Effects of seeding on phase transformation and mechanical properties in complex-alkoxide-derived cordierite gel powder
- PDF / 1,444,911 Bytes
- 7 Pages / 576 x 792 pts Page_size
- 85 Downloads / 200 Views
Cordierite gel powder prepared by the hydrolysis of an alkoxide complex was seeded with crystalline a-cordierite particles. The effects of seeding on the phase transformation and mechanical properties were studied. The unseeded cordierite gel powder crystallized into a-cordierite via /A-cordierite at 1050 °C. The fi-a cordierite transformation caused microcracks which led to poor flexural strengths of about 30 MPa, although the strength of /A-cordierite was 160 MPa. The seeding appeared to promote a direct formation of the a-cordierite from the amorphous state. The development of microcracks due to /m-a cordierite transformation could be prevented by the modified crystallization behavior, resulting in the improved flexural strength of a-cordierite of 190 MPa. The seeding also lowered the crystallization temperature of a-cordierite to 900 °C, which could be attributed to the epitaxial crystallization of the complex-alkoxide-derived cordierite gel powder.
I. INTRODUCTION Cordierite (2MgO-2Al 2 O 3 -5SiO 2 ) or cordieritebased glass-ceramics attract considerable interest as electronics packaging materials, because of their low dielectric constants, low thermal expansion coefficients, and other features.1 Alkoxide methods have been applied to the preparation of cordierite gel powders,2"8 which exhibited remarkable reductions in sintering and crystallization temperatures. These powders with high specific surface areas2"4 were usually aggregated particles consisting of nanometer-sized primary particles, which may cause difficulties in the subsequent powder processing. We studied the optimum synthesis and hydrolysis conditions of an alkoxide-complex precursor of cordierite in order to improve the morphology and chemical homogeneity of cordierite gel powder, and obtained an unagglomerated powder of about 0.2 /urn, which appeared chemically homogeneous. 910 The powder morphology may facilitate the uniform packing of particles, leading to enhanced sinterability and mechanical properties. When preparing a-cordierite ceramics from an alkoxide-derived gel powder, a major problem presents itself. When a-cordierite appears as a major crystalline phase, the crystallization occurs via a /3quartz structure (//--cordierite). The fi-a transformation causes a volume expansion and a change in thermal expansion coefficient. Thus, high stresses are formed which lead to cracking.11'12 To prevent microcracking,
a>Now
with NGK Spark Plug Co., Ltd., Komaki, Aichi, Japan. with Kurosaki Refractories Co., Ltd., Kitakyushu, Japan. c*Now with Nippon Steel Corporation, Kawasaki, Japan.
b*Now
J. Mater. Res., Vol. 7, No. 8, Aug 1992
modification of the whole crystallization process (i.e., /x-a transformation) may be essential. Recently, epitaxial seedings have been demonstrated to offer the potential to modify the crystallization of alkoxide-derived gels or powders. Kumagai and Messing et al.13~i5 found that epitaxial seeding modified the crystallization of boehmite gel and significantly lowered the transformation temperature by effectively eli
Data Loading...
