Study Of Surface Modification And Weatherability Of Nickel Powders For PPTC Materials

The polymeric positive temperature coefficient (PPTC) materials in which micron-sized nickel powders are used as electrically conductive fillers possess low electrical resistivity. However, the wide specific surface area and many flaws and bond suspended in the air on the surface of nickel powders, the surface energy of these powders is very high, nickel powders are in the thermodynamically unstable state, leading that the weaherability of PPTC materials decreases. Appropriate surface modification is used to overcome nickel powders’ flaws such as easy to oxide and agglomerate, the poor compatibility with organic polymers. The modification improves the physical and chemical properties of the nickel powders effectively and enlarges the application of nickel powders.Because appropriate amount of cobalt can improve the weatherability of nickel powders and silver-nickel alloy possesses great anti-oxidation, a layer of cobalt or silver was coated on the surface of nickel powders to avoid the direct contact between nickel powders and the air and to improve the anti-oxidation ability of nickel powders. The modified nickel powders were then used as the electrically conductive fillers in the polymer to improve the weatherability of PPTC materials.In this paper, the cobalt coated nickel powders were prepared through electroless plating in water, ethanol and ethylene glycol solvent system, respectively. The silver coated nickel powders were prepared through displacement reaction. The cobalt or silver coated nickel powders were investigated by SEM and EDS to analyze the morphology, surface structure and compositions. The results indicated that the cobalt coated nickel powders with good surface modification effect could be prepared as the reaction condition was modulated appropriately in all three solvent systems, the morphology and content of silver on the surface of nickel powders was greatly influenced by the reaction temperature.The modified nickel powders and polyethylene (PE) were prepared into PPTC materials through roll mill, hot pressing, radiated byγ-ray and punched into pieces. The PPTC materials were then underwent thermal aging treatment. The electrical resistivity of the PPTC materials was measured by four-wire method. The experimental results proved that when the coating attached tightly on the surface of nickel powders in both the cobalt and silver coated nickel powders, the weatherability of PPTC materials was improved. In addition, when the content of the silver on the surface of nickel powder was overdosed, the weatherability of PPTC materials will decrease.