• 한국초전도ㆍ저온공학회논문지
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• 제15권2호
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• pp.16-19
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• 2013

The magneto-resistivity and electric field-current density (E-J) curves were investigated up to a magnetic field 9 T in the optimally doped $BaFe_{1.8}Co_{0.2}As_2$ single crystal with a superconducting temperature ($T_c$) of 24.6 K. The E-J Scaling behaviors below and above vortex glass transition temperature ($T_g$) were found, confirming the existence of the vortex glass phase transition. The critical exponents for the diverging spatial and time correlations at $T_g$, were obtained as v = $1.1{\pm}0.1$ and z = $4.5{\pm}0.3$, respectively. The obtained critical exponents are in good agreement with the predicted values of v ~ 1 - 2 and z > 4 within the 3D vortex glass theory.

• Bulletin of the Korean Chemical Society
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• 제9권6호
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• pp.368-376
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• 1988

Transition metal PSSA ionomers containing Co(II), Ni(II), Cr(III), Ru(III), and Rh(III) are investigated by IR, Far-IR, UV-Vis and DSC. Reliable IR Spectroscopic criteria are established for assessing the degree of ion-exchange of PSSA ionomers and the local structures around metal cations in them. In the hydrated transition metal PSSA ionomers, the ionic groups are solvated by water molecules and there is no significant interactions between sulfonate group and metal cations. The visible spectra indicated that metal cations are present as [M$(H_2O)_6$]$^{n+}$ with Oh symmetry. Their $T_g$ values increase as the extent of ionic site concentration increases, but there is no direct dependence of $T_g$ on the nature of metal cations or their oxidation states. Thus, the water content in PSSA ionomer is found to have dominant influence on $T_g$ of hydrated transition metal PSSA ionomers. Dehydration of the hydrated transition metal PSSA ionomers results in direct interaction between ionic groups and significant color changes of the ionomers due to the changes of the local structures around metal cations. On the base of spectral data, their local structures are discussed. In case of dehydrated 12.8 and 15.8 mol % transition metal PSSA ionomers, no glass transition is observed in 25-$250^{\circ}C$ region and this is believed to arise from the formation of highly crosslinked structures caused by direct coordination of sulfonate groups of metal cations. In the 6.9 mol % transition metal PSSA ionomers, the glass transition is always observed whether they are hydrated or dehydrated and this is though to be caused by the sufficient segmental mobility of the polymer backbone.

• 한국재료학회지
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• 제23권6호
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• pp.309-315
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• 2013

$GdBa_2Cu_3O_{7-y}$(Gd123) powders were synthesized by the solid-state reaction method using $Gd_2O_3$ (99.9% purity), $BaCO_3$ (99.75%) and CuO (99.9%) powders. The synthesized Gd123 powder and the Gd123 powder with $Gd_2O_3$ addition ($Gd_{1.5}Ba_2Cu_3O_{7-y}$(Gd1.5)) were used as raw powders for the fabrication of Gd123 bulk superconductors. The Gd123 and Gd1.5 bulk superconductors were fabricated by sintering or a top-seeded melt growth (TSMG) process. The superconducting transition temperature ($T_{c,onset}$) of the sintered Gd123 was 93 K and the transition width was as large as 20 K. The $T_{c,onset}$ of the TSMG processed Gd123 was 82 K and the transition width was also as large as 12 K. The critical current density ($J_c$) at 77 K and 0 T of the sintered Gd123 and TSMG processed Gd123 were as low as a few hundreds A/$cm^2$. The addition of 0.25 mole $Gd_2O_3$ and 1 wt.% $CeO_2$ to Gd123 enhanced the $T_c$, $J_c$ and magnetic flux density (H) of the TSMG processed Gd123 sample owing to the formation of the superconducting phase with high flux pinning capability. The $T_c$ of the TSMG processed Gd1.5 was 92 K and the transition width was 1 K. The $J_cs$ at 77 K (0 T and 2 T) were $3.2{\times}10^4\;A/cm^2$ and $2.5{\times}10^4\;A/cm^2$, respectively. The H at 77 K of the TSMG-processed Gd1.5 was 1.96 kG, which is 54% of the applied magnetic field (3.45 kG).

