in the kiln. Their pore structure mainly consists of millimeter-scale hollow sphere cavities and the stacking gaps between the spheres; this relatively simple pore structure still has room for improvement in its ability to block radiative heat transfer at high temperatures. During long-term high-temperature service, microcracks are prone to form at the joints between the hollow spheres, which can negatively impact the material's long-term thermal shock resistance.

[0004] Patent CN121063923A discloses a lightweight alumina foam ceramic refractory material and its preparation method. The method involves first obtaining modified zirconia fibers through electrospinning, then mixing alumina micropowder, hollow alumina, a high-temperature foaming agent, fibers, and sintering aids, followed by molding and sintering. This technical solution attempts to reduce density and increase strength by introducing hollow aggregates and a foaming agent. However, in practical applications, the molding process struggles to precisely control the balance between achieving ultra-low density and maintaining the integrity of the pore structure. Furthermore, the introduction of electrospun fibers increases process complexity, leaving room for further optimization in achieving large-scale, low-cost preparation of ultralight materials.

[0005] Patent CN105541306B discloses an alumina fiber-reinforced alumina closed-cell foam ceramic and its preparation method. This method combines the addition of a pore-forming agent with a foaming process to prepare closed-cell foam ceramics with a bulk density between 0.7 and 1.0 g / cm³. This technical solution effectively reduces material density and improves thermal insulation performance. However, in terms of microstructure design, this solution mainly relies on the pores generated by foaming, lacking the support of a micron-level hollow framework. There is still room for improvement in terms of load softening temperature and volume stability under ultra-high temperature conditions. Furthermore, the single foamed pore structure is still insufficient compared to multi-level composite pore structures in terms of the synergistic effect of blocking high-temperature thermal radiation heat transfer.

[0006] Based on the above-mentioned shortcomings, this study aims to develop an alumina-based lightweight thermal insulation ceramic material and its preparation method that combines ultra-low density, high thermal insulation performance, excellent mechanical strength and thermal stability. This material addresses the problems of high density, weak high-temperature radiative heat insulation capacity, and short service life of existing materials, and has significant industrial application value and practical significance. Summary of the Invention

[0007] The purpose of this invention is to provide a novel low-density alumina hollow sphere microporous foam ceramic material and its preparation method. By introducing alumina hollow spheres with a particle size of 0.1-0.25 mm as a skeleton, and combining in-situ mullitization reaction and sintering process with composite silicon source, a composite pore structure is constructed inside the material, consisting of macropores of alumina hollow spheres, mesopores formed by foaming, and micropores generated in situ. This solves the problems of high density, weak high-temperature radiation heat barrier capacity, and short service life of existing materials.

[0008] The objective of this invention is achieved through the following technical solution: A method for preparing a novel low-density alumina hollow sphere microporous foam ceramic material includes the following steps: S1 Ingredients and Dispersion: Weigh 70-80 parts by weight of alumina powder, 15-20 parts by weight of hollow alumina spheres with a particle size of 0.1-0.25mm, and 15-20 parts by weight of silica powder and silica sol (on a dry basis). Add dispersant and water to the resulting mixture, and use ultrasonic-assisted dispersion combined with high-speed shear stirring to form a uniform slurry. S2 Foaming and Additives: Add a foaming agent to the slurry and mechanically stir to foam it. Then add a coagulant and a pore-forming agent and continue stirring to mix evenly to obtain a foamed slurry. S3 Molding and Drying: The foaming slurry is injected into the mold and allowed to solidify to form a green body, which is then dried. S4 High-Temperature Sintering: The dried green body is placed in a high-temperature kiln for heating and sintering and then kept at that temperature. It is then cooled to room temperature with the furnace to obtain a new low-density alumina hollow sphere microporous foam ceramic material.

[0009] As some possible implementations of this application, in step S1, the average particle size D50 of the alumina powder is 0.5-2 μm; the average particle size D50 of the silica powder is 0.1-1 μm; and the solid content of the silica sol is 25%-35%.

[0010] As some possible implementations of this application, in step S1, the dispersant is ammonium polyacrylate, and its addition amount is 0.1%-0.5% of the total mass of the dry powder mixture; the addition amount of water is 30%-50% of the total mass of the dry powder mixture. In step S2, the foaming agent is composed of sodium dodecyl sulfate and sodium fatty alcohol polyoxyethylene ether sulfate, with a mass ratio of 1:(1-3), and the total amount of foaming agent added is 0.5%-2% of the slurry mass; In step S2, the coagulant is modified starch, and its addition amount is 1%-3% of the total mass of the dry powder mixture; the pore-forming agent is polystyrene microspheres or organic polymer particles with a core-shell structure, the particle size of the pore-forming agent is 10-50 μm, and its addition amount is 2%-5% of the total mass of the dry powder mixture. Furthermore, the core-shell structured organic polymer particles are preferably microspheres with polymethyl methacrylate (PMMA) as the core and polyvinyl alcohol (PVA) as the shell.

