The microstructure is defined by the type, crystal structure, number, shape and topological arrangement of phases and defects such as point defects, dislocations, stacking faults or grain boundaries in a crystalline material. Nanocrystalline materials are polycrystalline solids with grain sizes below 100 nm. Grains as well as pores, interfaces and other defects are of similar dimensions. This nano-specific microstructure (nanostructure) leads to chemical and physical size effects. It is a prerequisite for the understanding of properties of nanomaterials to have a detailed knowledge of the structure from the atomic/molecular (local) to the crystal structure (long range order) and to the microstructure (mesoscopic scale and defect structure). Consequently, various analytical techniques are required to characterize the nanomaterials on all length scales.
Particle size and morphology
Scattering and imaging methods such as line broadening in X-ray diffraction, small angle X-ray and neutron scattering and transmission electron microscopy, are employed to determine average particle and grain size, their distributions and the morphology of the particles. Additionally, nitrogen adsorption, light scattering, atomic probe techniques and mass spectroscopy provide complementary information on these size related parameters. The method of choice depends on the type of material (powder, solid and liquid dispersion, consolidated and sintered ceramic, etc.). Care should be taken in comparing the average sizes determined by these methods because different size related parameters are determined (e.g. column length in XRD line broadening) and the average values are based on different weight functions.
Information on the local structure can be provided by spectroscopic methods such as NMR or EXAFS spectroscopy but is also contained in diffuse scattering. Nanocrystalline materials are heterogeneously disordered with a large fraction of atoms located in surfaces and interfaces. In case of pure, tetragonal nanocrystalline zirconia with a particle size of 5 nm it was shown that atoms located at particle surfaces have a higher degree of disorder compared to atoms in the core of the particles due to more degrees of freedom. Primary processes during the particle formation by CVS and the evolution of the microstructure during sintering occur on the local, molecular level. Segregation of aluminium atoms was identified as the mechanism for the grain growth inhibition during sintering of zirconia doped with alumina by Reverse Monte Carlo analysis of EXAFS spectra. Similarly, an inhomogeneous distribution of yttrium in zirconia was found and correlated to a low sinterability.
X-ray, electron and neutron diffraction are extensively used to characterize the crystal structure which has been observed to be size dependent in many cases. The line broadening at very small crystallite sizes limits the accuracy of crystal structure determination, i.e. difficulty in distinguishing between tetragonal and cubic zirconia. In general it is observed for CVS nanopowders that all diffraction reflexes are present and that the background is very low. This indicates a high degree of crystallinity and low defect density within the individual particles. An exemption is nanocrystalline silicon carbide which exhibits stacking faults and twinning, especially when synthesized at low temperature. From Rietveld refinement not only the phase composition, lattice parameters and positions of atoms in the unit cell, but also crystallite size and the microstrain can be extracted.
On a larger length scale electron microscopy (both scanning and transmission) is extensively used to characterize defects such as dislocations, grain boundaries, agglomerate size and structure, distribution of phases, grain and phase boundaries, pores etc. which are important for the properties of the nanoceramics. In case of CVS nanopowders, the primary nanoparticles are single crystalline with lattice fringes extending to the surface. In addition, it is frequently found that the nanoparticles exhibit crystallographic habitus planes. For the characterization of porosity in nanoceramics, nitrogen adsorption and small angle scattering are used to determine surface area, total pore volume and pore size distributions. Powders exhibit fractal structures typically observed for aerogels.
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