Abstract
(Mo,W)Si7 composite powders with the grain size of about 1 pm were synthesized at 1150°C for 2 h. The high purity molybdenum concentrate (with the main component of MoS), W and Si powders were utilized as raw materials and lime as desulfurizer. The graphite felt was laid between the compact (made of MoS7, W and Si) and the lime to facilitate the separation of the produced composite powders from desulfurization product. The phase composition, microstructure evolution, and residual sulfur content during the silicothermic reduction reaction were evaluated. The experimental results showed that the reaction could be completed after a reaction time of 1150 °C for 2 h, and the residual sulfur content of the product was 0.087 wt.%. It was concluded that the interactions between Si and the intermediate products of SiS and SiS7 are crucial for the preparationof (Mo, W)Si7 composite powders at low temperatures. The preparation process could be divided into three stages: solid state reactions between MoS„ W and Si to produce MoSi„ WSi7 and gaseous SiS; gas-solid reaction between MoS7, W and gaseous SiS to generate MoSi7, WSi7 and gaseous SiS7; and gas-solid reaction between gaseous SiS, and Si to form SiS gas. With this short flow process, fine-grained (Mo, W)Si7 composite powders can be produced at low cost at low temperature, which has great application potential.
Keywords: Molybdenum disilicide; Tungsten disilicide; Composite powders; Silicothermic reduction
Apstrakt
(Mo, W)Si7 kompozitní prahovi veličine zrna oko 1 pm sintetisani su na 1150°C u trajanju od 2 h. Kao sirovine su korišćeni koncentrat molibdena visoke čistoće (sa glavnom komponentom MoS7), prah W i Si, a kao sredstvo za odsumporavanje je koriščen kreč. Grafitni filc je položen između kompakta (napravljenog od MoS„ W i Si) i kreča da bi se olakšalo odvajanje proizvedenih kompozitnih prahova od proizvoda odsumporavanja. Procenjivani su fazni sastav, evolucija mikrostrukturę i sadržaj zaostalog sumpora tokom reakcije silikotermalne redukcije. Eksperimentalni rezultati su pokazali da se reakcija može završiti nakon reakcionog vremena od 2 sata na temperaturi od 1150°C, a sadržaj zaostalog sumpora u proizvodu je bio 0,087 tež.%. Zaključeno je da su interakcije između Si i međuproizvoda SiS i SiS, ključne za pripremu (Mo, W)Si7 kompozitnih prahova na niskim temperaturama. Proces pripreme se može podeliti u tri faze: reakcije u čvrstom stanju između MoS7, W i Si da bi se dobili MoSi7, WSi7 i gasoviti SiS; reakcija gas-čvrsto između MoS7, W i gasovitog SiS za stvaranje MoSi7, WSi7 i gasovitog SiS7; i reakcija gas-čvrsto između gasovitog SiS7 i Si da bi se formirao SiS gas. Ovim postupkom kratkog toka, fino zrnasti (Mo, W)Si7 kompozitní prahovi se mogu proizvesti po niskoj cení na niskoj temperaturi, što ima veliki potencijal primene.
Ključne reči: Molibden disilicid; Volfram disilicid; Kompozitní prahovi; Silikotermalna redukcija
(ProQuest: ... denotes formulae omitted.)
