Integration of CCTAO/PDMS composite films into proximity capacitive sensor devices

Flexible capacitive proximity sensors are promising for contactless sensing applications, but their performance is strongly influenced by the dielectric properties and microstructure of the sensing layer. In this work, CaCu3Ti4-xAxO12/polydimethylsiloxane (CCTAO/PDMS, A = Nd³⁺ or Gd³⁺) composite films were developed as flexible dielectric layers for interdigitated capacitive proximity sensors. Nd- and Gd-doped CCTO ceramics were synthesized by a solid-state reaction method and incorporated into a PDMS matrix at different filler loadings. Structural analysis confirmed that the CCTAO ceramics retained the cubic CCTO phase after rare-earth substitution, while the composite films preserved the characteristic amorphous structure of PDMS with embedded ceramic fillers. The FESEM, EDS mapping and X-ray tomographic microscopy analyses showed that the CCTNdO/PDMS composite had a more uniform distribution of ceramic particles than the CCTO/PDMS system. The dielectric measurements demonstrated the improvement in the dielectric constant of the PDMS-based composites upon CCTNdO incorporation and also indicated that the composites did not exhibit any significant changes in their dielectric properties across the range of frequencies examined. The CCTNdO/PDMS films were found to show the negative capacitance response as a function of distance due to the electric-field shunting mechanism when used in an interdigitated capacitor sensor. The sensor with composition 10 wt% CCTNdO/PDMS had excellent performance with a maximum normalized capacitance change equal to −8.70%, which corresponds to a proximity sensitivity of around 0.42%/mm and an effective sensing range of around 20 mm. It is concluded that the optimization of the loading of the rare-earth material in a flexible PDMS matrix is an effective approach to achieve a compromise between the dielectric enhancement of the sensor and the dispersion of the filler and fringing-field interaction in the contactless capacitive proximity sensor.