Arup Purkayastha

The effect of nanoparticles on the liquid-gas surface tension of Bi2Te3 nanofluids

This work investigates the effect of size and concentration of nanoparticles on the effective gas-liquid surface tension of aqueous solutions of bismuth telluride nanoparticles functionalized with thioglycolic acid. The gas-liquid surface tension is obtained by solving the Laplace-Young equation under experimentally measured boundary conditions and droplet parameters. The results demonstrate that the gas-liquid surface tension depends on concentration as well as nanoparticle size. Solutions containing 2.5 and 10.4 nm nanoparticle diameters have been tested. For both, a minimum surface tension exists within the range of tested mass concentrations. The largest reduction in the surface tension (>50% versus bulk liquid) occurred for the 2.5 nm nanoparticle nanofluid. Accumulation and assembly of the charged nanoparticles at the liquid-gas interface was assumed to be responsible for the surface tension of the nanofluids investigated in this work.

Surfactant-Directed Synthesis of Branched Bismuth Telluride/Sulfide Core/Shell Nanorods†

Branched core/shell bismuth telluride/bismuth sulfide nanorod heterostructures are prepared by using a biomimetic surfactant, L-glutathionic acid. Trigonal nanocrystals of bismuth telluride are encapsulated by nanoscopic shells of orthorhombic bismuth sulfide. Crystallographic twinning causes shell branching. Such heteronanostructures are attractive for thermoelectric power generation and cooling applications.

Electrowetting on dielectric-actuation of microdroplets of aqueous bismuth telluride nanoparticle suspensions

This work reports the actuation of droplets of nanofluid by the electrowetting on dielectric (EWOD) effect. The nanofluid is comprised of an aqueous (deionized water) suspension of 3?nm diameter bismuth telluride nanoparticles capped with thioglycolic acid (TGA). Microdroplets of nanofluid are cast on Si(001) wafers coated with 100?nm thick layers of silicon dioxide and AF Teflon. Applying an electric field between the substrate and an electrode immersed in the nanofluid droplet results in a strong change in the contact angle from 110? to 84? for a 0?60?V voltage range. The droplets of nanofluid exhibit enhanced stability and absence of contact angle saturation in the tested voltage range when compared with droplets of aqueous solutions of 0.01?M Na2SO4 or thioglycolic acid in deionized water. We propose that ion generation due to capping-agent desorption is a key factor determining the EWOD effect in the bismuth telluride nanofluid along with the nanoparticle contribution to charge transport. Our results open up new vistas for using nanofluids for microscale?actuator device applications.

High-coercivity FePt nanoparticle assemblies embedded in silica thin films

The ability to process assemblies using thin film techniques in a scalable fashion would be a key to transmuting the assemblies into manufacturable devices. Here, we embed FePt nanoparticle assemblies into a silica thin film by sol-gel processing. Annealing the thin film composite at 650 degrees C transforms the chemically disordered fcc FePt phase into the fct phase, yielding magnetic coercivity values H(c)>630 mT. The positional order of the particles is retained due to the protection offered by the silica host. Such films with assemblies of high-coercivity magnetic particles are attractive for realizing new types of ultra-high-density data storage devices and magneto-composites.

Magnetic properties of Sb-doped FePt nanoparticles

Sb-doped FePt nanoparticles with an average diameter of 8.5nm were prepared by thermal decomposition of platinum acetylacetonate, antimony acetate, and iron pentacarbonyl. Upon annealing to ∼300°C for 30min, nanoparticles with XSb=0.14 and 0.23 show Hc>500mT, and L10 ordering parameter S values of ∼0.83–0.87. Transmission electron microscopy of the annealed assemblies shows no observable nanoparticle coalescence at 300°C. Low-temperature coercivity measurements with a superconducting quantum interference device indicate the presence of particles that exhibit superparamagnetism, probably due to the large particle size distribution or inhomogeneity in Sb incorporation. Our results underscore the necessity to synthesize monodisperse FePt nanoparticles with controlled composition to maximize ferromagnetic behavior.