Processing-structure-property Relationship In Direct Laser Writing Carbonization Of Polyimide And Its Application For Solar Seawater Desalination

With high-energy laser beam(e.g.,CO2 laser)as a tool,direct laser writing carbonization(DLWc)is currently an intensively pursued method for photothermally patterning and carbonizing/graphitizing carbonizable polymers,such as polyimide to create carbon features/arrays.It has shown great potentials for large-scale fabrication of various carbon-based functional devices.The present thesis focuses on the critical photothermal conversion and laser processing issues involved in DLWc with an aim at establishing and understanding the processing-multi-scale structure-electrical property relationship of DLWc created carbon structures and exploring its use in solar seawater desalination.Firstly,the DLWc created "dot" carbon feature at various laser processing conditions were investigated by scanning electron microscopy and Raman spectroscopy.It reveals that the Gaussian beam shape,laser power,irradiation duration and laser focus distance all have significant effects on the size,morphology,pore structure and nonuniform distribution of the graphitic microstructures of the dot feature.To further understand the experimental observations,a photothermal model was established and numerically solved,which allows to predict the temporal temperature evolution of the PI film when under a Gaussian CO2 laser beam and also provide a good agreement with experimental results on the lateral size of the carbon dots.Secondly,to understand the porous structure evolution of DLWc created carbon structures,the carbon "line" features at various laser processing conditions were fabricated and their methylene blue adsorption behavior,both isotherm and kinetics,were investigated.This in-situ porous structure characterization method in conjunction with scanning electron microscope,Raman scattering spectroscopy,and electrical property measurements allows for a systematic investigation of the multiscale structure evolution and the electrical sheet resistance of the carbon lines fabricated by DLWc and establishing the related processing-multi-scale structure-electrical property relationship.The key processing parameters being investigated include:laser power-P,laser beam scanning speed-S,laser focus distance,and the averaged areal laser energy density.Quantitative relationships are established between these processing parameters and the specific surface area,the porosity,the degree of perfection of the layered carbon or graphitic basic structure units,as well as the electrical sheet resistance of the carbon lines created by DLWc.The comprehensive and quantitative processing-multiscale structure-electrical property relationships for DLWc established in this study expect to be useful for better understanding the complicated photo-thermally induced polyimide pyrolysis/carbonization process.Lastly,this thesis takes advantage of the low cost,highly-flexible characteristics of DLWc and utilizes its unique and tailorable porous carbon structure to explore its application in solar seawater desalination.A double-layer porous carbon sheet was fabricated by DLWc,which showed excellent sunlight absorption characteristic.A sandwiched structure,composed of double-layer porous carbon sheet-cellulose paper-polyurethane foam,was designed,fabricated and tested for its solar energy induced evaporation performance.Under one sunlight irradiation,this system showed solar evaporation rate of 1.55 kg m-2 h-1 and efficiency of 85%.It indeed reveals the great potential of DLWc in large-scale production of high-performance sun-light absorber for solar seawater desalination.