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Seunghyo Noh,Dohyun Kwak,Juseung Lee,강준희,한병찬 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.6
We utilized first-principles density-functional-theory (DFT) calculations to evaluate the thermodynamicfeasibility of a pyroprocessing methodology for reducing the volume of high-level radioactivematerials and recycling spent nuclear fuels. The thermodynamic properties of transuraniumelements (Pu, Np and Cm) were obtained in electrochemical equilibrium with a LiCl-KCl moltensalt as ionic phases and as adsorbates on a W(110) surface. To accomplish the goal, we rigorouslycalculated the double layer interface structures on an atomic resolution, on the thermodynamicallymost stable configurations on W(110) surfaces and the chemical activities of the transuranium elementsfor various coverages of those elements. Our results indicated that the electrodepositionprocess was very sensitive to the atomic level structures of Cl ions at the double-layer interface. Our studies are easily expandable to general electrochemical applications involving strong redoxreactions of transition metals in non-aqueous solutions.
Solution synthesis of GeS and GeSe nanosheets for high-sensitivity photodetectors
Ramasamy, Parthiban,Kwak, Dohyun,Lim, Da-Hye,Ra, Hyun-Soo,Lee, Jong-Soo The Royal Society of Chemistry 2016 Journal of Materials Chemistry C Vol.4 No.3
<▼1><P>High-sensitivity photodetectors are reported using solution synthesized GeS and GeSe nanosheets.</P></▼1><▼2><P>We report the synthesis of 2D nanosheets of GeS and GeSe by facile solution based approaches. The synthesized nanosheets are single-crystalline in nature with lateral dimensions in micrometers. Band structures calculated from DFT calculations predicted a direct bandgap value of 1.67 and 1.37 eV for GeS and GeSe, respectively. The experimental bandgap values (GeS, <I>E</I>g = 1.6 eV and GeSe, <I>E</I>g = 1.2 eV) determined from optical measurements are slightly smaller than the predicted ones. Photoresponse measurements of GeS and GeSe nanosheets revealed that the nanosheets are extremely photoresponsive toward the incident light and exhibit a high photoresponsivity of up to 173 and 870 A W<SUP>−1</SUP> under a 405 nm laser diode, respectively. These values are several orders of magnitude higher than those of previous reports for graphene and many other metal chalcogenide nanosheet photodetectors. In addition, the photodetectors show a fast photoresponse time and a specific detectivity on the order of 10<SUP>13</SUP> Jones. These results show that both the GeS and GeSe nanosheets are promising narrow bandgap semiconductors for high performance photodetectors.</P></▼2>
Grid Method 기법을 이용한 베이지안 비정상성 확률강수량 산정
곽도현,김광섭,Kwak, Dohyun,Kim, Gwangseob 한국수자원학회 2015 한국수자원학회논문집 Vol.48 No.1
본 연구에서는 Grid method를 사용하여 베이지안 비정상성 확률강우량 산정 모형을 확립하였다. 강우 극치자료의 분포로 Gumbel 분포를 채택하였으며, 분포형의 매개변수에 사전분포를 적용하고, 사전분포에 포함된 매개변수에는 초사전 분포를 적용하여 계층적 베이지안 모형을 구성하였다. Grid method는 매개변수의 발생가능 전 구간에 대하여 확률적으로 더 높은 뒷받침이 있는 하위 구간에서 난수를 직접 생성하여 집합을 구성함으로써 잘못된 결과를 도출할 수 가능성이 높은 상황에서도 보다 정확한 매개변수의 추정을 가능케 하므로 매개변수의 추정과정에서 비표준분포로 나타나는 조건부 확률밀도함수를 통한 난수의 추출은 기존에 사용해 온 Metropolis Hastings 알고리즘이 아닌 Grid method를 사용하였다. 개발된 모형은 서울의 1973년부터 2012년까지의 시강우자료를 이용하여 미래에 대한 재현기간에 따른 확률강수량을 산정하였으며, 그 결과로 기존 정상성 가정에 비해 목표연도에 따라 5%에서 8%정도의 증가율을 나타냈다. A Bayesian nonstationary probability rainfall estimation model using the Grid method is developed. A hierarchical Bayesian framework is consisted with prior and hyper-prior distributions associated with parameters of the Gumbel distribution which is selected for rainfall extreme data. In this study, the Grid method is adopted instead of the Matropolis Hastings algorithm for random number generation since it has advantage that it can provide a thorough sampling of parameter space. This method is good for situations where the best-fit parameter values are not easily inferred a priori, and where there is a high probability of false minima. The developed model was applied to estimated target year probability rainfall using hourly rainfall data of Seoul station from 1973 to 2012. Results demonstrated that the target year estimate using nonstationary assumption is about 5~8% larger than the estimate using stationary assumption.
Kwon, Choah,Kang, Joonhee,Kang, Woojong,Kwak, Dohyun,Han, Byungchan Elsevier 2016 ELECTROCHIMICA ACTA Vol.195 No.-
<P><B>Abstract</B></P> <P>Using first principles density functional theory (DFT) calculations we obtain thermodynamic and kinetic properties of U in an electrorefining process for spent nuclear fuels using a LiCl-KCl eutectic molten salt and Mo as a cathode. The thermodynamic stability of electrodeposited U from the molten salt onto the Mo(110) surface electrode is evaluated by activity coefficients as function of surface coverages of U and Cl. Additionally, ab-initio molecular dynamic simulations combined with the Stokes-Einstein-Sutherland relation enables us to calculate the viscosity of the LiCl-KCl eutectic molten salt. Our results well agree with previously reported experimental data endorsing the credibility. Based on our atomic-level mechanical understanding we propose that an accurate computational model system incorporating the electrochemical conditions of the electrorefining process essential for the purpose of establishing thermodynamic and kinetic database of U, otherwise critical deviations are inevitable. More interestingly, the effect of coadsorption of Cl with U on the Mo(110) surface plays a key role in stabilizing electrodeposited U on the cathode. Our approach can be useful for validating published experimental database and for identifying key factors guiding a rational design of highly efficient electrorefining system for spent nuclear fuels, and thus reducing high-level radioactive nuclear wastes.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>