Adsorption and Desorption Studies of Cesium on Sapphire Surfaces

Abstract

The adsorption/desorptioncharacteristicsof Cs on sapphiresurfaces have been studied using a combinationof surface analytical techniques. An approximateinitial sticking coefficient for Cs on sapphire has been measured using a reflection mass spectrometric technique and has been found to be 0.9. Thermal Desorption Mass Spectrometry(TDMS) and Auger El_tron Spectroscopy(AES) have been used to verify that a significant decrease in sticking coefficient occurs as ,e Cs coverage reaches a critical submonolayer value. TDMS analysis demonstrates that Cs is stabilized on a clean sapphire surface at temperatures (1200 K) in excess of the temperatures experiencedby sapphire in a TOPAZ-IIthermionic fuel element (TFE). Surface contaminants on sapphire can enhance Cs adsorptionrelative to the clean surface.C contamination eliminates the high temperature state of Cs desorptionfoundon clean sapphirebut shifts the bulk of the Cs desorption from 400 to 620 K. Surface C is a difficult contaminant to remove from sapphire, requiringannealing temperatures in excess of 1400 K. Whether Cs is stabilized on sapphirein a TFE environment will most likely dependon the relationship between surface contaminationand surface structure. I]Ib-'TRtBUTION 0F THISOOCUMEN1 IS ll_k.li_likll 4' DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government.

Electron induced surface chemistry at the Cs/sapphire interface

“…The (0oO1) surface shows a 50% increase in initial capacity with increased etch cycle whereas the (1102) surface shows a 5% decrease. This increase in Cs capacity for the (OOO1) surface is in contrast to previously reported measurements that show that surface de-oxygenation decreases the capacity for Cs adsorption on the c-plane (Zavadil and Ing 1994). XPS measurements show no significant increase in Cs surface concentration which indicates that the morphology of the deposit changes with etching.…”

“…Optimum annealing temperatures have been found to be 1200 K and !NO K for (OOO1) and (1 102) crystals, respectively, based on minimization of initial &n contamination to approximately 1 at.%. These crystals could be heated by temperature programming to 1400 K using resistive heating as previously described (Zavadil and Ing 1994). Thermal desorption data was collected using a heating rate of 5 Wsec.…”

Comparative analysis of recirculating and collimating cesium ovens

“…Similarly, at small coverage, alkali metals bound to oxide surfaces (such as SiO 2 or Al 2 O 3 ) by a strong chemisorption-ionic bond (Ea = 2.73 eV), and desorption of the alkali metal occurs between 600 and 1000 K. As the amount of Cs deposited increased, there is a reduction in the Cs desorption temperature to 350 K, reflecting a continuous decrease in the Cs adsorption energy to 0.8 eV with increasing Cs coverage. [117][118][119] Finally, melting most of the compound formed from the reaction of Cs with air that could have been formed due to an improper pumping is possible at high temperature (CsH at 170 ○ C, CsOH at 272 ○ C, and Cs 2 O and CsNO 3 below 610 ○ C 90 ). Thus, being able to heat the tube material to high-T looks like an important criterion to minimize Cs sticking.…”