United States

Strontium ferrite/polyamide12 (PA12) magnetic composite fused deposition modeling filaments made using a twin-screw extruder are being studied to be used for Magnetic Field Assisted Additive Manufacturing (MFAAM) of permanent magnets. A previous study shows that the magnetic viscosity (MV) varies with field and field angle. The M&lt;sub&gt;x&lt;/sub&gt; MV is maximum in the easy plane (1.1%) and decreases with a factor of two along the filament’s macroscopic hard axis [1]. The M&lt;sub&gt;y&lt;/sub&gt; MV is nearly zero along the hard axis and in the easy plane but maximum for intermediate angles. Here we study the time dependence as a function of temperature using a MicroSense EZ9 biaxial VSM to determine the melt viscosity.&lt;br/&gt;Samples were made by loading ~14 mg of SrO(Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;6&lt;/sub&gt;/PA12 (26 or 40 wt%) cylindrical-shaped filament parts in pyrex glass tubes. The filament is compacted with PEEK stoppers on both sides to secure its position and avoid direct exposure to the atmosphere while heating. Our aim is to study the rotation of the magnetic particles in the molten PA12 matrix. We first heat the sample to 200 &lt;sup&gt;o&lt;/sup&gt;C to orient all the particles in the y-direction using a field of 22 kOe. Then the sample temperature is lowered to the target temperature after which the field is lowered to zero. Finally, a field step is applied in the x-direction and M&lt;sub&gt;x&lt;/sub&gt; and M&lt;sub&gt;y&lt;/sub&gt; are monitored as a function of the time. The rotation of the moment vector with time is calculated from the data. Measurements are done for the fields of 500 and 2000 Oe, and at the temperatures of 120, 124, 128, 132, and 136°C.&lt;br/&gt;Up to 128°C, no strong time dependence was observed. Between 128 and 132°C, the magnetic particles orientations are no longer fixed in the matrix, but they can rotate and line up along the field direction. The time dependence of the rotation of the magnetic particles in the PA12 matrix is calculated from M&lt;sub&gt;x&lt;/sub&gt; and M&lt;sub&gt;y&lt;/sub&gt; (Fig. 1 and Fig. 2). The time constant decreases with temperature and field. The melt viscosity of the molten PA12 matrix at 136 &lt;sup&gt;o&lt;/sup&gt;C is determined from the data is 1.4E5 Pa.s at 500 Oe field.

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2022 Joint MMM-INTERMAG

Time Dependence Analysis as a Function of the Temperature of Magnetic Polymer Composite Filaments

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INTERMAG 2021

Strontium ferrite/polyamide12 (PA12) magnetic composite fused deposition modeling filaments made using a twin-screw extruder are being studied to be used for Magnetic Field Assisted Additive Manufacturing (MFAAM) of permanent magnets. A previous study shows that the magnetic viscosity (MV) varies with field and field angle. The Mx MV is maximum in the easy plane (1.1%) and decreases with a factor of two along the filament’s macroscopic hard axis [1]. The My MV is nearly zero along the hard axis and in the easy plane but maximum for intermediate angles. Here we study the time dependence as a function of temperature using a MicroSense EZ9 biaxial VSM to determine the melt viscosity. Samples were made by loading ~14 mg of SrO(Fe2O3)6/PA12 (26 or 40 wt%) cylindrical-shaped filament parts in pyrex glass tubes. The filament is compacted with PEEK stoppers on both sides to secure its position and avoid direct exposure to the atmosphere while heating. Our aim is to study the rotation of the magnetic particles in the molten PA12 matrix. We first heat the sample to 200 oC to orient all the particles in the y-direction using a field of 22 kOe. Then the sample temperature is lowered to the target temperature after which the field is lowered to zero. Finally, a field step is applied in the x-direction and Mx and My are monitored as a function of the time. The rotation of the moment vector with time is calculated from the data. Measurements are done for the fields of 500 and 2000 Oe, and at the temperatures of 120, 124, 128, 132, and 136°C. Up to 128°C, no strong time dependence was observed. Between 128 and 132°C, the magnetic particles orientations are no longer fixed in the matrix, but they can rotate and line up along the field direction. The time dependence of the rotation of the magnetic particles in the PA12 matrix is calculated from Mx and My (Fig. 1 and Fig. 2). The time constant decreases with temperature and field. The melt viscosity of the molten PA12 matrix at 136 oC is determined from the data is 1.4E5 Pa.s at 500 Oe field.

