Study On Liquid Desiccant Drying Mechanism Of Nano Al₂O₃Gelcasting Green Body

This dissertation aims to study the mechanism of liquid desiccant drying of greenbody prepared by gel-casting. The commercial nano-Al₂O₃powder was used as rawmaterial for preparing of gel-casting slurry. The effects of pH, dispersant content, solidloading and milling time on dispersion stability and fluidity of suspension was studied bymeans of Zeta potential, sedimentation method and viscosity test, respectively. Aftergelation, the samples were demolded, then dried using liquid desiccant of polyethyleneglycol（PEG） water solution. And the effects of liquid desiccant drying on green body werecompared with that of hot air（temperature of40℃, humidity of RH60%）. The mechanismof liquid desiccant drying and the drying process of gel-casting parts were discussed by ageneral Fickian diffusion model. The drying process is satisfactorily analyzed by thesimulation using the ANSYS analysis software. The influences of drying factors, includingtemperature, solid loading, particle size, molecular weight and concentration of desiccant,on the drying process and microstructure of gel-casting body were also tested.The results show that there is an optimum pH value and dispersant content for theslurry to reach the minimum viscosity when the solid loading is constant. Afteroptimization, the viscosity of the alumina suspension with solid loading of40vol.%is only34.62mpa·s, while the pH, dispersant content and milling time is10,0.28wt.%and12h,respectively.Liquid desiccant drying provides not only a faster drying rate but also a moreuniform medium to release the residual stresses and reduce the malformations incomparison with conventional hot air drying. During the process of desiccant drying in thesolution, the migration of water from the body surface to the PEG solution is mainlydriven by osmotic pressure of the hypertonic polymer solution, body contraction andcapillary force. The diffusion of moisture inside the body is driven by the concentrationgradient within the body and the drying rate of the process is limited by the diffusioninside, thus the mass transfer coefficient of moisture can be obtained by using the solutionof diffusion model under unsteady-state diffusion. According to the experimental data of unidirectional drying of cylindric green body（h=20mm, d=10mm） in the PEG10000solution of20wt.%, the average diffusion coefficient is0.576×10-9m2/s. With the help ofANSYS, the simulation profile of water content in the body during drying processpredicted by diffusion model fits the experimental profile very well with a deviation lessthan10%.The research about the liquid desiccant drying factors shows that the increase ofdesiccant concentration, PEG molecular weight, particle size of powder and dryingtemperature will speed up the drying rate, while the increase of solid loading of thegel-casting part will decrease the drying rate. Additionally, the change of drying factorswill result in some impacts on the internal microstructure of the green body.