Nonylphenol polyethoxylate coated body-center-cubic iron nanocrystals for ferrofluids with technical applications

Abstract

An approach to the design of a suitable system for technological applications, such as magneto-rheological fluids with controllable performance, showing high saturation magnetization and low coercivity and remanence, is presented. This approach is based on the synthesis of stable iron nanoparticles with a relatively thick polymeric coating—the non-ionic surfactant nonylphenol polyethoxylate—by a microemulsion method with NaBH4 as a reducing agent. X-ray diffractometry, Mössbauer spectroscopy, and high resolution electron microscopy reveal a body-center-cubic structure in the iron cores. The resulting nanoparticles are predominantly spherical, having an average core size of 7 nm and a constant shell thickness of 3 nm. Magnetic measurements reveal a higher saturation magnetization (127.4 Am2 kg−1 at 300 K and 153.2 Am2 kg−1 at 5 K) than in other approaches and a small coercive field of 12 mT. X-ray diffractometry results account for the presence of iron borate traces as a secondary phase, formed at the initial stages of the synthesis during the reduction process of precursors. Preliminary studies under oxidant conditions show a reduction in saturation magnetization lower than 9% over a six months period.