• 한국분말재료학회지
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• 제29권1호
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• pp.34-40
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• 2022

Recently, high-entropy carbides have attracted considerable attention owing to their excellent physical and chemical properties such as high hardness, fracture toughness, and conductivity. However, as an emerging class of novel materials, the synthesis methods, performance, and applications of high-entropy carbides have ample scope for further development. In this study, equiatomic (Hf-Ti-Ta-Zr-Nb)C high-entropy carbide powders have been prepared by an ultrahigh-energy ball-milling (UHEBM) process with different milling times (1, 5, 15, 30, and 60 min). Further, their refinement behavior and high-entropy synthesis potential have been investigated. With an increase in the milling time, the particle size rapidly reduces (under sub-micrometer size) and homogeneous mixing of the prepared powder is observed. The distortions in the crystal lattice, which occur as a result of the refinement process and the multicomponent effect, are found to improve the sintering, thereby notably enhancing the formation of a single-phase solid solution (high-entropy). Herein, we present a procedure for the bulk synthesis of highly pure, dense, and uniform FCC single-phase (Fm3m crystal structure) (Hf-Ti-Ta-Zr-Nb)C high-entropy carbide using a milling time of 60 min and a sintering temperature of 1,600℃.

• Geomechanics and Engineering
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• 제34권6호
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• pp.683-696
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• 2023

The destruction and fracture of rock masses are crucial components in engineering and there is an increasing demand for the study of the influence of rock mass heterogeneity on the safety of engineering projects. The numerical manifold method (NMM) has a unified solution format for continuous and discontinuous problems. In most NMM studies, material homogeneity has been assumed and despite this simplification, fracture mechanics remain complex and simulations are inefficient because of the complicated topology updating operations that are needed after crack propagation. These operations become computationally expensive especially in the cases of heterogeneous materials. In this study, a heterogeneous model algorithm based on stochastic theory was developed and introduced into the NMM. A new fracture algorithm was developed to simulate the rupture zone. The algorithm was validated for the examples of the four-point shear beam and semi-circular bend. Results show that the algorithm can efficiently simulate the rupture zone of heterogeneous rock masses. Heterogeneity has a powerful effect on the macroscopic failure characteristics and uniaxial compressive strength of rock masses. The peak strength of homogeneous material (with heterogeneity or standard deviation of 0) is 2.4 times that of heterogeneous material (with heterogeneity of 11.0). Moreover, the local distribution of parameter values can affect the configuration of rupture zones in rock masses. The local distribution also influences the peak value on the stress-strain curve and the residual strength. The post-peak stress-strain curve envelope from 60 random calculations can be used as an estimate of the strength of engineering rock masses.

• Nuclear Engineering and Technology
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• 제55권1호
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• pp.310-323
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• 2023

The k-eigenvalue neutronics/thermal-hydraulics coupling calculation is a key issue for reactor design and analysis. Jacobian-free Newton-Krylov (JFNK) method, featured with super-linear convergence rate and high efficiency, has been attracting more and more attention to solve the multi-physics coupling problem. However, it may converge to the high-order eigenmode because of the multiple solutions nature of the k-eigenvalue form of multi-physics coupling issue. Based on our previous work, a modified JFNK with a line search method is proposed in this work, which can find the fundamental eigenmode together with thermal-hydraulics feedback in a wide range of initial values. In detail, the existing modified JFNK method is combined with the line search strategy, so that the intermediate iterative solution can avoid a sudden divergence and be adjusted into a convergence basin smoothly. Two simplified 2-D homogeneous reactor models, a PWR model, and an HTR model, are utilized to evaluate the performance of the newly proposed JFNK method. The results show that the performance of this proposed JFNK is more robust than the existing JFNK-based methods.

