ABO3微波介電材料的光譜研究

Abstract

在這篇論文中利用拉曼光譜、x-ray光吸收譜以及x-ray繞射，來量測微觀狀態下鈣鈦礦結構(perovskite structure)的微波介電陶瓷材料結構性質與其微波性質之關連性。本文主要討論的鈣鈦礦結構陶瓷材料的是ABO3中之1：2結構，也就是A(B’1/3B”2/3)O3。其中主要探討的部份包含：Qf值與氧八面體結構之關係，以及微波介電係數是否受到陶瓷鈣鈦礦結構材料中陰陽離子間距的影響。 我們利用三組樣品來探討上述的微波特性與晶體結構的相關性。有兩組樣品是改變B晶體位置的樣品，第三組樣品是改變A位置的系列樣品。利用這兩個系列的樣品來說明A位置與B位置的取代對微波介電參數的影響。第一組樣品為xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3（縮寫為xBMT+(1-x)BMN），這組樣品是針對改變B”位置的離子，探討結構與微波關連性。我們發現第一組樣品中，圍繞在B”位置周圍的O離子所形成的氧八面體結構對於Qf值有極強烈相關性，而Ta離子與O離子的距離對於介電係數之也有影響。經由延伸X光吸收精細結構(Extended x-Ray Absorption Fine Structure)的擬合發現，在xBMT+(1-x)BMN中，當x增加時，即Ta離子在B”位置上的含量越來越多時，氧八面體會越趨於緊密，也就是Ta離子距O離子的距離越來越小，同時介電係數也有此趨勢，由此我們可以證明介電係數確實與陰陽離子的間距相關；經過計算後，我們可以獲得在此材料中站在B”位置上的離子與O離子平均偏移量與Qf值之關係，此偏移量可以看成是氧八面體的扭曲程度，可以發現當平均偏移量較大時Qf值會相對較小，所以未摻雜的Ba(Mg1/3Ta2/3)O3或是Ba(Mg1/3Nb2/3)O3會有較高的Qf值。在x=0.5時Qf值最小的原因是此時Ta與Nb各佔一半在B”位置上，1：2有序的情況最差，因而有最小值；在x=0.75的樣品具有較大的偏移量使得Qf值較x=0.25的樣品低相當多。 第二組樣品是xBaTiO3-(1-x) Ba(Mg1/3Ta2/3)O3；Ti離子同時取代Mg與Ta離子，在x-ray繞射的結果可看出，隨著濃度上升晶格常數及Qf值變小而介電係數卻上升；我們歸因於入雜質Ti會造成氧八面體之扭曲而造成介電係數以及A1g(O)聲子的半高寬增加，在拉曼光譜中，1比2的結構聲子也隨著濃度漸漸不明顯，使得Qf值下降。 在第三組樣品：xSr(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Ta2/3)O3（縮寫為xSMT+(1-x)BMT），由這組樣品可瞭解當A位置上的離子受到取代時，微波介電的特性與結構的關係，並與與B位置的取代的結果比較其異同。我們對xSMT+(1-x)BMT進行拉曼光譜的分析和x-ray吸收精細結構譜的擬合。在這兩項量測結果和微波量測的數據中，均發現有三段式的結構與微波特性的變化，其轉折點在x=0.5與0.75。拉曼測量中1：2比例結構的聲子在x>0.5後開始消失，並且在不同頻段增加很多新的聲子，而在x-ray吸收譜擬合，Ta-O距離傅利葉轉換圖譜也顯示出相對應的結構對稱變化。拉曼測量的結果可推測在x<0.5時Ba(Mg1/3Ta2/3)O3結構主導其結構特性與微波特性。我們發現離子半徑較小的Sr離子取代Ba離子時，造成整體結構變緊密使得大部分聲子有藍位移，但是對於TaO6氧八面體來說反而由於A位置取代成因電性較小的Sr離子讓氧八面體產生形變，使得A1g(O)聲子半高寬上升。入半徑較小的Sr離子，也造成A位置上的離子距O離子的平均距離增加，因此介電係數值也增加。x-ray吸收精細結構分析也有相似的結果。在x=0.75後， Sr(Mg1/3Ta2/3)O3結構主導材料特性，而微波特性也隨著改變。

In this thesis, we adopted the Extended X-ray Absorption Fine Structure (EXAFS), Raman scattering and x-ray diffraction techniques to detect the micro-structure of a series of perovskite microwave materials and the correlation with microwave dielectric properties were also investigated. The size of oxygen-octahedrons in perovskite ceramics can be revealed by these measurements, and can be directly correlated with the microwave dielectric properties. The EXAFS measurements of xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 (hereafter xBMT +(1-x)BMN) with Ta and Nb as core ions revealed the structural properties of the oxygen-octahedrons. The rigid oxygen-octahedrons give the tight oxygen bonds which imply the reluctant motion of the cations and anions due to external electromagnetic waves. We found the Ta-O bond length decrease with the Ta concentrations in xBMT +(1-x)BMN, therefore, the decreasing of dielectric constant as Ta concentration increases in xBMT +(1-x)BMN is expected. The distortion of the oxygen-octahedrons in xBMT +(1-x)BMN were also revealed by the EXAFS measurement, and the small distortion gives high Qxf values as expected, such as Ba(Mg1/3Ta2/3)O3 and Ba(Mg1/3Nb2/3)O3 ceramics. The largest distortion in xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 were found for x=0.75, and this causes its Qxf value is lower than the compound with x=0.25. The smallest Qxf value were expectedly found for the compound with x=0.5, due to the distortion of the oxygen-octahedrons and the B”-site ordering effect. However, the 1:2 ordering factors in these compounds can not be easily detected by EXAFS. The substitution of Ti ions for both Ta and Mg ions in xBaTiO3-(1-x) Ba(Mg1/3Ta2/3)O3 are the cause of small lattice constants, low quality values, and the high dielectric constants. The oxygen octahedrons were deteriorated due to Ti substitution, and this causes dielectric constant increase with Ti concentration. The result of Qf value decrease with Ti concentration is mainly due to the degrading of 1:2 ordered structure, which were determined by the increasing of A1g(O) phonon width. The result of complex perovskites of xSr(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Ta2/3)O3 ( hereafter xSMT+(1-x)BMT) clearly reveal a three-stage variation in the crystal structure due to the Sr substitution. For x≦0.5, the 1:2 ordered structure is still a dominate structure, and the dielectric constant increases and the Qf value declines as Sr substitution increases. This is mainly due to the smallness of the Sr2+ ion and the slightly twist of the TaO6 oxygen octahedrons caused by the Sr substitution. For 0.5≦x≦0.75, the sample structure is no longer dominated by the 1:2 ordered structure. At x≧0.75, the disappearance of the 1:2 ordered phonons reveals that the new crystal symmetry has appeared. Still, the characteristics of the oxygen octahedrons are strongly related to the microwave properties.