3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles

Journal Pre-proof 3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles ¨ Moritz Weiß, Patrick Salzler, Norbert Willenbacher, Erin Koos

PII:

S0955-2219(20)30163-1

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2020.02.055

Reference:

JECS 13102

To appear in:

Journal of the European Ceramic Society

Received Date:

7 October 2019

Revised Date:

22 January 2020

Accepted Date:

26 February 2020

¨ Please cite this article as: Weiß M, Salzler P, Willenbacher N, Koos E, 3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles, Journal of the European Ceramic Society (2020), doi: https://doi.org/10.1016/j.jeurceramsoc.2020.02.055

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3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles Moritz Weiß1,2, Patrick Sälzler2, Norbert Willenbacher2, Erin Koos1,*

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KU Leuven, Department of Chemical Engineering, Celestijnenlaan 200f, 3001 Leuven, Belgium 2

Karlsruhe Institute of Technology, Institute for Mechanical Engineering and Mechanics, Gotthart-Franz-Strasse 3, 76131 Karlsruhe, Germany

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* E-mail: [email protected]

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Abstract: A new versatile route is reported to produce tailored porous sintering materials with high mechanical strength. A self-assembly mechanism deposits trace amounts of nanoparticles at contact areas between coarse ceramic particles prior to sintering, resulting in large and uniformly dense sintering bridges. This increases the porosity up to 75 % and simultaneously offers higher mechanical strength in comparison to similar materials even when weaker silica nanoparticles are introduced. This route is ideally suited for robocasting as well as conventional extrusion and, thus, can be used for rapid prototyping and mass production. We report the highest compressive strength-to-weight ratios for lightweight macroporous ceramics, covering an unrivaled density range while offering exceptional flexibility in tailorable microstructure. The strength is dominated by the silica bridges and identical strength is achieved for alumina and aluminosilicate structures. Finally, this route can be used to build hierarchical structures by point-welding particles while preserving inner surface area.

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Keywords: porous ceramics; mechanical strength; lightweight materials; capillary suspensions; direct ink writing

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Introduction

Highly porous ceramics are widely used in catalysis, filtration, heat exchange, energy storage, and other various applications with hot or chemically reactive media, as well as in tissue engineering and as lightweight construction materials [1–6]. The resulting ceramic material characteristics vary strongly depending on the microstructure, namely open/closed porosity, pore size, pore orientation and any hierarchical structure [6,7]. For instance, hierarchical structuring offers high active surface area and low density while maintaining a high relative compressive strength and features fast accessibility of small pores at low flow resistance [8,9].

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The development of lightweight materials has been flourishing in the last years where preceramic polymers, capillary suspensions, replica, direct foaming, emulsion, and sacrificial template methods have been used in combination with different 3D printing techniques to fabricate hierarchically structured lightweight ceramics [10–15]. The ceramic-polymeric composites developed by Bauer et al are the state-of-the-art lightweight materials reaching a very high compressive strength of 110 MPa at a density of 463 g/cm³ [16]. Minas et al. achieved the highest strength-to-weight ratios for hierarchically structured ceramics (alumina), providing fully open porous struts featuring pore sizes of 5 µm [10]. Current methods, however, are limited in terms of their microstructure, feature size or morphology, and are often difficult to upscale due to their high cost and/or time-consuming multi-step processing [6,17]. For instance, the method of Bauer et al. relies on multiphoton lithography [16], which is limited to photosensitive materials and small (< 1 mm) sized bodies. The method of Minas et al. is limited in terms of possible microstructure (pore size and porosity). Therefore, manufacturing hierarchical porous ceramics is still a challenging topic [6,17].

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Ceramic capillary suspension processing is a versatile, low-cost approach to produce fully open macroporous ceramics with tailorable porosities > 50 % and pore size ranging from 0.5 – 100 µm [18,19]. The simplicity of the method and the combination of an adjustable homogeneous microstructure in combination with porosities above 50 % and pore sizes < 10 µm, especially elucidate the method’s benefits compared to other common processing routes [6,7,18,19]. Capillary suspensions are three-phase, solid-liquid-liquid, systems where one of the immiscible liquids is in a minority (typically < 5 %). This liquid forms capillary bridges between the particles and creates a sample spanning network of flocculated particles [20]. This particle network is mostly retained during debinding or sintering and, therefore, can serve as stable precursor for sintered ceramics with high open-porosity [19]. Recently, Dittmann et al. introduced the smart capillary suspension concept for fabricating highly porous materials with excellent mechanical properties [21]. They tripled the mechanical strength of porous alumina by replacing 20 vol% (hereafter written as %) of the total solids with zirconia particles that were about ten times smaller than the alumina particles [21]. Some of the fine particles were deposited between coarse alumina particles, where the increase in mechanical strength was attributed to an increased pore roundness and the formation of a so-called zirconia toughened alumina composite material [21]. The concept relies on the addition of a “smart” additive, a fraction of which forms a composite material at the sintering neck. Thus, the transfer of the concept to other sintering materials is not straightforward. 2

