(504g) A Novel Characterization of Pd/Ag Alloy Phase Nucleation and Growth Kinetics Via in-Situ Time-Resolved High Temperature X-Ray Diffraction Analysis

Authors: 
Ayturk, M. E., Worcester Polytechnic Institute
Payzant, E. A., Oak Ridge National Laboratories


Composite Pd/Ag alloy membranes have potential applications in H2 production via membrane reactors. The detailed reaction pathways and the alloy growth mechanism for the phase transformation between Pd and Ag are not fully understood. This is partly due to different preparation techniques and mainly due to incomplete alloying between co-existing metal-metal interfaces. It is therefore interesting to note that the conventional ex-situ studies, carried by conventional X-ray point scanning detectors, might fail to reveal key aspects of the Pd/Ag alloy formation and growth kinetics. A novel approach is to conduct an annealing study via in-situ using time-resolved, high-temperature X-ray diffraction analysis (HTXRD), by using linear position sensitive detectors (LPSD) and advanced optics, which infers kinetic information from changes observed in the XRD data as a function of time as the film is heated. LPSD collects the XRD data simultaneously over the 10° window, which dramatically shortens the data collection time and permits in-situ time-resolved studies. By the use of the advanced software JADE-6, a careful analysis of in-situ time-resolved HTXRD data permits qualitative observation of reaction pathways as well as quantitative investigation of phase transformations, alloy formation and grain growth kinetics. The objective of the present work is to study the formation of Pd/Ag alloys by in-situ observation of isothermal annealing using time-resolved HTXRD and to elucidate the mechanistic details of the Pd/Ag alloy phase growth by estimating the reaction order and the apparent activation energy for the FCC phase transformation. The time-resolved HTXRD experiments were performed using a Scintag PAD-X diffractometer equipped with a vertical Θ/Θ goniometer, a Buehler HDK 2.3 furnace, and an mBraun LPSD. Pd and Ag films were prepared by the electroless plating onto 1.5 cm2 pieces of porous metal substrates. The isothermal phase evolution of the Pd/Ag alloy at 500oC, 550oC and 600oC were quantified by both the Avrami kinetic model [1] and the parabolic rate law [2]. The value of the Avrami exponent, n, was found to increase with temperature with the values of 0.34, 0.39 and 0.67 at 500oC, 550oC and 600oC, respectively. This indicated that the nucleation of the Pd/Ag alloy phase was instantaneous where the growth mechanism was through diffusion-controlled one-dimensional thickening of the Pd/Ag alloy layer. For diffusion controlled processes, low values of the Avrami exponent such as those observed (n<1) were due to the rapid completion of the nucleation step [2,3]. Due to the fast nucleation kinetics, the overall growth rate of the alloy phase was governed by the mobility of the reactant species. This was in good agreement with the low Tamman temperature of Ag (344oC). In the case of parabolic rate law analysis, a noticeable deviation from the linear square root proportionality was observed, suggesting that the nucleation of the Pd/Ag alloy phase was through a heterogeneous nucleation mode, in which the nucleation sites were defined as the non-equilibrium defects. In addition, the cross-section SEM micrographs indicated that the Pd/Ag alloy phase growth was strongly dependent upon the deposition morphology of the as-synthesized Pd and Ag metals. Even though the presence of heterogeneous nucleation sites might substantially affect the growth of the alloy phase, regardless of how fast the nucleation process was, higher temperatures could overcome the nucleation barriers that might arise due to heterogeneous nucleation. Since the increase of the Avrami exponent, n, with temperature also indicated an enhancement in nucleation processes, both the Avrami and the parabolic rate law models were in good agreement in terms of revealing key aspects of the Pd/Ag alloy nucleation kinetics. However, it was found that the growth of the Pd/Ag alloy phase was better explained by the Avrami model than the parabolic rate law as evidenced by the linearity of the data. The estimated apparent activation energy based on the Avrami kinetic model and the parabolic rate law for the Pd/Ag alloy phase transformation was 236.5 kJ/mol and 185.6 kJ/mol, respectively. These were in the same order of magnitude and in reasonable agreement with the activation energy data reported for the self-diffusion of polycrystalline Pd/Ag alloys (0-20.2 wt% Pd), which were 239.5 kJ/mol and 183 kJ/mol for the temperature range 1123-1173 K and 988-1215 K, respectively[4]. Keywords: Pd/Ag alloys, electroless plating, composite membranes, H2 separation, isothermal annealing, in-situ time-resolved high temperature X-ray diffraction, phase transformation, nucleation and growth.