[0011] As some possible implementations of this application, in step S2, the mechanical stirring and foaming speed is 1000-1500 r / min, the stirring time is 3-5 min, until the slurry volume expands to 1.5-2.5 times the initial volume; the stirring time after adding coagulant and pore-forming agent is 5-10 min.

[0012] As some possible implementations of this application, in step S3, the ambient temperature for static solidification is 25-40℃; the temperature for the drying treatment is 100-120℃, and the drying time is 12-24h.

[0013] As some possible implementations of this application, in step S4, the sintering temperature is 1650-1750℃ and the holding time is 8-12h; When the temperature is raised to the range of 400-600℃, the heating rate is controlled at 0.5-1.5℃ / min, and the temperature is maintained within this range for 2-4 hours.

[0014] As some possible implementations of this application, in step S1, the alumina hollow spheres undergo pre-coating modification treatment before feeding. The modification process is as follows: the alumina hollow spheres are placed in a pseudo-boehmite sol with a mass fraction of 1%-2% and coated at a low speed of 50-80 r / min. Then, they are dried at 130-160℃ for 0.5-2 h to form an Al2O3 pre-coating layer with a thickness of 0.5-1 μm on the surface of the alumina hollow spheres.

[0015] In practical applications, the thin walls and low strength of 0.1-0.25mm small-particle-size hollow alumina spheres make them susceptible to breakage due to the mechanical action of ultrasonic-assisted dispersion and high-speed shear stirring. Furthermore, the low density of these hollow spheres makes them prone to floating and segregation in the slurry, leading to uneven composition of the green body and inconsistent product performance. Therefore, this application pre-coats and modifies the 0.1-0.25mm small-particle-size hollow alumina spheres by forming an alumina coating layer on their surface. This strengthens the sphere walls, reduces the dispersion breakage rate, controls the apparent density, alleviates floating and segregation, and ensures the uniformity of the green body structure.

[0016] As some possible implementations of this application, in step S1, the silica powder and silica sol are first premixed to form a silica source emulsion, the mass ratio of silica sol to silica powder is (2-4):(2-1), and the silica source emulsion is added to the mixture in two parts. The first part is 60%-70% of the silica source emulsion added and ultrasonic-assisted dispersion is carried out simultaneously. The second part is the remaining 30%-40% of the silica source emulsion added before foaming.

[0017] In practical applications, the mullitization reaction involving silica sol is accompanied by volume expansion, while the solid-phase mullitization reaction involving silica powder exhibits micro-volume shrinkage, showing a significant difference in volume effects. Adding either of these silicon sources individually or in a single step can easily lead to stress concentration at the contact points of small-diameter hollow alumina spheres, damaging the sphere walls and causing localized micropores within the matrix, reducing the material's density and mechanical strength. Furthermore, silica sol, being colloidal particles, easily adsorbs onto the surface of hollow spheres, forming silica-rich regions, resulting in uneven mullitization reaction rates and poor necking formation. Therefore, this application premixes the two silicon sources to form a silicon source emulsion, then adds it to the slurry in batches. This balances the reaction volume effects of the two silicon sources, alleviates stress concentration at contact points, reduces micropore formation in the matrix, and avoids localized silica sol enrichment. This ensures uniform mullitization, regular necking formation, and maintains the integrity of the hierarchical pore structure, giving the material both good mechanical strength and thermal insulation properties.

[0018] As some possible implementations of this application, in step S2, the mechanical stirring foaming adopts real-time coupling control of the stirring speed. In the initial stage of foaming, the stirring speed is 1200-1500 r / min. When the slurry volume expands to 1.5-2.5 times, the stirring speed is reduced to 800-1000 r / min. During the speed control process, the coagulant is slowly added to maintain the slurry viscosity at 5000-8000 mPa·s. Before foaming, 0.2%-0.3% by mass of nano-attapulgite and 0.05%-0.15% by mass of polyamide wax composite anti-segregation agent are added to the slurry. The porogen is pretreated with an ethanol aqueous solution before addition. The pretreatment process is as follows: the porogen is placed in an ethanol aqueous solution with a mass fraction of 3%-5% and ultrasonically dispersed for 5-10 min, and then dried before use.

[0019] In practice, conventional foaming processes have many shortcomings. For example, a single stirring speed can easily lead to uneven foaming, bubble merging and rupture, making it difficult to form uniform pores. Moreover, both bubbles and small-diameter hollow alumina spheres are low-density phases, and they are prone to synergistic floating during the static solidification of the slurry, resulting in uneven distribution of components in the green body. This leads to anisotropy in the mechanical and thermal insulation properties of the product, and insufficient structural stability during high-temperature service. In addition, the direct addition of pore-forming agents results in poor dispersibility. If the small molecules generated by low-temperature pyrolysis cannot be discharged in time, they are prone to carbonization and blockage of micropores at high temperatures, thereby increasing the thermal conductivity of the material. Based on this, this application couples and controls the foaming speed and slurry viscosity in real time. Combined with a composite anti-segregation additive and a pore-forming agent pretreatment process, the foaming speed is adjusted in stages to match the foaming rhythm, and the viscosity is controlled to stabilize the bubble structure. The thixotropic properties of the compound additives are used to alleviate the floating and segregation problem, improving the uniformity of product performance. At the same time, the pore-forming agent is pretreated with ethanol using ultrasound to improve its dispersibility and promote the rapid discharge of pyrolysis products, reducing the risk of carbon black blockage and ensuring the integrity of the multi-level pore structure.

