2023 Volume 64 Issue 3 Pages 643-649
In this paper, the influences of ROH (R = Li, Na, K) additions on the preparations of Tungsten (W)/Molybdenum (Mo) powders by hydrogen reduction of WO3/MoO2 in the temperature range of 800°C to 1100°C were studied in detail. In the absence of ROH addition, the prepared W/Mo powders basically maintained the morphology of raw oxide. However, with the assistance of ROH, the morphology and particle size of the products had been greatly changed. The grains of W and Mo were obviously coarsened, and micron-sized Mo powder and W powder with good dispersion can be prepared. In addition, the addition of ROH can promote the reduction rate of WO3 but inhibit the reduction of MoO2. Moreover, the effect of LiOH on grain growth is more significant than that of NaOH and KOH.
Fig. 8 SEM images of W prepared by reducing WO3 with the assistance of 0.1% ROH at 900°C: (a) LiOH, α = 0.58; (b) NaOH, α = 0.60; (c) KOH, α = 0.64, (α is reaction extent).
Tungsten and molybdenum, as the most frequently used refractory metals, possess high melting point, high creep and corrosion resistance, low expansion coefficient, high yielding strength and hardness at high temperature. Thus, they have been extensively utilized in chemical, metallurgy, military, aerospace and electrical industries.1–5) However, the melting-casting method is not suitable for the preparations of W/Mo alloys due to the limitation of their high melting points. Therefore, the preparations of qualified W/Mo metal powders are particularly important for the preparations of W/Mo alloys by the method of powder metallurgy.
There are many methods for producing metal W/Mo powders, including hydrogen reduction,6,7) chemical vapor synthesis,8,9) ball milling,10,11) molten salt synthesis,12–16) and self-propagating high-temperature synthesis,17,18) carbothermal pre-reduction and hydrogen deep reduction.19,20) Currently, W/Mo metals powders are prepared by the method of hydrogen reduction. However, the uncontrollable nucleation and growth during hydrogen reduction of tungsten/molybdenum oxide is one of its shortcomings. When the pseudomorphic transformation mechanism plays a dominant role at a lower reduction temperature, although a large number of nano-nuclei are formed, the prepared W/Mo products have bad dispersity due to the slow growth rate, and the overall morphology of product almost keeps the same as the raw oxide. When the CVT (Chemical Vapor Transport) mechanism plays a dominating role at a higher temperature, the nano-nuclei can grow by CVT, and the prepared W/Mo product has a larger particle size.21,22)
Many efforts have been made on hydrogen reduction of tungsten and molybdenum oxide with the assistance of alkali metal compounds,12–16,23) including chlorine salts (NaCl, KCl, CaCl2 and MgCl2),14,15) carbonate (Li2CO3, Na2CO3 and K2CO3),16) hydroxide and so on. In our previous research,14–16) it was found that the additions of alkali metal carbonate and alkali metal chlorine salts can improve the uniformity of molybdenum powder and refine the particle size of molybdenum powder. However, with the addition of chlorine salts, the chloride vapor in the high temperature reaction process has strong corrosiveness to instruments.14,15) With the addition of carbonate into molybdenum trioxides, the alkali metal oxides produced after the reaction will cause pollution to the product composition.16) However, there is no investigation on ROH (R = Li, Na, K) assisted hydrogen reduction of molybdenum oxide to prepare Mo powder. At the same time, the different influences of the ROH addition on the hydrogen reduction of tungsten oxide and molybdenum oxide are also rarely reported. Therefore, in the present work, ROH (R = Li, Na, K) was added as additive to assist the reductions of WO3 and MoO2. The effects of addition amount and reaction temperature were investigated in detail.
