Preparation method and application of composite magnesium-aluminum oxide ceramic precursor sol with low crystalline form conversion temperature

Abstract

The application provides a preparation method and application of a composite magnesium-aluminum oxide ceramic precursor sol with a low crystalline form conversion temperature, and belongs to the technical field of composite aluminum oxide-based ceramics and preparation of a precursor sol thereof. The method is as follows: aluminum powder is reacted with a strong acid aluminum salt aqueous solution to obtain a polymeric aluminum sol; a dopant is added to the polymeric aluminum sol to obtain sol A; a magnesium source compound is added to the sol A to form sol B; an alkaline earth metal salt is added to the sol B for modification to obtain a composite magnesium-aluminum sol C; after the composite magnesium-aluminum sol C is added with lactic acid, preliminary concentration is performed, then spinning aid is added for secondary concentration, and finally an acid adjusting agent is added to obtain a composite magnesium-aluminum oxide ceramic precursor sol. The sol is subjected to electrostatic spinning, and then dried and fired at 450 DEG C-650 DEG C to obtain a magnesium-aluminum spinel ceramic fiber membrane. The sol can be used to obtain a high-temperature stable ceramic fiber membrane through low-temperature firing.

Description

Technical Field

[0001] This invention belongs to the field of composite alumina-based ceramics and their precursor sol preparation technology, and relates to a method for preparing composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature and its application. Background Technology

[0002] Alumina possesses properties such as high temperature resistance, wear resistance, and corrosion resistance, and is widely used in industries such as ceramics, refractories, polishing, plastics, glass, chemicals and catalysts, electronics, and aerospace, becoming an indispensable raw material for some industries. Alumina has multiple crystal forms, including γ, η, δ, θ, ρ, and α, with the α phase being the thermodynamically stable phase at high temperatures. For high-temperature structural material applications, the alumina crystal form must be transformed into the α phase, and complete transformation to the α phase typically requires a calcination temperature above 1200℃.

[0003] Magnesium aluminum spinel has excellent wear resistance, corrosion resistance, good chemical stability, good insulation, high hardness, and low coefficient of thermal expansion. It is used as an insulating skeleton for electronic devices, a ceramic protective film for metal products, a refractory material, a fine ceramic, a catalytic material, an excellent short-wavelength laser crystal material, and a decorative material in many fields of life and production.

[0004] Due to its large specific surface area, γ-Al₂O₃ is used as a catalyst support material for alkane conversion. However, it undergoes a phase transition and sintering at temperatures above 700℃, thus limiting its industrial application as a catalyst support material. MgAl₂O₄ has a similar molecular structure to γ-Al₂O₃, both consisting of 32 oxygen atoms. 2- It forms a cubic close-packed structure, but γ-Al₂O₃ only has 8 Al atoms. 3+ The oxygen ion voids in MgAl2O4 are filled with 24 metal ions (16 aluminum ions and 8 magnesium ions). Therefore, compared with γ-Al2O3, MgAl2O4 is less prone to phase transformation at high temperatures, has higher thermal stability, and is more suitable as an excellent support for catalyst materials.

[0005] However, while magnesium aluminum spinel has excellent properties, its sintering and synthesis temperature is relatively high. The solid-state reaction between periclase and alumina to form MgAl₂O₄ results in a volume expansion of approximately 8%. As the thickness of the MgAl₂O₄ product layer increases, the Mg... 2+ And Al 3+ Diffusion of reactants and products to the reaction interface becomes increasingly difficult, thus requiring higher synthesis temperatures, specifically calcination of the raw material mixture at temperatures above 1500°C to obtain the spinel phase (>80%). Magnesium aluminum spinel powder synthesized by solid-state method has disadvantages such as numerous lattice defects, uneven powder distribution, and relatively low spinel purity. It is difficult to prepare ultrafine (<1μm) magnesium aluminum spinel powder using a high-temperature solid-state reaction method at 1300℃-1500℃.

[0006] Magnesium aluminum spinel produced by the electric melting method has a dense microstructure. However, due to the high arc melting temperature (approximately 2200℃), the power consumption is high, resulting in a relatively high product cost. It is generally used for producing high-purity magnesium aluminum spinel.

