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Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension
Journal Pre-proof Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension Taekyu An, Kwang-Taek Hwang, Jin-Ho Kim, Jihoon Kim PII:
S0272-8842(19)33317-6
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.127
Reference:
CERI 23496
To appear in:
Ceramics International
Received Date: 16 October 2019 Revised Date:
13 November 2019
Accepted Date: 15 November 2019
Please cite this article as: T. An, K.-T. Hwang, J.-H. Kim, J. Kim, Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.11.127. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension
Taekyu An1, Kwang-Taek Hwang2, Jin-Ho Kim2 and Jihoon Kim1 * 1
Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungchungnam-do 32588, Korea
2
Icheon Branch, Korea Institute of Ceramic Engineering & Technology (KICET), Icheon-si, Gyeonggi-do 17303, Korea
*Corresponding author Email address: [email protected] (J. Kim)
ABSTRACT
Advances in additive manufacturing (AM), represented as the instant evolution of design files into fully functional products, provide customer-centric smart manufacturing solutions that are increasingly replacing traditional manufacturing methods in various industries. However, the advantages of AM often diminish when applied to the production of ceramic components, owing to the demanding nature of ceramic materials. Extrusion-based direct ink writing (DIW) is considered one of the few AM technologies applicable to ceramic materials. Precise control of
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the ceramic suspension is required in extrusion-based 3D DIW to create 3D structures for various industrial products. This study describes a fundamental relationship between the rheological properties of the ceramic suspensions and their 3D DIW printability. The rheological properties determine flowability, DIW height, and the overhang angle that can be achieved in extrusionbased 3D DIW. The printability of NiZn-ferrite suspensions with various rheological properties was investigated, and the potential of 3D DIW amenable to manufacturing ceramic electronic components was demonstrated.
KEYWORDS: Direct ink writing, Auger-extrusion, NiZn-ferrite, Magnetic core, Ceramic 3D printing
1. INTRODUCTION Functional ceramic materials have been widely employed in various emerging technical applications because of their inherently superior electrical, optical, and magnetic properties, as well as their excellent mechanical properties such as high-temperature resistance, abrasion resistance, and high strength [1-11]. However, their use is often limited due to manufacturing constraints of ceramic components with geometric complexity [12-17]. Traditional manufacturing processes, represented by injection molding, isostatic pressing, and casting, are limited to forming relatively simple 3D shapes and 2D films, and are considered costly and timeconsuming [18-25]. This technical challenge has generated considerable industrial interest in exploring additive manufacturing technologies that replace or complement the traditional processes to form functional ceramic components having complex geometries.
2
Additive manufacturing (AM) is a widely adopted technology for realizing 3D structures with intricate geometries owing to its numerous advantages compared with conventional ceramic fabrication processes, such as freedom of design, low manufacturing cost, and short production runs [26-27]. AM techniques include stereolithography [28], selective laser sintering [29], powder bed printing [30], and direct ink writing (DIW) [31]. DIW is considered economically advantageous because it has a simple equipment setup that extrudes high-solid-loading ceramic suspensions through a computer-controlled nozzle to print complex 3D structures. However, extrusion-based DIW requires stringent control over the rheological behavior of the ceramic suspension; it must flow easily through the nozzle during the extrusion yet have sufficiently high strength and stiffness to be capable of maintaining the direct-ink-written (DIWn) shape. Such rheological behavior of the ceramic suspension involves a viscoelastic property having high shear thinning (liquid-like behavior) at high shear rates during the extrusion, as well as sufficient yield stress (solid-like behavior) to resist the gravitational slumping force that could deform the DIWn object. Effective use of the viscoelastic ceramic suspension also requires precise control of the extrusion process since the suspension responds slowly to changes in the extrusion force. This inevitably results in time delays when starting and stopping the extrusion process. An auger extruder is suitable for ensuring a rapid response to the extrusion force because it exerts an extrusion force on a relatively small suspension volume in a auger chamber (Figure 1), unlike a conventional ram extruder that uses a syringe and plunger where the entire suspension stored in a relatively large syringe barrel [32-33]. The present study investigated the rheological requirements of a high-solid-loading ceramic suspension for printing 3D structures by extrusion-based 3D DIW. A NiZn-ferrite (NZF)
3
suspension having viscoelastic characteristics was formulated and extruded to form 3D structures. The rheological properties of the viscoelastic NZF suspension were analyzed to predict the maximum cumulative height of 3D-DIWn structure that was achievable without structural deformation by the gravitational slumping force. The cumulative height of the 3D-DIWn structure at various overhang angles was also investigated by examining the yield stress and center of gravity (CoG) of the 3D structures. The magnetic properties, dimensional shrinkage, and density of the 3D-DIWn NZF structures were investigated after sintering at elevated temperatures. Finally, NZF magnetic cores having an economical flat design (EFD) were DIWn and applied to the fabrication of transformers. Performance of the transformer having the 3DDIWn EFD magnetic core was compared with that of a commercial core having the same design.
2. EXPERIMENTAL SECTION 2.1 Formulation of viscoelastic NZF suspension Polystyrene-polyisoprene-polystyrene (SIS, 22% styrene, Sigma Aldrich) stock solution was made by dissolving SIS powder in n-methyl-2-pyrrolidone (NMP, boiling point: 204 °C, density: 1.03 g/mL at 25 °C) at a weight ratio of SIS:NMP = 1:2.5. This stock solution was mixed in a Thinky mixer for 5 min at 2000 rpm and then defoamed for 1 min at 2200 rpm five times. NZF nanoparticles (Ni0.4Zn0.6Fe2O4, D50 = 500 nm, Amotech) were then added to the stock solution in a stepwise manner to prepare the NZF suspension. The NZF solid loading in the suspension was fixed at 58 vol %. To adjust the viscoelasticity, the SIS concentration in the NZF suspension was varied from 3 to 10 vol % with the addition of NMP. The final NZF suspensions were then mixed and defoamed in a Thinky mixer using the same method as that for preparing the SIS stock solution.
