Microstrains and Stresses Analysis in Electroless Deposited Thin Pd Films

Abstract

This work is aimed at the measurement with X-ray diffraction techniques of microstrains, intrinsic stresses, and thermal and hydrogen stresses in composite Pd porous metal (PM) structures and their release at high temperatures. In addition, the changes in the Pd microstructure upon heating were studied with SEM to determine the relation between stress release and microstructure changes. The initial microstrains, 0.29%, in the electroless deposited Pd layer were irreversibly released after annealing at 400 °C for 1 h in He atmosphere. The initial intrinsic stress, mostly due to the deposition method, was tensile in nature (104.7 MPa) and was also released at 400 °C for 1 h in He atmosphere. After the release of intrinsic stresses, the total stress was given by the sum of the thermal stresses (mismatch in coefficients of thermal expansion) and the H2 stresses due to the absorption of H2 in the Pd layer. The total stress (thermal + hydrogen stresses) was compressive and was released at temperatures higher than 400 °C with a significant change in Pd morphology. A model, valid in the 60−400 °C temperature range, was also developed to predict the total stress to which composite Pd membranes were exposed in the 250−400 °C temperature and the 1−5 bar H2 pressure range. The highest total stress, at 250 °C and 5 bar, was estimated to be equal to −260 MPa according to the model developed in this work. The lowest stress, at 400 °C and 1.5 bar, equaled −78 MPa. At temperatures higher than 400 °C, the model did not hold since stresses were released by plastic deformation; however, their value was lower than −78 MPa. The characterization of several composite Pd membranes prepared on porous metal (PM) supports showed that leaks formed at temperatures above 400−450 °C. Since leaks only formed at T > 400 °C, the magnitude of stresses only played a minor role in leak formation.