I want to thank my advisor, Dr. Sundaram, for his guidance and support throughout my Masters. I want to thank my committee members, Dr. Junjun Ding, for their support, encouragement and knowledge. I want to thank my friends and colleagues who were by my side both in the laboratory and outside it.
This investigation summarizes work on two materials of interest for energy generation and storage applications, fully stabilized 8 mol% yttria-stabilized zirconia (YSZ) and lithium silicate powders printed with a Lithoz CeraFab 8500 lithography-based ceramic slurry printer. Lithium silicate powders mixed with Lithoz MS13B binder at a solids loading of 51 vol % sintered at 800 °C resulted in dense parts with surface anomalies and no apparent changes in bulk resistance, conductivity or actuation due to layering.
INTRODUCTION
- Ink-Jet Printing of Thin Film Ceramics
- Lithography-based Ceramic Manufacturing (LCM)
- Solid Oxide Fuel Cell
- Yttria Stabilized Zirconia
- Lithium Silicate
- Lithium Silicate Synthesis
When a portion has been lifted out of the tub, holes in the slurry must be filled. The incoming flow of oxygen into the cathode supplies oxygen ions to the electrolyte from a reduction reaction occurring in the cathode. YSZ has a cubic fluorite structure; addition of yttria stabilizes zirconia in the cubic phase.
When the material is fully stabilized in the cubic phase, changes are no longer a problem. SSR is a relatively simple method for synthesizing lithium silicate because it works by heating a mixture of lithium carbonate and silicon oxide together into the correct stoichiometry.
EXPERIMENTAL PROCEDURE
- Powder Preparation
- X-Ray Diffraction (XRD)
- BET Measurements
- Slurry Development
- Post Printing Processing
- Scanning Electron Microscopy (SEM)
- Density and Porosity Testing
- Electrical Conductivity
The rotation of the tub and the curing energy were also changed depending on the characteristics of the slurry. The slurry tank itself also tilted down away from the part before the part was raised and lowered; this helped avoid air bubbles and keeps the parts firmly attached to the build plate. The speed of the tub was raised or lowered and adjusted depending on the viscosity of the slurry in the tub.
The build plate for the printer had a piece of LithaFoil (Lithoz America, LLC, Troy, NY, USA) applied to the surface along with a piece of electrical tape around the edge to prevent stray light from escaping. LithaFoil was a thin matte plastic that was applied to the build plate glass. The plastic allowed for better adhesion of the part to the build plate while still allowing the part to be removed once the print was complete.
Some of the thicker mixes had to be heated to a higher temperature of 30°C in the printer to improve the viscosity so that the slurry could be distributed evenly in the tub. The parts were carefully removed from the LithaFoil layer on the build plate and then cleaned with LithaSol 20. The parts were preconditioned at 120°C for 2-4 days depending on the thickness of the part, thicker parts required longer preconditioning.
AccuPyc was used to determine the actual density of the materials that make up the final parts. Impedance spectroscopy was used to determine the frequency-dependent conductivity of 3D-printed sintered materials. The "surface" orientation layers were printed with layers parallel to the disk surface where the impedance testing electrodes were placed.
RESULTS AND DISCUSSION
YSZ Powders and Slurry Development
MS13B was the binder that showed the most promise with the Inframat powders and was used as the primary binder used when testing the Tosoh powders. When the powders were mixed with MS13B, at best, they had a consistency similar to peanut butter. The yellow color in the images is from the yellow lighting inside the lab, limiting the sludge's exposure to blue light.
The SEM images of the powders shown in Figure 8 showed relatively large individual particle sizes for the Inframat superfine powders. However, the data contained additional peaks and broader peaks, which meant that the powders were of lower quality compared to the Tosoh powders. All three diffractograms of the Inframat powders are shown in Figure 10, with slightly sharper peaks in the nano and superfine powders.
The primary powder tested was TZ-8YS powder because it was found to have a lower surface area compared to the other 8 mols. BET surface area measurements for TZ-8YS resulted in a surface area of 5.57 m2/g, which allowed to achieve relatively high solids loading, needed later during thermal processing of the samples. The maximum solid loading at which the TZ-8YS powders were printed was 48.5 vol %, with 42.5 vol % being the lowest tested.
Grindometer measurements were used to determine the consistency of the slurry and whether it would flow well in the press. TZ-8Y powders were also tested with a lower solids loading than the TZ-8YS powders, but still achieved a liquid suspension within the ideal solids range. Lithium silicate was ground into the binder, but the powder reacted with the resin causing the slurry to become too thick and the viscose unusable.
