Journal of Manufacturing and Materials Processing

Article Development and Characterization of Stable Polymer Formulations for Manufacturing Magnetic Composites

Balakrishnan Nagarajan 1, Milad Kamkar 2 , Martin A.W. Schoen 3, Uttandaraman Sundararaj 2, Simon Trudel 3 , Ahmed Jawad Qureshi 1 and Pierre Mertiny 1,* 1 Department of Mechanical Engineering, University of Alberta, 9211-116 St. NW, Edmonton, AB T6G 1H9, Canada; [email protected] (B.N.); [email protected] (A.J.Q.) 2 Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada; [email protected] (M.K.); [email protected] (U.S.) 3 Department of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada; [email protected] (M.A.W.S.); [email protected] (S.T.) * Correspondence: [email protected]



Received: 20 December 2019; Accepted: 9 January 2020; Published: 12 January 2020


Abstract: Polymer bonded permanent magnets find significant applications in a multitude of electrical and electronic devices. In this study, magnetic particle-loaded epoxy resin formulations were developed for in-situ polymerization and material jetting based additive manufacturing processes. Fundamental material and process issues like particle settling at room temperature and elevated temperature curing, rheology control and geometric stability of the magnetic polymer during the thermal curing process are addressed. Control of particle settling, modifications in rheological behavior and geometric stability were accomplished using an additive that enabled the modification of the formulation behavior at different process conditions. The magnetic particle size and additive loading were found to influence the rheological properties significantly. The synergistic effect of the additive enabled the developing of composites with engineered magnetic filler loading. Morphological characterization using scanning electron microscopy revealed a homogenous particle distribution in composites. It was observed that the influence of temperature was profound on the coercive field and remanent magnetization of the magnetic composites. The characterization of magnetic polymers and composites using rheometry, scanning electron microscopy, X-ray diffraction and superconducting quantum interference device (SQUID) magnetometry analysis enabled the correlating of the behavior observed in different stages of the manufacturing processes. Furthermore, this fundamental research facilitates a pathway to construct robust materials and processes to develop magnetic composites with engineered properties.

Keywords: material jetting; magnetic composites; polymer formulation; particle settling; material behavior control; magnetic characterization

1. Introduction Permanent magnets are used in a wide range of consumer and industrial applications that involve the conversion of mechanical energy to electrical energy, and vice versa. Permanent magnets find applications in areas like factory automation, medical devices, household appliances, consumer electronics and automotive systems. Permanent magnets are utilized in electro-mechanical devices such as microwave generators, motors, dynamos, actuators, speakers and magnetic couplings [1,2]. Among many permanent magnet materials, alnico, ferrites, samarium cobalt, and neodymium iron boron (Nd2Fe14B, abbreviated herein as NdFeB) are predominantly used in the industry. The hard-magnetic

J. Manuf. Mater. Process. 2020, 4, 4; doi:10.3390/jmmp4010004 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 4 2 of 18 properties of these materials make them attractive for selective applications over other magnetic material options. A permanent magnet where the magnetic powder is mixed with a polymeric binder is called a bonded magnet. Commonly utilized magnetic powders include the aforementioned magnetic materials and hybrid mixtures thereof. The binders include polyamide (PA), polytetrafluoroethylene, epoxies, polyester and polyphenylene sulphide. Four traditional processes utilized to manufacture bonded magnets are extrusion, compression molding, injection molding and calendaring [3]. The properties of the polymer bonded magnets depend on the type of magnetic filler used, the polymer binder and the distribution of the filler [4]. Magnetic composites with magnetically hard and soft fillers find applications in electrical machines due to their favorable mechanical, magnetic and physical properties. Electric motors are devices utilized to convert electrical energy to mechanical energy with high conversion efficiency [5]. One such application is a flywheel energy storage system (FESS), which utilizes an electrical machine that functions both as a motor and generator [6]. Edwards et al. developed fiber-reinforced polymer composite laminates with both mechanical and magnetic functionality for use in electromechanical applications [7]. Mechanically stiff magnetic composites with high tensile elasticity and electrical resistivity were developed using bidisperse iron particles for flywheel lift magnet applications [8]. With the motive of developing anisotropic magnetic polymer composites with enhanced magnetic characteristics, particle structuring using uniaxial and biaxial fields has been adopted to fabricate composites with chain and sheet like particle structures, respectively. To prevent the sedimentation of magnetic particles during fabrication, a magnetic field supplied by permanent magnets during the room temperature gelling of resin combined with a multistage curing methodology was utilized [9]. Additive manufacturing (AM) is a rapid prototyping technique where parts are constructed by adding materials in layers, where each layer of the part is a thin cross-section derived from a computer-aided design (CAD) file. AM enables the production of complex three-dimensional (3D) objects directly from CAD data without having to consider tooling [10]. AM of magnetic components has been an extensive field of research to develop components for the electrical and electronic industry. Researchers have studied both metal and polymer-based AM systems to manufacture magnetic functional materials. Mikler et al. demonstrated laser additive processing of three soft magnetic alloys using direct energy deposition [11]. Nilsen et al. utilized powder bed fusion to develop ferromagnetic nickel–manganese–gallium alloy and studied the influence of process parameters using structured experimental design [12]. A commercial multi-extruder 3D printer was utilized by Yan et al. for fabricating magnetic components in an effort to simplify integration processes in power electronic circuits [13]. The feasibility of AM processes to fabricate complex magnetic components by printing a conductive winding along with a magnetic core has been reported in the technical literature [14]. Additionally, improvements in magnetic properties were reported by adjusting the feed paste formulation, its flow characteristics and AM process parameters. Direct-write AM of NdFeB polymer bonded permanent magnets using epoxy as a binder was demonstrated by Compton et al. [15]. In other research, big area additive manufacturing of isotropic NdFeB powder in PA binder enabled the manufacturing of magnets with enhanced remanence and coercive field compared to traditional injection-molded magnets [16]. Polymer bonded permanent magnets with anisotropic properties were developed by applying an external magnetic field with intensities varying from 5 kOe to 50 kOe using an electromagnet as a part of the post printing process [17]. Methodologies to orient ferromagnetic particles at user defined angles and the influence of external magnetic field strength on degree of particle alignment at lower filler loadings for AM process have been reported in the technical literature [18]. Magnetic products based on strontium ferrite (SrFe12O19, abbreviated herein as SrFeO) and NdFeB were fabricated using the extrusion of developed strips and filaments using ethylene ethyl acrylate as a binder [19]. Stainless steel microparticles in an acrylonitrile butadiene styrene polymer matrix were 3D printed with the motive of utilizing the resulting parts in the application of passive magnetic sensors and actuators [20]. Magnetic composites using a NdFeB/PA magnetic filament were printed for the prototype of a rotary blood pump and successfully integrated [21]. Research related to 3D J. Manuf. Mater. Process. 2020, 4, 4 3 of 18 printing polymer bonded magnets has mainly been focused on thermoplastic polymers and enhancing magnetic filler loading and efficiency. Thermoplastic filaments with engineered magnetic particle content for AM have already been reported in the technical literature [20]. In contrast, developing a methodology to engineer magnetic particle content in 3D printed thermoset composites has received limited attention and is thus one of the many challenges addressed in the present work. In this research, the authors develop and engineer magnetic pastes for in-situ polymerization and material jetting-based AM processes. First, magnetic pastes were engineered to prevent particle settling at room temperature and at elevated curing temperatures. X-ray diffraction (XRD) analysis was utilized to validate the ability of the material formulations to resist gravitational particle settling after the thermal curing process. The rheological properties of the magnetic pastes with multi modal magnetic particles and an additive material were characterized. These results were utilized to elucidate the material behavior encountered in different stages of the manufacturing process. Developed magnetic pastes were further used to print 3D magnetic structures using an in-house developed material jetting 3D printer. Tailoring the paste rheology enabled the printing of magnetic composites with engineered magnetic particle loading without significant deformation after the curing process. The particle distribution in 3D printed magnetic composites was characterized using scanning electron microscopy (SEM). Magnetic characterization of 3D-printed composites was conducted using a SQUID (superconducting quantum interference device) magnetometer. The results from this research enable an in-depth understanding of the role that engineered material formulations play in overcoming several manufacturing process issues and demonstrate the clear benefits of utilizing tailored rheology to one’s advantage.