[0007] Although coprecipitation is the simplest and most convenient chemical method, producing powders with uniform particles and no hard agglomerates, its fatal flaw is that impurities are easily introduced during water washing and filtration, preventing the prepared product from achieving a high purity. Its firing temperature is approximately 1000℃-1200℃.

[0008] The sol-gel method involves mixing raw materials with organic monomers, crosslinking agents, initiators, etc., to form a gel, which is then calcined to produce powder. The calcination temperature is approximately 1000℃. Calcination at 1200℃ using magnesium and aluminum alkoxides as raw materials via co-precipitation yields MgAl₂O₄. When synthesizing magnesium aluminum spinel using methods such as co-precipitation, the initial formation temperature is 800–900℃, and the complete formation temperature is 1100–1200℃.

[0009] Singh et al. prepared magnesium aluminum spinel powder using a gel-precipitation method, using Al2(SO4)3·6H2O and MgSO4·7H2O as raw materials in a 1:1 molar ratio of MgO to Al2O3. The results showed that the spinel phase appeared at 600℃, and spinel was completely synthesized at 1000℃. Shiono et al. synthesized MgAl2O4 spinel powder with high sintering performance using an isoalkoxide solution containing fine MgO at 1000℃.

[0010] Relatively pure magnesium aluminum spinel powder can be obtained by mechanical activation followed by calcination at 900℃-1000℃. Magnesium aluminum spinel powder can also be obtained by preparing precursors via hydrothermal method followed by calcination at 550℃-750℃, but the sintering activity is poor. Summary of the Invention

[0011] The purpose of this invention is to address the aforementioned problems in existing technologies by proposing a method for preparing and applying a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature, which can achieve high-temperature stable alumina-based ceramic fiber membranes through low-temperature sintering.

[0012] The objective of this invention can be achieved through the following technical solution: a method for preparing a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature, specifically including the following steps: S1 Aluminum Source Polymerization: Aluminum powder is reacted with an aqueous solution of a strong acid aluminum salt to obtain polymerized aluminum sol; S2 Sol Structure Regulation: Dopant is added to polyaluminum sol to regulate the sol structure and obtain preliminarily modified sol A; S3 Oligopolymerization Degradation and Repolymerization: A magnesium source compound is added to sol A to carry out oligopolymerization degradation and repolymerization reaction to form sol B; S4 Alkaline earth metal modification: Alkaline earth metal salts were added to sol B for modification to obtain composite magnesium-aluminum sol C; S5 Additives and Concentration: After adding lactic acid to the composite magnesium-aluminum sol C, a preliminary concentration is carried out. After the concentration is completed, a spinning aid is added for a second concentration. After the concentration is completed, an acid regulator is added and the mixture is stirred evenly to obtain the composite magnesium-aluminum oxide ceramic precursor sol.

[0013] Preferably, the dopant is one of water-soluble zirconium salt, water-soluble chromium salt, aqueous solution of H2ZrF6, aqueous solution of H2SiF6, and aqueous solution of HBF4. The water-soluble zirconium salt is ZrOCl2•8H2O or Zr(NO3)4•5H2O; the water-soluble chromium salt is CrCl3•6H2O or Cr(NO3)3•9H2O; the aqueous solution of strong acid aluminum salt is specifically aluminum trichloride aqueous solution; the magnesium source compound is one of hydrated magnesium chloride, hydrated magnesium nitrate, basic magnesium carbonate, magnesium hydroxide, and magnesium oxide; the alkaline earth metal salt is one of anhydrous CaCl2, Ca(NO3)2•4H2O, SrCl2•6H2O, and Sr(NO3)2; the spinning aid is polyvinyl alcohol with a molecular weight of 150,000-200,000 g / mol; and the acidity regulator is glacial acetic acid.

[0014] Preferably, in step S1, the ratio of the number of moles of aluminum in the aluminum powder to the number of moles of aluminum in aluminum trichloride is 1.2-4.6:1.

