## 3 Results and Discussion  
### Resistance Experiments  
Our previous study [21] showed that NiO thin film sensors cured in a furnace have extremely high resistance and can thus be considered insulators at atmospheric temperature. Therefore, the resistances of furnace-cured and UV laser-sintered NiO sensors were compared in this study. Figures 6 and 7 present the resistance of laser-sintered NiO thin films that were measured at 35 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, respectively; the sintered NiO thin film showed better sensitivity in the range of 35 \({}^{\circ}\)C to 100 \({}^{\circ}\)C.  
In Figure 6, the 1.5 W laser output with the low scanning speed resulted in low resistance. The resistance dramatically declined while increasing laser energy (lower scanning speed). It exhibited the adjacent nanoparticles rapidly and massively joined together. Therefore, slowing down the scanning speed or delivering more energy on the thin films efficiently reduced the resistance of the NiO thin film sensors. Additionally, the error bars of experimental replications were much larger than those of the NiO thin films sintered at other laser powers, which means the manufacturing process was not stable under the scanning speeds of 100 mm/s to 150 mm/s, the frequencies of 90 k to 120 k Hz, while the energy output was set at 1.5 W. Compared with parameters with 2W laser output, the NiO thin films was not fully sintered by 1.5 W laser output with any scanning speed or frequency. Figure 6: Resistance experimental results of all laser parameters that were measured at 35 \({}^{\circ}\)C and compared with furnace 260 \({}^{\circ}\)C calcination process (far right in each chart). In the 2 W laser output experiments (blue bar chart), as scanning speed increased, the decrease of resistances tendency was slowed and the energy absorption of NiO thin films approached the saturation state. The variations of experimental replications of NiO thin film resistance decreased significantly; it is inferred that 2 W laser output with a scanning speed of 100 mm/s to 150 mm/s was the adequate energy for the stable sintering process. The variations of experimental replications of parameters with 2.5 W laser output were also small; however, the trend of resistances increased while the scanning speed accelerated. The excess energy absorption resulted in diminished electrical performance. This phenomenon was also observed in previous research [22; 23]. Regarding the effect of the laser frequency setting, the variation of frequency did not affect any trend of resistances in Figures 6 and 7. Thus, a single-factor analysis of variance (ANOVA) was performed to evaluate the statistical difference of frequency variation (\(p\) value < 0.05). The conditions of single-factor analysis were fixed laser outputs and fixed scanning speeds; frequency was the only varied factor. Figures 6 and 7 show the \(p\) values of laser frequency settings (90 k to 120 k Hz) with identical laser output and speed; the result indicated all \(p\) values of frequencies were above 0.05, which means the influence of frequency variation on the resistance performance was insignificant in this study. The equation of laser pulse overlapping rate explained why the frequency variation did not affect the results--laser pulse overlapping rate directly affects the energy adsorption. The higher overlapping rate causes higher irradiation energy per area, while the lower overlapping rate represents lower energy absorption. The laser pulse overlapping rate equation can be defined as (Equation (5)) [29]:  
\[\text{Roverlapping}=(1-\frac{v}{D\times f})\;100\% \tag{5}\]  
where \(D\) is the spot size (mm), \(v\) is the scanning speed (mm/s), and \(f\) is the frequency (pulse repetition rate) in Hz (1/s). The distance between the focal lens and workpiece was fixed at 150 mm; thus, the laser spot size (\(D\)) was identical in each experiment. For the same scanning speed, the only factor that affects the overlapping rate is frequency in Equation (5).