RU2698689C1 - METHOD OF PRODUCING COMPOSITE LUTETIUM AND IRON OXIDE LuFe2O4±δ - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to the technology of producing complex oxides which have properties of multiferroic materials, exhibit a magnetoelectric effect, a magnetocaloric effect and can be used in multifunctional devices in information and energy-saving technologies. Method of producing composite lutetium oxide and iron LuFe₂O_4±δ includes preparation of a mixture of iron (III) and lutetium (III) oxides and annealing of the obtained mixture in a gaseous medium, wherein the initial oxides are mixed in a different from the stoichiometric ratio of Fe₂O₃:Lu₂O₃, making 1.00:0.39, homogenised by grinding for at least 60 minutes, and annealing of the prepared mixture is carried out at temperature of 1,090 °C in gas medium, reducing conditions of which are provided using a gas mixture consisting of argon and oxygen, while maintaining a given value of oxygen pressure in range Po₂=10^-11.24÷10^-12.04 atm.

EFFECT: invention enables to obtain a complex oxide of lutetium and iron LuFe₂O_4±δ with given value of oxygen index based on determination of stability interval relative to partial pressure of oxygen in gaseous medium during isothermal treatment.

1 cl, 4 dwg

Description

The invention relates to a technology for the production of complex oxides that possess the properties of multiferroic materials, exhibit a magnetoelectric effect, magnetocaloric effect. These properties are due to the presence of multivalent iron cations (Fe ^{+ 2} and Fe ⁺³ ) and enable the practical use of such materials in the field of multifunctional devices in information and energy-saving technologies. These substances have a wide range of homogeneity with respect to oxygen, which makes it possible to change their properties depending on the oxygen content in the structure; therefore, special attention must be paid to the quality of synthesis. Complex oxides of the type RFe ₂ CO ₄ (R is a rare-earth element) with a mixed valence of cations can be obtained only at low oxygen pressures.

A known method of producing complex metal oxides of the VB group of the periodic system of elements D.I. Mendeleev, in which the metal oxide of the VB group, for example V ₂ O ₅ , Nb ₂ O ₅ or Ta ₂ O ₅ , is mixed with alkali metal oxalate, for example Na ₂ C ₂ O ₄ . The mixture is placed in a platinum crucible and heated in a furnace in a vacuum of 10 ^-2 -10 ^-3 mm Hg to 625-650 ° C for 8-9 hours, then the resulting product is cooled together with the furnace. The result is the production of single-phase complex metal oxides of the VB group of various compositions, for example, Na ₀ , ₈₃ V ₂ O ₅ , Na _0,7 V ₂ O ₅ , Na ₂ Nb ₈ O ₂₁ , Na ₂ Ta ₄ O ₁₁ , with a metal content of VB group in various degrees of oxidation, for example, the ratio of V ⁴⁺ / V ⁵⁺ is 0.319-0.326, Nb ⁴⁺ / Nb ⁵⁺ is 0.01. (RF patent No. 2209769, IPC C01G 31/02, op. 08/10/2003).

The disadvantage of this method is the relatively high oxygen pressure during firing of the starting components and low synthesis temperatures at which it is impossible to obtain complex iron oxides and lanthanides, for example lutetium.

The most common way to obtain compounds with mixed valency of cations is the synthesis using a gas mixture, in which reduced pressure was achieved by mixing CO ₂ , CO and H ₂ in certain quantities.

A known method of producing complex oxides of RFe ₂ O ₄ (R = Tm, Yb, Lu) by the solid-phase method. The technology includes 2 stages, first stoichiometric amounts of oxides R ₂ O ₃ and Fe ₂ O ₃ are mixed, pressed and heated to a temperature of 1200 ° C in air for 12 hours. At the next stage, the samples are ground, pressed into tablets and calcined in the atmosphere CO / CO ₂ in a ratio of 2/3, also within 12 hours. The result is the production of single-phase complex oxides. (Blasco J., Lafuerza S., Garcia J., Subias G. Structural properties in RFe ₂ O ₄ compounds (R = Tm, Yb, and Lu) // Physical Review B, 2014, V.90, 094119).

The disadvantage of the method for producing complex lutetium oxide and iron oxide under the indicated conditions is the high synthesis temperature, multi-stage process and the duration of the process, when it is impossible to control the oxygen non-stoichiometry of the obtained complex oxide.

