IEEE Transactions on Electrical Insulation Vol. EI-22 No.4, August 1987 TEMPERATURE DEPENDENCE OF ION INJECTION BY METALLIC ELECTRODES INTO NON-POLAR DIELECTRIC LIQUIDS M. Nemamcha, J. P. Gosse, A. Denat and B. Gosse Laboratoire d'Electrostatique et de Materiaux Dielectriques C.N.R.S. Grenoble France ABSTRACT This study deals with the influence of temperature on the electrical conduction of solutions of electrolytes in n-decane. The temperature dependence of the ion mobility deduced from transit time measurements is due to the viscosity variation. The activation energy for the ion creation is rather well fore- seen from the Fuoss model of dissociation of ion-pairs and from the assumption that the distance between ions in an ion-pair decreases with temperature. From the injection current den- sity, we deduce that the charge density qA at the distance XA of closest approach from the metal follows the relation qA/qo=exp[eo/16'7rExAkT] whatever the liquid conductivity (a=2Kqo) and the temperature, and is entirely determined by the interactions of an ion with its image. This observation, in agreement with our previous ones, supports the assumption of a charge-transfer step involving only the electrolyte. INTRODUCTION It is now admitted that conduction of dielectric liq- uids is ionic for electric fields lower than 1 MV cm- at least. This result is deduced from charge carrier mobility measurements which have been generalized in liquids, whether pure [1,2,3] or containing different additives [4,5]. Electrohydrodynamic phenomena caused by ion injection have also been experimentally and theoretically investigated [6,7]. It was thus estab- lished that liquid motion is responsible for the high values of the ionic mobilities cited in the literature. Lastly, all doubt was erased when electronic mobilities measured in different pure liquids [8,9] were found about two or three orders of magnitude higher than the electrohydrodynamic mob ilities. The mechanisms of creation of ions in the bulk of the liquid or at the metal electrodes are also now known. Theoretical models have been experimentally verified in polar liquids and in non-polar ones [10,11,12]. We de- scribe ion injection by a two-step process, a step of creation of ions at the electrode (the charge transfer step) followed by a step of escape of ions out of the image-force region. This second step has been fully understood and interpreted, whereas the charge-transfer reaction at the electrode remains unknown. But, when the escape kinetic rate constant is far lower than the kinetic constants involved in the charge-transfer re- action, the field dependence of the injection current density can be established, i = qA exp(-e2/16ffExAkT) G(E) = qAG(E) (1) The function G(E) contains a tabulated modified Bessel function [12], xA is the distance of minimum approach of an ion from the metal, qA is the charge density at xA, eo is the electron charge and c is the liquid per- mittivity. The function G(E) originates from the es- cape step, whereas qA is determined by the charge-trans- fer reaction at the electrode. We have previously studied the dependence of the cur- rent density on the electric field, the electrode gap and the liquid conductivity [5]. With different liquids and electrolytes, we have ob- served that j follows the theoretical law (1) but is also proportional to the positive (or negative) charge density q0-a/2K, a being the liquid conductivity and K the ionic mobility (supposed to be equal for positive or negative ions). For instance in solutions of TIAP- at 7xlO-4 M in cyclohexane at room temperature a=5.5x1O-12 Qr'rrr', ql4=5.5x1 Cm3, qA=1 .46x107 Cm-3 if xA=O.3 nm, and qA/qo=2.8. To complete our knowledge of the ion injection process and especially of qA related to the charge-transfer re- action, we have investigated the influence of tempera- ture on the ion injection from metal electrodes into n- decane chosen for its high boiling point. 001 8-9367/87/0800-0459*0 1 .00 c@ 1987 IEEE 459