Preparation method of alpha-phase nanometer aluminum oxide particles based on gas-phase heterogeneous nucleation

Abstract

The invention discloses a method for preparing alpha-phase nanometer aluminum oxide particles based on gas phase heterogeneous nucleation, and belongs to the field of nanometer material preparation. The method comprises the following steps: mixing an aluminum salt precursor with an alpha-phase aluminum oxide seed crystal to form a solution, atomizing and spraying the solution into high-temperature flame; a seed crystal is introduced as a heterogeneous nucleation template, and the flame temperature and the particle retention time are cooperatively regulated, so that gas-phase species generated by pyrolysis of a precursor preferentially epitaxially grow on the surface of the seed crystal, and the alpha-phase nano aluminum oxide is directly synthesized in one step. The method solves the problems that the alpha phase is difficult to directly synthesize and subsequent calcination is needed in the traditional flame method, and is simple in process, suitable for continuous production, high in product purity and good in dispersity.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation technology, specifically relating to a method for directly preparing α-phase nano-alumina particles during flame spray pyrolysis based on a gas-phase heterogeneous nucleation mechanism. Background Technology

[0002] Alumina (Al₂O₃) has various crystal forms, including γ, θ, and α. Among them, α-phase alumina (corundum) is widely used in advanced ceramics, catalyst supports, wear-resistant coatings, composite material reinforcing phases, and optical devices due to its superior thermal stability, chemical inertness, hardness, and excellent mechanical, optical, and electrical properties. Nanoscale α-phase alumina, with its higher specific surface area and surface activity, can further enhance its performance in the aforementioned applications. Therefore, developing efficient nano-α-phase alumina preparation technology is of great significance.

[0003] Traditional methods for preparing α-phase alumina require high-temperature calcination (typically >1200℃) to convert the transition phases (γ and θ phases) into the α phase. This process easily leads to particle sintering, growth, and hard agglomeration, making it difficult to obtain finely dispersed nanoparticles. To address this issue, researchers have developed sol-gel methods, hydrothermal methods, and precipitation methods, but these methods suffer from drawbacks such as complex processes, long cycles, low yields, or the need for complex additives.

[0004] Flame spray pyrolysis (FSP), as a continuous and high-yield nanoparticle preparation technology, has the potential for one-step synthesis. However, the direct synthesis of pure α-phase alumina via conventional FSP still faces significant challenges. According to classical nucleation theory, in the high-temperature environment of a flame, the condensation and nucleation of alumina gaseous species mainly includes two pathways: homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation requires overcoming a high thermodynamic barrier, and within the extremely short residence time of the flame (typically <200 ms), the system tends to form metastable transition phases (such as γ and θ phases), which are more kinetically easier to form, making it difficult to directly form the thermodynamically stable but higher-barrier α phase. In contrast, heterogeneous nucleation, using pre-added α-phase nano-alumina seeds as the nucleation substrate, can significantly reduce the nucleation activation energy. Since the seeds provide a structurally matched growth interface, the newly formed alumina can directly form the α phase through an epitaxial growth mechanism, effectively avoiding the formation of metastable phases and providing the possibility of achieving highly selective synthesis of the α phase under instantaneous flame conditions.

[0005] Existing flame-based alumina preparation techniques do not fully utilize the heterogeneous nucleation mechanism and do not explicitly define seed crystals as the core inducing sites for gas-phase heterogeneous nucleation, resulting in low α-phase conversion and impure product crystalline phases. Therefore, developing a method for preparing high-purity α-phase nano-alumina in one step, using seed crystal-induced gas-phase heterogeneous nucleation as the core and combined with synergistic control of flame T-τ parameters, can effectively solve the pain points of existing technologies and has significant industrial application value and scientific theoretical significance. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flame combustion preparation method for producing high-purity, fine, and well-dispersed α-phase nano-alumina particles by using seed-induced gas-phase heterogeneous nucleation as the core process. This method is simple, continuous, efficient, and low-cost.

[0007] The core concept of this invention is based on the classical heterogeneous nucleation and crystal growth theory, and for the first time reveals a triple synergistic mechanism for achieving efficient α-phase heterogeneous nucleation in flame gas-phase synthesis: (1) Structural template mechanism: α-phase alumina seed crystals were selected as the active substrate. Its stable hexagonal crystal structure is homogeneous with the target product, providing a crystallographically compatible epitaxial growth template for newly formed alumina species in the flame, effectively reducing the interfacial energy barrier for heterogeneous nucleation; (2) T-τ synergistic mechanism: The synergistic regulation of temperature (T) and residence time (τ) in the flame field was discovered. In the lower temperature range, a longer residence time is required to drive the complete transformation of the precursor and the stable growth of the α phase. In the higher temperature range, a shorter residence time is required to promote nucleation with high temperature while avoiding grain sintering and abnormal growth caused by long-term exposure to high temperature. (3) Active site mechanism: The amount of seed crystals added must ensure that they provide a sufficiently high density of effective active sites in the reaction system. Too little addition will lead to insufficient heterogeneous nucleation sites, which will not dominate the process; too much addition may introduce seed crystal agglomeration, resulting in problems such as growth inhibition. This invention controls the amount of seed crystals added to 0.1%~20% of the theoretical alumina yield, ensuring that gaseous Al-O species can preferentially and efficiently adsorb onto the seed crystal surface, thereby significantly suppressing the competition for spontaneous homogeneous nucleation.