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technical paper

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Where the high-performance permanent magnets are needed (e.g. in the area of ‘green’ energy technology, such as electric vehicles and windmills) rare-earth (RE) magnetic materials dominate. At the same time, the heavier RE elements are quite expensive, some of them are rapidly decreasing in availability, and are often mined with the methods that leave an environmental footprint. Additionally, nowadays, there is a gap between the ‘strong’ RE-based magnets and their RE-free alternatives. We are using the data-mining approach in the search for high (and medium)-performance RE-free/lean magnetic materials. Filtering through a large number of known structures from ICSD database [1], we are looking for materials with high magnetization M &gt; 1 T, uniaxial MAE &gt; 1 MJ/m3, and Tc &gt;300 K. Sometimes, additional elements alterations are attempted to tweak the properties that are below the required values or make the material more cost-effective. Two searches have already been performed – a search amongst the materials containing a 3d- and a 5d-elements [3] and the materials that contain at least two 3d- metals [4]. New material was found (that had not been used as a permanent magnet before) and consequently synthesized by our experimental collaborators – Co3Mn2Ge [4]. From the ab-initio calculations, the defect-free material was predicted to have the saturation magnetization of 1.71 T, the uniaxial magnetocrystalline anisotropy of 1.44 MJ/m3, and the Curie temperature of 700 K. The samples synthesized, however, were found to have a partial disorder of Co and Ge. A saturation polarization of 0.86 T at 10 K was detected, together with a uniaxial magnetocrystalline anisotropy constant of 1.18 MJ/m3, and the TC = 359 K. These magnetic properties make Co3Mn2Ge a very promising material to be considered as a rare-earth free permanent magnet. Its magnetism depends critically on the amount of disorder of the Co and Ge atoms, a further improvement of the magnetism is possible. The next high-throughput search is being currently performed in collaboration with the experimental colleagues. We would like to acknowledge the financial support of the SSF and SNIC for the computation resources.

High-throughput and Data-mining Search for Novel Rare-Earth-Free Permanent Magnets

Permanent magnets (PMs) are indispensable in sectors as important as energy and electromobility. MnAlC is an excellent rare earth-free candidate to contribute to fill the gap existing between ferrites and NdFeB magnets, provided a successful development of its PM properties. This will be only possible by combining a high magnetization, through a maximized content of the only ferromagnetic phase in the MnAl system (τ-phase), and an enhanced coercivity, through efficient nanostructuring and phase transformation [1]. We have recently shown the importance of combining the formation of the β-Mn phase and Mn3AlC to increase coercivity -while maintaining a high remanence- in both powder and bulk MnAlC magnets produced by a simplified hot-pressing method [2]. The choice of specific additives may be a successful route to enhance magnetic properties [3]. This study shows the possibility of reaching a coercivity exceeding 2.5 kOe (200 kA/m) for 1 at.% Mo- and Cr-doped MnAlC powder and 3.3 kOe (263 kA/m) for V-doped MnAlC powder. This has been possible by combining gas-atomization and the self-developed flash-milling method (30-240 s) for efficient nanostructuring and promoting the creation of nanoprecipitates acting as pining centers [4]. A five-fold increase in coercivity has been achieved by comparison with that of the unmilled powder (see Fig. 2 for Mo-doped MnAlC). Furthermore, a proper choice of the milling time followed by an optimized annealing has allowed for combining high coercivity with an enhanced remanence (35 emu/g for a Cr-doped MnAlC sample) [4]. This approach opens a new path for engineering novel MnAlC doped nanostructures with promising PM properties. Acknowledgments. Gas-atomized powder was synthesized by Höganäs AB under the industrial project ECNanoManga between Höganäs and IMDEA Nanociencia. IMDEA acknowledges financial support by NEXUS project (PID2020-115215RB-C21) from MICINN.


