Advanced Structural and Functional Materials

Nanocrystalline solids are polycrystals the crystal size of which is a few (typically 1 to 10) nanometers so that 50% or more of the solid consists of incoherent interfaces between crystals of different crystallographic orientations. Materials consisting primarily of internal interfaces represent a separate state of solid matter because the atomic arrangements formed in the cores of interfaces are known to be arrangements of minimum energy in the potentials field of the adjacent crystal lattices. The boundary conditions imposed on the atoms in the interfacial cores by the adjacent crystal lattices, result in atomic structures in the interfacial cores which cannot be formed elsewhere (e.g. in glasses or perfect crystals). Nanocrystalline materials seem to be of interest for the following four reasons:

The development of ceramic materials for high performance structural applications is reviewed. Although many attractive properties of these materials are intrinsic features of the chemical bonding, reliable performance depends on the mastery of micro-structurally dependent properties and hence on the successful design and fabrication of microstructures. Progress in these two areas is reviewed and trends in current work are described. The ability of analytical and modelling methods to deal with increasing complexity and the tendency of refined processing methods to yield more controlled products has led to the point where improved interaction between modelling and experiment can be expected. The economic matching of these products to applications remains challenging particularly where materials substitution is involved.

Ceramic matrix composites (CMCs) consist of a ceramic matrix reinforced with ceramic fibers. They have been designed to be used in severe environments. With respect to monolithic ceramics, CMCs are characterized by a non-linear stress/strain mechanical behavior, a high resistance to crack propagation, a non-catastrophic failure and thus an improved reliability. CMCs are processed according to either a gas phase route (the matrix being deposited chemically from a gaseous precursor in the pores of a fiber preform) or a liquid phase route (by hot-pressing fiber tapes impregnated with a liquid precursor of the matrix). The most important CMCs are those made from SiC-based fibers embedded in glassceramic or SiC matrices. CMCs are inverse composites with the result that under loading the matrix fails first. They exhibit a tough behavior only when the fibers are weakly bonded to the matrix: the fiber/matrix interfaces arresting or deflecting the matrix microcracks preventing thus the early failure of the fibers and a catastrophic propagation of a macrocrack. The control of the fiber/matrix bonding is achieved through the use of an interphase material (e.g. a layer of carbon or BN) and a proper choice of the respective thermal expansion coefficients. Since long exposures to oxidizing atmospheres at high temperatures may have a detrimental effect (e.g. a tough/brittle transition due to the oxidation of the interphase), most CMCs receive a protective surface treatment, however, some of them exhibit a self-healing character.

An overview is given on intermetallics with respect to the possibilities for the development of new structural materials for high-temperature applications. In the first main section the fundamentals of intermetallics are discussed, i.e. the type and strength of bonding and its correlation with materials properties, in particular with elasticity and diffusion. In the second main section present materials developments are overviewed which are based on titanium aluminides, nickel aluminides and less-common phases. Characteristic data are presented, and the problems and prospects are discussed.

Shape memory is the ability of a material to remember its original shape, either after mechanical deformation (one-way effect) or by cooling and heating (two-way effect). This phenonemon is based on a structural phase transformation, which is diffusionless and connected with large amounts of homogeneous shear. It is known as martensitic transformation. Particularities of many bulk and microstructural properties are found in the environment of the transformation temperatures. It is explained how the alloys acquire their final shape, how they learn the two-way effect, and how they fatigue. Finally, a systematic survey is given on present and perspective applications in engineering and medicine.

Fibre reinforced polymers play an enormous role as structural materials in present day technology. The same is true for highly filled polymers, where the filler consists of isotropic particles which, however, provide impact strength, thermal conductivity and rigidity. The use of mixtures of form anisotropic (fibres) or form isotropic (powders) particles and polymers is linked to a number of serious problems in processing technology. Moreover, the presence of internal surfaces and phase boundaries in the composite materials is at the origin of the failure, ageing and other misbehaviors which limit the technical performance of these otherwise interesting materials.

Organic conjugated polymers form a new class of materials in which each macromolecular chain presents a high electrical conductivity. This unique property allows to represent these materials as a tridimensional network of molecular wires, able to carry informations. These wires can be chemically functionalized by the covalent binding of various prosthetic groups, presenting specific interactions whether with the chemical environment or with external physical quantities. Molecular transducers can thus be developed, and examples already exist on the molecular recognition exerted by functionalized conducting polymers toward ions or toward optically active species in solution. These structures open the promising field of molecular electronics, although the development of this new research area first requires the demonstration that the charge transport properties of these materials are efficient enough for them to be used as active layers in electronic devices. This step has been recently achieved, by the development of a new class of organic conjugated materials, conjugated oligomers, which present shorter conjugation length but much lower content of structural defects. The semiconducting properties of these oligomers have been determined through the characterization of field-effect transistors fabricated from these organic materials. A new all-organic thin film architecture has been proposed for these transistors, which shows comparable characteristics to those of a-Si:H based devices. These results confirm that organic conjugated polymers and oligomers can be used as active materials for the construction of electronic devices, and that their further functionalization will allow the development of new types of molecular transducers.

Piezoelectric composites have been developed to meet the requirements for the design of special electromechanic and electroacustic transducers. The main incentives are electrical and acoustic impedance matching. The composites are fabricated by very different preparation routes, the most important ones being tape casting and laminating. The properties of piezocomposites can be derived from the properties of ferroelectric ceramics and the connectivity of the composite material.