Laser-Induced Pressure Pulse (LIPP) Method
Fig. 1: Schematic representation of laser-induced pressure pulse (LIPP) method (left). The generated sample current shows peaks corresponding to the charge distribution, including the charges at the electrodes (right)

Pressure pulse methods can be used to determine the distribution of the charges in the thickness direction of dielectrics with µm resolution. One such method is the laser-induced pressure pulse method (LIPP method), which is illustrated in the left-hand part of Fig. 1 [1-3]. In this technique, a short laser pulse is absorbed at the sample surface. The heating of the surface layers generates a pressure pulse which propagates through the dielectric. Changes in geometry and permittivity caused by this pulse produce a current in the external circuit which is shown in the right-hand part of the Figure. The current at any instant is directly proportional to the charge density ρ(x), the polarization gradient dP/dx, and the gradient of the piezoelectric e-coefficient at the momentary location of the pressure pulse. The display at the oscilloscope shows therefore directly the distribution of these quantities as a function of location.

Fig. 2: Charge distribution in 25µm Teflon FEP films irradiated with electron beams of 20 and 30 keV as indicated. The first positive spike corresponds to the induction charge on the front electrode of the film while the large negative spike corresponds to the deposited electron charge [3]

The LIPP method is widely used to determine charge and polarization distributions in polymers. An example of such a study is the measurement of the deposition-profile of monoenergetic electrons in Teflon, as shown in Fig. 2 [3]. Measurements of this kind are important for basic investigations of charge storage and charge dynamics in dielectrics and have been used to test models of charge transport in polymers [4]. The LIPP method is also employed for practical applications such as the exploration of charge penetration into cable insulation [5].

Literature

  • [1] G. M. Sessler, J. E. West, and R. Gerhard, “Measurement of Charge Distribution in Polymer Electrets by a New Pressure-Pulse Method”, Polymer Bulletin 6, 109 – 111 (1981).
  • [2] G. M. Sessler, J. E. West, and R. Gerhard, “High-Resolution Laser-Pulse Method for measuring charge distributions in dielectrics”, Phys. Rev. Lett. 48, 563 – 566 (1982).
  • [3] G. M. Sessler, J. E. West, R. Gerhard-Multhaupt, and H. von Seggern, “Nondestructive Laser Method for Measuring Charge Profiles in Irradiated Polymer Films”, in IEEE Trans. Nucl. Sci. NS-29, 1644 – 1649 (1982).
  • [4] G. M. Sessler, M. T. Figueiredo, and G. F. Leal Ferreira, “Models of charge transport in electron-beam irradiated insulators”, IEEE Trans. Diel. and Electrical Insulation, 11, 192-202 (2004).
  • [5] G. M. Sessler, “Distribution and Transport of Charge in Polymers”, in Electrets, Vol. 2, 3rd Edition (R. Gerhard-Multhaupt, Ed., Laplacian Press, Morgan Hill, CA, 1999) Chapter 10, pp. 41 – 80.