Ultrafast laser driven spin generation in metallic ferromagnets

This dissertation presents experimental studies of spin generation in metallic ferromagnets (FM) driven by ultrafast laser light using a pump-probe technique. The pump light gives a driving force for spin generation by depositing energy or spin angular momentum on FM. The probe light measures spin responses by magneto-optical Kerr effect or temperature responses by time-domain thermoreflectance. I find that ultrafast laser light generates spins in FM in three distinct mechanisms: (i) demagnetization; (ii) spin-dependent Seebeck effect (SDSE); (iii) optical helicity. The demagnetization-driven spin generation is due to energy transport between electrons and magnons of FM and conservation of angular momentum for electron-magnon coupling. Ultrafast laser light deposits its energy in electrons of metallic layers and leads to a sharp increase of the electron temperature. The excited electrons transport energy to magnons of FM by the electron-magnon coupling. The magnon excitation results in ultrafast demagnetization of FM. I find that the spin loss by magnon excitations during the demagnetization process is converted to the spin generation in electrons of FM by the conservation of angular momentum for electron-magnon coupling. The generated spins diffuse to other layers and leads to spin accumulation in nonmagnetic metals (NM) or spin transfer torque on other FMs. I measure the demagnetization-driven spin accumulation in a NM/FM1/NM structure and spin transfer torque in a NM/FM1/NM/FM2 structure. The SDSE-driven spin generation is due to a heat current at FM/NM interfaces and spin-dependent Seebeck coefficient of FM. Ultrafast laser light deposits its energy in a heat absorbing layer of a multilayer structure and leads to a heat current from the heat absorbing layer to heat sinking layer. When an FM is incorporated in the multilayer structure, the spin-dependent Seebeck coefficient of FM converts the heat current to spin generation at interfaces between FM and NM. The interfacial spin generation rate is proportional to the heat current through FM and spin-dependent Seebeck coefficient of FM. I find that the heat current and spin-dependent Seebeck coefficient can be controlled by thickness of the heat sink layer and composition of FM, respectively. The generated spins diffuse to other layers and leads to spin accumulation on NM or spin transfer torque on other FM. I measure the SDSE-driven spin accumulation in a NM/FM1/NM structure and spin transfer torque in a NM/FM1/NM/FM2 structure. The optical helicity-driven spin generation is due to angular momentum transport between light and electrons of FM and spin-orbit splitting of FM. A circularly polarized light with a wavelength of 785 nm triggers a dipolar transition from occupied 3d to unoccupied 4p bands of 3d transition FMs. The selection rule predicts a significant spin polarization for the dipolar transition from spin-orbit 3d-sub-bands (3d3/2 and 3d5/2) to 4p band. However, energy degeneracy between 3d3/2 and 3d5/2 leads to zero spin polarization. I find that a small-but-finite spin-orbit splitting of the 3d bands leads to a finite spin generation from a circularly polarized light. The generated spins in electrons can be absorbed by magnetization of FM and lead to spin transfer torque. I measure the optical helicity-driven spin transfer torque in a single FM structure

Performance Characteristics of a Refrigerator-Freezer with Parallel Evaporators using a Linear Compressor

A linear compressor for a domestic refrigerator-freezer has energy saving potential compared with a reciprocating compressor because of a low friction loss and free piston system. A linear compressor can control the piston stroke since it does not have mechanical restriction of piston movement. Therefore, the energy consumption of a domestic refrigerator-freezer using a linear compressor can be reduced by changing the cooling capacity of the compressor. In order to investigate the performance of a refrigerator-freezer with parallel evaporators using a linear compressor and the relation between cooling capacity of the linear compressor and cooling load, experimental simulation is conducted with variation of the capacity of a linear compressor, an ambient temperature, and cooling load. In addition, the power consumption of a linear compressor is compared to that of an inverter reciprocating compressor in a refrigerator-freezer. The performance of a linear compressor is measured with variation of the capacity of a linear compressor from 60% to 100% of the maximum capacity in a refrigerator-freezer. Based on the experimental data, the power consumption of a linear compressor is reduced by 22.4% with 70% capacity compared to 100% but on-time ratio is increased by 12.8%

Effects of Operating Conditions on the Oxygen Removal Performance of the Deoxo Chamber in the Water Electrolysis System

Although the production of high-quality hydrogen from electrolysis systems is essential, research in this area is limited. In this study, we investigate the effect of operating conditions on the change in oxygen concentration through computational analysis for optimizing the deoxo chamber of a water electrolysis system. The test results of the water electrolysis system are simulated, and the oxygen concentration of the deoxo chamber is calculated through computational fluid dynamics analysis according to various conditions, such as the pressure, temperature, and flow rate. The O2 removal performance is significantly affected by the operating pressure and temperature, with an increase in both leading to a decrease in the O2 concentration in the water electrolysis system. Furthermore, we confirm that the change in the flow rate into the chamber has a minor effect on the change in the oxygen removal performance when the inlet flow rate was 1–1.5 kg/h and the length diameter ratio of the chamber is 38.4

