Mullite–Nickel Magnetic Nanocomposite Fibers Obtained from Electrospinning Followed by Thermal Reduction

Abstract

Mullite-nickel nanocomposite fibers with Ni nanoparticles of controllable size, dispersion, and consequent magnetic properties were fabricated using sol-gel/electrospinning method, followed by thermal reduction. The fibers were electrospun from an aqueous solution containing sol-gel mullite precursor and nickel nitrate. These fibers were then heat treated in the reducing atmosphere between 550°C and 750°C to achieve fine-dis persed metallic Ni nanoparticles (NPs). After the Ni 2+ was reduced to Ni NPs at 750°C for 10 h, the fibers were then directly transformed to the mullite fibers at 1000°C without the undesirable intermediate spinel phase. In many high-temperature applications, mullite is the desired phase than spinel. If not fully reduced, the Ni 2+ cations induce early precipitation of spinel phase before mullite can be formed. This spinel phase was a solid solution between Al 2 NiO 4 and Al-Si spinels, which later reacted with the residual silica and formed a mixture of mullite and spinel at 1400°C. The formation of spinel phase was suppressed or fully eliminated with chemically reducing Ni 2+ to metal NPs. The average size of nickel NPs within the fibers was~20 nm, insensitive of the Ni concentration and reducing temperature. However, the Ni NPs on the fiber surface grew as large as~80 nm due to fast surface diffusion. The magnetic nanocomposites exhibited ferromagnetism with saturation magnetization (M s ) close to pure nickel of the same nominal weight, but coercivity (H c ) much smaller than the bulk nickel, indicating the nature of bimodal magnetic nanoparticle distributions. The majority of small Ni NPs (~20 nm) within the fibers exhibited superparamagnetism, while the minor portion of relatively large NPs (50-80 nm) showed ferromagnetism.

A review on electrospun magnetic nanomaterials: methods, properties and applications

“…124 The calcination conditions are adapted to meet the varying demands of a given system. For example, air is an idealised atmosphere if metal oxide NFs 125 are targeted, whereas hydrogen (H 2 ), [126][127][128] often in combination with an inert gas such as argon (Ar), [129][130][131][132] and ammonia gas (NH 3 ) are adopted as a reduction (often referred to as de-oxidation in the literature) source. In some cases, the calcination atmosphere is selected to generate oxygen vacancies or eliminate impure phases.…”

“…Generally, for a given system, lower thermal treatment temperatures result in smaller particles being formed (down to a critical minimum size) and the smaller particles exhibit smaller M s values. 106,114,119,122,132,133,170,172,[176][177][178][179][180][181][182][183][184][185][186][187][188] Larger particles have less magnetic contribution from the surface (whilst the contribution from the interior concomitantly increases) and thus the M s value increases. 114 However, the smaller M s values observed at low calcination temperatures are not always only due to the decrease in size of the particles.…”

Fabrication and characterization of magnetic nanocomposites by electric fields assisted electrospinning

“…In recent years, polymeric nanocomposites embedded nanoparticles with properties of electricity and magnetism have attracted the great interest of many researchers due to their wide applications in biomedical [1,2], electrochemical [3], microwave absorption [4], and optical materials [5]. Electrospinning (ES) is an efficient method for the fabrication of magnetic composite nanofibers (MCNF) [6]. However, the obtained nanocomposites usually show random structures, which could not satisfy the application requirement.…”

Magnetic Submicron Mullite Coatings with Oriented SiC Whiskers

“…5,14,24,25 Recent studies demonstrated a robust and attractive methodology to make many desirable fillers magnetic by decorating ceramic particles with magnetic nanoparticles responding to sufficiently weak magnetic fields. 14,26,32 We will follow this methodology.…”