UV curing-assisted 3D plotting of ceramic feedstock containing thermo-regulated phase-separable,photocurable vehicle for dual-scale porosity structure

We propose a novel approach for manufacturing dual-scale porosity alumina structures by UV curing-assisted 3D plotting of a specially formulated alumina feedstock using a thermo-regulated phase separable, photocurable camphene/triethylene glycol dimethacrylate (TEGDMA) vehicle. In particular, 3D plotting process was conducted at - 5 °C, and thus an alumina suspension prepared using liquid camphene/TEGDMA at room temperature could undergo phase separation, resulting in camphene crystals surrounded by walls comprised of liquid photopolymer enclosing alumina particles. To enhance the shape retention ability of extruded filaments, polystyrene (PS) polymer was used as the tackifier. The phase-separated feedrod could be extruded favorably through a nozzle and rapidly photopolymerized by UV light during the 3D plotting process. Three-dimensionally interconnected macropores were tightly constructed, which were separated by microporous alumina filaments, where micropores were created by the removal of camphene crystals via freeze-dying. The macroporosity of porous alumina ceramics was controlled by adjusting the distance between deposited filaments, while their microporosity was kept constant, leading to tightly tailored overall porosity and mechanical properties.     

First printing of continuous fibers into ceramics

This article reports a novel method for three-dimensional (3D) printing of continuous fibers into ceramics to improve the mechanical properties of printed ceramics, which is difficult in other 3D printing technologies. The ceramics were derived by pyrolysis of thermoplastic ceramic precursor feedstocks, which were prepared by two methods. One is homogeneously mixing thermoplastic resins and ceramic precursors. The feedstocks prepared by this method exhibit good thermoplastic properties and can be extruded into filaments. Ceramics were obtained by heating the feedstocks to 1100°C in argon atmosphere. The ceramics were amorphous and remained stable during 1100-1300°C; at 1400°C they decomposed into β–SiC with simultaneous volatile gas generation. Above 1400°C, their quality decreased significantly due to cracking of ceramic skeletons. The other method is directly heating, extruding and printing the ceramic precursor. The precursors showed good printability and complex ceramic structures were printed with continuous carbon fibers inside. The continuous carbon fibers improved the flexural strength of pyrolytic ceramics, which is about 7.6 times better than that of the ceramics without fibers. The novel method unravels the potential of 3D printing of continuous fibers into ceramics with complex lightweight structures to improve the strength.     

Multi-material ceramic material extrusion 3D printing with granulated injection molding feedstocks

Material Extrusion (MEX) is an advanced technology for polymer 3D printing and countless printers are commercially available. MEX has also been demonstrated for ceramics. For that purpose, thermoplastic binders are filled with high loads (>40 vol%) of a ceramic powder. The printed parts are subsequently debound and sintered. In contrast to most MEX printers, the ceramic printer presented herein works with granulated feedstock instead of filaments. Therefore, the development of novel feedstocks is faster and more straightforward since the challenges associated with filament production are omitted. Furthermore, commercial ceramic injection molding (CIM) feedstocks can be used which allows fast prototyping with the same material that is later used in high-quantity industrial production by CIM.In this study, a method to fabricate multi-material ceramic parts using a granulate-fed printer is presented. Examples of multi-material printing include colored ZrO₂ parts as well as ceramic high-temperature heating elements in various shapes consisting of an electrically conductive and a non-conductive component. Light- and electron microscopy confirms that the layer adhesion before and after sintering is flawless, even between different materials if the material combination is chosen carefully. All feedstocks are based on a commercially available CIM binder filled with the desired ceramic powder. Consequently, the feedstock preparation as well as optimizing of debinding and sintering conditions are simple and reproducible.     

