• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.34 no.6
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• pp.449-453
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• 2021

Interest and investment in renewable energy have increased worldwide, highlighting the need for renewable energy. Solar energy was the most promising energy of all renewable energy sources, and it has the highest investment value. Because photovoltaics require a certain amount of area for installation, high density and high output performance are required. Shingled module is a promising technology in that they are featured by higher density and higher output compared to the conventional modules. Shingled technology uses a laser scribing to divide solar cells that are to be bonded with electrically conductive adhesive (ECA) to produce and connect strings, which has a higher output in the same area than the conventional modules. In the process of producing solar modules, metal ribbons are used to interconnect cells, but they are also needed for string connections in shingled solar cells. Accordingly, in this study, we researched the interconnection that best suits the connector that joins the string to the string. The module outputs produced under the conditions of the string interconnection were compared and analyzed.

• Journal of Microbiology and Biotechnology
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• v.29 no.6
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• pp.952-961
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• 2019

Chlorella spp. are green algae that are found across wide-ranging habitats from deserts to arctic regions, with various strains having adapted to survive under diverse environmental conditions. In this study, two novel Chlorella strains (ABC-002, ABC-008) were isolated from a freshwater lake in South Korea during the winter season and examined for possible use in the biofuel production process. The comparison of ABC-002 and ABC-008 strains with Chlorella vulgaris UTEX265 under two different temperatures ($10^{\circ}C$, $25^{\circ}C$) revealed their cold-tolerant phenotypes as well as high biomass yields. The maximum quantum yields of UTEX25, ABC-002, and ABC-008 at $10^{\circ}C$ were 0.5594, 0.6747, and 0.7150, respectively, providing evidence of the relatively higher cold-resistance capabilities of these two strains. Furthermore, both the biomass yields and lipid content of the two novel strains were found to be higher than those of UTEX265; the overall lipid productivities of ABC-002 and ABC-008 were 1.7 ~ 2.8 fold and 1.6 ~ 4.2 fold higher compared to that of UTEX265, respectively. Thus, the high biomass and lipid productivity over a wide range of temperatures indicate that C. vulgaris ABC-002 and ABC-008 are promising candidates for applications in biofuel productions via outdoor biomass cultivation.

• BioChip Journal
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• v.12 no.4
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• pp.332-339
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• 2018

The future neuro-prosthetic devices would be required spikes data monitoring through sub-nanoscale transistors that enables to neuroscientists and clinicals for scalable, wireless and implantable applications. This research investigates the spikes monitoring through integrated CNT front-end amplifier for neuro-prosthetic diagnosis. The proposed carbon nanotube-based architecture consists of front-end amplifier (FEA), integrate fire neuron and pseudo resistor technique that observed high electrical performance through neural activity. A pseudo resistor technique ensures large input impedance for integrated FEA by compensating the input leakage current. While carbon nanotube based FEA provides low-voltage operation with directly impacts on the power consumption and also give detector size that demonstrates fidelity of the neural signals. The observed neural activity shows amplitude of spiking in terms of action potential up to $80{\mu}V$ while local field potentials up to 40 mV by using proposed architecture. This fully integrated architecture is implemented in Analog cadence virtuoso using design kit of CNT process. The fabricated chip consumes less power consumption of $2{\mu}W$ under the supply voltage of 0.7 V. The experimental and simulated results of the integrated FEA achieves $60G{\Omega}$ of input impedance and input referred noise of $8.5nv/{\sqrt{Hz}}$ over the wide bandwidth. Moreover, measured gain of the amplifier achieves 75 dB midband from range of 1 KHz to 35 KHz. The proposed research provides refreshing neural recording data through nanotube integrated circuit and which could be beneficial for the next generation neuroscientists.

