A new material in the field of sound insulation and noise reduction-porous sound-absorbing ceramics

Noise, that is, noise, is usually defined as "everything that people don't want". In recent years, with the development of industrial modernization and the increase in the number of cars, industrial noise, road traffic noise, construction noise, family and social noise have seriously affected people's daily life. The problem of noise control has gradually aroused the close attention of scientists and relevant state departments. The use of sound-absorbing materials for sound-absorbing and noise-reducing treatment is currently an effective sound-absorbing and noise-reducing method.

Porous ceramics are a new type of ceramic matrix composites containing pores, and their manufacture began in the 1970s. Because of its excellent properties such as good permeability, low density, high hardness, large specific surface area, low thermal conductivity, high temperature resistance, and corrosion resistance, it is widely used in filtration and separation, chemical catalyst carriers, biomedical implants, heat preservation, and heat insulation. , sound absorption damping, combustion and fire resistance and other aspects have a good application.

Figure. Honeycomb structure ceramic sound-absorbing panel via network

1 Types of porous structure sound-absorbing materials

Porous sound-absorbing materials have many continuous, tiny pores. According to Huygens' principle, when the sound is incident on the surface of the material, part of it is reflected off the surface of the material, and the other part is transmitted into the porous body, causing the air in the pores to vibrate and rub against the ceramic tendons. Due to the viscous and heat conduction effects, the sound energy is converted into heat energy and consumed. After the sound wave is reflected by the rigid wall, when it passes through the material and returns to its surface, part of the sound wave is transmitted into the air, and part of it is reflected back to the inside of the material. Through this repeated propagation of the sound wave, the energy is continuously converted and consumed, and so on, so as to achieve the effect of sound absorption. Effect.

At present, porous structure sound-absorbing materials are usually divided into three categories, namely organic porous sound-absorbing materials, inorganic porous sound-absorbing materials and porous metal sound-absorbing materials.

2 Preparation method of porous sound-absorbing ceramics

In 1978, the United States successfully developed porous ceramic materials for the first time. They used alumina, kaolin and other ceramic materials to make porous ceramics for filtration in aluminum alloy casting, which can significantly improve the quality of castings and reduce the scrap rate. Since then, many countries have competed to carry out research on porous ceramics, forming a new industry. The researchers used the circular tube theoretical model to study the porosity, pore diameter, material thickness and theoretical model of porous materials, and studied the influence of porous material porosity, pore diameter, material thickness and structure factors on the sound absorption performance.

The results show:

The pore size and thickness of the porous material are controlled to be constant, and the sound absorption performance increases with the increase of the porosity;

The thickness and porosity of the porous material are controlled to remain unchanged, and the sound absorption performance increases with the decrease of the pore size;

Controlling the pore size and porosity of the porous material remains unchanged, its low-frequency sound absorption performance increases with the increase of material thickness, while the high-frequency sound absorption performance decreases;

When the material thickness, pore size and porosity remain unchanged, the structure factor has no obvious effect on the low-frequency sound absorption performance of the material, but the sound absorption coefficient changes periodically in the mid-high frequency range. The performance of porous ceramics prepared by different methods is very different, and the porosity, material thickness and pore size are important factors that affect the good sound absorption performance of porous ceramics.

At present, the commonly used methods for preparing porous sound-absorbing ceramic materials include particle stacking sintering method, adding pore-forming agent method, organic foam impregnation method, foaming method, sol-gel method, etc.

① Particle stacking sintering method

It uses the accumulation and sintering of aggregate particles to connect to form porous ceramics. The aggregate particles can be connected by adding fine particles with the same composition, and sintering at a certain temperature to connect large particles; The solid phase reaction occurs between the materials to connect the particles, or some can form the expansion coefficient and chemical composition in the sintering process to match the aggregate, and can form a liquid phase infiltrated with the aggregate phase at high temperature additives.

② Adding pore-forming agent method

This process is widely used in the preparation of porous ceramics. It adds volatile or combustible pore-forming agents, such as wood chips, coal powder, plastic powder, etc., to the ceramic ingredients, and uses these pore-forming agents to volatilize or Burn out leaving porosity in the ceramic matrix.

The key of this method lies in the selection of pore-forming agent, in order to improve the porosity of the porous material. Commonly used pore formers are generally divided into two categories: inorganic substances and organic substances. The former includes volatile inorganic substances such as ammonium bicarbonate, ammonium carbonate, and ammonium chloride. A large amount of gas is generated through the decomposition of inorganic substances at high temperatures, and the material will become porous after cooling; the latter includes some natural fibers such as starch and carbon powder, The high molecular polymer occupies a space of a certain size during the molding process of the mold, and then oxidizes under high-temperature sintering conditions to form certain pores.

③Organic foam impregnation method

The process was first patented in 1963, and has been greatly developed since then and has become one of the most widely used technologies for preparing porous ceramics. It is unique in that it relies on the special structure of the open-pore three-dimensional network skeleton of the organic foam, coats the prepared slurry on the organic foam, and then burns off the organic matter and sinters the ceramic body to obtain a porous ceramic product. .

The size of the porous body prepared by this process mainly depends on the pore size of the organic foam, and is also related to the coating thickness of the slurry on the organic body and the drying and sintering shrinkage of the slurry. In general, the pore size of the resulting porous ceramic will be slightly smaller than that of the original organic foam. At the same time, the method has simple process, convenient operation and low preparation cost, and is an economical and practical porous ceramic manufacturing process with broad development prospects.

④ Foaming method

This method was invented in the 1970s. It is to add inorganic or organic substances to the ceramic raw materials to generate volatile gas in the sintered device, so that it is produced in the ceramic, and it is dried and sintered to prepare porous ceramics. Usually calcium carbonate, sodium hydroxide, aluminum sulfate and hydrogen peroxide are used as foaming agents.

Compared with the organic foam impregnation method, the foaming method has the advantage that the shape, composition and density of the product are easy to control, and is especially suitable for the production of closed-cell ceramics. The disadvantage is that it has higher requirements for raw materials and is difficult to control in technology.

⑤ Sol-gel method

This process is mainly used to prepare microporous ceramic materials with nanoscale pore diameters, especially microporous ceramic films. At the same time, this method is also the main method for preparing porous materials with high regularity. The preparation process is to dissolve metal alkoxide in lower alcohol, slowly drop water to carry out hydrolysis-polycondensation reaction, make the solution into a gel, dry the gel and combine heat treatment to obtain a porous product.

The pore size distribution of porous ceramics prepared by this method is very narrow, and the pore size can be controlled by the solution composition and heat treatment process, but the disadvantage is that the shape of the porous product is limited, and the process is more complicated, the cost is higher, and the output is lower. . Some researchers prepared aluminum sol by hydrolyzing aluminum powder in aluminum chloride solution, and directly mixed pore forming agent with it to prepare alumina porous ceramics.

In addition to the above common methods, the new preparation methods of porous ceramics include freeze-drying, wood ceramicization, gel injection molding, self-propagating high-temperature synthesis, high-energy ball milling, carbothermal reduction, and solid-state replacement. law etc.

3 Outlook

With the improvement of people's requirements for the quality of life, sound-absorbing materials have been used in large conference rooms, concert halls, etc. to general family buildings. Compared with traditional sound-absorbing materials, porous sound-absorbing materials have their unique advantages, especially porous ceramic materials, which have the characteristics of three-dimensional mesh opening structure, weather resistance, corrosion resistance, heat resistance and earthquake resistance. However, due to the brittleness of ceramic materials, its application range is greatly limited. If the application field of porous ceramic materials is to be further improved, the research on the toughening of ceramic materials may become the focus of future research.