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Effect of Dopants (Mo, Cr, V) on the Magnetic Properties of Mn-Al-C nanostructured by Flash-Milling

Effect of Dopants (Mo, Cr, V) on the Magnetic Properties of Mn-Al-C nanostructured by Flash-Milling

Neodymium-based magnets are used dominantly in the industry due to their exceptional magnetic properties. However, because of regulations and environmental harm on rare earth elements, it is unsustainable to use Nd-Fe-B. Fe16N2 is proposed as an alternative to rare-earth-based magnets, as it shows a giant magnetic energy product and abundance of its elements. Here we demonstrate an improvement to our previously published low-cost synthesis method to produce highly coercive Fe16N2 flakes and a consolidation method to form it into a permanent magnet. The synthesis path begins with deforming iron powder into iron flakes by surfactant-assisted ball milling. Iron flakes are heat treated under air to achieve a porous morphology to increase Fe to Fe16N2 conversion yield. Porous flakes are reduced under Ar/H2 gas flow, followed by a low-temperature nitridation process under ammonia gas. Synthesized Fe16N2 flakes exhibit saturation magnetization up to 180 emu/g and coercivity up to 1.1 kOe. (Fig .1) Rietveld analysis of the X-ray diffraction measurement of flakes shows up to 93% volumetric fraction of Fe16N2 phase. To consolidate Fe16N2 cold press method is used. Consolidated flakes do not indicate any trace of phase decomposition, hence a solid Fe16N2 magnet is produced. (Fig.2.) A cost-effective method to bulk produce Fe16N2 magnet could be sustainable for next-generation magnet applications.

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Fe16N2 Permanent Magnet Production by High-Pressure Consolidation

Magnetic properties and phase transformations in the archetypal binary alloys Fe-Pt, Co-Pt, and Fe-Pd are typically studied at near-equimolar compositions, due to the high magnetocrystalline anisotropy of the ordered L10 intermetallic. But these systems also possess eutectoid triple points, and proper thermal treatments at these compositions can create fascinating two-phase microstructures, e.g., nanochessboards in Co-Pt. Phase transformation behavior in the Fe-Pt system associated with the Fe-rich A1-&gt; L12+L10 eutectoid has barely been examined. We have produced an Fe-Pt arc melt with 34 at% Pt, nominally in the L12+L10 coexistence region. Continuous cooling of this sample through the eutectoid at various rates decomposes the high-temperature A1 phase into two tetragonal phases with different c/a ratios, rather than forming a cubic and tetragonal phase as suggested by the equilibrium phase diagram. Analysis of the Bragg-Williams long-range order parameter leads us to hypothesize the coexistence of L10 (Fig. 1) with the L1’ (Fig. 2) phase that was proposed by Shockley1in 1938. Subsequently, L1’ has been shown using computational approaches to be a stable low-temperature ordering of off-stoichiometry L10. L1’ is a specially decorated L10 lattice that accommodates off-stoichiometric compositions only on the basal-plane center atom. Until recent work by our group in Fe-Pd thin films, L1’ had not been observed experimentally. This research will examine the ordering mechanisms and phase evolution in bulk Fe-Pt, and the effect of ordering kinetics and lengthscales on the magnetic properties in and near the two-phase region. Support of the National Science Foundation through grant DMR-1709914 is gratefully acknowledged.

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Fig 1. In these unit cell illustrations, Fe is orange and Pt is gray. L10 and L1’ are both tetragonal phases that share the P4/mmm space group, however, the two structures accommodate (off-stoichiometric) excess iron differently. In our Fe- 34 at% Pt composition sample, fully ordered L10 distributes excess Fe-atoms equally across all Pt sites.

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Fig 2. For the same composition, fully ordered L1’ places excess Fe-atoms only on the basal-plane center atom, while the corner Pt-site remains undisturbed.

