Celsian phase

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. 

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. 

D. Bahat, Kinetic Study on the Hexacelsian-Celsian Phase Transformation, J. Mater. Sci, 5, 805-810 (1970). 

Figure 6. Evaluation of (0.2SrO-BAS)/HTE (75/25wt.%) (a) XRD showing celsian phase, and (b) thermal shock test.

Lin and Foster indicated that above 1590°C, celsian would undergo a transformation to hexacelsian and the formed hexacelsian could metastably exist at temperatures below 1590°C. X-ray diffraction patterns in Figure 3. show the pure celsian phase without containing Li20 would convert to hexacelsian after heated at 1650°C. For 1 h at 1650°C, there is a lot of hexacelsian transformed from celsian (Figure 3(a)) and the conversion is almost completed for 3 h at 1650 C (Figure 3(b)). 

N. P. Bansal, Comment on Kiirctics study on the Hexacelsian-Celsian Phase Transformation, Material Science and Engineering A, 342,23 (2003). 

In spite of the thermodynamically favourable Sr-celsian phase formation, it is a kinetically complex process. Hence, no such phase formation is evident for the YS077/A1S1 441 microstmcture. The steady-state volatility data indicated lifetime weight loss (40,000 h) of the sealing glass <0.1 wt% indicating the potential viability of YS077 as a sealant material. 

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]. 

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. 

Tensile properties of unidirectional BN/SiC coated Hi-Nicalon fiber reinforced celsian matrix composites [36-37] from room temperature to 1200°C in air are shown in Table 6, The value of Young s modulus decreased with increase in test temperature indicating the presence of glassy phase in the matrix. The yield stress decreased from room temperature... 

G. Gouadec, P. Colomban, and N. P. Bansal, Raman Study of Hi-Nicalon Fiber-Reinforced Celsian Composites. Part I Distribution and Nanostrueture of Different Phases, J. Am. Ceram. Soc., 84 [5] 1129-1135 (2001). 

The most common phases obtained through recrystallization from the various zeolite types are feldspars, such as celsian and high- sanidine, feldspatoids, such as the above mentioned carnegieite and nepheline, and the high-temperature silica polymorphs (tridymite and cristobalite), which are usually the recrystallization products of high-silica zeolites, such as ZSM-5 [48]. 

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