Preparation method of porous ceramic filters
1. Organic (polymer) foam impregnation method
The organic foam impregnation process was invented by Schwartzwalder in 1963.
It is an impregnating ceramic slurry with organic foam and drying and burning the organic foam to obtain porous ceramic filters. Its uniqueness lies in that it relies on the special structure of the open-cell three-dimensional mesh skeleton of the organic foam body to evenly coat the prepared slurry on the organic foam mesh body. After burning the organic foam, a mesh-type pore is obtained.
The pore size of the final product mainly depends on the pore size of the organic foam body. Also, it has a certain relationship with the coating thickness of the slurry on the organic foam body. Still, the pore structure of the mesh porous body is almost the same as that of the organic foam matrix, presenting an open-cell three-dimensional mesh skeleton structure.
This special structure gives it significant advantages as a filter material:
(1) Small pressure loss when passing fluid. (2) Large surface area and high fluid contact efficiency. (3) Lightweight.
When this type of porous ceramic filter is used for fluid filtration, especially molten metal filtration, it is not only simple to operate, saves energy, and reduces costs, but also has higher filtration efficiency compared to the traditional ceramic particle sintered body and glass fiber cloth.
(1) It must be an open-pore mesh material to ensure that the ceramic slurry can penetrate freely and adhere to each other so that a porous three-dimensional network skeleton can be formed after firing;
(2) It must have a certain hydrophilicity and can firmly adsorb the ceramic slurry
(3) It should have sufficient resilience to ensure that it can quickly rebound and restore its shape after squeezing out excess slurry;
(4) It should volatilize at a temperature lower than the ceramic firing temperature and will not pollute the ceramic.
The ceramic powder is selected according to the different uses of the porous ceramic material it develops.
For example, cordierite, alumina, Al2O3, and Al2O3CrO3 series materials are generally selected for filtering nonferrous metals such as aluminum(alumina ceramic foam filter), copper, zinc, tin, and other low melting point alloys;
When smelting ferrous metals and their alloys, silicon carbide is used as a high-temperature filter due to the high chemical activity and casting temperature;
The use of partially stable zrO2-Al2O3 series materials and magnesium-aluminum series filter materials to filter molten steel can effectively reduce the inclusion content in the molten steel;
The use of cordierite-mullite composite as a ceramic filter can filter diesel engine exhaust particles.
In short, choosing the right ceramic powder is very important for preparing high-performance and practical mesh porous ceramic materials.
The slurry is mainly composed of ceramic powder, solvent, and additives.
Generally, water is used as a solvent, but organic solvents such as ethanol are also used.
In addition to the properties of general ceramic slurry, the slurry also needs to have as high a solid content as possible (the water content is generally 10~40wt%, and the slurry specific gravity is 18~2.2g/cm3) and good thixotropy. The high-performance slurry is not only conducive to molding but also plays an important role in ensuring the performance of the product. At the same time, to obtain a slurry that is more suitable for impregnation molding, a certain amount of additives must be added. The additives generally consist of the following:
(1) Binder. Adding a binder to the slurry for preparing mesh porous ceramics not only helps to improve the strength of the green body after drying but also prevents the body from collapsing during the removal of organic matter, thereby ensuring that the final sintered body has sufficient mechanical strength. Binders are divided into inorganic binders and organic binders. Commonly used inorganic binders include potassium, sodium silicates, borates, phosphates, aluminum hydroxide sols, and silica sols. The type and characteristics of the binder have a great influence on the performance of the product. Selecting a suitable binder is very important for improving the performance of mesh porous ceramics. For example, the use of aluminum hydroxide gel as a binder to prepare porous alumina greatly improves the filter's resistance to molten aluminum corrosion.
(2) Dispersant. To increase the solid content of the slurry, a dispersant must be added to both the water-based system and the non-water-based system. The dispersant can improve the stability of the slurry, prevent the particles from reaggregating, and thus increase the solid content of the slurry. However, it should be noted that the effect of the dispersant is generally different for different powder systems. For the water-based system of Al2O3, only 25wt% of polymethacrylic acid amine has a good dispersing effect; for the water-based system of SiC, PEI (polyethyleneimine) is ideal as a dispersant. In short, choosing a suitable dispersant is an important way to increase the solid content of the slurry.
