3.9YSZ粉末粒度對大氣電漿噴塗固態氧化物燃料電池電解質的影響及燒結對其微結構及導電率改進之研究

吳漢平

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3.9YSZ粉末粒度對大氣電漿噴塗固態氧化物燃料電池電解質的影響及燒結對其微結構及導電率改進之研究

Effects of Feedstock Powder Sizes on Atmospheric Plasma-Sprayed Solid Oxide Fuel Cell’s 3.9YSZ Electrolyte and Its Improvement by Sintering on Microstructure and Electrical Conductivities

吳漢平 , Ph.D　　Advisor：方冠榮　　

繁體中文

氧化釔安定氧化鋯；大氣電漿噴塗；固態氧化物燃料電池； Yttria-stabilized zirconia ； Atmospheric plasma spraying ； Solid oxide fuel cell

Abstract │ Reference (161) │ Altmetrics

Abstract 〈TOP〉

氟化鈣螢石結構的8 mol.%氧化釔完全安定立方相的氧化鋯，其做中溫固態氧化物燃料電池之電解質，是一良好的離子導體，但其機械強度則較差。使用含較少安定劑的部份安定氧化鋯(Partially-stabilized zirconia, PSZ)，其相較於8 mol.%氧化釔完全安定的立方相氧化鋯之離子的導電性則較差，但其電解質的機械強度、破斷靭性及耐熱衝擊性則會提高，其可製作較薄的電解質以降低其之阻抗，以減少整個燃料電池的歐姆損失。

本研究探討粒度範圍在20-45、45-75、75-106 μm等三種不同大小之3.9 mol.%氧化釔安定氧化鋯(3.9YSZ)粉末，以大氣電漿噴塗方式製作固態氧化物燃料電池的電解質。大氣電漿噴塗是以氬氣流量45 L/min為主要電漿噴塗氣體，再加入三種不同流量(2, 7, 12 L/min) 輔助性的高熱焓及高熱導的氫氣做大氣電漿噴塗氣體，以研究高熱焓及高熱導性噴塗氣體，對大氣電漿噴塗製作3.9YSZ堆積膜的影響。由於大氣電漿噴塗3.9YSZ試片的導電性較差，故本研究亦探討在1300-1400℃的燒結對其微結構、物性、相變化及導電性上等的影響。

本研究發現，當大氣電漿噴塗氣體裡的氫氣含量增加時，較高氫氣含量可提供較高的熱焓及較佳熱導性電漿氣體，其可增加粉末顆粒的熔融程度及提高融熔液滴的溫度以降低其黏滯性，以使大氣電漿噴塗的薄片狀結構在熔融液滴凝固前較容易向外延伸，使薄片狀結構較薄、較平順且有較大的面積，堆積膜的微結構則會有較多的柱狀晶成長並且較為緻密，材料含有較少的單斜晶相以使堆積膜有較高的導電性。在本研究的實驗結果顯示粒度為45-75 μm 的3.9YSZ粉末，在氬氣流量45 L/min及氫氣流量12 L/min的大氣電漿噴塗製作之電解質有最佳導電率，在800°C其約為2,860 (μS•cm-1)，其離子導電機構有相當低的分解活化能(0.21 eV)並有相當低的遷移活化能(0.89 eV)。

大氣電漿噴塗使用原料粉末粒度大小與堆積膜的表面粗糙度、硬度、體密度及視孔隙率等相關，當3.9YSZ粉末粒度愈大時，其大氣電漿噴塗的堆積膜表面也愈粗糙，若3.9YSZ粉末粒度較大在75-106 μm範圍，因其在電漿噴塗過程中並未使粉末顆粒完全熔融，及當3.9YSZ粉末粒度較小在20-45 μm範圍，因其在電漿噴塗過程中粉末顆粒被熔融且過度加熱，使這二種範圍的粉末粒度相對於粉末粒度範圍在45-75 μm，其堆積膜的硬度都會顯著的較低。當3.9YSZ原料粉末粒度為45-75 μm噴塗的堆積膜，不論輔助性的氫氣氣體流量大小(2, 7, 12 L/min)，其噴塗堆積膜體密度相對於其它二種粉末都較高，且視孔隙率相對於其它二種粉末粒度都較低。

大氣電漿噴塗3.9YSZ堆積膜，在1300及1400℃燒結2小時後，其堆積膜的體密度及視比重會下降，且其視孔隙率亦會下降，這應是由於部份的空孔在燒結中被封孔及部份 相轉成單斜相所致，但在延長燒結時間為10小時後，其體密度及視比重會上升至與原噴塗的堆積膜體密度相當，而試片的視孔隙率仍會維持比原噴塗的堆積膜低約5%。在1300及1400℃燒結10小時後電解質的硬度會顯著的增加約4-23 HR15YW左右，但表面粗糙度則改變不大，其柱狀晶及胞狀晶的微結構組織及薄片狀結構內及薄片狀結構間的微裂都仍存在燒結後的3.9YSZ電解質試片內。

