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26PREPARATION OF CHROMIUM POWDER AND ITS COMPOSITE WITH TUNGSTEN FROM COMPLEX OXIDES BY MAGNESIUM VAPOURS REDUCTION
V. N. Kolosov*", M. N. Miroshnichenko", and T. Yu. Prokhorova"
aTananaev Institute of Chemistry - Subdivision of the Federal Research Centre Kola Science Centre of the Russian Academy of Sciences, Apatity, 184209 Russia *e-mail: tantal@chemy.kolasc.net.ru
DOI: 10.24411/9999-0014A-2019-10066
Chromium powder is widely used for the production of high-temperature cermets, which are composites having high electrical conductivity, thermal stability, and corrosion resistance [1]. Along with chromium, tungsten is also often regarded as a high-temperature structural material. However, its use is largely limited by susceptibility to oxidation at moderate temperatures. One of the effective ways to suppress the oxidation of tungsten at high temperatures is doping with chromium [2]. The use of chromium as an anticorrosive element is due to the fact that W-Cr alloy is an isomorphic system, and the Gibbs free energy of Cr2O3 formation is more negative than the energy of WO3 formation [3]. Therefore, chromium is preferentially oxidized as compared to tungsten in the W-Cr alloy. If chromium activity in the alloy is high enough, then a stoichiometric and continuous protective oxide film Cr2O3 is formed. However, the consolidation of the alloy W-Cr is very difficult, because in this system there is a low interdiffusion of the components, even at high temperatures [4]. When sintering powders W and Cr, an activator element is usually added to their mixture. It allows to improve the sintering but has a negative impact on the performance properties of the material. At the same time, powders with high surface area provide a large driving force for sintering due to availability of abundant surface energy and results in dense compacts with significantly improved properties.
Production of chromium powder usually involves two main stages: metallothermic or electrolytic (extraction) reduction and mechanical milling [5, 6]. The milling process is energy-intensive and it also involves the risk of contamination of the powder by constituents from air and the mill tools material [7]. In addition, the crushed powder usually has a wide range of particle sizes. To obtain tungsten powders the most widely used method is hydrogen reduction. Thus, ammonium paratungstate or oxide WO3 typically use as precursors. Reduction of precursors to metal includes several stages, which are performed at different temperatures. At the same time, the thermodynamic conditions of metal formation are very unfavorable, which necessitates the use of almost tenfold excess hydrogen during the reduction.
Previously, it was found that the reduction of double tungsten and molybdenum oxides containing MgO or CaO in their composition by magnesium vapor allows obtaining powders of these metals with a specific surface area of up to 20 m2-g-1 [8, 9]. In this paper, the possibility of obtaining chromium powder and its composite with tungsten by reducing double Cr and W oxides with magnesium vapor is investigated.
As precursor for the preparation of chromium powder, chromite MgCr2O4 was used. For comparison, Cr2O3 oxide was reduced under similar conditions. To obtain a chromium-tungsten composite, a mixture of double CaCrO4-CaWO4 oxides was used. The process occured under an atmosphere of magnesium vapor and argon. A container with magnesium was mounted on the bottom of a reaction vessel. Weighed amounts (5 g) of precursor were loaded into metallic crucibles, which were placed over the magnesium-containing container. A shield was placed over the crucibles. The separation between the surface of the precursor and the shield was 27-30 mm. The assembly was mounted in stainless
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steel retort, which was hermetically sealed, pumped down, filled with an inert gas, and heated to a required temperature. The reaction vessel-was kept closed to avoid magnesium losses. The reduction process was in the temperature range 700-800°C under a residual argon pressure in the range of 5-15 kPa. The apparatus, methods of synthesis of double oxides and reduction are basically similar to those used earlier [8, 10]. The phase composition of the powders was determined by X-ray diffraction analysis using a Shimadzu XRD-6000 X-ray diffractometer (Cula radiation). The specific surface area and porosity of the powders were determined by BET and BJH measurements using a TriStar II 3020 VI.03 analyzer. The study of the morphology of the powders was carried out using a SEM LEO 420 scanning electron microscope. The reduction of chromium compounds takes place according to the reactions (1), (2) and in the selected temperature range is accompanied by a significant decrease in Gibbs energy (Table 1).
Table 1. Thermodynamic characteristics of reactions of chromium oxide compounds reduction by magnesium vapor._
№ Reaction -AHr, kJmol-1 AS r., J-(mol-K)-1 -AG973, kJmol-1 -AG1073, kJmol-1
1 &2O3 + 3Mg = 2Cr + 3MgO 1049 -445 615 571
2 MgCr2O4 + 3Mg = 2Cr + 4MgO 1004 -443 573 528
Crucibles with reaction products and corresponding shields after reduction of Cr2O3 and MgCr2O4 compounds are shown in Fig. 1. It can be seen that after the reduction of &2O3 deposits of a white substance were found on the surfaces of the reaction product and the shield, while after the reduction of MgCr2O4 under the same conditions, these surfaces are black.
Fig. 1. Crucibles with reaction products (a, c) and corresponding shields (b, d) after reduction; precursors: &2O3 (a), MgCr2O4 (c); temperature in the reactor of 750°C, pressure of 10 kPa.
According to XRD data, the crust of the white substance on the surface of the reaction product and on shield after &2O3 reduction is pure magnesium oxide. Under the crust there is chromium powder. After MgCr2O4 reduction under the same conditions, the reaction product is chromium powder with MgO content corresponding to its fraction according to the reaction 2 (Table 1). When magnesium vapor reduction of &2O3 specific surface area of chromium powders is 4-5 m2g-1, while the reduction of MgCr2O4 surface area reaches 15-33 m2g-1. The specific surface area of chromium powders obtained by MgCr2O4 reduction corresponds to the calculated average particle size of 25-50 nm. However, as can be estimated from the SEM image of the powder with a specific surface area of 33 m2g-1 (Fig. 2), the powder is represented by coarser particles with an average size of at least 200 nm.
