Theory And Application Of Liquid-Solid Transformation Of Poly (Vinyl Alcohol) In Freeform Fabrication Of Ceramics

Colloidal freeform fabrication of ceramics (CFFC) is the computerized layer-by-layer method to fabricate ceramic suspensions with or without binders according to the principle of Rapid Prototyping & Manufacturing (RP&M), while a single layer undergoes the liquid-solid transformation under fixed conditions before fabricating of another fresh layer. CFFC has been essential to the fabrication of ceramic components with complex structure in its higher flexibility and cost-effectiveness. However, for domestic development, there is still a long way to go for the restriction of development in relevant fields.In this dissertation, basic principles of the liquid-solid transformation of poly (vinyl alcohol) (PVA) aqueous solution induced by borax aqueous solution were investigated for the freeform fabrication of ceramic components. The application in glass-alumina functionally graded materials (G-A FGMs) and the pore graded glass with controlled pore size and distribution based on the authorized Chinese patent (No. ZL200510043174.7) was investigated. Basic models were established for the key techniques. Composition design, freeform fabrication process, sintering, and the characterization of the as-prepared ceramic materials were analyzed and discussed. The research will be essential to the theoretical and practical development of CFFC based on the gelation of PVA.Effects of spraying angles, time, and the elevation angle of the suspensions on the velocity of the borax aqueous solution drops were analyzed. Additionally, the motion of the borax aqueous solution right before and after gelation of the suspensions was analyzed. According to the numeric results, increasing falling height and spraying angles are favorable to the increase of the boundary layer and the velocity on top, which can efficiently increase the contact of the borax aqueous solution and the suspensions.Chebyshev regression of the aberration effect before the critical gelation of PVA aqueous solution induced by borax aqueous solution was investigated. Numeric results show that Chebyshev regression can aid the analysis of the data for the aberration effect with a maximum relative error less than or equal to 5%. A nonlinear equation of higher order is established for the effect of borax aqueous solution addition on the apparent viscosity of the PVA aqueous solution.Diffusion of Eosin Y aqueous solution and Eosin Y/borax aqueous solution in the PVA aqueous solution film before and after gelation was analyzed. Surface morphology of the PVA before and after gelation at different heights was observed by AFM. Reducing the falling height or removing mechanically can serve as good means to reduce the residual borax. Confirmed by AFM, the gelled structure differs significantly from the structure before gelation in its denser structure confirmed by smaller Ra from AFM.Effect of time and temperature on the relative water evaporation rate for alumina aqueous suspensions, alumina-PVA aqueous suspensions, and glass-alumina-PVA aqueous suspensions was investigated. Definition of drying process, stress evolution, and compositional effect on the drying process were discussed by DSC/TG analysis.Two linear increase periods are exhibited in the drying process before the falling rate period, and owing to the movement of small particles, the relative evaporation rate as a function of time for the second period is two times greater than that for the first period. Addition of PVA into the suspensions forms a polymer film onto the surface of the alumina particles which contributes to the greater relative water evaporation rate for both linear increase periods due to the bonding of water molecules and the hydroxyl groups on the surface of the alumina particles. For the glass+alumina suspensions, the glass network determines the evaporation of water and the effect of the ordered solvent layers can be neglected.The relative water evaporation rate has a determined effect on the pressure distribution in the suspensions. For the linear increase periods during heating, the stress increases with the increased depth and the evaporation rate, but it decreases with the increased coefficient of permeability.G-A FGMs with good physical and chemical properties were prepared based on the proposed method. The optional sintering temperature is 700-725℃for 30～50min.The shrinkage of the G-A FGMs as a function of temperature, layers, and alumina content was predicted by a back propagation artificial neural network (BP-ANN). Comparison of the shrinkage predicted for G-A FGMs with different compositions shows that the increase of layers in the G-A FGMs is beneficial to the shrinkage and expansion resistance.An integrated kinetics equation was established for the initial 20 minutes at 710℃for G-A FGMs. Linear shrinkage is due to the integrated working of both viscous flow of the glass particles and gravitation of the effective activated alumina grain boundaries towards the equilibrium state. The driving force for gravitation of the activated alumina grain boundaries originates from the coactions of passive flow of the alumina particles during viscous flow of the glass particles and the compressive stress formed during cooling. Confirmed by ESEM and the linear scanning EDS, the sintering process follows the solution-precipitation mechanism in the middle 10 minutes.The damage evolution in G-A FGMs was investigated. Pores with mean diameter between 20μm and 100μm are introduced by partial decomposition of PVA gel. A crack propagates through the weakly bonded grain boundaries in alumina particles, resulting in the final fracture of the glass matrix. The alumina domains may be considered as incoherent powder agglomerates that, due to their large size and irregular shape, favor the crack propagation. All the microstructures show that pores have no significant effect on crack generation and propagation.PGMs with Na₂O-CaO-SiO₂ glass and Al as ingredient materials were successfully designed and prepared based on the proposed method. Effect of sintering temperature, time, and composition on both the composite materials and PGMs was investigated.For PGMs, with the increased temperature, the flexural strength decreases while the water absorption rate increases. With the increased sintering time, the flexural strength increases in the initial stage and then decreases while the water absorption rate increases.Significant difference of the morphology and distribution of the pores was confirmed with the increased content of Al by ESEM. Three categories of pores are exhibited, including the residual small pores with mean diameters less than or equal to 15μm due to the sintering of the glass particles, medium-scale pores with mean diameters between 15μm and 60μm due to the encapsulated Al particles by glass, and the larger pores with mean diameters greater than 60μm due to the growth of the residual sintering pores in the viscous flow of the glass particles.Temperature and elongated sintering time have a strong effect on the formation of the unique structure that with the increased Al content, larger pores decrease gradually while those medium-scale ones increase gradually.α-Al₂O₃ platelets were prepared in the interface of Na₂O-CaO-SiO₂ glass induced by Al. At 1200℃, the prepared platelets have mean diameters between 400nm and 1000nm. Most of them are aggregated and part of them in intergrown. Theα-Al₂O₃ platelets can be formed at 740℃.