CRYSTALLIZATION KINETICS AND ANNEALING STUDIES ON ELECTROLESS AMORPHOUS TRANSITION METAL-METALLOID SYSTEM Ni-Co-P

Abstract

The Ternary amorphous Ni x Co y Pz system with composi- tional ranges of 15.3 < x < 55.5, 29.0 < Y < 70. 0 and 14. 7 < z < 15.5 has been prepared following an electroless deposition technique. The technique consist in using an alkaline bath containing nickel and cobalt sulphate, sodium citrate, ammonium sulphate and sodium hypophosphite through a chemical reduction on a substrates suitably sensetized by SnC12 and PdC12. The compositional analysis was done using Inductively Coupled Plasma (ICP) with an accuracy of better than +1%. The samples studied are categorized into following compositiOn ranges: Ni Co P Ni Co P 55.5 29.0 15.5' 39.0 46.0 15.0' Ni35.6Co49.1P15.3, Ni27.0Co58.0P15.0 and Ni15.3Co70.6P14.7 • Differential Scanning Calorimeter (DSC) is used for a study of the kinetics of crystallization and thermal stability aspects of the system. The structure of 'as-deposited' and annealed samples have been investigated using selected area mode of Transmission Electron' Microscope (TEM) and X-Ray Diffraction (XRD) techniques in the temperature range of 300 K to 823 K. The Magnetization and Electrical Resistivity behaviour of 'as-deposited' and on annealing have been investigated using Vibrating Sample Magnetometer (VSM) and Four Probe Resistivity measurements upto temperature of 850K. li The DSC response shows a small peak followed by two strong .exothermic reaction peaks (TPI TPII) in the temperature range of 550-800 K. The first small peak at about 500-550 K is attributed to structural stress. relaxation. The enthalpy change for first major reaction is maximum for nearly equal Nickel-Cobalt ratio where fluctuating type of trend is observed for second reaction. The limiting temperature (zero heating rate) for first reaction shows higher value of 595 K for sample containing 49.1 at% Cobalt. Non-isothermal Kissinger's method is applied for calculation of activation energies of first and second reactions with an accuracy of better than 5.3% . The activation energy for reaction under the first peak (TpI) is found to maximize to 189.7 KJ/mol for Ni35.6Co49.1P15.3 where that of second peak (TPII) tends to 177.0 KJ/mol for Ni39._uCo 46.0P15.0' The parameter 'n' is found to be 0.6<n<0.8 for first reaction and 1.1<n<1.4 for second reaction. The values of 'n' indicate that the- reaction under first peak taking place on pre-existing nuclie where the second one is eutectic type of transformation. The 'as-deposited' state of thin deposits and. bulk samples show diffuse ring and a broad' peek under SAD mode of TEM and XRD with d-spacing matching to (0002) and (111) iii planes of h.c.p. Cobalt (oc -Cobalt) and f.c.c. Nickel. The samples annealed to 628 K, the region of first strong DSC peak, show separation of h.c.p. Cobalt (0111, 0002, 0110) and f.c.c. Nickel (111) whereas the matrix remains still amorphous. When samples heated to 823 K, the second peak region of DSC, the matrix crystallization takes place through appearance of many metastable and stable phases. The structural annealing behaviour of amorphous Ni-Co-P system can be divided into two broad categories: 1) the alloys having the composition neat to that of binary system, that is sample Ni55.5Co29.0P15.5 and Ni15.3Co70.0P14.7 show the final matrix crystallization consisting of equilibrium phases Ni3P and Co2P. 2) The alloy8 having the intermediate compositions, Ni39.0C°46.01315.01 Ni35.6Co49.1P15.3 and Ni27.0 Co58.0 P15.0 transform to many non-equilibrium phases, Ni12P5' Ni7P3, Ni5P4, COP4 and CoP2 beside the equilibrium phases Co2P and Ni3P. Magnetization of 'as-deposited' samples increases from 3.33 emu/g for Ni55.5 Co29.0 P15.5 to 49.2 emu/g for Ni15.3Co70.0P14.7 and with 'nt' number of electrons transferred from metalloid to be 4.5 resona-bly good agreement is found between theoretical and experimental values. The annealing behaviour of magnetic moment reveals that the initial increase in magnetization at about 560 K is due to separation of h.c.p. Cobalt and f.c.c. Nickel as indicated by iv TEM and XRD studies. The matrix crystallization process causes a relatively lower rate of change in magnetization. The electrical resistivity of 'as-deposited' samples show the expected relation between resistivity ijp and Temperature Coefficient of Resistivity (TCR)_Jimul,confirms the mooij correlation for TM-M systems. The resistivity is found to be 114.5 pl. cm for Ni55.5Co29.0P1.5 sample and increases as Cobalt composition of the sample increases and tends to maximum value of 122.5 piL cm for Ni39.0Co 46.0P15.° with reduce in TCR from 13.06 x 10-5K-1 to 4.07 x 10-5K-1. On annealing, the minor drop in resistivity at temperature of about 473 K may be attributed to stress relaxation. The major drops of electrical resistivity are closely associated with the crystallization process. The ternary Ni-Co-P samples with 29.0, 58 and 70.0 at % Cobalt show the separation of primary phases of h.c.p. Cobalt and Nickel distinct from that of matrix crystallization. In case of samples having nearly equal ratios of Cobalt and Nickel the two stages of crystallization are not distinct. • To conclude, our study of the mode of crystallization in ternary amorphous Ni-Co-P shows that, crystallization takes place in two distinct _steps; the first step is, a primary crystallization, involving the separation 'of phases of M -Cobalt and Nickel from amorphous matrix. Second step, beleive to be an eutectic crystallization, shows that in case of Ni39.0 Co46.0 P15.0' Ni35.5 Co49.1 P15.3 and Ni27.°Co58.0P15.0 V matrix crystallization results in appearance of a large number of metastable phases, while Ni55.5Co29.0P15.5 and Ni15.0Co70.0P15.0 which are much closer to a corresponding binary system Ni-P or Co-P show predominantly formation of equilibrium crystalline phases only. The addition of Cobalt in equal proportion of that of Nickel enhances the stability of the Ni-CO-P alloy and phosphorous diffusion is the slowest step at these compositions.