Preparation Of Cordierite Porous Ceramics

Due to the low dielectric constant and low thermal expansion coefficient,cordierite ceramic is widely used in the field of high-frequency electronic insulatingmaterial, integrated circuits substrate, circuit templates and porous ceramic catalystcarrier, etc. In this study, magnesium chloride hexahydrate, aluminum chloridehexahydrate and silica sol were chosen as raw materials. Through the studies ofsynthetic cordierite porous ceramics, we investigated the effects of pH, addition ofmullite fiber for cordierite preparation and porous ceramic structure formation. X-raydiffraction, DSC-TG thermal analyzer, scanning electron microscopy and otheranalytical methods were used to characterize porous cordierite ceramics and we takethe following conclusions.(1)Using chemical co-precipitation to synthesize cordierite, it’s mainly basicchloride gel precipitation and silica particles in the gel under acidic conditions whileprecipitated hydroxides and the silica sol particles under alkaline conditions. Nocleaning to the gel, the ratio of each component of the material had no change. Itfinally generated cordierite phase after dehydration, decomposition, and solid-phasereaction during the heat treatment, and pH value had no significant impact on thecordierite material.(2)When using chemical co-precipitation experiments, with ammonia as theprecipitating agent, the recovery of Mg2+was less than10%under acidic conditionsafter cleaning. And excess ammonia, the recovery of Mg2+was only77%when theammonia was overdosed. The gel particle was of about20~30nm. At pH10, theamount of cordierite was80percent after heating at1350℃. It’s difficult to obtain purecordierite powder.(3)When using reverse phase chemical co-precipitation, Mg2+, Al3+werecompletely precipitated and we can get pure cordierite powders after heat treatment at1300℃.(4)Alcohol-water ratio have influence on the specific surface area of cordierite powder.(5)Mullite fiber can improve the cracking of cordierite porous materials, but itwould make the surface wrinkle when adding too much fiber, and the sample surfacewas not flat.(6)The samples exhibited the phenomenon of micro-expansion when addingsmall-size low content mullite fiber at900℃while contraction in higher temperature.The shrinkage of fiber-samples was significantly less than the non-fiber-samples. Theapparent porosity of fiber-samples was lower than those non-fiber-samples and thebulk density was higher.