Expanding the scope of multiphase-flash sintering: Multi-dogbone configurations and reactive processes

Ceramics International 50 (2024) 25210–25215 Available online 20 April 2024 0272-8842/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Expanding the scope of multiphase-flash sintering: Multi-dogbone configurations and reactive processes A.F. Manch´ on-Gord´ on a , * , S. Molina-Molina a , A. Perej´ on a , b , A. Alcalde-Conejo a , P.E. S´ anchez-Jim´ enez a , b , L.A. P´ erez-Maqueda a a Instituto de Ciencia de Materiales de Sevilla, ICMSE CSIC-Universidad de Sevilla, C. Am´ erico Vespucio 49, Sevilla, 41092, Spain b Departamento de Química Inorg´ anica, Facultad de Química, Universidad de Sevilla, 41012, Sevilla, Spain ARTICLE INFO Handling Editor: Dr P. Vincenzini Keywords: Flash sintering Multiphase-flash sintering Reaction flash sintering Yttria-stabilized zirconia Strontium ferrite ABSTRACT In this work, we have expanded the possibilities of the multiphase-flash sintering (MPFS) technique by investigating several configurations that involve multiple dogbone specimens (ranging from 1 to 3) and multiple phases (also ranging from 1 to 3). Unlike the traditional MPFS approach using complex 3D or cylindrical samples, this new method allows for a direct comparison with the established conventional flash sintering technique. Our experimental results with dense 8-mol% Yttria-stabilized zirconia demonstrate a significant reduction in the onset temperature as the number of phases and dogbones increases. Building on these findings, we achieved the preparation of pure bulk specimens of SrFe 12 O 19 for the first time through reactive multiphase-flash sintering. 1. Introduction The flash sintering (FS) technique has emerged as one of the most promising among the Field-Assisted Sintering Techniques (FAST) since its inception in 2010 [1]. This technique stands out for its environmental friendliness in comparison to conventional sintering methods, thanks to the significantly reduced time and temperature requirements [2–4]. What sets FS apart from other FAST methods is that the electric current flows exclusively through the green body throughout the process. This unique characteristic, in theory, should make FS more practical for real-world applications when compared to other FAST methods, where the electric current flows through both the sample and the graphite die [5]. Indeed, studies have demonstrated that FS can operate under continuous conditions using rolling electrodes [6]. This concept is currently being investigated on a larger scale by Lucideon in a pilot plant, where successful production of conventional ceramics, composite materials, and advanced ceramics for various technological sectors has been demonstrated [7]. However, there are limitations to upscaling FS, mainly due to the need for attaching electrodes to supply current to the workpiece and the formation of preferential current paths leading to "hot spots" [8]. Intensive research efforts have been focused on overcoming these limitations. For instance, it has been demonstrated that a flash event can be induced without the use of electrodes, a method referred to as "touch-free FS" [9], by superimposing magnetic fields onto the electrical field. Saunders et al. [10] employed arc plasma electrodes to establish a conductive medium for the current to pass through the pellet, achieving contactless FS. Additionally, Dong et al. [11] utilized dielectric barrier discharge atmospheric cold plasma to sinter zirconia samples, replacing metallic electrodes. Moreover, Johnson et al. [12] employed flames for both heating and as electrodes to prepare ceramic coatings on metallic substrates. A similar approach using flames was adopted by Lerdprom et al. [13] for glaze repair of sanitary ware bodies without the need for a kiln. Very recently, the touch-free technique was further developed to prepare a single-phase SrFe 12 O 19 compound from Fe 2 O 3 and SrCO 3 powders in