TRANSIENT THERMAL-GRADIENTS IN BARIUM-TITANATE POSITIVE TEMPERATURE-COEFFICIENT (PTC) THERMISTORS

Barium titanate positive temperature coefficient (PTC) ceramic disk th ermistors can suffer major mechanical damage if inhomogeneous heating occurs under voltage. The steady-state and transient temperature distr ibutions for thin disk samples (radius of 5 mm, thickness of 2 mm) hav e been studied with an infrared microscope, using a spatial resolution of 35 mu m. The transient temperature distribution is observed to be particularly sensitive to the electrical boundary conditions during th e initial heating period after application of a voltage. Small variati ons in electrode symmetry can lead to axial asymmetric thermal gradien ts up to 25 K/mm across the entire rim when an ac voltage of 100 V is applied, A finite-difference model in two dimensions, based on solutio n of the heat equation with a local temperature-dependent Joule heatin g-source term, has been developed to describe the axial and radial tra nsient temperature distributions in the cylindrical geometry. The pred ictions reveal current concentration at the edge of an electrode when the metal layer coverage is slightly smaller than that of the opposite face. This phenomenon results in stronger localized heating in a ring that initiates the thermal gradient.