The "SpongeBob SquarePants" of the ceramic world, this ceramic is both light and heat-retaining!

Produced by: Science Popularization China

Author: Li Kewei (Institute of Solid State Physics, Chinese Academy of Sciences)

Producer: China Science Expo

Speaking of ceramic materials, I believe everyone is familiar with them. The ceramic industry is one of the earliest industries. As early as 24,000 BC, the earliest ceramic industry was born. At first, people found that clay and water could be sintered to make objects of various shapes. This was the predecessor of ceramics. With the development and progress of firing technology, ceramics have gradually become an indispensable material in human life and production.

With the inheritance and continuation of ceramic preparation technology, ceramic products have gradually changed from rough to fine, and sintering conditions have developed from low temperature to high temperature. At the same time, the ceramic family has been further expanded and improved. In today's material system, all other materials except organic materials and metal materials, that is, inorganic non-metallic materials, are collectively referred to as ceramic materials.

There are many kinds of ceramic materials, porous ceramics are one of them

Ceramic materials can be divided into structural ceramics and functional ceramics according to their functions. Since ceramic materials are a type of solid material made by high-temperature sintering of inorganic non-metallic materials, they usually have high melting points, high hardness, and good chemical stability.

When we need to bring out the mechanical, thermal and chemical properties of materials, we have to use structural ceramics. Structural ceramics are very popular in the aerospace field. They can be seen on aircraft and rocket engine blades, as well as in key parts such as the nose cone and leading edge of the wing of high-speed aircraft.

Functional ceramics are ceramic materials made by utilizing their special functions on physical properties such as electricity, light, magnetism, sound, and heat. Its family is also huge. For example, insulating ceramics and dielectric ceramics used in radio devices and circuit elements, piezoelectric ceramics and transparent ceramics used in sensors, etc. Although inconspicuous, they are like thunder in silence.

Ceramic matrix composite components of the engine (left) and transparent functional ceramic materials (right)

(Photo source: Composites.com)

Often, when we mention ceramic materials, the first thing that comes to mind is that they are heavy and strong, because these common ceramic materials are very dense. However, in the large family of ceramic materials, porous ceramics have gradually come into everyone's sight with their unique structure and performance.

Porous ceramics, as the name implies, are ceramic materials with a large number of tiny pores. These pores can be closed or interconnected, which give porous ceramics a series of unique physical and chemical properties. The porosity of porous ceramics is usually between 10% and 90%, and the pore size can range from nanometers to micrometers.

Porous oxide ceramics

(Photo source: China Powder Network)

The application of porous ceramics began in the late nineteenth century. During the preparation process, by adjusting different production parameters and conditions, pores of different numbers, sizes and shapes can be produced inside the ceramics. These pores can be microscopic or macroscopic and have various shapes, thus producing a unique porous structure.

It not only has the advantages of high chemical stability, high temperature resistance, corrosion resistance, and high strength of ordinary ceramic materials, but also because of its large surface area and void structure, porous ceramics have the advantages of light weight, good adsorption performance, and good thermal insulation performance, which makes it widely used in environmental protection, energy, chemical industry, pharmaceuticals, biomedicine and other fields.

How are the pores in porous ceramics formed?

According to the complexity of the preparation process and the porosity of the prepared porous ceramics, the preparation methods of porous ceramics can generally be divided into partial sintering method, replica template method, sacrificial template method and direct foaming method.

Schematic diagram of porous ceramic preparation process

(Image source: Reference 1)

Partial sintering is currently a common method for preparing porous ceramic materials. Simply put, ceramic particles or ceramic fibers are pressed into a green body, and then sintered under high temperature conditions to form a bond between the particles or fibers. At the same time, the powder particles are bonded due to surface diffusion or evaporation and condensation. The sintering is terminated before complete density to form a uniform porous structure in the ceramic. By changing the particle size of the starting powder and the partial sintering procedure, the pore size and porosity of the porous ceramic can be changed. For ceramic materials prepared by this method, the sintering neck between the grains, the porosity and the size of the pore size will directly affect its performance.

The sacrificial template method is also called the pore-forming agent addition method. Its main advantages are low cost and convenient operation. The principle is to add a certain amount of pore-forming agent to the ceramic slurry. During the sintering process, the pore-forming agent is decomposed into gas by heat to form pores. Although this method is simple in process, it is difficult to control the pore distribution, which will result in poor uniformity of the finished product.

