Microstructure and reduction behavior of Mo powders doped with La2O3 and ZrO2 oxides using the spray drying method

Journal of Alloys and Compounds 964 (2023) 171307 Available online 11 July 2023 0925-8388/© 2023 Elsevier B.V. All rights reserved. Microstructure and reduction behavior of Mo powders doped with La 2 O 3 and ZrO 2 oxides using the spray drying method Serhii Tkachenko a , * , 1 , Vendula Bednaˇ ríkov´ a a , Olha Ksenzova a , Michaela Remeˇ sov´ a a , Karel Sl´ ameˇ cka a , b , Jaroslav Cihl´ aˇ r a , Matej Bal´ aˇ z c , Andrea De´ ak d , Ladislav ˇ Celko a a Research Group of High-Performance Materials and Coatings for Industry, Central European Institute of Technology, Brno University of Technology, Purkyˇ nova 123, 61200 Brno, Czech Republic b Institute of Physical Engineering, Faculty of Mechanical Engineering, Brno University of Technology, Technick´ a 2, 616 69 Brno, Czech Republic c Department of Mechanochemistry, Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 040 01 Koˇ sice, Slovakia d Supramolecular Chemistry Research Group, Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Magyar Tud´ osok k¨ orútja 2, Budapest 1117, Hungary ARTICLE INFO Keywords: Molybdenum Lanthanum ODS alloy Spray drying Oxide Reduction ABSTRACT In the present contribution, we used the spray drying method for the preparation of molybdenum-based composite powders, which belong to an important class of high-temperature structural materials – oxide dispersion strengthened molybdenum alloys (ODS-Mo). Three types of materials were prepared via spray drying and subsequent steps of calcination and reduction, including undoped reference Mo, ODS-Mo doped with 1 wt% of La 2 O 3 , and ODS-Mo doped with 1 wt% of La 2 O 3 and 1 wt% of ZrO 2 . The particular focus was given to precursor solution; specifically, the effects of type (aqueous or water-ethanol mixture) and chemical composition (the addition of La and Zr precursors) were studied in detail, which was rarely given attention in previous research. The morphological and microstructural changes of the particles during oxidation and reduction of sprayed-dried powders were extensively analysed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS). The morphology and crystallinity of spray dried powders were controlled by the state of spraying precursor solution, i.e. homogeneous solution or particle suspension. Calcination at the temperatures of 500 ◦C and 650 ◦C produced spherical polycrystalline and slab-shaped monocrystalline MoO 3 oxide particles. Subsequent reductive treatment of transformed calcined powders into metal-based powders took place with efficiency that depended mainly on the type of spraying solution (aqueous solution or water-ethanol suspension) and its chemical composition, in which case, the addition of La and Zr precursors resulted in lower efficiency than the addition of La alone. In terms of powder particle size, homogeneity of alloying elements, and reduction efficiency, the powders produced from water-ethanol suspensions were clearly superior to those produced from aqueous solutions. 1. Introduction Molybdenum (Mo) and its substitutional alloys are very promising refractory materials that are used in several important applications, especially in energy generation technologies, high-temperature structural parts, and electronics [16]. Their attractiveness is due to several advantageous properties, such as high melting point (2623 ◦C for pure Mo), excellent high-temperature strength, high creep resistance, good thermal conductivity, etc. [9,13]. However, Mo and its alloys exhibit some serious disadvantages, such as inadequate ductility at room temperature, low oxidation resistance, low recrystallization temperature, and high ductile-to-brittle transition temperature, which limits their applications at low temperatures. To improve ductility, the development of Mo alloys strengthened with thermodynamically stable oxide particles, typically rare-earth oxides, was employed by using powder metallurgical approaches based on high-energy ball milling [27, 29]. Doping the molybdenum grain boundaries with dispersed oxide phases promotes grain nucleation and hinders grain growth, leading to substantial microstructural refinement [4]. Extensive grain boundaries block dislocation movement by dispersion/dislocation interactions, * Correspondence to: Central European Institute of Technology, Brno University of Technology, Purkyˇ nova 123, 61200 Brno, Czech Republic. E-mail address: [email protected] (S. Tkachenko). 