3D printing of porous low-temperature in-situ mullite ceramic using waste rice husk ash-derived silica

The in-situ mullite (3Al₂O₃·2SiO₂) foams are fabricated by 3D printing (direct ink writing (DIW)) technique and utilize waste rice husk ash (RHA). The Al₂O₃-SiO₂ inks are prepared using an aqueous binder with α-alumina and two different silica sources, i.e., RHA extracted biogenic nano-silica (NS) and commercial silica (CS). The ink rheological features are first designed in terms of solid-to-liquid ratio and dispersant, and found that a higher amount of dispersant is needed for functionalization of NS-containing ink than CS (micro-sized) consisting of ink. Secondly, the DIW log-pile structures are fired at different temperatures (1200?1500 °C), and NS containing samples exhibited remarkable enhanced properties at a lower firing temperature than CS. At 1400 °C, alumina and RHA nano-silica entirely transformed into mullite and retained ～75 % porosity, ～8 MPa cold compressive strength, and thermal conductivity ～0.173 W/m·k that designate a simple and effective way to fabricate of mullite foamy structure.     

3D printing of open-porous cellular ceramics with high specific strength

We present a novel processing route for manufacturing highly open porous, hierarchically structured ceramics via direct ink writing. We manufactured cellular samples with overall porosities up to 88% that exhibit fully open-porous struts with porosities between 45 and 60% and pore sizes x_50,3 < 6 μm using capillary suspension based inks. An innovative processing strategy enabled manufacturing crack-free, undeformed cellular ceramic samples.We printed hexagonal honeycomb structures that showed exceptionally high specific strength under compression load and significantly enlarged the strength-density range that was covered by sintered capillary suspensions, so far. Without loss of mechanical strength the density of ceramic parts was decreased by about a factor of 2–3. Strength of in-plane and out-of-plane loaded hexagonal honeycomb structures varies according to common scaling laws for cellular structures. The honeycombs are mechanically more efficient than bulk specimens from capillary suspensions, since they show a distinctly lower sensitivity of strength on density.     

3D printing of SiC ceramic: Direct ink writing with a solution of preceramic polymers

3D structured SiC ceramics with varying feature sizes (100–400?μm) were achieved by direct ink writing of polycarbosilane (PCS)/n-hexane solution. The rheological properties of the PCS solution and printing parameters were tailored for optimum writing behaviour. The integrity and clear surface of the filaments indicated the printing ability of forming the self-supporting features along with the rapid evaporation of solvent. As-printed 3D structured PCS was processed by oxidative crosslinking and pyrolysis and converted to SiC ceramic. Although strong shrinkage occurred during the pyrolysis, SiC ceramic maintained the original 3D structure. Both proper viscoelasticity of printable solutions and the homogeneous shrinkage in the pyrolysis determine the integrity and feature characteristic of 3D structured SiC using direct ink writing preceramic polymer.     

Effect mechanism of multiple obstacles on non-Newtonian flow in ceramics 3D printing (linear elements)

Direct ink writing (DIW) provides a new way to mould ceramic parts. When a single screw extruder is used to extrude SiC slurry, the deposits caused by low viscosity and the agglomerations resulting from the nonuniform mixing, form the obstacles in the channel, which affect the normal flow of the slurry, theoretical outlet velocity, and interaction with other printing parameters. Therefore, it is necessary to study the mechanism responsible for the effects of the obstacles on the flow. The obstacles are always irregular, which makes it difficult to directly analyse them. Irregular geometries are always composed of linear and/or arcuate elements; therefore, the obstacles can be simplified into regular geometries. In the present work, linear elements are analysed first. Then, an improved MRT LBM (multiple relaxation time lattice Boltzmann method) with a pseudo external force is proposed for the flow analysis. The improved MRT LBM is combined with rheological test data to investigate cases with two obstacles, and the results are applied to reveal the general mechanism in cases with multiple obstacles. The results show that the angles, sizes, and positions of the obstacles are three important factors influencing the flow. To obtain a stable and controllable slurry flow, it is recommended that the first angle θ₁ be an acute angle. In addition, the number of obstacles should be minimized, and the position of the last obstacle should be far away from the outlet.     