• Transactions on Electrical and Electronic Materials
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• 제13권6호
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• pp.297-300
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• 2012

Lead-free (1-x)$(Na_{0.5}K_{0.5})NbO_3-xLiNbO_3$, i.e., NKN-LNx (x=0.0, 0.1, 0.2, 0.3, 0.4 mol) was prepared using the conventional solid state reaction method. The effects of LN mixing on the ferroelectric properties of NKN-LNx ceramics were studied using a dielectric constant and P-E (Polarization-electric field) measurements. Ferroelectricity was observed in the composition for x approximately varying between 0.0 and 0.4. Minimum remanent polarization $2P_r=5C/cm^2$ was achieved in the composition for x = 0.2. The ferroelectric phase transition temperature $T_C$ increased with increasing LN content. The ferroelectric phase transition of NKN-LNx ($x{\geq}0.1$) is a second-order phase transition, and that of NKN-LNx ($x{\leq}0.2$) is a first-order phase transition. These results indicate that the ferroelectric phase transition temperature of NKN-LNx change from that of second-order to weak first-order phase transition according to the LN content.

• Preventive Nutrition and Food Science
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• 제8권3호
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• pp.235-238
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• 2003

We investigated the effect of the phase transition temperature (T$_{c}$) of phospholipid in liposomes on the stability of incorporated retinol. Two kinds of phospholipid which have different T$_{c}$, L- $\alpha$ -dimyristoyl phosphatidyl choline (DMPC, T$_{c}$=22$^{\circ}$C) and D,L- $\alpha$ -dipalmitoyl phosphatidyl choline (DPPC, T$_{c}$=42$^{\circ}$), were used to prepare liposomes. Liposome with retinol was prepared as multilamella vesicles (MLVs) by the dehydration/rehydration method. The incorporation efficiency of retinol into liposomes prepared from DMPC and DPPC were 99.89$\pm$0.08% and 99.97$\pm$0.03, respectively. The average size of liposomes from DPPC were greater than that of DMPC. Two kinds of liposomes in phosphate buffer (10 mM, pH 7.0) were stored at 15, 30, and 5$0^{\circ}C$, and stability of incorporated retinol was analyzed. The stability of retinol in DMPC liposome was decreased, whereas the stability in DPPC liposome was increased as temperature increased, although the overall protection effect of liposome on the incorporated retinol was greater in DMPC liposomes than in DPPC liposomes.posomes.

• 열처리공학회지
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• 제10권1호
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• pp.63-70
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• 1997

A study was performed to examine the aging behaviors of Al-2.1Li-2.9Cu alloy by differential scanning calorimetry and transmission electron microscopy. DSC measurements were conducted over the temperature range of $25{\sim}550^{\circ}C$ at a heating rate of $5^{\circ}C$/min. for the specimens aged at 130, 160, $190^{\circ}C$ and $220^{\circ}C$ for various times after solution treatment at $540^{\circ}C$ for 30 minutes. The peaks due to the formation of G.P.zone were not detected in the specimens aged at 130 and $160^{\circ}C$, but those at 190 and $220^{\circ}C$ appeared in DSC curves. The heat absorption due to the dissolution of ${\delta}^{\prime}$ phase was increased with increasing aging time at $130^{\circ}C$ aging. In contrast, those values for the specimens aged at 160 and $190^{\circ}C$ were initially increased and inversely decreased at the transition time of 72 and 1 hour, respectively. The heat evolution due to the formation of $T_1$ phase was nearly unchanged at $130^{\circ}C$ aging, but at $160^{\circ}C$ and $190^{\circ}C$ aging, drastically decreased after the transition time. It can be considered that the increase of $T_1$ phase results in the decrease of ${\delta}^{\prime}$ phase when aging time is longer than the transition time. The hardness of the specimen aged at $190^{\circ}C$ is initially higher compared with that at $160^{\circ}C$, however, the peak hardness shows the lower value than that at $160^{\circ}C$.

• Macromolecular Research
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• 제10권1호
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• pp.34-39
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• 2002

Cure behavior of an epoxy resin was investigated at different cure temperatures (110, 120, 130, 140, and 150 $^{\circ}C$) and cure times in the presence of 2 wt% of an N-benzylpyrazinium hexafluoroantimonate (BPH) cationic catalyst by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The glass transition temperature ( $T_{g}$) and chemical conversion (x) at the different temperatures were determined from DSC thermograms. The $T_{g}$ and x vs. In time data were superposed up to $T_{g}$ = 10$0^{\circ}C$ and x = 0.70 by shifting horizontally at a reference temperature of $T_{g}$ = 13$0^{\circ}C$. It is interesting that the $T_{g}$ and x of the superposed data increase rather slowly in the early stage of cure and rapidly thereafter. Therefore, the increase of the $T_{g}$ and x can be divided into two regions; $R_{I}$= -18.4(= $T_{go}$ ) ~5$^{\circ}C$ and $R_{II}$ = 5 ~ 10$0^{\circ}C$ in $T_{g}$, and $R_{I}$ : 0~0.24 and $R_{II}$ : 0.24~0.70 in x. The $R_{I}$ is closely related to the initiation reactions between BPH and epoxy and between hydroxy group and epoxy in this epoxy/catalyst system. From the kinetic analysis of the $T_{g}$-shift, activation energy was 12.5 kcal/mol. The relationship between $T_{g}$ and x was also considered. The gelation and vitrification times for different cure temperatures were obtained from DMA curves.urves. DMA curves.urves.