1 ORCID ID: https://orcid.org/0000-0001-9111-1520 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2023.171307 Received 11 April 2023; Received in revised form 4 July 2023; Accepted 7 July 2023 Journal of Alloys and Compounds 964 (2023) 171307 2 hindering grain boundary sliding, and effectively decreasing oxygen concentration as a result of strong interactions between oxygen and rare-earth elements. This improves the low-temperature ductility and high-temperature microstructural stability of molybdenum-based alloys strengthened with dispersed oxides (ODS-Mo). Typically used dispersed rare-earth oxide particles are La 2 O 3 and CeO 2 phases, which are well-known for their grain-boundary purification and grain size reduction effects [17,27]. The shortcomings of traditional powder metallurgy methods used for the composite preparation are related to the non-uniform size distribution of reinforcement particles and the excessive grain growth due to the highly deformed structure obtained by high-energy ball milling. In recent years, the development of ODS-Mo composites has been attempted using the so-called ‘‘bottom-up’’ approach based on wet chemistry methods. This approach employs chemical processes to construct the composite product from basic units such as molecules and atoms, facilitating the uniform distribution of nanosized oxide second phase particles in composite powders and grain size retention to further improve the mechanical properties of the ODS alloy. Thus, the distribution of second phase particles either within the grain or at the grain boundary can be controlled by adopting different precursor materials and reaction procedures, while the grain size is usually kept at the nano-/micro-scale [11]. Subsequently, liquid-liquid doping has been introduced into the co-precipitation method [15,16,28] to prepare a high-quality Mo-La 2 O 3 composite with the improved strength and high ductility. Another method investigated for the preparation of ODS-Mo alloys was hydrothermal synthesis of oxide powder precursor followed by hydrogen reduction and subsequent sintering of bulk material. Various types of ODS-Mo alloys have been produced by this method, including Mo-Al 2 O 3 [31] and Mo-ZrO 2 [4]. Freeze drying was yet another chemical method used for the production of ultrafine Mo-Y 2 O 3 powders [11]. Finally, the spray drying method was used to synthesize composite powders based on the W-Mo system [8]. Among chemical methods, the spray drying technique provides the possibility to prepare spherical powders that are of great importance for many industrial applications due to improved powder flowability, thus providing high-packing density and good sinterability to compacts. Furthermore, spherical powder particles have a much lower surface energy than particles with acicular morphology, which can hinder abnormal grain growth during the sintering of the spray-dried powder in comparison with ball-milled powders. At the same time, spray drying has been used to prepare composite powders based mainly on tungsten alloys [5,8], and the employment of this method for ODS-Mo has not been reported yet. In addition, the effects of a number of process parameters, such as chemical composition of the sprayed solution, spraying temperature, and nozzle pressure, on the microstructure of the spray-dried products were mainly investigated, while the other important parameters, such as homogeneity of the sprayed solution (i.e., the solution or particle suspension), were not investigated at all. To fill the above-mentioned gaps, the following items were addressed in the present work, such as (i) the possibility to produce high-quality Mo-La 2 O 3 and Mo-La 2 O 3 -ZrO 2 composite powders from solutions of Mo, La, and Zr precursors by a combination of spray drying and subsequent calcination and reduction steps, and (ii) the effects of the