The challenge of ceramic/metal microcomposites and nanocomposites

Abstract

It is increasingly being recognized that new applications for materials require functions and properties that are not achievable with monolithic materials. The combination of dissimilar materials for these new applications creates interfaces whose properties and processing need to be understood before they can be applied commercially. In the present review paper we try to emphasize the important role and challenges of ceramic/metal micro/nanocomposites in the new technologies. In this respect we will study and review the exotic effects of metal particles embedded into matrix ceramics due to the dissimilar properties of the components, percolation laws, and the nature of the interfaces. From an electromagnetic point of view we have underlined the enormous enhancement of permittivity in the proximity of the percolation threshold, associated with an induced soft mode similar to para-ferroelectric transition. From a mechanical standpoint, the synergic effect of nanometer size, clustering addressed by the percolation theory and ceramic/metal interface features produces an unexpected enhancement in the hardness of the composite giving rise to superhard materials.

Introduction

Composite materials are used in numerous industries and for multiple product categories. For the last few years, the composite materials industry in Europe has undergone a period of consolidation and restructuring. In the European composite materials market, end-users are as follows [1]: 30% in the building and construction sector; 25% in the transportation sector; 15% in the industrial and agricultural equipment sectors; 15% in the electrical and electronics sectors; 10% in the sport and leisure goods and consumer goods sectors; and 5% in other, miscellaneous, sectors. Nowadays, it is increasingly being recognized that new applications for materials require functions and properties that are not achievable by monolithic materials. Combining dissimilar materials for these new applications creates interfaces whose properties and processing need to be understood to bridge the gap between the composite material microstructure and the end-product.

It is the object of this paper to emphasize the importance of composite materials formed by metallic particles dispersed within an insulating matrix. These composites, also known as cermets, have recently attracted much attention due to a singular combination of properties (including mechanical, optical, electrical and magnetic properties), which make them excellent candidates to fabricate multifunctional devices with unique features. For example, the improvement of mechanical properties by incorporating dispersed metal particles (e.g., W, Mo, Ni, Al, Cr, Ti, etc.) to a ceramic matrix is well reported nowadays and this approach has been successfully studied in numerous systems (i.e. mullite/Mo [2], [3], [4], Al₂O₃/Ni [5], [6], [7], [8], [9], [10], [11], [12], [13], ZrO₂/Ni/Al₂O₃ [14], Al₂O₃/ZrO₂–Ni [15], [16], AlN/Mo [17], AlN/Al [18], Al₂O₃/Al [19], spinel/W, etc.), particularly regarding fracture toughness. These selected examples represent typical composite materials which have been studied by our research team as well as by other different groups.

In a wider sense, this paper attempts a comprehensive review of a number of particular topics that merit special attention and in-depth research in this field. Such is the case of the properties of cermets in relation to their composition, metal concentration and microstructure. The experimental procedure followed to obtain the composites has received special attention because these features are directly related to the method of preparation, rheology, conforming and sintering of the ceramic and metal powders.

However, the dependence of the microstructure and, therefore, the properties of cermets on metal concentration are not well understood yet. This fact has prevented further technological development based on these materials. A number of theories describing the dependence of electrical and magnetic properties of heterogeneous media with metal fraction may be found in the literature. Each description approximately predicts the properties of such composites over a limited range of concentrations as a function of the intrinsic properties of the components: concentration and geometric parameters that depend on the shape and distribution of individual particles inside the heterogeneous material. In cermets where the difference in conductivity between the different components is large, a critical concentration named the percolation threshold can be found where a sharp change in the electrical properties takes place. In materials with a concentration of conductive phase below this threshold the properties are similar to those of the matrix, while above that they are mainly determined by the nature of the metallic inclusions. A less known feature of the percolation theory is the divergence of the imaginary part of the conductivity, i.e. the real part of the dielectric constant, at the percolation threshold. Likewise, these issues are thoroughly reviewed in this paper.

Ceramic/metal interfaces play a key role in the understanding of many fundamental material properties. Also the ceramic/metal interfaces play a crucial role in a wide range of technological applications such as coating, heterogeneous catalysis, fuel cells, microelectronics, optoelectronics, as well as structural components. The determination of the chemical basis of the metal–oxide interaction is a challenging task due to its inherent complexity. Recent ab initio calculations allow the identification of the metal adhesion mechanism in certain ceramic/metal interfaces [20], [21], [22], [23], suggesting that theoretical methods are useful tools to seek interfaces with good adhesion. From the standpoint of wetting, the contact angle of a liquid droplet on a solid is a key parameter determinant for many technological processes (e.g., brazing or infiltration) and for microstructure evolution during formation of multiphase materials. Numerous studies have been conducted on the subject.

