Jacky Wing Yip Lam

Hyperbranched Poly(ferrocenylene)s Containing Groups 14 and 15 Elements: Syntheses, Optical and Thermal Properties, and Pyrolytic Transformations into Nanostructured Magnetoceramics

A series of hyperbranched poly(ferrocenylene)s containing elements (E) of groups 14 [E=Si (hb-1), Ge (hb-2)] and 15 [E=P (hb-3), Sb (hb-4)] are prepared in good isolation yields (up to 82wt%) by the salt-eliminative polycoupling of dilithioferrocene with tri-(RECl3) or tetrachlorides of the elements (ECl4). While the polymers with no or small R groups are insoluble or partially soluble, those with long alkyl chains (R=CnH2n+1 with n ≥ 8) are completely soluble and film forming. The polymers exhibit solution properties characteristic of hyperbranched macromolecules: e.g. hb-1(18) shows a low intrinsic viscosity ([η]=0.02dL/g) despite its high absolute molecular weight (Mw=5 × 105). Spectroscopic analyses reveal that the polymers possess rigid skeleton structures with extended conjugations, with their absorption spectra tailing into the infrared region (>700nm). The polymers show good thermal stability with Td up to ~400°C and can be graphitized into iron-containing ceramics when pyrolyzed at high temperatures, with char yields up to ~60wt%. While calcinations of the Si-containing polymers (hb-1) at 1000°C under nitrogen give ceramics containing mostly α-Fe nanoparticles, those of Ge-(hb-2) and Sb-containing polymers (hb-4) are completely transformed into their iron-alloys. The ceramics from the P-containing polymers (hb-3) show diffraction patterns of iron phosphides. Iron silicide nanocrystals of “large” sizes are obtained when the pyrolysis of hb-1 is conducted at a high temperature of 1200°C under argon. This ceramic is highly magnetizable (Ms up to ~51emu/g) and shows near-zero remanence and coercivity; in other words, it is an outstanding soft ferromagnet with a high magnetic susceptibility and practically nil hysteresis loss.

Nanostructured magnetoceramics from hyperbranched polymer precursors

Abstract Controlled pyrolysis of a hyperbranched polysilyne, poly[1,1′-ferrocenylene(methyl)silyne] (1), at high temperature in inert atmosphere produces nonostructured ceramics (2) in ∼48–62% yields. The ceramic products 2 are characterized by SEM, XPS, EDX, XRD, and SQUID. It is found that the ceramics are electrically conductive and possess a mesoporous architecture. The iron contents of 2 estimated by EDX are 36–43%. The nanocrystals formed in the ceramics produced under nitrogen 2N are mainly α-Fe2O3 whereas those in the ceramics produced under argon 2A are mainly Fe3Si. When magnetized by an external field at room temperature, 2A exhibits a high saturation magnetization (Ms∼49 emu/g) and near-zero remanence and coercivity.