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Here, we demonstrate a more generic and versatile approach, yielding a similar strengthening effect, by simply suspending a small fraction of nanoparticles in the secondary fluid. These nanoparticles are exclusively deposited in the contact areas of the large particles during capillary network formation. Due to the small size of the particles and the corresponding high sinterability, higher porosities – up to 75 % – are accessible while a high level of mechanical strength is preserved. This concept is independent of the chosen ceramic chemical composition and, thus, applicable to different materials. Indeed, the silica bridging material used here produces bodies that are five times stronger than obtained from the pure ceramic material despite the weakness of the silica in comparison to the alumina coarse particles. This generic approach was demonstrated for alumina and aluminosilicate ceramics as well as bodies using zeolite particles. Robocasting, also called direct ink writing (DIW), yields cellular structures with a specific strength close to that of balsa wood, surpassing the state-of-the-art specific strength of porous ceramics and, therefore, making these materials ideal for lightweight applications. Moreover, capillary suspensions are suitable for extrusion, i.e. large scale mass production, without changes in the resulting microstructure [15,22]. Therefore, the presented materials can be easily developed on a small scale, where the properties are adjusted to specific applications, and then upscaled and transferred to production without requiring changes to the material composition or processing conditions. Experimental Sections Materials and sample preparation

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The raw alumina (α-Al2O3) powders have a median particle size of 𝑑50,3 = 5.5 µm,

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𝑑50,3 = 0.5 µm and 𝑑50,3 = 0.2 µm (CT19FG, CT3000SG from Almatis GmbH and AKP50 from Sumitomo Chemical, respectively) according to manufacturers. All alumina particle types have a density of 𝜌 = 3.94 g cm-3 and exhibit an arbitrary, isometric shape. The flexural strength of matrix alumina given in the literature is 𝜎f,0 = 400 MPa [23]. Aluminosilicate FAUtype zeolite powder (NaMSXP-TR, Chemiewerk Bad Köstritz GmbH) with particle size 𝑑50,3 = 3.6 µm, measured by Fraunhofer diffraction, was chosen as lightweight raw material. We chose FAU-type zeolite because it forms an amorphous aluminosilicate phase at lower temperatures, and it is easily available. The powder density is 𝜌powder = 2.05 g cm-3 and

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densified during sintering to 𝜌(1200 ℃) = 2.48 g cm-3. The matrix material flexural strength was estimated as 𝜎f,0 = 218 MPa from its chemical composition using the linear rule of mixtures (Voigt model [24])and literature values for the component flexural strengths [23,25]. The Voigt model provides an upper limit for estimated flexural strength, resulting in the lower limit of relative compressive strength for aluminosilicate-based samples. This smaller relative strength was chosen to avoid any overestimation of the true value. Low viscous paraffin oil (Carl Roth GmbH & Co. KGaA, 𝜌 = 0.85 g cm-3, 𝜂(20 ℃) = 0.035 Pa s) was used as the bulk phase for the monolithic bodies, and a mixture of 49 % high viscous paraffin oil (Carl Roth GmbH & Co. KGaA, 𝜌 = 0.87 g cm-3), 50 % mineral spirits (0.752 g cm-3, Sigma-Aldrich), and 1 % stearin wax (TrendLight Creativ GmbH, 0.92 g cm-3) were used as the bulk phase for 3D printing. A non-ionic wetting agent (Tween 20, Carl Roth GmbH & Co. KGaA) was added 3

∗ to the CT3000SG based samples. We used different volume fractions ( 𝜙np = 0– 19% ) colloidal amorphous silica suspension, composed of a commercial silica suspension (50 wt%, Ludox TM-50, Merck Kg, 𝑑50,3 = 15 nm) diluted with an aqueous sucrose solution (distilled water, MilliQ) or an alumina nanosuspension (Alumina Polishing Suspension, Deagglomerated, 0.05 micron, Allied High Tech Products, 𝑑50,3 = 50 nm) as secondary phase. All mixtures contained 30 vol% D(+)sucrose (Carl Roth GmbH & Co. KGaA). The flexural strength of matrix silica from the literature is approximately 50 MPa [26]. The three phase contact angles of all systems are < 90°, placing the systems in the pendular state [15,18,20]. For a summary of the compositions used, see the Error! Reference source not found..

