Sintering, crystallization and foaming of La2O3 center dot SrO center dot 5B(2)O(3) glass powders - effect of the holding temperature and the heating rate

Abstract

The peculiarities of sinter-crystallization of La2O3 center dot SrO center dot 5B(2)O(3) fine glass powder (below 1 mu m) are investigated and discussed. The sintering was studied by contactless optical dilatometry and porosity measurements. The crystallization process was evaluated by differential scanning calorimetry (DSC) and the crystalline phase formation was analyzed by X-ray diffraction (XRD). Finally the microstructures of the samples sintered at different temperatures were observed by field emission scanning electron microscope (FESEM). The specific aim of this study was to highlight the influence of sintering temperature and heating rate on the densification process and the final structure of glass-ceramic samples. It was shown that zero water adsorption and minimum closed porosity of similar to 3% were obtained at heating rate of 2 degrees C/min and at about 1 h holding at 690 degrees C. The temperature increase accelerated intense foaming process that resulted in formation of total porosity of 35-45%. It was also demonstrated that the foaming depended on the heating rate: the higher the heating rate - the more intensive was the bloating. The observed foaming process was related to intensive crystallization and re-crystallization of La2O3 center dot SrO center dot 5B(2)O(3) crystal phase. The phase formation processes entirely completed after 30-60 min at 730-740 degrees C with the formation of intriguing and unusual crystal structures.