Nanodielectric Properties of High Conductivity Carbon-Loaded Polyimide Under Electron-Beam Irradiation
Poster presented at the 2013 IEEE International Conference on Solid Dielectrics (ICSD), Bologna, Italy, June 30-July 4, 2014. PDF of poster is available for download through link above.
High conductivity Black KaptonTM is a common material, with a polyimide matrix that has been loaded with turbostatic carbon particles to increase its conductivity. On a macroscopic scale, Black KaptonTM acts as a good conductor. However, on the nanoscale the material exhibits both conducting and dielectric properties. The length scale is set by the size of the carbon soot particles (~100-500 nm) and the separation of carbon particles (~20-2000 nm) in the carbon-depleted surface layer (~1000-5000 nm depth). The separation distance range is comparable to the penetration depths of ~0.5-10 keV electrons.
Electron irradiation experiments were conducted to investigate the electron emission, transport, charging, discharging and cathodoluminescence properties of high conductivity Black KaptonTM. Measurements were conducted in an ultrahigh vacuum electron emission test chamber. A high energy electron gun, with monoenergetic beam energies ranging from 200 eV to 25 keV and flux densities from 0.1 nA/cm2 to 100 nA/cm2, deposited electrons in the surface layer of the material. Measurements were conducted from <40 K to>350 K, using both a l-N2 reservoir and a closed cycle He refrigerated cryostat. The dark current and radiation induced conductivities in polyimide are several orders of magnitude lower at the low end of this temperature range, leading to reduced charge dissipation and enhanced charging and electrostatic discharges at low temperatures.
Various experiments measured transport and displacement currents to a rear grounded electrode, absolute electron emission yields, electron-induced absolute photon emission yields and photon emission spectra (~250 nm to 1700 nm), and arcing rates and location. “High conductive” Black KaptonTM exhibited numerous arcing events from the material edge to an electrically isolated grounded sample holder (particularly at lower temperatures), which are indicative of charge accumulation within the insulating regions of the material. Three types of light emission were also observed: (i) long duration cathodoluminescence that turned on and off with the electron beam, (ii) short duration (<1 >s) arcing resulting from electrostatic discharge, and (iii) intermediate duration (~100 s) glow that dissipated exponentially with time after infrequent and rapid onset.
We discuss how the electron currents and arcing, as well as light emission absolute intensity, frequency, duration, location on the sample, and correlations with other phenomena depend on electron beam energy, power, flux and fluence and on temperature, charging and penetration depth. We discuss how these results are related to the nanoscale structure of the composite material. The results have important consequences wherever Black KaptonTM is used in a charging environment—particularly at low temperature vacuum environment where charge dissipation is minimized— such as for spacecraft charging concerns in the space industry where Black KaptonTM use is ubiquitous.