Sintering and Foaming of Barium Silicate Glass Powder Compacts

Boris Agea-Blanco,Ralf Müller,Stefan Reinsch

doi:10.3389/fmats.2016.00045

Abstract

The manufacture of sintered glasses and glass-ceramics, glass matrix composites and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, DTA, Vacuum Hot Extraction (VHE) and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere sig¬ni¬ficantly affected foaming. The strength of this effect increased in the order Ar  N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.

Highlights

Glass powders are widely used in fabricating sintered glass, sintered glass-ceramics, glass matrix composites, and glass-bonded ceramics or pastes when lower fabrication or processing temperatures, gas-tight seals, or complex shapes are required (Rabinovich, 1985; Schiller et al, 2008; Müller and Reinsch, 2012)
This foaming is most pronounced for milling in CO2, where it already starts at ≈770°C
Sintering and foaming of barium silicate glass powder compacts were studied for different milling times and atmospheres

Summary

Introduction

Glass powders are widely used in fabricating sintered glass, sintered glass-ceramics, glass matrix composites, and glass-bonded ceramics or pastes when lower fabrication or processing temperatures, gas-tight seals, or complex shapes are required (Rabinovich, 1985; Schiller et al, 2008; Müller and Reinsch, 2012). One problem often addressed within this context is concurrent crystallization and sintering (Müller, 1994; German, 1996; Prado and Zanotto, 2002; Pascual and Duran, 2003; Prado et al, 2003a,b, 2008). This effect is most pronounced in sintered glass-ceramics, where a large crystal fraction is desired and rapid crystallization starts from the powder surface (Müller and Reinsch, 2012). Many practical applications of sintered glass-ceramics, rely on slowly crystallizing glasses Such glasses are, for instance, used for low temperature co-fired ceramics (Imanaka, 2005), paste glasses (Hwang et al, 2002), or SOFC sealants (Fergus, 2005; Gross et al, 2005). Due to the low glass viscosity required for joining and gas-tight sealing or obtaining a desired crystallinity, gas bubble formation, and related sample swelling (“foaming”) often occur, even when organic aids are not used in powder processing

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