In the industrial production, the preparation of molybdenum powder is mainly a two-stage reduction process (MoO3 → MoO2; MoO2 → Mo), while the tungsten powder is generally prepared by a one-stage reduction process, by using the raw material of WO3. Therefore, MoO2 powders (Jinduicheng Molybdenum Co., Ltd., Xi’an, China) and WO3 (>99 mass%, Sinopharm Chemical Reagent Co., Ltd.) were chosen as the starting material to study the preparations of Mo and W powders in the current study. The morphologies of the WO3 and MoO2 powders are shown in Fig. 1. The block-shaped WO3 particles (about 50 µm) are composed of grains with the size of 100 nm. The MoO2 particles are platelet-shaped with a size of 2–3 µm. The additives used in the present work are reagent grade LiOH (99% purity), NaOH (99.9% purity) and KOH (99.9% purity), which were first dissolved in deionized water to form a ROH solution with the same molar concentration (0.25 mol/L). Subsequently, the ROH solution was added to the WO3/MoO2 surface by spraying, and the sample was continuously stirred during the spraying process to ensure that ROH could be evenly distributed on the oxide surface, and the ROH mass fraction (0.1 mass%, 0.02 mass% or 0.5 mass%) in the oxide samples was controlled by controlling the amount of ROH solution added. After drying at 100°C for 6 hours, the oxide material with fine ROH particles evenly loaded on the surface can be obtained for subsequent reduction experiments.
SEM images of (a) WO3; (b) MoO2.
To monitor the change of sample mass with reaction time, experiments were performed using a thermal analysis system (HCT-2, Beijing Hengjiu Instrument Ltd., Beijing), which includes a thermo-gravimetry (TG) microbalance with a precision of ±0.1 µg. In each experiment run, an alumina crucible containing about 100 mg of samples was placed in the furnace, and Ar gas (60 ml/min) was used to exclude the air prior to the experiment. When the furnace was heated to the desired reaction temperature at a heating rate of 20°C/min, Ar gas was switched to reducing gas H2 (60 ml/min). Finally, when the mass loss of the samples no longer changed, the H2 was switched to Ar gas and the sample was cooled to ambient temperature for further examination.
The phase composition of sample was examined via X-ray diffraction (XRD; TTR III, Rigaku Corporation, Japan). The morphology of product was observed by field-emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany). The particle sizes were measured by the statistics of >200 particles from three different fields in the FE-SEM images.
Figure 2 presents the mass loss curves of WO3 and MoO2 during the hydrogen reduction at 800°C. It can be obviously seen that the additions of ROH greatly affect the reaction rates of WO3 and MoO2. The kinetic curves of reducing WO3 and MoO2 are illustrated in Fig. 2, and the final mass loss rates are around 20.71% and 25%, corresponding to the theoretical mass loss rates from WO3 to W and MoO2 to Mo, respectively. In addition, it is observed that the slope of the curve is discontinuous at a mass loss rate near 6.9%, which corresponds to the reaction from WO3 to WO2. By comparing the different kinetic curves shown in Fig. 2(a), the reaction time of WO3 becomes shorter with the assistance of ROH, indicating that ROH promotes the reduction rate of WO3. However, for the reduction of MoO2, the reaction rate follows the opposite rule, as illustrated in Fig. 2(b). The reduction time becomes longer with the assistance of ROH, which indicates that ROH hinders the reduction of MoO2. The different effects of ROH on the reduction rates of tungsten and molybdenum oxides will be discussed in the following sections.
Mass loss curves of (a) WO3; (b) MoO2 during the hydrogen reduction at 800°C.
Figure 3 shows the XRD patterns of products by reducing pure oxides and ROH doped oxides. It can be concluded that only W or Mo phase exists in the products regardless of the addition of ROH. Thus, all reactants are completely reduced to metal W or Mo powders.
XRD patterns of products by reducing (a) MoO2 and ROH doped MoO2; (b) WO3 and ROH doped WO3.
