## 3 Results and Discussion  
Figure 1a depicts the DTA (differential thermal analysis) traces of PSAB glasses in dependence of CuO concentration increased from 0-2.0 wt%. The thermal properties, such as glass transition temperature (T\({}_{\rm g}\)) and crystallization temperature (T\({}_{\rm x}\)), were taken as illustrated in Figure 1a. It was noticed that the T\({}_{\rm g}\) remained almost unchanged at around 387-389 \({}^{\circ}\)C regardless of CuO dopant  
concentration. However, \(T_{x}\) considerably increased up to 509 \({}^{\circ}\)C from 480 \({}^{\circ}\)C with the addition of 0.5 wt% of CuO to the base glass, and it remained constant for further increase in CuO concentration. The thermal stability (\(\Delta\)T) is frequently characterized by the difference between \(T_{g}\) and \(T_{x}\), \(\Delta\)T = \(T_{x}\) - \(T_{g}\). Figure 1b shows the quantitative values of \(T_{g}\), \(T_{x}\), and \(\Delta\)T, dependent on CuO concentration. The \(\Delta\)T increased compared to the base PSAB glass, and it remained almost constant regardless of CuO dopant concentration. The change in \(T_{g}\) as a function of CuO concentration has been explained in other reported glasses [16; 17; 18]. On one hand, it was reported that the \(T_{g}\) increased with increase in CuO concentration in \(P_{2}O_{5}\)-Na\({}_{2}O\)-CuO glass network system. This was attributed to the increased formation of P-O-Cu bonds, resulting in an increased cross-linking density in the \(P_{2}O_{5}\)-Na\({}_{2}O\)-CuO glass network. Therefore, the thermal and mechanical properties of the glasses were enhanced. On the other hand, there were several reports of the opposite effect of CuO on \(T_{g}\), i.e., a decrease in \(T_{g}\). It was reported that the \(T_{g}\) decreased as the CuO concentration increased in the SiO\({}_{2}\)-Li\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-CuO glass system [19]. [PERSON] et al. presented that the \(T_{g}\) decreased as the CuO concentration increased in SiO\({}_{2}\)-\(P_{2}O_{5}\)-K\({}_{2}\)O-MgO-CaO-CuO glasses. Because the more covalent character of Cu\({}^{2+}\)-O bonds replaced the more ionic bonds, such as Ca-O bonds and Mg-O bonds, this caused the glass structure to be more rigid, which resulted in the increasing amount of stress in the glass and thus, the \(T_{g}\) decreased [20]. [PERSON] et al. reported that the \(T_{g}\), CTE, and molar volume decreased with increasing CuO concentration in the CuO-BaO-B\({}_{2}\)O\({}_{3}\)-\(P_{2}O_{5}\) glass system [21]. This behavior, i.e.,  
Figure 1: (a) Differential thermal analysis (DTA) traces; and (b) the quantitative values of glass transition temperature (\(T_{g}\)), crystallization temperature (\(T_{x}\)), and thermal stability \(\Delta\)T (\(T_{x}\) - \(T_{g}\)) in a PbO-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-B\({}_{2}\)O\({}_{3}\) (PSAB) glass system doped with various CuO concentrations. the decreasing trend of T\({}_{\rm g}\) and CTE simultaneously was not well explained. Therefore, a more systematic investigation of T\({}_{\rm g}\) and CTE according to CuO concentration was carried out using the TMA method, which uncovered microscopic trends on the effect of CuO in glass systems. Figure 2a shows the TMA traces of the PSAB glass system as a function of CuO dopant concentration. From TMA spectra, the CTE, T\({}_{\rm g}\), and softening temperature (T\({}_{\rm s}\)), were obtained, dependent on CuO concentration. The inset of Figure 2a showed that the T\({}_{\rm g}\) and T\({}_{\rm s}\) decreased from 404 to 392 \({}^{\circ}\)C and 435 to 429 \({}^{\circ}\)C as CuO concentration increased from 0 wt% to 2.0 wt%, respectively. Figure 2b shows the quantitative values of CTE as a function of CuO dopant concentration for different temperature ranges.