[0008] Based on the above mechanism, this invention successfully shifts the nucleation path in the flame from homogeneous to heterogeneous by introducing α-phase seed crystals with specific parameters and synergistically regulating the T-τ relationship, thereby achieving one-step direct synthesis of α-phase nano-alumina.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing α-phase nano-alumina particles includes the following steps: (1) Preparation of precursor solution: Dissolve the aluminum salt precursor in an organic solvent and stir thoroughly until completely dissolved to form a uniform and transparent aluminum salt solution. Then, add α-phase nano-alumina seeds to the solution and disperse the seeds uniformly in the solution by ultrasonic dispersion, magnetic stirring, etc., to form a stable precursor suspension (or precursor solution). (2) Flame combustion synthesis: The precursor solution obtained in step (1) is delivered to the flame spray pyrolysis device through the injection system. The precursor solution is sheared and atomized into fine droplets by the flame support gas and then sprayed into the high-temperature flame. In the high-temperature zone of the flame, the organic solvent evaporates rapidly. The aluminum salt precursor is pyrolyzed to generate gaseous Al-O clusters, which preferentially adsorb onto the active sites on the seed crystal surface to undergo heterogeneous nucleation and epitaxial growth, generating α-phase nano-alumina particles. Subsequently, they leave the high-temperature zone under the airflow and are collected by the collection device after rapid cooling.

[0010] In step (1), the aluminum salt precursor includes aluminum nitrate, aluminum chloride, aluminum alkoxide, or a combination thereof.

[0011] In step (1), the precursor organic solvent is an alcohol, ketone, carboxylic acid, ester, hydrocarbon or a combination thereof, or a system mixed with water.

[0012] In step (1), the α phase content of the α phase nano alumina seed crystal is ≥50%, the purity is ≥50%, the particle size is 1~200 nm, and the addition amount is 0.1%~20% of the theoretical alumina yield. This range covers the available industrial-grade seed crystals, and those skilled in the art can determine the minimum usable purity through routine experiments.

[0013] In step (2), the liquid flow rate of the injection system is 1~50 mL / min.

[0014] In step (2), the flame support gas is composed of small molecule hydrocarbons such as hydrogen, methane, and propane, or a mixture thereof with air or oxygen.

[0015] In step (2), the reaction temperature in the high-temperature zone of the flame is 800~2000 ℃, which is controlled by parameters such as the type of precursor organic solvent, the ratio of flame support gas type and volume flow rate.

[0016] In step (2), the flame spray pyrolysis device needs to provide a combustion environment that can maintain the high-temperature reaction zone and allow the particle residence time τ to be adjusted.

[0017] In step (2), the flow rate of the flame-supported gas shear atomization precursor solution is 1~50 L / min.

[0018] In step (2), the synthesized α-phase nano-alumina particles have an α-phase content ≥80%, a purity ≥50%, an adjustable primary particle size within the range of 20 nm to 5 μm, and a uniform grain size distribution.

[0019] The beneficial effects of this invention are as follows: (1) A new path for directional heterogeneous nucleation in flames has been pioneered: by combining α-phase seed template induction with T-τ parameter synergistic regulation, the technical bottleneck of the traditional flame method is difficult to directly synthesize α phase, realizing one-step, direct and highly selective synthesis of α-phase nano alumina, avoiding post-processing; (2) Excellent and controllable product performance: Through the above synergistic regulation, while obtaining a high α phase content, the hard agglomeration and abnormal growth of particles are effectively suppressed. The original particle size of the obtained product is adjustable in the range of 20 nm to 5 μm and has good dispersibility. (3) High efficiency and low cost: The entire process is completed in one step, suitable for continuous and large-scale production. It has relaxed requirements on the purity of seed crystals, low raw material costs, and no need to add complex reagents; (4) Broad application prospects: The obtained high-performance α-phase nano alumina can be directly used in high-end ceramics, catalysis, coating and other fields to improve the performance of downstream products. Detailed Implementation

[0020] To clarify the purpose, concept, technical solution, and beneficial effects of this application, the technical solution of this application is described in detail and completely below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments disclosed in this specification are only some exemplary implementations of this application and not all implementations of the technical solution of this application. Based on the embodiments disclosed in this application, all other technical solutions obtained by those skilled in the art through reasonable deduction, equivalent substitution, or modification without creative effort should be included within the protection scope of this application.

