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Electrolyte Effects on Energy Conversion and Storage of Dye-Sensitized Photo-Electrochemical Cells
No full textDepartment of ChemistryDye-sensitized photoelectrochemical cells have been utilized as a photo-energy conversion device in single photovoltaics and a photo-electrode for hybrid photo-rechargeable battery systems, because of their high performance under outdoor and indoor light conditions. They possess transparency, multi-color and low-cost fabrication. Furthermore, consistent progress and efforts in photosensitizers, redox mediators and device structures have improved the performance of DSCs and successfully developed the DSPBs, particularly under ambient light conditions. However, it is still unclear that how electrolytes affect the performances of the DSSC and the hybrid system (dye-sensitized photo-rechargeable batteries (DSPBs).
Here, I first investigated the recombination properties between electrolytes and TiO2 (Electron-electrolyte recombination, ERR) according to the donor size of photosensitizers. The findings suggest that the finely tuned donor unit in a photosensitizer can effectively prevent the ERR by physically blocking the electrolyte approach to the TiO2???s surface without other side effects. The co-sensitization study was carried out to further improve the device performances and the further improved photo-voltage and photo-current were obtained.
Next, the electrolyte concentration effects, especially Li-ion concentration, on the DSPB???s performances were explored to understand comprehensive operation properties of DSPBs and enhance the energy density to the maximum of a capacity of storage materials. This study shows that the charging/discharging processes of DSPBs are highly influenced by the Li-ion concentration, triggered by concentration change of electrolyte on the photo-electrode of DSPBs. As a results, increasing Li-ion concentration can effectively stabilize the photovoltaic power of photo-electrode and reduce the Li-ion diffusion resistance, resulting in increment of output capacity However, output voltage decreased, because of increased diffusion resistance of redox mediator during the discharging process. As a result, 12.64 mWh/cm3 of energy density was obtained, which is 1.6-fold increased energy density comparing to the previous work.clos
Growth of Nanocrystalline and Amorphous Boron Nitride
Get PDFDepartment of ChemistryHexagonal boron nitride (hBN) is the only insulator among two-dimensional materials and is essential to the success of 2D electronics. However, the growth procedure of hBN is not compatible with current semiconductor technology because of requirement of high temperature for its growth. Thus, new growth methods of hBN or different types of BN should be developed for practical applications. In this work, we introduce the growth of high-quality hBN, nanocrystalline hBN, and amorphous BN by varying the growth temperature. In general, growth temperature and crystallinity are proportional. Using this proportional relationship, the crystallinity of hBN can be adjusted. The growth on sapphire substrate at 1,400 oC showed wrinkle-free, unidirectionally aligned hBN multilayers. On the other hand, the growth at lower temperature induced nanocrystalline hBN (around 700 oC) and amorphous BN (below 400 oC). In nanocrystalline hBN, small nanocrystalline hBN crystals are embedded in an amorphous BN matrix. Amorphous BN consists of sp2 boron and nitrogen elements with random orientation of BN bonds. These results are important studies that are the basis for future BN application research. Also, BN films grown at low temperatures can contribute to industrial application research. An interesting property of amorphous BN is a very low dielectric constant of less than 2, indicating that it can be used as a ultra-low dielectric material in the back-end-of-line (BEOL) of integrated circuits. The dielectric property of nanocrystalline BN is between those of hBN and amorphous BN. Also, we compare the structure and dielectric property of hBN, nanocrystalline hBN, and amorphous BN. Therefore, in this study, BN films with various crystallinity were successfully deposited through the deposition temperature control, and the deposited BN films were analyzed to contribute to BN application research.ope
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Get PDFDepartment of Urban and Environmental Engineering (Environmental Science and Engineering)Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous toxic air pollutants, which are mainly emitted through incomplete combustion together with derivates PAHs such as OPAHs and NPAHs.
Additionally, NOPAHs are also formed through secondary formation by a reaction between existing PAHs and atmospheric oxidants such as nitrate, ozone, and OH radicals.
In this study, the level of concentration, seasonal and spatial variation, source-receptor relationship, and potential cancer risk of 21 PAHs, 9 OPAHs, and 17 NPAHs was investigated using passive air samplers (PASs) in Ulsan.
The air samples were pretreated through Soxhlet extraction, clean-up using silica gel column for PAHs and silica/alumina gel column for NOPAHs, and concentration.
After that, PAHs were analyzed using a gas chromatography/mass spectrometry-electron ionization (GC/MS-EI) meanwhile NOPAHs were analyzed using a gas chromatography/mass spectrometry-negative chemical ionization (GC/MSNCI).
The abundant compounds consisted of low and middle-molecular weight compounds (2-, 3-, and 4-rings). The level of concentration is similar compared to other studies.
