The relative density of glass compa

The relative density of glass compacts cold-sintered for only 1min dwell at 250 ˚C (10 M) was 91.2%. As the holding time increased from 1 min to 20 min then to 1 h, the relative density was enhanced first to 94.7% and then slightly more to 94.8%. That is to say, a rapid densification was observed at the beginning of the process, similar to borosilicate glass monoliths produced by HHP[19,22]. Comparable densification effect was observed with the increase in the NaOH concentration of the aqueous solution. Going from 5 M to 15 M, the relative density was improved from 92.4% to 95.2% (at 250°C for 20 min). Nishioka et.al. [19] reported that the activation energy required for densification of borosilicate glass powder by HHP decreases with increasing normality of the NaOH in the liquid phase. High concentration of NaOH is considered to improve dissolution and thereby assist the densification process. Furthermore, it was shown that increasing the processing temperature decreased the activation energy for densification as well [9].