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In a recent study published in the Journal of the European Ceramic Society, a unique method for producing mullite ceramic foam using nano-particle stabilized ceramic foam and boehmite combined with silica as a two-phase sol was proposed.

Research: Mullite ceramic foam with adjustable pores comes from a two-phase sol nanoparticle stabilized foam. Image source: socrates471/Shutterstock.com

Nanoparticles (NP) help increase the specific surface area of ​​ceramic foam. The produced mullite foam ceramics have layered pores, better compressive strength and a high level of open porosity.

As a stable compound, mullite (3Al2O3.2SiO2) is often used as a refractory material and high-temperature structural material due to its superior properties such as high melting point, low creep rate, low thermal conductivity, and strong electrical resistance. To thermal shock.

Because of its small dielectric constant, mullite is also a good electronic circuit substrate. On the other hand, its traditional compact structure hinders the development of certain features and limits its potential applications.

Adding high porosity to mullite ceramics may give them unexpected properties, including low bulk density, large specific surface area, reduced thermal conductivity, and reduced dielectric constant. Aerogels made of mullite nanofibers are more suitable for lightweight insulating materials.

Due to the open-cell structure of mullite, foam ceramics have been widely used as high-temperature gas and fluid screens, separation membranes and catalyst carriers.

Porous mullite ceramics have been manufactured using various techniques, such as reaction sintering, freeze casting, foam or emulsion templates, replicas of the method of adding pores, gel casting, and 3D printing.

Unfortunately, hardly any products made using these processes have a porosity greater than 80%. It is important to develop a process for successfully manufacturing high-porosity mullite foam ceramics without sacrificing compressive strength, but it is still difficult.

Recently, sol-NPs have been used to produce nanoparticle-stabilized foams. Compared with the traditionally used micro-scale ceramic particles, sol-NPs are more conducive to foam stability, creating porous ceramics with thin walls and small particle sizes, and generating greater compressive strength based on high porosity.

In addition, the specific surface area of ​​foam ceramics has also increased, which may be related to the increased reactivity of the frit and part of the frit.

Due to partial fragmentation, the use of NPs will create openings in the cell membrane. The interconnectivity of the open pores will increase the available surface area and porosity, making it suitable for use as filters, catalytic supports, and bio-scaffolds. These characteristics can be adjusted by adjusting the concentration of the surface modifier, which will change the electrostatic interaction energy between the bubbles and the colloidal particles, thereby changing the characteristics of the foam.

So far, single-phase sols have been effectively used to produce nanoparticle-stabilized foams, including alumina and silica. However, the performance of the produced porous mullite ceramic foam cannot meet the expectations of a highly porous structure, uniformity, and reduced pore size.

Two-phase sol-NPs as the building block of nano-particle stable foam may hopefully solve this problem, so as to obtain the best porosity microstructure and mechanical quality after reaction fragmentation.

Nevertheless, due to the difference between the surface states of the particles, obtaining stable foam through a two-phase sol is more complicated than a single-phase sol.

This research proposes a simple, direct and inexpensive method to produce mullite ceramic foam using dual-phase sol NP-supported foam.

Sol NP is modified to make its surface hydrophobic. By changing the frit temperature and solid load, the researchers created a mullite ceramic foam with layered porosity, increased porosity, significant compressive strength, and reduced thermal conductivity.

It is best to use a sol NP with a constant surface charge and pH value to ensure uniform mixing of the two-phase sol without aggregation. As a result, the starting ingredients were boehmite sol with an initial pH of 6.2 and silica sol with a comparable pH of 4.3.

The pH value is increased to the range of 5.0-6.0, which improves the foam stability and promotes the formation of the gel network, thereby obtaining the best rheological properties.

For the first time, boehmite sol and silica sol were used as two-phase sols to construct stable nano-particle foams, and to prepare mullite ceramic foams with hierarchical porosity.

In order to improve the foam stability, the pH value was changed during the foaming process to promote the gelation of the two-phase sol NP stabilized foam.

It was found that mullite ceramics have higher compressive strength and low thermal conductivity, making them suitable for use as filters, catalytic carriers and thermal insulation materials.

Continue reading: Why is it important to understand the rheology of nanofluids?

Yang J, ZX (2021) Two-phase sol nanoparticle stabilized foam. Mullite ceramic foam with adjustable pores. Journal of the European Ceramic Society. Website: https://www.sciencedirect.com/science/article/pii/S095522192100902X?via%3Dihub

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