Capillary suspensions were prepared by first dispersing the coarse particles in the bulk fluid (volume fraction 𝜙cp ) using a Hauschild SpeedMixer at 2000 rpm for 5 min. A secondary fluid ∗ containing 𝜙np nanoparticles was then added and mixed at 2000 rpm for 5 min, followed by a

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∗ homogenizing step. Thus, the total volume fraction of nanoparticles was 𝜙np = 𝜙sec ∙ 𝜙np . We performed a three-roll milling step (EXAKT E80, EXAKT Advanced Technologies GmbH) with a minimum gap width of 5 µm for the smaller alumina particles (CT3000SG and AKP50). Whereas, aluminosilicate and CT19FG based suspensions were homogenized by a ball milling process at 2000 rpm for 10 min.

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Samples for bulk mechanical characterization were molded into rectangular forms with the dimensions 50 mm × 12 mm × 5 mm and placed on an absorbent pad. Honeycomb structures were 3D-printed via direct ink writing (Developer’s Kit 3D Printer, Voxel8). The capillary suspensions were extruded at room temperature with constant volumetric flow rates through tapered nozzles (inner diameters of 410 µm, 250 µm, 200 µm, or 150µm, Nordson EFD) controlled by the applied pressure (30 – 90 kPa). Alumina based capillary suspensions were extruded on a PEG (PEG 1500, Sigma-Aldrich) coated substrate. After a heat treatment up to 95 °C and wetting of the plate and the printed body with water, as described in detail by Maurath et al. [15], the extruded bodies were placed onto a porous ceramic sintering plate. We used sintering plates infiltrated with highly viscous paraffin oil as a substrate for aluminosilicatebased inks and did not perform a heat treatment step prior to debinding. G-Codes of the printed cellular structures are presented in the Error! Reference source not found..

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All samples were thermally debinded in a furnance (Nabertherm LVT 05/11) using a protocol that heated the samples to 200 °C (30 min), 500 °C (70 min) and then 800 °C (15 min) to completely remove the remaining bulk and secondary liquids. We applied sintering temperatures between 1200 and 1650 °C for alumina samples and 1000 °C (15 min) for aluminosilicate samples in a sintering oven (Nabertherm LHT 04/17). Heating rates remained constant at 2 °C min-1. Details of the sintering conditions are shown in the Error! Reference source not found. for each of the samples. 2.2

Characterization

The strut porosity 𝜀s was determined using Archimedes’ principle according to DIN EN 933-1. The density of the printed cellular structures were determined from the structures’ volume and weight. The true porosity 𝜀 ∗ was calculated from the structure’s density 4

and ceramic material density. The filament widths of the cellular ceramics were analyzed using a digital microscope (KEYENCE VHX-950F). Scanning-electron-microscopy (SEM) micrographs (S-4500; Hitachi High Technologies Europe GmbH) of sintered part crosssections were used to analyze the microstructure. Pore sizes were calculated from the SEM images via the Line-Intercept-Method [27]. Combined TEM-EDX analysis was performed to investigate nanoparticle distribution in the capillary nanosuspensions. These investigations were carried out using an FEI Titan 80-300 cubed transmission electron microscope, equipped with EDXS system (EDAX). A salt-and-pepper median filter was applied using Matlab to minimize noise in the TEM-EDX images. TEM samples were prepared by standard preparation method (grinding, polishing, Ar-ion etching) after infiltrating the porous structures with resin. The aluminosilicate density after sintering at 1200 °C for 2 h was measured using a multivolume gas pycnometer (Model MP 1305 with helium) at 20 °C.

Results and discussion

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Compressive strength tests were performed using a TA.XT.plus (Stable Micro Systems). The bulk samples were ground to flat rectangular shapes (according to DIN EN ISO 604) and a constant displacement rate of 0.25 mm s-1 was used to ensure reproducible results. Cellular honeycomb structures were loaded at a displacement-controlled rate of 0.005 mm s-1 using a 30 kN load cell (Instron 5985). Samples were ground to a fully flat surface prior to testing. The relative compressive strength was calculated from the measured compressive strength divided by the matrix material’s flexural strength.

