Effect of boron milling on phase formation and critical current density of MgB₂ bulk superconductors

Kang, M.O.,Joo, J.,Jun, B.H.,Park, S.D.,Kim, C.S.,Kim, C.J. The Korea Institute of Applied Superconductivity a 2019 한국초전도저온공학회논문지 Vol.21 No.1

This study was carried out to investigate the effect of milling of boron (B), which is one of raw materials of $MgB_2$, on the critical current density ($J_c$) of $MgB_2$. B powder used in this study is semi-amorphous B (Pavezyum, Turkey, 97% purity, 1 micron). The size of B powder was reduced by planetary milling using $ZrO_2$ balls (a diameter of 2 mm). The B powder and balls with a ratio of 1:20 were charged in a ceramic jar and then the jar was filled with toluene. The milling time was varied from 0 to 8 h. The milled B powders were mixed with Mg powder in the composition of (Mg+2B), and the powder mixtures were uniaxially pressed at 3 tons. The powder compacts were heat-treated at $700^{\circ}C$ for 1 h in flowing argon gas. Powder X-ray diffraction and FWHM (Full width at half maximum) were used to analyze the phase formation and crystallinity of $MgB_2$. The superconducting transition temperature ($T_c$) and $J_c$ of $MgB_2$ were measured using a magnetic property measurement system (MPMS). It was found that $B_2O_3$ was formed by B milling and the subsequent drying process, and the volume fraction of $B_2O_3$ increased as milling time increased. The $T_c$ of $MgB_2$ decreased with increasing milling time, which was explained in terms of the decreased volume fraction of $MgB_2$, the line broadening of $MgB_2$ peaks and the formation of $B_2O_3$. The $J_c$ at 5 K increased with increasing milling time. The $J_c$ increase is more remarkable at the magnetic field higher than 3 T. The $J_c$ at 5 K and 4 T was the highest as $4.37{\times}10^4A/cm^2$ when milling time was 2 h. The $J_c$ at 20 K also increased with increasing milling time. However, The $J_c$ of the samples with the prolonged milling for 6 and 8 h were lower than that of the non-milled sample.

Effects of a compaction method for powder compacts on the critical current density of MgB₂ bulk superconductors

Kang, M.O.,Joo, J.,Jun, B.H.,Choo, K.N.,Kim, C.J. 한국초전도저온공학회 2019 한국초전도저온공학회논문지 Vol.21 No.2

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).

Superconducting properties of MgB₂ superconductors in-situ processed using various boron powder mixtures

Kang, M.O.,Joo, J.,Jun, B.H.,Kim, C.J. The Korean Society of Superconductivity and Cryoge 2021 한국초전도저온공학회논문지 Vol.23 No.3

In this study, the effect of the size of B powder on the critical current density (J_c) of MgB₂ prepared by an in situ reaction process was investigated. Various combinations of B powders were made using a micron B, ball-milled B and nano B powders. Micron B powder was reduced by ball milling and the milled B powder was mixed with the micron B or nano B powder. The mixing ratios of the milled B and micron or nano B were 100:0, 50:50 and 0:100. Non-milled micron B powder was also mixed with nano powder in the same ratios. Pellets of (2B+Mg) prepared with various B mixing ratios were heat-treated to form MgB₂. T_c of MgB₂ decreased slightly when the milled B was used, whereas the J_c of MgB₂ increased with increasing amount of the milled B or the nano powder. The used of the milled B and nano B power promoted the formation MgB₂ during heat treatment. In addition to the enhanced formation of MgB₂, the use of the powders reduced the grain size of MgB₂. The use of the milled and nano B powder increased the J_c of MgB₂. The highest J_c was achieved when 100% nano B powder was used. The J_c enhancement is attributed to the high volume fraction of the superconducting phase (MgB₂) and the large grain boundaries, which induces the flux pinning at the magnetic fields.

싱가포르의 주택 집단매각이 공공-민간주택 시장에 미치는 영향

구강모 ( Koo¸ Kang Mo ),김지나 ( Kim¸ Ji Na ),하서진 ( Ha¸ Seo Jin ) 한국부동산분석학회 2021 不動産學硏究 Vol.27 No.3

