A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

SILICIDESA metallic mixed-lanthanide-containing material, a ferro-silicon alloy called rare-earthsilicide, is produced in a submerged electric arc furnace by the direct reduction of bastnasiteconcentrate, together with iron ore and quartz. The resulting alloy is approximately 1/3mischmetal, 1/3 silicon, and 1/3 iron and is used as an additive to alloy steels[1] and to castirons[2]. (No benefit can be gained by separating the lanthanide precursor into individual Ln's priorto reduction as there is no major difference in behavior between different lanthanides. In ferrousmetallurgy, silicon is often present in the final product.)Rare Earth SilicideTypical Analysis %Fe ≈33Ce ≈16La ≈11Nd ≈2misc Ln’s ≈2Si ≈33The silicide additives provide lanthanides that, in cast irons, can control morphology of theprecipitated graphite that defines the iron's properties. However, such control is now moreeconomically supplied by Mg and the role of the Ln's has become that of specialist additive withinsuch silicides. Some magnesium ferro-silicon alloys contain ≈l % of Cerium, derived from ceriumconcentrate. The Cerium's role is particularly to capture, probably as intermetallics, contaminantssuch as lead and bismuth[3] and to increase the number of nuclei on which graphite grows.Several discrete intermetallic compounds of Ln and Si, e.g. Ln 5 Si 3 , LnSi, and LnSi 2 ⎯ inwhich the Si atoms form a three dimensional network⎯ and other compositions, are known, e.g.for Cerium[4] and Gadolinium[5]. The properties of some such materials are of interest for thebehavior of micro-electronic devices based on Si-SiO 2 systems. Lanthanide silicides are possiblelow-resistance contacts with silicon. Deposition of a thin film of say La, followed by a diffusionreaction in-situ, creates a silicide barrier that has a desirable low or high Schottky barrier,depending on the nature of the Si substrate.[6][1] Reducing Temper Embrittlement by Lanthanide Additions, C.I.Garcia et al., J.Metals, 1985, 37, 22[2] The Role of Rare Earth Elements in the Production of Nodular Iron, H.F.Linebarger et al., in "IndustrialApplications of Rare Earth Elements" ed. K.A.Gschneidner, ACS Symposium 164, pub Am.Chem.Soc., 1981,19[3] Neutralization of the Deleterious Effects of Bismuth and Lead in Gray Cast Iron by Lanthanide Additions,D.M.Stefanescu et al., in "Advanced Casting Technology", Conf.Proc., Publ. ASM, 1986, 167[4] The Cerium-Silicon System, A.B.Gokhale et al., Bulletin Alloy Phase Diag., 1989, 10(l), 73[5] The Gadolinium-Silicon System, ibid., 1988, 9(5), 574[6] Rare Earth Silicide Schottky Barriers, R.D.Thompson, U.S.Patent, 4,394,673, 19 July 198340