Sintering is a crucial step in the production of alumina ceramics, significantly impacting their physicochemical properties. Common sintering methods mainly include the following types:
I. Atmospheric Pressure Sintering
This involves sintering green bodies under atmospheric conditions. This method requires high sintering temperatures, demanding high-performance kilns, and results in significant energy waste.
II. Hot Pressing and Hot Isostatic Pressing
Hot pressing sintering temperatures are much lower than atmospheric pressure sintering temperatures, and the theoretical density of the sintered product can reach 99%. Hot pressing sintering uses pre-forming or direct powder loading into molds, making the process simple. However, it is unsuitable for producing products with excessive thickness or complex shapes, resulting in small-scale production and high costs. Hot isostatic pressing avoids uneven sintering pressure, leading to a more uniform structure and more stable properties, making it suitable for producing alumina ceramics with complex shapes.
III. Liquid Phase Sintering
In this method, low-melting-point additives are used to promote the sintering of materials, and the introduction of additives usually produces good results. Calcium oxide, magnesium oxide, and barium oxide are often added as fluxes. Liquid phase sintering promotes diffusion through chemical reactions, viscous flow, particle rearrangement, and mass transfer, thereby lowering the sintering temperature and effectively accelerating sintering.
IV. Other Sintering Methods
(1) Atmospheric Sintering. That is, for products that are difficult to sinter in air, a certain amount of gas is introduced into the furnace to form the required atmosphere to prevent oxidation, and sintering is carried out in this atmosphere.
(2) Electric Field Sintering. That is, the sintering of alumina ceramic bodies under the action of a DC electric field.
(3) Ultra-High Pressure Sintering. That is, sintering is carried out at pressures of hundreds of thousands of atmospheres or higher.