Monoclinic celsian

Celsian. One of the compounds important for ceramic materials science is monoclinic celsian, BaAljSijO, a compound with a feldspar structure. Flowever, its formation during solid phase synthesis occurs in several stages with the formation of a very stable hexagonal modification of BaAljSijO as an intermediate phase. Its subsequent transition into thermodynamically stable monoclinic celsian is hindered and requires calcination at elevated temperatures ( 1600°C) either the hydrothermal treatment, or the use of mineralizers [12-14]. 

KF) is practically completed at 1100°C within 2 h, while substantially higher temperatures and treatment duration are necessary for the formation of monoclinic celsian in the case when a usual synthesis procedure is used. 

Hexagonal celsian can be transformed into monoclinic modification by mechanical activation of the mixture with modifiers (KF, LiCl at a level of 1-5 mol. %) followed by annealing at 1100°C. 

FIGURE 1. Linear thermal expansion ofhexacelsian and monoclinic celsian BAS. (Reprinted, from reference 12, with kind permission of Elsevier). 

On heat treatment, both hexacelsian and monoclinic celsian phases crystallize in BAS glass [23-25]. Also, hot pressing of barium aluminosilicate (BAS) glass or its composites reinforced with large diameter silicon carbide SCS-6 monofilaments or small diameter multifilament Nicalon or Hi-Nicalon fibers resulted in the crystallization of both hexacelsian andmonoclinic celsian phases [12]. On doping BAS with 5 wt.% monoclinic celsian seeds or 10 wt.% strontium aluminosilicate (SAS), only the celsian phase was formed in hot pressed... 

X-ray diffraction (Fig. 5) from the surface ofahot pressed composite panel [27] showed only the desired monoclinic celsian. This indicates that the mixed oxide precursor is converted in situ into the monoclinic celsian phase during hot pressing ofthe FRC. The undesired hexacelsian phase was not detected from XRD. However, a small amount of hexacelsian was detected by Raman micro-spectroscopy [28]. The hot pressed composite panel was surface polished and sliced into test bars for mechanical testing. 

N. P. Bansal, Solid State Synthesis and Properties of Monoclinic Celsian, J. Mater. Sci, 33 [19] 4711-4715... 

Figure 1. Process flowchart for the production of monoclinic barium aluminosilicate (celsian).

M. Chen, W.E. Lee and P.F. James, Synthesis of Monoclinic Celsian Glass-Ceramic from Alkoxides, J. Non-Cryst. Solids, 147,532 (1992). 

These include celsian, Ba [Al2Si20g], and hyalophane, (K, Na, BaJ [Alj., Sig- Og], both of which are structurally very similar to the potassium feldspars. In celsian, which is truly monoclinic, the A1 and Si are ordered into alternate tetrahedral sites (Newnham and Megaw [I960]). 

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