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3D-printed nanoporous ceramics: Tunable feedstock for direct ink write and projection microstereolithography
Materials and Design 198 (2021) 109337
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3D-printed nanoporous ceramics: Tunable feedstock for direct ink write and projection microstereolithography Alyssa L. Troksa 1, Hannah V. Eshelman 1, Swetha Chandrasekaran, Nicholas Rodriguez, Samantha Ruelas, Eric B. Duoss, James P. Kelly, Maira R. Cerón ⁎, Patrick G. Campbell ⁎ Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA.
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A feedstock compatible with direct ink write and projection microstereolithography is developed to produce nanoporous ceramics with 250–500 μm features. • Pore sizes ranging from 0.10–500 μm are achieved through the introduction of designed structural porosity and partial sintering. • Printing by both techniques is enabled by modifying solids loading in the range from 55 to 70 wt% to tailor feedstock viscosity.
a r t i c l e
i n f o
Article history: Received 21 September 2020 Received in revised form 13 November 2020 Accepted 15 November 2020 Available online 17 November 2020 Keywords: Porous ceramics Additive manufacturing Direct ink writing Projection microstereolithography Versatile ceramic feedstock
a b s t r a c t Porous ceramic materials have a wide range of potential applications for which controlling the structure across multiple length scales is desirable. Additive manufacturing (AM) of porous ceramics is therefore of interest for design flexibility. Here, a ceramic ink compatible with 2 AM techniques, projection microstereolithography (PμSL) and direct ink write (DIW), was formulated and demonstrated printed parts with a range of controllable feature sizes as well as nanoporosity resulting from partial sintering. A diacrylate polymer was mixed with 3% yttria partially stabilized zirconia (3YZ) ceramic nanoparticles having different sizes and solids loadings to find formulations that meet the printing requirements of both AM techniques. Detailed rheological studies were used to determine optimal ink formulations to use for either printing method. The resulting 3YZ structures printed with DIW and PμSL have engineered macro cavities with span lengths greater than several millimeters, wall thicknesses of 200 to 540 μm, and porosity within the wall structure on the order of 100 nm. This study revealed that through facile composition changes to the 3YZ ink, it was feasible to use the same ink base for multiple AM techniques without the need for separate cumbersome ink development processes. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction Additive manufacturing (AM) techniques are currently used to create 3D structures from a digital model. Many industries including ⁎ Corresponding author. E-mail address: [email protected] (M.R. Cerón). 1 These authors contributed equally.
defense, medical, and aerospace have adopted AM as a means of rapidly manufacturing different materials [1,2]. The primary advantages of AM compared to conventional manufacturing methods include: (1) better economics for low-volume or custom production, such as rapid prototyping or to produce tooling or fixturing, (2) the ability to create complex near-net shape or net shape geometries that may be difficult or even impossible to fabricate by alternative forming methods, and in some cases, (3) to readily create 3D printed parts with spatial variance
https://doi.org/10.1016/j.matdes.2020.109337 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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Materials and Design 198 (2021) 109337
pore sizes on the order of 100 nm to maintain the liquid within the pores [36]. Here, further development of this slurry to be printed using PμSL and DIW to expand the possible geometries of the nanoporous ceramic is reported. The simplicity of this formulation, consisting of only the ceramic nanoparticles, the polymer, and initiator, allows it to be applied to both printing techniques, as well as maintain the same heating profile in the post-processing steps to remove the polymer and partially sinter the ceramic. Previously, ceramic slurries for additive manufacturing have been compatible with only one technique. Developing this nanoporous ceramic for both PμSL and DIW allows for more design freedom and customizability to create structures with engineered porosity that are beneficial for different types of fluid or gas flow such as filtration, catalysis, thermal insulation, and fluid treatment technologies [38]. In order to develop the ceramic ink for DIW and PμSL, the rheological properties of the ink, viscosity and storage modulus were measured. These properties were tuned by changing the ceramic particle sizes and solids loading. The resultant printed parts from both methods have engineered macropores while maintaining homogeneous internal nanoporosity and demonstrating feature sizes down to 250 μm, allowing for applications involving fluid or gas flow through both the macro- and nanopores. Yttria stabilized zirconium oxide (3YZ) ceramic nanoparticles were chosen for the current study because they showed desirable mechanical properties and chemical stability [37], but in principle this technique can be applied to a variety of ceramic nanoparticle precursors.
in structure (e.g. gradients in porosity and density) to alter the observed properties, characteristics, and performance [1,3–5]. AM has become a valuable tool to quickly produce complex parts with high accuracy and with feature sizes spanning hundreds of nanometers to meters [4]. Of the many types of AM, direct ink writing (DIW) [6–14] and projection microstereolithography (PμSL) [15–17] are two common techniques that use ink feedstocks. DIW is a technical evolution of methods originally used to make coil pottery and is typically done by extruding a shear thinning ink through a nozzle to write with a fine filament using a movable, programmable nozzle or stage. This technique is advantageous because it allows versatile ink compositions and complex geometries to be printed quickly. However, tuning the ink's rheological properties for shear-thinning behavior to enable flow through a small nozzle followed by thickening after extrusion for good shape retention is a challenge [6]. The PμSL technique builds structures from 2-D layers by using a UV-curable ink. Each layer can range in thickness from tens to hundreds of microns and is patterned using a digital light mask [18]. Like DIW, PμSL enables printing complex geometries, but with the additional advantage of printing shapes with more significant overhangs since the curing process imparts additional strength to the material. PμSL feature size and resolution, as small as tens of microns, are limited by ink viscosity as well as transparency to UV light; the formulation must be tuned to allow adequate light penetration while limiting scattering within the material, which can cause loss of resolution or incomplete curing [15,19–21]. Since the feedstock material requirements are different, inks are typically developed independently for these 2 AM processes; in this work an ink compatible with both processes was developed. Ceramic inks for AM are of interest because of the advantages AM offers over traditional methods of ceramic forming, such as tape casting [22–26], molding [12,27,28], and subtractive manufacturing [28,29]. Typically, ceramic inks for DIW are made from a highly viscous paste composed of ceramic nanoparticles, polymeric binder, and additives to control the rheological properties of the ink to enable flow through the nozzle and the ability to be self-supporting once printed [30]. Similarly, ceramic