• Current Optics and Photonics
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• v.6 no.6
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• pp.583-589
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• 2022

Vanadium dioxide (VO₂) is a well-known material that undergoes insulator-to-metal phase transition near room temperature. Since the conductivity of VO₂ changes several orders of magnitude in the terahertz (THz) spectral range during the phase transition, VO₂-based active metamaterials have been extensively studied. Experimentally, it is reported that the metal nanostructures on the VO₂ thin film lowers the critical temperature significantly compared to the bare film. Here, we theoretically studied such early transition phenomena by developing an analytical model. Unlike experimental work that only measures transmission, we calculate the reflection and absorption and demonstrate that the role of absorption is quite different for bare and patterned samples; the absorption gradually increases for bare film during the phase transition, while an absorption peak is observed at the critical temperature for the metamaterials. In addition, we also discuss the gap width and VO₂ thickness effects on the transition temperatures.

• Steel and Composite Structures
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• v.25 no.6
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• pp.639-648
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• 2017

Fracture energy values KV have been measured on cast steel, used in the container manufacture, by instrumented Charpy impact testing. This material has a large ductility on the upper transition region at $+20^{\circ}C$ and a ductile tearing with an expended plasticity before a brittle fracture on the lower transition region at $-20^{\circ}C$. To assess the fracture toughness of this material we use, the $K_{IC}$-KV correlations to measure the critical stress intensity factor $K_{IC}$ on the lower transition region and the dynamic force - displacement curves to measure the critical fracture toughness $J{\rho}_C$, the essential work of fracture ${\Gamma}_e$ on the upper transition region. It is found, using the $K_{IC}$-KV correlations, that the critical stress intensity factor $K_{IC}$ remains significant, on the lower transition region, which indicating that our testing material preserves his ductility at low temperature and it is apt to be used as a container's material. It is, also, found that the $J_{\rho}-{\rho}$ energetic criterion, used on the upper transition region, gives a good evaluation of the fracture toughness closest to those found in the literature. Finally, we show, by using the ${\Gamma}_e-K_{IC}$ relation, on the lower transition region, that the essential work of fracture is not suitable for the toughness measurement because the strong scatter of the experimental data. To complete this study by a numerical approach we used the ANSYS code to determine the critical fracture toughness $J_{ANSYS}$ on the upper transition region.

• Journal of the Korea Institute of Military Science and Technology
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• v.13 no.1
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• pp.50-56
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• 2010

Military technology transition is the process of transition from the science and technology environment to systems to supply effective weapon systems and support systems to the fighters. In case of technology transition decision, immatured technologies result in increasing acquisition cost and delaying schedule toward the objective system. In this paper, we proposed a method to identify and select critical technology elements by integration relations between technologies or components, for supporting technology transition and risk management of military R&D projects.

• Progress in Superconductivity
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• v.3 no.1
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• pp.5-12
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• 2001

Our experimental research focuses on manipulating pinning deflects to alter the phase diagram of vortex matter, creating new vortex phases. Vortex matter offers a unique opportunity for creating and studying these novel phase transitions through precise control of thermal, pinning and elastic energies. The vortex melting transition in untwinned YB $a_2$C $u_3$ $O_{7-}$ $\delta$/ crystals is investigated in the presence of disorder induced by particle irradiation. We focus on the low disorder regime, where a glassy state and a lattice state can be realized in the same phase diagram. We follow the evolution of the first order vortex melting transition line into a continuous transition line as disorder is increased by irradiation. The transformation is marked by an upward shift in the lower critical point on the melting line. With columnar deflects induced by heavy ion irradiation, we find a second order Bose glass transition line separating the vortex liquid from a Bose glass below the lower critical point. Furthermore, we find an upper threshold of columnar defect concentration beyond which the lower critical point and the first order melting line disappear together. With point deflect clusters induced by proton irradiation, we find evidence for a continuous thermodynamic transition below the lower critical point..

