Influence of powder feedstock characteristics on extrusion-based 3D printing of magnetocaloric structures

A significant barrier to the commercialization of magnetic heat pumping is the lack of scalable, low-cost manufacturing techniques that enable shaping brittle magnetocaloric materials into heat exchange structures with porous geometries, controlled chemical gradients, and advantageous anisotropic microstructures. Though direct ink writing additive manufacturing has the potential to expand into a viable net-shaping technology for functional magnetic alloys, it is typically challenging to fabricate dense parts—an observation ascribed to the constraint on powder particle size that inevitably impacts both green density of 3D printed parts and shrinkage during sintering. To this end, we report a comprehensive study on the influence of precursor powder characteristics on the magnetic and structural properties of 3D printed test coupons produced using La0.67Ca0.33MnO3 magnetocaloric particles. Ink formulations comprising powders with nano-scaled, micron-scaled, and bimodal size distributions were printed and sintered. The impact of particle size on part quality and magnetofunctional response was examined, and it was found that test coupon fabricated using nano-scaled powders (∼100–200 nm) demonstrated the lowest part porosity (∼17%) and the highest magnetocaloric response (8 J kg−1·K−1 at μ 0H = 5T). The results presented in this work address critical technical questions about the process feasibility of making magnetic heat pumps with additive manufacturing schemes.


Introduction
Since the first additive manufacturing (AM) concept was patented in 1986 by Charles Hull, the scientific community has developed numerous AM technologies for net-shaping engineering materials for diverse applications ranging from the aerospace and automotive to the biomedical and wireless communication industries [1].Over the last two decades, AM has rapidly transformed from a laboratory-scale rapid prototyping technique to an industrial-scale manufacturing technology for both structural and functional materials [2][3][4], and it is increasingly finding its way into the global magnet industry [5][6][7].Against this background, magnetic heat exchangers used in magnetocaloric heat pump (MCHP) technologies are exciting end-use applications for additive manufacturing processing schemes.
MCHPs are based on the magnetocaloric effect (MCE), a thermal response that magnetocaloric materials (MCM) demonstrate when exposed to a magnetic field change [8].Theoretically, MCHPs can achieve higher efficiencies than traditional vapor-compression cooling devices, and due to the complete absence of gas compressors, they also produce less noise -an important feature that restricts widespread adoption of these devices [9].From a manufacturing perspective, MCHPs involve three main elements: (i) a magnetic field source, (ii) a magnetic heat exchanger, also known as an active magnetocaloric regenerator (AMR), and (iii) a hydraulic circuit for the passage of a heat transfer fluid [8].Among these, the AMR is the indispensable component of an MCHP as it is built entirely from MCMs that demonstrate large isothermal entropy change (ΔS mag ) and an adiabatic temperature change (ΔT ad ) in response to a varying magnetic field [10].In addition to excellent magnetocaloric characteristics, the geometry of the AMR must be optimal such that it satisfies the following criteria: (i) low porosity to maximize the amount of working MCM material and reduce demagnetization effects during magnetic field cycling , (ii) a large surface-to-volume ratio to maximize the heat transfer area between the MCM and the heat exchange fluid, (iii) low surface roughness and uniform channel cross-section to minimize flow resistance that leads to pressure drop along the length of the AMR [10].While most of the magnetic refrigerator prototypes to date still utilize MRs with very simple geometries, such as packed particle bed and epoxy-bonded parallel-plate structures [8], in recent years, several studies have been devoted to the fabrication of channeled magnetocaloric structures using AM processing schemes (see detailed list in table 1 of [11]).Among these AM methods, direct ink writing DIW (extrusion-based 3D printing) stands out due to its unique ability to extrude material at room temperature and applicability to various materials, including ceramics, polymers, metals, and composites [8].In previous work, we have demonstrated the proof-of-concept feasibility of 3D printing spatially designed microchannelled magnetocaloric heat exchanger structures with minimum dimensions of ∼150 μm using a nozzle of diameter ∼200 μm [11].
In DIW, a liquid-phase 'ink' comprising primarily of powders of the working material and a sacrificial binder is dispensed out of small nozzles under controlled flow rates and deposited along digitally defined paths to fabricate 3D structures layer by layer.The initial printing process produces a green part that is dried, debinded, and sintered to obtain the final functional object.Like in any AM technique, both material properties and processing parameters impact the quality of the printed part.It is well known that the feedstock powder characteristics, such as particle size distribution and shape, directly affect the particle dispersion, particle packing, and rheological properties of the inks [12].While a spherical powder shape is preferred for feedstock powders in AM technologies to enhance flowability, layer spreading, and loose powder packing [13], the availability of spherical magnetocaloric powders for AM is strictly limited, mainly due to its prohibitively high cost in the commercial sector.
Against this background, this paper addresses the processing challenges associated with 3D printing magnetocaloric components using inks with high solid loading of non-spherical particles.Here, we examine the effect of precursor powder characteristics on the structural attributes and magnetocaloric properties of test coupons 3D-printed using La 0.67 Ca 0.33 MnO 3 magnetocaloric powders prepared via a wet chemical technique.The end goal is to maximize the performance of the printed magnets by understanding the underlying role of feedstock powder characteristics on the microstructure and functional response of the 3D-printed part.

