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Multiwalled carbon nanotubes (CNTs) were used as the conductive filler of composites for switching type humidity sensor. The CNTs were oxidized by mixed acids (H 2 SO 4 : HNO 3 ) at a mild temperature to modify carboxylic acid (COOH) groups on the surface of the nanotubes. The dispersibility of acid treated CNTs (CNTs-COOH) in water is much improved, which is beneficial for dispersing CNTs in the polyvinyl alcohol (PVA) matrix without external additives. The obtained CNTs-COOH/PVA sensors show nonlinear response to relative humidity (RH), that is, switching properties. The resistances of the sensors remain constant before 80% relative humidity (RH) and then increase sharply with RH, indicating excellent switching characteristic of the sensors. The 10 wt % CNTs-COOH/PVA sensor shows a sensitivity (DR/Ro) of 32.3 at 100% RH. The humidity switching properties of CNTs-COOH/PVA are much better than that of pristine CNTs/PVA. The improvements are attributed to the improved balance between the dispersibility of CNTs-COOH and electrical conductivity of the composite films.

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