Processing route

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The scheme of this advanced processing route to form capillary nanosuspensions, using sinterable nanoparticle suspensions instead of a pure liquid as a secondary phase, is shown in Figure 1a. The capillary bridges contain nanoparticles, which are still present between the coarse microparticles after debinding. Therefore, we exploit the secondary phase to deposit nanoparticles directly and exclusively at the contact regions of the coarse particles. These nanoparticles can function as sintering aids, either by having a chemical composition with lower sintering temperature or by using the size effect where smaller particles sinter at a lower temperatures than coarser ones due to their increased free surface [23,28]. The theoretical maximum nanoparticle volume fraction for capillary nanosuspensions is determined by the desired secondary phase fraction and the maximum packing fraction of the nanoparticles in the bridges. Typically, the fraction of added nanoparticles in the sintered body is 𝜙np ⁄(𝜙cp + 𝜙np ) < 5 %. The ceramic, shown in Figure 1b-d, was produced using capillary nanosuspensions consisting of coarse alumina particles (𝑑50,3 = 5.5 µm, 𝜙cp = 15 %) and a suspension of silica ∗ nanoparticles (𝑑50,3 = 15 nm, particle volume fraction 𝜙np = 18.5 %) as the secondary phase. The elements, Al and Si are presented in Figure 1b and Figure 1c, respectively, and an overlay of both elements is shown Figure 1d. The alumina signal (Figure 1b) shows a gap between the particles indicating that there is little if any connection between microparticles. This lack of alumina between particles is consistent with the used low sintering temperature of 1350 °C,

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where sintering between the 5.5 µm diameter alumina particles is negligible [23,29]. The incorporated silica nanoparticles are only present in a few locations (Figure 1c) and as shown in the composite image (Figure 1d), the silica acts as connecting material for the coarse alumina particles. The silica nanosuspension wets the alumina microparticles, depositing the silica in the gaps between the coarse alumina particles. The bridges are preserved by the nanoparticles during debinding and then as solid silica “bridges” in the sintered ceramic. The broad silica region between plate-shaped alumina particles is in accordance with the findings of Maurath et al. and Sun et al. showing that plate-shaped particles tend to orient in a face-face configuration incorporating large amounts of secondary phase in capillary suspensions [30,31]. Further TEMEDXS images are presented in the Supporting Information.

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Figure 1: (a) Schematic processing route to produce porous ceramics from capillary suspensions with incorporated nanoparticles as precursor. (b-d) TEM-EDXS mappings of a sintered capillary suspension using paraffin oil as bulk phase, ϕcp = 15 % alumina particles ( d50,3 = 5.5 µm) and ϕsec = 3 % silica nanosuspension

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(ϕ∗np = 18.5 % silica, d50,3 = 15 nm) as secondary phase. Sintered at 1350 °C for 2 h. Dark areas represent vacuum regions, Si areas are shown in yellow, and Al areas in blue. (b) showing only Si fractions, (c) presenting only the Al fraction and (d) showing both mapped elements.

3.2

Bulk ceramic samples

As is typical for capillary suspension derived ceramics, the bodies have a fully open pore structure [15,18,19,25]. We performed porosity and compressive strength measurements to characterize the resulting ceramic bodies. Figure 2a shows compressive strength versus porosity for capillary suspension derived ceramics with and without incorporated nanoparticles at two sintering temperatures. At the lower sintering temperature, the incorporated silica nanoparticles lead to a significant increase in porosity (from 65 % to 75 %) and strength (from 0.1 MPa to 2 MPa), simultaneously. Incorporating nanoparticles (silica as well as alumina) leads to higher 6

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porosities (an increase from 66 % up to 69 %) while retaining the same compressive strength (7 MPa) at a sintering temperature of 1650 °C. Therefore, this route offers an especially high potential for processing lightweight materials. For both temperatures, samples with incorporated silica nanoparticles show more significant impact on the resulting properties, which can be explained by the threefold smaller silica nanoparticles compared to the alumina nanoparticles (d50,3 = 14 nm, d50,3 = 50 nm) and the lower sintering temperature of silica. As shown in Figure 2b, the compressive strength is higher for the capillary nanosuspensions over the equivalent standard formulation at each porosity. The upper limit in porosity was also increased from 69 % to 75 %. The effect is more pronounced at the higher porosities where the incorporation of nanoparticles increases both the porosity and strength. For denser bodies, such as from small alumina particles (CT3000SG), the incorporation of nanoparticles increases the strength (from 26 MPa to 59 MPa) without changing porosity (51 %). Formation of a homogeneous capillary suspension with the used alumina nanosuspension was difficult since the system is more prone to agglomeration, limiting the achieved properties.

