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HRTEM processing investigation of nanocrystalline materials

Because knowledge of microstructure (defects, grain sizes, ..) is indispensable for understanding the macroscopic behavior of solids, the application of HREM image processing (HREM-IP) and Fourrier filtering could give a deeper insight into the particular process on the atomic level. It appears that HREM-IP could be very successfully applied in the analysis of series of the nanocrystalline materials prepared by ball milling (BM), sol-gel procedure or rapid quenching. The HRTEM-IP analysis of ball milled monoclinic (m) m-ZrO2 and m-ZrO2 + Y2O3 with different oxides revealed transition to tetragonal (t) t-ZrO2 and t-ZrO2 solid solution. The appearance of high-temperature phase at room temperature as result of milling treatment is now explained by thermodynamic consideration about the role of the defects and the value of the grain size in the transition from m-ZrO2 to t-ZrO2 (and appearance of high temperature phase at room temperature as a result of ball milling). It is now generally accepted that the defects (dislocations, vacancies, stacking faults, grain boundaries, interfaces) initiate these transitions. Those defects give the contributions to the free energy of defects  Gd. When free energy of monoclinic phase Gm and defects (strain energy) Gd are higher than free energy of tetragonal phase Gt, tetragonal phase can be formed even at room temperature. Here we present the results obtained: (a) BM ZrO2 (Figure 1) [1]. In FT of Fig. 1(l) diffraction spot 7 reveals (002) t-ZrO2 planes in Fig. 1(f) while diffraction spot 5 reveals (-111) m-ZrO2 planes of Fig. 1(d). It appears that superposition of tetragonal planes in Fig. 1(h) and superposition of m-ZrO2 is in Fig. 1(i). In the Fig. 1(a) to Fig. 1(g) the dislocations forming GB are observed. Such analysis gives evidence about the transition from m-ZrO2 to t-ZrO2 obtained in BM. Some other analysis gave as the result that the order-disorder process took place in BM of ZrO2 and ZrO2 + Y2O3 [1, 2]. (b) An analysis of nanocrystalline iron-doped ZrO2 powders [3] obtained from HRTEM-IP revealed the identification of different phases of TiO2 (rutile, brookite), the presence of poly(ethylene glycol) and beginning of   Fe2O3 precipitations in GB. One concludes that the HRTEM-IP is a sensitive and precise method for the analysis of grain boundaries, defects including SF and overlapping layers, as well as for obtaining results which are inaccessible with other methods.

nanomaterials; HRTEM processing; ZrO2; Y2O3

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