The effects of composition on the Thermal, Mechanical and Electrical Properties of Alumino-borosilicate Sealing Glasses for Solid Oxide Fuel Cell (SOFC) Applications

Abstract
Structural integrity and reliability of sealing materials for planar type solid oxide fuel cells (pSOFCs) is key to attaining the required functionality and subsequent commercialisation of such fuel cells. In this thesis a number of different series of alumino-borosilicate glasses containing alkaline earth modifiers, as well as ZnO and La2O3 are studied as potential sealant materials. The glass ceramics derived from these glasses were also studied. Vickers hardness indentation was used to assess the hardness and indentation fracture toughness of these glasses and acoustic measurements were used to determine their moduli. The results reveal a decrease in mechanical properties with modifier additions in all the series except for increasing La2O3 in xSi(20-x)La(Sr) with little variation of mechanical properties in the case of xB(15-x)Zn (10BaO-(15-x)ZnO-15La2O3-5Al2O3-(10+x)B2O3-45SiO2 (X= 2.5, 5, 7.5, 10)) and xSi(20-x)Zn (10BaO-(20-x)ZnO-15La2O3-5Al2O3-10B2O3-(40+x)SiO2 (X=2.5,5,7.5) mol%) hardness. Electrical conductivity of sealing glasses must be lower than 10-4 S cm-1 and or > 104Ω cm. Hence the electrical properties the electrical properties of these glasses were measured using impedance spectroscopy and the results indicated that all of the glass and glass ceramic samples studied are electrically resistive and show promise for use as sealing materials. Another important parameter is the thermal properties where the TEC of the sealing glass must be compatible with the other components because differences in TEC of sealing glasses and adjoining SOFC parts result in mismatch and induce thermal stresses during thermal cycling and this may generate cracks through which gas leakages occur. The TEC of xBa(10-x)Al series (10+x)BaO-5ZnO-20SrO-(10-x)Al2O3-20B2O3-35SiO2 (X= 0, 2, 3, 4, 5) and some of xBa(40-x)Si samples (15+x)BaO-5ZnO-15La2O3-5Al2O3-20B2O3-(40+x)SiO2 (X=2.5,5,7.5, 10) have fall within the requirement for sealing glasses. Apart from the TEC the Tg is also a determining factor as to the suitability or otherwise of a sealing glass to be a promising candidate due to the following reasons: (i) thermal stresses develop below the Tg where the glass is brittle therefore the Tg should be as low as possible; (ii) due to high temperature material degradation research efforts are on to reduce the operating temperatures of SOFCs to enhance materials service life and the opportunity for variety of materials selection to construct the SOFCs components. Heat treating series xB(15-x)Zn, xSi(20-x)Zn and xBa(40-x)Si lead to the formation of lanthanum borosilicate single phase. The evolution of these phase lead to not only increased in conductivity as mentioned above but also in the hardness as they are higher in the glass ceramics. However the TEC of the glass ceramics compared with the parent glasses were slightly lower and its reported in this study that this is a good sign of thermal stability as the TEC did not exhibit the possibility of continues increase.