Behavior of silver and palladium mixtures during heating

The behavior of mixtures of silver and palladium during heating in both air and an inert atmosphere was studied using X-ray diffractometry (XRD), ther mogravimetric analysis (TGA), differential scanning calorimetry (DSC), dila tometry, and scanning electron microscopy (SEM). In situ high-temperature X RD studies on a commercial 20% palladium material with submicrometer-sized particles indicated that an intermetallic phase, most likely Ag3Pd, formed in air between 300 degrees and 400 degreesC, the same temperature range whe re a 13% linear expansion was measured by dilatometry. The DSC data indicat ed an esothermic peak at 340 degreesC, a temperature where the TGA results indicated that the material had picked up only 0.2%, oxygen, compared with the maximum of 1.4% at 525 degreesC. No PdO was detected by XRD at 400 degr eesC, which suggests that oxygen was being incorporated in the intermetalli c. Microstructural examination using SEM indicated that larger particles, w ith internal pores, had formed after heating in air to 375 degreesC, When t he material was heated in argon for 1h at 400 degreesC, no intermetallic ph ase or alloy formed, and minimal expansion occurred. When mixtures of large r silver particles (5-30 mum) with palladium particles (1-3 mum) were heate d in air, the maximum amount of expansion that occurred increased from 0% f or pure palladium up to a maximum of 18% at 75% silver. This result support s the conclusion that expansion is a result of formation of this new phase, in the presence of oxygen, not of the oxidation of the palladium.