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No full textSchool of Energy and Chemical Engineering (Battery Science and Technology)clos
Low-Jitter, Low-Fractional-Spur and Low-Reference-Spur Ring-Oscillator Based Digital Fractional-N PLL
No full textDepartment of Electrical Engineeringclos
Multidimensional Ultrafast Electron Microscopy : Access to Light-Matter Interactions at the Space, Time, and Energy Limits
No full textDepartment of Chemistryclos
3D printing of Bi2Te3-based thermoelectric materials and modules
Get PDFDepartment of Materials Science and EngineeringThermoelectric power generation technology has attracted tremendous interest due to its ability to convert heat energy to electricity with no mechanical moving part. This technology, after nearly 60 years of development, now appears to be mature. However, the presence of TE devices in the marketplace is rather vague due to the stereotypical geometry and the low efficiencies. Although the recently developed strategies of nanostructuring have enhanced the inherent efficiencies of materials, these technologies do not meet the industrial needs in terms of reproducibility and processing cost. More importantly, the traditional manufacturing process cannot provide any geometrical design of TE devices to fit heat sources. To solve these issues, in this dissertation, the development of 3D printing to produce highly efficient TE materials and TE generators (TEG) is explored, which can provide an opportunity for TE materials and TEGs that are fully compatible to various heat sources.
The first part of the dissertation describes 3D printing of thermoelectric materials that are compatible to curved heat sources. Thermoelectric inks for pneumatic dispenser must flow under pressure and be able to retain its shape after the deposition. Also, it must have a homogeneous phase to prevent sedimentation of the particles. To give these rheological properties, organic binders such as conducting polymers and carbon materials are added. However, these organic additives are electrically insulating and do not contribute to the sintering of the printed material. As an alternative to these organic binders, I used molecular chalcogenidometallates ions as the ???inorganic binder,??? which provide strong electrostatic interaction between TE particles and the phase stability, allowing for tailoring rheological properties of inks according to types and compositions of TE materials and binders. I designed and fabricated ultrahigh-performance TEG with conformal geometry to heat sources by geometry-engineering of TE legs. I focused on controlling the shapes and dimensions of TE legs for securing the highest temperature differences and TE power output.
The second part of the dissertation introduces the development and fabrication of high-performance micro thermoelectric devices via direct ink writing. Wireless sensor network (WSN) devices are getting tremendous amount of attention due to its ability to mimic and collect personal physiological information. These devices are expected to be energy autonomous due to inaccessible structure. Micro thermoelectric modules are regarded as a potential power source for these devices. However, there are no established process to manufacture 3D micro TEG other than microelectromechanical system (MEMS) which produces only 2D structure. Low temperature difference due to a low aspect ratio of 2D structure give rise to a lower thermoelectric performance. In this regard, I aimed to create micro thermoelectric module with high aspect ratio by using direct ink writing process. With higher temperature difference across the module, higher thermoelectric performance can be expected. To formulate highly viscoelastic ink that is suitable for direct ink writing micro 3D TE materials, I explore a synergistic combination of particle size and surface engineering of TE particles. As a result, direct-written ??TEG demonstrated an order of magnitude higher thermoelectric performance due to its high aspect ratio of direct written materials.ope
ESSAYS ON EMPIRICAL CORPORATE FINANCE WITH TEXTUARE ANALYSIS
No full textSchool of Business Administration (Management Engineering)clos
Development of Transition Metal Dichalcogenides-Based Heterostructures for Electrochemical Applications in Hydrogen Economy
Get PDFSchool of Energy and Chemical Engineering (Energy Engineering)Energy demand has continuously increased with the development of global industries and rapid population growth. Among them, fossil fuels including coal, petroleum oil, and natural gas have been powering economies for over 150 years resulting in the inevitable emission of greenhouse gases. The heavy reliance on fossil fuels gives rise to serious trouble in the environment called global warming. Therefore, technologies for clean and sustainable energy have been developed such as solar, wind, and marine energy. However, they have some difficulties in substituting the current energy system due to regional and seasonal factors. In this respect, an efficient energy conversion and storage system are needed to resolve intermittent availability.