Effect mechanism of multiple obstacles on non-Newtonian flow in ceramic 3D printing (arcuate elements)

Direct ink writing (DIW), as an additive manufacturing method, can effectively mould ceramic parts. The single screw extruder is used here to extrude viscous SiC slurry. The deposits caused by the low viscosity and the agglomerations resulting from the nonuniform mixing, form the obstacles in the channel, which affect the normal theoretical flow of the slurry, and interaction with other printing parameters. Therefore, it is necessary to explore the effect mechanism of the obstacles on the flow. The obstacles are always irregular, which makes it difficult to directly analyse them. The irregular geometries are always composed of linear and/or arcuate elements; therefore, the obstacles can be simplified into regular geometries. In the previous work, linear elements have been analysed first. As the continuous work, arcuate elements are investigated in the current research. To conduct the required simulations, an improved MRT LBM (multiple relaxation time lattice Boltzmann method) with a pseudo external force is proposed. The above numerical method is combined with the rheological model to analyse cases with two obstacles, and the obtained results are used to reveal the general mechanism in cases with multiple obstacles. The results show that the central angles, radii, and positions of the obstacles are important factors affecting the flow. To obtain a stable and controllable slurry flow, it is recommended that the central angle and the radius should be small enough. The number of obstacles should be minimized, and the position of the last obstacle is expected to be far away from the outlet to avoid the negative velocity. In addition, the adjacent obstacles should maintain a certain distance to ensure the full development of the vortex and avoid affecting the following obstacles or back vortices.     

3D printing Si3N4-bonded SiC refractories fabricated using colloidal films-containing slurries based on non-spherical SiC and Si powders

Stable slurries for Si₃N₄-bonded SiC refractories for direct ink writing (DIW) were successfully prepared from a mixture of non-spherical silicon carbide (SiC) and silicon (Si) powders with an average particle size of D₅₀ = 41.98 μm. The rheological properties and printability of slurries prepared using polyvinyl alcohol (PVA; 4–16 wt %) or hydroxypropyl methylcellulose (HPMC, 0.5–2 wt.%) were investigated with the effect of sintering temperature on the mechanical performance, phase, and microstructure of Si₃N₄-bonded SiC refractory products. The results indicated that slurries prepared with the HPMC solution showed better printability than those prepared with the PVA solution because colloidal films formed by HPMC in slurries play a role in encasing particles, preventing solid−liquid separation and contributing to plasticity and lubrication, which guarantees the smooth extrusion and homogeneity of slurries. The successful printing of SiC–Si slurries is not only related to proper viscosity, yield value, and shear thinning characteristics but it is also crucial for maintaining the homogeneity of slurries under extrusion pressure. Optimal SiC–Si slurries containing 52 vol % SiC–Si and 1.5 wt% HPMC exhibited proper viscosity, shear thinning, and homogeneity characteristics during printing. The obtained specimens achieved the best printing performance with height and section retention rates of 98.7% and 97.6%, respectively. When sintered at 1450 °C, Si₃N₄ fibres grow further and reach a diameter of 342.5 nm, the nitriding rate is 92.43%, the fibres tend to form a full network structure, and the mechanical properties of Si₃N₄-bonded SiC products are the best.     

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

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.     

3D-SiC decorated with SiC whiskers: Chemical vapor infiltration on the porous 3D-SiC lattices derived from polycarbosilane-based suspensions

SiC_w/3D-SiC composites were fabricated by chemical vapor infiltration (CVI) of the 3D SiC lattices, which were prepared via direct ink writing of polycarbosilane-based suspensions. Microstructure, composition and tensile strength of the composites were investigated. Curing and pyrolysis temperature greatly affected the shrinkage, weight loss, density and composition of the 3D SiC. Although sound structure with spanning feature was achieved, cracks and pores in 3D SiC were formed during the pyrolysis owing to the large shrinkage. CVI process decreased the porosity and led to fully dense surface of the SiC_w/3D-SiC composites. After 60h of CVI, short β-SiC fibres or long SiC whiskers were deposited in the structural spacing of 3D lattices or spherical pores inside the filaments, respectively. The tensile strength of the composites by CVI increased from 3.3 MPa to 15.7 MPa (20 h) and 47.3 MPa (60 h), due to the high strength of dense CVI layers and in-situ formed SiC whiskers in pores. This work showed a way to strengthen the 3D SiC with in-situ formed whiskers via the polymer precursor routes.     

3D printing of ceramics: A review

Along with extensive research on the three-dimensional (3D) printing of polymers and metals, 3D printing of ceramics is now the latest trend to come under the spotlight. The ability to fabricate ceramic components of arbitrarily complex shapes has been extremely challenging without 3D printing. This review focuses on the latest advances in the 3D printing of ceramics and presents the historical origins and evolution of each related technique. The main technical aspects, including feedstock properties, process control, post-treatments and energy source–material interactions, are also discussed. The technical challenges and advice about how to address these are presented. Comparisons are made between the techniques to facilitate the selection of the best ones in practical use. In addition, representative applications of the 3D printing of various types of ceramics are surveyed. Future directions are pointed out on the advancement on materials and forming mechanism for the fabrication of high-performance ceramic components.     