• Restorative Dentistry and Endodontics
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• 제46권1호
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• pp.3.1-3.14
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• 2021

Objectives: This study aimed to evaluate the interface between a calcium silicate cement (CSC), Biodentine and dental adhesives in terms of sealing ability. Materials and Methods: Microleakage test: 160 standardized class II cavities were prepared on 80 extracted human molars. The cavities were filled with Biodentine and then divided into 2 experimental groups according to the time of restoration: composite resin obturation 15 minutes after Biodentine handling (D0); restoration after 7 days (D7). Each group was then divided into 8 subgroups (n = 5) according to the adhesive system used: etch-and-rinse adhesive (Prime & Bond); self-etch adhesive 2 steps (Optibond XTR and Clearfil SE Bond); self-etch adhesive 1 step (Xeno III, G-aenial Bond, and Clearfil Tri-S Bond); and universal used as etch-and-rinse or self-etch (ScotchBond Universal ER or SE). After thermocycling, the teeth were immersed in a silver nitrate solution, stained, longitudinally sectioned, and the Biodentine/adhesive percolation was quantified. Scanning electron microscopic observations: Biodentine/adhesive interfaces were observed. Results: A tendency towards less microleakage was observed when Biodentine was etched (2.47%) and when restorations were done without delay (D0: 4.31%, D7: 6.78%), but this was not significant. The adhesives containing 10-methacryloyloxydecyl dihydrogen phosphate monomer showed the most stable results at both times studied. All Biodentine/adhesive interfaces were homogeneous and regular. Conclusions: The good sealing of the CSC/adhesive interface is not a function of the system adhesive family used or the cement maturation before restoration. Biodentine can be used as a dentine substitute.

• Geomechanics and Engineering
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• 제31권1호
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• pp.99-112
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• 2022

The present study examines the natural frequencies (NFs) of perfect/imperfect functionally graded sandwich beams (P/IP-FGSBs), which are composed of a porous core constructed of functionally graded materials (FGMs) and a homogenous isotropic metal and ceramic face sheets resting on elastic foundations. To accomplish this, the material properties of the FGSBs are assumed to vary continuously along the thickness direction as a function of the volume fraction of constituents expressed by the modified rule of the mixture, which includes porosity volume fraction represented using four distinct types of porosity distribution models. Additionally, to characterize the reaction of the two-parameter elastic foundation to the Perfect/Imperfect (P/IP) FGSBs, the medium is assumed to be linear, homogeneous, and isotropic, and it is described using the Winkler-Pasternak model. Furthermore, the kinematic relationship of the P/IP-FGSBs resting on the Winkler-Pasternak elastic foundations (WPEFs) is described using trigonometric shear deformation theory (TrSDT), and the equations of motion are constructed using Hamilton's principle. A closed-form solution is developed for the free vibration analysis of P/IP-FGSBs resting on the WPEFs under four distinct boundary conditions (BCs). To validate the new formulation, extensive comparisons with existing data are made. A detailed investigation is carried out for the effects of the foundation coefficients, mode numbers (MNs), porosity volume fraction, power-law index, span to depth ratio, porosity distribution patterns (PDPs), skin core skin thickness ratios (SCSTR), and BCs on the values of the NFs of the P/IP-FGSBs.

• 한국결정성장학회지
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• 제11권2호
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• pp.49-55
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• 2001

sol-gel방법에 의해서 p-형 Si(100)웨이퍼와 ITO glass, quartz glass기판 위 ZnO박막을 형성시켰으며 출발 물질은 zinc acetate dihydrate를 사용하였다. Zinc acetate dihydrate를 2-methoxyethanol-monoethanolamine(MEA)용액에 녹여 :균질하고 안정된 용액을 만들었다. ZnO박막은 2800rpm에서 25초 동안 spin-coating하고 $250^{\circ}C$의 hot plate에서 10분 동안 중간 열처리한 후 이를 반복하여 형성시켰고 결정화를 위한 열처리는 $400^{\circ}C$~$800^{\circ}C$공기 분위기에서 1시간 동안 시행하였다. X-ray diffraction(XRD), scanning electron microscopy(SEM), UV-vis 투과 스펙트럼, IR 투과 스펙트럼, photoluminescence(PL)스펙트럼 등의 측정 결과로부터 박막의 구조적 특성과 광학적 성질에 대해서 논의하였다. 제조된 ZnO막은 (002)면으로 정렬되어 있으며 380nm파장에서 예리한 흡수단이 있고 가시광선 영역에서 투명(투과율 70% 이상)하였는데 이 흡수단은 ZnO의 밴드 갭(3.2eV)과 잘 일치한다. 저온에서의 띠끝 PL스펙트럼은 속박된 엑시톤 복합체와 포논 복제에 의한 다중선 구조를 보인다.