[0015] Preferably, in step S2, when a water-soluble zirconium salt is selected, the ZrO2 content is required to be 3-6% mol of the total oxides; when a water-soluble chromium salt is selected, the Cr2O3 content is required to be 1-3% mol of the total oxides; when an H2ZrF6 aqueous solution is selected, the ZrO2 concentration is required to be 1-3 mol of the total oxides; when an H2SiF6 aqueous solution is selected, the SiO2 concentration is required to be 1-5 mol of the total oxides; and when an HBF4 aqueous solution is selected, the B2O3 concentration is required to be 1-5 mol of the total oxides.

[0016] Preferably, in step S3, the ratio of the total number of moles of magnesium oxide to the total number of moles of aluminum oxide in sol B is 0.6-1.1:1.

[0017] Preferably, in step S4, the content of calcium oxide or strontium oxide in the composite magnesium-aluminum sol C is 2-7% mol of the total number of oxide moles.

[0018] Preferably, in step S5, the amount of lactic acid added is 8-16 ml of lactic acid per 100 g of oxide; the amount of polyvinyl alcohol added is 0.2-2 g of polyvinyl alcohol per 100 g of oxide.

[0019] Preferably, in step S1, the reaction temperature is 80℃-100℃; in step S2, the reaction temperature is 20-25℃, and the pH value of sol A is 2.5-3.0; in the preliminary concentration and the second concentration in step S5, the concentration temperature is 60-80℃, the target viscosity in the preliminary concentration is 10-40 Pa•s; in the second concentration, the target viscosity is 100-600 Pa•s, and the solid content is controlled at 55-70%; the amount of glacial acetic acid added is 5-10% of the weight of the product obtained after the second concentration.

[0020] An application of a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature is described. The composite magnesium alumina ceramic precursor sol is electrospun to obtain a ceramic green body, which is then dried and fired to obtain a nano-magnesium alumina spinel ceramic fiber membrane. The firing temperature is 450℃-650℃.

[0021] Compared with existing technologies, this invention has the following advantages: By adjusting the structure of the sol particles in the prepared polyaluminum sol, it undergoes oligomerization degradation to form oligomers with negative ion coordination polyhedra, which further undergo condensation reactions with hydrated magnesium ions to generate composite magnesium-aluminum sol particles. This allows for more precise crystal structure combination and growth at the molecular level. After preparing the magnesium-aluminum composite precursor sol, pure magnesium-aluminum spinel particles can be obtained at a lower firing temperature (450℃-650℃). Electrospinning using this magnesium-aluminum composite precursor sol can yield magnesium-aluminum spinel ceramic fiber membranes. This invention successfully solves key technical bottlenecks such as high synthesis temperature, single morphology, and insufficient thermal stability of the γ-Al2O3 carrier for magnesium-aluminum spinel materials through an innovative composite sol preparation technology. The core of its effectiveness lies in the precise design and control of precursor structures at the molecular level, resulting in significant technological advancements such as low-temperature synthesis, high-temperature stability, and controllable morphology. This provides new material solutions for the application of high-performance alumina-based ceramic materials in fields such as catalysis, filtration, and high-temperature insulation. Attached Figure Description

[0022] Figure 1 XRD pattern of the sample prepared by firing at 450℃ in Example 2 of this invention.

[0023] Figure 2 XRD pattern of the sample from Example 3 of this invention, which was fired at 550°C.

[0024] Figure 3XRD pattern of the sample from Example 4 of this invention, which was fired at 550°C.

[0025] Figure 4 XRD pattern of the sample from Example 5 of this invention, which was fired at 650°C.

[0026] Figure 5 XRD pattern of the sample from Example 6 of this invention, which was fired at 650°C.

[0027] Figure 6 SEM image of the magnesium aluminum spinel ceramic fiber membrane in Example 6 of this invention. Detailed Implementation

[0028] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0029] Example 1 A method for preparing a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature specifically includes the following steps: (1) Mix aluminum powder with aluminum trichloride aqueous solution at 80℃-100℃ to obtain polyaluminum sol. The ratio of the number of moles of aluminum in the aluminum powder to the number of moles of aluminum in aluminum trichloride is (4.6-1.2):1. Then, add a small amount of water-soluble zirconium salt, ZrOCl2•8H2O or Zr(NO3)4•5H2O, with the ZrO2 content being 3-6% mol of the total oxides, or a small amount of water-soluble chromium salt, CrCl3•6H2O, or Cr(NO3)4•5H2O to the polyaluminum sol. Add 3•9H2O, with Cr2O3 content at 1-3% mol of total oxides, and stir continuously at 20-25℃ until the reaction is complete to obtain sol A, pH 2.5-3.0; water-soluble zirconium salt can be replaced with H2ZrF6 aqueous solution, with ZrO2 concentration at 1-3 mol% of total oxides, or H2SiF6 aqueous solution, with SiO2 concentration at 1-5 mol% of total oxides, or HBF4 aqueous solution, with B2O3 concentration at 1-5 mol% of total oxides.

