| Fine, porous nickel powder with high specific surface area is currently in demand. Nickel powder is required as a conductive material for electrodes in nickel metal-hydride rechargeable batteries. In the future the major demand for nickel metal-hydride batteries will be for use in hybrid electric vehicles. The need will soon be great for a low cost and safe method for production of fine, porous nickel metal with good electrical conductivity. The aerosol flow thermal decomposition of nickel oxalate is an advantageous synthesis route for fine, porous nickel production. Nickel oxalate is the solid precursor and is fed as a dispersed dust cloud to a transport tube reactor. Particle heating rates are high in the transport tube, so reaction takes place on the order of seconds, limiting the degree of product growth and porosity loss. This dissertation work represents the first investigation into the aerosol flow production of fine nickel metal powder from a solid precursor. With a carrier gas mixture of 95% argon and 5% hydrogen, high purity (approaching 99% conversion) nickel product is synthesized at reactor wall temperatures of 885--980 K with residence times of ∼1.5--4.0 seconds. The global kinetics for the thermal decomposition of nickel oxalate within an aerosol flow reactor are studied by applying a one-dimensional particle phase model to the reactor. The first order solid state decomposition mechanism is interpreted as instantaneous nucleation of product nickel followed by two-dimensional growth of these nuclei controlled by diffusion of the product carbon dioxide. The best fit Arrhenius rate parameters along with 95% joint confidence interval limits are a pre-exponential factor of 1.25 x 109 +/- 0.73 x 109 s-1 and an activation energy of 1.30 x 105 +/- 0.20 x 10 4 J/mol. Feed precursor particles can be thought of as microcontainers. As the decomposition process begins, nickel primary particles, or grains, are nucleated within the microcontainer and continue to grow. The evolved product carbon dioxide gas leads to porosity increase and grain breakup. A model describing grain coalescence is developed based upon the self-diffusion coefficient of nickel. Product nickel particles are composed of spherical nanosized primary particles, or grains, held together in an overall particle matrix. |