Figure 4 presents the microscopic morphologies of Mo products prepared at 900°C. From Fig. 4(a), as reducing pure MoO2, the product shows a platelet morphology similar to MoO2, and cracks as well as pores appear on the surface of the sample due to tensile stress. The generation of tensile stress can be explained by the changes in the molar volume (defined as the volume corresponding to one mole of Mo atoms in a substance) of the species before and after phase transformation. The molar volume of MoO2 and Mo are 19.77 cm3/mol21) and 9.41 cm3/mol, respectively (Table 1). During the phase transformation from MoO2 to Mo, the molar volume decreases by about 52.4%, resulting in the generation of tension stress and cracks on the surface of the sample. From Fig. 4(b), it is found that when 0.02% NaOH is added, the morphology of Mo particles has a significant change, and the produced Mo particles are composed of many nanoparticles. However, the Mo particles have serious agglomeration. When the addition amount is increased to 0.1% and 0.5%, as illustrated in Figs. 4(c)–(d), the morphology of Mo powders dramatically changes. The grains are no longer closely connected and the gaps between the grains also increased, which make the agglomeration phenomenon is gradually weakened, even though the grain size of Mo is obviously larger as the increase of NaOH addition from 0% to 0.5%. In the following sections, 0.1% addition of ROH will be adopted for the further study.
The FE-SEM images of Mo prepared by reducing MoO2 with the assistance of different amounts of NaOH at 900°C: (a) 0%; (b) 0.02%; (c) 0.1%; (d) 0.5%.
Figure 5 presents the microscopic morphologies of Mo products prepared at different temperatures (800°C, 900°C, 1000°C, 1100°C). For the reduction of pure MoO2 shown in Figs. 5(a), (e), (i), (m), at all the four temperatures, the produced Mo particles inherit the morphology of raw oxide, even if they are composed of many nanoparticles. As shown in Figs. 5(b)–(d), when adding 0.1% ROH, the morphologies of produced Mo particles at 800°C still maintain the platelet-shape morphology as the raw MoO2. As increasing the reaction temperature to 900°C, from Figs. 5(f)–(h), the morphologies of Mo particles dramatically change, and the gaps between the grains increase. When the temperature is further increased to 1000°C or 1100°C (Figs. 5(j)–(l) and 5(n)–(p)), the grains size of Mo increases significantly and the connectivity between particles decreases obviously compared with the case without ROH addition. When reducing MoO2 with the assistance of 0.1% LiOH, the grain size of Mo is obviously larger than that with other additives (0.1% NaOH and KOH). From the above results, ROH significantly enhances grain growth rate during reduction process. In addition, the higher the reaction temperature, the better dispersion degree of Mo powder and the larger the size of Mo grains will be. At the same time, compared with NaOH and KOH, LiOH has a more significant effect on the growth of grains.
SEM images of Mo prepared by reducing pure MoO2 and ROH doped MoO2 at different temperatures: (a) pure MoO2 at 800°C; (b) 0.1% LiOH at 800°C; (c) 0.1% NaOH at 800°C; (d) 0.1% KOH at 800°C; (e) pure MoO2 at 900°C; (f) 0.1% LiOH at 900°C; (g) 0.1% NaOH at 900°C; (h) 0.1% KOH at 900°C; (i) pure MoO2 at 1000°C; (j) 0.1% LiOH at 1000°C; (k) 0.1% NaOH at 1000°C; (l) 0.1% KOH at 1000°C; (m) pure MoO2 at 1100°C; (n) 0.1% LiOH at 1100°C; (o) 0.1% NaOH at 1100°C; (p) 0.1% KOH at 1100°C.
In order to further study the formation of Mo particles during the reduction process, reduction products were obtained at 900°C with the reaction extent around 0.4 (0.42, 0.46 and 0.40 for products with the additions of 0.1 mass% of LiOH, NaOH and KOH, respectively). The FE-SEM micrographs of these samples are illustrated in Fig. 6. It can be found that many Mo nanoparticles are formed surrounding the unreacted MoO2. Therefore, the addition of ROH leads to the dispersed nucleation of Mo and improves the dispersity of the final Mo powder.
SEM images of Mo prepared by reducing MoO2 with the assistance of 0.1% ROH at 900°C: (a) LiOH, α = 0.42; (b) NaOH, α = 0.46; (c) KOH, α = 0.40, (α is reaction extent).
