Oriented Porous Materials Prepared By Ice Templating And Properties Of Their Composites

In recent years,by imitating the unique and excellent properties of natural porous materials（such as light density,high specific strength,high specific stiffness,good permeability,etc.）,the research of bioinspired porous materials has been rapidly developed.As one of the methods of artificial bioinspired porous materials,ice templating stands out because of its advantages such as simple operation,efficient preparation,low cost and environmental protection.Based on the ice template method,many different freezing techniques have been proposed to obtain bioinspired porous materials with light density,high strength,high fracture toughness and good heat insulation.However,with the development of science and technology and the increasing application demand,the design and control of porous structure have higher requirements,therefore,the innovation of the existing ice template technology to fabricate more controllable orientation,higher mechanical strength,more diversified application of bioinspired porous materials are urgent.In this paper,the preparation,regulation and performance of bioinspired porous materials fabricated by ice templating are studied as follows:By studying the effects of temperature gradients on ice nucleation in depth,the freezing precursor is poured into a copper mold with good thermal conductivity for ice templating experiment,we have got multi-orientation porous lamellar structure,and proposed a programmable local orientation of micro porous structure fabrication technology:Mold-Assisted Ice Templating technology（MIT）.By designing mold shapes,such as circle,square,triangle,star,Chinese Taiji,fish,etc.,programmable temperature field（direction controllable）is introduced in XY plane to realize the orientation control of local temperature gradient in three-dimensional space,and effectively edit the local micro orientation of porous structure,which fully confirms the universality of MIT technology.The solution concentration was changed to adjust the lamellar spacing（74μm to 42μm）and thickness（380 nm to 500 nm）to achieve further design and control of the material structure.The experimental equipment of MIT technology is simple,and it can prepare the local directional controllable microstructure that cannot be obtained by traditional molding method.The aerogel material obtained has the characteristic of negative Poisson’s ratio of compression（about-0.36）,which has a broad application prospect in the field of porous materials.Based on the principle of MIT technology,using polyethylene terephthalate（PET）material with poor thermal conductivity and copper material with good thermal conductivity to make composite material mold,we successfully prepared porous material with vertical arrangement like pine tower structure.With the increase of mold thickness t,the proportion of lamellate structure（near copper side）in the whole thickness in the XZ plane increases.When the thickness t is greater than 3 mm,the carbon nanotube（CNT）aerogel with vertical orientation like pine tower structure can be successfully fabricated.The lamellar structure of pure CNT aerogel is smooth,while the carbon nanotube/polyvinyl alcohol（CNT/PVA）aerogel has many tiny hairs,and the thickness of the lamellar layer is less than that of pure CNT aerogel.After composite with polydimethylsiloxane（PDMS）,PDMS was uniformly and continuously filled into the micron-scale porous structure designed by CNT aerogel to obtain CNT/PDMS composites without interface defects.The maximum bending stress of CNT/PDMS composites is about 1.06 MPa,which is 35.9%higher than that of pure PDMS.CNT/DMS composites have anisotropic conductivity,and the conductivity in the t direction is poor,and the resistivity is 5 times that in the H direction.It is found that the resistance change rate of CNT/PDMS composites is linearly correlated with the strain in the range of 60%compression strain.Based on the study of the excellent properties of inorganic porous materials,we successfully prepared micron-sized porous silica（SiO₂）materials with directional arrangement of pores by ice templating.The micropores of porous SiO₂ obtained by unidirectional freezing at-70℃showed random short-range ordered linear structure.In ethanol at-70℃,there was a continuous lamellar structure with obvious orientation.In liquid nitrogen,a part of SiO₂ particles were extruded out of the lamellar structure,and the spacing between the lamellar particles was smaller.The macroscopic morphology of porous SiO₂ shrinks after sintering at high temperature,and the sample changes from water soluble to water insoluble.With the increase of sintering temperature and sintering time,the sintering degree of the sample increases,the porosity decreases greatly,and the thickness of the pore wall increases gradually.After the porous SiO₂ material was combined with PDMS,we have got no obvious interface defects of SiO₂/PVA/PDMS composite materials.The existence of the porous SiO₂structure improves the compressive properties of PDMS.The higher the degree of sintering,the greater the modulus of composites.After 10 cycles of compression,no macroscopic cracks appeared in the composite SiO₂/PVA/PDMS.In the subsequent compressive strain of 0～57%range,the strength is still higher than pure PDMS,which have proved the SiO₂/PVA/PDMS has good flexibility and durability.