1.1 Alumina Crystal Structure
Alumina has a relatively complex structure, in which it is divided into two categories based on the arrangement of O2-: cubic structure (fcc) or hexagonal close-packed structure (hcp). Within these two major categories, further classification can be made based on Al3+, among which γ (cubic), η (cubic), θ (monoclinic), and δ (orthorhombic) structures are based on the fcc lattice, while α (trigonal), κ (orthorhombic), and χ (hexagonal) are hcp structures.
The α-Al2O3 crystal structure has O2- arranged in a hexagonal close-packed structure, with the remaining parts forming octahedral interstitial sites, and Al3+ filling 2/3 of the interstitial sites, thereby ensuring that the chemical formula of alumina is α-Al2O3. The filling principle of Al3+ in octahedral interstitial sites satisfies Pauling's rules, that is, the distance between Al3+ within the same layer and between layers should be kept as far apart as possible [1].
α-Al2O3 is further divided into flake-like α-Al2O3 and spherical α-Al2O3. Flake-like α-Al2O3 exhibits a two-dimensional planar structure in space, with small thickness and a large diameter-to-thickness ratio. It is usually prepared by three methods: wet chemistry, solid-state reaction, and molten salt method. The spherical α-Al2O3 structure is mainly arranged in three-dimensional space, and its growth in the two-dimensional direction must be suppressed during preparation, so methods such as homogeneous precipitation, hydrothermal method, drip ball method, template method, spraying method, and plasma electrolysis are mainly used here. Spherical α-Al2O3 is relatively more difficult to prepare.
The γ-Al2O3 crystal structure is characterized by cubic close packing of O2-, thereby forming an octahedral structure. This structure further evolves into a spinel structure, in which 32 cubic close-packed O atoms constitute the unit cell. In the spinel structure, 32 O atoms form 8 tetrahedral voids and 16 octahedral voids, in which 67/3 aluminum atoms are randomly distributed in the voids, while 8/3 vacancies exist [2].
The characteristic of the γ-Al2O3 crystal structure lies in its high specific surface area and widely distributed active centers. At suitable catalytic temperatures, this material exhibits good stability. Therefore, γ-Al2O3 catalysts have broad application prospects in industry and scientific research due to their high selectivity.
α-Al2O3 is the most stable crystal form of alumina, and the other crystal forms are unstable and are called transition phases. These transition phases will eventually transform into the stable α-Al2O3 structure under certain conditions, so α-Al2O3 is also the final transformation phase of alumina. Therefore, only α-Al2O3 exists in nature, in the form of natural minerals such as natural corundum, ruby, and sapphire. This structure is also called the corundum crystal structure [3-4].
1.2 Alumina Ceramics
Alumina ceramics are a high-performance ceramic material. Due to their excellent physical and chemical properties, they are widely used in harsh environments such as high temperature, high strength, high wear resistance, and high insulation. Their development history can be traced back to the end of the 19th century. Due to their high strength and stiffness, alumina ceramics are one of the most popular oxide ceramics. At the same time, due to their general mechanical, electrical, and optical properties, alumina ceramics are used in many modern industrial fields.
Alumina ceramics have high hardness, with a Rockwell hardness of HRA80-90, which is second only to diamond. The wear resistance of alumina ceramics is equivalent to 266 times that of manganese steel and 171.5 times that of high-chromium cast iron. Under the same working conditions, it can extend the service life of equipment by at least ten times. The density of alumina ceramic material is 3.7~3.95 g/cm³, only half that of steel, which can also greatly reduce equipment load. At the same time, it also has advantages such as high thermal conductivity, good resistivity and thermal stability, and a small dielectric constant. It is the preferred material for a new generation of microelectronic devices and systems and has become the main material for circuit substrates in fields such as aerospace, 5G communications, high-power semiconductors, and high-power LED lighting [5].
High-purity ultrafine alumina powder preparation technology is one of the key factors restricting the rapid development of special alumina ceramic materials and is also a key link in reducing costs and increasing efficiency in industrial production. The development of new methods for preparing high-quality ceramic powders is of great significance to the development of ceramic materials and related high-tech fields.
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Alumina Powder Preparation Technology
2.1 Classification of Alumina Precursors
The preparation of alumina usually involves the thermal decomposition of alumina hydrates, which are precipitated from solutions containing Al3+ ions or organometallic compounds. Studies have shown that this conversion process (such as the dehydration of boehmite to γ-Al2O3) is a topotactic reaction, allowing the microscopic morphology of the precursor to be retained in the alumina. Therefore, the characteristics of alumina are closely related to the hydrate precursor.