chemical composition and the state of spraying solution on the resulting powder microstructure, phase composition and reduction properties. Liquidliquid doping of Mo with La and Zr oxide particles according to the methodology below was chosen due to the availability of chemical precursors of La and Zr and well-known positive effect of La 2 O 3 and ZrO 2 reinforcements on the microstructure and properties of Mo [4,27–29]. The main focus was put on the effects of the state of (NH 4 ) 6 Mo 7 O 24 ⋅4 H 2 O precursor solution (i.e., homogeneous solution or suspension) and the presence of dissolved rare-earth La(NO 3 ) 3 ⋅6 H 2 O and ZrCl 2 O⋅8 H 2 O compounds on the ODS-Mo powders morphology, phase transformation and reduction efficiency in the sequence of spray drying → calcination → reduction processes. 2. Experimental 2.1. Fabrication of powders A liquid-liquid doping approach was used to prepare three types of powders by means of the spray drying technique, namely undoped Mo powder, composite Mo powder doped with La oxide, and composite Mo powder doped by both La and Zr oxides. Furthermore, solutions used for spray drying were of two types: aqueous solutions (henceforth designated by -w suffix) and water-ethanol mixtures (henceforth designated by -et suffix). The summary of prepared solutions for spray drying is presented in Table 1. The details of the preparation of aqueous solutions for the spraydrying process were as follows. Undoped Mo powder (denoted as Mow) was prepared using water-soluble ammonium heptamolybdate tetrahydrate (NH 4 ) 6 Mo 7 O 24 ⋅4 H 2 O (AHM; VWR International), which was dissolved in deionized water (50 ml) and stirred for 30 min to obtain a transparent solution. Next, based on preliminary experiments and previous literature data on spray drying technology [18], polyethylene glycol (PEG; Roth) was added as a polymeric dispersant to the solution to facilitate spray drying. For the preparation of ODS-Mo alloy doped with lanthanum oxide (denoted as MoLa-w), AHM and lanthanum nitrate hexahydrate La(NO 3 ) 3 ⋅6 H 2 O (LN; Alfa Aesar) were dissolved in deionized water. In addition, PEG was also used as a dispersant. For the preparation of ODS Mo alloy doped with oxides of both lanthanum and zirconium (denoted as MoLaZr-w), AHM, lanthanum nitrate hexahydrate (La(NO 3 ) 3 ⋅6 H 2 O), and zirconium oxychloride ZrOCl 2 ⋅8 H 2 O (ZO; Alfa Aesar) were dissolved in deionized water, again with the addition of PEG. The compositions of solutions were adjusted in such a way as to get Mo-1 wt% La 2 O 3 and Mo-1 wt% La 2 O 3 -1 wt% ZrO 2 . It was found that during preparation of Mo-w and MoLa-w, they formed transparent solutions (Fig. 1a), while the addition of the ZO to the aqueous solution of MoLaZr-w sample produced a suspension with white precipitation, which gave a solution a milky, opaque shade (Fig. 1b). In addition to the aqueous Mo-w, MoLa-w and MoLaZr-w solutions, the solutions with the precipitation of AHM component using ethanol as antisolvent were also spray dried. Mo-et, MoLa-et and MoLaZr-et suspensions were prepared by dropwise addition of 15 ml of ethanol, keeping the water/ethanol ratio of 7: 3, in order to produce AHM Table 1 Summary on the solutions prepared for spray drying. Sample designation Type of spraying solution Molar ratio of precursors Transparency pH Mo-w Aqueous solution (w) H 2 O: PEG: AHM 2.78: 0.0095: 0.013 Transparent solution 5.60 MoLa-w H 2 O: PEG: AHM: LN 2.78: 0.00095: 0.013: 0.0006 Transparent solution 5.56 MoLaZr-w H 2 O: PEG: AHM: LN: ZO 2.78: 0.00095: 0.013: 0.0006: 0.00039 Opaque suspension 5.47 Mo-et Water ethanol mixture (et) H 2 O: PEG: EtOH: AHM 2.64: 0.00095: 0.043: 0.013 Opaque suspension 5.56 MoLa-et H 2 O: PEG: EtOH: AHM: LN 2.64: 0.00095: 0.043: 0.0013: 0.0006 Opaque suspension 5.56 MoLaZr-et H 2 O: PEG: EtOH: AHM: LN: ZO 2.64: 0.00095: 0.043: 0.0013: 0.0006: 0.00039 Opaque suspension 5.49 S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 3 precipitation, which gives the produced Mo-et, MoLa-et and MoLaZr-et suspensions a milky shade, rendering them opaque (Fig. 1b), similarly to the effect produced by the ZO addition to aqueous MoLaZr-w sample. The addition of ethanol did not cause