含二氧化鈰之奈米鈦酸鋇之製備、微觀結構與介電性質之研究

Abstract

本研究將鈦酸鋇（BaTiO3）與二氧化鈰（CeO2）粉體混合研磨分散至奈米尺度，經700至1200□C高溫燒結後其微觀結構與及添加CeO2對介電性質之影響。X光繞射（X-ray Diffraction，XRD）分析結果顯示，CeO2並沒有和BaTiO3產生固溶現象；穿透式電子顯微鏡（Transmission Electron Microscopy，TEM）與能量散佈光譜儀（Energy Dispersive Spectrometer，EDS）之分析顯示CeO2以立方晶結構（Cubic；晶格常數a = 5.411 nm）分佈在BaTiO3正方晶結構中（Tetragonal；晶格常數a = b = 3.994 nm；c = 4.038 nm）。掃描式電子顯微鏡（Scanning Electron Microscopy，SEM）對表面形貌之觀察發現添加CeO2的試片，燒結溫度900至1200□C之晶粒大小介於1至2 μm，燒結密度約93至96%。由介電常數量測結果與Ang Chen等人以及實驗室之既往研究成果相比較，顯示添加CeO2的奈米BaTiO3可降低燒結溫度至900□C，而最佳摻雜量為10 mol.%，其介電常數和介電損失分別約為5369以及 0.83。介電常數隨溫度變化之量測與示差掃描熱量計（Differential Scanning Calorimetry，DSC）的分析結果顯示奈米BaTiO3的居禮溫度（Curie Point，TC）約介於120至130□C之間，其驗證了BaTiO3在TC之相轉換為二階相變化（Second-order Transition）。電滯曲線（Hysteresis Loop）的對極化量與燒結溫度之變化分析顯示，奈米尺度之BaTiO3的極化量較微米尺寸者為大，因此其介電常數也較大，其成因推測為細晶試樣中之晶界結構缺陷使相鄰電偶極間之耦合較低，反平行之方式排列之機率應較小，故電偶極間之極化量相互抵銷之機率亦小，造成總極化量較大，也因相鄰電偶極耦合較小，故當施予外加電場時，電偶極較容易呈順電場方向排列，故有較高之介電常數。

The addition of CeO2 to BaTiO3 and its effects on the improvement of dielectric properties and sintering process were studied. After the grinding and dispersion of nano-sized BaTiO3 and CeO2 powders, the sintering at temperatures ranging from 700 to 1200□C was carried out. X-ray diffraction (XRD) and transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometer (EDS) revealed the embedment of cubic CeO2 (lattice parameter a = 5.411 nm) phase in tetragonal BaTiO3 matrix (lattice parameters a = b = 3.994 nm; c = 4.038 nm). Scanning electron microscopy (SEM) observations showed that the average grain size of the samples containing CeO2, sintering at 900 to 1200□C, were about 1 to 2 μm and had a relative sintering density of 93 to 96%. Dielectric properties measurements revealed that the addition of 10 mol.% of CeO2 in BaTiO3 provides the best result with dielectric constant and dielectric loss equals to 5369 and 0.83, respectively. It also shows that the addition of CeO2 lowers the sintering temperature down to 900□C. The measurements of dielectric constant as a function of temperature and differential scanning calorimetry (DSC) analysis indicated that the Curie temperature (TC) of all samples are around 120 to 130□C, and verified that the the addition of CeO2 barely affect the phase transition temperature. The phenomena of polarizations relative to the dielectric constant of the samples were characterized by hysteresis loops, which also confirmed that the nano-sized BaTiO3 possesses higher dielectric constant than micro-sized one. This was attributed to the weaker coupling in between the electric dipoles induced by the existence of grain boundaries in fine-grained samples. The anti-parallel arrangement of dipoles, as seen in coarse-grained samples, hence less likely occurs in fine-grained samples. This reduces mutual annihilation of electric dipoles. Furthermore, the weak coupling between electric dipoles enables them to align along the direction of external field much easier than those in coarse-grained samples and, thereby, the net polarization and dielectric constant of the samples increase.