기계화학공정을 이용한 PLT 나노 분말의 제조 및 저온 소결 특성 평가

Abstract

본 연구에서는 기계적 에너지에 의해 상 형성 반응이 활성화되는 기계화학공정(Mechano Chemical Technique; MCT)을 이용하여 상온에서 산화물 원료 분말을 한 번의 공정으로 나노 크기의 perovskite 구조를 갖는 lanthanum으로 변형된 lead titanate 즉, PLT(Pb_(0.9)La_(0.1)TiO₃) 분말을 제조하였다. 원료 분말인 PbO, La₂O₃, TiO₂를 0.9:0.05:1의 몰 비로 0～12시간 동안 각각 밀링하였고, 제조된 PLT 나노 분말을 이용하여 성형 후 1050℃와 1150℃에서 각각 3시간 동안 온도를 유지하면서 소결하였다. 그리고 밀링한 분말은 밀링 시간에 따라 PLT 상을 포함한 전체적인 상 변화를 분석하였고, 분말의 크기와 형상 및 결정성을 관찰하였다. 소결한 시편은 PLT 상의 형성 및 표면과 단면의 미세구조를 관찰하였고, 소결 밀도를 측정한 후 유전율을 분석하였다. PLT 분말은 MCT를 이용하여 4시간 밀링을 통해 순수한 perovskite 구조를 가지는 나노 분말로 제조되었다. 마이크로 크기였던 원료 분말은 밀링이 진행됨에 따라 평균 ～20 nm 크기의 나노 분말로 이루어진 균일한 입도분포를 보였으며, 결정성이 좋음을 확인하였다. 또한 소결한 PLT는 1050℃의 낮은 소결 온도에서 비교적 높은 상대 밀도와 360 이상의 유전율을 상온에서 측정하여 얻었다. 한편 0.5～2 wt%의 Bi₂O₃ 분말을 첨가한 PLT-Bi₂O₃도 PLT와 동일한 방법으로 분말을 제조하고 소결하였는데, 이때 소결 온도는 1000℃와 1050℃로 PLT보다 낮은 온도에서 진행하였다. 소결한 PLT-Bi₂O₃ 시편 역시 PLT 상 형성을 분석하였고 미세구조를 관찰한 후 소결 밀도와 유전율을 측정하였다. 소결된 PLT-Bi₂O₃는 Bi₂O₃ 첨가량이 증가함에 따라 소결 밀도와 유전율이 그에 비례하여 증가하였다. 특히, PLT-Bi₂O₃를 1000℃에서 소결했을 때, 우수한 상대 밀도와 400 이상으로 향상된 유전 상수 값을 나타내었다.; Ferroelectric ceramics are important electrical materials that have a wide range of industrial and commercial applications, such as piezoelectric sonar, high-dielectric constant capacitors, ultrasonic transducers and ultrasonic motors. Among them, lanthanum modified lead titanate(PLT) is a perovskite structure type of ferroelectric ceramics. It has been considered for several demanding technological applications, including energy storage, non-volatile memory, sensors, and actuators. The purpose of this study is to directly synthesize nanosized PLT powder with a perovskite structure using oxide powders by a Mechano Chemical Technique(MCT) at room temperature. The most significant characteristic of the MCT is that the formation of desirable phase was triggered by mechanical energy, instead of calcination at elevated temperatures. Reaction occur to change from mechanical energy to chemical energy by impact between ball and powder in the vial using equipment of SPEX mill. A mixture of starting materials(PbO, La₂O₃, TiO₂) was weighed in 0.9:0.05:1 of molar ratio and then synthesis was carried out for 0～12 hrs, respectively. The synthesized PLT nano powder was pressed and then sintered at 1050℃ and 1150℃ during 3 hr, respectively. The milling powders were analyzed for phase present, powder morphology, particle size, and crystalline with increasing mechano-chemical treatment time. The sintered samples were characterized for formation of PLT phase, surface and cross section of microstructure. And samples were measured sintering density and dielectric properties. After 4 hr of mechano-chemical treatment, a pure PLT phase of perovskite structure was formed using the MCT. The average particle size was about 20 nm with a relatively narrow distribution up to 4 hr of milling time from raw powder of several micrometers with an inhomogeneous distribution. And the PLT powder was well-crystalled. Also, the sintered PLT has high dielectric constant of above 360 at relatively low sintering temperature of 1050℃. Its dielectric constant was higher than theoretical that of PbTiO3. Meanwhile Bi₂O₃ powder of 0.5～2 wt% was added PLT-Bi₂O₃ powder was synthesized and sintered in a same process of the PLT. At this time sintering temperature was 1000℃ and 1050℃, respectively which was lower than the PLT. Sintered samples of PLT-Bi₂O₃ were studied for PLT phase and microstructure and then measurements of relative density and dielectric constant were conducted. The sintered PLT-Bi₂O₃ increased properties of sintering density and dielectric constant with increasing Bi₂O₃ addition proportionally. Specially, sintering temperature could be decreased to 1000℃, and excellent relative density and improved dielectric constant of over 400 could be achieved by little Bi₂O₃ addition. To conclude, PLT nano powder was successfully synthesized at room temperature and also good sinterability and high dielectric constant of above 400 were accomplished at low sintering temperature of 1000℃ by Bi₂O₃ powder addition.