OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Fig. 9. Cyclic capacitance and humidity variation over 1,000 s, demonstrating sensor stability Capacitive Sensitivity and Frequency Response Capacitive sensitivity was greatly increased by NGM doping, reaching a peak of 65 a.u. for 5% NGM doping at 90% RH (Fig. 10, c). This is due to the high conductivity and high surface area of NGM, promoting increased charge storage and water molecule adsorption [8]. Doping concentrations higher than 5% caused decreased capacitance through agglomeration, limiting effective surface area and sites for adsorption. Frequency-dependent measurements (Figs. 10, a and 10, b) identified that lower frequencies (<10 kHz) had more capacitance change, which is enhanced by stronger polarization. However, higher frequencies (>100 kHz) were less sensitive due to quicker charge carrier relaxation, restraining the capacity of water molecules to align with the electric field [9]. Fig. 7, d also verifies that at 5% doping, the sensor recorded high RH sensitivity over a range of frequencies, particularly at lower frequencies. a b с d Fig. 10. (a) Capacitive response at 90% RH; (b) Response for 5 wt.% NGM doping; (c) Capacitance as a function of doping level at 90% RH; (d) Capacitance as a function of RH at different frequencies (5 wt.% NGM doping)
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