Zirconia ceramics that are only black and white are obviously unable to satisfy people's sensory needs, so how to make them"colorful" has drawn attention in the ceramics industry. Zirconia is said to be more valuable because it is "colorful."
The key to preparing colored zirconia is to evenly distribute the coloring hue (consisting of oxides of transition metals or rare earth metals or other compounds, such as CoO, Cr2O3, Fe2O3, etc.) throughout the ceramic matrix. However, nanopowders are susceptible to agglomeration due to their small particle size, large surface area, high surface energy, capillary force, electrostatic attraction, and van der Waals force between particles. This reduces the physical and chemical properties of ceramics. Therefore, a suitable dispersion method must be used in order to prepare zirconia ceramics with good performance and a variety of colors.
Commonly-Used Preparation Methods of Colored Zirconia Ceramics
Solid Phase Mixing
The most widely used technique at the present time is mixing ball milling technology to create colored zirconia powder. According to a specific chemical ratio, stable zirconia nanometer powder is combined and ball-milled with oxide particles like coloring agents and mineralizers. This process refines the solid particle grains, resulting in phenomena that are favorable for low-temperature chemical reactions, such as microcracks, lattice distortion, and surface energy increase. This method benefits from easy industrialization, low cost, convenient operation, and simple process, etc.
The flow chart of the solid phase mixing method is as follows:
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The biggest limitation of the solid phase mixing method is that it cannot overcome the phenomenon of nanoparticle agglomeration. The coloring phase and matrix nanoparticles are mixed unevenly. Additionally, the ball milling time is lengthy and the ball milling medium or the atmosphere may cause serious pollution to the powder. Furthermore, particle diffusion influences the solid phase reaction rate. It is challenging to fully carry out the coloring reaction of the coloring agent in the zirconia matrix because of the large mass transfer distance between the coloring reactants. This makes it difficult to control the coloring stability and causes chromatic aberration.
In addition, the high-temperature heat preservation time of this method is lengthy, the energy consumption is high, and the expensive colorant vaporizes, resulting in serious waste; during the drying process following ball milling, phenomena such as agglomeration and sedimentation of dispersed particles occur. Consequently, electrolyte dispersants or ultrasonic dispersion techniques are typically used on the basis of the solid-phase mixing method to prevent the aforementioned phenomena of powder particles and improve their dispersibility.
Chemical Co-Precipitation
Chemical co-precipitation entails combining zirconium salt, stabilizer salt, and coloring ion salt solution, reacting with alkali or carbonate, etc. to form hydroxide or carbonate precipitation, and then heating and decomposing them to obtain zirconia composite. The process flow of this method is depicted in the figure below:
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Although the chemical coprecipitation process is fairly complex, the powder that is produced is highly pure and performs superbly. The disadvantage of this method, however, is that the colored zirconia co-precipitated ions are complex, leading to complex reactions in the subsequent sintering process, and the zirconia stabilizer may have unanticipated reactions with the color ions, resulting in high consumption. In addition to affecting the color rendering optical properties of the colored ions, it also has an impact on the performance of the finished colored zirconia product.
The formation of hard agglomerates is another issue that must be taken into consideration when using the chemical precipitation method. To prevent this phenomenon, the following measures can be implemented: reduce the introduction of impurity ions; wash the precipitated product with alcohol, acetone, and other organic solvents to prevent powder agglomeration; in the drying stage of the powder, typically use supercritical drying and freeze-drying methods.
Liquid Phase Impregnation Method
The liquid phase impregnation method is capable of well achieving high uniformity doping, and it can endow the surface with new properties while maintaining the properties that were present in the material when it was initially created. The diagram below depicts the process flow:
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In order to create a green body with a connected pore structure after injection molding, the green body is first subjected to water extraction and degreasing. After that, it is placed in a solution containing coloring phase ions for impregnation. Along with the solution, the coloring phase ions move from the surface of the green body into its interior through the pores. The impregnation depth is regulated by the impregnation time, and a variety of colored zirconia ceramics with superior properties are eventually produced.
In order for the colored phase ions in the solution to slowly penetrate from the surface to the inside and then distribute evenly within the matrix, the impregnation matrix required by the liquid phase impregnation method must have a pore structure containing internally connected surface openings. The liquid phase impregnation method is more process-friendly than the conventional preparation method, and the colored zirconia ceramics produced by this method exhibit superior color uniformity and physical properties. Additionally, this method fully takes advantages of injection molding, allowing it to create a variety of colored zirconia ceramics with complex shapes by producing corresponding zirconia green bodies.