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Figure 2: (a) Compressive strength vs. porosity for bulk alumina ceramics derived from capillary suspensions with incorporated silica nanoparticles (red squares, 𝑑50,3 = 15 nm), incorporated alumina nanoparticles (red triangle, 𝑑50,3 = 50 nm) and without incorporated nanoparticles (black squares) at two different temperatures (open/closed squares). Sintering temperatures were constant for 2 h. Coarse particles (CT19FG, 𝑑50,3 = 5.5 µm) volume fraction was constant at 15 % for all samples. Secondary phases fraction ranges from 2-3 %. (b) Compressive strength versus porosity for capillary suspension derived alumina bodies with and without incorporated nanoparticles. Hollow black circles represent samples without nanoparticles [18] and samples with incorporated nanoparticles are presented as red squares. Also included are filled black diamonds denoting samples from alumina-alumina mixtures and half-filled blue diamonds alumina-zirconia mixtures using the smart capillary suspensions concept [21]. (c) Schematic sintering neck formation with incorporated nanoparticles (left) and without nanoparticles (right).

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This strength and porosity are superior to the alumina-reinforced alumina ceramic (Al2O3/Al2O3) produced using the smart capillary suspension method of Dittmann et al. [21] and is comparable to the zirconia-reinforced ceramic (Al2O3/ZrO2) despite the weaker silica bridging material used here. This significant increase in compressive strength by the capillary nanosuspension approach using weaker silica is attributed to the formation of larger and more uniform sintering necks between the coarse particles, as drawn schematically in Figure 2c. In the conventional method without incorporated nanoparticles, sintering necks are formed solely out of the coarse particles’ material and would be greatly affected by any surface roughness. In case of a capillary nanosuspension, the nanoparticles accumulate between two coarse particles during debinding and form the sintering necks between coarse particles during sintering (see TEM-images in Figure 1). After debinding, the coarse particles’ contact areas are filled with densely packed nanoparticles, increasing the local solid content to form a larger and more uniform contact area compared to the standard capillary suspension route. Therefore, the nanoparticles enhance the sintering neck formation, making them larger and favoring densification.

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This enhanced neck densification is especially pronounced for arbitrary shaped coarse particles as schematically drawn in Figure 2c. Here, the increase in strength is attributed to the formation of smooth silica sintering necks among particles and the increase in porosity is caused by the accompanying increased stability and hence reduced shrinkage during debinding, particularly at the low solids loading of 15 % (Figure 2a). Since we could not quantify an increased bridge area by our microstructural investigations, this increase must be rather small, but large enough to cause the observed difference in the resulting mechanical properties. Since the nanoparticles are deposited between every coarse particle contact area, a small increase in the area can have a substantial impact on the overall body’s mechanical strength. [19,21,25,32,33]. Raw data and conditions are presented in the Error! Reference source not found. Error! Reference source not found.. Interestingly, the nanoparticle fraction was varied between 0.37 - 3.57 % of the total solid phase without observing any differences in structural or mechanical properties (Error! Reference source not found.). This indicates that we only need a very small amount of nanoparticles in the secondary phase, possibly just enough to form a smooth interface between the coarse particles. Therefore, the capillary nanosuspension method is robust to variations in the nanoparticle fraction but can still facilitate tailoring mechanical and structural properties of ceramics derived from capillary suspensions. Using this approach broadens the achievable porosity range and significantly increases the mechanical strength of the capillary suspension based ceramic bodies. Finally, a desired porosity and strength may be achieved at lower sintering temperature thus saving energy and time. In the case of silica nanoparticles, we cannot rule out the formation of alumina-rich mullites [34–36]. However, the formation of alumina-rich mullites is unlikely to significantly impact the results since we observe the highest impact at a low sintering temperature (1350 °C) where a similar system did not show any mullite formation [37]. Furthermore, the amount of added silica, which should impact the mullite composition and/or volume has no impact on the strength or porosity. Finally, as shown in Figure 2, this method is not limited to silica (or other 8

components with lower melting temperatures) in the secondary phase. Instead, the nanoparticles need to be small enough be incorporated in the capillary bridges and are part of a homogeneous nanosuspension to assure incorporation of nanoparticles in every capillary bridge. Capillary suspensions, as capillary bridges, can be formed with preferentially and non-preferentially wetting secondary phases [20] and has a broad variety of applications in material creation. The direct influence of the contact angle of molten nanoparticles as well as their sintering activity on the material strength and porosity is currently unknown and should be investigated in future work. 3.3