Indications for Novel L1’ Ordering in Fe-Pt Alloys Near the Fe-rich Eutectoid Composition

Epsilon iron oxide (ε-Fe2O3) is a nanoscale iron oxide polymorph with a large coercive field value (&gt; 20 kOe at room temperature). High yield ε-Fe2O3 syntheses involve the use of a SiO2 matrix to prevent agglomeration during high temperature sintering (&gt; 900°C).1-3 Herein, we present a new synthesis method of ε-Fe2O3 at low temperature and ambient pressure which uses no protective matrix, via a precursor of Prussian blue with a small amount of embedded ε-Fe2O3 nanoparticles. Core ε-Fe2O3 nanoparticles, prepared using a standard method,4 were sonicated in K4[Fe(CN)6](aq) solution before 2M HCl(aq) was added. The obtained sample (denoted core-shell) was sintered in air at 300°C, 400°C and 500°C for 2 hours. Fig. 1a shows TEM images of the core-shell, 300°C, 400°C and 500°C sintered samples. ε-Fe2O3 nanoparticles are shown to have spherical morphology, dTEM = 23 ± 7 nm, whereas Prussian blue nanocrystals are larger cubic or spherical particles. The sintered samples at 300°C, 400°C and 500°C show aggregations of spherical and cubic nanoparticles which become more distinct as the temperature increases. Fig. 1b shows the XRD spectra with Rietveld analysis. The core-shell sample shows peaks assigned to ε-Fe2O3 (24%, orthorhombic, Pna21) and Prussian blue (76%, cubic, Fm3m). In the sintered samples, Prussian blue peaks are not present, though the peaks corresponding to the core ε-Fe2O3 remain with the particle size calculated via the first principles method consistent in all samples, ca. 25 nm. At 300°C, a second phase of ε-Fe2O3 and γ-Fe2O3 with particle sizes ca. 3 nm are present, alongside a small amount of α-Fe2O3. At 400°C, the proportion of small size Fe2O3 nanoparticles decreases, and the proportion of ca. 25 nm ε-Fe2O3 and α-Fe2O3 increases. At 500°C, the only phases present are ca. 25 nm ε-Fe2O3 (92%) with a small amount of macroscale α-Fe2O3 (8%). Prussian blue was shown to convert to ε-Fe2O3 at high yield when sintered at 500°C in the presence of ε-Fe2O3 core nanoparticles. This is the lowest temperature ε-Fe2O3 has been reportedly prepared at in ambient pressures and may pave the way to a lower energy synthesis method for this hard magnetic ferrite.

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Fig. 1 a) TEM images and b) XRD spectra of respective samples

The Formation of a Nanoscale Hard Magnetic Ferrite from Prussian Blue Nanocrystals

M-type hexaferrites is known to be sensitive to thier chemical composition, which can be an effective instrument to preciesly control magnetic properties of the materials. In this study, we demonstrate the synthesis and the investigation of the magnetostatic and magnetodynamic properties of single-domain gallium substituted strontium M-type hexaferrite particles SrFe12-xGaxO19 (x = 1 - 6). The synthetic approach is based on a precursor preparation via a facile citrate-nitrate auto-combusion method followed by thermal treatment at 1200 oC. The annealing of the porous precursor led to the formation of single-domain hexaferrite particles with mean sizes of 360 – 560 nm. Introduction of Ga3+ ions up to x = 4 into the hexaferrite structure resulted in a growth of coercivity from 5.1 to 6.4 kOe, and further increase in gallium concentration led to a decrease in the coercivity. The samples display natural ferromagnetic resonance frequency in the range of 49 – 57 GHz. Crystal structure refinement reveals, that Ga3+ ions preferably occupy 2a, 4f1, 12k, and 2b sites and cause a decrease in the magnetic anisotropy and a modest decrease in the saturation magnetization. The research was supported by RFBR grant no 21-53-12002.

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a) Magnetic hysteresis loops of SrFe12-xGaxO19 (x = 0 – 6) samples; b) The dependencies on the substitution degree (x) of coercivity (HC) and saturation magnetization (MS).

