Novel Ceramic Foams from Crystals of AlCl 3 (Pr i 2 O) complex
- PDF / 512,469 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 36 Downloads / 177 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Novel ceramic foams from crystals of AlCl3 (Pri 2 O) complex G. S. Grader, G. E. Shter, and Y. de Hazan Chemical Engineering Department, Technion, Haifa 32000, Israel (Received 27 April 1998; accepted 18 September 1998)
In this paper we report a novel process for the production of ultralight cellular ceramics. The foams are generated by the heat treatment of crystals of the aluminum chloride isopropyl ether complex [AlCl3 (Pri 2 O)]. The crystals, which are the only foam precursor, are obtained from concentrated solutions of AlCl3 , Pri 2 O, and CH2 Cl2 . The foams consist of an arrangement of closed cells, 50–300 mm in diameter, having cell walls approximately 1–2 mm thick. An exceptionally high porosity is obtained ranging from 94 to 99%, and the cellular structure is retained during heating at 1500 ±C. The foaming mechanism involves two consecutive nonhydrolytic sol-gel chemical reactions and physical processes including crystal dissociation, solvation, phase separation, and foaming. While other foaming mechanisms cited in the literature utilize one or more of the processes above, no analog mechanism exists in the organic, ceramic, or metal foam production processes. The effectiveness of the process originates from an initial precursor which contains all the necessary foaming components in such a way that the application of mild heating accelerates its transformation to a solid, dry, ultralight foamed material.
I. INTRODUCTION
There is an increasing interest in ceramic foams due to diverse applications in areas such as thermal insulation, separation processes, catalysis, and low dielectric constant materials. The most common method to produce ceramic foams involves the impregnation of an organic polymer foam (e.g., polyurethane) with a ceramic slurry. The wet impregnated foam is then dried, heated in air to burn the organic component, and finally sintered to obtain the ceramic foam, which is the “negative” replica of the organic foam precursor.1,2 Another method (HIPE: high internal phase emulsion) is based on the preparation of a high concentration emulsion with a continuous phase of a polymerizing monomer (e.g., sodium silicate) dispersed in a pore-forming phase (e.g., petroleum spirit) with the aid of a surfactant. The continuous phase is stabilized by polymerization, washed, and dried to obtain an open-cell foam.3 Both methods produce open-cell ceramic foams; however, they do not involve any gas phase blowing agent in their production. Successful production of cellular silica and SiCwhisker-reinforced cellular silica using physical blowing agents incorporated into a ceramic suspension was reported by Fujiu et al.,4 and Wu and Messing.5 The process starts from a stabilized aqueous suspension of colloidal silica. The blowing agent is dispersed in the suspension with the aid of a surfactant and methanol, in the form of small liquid droplets. The final stage involves the gelation by pH adjustment which is accompanied with a rapid viscosity increase to stabilize the
Data Loading...