UV curing–assisted 3D plotting of core-shelled feedrod for macroporous hydroxyapatite scaffolds comprised of microporous hollow filaments

The present study demonstrates the manufacturing of macroporous hydroxyapatite (HA) scaffolds, comprised of microporous hollow filaments with high shape retention, by UV curing-assisted 3D plotting using a feedrod comprised of a photocurable HA shell and a carbon black (CB) core. Two types of scaffolds with different filament interspaces (0.5 mm and 1 mm) were produced by depositing core-shelled filaments extruded through a 1.07-mm-diameter nozzle with in situ polymerization process. Both scaffolds exhibited that the hollow HA filaments were produced after the removal of CB core by heat-treatment, while micropores in the HA walls were created as the replica of camphene-camphor crystals. Overall porosity and macroporosity obtained using a camphene-camphor content of 60 vol% increased from 74.3 vol% to 79.3 vol% and from 50.7 vol% and 64.6 vol%, respectively, with an increase in filament interspace sizes from 0.5 mm to 1 mm. Both scaffolds exhibited reasonably high compressive strengths (2.36 ― 3.58 MPa) and modulus (68–86 MPa).     

Optimized preceramic polymer for 3D structured ceramics via fused deposition modeling

3D structured ceramics stemmed from preceramic polymers via additive manufacturing have attracted much attention recently. However, these polymers with high ceramic yield are so brittle that extrusion-based additive manufacturing techniques are hardly able to be utilized for assembling 3D structures. Herein, we developed a strategy to prepare feedstocks for these manufacturing techniques, i.e., utilizing a small amount of thermal-plastic polymer to optimize the preceramic polymer while good compatibility is required between the two polymers to ensure a homogeneous mixture. Polycarbosilane and polypropylene were selected as the representative materials. Polypropylene occupied a small proportion (≤5wt.%) and significantly improved the formability of the precursor. Three-dimensional SiC were obtained via fused deposition modeling combined with crosslinking and pyrolysis. The SiC ceramic filaments showed a mean tensile strength of 471 MPa. The strategy is also applicable to a large field of ceramic systems with corresponding precursor, such as sialon ceramic and multicomponent Si-based ceramics.     

Dual-scale porous biphasic calcium phosphate gyroid scaffolds using ceramic suspensions containing polymer microsphere porogen for digital light processing

This study demonstrates a novel type of biphasic calcium phosphate (BCP) gyroid scaffolds featuring of gyroid macroporous structure and micropous BCP walls using poly(methyl methacrylate) (PMMA) microspheres as the porogen for ceramic digital light processing (DLP) technique. To tailor the microporosity of the BCP walls and the overall porosity of the dual-scale porous BCP scaffolds, the PMMA content with regard to the BCP powder was controlled in the range of 40 vol% to 70 vol%. After debinding at 600 °C and sintering at 1200 °C for 3 h, micropores were uniformly created throughout each BCP framework, while preserving 3?dimensional gyroid macroporous structures. As the PMMA content increased from 40 vol% to 70 vol%, the microporosity remarkably increased from 31.9 (±2.5) vol% to 55.2 (±1.4) vol%. This approach allowed the achievement of very high overall porosities (82.2–89.7 vol%) for the dual-scale porous scaffolds. However, all the scaffolds showed reasonable compressive strengths (0.8 MPa ?2.1 MPa), which are comparable to those of cancellous bones.     

Fused deposition fabrication of high-quality zirconia ceramics using granular feedstock

Benefiting from the mature technology of ceramic injection molding, Fused deposition modeling based on highly-filled ceramic-polymer granular feedstocks has been showing great potential and advantage for fabricating 3D ceramics. Herein, 3D zirconia ceramics using granular feedstock were fabricated, and typical morphology, surface quality, and effect of the thermal accumulation on 3D structure were clarified. Typical morphology of printing steps on the surface were quantitatively characterized, and determined by the surface curvature and layer height of the printed structure. Aligned triangular pores were confirmed at the junction of the deposited filaments with elliptical cross-section morphology. Simple square plates with different size were used to illustrate the influence of thermal accumulation on the morphology of 3D structure. Small printing size increased the thermal accumulation during deposition, resulting in decreased printing quality caused by the secondary over-melting of former deposited layers. Except for the pores at the junctions, dense zirconia ceramics with uniform structure and smooth surface could be achieved. A low-cost and high-quality route for the preparation of 3D ceramics was demonstrated via FDM of highly-filled granular feedstocks.     