any notable change of pH. Spray drying of both aqueous solutions and water-ethanol mixtures was performed using a laboratory spray dryer (B-290, Büchi, Switzerland) equipped with a two-fluid nozzle of 1.5 mm nozzle and an inert loop. The spray drying parameters were: inlet/outlet temperature of hot nitrogen was 160/90 ◦C and inlet nitrogen flow rate was 357 l/h. The solution feed rate was fixed at about 20 ml/min. The use of selected spray-drying parameters (nozzle temperature, nitrogen gas flow, and solution feed rate) was based on the results of preliminary optimization experiments. Granulated particles were sieved through a 120-mesh sieve. 2.2. Powders calcination and reduction Calcination of the obtained powders was performed at temperatures of 500 ◦C for 5 h and 650 ◦C for 5 h. The selection of these temperatures was based on data available from the literature on the thermal decomposition of AHM, indicating that this compound completely transforms into molybdenum oxide MoO 3−x in the temperature range of 500–675 ◦C [2,14]. The powders placed in the aluminium oxide crucibles were calcined in the laboratory muffle furnace HT1200 M-b (Classic, Czech Republic) in air atmosphere. Reduction experiments were carried out in the GSL-1700 GLX tubular furnace (MTI, USA) (Fig. 3b) in an Ar/H 2 atmosphere (95:5 vol %), using two reduction cycles in total for all calcined powders, where each reduction cycle consisted of three temperature steps and is schematically described in Fig. 2. After each reduction cycle, the microstructure and phase composition of reduced powders were examined with scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, according to the description provided below in Section 2.3. The selection of the reduction procedure was based on the literature data [1,12,23], which found that MoO 3 is initially converted to MoO 2 at temperatures of 450–770 ◦C through the exothermic reaction path MoO 3 → Mo 4 O 11 → MoO 2 . The further reduction occurs at temperatures of 900–1400 ◦C and involves the endothermic reduction of MoO 2 to metallic Mo. 2.3. Materials characterization The crystalline composition of the produced (spray dried, calcined, and reduced) powders was evaluated by using X-ray diffractometer (XRD; SmartLab 3 kW CF2; Rigaku, Japan) in the Bragg-Brentano geometry, using Cu-Κ α radiation (40 kV, 30 mA) for a scanning 2θ range between 10◦and 90◦and a scanning speed of 3◦/min. The crystalline phases were identified based on the comparison of the obtained XRD patterns with the international centre for diffraction data (ICDD) powder diffraction files (PDF). Rietveld refinement of the obtained spectra was performed using High Score Plus software and the Inorganic Crystal Structure Database (ICSD) to quantify the composition of the present crystalline phases [22,26,30]. The surface morphology of powders particles was characterized with a scanning electron microscope (SEM; TESCAN Lyra3, Czech Republic) equipped with an energy dispersive X-ray (EDX) spectroscope (Max N 50, Oxford Instruments, UK). Samples were analysed in both secondary electrons (SE) and back-scattered electrons (BSE) detection modes at an accelerating voltage of 10 kV, while EDX analyses were performed at an accelerating voltage of 20 kV. To prevent electrical charging, the powder samples were coated with a carbon layer of approximately 25 nm thickness using a high-vacuum sputtering coater (Leica EM ACE600; Leica Microsystems, Austria). Powder particle size analysis was performed using ImageJ 1.53 t software. X-ray photoelectron spectroscopy (XPS, Kratos Axis Supra) measurements were performed with a monochromatic X-ray source (combined Al/Ag anode), an emission current of 15 mA, a hybrid lens mode, and charge neutraliser. Wide and highresolution spectra were recorded with a pass energy of 80 eV and 20 eV, using scanning steps of 1.0 and 0.1 eV, respectively. XPS spectra were analysed using CasaXPS software version 2.3.17PR1.1. The obtained spectra were calibrated using C 1 s peaks with a fixed value of 284.8 eV. 3. Results 3.1. Microstructure and chemistry of the spray-dried powders The microstructure of all spray-dried powders is presented in Fig. 3. The Mo-w, MoLa-w