Труды КНЦ вып.9 (ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ вып. 1/2018(9) часть 1)

Physical properties of Ni0,5Zn0,sFe2O4microtubes calcined at different temperatures Table 1 Sample NZF-873 NZF-973 NZF-1073 NZF-1173 NZF-1273 й ?311 interplanar spacing (nm) 2,5392 2,5391 2,5378 2,5363 2,5330 Unit cell parameter (nm) 0,8430 0,8429 0,8423 0,8423 0,8413 Average crystal size (nm) 25,3 37,2 73,6 95,4 112 Specific surface area (m2/g) 80,7 63,7 44,3 29,4 17,0 Fig. 2. SEM images of (a) NZF-873; (b) NZF-1073; (c) NZF-1273 microtubes. Scale bar is 1 ^m In order to study the structure transition during calcination, characteristic SEM images of the microtubes calcined at different temperatures were taken. As the temperature increases, the template and the gel absorbed in the surface layer of the template are decomposed causing an interfacial solid-state reaction which yields interconnected ferrite particles (Fig. 3, a). The tube diameter decreases after calcination at higher temperatures due to increased size of individual ferrite nanoparticles. The individual nanoparticles can be seen in a TEM image (Fig. 3, b). They have the shape of irregular polyhedrons with a mean size between 50 and 80 nm. The observed particle size is somehow larger than the crystallite size obtained from the XRD study due to the formation of connecting necks between the two neighboring particles during calcination. No large aggregated nanoparticles was observed confirming much higher degree of dispersion as compared with a non-templated synthesis method [6]. Due to this fact, the specific area of the Ni0,5Zn0,sFe2O4 microtubes was considerably enhanced in this study, * I Ш Ш Ш 50 nm ' Fig. 3. Characteristic SEM image of cross sectional view (scale bar is 4 ^m) (a) and TEM image of NZF-1073 microtubes (b) Room temperature hysteresis loops of the Ni0,5Zn0,sFe2O4microtubes calcined at different temperatures are shown in Figure 4 and their magnetic parameters are listed in Table 2. The maximum value of saturation magnetization exceeds that of the bulk Ni 0 , 5 Zn 0 , 5 Fe 2 O 4 ferrite (56 emu/g) in all samples. The saturation magnetization in nanoparticles is influenced by both the intrinsic (composition, preferential site occupancy of the cations, exchange effect) and extrinsic factors (microstructure and grain size) [6]. The increased mean crystal size increases saturation magnetization. However the exchange interaction between the Ni-Zn ferrite and the hematite impurity decreases the saturation magnetization in the samples calcined at higher temperatures [6]. The highest coercivity of 88.1 Oe is observed in NZF-973 as its grain size (37,2 nm, Table 1) is only slightly above the critical domain size (22,2 nm, Table 2). In the multidomain region, the coercivity decreases as the grain size increases from 37.2 to 112 nm. As the calcination temperature increases, the Curie temperature of the microtube increases from 532 to 549 K (Table 2). A similar trend was observed by Sepelak et al. for nanostructured Mg ferrites [7]. 102