Photocurable Suspensions Research Articles

Entering a New Dimension in Powder Processing for Advanced Ceramics Shaping.

Filigree structures can be manufactured via two-photon polymerization (2PP) operating in the regime of nonlinear light absorption. For the first time, it is possible to apply this technique to the powder processing of ceramic structures with a feature size in the range of the critical defect sizes responsible for brittle fracture and, thus, affecting fracture toughness of high-performance ceramics. In this way, tailoring of advanced properties can be achieved already in the shaping process. Traditionally, 2PP relies on transparent polymerizable resins, which are diametrically opposed to the usually completely opaque ceramic resins and slurries. Here a transparent and photocurable suspension of nanoparticles (resin) with very high mass fractions of yttria-stabilized zirconia particles (YSZ) is presented. Due to the extremely well-dispersed nanoparticles, scattering of light can be effectively suppressed at the process-relevant wavelength of 800nm. Sintered ceramic structures with a resolution of down to 500nm are obtained. Even at reduced densities of 1-4g cm-3 , the resulting compressive strength with 4.5GPa is equivalent or even exceeding bulk monolithic yttria-stabilized zirconia. A ceramic metamaterial is born, where the mechanical properties of yttria-stabilized zirconia are altered by changing geometrical parameters, and gives access to a new class of ceramic materials.

Three‐dimensional complex construct fabrication of illite by digital light processing‐based additive manufacturing technology

AbstractIllite is a group of clay minerals that are expected to be widely used in catalyst fabrication, radioactive element adsorption, and so forth, due to its excellent adsorption properties. However, the shape control limitation of the illite product should be overcome to maximize its utilization and properties. We herein propose additive manufacturing (AM) as one of the best solutions to solve this structural drawback. Digital light processing (DLP) technology with the film‐type of the material supplying system was adapted instead of the general vat‐type DLP system to increase illite printability. The photo‐curability and printability of illite‐contained photocurable suspension were optimized. The color effect due to different ferric oxide content in yellow‐ and white‐illite which influence the photopolymerization process also adjusted thoroughly. White illite showed better photo‐curability and could be increased solid loading than yellow illite. The defect‐free illite products with three‐dimensional complex structures, which cannot be produced by typical ceramic processes, were obtained by DLP technology for both yellow‐ and white‐illite after sintering at 1100°C. The overcoming of shape control limitation of illites by ceramic AM proved in this study has excellent potential for expanding illite utilities in various applications.

Study of mixing process of low temperature co-fired ceramics photocurable suspension for digital light processing stereolithography

The Low Temperature Co-fired Ceramic (LTCC) materials are highly used for high frequency devices required for high-speed data communications, representing an attractive material for electronic applications with a direct industrial applicability. The development of a photocurable LTCC suspension for Digital Light Processing Stereolithography (DLP-SLA) technology is presented in this work. The LTCC suspension, with an optimal solid load of 40.4 vol%, was characterized along its preparation regarding the rheological behaviour, dispersant content, particle size distribution in function of milling time and photocuring properties in a visible light range. The effect of the particle size change, through ball milling, on viscosity and photocuring behaviour was studied, achieving an optimal mixing range time, which highlights the importance of the manufacturing standardization of the photocurable suspensions. The optimized suspension presents a viscosity of 3.6 Pa s at shear rate of 2 s−1, a sensitivity of 41 μm and a critical energy dose of 15 mJ cm−2. The printing process was successfully achieved, demonstrated by some defect-free printed pieces.

Rapid three-dimensional structuring of transparent SiO2 glass using interparticle photo-cross-linkable suspensions

Photo-curable suspensions are important materials for shaping complex-structured ceramic and glass components. However, most systems undergo slow debinding and sintering so to avoid structural collapse by rapid gas generation from polymers. Here, we propose a new strategy to fabricate interparticle photo-cross-linkable suspensions for rapid three-dimensional structuring with short debinding and sintering times. SiO2 particles modified with polyethyleneimine complexed with oleic acid (PEI-OA) are dispersed into a refractive-index-tuned solvent, where a photo-radical initiator and a lower-than-typical amount of multifunctional acrylates (MAs) were dissolved. The SiO2 suspension is cured by the photo-radical polymerization of MA and the Michael additive reaction between polymerized MA and amino groups of PEI-OA on SiO2 particles. These photo-curable suspensions can be employed in various shaping processes, from micro-scale stereolithography to centimeter-scale silicone molding. The SiO2 green compacts with complex structures are also debinded and sintered into transparent glass components by rapid heating.