According to differences in structure and water content, alumina precursors mainly include gibbsite, bayerite, nordstrandite, and diaspore, among which diaspore is further divided into diaspore and boehmite. Gibbsite is the main component of bauxite in tropical regions. As the final product of the commercial Bayer process, it is produced on a large scale by alkaline precipitation of Al3+ and is widely used as a flame retardant, additive for polymers and paper, toothpaste filler, and stabilizer for titanium dioxide. The crystal structure of gibbsite belongs to the monoclinic system, with aluminum-oxygen octahedra connected by edges to form a layered structure, and the arrangement of double-layer stacking can be expressed as AB BA AB BA.... Boehmite, also known as pseudo-boehmite. It can be synthesized hydrothermally using gibbsite as a raw material, or obtained by controlled roasting of bayerite. Boehmite belongs to the orthorhombic system, with aluminum-oxygen octahedra connected by edges and arranged in a stepped manner, and its layers are connected by hydrogen bonds. In the octahedral structure, oxygen atoms are arranged in the manner ABC ABC....
Among various aluminum hydroxide precursors, pseudo-boehmite is a type of hydrate with relatively low crystallinity. It is characterized by high specific surface area, abundant pores, a positively charged surface, easy dispersion in water and hydroxyl-containing solvents, and good gel solubility under acidic conditions. These characteristics make pseudo-boehmite widely used as a catalyst carrier and binder, and it is also an important intermediate product for producing activated alumina [6].
2.2 Alumina Powder Preparation Methods
2.2.1 Gas Phase Method
The gas phase method directly uses gas or vaporizes reactants through different heating methods, causing physicochemical reactions to occur in the gaseous state, and finally condensing to form ultrafine powders. This type of method has mild reaction conditions, easy control of agglomeration, good particle dispersibility, small particle size, and narrow distribution; the disadvantage is extremely low yield, and production costs are difficult to control.
In the gas phase method, raw materials must be completely vaporized before reaction, which consumes a lot of energy, and the reaction requires a large amount of inert gas with very low humidity, resulting in relatively high cost for the gas phase method.
Chemical vapor deposition (CVD) is a commonly used gas phase method for preparing nano-alumina. This method uses aluminum chloride and water vapor to react in a reaction chamber to generate alumina nanopowder. Common CVD methods include flame CVD, laser pyrolysis CVD, and others. Han Shizhong et al. [7] prepared high-purity alumina powder with an average particle size of 5.6 nm by plasma organometallic CVD, reacting in an oxygen environment at 1000°C and 5.3 kPa to obtain spherical nano-alumina powder.
2.2.2 Liquid Phase Method
The liquid phase method involves dissolving aluminum salts in a certain solvent, separating the solute and solvent through evaporation, sublimation, addition of precipitant, hydrolysis, and other methods to obtain particles of a certain shape and size, that is, preparing the precursor of Al2O3 powder, and finally heating and decomposing the precursor.
First, the aluminum element exists in the solution in ionic form and can be dissolved in water solvents or organic solvents such as trichloromethyl, ethanol, and toluene through aluminum salts such as aluminum sulfate, aluminum sec-butoxide, and sodium metaaluminate. Subsequently, the aluminum ions can be deposited and precipitated in the form of covalent oxides through methods such as hydrolysis and evaporation of aluminum salts, or by using precipitants such as ammonium carbonate, sodium hydroxide, urea, and sulfuric acid. Finally, the precipitate is washed and dried or calcined at high temperature to obtain the desired alumina crystal form [8-9].
Spray Pyrolysis Technology
Spray pyrolysis technology, also known as flame spray pyrolysis technology, is a method for preparing ultrafine alumina powder based on ultrasonically generated micron-sized aerosol droplets that are heated and decomposed at 400°C~800°C. Laine et al. [10] successfully purified multiphase alumina powder using spray pyrolysis technology, obtaining high-purity nano-alumina powder with a particle size of 50 nm~80 nm and α-phase purity of 50%~85%.
Alkoxide Hydrolysis Method
The alkoxide hydrolysis method involves preparing a solution from a suitable water-soluble aluminum salt according to stoichiometric proportions, then selecting a precipitant that can react with metal ions to form a precipitate, and finally heating and dehydrating the precipitate to obtain ultrafine powder [11]. Common alkoxide hydrolysis methods include the sulfur-emission-free aluminum ammonium sulfate method and the aluminum ammonium carbonate method, etc. [12-13].
Zhong Lianyun et al. [14] used analytically pure aluminum nitrate and ammonium bicarbonate as the main raw materials to prepare nano-Al2O3 powder by the co-precipitation method. The average particle size of the prepared nano-Al2O3 powder was 20 nm~30 nm. Adding the surfactant PEG6000 improved the dispersion performance of α-Al2O3 powder.