(3) Rheological agent. According to the molding characteristics of this process, the slurry is required to have not only a certain fluidity, but also good thixotropy, that is, the slurry is required to be in a solidified state when stationary, but restores fluidity under the action of external force. The fluidity of the slurry ensures that the slurry penetrates the organic foam during the impregnation process and is evenly coated on the pore walls of the foam network. The thixotropy of the slurry can ensure that when the slurry is impregnated and the excess slurry is squeezed out, the viscosity is reduced under the shear action, the fluidity of the slurry is improved, and it is helpful for molding. At the end of molding, the viscosity of the slurry increases and the fluidity decreases. This makes it easy for the slurry attached to the pore wall to be rounded and fixed, to avoid serious pore blockage in the green body due to the flow of the slurry, which affects the uniformity of the pores of the product. To improve the rheological properties of the slurry, especially the thixotropy, the added rheological agent usually includes natural clay (0.1~12w%) such as bentonite, kaolin, and carboxymethyl cellulose.
(4) Surfactant. If the ceramic slurry is a water-based slurry, if the wettability between the organic foam precursor and the slurry is poor, a thicker slurry will appear at the cross-section of the foam structure when the slurry is impregnated. In severe cases, it will cause the green body to crack during sintering, significantly reducing the strength of the porous ceramic, or even collapse. Therefore, the method of adding surfactants is usually used to improve the adhesion between ceramic slurry and organic foam to solve this problem, such as adding PEI (polyethyleneimine).
(5) Defoamer. To prevent the slurry from foaming during the impregnation squeezing out excess slurry and affecting the performance of the product, a defoamer is required, generally a low molecular weight alcohol or silicone. The organic foam must be repeatedly squeezed to remove air before impregnation with the slurry, and then the slurry is impregnated. The methods include the normal pressure adsorption method, vacuum adsorption method, machine rolling method, and manual kneading method. Regardless of the impregnation method, the slurry is required to be fully coated on the organic foam. After the organic foam is impregnated with the slurry, the excess slurry needs to be removed. The key to this step is the uniformity of the extrusion force, which not only removes the excess slurry but also ensures that the slurry is evenly distributed on the network pore wall to prevent clogging. The bulk density of the green body after molding is more suitable in the range of 0.4~0.8g/cm3. Large-scale production should be completed using equipment such as centrifuges or roller mills.
The green body obtained by squeezing out the excess slurry needs to be dried, which can be done by shade drying, drying, or microwave drying. It can be fired when the moisture content is below 1.0wt%. To shorten the production cycle, it is generally required to formulate a reasonable drying system.
When formulating the sintering curve, it is necessary to consider both the sintering performance of the product and the economy of the sintering cycle, especially the industrialization of the process.
The sintering process is divided into two important stages, namely the low-temperature stage and the high-temperature stage.
In the low-temperature stage, if the temperature rises too fast, a large amount of gas will be generated in a short time due to the violent oxidation of organic matter, causing cracking and powdering of the green body. For larger parts, to prevent the green body from cracking during the sintering process, the slurry formula can be adjusted to optimize the slurry performance and increase the thickness of the slurry on the organic foam mesh. It is very important to choose a suitable binder to improve the high-temperature sintering strength of the green body. The problem of not burning through can generally be solved by extending the insulation time (1~5 hours) and using appropriate pads to increase the heating surface. The addition of sintering aids also has a significant impact on the sintering process.
The sintering of ceramics is divided into garden phase sintering and liquid phase sintering. The common point between the two is that the driving force of sintering is surface energy, and the sintering process is composed of stages such as particle rearrangement, pore filling, and grain growth. The difference is that since flow mass transfer is faster than diffusion mass transfer, the liquid phase sintering has a high densification rate, which can make the green body obtain a dense sintered body at a much lower temperature than solid phase sintering. In solid-phase sintering, a small amount of sintering aid can form a garden solution with the main crystal phase to promote the increase of defects. If a small amount of sintering aid is added to produce a liquid phase, liquid phase sintering is formed, which can greatly promote the sintering process.
Its functions are as follows:
(1) Forming a solid solution with the sintering phase: When the ion size, lattice type, and valence number of the additive and the sintering phase are close, they can form a solid solution, causing the main crystal phase lattice distortion and defect increase, which is convenient for diffusion mass transfer to promote sintering. Generally speaking, the formation of a limited solid solution between them can promote sintering more than the formation of a continuous solid solution. The greater the difference between the charge and radius of the additive ions and the charge and radius of the ions of the sintering phase, the greater the degree of lattice distortion and the more significant the effect of promoting sintering. Selecting an additive with a radius similar to that of the positive ions of the sintered body but a different charge can form a vacancy-type solid solution, or selecting positive ions with a small radius to form an interstitial solid solution to cause lattice distortion, increase activity, and thus promote sintering. For example, in the sintering of Al2O3, adding a small amount of TiO2 or CrO2 can promote sintering. Adding 3wt% CrO2 to Al2O3 forms a continuous solid solution, which can be sintered at 1860℃ while adding 1~2wt% Tio2 to Al2O3 can densify at 1600℃. This is because CrO2 and Al2O3 have similar positive ion radii and the same electric charge, so they can form a continuous solid solution, while Ti and AP have different electric charges. After substitution, positive ion vacancies will be generated, and Tri4 may be transformed into T with a larger radius at high temperatures, aggravating lattice distortion, increasing the activity of Al2O3, and more effectively promoting sintering.