and MoLaZr-w powders spray-dried from the aqueous solutions (Fig. 3a-c) were of spherical morphology and with smooth surface, although some particles exhibited dents on the surface. The size of particles ranged from 2 to 15 µm, with an average size of 8µm. The analysis revealed no principal difference between the three types of powders synthetized with different rare-earth precursors. The Mo-et, MoLa-et and MoLaZr-et powders spray-dried from the water-ethanol mixtures (Fig. 3d-f) show a distinctly different morphology, most of them having a surface with an "orange-peel" quality. The particle size was similar to that of the powders synthetized from aqueous solutions and ranged from 2 to 17 µm, again with an average size of 8 µm. Similarly, there was no significant difference between the surface morphology of the undoped Mo-et sample and the powders MoLa-et and MoLaZr-et doped with La and La/Zr mixture. XRD analysis revealed (Fig. 4) that the character of spraying solutions (transparent versus opaque with dispersed AHM precipitates) greatly influenced the crystallinity of the spray-dried powders. Thus, the Mo-w and MoLa-w powders spray-dried from transparent aqueous solutions were amorphous rather than crystalline, showing almost no peaks in their XRD patterns (Fig. 4a). At the same time, the XRD patterns Fig. 1. The appearance of (a) transparent solution (obtained during the preparation of aqueous Mo-w and MoLa-w products) and (b) opaque suspension of milky shade (obtained during the preparation of aqueous MoLaZr-w, and waterethanol mixture Mo-et, MoLa-et, and MoLaZr-et products) used for the following spray-drying. Fig. 2. The reduction cycle of calcined powders used in the study. S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 4 of the powders produced from opaque solutions, such as MoLaZr-w (sprayed-dried from aqueous solution with ZO acting as a precipitant) and Mo-et, MoLa-et and MoLaZr-et powders (sprayed-dried from waterethanol mixtures) were crystalline (Fig. 4b). All XRD patterns showed the peaks corresponding to the starting AHM precursor (NH 4 ) 6 Mo 7 O 24 ⋅4 H 2 O with the monoclinic crystal lattice (JCPDS 98–003–0642). 3.2. Microstructure and phase conversion of powders after calcination The microstructure of the calcined powders is presented in Fig. 5 and Fig. 6. Upon calcination at 500 ◦C, the Mo-w and MoLa-w powders obtained from transparent aqueous solutions partially retained the microspherical morphology of the former AHM powder (Fig. 3). SEM-SE micrographs (Figs. 5a and 5b) showed that these polycrystalline particles were composed of small equiaxed crystals, while the smaller initial AHM spherical particles were transformed to small plate-shaped crystals of elongated or regular morphology. In contrast, the MoLaZr-w powder obtained from aqueous suspension retained its spherical morphology to a greater extent, showing no evidence of plate-shaped crystals (Fig. 5c). Calcination at 650 ◦C led to the morphological changes from spherical to large 2D monocrystals of slab shape in all powders prepared from aqueous solutions. In the case of the Mo-w and MoLa-w samples (Figs. 5d and 5e), the average size of slab-shaped crystals was approximately 42 ×6µm, with a thickness of 0.6–1.0 µm, which was higher than that of slab-shaped crystals in the MoLaZr-w sample with an average crystal size of 36 ×5µm and 0.6–0.8 µm thickness (Fig. 5f). The SEM-SE micrographs of the Mo-et, MoLa-et, and MoLaZr-et powders calcined at 500 ◦C (Figs. 6a, 6b, and 6c) showed that these powders also retained the micro-spherical morphology of the initial spray-dried AHM particles (Figs. 3d, 3e and 3f), similarly to the powders sprayed from aqueous solutions. However, the nanocrystals composing the polycrystalline spherical particles were notably smaller than those in the Mo-w and MoLa-w powders. Calcination of the Mo-et, MoLa-et, and Fig. 3. SEM-SE micrographs of the (a) Mo-w, (b) MoLa-w, (c) MoLaZr-w powders spray-dried from the aqueous solutions, and (d) Mo-et, (e) MoLa-et, and (f) MoLaZret powders spray-dried from water-ethanol mixtures. Fig. 4. XRD patterns of spray-dried powders prepared from (a) aqueous and (b) water-ethanol mixtures. S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 5 MoLaZr-et at 650 ◦C led to the complete transformation from spherical morphology to the slab-shaped monocrystals in all three powders (Figs. 6d, 6e and 6f). The mean crystal size was about 30 ×4µm with a thickness of 0.5–0.8 µm, which was ~25–30% smaller than the slabshaped crystals in the powders sprayed from aqueous solutions (Figs. 5d and 5e). The detailed morphology of MoLa-w powder particles sprayed-dried from an aqueous solution and MoLa-et particles sprayed from waterFig. 