inks for PμSL consist of ceramic nanoparticles and a polymeric binder but may also include additives such as photoabsorbers to control the UV light or surfactants to help maintain a low viscosity and prevent nanoparticle agglomeration or sedimentation [28,31]. Since the additives that are often used to print ceramics on each of these printers have different roles, an ink developed for one method is typically not compatible with the other method. A ceramic ink that is compatible with both methods would be advantageous for utilizing the assets of each printer, which has not been demonstrated before. Ceramic AM can allow for creation of macroporous ceramics based on the geometry being printed, such as an octet truss, gyroid, or a lattice structure, but in order to create micro- or nanoporous ceramics, extra postprocessing steps are necessary. Porous ceramics are ideal for applications in filtration [22,32], catalysis [22,32,33], and thermal insulation [32,34], because of characteristics such as high surface area [32], and tunable porosity (including nano-porosity) and microstructure based on processing conditions and raw materials [34,35]. Common methods for making porous ceramics make use of sacrificial pore formers, direct foaming, partial sintering, and the replica technique [32,34]. Of these, the most straightforward approach to obtain porosity, that is also compatible with AM, is to partially sinter the ceramic, either by inhibiting active densifying sintering mechanisms to avoid final stage sintering or by promoting non-densifying sintering mechanisms to strengthen the material without affecting porosity [22]. Previously, our group developed a porous ceramic support for CO2 separation using a dual-phase (liquid/solid) membrane [36]. The support was made by tape casting a slurry of ceramic nanoparticles and a pre-polymer poly(ethylene glycol diacrylate) (PEGDA), followed by curing the pre-polymer and heating the green body under air to remove the polymer and partially sinter the ceramic [37]. The pores were then filled with molten hydroxide, which required
2. Materials and methods 2.1. Ink formulation All chemicals were purchased from Sigma Aldrich (St. Louis, MO) and were used as received unless otherwise noted. Ceramic inks were prepared by mixing 3% yttria partially stabilized zirconia (3YZ) nanoparticles (TZ-3Y-E or TZ-3YS-E, Tosoh, Tokyo, Japan), polyethylene glycol diacrylate (PEGDA, Mn 575), and a thermal initiator (Luperox® 231). Designations, particle sizes, specific surface areas, and density of the 3YZ nanoparticles (NPs) are given in Table 1. For notation simplicity throughout the rest of this article, the two different 3YZ NPs will be referred to by 3Y-55 and 3YS-120. The average particle diameters of 3Y-55 and 3YS-120 powders were measured using ImageJ analysis of SEM images (Fig. S1). The thresholding and particle analysis tools were used to measure the diameters of over 200 particles using ImageJ to determine the average particle diameter. Sample preparation for SEM was done by sonicating 5 mg of powder in 20 mL ethanol for 3 to 5 min to disperse agglomerates, followed by drop-casting onto a silicon wafer. Once the residual ethanol was completely evaporated from the silicon wafer, the wafers were attached to an SEM stub using copper tape. An Apreo SEM (Thermo Fisher Scientific, Waltham, MA) was used for imaging of particles and DIW parts. An accelerating voltage of 5 kV, working distance of 10.8 mm, 6.3 × 10–6 mbar high vacuum chamber, and Everhart-Thornley SE detector (ETD) were used for all SEM images. A Phenom Desktop SEM (Thermo Fisher Scientific, Waltham, MA) was used for imaging of the PμSL parts. An accelerating voltage of 10 kV, 0.6 mbar vacuum chamber, and backscattered electron (BSE) detector were used for Phenom images. The 3YZ inks were made using solids loading in PEGDA ranging from 55 to 70 wt% (18–30 vol%, Table 2). The ink was mixed with spherical zirconia grinding beads (4 mm diameter) in an acoustic mixer (LabRAM II, Resodyn Acoustic Mixers, Butte, MT) for 3 h at 70 g-force to break up agglomerates and disperse the particles. The LabRAM II mixing time was determined by SEM images of the inks at different time points to ensure homogeneous dispersion (Fig. S2). The grinding beads were removed and the thermal initiator, Luperox® 231 (1–2 wt% with respect to PEGDA), was then added to the ink before being mixed for 5 min 2
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Materials and Design 198 (2021) 109337
Table 1 Supplier specifications for Tosoh 3YZ NPs and comparison to measured particle sizes (density refers to fully dense ceramic). Supplier specifications
Measured
Designation
Grade
Particle Size (nm)
Specific Surface Area (m2/g)
Density (g/cm3)
Particle Size (nm)
3Y-55 3YS-120
TZ-3Y-E TZ-3YS-E
40 90
16 ± 3 7±2
6.05 6.05
55 ± 10 120 ± 25
printing. Multiple shapes and geometries were printed to demonstrate the versatility of the ceramic ink including the following three structures: (1) tubes, (2) face-centered cubic lattices, and (3) a ‘wagon wheel’/ co-axial cylinders with rectilinear infill structure.
Table 2 Measured wt% and vol% calculated using the densities of the PEGDA and 3YZ NPs – 1.12 g/cm3 and 6.05 g/cm3. 3YZ NP wt% of Ink
3YZ NP vol% of Ink
55 65 70
18 26 30
2.4. Projection microstereolithography of ceramic inks The Mk IV PμSL system is a custom designed printer created at Lawrence Livermore National Laboratory (LLNL) for use with highly viscous inks. The ink is spread into a rotating circular stage equipped with a wiper blade. A build plate is lowered into the stage to compress the ink to the pre-set layer thickness and a UV LED array (405 nm) illuminates a DLP6500 digital micromirror device (DMD) which patterns the uniform light field (Texas Instruments, Dallas, TX). The DMD is an array of 1920 × 1080 micromirrors with a pitch of 7.56 μm, where each mirror can be switched on or off with a refresh rate as high as 9523 Hz to reflect a patterned image. This reflected image passes through an aperture that can control the focus of the image and is magnified 2× to have an image size of 29 × 16 mm with 15 μm pixels. The mask is projected from below the stage through a transparent acrylic plate with a fluorinated ethylene propylene (FEP) film contacting the ink. As the layer cures, it adheres to the build plate which is lifted off the stage, and the stage rotates to spread fresh ink over the area to be cured (Fig. S4). Using this recoating system, the printer can handle inks with viscosities ranging up to at least 1000 Pa.s at a strain rate of 0.1 s−1. Printing parameters that can be controlled include aperture, light intensity, exposure time, and layer thickness. When the aperture is fully opened to 50 mm, the corresponding light intensity is 258 mW/cm2 at the build plane. An aperture of 12.2 mm was used in this study, therefore a light intensity of 100% corresponds to an intensity of 34 mW/cm2. Decreasing the aperture results in a more focused image, and at small apertures down to 7.5 mm diameter, the printer can project features as small as 50 μm. Throughout this manuscript, light intensity will be referred to in terms of percentage of full intensity. Further details on the parameters selected for printing can be found in Table S1. The primary geometries that were printed for the purpose of this study included the following three structures: an octet truss unit cell, an octet truss lattice, and a gyroid. These shapes were selected to demonstrate the capability of the PμSL to print complex shapes with overhangs, as well as to investigate the potential of the unique porous structure of these geometries, particularly the gyroid, because of its multiple interpenetrating flow channels and high surface area [39].