Experimental methods
2.1.Preparation of feedstock La 0.6 Ca 0.4 MnO 3 powders Precursor powders of nominal composition La 0.67 Ca 0.33 MnO 3 were prepared via a modified Pechini sol-gel synthesis method as described in [14].All reagents, lanthanum(III) nitrate hydrate (Sigma Aldrich 99.9%), calcium carbonate (Sigma Aldrich), manganese(II) nitrate hydrate (Sigma Aldrich 98%), citric acid (Sigma Aldrich 99.5%), (A.C.S. reagent), polyethylene glycol PEG (Sigma Aldrich), and nitric acid (Fischer Scientific) were used without any further purification.In a typical reaction, La(NO 3 )•xH 2 O, CaCO 3 , and Mn(No 3 ) 2 •xH 2 O serve as the metal precursors that are mixed with a chelating agent (citric acid) and a cross-linking agent (PEG) in a concentrated nitric acid solution (4M).The solution is heated to 100 °C for 12 h for gel polymerization -a reaction that yields a brown sticky gel that is dried and then annealed in the open air for 10 h at temperatures ranging from 900 °C to 1500 °C to obtain the final La 0.67 Ca 0.33 MnO 3 powder product.The powder is then crushed in a mortar and pestle and sieved into − 450 mesh using a sieve shaker.

3D printing process and post-processing heat treatment
The ink formulation consists of magnetic La 0.67 Ca 0.33 MnO 3 powder, sacrificial polymer binder (specifically, polyethylene oxide PEO of an average molecular weight of 1,000,000, Thermo Fisher Scientific) that are suspended within a tri-solvent system comprised of dichloromethane DCM (Sigma-Aldrich), ethylene glycol butyl ether EGBE (Sigma-Aldrich), and dibutyl phthalate (Sigma-Aldrich).All these materials were used in their as-received state without undergoing any additional purification steps.
Following guidelines obtained from [11], La 0.67 Ca 0.33 MnO 3 -based inks consisting of ∼80 wt% magnetocaloric powder and 20 wt% PEO were prepared.For every 1g of magnetic powder, 0.091 g EGBE, 0.006 g DBP, and 0.107 g DCM was added and mixed by a planetary centrifugal mixer (Thinky USA) for 1 min at 2000 RPM to form a particle slurry.PEO was dissolved in DCM with a 66.7 mg mL −1 concentration and retained for 24 h to ensure a uniform solution.The dynamic viscosity of the three ink formulations was characterized on a high precision rheometer (Anton Paar MCR301), and consistent with results presented in [11], the inks prepared in this study demonstrated optimal shear thinning and ink stability characteristics.The printing conditions kept in the range: printing speed = (1.6-2mm s −1 ), pressure = (0.7-1.2 bar), nozzle size − 400 μm.After printing, the samples were heat-treated in a Muffle furnace (MTI-1600 GX) in open-air following the two-stage protocol wherein the printed part was first heat-treated to ∼450 °C for 30 min to remove the sacrificial PEO polymer binder and then sintered at 1200 °C for 2 h and eventually to 1400 °C for an additional 2 h, to promote grain growth and densification (ramp rate during heating ∼5 °C min -1 ).

