Microstructure and properties of composites in the La2O3-Al2O3 system

Abstract

666o762~Ii+546,654 Interest in the study of the properties and structure of composites containing rareearth elements (REE) has increased particularly in the last two decades when these materials became more accessible and it became possible to make practical use of them. Oxides of REE and their compounds are materials of a high level of refractoriness and low volatility and have other properties which are necessary for use in high-temperature engineering.The system La203--AI203 is interesting for the formation of two high-temperature compounds: La20~'Al20s with a perovskite structure with N = 2.030, N = 2.025, and a pycnometric density of 5.84 g/cm 3 and La203.11AI=03 belonging to t~e 8-alumlnaPgroup and crystallizing in the form of hexagonal plates with No = 1o780, N c = 1.768 and a pycnometric density of 4.03 g/cm 3 (Fig~ 1) [i].We have studied the synthesis of lanthanum aluminates, their properties~ and the formation of the structure of other composites of the system.From the Curie--Wolf conditions, the minimum free energy is found on the plane face of the crystal and the grain has an hexagonal shape. The pore formed in the crystalline material under equZlibrium conditions also has a hexagonal shape ("negative hexagonal crystal"). Knowing that La2Os.llAl=Os crystallized in the form of hexagonal plates, we attempted to obtain a composite structure close to the equilibrium one, i.e., stable in use up to 1800~ (the temperature at which the melt appears in the system is 1830~We studied the compositions crystallizing in the fields of the primary crystallization of La2Os, La=Os.llAl2Os, and also a composition corresponding to the compound La203"AI203.Unlike the studies i, 2 based on pure materials, in our study we used finely milled electrosmelted corundum (EB) and a concentrate of lanthanum oxide with 98% of La=03. As the binder additive we used a 52 solution of polyvlnyl alcohol.It was established that composition which crystallized in the primary crystallization field of La20~ requires stabilization to prevent hydration damage. This is done by adding REE cations such as Ce 4+, Tb 3+, Nd 3+, and also Ca 2+, and A1 s+, with a high-temperature anneal at 1700~ Good results were obtained when 5% CeO= or CaF2 was added. The specimens with these additives and fired at 1500~ were stable for 6-10 days.Composites of the compositions 2-4 were extremely stable in relation to water after firing at 1700~Lanthanum concentrate and white electro-fired corundum are completely converted to lanthanum aluminate at 1700~ and lines of only LaAIO3 are observed on the x-ray patterns (Fig. 2a). The structure of this composite is isogranular and almost totally composed of crystals of a regular hexagonal form. The shape of the pores is also close to the equilibrium shape (Fig. 3). The properties of specimens of composition 2 are as follows: open porosity, 15,5%; apparent density. 5.06 g/cm3; and ultimate compressive strength, 84.1 MPa. During the isothermal dwell of 1.5-7.5 h at 1700~ these properties remain stable, confirming the fact ...