5. Microstructure of the Mo-w, MoLa-w and MoLaZr-w powders spray-dried from the aqueous solutions after calcination at 500 ◦C (a, b, c) and 650 ◦C (d, e, f), respectively (SEM-SE). Fig. 6. Microstructure of the Mo-et, MoLa-et and MoLaZr-et powders spray-dried from opaque water-ethanol mixtures after calcination at 500 ◦C (a, b, c) and 650 ◦C (d, e, f), respectively (SEM-SE). S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 6 ethanol mixture, after calcination at 500 ◦C is presented in Fig. 7. The micrographs show that the calcination converted the large Mo-La particles into small plate-shaped nanocrystals which were larger in powders sprayed from the transparent aqueous solution (Figs. 7a and 7b) than in powders produced from opaque water-ethanol suspension (Figs. 7c and 7d). These results highlight the difference between powders sprayeddried from solutions and from suspensions when calcined at 500 ◦C was. Specifically, the Mo-w powder obtained an aqueous solution was similar in appearance to the MoLa-w powder, while the MoLaZr-w, Moet, and MoLaZr-et powders sprayed from suspension were similar to MoLa-et powder. Phase analyses showed that the only detectable phases were orthorhombic MoO 3 (JCPDS 98–007–6651) and hexagonal La 2 O 3 (JCPDS 01–083–1345) in the case of doped ODS-Mo powders, Fig. 8 and Fig. 9. No other compounds of Mo, La, or Zr were identified. In contrast to the XRD patterns of powders calcined at 500 ◦C (Figs. 8a and 9a), the samples calcinated at 650 ◦C showed much higher intensities of the (2 0 0), (4 0 0), and (6 0 0) MoO 3 oxide diffraction peaks (Figs. 8b and 9b), which corroborates the microstructural observations of the formation of anisotropic slab-shaped monocrystals. Comparison of XPS spectra of Mo-w and Mo-et samples spray-dried from aqueous solution and water-ethanol mixture, respectively, showed that the XPS wide spectra provide the evidence of Mo, O and C elements (Fig. 10a). The high-resolution Mo 3d spectrum revealed two peaks located at 233.108 and 236.228 eV for the Mo-w sample and 233.192 and 236.317 eV for the Mo-et sample, corresponding to Mo3d5/2 and Mo3d3/2 of MoO 3 (Fig. 10b). The XPS spectra of MoLa-w, MoLaZr-w, MoLa-et and MoLaZr-et were similar to the spectra of Mo-w and Mo-et, and hence are not presented here. 3.3. Reduction studies As described in Section 2.3, two reduction cycles were applied to calcined powders to study the reduction effectiveness and phase transformation mechanisms. Reduction of Mo-w, MoLa-w, and MoLaZr-w powders sprayed from aqueous solutions was not completed after the first reduction cycle, as evidenced by both SEM characterisation (Fig. 11a) and XRD analyses (Fig. 14). XRD analysis showed that the MoO 3 oxide completely transformed to mixture of MoO 2 and Mo phases. The formation of metallic Mo, which is present in the form of small equiaxed grains, is evident on the surface of the MoO 2 powder particles (Fig. 11a). The formation of these equiaxed grains was inhomogeneous, with different amounts at different locations (Fig. 11a). The EDX mapping of powders doped with La showed the preferred precipitation of La 2 O 3 in the MO 2 intercrystallite boundaries (Fig. 13a), which was similar to MoLa-w and MoLaZr-w powders. The Rietveld quantification established that the amount of metallic Mo in Mo-w, MoLa-w, and MoLaZr-w powders after the first reduction cycle varied between 2.3 and 8.9 wt%. After the second reduction cycle, the extent of the reduction increased, however, it was not completed (Fig. 14a). The amount of Mo was higher in the MoLa-w powder. Comparison of reduction efficiency, i.e., the percentage of metallic Mo due to reduction, shows