Baixe fredericci 2000 e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! CRYSTAL GROWTH MEASUREMENTS ON A SODA-LIMF- SILICA GLASS SPHERE
USING FEG-SEM
Fredericci, C'"; Zanotto, E. D'"; Souza, L. A? and Chinellato, A. Lt?
Universidade Federal de São Carlos - DEMa, 13564-905 — São Carlos — SP
QUNESP- Departamento. de Química, 14800-000 — Araraquara — SP
GiUniversidade Estadual de Ponta Grossa — DEMa — 84031-510 - Ponta Grossa - PR
An alternative route to glass-ceramics technology is sintering and crystallization of glass powders.
However, many experiments must be carried out to know the best conditions to obtain a dense sintered body or
one having controlled porosity. In ongoing research we propose and use a computational simulation technique te
verify if a glass sinters before it crystallizes or vice-versa, before any sintering experiment is carried out. For this
purpose two models that describe the sintering of glass - the Frenkel and Mackenzie-Schuttleworth model - and
the Johnson-Mehl- Avrami (JMA) theory of crystallization were used. To test the simulation, soda-lime-silica
glass spheres with average diameters of about 200 um were used. The crystal growth rate (U) of spherulitic
crystals developed on the surface of the spheres was obtained through FEG-SEM. This technique was important
due the following reasons: i) the crystal nucleation rate on the surface spheres is high and the crystals are too
small (1-10 um diameter) to be measured with precision by optical microscopy, ii) examination at low voltage in
a FEG-SEM suppressed the need for coating the sample, permitting to treat the same sample at different times
without interference of the coating on the crystal growth rates.
Rabinovich [1] reported that glasses that show low surface crystallization rates are suitable for sintering.
1f a glass presents volume crystallization and/or high surface crystallization rate it crystallizes so fast that the
viscosity increases to near infinity and sintering (by viscous flow) stops [2]. [owever, itis common to find in the
literature that many experiments have to be made to achieve the best conditions to obtain a dense sintered body;
sometimes without success. Glass properties, which greatly affect sintering, such as viscosity, particle sizes and
nucleation and crystallization rates, can be measured in the laboratory. Thus, using this data it is possible to
predict by simulation, before any sintering experiment, if a glass system is favorable to produce a fully dense
glass-ceramic body. In this communication attention was given to crystal-growth rate measurements using FEG-
SEM.
The system chosen for this study was commercial soda-lime-silica glass spheres (Potters) since most
sintering models are based on spherical particles. A sample was placed in a stabilized furnace at 680 ºC, initially
for 6h. After this time it was removed from the fumace and analyzed, without coating im a XL-30 FEG —Philips
using 20KV. After obtaining an image, the sample was replaced in the furnace for additional 2h. This procedure
was repeatedly carried out until the impigement of the crystals.
Figure 1 shows two micrographs of the glass sphere heat-treated at 680 ºC for 8h and 10 h, respectively.
The same region was taken in order to measure the size of the same crystal, to obtain the crystal growth rate.
FEG-SEM is an important tool to measure the crystal growth rate of soda-limc-silica glass spheres and
inferentially of other glasses. Although the sample was not coated with a conductive material the quality of the
images were good for measuring lhis important parameter to be used in the simulation.
ACKNOWLEDMENTS
Funding by FAPESP, CNPq and PRONEX are deeply appreciated.
REFERENCE
1. RABINOVICH, EM -.. Mater. Sci., 20 (1985), 4259-97
2. KNICKERBOCKER, S., TUZZOLO, M.R,LAWHORNE, S- /. Am. Ceram. Soc., T2[10], 1989, 1873-79.
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