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ЭЛЕКТРОХИМИЧЕСКИЙ СИНТЕЗ ДИСПЕРСНОГО ФЕРРИТА БАРИЯ С ИСПОЛЬЗОВАНИЕМ АНОДНОГО РАСТВОРЕНИЯ МЕТАЛЛА
И.О. Григорьева, А.Ф. Дресвянников
Ирина Олеговна Григорьева*, Александр Федорович Дресвянников
Кафедра технологии электрохимических производств, Казанский национальный исследовательский технологический университет, ул. К. Маркса, 68, Казань, Российская Федерация, 420015 Е-mail: iren-grigor@mail.ru*, a.dresvyannikov@mail.ru
Исследован процесс анодного растворения железа (чистота не менее 99%) в водных растворах хлорида бария, нитрата бария и бинарных электролитах в гальваностатических условиях и методом снятия потенциодинамических поляризационных кривых. Показано влияние состава и концентрации раствора и величины приложенного постоянного тока на интенсивность анодного окисления железа. Выявлено, что скорость окисления металла в бинарных электролитах, содержащих хлорид и нитрат бария, сопоставима с интенсивностью анодного растворения в растворе на основе хлорида бария. Установлено, что анодная поляризационная кривая в растворах, содержащих BaCh и Ba(NO3)2, имеет сложную форму, типичную для пассивирующихся металлов; на ней, как и в растворе нитрата бария, имеет место четкий максимум тока анодного окисления, однако высота пика значительно выше (в 50-150 раз). Предложен способ синтеза дисперсного феррита бария, основанный на анодном окислении железа в водных растворах хлорида и нитрата бария с последующей термообработкой продукта электрохимического растворения. Фазовый и элементный состав и структурные характеристики полученных образцов прекурсора и феррита исследованы с применением рентгенофазового анализа. Показано влияние режима термической обработки на фазовый состав синтезированных образцов. Установлено, что электролиз с растворимым железным анодом при постоянном токе в растворе 0,05M BaCh + 0,5M Ba(NO3)2 и последующая термообработка продукта растворения (1200 °С) обеспечивает формирование дисперсной системы, фазовый состав которой представлен преимущественно Ba087Feu,08O17,15 (74%) и BaFe2O4 (17%).
Ключевые слова: анодное растворение, электролиз, BaCh, Ba(NO3)2, железный анод, дисперсная система, феррит бария
ELECTROCHEMICAL SYNTHESIS OF DISPERSED BARIUM FERRITE USING ANODIC DISSOLUTION OF METAL
I.O. Grigoryeva, A.F. Dresvyannikov
Irina O. Grigoryeva*, Alexander F. Dresvyannikov
Department of Technology of Electrochemical Industries, Kazan National Research Technological University,
Karl Marx st., 68, Kazan, 420015, Russia
E-mail: iren-grigor@mail.ru *, a.dresvyannikov@mail.ru
The process of anodic dissolution of iron (purity not less than 99%) in aqueous solutions of barium chloride, barium nitrate and binary electrolytes under galvanostatic conditions and by registration of potentiodynamic polarization curves was investigated. The influence of solution composition and concentration and the value of applied direct current on the intensity of anodic oxidation of iron was shown. It was revealed that the oxidation rate of metal in binary electrolytes containing barium chloride and barium nitrate is comparable with the intensity of anodic dissolution in a solution based on barium chloride. It was found that anodic polarization curve in solutions containing BaCh and Ba(NO3)2 has a complex form typical of passivated metals. There is a clear maximum of anodic oxidation current on this curve, as well as in barium nitrate solution, however the peak height is much higher (50-150 times). A method for the synthesis of dispersed barium
ferrite, based on the anodic oxidation of iron in aqueous barium chloride and barium nitrate solutions with subsequent thermal treatment of the product of electrochemical dissolution was suggested. The phase and elemental composition and structural characteristics of obtained precursor and ferrite samples were examined using X-ray phase analysis. The influence of the heat treatment mode on the phase composition of the synthesized samples is shown. It is found that electrolysis with a soluble iron electrode using direct anode current in 0.05M BaCh + 0.5M Ba(NO3h solution and subsequent thermal treatment of the dissolution product at 1200 ° C provide the formation of dispersed system, whose phase composition is predominantly Ba0.87Fe11.08O17.15 (74%) and BaFe2O4 (17%).
