Scientific Advisory Board - Erich Schmid Institute

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$dissertation global and local fracture properties of metal matrix ...$

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE Revealing the relationship of the lattice constant and nitrogen vacancy in CrN using atomic resolution electron microscopy (Advanced imaging and analytics) transition metal nitrides like crn have, besides applications as wear-resistant coatings, recently gained considerable interest for its unique antiferromagnetic configurations, and have been used as a prototype material for strong magneto-structural interactions. the study of the atomic and electronic structure of crn films is of vital importance for understanding the antiferromagnetic property. ordered n vacancies distributed at {111} planes were detected, which lead to a distorted lattice. Such ordered n vacancies can form at the interface crn–cr during the film synthesis. Advanced tem techniques, such as electron energy-loss spectroscopy (eeLS) and energy-loss near-edge structure (eLneS) analysis, spherical aberration (c S)-corrected high-resolution transmission electron microscopy (HRtem) and scanning tem (Stem) and geometrical phase analysis (gpA), are applied to characterize the atomic and electronic structures of the ordered n vacancies, furthermore, exploring the relationship between the lattice constant and nitrogen vacancy concentration and a comparison will be made with the theoretical calculations using the density functional theory (dft) (cooperation with prof. c. mitterer, department of materials physics, Univ Leoben, cooperation with prof. draxl claudia, Humboldt-Universität zu Berlin) independent of the substrates used, the crn films grow frequently in a fine grained randomly oriented microstructure during the initial nucleation stage and then continuously grow in a columnar manner. However, when a cr interlayer is introduced between the crn and the substrate, crn can grow directly on the cr layer. the columnar grain size is about 60 -100 nm in width. to examine the atomic and electronic structure of the interface in detail, a large columnar grain is selected for the atomic resolved imaging. A c S-corrected HRtem image (fig. 1) acquired in [110] direction shows some “stripe” features along the {111} plane adjacent to the cr-crn interface, forming a “defective layer” (crn x) in the film. the thickness of the defect layer is in the range of a few to tens of nanometers. the diffractograms illustrate that the defective layer possesses the identical pattern as the bulk crn except a slight expansion, which means that it retains the same crystal structure as the crn, of fcc lattice, (also corroborated by the HRtem image from the [010] direction), but possesses a slightly different lattice constant. A cs-corrected high resolution Stem images is also shown in fig.1, in which the stripe features are still visible (cooperation with dr. Herbert Schmid, Leibniz-institut für neue materialien, Saarbrücken, germany). the strong distortions at the region containing numerous ordered n vacancies can be visualized using the geometrical phase analysis. the distortion mapping performed at the same region (where the eeLS analysis was carried out) clearly shows three distinguishable regions (fig. 2a). the corresponding integrated line profile crossing the three layers shows a different magnitude of distortion. from the map, relative to the perfect crn layer, the defective layer is slightly compressed while the cr layer is somewhat expanded. the distortion distributions in the defective layer appears inhomogeneous, which could imply the presence of anisotropic distortion distributions attributed to the nitrogen vacancies located at the {111} planes. the displacement map (d 200) performed on the same region was acquired using the crn (200) reflection as shown in fig. 2b. As the defective layer retains the cubic structure and assuming a direct relationship between the structure and composition, the displacement map actually demonstrates the lattice constant (2×d 200) variations with the n vacancy, which gives displacement information at each individual position as comparing with the analysis of the page 16 Scientific RepoRt 2012
ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE Fig . 2 The distortion map using the reflections from the Cr layer clearly showing three distinct regions, a line profile crossing three layers integrated from a rectangular region from top to bottom attached below. (b) The d (200) displacement map acquired using the CrN (200) reflection, and an integrated line profile over a distance of ~ 12 nm as labeled by a rectangular region, starting at CrN about 3.0 nm away from CrN-CrN x interface and ending at the CrN x-Cr interface is shown below. debye-Scherrer ring obtained from fft (fast fourier transform) analysis of a certain area of the image. An integrated line profile spanning from the perfect crn layer to the interface of cr/ defective crn about 12 nm in length, which corresponds to the same distance as the eeLS linescan carried out (within the range the n/cr atomic ratio significantly changes), exhibits the distribution of the lattice constant. the strong fluctuations in the profile originate from the displacement singularities at the location of dislocations. However, the linear relationship still can be found by fit, which is d 200 = (0.2067 – 7.6736×10 -4 ×d) nm, where d represents the distance. this also reflects the change of the lattice constant (2×d 200) within the measured distances, signifying that the lattice constant and n vacancy variations are closely related. With the available data from the HRtem image and eeLS analysis performed at the same region crossing the defective layer, a generalized relationship between the lattice constant and n vacancy concentration (or the n/cr atomic ratio) can be further established. Using the relationship fit of lattice constant and distance (fig.2b), combined with the eeLS data acquired at the identical region, the variation of the lattice constant with the n vacancy concentration can be experimentally derived and plotted (fig.3). for comparison, the calculated relationship by dft (red) is also inserted, where, for convenience, the n vacancy concentration is expressed as (1-x)×100% for crn x and the lattice constant is described as a lattice constant ratio. Very interestingly, it reveals that the experimentally measured and theoretically calculated relationships present a similar tendency, and both display that the lattice constant gradually decreases with increasing the n concentration. it is clearly seen that within the error it reaches a reasonable agreement. in fact, it uncovers a generalized relationship between the lattice constant and its composition in crn, i.e. n vacancy. However, it should be pointed out that Scientific RepoRt 2012 page 17
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Scientific Advisory Board - Erich Schmid Institute

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