To solve the problem of drying gelcast green body, the thermoresponsive gel system which contains macromonomer graft chains was used in gelcasting of ZnO. The effects of the amount and length of graft chains macromonomer PIPAAm, the total amount of organic matters, and the solid loading on the rheological properties of suspensions were investigated, and the drying mechanism of gelcast green body was analyzed. The results show that ZnO suspensions with the gel system still display shear- thinning rheological behavior, but its viscosity increases with increasing the addition amount and relative molecular mass of PIPAAm graft chain, and the total organic matter content. The PIPAAm graft chains inhibit or even eliminate the formation of the "dense layer” on the surface of gelcast ZnO green bodies, and accelerate the drying of green bodies. The introduction of PIPAAm graft chains facilitates the shrinkage of the gelcast ZnO green bodies, which is a feasible method to increase the relative density of green bodies.
BeO gelcast green bodies were dried by liquid drying method with liquid desiccant ethanol.Effects of ethanol concentration,solids loading and aspect ratio of green body on the moisture and shrinkage of green bodies were studied through measuring mass and dimension.Additionally,liquid drying stress was analyzed and a model of the initial stage drying stress was established.The results show that higher ethanol concentration,lower solids loading and higher aspect ratio of gelcast green bodies increase the drying rate.Increasing the ethanol concentration decreases the shrinkage rate.Liquid drying stress is generated due to a non-uniform drying rate.During the process of liquid drying,the inner drying stress of the green body changes from compressive stress to tensile stress,while the outer drying stress changes from tensile stress to compressive stress.
Novel colloidal processing using thermosensitive poly(N-isopropylacrylamide) (PNIPAM) as a coagulating agent has beendeveloped to prepare complex-shaped ceramic components. In this work, the properties of PNIPAM aqueous solutions and therheological behavior of ZnO suspensions with PNIPAM were investigated. The results show that the PNIPAM solutions exhibitobvious thermosensitivity and its transition temperature is around 32℃. When the temperature is above 40℃ (Tc, the criticaltransition temperature of thermosensitive suspension), the 50% ZnO (volume fraction) suspension with 8 mg/mL PNIPAM has asharp increase in viscosity and reaches up to 11.49 Pa·s at 50℃, displaying strong elasticity. The main reasons are the increase ofeffective volume fraction attributed to precipitation of PNIPAM segments and the flocculation between ZnO powder particles. Inaddition, the maximum solid loading (volume fraction) at 20 ℃ is higher than that at 40℃, which proves that the phase transition ofPNIPAM can induce the flocculation of suspension.
To obtain porous alumina ceramic with high strength,a novel gelcasting system based on 2-hydroxyethyl methacrylate(HEMA)dissolved in tert-butyl alcohol(TBA)was developed.The polymerization of the HEMA-TBA gelcasting system,the thermal behavior of obtained green body,and the microstructures and mechanical properties of the sintered bodies were investigated by rheometer,TG-DSC,SEM and bending strength testing,respectively.The results show that,(1)10 mg/mL of the initiator(benzoyl peroxide)is the optimal amount for polymerization of this gelscasting system at 25 ℃;(2)The alumina suspension of the HEMA-TBA gelcasting system showing shear-thinning behavior is sufficiently low for gelcasting process;(3)The bending strength of porous alumina ceramic samples,whose porosities range from 42% to 56%,is from(8±0.5)to(91±4.5)MPa.