Kinetic Highlights of the Reduction of Silver Tungstate by Mg + C Combined Reducer

Abstract

The programmed reduction of tungstates and molybdates may yield the production of an intimate mixture of metals, pseudo-alloys or composite powders. As an extension of the study of obtaining powders of tungsten-copper, molybdenum-copper and tungsten-nickel from their respective salts, in the present study the reduction of silver tungstate was performed. Considering the extreme conditions for the synthesis of W-Ag alloys in the combustion wave and the limited toolkit for the study of the associated reduction mechanism, the interaction in the Ag₂WO₄-Mg-C system was modeled at high heating rates closer to the heating rates of reagents in the combustion wave, namely by the high-speed temperature scanner (HSTS). For the effective study of the interaction mechanism and calculation of the kinetic parameters of the individual stages, the heating rate of the reagents was changed in a wide range (from 100 to 1200 °C min⁻¹). The interaction scheme and the sequence of the reactions along with their starting temperatures were deduced; the nature of intermediates formed during the reduction process and the microstructure evolution were monitored.

1. Introduction

The low and alterable thermal expansion coefficient, enhanced welding and erosion characteristics of W-Ag pseudo-alloys render their wide application in semiconductor equipment powerful electrical contacts. These pseudo-alloys can also be used in microwave ovens, microelectronics, as heat-dispersing components and electronic switches for powerful circuit breakers [1,2,3,4,5,6]. As heat-conducting materials, they can replace the agents in microelectronic devices that tend to fail at higher temperatures [7,8]. The behavior of this material depends not only on the composition and distribution of phases, but also on the method of preparation. The particle size and shape, the homogeneity of the microstructure and the degree of porosity have a severe influence on the mechanical, electrical and thermophysical properties of the composite material [9,10,11,12].

The preparation of refractory metal-based alloys by conventional metallurgical methods is challenged by the large differences in the specific gravity and melting points of the constituents (T_melt. (Ag) = 962 °C, T_melt. (W) = 3422 °C). This is further complicated by the limited mutual solubility of the metals in both the solid and liquid states [13,14] and the associated extensive segregation of silver. The powder metallurgy [15], hydrogen reduction of silver tungstate [16], explosive consolidation [17], press-sinter infiltration [18] and combustion synthesis [19] are reported as W-Ag preparation methods. However, a huge challenge still lies in the lack of sufficient quality materials (high relative density, homogeneous dispersion of phases, etc.) impeding the technological advancement of tungsten-based composites in a variety of applications.

In order to produce high-density parts with net shape components, a homogeneous structure and enhanced physicomechanical properties, the very fine dispersion of both metals should be attained [20]. Thus, the development of cost-effective techniques of production of homogenous and fine-grained composite materials, as well as the optimization of their preparation conditions are of current relevance. In recent years, a number of researchers [21,22,23,24,25,26,27] have offered a self-propagating high-temperature synthesis (SHS) method via a thermo-kinetic coupling approach by using an Mg + C combined reducer for producing metal-composite materials. Its essence consists in the coupling of a low exothermic reduction reaction (e.g., Ag₂WO₄ + C) with a high caloric one (Ag₂WO₄ + Mg), with a possible change of reaction pathway. The choice of Ag₂WO₄ is driven by the significant benefits of the excellent chemical homogeneity of the final product due to the presence of both metals in the same crystal structure. This strategy allows for control over the combustion process parameters, regulation of the powder grain size and maintainance of compositional homogeneity. On the other hand, the extreme conditions of the synthesis of metal-based composites in the combustion wave (high temperatures, high heating rates of initial material) and the limited toolkit for the study of the associated interaction mechanism hinder the comprehensive management of the process. To meet the challenge, the processes in the combustion wave are modeled by a high-speed temperature scanner (HSTS) [28,29,30,31,32,33]. This technique allows one to perform detailed examinations in the reactive powder mixtures or compacts from RT up to 1700 °C and fast heating (up to 10,000 °C min⁻¹). For the investigation of the interaction mechanism in the system under study (Ag₂WO₄-Mg-C), the heating rates of the reagents were changed in the range from 100 to 1200 °C min⁻¹, which allowed one to split the main stages and to study the intermediates quenched at characteristic temperatures. As a result, the optimal conditions for the joint complete reduction of metals at linear heating were determined and the kinetic parameters were calculated.