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2.2 3D DIW and sintering The NZF suspensions were loaded in syringe barrels and defoamed in a Thinky mixer for 30 s at 2200 rpm before DIW. The NZF suspension was extruded using a commercially available auger dispenser (MSD-3, Musashi) having a nozzle with an orifice inner/outer diameter of 0.72/1.08 mm. The auger dispenser was mounted on a 3D printing stage (Prusa I3). The thickness of each layer of the 3D-DIWn structure was fixed at 0.5 mm. The DIW speed in the xand y-directions was between 5 and 10 mm/s, while the build rate in the z-direction (DIW speed in z-direction) was fixed at 2 mm/min. The 3D structures were DIWn at room temperature in air and then dried at 60 °C for 24 h before sintering. The dried green structures were sintered in a box furnace at 930 °C for 5 h after a temperature ramp of 20 °C/h. 2.3 Materials characterization Rheological properties of the NZF suspensions were characterized with an Anton Paar rheometer (MCR-92) equipped with a 25 mm parallel plate (PP25) and a 25 mm perforated plate (PP25-perforated). Flow behavior (shear stress vs shear rate) and amplitude sweep (complex shear modulus vs shear stress) were measured to determine the shear thinning, yield stress, and viscoelasticity of the NZF suspensions. Phase analysis of the 3D-DIWn structure before and after sintering were performed by X-ray diffraction (XRD) using Cu-Kα radiation (Rigaku Miniflex 600). The Archimedes method described in ASTM standard C373-18 was employed to determine the densities of the 3D-DIWn NZF structures before and after sintering at various temperatures. The microstructures of the 3D-DIWn structures were investigated using a field emission scanning electron microscope (FESEM, Nova 200). The magnetic hysteresis curves and frequency-dependent permeability of the 3D-DIWn NZF structures were characterized using a B-H analyzer (SY-8218, IWAST) and an impedance analyzer (Agilent 4294A), respectively. The
5
performance of a transformer with a 3D-DIWn NZF core was demonstrated using a signal generator (HP 432B) and an oscilloscope (GW Instek GDS-2072E) with a current probe (Tektronics P6022) and a multimeter (GW Instek GMD-8341).
3. RESULTS AND DISCUSSION 3.1. Rheological behavior of NZF suspensions for extrusion-based 3D DIW One of the most important rheological requirements for extrusion-based 3D DIW is the ceramic suspension having a viscoelastic property that exhibits sufficient shear thinning and yield stress during and after the extrusion, respectively. The shear-thinning flow behavior of the viscoelastic ceramic suspension can be modeled using the Hershel-Bulkley model [34]: = where σ is the shear stress (Pa), rate (s-1), and
+
(1)
is the yield stress,
is the viscosity parameter, γ is the share
is the shear thinning exponent. Figure 2(a,b) presents the viscosity and shear
stress as a function of shear rate for the NZF suspensions. The shear thinning exponents and the yield stress values for the NZF suspensions were determined by fitting the log( -
) vs log( )
curves and are summarized in Table 1. The shear thinning exponent ranges between 0 and 1, indicating that all the NZF suspensions show shear thinning characteristics. The viscoelastic properties of the NZF suspensions were also characterized by measuring the complex shear modulus [35]: where (
∗
∗
=
+
(2)
is the complex shear modulus consisting of the storage modulus ( ) and loss modulus
). The rheometer amplitude sweep provides the complex shear modulus, giving
and
vs
, as seen in Figure 2(c). The yield stress was determined by the limit of the linear viscoelastic
6
region (LVE) in the complex shear modulus curve. The limit of the LVE is defined as the shear stress where the
curve starts to deviate from the plateau value. The yield stress values of the
NZF suspensions increased with increasing SIS concentration, and are comparable to the values obtained from the Hershel-Bulkley model as shown in Figure 2(d) and Table 1. The maximum shear rate induced at the nozzle wall during extrusion of the ceramic suspension can be estimated by [36]: γ = where
4
=
4
=
4
is the volumetric suspension flow rate (mm3/s),
is the nozzle radius (mm). In this study,
= 8 mm/s and
(3) is the extrusion speed (mm/s), and r = 0.72 mm, so the maximum shear
rate during the extrusion was 88 s-1, which corresponds to a shear stress on the order of 103-105 Pa depending upon the SIS concentration. Therefore, all NZF suspensions were stressed beyond their yield stress points, indicating a shear-thinning flow during extrusion. This shear-thinning behavior was also confirmed by the complex shear modulus curves that show a liquid-like flow behavior (
>
) at the shear stress during the extrusion (Figure 2(c)). After extrusion from the
nozzle, the shear stress is removed from the NZF suspension and the extruded rod-like ceramic suspension exhibits solid-like behavior (
>
). From this point, the yield stress of the NZF
suspensions plays an important role in maintaining the 3D-DIWn structure against the gravitational slumping force.
3.2. Yield stress vs slumping force: predicting 3D DIW cumulative height As stated in the Introduction, the yield stress of the DIWn suspension is an important measure for estimating the height which a 3D-DIWn structure can achieve against the gravitational slumping force. The gravitational slumping force was calculated from a geometrical
7
model consisting of a simple packing of spherical ceramic particles, as seen in Figure 3. The calculation of the gravitational slumping force due to the mass of a single spherical particle acting on its own bottom surface was first defined, and then extended to a 3D-stacked particles model. The balance equation between the gravitational slumping force and the yield stress of the DIWn suspension was formulated as:
where
≈
≈
1 3
*+ * *, 1 = *, *+ * 3
1 3
!" # $
=
1 3
%& '# ( )%& (4)
ℎ !" # 3 $(#). (5)
is the diameter of a spherical particle, is the particle density,
acceleration, and *+,
,,
is the gravitational
is the number of particles in the x, y, and z dimensions. This model is an
approximation that gives a theoretical limit to the cumulative height of a 3D-DIWn structure, indicating that the 3D DIW height increases as the yield stress of the ceramic suspension increases. In this model, deformation arising from the surface tension acting on the DIWn suspension is excluded because the deformation force due to surface tension is negligible in comparison with the gravitational slumping force. The comparison between the surface tension and the slumping force from the yield stress of the NZF suspension is shown in Figure S1 of the supplementary informatioin. The final aspect to consider regarding the yield stress of the ceramic suspension for predicting the 3D DIW cumulative height is the yield stress variation with drying time. It was observed that the yield stress of the DIWn NZF suspension increased with increased drying time after its exposure to the outside air from the nozzle, as shown in Figure 4(a). This indicates that the build rate in the z-axis for a given 3D structure plays an important role in determining the 3D DIW cumulative height. The slower the z-axis build rate, the longer the 3D-DIWn NZF green
8
body will be exposed to the air for drying before the next layer is applied. The change in the yield stress over the z-axis build time (drying time) was converted to the predicted cumulative height using the balance equation (Equation 5) and plotted along with various z-axis build rates, as seen in Figure 4(b). The intersections of the lines in Figure 4(b) indicate the predicted cumulative heights of 3D-DIWn structures that can be achieved by each NZF suspension at a given z-axis build rate. This implies that the predicted 3D DIW cumulative height is inversely proportional to the z-axis build rate. The prediction of the 3D DIW cumulative height was compared with the actual height of a 3D-DIWn structure just before its collapse, as shown in Figure 5(a). Square hollow pillars having a basal plane area of 20 × 20 mm2 and a wall thickness of 2 mm (see Figure S2) were 3DDIWn for this comparison. The z-axis build rate was fixed at 2 mm/min. The yield stress of the 6 vol % SIS NZF suspension started at 279.79 Pa and increased to 497.86 Pa at a z-axis build rate of 2 mm/s, which corresponds to a cumulative DIW height of 46 mm, as shown in Figures 4(b) and 5(a). It was observed that a 3D-DIWn pillar with the same NZF suspension and z-axis build rate successfully reached 57 mm as shown in Figure 5(b), which is comparable to the height predicted from the yield stress. However, as the cumulative height increased to 59 mm, the lower portion of the pillar started to sink and the entire pillar eventually collapsed, as shown in Figure 5(b). Figure 5(c) shows the pillar built up to 200 mm using the 10 vol % SIS NZF suspension. Please note that, although the cumulative height of the 10 vol % SIS NZF suspension was predicted to be 240 mm, the DIW of the pillar was stopped at the height of 200 mm due to the limitation of the z-axis motion of the DIW printer used in this experiment.