3D printing of hydroxyapatite/tricalcium phosphate scaffold with hierarchical porous structure for bone regeneration

Three-dimensional (3D)-printed scaffolds have attracted considerable attention in recent years as they provide a suitable environment for bone cell tissue regeneration and can be customized in shape. Among many other challenges, the material composition and geometric structure have major impacts on the performance of scaffolds. Hydroxyapatite and tricalcium phosphate (HA/TCP), as the major constituents of natural bone and teeth, possess attractive biological properties and are widely used in bone scaffold fabrication. Many fabrication methods have been investigated in attempts to achieve HA/TCP scaffolds with microporous structure enabling cell growth and nutrient transport. However, current 3D printing methods can only achieve the fabrication of HA/TCP scaffolds with certain range of microporous structure. To overcome this challenge, we developed a slurry-based microscale mask image projection stereolithography, allowing us to form a HA/TCP-based photocurable suspension with complex geometry including biomimetic features and hierarchical porosity. Here, the curing performance and physical properties of the HA/TCP suspension were investigated, and a circular movement process for the fabrication of highly viscous HA/TCP suspension was developed. Based on these investigations, the scaffold composition was optimized. We determined that a 30 wt% HA/TCP scaffold with biomimetic hierarchical structure exhibited superior mechanical properties and porosity. Cell proliferation was investigated in vitro, and the surgery was conducted in a nude mouse in vivo model of long bone with cranial neural crest cells and bone marrow mesenchymal stem cells. The results showed our 3D-printed HA/TCP scaffold with biomimetic hierarchical structure is biocompatible and has sufficient mechanical strength for surgery.

Influence of Resin Composition on the Defect Formation in Alumina Manufactured by Stereolithography

Stereolithography (SL) is a technique allowing additive manufacturing of complex ceramic parts by selective photopolymerization of a photocurable suspension containing photocurable monomer, photoinitiator, and a ceramic powder. The manufactured three-dimensional object is cleaned and converted into a dense ceramic part by thermal debinding of the polymer network and subsequent sintering. The debinding is the most critical and time-consuming step, and often the source of cracks. In this study, photocurable alumina suspensions have been developed, and the influence of resin composition on defect formation has been investigated. The suspensions were characterized in terms of rheology and curing behaviour, and cross-sections of sintered specimens manufactured by SL were evaluated by SEM. It was found that the addition of a non-reactive component to the photocurable resin reduced polymerization shrinkage and altered the thermal decomposition of the polymer matrix, which led to a reduction in both delamination and intra-laminar cracks. Using a non-reactive component that decomposed rather than evaporated led to less residual porosity.

Polymeric Inverse Glasses for Development of Noniridescent Structural Colors in Full Visible Range.

Amorphous colloidal array with short-range order displays noniridescent structural colors due to the isotropic nature of the colloidal arrangement. The low angle dependence renders the colloidal glasses, which is promising for various coloration applications. Nevertheless, the colloidal glasses are difficult to develop red structural color due to strong cavity-like resonance from individual particles in the blue region. To suppress the cavity mode and develop the colors in the full visible range, we prepare inverse glasses composed of amorphous array of air cavities with short-range order. To produce the structures in a simple and reproducible manner, monodisperse silica particles are dispersed in a photocurable resin of poly(ethylene glycol) dimethacrylate (PEGDMA) at a volume fraction of 0.3. The particles spontaneously form the amorphous array with short-range order, which is rapidly captured in polymeric films by photopolymerization of the resin. Selective removal of silica particles from the polymerized resin leaves behind amorphous array of air cavities. The inverse glasses display structural colors with negligible backscattering in blue due to short optical path and low index in each cavity. Therefore, the colors can be tuned in full visible range by simply controlling the cavity size. The photocurable suspensions of silica particles can be patterned by photolithography, which enables the production of freestanding films containing patterned inverse glasses with noniridescent structural colors.

Microfluidic Design of Magnetoresponsive Photonic Microcylinders with Multicompartments.

