NANO-STRUCTURED MATERIALS

Part 1: Technical Annex

A. BACKGROUND

Nanophase materials and nano composites, characterised by an ultra fine grain size (< 50 nm) have created a high interest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. For example:

* Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures as compared to the coarse-grained ceramics.

* Nanostructured semiconductors are known to show various non-linear optical properties. Semiconductor Q-particles also show quantum confinement effects which may lead to special properties, like the luminescence in silicon powders and silicon germanium quantum dots as infrared optoelectronic devices. Nanostructured semiconductors are used as window layers in solar cells.

* Nanosized metallic powders have been used for the production of gas tight materials, dense parts and porous coatings. Cold welding properties combined with the ductility make them suitable for metal-metal bonding especially in the electronic industry.

* Single nanosized magnetic particles are mono-domains and one expects that also in magnetic nanophase materials the grains correspond with domains, while boundaries on the contrary to disordered walls. Very small particles have special atomic structures with discrete electronic states, which give rise to special properties in addition to the super-paramagnetism behaviour. Magnetic nano-composites have been used for mechanical force transfer (ferrofluids), for high density information storage and magnetic refrigeration.

* Nanostructured metal clusters and colloids of mono- or plurimetallic composition have a special impact in catalytic applications. They may serve as precursors for new type of heterogeneous catalysts (Cortex-catalysts) and have been shown to offer substantial advantages concerning activity, selectivity and lifetime in chemical transformations and electrocatalysis (fuel cells). Enantioselective catalysis were also achieved using chiral modifiers on the surface of nanoscale metal particles.

* Nanostructured metal-oxide thin films are receiving a growing attention for the realisation of gas sensors (NOx, CO, CO2, CH4 and aromatic hydrocarbons) with enhanced sensitivity and selectivity. Nanostructured metal-oxide (MnO2) find application for rechargeable batteries for cars or consumer goods. Nano-crystalline silicon films for highly transparent contacts in thin film solar cell and nano-structured titanium oxide porous films for its high transmission and significant surface area enhancement leading to strong absorption in dye sensitized solar cells.

* Polymer based composites with a high content of inorganic particles leading to a high dielectric constant are interesting materials for photonic band gap structure produced by the LIGA.

Nanophase engineering expands in a rapidly growing number of electronic materials, both inorganic and organic, allowing to manipulate optical and electronic functions. The production of nanophase or cluster-assembled materials, is usually based upon the creation of separated small clusters which then are fused into a bulk-like material or on their embedding into compact liquid or solid matrix materials. E.g. nanophase silicon, which differs from normal silicon in physical and electronic properties, could be applied to macroscopic semiconductor processes to create new devices. For instance, when ordinary glass is doped with quantized semiconductor ''colloids,'' it becomes a high-performance optical medium with potential applications in optical computing.

A.1: Influence on properties by "nano-structure induced effects"

For the synthesis of nanosized particles and for the fabrication of nanostructured materials, laser or plasma driven gas phase reactions, evaporation-condensation mechanisms, sol-gel-methods or other wet chemical routes like inverse micelle preparation of inorganic clusters have been used. Most of these methods result in very fine particles which are more or less agglomerated. The powders are amorphous, crystalline or show a metastable or an unexpected phase, the reasons for which is far from being clear. Due to the small sizes any surface coating of the nano-particles strongly influences the properties of the particles as a whole. Studies have shown that the crystallisation behaviour of nano-scaled silicon particles is quite different from micron-sized powders or thin films. It was observed that tiny polycrystallites are formed in every nano-particle, even at moderately high temperatures.