2. Materials and Methods For this study, anisotropic NdFeB powder (type MQA-38-14) was purchased from Magnequench Inc. (Singapore), and SrFeO powder was purchased from Dowa Electronics Materials Co. Ltd. (Tokyo, Japan). For the polymer, a bisphenol A based epoxy resin, EPON 826, with an aromatic amine curing agent, EPICURE W, were used (Hexion Inc., Columbus, OH, USA). Disparlon 6900-20X obtained from King Industries (Norwalk, CT, USA) was used as an anti-settling additive and shear thinning agent. Table1 lists the physical properties of the magnetic particles and the epoxy resin.

Table 1. Physical properties of materials utilized (as obtained from manufacturers).

Material Type Average Particle Size Density [g/cm3] MQA-38-14 (NdFeB) 90 µm 7.51 SF-500 (SrFeO) 1.41 µm 3.41 EPON 826 - 1.16

2.1. Scanning Electron Microscopy The morphology of the magnetic particles and the magnetic particle-reinforced composites was studied using a Zeiss Sigma 300 VP field-emission scanning electron microscope (Oberkochen, Germany) equipped with secondary and backscattered electron detectors. Prior to imaging, the composite samples were cut, polished, and finally coated with carbon using a Leica EM SCD005 evaporative carbon coater (Wetzlar, Germany) to prevent the charging of the composite surface.

2.2. Preparation of Magnetic Paste Formulations The composite mixtures were prepared from epoxy resin, which in some cases was modified with the rheological additive, and magnetic particles by mechanical mixing using an impeller agitator from Calframo Ltd. (Georgian Bluffs, ON, Canada). The additive material was premixed with the epoxy resin to create a polymer base (additive modified epoxy is herein abbreviated as aZEPX where ‘Z’ indicates the additive weight fraction). For rheological studies, the magnetic particles were then J. Manuf. Mater. Process. 2020, 4, 4 4 of 18 added in increments to the polymer base to meet the desired filler loading (i.e., the magnetic filler to base polymer ratio). For the 3D-printed and cured samples used in the magnetic characterization, the required quantity of the curing agent was included in the amount of base polymer when determining the mass of magnetic filler to ensure the desired filler loading was achieved. Acetone was used to aid in the dispersion process. The curing agent, when needed, was added as the last component of the magnetic paste formulation. After blending, the composite mixtures were degassed in vacuum to remove entrapped volatiles.

2.3. Rheological Characterization The rheological behavior of magnetic paste formulations was assessed using an Anton–Parr MCR 302 rheometer equipped with a 25 mm diameter parallel plate geometry. Table2 lists the material formulations that were characterized for their rheological properties. For all the rheological tests, the magnetic filler loading in the magnetic paste formulations was maintained at 50 wt%. Hy-EPX sample, which is a hybrid formulation, containing 30 wt% of NdFeB and 20 wt% of SrFeO.

Table 2. Materials characterized for rheological properties.

Sample Name Material Type Magnetic Filler Loading EPX Pure epoxy - NdFeB-EPX NdFeB + epoxy 50 wt% SrFeO-EPX SrFeO + epoxy 50 wt% Hy-EPX NdFeB + SrFeO + epoxy 50 wt% NdFeB-a5EPX NdFeB + epoxy modified with 5 wt% additive 50 wt% NdFeB-a10EPX NdFeB + epoxy modified with 10 wt% additive 50 wt%

The viscosity of the magnetic pastes was monitored as a function of shear rate at room temperature. A single rheological experiment was performed for each of the materials listed in Table2. The resulting flow curve data were constructive for deriving a fundamental understanding of how rheological properties are influenced by magnetic particle size and additive loading. The yield stress, defined as the shear stress at zero shear rate, was estimated by curve fitting of the Herschel–Bulkley model (as shown in Equation (1)) to flow curve data.

. n τ = τ0 + Cγ (1)

. Here, τ0 is the yield stress, C is the consistency index, τ is the shear stress, γ is the shear rate and n is the flow index. The value of the flow index enabled the classification of material behavior as shear thinning (n < 1), shear thickening (n > 1), or Bingham fluid (n = 1) [22]. The shear thinning index (STI)—the ratio of viscosity at two different shear rates—was used to estimate the extent of non-Newtonian behavior exhibited by the magnetic paste formulations [23]. Rheological analysis of magnetic pastes is essential for understanding the paste behavior at different shear rates experienced by the paste inside the cylinder barrel and nozzle for 3D printing processes [24]. Moreover, to understand the influence of temperature on flow behavior, the viscosity of the magnetic pastes was measured at different temperatures (i.e. 60, 80 and 100 ◦C) with increasing shear rates. The Arrhenius equation is widely used to describe the relationship between viscosity and temperature, i.e.,

η = A eEη/RT (2) · where Eη is the flow activation energy, R is the ideal gas constant, T is the absolute temperature, A is the regression coefficient and η is viscosity [25,26]. Oscillatory rheology measurements (e.g., frequency sweep tests) were also conducted on the magnetic pastes to determine the viscoelastic properties, such as storage modulus (G’), loss modulus (G”) and damping factor (tan δ) of the samples. Studying the viscoelastic response of the samples provided the opportunity of identifying whether the magnetic pastes exhibited liquid like or solid like J. Manuf. Mater. Process. 2020, 4, 4 5 of 18 behavior, which enabled the evaluation of the effects of the particulate microstructure in the polymeric matrix. Frequency sweep tests have previously been utilized to evaluate the material consistency at rest, storage stability, sedimentation, synergies and phase separation of polymeric dispersions [22].

2.4. Particle Settling Evaluation in Uncured and Cured Magnetic Polymer Composites The capability of the rheological additive to mitigate particle settling at room temperature was assessed using simple sedimentation experiments, where particle settling effects were captured using digital photographs. The magnetic paste formulations listed in Table2 were utilized for the experiments. The prepared pastes were transferred into transparent glass beakers where settling assessment experiments were conducted. To evaluate the capability of the material formulation to withstand particle settling at elevated curing temperatures, a measured quantity of the magnetic paste formulation was transferred to a small aluminum cuvette and subsequently cured in an oven at 80 ◦C for 4 hours. Additionally, particle settling was evaluated using XRD tests. XRD measurements were performed using a Geigerflex 2173 diffractometer (Rigaku Corporation, Tokyo, Japan) fitted with a Co-tube X-ray source (λ = 1.789 Å; 38 kV and 38 mA) and a graphite monochromator to filter the Co K-β wavelength. The samples were scanned over 2θ, ranging from 30◦ to 90◦ at a rate of 2 ◦/min. Both the top and bottom surface of the cured samples were analyzed. The cured samples were also analyzed over their cross-section via SEM to corroborate findings derived from XRD testing.

2.5. Additive Manufacturing of Magnetic Polymer Composites A material jetting-based additive manufacturing technique was utilized to deposit the magnetic pastes and fabricate the magnetic composites. For this purpose, an in-house developed material jetting platform was equipped with a precision dispensing system (Ultimus V, Nordson EFD, East Providence, RI, USA), controlled using the Labview software environment [27] (National Instruments, Austin, TX, USA). A graphical user interface enabled the adjusting of the dispensing pressure and deposition speed. The open source software Sli3cr was used to generate the g-code for the tool path based on given input parameters like layer thickness, extruded material width and infill pattern [28]. Deposition trials were conducted to determine the feasible parameter combinations prior to actual composite fabrication.

2.6. Magnetic Characterization The materials listed in Table3 were mixed, 3D printed, cured and then used for magnetic characterization. The target magnetic filler loading was 80 wt% for NdFeB80-a10EPX-C, SrFeO80-a10EPX-C and Hy80-a10EPX-C, and 50 wt% for NdFeB-a10EPX-C, with the base polymer for all samples comprising 10 wt% rheological additive (‘C’ in the material identifier indicates cured epoxy pastes). Small pieces from the 3Dprinted, not pre-magnetized samples were weighed and then placed in gelatin capsules, which were themselves inserted into clear and diamagnetic plastic straws.

Table 3. Materials 3D printed and characterized for magnetic properties.