The template replication method literally means replicating the template of other materials to prepare porous ceramics with high volume porosity and open pore walls. Taking sponge as an example, the template is first impregnated with a ceramic suspension, a precursor solution, etc., and the remaining part is discharged by centrifugation or the like. After the ceramic impregnated template is dried, it is heat treated to decompose the organic sponge, and finally the ceramic layer is densified at high temperature. These open units are interconnected, allowing the fluid to pass through the porous ceramic at a relatively low pressure drop. This method can produce porous ceramics with high porosity, high strength and low density, but it is difficult to prepare small-pore closed-pore ceramics, and the shape of the pores will be limited by the template.

The direct foaming method is to stabilize and dry the ceramic suspension first, and then sinter it to obtain a consolidated structure. Its advantage is that the prepared material has a strong pore structure and few defects. Gas is introduced into the suspension by mechanical stirring or adding a foaming agent to form a fine and stable foam, which is then dried and sintered to obtain a porous ceramic. This method is easy to industrialize and can prepare porous ceramics with open or closed pore structures and high porosity. Therefore, it is crucial to stabilize the bubbles in the ceramic suspension. The most commonly used stabilization method is to reduce the energy of the gas-liquid interface by adding a surfactant, thereby controlling the reaction conditions to a certain extent.

Porous ceramics have superior performance and are widely used in many fields

Since it is a porous material, we can naturally imagine that its mass will be lighter than ordinary ceramic materials. The large number of pore structures introduced into it will disperse the energy after the sound waves or heat are conducted into the ceramic, thereby achieving the purpose of sound absorption and heat insulation. Since porous ceramics have both the excellent mechanical properties and sound absorption and heat insulation properties of ceramic materials, they can be used in places with high sound insulation requirements such as high-rise buildings, tunnels, and cinemas.

Schematic diagram of heat transfer in porous ceramics:

(a) Randomly arranged porous microstructure and (b) closed channels perpendicular to the heat flow

(Image source: Reference 2)

The material is full of pores, so porous ceramics have a large surface area, which provides a natural advantage for gas adsorption and catalysis. Due to the high temperature resistance of the ceramic material itself, it is more suitable for use in high-temperature exhaust gas purifiers and sewage treatment equipment. For example, the black smoke emitted by diesel engines can not only be filtered, but also catalytically convert CO, NOX, CnHm and other gases into non-toxic CO2, N2 and H2O, etc., making a contribution to our environmental protection. [3]

In addition, porous bioceramics developed on the basis of traditional bioceramics, such as artificial bones and artificial joints, have a three-dimensional network structure that is almost the same as the human body's spongy bone. This porous network structure allows bone tissue to grow into the pores, resulting in a more secure fixation between the implant and the organism. It has better biocompatibility and more stable physical and chemical properties than most metal materials, and has no toxic side effects, which provides an excellent alternative for patients who need bone transplants. In recent years, porous bioceramics have also become a hot topic in the research of inorganic biomaterials. [3,4]

Porous ceramic sound-absorbing panels (a), ceramic carrier automobile exhaust purifiers (b), zirconia ceramic dentures (c), and porous ceramics for bone repair (d)

(Image source: References 5, 6 and 3D ScienceValley)

The power of porous ceramics goes far beyond this. Its complex internal structure makes it a natural insulator and can be used as an electromagnetic wave shielding material. In addition, due to its good stability and durability, it has been well applied as a micro-electromechanical system. It can be worn on the body, even on the wrist and the sole of the shoe, as a sensor to detect human activities, thereby better serving people. [7]

With the further development of science and technology, the preparation technology and performance of porous ceramic materials are also progressing steadily, and their development is in the ascendant. In the future, people will develop more porous ceramic materials with more complex pore structures and ideal porosity through precise control of their microstructure and pore size, and their application will be more economical, efficient and long-term reliable. Its excellent performance will surely shine in every corner of our lives. Porous ceramics have great potential!

References:

[1] Preparation methods and research status of porous ceramics

[2] Hammel EC, Ighodaro OLR, Okoli O I. Processing and properties of advanced porous ceramics: An application based review[J]. Ceramics International, 2014, 40(10): 15351-15370.

[3] Brief analysis of eight applications of porous ceramic materials

[4] Zhu Xinwen, Jiang Dongliang, Tan Shouhong. Preparation, properties and applications of porous ceramics: (II) Structure, properties and applications of porous ceramics [J]. Journal of Ceramics, 2003, 024(002):85-91.

[5] Porous sound-absorbing ceramics: a new type of material for sound insulation and noise reduction

[6] Summary of preparation technology and application of porous ceramics

[7] Chen Y, Wang N, Ola O, et al. Porous ceramics: Light in weight but heavy in energy and environment technologies[J]. Materials Science and Engineering: R: Reports, 2021, 143: 100589.