Views: 0 Author: Site Editor Publish Time: 2023-02-02 Origin: 粉体圈儿
High-performance ceramic materials with high hardness, high strength, stable physical and chemical properties, widely used in semiconductor, new energy, aerospace and other fields. The excellent properties of ceramic materials are closely related to its forming methods, so it becomes a research hotspot in various fields. In 1947, Howatt proposed Tapecasting, a asting method for preparing ceramic sheets, which opened up a new way to asting ceramic materials.
Casting molding is to pour ceramic slurry with a certain viscosity into the tank. Under the traction of the film belt, the slurry is spread into a thin casting green with a scraper, and the solvent in the green is removed in the heating environment. After drying, the ceramic sheet is obtained. Compared with other forming processes, casting molding has the following advantages: simple process, high efficiency, large scale, strong continuity, has been widely used in various manufacturing industries and scientific research, such as ceramic substrate, multilayer capacitor and fuel cell film. Although casting molding has been widely used, there are many defects in ceramic sheet during its drying process. Therefore, it is necessary to summarize the influencing factors of convection forming.
This paper describes the study on the casting technology of ceramic sheet preparation. The properties of powder materials used in casting molding, the action mechanism of organic additives on slurry properties, and the effects of drying environment on the apparent properties of ceramic sheets were discussed. The application status of ceramic sheet is summarized. The future development trend of ceramic sheet is prospected. Powder is one of the most important raw materials in any molding technology. The performance of ceramic products often depends on the characteristics of the powder itself. The influence of ceramic powder on convection casting is mainly considered as follows:
① Size and morphology. The finer the powder, the greater the driving force of the sintering process, which is conducive to densification;
② Specific surface area. The specific surface area affects the interaction between powder and organic additives, thus determining the optimal content of organic additives.
③ Powder density. The D50 of high density powder is small to ensure that the particles are suspended in the slurry. Ojalvo et al. used a grain size of 0.7μm, a specific surface area of 20.7㎡·g -- 1, a density of 2.559g·cm -- 3, and a diameter of 1.5μm. The original morphology of whisker was not damaged after flow casting, and the whisker was arranged in a directional manner, which was conducive to its toughening effect. He et al. used nano-al2o3 as raw material to prepare transparent ceramic sheets by casting molding. The ceramic sheet has uniform texture, no deformation, and thickness close to 0.1mm. Through the above research found that the selection of ceramic powder is determined by product properties. After the powder is determined, the next step is to find suitable solvents, binders and plasticizers to prepare ceramic slurry suitable for casting. 2 Solvent selection casting molding is a fluid forming process, powder should be like a fluid with fluidity, so it is necessary to choose the right solvent, powder preparation into slurry. The solvent needs to meet the following three characteristics: dissolve the powder, homogenize the powder and other additives, evaporate quickly and without pollution. Water, ethanol, toluene, trichloroethane, acetone, or a combination of solvents are usually selected.
Because the drying speed of casting green billet is closely related to the production capacity, it is the best choice to choose the solvent with fast volatilization and instantaneous evaporation during drying. Si3N4 ceramics are widely used in complex structures because of their excellent mechanical properties and high temperature properties. For the preparation of complex structures, the casting technology is usually chosen. According to different solvent choices, Wei et al. used ethanol as solvent and Li et al. used water as solvent to prepare Si3N4 ceramic flakes. Water has the disadvantages of high evaporation temperature and poor drying characteristics, so many quality problems need to be solved. Ethanol solvent dries quickly and can produce ceramic flakes with high density and good apparent properties, but it has the disadvantage of polluting the environment. Therefore, suitable solvents should be selected for different production requirements.
3. In casting molding technology, the binder is the most important organic additive in the whole system. The binder constructs a three-dimensional network between the powders, providing strength and toughness to the ceramic sheets, allowing the sheets to be processed and retain their shape. Commonly used binders are polyethylene, polyvinyl alcohol, polyvinyl butyraldehyde, methyl cellulose, etc., the selected binder must be able to dissolve in solvent. Polyvinyl butyral (PVB) is one of the most commonly used binders. PVB is a long chain polymer molecule whose backbone consists of covalently bonded atoms such as carbon, oxygen and hydrogen. Attached to the backbone are side groups distributed along the length intervals of the molecule, which determine its solubility in ethanol. The long chain of polymer forms a spatial network structure in the ethanol solution, and the ceramic powder is wrapped in the cell of the network. After drying, the casting green with certain strength and plasticity is formed.