a single step. This method involves flashing mixtures of powders of the constituent oxides and is known as the touch-free reactive flash sintering technique [14]. Another challenge in FS is the formation of the aforementioned hot spots, which is exacerbated by the limited variety of specimen shapes typically used [2]. In this regard, FS has been expanded to accommodate complex 3D-shaped specimens. This was achieved by employing a multiphase power supply and multiple electrodes, which is expected to prevent the formation of preferred current paths [15]. In this study, we have delved into the application of the multiphaseflash sintering technique to dogbone-shaped specimens made of 8 % mol Yttria-stabilized zirconia (8YSZ). The choice of this particular material * Corresponding author. E-mail address: [email protected] (A.F. Manch´ on-Gord´ on). Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint https://doi.org/10.1016/j.ceramint.2024.04.250 Received 1 March 2024; Received in revised form 5 April 2024; Accepted 19 April 2024 Ceramics International 50 (2024) 25210–25215 25211 for our experiments is rooted in the extensive analysis of its sintering behavior, making it a well-understood standard for studying flash sintering phenomena [16–19]. Moreover, the electrode effects usually described during FS of 8YSZ under DC fields [20] can be avoided in multiphase configurations, as they involve alternating fields. Despite the fact that dogbone-shaped specimens may not be the ideal choice for the technological implementation of FS, at an experimental level their ease of use and well-established procedures make them an attractive option for researchers in the field. Furthermore, employing these specimens allows for direct comparisons with previously published results, enhancing the reliability of our findings. Our results clearly demonstrate that the onset temperature can be significantly reduced with the right configuration of phases and dogbone specimens in multiphase-flash sintering. This approach has been further expanded to produce SrFe 12 O 19 from Fe 2 O 3 and SrCO 3 powders in a single step. To the best of our knowledge, this is the first instance where reactive flash sintering has been applied using multiphase configurations, showcasing the innovative potential of this technique. 2. Experimental 2.1. Materials and characterization Dogbone specimens were crafted from commercial powders of 8 mol % Yttria-stabilized zirconia (8YSZ) obtained from Tosoh Corporation (TZ-8Y). To create the pellets, the powder was blended with approximately 5 % w/w of a 4 % w/w PVA (Polyvinyl Alcohol) solution in distilled water (procured from Sigma-Aldrich, with a purity ranging from 98.0 % to 98.8 %). About 1 g of the precursor powder was uniaxially pressed into dogbone-shaped specimens using a pressure of 500 MPa for 3 min. Following this, the resultant dogbone specimens underwent a preheating process at 500 ◦C for 0.5 h in an air atmosphere to remove the binder, with a linear heating rate of 3 ◦C min −1 . Subsequently, the dogbones were sintered before the flash experiments at 1400 ◦C for 1 h, employing heating and cooling rates of 10 ◦C min −1 . This resulted in relative densities of 95 %, as determined using Archimedes’ principle. The use of already dense specimens is intended to prevent microstructural changes associated to sintering, which can alter the electrical conductivity and complicate the study. For the dense dogbones, the thickness and width are approximately 1.51 mm and 3.12 mm, respectively, with a length between the holes of the ears of 16.2 mm. The synthesis of the SrFe 12 O 19 compound involved the use of commercially available Fe 2 O 3 powder (procured from Sigma-Aldrich, with a purity of ≥99 %) and SrCO 3 powder (also from Sigma-Aldrich, with a purity of ≥98 %). To ensure the removal of any moisture, the precursor powders were dried in a warming oven and subsequently mixed for 20 min using an Emax ball mill from