Preparation method of aluminum oxide hollow microcrystals

Abstract

The invention provides a preparation method of aluminum oxide hollow microcrystals, and belongs to the field of aluminum oxide. The method comprises the following steps: carrying out first hydrothermal reaction on aluminum nitrate and urea to obtain an Al2O3.H2O precursor of which the {001} crystal face proportion is greater than or equal to 70%; taking the Al2O3.H2O precursor as a seed crystal, and carrying out second hydrothermal reaction on the Al2O3.H2O precursor and an aluminum nitrate solution to obtain alpha-Al2O3 microcrystals; the method comprises the following steps: soaking alpha-Al2O3 microcrystals in an etching solution formed by mixing hydrochloric acid and ethylenediaminetetraacetic acid so as to selectively etch the alpha-Al2O3 microcrystals to obtain mixed slurry; performing solid-liquid separation on the mixed slurry to obtain a filter cake; and carrying out annealing treatment on the filter cake to obtain the aluminum oxide hollow microcrystal. According to the method, the technologies of controlling crystal face growth through a hydrothermal reaction, inhibiting non-target crystal face growth, selectively inhibiting aluminum ion redeposition, forming a regular cavity, annealing to eliminate stress, repairing lattice defects and the like are utilized, and performance breakthrough and application expansion of the aluminum oxide hollow microcrystals are achieved.

Description

Technical Field

[0001] The present application relates to the technical field of aluminum oxide, and in particular to a method for preparing hollow aluminum oxide microcrystals. Background Art

[0002] Alumina microcrystals are widely used in catalysis, electronics, aerospace, and other fields due to their high strength, high-temperature resistance, and excellent dielectric properties. As market demand escalates, hollow-structured alumina microcrystals have become a research hotspot due to their lightweight, high specific surface area, and functional adjustability.

[0003] However, existing preparation technologies still face multiple bottlenecks: (1) Difficulties in controlling crystal form and cavity: Although the traditional hydrothermal method can synthesize alumina microcrystals, the crystal plane orientation is uncontrollable, and the cavity formation depends on high-temperature etching or complex templates, which can easily lead to grain coarsening (≥50nm) or insufficient hollowness (≤85%); (2) Process complexity and energy consumption: The template method requires a high temperature of more than 600°C to remove the template, which not only has high energy consumption (carbon emission intensity reaches 1.5kg / kg), but also easily causes lattice distortion; (3) Although the sol-gel method can customize nanoparticles, the equipment cost is high and large-scale production is limited; (4) Insufficient structural uniformity: High-temperature sintering and chemical vapor deposition methods are difficult to accurately control the cavity size and wall thickness, resulting in low specific surface area (<50m 2 / g) and mechanical strength fluctuations; (5) Environmental protection and cost contradiction: Existing technologies generally have the defects of high energy consumption, high carbon emissions (such as traditional calcination process) or low raw material utilization (such as powder metallurgy requires high-purity aluminum powder), which are contrary to the trend of green manufacturing. Therefore, it is urgent to develop a new method for preparing hollow alumina microcrystals through directional growth and selective etching. Summary of the Invention

[0004] The present application provides a method for preparing hollow alumina microcrystals, thereby providing a new method for preparing hollow alumina microcrystals through directional growth and selective etching.

[0005] The present invention provides a method for preparing hollow alumina microcrystals, the method comprising:

[0006] Aluminum nitrate and urea are subjected to a first hydrothermal reaction to obtain an Al2O3·H2O precursor having a {001} crystal plane ratio of ≥70%;

[0007] The Al2O3·H2O precursor is used as a seed crystal to undergo a second hydrothermal reaction with an aluminum nitrate solution to obtain α-Al2O3 microcrystals; the pH value of the second hydrothermal reaction is 8.0 to 10.0;

[0008] Immersing the α-Al2O3 microcrystals in an etching solution mixed with hydrochloric acid and ethylenediaminetetraacetic acid to selectively etch the α-Al2O3 microcrystals to obtain a mixed slurry;

[0009] performing solid-liquid separation on the mixed slurry to obtain a filter cake; and

[0010] The filter cake is annealed in an air atmosphere to obtain the alumina hollow microcrystals.

[0011] Optionally, the molar ratio of the aluminum nitrate to the urea is 1:(2-4).

[0012] Optionally, the reaction temperature of the first hydrothermal reaction is 150° C. to 220° C., and the reaction time of the first hydrothermal reaction is 10 h to 15 h.

[0013] Optionally, the molar concentration of the aluminum nitrate solution is 0.5 mol / L to 0.6 mol / L, and the added amount of the Al2O3·H2O precursor is 0.5 g / L to 0.8 g / L.

[0014] Optionally, the reaction temperature of the second hydrothermal reaction is 150° C. to 220° C., and the reaction time of the second hydrothermal reaction is 20 h to 24 h.

