• Current Applied Physics
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• v.18 no.12
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• pp.1534-1539
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• 2018

SnSe single crystal showed a high thermoelectric zT of 2.6 at 923 K mainly due to an extremely low thermal conductivity $0.23W\;m^{-1}\;K^{-1}$. It has anisotropic crystal structure resulting in deterioration of thermoelectric performance in polycrystalline SnSe, providing a low zT of 0.6 and 0.8 for Ag and Na-doped SnSe, respectively. Here, we presented the thermoelectric properties on the K-doped $K_xSn_{1-x}Se$ (x = 0, 0.1, 0.3, 0.5, 1.5, and 2.0%) polycrystals, synthesized by a high-temperature melting and hot-press sintering with annealing process. The K-doping in SnSe efficiently enhances the hole carrier concentration without significant degradation of carrier mobility. We find that there exist widespread Se-rich precipitates, inducing strong phonon scattering and thus resulting in a very low thermal conductivity. Due to low thermal conductivity and moderate power factor, the $K_{0.001}Sn_{0.999}Se$ sample shows an exceptionally high zT of 1.11 at 823 K which is significantly enhanced value in polycrystalline compounds.

• Korean Journal of Materials Research
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• v.5 no.2
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• pp.151-156
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• 1995

Low temperature firing $(Ba, Sr)TiO_{3}$ dielectrics were successfully prepared with lead based glass and those electrical properties were investigated. Different amount of PbO content glass materials were added to dielectrics to investigate the sinterability and its dielectric properties. Also, various compositions of ceramic capacitors were prepared to applicate in multilayer ceramic capacitors. A large amount of experiment has been done with various Pb contented glasses and different sintering temperatures. The sintering temperature of $(Ba,Sr)TiO_{3}$can be reduced from $1350^{\circ}C$to as low as $1050^{\circ}C$ with 4wt% addition of $PbO-ZnO-B_{2}O_{2}$ glass materials. Its dielectric constant at room temperature was up to 8100 with low dielectric loss, 0.005. This ceramic capacitor showed fully fired microstructures with its grain size of 1-3$\mu \textrm{m}$. The sintered body which was sintered at $1150^{\circ}C$ for 2hr with 4wt% $PbO-ZnO-B_{2}O_{2}$ glass material addition satisfied the Z5U specification of the EIAS.

• Korean Journal of Materials Research
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• v.22 no.1
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• pp.35-41
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• 2012

Presently, the most promising family of lead-free piezoelectric ceramics is based on $K_{0.5}Na_{0.5}NbO_3$(KNN). Lithium, silver and antimony co-doped KNN ceramics show high piezoelectric properties at room temperature, but often suffer from abnormal grain growth. In the present work, the $(Ba_{0.85}Ca_{0.15})(Ti_{0.88}Zr_{0.12})O_3$ component, which has relaxor ferroelectric characteristics, was doped to suppress the abnormal grain growth. To investigate this effect, Lead-Free $0.95(K_{0.5}Na_{0.5})_{0.95}Li_{0.05}NbO_3-(0.05-x)AgSbO_3-x(Ba_{0.85}Ca_{0.15})(Ti_{0.88}Zr_{0.12})O_3$[KNLN-AS-xBCTZ] piezoelectric ceramics were synthesized by ball mill and nanosized-milling processes in lead-Free $0.95(K_{0.5}Na_{0.5})_{0.95}Li_{0.05}NbO_3-(0.05-x)AgSbO_3$ in order to suppress the abnormal grain growth. The nanosized milling process of calcined powders enhanced the sintering density. The phase structure, microstructure, and ferroelectric and piezoelectric properties of the KNLN-AS ceramics were systematically investigated. XRD patterns for the doped and undoped samples showed perovskite phase while tetragonality was increased with increasing BCZT content, which increase was closely related to the decrease of TO-T. Dense and uniform microstructures were observed for all of the doped BCZT ceramics. After the addition of BCTZ, the tetragonal-cubic and orthorhombic-tetragonal phase transitions shifted to lower temperatures compared to those for the pure KNNL-AS. A coexistence of the orthorhombic and tetragonal phases was hence formed in the ceramics with x = 0.02 mol at room temperature, leading to a significant enhancement of the piezoelectric properties. For the composition with x = 0.02 mol, the piezoelectric properties showed optimum values of: $d_{33}$ = 185 pC/N, $k_P$ = 41%, $T_C=325^{\circ}C$, $T_{O-T}=-4^{\circ}C$.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.206-206
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• 2008

