Study On Gelcasting Method And Its Application Of Preparation For Bipolar Plates Of PEMFC

Bipolar plates for proton exchange membrane fuel cell were prepared withmesocarbon microbeads (MCMB) by Gelcasting, which is a near-net shape formingtechnology in ceramic processing. The main discussions that focused on are: the influenceson slurry's rheological properties and stability by MCMBs' characteristics such asparticles' shape and appearance, diameter and its' distribution and specific surface area; themixing manners also had effects on MCMB slurry's properties. An effective dispersants'selection methodology had been put forward. Firstly, some provably dispersants wereselected out by sedimentation experiments in low solid loadings, then the selecteddispersants could be used in higher solid loading slurries and, finally, the most promisefuldispersants were picked up. The results suggest that MCMBs' characteristics had muchinfluence on slurry and milling was more propitious to achieve higher solid loading slurrythan just stirrering. The selected dispersants were added to the slurries in the proportion of2wt.% to MCMBs' weight and then milled for 16 hours. The finally MCMB slurries withviscosity of40mPa.s and 60.06vol.% solid loadings were achieved.The gelling reaction dynamics was studied and the influences on gelling process andgel strength by initiator, catalyst, proportion of monomer/crosslinker and monomerconcentration were discussed. Results indicate that the initiator had much stronger effecton gelling and idle time than catalyst. The most promiseful strength could be achieved byadding proper crosslinker amounts, otherwise the worse results would be got only. Fromthe Gel Chemistry we know that the homogeneousness of gels' structures had effect onits' transparence. In other words, the more transparent the gels are, the morehomogeneous structure they have. The gelation experiments ofn-hydroxymethylacrylamide indicate that the unitary reaction system could get morehomogeneous structure than binary reaction system. The reaction stresses induced bytemperature grads are deleterious to green and sintered parts. It could be eliminated byadding no catalyst and substituting Polytetrafluroethylene (PTFE) for the steel ribs justbecause the PTFE has much lower heat conductivity than steel.The gelled green parts were dried by liquid desiccant method, which mean that thesaturated parts were submersed into the organic solvents which have low surface tension.The influences of the organic solvents' concentration on the shrinkages of green parts werediscussed. And a theoretical shrinkage model was put forward as: f((?)u／(?)t)=uâ–½²u+(K+1／3u)â–½(â–½u),F_sh=u∫[(u_xx-T／3)²+(u_yy-T／3)²+(u_zz-T／3)²]dVthe model can well anticipate the shrinkage in three-dimensional direction of the ball shapegels, column gels and disc shape gels and explained the shrinkages of the prepared greenparts. It was proved that the liquid desiccant method is safe arid fast to dry the gelled greenparts.Results of pure gel and MCMB green parts' TGA and DTA showed that the gelpolymer pyrolyzed at 233.3℃and ultimately ended before 444.7℃; and MCMB pyrolyzedat 392.3℃and ultimately ended before 678.8℃, respectively. The influences of greenparts' initial densities, tieating rate and sintering temperature on sintered parts' densitiesand shrinkages were discussed respectively. Based on the former discussion, a theoreticalshrinkage model was put forward as:λ_i=(ρ0th／ρith)^1／3(â–³_i)^1／3-1which only required descriptions of theoretical density(p0th, pith )and mass loss behavior(Δ_i) to estimate the shrinkage behavior at any sintering temperature. The calculatedshrinkages were similar to the measured results.Finally, the sinter mechanism for the MCMB was formulated, which consists of twoimportant processes: liquid-phase sintering mechanism at temperature below 800℃, andsolid-phase sintering mechanism between 800～1100℃. In liquid-phase sintering process,the low molecular weight organic compounds andβresin pyrolyzed, which produced theliquid phase and resulted in the MCMB particles crystallographic arrangement and formingthe necks. Sample's theoretical density increased after this stage. In solid-phase sinteringprocess, the samples' shrinkage and theoretical densities increased slowly just because thepoor mass-transfer characteristic in solid phase.The electrical properties of the prepared bipolar plates were performed with a fuel celltesting station and testing results showed that the bipolar plates had excellent electricalproperties and well stability under PEMFC working conditions.