[0020] As one possible implementation method of this application, in step S4, the high-temperature sintering is a five-stage speed-controlled and atmosphere-regulated sintering process, specifically as follows: Low temperature section: The room temperature is raised to 600℃, and a nitrogen-air mixture atmosphere with a nitrogen gas integral of 85%-95% is introduced. The heating rate is 0.8-1.2℃ / min, and the temperature is held at 600℃ for 1-3 hours. Medium temperature section: Heating from 600℃ to 1200℃, with pure air atmosphere introduced, heating rate 2-3℃ / min, and holding at 1200℃ for 2-4 hours; Mullite petrochemical section: Heating from 1200℃ to 1650℃, with pure air atmosphere introduced, heating rate 4-5℃ / min, without heat preservation. High-temperature sintering section: Heat from 1650℃ to 1750℃, introduce pure air atmosphere, and hold for 8-12 hours; Cooling section: Temperature drops from 1750℃ to 1000℃ at a rate of 1-2℃ / min, and then naturally cools to room temperature below 1000℃.

[0021] In practice, conventional high-temperature sintering processes employ a single heating rate and atmosphere, which is difficult to adapt to the sintering requirements of multi-level porous foam ceramics. On the one hand, if the decomposition products of pore-forming agents and organic additives are not fully discharged, they are prone to residues that affect the purity of the matrix. On the other hand, the mullitization reaction process around small-diameter hollow alumina spheres is uneven, resulting in differences in the degree of densification of the bonding neck. At the same time, excessively rapid heating and cooling rates can easily generate thermal stress, which may lead to cracking at the interface between the matrix and the bonding neck, damage to the multi-level porous structure, and affect the structural stability and high-temperature performance of the material. Based on this, this application adopts a five-stage sintering process with coordinated speed control and atmosphere regulation. In the low-temperature stage, a mixed atmosphere is used to promote the full discharge of pyrolysis products and reduce residues. In the medium-temperature stage, the temperature is slowly increased to release the stress in the green body and reduce the risk of microcrack initiation. Precise temperature control in the mullitization stage and the high-temperature sintering stage ensures sufficient reaction and uniform densification of the bonding neck. The gradient cooling stage reduces thermal stress, reduces the risk of interface cracking and pore structure damage, improves the structural stability and performance uniformity of the sintered material, and extends its high-temperature service life.

[0022] In addition, this application also provides a novel low-density alumina hollow sphere microporous foam ceramic material, which is prepared by any of the above preparation methods.

[0023] Compared with the prior art, the beneficial effects of the present invention are: The novel low-density alumina hollow sphere microporous foam ceramic material prepared by this method can effectively improve the problems of traditional alumina hollow sphere bricks, such as high density, simple pore structure, insufficient bonding strength, and strong dependence on raw materials. The specific technical effects are as follows: (1) It helps to reduce the bulk density of the material and reduce the heat storage loss of the kiln. This method uses 0.1-0.25mm small-diameter alumina hollow spheres as the skeleton, and works with foaming agent and pore-forming agent to create pores. The bulk density of the material can be controlled at 500-600kg / m³, which is lower than the bulk density of traditional alumina hollow sphere bricks of 1.4-1.7g / cm³. This can reduce the heat storage loss of the high-temperature kiln lining, shorten the kiln heating time, and meet the energy-saving and consumption-reducing requirements of high-temperature industrial kilns.

[0024] (2) It is beneficial to optimize the pore structure and improve the high-temperature radiation heat transfer barrier capacity. By using foaming agents to generate bubbles and pore-forming agents to decompose at high temperatures to leave pores, combined with the volume effect of the mullitization reaction during sintering at 1650-1750℃, a multi-level composite pore structure is formed inside the material, which is composed of macroscopic macropores (small-diameter hollow spheres) and in-situ micron- and submicron-level closed micropores at the neck and sphere walls. This structure can increase the tortuosity of the heat conduction path, increase the number of reflections and scatterings of high-temperature heat radiation at the pore wall interface, and help reduce the high-temperature thermal conductivity of the material and optimize the heat insulation performance.

[0025] (3) It is beneficial to improve the bonding strength and thermal shock resistance. This method uses silica powder and silica sol as composite silicon sources. During the high-temperature sintering stage, they undergo an in-situ mullite formation reaction with alumina, forming a needle-like mullite crystal interwoven network at the contact point of the hollow sphere, which constitutes the reaction sintering bonding neck, which can replace the traditional external binder. The bonding neck has good thermodynamic stability and good thermal expansion matching with the alumina matrix. The bonding neck enables the material to maintain a certain mechanical strength at a lower density. Under long-term high-temperature service and thermal cycling conditions, it can reduce the initiation of microcracks and structural spalling, which is beneficial to improving the thermal shock resistance and service life of the material.

