INFLUENCE OF THE PARAMETERS OF THERMAL DECOMPOSITION OF AMMONIUM TETRAVANADATE ON THE PROCESSES OF PHASE FORMATION OF HIGHLY DISPERSED VANADIUM OXIDES.
Introduction. Vanadium dioxide (V[O.sub.2]) as a material for optics and electronics has attracted the attention of researchers for many decades. The reason for this is the reversible phase transition of the semiconductor-metal (PTSM), which is accompanied by an extremely strong change in optical and electrical properties of the material. Unique characteristics of this connection allow it to be used in temperature sensors, optical switches, memory elements, energy-saving coatings for glass, optical data carriers and thermochromic indicators [1-3].
Despite the wide possibilities for the practical use of volumetric samples of vanadium oxides, many of them have not yet been implemented. The reason is the mechanical stress that occurs when PTSM, which leads to the destruction of crystals in the process of cyclic switching. In this connection, many modern experiments are aimed at developing methods for obtaining film materials [4-6] however, the use of films is limited because of the impossibility of their application in the field of significant electric currents.
Previous studies have shown  that the mechanical voltage in V[O.sub.2] is diminished with a decrease in the geometric sizes of crystals to 100 nm, which allows obtaining materials with stable characteristics in the process of thermocycling. For example, reducing the size of particles of vanadium oxide powder from micro to nanometers can not only improve the mechanical characteristics of ceramic materials on the basis of V[O.sub.2], but also substantially change the properties of the substance. In this case, the changes relate to the main characteristics of the solid: 1) the parameters of the grid; 2) electronic structure; 3) melting point and phase transformation; 4) diffusion rate and chemical reactions.
From the technological and economic point of view, one of the most promising ways of synthesizing of nanocrystalline vanadium oxides is the thermal decomposition of the precursor-highly dispersed salt of ammonium tetravanadate. The regulation of technological parameters at the stage of synthesis as well as at the stage of thermal decomposition of tetravanadate ammonium can control the physical and chemical properties of vanadium oxides .
Despite the unique properties of tetravanadate ammonium, the conditions for its thermal decomposition are not fully understood, which prevents the optimization of the technological process. Thus, the development of technology for the production of nanodispersed powders of vanadium oxides from tetravanadate ammonium is an urgent task.
The aim of the study. On the basis of experimental studies, to determine the influence of thermal decomposition parameters of tetravanadate ammonium on the processes of phase formation of anticrystalline products of vanadium oxides ([V.sub.2][O.sub.5], V[O.sub.2]). To achieve this goal, the following issues were solved:
--to determine the influence of the decomposition atmosphere on the phase composition of the product of vanadium oxides;
--to determine the optimum temperature of the thermal decomposition of tetravanadate ammonium to obtain a nanocrystalline V[O.sub.2] with the phase transition of the semiconductor-metal it is present.
Materials and methods. Salt--ammonium tetravanadate [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9], which is a precursor for the synthesis of vanadium oxides, was prepared by the interaction of [V.sub.2][O.sub.55] with an aqueous solution of oxalic acid during heating, followed by the precipitation of ammonium hydroxide to pH 126.96.36.199 by centrifugation, washing and drying the product in an atmosphere of argon . Drying in an inert atmosphere at 150 ... 180[degrees]C is required for salt storage, since the presence of water promotes the rapid oxidation of vanadium to a five valent state.
At the final stage of the technology, the dried salt of tetravanadate ammonium was thermally decomposed in air and in a neutral atmosphere of argon.
The decomposition of the precursor in the air flow was carried out in the electric furnace of the brand SNOL 8.2/1100 U4A to a temperature of 400 ... 500[degrees]C, and in the atmosphere of argon in a specially furnished furnace (Fig. 1) in the temperature interval of 380 ... 900[degrees]C.
The speed of heating in both cases was 5[degrees]C/min. with an endurance in time of 10 ... 15 min. To prevent the oxidation of vanadium dioxide, the cooling of the resulting powders was carried out in an inert atmosphere.
The total amount of vanadium ([V.sup.+4], [V.sup.+5]) in the synthesized powders was determined by titration of the Mora salt by the method .
