Effect of Processing on Electrical Properties of Calcium Aluminate-Based Chemically Bonded Ceramics
Borglum, Brian Philip
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Permalink
https://hdl.handle.net/2142/72171
Description
Title
Effect of Processing on Electrical Properties of Calcium Aluminate-Based Chemically Bonded Ceramics
Author(s)
Borglum, Brian Philip
Issue Date
1993
Doctoral Committee Chair(s)
Buchanan, Relva C.
Department of Study
Materials Science and Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
The cementitious reaction has been used to successfully develop a calcium aluminate ceramic material that has a low dielectric constant, low dielectric loss, and high resistivity and maintains these properties at high relative humidities. The electrical resistivity of typical cements change from 10$\sp $ ohm$\cdot$cm for strongly dried materials to 10$\sp6$ ohm$\cdot$cm upon exposure to high relative humidity environments. This is attributed to the development of parallel conduction pathways in the connected porosity of the materials as they fill with water. The developed material maintains its insulating ability upon exposure to 93% relative humidity, with resistivity changing only to 10$\sp $ ohm$\cdot$cm.
This material was formed at moderate temperatures (200$\sp\circ$C) utilizing particle packing strategies that densified the system. Submicron silica fume not only reduced water demand during processing, but reacted chemically to form a silica-substituted hydrogarnet matrix. This produces dense microstructures and increases thermal stability to temperatures greater than 300$\sp\circ$C.
Processing steps were developed that aided in the densification of these materials. A dispersant system was developed for calcium aluminates that reduced water content and provided a high degree of fluidity, without accelerating the hydration reaction. Additionally, uniaxial pressing the cement pastes was used to remove water from the paste and force particle rearrangement into densely packed structures.
The dielectric constant of the developed material is approximately 11.5 at 1 MHz. Hollow silica microspheres were introduced into the chemically bonded matrix and reduced the dielectric constant to 5.1 at 1 MHz, which is in the range of values desired for microelectronic packaging applications. The low temperature nature of chemically bonded ceramics makes possible the formation of composite structures that would be difficult using high temperature processes.
Electrical properties are dominated by the degree of connected porosity, and not by the reaction product phase. The ability to minimize porosity ultimately determines the electrical resistivity, dielectric constant, and dissipation factor of the chemically bonded matrix when water vapor is present. Anhydrous hexagonal anorthite $\rm(CaAl\sb2Si\sb2O\sb8)$ was synthesized at moderate temperatures ($>$200$\sp\circ$C) under hydrothermal conditions and has very desirable strength and thermal stability properties. However, because it is porous in nature it is not suited for use as an electrical insulator in humid environments.
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