Hindered densification as a porogenic mechanism for alumina-based thermal insulators

• Autor(a) principal: Emílio, Ana Beatriz Verdi
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/18/18158/tde-26012021-092249/
• topic: Alumina; Alumina; Aluminum hydroxide; Cerâmica porosa; Direct cast; Hidróxido de alumínio; Isolante térmico; Moldagem direta; Porous ceramics; Sinterização; Sinterization; Thermal insulator

Porous ceramics for application as high-temperature thermal insulators (600°C-1600°C) have been investigated as an alternative to the use of block and blankets of ceramic fibers. They present potential for such application because of the combination of low conductivity of porous structures (less than 5 W(m.K)-1 at 1200°C) with the refractoriness of ceramics. However, they do not resist densification as well as fibers after long periods at high temperatures. The presence of porosity reduces thermal conduction, because pores are filled with gases with low conductivity and also provoke scattering of photons, which are responsible for radiation process that occurs at high temperatures. To be effective it is also necessary that the pores be closed and small (< 4 mm diameter), in order to avoid the convection process. To maintain porosity and stability at high temperatures, it is necessary that the main sintering mechanism be surface diffusion, because it does not promote significant densification. This work is based on the principles of sintering and heat conduction to propose a novel porogenic mechanism. Thus, this work studied the compacting of pre-sintered aluminum hydroxide (Al(OH)3) because the particles obtained have internal pores, intrinsically resistant to densification due to their low surface area and curved shape. Three compaction methods were used: isostatic and uniaxial pressing and direct cast. The pressed samples resulted in lower values of total porosity at 1600°C (61.5% for uniaxial and 56.3% for isostatic), as they produced broken filaments during the application of pressure which increased the compaction of the particles and facilitated densification. The samples obtained by direct molding, on the other hand, did not compact well because the main diffusion mechanism was surface diffusion, maintaining porosity at high levels (74.4%). The physical properties obtained such as flexural strength (3.75 MPa) and rigity (4.57 GPa) are satisfactory for the application as thermal insulator (0.5-1 MPa and 2 GPa, respectively). In addition, the samples showed low thermal conductivity (0.49 W·m-1 ·°C-1 to 1400°C), low shrinkage after sintering (11,92% at 1600°C) and high resistance to thermal shock due to the low expansion coefficient obtained (5.7 10-6 ·°C-1 at 1200 ° C).