An intimate Ba-Al-Al 2 O 3 -SiO 2 powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edgeto-edge width). Heat treatments conducted from 300°to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl 2 Si 2 O 8 . Barium oxidation was completed, and a binary silicate compound, J ournal 3109 Prior work has revealed that novel reaction paths to Ba-CeO 3 , 62 BaFe 12 O 19 , 66 BaTiO 3 , 67,68 and YBa 2 Cu 3 O y 64 can be accessed by the oxidation of solid, alkaline-earth-metal-bearing precursors. The purpose of the present paper is to identify the reaction path(s) to celsian from a Ba-Al-Al 2 O 3 -SiO 2 precursor. Knowledge of the reaction path to celsian is considered an important first step in the processing of celsian-based composites. Before discussing the results of such work, prior analyses of phase evolution to celsian from mixed salt and metallic precursors is reviewed.
II. Reaction Paths to Celsian
(1) Mixed Salt PrecursorsSeveral authors have examined the formation of celsian from powder mixtures containing BaCO 3 and kaolin. [39][40][41][42][43]49 Heating such precursors at 6°-10°C/min in the temperature range ∼400°-665°C results in appreciable weight loss because of the dehydroxylation of kaolinite. 40,42,49 Weight-loss measurements also have indicated the onset of BaCO 3 decomposition above ∼600°C. 40,42,49 Further annealing in the temperature range 650°-800°C has yielded an amorphous oxide. 2,[41][42][43]49 Hexacelsian had been reported to form during annealing at Ն825°C, whereas the onset of monoclinic celsian formation has required further heat treatment at ∼1130°-1200°C. 2,39,[41][42][43]49 Based on infrared spectroscopy and X-ray diffraction (XRD) analyses, Planz and Muller-Hesse 39 have reported that the loss of H 2 O from kaolin leads to the formation of the metakaolin structure described by Brindley and Nakahira, 81 in which a layer of corner-sharing and edge-sharing AlO 4 tetrahedra is joined to a layer of SiO 4 tetrahedra. This metakaolin precursor is believed to yield an ordered form of hexacelsian with a layer of cornersharing AlO 4 tetrahedra joined to a layer of SiO 4 tetrahedra (as opposed to a -hexacelsian structure with aluminum and silicon distributed in a more random fashion within a double layer of corner-sharing tetrahedra 33 ).