using a Thinky mixer (ARE-310, Thinky Corporation, Laguna Hills, CA). Further additions, to the PμSL formulations only, included a photoinitiator comprised of 4-Methoxyphenol (MEHQ, 0.1 wt% with respect to PEGDA), isopropylthioxanone (ITX, 0.1–0.3 wt% with respect to PEGDA), and 2-ethylhexyl 4-(dimethylamino)benzoate (EHDA, 0.2–0.6 wt%), which were dissolved in 0.1 mL tetrahydrofuran (THF) and added with the Luperox® 231. Once the initiator was added, the ceramic ink was ready for subsequent characterization and printing trials. 2.2. Ink rheology The rheological behavior of the ceramic inks with the initiator added was studied using an AR2000ex rheometer (TA Instruments, New Castle, DE) as a tool to better quantify the properties of the ink. The viscosity of the ceramic ink was measured relative to shear rates ramping from 0.1–2000 s−1; the ink was pre-sheared at 0.1 rad/s for 10 s. Oscillatory stress sweeps were also performed to measure the storage modulus and determine yield stress from an inflection point in the storage modulus curves. Measurements were taken from 1 to 5000 Pa at a frequency of 1 Hz. Each experiment was performed on three different samples from the same batch of ink; numbers reported throughout the article are the average of the measurements. Stainless steel parallel plates with 20 mm diameter were used for all measurements. Gaps of 100 μm and 500 μm were used for shear sweep and shear modulus measurements, respectively. 2.3. Direct ink writing of ceramic inks The ceramic inks were loaded into a Nordson EFD clear Optimum® ZeroDraft™ syringe barrel, with volume ranging from 3 mL to 10 mL depending on the quantity of ink needed for printing. Once the ceramic ink was filled, an Optimum® plunger was pressed into the barrel and the syringe was degassed in the Thinky mixer to remove air bubbles. Next, an Optimum® SmoothFlow™ tapered dispensing PTFE tip was attached to the syringe using a luer-lock mechanism. The nozzle inner diameters ranged from 250 to 1190 μm depending on the desired line thickness of the printed part. The ceramic ink was then extruded while translating the nozzle with a custom Mechanical Gantry system with an Ultimus V, EFD air powered fluid dispenser (Fig. S3) which applies a controllable pressure ranging from 103 to 172 kPa that can be varied depending on both the ink composition and the nozzle diameter. For the smaller ceramic NPs (3Y-55) in an ink with 65 wt% solids and a 600 μm diameter nozzle, 124 kPa was required for extrusion whereas for the larger ceramic NPs (3YS-120), 124 kPa was required for 70 wt% solids and a 600 μm diameter nozzle. A summary of these optimized parameters can be found in Table S1. A writing speed of 3 mm/s was used for all
2.5. Post-processing of AM printed ceramic parts After printing parts with either technique, the ceramic green bodies were thermally cured at 100 °C for 12 h. The green bodies were then thermally processed to remove all polymers and partially sinter the 3YZ in a Neytech Vulcan box furnace to maintain porosity. Polymers were removed using a heating rate of 1 °C/min and dwell temperatures and times of 200 °C for 2 h, 300 °C for 4 h, and 400 °C for 2 h, followed directly by partial sintering using a heating rate of 2 °C/min and dwell temperatures and times of 800 °C for 4 h and 1090 °C for 15 h, followed directly by cooling at a rate of 2 °C/min [37]. These conditions were selected based on thermogravimetric analysis (TGA) showing mass loss associated with pyrolysis of the organic components at ~200 °C 3
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ceramic inks were performed to quantify the effect that NP size and solids loading have in the ink properties; the results are shown in Fig. 1. Fig. 1a shows the average measured viscosities of the inks over a range of shear rates for inks containing 3Y-55 or 3YS-120 NPs and with different solids loadings. All of the formulations demonstrate shear thinning behavior and also show a clear trend of increasing viscosity with increasing NP loading, especially at low shear rates. At a shear rate of 1 s−1 the viscosities of inks with 55 wt% and 70 wt% (Table 2) solids varies by an order of magnitude for both ceramic inks (Fig. 1b), which is consistent with previously reported ceramic NP systems [14,44]. When comparing the viscosity differences between the two NP inks at the same solids loading, the inks with 3Y-55 ceramic NPs are higher than that of the inks with 3YS-120 NPs (Fig. 1b and Table S2). This can be attributed to the increased specific surface area of the NPs in the 3Y-55 ink compared to the NPs in the 3YS-120 ink, and increased particle-particle interactions that inhibit ink flow [44]. The storage moduli of the inks were also measured as a function of shear stress (Fig. 1c). On this storage modulus curve, yield stress is reported as 90% of the storage modulus plateau at lower shear stresses and denoted by the circular markers in Fig. 1c. There is a marked increase in yield stress with increased solids loading shown in Fig. 1d. The smaller particle size in 3Y-55 showed an order of magnitude higher yield stress compared with the 3YS-120 inks at a given solids loading (Fig. 1d and Table S3), which can be attributed to increased particle-particle interactions and strong interference of the polymer
[40,41]. The sintering temperature was selected to maintain the nanoporosity, demonstrated by the BET data showing pore size distribution up to 1100 Å (Fig. S5).
3. Results and discussion 3.1. Rheology of ceramic inks Printing with the Mk IV PμSL system requires a UV curable ink that is spreadable and shear thinning to create uniform thin layers [42]. The property of spreadability is inversely related to the yield stress of a material [43]. Inks for DIW printing must be able to flow to extrude through a small nozzle, but stiff enough to self-support its shape once the ink is printed and any additional material printed on top of it. The basis of tuning the ceramic ink for various additive manufacturing techniques, both with different requirements, relies on two main characteristics: the nanoparticle size and the solids loading of the ceramic ink. Two different sizes of 3YZ NPs (measured average size of 55 ± 10 and 120 ± 25 nm) were investigated in order to: (1) demonstrate the effect of doubling the average NP diameter on viscosity and stiffness of the ceramic ink and (2) determine the impact of different NPs on light scattering in the PμSL system. Likewise, the varied solids loadings are important to optimize shear thinning inks for both DIW and PμSL and to tune the porosity and microstructure of the sintered porous ceramic [37]. A series of rheology experiments such as shear sweep and stress sweep of the
Fig. 1. (a) Log-log plot of the shear thinning viscosity profiles for inks with various NP solids loadings and (b) the viscosity measured at 1 s−1 for the 55, 65, and 70 wt% NP loaded inks. (c) Log-log plot of representative storage modulus curves as a function of shear stress for 55, 65, and 70 wt% NP loaded inks and (d) the yield strength estimated as 90% of the storage modulus plateau of the representative storage modulus vs. shear stress curve. 4
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of the part shown are 10 mm in height and 4 mm diameter. The wall thickness and layer height of the cylinder are both determined by the nozzle diameter, which is 1190 μm, and results in a wall thickness of 1050 ± 25 μm after sintering at 1090 °C. The printed tube shows that the ink can support the weight of repeated layering without significant shape distortion. The diameter of the tube remains constant around the circumference and height (10 mm) of the cylinder; it is clear by looking down the length of the tube that there is no sagging. Additionally, this simplistic structure can be printed quite easily and quickly by DIW (Fig. S7) in contrast to some other AM techniques such as inkjet printing or selective laser sintering [13, 51]. To calculate the shrinkage of the printed part, the dimensions of the cylinder were measured after each step of post-processing; for this ink (70 wt% 3Y-55), the total shrinkage from printing to a sintered part was determined to be ~15%. The SEM micrograph in Fig. 2b shows the internal nano-porosity achieved by partial sintering at 1090 °C. The optical image of the ‘wagon wheel’ structure shown in Fig. 2c demonstrates that the structural integrity of the ceramic ink is maintained after the curing and partial sintering post-processing steps. The structure is composed of alternating layers of concentric circles and angled lines from the center of the circle spanning the radius of the circle. The dimensions of the part are 20 mm in diameter and 5 mm in height. The diameters of the circles, the angle of the lines, and the layer height are all dependent on the nozzle diameter that is used; a 406 μm nozzle diameter was used for this print, which results in an outer diameter of
chains [44]. Since large storage modulus and yield strength are important for inks used in DIW printing, these results confirm that higher NP loading, such as 70 wt%, results in an ink with excellent stiffness while maintaining its shear thinning properties. Formulations of 3Y-55 or 3YS-120 can be used to create inks with a yield stress between 105 and 106 Pa, which is consistent with other DIW inks [4,6]. However, the 3Y-55 ink was chosen over the 3YS-120 ink because the smaller particle size produced less clogging in the nozzle as well as better shape retention after sintering. For the PμSL requirements of spreadability (low yield stress) and shear thinning behavior, the larger NPs (3YS-120) are superior, as is lower solids loading. PμSL inks made from both particle sizes were evaluated, with details discussed below. 3.2. Direct ink writing Based on the rheological analysis, 70 wt% (30 vol%) of 3Y-55 ceramic was used for the DIW studies. The ink was extruded through various nozzles with diameters ranging from 250 to 1190 μm (Fig. S6) using the optimized printing conditions described in the experimental section. Fig. 3 shows a selection of partially sintered parts printed by DIW, including tubes, a ‘wagon wheel’/ co-axial cylinder with rectilinear infill structure, and a simple cubic lattice. Each of these printed structures contains nano-porosity on the order of 100 nm. The partially sintered tube shown in Fig. 2a is printed by repeating layers of circles with a specified diameter and height; the dimensions
Fig. 2. DIW printed parts using 70 wt% (30 vol%) 3Y-55 ceramic ink with a 406 μm nozzle and SEM images of the corresponding internal porosity: (a, b) partially sintered tubes using a 1190 μm nozzle, (c, d) partially sintered ‘wagon wheel’, and (e, f) partially sintered simple cubic lattice. 5
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Fig. 3. Partially sintered porous zirconia gyroids printed from (a) 3Y-55 NPs and (b) 3YS-120 NPs using the Mk IV PμSL and (c) and (d) SEM of the internal microstructure of the corresponding sintered part.