Material characterization
Structural evaluation of the precursor powders and the 3D printed samples was achieved using x-ray diffraction (XRD) with Cu-Kα radiation (Rigaku, MiniFlex600/).Phase identification was accomplished by performing Rietveld refinement using Highscore software.This refinement involved optimizing major parameters such as scale factor, background, zero shift, lattice parameters, atomic coordinates (x, y, z), peak shape parameters.The microstructure of the 3D printed samples was examined using optical microscopy and field emission scanning electron microscopy (SEM) utilizing a Hitachi SU-70 instrument.While the apparent porosity of the powders was estimated by comparing the bulk density (measured using the ASTM B212-17 standard) with the theoretical density of La 0.6 Ca 0.4 MnO 3 , the volumetric porosity of the 3D printed samples was examined using x-ray microcomputed tomography XCT (Skyscan 1173).The XCT analysis was performed with the following settings: a voxel size of 3, a voltage of 130 kV, a current of 61 μA, an exposure time of 1500 ms, a rotation step of 0.3 deg, frame averaging of 4, and an Image Pixel Size of 7.91 μm.After conducting the scans on the 3D printed and sintered scaffolds, the obtained raw data underwent reconstruction using the N-Recon software (Skyscan) version 1.7.5.0, with the reconstruction engine GPU Recon server.Subsequently, the reconstructed images were centered for COR, TRA, and SAG VOI (Volume of Interest) using data viewer software and were further analyzed using CTan software.Magnetization measurements were carried out using a vibrating sample magnetometer in a Quantum Design physical property measurement system (PPMS).The temperature dependence of the magnetization M(T) data was obtained to measure the magnetization (M) of all the precursor powder samples and 3D printed samples before and after the sintering processes.The magnetic properties were investigated over a 150-400 K temperature range, utilizing applied magnetic fields varying from 0.1 T to 5 T. The Curie temperature was determined by identifying the inflection point on the M (magnetization) versus T (temperature) curve.The magnetocaloric behavior of the precursor powders and the 3D printed samples was indirectly evaluated from the magnetic entropy change under an applied magnetic field of μ 0 H 5 T.This change in entropy was calculated using Maxwell relationship [15][16][17], Here, μ 0 is the permeability of free space, (∂M)/(∂T) is the temperature derivative of the magnetization, and Hmax is the maximum applied magnetic field.

Results
In this section, the structural and magnetic attributes of feedstock La 0.67 Ca 0.33 MnO 3 magnetocaloric powders prepared via the modified Pechini sol-gel method [14] is presented first; the microstructural features and magnetocaloric properties of test coupons La 0.67 Ca 0.33 MnO 3 3D-printed via a direct ink writing scheme developed in [11] is discussed thereafter.

