• S Haldar

      Articles written in Bulletin of Materials Science

    • Synthesis of high-energy-density Pr2Fe14−xCoxB,x⩽3, magnets for practical applications

      S Haldar S Ram P Ramachandrarao H D Banerjee

      More Details Abstract Fulltext PDF

      Stable magnetic powders, of 1–2µm particle size, of partially Co-substituted, Pr2Fe14−xCoxB,x⩽3, alloys together with 2–4 at% excess Pr were prepared by rapidly quenching the associated melts into thin ribbons and then mechanical attriting the ribbons in the refined particle sizes. The saturation magnetizationMs, remanent magnetizationJr, intrinsic coercivityHci and Curie temperatureTc were studied in characterizing the powders for fabricating into sintered or polymer bonded magnets. It is found that the smallx=0·4–0·8 substitution of the Co on Fe sites in this series sensitively leads to an increase in the value ofHci, by as much as 40%, with the optimum value of 21 kOe atx ∼ 0·55, together with an improvement in theTc from 292°C to 325°C, without significantly diluting theMs∼150 emu/g andJr∼8·0 kG values. The Co-substituted Pr2Fe14B alloy particles are better stable and corrosion resistant in ambient atmosphere. The results are discussed with the microstructure and comparison with the data for Nd2Fe14B powders processed under the same conditions.

    • Formation of Nd2Fe14B hydride by milling of anhydride particles in toluene in a closed reactor

      S Ram H D Banerjee S Haldar P Ramachandrarao

      More Details Abstract Fulltext PDF

      When milling micrometer thin Nd2Fe14B platelets, of an average 1–2 mm diameter, in toluene in a closed reactor, part of the toluene decomposes at the surface of the platelets and yields nascent hydrogen and carbon/low hydrocarbons. The hydrogen diffuses into the Nd2Fe14B platelets and the carbon forms a thin surface passivation layer of the platelets, forming the stable Nd2Fe14BHx,x ≤ 5, hydride at room temperature. On heating in a calorimeter, the hydrogen desorbs off the sample with a well-defined endotherm between 370 and 425 K. An N2 gas atmosphere, if used during the heating, facilitates the H-desorption process with the modified kinetic parameters. For example, the enthalpy of the H-desorption ΔH and the related activation energyEa have the measured values ΔH = 153 J/g andEa = 58·2 kJ/mol in argon and ΔH = 256 J/g andEa = 41·6 kJ/mol in N2. It is argued that N2 gas has a fast reaction with the H atoms desorbing off the thin sample platelets and forms NH3 gas with an instantaneous decrease of the total external gas pressure at the sample. This supports the fast desorption of H atoms in the sample with the modified desorption kinetics in N2 gas.

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