Touch-free reactive flash sintering of dense strontium hexaferrite permanent magnet

Received: 29 May 2023 Revised: 17 July 2023 Accepted: 21 July 2023 DOI: 10.1111/jace.19389 RAPID COMMUNICATION Touch-free reactive flash sintering of dense strontium hexaferrite permanent magnet Syed I. A. Jalali1Alejandro F. Manchón-Gordón2Ricardo Chacartegui3 Pedro E. Sánchez-Jiménez2,4Javier S. Blázquez5Antonio Perejón2,4 Rishi Raj1Luis A. Pérez-Maqueda2 1Department of Mechanical Engineering, University of Colorado Boulder, Boulder, Colorado, USA 2Instituto de Ciencia de Materiales de Sevilla, ICMSE CSIC, Universidad de Sevilla, Sevilla, Spain 3Dpto. Ingeniería Energética, Universidad de Sevilla, Seville, Spain 4Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Sevilla, Spain 5Dpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, Sevilla, Spain Correspondence Luis A. Pérez-Maqueda, Instituto de Ciencia de Materiales de Sevilla, ICMSE CSIC-Universidad de Sevilla, C. Américo Vespucio 49, Sevilla 41092, Spain. Email: [email protected] Funding information EU Next Generation; Spanish Ministry of Science and Innovation, Grant/Award Numbers: TED2021-131839B-C22, PDC2021-121552-C21, PID2022-140815OB-C22; Junta de Andalucía-Consejería de Transformación Económica, Industria, Conocimiento y Universidades, Grant/Award Number: ProyExcel_00360; Office of Naval Research, Grant/Award Number: N00014-18-1-2270; Fulbright Association, Grant/Award Number: PRX21/00434 Abstract This work presents an extension of the touch-free flash sintering technique. In the proposed technique, chemical reaction and sintering occur in a single step, without the use of electrodes, in the presence of electric and magnetic fields. We show that a dense, single-phase strontium hexaferrite magnet can be produced from a mixture of commercial carbonate and oxide powders in a single step in a little more than a minute. This new technique implies significant reduction in energy and time consumption (primarily because of ultrafast processing) relative to conventional sintering. KEYWORDS permanent magnets, reactive flash sintering, strontium ferrite, touch-free flash sintering 1 INTRODUCTION The flash sintering (FS) technique, first proposed in 2010,1 has drawn attention due to its environmental-friendly character, low sintering furnace temperatures, and short dwell times as compared to conventional sintering techniques.2–5 However, a real application of this technique is limited by the necessity of attaching electrodes that supply current to the workpiece. Since densification This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2023 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society. is related to the current density,6free form sintering would require a system of electrodes that maintain spatially uniform current in the workpiece. Recently, it has been shown that multiple electrodes can be used to sinter objects of an arbitrary shape.7Very recently, it has been shown that flash can be induced without the use of electrodes, called touch-free FS,8by the superposition of magnetic fields to the electrical field. Free form objects made from three yttria-stabilized zirconia (YSZ) were 7202 wileyonlinelibrary.com/journal/jace J Am Ceram Soc. 2023;106:7202–7208. JALALI et al. 7203 sintered in this way. In a different approach, Saunders et al.9used arc plasma electrodes for creating a conductive media for the current to pass to the pellet, achieving contactless FS. This method was used for sintering B4C and SiC:B4C materials. Moreover, Dong et al.10 used a dielectric barrier discharge atmospheric cold plasma to sinter zirconia samples replacing the metallic electrodes. Besides, Johnson et al.11 used flames both for heating and as electrode to prepare ceramic coatings on metallic substrates. A similar approach using flames was used by Lerdprom et al.12 for glaze repair of sanitaryware bodies without the need for a kiln. Separately, we have shown that chemical reaction and sintering in multicomponent ceramics can occur in a single step by flashing mixtures of powders of constituent oxides: so-called reactive flash sintering (RFS).13 Since then, this technique has been applied to process ferrites,14,15 ferroelectrics,16,17 high-entropy oxides,18–21 and Li-ion ceramic electrolytes.22,23 Here, we explore the application of the touch-free FS technique to RFS (TF-RFS), that is, preparation of multicomponent oxides by flash without the use of electrodes. We demonstrate thisresult to fabricate single-phase SrFe12O19 magnet from Fe2O3and SrCO3powders. Strontium ferrite is considered a promising substitute for rare earth-based permanent magnets; it has high coercivity and good resistance to corrosion.24 2EXPERIMENTAL SrFe12O19 compound was synthesized from commercially available Fe2O3and SrCO3powders (Sigma–Aldrich, ≥99% and ≥98% purity, and ≤5and∼1μm particle size, respectively). The precursor powders were dried in a warming oven to remove any moisture and mixed for 20 min in an Emax (Retsch) ball mill. Hardened steel balls and vials were employed as milling media. Powder mixtures were compressed into small pellets in a die at a pressure of 