Characterization of polyimide irradiated in silver solution with a pulsed electron beam

Abstract

Polyimide samples (50 μm × 12 mm × 12 mm) were immersed in AgNO 3 solution and exposed to a pulsed beam of 6-MeV electrons, with pulse width ∼1.6 μs and repetition rate 50 pps. The exposure time was changed from 20 and 50 min. Over this period the electron fluence received by the polyimide samples varied in the range of 2 × 10 15–5 × 10 15 e/cm 2. The presence of silver in the polyimide sample was confirmed by the XRF technique. The relative concentration of silver in these polyimide samples was found to increase with increasing irradiation time. These irradiated polyimide samples were also characterized by the RBS technique. The results showed the diffusion of silver in the polyimide samples to a depth of 2.5 μm, at room temperature. The measured value of the diffusion coefficient, D ′, of silver in the polyimide varied from 1.26 × 10 −13 to 9.3 × 10 −14 m 2/s over a range of electron fluence 2 × 10 15–5 × 10 15 e/cm 2. This value of the diffusion coefficient, D ′, is around six orders of magnitude higher than that of the thermal diffusion coefficient, D, of silver in polyimide at ∼300 °C. Clustering of the silver atoms and bridging between the silver atoms and the polyimide segments may be the causes for the observed decrease in the diffusion coefficient, D ′, at higher electron fluences. In addition, the polyimide samples were characterized by dielectric relaxation spectroscopy. The dielectric constant, ε ′, of the polyimide samples was found to increase with increasing silver concentration and reached a value of 4.12, as compared to 3.15 of the virgin polyimide, measured at 1 kHz. The increase in the dielectric constant, ε ′, is, however, not commensurate with the number of silver atoms diffused in the polyimide, as per the Clausis–Mossotti equation. This indicates that the free volume produced in the polyimide by electrons acts in the opposite direction to that of the silver atoms which enhance the dielectric constant, ε ′, through dipolar interactions with the carbonyl group of the polyimide. The diffusion of an appreciable amount of silver into polyimide at room temperature in a period of 0.3 s is an unique example of the radiation-enhanced diffusion of silver in polyimide by employing pulsed electron beams.