Exploitation Of Continuously Immersing Ice-templating Technique And Its Application In Fabricating Wood-inspired Porous Zirconia

Macro-porous materials have broad applications,e.g.,for structural weight reduction,noise reduction,thermal insulation,catalyst support and absorption,owing to their outstanding property advantages,such as low density,high specific strength,low thermal conductivity,and large specific surface area.The macro-porous materials with ordered architectures,where the pores are preferentially aligned along a specific orientation,tend to exhibit anisotropic and enhanced properties as compared to those having randomly distributed pores and lacking specific orientations.Wood,as a notable example of ordered porous materials evolved in nature,has been regarded as a prototype for exploiting new man-made macro-porous materials towards enhanced properties.Ice-templating technique offers a viable means for constructing ordered macro-porous architectures in materials;nevertheless,it is generally limited by a low efficiency for fabrication,large difficulty for manipulation,along with the small dimension and poor structural uniformity of ice-templated materials.Here,in order to solve the above problems,a new efficient approach is exploited toward efficient fabrication of large-sized macro-porous materials based on the achievement of a high,stable freezing rate by descending the freezing front into the cooling medium to accommodate its upward growth.A continuous ice-templating equipment is also invented independently for accomplishing the above technique.By designing the mold into a shape with one side thinner than the other three sides and assembling a wedge with low thermal conductivity at the bottom of the mold,a large and stable bidirectional temperature gradient is successfully established at the front of the solid-liquid interface,thereby achieving continuous ice-templating of uniformly ordered macro-porous materials.A series of porous zirconia ceramic scaffolds are prepared by static and continuous ice-templating techniques with different descending speeds of mold;and the processing-structure-property relationships for the scaffolds are analyzed.The main research contents and results are as follows:（1）The continuously immersing ice-templating technique can increase the size of the sample,accelerate the solidification speed,refine the structure,and realize the homogeneous and oriented structure.The height of the sample exceeds 15 cm,breaking the height limit for the static ice-templating technique.The solidification speed of the slurry exceeds 1.5 mm min^-1,leading to a thickness of lamellae of less than 5μm,thereby refining the structure of the material.The difference in the thickness of the lamellae along the longitudinal direction is less than 3μm;and the difference in the orientation of the lamellae in the central area is less than 5°.As such,the uniformity and ordering of alignment of lamellae can be simultaneously improved.（2）By measuring the temperature of the slurry during solidification process and based on the Fourier one-dimensional heat conduction equation,the temperature distribution function of the liquid phase ahead of the freezing front in the ice-templating process has been derived to obtain the solidification parameters such as the solidification rate and the temperature gradient at the interface.In the static ice-templating technique,as the solidification advances,the temperature gradient gradually decreases with the solidification rate gradually slowed down.Larger and more stable solidification rate and bidirectional temperature gradient can be obtained in the continuous ice-templating process by adjusting the descending speeds of the mold,as compared to the conventional static ice-templating technique.（3）The structural parameters of porous zirconia ceramics such as the thickness of lamellae,inter-spacing of lamellae,and bridging density are analyzed through morphological characterization of the porous scaffolds.In case of the static ice-templating technique,the thickness of lamellae and inter-spacing of lamellae are obviously increased from the bottom to the top with the bridging density reduced.In case of the continuous ice-templating process,the macro-porous architecture of materials is made much finer and more uniform over the entire sample,and exhibited better ordering of alignment and enhanced inter-connectivity between lamellae.The thickness of lamellae and inter-spacing of lamellae along the longitudinal direction of porous material demonstrate a decreasing trend with the increase in temperature gradient at the freezing front.The bridging density tends to increase with the increase of temperature gradient.（4）The compressive properties of porous zirconia ceramics are analyzed through the uniaxial compression tests of the scaffolds.The compressive strength is markedly decreased from the bottom to the top in case of the static ice-templating technique.The obtained porous material has low compressive strength and poor uniformity of strength along the height direction of the sample.There is an improvement in the compressive strength and its stability along the height direction for continuous ice-templated materials than those made by the static ice-templating technique at a similar porosity.The compressive strength demonstrates a constant decreasing trend with the increase in the thickness of lamellae or inter-spacing of lamellae,and increases with increasing bridging density for all the macro-porous ceramics.To summarize,there is a simultaneous improvement in both the freezing capacity,i.e.,indicated by the enlarged dimension of frozen body,and the freezing efficiency,i.e.,indicated by the much higher freezing rate.The ice-templated architectures are made more uniform and much finer,and exhibited higher extent of inter-connectivity between lamellae owing to the increased density of bridges.This study may facilitate the scale up of ice-templating technique and promote the exploitation and application of new high-performance materials.