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Ryu, Byungki,Chung, Jaywan,Choi, Eun-Ae,Kim, Bong-Seo,Park, Su-Dong Elsevier 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.727 No.-
<P><B>Abstract</B></P> <P>The Bi<SUB>2</SUB>Te<SUB>3</SUB>-related binary compounds (Bi<SUB>2</SUB>Te<SUB>3</SUB>, Sb<SUB>2</SUB>Te<SUB>3</SUB>, Bi<SUB>2</SUB>Se<SUB>3</SUB>, and Sb<SUB>2</SUB>Se<SUB>3</SUB>) and ternary alloys [(Bi<SUB>1-x</SUB>Sb<SUB>x</SUB>)<SUB>2</SUB>Te<SUB>3</SUB> (BST, 0 ≤ x ≤ 1) and Bi<SUB>2</SUB>(Te<SUB>1-y</SUB>Se<SUB>y</SUB>)<SUB>3</SUB> (BTSe, 0 ≤ y ≤ 1)] are known as the high performance room temperature thermoelectric materials. Here, for the first time, we systematically study the thermoelectric transport properties of BST and BTSe alloys by calculating their thermoelectric power factor (PF) as a function of alloy composition ratio x and y, carrier concentration <B>n</B>, and the absolute temperature T. The band valley degeneracy and the band gap are critical to determine the thermoelectric transport properties of ternaries. We find that PFs of p-type BST are comparable to those of binaries, while those of n-type BTSe are not, due to the band structure similarity. And the p-type BST performances are superior to the n-type BTSe due to the longer carrier relaxation time, transport anisotropy, and the band valley degeneracy. We also find that the optimal carrier concentrations which maximize the PFs (<B>n</B> <SUB>opt</SUB>) depend on the ternary composition and the transport direction. The bipolar effect is found to be less significant for n-type BTSe due to the large band gap and the large n<SUB>opt</SUB>. For p-type poly-crystalline BST, the <B>n</B> <SUB>opt</SUB> is between 3 and 4 × 10<SUP>19</SUP> cm<SUP>−3</SUP> and it is achievable by Sb alloying and controlling the concentration of intrinsic defect. However, for n-type polycrystalline BTSe, n<SUB>opt</SUB> is ranging between 6 × 10<SUP>19</SUP> cm<SUP>−3</SUP> and 1 × 10<SUP>20</SUP> cm<SUP>−3</SUP> and thereby we need additional extrinsic dopant beyond Se alloying. The defect formation energy calculations reveal that Cl, Br, and I impurities are potential candidates for n-type carrier sources without forming any compensating defect, while F as well as Au is the compensating defect.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Calculation of power factor (PF) for p-type Bi-Sb-Te and n-type Bi-Te-Se alloys. </LI> <LI> PF as the function of composition ratio, carrier density, temperature. </LI> <LI> Valley degeneracy and band gap responsible for high PF in p-type alloys. </LI> <LI> Prediction of optimal carrier concentration to maximize PF. </LI> <LI> Suggestion of n-type dopant (Cl, Br, I) to maximize n-type PF. </LI> </UL> </P>
Byungki Ryu,Sungjin Park,Eun-Ae Choi,Johannes de Boor,Pawel Ziolkowski,Jaywan Chung,Su Dong Park 한국물리학회 2019 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.75 No.2
We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg$_2$Si, Mg$_2$Ge, and Mg$_2$Sn. For all Mg$_2$X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, \textit{i.e.}, band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg$_2$Si, Mg$_2$Ge, and Mg$_2$Sn are 0.191, 0.090, and $-$0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35\% fraction of the exact Hartree-Fock exchange energy (HSE-35\%), we overcame the negative band gap issue in Mg$_2$Sn. Finally, in quasi-particle calculations on top of the HSE-35\% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg$_2$Si, Mg$_2$Ge, and Mg$_2$Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.
First-principles study of the electronic structure of aluminate nanotubes
Ryu, Byungki,Kang, Yong-Ju,Chang, K J Institute of Physics Publishing Ltd 2007 Journal of Physics: Conference Series Vol.61 No.1
<P>We report the results of first-principles theoretical calculations for the electronic structure of aluminate nanotubes. A tubular structure in the form of AlO<SUB>2</SUB> is energetically stable and exhibits metallic conduction. Due to weak interactions between Li atoms and nanotubes, Li doping does not alter the stability of AlO<SUB>2</SUB> nanotubes and only increases the Fermi level. On the other hand, stable AlO nanotubes can be obtained by hole doping with Be and Mg impurities.</P>
Computational Simulations of Thermoelectric Transport Properties
Ryu, Byungki,Oh, Min-Wook The Korean Ceramic Society 2016 한국세라믹학회지 Vol.53 No.3
This review examines computational simulations of thermoelectric properties, such as electrical conductivity, Seebeck coefficient, and thermal conductivity. With increasing computing power and the development of several efficient simulation codes for electronic structure and transport properties calculations, we can evaluate all the thermoelectric properties within the first-principles calculations with the relaxation time approximation. This review presents the basic principles of electrical and thermal transport equations and how they evaluate properties from the first-principles calculations. As a model case, this review presents results on $Bi_2Te_3$ and Si. Even though there is still an unsolved parameter such as the relaxation time, the effectiveness of the computational simulations on the transport properties will provide much help to experimental scientist researching novel thermoelectric materials.
Work function of bismuth telluride: First-principles approach
한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol. No.
<P>First-principles approach is demonstrated to calculate the work function of Bi2Te3. The reference potential and the vacuum energy levels are extracted from the Bi2Te3 (0001) surface structure using the reference potential method based on the density functional theory. The one-shot G(O)W(O) many-body perturbation theory is used to place the bulk band edge energies with respect to the reference level and the vacuum energy. At last, the work function of 5.301 - 5.131 eV is predicted for Bi2Te3 (0001) surface and compared to various elements.</P>
Relationships of Musculoskeletal Disorder Symptoms and Perceived Workload among Hospital Workers
Byungki Jin,Joobong Song,Taebeum Ryu,Myung Hwan Yun 대한인간공학회 2011 대한인간공학회 학술대회논문집 Vol.2011 No.10
Objective: This study attempts to investigate the prevalence of musculoskeletal disorders (MSD) among Korean hospital workers and to analyze the relationship of MSD symptoms and workload perceived by workers. Background: Despite of high exposure to the MSD risk factors and high MSD symptom prevalence among health care workers, there were not enough studies of MSD prevalence among the hospital workers in South Korea. Method: This study designed a survey based on Nordic questionnaire to obtain MSD symptoms and the degree of four perceived workloads: work repetition, urgency, physical exertion and satisfaction. In this survey, more than 1,800 workers in a hospital participated. The prevalence of MSD was analyzed for each body part, and MSD cases, which were predetermined in this study, were identified. The relationship between the MSD cases and each perceived workload was analyzed using chi-square test. Results: The pain in the shoulder was the most prevalent among the workers as 52%, and the low back (37%) and leg discomfort (36%) followed. The MSD cases of this study were also the most prevalent in the shoulder (13%). Female workers had higher rate of MSD cases than the males. Among the four workload variables, the physical exertion was the statistically related to MSD cases for all the body parts. In addition, the others also had significant relation to MSD cases except one or two body parts. Conclusion: This study found that Korean hospital workers had MSD symptoms mainly in the shoulder, low back and legs in order, which was different from those in North America or Europe. Application: This study provides another evidence that subjective physical exertion perceived by workers is an important factor to explain MSD cases as same as the objective one.