[0026] (4) It helps reduce reliance on raw materials and control the cost of large-scale production. This method reduces the reliance on high-quality, large-diameter pre-fabricated alumina hollow spheres, and the skeleton can be constructed using small-diameter hollow spheres of 0.1-0.25mm. At the same time, the ultrasonic-assisted dispersion combined with high-speed shear stirring can alleviate the problem of easy agglomeration of small-diameter hollow spheres, making the raw material adaptability wider and reducing the stringent requirements on raw material quality, which is conducive to cost control in the process of large-scale production.

[0027] (5) It is beneficial to improve the overall performance of the material. The small-diameter hollow spheres have a larger specific surface area and are more tightly bonded to the alumina matrix. Combined with the uniform dispersion of raw materials achieved in each process step, it can improve the uniformity of the material structure. The continuous network structure formed by the in-situ reaction also helps to improve the volume stability and high-temperature erosion resistance of the material, making up for some of the application limitations of traditional alumina hollow sphere bricks and meeting the requirements of high-temperature (≥1600℃) industrial kilns for lightweight heat-insulating refractory materials. Attached Figure Description

[0028] Figure 1 is a scanning electron microscope image (100x) of the novel low-density alumina hollow sphere microporous foam ceramic material prepared in Example 1 of the present invention. Figure 2 is a scanning electron microscope image (2000x) of the novel low-density alumina hollow sphere microporous foam ceramic material prepared in Example 1 of the present invention. Figure 3 is a scanning electron microscope image (4000x) of the novel low-density alumina hollow sphere microporous foam ceramic material prepared in Example 1 of the present invention. Figure 4. Scanning electron microscope image (300x) of the novel low-density alumina hollow sphere microporous foam ceramic material prepared in Example 1 of the present invention. Detailed Implementation

[0029] Example 1 S1 Ingredients and Dispersion: Weigh out 75 parts by mass of alumina powder (average particle size D50=1μm), 18 parts by mass of 0.1-0.25mm alumina hollow spheres, 17 parts by mass of silica powder (average particle size D50=0.5μm) and silica sol (solid content 30%) (dry basis, dry basis mass ratio 1:1). Add 0.3% of the total mass of dry powder of the mixture of ammonium polyacrylate and 40% of deionized water to the mixture. First, use ultrasonic-assisted dispersion (power 300W, frequency 28kHz, time 20min), and then use high-speed shear stirring (speed 1200r / min, shear gap 0.5mm, time 15min) to form a slurry.

[0030] S2 Foaming and Additives: Add 1.2% by weight of the compound foaming agent (composed of sodium dodecyl sulfate and sodium fatty alcohol polyoxyethylene ether sulfate in a mass ratio of 1:2) to the slurry, and mechanically stir at 1200 r / min for 4 min until the slurry volume expands to 2.0 times the initial volume; then add 2% by weight of the total dry powder of the mixture of modified starch (commercially available oxidized modified corn starch, viscosity ≥800 mPa・s, 25℃) and 3.5% of polystyrene microspheres (particle size 30 μm), and continue stirring at 800 r / min for 8 min to obtain the foamed slurry.

[0031] S3 Molding and Drying: Slowly inject the foaming slurry into the polytetrafluoroethylene mold (to prevent the bubbles from bursting), and let it stand in a constant temperature environment of 30℃ for 6 hours to solidify and obtain the green body; place the green body in a 110℃ forced-air drying oven for 18 hours to dry until the moisture content of the green body is less than 1%.

[0032] S4 High-Temperature Sintering: The dried green body is placed in a high-temperature kiln and heated at a rate of 1.5℃ / min. When the temperature reaches 500℃, the heating rate is adjusted to 0.8℃ / min and held at this temperature for 3 hours. The temperature is then increased to 1700℃ at a rate of 1.5℃ / min and held for 10 hours. After sintering, the green body is naturally cooled to room temperature in the furnace to obtain a new low-density alumina hollow sphere microporous foam ceramic material.

[0033] The novel low-density alumina hollow sphere microporous foam ceramic material prepared in this embodiment was characterized by scanning electron microscopy (SEM), and the results are shown in Figure 1: [Figure 1 shows the results at low magnification (100x, 300x)]. Figure 4 As can be seen, the material forms a three-dimensional porous network structure with foamed ceramic ribs as the framework. Hollow alumina spheres are uniformly semi-embedded in the ribs, and some hollow spheres are damaged or detached during sample preparation, forming pits. The large pores formed by foaming are uniform in size and mainly distributed between 500-1200 μm, nesting with the macropores of the hollow spheres to form a multi-level composite pore structure. At high magnification (2000x, 4000x, Figure 2, ... Figure 3The uniform thickness of the hollow alumina spheres (approximately 5-8 μm) is clearly observable. A seamless, dense sintered bonding neck and interface are formed between the sphere walls and the foamed ceramic matrix. The hollow cavity structure of the spheres is evident, and the two are tightly bonded without cracks or gaps. This microstructure directly proves that the process of this invention can achieve uniform dispersion of raw materials and a firm bond with the matrix, confirming that the composite silicon source and sintering process provide reliable microscopic support for the high-temperature structural stability of the material.