According to the previously proposed model of powder particle formation during the reduction of oxide compounds of refractory metals by magnesium vapor, the high specific surface area of the powder is a consequence of its nanoporous structure [11]. For the obtained chromium powders, this is confirmed by the results of porosity measurement. The total surface area of the powder is almost equal to the total surface area of the pores (Fig. 3).
35
*
.....
1 0 2 0 3 0 40
Pore Diameter (nm)
Fig. 2. SEM image of the chromium powder with specific surface area of 33 m2-g_1.
Fig. 3. Cumulative distribution curves of the pore area of chromium powders, specific surface area of powder: 1 13; 2 33 m2-g-1.
View of the precursor, crucible with the reaction product and the shield after reduction of the mixture CaCr04-CaW04 shown in Fig. 4. It can be seen that the surface of the reaction product is mainly dark with small white inclusions. At the same time, there is white sediment in the central part of the shield, on the side facing the precursor. According to XRD data, it is a pure magnesium oxide (Fig. 5a). The reaction product consisted of tungsten, chromium and calcium and magnesium oxides (Fig. 5b). The washed powder contains only a mechanical mixture of Cr and W (Fig. 5c). The formation of the alloy during the reduction of compounds is difficult for the following reasons. First, mutual diffusion and atomic mobility in the Cr-W system are low even at higher temperatures [4]. In addition, the layers of MgO and CaO formed as a result of the reduction of oxide compounds serve as a diffusion barrier between metals.
Fig. 4. View of the precursor (a), crucible with reaction product (b) and shield (c) after reduction CaCr04-CaW04; temperature in the reactor of 750°C, pressure of 10 kPa.
(a) (b) (c)
Fig. 5. XRD patterns of the white matter from the screen (a), the reaction product (b) and the washed powder (c) after reduction of the mixture CaCr04-CaW04. T = 750°C, P =10 kPa.
The specific surface area of the Cr-W powder mixture, depending on the reduction conditions, was 24-34 m2-g-1. The type of adsorption-desorption isotherms of these powders corresponds to type IV according to IUPAC classification, which is typical for mesoporous powders (Fig. 6).
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Thus, the study confirms the previously established possibility for tungsten and molybdenum of increasing the specific surface area of powders obtained by the magnesium vapor reduction using compounds having refractory oxides in their composition as a precursor. By the reduction of such compound powders of chromium and Cr-W composite with tungsten with a specific surface area of 15-34 m2g-1 were produced, which one is 3-6 times higher than that for powders obtained by reduction in similar oxides &2O3 and WO3.
- Adsorption
- Desorption
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J
ji
/ /
. [ g -" -js^f" w
0.2 0.4 0.6 0.8 Relative Pressure (p/p1)
—t- - Adsorption -e- - Desorption
-e 40
■ 30
i 20
o 10
—f
Â
Ss
1.0
0.2 0.4 0.6 O.i Relative Pressure (p/p°)
1.0
b
Fig. 6. Adsorption-desorption isotherms of nitrogen for chromium powders obtained from CaCr04-CaW04 at 800°C (a), 720°C (b); specific surface area of powders, m2-g-1: (a) 24; (b) 34.
a
The research was supported by the Russian Foundation for Basic Research, (project no. 18-0300248).
1. W. Schatt, K.-P. Wieters, Powder Metallurgy—Processing and Materials, European Powder Metallurgy Association, Shrewsbury, 1997.
2. S. Telu, A. Patra, M. Sankaranarayana, R.Mitra, S.K. Pabi, Microstructure and cyclic oxidation behavior of W-Cr alloys prepared by sintering of mechanically alloyed nanocrystalline powders, Int. J. Refract. Met. Hard Mat., 2013, vol. 36, no. 1, pp. 191-203.
3. Barin, G. Platzki, Thermochemical Data of Pure Substances, New York: VCH, Weinheim, 1995.
4. F.J.A. Den Broeder, Interface reaction and a special form of grain boundary diffusion in the Cr-W system, Acta Metall, 1972, vol. 20. pp. 319-332.
5. Handbook of Extractive Metallurgy, F. Habashi, ed., Wiley-VCH, Weinheim, 1997, vol. IV, pp. 1761-1811.
6. N.P. Lyakishev, M.I. Gasik, Metallurgy of Chromium, Allerton Press, NY, 1998.
7. W. Abdul-Razzaq, M. Seehra, Observation of oxidation and mechanical strain in Cr nanoparticles produced by ball milling, Phys. Stat. Sol.(a), 2002, vol. 193, pp. 94-102.
8. V.N. Kolosov, M.N. Miroshnichenko, V.M. Orlov, Influence of the chemical composition of precursors and reduction conditions on the properties of magnesiothermic tungsten powders, Inorg. Mater., 2016, vol. 52, no. 8, pp. 783-790.
9. V.N. Kolosov, M.N. Miroshnichenko, V.M. Orlov, Influence of the composition of precursors and reduction conditions on the properties of magnesiothermic molybdenum powders, Inorg. Mater., 2017, vol. 53, no. 10, pp. 1058-1063.
10. M.N. Miroshnichenko, V.N. Kolosov, T.I. Makarova, V.M. Orlov, Synthesis of molybdates and tungstanates of calcium and magnesium, Isvestija S.-PbGTI(TU), 2017, no.38, pp. 44-47.
11. V.M. Orlov, M.V. Kryzhanov, V.T. Kalinnikov, Magnesium reduction of tantalum oxide compounds, Dokl. Chem., 2014, vol. 457, part 1, pp. 161-163.
