• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.12
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• pp.1521-1527
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• 2004

Thermocapillary migration of a liquid slug is caused by temperature difference between the ends of a slug. The temperature difference induces the difference of the surface tension coefficient and consequently of capillary pressure between the ends of the slug. Presently available model to predict a velocity of thermocapillary migration adopts the Poiseuille equation which is valid only for a very long slug and neglects the shear stress near the contact line. In the present study, a new model has been developed to consider the shear stress near the contact line so that it can be applied to slugs or drops of general configuration. The experiments using mineral oil with the length to diameter ratio being 10 and a glass capillary were performed. It was found that the liquid slug began to move upon overcoming contact angle hysteresis when the temperature difference reached 35$^{\circ}C$. The results indicate that the new model well predicts the velocity of the liquid slug.

• Journal of Information Display
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• v.11 no.3
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• pp.109-112
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• 2010

The solid phase compositions and dielectric properties of silicon nitride ($SiN_x$) films prepared using the plasma enhanced chemical vapor deposition (PECVD) technique at a low temperature ($200^{\circ}C$) were studied. Controlling the source gas mixing ratio, R = $[N_2]/[SiH_4]$, and the plasma power successfully produced both silicon-rich and nitrogen-rich compositions in the final films. The composition parameter, X, varied from 0.83 to 1.62. Depending on the film composition, the dielectric properties of the $SiN_x$ films also varied substantially. Silicon-rich silicon nitride (SRSN) films were obtained at a low plasma power and a low R. The photoluminescence (PL) spectra of these films revealed the existence of nano-sized silicon particles even in the absence of a post-annealing process. Nitrogen-rich silicon nitride (NRSN) films were obtained at a high plasma power and a high R. These films showed a fairly high dielectric constant ($\kappa$ = 7.1) and a suppressed hysteresis window in their capacitance-voltage (C-V) characteristics.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.15 no.12
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• pp.1060-1064
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• 2002

We have studied the effect of crystallization temperature, composition and film thickness, which are the fundamental processing parameters of lead zirconate titanate(PZT) thin film fabrication, in the respect of the piezoelectric properties by our pneumatic loading method(PLM). A great deal of research has been done in the field of characterization for piezoelectric thin films after the first report on the measurement for the piezoelectric coefficient of thin films in 1990. Even though the piezoelectric properties of thin films are very critical factors in the micro-electro mechanical system(MEMS) and thin film sensor devices, a few reports for the piezoelectric characterization are provided for the last decade unlikely the bulk piezoelectric devices. We have found that the piezoelectric properties of thin films are improved as the increase of crystallization temperature up to 750$\^{C}$ and this behavior can be also explained by the analysis of dielectric polarization hysteresis loop, X-ray diffraction and scanning electron microscopy. The effect of Zr/Ti composition has been also studied. This gives us the fact that the maximum piezoelectricity is found near Morphotropic Phase Boundary(MPB) as bulk PZT system does.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.45 no.2
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• pp.235-241
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• 1996

In this paper dielectric and electrostrictive strain properties of (1-y-x)Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-PbTiO$_{3}$$-yPbTiO_{3}-xSrTiO_{3}[(1-y-x)PMN-yPT-xST]$ ceramics fabricated by using columbite precursor method have been investigated with the substitution of SrTiO$_{3}$(ST). Dielectric constant of the specimens increased with the increase of ST content up to 5[m/o] and decreased with further substitution of ST. And the pyrochlore phase decreased with the increase of ST content up to 5[m/o] in XRD analysis. The elimination of the pyrochlore phase improved dielectric constants. The electrostrictive strains generated by AC electric field have the highest value at 5[m/o] SrTiO$_{3}$ addition and the hysteresis of strain ranged from 12 to 20[%]. The electrostrictive strain at various temperature investigated in the temperature range of $-50[^{\circ}C]~74[^{\circ}C].$ In higher temperature than phase transition region, it showed paraelectric property which shows very small hystersis.

• Journal of Magnetics
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• v.13 no.3
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• pp.110-113
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• 2008

In this study, heavy rare earth garnet $Tb_2Bi_1Ga_1Fe_4O_{12}$ powders were fabricated by a sol-gel and vacuum annealing process. The crystal structure was found to be single-phase garnet with a space group of Ia3d. The lattice constant $a_0$ was determined to be 12.465 ${\AA}$. From the analysis of the vibrating sample magnetometer (VSM) hysteresis loop at room temperature, the saturation magnetization and coercivity of the sample are 7.64 emu/g and 229 Oe, respectively. The N$\acute{e}$el temperature($T_N$) was determined to be 525 K. The M$\ddot{o}$ssbauer spectrum of $Tb_2Bi_1Ga_1Fe_4O_{12}$ at room temperature consists of 2 sets of 6 Lorentzians, which is the pattern of single-phase garnet. From the results of the M$\ddot{o}$ssbauer spectrum at room temperature, the absorption area ratios of Fe ions on 24d and 16a sites are 74.7% and 25.3%(approximately 3:1), respectively. These results show that all of the non-magnetic Ga atoms occupy the 16a site by a vacuum annealing process. Absorption area ratios of Fe ions are dependent not only on a sintering condition but also on the temperature of the sample. It can then be interpreted that the Ga ion distribution is dependent on the temperature of the sample. The M$\ddot{o}$ssbauer measurement was carried out in order to investigate the atomic migration in $Tb_2Bi_1Ga_1Fe_4O_{12}$.

