Is the morphology of ultrafine alumina powder difficult to control?
The particle size and morphology of ultrafine powders have a great influence on their physical and chemical properties, and their morphology basically determines the overall and surface characteristics of the powders. In addition, the structure and morphological characteristics of the powder include the shape, chemical composition, material composition, internal and external surface area, volume and surface defects of the powder, which together determine the comprehensive performance of the ultrafine powder.
The structure and morphology requirements of ultra-fine alumina powder vary according to the application. For example, ultra-fine alumina powder with small particle size and spherical shape is suitable for preparing ceramic materials; and flake ultra-fine alumina powder is aluminate The ideal raw material for phosphors. Therefore, in the preparation process, it is very important to control the structure and morphology of the powder according to its application needs.
Various morphological structures and characteristics of alumina powder
At present, according to the morphological characteristics of alumina powder, people have synthesized several shapes of alumina powder, such as spherical, hollow spherical, flower-shaped, cubic, plate-shaped, diamond-shaped, needle-shaped or fibrous. They have the following characteristics:
1. Spherical Al2O3 powder can be used as a support for porous Al2O3. Since the pores formed are regular, it is easy to make the whole support uniform. The dispersed α-Al2O3 spherical particles have good compression molding and sintering characteristics, which is beneficial to the production of high-quality ceramic products.
2. There are countless small pores on the particles of the hollow spherical powder, which has a large specific surface area, and a very important requirement of the catalyst carrier is to have a large specific surface area, so this product can be used as an excellent catalyst Carrier.
As one of the effective methods for preparing hollow spherical alumina powder, spray drying method has received more and more attention in recent years. In the spray-drying drying tower, a slurry drop is sprayed out by the spray gun and falls at a certain height, in the process it is dried. Due to the high temperature, the surface of the slurry dries quickly. The binder on the surface and the fine powder particles combine to form a hard shell, which closes the surface of the sphere, while the internal moisture continues to vaporize rapidly, forming the air pressure inside the sphere. When the air pressure reaches a certain level, the gas bursts out of the sphere from the weak place, making the sphere appear a hole. Because the strength of the surface hard shell and the air pressure in the ball are different, its bursting force is also different, so it has formed an apple shape slightly, and a hollow shape and a bead shape.
3. Flower-shaped and cubic alumina powder can be used as a catalyst carrier, and has important application value in biological materials, composite materials or structural reinforcement materials, so whether it is its synthesis method or morphology generation mechanism The research is worth further exploration.
4. The plate-shaped alumina powder has a two-dimensional planar structure, so it has good adhesion, significant shielding effect and the ability to reflect light. It also has acid and alkali resistance, high temperature resistance, high hardness, high melting point, and thermal conductivity. High and high resistivity and many other excellent properties. The plate-shaped alumina powder with small particle size and good dispersibility can be widely used in many fields such as pigments, coatings, automotive topcoats, phosphor raw materials, cosmetics, inks and abrasives.
Research has found that when plate-shaped alumina is used as a filler for polymers, it can enhance its thermal conductivity. Adding a certain amount of plate-like alumina to the polymer can form an alumina network, which can transfer most of the heat. The larger the diameter of the plate-shaped alumina, the fewer the nodes forming the network, and the better the thermal conductivity.
In addition, the addition of alumina plate crystals as a second-phase toughener to alumina ceramics can effectively increase crack reflection and bridging, and has a significant effect on improving the fracture toughness of ceramics, overcoming the general alumina ceramics. The low mechanical performance restricts its lack of wider application. In short, plate-shaped alumina powder has broad application prospects.
5. Diamond-shaped alumina products can be used as alumina for stainless steel polishing. The grinding rate and light extraction rate are better than those of the original crystal in other shapes. The abrasive particles have self-sharpness, so they can be used as stainless steel polishing alumina.
6. As a high-performance inorganic fiber, alumina fiber has good high temperature resistance, wear resistance and oxidation resistance, but also has low thermal conductivity, small thermal expansion coefficient, good seismic performance, high modulus, high plasticity, high toughness, High strength, high insulation and high dielectric constant and other outstanding advantages, so it is widely used in insulating materials, fiber protection, and reinforcement materials.
Micro-morphology control of alumina powder
For the preparation of ultrafine powder, the high performance of the powder is the economic benefit of the production. The microstructure characteristics of the particles directly determine the application performance of the final product, and the control of the microscopic morphology of the particles is particularly important. Achieving control of powder morphology is a complex process involving multiple disciplines such as solid chemistry, interfacial reaction and kinetics.
Essentially, controlling the morphology of nanoparticles is controlling the dynamics of crystal growth. Because the morphology of the crystal depends on the growth rate of different crystal phase surfaces, and the growth degree of a certain surface of the crystal is controlled by the crystal structure and crystal defects on the one hand, and by the surrounding environmental conditions on the other hand. Therefore, the morphology of the nano-powder can be controlled from two angles.
