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oai:www.ideals.illinois.edu:2142/50388 2023-07-11 col_2142_5131 col_2142_13836 com_2142_5130 com_2142_13835 com_2142_234

Martin, Lane W. Martin, Lane W. Cahill, David G. Zuo, Jian-Min Cooper, S. Lance Rama Damodaran, Anoop 2014-09-16T17:12:28Z 2014-09-16T17:12:28Z 2016-09-22T20:59:22Z 2014-08 2014-09-16 2014-08 Over the last few decades, considerable attention has been given to the development of lead-based ferroelectric systems such as PbZr1-xTixO3 due to their robust high temperature ferroelectric properties, as well as the presence of the so-called morphotropic phase boundary (MPB) ̶ a temperature-independent composition-driven structural instability that results in superior dielectric and piezoelectric properties. However, increasing environmental concerns are driving efforts towards the development of lead-free ferroelectrics such as BiFeO3, BaTiO3, and others. Previous work on epitaxial BiFeO3 thin films have shown that large compressive strains can drive the formation of complex mixed-phase structures with enhanced electromechanical responses (4-5% strains). In this work, we probe the nanoscale distribution of phases present in these mixed-phase structures using a combination of epitaxial thin-film growth and characterization techniques such as x-ray diffraction and piezoresponse force microscopy. We show, for the first time, the presence of monoclinic distortions and intermediate phases (akin to conventional MPB systems) in the mixed-phase films that are crucial for enhanced electromechanical responses. We then present thickness- and temperature- dependent phase-evolution studies that indicate the presence of a strain-spinodal between the various structural polymorphs of BiFeO3 to be the origin of mixed-phase formation. Finally, we discuss limitations due to a breakdown in epitaxy that occurs in thicker films and present chemical alloying-based approaches to mitigate these challenges. Having highlighted strain relaxation with increasing film thickness as a fundamental limitation to epitaxial-strain control, we explore an alternative route involving the use of a combination of defect-engineering and epitaxial strain to stabilize enhanced deformation states in materials. For this, we present a systematic study of BaTiO3 thin films, and show that epitaxial strain can be used to control the ordering of growth-induced defects driving deterministic additional out-of-plane strains that can enhance the ferroelectric Curie temperature to values exceeding 800°C without any need to change substrates. Such a combined control of epitaxial strain and engineered defect-structures provides a new pathway to extend the limits of strain-control of materials and properties. Lastly, we investigate a new route involving the use of epitaxial strain in conjunction with controlled composition- and strain- gradients to tune the thermal stability of dielectric responses of ferroelectric thin films. We present preliminary studies that reveal enhanced relative dielectric permittivity values of ~750, that change by less than 10% over a wide temperature range from 25-350ºC in compositionally-graded epitaxial BaxSr1-xTiO3 thin films, which is promising for next-generation microwave applications. Item withdrawn by Laura Spradlin (lspradl2@illinois.edu) on 2014-07-15T17:31:50Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 1 Rama Damodaran_Anoop.pdf: 7610290 bytes, checksum: 170c6437e2866ece2f44ad038f192a64 (MD5) Made available in DSpace on 2014-09-16T17:12:28Z (GMT). No. of bitstreams: 2 Anoop_Rama Damodaran.pdf: 7610290 bytes, checksum: 170c6437e2866ece2f44ad038f192a64 (MD5) license.txt: 4069 bytes, checksum: fb3261712cbff2c5fb17ad5400f13a9f (MD5) Embargo set by: Seth Robbins for item 50499 Lift date: 2016-09-16T17:13:01Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system Limited Restriction Lifted for Item 50499 on 2016-09-22T20:59:22Z. http://hdl.handle.net/2142/50388 en Copyright 2014 Anoop Rama Damodaran bismuth ferrite (BiFeO3) thin films electromechanical responses phase transitions barium titanate (BaTiO3) BaxSr1-xTiO3 Lead-free Epitaxy Crystal structure New modalities for strain engineering of lead-free perovskite ferroelectric thin films text Materials Science & Engineerng 1919 Materials Science & Engr 0130 University of Illinois at Urbana-Champaign Dissertation Ph.D. PHD:Materials Sci & Engr -UIUC 10KS0130PHD