Experimental Characterization and Physics-Based Modeling of Optical Property Temperature Dependencies in Doped Metal Oxides

high temperature optical properties;supercontinuum laser spectroscopy;Materials Science & Engineering

Brupbacher, Michael CollinsHemker, Kevin J. ;

Johns Hopkins University

关键词: high temperature optical properties;    supercontinuum laser spectroscopy;    Materials Science & Engineering;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/58644/BRUPBACHER-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

The technical significance of doped metal oxides, and in particular oxides of the ;;poor” metal aluminum and the transition metal zirconium, is apparent in the abundance of literature regarding their synthesis and optical property characterizations.Processing methods for these doped metal oxides have ranged from traditional solid-state techniques to sol-gel chemistries and innovative thermal spray preparations.Despite the breadth of synthesis routes used to make these materials, a commonality of the importance in the materials science paradigm is undoubtedly observed across a wide variety of applications.As modern-day applications continue to expand the uses for these doped metal oxides, the ability to extend the fundamental and phenomenological theories describing optical properties to include the effects of extreme environmental conditions (e.g. high temperatures) will certainly be of increasing importance.One such emerging application is found in the optical characterization of aluminized solid rocket propellant fires.Deep space exploration often necessitates the use of radioisotope thermoelectric generators to power instrumentation in the absence of sufficient solar irradiance for conventional solar-cell power generation.In the event of a launch accident there is potential for the release of radioactive material through several pathways, and transition metal oxides have been proposed as surrogates to simulate these various release mechanisms.While the appropriate measurement technique is dependent on the fire region of interest, each proposed dopant transition metal ion (e.g., Cr3+) in an alumina host presents the prospect of capturing unique temperature-dependent optical signatures.The spectral locations of the absorption and fluorescence features provide identification of the dopant in the probed physical location, given that the temperature-dependent spectral location of the features has either been previously measured or accurately modeled at expected fire temperatures (T ≤ 3100 K).Thermal barrier coatings involving oxides of zirconium have found a multitude of applications over several critical industries, including both aerospace and industrial power production.In order to achieve lower thermal conductivities than are intrinsic to the zirconias themselves, these materials are typically thermally-sprayed to introduce significant levels of porosity.Additionally, these zirconias often incorporate transition metal dopants (e.g., yttrium) to achieve phase stability in the most extreme environmental conditions.As the definition of ;;extreme environmental conditions” continues to evolve to higher temperatures (T ≥ 2273 K) in response to aerospace application demands, the role of dopants will naturally extend to include the capacity to radiate energy into the surroundings.One material subset that has received little attention in this regard are the rare earth zirconates of the form Ln2Zr2O7 (where Ln = La, Nd, Sm).Knowledge of temperature-dependent optical properties for these materials will be integral in effectively designing next-generation coating systems for thermal management.Owing to the importance of optical properties of various doped metal oxides at elevated temperatures for a variety of applications, this dissertation focuses on experimental characterization and physics-based modeling of optical property temperature dependencies for two specific doped-metal-oxide systems: the transition-metal-doped aluminas (i.e. Cr-doped alumina) and the rare earth zirconates (i.e. Nd2Zr2O7).While the optical property measurement techniques employed are of the same for both doped-metal-oxide systems, processing methods to produce these materials vary.Transition-metal-doped alumina powders are prepared via sol-gel chemistries, whereas rare-earth zirconate powders are obtained commercially and are derived from traditional powder-processing techniques.These doped-metal-oxide powders are thermally-sprayed as thick deposits using atmospheric plasma spray, and from these monolithic coatings are obtained which are suitable for CO2 laser heating.The facility to produce these materials in their desired physical/structural forms and accurately measure their room-temperature optical properties without debilitating spectral interference from impurities is demonstrated.A supercontinuum-laser-based technique for determination of temperature-dependent optical properties and physics-based modeling of these measurements will also be described.Previously-unreported optical phenomena have been observed at high temperatures in both doped-metal-oxide systems using supercontinuum-laser-based spectroscopy.For the transition-metal-doped aluminas, these relate to absorption band splitting due to thermal and optical anisotropy in the host material, as well as fluorescence quenching due to increased phonon interaction at high temperatures.In the case of the rare earth zirconates, these relate to free-carrier absorption due to extrinsic semiconductor behavior at high temperatures in non-equilibrium phases.