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Description
Natural hydroxyapatite (HAp) derived from bovine bone waste is a promising sustainable dielectric material for humidity sensing , but its sensing performance can be tuned through simple post-synthesis processing such as mechanical milling. This study examines the influence of mechanical milling duration on its structure and humidity-sensing performance. HAp was extracted by deproteinization and calcination at 900 °C, then mechanically milled for 0, 1, or 2 h. X-ray diffraction confirmed a hexagonal HAp phase with lattice parameters in >99% agreement with the ICDD reference, a crystallite size of ~49 nm, and crystallinity of 78.1%, while scanning electron microscopy showed submicron agglomerates with morphology preserved across all milling conditions. Despite this structural similarity at the bulk level, milling was expected to alter surface area and particle packing within the sensing film, factors that directly govern moisture adsorption and charge transport. To test this, HAp was blended with 10 wt% PVA, drop-cast onto interdigital copper electrodes, and characterized by impedance and capacitance measurements from 0.1 to 1000 kHz under humidity conditions generated by MgCl₂ and MgSO₄ salt exposure, spanning approximately 35–79% RH. All samples showed decreasing impedance and increasing capacitance with rising humidity, with the strongest response at 0.1 kHz. The 1-h milled sample exhibited the highest capacitive sensitivity (9.4%) with a linear response, while further milling to 2 h reduced the response, indicating that an intermediate milling duration provides the optimal sensing performance. These findings support waste-derived natural HAp as a viable platform for low-cost humidity sensor applications.
