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Feasibility of Room Temperature Fabrication for 3D printing of Li2MoO4 ceramics
Keywords: Room temperature fabrication, Stencil printing, Capacitor
In this work, the possibility to fabricate solid ceramic parts at room temperature directly by 3D printing without organic additives or sintering is demonstrated for the first time. This new approach is based on Room Temperature Fabrication (RTF) (Kähäri 2014, Kähäri 2015) which utilizes the water-solubility of the ceramic material, lithium molybdate (Li2MoO4) in this case, to create a small amount of aqueous phase to aid the particle packing during compression. The drying of the sample and related recrystallization of the dissolved Li2MoO4 can be speeded up with a post-processing at 120 °C. Additive manufacturing, or 3D printing, allows fast production of parts even with complex shapes without a requirement for special molds or material-subtractive post-processing. However, 3D printed ceramics in general need a secondary sintering step at high temperature to densify the product. Now the RTF method was applied to the 3D printing with material extrusion. To enable the printing, the amount of added water was increased to a point where the Li2MoO4-water-mixture reached a viscous paste-like state. The analysis of the rheological properties which determine the flow characteristics of the paste was done using a rheometer (Discovery HR-1, TA Instruments, New Castle, DE) and showed shear thinning and yielding behaviors. These facilitate a good flow of the paste under the printing pressure and subsequent setting after its deposition. Simple disc-shaped samples with three layers were printed using heated printing platform to aid shape retention of the paste as more layers were printed. The samples were dried first at room temperature followed by heat-treatments at 60 °C and 120 °C to speed up the water evaporation and to ensure complete drying, respectively. As no sintering or other pressure than that resulting from extrusion was applied, the consolidation and densification of the samples occurred during both printing and drying of the paste due to extrusion pressure, capillary forces, and recrystallization of the dissolved Li2MoO4. The microstructure, its uniformity and porosity, were studied using a field emission scanning electron microscope (Zeiss ULTRA Plus, Carl Zeiss SMT AG, Germany). The high frequency dielectric properties were measured with a non-contact method using a Split Post Dielectric Resonator (QWED, Warsaw, Poland), with a nominal resonant frequency of 9.97 GHz. Densities were measured with a liquid impregnation using the vacuum method according to the standard for characterization of parts fabricated by additive manufacturing (EN ISO/ASTM Standard 17296-3). The measured properties were compared to Li2MoO4 samples fabricated with RTF method (Kähäri 2015) and sintering (Zhou 2010, He 2014) in addition to theoretical relative permittivity calculated using porosity correction and a mixing rule. As a result, relatively high densities and good dielectric properties were achieved. No delamination of the printed layers were observed in the microstructure. As a conclusion, the results indicate that other similar ceramics or ceramic composites could be fabricated with 3D printing in the future. The use of additive manufacturing technology by the printing of soluble ceramics and corresponding composites enables a vast number of applications, for example in electronics and telecommunication, and allows significant time, cost, and energy savings compared to conventional ceramic processing.
Maria Väätäjä, Doctoral student
Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu
Oulu, Finland

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