An investigation of corrosion mechanisms of constructional alloys in supercritical water oxidation (SCWO) systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (leaves 179-186).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Supercritical water oxidation (SCWO) is a technology that can effectively destroy aqueous organic waste above the critical point of pure water. These waste feed streams are very aggressive and pose material performance issues. As potential alloys in construction of SCWO systems, nickel-base alloys are tested. Corrosion in aqueous feed streams of ambient pH values of 2, 1 and 7 is studied both at supercritical (-425⁰C) and subcritical (-300-360⁰C) temperatures with a constant pressure of 24.1MPa. Dealloying of Ni and Fe, and oxidation of Cr and Mo are observed at subcritical temperatures at a pH value of 2. At a pH value of 1, even chromium is selectively dissolved and only molybdenum forms a stable oxide at the subcritical temperature. At supercritical temperatures, normal thin oxidation occurs at both pH values of pH 2 and 7. In contrast, in the neutral pH solution, dealloying is not observed at any temperature. Stress corrosion cracking (SCC) in acidic feed streams is observed both at the supercritical and subcritical temperatures. In order to understand the corrosion mechanisms, the chemistry of a feed stream, the formation of the dealloyed oxide layer, and the level of stress are investigated.(cont.) The suppression of dealloying at supercritical temperatures comes from the low proton concentration associated with the low dissociation constant of HCl and water. However, the growth rate of the dealloyed oxide layer at subcritical temperatures is very fast, which is primarily due to the dealloying and the high diffusivity of the nickel in this defective oxide layer. SCC at subcritical temperatures results from the dealloyed oxide layer formation along the grain boundary as intrusions, which act as a precursor to the crack initiation and propagation. SCC at the supercritical temperature is thought to result from the direct chemical attack of associated HC1 molecules. SCC is not observed in the neutral solution.by Hojong Kim.Ph.D

Calcium-Antimony Alloys as Electrodes for Liquid Metal Batteries

The performance of a calcium-antimony (Ca-Sb) alloy serving as the positive electrode in a Ca∥Sb liquid metal battery was investigated in an electrochemical cell, Ca(in Bi) | LiCl-NaCl-CaCl[subscript 2] | Ca(in Sb). The equilibrium potential of the Ca-Sb electrode was found to lie on the interval, 1.2–0.95 V versus Ca, in good agreement with electromotive force (emf) measurements in the literature. During both alloying and dealloying of Ca at the Sb electrode, the charge transfer and mass transport at the interface are facile enough that the electrode potential varies linearly from 0.95 to 0.75 V vs Ca(s) as current density varies from 50 to 500 mA cm[superscript −2]. The discharge capacity of the Ca∥Sb cells increases as the operating temperature increases due to the higher solubility and diffusivity of Ca in Sb. The cell was successfully cycled with high coulombic efficiency (∼100%) and small fade rate (<0.01% cycle[superscript −1]). These data combined with the favorable costs of these metals and salts make the Ca∥Sb liquid metal battery attractive for grid-scale energy storage.United States. Advanced Research Projects Agency-Energy (Award DE-AR0000047)TOTAL (Firm)Marubun Research Promotion FoundationMurata Overseas Scholarship Foundatio

Calcium-based multi-element chemistry for grid-scale electrochemical energy storage

Calcium is an attractive material for the negative electrode in a rechargeable battery due to its low electronegativity (high cell voltage), double valence, earth abundance and low cost; however, the use of calcium has historically eluded researchers due to its high melting temperature, high reactivity and unfavorably high solubility in molten salts. Here we demonstrate a long-cycle-life calcium-metal-based rechargeable battery for grid-scale energy storage. By deploying a multi-cation binary electrolyte in concert with an alloyed negative electrode, calcium solubility in the electrolyte is suppressed and operating temperature is reduced. These chemical mitigation strategies also engage another element in energy storage reactions resulting in a multi-element battery. These initial results demonstrate how the synergistic effects of deploying multiple chemical mitigation strategies coupled with the relaxation of the requirement of a single itinerant ion can unlock calcium-based chemistries and produce a battery with enhanced performance.United States. Advanced Research Projects Agency-EnergyTOTAL (Firm

Electrolysis of Molten Iron Oxide with an Iridium Anode: The Role of Electrolyte Basicity

Molten oxide electrolysis (MOE) is a carbon-free, electrochemical technique to decompose a metal oxide directly into liquid metal and oxygen gas. From an environmental perspective what makes MOE attractive is its ability to extract metal without generating greenhouse gases. Hence, an inert anode capable of sustained oxygen evolution is a critical enabling component for the technology. To this end, iridium has been evaluated in ironmaking cells operated with two different electrolytes. The basicity of the electrolyte has been found to have a dramatic effect on the stability of the iridium anode. The rate of iridium loss in an acidic melt with high silica content has been measured to be much less than that in a basic melt with high calcia content.American Iron and Steel Institute ((AISI) CO2 breakthrough program