3.3. 3D DIW cumulative height vs overhang angles
9
It is important to be able to form overhang structures in complex 3D structures. Most 3D printing technologies that build up 3D objects on a layer-by-layer basis require additional support structures for extreme overhangs. In this study, the maximum DIW height of the overhang at a given angle without a support structure was investigated by taking into account two aspects: the CoG of the overhang and the yield stress of the NZF suspension. To simplify our discussion, the same square hollow pillars as described in section 3.2 were DIWn at various inclination angles. In general, pillars inclined at a certain angle can be built up to a geometrically allowed height (defined as the critical height in this study) where its CoG lies just above the edge of the base plane. As the height increases further, the CoG begins to deviate from the base plane, creating torque that causes the pillar to fall over, as illustrated in Figure S3. Since the base plane was fixed at 20 × 20 mm2, the critical heights of the square hollow pillars inclined at different angles can be calculated from ℎ12343156 = 7 × tan(<3 the base length, and <3
16
16 ), where
ℎ12343156 is the critical height, 7 is
is the inclination angle. It was then investigated whether these critical
heights could be predicted from the yield stress, which should be high enough to achieve these critical heights. Since the z-axis build rate was fixed at 2 mm/min, the z-axis build time (drying time) up to the critical heights of the pillars inclined at different angles can be calculated as zaxis build time = critical height/z-axis build rate. With this z-axis build time, the maximum cumulative height of the NZF suspension pillar, predicted from the yield stress, can be seen in Figure 4(b). The cumulative heights estimated using both the CoG and the yield stress were plotted together in Figure 6(a) for the NZF suspensions with 3 and 10 vol % SIS. This plot predicts that the critical height can be achieved using the NZF suspension with 10 vol % SIS, but not that with 3 vol % SIS. To confirm this prediction, pillars inclined at angles ranging from 80° to 45° were 3D-DIWn with the 10 vol % SIS NZF suspension. The heights of the 3D-DIWn
10
pillars were measured and are included in Figure 6(a), which shows that the experimental observation was comparable to the predicted critical height. Figure 6(b) shows the images of pillars DIWn with the 10 vol % SIS NZF suspension at a 60° inclination angle. This pillar was built up to 36 mm without any deflection, comparable to the critical height of 33 mm. However, as the DIW progressed further, the inclination increased, and eventually collapsed due to torque even though the pillar had enough yield stress to go higher. Figure 6(c) shows Moai statues DIWn with the 10 vol % SIS NZF suspension. The neck was designed to have a cross-sectional area close to 20 × 20 mm2 and the angle at the neck was maintained at 60°, similar to the previous pillar structure inclined at the same angle. The head was designed to have two different heights as shown in Figure 6(c): 33 and 43 mm. The Moai statue with a head height of 33 mm was successfully DIWn without any deformation. However, in the case of the statue with a head height of 43 mm, the head was inclined more than its design angle of 60° since the head height exceeded the critical height of 33 mm, causing cracks near the neck. The Moai statues after sintering at 930 ºC are shown at the bottom of Figure 6(c). Shrinkage of the 3D-DIWn structure was observed in both Moai statues. However, the shrinkage enlarged the cracks on the Moai statue with a head height of 43 mm and also increased the inclination angle of the head.
3.4. Structural and magnetic properties of 3D-DIWn NZF structures The structural properties of the 3D-DIWn NZF structures were characterized by XRD and FESEM. Figure 7(a) shows the XRD diffractograms of 3D-DIWn NZF cubes with the 10 vol % SIS NZF suspension before and after sintering at 930 °C. The diffraction peaks indicate both samples have a spinel ferrite structure. The increase in peak intensity after sintering indicates an increase in crystallinity, which was confirmed from the densified cross-sectional
11
SEM image of the sintered NZF samples as compared with that of the unsintered-DIWn sample, as shown in Figure 7(c,d). The shrinkage and density of the 3D-DIWn NZF cubes were measured before and after sintering, as seen in Figure 7(b). The shrinkage increased with increasing sintering temperature, resulting in a final shrinkage in all three axes of 16–18% at 930 °C. The increase in shrinkage implies that the density of the 3D-DIWn NZF cubes was increased as a result of the removal of solvent, organic additives, and pores during the sintering process. The density of the 3D-DIWn structure sintered at 930 °C was 5.18 g/cc, which was 97% of the theoretical density (5.34 g/cc) [37]. Magnetic hysteresis curves measuring flux density (B) against magnetic field (H), known as B-H curves, were obtained from 3D-DIWn NZF toroidal samples before and after sintering (930 °C), as shown in Figure 8(a). The magnetization increased after sintering of the 3D-DIWn toroid sample, exhibiting a soft magnetic characteristic with narrow hysteresis loops. The saturation magnetization (Bs) of the 3D-DIWn toroid after sintering was 0.39 T. The frequencydependent permeabilities of the 3D-DIWn toroid samples before and after sintering were compared (Figure 8(b)). The complex permeability, described by = = = + >= , was measured at frequencies between 1 and 10 MHz before and after sintering, indicating = increased to 124–128 after sintering, while = was negligibly small (less than 5) in both cases. The magnetization and permeability of the 3D-DIWn NZF samples after sintering are similar to the values in previously reported studies [38-40]. To confirm whether the 3D-DIWn NZF structure can be applied to the manufacture of electronic ceramic components, a transformer core having an EFD design was 3D-DIWn and the performance of the transformer using it was investigated. The dimensional details of the EFD transformer core are illustrated in Figure S4. Figure 8(c) shows the 3D-DIWn EFD transformer cores and the final assembly of the ECD transformer with primary and
12
secondary coils wound around the 3D-DIWn core. A step-down transformer was fabricated with a transformer turns ratio of *? /* = 90/40, where *? is the number of turns of the primary coil and * is the number of turns of the secondary coil. Figure 8(d) shows that the input voltage (C2DE = 350 mV) from the primary coil was stepped down to the output voltage (C2DE = 151 mV) in the secondary coil. The efficiency of the transformer using the 3D-DIWn EFD core was calculated using a transformer turns ratio of 1 from the following equation: F ≈ where F is the transformer efficiency, GK
GHI4JI4 × 100 (6) GK JI4
JI4 is
the input power in the primary coil, and GHI4JI4
is the output power in the secondary coil. The transformer efficiency was around 92% over an input power of 1 to 20 W.
