Additive Manufacturing of Ceramics with Multiscale Heterogeneous Microstructure

The additive manufacturing (AM) of ceramic has gained significant attentions because of the difficulties in fabricating hard and brittle ceramics into complex geometry. In addition, AM provides the possibility of fabricating ceramic parts with heterogeneous microstructure on the multiscale, which cannot be achieved using the traditional methods. In today’s AM technologies, ceramics are often extruded for form a green body and then fired to a high density. However, two challenges are faced by the extrusion-based AM of ceramics with multiscale features. One is to depositing ceramics with submicrometer thickness. The other one is to embed microchannels in large-scale ceramic parts. To expand the current extrusion-based AM technology, in this thesis, new approaches of AM ceramics with multiscale heterogeneous features are demonstrated. To deposit ceramics with nano-meter thickness, the inkjet printing of sol-gel ink was developed. Dense mullite nano-ribbon was fabricated using this method with post-heat treatment. The novel single-phasic ink from the water-based mullite sol-gel precursor without any solid particles was developed to avoid the coffee ring effect. Line stability during printing is highly dependent on the printing frequency, drop spacing and substrate temperature and the sol-gel ink. It is demonstrated in this thesis that inkjet printing of sol-gel ink can have a stable processing window which would be non-existing according to the previous theoretical studies. Taking advantage of the solvent evaporation and sol-gel transition upon substrate heating, we were able to print stable lines. It is shown that the crack-free mullite nano-ribbon of the thickness can be printed directly on substrates. In Chapter III, the direct inkjet printing technique was extended to the fabrication of miniaturized scintillators based on Erbium activated YAG ceramics. Again, line instability was observed at 25°C and was significantly suppressed when the substrates were slightly heated. Dense and crack-free YAG lines were obtained after firing by adding polyvinylpyrrolidone to the precursor inks. The photoluminescence of YAG:Er3+ ceramics was optimized. In Chapter IV, an innovative method, a hybrid of extrusion freeforming and picosecond laser machining, was developed to flexibly-embedded microchannels in bulk ceramics of complex geometries. The bulk ceramic green body of complex geometries was fabricated using the extrusion method. After one green layer was extruded, in-plane microchannels with variable cross-section sizes and aspect ratios were fabricated using a picosecond laser. After the microchannels were fabricated, a cover layer was extruded. The green state processed structures were pressurelessly sintered to a bulk density of ~94%. Complex channel patterns and networks were demonstrated. During extrusion, the gaps between the adjacent filaments can be eliminated by controlling the filament spacing and the distance between needle tip and substrate. With the correct paste rheology, the cover layer did not sag into the microchannels and the bonding between layers was excellent. The laser can cut through multiple layers without damage to the bonding between layers. Due to the uniform shrinkage during pressureless sintering, the green shapes of the microchannels were well preserved. A functional multi-oxide ceramic oxygen separator was designed and fabricated in Chapter V using the hybrid method. The oxygen separator was realized by embedding 3D microchannels into the bulk BaCo0.4Fe0.4Zr0.1Y0.1O3-δ ceramic. The wall between neighboring channels was ~200 µm without any observable defects, enabling the permeation of oxygen through channel walls. Correct laser repetition rate was required to guarantee that the laser removed paste rather than partial sintered the green body