Low density ceramic foams from alumina-sucrose using magnesium nitrate as a blowing and setting agent
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The effect of magnesium nitrate on the thermo-foaming of powder dispersions in molten sucrose for the preparation of alumina foams has been studied. The magnesium nitrate decreases the melting point of sucrose from 180 to 160 °C and acts as a blowing and setting agent. The foaming time and setting time decreases with an increase in foaming temperature as well as an increase in magnesium nitrate concentration. The rapid foaming followed by foam collapse is observed beyond 140 °C. The accelerated foaming and setting is due to an increase in the rate of –OH condensation because of the catalytic effect of H1 generated by the hydrolysis of magnesium nitrate. The porosity of alumina foam increases while cell size and grain size decrease with an increase in magnesium nitrate concentration. A change in foam structure, from partially interconnected cellular to completely interconnected reticulate-like, occurs when the magnesium nitrate concentration increase from 4 to 8 wt%.
I. INTRODUCTION
Ceramic foams are getting renewed interest due to their wide spread applications as high temperature thermal insulation, filter media for molten metal’s, catalyst support, bio-implant and pre-form for metal-ceramic and polymer-ceramic composites.1–5 The polymer foam replication, foaming, and setting of ceramic powder suspensions, emulsion-based methods and freeze-casting are well studied for the preparation of ceramic foams.6–30 In polymer foam replication method, a ceramic replica of a reticulated polyurethane foam is prepared by coating the webs of the polyurethane foam with a ceramic powder suspension followed by drying, burnout of the polymer foam template, and sintering.6–9 The ceramic foams with interconnected cellular structure are prepared by foaming and setting of aqueous ceramic powder suspensions. In this, the foaming of powder suspensions is accomplished by stabilizing gas bubbles in the slurry medium with the help of a surfactant or a particle with suitable wetting characteristics. The aqueous foams thus obtained are set either by in situ polymerization of organic monomers or by coagulation of the ceramic particles. The wet foams obtained are then dried and sintered to produce ceramic foams.10–18 In the emulsionbased method, fine droplets of immiscible liquid (oil) are dispersed in an aqueous ceramic powder suspension with the help of an emulsifying agent to produce high internal phase emulsions. The emulsions thus obtained are Contributing Editor: Paolo Colombo a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.300
subsequently dried, oil-removed, and sintered to produce ceramic foams.19–25 In the freeze-casting, ceramic powder suspensions cast in a mold is set by freezing the suspension medium. The subsequent removal of the frozen medium by sublimation and sintering of the resultant porous body produces ceramic foams.26–30 Recently we have reported a thermo-foaming of alumina powder dispersion in molten sucrose for the preparation of alumina foams. In this, the sucrose polymer–
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