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Knudsen, Cecilie Grønvaldt
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document
‘Need for Speed’: Sub-second in situ diffraction to unravel rapid sintering & texture evolution in ferrite magnets
Abstract
The magnetic behaviour of a permanent magnet emerges hierarchically over 6 orders of magnitude in length scales – from the atomic to the macro scale.[1] Intrinsic magnetic properties are determined by the underlying atomic & crystalline structure. Microstructural features like crystallite morphologies/sizes influence extrinsic magnetic behaviour.[2] The structure at the atomic- & nanoscale are effectively controlled by the material synthesis and numerous studies have focused on controlling these structures to tune magnetic properties.[3] However, for real-world applications, the nanoparticulate powders must be compacted. The crystallographic texture of the consolidated product is a key influencer on final performance. The proverbial “last mile” consolidation of loose nano powders with optimal magnetic properties to dense magnets is barely studied in literature and the effect of resultant texture on magnetic properties is even less understood. [2,4] Therefore, to truly optimize the magnetic performance of the final material, it is important to understand & control the structure at all hierarchical length scales.<br/>The consolidation of magnets is typically done either by mechanical compaction followed by sintering in conventional furnaces OR via hot compaction using complicated setups e.g., spark plasma sintering. The influence of the consolidation process on the final magnet’s structure and resultant properties has been conventionally studied in a ‘black-box’ manner. The structure of the final magnet is studied ex-situ, post-mortem via diffraction techniques and the impact on the magnetic performance is assessed.[4] Control over the composition & structure over multiple length scales is limited. Adding to it, consolidation affects the magnetic properties non-trivially, making optimization a tedious ‘trial-and-error’ process. A recently developed method – ultrafast high-temperature sintering (UHS) – reported by Wang et.al. (Science, May 2020) holds the potential to overcome these problems.[5] It operates on the principle of resistive Joule heating of carbon strips in an inert atmosphere to sinter compacted pellets. It allows for mechanical stability after fast sintering (~10s) at high temperatures (~3000ºC) with rapid heating rates (~104 ºC/min) and has been shown to successfully sinter ceramic materials with multiple phases while retaining stoichiometry and optimal grain sizes. However, knowledge of the structural evolution processes during sintering is still limited and the ‘black-box’ problem remains! <br/>To address this gap in knowledge, our group at Aarhus University has developed a custom-built furnace and sample environment - the Aarhus Rapid Ωhmic Sintering (AROS) sample environment - based on the UHS principle. The AROS setup combines the high temperature, rapid heating capabilities with the ability to use high-energy transmitted X-rays to probe the bulk structural evolution processes in situ & in real-time during sintering (Fig. 1). In this presentation, the capabilities of the AROS setup will be discussed. In August 2021, the AROS setup was tested at P02.1 to sinter pre-compacted pellets of iron oxide nanoparticles at elevated temperatures (900°C to 1200°C) in vacuum/inert atmosphere. The experiment investigated the formation of magnetic SrFe12O19 from non-magnetic anisotropic shaped nanocrystallites: FexOy (nano-shaped) + Sr2+ → SrFe12O19.[6] Continuously collected diffraction data at P02.1 provided information on phase transformation, crystallite growth & induced texture evolution processes in situ & in real-time. Results & insights from this test experiment probing the sintering process with unprecedented detail will also be presented. These investigations on the sintering process in situ provide crucial details towards plugging the “last mile” consolidation gap and enable the development of stronger permanent magnets in the future. Beyond this, the results help shed light on the dynamics of the densification/consolidation process with significant relevance to the science of materials sintering.<br/><br/>References:[1] (a) Leslie-Pelecky, D.L., & Rieke, R.D.,Chem. Mater. 8, 1770 (1996). (DOI:10.1021/cm960077f) (b) Skomski, R., J. Phys. Condens. Matter. 15, R841 (2003). (DOI:10.1088/0953-8984/15/20/202)<br/>[2] Sander, D. et.al., Phys. D. Appl. Phys. 50, 363001 (2017). (DOI:10.1088/1361-6463/aa81a1)<br/>[3] (a) Mohseni, F. et.al., J. Alloys Compd. 806, 120 (2019). (DOI:10.1016/j.jallcom.2019.07.162) (b) Volodchenkov, A.D. et.al., J. Mater. Sci. 54, 8276 (2019). (DOI:10.1007/s10853-019-03323-z) (c) Volodchenkov, A.D. et. al., J. Mater. Chem. C. 4, 5593 (2016). (DOI:10.1039/C6TC01300G) (d) Liu, F. et. al., Chem. Soc. Rev. 43, 8098 (2014). (DOI:10.1039/C4CS00162A)<br/>[4] (a) Skokov, K.P., & Gutfleisch, O.,Scr. Mater. 154, 289 (2018). (DOI:10.1016/j.scriptamat.2018.01.032) (b) Saura-Múzquiz, M. et.al., Nanoscale. 12, 9481 (2020). (DOI:10.1039/D0NR01728K...