Key words: anodic dissolution, electrolysis, BaCh, Ba(NO3)2, iron anode, dispersed system, barium ferrite
Для цитирования:
Григорьева И.О., Дресвянников А.Ф. Электрохимический синтез дисперсного феррита бария с использованием анодного растворения металла. Изв. вузов. Химия и хим. технология. 2021. Т. 64. Вып. 4. С. 59-66 For citation:
Grigoryeva I.O., Dresvyannikov A.F. Electrochemical synthesis of dispersed barium ferrite using anodic dissolution of metal. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. [ChemChemTech]. 2021. V. 64. N 4. P. 59-66
INTRODUCTION
Ferrite-based magnetic materials have a wide range of industrial and technological applications, such as radio and computer engineering, telecommunication, various electronic and electrical equipment devices. Most of the ferrites are ferrimagnets and combine high magnetic susceptibility with semiconductor or dielectric properties, which is why they are broadly used as magnetic materials. Ferrites with a hexagonal crystal structure (M-type) occupy a special place among them, in particular, barium and strontium hex-aferrites, which dominate the world market of permanent magnets; they account for more than 90% of the global production of ceramic magnets [1].
At present, M-type hexagonal barium ferrites with the structure of magnetoplumbite ВаFеl2Оl9 are important magnetic materials [2] and have versatile properties: high values of coercive force, saturation magnetization, magneto-crystalline anisotropy, electrical resistivity, excellent chemical stability, low eddy current loss, relatively low cost, corrosion resistance [3-5].
Such a variety of functional characteristics makes it possible to use barium hexaferrites to obtain new-generation permanent magnets [2], as information carriers with a high recording density [6-7], as absorbing material in various microwave devices and electromagnetic shielding systems [8-9], in medicine, in particular for hyperthermia of malignant cells, treatment of liver cancer, delivery and retention of medicines in the body [7, 10-11].
There are various methods for the synthesis of dispersed powders of hexagonal barium ferrites, in most cases mainly chemical methods are used. The most common and popular methods are chemical co-precipitation [12-13], hydrothermal synthesis [14-15], sol-gel technique [16-17]. Nanosized BaFe^O^ has been obtained in the form of ultrafine powders [18],
nanocrystals [16], hollow microspheres [19], nano-plates [20], solid [21] and hollow fibers [22]. Traditional methods for producing ferrites are multi-stage processes, characterized by the laboriousness, complexity of the technology and high duration of individual process steps, require high temperatures and high consumption of reagents.
In recent years, a number of electrochemical methods have been proposed [23-26] to be used for obtaining highly dispersed iron-containing oxide systems involving AC and DC electrolysis. For instance, in a study [24] nanodispersed powders of iron oxides were obtained by applying an alternating sinusoidal current to iron electrodes in 17 M NaOH solution. In this case, the maximum powder production rate has been achieved at a current density of 2.5 A/cm2, a frequency of 20 Hz and an electrolyte temperature of 70 °C Another study [25] reported on the synthesis of ma-ghemite nanoparticles by electrochemical oxidation of iron electrodes in an aqueous solution of dimethylfor-mamide (DMF) with the addition of cationic surfactants. The electrochemical method for the synthesis of metal oxide powders is the most environmentally friendly and clean. Moreover, such approach allows one to obtain highly dispersed oxide systems and efficiently control the synthesis process and product characteristics (particle sizes, phase and chemical composition) by varying the electrolysis parameters.
The present work focused on the results of a study of the anodic oxidation of Fe in barium chloride and barium nitrate solutions. The objective of this study is to suggest an electrochemical method for producing dispersed ferrites using electrolysis with a soluble anode, as well as to investigate the influence of electrolysis conditions (solution composition and concentration, current density) on the intensity of anodic dissolution of iron and characteristics of the obtained dispersed products.
