Robocasting of complex structural ceramics

Additive manufacturing (AM) has been recognized as a significant manufacturing method since it was developed in early 1980s. It is a technology usually based on 3D model files (CAD model) and layer by layer fabrication. Many AM technologies have been developed for manufacturing different range of materials such as metals and polymers while only a few of them are suitable for producing ceramic objects. Robocasting (also called direct ink writing) is a ceramic AM technology which was developed to produce complex ceramic parts. The technology is based on extrusion of a paste that has to exhibit a shear thinning behaviour. However, one of the remaining challenges is the fabrication of parts with complex hierarchical structures or combining different materials. In this thesis, porous ceramics with porosity from 10 % to 60 % and average pore size from 3 μm to 8 μm were produced by robocasting emulsion-based inks. Ceramic matrix composites with continuous steel fibres were fabricated successfully by extruding filaments with a novel core/shell structure. The composites reinforced with the steel fibres have an increasing of toughness without sacrificing the strength of the products. Density, strength and toughness of the products were measured to analyse their reliability.Open Acces

Enhanced and shortened Mn 2+ emissions by Cu + co-doping in borosilicate glasses for W-LEDs

A novel pair of transition metal ions Cu+, Mn2+ is co-doped in borosilicate glasses. Both copper and manganese ions exist in lower valence states (Cu+, Mn2+) in the as-prepared glasses. Around 5-time enhanced Mn2+ emission under the UV excitation is observed, which, as demonstrated by excitation spectra and emission decay curves, is due to an energy transfer from Cu+ ions resulting in greatly increased absorption of Mn2+ ions in the UV region, and relaxation on doubly-forbidden transition of Mn2+ leading to the much shortened Mn2+ emission lifetime from millisecond to microsecond level. Besides, a composite white emission is generated by combining the blue-green part from Cu+ ions with the green-red part from Mn2+ ions and it can be effectively tuned from cold to warm by adjusting host glass composition and altering excitation wavelength. Relevant mechanisms are discussed

Additive manufacturing of Al2O3 with engineered interlayers and high toughness through multi-material co-extrusion

The additive manufacturing of ceramic composites with tailored microstructures is still challenging and time-consuming. However, there is great interest as it may enable the implementation of novel materials architectures following computer designs. In this work, we demonstrate a straightforward method to obtain ceramics with a network of continuous weak interlayers designed to increase fracture resistance using alumina as a model system. This is achieved by combining direct ink writing with the coextrusion of multi-material pastes with carefully matched rheology based on thermally reversible hydrogels and inorganic powders. The printed Al2O3 bars with and without weak interlayers exhibit strengths ranging between 180 and 360 MPa and KIC ∼ 3 MPa∙m1/2. The introduction of weak interlayers using different raster patterns, such as length wise and Bouligand alignments can be used to direct crack propagation and promote gradual failure. The result is an improvement in the fracture energy up to 230 J/m2 and Kj up to 9 MPa·m1/2. These results suggest the potential of manufacturing ceramics with enhanced mechanical properties by using robocasting with multi-material inks to engineer complex interlayer networks

Symmetrical triangular LFM for underwater acoustic communications

Synchronous detection acts as a key role in underwater acoustic communications. It is discovered that the Symmetrical Triangular Linear Frequency Modulation (STLFM) signal has double energy peaks in their fractional Fourier transform domain and these two peaks have symmetry in their amplitude if the STLFM signal is captured in the accurate time windows for the synchronization. In this paper, we propose a synchronization detection method based on the STLFM. To verify the effectiveness of the proposed method, we apply simulations based on the Bellhop program and sea trial data collected at Wuyuan Bay, Xiamen, China. The results show that the proposed STLFM method outperforms the LFM method