Extrusion-based 3D printing of fully dense zirconia ceramics for dental restorations

Extrusion-based 3D printing using zirconia ceramics shows promising results in biomedical applications, but the challenges in deployment are still significant. In this study, we developed an optimized extrusion-based 3D printing method for fabrication of intricate, fully dense ceramics based on gelled zirconia suspensions. The viscoelastic properties of the suspensions were tailored to facilitate smooth flow through a 150–250 µm nozzle at high solid volume fraction (52%) and produce self-supporting features. Because of the minimal organic content, the as-dried zirconia specimens could be directly sintered without binder-removal steps. The maximum relative density, flexural strength, and hardness of the sintered zirconia ceramics were 99.3%, 1010 MPa, and 15.9 GPa, respectively. Zirconia dental restorations were fabricated, and the occlusal details were preserved by co-printing a novel polyacrylate supporting material which can be removed by cooling owing to its temperature-dependent rheological behavior.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.     

Formation of a Polymer-Coated Inclusion Complex of D-Limonene and β-Cyclodextrin by Spray Drying

The polymer-coated inclusion complex powder formation of D-limonene and β-cyclodextrin obtained by spray drying was investigated with respect to the effects of various types of polymer coating agents on the powder particle size and morphology. The addition of the polymer coating agent affected the average particle size, morphology, and internal structures of the spray-dried powders. The average particle diameter of the uncoated spray-dried powders was approximately 5 µm. The powder particle size increased upon the addition of a polymer coating reagent. With the addition of 9 wt% of the polymer coating agent, an average diameter of approximately 80 µm was obtained for the spray-dried powder particles. However, further addition showed a negligible effect on the particle size. Inclusion complex crystals were observed on the surface and inside of the powder particles.     

Fabrication of zirconia all-ceramic crown via DLP-based stereolithography

Zirconia ceramics have shown a wide applicable prospect in dental prosthetics because it possesses excellent mechanical performance and biocompatibility. The rheological behavior and curing properties of zirconia stereolithography slurry were studied, as well as the microstructure of the green, pyrolyzed, and sintered body. Variquat CC-42 NS was proved to be effective for zirconia powder dispersing in photocurable resin (SP-RC700). The curing characteristic of slurry showed that the depth of penetration tended to increase with the solid loading. Finally, the zirconia all-ceramic crowns were fabricated via DLP-based stereolithography printer. Results showed that the particles were evenly distributed in the cured resin matrix without obvious agglomeration, and the interlayered structure disappeared after binder burnout. The sample with a major crystalline phase of tetragonal zirconia and a porosity of 10(1)% were obtained after sintering at 1550°C.     

Enhanced Activation Energy of Crystallization of Pure Zirconia Nanopowders Prepared via an Efficient Way of Synthesis Using NaBH4

An efficient way through borohydride synthesis route using NaBH₄ was performed to prepare pure zirconia nanopowders via three different conditions such as gelation, precipitation, and constant pH. Zirconia powders prepared through constant pH route show highest activation energy of crystallization (E_a = 260 kJ/mol) or higher exothermic peak temperature (717°C), when compared with gelation or precipitation route due to its controlled growth of smaller crystallites. The released huge amount of H₂ gas bubbles during borohydride synthesis via constant pH route play a major role for formation of loose smaller crystallites and thus enhances the activation energy of crystallization of pure zirconia. So, the as‐prepared zirconia powders prepared through constant pH route remain amorphous up to 600°C and pure t‐ZrO₂ (~20 nm) was stable up to 800°C.     

