At present, lightweight refractory products are mainly composed of lightweight aggregate and dense matrix, and light aggregate accounts for about 50% of the large proportion, so the performance of lightweight aggregate will directly affect the overall performance of lightweight refractory product.
It is the key to the preparation technology to form pores of appropriate size and quantity and uniform distribution in the refractory aggregate. Porous ceramics have excellent properties such as low density, high temperature resistance, thermal shock resistance, corrosion resistance, uniform permeability, and high mechanical strength. Refractory bone the development of materials mostly adopts the preparation method of this material.
Porous ceramics is a new type of ceramic material with a large number of interconnected or closed pores after high-temperature sintering. Controlling the formation of pores is the core step in the preparation of porous ceramics. The preparation methods generally include: particle accumulation method, burn-off addition method, Organic foam impregnation method, foaming method, sol-gel method, etc. The above several preparation methods are briefly introduced below.
01 Particle packing method
Particle accumulation method is also called solid-state sintering method or aggregate accumulation method. This method uses fine powder as aggregate and utilizes the characteristics of fine powder for easy sintering to generate part of the liquid phase at high temperature and make it interconnected. Since each fine powder particle is only connected to other particles at several points, there are many interpenetrating pores between each other, and a three-dimensional pore channel is formed after the material is cooled. This process can control the pore structure by adjusting the particle size distribution, but its porosity is only related to the accumulation mode of the aggregate and has nothing to do with the particle size of the aggregate, so the porosity of the products made by this method is often not high. Generally, it is 20%~30%. Therefore, this method is gradually replaced by other preparation methods or used in combination with other preparation methods. For example, adding carbon powder, sawdust, starch, plastic and other pore-forming agents to the raw materials and burning them at high temperature can make the overall pores rate increased to around 75%.
02 Burn-off addition
The process is to add a certain amount of pore-forming substances such as carbon powder, wood chips, coal powder, starch, polymethyl methacrylate, polyvinyl butyral, polystyrene, polyvinyl alcohol, etc. to the ingredients. Such materials are often used, they are called pore-forming agents or pore-forming agents, which occupy a certain space in the green body and are burned out during the firing process to form pores. The process of this method is simple. The size and shape of the pores of the product can be adjusted according to the pore-forming agent, and products with complex shapes can be produced. The porous ceramics prepared by this method have both high porosity and high strength. It is a preparation method for porous ceramics. The very most common method of production. However, it also has certain limitations, such as the distribution of pores is not uniform and it is difficult to obtain products with high porosity.
03 Organic foam impregnation
In 1963, American scholar Schwartzwalder et al. invented the organic foam impregnation method. The principle is simple. The three-dimensional open-pore network skeleton structure of the organic foam is used as a template to uniformly coat the ceramic slurry on the surface of the template to form a coating. After drying, the organic foam is burned to obtain. The porosity of porous ceramics with an organic foam inversion structure can be as high as 70% to 95%. The product obtained by this method has uniform void distribution and low cost, and the process is simple and suitable for large-scale industrial production. However, the burning of organic foam will form a small amount of carbon residue in the porous ceramics, which is difficult to form closed cells with small gaps, thereby reducing the strength of the porous ceramics. The harmful gases produced during the combustion process will also pollute the environment.
04 Foaming method
This method is to mix foaming agent and foam stabilizer with a certain proportion of water to make foam liquid and mud, and then pouring, curing, drying, and firing to make products. The preparation and stabilization of foam mud in the production process is foaming is the key to making lightweight materials.
Compared with the foam impregnation process, this method is easier to control the shape, composition and density of the product, and prepares porous ceramics with various pore sizes and shapes. It is especially suitable for the production of closed-cell ceramic products. Compared with adding burnout, this method is easier to produce. Thermal insulation products with low bulk density are mostly used in the production of ultra-light thermal insulation and refractory products. However, the foam method also has certain disadvantages, and its production process is more complicated, production control is more difficult and production efficiency is lower.
05 Sol-gel method
The Sol-Gel method is mainly used to prepare microporous ceramics, especially microporous ceramic films. Its characteristic is to use the unique three-dimensional network structure of the gel to obtain a porous ceramic membrane with a pore size of 2-100nm. Its simple preparation process is to carry out the hydrolysis-polycondensation reaction using metal alkoxide and its compounds as raw materials under the action of a certain medium and catalyst, so that the solution changes from sol to gel colloid particles and connects with each other to form a network during the sol gelation process. The structure is filled with the solution, and then the solution is volatilized through the drying and sintering process to obtain a porous product with nano-scale voids.
Porous ceramics prepared by Sol-Gel process have a very narrow pore size distribution range. The pore size can be controlled by adjusting the solution composition and heat treatment process, but the raw materials are limited and the production efficiency is low. It is the most active research field at present.