Effect Of SnO 2 on the Mechanical Properties of SiO 2 /SnO 2 Gel-Derived Composites
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EFFECT OF SnO 2 ON THE MECHANICAL PROPERTIES OF SiO 2/SnO
2
GEL-DERIVED COMPOSITES
R. DAL MASCHIO, S. DIRE', R. CAMPOSTRINI, G.D. SORARU AND G. CARTURAN
Dipartinento di Ingegneria dei Materiali, University of Trento, 38050 Mesiano, Trento, Italy.
ABSTRACT
The morphological evolution and Sn02 crystallization of various gel-derived materials, composed of Si02-SnO2 in the range from 34 to 5 wt % SnO2, are studied in function of the thermal treatment in the interval 200-800 *C. Sn02 phase separation occurring above 400 °C affects the density, the apparent porosity and mechanical properties like hardness and Young's modulus. At the lowest SnO2 % a continuous density increase is observed on heating at variance with high tin dioxide contents for which the apparent density is almost constant. The results are discussed on the basis of Sn02 crystallization, which occurs by two different mechanisms, evidenciated by X-ray analyses. The data will be compared with those of pure SiO 2 . INTRODUCTION The synthesis of inorganic materials from solution by the sol-gel method allows complete phase homogeneity of precursors before chemical and thermal processes afford product formation. This homogeneous state is required for as long as possible during gel occurrence and subsequent heat treatments: thus,glass formation simply involves a continuous evolution of chemical and physical properties, whereas ceramics are born from sudden homogeneous nucleation. In the latter case, the absence of any crystalline phase detectable by X-ray analysis may not be a definite criterion for accepting the homogeneity of xerogels in the amorphous state, because of possible phase separation without crystallization. In the case of systems composed of different oxides and obtained from alkoxide precursors subjected to acid-catalyzed hydrolysis and condensation, phase separation without crystallization may be favoured by electronegativity differences among metal cations. The SiO2/SnO2 system a priori presents the most favourable features for observing separation of amorphous Sn(IV) oxides starting from Si(OC 2 H 5 )4 and Sn(IV) species in strongly acidic alcohol solutions. The morphological evolution and Sn02 crystallization of various gel-derived materials, SiO 2 -SnO 2 in the compositional range from 34 to 5 wt % Sn02, are studied in function of the thermal treatment in the interval 200-800 *C. Sn02 phase separation and crystallization, occurring above 4000 C, affects the density, the apparent porosity and mechanical properties like hardness and Young's modulus. EXPERIMENTALS Four compositions, corresponding to the molar ratios SiO2 / SnO 2 1: 0.02, 1: 0.05, 1: 0.1 and 1: 0.2, were prepared by reacting SnCI 4 at 0 'C in an Ar flow with absolute ethanol. Si(OC 2 H 5 )4 was added to the solutions at room temperature; the mixtures were refluxed for 2 h under vigorous stirring and subsequently hydrolized at room temperature. After about fifteen Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
352
days the solutions, held in an open
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