Submicron particles of Ga-substituted strontium hexaferrite obtained by citrate auto-combustion method

Ferrites are the most used permanent magnets (PMs) due to their availability and low cost, being an alternative to PMs based on critical rare-earths in new applications such as electromobility [1]. In this work we will show the possibility of 3D-printing objects using strontium ferrite powder obtained by successful recycling of the residue generated from manufacturing commercial ferrite magnets. Tuning the morphology and microstructure of the Sr-ferrite residue by post-processing made possible to end with Sr-ferrite powder with PM properties superior, not only to the residue precursor, but to those of the brand new commercial powder (3.5 fold increase in coercivity and a 25% increase in remanence) [2]. The recycled ferrite was used as precursor for synthesizing composites by the solution casting method [3], and extruding filaments for advanced manufacturing of magnets (with a particle content up to 65 wt%) by Fused Filament Fabrication. Magnetic characterization of composites, filaments and printed objects was performed by Vibrating Sample Magnetometer (VSM), showing that the magnetization scales accordingly to the content of ferrite particles and that there is no deterioration of the PM properties of the starting particles along processing (Fig. 1). Scanning Electron Microscopy (SEM) analysis showed a homogeneous distribution of the particles in the polymer matrix (Fig. 2) [4]. This work has shown an efficient path for developing new and alternative PMs by the combination of recycling and 3D-printing. Acknowledgements. Authors acknowledge financial support from MICINN by NEXUS project (PID2020-115215RB-C21), industrial collaboration PLASMAG with IMA S.L.U. and from the Regional Government of Madrid by NanoMagCOST project (P2018/NMT-4321).

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Fig. 1. (a) First and second quadrants of the hysteresis loops of starting powder and composites with different content of ferrite. (b) Normalized hysteresis loops of the starting powder, composite and 3D-printed object. Insets show a 3D-printed piece based on ferrite particles.

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Fig. 2. SEM images of: (a) cross section and (b) closer view of a magnetic filament with a 50 wt% of ferrite; and (c) and (d) inner morphology of a 3D-printed object with a particle content of 65 wt%.

A Sustainable Route for Permanent Magnets Fabrication: Additive Manufacturing Applied to Recycled Ferrite Residues

Additive manufacturing (AM) of composites is attracting much interest in high-tech sectors for fabricating complex high-performance objects with tailored properties [1]. For permanent magnets (PMs), the challenge is to develop magnets by AM with no geometrical restrictions, high filling factor (FF), and non-deteriorated PM properties [2], together with finding alternatives (e.g., improved ferrites and promising MnAlC-based alloys) to rare earth-based magnets [3]. Composites (PM particles/polymer) were synthesized by solution casting, followed by extrusion of filaments for 3D-printing, being reported for the very first time (Fig. 1). Several alternative PM materials were studied (gas-atomized τ-MnAlC, Sr-ferrite and hybrids —Sr-ferrite/NdFeB). Particle size and fine-to-coarse particles ratio (FP/CP) play a key role on the flexibility and powder loading of MnAlC filaments (length&gt;10 m), reaching FF&gt;80% and non-deteriorated PMs properties (Fig. 1) [4]. These results will be compared to the obtained for filaments based on Sr-ferrite (Hc=239 kA/m, FF=92%), NdFeB (Hc=812 kA/m, FF=93%) and hybrids (Hc=629 kA/m, FF=90%) (Fig. 2). MnAlC-based objects were 3D-printed under controlled temperature, proving that alternative PM materials can be efficiently synthesized and processed to develop novel PMs by AM (Fig. 1) [4]. Acknowledgements. Authors acknowledge collaborations with B. Skårman, H. Vidarsson and P.-O. Larsson (Höganäs, Sweden) by the industrial contract GAMMA, and A. Nieto and R. Altimira (IMA, Spain), and financial support from: EU M-ERA.NET by COSMAG project (PCI2020-112143); MICINN by NEXUS project (PID2020-115215RB-C21); CM by NanoMagCOST project (P2018/NMT-4321).

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Fig. 1. (a) Gas-atomized MnAlC, polymer, and composite; (b) SEM image of MnAlC particles and size distribution; (c), (d) image and SEM cross section of a MnAlC/ABS filament; (e) MnAlC-based 3D-printed object; (f) normalized magnetic response of MnAlC-based samples; and (g) magnetic flux density of a 3D-printed MnAlC-based disc.


