氧離子固態電解質為具有高氧離子導電率之離子導體，導電的機構主要為氧離子藉晶體中的氧空缺達到移動傳導的目的。鈣鈦礦型(perovskite type，ABO3)的氧化物，ABO3，具有穩定的晶體結構，而且藉由加入較低價數的陽離子來取代A-site或B-site的離子，可以有效地增加氧空缺(oxygen vacancy)的數目，進而提升氧離子導電率。在前人的研究中，La1-xSrxGa1-yMgyO3-δ (LSGM)是目前最具有發展潛力的固態電解質。LSGM的晶體結構穩定而且導電率高於傳統的釔安定化氧化鋯固態電解質。由於Ga2O3為一非常昂貴的金屬氧化物，不利於實際上的應用，因此本實驗以同樣具有perovskite結構且成本較低的LaAlO3來作研究，以探討不同的異價添加劑對LaAlO3氧離子導體晶體結構與導電率的影響。
在取代A-site陽離子方面，LaAlO3對添加Ca離子的固溶量為5mol%，對Sr離子則有高達20mol%的固溶量，而只有~1mol%的Ba離子可以完全固溶到LaAlO3中。在B-site的Al離子則是以Mg離子來取代之，LaAlO3對Mg離子的固溶量為3mol%。當同時取代A、B離子時，Ca、Sr、Ba離子的添加有助於提升Mg離子在LaAlO3中的固溶量；相對而言，Mg離子的加入則會對Ca離子的固溶有所幫助，但卻會降低LaAlO3對Sr和Ba離子的固溶量。
導電率方面，在系統中，添加雙價陽離子後，在系統所能固溶的範圍內，導電率會隨著添加劑含量的增加而上升，這是由於異價離子的添加，可以有效地增加氧空缺濃度，提高離子導電率，但不含氧空缺的第二相則會使得導電率下降。例如undoped- LaAlO3在900℃時，σ=0.58×10-3S/cm，而在Ca-doped系統中，La0.95Ca0.05AlO2.975的導電率則提升到σ=1.21×10-2 S/cm，當添加10mol%的Ca離子時即有第二相的生成而使得導電率下降，900℃時σ=7.5×10-3 S/cm。在Sr-doped系統中，La0.8Sr0.2AlO2.9在900℃時σ=1.07×10-2 S/cm，而當Sr離子添加量在30mol%時，因為第二相的出現而使得導電率下降，900℃時σ=9.8×10-3 S/cm。在Ba-doped系統中，LaAlO3添加5mol%Ba離子，在900℃時σ=6.8×10-3 S/cm，當LaAlO3添加15mol%Ba離子時，第二相的生成有明顯地增加而使得導電率下降，900℃時σ=6.8×10-3 S/cm。Mg-doped系統中，LaAl0.95Mg0.05O2.975在900℃時σ=3.3×10-3 S/cm。當LaAlO3添加10mol%Mg離子時，會使得第二相的生成有明顯地增加而造成導電率的下降，900℃時σ=2.9×10-3 S/cm。
添加劑對於氧離子傳導的活化能也有顯著的影響。在LaAlO3系統所能固溶的範圍內，活化能會隨著添加劑含量的增加而下降，主要是由於異價添加劑的加入會增加氧空缺濃度，而氧空缺有助於氧離子的傳導，使得活化能下降。例如，undoped- LaAlO3的活化能為126.46 kJ/mole，而La0.95Ca0.05AlO2.975、La0.95Sr0.05AlO2.975、La0.95Ba0.05AlO2.975、LaAl0.95Mg0.05O2.975的活化能分別為102.04 kJ/mole、99.26 kJ/mole、91.29 kJ/mole和95.84 kJ/mole，均較undoped- LaAlO3的活化能低。
因而在同一添加劑取代系統中，若陽離子的取代造成saddle-point的縮小，則活化能會有上升的現象。例如La0.9Ca0.1Al0.95Mg0.05O2.975 (rcrit=0.904Å，Ea=116.5kJ/mole)和La0.95Ca0.05Al0.9Mg0.1O2.975(rcrit=0.901Å Ea=127.1kJ/mole)所產生的氧空缺濃度相同，但前者的saddle-point的臨界半徑(rcrit)較大，因此活化能較低。
由SEM觀察，LaAlO3試片為一非常不緻密的燒結體，相對密度為62.17%，而異價添加劑的加入則可以明顯促進LaAlO3的燒結緻密化。例如La0.95Ca0.05AlO2.975的相對密度高達92.16%，而La0.9Sr0.1AlO2.95的相對密度也有74.43%。由於LaAlO3對Ba離子或Mg離子的固溶度非常低，因此的添加對燒結緻密化的影響不大，LaAlO3添加Ba離子或Mg離子的相對密度均約為65%左右。
添加劑對於LaAlO3的燒結行為，可由相對密度及顯微結構的結果得知，系統中氧離子的擴散速率是最慢，而位於A-site和B-site的離子移動速度較快。因此異價離子的添加可以提升氧空缺濃度，增加氧離子的擴散速率，並促進燒結的緻密化。