Retsch. Hardened steel balls and vials were employed as the milling media. For the milling process, the initial powder mass used was 10 g, maintaining a ball-to- powder ratio of 10:1. The vials were rotated at a speed of 500 rpm. To prevent the vials from overheating, a cycling approach was adopted, involving 5 min of milling followed by 1 min of rest. For the green body dogbones, the thickness and width are approximately 1.82 mm and 3.6 mm, respectively, with a length between the holes of the ears of 20.0 mm. The SrFe 12 O 19 phase was characterized through X-ray diffraction analysis conducted at room temperature. A Rigaku MiniFlex diffractometer was utilized, and Cu-K α radiation was employed as the X-ray source. 2.2. Experimental setup The experimental setup used in this study closely resembles the configurations typically employed in conventional flash experiments. It includes essential components such as a furnace, camera, computer, power supply, and power analyzer. However, the primary distinction lies in the type of power supply utilized. Conventional FS makes use of single-phase electric power in a two-electrode configuration where the applied voltage is continuous (DC-FS) or a sole sine wave oscillating at a fixed frequency (AC-FS), resulting in unidirectional current flow. In this work, a 3-phase AC voltage source is employed. The dogbone-shaped specimens are connected to the power supply using the two 3-phase fundamental wiring configurations: Delta and Star, which are illustrated schematically in Fig. 1. In both setups, the waveforms are phase-shifted by 120◦and the dogbone specimens are attached by the ends of NiCr wires. In the Star configuration (Fig. 1a), the individual dogbone specimens are connected to a common point where the neutral wire is attached. In other words, the Star configuration functions as a 3-phase 4-wire system. Conversely, the Delta configuration (Fig. 1b) employs an equal number of wires and phases, i. e. a 3-phase 3-wire system, and these wires are positioned at the holes of the ears of the dogbone specimens. A 2-phase 3-wire system was additionally tested (Fig. 1c), with the neutral wire taken from the junction between individual dogbones whose ends are attached to the live wires. A particular case of 2-phase configuration where a single dogbone is placed between the two live wires was evaluated as well (Fig. 1d). In both cases, various phase shifts were examined. The specimens were connected to a 3-phase AC power supply (AST1503A1 AMETEK Asterion, US) according to the different multiphase configurations described above. Platinum paste was used to improve electrical contact between the sample and the NiCr wires employed as electrodes. A tubular furnace, regulated by a Eurotherm 3216 PID controller, was used to heat the samples at a constant rate of 5 ◦C min −1 . Electric fields were applied by utilizing individual AC voltage signals with the same amplitude and frequency (50 Hz). The phase of these signals was shifted depending on the configuration used in the experiment. The power supply operated in a voltage-controlled mode until the critical flash event occurred. At this point, the power supply automatically switched to a current-controlled mode to counteract the abrupt, non-linear increase in conductivity. The flash event was maintained for a specific time interval before the power supply was deactivated. Throughout the experiments, various parameters, including applied electric field, current density, resistance, and power density, were continuously monitored and recorded using a 3-phase power analyzer (PPA1500 Newtons 4th Ltd, UK). The provided electric field and current density correspond to RMS values, establishing the equivalence between the applied AC voltages employed in this work and the DC voltages used elsewhere. The material temperature was monitored using an infrared camera PI 1M, Optris GmbH, Germany) calibrated with a selected emissivity of 0.9. 