Elemental strain and trapped space charge in thermoelectrical aging of insulating materials Part 1: Elemental strain under thermo-electrical-mechanical stress

The life model proposed previously by the authors ascribes electrothermal aging to the formation and/or enlargement of nanovoids driven by electromechanical energy stored in local concentrations of space charge. In the previous work, the strain associated with the mechanical stress produced by the space charge center was derived using a macroscopic stress-strain relationship. For chain conformation changes on the molecular scale, these macroscopic concepts may no longer be valid. In this paper, we have investigated the possibility that the local strain is an elemental property of the polymer morphology, as suggested by molecular quantum mechanical calculations. The elemental strain has been related to the bulk modulus through the elemental volume of the affected chain section and the number density of contributing centers. In this way an upper bound has been obtained for the elemental strain as a function of these variables. Under the assumption that the charge concentrations are formed by charges trapped in nanovoids, estimates have been obtained for the smallest magnitude of the local field that is able to initiate aging, and these estimates have been correlated with the maximum number of electronic charges that such a void may trap. By doing so, a connection is found between the aging model and experimental space charge densities. This connection is the subject of Part 2 of this paper.