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

properties of ferrite materials are rather sensitive to their preparation method, many methods are not commercially viable at a meaningful scale due to complexity, long synthesis time, or cross contaminationwith impurities. In this way, the template approach is very simple and cost-effective preparation method compared to the other synthesis methods. The goal of this study was to develop a template assisted sol-gel method for the synthesis of one dimensional isolated Ni-Zn ferrite microtubes (NZF microtubes) with high specific heating rate using natural cotton fiber templates. The influence of calcination temperature on the structural, magnetic and RF heating properties of the materials obtained was investigated. Experimental Ni-Zn ferrite microtubes with a nominal composition of Ni0,5Zn0,5Fe2O4 were prepared by the template assisted sol-gel synthesis following the approach described in our prevous study [5]. Solution A was prepared by dissolution of the corresponding metal nitrates in ethanol (all from Aldrich Co., ACS grade). Citric acid was dissolved in ethanol in a separate vessel to produce solution B which was added into solution A and the resulting mixture was stirred for 4 h. Then an ammonia solution was added dropwise till a pH of 2,4, the mixture was stirred for 24 h and then it was absorbed by cotton fibers. These cotton fibers were dried in 353 K. The samples were calcined at a desired temperature in the 873-1273 K range for 1 h to produce the corresponding ferrite microtubes. The microtubes are labeled as NZF-T hereafter, where index T represents the calcination temperature in K. The phase composition of the samples was determined using an X-ray diffractometer (X’Pert PRO) with Cu Ka radiation produced at 40 kV and 27,5 mA, at a scanning rate of 5 o2theta /min and a step of 0,02 o. The morphology was characterized by scanning electron microscopy (JSM-6700F, Jeol, Oxford) equipped with an energy dispersive spectrometer (EDS). The specific surface area was determined by nitrogen adsorption at 77 K on a nitrogen adsorption apparatus (Micromeritics NOVA 1000E). The magnetization curves of the as-prepared samples were measured by a vibrating sample magnetometer (Princeton Measurements Corporation MicroMag 3900 VSM) equipped with a 2 Tesla electromagnet at several temperatures in the range from 298 to 833 K. The saturation magnetization (M), remnant magnetization (Mr), coercivity (H ) and hysteretic losses were evaluated from the magnetization curves. The RF properties of the samples were characterized by their specific absorption rate (SAR). The SAR is determined by the thermal energy released by the material in a external magnetic field with a frequency o f295 kHz and an intensity of 500 Oe. Results and discussion XRD spectra of Ni0,5Zn0,5Fe2O4 microtubes calcined at different temperatures are shown in Figure 1. It can be seen that the spinel phase is formed in the entire temperature range studied (873-1273 K). з cd Й(D Ссс H 1 )02 2( •o ^ Я ^ 2 4( 2 <N ^ Г р ! h h ) H Hematite 4( S ^<N m .. i А. V . i .1 . i. NZF-1273 . . . A . . » NZF-1173 . JL J , 1 NZF-1073 1 . NZF-973 NZF-873 < 1 1 , 1 . 1 1 . Standard NZF .......................................................................................................... 20 30 40 50 60 70 80 20 (degree) Fig. 1. XRD patterns of the Ni0,5Zn0,5Fe2O4 microtubes The unit cell parameter, interplanar spacing, average crystal size and specific surface area of the microtubes calcined at different temperatures are listed in Table 1. As the temperature increases, the specific surface area decreases by a factor of nearly five due to progressive aggregation of small crystallites into larger particles. Figure 2 shows characteristic SEM images of the template and Ni 0 , 5 Zn 0 , 5 Fe 2 O 4 microtubes calcined at different temperatures. It can be seen that one-dimensional isolated ferrite microtubes are obtained. The microtubes with a low degree of crystallinity are obtained after calcination at 873 K which agrees with the XRD data. Their mean diameter is 6 ^m (Figure 2, a). The microtubes with a mean diameter of 3,7 ± 0,2 ^m and with a higher degree of crystallinity were obtained after calcination at 1073 K (Figure 2, b). They are formed by rather uniform nanoparticles with a size of 80 nm and they have a higher length-to- diameter ratio of 12. After calcination at 1273 K, the microtubes with a mean diameter of 2,5 ± 0,2 ^m were obtained. A characteristic feature of the fibers is the presence of small channels along the radial direction. 101