March 1998 Ž . Materials Letters 34 1998 364–371 Analysis of electric conduction mechanisms in polyethylene terephthalate Eugen R. Neagu ) , Rodica M. Neagu Physics Department, Technical UniÕersity of Iasi, Str. Copou, N 22, Iasi 6600, Romania Received 3 March 1997; revised 22 July 1997; accepted 23 July 1997 Abstract The isothermal current–voltage measurements on insulating materials can be improved by using a symmetric triangular signal. The measured current is field dependent and the Richardson–Schottky, the Fowler–Nordheim, the Poole–Frenkel, Ohm, space charge limited current and ionic current mechanisms can be used for the interpretation of the results. The experimental data can fit with two or more from the presented mechanisms. In deciding which is the best mechanism, it is necessary to use a second condition. We propose to use the slope of the Richardson–Schottky and the Poole–Frenkel plots to calculate the dielectric permittivity of the sample. For high fields the work function can be determined from the Fowler–Nordheim plot and can be used as a control parameter. Polyethylene terephthalate was used as standard material to Ž y1 y1 . test the validity of these models under moderate electric fields 0.5 MV m -E -20 MV m . For low fields Ohm’s law describes the current flow. The Richardson–Schottky law is adequate only for a small field range. The Poole–Frenkel law fits best the experimental data. For high fields the Fowler–Nordheim law fits as well the experimental results. The charge accumulated in the sample strongly influences the measured current. q 1998 Elsevier Science B.V. Keywords: Electrical conduction; polyethylene terephthalate 1. Introduction Though the insulating organic materials are used in many applications with good results, few things are exactly known about electric conduction mecha- nism in these materials. This situation is determined on one hand by the fact that little experimental information can be obtained, and on the other hand the measured quantities are complex being time, temperature, electric field and structure dependent. The conductivity of insulating solids has received considerable attention and much literature on the ) Corresponding author. E-mail: eneagu@sb.tuiasi.ro. w x topic exists 1–4 . We have studied the contribution of the conduction and the displacement currents to Ž . space charge SC accumulation in the insulating w x materials 5–7 . The result was that the charge that flows through the circuit gives rise to a SC cloud in the sample at the anode and at the cathode. The wx isothermal charging current decay for a long time 8 and subsequently it starts to increase. It is not clear how long the time must be till the current has a steady state value. To overcome this situation, we proposed to use a positive triangular symmetric sig- nal with high repetition period to measure the current that flows through an insulating material as a func- tion of the applied electric field at different constant 00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0167-577X 97 00192-4