therefore an increased print time, or inadequate depth of cure and poor adhesion [19–21]. Scattering can also cause excess cure beyond the bounds of the projected mask, and thus decrease the print resolution [20]. The higher the NP loading within the ink, the more the light will be scattered [28], but higher particle loading is needed to increase mechanical strength of the part and reduce shrinkage during sintering [37,47]. Creating an ink that prints a mechanically robust 3D part while maintaining good resolution requires balancing these competing factors. Cure depth is dependent on several parameters: mean particle size, solids loading, light intensity, exposure time, difference in refractive indices of the monomer solution and particle filler, and wavelength of the light source [19]. Cure depth and width are proportional to mean particle size and inversely proportional to solids loading [19,28]. Several studies have investigated the impact of changing the particle sizes, the refractive index of the particles, and the refractive index of the resin, as well as external parameters such as energy dose [20,21,47,49]. For the purpose of developing this ink, the primary parameters of interest were the mean particle size and solids loading. Initial ink development for optimization of the initiator and inhibitors was done using 3Y-55 NPs at loadings ranging from 50 to 60 wt%. The lower loading was selected because of the lower yield stress of the inks with lower solids loadings (Fig. 1c). While an ink of 3Y-55 is printable (Fig. S8 and S9 show parts printed during initial development), 3YS-120 was of interest due to the potential of the larger particles to decrease scattering and therefore increase cure depth and reduce excess cure width. Fig. 3 shows printed and sintered gyroids from a 55 wt% (18 vol%) 3Y-55 ink (Fig. 3a) and 3YS-120 ink (Fig. 3b) using otherwise identical conditions. Fig. 3 clearly shows that using larger NPs provided superior layer adhesion, which enabled printing taller parts and with better resolution (determined based on the wall
20 mm, inner diameter of 4 mm, angles of 6 degrees, and a layer height of 0.4 mm. The SEM image in Fig. 2d reveals the homogeneous nanoporosity of the resulting printed structure. The partially sintered simple cubic lattice (Fig. 2e) consists of orthogonal layers made of parallel cylindrical rods, with each layer rotated 90 degrees. The dimensions of the green body are 15 mm by 15 mm with approximately 450 ± 7 μm wide struts. After partially sintering the dimensions are 12.75 mm by 12.75 mm with 380 ± 5 μm wide struts. This structure demonstrates the achievable resolution and feature size of the ceramic ink and the ink's self-supporting ability across an unsupported span. The layer height is dependent on the nozzle diameter used for printing. The cylindrical rod diameter and the spacing between the parallel cylindrical rods are equal to the nozzle diameter (406 μm). The SEM micrograph of this structure also reveals consistent internal morphology with the other printed parts (Fig. 2f). 3.3. Projection microstereolithography Although appropriate ink rheology is an important factor for spreading thin layers in the PμSL system, it is not the only factor that must be considered. An additional challenge of printing ceramic inks using PμSL comes from the presence of NPs with a high refractive index. The presence of NPs causes the ink to be opaque and prevent it from being compatible with other light-based AM techniques, such as two-photon polymerization, which allows for smaller resolution than PμSL [45]. NP-filled inks can be printed using PμSL but special consideration must be taken to factor in the impact of scattering on the cure depth and resolution. Light scattering within the ink depends on the difference in refractive index between the 3YZ NPs (n = 2.19 at 633 nm) [46] and the UV-curable resin base (n = 1.467 at 589 nm) [47,48]. Scattering reduces light penetration depth, which can cause thinner layers and 6
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Declaration of Competing Interest
thicknesses of the sintered parts). Fig. S10 shows a side view of gyroids where the difference in wall thickness is apparent. For instance, when gyroids with 55 wt% 3Y-55 ink and a projected wall thickness of 750 μm were printed, a sintered part was obtained with a wall thickness closer to 1050 ± 100 μm (Fig. S10a), indicating the ceramic resin over cured. On the other hand, when the 55 wt% 3YS-120 ink and a projected wall thickness of 666 μm was printed, a sintered part with a wall thickness of 540 ± 60 μm was observed (Fig. S10b). Shrinkage of the part during sintering accounts for the difference between the projected wall thickness and measured wall thickness in the 3YS-120 part. Fig. 3c and d show SEM of the microstructure of both 3Y-55 and 3YS120 printed with the Mk IV PμSL. The morphology is consistent with that seen from the DIW parts in Fig. 2.
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. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2020.109337. References [1] T.D. Ngo, A. Kashani, G. Imbalzano, K.T.Q. Nguyen, D. Hui, Additive manufacturing (3D printing): a review of materials, methods, applications and challenges, Compos. Part B-Eng. 143 (2018) 172–196. [2] M.R. Khosravani, T. Reinicke, On the environmental impact of 3D printing technology, Appl. Mater. Today 20 (2020) 100689. [3] L.C. Hwa, S. Rajoo, A.M. Noor, N. Ahmad, M.B. Uday, Recent advances in 3D printing of porous ceramics: a review, Curr. Opin. Solid State Mater. Sci. 21 (2017) 323–347. [4] N. Travitzky, A. Bonet, B. Dermeik, T. Fey, I. Filbert-Demut, L. Schlier, T. Schlordt, P. Greil, Additive manufacturing of ceramic-based materials, Adv. Eng. 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4. Conclusions In this work, a versatile ceramic ink feedstock consisting of ceramic nanoparticles suspended in PEGDA resin was developed to produce porous ceramic structures using two different AM techniques, PμSL and DIW. The ink's viscosity and storage moduli were measured as a function of different ceramic NP particle size and loading. Reducing the mean NP diameter of the 3YZ powder from ~120 nm to ~55 nm and increasing the solids loading from 55 to 70 wt% increased ink viscosity by an order of magnitude (at a shear rate of 1 s−1) and the yield stress calculated from the storage modulus curves by two orders of magnitude (104 Pa to 106 Pa). The 3Y-120 NPs at a 55 wt% (18 vol%) loading was chosen for PμSL because of the low yield stress, imparting good spreading characteristics, as well as the decrease in scattering that improved adhesion of the layers and print resolution. The 3Y-55 NPs at 70 wt% (30 vol%) was chosen for DIW because of the high yield stress and shear thinning behavior that allow the ink to be extruded effectively while retaining its shape after extrusion. The resulting printed structures demonstrated the control of features on macro-, micro-, and nanoscales; DIW printing capabilities are showcased in the cylinder height (10 mm) and concentric circle line spacing in the wagon wheel structure (using a 406 μm nozzle); the smallest feature size printed was 250 μm. The PμSL printed gyroid has a print height of 12 mm and wall thickness of 540 μm. All printed structures show internal porosity on the order of 100 nm. This collection of printed nanoporous ceramic structures demonstrates the variety of engineered porosity that can be accomplished using these techniques. The combination of the internal nanoporosity and the engineered macroporosity derived from the structure makes these ceramic parts ideal for applications requiring fluid or gas flow in high temperature or corrosive conditions.