35 MPa. The dimensions of the green cylindrical pellet were ∼6mmindiameterand2mm in thickness. The pellets were placed within the reactor, and then flashed to full density by the application of magnetic field. The reactor consisted of a pre-sintered dog-bone specimen made from 8 mol% yttria-doped zirconia powder (Tosoh Corp.). The system configuration is shown in Figure 1a. The green sample pellet, that is, the workpiece, was held about 2 mm above the dog-bone using Kanthal wires that were electrically insulated from the electrical lines of the dog bones and the induction coil. The induction coil consisted of Kanthal wire with a diameter of 1.8 mm wrapped into 30 turns over a linear distance of 150 mm. The reactor and coil assembly were placed on a linear motion assembly to be fed into the hot zone of a furnace heated up to 900◦Catarateof10–25 ◦C/min. The electrical circuitry of the touch-free sintering apparatus used here consisted of two power supplies, one, a DC power supply to instigate flash in the dog-bone, and the other, the AC line voltage at 60 Hz, attenuated through a variac transformer, to power the induction coil. The flash in the zirconia dog-bones was initiated by applying a DC field of100V/cm whileholdingthe furnaceat 900◦C.Flash is signaled by a sudden rise in conductivity. The maximum current flowing through the dog-bone was limited to a current density of 200 mA/mm2. The current in the induction coil was increased from 0 to 50 A (AC) by turning the knob of the variac transformer. This process takes about 2–3 s. The SrFe12O19 phase was characterized by X-ray diffraction using a Rigaku MiniFlex diffractometer (Cu-Kαradia- tion) at room temperature. Le Bail fitting was performed using Topas Software. Scanning electron microscopy (SEM) images were obtained using a Hitachi S4800 SEMFEG microscope operated at 10 kV. Magnetic hysteresis loops were obtained at room temperature using a maximum applied magnetic field of ±1.5 T in a vibrating sample magnetometer(Lakeshore7407).Thesamples wererotated in the magnetic field to attain hysteresis loops at different conditions. 3RESULTS AND DISCUSSION 3.1 Electrical activation parameters The stages of the TF-RFS experiment are shown in Figure 1b–e. The evolution of electrical parameters (electric field and current density) are given as a function of time. The clock begins once the furnace reaches 900◦C (time zero at point A). A field of 100 V/cm was applied to the zirconia dog-bone specimens with the DC power supply (point B). The sample conductivity rises nonlinearly, signaling the onset of flash, until the set current density limit of 200 mA/mm2is reached25 (in about 30 s). At this point the power supply is switched from voltage control to current control. After holding the constant current density for 70 s, the current in the magnetic coil was instantaneously increased to 50 A (point C), which corresponds to an estimated magnetic field of ∼0.125 T.8 At this point, magnetic field helps to concentrate plasma around the sample with the mixture of reactans,8which, at this transition (point D), luminesces (as seen in the optical image). This is characteristic of a flash event. Note that the electric field required to maintain a constant value of the current density in the dog-bone specimen decreases. 15512916, 2023, 12, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.19389 by Universidad De Sevilla, Wiley Online Library on [29/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 7204 JALALI et al. FIGURE 1 Touch-free sintering experiment using a DC power supply for the dog-bone and line AC (60 Hz) for induction. Schematic representation of (A) the touch-free setup and (B) the different stages of the experiment. (C) In operando images showing the onset of flash in the workpiece. (d) The pellet before and after the experiment. (E) The application of voltage to the dog-bones (A), switch to current control (B), application of magnetic field (C), and experiment ends at (D). 15512916, 2023, 12, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.19389 by Universidad De Sevilla, Wiley Online Library on [29/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License JALALI et al. 7205 The sample is held in this stage for 55 s, and then, the system is switched off. Note that the original, red color of the pellet (the workpiece), made from a mixture of Fe2O3and SrCO3powders, transforms into a shrunken and sintered black pellet due to RFS. The linear shrinkage in the diameter of the pellet is about 20%, indicating nearly full densification. Thus, reaction into a single phase and densification occurred in asinglestep. 