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), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dilatometry, and scanning electron microscopy (SEM). In situ high-temperature XRD studies on a commercial 20% palladium material with submicrometer-sized particles indicated that an intermetallic phase, most likely Ag 3 Pd, formed in air between 300° and 400°C, the same temperature range where a 13% linear expansion was measured by dilatometry. The DSC data indicated an exothermic peak at 340°C, 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°C. No PdO was detected by XRD at 400°C, which suggests that oxygen was being incorporated in the intermetallic. Microstructural examination using SEM indicated that larger particles, with internal pores, had formed after heating in air to 375°C. When the material was heated in argon for 1 h at 400°C, no intermetallic phase or alloy formed, and minimal expansion occurred. When mixtures of larger silver particles (5-30 μm) with palladium particles (1-3 μm) were heated in air, the maximum amount of expansion that occurred increased from 0% for pure palladium up to a maximum of 18% at 75% silver. This result supports 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.