3. Results and discussion Flash experiments were conducted on dense dogbone pellets made of 8YSZ, utilizing different combinations of the number of dogbones and phases as explained in the experimental section. The experiments followed the conventional flash sintering method, employing a controlled temperature ramp at 5 ◦C min −1 and consistent electrical parameters for each phase involved in the study: an applied voltage of 62 V cm −1 , a maximum current density of 107 mA mm −2 , and a frequency of 50 Hz. When only one phase was used, even if multiple dogbones were employed, the approach was considered conventional AC flash sintering (AC voltage is applied), denoted as AC-FS. In contrast, when multiple phases were utilized, it was referred to as multiphase-flash sintering, or MPFS. Fig. 2 illustrates the Arrhenius total power density profile plots for dense 8YSZ dogbones, employing various configurations of AC-FS and MPFS methodologies. The power profiles exhibited the anticipated behavior in both scenarios. They displayed a linear rise in power density until the flash initiation point was reached, which was then succeeded by a rapid power surge. An important differentiation between MPFS and traditional AC-FS lies in the fact that, under equivalent applied voltage A.F. Manch´ on-Gord´ on et al. Ceramics International 50 (2024) 25210–25215 25212 and at a specific temperature, MPFS delivers substantially greater power to the material. Consequently, the flash event occurs at substantially lower temperatures. The onset temperatures for the flash events in different combinations of dogbones and phases are compiled in Table 1. Notably, a greater number of phases corresponded to lower flash onset temperatures, even for the same phase-to-dogbone ratio. Indeed, the 3-phase 3-dogbone systems provided the highest delivered power and reduction in onset temperature. Both Star and Delta configurations exhibited the same flash onset temperature. However, in the Delta configuration, once the flash initiated, the system became unbalanced, losing the 120◦phase-shift between phases. This led to significant disparities in the current densities flowing through each specimen, resulting in uneven heating (for this reason, this result is not included in Fig. 2). The presence of a neutral wire ensured that similar current densities passed through the three dogbones in the Star configuration. This effect represents a distinct contrast to what was observed in MPFS when using 3-D complex specimens for flashing. In that scenario, it was evident that the Delta configuration resulted in a greater microstructural homogeneity of the sintered specimen when compared to the Star configuration [15]. The role of the phase-shift is also noteworthy in 2-phase systems. In a 2-phase 2-dogbone configuration with a common neutral wire, a 180◦ phase-shift resulted in a slight increase in the onset temperature compared to a 90◦phase-shift (due to the slight difference between both experiments, it has been only shown that corresponding to the 180◦ phase-shift for a better observation of the figure). When the two voltage signals are 180◦apart, they both reach 0 V simultaneously, whereas a Fig. 1. Experimental setup and sample connections for MPFS configurations of (a) 3-phase 3-dogbone (Star), (b) 3-phase 3-dogbone (Delta), (c) 2-phase 2-dogbone and (d) 2-phase 1-dogbone. Infrared thermographic images of the specimens for each configuration have also been included. Fig. 2. Power density as a function of the inverse of temperature during temperature ramping flash experiments of dense 8YSZ using different combinations of dogbones and phases in conventional single-phase flash (AC-FS) and MPFS configurations. Table 1 Onset temperature for both processes studied in this work (flash sintering of 8YSZ and reaction flash sintering of Fe 2 O 3 and SrCO 3 mixed powders) under different combinations of dogbones and number of phases. Material Configuration Dogbones Phases φ (◦) Tonset (◦C) 8YSZ AC-FS 1 1 – 679 2 1 – 819 MPFS 1 2 90 612 180 563 2 2 90 624 180 632 3 3 120 (STAR) 556 120 (DELTA) 556 Fe 2 O 3 +SrCO 3 AC-RFS 1 1 – 762 MPRFS 2 2 180 740 3 3 120 (DELTA) 686 A.F. Manch´ on-Gord´ on et al. Ceramics International 50 (2024) 25210–25215 25213 90◦phase-shift provides a consistent power delivery, which is preferred in a flash sintering process. However, this effect is not as significant in a 2-phase 1-dogbone configuration. Removing the neutral wire and connecting the sample directly between the two phases greatly reduces the onset temperature because the phase-to-phase voltage is higher than the phase-to-neutral voltage, especially when maximized for a 180◦phaseshift. Even in the case of a single dogbone, the substantial decrease in onset temperature provides further evidence of the potential and superiority of MPFS over conventional AC-FS. In that approach, the addition of more dogbones (specifically, in a 1-phase 2-dogbone system) evidently raises the onset temperature owing to the increased resistivity of the system. Fig. 3 shows the changes in electric field and current density with respect to the furnace temperature during a MPFS experiment of 8YSZ, specifically employing the 2-phase 2-dogbone configuration as an example. The typical three stages of the flash sintering process are prominently evident. The current density sharply increases, reaching a value of 107 mA mm −2 at around 645 ◦C. During this phase, the sample is held in a flash-activated state under controlled current for several seconds. Notably, the flash event commences simultaneously in both phases utilized in the experiment. As mentioned earlier, under equivalent applied voltage and at a specific temperature, MPFS delivers significantly higher power to the material compared to conventional ACFS experiments. While the impact of increasing the number of phases is evident, the reduction in onset temperature may also be influenced by other factors, such as the heat radiated by each dogbone relative to the others. Although this effect could potentially affect the initiation of the flash event, it becomes negligible towards the end of the flash event, as all specimens reach the same maximum temperature, determined by the power density. The primary advantage of FS, when compared to conventional techniques, lies in the significantly reduced processing times and temperatures. Furthermore, recent advancements have demonstrated the capability to simultaneously initiate reactions and densify compounds using a mixture of precursor oxide powders in their green state. This innovative approach is referred to as reaction flash sintering [21] and has been applied to prepare various materials [22]. The latest endeavors in RFS have focused on producing a broad spectrum of different materials such as ferrites [23,24], oxide compounds [25], solid electrolytes [26] and high-entropy oxides [27,28]. Our study also explores the potential expansion of the MPFS technique to encompass RFS. We have employed both conventional and multiphase-reactive flash sintering methodologies to investigate the production of strontium ferrite from a mixture of commercially available SrCO 3 and Fe 2 O 3 powders. It is worth noting that this compound has previously been prepared using the conventional reactive flash sintering technique, DC-RFS [29], which enables direct comparisons with samples prepared through multiphase-reactive flash sintering (MPRFS) in this study, as the experimental conditions pertaining to applied electric field and current density limits are well-established (40 Vcm −1 and 100 mA mm −2 ) [29]. Indeed, it is widely recognized that the careful selection of experimental parameters is essential to achieve sample homogeneity [30]. Fig. 4 displays the Arrhenius total power density profiles for reaction flash sintering of dogbones containing a mixture of SrCO 3 and Fe 2 O 3 powders. These profiles were generated using different configurations of AC-reactive