Hydrogen as an energy carrier would be one of the leading options for future carbon-neutral energy systems in the sense that it has the highest gravimetric energy density. Hydrogen is not only being used in many existing industries such as refining petroleum, producing fertilizer, but also can be used as the primary energy carrier for clean energy technologies like fuel cells in the hydrogen economy. As the worldwide energy trend gradually moves toward a carbon-neutral society, the demand for hydrogen is increasing. However, most hydrogen is still being produced from fossil fuels emitting harmful CO2 with a low production rate as well as containing CO2 as an impurity leading to a decline in the durability of the fuel cell due to poisoning the noble metal-based electrocatalysts. Considering both the environmental and the efficiency aspects, it is necessary to advance the water electrolysis technology combined with an eco-friendly energy grid such as solar energy. The main half-reactions of water electrolysis, HER and OER, require efficient and durable electrocatalyst with low cost to replace commercial Pt or Ir-based expensive catalyst. Two-dimensional transition metal dichalcogenides (TMD) are one of the most promising candidates due to their proper hydrogen adsorption/desorption properties. However, the limited number of active edge sites and poor conductivity of the basal plane make it difficult to use TMD as a practical electrocatalyst in water electrolysis. In this dissertation, MoS2, a typical TMD, is used as a bifunctional electrocatalyst by heterostructuring with multiple active components.
In Chapter 1, the broad introduction of the hydrogen economy would be addressed. In addition, the water electrolysis, the mechanism of each half-reaction, and several evaluation parameters would be presented. The structure of TMD and some strategies to improve the electrochemical application are also explained.
Constructing TMD-based heterostructure, the transition metal phosphides, other sulfides are introduced and grafted on conductive substrates in this work. In Chapter 2, nickel phosphide (Ni2P) and nitrogen-doped carbon (N:C) support are used to adjust the electronic structure of MoS2 as well as increase the number of active sites. As a result, the electronic conductivity of Ni2P/MoS2/N:C catalyst was highly enhanced making phosphosulfide bonding. Besides, the number of active sites is greatly increased. Due to the synergistic effect between Ni2P and MoS2, Ni2P/MoS2/N:C shows Pt-like HER performance in the acidic electrolyte.
Chapter 3 is focused on the electrocatalyst working in wide pH electrolytes. The other sulfide, cobalt sulfide (Co9S8), was chosen for the formation of heterostructure with MoS2 because Co9S8 has been studied for hydrodesulfurization (HDS) which has similar reaction properties to HER in the sense that the key factor is adsorption of hydrogen species. Besides, Co9S8 can be applied to OER as well as HER. The electronic structure was reconstructed, and the number of active sites is increased by heterostructuring. Therefore, the Co9S8-MoS2 shows remarkable HER performance in acidic, neutral, and alkaline electrolytes. The actual active site in Co9S8-MoS2 heterostructure was also discussed in detail.
In line with research in Chapter 3, the overall water splitting system is realized in Chapter 4. As mentioned, Co9S8 has OER activity as well as HER activity. Thus, Co9S8-MoS2 was also tested as a bifunctional electrocatalyst for overall water electrolysis. The Co9S8-MoS2 as a pre-catalyst for OER shows exceptional activity and stability in alkaline media. The Co9S8 in the heterostructure transforms to cobalt oxide species as an OER active site during the oxidative OER while MoS2 in the heterostructure forms porosity leading to an increase in the electrochemically active surface area. Combining HER system, the Co9S8-MoS2 exhibits excellent overall water splitting performance in a two-electrode system. Finally, the electrochemical system was connected with a photovoltaic cell called PV-EC system using Co9S8-MoS2/NF for both cathode and anode.