본 연구는 HDB로 알려진 싱가포르의 공공주택에 대한 정책의 변화에 대하여 살펴보고, 민간주택시장과 공공주택시장의 유사한 가격 트렌드가 최근의 시장에서도 유효한지 분석하는 것을 목표로 하였다. 싱가포르 주택 시장에 관한 기존 논문들은 공공주택시장과 민간주택시장이 서로 유효한 상관관계를 갖고 있으며, 가격의 변화가 유사한 변화를 보인다고 주장해왔다. 하지만 최근 싱가포르 주택 시장은 이전에 보이지 않던 시장 변화가 감지되었으며, 이는 민간주택의 수요에 영향을 주어 가격 변동요인이 발생하였다. 공공주택 소유자들이 자본소득을 추구할 수 있도록 도와주어 주거 사다리의 낮은 계층에서 상위 계층으로 이동할 수 있도록 하는 정부의 노력이 중요하지만, 외부의 요인으로 인하여 두 시장 사이의 가격 차이가 더 크게 벌어지게 되는 경우, 견고한 주거 사다리를 제공하는 것은 어려워진다. 본 논문은 대규모의 민간주택 집단매각이 발생할 경우, 민간 주택 시장의 수급의 균형이 깨지게 되어 수요의 증가를 일으키고, 이로 인하여 공공·민간 주택 시장의 가격 차이가 심화될 수 있다는 점을 발견하였다. This paper aims to introduce the Singapore’s residential policy related with public house, which is known as HDB flats, and investigate if the similar trend in price between public and private residential market still holds in recent years with unprecedented market conditions that have affected the demand for houses in the context of Singapore. While it is important to setup solid housing ladder that enables residents to move from lower tiers to higher level of the ladder through realizing capital gains from owning public houses, it becomes harder to secure the housing ladder when the price gap between public and private market widens for any reasons. This paper finds that both public and private house market show a widened price gap after experiencing sudden change in demand for private houses due to unprecedentedly high level of collective sales that deters the balance of supply and demand in the private housing market.

Metallic Phases on a Pristine Si(111) Surface

Kangmo Yeo,Sukmin Jeong 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.76 No.8

We present the surface structures of pristine silicon with novel structural and electronic properties based on first-principles calculations. The surfaces are composed of connected armchair ribbons, resulting in quadrilateral, hexagonal, and elongated octagonal structures, in which the constituent Si atoms are threefold (dangling bonds) or fourfold coordinated. Some of the subsurface Si atoms below the surface layer are fivefold-coordinated (floating bonds). Despite having higher formation energies than the conventional Si(111) surfaces (e.g., 0.5 eV/(1 × 1) higher than the Pandey chain model), the structures have unique structural and electronic properties. One of the interesting structural properties is that the surface is nearly flat (average height difference of ~0.02 Å) unlike the typical Si surfaces with large atomic bucklings. The calculated electronic structures show that the surfaces are metallic, as in topological insulators, in which the insulating bulk is surrounded by metallic surfaces.

신경망 학습 알고리즘을 이용한 공정 최적화 방법에 관한 연구

정강모(Kangmo Jung),이강석(Kangsuk Lee) 한국정보과학회 1990 한국정보과학회 학술발표논문집 Vol.17 No.1

본 논문에서는 신경망 학습 알고리즘을 공정 최적화 시스템에 적용시킬 수 있음을 보여준다. 공정시스템의 데이타를 훈련시킴으로써 공정시스템의 상태를 나타내는 방정식을 구하고 그 결과에 영향을 많이 미치는 변수를 찾아낸다. 또한 공정시스템의 상태를 최적으로 하기위한 변수의 조정값을 구한다. 본 논문에서 제시된 신경망 학습 알고리즘을 이용한 공정 방법은 관찰의 대상이 되는 데이타가 방대하고, on_line으로 생성될 때 특히 유용한 잇점을 가지고 있다.

The Effect of Prebiotics on Atopic Dermatitis

Ahn Kangmo 대한천식알레르기학회 2023 Allergy, Asthma & Immunology Research Vol.15 No.3

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아토피피부염에서의 세균감염의 치료

안강모(Kangmo Ahn) 한국환경독성학회 2010 한국독성학회 심포지움 및 학술발표회 Vol.2010 No.11

The Detrimental Effects of Phthalates on Allergic Diseases

Ahn Kangmo 대한천식알레르기학회 2022 Allergy, Asthma & Immunology Research Vol.14 No.3

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한국 소아 식품알레르기의 연구 현황과 과제

안강모 ( Kangmo Ahn ) 대한천식알레르기학회(구 대한알레르기학회) 2018 Allergy Asthma & Respiratory Disease Vol.6 No.S

Food allergy is an immune-mediated adverse reaction that occurs mainly by consumption of food. Some of the children with food allergies have mild symptoms like urticaria and pruritus, while others manifest fatal symptoms like anaphylaxis. Many cases develop in infants and children, and disappear spontaneously over time. The prevalence of food allergy is known to be up to 6% of children and 3%-4% of adults. Growing concern is that prevalence of food allergy seems to be increasing. The prevalence, common allergens and natural course of food allergy vary according to the race, age, eating habits, and unknown factors. In Korea, many researchers have reported regional data on food allergy for comparison with those of foreign countries. Awareness is increasing and food labelling is being established to help people avoid allergenic foods. In the future, monitoring of prevalence and common food allergens at regular intervals is needed to make appropriate policies for children with food allergy. There are still lack of data on pathogenesis, natural course and clinical trials of immunotherapy in Korean children. In addition, it is necessary to form a consensus on primary prevention of food allergy by early exposure to allergenic foods. Further regional studies should be performed to expand our understanding of food allergy and to lead to proper management for Korean children with food allergy. (Allergy Asthma Respir Dis 2018;6 Suppl 1:S44-51)