Material Number Material Type Magnetic Filler Loading NdFeB80-a10EPX-C NdFeB + epoxy modified with 10 wt% additive 80.2 wt% SrFeO80-a10EPX-C SrFeO + epoxy modified with 10 wt% additive 80.1 wt% Hy80-a10EPX-C NdFeB + SrFeO + epoxy modified with 10 wt% additive 80.8 wt% NdFeB-a10EPX-C NdFeB + epoxy modified with 10 wt% additive 50.1 wt%

The magnetic properties of the polymer composites were measured using a SQUID magnetometer (MPMS XL-7 Evercool, Quantum Design, San Diego, CA, USA). Magnetization reversal loops were measured for each composite listed in Table3 in magnetic field strengths of µ H = 7 T at temperatures 0 ± of 325, 350, 375 and 395 K. The saturation magnetization was determined at a field of 7 T. The remanence and coercivity were quantified by the linear interpolation of the magnetization at zero applied field, J. Manuf. Mater. Process. 2020, 4, 4 6 of 18

andJ. Manuf. the appliedMater. Process. field 2020 at, zero 4, x FOR magnetization, PEER REVIEW respectively. The magnetic parameters were determined 6 of 19 by averaging the values for both field sweep directions. 3. Results 3. Results 3.1. SEM Characterization of Magnetic Fillers 3.1. SEM Characterization of Magnetic Fillers Figure 1 shows the morphological features of magnetic particles investigated via SEM. The imagesFigure of 1NdFeB shows and the SrFeO morphological particles featuresindicate of irregu magneticlar morphology. particles investigated Even though via particle SEM. The size images data ofwas NdFeB obtained and SrFeOfrom material particles data indicate sheets, irregular particles morphology. dimensions were Even measured though particle in the sizeSEM data images was obtainedusing the from ImageJ material software data (National sheets, particles Institutes dimensions of Health, were Bethesda, measured MD, in USA) the SEM [29]. images Particle using linear the ImageJdimensions software ranging (National from Institutes5 to 120 μ ofm Health,were observed Bethesda, for MD, anisotropic USA) [29 NdFeB,]. Particle and linear 0.5 to dimensions 5 μm for ranginganisotropic from SrFeO, 5 to 120 indicatingµm were a observed wide particle for anisotropic size distribution NdFeB, of and the 0.5 magnetic to 5 µm forfillers. anisotropic It is assumed SrFeO, indicatingthat smaller a wide particles particle enabled size distributionthe enhancing of of the the magnetic loading fillers.fraction It by is assumedfilling in thatgaps smaller between particles larger enabledparticles. the enhancing of the loading fraction by filling in gaps between larger particles.

FigureFigure 1. 1.SEM SEM imagesimages ofof magneticmagnetic particles: ( A)) NdFeB NdFeB and and ( (BB)) SrFeO. SrFeO.

3.2.3.2. Rheological Rheological AnalysisAnalysis ofof MagneticMagnetic PastesPastes

3.2.1.3.2.1. Viscosity Viscosity and and FlowFlow CurveCurve AnalysisAnalysis RheologicalRheological properties properties of of magnetic magnetic pastes pastes play play a significanta significant role role in in controlling controlling and and optimizing optimizing the manufacturingthe manufacturing process process conditions. conditions. The rheological The rheolo behaviorgical behavior of complex of complex material material systems dependssystems ondepends a multitude on a ofmultitude parameters, of parameters, including particleincluding size, particle shape, size, filler shape, volume filler fraction, volume inter-particle fraction, inter- and filler-matrixparticle and interactions filler-matrix [30 interactions]. The rheological [30]. The properties rheological of theproperties samples of under the samples rotational under and rotational oscillatory flowand fieldsoscillatory were flow analyzed fields to were study analyzed the effects to study of particle the effects size and of particle rheological size additiveand rheological on flow additive behavior andon flow network behavior structure and network of the magnetic structure pastes. of the magnetic pastes. ApartApart fromfrom thethe purepure epoxyepoxy thatthat exhibitsexhibits aa NewtonianNewtonian flow flow behavior (shear (shear rate rate independent), independent), allall magnetic magnetic pastespastes exhibitexhibit non-Newtoniannon-Newtonian behavior, which can can be be identified identified by by a a decrease decrease in in the the viscosityviscosity of of the the magnetic magnetic pastespastes withwith increasingincreasing shearshear rate (see Figure 22).). UtilizingUtilizing Equation (1), the rheologicalrheological properties properties ofof thethe magneticmagnetic pastespastes (i.e.,(i.e., yield strength, flow flow index index and and consistency consistency index, index, listedlisted in in Table Table4) were4) were derived derived in order in order to investigate to investigate the influence the influence of the constituentof the constituent materials materials utilized toutilized formulate to formulate the magnetic the magnetic pastes. Inpastes. general, In general, for all the for magneticall the magnetic paste formulations,paste formulations, the flow the index,flow whichindex, represents which represents the degree the of degree pseudo-plasticity, of pseudo-plast wasicity, found was to befound less to than be unityless than (i.e., unityn < 1), (i.e., indicating n < 1), shearindicating thinning shear behavior. thinning Additionally, behavior. Additionally, STI values greater STI values than unity greater confirm than pseudo-plasticunity confirm behavior,pseudo- andplastic the magnitudebehavior, and of thethe STI magnitude values for of all the the STI magnetic values pastesfor all enabledthe magnetic us to understandpastes enabled the extentus to ofunderstand pseudo-plasticity the extent within of pseudo-plasticity the probed range within of shear the rate probed window. range It of was shear observed rate window. that fine It SrFeO was particlesobserved and that rheological fine SrFeO particles additives and significantly rheological enhanced additives thesignificantly yield strength enhanced of the the magnetic yield strength paste formulations.of the magnetic To paste elaborate formulations. on this observation, To elaborate NdFeB-EPX on this observation, sample, compared NdFeB-EPX to SrFeO-EPXsample, compared sample, exhibitsto SrFeO-EPX lower viscositysample, exhibits at low shear lower rates viscosity (seeFigure at low2 shear), which rates is attributed(see Figure to 2), the which higher is attributed surface area to tothe volume higher ratio surface of SrFeO area to as volume compared ratio to of NdFeB. SrFeO Hence,as compared for the to same NdFeB. filler Hence, loading for inthe the same magnetic filler loading in the magnetic pastes, SrFeO particles feature a considerably greater surface area compared to NdFeB. The effect of fine SrFeO particles is clearly observed in the yield strength of the magnetic pastes, which is 30 times higher than that of samples containing NdFeB particles. The hybrid

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 19

magnetic paste (Hy-EPX) exhibited significant enhancement in low-shear viscosity and yield strength compared to NdFeB-EPX sample. Again, this enhancement is attributed primarily to the presence of fine SrFeO particles. A decrease in flow index for materials SrFeO-EPX and Hy-EPX compared to NdFeB-EPX is congruent to findings in the technical literature, where a reduction in flow index was observed with decreasing particle size [26]. Referring to Figure 2 and Table 4, comparing sample NdFeB-EPX with NdFeB-a5EPX and NdFeB-a10EPX reveals that the addition of the rheological additive (Disparlon 6900-20X) J.significantly Manuf. Mater. Process. increased2020, 4,the 4 low shear viscosity and yield strength of the magnetic pastes. 7This of 18 rheological additive is known to create a 3D network through hydrogen bonding with epoxy polymer chains resulting in a gel-like structure. Epoxy resins containing the rheological additive exhibit shear pastes, SrFeO particles feature a considerably greater surface area compared to NdFeB. The effect of thinning behavior through the disruption of the network structure, while the formation of hydrogen finebonds SrFeO imparts particles thixotropic is clearly properties, observed enabling in the yield time strength-dependent of the changes magnetic in viscosity pastes, which[31]. Comparing is 30 times higherflow behavior than that of of samples samples NdFeB-a5EPX containing NdFeB and particles.NdFeB-a10EPX, The hybrid it can magnetic be concluded paste (Hy-EPX)that an increase exhibited in significantrheological enhancement additive content in low-shear fostered viscositystrong shear and thinning yield strength behavi comparedor, which is to supported NdFeB-EPX by sample. a low Again,flow index this enhancement value. Overall, is the attributed addition primarily of fine SrFeO to the particles presence and of finethe rheological SrFeO particles. additive Adecrease raised the in flowyield index strength for materials compared SrFeO-EPX to samples and containing Hy-EPX only compared NdFeB to particles, NdFeB-EPX which is congruent is highly desirable to findings for in thethe technical considered literature, 3D printing where processes. a reduction in flow index was observed with decreasing particle size [26].