A known method of producing complex oxides of LuFe _2-x Mn _x O _{4 + δ} (x = 0, 0.05, 0.12) by chemical homogenization from solution. At the preliminary stage, a powder mixture of a given composition is prepared. To do this, anesthetized paper filters are impregnated with a mixture of nitrate solutions with the necessary ratio of cations, dried and burned, the remaining carbon is removed by annealing at 600 ° C for 2 hours in air. The resulting powder is pressed into tablets, which are reduced in the first stage in sealed quartz ampoules in the presence of an Fe / FeO getter at a temperature of 1000 ° C for 30 hours, then in the second stage they are oxidized using an FeO / Fe ₃ O ₄ getter at a temperature of 1000 ° C for 30 hours. The approach used makes it possible to obtain single-phase ceramic samples. (Gamzatov A.G., Aliev AM, Markelova M.N., Burunova N.A., Kaul A.R., Semisalova A.S., Perov N.S. Magnetic and magnetocaloric properties of multiferroics // Solid State Physics. 2016.V.58. Issue 6.p. 1107-1111).

The disadvantage of the method for producing lutetium ferrite (complex oxide) under the specified conditions is the multi-stage process and the duration of the process, as well as the effect of the intermediate quenching of the sample between synthesis steps on the formation of the required oxygen non-stoichiometry of the obtained complex oxide, low synthesis temperature, and the inability to control the partial pressure of oxygen in the system at thermal treatment of the sample due to the presence of getters and, as a consequence, the lack of control over the formation of the oxygen non-technical value Iometrics do not allow the use of this method to obtain complex lutetium ferrite.

The closest set of essential features is a method for producing a polycrystalline complex lutetium ferrite oxide LuFe ₂ O ₄ using the solid-phase method. For this, the initial components in the form of oxides Lu ₂ O ₃ , Fe ₂ O ₃ and metallic iron Fe in a ratio of 0.485: 0.815: 0.37 (respectively) are mixed and ground in an agate mortar, then pressed. After this, the sample is placed in a sealed ampoule of silicon dioxide, from which air is removed. Heat treatment is carried out at a temperature of 1180 ° C for 12 hours. According to x-ray data, the result of the method is to obtain a single-phase sample (Bourgeois J., Andre G., Petit S., Robert J., Poienar M., Rouquette J., Elkaim E., Hervieu M., Maignan A., Martin C, and Damay F. Evidence of magnetic phase separation in LuFe ₂ O ₄ // Physical Review B. 2012. V.86. 024413 (9)).

The disadvantage of the method for producing complex lutetium oxide and iron oxide under the indicated conditions is a rather high synthesis temperature and the inability to control and regulate the partial oxygen pressure to form the required oxygen non-stoichiometry.

The technical result of the proposed method is to improve the quality of the obtained material lutetium ferrite LuFe ₂ O _{4 ± δ} with a given value of the oxygen index based on the determination of the interval of its stability with respect to the partial pressure of oxygen in the gas medium during isothermal treatment.

The specified technical result is achieved by the fact that in the method for producing complex lutetium and iron oxide LuFe ₂ O _{4 ± δ} , comprising preparing a mixture of iron (III) and lutetium (III) oxides, calcining the resulting mixture in a gas medium, according to the invention, the starting oxides are mixed in non-stoichiometric ratio of Fe ₂ O ₃ : Lu ₂ O _{3 of} 1.00: 0.39; non-stoichiometric ratio of Fe ₂ O ₃ : Lu ₂ O _{3 of} 1.00: 0.39, homogenized by grinding not less than 60 minutes, and firing the prepared mixture is carried out at a temperature of 1090 ° C in a gas medium, which reducing conditions are provided by using a gas mixture consisting of argon and oxygen, while maintaining a predetermined oxygen pressure value in the range Ro = 10 ₂ ^-11,24 ÷ 10- ^12,04 atm.

Since the spatial structure of the LuFe ₂ O _{4 ± δ} compound contains disordered iron cations located in anionic polyhedra, it is necessary to conduct heat treatment in the synthesis of such compounds while maintaining the specified value of the oxygen partial pressure. This approach allows one to form a certain ratio of multivalent iron cations in the compound and, as a result, to regulate the value of oxygen non-stoichiometry. A significant parameter for the formation of the ratio of heterovalent cations is the temperature of the heat treatment. A significant decrease in the processing temperature while maintaining a certain value of the partial pressure of oxygen in the gas mixture leads to the formation of Fe ³⁺ ions in the material and the impossibility of obtaining ions

Fe ⁴⁺ . Therefore, the choice of synthesis temperature is due to the possibility of forming the necessary ratio of multivalent iron ions in the material.

