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Kinetic Research of Solid-Phase Synthesis of Lithium Pentaferrite A.P. Surzhikov, A.M. Pritulov, O.V. Galtseva, R.U. Usmanov, Tomsk Polytechnic University, 30, Lenin ave., Tomsk, 634034, Russia Tel./Fax: +8(3822) 53-37-87, E-mail: [email protected] Abstract – In the context of kinetic model of nu- cleation the analysis of kinetic dependences of ra-diation – thermal synthesis of lithium pentaferrite The synthesis was carried out from a mechanical mix- was carried out. It’s shown that the solid-phase ture of lithium carbonate and iron oxide. It was pressed synthesis in diffusion – kinetic mode was carried as tablets by mass thickness 0.6 g/cm2. Radiation- out. At increasing of roasting temperature the syn- thermal roasting of specimens was carried out on thesis changes from diffusion – control into kinetic pulse electron accelerator ILU-6 (Institute of nuclear mode of reaction. In conditions of radiation the Physics, SB RAS, Novosibirsk). Electronic energy was decrease of energy of activation of process ob- 2.4 MeV, beam current in the pulse – 400 mА, pulse serves, which connecting with dynamic recharging duration – 500 µs, pulse repetition rate – 7÷15 Hz. Average irradiation power of heating ~ 5 kGr/s, in mode of isothermal roasting ~ 3 kGr/s. Local power of absorbed doze is 800 Gr/s within of action of one pulse. For revealing radiation effects, the thermal synthesis In material science of multicomponent connections the was carried out at the same temperatures and durations problem of activation of solid-phase reactions is ur- of roasting. Duration of nonisothermal stages (heating gent. The decision of this problem would allow to reduce temperature of ceramic synthesis and to reduce The determination of phase structure and parameters its duration. This problem in the relation to lithium of a crystal lattice of researched samples was carried containing spinels is especially urgent which concern out by a method of the full-profile analysis of X-ray to number of the most widespread magnetic materials pictures with use of a program complex Powder Cell 2.4. These materials are used in discrete high-speed 3. Results and discussion phase shifters and are perspective inexpensive cathode Figure 1 represents the fragments of X-ray pictures, materials of rechargeable lithium batteries [1]. which were measured after roasting of reactionary The available data testify, that the influence on sub- mixtures in thermal and radiation-thermal modes dur- stance of intensive flows of accelerated electrons is possible to relate to perspective methods of activation of solid-phase reactions, which capable to warm up it to high temperatures at the braking in substance (radiation – thermal or (RТ) of influence) [1, 2]. In particular, it was shown in works [1, 2], that at isochronic roasting of stoichiometric mixture of lith- ium carbonate and iron oxide in conditions of elec-tronic irradiation the increase of speed of synthesis of lithium pentaferrite is observed in comparison with synthesis at thermal roasting. These data practically do not carry the information about a nature of processes responsible for observable effect, but proving a pres-ence of radiation stimulation of solid-phase synthesis of lithium pentaferrite. In this plan the methods of kinetic analysis is sig- nificant informative, allowing to concretize the mecha- nism of solid-phase reaction. In the present work, the results of kinetic researches of RТ synthesis of lithium Fig. 1. The difractograms of mixture Li Fe5O8 after thermal pentaferrite are submitted in a range of temperatures (Т) and radiation-thermal (RТ) roasting at 600 and 700 °С, during 60 min (О − Li Fe5O8; Х − α-Fe2O3) There are reflections of a spinel phase and α-Fe2O3 diffusion and kinetic stages of process the model can at difractogram structure. By us a spinel is identified be described by the combined equation [4] as lithium pentaferrite (s.g. Р4332) with a lattice pa- rameter is 8.329 Å. The presence of superstructural reflections (110), (210), (211), (310), and (320), char-acteristic for ordered lithium ferrospinel, confirms Kd and Kr are the constants of reaction speed, which an identification of spinel with lithium pentaferrite. are proceeding on the diffusion and kinetic mechanism In addition, the full-profile analysis has revealed weak correspondingly; (1 – с) and с are the partial contribu- reflections from phase planes of lithium orthoferrite tions of one-dimensional diffusion increase and one- 2, which were imposed on reflexes of spinel Corrected experimental kinetic curves were ap- Comparing difractograms of Т and RТ roasting proximated by equation (2) that has allowed determin- mixtures, it is visible what the intensity of spinel re- ing a constant of reaction speeds and partial contribu- flections essentially increase at increasing