Detailed description: Porous ceramics are commonly used as thermal insulators thanks to an advantageous combination of the high refractoriness and chemical inertness of ceramics with the low thermal conductivity of the porous phase. Due to this and the growing demand for solutions that minimize energy expenditure in industrial processes, these materials' scientific and technological potential has been widely explored in the specialized literature, where several alternative materials, production methods, and their characterizations are presented. However, the expansion of its use in environments involving prolonged exposure to high temperatures (above 1000°C, as in the case of applications in the steel and cement industries, for several months) still faces two main limitations: the low thermomechanical resistance due to the high porosity (especially in those materials with a volumetric pore fraction above 50%) and the reduction in porosity and consequently the loss of thermal insulation capacity caused by the phenomena of densification and grain growth that accelerate and intensify strongly above 1000°C.

Several studies show that these negative aspects can be minimized separately, but not simultaneously, most of the time. The thermomechanical strength of ceramics can be significantly increased with greater particle packing efficiency, the improvement of processing methods to reduce the introduction of defects, or even the use of additives to control sintering. On the other hand, maintaining porosity at high temperatures can be achieved by increasing the initial fraction of pores in the green material with controlled heating and sintering ramps or even introducing compounds that minimize grain growth and densification. However, when observing the various solutions used in each case, it is clear that the results obtained to improve thermomechanical strength act concurrently in relation to the increase in porosity and insulation capacity and vice versa. This fact occurs because these properties have opposite behaviors in relation to the presence of pores in the material structure: more pores lead to better performance as a thermal insulator but considerably reduce the thermomechanical resistance; on the other hand, greater densification makes the insulator more resistant, but reduces its ability to retain the heat flow. Due to these limitations, traditional methods are based and limited on finding an optimal point where neither of the two properties is seriously harmed, resulting in average performances. This analysis suggests the need for a new approach that simultaneously considers the generation of improvements in these two properties. This line of research proposes that the use of nanoporous ceramics with hierarchical structures is this approach.

Hierarchical structures can be defined as structures that contain elements formed by other structures organized at different levels. An important consequence of this organization is the optimization of several properties simultaneously. As a classic example of a hierarchical structure, one can mention the structure formed by the combinations of calcium carbonate plates and natural polymers (such as collagen and chitin) found in the shells and carapaces of crustaceans and mollusks. In these materials, various levels of the organization can be observed. At the atomic-molecular level, there is the periodic arrangement of atoms in calcium carbonate crystals and the orientation of polymer chains; the good adhesion between these nanometric elements allows the carbonate plates to be strongly bonded with a small amount of polymer, generating a lamellar structure; several layers of this structure (now with micrometric dimensions) are joined to form the exoskeleton of these animals. The hierarchy of the structure allows for obtaining a unique combination of biomechanical properties (high rigidity, hardness, fracture energy, and resistance to the action of microorganisms) and low density (in the order of 1.0 to 1.5 g.cm-3).

Inspired by this type of structure and its remarkable performance, the main objective of this line of research is to simultaneously maximize the thermomechanical and thermal insulation properties of porous ceramics for use in refractory applications (above 1000°C), based on: a) hierarchization of the structure of these materials, starting from the most elementary levels of microstructure, b) the combination of various methods of pore incorporation and ceramic processing and c) the constant search for new materials where excellent performance can be combined primarily with ease of processing, allowing for use of national raw materials, at competitive production costs.

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