The interaction mechanism was investigated in the binary Ag₂WO₄-Mg and Ag₂WO₄-C systems, and then in a complex ternary Ag₂WO₄-Mg-C system under identical heating conditions.

2. Materials and Methods

Silver nitrate (AgNO₃, 7761-88-8, chemically pure grade, PPD, Prague, Czech Republic), sodium tungstate (Na₂WO₄·2H₂O, chemically pure grade, Pobedit Company, Vladikavkaz, Russia), magnesium (MPF-3, pure grade, 0.15 < μ < 0.3 mm, Ruskhim, Moscow, Russia) and carbon black (P-803, pure grade, µ < 0.1 μm, Ivanovo carbon black and rubber JSC, Ivanovo, Russia) were used in HSTS experiments as initial reagents. A chemical co-precipitation method was utilized for the preparation of silver tungstate by reacting aqueous solutions of AgNO₃ and Na₂WO₄ [34]. These solutions were mixed with a 2:1 molar ratio at continuous stirring for 20 min at room temperature and fixed pH (pH = 7). The precipitate formed was centrifuged, filtered and washed by ethanol and distilled water several times. Then, it was dried in a vacuum oven at 70 °C for 4 h. A light-yellow single phase α-Ag₂WO₄ was obtained with an average particle size of 10–20 nm [34].

The HSTS-1 setup (Figure 1) [28,29,30,31,32,33] was utilized for the kinetic investigations of the Ag₂WO₄-Mg-C system under the programmed linear heating rates up to 1200 °C min⁻¹, which are closer to the heating rates of reagents in the combustion wave at the synthesis of tungsten-based composite materials. The principle of operation of the device is based on the direct electric heating of a cell made of thin metal foil containing reactive powder. The required amounts of the reagents (Ag₂WO₄, Mg, C) were mixed in a ceramic mortar for ~15 min to obtain a homogeneous reactive mixture. The test sample (50 mg) was poured into a thin (~100 μm) nickel foil and wrapped, then placed in a HSTS reactor and fixed to massive electrical contacts. Before heating, the reactor was vacuumed and filled with 0.1 MPa of argon (99.8 % purity, oxygen content < 0.1 %, Airgas, Denver, CO, USA). The maximum heating temperature did not exceed 1300 °C. The interaction of the nickel foil with the reactive mixture under the studied temperature–time area was not observed.

In the case when the reaction did not occur (Al₂O₃ instead of reactive mixture), the inert experiment produced a linear temperature–time history, which determined the heating rate. Exothermic or endothermic reactions were shown by the deviation of the reactive profile from the inert one. The heating curves were used to estimate the reaction onset temperature (T_o), the maximum peak temperature recorded during the self-heating (T_max), and the temperature specified by linear heating, where the maximal exothermic effect was observed (T*). The HSTS system interrupted the process by automatically switching off the electrical current at several characteristic stages. Extremely rapid cooling (up to 12,000 °C min⁻¹) occurred at that point to exclude possible interactions at the quenching process.

The characterization of powders was performed by scanning electron microscope (SEM, Prisma E, Thermo Fisher Scientific, Hillsboro, OR, USA) with secondary electrons (SE) regime and an X-ray diffractometer (D8 Advance, Bruker, Billerica, MA, USA, with CuKα radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA at the 20–80° (2θ) angular range with an increment of 0.02° and a counting time of 2 s per step).

The calculation of the effective activation energies for the exothermic stages in the studied systems was performed by Kissinger’s isoconversional method [35] according to the following expression:

$$\ln \left(\frac{\beta }{{\left(T*\right)}^2}\right)=\ln A-\frac{E}{\mathrm{R}}\left(\frac{1}{T^{*}}\right)$$

(1)

The equation considers A as a pre-exponential factor, E—effective activation energy (kJ mol⁻¹), β—the heating rate (K min⁻¹), T*—temperature shift matching to the maximum advance in the heating curve at a fixed heating rate, R—universal gas constant (8.314 J mol⁻¹ K⁻¹).

Gibbs free energies of the reactions under study were calculated by using the HSC-10 software package (Version 10, Metso Outotec, Pori, Finland).

3. Results and Discussion

3.1. Ag₂WO₄–Mg System

Figure 2a shows the heating curve of the Ag₂WO₄ + 4Mg mixture (β = 300 °C min⁻¹), according to which the interaction proceeds in the form of a single-stage exothermic reaction, which starts at 710 °C covering the range up to 820 °C (T* = 722 °C, T_max = 1040 °C). It is remarkable that Mg melting was not observed.

