Model studies of colloidal silica precipitation using biosilica extracts from<Emphasis Type="Italic"> Equisetum telmateia</Emphasis>

Structural materials containing silicon are produced in single celled organisms through to higher plants and animals. Hydrated amorphous silica is a colloidal mineral of infinite functionality that is formed into structures with microscopic and macroscopic form. Proteins and proteoglycans are suggested to play a critical role in the catalysis of silica polycondensation and in structure direction during the formation of these magnificent structures. This article extends knowledge on the effect of protein containing biosilica extracts from Equisetum telmateia on the kinetics of silica formation and structure regulation. Utilising potassium silicon catecholate as the source of soluble silicon, bioextracts obtained from plant silica by dissolution of the siliceous phase with aqueous HF following extensive acid digestion of the plant cell wall were found to modify the kinetic rate constants for the formation of small silicic acid oligomers under circumneutral pH conditions and to modify the solubility of silicic acid in solution. Addition of the bioextracts at ca. 1 wt% to the reaction medium reduced the sizes and range of sizes of the fundamental silica particles formed and led to the formation of crystalline polymorphs of silica under conditions of ca. neutral pH, room temperature and in the absence of multiply charged cations, conditions assumed to be relevant to the biological mineralization environment. The ability of biological organisms to regulate the formation of silica structures with prevention of crystallinity is discussed as are the implications of this study in terms of the generation of new materials with specific form and function for industrial application.     

Singularities of the solution of a mixed problem for a general second order elliptic equation and boundary stabilizationof the wave equation

We deal here with a second order elliptic mixed problem which is posed in a regular open bounded domain of . We study the regularity of its solution. We apply our results to the boundary stabilization of the wave equation.     

Kaolinite-mullite series: Firing vs. reactive hot-pressing

The effect of an applied pressure on the transformation of metakaolin to spinel has been studied under reactive hot-pressing conditions and the results are compared with the conventionally fired clays. For the experiments both Georgia kaolin and a Canadian fireclay (kaolinitic) were used. The results indicated that the transformation temperature of metakaolin to spinel can be lowered by the application of an external pressure, confirming the earlier work of Carruthers and Wheat. The application of Clapeyron-Clausius equation showed that in order to lower the transformation temperature, the enthalpy change (H) for this transformation has to be positive, contrary to experimental DTA plots, where an exothermic reaction is generally encountered. This anomalous behaviour can be explained, if it is accepted that the exothermic peak at about 900°C in the kaolinite to mullite transformation sequence is due to the crystallization of amorphous silica, as suggested by Nicholson and Fulrath.

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Zusammenfassung Der Einfluß eines geeigneten Druckes auf die Umwandlung von Metakaolin in Spinell wurde unter den Bedingungen des reaktiven Warmpressens untersucht und die Resultate mit den für konventionell gebrannten Ton erhaltenen verglichen. Die Versuche erfolgten mit einem Kaolin aus Georgia und einer Tonart aus Kanada. Die Ergebnisse zeigen, daß die Übergangstemperaturen Metakaolin-Spinell durch Anwendung eines äußeren Druckes erheblich vermindert werden können. Mit Hilfe der Clausius-Clapeyronschen Gleichung wurde für die Umwandlung eine positive Enthalpieänderung errechnet, hingegen deuteten die experimentellen DTA Spitzen eine exothermische Reaktion an. Dieses anomalische Verhalten läßt sich durch die Annahme von Nicholson und Fulrath erklären, nach der die exothermische Spitze bei 900° beim Kaolinite-Mullit Übergang der Kristallisation des amorphen Siliziumdioxyds zuzuschreiben ist.

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Résumé Etude de l'influence de la pression exercée à chaud sur la transformation du métakaolin en spinelle et comparaison des résultats avec ceux obtenus avec les argiles calcinées de manière conventionnelle. Les expériences ont été pratiquées sur un kaolin de Géorgie et sur une argile kaolinitique du Canada. Les résultats montrent que le fait d'exercer une pression extérieure abaisse d'une manière importante la température de la transformation du métakaolin en spinelle. La variation d'enthalpie correspondante a été calculée à l'aide de l'équation de Clausius-Clapeyron. On l'a calculé positif, cependant les pics ATD montrent une réaction exothermique. On peut expliquer ce comportement anomalistique par la suggestion de Nicholson et Fulrath, selon laquelle le pic exothermique de 900° dans la transformation kaolinite/mullite est due à la cristallisation de l'oxyde de silice amorphe.

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, . . , . (+H) - .

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Thanks are due to Dr. D. Coates for his helpful suggestions on the applicability of the Clapeyron-Clausius equation in this transformation sequence.