Nanocrystalline grains

The nanohardness of 400 nm thickness chromium coatings with ultra-fine grain (nanocrystalline) structure is equal to 21.61 GPa (Table 1), which is 8-9 times higher than that of cast chromium. At the same time the nanohardness of molybdenum coatings, which characterized even more fine-grain structure, is equivalent to 9.98 GPa. It is necessary to note, that in a massive state hardness of polycrystalline chromium is lower than hardness of polycrystalline molybdenum (2 GPa and 1.3 GPa for poly crystalline molybdenum and chromium accordingly [7]). 

Simple approach based on the effective mass theory has been developed and successfully applied to simulate electronic properties of monocrystalline and grained nanocrystalline films accounting for the confinement effect and interactions between the grains. Quantum confinement was found to influence band gap values only for the films with the thickness less than 5 nm. The highest gap varied from 0.63 to 0.91 eV depending on the film thickness as well as on the lateral size of the grains. Inclusion of the grains inside the film induces a eonsiderable increase of the gap as compared to the monocrystalline film of the same effective thickness. 

Figure 2. Band gap versus film thickness for monocrystalline /sEg = 0) and grained nanocrystalline (A g = 0.21 eV) CrSij films.

The electronic properties of monocrystalline and grained nanocrystalline CrSi2 films were estimated within the Effective Mass Theory. Inclusion of the grains inside the film increases the energy gap up to 60% compared to the monocrystalline film of the same effective thickness. 

Figure C2.17.9. Size-dependent changes in PXRD linewidtlis. PXRD can be used to evaluate tire average size of a sample. In tliese cases, different samples of nanocrystalline titania were analysed for tlieir grain size using tire Debye-Scherr fonnula. As tire domain size increases, tire widtlis of tire diffraction peaks decrease.

Various microstructures and configurations are possible for useful solid materials, including bulk single crystals and epitaxial layers, polycrystalline articles or thin films with controlled grain size (including micro- and nanocrystalline... 

In addition to microwave plasma, direct current (dc) plasma [19], hot-filament [20], magnetron sputtering [21], and radiofrequency (rf) [22-24] plasmas were utilized for nanocrystalline diamond deposition. Amaratunga et al. [23, 24], using CH4/Ar rf plasma, reported that single-crystal diffraction patterns obtained from nanocrystalline diamond grains all show 111 twinning. 

More serious errors may result when the grain-size of a specimen is small compared with the size of an indentation. Then, since all crystals are elastically anisotropic a rigid indenter will produce differing amounts of elastic strain in the grains depending on their orientations. This will create an effective roughening of the surface and increase the friction coefficient. This may result in overestimates of hardnesses. For example, this may underlie reports of nanocrystalline materials being harder than diamond. 

Hydrogenation is sensitive to the surface modifications. For example, the ball-milled powders require less activation compared to the conventional powders. For nanocrystalline hydrides, the grain boundary does not dramatically affect the PCI, which describes the thermodynamic aspects of hydride formation. However, the pressure for hydrogen desorption of the unmilled MgH2 is lower than that of the milled one as seen in Figure 11.7 [66]. 

Nanocarbon emitters behave like variants of carbon nanotube emitters. The nanocarbons can be made by a range of techniques. Often this is a form of plasma deposition which is forming nanocrystalline diamond with very small grain sizes. Or it can be deposition on pyrolytic carbon or DLC run on the borderline of forming diamond grains. A third way is to run a vacuum arc system with ballast gas so that it deposits a porous sp2 rich material. In each case, the material has a moderate to high fraction of sp2 carbon, but is structurally very inhomogeneous [29]. The material is moderately conductive. The result is that the field emission is determined by the field enhancement distribution, and not by the sp2/sp3 ratio. The enhancement distribution is broad due to the disorder, so that it follows the Nilsson model [26] of emission site distributions. The disorder on nanocarbons makes the distribution broader. Effectively, this means that emission site density tends to be lower than for a CNT array, and is less controllable. Thus, while it is lower cost to produce nanocarbon films, they tend to have lower performance. 

Las but not least, sample preparation is also an important issue. If we want to examine nanocrystalline powder samples. The grain size must be just a few nanometers, the layer, formed by these nanocrystals must be as thin as possible (to minimize dynamic difiraction), continuous and self-supporting. In many cases not all these requirements are fulfilled simultaneously. The nanocrystalline material to be studied is frequently present on a thin supporting carbon layer. In such cases peak decomposition can not yield an acceptable fit unless the presence of the amorphous material (in the form of a few diffuse rings) is taken explicitly into account in the model to be fitted. The size of the background is also affected by scattering in such a carbon support. 

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]. 

R.A. Vatin, T. Czujko, Z.S. Wronski, Particle size, grain size and g-MgH effects on the desorption properties of nanocrystalline commercial magnesium hydride processes by controlled mechanical milling, Nanotechnology 17 (2006) 3856-3865. 

The estimated value of -67 nm (no lattice strains) [6] is at the border of nanocrystallinity if one defines it as the grain size smaller than 100 nm [7]. Our result correlates very well with the grain size of 78 nm reported very recently by Kojima et al. [8] for their commercial MgH. Both resnlts suggest that certain commercial varieties of MgH could be subjected to either ball milling or other type of postdeformation in the proprietary mannfactnring process, which results in the final nearly nanosize grains (crystallites). 

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