EXPERIMENTAL PART
Polarization measurements were carried out in a conventional three-electrode electrochemical cell using a platinum auxiliary electrode, commercial Ag/AgCl reference electrode (E = 0.222 V) and working iron electrode under standard conditions (101.3 kPa, 298 K) with natural aeration in the potentiodynamic regime (potential scanning speed of 1 mV/s) by means of
a computer-controlled potentiostat-galvanostat model P-301M (Elins). The test iron electrode was of 99.1% purity (elemental composition is given in Table 1) with exposed surface area of 0.36 cm2. The surface of the test electrode was mechanically polished using diamond paste, degreased with ethanol, and rinsed with double-distilled water before to the experiment.
Table 1
Elemental composition of iron samples (%)
Fe C Si Mn P S Cr Mo Ni Al Co Cu Ti V W
99.1 0.12 0.07 0.45 0.01 0.01 0.01 0.005 0.03 0.005 0.02 0.06 0.002 0.005 0.02
Synthesis of a highly dispersed precipitate was performed via electrolysis in a rectangular cell (80 mm*50 mm*50 mm) of 200 ml using direct current (DC) power supply (MATRIX, model MPS-7101). The plates made of technical iron (99.1% purity) and 12Cr18Ni10Ti stainless steel served as soluble anode and cathode, respectively. The working surface area of the cathode and anode was the same (5 cm2), the electrodes were located near the opposite walls of the cell, the distance between them was 60 mm. The experiments were carried out at room temperature and natural aeration in aqueous solutions of 0.5 M BaCh and Ba(NO3)2 and binary solutions with different ratio of these components - BaCh: Ba(NO3)2 = 1: 1; 1: 2; 1: 3; 1: 4; 1: 5; 1:10. The electrolysis was performed under the following conditions: current density 100200 mA/cm2, voltage 8-10 V, electrolysis time 60 min.
For crystallization, the precipitate obtained during electrolysis process was kept in a matrix solution for 48 h; then, it was filtered and dried at a temperature of 80 °C for 2 h. Further, the dried powder was subjected to thermal treatment at a temperature of 1200 °C for 2 h.
X-ray diffraction analysis of the electrolysis and thermal treatment products was carried out by standard technique with a Rigaku SmartLab multifunctional diffractometer using monochromated Cu Ka radiation (A = 1,5418 A) operating at 20-60 kV voltage and 2-60 mA current. Phase analysis of the samples was conducted in the step scanning mode (the 29 scanning step was 0.02°, exposure time at the point was 1 s, the 29 shooting range varied from 3° to 65°). X-ray diffraction patterns were analyzed using the JCPDS database [27].
A profile analysis of the XRD patterns (phase composition, structural and microstructural parameters of the detected phases) was carried out with PDXL-2 software package. The diffraction peaks were described by the pseudo-Voight function, the background
- by B-splines. Due to the fact, that some of the detected phases did not have free access to files containing crystallographic information, the semi-quantitative phase analysis was carried out by fitting the theoretical values of the reflection intensities to the experimental ones and subsequent calculation using corundum coefficients (RIR) in PDXL-2 software package. The refinement of the crystal lattice and microstructure parameters (the size of coherent scattering regions -CSR, the magnitude of microstresses) was performed after a full profile description of the experimental XRD patterns. The calculation of the microstructural parameters was carried out in two ways: the Halder-Wagner method [28] and the method based on the Cagliotti formula [29]. In both cases, the instrumental broadening of diffraction peaks was taken into account, which was determined by shooting a standard sample of silicon powder free of microstresses and dimensional broadening. The lattice parameters and microstructural parameters were refined for phases that had sufficiently intense diffraction maxima.
X-ray fluorescence analysis of the iron anode and obtained precipitate (preliminary investigation) was carried out using a portable X-ray fluorescence spectrometer S1 TITAN (Bruker) with maximum voltage of 50 kV via data packet GeoChem General.
RESULTS AND DISCUSSION
In order to gain an insight into the intensity of anodic dissolution of iron under the DC effect with a maximum yield of its ionization products, we carried out preliminary studies in aqueous solutions of barium chloride and barium nitrate and determined the values of the oxidation rate for iron electrode by the gravimetric method. For the oxidation rate of the metal its weight loss per unit of time (1 h), related to the unit of its working surface, has been taken. According to the obtained data (Table 2), a quite intense dissolution oc-
curs on the surface of iron anode during DC electrolysis in the barium chloride solutions (experiments 1-4, Table 2), which begins along the edges of the plate; the surface of the electrode is covered with a dark brown film, easily washed off with water. The oxidation rate of iron varies slightly with increasing current density from 100 to 200 mA/cm2 (experiments 1-3, Table 2). On the contrary, an increase in the BaCl2 concentration promotes a more intense dissolution of the metal (experiments 1 and 4, Table 2).