4. CONCLUSIONS The rheological properties of NZF suspensions were investigated to ensure 3Dprintability of the suspension in extrusion-based 3D DIW. Such NZF suspensions with high 3Dprintability are expected to flow easily through the nozzle during DIW and have enough viscoelasticity to withstand gravitational slumping after DIW. The maximum 3D DIW height that the NZF suspension can reach was predicted by comparing the yield stress of the suspension with the gravitational slumping force. If the yield stress of the suspension as a function of z-axis build time (drying time) was premeasured, the 3D DIW cumulative height could be predicted even before DIW. The 3D DIW cumulative height of the NZF suspension at various overhang angles was also investigated by considering the yield stress of the suspension and the CoG of the inclined structure. It was found that the yield stress of the suspension needs to be high enough to
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achieve the critical height defined by the CoG of the 3D-DIWn structure inclined at a specified angle. The magnetic properties of the 3D-DIWn NZF structure, including the magnetization and the complex permeability, were investigated to confirm that 3D DIW of an NZF suspension could be applied to electronic components. Transformers having 3D-DIWn NZF magnetic cores were fabricated to determine their applicability to electronic components. The successful formation of a transformer with the 3D-DIWn NZF magnetic core indicates that 3D DIW of ceramic materials can be used in a variety of electronic applications.
ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation
of
Korea
(NRF)
funded
by
the
Ministry
of
Education
(NRF-
2018R1D1A1B07043117) and the Nano R&D program through the Korea Science and Engineering Foundation funded by the Ministry of Science, ICT and Future Planning (NRF2015M3A7B4050307). This work was also supported by the “Infrastructure Program for New Value Creation of Traditional Ceramic Industry” (BUS010025000) under the Ministry of Trade, Industry and Energy. REFERENCES [1] R. E. Newnham, Electroceramics, Rep. Prog. Phys. 52 (1989) 123-156. https://doi.org/10.1088/0034-4885/52/2/001 [2] J. –S. Park, H. Kim, I. –D. Kim, Overview of electroceramic materials for oxide semiconductor thin film transistors, J. Electroceram. 32 (2014) 117-140. http://dx.doi.org/10.1007/s10832-013-9858-0
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[27] J. W. Stansbury, M. J. Idacavage, 3D printing with polymers: challenges among expanding options and opportunities, Dent. Mater. 32 (2016) 54-64. http://dx.doi.org/10.1016/j.dental.2015.09.018 [28] T. Chartier, C. Chaput, F. Doreau, M. Loiseau, Stereolithography of structural complex ceramic parts, J. Mater. Sci. 7 (2002) 3141-3147. https://doi.org/10.1023/A:1016102210277 [29] K. Shahzad, J. Deckers, J. -P. Kruth, J. Vleugels, Additive manufacturing of alumina parts by indirect selective laser sintering and post processing, J. Mater. Process Tech. 213 (2013) 1484-1497. http://dx.doi.org/10.1016/j.jmatprotec.2013.03.014 [30] Z. Chen, Z. Li, J. Li, C. Liu, C. Lao, Y. Fu, C. Liu, Y. Li, P. Wang, Y. He, 3D printing of ceramics: A review, J. Eur. Ceram. Soc. 39 (2019) 661-687. http://dx.doi.org/10.1016/j.jeurceramsoc.2018.11.013 [31] J. A. Lewis, Direct ink writing of three-dimensional ceramic structure, J. Am. Ceram. Soc. 89 (2006) 3599-3609. http://dx.doi.org/10.1111/j.1551-2916.2006.01382.x [32] W. Li, A. Ghazanfari, M. C. Leu, R. G. Landers, Extrusion-on-demand methods for high solids loading ceramic paste in freeform extrusion fabrication, Virtual Phys. Prototy. 12 (2017) 193-205. http://dx.doi.org/10.1080/17452759.2017.1312735 [33] Y. Jo, J. Y. Kim, S. –Y. Kim, Y. –H. Seo, K. –S. Jang, S. Y. Lee, S. Jung, B. –H. Ryu, H. – S. Kim, J. –U. Park, Y. Choi, S. Jeong, 3D-printable, highly conductive hybrid composites employing chemically-reinforced, complex dimensional fillers and thermoplastic triblock copolymers, Nanoscale 9 (2017) 5072-5084. http://dx.doi.org/10.1039/C6NR09610G [34] W. H. Herschel, R. Bulkley, Konsistenzmessungen von Gummi-Benzollosungen, Kolloid Zeitschrift 39 (1926) 291-300. http://dx.doi.org/10.1007/BF01432034
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[35] J. A. Lewis, Colloidal processing of ceramics, J. Am. Ceram. Soc. 83 (2000) 2341-2359. http://dx.doi.org/10.1111/j.1151-2916.2000.tb01560.x [36] A. M’Barki, L. Bocquet, A. Stevenson, Linking rheology and printability for dense and strong ceramics by direct ink writing, Scientific reports 7 (2017) 6017. https://doi.org/10.1038/s41598-017-06115-0 [37] M. P. Reddy, I. G. Kim, D. S. Yoo, W. Madhuri, N. R. Reddy, K. V. S. Kumar, R. R. Reddy, Characterization and electromagnetic studies on NiZn and NiCuZn ferrites prepared by microwave sintering technique, Mater. Sci. Appl. 3 (2012) 628-632. http://dx.doi.org/10.4236/msa.2012.39091 [38] R. Kaveti, J. Kim, Flexible wireless power transfer module implemented with aerojetprinting and laser-sintering of rigid NiZn-ferrite ceramic films, Nano Energy 57 (2019) 317-326. https://doi.org/10.1016/j.nanoen.2018.12.021 [39] M. Bissannagari, T. –H. Kim, J. –G. Yook, J. Kim, All inkjet-printed flexible wireless power transfer module: PI/Ag hybrid spiral coil built into 3D NiZn-ferrite trench structure with a resonance capacitor, Nano Energy 62 (2019) 645-652. http://dx.doi.org/10.1016/j.nanoen.2019.05.075 [40] M. Bissannagari, W. Lee, W. Y. Lee, J. H. Jeong, J. Kim, Fully-inkjet-printed Agcoil/NiZn-ferrite for flexible wireless power transfer module: rigid sintered ceramic body into flexible form, Adv. Funct. Mater. 27 (2017) 1701766. http://dx.doi.org/10.1002/adfm.201701766
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List of Figure Caption
Fig. 1. Schematic illustration of extrusion-based 3D DIW. Fig. 2. Rheological properties of the NZF suspensions having various SIS concentrations. Flow behavior: (a) evolution of viscosity and (b) evolution of shear stress. Amplitude sweep: (c) Complex shear modulus. (d) Yield stress comparison obtained from both flow behavior measurements and amplitude sweep. Fig. 3. Geometrical model consisting of simple packing of spherical ceramic particles. Fig. 4. (a) Yiled stress of the NZF-suspension with various SIS concentration as a function of drying time. (b) The predicted cmumulative height as a function of z-axis build time. Fig. 5. (a) Comparison of the predicted cumulative height and the experimentally measured height of NZF suspensions. (b) Square hollow pillars DIWn with the 6 vol % SIS NZF suspension. (c) Square hollow pillar DIWn with the 10 vol % SIS NZF suspension. Fig. 6. . (a) Cumulative heights estimated from the yield stress and CoG. NZF suspensions with 3 and 10 vol % SIS were used in this study. (b) The pillar DIWn with the 10 vol % SIS NZF suspension at an inclination angle of 60°. (c) Moai statues DIWn with the 10 vol % SIS NZF suspension. Fig. 7. (a) X-ray diffraction spectra of the 3D-DIWn NZF structure before and after sintering at 930 °C. (b) Shrinkage and density of the 3D-DIWn NZF cube samples. The microsctructure was observed by SEM (c) before and (d) after sintering at 930 °C. Fig. 8. (a) Magnetization hysteresis curves and (b) frequency-dependent permeabilities of the 3D-DIWn NZF toroid samples before and after sintering at 930 °C. (c) Transformer with the 3D DIWn EFD-type magnetic core. (d) Performance of the transformer.
