Celsian hexacelsian

The celsian structure (Fig. 3b) [17] is similar to the feldspar structure in which all four vertices of the silica tetrahedral are shared, forming a three-dimensional network. As in hexacelsian, A1 substitutes for Si with charge compensation by Ba in the larger interstices of the structure. Gay [18] and Newnham and Megaw [17] considered the formation of a superlattice in celsian associated with ordering of the Al-Si atoms. 

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). 

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)). 

Fig. 4(a) shows the pure celsian BAS was made from the mixture of hexacelsian and 5 mol% Li 0 heated at 1100°C for 4h. The Li20-containing celsian was then heated at 1650°C for 1 h, a temperature where celsian BAS becomes unstable. As shown in Fig. 4(b), there is no reaction happened, implying that the additive Li20 not only could promote the celsian formation fix>m hexacelsian BAS below 1590°C, but could also stabilize the structure of celsian above 1590°C. 

Figure 1. X-ray diffraction patterns show the effect of starting materials on the celsian formation with or without doping with LiiO. (a) Complete celsian formed from hexacelsian powder heated at 1530°C tor 50h (b) Insignificant amount of celsian formed from a mixture of BaCOi, A Oj and SiO powders heated at 1530 C for 50h (c) Complete celsian fanned from hexacelsian powder with the presence of 5 mol% Li20 heated at 900°C for 4h (d) Incomplete celsian formed from a mixture of BaCOj, AI2O3 and Si02 powders with 5 mol% L12O heated at lOSO C for 4h. (H) represents peaks from hexacelsian BAS, umnaffced peaks and (C) celsian BAS, (B) BaAl204 and ( ) unidentified phase.

Figure 3, X-ray diffraction patterns show the instability of celsian without containing Li20 when heated at 1650°C. Pure clesian heated (a) at 1650°C for 1 h and (b) at 1650°C for 3 h. (H) hexacelsian BAS peaks, unmarked peaks and (C) celsian BAS peaks.

When in-situ 70 vol% Si3N4 reinforced BAS composites are synthesized, the formation of celsian is suppressed, even respectively with the effective mineralizers of LijO, NaF. SrFi and ZxOi, and hexacelsian exists as a metastable phase below 1590°C. 

The XRD experiments show that in monolithic system the LijO-containing celsian can persistent exist at 16S0°C however, in BAS/S13N4 composite it transforms into hexacelsian phase at 1600. It is found that the presence of Si3N4 could enhance the instability of the Li20-containing celsian above 1590 C, in which the hexacelsian is a stable phase in thermodynamics. 

LijO-containing celsian powders added with 5 or 10 mol% of SiC>2 and then heated at temperatures above 1600°C would become instable and transform into hexacelsian phase. 

The celsian BAS containing U2O would resist the transformation to hexacelsian at temperatures above 1590°C and would lower the melt point at about 1500°C, much lower than I760 C of the celsian BAS without containing Li O. 

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

K-T. Lee and P.B. Aswath, Kinetics of the hexacelsian to celsian transformation in barium aluminosilicates doped with CaO, International Joumed of Inorganic Materials, 3,687 (2001). 

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