BiMn and BiMn-Based Ternary Solid Solutions or Compounds

The majority of currently used permanent magnets are based on compounds of rare earth elements (e.g., Co5 Sm, Nd2 Fe14B); however, the limited supply and constantly increasing price of rare earth elements start to pose a severe problem, especially in view of the application of permanent magnets in various environmentally friendly technologies. This initiated the search for other, rare earth-free materials that could serve as permanent magnets. Among the possible candidates is the intermetallic compound aBiMn which shows a number of highly interesting and promising magnetic properties: it is ferromagnetic with a critical temperature Tc above its peritectic decomposition, it shows magnetic anisotropy, and its coercivity increases with temperature. Yet the key problem in the study of BiMn and BiMn-based materials has been the extreme difficulty to synthesize the bulk compound in phase-pure form due to the peculiar phase relationships in the Bi-Mn system, the difference in melting temperatures and the slow diffusion kinetics of Bi. Attempts to improve the properties of BiMn by adding other elements have also not been overly successful in the past. Therefore it is proposed to study the preparation of phase-pure BiMn by different methods, i.e. by standard metallurgical methods and by various chemical methods (reduction from aqueous solution, reduction of oxides, and reduction of other chemical compounds containing Bi and Mn in an equimolar ratio). The obtained materials are to be characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), and magnetic measurements; thermochemical properties (heat capacity, enthalpy of formation) are to be determined by calorimetric methods. In a next step, partial phase equilibria in the three ternary systems Bi-Mn-Ni, Bi-Mn-Pt, and Bi-Mn-Rh are to be studied, especially along the isopleths connecting the AsNi-type phases in the corresponding binary systems with BiMn. In view of the possibility to modify/improve the magnetic properties of BiMn itself, the magnetic behavior of the ternary solid solutions or any ternary intermetallic compounds with AsNi-type structure will be measured. Although other transition metals like Ti, V, Cr, Fe, and Co apparently do not form compounds with Bi it is still considered worthwhile to investigate their solubility in the compound BiMn and to characterize the corresponding solid solutions, again with particular regard to their magnetic properties. Finally it is proposed to use the obtained experimental information on phase equilibria and thermochemical properties together with available literature data as an input in a CALPHAD-type optimization of the binary Bi-Mn system and the ternary Bi-Mn-Ni, Bi-Mn-Rh, and Bi-Mn-Pt systems.

Magnetic materials play an important role in modern society, especially for electric machines, i.e. motors and generators. Nearly all of the powerful permanent magnets in use contain so-called rare earth elements. These are currently mostly imported from China, their price is subject to considerable fluctuations, and an impending shortage in the near future cannot be excluded. Therefore, the search is on for possible substitutions that are free of rare earth elements. One candidate has been the compound BiMn that consists of the two metallic elements bismuth and manganese. However, up to now it had been impossible to synthesize this compound as pure bulk material.Different methods to prepare BiMn in pure form were tested in the present project; unfortunately, all of them failed, mainly due to the rather low decomposition temperature of the compound (355C). At these low temperatures, the reaction between bismuth and manganese is simply too slow and comes to a complete stop after some time (two to three weeks). The best result, with a purity of up to 87 % BiMn, was obtained by melting together the two starting materials and centrifuging off the supernatant liquid (which virtually always remains) at a temperature of 320C. All attempts to obtain BiMn by chemical reduction from an aqueous solution of the two components failed likewise, due to the very different chemical behavior of bismuth and manganese.As a possible alternative, it was attempted to stabilize BiMn by adding a third element, which forms either with manganese or with bismuth a chemical compound with the same crystal structure as BiMn. For this purpose the elements antimony or nickel, platinum, and rhodium, respectively, were considered (both MnSb as well as BiNi, BiPt, and BiRh crystallize in the same structure as BiMn). However, during a study of the corresponding alloy systems it turned out that none of these elements dissolves in BiMn in any significant amount. Furthermore, in two of the cases (manganese-antimony and bismuth-rhodium) it became necessary to investigate the phase diagrams of the two-component systems in detail.Although several new ternary compounds were discovered in the three-component systems they all were either nonmagnetic or their magnetism was found to disappear at rather low temperatures.Possibly one of the most important results of this project was the insight that the compound BiMn will never play a significant role as magnetic material, at least in bulk form, leaving aside possible applications as thin films or other very special forms.