Design And Direct Ink Writing Of SiC/Gypsum Paste

Direct ink writing is a novel method of forming 3D complex structures.In this dissertation,printable silicon carbide and gypsum pastes were designed and printed to obtain three types of 3D dot-matrix structures: silicon carbide,gypsum and silicon carbide/gypsum,respectively.The main conclusions were followed:(1)Printable silicon carbide pastes were obtained by adjusting the dispersant content,p H,solid phase volume fraction and methyl cellulose.The optimum dispersant content results in the lowest paste viscosity;p H affects the dissociation of the dispersant to change the viscosity of the paste;the higher the solid phase volume fraction,the greater the structural integrity of the print;and the addition of methyl cellulose increases the viscosity and shear modulus of the paste,making it printable.The optimized silicon carbide paste formulations: 0.01% PAA,0.4 % TMAH and 1.4 % PAA of dispersant(in mass),63 %,60 %,55 % of solid loading and 0.04 %,0.06 % and 0.05 % of MC(in mass),all at p H >10 to fabricate 1 μm,10 μm and 44 μm printable silicon carbide pastes respectively.(2)The addition of citric acid and HPMC can adjust the setting time and rheological properties of the gypsum paste,making it ready for direct writing into shape.Citric acid delays the hydration of the gypsum paste,which prolongs the setting time of the paste and provides sufficient printing time;HPMC increases the shear modulus of the paste,allowing it to pass smoothly through the print nozzle and keep the shape.The optimized addition of citric acid and HPMC were 0.6 wt.% and 0.3 wt.%respectively.The 3D structure of gypsum obtained by direct-write forming had a high compressive strength(20 MPa).(3)The rheological properties of the silicon carbide and gypsum pastes were further adjusted to optimize the printing process parameters(movement speed and extrusion pressure,etc.),and the silicon carbide/gypsum 3D structure was obtained by two-head direct ink writing.The rheological properties of the paste determine the stiffness of the printed filaments,while the print speed and extrusion pressure affect the amount of two pastes extrusion.The highest print stiffness was achieved with 44 μm silicon carbide,followed by 10 μm silicon carbide and the lowest with gypsum,so the extrusion pressures were set at 72 Pa,47 Pa and 37 Pa,and the print speeds were set at 8-10 mm/s,7-9 mm/s and 7 mm/s,respectively.