3.2 Structural, microstructural, and magnetic characterization The influence of the induction current on phase evolution was determined. In these experiments, the field and current density in the dog-bones were kept constant: DC field of 100 V/cm, furnace temperature of 900◦C, and the current density limit of 200 mA/mm2.Inonecase,an induction current of 20 A was employed, while in the second case, the current was increased to 50 A. Figure 2a shows the X-ray diffraction (XRD) patterns from these two cases. The presence of unreacted 𝛼-Fe2O3phase (𝑅 3𝑐 space group) in the sample prepared by applying 20 A in the induction coil indicates that for this intensity value, the induced magnetic field is insufficient for transferring the energy to form the SrFe12O19 phase. Interestingly, in a studyonTF-RFS, itwasobservedthatthehigher theinduction current was, the higher the density of the sample.8 Here, it is concluded that there is also a minimum value of induction current to induce the solid-state reaction in the touch-free experiment. When the current was increased to 50 A, no traces of the α-Fe2O3phase were detected and all reactants were converted, yielding phase-pure SrFe12O19 (𝑃63∕𝑚𝑚𝑐 space group). Le Bail fit to the XRD pattern of the monophasic sample is shown in Figure 2b. The “goodness” value of 1.2 illustrates the good quality of the fitting. The obtained lattice parameters were 𝑎=5.8796 (2) Å and c=23.0674 (3) Å, which are similar to those obtained for pure SrFe12O19 compound.26 The microstructure of the cross-section fracture of the TF-RFSed pellet at an induction current of 50 A with a 1 min dwell time is shown in Figure 3a. The microstructure is a fairly homogeneous. Thus, the grain size distribution determined by averaging over 230 particles shows a mean value of 𝑑∼0.8 ±0.1 μm with a narrow distribution. The microstructure represents a well-sintered, dense material, in agreement with the linear shrinkage in the pellet during FS. The relative density of the sintered sample was measuredbythe Archimedes’methodusing distillatedwateras the immersion liquid, obtaining a value of ∼96% of the theoretical density (5.1 g/cm3was employed as the theoretical density of SrFe12O1926). FIGURE 2 (A) X-ray diffraction patterns from pellets flash sintered with an induction current of 20 and 50 A. The indexed peaks correspond to the hematite (α-Fe2O3) phase. (B) Le Bail fitting of the XRD pattern from the pellet obtained with an induction current of 50 A, that is, monophasic SrFe12O19 compound (JCPDS 33-1340). The hysteresis loop 𝜎(𝐻), where 𝜎is the specific magnetization and 𝐻is the applied field, is shown in Figure 3b. The data were obtained at room temperature. Loops show high coercive field, 𝐻C: a characteristic of hard magnets. The slope at high fields (which was limited by the equipment) gives information regarding saturation magnetization, 𝜎s. The reversibility confirms saturation at high fields. In order to determine the existence of a preferred grain orientation, measurements were made by modifying the angle, 𝜃, between the applied magnetic field, 𝐻,andsample’s in-plane direction. A total of five hysteresis curves, carried out with a tilt angle increment of 15◦from 0◦to 90◦, are included in Figure 3b (for simplicity, no demagnetizing factor has been considered). No significant differences can be observed in the loops; apparently, the application 15512916, 2023, 12, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.19389 by Universidad De Sevilla, Wiley Online Library on [29/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 7206 JALALI et al. FIGURE 3 (a) Scanning electron microscopy (SEM) image of SrFe12O19 touch-free reactive flash sintered for 1 min at 900◦Cwith an induction current of 50 A. Inset shows the histograms for the grain size. (b) Hysteresis loops at room temperature of the SrFe12O19 sample with the pellet tilted at the different angles relative to the external field. of the magnetic induction current during TF-RFS did not produce preferred grain orientation.8 The saturation magnetization, 𝜎s, was derived from the Stoner–Wohlfarth model,27 obtaining a value of 71.9 emu/g, a value higher than magnetization at 1.5 T, 𝜎1.5T =66.1 emu/g. Furthermore, values of remnant magnetization, 𝜎R=37.2 emu/g, and a coercive field, 𝐻C=3.9 kOe, were obtained. Note that the ratio 𝜎R/𝜎sis close to 0.5, which is characteristic of samples with complete randomly oriented crystallites.28 Similar values for 𝜎R/𝜎s have been obtained in compounds prepared by conventional methods.29 The energy product 𝐵𝐻max is the figure of merit for hard magnets. It is associated with the energy stored in the magnetic field lines. This parameter, 𝐵𝐻max =26 kJ/m3, is comparable to the highest 𝐵𝐻max value obtained for pre-aligned powders sintered by spark plasma sintering30 and prepared by conventional RFS.15 In order to maximize the maximum energy product in permanent magnets, it is necessary to achieve a high degree of densification while retaining significant 𝐻Cand 𝜎R. In the case of ferrites, sintering process often results in the loss of their hard magnetic properties, since the sintering temperatures required to achieve optimum densification, located between 1100◦C and 1300◦C, induce massive grain growth and significant loss in 𝐻C.31 Therefore, the 𝐵𝐻max obtained values after sintering are often far from the theoretical maximum value predicted for strontium ferrites (45 kJ/m3).32 The obtained results in this work indicate the potential of the proposed technique to circumvent this problem due to the lowest temperatures required to obtain pure ferrite pellets, although there is room for improvement as the achieved value for 𝐵𝐻max is still lower than the theoretical one for this material. 