BaTiO3 is one of the most attractive materials among perovskite materials, such as PZT, Pb(ZrxTi1-x)O3 because of its superior dielectric properties, which makes it to be used as key material for multi-layer ceramic capacitor (MLCC) application. Nowadays, very fine BaTiO3 particles which have outsta...
BaTiO3 is one of the most attractive materials among perovskite materials, such as PZT, Pb(ZrxTi1-x)O3 because of its superior dielectric properties, which makes it to be used as key material for multi-layer ceramic capacitor (MLCC) application. Nowadays, very fine BaTiO3 particles which have outstanding dielectric properties are needed to meet the requirement for small sized and highly functional electric devices. Even though many attempts have been made to develop nano-sized BaTiO3 powder by other researcher, many results show that the dielectric properties of nano-sized BaTiO3 powder have been reduced with decreasing particle size of BaTiO3 due to its size effect. It is generally believed that the size effect of BaTiO3 is affected by various defects such as impurities, vacancies as well as other powder characteristics. And those defects have an effect on the volume of gradient lattice strain layer (GLSL). Therefore, reported critical sizes for tetragonal to cubic transition of BaTiO3 are different from each other, depending on the various processes and inherent defects level. So, the exact explanation for the size effect can be applied for different synthetic methods for the fabrication of fine BaTiO3 powder. The objective of this research is to enhance dielectric properties of nano-sized BaTiO3 powder by controlling various variables which affect to the size effect of BaTiO3 powder synthesized by oxalate process.
In the first part, basic examinations on the starting materials carried out to understand fundamental features, including phase transition behavior of starting materials by electron microscopy, TG-DTA, high temperature XRD (HT-XRD), pyrolyzer gas chromatography and mass spectroscopy (Py-GC-MS). The investigations on the intermediate phase of BTO were conducted using the high resolution transmission electron microscopy, including the phase analysis by electron energy loss spectroscopy (EELS). Especially, BaCO3 and TiO2 were founded as intermediate phases during the thermal decomposition of BTO, and newly modified BTO transition model using the previous popular model has been suggested. Moreover, various analysis results have been directly reflected to the new model.
In the second part, the effects of controlling variables associated with the condition of starting materials, such as initial particle size of starting materials, degree of the milling and the contents of impurities induced from processing, were studied. The effects of milling condition were examined using different sized ZrO2 beads for different time. The quantity of incorporated impurities, such as H+, Cl- and Zr4+, was adjusted by controlling the milling and washing processes, which are essential to the preparation of BaTiO3. It was shown that reducing amount of impurities in BTO is helpful to enhance the dielectric properties of BaTiO3 powder.
In the third part, the effects of variables related to calcination were studied, including atmosphere, nucleation enhancement by varying the calcination schedule, heating rate and temperature. In order to decrease level of oxygen vacancy incorporated in the lattice of BaTiO3 powder by enhancing the removal of evolving gases such as CO, CO2, vacuum calcination was conducted. Most of the calcined BaTiO3 in vacuum shows higher tetragonality than those in air. 2-step calcination has been carried out to control the nucleation rate and growth during the calcinations. XRD, HT-XRD and TG-DTA were utilized to determine the effects of 2-step calcination. Although more heat was consumed by 2-step calcinations, the powders synthesized by 2-step heat treatment showed a similar particle size with higher tetragonality than those produced by direct heating. The effects of rapid heating up to 100℃ higher than the final soaking temperature was examined, which was performed to minimize the incorporated defect and second phase by offering larger amount of heat in a moment. Even though BTO was exposed at higher temperature during rapid calcination, particle size was almost same with those produced by normal calcination process. On the other hands, tetragonality of powder using rapid calcination showed higher value than prepared powder by normal calcination method.
In the last part, the effects of post-treatments combination were examined to compare the sum of each effect. All of the particles prepared by different processes in each part were examined by electron microscopy and XRD, which was carried out to get the information on crystallographic, such as d-spacing of a-axis and c-axis, amorphous phase contents with respect to crystalline phase.
And to conclude, many post-treatment methods were suggested, which can enhance the dielectric properties of BaTiO3 by parameter optimization related with process and calcination control. We believe that these improved properties came from the minimized defect levels which resulted in the smaller fraction of GLSL. Fine BaTiO3 particles with improved dielectric property by theses various treatments can be practically applicable for the development of high-end MLCC.