Roughly two kinds of "nano-structure induced effects" can be distinguished: First the size effect, in particular the quantum size effects where the normal bulk electronic structure is replaced by a series of discrete electronic levels and second, the surface or interface induced effect, which is important because of the enormously increased specific surface in particle systems. While the size effect is mainly considered to describe physical properties, the surface or interface induced effect, plays an eminent role for chemical processing, in particular in connection with heterogeneous catalysis. Experimental evidence of the quantum size effect in small particles has been provided by different methods, while the surface induced effect could be evidenced by measurement of thermodynamic properties like vapour pressure, specific heat, thermal conductivity and melting point of small metallic particles. Both types of size effects have also been clearly separated in the optical properties of metal cluster composites. Very small semiconductor (

Table 1: Some typical properties of nano-structured materials and possibilities of future applications

Property Application

Bulk

Single magnetic domain Magnetic recording

Small mean free path of electrons in a solid Special conductors.

High & selective optical absorption of metal particles

Colours, filters, solar absorbers, photovoltaics, photographic material, phototropic material, Molecular Filters

Formation of ultra fine pores due to superfine agglomeration of particles Molecular Filters

Uniform mixture of different kinds of superfine particles R&D of New Materials

Grain size too small for stable dislocation High strength and hardness of metallic materials

Surface/ Interface

Large specific surface area Catalysis, sensors

Large surface area, small heat capacity Heat-exchange materials, Combustion Catalysts Lower sintering temperature Sintering accelerators Superplastic behaviour of ceramics ductile ceramics Cluster coating and metallization Special resistors, temperature sensors Multi-shell particles Chemical activity of catalysts

A.2: Research and Development on nano-structured materials in the world

Research on nanoscale science and technology is carried out in all major industrialised countries. Although financing, organisation, work distribution between industry and universities and programmes vary from country to country, some examples are given below :

* In the USA the scientific activities are more widespread and better financed than in Europe. For example, NIST is co-ordinating the activities in the area of giant magnetoresitance in the USA.

* The major research activities in Japan are focused on the refinement of advanced manufacturing processes and instrumentation in diverse fields such as microelectronics, optics, ultra-precision machining, as well as the progress in fundamental nanometer-scale materials sciences research. MITI has identified nanotechnology as a very important discipline for which MITI has announced a budget of 210 Mio. US-$ for the next ten years for foreign and Japanese academic, government and corporate R&D.

* Europe has no uniform programme on nanomaterials. In contrary nearly each major industrialised country has its own nanomaterials program. Therefore the progress on a common European program is relatively small, despite important programs in each of the following countries :

In Germany, the DFG presently funds a package project involving seven academic institutions on nanocrystalline materials. A larger DFG research on the same topic is presently under consideration. Nanotechnology is being supported by on-going BMBF projects as well as by DFG and Volkswagen Foundation.

In Great-Britain the Advanced Magnetic Programme of EPSRC also covers nanomagnetism. Additionally, research programmes regarding nanostructured materials are presently in evaluation.

In Sweden a national consortium for nanostructured materials exists whereas in Finland the national research programme "Ultrafine Particles (UFP)" has been started on nanomaterials at VTT Chemical Technology (1995-1998) with funds ~1Mio ECU/year.

In Switzerland, parts of the three research programmes in the area of Nanotechnology and nanomaterials (MINAST and Swiss Science Foundation NFP36) support some activities.

* In France CNRS (Centre National de la Recherche Scientifique) co-ordinates specific governmental research actions (GdR-CNRS) about :

- Wave propagation in random linear and/or non-linear media (resp. A. Migus)

- Aggregates (resp. C. Brchignac)

- Magnetic Nanostructures (resp. A. Fert)

- Physico-chemical study of Si based nanophase ceramic powders (resp. C. Snmaud)

- Nanocomposites and cermets by chemical process (resp. Y. Laurent)

- C60 and its derivatives (P. Bernier)

Periodic workshops (Nano95, Nano96) were organised in Odeillo (3 days, around 70 participants from governmental research and industry) about "Preparation, characterisation, properties and application of nanomaterials".

As well as in other domains of Materials Science, the projects and research programmes are characterised by a strong interdisciplinarity. Nevertheless, Europe shows a lack of a coherent inter-state strategy to support and to subsidise this important interdisciplinary co-operation in nanomaterials.