In order to find out the mechanism of the interaction, the process was interrupted at different characteristic temperatures (A—670 °C, B—900 °C, C—1100 °C, D—1300 °C); the samples were cooled and subjected to X-ray diffraction (XRD) examinations (Figure 2b).

According to the results obtained, at 670 °C only the starting materials were present in the XRD pattern (Figure 2b, pattern A), which means that there was no interaction in the system before the melting point of Mg. After the exothermic effect (B—900 °C), the interrupted sample contained reduced Ag, W, as well as MgO and MgWO₄ (pattern B), which means that the magnesiothermic reduction of both metals occurred simultaneously. In this system, complete reduction was achieved only at 1300 °C (pattern D), as at 1100 °C the partially reduced W₁₈O₄₉ was also identified (pattern C).

The Ag₂WO₄ + Mg interaction during the heating can be represented in a complex scheme, Ag₂WO₄ + 2.5Mg = 2MgO + 2Ag + 0.5W + 0.5MgWO₄ (∆G° = –1012 kJ mol⁻¹), which is a summation of two parallel interactions: single replacement reaction: 0.5Ag₂WO₄ + 0.5Mg = Ag + 0.5MgWO₄ (∆G° = –289 kJ mol⁻¹) and the reduction stage: 0.5Ag₂WO₄ + 2Mg = 2MgO + Ag + 0.5W (∆G° = –724 kJ mol⁻¹). Hence, the silver tungstate was consumed in the first stage of interaction and then magnesium tungstate continued the interaction with Mg: 3.6MgWO₄ + Mg = 4.6MgO + 0.2W₁₈O₄₉ (∆G° = –268 kJ mol⁻¹). Next, the partially reduced oxide (W₁₈O₄₉) continued to be reduced with the rest of Mg up to tungsten: 0.055W₁₈O₄₉ + 2.7Mg = W + 2.7MgO (∆G° = –606 kJ mol⁻¹).

The samples quenched at 900 °C (sample B) and 1300 °C (sample D) were subjected to microstructural examinations.

As can be seen from Figure 3, immediately after the exothermic reaction (sample B), when, according to the XRD results, MgWO₄ predominated in the product, the powder was more micro-dispersed, and the grains were round, homogeneous and submicron-sized. At 1300 °C (sample D), the powder comprised particles of well-defined grain boundaries: there were both round and rectangular grains, all of which exceeded 2–3 μm size. In fact, as the temperature increased, that is, at the end of the reduction process, particles became larger, reaching a few micrometers.

Compared to low heating rates, β = 5−20 °C min⁻¹ [34], where the interaction starts with a solid + solid mechanism and ends with a liquid + liquid one, in this case, the process began with molten magnesium and silver tungstate.

As was foreseeable, with the increasing of the heating rate (from 100 to 1200 °C min⁻¹) the exothermic interaction shifted to a higher temperature area (Figure 4a). Based on the data of the maximum deviation temperatures of the exothermic effect at different heating rates, the Kissinger Equation (1) was used to calculate the effective activation energy value for the Ag₂WO₄ + 4Mg reaction: 119 ± 13 kJ mol⁻¹ (Figure 4b).

3.2. Ag₂WO₄–C System

Figure 5a shows the heating curve of the Ag₂WO₄ + 3C mixture (selected in accordance with the following scheme: Ag₂WO₄ + 3C = 2Ag + W + 2CO↑ + CO₂↑; β = 300 °C min⁻¹), according to which two weak exothermic reactions are detected within the 420–700 °C temperature range, as well as a weak endothermic effect at the 1100–1250 °C temperature domain.

In order to ascertain the sequence of reactions in the Ag₂WO₄ + 3C mixture, the samples quenched at different characteristic temperatures (A—420 °C, B—520 °C, C—700 °C, D—1050 °C, E—1150 °C, F—1300 °C) were subjected to XRD examinations (Figure 5b).