• Current Applied Physics
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• 제18권11호
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• pp.1248-1254
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• 2018

Magnetization versus temperature and magnetic-field measurements, M(T, $H_a$), have been carried out to study the magnetic and critical properties of polycrystalline $La_{1-x}Ba_xCoO_3$ (x = 0.3 and 0.5) cobaltites. These compounds with the density of ${\sim}6.2g/cm^3$ crystallized in the $R{\bar{3}}c$ rhombohedral and $Pm{\bar{3}}m$ cubic structures, respectively. With an applied field $H_a=200Oe$, M(T) data have revealed that the samples with x = 0.3 and 0.5 exhibit the ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature points $T_C=202$ and 157 K, respectively. At 4.2 K, the saturation magnetization ($M_{sat}$) decreases from 35.9 emu/g for x = 0.3-26.1 emu/g for x = 0.5. Particularly, the critical-behavior analyses in the vicinity of $T_C$ reveal all samples undergoing a second-order phase transition, with critical exponent values (${\beta}=0.328$ and ${\gamma}=1.251$ for x = 0.3, and ${\beta}=0.331$ and ${\gamma}=1.246$ for x = 0.5) close to those expected for the 3D Ising model. This proves short-range magnetic order existing in $La_{1-x}Ba_xCoO_3$. We believe that magnetic inhomogeneities due to the mixture of hole-rich FM regions (confined in the trivalent-cobalt hole-poor anti-FM matrix) and uniaxial anisotropy prevent long-range order in $La_{1-x}Ba_xCoO_3$.

• Macromolecular Research
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• 제17권7호
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• pp.528-532
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• 2009

The size and shape of free volume (FV) holes available in membrane materials control the rate of gas diffusion and its permeability. Based on this principle, a segmented, thermo-sensitive polyurethane (TSPU) membrane with functional gate, i.e., the ability to sense and respond to external thermo-stimuli, was synthesized. This smart membrane exhibited close-open characteristics to the size of the FV hole and water vapor permeation and thus can be used as smart food packaging materials. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), positron annihilation lifetimes (PAL) and water vapor permeability (WVP) were used to evaluate how the morphological structure of TSPU and the temperature influence the FV holes size. In DSC and DMA studies, TSPU with a crystalline transition reversible phase showed an obvious phase-separated structure and a phase transition temperature at $53^{\circ}C$ (defined as the switch temperature and used as a functional gate). Moreover, the switch temperature ($T_s$) and the thermal-sensitivity of TSPU remained available after two or three thermal cyclic processes. The PAL study indicated that the FV hole size of TSPU is closely related to the $T_s$. When the temperature varied cyclically from $T_s-10{\circ}C$ to $T_s+10^{\circ}C$, the average radius (R) of the FV holes of the TSPU membrane also shifted cyclically from 0.23 to 0.467 nm, exhibiting an "open-close" feature. As a result, the WVP of the TSPU membrane also shifted cyclically from 4.30 to $8.58\;kg/m^2{\cdot}d$, which produced an "increase-decrease" response to the thermo-stimuli. This phase transition accompanying significant changes in the FV hole size and WVP can be used to develop "smart materials" with functional gates and controllable water vapor permeation, which support the possible applications of TSPU for food packaging.

• 한국결정성장학회지
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• 제7권4호
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• pp.548-554
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• 1997

수직 Bridgman방법으로 $Mg_{0.18}Zn_{0.84}$Te:Co(Co:0.01 mole%)단결정을 성장시켰다. 성장된 단결정의 결정구조와 광흡수 spectra를 연구하였다. X 선 회절무늬로부터 성장된 단결정의 구조는 cubic구조이었고 격자상수 a=6.1422 $\AA$이었다. 광흡수 측정결과 $Co^{2+}$ 이온에 기인된 $A-band:^4A_2(^4F){\to}^4T_2(^4F),\; B-band:^4A_2(^4F){\to}^4T_1(^4F), C- band:^4A_2(^4F){\to}^4T_1(^4P)$의 intracenter transition과, 흡수단 근처의 charge transfer에 의한 photoionization transition에 관계된 D-band를 550-770 nm의 파장영역에서 관측하였다. 또한 결정장 이론에 의해 결정매개변수(Dq)와 Racah parameter(B)를 결정하였다.