flash sintering (AC-RFS) and multiphase-reactive flash sintering (MP-RFS) methodologies. The observed behavior aligns with that seen in AC-FS and MPFS techniques (refer to Fig. 2) for 8YSZ: a higher number of phases corresponded to lower flash onset temperatures (as indicated in Table 1). To assess the capacity of these techniques to produce single-phase SrFe 12 O 19 , ex-situ X-ray diffraction (XRD) patterns were registered at room temperature, as shown in Fig. 5. The XRD analysis of the as-milled powders indicates a mixture of the initial materials, with all diffraction peaks corresponding to Fe 2 O 3 and SrCO 3 phases. Upon flash sintering, the diffraction peaks from the asmilled powders are replaced by characteristic XRD peaks attributed to the SrFe 12 O 19 phase, which is characterized by a hexagonal crystal lattice structure with the P63/mmc space group [29]. Fig. 3. Evolution of the electric parameters a) applied field and b) current density as function of temperature in the case of the 2-phase 2-dogbone experiment with φ =90 ◦. Inset included in panel b) presents an infrared thermographic image of the dogbone specimens during the flash event. Fig. 4. Power density as a function of the inverse of temperature during temperature ramping experiments using conventional flash and different combinations of dogbones and phases for the production of SrFe 12 O 19 by reaction flash sintering. A.F. Manch´ on-Gord´ on et al. Ceramics International 50 (2024) 25210–25215 25214 The synthesis of a single-phase SrFe 12 O 19 compound via conventional DC reaction flash sintering [29] and touch-free reaction flash sintering [14] has been previously documented by some of the authors. In these studies, the pellet density was approximately 90 %, achieved using DC fields. The density of the prepared samples was estimated from Archimedes method, obtaining a relative density around 85 % using 5.1 g cm −3 as the theoretical density value of SrFe 12 O 19 [31]. Future investigations will focus on analyzing the impact of AC fields on relative density and grain size. It has been demonstrated that AC fields [32,33] can effectively address issues encountered in bodies sintered via flash sintering under DC fields, such as non-uniform microstructure [34,35] and blackening due to reduction [36]. 4. Conclusions This study represents a significant advancement in the multiphaseflash sintering technique on two fronts. Firstly, it illustrates the adaptability of this technique to various configurations, including multiple dogbones and phases. We observed that increasing the number of phases led to lower flash onset temperatures, even when maintaining the same phase-to-dogbone ratio. Consequently, selecting the appropriate phasedogbone configuration can optimize the flash sintering process in terms of temperature and material quantity produced in a single experiment. Secondly, we integrated the flash reaction method into multiphase-flash sintering, exemplified by synthesizing a single-phase SrFe 12 O 19 compound from a mixture of SrCO 3 and Fe 2 O 3 powders. Future studies are encouraged to analyse the impact of multiphase electric fields on the microstructure of the prepared ceramics, which will further demonstrate the potential of the proposed technique. CRediT authorship contribution statement A.F. Manch´ on-Gord´ on: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Validation, Writing – original draft. S. Molina-Molina: Conceptualization, Data curation, Investigation, Writing – original draft, Validation, Visualization. A. Perej´ on: Supervision, Writing – review & editing. A. Alcalde-Conejo: Data curation. P. E. S´ anchez-Jim´ enez: Writing – review & editing. L.A. P´ erez-Maqueda: Methodology, Project administration, Resources, Writing – review & editing, Supervision. Declaration of generative AI and AI-assisted technologies in the writing process During the preparation of this work the authors used ChatGPT- OpenAI in order to improve the language. