Reimagining the Shape of Porous Tubular Ceramics Using 3D Printing Technology

Recently, researchers from the Department of Chemical Engineering at the University of Bath, led by Garyfalia A. Zoumpouli, published a paper in Applied Materials Today titled "Reimagining the shape of porous tubular ceramics using 3D printing." The study demonstrates that 3D printing technology enables the fabrication of porous ceramic tubes with unconventional geometries, offering significant potential for enhancing the efficiency of separation, contact, and catalytic processes.

Original article：https://doi.org/10.1016/j.apmt.2024.102136

Research Overview

Porous tubular ceramic structures have found widespread application in separation, catalytic reactions, and gas–liquid contact processes due to their mechanical strength, chemical resistance, and high permeability to gases and liquids. The study highlights that the geometry of the porous structure—including parameters such as wall thickness, number, and diameter of channels—is crucial to process performance, as it influences surface area, flow patterns, and mass transfer resistance

By adopting additive manufacturing (3D printing), it becomes possible to design and fabricate structures with tailored geometries. Both Digital Light Processing (DLP) and Stereolithography (SLA) can be employed for ceramic fabrication. This paper presents a novel 3D printing approach that enables the production of custom-shaped porous tubular ceramic structures. The study focuses on two specific geometrical designs—sinusoidal and twisted profiles—aimed at enhancing fluid mixing

Using Computational Fluid Dynamics (CFD) simulations, the researchers systematically investigated how various design parameters influence fluid velocity patterns and wall shear stress. A particle-free DLP printing process based on titanium acrylate resin was utilized to fabricate composite structures with diverse designs. Subsequent thermal post-treatment at high temperatures yielded porous TiO₂ tubes, which successfully retained the designed sinusoidal or twisted morphologies

As shown in Figure 1A, sinusoidal tubes with varying wavelengths (λ) and peak amplitudes (α) were compared while maintaining a constant maximum outer diameter (Dₘₐₓ = 1 cm). CFD simulations confirmed that the sinusoidal geometry promotes enhanced fluid mixing by generating vortices (recirculation zones) near the tube surface (Figure 1B). Generally, higher α values and/or lower λ values increased the shear stress exerted on both the inner and outer walls, although the observed trend was not strictly monotonic (Figure 1C).

Figure 1. (A) Three-dimensional view of the axisymmetric geometry used for CFD simulations of the sinusoidal tube. (B) Velocity streamlines illustrating the effect of the peak amplitude (α) on vortex formation for a fixed wavelength (λ = 10 mm). (C) Contour plots showing the maximum shear stress (absolute values) on the inner (core) and outer (shell) tube walls, generated from 31 data points with varying design parameters. In all simulations shown, the inlet flow velocity was 0.1 m/s for both core and shell under counter-current conditions.

The twisted tubular design was generated by twisting a square channel around its central axis. The square width was selected such that its inscribed circle matched the diameter of the sinusoidal tube, ensuring both designs shared an identical cross-sectional footprint (Figure 2A). The twisted geometry markedly enhanced fluid mixing in directions perpendicular to the main flow (Ux and Uy, with the primary flow along the Z-axis), as shown in Figure 2B

CFD simulation results revealed that the mixing mechanism in the twisted tubes shares similarities with, yet also differs from, that of the sinusoidal tubes. Increasing the twist angle led to a rise in shear stress on both the inner and outer tube walls (Figure 2C). Regions of high Ux velocity components moved and merged more rapidly, while the flow vorticity increased along the main flow direction, indicating pronounced recirculation zones (Figure 2D)

Overall, the custom-designed twisted tubes exhibit rotational flow behavior that enhances fluid mixing without the need for additional baffles or static mixers.

