In situ study of glass foaming using heating microscope
R.R. Petersen1, J. König1,2, Y.Z. Yue1
1
Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej 7H, DK-9220 Aalborg Øst, Denmark
2
Advanced Materials Department, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
rp@bio.aau.dk
Optical monitoring techniques are useful for recording the sample size as a function of temperature or time. The
images recorded are processed and direct measurement of the variation in sample size allows us to monitor the
expansion of foam glass in situ during dynamic heating.1) We evaluate the potential of using heating microscope
to assess foaming ability and optimising the chemistry of powder mixture for foam glass production. The
silhouette area curves are assessed (Fig. 1a) and for the first time the derived expansion rate values are
determined (Fig. 1b). Using panel glass from cathode ray tubes and MnO2 as a foaming agent we show that the
expansion rate has a maximum (Fig. 1b). The maximum is reached when the foam glass becomes percolated.2)
Crystallisation is not affecting the expansion curve, since the panel glass has a very high glass stability. 3) In
acoustic and catalytic applications percolated foam glass is preferred. However, as thermal insulation material
and lightweight filling material the foam glass needs to have closed pores. Since maximum expansion rate is
correlated to a percolation threshold of foam glass, the maximum expansion rate can define an upper
temperature limit for making foam glass with closed pores. The lower limit is defined as the foaming onset. The
production of foam glass should be performed near the temperature of percolation threshold to maximise
expansion. Therefore, we can use the percolation threshold to determine the maximum allowable expansion of
foam glass during dynamic heating. We use the data from Bayer and Köse4) which were measured during the
foaming of bottle glass using different concentrations of SiC. They observed the foaming with heating
microscope. We demonstrate that the foam glass with lowest density and high closed porosity is gained at 2 wt%
SiC.2) This value is in agreement with industrial ones. The use of heating microscope to assess foaming ability
and quantify the optimal foaming conditions proves to be a powerful tool.
(a)
(b)
Figure 1 – (a) Characteristic pictures showing the silhouette of a sample placed on an Al2O3 plate at different
temperatures. (b) The normalised area (A/A0) and the expansion rate (rA) as a function of treatment temperature.
The definition of the characteristic temperatures can be found in ref [2].
1)
2)
3)
4)
C. Steiner, Foam glass production from vitrified municipal waste fly ashes, (PhD thesis) Eindhoven University, 2006.
R.R. Petersen, J. König, Y.-Z. Yue, In situ evaluation of foaming ability of glass melts using heating microscope, Int. J. Appl. Glass
Sci. (2016), doi: 10.1111/ijag.12189
R.R. Petersen, J. König, Y.-Z. Yue, The mechanism of foaming and thermal conductivity of glasses foamed with MnO2, J. Non-Cryst.
Solids 425 (2015) 74–82.
G. Bayer and S. Köse, Reaction of foaming additives with waste glass powders in the preparation of lightweight materials, Riv. della
Staz. Sper. Vetro 5 (1979) 310-320.