(2) Crystalline transformation of the structure: Some oxides undergo a crystallographic transformation during sintering, accompanied by a large volume change, which can easily cause cracking of the green body and make sintering densification difficult. Selecting appropriate additives to inhibit this can promote sintering.
(3) Inhibit grain growth: Grain growth in the late stage of sintering plays an important role in sintering densification. However, if the secondary recrystallization or intermittent grain growth is too fast, the grains will become coarser and the grain boundaries will become wider, resulting in anti-densification and affecting the microstructure of the product. The addition of additives can inhibit abnormal grain growth and promote the sintering process. When sintering Al2O3, MgO, or MgF2 is generally added to inhibit secondary recrystallization. At high temperatures, MgO will react with AlO3 to form spinel (MgO. Al2O3) distributed between AlO3 particles, inhibiting grain growth and promoting densification.
(4) Generate liquid phase: If there is a suitable liquid phase during sintering, it will greatly promote particle rearrangement and mass transfer. The addition of additives can make the green body produce a liquid phase at a lower temperature to promote sintering. The reason for the appearance of the liquid phase may be due to the low melting point of the additive itself, or it may be that it forms a multi-element eutectic with the sintered material. When a small amount of CaO and SiO2 is added to the sintering of Al2O3 (at a molar ratio of 2.5-1.0), the material can be sintered at a lower temperature due to the formation of CaO-AlO3-SiO2 liquid phase. This is the latter case. Adding CeO2 to the sintering process of cordierite ceramics can promote the formation of the liquid phase, accelerate the formation rate of α-phase cordierite, and promote the transformation of the intermediate phase to cordierite.
(5) Expanding the sintering temperature range: Adding an appropriate amount of additives can also expand the sintering temperature range of ceramics, which is beneficial to the temperature control of the firing process. This is because the additives produce vacancies in the lattice, which is beneficial to the densification of the ceramic blank and widens the upper limit of the sintering temperature.
It should be noted that additives can only promote sintering when added in appropriate amounts. If the additives are not selected properly or added in excessive amounts, the sintering process will be hindered, because excessive additives will hinder the direct contact of the sintering phase particles and affect the mass transfer process. For example, adding 2wt% MgO reduces the sintering activation energy of AO3 from 502Jmol to 3975J/mol, thus promoting the sintering process. When 5wt% MgO is added, the sintering activation energy of Ak2O3 increases to 544J/mol, which inhibits sintering.
2. Foaming method
The foaming process is to add organic or inorganic chemicals to the ceramic components, produce volatile gases through chemical reactions, etc., and make porous ceramics η after drying and firing. Compared with the foam impregnation process, the foaming process makes it easier to control the shape, composition, and density of the product and can prepare porous ceramics with various pore shapes and sizes, especially suitable for preparing ceramic materials with closed pores. There are many types of chemicals used as foaming agents, for example, calcium carbide, calcium hydroxide aluminum powder, aluminum sulfate, and hydrogen peroxide are used as foaming agents; porous ceramics are prepared by foaming hydrophilic polyurethane plastics and ceramic slurry at the same time; sulfide and sulfate are mixed as foaming agents, etc.
3. Adding pore-forming agent method
This process is to prepare porous ceramics by adding the pore-forming agent to the ceramic ingredients, using the pore-forming agent to occupy a certain space in the green body, and then after sintering, the pore-forming agent leaves the base to form pores. The process flow of preparing porous ceramics by adding pore-forming agent is similar to that of ordinary ceramics. There are two types of pore-forming agents: inorganic and organic. Inorganic pore-forming agents include high-temperature decomposable salts such as ammonium carbonate, ammonium bicarbonate, and ammonium chloride, as well as coal powder, carbon powder, etc. Organic pore-forming agents are mainly natural fibers, polymers, and organic acids. The shape and size of the pore-forming agent particles determine the shape and size of the pores of the porous ceramic material. The molding method of porous ceramic materials is similar to that of ordinary ceramics, mainly including molding, extrusion, isostatic pressing, rolling, injection, and slurry casting, etc.
4. Extrusion molding method
There are many molding methods for honeycomb ceramics, and extrusion molding is one of the most commonly used manufacturing methods. Its process flow is: raw material synthesis → mixing → extrusion molding → drying → firing → finished product. In the production process, one of the core processes is extrusion molding, and the extrusion molding die is the core technology of extrusion molding. Recently, my country has developed and produced honeycomb ceramic extrusion molding dies with a specification of 400 holes/in2. The United States and Japan have developed high-pore density, ultra-thin-wall honeycomb ceramics with 100 holes/in2 and 900 holes/in2. my country has also begun research on 600 holes/in2 extrusion molding dies and has achieved initial success.
