Hello everyone. In the previous three lectures, we talked about the ceramic powder processing technology. In this lecture, we are talking about the ceramic powder compaction and sintering technology because the most common form of ceramic materials are bulk form material. The uniaxial pressing, the die pressing is widely used ceramic powder compression technique. This technique is suitable for mass production of simple ceramic part. Major components of a simple dies include steel sleeve with containing cavity and collar with opened steel cylinder and pushing rod. This uniaxial pressing just included the filling of the die, compaction, and ejection as shown here. Sometimes we can use additives for uniaxial pressing such as binder, lubricant, and plasticizer in order to improve the properties of ceramic part. The packing at relatively low pressure makes the particles rearrangement into a higher packing order. Deformation. The particle cannot move into higher packing order, but we can obtain the primary densification. Rearrangement at highest pressure, we can obtain the particles that arranged into a higher packing order. Due to the frictional force during the uniaxial pressing it makes pressure gradient, and then makes defect structure such as lamination, end capping, and ring capping. Due to the relatively low packing density of uniaxial press, sometimes we can use this cold isostatic pressing. Isostatic means the applying pressure from multiple directions. We can use pressure mediums such as liquid and gas. Also, we need sample containers such as flexible mold, the collapsible bag, polyurethane mold. Then the final stage to make the bulk sample is sintering. Sintering is a process involved in the heat treatment of powder compacts at elevated temperature, by diffusional mass transport and this diffusion is much faster at higher temperatures, so we need high temperature for sintering. The powdered material is heated to a temperature below the melting point, normally two-third of the melting temperature. Then it provoke the diffusion of normally cation across the boundary of the particles. Then fusing the particles together and creating one solid piece. So successful sintering usually result in dense polycrystalline solid. So we can understand this sintering process as densification by heat treatment. Let's think about the thermodynamics of sintering. The sintering is an irriversible process because free energy decreased during the sintering due to the decrease in surface area of particles. The driving force for sintering is decrease in surface free energy of powdered compacts by replacing solid-vapor interfaces, Gamma_sv, with solid-solid interfaces, Gamma_ss. The change of system energy, Delta E due to the sintering is composed of the increase due to the creation of new grain boundary area, Delta A_ss, positive value, and due to the annihilation of vapour-solid interface, Delta A_sv, with negative value. So the necessarily thermodynamic condition for the sintering is negative value of Delta E. We can find two different types of sintering mechanism. The first is solid state sintering. It includes the initial stage, width formation of necks and intermediate stage with the evolution of necks and elimination of pores, and the final stages with isolation of pores as shown here. Another type of sintering mechanism is liquid state sintering. The mechanism is a small amount of liquid, typically less than a few volume percent of the original solid mixture is present at the sintering temperatures. The liquid phase allow enhanced densification at relatively low temperatures. Maximum use temperature is controlled by the softening temperature of liquid phase. The stage of sintering of liquid state sintering includes the initial stage which is related with, the heating, melting, particle rearrangement, and intermediate stage, which is related with solution and precipitation, contact flattening, grain shape accommodation, Ostwald ripening and coalescence. The final stage, which is related with the skeletal sintering and coarsening, reaction-controlled or diffusion-controlled grain growth. Typical liquid phase sintering has a liquid contents of a maximum 15 volume percent. When the liquid content is higher, at least 25 of volume percent, the vitrification problem can be occurred. The liquid volume is sufficient to fill the volume of the remaining pores. A dense product can be achieved by the formation of a liquid phase and flow of the liquid into pores, crystallization and vitrification of the liquid cooling can be occurred. The factors affecting liquid phase sintering are small particle, and viscosity, and surface tension, and temperature. The small size particle result in high surface area energy and makes high capillary pressure. Low viscosity is related with the good wetting, and high surface tension related with high capillary force. Another important sintering process is transient liquid phase sintering. As shown here, during the sintering phase, the solid phase bismuth antimony telluride and liquid phase tellurium can be found in the figure. Then during the sintering process, if we apply the pressure to this sample, we can eliminate the liquid phase. So this is the transient liquid phase sintering. In the presence of liquid phase during the sintering we can activate the grain rearrangement and as showed here, sometimes we can obtain this high density dislocation array along the grain boundaries. The potential defect structure after sinterings are intra-granular pores due to the sintering aids, and warpage due to the density variation from overfiring. Excessive grain growth due to overfiring, and cracking due to high heating and cooling rate. Sometimes we can find incomplete decomposition reaction after sintering. Then let's think about the factors affecting sintering. So they are packing density, and particle shape, and particle size distribution. Poor packing density results in high porosity. Irregular particle shape result in large irregular pores and the particle size is too large, this results in high porosity. If we use the fine particle, we can effectively lower the sintering temperature material. If we use the bimodal, particle size, we can improve the packing efficiency of sintered [inaudible]. There are two different types of sintering variables. The first is materials variables. The variables relate to raw materials including chemical composition of powder, compact, powders, size, powder shape, powder size distribution, degree of powder agglomeration, and influence the powder compressibility and sinterability. Another sintering variables is processing variables including thermodynamic variables, temperature, time, atmosphere, the pressure, and heating and cooling rate. Sometimes we need to fabricate the nanomaterial by sintering. So the bulk sintered material with nanoscale structure has the potential to offer nover or enhanced properties. So the physical property change in three-dimensional nanomaterials, so already discussed. Change in processing nanocrystalline powders, how can we obtain the fully densified bulk material without losing the initial metastable features, such as nanoscale characteristics, size, and metastable phases? Sintering process is driven by the tendency to reduce the excessively large surface area per unit volume. So extra energy of a surface with the radius of curvature R, can be expressed as a stress Sigma in a Laplace equation, which is given by Sigma equal Gamma over R. So based on this equation we can obtain the very large stress, about 300 megapascal in 10 nanometer particles, but only three megapascal in one micro particles. So we should develop some fast sintering technique in order to make three-dimensional nanomaterials. Thank you.