압전소자를 이용한 초음파 모터는 전자기적 원리로 동작하는 기존의 모터에 비해 구조가 간단하고 소형, 경량화가 가능하며 저속에서 큰 토크가 가능하고 ${\mu}m$단위 까지 정밀제어가 가능하다는 장점 등으로 인해 그 응용분야가 점차 확대되고 있다. 초음파 모터의 원리는 수평과 수직방향에서 변위가 타원형 운동을 형성하는 것이다. 따라서 선택한 타원운동의 방식에 의해서 모터의 형상이 달라진다. 초음파 모터는 액츄에이터를 사용하여 만들기 때문에 액츄에이터의 특성은 모터의 타원변위나 토크에 영향을 미친다. 단판형 액츄에이터에 비하여 적층 액츄에이터는 입력 임피던스를 낮추어 낮은 구동전압에서 구동이 가능하며 큰 변위와 토크를 발생하기 때문에 진동자의 수명 향상과 구동전압을 낮추기에 적합하다. 적층 액츄에이터는 변위량이나 응력 등을 개선하기 위해서 전기기계 결합계수(kp) 및 압전 d상수가 큰 재료가 요구되며, 고전압에서 장시간 구동 시 마찰에 의한 열손실을 감소시키기 위해 높은 기계적 품질계수(Qm)를 가져야한다. 적층 시 내부전극으로 사용하는 Pd, Pt가 함유된 전극은 가격이 비싸 제조비용을 상승시킨다. 상대적으로 값싼 Ag전극을 사용하면 비용절감을 할 수 있지만 융점이 낮아서 저온소결이 불가피하다. 따라서, 특성이 우수한 적층 액츄에이터를 제조하기 위해서 저손실, 저온소결 할 수 있는 액츄에이터 재료가 필요한 실정이다. L1-B4 혈 선혈 초음파 모터는 L1모드와 B4모드의 공진 주파수가 일치하여야 큰 변위를 얻을 수 있는데 이전의 논문에서 Atila를 이용한 시뮬레이션 결과를 분석한 봐 있다. 적층 액츄에이터의 층수를 5,7,9,11,13,15층으로 하여 L1-B4모드에서의 공진주파수를 비교한 결과 13 층일 때 두 모드가 비슷한 공진주파수를 보였고, 티원변위궤적도 다른 층수에 비해 크게 나타났다. 본 연구에서는 시뮬레이션 결과 가장 좋은 특성을 보인 13층 액츄에이터로 선형 초음파 모터를 제작하였다. 또한, 액츄에이터는 압전 및 유전특성이 우수한 저온소결 PZW-PMN-PZT세라믹을 이용하여 제작하였고, 내부전극으로 Ag전극을 사용하였다. 제작된 13 층 선형초음파모터를 가지고 프리로드 및 전압에 따른 속도를 조사하였고, 시뮬레이션 결과와 비교해 보았다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.29 no.10
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• pp.601-607
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• 2016

Piezoelectric thick films of soft $Pb(Zr,Ti)O_3$ (PZT) based commercial material (S55) were fabricated using a conventional tape casting method. Ag-Pd electrodes were printed on the piezoelectric film at room temperature and all 5 layered films with a dimension of $12mm{\times}16mm$ were successfully laminated for a multi-layered piezoelectric ceramic actuator. The laminated specimens were co-fired at $1,100^{\circ}C$ for 1 h. A flat layered and dense microstructure was obtained for the $112{\mu}m$ thick piezoelectric actuator after sintering process. Thereafter, a prototype piezoelectric speaker was fabricated using the multi-layered piezoelectric ceramic actuator which can operate as a bimorph. Its SPL (sound pressure level) characteristic was also evaluated for speaker application. Frequency response revealed that the output SPL with a root mean square voltage of 10 V increased gradually to the highest peak of 87.5 dB for 1.5 kHz and exhibited a relatively stable behavior over the measured frequency range (${\leq}20kHz$) at a distance of 10 cm, implying that the fabricated piezoelectric speaker is potential for speaker applications.