• Korean Journal of Materials Research
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• v.28 no.11
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• pp.680-684
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• 2018

We investigate the effect of light intensity and wavelength of a solar cell device using photoconductive atomic force microscopy(PC-AFM). A $POCl_3$ diffusion doping process is used to produce a p-n junction solar cell device based on a polySi wafer, and the electrical properties of prepared solar cells are measured using a solar cell simulator system. The measured open circuit voltage($V_{oc}$) is 0.59 V and the short circuit current($I_{sc}$) is 48.5 mA. Moreover, the values of the fill factors and efficiencies of the devices are 0.7 and approximately 13.6 %, respectively. In addition, PC-AFM, a recent notable method for nano-scale characterization of photovoltaic elements, is used for direct measurements of photoelectric characteristics in limited areas instead of large areas. The effects of changes in the intensity and wavelength of light shining on the element on the photoelectric characteristics are observed. Results obtained through PC-AFM are compared with the electric/optical characteristics data obtained through a solar simulator. The voltage($V_{PC-AFM}$) at which the current is 0 A in the I-V characteristic curves increases sharply up to $18W/m^2$, peaking and slowly falling as light intensity increases. Here, $V_{PC-AFM}$ at $18W/m^2$ is 0.29 V, which corresponds to 59 % of the average $V_{oc}$ value, as measured with the solar simulator. Furthermore, while the light wavelength increases from 300 nm to 1,100 nm, the external quantum efficiency(EQE) and results from PC-AFM show similar trends at the macro scale but reveal different results in several sections, indicating the need for detailed analysis and improvement in the future.

• Journal of Powder Materials
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• v.26 no.3
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• pp.201-207
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• 2019

Tungsten heavy alloys (W-Ni-Fe) play an important role in various industries because of their excellent mechanical properties, such as the excellent hardness of tungsten, low thermal expansion, corrosion resistance of nickel, and ductility of iron. In tungsten heavy alloys, tungsten nanoparticles allow the relatively low-temperature molding of high-melting-point tungsten and can improve densification. In this study, to improve the densification of tungsten heavy alloy, nanoparticles are manufactured by ultrasonic milling of metal oxide. The physical properties of the metal oxide and the solvent viscosity are selected as the main parameters. When the density is low and the Mohs hardness is high, the particle size distribution is relatively high. When the density is high and the Mohs hardness is low, the particle size distribution is relatively low. Additionally, the average particle size tends to decrease with increasing viscosity. Metal oxides prepared by ultrasonic milling in high-viscosity solvent show an average particle size of less than 300 nm based on the dynamic light scattering and scanning electron microscopy analysis. The effects of the physical properties of the metal oxide and the solvent viscosity on the pulverization are analyzed experimentally.

• Progress in Superconductivity and Cryogenics
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• v.21 no.2
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• pp.40-44
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• 2019

In this study, the effects of the compaction method for (Mg+2B) powders on the apparent density and superconducting properties of $MgB_2$ bulk superconductor were investigated. The raw powders used in this study were nano-sized boron (B) and spherical magnesium (Mg). A batch of a powder mixture of (Mg+2B) was put in a steel mold and uniaxially pressed at 1 ton or 3 tons into pellets. Another batch of the powder mixture was uniaxially pressed at 1 ton and then pressed isostatically at $1800kg/cm^2$ in the water chamber. All pellets were heat-treated at $650^{\circ}C$ for 1 h in flowing argon gas for the formation of $MgB_2$. The apparent density of powder compacts pressed at 3 ton was higher than that at 1 ton. The cold isostatic pressing (CIP) in a water chamber allowed further increase of the apparent density of powder compacts, which influenced the pellet density of the final products ($MgB_2$). The compaction methods (uniaxial pressing and CIP) did not affect the formation of $MgB_2$ and superconducting critical temperature ($T_c$) of $MgB_2$, but affected the critical current density ($J_c$) of $MgB_2$ significantly. The sample with the high apparent density showed high $J_c$ at 5 K and 20 K at applied magnetic fields (0-5 T).

