Bi₂(C₂O₄)₃·7H₂O and Bi(C₂O₄)OH Oxalates Thermal Decomposition Revisited. Formation of Nanoparticles with a Lower Melting Point than Bulk Bismuth

Abstract

Two bismuth oxalates, namely, Bi 2 (C 2 O 4 ) 3 ·7H 2 O and Bi(C 2 O 4 )OH, were studied in terms of synthesis, structural characterization, particle morphology, and thermal behavior under several atmospheres. The oxalate powders were produced by chemical precipitation from bismuth nitrate and oxalic acid solutions under controlled pH, then characterized by X-ray diffraction (XRD), temperature-dependent XRD, IR spectroscopy, scanning electron microscopy, and thermogravimetric differential thermal analyses. New results on the thermal decomposition of bismuth oxalates under inert or reducing atmospheres are provided. On heating in nitrogen, both studied compounds decompose into small bismuth particles. Thermal properties of the metallic products were investigated. The Bi(C 2 O 4 )OH decomposition leads to a Bi–Bi 2 O 3 metal–oxide composite product in which bismuth is confined in a nanometric size, due to surface oxidation. The melting point of such bismuth particles is strongly related to their crystallite size. The nanometric bismuth melting has thus been evidenced ∼40 °C lower than for bulk bismuth. These results should contribute to the development of the oxalate precursor route for low-temperature soldering applications.