• Journal of Powder Materials
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• v.24 no.5
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• pp.377-383
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• 2017

Microstructure, electric, and thermal properties of the Ta-Cu composite is evaluated for the application in electric contact materials. This material has the potential to be used in a medium for a high current range of current conditions, replacing Ag-MO, W, and WC containing materials. The optimized SPS process conditions are a temperature of $900^{\circ}C$ for a 5 min holding time under a 30 MPa mechanical pressure. Comparative research is carried out for the calculated and actual values of the thermal and electric properties. The range of actual thermal and electric properties of the Ta-Cu composite are 50~300 W/mk and 10~90 %IACS, respectively, according to the compositional change of the 90 to 10 wt% Ta-Cu system. The results related to the electric contact properties, suggest that less than 50 wt% of Ta compositions are possible in applications of electric contact materials.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.281-281
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• 2007

Low Temperature Co-fired Ceramic (LTCC) technology has been used in electronic device for various functions. LTCC technology is to fire dielectric ceramic and a conductive electrode such as Ag or Cu thick film below the temperature of $900^{\circ}C$ simultaneously. The glass-ceramic has been widely used for LTCC materials due to its low sintering temperature, high mechanical properties and low dielectric constants. To obtain the high strength, addition of filler, the microstructure should have various crystals and low pores in a composite. In this study, two glass frits were mixed with different alumina size(0.5, 2, 3.7um) and sintered at the range of $850{\sim}950^{\circ}C$. The microstructure, crystal phases, thermal and mechanical properties of the composites were investigated using FE-SEM, XRD, TG-DTA, Dilatomer. When the particle size of $Al_2O_3$ filler increased, the starting temperatures for the densification of the sintered bodies, onset point of crystallization, peak crystallization temperature in the glass-ceramic composites decreased gradually. After sintered at $900^{\circ}C$, the glass frits were crystallized as $CaAl_2Si_2O_8\;and\;CaMgSi_2O_6$. The purpose of our study is to understand the relationship between the $Al_2O_3$ particle size and thermal properties in composites.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.26 no.3
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• pp.246-251
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• 2013

Screen printing is commonly used to form the front/back electrodes in silicon solar cell. But it has caused high resistance and low aspect ratio, resulting in decreased conversion efficiency in solar cell. Recently the plating method has been combined with screen-printed c-Si solar cell to reduce the resistance and improve the aspect ratio. In this paper, we investigated the effect of light induced silver plating with screen-printed c-Si solar cells and compared their electrical properties. All wafers were textured, doped, and coated with anti-reflection layer. The metallization process was carried out with screen-printing, followed by co-fired. Then we performed light induced Ag plating by changing the plating time in the range of 20 sec~5min with/without external light. For comparison, we measured the light I-V characteristics and electrode width by optical microscope. During plating, silver ions fill the porous structure established in rapid silver particle sintering during co-firing step, which results in resistance decrease and efficiency improvement. The plating rate was increased in presence of light lamp, resulting in widening the electrode with and reducing the short-circuit current by shadowing loss. With the optimized plating condition, the conversion efficiency of solar cells was increased by 0.4% due to decreased series resistance. Finally we obtained the short-circuit current of 8.66 A, open-circuit voltage of 0.632 V, fill factor of 78.2%, and efficiency of 17.8% on a silicon solar cell.

• Tribology and Lubricants
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• v.39 no.5
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• pp.197-202
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• 2023

Recently, research on experimental and analytical techniques utilizing microfluidic devices has been pursued. For example, lab-on-a-chip devices that integrate micro-devices onto a single chip for processing small sample quantities have gained significant attention. However, during sample preparation, unnecessary gases can be introduced into the internal channels, thus, impeding device flow and compromising specific function efficiency, including that of analysis and separation. Several methods have been proposed to mitigate this issue, however, many involve cumbersome procedures or suffer from complexities owing to intricate structures. Recently, some approaches have been introduced that utilize hydrophobic device structures to remove gases within channels. In such cases, the permeability of gases passing through the structure becomes a crucial performance factor. In this study, a method involving the deposition and sintering of diluted Ag-ink onto a silicon wafer surface is presented. This is followed by unstructured nano-pattern creation using a Metal Assisted Chemical Etching (MACE) process, which yields a nanostructured surface with unstructured pillar shapes. Subsequently, gas permeability in the spaces formed by these surface structures is investigated. This is achieved by experiments conducted to incorporate a pressure chamber and measure gas permeability. Trends are subsequently analyzed by comparing the results with existing theories. Finally, it can be confirmed that the significance of this study primarily lies in its capability to effectively evaluate gas permeability through unstructured pillar-like nanostructures, thus, providing quantitative values for the appropriate driving pressure and expected gas removal time in practical device operation.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2000.04a
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• pp.9-10
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• 2000

