以亞穩相中間產物法合成 鍶鎂摻雜之鑭鎵氧化物電解質材料及其於中溫固態氧化物燃料電池應用之研究

Abstract

本研究目的為合成純鈣鈦礦結構的鑭鍶鎵鎂氧化物(La1-xSrxGa1-yMgyO3-δ)，以應用作為中溫固態氧化物燃料電池之電解質材料。嘗試以Pechini 一步法合成La1-xSrxGa1-yMgyO3-δ，但發現改變多項參數皆無法得到純相，於樣品中皆會有雜相的出現。又發現不添加Sr元素之LaGa0.9Mg0.1O3-δ可以得到純鈣鈦礦相，因此推測原因為Sr2+之離子半徑(158pm)過大，不易進入晶格中取代La3+ (150pm)的位置。另外觀察一步法合成La1-xSrxGa1-yMgyO3-δ時，常見雜相為LaSrGaO4、LaSrGa3O7、La4Ga2O9，其中La4Ga2O9可經由重複燒結而消除，然而其餘兩種含Sr雜相則無法經由重複燒結而完全消除。因此推論Sr、Ga易生成Sr、Ga為主要成分的氧化物，並且形成後不易分解。參考文獻上另一鈣鈦礦結構的鐵電陶瓷材料Pb0.8La0.2(Mg0.4Nb0.6)O3，一樣有難以得到純相的問題，但在將金屬元素分開來合成後便可得到純相。於是擬用相同模式，將La1-xSrxGa1-yMgyO3-δ所需金屬元素分開來做合成，擬出Metastable Intermediate Method將Sr、Ga兩元素分開來做合成。以僅含Ga不含Sr的條件下，排列組合可得中間產物分別是MgGa2O4、LaGaO3的兩種配方法，兩者再摻雜其他兩元素後皆可得到純相， 其中La0.75Sr0.25Ga0.67Mg0.33O3-δ 與La0.75Sr0.25Ga0.75Mg0.25O3-δ分別為兩種中間產物法合成可得到最大氧離子導電度值0.044、0.075 S•cm-1的組成份。

The purpose of this thesis is to prepare La1-xSrxGa1-yMgyO3-δ (LSGM) of pure perovskite phase for the use as the electrolyte material of intermediate temperature solid oxide fuel cell. However, it has been difficult to prepare LSGM of pure phase by one pot method. Several secondary phases, such as LaSrGaO4, LaSrGa3O7, and La4Ga2O9, were frequently formed. Nevertheless, pure perovskite phase of LaGa0.9Mg0.1O3-δ material could be obtained, where Sr was not present, suggesting that the Sr dopant is probably the reason of the formation of secondary phases. Among the secondary phases, La4Ga2O9 could be removed by sintering repeatedly at high temperatures, while the other two phases containing Sr could not. These results suggested that Sr and Ga prefer to react and form stable secondary phases, which cannot further react. Therefore, a metastable intermediate method was developed to prepare La1-xSrxGa1-yMgyO3-δ of pure perovskite phase by two-steps reactions. Electrolyte materials were prepared by separating Sr and Ga elementsand synthesizing the intermediate phases first. Then, the other components were incorporated in the second steps. By choosing the intermediate phases with Ga but without Sr, two intermediate phases, LaGaO3 and MgGa2O4, were prepared. Indeed, La1-xSrxGa1-yMgyO3-δ of pure perovskite phase was successfully prepared by using these two intermediate. The highest ionic conductivities were observed on samples La0.75Sr0.25Ga0.67Mg0.33O3-δ and La0.75Sr0.25Ga0.75Mg0.25O3-δ prepared by LaGaO3 and MgGa2O4 intermediate phases with conductivities of 0.044 and 0.075 Scm-1, respectively.