Papers by Author: Ramalinga V. Mangalaraja

Abstract: Lower energy-ball milling was used to prepare magnetic granular Ni5CoXCu95-X alloys produced by mechanical alloying through a milling process and subsequent annealing process, have been investigated. The pure copper shows high electrical conductivity and malleability, however the Cu-Co system in the thermodynamic equilibrium is non-soluble below 500°C. Nevertheless, mechanical alloyed particles of Cu with 5-7%Co and 5%Ni can be subjected to annealing at 500°C or consolidation-sintering treatments to obtain composite materials thereby improving their mechanical and magnetic properties suitable for electronic devices. The ultrafine Co and (Co,Ni) particles reduced and dispersed in the copper powder matrix with milling times of 20 to 60 h and thus affected the magnetic properties of the as-milled Ni5CoXCu95-X powder obtained from this non-equilibrium phases synthesis. The magnetic properties of the supersaturated solid solutions are strongly dependent on the interactions among the magnetic particles and the nanometric size of these particles. The morphology, structure and size of as-milled and sintered powders were characterized by SEM, HRTEM and XRD techniques. The results show that the microstructure, hardness and magnetic properties of the granular Ni5CoXCu95-X alloy have strong dependence of milling time. The continuous decrement of Ms as a function of milling time is a consequence to the variation of phase in the composition with formation of CoNi particle and the partial change of fcc-Co to hcp-Co. Super-paramagnetic behavior is observed in both as-milled and annealed powders, with a maximum Hc of 250-260 Oe obtained for 7%Co after 60h of milling. The effect of Nickel on the Ni5CoXCu95-X can be explained as Ni content inhibit the two-solid (Cu-Co) phases segregation of the alloys when annealed at high temperature, leading to a grained structure with precipitated Co particles in homogeneous Cu-Ni strengthened solid solution matrix.

Abstract: The structure, mechanical and magnetic properties of Cu90Co5Ni5 alloys produced by mechanical alloying and subsequent cold consolidation and sintering behavior, have been investigated. A system of small Co and Ni magnetic particles embedded in the non-magnetic copper matrix were prepared through a mechanical milling process by using a planetary ball mill under argon atmosphere for 20 to 60 h. The morphology and particles size, phase formation and chemical composition of the alloyed powder samples for each milling time were characterized by scanning electron microscope and powder X-ray diffraction techniques, respectively. After milling for 60 h, a supersaturated solid solution with coercive field Hc with maximum value of 235Oe was obtained. The continuous decreasing trend of saturation magnetization (Ms) with increasing of milling time can be explained by the reduction of copper oxide by (CoNi) oxide formation, confirmed by powder XRD patterns. The XRD analyses of the as-milled samples revealed that the Bragg peaks of FCC-Co changed partially to HCP-Co on increasing the milling time. Cu90Co5Ni5 powders cold consolidated and sintered at 650°C for 1h segregated mainly into two-phases of mixed (fcc,hc)-Co and fcc-CuNi. After sintering, the mechanical properties for 60h milling reached its optimum, 26HV in hardness corresponding to 250MPa as compressive strength. TEM microanalysis of sintered alloys revealed Co cluster of 2 to 5 nm in size separated each one by 10 to 20 nm in size. The variation of magnetic properties and its dependence on structural-precipitation change with milling time are discussed.

Abstract: Micro-tube type Gd-doped CeO2(CGO) reactor were prepared by hydrolysis of metal acetate gel at room temperature. The size of anode tube was ca. 0.8mm in diameter, and length was 20mm after sintering at 1400°C. DC conductivity of the prepared dense CGO cell consisting of 500nm grains was about 10 Ohm-cm at 600°C. Electrical power density of the micro-tube fuel cell, which was coated both CGO electrolyte and CGO/La0.6Sr0.4Co0.2Fe0.8O3 cathode on the surface of 40vol%NiO-CGO anode tube, was about 900mW/cm2 at 600°C under 30ml/min H2 gas flowing.