[0034] Example 2 Compared to Example 1, the following parameters were adjusted (unless otherwise stated, they are the same as in Example 1): S1 Ingredients and Dispersion: 70 parts alumina powder, 20 parts 0.1-0.25mm hollow alumina spheres, 20 parts total of silica powder and silica sol (dry basis mass ratio 1:1.2); 0.5% ammonium polyacrylate, 50% deionized water; S2 foaming and additives: foaming agent addition 2%, rotation speed 1500r / min, time 3min (expansion to 2.5 times), modified starch 3%, pore-forming agent 5%; S3 molding and drying: stand at 40℃ for 4 hours, then dry at 120℃ for 12 hours; S4 High-Temperature Sintering: Sintering temperature 1750℃, holding time 8h.

[0035] Example 3 Based on Example 1, the 0.1-0.25mm hollow alumina spheres were replaced with an equal mass of coated alumina. All other ingredients, parameters, and steps were completely consistent with Example 1. The method for preparing coated alumina is as follows: Hollow alumina spheres with a thickness of 0.1-0.25 mm are placed in a pseudo-boehmite sol with a mass fraction of 1.5%, and coated at a low speed of 65 r / min for 30 min using a roller coater. After drying at 145℃ for 1.2 h, a uniform Al2O3 pre-coating layer with a thickness of 0.8 μm is formed on the surface of the hollow spheres, which is the coated alumina.

[0036] Example 4 Based on Example 1, only the silicon source was optimized by emulsion premixing and batch addition. All other ingredients, parameters, and steps were completely consistent with Example 1. The optimized silicon source process is as follows: Premix silica powder and silica sol with the same proportion and mass as in Example 1 at a dry basis mass ratio of 1:3, and stir at 500 r / min for 30 min to form a uniform silica source emulsion; add in two parts: the first 65% emulsion is added simultaneously with the ultrasonic-assisted dispersion in Example 1; the second 35% emulsion is added before foaming, and after stirring for 5 min to mix evenly, the subsequent operations are carried out according to the foaming parameters of Example 1.

[0037] Example 5 This embodiment is based on Example 1, but only the S2 foaming and additive addition steps are optimized. All other ingredients, parameters, and steps are completely consistent with Example 1. The optimized process is as follows: S2 Foaming and Additive Additives: Add 1.2% (by weight of the slurry) of compound foaming agent to the slurry and stir rapidly at 1350 r / min to initiate foaming. During foaming, take a 5 mL sample every 30 seconds through the sampling port on the side wall of the mixing vessel and test the viscosity offline using a rotational viscometer. When the slurry volume expands to 1.8 times its initial volume, reduce the rotation speed to 900 r / min and simultaneously start the modified starch feeding pump. Adjust the feeding rate according to the logic of "reducing the feeding rate to 1 mL / min when the viscosity is higher than 8000 mPa·s and increasing the feeding rate to 3 mL / min when the viscosity is lower than 5000 mPa·s" to stabilize the slurry viscosity at 6000 mPa·s. Before foaming, add 0.25% (by weight of the slurry) of nano-attapulgite clay (particle size 50-100 nm) and 0.1% (by weight of the slurry) to the slurry. The polyamide wax (commercially available polyamide wax slurry with a solid content of 55%, added by weight of slurry) was mixed with an anti-segregation additive at 800 r / min for 5 min. At the same time, the polystyrene microsphere pore maker prepared in Example 1 was placed in a 4% (w / w) ethanol aqueous solution, ultrasonically dispersed at 200W for 8 min, dried at 70℃ to constant weight, and then added to the slurry according to the addition amount in Example 1. The mixture was stirred at 800 r / min for 8 min to obtain a uniform foamed slurry.

[0038] Example 6 This embodiment is based on Embodiment 1, but only the S4 high-temperature sintering process is optimized. All other ingredients, parameters, and steps are completely consistent with Embodiment 1. The optimized process is as follows: S4 high-temperature sintering: Low temperature section: The room temperature is raised to 600℃, and a nitrogen-air mixture atmosphere with a nitrogen gas volume of 90% is introduced (achieved by supplementing the air with nitrogen gas), the heating rate is 1.0℃ / min, and the temperature is held at 600℃ for 2 hours; Medium temperature range: Heat from 600℃ to 1200℃, introduce pure air atmosphere, heating rate 2.5℃ / min, hold at 1200℃ for 3 hours; Molai Petrochemical Section: Heating from 1200℃ to 1650℃, pure air atmosphere is introduced, heating rate is 4.5℃ / min, no heat preservation; High-temperature sintering section: The temperature is slowly increased to 1700℃ at a rate of 1℃ / min, and held at that temperature for 10 hours in a pure air atmosphere to ensure that the green body is fully densified; Cooling section: The temperature is reduced to 1000℃ at a rate of 1.5℃ / min. The heating device is turned off below 1000℃, and the furnace is allowed to cool naturally to room temperature.

