Fabrication Of Hierarchically Structured Carbon Array By Direct Laser Writing Polyimide And Its Use For Piezoresistive Sensing

As compared to their homogeneous counterparts,the heterogeneous piezoresistive materials,such as aggregates and assemblies of conductive particles or conductive polymeric composites,possess a variety of unique characteristics,including ease of processing,low-cost and in particular tailorable electrical,mechanical,and piezoresistive performance.This makes them highly promising for the application of ultrasensitive piezoresistive sensors to meet the emerging needs in the rapidly developed next-generation technologies,such as flexible electronics,intermet of things,soft robots,etc..To achieve this goal,it is desired to have a generally applicable design principle and the corresponding implementation method for tailoring and enhancing the piezoresistive sensitivity of heterogeneous piezoresistive materials.On the basis of direct laser writing carbonization(DLWc)of polyimide(PI)film,in this work,a hierarchical contact design concept was proposed and being successfully implemented for making ultrasensitive piezoresistive sensors.This concept relies on the DLWc generated dot and line features as basic geometric units(BGUs).Through controlling the overlapping extent between BGUs and their various fashions of arrangement and assembly,the hierarchical contacts of the graphitic basic structure units(BSUs)in the BGUs can be adjusted to allow for the piezoresistivity of the DLWc generated carbon patterms being tailored.Guided by this concept,four different carbon patterns with increasing levels of hierarchical contacts including plain line,dot-dot contact line,line-line contact area and circle-circle contact area were designed and fabricated by DLWc on PI film,which manifested a three-order of magnitude ehange on gauge factor.An ultrahigh gauge factor of～5700-10,000 under small deformation(ε<0.1%)for the carbon pattern with a circle-circle contact area design has been achieved,which is 5000 times higher than the commercial metal-foil based strain gauge.The newly developed ultrasensitive piezoresistive sensor has also been demonstrated for its use in sensing both static and high-frequency(20-30 kHz)mechanical stimuli.By considering a fractal network formed by random packing of the graphitic BSUs with distributed contact strength,a phenomenological model was developed,which can nicely capture the piezoresistive behavior of all the heterogeneous carbon patterns with varied hierarchical contact structures.Moreover,the proposed fractal network piezoresistive model suggests that a low fractal dimension and a small BSU contact strength are the keys for achieving high piezoresistive sensitivity.In addition to PI film,this thesis also takes advantage of the flexibility of DLWc to directly fabricate the helical carbonized line and dot-dot square area sensing elements on the external and internal surfaces of PI tubes.This approach allows for a new strategy for integration of the design of the sensor structural component and the sensing element fabrication process,which makes it highly versatile for implementation of various sensing functions.Through utilizing the different forms of mechanical deformation modes of the tubular structure such as radial expansion,axial compression,torsion,and out-of-plane bending and with assistance of finite element analysis(FEA),the PI tubular sensing system has been successfully designed and fabricated in sensing gas pressure,contact pressure,liquid viscosity,ultrasound,wind speed and direction,and gait performance and foot pressure distribution.The concept of previously proposed hierarchical contact design was also applied to the tubular sensing system,which enabled the GF doubled for the dot-dot helical line sensing element as compared to the plain line sensing element.The hierarchical contact design concept and the fractal network piezoresistive model proposed in this thesis establish a generally applicable principle for developing ultrasensitive piezoresistive sensors with heterogeneous granular matters and composite materials.In addition,the new strategy to combine DLWc with complex structured sensitive components allows for a novel approach for low-cost,high-throughput and versatile development of piezoresistive sensing system that can meet the highly demanding and customized force and strain measurement applications.