Chapter 10:Modeling of Stiffness,Strength, and Structure-Property Relationship in Crosslinked Silica Aerogel Samit Roy and Awlad Hossain Department of Aerospace Engineering and Mechanics,The University of Alabama,Tuscaloosa,AL 35487.USA 10.1 Introduction Mechanically stable forms of lightweight materials with porosities up to 98%were first introduced in the form of silica aerogels in the 1930s. Recently,interest in aerogels and other lightweight materials in engineering applications have increased tremendously.Native silica aerogels are chemic- ally inert,low-density,nanostructured porous materials with poor mechanical properties.They are the product of the sol-gel process whose final step involves extracting the pore-filled solvent with liquid carbon dioxide through supercritical drying.Practical applications of native aerogels are somewhat limited as they are brittle and hygroscopic,absorbing moisture from the environment which eventually leads to aerogel collapse due to capillary forces in the pores.Nevertheless,it has been recently discovered that crosslinking the nanoparticle building blocks of silica aerogels with polymeric tethers increases both modulus and strength significantly [4]. Along these lines,a novel,multifunctional,crosslinked silica aerogel,to be
Chapter 10: Modeling of Stiffness, Strength, and Structure–Property Relationship in Crosslinked Silica Aerogel Samit Roy and Awlad Hossain Department of Aerospace Engineering and Mechanics, The University of Alabama, Tuscaloosa, AL 35487, USA 10.1 Introduction Mechanically stable forms of lightweight materials with porosities up to 98% were first introduced in the form of silica aerogels in the 1930s. Recently, interest in aerogels and other lightweight materials in engineering applications have increased tremendously. Native silica aerogels are chemically inert, low-density, nanostructured porous materials with poor mechanical properties. They are the product of the sol–gel process whose final step involves extracting the pore-filled solvent with liquid carbon dioxide through supercritical drying. Practical applications of native aerogels are somewhat limited as they are brittle and hygroscopic, absorbing moisture from the environment which eventually leads to aerogel collapse due to capillary forces in the pores. Nevertheless, it has been recently discovered that crosslinking the nanoparticle building blocks of silica aerogels with polymeric tethers increases both modulus and strength significantly [4]. Along these lines, a novel, multifunctional, crosslinked silica aerogel, to be
464 S.Roy and A.Hossain Nonporous secondary particles Surface carbonate (diamete灯<1nm dense silica) Channels to (OCN-R-NM(CO)O-Si-O-Si-) micropores Channels partially Mesopores blocked 5-10nm Porous secondary particles (density~2 silica0）(间 (b) Fig.10.1.(a)Silica aerogel structure before crosslinking and (b)x-aerogel structure after crosslinking [6] referred to as x-aerogel,is derived by coating and encapsulating the skeletal framework of amine-modified silica aerogels with polyurea as depicted in Fig.10.1 Aerogels are reported to be one of the best thermal insulators.When sandwiched between two glass layers,aerogels reduce heat loss coefficient by more than a factor of 10,while preserving the capability of moderately high light transmission [5].Aerogels are being considered for different aerospace applications,such as a thermal protection system (TPS),catalyst support,or as hosts for a variety of functional materials for chemical, optical,and electronic devices. Cylindrical samples of x-aerogel manufactured in the author's labo- ratory are shown in Fig.10.2.It was observed from the mechanical char- acterization tests that x-aerogel has very good compressive,tensile,and shear properties,in addition to its low thermal conductivity [4,6,8]. Recently,manufacturing of a lightweight cryogenic propellant tank with low thermal conductivity has been proposed using novel x-aerogel material. While the use of composite sandwich panels for cryotanks is not novel,it is feasible that the delamination of facesheet from the core due to cryo- pumping,analogous to the failure that occurred in the X-33 prototype,could perhaps be prevented through the use of x-aerogel core instead of a standard honeycomb core
