• Journal of Sensor Science and Technology
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• v.1 no.1
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• pp.67-75
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• 1992

PTC thermistor has been fabricated with as-received $BaTiO_{3}$ powder and its electrical properties were investigated. The resistivity of the PTC thermistor was measured at $20^{\circ}C$ intervals from $20^{\circ}C$ to $200^{\circ}C$. The electrical characteristics of the PTC thermistor are determined by the ac complex impedance analysis. The average grain size measured with a scanning electron microscope increased from $3.8{\mu}m$ to $8.8{\mu}m$ with increasing sintering temperature between $1280^{\circ}C$ and $1400^{\circ}C$. The maximum resistivity jump was $4{\times}10^{5}$. The bulk resistivity of the thermistor sintered above $1340^{\circ}C$ decreased with increasing temperature of the measurement. The grain boundary resistance increased exponentially, the grain boundary capacitance decreased, and the built-in potential at the grain boundary increased with increasing temperature of the measurement. The charge densiy at the grain boundary increased with increasing temperature up to $110^{\circ}C$, which leveled off with further increase in measuring temperature.

• Journal of the Korean Ceramic Society
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• v.53 no.2
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• pp.150-161
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• 2016

The conventional brick-layer model is not satisfactory either in theory or in practice for the description of dispersive responses of polycrystalline solid electrolytes with current-constriction effects at the grain boundaries. Parallel networks of complex dielectric functions have been shown to successfully describe the AC responses of polycrystalline sodium conductors over a wide temperature and frequency range using only around ten model parameters of well-defined physical significance. The approach can be generally applied to many solid electrolyte systems. The present work illustrates the approach by simulation. Problems of bricklayer model analysis are demonstrated by fitting analysis of the simulated data under experimental conditions.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.4
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• pp.343-348
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• 2000

NCZF ferrites doped with B-Bi-Zn(35-25-40) glass ceramics were prepared to investigate the magnetic properties. The XRD peaks of all of samples were observed only spinel phase. As the additive increased at sintering temperature 750$^{\circ}C$ and 850$^{\circ}C$ for 3 hours, the density and shrinkage of the samples increased until 5.28 g/cm$^3$ and 20% respectively. And other samples showed constant trends having the density about 5.4g/cm$^3$. According to SEM images the growth of grain progressed rapidly at sintering temperature of 850$^{\circ}C$ for 3 hours. Increasing the additive initial permeability and complex permeability decreased. The high values of complex permeability as a function of frequency showed that NCZF doped with 3 wt%, 5wt%, samples at sintered temperature of 850$^{\circ}C$/3h and 900$^{\circ}C$/3h.

• Proceedings of the Korean Geotechical Society Conference
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• 2002.10a
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• pp.271-278
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• 2002

Soft ground has complex features in mechanic character of ground. Some problems about the settlement and transformation occur if the ground strength is comparatively weak and the depth is large. Therefore, we should consider physical and mechanical characters for safe, economical design and management. As the result of the course, we can compare them with those of field then solve the limitations which were came from the complex character of the soft ground. I have considered the soil's physical character (specific gravity of soil particles, moisture content, grain-size analysis etc) and mechanical character (direct shear test, consolidation, triaxial shear test etc), and then make out a linear interpolation by regression using the two, those and connection of the depth

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1999.11a
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• pp.89-92
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• 1999

NCZF ferrites doped with B-Bi-Zn(35-25-40) glass ceramics were prepared to investigate the magnetic properties. The XRD peaks of all of samples were observed only spinel phase. As the additive increased at sintering temperature 750$^{\circ}C$ and 850$^{\circ}C$ for 3 hours, the density and shrinkage increased until 5.28g/㎤ and 20%, respectively. According to SEM images, the growth of grain progressed rapidly at sintering temperature 850$^{\circ}C$ for 3 hours. Increasing the additive, initial permeability and complex permeability decreased. The complex permeabilities as a function of frequency showed high values at a12, a16, b11 and b16 sarmples.

• Korean Journal of Materials Research
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• v.26 no.3
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• pp.154-159
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• 2016

A roll-bonding process was applied to fabricate an AA1050/AZ91/AA1050 laminate complex sheet. Two AA1050 and one AZ91 magnesium sheets of 2 mm thickness, 30 mm width and 200 mm length were stacked up after surface treatment that included degreasing and wire brushing; material was then reduced to a thickness of 3 mm by one-pass cold rolling. The laminate sheet bonded by the rolling was further reduced to 2 mm in thickness by conventional rolling. The rolling was performed at 623K without lubricant using a 2-high mill with a roll diameter of 210 mm. The rolling speed was 15.9 m/min. The AA1050/AZ91/AA1050 laminate complex sheet fabricated by roll bonding was then annealed at 373~573K for 0.5h. The microstructure of the complex sheets was revealed by electron back scatter diffraction (EBSD) measurement; the mechanical properties were investigated by tensile testing and hardness testing. The strength of the complex sheet was found to increase by 11 % and the tensile elongation decreased by 7%, compared to those values of the starting material. In addition, the hardness of the AZ91 Mg region was slightly higher than those of the AA1050 regions. Both AA1050 and AZ91 showed a typical deformation structure in which the grains were elongated in the rolling direction; however, the mis-orientation distribution of grain boundaries varied greatly between the two materials.