B-doped hardystonite bioceramics from preceramic polymers and fillers: Synthesis and application to foams and 3D-printed scaffolds

Highly porous hardystonite-based bioceramics, in the form of foams and 3D scaffolds, were obtained by the thermal treatment, in air, of silicone resins and engineered micro-sized oxide fillers. Besides CaO and ZnO precursors (CaCO₃ and ZnO powders), calcium borate, in both hydrated and anhydrous form (Ca₂B₆O₁₁·5H₂O and Ca₂B₆O₁₁, respectively), was added to commercial silicone resins, with a significant impact on the microstructural evolution. In hydrated form, calcium borate led to a substantial foaming of silicone-based mixtures, at low temperature (420 °C); after dehydration, upon firing, the salt provided a liquid phase, favouring ionic interdiffusion, with the development of novel B-contaning hardystonite-based solid solutions (Ca₂Zn_1-xB_2xSi_2-xO₇). Although fired at lower temperature than previously developed silicone-derived hardystonite cellular ceramics (950 °C, instead of 1200 °C), the newly obtained foams and scaffold exhibit substantial improvements in the mechanical properties.     

Direct ink writing of 3D piezoelectric ceramics with complex unsupported structures

Piezoelectric ceramic, as a typical smart material, shows great potential in applications of sensing & actuation, smart structure and energy harvesting. However, the use of traditional methods to fabricate complex and high-precision piezoelectric ceramic devices faces huge technical difficulties and high molding costs. Direct ink writing is a typical additive manufacturing technology, but has limited capabilities when preparing ceramic products with complex unsupported structures. In this work, a combined process of direct ink writing (DIW) and secondary shaping of flexible ceramic green body has been developed and proved to produce complex piezoelectric ceramics. The PZT ink shows shear thinning behavior and appropriate viscoelasticity such as moderate viscosity and high storage modulus. The printed green body can be flexibly deformed and the samples after polarization show good piezoelectric properties, with an average d₃₃ up to 265 pC/N. This work demonstrates an attractive method for geometrically complex piezoceramic with unique macro structure due to its simplicity and low cost, and provides a solution to the key problems in the existing manufacturing technology of 3D ceramics.     

3D printable ceramic pastes design: Correlating rheology & printability

Binder-free ceramic pastes based on TiO₂ nanoparticles (64 wt%) with different solvent:co-solvent volumetric ratios (water:glycol, W:G) are disclosed for the first time. Ceramic pastes were prepared without binders (polymers or gel-precursors) using a straightforward, and environmentally respectful method. Two criteria to achieve three-dimensional and complex structures were proposed. 1.- shear-thinning ability and 2.- G' = G'' > 1 × 10³ Pa. Pseudoplastic behaviors were found and were suitably modeled for all pastes analyzed, fulfilling the first criterion. Nonetheless, only those pastes with 60:40, 50:50, and 40:60 (W:G) fulfilled the second criterion. These latter exhibited a good balance between rigidity to form long and horizontal bridges but fluid enough to avoid flow cuts or clogs. Thus, 3D structures (20 layers of stacking) were obtained with outstanding self-supporting and excellent shape-retention. The complex structures were dried at 400 °C, thus obtaining binder-free and mechanically stable ceramic structures preserving their printed shape.     

Effect of internal lattice structure on the flexural strength of 3D printed hierarchical porous ultra-high temperature ceramic (ZrB2)

3D printing of technical ceramics using direct ink writing (DIW) of multiphase colloidal inks has the unique ability to create structures with hierarchical features. To facilitate the application of 3D printed hierarchical porous ultra-high temperature ceramics (UHTCs), additional limiting factors such as strength and the effect of 3D printed internal lattice structure need to be better understood. This study reports on the strength dependence of common DIW print parameters including internal lattice structure shape, nozzle diameter and spacings between adjacent filaments. The present study applies Weibull statistics to the experimental array that considers macro features introduced through print parameters as flaw types, which shows strength of 3D printed hierarchical ZrB₂ is highly dependent on the introduced 3D printed structure, size and the stressed volume. This work provides essential information that can be used in the initial stages of design when considering implementation of additively manufactured hierarchical porous UHTCs.     

