Thermal Effects on Nano-Energy Absorption Systems (Nano-EAS)

Development of intelligent and ecological energy absorption systems (EAS) is important to various fields such as automotive (vehicle suspensions, bumpers, engine mounts), construction (protections against seismic and wind-induced vibrations), and defense (parachuted objects, armors). Usual EAS use composites, shape-memory alloys and foams. Recently, liquid adsorption/desorption in/from nanoporous solids was employed to develop high-performance nano-EAS. Energy loss is based on the well-known capillary phenomenon: external work must be done to spread a non-wetting liquid on a solid surface. Nano-EAS provide considerably higher dissipated energies, about 1–10J/g at deformability of 30–70%, compared with the energy absorption of Ti-Ni alloys, about 0.01–0.05J/g at deformability of 5–8%. For water against hydrophobic nanoporous silica gel (artificial sand), the nano-EAS become ecological; they can be also made intelligent by thermo-electrical control. Relative to thermal effects, Qiao et al. have investigated, for nanoporous silica gel with insufficient coverage of the alkyl-based hydrophobic coating, the problem of hysteresis recovery by increasing the temperature in the range 20∼80°C. Energy loss capacity reduced severely after the first loading-unloading cycle, so, the hysteresis was found as irreversible. Shape of the first hysteresis, the accessible specific pore volume and the desorption pressure were almost unaffected by the temperature change. At temperature augmentation the second hysteresis was partially recovered and when the temperature exceeded 50°C the system became almost fully reusable. Water inflow was found as governed by Laplace-Washburn equation but the outflow process was perceived as thermally aided. On the other hand, Eroshenko et al. have contradictorily obtained for nanoporous silica gel with full coverage of the alkyl-based hydrophobic coating, a stable hysteresis at repeated working cycles. Adsorption pressure decreased and desorption pressure increased at temperature augmentation, this producing a reduction of the hysteresis area and damping. However, the accessible specific pore volume was found as thermally unaffected. Oppositely, both the in- and out-flows were found as governed by Laplace-Washburn equation. In this work, for nanoporous silica gels with partial and full coverage of the alkyl and fluorocarbon based hydrophobic coatings, the thermal effects on the hysteresis and damping performances are studied. Test rig used is a compression-decompression chamber introduced inside of an incubator that allows temperature adjustment in the range of −10∼50°C. Results reveal that, depending on the hydrophobic coating coverage, findings reported by Qiao et al. and Eroshenko et al. are in fact not contradictory but complementary. However, as expected, the accessible specific pore volume was found to decrease at temperature reduction. In order to explain all these apparently opposite experimental findings, a model based on the water cluster size distribution versus temperature, the pore size distribution of silica gel and the ability of water molecules to form hydrogen bonds with the uncovered hydroxyl groups on the solid surface is proposed.