Mechanisms Of Interface Stability And Microstructure Formation In Solidification Of Particle Suspensions

Solidification of particle suspensions is a new interdisciplinary subject,involving the fields of freeze-casting porous materials,frost heaving,sea ice and biological tissue engineering etc.Especially in recent years,many advanced materials with excellent properties were developed based on the processing of suspension solidification.Experimental phenomena in suspension solidification were different from those in alloy solidification,such as the close-packed particle layer,the ice bridge structure and the periodic ice lenses etc.But the formation mechanisms of these microstructures are still unclear.In this paper,the interfacial stabilities and the mechanisms of microstructure formation in solidification of particle suspension were systematically studied by the methods of in-situ experimental observation and theoretical analysis.The main conclusions of the thesis are as follows:(1)The magnitude of particulate constitutional supercooling(PCS)was revealed by a new method of undercooling measurement,i.e.differential visualization method which transforms the interface undercooling into a distance gap under a linear temperature gradient.By comparing the interfacial positions of freezing suspension and its supernatant,we distinguished PCS and SCS(solutal constitutional supercooling)in different systems(alumina and polystyrene).The experimental data confirmed the validity of original parameters in PCS theory and both experiments and the theory showed that PCS is small.(2)By increasing the initial volume fraction of particles,we obtained three different initial instability modes: 1)Mullins-Sekerka(MS)instability,forming ice lamellae;2)local split instability,forming ice lamellae within particle clusters;3)banded instability,forming periodic ice lenses.These instability modes come from the competition of solute boundary layer and close-packed particle layer.When the effect of the particle layer is dominant,banded instability appears.When the effect of the solute layer is dominant,MS instability appears.When their effects are identical,local split instability appears.(3)We studied the effect of interactions between nanoparticles and polymers on the initial instabilities and steady-state microstructures in suspension solidification.We found that at a low pulling speed(3 ?m/s),small nanoparticles(20 nm)accumulated in front of the solidifying interface and formed long narrow nanoholes,blocking the polymer diffusion,which induced banded instability and ice lenses formed.When both the size of the nanoparticles(100 nm)and the pulling speed(30 ?m/s)increased,the blocking effect disappeared and the solidifying interface appeared MS instability and formed dendrites.(4)The formation mechanism of ice bridges in the mushy zone was studied.Through comparing the length of mushy zone between freezing suspension and its supernatant,we found that the formation of ice bridges is related to solute diffusion and particle packing.Low pulling speed induced shallow cells,and solutes in the mushy zone were few.Nanoparticles accumulated firmly in the mushy zone,which bringed extra curvature undercooling and increased the mushy zone.The supercooled liquid in this increased zone can grow laterally and connect two adjacent cells to form ice bridges.(5)We explored thermodynamics of ice spear-induced periodic ice lenses and its interfacial movement characteristics.Through experiments of different systems of freezing suspension(alumina and polystyrene),we found that ice spears were related to the weak undercooling induced by particles.After a new ice lens was formed,the interfacial speed decreased gradually.Because the premelting film at the interface became thin which decreased the growth rate with time.(6)We revealed the speed-dependent particle packing and pore ice-induced ice lenses.Particulate Brownian motion helps close-packed particle packing which causes a big curvature undercooling of pore ice and then brings a large characteristic width of ice lenses.The rapid growth of the particle layer drives particle packing to be nonequilibrium,forming amorphous defects and the density of the particle layer decreases,which causes a small curvature undercooling of pore ice and then brings a small width of ice lenses.(7)We established a mathematical model to predict the formation of periodic ice lenses based on Darcy flow and premelting film flow.Using force balance and thermodynamics of the particle layer in front of the interface,we obtained the effects of particle-interface mechanical interaction and particle thermodynamics on the microstructure evolution of suspension solidification.Two dimensionless parameters(Darcy coefficient D and premelting coefficient F)was used to describe experimental conditions of the periodic ice lenses induced by the pore ice and the ice spears.Finally,we obtained a theoretical microstructural selections of periodic ice lenses.