鈦酸鋇的微結構在直流電場下對其可靠度之影響研究

Abstract

鈦酸鋇為基底的陶瓷材料因為其優越的鐵電性質而廣泛地應用於電子工業中，微結構在鐵電性能的表現上扮演了很重要的角色。本研究即利用不同的燒結曲線製備出具有不同微結構之鈦酸鋇試樣。在微結構的觀察中，發現鈦酸鋇的試樣中呈現出非連續性的晶粒成長（亦稱為異常晶粒成長）。在燒結溫度為1320℃-1360℃且持溫時間2小時的試樣中，其微結構呈現了典型的混合結構：同時存在正常小晶粒和異常大晶粒。透過定量的顯微結構特徵分析，可以得到正常晶粒和異常晶粒的粒徑變化情形。對小晶粒而言，其平均粒徑從1.9 μm稍微增加到 2.2 μm，對大晶粒而言則是從122 μm成長到211 μm。大小晶粒之間的晶粒尺寸差異，相差了將近100倍，而且沒有發現到任何具有中間尺寸的晶粒。藉由進一步的顯微觀察可以發現到有一個我們稱為「擬-異常晶粒」的區域，是由許多群聚的小晶粒所組成。推測對鈦酸鋇而言，此「擬-異常晶粒」區域的形成，可以提供一個從小晶粒到大晶粒的過渡轉變。 本實驗亦對鈦酸鋇在直流電場下的可靠度進行研究。我們利用高加速壽命測試來模擬長時間使用下之介電崩潰並進一步估算鈦酸鋇的可靠度，同時利用韋伯統計的技巧對破壞時間的分佈進行分析。鈦酸鋇的韋伯模數分布在0.21-0.52之間。並透過直流崩潰電場測試來得到鈦酸鋇的介電強度。具有越多數目大晶粒的試樣，其介電強度越低。透過顯微結構的觀察，對於在直流電場下不同的測試方式：高加速壽命破壞測試以及直流崩潰電場測試，其介電崩潰的破壞機制也不相同。對於直流電場崩潰測試而言，微裂縫在高電場誘發應變的影響下所造成之裂縫成長是造成介電崩潰的原因。而因為局部熱集中所造成的熱崩潰 (thermal runaway)現象，則被視為在高加速壽命測試下介電崩潰的主要機制。

Barium titanate-based ceramics are used extensively because of their superior ferroelectric properties. The microstructure characteristics of BaTiO3 play important roles on the ferroelectric performance. In the present study, BaTiO3 specimens with different microstructure are prepared by using various sintering profiles. A discontinuous grain growth, also called abnormal grain growth (AGG) or exaggerated grain growth (EGG), is observed in some specimens. The microstructure of the specimens sintered at 1320-1360℃ exhibits a typical bimodal structure. Through the quantitative characterization, the size variation of normal and abnormal grains is determined. The average size of normal grains varies from 1.9 to 2.2 μm and abnormal grains from 122 to 211 μm. The size difference between the normal and abnormal grains is two orders of magnitude. No grain with an intermediate size is observed. A “pseudo-abnormal” region with a cluster of fine grains is found during the microstructure observation. The formation of pseudo-abnormal regions may provide a microstructure transition from normal to abnormal grains. The reliability of barium titanate under DC electric field is also investigated in the present study. The highly accelerated life test (HALT) method is used to estimate the reliability for long-term usage. The distribution of lifetime is analyzed by using the Weibull statistics. The Weibull modulus of the BaTiO3 ceramics is in the range of 0.21-0.52. The DC breakdown voltage test is also applied to obtain the dielectric strength (DS) of the BATiO3 specimens. The dielectric strength is in the range of 5.7- 11 MV/m. When the area fraction of abnormal grains is higher, the dielectric strength is smaller. Through the microstructure observation on the fracture surface, the mechanism of the dielectric breakdown is different from that of specimens under the breakdown tests. For the DC breakdown voltage test, the growth of microcracks caused by the electrically induced effect is the reason for the breakdown. And for the HALT, it is thought to be dominated by thermal runaway (TRA) which is caused by the localized heating.