Thermal properties of expanded perlite fillers modified with alumino-silicate gel
Waldemar Pichór AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Mickiewicza 30, 30-059 Cracow, Poland
Quarterly No. 1, 2005 pages 35-40
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abstract Expanded perlite is commonly used in cryogenic and high temperature applications. This lightweight filler has excellent thermal conductivity but its use as the insulation in high temperature applications is limited to about 900oC. In this paper, the investigation’s results of the thermal properties of expanded perlite modified by spraying with alumino-silicate gel are presented. The sol-gel method allows to prepare very homogenous and reactive gel that enables synthesizing mullite phase at relatively low temperatures. The gel is also easy to be sprayed or used as coating of different materials e.g. lightweight fillers like expanded perlite. Such fillers modified with alumino-silicate gel after firing above temperature of forming mullite have better high temperature stability and durability. The DTA analysis shows that in case of alumino-silicate gel and gel modified perlite at 980oC the mullite phase is formed (Fig. 1). The powder X-ray patterns indicates that in case of gel and modified perlite the crystalline mullite phase is formed only above 1100oC (Figs 2 and 3). The physical thermal properties were also obtained. The thermal conductivity of samples were realized by steady-state method (20 and 50oC) and by hot wire method up to 1000oC. Obtained results show that modified perlite have still good thermal conductivity (Fig. 5). Figures 8 and 9 show saved porous structure of perlite grains reinforced with mullite crystals. Consequently, the thermal resistance and durability in high temperatures of such composites are significantly improved, up to 1300oC. Key words: high-temperature insulation, perlite, mullite, sol-gel