Robocasting and surface functionalization with highly bioactive glass of ZrO2 scaffolds for load bearing applications

This work is a proof of concept for making load bearing implants with osseointegration and bone bonding ability. Yttria-stabilized zirconia (YSZ) scaffolds with a percentage of macro porosity of about 70% were fabricated by robocasting. Although a maximum solids volume fraction of 50 vol.% could be achieved, the 3D-printing process revealed to be more reliable when using inks with 48 vol.% solids. The sintered porous structures exhibited an average compressive strength of ~236 MPa. After some preliminary coating experiments, an ethanol slurry of fine bioactive glass (BG) particles (10 wt.%) stabilized with polyvinylpyrrolidone was used to deposit a uniform surface coating onto the filaments, followed by glazing at 850°C. The functionalized scaffolds showed a relatively uniform surface coverage by the bioactive glass. The results of in vitro testing by immersing the scaffolds in simulated body fluid (SBF) showed remarkable morphological surface changes and an extensive deposition of hydroxyapatite layer. The overall results demonstrate the viability of producing porous YSZ scaffolds with excellent bioactivity, which are promising for bone tissue engineering under load bearing applications.     

3D gel-printing of hierarchically porous BCP scaffolds for bone tissue engineering

In this study, a conventional technique of porous preparation was used to improve the constructive capability of direct ink writing on microstructures, and the hierarchically porous scaffolds were successfully prepared by 3D gel printing (3DGP). Micron-sized hydroxyapatite (HA) was coated with tricalcium phosphate (TCP) nanopowders synthesized by chemical co-precipitation to form biphasic calcium phosphate (BCP). The random structure of concave micropores was achieved by filling the BCP slurry with PMMA microspheres while successfully controlling the internal porosity of printed filaments. The results showed that the three-stage porous structure was successfully constructed, i.e., macroscopic pores of 1.50–2.00 mm, spherical micropores of 100–200 µm, and inter-powder interstices of 1.00–10.00 µm. Nano-TCP coated micron-HA powders improved the sintering activity of BCP particles. The compressive strength and porosity of the scaffolds sintered at 1400 °C were 2.78 MPa and 84.98%. The hierarchically porous BCP scaffolds had bright applications in bone tissue engineering.     

Electrophoretic deposition of titania nanostructured coatings for photodegradation of methylene blue

The suspensions of titania nanoparticles were prepared in different alcohols (methanol, ethanol, isopropanol and butanol) using polyethyleneimine (PEI) as dispersant. The results of sedimentation, conductivity, zeta potential, FTIR and thermal analysis showed that PEI macromolecules are protonated in the suspensions and then adsorbed on the particles enhancing their positive surface charge and so colloidal stability. Optimum concentration of PEI (PEI^*) was lower in large molecular size alcohols due to its higher adsorption efficiency. PEI^* was 0.75, 0.5, 0.5 and 0.25?g/l in methanolic, ethanolic, isopropanolic and butanolic suspensions, respectively. Electrophoretic deposition (EPD) was performed at 60?V in different alcoholic suspensions. EPD rate was the fastest in the suspensions with PEI^* due to the highest mobility of particles. In contrast to the coatings deposited for 1?min from isopropanolic and butanolic suspensions, those deposited at same conditions from methanolic and ethanolic ones severely cracked during drying owing to their higher thickness and lower PEI contents (acting as the binder). The coatings deposited from methanolic and ethanolic suspensions with PEI^* had finer and rougher microstructures. The sintered coating deposited at 60?V for 10?s from ethanolic suspension with PEI^* had crack-free microstructure with the thickness of ≈?130?μm. This coating degraded ≈?24% of methylene blue from its aqueous solution (30?ml with the concentration of 5?mg/l) within 2?h under UV illumination. Photodegradation of MB on the surface of titania coating obeyed the first order kinetics law.     

义眼台用羟基磷灰石的制备和性能

采用均相共沉淀法直接合成了高纯度羟基磷灰石粉体,通过改变反应物加入方式和反应时间调控粉体形貌和组成,分析了体系pH值对产物纯度的影响;以硅溶胶为粘结剂、尿素为造孔剂,室温下压制成型,高温烧结制备了孔隙度高、力学性能良好的多孔羟基磷灰石块体,考察了粉体粒径、造孔剂含量对孔隙度和抗压强度等性能的影响。结果表明,纳米粉有利于块体成型,随造孔剂含量增加,块体密度减小、孔隙度增加,当羟基磷灰石与尿素质量比为1.5:1时,孔隙度达69%,抗压强度达8 MPa,满足义眼台应用需求。     