[0015] Optionally, the selective etching includes a first-stage etching and a second-stage etching; wherein,

[0016] The temperature of the first stage etching is 25°C to 40°C, and the stirring time of the first stage etching is 1 hour;

[0017] The temperature of the second stage etching is 40° C. to 50° C., and the stirring time of the second stage etching is 0.5 h to 1 h.

[0018] Optionally, the molar ratio of the hydrochloric acid to EDTA is (0.8-2.5):1, the molar concentration of the hydrochloric acid is 0.1 mol / L-0.15 mol / L, and the molar concentration of EDTA is 0.05 mol / L-0.08 mol / L.

[0019] Optionally, the annealing treatment includes the following parameters: a heating rate of 3°C / min to 5°C / min, a temperature of 500°C to 550°C, and a holding time of 2h to 3h.

[0020] Optionally, the hollow alumina microcrystals meet the following size parameters: grain size ≤ 50 nm, cavity diameter 400 nm to 800 nm, and wall thickness 30 nm to 80 nm.

[0021] Optionally, the alumina hollow microcrystals meet the following performance parameters: a {001} crystal plane ratio of ≥70%, a dielectric constant of 6.5 to 8.0, and a compressive strength of ≥120 MPa.

[0022] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0023] An embodiment of the present application provides a method for preparing hollow alumina microcrystals, the method comprising: subjecting aluminum nitrate and urea to a first hydrothermal reaction to obtain an Al2O3·H2O precursor having a {001} crystal plane ratio of ≥70%; using the Al2O3·H2O precursor as a seed crystal, subjecting it to a second hydrothermal reaction with an aluminum nitrate solution to obtain α-Al2O3 microcrystals; the pH value of the second hydrothermal reaction is 8.0-10.0; immersing the α-Al2O3 microcrystals in an etching solution mixed with hydrochloric acid and ethylenediaminetetraacetic acid to selectively etch the α-Al2O3 microcrystals to obtain a mixed slurry; subjecting the mixed slurry to solid-liquid separation to obtain a filter cake; and annealing the filter cake in an air atmosphere to obtain the hollow alumina microcrystals. First, a hydrothermal reaction is used to control crystal face growth and inhibit the growth of non-target crystal faces. Second, an Al2O3·H2O precursor is used as a seed to guide the preferential growth of α-Al2O3 microcrystals along the {001} crystal plane, forming a regular crystal structure. Third, selective etching is used to inhibit aluminum ion redeposition and form regular cavities. Finally, annealing is performed to eliminate stress and repair lattice defects. This provides a new method for preparing hollow alumina microcrystals through directional growth and selective etching. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0025] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 A schematic flow chart of a method for preparing hollow alumina microcrystals provided in an embodiment of the present application. DETAILED DESCRIPTION

[0027] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0028] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a hard limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numbers within the range. For example, the description of a range from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0029] In addition, in the description of the specification of this application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second" are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" means one or more, and "plurality" means two or more. "At least one", "at least one of the following items (individuals)" or similar expressions refer to any combination of these items, including any combination of single items (individuals) or plural items (individuals). For example, "at least one of a, b, or c", or "at least one of a, b, and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple. "Parts" such as parts by weight and parts by mass represent the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one-to-one to the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0030] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0031] Figure 1 A schematic flow chart of a method for preparing hollow alumina microcrystals provided in an embodiment of the present application.

[0032] like Figure 1 As shown, the present application provides a method for preparing hollow alumina microcrystals, the method comprising:

[0033] S1. performing a first hydrothermal reaction of aluminum nitrate and urea to obtain an Al2O3·H2O precursor having a {001} crystal plane ratio of ≥70%;

[0034] In step S1, an Al2O3·H2O precursor with a high {001} crystal plane ratio (≥70%) is generated by the hydrothermal reaction of aluminum nitrate and urea to provide seeds for subsequent directional growth. The {001} crystal plane of alumina is directly related to the mechanical strength, catalytic activity and dielectric properties of the product. The {001} crystal plane appears as a close-packed plane in hexagonal alumina (such as α-Al2O3), and the atomic arrangement is highly ordered, giving it excellent mechanical stability and thermal conductivity. At the same time, the {001} crystal plane is more likely to form acidic sites (such as Lewis acids) due to the high density of surface hydroxyl groups, making it suitable for the design of catalytic reaction carriers. For example, the {001} crystal plane ratio of the Al2O3·H2O precursor can be 70%, 72%, 74%, 76%, 80%, 85%, etc.

[0035] It should be noted that before the first hydrothermal reaction, aluminum nitrate and urea need to be dissolved in an appropriate amount of deionized water.

[0036] In some embodiments, the molar ratio of the aluminum nitrate to the urea is 1:(2-4).

