Pd/Cu(220) interdiffusion under compressive stress

Highlights: • The applied compressive stress enhances the interdiffusion of the Pd/Cu(220) diffusion couples. • Activation enthalpy of Pd/Cu interdiffusion can be calculated by Matano approach and ESCA atom profiles. • The external compressive stress would lower the activation enthalpy for the Pd/Cu(220) interdiffusion. This work investigates the effect of the external compressive pressure on the interdiffsuion between Pd and Cu. By analyzing ESCA atomic depth profiling without/with the external compressive pressure, two important findings were observed: (1) Pd/Cu(220) interdiffusion was found to be promoted by the external compressive pressure. (2) Under external compressive pressure, Pd atoms diffuse across the interdiffusion interface into the Cu(220) side more than the Cu atoms across the interdiffusion interface into the Pd side. Combining Matano approach and the obtained ESCA atomic profiles, the interdiffusion diffusivities and the activation enthalpy (H) of Pd/Cu(220) interdiffusion can be calculated. The activation enthalpy (0.694 eV) of Pd/Cu(220) interdiffusion is reduced to 0.625 eV and 0.604 eV with applying external pressures of 125 MPa and 250 MPa, respectively. The present calculation of the activation enthalpy implies that the applying compressive stress results in negative changes of the activation enthalpy (δH) and the free energy of activation (δG) for the Pd/Cu(220) interdiffusion. With Gibbs equation, the negative changes of δG and δH can further deduce the relation of the entropy change (δS) and the activation of enthalpy (δH) for the Pd/Cu(220) interdiffusion system under the external compressive stress, which are (1) the change of the entropy has to be positive (δS > 0) or (2) the change of the entropy (δS) has to be smaller than the term of δH/T., i.e., δS.