טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBar Hedva
SubjectThe Electrical Behavior of Thermosetting Polymers
Containing Metal Plated Ceramic Filler
DepartmentDepartment of Chemical Engineering
Supervisor Professor Emeritus Moshe Narkis


Abstract

The objective of this research was to produce conductive composites based on highly crosslinked epoxy matrices and copper coated ceramic filler, to characterize the electrical resistivity and the PTC effect in these systems, and understand the relationship of these features and the materials' morphology.

Several ceramic fillers, such as glass fibers and mica flakes, were coated with copper by electroless plating. The conductive particles were incorporated at different loadings into an epoxy resin based on DGEBA with TETA curing agent. Percolation threshold for these systems was 1.7%-6% vol. of filler, corresponding to copper concentrations of 0.13%-0.44% vol. These materials can offer an improved alternative for conductive adhesives or conductive composites, characterized by low density and low filler content, which maintains the polymer properties.

Epoxy system containing copper coated mica particles is characterized by an extremely large PTC effect, changing the material's resistivity by more than 9 orders of magnitude to become an insulator. This effect is not followed by an NTC effect (resistivity reduction at higher temperatures), which makes the material suitable for applications such as current limiters and thermoelectric switching devices. Increasing the copper coated mica concentration raises the PTC temperature. Exposing the material to consecutive heating-cooling cycles results in a decrease in PTC temperature and increase of the room temperature resistivity.

Supported by TMA analysis, inverse relations were found between the coefficient of thermal expansion and the PTC temperature. No relation between glass transition temperature and PTC temperature was found. Accordingly, the main mechanism responsible for the PTC effect in the epoxy-mica composites is based on breakage of electrical contacts in the conductive network due to larger thermal expansion of the polymeric matrix compared to the ceramic filler.