Tape casting, also known as belt casting method and doctor blade method, is an important molding process for thin ceramic materials. The process was first used by GN. Howatt is used in the field of ceramic forming and has been used in the production of single-layer or multi-layer thin-sheet ceramic materials since it was patented in 1952. The tape casting molding equipment is relatively simple, the process is stable, the continuous operation is possible, the production automation is convenient, and the production efficiency is high. The disadvantage of tape casting is that the binder content is high, so the shrinkage rate is large (up to 20%).
At present, this process is a relatively mature molding method that can obtain high-quality, ultra-thin ceramic products, and has been widely used in the production of monolithic capacitor ceramics, thick-film and thin-film circuit substrates and other advanced ceramics.
The Preparation Of Casting Slurry
Since the casting molding method is mainly used to manufacture ultra-thin products, the selection of ceramic powder, solvent, dispersant, binder and plasticizer in the preparation process of casting slurry Very important. Selecting suitable ceramic powder and additives to prepare casting slurry can obtain casting green body with good quality. Because in this way, the slurry can have good fluidity, and at the same time, a certain number of powder particles can be kept in the thickness direction. For example, when making a thin billet with a thickness of 40 lim, the stacking number in the thickness direction generally requires more than 20 particles, then the powder with a particle size of less than 2 μm is required to account for more than 90% to ensure the quality of the thin billet. Therefore, the casting method usually uses micron-sized particles. The main factors to be considered in the selection of solvents are:
- Dissolving dispersants, binders and plasticizers;
- Maintaining chemical stability in the slurry and not chemically reacting with the powder;
- Providing suitable materials for the slurry Viscosity;
- It can be evaporated and burned off at a proper temperature;
- It is safe and hygienic to use and has less pollution to the environment and is cheap.
In practice, it is very common to use mixed solvents in the choice of solvents.This not only facilitates greater applicability of the various organic components, but also because the polymer is more soluble in the optimal mixture of solvents than in any one liquid alone. Of these, mixtures of “kinetic solvents” (small molecules such as ethanol) and “thermodynamic solvents” (esters, ketones, etc.) are the most effective, since their use can reduce the amount of necessary solvent (excess solvent can cause excessive shrinkage) rate, thereby causing cracking of the formed body).
In general, azeotropic mixtures of solvents are suitable for use in organic binder systems, they provide good solubility characteristics and the solvent evaporates with constant composition. Non-azeotropic solvent mixtures have different evaporation rates and are capable of continuous evaporation. Their variety of boiling problems can impart variability in the drying conditions of the green bodies, and are therefore often used as solvents for casting slurries. The dispersing effect of the dispersant is the key to the success or failure of the casting film.
The dispersion uniformity of ceramic powder in the casting slurry directly affects the quality of the green film and its sintering characteristics, thereby affecting a series of characteristics of the sintered film. There are four types of dispersants commonly used in casting film production: anionic, cationic, nonionic and zwitterionic. Generally speaking, anionic surfactants are mainly used for neutral and weakly alkaline slurries with positively charged particle surfaces, while cationic surfactants are mainly used for neutral or weakly acidic slurries with negatively charged particle surfaces. The main function of the binder is to solidify itself to form a three-dimensional interconnected network skeleton by wrapping the powder particles, giving the green body a certain strength and toughness. Factors to be considered when choosing a binder are:
- Thickness of the green body;
- Compatibility with the selected solvent (whether it prevents the solvent from volatilizing and does not produce bubbles);
- Easy to burn off without leaving residues;
- It should be low The plastic transition temperature to ensure that no condensation occurs at room temperature;
- Consider the nature of the gasket material used, whether it is sticky and easy to separate.
The addition of plasticizer is mainly used to adjust the plastic limit temperature. Because the binders are cross-linked, the plastic limit temperature is often higher than room temperature. That is to say, the binder can ensure the strength of the china at room temperature, but it cannot make the china tough enough for machining. The addition of plasticizer can adjust the plastic limit temperature, make it close to or lower than room temperature, and improve the toughness of the green body. However, the addition of plasticizers also reduces the strength of the green body.
Therefore, the selection of plasticizers should be carried out on the premise of ensuring the strength of the china. It is required that the selected plasticizer has as little content as possible, is easy to burn off, has stable chemical and physical properties, is cheap, and is non-toxic. Based on the above principles of raw material selection, suitable raw materials are selected for the preparation of casting slurry. The typical slurry formulation and technological process are shown in Table 4-8 and Figure 4-11.
Introduction To The Tape Casting Process
Since its appearance, tape molding has been continuously developing both in terms of theory and technical equipment. The molding process has developed from the original non-water-based tape casting process to the current water-based Tape molding I process and some new molding processes derived therefrom, such as tape casting isostatic pressing composite molding process, UV-initiated polymer molding process and gel casting molding process. The maturity of traditional technology and the research and development of new technology have made the tape casting process more perfect and promoted the further application of this technology in the field of ceramics.
Non-aqueous-based tape casting process
Non-aqueous-based tape-casting is the traditional tape-casting method, and the molding process is as follows: adding a binder, a plasticizer, and a dispersant to the prepared powder. , solvent, etc., and then mix them evenly. Then put the slurry into the hopper of the casting machine, and the slurry flows from the lower part of the hopper to the film carrier (conveyor belt) of the casting machine, and the thickness of the film is controlled by a scraper. The prepared film is dried to obtain a film blank. The film blanks are spooled together with the carrier ready for use, or processed as required. At present, the traditional non-aqueous-based tape casting has been relatively mature and has been widely used in the field of ceramics, such as the preparation of A12 03 ceramic membranes, the preparation of Al membranes, and the preparation of YSZ thin films by means of tape casting.