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.10a
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• pp.19.1-19.1
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• 2011

We report on the metal-insulator transition that occurs as a function of film thickness in ultrathin $La_{0.75}Sr_{0.25}VO_3$ films. The metal-insulator transition displays a critical thickness of 5 unit cell. Above the critical thickness, metallic films exhibit a temperature driven metal-insulator transition with weak localization behavior. With decreasing film thickness, oxygen octahedron rotation in the films increases, causing enhanced electron-electron correlation. The electron-electron correlations in ultrathin films induce the transition from metal to insulator in addition to Anderson localization.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.10 no.5
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• pp.104-109
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• 2011

Bulk metallic glasses (BMG) have been greatly improved by the advance of synthesis process during last three decades. It was also found that the Glass Forming Ability (GFA) strongly depends on the glass transition temperature. When the temperature approaches to a critical value, the crystals nucleation from the supercooled liquid can be suppressed so that bulk glass formation possible. Egami and others found that the local glass transition temperature depends on the volumetric strain of each atom and suggested the critical transition temperature. In this paper, we explore the strain dependency of local glass transition temperature using the atomic strain defined by the deformation tensor for the Voronoi polyhedra.

• Progress in Superconductivity
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• v.14 no.1
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• pp.34-38
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• 2012

The effect of graphene inclusion in the ex-situ $MgB_2$ was analyzed with the help of resistivity behavior and critical current density studies. Amount of graphene was systematically varied from 0% for pristine sample to 3% by the weight of $MgB_2$. Graphene that is considered as a good source of carbon was found to be intact without any significant carbon doping in $MgB_2$ structure as reveled by XRD measurements. There was no signature of graphene inclusion as far as the superconducting transition is concerned which remained same at 39 K for all the samples. The transition width being sensitive to defect doping remained more or less about 2 K for all the samples showing no variation due to doping. Although there was no change in the superconducting transition or transition width, the graphene doped sample showed noticeable decrease in the overall resistivity behavior with respect to decrease in temperature. The graphene inclusion acted as effective pinning centers which have enhanced the upper critical field of these samples.

• Proceedings of the CALSEC Conference
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• 2004.02a
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• pp.89-97
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• 2004

1. Introduction; Fundamental Difference between conventional old commerce and e-commerce？ 2. "Quantity changes into Quality"; Phase transition and Critical phenomena Logarithmic Convergence and Emergence of Quality 3. Networked Small World; Indications from Genomics; Power-Law Ordered Plain Structure of Super-Complex System ⇔ The Structure of e-Biz. 4. Uniquitous Crisis to Ubiquitous Critical Points for the Emergence of Qualified Business with e-strucrure

• Fire Science and Engineering
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• v.28 no.1
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• pp.26-30
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• 2014

A Compensation-type fire detector (CFD) is operated with two functions of a differential-temperature detector and as a fixed-temperature detector. The differential-temperature detector observes a rate of temperature increase, and the fixed-temperature detector measures a given fixed temperature. The differential-temperature detector does not observe the outbreak of fire in slowly increasing temperature conditions, whereas the fixed-temperature detector is not able to observe the outbreak of fire in conditions under predetermined temperature level. We developed a CFD to compensate for weaknesses of both detectors. To compensate for the disadvantages, a sensor of the sensor metal-insulator-transition critical-temperature sensor was used. Temperature coefficient of resistance is the sensitivity for sensor. At $55^{\circ}C$, temperature coefficient of resistance of metal-insulator-transition critical-temperature sensor was 14.15%. Temperature coefficient of resistance of thermistor was about 0.5%. This CFD was operated as two ways that fixed-temperature detector and differential-temperature detector in one sensor.

• Transactions of the Korean Society of Automotive Engineers
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• v.12 no.3
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• pp.109-121
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• 2004

This paper presents an optimal design method for structure-borne durability of a vehicle body structure. Structure-borne durability design requires a new design that can increase fatigue lives of critical areas in a structure and must prohibit transition phenomenon of critical areas that results from modification of the structure at the same time. Therefore, the optimization problem fur structure-borne durability design are consists of an objective function and design constraints of 2 types; type 1-constraint that increases fatigue lives of the critical areas to the required design limits and type 2-constraint that prohibits transition phenomenon of critical areas. The durability design problem is generally dynamic because a designer must consider the dynamic behavior such as fatigue analyses according to the structure modification during the optimal design process. This design scheme, however, requires such high computational cost that the design method cannot be applicable. For the purpose of efficiency of the durability design, we presents a method which carry out the equivalent static design problem instead of the dynamic one. In the proposed method, dynamic design constraints for fatigue life, are replaced to the equivalent static design constraints for stress/strain coefficients. The equivalent static design constraints are computed from static or eigen-value analyses. We carry out an optimal design for structure-borne durability of the newly developed bus and verify the effectiveness of the proposed method by examination of the result.