| With the rapid development of intelligent wearable technology,flexible electronic products have been widely used in various electronic components.Anisotropic flexible sensors with high precision,complex microstructure and functional macro structure can be obtained with 3D printing technology.Anisotropic sensors can be applied in the human-machine engineering in space environment to realize the force monitoring,reconstruction of space complex surface,and can also greatly improve the tactile perception and feedback accuracy of bionic robot gripper.At present,there is little research on anisotropic sensors.In this project,the anisotropic sensors with excellent performance were prepared by 3D direct writing.The direct writing process of the flexible sensor was designed and explored to further characterize the inherent performance of the printed samples.The sensing characteristics of the CF/CNT filled anisotropic sensor under different stress conditions were then studied.The sensor with functional shape was further designed through printing technology and pre-stretching technology.The resistance relaxation phenomenon of the anisotropic composite material under the external electric field was finally revealed.The main research contents are as follows:(1)3D printing process design of the filled polymer matrix compositesThe 3D direct writing printing process of liquid polymer conductive materials was explored and optimized,including material selection and design,printing parameters and curing process design.Liquid printable materials are supposed to own the characteristics of stable distribution of filler and no settlement in a certain period.The settlement behavior of the filler is related to the density difference between the filler and the matrix and the viscosity of the suspension.In this paper,0.6wt% CNT filler and 35wt% CF were selected as fillers,and PDMS with low viscosity was selected as the polymer matrix.The addition of inhibitors and catalysts greatly improved the service window of printing materials.0.03 wt% inhibitor and 0.5 wt% catalyst was selected to improve the fluidity of the material and improve the printing window period of the material.The flow characteristics and viscosity of the printing material remained stable within 15 days.The printing parameters and curing process of flexible sensor would affect the forming quality of flexible sensor.The printing parameters of the direct writing process were determined as D was 0.34 mm,V was 5cm/s,h was 0.5mm,and η 50～60%.Two-stage curing process of PDMS matrix composites were adopted,the first and the post curing were carried out at 100 ℃ for 10 min.(2)Study on the inherent properties of the CF/CNT filled PDMS compositesThe characterization and observation of the inherent properties of the printed samples mainly included the observation of micro morphology,the characterization of anisotropic electrical and mechanical properties.CFs in the composite,under the printing shear force,were aligned along the printing direction.By observing the micro distribution of the filler of the printing material and calculating the orientation rate of the filler with MATLAB,the orientation rates of CF on the upper and below surface were 78.53% and 54.51%,respectively.The distribution of CF in the printing sample was anisotropic in the horizontal direction and thickness direction.The orientation behavior of the filler gave the printed samples anisotropic electrical and mechanical properties.The resistance of the printed sample in the vertical direction was about ten times of that in the parallel direction.The printed samples can maintain stable electrical performance after being stored in the atmospheric environment for15 days.The orientation of carbon fiber can directionally enhance the tensile strength and elongation of the material,and the material has a higher tensile strength and elongation at break along the orientation direction.The crack propagation mode and fracture morphology of the printed samples were also various in different tensile directions,which is due to the different interactions between fibers and the matrix.(3)Study on the sensing performance of the CF/CNT filled anisotropic sensorThe anisotropic sensor has a sensitive,highly repeatable resistance response to tensile stress and compressive stress,as well as an extremely wide linear sensing range.The tensile sensing characteristics of the sensor are related to the loading speed.The sensing characteristic curve was linear between the strain of 0～ 20%,as well as the pressure of 0～200k Pa.The linear relationship of the sensing curve was related to the linear stress-strain relationship and the oriented fillers rarely deflects under load.The double peak phenomenon in the cyclic stretching process of flexible sensor was revealed by video observation and Fourier frequency domain signal analysis,which was attributed to the viscoelasticity of filled flexible composites.Anisotropic sensor can realize multi signal sensing and characterize the stress of materials.The load on the sensor can be comprehensively evaluated by the changes of electrical signals in different orientation directions,when the anisotropic sensor is loaded.(4)The sensors with functional shape designed based on 3D printing process and pre-stretching preparationThe CF/CNT filled anisotropic flexible sensors can be pre-stretched along different directions to obtain functional sensors with three-dimensional structure.A three-dimensional spiral sensor was prepared by stretching the sample along the 45 ° direction and pre-stretching60% strain.The sensor with three-dimensional spiral structure has good linear sensing characteristics and cyclic response stability.The three-dimensional spiral structure sensor was less affected by the viscoelasticity of polymer materials in the process of cyclic tension,which makd the sensors with good wearable characteristics and stable work duration.It can be used in the connection of robot joints and human-computer interaction.3D direct writing technology gives a lot of design freedom to the flexible sensor.The prepared humanoid finger inspired sensors would provide a new idea for the design of bionic structure sensor circuit.(5)Study on the resistance relaxation of anisotropic sensor under electric fieldThe resistance value of anisotropic sensor would relax under external power supply.After the loading of 12 hours of constant DC voltage of 3～10V,the resistance value of the flexible sensor finally reduced to 0.9 times of the initial resistance value.The Joule heating effect of flexible conductive composites hinders the resistance relaxation process of the sensor,while the movement and deflection of the filler promote the resistance relaxation.Through the microscopic observation and the analysis of the interaction force between the fillers in the composite,the CF tends to deflect and approach to each other during the electrifying process.This work will contribute to a better understanding of the static sensing characteristics of flexible sensors.This study provides a comprehensive consideration for 3D printing anisotropic sensor,from the preparation,characterization to sensors design.The material and preparation process design,the structural design of the flexible sensor,the electromechanical response performance,as well as the working characteristics under the external power supply was systematically completed. |
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