鈦摻雜鉻酸鑭氧化物材料合成與特性分析

Abstract

本論文試將鈦摻雜鉻酸鑭氧化物材料應用在固態氧化物燃料電池電極上。以溶膠凝膠法(sol-gel method)合成鈦摻雜鉻酸鑭氧化物(Ti-doped LaCrO3)粉末，並以不同比例的鈦取代晶格中的鉻進行鈦摻雜對晶體結構的影響。在X光粉末繞射圖中發現，無論在酸性、中性與鹼性環境下合成鈦摻雜鉻酸鑭氧化物皆可形成與LaCrO3相同的斜方晶(orthorhombic)結構，且其形成純相的燒結溫度僅為800°C，低於一般文獻上所使用的1000°C至1400°C，而其鈦摻雜的固溶度可達15%，摻雜超過15%的鈦便會在結構中產生燒綠石(pyrochlore)結構的La2Ti2O7的第二相。掃描式電子顯微鏡影像顯示摻雜不同鈦的量對其形貌並無影響。同時EDX與ICP-AES檢測後，在最大摻雜固溶度內，實際摻雜量與理論量相符。更進一步，以X射線光電子能譜儀(XPS)分析摻雜後各離子的氧化態與鍵結環境，摻雜Ti後，La離子與Cr離子氧化態下降，而氫氣還原後，又以Ti離子氧化態變化最為明顯，並以四點探針進行導電率之研究，未摻雜鈦的LaCrO3為P型半導體，藉由Cr離子中3d能帶上的電洞導電。摻雜鈦後的導電率隨著摻雜比例而逐漸提高，可知摻雜Ti後，電荷補償作用下產生的晶格缺陷有助於提升電子導電率，但其電子導電度並未達到一般對電極材料所要求的1S/cm。然而交流阻抗頻譜儀所量測的15%鈦摻雜鉻酸鑭氧化物，其離子導電度0.107S/cm已達接近電解質的程度，兩者相互彌補，整體導電率而言可能可作為電極材料使用，還須進行半電池與全電池測試做為更仔細的評估。

In the present study, Ti-doped LaCrO3 perovskite materials were successfully synthesized from EDTA-chelating precursor solution containing the respective aqueous solution of metal nitrate, nitric acid, and EDTA. The phase presence and redox stability of Ti-doped LaCrO3 was characterized by powder X-ray diffraction. Ti-doped LaCrO3 powder with single phase orthorhombic perovskite were synthesized under 800°C, which is lower than the one reported to form other perovskite materials. The structure remains its orthorhombic phase when the content of dopant is less than 0.15 as observed by PXRD. As the content of dopant is more than 0.15, it tends to form La2Ti2O7 pyrochlore second phase. Furthermore, the electronic conductivity investigated by four-probe method increased with raising amount of Ti, but it is still lower than the requirement, 1S/cm. Yet, outstanding ionic conductivity of 15% Ti-doped LaCrO3, 0.107S/cm, is observed by electrochemical impedance spectroscopy. Also, it’s shown that electronic and ionic conductivity both in O2 and H2 increases with raising amount of Ti, indicating Ti-doped LaCrO3 is a p-type semiconductor. Further study on surface characteristics by X-ray photoelectron spectroscopy exhibits that the change in conductivity with concentration of charge carriers is closely related to the altering in oxidation states of ions. Dropping conductivity in reducing atmosphere and low electronic conductivity are great concerns for its application on anode for SOFCs, hence further investigation in cell testing needs to be carried out.