[0037] The molar ratio of aluminum nitrate to urea is limited to 1:(2-4), which can regulate the seed crystal morphology and crystal facet ratio. Within this range, the {001} facet ratio exceeds 70%. Excessive urea will form Al2(CO3)3 impurity phase, reducing the seed crystal purity. Exemplary molar ratios of aluminum nitrate to urea can be 1:2, 1:2.5, 1:3, 1:3.5, 1:4, etc.

[0038] In some embodiments, the reaction temperature of the first hydrothermal reaction is 150° C. to 220° C., and the reaction time of the first hydrothermal reaction is 10 h to 15 h.

[0039] Limiting the temperature of the first hydrothermal reaction to 150°C to 220°C can control the crystallinity and size of the seed crystals. Below 150°C, the reaction rate is slow, the reaction cycle is long, and the seed crystallinity is low. Above 220°C, the reaction is too rapid, the seed crystal size is uneven, and surface defects increase. Furthermore, excessively high temperatures also increase energy consumption. Limiting the reaction time of the first hydrothermal reaction to 10 hours to 15 hours can balance crystal growth and side reactions. If the reaction time is less than 10 hours, the reaction is incomplete, the seed crystal size is small, and {001} face adsorption is insufficient. If the reaction time is greater than 15 hours, excessive seed crystal growth can lead to seed crystal agglomeration, a decrease in specific surface area, and reduced seed effectiveness. For example, the reaction temperature of the first hydrothermal reaction can be 150°C, 170°C, 190°C, 210°C, 220°C, etc., and the reaction time of the first hydrothermal reaction can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, etc.

[0040] S2, using the Al2O3·H2O precursor as a seed crystal, and conducting a second hydrothermal reaction with an aluminum nitrate solution to obtain α-Al2O3 microcrystals; the pH value of the second hydrothermal reaction is 8.0-10.0;

[0041] In step S2, an Al2O3·H2O precursor is used as a seed crystal to guide the preferential growth of α-Al2O3 crystallites along the {001} crystal plane, forming a regular crystal structure. The pH value of the second hydrothermal reaction is limited to 8.0-10.0, promoting the formation of Al(OH)3 colloids in an alkaline environment, providing an aluminum source for α-Al2O3. Exemplary pH values for the second hydrothermal reaction can be 8.0, 8.5, 8.7, 9, 9.5, 9.8, 10.0, etc.

[0042] In some embodiments, the molar concentration of the aluminum nitrate solution is 0.5 mol / L to 0.6 mol / L, and the amount of the Al2O3·H2O precursor added is 0.5 g / L to 0.8 g / L.

[0043] The molar concentration of the aluminum nitrate solution is limited to 0.5mol / L to 0.6mol / L, which can provide an aluminum source. Aluminum nitrate hydrolyzes to form Al(OH)3 colloid, which provides an aluminum source for the growth of α-Al2O3 crystals. If it is less than 0.5mol / L, it will lead to insufficient Al ions, slow crystallite growth rate, small size, and reduced crystal face ratio; if it is greater than 0.6mol / L, the solution will be oversaturated, and γ-Al2O3 impurity phase will be generated, reducing the purity of the final product. For example, the molar concentration of the aluminum nitrate solution can be 0.5mol / L, 0.52mol / L, 0.54mol / L, 0.56mol / L, 0.58mol / L, 0.6mol / L, etc.

[0044] The addition amount of Al2O3·H2O precursor is limited to 0.5g / L to 0.8g / L, which can control the seed density. Al2O3·H2O precursor serves as a growth template, and its amount directly affects the nucleation density and growth uniformity of the microcrystals. If the amount is less than 0.5g / L, there are too few seeds, the growth spacing of the microcrystals is large, and the size distribution is too wide; if the amount is greater than 0.8g / L, the seeds agglomerate, and competitive growth between the microcrystals leads to holes and cracks in the structure. For example, the addition amount of Al2O3·H2O precursor can be 0.5g / L, 0.55g / L, 0.6g / L, 0.65g / L, 0.7g / L, 0.75g / L, 0.8g / L, etc.

[0045] In some embodiments, the reaction temperature of the second hydrothermal reaction is 150° C. to 220° C., and the reaction time of the second hydrothermal reaction is 20 h to 24 h.

[0046] Limiting the reaction temperature of the second hydrothermal reaction to 150℃~220℃ can control the reaction kinetics. 3+ Hydrolysis and crystal growth rate, but high temperature should be avoided to cause side reactions. Above 220°C, the reaction rate is too fast, the crystallite size is uneven, the lattice stress increases, and the compressive strength decreases. Below 150°C, the reaction rate will slow down. Limiting the reaction time of the second hydrothermal reaction to 20h~24h can balance growth and defect control. If the reaction time is less than 20h, the crystal growth is incomplete, the crystallite size will be small, and the crystal face ratio will fluctuate; if it is greater than 24h, too long a time will cause excessive crystal growth, resulting in coarsening of the crystallite and a decrease in specific surface area. For example, the reaction temperature of the second hydrothermal reaction can be 150°C, 170°C, 190°C, 210°C, 220°C, etc., and the reaction time of the first hydrothermal reaction can be 20h, 21h, 22h, 23h, 24h, etc.