3D-printed samples

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Based on the outstanding mechanical properties of porous ceramics derived from capillary suspensions with incorporated nanoparticles, we investigated their suitability as lightweight construction materials. Honeycomb structures were produced using a 3D-printing process because of the high out-of-plane compressive strength of this low density cellular structure [9,10,40–43,11–16,38,39]. Capillary suspensions are particularly suitable for extrusion and direct ink writing due to their distinct rheological properties comprising a high yield stress and strong shear thinning. Maurath et al. successfully printed cellular ceramics from standard capillary suspensions with high specific strength at low densities [15]. Here, we produced honeycomb structures with varying strut width, cell-area ratio, as well as total areas to modify the resulting total density. Images of 3D-printed alumina bodies with different strut widths and cell-area ratios are shown in Figure 3a and Figure 3b. Structure shrinkage was observed to be 38 vol% and 67 vol% for alumina based and aluminosilicate based pastes, respectively, and almost homogeneous for all samples. The high shrinkage for aluminosilicate-based samples is related to the density change during sintering. Even though shrinkage is, due to adhesion, lower on the substrate than on the free surface [22], the samples did not crack during debinding and sintering. Further illustrating the versatility of this method, a successfully extruded capillary suspension based hollow tube is shown in Figure 3c. The detailed extrusion process is described elsewhere [22].

Figure 3: Images of sintered cellular alumina specimens with different dimensions from capillary suspensions with incorporated nanoparticles. (a) CT3000SG based large honeycomb geometry and (b) top view of an AKP50 based small honeycomb with thick strut width. All geometries were printed with all inks. (c) Extruded hollow-tube ceramic derived from capillary suspension (further information can be obtained elsewhere [22]). (d) Crosscut SEM image (SE) of an AKP-50 based sintered honeycomb strut (sintering condition: 𝑇𝑠 = 1200 ℃, 2h).

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SEM images (Figure 3d, Error! Reference source not found.) as well as laser scanning microscopic (LSM) images (Error! Reference source not found.) were made to evaluate printing quality. LSM images reveal strut widths of the sintered parts to be 327 µm and 527 µm, for the 200 µm and 410 µm nozzle diameters, respectively. The increased strut width beyond the nozzle diameter may be related to material overflow during extrusion [15]. Lower volumetric flow rates or higher translation velocities led to ink sputtering, so could not be applied for reliable manufacturing but a higher paste yield stress should lead to narrower struts and enable spanning larger open areas without pattern deformation. A characteristic SEM image (Figure 3d) of a printed AKP-50 based sample shows a homogeneous microstructure without agglomeration or air entrainment. The printed struts offer a fully open porosity of 55 % with an average pore size 𝑑pore = 6.0 µm in the sample, identical to the corresponding bulk samples (see Error! Reference source not found.). Therefore, extrusion during direct ink writing of capillary nanosuspensions does not affect the resulting microstructure. The density of the printed alumina-based parts ranges from 192 g cm-³ to 844 g cm-³ while the aluminosilicatebased parts provide densities of 352 - 623 g cm-³. Relative density ranges from 0.05 up to 0.25 and can be further adjusted easily for all used materials by tuning the strut porosity and/or feature size. A summary of microstructural properties of these sintered ceramic parts is presented in the Error! Reference source not found. (Error! Reference source not found.). The combination of direct ink writing and capillary suspension processing enables the tailoring of resulting pore size in the mm-range through printing and in the µm-range through the capillary suspension paste structure, while simultaneously maintaining a fully open porous hierarchically structured body. This hierarchy introduces load-bearing structural units at different length scales leading to high mechanical efficiency [9,44,45].

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The compressive strength of the printed honeycomb structures is given in Figure 4a, plotted against their density. For comparison, we added balsa wood [40], 3D-printed ceramics from normal capillary suspensions [15], lightweight porous ceramics from Minas et. al [10], as well as the data of Bauer et al. [16] and Eckel et al. [14] The structures from Bauer et al. capitalize on size-affected material properties, where polymeric scaffolds were fabricated using photon lithography direct laser writing (3D-DLW) offering layer thicknesses of 290 - 580 nm and were subsequently coated with 10 – 200 nm thick alumina layers [16]. Eckel et al. used stereolithographic 3D printing to build porous ceramic structures, e.g. honeycombs, consisting of dense struts derived from preceramic polymers [14]. The resulting mechanical stability is attributed to printed honeycomb structure [9,46] and nearly flawless dense struts [14]. However, both methods have their technical and/or economical drawbacks: 3D-DLW is limited to very small sample volumes (~1 mm³); only a few applicable materials; and the parts cannot be exposed to high temperatures or harsh environments. Stereolithography is restrained to liquid, curable polymers, thus only preceramic polymers lead to ceramic structures [16,47–49]. Furthermore, both production methods are expensive and, hence, only relevant for small, specialized applications [48,49]. Capillary suspension derived ceramics, on the other hand, are inexpensive, temperature stable, chemically inert, the manufacturing is fast, and this process can be readily upscaled [19]. Minas et al. combine DIW with an alumina-foam template process to achieve lightweight cellular structured ceramics [10]. While their results are the top-of-theedge lightweight hierarchical structured ceramics, the presented method is highly limited in 10

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achievable strut microstructures. Furthermore, upscaling, e.g. by an extrusion process, is not straight forward. These two drawbacks are not present with the capillary suspension route.