Printing and Sintering
For parts printed with the standard TZ-8YS MS13B 42.5% solid loading disc, the parts achieved a density of 97.2% versus the Tosoh TZ-8YS MS13B 48.5 vol % sludge was used to determine a maximum relative loading solid printable using Tosoh TZ- powders. Tosoh TZ-8Y powders have a BET surface area over twice that of the TZ-8YS powders tested.
Some of the smaller standard 10mm 3mm discs stuck together or had some minor cracks. XRD plots of Tosoh TZ-8YS were almost identical to TZ-8Y plots, with a cubic fluorite structure identified. The TZ-8YS powders, as shown in Figure 21, have nice relatively spherical particles with no large agglomerates present.
In the Tosoh TZ-8YS samples, cracks were observed between some of the printed layers, which did not compact together during sintering. Figures 22-30 are the TZ-8YS samples with smaller grain sizes compared to the TZ-8Y samples in Figures 31-34. Higher solids loading and the switch from TZ-8YS to TZ-8Y powders resulted in a decrease in identifiable cracks.
The preconditioned powders were not as clear as the powders in the TZ-8YS samples. The sintered TZ-8Y parts had smaller surface features compared to the TZ-8YS parts. The TZ-8YS grains had larger features compared to the smaller features seen on the surface of the TZ-8Y samples.
The change in surface area between the TZ-8YS and TZ-8Y parts shows some correlation between the solid loading and the particle sizes of the base powder. The TZ-8Y samples had larger grains with a slightly lower density of 93% versus 97% for the TZ-8YS samples.
Lithium Silicate
The final sintered part had sharper peaks with fewer small peaks from unreacted precursors in the material. Pressed lithium silicate powder from batch 5 was used to test the ideal sintering temperature for the printed parts. The lithium silicate slurry was initially printed with relatively thick layers of 100 μm, because the lithium silicate slurry had relatively low light absorption, which allowed the light energy from the light source to penetrate deeper into the slurry.
There was a partially cured slurry on the edges of parts that was not seen on the zirconium parts, and this was the result of light scattering in the slurry. The lithium silicate powder from batch five was used to make slurry batch number MD-208, which resulted in successful printed parts. The high solids content resulted in less binder in the slurry resulting in poorly bonded parts.
The image to the right of Figure 42 is illuminated from the yellow lighting in the Lithoz cleaning station. Sintering temperature and residence times were varied over a narrow range to improve density in the final parts. The fourth and fifth profiles included a step between binder burnout and sintering to heat any unreacted precursors in the material.
The segment held at temperatures in the SSR range is used to prepare the powder base of the samples before ramping to a final sintering temperature. The samples with a lower surface area between the layers in relation to the size resulted in the best density. The layers of materials helped release CO2 gas from the lithium carbonate through lithium silicate reaction during sintering; this is evident because samples with lower layer surface area had higher density and more open pores can be seen on a layered surface as seen in Figures 46-50.
Electrical Conductivity
The first fitted curve is the bulk resistance and the rest of the curves represent the grain boundary and electrode interface resistance. The bulk conductivity was then used to generate Arrhenius plots, which were used to find the activation energy of the printed parts. Key features tested were layer orientation to determine if there were any effects on the material's conductivity and activation energy.
The calculated average bulk activation energy for YSZ was 0.34 eV and the average for lithium silicate was 0.22 eV. Figures 55 and 56 also show Arrhenius plots for the average grain boundary activation energy for both YSZ and lithium silicate. The grain boundary activation energy for YSZ was 0.48 eV and 0.40 eV for lithium silicate.
The experimental activation energy of lithium silicate was determined for a temperature range of 125–300 °C with a printed layer thickness of 100 μm. The lowest recorded bulk activation energy found was by Adnan et al.12, which was 0.19 eV for samples prepared from sol-gel and pelletized and sintered. The 3D-printed lithium silicate samples had high porosity with an average density of approximately 85%, as well as large outgassing of unreacted precursors, creating large-scale discontinuities and defects in the materials, thereby increasing activation energy.
YSZ experimental activation energy was determined for a temperature range from 300 °C to 500 °C with a printed layer thickness of 25 μm. The experimental average bulk activation energy was 0.34 eV with Kwon et al.24 reporting bulk activation energies of 1.16 eV. The lower activation energy for the experimental 3D printed and sintered material can be attributed to electrons potentially finding a path of least resistance not found in traditionally formed parts.
SUMMARY AND CONCLUSIONS …
FUTURE WORK
