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Solid-phase interaction in ZrO2-Fe2 O3 nanocrystalline system
S. A. Kirillova1, O. V. Almjasheva1, V.V. Panchuk2, V.G. Semenov2
xSt. Petersburg State Electrotechnical University "LETI", ul. Professora Popova, 5, St. Petersburg, 197376, Russia 2 St. Petersburg State University, Universitetskaya nab., 7-9, St. Petersburg, 199034, Russia refractory-sveta@mail.ru, almjasheva@mail.ru, vitpan@mail.ru, vaLsem@mail.ru
PACS 64.75.+g; 76.20.+q DOI 10.17586/2220-8054-2018-9-6-763-769
Based on the results of X-ray phase analysis and Mossbauer spectroscopy, it was demonstrated that in the ZrO2-Fe2O3 system, represented by the mechanical mixture of m-ZrO2 (14 ± 2 nm) and a-Fe2O3 (43 ± 2 nm) nanoparticles, being heated above the temperature corresponding to the melting temperature of the two-dimensional nonautonomous phase, transformation of a-Fe2O3 occurs resulting in appearing of the X-ray amorphous magnetically disordered state localized on the surface of ZrO2 nanoparticles in the form of a thin layer. Transformation pattern in ZrO2-Fe2O3 nanocrystalline system has been introduced.
Keywords: nanostructures, iron-zirconia, surface, X-ray diffraction, Mossbauer spectroscopy.
Received: 2 October 2018 Revised: 24 November 2018
1. Introduction
Interest in systems containing zirconium dioxide is caused by high refractory properties, strength, crack resistance, chemical inertness, superionic oxygen conductivity, biocompatibility, catalytic activity and other important characteristics of the materials, based on this substance [1,2]. Ceramic and composite materials, based on ZrO2-Fe2O3 system, are widely used as catalysts or catalyst carriers for hydrocarbons isomerization, carbon monoxide hydrogenization, selective hydrogenation (Fischer-Tropsch reaction) and ammonia synthesis [3,4], as well as the magnetic materials [5,6].
Different contradictory data, published in the scientific literature, are pertinent to possibilities and ranges of solid solutions formation in ZrO2-Fe2O3 system. The study of phase formation processes in the considered system is connected with definite methodological difficulties, related to the behavior of the studied system components [7-16].
While extremely localized areas of the iron oxide-based solid solutions in zirconium dioxide may exist for the macroscopic particles, as it follows from data [17-26] (which is most pronounced for the low-temperature range), more wide solubility range of iron oxide in ZrO2 -based nanocrystals was detected, which is shown by data [27-36], in case of nano-scale particles. In this case, the possibility and the ranges of solid solutions existence in this system to a great degree are determined by the synthesis method of considered compositions.
Composition synthesis and subsequent study of their behavior during heating for ZrO2-Fe2O3 system are generally realized with application of sol-gel synthesis followed by the annealing-quenching heat treatment of received precursors [16,22-24,31-33]; hydrothermal synthesis [5]; thermal decomposition of iron (III) and zirconium salts [27]; ZrO2 impregnating with aqueous solution of ferric (III) nitrate and subsequent drying and incineration [3,4]; solid-phase synthesis method [25,37]; reaction media burning method (solid-phase flame method, solution burning) [26,38,39], etc. [6,34-36].
Inconsistencies pertinent to ranges of solid solutions areas, a state of ZrO2 surface and iron ions location in different positions still exist and may be found almost in any new paper devoted to this system. Therefore, it is of interest to study the behavior of ZrO2 and Fe2O3 nano-scale particles during their thermal treatment.
2. Experimental procedures
In order to study the solid-phase interaction of the components, ZrO2 and Fe2O3 nano-scale powders, which have been obtained by hydrothermal treatment method (refer to [40]), were mixed in ethyl alcohol. Specimens were formed by pressing under pressure of 5 MPa, after which these specimens were heat treated in the air following the annealing-quenching mode with exposure during 30 minutes at temperatures of 800, 900, 1000 and 1100 °C.
The phase composition of the specimens was studied by X-ray phase analysis using XRD-7000 Shimadzu diffractometer (CuKa-radiation). The X-ray diffraction pattern peak identification was performed using the PDWin 4.0 software solution and Crystallographica Search-Match package. Based on the data analysis pertinent to X-ray
diffraction pattern peak tailing, the average crystalline particle size has been calculated. Calculation was performed using the Sherrer equation [41].
The specimens' elemental composition was determined by electron probe microanalysis using Hitachi S-570 scanning electron microscope, equipped with Bruker Quantax 200 microprobe system.
Exposure of Mossbauer spectra was realized using WISSEL spectrometer operating in the constant acceleration mode, at the room temperature and at the liquid nitrogen boiling point. 57Co in Rh-matrix, characterized by the radioactivity of 30 mC, was used as the source. Mathematical treatment of the experimental spectra was performed using the MOSSFIT software package [42]. The spectrometer velocity scale was calibrated using a-Fe foil at the room temperature. The chemical shift values were presented relative a-Fe.