[0030] (2) Add hydrated magnesium chloride, hydrated magnesium nitrate, basic magnesium carbonate, magnesium hydroxide or magnesium oxide to sol A in proportion, stir until the mixture is uniform, clear and transparent, to obtain sol B; the ratio of the total number of moles of magnesium oxide to the number of moles of aluminum oxide in sol B is (0.6-1.1):1.

[0031] (3) Alkaline earth metal salts are added to sol B for modification to obtain sol C. The alkaline earth metal salts added to sol B are any one of anhydrous CaCl2, Ca(NO3)2•4H2O, SrCl2•6H2O or Sr(NO3)2. The content of calcium oxide or strontium oxide in sol C is 2-7%mol of the total number of oxide moles.

[0032] (4) Add lactic acid to sol C, heat and stir at 60-80℃ to concentrate to a viscosity of 10-40 Pa•s; add polyvinyl alcohol and concentrate to a viscosity of 100-600 Pa•s. The amount of lactic acid added to sol C is 8-16 ml of lactic acid per 100 g of oxide, and the amount of polyvinyl alcohol added is 0.2-2 g of polyvinyl alcohol per 100 g of oxide. The molecular weight is 150,000-200,000, and the solid content is controlled at 55-70%. Add 5-10% glacial acetic acid, stir evenly, and prepare the composite magnesium alumina ceramic precursor sol, i.e. spinning sol.

[0033] The prepared composite magnesium alumina ceramic precursor sol was electrospun, and the resulting ceramic green body was dried and fired at 350℃-650℃ to obtain a ceramic fiber membrane.

[0034] Example 2 like Figure 1 As shown, following the technical route of Example 1, 24.1 g of aluminum chloride hexahydrate was dissolved in 200 mL of water to prepare an aluminum chloride aqueous solution. 12.1 g of aluminum powder was added to this solution, and the reaction was carried out under gentle boiling and reflux conditions for 2 hours until the solution was clear and transparent, indicating complete reaction. 4.68 g of zirconium nitrate pentahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 5.0 mL of lactic acid was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 55.9 g of magnesium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. The reaction was carried out in a constant-temperature heated magnetically stirred water bath at approximately 25°C, thereby obtaining a modified magnesium-aluminum composite sol with a pH of approximately 3.0. This composite sol was then concentrated with a stabilizer and a spinning aid was added. Electrospinning was then performed to obtain a composite magnesium-aluminum hydroxide ceramic preform, which was dried and fired at 450°C for 1 hour to obtain a nano-magnesium-aluminum spinel ceramic fiber membrane.

[0035] Example 3 like Figure 2As shown, 24.1 g of aluminum chloride hexahydrate was dissolved in 200 mL of water to prepare an aluminum chloride aqueous solution. 12.1 g of aluminum powder was added to this solution, and the reaction was carried out under gentle boiling and reflux conditions for 2 hours until the solution was clear and transparent, indicating complete reaction. 55.9 g of magnesium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 3.58 mL of fluorotitanic acid aqueous solution (60% wt) was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. The reaction was carried out in a constant-temperature heated magnetically stirred water bath at approximately 25°C, thus obtaining a modified magnesium-aluminum composite sol with a pH of approximately 3.0. This composite sol was then concentrated by adding a stabilizer and a spinning aid, followed by electrospinning to obtain a composite magnesium-aluminum hydroxide ceramic preform. After drying and firing at 550°C for 1 hour, a nano-magnesium-aluminum spinel ceramic fiber membrane was obtained.