For these reasons, the proposed COST-Action will be the ideal frame to implement an European co-operation in the domain of nanomaterials. The action proposer support a co-ordination with the existing COST-Action D5 "Nanochemistry at Surfaces and Interfaces" especially with the part " Nanomaterials ", as well as with the planned Cost-Actions of the Ad-Hoc working group " Technology driven Physics " namely " Nanophysics, Nanostructured Materials and Nanotechnology " and " Modelling of Physical Phenomena in Technological Application ". A more detailed description of this co-ordination will be given in the part D "Organisation and Timetable".

B. OBJECTIVES AND BENEFITS

The main objective of the COST-action is to develop nanostructured materials with new and unique structural and functional properties. This should be carried out in European industries but it has to be combined with fundamental research in order to solve technological problems. The last are at the origin of the present limited commercial diffusion of these materials.

The fundamental issues in nanostructured materials are

(1) ability to control the scale (size) of the system,

(2) ability to obtain the required composition -

not just the average composition - but details such as defects, concentration gradients, etc.,

(3) ability to control the modulation dimensionality,

(4) during the assembly of the nano-sized building blocks, one should be able to control the extent of the interaction between the building blocks as well as the architecture of the material itself.

Therefore, the more specific objectives are the following:

* Development of synthesis and/or fabrication methods for raw materials (powders) as well as for the nanostructured materials.

* Better understanding of the influence of the size of building blocks in nano structured materials as well as the influence of microstructure on the physical, chemical and mechanical properties of this material.

* Better understanding of the influence of interfaces on the properties of nano-structured material.

* Development of concepts for nanostructured materials and in particular their elaboration.

* Investigation of catalytic applications of mono- and plurimetallic nanomaterials

* Transfer of developed technologies into industrial applications including the development of the industrial scale of synthesis methods of nanomaterials and nanostructured systems.

C. SCIENTIFIC PROGRAMME

The scientific program can be structured in the following tasks :* Materials Synthesis * Nanostructured materials for structural applications * Nanostructured materials for functional applications - Chemical (catalysis) Electrical Optical - Magnetic

It is evident, that this classification of the scientific programme in these " classical tasks " of materials research programmes is an artificial one. Especially the first task group will have strong interactions with all other task groups, because these works fundamentally influence the following materials applications. Both groups of nanostructured materials have to be investigated on their mechanical and physical needs but in different evaluations as carried out by the following paragraphs.

C.1: Material Synthesis and Processing

Synthesis techniques for nanostructured materials can be basically divided into the following three categories:

- Atomic or molecular precursors form a basic from which larger building blocks can be constructed. The commonly used techniques in this class are the gas condensation, chemical precipitation, aerosol reactions, biomimetics, etc.

- Another useful approach is to start from conventional coarse-grained precursors and break them down to ultrafine grains by high energy mechanical attrition.

- Crystallisation of an amorphous precursor at low temperatures to obtain nanocrystalline material,

In experiments designed for basic understanding, particles of controlled size, shape, crystallinity and surface-pureness need to be studied. New techniques of particle production and coating as well as methods of determining and selecting particle properties before compacting must be applied or developed for this purpose. The major draw backs is the availability of high performance low-cost starting materials (e.g. nano-sized agglomerate free powders) and appropriate processing methods, which allow the fabrication of components, parts, coatings or micro-patterned parts by industrially applicable processing techniques. Meanwhile, chemical routes to solve these problems become interesting, but, especially with respect to the materials' fabrication, the potential of chemistry is only exploited to an insufficient extent. Similar observation can be made for the processing step. On the other hand, as already shown in a variety of examples, processes like chemical vapour reaction (CVR), Aerosol-reactions, chemical precipitation using control of growth processes or microemulsion techniques are able to produce large quantities of high-grade powders at comparably low costs.

It is proposed to investigate and to develop chemistry based production and processing methods since it has already been shown that these processes allow to handle nanoparticles with techniques suitable for industry. It is quite necessary to exploit these fields to develop broadly applicable industrial processes. These developments focus on optical materials, for components of integrated optics and elektrooptical devices as well as for new shaping techniques for nanosized ceramic parts or direct use of nanoparticles for drug targeting or magnetic imaging.