• 대한치과보철학회지
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• 제43권3호
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• pp.363-378
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• 2005

Statement of problem. Hydroxyapatite(HA) coated titanium surfaces have not yet showed the reliable osseointegration in various conditions. Purpose. This study was aimed to investigate microstructures, chemical composition, and surface roughness of the surface coated by the hydrothermal method and to evaluate the effect of hydrothermal coating on the cell attachment, as well as cell proliferation. Material and Methods. Commercially pure(c.p.) titanium discs were used as substrates. The HA coating on c.p. titanium discs by hydrothermal method was performed in 0.12M HCl solution mixed with HA(group I) and 0.1M NaOH solution mixed with HA(group II). GroupⅠ was heated at 180 $^{\circ}C$ for 24, 48, and 72 hours. GroupⅡ was heated at 180 $^{\circ}C$ for 12, 24, and 36 hours. And the treated surfaces were evaluated by Scanning electron microscopy(SEM), Energy dispersive X-ray spectroscopy(EDS), X-ray photoelectron spectroscopy(XPS), X-ray diffraction method(XRD), Confocal laser scanning microscopy(CLSM). And SEM of fibroblast and 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide(MTT) assay were used for cellular responses of the treated surfaces. Results. The color of surface changed in both groups after the hydrothermal process. SEM images showed that coating pattern was homogeneous in group II, while inhomogeneous in group I. H72 had rosette-like precipitates. The crystalline structure grew gradually in group II, according to extending treatment period. The long needle-like crystals were prominent in N36. Calcium(Ca) and phosphorus(P) were not detected in H24 and H48 in EDS. In all specimens of group II and H72, Ca was found. Ca and P were identified in all treated groups through the analysis of XPS, but they were amorphous. Surface roughness did not increase in both groups after hydrothermal treatment. The values of surface roughness were not significantly different between groups I and II. According to the SEM images of fibroblasts, cell attachments were oriented and spread well in both treated groups, while they were not in the control group. However, no substantial amount of difference was found between groups I and II. Conclusions. In this study during the hydrothermal process procedure, coating characteristics, including the HA precipitates, crystal growth, and crystalline phases, were more satisfactory in NaOH treated group than in HCl treated group. Still, the biological responses of the modified surface by this method were not fully understood for the two tested groups did not differ significantly. Therefore, more continuous research on the relationship between the surface features and cellular responses seems to be in need.

• 한국세라믹학회지
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• 제41권12호
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• pp.921-928
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• 2004

수용액의 pH, 소성온도, 반응시간 등을 변화한 조건하에서 $Ca(OH)_2$와 $H_{3}PO_4$ 수용액을 이용하여 인산칼슘계 화합물을 제조하였다. 산성분위기(pH=5.21)에서는 ${\beta}-TCP({\beta}-tricalcium phosphate)$가, pH>7.62에서는 HAp(hydroxyapatite)가 주 결정상으로 관찰되었다. XRD 분석결과, $Ca(OH)_2$와 $H_{3}PO_4$는 빠르게 반응(5분이내)하여 HAp를 생성하였고, 반응 16시간까지 안정적으로 구조를 유지하였다. 우수한 결정성을 나타내는 소성온도는 $1200^{\circ}C$로 관찰되었다. 주사전자현미경(SEM)에 의한 미세구조 관찰 결과, pH가 5.21일 경우 침상 형태의 ${\beta}-TCP$ 입자가, pH 7.62에서는 침상 형태의 HAp가 합성되었다. 그러나 pH 9.16에서는 매우 작고 균일한 구형의 HAp 입자들이 생성되었고, 다시 작은 입자들이 침상으로 재배열된 형상을 관찰할 수 있었다. 합성된 HAp(pH 9.16)를 본차이나 소지에 일정량 혼합하여 본차이나 물성, 즉 결정성, 흡수율, 색도, 파단면의 형상을 측정${\cdot}$분석하였다.