Investigation of the phase composition of vanadium oxide products formed after thermal treatment of ammonium tetrabutanate salt in an inert atmosphere at different temperatures (380 ... 900[degrees]C) was carried out by X-ray diffraction analysis.
X-ray diffraction analysis was performed on a DRON-2.0 diffractometer using CuK a radiation (l=0.15418 nm.) In the mode: voltage 35 kV, current 10 mA.
The morphology of the powders was conducted by transducing electron microscopy using a JEM-100 cCXII microscope.
The study of the temperature of the phase transition in the final product V[O.sub.2] was carried out by the method of DTA by the method .
The solubility of heat-treated powders was determined by obtaining a saturated solution of vanadium oxide followed by filtration of the mixture and determining the total amount of vanadium in the filtrate by the method .
Results. The salt [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] synthesized by us represents a crystalline powder of dark color with a density of 2.1 g/[sm.sup.3] and a bulk density of 1.1 g/[sm.sup.3], the water solubility is 5 x [10.sup.-4] mol/l [8, 12].
The final stage of the technological process is the thermal treatment of tetravanodate ammonium to produce vanadium oxides with an available phase transition of a semiconductor-metal. Assume that thermal treatment at temperature intervals of 380 ... 900[degrees]C can affect the physical and chemical properties of the final products (phase composition, particle size, solubility, etc.). Therefore, an important step is to determine the optimal conditions for the thermal treatment of tetravanadate ammonium.
Determination of the influence of the oxidative atmosphere in the thermal decomposition of the precursor on the presence of vanadium ions ([V.sup.+4], [V.sup.+5]) is shown in Fig. 2.
Significant increase in the concentration of [V.sup.+5] in the powder of ammonium tetravanadate at temperature treatment above 200[degrees] C is due to the oxidation of [V.sup.+4] to [V.sup.+5] by reaction 1
[(N[H.sub.4]).sub.2][V.sub.4][O.sub.9]+[O.sub.2] [right arrow] 2N[H.sub.3] [up arrow] +[H.sub.2]O+2[V.sub.2][O.sub.5]. (1)
According to reaction 1, to obtain V[O.sub.2] decomposition of tetravanadate ammonium must be carried out in an argon stream (reaction 2):
[(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] [right arrow] 4V[O.sub.2]+2N[H.sub.3]+[H.sub.2]O. (2)
The diffraction patterns of the powders are obtained by the thermal decomposition of tetravanadate ammonium at temperatures of 380, 450, 650, 900[degrees]C shown in Fig. 3.
As can be seen from Fig. 3. thermodegradation of powders at temperature intervals 380 ... 450[degrees]C (curve 1, 2) forms a mixture of various oxides of vanadium [V.sub.2][O.sub.3], V[O.sub.2], [V.sub.5][O.sub.9], [V.sub.2][O.sub.5]. With an increase in temperature to 650[degrees]C (curve 3) there are low-intensity reflexes characteristic of the phase V[O.sub.2]. The temperature increase to 900[degrees]C is characterized by the appearance of a crystalline phase V[O.sub.2] with a monoclinic angle of 106.9[degrees]C.
The study of the temperature of the phase transition of the obtained samples V[O.sub.2] (with different thermal processing) is shown in the Fig. 4.
Thermal degradation of [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] at 900[degrees]C indicates the formation of vanadium dioxide with its corresponding electro-functional properties and an expressive endothermic peak at 68[degrees]C (Fig. 4).
The method of transmission electron microscopy determines the size of the vanadium oxide particles (Fig. 5, a) which does not exceed 80 ... 82 nm.
Fig. 5, b shows the dependence of the size of the particles of vanadium oxide products obtained from the heat treatment temperature (380 ... 900[degrees]C).
Fig. 6 shows the dependence of solubility in distilled water of V[O.sub.2] powders on the temperature of thermal degradation
The solubility of vanadium dioxide powder obtained at a temperature of 650[degrees]C is 2.3 x [10.sup.-4] mol/l and at 900[degrees]C the powder is not soluble in distilled water.