Declaration of Competing Interest None. Acknowledgements Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Funding was provided by the Lawrence Livermore National Laboratory Directed Research and Development (LDRD) [grant numbers 16-LW-013 and ISCP 19-PLS-007]. This work was supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences [grant number DE-SC0017124] and the U.S. Department of Energy's Office of Environment, Health, Safety and Security (AU-30) under the Nuclear Safety Research and Development Program (NSRD-20). IM release: LLNL-JRNL-811089. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. 7
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
3D-printed nanoporous ceramics: Tunable feedstock for direct ink write and projection microstereolithography Alyssa L. Troksa 1, Hannah V. Eshelman 1, Swetha Chandrasekaran, Nicholas Rodriguez, Samantha Ruelas, Eric B. Duoss, James P. Kelly, Maira R. Cerón ⁎, Patrick G. Campbell ⁎ Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA.
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A feedstock compatible with direct ink write and projection microstereolithography is developed to produce nanoporous ceramics with 250–500 μm features. • Pore sizes ranging from 0.10–500 μm are achieved through the introduction of designed structural porosity and partial sintering. • Printing by both techniques is enabled by modifying solids loading in the range from 55 to 70 wt% to tailor feedstock viscosity.
a r t i c l e
i n f o
Article history: Received 21 September 2020 Received in revised form 13 November 2020 Accepted 15 November 2020 Available online 17 November 2020 Keywords: Porous ceramics Additive manufacturing Direct ink writing Projection microstereolithography Versatile ceramic feedstock
a b s t r a c t Porous ceramic materials have a wide range of potential applications for which controlling the structure across multiple length scales is desirable. Additive manufacturing (AM) of porous ceramics is therefore of interest for design flexibility. Here, a ceramic ink compatible with 2 AM techniques, projection microstereolithography (PμSL) and direct ink write (DIW), was formulated and demonstrated printed parts with a range of controllable feature sizes as well as nanoporosity resulting from partial sintering. A diacrylate polymer was mixed with 3% yttria partially stabilized zirconia (3YZ) ceramic nanoparticles having different sizes and solids loadings to find formulations that meet the printing requirements of both AM techniques. Detailed rheological studies were used to determine optimal ink formulations to use for either printing method. The resulting 3YZ structures printed with DIW and PμSL have engineered macro cavities with span lengths greater than several millimeters, wall thicknesses of 200 to 540 μm, and porosity within the wall structure on the order of 100 nm. This study revealed that through facile composition changes to the 3YZ ink, it was feasible to use the same ink base for multiple AM techniques without the need for separate cumbersome ink development processes. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction Additive manufacturing (AM) techniques are currently used to create 3D structures from a digital model. Many industries including ⁎ Corresponding author. E-mail address: [email protected] (M.R. Cerón). 1 These authors contributed equally.
defense, medical, and aerospace have adopted AM as a means of rapidly manufacturing different materials [1,2]. The primary advantages of AM compared to conventional manufacturing methods include: (1) better economics for low-volume or custom production, such as rapid prototyping or to produce tooling or fixturing, (2) the ability to create complex near-net shape or net shape geometries that may be difficult or even impossible to fabricate by alternative forming methods, and in some cases, (3) to readily create 3D printed parts with spatial variance
https://doi.org/10.1016/j.matdes.2020.109337 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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Materials and Design 198 (2021) 109337
pore sizes on the order of 100 nm to maintain the liquid within the pores [36]. Here, further development of this slurry to be printed using PμSL and DIW to expand the possible geometries of the nanoporous ceramic is reported. The simplicity of this formulation, consisting of only the ceramic nanoparticles, the polymer, and initiator, allows it to be applied to both printing techniques, as well as maintain the same heating profile in the post-processing steps to remove the polymer and partially sinter the ceramic. Previously, ceramic slurries for additive manufacturing have been compatible with only one technique. Developing this nanoporous ceramic for both PμSL and DIW allows for more design freedom and customizability to create structures with engineered porosity that are beneficial for different types of fluid or gas flow such as filtration, catalysis, thermal insulation, and fluid treatment technologies [38]. In order to develop the ceramic ink for DIW and PμSL, the rheological properties of the ink, viscosity and storage modulus were measured. These properties were tuned by changing the ceramic particle sizes and solids loading. The resultant printed parts from both methods have engineered macropores while maintaining homogeneous internal nanoporosity and demonstrating feature sizes down to 250 μm, allowing for applications involving fluid or gas flow through both the macro- and nanopores. Yttria stabilized zirconium oxide (3YZ) ceramic nanoparticles were chosen for the current study because they showed desirable mechanical properties and chemical stability [37], but in principle this technique can be applied to a variety of ceramic nanoparticle precursors.
in structure (e.g. gradients in porosity and density) to alter the observed properties, characteristics, and performance [1,3–5]. AM has become a valuable tool to quickly produce complex parts with high accuracy and with feature sizes spanning hundreds of nanometers to meters [4]. Of the many types of AM, direct ink writing (DIW) [6–14] and projection microstereolithography (PμSL) [15–17] are two common techniques that use ink feedstocks. DIW is a technical evolution of methods originally used to make coil pottery and is typically done by extruding a shear thinning ink through a nozzle to write with a fine filament using a movable, programmable nozzle or stage. This technique is advantageous because it allows versatile ink compositions and complex geometries to be printed quickly. However, tuning the ink's rheological properties for shear-thinning behavior to enable flow through a small nozzle followed by thickening after extrusion for good shape retention is a challenge [6]. The PμSL technique builds structures from 2-D layers by using a UV-curable ink. Each layer can range in thickness from tens to hundreds of microns and is patterned using a digital light mask [18]. Like DIW, PμSL enables printing complex geometries, but with the additional advantage of printing shapes with more significant overhangs since the curing process imparts additional strength to the material. PμSL feature size and resolution, as small as tens of microns, are limited by ink viscosity as well as transparency to UV light; the formulation must be tuned to allow adequate light penetration while limiting scattering within the material, which can cause loss of resolution or incomplete curing [15,19–21]. Since the feedstock material requirements are different, inks are typically developed independently for these 2 AM processes; in this work an ink compatible with both processes was developed. Ceramic inks for AM are of interest because of the advantages AM offers over traditional methods of ceramic forming, such as tape casting [22–26], molding [12,27,28], and subtractive manufacturing [28,29]. Typically, ceramic inks for DIW are made from a highly viscous paste