1. Introduction
Molybdenum disilicide (MoSi2) has attracted considerable interest owing to its high melting point (above 2000°C), moderate density (about 6.28 g/cm3) and excellent thermal/electrical conductivity. Other advantages include dramatic oxidation and corrosion resistance in oxidizing atmospheres, primarily due to a thin, coherent, adherent, and protective silica layer that forms on the surface. MoSi2 is now used in manyapplications, such as heating elements, high temperature oxidation resistant coating materials, aerospace gas turbine engines and industrial gas burners. However, monolithic MoSi2 has weak fracture toughness at room temperature and low strength at high temperatures (above 1000°C) [1-7]. Hence, many studies have been carried out in recent years to improve the mechanical characteristics of MoSi2-based materials by fine grain strengthening and nanoparticle reinforcement [8,9]. MoSi2 has thermodynamic stability with numerous carbides, nitrides, oxides, and boride ceramic reinforcements such as SiC, Si3N4, ZrO2, A12O3, Y2O3 and ZrB2 at high temperatures [1,10-19]. It has been demonstrated that the addition of a second phase can improve hardness, fracture toughness and bending strength could be achieved by adding a second phase. In addition, alloying elements such as Al, Cr, W, V and Ta have been used to improve the mechanical properties [2023]. Since both Mo and W belong to the VIB group, and WSi2 is most similar to MoSi2 in terms of crystal structure, alloying MoSi2 with WSi2 can significantly improve theoverall performance [24-26]. Zhang et al. [24] reported that the MoSi2-based composite with 50 mol.%WSi2 has fracture toughness about 86% higher than that of monolithic MoSi2. Similarly, Chen et al. [25] investigated WSi2 reinforced MoSi2 composites and found that a MoSi2 matrix with 5 vol.% WSi2 sample exhibited the optimum mechanical properties. Bose et al. [26] illustrated that alloying MoSi2 with WSi2 significantly reduced the high-temperature creep of MoSi2.
Due to the high melting points of MoSi2 and WSi2 (2030°C and 2165°C, respectively) the powder sintering method is generally used for preparing MoSi2-WSi2 composites. Thus, a powder with fine particle size and homogeneous composition is the foundation for the production of high-performance MoSi2-based composite materials. MoSi2/WSi2 powders were produced by mechanical alloying (MA) [27,28], the self-propagating high-temperature synthesis method (SHS) [29], the solid phase replacement method [30] and plasma spray processing [31]. Schwarz et al. [27] synthesized MoSi2 composite powders with different W contents from elemental powders by MA. Zamani et al. [28] fabricated the (Mo,W)Si2-WSi2 nanocomposite by MA. It should be noted that long-term ball milling inevitably introduces impurities into the product. Jo et al. [29] prepared MoSi2 powder by SHS using Mo and Si powders as primary materials. Even though lots of efforts have been made, most studiesin the literature selected Mo and W powders as the molybdenum and tungsten source, respectively.
It is worth mentioning that the grain size of the powder has a considerable impact on the sintering activity [9]. However, it is difficult to prepare ultrafine Mo powder by the normal hydrogen reduction process due to the presence of chemical vapor transport (CVT) mechanism, which limits the production of ultrafine MoSi2 powder with low cost. Generally, the Mo element occurs naturally in the form of sulfide minerals, which can be easily purified to get MoS2 [32-34]. Molybdenite concentrate is an essential ore mineral in the molybdenum industry for the production of molybdenum. Firstly, molybdenite concentrate is oxidation roasted to produce commercial molybdenum oxides. Secondly, the ammonium molybdate is prepared via ammonia leaching of commercial molybdenum oxides. Thirdly, the ammonium molybdate is oxidation roasted to generate pure MoO3. Finally, pure MoO3 is reduced to MoO2 and then to Mo by H2. As discussed above, the preparation of Mo is complicated and involves highenergy consumption, resulting in relatively high costs. On the contrary, the cost of pure MoS2 powder, which can be prepared from molybdenite concentrate after acid leaching, is much lower than that of pure Mo powder.
Therefore, it would be both energy and cost saving if MoS2 could be directly used as the molybdenum source to prepare MoSi2. In a previous study [35], it was proposed that MoSi2 powder can be successfully prepared by silicothermic reduction of MoS2, which could significantly reduce the production cost. Meanwhile, a sulfur-emission free route was also used to treat sulfur-containing by-products during the silicothermic reduction process [36].
Hence, the aim of this work is to synthesize ultrafine MoSi2-WSi2 composite powders by silicothermic reduction method using high purity molybdenum concentrate (MoS2), Si and W powders as raw materials. The samples were coated with desulfurizer (CaO) to capture the sulfur-containing gaseous by-product. The phase transition, microstructure evolution, residual sulfur content and reaction mechanism during the silicothermic reduction reaction process were evaluated in detail.