[0036] Example 4 like Figure 3 As shown, 24.1 g of aluminum chloride hexahydrate was dissolved in 200 mL of water to prepare an aluminum chloride aqueous solution. 12.1 g of aluminum powder was added to this solution, and the reaction was carried out under gentle boiling and reflux conditions for 2 hours until the solution was clear and transparent, indicating complete reaction. 55.9 g of magnesium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 4.68 g of zirconium nitrate pentahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 4.64 g of anhydrous calcium chloride was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. The reaction was carried out in a constant-temperature heated magnetically stirred water bath at approximately 25°C, thus obtaining a modified magnesium-aluminum composite sol with a pH of approximately 3.0. This composite sol was then concentrated by adding a stabilizer and a spinning aid, followed by electrospinning to obtain a composite magnesium-aluminum hydroxide ceramic preform. After drying and firing at 550°C for 1 hour, a nano-magnesium-aluminum spinel ceramic fiber membrane was obtained.

[0037] Example 5 like Figure 4As shown, 24.1 g of aluminum chloride hexahydrate was dissolved in 200 mL of water to prepare an aluminum chloride aqueous solution. 12.1 g of aluminum powder was added to this solution, and the mixture was reacted for 2 hours under gentle boiling and reflux conditions until the solution was clear and transparent, indicating complete reaction. 4.17 g of chromium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 5.0 mL of lactic acid was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 55.9 g of magnesium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. The reaction was carried out in a constant-temperature heated magnetically stirred water bath at approximately 25°C, thus obtaining a modified magnesium-aluminum composite sol with a pH of approximately 3.0. This composite sol was then concentrated by adding a stabilizer and a spinning aid, followed by electrospinning to obtain a composite magnesium-aluminum hydroxide ceramic preform. After drying and firing at 650°C for 1 hour, a nano-magnesium-aluminum spinel ceramic fiber membrane was obtained.

[0038] Example 6 like Figure 5 and Figure 6 As shown, 24.1 g of aluminum chloride hexahydrate was dissolved in 200 mL of water to prepare an aluminum chloride aqueous solution. 12.1 g of aluminum powder was added to this solution, and the mixture was reacted for 2 hours under gentle boiling and reflux conditions until the solution was clear and transparent, indicating complete reaction. 4.68 g of zirconium oxychloride octahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 5.0 mL of lactic acid was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. 55.9 g of magnesium chloride hexahydrate was added at room temperature, and the mixture was stirred for 2 hours until complete reaction, at a stirring rate of 300 rpm. The reaction was carried out in a constant-temperature heated magnetically stirred water bath at approximately 25°C, thus obtaining a modified magnesium-aluminum composite sol with a pH of approximately 3.0. This composite sol was then concentrated with a stabilizer and a spinning aid was added. Electrospinning was then performed to obtain a composite magnesium-aluminum hydroxide ceramic preform. After drying and firing at 650°C for 1 hour, a nano-magnesium-aluminum spinel ceramic fiber membrane was obtained.

[0039] The specific embodiments described herein are merely illustrative examples illustrating the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or substitute them with similar methods, without departing from the spirit of the invention or exceeding its defined scope. Although the invention has been detailed and described in the accompanying drawings and foregoing description, such descriptions are considered illustrative or exemplary rather than restrictive. It should be understood that changes and modifications can be made by those skilled in the art within the scope of the following claims.

Claims

1. A method for preparing a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature, characterized in that... Specifically, it includes the following steps: S1 Aluminum Source Polymerization: Aluminum powder is reacted with an aqueous solution of a strong acid aluminum salt to obtain polymerized aluminum sol; S2 Sol Structure Regulation: Dopant is added to polyaluminum sol to regulate the sol structure and obtain preliminarily modified sol A; S3 Oligopolymerization Degradation and Repolymerization: A magnesium source compound is added to sol A to carry out oligopolymerization degradation and repolymerization reaction to form sol B; S4 Alkaline earth metal modification: Alkaline earth metal salts were added to sol B for modification to obtain composite magnesium-aluminum sol C; S5 Additives and Concentration: After adding lactic acid to the composite magnesium-aluminum sol C, a preliminary concentration is carried out. After the concentration is completed, a spinning aid is added for a second concentration. After the concentration is completed, an acid regulator is added and the mixture is stirred evenly to obtain the composite magnesium-aluminum oxide ceramic precursor sol.