FigureFigure 2. 2.Viscosity Viscosity asas aa functionfunction ofof shearshear rate for the magnetic pastes as listed listed in in Table Table 22..

TableTable 4. 4.Derived Derived rheologicalrheological propertiesproperties ofof thethe magneticmagnetic pastes as listed in Table 22..

MaterialMaterial Number Number Yield StrengthStrength [Pa] [Pa] Flow Flow Index- Index-n n ConsistencyConsistency Index- Index-C C STISTI EPXEPX ------1 1 NdFeB-EPXNdFeB-EPX 7.877.87 0.96 0.96 17.02 17.02 3 3 SrFeO-EPXSrFeO-EPX 235.16235.16 0.82 0.82 43.48 43.48 3030 Hy-EPX 67.77 0.84 36.92 10 Hy-EPX 67.77 0.84 36.92 10 NdFeB-a5EPX 159.40 0.49 63.24 26 NdFeB-a5EPXNdFeB-a10EPX 232.66159.40 0.19 0.49 302.37 63.24 4626 NdFeB-a10EPX 232.66 0.19 302.37 46

3.2.2.Referring Influence to of FigureTemperature2 and Tableon Viscosity4, comparing sample NdFeB-EPX with NdFeB-a5EPX and NdFeB-a10EPX reveals that the addition of the rheological additive (Disparlon 6900-20X) significantly increasedThe thetemperature low shear viscositydependence and yieldof viscosity strength must of the be magnetic carefully pastes. considered This rheological to control additive particle is knownsettling to at create elevated a 3D curing network temperature through hydrogenin in-situ polymerization bonding with epoxyand to polymer ensure geometric chains resulting stability in in a gel-likematerial structure. jetting processes. Epoxy resins As containingis observed the in rheologicalFigure 3A, additivethe viscosity exhibit of shearall the thinning magnetic behavior paste throughformulations the disruption decreased ofwith the increasing network structure,temperature. while It is theclearly formation observed of that hydrogen rheological bonds additives imparts thixotropicin samples properties, NdFeB-a5EPX enabling and NdFeB-a10EPX time-dependent enabled changes maintaining in viscosity higher [31]. Comparing shear viscosity flow up behavior to 100 of°C samples compared NdFeB-a5EPX to formulations and without NdFeB-a10EPX, an additive. it can be concluded that an increase in rheological additive content fostered strong shear thinning behavior, which is supported by a low flow index value. Overall, the addition of fine SrFeO particles and the rheological additive raised the yield strength compared to samples containing only NdFeB particles, which is highly desirable for the considered 3D printing processes.

3.2.2. Influence of Temperature on Viscosity The temperature dependence of viscosity must be carefully considered to control particle settling at elevated curing temperature in in-situ polymerization and to ensure geometric stability in material J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 19

The observed material behavior is consistent with the Arrhenius law presented in Equation (2) where viscosity has a negative correlation with temperature. Modifying Equation (2), it can be shown that ln(η) has a linear relationship with 1/T. For example, the magnetic pastes containing NdFeB (samples NdFeB-EPX, Hy-EPX and NdFeB-a10EPX) exhibit such a linear relation with good agreement, as indicated by Figure 3B. It can be observed that the rate of viscosity decrease is substantially lower for the formulation engineered with the rheological additive (NdFeB-a10EPX) as compared to the other NdFeB composites (NdFeB-EPX and Hy-EPX), confirming that formulations engineered with the rheological additive are less susceptible to temperature changes. Furthermore, the activation energy was calculated from the slope of ln (η)–(1/T) curves with the known R value according to Equation (2). The activation energies (Eη) obtained for NdFeB-EPX, Hy-EPX and NdFeB- a10EPX are 72.2 kJ/mol, 54.9 kJ/mol and 34.9 kJ/mol, respectively. It is observed that the formulation J. Manuf. Mater. Process. 2020, 4, 4 8 of 18 engineered with rheological additive (NdFeB-a10EPX) exhibited a lower activation energy compared to the formulation without additive (NdFeB-EPX). Activation energy is an indicator used to jettingcharacterize processes. the As thermal is observed susceptibility in Figure of3 A,materi theals. viscosity Materials of allwith the low magnetic activation paste energy formulations were decreasedobserved with to be increasing less susceptible temperature. to temperature It is clearly changes observed [26,32]. thatAdditionally, rheological the additiveschosen rheological in samples additive (Disparlon 6900-20x) is PA based with a high melting point, which extends its activity to NdFeB-a5EPX and NdFeB-a10EPX enabled maintaining higher shear viscosity up to 100 ◦C compared high temperature ranges [33]. to formulations without an additive.

1 FigureFigure 3. ( A3. )(A Viscosity) Viscosity as as a functiona function of of temperature temperature atat shearshear rate of of 1 1 s s−−1 forfor magnetic magnetic paste paste materials materials listedlisted in Table in Table2.( B )2. Viscosity–temperature (B) Viscosity–temperature data lineardata linear curve cu fittingrve fitting with linearizedwith linearized Arrhenius Arrhenius equation (Equationequation (2)) (Equation for NdFeB (2)) based for NdFeB magnetic based paste magnetic materials paste NdFeB-EPX,materials NdFeB-EPX, Hy-EPX andHy-EPX NdFeB-a10EPX and NdFeB- (as pera10EPX Table2). (as per Table 2).

3.2.3.The Oscillatory observed materialRheology behavior Analysis isof consistentMagnetic Paste with Formulations the Arrhenius law presented in Equation (2) where viscosityOscillatory has frequency a negative sweeps correlation have withbeen temperature.widely used to Modifying characterize Equation polymer (2), filled it can samples be shown that(polymer ln(η) has composites, a linear relationship polymer solu withtions, 1/ Tetc.). For [34,35]. example, Informatio the magneticn about pastesa sample’s containing rigidity NdFeBand (samplesnetwork NdFeB-EPX, structure can Hy-EPX be obtained and NdFeB-a10EPX) by studying viscoelastic exhibit suchparameters, a linear such relation as the with storage good modulus agreement, as indicated(G’), loss modulus by Figure (3GB.’’) Itand can damping be observed factor that (tan theδ) [36]. rate The of viscosity storage and decrease loss moduli is substantially are indicative lower for theof the formulation energy that engineered is stored and with dissipated the rheological in the material, additive respectively. (NdFeB-a10EPX) In the present as compared study, the to the otherviscoelastic NdFeB composites characteristics (NdFeB-EPX of the pastes and were Hy-EPX), probed confirming using frequency that formulations sweep tests conducted engineered over with an the rheologicalangular frequency additive are (ω) less ranging susceptible from 0.1 to to temperature 100 rad/s at changes.a constantFurthermore, shear strain γ0 the= 1%. activation Figure 4A,B energy wasdepict calculated the frequency from the dependence slope of ln (ofη)–(1 G’ /andT) curves G’’ on withω, respectively. the known ForR value samples according NdFeB-EPX, to Equation SrFeO- (2). EPX and Hy-EPX, the loss modulus was greater than the storage modulus (G’’ > G’) for the entire The activation energies (Eη) obtained for NdFeB-EPX, Hy-EPX and NdFeB-a10EPX are 72.2 kJ/mol, 54.9range kJ/mol of and probed 34.9 kJfrequencies,/mol, respectively. which indicates It is observed that the that viscous the formulation character is engineered dominant. withFor samples rheological NdFeB-a5EPX and NdFeB-a10EPX, which contain the rheological additive, the opposite behavior was additive (NdFeB-a10EPX) exhibited a lower activation energy compared to the formulation without observed, i.e., the storage modulus was higher than the loss modulus (G’ > G’’), signaling a solid-like additive (NdFeB-EPX). Activation energy is an indicator used to characterize the thermal susceptibility of materials. Materials with low activation energy were observed to be less susceptible to temperature changes [26,32]. Additionally, the chosen rheological additive (Disparlon 6900-20x) is PA based with a high melting point, which extends its activity to high temperature ranges [33].

