Abstract
Calcium aluminate cement (SECAR TM 80) was loaded with 0, 27.5, 40, 50, 60 and 70 vol% hollow glass microspheres and processed macro-defect free by high shear mixing with polyvinyl alcohol as a rheological aid. Discs were formed by die-pressing and dielectric measurements obtained from 100 Hz to 2 MHz. Relative dielectric permittivity was lowered for all microsphere loadings and was lowest at 40 vol%, corresponding to an effective porosity of 21.7%. A direct correlation was found to exist between microsphere loading (effective porosity) and the relative dielectric permittivity. Volume loadings of 27.5 and 40% microspheres resulted in the largest number of unbroken microspheres after processing. When compared with four ideal dielectric mixing models, the cement-microsphere composites were found to correlate best with Maxwell and Lichtenecker mixing. Dissipation factor was lower over the measured frequency range for all microsphere loadings. No correlation was found to exist between effective porosity and dissipation factor. Dielectric losses at low frequencies can be attributed to unbound water and polyvinyl alcohol.
Published Version
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