Effect of zirconia content and particle size on the properties of 3D-printed alumina-based ceramic cores

Alumina-based ceramic cores are used to manufacture the internal structures of hollow alloy blades, requiring both high precision and moderate properties. In this work, zirconia is regarded as a promoter to improve the mechanical properties of sintered ceramic. The effect of zirconia content and particle size on the microstructure and mechanical properties of ceramics was evaluated. The results indicate that the flexural strength of sintered ceramics reached the maximum of 14.5 ± 0.5 MPa when 20 wt% micron-sized (10 μm) zirconia (agglomerate size, consistent with the alumina particle size) was added, and 26.5±2.5 MPa when 15 wt% 0.3 μm zirconia was added. Zirconia with submicron-sized (0.3 μm) particles effectively filled the pores between alumina particles, thus leading to the maximum flexural strength with a relatively low content. The corresponding sintered ceramics had a bulk density of 2.0 g/cm³ and open porosity of 59.6%.     

Thermoplastic 3D Printing—An Additive Manufacturing Method for Producing Dense Ceramics

In our new approach—thermoplastic 3D printing—a high‐filled ceramic suspension based on thermoplastic binder systems is used to produce dense ceramic components by additive manufacturing. Alumina (67 vol%) and zirconia (45 vol%) suspensions were prepared by ball milling at a temperature of about 100°C to adjust a low viscosity. After the preparation the suspension solidified at cooling. For the sintered samples (alumina at 1600°C, zirconia at 1500°C), a density of about 99% and higher was obtained. FESEM studies of the samples' cross section showed a homogenous microstructure and a very good bond between the single printed layers.     

Three-dimensional printing of zirconia scaffolds for load bearing applications: Study of the optimal fabrication conditions

Yttria-stabilized zirconia (YSZ) scaffolds with a planned macroporosity fraction of about 70% were fabricated by Robocasting from inks with high solid loadings. The effects of solids loading and the concentrations of processing additives on the flow behavior of the starting suspensions and the viscoelastic properties of the resulting inks were investigated aiming at optimizing the printing process. The shear thinning flow behavior of the starting suspensions containing 45‒48 vol% solids and dispersant concentrations varying within 0.2‒0.8 wt% could be well described by the four-parameter Cross model. The flow behavior of the suspensions could be correlated with the interaction forces, and the ad-layer thickness formed around the YSZ particles. Further adding suitable amounts of a binder and a coagulating agent enabled optimizing the viscoelastic properties of inks for 3D printing. Good shape retention was observed for inks with elastic modulus, G′ ≥ 10 MPa. The green scaffolds were dried, sintered at 1350°C, and then used for the assessment of porosity and mechanical properties under compression tests. The porous structures exhibit average compressive strength (σ) of ~70 MPa. Weibull statistics applied to σ data revealed good reliability of the process, which can be used to fabricate YSZ scaffolds for orthopedic applications.     

Effect of the solvent composition and annealing process on the preparation of spray freeze-dried acetaminophen powder

The analgesic and physical properties of acetaminophen powder suitable as an inhaled drug produced by spray freeze-drying (SFD) were compared with those of raw acetaminophen. A laser particle size analyzer and scanning electron microscopy (SEM) were applied to estimate physical structure and properties of the particles. A cyclooxygenase (COX) inhibitor screening assay was used to compare the antipyretic and analgesic activity of raw and SFD acetaminophen. According to SEM, SFD acetaminophen particles had various shapes and sizes with porous structures. The optimized conditions for solvent, annealing temperature, and annealing time were water/ethanol mixture (60% water and 40% ethanol), ?40°C, and 7?h, respectively. The diameter of optimized acetaminophen powder was 7.33?µm, and the aerodynamic particle size was 3.38?µm. The antipyretic and analgesic activities of acetaminophen after SFD were from 84.3 to 97.1% for COX-1 and from 91.6 to 102.9% for COX-2 compared to those of raw acetaminophen, respectively.     

