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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices

The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. Furthermore, the reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Large tetragonality and room temperature ferroelectricity in compressively strained CaTiO 3 thin films

We report that ferroelectricity and piezoelectricity are desirable for a variety of high-temperature applications such as actuators and sensors in heat engines, high-temperature manufacturing, and space technologies; however, the number of such material candidates is currently limited. Here, we demonstrate that CaTiO 3 , the first discovered perovskite mineral that is abundantly found in the Earth, as a non-polar bulk form, becomes a high-temperature ferroelectric oxide under compression. A strain-phase-temperature diagram of CaTiO 3 is created by growing a series of thin films with a range of compressive and tensile strains. Using temperature dependent optical second harmonic generation analysis, we show that tensile strained films exhibit predominantly in-plane polarization with orthorhombic-like point group symmetry with a phase transition below room temperature. On the other hand, compressively strained CaTiO 3 films exhibit a neartetragonal unit cell with a c/a ratio of 1.03, larger than that of classic ferroelectric, e.g. BaTiO 3 (c/a~1.01). These films exhibit a robust and switchable out-of-plane polarization at room temperature, with a ferroelectric transition temperature up to ~800 K. Density functional theory calculations reveal that compressive strain gives rise to a large out-of-plane displacement of Ti-cations inside the TiO 6 octahedral cages responsible for the strain induced polarization of ~ 9μC/cm 2 . Given that nearly half of the perovskites exhibit the bulk symmetry of CaTiO 3 , compressive strain tuning of this family may prove to be a fertile ground for the discovery of strain-induced piezoelectrics and ferroelectrics at high-temperatures. High-temperature piezoelectric sensors are important for many industries such as automotive, aircraft, and aerospace, as well as nuclear and geothermal energy, where dynamic structural monitoring of high-speed rotors in engines, turbines, and power generators is critical for safety and reliability.[1, 2] Modern sensors are however limited, where the main challenges are maximizing operating temperatures while maintaining good sensing capability and low electrical leakage.[3] High-temperature piezoelectric materials are typically derived from BaTiO 3 or PbTiO 3 based ceramics and single crystals, as these materials are also ferroelectric, meaning that they have a built-in spontaneous polarization (Ps). In bulk, these standard ferroelectric perovskite materials yield maximum operating temperatures ranging from 250-450 °C.[4], where above these temperatures the spontaneous polarization (P) and piezoelectric properties disappear due to the onset of a ferroelectric-to-paraelectric phase transition (TC). As next-generation engines, turbines, and reactors require hotter operating temperatures above 500 °C, materials with enhanced ferroelectric properties and increased ferroelectric transition temperatures will be needed in such extreme conditions. A promising way of boosting ferroelectric transition temperatures and polarization is through epitaxial strain engineering,[5] where tremendous in-plane distortions can be applied to thin film crystal materials, well beyond the point at which their bulk ceramic and single crystal counterparts would crack. Strain engineering has proven to drastically enhance ferroelectric polarizations and transition temperatures in classic ferroelectric materials such as BaTiO 3 [6] and PbTiO 3 [7], and strain can even turn on ferroelectricity in otherwise non-polar materials such as SrTiO 3 and CaTiO 3 ,[8] although reported strain-induced ferroelectric transition temperatures are only at room temperature or below.[9-12] In this work, we will focus on strain-induced ferroelectricity in CaTiO 3 , the first discovered perovskite mineral and long known to be a simple nonpolar insulating crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Thermodynamic Theory of Proximity Ferroelectricity

Proximity ferroelectricity has recently been reported as a new design paradigm for inducing ferroelectricity, where a nonferroelectric polar material becomes a ferroelectric one by interfacing with a thin ferroelectric layer. Strongly polar materials, such as AlN and ZnO, which were previously unswitchable with an external field below their dielectric breakdown fields, can now be switched with practical coercive fields when they are in intimate proximity to a switchable ferroelectric. Here, we develop a general Landau-Ginzburg theory of proximity ferroelectricity in multilayers of nonferroelectrics and ferroelectrics to analyze their switchability and coercive fields. The theory predicts regimes of both “proximity switching,” where the multilayers collectively switch, and “proximity suppression,” where they collectively do not switch. The mechanism of the proximity ferroelectricity is an internal electric field determined by the polarization of the layers and their relative thickness in a self-consistent manner that renormalizes the double-well ferroelectric potential to lower the steepness of the switching barrier. Further reduction in the coercive field emerges from charged defects in the bulk that act as nucleation centers. The application of the theory to proximity ferroelectricity in Al x−1⁢ Sc x ⁢N/AlN and Zn 1−x ⁢Mg x ⁢O/ZnO bilayers is demonstrated. The theory further predicts that dielectric-ferroelectric and paraelectric-ferroelectric multilayers can potentially lead to induced ferroelectricity in the dielectric or paraelectric layers, resulting in the entire stack being switched, an exciting avenue for new discoveries. This thawing of “frozen ferroelectrics,” paraelectrics, and potentially dielectrics with high dielectric constants promises a large class of new ferroelectrics with exciting prospects for previously unrealizable domain-patterned optoelectronic and memory technologies.

36 MATERIALS SCIENCE↗