Effect of MgO sintering additive on mullite structures manufactured by fused deposition modeling (FDM) technology

An optimized recipe for 3D printing of Mullite-based structures was used to investigate the effect of MgO sintering additive on the processing stages and final ceramic properties. To achieve dense 3:2 mullite, ceramic filaments were prepared based on an alumina powder, a methyl silicone resin, EVA elastomeric binder and MgO powder. Using 1 wt% MgO and a dwell time of 5 h at 1600 °C, a dense mullite structure could be obtained from filaments with a diameter of 1.75 mm. Ceramic structures with and without sintering additive were printed in vertical and horizontal direction, to investigate the effect of printing direction on mechanical strength after sintering. Using four-point bending test, it was demonstrated that by using MgO, the printing orientation did not affect the mechanical strength significantly anymore. The low Weibull modulus could be explained by the closed porosity that emerge during the degassing of the preceramic polymer due to cross-linking.     

Digital light processing of ceramic components from polysiloxanes

Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.     

Calcium phosphate ceramics with continuously gradient macrochannels using three-dimensional extrusion of bilayered ceramic-camphene mixture/pure camphene feedrod

We herein demonstrate a novel, versatile approach to produce calcium phosphate (CaP) ceramics with continuously gradient macrochannels using three-dimensional extrusion of a bilayered ceramic-camphene mixture/pure camphene feedrod. In this technique, the pure camphene used as the upper part could be preferentially extruded because of the wall slip phenomenon. This enabled the formation of green filaments comprised of a camphene core surrounded by a ceramic/camphene shell, where the core/shell thickness ratio increased gradually as extrusion proceeded. CaP ceramics with continuously gradient macrochannels could be successfully produced by three-dimensionally depositing the extruded filaments layer-by-layer. With increasing the distance from the dense bottom layer, macrochannels created after the removal of the camphene cores via freeze-drying became larger, while the CaP walls became thinner. The local porosity could increase gradually and continuously from the dense bottom and reach up to ~72 vol%.     

Processing of dense bioinspired ceramics with deliberate microstructure

The architectures of biological hard materials reveal finely tailored complex assemblies of mineral crystals. Numerous recent studies associate the design of these local assemblies with impressive macroscopic response. Reproducing such exquisite control in technical ceramics conflicts with commonly used processing methods. Here, we circumvent this issue by combining the recently developed Magnetically Assisted Slip Casting (MASC) technique with the well-established process of templated grain growth (TGG). MASC enables the local control over the orientation of platelets dispersed among smaller isotropic particles. After a high-temperature pressureless heat treatment, the grains of the final ceramic follow the same orientation as of the initial platelets. This combination allows us to produce 95% dense alumina part with a grain orientation following any deliberate orientation. We successfully fabricated microstructures inspired from biological materials with ceramics that present periodically varying patterns with a programmable pitch down to a few tens of micrometers. The periodically textured dense ceramics exhibit matching variation of local hardness, confirming the capacity of the process to tailor local properties. This unique micrometer scale control over the local mechanical properties could be applied to adapt ceramic structures to complex loads using this inexpensive and scalable process. In systems where functional properties also depend on anisotropic grain orientation, the principle presented here could enable the creation of new multifunctional ceramics.     

