FY2010 - Oak Ridge National Laboratory

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Director’s R&D Fund— Neutron Sciences Results and Accomplishments During FY 2010, the first year of the project, we have made much progress in setting up materials synthesis of various components of fuel cells such as cathodes and electrolytes. We have also made great strides in setting up equipment to synthesize solid materials (Ball-mills and high-temperature furnaces) and to characterize electrochemical properties such as AC impedance spectroscopy, four probe conductance, thermogravimetric analysis, dilatometry, etc., which are crucial for new materials development for SOFC. On the electrolyte materials end, we have concentrated on improving the structural stability of a recently proposed rare earth ortho niobate proton conductor family (1.0% calciumdoped LaNbO 4 ). By substituting tantalum in the niobium site we have been able to increase the phase transition temperature of this material, which is the principal cause of thermal instability of this family of electrolytes. We have also studied the phase diagram of the Ln(Ba,Sr)Co 2 O 5+δ layered perovskite system, which is a candidate for cathode material for intermediate-temperature SOFC. We found that the increasing strontium solubility in Ba-site with decreasing size of Ln 3+ ion from Ln = Pr to Ho. The substitution of strontium for barium in YBaCo 2 O 5+δ prevents the phase decomposition at high temperatures, which allows the use of Y(Ba,Sr)Co 2 O 5+δ as a cathode in SOFC. Neutron beam time has been allotted this cycle (December 2010) and the next cycle (Feb 2011–June 2011) to carry out in situ neutron diffraction studies for both these systems. The principal accomplishment in the first year of this project was the construction of an integrated sample environment dedicated for in situ powder diffraction experiment at the Powgen instrument at SNS. An old vanadium furnace, obtained from the Intense Pulsed Neutron Source (IPNS), was modified, and measurements were made in the temperature range of (200–1000°C) using a powder sample of cathode material for SOFC in vanadium can. This furnace was then modified to an atmosphere furnace, which will allow the flow of various different gases using a quartz tube sample insert. The complete ensemble includes a permanent gas manifold system at the instrument equipped with 11 mass flow controllers housed in two separate gas cabinets, one of which is for hazardous and flammable gases. An oxygen sensor is available for measurement of pO 2 , and a thermogravimetric analyzer has been added to the furnace for independent measurements of weight gain or loss by the sample, and a residual gas analyzer is connected to monitor the out flowing gas mixture. Each of the separate components has been tested individually. The whole system is expected to be connected and tested at the instrument by the end of the first week of December. The initial commissioning measurements using this multi-probe ensemble are expected to take place in December of 2010. The results from the integrated in situ ensemble will be reported in the future. In FY 2010 two postdocs, Jung-Hyun Kim and Zhonghe Bi, were identified and hired to work on oxygen conductors (for cathode materials) and proton conductors (for electrolyte materials). 05511 Addressing Fundamental Challenges in Modeling the Recrystallization of Metallic Polycrystals through In Situ Neutron Diffraction Studies B. Radhakrishnan, S. B. Gorti, X-L.Wang, G. M. Stoica, and G. Muralidharan Project Description Structural and functional materials used in energy applications often derive their unique properties by preferred grain orientation (texture) obtained through precise thermomechanical processing routes. An 58
Director’s R&D Fund— Neutron Sciences important processing step is recrystallization, the formation and growth of specific texture components from a deformation substructure consisting of many dislocations. Despite years of research, a comprehensive understanding of the characteristics of recrystallization has been elusive. Even with the availability of the most sophisticated computers and simulation tools, a direct dislocation level simulation of recrystallization is nearly impossible. The project seeks to exploit the unique features of the VULCAN diffractometer at the Spallation Neutron Source (SNS) to perform in situ neutron diffraction investigation of the kinetics of texture evolution. The texture data in conjunction with the existing advanced microstructure evolution modeling and high-performance computing capabilities at ORNL will be used to develop an integrated, predictive, process-modeling tool for structural materials. The tool will be used to devise a new annealing procedure for wrought magnesium alloys by inducing and controlling the development of unique texture components for enhancing their room temperature formability. The successful completion of this research will lead to enhanced use of wrought magnesium sheets in automobiles resulting in significant energy savings through weight reduction. Mission Relevance The project focuses on the development of lightweight materials for structural and energy applications and is therefore of relevance to the mission of DOE’s Industrial Technology Program and Vehicle Technology Program. It is also of interest to the Department of Defense (DoD) and the <strong>National</strong> Aeronautics and Space Administration (NASA) because of the unique applications that the proposed approach might generate in defense and aerospace components. Results and Accomplishments We have performed the initial, in situ neutron diffraction measurements of recovery and recrystallization in Al-2Mg alloy and a commercial Mg alloy AZ31 using the engineering spectrometer, VULCAN, at SNS in order to understand the kinetics of the processes and the evolution of texture. In Al-2Mg the measurements clearly indicate the strengthening of Cube texture associated with recrystallization. In AZ31, the process that occurred predominantly at the test temperature was recovery associated with reduction in peak width. No significant difference in recovery kinetics was observed in Al-2Mg between different texture components. However, this conclusion must be validated using optimized experiments that will allow more efficient data collection at early times. We have extended the existing crystal plasticity models to Hexagonal Close Packed systems and also successfully mapped microstructure and texture from experimental samples to three-dimensional simulation domains. The crystal plasticity deformation model captures the texture evolution during deformation plane strain compression as well as the weakening of the basal texture in the presence of shear. An existing nucleation model based on “Excess dislocations” to explain Cube texture formation during recrystallization in Face Centered Cubic polycrystals was used to investigate the weakening of Cube texture during cross-rolling of aluminum. Since the in situ recrystallization experiments are the first of their kind at SNS, we are learning as we go along to better optimize the experimental conditions to improve the quality of our results. One such area is in rapid specimen heating and the control of sample temperature. We have initiated activities at SNS for exploring various specimen heating techniques which we will exploit in the second round of experiments. 59
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FY2010 - Oak Ridge National Laboratory

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