3. Results and their discussion
Results of X-ray phase analysis of specimens, containing 6.4 ± 0.3 and 9.0 ± 0.6 mol.% FeOi.5, which have been annealed at a temperature of 800, 900, 1000 and 1100 °C over 30 minutes, are presented in Figs. 1(a) and 2(a). Initial specimens were represented by the mechanical mixture of preliminary received nano-scale powders of ZrO2 (monoclinic and tetragonal modification groups) and Fe2O3 [hematite, (104) peak, I - 100 %]. After heating at a temperature of 800 - 900 °C, a decrease in the intensity of the t-ZrO2 peaks was observed, and after heating at a temperature above 900 °C, diffractograms contained only those reflections, which corresponded to monoclinic modification group of ZrO2. For the specimen containing 6.4 ± 0.3 mol.% FeO1.5, which was annealed at 800 and 900 °C, a peak on diffractograms, corresponding to a-Fe2O3, disappeared. During further increase of the thermal treatment temperature, (104) peak on diffractograms appeared again, however, its intensity was substantially less as compared with the initial composition. For the specimen, containing 9.0 ± 0.6 mol.% FeO1.5, which has been annealed at a temperature of 800 - 900 °C, the relative intensity of (104) peak on diffractograms decreased as compared with the initial composition, while at a temperature of 1000 - 1100 °C the integral intensity of this peak has increased, becoming comparable with the initial composition.
It should be noted that sizes of m-ZrO2 crystallites during heating up to a temperature of 900 °C have changed only slightly both for the specimen, containing 6.4 ± 0.3 mol.% FeO1.5 (Fig. 3), as well as for the specimen, containing 9.0 ± 0.6 mol.% FeO1.5. Analysis of m-ZrO2 unit cell parameters dependence on temperature has demonstrated their invariability within the measurement accuracy (Fig. 4). Absence of variation of ZrO2 unit cell parameters may be indicative of the fact that heat treatment during 30 minutes at a temperature of up to 1000 and 1100 °C did result in m-ZrO2-based solid solution formation.
Mossbauer spectra for initial compositions of specimens, containing 6.4 ± 0.3 and 9.0 ± 0.6 mol.% FeO1.5, as well as for specimens, annealed at 800, 900, 1000 and 1100 °C, are presented in Figs. 1(b) and 2(b), respectively. The results of experimental spectra analysis with detection of sextet and doublet components, corresponding to iron atoms in magnetically ordered and magnetically disordered phases, are also presented in Figs. 1(b) and 2(b). Mossbauer parameters for sextet and doublet components of the iron-containing phases are presented in Table 1. The quantitative estimation of the iron relative content in different phases was performed with precision determined by difference in values of fM Mossbauer factor (probability of resonance absorption in each phase).
For the initial specimens, which were represented by the mechanical mixture of the preliminary produced t-, m-ZrO2 and a-Fe2 O3 nano-scale powders, the Mossbauer spectrum was characterized by sextet, parameters of which have corresponded to the sextet parameters for a-Fe2O3 [43].
For the specimens being thermally treated at 800 ° C, the Mossbauer spectrum was characterized by doublet (6.4 ± 0.3 mol.% FeO1.5) or doublet and sextet superposition (9.0 ± 0.6 mol.% FeO1.5). It should be noted that for the specimen containing 6.4 mol.% FeO1.5 only doublet components have been commonly observed, while these doublet components are incidental to the atoms of iron in 3+ oxidation rate located in the form of thin layer on the surface of another oxide [44,45]. With a temperature increase, the sextet appeared again and the doublet share in experimental spectrum has decreased; at a temperature of 1100 °C a share of the magnetically disordered phase was about 23 %. Such an effect was observed in case of increasing of particles sizes [43]. For the specimen, containing 9.0 mol.% FeO1.5, the sextet component is common for the whole heat treatment temperature range.
While comparing the data of X-ray phase analysis and Mssbauer spectroscopy, it may be concluded, that at a temperature of 800 °C, transformation of a-Fe2O3 begins, which results in its transfer into X-ray amorphous magnetically disordered phase, localized on the surface of zirconium dioxide particles, as shown by the absence of (104) a-Fe2O3 peak on the diffractogram, by sextet disappearance and doublet appearance in Mossbauer spectrum of the specimen, containing 6.4 ± 0.3 mol.% FeO1.5. With increased thermal treatment temperatures, the sizes of m-ZrO2 particles are increased (Fig. 3) that causes decrease of their surface area and, therefore, aggregation of the iron oxide surface layer, accompanied with a-Fe2O3 bulk phase particles formation. Formation of a-Fe2O3 particles is confirmed by the data of X-ray phase analysis and Mossbauer spectroscopy, i.e. (104) a-Fe2O3 peak
Fig. 1. X-ray diffractograms (a) and Mossbauer spectra series, measured at 25 °C (b) of the specimen, containing 6.4 ± 0.3 mol.% FeOi.s, which has been annealed at a temperature of 800, 900, 1000 and 1100 °C during 30 minutes
on the diffractogram appears again, in Mossbauer spectrum for the specimen, containing 6.4 ± 0.3 mol.% FeO15 sextet corresponding to a-Fe2 O3, may be revealed against the doublet background.