[0039] Example 7 Based on Example 1, the optimized processes of Examples 3-6 are introduced simultaneously, while the remaining basic ingredients and steps remain consistent with Example 1.

[0040] Comparative Example 1 The traditional alumina hollow sphere brick process is adopted: using alumina hollow spheres with a particle size of 0.2-5mm as aggregate (70% by mass), adding 25% by mass of alumina micro powder (D50=1μm) and 5% by mass of aluminum dihydrogen phosphate binder, adding 30% by mass of deionized water of the total mixture and mixing for 30min, vibrating and pressing to form (5MPa, holding pressure for 2min), drying at 110℃ for 24h, sintering at 1600℃ for 6h, and cooling in the furnace.

[0041] Comparative Example 2 Based on Example 1, only the foaming agent and pore-forming agent were completely removed, while all other ingredients, parameters, and process steps were completely consistent with Example 1. The dispersed slurry was directly molded, dried, and sintered according to the conditions of Example 1.

[0042] Comparative Example 3 Based on Example 1, only two routine adjustments were made to the silicon source; all other ingredients, parameters, and steps were completely identical to those in Example 1. (1) Remove the silica sol and use only silica powder as the silicon source. The amount added is the total SiO2 equivalent of the silicon source in Example 1; (2) Adjust the mass fraction of silica powder to 18% of the total mass of the dry powder of the mixture.

[0043] Comparative Example 4 Based on Example 1, the particle size of the hollow spheres was adjusted to 0.3-0.5 mm, while all other ingredients, parameters, and steps were completely consistent with Example 1.

[0044] Comparative Example 5 Based on Example 1, the mass fraction of hollow alumina spheres with a diameter of 0.1-0.25 mm was adjusted to 30 parts, and all other ingredients, parameters, and steps were completely consistent with Example 1.

[0045] Experimental Example Performance testing experiments were conducted on the materials from Examples 1-7 and Comparative Examples 1-5 (all prepared as rectangular plate-shaped samples with standard dimensions of 100mm × 50mm × 20mm; the sample surfaces were water-polished to remove burrs, missing corners, and cracks). All experiments were repeated three times in parallel, with a relative standard deviation (RSD) ≤ 5%. The average value was taken as the final result (as shown in Table 1). Specific testing indicators and methods followed the general national standards for the refractory materials industry, as detailed below: (1) Basic physical property testing: ① Bulk density: The density was determined by the water displacement method in the "Test Method for Bulk Density, Apparent Porosity and True Porosity of Refractory Materials", with the unit being kg / m³. The lower the value, the better the material's lightweight effect, which is more conducive to reducing heat storage loss in the kiln. ② Apparent porosity: The apparent porosity was determined by the "Test Method for Bulk Density, Apparent Porosity and True Porosity of Refractory Materials", with the unit being %.

[0046] (2) High temperature insulation performance test: thermal conductivity at 1600℃: The thermal conductivity of the material was measured at 1600℃ using the "Test Method for Thermal Conductivity of Refractory Materials (Hot Wire Method)". The unit is W / (m·K). The lower the value, the stronger the material's ability to block high temperature heat conduction and heat radiation, and the better the insulation performance.

[0047] (3) Structural stability and uniformity testing: ① Thermal shock resistance: The water cooling method in the "Test Method for Thermal Shock Resistance of Refractory Materials" is adopted. The sample is heated to 1600℃ and kept at that temperature for 30 minutes. Then it is quickly immersed in 25℃ deionized water for cooling. The cycle is repeated until the sample shows visible cracks or structural spalling. The number of cycles is recorded. The more cycles, the better the thermal shock resistance of the material and the stronger the structural stability during high-temperature service.

[0048] ② Uniformity of the green body: The metallographic microscope was used for microscopic analysis. The observation points were fixed at the center of the upper surface of the sample and the four corners of the upper surface (a total of 5 fixed points). The magnification of the metallographic microscope was set to 200x. The distribution of hollow alumina spheres, the density of the matrix phase, and the pore arrangement were observed simultaneously at the 5 fixed points. Segregation was judged according to the general quantitative standard for microscopic observation of refractory materials: Level 1 (no segregation, the hollow spheres at the 5 points are arranged in a regular manner, the matrix phase is continuous, the pore size is uniform, and there is no local enrichment / deficiency); Level 2 (slight segregation, a small amount of hollow spheres are aggregated or the matrix phase is thin at a single point, and there is no large-area stratification); Level 3 (severe segregation, hollow spheres float and accumulate at 2 or more points, the matrix phase is discontinuous, the pore density is significantly different, and the components of the upper and lower layers are unevenly distributed).