Improved Bayer Process
The Bayer process utilizes the change in solubility of aluminum oxide in alkaline solution, using concentrated sodium hydroxide solution to convert aluminum hydroxide into sodium aluminate, then separating insoluble substances through precipitation, and finally re-precipitating aluminum hydroxide by dilution and addition of aluminum hydroxide seed crystals, followed by roasting and dehydration to obtain alumina powder.
In recent years, through continuous refinement of the Bayer process, new methods capable of efficient desilication have been developed [15]. The improved Bayer process mainly improves the preparation of sodium hydroxide and sodium removal, performs desilication and iron removal on sodium aluminate, and regulates decomposition conditions to obtain high-purity aluminum hydroxide. During this process, aluminum hydroxide precipitates slowly, effectively reducing the occurrence of abnormal crystal nuclei and also reducing the incorporation of sodium and silicon impurities. Finally, high-purity alumina is obtained through high-temperature calcination and grinding.
Sol-Gel Method
The sol-gel method is a simple and effective method for synthesizing nano-alumina. Unlike the precipitation method, which initially converts to sol and gel, aluminum salts and high-purity amine or ammonium salt solutions undergo hydrolysis or polymerization to form a precursor gel, which is then alcohol-washed, aged, and finally calcined to obtain alumina powder.
Some researchers have also made technical improvements based on the traditional sol-gel method. For example, Zhu Zuoyuan [16] adopted a hybrid process of metal alkoxide hydrolysis and sol-gel method to prepare high-purity ultrafine alumina powder. First, aluminum powder with purity ≥2N was heated to synthesize aluminum alkoxide solution, insoluble particulate impurities were removed by microfiltration, and after aging, hydrolysis was carried out. A dry gel was obtained by distillation, and finally high-purity alumina powder was obtained by high-temperature calcination.
Aluminum Alkoxide Method
The aluminum alkoxide method belongs to the technological iteration of the alkoxide hydrolysis method. It involves adding metal aluminum flakes to isopropanol solution for reaction to generate aluminum isopropoxide, then hydrolyzing aluminum isopropoxide to generate hydrated alumina, and after steps such as aging, filtration, drying, and dehydration activation, finally obtaining alumina powder with relatively high sintering activity.
Yang Conglin et al. [17] used metal aluminum and carbon alcohol as raw materials to synthesize high-purity aluminum alkoxide, and prepared hydrated alumina by alkoxide hydrolysis, eliminating the vacuum distillation process with high equipment requirements, and finally obtained 5N-grade high-purity ultrafine alumina powder by calcination, with powder purity ≥99.999% and particle size of 0.1 µm~0.3 µm. The alumina powder prepared by this method has high purity, and the preparation process is simple with strong controllability of the product.
2.2.3 Solid Phase Method
The solid phase method is a method in which two or more powders are mixed and directly reacted under a certain temperature and atmosphere to generate nano-alumina.
Mechanical Crushing Method
The mechanical crushing method uses various ultrafine crushers (high-energy ball mills, planetary ball mills, tower mills, jet mills, etc.) to directly crush and grind raw materials into ultrafine powder. Among them, high-energy ball mills are widely used. They mechanically crush alumina powder in water or alcohol media to prepare high-purity ultrafine alumina powder. Through the vibration and rotation of the ball mill, energy is provided to the raw materials, causing the raw materials to be strongly impacted by hard balls, crushed into fine particles, or activated to undergo chemical reactions, thereby preparing ultrafine powder.
Detonation Method
The detonation method can also be used as a solid-phase method for preparing alumina powder. This method performs combustion reactions inside a constant-volume vessel and generates a large amount of gas. Therefore, the instantaneous reaction often proceeds under high pressure, suppressing the saturated vapor pressure of the reactants and raising the phase transition temperature under normal pressure, which is conducive to the stable formation of single-phase metastable alumina powder. The detonation method has high requirements for production equipment, difficult production process control, low yield, and relatively high production cost.
Peng et al. [18] obtained different detonation results by controlling the pH value of the glycine-aluminum nitrate system. The results showed that an alkaline environment can effectively control grain size, reduce the proportion of amorphous phases, and increase specific surface area, while agglomeration of varying degrees occurs in environments with pH≤6.
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Summary
High-purity ultrafine alumina powder, due to its advanced physical and chemical properties, has greatly expanded the application scenarios of alumina powder and is a key raw material in modern advanced manufacturing. At present, the main production methods of high-purity ultrafine alumina powder in China have disadvantages such as complex processes, difficult control of technical conditions, high cost, and serious pollution. With the continuous development of China's economy, the demand for high-purity ultrafine alumina will continue to increase, and future requirements for equipment precision, preparation processes, control technology, energy saving, and environmental protection will also become higher and higher.