Synthesis, Mechanical And Chemical, Characterization Of Vanadium- Based Aerogels

Monolithic aerogels are highly mesoporous materials that have low density, low thermal conductivity, low dielectric constant as well as high acoustic impendence, a few of the properties that make them attractive for wide range of applications in thermal and acoustic insulation, electronics, separations and catalysis. However, fragility, hydrophilicity, as well as the requirement for drying using supercritical fluid extraction has limited the actual use to only specialized space applications or as Cerenkov radiation detectors in some types of nuclear reactors. Recently, the fragility problem was solved by casting a conformal polymer coating over the skeletal framework of typical silica aerogels prepared via a base-catalyzed sol-gel method (Leventis et al. 2002). That framework consists of a pearl-necklace like three-dimensional assembly of nanoparticles. The applied polymer coating cross-links the nanoparticles by developing covalent bonding with their surface and reinforces the structure without clogging the pores. Thus, the density typically increases by a factor of 3, while the strength at failure increases by a factor of 300 with a remaining porosity at 70% (Leventis et al. 2002; Zhang et al. 2004; Bertino et al. 2004). Cross-linked samples are able to deform by over 77% compressive strain without developing surface cracks, and remain stable when saturated with water.Mechanical & Aerospace Engineerin

Scalable, Hydrophobic and Highly-Stretchable Poly(isocyanurate-Urethane) Aerogels

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability. Herein, we report the fabrication of a family of low-density, ambient-dried and hydrophobic poly(isocyanurate-urethane) aerogels derived from a triisocyanate precursor. The bulk densities ranged from 0.28 to 0.37 g cm-3 with porosities above 70% v/v. The aerogels exhibit a highly stretchable behavior with a rapid increase in the Young\u27s modulus with bulk density (slope of log-log plot \u3e 6.0). In addition, the aerogels are very compressible (more than 80% compressive strain) with high shape recovery rate (more than 80% recovery in 30 s). Under tension even at high strains (e.g., more than 100% tensile strain), the aerogels at lower densities do not display a significant lateral contraction and have a Poisson\u27s ratio of only 0.22. Under dynamic conditions, the properties (e.g., complex moduli and dynamic stress-strain curves) are highly frequency- and rate-dependent, particularly in the Hopkinson pressure bar experiment where in comparison with quasi-static compression results, the properties such as mechanical strength were three orders of magnitude stiffer. The attained outcome of this work supports a basis on the understanding of the fundamental mechanical behavior of a scalable organic aerogel with potential in engineering applications including damping, energy absorption, and substrates for flexible devices

Mechanical Characterization Of Aerogels

As multifunctional porous nanostructured materials (e.g., thermally/acoustically insulating), aerogels are derived from their vast porosity and their high specific surface area and may also hold exceptional specific mechanical properties under certain conditions as well. In this chapter, the mechanical characteristics of aerogels are discussed in detail. First, the mechanical characterization of traditional aerogels is summarized, and then, the mechanical behavior of polymer crosslinked silica and vanadia (X-aerogels), as well as organic aerogels, is presented. Finally, the acoustic attenuation property is briefly discussed for polyurea aerogel. In polymer crosslinked aerogels, a few-nanometer-thick conformal polymer is coating on secondary particles, while the pores is not clogging, which thus preserves the multifunctionality of the native framework and improves the mechanical strength. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression, and flexural bending testing) and high-strain-rate loading conditions using a split Hopkinson pressure bar. We evaluated the effects of strain rate, mass density, loading–unloading, moisture concentration, and low temperature on the mechanical properties of aerogels. Digital image correlation was used to analyze the surface strains through ultrahigh-speed images for calculation of properties such as dynamic Poisson\u27s ratio. A remarkable result is that crosslinked vanadia aerogels remain ductile even at −180 °C, indicating a property derived from interlocking and sintering-like fusion of skeletal nanoworms during compression. Due to the substantial improvement in the mechanical properties of X-aerogels with a small amount of polymeric crosslinking agent, purely polymeric aerogels with similar X-aerogel nanostructures were investigated. Therefore, in this chapter, the mechanical properties of organic aerogels including polyurea and polyurethane aerogels were also studied. Furthermore, a special attention has been carried out on the acoustic attenuation of polyurea aerogels by means of normal incidence sound transmission loss measurements. Polyurea aerogels showed unprecedented high sound transmission losses over a broad range of frequencies, a trend that clearly breaks the empirical Mass Law nature of the conventional acoustic materials

Sound Insulation Properties in Low-Density, Mechanically Strong and Ductile Nanoporous Polyurea Aerogels

Aerogels are quasi-stable, nanoporous, low-density, three-dimensional assemblies of nanoparticles. In this paper, an extremely high sound transmission loss for a family of ductile polyurea aerogels (e.g., over 30 dB within 1 to 4 kHz at bulk density 0.25 g/cm3 and 5 mm thickness) is reported. The fundamental mechanisms behind the aerogel acoustic attenuations are investigated. Sharing striking similarities with acoustic metamaterials, initially, aerogels are studied via a one-dimensional multi degree-of-freedom mass-spring system. Different effects such as spring constant disparity are investigated in regards to the structural vibration wave transmission loss. Results are given for different configurations consistent with the aerogel nano/microstructures. A significant wave attenuation is observed by considering a random spring distribution. In the next step towards modeling such a complex hierarchical and random structural material, the continuum Biot\u27s dynamic theory of poroelasticity is implemented to analyze the experimental sound transmission loss results. In this framework, a two-dimensional plane strain analysis is considered for the interaction of a steady state time-harmonic plane acoustic wave with an infinite aerogel layer immersed in and saturated with air. The effects of bulk density and thickness on the aerogel sound transmission loss are elucidated. By comparing the theoretical results with the experimental observations, this study develops a qualitative/quantitative basis for the dynamics of the aerogel nanoparticle network as well as the air flow and solid vibroacoustic interactions. This basis provides a better understanding on the overall acoustic properties of the aerogels that might also be helpful in the design of the future hierarchical materials

Sound Transmission Loss Enhancement in an Inorganic-Organic Laminated Wall Panel Using Multifunctional Low-Density Nanoporous Polyurea Aerogels: Experiment and Modeling

Recently, the authors have reported an exceptional normal incidence sound transmission loss characteristic for a class of low density, highly porous, and mechanically strong polyurea aerogels. Herein, a laminated composite comprising the organic low-density aerogels bonded with an inorganic compound (e.g., gypsum materials) is considered to investigate the constrained damping effects of the aerogels on the airborne sound insulation behavior of the composite using the standard chamber-based diffuse sound field measurements. Huge improvement in the sound transmission loss is obtained due to the use of aerogel without a significant increase in the overall weight and thickness of the composite panel (e.g., more than 10 dB increase by reaching 40 dB sound transmission loss at 2 kHz after the implementation of only two 5 mm-thick aerogel layers at bulk densities 0.15 and 0.25 g cm-3). This uncommon behavior breaks the empirical Mass Law nature of the most conventional acoustic materials. In addition, an exact analytical time-harmonic plane-strain solution for the diffused wave propagation through the multilayered structure is provided using theories of linear elasticity and Biot\u27s dynamic poroelasticity. The theoretical results are well supported by the experiments which can be utilized for the design of the future light-weight multifunctional composite structures