Influence of Ru doping on the conductivity of LaCrO3 ceramic prepared by SPS and the feasibility of the doped ceramic for an inert anode of molten salt electrolysis
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摘要: 鉻酸鑭（LaCrO3）陶瓷材料在高溫熱電和固體氧化物燃料電池（SOFC）等領域具有廣泛的應用價值，然而其燒結性能差、導電率低等不足卻限制了LaCrO3陶瓷的高性能應用。針對上述問題，采用放電等離子燒結（SPS）方式制備致密的LaCrO3塊體。同時，通過A位摻雜Ru元素，以期實現高電導率的摻雜態鉻酸鑭（La1?xRuxCrO3）致密陶瓷。所得樣品的X射線衍射（XRD）及掃描電子顯微（SEM）分析結果表明，無論A位Ru元素含量多少（x=0～0.25， x為Ru的原子含量），SPS所得樣品均為單相鈣鈦礦結構，且具有較高的致密度。此外，高溫電導率測試結果顯示，摻雜態La1?xRuxCrO3的電導率隨著溫度和Ru摻雜量的增加而增加。同時，摻雜前后La1?xRuxCrO3導電性均滿足Arrhenius公式，且摻雜態La1?xRuxCrO3陶瓷的電導活化能明顯低于未摻雜的LaCrO3陶瓷。隨后，將La1?xRuxCrO3置于800 °C熔融CaCl2熔體中，研究其作為熔鹽電解用惰性陽極材料的可行性。結果顯示，摻雜態La1?xRuxCrO3具有較高的抗熔鹽化學腐蝕性，然而其抗熱振性較差，電解之后出現明顯的表層機械脫落現象。上述結果表明，摻雜態La1?xRuxCrO3具備作為惰性析氧陽極材料的化學穩定性，然而需要進一步提高其熱穩定性才能適用于熔鹽電解用惰性陽極。Abstract: LaCrO3 ceramic is a promising function material in areas such as high temperature piezoelectric materials and solid oxide fuel cells (SOFC). However, its practical applications are limited by fatal flaws including their low density and poor conductivity. To address these challenges, spark plasma sintering (SPS) was used to prepare the high-density LaCrO3 ceramic. Additionally, Ru, a multivalent metallic element, was doped in the A site of the LaCrO3 ceramic to investigate the conductivity of the La1?xRuxCrO3 (x=0?0.25). X-ray power diffraction (XRD) results and scanning electron microscope images show that the sintered La1?xRuxCrO3 ceramic has a single perovskite phase and high density. The characteristic peak shifting observed in the XRD pattern indicates that the Ru element has been successfully doped in the A site of the LaCrO3 ceramic. Whereas, the results of the Energy dispersive spectrometer (EDS) prove that there is no obvious change in the Ru content before and after sintering by SPS, which indicates that no actual Ru loss can occur during the SPS process at 1600 °C. Moreover, the conductivity of the sintered La1?xRuxCrO3 increases with increasing Ru content and temperature. The results also indicate that there is good linear relationship between ln(σT) and 1/T, demonstrating that the conductivity of the La1?xRuxCrO3 obeys the Arrhenius law. The activation energy of the doped La1?xRuxCrO3 ceramic is smaller than that of the LaCrO3 ceramic. Lastly, the feasibility of the application of doped La1?xRuxCrO3 ceramics as the inert anode of molten salt electrolysis in CaCl2 melt has been investigated at the temperature of 800 °C. These findings demonstrate that the doped La1?xRuxCrO3 ceramic has an excellent chemical corrosion-resistant property. However, it has poor thermal stability, which inhibits its application as an inert anode. Future studies focusing on the improvement of the heat-shock resistance and elucidating the corrosion resistance mechanism of La1?xRuxCrO3 in CaCl2 melt is recommended.
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Key words:
- Ru doping /
- LaCrO3 ceramic /
- conductivity /
- molten salt electrolysis /
- inert anode
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圖 1 摻雜態La1?xRuxCrO3陶瓷制備示意圖
Figure 1. Schematic illustration of the preparation of doped La1?xRuxCrO3 ceramic
圖 2 SPS制備過程中軸向壓縮形變量和燒結溫度隨時間的變化關系
Figure 2. Changing law of axial direction shrinkage with temperature during the SPS
圖 4 SPS制備所得致密陶瓷SEM 圖譜及Ru元素EDS分析結果。（a）Ru摩爾分數為0的樣品的SEM形貌；（b）不同Ru含量樣品所得EDS結果；（c）Ru摩爾分數為0.05的樣品的SEM（上）及對應的EDS（下）面掃描結果；（d）Ru摩爾分數為0.10的樣品的SEM（上）及對應的EDS（下）面掃描結果；（e）Ru摩爾分數為0.15的樣品的SEM（上）及對應的EDS（下）面掃描結果；（f）Ru摩爾分數為0.20的樣品的SEM（上）及對應的EDS（下）面掃描結果；（g）Ru摩爾分數為0.25的樣品的SEM（上）及對應的EDS（下）面掃描結果
Figure 4. SEM images of the ceramics with different Ru contents and EDS mapping of Ru element: (a) SEM image of the SEM without Ru; (b) EDS results of the sample with different Ru contents; (c) SEM (above) and EDS mapping (below) of the sample with Ru of 0.05 mole fraction; (d) SEM (above) and EDS mapping (below) of the sample with Ru of 0.10 mole fraction; (e) SEM (above) and EDS mapping (below) of the sample with Ru of 0.15 mole fraction; (f) SEM (above) and EDS mapping (below) of the sample with Ru of 0.20 mole fraction; (g) SEM (above) and EDS mapping (below) of the sample with Ru of 0.25 mole fraction
圖 5 樣品性能表征。（a）UV測試試樣禁帶寬度變化；（b）四探針測試用試樣（表面涂布導電銀漿并連接了鉑絲）光學照片；（c和d）試樣電導率隨溫度的變化
Figure 5. Performance of the samples: (a) UV results of the samples; (b) digital photos of the samples coating with silver paste and platinum wire; (c and d) conductivity of the samples changing with temperature
圖 6 La0.85Ru0.15CrO3陶瓷浸入CaCl2熔體中保溫72 h前（a）后（b）的光學照片和SEM圖譜
Figure 6. Photos and SEM images of the La0.85Ru0.15CrO3 ceramic before (a) and after (b) immersed in CaCl2 melt for 72 h
圖 7 （a）電解過程示意圖；（b）La0.85Ru0.15CrO3陶瓷在CaCl2熔體中，2.8 V下電解48 h前（左）、后（右）光學照片
Figure 7. (a) Schematic illustration of the cell; (b) digital photos of the La0.85Ru0.15CrO3 ceramic before and after electrolysis for 48 h at 2.8 V
表 1 摻雜態La1?xRuxCrO3陶瓷電導活化能
Table 1. Activation energy of the doped La1?xRuxCrO3 ceramic
Sample LaCrO3 La0.95Ru0.05CrO3 La0.90Ru0.10CrO3 La0.85Ru0.15CrO3 La0.80Ru0.20CrO3 La0.75Ru0.25CrO3 Activation energy/eV 0.58 0.13 0.14 0.14 0.13 0.14 
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