Diffussion implemented silver and cesium in glassy carbon

Abstract

Glassy carbon also known as vitreous carbon has a number of special properties making it a very interesting material with many applications. It is a non-graphitizing carbon which combines glassy and ceramic properties with those of graphite. Among the several interesting and unique properties of glassy carbon, are: extremely high corrosion resistance, high temperature resistance up to 3000C, and impermeable to gases and liquids. Glassy carbon is manufactured by the carbonization (pyrolysis) of an organic precursor. It consists of very small crystallites randomly orientated. The bonds are mostly sp2 but also with some sp3. The effects of ion implantation and subsequent annealing on the surface topography, structural changes and on the diffusion of cesium and silver into glassy carbon (Sigrador® G) are reported. The in-diffusion investigation was only done for the silver into the glassy carbon. The glassy carbon samples with 100 nm thickness layer of naturally occurring silver on their surface were vacuum annealed for 5 hours at temperatures of 600C and 750C. No silver in-diffusion was observed but the silver conglomerated on the surface into island structures Cesium and silver ions were separately implanted into glassy carbon at 360 keV to a fluence of 2 × 1016 ions/cm2 at room temperature, 350C and 600C. To remove surface irregularities before implantation the glassy carbon samples were mechanically polished with diamond paste to a mirror finish and chemically cleaned.

The cesium and silver implanted glassy carbon samples were investigated under different annealing conditions. The influence of ion implantation and annealing on the surface topography was investigated by the scanning electron microscopy (SEM). The SEM results of the samples showed that implantation and subsequent annealing strongly influenced the surface morphology of the glassy carbon. The SEM also showed a great difference in the surface topography of the polished virgin glassy carbon surface as compared to the as-implanted samples. The effects of laser beam annealing under ambient air and moisture during Raman measurements at high power on the cesium implanted glassy carbon sample are also reported. The SEM study on the spot where the laser beam was focused showed that the damaged carbon layers were removed, exposing the bulk of the material that was not affected by cesium implantation. The Raman spectroscopy was used to monitor the corresponding structural changes induced in glassy carbon due to ion implantation and subsequent annealing. This investigation was only done for the cesium implanted glassy carbon. The Raman spectrum of the polished virgin glassy carbon surface, which is a disordered carbon, showed two sharp peaks, namely the D (disorder) and G (graphite) peaks at 1350 cmand cm1, respectively. The Raman study showed that there was less damage in to glassy carbon when cesium was implanted at high temperatures than at room temperature. Upon annealing the room temperature cesium implanted glassy carbon sample, Raman spectroscopy showed that there was some recrystallization of the glassy carbon. This means that some of the damage due to implantation were annealed out. The depth profiles of the as-implanted samples and subsequent annealing were obtained by Rutherford backscattering spectrometry (RBS). RBS-determined depth profiles of the glassy carbon implanted with cesium ions revealed that there is a strong redistribution of the cesium ions towards the surface. The redistribution of the cesium occurred already at room temperature implantation and enhanced at elevated substrate implantation temperature. However, no implanted cesium atoms were lost in that process, but rather accumulated on the surface of glassy carbon. Contrary, annealing the glassy carbon implanted with cesium at room temperature, resulted not only in the diffusion and redistribution of cesium but also in a significant sublimation/evaporation of cesium into the vacuum. The RBS determined depth profiles of the glassy carbon implanted with silver at room temperature, then annealed isothermally at 350C at times ranging from 30 minutes to 3 hours showed not much diffusion of silver into the glassy carbon. There was no real broadening of the Ag implanted profile, indicating no or little diffusion of the silver into the bulk of the substrate material and towards the surface. Isochronal annealing of the room temperature silver implanted glassy carbon sample for 1 hour at temperatures ranging from 400C to 700C showed continuous loss of silver. After annealing at 700C, the silver disappeared completely from the glassy carbon being lost in the vacuum. To calculate the diffusion coefficient of silver in glassy carbon, the optimum temperature of 575 C was chosen. The diffusion coefficient of silver into glassy carbon was calculated to be D = 5.30 × 102 nm2/s.