2. Experiment details
In this study, high purity molybdenite concentrate (MoS2, 98.5 % purity, average size of about 2 pm), Si (99.99 % purity, average size of about 5 pm) and W powder (99 % purity, average size of about 300 nm) were used as raw materials, with corresponding morphologies shown in Figures l(A-C). Calcium carbonate (CaCO3, 99% purity) was pyrolyzed at 1000 °C for 10 h to create lime powder (CaO), which was then utilized as a desulfurizer.
W powder was prepared via a two-stage reduction approach of WO3 [37]. The powders of MoS2, Si and W were accurately weighed and homogeneously mixed in a mortar with alcohol as the medium according to the desired stochiometric composition. The slurry was dried in an oven at 90°C for 30 min. Following that, about 5 g of the as-mixed powder was pressed into a cuboid stainless steel mold under a uniaxial charge of 64 MPa for 5 min. The compositions and nomenclatures of different powder mixtures are summarized in Table 1. For comparison, powders of Mo, W, and Si are also used to directly synthesize MoSi2-WSi2 composite powder.
The green compact was precisely weighed and then put in an alumina crucible. In oreder to easily removethe target product from desulfurized layer after the reaction, graphite felt was laid between the green compact and the desulfurizer lime. The quality of the added lime was 1.5 times the theoretical consumption calculated by reaction (1). When the temperature reached the designated temperature (1100°C or 1150°C), the quartz tube with the sample was inserted into the furnace chamber and kept under the argon atmosphere (Ar, 200 ml/min) for variious periods of time. Once the reaction was finished, the sample was removed and allowed to cool naturally to room temperature. The target product was accurately weighed again to calculate the mass loss. Then, the bulk product was crushed into powder and sieved through a 200-mesh sieve for the subsequent analysis.
...
To determine the phase composition, X-ray diffraction analysis (XRD, TTR III, Rigaku Corporation, Japan) wasperformed , with Cu Ka radiation in the 20 range of 10-90°, a scan rate of 0.2 s/step and a step-size of 0.02 deg/step. The microstructures of the raw materials and the obtained composite powders were characterized by fieldemission scanning electron microscopy (FE-SEM, ZEISS SUPRA 55, Oberkochen, Germany) with an energy dispersive X-ray spectrometer (EDS). The residual sulfur content of the sample was measured by an infrared carbon-sulfur analyzer (EMIA-920V2; HORIBA).
3. Results and discussion
After the reaction, the surface of the desulfurization layer remained white (color of CaO), indicating that the sulfur-containing by-products were completely captured. Clearly, there is a classic sandwich structure (top desulfurization layer-graphite felt-products). Duc to the presence of graphite felt as an intermediate layer, the product layer can be easily removed from the desulfurization layer. Simultaneously, due to the porous characteristics of the graphite felt, the SiS gas can cross the graphite felt and react with CaO. Meanwhile, the graphite felt is not involvcdin any reaction and can be used repeatedly.
3.1. Thermodynamic analyses
The main reactions for the preparations of MoSi2WSi2 composite powders could be described by Eqs. (2-3). The changes in the standard Gibbs free energy of these two reactions with temperature are indicated in Figure 2. Obviously, the changes of standard Gibbs free energy of Eqs. (2-3) are all negative in the temperature range of 1100-1150°C, suggesting that these reactions are all possible from the viewpoint of thermodynamics. However, the initial reaction should occur between the solid MoS2 and solid Si since the reaction temperature is lower than the melting points ofthe two.
...
According to previous work [36,38,39], the SiS produced in reaction (2) was not directly captured by the top layer of CaO. The produced SiS is a gaseous phase above 1100°C and reacts with the nearby MoS2 immediately (reaction 4). Simultaneously, the formed SiS2 transfers to the surface of the Si particle to react and produce gaseous SiS again which participates in the MoSi2 formation process (reaction 5). According to Figure 2, the change in standard Gibbs free energy of reaction (4) at temperatures ranging from 1100°C to 1150°C is higher than that of reaction (2). However, when gaseous SiS participate in the process, the mass transfer rate increases considerably.