[0049] ③ Cell uniformity: Observation was performed using a metallographic microscope at a magnification of 100x. The observation points were fixed at the center, left, and right sections of the sample (a total of 3 fixed sections). Three fixed sub-points were selected at the top, middle, and bottom of each section. Three clear photomicrographs were taken at each sub-point. ImageJ image analysis software was used to analyze the pore diameter of ≥50 cells in each photograph, and the coefficient of variation (CV value = standard deviation / mean × 100%) of the pore diameter at a single sub-point was calculated. Uniformity was judged according to the following standards: Level 1 (CV value ≤ 8% for all sub-points); Level 2 (8% < CV value ≤ 15%); Level 3 (CV value > 15%), visually demonstrating the optimization effect of foaming speed and viscosity control on the cell structure.

[0050] (4) High-temperature service core performance test: The "Test method for softening temperature of refractory materials under load (heating method)" is adopted to determine the temperature when the deformation of the material reaches 4% under 0.2MPa compressive stress (i.e., the softening temperature under load). The unit is °C. The higher the value, the better the volume stability of the material under high-temperature load conditions.

[0051] Table 1: In the table, '-' indicates that there is no corresponding structure or the core performance does not meet the standards, and the testing of this item is not required.

[0052] As can be seen from Table 1: Examples 1-2, serving as the basic preparation scheme of this invention, effectively solve the problems of high density, simple pore structure, poor high-temperature insulation, and insufficient service stability of traditional alumina hollow spherical bricks, making them suitable for the use of high-temperature kiln hot surface linings. Specifically, the bulk density is ≤579kg / m³, and the apparent porosity is ≥78%, successfully achieving material lightweighting and constructing a basic porous structure; the thermal conductivity at 1600℃ is ≤0.23W / (m・K), effectively improving high-temperature insulation capacity and reducing kiln heat storage loss; the thermal shock resistance is ≥14 cycles, and the load softening temperature is ≥1675℃, ensuring the structural stability of the material under high-temperature thermal cycling conditions; the uniformity of the green body and the uniformity of the pores are both grade II, achieving a basic uniform distribution of raw materials and pore structure, laying a good performance foundation for subsequent process optimization.

[0053] Example 3: Based on Example 1, alumina hollow spheres were pre-coated and modified, which specifically solved the problems of easy breakage of small-diameter hollow spheres during dispersion and easy floating and segregation in slurry. The uniformity of the green body was improved from level two to level one, which improved the uniformity of the green body structure and the stability of high-temperature service.

[0054] Example 4: Based on Example 1, the silicon source was optimized by emulsion premixing and batch addition, which effectively balanced the volume effect of the two silicon sources participating in the mullitization reaction, avoided the problem of uneven reaction caused by local enrichment of silica sol, improved the uniformity of the green body to level 1, and optimized the high-temperature structural stability and thermal insulation performance of the material.

[0055] Example 5: Based on Example 1, the foaming and additive addition process was optimized, and the uniformity of the foam cells and the uniformity of the green body were improved from level 2 to level 1, thus optimizing the porous structure and high-temperature insulation performance of the material.

[0056] Example 6: Based on Example 1, the sintering process was optimized, the uniformity of the green body was improved to level one, and the structural stability and service life of the material under high temperature conditions were improved.

[0057] Example 7 combines all the process features of hollow sphere pre-coating, batch addition of silicon source emulsion premixing, foaming and pore formation optimization, and five-stage controlled-speed atmosphere sintering based on Example 1. The optimized processes have a synergistic effect, and all the material properties have reached the optimal level, achieving a balance between lightweight, high thermal insulation, high structural uniformity and high-temperature service stability.

[0058] Comparative Example 1 uses the traditional alumina hollow sphere brick preparation process, which is limited by the inherent defects of "large-diameter hollow spheres + stacking molding + single-pore structure". It cannot solve the problems of high density, poor thermal insulation and low structural uniformity. All performance indicators are inferior to those of the embodiments of the present invention, which reflects the technical limitations of the traditional process.

[0059] Comparative Example 2 removes the foaming agent and pore-forming agent from Example 1, thus losing the synergistic pore-forming effect of foaming and pore formation, making it impossible to construct a complete composite porous structure, resulting in a decline in the core performance of the material.

[0060] Comparative Example 3 uses a single silicon source based on Example 1, eliminating the combination of two silicon sources. This makes it impossible to balance the reaction volume effect of the two sources, resulting in uneven mullite reaction and poor necking effect, which in turn affects the overall performance of the material.

[0061] Comparative Example 4: The particle size of the hollow spheres was adjusted based on Example 1, resulting in uneven distribution of the hollow spheres in the slurry and segregation of the green body. At the same time, it affected the porous structure and matrix continuity of the material, ultimately making the material's various properties inferior to those of Example 1.

[0062] Comparative Example 5: Based on Example 1, an excessive amount of alumina hollow spheres was added, which disrupted the raw material ratio balance defined by the present invention, affected the bonding strength between the porous structure of the material and the matrix, and thus led to a decrease in the overall performance of the material.