Atomistic modeling of dopant segregation in α-alumina ceramics: Coverage dependent energy of segregation and nominal dopant solubility

Microstructural control is a key aspect in producing ceramics with tailored properties and is often achieved by using dopants in a rather empirical fashion. Atomic scale simulations could provide much needed insight but the long-standing challenge of linking simulation results on isolated grain boundaries to those measured in real ceramics needs to be resolved. Here a novel Monte-Carlo simulation method based on a microstructural model in combination with energies obtained from atomic scale energy minimization is presented. This approach allows, for the first time, the prediction of the nominal solubility of dopants (Y, La and Mg) in a ceramic purely from theory.Results compare well with segregation/precipitation data as a function of grain size, found in the literature. The method can therefore be used in developing experimental guidelines for the effective use of dopants in ceramic production, thus accelerating the development of novel materials required for innovative applications.     

Enhanced Bone Regeneration in Variable-Type Biphasic Ceramic Phosphate Scaffolds Using rhBMP-2

Our aim was to investigate the bone regeneration capacity of powder-type biphasic ceramic scaffold (BCP powder), block-type BCP (BCP block), and collagen-added block-type BCP (BCP collagen) with different concentrations of recombinant human bone morphogenetic protein 2 (rhBMP-2) in an animal model. Four rabbits were assigned to each of the following groups: no graft + rhBMP-2 (0.1/0.2 mg/mL), BCP powder + rhBMP-2 (0.1/0.2 mg/mL), BCP block + rhBMP-2 (0.1/0.2 mg/mL), and BCP collagen + rhBMP-2 (0.1/0.2 mg/mL), i.e., a total of 32 rabbits. Polycarbonate tubes (Φ 7 mm × 5 mm) for supporting scaffolds were fixed into a 7 mm round border. Subsequently, 0.1 mL of rhBMP-2 solutions with different concentrations was injected into the tubes. Both radiological and histomorphometric analyses showed that osteogenesis was not enhanced by increasing the concentration of rhBMP-2 in all groups at both 3 and 6 weeks. Radiological analysis showed that bone formation was higher in the BCP collagen group than in the BCP powder and BCP block groups at both rhBMP-2 concentrations at 3 weeks. rhBMP-2 enhanced bone formation; however, as the concentration increased, bone formation could not be enhanced infinitely. Collagen-added alloplastic graft material may be useful for mediating rapid bone formation in initial stages.     

Robotic deposition of 3d nanocomposite and ceramic fiber architectures via UV curable colloidal inks

A novel approach for producing predetermined, complex 3d ceramic architectures by robotic deposition where UV radiation is used for solidification is presented. Homogeneous, highly loaded, solvent free colloidal inks with controlled viscoelastic properties are achieved by proper selection of monomers and surfactants. Room temperature deposition of complex 3d fiber networks having filaments in the 100 μm range is demonstrated for Al₂O₃ and hydroxyapatite model particles for structural and biomedical applications. Solidification of the structures by UV radiation allows additional shaping of the structures by post-printing processes such as cutting, folding and bonding. 2d and 3d architectures with high aspect ratios retain their shape and transform to macroscopic ceramics after thermal debinding and sintering procedures. Sintered alumina fiber networks functionalized with a 3-5 μm layer of TiO₂ nanoparticles exhibit photocatalytic activity for the decomposition of formaldehyde as a similar weight of loose powder, indicating possible applications in catalytic reactors prototypes.     

Fabrication of Macroporous Alumina with Tailored Porosity

Macroporous alumina materials were fabricated via colloidal processing using polymer spheres as the template and ceramic particles as the building blocks. The influence of the suspension conditions and volume ratio of the polymer/ceramic particles on the formation of the pore structure has been investigated. The results showed that the suspension conditions have a significant effect on the pore morphology. A well-defined three-dimensional, ordered porous structure with a controllable pore size and porosity could be obtained through the hetero-coagulation, self-assembled processing of the polymer/ceramic particles. The pore size and porosity could be easily tailored by varying the polymer size and the volume ratio of the polymer/ceramic particles.     