[0047] S3, immersing the α-Al2O3 microcrystals in an etching solution mixed with hydrochloric acid and ethylenediaminetetraacetic acid to selectively etch the α-Al2O3 microcrystals to obtain a mixed slurry;

[0048] In step S3, α-Al2O3 microcrystals are etched in conjunction with hydrochloric acid and EDTA to form a hollow structure.

[0049] In some embodiments, the molar ratio of hydrochloric acid to EDTA is (0.8-2.5):1, the molar concentration of hydrochloric acid is 0.1 mol / L-0.15 mol / L, and the molar concentration of EDTA is 0.05 mol / L-0.08 mol / L.

[0050] The molar ratio of hydrochloric acid to EDTA is limited to (0.8-2.5):1. If the molar ratio is lower than 0.8, it means that there is an excess of EDTA.3+ It will redeposit on the surface of the microcrystals, causing changes in wall thickness deviation and uneven etching. A molar ratio greater than 2.5 means that there is an excess of HCl, the etching rate will be very fast, and it will cause local over-etching, resulting in increased wall thickness deviation. For example, the molar ratio of hydrochloric acid to EDTA can be 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.5:1, etc.

[0051] The role of hydrochloric acid is etching, and the role of EDTA is to complex with aluminum ions. The concentration of hydrochloric acid directly affects the etching rate of Al2O3. If the concentration of hydrochloric acid is lower than 0.1mol / L, the etching rate will be very slow and the production cycle will be very long. If the concentration of hydrochloric acid is higher than 0.1mol / L, the etching time will be shortened. However, if the concentration of hydrochloric acid is greater than 0.15mol / L, the etching rate will be too fast, which will cause over-etching, resulting in uneven wall thickness or microcrystal breakage. EDTA forms a stable complex with aluminum ions to prevent aluminum ions from redepositing on the surface of the microcrystal and ensure etching uniformity. If the concentration is too low, lower than 0.05mol / L, the degree of complexing aluminum ions is insufficient; if it is too high, greater than 0.08mol / L, the H + The activity is partially neutralized, which reduces the etching efficiency. For example, the molar concentration of hydrochloric acid can be 0.1 mol / L, 0.11 mol / L, 0.12 mol / L, 0.13 mol / L, 0.14 mol / L, 0.15 mol / L, etc., and the molar concentration of EDTA can be 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.075 mol / L, 0.08 mol / L, etc.

[0052] In some embodiments, the selective etching temperature is 25° C. to 50° C., and the etching time is 1.5 h to 2.5 h.

[0053] Limiting the etching temperature to 25°C to 50°C can control the reaction rate. Temperatures below 25°C will result in a very slow reaction rate, significantly increasing the etching time. Temperatures above 50°C are too high, resulting in a too fast reaction rate, causing the reaction to get out of control and uneven etching. Furthermore, temperatures above 60°C can cause EDTA to decompose. Limiting the etching time to 1.5h to 2.5h can control the cavity size and wall thickness. If the etching time is less than 1.5 hours, the etching will be incomplete, the cavity diameter will be too small, and the wall thickness will be too large. If the etching time is greater than 2.5 hours, the etching will be excessive, reducing the wall thickness and significantly reducing the compressive strength. For example, the etching temperature can be 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, etc., and the etching time can be 1.5h, 1.7h, 1.9h, 2.1h, 2.3h, 2.5h, etc.

[0054] In some embodiments, the selective etching includes a first stage etching and a second stage etching; wherein,

[0055] The temperature of the first stage etching is 25°C to 40°C, and the stirring time of the first stage etching is 1 hour;

[0056] The temperature of the second stage etching is 40° C. to 50° C., and the stirring time of the second stage etching is 0.5 h to 1 h.

[0057] The etching process is divided into two stages: the first stage is a low-temperature etching stage. At low temperatures, the etching rate is slow, mainly to control the initial cavity, avoid local stress concentration, and ensure uniform wall thickness. Another reason is that the diffusion rate of aluminum ions is low at low temperatures, which allows EDTA to fully complex and reduce surface defects. The second stage is to increase the etching rate. Increasing the temperature helps to increase the reaction rate and shorten the total process time. Another reason is that increasing the temperature enhances the complexing ability of EDTA, preventing aluminum ion redeposition.

[0058] S4, performing solid-liquid separation on the mixed slurry to obtain a filter cake; and

[0059] S5. Annealing the filter cake in an air atmosphere to obtain the hollow alumina microcrystals.

[0060] In step S5, annealing treatment can be used to eliminate etching stress, repair lattice defects, and improve the mechanical strength and thermal stability of the material.

