Conventional alumina t ransparent ceramics are typically sintered at temperatures above 1700°C in a hydrogen atmosphere. High sintering temperatures cause grains to grow too quickly, which has a negative impact on the mechanical characteristics of ceramics and the hardness of materials. Alumina is a hexagonal material, so if the grain size is large enough (>410 um), there will be two optical axes, which will result in light scattering and a decrease in the light transmittance of ceramics due to the birefringence effect of the grains.
Alumina transparent ceramics made using conventional techniques generally have an in-line transmittance of less than 10%, which severely restricts their use. Therefore, increasing the light transmittance of alumina ceramics is crucial.
Material Selection of Alumina Ceramics
High Purity Raw Material Selection
Low-purity raw materials will have more impurities, second phases, and other different structural defects. The different refractive indices of various defects result in a substantial amount of light scattering and absorption, which drastically reduces the light transmission of ceramic materials. For transparent ceramics, the purity of the raw materials must typically be higher than 99.9% to prevent light loss from impurities.
Additive Selection
High-purity raw materials are frequently supplemented with additives (doped with trace elements, especially rare earth elements) to promote the sintering and densification of ceramics in order to achieve the lowest porosity. The primary function of the additives added to the raw materials is to concentrate at the grain boundary, preventing the crystal from growing too large during the sintering process and assisting in the reduction of pores. The mass transfer process from grain boundaries to pores can be sped up by using additives with cations that differ from those of the main sintered oxide and sintering in vacuum or hydrogen to encourage the degassing of the blank and increase the defectivity of the structure. Ceramics may also sinter in liquid phase as a result of the addition of additives, which lowers the sintering temperature.
Forming of Transparent Ceramics
For transparent ceramics, a combination of dry pressing and cold isostatic pressing is the most widely used molding technique. The green body is first compressed using dry pressing to achieve the desired shape and strength, and then further compressed using cold isostatic pressing to achieve greater density at a lower cost. New types of molding include gel casting and injection molding.
Gel molding, a type of fluid molding, is fundamentally distinct from dry pressing and cold isostatic pressing. It has drawn increasing attention because of its high level of dimensional accuracy, high level of green body strength, and capacity to create devices with intricate shapes. This method's drawback is that it requires debinding after molding.
Injection molding is a type of rapid precision molding that is also related to fluid molding. Generally speaking, the sintered parts can be produced automatically and do not need any additional processing. The injection molding green body has a high concentration of organic solvents, which necessitates a challenging debinding treatment prior to sintering (it's more difficult for large parts because they are more likely to crack). This technique is currently being used to create transparent ceramic arc tubes for high-voltage halogen lamps.
Sintering Methods of Alumina and Other Transparent Ceramics
Hot Pressing Sintering
By applying external pressure during the sintering process, hot pressing sintering encourages material densification in order to produce densified ceramics with fine grains. It is frequently used in transparent ceramics and has a lower sintering temperature than vacuum sintering. The drawbacks of this sintering method include the difficulty in producing devices with complex shapes, the high cost of the equipment needed, the small production scale, and the ease with which impurities and structural flaws can be introduced. Presently, transparent ceramics such as YAG, AIN, Lu2O3, and others have been prepared by this method.
Hot Isostatic Pressing
Hot isostatic pressing differs from cold isostatic pressing in that it heats the green body while applying equal pressure from all directions so that it can finish sintering under the combined action of high temperature and high pressure. In this process, gas is used as the pressurized medium. Hot isostatic pressing has a low sintering temperature and can produce parts with complex shapes or large sizes directly, but the machinery is complicated, expensive, and difficult to operate. It has currently been used to create transparent ceramics like alumina, yttrium oxide, and PNNZT.
Oxidizing Atmosphere Sintering
Sintering in vacuum or a reducing atmosphere is now frequently practiced, which is advantageous for removing pores from the sintering process and enhancing the densification of ceramics. Additionally, the equipment is simple and the cost of production is low. Currently, it serves as the primary sintering technique for transparent ceramics. A relatively recent sintering technique involves sintering transparent ceramics in an oxidizing environment. Defects, such as vacancies brought on by the volatilization of material components, can be eliminated by the oxygen atmosphere. In addition, oxygen will split into oxygen ions at high temperatures, creating a convenient pathway for pore removal, which can achieve the goal of increasing the density of ceramics.
The oxygen atmosphere sintering apparatus is comparable to the reducing atmosphere, but it is significantly safer. Currently, this sintering technique has been used to create transparent ceramics like alumina, YAG, and Y2O3.
Microwave Sintering
Another novel transparent ceramic sintering technique is microwave sintering, which has a quick heating rate, even heat distribution, and a short sintering time. This technique can quicken the densification of ceramics. It has been successfully used in the preparation of transparent ceramics like AlON, aluminum nitride, alumina, aluminum magnesium spinel, etc.
Spark Plasma Sintering
Spark plasma sintering achieves the activated sintering of ceramic powder particles by utilizing the discharge pulse pressure, pulse energy, and Joule heat produced by instantaneous pulse current. Plasma sintering is advantageous because of its rapid processing time, brief holding period, low sintering temperature(lower than hot isostatic pressing), uniform temperature distribution, high purity of the sintered body, and small grain size. However, the ceramics made using this method crack easily because of the extremely rapid heating rate. Presently, SPS can produce transparent ceramics such as MgO, YAG, ZrO2, MgAl2O4, Al2O3, and AIN.
Sintering Process of Transparent Ceramics
The densification of ceramics is greatly affected by the sintering temperature. Generally, the size of the ceramic grains increases with the increasing sintering temperature. As the sintering temperature rises, the grains expand significantly, the pores diminish, and the grain boundary thickness decreases.
The effect of the holding time is comparable to that of the sintering temperature. The density of the ceramics increases, the grains get bigger, the grain boundaries get cleaner, and the grain uniformity gets better as the holding time is extended. The holding time is related to the specific sintering method in order to get better performance. The holding time for transparent ceramic vacuum sintering typically exceeds 10 hours, whereas the holding time for plasma sintering typically falls below 30 minutes.
But in order for alumina transparent ceramics to have better light transmittance, the crystal grains must not be excessively large, and there must be as few pores as possible. Strict requirements are placed on the process design in order to balance the two factors.