referred to as x-aerogel, is derived by coating and encapsulating the skeletal framework of amine-modified silica aerogels with polyurea as depicted in Fig. 10.1. Fig. 10.1. (a) Silica aerogel structure before crosslinking and (b) x-aerogel structure after crosslinking [6] Aerogels are reported to be one of the best thermal insulators. When sandwiched between two glass layers, aerogels reduce heat loss coefficient by more than a factor of 10, while preserving the capability of moderately high light transmission [5]. Aerogels are being considered for different aerospace applications, such as a thermal protection system (TPS), catalyst support, or as hosts for a variety of functional materials for chemical, optical, and electronic devices. Cylindrical samples of x-aerogel manufactured in the author’s laboratory are shown in Fig. 10.2. It was observed from the mechanical characterization tests that x-aerogel has very good compressive, tensile, and shear properties, in addition to its low thermal conductivity [4, 6, 8]. Recently, manufacturing of a lightweight cryogenic propellant tank with low thermal conductivity has been proposed using novel x-aerogel material. While the use of composite sandwich panels for cryotanks is not novel, it is feasible that the delamination of facesheet from the core due to cryopumping, analogous to the failure that occurred in the X-33 prototype, could perhaps be prevented through the use of x-aerogel core instead of a standard honeycomb core. 464 S. Roy and A. Hossain
Chapter 10:Modeling of Stiffness,Strength,and Structure 465 江 可 Fig.10.2.Cylindrical samples of x-aerogel It is envisioned that aerogel material can be used as the central core bonded between two facesheets of a sandwich plate.As aerogels are highly porous,facesheets will be necessary to make the sandwich composite panels impermeable for storing cryogenic fuels.Facesheets can be made of carbon fiber-reinforced polymer(CFRP)having high tensile load-bearing capability.Schematics of a typical sandwich plate with a standard honey- comb core and an x-aerogel core at the center are shown in Fig.10.3.As another example of a practical application of x-aerogel,a conceptual design of a prototype cryogenic propellant tank using x-aerogel material is shown in Fig.10.4. Honeycomb core Facesheet Facesheet x-aerogel (a) (b) Fig.10.3.(a)Traditional sandwich panel with honeycomb core and (b)novel sandwich panel with x-aerogel as central core material
Fig. 10.2. Cylindrical samples of x-aerogel It is envisioned that aerogel material can be used as the central core bonded between two facesheets of a sandwich plate. As aerogels are highly porous, facesheets will be necessary to make the sandwich composite panels impermeable for storing cryogenic fuels. Facesheets can be made of carbon fiber-reinforced polymer (CFRP) having high tensile load-bearing design of a prototype cryogenic propellant tank using x-aerogel material is shown in Fig. 10.4. Fig. 10.3. (a) Traditional sandwich panel with honeycomb core and (b) novel sandwich panel with x-aerogel as central core material Chapter 10: Modeling of Stiffness, Strength, and Structure capability. Schematics of a typical sandwich plate with a standard honeycomb core and an x-aerogel core at the center are shown in Fig. 10.3. As another example of a practical application of x-aerogel, a conceptual 465
466 S.Roy and A.Hossain Inlet Port Sensor Port Vacuum Port -Outlet Port Aluminum Liner Aluminized Mylar Storage Tank X-Aerogel Maintained at Vacuum Outlet Jacket- Fig.10.4.Prototype cryotank design concept with x-aerogel material 10.2 Nanostructural Features of Silica Aerogel Aerogel is a class of monolithic material that possesses porous structure Scanning electron microscopy (SEM)[1]and transmission electron micro- scopy (TEM)[9]are widely used to produce direct images of mesoporous structures.For the crosslinked silica aerogel,SEM imaging was conducted for different loading stages,as shown in Fig.10.5.In this figure,the clusters of secondary nanoparticles are clearly visible along with the mesopores The mechanical,thermal,electrical,and optical properties exhibited by x-aerogels are related to their mesoporous cluster assemblies,as depicted earlier in Fig.10.1.The sol-gel manufacturing process can control the geo- metry,porosity,and physical properties of mesoporous silica aerogels by manipulating its chemistry and processing parameters.The stiffness and strength of x-aerogels strongly depend on their microstructural features, such as particle connectivity.However,there is no experimental technique currently available to measure this connectivity directly.As an alternative metric,the self-similar characteristics of aerogel structures can be investi- gated by evaluating their fractal dimension from geometric correlations