[0021] Example 1 12.07 g of aluminum chloride hexahydrate (aluminum salt precursor) was weighed and dissolved in 100 ml of ethanol / water mixed solvent with a volume ratio of 1:1. The solution was magnetically stirred for 30 minutes until completely dissolved. 0.051 g of α-Al2O3 seed crystals with a purity of 99%, an α-phase content of 98%, and a primary particle size of approximately 20 nm (theoretical alumina production is approximately 2.55 g, seed crystal addition is 2%) was added to the above solution. After ultrasonic dispersion for 30 minutes, the solution was magnetically stirred for 2 hours to obtain the precursor solution. The precursor solution was delivered to a flame spray pyrolysis device at a flow rate of 5 mL / min. After being atomized by flame support gases (H2 6 L / min, O2 6 L / min, Air 20 L / min), the solution was sprayed into the high-temperature zone of the flame (the measured instantaneous maximum temperature was approximately 1802 ℃). The residence time was calculated to be approximately 38 ms. Subsequently, the particles left the high-temperature zone under the airflow and were collected after rapid cooling. The product was found to have an α phase content of 94%, a primary particle size of 40-90 nm, a purity of 97.2%, and good dispersibility.

[0022] Example 2 12.07 g of aluminum chloride hexahydrate was weighed and dissolved in 100 ml of a 9:1 volume ratio ethanol / isooctanoic acid mixed solvent. The mixture was magnetically stirred for 30 minutes until completely dissolved. 0.025 g of α-Al2O3 seed crystals with a purity of 99%, an α-phase content of 90%, and a primary particle size of approximately 30 nm (addition amount of 1%) was added and ultrasonically dispersed for 30 minutes, followed by magnetic stirring for 2 hours to obtain a precursor solution. The precursor solution was delivered to a flame spray pyrolysis device at a flow rate of 10 mL / min. After being atomized by flame support gases (H2 10 L / min, O2 10 L / min, Air 30 L / min), the solution was sprayed into the high-temperature zone of the flame (the measured instantaneous maximum temperature was approximately 2146 ℃). The residence time in the high-temperature zone was calculated to be approximately 18 ms. Subsequently, the particles left the high-temperature zone under the airflow and were collected after rapid cooling. The product was found to have an α-phase content of 86%, a primary particle size of 500 nm to 1.5 μm, a purity of 95.5%, and good dispersibility.

[0023] Comparative Example 1 12.07 g of aluminum chloride hexahydrate was weighed and dissolved in 100 ml of a 1:1 volume ratio ethanol / water mixture. The solution was magnetically stirred for 30 minutes until completely dissolved. No seed crystals were added, and the precursor solution was obtained directly. All other process parameters were identical to those in Example 1. Analysis showed that the α-phase content of the product was 0, the crystal form was mainly γ-phase, the primary particle size was 1 μm to 2 μm, and severe hard agglomeration was observed. This indicates that in the absence of seed crystals, the system will be dominated by homogeneous nucleation, generating a metastable product.

[0024] Comparative Example 2 12.07 g of aluminum chloride hexahydrate was dissolved in 100 ml of a 1:1 volume ratio ethanol / water mixture, and 0.051 g of the same seed crystals as in Example 1 was added to prepare a precursor solution. To investigate the effect of excessively long residence time under non-optimized conditions, the injection flow rate was adjusted to 1 mL / min, and the flame support gas was adjusted to 1 L / min H2, 1 L / min O2, and 5 L / min Air. The simulated residence time was approximately 156 ms. The product showed an α-phase content of 51%, but the particles were severely sintered and poorly dispersed. This indicates that even in the presence of seed crystals, unsuitable process conditions can lead to deterioration of product performance.

[0025] Comparative Example 3 12.07 g of aluminum chloride hexahydrate was dissolved in 100 ml of a 1:1 volume ratio ethanol / water mixture. 0.051 g of α-Al₂O₃ seed crystals (99% purity, >99% α-phase content, and a native particle size of 400 nm, exceeding the 1-200 nm range) were added. The remaining process parameters were the same as in Example 1. Analysis showed that the product contained 20% α-phase with a native particle size of 1-3 μm and contained a large amount of γ-phase. This indicates that when seed crystals exceeding the specified size are added, the seed crystal surface area is insufficient, resulting in a scarcity of effective nucleation sites. This leads to the failure of the heterogeneous nucleation mechanism, and the system reverts to an inefficient homogeneous nucleation pathway.