composed of ceramic nanoparticles, polymeric binder, and additives to control the rheological properties of the ink to enable flow through the nozzle and the ability to be self-supporting once printed [30]. Similarly, ceramic inks for PμSL consist of ceramic nanoparticles and a polymeric binder but may also include additives such as photoabsorbers to control the UV light or surfactants to help maintain a low viscosity and prevent nanoparticle agglomeration or sedimentation [28,31]. Since the additives that are often used to print ceramics on each of these printers have different roles, an ink developed for one method is typically not compatible with the other method. A ceramic ink that is compatible with both methods would be advantageous for utilizing the assets of each printer, which has not been demonstrated before. Ceramic AM can allow for creation of macroporous ceramics based on the geometry being printed, such as an octet truss, gyroid, or a lattice structure, but in order to create micro- or nanoporous ceramics, extra postprocessing steps are necessary. Porous ceramics are ideal for applications in filtration [22,32], catalysis [22,32,33], and thermal insulation [32,34], because of characteristics such as high surface area [32], and tunable porosity (including nano-porosity) and microstructure based on processing conditions and raw materials [34,35]. Common methods for making porous ceramics make use of sacrificial pore formers, direct foaming, partial sintering, and the replica technique [32,34]. Of these, the most straightforward approach to obtain porosity, that is also compatible with AM, is to partially sinter the ceramic, either by inhibiting active densifying sintering mechanisms to avoid final stage sintering or by promoting non-densifying sintering mechanisms to strengthen the material without affecting porosity [22]. Previously, our group developed a porous ceramic support for CO2 separation using a dual-phase (liquid/solid) membrane [36]. The support was made by tape casting a slurry of ceramic nanoparticles and a pre-polymer poly(ethylene glycol diacrylate) (PEGDA), followed by curing the pre-polymer and heating the green body under air to remove the polymer and partially sinter the ceramic [37]. The pores were then filled with molten hydroxide, which required
2. Materials and methods 2.1. Ink formulation All chemicals were purchased from Sigma Aldrich (St. Louis, MO) and were used as received unless otherwise noted. Ceramic inks were prepared by mixing 3% yttria partially stabilized zirconia (3YZ) nanoparticles (TZ-3Y-E or TZ-3YS-E, Tosoh, Tokyo, Japan), polyethylene glycol diacrylate (PEGDA, Mn 575), and a thermal initiator (Luperox® 231). Designations, particle sizes, specific surface areas, and density of the 3YZ nanoparticles (NPs) are given in Table 1. For notation simplicity throughout the rest of this article, the two different 3YZ NPs will be referred to by 3Y-55 and 3YS-120. The average particle diameters of 3Y-55 and 3YS-120 powders were measured using ImageJ analysis of SEM images (Fig. S1). The thresholding and particle analysis tools were used to measure the diameters of over 200 particles using ImageJ to determine the average particle diameter. Sample preparation for SEM was done by sonicating 5 mg of powder in 20 mL ethanol for 3 to 5 min to disperse agglomerates, followed by drop-casting onto a silicon wafer. Once the residual ethanol was completely evaporated from the silicon wafer, the wafers were attached to an SEM stub using copper tape. An Apreo SEM (Thermo Fisher Scientific, Waltham, MA) was used for imaging of particles and DIW parts. An accelerating voltage of 5 kV, working distance of 10.8 mm, 6.3 × 10–6 mbar high vacuum chamber, and Everhart-Thornley SE detector (ETD) were used for all SEM images. A Phenom Desktop SEM (Thermo Fisher Scientific, Waltham, MA) was used for imaging of the PμSL parts. An accelerating voltage of 10 kV, 0.6 mbar vacuum chamber, and backscattered electron (BSE) detector were used for Phenom images. The 3YZ inks were made using solids loading in PEGDA ranging from 55 to 70 wt% (18–30 vol%, Table 2). The ink was mixed with spherical zirconia grinding beads (4 mm diameter) in an acoustic mixer (LabRAM II, Resodyn Acoustic Mixers, Butte, MT) for 3 h at 70 g-force to break up agglomerates and disperse the particles. The LabRAM II mixing time was determined by SEM images of the inks at different time points to ensure homogeneous dispersion (Fig. S2). The grinding beads were removed and the thermal initiator, Luperox® 231 (1–2 wt% with respect to PEGDA), was then added to the ink before being mixed for 5 min 2
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Table 1 Supplier specifications for Tosoh 3YZ NPs and comparison to measured particle sizes (density refers to fully dense ceramic). Supplier specifications
Measured
Designation
Grade
Particle Size (nm)
Specific Surface Area (m2/g)
Density (g/cm3)
Particle Size (nm)
3Y-55 3YS-120
TZ-3Y-E TZ-3YS-E
40 90
16 ± 3 7±2
6.05 6.05
55 ± 10 120 ± 25
printing. Multiple shapes and geometries were printed to demonstrate the versatility of the ceramic ink including the following three structures: (1) tubes, (2) face-centered cubic lattices, and (3) a ‘wagon wheel’/ co-axial cylinders with rectilinear infill structure.
Table 2 Measured wt% and vol% calculated using the densities of the PEGDA and 3YZ NPs – 1.12 g/cm3 and 6.05 g/cm3. 3YZ NP wt% of Ink
3YZ NP vol% of Ink
55 65 70
18 26 30
2.4. Projection microstereolithography of ceramic inks The Mk IV PμSL system is a custom designed printer created at Lawrence Livermore National Laboratory (LLNL) for use with highly viscous inks. The ink is spread into a rotating circular stage equipped with a wiper blade. A build plate is lowered into the stage to compress the ink to the pre-set layer thickness and a UV LED array (405 nm) illuminates a DLP6500 digital micromirror device (DMD) which patterns the uniform light field (Texas Instruments, Dallas, TX). The DMD is an array of 1920 × 1080 micromirrors with a pitch of 7.56 μm, where each mirror can be switched on or off with a refresh rate as high as 9523 Hz to reflect a patterned image. This reflected image passes through an aperture that can control the focus of the image and is magnified 2× to have an image size of 29 × 16 mm with 15 μm pixels. The mask is projected from below the stage through a transparent acrylic plate with a fluorinated ethylene propylene (FEP) film contacting the ink. As the layer cures, it adheres to the build plate which is lifted off the stage, and the stage rotates to spread fresh ink over the area to be cured (Fig. S4). Using this recoating system, the printer can handle inks with viscosities ranging up to at least 1000 Pa.s at a strain rate of 0.1 s−1. Printing parameters that can be controlled include aperture, light intensity, exposure time, and layer thickness. When the aperture is fully opened to 50 mm, the corresponding light intensity is 258 mW/cm2 at the build plane. An aperture of 12.2 mm was used in this study, therefore a light intensity of 100% corresponds to an intensity of 34 mW/cm2. Decreasing the aperture results in a more focused image, and at small apertures down to 7.5 mm diameter, the printer can project features as small as 50 μm. Throughout this manuscript, light intensity will be referred to in terms of percentage of full intensity. Further details on the parameters selected for printing can be found in Table S1. The primary geometries that were printed for the purpose of this study included the following three structures: an octet truss unit cell, an octet truss lattice, and a gyroid. These shapes were selected to demonstrate the capability of the PμSL to print complex shapes with overhangs, as well as to investigate the potential of the unique porous structure of these geometries, particularly the gyroid, because of its multiple interpenetrating flow channels and high surface area [39].
