Applications of high-intensity ultrasound for synthesis and modification of advanced materials.

摘要

Irradiation of liquids with powerful ultrasound produces transient cavitation: nucleation, growth and violent collapse of bubbles. The implosive bubble collapse generates localized hot spots with temperatures as high as 5000 K, pressures of about 800 atm, and cooling rates exceeding 1010 K/s. This induces intense shock waves, which propagate in the liquid at velocities well above the speed of sound. In the case of slurries, these shockwaves lead to an extremely rapid mass transfer and induce high velocity collisions among solid particles suspended in ultrasonically irradiated liquids. Such interparticle collisions result in extreme heating at the point of impact, which can lead to effective localized melting and significant increase in the rates of many solid-liquid reactions.; Ultrasound irradiation of the decane slurries containing various loadings of ∼5 mum zinc powder produces dense 50--70 mum zinc agglomerates. The size of the initial solid particle is critical for effective interparticle collisions. Particles smaller than a few microns or larger than a few tens of microns do not collide with sufficient energy to cause a localized melting. Ultrasonic irradiation of mixtures of appropriate size particles with those too large to undergo fusion on impact, still lead to efficient agglomeration.; The effects of cavitation in this phenomenon of interparticle collisions are due to shockwaves generated in the liquid, and not to extreme temperature of the sonochemical hot-spot.; Applications of superconductors are determined by the critical current density, above which a superconductor becomes resistive and dissipates energy. To increase the critical value, small defects are introduced into the superconductor's bulk ("pinning"). Ultrasonic irradiation of alkane slurries containing polycrystalline superconducting materials, with Fe(CO)5 or MO(CO) 6 leads to embedding of produced in situ ferromagnetic Fe2O3 or Mo2O5 nanoparticles into the bulk of superconducting material, thus creating novel nanocomposite materials. Study conducted on obtained Fe2O3-MgB2 and Mo2O5-MgB2 nanocomposite material exhibit significant enhancement of magnetic flux pinning. Similar study focused on such promising high-Tc superconductors as YBa 2Ca3CuO7-delta and Bi2Sr 2CaCu2O8+x. (Abstract shortened by UMI.)