Abstract

Oxide ion solid electrolytes are materials which exhibit high oxide ion conductivity. In oxide ion conductors, the oxide ion usually migrates in the lattice via oxygen vacancies, and become the main charge carriers. Perovskite type oxides, ABO3, have stable crystal structures. Also, a large number of oxygen vacancies can be introduced into the lattice by the partial substitution of cation A or B with lower valence cations[1]. Thus,the oxide ion conductivity can be enhanced.
In the last decade , LaGaO3-based oxides have been extensively investigated. LaGaO3 doped with Sr and Mg ions is the most potential material as an electrolyte for SOFCs. However, the high cost of Ga2O3 prohibits its practical applications. For the cost-effective point of view, in this study, much inexpensive perovskite-type oxides, LaAlO3 was selected. The objective of this study was to investigate the effect of dopants on the crystal structure and conductivity of LaAlO3.
For replacement of A-site cations in LaAlO3, the solubility of Ca ion into the La cation sublattice was found to be 5 mol% and the solubility of Sr ion could reach 20 mol%. However, only less than 1 mol% Ba ion could be dissolved . In this study, Mg ion was substituted for Al ion in the B-site. The solubility of Mg ion was about 3 mol%. With double doping for A and B parent cations, addition of Ca, Sr, Ba ion tend to enhance the solubility of Mg ion from 3mol% to 10 mol%. Although the addition of Mg ion could increase the solubility of Ca ion, the solubility of Sr and Ba ion was suppressed.
The concentration of oxygen vacancy increases with the increasing amount of divalent cation dopants, within their solubility limits. However, the second phases were found to be stoichiometric compounds without oxygen defects. Thus, the conductivity of second phases was significantly lowered.
The dopants also affect the activation energy of ionic conduction. In the perovskite systems, the "saddle point" is used to describe the critical size (rcrit) which is allowed for oxygen transport in the direction perpendicular to (110) plane. As expected, the decrease of rcrit at the saddle point would increase the activation energy (Ea) of the ionic conduction.
The microstructure of undoped and doped LaAlO3 samples were examined using SEM.
The undoped LaAlO3 shows very porous microstructure with a relativity density of 62.17%. The divalent cation-doped LaAlO3 samples, on the other hand, exhibit much denser microstructure. For example, the 5mole% Ca-doped LaAlO3 has a relative density as high as 92.16%.
It is known that the sintering kinetics is controlled by the diffusion of ion with the lowest diffusivity. Thus, it is believed that the densification enhancement of doped LaAlO3 is caused by the increasing oxygen ion diffusivity.Consequently, the diffusivity of oxygen ions is the lowest among the ions in LaAlO3.

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