Besides the size of building blocks of the nanostructured material, the microstructure of the materials has an extremely important influence on the properties. Therefore, the aim of this task will be to develop " integral synthesis " of nanostructured materials. Depending on the application and the properties of the material, different synthesis methods will be developed in co-operation with the other task groups. It is also important to note, that especially for these task a strong co-operation with the related COST-Action D5 is foreseen.

C.2 Structural Application (mechanical properties)

Fundamentally, the mechanical behaviour of a material is determined by the type of bonding and defect structure. Metals - with non-directional bonds - are highly ductile. Ionic solids are more difficult to deform because charge neutrality conditions have to be satisfied. Covalent solids have strong, directional bonds. Thus, ceramics and inter-metallics are subject to brittle fracture, while metals are soft and easily deformable due to dislocations. In the nanophase, both types of "bulk-like" behaviour are altered.

Nanophase metals show increasing hardness with decreasing grain size. For example, there is a five-fold increase in hardness in nanophase Cu (grain size = 6 nm) as compared to coarse grained Cu (grain size 50 mm). In nanophase Pd (grain size = 7 nm), the yield stress goes up by a factor of five from the bulk metal (grain size = 100 mm).

In the case of intermetallics - at large grain sizes: the hardness increases with decreasing grain sizes, but often decreases or saturates below a limiting size - thus showing a transitional behaviour. The stress-strain curve for the very brittle alloy: Fe28Al2Cr, shows macroscopic failure at a relatively low strain ( 0.4). But, in the corresponding nanophase material (grain size = 80 nm): compression produces a continuous plastic deformation and fracture does not occur.

Ceramics are normally the most brittle of the three classes. The brittleness of ceramics can be quantified in terms of the strain rate sensitivity, m, which is the exponent in the equation: s=k(de/dt)m, and can vary from 0 (perfectly brittle) to 1 (perfectly ductile). For conventional ceramics such as ZrO2 and TiO2, m < 0.01. For nanophase ZrO2 and TiO2 (grain size = 100 nm) m 0.02 at 300 K and m increases almost exponentially below 50 nm for both of the oxides. Fully dense nanophase TiO2 becomes highly ductile at 800 C.

Much efforts will have to be dedicated to explaining in details the mechanical properties in an atomistic scale. We believe, that only an approach, which includes all three classes of materials (metals, intermetallics, ceramics), can provide a complete overview on how the size of building blocks of nanostructured materials can influence the mechanical properties.

C.3 Functional application (electrical, optical, chemical and magnetic properties)

Electrical properties

The investigation of electrical properties of nanostructured and composite materials has long tradition and products of interesting resistance properties have been performed. However, it is shown that this field has still a large potential for further development, in particular by more directed topology manipulation, by inclusion of " zero dimensional " units (quantum dots), ballistic transport, electroluminescence of clusters, electrolytic cluster cells, etc.

Examples are:

1. Varistors with non-linear dependence of electrical conductivity on electric field.

2. Cermets (ceramic-metal composites) with extreme temperature dependencies and non-linear behaviour due to (single electron) tunnelling currents over Coulomb barriers between adjacent clusters.

3. PTC thermestors which use the temperature dependence of the conductivity as thermometers, current sensors and for current control.

4. Current sensitive conductors which have applications as current limiters and sensors.

5. Non-linear insulators with non-linear dependence of the dielectric constant on electric field and temperature. They are applied for high voltage shielding.

6. Piezoresistors with pressure dependent electrical resistivity which are used as pressure sensors and switches.

The scientific goal of this working group will be the understanding of the influence of the microstructure (or topology) of nanostructured materials, mainly of nanostructured composites on the properties. Besides the characterisation of such new materials, this fundamental understanding as well as the methods for these synthesis need to be developed.

