• Journal of Powder Materials
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• v.28 no.2
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• pp.103-109
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• 2021

High-entropy alloys have excellent mechanical properties under extreme environments, rendering them promising candidates for next-generation structural materials. It is desirable to develop non-equiatomic high-entropy alloys that do not require many expensive or heavy elements, contrary to the requirements of typical high-entropy alloys. In this study, a non-equiatomic high-entropy alloy powder Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5 (at.%) is prepared by high energy ball milling and fabricated by spark plasma sintering. By combining different ball milling times and ball-to-powder ratios, we attempt to find a proper mechanical alloying condition to achieve improved mechanical properties. The milled powder and sintered specimens are examined using X-ray diffraction to investigate the progress of mechanical alloying and microstructural changes. A miniature tensile specimen after sintering is used to investigate the mechanical properties. Furthermore, quantitative analysis of the microstructure is performed using electron backscatter diffraction.

• Nuclear Engineering and Technology
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• v.53 no.1
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• pp.164-171
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• 2021

The europium sorption on Tamusu clay was investigated by batch sorption experiments and spectroscopic study under the condition of strong ionic strength. The results demonstrated that europium sorption on Tamusu clay increased rapidly with pH value, but decreased with the ionic strength of solution increased. The europium sorption also increased in the presence of humic acid, especially at low pH value. The sorption could be fitted by Freundlich isotherm model and the europium sorption on clay was spontaneous and endothermic reaction. Besides, the result indicates that ion exchange was the main process at low pH value, while inner-sphere surface complexation dominated the sorption process at high pH value. The Backscatter electron scanning/Energy Dispersive Spectrometer(BSE/EDS) and the effect of Na for europium sorption results further suggested that europium sorption on Tamusu clay mainly competed with Na at low pH value. Overall, the results in this research were of significance to understand the sorption behavior of europium on the geological media under high ionic strength.

• Journal of Powder Materials
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• v.28 no.4
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• pp.336-341
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• 2021

In this study, a nanocrystalline FeNiCrMoMnSiC alloy was fabricated, and its austenite stability, microstructure, and mechanical properties were investigated. A sintered FeNiCrMoMnSiC alloy sample with nanosized crystal was obtained by high-energy ball milling and spark plasma sintering. The sintering behavior was investigated by measuring the displacement according to the temperature of the sintered body. Through microstructural analysis, it was confirmed that a compact sintered body with few pores was produced, and cementite was formed. The stability of the austenite phase in the sintered samples was evaluated by X-ray diffraction analysis and electron backscatter diffraction. Results revealed a measured value of 51.6% and that the alloy had seven times more austenite stability than AISI 4340 wrought steel. The hardness of the sintered alloy was 60.4 HRC, which was up to 2.4 times higher than that of wrought steel.

• Journal of Powder Materials
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• v.28 no.3
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• pp.246-252
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• 2021

The effect of sintering conditions on the austenite stability and strain-induced martensitic transformation of nanocrystalline FeCrC alloy is investigated. Nanocrystalline FeCrC alloys are successfully fabricated by spark plasma sintering with an extremely short densification time to obtain the theoretical density value and prevent grain growth. The nanocrystallite size in the sintered alloys contributes to increased austenite stability. The phase fraction of the FeCrC sintered alloy before and after deformation according to the sintering holding time is measured using X-ray diffraction and electron backscatter diffraction analysis. During compressive deformation, the volume fraction of strain-induced martensite resulting from austenite decomposition is increased. The transformation kinetics of the strain-induced martensite is evaluated using an empirical equation considering the austenite stability factor. The hardness of the S0W and S10W samples increase to 62.4-67.5 and 58.9-63.4 HRC before and after deformation. The hardness results confirmed that the mechanical properties are improved owing to the effects of grain refinement and strain-induced martensitic transformation in the nanocrystalline FeCrC alloy.

