SiO[sub 2]-Tolerant Grain-Boundary Conduction in Sr- and Mg-Doped Lanthanum Gallate

Abstract

The effect of SiO 2 impurity on the grain-boundary conduction in Sr-and Mg-doped lanthanum gallate ͑La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3−␦ , LSGM͒ was studied. The grain-boundary conduction of LSGM was slightly affected, even by the addition of 500-2000 ppm SiO 2 , while the apparent grain-boundary resistivity was increased up to ϳ780 times in gadolinia-doped ceria by the doping of 500-2000 ppm SiO 2 . The high tolerance of the grain-boundary conduction against siliceous impurity in the LSGM specimen was explained by the gathering of the acidic siliceous phase near the basic SrO component.The grain boundary of stabilized zirconia is typically 10 2 -10 4 times more resistive than the grain interior. 1,2 The depletion of oxygen vacancies near the grain boundary is dominant in highly pure specimens. 1,3 SiO 2 impurities even at low concentrations of hundreds of ppm deteriorate the grain-boundary conduction to a great extent. 4-6 Siliceous impurity is a ubiquitous element that can easily be incorporated during ceramic processing, sintering, and fuel cell operation. 7 Accordingly, the negative role of the siliceous intergranular phase cannot be excluded even in relatively pure materials.The operation of solid oxide fuel cells ͑SOFCs͒ at low temperature ͑LT͒ or intermediate temperature ͑IT͒ ͑400-700°C͒ is advantageous from the viewpoint of the operational stability, the quick startup, and the use of cost-effective gas sealing and interconnecting materials. 8-10 Gadolinia-doped ceria ͑GDC͒ and Sr-and Mg-doped lanthanum gallate ͑La 1−x Sr x Ga 1−y Mg y O 3−␦ , LSGM͒ are the two most representative electrolytes for LT-and IT-SOFCs on account of their high ionic conductivities. 7,11 The extrinsic deterioration of the grain-boundary conduction by siliceous contamination becomes more significant in the LT and IT regimes.It has been reported that the role of siliceous impurity in the grain-boundary conduction of GDC is analogous to that in stabilized zirconia. 12-16 Gerhardt and Nowick 12 reported that a 6 mol % Gd 2 O 3 -doped CeO 2 specimen containing 1000 ppm of SiO 2 showed a significantly larger grain-boundary arc in the impedance spectra than the same specimen with Ͻ10 ppm of SiO 2 . Zhang et al. 14 and Jasinski et al. 16 reported that the grain-boundary resistivity of a Gd 0.2 Ce 0.8 O 1.9 specimen was increased 6-30 times by the addition of 200-3000 ppm SiO 2 . Lane et al. 17 suggested that the addition of CaO or SrO improves the grain-boundary conduction of GDC containing SiO 2 impurity. The authors 18-20 further investigated the mechanism underlying the enhancement of the grain-boundary conduction upon doping with CaO and SrO. In addition, the authors 21,22 suggested MgO and BaO as other promising additives to mitigate the effect of the harmful siliceous intergranular phase.LSGM contains Sr and Mg elements that might play the role of scavenger materials for the siliceous impurity in the GDC system. Thus, the effect of the SiO 2 impurity on the grain-boundary conduction in LSGM is expected to be different from that in GDC...

Low-temperature sintering and electrical properties of strontium- and magnesium-doped lanthanum gallate with V2O5 additive

“…Previously, it was reported [51] that the grain-boundary conduction of LSGM is barely affected by the addition of even 2000 ppm SiO 2 to LSGM, while that of GDC is significantly deteriorated. The siliceous phase is scavenged near the Sr-rich phase rather than the Mg-rich one, which is attributed to the higher chemical affinity between SrO and the acidic siliceous intergranular phase as a result of the greater basicity, according to cationic field strength, of SrO compared with MgO [52].…”

Effect of oxide additives on the sintering behavior and electrical properties of strontium- and magnesium-doped lanthanum gallate

“…By contrast, the ρ gb app values of the LSGM-Si specimens in the present study were slightly smaller than those of the LSGM specimens. In the previous contribution, the present authors 57 reported that the grain-boundary conduction of LSGM sintered at 1400 • C was hardly affected even by the addition of 500-2000 ppm of SiO 2 impurities. This was attributed to the gathering of the acidic siliceous phase near the more basic SrO component among the two basic components in LSGM: SrO and MgO.…”

Effects of La0.9Sr0.1Ga0.9Mg0.1O3−δon the microstructure and ionic conductivity of purity/impure Ce0.8Nd0.2O1.9electrolytes

“…2f shows that the concentrations of Si and Sr are higher than those of Mg and Ce, the line shows that Si accumulates at the grain boundaries of the LSGM-enriched phase, where its higher chemical affinity for SrO (vs. MgO) increases the Si concentration (Spot1). 33 These results also suggest that no Srcontaining or Mg 2 SiO 4 phases form at the grain boundaries with this composition. 14,16 This is presumably because the MgO and SrO content of LSGM is too low, and because the mechanism governing A/B (SrO/MgO) substitution in LaGaO 3 (ABO 3 ), is different from the one report for SrO/MgO doped CeO 2 -based electrolytes, 17,18 in which these phases have been shown to improve the grain boundary conductivity.…”