In Chapter 5, the development of transition metal dichalcogenides by heterostructuring for water electrolysis is summarized. After that, a mixed phase of MoS2 would be introduced for future work which can be used in spray or printing methods. Thus, the electrode can be used for green ammonia production as a hydrogen carrier.clos
The Impact of Expected and Unexpected Socio-economic Change on Air Pollutants and Carbon
Get PDFDepartment of Urban and Environmental Engineering (Disaster Management Engineering)This study investigates the impact of unexpected and expected socio-economic changes on air pollutants and carbon. For the impact of socio-economic changes on air pollutants, the analysis used the air quality, thermal power generation, and macroeconomic indices data to identify the effect of thermal power generation and COVID-19 on air quality. The findings are that thermal power generation had worsened the level of CO from 2015 to 2019 and that the impact of COVID-19 improves the air quality in China and South Korea. For the impact of socio-economic changes on carbon, the analysis used firm-level emissions and corporate variables to identify how ETS implementation has influenced carbon productivity (a firm-level revenue created per unit of carbon emission). The findings are that the firms increased carbon productivity after participating in the national-level Emission Trading Scheme (ETS) of South Korea, especially for high-emitting industries, and firms that are profitable, innovative, and managed by CEOs with environment-related educational or working experience successfully increase their carbon productivity. These findings suggest that to improve air pollution and global warming, governments have to establish appropriate air quality control measures on each air pollutant and assist green-minded firms and firms have to increase profitability and invest in eco-friendly technologies by green-minded executives with regulations on high-emitting firms.ope
Estimating VS30 for Seismic Stations in South Korea Using Initial Peaks of Primary-wave Seismograms
Get PDFDepartment of Urban and Environmental Engineering (Disaster Management Engineering)The confirmation of geotechnical condition is critical when engineers assess seismic hazard for various structures. The shear-wave velocity averaged by propagation time for upper 30-m soil (VS30) is one of index for ground classification and widely used for structural design. In general, VS30 can be measured by various field test but it is hard to aggregate VS30 because of environmental and geological constraint for performing tests. The P-wave method is one of proxy method to estimate VS30 using the first peak of P-wave velocity time series measured at surficial seismic station. 627 seismogram data measured at 53 stations were used for validation the method in South Korea region. The shear-wave velocity averaged from a certain depth z to surface (VSZ) were calculated using the ratio of radial to vertical component using the first peak of velocity time series. A calculated VSZ was converted to VS30 by using an empirical relationship from previous literature. The additional residuals between the measured VS30 and estimated VS30 values such as between-site residuals and within-site residuals and mean overall bias were considered to evaluate the availability of the method in South Korea region. The standard deviation of between-site residuals in this study was calculated as 0.32 as minimum which is lower than previous studies. After adjusting the estimated VS30 result data, over 72% of measured VS30 and estimated VS30 are within ??50% of each other. Using revised process, this study estimated 122 VS30 values at the stations where VS30 is not available.ope
A Novel Standard Ternary Inverter Design Using Field-Effect Transistors and PN Junctions
Get PDFDepartment of Electrical EngineeringOver the past few decades, silicon-based CMOS technology has been steadily developed. However, the power density limit is faced to process large amounts of information, and new approaches for a breakthrough are required. Multi-valued logic (MVL) is one of the new approaches to overcome the power density limit while reducing the complexity of the circuit by shifting from a binary to a ternary. The MVL circuit has fewer logic gates and interconnect compared to the binary logic for the same function by calculating multiple values in a logic gate, so it is more efficient in power consumption, interconnect complexity, and the gate counts. There are many studies about ternary logic circuit. The challenge of ternary circuit is standby power caused by additional intermediate state.
In this thesis, I propose a new design of a standard ternary inverter (STI) circuit with a minimum number of transistors which is consisting of 4 transistors and 2 pn junctions. By exploiting pn junctions, proposed STI reduces the power consumption when the output voltage is VDD/2. Thus, the standby
power issue of additional intermediate state is overcome. In addition, the number of interconnect and area are reduced by replacing transistors (4-terminal device) with pn junctions (2-terminal device). The STI circuit has been simulated with 110-nm technology node model library using HSPICE and Virtuoso. Furthermore, exploiting heavily doped pn junctions enables to operate at 10MHz. From simulation results, proposed STI shows significant power-delay product (PDP) reductions38% compared to a state-of-the-art STI of existing STI designs.ope