The proposed method is as follows:

The dried starting iron (III) oxides of iron and lutetium (III), taken in the following proportion of 1.00: 0.39, were mixed and calcined at a temperature of 1090 ° C in a gas medium consisting of an inert gas of argon and oxygen, while maintaining the set value oxygen pressure in the mixture. When using the starting components in a stoichiometric ratio (Fe ₂ O ₃ : Lu ₂ O ₃ = 1.00: 0.50), a small amount of lutetium oxide is present in the final product of the synthesis. The composition of the charge used by us allows us to obtain a single-phase product LuFe ₂ O _{4 ± δ} .

The claimed method is tested in laboratory conditions.

At the first stage, the task was to obtain a sample homogeneous in composition in the entire volume of the product. Samples of the initial oxides of lutetium (III) and iron (III), taken in the indicated proportion, were thoroughly mixed and ground in an agate mortar to homogenize the composition of the mixture and achieve the required particle size. To determine the required particle size, the initial mixture of the components was divided into two batches, the first of which was triturated for 10 minutes, the second for 60 minutes. To measure the particle size, the dynamic light scattering method was used, for which the fluctuation of light scattering of particles in a state of Brownian motion is measured, which leads to a broadening of the spectrum of the scattered light wave. The half-width of the scattered spectrum G (damping constant) is proportional to the diffusion coefficient:

where D is the diffusion coefficient; q is the scattering vector (= 4πnsin (θ / 2) / λ, where n is the refractive index of the medium; λ is the wavelength of the incident light; θ is the light scattering angle). The diffusion coefficient in a monodisperse system is related to the hydrodynamic particle radius (R) by the Stokes-Einstein equation

where k _B is the Boltzmann constant; η is the viscosity of the solvent; T is the temperature. One of the methods for determining the value of Г is to calculate the autocorrelation function of the intensity of the scattered light g ⁽¹⁾ (τ). For a monodisperse medium g ⁽¹⁾ (τ):

where B is a constant depending on the parameters of the device, such as the size of the aperture; and G is the attenuation constant (half-width of the scattered spectrum). In the case of a Brownian motion of a mixture of particles (i.e., a polydisperse solution), the intensity fluctuations will depend on the diffusion coefficients of the particles, and the autocorrelation function will be the sum (integral) of exponential terms with different attenuation constants:

where A _i is the relative intensity of light scattered by particles with an attenuation constant Г _i ⋅ Г _i , is proportional to the diffusion coefficient of particles of a certain size and depends on the relative number of such particles.

During the measurement cycle, the intensity of the scattered light is recorded as a sequence of data on the number of light pulses for the reference period Δτ. Then the correlation function is calculated. Two methods were used to process the autocorrelation function: the cumulant method and the regularization method. When using the cumulant method to determine the coefficients K _{m, the} logarithm g ⁽¹⁾ (τ) is approximated by the polynomial:

The first-order coefficient (or slope ln (g ⁽¹⁾ (τ)) is the average attenuation constant <Г>, knowing which, using equations (1) and (2), we can calculate the average diffusion coefficient and particle diameter.

. Для монодисперсных образцов значение показателя полидисперсности, как правило, меньше 0.1. Для полидисперсных образцов его значение увеличивается. Определение вклада разных фракций в интенсивность рассеянного света и распределения частиц по размерам на основании установленной автокорреляционной функции проводилось методом NNLS (Non-NegativeLeastSquares) При использовании растирания в течение 10 мин. присутствуют достаточно крупные частицы исходных компонентов. Показатель полидисперсности образца после растирания в течение 60 мин. (фиг.1) увеличивается в 1,5 раза, что согласуется с появлением большего количества фракций, в т.ч. размеров частиц менее 200 нм. Это способствует лучшему компактированию образца. Из полученных после перетирания смесей обеих партий прессовали таблетки диаметром 10 мм на гидравлическом прессе при давлении 150 кПа/см², которые подвергали обжигу в газовой среде, состоящей из аргона и кислорода, с контролируемым парциальным давлением кислорода при 1090°С в течение 19 часов. Выявлено, что увеличение полидисперсности частиц во второй партии облегчает спекание, поскольку мелкие частицы заполняют имеющиеся пустоты между более крупными частицами, а развитая поверхность предоставляет возможность лучшего контакта газовой и твердой фаз и обеспечивает однородность состава по объему спеченного продукта.The second-order coefficient divided by the square <G> is an indicator of polydispersity,

