• 한국세라믹학회지
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• 제31권7호
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• pp.745-752
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• 1994

A polymer reinforced grouts using ordinary portland cement mortar and water soluble polymer{polyvinyl alcohol(PVA), styrene-butadiene rubbre(SBR), etylene-vinyl acetate copolymer(EVA)} were made. The mechanical properties of the hardened specimens were investigated through the observation of the microstructure and application of fracture mechanics. When the PVA, SBR and EVA was added with 1.5 wt% to the grouts, the compressive strength were about 54 MPa, 63 MPa and 68 MPa respectively, and the flexural strength was about 11 MPa, 12.8 MPa, and 13.6 MPa respectively, and Young's modulus was about 3.8 GPa, 4.4 GPa and 4.6 GPa respectively, and critical stress intensity was about 0.73 MNm-1.5, 0.85 MNm-1.5 and 0.9 MNm-1.5 respectively. It can be considered that the strength improvement of polymer mortar grouts may be due to the removal of macropores and the increase of various fracture toughness effects, such as grain bridging, frictional interlocking and polymer bridging.

• Animal Bioscience
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• 제35권4호
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• pp.587-595
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• 2022

Objective: The present study was conducted to evaluate the effects of dietary supplementation with Bifidobacterium animalis, Agave fructans, and symbiotic of both encapsulated on growth performance, feed efficiency, blood parameters, and immune status in broiler chickens, and to compare these with diets including antibiotic growth promoters and without additives. Methods: A comparative experimental study was carried out with 135 male Ross 308 broiler chickens. Each trial was divided into 5 equal groups. Control group (CON) received a standard diet without growth promoter; GPA, a standard diet with colistin sulfate and zinc bacitracin (0.25 g/kg of feed); PRE, a standard diet with 1% Agave fructans; PRO, a standard diet with Bifidobacterium animalis (11.14±0.70 log CFU/g); SYM, a standard diet with B. animalis and Agave fructans. Results: A significant decrease in food consumption was found for the GPA, PRE, and SYM, compared to the CON group. The results show a better feed conversion index in PRE and GPA with respect to the CON group with the highest conversion index. Interestingly, the weight of the gastrointestinal tract shows a statistically significant difference between GPA and PRE groups. Moreover, the length of the gastrointestinal tract of the GPA group was less than the PRE group. In the total leukocyte count, there was a statistically significant increase in the GPA group compared to the CON, PRE, and PRO groups, and the heterophiles-lymphocytes index was lower in PRO. Regarding the cytokines, interleukin 10 (IL-10) decreased in PRO compared to CON and PRE, while IL-1β increased in the SYM group. Conclusion: Alternative treatments were shown to achieve similar productive results as growth-promoting antibiotics and showed improvement over diet without additives; however, they have immunomodulatory properties and improved the development of the gastrointestinal tract compared to the treatment of growth-promoting antibiotics.

• 대한두경부종양학회지
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• 제39권1호
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• pp.27-31
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• 2023

Granulomatosis with polyangiitis (GPA), formerly known as Wegener's granulomatosis, is a rare immunological small vessel disease which usually affects respiratory tract and kidneys. However, salivary gland involvement in GPA is rare as a primary manifestation. We report a case of GPA with the primary presentation of submandibular gland involvement. A 48-year-old female patient presented submandibular swelling with a skin defect that lasted for 1 month. Although the biopsy result was chronic inflammation, the skin defect did not heal for a month. Further imaging study revealed multiple lung and renal masses. More clinical manifestations such as gingivitis, ischemic change of finger joint and nasal tip skin, and positive c-ANCA test was presented. Additional biopsy was made at the submandibular gland, lung, and finger skin. The patient was finally diagnosed with GPA and treated with steroid pulse therapy and cyclophosphamide. The patient showed improvement of prior clinical symptoms.

• 한국세라믹학회지
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• 제31권5호
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• pp.491-498
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• 1994

A fiber reinforced grouts were made using ordinary cement mortar and high effective water reducing agent (naphthalene sulfonate) were made by addition polypropylene fiber and carbon fiber. The physical properties of the grouts were investigated through the observation of the microstructure and the application of fracture mechanics. When the polypropylene fiber and carbon fiber were added respectively with 0.03 wt% to the grouts the compressive strength, flexural strength and Young's modulus were about 60∼63 MPa, 12.2∼12.4 MPa, 4.2∼4.8 GPa and 63∼68 MPa, 12.2∼12.6 MPa, 4.8∼5.1 GPa, and critical stress intensity were about 0.77∼0.82 MNm-1.5, and 0.80∼0.87 MNm-1.5 respectively, It can be considered that the strength improvement of fiber reinforced grouts (FRG) may be due to the removal of macropores and the increase of various fracture toughness, polymer fibril bridging and fiber bridging.

• 한국세라믹학회지
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• 제31권3호
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• pp.330-336
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• 1994

High strength cement composites (W/C=0.1) were prepared by using various blending materials such as SiC whisker and white carbon (hydrated silica: SiO2·nH2O). The effect of various blending materials on the microstructure and strength of the hardened cement paste were investigated in the view of fracture mechanics. The plain specimen showed 101 MPa of flexural strength, 81 GPa of Young's modulus and 1.32 MPam1/2 of fracture toughness. When the blending materials were added to the composites, their values were enhanced to about 110∼138 MPa, 95∼146 GPa and 1.32∼1.87MPam1/2 respectively. The improvement of the mechanical strength for the hardened cement paste may be due to the removal of macropores, the reduction of total porosity, pozzolanic reaction and the increase of various fracture toughening effect.