Optical Properties

Noble metal colloids have been used for staining or colouring glasses for hundreds of years. e.g. for ruby-red glass, Au-colloids are used while for the yellow glass one takes Ag-colloids, both typically about 10 nm in size. The unusual linear optical properties of these materials are ascribed to surface plasmon resonance of the conducting electrons induced by light which dramatically increase the local fields.

These resonance's occurring at particular frequencies of light lead to selective absorption bands. Surface resonance's of metal clusters are sensitive to the particle material, the particle geometry's (shape, size, etc.), the interface between particle and surrounding and the topology of the clusters in many-particle-systems. Hence, by directed manipulation of these features, the optical properties can be modified over broad regions of frequencies. High efficiency optical colours, absorbers, and filters of widely varying properties can thus be fabricated, e.g. they can be outstandingly stable for a large range of temperatures.

The non-linear optical properties of clusters, as well, are of general interest. If local field resonance's as described above, occur in metal clusters or many-cluster-samples, this induces dramatically enhanced non-linearity compared to the bulk. e.g. the third-order susceptibility increases, compared to the bulk value proportional to the fifth order of the resonance factor. Effects are both described for non-linear clusters and non-linear embedding media. They were observed, e.g. in noble metal colloids embedded in glass. It is also possible to obtain strong non-linear behaviour by embedding in matrices like barium titanate. Thus, the aim of this task is the development of new nanostructured composite materials with unusual linear or non-linear optical properties. Because the sizes as well as the shapes of the particles play an important role, a close co-operation with the working group " Synthesis " is planned.

Chemical properties

Nanostructured metal clusters and colloids of mono- or plurimetallic composition have a special impact in catalytic applications. They may serve as precursors for a new type of heterogeneous catalysts (Cortex-catalysts and were shown to offer substantial advantages concerning activity, selectivity and lifetime in chemical transformations and electrocatalysis (fuel cells). Enantioselective catalysis was also achieved using chiral modifiers on the surface of nanoscale metal particles.

Industries (chemical and oil companies) are strongly interested in better controlled ways for the preparation of heterogeneous catalysts. Nanoscale metals and metaloxides offer new ways to improve activity, selectivity, and stability. By sequential reduction or co-reduction of metal-salts in presence of protecting groups, mono- and bi-metallic nanoparticles can be prepared and the resulting colloid particles may be supported by carbon, oxides, polymers or ceramics yielding highly effective catalysts of a new type. Hydrogenation catalysts based on nanoscale noble metal precursors showed significant advantages when compared with conventionally prepared industrial catalysts.

The aim of this task is therefore the development of nanoscale catalyst precursors which could be exploited for the design of highly efficient, selective and long-time-stable catalysts for chemical and electrochemical processes. The amount of catalyst applied is tiny compared to the huge amounts of products used, and hence the manufacturing cost of these advanced systems are not an obstacle for practical uses.

Magnetic properties

Giant Magneto-Resistance (GMR) is observed in 10 nm thick multilayers of Fe separated by a suitable thickness of non-magnetic layers of Cr. Depending on the layer-repeat distance, the magnetic layers may be coupled either ferromagnetically (parallel) or anti-ferromagnetically (anti-parallel). When the resistance R of an antiferro-magnetically coupled multilayer system is measured as dependent on a magnetic field H normal to the current direction, the change of the magneto resistance (RH - Ro) / H, which in normal metals amount to some percent, can be 40% or even higher.

GMR materials are promising as highly sensitive sensors and next generation read/write devices in information technology. Materials with a large value of GMR (dR/dH) at low fields is already in test use in anti-locking automobile devices. It is also expected that these elements would allow the data storage density to be raised to much higher values (~1 Gb).

GMR is very sensitive not only to the nature of the magnetic coupling but also to the nature of the interface and intermediate layers. It is thus possible to replace the multilayer system by a system of fine particles dispersed in a non-magnetic matrix. The aim of this work is therefore the fundamental understanding of the effect of nanostructuring and the interfaces on the GMR-effect. Based on this knowledge, the development of new particulate materials with unusual magnetic properties will be possibly a main target of this group