Fortalecimento e tensões residuais em selantes vitrocerâmicos para células a combustível de óxido sólido

Abstract

Glass-ceramics based on the system BaO/SrO-MgO-B2O3-SiO2 were studied. For the BaO-MgO-B2O3-SiO2 system, the variation in hardness and elastic modulus was measured after an initial heat treatment and after being subjected to thermal cycles of 750°C/5h. For heat treatments of 800°C/100h, the hardness does not change after thermal cycling. By increasing the temperature of the initial treatment to 850°C, the hardness decreases after thermal cycling. The elastic modulus remains constant. The fracture toughness increases with heat treatments of up to 100h at 800°C. With increasing the time further at 800°C, it remains constant due to the crystallization of the glass-ceramic and is equal to 1.4 MPa.m1 / 2. Residual stresses were measured in Crofer22APU steel/glass joints produced by laser cladding. In glass, the stress measured by Vickers microindentation was -290 ± 40 MPa. The stresses in the steel measured by X-ray diffraction ranged from 210 to 355 MPa, with values close to its yield stress. Residual stresses measured in the electrolyte/anode joint by Raman spectroscopy were -480 ± 90 MPa. In the steel/7.5B (Ba) glass-ceramic / electrolyte /anode joints heat treated at 850 °C/10h and cycled, the stresses in the electrolyte were lower, from -150 to -270 MPa. For the SrO-MgO-B2O3-SiO2 system, the SrO / MgO ratio was kept constant and equals to 1.5 and the SiO2 concentration varied from 50 to 65 mol%. In another study, 5% and 10% Al2O3 (mol%) were added. The last composition studied was with the addition of 15% ZrO2 fibers (wt%). Dilatometry, DTA and sintering measurements using hot stage microscopy were performed. The best composition was Mg1.5-50(Sr). The compositions 10B(Sr), 10B(Sr)+5Al and 10B(Sr)+ZrO2 were mechanically characterized by measurements of hardness, elastic modulus, biaxial fracture strength, fracture toughness, residual stresses, adhesion and self-healing. X-ray diffraction indicated the crystallization of Sr2MgSi2O7 phase in composition 10B(Sr) when heat treated at 750°C up to 800h. This phase also appears at the beginning of the heat treatment for the composition 10B(Sr)+5Al. After 100h, the SrAl2Si2O8 phase also appears. In both compositions, analysis by EDS revealed the appearance of phases with composition (1-x)SrO.xMgO.SiO2 and Mg and Sr silicates.The fracture toughness of composition 10B(Sr) was slightly higher than that of composition 7,5B(Ba). In addition, the crystallization of composition 10B(Sr) is lower, making it more favorable for use as a sealant than composition 7.5B(Ba). Fracture strength tests at room temperature and 750°C indicated that the addition of ZrO2 fibers increased the mechanical resistance in relation to the composition 10B (Sr). The composition 10B(Sr)+5Al also showed a fracture resistance comparable to the samples 10B(Sr)+15ZrO2 after 800h of heat treatment. The best adhesion with Crofer22APU steel after heat treatment of 850°C/10h was that of composition 10B(Sr),being equal to 17 ± 2 J.m². Porosity affected the adhesion of other compositions.Compositions 10B(Sr)+15ZrO2 and 10B(Sr)+5Al showed better self-healing than composition 10B(Sr), being superior to the composition 10B(Sr)+5Al. The highest residual stress in the electrolyte in Crofer22APU/glass-ceramic/electrolyte/anode joints heat treated at 850°C/10h was composition 10B (Sr), of -620 MPa.