Microstructure Development in Silica Glass Ceramic by Rapid Sintering ofSubmillimeter Wave Gyrotron
H. Aripin1, S. Mitsudo2, I.N. Sudiana2, S. Tani2, K. Sako2, Y. Fujii2, T. Saito2 and T. Idehara2
1

Department of Physics, Faculty of Mathematics and Natural Science, University of Haluoleo
Kampus Bumi Tridharma Anduonohu, Kendari 93232 Indonesia
2
Research Center for Development of Far Infrared Region, University of Fukui
Bunkyo 3-9-1 Fukui 910-8507, Japan

Abstract - In this paper, we present and discuss experimental results on sintering of silicaglass ceramic, produced from a silica xerogel extracted from sago waste ash. The pressedsilica xerogels were sintered using submillimeter wave processing of a 300 GHz CW gyrotron
at the temperature ranging from 300oC to 1500oC. The microstructural changes are observed
using SEM. The results show that the presence of dense layer on the sample surface increases
while the presence of pore acts in the opposite direction with increasing temperature.
Keywords: silica xerogel, submillimeter wave gyroton, silica glass ceramic, porosity
1. Introduction
The production of silica glass ceramic at higher sintering temperature by conventional
heating needs longer time, which promotes grain growth. In contrast to the conventional(surface) heating the microwave heating is volumetric, since the electromagnetic energy is
dissipated simultaneously in the whole irradiated volume. Additionally, the microwave
treatment offers higher densities of the heating power, better precision in controlling theprocess of energy deposition etc. A successful use of millimeter wave heating to sintermaterials such as boron carbide [1] and alumina [2] has been reported recently. It has beenfound that the irradiation by millimeter waves enables very high heating rates to be achieved
and reduces the time for grain growth. The smaller grains make the microstructure more
uniform and increase the strength of the material. In this paper we present the results of ourstudy and discuss the influence of the sintering temperature using a 300 GHz CW gyrotron on
the microstructure of silica glass ceramic.
2. Description of the experimental procedure
Silica xerogel was extracted from a sago waste (solid residue, which is left behind after
the starch has been washed out) obtained from the sago processing plant in Kendari,
Indonesia. The detailed extraction procedures are described elsewhere [3]. The molding ofsample and sintering procedures using a sub-millimeter wave gyrotron (Gyrotron FU CW I)
are described in the literatures [4, 5]. Furthermore, the microstructure of the sintered samples
was characterized by the scanning electron microscope (SEM).
3. Experimental results and discussion
Figure 1 shows SEM image of the silica xerogel samples without calcination and such
sintered at different temperatures using gyrotron heating. In the sample without sintering, bothrough surfaces and uniform pores can be observed. With increasing the sintering temperature
up to 362oC, the roughness and pores appear to be reduced from the sample surfaces. Thesepores are ascribed to the empty sites left by evaporated water and to some residues of polyvinyl alcohol [4]. The incorporation of the particles and the reduction of porosity in thesintered samples can be observed at 800oC. Thus it is likely that at this temperature, in thecondensation reactions, more hydroxyls on the pore walls are activated and condensatesimultaneously. During sintering process, silica begins to melt into pockets of a viscous liquid
phase within the pores. It was confirmed from the measured XRD pattern that the silica
xerogel starts to crystallize at the temperature [4]. At temperature from 1200oC to 1500oC, thelarge amount of liquid phase fills more pores, and therefore, the dense layer on the sample
surface was observed. The appearance of pores on the sample surfaces sintered at 1200oC and1500oC is not distinguishable. This can be due to the very low rate of densification and poreremoval as a result of the transformation of the amorphous materials into crystalline phase.

Figure 1. SEM picture of the silica xerogel samples without sintering (a) and sintered at (b)300oC, (c) 362oC, (d) 800oC, (e) 1200oC, and (f) 1500oC.
4. Conclusions
Xerogel powders have been used successfully for producing glass silica ceramics
controlling accurately the sintering temperature. From observed SEM image, an increase thedense layer and pore removal on the glass silica ceramic surface was attributed to threedifferent mechanisms. They are the empty sites left by evaporated water and to some residuesof PVA as well as to combusted residual organics at 300oC, partially crystallized sample at800oC, and fully crystallized sample at temperatures greater than or equal to 1200oC.
Acknowledgements
The authors would like to acknowledge the financial support from Directorate of Researchand Community Service, Directorate of Higher Education, at the Department of National
Education, Republic of Indonesia through Project of Overseas Collaboration Research andInternational Publication, Number: 1140/D3/PL/2010 and the Research Center forDevelopment of Far-Infrared Region (FIR FU Center), University of Fukui, Japan.
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