• Journal of the Korean Ceramic Society
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• v.32 no.3
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• pp.313-320
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• 1995

We investigated the preferred orientation, electrical and magnetic properties of the Mn-Zn ferrite thin films deposited on SiO2/Si(100) by ion beam sputtering. The Cu-added Mn-Zn ferrite thin films had a preferred orientation of (111) with a weak orientation, (311). While the Zn-added one had a strong (111) preferred orientation. The saturation magnetization of the Cu- or Zn-doped Mn-Zn ferrite films increased with increasing substrate temperature (Ts) due to the increase of grain size and the enhancement of crystallinity. For the same reason the coercivity of Cu- or Zn-doped Mn-Zn ferrite films deposited at low Ts increased with increasing Ts, but those of the films deposited at high Ts slightly decreased not only because the defect density of the films decreases but because more grains have multi-domains with increasing Ts. The resistivity of Cu- or Zn-added Mn-Zn ferrite thin fims measured by complex impedance method decreased with increasing Ts due to the ehhancement of crystallinity as well as due to the increase of grain size.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.6
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• pp.538-548
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• 2003

We studied effects by Re$_2$O$_3$(R=Dy, Gd, Ho) addition on the properties of Mn-Zn ferrite. The doping concentration range from 0.05 wt% to 0.25 wt%. All samples were prepared by standard fabrication of ceramics. With increasing the rare earth oxides, specific density and initial permeability increased on the whole. But, the tendencies such as upper result had the measured value on limitation and characteristics saturated or decreased properties after that. In case of excessive addition of additive beyond some level, initial permeability properties of ferrite have gone down in spite of anomalous grain. With increasing the content of additive, both the real and imaginary component of complex permeability and the magnetic loss (tan$\delta$) increased. Because the increased rate of real component had higher than imaginary component, magnetic loss increased none the less for increasing the real component related with magnetic permeability. But, the magnetic loss of ferrite doped with the rare earth oxides was lower than that of Mn-Zn ferrite at any rate. The small amount of present rare earth oxides in Mn-Zn ferrite composition led to enhancement of resistivity in bulk, and more so in the grain boundary. It was seem to be due to the formation of mutual reaction such as between iron ions and rare earth element ions.

• Journal of Sensor Science and Technology
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• v.9 no.1
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• pp.76-82
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• 2000

PTC thermistors with multilayer structure were fabricated by internal electrode bonding technique in order to realize low resistance. MLPTC (Multilayer Positive Temperature Coefficient) possess various features, such as small size, low resistivity and large current. We describe the effect of additives on the PTC characteristics, voltage - current characteristics, temperature dependence of resistance and complex impedance spectra as a function of frequency range 100 Hz to 13MHz to determine grain boundary resistance. It was found that MLPTC thermistor has both highly nonlinear effects of temperature dependent resistance and voltage dependent current behaviors, which act as passive element with self-repair mechanisms. Decrease of room temperature resistance with increasing the number of layers was demonstrated to be a grain boundary effect. Switching characteristics of current were caused by heat capacity of PTC thermistor with multilayer structure. Switching times are lengthened by increasing the number of layers.

• Applied Microscopy
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• v.45 no.3
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• pp.170-176
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• 2015

In this work, various electron microscopy and analysis techniques were used to investigate the microstructural evolution of a 9% Cr tempered martensite ferritic (TMF) steel T91 upon ultrasonic nanocrystalline surface modification (UNSM) treatment. The micro-dimpled surface was analyzed by scanning electron microscopy. The characteristics of plastic deformation and gradient microstructure of the UNSM treated specimens were clearly revealed by crystal orientation mapping of electron backscatter diffraction (EBSD), with flexible use of the inverse pole figure, image quality, and grain boundary misorientation images. Transmission electron microscope (TEM) observation of the specimens at different depths showed the formation of dislocations, dense dislocation walls, subgrains, and grains in the lower, middle, upper, and top layers of the treated specimens. Refinement of the $M_{23}C_6$ precipitates was also observed, the size and the number density of which were found to decrease as depth from the top surface decreased. The complex microstructure and microstructural evolution of the TMF steel samples upon the UNSM treatment were well-characterized by combined use of EBSD and TEM techniques.