• Korean Journal of Remote Sensing
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• v.36 no.6_3
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• pp.1681-1690
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• 2020

A scanning lidar system has been developed. The system has two wavelength observation channels of 532 and 1064 nm and is capable of 360-degree horizontal scanning observation. In addition, an analysis method that can classify the measured particle as an indicator of coarse-mode particle (PM2.5-10) and an indicator of fine-mode particles (PM2.5) and calculate the mass concentration of each has been developed by using the backscatter coefficient at two wavelengths. It was applied to the data calculated by observation. The mass concentrations of PM10 and PM2.5, which showed a distribution of 22-110 ㎍/㎥ and 7-78 ㎍/㎥, respectively, were successfully calculated in the Ulsan Onsan Industrial Complex using the developed scanning lidar system. The analyzed results showed similar values to the mass concentrations measured on the ground around the lidar observation area, and it was confirmed that high concentrations of 80-110 ㎍/㎥ and 60-78 ㎍/㎥ were measured at points discharged from factories, respectively.

• Journal of Powder Materials
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• v.28 no.3
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• pp.221-226
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• 2021

We fabricate the non-equiatomic high-entropy alloy (NE-HEA) Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5 (at.%) using spark plasma sintering under various sintering conditions. Each elemental pure powder is milled by high-energy ball milling to prepare NE-HEA powder. The microstructure and mechanical properties of the sintered samples are investigated using various methods. We use the X-ray diffraction (XRD) method to investigate the microstructural characteristics. Quantitative phase analysis is performed by direct comparison of the XRD results. A tensile test is used to compare the mechanical properties of small samples. Next, electron backscatter diffraction analysis is performed to analyze the phase fraction, and the results are compared to those of XRD analysis. By combining different sintering durations and temperature conditions, we attempt to identify suitable spark plasma sintering conditions that yield mechanical properties comparable with previously reported values. The samples sintered at 900 and 1000℃ with no holding time have a tensile strength of over 1000 MPa.

• Journal of Powder Materials
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• v.29 no.5
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• pp.382-389
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• 2022

A typical trade-off relationship exists between strength and elongation in face-centered cubic metals. Studies have recently been conducted to enhance strength without ductility reduction through surface-treatment-based ultrasonic nanocrystalline surface modification (UNSM), which creates a gradient microstructure in which grains become smaller from the inside to the surface. The transformation-induced plasticity effect in Fe-Mn alloys results in excellent strength and ductility due to their high work-hardening rate. This rate is achieved through strain-induced martensitic transformation when an alloy is plastically deformed. In this study, Fe-6%Mn powders with different sizes were prepared by high-energy ball milling and sintered through spark plasma sintering to produce Fe-6%Mn samples. A gradient microstructure was obtained by stacking the different-sized powders to achieve similar effects as those derived from UNSM. A compressive test was performed to investigate the mechanical properties, including the yielding behavior. The deformed microstructure was observed through electron backscatter diffraction to determine the effects of gradient plastic deformation.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.489-489
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• 2023

최근 유량 측정을 위해 사용되는 ADCP를 통해 부가적으로 측정되는 초음파산란도 자료를 활용하여 부유사농도를 측정하는 연구가 수행되고 있다. 이에 국내에서는 국가하천에 설치되어 있는 자동유량관측소의 초음파산란도를 활용하여 연속적인 부유사농도를 측정하는 연구가 수행되고 있다. 이를 통해 10분 단위로 연속적인 유사량 자료를 생산할 수 있을 것으로 기대되며, 현재 유사량 측정결과의 제공을 위해 사용되는 유량-유사량 관계곡선의 산포로 인한 신뢰도 문제를 개선할 수 있을 것으로 기대되고 있다. 그러나, 이미 설치된 자동유량관측소의 H-ADCP 원시데이터를 활용하여 다지점에서 부유사농도를 측정에 대한 분석을 수행하기 위해서는 초음파산란도의 보정, 관계식 개발, 관계식 적용을 통한 유사량 측정 결과의 분석을 위한 소프트웨어 개발이 필요하다. 이에 본 연구에서는 초음파산란도 자료를 이용하여 부유사농도를 분석할 수 있는 소프트웨어 개발하고자 하였다. 개발된 소프트웨어는 Microsoft Visual Studio를 이용하여 C# 언어를 사용하여 개발하였으며, ComponentOne 라이브러리를 활용하여 그래픽 사용자 인터페이스(GUI)를 구현하였다. 소프트웨어의 구성은 H-ADCP 원시자료와 실측 부유사농도 자료와의 시간동기화를 통해 동일시간에서 측정된 자료를 획득, 초음파산란도의 보정과 지표로 활용할 초음파산란도의 측정영역 분석, 초음파산란도-부유사농도와의 다중 회귀를 통한 관계식 개발 및 통계 분석결과 도출, 관계식을 활용한 부유사농도 계산을 수행할 수 있도록 구성하였다. 본 연구를 통해 개발된 소프트웨어를 통해 추후에 시범적용 예정인 자동유량관측소의 초음파산란도를 활용 부유사농도 측정 방법에 대한 분석 효율성을 향상시키고, 지속적인 개선을 통해서 실제 실무에서 활용이 가능할 것으로 기대된다.

