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K<sub>2</sub>WO<sub>4</sub> flux 에 의한 K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker 의 합성
이진식 ( J. S. Lee ),이상문 ( S. M. Lee ),이철태 ( C. T. Lee ),권긍택 ( K. T. Kohn ),김영명 ( Y. M. Kim ) 한국공업화학회 1993 한국공업화학회 연구논문 초록집 Vol.1993 No.0
K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker는 구조적인 특성 때문에 물리ㆍ화학적으로 매우 안정하며 보강재, 마찰재, 단열재 등의 많은 용도를 갖게된다. 특히 최근에는 석면이 발암물질로 인한 자동차 브레이크 마찰재료의 사용이 금지됨에 따라 K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker는 이의 대체 섬유로서 주목을 받고 있다. 이러한 K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker의 합성방법으로는 TiO<sub>2</sub>와 K<sub>2</sub>CO<sub>3</sub>의 화학양론적 조성의 혼합들을 설정온도에서 소성(calcination method)시키는 방법을 비롯하여 응용법(melting method), 수열법(hydrothermal method), 융제법(flux) 및 KDC법(kneading-drying-calcination method) 등의 방법이 있다. 그러나 수열법의 경우 양질의 whisker를 얻을 수 있으나 고압합성 이므로 위험하고 가격이 비싼 결점이 있으며 공업상 제조에 필요한 조건이 복잡하고 연속조작이 어려워 비현실적인 방법이다. 또한 서냉소성법의 경우 공정이 단순하며 공업화가 쉬우나 비교적 장섬유가 얻어지게 된다. 따라서 본 연구에서는 융제법을 이용하여 K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker를 합성하였으며. 과거에 용제로 사용된 KC1-KF계. K<sub>2</sub>O-Na<sub>2</sub>O-B<sub>2</sub>O<sub>3</sub>계 등의 높은 volatility와 viscosity 그리고 낮은 solubility에 대한 문제점을 개선하기 위해 K<sub>2</sub>WO<sub>4</sub>를 flux로 선정하여 K<sub>2</sub>Oㆍ6TiO<sub>2</sub> whisker를 합성하였다.
Hiroyuki Kimura,K Kohn,K Kakurai,K Kaneko,N Metoki,S Wakimoto,Y Kamada,Y Noda 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
A microscopic magnetic phase diagram as functions of temperature and magnetic .eld was mapped for multiferroic compounds of RMn2O5 (R = Er, Ho) by using neutron diraction mea- surements under high magnetic .eld. The unambiguous relation between the ferroelectric phase transition and the magnetic phase transition from a two-dimensional incommensurate to a com- mensurate phase, where both the transitions are concomitantly induced by the magnetic .eld, indicates that the electric polarization is magnetically induced and the commensurate spin state is important for stabilizing the ferroelectricity in the multiferroic RMn2O5 system. The origin of the .eld-induced magnetic phase transition is discussed based on the magnetic structure. j_si
Review and Prospect of Ferroelectricity and Magnetism in YMn2O5
Y. Noda,H. Kimura,I. Kagomiya,K. Kohn,N. Ikeda,S. Matumoto,T. Shobu,Y. Fukuda 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.IV
YMn2O5 undergoes ferroelectric successive phase transitions at 39 K and 19 K, in which a dielectric constant shows anomalies. Spontaneous polarization was also observed. Just above these ferroelectric phase transition temperatures, at 45 K, antiferromagnetic order occurs. Neutron elastic measurements and magnetic susceptibility measurements reveal that the ferroelectric phase transition at 39 K and 19 K occurs simultaneously with the change of magnetic ordering pattern. Review of experimental results and conjecture for the origin of spontaneous polarization are given.??
Lattice Modulation in YMn2O5 and TbMn2O5 Studied by Using Synchrotron Radiation X-rays
Yukio Noda,H Kimura,H Sawa,K Kohn,N Ikeda,S Kobayashi,Y Ishikawa,Y Kamada,Y Wakabayashi 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
RMn2O5 (R = Y and rare earth) shows successive magnetic and ferroelectric phase transitions at about 45 K, 40 K, 39 K, 20 K and 10 K and is called a multiferroic compound. We have investigated the lattice modulation vectors (qL) of YMn2O5 and TbMn2O5 by using single crystal Synchrotron Radiation X-ray diraction and compared them with the magnetic propagation vectors (qM) studied by neutron diraction. The relation of the propagation vectors between magnetic materials and dielectrics is found to be qL = 2qM. A preliminary structure analysis of the atomic displacement pattern in the commensurate ferroelectric phase of TbMn2O5 is discussed.fb
Temperature Dependences of Electric Polarization and Magnetic Ordering of TmMn2O5
Mamoru Fukunaga,H Kimura,K Kohn,Y Noda 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
In order to reveal the relationship between the appearance of ferroelectricity of RMn2O5 and magnetic ordering, we have measured the ferroelectric P-E hysteresis loops and investigated the temperature dependence of the electric polarization of TmMn2O5. Strange P-E loops were ob- served, and a pyroelectric response was observed even on zero .eld cooling. The causes of both strange phenomena are unknown, but the temperature-dependent tendencies of the polarization and of the coercive .eld corresponding to the magnetic phase diagram were observed.