Synthesis of Ferromagnetic Cobalt Nanofibers By Electrospinning

Abstract

The increase in coercivity of materials composed of fine powders (10 nm to 100 nm) of important ferromagnetic materials, such as cobalt or iron, is well understood where the flux-closure configuration is energetically unfavorable[3].[4]. Thus, it is not too surprising that 1-D nanofiber structures of cobalt synthesized by electrospinning display enhanced coercivity over current bulk methods[1]. The exclusively high surface area-to-volume ratio of nanostructures and the rotation being opposed by the shape anisotropy of the demagnetization energy play a key role in achieving high coercivity[3]. Likewise, the synthesis method using electrospinning offers many advantages: ease of control over nanofiber dimensions, repeatability, scalability, and ease of assembly[2]. Electrospinning was used to fabricate nanofibrous cobalt structures by electrospinning a cobalt acetate (CoAc2)/poly (vinyl pyrrolidone) (PVP) composite solution followed by reduction in hydrogen environment. Two separate 2-level, full-factorial, design of experiments (DOEs) were conducted to control and optimize the process to minimize fiber diameter and eliminate beading defects. A 3-factor DOE was run on solution properties (viscosity, surface tension, electrical conductivity) and a 2-factor DOE was run on electrospinning parameters (voltage, flow rate). Solution viscosity was found to be the main factor in controlling fiber diameter. The high and low values in viscosity were achieved by changing both the molecular weight of PVP and the weight percentage of PVP in the solution. In the second DOE, the applied potential had a larger effect on fiber diameter than the flow rate. However, both were found to be significant factors and a balance between these two parameters is needed to prevent clogging of the needle as a function of evaporation rate. The as-spun nanofiber mats were subsequently annealed in pure H2 gas to produce pure cobalt nanofibers. Scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and X-ray powder diffraction (XRD) were all used to characterize fiber diameter, morphology, and crystal structure. The smallest, average, as-spun fiber diameter achieved was 126 nm with minimal beading. We will be investigating the magnetic properties of these cobalt nanofibers as a function of fiber size using a vibrating sample magnetometer.