**2. Nanostructure materials**

The size control of microstructural elements has been always considered as one of the most important factors in control of several properties in the development of new materials or design new microstructures. As a historical example, it can be mentioned that the grain refining of metallic materials, which results in increased mechanical strength, tenacity, occurrence of superplastic, etc. New materials with sub-micron grain size have been developed recently as commercial materials and the latest generation of this development is the nanostructured materials (Inoue & Masumoto, 1993).

Nanostructured materials (also called nanocrystalline materials, nanophasics materials or nanometer-sized crystalline solids) are known to have properties or combinations of properties, which may be new or even superior to conventional materials (Greer, 1998). Nanostructured materials can be defined as a system containing at least one microstructural nano characteristic (with sizes ranging up to 100-150nm). Due the extremely small dimensions, a large volume fractions atoms located in grain boundaries (Morris, 1998), which gives them a unique combination of composition and microstructure (Suryanarayana, 1995). Generally, these materials exhibit high strength and hardness, increased diffusivity, improved ductility and toughness, reduced elastic modulus, lower thermal conductivity when compared to larger grain size materials (~10μm) (Suryanarayana, 1995).

Since nanocrystalline materials contain a large fraction of atoms in grain boundaries, many of these interfaces provide high density of short diffusion paths. Therefore, it is expected that these materials show increased diffusivity compared to polycrystalline materials of the same composition and conventional particle size (of the order of microns) (He & Ma, 2000). The consequence of such increased diffusivity is increased sinterability of nanometric powders, which causes decrease in sintering temperature of these powders when compared to the same material with conventional particle size (Porat et al., 1996).

The driving force for sintering or "sintering stress" of nanocrystalline ceramics with pores in the range of 5nm is about 400MPa (considering γ about 1Jm-2), while for conventional ceramics with pores around 1 μm it is 2 MPa. Thus, a nanocrystalline ceramic must have a great thermodynamic driving force for retraction, which must densify extremely well even under unfavorable kinetic conditions such as low temperatures (Suryanarayana, 1995).

The interest in this nanostructured materials area has grown due to the availability of nanocrystalline ceramic powders. These nanocrystalline powders can be synthesized using different techniques, but its consolidation into dense ceramics without significant grain growth is still a challenge.

Compaction and sintering of ultra fine and/or nanoscale powder have a positive set of aspects over behavior during processing and final properties of products; however, there are also

densification could occur without grain growth. The aforementioned hypothesis was proposed by Chen and Wang (Chen & Wang, 2000) and has been successfully applied to different types of materials. A second phase can be added to preserve fine grains. In this case, grain boundary inhibition can be due to the pinning effect, which is associated with particles locations at grain boundaries or triple junctions (Chaim e al. 1998; Trombini et al., 2007). This drag pinning effect associated with heating curve control can be more effective to

The size control of microstructural elements has been always considered as one of the most important factors in control of several properties in the development of new materials or design new microstructures. As a historical example, it can be mentioned that the grain refining of metallic materials, which results in increased mechanical strength, tenacity, occurrence of superplastic, etc. New materials with sub-micron grain size have been developed recently as commercial materials and the latest generation of this development is

Nanostructured materials (also called nanocrystalline materials, nanophasics materials or nanometer-sized crystalline solids) are known to have properties or combinations of properties, which may be new or even superior to conventional materials (Greer, 1998). Nanostructured materials can be defined as a system containing at least one microstructural nano characteristic (with sizes ranging up to 100-150nm). Due the extremely small dimensions, a large volume fractions atoms located in grain boundaries (Morris, 1998), which gives them a unique combination of composition and microstructure (Suryanarayana, 1995). Generally, these materials exhibit high strength and hardness, increased diffusivity, improved ductility and toughness, reduced elastic modulus, lower thermal conductivity

Since nanocrystalline materials contain a large fraction of atoms in grain boundaries, many of these interfaces provide high density of short diffusion paths. Therefore, it is expected that these materials show increased diffusivity compared to polycrystalline materials of the same composition and conventional particle size (of the order of microns) (He & Ma, 2000). The consequence of such increased diffusivity is increased sinterability of nanometric powders, which causes decrease in sintering temperature of these powders when compared

The driving force for sintering or "sintering stress" of nanocrystalline ceramics with pores in the range of 5nm is about 400MPa (considering γ about 1Jm-2), while for conventional ceramics with pores around 1 μm it is 2 MPa. Thus, a nanocrystalline ceramic must have a great thermodynamic driving force for retraction, which must densify extremely well even under unfavorable kinetic conditions such as low temperatures (Suryanarayana, 1995).

The interest in this nanostructured materials area has grown due to the availability of nanocrystalline ceramic powders. These nanocrystalline powders can be synthesized using different techniques, but its consolidation into dense ceramics without significant grain

Compaction and sintering of ultra fine and/or nanoscale powder have a positive set of aspects over behavior during processing and final properties of products; however, there are also

when compared to larger grain size materials (~10μm) (Suryanarayana, 1995).

to the same material with conventional particle size (Porat et al., 1996).

suppress the grain growth.

growth is still a challenge.

**2. Nanostructure materials** 

the nanostructured materials (Inoue & Masumoto, 1993).

several processing difficulties. Main positive aspects include: increased reactivity between reagents and solid particles and between particles and the gas phase, which are important processes in synthesis; increased sintering rate and particularly lowering of sintering temperature, which can reduced by half the material's melting point (Hahn, 1993; Mayo, 1996).

On the other hand, also due to the large surface area and large excess of free energy nanometric powder systems, there are many detrimental aspects to the processing and obtaining the refined and homogeneous microstructures. Some of these aspects are: a very strong tendency to agglomeration of primary particles of nanometric powders; difficulties of mixing and homogenization of compression due to the strong attraction between particles; demand for greater sintering atmosphere control, not only due to the higher reactivity, but also the possibility of formation of thermodynamically unstable phases and appearance of strong effect of adsorbed gases on the surface (Allen et al., 1996; Averback et al., 1992).

Many studies (Chen & Chen, 1996, 1997) on nanometric size particles have shown reduction of sintering temperature. Hahn et al. (Hanh, 1990), studying the sintering of nanometric TiO2 (12nm), Y2O3 (4 nm) and ZrO2 (8nm), found lower sintering temperatures than those conventional. The sintering of TiO2 occurred at 1000ºC while conventional TiO2 sintering requires temperatures above 1400ºC. The same pattern of reduced sintering temperature was observed for Y2O3 and ZrO2. In spite of the proven decrease in sintering temperature of nanometric powders, its densification is often accompanied by a large grain growth, causing lost of their nanocrystalline ceramics characteristics.