Sintering behaviors and properties of porous ceramics derived from artificially cultured diatom frustules

Bimodal porous ceramics with high strength have been fabricated by conventional powder metallurgy utilizing artificially cultured diatom frustules (DFs). The effect of sintering temperature on thermal behaviors, phase transition, and pore structures features of DFs-based porous ceramics is investigated between 800 and 1200°C. The phase evolution of DFs powders is investigated with thermal analysis (DIL and DSC-TG). Phase transition behaviors analyzed with XRD, Raman, and FT-IR spectra confirm the transformation of quartz into cristobalite phases occurs under 1050°C. Sintering under 950°C could bind DFs powders tightly into high strength porous ceramics while maintain the multilayer pore structures simultaneously, having porosity of 56.4%, compressive strength of 15.0 MPa and surface area of 50.9 m²/g, respectively. Slit-shaped microstructures and mesopores (2-50 nm) are observed in DFs-based porous ceramics sintered under 1050°C. Collapse and blockage of pore structures as well as partial fusion of DFs particles happened at the temperature of 1100°C, indicating the presence of diminished multilayers and particle agglomeration.     

Sol-gel infiltration of complex cellular indirect 3D printed alumina

Gelled aqueous solutions containing the soluble precursor aluminum chlorohydrate were developed for the infiltration of porous indirect 3D printed alumina. Viscosity and amplitude sweep tests confirmed the gel formation and sheer thinning behavior beneficial for the subsequent coating and infiltration process. High temperature XRD confirmed the formation of corundum at a temperature of 1000?°C. Complex alumina structures with high surface area and isotropic pore channels were achieved by indirect 3D printing. Coating and infiltration of the pre sintered alumina with a subsequent sintering step transformed the precursor to corundum and partially filled the residual porosity and decreased surface defects after 3D printing. With the gel coating a pronounced improvement up to a maximum value of Δσ_compr?=?61.9?MPa was observed and with the gel infiltration a maximum improvement of ΔE?=?136.2?GPa. The results show the possibility to infiltrate even complex alumina structures with aqueous alumina precursors without the need to disperse ceramic particles.     

An investigation of atomic force microscopy,surface topography and adhesion of luting cements to zirconia: effect of silica coating,zirconia primer and laser

This study evaluated the effect various surface conditioning methods on the surface topography and adhesion of luting cements to zirconia. Zirconia blocks (N?=?25) were randomly assigned to five groups according to the surface conditioning methods: (a) No conditioning, control (CON), (b) tribochemical silica coating (TSC), (c) MDP-based zirconia primer (ZRP), (d) coating with nano aluminum nitride (ALN) (e) etching with Er: YAG laser (LAS). The conditioned zirconia blocks were further divided into five subgroups to receive the luting cements: (a) MDP-based resin cement (Panavia F2.0) (PAN), (b) 4-META-based cement (Super Bond) (SUB), (c) UDMA-based (GCem) (GCE), (d) bis-GMA based (Bifix QM) (BIF) and (e) polycarboxylate cement (Poly-F) (POL). Cements were applied in polyethylene moulds (diameter: 3?mm; height: 2?mm). The bonded specimens were first thermocycled for 5500 cycles (5–55?°C) and then adhesive interface was loaded under shear (0.5?mm/min). The data (MPa) were analyzed using 2-way ANOVA, Tukey’s and Bonneferroni tests (alpha?=?0.05). Regardless of the cement type, TSC resulted in significantly higher bond strength (p???0.05) (13.3?±?4.35–25.3?±?6.3) compared to other conditioning methods (2.96?±?1.5–5.4?±?5.47). Regardless of the surface conditioning method, no significant difference was found between MDP, 4-META and UDMA based cements (p?>?0.05) being significantly higher than those of bis-GMA and polycarboxylate cements (p???0.05). Failure types were frequently adhesive in all groups. Tribochemical silica coating provided superior bond results compared to other conditioning methods tested on zirconia especially in conjunction with UDMA- and 4-META-based resin cements.     