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Fig. 2. (a) SEM and EDX of a hybrid composite; (b) second quadrant of the hysteresis loops for the hybrid, Sr ferrite- and NdFeB-based materials. Insets show composite pellets and SEM cross section of a Sr ferrite-based filament.

Optimizing the Particle Size Distribution in Permanent Magnet Composites to Extrude Flexible Filaments for Additive Manufacturing

3D printing of permanent magnets has recently received considerable attention [1,2]. In previous work the realization of isotropic magnets was demonstrated using fused filament fabrication, which results in the fabrication of polymer bonded magnets, with properties very similar to state-of-the-art magnets fabricated by injection molding [1]. In Ref [3] the easy axis of the printed magnet was aligned by printing the magnetic filament on magnetized hard magnets. Within this talk we will present a significant step forward by producing magnets where the easy axis can be locally aligned in-situ during the printing process. This is realized by redesigning the printer nozzle of a fused filament fabrication printer (FFF) so that permanent magnets can be placed next to the nozzle. The magnetic strayfield of the permanent magnets is optimized so that it aligns the easy axis of anisotropic ferromagnetic particles inside a paste-like compound material when it is in the melted state. Two different magnetic materials are investigated: (i) Strontium hexaferrite inside a PA6 matrix (Sprox® 10/20p), fill grade: 49 vol% and (ii) Sm2Fe17N3 inside a PA12 matrix, fill grade: 44 vol%. In the presence of the external alignment field, the Sr ferrite particles inside the PA6 matrix can be well aligned as shown in Fig. 1, where the hard axis hysteresis loop (blue) and easy axis hysteresis loop (orange) is shown. The ratio of the remanence of the hard axis loop and the easy axis hysteresis loop is 2.2, which shows a significant alignment. No significant alignment for Sm2Fe17N3 could be achieved, where the ratio of the easy and hard axis remanence is just 1.15. The results show the feasibility to fabricate magnets with arbitrary and locally defined easy axis, since the permanent magnets (or alternatively electromagnet) can be mounted on a rotatable platform. Hence, highly optimized magnets can be produced such as Hallbach arrays which can not be produced by any other method.

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Hysteresis loops in hard and easy axes of a hard magnetic ferrite (Sprox) measured with the VSM. The 3d printed magnetic material is aligned in-situ during the print process by permanent magnets next to the printer nozzle.

In-situ alignment of anisotropic hard magnets of 3D printed magnets

Additive manufacturing (AM) via 3D printing technologies have become a frontier in materials research, including its application in the development and recycling of permanent magnets1,2. The limited availability of the rare earth elements (REEs) for magnets present a demanding challenge and necessitates reduction in the losses of REEs from conventional or subtractive manufacturing. AM allows for a minimal waste production and re-utilization of recovered rare earths in the magnet processing. In-situ alignment during 3D printing of magnetic materials has opened new horizons for manufacturing of complex permanent magnets3,4. The introduction of functionalized magnetic 3D printing as an AM process allows rapid prototyping of multi-directionally aligned magnetic systems, and reduced energy requirement due to the elimination of post-production alignment3. In this work, we have developed a robust multiphysics model to simulate the alignment of magnetic particles in a polymer matrix, during 3D printing. Magnetization vs. field measurements of extruded and printed Sm-Co bonded in PLA (15 vol.%) and Nd-Fe-B+Sm-Fe-N bonded in Nylon 12 (65 vol.%), with and without alignment, confirmed the alignment of magnetic particles. A parametric analysis was performed to understand the effect of process variables, like alignment field, loading fraction, printing speed and particle size, on the degree of alignment of the samples (Fig.1)5. The model is upgraded to consider variable particle size distributions, non-homogenous particle spacing and different printing temperature. The model predicts a competing behavior between particle-fluid and particle-particle interactions under magnetic field. The model provides a framework to efficiently predict the DoA in tandem with a functionalized-magnetic 3D printer and allows the user to adjust the operating parameters according to the desired DoA. This work is supported by the Critical Materials Institute (CMI), an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Ames Laboratory is operated for the U.S. DOE by Iowa State University of Science and Technology under Contract No. DE-AC02-07CH11358.

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