Stereolitography of ceramic suspensions

The need of fast production of prototypes of complex shapes in very short time lead to the development in the last years of many additive rapid prototyping (RP) technologies for the production of single objects or of very limited series. The new fabrication concept allowed the construction of complex parts, starting from a 3D-CAD model, without using a mould. However, most of these additive processes produce polymeric objects and only recently the laser sintering of metal powders has been commercially introduced. In this work the production of ceramic objects by stereolithography is presented starting from the development of UV curable pre-ceramic suspensions for free form fabrication of alumino-silicate parts. The suspensions are characterized by 40%–50% by volume of powder content and by a reactivity and a viscosity compatible with their application in stereolithography. The ceramic green is built in a stereolithographic system operating with a He-Cd laser (325 nm). Then, the ceramic objects are obtained by pyrolisis of the organic binder and subsequent sintering of the green at 1600 °C. Finally, a characterisation of the mechanical properties and of the microstructure of the samples is presented.     

Rheology of composite ceramic injection moulding suspensions

Ceramic injection moulding compositions incorporating sinterable silicon nitride powder and silicon carbide whiskers were prepared by twin screw extrusion using a polypropylene-based organic vehicle. Their viscosities in the shear rate range 100 to 1400 sec^–1 were measured by capillary rheometry. The relative viscosity ( _r)-ceramic volume loading (V) curve fitted the Chong equation

Particle network re-structuring in highly-concentrated ceramic suspensions

In this study, the restructuring behavior of particle networks in highly-concentrated zirconia-wax suspensions as model suspensions is investigated in terms of temperature and solids concentration. The restructuring kinetics is analytically determined such that it can be formulated by a linear correlation between the relative viscosity (_r) of the suspensions and the logarithmic time (t) of isothermal annealing. Upon restructuring, a gelling point t _gel, defined as the time period of which the discrete particle networks in the pre-sheared suspensions are observed to start to form a continuous structure. Above t _gel, a sharp increase in suspension viscosity is commonly observed. The t _gel varies as a function of both temperature and solids concentration, The temperature effect provides valuable information for a better understanding of the restructuring behavior from an energy viewpoint. The influence of solids concentration, i.e., the t _gel decreased linearly with increasing solid concentration, suggests a stronger interparticle attraction that further accelerates flocculation.     

Investigation of the electrophoretic deposition behaviour of non-aqueous ceramic suspensions

The topic of the investigations was the electrophoretic shaping of AlN, SiC and Al₂O₃, respectively, using ethanolic suspensions of the powders. Suitable organic additives were chosen for achieving an effective suspension stabilisation and a sufficiently high electrophoretic mobility of the particles, which are the basic requirements for a successful electrophoretic deposition. Most of the EPD experiments were carried out in the direction of the gravitational force. Either the voltage or the current was kept constant during the deposition. In the case of the EPD of AlN, the dependence of the deposit mass on the experimental parameters has been investigated in detail and explained on the basis of changes in the effective electric field strength. Several factors influencing the effective field strength are discussed in this paper. By EPD from the different suspensions, green bodies with monomodal pore size distributions reflecting a high homogeneity of the particle packing were produced. It was shown that by the simultaneous deposition of non-oxide powders and sintering additives from suspensions with high solid contents deposits with a homogeneous distribution of the sintering additive can be obtained, whereas in diluted suspensions different electrophoretic mobilities cause a separation of the components.     

Influence of dispersant size on rheology of non-aqueous ceramic particle suspensions

The surface of alumina, which is hydrophilic in general, was made hydrophobic either by adsorption of polymer (phys-adsorption) or by an alkylation reaction with alcohol (chem-adsorption) to enable dispersion in dodecane. Hypermer A70 (8.2 nm) was used as the polymer and 1-octanol (1.2 nm), 1-decanol (1.5 nm) and 1-hexadecanol (2.5 nm) were used as the alcohol (values in brackets are the approximate thickness of the steric barrier). Rheological measurements of ceramic suspensions indicate that it is possible to achieve a high solid loading (50 vol.%) with relatively low viscosity (0.25 Pa s at 100 s^?1, the typical shear rate for pumping of liquids in pipes) as long as the stabilising molecule is large enough. The observed rheological behaviour fitted the Quemada viscosity model quite well when excluded volume effects were taken into account. Addition of 2.8 wt.% of Hypermer A70 with respect to weight of alumina was enough to stabilise the particles.     

