Residual Gases Present during the Outgassing and OAOR Cycling of Silver

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Abstract
High-purity silver powder (99.97%) subjected to isothermal outgassing, oxygen adsorption, outgassing, and chemical reduction (OAOR cycling) at 320 °C has been shown to contain 750 ppm of unreported volatile impurities and to be reproducible to the chemisorption of oxygen and other gases. To determine some of the volatile contaminants present before cycling and the effect of cycling on their release by the sample, a residual gas analyzer was used to study the gases which were desorbed by heating a sample of silver powder in stages to 900 °C in vacuo. A second sample was subjected to five OAOR cycles at 320 °C and then outgassed in stages up to 900 °C. During the isothermal outgassing of the uncycled sample, the partial pressures of carbon monoxide, carbon dioxide, oxygen, water, argon, and sulfur dioxide increased by as much as ten times between 300 ° and 410 °C and again between 540 ° and 565 °C. The increase in the residual gases results from a combination of bulk diffusion, surface reaction and desorption, and possibly the release of gases trapped in voids in the powder. Similar increases were seen for the cycled sample, except that sharper peaks were found in the temperature ranges of 505 °–535 °C and 720 °–730 °C. A third sample was OA cycled at 500 °C and outgassed in the same manner. Similar increases of the same impurities were found between 740 °–760 °C. Since silver powder sinters on outgassing above 200 °C, cleaning by chemical reduction cannot be carried out without strongly sintering the sample.

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