During DC electrolysis in the BaCl2 solution, a suspension of brown, swamp or gray-green color is formed in the electrolyte volume, and dense brown film is formed on its surface. After exposure to the mother liquor (48 hours), filtering and drying for 2 h at 80 °C the powder of brown color is formed. A preliminary X-ray fluorescence analysis revealed the presence of a significant amount of barium, chlorine and a low iron content in the obtained samples (experiments 1-3, Table 2).
During electrolysis in barium nitrate solutions (experiments 5-6, Table 2), the metal oxidation with the subsequent formation of the precipitate in the bulk of the solution does not occur with the weight loss of the iron electrode tending to zero. Since active ioniza-tion of iron does not occur, we can assume that in this case there is passivation of the anode surface due to the formation of an oxide film.
Table 2
Oxidation rate of iron in the DC mode Таблица 2. Скорость окисления железа при постоянном токе
№ п/п Solution composition Current density, mA/cm2 Üxidation rate, (mg/cm2h)
1 0.5 M BaCl2 100 164
2 0.5 M BaCl2 150 178
3 0.5 M BaCl2 200 189
4 1.0 M BaCl2 100 225
5 0.5 M Ba(NÜ3)2 100 0
6 0.5 M Ba(NÜ3)2 150 0
7 0.5 M BaCl2 + 0.5 M Ba(NÜ3)2 150 158
8 0.2 M BaCl2 + 0.6 M Ba(NÜ3)2 150 145
9 0.1 M BaCl2 + 0.4 M Ba(NÜ3)2 150 142
10 0.05 M BaCl2 + 0.25 M Ba(NÜ3)2 150 162
11 0.05 M BaCl2 + 0.5 M Ba(NÜ3)2 150 166
Fig. 1 presents the anodic potentiodynamic curves (potential-current, E = f(j) curves) of iron in the
studied single-component solutions. One can see relatively low values of the corrosion potential in these media: from (-90) - (-115) mV in BaCh to (-110) - (-130) mV in Ba(NO3)2. The presence of chloride ions, which are classical activators of the anode process, as well as an increase in their concentration leads to an increase in current density and the rate of dissolution of iron (Fig. 1a).
A different picture is observed in the barium nitrate solutions (Fig. 1b). The E = f(j) curves have a typical form that is characteristic of passivating metals - an active zone of anodic dissolution (or active-passive transitions) with the presence of current maximum (peak), passivity region and region of oxygen evolution are clearly visible on these dependences. The anodic curve exhibits current peak near 0-30 mV (current values do not exceed 10 mA/cm2), which should be associated to formation of a passive layer on iron. The current peak reaches its highest values in 0.1 M Ba(NO3)2 solution; at other concentrations, its value decreases by about half. The extent of the passivity region is 475-600 mV (Fig. 1b).
-150 -50 50 150
Е, mV
a
b
Fig. 1. Anodic potentiodynamic (1 mV/s) curves of iron in (a) BaCl2 and (б) Ba(NÜ3)2. Solution concentration, mol/l: 1 - 0.05; 2 - 0.1; 3 - 0.5
Рис. 1. Анодные потенциодинамические (1 мВ/с) кривые железа в BaCl2 (а) и Ba(NO3)2 (б). Концентрация раствора, моль/л: 1 - 0,05; 2 - 0,1; 3 - 0,5
In order to determine the optimal electrolysis conditions for obtaining the product, two-component solutions, BaCl2 + Ba(NO3)2, were also investigated. In these media, the anodic polarization curve also has a complex shape typical of passivated metals (Fig. 2a and curve 3 in Fig. 2b). It can be seen a clear current maximum of the anodic oxidation, as well as in the Ba(NO3)2 solution (Fig. 1b), however, due to activation of the electrode surface by chloride ions, the peak height increases significantly and corresponding current density values are 50-150 times higher than in Ba(NO3)2. In addition, the oxidation peak becomes much more extended in width (on average in the range from -200 to 1000 mV) and shifted to higher potential. Next, for higher potential values (above 900-1000 mV) a relatively short passivity region (length 350-400 mV) is observed (Fig. 2a).