• Advances in nano research
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• v.7 no.3
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• pp.209-221
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• 2019

Graphene, with impressive electronic properties, have high potential in the microelectronic field. However, graphene itself is a zero bandgap material which is not suitable for digital logic gates and its application. Thus, much focus is on graphene nanoribbons (GNRs) that are narrow strips of graphene. During GNRs fabrication process, the occurrence of defects that ultimately change electronic properties of graphene is difficult to avoid. The modelling of GNRs with defects is crucial to study the non-idealities effects. In this work, nearest-neighbor tight-binding (TB) model for GNRs is presented with three main simplifying assumptions. They are utilization of basis function, Hamiltonian operator discretization and plane wave approximation. Two major edges of GNRs, armchair-edged GNRs (AGNRs) and zigzag-edged GNRs (ZGNRs) are explored. With single vacancy (SV) defects, the components within the Hamiltonian operator are transformed due to the disappearance of tight-binding energies around the missing carbon atoms in GNRs. The size of the lattices namely width and length are varied and studied. Non-equilibrium Green's function (NEGF) formalism is employed to obtain the electronics structure namely band structure and density of states (DOS) and all simulation is implemented in MATLAB. The band structure and DOS plot are then compared between pristine and defected GNRs under varying length and width of GNRs. It is revealed that there are clear distinctions between band structure, numerical DOS and Green's function DOS of pristine and defective GNRs.

• Journal of the Korean Society of Industry Convergence
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• v.21 no.6
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• pp.265-271
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• 2018

DC-DC buck converter is a critical building block in the power management integrated circuit (PMIC) architecture for the portable devices such as cellular phone, personal digital assistance (PDA) because of its power efficiency over a wide range of conversion ratio. To ensure a safe operation, avoid unexpected damages and enhance the reliability of the converter, fully-integrated protection circuits such as over voltage protection (OVP), under voltage lock out (UVLO), startup, and thermal shutdown (TSD) blocks are designed. In this paper, these three fully-integrated protection circuit blocks are proposed for use in the DC-DC buck converter. The buck converter with proposed protection blocks is operated with a switching frequency of 1 MHz in continuous conduction mode (CCM). In order to verify the proposed scheme, the buck converter has been designed using a 180 nm CMOS technology. The UVLO circuit is designed to track the input voltage and turns on/off the buck converter when the input voltage is higher/lower than 2.6 V, respectively. The OVP circuit blocks the buck converter's operation when the input voltage is over 3.3 V, thereby preventing the destruction of the devices inside the controller IC. The TSD circuit shuts down the converter's operation when the temperature is over $85^{\circ}C$. In order to verify the proposed scheme, these protection circuits were firstly verified through the simulation in SPICE. The proposed protection circuits were then fabricated and the measured results showed a good matching with the simulation results.

• Composites Research
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• v.31 no.6
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• pp.398-403
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• 2018

Recently, environmental pollution caused by plastic waste, ecosystem disturbance of micro-plastics and human body accumulation are becoming big problems. In order to replace the traditional plastic, eco-friendly resin and natural fiber-based composite materials have been developed, but they have a disadvantage that their mechanical properties are significantly lower than those of synthetic fiber-based composites. In this study, eco - friendly nanofiber was extracted from seaweed and used as an additive in order to improve the mechanical properties of jute fiber-reinforced composites. Through the hand lay-up process, the composites were fabricated, and it was confirmed that the nanofiber was effective in improving the mechanical properties of natural fiber composites through tensile, bending and drop weight impact tests.

• Journal of Surface Science and Engineering
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• v.52 no.1
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• pp.16-22
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• 2019

The structural change of DLC coatings during long-term wear test and dicing test under the low loading condition was investigated. DLC coatings were applied for the precision injection molds of a modified SNCM steel for the extension of life and the micro-diamond blades for the high cutting efficiency and the increase in life. A ball-on-disc wear tests in the mold steel and a dicing tests in the micro-diamond blades were conducted to understand degradation of DLC coatings. The degradation of DLC coatings for the injection mold steel and the micro-diamond blades during the wear and dicing tests were studied with Raman Spectroscopy. Raman peaks were divided two bands(D band and G band) to study the degradation process of DLC structure. By the wear test, polished condition of wear marks were observed to be maintained until 10 hrs of wear test period is given, but small striation marks appeared in 20 hours wear test. It was observed that $I_D/I_G$ ratios changed as the degradation of DLC coatings is proceeded during the wear tests and the dicing tests. It is suggested that the change in $I_D/I_G$ value possibly reflected from the composition of $sp^2$ and $sp^3$ bondings in DLC layers relevant to the change in mechanical and physical property.