According to the results (Figure 5b), at 520 °C a reduced silver was detected along with Ag₂W₂O₇ (pattern B), whereas before that temperature there was no interaction (pattern A). At 700 °C, there was no raw salt (pattern C); instead of the latter, there was WO₃, which was reduced up to W₁₈O₄₉ (pattern D) at 1050 °C, and at 1150 °C—up to WO₂ (pattern E). At higher temperatures (1300 °C), the reduced tungsten interacted with excess carbon with the formation of W₂C (pattern F). Combining the results, it can be concluded that the weakly expressed exothermic effects at the 420–700 °C temperature domain corresponded to the partial reduction reactions of metals (Ag, W). It should be noted that at low heating rates, β = 5−20 °C min⁻¹ [34], the carbothermic reduction of the silver tungstate proceeded again in the form of weak exothermic sequential reactions in the temperature ranges of 460–535 °C and 540–610 °C, respectively, but the mass decrease in the mixture continued up to the studied temperature limit (1000 °C).

In this system, silver was reduced firstly, followed by a stepwise reduction of tungsten in accordance with this scheme: Ag₂WO₄ → WO₃ → W₁₈O₄₉. The main difference is that with the HSTS device it was possible to detect the transition from W₁₈O₄₉ to WO₂, and then the carbidization of the reduced metal at 1300 °C. In addition, it should be noted that the reduction of silver tungstate began without its preliminary decomposition.

Figure 6 shows the SEM images of the samples interrupted at different characteristic temperatures (B—520 °C, C—700 °C, D—1050 °C, F—1300 °C).

According to the XRD examinations, at point B, the sample is mainly composed of Ag₂W₂O₇ salt of triclinic structure (phase transition of Ag₂WO₄ into Ag₂W₂O₇ occurs), which, according to the SEM image (sample B), is a fine-grained foam-like powder. At 700 °C (sample C), the powder becomes finer, and at the same time, the foaming effect becomes more pronounced. At this point, Ag₂W₂O₇ turns into WO₃ (Figure 5b, pattern C). With the increasing of the temperature (1050 °C, sample D), rod-like particles appear, which are already absent at 1300 °C (sample F).

Combining the results with the XRD examinations (Figure 5b, pattern D), it could be affirmed that these rod-shaped species represented mainly W₁₈O₄₉. This sample also contained very small foam-granules, which represented tungsten. Among the images were round beads of silver (as a result of melting) with very expressed contours, which were also present in the previous system (Ag₂WO₄−Mg). In this case too, as the temperature increased the size of round-shaped grains increased up to ~2 μm.

3.3. Ag₂WO₄–Mg–C System

Figure 7a demonstrates the heating curve of the Ag₂WO₄ + 2Mg + 2C mixture (β = 300 °C min⁻¹). It testifies that in the temperature range of 450–670 °C there was a weak exothermic reaction effect, followed by a stronger one (720–840 °C, T* = 730 °C, T_max = 950 °C).

According to the results of XRD examinations (Figure 7b) of quenched samples at different characteristic temperatures (A—420 °C, B—670 °C, C—850 °C, D—1000 °C, E—1300 °C), at 420 °C there was no interaction in the system (pattern A), but at 670 °C the formation of Ag and WO₃ was registered, and MgO was not observed (pattern B). This suggests that reduction in this system starts with a weaker reducing agent (carbon), according to the following scheme: Ag₂WO₄ + C → Ag + WO₃ + CO₂↑ + CO↑. At this stage, the Ag₂WO₄ is completely consumed. Thus, the weak exothermic effect observed in the temperature range of 450–670 °C (Figure 7a) corresponded to the carbothermic reduction of Ag. The exothermic effect recorded at the 720–840 °C temperature domain corresponded to the magnesiothermic reduction of W by the following scheme: WO₃ + Mg → W + WO₂ + MgO, after which the quenched sample contained WO₃, WO₂, W, MgO and Ag (pattern C). Starting at 1000 °C, the formation of W₂C was observed (pattern D). It should be noted that in the case of magnesio-carbothermic reduction, there was no significant temperature shift for reduction reactions compared to the binary Ag₂WO₄ + 4Mg and Ag₂WO₄ + 3C interactions.

The studies have also been carried out for 1.5 mol ≤ C (carbon) ≤ 2 mol compositions, and according to the results, the Ag₂WO₄ + 2Mg + 1.7C mixture made it feasible to avoid the formation of tungsten carbide and to ensure a complete reduction in both metals at 1300 °C (Figure 8).

