Aluminas beta phases

The aim of the present paper is double. Firstly, we wish to question more precisely the role of TEA+ ions in competition with the Na+ cations and possibly in close relation with other synthesis parameters such as the silica source, or the alumina content, by comparing a series of other physicochemical characterizations(chemical composition, nature of the occluded organics, void volume...) of zeolites ZSM-20 and Beta. In a second step, we conduct a more in depth investigation of the whole synthesis conditions and their modification in order to propose selective preparation routes for both zeolites and to possibly define further favorable conditions for the formation of other potential open phases. 

A.T. Hunt, CCVD Processing of Lanthanum Phosphate and Beta-Alumina Fiber Coatings for Mullite Matrix Composites, Contract DM19561712, NSF SBIR Phase 1 Grant, 1996. 

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

From most of the compositions in the field of corundum, both corundum and beta-alumina crystallize at about 100° below liquidus temperatures. For liquidus temperatures below 1400° crystallization of both of these phases proved difficult. 

Alkali loss has been observed in compositions of low silica content in several alkali-silica systems with other oxides. Goldsmith [16] found a loss of Na20 by volatilization from melts in the NaAlSi04-CaO A1203 system, particularly in those melts rich in the latter component. He noted that the loss of Na20 influenced the amount and even the presence of nonsodic or soda-poor phases such as CaO 2A1203 and beta alumina. Schairer and Bowen [4] determined that in multi-component silicate systems, alkali losses are not controlled by alkali content alone. Rather the presence of other oxides has been observed to affect the alkali losses. For example, silica and alumina tend to stabilize compositions and prevent or minimize alkali losses, whereas CaO or MgO tends to increase the losses. 

R. C. DeVries and W. L. Roth, Critical evaluation of the literature data on /3-alumina and related phases. 1. Phase equilibria and characterization of beta alumina phases. J. Am. Ceram. Soc. 52(7), 367 (1969). 

Common intermediate phase in the processing of beta- and beta -alumina ... 

The alkylation of phenol with methanol, in the liquid phase, has been investigated using zeolites H-ZSM-5, H-beta, H-MCM-22, H-mordenite, H-USY as well as amorphous silica alumina. At the low temperature of 200°C, anisole was the major product over all the catalyst investigated, second was cresols. H-Beta, H-USY, H-ZSM-5, H-mordenite and amorphous silica alumina showed similar cresol distributions. H-MCM-22, which has the smallest pore openings and the narrowest channel system among all zeolites studied, showed the highest preference for p-cresol. 

For research into the manufacture of gas sensors of a potentiometric type, we have tested different solid electrolytes. The unique property of these sensors is that the two different metalhc electrodes ate located in the same gaseous phase. This property has prompted us to study particularly the beta-alumina and calcium sulfate. 

In 1982, D.E. Williams proposed a potentiometric sensor, elaborated using beta-alumina and two metalhc electrodes located in the same gaseous phase. Furthermore, these two electrodes were of different natures and sizes. 

This class of materials was developed to combine the chemical stability of alumina and properties of Si3N4 with ease of sintering. A beta Sialon is formed by substitution of Al and O ions for Si and N in Si3N4 lattice, as Sie-zAlzOzNg-z- The resultant structure contains elongated beta-Sialon grains, typically with some intergranular phase, which could be amorphous or crystalline. Substitution of alumina adds to the chemical stability... 

Figure 25. Etching played an important role in the growth when only metal beta-diketonates were used. Despite this the growth was found to be reproducible and thus surface controlled. The etching did not occur when LaSiO/alumina samples were prepared from alternating reactions of La(thd)3, HMDS, and air. This is because the thd released in the reaction of La(thd),3 cannot release silicon into the vapor phase, and similarly the NHs formed in the HMDS reaction cannot release lanthanum.

Sodium beta alumina is not a single compound but exists as two distinct phases (designated p and p ), each of variable composition The idealised formulae are... 

Two general approaches have been adopted to the fabrication of electrolyte tubes (l) isostatic pressing (2) electrophoretic deposition. In either case the green tube must be sintered at high temperature in air in order to form and densify the beta alumina phase. 

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