A Study of the Kinetics and Energetics of Solid State Reactions in Pd/Sn Diffusion Couples
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at. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
relatively slowly as a function of rolling. For samples of average stoichiometry PdSn 4, layer thicknesses of the two metals after deformation were approximately 10.tm for Pd and 751.lm for Sn. The diffusion couples were hermetically sealed in aluminum pans for heating in a differential scanning calorimeter. Sealing took place in an Ar atmosphere of lOkPa. Next, a series of three identical differential scanning calorimetry (DSC) runs were performed where each sample was heated at 20K/min to a desired isotherm temperature and held there for 10 to 30 minutes. Heat flow data was taken during all three runs with the data of the third cycle being subtracted from the first for data analysis. After heating, most samples were examined by x-ray diffraction analysis to determine which intermetallics had formed. Some reacted samples were examined further with scanning electron microscopy (SEM) to measure grown intermetallic thicknesses and energy dispersive spectroscopy (EDS) analysis to identify these intermetallics. RESULTS AND DISCUSSION Phase Formation Sequence A sketch of the Pd-Sn phase diagram [14,15] is shown in Fig. 1. The phase diagram indicates that a number of equilibrium alloys can exist at room temperature. To determine whether certain intermetallics are more likely to form in the temperature range of our study (290-500K), we examined the phase formation sequence in solid state diffusion couples using x-ray diffraction and EDS analysis. Composites were examined before and after annealing. The presence of the most Sn rich intermetallic, PdSn 4, in as rolled composites was indicated by x-ray diffraction analysis. Figure 1- A sketch of the phase diagram for the system 114-15].
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1400
LPd/Sn
1287
So_1000-l~f 0010 200-0
Pd
20
820
40
1ooL
1tU I
• l 1k
Atomic Percent Sn
po
Sn
Upon heating the Pd/Sn diffusion couples to temperatures up to the melting point of Sn (505K), further growth of PdSn4 was observed. At temperatures greater than 470K, the growth of PdSn 3 was also observed with both the PdSn3 and PdSn 4 alloys growing in the temperature range of 470-500 K. At temperatures between 520 K and 620K, evidence for the formation of PdSn 2 was found. Figure 2 shows a plot of heat flow versus temperature for a rolled sample of average stoichiometry of Pd 3 Sn that was heated at 20 K/mmn from room temperature to 750K. The three reaction peaks are labeled to indicate that these temperature regions are associated with the initial 308
growth of a particular phase. This information was determined by heating remaining sample into the beginning portion of one of the three temperature regions and then immediately cooling to room temperature. The reacted sample was then examined with x-ray diffraction and EDS analysis to determine the intermetallic formations. As indicated in Fig. 2, these analyses showed that PdSn 4 was the first phase to form. The other two Sn rich phases were found to form next; PdSn 3 near 470K and PdSn 2 near 520K. '
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