Through the spatial distribution, it was confirmed that each target compound had other emission sources by season.
PAHs are mainly emitted by petrochemical industrial and non-ferrous metal industrial complexes in the warm season and by the automobile industrial complex in the cold season.
OPAHs were mainly emitted near the automobile industrial complex during all seasons.
On the other hand, it was difficult to find clear emission sources for NPAHs.
To identify the relationship between emission source and receptor, correlation analysis and diagnostic were performed.
As a result of correlation analysis between PAHs and derivative PAHs, there are significant positive correlations suggesting they were emitted from similar emission sources.
The emission sources and formation pathways of target compounds were investigated using the diagnostic ratio.
PAHs were mainly affected by liquid fossil fuel combustion during total sampling period, greatly influenced by petroleum evaporation in warm seasons, and clearly influenced by biomass burning and coal combustion in cold seasons. For NPAHs, it was generated through secondary generation by OH radicals.
Potential cancer risk was assessed by using the incremental lifetime cancer risk (ILCR) model. In advance, BaPeq was calculated using individual concentrations (Ci) and toxic equivalent factors (TEFi).
Through the spatial distribution of risk, PAHs were higher in industrial complexes and NPAHs in non-ferrous metal industrial complexes. It is the first study to investigate source identification and health risk assessment for PAHs, NPAHs, and OPAHs using PUF-PAS in Korea.ope
MANIPULATION OF NANOSCALE WRINKLES ON TRANSPARNET AND FLEXILBE FILMS FOR MULTI-MODAL STRUCTURL COLORATION
Get PDFDepartment of Mechanical EngineeringMost materials and devices with structurally switchable color features responsive to external stimuli can actively and flexibly display various colors. However, realizing covert???overt transformation behavior, especially switching between transparent and colored states, is more challenging. Current stimulus-response pattern hiding and displaying technology responds to numerous external stimuli (temperature, light, mechanical stimulation, etc.) and causes a change in dye properties and in the spacing of nanostructures. In this phase change method, it is difficult to completely hide information because patterns are easily expressed due to limited viewing angles or irregular external conditions. Therefore, there is a need for a method of controlling a transparent nanoscale surface that can completely hide information. Thus, we introduce a method of forming traditional buckling-type corrugations using bilayer-like or trilayer film. In addition, the photonic structure is fabricated using an inkjet printing method and completely laminated into the film. Moreover, in this dissertation , we study the principle of structural color in which living organisms have color by a nanostructure without pigment and the characteristics of the nanostructured photonic crystal hidden in it and design a color-changing nanostructure for single and complex structural colors. Nanoscale wrinkles are generated on the ductile top surface of various multilayered substrates by external stimuli, and their geometrical and optical features are determined by the material and structural properties of the laminated films.
First, we develop a bilayer-like laminated film with a rigid SiO2-nanoparticle (NP)-encapsulated poly(dimethylsiloxane) (PDMS) composite structure surrounded by soft PDMS as a multidimensional structural color platform. Owing to the similarity in the optical properties of PDMS and SiO2 NPs, this device is fully transparent in the normal state. However, as their mechanical strengths differ considerably, upon compressive loading, buckling-type instability arises on the surface of the laminate, leading to the generation of 1D or 2D wrinkled patterns in the form of gratings. As a result, we demonstrate an application of the device in which quick response codes are displayed or hidden as covert???overt convertible-colored patterns for optical encryption/decryption, showing their remarkable potential for anti-counterfeiting applications.
Second, we describe a thin trilayer film that can generate various wrinkles on transparent and flexible films in the presence of external mechanical bending. In particular, the wrinkle wavelength can be controlled on a tens of nanometer scale by modulating the material properties of each layer. This active modulation plays a critical role in determining resulting structural color spectra. In other words, the wrinkles function as a diffraction grating so that the film displays bright structural colors under bending conditions. After the bending stress is released, the wrinkles disappear and the film becomes transparent again.