Having analyzed the ability of Vanadium oxide powders to store it (Fig. 7) and the above data, it can be concluded that the temperature treatment at 900[degrees]C is optimal for obtaining a chemically pure (98 %) nanocrystalline product with a characteristic phase transition of a semiconductor-metal.
Conclusion. Thus, on the basis of experimental studies, the influence of the parameters of thermal decomposition (temperature and decomposition atmosphere) of tetravanadate ammonium on the phase composition, particle size, solubility and oxidative ability during storage of highly dispersed vanadium oxides has been established.
Radiographs and results of transmitted electron microscopy confirmed and clarified the picture of phase transformations.
The high probability of oxidation of tetravanadate ammonium with its decomposition in the atmosphere of air with the formation of [V.sub.2][O.sub.5] at a temperature of 450 ... 500[degrees]C was revealed.
It was found that thermal treatment [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] should be carried out in an inert atmosphere of argon.
It is shown that at temperature processing 380 ... 650[degrees]C a mixture of various vanadium oxides with a water-solubility of 2.3 ... 4.6 x [10.sup.-4] mol/l is formed and which are capable of oxidizing during storage. The nanocrystalline V[O.sub.2] with the particle size 80 ... 82 nm and its tributary PTSM (at a temperature of 68[degrees]C) is formed as a result of thermal decomposition of 850 ... 900[degrees]C for 10 minutes. Next, it is planned to conduct research on the effects of the conditions for the synthesis of ammonium tetravanadatuna, the size of the particles of vanadium dioxide.
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[phrase omitted]; Luskan Katerina, ORCID: http//orcid.org/0000-0001-8435-185X
[phrase omitted]; Girenko Alena, ORCID: http//orcid.org/0000-000!-1655-816X
[phrase omitted]; Misov Oleg, ORCID: http//orcid.org/0000-0003-1114-1381
[phrase omitted]; Savchenko Maria, ORCID: https://orcid.org/0000-0003-0908-1604
[phrase omitted]; Klymenko Olexander, ORCID: http://orcid.org/ 0000-0003-4332-2790/
Received March 05, 2018
Accepted March 28, 2018
K. Luskan, A. Gyrenko, PhD, Assoc. Prof., O. Musov, PhD, Assoc. Prof., M. Savchenko, PhD, Assoc. Prof., O. Klimenko, PhD
Ukrainian State University of Chemical Technology, 8 Gagarin Ave., Dnipropetrovsk, Ukraine, 49005; e-mail: email@example.com
Caption: Fig. 1. Designed furnace for decomposition of tetravanadate ammonium: 1--quartz reactor, 2--oven, 3--porcelain tigel, 4--weight [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9], 5--weight Si[O.sub.2], 6--capacity for supply of argon, 7--capacity for gas exhaust in decomposition process, 8, 9--thermocouples
Caption: Fig. 2. Dependence of the content [V.sup.+4][V.sup.+5] in the synthesis product on the decomposition temperature
Caption: Fig. 3. The diffractograms of the heat treatment powder ammonium tetrabutanate for 10 minutes. with decomposition temperature: 1-380; 2-450; 3-650; 4-900[degrees]C
Caption: Fig. 4 DTA curves of vanadium dioxide obtained by thermal decomposition (NH4])2V4O9 for 10 minutes at temperatures (380 ... 900[degrees]C)
Caption: Fig. 5. Microphotography of V[O.sub.2] obtained by thermal degradation [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] at 900[degrees]C for 10 minutes with a particle size of 80 ... 82 nm (a); dependence of the size of vanadium oxide particles on the temperature of thermal decomposition [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] (b)
Caption: Fig.6. The dependence of solubility of V[O.sub.2] on the temperature of thermal degradation (380 ... 900[degrees]C) at 25[degrees]C
Caption: Fig. 7. The oxidative ability of vanadium dioxide powder obtained by thermal degradation [(N[H.sub.4]).sub.2][V.sub.4][O.sub.9] at 600 ... 900[degrees]C with an exposure time of 10 minutes when stored for two years
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|Author:||Luskan, K.; Gyrenko, A.; Musov, O.; Savchenko, M.; Klimenko, O.|
|Publication:||Odes'kyi Politechnichnyi Universytet. Pratsi|
|Date:||Mar 1, 2018|
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