Effects of gas phase on tree initiation in polyethylene

Abstract

A simple model was proposed by two of the authors for tree initiation in polyethylene, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> » <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in which the repeated process of charge injection and extraction between the electrode and the dielectric is responsible for tree initiation. It is made either directly between the two or indirectly via a certain gas phase which is considered to exist inevitably between the two. The former process is related to an electronic barrier which electrons are forced to jump over or tunnel through, i.e., the work function of a metal minus electron affinity of a dielectric. This paper deals with the latter process. Two kinds of experiments were carried out. First, a hypodermic needle was inserted into a polyethylene slab so that any kind of gas might be introduced to the tip of the needle. The test specimen referred to as the treeing specimen in this paper was set in a stainless steel cell filled with slightly pressurized SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> . Gases such as air, N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> were introduced from an outer gas reservoir to the needle system. Tree initiation voltages, corona pulse height spectra and shapes of the generated trees in their early stage were investigated. Second, a sewing-needle was used facing a polyethylene slab grounded on its opposite side. This is called the free space specimen in the paper for convenience. Corona starting voltages and pulse height distribution characteristics were measured when the stainless steel cell was filled with either air, N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> or SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> at atmospheric pressure. It is concluded that the existence of a thin gas-layer between a metal electrode and a dielectric will affect the tree initiation voltage in a typical treeing experiment, but that it is not caused by any preceding gaseous discharge. A process may be involved that the work function of a dielectric at its surface is affected by a gas through adsorption or chemical reaction, resulting in the change in the probability of electron injection.