Lastly, we demonstrate that the material and structural patterning technique shows remarkable potential for structural coloration applications such as multimodal displays and novel barcode-based anti-counterfeiting techniques.ope
On the Existence and Temporal Asymptotics of Solutions for the Two and Half Dimensional Hall MHD
No full textIn this paper, we deal with the 212
dimensional Hall MHD by taking the velocity field u and the magnetic field B of the form u(t,x,y)=(????????(t,x,y),W(t,x,y))
and B(t,x,y)=(????????(t,x,y),Z(t,x,y))
. We begin with the Hall equations (without the effect of the fluid part). In this case, we provide several results such as the long time behavior of weak solutions, weak-strong uniqueness, the existence of local and global in time strong solutions, decay rates of (??,Z)
, the asymptotic profiles of (??,Z)
, and the perturbation around harmonic functions. In the presence of the fluid field, the results, by comparison, fall short of the previous ones in the absence of the fluid part and we show the existence of local and global in time strong solutions
On the local and global existence of unique solutions to the L??st model
No full textThe Magnetohydrodynamics equations (MHD) provide a macroscopic description of a plasma and it provides a relevant description many physical phenomena. However, MHD is deficient in many respect and some of these deficiencies are accounted for by extending Ohm???s law with the momentum equations being usually unchanged. But, this approach does not give conservation of the energy for some cases. In this paper, we take a model proposed by L??st who appears to be the first to correct the generalized Ohm???s law to guarantee conservation of the energy. We first show the existence of unique solutions and establish a blow-up criterion to the L??st model with initial data in
. We then prove that this solution is defined globally-in-time and decays algebraically when the initial data is sufficiently small
Security defense against long-term and stealthy cyberattacks
No full textModern cyberattacks such as advanced persistent threats have become sophisticated. Hackers can stay undetected for an extended time and defenders do not have sufficient countermeasures to prevent advanced cyberattacks. Reflecting on this phenomenon, we propose a game-theoretic model to analyze strategic decisions made by a hacker and a defender in equilibrium. In our game model, the hacker launches stealthy cyberattacks for a long time and the defender decides when to disable a suspicious user based on noisy observations of the user???s activities. Damages caused by the hacker can be enormous if the defender does not immediately ban a suspicious user under certain circumstances, which can explain the emerging sophisticated cyberattacks with detrimental consequences. Our model also predicts that the hacker may opt to be behavioral to avoid worst cases. This is because behavioral cyberattacks are less threatening and the defender decides not to immediately block a suspicious user to reduce cost of false detection
High-temperature steam oxidation of (Ti, Mo)C-forming FeCrAlY alloy
No full textHigh-temperature steam oxidation testing is conducted at 1200 and 1300 degrees C to evaluate the oxidation resistance and characterize the oxide of the (Ti, Mo)C-forming FeCrAlY alloy developed as a candidate for accident-tolerant fuel-cladding materials in nuclear power plants. The oxidation behavior on both tem-peratures was investigated using X-ray diffraction, scanning, and transmission electron microscopies. The oxidation behavior of the new alloy indicates a temperature dependency: at 1200 degrees C, TiO2 and Y2Ti2O7 are seen on the outermost oxide layer, whereas no Ti-rich oxide is detected at 1300 degrees C, but and Y3Al5O12 are observed, which are typical oxides in carbide-free FeCrAlY alloys. On the TiO2 surface, water molecules can become dissociated, and the products of the dissociation can create more oxygen vacancies, which in turn can lead to a porous oxide layer with voids and a higher oxidation rate when compared with FeCrAlY alloys. However, at 1300 degrees C, Ti diffusion is efficiently blocked by Y segregation along oxide grain boundaries due to larger oxide grains and less intersections between the oxide grain boundaries and the oxide/metal interface. The oxidation resistance of (Ti, Mo)C-forming FeCrAlY is superior to that of the Zr alloy, which makes it a suitable candidate for accident-tolerant fuel-cladding materials from the perspec-tive of high-temperature steam oxidation.(c) 2022 Elsevier B.V. All rights reserved
Separation and purification of syngas-derived hydrogen: A comparative evaluation of membrane- and cryogenic-assisted approaches
No full textHydrogen (H2) separation and purification is challenging because of the high purity and recovery requirements in particular applications, as well as the critical properties of H2 and its associated components. Unlike pressure swing adsorption, cryogenic- and membrane-based technologies are currently employed for H2 separation. Membraneassisted (case-I) and cryogenic-assisted (case-II) separation and purification of H2 were evaluated in this study in terms of the energy, exergy, and economic aspects of the processes. In case-I and case-II, H2 was first produced from synthesis gas via the water-gas shift reaction and was then separated from other components using membrane and cryogenic systems, respectively. Additionally, an organic Rankine cycle was integrated with the water-gas shift reactors to recover the waste heat. A well-known commercial process simulation software, Aspen Hysys (R) v11, was employed to simulate both processes. Energy analysis revealed that case-I has a lower energy consumption (0.50 kWh/ kg) than case-II (2.01 kWh/kg). However, low H2 purity and recovery rates are the main limitations of case-I. In terms of exergy, the H2 separation section in case-I exhibited a higher efficiency (28.4%) than case-II (14.7%). Furthermore, the economic evaluation showed that case-I was more expensive (10.2 M) because of the high cost of the compressors required. In conclusion, this study could assist industry practitioners and academic researchers in selecting optimal H2 separation and purification technologies for improving the overall H2 economy