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Figure 4: Ashby charts showing the 3D printed cellular honeycomb structures compared with data found in literature. (a) Compressive strength versus density comparing ceramics derived from capillary suspensions with incorporated nanoparticles to classical ceramics from capillary suspension [15], engineered materials from Bauer et al. [16], Eckel et al. [14] and Minas et al. [10] as well as balsa wood [40]. (b) Relative compressive strength 𝜎c ⁄𝜎f,0 as a function of relative density for our structures and other 3D-printed cellular ceramics and glasses offering open porosities [10–13,15].

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As previously shown for bulk materials (Figure 2a), the incorporation of nanoparticles drastically increases the resulting compressive strength and porosity (Figure 4a). This bridge reinforcing mechanism is evident for both the alumina and aluminosilicate materials, but is more pronounced for the aluminosilicate samples, which increase from 0.4-1.4 MPa to 5.36.6 MPa at the same density of 360±20 kg m-³. The printed honeycomb alumina structures showed a reduced density through the introduction of nanoparticles, for example from 490 kg m-³ to 192 kg m-³ while the compressive strength increased from 3.3 MPa to 4.0 MPa. This sample was our lightest fabricated sample and close to the mechanical stability-density area covered by balsa wood. Our strongest sample, made from aluminosilicate, offers a compressive strength of 60 MPa at a density of 571 kg m-³, which is comparable to those of Bauer et al. whose engineered cellular materials had a similar density (519 kg m-³) and a compressive strength of 166 MPa [16]. Our results surpass the structures of Minas et al. in terms of mechanical strength over the whole density range, which is even broadened in our study. Furthermore, Minas’s samples reveal a drastic drop in mechanical strength for lower densities [10]. This behavior is likely attributed to the thinning interface between the interconnected bubbles (pores), which translates into a structure that dramatically weakens at high air entrainment (low density). The low-density samples made from capillary suspensions, on the other hand, follow the same trend over the whole density range investigated. The compressive strength of ceramics manufactured with incorporated nanoparticles strongly depends on density but is nearly identical for both ceramic materials investigated despite their vastly different flexural strength (𝜎f,0 = 400 MPa for alumina and only 𝜎f,0 = 11

218 MPa for aluminosilicate). The strength of the porous parts is mainly controlled by solid bridges between coarse particles created by sintered nanoparticles. This is further emphasized by our strongest sample, which was achieved using the weaker aluminosilicate particles. Remarkably, this structural reinforcement is achieved by adding nanoparticles which possess a much lower mechanical strength (𝜎f,0 = 50 MPa for silica) than the ceramic materials. This strength is due to the increase in strength of the struts caused by the reinforced sintering necks, as sketched in Figure 2c. The fact that this method can be used for different materials without affecting the strength illustrates its flexibility and sets it apart from many common processing routes of porous ceramics [6,7].

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Figure 4b shows compressive strength normalized to the tensile strength of the matrix material versus their relative densities, and compares our results to other 3D-printed porous ceramics [10–13,15]. Our printed structures’ relative density varies from 0.05 up to 0.25 and provides relative compressive strength ranging from 0.01 to 0.28. Thereby, we extend the density range published for porous ceramics towards lower densities, while offering higher specific compressive strengths [10–13,15]. Only the results of Minas et al [10] can compete in terms of their compressive strength-to-weight ratio in a portion of this density range, but, as mentioned earlier, their samples lose their compressive strength at lower densities. Porous ceramics from capillary nanosuspensions can be fabricated with a tailored microstructure (porosity and pore size) in a broad range and can be made from a large variety of different materials [10,15,18,21,25]. Furthermore, we achieved compressive strength of 60 MPa at a relative density of 0.3 for an aluminosilicate structure, highlighting the unique mechanical properties accessible with our material design since compressive strength values for cellular ceramics typically do not exceed 30 MPa at this relative density [3].

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Since there is no dependence of strength on the two tested types of used coarse material, we can assume that the bodies are only sintered by the nanoparticles and the coarse particles likely do not undergo a significant change. These sintering necks created from the nanoparticles define the mechanical strength of the manufactured parts. Scattering within our data is most likely the result of geometric imperfection caused by printing or grinding, and suggests that we did not reach the full potential of the material [9,38,39,50,51].