If some surface modifiers that can selectively act on the surface of the crystallite are added during the formation of the crystal, the growth rate of the crystal on different crystal axes can be adjusted to achieve the purpose of controlling the morphology of the nanoparticles. At the same time, it is also possible to control the morphology of the nanoparticles by changing many factors of the surrounding environmental conditions during the crystallization process, such as the pH value, temperature, ionic strength, solvent or organic additives, and the ratio of reactants.
1. Effect of additives on powder microstructure
The microstructure of Al2O3, especially the morphology of grains, has a great influence on the performance of Al2O3 ceramics. Therefore, seeking new additives to improve the morphology of Al2O3 grains has been one of the goals of people's research. Appropriate additives not only affect the morphology and size of the particles, but also promote the alpha phase conversion of alumina, but the effects of different additives are obviously different. The influence of additives is mainly attributed to the adsorption of the growing crystal on the additive, which leads to the change of the crystal growth rate, which is finally reflected in the crystal morphology; the second is the degree of activity of the additive in the solution. At present, the mechanism of the effect of additives on the microstructure of powders has remained at the initial stage, and there is a lack of in-depth research.
2. The effect of hydrolysis and gel technology on the microstructure of powder
Factors that affect particle morphology during liquid precipitation include supersaturation, hydrodynamics, reaction time, etc. In solution, the habit surface with the smallest crystal growth rate determines the crystal growth morphology. The interaction between the solvent and the precipitate determines the lowest energy surface, and the dynamic conditions of grain growth have an effect on the growth of the habit surface that affects the crystal morphology.
The temperature control during the hydrolysis and gelation process will affect the morphology of the powder. When the reaction temperature is higher, the nucleation rate and growth rate increase correspondingly. Crystals often develop into slender columnar, needle-like, and scale-like aggregates, and sometimes even grow into skeletal crystals with special shapes. This is because the crystals grow in an extremely unbalanced state, and the interface of the crystal has a large The surface energy is also unstable on its own. As a result, it grows along some crystal edges or apex angles to form a skeletal crystal. If the crystal grows in a near-equilibrium state, the growth rate is relatively slow. Generally, the crystal can obtain a relatively complete crystalline polyhedron.
The anisotropy of Al2O3 shows that the growth rate of the C axis is much larger than that of the other two coordinate axes. That is to say, when the temperature is higher and the overall growth rate is larger, the growth rate on the C axis is greater than the overall growth rate. At this time, rod-shaped and needle-shaped particles are easily formed; when the ice water bath is used, the overall growth rate is small, The growth rate on the C axis is not much different from the overall growth rate, and spherical particles are easily formed at this time.
During the growth process of Al2O3 nanograins, when the obtained colloid is subjected to a long-term high temperature gel, the higher temperature helps the microscopic thermal movement of Al2O3 nanoparticles, coupled with the extremely small size of Al2O3 nanoparticles The surface energy is very high, and the longer reaction time is conducive to the reorientation and arrangement of the nanocrystallites. The result is that many spherical Al2O3 nanoparticles are close to each other, combined and arranged on a unified crystal plane, and eventually form a larger sheet structure.
3. Effect of drying method on powder microstructure
According to research by scientific researchers, the same raw materials, surfactants and preparation methods are used, and when the drying methods are different, the morphology of the obtained alumina powder is very different. When using natural drying at room temperature, the solvent slowly evaporates from the surface, so that the particles have sufficient time to contact each other, and the particles are cross-linked by the action of the surfactant. Due to the static drying, flake crystals are formed Or stacked body.
When drying with a self-made simple spray device, the diluted solution of sol is sprayed into fine small droplets. The small droplets are dried in the process of falling and the surface shrinks, but the sample is not completely dried in this process. After reaching the receiving dish, continue to evaporate the solvent on the surface. Due to the presence of surfactant, when the solvent evaporates, the surfactant cross-links aluminum hydroxide to form a long chain. After the surface solvent is completely evaporated, with the help of the surfactant The effect is to form a long strip of aluminum hydroxide. However, there are also parts that cannot be cross-linked, forming droplet-like spheres.
4. Effect of calcination process on powder microstructure
1 Influence of temperature
By calcining at different temperatures, powders with different morphologies can be obtained. When the researchers prepared nano-fibrous Al2O3 powder, the microstructure of the powder calcined at 500 ℃ was a slightly stretched fiber network; when the calcination temperature rose to 1200 ℃, the sample no longer appeared fibrous, but transformed It is spherical. This is because the surface energy of the fibrous object is higher, and as the temperature increases, it will transition to a sphere that can reduce the surface energy.
2 Impact of operating system
The operation system also affects the morphology and particle size of Al2O3 powder. According to relevant literature reports: adding seed crystals or alumina colloids to the calcination process is the most common and effective method to reduce the alpha transformation temperature of alumina. In addition to α-Al2O3, the selected seed crystals can also be α-Fe2O3, α-Cr2O3 and MgO, etc. They have the same or similar crystal structure as α-Al2O3, otherwise it has no or little effect on the α phase transition.