[0061] In some embodiments, the annealing treatment includes the following parameters: a heating rate of 3° C. / min to 5° C. / min, a temperature of 500° C. to 550° C., and a holding time of 2 h to 3 h.

[0062] The heating rate is limited to 3°C / min to 5°C / min. The primary purpose of a slow heating rate of 3°C / min to 5°C / min is to release gradient stress, avoid thermal stress concentration within the microcrystals caused by sudden temperature changes, and prevent microcrystal cracking or structural collapse. Secondly, it is to control uniform grain growth. A heating rate too fast (>5°C / min) may lead to uneven grain growth; a slow rate (<3°C / min) will extend the process cycle and reduce efficiency. The annealing temperature is limited to 500°C to 550°C to repair lattice defects, eliminate dislocations and point defects generated during etching, improve crystal plane integrity, promote densification of α-Al2O3 grains, and enhance mechanical strength. However, temperatures exceeding 550°C decrease the activation energy for grain growth, leading to rapid grain size growth and a significant decrease in specific surface area. A holding time of 2 to 3 hours allows for complete lattice defect repair, further improving density and compressive strength. However, this time should be limited to 3 hours, as overdensification will occur, accelerating grain boundary migration, increasing grain size, and decreasing specific surface area. Exemplarily, the heating rate can be 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min, etc., the temperature can be 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, etc., and the holding time can be 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, etc.

[0063] In some embodiments, the hollow alumina microcrystals meet the following size parameters: grain size ≤ 50 nm, cavity diameter 400 nm to 800 nm, and wall thickness 30 nm to 80 nm.

[0064] In some embodiments, the hollow alumina microcrystals meet the following performance parameters: a {001} crystal plane ratio of ≥70%, a dielectric constant of 6.5 to 8.0, and a compressive strength of ≥120 MPa.

[0065] The embodiments of the present application provide a method for preparing hollow alumina microcrystals based on induced growth, selective etching, and annealing strengthening. Advanced technologies such as hydrothermal reaction are used to control crystal face growth, inhibit non-target crystal face growth, selectively inhibit aluminum ion redeposition, form regular cavities, and anneal to eliminate stress and repair lattice defects, achieving performance breakthroughs and application expansion. The hollow alumina microcrystal products prepared by this method can be used in high-frequency circuit substrates and power device packaging in the field of electronic packaging, high-temperature resistant components of aircraft engines in the field of structural ceramics, and in the field of photoelectrocatalysis to enhance the efficiency of photogenerated carrier separation, as well as in the reduction of CO2 and hydrogen production by hydrolysis.

[0066] In summary, this application has achieved breakthroughs in the crystal plane ratio, structural uniformity, and overall performance of hollow alumina microcrystals through the innovative process of crystal plane oriented growth-stage etching-low temperature annealing. Its core advantages are:

[0067] (1) High {001} crystal plane ratio and performance improvement: Through the hydrothermal reaction of aluminum nitrate and urea (molar ratio 1:2-4, 150-220℃, 10-15h), an Al2O3·H2O precursor with a {001} crystal plane ratio of ≥70% is generated. This crystal plane is a hexagonal close-packed plane with a highly ordered atomic arrangement, which gives the material high mechanical strength: compressive strength ≥120MPa, suitable for high-temperature resistant structural ceramics (such as aircraft engine components). Excellent dielectric properties: dielectric constant 6.5-8.0, suitable for high-frequency electronic device packaging requirements. Enhanced catalytic activity: The high density of hydroxyl groups on the {001} crystal plane forms abundant Lewis acid sites, improving the efficiency of photocatalytic / electrocatalytic reactions (such as CO2 reduction and hydrolysis to produce hydrogen).

[0068] (2) Precise control of hollow structure: Through the staged etching process, in the low temperature etching stage (25-40℃, 1h), hydrochloric acid (0.1-0.15mol / L) and EDTA (0.05-0.08mol / L) work together to preferentially etch the grain boundary defect area to form an initial cavity and avoid stress concentration. In the high temperature etching stage (40-50℃, 0.5-1h), the etching rate can be accelerated, and EDTA can complex the free Al 3+ , preventing redeposition and ensuring uniform wall thickness (30-80nm). As a result, the cavity / wall thickness ratio of the hollow microcrystals can be adjusted, increasing the specific surface area, enhancing mass transfer efficiency and active site exposure.

[0069] (3) Optimization of grain size and uniformity: The Al2O3·H2O precursor (0.5-0.8 g / L) is used as a template to control the nucleation density of α-Al2O3 microcrystals, resulting in a grain size of ≤50 nm and a narrow size distribution (D90 <20%), thus avoiding the coarsening and agglomeration problems encountered in traditional processes. Furthermore, the concentration of the aluminum nitrate solution (0.5-0.6 mol / L) and the pH (8.0-10.0) synergistically inhibit the formation of the γ-Al2O3 impurity phase, thereby improving product purity.