• 한국신재생에너지학회:학술대회논문집
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• 2010.11a
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• pp.73.1-73.1
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• 2010

For the successful cold starting of a fuel cell engine, either internal of external heat supply must be made to overcome the formation of ice from water below the freezing point of water. In the present study, switchable vanadium oxide compounds as variable temperature-electrical resistance materials onto the surface of flat metallic bipolar plates have been prepared by a dip-coating technique via an aqueous sol-gel method. Subsequently, the chemical composition and micro-structure of the polycrystalline solid thin films were analyzed by X-ray diffraction, X-ray fluorescence spectroscopy, and field emission scanning electron microscopy. In addition, it was carefully measured electrical resistance hysteresis loop over a temperature range from $-20^{\circ}C$ to $80^{\circ}C$ using the four-point probe method. The experimental results revealed that the thin films was mainly composed of Karelianite $V_2O_3$ which acts as negative temperature coefficient materials. Also, it was found that thermal dissipation rate of the vanadium oxide thin films partially satisfy about 50% saving of the substantial amount of energy required for ice melting at $-20^{\circ}C$. Moreover, electrical resistances of the vanadium-based materials converge on an extremely small value similar to that of pure flat metallic bipolar plates at higher temperature, i.e. $T{\geq}40^{\circ}C$. As a consequence, experimental studies proved that it is possible to apply the variable temperature-electrical resistance material based on vanadium oxides for the cold starting enhancement of a fuel cell vehicle and minimize parasitic power loss and eliminate any necessity for external equipment for heat supply in freezing conditions.

• Journal of the Korean Magnetics Society
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• v.12 no.2
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• pp.46-50
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• 2002

Soft magnetic Mumetal thin film was fabricated under magnetic field at various substrate temperatures. High vacuum annealing was carried out at 200$\^{C}$ during 1 hr. The in-plane anisotropy of Mumetal thin film was determined from hysteresis loops measured by VSM when the sample axis varied from the field direction from 0°to 180°. As the substrate temperature increases, the coercivity in easy direction decreases, but uniaxial anisotropy deviates from the field direction. After vacuum annealing at 200$\^{C}$ for 1 hr, the uniaxial anisotropy is improved irrespective of substrate temperature. When the substrate temperature was 50$\^{C}$, the anisotropy field is 4.3 Oe. As the substrate temperature increases anisotropy field decreases. Uniaxial anisotropy of Mumetal thin film was formed best at 50$\^{C}$ before and after annealing.

• Journal of the Korean Magnetics Society
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• v.16 no.1
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• pp.14-17
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• 2006

[ $AIFeO_3$ ]has been studied by x-ray diffraction (XRD), vibrating sample magnetometer, Mossbauer spectroscopy. The crystal structure is found to orthorhombic with the lattice parameters being $a_0=4.983\;{\AA},\;b_0=8.554\;{\AA},\;c_0=9.239\;{\AA}$, Magnetic hysteresis curve for $AIFeO_3$ showed weakly ferromagnetic phase at room temperature and a asymmetric shape dependent on the direction of applied field at low temperature. The Curie temperature determined by the temperature dependence of magnetization is 250 K. Mossbauer spectra of $AIFeO_3$ have been taken from 4.2 K to 295 K. Isomer shift at room temperature are found to be $0.11\~0.32\;mm/s$, which is consistent with ferric state. The absorption lines widths become broader with increasing temperature, which is attributed to the Fe ions distribution of each cation site and anisotropy energy difference of each sublattice.

• Korean Journal of Metals and Materials
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• v.49 no.2
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• pp.112-120
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• 2011

The out-of-phase thermo-mechanical fatigue (OP TMF) in a <001> oriented single crystal nickel-based superalloy CMSX-4 has been studied. OP TMF life was less than a half of low cycle fatigue(LCF) life in spite of a small hysteresis loop area of OP TMF compared to that of LCF. The failure was caused by the initiation of a crack at the oxide-layered surface followed by its planar growth along the <100> ${\gamma}$ channel in both LCF and OP TMF. However, deformation twins appeared near the major crack of OP TMF. The multiple groups of parallel twin plates on {111} planes provided a preferential path for crack propagation, which caused a significant decrease in OP TMF life. Additionally, the analysis on the surface crack morphology revealed that the tensile strain at the minimum temperature of OP TMF was found to accelerate the crack propagation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.11
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• pp.1178-1185
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• 2004

In the atmosphere of $N_2$ gas, BaZ $r_{0.08}$ $Ti_{0.92}$ $O_3$ polycrystal was grown by floating zone technique using BaZ $r_{0.08}$ $Ti_{0.92}$ $O_3$ ceramics as a feed and SrTi $O_3$(1l0) single cystal as a seed. The dielectric constant and loss at 10 kHz, 100 kHz, and 1 MHz for the as-grown sample were measured as a function of temperature in the temperature range between -10$0^{\circ}C$ and 150 $^{\circ}C$ to find a dielectric peak with frequency dispersion at Curie point. The hysteresis loop showed that the grown sample had very small polarization which was 0-0.01 $\mu$C/$\textrm{cm}^2$ for the applied dc-electric fields from -7 kV/cm to +7 kV/cm. However, the normal hysteresis loop was appeared after oxygen annealing. The electric-field dependence of the dielectric constant for both the as-grown and the post-annealed samples was studied by measuring the dielectric constants as a function of the biased-electric fields and their tunability was figured out from it at room temperature(27 $^{\circ}C$) and cryotemperature( -73$^{\circ}C$). Tunability for the as-grown sample was 51 % and the figure of merit 20.4 at 10kHz with the biased electric-field of 12 kV/cm. The tunability for the grown sample may be increased up to 80 % if the electric field of 25 kV/cm is applied. Tunability for the post-annealed sample was 41 % and the figure of merit 10.3 at 10 kHz with the biased electric-field of 12 kV /cm. Post-annealing improved the crystallinity of the as-grown sample but decreased its tunability.ability.