Further, in order to obtain a product of the required composition and with a maximum yield, electrochemical synthesis of dispersed samples was carried out in binary electrolytes with different contents of barium chloride and barium nitrate, ratio of components is BaCh : Ba(NO3)2 = 1:1; 1:3; 1:4; 1:5; 1:10. According to the data (experiments 7-11, Table 2), a change in the ratio of components weakly affects the intensity of the anodic dissolution of iron, the values of the oxidation rate in all cases are comparable. In these media, unlike the BaCl2 solutions, the anodic dissolution of metal, as a rule, starts from the 5-10th min of the process.
The product of DC electrolysis in the 0.05M BaCl2 + 0.5M Ba(NO3)2 solution is initially a suspension of marsh color. After its holding in the matrix solution for 48 h, a two-layer precipitate is formed with clear layer boundaries: brownish-green lower layer with a height of 10-25 mm (with a cell width of 50 mm), and the upper layer (5-10 mm high) is brown or red-brown in color. This indicates the spatial separation of the oxidation products of metallic iron, which are present in the form of compounds with a predominance of iron
(II) in the lower layer of the precipitate, and, conversely, iron (III) - in the upper one. This indicates the spatial separation of the products of oxidation of metallic iron, which are present in the form of compounds with a predominance of iron (II) in the lower layer of the precipitate, and, conversely, iron
(III) in the upper one. After precipitate filtering, drying and mechanical grinding at room temperature, a light-brown product is formed. After precipitate drying at 80 °C for 2 h, its color becomes more saturated and changes to dark brown. After thermal treatment of the sample at 1200 °C for 2 h, the color of the product becomes darker, almost black.
600 -|
Ъ 500 -о
400 300 200 100 0
-700 -200 300 800 1300 1800 Е, mV
a
b
Fig.2. Anodic potentiodynamic (1 mV/s) curves of iron in (a) two-component solution and (б) in semilogarithmic coordinates. 1 - 0.05М BaCk; 2 - 0.5М Ba(NO3>; 3 - 0.05М BaCk + 0.5M Ba(NO3)2
Рис. 2. Анодные потенциодинамические (1 мВ/с) кривые железа в двухкомпонентном растворе (а) и в полулогарифмических координатах (б): 1 - 0,05М BaCk; 2 - 0,5М Ba(NO3)2;
3 - 0,05М BaCl2 + 0,5M Ba(NO3>
The XRD patterns of the samples obtained by means of anodic oxidation of iron in the 0.05М BaCh + 0.5M Ba(NO3)2 solution and subsequent thermal treatment are shown in Fig. 3, and the results of XRD study as a function of heating temperature are given in Table 3. According to these data, by the DC electrolysis and subsequent drying of the precipitate at 80 °C, a dispersed material is formed, whose phase composition is a mixture of complex iron hydroxide Fe0.96O0.ss(OH)u2 of goethite structure (54 wt%), witerite BaCO3 (21 wt%), iron nitride Fe4N (15 wt%), and also iron oxides -magnetite (5 wt%) and hematite (5 wt%).
The high-temperature treatment at 1200 °C of the precipitate obtained by electrolysis leads to structural ordering and the formation of barium hexaferrite Ba0.87Fe11.0sO17.15 (74 wt%) and barium monoferrite BaFe2O4 (17 wt%) (Fig. 3b, Table 3). In this case, the phase of iron oxide with hematite structure (5 wt%) remains unchanged in the powder composition, a-Fe phase (5 wt%) forms, the content of iron nitride significantly decreases (from 15 wt% to 1 wt%).