Structural and magnetic properties of feedstock powders
The XRD pattern of La 0.67 Ca 0.33 MnO 3 powders annealed in the temperature range 900 °C to 1500 °C, shown in figure 1(a), indicates the presence of Bragg peaks corresponding to a single-phase material with an orthorhombic crystal structure with a space group of Pnma.With an increase in annealing temperature (T a ), the diffraction intensity increases, and the linewidth (β) of XRD peaks becomes narrower, indicating an increase in the crystallinity of the powders.Based on the Williamson-Hall equation of β cos θ = (Kλ/d) + 2ε sin θ (where K = 0.9 and θ are the shape factor and the diffraction angle, respectively), the average values of crystallite size (d) and lattice strain (ε) was determined, as reported in table 1.With increasing T a , d gradually increases from ∼62 nm to 851 nm, and concurrently, ε decreases from 0.22% to 0.07%, figure 1(b).It is worth noting that the crystallite sizes reported for annealing temperatures greater than 1000 °C are a mere estimated value as the Williamson-Hall method is more accurate for nanoparticles with d < 100 nm [18].The lattice parameters (a), (b), (c) and unit cell volume (V) are also reported in table 1, and overall, a monotonic increase in unit cell volume is noted with an increase in T a .
Consistent with the trends observed in the XRD data, a systematic increase in the size of the particles is observed with an increase in T a using SEM. Figure 2 shows representative SEM micrographs of crystalline La 0.67 Ca 0.33 MnO 3 powders annealed at 1100 °C and 1500 °C, respectively.The powders in both samples are irregular in shape and agglomerated.While powders annealed at 1100 °C show nano-scaled particle sizes ranging from 100 nm to 200 nm, annealing at 1500 °C results in a massive growth in particle size, resulting in particles ranging between 2-4 μm in size.Following this initial investigation, the La 0.67 Ca 0.33 MnO 3 powders were grouped into two categories with two well-differentiated particle sizes.Nanoscaled powders obtained via  heat treatment at 1100 °C, demonstrating a bulk density of 4.47 g cm −3 and a porosity of ∼40%, will be referred to as fine particles hereafter.Micron-scaled powders obtained via heat treatment at 1500 °C demonstrate a bulk density of 5.50 g cm −3 and a porosity of ∼27%, will be referred to as coarse particles hereafter.
Temperature-dependent magnetization curves measured at an applied magnetic field of 20 kOe (μ 0 H app = 2 T), shown in figure 3(a), demonstrate that both the fine and the coarse particles undergo a ferromagnetic (FM) → Paramagnetic (PM) transition upon heating.While the overall saturation magnetization of both powders is comparable (Ms∼80 emu g −1 ), the Tc of the coarse powders was found to be slightly higher than that of the fine powders (T c,fine = 252 K, T c,coarse = 266 K).Magnetic entropy change curves (ΔSmag versus T plots) measured at μ 0 H app = 5 T, shown in figure 3(b), indicate a peak magnetic entropy change (ΔS max ) corresponding to 5.6 J.kgK and 6.3 J.kgK in the fine and coarse powders respectively.The temperatures corresponding to ΔS max are consistent with the T c of the powder samples.As such it is surmised that the Tc and ΔS max of the La 0.67 Ca 0.33 MnO 3 powders increase with particle size-an observation consistent with previous reports in the literature.

Structural and magnetic properties of 3D printed test coupons
To evaluate the effect of precursor powder size and its distribution on the functional response of magnetocaloric components fabricated using DIW 3D printing, three ink formulations consisting of ∼80 wt% La 0.67 Ca 0.33 MnO 3 powder combined with 20 wt% sacrificial PEO binder were prepared-one using fine particles, a second using coarse particles, and finally, a third using both fine and coarse particles mixed in the ratio of 3:1 by weight.The printability of the inks was confirmed by assessing the rheological behavior of the ink formulation via dynamic viscosity measurements (see [11] for details).Subsequently, three test coupons were 3D printed and sintered, and depending upon the particle size of the magnetocaloric powders used in the precursor ink, they will be referred to as Samples S fine , S coarse , and S mixed subsequently in the manuscript.
The XRD patterns of all three 3D printed samples post-sintering indicate a highly crystalline structure without any impurity phases (see figure S1 in the supplemental section).All the Bragg peaks in the samples index to the standard reflection pattern of the orthorhombic perovskite crystal structure, with comparable lattice parameters, table 2. In the as-printed state, the particles are uniformly dispersed in the binder but highly aggregated.The volumetric porosity of the samples S fine , S coarse , and S mixed was determined as ∼21.1%, 15.9%, and 14.8%, respectively.During thermal polymer de-binding, there is a mass transport phenomenon wherein the binder migrates outward, leaving significant voids at the center of the sample During the sintering process, the La 0.67 Ca 0.33 MnO 3 particles coalesce, and a volumetric shrinkage of ∼57%, ∼50%, and 32% was observed in  samples S fine , S mixed , and S coarse , respectively.While sintering decreases the volumetric porosity of Sample S fine by 4% to a value of ∼17%, the volumetric porosity of Samples S coarse and S mixed were found to increase by 3% to a final value of ∼18.8% and 17.9%, respectively.For reference, coronal cross-sectional view of CT scans of the test coupons in their green and sintered state are shown in figure 4. The images shown here correspond to a slice with a cross sectional area ∼7.5 × 7.5 mm taken from the middle of the sample.Video 1 in the supplement shows the porosity distribution across the thickness of the 3D printed S coarse sample.The magnetic properties of the 3D-printed test coupons are reported in table 2. Figure 5 shows the temperature-curves measured at μ 0 H = 2 T and the magnetic entropy change curves measured at μ 0 H app dependent magnetization = 5 T for the 3D printed samples post-sintering.The overall magnetization of S coarse is slightly smaller than that of the Samples S fine .While the T c of S fine and S Coarse sintered samples was determined as ∼266 K, that of S mixed is slightly lower at 258 K.In all the sintered samples, the temperature at which ΔS max is observed corresponds to their T c .The ΔS max of sample S fine (8 J/kg-K) was found to be higher than that of samples S coarse (6.38 J/kg-K) and S mixed (6.73 J/kg-K).As such, ΔS max of the 3D printed samples increases with decreased part porosity, figure 6.