Figure 9 represents the SEM images of the B (670 °C), C (850 °C) and E (1300 °C) samples. The powder of the sample B was mainly composed of WO₃ (Figure 7b pattern B), which, like the previous system (Ag₂WO₄−C), had a fine-grained and foam-like appearance. Immediately after the exothermic reaction (magnesiothermic reduction), at 850 °C (sample C) the grains became more contoured, the foaming effect almost disappeared, and WO₂ was formed in the mixture. There were no rod-shaped grains in this system, and according to XRD patterns, W₁₈O₄₉ was not detected here, which once again proves the assumption that the elongated rod-type grains were W₁₈O₄₉. At 1300 °C (sample E), the product of the complete reduction reaction was a powder with a homogeneous microstructure with particles of 300–700 nm in size and spherical morphology.

As can be seen from Figure 10a, the exothermic interactions shifted to a higher temperature range when switching to a higher heating rate (from 100 to 1200 °C min⁻¹). Moreover, it is worthy to note that the carbothermic reduction stage became much more obvious in the heating curve at 1200 °C min⁻¹ (Figure 10a, curve A).

It should be noted that at low heating rates, β = 5−20 °C min⁻¹ [34], it was not possible to ensure the complete reduction of tungsten. This might be caused by the limited temperature range of the DTA/TG device used (1000 °C).

As in the previous case (Ag₂WO₄ + 4Mg reaction), here the effective activation energy value for the magnesiothermic reduction stage in Ag₂WO₄ + 2Mg + 2C mixture was calculated using the Kissinger method (Equation (1)). The E_a value made 77 ± 4 kJ mol⁻¹ (Figure 7b) for the magnesiothermic stage in the ternary system.

In comparison, at low heating rates, β = 5−20 °C min⁻¹ [34], in the 569–615 °C (stage I) and 600–671 °C (stage II) temperature domains the effective activation energy values for the Ag₂WO₄ + 4Mg reaction were 58 kJ mol⁻¹ and 36 kJ mol⁻¹, respectively, and for the magnesiothermic reduction stage in the Ag₂WO₄ + 2Mg + 2C mixture: 39 kJ mol⁻¹ (617–689 °C temperature range). The difference in the activation energy values for the magnesiothermic reduction stages for Ag₂WO₄ + 4Mg and Ag₂WO₄ + 2Mg + 2C mixtures at low and high heating rates is caused by the fact that at low heating rates, in contrast to high heating ones, the reactions take place with different pathways, which lead to variations in the effective activation energy value. All values of effective activation energies calculated in this work and reported in [34] are given in Table S1.

Thus, at all investigated heating rates, the reduction of Ag₂WO₄ begins with the weaker reducing agent-carbon. Moreover, first the silver is reduced, then the reduction of tungsten with magnesium or Mg/C begins. Reduction with carbon continues along the entire heating range.

4. Conclusions

The HSTS technique was used to successfully analyze the stepwise character of complex reactions in the multicomponent Ag₂WO₄-Mg-C system, revealing phase and microstructure evolution at high heating rates (up to 1200 °C min⁻¹) and determining effective activation energy values.

It was concluded that:

• In the Ag₂WO₄-Mg system, in contrast to low heating rates, β = 5−20 °C min⁻¹, there is no interaction before the melting point of Mg. The magnesiothermic reduction of both metals occurs simultaneously and a complete reduction is confirmed only at 1300 °C.
• In the Ag₂WO₄-C system, silver is reduced first, followed by a stepwise reduction of tungsten in accordance with this scheme: Ag₂WO₄ → WO₃ → W₁₈O₄₉ → WO₂ → W. These intermediate products are completely different by their microstructures. At 520 °C, Ag₂W₂O₇ is detected: the reduction of silver tungstate begins without its preliminary decomposition.
• The reduction in the Ag₂WO₄-Mg-C system starts with a weaker reducing agent: carbon. The reduction temperatures are commensurate with the temperatures of binary Ag₂WO₄ + 4Mg and Ag₂WO₄ + 3C interactions. The product of the complete reduction reaction is a powder with a round granular homogeneous microstructure with a particle size of 300−700 nm.
• The effective activation energy value calculated by the Kissinger method for the Ag₂WO₄ + 4Mg reaction is 119 ± 13 kJ mol⁻¹, and for the Ag₂WO₄ + 2Mg + 2C reaction—77 ± 4 kJ mol⁻¹. Thus, the addition of carbon facilitates the reduction reaction with decreasing the effective activation energy value.