...(6)
Furthermore, reaction (6) is also a gas-solid reaction with a negative standard Gibbs free energy change in the temperature range of 1100-1150°C. Another experiment was carried out to investigate the feasibility of reaction (6) at 1150°C. Firstly, a green compact (about 4 g) consisting of MoS2 and Si mixture with a molar ratio of 4:1 was placed on the left side of the alumina crucible to offer SiS gas, and about 0.2 g of W powder was placed on the other side. Then, the crucible was covered with a graphite sheet to ensure that the SiS gas generated by reaction (2) could contact and react with the W powders on the right side. The quartz tube with the alumina crucible was heated for 2 h in argon atmosphere when the temperature reached 1150°C. Figure 3(A) illustrates the schematic diagram of the verification experiment. Finally, the products on the right side of the crucible were characterized, and their XRD results were plotted in Figure 3(B). Obviously, both W5Si3 (PDF 51-941) and SiS2 (PDF 47-1375) can be confirmed after reacting at 1150°C for 2 h, which reveals that the formed gaseous SiS can react with W, as represented by reaction (6). The reason for the occurence of W5Si3 can be explained as the insufficient contact in the gassolid interaction between W and SiS(g), the reaction between W and SiS is not complete. Thus, many silicon-poor phases (W5Si3) and unreacted W are detected in the XRD patterns of the product. In summary, the method for synthesizing MoSi2-WSi2 composite powders from MoS2, Si and W powders by silicothermic reduction is thermodynamically feasible.
Interestingly, the relationship between SiS, Si and SiS2 is similar to the relationship with CO, C and CO2 during the ironmaking process in high blast furnace, as described by Eqs. (7-9). SiS plays the same role as CO, while Si and SiS2 play the same role as C and CO2, respectively. In the top part of the blast furnace, most of the reactions are carried out by reaction (8). This is comparable to the current experimental situation.
...
It should be pointed out that not all the generated SiS reacts with MoS2 and W. A part of them escapes from the product layer and are captured by the surface-covered CaO. The desulfurization reaction between CaO and SiS is a highly exothermic process with a great enthalpy change of about -874 KJ/mol at 1150°C, which is of great significance to the process of preparing composite powders at low temperatures. Firstly, the sulfur-containing by-products are not released and do not cause environmental pollution. Next, a large amount of heat generated by the desulfurization reaction (reaction 1) can accelerate the reaction but it could also lead to the grain size growth.
3.2. Phase analysis of products prepared under different reaction temperatures and times
Figure 4(A) illustrates the XRD patterns of products with different W contents obtained after reacting at 1100°C for 2 h. The powder diffraction data, including PDF 41-0612 (MoSi2), PDF 44-1055 (WSi2), PDF 49-1491 (Mo,W)Si2 PDF 89-3659 (W), were used to identify crystalline phases. Clearly, the main phases of the products are (Mo,W)Si2, WSi2 and a small amount of unreacted W. The diffraction peak of Si disappears completely under the present conditions. Besides, the sulfur-containing phase cannot be detected in all samples, which shows that MoS2 and Si were completely reacted. It should be stated that the main products are (Mo,W)Si2 and WSi2. It should be noted that, in contrast to Figure 3, the W5Si3 diffraction peak is not found. This is due to the fact that W thoroughly reacted with Si and gaseous SiS to form WSi2 after pressing the raw materials into green compacts.