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Table 1. Rheological properties of the NZF suspension with various SIS concentration from Hershel-Bulkley model. SIS (Vol %) 3 6 9 10
Yield stress (Pa) from Hershel Bulkley 18.31 279.79 597.88 683.02
(Pa⋅sn) 0.63 0.84 0.92 0.89
228.95 2240.13 2619.55 3539.68
Yield sress (Pa) from 45.21 315.76 542.29 721.73
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Fig. 1. TaeKyu An, et al.
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Fig. 2. TaeKyu An, et al.
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Fig. 3. TaeKyu An, et al.
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Fig. 4. TaeKyu An, et al.
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Fig. 5. TaeKyu An, et al.
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Fig. 6. TaeKyu An, et al.
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Fig. 7. TaeKyu An, et al.
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Fig. 8. TaeKyu An, et al.
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Table of Contents Graphic and Synopsis
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33317-6
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.127
Reference:
CERI 23496
To appear in:
Ceramics International
Received Date: 16 October 2019 Revised Date:
13 November 2019
Accepted Date: 15 November 2019
Please cite this article as: T. An, K.-T. Hwang, J.-H. Kim, J. Kim, Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.11.127. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Extrusion-based 3D direct ink writing of NiZn-ferrite structures with viscoelastic ceramic suspension
Taekyu An1, Kwang-Taek Hwang2, Jin-Ho Kim2 and Jihoon Kim1 * 1
Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungchungnam-do 32588, Korea
2
Icheon Branch, Korea Institute of Ceramic Engineering & Technology (KICET), Icheon-si, Gyeonggi-do 17303, Korea
*Corresponding author Email address: [email protected] (J. Kim)
ABSTRACT
Advances in additive manufacturing (AM), represented as the instant evolution of design files into fully functional products, provide customer-centric smart manufacturing solutions that are increasingly replacing traditional manufacturing methods in various industries. However, the advantages of AM often diminish when applied to the production of ceramic components, owing to the demanding nature of ceramic materials. Extrusion-based direct ink writing (DIW) is considered one of the few AM technologies applicable to ceramic materials. Precise control of
1
the ceramic suspension is required in extrusion-based 3D DIW to create 3D structures for various industrial products. This study describes a fundamental relationship between the rheological properties of the ceramic suspensions and their 3D DIW printability. The rheological properties determine flowability, DIW height, and the overhang angle that can be achieved in extrusionbased 3D DIW. The printability of NiZn-ferrite suspensions with various rheological properties was investigated, and the potential of 3D DIW amenable to manufacturing ceramic electronic components was demonstrated.
KEYWORDS: Direct ink writing, Auger-extrusion, NiZn-ferrite, Magnetic core, Ceramic 3D printing
1. INTRODUCTION Functional ceramic materials have been widely employed in various emerging technical applications because of their inherently superior electrical, optical, and magnetic properties, as well as their excellent mechanical properties such as high-temperature resistance, abrasion resistance, and high strength [1-11]. However, their use is often limited due to manufacturing constraints of ceramic components with geometric complexity [12-17]. Traditional manufacturing processes, represented by injection molding, isostatic pressing, and casting, are limited to forming relatively simple 3D shapes and 2D films, and are considered costly and timeconsuming [18-25]. This technical challenge has generated considerable industrial interest in exploring additive manufacturing technologies that replace or complement the traditional processes to form functional ceramic components having complex geometries.
2
Additive manufacturing (AM) is a widely adopted technology for realizing 3D structures with intricate geometries owing to its numerous advantages compared with conventional ceramic fabrication processes, such as freedom of design, low manufacturing cost, and short production runs [26-27]. AM techniques include stereolithography [28], selective laser sintering [29], powder bed printing [30], and direct ink writing (DIW) [31]. DIW is considered economically advantageous because it has a simple equipment setup that extrudes high-solid-loading ceramic suspensions through a computer-controlled nozzle to print complex 3D structures. However, extrusion-based DIW requires stringent control over the rheological behavior of the ceramic suspension; it must flow easily through the nozzle during the extrusion yet have sufficiently high strength and stiffness to be capable of maintaining the direct-ink-written (DIWn) shape. Such rheological behavior of the ceramic suspension involves a viscoelastic property having high shear thinning (liquid-like behavior) at high shear rates during the extrusion, as well as sufficient yield stress (solid-like behavior) to resist the gravitational slumping force that could deform the DIWn object. Effective use of the viscoelastic ceramic suspension also requires precise control of the extrusion process since the suspension responds slowly to changes in the extrusion force. This inevitably results in time delays when starting and stopping the extrusion process. An auger extruder is suitable for ensuring a rapid response to the extrusion force because it exerts an extrusion force on a relatively small suspension volume in a auger chamber (Figure 1), unlike a conventional ram extruder that uses a syringe and plunger where the entire suspension stored in a relatively large syringe barrel [32-33]. The present study investigated the rheological requirements of a high-solid-loading ceramic suspension for printing 3D structures by extrusion-based 3D DIW. A NiZn-ferrite (NZF)
3
suspension having viscoelastic characteristics was formulated and extruded to form 3D structures. The rheological properties of the viscoelastic NZF suspension were analyzed to predict the maximum cumulative height of 3D-DIWn structure that was achievable without structural deformation by the gravitational slumping force. The cumulative height of the 3D-DIWn structure at various overhang angles was also investigated by examining the yield stress and center of gravity (CoG) of the 3D structures. The magnetic properties, dimensional shrinkage, and density of the 3D-DIWn NZF structures were investigated after sintering at elevated temperatures. Finally, NZF magnetic cores having an economical flat design (EFD) were DIWn and applied to the fabrication of transformers. Performance of the transformer having the 3DDIWn EFD magnetic core was compared with that of a commercial core having the same design.
