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

Engineering Education in Developing Nations: Progress on the School of Engineering at Northrise University in Ndola, Zambia

The engineering and technology capabilities of developed nations continues to advance and to be a major driver of the economies of those nations. Developing nations recognize this reality, and they accordingly recognize the importance of nurturing the growth of their own countries’ STEM capabilities. Engineering education in developing nations is thus a critical need; but it is work that, for a variety of reasons, tends to be under-resourced by developing nations themselves as well as by potential participants from developed nations. As followers of Jesus, we sometimes find our hearts stirred on behalf of our brothers and sisters in Christ in developing nations by needs along these lines, yet we also sense the staggering magnitude of the challenge. Thus, when the Lord opens an avenue for making a practical and lasting contribution, we might find it hard to resist getting involved. Over the past 5+ years, just such an avenue has been opened at Northrise University in Ndola, Zambia, and four professors—two from Dordt University and two from LeTourneau University—have indeed found the opportunity hard to resist. Led by these four, the development of a school of engineering at Northrise —specifically a 5-year bachelor’s degree program in Civil Engineering—is well along and is on track for a target opening date of February 2024. This paper will focus mostly on the practical aspects of the project, divided into three areas of activity related to the development of curriculum, facilities, and faculty. Where appropriate, attention is also given to some of the philosophical and cross-cultural questions that have naturally arisen as the endeavor has progressed

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

Nano-engineering Silica Aerogel Structure to Determine the Property-Structure Relationship

We characterize mechanically strong nano/meso-porous cross-linked templated silica aerogels that were synthesized through the sol gel process and reinforced by nano casting a 4-1 Onm thick conformai layer of isocynate derived polymer. Tri-block co-polymer (pluronic P123) was used as a structure directing agent to produce ordered mesoporous walls while 1, 3, 5 trimethylbenzene (TMB) was added as micelle-swelling reagent to regulate the size of the pores. The shape and size of the micro and meso pores were nano engineered by varying the amount of chemical surfactant as well as the concentration of the cross-linking solution used to form the polymer nano layer. In so doing we manipulated the structure at the molecular level to develop an optimized structure that closely resembles the honeycomb structure found in nature. Dynamic mechanical analysis (DMA) test results established that the material had an a-grass transition temperature of about 130°C while quasi-static compression tests showed that the optimized nano-structured silica aerogel had a Young\u27s modulus of about 800MPa. We present the synthesis protocol as well as chemical, physical and mechanical characterization of cross-linked templated silica aerogel (CTSA). ). In addition, material point method (MPM) simulation results are highlighted

Compressive Behavior of Mesoporous Silica Aerogels at High Strain Rates

Silica aerogel is the lightest solid on earth and has been considered for use in a variety of applications such as structural materials with good thermal/acoustic insulation capability. The actual applications, however, have been slow primarily because it is fragile. Recently, a crosslinking method has been developed to encapsulate silica aerogels to form a new class of aerogels called crosslinked silica aerogels (X-CSA). In this paper, we present results on the mechanical behavior of crosslinked mesoporous aerogels, MP4-X-1-T310. Using the newly developed SHPB, the mechanical behavior under impact for MP4-X-1-T310 was investigated at different strain rates. Pulse shaper was employed to achieve dynamic stress equilibrium and a constant strain rate under valid SHPB experiments. Results are compared with those of conventional engineering polymers, PMMA and PC Results indicate that the new templated aerogels have higher specific energy absorption than these conventional materials. In addition, due to the high porosity of the templated aerogels, the materials are absorbed by their pores during compression, making the templated aerogels an ideal multifunctional material for energy absorption and thermal/acoustic insulation. Using ultra-high speed camera, the dynamic deformation and failure behaviors of X-MP4-T310 were observed and discussed, which provides a comprehension understanding of the mechanical behavior for future structural application in armor systems

Dynamic Compressive Behavior of Crosslinked Silica Aerogels

Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework, and the thermal conductivity remains low at 0.041±0.001 W m -1K -1. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). The stress-strain relation is determined at high strain rates. The deformation and failure behaviors of the CSA are observed with a high-speed camera. The dynamic mechanical behavior of the CSA are discussed and compared with results from quasi-static experiments

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

Cross-linking 3D Assemblies of Nanoparticles into Mechanically Strong Aerogels by Surface-initiated Free-radical Polymerization

Skeletal nanoparticles of porous low-density materials formally classified as aerogels are cross-linked by surface-initiated polymerization (SIP) using a new surface-confined bidentate free-radical initiator structurally related to azobisisobutyronitrile (AIBN). Methylmethacrylate, styrene, and divinylbenzene are introduced in the mesopores, and upon heating at 70 °C, all mesoporous surfaces throughout the entire skeletal framework are coated conformally with a 10−12 nm thick polymer layer indistinguishable spectroscopically from the respective commercial bulk materials. the amount of polymer incorporated in the structure is controlled by the concentration of the monomer in the mesopores, and albeit an up to a 3-fold increase in bulk density (up to 0.6−0.8 g cm-3) and a decrease in the porosity even down to 40%, the materials remain mesoporous with average pore diameters increasing from 20 nm in the native samples to 41 and 62 nm in PMMA and polystyrene cross-linked samples, respectively. the new materials combine hydrophobicity with vastly improved mechanical properties in terms of strength, modulus, and toughness relative to their native (non-cross-linked) counterparts. the effect of polymer accumulation on the modulus has been also simulated numerically. Being able to use SIP for cross-linking 3D assemblies of nanoparticles comprising the skeletal framework of typical aerogels paves the way for the deconvolution of cross-linking from gelation (a free-radical versus an ionic process, respectively), so that ultimately all gelation and cross-linking reagents can be included together in one pot, leading to great process simplification. the mechanical properties of the new materials render them appropriate for anti-ballistic applications (e.g., armor)

Crosslinked Templated Mesoporous Silica Aerogels as Multifunctional Materials

Aerogels have high thermal insulation, high acoustic attenuation, and high specific strength. Their applications, however, are limited due to such problems as fragility and hydrophilicity. To resolve these problems, polymers are used to nanoencapsulate the templated silica nanoparticles, forming crosslinked templated aerogels. As a result, the mesoporous structure of the aerogels is maintained, thus providing high thermal insulation and acoustic attenuation associated with the bi-continuous mesoporous structures. The mechanical properties of these aerogels are measured. Results indicate that templated mesoporous aerogels have superior mechanical properties, with the specific energy absorption reaching 197 J/g. We perceive this work as a paradigm in the design of porous nanostructured materials, comprising three degrees of freedom, namely the chemical identity of the nanoparticles, the crosslinking polymer and the nanostructure morphology

Polybenzoxazine Aerogels: Synthesis, Characterization, and Conversion to Carbon and Graphite Aerogels

Polybenzoxazines (PBOs) are phenolic resin type polymers with high strength and high temperature stability comparable to that of polyimides. Here PBO aerogels were synthesized from bisphenol A, paraformaldehyde, aniline and DMSO. Resultant aerogels are macroporous materials and were characterized chemically (13C NMR), nanoscopically (SAXS, SEM) and macroscopically (uniaxial compression, thermal conductivity). Pyrolysis at 800 ⁰C yields robust, mainly microporous carbon aerogels, while further pyrolysis at 2300 ⁰C yields highly conducting graphite aerogels (50 mho cm-1) with a rod-like microstructure. The latter are evaluated for lithium intercalation