The effect of 0.1% ROH addition on morphologies and size of prepared W was investigated and the results are illustrated in Fig. 7. When reducing pure WO3 by hydrogen at 800°C (Fig. 7(a)), the large W particles maintain the blocky-shape morphology, which are composed of many small W grains. Similar to molybdenum, the pores and fissures can be seen on the surface of W particles. At 900°C, 1000°C and 1100°C (Figs. 7(b)–(d)), the agglomeration degree of the prepared W powders decreases and the particle size gradually increases. However, when 0.1% ROH was added, as illustrated in Figs. 7(e)–(p), the blocky-shaped W particles are transformed to W grains with good dispersion degree. It can be also concluded in Figs. 7(e)–(p) that as the temperature increases from 800°C to 1100°C with the additions of LiOH, NaOH or KOH, the grain size of W increases from 1.72 µm to 15.34 µm, 1.28 µm to 13.17 µm, and 1.03 µm to 10.23 µm, respectively. Therefore, the addition of ROH has a significant effect on the coarsening of W grains. The same phenomenon is also observed for the reduction of molybdenum oxide.
SEM images of W prepared by reducing pure WO3 and ROH doped WO3 at different temperatures: (a) pure WO3 at 800°C; (b) pure WO3 at 900°C; (c) pure WO3 at 1000°C; (d) pure WO3 at 1100°C; (e) 0.1% LiOH at 800°C; (f) 0.1% LiOH at 900°C; (g) 0.1% LiOH at 1000°C; (h) 0.1% LiOH at 1100°C; (i) 0.1% NaOH at 800°C; (j) 0.1% NaOH at 900°C; (k) 0.1% NaOH at 1000°C; (l) 0.1% NaOH at 1100°C; (m) 0.1% KOH at 800°C; (n) 0.1% KOH at 900°C; (o) 0.1% KOH at 1000°C; (p) 0.1% KOH at 1100°C.
In order to further investigate the distribution of W nucleus in the reduction process of tungsten, reduction products were prepared at 900°C with the reaction extent around 0.6 (0.58, 0.60 and 0.64 for products with the additions of 0.1 mass% of LiOH, NaOH and KOH, respectively). The FE-SEM micrographs of these samples are shown in Fig. 8. The results show that the W nanoparticles are also uniformly dispersed around the unreacted WO3.
SEM images of W prepared by reducing WO3 with the assistance of 0.1% ROH at 900°C: (a) LiOH, α = 0.58; (b) NaOH, α = 0.60; (c) KOH, α = 0.64, (α is reaction extent).
In the hydrogen reduction experiment, water vapor could be generated, which inevitably leads to the formation of gaseous transport phase WO2(OH)2/MoO2(OH)2 (as shown by reactions (1) and (2)). Due to the existence of the transport phase, the nucleation and growth of tungsten and molybdenum are uncontrollable during the hydrogen reaction process.7,24–26) Under the condition of a low concentration of WO2(OH)2/MoO2(OH)2 at a lower reaction temperature, the pseudomorphic transformation mechanism plays a dominant role. Even though a large number of nano nuclei can be obtained, but the growth of nuclei is difficult through the gaseous transport phase, so the produced W/Mo particles are prone to agglomerate. However, under the condition of high concentration of WO2(OH)2/MoO2(OH)2 at a higher temperature, the CVT mechanism plays a leading role, and only a few stable nuclei can be formed, because small particles are not stable and will be re-oxidized into the gaseous transport phase.27–29) Therefore, the sizes of the products are relatively large by the reduction and deposition of WO2(OH)2/MoO2(OH)2 (as shown by reactions (3) and (4)).
