Author: Yue, I-hung Raymond
Title: BaPbO3/PE composite for PTCR applications
Degree: M.Sc.
Year: 1999
Subject: Composite materials
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Applied Physics
Pages: xiii, 84 leaves : ill. (some col.) ; 30 cm
Language: English
Abstract: The positive temperature coefficient of resistivity (PTCR) effect of a barium metaplumbate / polyethylene (BaPbO3 / PE) composites with 12 vol% of BaPbO3 were studied. The composite samples were prepared by hot-pressing a mixture of BaPbO3 ceramic and high-density polyethylene powders around the melting point of polyethylene. The composites exhibit a pronounced PTCR effect of up to a eight-decade increase in resistivity within a narrow range of temperature (~ 10 C). The dependence of the room temperature resistivity and the magnitude of the resistivity jump on the pressing and annealing temperature, and the electrical behavior after repeated heating-cooling cycles were investigated. The fracture surfaces of the composite samples were examined in a scanning electron microscope in order to correlate the electrical behavior with the microstructures. Topics discuss in these chapters are as follows: 1. The formation of traditional barium titanate-based PTCR materials (BaTiO3) and their applications were introduced. The analysis of a typical PTCR curve with an abrupt jump of resistivity at the Curie point and the limitation on the electrical conductivity change in this type of PTCR thermistors were also discussed. Because of the particular characteristics of the PTCR materials, they can be widely used for the control of power consumption, circuit protection, overcurent sensor, and in the electrical heating, in many electrical appliances. 2. Other than ferroelectric BaTiO3, BaPbO3 conductive ceramic with a much lower room temperature resistivity was found to be an alternative choice of PTCR material when it is mixed with insulated polymer. Both the BaTiO3 and BaPbO3 have similar perovskite structure except for the difference in the the central atoms, namely, Ti and Pb. The origins of the electrical conductivity in BaTiO3 and BaPbO3 ceramic materials were interpreted by a grain boundary model which give rise to an accumulation of the space charge layer and by an impurity conduction theory, respectively. 3. The basic linear structure of the polymer polyethylene(PE) was introduced and the melting point of the polyethylene used in the present study was found by differential scanning calorimetry (DSC). The effect of polyethylene melting on the electrical conduction in a composite was evaluated. Bulk polymers crystallized from the melt form spherulites which composed of lamellae. When crystallization takes place, lamellar becomes thicker that would give rise to the resistivity increase in a composite system and enhance the PTCR effects. It was found that crystallization of the polymer host is an important factor for the PTCR effects because of the breaking of the conductive network at the crystalline melting point due to thermal expansion of the polymer. 4. When the conducting ceramic particles in the polymer forms a continuos path or are separated by small intergrain gaps at the percolation threshold, the resistivity of the composite decreases sharply. The concept of how tunneling current depends on the filler concentration is also described. Fabrication and measurement of the BaPbO3/polyethylene were also reported. 5. The effect of processing on the BaPbO3/polyethylene composites was analyzed. It was found that the lower room-temperature resistivity occurs in sample hot pressed at 130 C. SEM micrographs showing the formation of conductive network was presented. A large PTCR effect ~6 orders of magnitude was obtained by annealing the composite at 90 C for 3 hours after being hot pressed at 135 C. This sample has an uniform distribution of BaPbO3 particles. As measurements were repeated, the room-temperature resistivity of the composite increased and the PTCR effect decreased gradually because of the aging effect.
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Access: restricted access

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