No abstract

A circular disk resonator is used to study the gas sensing properties of carbon nanotubes. It detects the presence of gases based on the change in the dielectric constant rather than electrical conductivity of single walled carbon nanotubes (SWNTs) upon gas exposure. A conducting circular disk is coated with electric arc prepared SWNTs and degassed by heating under a high vacuum. It exhibits noticeable shifts in resonant frequency to both polar (NH3 and CO) and nonpolar gases (He, Ar, N2, and O2). Gas concentrations as low as 100 ppm can be detected using this sensor configuration.

We demonstrate a bulk process for the synthesis of Y-junction carbon nanotubes using Ti-doped Fe catalysts. It is shown that the nanotube branching can be induced or stopped at will by tuning the Ti composition in the catalyst particle that seeds the growth of nanotubes. Detailed electron microscopic studies suggest that the mechanism for the observed Y-junction formation is mediated via catalyst particle attachment on the walls of a growing MWNT, from which the branching nanotubes nucleate and grow. By controlling the Ti concentration in the precursor, cascading Y-junction series as well as quadruple junctions have been successfully synthesized, offering the possibility for interesting device applications. The simplicity and controllability of such an in-situ Y-junction fabrication technique make it an excellent source of ready-made networks for potential nanoscale devices.

A hydrothermal nanoparticle-plating technique has been employed in order to grow a layer of CoSb3 nanoparticles on the surface of La0.9CoFe3Sb12 bulk matrix grains. The nanoparticles have a typical size of 30–40 nm while the nano-layer can be up to several hundreds of nanometer thick. The nanoparticle layer, which resides at the grain boundary after hotpressing, provides an extra scattering channel for phonons, in addition to the “rattler” atoms (La), grain boundary scattering, and mass fluctuation mechanisms found within the bulk matrix grain. The electrical resistivity, thermopower, thermal conductivity, and Hall coefficient have been investigated as a function of temperature and the weight percentage (%) of nanoparticles. Largely due to the reduced lattice thermal conductivity, a ZT value of ∼0.5 is attained at 725 K on the sample with 5 wt % of nanoparticles showing a 15% improvement of the ZT from that of the sample without nanoparticles and comparable to the best value reported at this temperature.