Celsian

Many barium aluminosilicate-based compositions will eventually react with the chromium oxide or aluminum oxide scales on the metal interconnect or metal edge rails to form barium chromate or a celsian phase at the interface [6], This can cause a mechanical weakness that is easily delaminated. Also, compositions that contain boron can react over time with water (steam) to produce B2(OH)2 or B(OH)3 gas. This can decompose the glass and greatly limit the lifetime of the seal. Thus many of the new investigations have emphasized low or no boron glass compositions. 

Ba 100-1000 ppm Forms pure component BaAl2Si208 (celsian cf. table 5.63) positively correlated with K. Exceptionally high values observed in Mn deposit, where celsian occurs as main component. 

Table 5.63 lists the structural characters of the various feldspar polymorphs, according to Smyth and Bish (1988). Note duplication of cell edge c in anorthite and celsian and the progressive increase in volume that accompanies the lowering of symmetry in KAlSi30g. 

Name High Albite Low Albite Anorthite Celsian... 

Newnham R. E. and Megaw H. D. (1960). The crystal structure of celsian (barium feldspar). Acta Cryst., 13 303-312. 

In addition to the conventional Synroc-C, other Synroc formulations were designed. The first Synroc variety, Synroc-A, contained the silicates kalsilite, leucite and celsian, and was... 

The anions in figure 7.12 occur, for example, in the minerals albite Na[AlSi308], celsian Ba[Al2Si208] and the group of the feldspars. 

Referring in particular to Ba++ and NH4, inasmuch as their anhydrous radii are the closest among those of the cations examined, to Rb+ and Tl+, it has been considered useful to investigate more widely on the phases synthesizable, when they are coupled with lithium (see experimental). Figures 3 and 4 report the respective crystallization fields, from which it is possible to notice that no cancrinite forms at any composition of the reaction system. The phases obtained refer to Li-A, ABW (A) and Li-K (K) (9) in both fields, Ba,Li-M, PHI (4) and Ba-P, analogous to celsian or cymrite (15) in Ba,Li field. Am denotes no crystallization. 

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. 

SiO2 0.69H2O also intensifies the synthesis of celsian [11]. Practically monophase hexagonal celsian was obtained by mechanical activation of the mixture for 10 min and thermal treatment at 1200°C for 3 h (Fig. 7.4). 

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. 

Ovcharenko N.A., Komilovich B.Yu., Ovcharenko F.D. Investigation of the kinetics of hydrothermal synthesis ofmonochnic celsian. Dokl. AN USSR B 1978 1 37-39. 

Gouadec, G., Colomban, R, and Bansal, N.R, Raman study of Hi-nicalon fiber reinforced celsian composites. Rart I Distribution and nanostructure of different phases, J. Am. Ceram. Soc., 84, 1129, 2001. 

Si MAS NMR has also been used to study the gel synthesis of other silicates, including various magnesium silicates (Hartman and Millard 1990), glass-like cordierite (Selvaraj et al. 1990) and barium aluminosilicate (celsian) (MacKenzie and Kemmitt 1999). 

The crystal structures of the synthetic feldspars SrGa2Si20g and BaGa2Si20g are both similar to that of celsian. There is complete Ga-Si ordering in the former, and probably in the latter also. ... 

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