• Journal of Aerospace System Engineering
• /
• v.13 no.2
• /
• pp.43-50
• /
• 2019

Satellite and aircraft components are greatly affected by the possibility for missions and safety due to temperature effects. In the field of fuel cells, research is actively carried out for UAV. For the efficiency and stability of the fuel cells, the temperature for operations must be confirmed. In this paper, a smart paint was fabricated with $BaTiO_3$ and $SrTiO_3$ ceramics in order to take advantage of the rapid characteristics of the PTC thermistor, which is the resistance changes abruptly above the Curie point. A coating agent was prepared to prevent the paint from peeling off the samples and the coated models were checked for resistance changes. Moreover, the resistance change of the $BaTiO_3$ and $SrTiO_3$ with temperature conditions was measured before and after coating.

• Journal of the Korean Magnetics Society
• /
• v.12 no.1
• /
• pp.14-19
• /
• 2002

The iron-doped perovskite La$_{0.67}$Sr$_{0.33}$Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$compound has been studied by x-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. The single phase of the polycrystalline La$_{0.67}$Sr$_{0.33}$Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$powder has been prepared by a waterbased solgel method. Crystalline La$_{0.67}$Sr$_{0.33}$Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$was a rombohedral structure with lattice parameters a$_{0}$=5.480 $AA$, $alpha$=60.259$^{circ}$. Mossbauer spectra of La$_{0.67}$Sr$_{0.3}$/Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$have been taken at various temperatures ranging from 20 to 400 K. As the temperature increases toward the Curie temperature, T$_{c}$=375 K, the Mossbauer spectra show line broadening and the difference between the 1,6 and 3,4 linewidths is caused by the anisotropic hyperfine field fluctuation. The anisotropic field fluctuation of +H (P$_{+}$=0.80) is greater than -H (P$_{-}$=0.20). We calculated that the anisotropy energy was 124.01 erg/cm$^3$for T=150 K which is associated with the large line broadening.

• Journal of the Korean Magnetics Society
• /
• v.4 no.3
• /
• pp.226-232
• /
• 1994

Rapidly solidified $SmFe_{7+x}M_{x}(M=Mo,\;V,\;Ti)$ compound were found to crystallize in the ${Sm(Fe,\;M)}_{7}$ based stable magnetic phase by introducing a second transition element into the Sm-Fe binary system. The ${Sm(Fe,\;M)}_{7}$ phase exhibits the highest Curie temperatuer ($T_{c}=355^{\circ}C$) ever Known in the Sm-Fe magnetic systems with a quite high intrinsic coercivity($_{i}H_{c}=3~6\;kOe $). The ${Sm(Fe,\;M)}_{7}$ phase remains stable even after annealing if once form during the rapid solidification. The primary reason for the high coercive force is due to the fine grain size($2000~8000\;{\AA}$)of the magnetic ${Sm(Fe,\;M)}_{7}$ matrix phase, and the enhanced Curie temperature is attributed to the extended solid-solubility of the additive transition elements in Fe matrix, which leads to volume expansion of the ${Sm(Fe,\;M)}_{7}$ cell causing an enhanced coupling constant of Fe atoms.

• Journal of the Korean Ceramic Society
• /
• v.34 no.2
• /
• pp.202-208
• /
• 1997

Bi-layered SrBi2Ta2O9(SBT) films were deposited on Pt/Ti/SiO2/Si sibstrates by rf magnetron sputt-ering at room temperature and then were annealed at 75$0^{\circ}C$, 80$0^{\circ}C$ and 85$0^{\circ}C$ for 1 hour in oxygen at-mosphere. The film composition of SrBi2Ta2O9 was obtained after depositing at room temperature and annealing at 80$0^{\circ}C$. Excess 20mole% Bi2O3 and 30 mole% SrCO3 were added to the target to compensate for the lack of Bi and Sr in SBT film. 200 nm thick SBT film exhibited and dense microstructure, adielectric constant of 210, and a dissipation factor of 0.05 at 1 MHz frequency. The films exhibited Curie temperature of 32$0^{\circ}C$ and a dielectric constant of 314 at that temperature under 100 kHz frequency. The remanent polarization(2Pr) and the coercive field(2Ec) of the SBT films were 9.1 $\mu$C/$\textrm{cm}^2$ and 85 kV/cm at an applied voltage of 3V, resspectively and the SBT film showed a fatigue-free characteristics up to 1010 cy-cles under 5V bipolar pulse. The leakage current density of the SBT film was about 7$\times$10-7A/$\textrm{cm}^2$ at 150 kV/cm. Fatigue-free SBT films prepared by rf magnetron sputtering can be suitable for application to non-volatile memory device.

• Journal of the Korean Magnetics Society
• /
• v.17 no.1
• /
• pp.10-13
• /
• 2007

Single crystalline, $LuFe_2O_4$, was grown by the floating zone method. The crystal structure was a two-dimensional layered-type rhombohedral($R\={3}mh$) structure, with an $a_0=3.440(2)\;{\AA}\;and\;a\;c_0=25.263(2)\;{\AA}$. The magnetic $N\'{e}el$ temperature($T_N$) was determined to be 250 K. The $M\"{o}ssbauer$ spectrum at 12 K was fitted with four sextet sets which was resulted from the crystal structure. The spectrm at room temperature consisted of three singlets and a doublet with the electric quadrupole splitting. The isomer shift($\delta$) value of the singlet was $0.20{\pm}0.01mm/s$ relative to the Fe metal indicating the $Fe^{3+}$ valence state, and the value of the doublet was $0.70{\pm}0.01mm/s$ indicating $Fe^{2+}$. The $M\"{o}ssbauer$ absorption area ratio between $Fe^{3+}$ and $Fe^{2+}$ at room temperature was 1:1. The doublet phase of spectra gradually disappears by up to 360 K. At 360 K, the spectrum shows the singlet phase. We suggested that the spontaneous polarization effect of $LuFe_2O_4$ was caused by the change of iron behavior.