5. Sol-gel method
The sol-gel process mainly uses the accumulation of colloidal particles during the gelation process and the small pores left during the gel treatment and heat treatment to form a controllable porous structure. This method mostly produces nano-scale pores and is mostly used to produce microporous ceramics. The sol-gel process is a new process for preparing porous ceramics, which is unique compared to other processes. For example, the sol-gel method can further improve the control of pore size distribution, phase change, purity, and microstructure of porous alumina ceramics compared with particle mixing, foam impregnation, spray drying, and other methods. The table compares the characteristics of the above four process methods.
6. Freeze-drying method Freeze
The full name of the drying method is vacuum freeze drying. The technology was invented by the British Wollaston in 1813. The principle of freeze drying is to freeze the material to be dried at a low temperature below its eutectic point, so that the water in the material becomes solid ice, and then in an appropriate vacuum environment, the ice is directly sublimated into water vapor by heating and removed, to obtain a dry product. At present, people have applied the freezing principle to prepare porous ceramics. For example, Bethtold and Mahler reported a freeze-forming process for ceramic fibers, which used phase separation for colloidal hydrates (such as silicate).
Technology. In this process, ice is allowed to surround and isolate the columnar gel, and the growth direction of ice in the solution is controlled to be unidirectional. When the ice melts, the fiber is formed. In another freeze-drying process for preparing porous ceramics, the solvent is directly sublimated from the solid state to the gas state and removed. By controlling the freezing direction of the metal salt solution, porous ceramics with good directionality and high porosity (>90vol%) are obtained.
7. Template method
The template method is a technology that can accurately control the pore structure, pore size, and distribution. At present, there are mainly the following ways:
①Porous body-in-situ reaction method: The most typical one is porous SiC. First, porous carbon is prepared and then siliconized to form porous SiC. Aoki directly reacted silicon gas with porous carbon to prepare porous SiC that maintains the shape of porous carbon. Zhang et al. prepared unidirectionally arranged porous AlO3 by impregnating cotton thread into the slurry. Its bending strength can reach 155±20MPa, the pore size is 165μm, and the porosity is 35wol%.
②Polymer template method: The core-shell structure with ceramic as the shell and polymer as the core is used as a template, and the polymer is removed by calcination to generate porous ceramics. The polymer template method is the latest technology for preparing porous ceramics. It uses the core-shell structure with polymer as the core and ceramic as the shell made by the colloidal flocculation method. Tang used monodispersed polymethyl methacrylate polymer spheres with a particle size of several hundred nanometers as templates and ceramic nanoparticles (AlO3, TO2, and zrO2) modified by polypropylene imine as ceramic materials to prepare polymer/ceramic core-shell composite materials, which were sintered into nanoscale porous ceramics with controllable pore size. Among the methods of preparing porous ceramics by template method, the preparation of porous ceramics using wood as template is a new method. The porous ceramics obtained by this method are mainly wood ceramics and carbide ceramics with wood microstructure. The former is mainly a new type of porous carbon material made by impregnating wood or wood materials with thermosetting resin or liquefied wood and then carbonizing them at high temperatures. The latter is prepared by reactive melt silicification, gas phase reactive infiltration, and carbon thermal reduction with charcoal as a template, and the porosity ranges from 20 to 80 vol% depending on the wood species and preparation method.
8. Bionic synthesis method
Surfactants can self-assemble in water to form liquid crystals and vesicles, which can be used as templates for biomimetic synthesis. Taney used neutral surfactants with polar groups at both ends and hydrophobic chains in the middle as template molecules to form vesicles as templates, and synthesized layered porous SiO2 with a pore size of 0.5 to 10 mm. When the surfactant forms spherical micelles in water, spherical porous SiO2 can be synthesized.
9 Hydrothermal-hot static pressing method
This process uses water as a pressure transmission medium to prepare porous ceramics of various pore sizes. The preparation steps are: silica gel and 10wt% water are mixed and placed in an autoclave at a pressure of 10 to 15 MPa and a temperature of 300°C. Porous ceramics are made by the volatilization of water vapor. In the hydrothermal-hot static pressing process, the reaction time is generally 10-180 min, and the porous ceramic volume density can be obtained by treating it at 25 MPa for 60 min, and the pore volume is 0.88 g (cm3, the pore volume is 0.59 cm3/g, the pore size distribution range is 30-50 m, and the compressive strength is as high as 80 MPa. The porous ceramic hydrothermal-hot static pressing process has the following advantages: the porous ceramics have high compressive strength, stable performance, and a wide range of pore size distribution.