Transmission Delay-Based Uplink Multi-User Scheduling in IEEE 802.11ax Networks

As the demands for uplink traffic increase, improving the uplink throughput has attracted research attention in IEEE 802.11 networks. To avoid excessive competition among stations and enhance the uplink throughput performance, the IEEE 802.11ax standard supports uplink multi-user transmission scenarios, in which AP triggers certain stations in a network to transmit uplink data simultaneously. The performance of uplink multi-user transmissions highly depends on the scheduler, and station scheduling is still an open research area in IEEE-802.11ax-based networks. In this paper, we propose a transmission delay-based uplink multi-user scheduling method. The proposed method consists of two steps. In the first step, the proposed method makcreateses station clusters so that stations in each cluster have similar expected transmission delays. The transmission delay-based station clustering increases the ues of uplink data channels during the uplink multi-user transmission scenario specified in IEEE 802.11ax. In the second step, the proposed method selects cluster for uplink multi-user transmissions. The cluster selection can be performed with a proportional fair-based approach. With the highly channel-efficient station cluster, the proposed scheduling method increases network throughput performance. Through the IEEE 802.11ax standard compliant simulations, we verify the network throughput performance of the proposed uplink scheduling method

Effects of n-Heptane/Methane Blended Fuel on Ignition Delay Time in Pre-Mixed Compressed Combustion

This study analyzed factors that influence the ignition delay characteristics of n-heptane/methane-blended fuel. The effects of chemical species, exhaust gas recirculation rate, compression ratio, cool/hot flames, and combustion chamber conditions (temperature, pressure, and O2 concentration) were determined and analyzed using CHEMKIN Pro. The experiment conditions for verification were 550–1000 K at 15 bar with 50% H2/50% CH4 fuel. The main combustion reactions were confirmed through reactivity analysis and sensitivity analysis on the ignition delay time. The ignition delay time at 14.7% O2 concentration was significantly higher than that at 21% O2 concentration by more than 30%. In addition, a higher ratio of methane in the blended fuel increased the ignition delay time as a result of methane dehydrogenation, delaying the ignition of heptane

Effects of n-Heptane/Methane Blended Fuel on Ignition Delay Time in Pre-Mixed Compressed Combustion

This study analyzed factors that influence the ignition delay characteristics of n-heptane/methane-blended fuel. The effects of chemical species, exhaust gas recirculation rate, compression ratio, cool/hot flames, and combustion chamber conditions (temperature, pressure, and O2 concentration) were determined and analyzed using CHEMKIN Pro. The experiment conditions for verification were 550–1000 K at 15 bar with 50% H2/50% CH4 fuel. The main combustion reactions were confirmed through reactivity analysis and sensitivity analysis on the ignition delay time. The ignition delay time at 14.7% O2 concentration was significantly higher than that at 21% O2 concentration by more than 30%. In addition, a higher ratio of methane in the blended fuel increased the ignition delay time as a result of methane dehydrogenation, delaying the ignition of heptane

Spraying and Mixing Characteristics of Urea in a Static Mixer Applied Marine SCR System

The most effective de-NOx technology in marine diesel applications is the urea-based selective catalytic reduction (SCR) system. The urea-SCR system works by injecting a urea solution into exhaust gas and converting this to NH3 and CO2. The injection, mixing, and NH3 conversion reaction behavior of the urea-water solution all have a decisive effect on the performance of the system. To improve de-NOx efficiency, it is important to provide enough time and distance for NH3 conversion and uniform distribution prior to the solution entering the catalyst. In this study, therefore, the characteristics of gas flow, NH3 conversion, and its distribution are investigated with a static mixer by means of numerical methods, providing a special advantage to ship manufacturing companies through the optimization of the urea-SCR system. The results show that the inclusion of the mixer induces strong turbulence and promotes the NH3 conversion reaction across a wider region compared to the case without the mixer. The mean temperature is 10 °C lower due to the activated endothermic urea-NH3 conversion reaction and the NH3 concentration is 80 PPM higher at 1D than those without the mixer. Moreover, the uniformity of NH3 distribution improved by 25% with the mixer, meaning that the de-NOx reaction can take place across all aspects of the catalyst thus maximizing performance. In other words, ship manufacturing companies have degrees of freedom in designing post-processing solutions for emissions by minimizing the use of the reduction agent or the size of the SCR system

Effect of a Plasma Burner on NOx Reduction and Catalyst Regeneration in a Marine SCR System

The problem of environmental pollution by the combustion of fossil fuels in diesel engines, to which NOx emission is a dominant culprit, has accelerated global environmental pollution and global and local health problems such as lung disease, cancer, and acid rain. Among various De-NOx technologies, SCR (Selective Catalytic Reduction) systems are known to be the most effective technology for actively responding to environmental regulations set by the IMO (International Maritime Organization) in marine diesel applications. The ammonia mixes with the exhaust gas and reacts with the NOx molecules on the catalyst surface to form harmless N2 and H2O. However, since the denitrification efficiency of NOx can be rapidly changed depending on the operating temperature from 250 °C to 350 °C at 0.1% sur contents of the catalyst used in the SCR, a device capable of controlling the exhaust gas temperature is essential for the normal operation of the catalyst. In addition, when the catalyst is exposed to SOx in a low exhaust gas temperature environment, the catalyst is unable to reduce the oxidation reaction of the catalyst, thereby remarkably lowering the De-NOx efficiency. However, if the exhaust gas temperature is set to a high temperature of 360–410 °C, the poisoned catalyst can be regenerated through a reduction process, so that a burner capable of producing a high temperature condition is essential. In this study, a plasma burner system was applied to control the exhaust gas temperature, improving the De-NOx efficiency from the engine and regenerating catalysts from PM (Particulate Matter), SOOT and ABS (ammonia bisulfate), i.e., catalyst poisoning. Through the burner system, the optimum De-NOx performance was experimentally investigated by controlling the temperature to the operating region of the catalyst, and it was shown that the regeneration efficiency in each high temperature (360/410 °C) environment was about 95% or more as compared with the initial performance. From the results of this study, it can be concluded that this technology can positively contribute to the enhancement of catalyst durability and De-NOx performance