3.3 Energy consumption analyses The conventional preparation of SrFe12O19 magnets is an energetically intensive process. It usually requires two heating steps: first, the SrFe12O19 powder is prepared by heating a mixture of strontium carbonate and iron oxide at 1100◦C for 2 h.33 In the second step, the prepared powders aresinteredat1300 ◦C for 4 h.34 Figure 4includes the values of energy and time consumption of a tubular furnace (GSL-1500X, MTI Corporation) for both conventional processes as measured with a power analyzer (PPA1500, Newtons4th Ltd.) directly connected to the furnace during the heating cycles. The resulting energy consumption value, as determined by integrating the power consumed by the furnace for the entire synthesis +sintering cycle, was 3 ×108J. Incase ofthe TF-RFSprocess,the energyconsumption is assigned (i) to heating the furnace up to 900◦C andmaintaining thefurnacetemperatureduringtheentire process, and (ii) to the energy required for inducing the flash in the zirconia dog-bone and the energy consumed by the coil. In this case, the total energy consumed for preparing the material was 7 ×106J, that is, only about 2.5% of the energy consumed in conventional processing. The total time for the TF-RFS process, including furnace heating was approximately 15% of that required in the conventional method. In actual practice, the furnace will be held at temperature, and the samples will be flash sintered, one after another in quick succession. Therefore, the process time will be much shorter than estimated just above; much lower energy consumption would be expected in large-scale production. In the same way, in a large-scale production of the conventional synthesis-sintering process, it should be expected the production of several tens to hundred samples in one run. Thus, a significant reduction in the energy consumption of the conventional procedures is expected due to the easily scalable technique. However, 15512916, 2023, 12, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.19389 by Universidad De Sevilla, Wiley Online Library on [29/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License JALALI et al. 7207 FIGURE 4 A comparison of (a) the energy and (b) the time needed for conventional and touch-free reactive flash sintering (TF-RFS) preparation of SrFe12O19. In the latter case, the energy needed for furnace heating, and for inducing the flash in the zirconia dog-bone and the energy consumed by the magnetic coil were considered. there is no doubt that TF-RFS technique drastically reduces energy consumption when one-to-one sample is considered. Further studies will be required for precise energy consumption in real industrial applications. 4 CONCLUSIONS The viability of TF-RFS has been presented for the first time. In this process, a mixture of elemental oxides and carbonates is transformed into dense single-phase ferrite. The magnetic field enables “touch-free” processing without applying electrodes to the workpiece. A mixture of strontium carbonate and iron oxide was used to produce a dense pure SrFe12O19 magnet within seconds and with a very narrow grain size distribution. The energy consumption in this process is <15% of the conventional processing. The flash sintered sample shows magnetic properties comparable to those of samples prepared by conventional and spark plasma sintering procedures. ACKNOWLEDGMENTS This work was funded by EU Next Generation funds and/or the Spanish Ministry of Science and Innovation (grants TED2021-131839B-C22, PDC2021-121552-C21, and PID2022-140815OB-C22). Financial support received from Junta de Andalucía-Consejería de Transformación Económica, Industria, Conocimiento y Universidades (ProyExcel_00360) is also acknowledged. RR is grateful to the Office of Naval Research for supporting this research under the grant N00014-18-1-2270. RC acknowledges support from Fulbright Association (grant number PRX21/00434) for supporting his visit to Boulder. ORCID Pedro E.Sánchez-Jiménez https://orcid.org/0000-0001- 6982-1411 AntonioPerejón https://orcid.org/0000-0002-5525-2227 RishiRaj https://orcid.org/0000-0001-8556-9797 LuisA.Pérez-Maqueda https://orcid.org/0000-0002- 8267-3457 REFERENCES 1. Cologna M, Rashkova B, Raj R. Flash sintering of nanograin zirconia in <5sat850 ◦C. J Am Ceram Soc. 2010;93(11):3556–9. 2. Biesuz M, Sglavo VM. Flash sintering of ceramics. J Eur Ceram Soc. 2019;39(2):115–43. 3. Raj R, Kulkarni A, Lebrun J-M, Jha S. Flash sintering: a new frontier in defect physics and materials science. MRS Bull. 2021;46(1):36–43. 4. 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How to cite this article: Jalali SIA, Manchón-Gordón AF, Chacartegui R, Sánchez-Jiménez PE, Blázquez JS, Perejón A, et al. Touch-free reactive flash sintering of dense strontium hexaferrite permanent magnet. J Am Ceram Soc. 2023;106:7202–7208. https://doi.org/10.1111/jace.19389 15512916, 2023, 12, Downloaded from https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.19389 by Universidad De Sevilla, Wiley Online Library on [29/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License