Anomalous Charging Behavior of Inorganic Materials

Surface charge on materials is important due to its wide range of consequences and applications in many industries (e.g., semiconductor). One important parameter needed for the design of devices and selection of materials is the polarity of charge of solid surfaces after the surfaces are charged by contact electrification. Currently, it is only known that one surface charges positively, whereas the other surface charges negatively after contact (i.e., in accordance to the law of conservation of charge). This manuscript describes a previously unreported anomalous charging behavior of a general class of materials: the inorganic materials. Both contacting inorganic materials charged either both positively (e.g., mica and NaCl) or both negatively (e.g., quartz and CaF₂). A close monitoring of the charge in real time showed that charge was conserved immediately after contact; however, an active interaction of the separated surfaces and the surrounding atmosphere unexpectedly changed the polarity of one surface rapidly (e.g., 1 s), thus resulting in both positively or both negatively charged surfaces. This anomalous charging behavior fundamentally changes our understanding of the design of devices related to contact electrification of inorganic materials and the triboelectric series.