Tang Guowei et al. studied the influence of PVB on the density, mechanical properties, microstructure and sintering properties of ceramic casting green billet, and showed that the molecular weight of PVB is the main influencing factor. Feng et al. used PVB as binder to characterize the rheological behavior of grout. The viscosity of the slurry decreases with the increase of rotational speed, showing a typical shear thinning behavior. Xie Yupeng studied the relationship between the viscosity of SiCw slurry and the content of PVB. The viscosity of slurry increases with the increase of binder content. At the same time, Salam et al. found that the amount of binder added would affect the subsequent degreasing process. The more binder, the higher degreasing temperature and time is needed, and the lipid is not easy to completely remove, thus affecting the subsequent process. Liu et al. confirmed that the addition of excessive binder resulted in the reduction of green density of ceramic sheets. After high temperature sintering, the green billet produces too many holes, which leads to the increase of shrinkage of ceramic sheet. Therefore, it is very important to find the right amount of binder to add. However, the effect of the apparent morphology, processing characteristics and the porosity of the billet after degreasing on the product performance should be considered in the amount of binder addition.
4. Effect of Plasticizer on plastic properties Plasticizer is an additive that softens the binder in a dry or semi-dry state. Compared with binders, they are organics with lower molecular weight and soluble in the same solvent. The commonly used plasticizers are glycerin, polyethylene and dibutyl phthalate. The action mechanism of the plasticizer is to shorten or partially dissolve the main chain of the binder to achieve the purpose of reducing the conversion temperature of the binder glass, so that the long chain of the polymer can be extended or shortened without fracture, and the ceramic sheet shows a certain elastic behavior. At the same time, the addition of plasticizer can improve the interaction between polymer chains, avoid the cross-linking reaction between chain and chain, which is conducive to the migration of powder in ceramic sheet, so that the green ceramic sheet can be bent without breaking, which lays a foundation for the subsequent drying and processing process.
5. Under the traction of the film belt, the slurry is formed into a thin layer of slurry by the scraper. The drying process is to wait for the volatilization of the solvent. Due to the large amount of additives and solvents in the slurry, the raw ceramic sheet will show different drying behavior during the drying process. The drying process is mainly controlled by two factors: the rate of solvent evaporation on the green surface and the rate of solvent diffusion from inside the green to the surface. The evaporation rate is closely related to the drying environment, while the diffusion rate is related to the slurry system. Only when the two factors reach a relative balance, can the defect-free ceramic sheet be obtained.
6. After the ceramic sheet processing and molding is finished, the required shape shall be pressed or cut in the specified area in the direction of x and y. Stamping can be made into simple square or rectangular holes, while cutting can be made into a wide variety of other shapes by laminating the cut sheets. For example, a multilayer ceramic capacitor is a single solid ceramic body formed by sintering several layers of thin sheets. The basic parameters that must be controlled in the lamination are temperature, pressure and time. The selection of temperature mainly considers the glass conversion temperature after adding organic additives, as well as the thickness and quantity of the product lamination, so as to ensure the uniform heating of all ceramic sheets. The selection of pressure mainly considers the close contact between layers to meet the molding requirements, and the pressure will not deform the ceramic sheet and affect the final size.
7. After more than 70 years of development, the casting process is more and more widely used in the ceramic industry. For example: porcelain substrate, multilayer ceramic capacitors, fuel cells, functional gradient materials, especially in the electronics industry, are the use of casting molding method to prepare almost all the substrate materials.
The substrate is the carrier and pillar of the electronic circuit used to deposit and lay out the circuit. The common feature of these substrates is that they are very thin, usually 1.5mm or thinner. With the rapid development of 5G application, higher requirements will be put forward for the use of the substrate. Junction casting is a common method for preparing ceramic sheet because of its simple process and continuous production. However, the process is limited by many factors, and the content of organic additives is often determined by experience, rather than by the physical and chemical processes occurring on the surface of the particles and their interactions. Ceramic sheet is very sensitive to environmental variables, and the change of the variables in the process often affects the product performance. The thickness of ceramic sheet is not easy to be controlled, so the factors affecting the thickness of ceramic sheet should be carefully analyzed, and the thin sheet with uniform thickness and excellent quality can be prepared by means of finite element simulation. The application of casting molding paves the way for the wide application of electronic components and integrated circuits. The next research should be combined with the needs of engineering ceramics application, design of functional integrated materials with micro structure, porous structure and bionic structure.