Both Mo and W have a body-centered cubic structure with nearly identical lattice parameters (aMo = 3.14700 Å and aW = 3.16540 Å), resulting in very close Bragg peaks that are difficult to distinguish [40]. Moreover, the similar lattice constants and identical tetragonal structures of the MoSi2 and WSi2 phases enable the potential for many Mo atoms to be substituted by W atoms [24]. Figure 4(D) gives the drawings of yellow region enlargement in Figure 4(C). It can be clearly seen that the diffraction peaks of the MoSi2 samples doped with W shift to a small angle compared to the MoSi2 samples without W doped, indicating that the W atoms are dissolved in the MoSi2 lattice. It is worth mentioning that the W powder used in this work is about 300 nm in size, as shown in Figure 1, which is conducive to the uniform distribution of W in the (Mo,W)Si2 composite powder. According to the chemical composition of the product, they can be labeled as (Mo0.975,W0.025)Si2, (Mo0.95,W0.05)Si2 and (Mo0.9,W0.1)Si2, respectively. Additionally, the lattices of MoSi2 and WSi2 are so comparable that their diffraction peaks are extremely close to each other in XRD results. Therefore, MoSi2 peaks cannot be found in XRD patterns. The same phenomenon has also been reported in the literature [41-43]. Stergiou et al. [41] also prepared (Mo,W)Si2 composite with a low W content (2.1 at% W), and the major phase present was MoSi2 with W in solid solution according to the results of XRD and SEM-EDS. Furthermore, the diffraction peak of hexagonal WSi2 is also observed. The only structural distinction between tetragonal WSi2 (a=0.321 nm, c=0.786 nm) and hexagonal WSi2 (a=0.461 nm, c=0.641 nm) is the stacking order: tetragonal-WSi2 has an AB AB stacking sequence while hexagonal-WSi2 has an ABCABC order [45]. Zamani et al. and Ovali et al. [28,46,47] reported the formation of hexagonal WSi2 during the fabrication of W silicides by MA. Based on the above analyses, the overall reaction taking place in the system can be expressed in reaction (10). Moreover, the diffraction peak intensity of the hexagonal WSi2 is obviously enhanced as W content increases.
...
The XRD patterns of resultant powders with various W contents obtained at 1100 °C for 4 hours are plotted in Figure 4(B). The peak intensity of WSi2 is enhanced with the increase in W content. At the same time , the weak diffraction peak of W can still be observed. Figure 4(C) presents the XRD patterns of resultant powders with various contents of W obtained at 1150°C for 2 h. Only diffraction peaks for (Mo,W)Si2 and WSi2 are present. Thus, all reactants were reacted completely under the present conditions.
In the current method, MoS2, W and Si are used to prepare (Mo,W)Si2 composite powders at very low temperature. In order to verify the advantages of this experimental system, reactions among Mo, W and Si powders were conducted at 1150°C for 2 h. In this case, Mo, W and Si powders were mixed according to the MWS3 composition. The experimental process was the same as the above experiments. The possible reaction of the system can be demonstrated as reaction (11), and the XRD result of the product is shown in Figure 5. The characteristic peaks of Mo (PDF 657442), W (PDF 89-3659) and Si (PDF 77-21108) can be observed, revealing that most of the raw materials do not react. Additionally, only a small amount of Mo3Si (PDF 75-1182) was produced. It is worth noting that the reaction (11) has a negative change in standard Gibbs free energy at 1150°C. However, compared to the MoS2-W-Si system, there is no gaseous intermediate product in the Mo-W-Si system, which leads to a low reaction rate and extent. Generally, the current process has great advantages and enable the preparation of (Mo,W)Si2 composite powders at low temperatures.
...
3.3. Weight loss investigation at temperature of 1150°C
The sample MWS3 was selected for further analysis in order to investigate the reaction process. Six cuboids (about 5 g) were reacted at 1150°C for 2, 5, 10, 15, 30 and 60 min, respectively. The quartz tube with the sample was removed from the horizontal furnace immediately after the specific reaction time. The masses of the samples were weighed after the product had cooled naturally to room temperature. The weight loss rate W is defined as equation (12):
...
where m0 and m. are the initial and final weights of the sample, respectively. According to reactions (2-3), and assuming that SiS has escaped completely from the sample, the mass loss of the experimental system is therefore the mass of SiS captured by CaO. Therefore, the theoretical weight loss rate is about 39.85 %. The curve of the reaction extent varying with time can be drawn by the following equation:
...
The effects of the holding time on the reaction extent are shown in Figure 6. The value of X is 1 when the reaction is completed.