2. EXPERIMENTAL SECTION 2.1 Formulation of viscoelastic NZF suspension Polystyrene-polyisoprene-polystyrene (SIS, 22% styrene, Sigma Aldrich) stock solution was made by dissolving SIS powder in n-methyl-2-pyrrolidone (NMP, boiling point: 204 °C, density: 1.03 g/mL at 25 °C) at a weight ratio of SIS:NMP = 1:2.5. This stock solution was mixed in a Thinky mixer for 5 min at 2000 rpm and then defoamed for 1 min at 2200 rpm five times. NZF nanoparticles (Ni0.4Zn0.6Fe2O4, D50 = 500 nm, Amotech) were then added to the stock solution in a stepwise manner to prepare the NZF suspension. The NZF solid loading in the suspension was fixed at 58 vol %. To adjust the viscoelasticity, the SIS concentration in the NZF suspension was varied from 3 to 10 vol % with the addition of NMP. The final NZF suspensions were then mixed and defoamed in a Thinky mixer using the same method as that for preparing the SIS stock solution.
4
2.2 3D DIW and sintering The NZF suspensions were loaded in syringe barrels and defoamed in a Thinky mixer for 30 s at 2200 rpm before DIW. The NZF suspension was extruded using a commercially available auger dispenser (MSD-3, Musashi) having a nozzle with an orifice inner/outer diameter of 0.72/1.08 mm. The auger dispenser was mounted on a 3D printing stage (Prusa I3). The thickness of each layer of the 3D-DIWn structure was fixed at 0.5 mm. The DIW speed in the xand y-directions was between 5 and 10 mm/s, while the build rate in the z-direction (DIW speed in z-direction) was fixed at 2 mm/min. The 3D structures were DIWn at room temperature in air and then dried at 60 °C for 24 h before sintering. The dried green structures were sintered in a box furnace at 930 °C for 5 h after a temperature ramp of 20 °C/h. 2.3 Materials characterization Rheological properties of the NZF suspensions were characterized with an Anton Paar rheometer (MCR-92) equipped with a 25 mm parallel plate (PP25) and a 25 mm perforated plate (PP25-perforated). Flow behavior (shear stress vs shear rate) and amplitude sweep (complex shear modulus vs shear stress) were measured to determine the shear thinning, yield stress, and viscoelasticity of the NZF suspensions. Phase analysis of the 3D-DIWn structure before and after sintering were performed by X-ray diffraction (XRD) using Cu-Kα radiation (Rigaku Miniflex 600). The Archimedes method described in ASTM standard C373-18 was employed to determine the densities of the 3D-DIWn NZF structures before and after sintering at various temperatures. The microstructures of the 3D-DIWn structures were investigated using a field emission scanning electron microscope (FESEM, Nova 200). The magnetic hysteresis curves and frequency-dependent permeability of the 3D-DIWn NZF structures were characterized using a B-H analyzer (SY-8218, IWAST) and an impedance analyzer (Agilent 4294A), respectively. The
5
performance of a transformer with a 3D-DIWn NZF core was demonstrated using a signal generator (HP 432B) and an oscilloscope (GW Instek GDS-2072E) with a current probe (Tektronics P6022) and a multimeter (GW Instek GMD-8341).
3. RESULTS AND DISCUSSION 3.1. Rheological behavior of NZF suspensions for extrusion-based 3D DIW One of the most important rheological requirements for extrusion-based 3D DIW is the ceramic suspension having a viscoelastic property that exhibits sufficient shear thinning and yield stress during and after the extrusion, respectively. The shear-thinning flow behavior of the viscoelastic ceramic suspension can be modeled using the Hershel-Bulkley model [34]: = where σ is the shear stress (Pa), rate (s-1), and
+
(1)
is the yield stress,
is the viscosity parameter, γ is the share
is the shear thinning exponent. Figure 2(a,b) presents the viscosity and shear
stress as a function of shear rate for the NZF suspensions. The shear thinning exponents and the yield stress values for the NZF suspensions were determined by fitting the log( -
) vs log( )
curves and are summarized in Table 1. The shear thinning exponent ranges between 0 and 1, indicating that all the NZF suspensions show shear thinning characteristics. The viscoelastic properties of the NZF suspensions were also characterized by measuring the complex shear modulus [35]: where (
∗
∗
=
+
(2)
is the complex shear modulus consisting of the storage modulus ( ) and loss modulus
). The rheometer amplitude sweep provides the complex shear modulus, giving
and
vs
, as seen in Figure 2(c). The yield stress was determined by the limit of the linear viscoelastic
6
region (LVE) in the complex shear modulus curve. The limit of the LVE is defined as the shear stress where the
curve starts to deviate from the plateau value. The yield stress values of the
NZF suspensions increased with increasing SIS concentration, and are comparable to the values obtained from the Hershel-Bulkley model as shown in Figure 2(d) and Table 1. The maximum shear rate induced at the nozzle wall during extrusion of the ceramic suspension can be estimated by [36]: γ = where
4
=
4
=
4
is the volumetric suspension flow rate (mm3/s),
is the nozzle radius (mm). In this study,
= 8 mm/s and
(3) is the extrusion speed (mm/s), and r = 0.72 mm, so the maximum shear
rate during the extrusion was 88 s-1, which corresponds to a shear stress on the order of 103-105 Pa depending upon the SIS concentration. Therefore, all NZF suspensions were stressed beyond their yield stress points, indicating a shear-thinning flow during extrusion. This shear-thinning behavior was also confirmed by the complex shear modulus curves that show a liquid-like flow behavior (
>
) at the shear stress during the extrusion (Figure 2(c)). After extrusion from the
nozzle, the shear stress is removed from the NZF suspension and the extruded rod-like ceramic suspension exhibits solid-like behavior (
>
). From this point, the yield stress of the NZF
suspensions plays an important role in maintaining the 3D-DIWn structure against the gravitational slumping force.
