Electrical Properties of Graphite Nanoparticles in Silicone
: Flexible Oscillators and Electromechanical Sensing

  • Samuel Littlejohn

Student thesis: Doctoral ThesisPhD


This thesis reports the discovery of a wide negative di↵erential resistance (NDR)region in a graphite-silicone composite that was utilized to create a strain-tuned flexible oscillator. Encoding the strain into frequency mimics the behavior of mechanoreceptor neurons in the skin and demonstrates a flexible and electronically active material suitable for state of the art bio-electronic applications. The NDR was investigated over a range of composite filling fractions andtemperatures; alongside theoretical modelling to calculate the tunneling currentthrough a graphite-silicone barrier. This led to the understanding that the NDRis the result of a semi-metal to insulator transition of embedded graphene bilayers within the graphite nanoparticles. The transition, brought about by a transverse bias across specifically orientated particles, opens a partial band-gap at the Fermi level of the bilayer. NDR in a flexible material has not been observed before and has potential for creating a flexible active device.The electromechanical properties of the composite were considered througha bend induced bilayer strain. The piezoresistance was found to be dominatedby transient resistance spiking from the breaking of conduction lines, which then reform according to the viscoelasticity of the polymer matrix. The resistance spiking was embraced as a novel method for sensitive di↵erential pressure detection, used in the development of two applications. Firstly, it was employed for the detection of ultrasound waves and found to have an acoustic pressure detection threshold as low as 48 Pa. A commensurability was observed between the composite width and ultrasound wavelength which was shown to be consistent with the formation of standing waves, described by Bragg’s law. Secondly, a differential pressure array of 64 composite pixels was fabricated and demonstrated to image pressures under 3.8 kPa at a resolution of 10 dpi.The NDR active region was incorporated into an LC circuit where it wasdemonstrated to sustain oscillations of up to 12.5 kHz. The composite was thenstrained and an intrinsic frequency was observed which had a linear dependenceon the strain with a frequency shift of 84 Hz / % strain. Lastly the compositewas used in a strain-tuned amplifier circuit and shown to provide a gain of up to 4.5. This thesis provided the groundwork for a completely flexible electronically active device for futuristic bio-electronic skins with resolutions and sensitivities rivalling those of human tactile sensing.
Date of Award31 Dec 2013
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAlain Nogaret (Supervisor)


  • flexible electronics
  • active pressure sensors
  • artificial skin
  • bilayer graphene
  • negative differential resistance
  • NDR
  • bilayer strain

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