In-line monitoring of printing processes in an offset printing press by NIR spectroscopy: Correlation between the conversion and the content of extractable acrylate in UV-cured printing inks

Near-infrared (NIR) reflection spectroscopy was used for in-line monitoring of the conversion in printed layers of a UV-curable cyan printing ink at an offset printing press. Quantitative analysis of the spectra was based on the calibration of the method with chemometric approaches using reference data for the conversion obtained by FTIR/ATR spectroscopy. In this way, the conversion in ink layers with a thickness of 0.8 g m⁻² was determined with a precision of 5%. Moreover, it was shown that the specific migration of the acrylate components in the binder of the printing ink was linearly dependent on the conversion in the studied range between about 80% and 100%, which allowed an estimation of the specific migration from conversion data. The strict linear relation between both parameters was used for indirect in-line monitoring of the acrylate migration during the printing process by predicting it from the conversion data derived from the NIR spectra. For comparison, off-line measurements on random samples by HPLC were carried out. The error of the predicted migration data in comparison with the results of the analytic measurements was found to be about 0.03 mg dm⁻². The proposed indirect prediction of the migration will allow a better and more direct control of technical printing processes.     

An ecological approach to printing industry: Development of ecofriendly offset printing inks using vegetable oils and pine resin as renewable raw materials and evaluation of printability

Petroleum-based solvents and synthetic resins are used as raw materials for the production of varnish, which is one of the main components of offset printing inks. These petrochemical compounds that are released to the surrounding in printing process demonstrates harmful effects on environment and human health. In the light of these facts, this study was carried out to investigate the use of renewable natural resources for offset printing inks production to lower environmental impacts and protect human health. In this study, different vegetable-based offset printing inks were developed with safflower oil (SO); grape seed oil (GSO) and Pinus pinaster resin (PPR) and printability analysis were performed. To understand the effect of pine resin in the ink formulation an ink sample with mineral oil (MO) was also produced. Printing tests were carried out with coated papers and the IGT C1 offset printability tester. Color, gloss, rub resistance, viscosity and tack values were measured on the test prints. Comparison of results with standard values figured out the suitability of using the newly developed offset printing inks for printing industry. The color difference values of color differences were in range of tolerance value. The density value of SO-PPR ink was found as same with standard values while the density value GSO-PPR and MO-PPR inks were measured close to standards as 1.45, 1.42, and 1.46, respectively. The results of printability analysis demonstrated appropriate utilization of the renewable sources for ecological development of offset printing inks.     

3D printing of shape memory poly(d,l-lactide-co-trimethylene carbonate) by direct ink writing for shape-changing structures

Four-dimensional (4D) printing of shape memory materials has attracted increasing interests for personalized structures. In this study, a biocompatible poly(d ,l -lactide-co-trimethylene carbonate) (PLMC) is utilized to fabricate 4D shape-changing structures with customized geometries through direct ink writing. The printed objects show shape transformations at different dimensions under thermal programming. The influence of the printing parameters on the properties including rheological, solvent evaporation, and static mechanical behavior are systematically investigated. A printing map is further depicted to achieve high-quality printing with high viscous ink flowed from micronozzle to construct various structures. The printed structures in one-dimensional, two-dimensional, and three-dimensional (3D) exhibit shape-changing behavior with fast response around body temperature. The fast responsive time shows potential in the field of surgical suture (4 s), nonwoven fabric (3 s), and self-expandable stent (35 s). The feasibility of 3D printing of PLMC opens the way for applications in shape-changing devices with small diameter. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48177.     

Dimensional retention of photocured ceramic units during 3D printing and sintering processes

In this study, photocuring-based digital light processing (DLP) 3D-printing technology was used to prepare basic photocuring units from cordierite ceramic slurries loaded with three different average particle sizes. Exposure time was varied to realize a range of ultraviolet light-energy dosages. Basic units, including single lines, apertures, and single walls, were printed with different feature dimensions such as single-line width and thickness, aperture diameter, and single-wall thickness. The morphologies and structures of the units were studied after printing and sintering. Their dimensions were measured, and the relative and absolute differences before and after each processing step were calculated. The dimension-retention levels were finally determined and analyzed across the ranges of slurries, exposure times, and designed dimensions. Detailed insights into the printing and sintering behaviors and performances of the ceramic slurries and printed units were gained. This study contributes to the understanding and analysis of potential dimensional errors and the printed and sintered quality of ceramic components prepared based on photocuring-based DLP 3D printing.