Electromagnetic properties of ceramic/metal composites merit a separate section for the variety of different phenomena associated to them. In fact, an oxide ceramic is usually an excellent insulator, while metallic particles are very good conductors. Additionally, the sign of the real part of the dielectric constant of the ceramic is positive, while in the case of a metal it is negative and very large. Finally, the electromagnetic properties of ceramics do not change very much with the frequency, whereas metals, which follow the Drude model, have dielectric constants that are strongly dependent on frequency.

Most of the remarkable properties take place in the range of low to moderate metal concentration. This is why larger variations of the relative conductivity can be measured vs. metal concentration. Most interesting features appear at two specific metal concentration ranges: (i) very low concentration, in which optical surface plasmon resonances take place, and (ii) concentrations close to the percolation threshold. At this point, the conductivity of the composite increases up to 10 orders of magnitude (in the low frequency or static regime) in a very narrow range of concentrations. Additionally, huge increments of electric capacitance can be measured in the composite. We will show how both phenomena are due to the same reason, that is, the very large enhancement of the dielectric field of a particle embedded in a medium with a dielectric constant of a different sign. A review of different theories and numerical calculations will be included in addition to experimental results. Finally, at present, several devices which make use of the anomalous properties of the ceramic/metal electromagnetic properties are being introduced, as will be reported.

The metallic particle size effect is an important issue in the mechanical properties of ceramic/metal composites to be taken into consideration in this review. The design of new materials with hardness comparable to diamond is a current challenge for scientists and engineers. Among other fascinating properties, such as its very high refraction index and thermal conductivity, diamond is the hardest known substance. Because of this it is irreplaceable for grinding tools, drilling rocks, cutting concretes, polishing stones, machining and honing. However diamond has a major drawback, namely the fact that it reacts with Fe, Ti and Si, after which it cannot be used, for instance, for machining steel. This strong detrimental condition has promoted during the last 20 years the synthesis of other alternative superhard materials such as carbides, nitrides and borides. All these materials have a common feature, which is their directional covalent bonding and their very high shear modulus. However, the synthesis of intrinsically hard materials requires extreme conditions of high temperature and pressure. Thus, efforts have been devoted to develop superhard materials based on the singular properties of nanoparticles.

Previous experimental results of hardness of single-phase nanostructured metals or metallic superlattices [24], [25] clearly indicate that hardness increases with decreasing grain size (below 100 nm) up to 5–7 times following a d^−1/2 dependence known as the Hall–Petch effect [26], [27], [28], [29], [30]. However, this trend is inverted for particle sizes below 20 nm (inverse Hall–Petch effect [28], [31]), for which hardness decreases due to a grain sliding process along particle boundaries. The origin of superhardness in these composites is attributed to the following factors: (i) the suppression of dislocations due to the small crystal size of nanoparticles; [31] (ii) the supermodulus effect in the nanocrystal core due to the compressive stress of the non-crystalline shell; [28] and (iii) a strong interaction in the interface between different components [32]. In this context, the optimization of microstructural parameters of nanocomposites is a crucial subject that has been treated in many studies (for example, see Refs. [33], [34], [35], [36], [37], [38], [39]). In these articles, both hardening and softening were found dependent on the type of matrix, nanoparticles, and preparation method. However, a quite unexpected observation is that in most of the systems [33], [34], [35], [36] there is a critical volume concentration (f_c) where hardness reaches a maximum. With this in mind, it must also be taken into consideration that the largest number of publications on superhard nanocomposites deal with thin films made by plasma induced chemical and physical vapour deposition and that these processes are expensive and very dependent on the local fluctuation of the composition. Therefore, bulk composites made by metal nanoparticles embedded in a hard ceramic matrix have become the new promising approach to obtain superhard materials.

Finally, the electromagnetic properties of ceramic/metal nanocomposites (metal nanoparticles in the range from 3 to 10 nm) exhibit unique optical properties associated to electric field enhancement phenomena which are interesting for a wide range of applications. Whereas their linear optical properties, i.e. surface plasmon resonance, have long been used in decorative glass work and more recently in functional coatings, filters or polarisers, their non-linear optical (nlo) properties have not yet been used technologically. The efficiency of conventional non-linear materials is poor and they are very expensive compared to ceramic/metal nanostructured materials. According to the published experimental results, very large values of non-linear susceptibility have been obtained. In this respect, prototypes of new optical non-linear devices are being fabricated using nanocermets instead of the more expensive conventional homogeneous materials.