using a Thinky mixer (ARE-310, Thinky Corporation, Laguna Hills, CA). Further additions, to the PμSL formulations only, included a photoinitiator comprised of 4-Methoxyphenol (MEHQ, 0.1 wt% with respect to PEGDA), isopropylthioxanone (ITX, 0.1–0.3 wt% with respect to PEGDA), and 2-ethylhexyl 4-(dimethylamino)benzoate (EHDA, 0.2–0.6 wt%), which were dissolved in 0.1 mL tetrahydrofuran (THF) and added with the Luperox® 231. Once the initiator was added, the ceramic ink was ready for subsequent characterization and printing trials. 2.2. Ink rheology The rheological behavior of the ceramic inks with the initiator added was studied using an AR2000ex rheometer (TA Instruments, New Castle, DE) as a tool to better quantify the properties of the ink. The viscosity of the ceramic ink was measured relative to shear rates ramping from 0.1–2000 s−1; the ink was pre-sheared at 0.1 rad/s for 10 s. Oscillatory stress sweeps were also performed to measure the storage modulus and determine yield stress from an inflection point in the storage modulus curves. Measurements were taken from 1 to 5000 Pa at a frequency of 1 Hz. Each experiment was performed on three different samples from the same batch of ink; numbers reported throughout the article are the average of the measurements. Stainless steel parallel plates with 20 mm diameter were used for all measurements. Gaps of 100 μm and 500 μm were used for shear sweep and shear modulus measurements, respectively. 2.3. Direct ink writing of ceramic inks The ceramic inks were loaded into a Nordson EFD clear Optimum® ZeroDraft™ syringe barrel, with volume ranging from 3 mL to 10 mL depending on the quantity of ink needed for printing. Once the ceramic ink was filled, an Optimum® plunger was pressed into the barrel and the syringe was degassed in the Thinky mixer to remove air bubbles. Next, an Optimum® SmoothFlow™ tapered dispensing PTFE tip was attached to the syringe using a luer-lock mechanism. The nozzle inner diameters ranged from 250 to 1190 μm depending on the desired line thickness of the printed part. The ceramic ink was then extruded while translating the nozzle with a custom Mechanical Gantry system with an Ultimus V, EFD air powered fluid dispenser (Fig. S3) which applies a controllable pressure ranging from 103 to 172 kPa that can be varied depending on both the ink composition and the nozzle diameter. For the smaller ceramic NPs (3Y-55) in an ink with 65 wt% solids and a 600 μm diameter nozzle, 124 kPa was required for extrusion whereas for the larger ceramic NPs (3YS-120), 124 kPa was required for 70 wt% solids and a 600 μm diameter nozzle. A summary of these optimized parameters can be found in Table S1. A writing speed of 3 mm/s was used for all
2.5. Post-processing of AM printed ceramic parts After printing parts with either technique, the ceramic green bodies were thermally cured at 100 °C for 12 h. The green bodies were then thermally processed to remove all polymers and partially sinter the 3YZ in a Neytech Vulcan box furnace to maintain porosity. Polymers were removed using a heating rate of 1 °C/min and dwell temperatures and times of 200 °C for 2 h, 300 °C for 4 h, and 400 °C for 2 h, followed directly by partial sintering using a heating rate of 2 °C/min and dwell temperatures and times of 800 °C for 4 h and 1090 °C for 15 h, followed directly by cooling at a rate of 2 °C/min [37]. These conditions were selected based on thermogravimetric analysis (TGA) showing mass loss associated with pyrolysis of the organic components at ~200 °C 3
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ceramic inks were performed to quantify the effect that NP size and solids loading have in the ink properties; the results are shown in Fig. 1. Fig. 1a shows the average measured viscosities of the inks over a range of shear rates for inks containing 3Y-55 or 3YS-120 NPs and with different solids loadings. All of the formulations demonstrate shear thinning behavior and also show a clear trend of increasing viscosity with increasing NP loading, especially at low shear rates. At a shear rate of 1 s−1 the viscosities of inks with 55 wt% and 70 wt% (Table 2) solids varies by an order of magnitude for both ceramic inks (Fig. 1b), which is consistent with previously reported ceramic NP systems [14,44]. When comparing the viscosity differences between the two NP inks at the same solids loading, the inks with 3Y-55 ceramic NPs are higher than that of the inks with 3YS-120 NPs (Fig. 1b and Table S2). This can be attributed to the increased specific surface area of the NPs in the 3Y-55 ink compared to the NPs in the 3YS-120 ink, and increased particle-particle interactions that inhibit ink flow [44]. The storage moduli of the inks were also measured as a function of shear stress (Fig. 1c). On this storage modulus curve, yield stress is reported as 90% of the storage modulus plateau at lower shear stresses and denoted by the circular markers in Fig. 1c. There is a marked increase in yield stress with increased solids loading shown in Fig. 1d. The smaller particle size in 3Y-55 showed an order of magnitude higher yield stress compared with the 3YS-120 inks at a given solids loading (Fig. 1d and Table S3), which can be attributed to increased particle-particle interactions and strong interference of the polymer
[40,41]. The sintering temperature was selected to maintain the nanoporosity, demonstrated by the BET data showing pore size distribution up to 1100 Å (Fig. S5).
3. Results and discussion 3.1. Rheology of ceramic inks Printing with the Mk IV PμSL system requires a UV curable ink that is spreadable and shear thinning to create uniform thin layers [42]. The property of spreadability is inversely related to the yield stress of a material [43]. Inks for DIW printing must be able to flow to extrude through a small nozzle, but stiff enough to self-support its shape once the ink is printed and any additional material printed on top of it. The basis of tuning the ceramic ink for various additive manufacturing techniques, both with different requirements, relies on two main characteristics: the nanoparticle size and the solids loading of the ceramic ink. Two different sizes of 3YZ NPs (measured average size of 55 ± 10 and 120 ± 25 nm) were investigated in order to: (1) demonstrate the effect of doubling the average NP diameter on viscosity and stiffness of the ceramic ink and (2) determine the impact of different NPs on light scattering in the PμSL system. Likewise, the varied solids loadings are important to optimize shear thinning inks for both DIW and PμSL and to tune the porosity and microstructure of the sintered porous ceramic [37]. A series of rheology experiments such as shear sweep and stress sweep of the
Fig. 1. (a) Log-log plot of the shear thinning viscosity profiles for inks with various NP solids loadings and (b) the viscosity measured at 1 s−1 for the 55, 65, and 70 wt% NP loaded inks. (c) Log-log plot of representative storage modulus curves as a function of shear stress for 55, 65, and 70 wt% NP loaded inks and (d) the yield strength estimated as 90% of the storage modulus plateau of the representative storage modulus vs. shear stress curve. 4
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of the part shown are 10 mm in height and 4 mm diameter. The wall thickness and layer height of the cylinder are both determined by the nozzle diameter, which is 1190 μm, and results in a wall thickness of 1050 ± 25 μm after sintering at 1090 °C. The printed tube shows that the ink can support the weight of repeated layering without significant shape distortion. The diameter of the tube remains constant around the circumference and height (10 mm) of the cylinder; it is clear by looking down the length of the tube that there is no sagging. Additionally, this simplistic structure can be printed quite easily and quickly by DIW (Fig. S7) in contrast to some other AM techniques such as inkjet printing or selective laser sintering [13, 51]. To calculate the shrinkage of the printed part, the dimensions of the cylinder were measured after each step of post-processing; for this ink (70 wt% 3Y-55), the total shrinkage from printing to a sintered part was determined to be ~15%. The SEM micrograph in Fig. 2b shows the internal nano-porosity achieved by partial sintering at 1090 °C. The optical image of the ‘wagon wheel’ structure shown in Fig. 2c demonstrates that the structural integrity of the ceramic ink is maintained after the curing and partial sintering post-processing steps. The structure is composed of alternating layers of concentric circles and angled lines from the center of the circle spanning the radius of the circle. The dimensions of the part are 20 mm in diameter and 5 mm in height. The diameters of the circles, the angle of the lines, and the layer height are all dependent on the nozzle diameter that is used; a 406 μm nozzle diameter was used for this print, which results in an outer diameter of
chains [44]. Since large storage modulus and yield strength are important for inks used in DIW printing, these results confirm that higher NP loading, such as 70 wt%, results in an ink with excellent stiffness while maintaining its shear thinning properties. Formulations of 3Y-55 or 3YS-120 can be used to create inks with a yield stress between 105 and 106 Pa, which is consistent with other DIW inks [4,6]. However, the 3Y-55 ink was chosen over the 3YS-120 ink because the smaller particle size produced less clogging in the nozzle as well as better shape retention after sintering. For the PμSL requirements of spreadability (low yield stress) and shear thinning behavior, the larger NPs (3YS-120) are superior, as is lower solids loading. PμSL inks made from both particle sizes were evaluated, with details discussed below. 3.2. Direct ink writing Based on the rheological analysis, 70 wt% (30 vol%) of 3Y-55 ceramic was used for the DIW studies. The ink was extruded through various nozzles with diameters ranging from 250 to 1190 μm (Fig. S6) using the optimized printing conditions described in the experimental section. Fig. 3 shows a selection of partially sintered parts printed by DIW, including tubes, a ‘wagon wheel’/ co-axial cylinder with rectilinear infill structure, and a simple cubic lattice. Each of these printed structures contains nano-porosity on the order of 100 nm. The partially sintered tube shown in Fig. 2a is printed by repeating layers of circles with a specified diameter and height; the dimensions
Fig. 2. DIW printed parts using 70 wt% (30 vol%) 3Y-55 ceramic ink with a 406 μm nozzle and SEM images of the corresponding internal porosity: (a, b) partially sintered tubes using a 1190 μm nozzle, (c, d) partially sintered ‘wagon wheel’, and (e, f) partially sintered simple cubic lattice. 5
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Fig. 3. Partially sintered porous zirconia gyroids printed from (a) 3Y-55 NPs and (b) 3YS-120 NPs using the Mk IV PμSL and (c) and (d) SEM of the internal microstructure of the corresponding sintered part.
