Alumina Ceramics for Various Applications
The α-phase of alumina is the phase with the most stable physical and chemical properties. It is used extensively in refractory materials, ceramic materials, grinding and polishing materials, chemical materials, and high-strength glass, etc. Different alumina properties will be emphasized in various application situations. Multi-functional aluminas have emerged because there is no high-performance alumina that can satisfy all application requirements. In real-world applications, "alumina with specific properties" must be designed to meet the demands of the intended use.
In the field of grinding and polishing, indicators like the crystal size, particle shape, powder particle size and distribution, maximum particle size and agglomeration situation, wear resistance, and even its stability in the polishing solution are of the utmost importance. The precision and grinding force used in the grinding and polishing processes will be impacted by these indicators. In the refractory industry, the influence of alumina's α-phase conversion rate, impurity content, and average particle size on the refractoriness, shrinkage rate, and thermal shock resistance of refractory products is given greater consideration.
More than 400 different varieties of special alumina have been created in accordance with various application requirements. The impurity content of alumina, the conversion rate of α-alumina, the primary crystal size, and the particle size distribution after grinding are all common important indicators that influence alumina's application effect.
Effects of Temperature on α-Phase Transition Rate
It is generally acknowledged that the alumina α-phase transition is a two-stage process of nuclei growth. The α-phase nuclei come first, followed by the growth of particles (crystals). Scanning electron micrographs of α-alumina produced at various temperatures can be used to track this process. The entire process requires a lot of energy, most of which is used to break through the nuclear barrier and create the α crystal phase. The remaining energy is then used to push through the activation energy of grain growth and encourage the growth of the crystal nucleus. In other words, by lowering the phase transition activation energy, the phase transition temperature can be lowered and the growth of the particle size can be regulated.
Alumina can be found in a variety of crystal forms, including α, γ, η, δ, θ, κ, χ, etc. The crystal form will alter as the environment changes. Other metastable transition crystal phases will change into the α-phase as the temperature rises, with the exception of the α-phase, which is a thermodynamically stable phase. For instance, after calcination, γ-alumina can be changed into α-alumina, but at the same time, there is a significant volume shrinkage (the density of γ-alumina is 3.65g/cm3 and the density of α-alumina is 3.99g/cm3).
Alumina's α-phase transition temperature is very high, typically between 1200 and 1400 °C (at such a high temperature, alumina's sintering process has already started; once α-Al2O3 particles are formed, they will grow up right away and agglomerate with one another, forming sintering necks and a "vermiculite-like" hard agglomerate structure, and the removal of this hard agglomerate is not only expensive but also challenging. In order to prepare ultra-fine α-Al2O3 powder and ensure uniform dispersion, one of the key factors is to lower the temperature of α-phase transition (allowing it to grow slowly while the phase transition occurs).
In general, industrial alumina or aluminum hydroxide begins to form α-alumina at 1200°C in the absence of mineralizers. The conversion rate and the size of the α-alumina crystals increase with temperature under the same circumstances. For example, at 1300°C in a tunnel kiln, the conversion rate of a certain raw material is 90%, and the α-alumina single crystal is 0.7-0.8 um; while at 1520°C, the conversion rate can reach 96%, and the α-alumina single crystal can grow to 1.3 to 1.5 um.
Another example is the rotary kiln, whose holding time is relatively brief and is constrained by the technical conditions of the equipment itself. Under the same conditions, using aluminum hydroxide as the raw material, the conversion rate of α-alumina produced at a temperature of 1300°C is approximately 87%, and the α-alumina crystal is 0.6 um; at a temperature of 1420°C, the conversion rate of α-alumina is approximately 87%, and the primary crystal of α-alumina is about 1 um.
The α-alumina micro-powder (with a high α-phase conversion rate) that has been calcined at high temperature without shrinkage can make the castable basically not shrink and crack at high temperature, and can significantly improve the compressive strength of the castable (≥70MPa), thereby avoiding microcracks caused by high temperature shrinkage, which will lead to peeling.
