Suspension-enclosing Projection Stereolithography of Bi-continuous Piezoelectric Ceramic Composites

Li He

doi:10.17077/etd.005494

Mechanical flexibility and piezoelectricity are two seemingly conflicting properties existing in state-of-the-art piezoelectric materials, i.e., a type of material producing electrical signals in response to applied mechanical stress or vice versa. A typical strategy to achieve a piezoelectric material with both mechanical flexibility and piezoelectricity is to combine ferroelectric ceramics with elastic polymers in a certain connectivity pattern (including 0–3, 1–3, 2–2, and 3–3) and thus form a piezocomposite. Among the most-studied connectivity patterns, the 3–3 piezocomposites (i.e., bi-continuous piezocomposites) appear to be a promising architecture, with its two phases continuously interconnected in three dimensions (3D). Bi-continuous piezocomposites can be constructed by first building a ceramic lattice structure and then infiltrating the lattice pores with a flexible polymer. However, due to the extremely poor machinability, shaping ceramics of complex geometry is substantially challenging for traditional ceramic manufacturing processes. Efforts have been made to fabricate the ceramic phase with various additive manufacturing (AM) processes, among which the projection-based ceramic stereolithography (CSL) process has the advantages of high speed and high accuracy. The main challenges in fabricating piezocomposites through CSL include the high viscosity of feedstock, limitation in printable geometry, microstructure control during sintering, and the structure and interface design for piezocomposites. First, it is well known that a high solid loading slurry is desired to produce a high-density final part after sintering, but this slurry exhibits high viscosity which makes it extremely difficult to coat a thin layer during CSL fabrication. Second, when fabricating complex geometries (e.g., thin wall or overhanging structures), CSL requires building extra support structures to prevent damage or deformation of the fabricated features. Removing the support structures can result in issues such as poor surface quality, high risk of cracking. Furthermore, printed green parts will be post-processed by heat treatment, i.e., de-binding and sintering, where the microstructures of ceramics undergo significant changes, e.g., densification and grain growth. It remains unclear how processing parameters (such as green density, dopant, etc.) and microstructures (such as grain size, porosity, etc.) are correlated. Finally, after a ceramic structure is obtained, polymer infiltration is performed to achieve a bi-continuous piezocomposite in which a 3D continuous phase interface is formed between the ceramic phase and the polymer phase. How the architecture of the interface influences mechanical flexibility and piezoelectricity of a bi-continuous piezocomposite has not yet been fully exploited to date. To overcome these challenges, three major research tasks have been performed in this thesis and are summarized as follows. First, the rheological behaviors of slurries with different solids loading were experimentally studied. Measurement results from frequency sweep testing, amplitude sweep test and flow curves indicate that a feedstock suitable for printing should exhibit elasto-viscoelasticity behaviors. Its viscosity can be reduced by heating during the recoating process for a precise thin-layer recoating and lower shear force. To handle this high-viscosity slurry and mitigate the requirement of building the support structure, the properties of elasto-viscoelasticity slurries were used to guide the design of a new printing system, named the suspension-enclosing projection-stereolithography process (SEPS). The suspension’s inherently strong interparticle resistive force, i.e., yield strength, was utilized to support overhangs without the need for building additional support structures. A temperature-controlled layer-coating module was designed to dynamically form a localized suspension bridge above the free surface of previously deposited materials, which allows for the application of fresh thin layers with a controlled shear force. This new process provides the potential for fabricating ceramic 3D objects with any complex overhangs, such as vascular networks, biomimetic heat exchangers, and micro-reactors, and allows for the construction of a ceramic phase of arbitrary complexity in bi-continuous piezocomposites. Second, the process-microstructure correlations were studied through microstructure characterization and piezoelectric property measurement, and a complex piezoelectric ceramic architectural was fabricated through local doping control. Samples with different weight ratios of ceramics and dopants were prepared. The results show that with a higher green density, greater grain size and lower porosity can be achieved. Different dopant materials, including ZnO and ZrO2, have been tested and perform well as a grain growth inhibitor. The results also show that the grain growth inhibition is dependent on the type of dopant and the dopant concentration. The sample with 2wt% ZrO2 showed clear boundaries between doped and undoped regions due to its high melting point, low vapor pressure, and high carbothermic reaction temperature. Additionally, piezoelectric properties of printed specimens reveal a very good correlation with the average grain size. Decreasing grain size leads to a lower piezoelectric charge constant d33 and dielectric permittivity ɛ33 but results in an increase in dielectric losses and the piezoelectric voltage constant g33. Finally, a SEPS-enabled bi-continuous piezocomposite architecture is presented to achieve mechanical flexibility and piezoelectricity simultaneously in piezoelectric materials. This architecture comprises an active ferroelectric ceramic phase and a passive flexible polymer phase, which are separated by a tailorable phase interface. Triply periodic minimal surfaces were used to define the phase interface due to their excellent elastic properties and load transfer efficiency. The SEPS process was used to manufacture this material. Postprocesses including polymer infiltration, electroding, and poling are introduced. The piezoelectric properties of the piezocomposites are numerically and experimentally studied. The results highlight the role of tailorable triply periodic phase interfaces in promoting the mechanical flexibility and piezoelectricity of bi-continuous piezocomposites.

Suspension-enclosing Projection Stereolithography of Bi-continuous Piezoelectric Ceramic Composites

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