超声在陶瓷膜过滤氧化铝悬浮体系中的应用

超声技术可有助于控制膜污染的形成,采用超声波辅助陶瓷膜过滤氧化铝悬浮液体系,考察超声在不同颗粒的粒径、浓度以及膜过滤操作压力下对膜过程的影响规律,以及超声功率、超声方式等对膜过滤过程的影响;分析了超声波对强化颗粒体系过滤的机理.结果表明,超声可使陶瓷膜微滤通量提高10%左右,且对粒径大的颗粒体系影响增大;合适的超声功率为8 W,连续超声对通量的提高率大于间歇超声.     

The optical elongation of lightguides in photopolymerizable compositions

We have studied the possibility of forming a waveguide channel in a transparent weakly absorbing photopolymerizable composition by an optical method based on the self-channeling of a radiation emitted from the edge of an optical fiber. The influence of the directivity pattern of emission from the fiber edge (i.e., of the radiation mode composition) on the channel formation is considered. In the experiment, the length of a multimodal optical fiber with a diameter of 50 μm was increased by up to 1 cm.     

Application of ionic liquids in photopolymerizable holographic materials

The influence of ionic liquids in photopolymerizable holographic materials was investigated extensively. The structures of ionic liquids have important effect on the properties of the materials. Although not all tested ionic liquids can improve the properties of the materials, the ionic liquids based on imidazolium, pyridium, or phosphonium with appropriate counter anions can be used as additives to increase the sensitivity, the diffraction efficiency, and the resolution of the materials in the thin hologram. Polymerizable ionic liquids have also been used as additives. Higher sensitivity, higher diffraction efficiency and higher resolution were obtained as well. These ionic liquids can carry out the photopolymerization during exposure to UV light to recording the hologram. They may assist to form a more stable hologram.     

Optical characterization and applications of a dual-cure photopolymerizable system

The optical response of a photopolymerizable formulation consisting of a bisphenol A epoxy acrylate oligomer, a divinyl ether, and a photoinitiator system containing Rose Bengal was studied by recording holographic gratings. This blend is sensitive to blue-green light. Single- and double-exposure volume phase holograms were recorded. In addition to these examples, surface depth measurements were made by means of a holographic contour technique.     

Modified holographic recording setup for photopolymerizable/flexible recording materials

An improvement in the conventional holographic recording setup has been done for liquid (photopolymerizable) or flexible recording materials. The new vertical recording setup that comprises a horizontal substrate holder permits holographic recording on such materials without positioning them between two substrates, as has been done until now. This setup can be conveniently used for the fabrication of transmission, reflection, Fourier transform, and computer-generated holograms with few simple modifications as indicated.     

Macroporous polyacrylates from terpene–acrylate thermoreversible photopolymerizable vehicle

Novel thermoreversible polymerizable vehicle was prepared from terpenes, such as camphene, camphor and menthol and acrylate monomers. The vehicle exhibits liquidity above room temperature and rigidity at room temperature, where the terpenes are crystalline dendrites and the acrylate monomer fills up the interdendritic space. The rigid vehicle polymerizes on UV illumination and the terpenes sublimed subsequently from the polymerized samples leaving highly porous polymeric samples with dendritic-shaped interconnected porosity up to ~92 vol%. Increasing the terpenes content, the thermoreversible component, results in coarser dendritic structures. Lower recrystallization temperatures produce finer dendritic network, whereas recrystallization close to terpenes melting point results in larger continuous macropores. The proposed advantages include fast manufacturing because of fast recrystallization, achieving large pore size up to 13 μm and tailoring of the porosity through the portion of the sublimable vehicle and recrystallization temperature. The applicability of this novel technique in the biomedical field is demonstrated on a composition containing triethylene glycol dimethacrylate (TEGDMA), a monomer widely used for biomedical applications.     

Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording

We report the correction of the shrinkage observed during UV postrecording curing in a holographic solgel material that was recently achieved by the use of various chemical formulations for the composition of the hybrid supporting matrix. We found that a chemical modification of the matrix noticeably attenuates the shrinkage (from 1.3% to 0.4% of the material's initial thickness with the inclusion of just 20% tetramethylorthosilicate), providing a material with improved stability for permanent data storage applications. The holographic properties of samples with different binders are also reported. In addition, a theoretical study has revealed the way by which to compensate for angular deviation in the Bragg condition during UV postrecording by tailoring the binder shrinkage (s), the maximum refractive-index modulation capability of the photosensitive mixture (deltan), or both.     

Hydromechanics of brownian suspensions

The values of effective rheological characteristics and of the particle self-diffusion coefficient in concentrated Brownian suspensions are estimated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 4, pp. 577–585, April, 1990.     

Non-newtonian behavior of suspensions

Summary A theory is derived for the non-Newtonian behavior of a dilute suspension of neutrally buoyant, rigid, spherical inclusions in an incompressible Newtonian fluid. The source of the non-Newtonian effects is shown to be that portion of the kinetic energy associated with the flow perturbation caused by the presence of the particles. Explicit forms are given for the average stress constitutive relation under the restriction of slowly varying flow conditions. Related normal stress effects are considered in some detail for the case of simple shearing flow.

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Nicht Newtonsches Verhalten von Suspensionen

Zusammenfassung Es wird eine Theorie für nicht Newtonsches Verhalten von verdünnten Suspensionen gleichmäßig schwebender, starrer, kugeliger Teilchen in inkompressibler, Newtonscher Flüssigkeit hergeleitet. Es wird gezeigt, daß die Quelle nicht Newtonscher Effekte der Anteil der kinetischen Energie ist, der mit den Störungen in der Strömung zufolge der Anwesenheit der Teilchen verbunden ist. Es werden explizite Ausdrücke für die Stoffgleichungen der mittleren Spannung unter der Einschränkung langsam veränderlicher Strömungsbedingungen angegeben. Einflüsse der bezogenen Normalspannung werden in einigen Einzelheiten für den Fall der einfachen Scherströmung betrachtet.

     

Dilute fluid suspensions

Summary A special case of the theory of oriented fluids has been proposed as a model for the flow of dilute suspensions. The paper shows how the proposed theory may be regarded as an analogy of the classical theory of binary mixtures.     

Optimization of a photopolymerizable holographic recording material based on polyvinylalcohol using angular responses

Acrylamide-based holographic recording materials have significant advantages and the composition of these materials has been optimized in terms of energetic sensitivity and diffraction efficiency. As a result, diffraction gratings with an efficiency of almost 80% for energetic exposures of 35 mJ/cm² and a spatial frequency of 1000 lines/mm in photosensitive films 65 μm thick have been obtained. In this paper we present the effects of intensity, thickness, and variation in the concentration of each component by studying the angular responses of the diffraction gratings recorded in each composition.     

Synthesis and characterization of photopolymerizable triblocks for 3D printing tissue engineering scaffolds

While previous research on polycaprolactone (PCL) and polyethylene glycol (PEG) triblock copolymers has focused on their use as hydrogels or with conventional scaffold fabrication methods, this work concentrates on producing viable photocurable resins from synthesized triblocks for use in a layer-by-layer 3D printer. After successful synthesis of PCL-PEG-PCL and PCL-PEG-PCL-diacrylate triblocks, they were combined with (hydroxyethyl)methacrylated polyethylene glycol (PEG-HEMA) and used as biomaterials in a dynamic masking 3D printing system to fabricate porous scaffolds. Diacrylation of the polymer (PCL-PEG-PCL-DA) revealed a substantial increase in mechanical strength and resulting compound resolved the re-dissolving issue significantly during the 3D printing process. Degradation tests were carried out by incubation in phosphate-buffered saline, and both biomaterials demonstrated their degradation resistance with steady pH levels and mass loss plateauing at 20% over a sixty day timeframe. Preliminary MG63 cell culture tests on the cross-linked 3D porous structures showed no significant cytotoxicity and MTT assay data verified cell proliferation on the photocured samples after three days. As a result, end-capping PCL-PEG-PCL with acrylates demonstrated advantages over PCL-PEG-PCL while keeping similar performance in degradation and biocompatibility. Overall results from this work demonstrate the suitability of the novel triblocks for use as biomaterials in tissue engineering scaffolds.