[0070] (4) Annealing strengthening process: By increasing the temperature gradually (3-5°C / min), the etching stress can be released to avoid microcrystal cracking or structural collapse. By low-temperature annealing (500-550°C, 2-3h), lattice defects (dislocations, oxygen vacancies) can be repaired, the density can be improved, and the excessive growth of grains can be suppressed (the grain size is kept ≤50nm), thereby improving the specific surface area retention rate.

[0071] (5) Process economy and scalability: The hydrothermal reaction temperature (≤220°C) and annealing temperature (≤550°C) are lower than those of traditional solid-phase sintering methods (>1200°C), reducing overall energy consumption. Furthermore, the process is compatible with industrial-grade aluminum nitrate and urea, eliminating the need for high-purity raw materials and reducing production costs.

[0072] (6) Application scenario expansion: High dielectric constant and low dielectric loss make it suitable for 5G high-frequency circuit substrates and power device packaging. At the same time, high compressive strength and high temperature resistance (>1000℃) make it suitable for aircraft engine hot end components. The hollow structure enhances the efficiency of photogenerated carrier separation and improves the efficiency of photocatalytic water splitting to produce hydrogen. In addition, the large specific surface area and acidic sites work synergistically to improve the selectivity of CO2 reduction.

[0073] The present application will be further described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present application only and are not intended to limit the scope of the present application. The experimental methods in the following examples where specific conditions are not specified are generally measured according to industry standards. If there are no corresponding industry standards, then the methods are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0074] Example 1

[0075] 0.1 mol / L aluminum nitrate and 0.3 mol / L urea were dissolved in deionized water and magnetically stirred for 30 minutes; the mixture was transferred to a high-pressure reactor and reacted at 180°C for 12 hours; the slurry was centrifuged and washed at 8000 rpm for 10 minutes and vacuum dried at 60°C to obtain a precursor powder.

[0076] 0.5 g of precursor powder was mixed with 0.5 mol / L aluminum nitrate, and 25% ammonia water was added dropwise to adjust the pH to 9.0; a hydrothermal reaction was carried out in a high-pressure reactor at 200° C. for 24 hours to generate α-Al2O3.

[0077] A mixed solution is prepared with 0.1 mol / L hydrochloric acid and 0.05 mol / L ethylenediaminetetraacetic acid; α-Al2O3 is immersed in the mixed solution and stirred at 25°C for 1 hour; the temperature is raised to 50°C and etching is continued for 1 hour to form an internal cavity with a diameter of 500 to 800 nm.

[0078] The muffle furnace was heated to 500° C. at a heating rate of 4° C. / min, maintained in an air atmosphere, and kept warm for 2 hours to obtain alumina hollow microcrystal products.

[0079] The performance indicators of alumina hollow microcrystal products are as follows:

[0080] The {001} crystal plane accounts for 75%, the average grain size is 45nm; the cavity diameter is 600nm, the wall thickness is 47nm~50nm; the compressive strength is 125MPa; the dielectric constant ε=6.5.

[0081] Example 2

[0082] 0.1 mol / L aluminum nitrate and 0.4 mol / L urea were dissolved in deionized water and magnetically stirred for 30 minutes; the mixture was transferred to a high-pressure reactor and reacted at 180°C for 12 hours; the slurry was centrifuged and washed at 8000 rpm for 10 minutes and vacuum dried at 60°C to obtain a precursor powder.

[0083] 0.5 g of the precursor powder was mixed with 0.5 mol / L aluminum nitrate, and 25% ammonia water was added dropwise to adjust the pH to 9.0. The mixture was hydrothermally reacted at 200° C. for 24 hours to generate α-Al 2 O 3 cubic microcrystals.

[0084] Prepare a mixture of 0.08 mol / L hydrochloric acid and 0.1 mol / L ethylenediaminetetraacetic acid. Immerse α-Al2O3 in the mixture and stir at 25°C for 1 hour. Raise the temperature to 50°C and continue stirring for 1 hour to form cavities with a diameter of 500-800 nm and a wall thickness of approximately 80 nm.

[0085] The muffle furnace was heated to 500° C. at a heating rate of 3° C. / min, maintained in an air atmosphere, and kept warm for 2 hours to obtain alumina hollow microcrystal products.

[0086] The performance indicators of alumina hollow microcrystal products are as follows:

[0087] The {001} crystal plane accounts for 80%, the average grain size is 48nm; the cavity diameter is 600nm, the wall thickness is 77nm~80nm; the compressive strength is 150MPa; and the dielectric constant ε=7.2.

[0088] Example 3

[0089] 0.08 mol / L aluminum nitrate and 0.25 mol / L urea were dissolved in deionized water and magnetically stirred for 30 minutes; the mixture was transferred to a high-pressure reactor and reacted at 180°C for 10 hours; the mixture was centrifuged and washed at 8000 rpm for 10 minutes, and vacuum dried at 60°C to obtain a precursor powder.