As depicted in Figure 6, three stages of the reaction process can t be identified. The preliminary stage is a slow reaction stage in 0-2 min. The weight loss change can be neglected, and the value of X is only 0.012. This phenomenon could be caused by slow initial reaction rate between solid MoS2 and solid Si. The second stage (2-15 min) is a rapid reaction stage. The reaction extent increases linearly and reaches 0.94 in 15 min. At this stage, the possible reaction is the gas-solid reaction among MoS2, W and SiS. The produced SiS2 transfers to the surface of Si particles to react and produce SiS gas again to take part in the next round of MoS2 formation process and sulfuration reaction of W. The third stage is the deep desulfurization stage. Only a small amount of reaction is stillin progress. When the holding time is prolonged to 60 min, the reaction extent X is infinitely close to 1.
Figure 7 shows the XRD patterns of the products for samples MWS3 acquired after reacting at 1150°C for various holding times. It can be found that the main phase is (Mo,W)Si2, besides small amounts of WSi2 and Si in the initial stage. This is in accordance with the low reaction extent after 5 min in the above analysis. The diffraction peaks of Si disappear and only the peaks of (Mo,W)Si2 and WSi2 arc observed when the reaction time is prolonged to 15 min, which is consistent with the fact that the reaction is basically completed after 15 min. After further extending the reaction time, there are no obvious changes in the XRD results except for the increase in the intensity of the diffraction peak of WSi2. It can be presumed that most of the MoSi2 and WSi2 were formed in the first 15 min of the reaction.
3.4. Residual sulfur content analysis
Due to the minor fraction of sulfide in the product, detecting the corresponding diffraction peaks by XRD is difficult. To investigate the effect of reaction time on the residual sulfur content in product, the samples MWS3 fabricated at 1150°C for 15, 30, 60 and 120 min were examined using an infrared carbon-sulfur analyzer, and the corresponding results are present in Figure 8.
Obviously, the residual sulfur content decreases with increasing durationAfter reacting for 15 min, the residual sulfur content is 1.054 wt.%. When the reaction is prolonged to 30 min, the remaining sulfur content drops to 0.510 wt.%. After reacting for 60 min and 120 min, the residual sulfur content is further minimized to 0.202 wt.% and 0.087 wt.%, respectively, indicating that extending the holding time is beneficial to reduce the residual sulfur content. In fact, according to our previous research, the residual sulfur content can be further decreased by increasing the reaction temperature and prolonging the reaction time [35].
3.5. Microstructure and morphologies evolution
Figure 9 illustrates the FE-SEM images of the composite powders acquired at 1150°C for different times (5, 10, 30 and 120 min). Interestingly, the synthesized powders preserve the stacked sheet-like structure of the raw MoS2 materials shown in Figure 1(A). Chang et al. [39] reported that ultra-fine MoSi2 powders synthesized by silicothermic reduction of MoS2 maintained the morphology, size, and sheet-like structure of MoS2. Moreover, a small number of blocky particles can be seen in Figures 9(A1) and (Bl), which are marked with red circles. Combining Figure 1(C) and Figure 9 it can be assumed that these particles are the unrcactcd Si. However, they disappear after 30 and 120 minutes, as shown in Figures 9(C1) and (DI).
Figures 9(A2), (B2), (C2) and (D2) show the local, highly magnified views of (Al), (Bl), (Cl) and (DI), respectively. In particular, the acquired powder grains have a spherical or elliptical morphology and obvious sintering necks. The average grain size of the synthesized powder is about 300 nm, as seen in Figures 9(A2-B2). However, when the annealing time is prolonged to 60 min (Figure 9(C2)), the grain size increases significantly, with an average grain size of around 1 pm. Interestingly, the grain size does not change significantly when the annealing duration is prolonged to 120 min (Figure 9(D2). It can be interpreted that the dependence of the grain size on the annealing time decreases when the holding time exceeds 60 min. On the whole, increasing the reaction time causes grain growth.