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This method also offers the possibility to point weld coarse particles using the capillary nanosuspension concept to integrate a third level of pore hierarchy. We investigated this possibility using MFI-type zeolite powder (NH4CZP 800, Clariant AG, 𝑑50,3 = 4.75 µm, 𝑑pore = 0.55 nm) as coarse particles in the capillary nanosuspension and sintering them at 1000 °C for 24 h after molding and debinding to obtain mechanical stability. The specific surface area (BET, N2-adsorption) decreased slightly, from 388 cm² g-1 to 343 cm² g-1 for the initial powder and the temperature treated capillary nanosuspension, respectively. This small reduction in specific surface area is a consequence of the lower specific surface area of the incorporated silica, and/or a decrease in the zeolite porosity due to the heat treatment. Using zeolite, strong hierarchically organized, fully open pore structures with three characteristic length scales (5.5 Å pores inside the zeolite particles, 0.5 – 50 m inside the struts, 0.1 - 1 mm printed pores) can be manufactured, thus opening additional fields, especially for high temperature catalytic applications. 12

4

Conclusion

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In summary, we have presented an advanced processing route for porous ceramics derived from capillary suspensions with incorporated nanoparticles and highlighted their unique properties as a lightweight engineering material. Using a self-assembly process driven by capillary forces we deposit nanoparticles exclusively at the contact regions between coarse particles, debinding and sintering turns this liquid capillary bridges into solid contacts. We significantly increase the compressive strength of the resulting porous ceramic through this approach and push the limit for the maximum accessible porosity to 75 %. For porosities of 51 %, the compressive strength is more than doubled from 26 MPa to 59 MPa compared to the ceramic without nanoparticles. This is even more pronounced for higher porosities, i.e. we observe a five-fold increase in compressive strength for porosities around 70 %. The fraction of nanoparticles in the final sintered part is less than 5 % and reinforcement is achieved despite the lower mechanical strength of the added nanoparticles compared to the surrounding ceramic material. This increased mechanical strength increases the degrees of freedom in lightweight construction. Higher mechanical strength is available at a chosen porosity, or higher porosity is accessible without sacrificing mechanical integrity. The combination with direct ink writing enables fully open porous cellular structures to be printed, offering exceptional strength-toweight ratio in a wide range of densities. The minimum obtained printed structure’s density was 192 kg m-³ with a compressive strength of 4.0 MPa, close to the values reported for balsa wood or state-of-the-art photolithographically fabricated small-scale structures. Notably, balsa wood and the photolytic composite structures cannot resist high temperatures or harsh chemical environments, which set the ceramics presented here apart. The resulting compressive strength of the printed honeycomb structures only depends on their density and does not vary with the coarse particle material, demonstrating that the solid bridges formed by the incorporated nanoparticles primarily control the ceramic’s mechanical stability. We achieved compressive strength of 60 MPa at a relative density of 0.3, doubling the compressive strength of common cellular ceramics at this relative density. Furthermore, no decrease in strength is observed at low relative densities. We present the best performing macroporous ceramic lightweight materials, combined with an unrivaled broad density range as well as tailorable microstructure. Additionally, we introduced a third level of hierarchy by using microporous zeolites as coarse particles and utilizing the nanoparticles as bonding agents. The obtained monolith offers almost identical specific surface area as the starting powder.

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Capillary suspension processing, including these capillary nanosuspension pastes, is a fast, low cost and, due to the absence of conventional volume displacer, an environmentally-friendly processing route for porous sintering materials [19]. The fully open, tunable porous structure and applicability to a wide range of sinter materials provides manifold opportunities for targeted design of lightweight materials to be used at high temperature or in chemically harsh environment, as well as construction and/or thermal insulation materials. Introducing a hierarchical structure offers additional functionality and possible applications in biomedical engineering, filtration, heat exchange, gas adsorption, energy storage or catalytic processes. Furthermore, these pastes are suitable not only for 3D printing but also for extrusion processing. Thus, rapid prototyping as well as mass production is enabled without change in the resulting 13

material properties. In summary, the method demonstrated here can be considered unique: cutting-edge mechanical properties without sacrificing versatility and tunability.

Declarations of interest: none

Acknowledgements

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The authors would like to thank Almatis GmbH and Chemiewerke Bad Köstritz GmbH for the donation of the alumina, the aluminosilicate particles and the smooth collaboration. Felix Ruß and Vanessa Reiter are thanked for contributing to the bulk mechanical experiments. Finally, we acknowledge financial support from the Research Foundation Flanders (FWO) Odysseus Program (grant agreement no. G0H9518N) and support from the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013, ERC grant agreement no. 335380).

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