• Journal of Microbiology and Biotechnology
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• 제8권2호
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• pp.171-177
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• 1998

The strain A49, which produces a new type of extracellular polysaccharide was isolated from soil samples. From morphological, physiological and biochemical tests, the strain A49 was identified as a Bacillus polymyxa and named Bacillus polymyxa A49. Bacillus polymyxa A49 was found to produce a highly viscous extracellular polysaccharide when grown aerobically in a medium containing glucose as the sole source of carbon. The polysaccharide (A49 POL) showed a homogeneous pattern on gel permeation chromatography (GPC) and its molecular weight was estimated to be about 1.6 mega dalton (mDa). The FT-IR spectrum of A49-POL revealed typical characteristics of polysaccharides. As a result of investigations with HPLC and carbozole assay, A49-POL was found to consist of L-fucose, D-galactose, D-glucose, D-mannose, and D-glucuronic acid, with the molar ratio of these sugars being approximately 1:2:7:50:12. Rheological analysis of A49 POL revealed that it is pseudoplastic and has a higher apparent viscosity at dilute concentrations than does xanthan gum. The consistancy factor of A49 POL was found to be higher, and the flow index of A49 POL lower, than xanthan gum. Its apparent viscosity was comparatively unstable at various temperatures. the A49 POL showed the highest apparent viscosity at pH 3. When salts were added to A49 POL solution, the solution was compatible with up to 10% KCl, 35% NaCl, 55% $CaCl_2$, 55% $MgCl_2$, 55% $K_2HPO_4$, and 110% $Ca({NO_3})_2$, respectively.

• 대한화학회지
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• 제62권1호
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• pp.24-29
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• 2018

대표적인 양이온성 폴리사카라이드인 키토산은 생체 친화적인 특성으로 인하여 식품 및 의약품으로의 다양한 응용성이 제시되고 있으나, 용액의 pH에 따른 급격한 용해도 변화로 인하여 실제적인 사용에 많은 제약이 따른다. 본 연구에서는 양이온성 키토산 고분자체와 음이온성 고분자 전해질인 PAA의 이온성 상호작용을 이용하여 두 고분자 전해질의 구성 비율 및 용액의 pH에 따른 polyplex 형성 과정을 관찰하였다. 특히, 두 고분자체의 조성 비율에 따라 나타나는 입자 표면의 전하량은 입자 간정전기적 반발력을 제공하여 균일한 입자 크기와 높은 콜로이드 안정성을 제공하였으며, 이와 같이 안정화된 polyplex 입자는 추가적인 가교화 반응을 통하여 하이드로젤 입자로의 형성이 가능하였다. 두 고분자 전해질의 부분적인 교차 결합으로 형성된 하이드로젤 입자 내부의 acryl amide 작용기는 특징적인 저임계 용액 온도(LCST)를 나타냄으로써 입자의 온도에 따른 수화 작용에 차이를 보이며 그에 따른 입자의 수력학적 직경 변화가 관찰되어 온도에 따른 하이드로젤 입자의 가역적인 팽윤 작용이 관찰되었다. 이와 같은 하이드로젤 입자는 생리활성적인 환경에서 제한된 용해도를 보이는 키토산의 응용성을 한 층 더 높일 수 있을 것으로 기대된다.