• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.3 no.2
• /
• pp.99-106
• /
• 1993

A relationship $(T_c = -17.869C^2+1840.2C-46623)$ between Curie Temperature$(T_c)$ and (C), $Li_2O$ mole percent(%) was established from the measurement results of Curie temperature $(T_c)$) analysed by DTA(Differential Thermal Analysis) in the range from 48.50 to 49.00 $Li_2O$ mole %. Congruent melt composition of $LiTaO_3$ single crystal was to be 48.65 $Li_2O$ mole % and its Curie temperature was also determined to be $610{\pm}1^{\circ}C$ from the results of Curie temperature difference, ${\Delta}T (T_{c(Top)}-T{_c(Tail)})$ of Czochralski grown $LiTaO_3$ crystals and the distribution coefficient(k). The k was calculated from $LiO_2$ mole ratio of initial melt to final melt and initial crystal to final crystal in the range from 48.60 to 48.70 $Li_2O$ mole %.

• Journal of the Korean Magnetics Society
• /
• v.7 no.3
• /
• pp.140-145
• /
• 1997

The amorphous state of $NdFe_{10.7}TiB_{0.3}$ and its nanocrystallization have been studied by X-ray diffraction, 모스바우어 spectroscopy, and a vibrating sample magnetometer (VSM), $NdFe_{10.7}TiB_{0.3}$ amorphous ribbons were fabricated by a sigle-roll melt-spinning method. The average hyperfine field $H_{hf}$(T) of the amorphous state shows a temperature dependence of $[H_{hf}(T)-H_{hf}(0)]/H_{hf}(0)=-0.46(T/T_c)^{3/2}-0.34(T/T_c)^{5/2}$ for $T/T_c<0.7$ indicative of spin wave excitation. The quadrupole splitting just above the Curie temperature $T_c$ is 0.46 mm/s, whereas the average quadrupole shift below $T_c$ is zero. The Curie and crystallization temperatures are determined to be $T_c$=380K and $T_x=490K$, respectively, for a heating rate of 5 K/min. The occupied area of nanocrystalline phase at around 770K is about 65%. Above the Curie temperature, VSM data show magnetic moments increases again. The formation of $\alpha$-Fe is the main reason for the increasing moment as conformed with the 모스바우어 measurements.

• Journal of the Korean Ceramic Society
• /
• v.43 no.7 s.290
• /
• pp.421-426
• /
• 2006

[ $(BaSr)TiO_3$ ] thick films were prepared by tape casting method, using $BaTiO_3\;and\;SrTiO_3$ powder slurry in order to investigate dielectric properties i.e. dielectric constant, ${\varepsilon}_r$, Curie temperature, $T_c$. Grain growth within $(BaSr)TiO_3$ thick films was observed with increasing weight ratio of $BaTiO_3$. This observation can be explained by phenomena of substitution of $Sr^{2+}$ ion for $Bi^{2+}$ ion in the $BaTiO_3$ system. Also, the Curie temperature in $(BaSr)TiO_3$ thick films was shifted to lower temperature range with increasing $ SrTiO_3$. Furthermore, Curie temperature having maximum dielectric constant was in the range of $-40^{\circ}C\;to\;30^{\circ}C$, and hence sharper phase transformation occurred at Curie temperature. There occurred decrease in tunability and k-factor of $(Ba_{0.6}Sr_{0.4})TiO_3$ calculated from the dielectric constant, ${\varepsilon}_r$ above Curie temperature. In addition, above the $60^{\circ}C$, phase fixation was observed. This means that internal stress relief occurred with increasing $90^{\circ}$ domains.

• Journal of Magnetics
• /
• v.12 no.4
• /
• pp.161-164
• /
• 2007

Constant temperature regulation of a hybrid heater which is composed of a temperature sensitive ferrite with low permeability and a Cu tube is investigated for hyperthermia. The temperature sensitive ferrite is inserted into a Cu tube and its length and diameter are 10 mm and 3 mm. Below B=0.05 T, the measured temperature and the calculated one increased with the ratio of $B^{1/2}$ and agreed well with each other. Above B=0.05 T, the measured temperatures maintained constantly almost $50^{\circ}C{\pm}1.5^{\circ}C$ because of the influence of Curie temperature of the temperature sensitive ferrite. This result shows that the hybrid heater is able to regulate the temperature constantly at the rate of $50^{\circ}C{\pm}1.5^{\circ}C$.

• Proceedings of the KIEE Conference
• /
• 2001.07c
• /
• pp.1346-1348
• /
• 2001

Generally, Invar alloy shows very low thermal expansion characteristics, lower than $2{\times}10^{-6}$/K approximately. To apply Fe-Ni-Co-C Invar alloy as a core material for large ampacity transmission line we studied the effects of magnetic properties on coefficient of thermal expansion. The coefficient of thermal expansion(CTE) suddenly decreases with addition of a little carbon(0.08%), increases with the increasing carbon and has a constant value at the composition over than 1.0%C. The trend of Curie temperature change with carbon is similar with that of CTE. Therefore, the CTE has a linear relationship with Curie temperature. However, the CTE linearly decreases with the ratio of saturation magnetization and Curie temperature(${\sigma}_s/T_c$).