Claims

1. A method for preparing a novel low-density alumina hollow sphere microporous foam ceramic material, characterized in that, Includes the following steps: S1 Ingredients and Dispersion: Weigh 70-80 parts by weight of alumina powder, 15-20 parts by weight of hollow alumina spheres with a particle size of 0.1-0.25mm, and 15-20 parts by weight of silica powder and silica sol (on a dry basis). Add dispersant and water to the resulting mixture, and use ultrasonic-assisted dispersion combined with high-speed shear stirring to form a uniform slurry. S2 Foaming and Additives: Add a foaming agent to the slurry and mechanically stir to foam it. Then add a coagulant and a pore-forming agent and continue stirring to mix evenly to obtain a foamed slurry. S3 Molding and Drying: The foaming slurry is injected into the mold and allowed to solidify to form a green body, which is then dried. S4 High-Temperature Sintering: The dried green body is placed in a high-temperature kiln for heating and sintering and then kept at that temperature. It is then cooled to room temperature with the furnace to obtain a new low-density alumina hollow sphere microporous foam ceramic material.

2. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 1, characterized in that, In step S1, the average particle size D50 of the alumina powder is 0.5-2 μm; the average particle size D50 of the silica powder is 0.1-1 μm; and the solid content of the silica sol is 25%-35%.

3. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 1, characterized in that, In step S1, the dispersant is ammonium polyacrylate, and its addition amount is 0.1%-0.5% of the total mass of the dry powder mixture; the addition amount of water is 30%-50% of the total mass of the dry powder mixture. In step S2, the foaming agent is composed of sodium dodecyl sulfate and sodium fatty alcohol polyoxyethylene ether sulfate, with a mass ratio of 1:(1-3), and the total amount of foaming agent added is 0.5%-2% of the slurry mass; In step S2, the coagulant is modified starch, and its addition amount is 1%-3% of the total mass of the dry powder of the mixture; the pore-forming agent is polystyrene microspheres or organic polymer particles with a core-shell structure, the particle size of the pore-forming agent is 10-50μm, and its addition amount is 2%-5% of the total mass of the dry powder of the mixture.

4. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 1, characterized in that, In step S3, the ambient temperature for static solidification is 25-40℃; the temperature for drying is 100-120℃, and the drying time is 12-24h.

5. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 1, characterized in that, In step S4, the sintering temperature is 1650-1750℃, and the holding time is 8-12h; When the temperature is raised to the range of 400-600℃, the heating rate is controlled at 0.5-1.5℃ / min, and the temperature is maintained within this range for 1-4 hours.

6. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 1, characterized in that, In step S1, the alumina hollow spheres undergo pre-coating modification treatment before material preparation. The modification process is as follows: the alumina hollow spheres are coated at low speed in a pseudo-boehmite sol with a mass fraction of 1%-2% at a speed of 50-80 r / min, and then dried at 130-160℃ for 0.5-2 h to form an Al2O3 pre-coating layer with a thickness of 0.5-1 μm on the surface of the alumina hollow spheres.

7. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 6, characterized in that, In step S1, the silica powder and silica sol are premixed to form a silica source emulsion. The mass ratio of silica sol to silica powder is (2-4):(2-1). The silica source emulsion is added to the mixture in two parts. The first part is 60%-70% of the silica source emulsion, which is added simultaneously with ultrasonic-assisted dispersion. The second part is the remaining 30%-40% of the silica source emulsion, which is added before foaming.

8. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 7, characterized in that, In step S2, the mechanical stirring foaming adopts real-time coupling control of the stirring speed. In the initial stage of foaming, the stirring speed is 1200-1500 r / min. When the slurry volume expands to 1.5-2.5 times, the stirring speed is reduced to 800-1000 r / min. During the speed control process, the coagulant is slowly added to maintain the slurry viscosity at 5000-8000 mPa·s. Before foaming, 0.2%-0.3% by mass of nano-attapulgite and 0.05%-0.15% by mass of polyamide wax composite anti-segregation agent are added to the slurry. The porogen is pretreated with ethanol aqueous solution before addition. The pretreatment process is as follows: the porogen is placed in 3%-5% by mass of ethanol aqueous solution and ultrasonically dispersed for 5-10 min, and then dried before use.

9. The method for preparing the novel low-density alumina hollow sphere microporous foam ceramic material according to claim 8, characterized in that, In step S4, the high-temperature sintering is a five-stage speed-controlled and atmosphere-regulated sintering process, specifically as follows: Low temperature section: The room temperature is raised to 600℃, and a nitrogen-air mixture atmosphere with a nitrogen gas integral of 85%-95% is introduced. The heating rate is 0.8-1.2℃ / min, and the temperature is held at 600℃ for 1-3 hours. Medium temperature range: Heat from 600℃ to 1200℃, introduce pure air atmosphere, heating rate 2-3℃ / min, hold at 1200℃ for 2-4 hours; Molai Petrochemical Section: Heating from 1200℃ to 1650℃, pure air atmosphere is introduced, heating rate is 4-5℃ / min, no heat preservation; High-temperature sintering section: Heat from 1650℃ to 1750℃, introduce pure air atmosphere, and hold for 8-12 hours; Cooling section: Temperature drops from 1750℃ to 1000℃ at a rate of 1-2℃ / min, and then naturally cools to room temperature below 1000℃.

10. A novel low-density alumina hollow sphere microporous foam ceramic material, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

Images