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research has received funding from the Junta de Andalucía- Consejería de Universidad, Investigaci´ on e Innovaci´ on under the ProyExcel_00360 project. Financial support is also acknowledged from the grant TED2021- 131839B–C22 funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR, and the grant PID2022- 140815OB-C22 funded by MCIN/AEI/10.13039/501100011033 and ERDF A way of making Europe. References [1] M. Cologna, B. Rashkova, R. Raj, Flash sintering of Nanograin zirconia in <5 s at 850◦C, J. Am. Ceram. Soc. 93 (11) (2010) 3556–3559. [2] M. Biesuz, V.M. Sglavo, Flash sintering of ceramics, J. Eur. Ceram. Soc. 39 (2) (2019) 115–143. [3] E. Gil-Gonz´ alez, L.A. P´ erez-Maqueda, P.E. S´ anchez-Jim´ enez, A. Perej´ on, Flash sintering research perspective: a bibliometric analysis, Materials 15 (2) (2022). [4] A.F. Manch´ on-Gord´ on, A. Perej´ on, E. Gil-Gonz´ alez, M. Kowalczyk, P.E. S´ anchez- Jim´ enez, L.A. P´ erez-Maqueda, Low temperature magnetic transition of BiFeO3 ceramics sintered by electric field-assisted methods: flash and spark plasma sintering, Materials 16 (2023) 189. https://doi.org/10.3390/ma16010189. [5] C. Wang, W. Ping, Q. Bai, H. Cui, R. Hensleigh, R. Wang, A.H. Brozena, Z. Xu, J. Dai, Y. Pei, C. Zheng, G. Pastel, J. Gao, X. Wang, H. Wang, J.-C. Zhao, B. Yang, X. Zheng, J. Luo, Y. Mo, B. Dunn, L. Hu, A general method to synthesize and sinter bulk ceramics in seconds, Science 368 (6490) (2020) 521–526. [6] E. Sortino, J.-M. Lebrun, A. Sansone, R. Raj, Continuous flash sintering, J. Am. Ceram. Soc. 101 (4) (2018) 1432–1440. [7] https://www.lucideon.com/technologies/flash-sintering. [8] J.-M. Chaix, R. Bouchet, D. Bouvard, T. Fabre, T. Garnault, C. Harnois, K. Koutoati, M. Lachal, S. Marinel, M.C. Steil, A viewpoint on hot spots in microwave sintering and flash sintering, Adv. Eng. Mater. 25 (18) (2023) 2201742. [9] S.I.A. Jalali, R. Raj, Touch-free flash sintering with magnetic induction within a reactor activated by the usual flash method, J. Am. Ceram. Soc. 105 (11) (2022) 6517–6522. [10] T. Saunders, S. Grasso, M.J. Reece, Ultrafast-contactless flash sintering using plasma electrodes, Sci. Rep. 6 (1) (2016) 27222. [11] J. Dong, Z. Wang, X. Zhao, M. Biesuz, T. Saunders, Z. Zhang, C. Hu, S. Grasso, Contactless flash sintering based on cold plasma, Scripta Mater. 175 (2020) 20–23. [12] S.L. Johnson, G. Venugopal, A.T. Hunt, Flame-assisted flash sintering: a noncontact method to flash sinter coatings on conductive substrates, J. Am. Ceram. Soc. 101 (2) (2018) 536–541. [13] W. Lerdprom, C. Green, D. Pearmain, Contactless field enhanced sintering (cFES): a spot glaze repair technology (SGRT), J. Eur. Ceram. Soc. 42 (5) (2022) 2556–2560. [14] S.I.A. Jalali, A.F. Manch´ on-Gord´ on, R. Chacartegui, P.E. S´ anchez-Jim´ enez, J. S. Bl´ azquez, A. Perej´ on, R. Raj, L.A. P´ erez-Maqueda, Touch-free reactive flash sintering of dense strontium hexaferrite permanent magnet, J. Am. Ceram. Soc. 106 (12) (2023) 7202–7208. [15] S. Molina-Molina, E. Gil-Gonz´ alez, F.J. Dur´ an-Olivencia, J.M. Valverde, A. Perej´ on, P.E. S´ anchez-Jim´ enez, L.A. P´ erez-Maqueda, A novel Multi-Phase Flash Sintering (MPFS) technique for 3D complex-shaped ceramics, Appl. Mater. Today 26 (2022) 101274. [16] S. Molina-Molina, A. Perej´ on, L.A. P´ erez-Maqueda, P.E. S´ anchez-Jim´ enez, Influence of AC fields and electrical conduction mechanisms on the flash-onset temperature: electronic (BiFeO3) vs. ionic conductors (8YSZ), Ceram. Int. 49 (9) (2023) 14834–14843. Part B. [17] Y. Du, A.J. Stevenson, D. Vernat, M. Diaz, D. Marinha, Estimating Joule heating and ionic conductivity during flash sintering of 8YSZ, J. Eur. Ceram. Soc. 36 (3) (2016) 749–759. [18] W. Xu, A. Maksymenko, S. Hasan, J.J. Mel´ endez, E. Olevsky, Effect of external electric field on diffusivity and flash sintering of 8YSZ: a molecular dynamics study, Acta Mater. 