Spark Plasma Sintering for synthesis of transition metal oxides

The solid-state synthesis of transition metal oxides (TMO’s) is a challenging task. Slow diffusion and mass transfer of reagents are characteristic of such solid-state reactions (SSR’s). In this context, spark plasma sintering (SPS) seems to emerge a promising and technologically applicable synthetic route to obtain TMO’s. We successfully conducted SSR of Ti2O3 synthesis using SPS with dc-current acting as an accelerator of the diffusion-controlled processes between TiO2 and Ti [1]. Further, this approach was applied to directly synthesize different TMO’s (titanium oxides [2], molybdenum oxides [3], tungsten oxides [4], and chromium oxides [5]). Among the advantages of such synthetic routes, we would like to stress: i) simple pre-experiment preparation (i.e., mixing of the initial powders); ii) simultaneous compaction and shaping of products; iii) short synthesis time (i.e., from minutes to about the few hours), iv) enormous accuracy (i.e., ≅ 0.1 at % of oxygen) as well as v) high degree of reproducibility.
New types of electrochemical using SPS SSR was recently performed in our laboratories using SPS. By sintering TiO2 (insulator) with WO2 (metal) mixed in different proportions, we obtained solid solution based on rutile (i.e., TiO2) structure. However, an appearance of elemental tungsten cannot be avoided while performing the synthesis with graphite foils as separators between the reacting mixture and the punches. To shed light on the mechanism of such an electrochemical process we performed two reactions, applying the opposite polarity of dc-current pulses, to the placed in graphite die layers of unmixed TiO2 and WO2. The further combined metallographic-EDX investigation of the polished cuts of the reacted specimens revealed that in the case when WO2 was under positive pole (i.e., being an anode) free W is forming at “+”-electrode, whereas the switch of the polarity results in the formation of tungsten inclusion on the phases border between reactants. Thus, elemental tungsten seems to be the product of electrochemical reduction of WO2. Avoiding this reaction, a single phase Ti1-xWxO2 is obtained replacing the graphite foils by tungsten ones.

[1] Veremchuk I., Antonyshyn I., Candolfi C., et al. Inorg. Chem. (2013) 52, 4458.
[2] Feng B., Martin H.‐P., Börner F.‐D., Veremchuk I., et al. Adv. Eng. Mat. (2014) 16, 1252.
[3] Kaiser F., Schmidt M., Grin Yu., Veremchuk I., Chem. of Mater. (2020) 32, 2025.
[4] Kaiser F., Simon P., Burkhardt U., Kieback B., Grin Yu., Veremchuk I. Crystals (2017)7, 271.
[5] Veremchuk I., et al. ACS Appl. Electron. Mater. (2022) 4, 2943.