Thermal Shock Induced Phases Transformation and Microstructural Changes in a Novel Hydrogen Transport Membrane
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Thermal Shock Induced Phases Transformation and Microstructural Changes in a Novel Hydrogen Transport Membrane Lily, Yongjun Zhang1, Sukumar Bandopadhyay1, U. (Balu) Balachandran2, and Nagendra Nag3 1 College of Engineering and Mines, University of Alaska Fairbanks, AK 99775, U.S.A 2 Energy Systems Division, Argonne National Laboratory, Chicago, IL 60439, U.S.A 3 Group Manager, Advanced Process Development, Surmet Corporation, Buffalo, NY 14207, U.S.A ABSTRACT Bulk samples of a novel cermet (ceramic/metal composite) hydrogen transport membrane (HTM) were subjected to thermal cycling in the temperature range between 25-850°C to study phase transformations and microstructural changes under thermal shock. Scanning electron microscopy (SEM) and electron probe micro analyzer (EPMA) with energy dispersive spectroscopy (EDS) were used to characterize the microstructural and chemical changes in the membrane upon thermal cycling. SEM & EPMA analyses indicated that the temperature gradient during thermal cycling produced more micro-cracks inside the HTM disc, whereas, the chemical reaction between Pd and oxygen to form PdO disturbed the continuity of the metal palladium (Pd) - Yttria Stabilized Zirconia (YSZ) dual phases interconnection system from surface down. The agglomerates of un-crystallized YSZ grains found to be the inherent in the cracks of the asreceived HTM. A combination of trans-granular and inter-granular crack propagation results around the YSZ grains and the new precipitates. Based on the electron fractography analyses by both SEM and EPMA, the micro voids coalescence develops ahead of the crack tips in the crosssection of the HTM after 500 thermal cycles. INTRODUCTION A key part of U.S. Department of Energy’s FutureGen concept is to support the production of hydrogen to fuel a “hydrogen economy” with the use of clean burning hydrogen in powerproducing fuel cells, as well as for use as a transportation fuel. One of the key technical barriers to FutureGen deployment is a reliable and efficient hydrogen separation technology. Most Hydrogen Transport Membrane (HTM) research has currently focused on separation technology and hydrogen flux characterization [1-4]. One of the problems that exists with all composite hydrogen transport membranes is of strength and durability during operation. Mechanical properties are important for real-world applications. This is an area where data is insufficient or non-existent, particularly at high temperature. No significant work has been performed to determine the effects of the intrinsic factors (such as grain size and phase distribution) and extrinsic factors (such as temperature and atmosphere) on the micro-structural and thermosmechanical properties of HTM. In addition to the thermal expansion and the problem of thermal cycling, membrane standing in chemical gradients may suffer from chemical expansion, so that one side expands relative to the other as a result of a gradient in defect concentrations. This results in stresses and bending of the membrane, and is well-known in o