206 (2021) 116596. Fig. 5. X-ray diffraction patterns recorded at room temperature of the as-milled powder and those obtained after the indicated reaction flash experiments. Main reflection peaks of hematite (crosses) and SrCO 3 (circles) have been indicated in the figure. A.F. Manch´ on-Gord´ on et al. Ceramics International 50 (2024) 25210–25215 25215 [19] S. Wang, T.P. Mishra, Y. Deng, A. Kaletsch, M. Bram, C. Broeckmann, Experimental and numerical studies of densification and grain growth of 8YSZ during flash sintering, Adv. Eng. Mater. 25 (18) (2023) 2201744. [20] W. Qin, H. Majidi, J. Yun, K. van Benthem, Electrode effects on microstructure formation during FLASH sintering of yttrium-stabilized zirconia, J. Am. Ceram. Soc. 99 (7) (2016) 2253–2259. [21] E. Gil-Gonz´ alez, A. Perej´ on, P.E. S´ anchez-Jim´ enez, M.J. Sayagu´ es, R. Raj, L. A. P´ erez-Maqueda, Phase-pure BiFeO3 produced by reaction flash-sintering of Bi2O3 and Fe2O3, J. Mater. Chem. A 6 (13) (2018) 5356–5366. [22] B. Yoon, V. Avila, I.R. Lavagnini, J.V. Campos, L.M. Jesus, Reactive flash sintering of ceramics: a review, Adv. Eng. Mater. 25 (5) (2023) 2200731. [23] E. Gil-Gonz´ alez, A. Perej´ on, P.E. S´ anchez-Jim´ enez, D. Rom´ an-Gonz´ alez, L.A. P´ erez- Maqueda, Control of experimental conditions in reaction flash-sintering of complex stoichiometry ceramics, Ceram. Int. 46 (18) (2020) 29413–29420. [24] A.F. Manch´ on-Gord´ on, P.E. S´ anchez-Jim´ enez, J.S. Bl´ azquez, A. Perej´ on, L.A. P´ erez- Maqueda, Structural, vibrational, and magnetic characterization of orthoferrite LaFeO3 ceramic prepared by reaction flash sintering, Materials 16 (2023) 1019. https://doi.org/10.3390/ma16031019. [25] Y. Wu, X. Su, G. An, W. Hong, Dense Na0. 5K0. 5NbO3 ceramics produced by reactive flash sintering of NaNbO3-KNbO3 mixed powders, Scripta Mater. 174 (2020) 49–52. [26] V. Avila, B. Yoon, R.R.I. Neto, R.S. Silva, S. Ghose, R. Raj, L.M. Jesus, Reactive flash sintering of the complex oxide Li0. 5La0. 5TiO3 starting from an amorphous precursor powder, Scripta Mater. 176 (2020) 78–82. [27] H.-R. Mao, R.-F. Guo, Y. Cao, S.-B. Jin, X.-M. Qiu, P. Shen, Ultrafast densification of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 by reactive flash sintering, J. Eur. Ceram. Soc. 41 (4) (2021) 2855–2860. [28] K. Wang, B. Ma, T. Li, C. Xie, Z. Sun, D. Liu, J. Liu, L. An, Fabrication of highentropy perovskite oxide by reactive flash sintering, Ceram. Int. 46 (11) (2020) 18358–18361. [29] A.F. Manch´ on-Gord´ on, P.E. S´ anchez-Jim´ enez, J. Bl´ azquez, A. Perej´ on, L.A. P´ erez- Maqueda, Reactive flash sintering of SrFe12O19 ceramic permanent magnets, J. Alloys Compd. 922 (2022) 166203. [30] E. Gil-Gonz´ alez, A. Perej´ on, P.E. S´ anchez-Jim´ enez, D. Rom´ an-Gonz´ alez, L.A. P´ erez- Maqueda, Control of experimental conditions in reaction flash-sintering of complex stoichiometry ceramics, Ceram. Int. 46 (18, Part B) (2020) 29413–29420. [31] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (7) (2012) 1191–1334. [32] X. Su, Y. Jia, C. Han, Y. Hu, Z. Fu, K. Liu, Y. Yu, X. Yan, Y. Wang, Flash sintering of lead zirconate titanate ceramics under an alternating current electrical field, Ceram. Int. 45 (4) (2019) 5168–5173. [33] J. Liu, X. Li, X. Wang, R. Huang, Z. Jia, Alternating current field flash sintering 99% relative density ZnO ceramics at room temperature, Scripta Mater. 176 (2020) 28–31. [34] I. Bajpai, Y.-H. Han, J. Yun, J. Francis, S. Kim, R. Raj, Preliminary investigation of hydroxyapatite microstructures prepared by flash sintering, Adv. Appl. Ceram. 115 (5) (2016) 276–281. [35] M. Yoshida, S. Falco, R.I. Todd, Measurement and modelling of electrical resistivity by four-terminal method during flash sintering of 3YSZ, J. Ceram. Soc. Jpn. 126 (7) (2018) 579–590. [36] M. Biesuz, L. Pinter, T. Saunders, M. Reece, J. Binner, V.M. Sglavo, S. Grasso, Investigation of electrochemical, optical and thermal effects during flash sintering of 8YSZ, Materials 11 (7) (2018) 1214. A.F. Manch´ on-Gord´ on et al.