3.2.3. Oscillatory Rheology Analysis of Magnetic Paste Formulations Oscillatory frequency sweeps have been widely used to characterize polymer filled samples (polymer composites, polymer solutions, etc.) [34,35]. Information about a sample’s rigidity and network structure can be obtained by studying viscoelastic parameters, such as the storage modulus (G’), loss modulus (G”) and damping factor (tan δ)[36]. The storage and loss moduli are indicative of the energy that is stored and dissipated in the material, respectively. In the present study, the viscoelastic characteristics of the pastes were probed using frequency sweep tests conducted over an angular frequency (ω) ranging from 0.1 to 100 rad/s at a constant shear strain γ0 = 1%. Figure4A,B depict the frequency dependence of G’ and G” on ω, respectively. For samples NdFeB-EPX, SrFeO-EPX J. Manuf. Mater. Process. 2020, 4, 4 9 of 18 and Hy-EPX, the loss modulus was greater than the storage modulus (G” > G’) for the entire range of probed frequencies, which indicates that the viscous character is dominant. For samples NdFeB-a5EPX and NdFeB-a10EPX, which contain the rheological additive, the opposite behavior was observed, J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 9 of 19 i.e., the storage modulus was higher than the loss modulus (G’ > G”), signaling a solid-like behavior. Thisbehavior. change inThis the change material in behaviorthe material is imparted behavior byis imparted the rheological by the additiverheological due additive to the formationdue to the of a gel characterformation viaof a hydrogen gel character bonding. via hydrogen Moreover, bonding. it is observed Moreover, that it the is gradientobservedin that modulus the gradient curves in over themodulus frequency curves range over for magneticthe frequency pastes range with for the magn rheologicaletic pastes additive with the is considerablyrheological additive less than is for theconsiderably pastes without less additive. than for the This pastes characteristic without additi indicatesve. This reduced characteristic material indicates flowability, reduced which material further corroboratesflowability, the which existence further of corroborates gel-like behavior the existence in pastes of containinggel-like behavior the rheological in pastes containing additive. the Given therheological observations additive. made forGiven the the storage observations and loss made moduli, for the the storage ratio of and loss loss modulus moduli, to the storage ratio of modulus loss (tanmodulusδ = G”/G to’) isstorage less than modulus unity (tan for magneticδ = G’’/G paste’) is less formulations than unity containingfor magnetic the paste rheological formulations additive (NdFeB-a5EPXcontaining the and rheological NdFeB-a10EPX) additive (N whereasdFeB-a5EPX tan δ and> 1 NdFeB-a10EPX) for the other paste whereas formulations. tan δ > 1 for As the such, other paste formulations. As such, the findings are congruent with the notion that higher tan δ values the findings are congruent with the notion that higher tan δ values are indicative of greater energy are indicative of greater energy dissipation, as in the case of the magnetic paste formulations without dissipation, as in the case of the magnetic paste formulations without rheological additive. Hence, rheological additive. Hence, oscillatory and rotational rheometry confirm that the used additive oscillatory and rotational rheometry confirm that the used additive confers the vital prerequisites for confers the vital prerequisites for 3D printing of the magnetic pastes. 3D printing of the magnetic pastes.

FigureFigure 4. (4.A )(A Storage) Storage modulus modulusG G’’ andand ((BB)) lossloss modulus GG’’” as as a afunction function ofof angular angular frequency frequency for for magneticmagnetic paste paste materials materials listed listed in in Table Table2 .2.

3.3.3.3. Analysis Analysis of Magneticof Magnetic Particle Particle Settling Settling inin UncuredUncured Resin and and Cured Cured Polymer Polymer Composites Composites

3.3.1.3.3.1. Particle Particle Settling Settling at at Stationary Stationary Conditions Conditions MagneticMagnetic particle particle settling settling inin polymers is is primarily primarily due due to gravity to gravity and affects and aff theects stability the stability of the of thepaste paste formulation. formulation. In the In thecase case of material of material jetting jetting processes, processes, where paste where deposition paste deposition through a throughnozzle a nozzleis driven is driven by pressure, by pressure, additional additional inertia is inertia exerted is on exerted particles, on typically particles, in typicallythe downward in the direction, downward direction,which whichmay further may further intensify intensify particles’ particles’ settling settlingeffects. eMoreover,ffects. Moreover, particle particlesettling settlingmay lead may to leadthe to clogging of the nozzle and thus create a disruption in the manufacturing process. In this study, the the clogging of the nozzle and thus create a disruption in the manufacturing process. In this study, means to address particle settling was adding the rheological additive to the magnetic paste the means to address particle settling was adding the rheological additive to the magnetic paste formulations. Figure 5 shows the ability of the rheological additive to mitigate gravitational particle formulations. Figure5 shows the ability of the rheological additive to mitigate gravitational particle settling. Eight hours after filling the beaker, filler sedimentation and a clear supernatant is visible for settling.material Eight NdFeB-EPX, hours after while filling NdFeB-a5EPX the beaker, and filler NdFe sedimentationB-a10EPX are and still a clearuniformly supernatant mixed, exhibiting is visible for materialan anti-settling NdFeB-EPX, characteristic. while NdFeB-a5EPX The rheological and tests NdFeB-a10EPX indicated that are thestill additive uniformly causes mixed,a gelling exhibiting effect an anti-settlingin the magnetic characteristic. paste formulations, The rheological presumably tests due indicated to the development that the additive of a thixotropic causes a gelling network eff ect in thestructure. magnetic Hence, paste the formulations, rheological additive, presumably providing due tohigh the viscosity development at low ofshear a thixotropic rates, high networkyield structure.strength, Hence, and a greater the rheological storage modu additive,lus, effectively providing mitigated high viscosityparticle settling at low and shear promoted rates, higha stable yield paste formulation in stationary conditions.

J. Manuf. Mater. Process. 2020, 4, 4 10 of 18 strength, and a greater storage modulus, effectively mitigated particle settling and promoted a stable paste formulation in stationary conditions. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 19 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 19

FigureFigure 5. 5.Photograph Photograph of of clear clear supernatant supernatant in materi in materialal NdFeB-EPX, NdFeB-EPX, while NdFeB-a5EPX while NdFeB-a5EPX and NdFeB- and NdFeB-a10EPXFigurea10EPX 5. exhibit Photograph exhibit an anti-settling anof clear anti-settling supernatant characteristic. characteristic. in materi al NdFeB-EPX, while NdFeB-a5EPX and NdFeB- a10EPX exhibit an anti-settling characteristic. 3.3.2.3.3.2. Particle Particle Settling Settling in in Cured Cured Magnetic Magnetic Polymer Polymer Composites—In-Situ Polymerization Polymerization 3.3.2. Particle Settling in Cured Magnetic Polymer Composites—In-Situ Polymerization MagneticMagnetic particle particle settling settling during during thethe thermalthermal curing process process was was investigated investigated in insamples samples manufacturedmanufacturedMagnetic using usingparticle in in situ settlingsitu polymerization, polymerization, during the wherethermalwhere the curing magnetic process paste paste was was was investigated transferred transferred toin toasamples cuvette a cuvette manufacturedand cured at 80 using °C. Paste in situ formulations polymerization, NdFeB-a5EPX where the and magnetic NdFeB-a10EPX paste was were transferred utilized toin thisa cuvette set of and cured at 80 ◦C. Paste formulations NdFeB-a5EPX and NdFeB-a10EPX were utilized in this set of experiments.andexperiments. cured at The 80 The top°C. top andPaste and bottom formulations bottom surfaces surfaces NdFeB-a5EPX of of the the cured cured and samples samples NdFeB-a10EPX were were examinedexamined were utilized via XRD in (see (see this Figure Figureset of 6). experiments. The top and bottom surfaces of the cured samples were examined via XRD (see Figure Strong6). Strong XRD peaksXRD peaks of crystalline of crystalline NdFeB NdFeB were were obtained obtained from from thethe bottom surfaces surfaces of ofthe the samples samples 6).fabricated Strong XRDusing peaks both ofNdFeB-a5EPX crystalline NdFeB and NdFeB-a10E were obtainedPX, fromsuggesting the bottom the presence surfaces of near-surfacethe samples fabricated using both NdFeB-a5EPX and NdFeB-a10EPX, suggesting the presence of near-surface fabricatedNdFeB particles using (Figureboth NdFeB-a5EPX 6B). Similarly, and the NdFeB-a10E top surface ofPX, the suggesting sample fabricated the presence from ofNdFeB-a10EPX near-surface NdFeB particles (Figure6B). Similarly, the top surface of the sample fabricated from NdFeB-a10EPX NdFeBalso exhibited particles strong (Figure crystalline 6B). Similarly, peaks the (Figure top surface 6A). of In the contrast, sample fabricatedthe amorphous from NdFeB-a10EPX polymer peak also exhibited strong crystalline peaks (Figure6A). In contrast, the amorphous polymer peak dominants alsodominants exhibited for strongthe top crystalline surface of peaks the NdFeB-a5EPX (Figure 6A). Insample contrast, with theother amorphous peaks having polymer markedly peak fordominants thereduced top surface intensity, for the of which thetop NdFeB-a5EPXsurface suggests of that the near-surface sampleNdFeB-a5EPX with NdFeB other sample particles peaks with having are other largely markedlypeaks absent having reduceddue tomarkedly settling. intensity, whichreducedThe suggests XRD intensity, results that thus near-surfacewhich reflect suggests the NdFeB effectiveness that near-surface particles of the are NdFeB rheological largely particles absent additive are due largely to to mitigate settling. absent particle due The to XRD settling. settling results thusTheduring reflect XRD the the results thermal effectiveness thus curing reflect ofprocess. the the effectiveness rheological additiveof the rheological to mitigate additive particle to settlingmitigate duringparticle the settling thermal curingduring process. the thermal curing process.

