Observation of filament formation process of Cu/HfO 2 /Pt ReRAM structure by hard x-ray photoelectron spectroscopy under
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Sergey V. Ushakov Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit (NEAT ORU), University of California–Davis, Davis, California 95616
Meng Gu Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616
Nicole Schichtel and Carsten Korte Physikalisch-Chemisches Institut, Justus-Liebig Universität Gießen, Gießen, Germany. D-35392
Nigel D. Browning Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616; Pacific Northwest National Laboratory, MSIN K8-87, Richland, WA 99352
Yayoi Takamura Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616
Alexandra Navrotskya) Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit (NEAT ORU), University of California–Davis, Davis, California; and Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616 (Received 30 June 2011; accepted 6 December 2011)
Yttria-stabilized zirconia (YSZ)/Al2O3 multilayers deposited on Pt foil were studied by differential scanning calorimetry. Observed thermal effects were interpreted using additional evidence from x-ray diffraction and transmission electron microscopy. The crystallization temperature of YSZ increases from 344 to 404 °C as the layer thickness decreases from 15 to 4 nm. The enthalpy of crystallization becomes more exothermic with decreasing thickness, and it was measured to be 26 kJ/mol YSZ for 4-nm-thick layers and 12 kJ/mol for 15-nm-thick layers. The latter value is consistent with the reported crystallization enthalpy for YSZ powder of the same composition prepared by precipitation from aqueous solution. The more exothermic crystallization enthalpies for thinner films are indicative of a decrease in their degree of crystallinity. The 2–6-nm-thick Al2O3 layers remain amorphous when heated to 1000 °C. The described methodology enables thermal analysis of oxide thin films using commercial instruments. I. INTRODUCTION
Crystalline yttria-stabilized zirconia (YSZ) is commonly used as the electrolyte material in solid oxide fuel cells (SOFC) due to its high oxygen ion conductivity and its stability in both oxidant and fuel environments. However, problems related to cost and reliability require lowering SOFC operating temperatures. To maintain optimal SOFC performance, a number of ways to improve the conductivity of YSZ at these reduced temperatures are being investigated. Doping YSZ with Al2O3 was reported to enhance the low-temperature sintering behavior1,2 and scavenge a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.439 J. Mater. Res., Vol. 27, No. 6, Mar 28, 2012
Si impurities,3 leading to improvements of mechanical stability4 and to a lowering of the thermal expansion coefficient.5 There are also studies
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