Nanocrystalline phase

Addition of Nb into the Fe-Co-Si-B system from the previous case leads to transformation of 40 vol.% into grains of bcc-Fe(Co) with dimensions 30nm (Fig. 2). Even smaller nanograins of bcc-Fe(Mo), not exceeding 8nm are obtained by crystallization of Fe-Mo-Cu-B (Fig. 3), where the stability of the clustered amorphous remains keeps the content of nanocrystalline phase lower than 25 vol.% till almost lOOOK. The reasons for this behavior can be traced to drastically enhanced nucleation rate via heterogeneous or instantaneous nucleation, which can decrease the amount of nanocrystallized volume in the first transformation stage even below 20 vol.% [5]. 

While interest in amorphous alloys was becoming vane over the time, the paradigm shift in materials science brought about by the discovery of amorphous metals and rapid solidification cannot be underestimated. The interest of physicists, chemists, materials scientists, and engineers became refocused from well-ordered crystalline materials to disordered and nanocrystalline phases. Shortly, substantial R D programs were initiated in the USA, Japan, and worldwide both at universities and national government laboratories. Foundations have been established for the advance of the science of nanomaterials. 

The formation of mullite at ca 1000 °C is supported by the XRD data shown in Figure 12. Thus, based on DTA, TGA and XRD, one might decide that phase pure mullite without carbon contamination is produced by pyrolysis of precursor to 1000 °C. This then would represent a processing goal for making mullite materials. Also, it appears to be similar to the SiC precursor results, where nanocrystalline, phase pure SiC is obtained... 

Diffraction experiments combined with other techniques, such as different spectroscopic methods, will develop in the coming years. At present it is possible to carry out synchrotron X-ray diffraction experiments combined with for example Raman spectroscopy and extended X-ray absorption fine structure (EXAES) spectroscopy. Such experiments will help to clarify hydrogenation/dehydrogenation reactions involving for example amorphous/ nanocrystalline phases, as for example observed in several borohydrides. 

Large-angle X-ray powder diffraction (XRD) has been one of the most versatile techniques utilized for the structural characterization of nanocrystalline metal powders. The modern improvements in electronics, computers, and X-ray sources have allowed XRD to become an indispensable tool for identily-ing nanocrystalline phases as well as crystal size and crystal strain. The comparison of the crystallite size obtained by the XRD difffactogram using the Scherrer formula with the grain size obtained from the TEM image allows us to establish if the nanoparticles have a mono- or polycrystalline nature. 

Bulk as well as nanocrystalline phases of metal oxides, nitrides and carbides are often synthesized by employing carbothermal reactions. For example, carbon (activated carbon or carbon nanotubes) mixed with an oxide produces sub-oxide or metal vapour species that react with C, 0, or NHj to produce the desired product. Thus, heating a mixture of Ga Oj and carbon in or NHj produces GaN. Carbothermal reactions generally involve the following steps ... 

M. Urban ova, L. Kobera,J. Brus, Factor analysis ofAl-27 MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers, Magn. Reson. Chem. 51 (2013) 734-742. 

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