Colloidal photonic structures have been designed to have granular format to use them for paint pigments, encoded carriers, and display pixels. However, conventional approaches only provide spherical or discoid shapes, restricting their applications. Cylindrical granules with fan-shaped compartments in the cross section are appealing for microcarriers with abundant optical codes and active display pigments for color switching. In this work, a stratified laminar flow of concentrated silica particles is employed, formed in a cylindrical microchannel, to produce cylindrical photonic microparticles with multiple compartments. To accomplish this, a microfluidic device is designed to have one cylindrical main channel connected with four branch channels. Four different photocurable suspensions are independently injected through the branches to form quarter-cylindrically compartmentalized streams in the main channel. Local ultraviolet irradiation on the main channel polymerizes the suspension, thereby forming cylindrical microparticles with four compartments. In each compartment, silica particles form ordered array which develops particle size-dependent structural color. Therefore, different colors can be incorporated into single microcylinder by employing different sizes of silica particles. Moreover, one of the compartments can be rendered to be magnetoresponsive by embedding aligned magnetic particles, which enables the remote control of microcylinder orientation and therefore the switching of structural colors.

Magnetoresponsive discoidal photonic crystals toward active color pigments.

Photonic microdisks with a multilayered structure are designed from photocurable suspensions by step-by-step photolithography. In each step of photolithography, either a colloidal photonic crystal or a magnetic-particle-laden layer is stacked over the windows of a photomask. Sequential photolithography enables the creation of multilayered photonic microdisks that have brilliant structural colors that can be switched by an external magnetic field.

Low-Threshold Lasing in 3D Dye-Doped Photonic Crystals Derived from Colloidal Self-Assemblies

In this article, we demonstrated low-threshold lasing in three-dimensional (3D) polymeric photonic crystals derived from colloidal suspensions. To achieve this, we used monodisperse silica beads in a photocurable refractive index matched medium with high viscosity and polarity. In this system, the colloidal silica beads rapidly self-organized into nonclose-packed fcc crystals because of strong repulsive interparticle potential relative to diminishing van der Waals attraction, and subsequently the colloid crystals were solidified by UV irradiation. Dye molecules as optical gain medium were incorporated in the polymeric matrix by simply mixing the dye molecules and the photocurable suspension before casting the photonic crystal films. The translucent composite photonic films showed emission inhibition and enhancement due to the low photon density of states (DOS) at the stop band and high DOS at the band edge, respectively. On the other hand, the porous photonic films, which were prepared by removal of silica particles from the composite films, exhibited larger bandwidth and higher reflectivity (>80%) due to the enhanced refractive index contrast. Under irradiation of excitation light source, the porous photonic film showed strongly enhanced stimulated emission at the band edge by a factor of more than 300 with respect to the spontaneous emission of dye molecules embedded in a bulk film without nanostructure. In addition, the lasing wavelength could be controlled by simply changing the particle volume fraction in the composite films from which the porous films were prepared. Moreover, the threshold excitation intensity was reduced by a factor of one-tenth relative to the previously reported values. The simple method for preparing 3D photonic crystals described here and subsequent lasing characteristics have great potential in a broad range of applications including displays, μ-TAS, and optofluidic light sources.

Patterned Colloidal Photonic Domes and Balls Derived from Viscous Photocurable Suspensions

Many-body colloidal systems have been studied intensively on account of their phase behavior and photonic characteristics. Direct visualization of colloids using optical microscopy makes it possible to analyze colloidal crystal structures with ease; this allows such systems to be used as ‘‘visible’’ models of atomic or molecular assemblies. The periodic modulation of the refractive index within a colloidal crystal induces photonic bandgaps at wavelengths comparable to the periodicity or lattice constant. Generally, colloidal crystals are prepared by the self-organization of colloidal particles, and therefore the photonic bandgap properties can readily be controlled by adjusting the size and concentration of the particles. More importantly, the fabrication of colloidal photonic crystals via self-organization is simple and cheap compared with the conventional lithography-based approach. However, evaporation-induced self-assembly, the most popular method for fabricating close-packed colloidal crystals, requires long times and subtle fabrication conditions, and inevitably produces crystals with cracks due to non-uniformities in the capillary force. Although dispersion of charged colloidal particles in an aqueous medium and subsequent infiltration of a non-ionic monomer can produce crack-free non-close packed crystals, this approach still requires complicated processes to achieve large scale production. Recently, we developed a novel and simple method for spontaneous crystallization of charged colloidal particles in a photocurable resin. In this method, the viscous suspension is brought into contact with a template and crystallization occurs, starting at the interface between the colloidal suspension and the template. More importantly, the high viscosity of the colloidal suspension and