• 의학교육논단
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• 제25권3호
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• pp.243-257
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• 2023

This study aimed to analyze trends in research on depression among medical students in Korea and to conduct a meta-analysis to determine the average correlation coefficients between depression and related variables. In total, 38 quantitative studies (four theses and 34 journal articles) published between January 1995 and February 2023 were analyzed according to publication year, subjects, analysis methods, and measurement tools. Among them, 15 studies that provided numerical information on the relationships between depression and variables such as self-esteem, social support, grade point average (GPA), stress, and academic stress were selected for meta-analysis. The main findings of this study were as follows. First, quantitative research on depression among medical students began in earnest in 2009, and cross-sectional studies targeting first-year and second-year medical students were the most prevalent. Furthermore, the most commonly used analysis method was difference testing, and the Beck Depression Inventory was the most frequently used measurement tool. Second, the mean correlation coefficients between depression and stress, self-esteem, social support, academic stress, and GPA were 0.534, 0.532, 0.465, 0.390, and 0.102, respectively. The results for self-esteem, stress, and academic stress showed substantial heterogeneity, while those for social support and GPA showed little heterogeneity. These findings suggest that educational interventions, such as social support improvement programs, are necessary to prevent depression among medical students.

• Carbon letters
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• 제13권4호
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• pp.230-235
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• 2012

In this study, reduced graphene oxide/polyimide (r-GO/PI) composite films, which showed significant enhancement in their electrical conductivity, were successfully fabricated. GO was prepared from graphite using a modified Hummers method. The GO was used as a nanofiller material for the preparation of r-GO/PI composites by in-situ polymerization. An addition of 20 wt% of GO led to a significant decrease in the volume resistivity of composite films by less than nine orders of magnitude compared to that of pure PI films due to the electrical percolation networks of reduced GO created during imidization within the films. A tensile test indicated that the Young's modulus of the r-GO/PI composite film containing 20 wt% GO increased drastically from 2.3 GPa to 4.4 GPa, which was an improvement of approximately 84% compared to that of pure PI film. In addition, the corresponding tensile strength was found to have decreased only by 12%, from 113 MPa to 99 MPa.

• Composites Research
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• 제28권4호
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• pp.197-203
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• 2015

폴리프로필렌 기반 자기보강 복합재는 기지재와 보강재가 동일한 소재로 구성되어 기존 섬유강화 복합재와 달리 섬유와 기지재의 분리없이 바로 재활용이 가능할 뿐 아니라 폴리프로필렌 수준까지 저 비중화가 가능한 고분자 복합재이다. 본 연구에서는 더블벨트 라미네이터 공정기술을 이용하여 압력, 온도, 냉각조건과 같은 공정 변수가 폴리프로필렌으로 제조된 자기보강 복합재의 인장 물성에 미치는 영향을 연구하였다. XRD, DSC를 이용하여 결정구조의 변화에 따라 인장 탄성계수는 0.2~1.2 GPa로 약 6배까지 향상되는 것을 확인하였다. SEM 분석을 통해 함침 정도를 확인함으로써 자기보강 복합재의 함침 후 형상 변화 또한 고찰하였다.

• 한국세라믹학회지
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• 제46권5호
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• pp.505-509
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• 2009

In this study, bulk nano-crystalline $Si_3N_4$ ceramics were fabricated by spark plasma sintering (SPS) and their mechanical properties, in particular wear, were investigated. A wide range of grain sizes, from 80 nm and 250 nm were obtained by varying sintering conditions ($1550^{\circ}C$-5 min to $1650^{\circ}C$-20 min). The elastic modulus of obtained ceramics was ${\sim}250$ GPa and hardness was in the range of $13{\sim}14$ GPa. The indentation fracture toughness increased from $2.58MPa{\cdot}m^{1/2}$ to $3.24MPa{\cdot}m^{1/2}$ with increasing sintering temperature possibly due to the elongated grains. Sliding wear tests revealed at least an order magnitude improvement in wear resistance with grain refinement. Microstructure analysis indicated that nano-$Si_3N_4$ specimens worn mainly through delamination and microcracking, while that of coarser specimens revealed severe wear with grain debonding and fracture.

• 한국표면공학회지
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• 제54권3호
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• pp.133-138
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• 2021

Ternary Ti-X-N coatings, where X = Al, Si, Cr, O, etc., have been widely used for machining tools and cutting tools such as inserts, end-mills, and etc. Ti-Al-N-O coatings were deposited onto silicon wafer and WC-Co substrates by a cathodic arc evaporation (CAE) technique at various negative substrate bias voltages. In this study, the influence of substrate bias voltages during deposition on the microstructure and mechanical properties of Ti-Al-N-O coatings were systematically investigated to optimize the CAE deposition condition. Based on results from various analyses, the Ti-Al-N-O coatings prepared at substrate bias voltage of -80 V in the process exhibited excellent mechanical properties with a higher compressive residual stress. The Ti-Al-N-O (-80 V) coating exhibited the highest hardness around 30 GPa and elastic modulus around 303 GPa. The improvement of mechanical properties with optimized bias voltage of -80 V can be explained with the diminution of macroparticles, film densification and residual stress induced by ion bombardment effect. However, the increasing bias voltage above -80 V caused reduction in film deposition rate in the Ti-Al-N-O coatings due to re-sputtering and ion bombardment phenomenon.