Metal casting into NaCl molds fabricated by material extrusion 3D printing

Aluminum die casting is a well-established industrial process for mass producing aluminum parts with complex shapes, but design restrictions exclude some features like undercuts and hollow structures from being produced with this method. Water-soluble casting molds offer a promising solution to overcome those restrains, for example by hot pressing of salt cores or 3D printing of NaCl molds. Presently, 3D printing techniques available for NaCl are limited to direct ink writing (DIW) and photopolymerization. This study presents an approach to prepare NaCl parts by thermoplastic material extrusion (MEX) 3D printing. Firstly, a 3D printable feedstock is developed consisting of an organic binder, which is usually used for ceramic injection molding, and sodium chloride (NaCl) salt crystals. Various molds are then printed on a granulate-fed MEX printer. After thermal debinding and sintering at 690 °C, the 3D printed parts consist of pure NaCl. Furthermore, the same NaCl feedstock is used for injection molding. The bending strength of 3D printed samples with and without post-treatment are measured and compared to injection molded test specimens. Finally, metal casting in 3D printed NaCl molds is shown with tin or aluminum and the metal demonstrator parts with complex geometries such as gyroid structures and turbine wheels are released by dissolving the NaCl molds in water

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

Operando deconvolution of photovoltaic and electrocatalytic performance in ALD TiO2 protected water splitting photocathodes

The dual-working-electrode technique enables the deconvolution of the intrinsic properties of the buried p–n junction and the electrocatalyst on the surface for water splitting photocathodes

Water-soluble sacrificial 3D printed molds for fast prototyping in ceramic injection molding

Fabrication of steel molds is a major expense (time and cost) in ceramic injection molding research and development. 3D printed resin molds for fast prototyping are therefore highly attractive and have gained increasing attention. This paper reports strategies to use sacrificial molds 3D printed by fused deposition modeling (FDM) from PVA or digital light processing (DLP) from water soluble resin. Usage of sacrificial molds allows injection molding of complex geometries, which are not accessible for simple two-part molds. Ceramic heating elements in diverse geometries were injection molded using a composite feedstock containing MoSi2, Al2O3 and feldspar. More parts with various geometries were produced from Al2O3 feedstock. A comparison revealed that DLP printed molds are better suited for parts with very small structural features due to the higher resolution of the DLP process as compared to FDM. Finally, ceramic heaters were fabricated using two-component ceramic injection molding and successfully tested

MoSi2/Al2O3/feldspar composites for injection‐molded ceramic heating elements

MoSi2 is an electrically conductive material with numerous applications mostly in high-temperature environments. Herein, the production of MoSi2-containing resistive heating elements by ceramic injection molding (CIM) is described. The sintered parts consist of MoSi2 particles embedded in a matrix of vitrified feldspar and Al2O3. The conductivity of sintered parts can be tuned precisely by varying the content of the conductive phase. For the development of the injection-molding feedstock, four binder systems are evaluated. The corresponding feedstocks are injection molded into different geometries in traditional molds as well as in additively manufactured, soluble molds. For each feedstock, a debinding and sintering routine is elaborated based on thermogravimetric measurements. Higher debinding temperature leads to more oxidation of MoSi2 and less conductive samples. Therefore, the conductivity as well as density of sintered parts is used to evaluate the applicability of the feedstocks. Finally, glow tests prove that MoSi2/Al2O3/feldspar composite parts can be used as heating elements and by combining infrared temperature measurement data with computational simulations important material data such as thermal and electrical conductivity and thermal capacity can be obtained reliably

Resistance-based analysis of limiting interfaces in multilayer water splitting photocathodes by impedance spectroscopy

Photoabsorber materials such as Cu2O, which are normally prone to degradation reactions in aqueous environment, have regained attention for photoelectrochemical water splitting (PEC) due to the use of protective overlayers. Furthermore, by implementing an additional interlayer between the photoabsorber and protective layer, the photovoltage and the overall device efficiency can be enhanced due to the increased built-in voltage at the p–n junction. The detailed analysis of these multilayer PEC photoelectrodes under operando conditions is challenging due to the multiplicity of interfaces and charge carrier processes. To facilitate routine investigation of such multilayer systems, we have established a resistance-based method using electrochemical impedance spectroscopy (EIS) to identify the underlying potential-dependent processes of water splitting photocathodes under operation, which quickly reveals the problematic interfaces in these structures. Cu2O/Ga2O3/TiO2/RuOx and p–Si/TiO2/RuOx photocathodes were investigated, with varying thicknesses of both interlayer and protection layer. The main limitations in the Cu2O devices were found to be the Ga2O3/TiO2 interface and the surface properties of the cuprous oxide absorber (most likely Cu2+ at the surface). We demonstrate that a commonly applied etching procedure of the Cu2O to remove surface impurities reduced the associated resistance, but was not sufficient to achieve an ideal interface to the electron accepting layer. The analysis scheme enabled us to shed light on most of the involved charge carrier processes taking place in these complex systems, and we expect that this method will be applicable to other multilayer systems, facilitating a more routine and powerful operando characterization method for water splitting photoelectrodes. Furthermore the knowledge gained in this investigation will pave the way for the development of a complete equivalent circuit model of these protected buried heterojunction photocathodes

Shaping using additively printed soluble molds and support material

Additive manufacturing enables the introduction of new shaping concepts, not only through direct printing of ceramic green bodies, but also through the production of molds. The particular advantage of the almost unlimited geometry complexity of additive manufacturing unfolds when soluble molds are used. This can be used to produce undercut shapes, threads, channels, and other otherwise complicated shape elements. The use of molds instead of direct 3D printing of ceramic green bodies offers the opportunity to circumvent existing limitations of currently available technologies and materials. For example, direct printing using DLP (digital light processing) processes is limited by the optical properties of raw materials and by the properties of available resins. Even fewer ceramic/thermoplastic filaments are commercially available for the FDM (fused deposition modeling) process. These limitations can be circumvented by printing the molds using DLP and filling and curing them with ceramic materials, e.g. as thermoplastic feedstocks in injection molding or as thermosetting gelcasting slurries. Subsequently, the mold is dissolved in solvents, but preferably in water, and a precisely molded green body is obtained. A similar material concept can of course also be used for the production of soluble support structures in 3D printing. A wide variety of examples for the use of soluble molds and support structures in DLP or FDM processes will be presented

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

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction

The reductive part of artificial photosynthesis, the reduction of protons into H2, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized