Ferromagnetic microwires enabled multifunctional composite materials

Abstract

The last two decades have witnessed increasing international interest in ferromagnetic microwires research. Recent attention has turned to the development of innovative materials and composites derived from these microwires, such as microwire polymer composites. Through incorporating an extremely small concentration of microwires (10⁻² vol.%), the resultant composite exhibits a multitude of functionalities which are desirable for a range of technological applications. This article aims to provide a comprehensive review of current microwire composites research, from processing to structural and property evaluations with a focus on the multi-functionalities presented in these microwire composites. Starting with an introduction to multifunctional composites and the theories pertinent to the multiple functionalities of microwire composites, a detailed description of fabrication methods of microwire composites is given with a comparison of different processing techniques. Two fundamental effects, namely, giant magnetoimpedance (GMI) and giant stress-impedance (GSI) of microwire composites, are discussed in relation to monolithic microwires. Microwave tunable properties in the presence of a dc magnetic field, stress or temperature field are presented and analysed in depth. The ferromagnetic wire composites have also been shown to possess metamaterial characteristics and microwave absorption capability. A detailed discussion of the influence of composite architecture, such as local properties of microwires and topology of wire arrangements, on the performance of resultant composites, provides useful insights for an effective design of smart composites for specific engineering applications, such as structural health monitoring, stress sensing, invisible cloaking, microwave absorption and biomedical applications.

Introduction

Materials we are dealing with today can be classified into structural materials and functional materials in terms of their functionalities. Conventionally, composite materials are designed to be used as lightweight structural materials alternative to their metallic counterparts for structural functions only. In another aspect, functional materials have found a range of domestic and engineering applications with significant impacts. Naturally, an integration of these two categories of functionalities is much aspired for yielding the so-called multifunctional composites as against to the unifunctional ones, in that a wealth of applications can be explored ranging from aircraft wings to drugs dispensing [1]. A multifunctional composite must essentially meet the criteria of good mechanical performance and another one or more value-added physical or chemical functionalities, such as good electrical or thermal conductivity and peculiar electromagnetic behaviours.

To materialise the multifunctional concept, adding functional fillers is considered as one of, if not the only, the most effective routes. To name but a few examples, one of the most intensive research subjects is nanocomposites incorporating carbon nanotubes or nanoclays inside the polymer matrix to obtain favourable thermal and electrical conductivity for some specific applications such as de-icing, electromagnetic interference shielding and sensing applications [2], [3]. Sufficient carbon black is capable of turning an insulating polymer into a conductive one when it is added into the polymer in a proper manner [4]. The bioactive fillers can make a dull matrix biologically sensible in vivo and greatly extend the scope of tissue engineering applications [5].

In the past two decades, the ferromagnetic microwires have been intensively studied due to their particular magnetic properties which are of strong application interest [6], [7], [8], [9], [10]. Consisting of a metallic core and glass coat, these microwires are primarily of CoFeSiB in composition (often doped with transition metal elements e.g., Mo, Cr, Nb), thereby giving very good ferromagnetic properties. They are structurally amorphous thanks to the existence of metalloid elements and the fast-cooling process, but magnetically anisotropic in part due to their specific domain structure varying with their composition and in part due to their geometry, such as microscale diameter and large aspect ratio. The most important properties of these wires are the so-called giant magnetoimpedance (GMI) and stress-impedance (SI) effects, by virtue of which they can be used as functional fillers to enable their resultant composites to have particular properties such as field/stress tunable properties, microwave absorption functionality whilst retaining the mechanical performance of the composite.

Among the extensive literature on the multifunctional composite topic, there have been some excellent monographs published: the book edited by Xanthos [11] gives a panorama of functional fillers and their composites and the latest review paper by Gibson [1] surveyed different classes of multifunctional composites for hot topics such as self-healing composites and energy harvesting composites. Interested readers are referred to these works and the references therein. In these publications, however, there is no mention of the microwires composites. Although Makhnovskiy and Panina [12] contributed an outstanding book chapter on ferromagnetic microwires-based composites, it is focused on the theoretical treatment of microwave tunable properties. In this context, it would be of significant scientific interest to have a dedicated monograph on the microwire composites to summarise the on-going research on their fabrication, characterisation and perspective applications from an engineering perspective. The aim of this work is exactly to fill this gap by presenting a systematic review of microwire composites from their fabrication to the characterisation of the structural and electromagnetic functionalities of the composites. With this understanding of the structure–property relationship, they can be best exploited to meet a range of specific applications.

The rest of the review is organised as follows: Section 2 introduces the fundamentals of multifunctional composite from fundamental concept to expected properties. The fundamentals and theories of GMI/GSI, microwave tunable properties, metamaterial behaviour and microwave absorption capacity are elucidated. Section 3 details the fabrication techniques of microwires and their composites. The GMI/GSI effects and mechanical properties of microwire composites are elaborated in Section 4. Section 5 is devoted to the study of microwave tunable properties including magnetic field tunable properties, stress tunable properties and temperature tunable properties. The high frequency behaviours of microwires for magnetic and stress sensing are also expounded there. Section 6 is targeted on the singular microwave absorption capacity of the microwire composite. Corresponding to the properties of microwires and their composites, potential applications are outlined in Section 7. The main conclusions of the whole review are summarised in Section 8 together with an outlook.