本研究的三種3.9YSZ粉末粒度，在電漿噴塗氣體裡添加輔助性的氫氣愈多，其所製作的電解質的導電率愈佳。但在燒結10小時後，則以粉末粒度為45-75 μm及使用氫氣含量7 L/min噴塗的試片，在1400℃燒結的最佳導電率約為11,584 (μS•cm-1)，其之離子導電機構具有相當低的分解活化能(0.01 eV) ，但其遷移活化能(1.06 eV)則相當高，表示釔離子與氧孔缺的分解在導電機構的控制上是其最重要部份。在高的溫度(1300/1400℃)燒結10小時後，其堆積膜的晶粒在高溫(700-800℃)較原大氣電漿噴塗的3.9YSZ堆積膜的晶粒有較低的活化能，且燒結溫度愈高其活化能愈低亦即其晶粒有更多的氧孔缺。在低溫(500-700℃)則較原大氣電漿噴塗的3.9YSZ堆積膜的晶粒有較高的活化能，且燒結溫度愈高其活化能愈高亦即晶粒有更少的氧孔缺。其堆積膜的晶粒內的氧空缺複合體的整體平均分解(或結合)活化能較原大氣電漿噴塗的3.9YSZ堆積膜降低，但其隨著不同的大氣電漿噴塗條件及粉末粒度的不同而有很大的變異。

Parallel Abstract 〈TOP〉

The cubic fluorite structure of the 8 mol.% fully yttria-stabilized zirconia (8YSZ) is a good ionic conductor for the electrolyte of the intermeidiate-temperature solid oxide fuel cells (SOFCs). However, its mechanical properties are poor. Partially-stabilized zirconia (PSZ) with less phase stabilizer has lower electrical conductivities when compared with 8YSZ; however, its mechanical strength, fracture toughness, and thermal shock resistance are better than 8YSZ’s. Consequently it could be fabricated in a thinner form for the SOFC’s electrolyte to lower its electrical resistance to be able to lessen the whole cell’s ohm’s losses during operation.
In this study, 3.9 mol.% yttria-stabilized zirconia (3.9YSZ) with feedstock powder particle sizes of 20-45、45-75、75-106 μm were sprayed by atmospheric plasma to fabricate 3.9YSZ solid electrolyte to investigate their microstructural, physical and electrical characteristics. The primary plasma spraying gas is argon gas at the flow rate of 45 L/min and the auxiliary high enthalpy-enhancing plasma gas is the hydrogen gas at the flow rates of 2, 7, 12 L/min. 
It is found that when increasing the flow rates of the hydrogen gas in the plasma spraying gas, it could enhance the melting extent of the feedstock powder and raise the temperatures of the molten droplets to lower their viscosity to let the deposited splats expand wider prior to solidification. It could make the deposits have more columnar grains, better physical characteristics and higher electrical conductivities. In this study, the deposit of the best electrical conductivity, 2,860 (μS•cm-1), was sprayed by the plasma spraying gas consisting of argon gas flow rate at 45 L/min and hydrogen gas flow rate at 12 L/min with the feedstock powder sizes 45-75 μm. Its electrical conductivity mechanism has relatively low dissociative activation energy (0.21 eV) and migratory activation energy (0.89 eV). 
The characteristics of the roughness of the deposit surfaces, the hardness, the bulk density and the apparent porosity were closely related with the feedstock powder sizes. The larger the feedstock powder was, the coarser the deposit surface appeared. The decrease of the hardness of the deposits was due to the incompletely melting of the coarser particles and the overheating of the smaller particles. When the particle sizes of 45-75 μm were employed, all the deposits had relatively lower apparent porosity and higher bulk density compared with the other two feedstock powder sizes. After spraying, most of the monoclinic phase originally contained in the raw feedstock powders were suppressed to be transformed into tetragonal (T) , cubic (F) phases and/or nontransformable tetragonal phase ( ).
The sintering of atmospheric plasma sprayed 3.9YSZ deposits at 1300 and 1400°C for two hours would cause the deposits’ bulk densities and apparent specific gravities to be lowered. Nevertheless, the apparent porosities were reduced too. It should be caused by the sealing of some open porosity and the phase transformation of the nontransformable tetragonal phase ( ) into monoclinic phase. However, when the sintering time was extended into ten hours, the bulk densities and apparent specific gravities would rise to the same levels of the as-sprayed deposits, while the apparent porosities were still remained lower than the as-sprayed deposits for about 5%. The hardness of the sintered deposits increased about 4-23 HR15YW but the roughness were not affected much. The columnar and cellular grains still existed after sintering and the porosity and the microcracks among neighboring splats and within the splats still existed after being sintered. 
For all three different particle sizes, the more the hydrogen gas flow rate was, the higher electrical conductivity the deposit had. However, after being sintered, the best ionic conductivity, 11,584 (μS•cm-1), in this study was obtained at the deposit prepared by the particle sizes of 45-75 μm which was sprayed at the hydrogen gas flow rate at 7 L/min and sintered at 1400℃ for ten hours. Its mechanism of the electrical conductivity has relatively low dissociative activation energy (0.01 eV) but with quite high migratory activation energy (1.06 eV). It represents that the dissociation of the yttrium ions and oxygen vacancy is the main electrical conductivity control mechanism. At high temperatures (700-800 °C), the intragrain activation energy of the deposit decreased with increasing sintering temperature. In contrast, at low temperatures (500-700 °C), the intragrain activation energy of the deposit increased with increasing sintering temperature. The average dissociative/combinative activation energies decreased with increasing sintering temperature. However, there was great variation with spraying parameters and feedstock powder size.

Reference ( 161 ) 〈TOP〉

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