Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then co-polymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications.

This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.

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An overview on silica aerogels synthesis and different mechanical reinforcing strategies

Applications Rosaria Ciriminna,† Alexandra Fidalgo,‡ Valerica Pandarus, Franco̧is Beĺand, Laura M. Ilharco,*,‡ and Mario Pagliaro*,† †Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy ‡Centro de Química-Física Molecular and IN-Institute of Nanoscience and Nanotechnology, Instituto Superior Tećnico, Complexo I, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal SiliCycle Inc., 2500, Parc-Technologique Boulevard, Quebec City, Quebec G1P 4S6, Canada

The Sol–Gel Route to Advanced Silica-Based Materials and Recent Applications

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

Tailoring Mechanical Properties of Aerogels for Aerospace Applications

Rotating detonation engines (RDEs), also known as continuous detonation engines, have gained much worldwide interest lately. Such engines have huge potential benefits arising from their simplicity of design and manufacture, lack of moving parts, high thermodynamic efficiency and high rate of energy conversion that may be even more superior than pulse detonation engines, themselves the subject of great interest. However, due to the novelty of the concept, substantial work remains to demonstrate feasibility and bring the RDE to reality. An assessment of the challenges, ranging from understanding basic physics through utilizing rotating detonations in aerospace platforms, is provided.

https://arc.uta.edu/publications/cp_files/AIAA%202011-6043%20Rotating%20detonation%20wave%20propulsion%20experimental%20challenges,%20modeling,%20and%20engine%20concepts%20(invited).pdf

Rotating detonation wave propulsion: Experimental challenges, modeling, and engine concepts

Mixed mode fatigue crack growth: A literature survey

A general tetrakaidecahedron model for open-celled foams

We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacted with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Herein, we examine the effect of including up to 5% (w/w) carbon nanofibers in the silica backbone before cross-linking. The addition of 5% carbon nanofibers to the lowest density aerogels studied triples the compressive modulus and the tensile stress at break is increased five-fold with no density penalty. The carbon fiber also improves the strength of the initial hydrogels before cross-linking, which may have implications in manufacturing.

Reinforcing polymer cross-linked aerogels with carbon nanofibers

A study was done to determine the fatigue crack growth behavior of a PWA 1484 single-crystal nickel-base superalloy in a temperature range of 427 C to 871 C. Two distinctive failure modes were observed, which were a function of both temperature and frequency. At lower temperatures and higher frequencies crack growth occurred on the {l_brace}111{r_brace} octahedral slip planes at an oblique angle to the loading direction. Higher temperatures and decrease in frequencies favored a Mode I type failure process. The failure mode transitions were explained by invoking arguments based on environmental damage mechanisms. The fatigue crack growth rate data were analyzed using three different crack driving force parameters. The parameters investigated consisted of the Mode I stress intensity parameter corrected for the inclined crack trajectory, and two different octahedral Mode II parameters, which are based on the calculation of resolved shear stresses on the {l_brace}111{r_brace} slip systems. The Mode I {Delta}K parameter did a fair job in correlating the data but did not collapse it into a single narrow band. The two octahedral crack driving force parameters, {Delta}K{sub RSS} and a newly proposed {Delta}K{sub OCT}, collapsed all the data into a single narrow band. In addition to correlating the fatiguemore » crack growth rates, the two octahedral parameters also predicted the {l_brace}111{r_brace} planes on which the crack growth took place.« less

Fatigue Crack Growth Behavior of PWA 1484 Single Crystal Superalloy at Elevated Temperatures

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

Reliable engine-weight estimation at the conceptual design stage is critical to the development of new aircraft engines. It helps to identify the best engine concept amongst several candidates. In this paper, the major enhancements to NASA's engine-weight estimate computer code (WATE) are described. These enhancements include the incorporation of improved weight-calculation routines for the compressor and turbine disks using the finite difference technique. Furthermore, the stress distribution for various disk geometries was also incorporated, for a life-prediction module to calculate disk life. A material database, consisting of the material data of most of the commonly used aerospace materials, has also been incorporated into WATE. Collectively, these enhancements provide a more realistic and systematic way to calculate the engine weight. They also provide additional insight into the design tradeoff between engine life and engine weight. To demonstrate the new capabilities, the enhanced WATE code is used to perform an engine weight/life tradeoff assessment on a production aircraft engine.

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Louis J. Ghosn

Papers

A general tetrakaidecahedron model for open-celled foams

Reinforcing polymer cross-linked aerogels with carbon nanofibers

Fatigue Crack Growth Behavior of PWA 1484 Single Crystal Superalloy at Elevated Temperatures

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

A Computer Code for Gas Turbine Engine Weight and Disk Life Estimation