• Applied Science and Convergence Technology
• /
• v.26 no.5
• /
• pp.143-147
• /
• 2017

Structural studies on the addition characteristics of Nb ions to $BaTiO_3$ solid solutions were performed by XRD and SEM/EDS technique. The X-ray diffraction peaks of the (111), (200) and (002) planes of Nb-doped $BaTiO_3$ solid solutions with different mole% of Nb were analyzed. We also investigated the relationship between the dielectric and structural properties of Nb-doped $BaTiO_3$. The transition temperatures of $BaTiO_3$ solid solution doped with 0.5mole%Nb and 1.0 mole%Nb were ${\sim}116^{\circ}C$ and ${\sim}87^{\circ}C$, respectively, which were found to be shifted to very low temperature from the transition temperature of pure $BaTiO_3$ (about $125^{\circ}C$). As a result of analysis of 1/K versus T and ln[$(1/K)-(1/K_m)$ versus ($T-T_m$)] of the two compositions used in this experiment, the diffusivity slightly differs from that of pure $BaTiO_3$ at temperatures above Curie temperature. And this characteristic was analyzed by applying the modified Curie-Weiss law.

• Journal of the Korean Magnetics Society
• /
• v.5 no.3
• /
• pp.191-196
• /
• 1995

NiCuZn ferrites were prepared by a solid-state reaction and sintered at $900^{\circ}C$ for 5 hours. Its properties were investigated by controlling the ferrite composition and processing. NiCuZn ferrite with a composition of ${(Ni_{0.2}Cu_{0.2}Zn_{0.6}O)}_{1.02}{(Fe_{2}O_{3})}_{0.98}$ was found to have the maximum initial permeability as a result of the variation of Cu content and the (Ni+Cu)/Zn ratio. Curie temperature($T_{c}$) of NiCuZn ferrite was decreased with the larger Cu content and increased with the larger Ni content. NiCuZn ferrites of ${(Ni_{0.2}Cu_{0.2}Zn_{0.6}O)}_{1-w}{(Fe_{2}O_{3})}_{1+w}$ composition milled for 20~80 hours had the maximum initial permeability at w=-0.015 and Curie temperature ($T_{c}$) was decreased with the increasing of $Fe_{2}O_{3}$ deficiency(w).

• Proceedings of the Korean Magnestics Society Conference
• /
• 2003.06a
• /
• pp.90-91
• /
• 2003

Amorphous magnetic materials with competing magnetic interactions are the subject of current interest. Critical behaviour studies have been performed in order to understand the nature of the phase transition at the Curie point (T$\sub$c/) and type of magnetic ordering below the T$\sub$c/. In some cases there exists a temperature interval in which the magnetic system consists of ferromagnetic grains separated by the paramagnetic interlayers. Magnetic properties of nanoparticles embedded in amorphous matrix also are the subject of recent interest. While these materials exhibit excellent soft magnetic properties at room temperature, some of them have been found to be superparamagnetic in the temperature range above the T$\sub$c/ of the matrix. Thus the role of different magnetic phases in the intergrain magnetic coupling can possibly be taken apart in a sufficiently broad temperature range and investigated separately. In particular materials with competing magnetic exchange interactions show characteristics of enhanced magnetoresistance and softer magnetic properties when magnetic nanocrystals are dispersed in amorphous matrix. We expect careful magnetic measurements in the vicinity of T$\sub$c/ would throw some light on magnetic behaviour of above materials. We present here the FMR analysis of Fe$\sub$82/Mn$\sub$8-x/B$\sub$x/Zr$\sub$10/ alloy near the Curie point.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2007.11a
• /
• pp.91-92
• /
• 2007

An anomalous positive temperature coefficient of electrical resistivity (PTCR) was investigated in a ferroelectric lead-free perovskite-type compound $(Bi_{0.5}Na_{0.5})TiO_3$ within $BaTiO_3$-based solid solution ceramics. The effect of $Nb_2O_5$ content on the electrical properties and the microstructure of (1 - x) $BaTiO_3-x\;(Bi_{0.5}Na_{0.5})TiO_3$ (BNT) ceramics made using a conventional mixed oxide process also has been studied. The Curie temperature was obviously increased with the increasing of $(Bi_{0.5}Na_{0.5})TIO_3$ content. The Nb - doped BNT ceramics (x=0.01) display low resistivity values of $10^{1{\circ}}C-10^{2{\circ}}C$ ohm.cm at room temperature and the Curie Temperature of $T_c=160^{\circ}C$.

• Journal of Magnetics
• /
• v.10 no.4
• /
• pp.142-144
• /
• 2005

The Magnetization behaviour has been measured for amorphous $Fe_{80-x}Mn_xZr_{10}$ (x = 4,6,8,10) alloys. The Curie temperature decreased from 236 K to 195 K with increasing Mn concentration (x = 4 to x = 10). The magnetization measurements were conducted at temperatures above the Curie temperature in the paramagnetic region. In all samples, the magnetic properties showed superparamagnetic behavior above $T_c$ where the mean magnetic moment of the superparamagnetic spin clusters decreased with increasing temperature. A large magnetic entropy change, ${\Delta}S_M$, which is calculated from H vs M curves associated with the ferromagnetic-paramagnetic transitions in amorphous, has been observed. With Mn concentration increasing, ${\Delta}S_M$ decreases 1.04, 0.95, 0.87 J/kg K at 222, 210, 195 K (the Curie temperature), respectively.