| \begin{equation} \text{WO$_{2}$(s)} + \text{2 H$_{2}$O(g)} = \text{WO$_{2}$(OH)$_{2}$(g)} + \text{H$_{2}$(g)} \end{equation} | (1) |
| \begin{equation} \text{MoO$_{2}$(s)} + \text{2 H$_{2}$O(g)} = \text{MoO$_{2}$(OH)$_{2}$(g)} + \text{H$_{2}$(g)} \end{equation} | (2) |
| \begin{equation} \text{WO$_{2}$(OH)$_{2}$(g)} + \text{3 H$_{2}$(g)} = \text{W(s)} + \text{4 H$_{2}$O(g)} \end{equation} | (3) |
| \begin{equation} \text{MoO$_{2}$(OH)$_{2}$(g)} + \text{3 H$_{2}$(g)} = \text{Mo(s)} + \text{4 H$_{2}$O(g)} \end{equation} | (4) |
In this present work, with the assistance of a small amount of ROH, the influences on the morphology and size of W/Mo particles are considerable, from large platelet particles to small-sized W/Mo grains with good dispersion degree, even if the grain size of the W/Mo products are larger compared to that prepared without ROH addition. In previous studies,30,31) it can be known that when pure tungsten trioxide is reduced by hydrogen, the formation of tungsten nuclei only occurs at the stage from WO2 to W. But, with the assistance of ROH, the formation of tungsten nuclei becomes possible before the WO2 is reduced. Neugebauer32) observed that potassium tungsten bronzes (KxWO3) can be reduced to metallic tungsten before it is reduced to tungsten dioxide. Zimmerl30) also showed that alkali metals react with the tungsten oxide to form tungsten bronzes, and these bronzes could be reduced to tungsten nuclei during the stage from WO2.72 to WO2. Therefore, when ROH is added to WO3, it will react with tungsten oxide to form tungstate, which will be reduced by hydrogen to generate the W nuclei before WO2 is formed. After that, WO2 react with the water vapor in the subsequent reaction process to generate the volatile WO2(OH)2/MoO2(OH)2, which is transported to W nuclei formed in advance for rapid growth via CVT mechanism. But for molybdenum, since the raw material is MoO2, the effect of adding ROH to promote nucleation in advance is not so significant, so the effect of promoting grain growth is relatively weak. Moreover, the effect of adding ROH on the reduction of tungsten/molybdenum oxide may be explained by the catalytic action.14–16) The catalytic effects on tungsten oxide and molybdenum oxide occur at the reaction stages from WO2 to W and MoO2 to Mo.33,34) In this process, the presence of the liquid alkali compounds can increase the concentration of the gaseous transport phase WO2(OH)2/MoO2(OH)2, thereby promoting the growth of W/Mo grains.33,34) Therefore, from the above analyses, the addition of ROH promotes the growth of grains, and the grain sizes of the products are larger compared to the W/Mo powders prepared without ROH addition.
It is also concluded that the addition of LiOH has a more obvious growth effect on W/Mo grain than that of other additives (0.1% NaOH and KOH). The reason may be that at the end of the reaction, tungstate/molybdate are reduced and the metal is evaporated. Compared with Na and K, Li is not easy to volatilize due to its high boiling point during reaction process, so the residual amount is the highest and the effect on the reaction is more significant.24)
4.2 Effects of ROH on the reduction rate of tungsten and molybdenumBased on the above discussion, the addition of ROH can promote the nucleation of tungsten and molybdenum in advance, thereby increasing the reduction rate of tungsten oxide and molybdenum oxide. However, for the case of MoO2, owing to its small particle size, the molten ROH or the generated molybdates surrounded the MoO2 particles more homogeneously, which hindered the diffusion of water vapor and hydrogen, and led to the decrease of reduction rate.15,16) However, compared with the particle size of MoO2, the average size of WO3 reaches 50 µm, so the inhibition effect of the liquid phase on WO3 may be negligible. In addition, MoO2 is difficult to sublimate and migrate due to its high melting point (2500°C), while for WO3, when the temperature is higher than 800°C, it could sublimate. When the ROH was added, it reacted with WO3 to form tungstate, which increased the concentration of WO2(OH)2 and improved the reaction kinetics of WO3.
Therefore, the addition of ROH has different effects on the reduction rates of WO3 and MoO2, leading to that the grain sizes of the prepared W and Mo are also much different. The addition of ROH can promote the growth of grains, but the inhibition effect of molten salt on the reduction rate of MoO2 limits the excessive growth of Mo grains. However, for tungsten oxide, the addition of ROH promotes its reduction rate, so the grain growth is more significant.
In the present study, the ROH-assisted hydrogen reductions of WO3/MoO2 powders were investigated. The following conclusions can be drawn.
The authors gratefully acknowledge financial support from the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing.