Water-based tape casting process
The disadvantage of the traditional non-water-based tape casting process is that the organic solvent used has a certain toxicity, which deteriorates the production conditions and causes environmental pollution, and the production cost is high. In addition, due to the high organic content in the slurry, the green density is low, and it is easy to deform and crack. In view of the above shortcomings, people began to try to replace the organic solvent system with water-based solvent system.
Water-based tape casting
The process uses water-based solvents instead of organic solvents, so the selection of additives in the process of casting slurry preparation is different from the traditional non-water-based tape casting process. Since water molecules are polar molecules, and binders, plasticizers and dispersants are organic additives, there is a problem of compatibility with water molecules. Therefore, in the selection of additives, it is necessary to choose water-soluble or can form in water. The organics of the emulsion are stabilized to ensure a uniform and stable slurry. Under the premise of maintaining the stable suspension of the slurry, the dispersant should be used as little as possible; the organic substances such as binder and plasticizer should be used as little as possible under the premise of maintaining the strength and flexibility of the green body. Water-based tape casting has the advantages of low price, non-toxicity, non-flammability, etc., but there are also problems such as low solvent evaporation rate, high binder concentration, agglomeration of ceramic powder caused by oxygen bonds, slurry sensitive to changes in process parameters, green body Defects such as drying and easy cracking and deformation.
Ultraviolet-initiated polymerization casting molding
Art Based on the shortcomings of water-based casting molding, people have improved the molding mechanism. T. Chariier et al. applied the mechanism of UV-initiated in-situ polymerization, adding UV-sensitive monomers and UV-light polymerization initiators to the slurry to initiate a polymerization reaction during molding, so that the slurry can be cured in-situ to achieve the purpose of molding, thereby eliminating the need for the most Complicated and most likely to lead to molding failure drying I: art. The slurry used in UV-initiated polymerization tape casting is composed of ceramic powder, dispersant, photosensitive monomer and initiator, and can be used for molding after being mixed by ball milling. In order to maintain the viscosity of the slurry required for tape casting, the slurry can be heated above 50°C.
In order to maintain good fluidity of the slurry, photosensitive monomers with lower viscosity should be selected. Acrylic resin has high sensitivity to ultraviolet light and is easy to initiate photopolymerization. Its viscosity is low, it is suitable for preparing ceramic slurry with high solid content, and it is beneficial to improve the density and strength of green body. After the prepared ceramic slurry was cast and formed into a film, under the irradiation of a certain intensity of ultraviolet light, the initiator initiated the polymerization of the photosensitive monomer. A network structure is formed in which the ceramic particles are fixed and combined into a green body with a certain strength. Compared with the traditional tape casting process, the biggest feature of the UV-induced polymerization tape casting process is that it does not use solvents, so it does not require time-consuming and complicated drying procedures, which can avoid drying shrinkage and cracking, and improve the production crystallization rate.
The disadvantage of this molding T process is that the entire process needs to be maintained at a temperature above 50 C to ensure the necessary fluidity of the slurry, which brings certain inconvenience to the operation. In addition, the high intensity of UV light during polymerization can also be harmful to the human body.
Gel casting molding process
The gel casting molding process uses the polymerization principle of organic monomers to carry out casting molding. The method is to disperse ceramic powder in an aqueous solution containing organic monomers and cross-linking agents to prepare a suspension with low viscosity and high solid volume fraction, and then add initiators and catalysts to initiate organic reactions at a certain temperature. The monomers are combined to increase the viscosity of the suspension, which leads to in-situ solidification and molding, and a green body with a certain strength that can be machined is obtained. The slurry used in gel casting is prepared from ceramic powder, organic monomer, cross-linking agent, solvent, dispersant, plasticizer and other components. The advantage of the gel tape casting process is that it can greatly reduce the amount of organic matter used in the slurry, increase the solid content of the slurry, and thus improve the density and strength of the green body. In addition, the process also has the advantages of less pollution and low cost. At present, the gel casting process has been applied to the development of ceramic products such as sheet ceramics and fuel cell YSZ.
Casting isostatic pressing composite molding process
Casting isostatic pressing composite molding process is a new type of molding process developed on the basis of non-water-based and water-based tape-casting molding techniques. The solid phase content of the casting slurry is low, and the relative density of the prepared china is low. Although the solid content of the slurry and the density of the green body can be improved by increasing the particle size, the sintering performance of the powder particles will decrease if the particle size is too large. In addition, during the drying process of the china, many pits and cavities are left on the surface and inside due to the volatilization of the solvent, which makes the china structure loose. The small thickness of the cast green also determines that it cannot use some unconventional sintering methods (such as hot pressing sintering), and a large number of organic additives are burned off during the sintering process, so it is difficult to obtain a dense cast sintered film. The tape casting isostatic pressing composite molding process effectively combines the tape casting process and the isostatic pressing process, that is, the isostatic pressing secondary molding is performed on the green body with lower density but better ductility to improve the molding density of the green body. , thereby increasing the density of the sintered membrane. The molding process is relatively simple, and is easy to industrialize the production of the ceramic membrane. Chen Ming et al. prepared YSZ films by using the I: process of cast isostatic pressing, and achieved an increase of 8% – 11% in the density of the green body and 5% – 10% in the density of the sintered body.
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