Fabrication of large alumina foams by pyrolysis of thermo-foamed alumina-sucrose
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The aim of this study is to prevent cracks in large foam bodies prepared by thermo-foaming of alumina powder dispersions in molten sucrose. Cracks initiate in the binder burnout stage during which the bodies undergo shrinkage in the range of 32–49 vol% depending on sucrose content. Intermediate pyrolysis of the sucrose polymer binder prevents the cracking of large foam bodies as the carbon produced by pyrolysis binds the alumina particles during the initial stage of shrinkage and provides adequate strength to withstand the internal stresses produced during the pyrolysis and subsequent carbon burnout. The carbon bonded alumina foam bodies obtained after pyrolysis do not show any visible cracks during subsequent carbon burnout and sintering because the alumina particles establish a firm network with each other due to particle drag and rearrangement during pyrolysis of the sucrose polymer binder as evidenced from microstructure analysis. The carbon bonded alumina foam bodies show high compressive strength (2–1.3 MPa) and are amenable to machining operations such as milling and drilling without cracking.
I. INTRODUCTION
Ceramic foams are used in a variety of applications such as high temperature thermal insulation, molten metal filtration, catalyst support, lightweight structural components and preforms for polymer–ceramic and metal– ceramic composites.1–5 Foaming and setting of ceramic powder suspensions, polymer foam replication, and emulsion templating are three well known methods for the preparation of ceramic foams. In the first method, foamed powder suspensions, produced by stabilizing gas bubbles with either surfactant or particles, are cast in a mold.6–16 The cast foamed suspensions are then set by either in situ polymerization of organic monomers or by coagulation of the powder suspensions. The wet foam bodies are subsequently dried, binder removed, and sintered to produce the ceramic foams. In the second method, ceramic replica of a polymer foam template is produced by coating the polymer foam with a ceramic powder suspension followed by drying, burnout of the polymer foam template and sintering.17–19 In the third method, a high internal phase emulsion is made by dispersing water immiscible liquid droplets in an aqueous ceramic powder suspension using an emulsifying agent. The emulsion cast in a mold is subsequently dried, oil removed, and sintered to produce ceramic foam.20–23 Contributing Editor: Gary L. Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.395 302
J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
Formation of cracks is likely during drying, binder removal, and sintering of large bodies.24 Methods, such as water retention additives, freeze drying, and crosslinking of the polymeric binder, are used to prevent crack formation during drying of the wet-foams.16,24 Recently, we reported the thermo-foaming of powder dispersions in molten sucrose for the preparation of alumina foams.25 The method produces interconnected cellular ceramics with porosity in
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