[0090] 0.3 g of precursor powder was mixed with 0.7 mol / L aluminum nitrate, and 25% ammonia water was added dropwise to adjust the pH to 9.5; a hydrothermal reaction was carried out at 200° C. for 24 hours to generate α-Al 2 O 3 cubic microcrystals.

[0091] Prepare a mixture of 0.15 mol / L hydrochloric acid and 0.02 mol / L ethylenediaminetetraacetic acid. Immerse α-Al2O3 in the mixture and stir at 25°C for 1.5 hours. Raise the temperature to 50°C and continue stirring for another 1.5 hours to form cavities with a diameter of 500-800 nm and a wall thickness of approximately 80 nm.

[0092] The muffle furnace was heated to 450°C at a heating rate of 5°C / min. The temperature was kept in an air atmosphere for 1 hour to obtain hollow alumina microcrystals.

[0093] The performance indicators of alumina hollow microcrystal products are as follows:

[0094] The {001} crystal plane accounts for 80%, the grain size is 42nm; the cavity diameter is 800nm, the wall thickness is 47nm~50nm; the compressive strength is 150MPa, and the dielectric constant ε=7.2.

[0095] Example 4

[0096] 0.15 mol / L aluminum nitrate and 0.4 mol / L urea were dissolved in deionized water and magnetically stirred for 30 minutes; the mixture was transferred to a high-pressure reactor and reacted at 200°C for 10 hours; the mixture was centrifuged and washed at 8000 rpm for 10 minutes, and vacuum dried at 60°C to obtain a precursor powder.

[0097] 1.0 g of the precursor powder was mixed with 0.4 mol / L aluminum nitrate, and 25% ammonia water was added dropwise to adjust the pH to 8.5. The mixture was hydrothermally reacted at 200° C. for 24 hours to generate α-Al 2 O 3 microcrystals.

[0098] Prepare a mixture of 0.05 mol / L hydrochloric acid and 0.1 mol / L ethylenediaminetetraacetic acid. Immerse α-Al2O3 in the mixture and stir at 30°C for 1.5 hours. Raise the temperature to 50°C and continue stirring for another 1.5 hours to form cavities with a diameter of 500-800 nm and a wall thickness of approximately 80 nm.

[0099] The muffle furnace was heated to 600°C at a heating rate of 4°C / min. The temperature was kept at 600°C in an air atmosphere for 3 hours to obtain hollow alumina microcrystals.

[0100] The performance indicators of alumina hollow microcrystal products are as follows:

[0101] The α-Al2O3{001} crystal plane accounts for 85%, the grain size is 46nm; the cavity diameter is 450nm, the wall thickness is 47nm~50nm; the compressive strength is 180Mpa; the dielectric constant ε=7.6.

[0102] Comparative Example 1

[0103] 0.05 mol / L aluminum nitrate and 0.45 mol / L urea were dissolved in deionized water at a molar ratio of 1:6 and magnetically stirred for 30 minutes; transferred to a high-pressure reactor and reacted at 180°C for 12 hours; centrifuged and washed at 8000 rpm for 10 minutes, and vacuum dried at 60°C to obtain a precursor powder and an impurity phase.

[0104] 0.3 g of the precursor powder and the impurity phase were mixed with 0.3 mol / L aluminum nitrate at a molar ratio of 1:30, and 25% ammonia water was added dropwise to adjust the pH to 9.0; the mixture was hydrothermally reacted at 200° C. in a high-pressure reactor for 24 hours to generate a mixed phase of α-Al2O3 and γ-Al2O3.

[0105] A mixed solution was prepared with 0.05 mol / L hydrochloric acid and 0.02 mol / L ethylenediaminetetraacetic acid in a molar ratio of 2.5:1. The mixed phase of α-Al2O3 and γ-Al2O3 was immersed in the mixed solution and stirred at 25°C for 1 hour. The temperature was raised to 50°C and the stirring was continued for 1 hour to form a cavity with a diameter of about 500-800 nm.

[0106] The muffle furnace was heated to 450°C at a heating rate of 4°C / min, maintained in air atmosphere, and kept warm for 1 hour to obtain α-Al2O3 and γ-Al2O3 impurity phases.

[0107] Product performance indicators are as follows:

[0108] The {001} crystal plane accounts for 55%, the grain size is 48nm, the cavity diameter is 350nm, and the wall thickness is 30nm; the compressive strength is 70Mpa; and the dielectric constant ε is 5.8.

[0109] Comparative Example 2

[0110] 0.1 mol / L aluminum nitrate and 0.5 mol / L urea were dissolved in deionized water at a molar ratio of 1:5 and magnetically stirred for 30 minutes; transferred to a high-pressure reactor and reacted at 180°C for 12 hours; centrifuged and washed at 8000 rpm for 10 minutes, and vacuum dried at 60°C to generate a precursor powder + impurity phase.