. For monodisperse samples, the value of the polydispersity index is usually less than 0.1. For polydisperse samples, its value increases. The contribution of different fractions to the scattered light intensity and particle size distribution was determined based on the established autocorrelation function using the NNLS (Non-NegativeLeastSquares) method using grinding for 10 min. sufficiently large particles of the starting components are present. The polydispersity index of the sample after grinding for 60 minutes (figure 1) increases by 1.5 times, which is consistent with the appearance of a larger number of fractions, incl. particle sizes less than 200 nm. This contributes to better compaction of the sample. From mixtures of both batches obtained after grinding, tablets of diameter 10 mm were pressed on a hydraulic press at a pressure of 150 kPa / cm ² , which were fired in a gas medium consisting of argon and oxygen with a controlled partial pressure of oxygen at 1090 ° C for 19 hours. It was found that an increase in the polydispersity of particles in the second batch facilitates sintering, since small particles fill the voids between the larger particles, and the developed surface provides the opportunity for better contact between the gas and solid phases and ensures uniform composition over the volume of the sintered product.

The phase composition of the obtained samples was studied using the X-ray diffraction method on a Shimadzu XRD 7000C diffractometer. In FIG. Figure 2 shows the diffraction pattern of a sample of nominal composition LuFe ₂ O _{4 ± δ} obtained in an Ar + O ₂ gas mixture ( ^Ро ₂ = 10 ^-11.54 atm.) At a temperature of 1090 ° С, on which there are no reflections of extraneous phases, i.e. This oxygen pressure in the gas phase allows one to obtain LuFe ₂ O _{4 ± δ} oxide in a homogeneous state throughout the volume, with δ = -0.009.

Moreover, the stability limit of LuFe ₂ O _{4 ± δ} at a temperature of 1090 ° C with respect to the partial pressure of oxygen is limited by the values of oxygen pressures in the range 10 ^11.24 > Po ₂ > 10 ^-12.04 atm. X-ray diffraction pattern of a sample of nominal composition LuFe ₂ O _{4 ± δ} obtained in an Ar + O ₂ gas mixture while maintaining a stable pressure of Po ₂ = 10 ^-11.24 atm. (Fig. 3), shows that along with the reflexes of the LuFe ₂ O _{4 ± δ} main phase, reflexes of the oxidized phases (Fe ₃ O ₄ and LuFeO ₃ ) are present, i.e. the equilibrium was determined at the high oxygen boundary of the LuFe ₂ O _{4 ± δ} homogeneity region, with δ = + 0.024. In the sample synthesized while maintaining a stable pressure Po ₂ = 10 ^-12.04 atm. (Fig. 4) along with the reflexes of the main phase, reflections of the reduced phases (FeO and Lu ₂ O ₃ ) are fixed, i.e. equilibrium was established at the low oxygen boundary of the homogeneity region of the LuFe ₂ O _{4 ± δ} compound, with δ = -0.084. The obtained experimental data allow us to fix the boundary conditions for the existence of the LuFe ₂ O _{4 ± δ} compound under conditions of reduced oxygen pressure with fixing the temperature of the heat treatment.

Claims (1)

A method of producing a complex oxide of lutetium and iron LuFe ₂ O _{4 ± δ} , comprising preparing a mixture of iron (III) and lutetium (III) oxides, calcining the resulting mixture in a gas medium, characterized in that the starting oxides are mixed in a Fe ₂ different stoichiometric ratio O ₃ : Lu ₂ O ₃ , comprising 1.00: 0.39, is homogenized by grinding for at least 60 minutes, and the prepared mixture is fired at a temperature of 1090 ° C in a gas medium, the reducing conditions of which are ensured by using a gas mixture consisting of argon and oxygen supported SRI predetermined oxygen pressure value Po in the range of ₂ 10 = ^{-12.04 -11,24} ÷ 10 atm.

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