2. The method for preparing the low-crystal-transformation-temperature composite magnesium alumina ceramic precursor sol as described in claim 1, characterized in that... The dopant is one of the following: water-soluble zirconium salt, water-soluble chromium salt, aqueous solution of H2ZrF6, aqueous solution of H2SiF6, and aqueous solution of HBF4. The water-soluble zirconium salt is ZrOCl2•8H2O or Zr(NO3)4•5H2O; the water-soluble chromium salt is CrCl3•6H2O or Cr(NO3)3•9H2O; the aqueous solution of strong acid aluminum salt is specifically aluminum trichloride aqueous solution; the magnesium source compound is one of hydrated magnesium chloride, hydrated magnesium nitrate, basic magnesium carbonate, magnesium hydroxide, and magnesium oxide; the alkaline earth metal salt is one of anhydrous CaCl2, Ca(NO3)2•4H2O, SrCl2•6H2O, and Sr(NO3)2; the spinning aid is polyvinyl alcohol with a molecular weight of 150,000-200,000 g / mol; and the acidity regulator is glacial acetic acid.

3. The method for preparing the low-crystal-transformation-temperature composite magnesium alumina ceramic precursor sol as described in claim 2, characterized in that... In step S1, the ratio of the number of moles of aluminum in the aluminum powder to the number of moles of aluminum in aluminum trichloride is 1.2-4.6:

1.

4. The method for preparing the low-crystal-transformation-temperature composite magnesium alumina ceramic precursor sol as described in claim 2, characterized in that, In step S2, when a water-soluble zirconium salt is selected, the ZrO2 content is required to be 3-6% mol of the total oxides; when a water-soluble chromium salt is selected, the Cr2O3 content is required to be 1-3% mol of the total oxides; when an H2ZrF6 aqueous solution is selected, the ZrO2 concentration is required to be 1-3 mol of the total oxides; when an H2SiF6 aqueous solution is selected, the SiO2 concentration is required to be 1-5 mol of the total oxides; when an HBF4 aqueous solution is selected, the B2O3 concentration is required to be 1-5 mol of the total oxides.

5. The method for preparing the composite magnesium alumina ceramic precursor sol with low crystal transformation temperature as described in claim 2, characterized in that... In step S3, the ratio of the total number of moles of magnesium oxide to the total number of moles of aluminum oxide in sol B is 0.6-1.1:

1.

6. The method for preparing the composite magnesium alumina ceramic precursor sol with low crystal transformation temperature as described in claim 2, characterized in that... In step S4, the content of calcium oxide or strontium oxide in the composite magnesium-aluminum sol C is 2-7% mol of the total number of oxide moles.

7. The method for preparing the composite magnesium alumina ceramic precursor sol with low crystal transformation temperature as described in claim 2, characterized in that... In step S5, the amount of lactic acid added is 8-16 ml of lactic acid per 100 g of oxide; the amount of polyvinyl alcohol added is 0.2-2 g of polyvinyl alcohol per 100 g of oxide.

8. The method for preparing the composite magnesium alumina ceramic precursor sol with low crystal transformation temperature as described in claim 2, characterized in that... In step S1, the reaction temperature is 80℃-100℃; in step S2, the reaction temperature is 20-25℃, and the pH value of sol A is 2.5-3.0; in the preliminary concentration and the second concentration in step S5, the concentration temperature is 60-80℃, and in the preliminary concentration, the target viscosity is 10-40 Pa•s. In the second concentration, the target viscosity is 100-600 Pa•s, and the solid content is controlled at 55-70%; the amount of glacial acetic acid added is 5-10% of the weight of the product obtained after the second concentration.

9. Application of a composite magnesium alumina ceramic precursor sol with a low crystal transformation temperature, wherein the composite magnesium alumina ceramic precursor sol is a composite magnesium alumina ceramic precursor sol prepared by any of the methods in claims 1-8, characterized in that the composite magnesium alumina ceramic precursor sol is electrospun to obtain a ceramic green body, and after drying and firing, a nano-magnesium alumina spinel ceramic fiber membrane is obtained.

10. The application of the low-crystal-transformation-temperature composite magnesium alumina ceramic precursor sol as described in claim 9, characterized in that, The firing temperature is 450℃-650℃.

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