DFTTK: Density Functional Theory Tool Kit for High-throughput Calculations of Thermodynamic Properties at Finite Temperatures

In this work, we present a software package in Python for high-throughput first-principles calculations of thermodynamic properties at finite temperatures, which we refer to as DFTTK (Density Functional Theory Tool Kit). DFTTK is based on the atomate package and integrates our experiences in the last decades on the development of theoretical methods and computational software. It includes task submissions on all major operating systems and task execution on high-performance computing environments. The distribution of the DFTTK package comes with examples of calculations of phonon density of states, heat capacity, entropy, enthalpy, and free energy under the quasi-harmonic phonon scheme for the stoichiometric phases of Al, Ni, Al3Ni, AlNi, AlNi3, Al3Ni4, and Al3Ni5, and the fcc solution phases treated using the special quasirandom structures at the compositions of Al3Ni, AlNi, and AlNi3.Comment: 49 pages, 18 figure

Novel Bandwidth Expander Supported Power Amplifier for Wideband Ultrasound Transducer Devices

Ultrasound transducer devices have their own frequency ranges, depending on the applications and specifications, due to penetration depth, sensitivity, and image resolution. For imaging applications, in particular, the transducer devices are preferable to have a wide bandwidth due to the specific information generated by the tissue or blood vessel structures. To support these ultrasound transducer devices, ultrasound power amplifier hardware with a wide bandwidth can improve the transducer performance. Therefore, we developed a new bandwidth expander circuit using specially designed switching architectures to increase the power amplifier bandwidth. The measured bandwidth of the power amplifier with the help of the bandwidth expander circuit increased by 56.9%. In addition, the measured echo bandwidths of the 15-, 20-, and 25-MHz transducer devices were increased by 8.1%, 6.0%, and 9.8%, respectively, with the help of the designed bandwidth expander circuit. Therefore, the designed architecture could help an ultrasound system hardware with a wider bandwidth, thus supporting the use of different frequency ultrasound transducer devices with a single developed ultrasound system

High-efficiency high-voltage class F amplifier for high-frequency wireless ultrasound systems.

This paper presents a novel amplifier that satisfies both low distortion and high efficiency for high-frequency wireless ultrasound systems with limited battery life and size. While increasing the amplifier efficiency helps to address the problems for wireless ultrasound systems, it can cause signal distortion owing to harmonic components. Therefore, a new type of class F amplifier is designed to achieve high efficiency and low distortion. In the amplifier, the resonant circuit at each stage controls the harmonic components to reduce distortion and improve efficiency. Transformers with a large shunt resistor are also helpful to reduce the remaining noise in the input signal. The proposed class F amplifier is tested using simulations, and the voltage and current waveforms are analyzed to achieve correct operation with adequate efficiency and distortion. The measured performance of the class F amplifier has a gain of 23.2 dB and a power added efficiency (PAE) of 88.9% at 25 MHz. The measured DC current is 121 mA with a variance of less than 1% when the PA is operating. We measured the received echo signal through the pulse-echo response using a 25-MHz transducer owing to the compatibility of the designed class F amplifier with high- frequency transducers. The measured total harmonic distortion (THD) of the echo signal was obtained as 4.5% with a slightly low ring-down. The results show that the low THD and high PAE of the new high-efficiency and high-voltage amplifier may increase battery life and reduce the cooling fan size, thus providing a suitable environment for high-frequency wireless ultrasound systems

A New Approach to Power Efficiency Improvement of Ultrasonic Transmitters via a Dynamic Bias Technique

To obtain a high-quality signal from an ultrasound system through the transmitter, it is necessary to achieve an appropriate operating point of the power amplifier in the ultrasonic transmitter by applying high static bias voltage. However, the power amplifier needs to be operated at low bias voltage, because a power amplifier operating at high bias voltage may consume a large amount of power and increase the temperature of the active devices, worsening the signal characteristics of the ultrasound systems. Therefore, we propose a new method of increasing the bias voltage for a specific period to solve this problem by reducing the output signal distortion of the power amplifier and decreasing the load on the active device. To compare the performance of the proposed method, we measured and compared the signals of the amplifier with the proposed technique and the amplifier only. Notably, improvement was achieved with 11.1% of the power added efficiency and 3.23% of the total harmonic distortion (THD). Additionally, the echo signal generated by the ultrasonic transducer was improved by 2.73 dB of amplitude and 0.028% of THD under the conditions of an input signal of 10 mW. Therefore, the proposed method could be useful for improving ultrasonic transmitter performance using the developed technique