An important business-field of world-wide steel-industry is the coating of thin metal-sheets with zinc, zinc-aluminum and aluminum based materials. These products mostly go into automotive industry. in particular for the car-body. into building and construction industry as well as household appliances. Due to mass-production, the processing is done in large continuously operating plants where the mostly cold-rolled metal-strip as the substrate is handled in coils up to 40 tons unwind before and rolled up again after passing the processing plant which includes cleaning, annealing, hot-dip galvanizing / aluminizing and chemical treatment. In the liquid Zn, Zn-AI, AI-Zn and AI-Si bathes a combined action of corrosion and wear under high temperature and high stress onto the transfer components (rolls) accounts for major economic losses. Most critical here are the bearing systems of these rolls operating in the liquid system. Rolls in liquid system can not be avoided as they are needed to transfer the steel-strip into and out of the crucible. Since several years, ceramic roller bearings are tested here [1.2], however, in particular due to uncontrollable Slag-impurities within the hot bath [3], slide bearings are still expected to be of a higher potential [4]. The today's state of the art is the application of slide bearings based on Stellite\ulcorneragainst Stellite which is in general a 50-60 wt% Co-matrix with incorporated Cr- and W-carbides and other composites. Indeed Stellite is used as the bearing-material as of it's chemical properties (does not go into solution), the physical properties in particular with poor lubricating properties are not satisfying at all. To increase the Sliding behavior in the bearing system, about 0.15-0.2 wt% of lead has been added into the hot-bath in the past. Due to environmental regulations. this had to be reduced dramatically_ This together with the heavily increasing production rates expressed by increased velocity of the substrate-steel-band up to 200 m/min and increased tractate power up to 10 tons in modern plants. leads to life times of the bearings of a few up to several days only. To improve this situation. the Mechanical Alloying (MA) TeChnique [5.6.7.8] is used to prOduce advanced Stellite-based bearing materials. A lubricating phase is introduced into Stellite-powder-material by MA, the composite-powder-particles are coated by High Energy Milling (HEM) in order to produce bearing-bushes of approximately 12 kg by Sintering, Liquid Phase Sintering (LPS) and Hot Isostatic Pressing (HIP). The chemical and physical behavior of samples as well as the bearing systems in the hot galvanizing / aluminizing plant are discussed. DependenCies like lubricant material and composite, LPS-binder and composite, particle shape and PM-route with respect to achievable density. (temperature--) shock-reSistibility and corrosive-wear behavior will be described. The materials are characterized by particle size analysis (laser diffraction), scanning electron microscopy and X-ray diffraction. corrosive-wear behavior is determined using a special cylinder-in-bush apparatus (CIBA) as well as field-test in real production condition. Part I of this work describes the initial testing phase where different sample materials are produced, characterized, consolidated and tested in the CIBA under a common AI-Zn-system. The results are discussed and the material-system for the large components to be produced for the field test in real production condition is decided. Outlook: Part II of this work will describe the field test in a hot-dip-galvanizing/aluminizing plant of the mechanically alloyed bearing bushes under aluminum-rich liquid metal. Alter testing, the bushes will be characterized and obtained results with respect to wear. expected lifetime, surface roughness and infiltration will be discussed. Part III of this project will describe a second initial testing phase where the won results of part 1+11 will be transferred to the AI-Si system. Part IV of this project will describe the field test in a hot-dip-aluminizing plant of the mechanically alloyed bearing bushes under aluminum liquid metal. After testing. the bushes will be characterized and obtained results with respect to wear. expected lifetime, surface roughness and infiltration will be discussed.