Measurements of Thermochemical Properties of Alloys for Applications in Energy Systems
Open Access
- Author:
- Nigl, Thomas
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 01, 2021
- Committee Members:
- Robert Hickey, Major Field Member
Arthur Motta, Outside Unit & Field Member
Hojong Kim, Chair & Dissertation Advisor
Ismaila Dabo, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- Thermodynamics
Electromotive force measurements
Electrochemical separation
Electrorefining
Sr-Pb
Alkaline-earth metals
Ni-Al
Hot corrosion
Combustion - Abstract:
- To meet energy efficiency standards, energy system operating temperatures have rapidly increased in recent decades. Systems, such as gas turbines and nuclear pyroprocessing plants, require materials that resist degradation in chemically aggressive environments at elevated (> 500°C) temperatures. The governing mechanisms, especially thermodynamic properties, are crucial to material development and integration. However thermodynamic properties (e.g. activity) are not well explored for many materials. This dissertation focuses on evaluating the thermodynamics of two binary alloys (Sr-Pb and Al-Ni) and their application in two high temperature energy systems. Closing the nuclear fuel cycle requires establishing a reprocessing facility that can recycle used nuclear fuel. However, additional waste is generated during recycling due to fission products accumulating in processing salts (LiCl-KCl). Reducing the waste generated during recycling entails recovering alkaline-earth fission products (Ba2+/Sr2+) from molten chlorides with a minimal loss of bulk electrolyte constituents (Li+/K+). The first aim of the dissertation is to investigate the thermodynamic properties of Sr-Pb alloys to determine the viability of using a liquid Pb cathode to remove Sr2+ ions from the electrolyte. The thermodynamic properties of Sr-Pb alloys were determined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electromotive (emf) force measurements. Equilibrium and non-equilibrium intermetallic compounds of Sr-Pb alloys (0.07 < xSr < 0.75) were identified by XRD and the phase transition temperatures were determined by DSC (0.07 < xSr < 0.34), which were compared and contrasted to an existing assessed phase diagram of the Sr-Pb system. Additionally, thermochemical solution properties (i.e., activity and partial molar properties (Gibbs excess energy, entropy, enthalpy)) were calculated by measuring emf values of Sr-Pb alloys (0.07 < xSr < 0.59) using the following electrochemical cell: Sr(s) | CaF2-SrF2 | Sr(in Pb). By integrating solution properties from emf measurements with the phase behavior determined via DSC and XRD, a reliable thermodynamic description of the Sr-Pb system was established and the low activity values of Sr in Pb provided a thermodynamic basis for the successful separation of Sr2+ ions from molten LiCl-KCl electrolytes. Electrochemical codeposition of Ba2+/Li+ and Sr2+/Li+ into liquid metal (Bi, Sb, Sn, and Pb) and alloy (Bi-Sb) electrodes was investigated in LiCl-KCl-(SrCl2,BaCl2) electrolytes at 500 and 650 °C. For the pure Bi (500 °C) and Sb (650 °C) electrodes, the greatest percentage of charge was used to deposit Ba and Sr. Effective recovery of Ba/Sr by liquid Bi and Sb electrodes is supported via experimentally determined activity values of Ba/Sr in Bi and Sb. Alloying Sb with Bi increased Ba recovery but decreased Sr recovery, compared to the recovery using a liquid Bi electrode. The results suggest that alkaline-earth fission products can be recovered from molten chlorides using liquid metal electrodes via electrochemical separation, thereby providing a method to reduce the generation of nuclear waste from nuclear fuel recycling. Ni alloys used for gas turbine components are often exposed to sulfuric gases (i.e., SO2, SO3) and salt species (e.g., NaCl) during operation due to contaminated fuel and air sources. At high operating temperatures (> 650°C), molten sulfate salts (e.g., Na2SO4) form in the combustion zone of the turbine and deposit on metallic surfaces, leading to significant corrosion. NiAl alloys are used as coatings due to the formation of a stable, protective surface oxide (Al2O3). Alloying elements have also been investigated to improve the corrosion resistance of these coatings. The second aim of the dissertation seeks to provide a thermodynamic basis for the improvement of multicomponent alloy coatings and their effects on Ni alloy corrosion in sulfate environments by investigating the thermodynamic properties of Ni-Al alloys to understand the high temperature chemical interactions within the binary system. Thermodynamic properties of Al-Ni alloys were determined by XRD, emf measurements, and DSC. Equilibrium phases of Al-Ni alloys after annealing were confirmed by XRD. Emf values of Al-Ni alloys (0.05 < xAl < 0.90) were measured using the following electrochemical cell: Al | LiCl-KCl-AlF3 | Al(in Ni). From emf values of the Al-Ni alloys, thermochemical properties such as activity and partial molar properties were calculated and compared to an existing assessed phase diagram of the Al-Ni system. Phase transition temperatures of Al-rich alloys observed via emf measurements were corroborated by DSC measurements. By integrating activity values and other solution properties from emf measurements with phase behavior determined via XRD and DSC, a reliable thermodynamic description of the Al-Ni system was established and the range of activity values of Al in Ni in ß-NiAl alloys provided a thermodynamic basis for the subsequent investigation of thermodynamic properties of other ß-NiAl based ternary alloys and their corrosion behavior. Overall, knowledge of fundamental thermochemical properties of materials at high temperatures allows us to develop more effective and longer-lasting materials for energy systems.