• Journal of Sensor Science and Technology
• /
• v.5 no.3
• /
• pp.93-100
• /
• 1996

Polycrystalline $Sr_{2}(Ta_{1-x}Nbx)_{2}O_{7}$ and $La_{2}Ti_{2}O_{7}$ ceramics having very high Curie temperatures were synthesized by the conventional oxide mixing method (CON) and the molten salt synthesis method (MSS). Physical characteristics and phase relationship of calcined powders, as well as the sintering behavior, the grain-orientation and dielectric properties of sintered ceramics were investigated as a function of composition and firing temperature. The single $A_{2}B_{2}O_{7}$ phase was synthesized by using the MSS method at 100 - $150^{\circ}C$ lower temperature compared to the CON method. As Nb content increased in $Sr_{2}(Ta_{1-x}Nbx)_{2}O_{7}$ ceramics, the Curie temperature and the dielectric constant at Curie temperature were raised, and the sintering behavior and the degree of grain-orientation were also improved at the same time. The use of MSS-derived powders made it possible to lower the sintering temperature and to improve the dielectric properties of the sintering samples. However, the piezoelectric properties as well as the grain-orientation were not improved any further by the MSS route.

• Journal of the Korean Magnetics Society
• /
• v.7 no.1
• /
• pp.7-12
• /
• 1997

Amorphous $Fe_{50}Zr_{50}$ alloy has been manufactured by mechanical alloying from pure elemental powders of Fe and Zr in conventional ball mill under an Ar atmosphere. Structure and magnetic properties of the amorphous phase were studied by transmission electron microscopy and SQUID magnetometry. Selected area diffraction patterns taken from the mechanically alloyed powders showed two halo rings, indicating coexistence of Fe rich and Zr rich amorphous phases in mechanically alloyed powder. Curie temperature of the Fe rich amorphous phase, measured by Arrot plot, was 195 K. Fe content in the ferromagnetic amorphous phase, estimated from the Curie temperature, was about 65 at%. Spin wave stiffness constant of $Fe_{50} Zr_{50}$ alloys processed for 100 and 200 hrs were 52.2 and 63.8 meV, respectively. The higher spin wave stiffness constant in 200 hrs milled powders may arise from the precipitation of $\alpha$-Fe by partial crystallization of amorphous phase.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2007.11a
• /
• pp.258-258
• /
• 2007

KNN계 세라믹스는 $400^{\circ}C$ 이상의 높은 큐리온도와 압전특성을 가져 Pb대체용 소재로 각광을 받고 있고 많은 연구들이 진행중이다. 하지만 potassium의 수분과의 반응성이 강하고 녹는점이 낮아 소결시 K2O의 휘발로 인해 화학양론적인 정확도를 맞추기가 어려운 단점을 가지고 있다. 이러한 단점을 보완하기 위하여 Hot pressing, Hot forging, SPS등 여러 가지 방법을 이용한 연구가 보고되었다. 본 연구에서는 낮은 소결밀도를 향상시키기 위하여 성형압과 성형방법을 달리하여 성형밀도에 따른 소결 밀도와 압전 특성 변화에 대하여 관찰하였다. 넓은 범위의 성형밀도를 얻기 위하여 낮은 성형압을 가할 수 있는 기계적으로 작동하는 press와 높은 성형밀도를 가할 수 있는 유압식 press를 사용하였고, 양산성을 고려하여 일반적인 세라믹 제조공정을 이용하였다.

• Applied Chemistry for Engineering
• /
• v.22 no.3
• /
• pp.249-254
• /
• 2011

A nonthermal plasma reactor in conjunction with a tubular type with a ferroelectric (high-dielectric ceramic) pellet layer was designed and constructed. $SrBiTaO_9$ (SBT) pellets with 2.0 mm in diameter were held within the tube arrangement by two metal mesh electrodes (20 mm separation) connected to a high-voltage AC power supply. The dielectric constant of SBT pellets was 150 at room temperature and 500 at curie temperature ($335^{\circ}C$). The generation rate of ozone in the plasma reactor almost linearly increased with increasing applied voltage. In the case of the plasma reactor packed with SBT pellets the generation rate of ozone sharply increased at the applied voltage more than 20 kV. The ozone generation rate at the negative corona discharge was higher than that of the positive corona discharge. However, the destruction efficiency of toluene and methylene chloride was not increased in proportion to ozone concentration.

• Proceedings of the KIEE Conference
• /
• 2008.10a
• /
• pp.173-174
• /
• 2008

페로브스카이트 구조의 (Ba,Sr,Ca)$TiO_3$ 분말에 $Dy_2O_3$ 불순물을 첨가하여 첨가량에 따른 영향을 연구하였다. 시편의 제작은 Screen-printing을 이용하여 후막으로 제작하였으며, 구조적인 특성과 전기적인 특성을 관찰하였다. XRD 회절 분석을 통하여 $Dy_2O_3$가 첨가된 모든 시편에서 이차상이 없는 전형적인 페로브스카이트 구조를 나타내었다. 시편의 미세구조를 관찰한 결과 grain size는 $Dy_2O_3$ 첨가량이 증가 할수록 감소하였으며, 기공은 증가하는 것을 알 수 있었다. 후막의 두께는 $Dy_2O_3$ 첨가량에 영향을 받지 않았으며 평균 두께는 $69{\mu}m$이었다. 큐리 온도는 $Dy_2O_3$ 첨가량에 따라 감소하였으며, 유전 손실은 상온 이상에서 1%이하로 크게 감소하였다.