[0111] 1.0g / L of the precursor powder plus impurities was mixed with 0.7mol / L aluminum nitrate at a molar ratio of 1:90. 25% aqueous ammonia was added dropwise to adjust the pH to 9.0. A hydrothermal reaction was conducted in an autoclave at 200°C for 24 hours to generate crystals of the precursor α-Al2O3 and γ-Al2O3 impurity phases. A mixed solution was prepared using 0.2mol / L hydrochloric acid and 0.08mol / L ethylenediaminetetraacetic acid at a molar ratio of 2.5:1. The mixed phase crystals were immersed in the solution and stirred at 25°C for 1 hour. The temperature was then raised to 50°C and stirred for another hour, forming cavities with diameters of 500-800nm.

[0112] The muffle furnace was heated to 600°C at a heating rate of 4°C / min, and air atmosphere was maintained for 3 hours to obtain α-Al2O3 and γ-Al2O3 impurity phases.

[0113] Product performance indicators are as follows:

[0114] The {001} crystal plane accounts for 52%, the grain size is 44nm; the cavity diameter is 380nm, the wall thickness is 20nm; the compressive strength is 72MPa, and the dielectric constant ε=6.0.

[0115] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0116] In the embodiments of the present application, a method for controllable synthesis and high-strength preparation of hollow alumina microcrystals is provided. This method not only achieves high-precision structural control of hollow alumina microcrystals through processes such as precursor powder preparation, growth control, cavity formation, and calcination annealing, but also significantly outperforms existing patented technologies in terms of crystal plane ratio, mechanical strength, and process efficiency. It has broad application prospects in the fields of electronic packaging, structural ceramics, etc.

[0117] In the embodiments of the present application, it is in a leading position in terms of crystal plane control, cavity accuracy, process efficiency and mechanical properties. Its hydrothermal induced directional growth technology and gradient etching core technology are far superior to the level of the same industry.

[0118] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A method for preparing hollow alumina microcrystals, the method comprising: Aluminum nitrate and urea are subjected to a first hydrothermal reaction to obtain Al2O3 with a {001} crystal plane ratio of ≥70%. 3· H2O precursor; The Al2O3·H2O precursor is used as a seed crystal to undergo a second hydrothermal reaction with an aluminum nitrate solution to obtain α-Al2O3 microcrystals; the pH value of the second hydrothermal reaction is 8.0 to 10.0; Immersing the α-Al2O3 microcrystals in an etching solution mixed with hydrochloric acid and ethylenediaminetetraacetic acid to selectively etch the α-Al2O3 microcrystals to obtain a mixed slurry; performing solid-liquid separation on the mixed slurry to obtain a filter cake; as well as The filter cake is annealed in an air atmosphere to obtain the alumina hollow microcrystals.

2. The method according to claim 1, characterized in that The molar ratio of the aluminum nitrate to the urea is 1:(2-4).

3. The method according to claim 1, characterized in that The reaction temperature of the first hydrothermal reaction is 150° C. to 220° C., and the reaction time of the first hydrothermal reaction is 10 h to 15 h.

4. The method according to claim 1, wherein The molar concentration of the aluminum nitrate solution is 0.5 mol / L to 0.6 mol / L, and the added amount of the Al2O3·H2O precursor is 0.5 g / L to 0.8 g / L.

5. The method according to claim 1, characterized in that The reaction temperature of the second hydrothermal reaction is 150° C. to 220° C., and the reaction time of the second hydrothermal reaction is 20 h to 24 h.

6. The method according to claim 1, characterized in that The selective etching includes a first stage etching and a second stage etching; wherein, The temperature of the first stage etching is 25°C to 40°C, and the stirring time of the first stage etching is 1 hour; The temperature of the second stage etching is 40° C. to 50° C., and the stirring time of the second stage etching is 0.5 h to 1 h.

7. The method according to claim 1, characterized in that The molar ratio of the hydrochloric acid to the EDTA is (0.8-2.5):1, the molar concentration of the hydrochloric acid is 0.1 mol / L-0.15 mol / L, and the molar concentration of the EDTA is 0.05 mol / L-0.08 mol / L.

8. The method according to claim 1, characterized in that The annealing treatment includes the following parameters: a heating rate of 3°C / min to 5°C / min, a temperature of 500°C to 550°C, and a holding time of 2h to 3h.

9. The method according to claim 1, characterized in that The hollow alumina microcrystals meet the following size parameters: grain size ≤ 50 nm, cavity diameter 400 nm to 800 nm, and wall thickness 30 nm to 80 nm.

10. The method according to claim 1, characterized in that The hollow alumina microcrystals meet the following performance parameters: a {001} crystal plane ratio of ≥70%, a dielectric constant of 6.5 to 8.0, and a compressive strength of ≥120 MPa.

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