И.О. Григорьева, А.Ф. Дресвянников Intensity (cps)
.10 40
2-iheia (deg)
a
30 40 2-thcta (deg)
b
Fig. 3. XRD patterns of the samples obtained by DC electrolysis in 0.05М BaCk + 0.5M Ba(NO3> solution and thermally treated at (a) 80 °C: 1 - Feo.96Oo.88(OH)i.i2, 2 - BaCO3, 3 - Fe4N, 4 - Fe3O4, 5 - a-Fe2O3, and (b) 1200 °C: 1 - Ba0.s7Fe11.0sO17.15, 2 - BaFe2O4, 3 - a-Fe2O3,
4 - a-Fe, 5 - Fe3N
Рис. 3. Рентгено-дифрактограммы продукта электролиза в 0,05М BaCl2 + 0,5M Ba(NO3)2 и термообработанного при 80 °С: 1 - Fe0.96O0.ss(OH)1.12, 2 - BaCO3, 3 - Fe4N, 4 - Fe3O4, 5 - a-Fe2O3, (а) и 1200 °С (b) : 1 - Ba0.s7Fe11.0sO17.15, 2 - BaFe2O4, 3 - a-Fe2O3,
4 - a-Fe, 5 - Fe3N
Table 3
Phase composition of electrolysis products (0.05М BaCl2 + 0.5M Ba(NO3)2, j=150 mA/cm2, U=9 V, т=60 min)
Heating temperature, °C Phase composition, wt.%
Ba0.87Fe11.08O17.15 BaFe2O4 Fe0.96O0.88(OH)1.12 BaCOs Fe4N FesO4 a-Fe2O3 a-Fe
80 - - 54.0 21.0 15.0 5.0 5.0 -
1200 74.0 17.0 - - 1.0 - 5.0 3.0
Table 4
Results of full-profile analysis of Ba0.87Fe11.08O17.15 phase Таблица 4. Результаты полнопрофильного анализа _фазы Ba0,87Fen,08Or7,15 _
Size of OCD, nm (Cagliotti formula) 5990 ± 272
Magnitude of microstresses, % (Cagliotti formula) 0.14 ± 0.08
Size of OCD, nm (Halder-Wagner method) 3400 ± 23
Magnitude of microstresses, % (Halder-Wagner method) 0
Refined crystal lattice parameters a = 5.892 Â c = 23.203 Â
Thus, DC electrolysis and subsequent high-temperature treatment (1200 °C) make it possible to obtain a dispersed product in which barium hexaferrite phase Ba0.87Fe11.08O17.15 (74 wt%) dominates. The crystal lattice and microstructure parameters of barium hexaferrite phase are given in Table 4. To estimate the size of OCD, it is proposed to use the values obtained by the Halder-Wagner method, since they are less sensitive to diffraction fitting errors.
CONCLUSIONS
The anodic behavior of iron electrode in aqueous solutions of barium chloride, barium nitrate and bi
ЛИТЕРАТУРА
1. Канева И.И., Костишин В.Г., Андреев В.Г., Читанов Д.Н., Николаев А.Н., Кислякова Е.И. Получение гекса-феррита бария с повышенными изотропными свойствами. Изв. вузов. Матер. электрон. техники. 2014. Т. 17. № 3. С.183-188. DOI: 10.17073/1609-3577-2014-3-183-188.
nary electrolyte of 0.05M BaCh + 0.5M Ba(NO3)2 was studied. The influence of electrolysis conditions (solution composition and concentration, DC value) on the intensity of anodic dissolution of technical iron (99.1%) was investigated. It was shown that the rate of anodic dissolution of iron in two-component solutions of BaCh + Ba(NO3)2 is of the same order as in the case of dissolution in an BaCh-based electrolyte of an adequate concentration.
The possibility of synthesizing barium ferrites by anodic dissolution of technical iron (99.1%) in binary electrolyte based on barium chloride and barium nitrate and subsequent thermal treatment of the electrolysis products precipitate was shown. A dispersed system containing predominantly ferrite phases -Ba0.87Fe11.08O17.15 and BaFe2O4, as well as a-Fe2O3, a-Fe, and Fe3N in relatively small amounts (in total, about 11 %wt.), has been formed during DC electrolysis (ja = 150 mA/cm2, t = 60 min) in 0.05 M BaCh +
0.5 M Ba(NO3)2 solution and after the high temperature treatment (1200 °C).
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Поступила в редакцию 21.09.2020 Принята к опубликованию 11.01.2021
Received 21.09.2020 Accepted 11.01.2021