新型亚磷酸铀的合成与表征

由于无机微孔晶体具有规则的孔道结构和丰富的结构类型,在吸附、分离、催化、主客体化学等领域的广泛应用,具有新颖结构的微孔晶体的合成一直备受关注。近年来,以假四面体的HPO3基团取代的四面体PO4基团构筑微孔晶体在超大孔微孔晶体的合成领域取得很多成果。ZZ-1的化学式为[C10H24N2][(UO2)2(HPO3)3].H2O,是以佛尔酮二胺为模板剂合成出的层状亚磷酸铀,其结构是由UO7五双角锥和HPO3假四面体连接形成的4,8元环的网络片层,这些片层的堆积形成了层状结构。层与层之间的嵌有顺式的异佛尔酮二胺阳离子和水分子。ZZ-1在266nm激光激发下发出强烈的绿光。     

Flexible interconnected ceramic parts 3D printed by two-component material extrusion with water-soluble support structures

Material extrusion (MEX) of complex thermoplastic structures often depends on the reliable printing of a water-soluble support structure. The material of choice is typically polyvinyl alcohol (PVA), which is not used in ceramic MEX printing due to a limited printing compatibility with most ceramic feedstocks (poor layer adhesion). Herein, a new thermoplastic feedstock was developed as temporary support material on the basis of NaCl mixed with a commercial injection molding binder system. The NaCl feedstock is fully compatible for MEX printing with ceramic feedstocks and showed excellent printing properties and high green body strength. The support structure is mostly dissolved in water and the rest can be removed manually or during thermal debinding. The NaCl support material was used to print flexible Al₂O₃ samples with hinges or chainmail samples. This strategy is an attractive way to introduce additional functionality and new applications which were so far inaccessible to technical ceramics.     

In vitro biodegradable and mechanical performance of biphasic calcium phosphate porous scaffolds with unidirectional macro-pore structure

In this study, porous biphasic calcium phosphate (BCP) scaffolds were fabricated by a freeze–gel casting technique using a tertiary-butyl alcohol (TBA) based slurry. After sintering, unidirectional macropore channels of scaffolds aligned regularly along the TBA ice growth direction were tailored simultaneously with micropores formed in the outer wall of the pore channels. The synthesized porous BCP scaffolds (two different sintering temperatures) exhibit compressive strength of 46.8 MPa for 43.0% porosity and 33.1 MPa for 45.9% porosity, respectively. After immersion in Hank's balanced salt solution (HBSS) for 1, 2, 4, 8 weeks, a precipitation started to be formed with individual small granules on the scaffolds surface. In the case of BCP scaffolds sintered at 1200 °C, β-TCP were slowly degraded with increasing the immersing time; on the other hand, α-TCP (from BCP scaffolds sintered at 1300 °C) was extremely degraded within 1 week of immersing. This behavior could be due to a fast hydrolysis (dissolution–reprecipitation) as a phase transformation from α-TCP to brushite or apatite compared to the β-TCP. After immersion in HBSS, overall the compressive strength of the scaffolds reduced by the gradual degradation in biological environment solution. This behavior is consistent with the degradation behavior of scaffolds after immersion in HBSS.     

SiC and SiOC ceramic articles produced by stereolithography of acrylate modified polycarbosilane systems

3D printed ceramic articles are receiving increased interest recently. Stereolithography (STL) is the method of choice where surface quality, high resolution and high aspect ratio architectures are concerned. Recently, we have developed a UV curable system consisting of allylhydridopolycarbosilane (AHPCS) and multifunctional acrylates. In our present work we investigate the photo-crosslinking mechanism and use selected formulations for the 3D printing of SiC rich ceramic articles using a desktop STL device. High resolution and complex shape articles are demonstrated. The versatile curing method can be used for the STL of practically most other vinyl/allyl modified preceramic polymers. The nano-porosity as well as SiOxCy composition can be tailored in a wide range for specific applications by the ratio of acrylate to AHPCS and by the type of acrylate and AHPCS used.