Figure 10 presents the results of the FE-SEM/EDS of the MWS3 samples obtained at 1150°C after various times (5, 10, 30 and 120 min). As shown in Figure 10(A), the elemental Mo map almost completely coincides with the elemental Si map, indicating the presence of MoSi2. However, there are accumulation zones (white circles) of Si in the elemental Si map but no enrichment of Mo or W, indicating that these regions are unreacted element Si. Moreover, element W is almost uniformly distributed, implying that the majority of W has been dissolved into MoSi2. It should be noted that there are some regions (red circle) where only W and Si are enriched. The red circle regions are WSi2, which does not dissolve into MoSi2. The Si-enriched areas in Figure 10(B) are reduced, indicating that the amount of unreacted Si content has decreased. After reacting for 30 and 120 min, the independent Si-enriched regions are no longer present (Figures 10(C) and (D)), which is consistent with the XRD findings that the Si has been exhausted when the annealing duration is extended to 30 min. Figures 10(C) and (D) show the simultaneous enrichment of the elements Mo, W, and Si, and the elements W and Si, corresponding to tetragonal (Mo,W)Si2 and hexagonal WSi2, respectively.
4. Conclusions
In the present work, the (Mo,W)Si2 composite powder was successfully prepared using MoS2, W and Si powders as raw materials and lime as desulfurization agent. The main conclusions can be summarized as follows:
(1) The composite powder with the main phases of (Mo,W)Si2 and WSi2 and a grain size of about 1 pm were obtained at 1150°C for 2 h.
(2) The residual sulfur content could be decreased to 0.084 wt.% after reacting at 1150°C for 120 min.
(3) The macro morphology and microstructure of the products were mainly inherited from the raw material MoS2.
(4) The formation of the gaseous intermediate product SiS changed the reaction from a solid-solid reaction to a gas-solid reaction, effectively accelerating the reaction process.
Acknowledgements
The authors gratefully acknowledge financial support from the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, China.
Author's Contributions
Jia-Bing Huang; Methodology, Investigation, Writing - original draft. Guo-Hua Zhang; Resources, Supervision, Methodology, Writing - review & editing.
Data availability Statement
All data generated or analyzed during this study are included in this published article.
Conflict of interest
The authors declare that they have no conflict of interest.
(Received 25 May 2023; Accepted 19 December 2023)
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Abstract
(Mo, W)Si7 kompozitní prahovi veličine zrna oko 1 pm sintetisani su na 1150°C u trajanju od 2 h. Kao sirovine su korišćeni koncentrat molibdena visoke čistoće (sa glavnom komponentom MoS7), prah W i Si, a kao sredstvo za odsumporavanje je koriščen kreč. Grafitni filc je položen između kompakta (napravljenog od MoS„ W i Si) i kreča da bi se olakšalo odvajanje proizvedenih kompozitnih prahova od proizvoda odsumporavanja. Procenjivani su fazni sastav, evolucija mikrostrukturę i sadržaj zaostalog sumpora tokom reakcije silikotermalne redukcije. Eksperimentalni rezultati su pokazali da se reakcija može završiti nakon reakcionog vremena od 2 sata na temperaturi od 1150°C, a sadržaj zaostalog sumpora u proizvodu je bio 0,087 tež.%. Zaključeno je da su interakcije između Si i međuproizvoda SiS i SiS, ključne za pripremu (Mo, W)Si7 kompozitnih prahova na niskim temperaturama. Proces pripreme se može podeliti u tri faze: reakcije u čvrstom stanju između MoS7, W i Si da bi se dobili MoSi7, WSi7 i gasoviti SiS; reakcija gas-čvrsto između MoS7, W i gasovitog SiS za stvaranje MoSi7, WSi7 i gasovitog SiS7; i reakcija gas-čvrsto između gasovitog SiS7 i Si da bi se formirao SiS gas. Ovim postupkom kratkog toka, fino zrnasti (Mo, W)Si7 kompozitní prahovi se mogu proizvesti po niskoj cení na niskoj temperaturi, što ima veliki potencijal primene.