Fig. 10.4. Prototype cryotank design concept with x-aerogel material 10.2 Nanostructural Features of Silica Aerogel Aerogel is a class of monolithic material that possesses porous structure. Scanning electron microscopy (SEM) [1] and transmission electron microscopy (TEM) [9] are widely used to produce direct images of mesoporous structures. For the crosslinked silica aerogel, SEM imaging was conducted for different loading stages, as shown in Fig. 10.5. In this figure, the clusters of secondary nanoparticles are clearly visible along with the mesopores. S. Roy and A. Hossain The mechanical, thermal, electrical, and optical properties exhibited by x-aerogels are related to their mesoporous cluster assemblies, as depicted earlier in Fig. 10.1. The sol–gel manufacturing process can control the geometry, porosity, and physical properties of mesoporous silica aerogels by manipulating its chemistry and processing parameters. The stiffness and strength of x-aerogels strongly depend on their microstructural features, such as particle connectivity. However, there is no experimental technique currently available to measure this connectivity directly. As an alternative metric, the self-similar characteristics of aerogel structures can be investigated by evaluating their fractal dimension from geometric correlations. 466
Chapter 10:Modeling of Stiffness,Strength,and Structure 467 Fig.10.5.SEM images of crosslinked silica aerogels.The clusters of secondary nanoparticles (round particles)and the mesopores (dark spots)are clearly visible: (a)30%strain-no appreciable change in mesoporous structure,(b)45%strain- gradual decrease in the mesoporosity,few dark spots,and (c)77%strain appreciable loss of porosity,particles are squeezed closer to one another The shape of clusters or cluster configuration,the existence of voids of all sizes,and the gradual loss of connectivity among mesoporous particles suggest that a fractal dimension can be attributed to the x-aerogel structures as a useful descriptive parameter [5].The fractal dimension of a mesoporous structure can be determined from its particle orientation within a sphere of a given radius or from the slope of a radial distribution function.It was reported in the literature [5]that not only the mass of aerogel but also other properties,such as vibrational dynamics,scale according to aerogel's fractal dimension.In general,due to its inherent porosity,the aerogel morphology represents a fractal dimension of less than 3;and its fractal dimension decreases with decreasing cluster densities,as presented later in this chapter. In this study,a three-dimensional distinct element analysis (DEA) simulation was performed to determine the structure-property relationship of nanostructured x-aerogel material.The model attempted to incorporate microscale effects-such as particle bond stiffness,bond strength,particle frictional coefficient,initial cluster porosity (or density),and density of
Fig. 10.5. SEM images of crosslinked silica aerogels. The clusters of secondary nanoparticles (round particles) and the mesopores (dark spots) are clearly visible: (a) 30% strain – no appreciable change in mesoporous structure, (b) 45% strain – gradual decrease in the mesoporosity, few dark spots, and (c) 77% strain – appreciable loss of porosity, particles are squeezed closer to one another In this study, a three-dimensional distinct element analysis (DEA) simulation was performed to determine the structure–property relationship of nanostructured x-aerogel material. The model attempted to incorporate microscale effects – such as particle bond stiffness, bond strength, particle frictional coefficient, initial cluster porosity (or density), and density of Chapter 10: Modeling of Stiffness, Strength, and Structure The shape of clusters or cluster configuration, the existence of voids of all sizes, and the gradual loss of connectivity among mesoporous particles suggest that a fractal dimension can be attributed to the x-aerogel structures as a useful descriptive parameter [5]. The fractal dimension of a mesoporous structure can be determined from its particle orientation within a sphere of a given radius or from the slope of a radial distribution function. It was reported in the literature [5] that not only the mass of aerogel but also other properties, such as vibrational dynamics, scale according to aerogel’s fractal dimension. In general, due to its inherent porosity, the aerogel morphology represents a fractal dimension of less than 3; and its fractal dimension decreases with decreasing cluster densities, as presented later in this chapter. 467