Investigation on electrical properties of polyvinyl acetate/graphite adhesive by joule heating and hall effect tests

decreasing and increasing electrical resistivity of materials by increasing temperature named Negative thermal coefficient (NTC) and positive thermal coefficient (PTC), respectively, are two critical properties for thermistor materials. In this research, the PTC/NTC behavior of conductive adhesive are investigated by performing two tests: Hall Effect and Joule heating tests. In Hall Effect, the temperature increases by the heating element, while in Joule Heating, the ohmic resistance of adhesive leads to increasing the temperature. The adhesives are made of polyvinyl acetate as binder and 40, 50, 60, and 70 wt.% Graphite (G) as filler. Characterizations include the lap-shear strength, electrical conductivity, and Archimedes porosimetry tests, as well as Fourier-transform infrared spectroscopy, Raman spectroscopy, and Scanning Electron Microscopy analysis to investigate the change of chemical composition and crystallinity of carbon adhesives before and after the Joule Heating test. There are little information about the physical mechanisms of NTC/PTC behavior in literature. NTC behavior of carbon adhesives is interpreted by two physical mechanisms. The first is based on increasing temperature on the regions named hot spots, thereby decomposing polymer to amorphous Carbon (validate by experimental analyses) and the second, decreasing polymer viscosity and, consequently, s rotating and aligning the G particles, validated by mathematical modeling.

Automated summary

The PTC/NTC behavior of conductive adhesive was investigated through Hall Effect and Joule heating tests, revealing two physical mechanisms for the NTC behavior of carbon adhesives.