BaTiO3 is one of the most attractive materials among perovskite materials, such as PZT, Pb(ZrxTi1-x)O3 because of its superior dielectric properties, which makes it to be used as key material for multi-layer ceramic capacitor (MLCC) application. Nowadays, very fine BaTiO3 particles which have outstanding dielectric properties are needed to meet the requirement for small sized and highly functional electric devices. Even though many attempts have been made to develop nano-sized BaTiO3 powder by other researcher, many results show that the dielectric properties of nano-sized BaTiO3 powder have been reduced with decreasing particle size of BaTiO3 due to its size effect. It is generally believed that the size effect of BaTiO3 is affected by various defects such as impurities, vacancies as well as other powder characteristics. And those defects have an effect on the volume of gradient lattice strain layer (GLSL). Therefore, reported critical sizes for tetragonal to cubic transition of BaTiO3 are different from each other, depending on the various processes and inherent defects level. So, the exact explanation for the size effect can be applied for different synthetic methods for the fabrication of fine BaTiO3 powder. The objective of this research is to enhance dielectric properties of nano-sized BaTiO3 powder by controlling various variables which affect to the size effect of BaTiO3 powder synthesized by oxalate process.
In the first part, basic examinations on the starting materials carried out to understand fundamental features, including phase transition behavior of starting materials by electron microscopy, TG-DTA, high temperature XRD (HT-XRD), pyrolyzer gas chromatography and mass spectroscopy (Py-GC-MS). The investigations on the intermediate phase of BTO were conducted using the high resolution transmission electron microscopy, including the phase analysis by electron energy loss spectroscopy (EELS). Especially, BaCO3 and TiO2 were founded as intermediate phases during the thermal decomposition of BTO, and newly modified BTO transition model using the previous popular model has been suggested. Moreover, various analysis results have been directly reflected to the new model.
In the second part, the effects of controlling variables associated with the condition of starting materials, such as initial particle size of starting materials, degree of the milling and the contents of impurities induced from processing, were studied. The effects of milling condition were examined using different sized ZrO2 beads for different time. The quantity of incorporated impurities, such as H+, Cl- and Zr4+, was adjusted by controlling the milling and washing processes, which are essential to the preparation of BaTiO3. It was shown that reducing amount of impurities in BTO is helpful to enhance the dielectric properties of BaTiO3 powder.
In the third part, the effects of variables related to calcination were studied, including atmosphere, nucleation enhancement by varying the calcination schedule, heating rate and temperature. In order to decrease level of oxygen vacancy incorporated in the lattice of BaTiO3 powder by enhancing the removal of evolving gases such as CO, CO2, vacuum calcination was conducted. Most of the calcined BaTiO3 in vacuum shows higher tetragonality than those in air. 2-step calcination has been carried out to control the nucleation rate and growth during the calcinations. XRD, HT-XRD and TG-DTA were utilized to determine the effects of 2-step calcination. Although more heat was consumed by 2-step calcinations, the powders synthesized by 2-step heat treatment showed a similar particle size with higher tetragonality than those produced by direct heating. The effects of rapid heating up to 100℃ higher than the final soaking temperature was examined, which was performed to minimize the incorporated defect and second phase by offering larger amount of heat in a moment. Even though BTO was exposed at higher temperature during rapid calcination, particle size was almost same with those produced by normal calcination process. On the other hands, tetragonality of powder using rapid calcination showed higher value than prepared powder by normal calcination method.
In the last part, the effects of post-treatments combination were examined to compare the sum of each effect. All of the particles prepared by different processes in each part were examined by electron microscopy and XRD, which was carried out to get the information on crystallographic, such as d-spacing of a-axis and c-axis, amorphous phase contents with respect to crystalline phase.
And to conclude, many post-treatment methods were suggested, which can enhance the dielectric properties of BaTiO3 by parameter optimization related with process and calcination control. We believe that these improved properties came from the minimized defect levels which resulted in the smaller fraction of GLSL. Fine BaTiO3 particles with improved dielectric property by theses various treatments can be practically applicable for the development of high-end MLCC.
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