Measures to Lower Phase Transition Temperature of α-Alumina Ceramics
Ball Milling
Many unique phenomena, including grain refinement, generation of defects within grains, occurrence of phase transformation, crystallization of amorphous states, etc., take place during the mechanical activation process known as ball milling. When Y-AlOOH is subjected to high-energy ball milling for 250 minutes, it will completely changed into Y-Al2O3. People can draw the conclusion that the goals of α-phase transition caused by heating and ball milling are identical.
Other research has demonstrated that ball milling does not convert the precursor into the α-phase at room temperature, but rather changes the precursor from one to another, or causes the ball milled precursor to undergo lower temperature calcination and turn into the α-phase. Additionally, it was discovered that the alumina precursor completely transformed into γ-AlOOH after being ball milled for 40 hours, and that when the precursor was calcined, the transformation temperature of the α-phase decreased by 300 °C.
Therefore, some believe that the release of strain energy stored by ball milling at high temperature can be regarded as a heating process; secondly, mechanical treatment such as ball milling will generate numerous nucleation sites in the powder, thereby increasing the nucleation density; and thirdly, ball milling can produce α-phase grains with extremely small particle sizes (about 3nm) in the precursor, which can be used as seed crystals to increase the density of nucleation.
Add Mineral Agent
Mineralizers are substances added to ingredients in order to facilitate or regulate the formation or reaction of crystalline compounds. Mineralizer can help with sintering and enhance some product qualities when added in small amounts. Alumina ceramics with a tiny quantity of magnesia as the mineralizer to suppress abnormal grain growth and maintain flexural strength. Additionally, the additional mineralizer can interact with the reactant to activate the crystal lattice, enhancing the responsiveness and speeding up the solid phase reaction.
In the absence of mineralizers, even though the calcination temperature is high and the time is long , the conversion rate is low, and the primary crystal is also small. The method of adding mineralizers is typically used in production in order to lower the conversion temperature and raise the conversion rate. Mineralizers like boric acid, ammonium chloride, and fluoride (such as aluminum fluoride, calcium fluoride, ammonium fluoride, etc.) are frequently used.
Mineralizers can be added to products to decrease the amount of Na2O present while also accelerating conversion and lowering conversion temperature. The effects of different mineralizers vary, so that the utilization and process performance of the obtained α-alumina products also vary significantly. Mineralizers can be used separately or in combination, and what needs to be addressed is how to regulate the residual amount of mineralizer in α-alumina products.
Introduce Seed Crystals
Utilize tiny Al2O3 particles to lower the temperature of α-phase transition by adding them to the precursor. The α-Al2O3 seed crystal, in general, has two effects on the transition kinetics of the transformation process from the transition phase to the α-phase. On the one hand, the seed crystal increases the nucleation density of the α-phase in the transition phase. On the other hand, the presence of α-Al2O3 seed crystals can considerably lower the α-phase's nucleation barrier.
From a crystallographic standpoint, the addition of seed crystals can also alter the powder's microstructure, reduce grain size, and control particle shape. A seed crystal particle creates an α-phase particle. When it grows up, it will collide with particles created by nearby seed crystals, preventing the formation of a "dendritic" hard agglomeration structure.
Besides, a special atmosphere is introduced during the calcination of the precursor to cause it to interact with the surface of alumina, thereby accelerating atom diffusion and promoting the α-phase transition.
Impurities in Alumina Ceramics
The performance of alumina can be affected by up to 14–15 different types of impurity elements, though not all of them are harmful. The main 4-5 types of impurity elements are Na2O, Fe2O3, SiO2, CaO, and MgO. Among them, sodium oxide impurities can have a significant impact on the dielectric loss of the sintered alumina ceramics, and an increase in sodium oxide content is typically accompanied by a significant increase in the tgδ value (tgδ is the tangent of the dielectric loss angle). Additionally, sodium peraluminate during the calcination process will influence the conversion speed and conversion rate of Al2O3, as well as thicken the alumina grains, decrease the specific surface area, and make the crystal shape irregular. What's worse , its presence lessens the refractoriness of ceramics and harms their electrical properties.
Because of the presence of Fe2O3 impurities, the temperature at which alumina ceramics sinter will become limited, the thermal performance of ceramics will decrease, the electrical breakdown strength and flexural strength decrease as well, and the whiteness of ceramics will be affected, etc.