that powders with slab-shaped morphology calcined at 650 ◦C were reduced more effectively than powders calcined at 500 ◦C, as expected. Reduction of powders sprayed from water-ethanol mixtures (Fig. 12, Fig. 14b) was also incomplete after the first reduction cycle. However, the efficiency was higher as compared to the aqueous solutions and the surface of these products was covered with more particles (Fig. 11a). As opposed to the powders sprayed from aqueous solutions, the Rietveld quantification showed that the spherical Mo-et, MoLa-et, and MoLaZr-et powders calcinated at 500 ◦C were reduced more extensively than the powders with slab-shaped morphology produced by calcination at 650 ◦C (Fig. 14b). After the second reduction cycle, the powders produced from water-ethanol mixtures were completely reduced to metallic Mo. The resulting composite Mo-based powders were composed of small Fig. 7. The detailed SEM-SE morphology of spray-dried and calcined at 500 ◦C powders obtained from transparent aqueous solution (MoLa-w) (a,b) and from the opaque water-ethanol suspension (MoLa-et) (c,d) with AHM precipitation. S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 7 powder particles of regular shape and uniform distribution of La and Zr doping elements (Fig. 12, Fig. 13b). 4. Discussion 4.1. Microstructure of spray-dried powders In the present study, the ODS-Mo composite powder production process comprised several stages, including solution preparation, spray drying process, calcination step, and reduction step. In the first step, liquid-liquid doping started with the dissolution of the solid precursors in the deionized water, which enables homogeneous mixing of the precursor [MoO 4 ] 2– , La 3+ and ZrO 2+ ions: (NH 4 ) 6 Mo 7 O 24 ⋅4 H 2 O → 7[MoO 4 ] 2– +6NH 4 + +8 H + (1) La(NO 3 ) 3 ⋅6 H 2 O → La 3+ +3NO 3 – +6 H 2 O (2) ZrOCl 2 ⋅8 H 2 O → ZrO(OH) 2 +2Cl – +6 H 2 O (3) Fig. 8. XRD patterns of the Mo-w, MoLa-w and MoLaZr-w powders spray-dried from aqueous solutions and calcined at (a) 500 ◦C and (b) 650 ◦C, respectively. Fig. 9. XRD patterns of the Mo-et, MoLa-et and MoLaZr-et powders spray-dried from water-ethanol mixtures and calcined at (a) 500 ◦C and (b) 650 ◦C, respectively. Fig. 10. XPS spectra of Mo-w and Mo-et samples calcinated at 500 ◦C: (a) wide spectrum and (b) high-resolution spectrum of Mo 3d. S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 8 During spray drying, due to the rapid evaporation of the solvent, solid particles containing homogeneously dispersed Mo, La and Zr compounds were formed, forming a composite [(NH 4 ) 6 Mo 7 O 24 ; La (NO 3 ) 3 ; ZrOCl 2 ]⋅xH 2 O. As shown by energy-dispersive X-ray microanalyses (Fig. 13), the distribution of the doping La and Zr elements in the produced AHM powder particles was uniform and did not show showed inhomogeneities. Due to the high drying rate, the produced Mow and MoLa-w powders acquired an amorphous structure. In contrast, the ethanol addition induced the precipitation of AHM in the solutions used for spray drying. A similar behaviour was observed Fig. 11. Microstructure of the reduced Mo-w, MoLa-w and MoLaZr-w powders sprayed from aqueous solutions (SEM-BSE). S. Tkachenko et al. Journal of Alloys and Compounds 964 (2023) 171307 9 due to ZO presence in the MoLaZr-w sample. Ethanol was previously used as an AHM antisolvent during the production of AHM powder by spray drying from water solutions [24]. However, no notable changes in powder microstructure or crystallinity were reported. Precipitation of Mo and La nanoparticles from solution was previously utilized by Liu et al. (G. [16]) to produce core-shell composite powder particles, consisting of a (NH 4 ) 2 Mo 2 O 7 heterogeneously deposited on the surface of precipitated NH 4 La(Mo 2 O 7 ) 2 nuclei. Nevertheless, the authors did not report details of the precipitation procedure. In our case, the effects of precipitation on the resulting microstructure and crystallinity of the composite Mo-based powders were pronounced, since the addition of ethanol or ZO reduces the solubility of Fig. 12. Microstructure of the reduced Mo-et, MoLa-et and MoLaZr-et powders sprayed from water-ethanol mixtures (SEM-BSE). S. Tkachenko et al.