Apparently, during thermal treatment at a temperature of 800 and 900 °C with exposure for 30 minutes, a-Fe2O3 is transformed with the formation of the iron oxide layer on a surface of ZrO2 nanoparticles. At temperatures of 800 - 900 °C, for a-Fe2O3 are corresponding to the melting temperature of the surface (two-dimensional nonautonomous) phase [46]. It appears that transfer of the two-dimensional nonautonomous phase into a liquid (liquid-like) state initiates active mass transfer of the iron-containing component to a surface of ZrO2 nanoparticles. It should be noted that similar processes were observed earlier in BeO-Fe2O3, MgO-Fe2O3, Al2O3-Fe2O3, SiO2-Fe2O3 systems [44,45]. The transformation pattern, corresponding to the described processes in ZrO2-Fe2O3 system, in which zirconium dioxide is initially represented by m-ZrO2 and by a-Fe2O3 nanoparticles, characterized by the crystallites sizes of 14 ± 2 nm and 43 ± 2 nm, respectively, is presented in Fig. 5.
4. Conclusion
As distinct from the cases, in which compositions are received via codeposition, hydrothermal synthesis, etc., and in which mixing of components is initially possible at the atomic level, in the case of the solid-phase interaction of the nano-scale particles in ZrO2-Fe2O3 system in the range of low contents of the iron-containing component increasing of the treatment temperature up to 800 - 900 °C does not cause stabilization of zirconium dioxide tetragonal modification and the formation of tetragonal solid solution, as well as Fe3+ ions are not included into m-ZrO2 structure.
During the thermal treatment of ZrO2 and Fe2O3 nanoparticles, at definite critical value of temperature in range of 800 - 900 °C, a-Fe2O3 transformation occurs with thin layer formation on m-ZrO2 nanoparticles surface, i.e. formation of the composite nanoparticles of "nucleus (m-ZrO2 nanoparticles) - shell (amorphous Fe2O3)" type takes place.
Fig. 2. X-ray diffractograms (a) and Mossbauer spectra series, measured at 25 °C (b) of the specimen, containing 9.0 ± 0.6 mol.% FeOi.s, which has been annealed at a temperature of 800, 900, 1000 and 1100 °C during 30 minutes
Fig. 3. i-Zr02 (■) and m-Zr02 (♦) crystallites sizes dependence on the heat treatment temperature for the specimen, containing 6.4 ± 0.3 mol.% FeO1.5
Fig. 4. m-ZrO2 unit cell parameters dependence on the heat treatment temperature for the specimen, containing 6.4 ± 0.3 mol.% FeOi.s
Table 1. Mossbauer parameters of spectra, measured for the studied specimens at a temperature of 25 °C
FeOi.5 content, mol.% Annealing temperature, °C Parameters of Mossbauer spectra
sextet doublet Paramagnetic component content, %
Isomer shift, mm/s Quadrupole splitting, mm/s Effective field, T Isomer shift, mm/s Quadrupole splitting, mm/s
6.4±0.3 - 0.378±0.013 0.218±0.026 50.871±0.099 - - 0
800 - - - 0.326±0.000 1.032±0.000 100
900 0.375±0.018 0.221±0.037 51.734±0.122 0.331 ±0.000 1.092±0.011 84.85
1000 0.376±0.005 0.188±0.010 51.693±0.038 0.340±0.014 1.152±0.029 41.51
1100 0.376±0.004 0.185±0.007 51.594±0.028 0.337±0.021 1.123±0.042 22.62
9.0±0.6 - 0.357±0.005 0.228±0.010 50.905±0.040 - - 0
800 0.383±0.016 0.192±0.033 51.458±0.136 0.324±0.042 1.128±0.080 52.54
900 0.374±0.008 0.240±0.015 51.984±0.060 0.342±0.040 1.093±0.078 35.01
1000 0.372±0.005 0.193±0.009 51.976±0.037 0.203±0.077 1.292±0.169 14.18
1100 0.373±0.006 0.211±0.011 51.938±0.044 0.193±0.087 1.282±0.179 - 5
Acknowledgements
The authors express their gratitude to V. V. Gusarov for the attention he paid to the work and for his help in the interpretation of the results.
This work was supported by Ministry of education and science of the Russian Federation (Project 3.6522.2017).
D = 43±2 nm
Fig. 5. Transformation pattern in ZrO2-Fe2O3 system
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