Pore orientation of the gadolinia‐doped ceria cathode interlayer for a tubular SOFC using dip‐coating

An active region of cathode interlayer in a tubular solid oxide fuel cell (SOFC) is structurally analyzed using a dual‐beam focused ion beam/scanning electron microscope (FIB/SEM). The GDC (10 mol% gadolinia‐doped ceria) cathode interlayer (about 1 μm in thickness) is dip‐coated, and then sintered on YSZ (8 mol% yttria‐stabilized zirconia) electrolyte. At 1150°C sintering temperature, the pores oriented more along the axial direction than the radial direction. The anisotropy of pore shape is accounted for the withdrawal force during the dip‐coating of the GDC interlayer.     

Properties of alumina 10 vol% zirconia composites—The role of zirconia starting powders

Zirconia toughened alumina (ZTA) materials are applied for cutting tools, wear parts and in biomedical applications. Due to the constraint of the rigid alumina matrix, ZTA materials with up to 10 vol% zirconia addition (AZ10) do not require addition of stabilizer oxides. AZ10 materials based on submicron sized alumina and four different submicron to nanoscale zirconia powders were manufactured by hot pressing at temperatures between 1475?1600 °C. Results show that the powder choice has a strong influence on mechanical properties, evolution of microstructure and phase composition. Best results with strength up to 850 MPa, fracture toughness values of 8.5 MPa√m and invulnerability to overfiring were obtained with zirconia powders showing the coarsest yet most homogeneous primary particle size and a low degree of agglomeration. Ultrafine but hard agglomerated powders lead to materials with extremely inhomogeneous microstructure and inferior properties.     

The effects of laser patterning 10CeTZP-Al2O3 nanocomposite disc surfaces: Osseous differentiation and cellular arrangement in vitro

Customized square grid arrangements of different groove depths (1.0, 1.5 and 3.0?µm) and separations (10 and 30?µm) were successfully laser patterned, using a nanosecond pulsed fibre laser, on the surface of 10?mol% ceria-stabilized zirconia and alumina (10CeTZP-Al₂O₃) nanocomposite discs (diameter: 10?mm; thickness: 1.5?mm). The patterned surfaces and the in vitro biological response of osteoblasts (SAOS-2) towards them were thoroughly analysed. In terms of composition, the laser treatment was found to cause superficial monoclinic-tetragonal zirconia phase transformation and alumina evaporation. In vitro, the most effective grid configuration for osseous differentiation was found to be 1.5?µm groove depth and 10?µm groove separation, and confocal microscopy revealed that the cells show a tendency to be sorted as groove depth increases. It is thought that custom-made patterns could be produced to guide cell attachment in vivo, which could favour implant integration and reduce healing time.     

Selection of Suitable Dispersants for Aqueous Suspensions of Zirconia and Titania Powders using Acoustophoresis

Acoustophoresis was used to study the effect of adding various commercially available dispersants onto aqueous suspensions of two zirconia and two titania powders. These powders were characterised for elemental composition by X-ray fluorescence (XRF) spectroscopy and for surface area by BET single point nitrogen adsorption. From the maximum value of the zeta potential, it was possible to select the most promising dispersants. From the shape of the curve the minimum amount of dispersant required to stabilise the powder particles was noted. The iso electric point (i.e.p) of the powders was also identified. Several dispersants can be recommended for the first titania powder, whilst none can be recommended for the second titania as the final zeta potentials on addition of the dispersants were low. The two powders had different surface chemistries which was reflected in a large difference in their i.e.p; the first at pH 7·5 and the second at pH 6·1. This was due to different coatings on the powder surfaces; alumina and an organic material respectively. Removal of this organic coating by calcinatian then enabled the dispersants to fully adsorb. Similarly dispersants for the first zirconia powder could be identified and the i.e.p identified at pH 5·4. However, no dispersants can be recommended for the second zirconia powder as yttria dissolves out of the powder under the naturally occurring weakly acidic conditions. The i.e.p was estimated to be pH _i.e.p 7–7·5.