of ferritiza- tions of mechanisms at 700, 800, and 1000 °C. The tion temperature, and intensity of reflexes of initial temperature dependences of constants of reaction RТ roasting is actually equivalent to thermal, but for conducted roasting at higher temperature. Figure 2 illustrates kinetic curves of accumulation of lithium pentaferrite at roasting of reaction mixture at 800 °C. Fig. 2. Kinetic dependences of degree of transformation α of reaction mixture: RT – radiation-thermal synthesis; T – thermal synthesis; solid lines – approximation by equation (1); a similar kind (therefore we do not give them here). All established kinetic dependences are adequately αi is the degree of transformation; τі is the duration of process; a, b, and α0 are the adjusting parameters. Using adjusting results and the approach, which was stated in work [3], the amendment to duration of isothermal roasting and the levels of apparent satura- Arrenius's dependences of constants of reaction tion of kinetic curves were determined. It has allowed speeds, leaking at kinetic Kr and diffusion Kd mechanisms describing results by more simple equation, in which the initial non-stationary site is absent. In the context of kinetic model of nucleation, the The effective energy of activation of process of mathematical processing of experimental kinetic de- synthesis was determined on an inclination of straight pendences was carried out. For transitive diffusion– lines. This values and partial contributions of mecha- kinetic area and in the assumption of independence of Table. Partial contributions of mechanisms and energies electron beam there is an essential decrease of activa-of activation of speed constants of reactions of diffusion – tion energies Er and Ed. The decrease of effective en- kinetic mode of synthesis of lithium pentaferrite ergy of activation at radiation-thermal synthesis is possible to explain by short-term decrease of a cation Diffusion Kinetic charges from Fe3+ into Fe2+ owing to creation of the large concentration of free electrons by accelerated electron beam in material. As the ions Fe3+ have higher charge in comparison with ions Fe2+, the movement of cation Fe3+ on a lattice of spinel is more complicated, than movement of ions Fe2+. The decrease of activation energy for the specified ions is possible to explain by 0.69 0.31 reduction of energy of electrostatic interaction with environmental anions, i.e., the decrease of Madelung 0.38 0.62 component of connection energy. The dynamic over- charge of cations Fe3+ at radiation – thermal condi- tions is equivalent to decrease of their effective 0.14 0.86 charge, also causes an increase of mobility of iron ions 0.70 0.65 and an acceleration of reaction as a whole. Cations Li+ concern to ions with constant valency, it is difficult to 0.55 0.48 expect of decrease of their charge even at presence of superequilibrium concentration of free electrons. The data of Table show, that despite of compara- Therefore offered mechanism of radiation intensifyca- bility of the partial contributions as a whole, the most tion of synthesis is most probable, if a limiting stage powerful contribution brought a mechanism of diffu- of synthesis is a migration of ions Fe3+. sion–kinetic reaction. At 800 °С, the contributions of mechanisms are approximately equal. At 1000 °С the kinetic mechanism dominates. The increase of kinetic Radiation-thermal synthesis of lithium pentaferrite component is evidence about significant decrease of realized with the same succession of sold-phase trans- diffusion impediment with increase of temperature formations, as at thermal ferritization. Reaction carries of process and at bridge into RT mode of synthesis. in diffusion–kinetic mode with decreased energy of At the same time, the close values of activation energy activation of process. The explanations of observed of process in a formation zone of lithium pentaferrite Er and activation energy of transport of mobile reagent Ed testify about practical concurrence of values of po- The overcoming of potential barriers is necessary to reagent for moving into a reactionary zone and for Hui Yang, Solid State Ionics 178, 1590 (2007). subsequent formation of chemical connection. It is [2] A.P. Surzhikov and A.M. Pritulov, Radiation- possible to assume, that Er and Ed have an identical thermal sintering of ferrite ceramic, Moscow, En- nature, and the observable distinctions are caused by various distortion of a crystal lattice in a reaction zone [3] P.N. Oleinikov, G.P. Muraviova, and N.N. Oleini- kov, Izv. AN SSSR. Inogr. Mater. 31, 1572 (1995). Concerning the mechanism radiation stimulation [4] A.A. Buruchin, N.N. Oleynikov, B.R. Churagulov, of synthesis the following is possible to note. From the table it is visible, that in conditions of influence of an

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