Occurrence of SiO/sub 2/ on the contact surface and its dependence on electrodes in silicone vapor

As products made from silicone such as rubbers or oils have excellent characteristics, they are applied to various fields. Silicone vapor evaporates from silicone compounds. However, adsorbed silicone molecules on surfaces decompose to SiO/sub 2/ under high temperature. Since SiO/sub 2/ is a typical insulator, if SiO/sub 2/ is interposed between electrical contacts, contact failure easily occurs. In this paper, to clarify occurrence of SiO/sub 2/on the contact electrodes, both statically heating of the electrode applied by high voltage and dynamic condition of contacts for make-break switching operation in actual relays were studied. In the decomposition process, SiO/sub 2+/ ions occur. As the ions attract to cathode, it is expected that SiO/sub 2/ formed only on the cathode. Occurrence of SiO/sub 2/ was clearly found only on the surface of cathode in the statically heating the electrode under high DC voltage. On the contrary, for the dynamic operation of switching electrical load, SiO/sub 2/ was detected both electrode; however, amount of the products on the cathode rather lesser than anode. This difference in occurrence of SiO/sub 2/ between static and dynamic condition of contacts was explained by discharge phenomena. Namely, in the arc discharge, metallic and gaseous ions attack to the cathode surface and clean it. Also transfer of SiO/sub 2/ to the anode under the switching operation occurs.

I. Introduction

Silicones both as vapors and migration films greatly affect electrical contact performance in electromechanical devices. Silicone products are applied widely to various electronics because of their excellent characteristics. However, it has been reported that contact resistance of electromehanical devices with electrical contacts were degraded by the silicone which coats contacts surfaces in such a manner as migration, vapor transport, and direct coating [1] – [4]. Particularly, the silicone of low molecular weight evaporates from these silicone products and adsorbs easily on surfaces. Moreover residual low molecular weight silicone also evaporates gradually from silicone compounds such as a silicone rubber. In the presence of air, adsorbed silicone vapor decomposes to SiO₂ under elevated temperature. As SiO₂ is a typical insulator, if SiO₂ interposed between contacts, the contact resistance increases and its reliability degrade. This degradation is called as silicone contamination [5] – [11]. The silicone contamination for electrical contacts is very important problems to be solved its mechanism and protections.