therefore an increased print time, or inadequate depth of cure and poor adhesion [19–21]. Scattering can also cause excess cure beyond the bounds of the projected mask, and thus decrease the print resolution [20]. The higher the NP loading within the ink, the more the light will be scattered [28], but higher particle loading is needed to increase mechanical strength of the part and reduce shrinkage during sintering [37,47]. Creating an ink that prints a mechanically robust 3D part while maintaining good resolution requires balancing these competing factors. Cure depth is dependent on several parameters: mean particle size, solids loading, light intensity, exposure time, difference in refractive indices of the monomer solution and particle filler, and wavelength of the light source [19]. Cure depth and width are proportional to mean particle size and inversely proportional to solids loading [19,28]. Several studies have investigated the impact of changing the particle sizes, the refractive index of the particles, and the refractive index of the resin, as well as external parameters such as energy dose [20,21,47,49]. For the purpose of developing this ink, the primary parameters of interest were the mean particle size and solids loading. Initial ink development for optimization of the initiator and inhibitors was done using 3Y-55 NPs at loadings ranging from 50 to 60 wt%. The lower loading was selected because of the lower yield stress of the inks with lower solids loadings (Fig. 1c). While an ink of 3Y-55 is printable (Fig. S8 and S9 show parts printed during initial development), 3YS-120 was of interest due to the potential of the larger particles to decrease scattering and therefore increase cure depth and reduce excess cure width. Fig. 3 shows printed and sintered gyroids from a 55 wt% (18 vol%) 3Y-55 ink (Fig. 3a) and 3YS-120 ink (Fig. 3b) using otherwise identical conditions. Fig. 3 clearly shows that using larger NPs provided superior layer adhesion, which enabled printing taller parts and with better resolution (determined based on the wall
20 mm, inner diameter of 4 mm, angles of 6 degrees, and a layer height of 0.4 mm. The SEM image in Fig. 2d reveals the homogeneous nanoporosity of the resulting printed structure. The partially sintered simple cubic lattice (Fig. 2e) consists of orthogonal layers made of parallel cylindrical rods, with each layer rotated 90 degrees. The dimensions of the green body are 15 mm by 15 mm with approximately 450 ± 7 μm wide struts. After partially sintering the dimensions are 12.75 mm by 12.75 mm with 380 ± 5 μm wide struts. This structure demonstrates the achievable resolution and feature size of the ceramic ink and the ink's self-supporting ability across an unsupported span. The layer height is dependent on the nozzle diameter used for printing. The cylindrical rod diameter and the spacing between the parallel cylindrical rods are equal to the nozzle diameter (406 μm). The SEM micrograph of this structure also reveals consistent internal morphology with the other printed parts (Fig. 2f). 3.3. Projection microstereolithography Although appropriate ink rheology is an important factor for spreading thin layers in the PμSL system, it is not the only factor that must be considered. An additional challenge of printing ceramic inks using PμSL comes from the presence of NPs with a high refractive index. The presence of NPs causes the ink to be opaque and prevent it from being compatible with other light-based AM techniques, such as two-photon polymerization, which allows for smaller resolution than PμSL [45]. NP-filled inks can be printed using PμSL but special consideration must be taken to factor in the impact of scattering on the cure depth and resolution. Light scattering within the ink depends on the difference in refractive index between the 3YZ NPs (n = 2.19 at 633 nm) [46] and the UV-curable resin base (n = 1.467 at 589 nm) [47,48]. Scattering reduces light penetration depth, which can cause thinner layers and 6
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Declaration of Competing Interest
thicknesses of the sintered parts). Fig. S10 shows a side view of gyroids where the difference in wall thickness is apparent. For instance, when gyroids with 55 wt% 3Y-55 ink and a projected wall thickness of 750 μm were printed, a sintered part was obtained with a wall thickness closer to 1050 ± 100 μm (Fig. S10a), indicating the ceramic resin over cured. On the other hand, when the 55 wt% 3YS-120 ink and a projected wall thickness of 666 μm was printed, a sintered part with a wall thickness of 540 ± 60 μm was observed (Fig. S10b). Shrinkage of the part during sintering accounts for the difference between the projected wall thickness and measured wall thickness in the 3YS-120 part. Fig. 3c and d show SEM of the microstructure of both 3Y-55 and 3YS120 printed with the Mk IV PμSL. The morphology is consistent with that seen from the DIW parts in Fig. 2.
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4. Conclusions In this work, a versatile ceramic ink feedstock consisting of ceramic nanoparticles suspended in PEGDA resin was developed to produce porous ceramic structures using two different AM techniques, PμSL and DIW. The ink's viscosity and storage moduli were measured as a function of different ceramic NP particle size and loading. Reducing the mean NP diameter of the 3YZ powder from ~120 nm to ~55 nm and increasing the solids loading from 55 to 70 wt% increased ink viscosity by an order of magnitude (at a shear rate of 1 s−1) and the yield stress calculated from the storage modulus curves by two orders of magnitude (104 Pa to 106 Pa). The 3Y-120 NPs at a 55 wt% (18 vol%) loading was chosen for PμSL because of the low yield stress, imparting good spreading characteristics, as well as the decrease in scattering that improved adhesion of the layers and print resolution. The 3Y-55 NPs at 70 wt% (30 vol%) was chosen for DIW because of the high yield stress and shear thinning behavior that allow the ink to be extruded effectively while retaining its shape after extrusion. The resulting printed structures demonstrated the control of features on macro-, micro-, and nanoscales; DIW printing capabilities are showcased in the cylinder height (10 mm) and concentric circle line spacing in the wagon wheel structure (using a 406 μm nozzle); the smallest feature size printed was 250 μm. The PμSL printed gyroid has a print height of 12 mm and wall thickness of 540 μm. All printed structures show internal porosity on the order of 100 nm. This collection of printed nanoporous ceramic structures demonstrates the variety of engineered porosity that can be accomplished using these techniques. The combination of the internal nanoporosity and the engineered macroporosity derived from the structure makes these ceramic parts ideal for applications requiring fluid or gas flow in high temperature or corrosive conditions.
Declaration of Competing Interest None. Acknowledgements Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Funding was provided by the Lawrence Livermore National Laboratory Directed Research and Development (LDRD) [grant numbers 16-LW-013 and ISCP 19-PLS-007]. This work was supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences [grant number DE-SC0017124] and the U.S. Department of Energy's Office of Environment, Health, Safety and Security (AU-30) under the Nuclear Safety Research and Development Program (NSRD-20). IM release: LLNL-JRNL-811089. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. 7
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