Preparation of slurry with low viscosity and high solid content
The preparation of stable slurry with low viscosity and high solid content is the key to the preparation of high performance ceramic parts. The solid content of the slurry determines the bulk density of the green billet, and the viscosity of the slurry determines whether it is formed, so it is very important to study the preparation and performance of the slurry.
The content of dispersant is one of the key technologies for preparing ceramic slurry with high solid content. The addition of dispersant is to achieve high solid phase slurry through the stabilization effect of electrostatic repulsion and steric hindrance. The dispersant has a strong affinity for ceramic particles and a high compatibility with the solution. It can fully stretch to form a steric hindrance stable layer, and can ionize ions to produce electrostatic repulsion.
Schematic diagram of steric hindrance stabilization mechanism
The viscosity of ceramic suspension is affected by the content of dispersant and solid. Too little or too much dispersant will cause the stability of the suspension to deteriorate. Only when the appropriate amount is selected to make the particle surface in the saturated adsorption state, the dispersion is the best and the suspension stability can be made. With the increase of solid content, the viscosity of ceramic suspension will also increase. Determining the optimal solid content is the key to the successful preparation of low viscosity slurry.
1. Effect of solid content on viscosity of slurry
The pH value also has great influence on the stability of the slurry. Different pH values and different charging conditions on the surface of the powder will change its Zeta potential. The change of Zeta potential directly affects the electrostatic repulsion between particles, thus affecting the stability of the slurry.
Particle size also has an effect on slurry viscosity. When the particle size decreases, the corresponding specific surface area increases, and the total amount of water adsorbed will increase. Therefore, the free water in the slurry will decrease, and the slurry viscosity will increase. In addition, the finer the powder, the easier it is to agglomerate, and the agglomerated particles wrap water, resulting in a part of the water loss, which is also an important factor in the increase of slurry viscosity.
2. Safe drying of ceramic body
In the process of drying, the water evaporates quickly and the shrinkage rate is high, so the body is easy to crack. In addition, the gelatinized body drying process is a complex and slow process, especially the large size of the body, more prone to non-uniform drying shrinkage caused by the concentration of structural stress and residual stress, resulting in deformation of the body, warping and other problems.
Drying conditions of the injected zirconia billet in the PEG10000 solution (a) and in the air (b)
The safe drying of the body is very important to solve the problems of dimensional accuracy and the formation and propagation of cracks. According to the different ways of obtaining heat energy from the evaporation of water in the body of the billet, the drying methods can be divided into hot air drying, electric heating drying, radiation drying, freeze drying, etc., and new liquid drying.
Hot air drying: the equipment is simple, easy to operate, but the thermal efficiency is low, the drying cycle is long, the drying process is not easy to control, and it is easy to crack, warping and so on, especially in the case of large green parts.
Electric heating drying: the drying speed is fast, because the high water content in the billet current is large, drying fast, whereas the low water content of the place drying slowly, the overall drying process is very uniform.
Radiation drying: does not need any drying medium, to ensure the clean body; Simple equipment, easy to operate, easy to automate; The drying speed is fast and the drying is more uniform.
Liquid drying: moisture can be evenly precipitated to avoid uneven drying and cracking; The body is completely immersed in the liquid, without capillary force, reducing warping and cracking.
3. The mechanical processing of the body
Most of the ceramics have high hardness, high strength performance characteristics, resulting in it is not easy to deformation; In addition, the brittleness of ceramics makes it difficult to process. Ceramic finishing is completed by micro-deformation or removal of material at the processing point, there are a variety of processing methods, but the main processing method is mechanical processing.
The injection type can produce a complex shape, high density, good uniformity, relatively high strength, and a certain plasticity of the body, which meets the requirements of green mechanical processing, suitable for industrial production, and can accurately control the size and shape of the green, reduce the processing amount of green, processing difficulty and waste of raw materials. Thus, the processing amount of sintered products and the defects and crack propagation caused by the processing process are reduced, the processing safety is improved, and the reliability of ceramics is improved.
Injection molding has been widely studied in the field of porous materials, composite materials and functional materials. As a new near net size molding process, it has many advantages that traditional grouting molding cannot match. In the future, research on the preparation of low viscosity and high solid phase volume fraction suspended slurry, improvement of the existing gel system, search for efficient and non-toxic new gel system, open up new application fields, accelerate industrial production and the development of automatic control equipment will continue to be the focus of the injection condensation research.