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텅스텐 브론즈 Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> 단결정의 성장 특성
주기태,강봉훈,Joo, Gi-Tae,Kang, Bonghoon 한국재료학회 2012 한국재료학회지 Vol.22 No.12
Tungsten bronze structure $Sr_{1-x}Ba_xNb_2O_6$ (SBN) single crystals were grown primarily using the Czochralski method, in which several difficulties were encountered: striation formation and diameter control. Striation formation occurred mainly because of crystal rotation in an asymmetric thermal field and unsteady melt convection driven by thermal buoyancy forces. To optimize the growth conditions, bulk SBN crystals were grown in a furnace with resistance heating elements. The zone of $O_2$ atmosphere for crystal growth is 9.0 cm and the difference of temperature between the melt and the top is $70^{\circ}C$. According to the growth conditions of the rotation rate, grown SBN became either polycrystalline or composed of single crystals. In the case of as-grown $Sr_{1-x}Ba_xNb_2O_6$ (x = 0.4; 60SBN) single crystals, the color of the crystals was transparent yellowish and the growth axis was the c-axis. The facets of the crystals were of various shapes. The length and diameter of the single crystals was 50~70 mm and 5~10 mm, respectively. Tungsten bronze SBN growth is affected by the temperature profile and the atmosphere of the growing zone. The thermal expansion coefficients on heating and on cooling of the grown SBN single crystals were not matched. These coefficients were thought to influence the phase transition phenomena of SBN.
Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub>의 유전 특성에 대한 산화/환원 열처리의 영향
강봉훈,백영섭,주기태,Kang, Bong-Hoon,Paek, Young-Sop,Joo, Gi-Tae 한국세라믹학회 2006 한국세라믹학회지 Vol.43 No.3
${Sr_{1-x}Ba_xNb_2O_6$(SBN) have been obtained in Pt crucible by melting and cooling in air atmosphere. Some SBNs being at the bottom of the crucible are black and transparent, and the other SBNs colorless. The black SBN became to be colorless by oxidation heat treatment $1,300^{\circ}C$ for 4 h, Curie temperature is changed by colorless change of black SBN. The reason seems to be $Nb^{5+}$ oxidation of some $Nb^{4+}$ ions in SBN or effect of unknown impurities. Diffused Phase Transition (DPT) was appeared during heating and cooling process. Various sintered SBN ceramics specimen showed relaxor characteristics.
Raman 분광법을 이용한 $Li_{1-X}Al_{2X}Ta_{1-X}O_3$ 고용한계 분석
김정돈,홍국선,주기태,Kim, Chong-Don,Hong, Kug-Sun,Joo, Gi-Tae 한국분석과학회 1992 분석과학 Vol.5 No.1
강유전체인 $LiTaO_3$ 소재는 SAW filters나 IR sensors의 기본재료로 사용되고 있다. Dopant로서 $Al_2O_3$를 $LiTaO_3$에 일부 치환함으로써 유전 특성을 변화시키고 특히 용융점을 낮춤으로써 단결정 제조를 용이하게 한다. X-선 회절분석에 의한 격자상수 변화와 Raman spectroscopy의 band broadening을 측정한 결과 $LiTaO_3$에 대한 $Al_2O_3$의 고용한계가 $Li_{1-X}Al_{2X}Ta{1-X}O_3$에서 X=0.25mol이었으며, 고용한계 이상에서는 2차상인 $Al_2O_3$상이 XRD로 관찰되었다. Grain size에 의한 Raman band의 broadening을 고려하기 위하여 단결정과 소결체 $LiTaO_3$를 측정 비교하였다. The upper limit of solid solution of $Al_2O_3$ in $LiTaO_3$ was investigated using X-ray diffraction and Raman spectroscopy. By substituting cations in $LiTaO_3$ with $Al^{3+}$, the melting temperature was lowed and the ferroelectric properties can be improved. It is easier at lower temperature to fabricate the single crystal used for SAW filters and IR sensors. From the measured lattice constants and Raman band broadening, the solubility limit was X=0.25mol in $Li_{1-X}Al_{2X}Ta{1-X}O_3$, above which $Al_2O_3$ was obsered as a second phase. The Raman band of sintered $LiTaO_3$ was compared with that of the single crystal to see the effect of grain size on the band broadening.
LiNbO<sub>3</sub> 섬유 단결정의 분극에 관한 연구
강봉훈,장재혁,최덕균,신태희,주기태,Kang, Bong-Hoon,Jang, Jae-Hyuk,Choi, Duck-Kyun,Shin, Tae-Hee,Joo, Gi-Tae 한국세라믹학회 2009 한국세라믹학회지 Vol.46 No.4
Congruent or stoichiometric $LiNbO_3$ fiber single crystals were grown by the $\mu$-PD method, and the grown fiber crystals have the several (2 or 3) ridges with a diameter of $1.35{\sim}1.5\;mm$ and a length of $40{\sim}100\;mm$. In this $\mu$-PD process, different growth rates ($10{\sim}60\;mm/h$) were applied. Pt wire or $LiNbO_3$ crystal was used as a seed. The properties of grown $LiNbO_3$ fiber single crystals having a-axis or c-axis according to seeds were effected by the grown conditions(Pt tube diameter, pulling speed, after heater etc.). Disk-type $LiNbO_3$ samples were poled in condition of DC 5 V/cm at 1050, 1075 or $1100^{\circ}C$. XRD, SEM, conoscope image through the polarized microscope, $T_C$ measuring apparatus, optical transmittance measuring instrument are used to identify the properties of $LiNbO_3$.
제2고조파 발생특성을 통한 MgO가 2 mole% 첨가된 LiNbO₃ 단결정의 poling 조건연구
이종수(Jong Soo Lee),이범구(Bum Ku Rhee),주기태(Gi Tae Joo) 한국광학회 1997 한국광학회지 Vol.8 No.5
MgO가 2 mole% 첨가된 LiNbO₃ 결정을 3 V/㎝, 5 V/㎝, 7 V/㎝의 전기장에서 poling하여, 5 V/㎝의 전기장이 최적의 조건임을 제2고조파발생의 온도의존도특성을 관찰하여 알 수 있었다. 결정에 공간적으로 변조된 전기장을 인가하여 온도위상 정합 모양이 찌그러짐을 관찰하였으며, 이 결과와 잘 일치하는 계산값을 정량적으로 구할 수 있었다. 이것으로부터 제2고조파의 온도위상정합모양을 이용하여 결정의 균일성을 판단할 수 있음을 증명하였다. 5 V/㎝로 poling된 단결정의 온도 위상 정합 모양으로부터 온도에 따른 복굴절의 변화율, c-축으로 전기장을 인가하여 변화된 위상 정합 온도로부터 전기장에 따른 복굴절의 변화율을 계산하였다. Poling condition in 2 mol% MgO-doped LiNbO₃ crystals is investigated by varying the amplitude of poling electric field from 3 V/㎝ to 7 V/㎝. It is found that 5 V/㎝ is the best value for the formation of single domain by analysing the characteristics of the second harmonic generation(SHG) as the function of temperature. The temperature dependence of the phase-matching profile of SHG for the crystal applied by a spatially modulated electric field is observed to be very different from the simple sinc function. The distorted profile can be consistently fitted with the numerical simulations. This shows that the crystal homogeneity can be tested by the SHG temperature phase-matching profile. In addition, the thermo birefringence coefficient and electro birefringence coefficient of SHG were measured from the temperature dependence of phase-matching profile and shifts of phase-matching temperature by appling electric field along c-axis.