Figure 6. XRD diffractograms from (A) top surface (B) bottom surface of cured materials NdFeB- FigureFigurea5EPX 6. XRD and6. XRD NdFeB-a10EPX diff diffractogramsractograms (as from perfrom ( ATable )(A top) top2). surface surface (B) ( bottomB) bottom surface surface of curedof cured materials materials NdFeB-a5EPX NdFeB- anda5EPX NdFeB-a10EPX and NdFeB-a10EPX (as per Table (as per2). Table 2).

J. Manuf. Mater. Process. 2020, 4, 4 11 of 18

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 19 Cross-sections of the cured samples were further studied via SEM to validate the suppositions Cross-sections of the cured samples were further studied via SEM to validate the suppositions derived from the XRD study. In Figure7, the sample fabricated using material NdFeB-a5EPX features derived from the XRD study. In Figure 7, the sample fabricated using material NdFeB-a5EPX features a polymer-rich layer devoid of particles with an approximate thickness of 200 µm; in the sample made a polymer-rich layer devoid of particles with an approximate thickness of 200 μm; in the sample from NdFeB-a10EPX, particles are visible at and near the sample surface. It thus appears that some made from NdFeB-a10EPX, particles are visible at and near the sample surface. It thus appears that particle settling occurred in the material NdFeB-a5EPX sample, causing the observed suppression of some particle settling occurred in the material NdFeB-a5EPX sample, causing the observed XRDsuppression peaks. of XRD peaks.

FigureFigure 7. 7.SEM SEM cross-section cross-section images images illustratingillustrating reduced part particleicle settling settling in in a asample sample ofof cured cured material material NdFeB-a10EPXNdFeB-a10EPX (right) (right) compared compared to to NdFeB-a5EPXNdFeB-a5EPX (lef (left)t) (as (as per per Table Table 2).2). The The top top of of the the micrographs micrographs correspondscorresponds to to the the sample’s sample’s upper upper surface. surface. SomeSome entrapped air air bubbles bubbles are are visible visible as as well. well.

3.4.3.4. Additive Additive Manufacturing Manufacturing and and Characterization Characterization of Magnetic Composites Composites

3.4.1.3.4.1. Geometric Geometric Stability Stability Observations Observations inin 3D3D PrintedPrinted Magnetic Composites Composites OneOne of of the the fundamental fundamental goals goals ofof thisthis workwork waswas to print magnetic magnetic composites composites using using engineered engineered pastepaste formulations formulations containing containing magnetic magnetic particles.particles. Magnetically loaded loaded polymer polymer composites composites were were thusthus fabricated fabricated from from the the developed developed paste paste formulationsformulations using using an an in-house in-house developed developed material material jetting jetting 3D3D printer. printer. Prior Prior to to 3D 3D printing, printing, trialstrials werewere conducted to to determine determine the the apt apt printing printing parameters, parameters, includingincluding deposition deposition pressure, pressure, layer layer thicknessthickness andand depositiondeposition speed. speed. The The utilized utilized layer layer thickness thickness settingsetting varied varied between between 0.5 0.5 andand 1.2 mm mm and and the the ad adoptedopted printing printing speed speed range range was wasbetween between 5 and 5 10 and mm/s. The deposition pressure for printing composites was observed to depend on the printed 10 mm/s. The deposition pressure for printing composites was observed to depend on the printed magnetic pastes. The formulations containing SrFeO demanded high deposition pressures, ranging magnetic pastes. The formulations containing SrFeO demanded high deposition pressures, ranging from 137 to 275 kPa; for the NdFeB formulations, the required deposition pressures were between 21 from 137 to 275 kPa; for the NdFeB formulations, the required deposition pressures were between and 103 kPa. In some cases, it was required to adjust the deposition pressure and layer thickness 21 and 103 kPa. In some cases, it was required to adjust the deposition pressure and layer thickness settlings during part fabrication, as adopting the same parameters for multilayer structures resulted settlings during part fabrication, as adopting the same parameters for multilayer structures resulted in in gaps between the deposited layers and discontinuous prints. Initially, samples were manufactured gapsusing between materials the NdFeB-EPX, deposited layers NdFeB-a5EPX and discontinuous and NdFeB-a10EPX. prints. Initially, Figure samples8 shows the were final manufactured shapes of usingthe materials3D-printed NdFeB-EPX, magnetic composites NdFeB-a5EPX after thermal and NdFeB-a10EPX. curing. It can Figurebe observed8 shows that the only final the shapes paste of theformulations 3D-printed engineered magnetic compositesusing the rheological after thermal additive curing. (NdFeB-a5EPX It can be and observed NdFeB-a10EPX) that only retained the paste formulationsthe printed engineered shape adequately. using the From rheological rheologica additivel measurements, (NdFeB-a5EPX the and formulation NdFeB-a10EPX) without retained the therheological printed shape additive adequately. (NdFeB-EPX) From rheologicalhad low yield measurements, strength and theexhibited formulation viscoelastic without fluid the behavior, rheological additivein contrast (NdFeB-EPX) to materials had NdFeB- low yielda5EPX strength and NdFeB-a10EPX. and exhibited The viscoelastic ability of fluidthe rheologically behavior, in modified contrast to materialsresin to NdFeB-a5EPXmaintain a comparatively and NdFeB-a10EPX. high viscosity The over ability a broad of the temperature rheologically range, modified especially resin toat maintaincuring a comparativelytemperatures, enabled high viscosity the materials over ato broadmaintain temperature the printed range, geometry. especially at curing temperatures, enabled the materials to maintain the printed geometry.

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Figure 8.8. GeometryGeometry ofof 3D-printed 3D-printed composites composites (30 (30 mm mm by 30by mm 30 CADmm CAD dimensions) dimensions) afterthermal after thermal curing curingfor paste for formulations paste formulations NdFeB-EPX, NdFeB-EPX, NdFeB-a5EPX NdFeB-a5EPX and NdFeB-a10EPX and NdFeB-a10EPX (as per (as Table per2). Table 2).