• The Journal of the Acoustical Society of Korea
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• v.42 no.5
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• pp.422-428
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• 2023

Ultrafast ultrasound imaging has been applied to various imaging approaches, including shear wave elastography, ultrafast Doppler, and super-resolution imaging. However, these methods are still challenging in real-time implementation for three Dimension (3D) or portable applications because of their massive data rate required. In this paper, we proposed an adaptive quantization method that effectively reduces the data rate of large Radio Frequency (RF) data. In soft tissue, ultrasound backscatter signals require a high dynamic range, and thus typical quantization used in the current systems uses the quantization level of 10 bits to 14 bits. To alleviate the quantization level to expand the application of ultrafast ultrasound imaging, this study proposed a depth-sectional quantization approach that reduces the quantization errors. For quantitative evaluation, Field II simulations, phantom experiments, and in vivo imaging were conducted and CNR, spatial resolution, and SSIM values were compared with the proposed method and fixed quantization method. We demonstrated that our proposed method is capable of effectively reducing the quantization level down to 3-bit while minimizing the image quality degradation.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.252-252
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• 2022

최근 기존의 전통적인 부유사채집기를 활용한 유사량 측정방법의 대안으로 지속적인 유량측정을 목적으로 활용되고 있는 횡방향 초음파도플러유속계의 초음파산란도를 활용하여 부유사농도를 측정에 대한 연구가 수행되고 있다. 하지만, 하천에서 횡방향 초음파도플러유속계를 활용한 유사량 측정 연구는 현장에서 측정된 자료를 기반으로 분석이 수행되기 때문에 통제된 유사 조건에서의 유사의 농도 및 입도분포에 대한 사전 정보를 바탕으로 정밀한 분석의 어려움이 있다. 이에 본 연구에서는 안동하천실험센터 순환수로에서 균일한 입도를 가지고 있는 황토를 활용하여 주입량에 따른 초음파산란도의 변화와 동시에 측정된 LISST(laser in-situ scattering and transmissometry)의 부유사 농도와 입도 자료를 활용하여 유사공급에 따른 3Mhz의 단일 주파수를 사용하는 횡방향 초음파도플러유속계의 초음파산란도를 활용한 부유사농도 및 평균입경의 측정에 대한 연구를 수행하였다. 측정결과, 유사공급에 따라서 초음파산란도를 활용하여 부유사농도가 증가하는 경향을 나타내고 있었으며, 횡방향 초음파도플러유속계로부터 측정된 셀별 초음파산란도를 활용하여 부유사농도의 공간적인 분포의 분석이 가능함을 확인할 수 있었다. 그리고 유사감쇄계수를 활용하여 LISST로부터 측정된 평균입경과의 관계식을 개발하여 유사공급에 따른 평균입경의 변화를 측정할 수 있을 것으로 나타났다. 추후, 통제된 조건에서의 다양한 입도분포에 따른 실험을 통해 횡방향 초음파도플러유속계의 초음파산란도를 활용한 부유사농도 측정 정확도를 개선과 유사감쇄계수를 통해 평균입경을 측정하는데 활용이 될 것으로 판단된다.