Temperature effect on nonhydrolytic foaming process
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Temperature effect on nonhydrolytic foaming process G.S. Grader, Y. de Hazan, G. Natali, T. Dadosh, and G.E. Shter Chemical Engineering Department, Technion, Haifa 32000, Israel (Received 18 March 1999; accepted 8 July 1999)
This paper describes the effect of temperature on the formation of nonhydrolytic alumina foams. The foams are generated by heat treatment of crystals of the aluminum chloride isopropyl ether complex [AlCl3(Pri2O)], with the release of isopropyl chloride (PriCl). The chlorine content in the foams was determined by titration, and their weight loss during sintering was measured by thermogravimetric and differential thermal analysis. Based on these measurements, the condensation degree (CD) in the foams was modeled. The foaming time ranged from several minutes at 70 °C to several seconds at 160 °C. It was found that the chlorine-to-aluminum ratio of the foam (Cl/Al) decreased from 1.79 at 70 °C to 1.56 at 160 °C. Thermogravimetric analysis data confirm that the smaller Cl content gives rise to a smaller weight loss during thermal decomposition, consistent with a higher CD in the foams created at higher temperatures. Finally, about 80% of the PriCl produced during complex decomposition and subsequent –Al–O–Al– condensation reactions is lost during foaming.
I. INTRODUCTION
We have recently reported on the synthesis of ultralight ceramic foams by a novel nonhydrolytic route.1–3 The method allows a simple in situ preparation of monolithic foams in complicated shapes by a one-step procedure. The only foam precursor is the complex of aluminum chloride and isopropyl ether, made from concentrated solutions of AlCl3, Pri2O and CH2Cl2. The foams are characterized by a extremely large porosity of 94–99%, with cells in the 50–2000-m range. Most ceramic foams are produced by impregnation,4,5 which does not require any blowing agent. Other foaming processes include physical blowing agents6,7 coupled with gelation or chemical blowing agent coupled to organic polymerization,8 or emulsion.9 The nonhydrolytic foams studied here are distinguished by a phase separation that triggers release of a chemical blowing agent and by a gelation process that stabilizes the foam structure.1–3 Furthermore, the method is apparently self-regulating and leads to foams with the highest porosity ever reported for cellular ceramics. Although the basic underlying mechanism involved in the nonhydrolytic foaming was studied, the optimal conditions of foam preparation have not been determined. In particular, the effects of various parameters such as temperature and pressure on the foaming process are not known. In this manuscript we report on the effect of temperature on the foaming process. II. EXPERIMENTAL
AlCl3–Pri2O complex crystals preparation was reported previously in detail.1–3 The dried complex was 4020
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 10, Oct 1999 Downloaded: 16 Mar 2015
kept in sealed vials under argon at 5 °C. Foaming was carried out by placing a kno
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