Figure 2. (A) Three-dimensional geometry used for CFD simulations of the twisted tube. (B) Velocity contour plots showing the x-component of velocity in the horizontal plane for both straight and twisted tubes. The inset illustrates the velocity distribution at the mid-plane of the tube lumen, shown with an alternative color scale. (C) Effect of twist angle on the maximum shear stress (absolute values) along the inner (core) and outer (shell) tube walls. (D) Effect of twist angle on the maximum streamwise vorticity (absolute values) in the core and shell regions. In all simulations presented, counter-current flow was used with an inlet velocity of 0.1 m/s for both core and shell streams.
 

Initial 3D printing trials were performed using a titanium acrylate–modified resin containing 15 wt% Ti. After optimizing key printing parameters—particularly the exposure time—the researchers successfully fabricated sinusoidal and twisted tubular structures up to 10 cm in length, covering a wide range of geometric design parameters (Figures 3B and 4B). The axial (length) and radial (diameter/width and wall thickness) shrinkages showed no significant difference, indicating isotropic dimensional changes. Despite the relatively high overall shrinkage, the printed structures retained their designed shapes and aspect ratios (Figures 3C and 4C).

Figure 3. (A) CFD-guided design of the sinusoidal tubular structures. (B) 3D-printed composite structures fabricated using a titanium acrylate–modified resin containing 25 wt% Ti. (C) Sintered TiO₂ tubular structures with sinusoidal geometries featuring different wavelengths (λ) and peak amplitudes (α).

Figure 4. (A) CFD-guided design of the twisted tubular structures. (B) 3D-printed composite structures fabricated using a titanium acrylate–modified resin containing 25 wt% Ti. (C) Sintered TiO₂ tubular structures with twisted geometries featuring different twist angles.

Thermogravimetric analysis (TGA) (Figure 5A) revealed that the decomposition of organic components in the composite structures—with and without titanium acrylate—was completed within the 500–600 °C temperature range. X-ray diffraction (XRD) analysis confirmed the presence of a pure rutile crystalline phase in the sintered structures (Figure 5B), with all detected peaks corresponding precisely to those of the reference pattern.

Thermogravimetric analysis (TGA) (Figure 5A) revealed that the decomposition of organic components in the composite structures—with and without titanium acrylate—was completed within the 500–600 °C temperature range. X-ray diffraction (XRD) analysis confirmed the presence of a pure rutile crystalline phase in the sintered structures (Figure 5B), with all detected peaks corresponding precisely to those of the reference pattern.

Table 1. Mass loss and shrinkage of tubular structures printed using titanium acrylate–modified resins with different compositions during the sintering process. The table reports mean values and standard deviations of samples (n = sample size) sintered under various heating rates (2–15 °C/min) and holding times (1–15 h).

As shown in Figure 6A, micro-CT imaging reveals the internal architecture of the sintered structures. Figures 6B–E combine micro-CT reconstructions with scanning electron microscopy (SEM) micrographs, illustrating different degrees of porosity, including large voids and interconnected macropore networks.

Figure 6. (A) Different views of a micro-CT volume rendering of a segment of the sinusoidal tube. (B, C) Top-section views of the corresponding micro-CT reconstructions at the positions indicated by red dashed lines. (D, E) FESEM micrographs of the same structures. Printing and sintering conditions: 3D printing using 25 wt% titanium acrylate resin, sintered at 1050 °C with a heating rate of 5 °C/min for 3 hours.

As shown in Table 2, the combination of a porous architecture with a hollow tubular design significantly influenced the mechanical properties of the specimens, particularly their compressive strength.

Table 2. Porosity and mechanical properties of 3D-printed ceramic tubes (average of two samples).

Conclusions
In this study, high-resolution DLP 3D printing was successfully employed to fabricate highly porous, nonconventional TiO₂ tubular structures. The designs were optimized using CFD simulations, demonstrating superior fluid mixing performance compared to conventional straight tubes. The structures retained their shape during thermal conversion, and their properties were found to be closely dependent on the titanium acrylate content in the starting resin. These optimized tubular architectures hold significant potential for reducing contamination and enhancing mass transfer efficiency, contributing to energy savings across multiple applications.