Discussion
Additive manufacturing (AM) is widely recognized as a disruptive technology poised to bring a paradigm shift in design and manufacturing by facilitating the production of customized products with considerable geometric complexity, extended capabilities, and functional performances.Yet, despite the promise, AM at the commercial scale is limited to only a few weldable structural materials.In particular, AM for available magnetocaloric materials is still in its initial research and development phase and is focused mainly on Ni-Mn-Ga Heuslers [19] and La(Fe, Si) 13 -based alloys [20,21].Laser-based powder bed fusion (PBF) and direct energy deposition (DED) techniques are fraught with challenges related to phase stability due to melting and re-crystallizing of the compositionally sensitive magnetocaloric particles during shaping [11,22], the polymer binder in the composite precursor filaments used in FDM dilutes the functional response of the magnetocaloric heat exchanger structure [23].Though low-temperature processing binder jetting and direct ink writing results in 3D-printed parts with controlled chemical homogeneity, they exhibit significant porosity due to the absence of compacting forces during the printing and subsequent debinding/sintering process [11,[24][25][26].To reduce porosity, comprehensive knowledge of powder characteristics (mainly particle size, morphology, and density) is critical.Within the context of DIW 3D printing, we provide a powder optimization framework to improve part density and functional response in additively manufactured magnetocaloric components.
As a starting point, it is worthwhile to note that instead of Ni-Mn-Ga Heuslers and La(Fe,Si) 13 -based alloys that demonstrate promising magnetocaloric response in the range of (3.5 to 15.1 J/kgK and 4 to 30 J/kgK at 2 T) [27,28], the candidate materials system for this study is calcium-substituted lanthanum manganite-a chemically stable perovskite oxide that shows a moderate delta S value of 1.38 to 5.5 J/kgK at 1.5 T [29,30].Relative to intermetallic magnetocaloric materials, the ease of synthesis of LCMO is unparalleled, and over the last few decades, several chemical methods have been used to obtain substituted lanthanum manganites: solid  state method, sol-gel, chemical homogenization method, ball milling, flux method, heteronuclear complexing, and microwave heating [31].Among these methods, the Pechini method (a variation of the sol-gel method) was down-selected for this study as it inexpensively effectively produces powders with particle sizes ranging from nano to sub-micrometers depending on the annealing temperatures during processing [32].The La 0.67 Ca 0.33 MnO 3 powders used in this study demonstrate an irregular morphology, figure 2, and the average grain size increases due to increasing heat treatment temperature, T a .Consistent with the literature [14], the powders' overall bulk density, unit cell volume, Tc, and ΔS max increase with particle size, table 1.
Powder selection for DIW requires a thorough understanding of the whole manufacturing process.An adequate ink for DIW has to be chemically homogeneous, extrudable, have shape-retention capacity, and be self-supporting -features that depend on solid volume, geometry, size distribution, and surface chemistry of the particles and binder [33].In this study, inks with good processability and optimum shear thinning behavior were obtained with a solid loading of ∼80 wt% magnetocaloric powder and 20 wt% PEO [11].During printing, solid particles are deposited almost without external forces, and consequently, before sintering, the green parts are inherently porous.Sample S coarse demonstrates lower part porosity than Sample S fine due to the larger magnetocaloric particles used in its corresponding ink formulation, figure 7. One well-established theory for improving green density in DIW is using bimodal powder mixtures wherein the small particles fill the interstitial voids between large particles [34].Indeed, we find that before sintering, sample S mixed demonstrates an even lower part porosity than Sample S fine , figure 7.
While the packing density in 3D printed parts fabricated using DIW can be predicted with reasonable accuracy, there is significant uncertainty in the sintered density of a bimodal mixture [35] because powder characteristics that improve its processability during 3D shaping are detrimental to densification during debinding and sintering [36].Typically, fine powders are more active in sintering as there is more surface interaction between particles per volume, large particles significantly inhibit sintering densification due to the low sintering driving force.Adding coarse powders to an acceptable powder matrix constrains the sintering of the small powders near the large particles, leading to cracks or pore growth, figure 8.In [36], German et al developed a theoretical model to predict the sintered density of bimodal powder mixtures and suggested a critical connection between the part shrinkage and the design of powder mixtures for minimal sintered porosity.Generally, when a powder mixture contains fine particles with large sintering shrinkage and coarse particles with small sintering shrinkage, the highest density is achieved by only using the fine particles.Conversely, when a powder mixture combines fine particles with minor and coarse particles with significant sintering shrinkage, the highest density is achieved with a bimodal mixture.Over the last three decades, the German model has been validated experimentally using a wide range of spherical and irregular-shaped powders of various metals, alloys, and ceramics (examples include Mo, Fe, Cu, W, 316L stainless steel, BaTiO 3 , etc) [36].Against this context, it is essential to note that in this DIW study with chemically synthesized La 0.67 Ca 0.33 MnO 3 powders, the sintering shrinkage of the fine powders in sample S fine (57%) is significantly higher than that of the coarse powders in sample S coarse (32%).Thus, adding coarse powders to a fine powder matrix offers no improvement to the overall densification of the sample S mixed during sintering, figure 7.
Last, it is worthwhile to note that the influence of particle size on the magnetocaloric response of lanthanum manganites has been extensively studied in the literature, and it is well-known that in bulk powders, an increase in grain size improves the magnetocaloric response [29,31].Indeed, the micron-scaled coarse precursor powders examined in this work demonstrate a larger ΔS max than the nano-scaled fine powders, figure 3.Among the 3D printed test coupons, the sintered S fine sample shows the lowest part porosity; hence, it exhibits the highest ΔS mag (figure 6).The magnetocaloric response of the 3D-printed test coupons can likely be further improved by carefully tuning the process parameters during sintering (heat treatment temperature, time, ramp rate, etc).

