EFFECTS OF ALLOYING ADDITIVES ON STRUCTURAL, THERMAL AND MAGNETIC PROPERTIES IN EXCHANGE COUPLED Nd-Fe-B NANOCOMPOSITES

Abstract

The research work included in this thesis deals with studies of the effects of a partial substitution of Fe atoms by alloying elements Mn and Sn on the thermal, magnetic and microstructure properties of exchange coupled Nd2Fe14B/Fe3B nanocomposite alloys. Part of Nd is replaced by Pr in elucidating the role of the hard magnetic phase in the final magnetic properties. Aim is two fold; one from the applied point of view and other from fundamental aspects. In the application part, the main objective is to develop a nanocomposite magnetic alloy with large coercivity (iHc) and energy-product (BH)max, whereas in the other part, the motive is to develop basic understanding of the phase formation and magnetic properties in correlation to the microstructure and Mössbauer spectra. Three series of Nd-lean alloys (i) Nd4.5Fe77-xMnxB18.5 (x = 0, 1 and 2), (ii) B Nd4.5Fe77-xSnxB18.5 (x = 0, 0.5, 1.0 and 1.5), and (iii) (Nd1-yPry)4.5Fe77B18.5 (y = 0, 0.22, B B 0.45 and 0.66) are chosen in developing useful magnetic properties. Results and analysis of the thermal, magnetic, thermomagnetic, microstructure, and Mössbauer studies obtained on these alloy series under selective experimental conditions are described in four different Chapters. Chapter-1 provides a general introduction to the exchange coupled Nd2Fe14B/Fe3B nanocomposites, with statement of the problem, review of the literature along with the motivation behind selecting this specific series of the alloys, and typical physical properties and applications of such alloys. Chapter-2 describes experimental details of the alloy formation and sample preparations in part of the measurements and analysis of the different properties. Chapters-3 deals with studies of the effects of a partial Mn → Fe substitution on the magnetic properties of the Nd2Fe14B/Fe3B nanocomposite alloys in Nd4.5Fe77-xMnxB18.5 (xB = 0, 1, and 2) alloys. Optimum values iHc = 3.130 kOe and (BH)max = 10.34 MGOe could be obtained upon an optimal 1 at% Mn-content. Chapter-4 presents the crystallization kinetics of nanocrystals in liquid-liquid phase separation regions in the supercooled Nd4.5Fe77-xMnxB18.5 (x = 0, 1) alloys. B The effect of a partial Sn → Fe substitution on the crystallization sequence and magnetic properties in Nd4.5Fe77-xSnxB18.5 (x = 0, 0.5, 1.0, 1.5) nanocomposite alloys has B been discussed in Chapter-5. The Sn addition affects markedly the formation sequence of the Nd2Fe14B/Fe3B nanocomposite so that the supercooled alloy crystallizes in a single step subject to thermal annealing. At an optimal 1.5% Sn-content, improved Mr, Mr/Ms, and (BH)max-values stand to be 12.642 kG, 0.82, and 12.83 MGOe relative to the parent alloy of 10.165 kG, 0.70 and 9.12 MGOe, respectively. The Sn substitution leads to increase the TC-value 588 K of the hard magnetic phase Nd2Fe14B to as large value as 602 K. Chapter-6 describes magnetic properties of (Nd1-yPry)4.5Fe77B18.5 (y = 0, 0.22, 0.45 and B 0.66) nanocomposite alloys. Optimum values iHc = 3.624 kOe and (BH)max = 13.236 MGOe have been obtained on y = 0.45 Pr-content. A summary and conclusion of the major findings in this work are briefed in the last Chapter-7 Keywords: Exchange coupling, Nd2Fe14B/Fe3B nanocomposite, Permanent magnet, Coercicity, Mössbauer effect