Figure 2 - 3 The schematic diagram for processing overview of TGG process.
Figure 2 - 3 shows the schematic diagram for overview of TGG processes. Generally, the whole TGG process can be divided into the 4 steps, which should be carefully proceeded for successful fabrications of textured ceramics.
At the first step, to produce high-quality green sheets, the matrix powder must first be homogeneously produced for a homogenous composition. In second step, it is important to synthesis of template with high quality and uniform size through Molten Salt Synthesis (MSS) and topochemical reaction. The selecting a proper composition of templates for matrix system is one of critical point to make textured ceramics. It needs to consider more carefully for preparing textured ceramics that fit to intentions Texturing types are categorized into three groups, 1) TGG, 2) Reactive Templated Grain Growth (RTGG) and 3) Hetero-Templated Grain Growth (HTGG), that determined by the combinations of templates and matrix. To promote epitaxial growth of the matrix on the template in texturing process, the structure both of the templates and matrix system must be comparable crystallographically. The higher lattice mismatch between the matrix system and the templates result to lattice distortion of matrix system after the texturing.
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The third step is making a slurry for tape-casting. In general, the slurry is made with solvent, dispersant, binder, plasticizer, and calcined matrix powders, the mixture ratio of them should be optimized for matrix system since slurry conditions affect not only whole process from tape-casting to end process but also electrical and mechanical properties of textured samples. It is possible to control the slurry properties through adjusting or changing ratio of the components. Optimal conditions for slurry production should be adjusted to the properties of the calcined powder (e.g., composition, density, particle size, solubility, specific surface area and so on...). There are two kind of solvent systems. One is water-based system which is widely used in industry because it is suitable to mass productivity due to the cost effect and safety problem. The other is non-water-based system which is used in academia usually due to advantages of convenient manufactural processes. Most commonly used solvents are mixtures of two or more among ethanol, n-butanol, methyl ethyl ketone, toluene etc.
The solvent regardless of base system must be dissolve or disperse uniformly all the organics in slurry to avoid chemical inhomogeneity. Therefore, other additives must be prepared as water-soluble or water-insoluble substances depending on the solvent system.
The binders form a network in the slurry by binding all of the chemical components together. There are various type of binder which have different properties such as glass temperature (Tg), annealing temperature, mechanical elasticity, and the residual carbon ratio after the burnout process. The polyvinyl type and polyacrylate type are usually utilized for slurry. Generally, the polyvinyl-based binders in green body can be completely decomposed at a low temperature, 300 °C, in oxygen atmosphere due to their great mechanical elasticity. Almost no residual carbon remains after burnout process when the Polyvinyl butyral (PVB) based system applying. If residual carbon remains after the burnout process, it will adversely affect the sample properties. The advantages of acrylic binders are adjustable Tg and relatively lower decomposition temperature.
A role of dispersant is preventing the aggregation of components in slurry to avoid an inhomogeneity of green sheet after tape-casting. As a result of addition of dispersant, there is no intergranular aggregation of the particles in the slurry. There are two kind of dispersant according to dispersion mechanisms. The first type of dispersant uses the electrostatic mechanism which is mainly described dispersion behaviors in polar solvents. This mechanism is explained by the 'Boris Derjaguin and Lev Landau' (DLVO) theory which as the sum of van der Waals attraction and electrostatic repulsion. Here, the electric double layer produced by the dispersion of charged ions on the particle surface generates an electrically repulsive force, causing the particles to scatter without agglomeration. The second type of dispersant uses steric hindrance mechanism which is usually used to explain dispersing behavior in non- aqueous nonpolar solvents. Consequently, the powder in slurry is dispersed uniformly without aggregation through the steric hindrance due to polymer chains on the surface of particle.
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The plasticizer be able to induce the elasticity to green sheets which need a proper level of tensil strength to ease for handling. Processes after tape-casting such as a cutting, hole punching, lamination of green sheet, screen printing of electrode can be performed easier if the green sheets have an elasticity by plasticizer. The plasticizer also has an ability to dissolve the binder, loosening the organic chain and improving the flexibility of green sheets and increasing the viscosity of slurry. on the other hand, excessive amount of plasticizer more than necessary cause negative effect on both of slurry and green sheets to have vulnerable mechanical properties.
The fourth step is tape-casting, which is the best technique for aligning textured ceramics. Commonly, the tape-casting method is used for making multilayer ceramics devices such as Multilayer Actuators (MLA), Multilayer Ceramic Capacitor (MLCC) by stacking green sheets which is a single ceramic layer having tens to hundreds of micrometer scale. The TGG process is a tape-casting process with templates applied. The templates are single crystal with micro-scale having plate-like shape and they are aligned one direction by shear stress when they pass through the narrow tunnel. After the casting, we can get the green body through the cutting, stacking and lamination. Finally, the epitaxial growth take plate around template during the sintering.
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