[0026] Figure 1 The XRD patterns are those of the alumina products obtained in Examples 1 and 2 and Comparative Examples 1, 2 and 3. The diffraction peaks of Examples 1 and 2 mainly coincide with the standard peak positions of α-Al2O3, indicating that their products are mainly composed of high-purity α phase. The diffraction peaks of Comparative Example 1 mainly correspond to transitional phase alumina such as γ, δ, and θ, and no obvious α phase characteristic peaks are observed. The diffraction peaks of Comparative Example 2 and Comparative Example 3 correspond to α phase and transitional phase alumina, and the products contain some α phase alumina.

[0027] Figure 2 These are electron microscope (TEM) images of the alumina products obtained in Examples 1 and 2, and Comparative Examples 1 and 2, respectively. (a) is a TEM image of the product of Example 1, and (b), (c), and (d) are SEM images of the products of Example 2, Comparative Examples 1 and 2, respectively. It can be clearly observed from the images that the products of Examples 1 and 2 have uniform particle size and good dispersibility, with no obvious agglomeration; while the products of Comparative Examples 1 and 2 exhibit severe hard agglomeration, uneven particle size distribution, and irregular agglomerate structure.

[0028] Figure 3The figures show the flow and thermal environment characterization of particles during the flame spray pyrolysis process in Example 1. (a) is a two-dimensional temperature field cloud map inside the flame, clearly showing the spatial distribution of the high-temperature flame zone; (b) is the velocity-position distribution curve along the flame axis; (c) is the temperature-position distribution curve along the flame axis; and (d) is the T-τ curve of the particles, with the horizontal axis representing the cumulative residence time (s) and the vertical axis representing the corresponding temperature (K), visually presenting the complete thermal history of the particle flow process. The figures mark the start (τ1,T1), peak (τ2,T2), and end (τ3,T3) of the high-temperature zone of the flame (800~2000 ℃, i.e., 1073~2073 K). Based on this, the total residence time of the particles in the high-temperature zone is calculated to be approximately 38 ms, providing direct quantitative evidence for the core mechanism of T-τ synergistic regulation. Attached Figure Description

[0029] Figure 1 These are XRD patterns of the alumina products synthesized in the examples and comparative examples.

[0030] Figure 2 These are electron microscope images of the alumina products synthesized in the examples and comparative examples.

[0031] Figure 3 This is a characterization diagram of particle flow and thermal environment during the flame spray pyrolysis process of Example 1.

Claims

1. A method for preparing α-phase nano-alumina particles based on gas-phase heterogeneous nucleation, characterized in that: An aluminum salt precursor is dissolved in an organic solvent, and α-phase nano-alumina seeds are added to form a precursor solution. This solution is then atomized and sprayed into a high-temperature flame for reaction. By synergistically controlling the reaction temperature and the effective residence time of the particles in the high-temperature zone of the flame, the gaseous Al-O clusters generated by the pyrolysis of the aluminum salt precursor are preferentially adsorbed onto the active sites on the seed surface, resulting in heterogeneous nucleation and epitaxial growth, directly generating α-phase nano-alumina particles. The synthesized α-phase nano-alumina particles have an α-phase content ≥80%, an adjustable primary particle size within the range of 20 nm to 5 μm, and a uniform grain size distribution.

2. The method according to claim 1, characterized in that, The α-phase nano-alumina seed crystals have a particle size of 1~200 nm and are added at a rate of 0.1%~20% of the theoretical alumina yield.

3. The method according to claim 1, characterized in that, The effective residence time of particles in the high-temperature zone of the flame is 1~100 ms, which is jointly controlled by parameters such as the effective reaction volume of the high-temperature zone of the flame, the feed volume flow rate of the precursor, and the support gas volume flow rate of the flame.

4. The method according to claim 1, characterized in that, The reaction temperature in the high-temperature zone of the flame is 800~2000℃, which is controlled by parameters such as the type of precursor organic solvent, the type and ratio of flame support gas, and the volumetric flow rate.

5. The method according to any one of claims 1 to 4, characterized in that, The aluminum salt precursor includes aluminum nitrate, aluminum chloride, aluminum alkoxide, or combinations thereof.

6. The method according to any one of claims 1 to 4, characterized in that, The precursor organic solvent is an alcohol, ketone, carboxylic acid, ester, hydrocarbon or a combination thereof, or a system mixed with water.

7. The method according to any one of claims 1 to 4, characterized in that, The flame support gas consists of small-molecule hydrocarbons such as hydrogen, methane, and propane, or a mixture thereof with air or oxygen.

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