3.4.2. SEM SEM and and Magnetic Magnetic Characterization Characterization of 3D Printed Composite Magnets Using SEM and the the SQUID SQUID magnetometer, magnetometer, composites printed using the material jetting process were characterized for their microstructure and magn magneticetic properties (e.g., saturation magnetization, magnetization, remanence and coercivity), respectively. Note Note that that even even though the deformation of the deposited pastes was adequatelyadequately controlledcontrolled using using 5 5 wt% wt% rheological rheological additive, additive, 10 10 wt% wt% additive additive was was employed employed for forthe the analyses, analyses, as this as this additive additive loading loading was was more more effective effective in controlling in controlling particle particle settling. settling. The SEM images from the (cut and polished) surf surfaceace of the printed magnetic composites exhibit a homogenous microstructure with well distributed particles, as seen in Figure9 9.. TheThe presencepresence ofof resin-rich regions in sample NdFeB80-a10EPX-C (80.2 wt% NdFeB) is an indication that the magnetic fillerfiller loading can be further enhanced to fillfill gaps between particles. Some porosity is also observed in the composites.composites. The The presence presence of of porosities porosities indicates indicates an an opportunity opportunity to to improve improve mixing and degassing processes.processes. Less Less porosity porosity could could be observedbe observed for composite for composite SrFeO80-a10EPX-C, SrFeO80-a10EPX-C, which features which featuresthe smaller the SrFeOsmaller particles. SrFeO particles. Material Material Hy80-a10EPX-C, Hy80-a10EPX-C, which which is a mixture is a mixture of 60.1 of wt%60.1 wt% NdFeB NdFeB and and20.7 wt%20.7 wt% SrFeO, SrFeO, indicates indicates good distributiongood distribution of smaller of smaller and larger and particles larger particles in the composite. in the composite. Material MaterialNdFeB-a10EPX-C, NdFeB-a10EPX-C, which has which the lower has NdFeBthe lower filler NdFeB loading filler of 50 loading wt%, exhibits of 50 wt%, good distributionexhibits good of distributionmagnetic particles; of magnetic no signs particles; of particle no signs agglomeration of particle are agglomeration observed. Utilizing are observed. the ImageJ Utilizing software, the ImageJthe sizes software, of the NdFeB the sizes and of SrFeO the NdFeB particles and were SrFeO derived particles from were SEM derived images offrom composite SEM images samples. of compositeThe measured samples. linear dimensionsThe measured of particles, linear dimensions expressed inof theparticles, histograms expressed in Figure in 10the, indicatehistograms a wide in Figureparticle 10, size indicate distribution, a wide as particle indicated size by distribu the powdertion, as manufacturers. indicated by the powder manufacturers. Permanent magnets are are characterized by by remanence remanence and and coercivity, coercivity, i.e., their ability to supply sufficientsufficient magnetic flux, flux, and withstand demagnetizat demagnetization,ion, respectively. High values of remanence remanence and coercivity indicateindicate strongstrong magneticmagnetic performance. performance. Figure Figure 11 11 shows shows the the hysteresis hysteresis data data obtained obtained for thefor thepowder powder and and magnetic magnetic composite composite samples samples fabricated fabricated in this in research.this research. Obtained Obtained hysteresis hysteresis loops loops were wereobserved observed to be similarto be similar to the to ones the reported ones reported in technical in technical literature literature for isotropic for isotropic bonded bonded magnets magnets [15,37]. [15,37].Figures 12 and 13 show, respectively, the saturation magnetization (Ms), remanence (Mr) and coercive field (H ) extracted from hysteresis loops at temperatures between 300 and 400 K for the Figures 12 andc 13 show, respectively, the saturation magnetization (Ms), remanence (Mr) and composite samples and powders. In these figures, the extracted data from the hysteresis loops are coercive field (Hc) extracted from hysteresis loops at temperatures between 300 and 400 K for the compositedepicted with samples best linearand powders. fits to ease In understanding these figures, the overallextracted influence data from of temperature the hysteresis on loops magnetic are properties. The errors are estimated to be 5 emu/g for remanence and saturation magnetization and 0.1 depicted with best linear fits to ease understandin± g the overall influence of temperature on magnetic± properties.kOe for the coerciveThe errors field are for estimated all samples. to be The ±5 valuesemu/g of for magnetic remanence saturation and saturation for all the magnetization composite samples and ±match0.1 kOe within for the the coercive error band field of for the all loading-adjusted samples. The va powderlues of saturationmagnetic saturation magnetization, for all which the composite indicates samplesthat the saturationmatch within magnetization the error isband not influencedof the load bying-adjusted the composite powder fabrication saturation process. magnetization, Remanence whichfor samples indicates containing that the NdFeB saturation (NdFeB80-a10EPX-C, magnetization is Hy80-a10EPX-C not influenced and by NdFeB-a10EPX-C)the composite fabrication behaves process.similarly Remanence and follows for the loadingsamples adjustedcontaining powder NdFeB remanence. (NdFeB80-a The10EPX-C, composite Hy80-a10EPX-C containing only and the NdFeB-a10EPX-C)SrFeO filler (SrFeO80-a10EPX-C), behaves similarly however, and exhibited follows athe decrease loading in remanenceadjusted powder that exceeds remanence. the expected The compositedecrease due containing to the loading only the factor. SrFeO It isfiller thus (SrF deducedeO80-a10EPX-C), that magnetic however, properties exhibited not only a decrease depend onin remanence that exceeds the expected decrease due to the loading factor. It is thus deduced that magnetic properties not only depend on the magnetic characteristics of the powder but are also a

J. Manuf. Mater. Process. 2020, 4, 4 13 of 18 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 13 of 19 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 13 of 19 functionthe magnetic of filler characteristics distribution of and the powderdispersion but within are also the a function epoxy polymer. of filler distribution In general, andthe dispersionmeasured function of filler distribution and dispersion within the epoxy polymer. In general, the measured valueswithin of the remanence epoxy polymer. are comparable In general, with the the measured values reported values of in remanence the technical are literature comparable for withNdFeB- the values of remanence are comparable with the values reported in the technical literature for NdFeB- andvalues SrFeO-bonded reported in themagnets technical [19]. literature for NdFeB- and SrFeO-bonded magnets [19]. and SrFeO-bonded magnets [19].

FigureFigureFigure 9. 9.9. SEM SEMSEM images images of of magneticmagnetic comp compositesosites listed listed inin in TableTable Table 3. 33..

FigureFigure 10. 10.Size Size distribution distribution of (Aof) NdFeB(A) NdFeB and ( Band) SrFeO (B) particlesSrFeO particles in manufactured in manufactured magnetic compositesmagnetic Figureobtainedcomposites 10. from Size obtained SEM distribution micrographs. from SEM of micrographs.(A) NdFeB and (B) SrFeO particles in manufactured magnetic composites obtained from SEM micrographs.

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FigureFigure 11. 11.Hysteresis Hysteresis data data for for magnetizationmagnetization ve versusrsus applied applied magnetic magnetic field, field, for for (A ()A NdFeB) NdFeB powder, powder, (B)(B SrFeO) SrFeO powder, powder, cured cured materials: materials: (C) NdFeB80-a10EPX-C, (C) NdFeB80-a10EPX-C, (D) SrFeO80-a10EPX-C, (D) SrFeO80-a10EPX-C, (E) Hy80-a10EPX-C (E) Hy80- anda10EPX-C (F) NdFeB-a10EPX-C and (F) NdFeB-a10EPX-C (as per Table (as3). per Table 3).

J. Manuf. Mater. Process. 2020, 4, 4 15 of 18 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 15 of 19 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 15 of 19

Figure 12. (A) Saturation magnetization and (B) remanence of magnetic fillers and 3D-printed FigureFigure 12.12.( A(A) Saturation) Saturation magnetization magnetization and and (B) remanence (B) remanence of magnetic of magnetic fillers and fillers 3D-printed and 3D-printed magnetic magnetic composites (as per Table 3). compositesmagnetic composites (as per Table (as3 ).per Table 3).