Conclusions
Additive manufacturing (AM) is a promising route for shaping magnetocaloric heat exchangers in magnetic heat pump devices, and over the past decade several AM schemes have been explored to 3D print magnetocaloric structures.Most studies in the literature to date are focused on laser-based powder bed fusion (PBF) and direct energy deposition (DED) AM methods wherein melting of a MCM complicates phase formation, which occasionally leads to loss of functionality [37][38][39].The importance of metal powder characteristics in the PBF and DED processes is well documented [5,[39][40][41].How the powder flows and packs, can have a significant impact on powder bed formation, and hence the development of melt pools and microscopic homogeneity.Excessive variations in powder characteristics can lead to nonuniform deposition, inconsistent bulk density, increased defects, undesired mechanical properties, and poor surface finish.As a result, it is essential to be able to identify the various powder characteristics that can ensure consistent and reliable performance, particularly when a lower cost, less spherical powder is intended for AM.Binder Jetting (BJ) for 3D printing magnetocaloric structures has been examined in [5,26,42] and reports indicate that as in PBF technology, continued reuse of the powder material in BJ causes changes in composition, size distribution and surface characteristics of the powder particles which complicates the manufacturing process.No study to date has focused on examining the influence of precursor powder characteristics on magnetic and structural properties of 3D printed test coupons fabricated using DIW, and to this end, this study aims to close this gap in the literature.
Precursor powders were synthesized via a modified Pechini sol-gel synthesis method and annealed at temperatures in the range 900 °C to 1500 °C.Consistent with literature, it was found that La 0.67 Ca 0.33 MnO 3 particle size and magntocaloric response (as quantified by ΔSmax) increases with annealing temperature [14].Subsequently, three ink formulations consisting of ~80 wt% La 0.67 Ca 0.33 MnO 3 powder combined with 20 wt% sacrificial PEO binder were prepared-one using nano-scaled particles (100-200 nm), a second using micron-scaled particles (1-3 μm), and finally, a third using both fine and coarse particles mixed in the ratio of 3:1 by weight.The impact of particle size on part quality and magnetofunctional response was examined using a range of structural and magnetic probes, and it was ultimately found that the test coupon 3d printed using nano-scaled powders (∼100-200 nm) demonstrated the lowest part porosity (∼17%) and the highest magnetocaloric response (8 J kg −1 •K −1 at μ 0 H = 5T).Porosity is known to have a profound effect on a material's mechanical properties, often weakening the material.Highly porous 3D printed parts are undesirable for supporting a load-based structure due to the voids that are present throughout the microstructure of the material, and will likely lead to mechanical fatigue in a magnetic heat pump device wherein the AMR is subjected to a periodic magnetic field at frequencies up to ∼10 Ghz.
In summary, this work provides insight on how powder size distribution influences functional properties of magnetic alloys shaped using DIW-based additive manufacturing.Further, guidelines for powder optimization in order to achieve high sintered density in 3D printed magnetocaloric components is obtained.Since the magnetocaloric response of La 0.6 Ca 0.4 MnO 3 is rather modest [11,14], this framework will be applied to ongoing research focused on 3D printing commercially viable magnetocaloric alloys belonging to the La(Fe,Si) 13 family.Investigations to this end are ongoing and will be reported elsewhere.

Figure 1 .
Figure 1.(a) Room temperature XRD pattern of precursor powders annealed at different temperatures; (b) Comparison between crystallite size versus annealed temperature.

Figure 2 .
Figure 2. SEM image of precursor LCMO nanoparticles annealed at 1100 °C and 1500 °C for 10 h.

Figure 3 .
Figure 3. (a) Magnetothermal characteristics of the precursor powder sample A and sample B at an applied magnetic field of μ 0 H = 2 T; (b) Magnetic entropy change of the precursor powder sample A and sample B at an applied magnetic field of μ 0 H = 5 T.

Figure 4 .
Figure 4. Coronal cross-sectional view of CT scans (a) Green body of sample S fine .(b) Green body of sample S Coarse .(c) Sintered sample S fine (d) Sintered sample S Coarse .During de-binding, there is a mass transport phenomenon wherein the binder migrates outward, leaving significant voids at the center of the sample.The images shown here correspond to a slice with a cross sectional area ∼7.5 × 7.5 mm taken from the middle of the sample.

Figure 5 .
Figure 5. (a) Magnetothermal characteristics of the sintered 3D part sample A, sample B and sample C at an applied magnetic field of μ 0 H = 2 T; (b) Magnetic entropy change of the sintered 3D part sample A, sample B and sample C at an applied magnetic field of μ 0 H = 5 T.

Figure 7 .
Figure 7. (a) Porosity versus 3d printed green body and sintered parts (b) Volume shrinkage versus porosity of sintered 3d parts.

Figure 8 .
Figure 8. Schematic of particle size distribution in green and sintered test coupons obtained using fine, coarse and bimodal powder mixtures.Particles in the green body are constrained as they are combined with the sacrificial binder (shown in light pink).During the sintering process, the binder is removed and subsequently the particles coalesce and grow in size, resulting in part densification.

Table 1 .
Comparison between lattice parameters, crystallite size, strain, magnetic and magnetocaloric properties of LCMO precursor powders annealed at different annealing temperatures.

Table 2 .
Comparison between lattice parameters, magnetic and magnetocaloric properties of sintered parts with different particles sizes.