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

Fig. 4. Magnetization curves of Ni0,sZn0,sFe2O4 microtubes calcined at different temperatures (a), hysteresis loops at a larger magnification (b) Table 2 Magnetic properties of Ni 0 , 5 Zn 0 , 5 Fe 2 O 4 microtubes calcined at different temperatures Sample NZF-873 NZF-973 NZF-1073 NZF-1173 NZF-1273 Saturation magnetization (emu/g) 56,6 62,8 67,8 67,1 66,6 Coercivity (Oe) 56,9 88,1 56,6 31,4 26,1 Curie temperature (K) 532 534 535 537 549 Critical domain size (nm) 27,3 22,2 19,1 19,5 20,1 Specific heating rate (W/g) 2,45 4,36 3,10 2,30 2,10 Figure 5, a shows the temperature dependence of the coercivity of the NiasZnasFe^ microtubes. The coercivity shows a non-monotonous temperature dependence with a local maximum in the range between 573 and 823 K, which is slightly above their Curie temperature. Below the Curie temperature, the coercivity decreases with an increase in temperature in accordance with the ferromagnetism theory as the degree of atomic thermal vibration increases. Above the Curie temperature, an internal induced magnetic field is formed via a preferred orientation of the magnetic moments in the microtubes in the direction opposite to that of the applied magnetic field. The magnitude of this effect is proportional to the magnetic susceptibility, which decreases with temperature resulting in a decrease of coercivity. However such coercivity behavior cannot be accounted for by exclusively considering the temperature dependence. The characteristic internal stress in the isolated microtubes may result in the anomalous coercivity behavior since the internal stress induces anisotropy in the microtubes. The presence of temperature-dependent mechanical stresses acting on the domain walls must be considered as a cause of the observedbehavior. 300 400 500 600 700 800 300 400 500 600 700 800 900 Tested temperature (K) Tested temperature(K) Fig. 5. Coercivity(a) and hysteresis loss (b) of Ni0,5Zn0,sFe2O4 microtubes as a function of temperature Figure 5, b shows hysteresis loss of the microtubes as a function of tested temperature. Due to a non-zero saturation magnetizationand a moderate coercivity, the microtubes could be heatedby RF heating in the temperature range above their Curie temperature. This is a unique feature ofthese materials whichwas not observed in the nanoparticles ofthe same phase composition. From the viewpoint of application of the obtained materials in catalytic reactors under radiofrequency heating, it is important that they possess high specific surface area and demonstrate high specific heating rate which allows to maintain desired temperature inside the reactor. It can be seen that the sample with a mean particle size of 37 nm demonstrated the best combination of specific surface area and the largest specific heating rate in RF field of 295 kHz. To conclude, we report here the synthesis of a new generation of one-dimensional isolated Ni-Zn ferrite microtubes displaying heating properties at elevated temperatures considerably larger than any previously described material. The unique heating properties of these materials was shown to result from the formation of an isolated one-dimensional ferromagnetic tubular structure with a diameter in the 4-6 micron range and a mean particle size close to the critical domain size. These materials can be 103