The effect of alkaline and alkaline-earth metal oxides addition to the ZnO-Al2O3-SiO2 system on the glass formation has been studied and the crystallization behaviour of these glasses was investigated by DTA and XRD. The addition of alkaline metal oxides tends to hinder the glass formation and only the system containing Li2O forms glass. The addition of Li2O changes the crystallization behaviour and the precipitation of LiAlSi3O8 is caused together with gahnite which is a typical precipitation phase in the ZnO-Al2O3-SiO2 system. TiO2 added as a nucleating agent tends to form a compound with ZnO when extra ZnO exists. All glasses could be formed when alkalineearth metal oxides were added and the colour was changed to be brown by the addition of TiO2. In case of no TiO2 addition, glasses changed the form and melted before finish DTA because of low viscosity or high nucleation rate. However, crystallization peaks were clearer after added TiO2 due to the effect of nucleating agent. To conclusion, some glasses could not be formed when alkaline metal oxide was added because crystal growth rate was higher than alkaline-earth metal oxide addition; however, transparent glasses could not be obtained in all samples.

The effects of B 2 O 3 on the crystallization behavior of ZnOAl 2 O 3 SiO 2 glasses were investigated by DTA and XRD measurements after nucleation heat-treatment (NHT) at various temperatures for 24 h. The addition of B 2 O 3 also decreased T c of glasses. According to XRD measurements, gahnite precipitated as main phase in every composition. Precipitation of quartz or willemite was observed for the glass in which little nucleation occurred because of low NHT temperature. Addition of B 2 O 3 promoted the precipitation of willemite instead of quartz. However, the precipitation of these phases was not observed when the nucleation was enough. Crystal sizes became smaller with the increase of the number density of nuclei. On the other hand, they became larger by the addition of B 2 O 3 . It was found that B 2 O 3 addition decreases the nucleation rate and increases the crystal growth rate.

The effect of HfO 2 addition on the crystallization behavior of ZnO-Al 2 O 3 -SiO 2 glasses, which shows volume crystallization, was studied. The crystallization peak temperature (T C ) obtained by DTA for as cast glasses decreased with the addition of HfO 2 . The crystallization peak temperature also decreased by nucleation heat treatments (NHT), and the difference of T C among samples depending on HfO 2 content became less with the increase of NHT time. It was found that HfO 2 works as a nucleation agent because the addition of HfO 2 suppressed the precipitation of SiO 2 by promoting the precipitation of Gahnite(ZnAl 2 O 4 ). The effect of HfO 2 on nucleation is less than TiO 2 or ZrO 2 which are known well as nucleation agents. The low diffusion rate of Hf due to its heavier atomic weight than Ti and Zr would be a reason for the low nucleation ability of HfO 2 .Key words: