Rheology Of The SiO₂ Suspensions And The It's Application In The Defensive Materials

The shear thickening fluid(STF)can be defined as the system whose viscosity increases when certain force is imposed on it.STF is the key material to "fluid body armor materials" reported by American recently,and it is also used in the design of damping and control devices. So it is significant to investigate STF and the factors influencing its shear thickening behavior.The influences of several factors,such as the mass fraction of SiO₂, the temperature,the frequency,the property of suspending medium and the electric charge on the rheological and viscoelastic properties for the SiO₂ suspension have been studied by stress controlled rheometer.The energy dissipated(E_d)during stress shear for SiO₂ suspensions has been evalulated.The relation between E_d and the maximum strain(γ_o)has been studied.Besides,the defensive properties of the STF-Kevlar have been studied through stab and shooting experiments.The rheological experiments show that the nano-SiO₂ suspensions in polar organic fluids present reversible shear-thinning and shear-thickening behavior which can be explained by "clustering" theory.This is an increasing indication that "clustering" theory is the primary principle for shear-thickening behavior while the "order-disorder transition" mechanism is only a possible concomitant effect in the shear-thickening fluids.The rheological properties of nano-SiO₂ suspension in polyethylene glycol(PEG)show that the mass fraction,the diameter of the SiO₂,the temperature,the distance between the cone and the plate,and the average molecular weight of PEG have no influences on the critical stress(σ_c). The higher frequency would bring larger critical stress and the viscosity. The shear-thickening behavior of the system strongly depends on the mass fraction of SiO₂,the temperature,the molecular weigh of the PEG and the frequency.The viscoelastic properties at high frequency for the nano-SiO₂/PEG suspensions have been investigated for the first time.The systems present linear viscoelastic,shear-thinning and shear-thickening behavior as the shear stress increases.In linear viscoelastic region,the storage modulus (G')and loss modulus(G")almost remain unchanged,and increase as the SiO₂ fraction increases with the power law exponent of 4.43 and 2.26 respectively,and the linear viscoelastic regions broaden as the frequency increases.In shear-thinning region,G' decreases,but G" almost remains unchanged.In shear-thickening region,G' and G" increase simultaneously.Meanwhile the larger G" over G' in all the stress indicates that the suspensions mainly possess viscous property,and the energy dissipated is larger than the energy stored.E_d can be obtained by integrating the area contained in a plot of stress vs.strain.The relation between E_d andγ_o can be expressed by the equation:E_d = Aγ_oⁿ,where A,the pre-exponent factor;n,the power law exponent,are constants. Before the shear-thickening behavior takes place,n is close to 2,while in shear-thickening region,n increases with the increasing molecular weight of PEG.The polarity effects of the polar suspending medium(ethylene glycol, propylene glycol and butylene glycol et al.)on the rheological and viscoelastic properties for SiO₂ suspensions have also been investigated for the first time.A conclusion can be obtained:First,the critical stress connects with not only the viscosity of the medium,but also the forces between the SiO₂ and the medium.The stronger forces between the SiO₂ and the medium,and the stronger Brownian motion will bring larger critical stress.Second,E_d of the systems show an exponential increase withγ_o by the exponents of about 2 under low shear stress,however,in the shear-thickening region,the power law exponents become larger with the medium viscosity increasing.Meanwhile,compared with that of the nano-SiO₂ suspension,the rheological and viscoelastic properties of ultrafine SiO₂ suspension in ethylene glycon(EG)have been explored.For the ultrafine SiO₂/EG suspension,the increase degrees of complex viscosity and moduli(G' and G")are larger than those of the nano-SiO₂ system.Though the power law exponent of E_d versusγ_o of the ultrafine SiO₂/EG system is larger during shear-thickening region,E_d is smaller than that of the nano-SiO₂ system.The larger diameter of the SiO₂ makes the shear thickening take place more easily and the power law regression yields theσ_c dependence on the particle size(a)asσ_c=Aa^{-2.91±0.04}.Additionally,the effect of the electric charge on the critical stress of the untrafine SiO₂ system was investigated.The more electric charge,the larger the critical stress.At last,the defensive properties of nano-SiO₂ STF-Kevlar woven fabrics have been investigated through stab and shooting experiments. Four layers of the nano-SiO₂ STF-Kevlar can withstand the impact of the spike dropping from 1.07 m with the energy of 24.0 J,while 12 layers of the ultrafine SiO₂ STF-Kevlar were used for the spike dropping test from 0.75 m with the energy of 17.5 J reported by American.For the bullet with high speed of 450 m/s,all the nano-SiO₂ STF-Kevlar targets present less stabbed depth and wider area of the impact zone.The stabbed layers of the STF-Kevlar targets with the nano-SiO₂(11 nm)is less than that of the neat Kevlar targets,while the STF-Kevlar targets with ultrafine SiO₂ have no obvious deduction in the stabbed layers. Hence the nano-SiO₂ STF-Kevlar targets have better defensive quality than that of ultrafine SiO₂ ones.