3.2. Yield stress vs slumping force: predicting 3D DIW cumulative height As stated in the Introduction, the yield stress of the DIWn suspension is an important measure for estimating the height which a 3D-DIWn structure can achieve against the gravitational slumping force. The gravitational slumping force was calculated from a geometrical
7
model consisting of a simple packing of spherical ceramic particles, as seen in Figure 3. The calculation of the gravitational slumping force due to the mass of a single spherical particle acting on its own bottom surface was first defined, and then extended to a 3D-stacked particles model. The balance equation between the gravitational slumping force and the yield stress of the DIWn suspension was formulated as:
where
≈
≈
1 3
*+ * *, 1 = *, *+ * 3
1 3
!" # $
=
1 3
%& '# ( )%& (4)
ℎ !" # 3 $(#). (5)
is the diameter of a spherical particle, is the particle density,
acceleration, and *+,
,,
is the gravitational
is the number of particles in the x, y, and z dimensions. This model is an
approximation that gives a theoretical limit to the cumulative height of a 3D-DIWn structure, indicating that the 3D DIW height increases as the yield stress of the ceramic suspension increases. In this model, deformation arising from the surface tension acting on the DIWn suspension is excluded because the deformation force due to surface tension is negligible in comparison with the gravitational slumping force. The comparison between the surface tension and the slumping force from the yield stress of the NZF suspension is shown in Figure S1 of the supplementary informatioin. The final aspect to consider regarding the yield stress of the ceramic suspension for predicting the 3D DIW cumulative height is the yield stress variation with drying time. It was observed that the yield stress of the DIWn NZF suspension increased with increased drying time after its exposure to the outside air from the nozzle, as shown in Figure 4(a). This indicates that the build rate in the z-axis for a given 3D structure plays an important role in determining the 3D DIW cumulative height. The slower the z-axis build rate, the longer the 3D-DIWn NZF green
8
body will be exposed to the air for drying before the next layer is applied. The change in the yield stress over the z-axis build time (drying time) was converted to the predicted cumulative height using the balance equation (Equation 5) and plotted along with various z-axis build rates, as seen in Figure 4(b). The intersections of the lines in Figure 4(b) indicate the predicted cumulative heights of 3D-DIWn structures that can be achieved by each NZF suspension at a given z-axis build rate. This implies that the predicted 3D DIW cumulative height is inversely proportional to the z-axis build rate. The prediction of the 3D DIW cumulative height was compared with the actual height of a 3D-DIWn structure just before its collapse, as shown in Figure 5(a). Square hollow pillars having a basal plane area of 20 × 20 mm2 and a wall thickness of 2 mm (see Figure S2) were 3DDIWn for this comparison. The z-axis build rate was fixed at 2 mm/min. The yield stress of the 6 vol % SIS NZF suspension started at 279.79 Pa and increased to 497.86 Pa at a z-axis build rate of 2 mm/s, which corresponds to a cumulative DIW height of 46 mm, as shown in Figures 4(b) and 5(a). It was observed that a 3D-DIWn pillar with the same NZF suspension and z-axis build rate successfully reached 57 mm as shown in Figure 5(b), which is comparable to the height predicted from the yield stress. However, as the cumulative height increased to 59 mm, the lower portion of the pillar started to sink and the entire pillar eventually collapsed, as shown in Figure 5(b). Figure 5(c) shows the pillar built up to 200 mm using the 10 vol % SIS NZF suspension. Please note that, although the cumulative height of the 10 vol % SIS NZF suspension was predicted to be 240 mm, the DIW of the pillar was stopped at the height of 200 mm due to the limitation of the z-axis motion of the DIW printer used in this experiment.
3.3. 3D DIW cumulative height vs overhang angles
9
It is important to be able to form overhang structures in complex 3D structures. Most 3D printing technologies that build up 3D objects on a layer-by-layer basis require additional support structures for extreme overhangs. In this study, the maximum DIW height of the overhang at a given angle without a support structure was investigated by taking into account two aspects: the CoG of the overhang and the yield stress of the NZF suspension. To simplify our discussion, the same square hollow pillars as described in section 3.2 were DIWn at various inclination angles. In general, pillars inclined at a certain angle can be built up to a geometrically allowed height (defined as the critical height in this study) where its CoG lies just above the edge of the base plane. As the height increases further, the CoG begins to deviate from the base plane, creating torque that causes the pillar to fall over, as illustrated in Figure S3. Since the base plane was fixed at 20 × 20 mm2, the critical heights of the square hollow pillars inclined at different angles can be calculated from ℎ12343156 = 7 × tan(<3 the base length, and <3
16
16 ), where
ℎ12343156 is the critical height, 7 is
is the inclination angle. It was then investigated whether these critical
heights could be predicted from the yield stress, which should be high enough to achieve these critical heights. Since the z-axis build rate was fixed at 2 mm/min, the z-axis build time (drying time) up to the critical heights of the pillars inclined at different angles can be calculated as zaxis build time = critical height/z-axis build rate. With this z-axis build time, the maximum cumulative height of the NZF suspension pillar, predicted from the yield stress, can be seen in Figure 4(b). The cumulative heights estimated using both the CoG and the yield stress were plotted together in Figure 6(a) for the NZF suspensions with 3 and 10 vol % SIS. This plot predicts that the critical height can be achieved using the NZF suspension with 10 vol % SIS, but not that with 3 vol % SIS. To confirm this prediction, pillars inclined at angles ranging from 80° to 45° were 3D-DIWn with the 10 vol % SIS NZF suspension. The heights of the 3D-DIWn
10
pillars were measured and are included in Figure 6(a), which shows that the experimental observation was comparable to the predicted critical height. Figure 6(b) shows the images of pillars DIWn with the 10 vol % SIS NZF suspension at a 60° inclination angle. This pillar was built up to 36 mm without any deflection, comparable to the critical height of 33 mm. However, as the DIW progressed further, the inclination increased, and eventually collapsed due to torque even though the pillar had enough yield stress to go higher. Figure 6(c) shows Moai statues DIWn with the 10 vol % SIS NZF suspension. The neck was designed to have a cross-sectional area close to 20 × 20 mm2 and the angle at the neck was maintained at 60°, similar to the previous pillar structure inclined at the same angle. The head was designed to have two different heights as shown in Figure 6(c): 33 and 43 mm. The Moai statue with a head height of 33 mm was successfully DIWn without any deformation. However, in the case of the statue with a head height of 43 mm, the head was inclined more than its design angle of 60° since the head height exceeded the critical height of 33 mm, causing cracks near the neck. The Moai statues after sintering at 930 ºC are shown at the bottom of Figure 6(c). Shrinkage of the 3D-DIWn structure was observed in both Moai statues. However, the shrinkage enlarged the cracks on the Moai statue with a head height of 43 mm and also increased the inclination angle of the head.