FigureFigure 13. 13. CoerciveCoercive field fieldfield of of magnetic magnetic fillers fillersfillers andand 3D-printed3D-printed magneticmagnetic compositescomposites (as(as perper TableTable3 3).).3). The remanence to saturation ratio, which determines the degree of anisotropy, varies between TheThe remanenceremanence toto saturationsaturation ratio,ratio, whichwhich deterdeterminesmines thethe degreedegree ofof anisotropy,anisotropy, variesvaries betweenbetween 0.35 and 0.59 for all samples in the studied temperature range. Note that a remanence to saturation 0.350.35 andand 0.590.59 forfor allall samplessamples inin thethe studiedstudied temperattemperatureure range.range. NoteNote thatthat aa remanenceremanence toto saturationsaturation ratio greater than 0.5 indicates the transition in magnetic properties from isotropic to anisotropic [37]. ratioratio greatergreater thanthan 0.50.5 indicatesindicates thethe transitiontransition inin magnmagneticetic propertiesproperties fromfrom isotropicisotropic toto anisotropicanisotropic [37].[37]. It is unclear whether ratios greater than 0.5, as observed herein, are caused by interparticle interactions. ItIt isis unclearunclear whetherwhether ratiosratios greatergreater thanthan 0.5,0.5, asas observedobserved herein,herein, areare causedcaused byby interparticleinterparticle In terms of temperature, an increase resulted in reductions in saturation, remanence and coercivity (see interactions.interactions. InIn termsterms ofof temperature,temperature, anan increaseincrease resultedresulted inin reductionsreductions inin saturation,saturation, remanenceremanence andand Figures 11–13). Within the given error band, composites and powders exhibited similar temperature coercivitycoercivity (see(see FiguresFigures 11–13).11–13). WithinWithin thethe givegivenn errorerror band,band, compositescomposites andand powderspowders exhibitedexhibited dependencies.similar temperature Note thatdependencies a reduction. Note in M sthat, Mr anda reductionHc with increasingin Ms, Mr temperatureand Hc with is commonlyincreasing similar temperature dependencies. Note that a reduction in Ms, Mr and Hc with increasing observed in magnetic materials. Notably, material SrFeO80-a10EPX-C was the most stable of the temperaturetemperature isis commonlycommonly observedobserved inin magneticmagnetic matematerials.rials. Notably,Notably, materialmaterial SrFeO80-a10EPX-CSrFeO80-a10EPX-C waswas magnetic composites over the temperature range, which is consistent with the properties of the thethe mostmost stablestable ofof thethe magneticmagnetic cocompositesmposites overover thethe temperaturetemperature rangrange,e, whichwhich isis consistentconsistent withwith thethe powder. It was observed that material NdFeB80-a10EPX-C exhibited a stronger dependence on propertiesproperties ofof thethe powder.powder. ItIt waswas observedobserved thatthat materialmaterial NdFeB80-a10EPX-CNdFeB80-a10EPX-C exhibitedexhibited aa strongerstronger temperature with respect to magnetic saturation and remanence compared to all other materials. dependencedependence onon temperaturetemperature withwith respectrespect toto magnetmagneticic saturationsaturation andand remanenceremanence comparedcompared toto allall It is noted from the technical literature that the NdFeB phase undergoes spin reorientations due to otherother materials.materials. ItIt isis notednoted fromfrom thethe technicaltechnical literatureliterature thatthat thethe NdFeBNdFeB phasephase undergoesundergoes spinspin the temperature dependence of anisotropy [38]. In general, magnetic filler loading is observed to reorientationsreorientations duedue toto thethe temperaturetemperature dependencedependence ofof anisotropyanisotropy [38].[38]. InIn general,general, magneticmagnetic fillerfiller influence the coercive field. Material NdFeB-a10EPX-C (50.1 wt% magnetic filler) exhibited lower loadingloading isis observedobserved toto influenceinfluence thethe coercivecoercive fifield.eld. MaterialMaterial NdFeB-a10EPX-CNdFeB-a10EPX-C (50.1(50.1 wt%wt% magneticmagnetic coercivity compared to NdFeB80-a10EPX-C (80.2 wt% magnetic filler). Overall, the latter magnetic filler)filler) exhibitedexhibited lowerlower coercivitycoercivity comparedcompared toto NdFeB80-a10EPX-CNdFeB80-a10EPX-C (80.2(80.2 wt%wt% magneticmagnetic filler).filler). Overall,Overall, thethe latterlatter magneticmagnetic compositecomposite (NdFeB80-a10EPX-C)(NdFeB80-a10EPX-C) exhibitedexhibited thethe bestbest magneticmagnetic characteristicscharacteristics amongamong thethe testedtested materialmaterial formulations.formulations. Notably,Notably, ththee coercivecoercive fieldfield ofof NdFeB80-a10EPX-CNdFeB80-a10EPX-C atat roomroom

J. Manuf. Mater. Process. 2020, 4, 4 16 of 18 composite (NdFeB80-a10EPX-C) exhibited the best magnetic characteristics among the tested material formulations. Notably, the coercive field of NdFeB80-a10EPX-C at room temperature exceeds the properties of injection molded magnets from the same powder as mentioned in technical data sheets. No benefits in terms of Ms and Mr could be ascertained by mixing SrFeO with NdFeB powders (Hy80-a10EPX-C), as this material behaved as expected from a simple mass weighted addition of constituents’ properties. This extends to the behavior for Hc, where the addition of SrFeO did not achieve an increase at high temperatures. These findings are consistent with other research [39].

4. Conclusions In this study, stable magnetic powder-based material formulations were developed and tested that can be utilized for in situ polymerization and material jetting-based additive manufacturing processes. It was observed that the addition of a rheological additive into epoxy enhanced the desired rheological properties, i.e., viscosity at low shear rates, yield strength, degree of shear thinning and storage modulus of the magnetic paste formulation. A transition in the material behavior from viscoelastic fluid to viscoelastic solid was observed in formulations that contained the rheological additive. The material modified with fine strontium ferrite particles, while having favorable rheological properties, exhibited undesirable viscoelastic fluid behavior. All the magnetic paste formulations experienced reductions in viscosity with an increase in temperature, yet the formulation modified with the highest amount of rheological additive resulted in a viscosity for low shear rates at the epoxy curing temperature that was over 500 times greater in magnitude compared to the comparable paste without additive. The ability of the material to maintain high viscosity at low shear rates at curing temperatures enabled the control of particle settling, which was validated using XRD and SEM analyses. A rheological additive loading of 10 wt% was observed to be efficient in controlling magnetic particle settling. The developed material formulation enabled the printing of magnetic composites with a filler loading of 50 wt% and 80 wt% using a material jetting based 3D printer. It was observed that the formulations engineered with the additive maintained the printed shape, whereas significant material deformation was observed in unmodified resins. SEM images indicated good distribution and dispersion of magnetic particles within the composite. Magnetic characterization enabled understanding that the resultant magnetic properties are highly dependent on the magnetic powder characteristics, filler loading and the distribution of magnetic particle within the composite. Coercivity was observed to notably decrease with increasing temperature, whereas only a moderate decrease was observed in magnetic remanence. Even through the addition of strontium ferrite enhanced certain rheological characteristics, the magnetic performance was found to be inferior. Overall, this fundamental work yielded thermally stable magnetic material formulations, which enabled the solving of fundamental material and process issues related to additive manufacturing. Future work will include utilizing the developed material formulations to print magnetic composites and characterize cure and mechanical (elastic, fracture) behavior. Additionally, blending processes shall be optimized to reduce porosity and further engineer formulations to reach higher magnetic filler loadings.

Author Contributions: B.N., A.J.Q. and P.M.conceptualized the research framework. B.N. developed formulations, prepared feedstock materials, conducted settling experiments, 3D printed samples, conducted SEM and XRD work, interpreted results and wrote the manuscript. P.M., M.K., S.T., and M.A.W.S. reviewed and revised the manuscript. M.K. conducted rheological tests and M.A.W.S. conducted the magnetic characterization. U.S. and S.T. assisted in providing rheometer and SQUID facilities for the characterization and assisted B.N. in characterizing polymer formulations and magnetic composites. All authors have read and agreed to the published version of the manuscript. Funding: This work is a part of the University of Alberta’s Future Energy Systems research initiative and the Canada First Research Excellence Fund (CFREF). The authors are thankful for the gracious funding to make this research possible. Funding to M.A.W.S. was provided by the Alexander von Humboldt Foundation Feodor Lynen Research Fellowship and the University of Calgary’s Global Research Initiative in Sustainable Low Carbon Unconventional Resources funded by the Canada First Research Excellence Fund (CFREF). This research used facilities funded through a grant from the Canadian Foundation for Innovation John R. Evans Leaders Fund (CFI-JELF). J. Manuf. Mater. Process. 2020, 4, 4 17 of 18

Acknowledgments: The authors would like to express their gratitude to the staff at the nanoFAB and the Department of Earth and Atmospheric Sciences at the University of Alberta for supporting this research and providing access to XRD and SEM facilities. Conflicts of Interest: The authors declare no conflict of interest.

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