Multi-dimensional Al2O3/Ni nanocomposites with preserved multifunctional magnetic properties are successfully fabricated by an aqueous monomer suspension casting and coating processes. Based on an investigation of the effects of suspension composition on the drying, debinding, and reduction-pressureless sintering processes, a stable Al2O3/NiO binary oxide aqueous suspension with optimal dispersant, and solid loading is defined. The interconnected void structure of the debound multi-dimensional Al2O3/NiO composites provides hydrogen reduction of dispersed NiO without metamorphosis or shrinkage. Bulky and film structured Al2O3/Ni nanocomposites are successfully produced by pressureless sintering. Dispersed Ni particles exhibit typical ferromagnetic behavior in the Al2O3 matrix. This study confirms that aqueous suspension process is a viable method for fabricating industrially applicable multi-dimensional Al2O3/Ni nanocomposites with balanced mechanical and magnetic properties.
Keywords

Aqueous Suspension, Multi-Dimension, Nanocomposites, Alumina, Nickel
Introduction

Much attention has been focused on the development of ceramic/metal nanocomposites because such materials offer outstanding mechanical and multifunctional properties [1,2]. Multifunctional ceramic/metal nanocomposites such as Al2O3/Ni nanocomposites exhibited unique mechanical properties and peculiar magnetic functions [3]. These cylinder or rectangular shaped 3-dimensional (3D) bulky nanocomposites have been fabricated by powder metallurgy processes. However, these kinds of materials have previously been developed by powder metallurgy process, and are difficult to machine as a consequence of the improved mechanical properties. The high machining costs for these materials therefore preclude widespread industrial application.

Typically, 2-dimensional (2D) ceramic/metal nanocomposite film with fine microstructure displays unusual transport and magnetic properties depending on the metal content in the film. Film type ceramic/ metal nanocomposites can be applied to solar thermal collectors [4], electrodes of solid oxide fuel cells [5], remote sensors of applied stress [6], porous membranes [7], and ceramic-metal joint bonded materials [8]. However, it is difficult to fabricate nanocomposite-structured films with conventional powder process.

In order to overcome shape and dimensional limitations of ceramic/metal nanocomposites, it is required to develop novel process of multi-dimensional ceramic/metal nanocomposites. Recently, many novel forming processes discussed begin with a monomer based aqueous suspension as a starting state. In these processes, it is important to control the particle interaction to synthesize a stable aqueous suspension. The monomer based aqueous suspension properties play a substantial role in determining the final properties of the specimen and the range of applications [9,10]. This stable suspension determines the properties of the final products with overcoming shape limitation.

In the present study, the optimum aqueous monomer suspension conditions and process routes (casting and coating) for the fabrication of multi-dimensional (3D complex-shaped and 2D film) Al2O3/Ni nanocomposites are investigated. The relationship between the final specimen properties and the preparation conditions is investigated through systematic fabrication of 3D Al2O3/Ni nanocomposite under various conditions of suspension synthesis and reduction-sintering. Also, it is focused on the novel 2D spin coating process of stable Al2O3/NiO aqueous monomer suspensions, reduction of dispersed NiO on the film, and densification behaviors of 2D Al2O3/Ni nanocomposite films in the present study.
Experimental

The solution chemical route was selected to obtain Al2O3/NiO (for 5 vol% Ni) powder mixtures [3]. As a monomer and cross-linker of 3D casting and 2D coating, methacrylamide and N,N´,- methylenebisacrylamide (5 wt%, ratio 5:1) were dissolved in deionized water. Poly-acrylic acid (PAA) was added to this solution as a dispersant. A prepared Al2O3/NiO mixture was then added to that solution and ball milled for 24 h. Rheological measurements were performed on a rotational stress-controlled rheometer to estimate the relationship between rheology behavior and chemicals quantity.