3.4. Structural and magnetic properties of 3D-DIWn NZF structures The structural properties of the 3D-DIWn NZF structures were characterized by XRD and FESEM. Figure 7(a) shows the XRD diffractograms of 3D-DIWn NZF cubes with the 10 vol % SIS NZF suspension before and after sintering at 930 °C. The diffraction peaks indicate both samples have a spinel ferrite structure. The increase in peak intensity after sintering indicates an increase in crystallinity, which was confirmed from the densified cross-sectional
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SEM image of the sintered NZF samples as compared with that of the unsintered-DIWn sample, as shown in Figure 7(c,d). The shrinkage and density of the 3D-DIWn NZF cubes were measured before and after sintering, as seen in Figure 7(b). The shrinkage increased with increasing sintering temperature, resulting in a final shrinkage in all three axes of 16–18% at 930 °C. The increase in shrinkage implies that the density of the 3D-DIWn NZF cubes was increased as a result of the removal of solvent, organic additives, and pores during the sintering process. The density of the 3D-DIWn structure sintered at 930 °C was 5.18 g/cc, which was 97% of the theoretical density (5.34 g/cc) [37]. Magnetic hysteresis curves measuring flux density (B) against magnetic field (H), known as B-H curves, were obtained from 3D-DIWn NZF toroidal samples before and after sintering (930 °C), as shown in Figure 8(a). The magnetization increased after sintering of the 3D-DIWn toroid sample, exhibiting a soft magnetic characteristic with narrow hysteresis loops. The saturation magnetization (Bs) of the 3D-DIWn toroid after sintering was 0.39 T. The frequencydependent permeabilities of the 3D-DIWn toroid samples before and after sintering were compared (Figure 8(b)). The complex permeability, described by = = = + >= , was measured at frequencies between 1 and 10 MHz before and after sintering, indicating = increased to 124–128 after sintering, while = was negligibly small (less than 5) in both cases. The magnetization and permeability of the 3D-DIWn NZF samples after sintering are similar to the values in previously reported studies [38-40]. To confirm whether the 3D-DIWn NZF structure can be applied to the manufacture of electronic ceramic components, a transformer core having an EFD design was 3D-DIWn and the performance of the transformer using it was investigated. The dimensional details of the EFD transformer core are illustrated in Figure S4. Figure 8(c) shows the 3D-DIWn EFD transformer cores and the final assembly of the ECD transformer with primary and
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secondary coils wound around the 3D-DIWn core. A step-down transformer was fabricated with a transformer turns ratio of *? /* = 90/40, where *? is the number of turns of the primary coil and * is the number of turns of the secondary coil. Figure 8(d) shows that the input voltage (C2DE = 350 mV) from the primary coil was stepped down to the output voltage (C2DE = 151 mV) in the secondary coil. The efficiency of the transformer using the 3D-DIWn EFD core was calculated using a transformer turns ratio of 1 from the following equation: F ≈ where F is the transformer efficiency, GK
GHI4JI4 × 100 (6) GK JI4
JI4 is
the input power in the primary coil, and GHI4JI4
is the output power in the secondary coil. The transformer efficiency was around 92% over an input power of 1 to 20 W.
4. CONCLUSIONS The rheological properties of NZF suspensions were investigated to ensure 3Dprintability of the suspension in extrusion-based 3D DIW. Such NZF suspensions with high 3Dprintability are expected to flow easily through the nozzle during DIW and have enough viscoelasticity to withstand gravitational slumping after DIW. The maximum 3D DIW height that the NZF suspension can reach was predicted by comparing the yield stress of the suspension with the gravitational slumping force. If the yield stress of the suspension as a function of z-axis build time (drying time) was premeasured, the 3D DIW cumulative height could be predicted even before DIW. The 3D DIW cumulative height of the NZF suspension at various overhang angles was also investigated by considering the yield stress of the suspension and the CoG of the inclined structure. It was found that the yield stress of the suspension needs to be high enough to
13
achieve the critical height defined by the CoG of the 3D-DIWn structure inclined at a specified angle. The magnetic properties of the 3D-DIWn NZF structure, including the magnetization and the complex permeability, were investigated to confirm that 3D DIW of an NZF suspension could be applied to electronic components. Transformers having 3D-DIWn NZF magnetic cores were fabricated to determine their applicability to electronic components. The successful formation of a transformer with the 3D-DIWn NZF magnetic core indicates that 3D DIW of ceramic materials can be used in a variety of electronic applications.
ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation
of
Korea
(NRF)
funded
by
the
Ministry
of
Education
(NRF-
2018R1D1A1B07043117) and the Nano R&D program through the Korea Science and Engineering Foundation funded by the Ministry of Science, ICT and Future Planning (NRF2015M3A7B4050307). This work was also supported by the “Infrastructure Program for New Value Creation of Traditional Ceramic Industry” (BUS010025000) under the Ministry of Trade, Industry and Energy. REFERENCES [1] R. E. Newnham, Electroceramics, Rep. Prog. Phys. 52 (1989) 123-156. https://doi.org/10.1088/0034-4885/52/2/001 [2] J. –S. Park, H. Kim, I. –D. Kim, Overview of electroceramic materials for oxide semiconductor thin film transistors, J. Electroceram. 32 (2014) 117-140. http://dx.doi.org/10.1007/s10832-013-9858-0
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List of Figure Caption
Fig. 1. Schematic illustration of extrusion-based 3D DIW. Fig. 2. Rheological properties of the NZF suspensions having various SIS concentrations. Flow behavior: (a) evolution of viscosity and (b) evolution of shear stress. Amplitude sweep: (c) Complex shear modulus. (d) Yield stress comparison obtained from both flow behavior measurements and amplitude sweep. Fig. 3. Geometrical model consisting of simple packing of spherical ceramic particles. Fig. 4. (a) Yiled stress of the NZF-suspension with various SIS concentration as a function of drying time. (b) The predicted cmumulative height as a function of z-axis build time. Fig. 5. (a) Comparison of the predicted cumulative height and the experimentally measured height of NZF suspensions. (b) Square hollow pillars DIWn with the 6 vol % SIS NZF suspension. (c) Square hollow pillar DIWn with the 10 vol % SIS NZF suspension. Fig. 6. . (a) Cumulative heights estimated from the yield stress and CoG. NZF suspensions with 3 and 10 vol % SIS were used in this study. (b) The pillar DIWn with the 10 vol % SIS NZF suspension at an inclination angle of 60°. (c) Moai statues DIWn with the 10 vol % SIS NZF suspension. Fig. 7. (a) X-ray diffraction spectra of the 3D-DIWn NZF structure before and after sintering at 930 °C. (b) Shrinkage and density of the 3D-DIWn NZF cube samples. The microsctructure was observed by SEM (c) before and (d) after sintering at 930 °C. Fig. 8. (a) Magnetization hysteresis curves and (b) frequency-dependent permeabilities of the 3D-DIWn NZF toroid samples before and after sintering at 930 °C. (c) Transformer with the 3D DIWn EFD-type magnetic core. (d) Performance of the transformer.
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Table 1. Rheological properties of the NZF suspension with various SIS concentration from Hershel-Bulkley model. SIS (Vol %) 3 6 9 10
Yield stress (Pa) from Hershel Bulkley 18.31 279.79 597.88 683.02
(Pa⋅sn) 0.63 0.84 0.92 0.89
228.95 2240.13 2619.55 3539.68
Yield sress (Pa) from 45.21 315.76 542.29 721.73
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Fig. 1. TaeKyu An, et al.
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Fig. 7. TaeKyu An, et al.
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Fig. 8. TaeKyu An, et al.
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Table of Contents Graphic and Synopsis
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: