Solid state synthesis and its characterization of high density cordierite ceramics using fine oxide powders

Abstract

First, we succeeded in synthesizing and sintering cordierite ceramics via the solid state reaction route without the melting process by using single phase oxide powders. They are highly pure and fine amorphous silica, α-alumina and magnesia powders. Usually, cordierite ceramics was synthesized by a sol-gel route. Alternatively, it was prepared by a solid state reaction route using impure minerals such as talc, or the other which required melt-quenching and crystallization processes, though using single phase raw material powders. This is probably because single phase raw material powders especially alumina are deficient in reactivity. We have succeeded in synthesizing cordierite ceramics via the solid state reaction route without melt-quenching and crystallization processes, by adopting a reactive α-alumina as one of raw material powders. As a result, cordierite formation of the starting raw material mixture went to completion at 1270°C. The mixture was sintered at 1430°C for 2 h. Its bulk density reached 98% theoretical, its flexural strength reached 243 MPa, which agreed with the best value already reported, its fracture toughness was 3.5 MPa·m1/2 far larger than the reported value of 2.3 MPa·m1/2, its thermal expansion coefficient (from RT. to 800°C) is 1.8 × 10-6/K, which might be probably the ultimately smallest value experimentally expected and its dielectric constant was sufficiently small as 4.8 to 5.2 between extremely a wide frequency range, namely, from 1 MHz to 1 GHz with low loss tangent of < 6 × 10-3. These properties were found more excellent or at least comparable compared with the best properties reported already and suitable for substrate materials on micro-electronic applications.