Synthesis and characterization of ceramic composites of the binary system Ba 0.75 Sr 0.25 AlSi 2 O 8 - Al 2 O 3
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Synthesis and characterization of ceramic composites of the binary system Ba0.75Sr0.25AlSi2O8 - Al2O3 Jorge López-Cuevas, Magaly V. Ramos-Ramírez and José L. Rodríguez-Galicia CINVESTAV-IPN, Unidad Saltillo, Carretera Saltillo Monterrey, Km. 13.5, C.P. 25900, Ramos Arizpe, Coahuila, México. ABSTRACT Ba0.75Sr0.25AlSi2O8 (SBAS) - Al2O3 composites, with SBAS/Al2O3 weight ratios of: (a) 90/10, (b) 70/30, and (c) 50/50, are in situ synthesized by reactive sintering at 900-1500°C/5h. The effect of mechanical activation of the precursor mixtures for 0, 4 or 8h in an attrition milling device on the microstructure and phase composition of the composites is studied. Only SBAS and Al2O3 phases are obtained at 1300-1500°C, independently of milling time. In general, the relative proportion of the desirable monoclinic SBAS (Celsian) phase increases in the materials with increasing milling time and sintering temperature, which is enhanced by their SrO content. The promotion of surface nucleation of the undesirable hexagonal SBAS (Hexacelsian) phase by mechanical activation results in a maximum Hexacelsian to Celsian conversion fraction of only 81.4%, obtained for composition 2 milled for 8h and sintered at 1500°C/5h. Under these synthesis conditions, an increment in the amount and size of the Al2O3 particles in the composites is detrimental for the Hexacelsian to Celsian conversion. INTRODUCTION Monoclinic Ba1-xSrxAl2Si2O8 (SBAS) solid solutions are interesting as matrixes for ceramic composites for structural applications at high temperatures, due to their high refractoriness, low coefficient of thermal expansion (CET), good resistance to oxidation and to slag attack, as well as good thermal shock resistance [1,2]. However, hexagonal SBAS tends to appear prior to monoclinic SBAS, frequently remaining in a metastable state at low temperatures. Hexagonal SBAS is undesirable due to its relatively high CET and because it transforms into an orthorhombic form during cooling, which causes differential volume changes that lead to microcracking and weakening of the material [1-3]. The formation of SBAS, and its hexagonal to monoclinic conversion, can be promoted by partial substitution of BaO by SrO in BaAl2Si2O8 (BAS) [1,2]. Mixtures of BAS and SrAl2Si2O8 (SAS) could also be used for this [4]. Due to their similar properties, when both BAS and SBAS have monoclinic structure, they are indistinctly denominated as Celsian (or Monocelsian); similarly, the corresponding hexagonal forms are both known as Hexacelsian. The formation of SBAS obeys Vegard’s law and it can occur over the entire concentration range (0 x 1) [2,5]. D. Long-González et al. [2] recommend using 0.25 x 0.375 in order to obtain Celsian with optimum properties. There are reasons to believe that SBAS and Al2O3 are chemically compatible, in such a way that they are able to coexist at equilibrium in a wide range of temperatures [6-8]. However, this has not been studied in detail so far. In this work, we study the effect of mechanical activation of the precursor mixture
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